The recent discovery of black titanium dioxide (TiO2) has spurred intense and wide interests in the fundamental and applied properties of this material. A general overview as well as the recent progress will be presented, on both experimental findings and theoretical studies.

Titanates are a remarkable class of compounds with properties that have made them the materials of choice for many, diverse, applications. The presentation will focus on nanostructured TiO2, especially the anatase and TiO2-B polymorphs in the form of nanotubes, nanowires, mesoporous solids and nanoparticles, with dimensions as low as 3-4nm. The synthesis, characterisation and applications of these materials will be discussed. Consideration will be given to the challenge of characterising the atomic arrangement in nanomaterials with dimensions below the limits of conventional crystallography. The relationship between the structure/morphology and the behaviour of the materials as anodes in rechargeable lithium batteries will be considered in some detail.

Self-organized TiO2 nanotubes can be fabricated by electrochemical anodisation of titanium metal layers. The nanotube diameter and length can be adjusted by the anodisation conditions (current, Time and electrolyte composition). The average diameter is around 80 nm, the wall thickness 20 nm and the length is up to 2 micron. The 1D structure of the nanotubes can be very useful for the development of 3D microbatteries, because the Li insertion rate and capacity can be significantly enhanced. In this presentation, we will discuss the Li-ion insertion capacity as function of cycle number and discharge current in pure TiO2 nanotubes, in doped and composite systems. Furthermore, we will present the conformal electrodeposition of Li-ion conducting polymer inside the nanotubular TiO2. This approach allows now to envisage the fully electrochemical fabrication of a 3D Li-ion microbattery.
References:
1. G. Ortiz, I. Hanzu, T. Djenizian, P. Lavela, J.-L. Tirado, P. Knauth, Alternative Li-ion battery electrode based on self-organized titania nanotubes, Chem. Mater., 21, 63-67 (2009).
2. I. Hanzu, T. Djenizian, G. F. Ortiz, P. Knauth, Mechanistic Study of Sn Electrodeposition on TiO2 Nanotube Layers: Thermodynamics, Kinetics, Nucleation, and Growth Modes, J. Phys. Chem. C, 113, 20568-20575 (2009).
3. T. Djenizian, I. Hanzu, P. Knauth, Nanostructured negative electrodes based on titania for Li-ion microbatteries, J. Mater. Chem. 21, 9925-9937 (2011).
4. I. Hanzu, T. Djenizian, P. Knauth, Electrical and Point Defect Properties of TiO2 Nanotubes Fabricated by Electrochemical Anodization, J. Phys. Chem. C, 115, 5989-5996 (2011).

Heterogeneous photocatalysis is widely recognized as one of the cheapest and most efficient methods for solar decontamination of water and air from toxic organic pollutants. The most commonly used semiconductor is titanium dioxide which excels by the unique combination of low cost, non-toxicity, excellent stability against photocorrosion, and possibility for further functionalization. Moreover, apart from photocatalytic degradation reactions, in recent years we have become also increasingly aware of the great potential of TiO₂ for photocatalytic synthetic reactions, including selective photooxidation of organic compounds like alkanes or alcohols. Recently we reported an interesting example of successful unification of these two concepts: a novel type of surface-modified TiO₂ with improved photocatalytic degradation performance is prepared by a photosynthetic route involving visible light-induced activation of benzene or toluene [1,2].
It is well-known that irradiation of TiO₂ by UV light in the presence of benzene or toluene leads to brownish coloration of TiO₂ due to formation of stable polymeric products of photooxidation of the aromatic compound. Surprisingly, we observed that even irradiation by visible light (wavelength > 455 nm) leads to brown coloration of TiO₂. Moreover, the thus prepared modified TiO₂ was found to be more active in visible light-induced degradation of 4-chlorophenol than unmodified TiO₂. Detailed mechanistic investigations have shown that the better photodegradation properties are due to changes of adsorption/desorption properties of TiO₂ modified at the surface with carbonaceous deposits. This study thus presents one of the rare examples in which the modification of TiO₂ for enhanced photoactivity was achieved via an artificial photosynthesis route. At the same time it highlights the importance of surface engineering of heterogeneous photocatalysts in order to enhance photodegradation efficiencies. The talk will also discuss in detail the mechanistic aspects of the visible light-driven photosynthetic route involved.
References
[1] A. Ramakrishnan, S. Neubert, B. Mei, J. Strunk, L. Wang, M. Bledowski, M. Muhler, R. Beranek, Chem. Commun.2012, 48, 8556.
[2] A. Ramakrishnan, S. Neubert, B. Mei, J. Strunk, L. Wang, M. Bledowski, D. A. Guschin, M. Muhler, R. Beranek, in preparation.

A three-dimensional hierarchical TiO2 nanostructured array is constructed for photoelectrochemical (PEC) water splitting. The photoelectrode is composed of a core-shell structure where the core portion is a rutile TiO2 ND array and the shell portion is rutile and anatase TiO2 nanoparticles (NPs) sequentially located on the surface. The ND array provides a fast electron transport pathway due to its quasi-single-crystalline structure. NPs in the shell portion provide a larger surface area for more efficient photocharge separation. The anatase TiO2 NPs constructed on the surface of the ND/rutile TiO2 NP nanostructured array further enhance charge separation and suppress charge recombination at the interfacial region due to the higher conduction band edge of anatase TiO2 compared to that of rutile TiO2. Through both morphology and interfacial energetics controls for the PEC photoelectrode, a photocurrent density and photoconversion efficiency of 2.08 mA cm-2 at 1.23 V vs. RHE and 1.39% at 0.55 V vs. RHE are respectively attained using the hierarchical TiO2 nanostructured array photoelectrochemical cell under illumination of AM 1.5G (100 mW cm-2).

Nanostructured WO3/TiO2 composite materials were developed, characterized and tested for solar photoconversion and electrochromic applications. This resulted in a low-cost synthetic approach to prepare composite WO3/TiO2 nanotubes that can be detached from a Ti foil substrate and transferred to different substrates, including glass. SEM images show that these materials have the same ordered structure as TiO2 nanotubes, with an external nanostructured WO3 layer. Diffuse reflectance spectra show an improvement in the visible absorption relative to bare TiO2 nanotubes and in the UV absorption relative to bare WO3 films. Photo-electrochemical studies were conducted by employing these materials as photoanodes. Incident photon-to-current efficiency (IPCE) increased from 30% (for bare WO3) to 50% (for WO3/TiO2 composites) and extended up to the visible region (575 nm). With the addition of methanol, a model organic pollutant, the photo-currents exhibited more than a 5-fold increase. Chemical oxygen demand (COD) measurements showed the simultaneous photo-degradation of methanol. In addition, the materials were tested for electrochromic applications. The composite WO3/TiO2 nanostructures showed 10 times higher ion storage capacity and enhanced electrochromic contrast compared with the pure WO3 and TiO2. The cycling stability of these composite nanostructures was superior compared with WO3 films. The results of this work indicate that the unique structure and composition of these composites materials enhance the charge carrier transport and optical properties compared with the parent materials. In particular these materials showed a great potential for solar photoconversion including wastewater treatment and water splitting, in addition to electrochromic applications, including smart windows.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

In recent years, solar cells incorporating colloidal nanocrystal quantum dots (QDs) as the absorbing material have emerged as leading class of third generation photovoltaics. While the highest efficiencies are somewhat low today (~7%), the rate of improvement is faster than any other type of solar cell technology with a slope of ~1.5%/yr. With bulk band gaps of 1.0-1.5eV, one of the largest extinction coefficients among direct gap semiconductors, high carrier mobilities, and lack of toxic components, CuInSexS2-x shows great promise as the absorbing component of solar cells. We have shown that gram scale quantities of CuInS2 quantum dots can be synthesized with a range of sizes (and increased band gaps) with 90%+ chemical yield. More recently, we have modified the synthesis to incorporate controlled amounts of Se to shift the band gap to the near-IR without increasing quantum dot size. By applying mild Cd or Zn treatments to the dots, their photoluminescence (PL) can be enhanced more than 400-fold and the exciton lifetime increases by an order of magnitude without significant changes in absorption. High PL quantum yields imply suppressed non-radiative recombination pathways, crucial for high efficiency photovoltaics. Composition analysis indicates a selective surface-cation exchange gives rise to the enhanced PL. Electron transfer from CuInSexS2-x QDs to meso-porous TiO2 is not significantly hindered by the mild cation-exchange while the increased exciton lifetime results in higher photocurrent. In collaboration with Sharp Corporation, we are sensitizing meso-porous TiO2 with CuInSexS2-x QDs to fabricate high efficiency QD-sensitized solar cells (QDSSCs). On-going device studies are giving new insights into strategies for enhancing the performance and durability of QDSSCs including the importance of reducing recombination pathways such as surface traps without deterring charge transfer. For achieving record high efficiencies, reducing series resistance with an improved counter electrode and increasing light&’s path length with a back scattering layer of larger sized TiO2 nanocrystals are also critical.

Dye-sensitized solar cells (DSC) are attractive alternatives to conventional solid-state photovoltaic devices because of performance, stability, environmental compatibility and cost. In contrast to the conventional systems where the semiconductor assumes both the task of light absorption and charge carrier transport, these two functions are separate in DSSC and, therefore, efficiency is very sensitive to the cell structure/composition. High-efficiency DSCs based on mesoporous nanocrystalline titanium dioxide (TiO2) electrodes have received considerable research attention in the past decade. Grain size and thickness of the mesoporous TiO2 film have shown a dominant effect on the efficiency of the photovoltaic devices.
We have investigated screen printed TiO2 films deposited on a textured fluorinated SnO2 (FTO) glass substrate, using Raman spectroscopy and spectroscopic ellipsometry. Materials were prepared by repeating a same screen-printing procedure once, twice and three times, using TiO2 paste with 10nm, 20 nm and 200nm particles sequentially. All samples have been annealed at 450C in air. Raman spectra of PV devices cells taken at different excitation (266 nm, 325 nm, 364 nm, 532 nm, 633 nm, and 1064 nm) as well as pure TiO2 oxides are presented. Different excitation wavelengths allow to probe different depth of the sample. It was found that there is strong correlation of the position and the width of E2g mode of anatase at 146 cm-1 and size of TiO2 particles. The samples show that this peak shifts to the high frequency region and becomes broader for small size particles. The position and broadening of the peak can be described by optical confinement model that depends on the size of nano-crystals. Results showed varying grain sizes as correlated with different TiO2 paste applied. Thickness, optical constants and porosity of TiO2 films were determined by spectroscopic ellipsometry. In this work, we have demonstrated the use of Raman Spectroscopy and Spectroscopic Ellipsometry for non-destructive characterization of nanocrystalline TiO2 films for Dye-sensitized solar cells.

Narrowing the bandgap of wide-bandgap semiconductor photocatalysts like anatase TiO2 by introducing suitable heteroatoms has been actively pursued for increasing solar absorption. We found that the spatial distribution of dopants can play a vital role in controlling electronic structures and thus light absorption range/absorbance. It is concluded that surface doping only leads to a small shoulder-like visible light absorption band and homogeneous doping can result in the effective red-shift of the whole light absorption range. Homogeneous nitrogen doping has the merits of obtaining high visible light absorbance and high charge-carrier mobility. To overcome the difficulty of introducing dopants into the bulk, we used a layered structure to realize homogeneous doping. However, introducing dopants into the bulk of photocatalysts remains a challenge for nonlayered photocatalysts, for example TiO2.
To solve this problem, we developed a new synthesis strategy for obtaining {001} terminated anatase TiO2 microspheres with a boron contained core. The subsequent thermal treatment can lead to the diffusion of boron from the core to the surface shell. It is found that the preference towards important photocatalytic hydrogen and oxygen producing reactions from splitting water can be sensitively switched by creating such a shell with an interstitial boron ion gradient in the TiO2 microsphere. This switching stems from the downward-shift of electronic band edges of the shell by a band bending effect, whose origin are the extra electrons coming from the interstitial boron ions. Similar strategies can also be used for the preparation of boron doped rutile TiO2 microspheres.
With the above anatase TiO2 microspheres, we obtained red anatase TiO2 microspheres with a bandgap gradient varying from 1.94 eV on its surface to 3.22 eV in its core for harvesting the full spectrum of visible light. This approach uses the pre-doped interstitial boron gradient to weaken the nearby Ti-O bonds for the easy substitution of oxygen by nitrogen, and consequently it improves the nitrogen solubility. Furthermore, no nitrogen-related Ti3+ was formed in red TiO2 due to a charge compensation effect by boron. The red anatase TiO2 exhibits photoelectrochemical water splitting activity under visible light irradiation. The results obtained may shed light on how to increase high visible light absorbance of wide-bandgap photocatalysts.
References
(1) Liu, G.; Wang, L. Z.; Sun, C. H.; Yan, X. Wang, X. Chen, Z.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Chem. Mater. 2009, 21 (7), 1266.
(2) Liu, G.; Pan, J.; Yin, L. C.; Irvine, J.; Li, F.; Tan, J.; Wormald, P.; Cheng, H. M. Adv. Funct. Mater., 2012, 22 (15), 3233.
(3) Liu, G.; Yin, L. C.; Wang, J. Q.; Niu, P.; Zhen, C.; Xie, Y. P.; Cheng, H. M. Energy Environ. Sci., 2012, Accepted.
(4) Liu, G.; Lu, G. Q.; Cheng, H. M. and et al. Submitted.

Titanium dioxide has a wide range of potential and realized applications. The ability to control the structural characteristics of the titanium dioxide influences the properties of the material. For example, controlling the particle size and the porous structure affects the surface area and surface accessibility. In this talk the use of templating techniques to engineer the morphological features of titanium dioxide will be discussed. Variation in material shape, size, and pore structure can be achieved using different templates and synthesis techniques. The overall aim of the carefully controlled structural engineering is to improve the efficiency of the titanium dioxide in applications. This will be demonstrated in photocatalytic, biomaterial, and dye-sensitized solar cell applications.

TiO₂ nanoparticles were decorated onto the step edges of highly oriented pyrolytic graphite (HOPG) via physical vapor deposition. Photodeposition of Pt was accomplished by submerging a TiO₂ sample in an aqueous solution of 0.25 mM K₂PtCl₄ and 0.5 mM sodium citrate followed by photolysis with TiO₂ bandgap radiation (365 nm radiation from a 200 W Mercury lamp). Similarly, Ag nanoparticles were deposited by photolysis of a TiO₂ sample in an aqueous solution of 0.25 mM Ag(NO)₃ and 0.5 mM sodium citrate. Gold nanoparticles were deposited on TiO₂ nanoparticles using a photoelectrochemical cell whereupon a photocatalytic reduction mechanism was verified by photocurrent measurements. Samples of TiO₂ nanoparticles on HOPG, acting as a photoelectrode, were placed in a half-cell and immersed in either an electrolyte solution of 1.0 M NaCl or 1.0 M NaNO₃. Bare HOPG, acting as a counter electrode, was placed in a second half-cell and immersed in the same electrolyte solution. The two half-cells were connected by a salt bridge and the electrodes by a picoammeter. Upon irradiation of the TiO₂ nanoparticles by 365 nm UV light, photogenerated electrons produced a photocurrent. Subsequent to introducing 1 mL of le;15 mu;M HAuCl₄ into the cell containing the TiO₂ nanoparticles, the photocurrent decreased as a result of the reduction of Au³⁺ to Au on the TiO₂ nanoparticles or nitrate reduction. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray dispersive spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS) were used to characterize the morphology, crystal structure, and chemical identity of the nanoparticles.

Titanium(IV) oxide (TiO2), as a photoactive support for photocatalysts and sensitizers, has applications in catalysis and energy science. Metal nanoparticles supported on TiO2 have been shown to have unusual reactivity. To understand the influence of metal deposition, we photodeposit platinum (Pt) onto ordered linear arrays of TiO2 nanoparticles grown at the step edges of highly oriented pyrolytic graphite. Utilizing in-situ x-ray spectroscopies, we explore the electronic structure of the TiO2 nanoparticle arrays with and without photodeposited Pt. The Ti L-edge and O K-edge absorption spectra combined with atomic multiplet calculations provide information on crystal field effects and multiplet interactions, helping to determine the phases of the TiO2 particles. Valence photoemission results and band structure calculations indicate a narrowing of the TiO2 band gap when Pt is loaded onto the TiO2 nanoparticle surface. This suggests that Pt photodeposition onto linear TiO2 nanoparticle arrays may enhance the overall solar absorption.

In order to develop efficient dye-sensitized TiO2 photoelectrodes, both larger surface area and longer optical path length of the incident photon must be required. Therefore, most of the TiO2 films photoelectrodes are composed of not only nanoparticles (about 20 nm in diameter) but also light-scarttering particles (100~400 nm in diameter). Recently, it was reported that conversion efficiency increased by using nanoporous TiO2 spheres, which were made up of 10 nm particles and have a diameter of 250~450 nm, instead of simple large light-scarttering particles.[1] It is also reported that nanoporous TiO2 spheres have larger surface area and they can serve as light-scarrtering particles. [2]
In this study, nanoporous TiO2 spheres were firstly applied to Black-dye-based DSCs as light-scattering particles, and the effects on the solar cell performance were investigated in detail. Optical properties of the black-dye-sensitized TiO2 photoelectrodes drastically changed when nanoporous TiO2 spheres were used, and the conversion efficiency was improved up to 10.5%. In this case, Jsc value increased even though the amount of dye adsorption was not largely changed. Since the transmittance at 800 nm was decreased with increasing the light harvesting efficiency (LHE), this enhancement of the Jsc value was mainly attributed to the superior light-scattering ability of TiO2 spheres. Further improvement of the conversion efficiency up to 10.9% was achieved by using the TiO2 film having thicker light-scattering layer of nanoporous TiO2 spheres. These results clearly indicate that nanoporous TiO2 sphere is effective as a light-scattering layer component in terms of efficiency improvement. Furthermore, a significant efficiency improvement of the Black-dye-based DSC was observed by dye-cocktail system [3] of Black dye and organic dye D131 with co-adsorbent, deoxycholic acid (DCA). Efficiency of DSC with Black dye, D131 organic dye and DCA increased up to 11.5%. The presence of D131 with Black dye improved IPCE value at around 500 nm, where IPCE of single Black dye system is quite low. Co-sensitization of Black dye and D131 occurred very effectively, however, aggregation of Black dye did still occur. DCA addition reduced the aggregation of Black dye. Therefore, three components, Black dye, D131 and DCA are essential for significant efficiency improvement. The mechanism of efficiency improvement will be discussed in detail.
Acknowledgement
This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.
References:
[1]W. I. Lee, et al., Adv. Mater. 21, 3668 (2009).
[2] W. I. Lee, et al., J. Mater. Chem., 21, 532 (2011).
[3] H.Arakawa, et al., RSC Advances, 2, 3198(2012).

Titanium dioxide is the most studied material for solar energy-induced water splitting. However, its band gap of 3-3.2 eV constrains it to absorbing less than 5 % of the integral energy in the solar spectrum. Sensitization of TiO₂ to visible light by localized surface plasmon resonance (LSPR) on gold nanoparticles has been investigated as a means of enhancing energy conversion efficiencies and photocurrent densities / specific rates of water splitting.
Photon absorption and scattering by LSPR can be tuned by changing the size and shape of the nanoparticles on TiO₂. Recent reports have claimed such TiO₂/Au photo-anodes generated photocurrent with efficiencies of 5-10% both in visible light and infrared [1], whereas others have reported little or no effect [2]. Literature suggests that the effect may be present with doped rutile and TiO₂ nanotubes with impurities, but not with pure anatase. Furthermore, different mechanisms , such as electron injection from the metal nanoparticle, plasmonic resonant energy transfer (PRET), and the effect of the electric near-field have all been suggested for TiO₂ and other oxides [3].
We have studied 10-200 nm gold nanoparticles deposited on TiO₂ by: annealing metal thin films, micellar nanolithography, and nanosphere lithography, to probe the dependence of photocurrent on nanoparticle architecture, such as the presence of a titanium interlayer. Single crystals of Nb-doped (0.05% wt) rutile with the (110) orientation and undoped rutile (110) reduced in hydrogen were used as the substrates. Interestingly, though both materials exhibited absorption of visible light, only the Nb-doped rutile produced photocurrents with visible light corresponding to the plasmon resonance of the gold nanoparticle. Whilst measured incident photon to current efficiencies of the Nb-doped TiO₂ / Au system were only ca. 0.01% at 600 nm, an effect was clearly absent with the undoped rutile, implying the mechanism may depend on dopant type and concentration, so suggesting directions for further analysis.
The presence of the gold nanoparticles significantly decreased photocurrents under UV light and hence overall performance. Therefore, it is necessary to try to understand the mechanism to be able to optimize the system and achieve an overall gain in solar to hydrogen efficiency: the various mechanisms proposed in the literature will be discussed.
1. Nishijima, Y., et al., Plasmon-Assisted Photocurrent Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO₂ Electrode. The Journal of Physical Chemistry Letters, 2010. 1(13): p. 2031-2036.
2. Liu, Z., et al., Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination. Nano Letters, 2011. 11(3): p. 1111-1116.
3. Warren, S.C. and E. Thimsen, Plasmonic solar water splitting. Energy & Environmental Science, 2012. 5(1): p. 5133.

The controlled synthesis of anatase titaniumdioxide (TiO2) with both high surface area and defined facet is technologically important for its application in photocatalysis, photoelectrochemical cells, and solar cells. Recently, Yang et al reported anatase TiO2 crystals with exposed 47% {001} facet by using HF as capping group via simple hydrothermal route. After that, many effort have been worked on synthesizing TiO2 anatase crystals with high exposure percentage of high surface energy using containing F compound. Due to highly toxicity and corrosion of fluoride compound, fluorine free synthesis route is necessary. Herein, we report a simple and fluorine free hydrothermal method to synthesize hierarchically nanostructured mesoporous anatase TiO2 spheres (MATS), which were covered with {001} facets. Mild H2SO4 was used as both a phase-inducer for the formation of the anatase phase and a capping agent to promote oriented growth and formation of {001} facets. Detailed XRD and SEM studies suggested that formation of MATS follows a typical nucleation and growth process. The refining or reconstruction of TiO2 crystal structure during growth resulted in a mesoporous crystalline framework that exhibits enhanced adsorption and photocatalytic degradation of rhodamine B in comparison with that of commercial Degussa P25 TiO2. In order to further improve the photocatalytic activity of TiO2 spheres, submicron TiO2 spheres were obtained by adding the polymer into the reaction. The photocatalytic activity was improved due to high surface area. On the other hand, noble metal nanoparticles like Ag, Au were deposit on the surface of TiO2 crystal, the photocatalytic activity under visible light was observed.

The photoactive properties of titanium dioxide (TiO2) nanomaterials are very attractive allowing many applications in the environmental field including photocatalysis, super-hydrophilicity, energy storage and photovoltaics. In the latter domain, optimizing collection and transport of photogenerated carriers requires the elaboration of a nanostructured and/or porous layers, therefore the surface/volume ratio increases. Most of these coatings are commonly elaborated by sol-gel technology which generally requires an annealing step at high temperature (at least 450°C) for the crystallisation of TiO2. This thermal treatment has many disadvantages: decrease of the adherence to the substrate, partial destruction of the micro /nano-porosity and incompatibility with all-plastic substrates. We present an original procedure for synthesizing TiO2 anatase nanocrystals (NCs) from the hydrolysis of the precursor [Ti8O12(H2O)24]Cl8.HCl.7H2O [1] in an ethanol-based mixture with the presence of surfactants in mild solvothermal conditions (le;180°C). Particularly, it has been possible to obtain monodisperse anatase NCs with controlled shape and size by properly adjusting the molar ratio between Ti and the surfactants, the nature of surfactants and the temperature. Indeed, surfactants can selectively be adsorbed on specific crystallographic plans allowing an accurate control over the morphology of the NCs by inhibiting some growth directions. Thus, TiO2 NCs with different shapes, like rhombic and truncated rhombic, were obtained with preferential (101) and (001) orientations. In addition, these NCs have been stabilized as colloidal solutions which allowed us to carry out electrophoretic deposition (EPD). Finally, with this approach, we have successfully deposited dense or porous nanostructured films of crystallized TiO2 at room temperature on various substrates (FTO/glass, ITO/PET hellip;). The TiO2 NCs were characterized by TEM, XRD, Raman diffusion and IR spectroscopy. The size of colloids was investigated by photon correlation spectroscopy and the stability of colloidal solutions was evaluated by zetametry. Finally, the deposits were studied by SEM, EDX and XPS.
[1] Titanium aquo-oxochloride and preparation method thereof. L. Brohan, H. Sutrisno, E. Puzenat, A. Rouet, H. Terrisse. - French CNRS patent delivered N° 0305619 (07/07/2006) ; International Publication n° WO 2004/101436 A2 ( 25/11/2004) ; European (EP) CNRS patent n° 04 742 604.4 (24/11/2005) ; Japan (JP) CNRS patent n°2006-530327 (16/10/2006), delivered January 25, 2011.; United States (US) CNRS patent n° 018344/0578 (04/02/2006), delivered 18/08/2010.

Titanium oxide is one of the most widely studied materials for photochemical electrodes because of its high photocatalytic activity, which could be exploited for photovoltaic cells^1-3,water splitting to produce hydrogen⁴, and other uses including environmental purifications. In particular, nanostructured TiO₂ is a very attractive material due to its large surface area and high electrochemical catalytic activity. One-dimensional TiO₂ nanostructures such as nanorods, nanotubes, and nanowires have recently emerged as promising building blocks for the new generation of nanoscale devices. However, there is a fundamental problem in that the relatively large band gap of TiO₂ (i.e., 3.2 eV for TiO₂ anatase) permits the absorbance of only the UV component of the solar light. To utilize a wider range of solar spectrum for energy harvesting, narrowing of the band gap of TiO₂ is important. Thus, an effective TiO₂ photocatalyst can be developed if both the electronic structure (material band-gap) and the geometric structure (nano dimension) are considered.
We have investigated nitrogen-doped TiO₂ electrodes consisting of extremely small (~8 nm) diameter nanotubes, which allows an easier nitrogen incorporation. The hydrothermally synthesized nanotubular structure provides a much larger surface area, and also, the one-dimensional nanogeometry is beneficial for light-harvesting because of the continuous electron conducting path. We have successfully introduced a substantial degree of nitrogen-doping into the 8nm TiO₂ nanotubes for effective light harvesting, as the band gap could be noticeably reduced, thus enabling a higher photocatalytic activity under visible light irradiation.
We have conducted water splitting experiments using the nitrogen-doped, 8 nm TiO₂ nanotubes and performed comparative measurements of electric properties, photo-responses, and hydrogen generation. These highly nitrogen-doped, band-gap adjusted TiO₂ nanotubes seem to be active photocatalysts well suited for solar fuel hydrogen production.
References
1. B. O&’Regan, and M. Grätzel, Nature, 1991, 6346, 737.
2. J. Khamwannah, S. Y. Noh, C. Frandsen, Y. Zhang, H. Kim, S. D. Kong, and S. Jin, J. Renewable and Sustainable Energy, 2012, 4, 023116.
3. J. Khamwannah, Y. Zhang, S. Y. Noh, H. Kim, C. Frandsen, S. D. Kong, and S. Jin, NanoEnergy 2012, 1, 411.
4. J. Nowotny, T. Bak, M. K. Novotny and L. R. Sheppard, Int. J. Hydrogen Energy, 2007, 32, 2609.

TiO2 nanostructures are being extensively studied for the solar energy applications, such as DSSC, QDSSC, photocatalysts and solar water splittings. When working as photoelectrodes in these energy devices, TiO2 nanomaterials with tailored structural complexity and appropriate heterojunction formation will provide new pathways for performance enhancement. In this talk I will discuss the application of two types of TiO2 nanostructures (1D nanorods and 3D inverse opals) in photoelectrochemical (PEC) sacrificial water splitting. Chalcogenide quantum dots (QDs) and nanorods are fabricated using either SILAR, ion-exchange reaction, or CVD methods. Their sensitizer properties to TiO2 will be presented and systemically discussed in terms of bandgap and band alignments at the TiO2-sensitzer interfaces. A novel method by combining atomic layer deposition and ion exchange reaction (ALDIER) towards homogeneous coating of photosensitzer on arbitrary TiO2 substrates will be introduced. Furthermore, a smart combination of 3D inverse opal with 1D nanorods allows enhanced light harvesting and higher sensitizer loading, and subsequently higher photocurrent levels.
References
1. Li, H. X.; Cheng, C. W.; Li, X. L.; Liu, J. P.; Guan, C.; Tay, Y. Y.; Fan, H. J., Composition-Graded ZnxCd1-xSe@ZnO Core-Shell Nanowire Array Electrodes for Photoelectrochemical Hydrogen Generation. J Phys Chem C 2012, 116, 3802-3807.
2. Cheng, C. W.; Karuturi, S. K.; Liu, L. J.; Liu, J. P.; Li, H. X.; Su, L. T.; Tok, A. I. Y.; Fan, H. J., Quantum-Dot-Sensitized TiO2 Inverse Opals for Photoelectrochemical Hydrogen Generation. Small 2012, 8, 37-42.
3. Luo, J. S.; Ma, L.; He, T. C.; Ng, C. F.; Wang, S. J.; Sun, H. D.; Fan, H. J., TiO2/(CdS, CdSe, CdSeS) Nanorod Heterostructures and Photoelectrochemical Properties. J Phys Chem C 2012, 116, 11956-11963.
4. Karuturi, S. K.; Luo, J. S.; Cheng, C. W.; Liu, L. J.; Su, L. T.; Tok, A. I. Y.; Fan, H. J., A Novel Photoanode with Three-Dimensionally, Hierarchically Ordered Nanobushes for Highly Efficient Photoelectrochemical Cells. Adv Mater 2012, 24, 4157-4162.
5. Luo, J. S.; Karuturi, S. K.; Liu, L.; Su, L. T.; Tok, A. I. Y.; Fan, H. J., Homogeneous Photosensitization of Complex TiO2 Nanostructures for Efficient Solar Energy Conversion. Sci Rep 2012, 2, Artn 451.

Titanium oxide materials are of great interest in the conversion and storage of energy. Often the performance in such applications is critically dependent on the nanoscale morphology and the interfaces present in the respective materials. Here we discuss different strategies for controlling the nanoscale morphology of titania-based materials, including synthesis and assembly of ultrasmall nanoparticles to create mesoporous materials with extremely high surface areas, hierarchical scaffold structures (1), and biotemplating with shape-persistent templates. The resulting porous titania structures are being investigated in excitonic solar cells with different hole transport materials and show high efficiency in very thin film structures. On the other hand, nanoscale porous networks of lithium titanate created according to the above strategies feature ultrafast lithium insertion in electrochemical energy storage (2). We will discuss the impact of nano-morphology on transport behavior and efficiency of titania-based materials for energy conversion and storage.
(1) Mandlmeier, B., Szeifert, J., Fattakhova-Rohlfing, D., Amenitsch, H., Bein, T., J. Am. Chem. Soc. 133 (2011) 17274-17282.
(2) Feckl, J. M., Fominykh, K., Döblinger, M., Fattakhova-Rohlfing, D., Bein, T., Angew. Chem. 51 (2012) 7459-7463.

Noble metal incorporated titanium dioxide structure has been studied as enhanced localized surface plasmon resonance (LSPR) which improves the photo-catalytic activity. In this research, silver/titanium dioxide open core shell nanostructure was designed and fabricated as one of the best candidates for photo-catalytic nanostructure, because of its unique structure for enhanced LSPR near the surface reaction sites. Three different geometry of plasmonic inserted silver in the open core shell nano structure were fabricated by nano-imprint technology, electrodeposition and annealing process with unique fabrication design. Various silver nano structures (silver nanowire, nanorod and nanodot (sphere)) shelled with titanium dioxide thin film were formed by control of silver coverage area and thermal expansion of imprinted resin during annealing process. Photo-catalytic behavior of each different nanostructure was investigated that can be altered by geometry change as expected though simulation and absorption results. Photo-catalytic activity of each different nano structure array was examined by photo-catalytic decolorization of methylene blue solution. Electric near field amplitude of silver/titanium dioxide open core shell nanowires, which shows the enhanced photo-catalytic behavior were simulated. Titanium dioxide single layer film and silver/ titanium dioxide dual layer film were fabricated in same condition and investigated to understand the geometrical effect and junction effect though comparison analysis. Phase change of gel-like titanium dioxide to anatase was also considered to confirm the photo-catalytic behavior enhancement. Enhanced photo-catalytic behavior is observed and confirmed by improvement of LSPR at near of surface as well as increased absorption by LSPR red shift and phase change with open core shell nano structure in this research.

TiO2-based materials are widely researched for environmental remediation applications as titania is non-toxic, readily available and inexpensive.1 However, TiO2 suffers from a relatively low photocatalytic activity caused by (i) a wide band gap which requires high energy UV radiation (only 5% of sun light) to induce the reaction; (ii) the recombination of photon induced electron and hole pairs; and (iii) the recovery issues of photocatalyst nanoparticles after the photocatalytic reaction.2 Hence the ability to either decrease the band-gap of titania to allow photoactivity on irradiation with visible light or decrease the electron/hole recombination rate is attracting more attention.3 It has also been reported that the preparation method plays an important role in determining the photocatalytic activity of the final product.4
In this presentation, a rational design of titania materials using composition control together with morphology control to address above issues will be discussed. In the main content, it will be focused on solvothermal treatments combined with sol gel chemistry to prepare the mesoporous titania materials. The procedure involves using sodium hydroxide to control the hydrolysis of titania and the resultant salt as a template to produce the porous structure, coupled with sol-gel technique to obtain the titania materials. Various morphologies of fibres, rods and spherical nanoparticles were achieved depending on the hydrolysis and condensation rate of titania. The properties of the resultant materials, including surface hydroxyl group, porosity, crystal phase and crystal size, optical property and morphology were examined. The photocatalytic efficiency of the materials was assessed by studying the photodecomposition of methylene blue under visible light. Both the material properties and photocatalytic activity varied as a function of pH during the synthesis. The material with a fibre structure prepared at pH 11.5 showed highest photocatalytic activity as compared with rods and spherical nanoparticles. Furthermore, the fibre like structure was beneficial for the recovery after photocatalysis reaction.
1.X. Chen and S. S. Mao, Chemical Reviews, 2007, 107, 2891-2959.
2.X. D. Wang and R. A. Caruso, Journal of Materials Chemistry, 2011, 21, 20-28.
3.X. D. Wang, M. Blackford, K. Prince and R. A. Caruso, ACS Applied Materials & Interfaces, 2012, 4, 476-482.
4.X.D. Wang, D. R. G. Mitchell, K. Prince, A. J. Atanacio and R. A. Caruso, Chemistry of Materials, 2008, 20, 3917-3926.

Owing to the inherent advantages of high photocatalytic activity, strong oxidation power, low cost, environmental benignity and excellent stability, nanostructured TiO2 photocatalysts have been widely used for environmental remediation, energy conversion and storage. These advantageous properties of nanostructured TiO2 photocatalysts have recently attracted an extensive interest for their sensing applications. In this regard, the determination of organic pollutants in aquatic environment is one of the most noticeable sensing applications. This type of sensing applications normally utilises the strong oxidation power of the illuminated TiO2 photocatalyst to degrade the organic pollutants and simultaneously quantify the transferred electrons originated from the photocatalytic oxidation process to determine the total organic pollutants. This work reviews the recent advancement in photocatalysis and photoelectrocatalysis based sensing systems with a focus on the determination of aggregative organic parameters such as chemical oxygen demand (COD).

Recent density-functional theory calculations suggested that donor-acceptor codoping TiO2 is more effective than mono-doping for improving photoelectrochemical (PEC) water splitting performance because the donor-acceptor codoping can reduce recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. However, so far, there are limited codoping methods available, hence hinders further experimental studies. Here, we report a novel and simple ex-situ method for the donor-acceptor codoping of TiO2 nanowires by a sequential annealing of dopant-precursor coated TiO2 nanowires with flame and gas environments. The unique advantages of flame annealing are that the high temperature (>1000 oC) and ultra-fast heating rate of the flame enable rapid diffusion of metal dopants into TiO2 without damaging the TiO2 nanowires morphology and crystallinity, and the delicate glass substrate. Using this method, we successfully codoped the TiO2 nanowires with tungsten (W) and carbon (C). More importantly, this is the first experimental demonstration that the codoped TiO2:(W, C) nanowires outperform the mono-doped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for PEC. Such significant PEC performance enhancement is originated from greatly improved conductivity and activity for oxygen evolution reaction due to the synergistic effects of (W, C) codoping. We believe that the effectiveness of our doping method and codoping strategies, together with theoretic guidance, will enable further improvement of PEC water splitting performance of metal oxide photoanodes by various donor-acceptor pairs.

Over the past decade, TiO2 nanostructures have become a focus of intense research for exploration in numerous applications, such as bio-medical coatings, solar cells, photocatalysts, or sensing materials. TiO2 has various functional properties such as a wide band-gap which “optimum” band-edges to water, a high chemical stability, and considerably good electronic properties. In order to fully exploit or even enhance the properties low dimensional nanoscale geometries like 1D nanostructures such as nanorods, nanowires and nanotubes have been explored. These 1D TiO2 nanostructures can be fabricated by various techniques, and in many cases these structures show an enhanced chemical, physical and biological response compared with traditional bulk or nanoparticle structures. A very straightforward synthesis approach for ordered arrays of TiO2 nanotubes is their formation by self-organizing anodization of a metallic Ti substrate in a “dilute fluoride” containing electrolyte. In this case the oxide is directly back-contacted which is particularly useful for any application of the nanotube layers as electrodes.
In this presentation we give an overview of the latest results on TiO2 nanotube and porous structures. We will discuss critical factors to grow highly aligned and high aspect ratio anodic TiO2 nanostructures and the morphological variation by anodization parameters and their advantage for applications.

Recently, hybridization with graphene has received much interest as a proper method to overcome the drawbacks of TiO2. Graphene has many unique properties such as high conductivity, high specific surface area, etc. When graphene is combined with TiO2, the photogenerated charges in TiO2 are easily separated and transferred to graphene which induces retarded charge recombination. Although numerous papers have presented the enhanced photocatalytic and photoelectrochemical (PEC) activities with graphene/TiO2 hybrids, the effect of the hybrid graphene/TiO2 geometries has not been well studied.
In this work, size controlled nanographene oxides (NGOs; < 50 nm) were prepared by a two-step oxidation process and NGOs were self-assembled with TiO2 nanoparticles to form a core/shell structure. Nanosized GO-coated TiO2 nanoparticles (NGOTs) were then photocatalytically reduced under UV irradiation to obtain graphene-coated TiO2. This is a clearly different approach from the typical graphene/TiO2 composite with the particles-on-a-sheet geometry and is the first study on the core/shell structure of its kind. The physicochemical properties of NGOs and the reduced NGOTs (r-NGOTs) were characterized by various analytical and spectroscopic methods (AFM, FT-IR, XPS, TEM, EELS, etc.). The photocatalytic and photoelectrochemical activities of r-NGOT were compared with a composite of r-GO/TiO2 that has TiO2 nanoparticles loaded on a larger graphene sheet (r-LGOT).

In the past decades, numerous semiconducting materials, especially oxide semiconductors, have been investigated as potential photoelectrodes in photoelectrochemical system with a view to efficient light-induced water splitting for solar-hydrogen conversion. Compared to other metal oxide semiconductors, α-Fe2O3 (hematite) has the advantage such as the small band gap energy of ~2.0 eV, which enables it to absorb most of the photons of solar spectrum. Furthermore, its chemical stability, widespread availability and innocuity make it promising to be employed in a large scale. Unfortunately, the ultrafast recombination of the photogenerated carriers and the poor minority charge carrier mobility lead to a short hole diffusion length in α-Fe2O3, which allows only holes created close to the electrolyte interface to oxidize water. Hence, the overall photocurrents produced by solar light have been severely limited.
In this study, we used aqueous solution growth followed by ultrasonic-assisted sol-gel process to prepare TiO2/α-Fe2O3 shell/core nanorods. The thickness of TiO2 shell could be controllably adjusted by the concentration of TBOT precursor solution. The PEC tests showed that TiO2 shell coating could efficiently enhance the activity for solar water splitting. This should be due to the effective extraction of electrons from α-Fe2O3 core by TiO2 shell, which reduced the possibility of electron-hole recombination.

Anatase TiO2(101) is the most stable surface of the anatase polymorph of TiO2, and its interactions with water and molecular oxygen play a key role in the photocatalytic reactions occurring on this surface. At variance with rutile, intrinsic reducing defects, i.e. O vacancies and Ti interstitials, at anatase (101) reside preferentially in subsurface sites. In this talk I shall present results of recent first principle calculations of the interaction of water and O2 with reduced anatase (101). I shall discuss the influence of subsurface defects on water adsorption and dissociation as well as the mechanisms by which these defects can be healed by O2 adsorption. Extensive comparison with experiment will be also presented.

Titanium dioxide (TiO2) has the appropriate thermodynamic energy for both water oxidation and water reduction upon photoexcitation. However, appropriate charge separation has to take place in order to achieve the desired water splitting. In addition, catalysis should be fast and selective for the product of concern. To fulfill these criteria, the cobalt phosphate (CoPi) oxygen evolving catalyst was photochemically assembled on the surface of TiO2 making use of the large oxidative potential of photogenerated valence band holes. CoPi was extensively characterized showing structural and electrochemical properties similar to the electrodeposited materials. In addition, hydrogen evolving catalysts composed of highly segregated nanoparticles were assembled on the surface of TiO2 making use of the reduction potential of the conduction band electrons. Interestingly, Pt(dcbpy)Cl2 and Pd(dcbpy)Cl2 [dcbpy = 4,4prime;-dicarboxylic acid-2,2prime;-bipyridine] long thought to be efficient molecular catalysts for hydrogen production were used as precursors for nanoparticle formation. Encouraged by these results, an electrosynthetic device was constructed from these catalysts that are collectively capable of splitting water into hydrogen and oxygen, desirable feedstocks for fuel cells.

Pure and modified TiO2 nanoparticles have been extensively investigated as the most successful and practical photocatalyst for solar energy utilization such as the photodegradation of environmental pollutants and the production of solar fuels. Despite their popularity as photocatalytic nanomaterial, breakthroughs in materials development have yet to be achieved for practical applications. In particular, the lack in the visible light activity is the most critical drawback of TiO2. A variety of approaches have been tested to endow titania with the visible light absorbing ability. In this talk, various examples related with visible light activation of TiO2 (e.g., dye-sensitized TiO2, doped TiO2) will be introduced. In particular, ligand-to-metal charge transfer (LMCT) on surface complexed TiO2 and its application for the development of visible light photocatalysts will be discussed in detail. LMCT-based photocatalysis can be successfully developed by using low-cost phenolic resin and related materials. The interparticle charge transfer within TiO2 nanoparticles and their agglomerates (nanoclusters) was also investigated for the dye-sensitized system under visible light irradiation to probe the role of interparticle charge transfer. The visible light activity and reaction mechanism closely depend on how the charge transfer on TiO2 is modified. Their applications to the degradation of pollutants and hydrogen production under visible light will be discussed.

There has been a considerable interest in one-dimensional nanostructures owing to their remarkable characteristics in particular these electronic properties which have been considered to significantly improve the electron transport time and reduce the recombination rate. The range of research area of one-dimensional nanostructures covers meanwhile a wide rage of transition metal oxides. Among them, TiO2 is the most widely studied material because of its promising applications in photocatalysts, biomedical devices and solar cells. In order to form such one-dimensional TiO2 nanostructures, several approaches have been explored such as electrodeposition, sol-gel synthesis, hydrothermal and atomic layer deposition into ordered templates. However, these template assisted synthesis methods contain several drawbacks such as limitation size control (diameter, length), separation of the individual structures, agglomeration or bundling of structures. On the other hand, self-organized anodic formation of TiO2 is easy to control and also provides directly a back contact.
This presentation shows several types of TiO2 nanostructures such as nanotubes, ordered nanochannel, mesoporous and fishbone structures and their applications. We will also show that self-organizing anodization can expand other valuable metal such as Nb, Ta, V, W and alloys and discuss the key parameters for growth and application.

Conventional method to hydrothermally synthesize large scale TiO2 nanowires involves in a 3 day to one week treatment at 160oC-190oC with 10 M NaOH solution. We develop an innovative method to grow high quality TiO2 nanowires for only 15 hours. The experiments display that the processing at a higher temperature can dramatically speed up the growth of TiO2 nanowires. The proper temperature for anatase phase is 250oC. The synthesis at a higher temperature results in a rutile phase. For TiO2 nanowires fabricated at 250oC the photocatalytic degradation rate of methylene blue is about 2 times higher than that synthesized at 190oC and 3 times higher than commercial TiO2 nanoparticles (P25). Microstructure characterization shows that TiO2 nanowires have a higher crystallinity when growing at a higher temperature. Small amount of monoclinic phase embedded in anatase phase could trap free charged carriers and reduce their recombination rate. The newly developed TiO2 nanowires are also effective for degradation of other dyes and pharmaceuticals, including carbamazepine, naproxen and theophylline. High performance TiO2 nanowire membranes also display a high efficiency to deactivate E. coli in photocatalytic antibacterial experiments.

A high-quality mesoporous TiO2 layer is important for high-performance dye-sensitized solar cells. In this paper, we report a novel method to effectively fabricate the mesoporous TiO2 films of dyesensitized solar cells by formulating new TiO2 pastes. Graphene oxide (GO) is added into TiO2 nanoparticles pastes as an auxiliary binder. Thick mesoporous TiO2 films free of crack can be prepared by only single printing. GO helps bind TiO2 nanoparticles together through the interactions between functional groups on GO and the surface species of TiO2 nanoparticles. The presence of 0.8 wt.% GO in the TiO2 paste (GO weight percentage with respect to the weight of TiO2) is sufficient to fabricate thick and crack-free TiO2 films via single printing. These mesoporous TiO2 films fabricated from the TiO2eGO pastes are investigated as the anode of DSCs. They can give rise to a power conversion efficiency (PCE) of 7.70% for DSCs under AM1.5G illumination, which is almost the same as that of control devices with the TiO2 mesoporous electrode fabricated from the conventional TiO2 paste without GO via four-fold printings.

Heterostructures composed of LaAlO₃ (LAO) overlayers on anatase TiO₂ attract much attention as functional components in field effect transistors [1], magnetic tunnel junctions [2] and transparent conductors [3]. However, the growth conditions of the LAO overlayer strongly affect the crystalline properties of the underlying anatase TiO₂, impacting the film quality in some cases [3]. In this research, we focus on the oxygen stoichiometry of the LAO overlayer and study its influence on the properties of the anatase TiO₂.
LAO(10 nm)/TiO₂(40 nm)/LAO{001} and LAO(10 nm)/LAO{001} structures were grown by pulsed laser deposition. We systematically varied the oxygen partial pressure P_O2 and the deposition rate r during LAO growth, while fixing the previously optimized TiO₂ growth conditions [4]. The c-axis lattice constant c_LAO of the LAO thin films grown directly on LAO(001) was expanded compared to the substrate with decreasing P_O2 and increasing in r, while no deviation in c_LAO was observed for the LAO/TiO₂/LAO{001} heterostructures. The n-type carrier density of the TiO₂ layer obtained from Hall measurements in the heterostructures increased systematically for lower P_O2 and higher r. Concomitantly the x-ray diffraction intensity of the anatase TiO₂ (004) peak decreased. These results suggest that oxygen vacancies generated in the LAO thin films are compensated by oxygen ions which diffuse from the neighboring TiO₂ under reducing and high growth rate conditions, consequently degrading the anatase structural quality. By optimizing P_O2 and r, we show that high quality LAO/TiO₂ heterostructures can be successfully grown, providing opportunities for further developments of their functionality.
[1] M. Katayama et al., Appl. Phys. Lett. 92, 132107 (2008).
[2] J. Y. Yang et al., Appl. Phys. A 105, 1017 (2011).
[3] K. S. Takahashi and H. Y. Hwang, Appl. Phys. Lett. 93, 082112 (2008).
[4] T. Tachikawa et al., Appl. Phys. Lett. 101, 022104 (2012).

TiO2 possesses a wide range of application potential in hydrogen production, lithium-ion batteries, fuel cells, gas sensors, detoxification, photovoltaic, photocatalysts, and supercapacitors. Its one-dimensional (1D) morphology is expected to exhibiting higher performance in those applications compared to the bulk form. First of all, to synthesize TiO2 1D nanomaterials with defined phase, facet, shape, dimension, and high quality crystallinity is of fundamental importance for achieving desired functionality and performance. Recently, we demonstrated a surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique that can grow highly uniform single-crystalline TiO2 nanorods over a large area, even inside highly confined submicron-sized spaces. SPCVD uses separated TiCl4 and H2O precursor pulses at 600 degree C. The anisotropic growth of TiO2 crystals is attributed to the combined effects of surface recombination and HCl restructuring at high temperature during elongated purging time. Therefore, the crystal growth is effectively decoupled from precursor vapor concentration, which allows uniform growth of TiO2 NRs inside highly-confined spaces, such as inside anodized aluminum oxide (AAO) channels and among dense Si nanowires. The phase of TiO2 NRs can be tuned from anatase to rutile by raising the deposition temperature. By adjusting the precursor ratio and exposure time, intrinsic defects in TiO2 crystals could be engineered thus narrowing the bandgap of as-synthesized TiO2 NRs and allowing visible light absorption. For the first time, SPCVD technique realized a high-density 3D NW architecture by growing TiO2 NR arrays inside dense and deep Si NW forests. Such 3D structures offered super large surface area as well as excellent charge transport property. Therefore, dramatically enhanced efficiency was obtained when they were used as photoelectrochemical (PEC) anode for water splitting. 3D TiO2 NR arrays grown by SPCVD opened a new avenue toward high-performance electrodes design for advancing the performance of PEC and photovoltaic devices.

This talk will present highlights of the latest results of studies directed at developing oxygen-deficient metal oxides, including TiO2, WO3, and a-Fe2O3, nanostructures as electrode materials, which show significantly enhanced performance in applications for photoelectrochemical water oxidation. The enhanced photoelectrochemical performance is attributed to improved electrical conductivities by controlled incorporation of oxygen vacancies as shallow donors for metal oxides. The potential of these oxygen-deficient metal oxides for other energy conversion applications, such as photocatalytic reactions will also be discussed.

It has been reported that low level doping of TiO2 with Sn(IV) enhances its photocatalytic activity under UV light and visible light. Based on electronic structure studies of rutile SnxTi1-xO2 solid solutions, the improved photocatalytic activity has been attributed to a narrowing of the bulk bandgap at low Sn concentration and surface states associated with segregated Sn (II). The latest lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation[1]. Trapping could be considered detrimental if trap sites were far from preferred electron transfer sites or led to recombination[2]. Charge trapping at the surface may however bring benefits to the photocatalytic performance of a semiconductor provided the trapping energies do not diminish the redox power of a charge carrier in a large extend nor inhibit rapid transport. In this contribution we report a study of the effect of Sn(II)-doping of TiO2 restricted to the surface. Surface Sn(II)-doped TiO2 samples were prepared by a controlled reduction of Sn(IV)-grafted anatase TiO2, based on combined analysis of temperature programmed reduction (TPR) profiles and X-ray photoelectron spectra (XPS). Photocatalytic characterization based on methylene blue degradation and terephthalic acid hydroxylation shows that surface Sn(II)-doping greatly enhances the activity and overall efficiency of TiO2 in degradation of organics through direct oxidation by surface trapped holes.
The grafting procedure leads to the formation of isolated Sn(IV) sites on the surface of anatase TiO2 powders as gauged by structural characterization based on XRD, Raman spectroscopy and XAS. Studies of the surface reduction based on TPR experiments and XPS provide the conditions for a selective reduction of surface Sn(IV) to the divalent oxidation state. Electronic structure characterization based on valence band XPS and DRS shows that there is a slight widening of the band gap upon Sn(IV)-grafting, but Sn(II) related states emerge at the top of the main valence band upon reduction at temperatures up to 350°C, and this induces visible light absorption.
Grafting of TiO2 with Sn(IV) increases the formation rate of OH radicals on the surface of the material. Reduction of the Sn(IV)-grafted TiO2 to form surface Sn(II) brings about substantial increase of the photocatalytic efficiency for the methylene blue degradation under irradiation with lambda; ge; 320nm compared with Sn(IV)-grafted and pure anatase TiO2.
We are currently extending the concept of surface doping to other semiconductors with relevance for photocatalysis, in particular to zinc oxide.
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[1] F. E. Oropeza, B. Davies, R. G. Palgrave, R. G. Egdell, Phys. Chem. Chem. Phys. 2011, 13, 7882.
[2] M. A. Henderson, Surface Science Reports 2011, 66, 185-297.

TiO2 is one the most studied metal oxides because of its usefulness in a wide variety of applications. One application of TiO2 nanotubes (NTs) is in photoanodes for excitonic solar cells, where the straight tubular structure acts as transparent electron conductor and, at the same time, hosts dye molecules responsible for light absorption. In this context, TiO2 NTs are required to possess high specific surface to obtain high dye uptake and, as a consequence, high optical density of the photoanode in the range of light absorption of the dye. The specific surface critically depends upon the preparation conditions, which regulate the geometry of tube array (length and diameter of the tube), so optimization of tube geometry is a major issue.
The improvement of basic properties of TiO2 for photocatalytic devices is also important for the fabrication of metal oxide gas sensors. TiO2 thin films can monitor levels of gaseous species in the atmosphere through changes in the resistivity. These resistivity changes are caused by electronic transfer upon adsorption of gas molecules to the thin film surface. Small band-gap energies and high electrical mobility means that will be possible to affect the performances of gas sensing devices.
We have synthesized Nb containing TiO2 NTs by means of electrochemical anodization method. NTs have been obtained on stiff (glass, silicon, alumina) and flexible polymeric substrates (Kapton HN®). The morphology and shape of obtained TiO2 NTs have been tuned by variation of anodization parameters such as the type and concentration of electrolytes, the applied voltage and current and the anodization time. The morphological characterization of anodized pure and Nb containing titanium films have shown that well-aligned NTs with homogeneous distribution have been obtained on stiff and flexible substrates. The functional properties of prepared nanotubular structures have been tested for chemical sensing and dye-sensitized solar cells. A wide range of operating temperatures and different gases such as CO, H2, NO2, ethanol and acetone has been investigated.
The tubular arrays have been successfully integrated in flexible dye sensitized solar cells architecture. Improvement of functional properties of TiO2 NTs by introduction of Nb will be presented.
Investigations have shown the synthesis of TiO2 and Nb-TiO2 nanotubular structures on stiff and flexible substrates at room temperature is promising for the development of chemoresistive gas sensors and flexible dye sensitized solar cells.

Nanostructured hybrid metallic/semiconducting photocatalysts are extensively studied for energy conversion applications. In these systems light absorption in the semiconducting catalyst is followed by fast electron transfer to the metallic co-catalyst, favoring charge separation and reducing recombination losses.
Besides acting as catalysts, metallic nanoparticles also exhibit strong light scattering and absorption, due to the collective oscillation of their surface electrons. For most commonly used metals, such as gold or silver, these plasmon resonances occur in the visible part of the spectrum and are therefore easily detected by standard optical spectroscopy. Furthermore their spectral position depends strongly on the electron density of the particle. It is therefore possible to follow in real-time the activity of a hybrid photocatalyst by monitoring the resonance shift in the metallic co-catalyst.
Although the chemical and physical properties of metallic nanoparticles depend strongly on their size and shape, most studies of hybrid metal/semiconductor nanostructures rely on ensemble measurements and are therefore unable to unveil the details of their structure-function interplay. Here we study the photocatalytic activity of individual core@shell Ag@TiO2 nanoparticles, by monitoring both real-time and in-situ the LSPR shifts induced by the addition or removal of electrons into the metallic core.
Well-dispersed Ag-core TiO2-shell nanostructures are prepared by a two-step colloidal synthesis. In the first step Ag nanoparticles of various shapes and sizes ranging from 20 to 30 nm are formed from the reduction of a silver salt in presence of a surfactant. In the second step the Ag nanoparticles are coated with a 10-20 nm thick TiO2 shell via a sol-gel process, involving the hydrolysis of a titanium alkoxyde. Shell formation is confirmed both by TEM and optical spectroscopy, which shows a 20 nm shift in the plasmon resonance of the metallic cores.
UV excitation of Ag@TiO2 generates electron-hole pairs in the TiO2 shell with subsequent electron transfer to the Ag core, while the holes are scavenged by the solvent. As a consequence the plasmon resonance of the Ag core blue shifts by about 15 nm in an ensemble measurement. A similar shift is observed when following the evolution of the scattering spectra of individual nanoparticles during irradiation with UV light in an inverted dark-field optical microscope. The optical spectra of individual nanoparticles are then correlated with their electron microscopy images.
Our results show the potential for plasmonic nanoparticles to enhance charge separation in existing photocatalysts and enable new insight into charge-separation mechanisms in multi-electron redox reactions.

TiO2 nanorods can assist the reduction of AuCl4-ions in organic solvent under UV irradiation, which results in the generation of Au nanoparticles soluble in organic solvents. The gold nanoparticles have an average diameter of 7.2 nm and display a surface plasmon band at 530 nm, as confirmed by TEM and UV-Vis spectroscopy studies. The rate of the reaction can be accelerated by the addition of 1-hexadecanol as a hole scavenger. The effect of different concentration of TiO2 was also studied. The observed kinetic behavior suggests that the formation of the gold nanoparticles with the assistance of TiO2 nanorods involves two main steps: 1) reduction of the Au3+ ions by electrons to form Au0 which nucleates into gold seeds. 2) Growth of the gold seeds to form larger gold particles with the assistance of TiO2. The mechanism of the underlying multistep electron-transfer process has been discussed in detail. This method represents an easy process for the large-scale preparation of nanostructured semiconductor-metal composite system with relevant technological potential in photocatalysis and charge storage processes.

Control over the physical properties of TiO2 nanostructures is necessary to rationally design a material with desired properties such as morphology, surface area, crystal grain size and crystal phase. Good control over both the surface area and crystallinity has been shown to be of importance to the photocatalytic activity of TiO2 materials. Here we demonstrate the synthesis of TiO2 structures using controlled hydrolysis-condensation reactions of titanium-alkoxide precursors. These materials can be synthesized with a high degree of crystallinity which can be obtained through metal doping, water induced crystallization or calcination. The grain sizes of these materials can be tuned from 4 nm to over 20 nm. Further, the surface areas of these materials can be tuned across a wide range. The photocatalytic performance of microspheres with enhanced crystallinity is tested using the model system of the degradation of Rhodamine B under UV irradiation.

In this study, we have performed first principles density functional theory calculations to investigate the physical origins of the enhanced visible-light absorption of the N-doped anatase TiO2 in order to resolve the long-term controversy regarding the effect of nitrogen doping on the electronic and optical properties of TiO2, i.e. is the enhanced optical property mainly attributed to the band gap narrowing or simply by the excitation of the localized states in the band gap? Regarding this aspect, we have developed a new analysis method to unambiguously determine the degree of localization of the nitrogen-induced energy states in the electronic band structure of the N-doped TiO2 systems (TiO2-xNx). Our results showed that the nitrogen-induced energy states can only appear as the localized defect states in the band gap region when TiO2 is still under very low N-doping level. Our results showed that the nitrogen-induced energy states can only appear as the localized defect states in the band gap region when TiO2 is still under very low N-doping level. As the nitrogen concentration becomes greater than x = 0.125, however, we started to observe the hybridization of some N-induced states with the O 2p-derived valence band top of TiO2, which make the N-induced level appear like an extended state to achieve an effective band gap narrowing on the N-doped TiO2 system. On the other hand, our charge density difference calculations also showed that nitrogen doping in TiO2 can disturb the O 2p valence band top to induce significant valence charge redistribution in that region which can have strong influence on the optical response and the light absorption behavior of TiO2. Most interestingly, our calculations showed that in some cases the red-shift of the light absorption edge can be attributed to such an induced charge density redistribution in the valence top region rather than due to the band gap narrowing effect. We believe that our results has brought a perspective to this field of study.

Among variety of alternative energy sources, solar light is regarded one of the most viable to meet a prodigiously large amount of global energy demand. However, fluctuation of incident solar light resulting from climate, weather, locality, and diurnal variation requires its in-situ or ex-situ storage for further use. In this regard, this presentation contains very interesting and scientifically important experimental results that solar light could be harnessed and stored photoelectrochemically using TiO2 and WO3 mixed electrodes. This study particularly attempted to provide direct evidences that the electrons of TiO2 are transported to and simultaneously stored at WO3 upon light on (charging process), which are flowed back from WO3 to TiO2 upon light off (discharging process). Also it has been demonstrated that such the stored electrons could be used for anticorrosion of steel and remediation of water pollutants even in the absence of light by inserting Al2O3 layer between TiO2 and WO3.

In dye sensitized solar cells the photoanode is typically composed of a wide band gap semiconductor which acts as electron transporter for the photoelectrochemical system. Anatase TiO2 nanoparticles are one of the most used oxides and are able to deliver the highest photoconversion efficiency in this kind of solar cells. Modulation of the composition and shape of nanostructured photoanodes is key element to tailor the physical chemical processes regulating charge dynamics and, ultimately, to boost the efficiency of the end user device. Addition of one dimensional nanostructures like nanowires, nanotubes and nanorods in particulate photoanodes can favor charge transport and collection, reducing charge recombination.
We investigated several systems: (i) TiO2 nanoparticles / ZnO nanowires [1]: The system is fabricated by mixing single crystal ZnO nanowires and polycrystalline TiO2. The almost similar electronic band structure of ZnO and TiO2 guarantees perfect compatibility of such oxides from the point of view of the electron transport, without formation of detrimental electric fields which could affect electron mobility. (ii) Multiwall carbon nanotubes (MWCNTs) / TiO2 nanoparticles: The presence of MWCNTs favors electron collection during solar cell operations, inhibiting the recombination processes which lead to loss of photogenerated charges. (iii) TiO2 nanotubes [2-4]: The growth and processing of anatase nanotubes was obtained via sequential anodization and annealing of titanium thick film on plastic substrate for flexible solar cells. The choice of heat resistant substrate (Kapton HN ®) allows post growth annealing that induces the amorphous to crystalline transition of titanium oxide leading to high efficiency devices without affecting substrate shape.
Possible tailoring of structure and morphology of TiO2-based photoanodes, and their implication in improving the functional properties of solar cells will be discussed in detail.
[1] A. Vomiero, I. Concina, M.M. Natile, E. Comini, G. Faglia, M. Ferroni, I. Kholmanov, G. Sberveglieri, Applied Physics Letters 95 (2009) 193104.
[2] V. Galstyan, A. Vomiero, E. Comini, G. Faglia, G. Sberveglieri, RSC Advances 1 (2011) 1038-1044.
[3] A. Vomiero, V. Galstyan, A. Braga, I. Concina, M. Brisotto, E. Bontempi, G. Sberveglieri, Energy and Environmental Science 4 (2011) 3408-3413.
[4] V. Galstyan, A. Vomiero, I. Concina, A. Braga, M. Brisotto, E. Bontempi, G. Faglia, G. Sberveglieri, Small 7 (2011) 2437-2442.

The resistivity of transparent conductive oxide (TCO) in dye-sensitized solar cells (DSCs) is well-known to limit the size of individual modules. The TCO layers also account for 40% of the construction cost of these solar cells. We and others have investigated anodes and cathodes made of metal substrates as an approach for making large-area single modules and for reducing costs. Here, we report the optimized construction of a cathode consisting of a Ti metal mesh or Ti wire substrates. We report mesh and wire dimensions that are an optimum balance between high light transmission (for back illumination of the opaque anode), and high conductivity. Commercial mesh substrates were etched with hydrofluoric acid so that the light transmission exceeded 80% and therefore approached the optimum determined from our calculations. Pt nanoclusters with diameters of approximately 100 nm were electrodeposited on the Ti substrates. The conditions for electrodeposition of Pt were optimized to achieve dense nucleation of nanoparticles, as well as performance levels for the solar cells that were at least 5% efficient under 1 sun illumination. The advances described here and others are important in the broader effort to construct DSCs with inexpensive, all-metal substrates.

Because of versatility in photocatalytic, water splitting, photovoltaic, gas sensing, and lithium storage characteristics, titanium dioxide has been intensively researched for the past decades. To further enhance its surface photochemical reactivity, variety of nanostructures including nanoparticles, nanorods, nanowires, nanotubes, nanosheets, nanocomposites, etc. have been exploited. Recently, special attention has been paid to the surface engineering of anatase nanosheets to obtain more photoreactive surfaces. It is obvious that high energy facets are necessary to maximize reactivity. However, it is hard to obtain the high energy facets of anatase such as (010) and (001) because anatase (101) is thermodynamically more stable compared to other planes. In this study high-density anatase titanium dioxide (TiO2) nanosheets with high-energy (001) surfaces were successfully synthesized on silicon and silicon-coated substrates via chemical vapor deposition (CVD). Randomly oriented nanosheets (RNSs) and aligned nanosheets (ANSs) were synthesized depending upon gas flow conditions, and different growth mechanisms were proposed for each structure. To prevent anatase-to-rutile phase transformation, the substrate temperature was maintained as low as 450 oC and instead, hydrogen (H2) auto-ignition was induced to provide additional heat and pressure to the substrates in a moment. It is obvious that silicon vapor can suppress the growth of anatase crystals into [001] orientation, resulting in the formation of two-dimensional (001) nanosheets. This strategy of passivating specific crystal facets using silicon can be simply extended to the tailoring of other nanosheet structures that are impossible to be obtained via general crystal growth approaches.

Titanium oxide has shown to be an excellent material for removing sulfur heterocycles from hydrocarbon fuels at room temperature and atmospheric pressure. The Ag-TiO2 adsorbent can desulfurize commercial fuels down to ppmw level and can supplement conventional HDS process in producing cheap ultra clean fuel [1, 2]. The inherent surface acid sites of TiO2 act as the adsorption centers for sulfur adsorption. In this work, the role of titanium oxide in Ag-TiOx-Al2O3 adsorbent was investigated in desulfurizing JP5, JP8, and Off-Road Diesel (ORD). The material was prepared by dispersing Ag-TiOx on alumina support followed by calcination. Sulfur adsorption capacities were obtained through fixed bed continuous adsorption (Breakthrough) experiments using the commercial fuels. N2 physisorption, NH3 and O2 chemisorption, XRD, UV-DRS, and in situ IR spectroscopy were employed to characterize the adsorbent. Ammonia, 2, 6-Lutidine, and trimethyl chlorosilane (TMCS) were used as probe molecules to identify and quantify the surface acid sites of the adsorbents. Titanium oxide dispersed mixed oxides enhanced the sulfur adsorption capacity from the commercial fuels (>10 mg S/g adsorbent). TiOx dispersion on high surface area supports increased overall surface area and surface defect sites. Moreover, the defect sites on titanium oxide were able to host more dispersed silver active sites (up to 12 wt% Ag) intended for increase in sulfur capacity. Analysis via UV-DRS revealed lower Ti band gaps for the adsorbent material. NH3 chemisorption and IR spectroscopy of the samples demonstrated enhanced surface acidity and consequent sulfur adsorption capacity. The acid sites on defected TiOx surface were able to adsorb refractory sulfur heterocycles from hydrocarbon fuels.
[1] B. Tatarchuk, H. Yang, S. Nair, Silver-Based Sorbents, US Patent Application, 2008.
[2] S. Nair, B.J. Tatarchuk, Fuel 89 (2010) 3218-3225.

TiO₂ is one of the most studied semiconductor oxides (band gap of 3 eV) due to its chemical stability, low toxicity and unique optical, electric and photocalytic properties. The nanostructuring of TiO₂-based materials is desired in order to improve such properties.
Nanotubes (NTs) derived from TiO₂ can be made by treating TiO₂ with a concentrated NaOH solution under hydrothermal (HT) conditions. Several studies show that both anatase and rutile phases can be transformed in NaOH 10 mol.L^-1 under autogenous pressure inan autoclave for several hours at 100 - 150°C. However, little has been shown about the formation of such nanotubes from amorphous TiO2.
Amorphous TiO₂ was produced by hydrolyzing titanium tetraisoproxide (TTIP) under vigorous stirring for 15 min at 25°C. The powder was filtered and dried at 150 °C for 90 min. This material (1.8 g) was suspended in a 10 mol.L^-1 NaOH solution, which was placed in an autoclave and kept at 115°C for 24 or 72 h, then washed and dried at 80 °C. The resulting NTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. For comparison, the same procedures were employed using rutile TiO₂ instead of the amorphous phase. The BET surface area and pore volume were also determined for these materials.
TEM shows that that the 24 h HT product (Am-24h) is composed of both spherical nanoparticles and nanotubes having about 9 nm in diameter and lengths ranging from hundreds of nanometers to a few micrometers. Similarly, SEM reveals large aggregates of round particles as well as nanowires about 80 nm thick and hundreds of nm long. The latter structures were correlated to aggregates of a large number of nanotubes. Conversely, the 72 h product (Am-72h) presented only aggregates of round particles, which seem to be formed of short tubes and partially rolled sheets. These nanostructures also have less than 10 nm in diameter, but are shorter than 70 nm. The sample derived from the HT treatment of rutile for 72 h (Rut-72h), on the other hand, showed only homogenous aspect nanotubes. Rutile-derived nanotubes have the same diameter found in amorphous TiO₂-derived tubes, but have well defined walls and do not aggregate. SEM micrographs of this sample showed clusters of round particles.
The BET surface areas and BJH pore volumes for the amorphous-derived NTs are significantly larger than those observed for the rutile-drived NTs.
We conclude that nanotubes can be formed both from rutile and from non-crystalline TiO₂ using a HT treatment with concentrated NaOH, although rutile-derived tubes are regular and do not aggregate, whereas amorphous TiO₂-derived nanotubes are irregularly shaped and aggregated in bundles.

Solar energy utilization is an attractive option for new energy technology and economic development. Our research is the formulation of catalyst materials for solar production of hydrogen from water. Titanium(IV) oxide has been explored for water splitting; however, a major challenge is that titanium(IV) oxide can only absorb UV light. Visible light absorption can be increased by metal ion or anion doping by creating interband states. Most dopant protocols lead to deposition of dopant ions throughout the solid, and interfacial deposition has received very little attention. We have developed a method to selectively attach transition metal ions on the surface of titanium(IV) oxide nanorods using metal chlorides as precursors. The present study demonstrates that Cr(III), Mn(II), Fe(II), Co(II), Ni(II), and Cu (II) were coordinated to the surface of oleic acid capped TiO2 nanorods (NRs) by post-synthesis method without any phase or morphology transformation. Metal ion loading could be carefully controlled, and we show a titration curve for addition of transition metal ions to the nanorod surface. The materials were characterized with UV-visible spectroscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and powder X-ray diffraction.

Epitaxial rutile and anatase TiO2 thin films were grown on silicon substrates by using cubic and tetragonal yttria stabilized zirconia (YSZ) buffers, respectively. X-ray diffractometry (2theta;/theta; and phi; scans) and electron microscopy were employed for detail microstructural studies. The epitaxial relationship in the rutile/cYSZ/Si sample was shown to be: (100)[01 -1]rutile||(001)[010]cYSZ||(001)[100]Si. The epitaxial relationship in the anatase/tYSZ/Si heteroepitaxy was established as (001)[110]anatase||(001)[110]tYSZ||(001)[001]Si. Clean and atomically sharp interfaces were observed in both heterostructures. Subsequently, the samples were irradiated by a single pulse of a KrF excimer laser at the energy density of about 0.35 J.cm-2. The wettability of water on the surfaces of the rutile films was evaluated at room temperature in air. The samples were initially hydrophobic, but a superhydrophilic surface was obtained after laser annealing. Photocatalytic activity of the pristine and the laser annealed anatase sample was assessed through measurement of the decomposition rate of 4-chlorophenol under UV light. The photocatalytic reaction rate constants were determined as 0.0077 and 0.0138 min.-1 for the as-deposited and the laser treated samples, respectively. Thus, the efficiency was interestingly enhanced by about 2 times through nanosecond laser annealing. The stoichiometries of the surface regions of the samples before and after laser annealing were examined using XPS. The results revealed the formation of oxygen vacancies near the surface, which are surmised to be responsible for the observed enhancement of photochemical characteristics. Oxygen vacancies trap the photogenerated electrons and, hence, suppress the charge recombination. Such a nanosecond laser treatment opens new avenues for use of TiO2 in intelligent membranes, sensors, smart catalysts, and multifunctional devices.

Tuning the Semiconductor-to-Metal Transition Characteristics of VO2 Based Heterostructures through Microstructure Engineering
M.R. Bayati1, R. Molaei1, R.J. Narayan1,2, J. Narayan1
1. Department of Materials Science and Engineering, North Carolina State University, EB-1, Raleigh 27695-7907, NC, USA.
2. Joint Department of Biomedical Engineering, UNC Chapel Hill and North Carolina State University EB-3, Raleigh 27695-7115, NC, USA.
Abstract
Vanadium dioxide exhibits an ultrafast and first-order phase transformation from its monoclinic state to its tetragonal state at about 68 °C. This phase transformation is responsible for the semiconductor to metal transition or abrupt change in electrical resistivity and infrared transmittance. The transition temperature can be tuned through manipulating the microstructure. The control over the epitaxial strains was the idea behind using rutile TiO2 with different crystallographic orientations as a platform to tune the semiconductor to metal transition characteristics of the VO2 layer. We could successfully grow VO2[100], VO2[101], and VO2[001] epitaxial thin films on TiO2[100]/cYSZ/Si(100), TiO2[101]/r-sapphire, and TiO2[001]/m-sapphire templates, respectively. The epitaxial relationships were established and the atomic arrangement across the interfaces was modeled and studied in detail using X-ray diffraction (2theta;/theta; and phi; scans) and electron microscopy imaging and diffraction. In all cases, a cube-on-cube growth was observed across the VO2/TiO2 interface. It was observed that tetragonal phase of VO2 was stabilized at lower temperatures leading to a significant decrease in the semiconductor to metal transition temperature. The change in semiconductor to metal transition characteristics was attributed to the epitaxial strain along the c-axis of VO2. It was found that compressive strain results in decreasing the transition temperature. In summary, we were able to tune the transition temperature making the VO2 epitaxial heterostructures useful for a wide range of applications where different transition temperatures are required.

Heterogeneous photocatalysis is widely recognized as one of the cheapest and most efficient methods for solar decontamination of water and air from toxic organic pollutants. The most commonly used semiconductor is titanium dioxide which excels by the unique combination of low cost, non-toxicity, excellent stability against photocorrosion, and possibility for further functionalization. Moreover, apart from photocatalytic degradation reactions, in recent years we have become also increasingly aware of the great potential of TiO₂ for photocatalytic synthetic reactions, including selective photooxidation of organic compounds like alkanes or alcohols. Recently we reported an interesting example of successful unification of these two concepts: a novel type of surface-modified TiO₂ with improved photocatalytic degradation performance is prepared by a photosynthetic route involving visible light-induced activation of benzene or toluene [1,2].
It is well-known that irradiation of TiO₂ by UV light in the presence of benzene or toluene leads to brownish coloration of TiO₂ due to formation of stable polymeric products of photooxidation of the aromatic compound. Surprisingly, we observed that even irradiation by visible light (wavelength > 455 nm) leads to brown coloration of TiO₂. Moreover, the thus prepared modified TiO₂ was found to be more active in visible light-induced degradation of 4-chlorophenol than unmodified TiO₂. Detailed mechanistic investigations have shown that the better photodegradation properties are due to changes of adsorption/desorption properties of TiO₂ modified at the surface with carbonaceous deposits. This study thus presents one of the rare examples in which the modification of TiO₂ for enhanced photoactivity was achieved via an artificial photosynthesis route. At the same time it highlights the importance of surface engineering of heterogeneous photocatalysts in order to enhance photodegradation efficiencies.
References
[1] A. Ramakrishnan, S. Neubert, B. Mei, J. Strunk, L. Wang, M. Bledowski, M. Muhler, R. Beranek, Chem. Commun.2012, 48, 8556.
[2] A. Ramakrishnan, S. Neubert, B. Mei, J. Strunk, L. Wang, D. A. Guschin, M. Bledowski, M. Muhler, R. Beranek, in preparation.

A quick electrochemical Li ion insertion into TiO₂ nanotube arrays (TNT) markedly enhances the photoelectrochemical and photoelectrocatalytic performance. Potential pulses (-1.0 ~ -1.7 V _SCE for 1 ~ 11 seconds in 1 M LiClO₄ ) to pre-annealed TNT effectively insert Li ions (pre-annealed Li-TNT) into the mouth/wall and bottom TiO2 depending on the insertion condition. Pre-annealed Li-TNT prepared under an optimal Li ion insertion condition (-1.4 V_SCE for 3 seconds) exhibit ~70%-enhanced photocurrent generation, ~2.5 fold-higher incident photon-to-current efficiency, and an improved photoelectrocatalytic activity for the degradation of phenolic compounds in 1 M KOH electrolyte. A change in photoluminescence (PL) emission spectra and decrease in charge transfer resistance by Li ion insertion suggest that the inserted Li ions play a role in inhibiting charge recombination by compensating for Ti³⁺ charges (Li⁺-Ti³⁺-OH). As KOH concentration is diluted, however, such enhanced Li⁺ effects gradually vanish primarily due to liberation of reversibly inserted Li ions. To insert Li ions irreversibly, the potential pulses were applied to non-annealed TNT followed by annealing (post-annealed Li-TNT). Comparison between pre-annealed and post-annealed Li-TNTs in circum-neutral pH (0.1 M Na₂SO₄ at pH ~6) indicates that the former exhibits a similar performance to bare TNT (absence of Li ion effect), whereas the latter shows a superior performance with ca. 2.5-fold higher photoelectrochemical and photoelectrocatalytic activities. Detailed surface analyses (XPS, XRD, PL, SEM, ICP-MS, etc) and Li⁺-induced reaction mechanism were discussed.

Titanium dioxide (TiO2) is one of the most commonly used materials in many light induced processes such as a semiconductor for working electrode in dye sensitized solar cells (DSSCs), and a photocatalyst for degrading the harmful molecules, and a sensor material for molecule detection. The functional properties of TiO2 is governed by many factors: crystallinity, particle morphology, particle size, and surface area which are all affected by the TiO2 preparation method and condition. The growth of one dimensional TiO2 nanotubes has been proved to be very promising in the application of DSSCs due to the performance of higher electron transport. Especially, the application of freestanding TiO2 nanotubes on the photovoltaic performance of DSSCs-based solar cells using front side illumination has been investigated in this work. TiO2 nanotubes were grown on titanium (Ti) foils using the anodization process at a constant voltage of 60 V for various anodization times: 2, 4, 6, and 8 h, and then annealed at 450 °C for particular period of time. Subsequently, the formation of freestanding close-ended and open-ended TiO2 nanotubes was carried out via a second anodization step at 40 V and 80 V, respectively followed by immersing in 30 % H2O2 solution. X- ray diffraction (XRD) revealed the formation of amorphous TiO2 nanotubes before annealing while after annealing at 450 °C, it transformed into crystalline anatase phase TiO2. The thickness, inner diameter, outer diameter, and wall thickness of TiO2 nanotubes were analyzed from scanning electron microscopy (SEM) and transmission electron microscopic (TEM) images. The detached TiO2 nanotubes freestanding film was affixed onto FTO substrate by employing anatase TiO2 paste. The fixation of freestanding TiO2 nanotubes on FTO substrate can be categorized as three parts: fixing of downside close-ended nanotubes, fixing of upside close-ended nanotubes, and fixing of open-ended nanotubes. The above three various TiO2 nanotubes/FTO were utilized as photoanodes in DSSCs application. Among the various anodization times, the cell fabricated using 6 h anodized TiO2 nanotubes photoelectrode attained a higher photoelectric efficiency of 5.65 %, 6.93 %, and 6.42 % for close-ended-downside, close-ended-upside and opened end, respectively. Incident to photon-current conversion efficiency (IPCE) measurement shows a higher percentage at 530 nm wavelength for 8 h anodized photoanodes. Dye adsorption analysis utilizing UV-Visible Spectroscopy demonstrated a higher value for the same condition. Comparatively, the results indicate that the close-ended-upside photoelectrode achieves higher photoelectrical conversion efficiency than those of closed end - downside and opened end TiO2 nanotube electrodes. But the opened end samples performed a higher dye adsorption with higher percentage of IPCE values.

For several years now the CESES group at the Institut des Matériaux Jean Rouxel has been working to develop a third generation solar cell device showing intermediate bands (IB). The inter-mediate band concept, introduced by Luque and Marti in 1997(1), could be a way to overcome the Queisser-Schockley theoretical yield (about 32%) of classical Photovoltaic cells consisting of a simple p-n junction involving monogap semiconductors. The existence of partially filled intermediate bands should allow additional transitions from the valence band to IBs and IBs to conduction band, in principle increasing the amount of absorbed photons compared to monogap PV cells, and thus the photo-current obtained.
Our group has developed photosensitive sol-gels of titanium oxide in dimethylformamide (DMF)(2). They are highly promising for use in solar cells and photobatteries thanks to their original electrochemical characteristics. Under UV illumination, sols and gels are photosensitive and remain coloured in controlled atmosphere. Initially transparent, they become red or blue depending on the surrounding Red-Ox potential. This property is related to the nanometric size of particles, their structure(3), and the chemical composition of the electrolyte. The resulting energy storage is due to charge transfer and may be controlled chemically, electrochemically or by UV illumination(4). Under an applied electric field, by UV illumination or by addition of oxidizing or reducing agents, reduced or peroxidized species are generated at the surface of the nanoparticles. Their presence can be monitored by the appearance of absorption bands in the visible range, whose position and intensity depend on time/power of the UV illumination, on the composition/structure, but also on the surrounding Redox potential.
These Redox reactions occurring at the nanoparticle-adsorbed species interface generate intermediate band(s) in the gap of the titanium oxide.
Structural photo-induced changes have mainly been identified by Mass Spectrometry, Raman diffusion and UV-visible absorption. Structural models have been relaxed using DFT calculations to establish the associated band structure. This was done in order to understand and quantify the Redox reactions at the interface nanoparticles/electrolyte, a crucial step for the development of new devices.
(1) A. Luque and A. Martí, Phys. Rev. Lett, 1997, 78, pp. 5014-5017.
(2) Titanium Oxide-based Gel Polymer, L. Brohan, H. Sutrisno, O. Joubert, M. T. Caldes Rouillon, E. Puzenat, A. Rouet, Y. Piffard, CNRS patents: FR2835246 (March 9, 2004) ; EP1470078 (May 21, 2008); JP 4583758 (November 17, 2010) ; US7524482 (January 14, 2010) and US7723610 (May 25, 2010).
(3) T. Cottineau, M. Richard-Plouet, A. Rouet, E. Puzenat, H. Sutrisno, Y. Piffard, P.-E. Petit and L. Brohan. Chem. Ma-ter, 2008, 20, pp. 1421-1430.
(4) T. Cottineau, L. Brohan, M. Pregelj, P. Cevc and M. Richard-Plouet, D. Ar#269;on, Advanced Functional Materials, 2008, 18, 1-9.

It is well known that the coupling of TiO2 with other metal oxides, known as heterostructures, leads to an increase in its photocatalytic activity by reducing the effects of electron-hole pair recombination. Many reports have shown that the strong interaction between the units can lead to novel or improved physical or chemical performance relative to the individual components and the future development of such heterostructure materials will require deeper understanding of the components interface on the nanoscale, colletive phenomena and interparticle coupling. Then, it is still necessary develop stable and reproducible methods to obtain a desired heterostructure, in order to study those effects. In this research WO3/TiO2 heterostructures were produced by the decomposition of a stable peroxotungstate in solution under hydrothermal conditions for crystallization, and the heterostructures were crystallized by the same method in the presence of commercial TiO2 nanostructures. The characterizations were performed by X-ray diffraction, Raman spectroscopy, Electron Microscopy, photoluminescence spectroscopy (PL) and diffuse reflectance spectra (DRS). According to the results, the preparation of these nano-heterostructures is simple and fast compared to several examples from the literature. The orthorhombic WO3 phase and both anatase and rutile phases of TiO2 were found for the synthesized heterostructures. However, with WO3 content < 20 wt%, the tungsten species could not be detected by XRD or Raman spectroscopy. The microscopy images showed the WO3 were mainly formed by sub-micron plates recovered with TiO2 nanoparticles. HRTEM of WO3 results and FFT confirms the single-crystalline nature with the plates orientation along the [100] zone axis. The photocatalytic activity was studied in Rhodamine B (Rho-B) degradation in aqueous solution under UV and visible light. In both conditions the formed materials presented interesting photocatalytic properties when compared to TiO2 pre-formed particles, proving the beneficial effect associated to the heterostructure formation. We observed an increase in the photocatalytic activity with intermediate contents of WO3 in the heterostructures, especially for the 30 and 40 wt% WO3/TiO2, which characterizes an optimal ratio between these oxides, allowing the electron migration from one phase to another, consequently inhibiting the recombination of electrons and holes pairs. The PL spectrum showed a shift in the maximum emission to higher wavelengths according to the WO3 content, as well as a variation in the spectrum intensity, which confirms that the emission observed is a product of the WO3 and TiO2 interaction. This is an indicative that the electrons and holes are actually migrating to different phases. The lower bandgap energy was found for the 40 wt% heterostructure, where the better WO3/TiO2 ratio was found. It is in agreement with the other results, i.e., as expected.

Photoelectrochemical solar energy conversion on semiconductor surface has been extensively studied since the Fujisima&’s first report in 1972 using titanium dioxide (TiO2) photoanode [1]. Titanium dioxide (TiO2) is well-known as a candidate for water photocatalyst as it is abundant, stable in aqueous solution under irradiation, and has strong photocatalytic activity. However, due to its large band gap (~3.2 eV), it is only active in the ultraviolet (UV) region which contributes less than 5% of the total energy of the solar spectrum. One of the prerequisites of enhancing the solar energy conversion efficiency of titania is to enhance the visible light activity of TiO2, which composes a greater portion of the solar spectrum (45%) [2]. Another requirement of an effective photo-material is to obtain good electron-hole separation characteristics, which can be improved by reducing recombination centers and increasing charge transfer. Moreover, in a photoelectrochemical (PEC) cell, electrons generated in TiO2 photo-anodes have to travel within the TiO2 to the FTO contact and then transfer to the cathode. Therefore one-dimensional, single crystalline structures on conducting substrate are considered as an optimum morphology to enable electrons to travel to the FTO contact and holes to diffuse to the electrode/electrolyte interface in the easiest manner without any recombination lost at a grain boundary. Recently, low band gap semiconductors such as CdS, CdSe and CdTe have been successfully studied as a sensitizer for TiO2 nanosturtures using conventional successive ionic layer adsorbtion and reaction (SILAR) method . Ching-Fa Chi et. al. reported [3] that additional heat treatment of the TiO2 nanoparticulate film sensitized CdS or CdSe increased photocurrent response of the electrode due to the better crystalline between TiO2 and CdS and decreased band gap of the CdS particles. In this work polycrystalline TiO2 nanotubes by anodization of Ti foil and hydrothermally grown TiO2 nanorods (single crystalline) on F-doped SnO (FTO) glass substrates were fabricated in the same length (~1µm), respectively, and compared in order to examine crystalline effects of TiO2 on the photocatalytic performance. Furthermore, to enhance the visible light activity of the TiO2, the photoelectrodes sensitized with CdS and were heat- treated subsequently.

TiO2 based water-splitting technology has great potential for the production of hydrogen from solar energy. Fe2O3 doped TiO2 thin films are of particular interest for water splitting as iron doping allows a shift of the spectral response of TiO2 into the visible and so leads to a better conversion rate. Moreover, microstructure of the photo-electrode films is very important as charge transport is better in mesoporous systems.
In this study, Fe2O3 doped TiO2 thin films are synthesized by an optimized template-directed sol-gel process using dip-coating on conducting Indium thin Oxide (ITO) supports. Films are fired at 500°C. At this temperature, the sol-gel coatings are crystallized and possess a mesoporous structure that allows a large interfacial area in contact with the electrolyte. Different Fe3+ doping concentrations are evaluated up to 30 mol%. The synthesized films are characterized by UV-VIS spectrophotometry to study the influence of the doping on the absorption shift into the visible light. The optical band gap of the films decreases with increasing iron doping (3.3 eV for undoped TiO2 and 2.0 eV for 30 mol% Fe doped TiO2).
The influence of iron content on the film microstructure and on the crystallographic phases is studied to better understand the photo-electrochemical mechanisms. According to X-Ray Diffraction (XRD) and Raman spectroscopy, depending on the Fe3+ doping concentrations, samples with the lower Fe content possess the anatase phase. When the iron content increases, pseudo-brookite and then a mixture of pseudo-brookite and hematite phases is obtained at 500°C. XPS and Mossbauer spectroscopy were carried out to determine the exact composition and Iron valence in the structure. It shows that the structures contain a mixture of Fe2+ (40 %) and Fe3+ (60%).
Photocurrent measurements were carried out on these as-synthesized mesoporous semiconductor photo-electrode under visible light radiation (lambda; > 400 nm) and compared for different electrolytes. In the absence of iron, TiO2 shows negligible photocurrent. The addition of Fe3+ leads to an increase of the photocurrent up to an optimum doping concentration for 10 mol% Fe. For this composition, photocurrents of ~ 30 µAmps/cm2 are obtained in a 1 M NaOH solution.

In photocatalytic devices one of the most important goal of the recent research is to be able to prepare photocatalyst which can be active by absorbing the visible light. This could be very useful in order to use solar energy to promote photocatalytic processes, as, for example, for removing pollutants in waste water in poor regions (where it could be difficult to use UV light sources) or to reduce the application cost.
Titanium dioxide (TiO2), thanks to its interesting properties as nontoxicity, low cost and high chemical stability, has been extensively investigated for several application in which, following light absorption, the generated charges can be usefully applied, as for photovoltaic applications or for photocatalytic devices. However, TiO2 has a wide intrinsic energy gap, between 3.0 and 3.2 eV, depending on the crystalline structure. Therefore only a small fraction of the solar spectrum can be used to promote the light absorption
In order to increase the fraction of the solar spectrum that can be absorbed, different approaches have been used, mainly by doping TiO2 with metals or anions.
In this paper we will show the photocatalytic results obtained depositing N-doped TiO2 thin films by using sol-gel technique onto glass slides. The samples after the deposition have been annealed in air. These films have been characterized by XPS (chemical structure), XRD (crystalline structure), SEM (morphological analysis). The XPS analysis has shown the presence of a small concentration of nitrogen, whose binding energy suggests that N atoms are in interstitial position, while the XRD spectra have shown the presence of the anatase crystalline phase.
For photocatalytic experiments the photo-degradation of pharmaceutical pollutant carbamazepine (CBZ) dissolved in water was followed using a solar simulator as light source. The photocatalytic experiments have shown that these films are able to strongly reduce the CBZ concentration and this degradation depends on the pH and on the water quality.

After the first report by Fujishima and Honda on the photoelectrochemical splitting of water into H2 and O2 using a TiO2 anode, intense efforts have been devoted to the conversion of solar energy into “solar-fuels”. In spite of wide attempts to improve the water splitting performance using alternative semiconductive materials with more suitable light absorption (band-gap engineered materials) and enhanced charge transfer properties, TiO2 remains one of the most investigated platforms for photocatalytic applications. This is to a large extent due to its chemical stability (i.e., high photocorrosion resistance), a comparably long electron life time, and the low cost of the material.
In order to achieve better photocatalytic features with TiO2, mainly, pathways such as doping, co-catalyst decoration and nanostructuring have been widely explored. In this context, one-dimensional architectures, e.g., nanotubes arrays, may grant unidirectional percolation paths for the photoexcited charge carriers and can overall yield an inhibition of the electron-hole recombination processes.
Another promising way to improve the photo-electrochemical properties of a semiconductor is based on the introduction of dopants such as Ru, Nb or Ta into its structure, to improve either charge transfer or charge transport, and consequently reduce charge carrier recombination.
Indeed, an elegant approach to combine directional with doping features is the growth of self-organized nanotube layers from Ti-alloy substrates. Recently, this was successfully demonstrated for TiO2 nanotube (TiNT) arrays doped with low amounts (< 0.1 at%) of Ru or Nb where a dramatically enhanced water splitting behavior was found. Promising results were also attained for Ru-, Nb- and Ta-doped TiNT arrays employed in DSSC.
In the present work, we demonstrate the formation of Ta-doped TiNT arrays, directly grown on low Ta-concentration (0.03 - 0.4 at% Ta) Ti-Ta alloys by electrochemical anodization. Under optimized conditions (0.1 at% Ta, annealed at 650°C and 7 µm thick), Ta-doped TiO2 NT arrays show a significantly enhanced ability for the photoelectrochemical water splitting under simulated sunlight conditions.

A series of dielectric elastomer nanocomposites with enhanced electromechanical response have been synthesized by loading TiO2 nanoparticles into polydimethylsilane (PDMS) polymer. The maximum loading capacity of TiO2 nanoparticles in the PDMS is about 37.5 wt% without obvious agglomeration. Scanning Electrone Microscopy (SEM) and nanoparticle analysis show that the average diameter of the nanoparticles/clusters is around 90 nm which is uniformly distributed in the PDMS matrix. Among the elastomer nanocomposites, the 15 wt% TiO2 loaded PDMS shows relatively low elastic modules (77 kPa), fast response (27 mu;s) and high dielectric constants (6.4). The enhanced dielectric constant and relatively low elastic modulus are critical parameters for developing low driving voltage and high speed light modulators.

The capability of tuning the functional properties of nanosize TiO2 nanoparticles (NPs) by suitable control of surface chemistry, phase stability and crystal size plays a key role on their safe use and enhanced efficacy in actual and envisioned applications, including nanomedicine, environmental remediation, and food safety, among others. On this basis, any attempt to develop a size-controlled synthesis method and an efficient surface treatment protocol becomes indispensable. Accordingly, we have synthesized TiO2 NPs via a modified aqueous processing route using HNO3 as a catalyst, polyvinylpyrrolidone as particle size controller and a dispersing agent. The surface functionalization was carried out by using Ethylenediamine (EDA) as a source of amine species. Bare and functionalized TiO2 NPs were characterized by X-ray diffraction (XRD), Electron Microscopy (SEM and TEM) and FT-IR spectroscopy. The photocatalytic activity of TiO2 NPs was assessed by irradiating an aqueous solution of using Methylene blue (MB) dye containing different amounts of the NPs. XRD analyses evidenced the formation of anatase tetragonal TiO2 with an average crystallite size estimated at 10 nm. Bare and functionalized TiO2 NPs exhibited significant activity under UV light illumination (365 nm). Bare NPs exhibited a dye photo degradation capability of about 89.0-92.31% while functionalized NPs reported 94.58-98.42% dye photo degradation capability.

*To whom correspondence should be addressed. E-mail: [email protected]

Dye-sensitized solar cells (DSCs) have been studied extensively in recent years due to their high conversion efficiency and low cost in fabrication. In recent times, extensive research has been focused on novel bifacial and flexible solar cells for facilitating them in to advanced applications. Bifacial solar cell could produce up to 50 % more electric power by collecting albedo radiation from the surroundings. The lightweight and flexible metal foil, such as Ti foil and stainless steel, enable roll-to-roll mass production and make it possible to extend DSCs to newer applications. This present study focuses on enhancing the efficiency of bifacial and back-illuminated dye-sensitized solar cells (DSCs) by incorporating SiO2 mesoporous layer on TiO2 electrode. The performance of DSCs is investigated by UV-visible spectroscopy, incident photon conversion efficiency (IPCE) and electrochemical impedance spectroscopy (EIS). It is observed that the current ratio of back- to front-DSC increases from 0.72 to 0.78 with the increase in the thickness of SiO2 layer, which in turn influences the incident light to dyed-TiO2 electrode, especially in the wavelength of 400-600 nm. The increase in the JSC is due to the presence of SiO2 layer which contributes for the excellent light transmission. More the number of SiO2 nano-particles present in bulk electrolyte, higher will be the incident light passing through the dyed-TiOnot;not;2 electrode, which can reduce the light absorption (350-600 nm), leading to an increase of JSC by approximately 17.6 %. A small decrease of FF (ca. 1 %) is due to the increase of series resistance of DSC with SiO2 layer. The Ti foil based flexible small (0.28 cm2) and sub-module (5cm×10cm) DSCs having this modification show high conversion efficiencies of 6.76 and 5.54 % respectively under 100 mW cm-2 (AM 1.5).

We demonstrate the plasmon-enhanced photoelectrochemical (PEC) properties of a visible-light responsive composite material, which was fabricated by delicately depositing Au nanocrystals on the surface of TiO₂ nanowire (NW) array. The oscillation frequency of the surface plasmon resonance (SPR) of Au is strongly sensitive to the particle size and shape, with which Au nanocrystals may display effective light absorption from visible (Au nanoparticles) to IR (Au nanorods) region. As compared to pristine TiO₂ NW array, a significantly improved photoresponse was observed in both Au nanoparticle- and Au nanorod-decorated TiO₂ samples. The photoresponse improvement of Au-decorated TiO₂ was strongly correlated with the SPR absorption spectra of Au, manifesting the plasmonic effect of Au. Under visible light illumination, Au-decorated TiO₂ NW array exhibited a 10-fold performance enhancement in photocatalytic water splitting. The enhancement is attributed to the prominent SPR absorption of Au which acts as an effective sensitizer to produce hot electrons injecting into the conduction band of TiO₂. The current results may provide a new paradigm for designing plasmonic metal-TiO₂ nanocomposites that can effectively absorb the entire solar spectrum and beyond for solar fuel production.

The synthesis of InP/ZnS core/shell nanocrystals and TiO2 nanotubes and the optimization study to couple them together were explored in the quantum dot sensitized solar cells. InP/ZnS nanocrystals have the advantage of tunable optical properties and intrinsic nontoxicity. Highly luminescent InP/ZnS nanocrystals were produced by precursor-based colloidal synthesis for a photosensitizer. By controlling synthesis scheme and reaction parameters, different sized InP/ZnS nanocrystals were obtained. In order to improve on air stability, ZnS shell was grown on InP core. The emission peaks of the different sized nanocrystals were observed ranging from 520 to 620 nm. TiO2 nanotube is core inorganic material which is capable of efficiently harvesting light as well as being a prominent anode electrode in solar cells. The nanotubular form of TiO2 enhances charge transfer and reduces interfacial charge recombination. Free-standing TiO2 nanotubes with 10 um in length were produced by anodization using ammonium fluoride. The free-standing nanotubes were formed under the condition that chemical dissolution speed affected by fluoride concentration was faster than the speed of Ti oxidation. Electrophoretic deposition was carried out to couple the synthesized InP/ZnS nanocrystals with the TiO2 nanotubes. The nanoparticles suspended in a solution were forced to move toward the nanotubes electrode by applied electric field. Under constant voltage conditions, the current during the electrophoretic deposition decreased continuously with time. This indicates that electric resistance increased as the deposition progressed. An organic p-type charge transport material was used as the hole-transport material. For the InP/ZnS sensitized TiO2 solar cell, open circuit photovoltage, and a short-circuit current was measured under the illumination of a solar simulator.

Photocatalytic processes can be used to address several aspects of modern renewable energy production and management of environmental pollution. Titanium dioxide (TiO₂) is probably the most promising photocatalyst being environmentally friendly, with low cost, good photocatalytic activity and excellent photostability as demonstrated in electrochemical photolysis of water to produce hydrogen, and in abatement of volatile organic compounds from air. In this study, TiO₂ nanofibers were synthesized by hydrothermal method from TiO₂ powders in alkaline solutions and simple production of TiO₂ nanofibers by simply thermal annealing in air. In order to prepare the catalytically active TiO₂ nanofibers from the sodium hydrogen titanate nanofibers, a quick screening was carried out to find the appropriate calcination temperature. The samples were heated in air at 250, 400, 550, 600, 700, 850 and 1000 °C for 4 hours then analyzed by X-ray diffraction. Calcination at temperatures up to 850 °C for 4 hours resulted in the mixture phase of anatase and rutile. The TiO₂ catalyzed photodegradation of organic dyes obeys well the Langmuir-Hinshelwood mechanism. For low dye concentrations, the kinetics is of first-order and can be described as ln(C_o/C) = kt, where C is the concentration of the dye at time t, C_o denotes the initial concentration and k is the apparent reaction rate constant. The brilliant green decoloration over both the TiO₂ nanofibers calcined at 850 °C for 4 hours and the commercial Degussa P25 shows better rates with calculated rate constants of sim;1.96 × 10^-3 s^-1 and 1.75 × 10^-3 s^-1 , respectively. Consequently, we may assume that the obtained TiO2 nanofibers consisted of a mixture of anatase/rutile crystalline might be reasonable alternatives of the traditional Degussa P25 for photocatalytic applications.

TiO2 is the most used semiconductor for photocatalytic applications; despite its positive characteristics of non-toxicity, photostability, biological and chemical inactivity, it is known to be a high energy band gap material (Eg asymp; 3.2 eV) that can be excited only under UV irradiation, ruling out the use of visible light to promote photocatalytic reactions; nevertheless it is also characterised by low rate of e- transfer to reducible species and finally by high recombination rate of photoproduced charge carriers (e-/h+ pairs). In order to overwhelm the last one of the limits affecting TiO2 as photocatalyst, one of the most widely adopted strategy consists in producing TiO2-based mixed oxides photocatalysts. For that purpose, WO3-TiO2 mixed oxides were synthesized by a sol-gel method employing titanium tetra-isopropoxide (Ti(OC3H7)4) as titanium source and tungsten hexa-ethoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti molar ratios. The so prepared photocatalysts were then calcined at two different temperatures (500° and 700°C), in order to study the effect induced by sintering processes, and they were also modified by photodeposition of 0.5wt.% of Pt, to further enhance the e-/h+ pairs separation. Formic acid was used as a model compound for the abatement of organic pollutants in water under UV-visible irradiation, whereas the photocatalytic hydrogen production was achieved by photocatalytic steam reforming of methanol. Furthermore, the so-obtained materials were characterized by XRPD, BET, UV-vis reflectance and XPS analyses, to relate the physicochemical properties to the photocatalytic activity of the prepared samples.
Materials with an intimate contact between the two oxides led to an efficient photoproduced charge separation and a consequent enhanced photoactivity, further increased by Pt photodeposition. The optimal tungsten content was fixed at 1% W/Ti molar ratio, even ensuring the persistence of highly active crystalline anatase phase in TiO2, after calcination at high temperature. Since WO3 is not a photocatalyst as effective as TiO2, the photocatalytic activity of the mixed oxides decreased with W amount higher than 1 mol.% as a consequence of the increased extent of defects in the oxide structure, acting as recombination centres of photogenerated charge carriers, further proving that the photoactivity increase observed upon tungsten addition is to be mainly related to an improved electron-hole separation in TiO2 and it is not originated by the action of WO3 as photocatalyst.

Anatase nanocrystalline TiO2 (NC-TiO2) is one of the most versatile inorganic wide band-gap semiconductors, utilized for a variety of applications including photocatalysis, water splitting and solar cells. Since the performance of these devices depends strongly on the charge transport properties through the TiO2 matrix, more quantitative information on e.g. electron trap densities, and intra- and inter-particle charge motion is required. Despite extensive investigations, these properties are only partially known for NC-TiO2.
In this work the change in conductance in NC-TiO2 and in polycrystalline TiO2 (Pol-TiO2) were recorded using the time-dependent microwave conductance technique (TRMC) on pulsed UV excitation. Analysis of the transients yields the real and imaginary components of the conductance. Unexpectedly, these two components decay with identical time profiles, showing that they are determined by one transient species, i.e. conduction band electrons. Using the Drude-Smith model the same scattering momentum time (tau;) of 3.2x10-13 s and effective electron mass of ca 25 Me were found for both the NC-TiO2 and the Pol-TiO2 sample. This means that in the two materials the mobile electrons experience scattering events at comparable time intervals. Hence the presence of grain boundaries and nanoparticle surfaces in the NC-TiO2 does not affect tau; and the electron mobility. The yield for mobile charge carriers, however, is two orders of magnitude smaller in the NC material due to deep trapping of electrons. The TRMC measurements demonstrate that the large amounts of trapped electrons do not affect the motion of conduction band electrons and do not contribute to an additional response in our microwave measurements at least at the used frequency of 9 GHz. Next TRMC measurements on dye sensitized NC-TiO2 using pulsed excitation at 550nm were carried out. From the correspondence with results on the bare NC-TiO2 we conclude that photogenerated holes do not form any bound pairs with conduction band electrons or form any positive polaron influencing the complex photoconductance.

Nanostructured titanium dioxide (TiO₂) materials show an enhanced coupling to the environment due to their characteristic high surface-to-volume ratio. This property has been used to design improved electrochromic, photochemical and photoconductive devices. In this context the coupling between TiO₂ and selected dopants has been the key to the development of hybrid TiO₂-based active materials for optimized photovoltaic solar cells [1] and for photochromic paper [2]. Both materials are based in a nanostructured TiO₂ matrix doped either with a dye or Ag nanoparticles, respectively. Other interesting type of dopants are the trivalent rare-earth (RE) ions used for the development of light emitting devices both in the infrared and visible [3].
In the present work, we study the conditions to obtain efficient light-emission from Er³⁺-doped TiO₂ nanoparticulate materials prepared by a colloidal sol-gel method. In a previous work we have already reported the preparation and linear optical response of Er-doped TiO₂ films by electrophoretic depositon using the low cost and large scale colloidal sol-gel technique [4]. Sols with Er concentrations ranging from 0.5 to 6 mol% have been prepared. Structural characterization techniques and optical spectroscopy techniques have been used to monitor their properties upon annealing treatments up to 900 ^oC. It is shown that the colloidal synthesis leads to the formation of monomodal particulate sols with average nanoparticle sizes around 4 nm. It is observed that the anatase to rutile phase transition temperature increases as a function of the Er content. All the samples annealed above 300 ^oC show the characteristic and well defined Er photoluminescence spectra with a maximum intensity around 1.5 microns. It has been found that the samples with concentrations below 1 mol% show the highest emission efficiency (photoluminescence intensity and lifetime) and have promising properties for the development of integrated gain devices in the infrared. For higher concentrations Er-Er energy transfer processes take place, leading most likely to up-conversion processes that have potential applications for solid-state lighting devices in the visible. The influence of the Er ions concentration and of the anatase-rutile phase on the photoluminescence response will be discussed.
[1] I. Chung, B. Lee, J. He, R. P. H. Chang and M. G. Kanatzidis, Nature 485, 486 (2012).
[2] Y. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota and A. Fujishima, Nat Mater 2, 29 (2003).
[3] W. Luo, C. Fu, R. Li, Y. Liu, H. Zhu and X. Chen, Small 7, 3046 (2011).
[4] M. Borlaf, M. T. Colomer, F. Cabello, R. Serna and R. Moreno, J. Phys. Chem. B (2012) DOI: 10.1021/jp304044w.

We demonstrate continuous, self-sustained hydrogen gas production solely based on solar light and biodegradable biomass recycling, by coupling solar water splitting and microbial electrohydrogenesis in a photoelectrochemical cell-microbial fuel cell (PEC-MFC) device assembly. The PEC device utilizes a TiO2 nanowire-array photoanode and a Pt cathode for solar-to-hydrogen conversion. The MFC is an air-cathode dual-chamber device, inoculated with either Shewanella oneidensis MR-1 (batch-fed on growth medium) or natural microbial communities (batch-fed on local municipal wastewater). Under light illumination, the PEC TiO2 photoanode generates a photovoltage of ~0.7 V that shifts the potential of MFC bioanode to overcome the potential barrier for microbial electrohydrogenesis. With light illumination (AM 1.5G, 100 mW/cm2) and wastewater as the energy source, we demonstrate pronounced current generation of 1.25 mA/cm2 and a continuous hydrogen gas production at zero external bias (0 V vs. Pt). Stability tests on the device show continuous current flow and hydrogen generation over an interval of eight days. In addition to the measurement results, the cost-effectiveness and wide availability of the device materials suggest a promising method for self-sustained hydrogen gas production from renewable energy sources. The successful hydrogen generation from a self-biased, sustainable microbial device demonstrates a new solution that is able to address wastewater treatment and clean energy generation at the same time.

In this investigation, thin films of polyvinylidene fluoride (PVDF) containing nanoparticles of the ceramic titanium dioxide (TiO₂) are synthesized using physical vapor deposition techniques. This combination of materials shows promise for possible use as the dielectric in capacitors, with a particular eye toward energy storage. This composite approach allows for the integration of complimentary features such as high dielectric permittivity from the integrated nanoparticles and high breakdown strength from the polymer matrix, resulting in a greatly enhanced energy density. Co-deposited films with a TiO₂ content up to about 8 % have been synthesized and intermittent contact AFM and elemental mapping from EDS show that the dispersion of the nanoparticles in the material is homogeneous. Results concerning the composition and structure of the films using XPS and SEM are also presented. In addition, parameters such as the dielectric constant and the breakdown voltage are given.

Titanium dioxide is a material of choice in numerous traditional and emerging applications. In particular, this semiconductor is commonly used as a photocatalyst due to its relatively high activity and resistance to corrosion. Control of the electronic structure in the band gap is vital since efficiency of photocatalytic generation of electron-hole pairs strongly depends on the gap states. Moreover, for practical photocatalytic applications, it is necessary to sensitize this wide band-gap material to visible light. Thus, to address these issues, doping and irradiation were implemented in our studies. Thin (~ 200 nm) films of TiO₂, TiO₂/Ag, TiO₂/Au, TiO₂/ZrO₂ and TiO₂/ZrO₂/SiO₂ were grown by the sol-gel method on Si substrates. ZrO₂ and SiO₂ as well as Ag and Au nanoparticles were added to the titanium dioxide system to augment the gap structure. Composition of the films was analyzed with Auger electron spectroscopy. Additionally, the films were irradiated with a flux of Ti⁺ ions (140 KeV, 10¹² ions/cm²). The effects of irradiation and doping were probed using two defect-sensitive optical methods: surface photovoltage (SPV) spectroscopy and photoluminescence (PL) spectroscopy. The obtained results demonstrated, firstly, a good correlation between the data acquired by both techniques and, secondly, established that addition of metal oxides and noble metal nanoparticles introduces additional levels within the band gap. For example, addition of noble metals to the pure TiO₂ system results in a new state at ~ 1.8 eV above the top of the valence band. For some samples exposure to Ti⁺ ions resulted in a significant increase of the PL signal intensity indicating that irradiation could be a possible way of controlled generation of gap states available for visible light activation. Gaussian fits of the PL spectra reveal that the dominant defects are bulk states with energies ~ 2.3 eV, ~ 2.5 eV, and ~ 2.9 eV above the top of the valence band (both before and after irradiation). SPV results show, however, that additional gap states are present at the surface. Our results suggest that greater photocatalytic efficiencies in visible light could be achieved due to a greater number of gap states in specimens with more complex compositions.

Previous studies have shown that anatase (titanium dioxide), particularly with exposed (001) facets, is a highly photochemically active material with potential uses as catalyst for surface chemistry and as a semiconductor for electrical components. Due to the natural ubiquity of titanium and the ease of synthetic preparation, anatase shows great promise for the development of sustainable technologies for energy and the environment. In this study, highly [001] oriented anatase thin films (<1 µm thick) with ~100% exposed {001} facets were grown on gold coated silicon wafers and sapphire substrates using a fluoride mediated hydrothermal synthesis1. Due to the adsorption of fluorine in the synthetic mechanism, the stoichiometry of the as synthesized film is approximately TiO1.8F0.2(H20)0.2. Fluorine terminated {001} surfaces are less photochemically active then surfaces with fluorine has been removed. Post synthesis annealing is required to remove fluorine and water from the film to achieve the desired TiO2 stoichiometry and improve the film&’s crystallinity. A set of similarly prepared films were subjected to annealing temperatures from 80-1000°C to study the effect this process has on the phase, morphology and elemental composition. The resulting films were analyzed using diffuse reflectance FTIR, XRD, Raman, SEM and EDS. It was found that liquid water trapped within the film is removed between 170 -200°C, erupting out of the bulk of the material creating square shaped holes. Fluorine is removed gradually and is almost completely gone above 500°C after two hours. The films annealed above 500°C were shown to be more crystalline, exhibiting sharper XRD line widths, and greater transmissivity than as prepared samples. {001} anatase films heated above 800°C showed no evidence of rutile phase TiO2 but exhibited different crystallographic texture compared to the highly [001] oriented as-synthesized films. By contrast, anatase powder synthesized from sol-gels without fluoride with typical truncated bipyramidal morphologies and dominant (101) facets converts readily to rutile above 600°C, suggesting that this alternated crystal morphology inhibits the phase transition until higher temperatures and has higher crystallographic stability. Understanding the effects of post synthetic treatment of these oriented films will allow for better customization for practical use. A future study will include the effects of annealing temperature on the photochemistry of the material.
References:
1. Ichimura, AS; Mack, BM; Usmani, SM; Mars, DG Chem. Mater. 2012, 24, 2324-2329.

Great emphasis in research of composites materials synthesis has drawn the attention of scientists in recent decades due to their wide range of potential use [1-3]. The use of polymers and inorganic materials has given opportunity of developing different electro-chemical devices with applications in gas and humidity sensors, biosensors, electronic devices, optical devices, batteries and biological tissue engineering. In this work, we have examined composite films of polyaniline (PANI) and titanium oxide (TiO₂) produced by chemical synthesis and applied as pH sensors in the EGFET configuration. To obtain PANI-TiO₂ salt, an TiO₂ was dissolved in 20 mL of HCl solution 1M. After that a sonication process during 10 min was used for dissolving the salt completely. The mass of TiO₂ salt was 50mg, 100mg, 500mg and 1g. PANI was doped with DBSA: 1.50g of DBSA and it was dissolved in 20 mL of HCl 1M and further added to TiO₂ solution. Then, 1mL of aniline was added in TiO₂-DBSA solution, in ice bath with a temperature of 2°C in constant stirring. 1.2g of ammonium persulfate (APS) was dissolved in 40mL of HCl 1 M and this solution was slowly dropped in Aniline-TiO₂-DBSA solution. The reaction proceeded at 2 °C for 24 h and the solution was washed with water and acetone for several times, and finally dried at room temperature. The films of PANI-TiO₂ were prepared on glassy carbon dissolving 50 mg in 2.5 mL of chloroform and dried at room temperature 24h. The films were characterized as extended gate field effect transistor (EGFET) pH sensors. Linear responses as a function of pH were obtained for two different ranges: acid and basic. Basic range response shows more sensibility than acid range. Thus, PANI-TiO₂ are promising films for this pH range. In future, electrospinning fibers will be prepared with this synthesis and then they will be applied as gas and humidity sensor. We expect to discuss these things in this presentation. Work supported by CAPES, CNPq and FAPESP. References:
[1] P. Li, Y.Li, B. Yinga, M.Yanga. Sensors and Actuators B 141, 390-395 (2009).
[2] Q. Yub, M.Wanga, H. Chena, Z. Daib. Materials Chemistry and Physics 129, 666- 672 (2011)
[3] S. Neuberta, D. Pliszkaa, V. Thavasia, E. Wintermantel, S. Ramakrishna. Materials Science and Engineering B 176, 640-646(2011).

Complex TiO2 nanostructures composed of an arbitrary TiO2 nanocrystal “core” and anatase TiO2 nanowhisker “antennas” are fabricated by the strategy of colloidal epitaxial growth. We first show the growth of TiO2 nanocrystals with various morphologies by using a number of different methods, and discuss the key factors that control the morphology of the products. By using these shape controlled TiO2 nanocrystals as seeds, we further demonstrate the growth of secondary anatase TiO2 structures on these TiO2 nanocrystal seeds through a pyrolysis reaction. The structural evolution during the growth is studied by HRTEM and revealed TiO2 secondary structures such as nanowhiskers were epitaxially grown from the seeds along specific directions. As a result, well-defined TiO2 nanostructures with highly complex secondary structures are produced. By systematically tuning the structures of original seeds and the epitaxial growth condition, we are able to tailor the secondary structures and produce complex three-dimensional structural configurations in a highly predictable manner.

Titanium dioxide (TiO2) is a wide bandgap (3.2 eV) semiconductor that has important environmental and commercial applications in water remediation, photooxidation, and dye-sensitized solar cells [1, 2]. In this study, the photocatalytic ability of two types of TiO2 films were investigated. One set of films consisted of anatase films with a high fraction of {001} facets and the other utilized Degussa P-25 (a mixed phase of 80% anatase and 20% rutile). Anatase films with exposed {001} facets are of particular interest due to the apparent high reactivity of this facet compared to the {101} facet [3]. Recently, a method to prepare ~100% exposed {001} faceted anatase films was reported [4]. Thus, the primary objective of this work is to compare the photooxidative ability of {001} faceted anatase films to the P-25 films.
Anatase films were hydrothermally synthesized on the gold-coated silicon wafers from a solution of TiF4 and HF [4]. Annealing at 600 °C removes surface and bulk fluorine and improves the crystallinity of the film. P-25 films were prepared by spin-coating a P-25-ethanol slurry onto Pyrex substrates, which were then sintered at 450 °C. The films were positioned vertically in quartz cuvettes containing dilute, aqueous solutions of the compounds of interest, which included phenolic derivatives and colored dyes such as phenol, 4-nitrophenol, methylene blue (MB), and rhodamine B (RB). The solutions were photolyzed with 365 nm light and the concentrations were monitored by UV-vis spectroscopy to quantify the rate of photodegradation. It was found that sintered P-25 and (001) faceted anatase films showed comparable photocatalytic efficiencies when normalized for surface area, but not the enhancement expected for reactive (001) facets. A second set of experiments sought to extend the absorption of TiO2 into the visible region. In this work, TiO2 films were hydrogen-implanted using a low energy ~1 keV proton beam. The films absorbed visible light and both P-25 and {001} faceted anatase produced hydroxyl (*OH) radicals upon irradiation as indicated by terephthalic acid (TA) fluorescence assay. Our preliminary results on the effects of H+ implantation on photo-oxidation efficiency will be reported.
References
1. Hashimoto, K.; Fujishima, A.; Irie, Hiroshi. TiO2 Photocatalysis: A Historical Overview and Future Prospects. Japanese Journal of Applied Physics 2005, 44, 8269-8285.
2. Hoffman, M. R.; Martin, S. T.; Choi, W. Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96.
3. Fang, W. Q.; Gong, X-Q.; Yang, H. G. On the Unusual Properties of Anatase TiO2 Exposed by Highly Reactive Facets. The Journal of Physical Chemistry Letters 2011, 2, 725-734.
4. Ichimura, A.; Mack, B.; Usmani, S. M.; Mars, D. Direct Synthesis of Anatase Films with ~ 100% Exposed {001} Facets and [001] Preferred Orientation. Chemistry of Materials 2012, 24, 2324-2329.

One dimensional nanostructure is an impressive part of energy conversion devices due to its efficient electrical transport and exceptionally large surface area. We present photovoltaic devices with TiO₂ anatase nanotubular structured photoanodes that fabricated by template directed atomic layer deposition (ALD) method. The dimension of TiO₂ nanotube such as wall thickness, length and diameter is precisely controlled and consisted of quasi-single crystalline elongated grains of anatase. We firstly evaluated nanotube&’s crystal structure by transmission electron microscopy (TEM) and electrical conductivity. As a result, electrical conductivity of nanotubes shows almost 10³times higher than anodized nanotubes and nanoparticulated films. Photoluminiscence (PL) and Raman spectroscopy were performed for the understanding of their charge transport properties. As strong ionic bonded crystals with a soft phonon vibration (Eg), small polaronsare expected in anatase crystals, unlike to Rutile. We observed fingerprints for the small polarons from our well organized and crystallized anatase nanotubes using low temperature - PL and Raman study. These observations strongly support small polaron hopping mechanism with longer life time in anatase crystals as photoanodes. With our nanotubular TiO₂ photoanodes, we obtained photon-to-electricity conversion efficiency of DSCs (Dye-Sensitized Solar Cells) over 7% under AM 1.5 illumination.

Titanium dioxide (TiO2) has been regarded as one of the most promising anode material for lithium ion batteries due to its environmentally benign, abundance, low cost, and structural stability during Li insertion/extraction. However, poor rate capability of TiO2 based anode, caused by inherently low electronic conductivity, limits its practical use. Recent research strategies have mainly focused on improving the electronic conductivity of the active materials (TiO2) for lithium ion batteries to overcome poor rate capability problem. On the other hand, other key factors governing the rate capability of TiO2 based electrode have not been explored to its fullness. Here, we systematically demonstrate, for the first time, the role of factors governing the rate capability of TiO2 based electrode. The vertically aligned TiO2 nanotubes array with sealed cap and unsealed cap, directly grown on the current collector, and conventional randomly oriented TiO2 nanotubes electrode using paste process were prepared to explore the role of conductivity of active materials, kinetics and resistance between the active material and the current collector. The vertically aligned TiO2 nanotubes array electrode with unsealed cap showed superior performance with six times higher capacity at 10 C rate compared to conventional randomly oriented TiO2 nanotubes electrode with 10 wt% conducting agent. Based on detailed experimental results and theoretical analysis, we report that the reduction of the electronic resistance between electrode and current collector plays an important role on improving the electronic conductivity of the overall electrode system.

Porous materials in relation to environment and energy such as air, water filter for waste disposal and toxic substance, solar cell and secondary battery, are actively studied these days. Open and closed pore structures in porous materials increase its surface area to improve absorption and diffusivity as well as improving elevation elasticity and strength of materials. Therefore, porous materials can be used for shock and sound absorber, as well as light structural materials. Conventional process for fabricating porous materials has many disadvantages such as high temperature process, and its difficulty to control pore size and distributions. In this study, novel method of porous materials fabrication for the application of dye sensitized solar cell (DSSC) is being introduced. DSSC has many advantages such as simple fabrication process and low cost over those for Si Solar Cell. In addition, flexible substrates can be used to fabricate flexible solar cell. Nano particle deposition system (NPDS), which is a dry and room temperature powder deposition system, is used to form porous photoelectrode by simultaneous deposition of metal and ceramic powders. Sn powders were used as low melting point metal to burn off after sintering at 500°C and TiO2 powders were deposited simultaneously. It was confirmed that the amount of dye absorption and scattering effect of light using this porous photoelectrode, was improved due to its porous structure. This porous photoelectrode structure for DSSC was confirmed by several analytical tools such as FE-SEM, BET and UV-Visible spectroscopy. Therefore, fabrication of porous photoelectrode has been demonstrated using NPDS.

One of the most effective way to increase the lifetime of photogenerated charges in anatase TiO2 during a photocatalytic process is to couple the oxide with a different band gap semiconductor, in order to form an heterostructure. SnO2 is suitable for this purpose since this oxide belongs to the same crystal symmetry (tetragonal) of anatase TiO2, and both present two molecular units per primitive unit cell. These facts make the association between those materials easier. In this research, we investigated two different synthesis methods to produce TiO2-SnO2 heterostructures in three different proportion, and their effects regarding the particle photoactivity: hydrolitic sol-gel (HSM) and polymeric precursor method (PPM). Thermal treatment at 200°C for HSM and at 400°C for PPM was required to obtain desired crystalline phases. X-ray diffraction analysis showed that both methods were able to promotes rutile SnO2 growth over pre formed anatase TiO2. Even after thermal treatment at different temperatures, crystallite size obtained by Scherrer equation didn&’t show remarkable difference to the samples with lower proportion of SnO2. Using Raman spectroscopy analysis, we suggest that higher dispersion of SnO2 over TiO2 was obtained by the PPM. Rhodamine B photodegradation in water under UV radiation was used as a probe reaction to measure the particles photoactivity. Samples with similar proportion in mass of TiO2 and SnO2 showed higher photoactivity in both methods applied in this research. The results show that there is an optimum value for mass ratio independent of the synthesis method. This occurs since this proportion promotes longer lifetimes of photogenerated charges, due to the heterojunction formed. Furthermore, these samples present enough free surface composed of TiO2, which actually leads to the photodegradation process due to high oxidative positive hole in its valence band. Despite the difference in annealing temperature, there is no remarkable differences in materials properties obtained by different synthesis method, as specific surface area and photoactivity, when comparing samples with the same covering degree, showing that oxides proportion is the most important factor regarding photocatalysis. In this sense, we suggest that different annealing temperature, in the range applied here, does not affect the coupling between oxides, a main factor acting in the systems. For both methods, the interface affect the charge separation, leading a better photocatalytic response.

Thanks to the size of its band gap and its relative position toward HO-/HO#9679; and O2/O2#9679;- redox potentials, TiO2 is one of the most efficient photocatalyst under UV irradiation and anatase is the active polymorph. The particles used must have a relatively high specific surface tailored to efficiently interact with the pollutant. Moreover, a good stacking of atoms constituting the crystalline structure is mandatory to display high activity since defects may act as recombination centre between photogenerated electrons and holes.1 Recent works on that phase demonstrated in addition to an influence of the nanoparticles size, an effect related to the nature of the exposed surfaces.2
We used the sol-gel method to obtain a wide range of anatase nanoparticles sizes and morphologies by changing the concentrations, ions in solution, solution acidity, and aging parameters and by new activation methods such as microwave heating.3
Especially, different shapes of pure anatase nanoparticles have been synthesized by selective adsorption of organic molecules during the syntheses.4 The influence of synthesis parameters and the organic molecules nature on the nanoparticles structure and morphology have been analysed with various techniques, such as XRD and HRTEM.
The photocatalytic efficiency of the different morphologies was probed by the degradation of Rhodamine B in aqueous solution and the activity was correlated to the surface composition. The adsorption of the dye as well as his degradation has been discussed.
Pyridine adsorption on anatase surfaces in order to get acidity properties have been done and compared to MUSIC model.5 Reactive Oxygen Species production of each morphology has been studied.
The approach presented here can be extended to other application in which the nature of the material surface is a key parameter., such as Dye-Sensitized Solar Cells.6
[1] 1. G. Benko, B. Skarman, R. Wallenberg, A. Hagfeldt, V. Sundstrom, and A.P. Yartsev, J. Phys. Chem. B 107, 1370 (2003),
[2] D. Q. Zhang, G. S. Li, H. B. Wang, K. M. Chan, J. C. Yu, Cryst. Growth Des., 2010, 10, 1130
[3] F. Dufour, S. Cassaignon, O. Durupthy, C. Chanéac, Eur. J. Inorg. Chem, 2012, accepted
[4] T. Sugimoto, X.P. Zhou, A. Muramatsu, Journal of Colloid and Interface Science, 2003, 259, 53
[5] T. Hiemstra, P. Venema and W. H. V. Riemsdijk, J. Colloid Interface Sci. 184, 680 (1996)
[6] C. Magne, F. Dufour, F. Labat, G. Lancel, O. Durupthy , S. Cassaignon, T. Pauporté, J. Photochem. Photobiol. A, 2012, 232, pp 22-31

Various nanostructures of titanium dioxide (nanoparticles, nanotubes and nanorods) are widely used as electrode material in diversity of applications, such as dye-sensitised solar cells (DSSCs), photocatalysis and sensors. This paper focuses on how the unusual crystallography and shape of new, two-dimensional TiO₂ nanoplatelet materials can be exploited in such applications. We will discuss the fabrication of hybrid films from two-dimensional nanoplatelets and other anisotropic nanostructures through layer-by-layer deposition and post-treatment of doctor-blading techniques in different conditions, the characterisation of the films via electron microscopy, X-ray diffraction, thickness measurements and adsorption measurements, and the assessment of the hybrid film performance in photovoltaic devices.

In the on-going technological efforts for sustaining supercapacitors effectiveness, considerable efforts are being focused on advanced techniques for synthesis and characterization of inexpensive and effective electrode materials with improved performances. This study explores a compositional formulation synergy of incorporating the relatively highest specific capacitance but expensive, ruthenium oxide and a high specific capacitance, less expensive, but chemically unstable manganese oxide, with relatively lower specific capacitance but chemical stable and readily available titanium oxide. Submicron sized titanium oxide powder was mixed with varying compositions with micron sized manganese powders and submicron sized ruthenium powders using a turbula mixer. Blended powders were consolidated using spark plasma sintering, at temperatures ranges of 1200 to 1400oC, with a holding time of 5min, heating rate of 50k/min and a pressure of 25MPa. XRD studies on the as-consolidated composite and SEM-EDX analysis of the polished sections were used to study the metallurgical interactions that occurred during spark plasma sintering. Diffraction studies and SEM analysis confirmed different phase formation during the sintering process. SEM analysis of polished sections revealed different pore morphologies that were dependent on compositional variations and sintering parameters. The supercapacitive behaviour of the composite electrodes can therefore be consequential on the variation in compositional variation as well as pore morphologies.

This work investigates the spectroscopic and photocatalytic properties of a series of Au@TiO2 yolk-shell nanostructures, where the crystallinity of the TiO2 shells was varied by changing the etching and calcination conditions. The goal of the work is to identify spectroscopic observables that correlate with the measured photocatalytic activity of the particles. By measuring the H2 production under ultraviolet illumination, a strong correlation between TiO2 shell crystallinity and the H2 production rate was found. Time-resolved photoluminescence experiments on these samples showed that the lifetime of the photoluminescence increases with sample crystallinity, providing a good correlation with photocatalytic activity. But this correlation is only found when the excitation wavelength is less than or equal to 300 nm. Transient absorption experiments on the samples showed no correlation of the decay with photocatalytic activity. Our results imply that photoexcitation with high energy photons can generate both reactive surface sites and photoluminescent surface sites in parallel. Both types of sites then undergo similar electron-hole recombination processes that depend on the crystallinity of the TiO2 shell. Surface sites created by low energy photons, as well as bulk TiO2 carrier dynamics that are probed by transient absorption, do not appear to be sensitive to the same dynamics that determine chemical reactivity. These results provide a preliminary way to connect spectroscopic observables with photocatalytic activity in these nanomaterials.

Titanium dioxide films find applications in photodegradation of contaminants, dye sensitized solar cells (DSSC), and hydrogen production1. Recently, Yang et al. synthesized anatase (TiO2) powders with the high energy {001} facets preferentially exposed.2 In our work, we have synthesized oriented anatase thin films with ~100% (001) exposed facets that also show [001] preferred orientation.3 Synthesis of the films was accomplished by controlling the crystal morphology during growth by the addition of a capping agent. Fluorine binds to the {101} and {001} facets but lowers the surface energy of the minority {001} surface relative to the {101}. Thus the crystal grows isotropically along the [100] and [010] zone axes leading to highly truncated bipyramid morphologies whose surfaces are dominated by the {001} facets.
It is believed that the growth of the films proceeds by nucleation on the surface followed by a competitive growth mechanism. In this experiment we will study the proposed competitive growth mechanism of the synthesized films via electron backscatter diffraction (EBSD). This is accomplished by taking a film ~900 nm thick and using a vibratory polisher to polish and thin the film to a thickness 800 , 600 and 400 nm,. Grazing angle-XRD and EBSD both show that the {001} facets are normal to the c axis with an angular distribution of ~8 degrees. Inverse Poll Figure data suggests that the growth of neighboring grains are not independent one of another. This evidence is highly suggestive of a competitive growth mechanism.
1. Baddour-Hadjean, R.;; Pereira-Ramos, J-.P. Chem. Rev. 2010, 110, 1278-1319.
2. Yang, H.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638-641.
3. Ichimura, A. S.; Mack, B.; Usmani, S.M.; Mars, D. Chem. Mater., 2012, accepted.

Bi20TiO32 nanobelts were obtained through a nano-scale solid state reaction by using Na2Ti3O7 nanobelts as a solid template for the first time. This methodology provides a one-step, convenient, low-cost, nontoxic and mass-production route for the synthesis of nanobelt-structured functional matarials. The Bi20TiO32 nanobelts were characterized by Scanning Electron Microscope, X-ray Powder Diffraction and High-Resolution Transmission Electron Microscope examination, UV-Vis diffuse reflectance spectrum and so on. In this study, the mechanism of the reaction was investigated. Bi2O3 can be got from the decomposition of BiONO3 under lower temperature because of the existence of activated TiO2. Inspiringly, the as-prepared Bi20TiO32 nanobelts displays photocatalytic activity that is about 60% higher than the pure TiO2 nanobelts under the same conditions in the decomposition of methyl orange (MO) aqueous solution, and the reasons for the high photocatalytic activity were also discussed.

Nano-sized gold supported on crystalline titanium dioxide represents a promising new catalyst for a number of reactions, but is especially effective for the photocatalyzed production of hydrogen from ethanol, which is a potential renewable source of hydrogen. Typically, a high temperature (~500 °C) is required to produce hydrogen from ethanol, but under UV irradiation titania-supported gold can enable this reaction to occur at room temperature. Both the metal and support are active in this reaction, in which the titania serves to absorb light and generate electrons to initiate the reaction and gold enhances the electron lifetime such that the electrons are available to drive the reduction of hydrogen.
Typical catalyst designs feature gold nanoparticles reduced on the surface of titania, which although effective, are limited in that they are vulnerable to sintering during processing. This issue creates difficulty for tuning the nanoparticle size independently of the loading percent, as higher loading percents will lead to increased particle size, which complicates the study of the parameters of this system as well as attempts to determine an optimal catalyst configuration. To avoid this issue, we have designed a method for synthesizing a gold@titania core/shell nanoparticles, in which we can vary the core size as well as the shell thickness. In this synthesis, a highly concentrated aqueous solution of previously synthesized gold nanoparticles is dispersed in an ethanolic solution containing surfactant and an acid catalyst, to which a titanium alkoxide is slowly added to produce a controlled coating on the gold. Additional coating steps may be performed to increase the titania shell thickness. The as-synthesized core/shell nanoparticles possess an amorphous titania shell, which is subsequently rendered highly crystalline through a high temperature calcination process.
The titania shell presents a physical barrier to reduce sintering during the high temperature processing, which enables us to obtain both a photoactive crystalline shell as well as excellent contact between the gold and titania. The core/shell structure also increases the degree of gold-titania interfacing in the system, which is beneficial as recent studies have suggested that the metal-support interface is the most active portion of a number of catalysts. Since the stability of this catalyst system enables us to control the size and the loading percent of the gold nanoparticles independently, we are able to perform a catalysis study comparing the effect of different shell thicknesses, and corresponding gold loading percents, without significantly changing the gold size during processing. We expect this catalysis data to shed further light on the mechanism of this system and core/shell metal/metal oxide catalysts in general, as well as aid in identifying the optimal configuration for a titania-supported gold catalyst for photocatalytic hydrogen production from ethanol.

Nano photocatalytic materials have shown great potentials not only in environmental remediation, but also in solar-chemical conversion by photocatalytic water-splitting as well as CO2 reduction. Up to now, aiming at realizing a more efficient utilization of solar energy, we have paid extensive attention on the fundamental research and development of nano-structured photocatalysts which are highly efficient under not only UV but also visible-light irradiation1-4). In this talk, our recent works in challenging the possibilities of the nano-structured photocatalytic materials will be introduced5-13). Specifically, design and control of surface/interface nano-structures of TiO2 and novel oxide photocatalysts such as Ag3PO4, from both experimental and theoretical approaches for higher photocatalytic activity will be introduced and discussed in detail.
References
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[2] D. Chen, J. Ye, Adv. Funct. Mater. 18, 1922-1928 (2008).
[3] D. Wang, T. Kako, J. Ye, J. Am. Chem. Soc. 130, 2724-2725 (2008).
[4] X. Li, N. Kikugawa, J. Ye, Adv. Mater. 20, 3816-3819 (2008).
[5] Z. Yi, J. Ye, N. Kikugawa, T. Kako, et al., Nature Mater. 9, 559-564 (2010).
[6] S. Ouyang and J. Ye, J. Am. Chem. Soc. 133, 7757-7763 (2011).
[7] Y. Bi, S. Ouyang, N. Umezawa, J. Cao, J. Ye, J. Am. Chem. Soc. 133, 6490-6492 (2011).
[8] G. Xi, S. Ouyang, P. Li, J.Ye, et al., Angew Chem Int. Ed., 51, 2395 -2399(2012).
[9]Y. Bi, S. Ouyang, J. Cao, J. Ye, Phys. Chem. Chem. Phys.13, 10071-10075(2011).
[10] X Chen, J. Ye, S Ouyang, T. Kako, Z. Li, and Z. Zou, ACS Nano, 5(6), 4310-4328(2011).
[11] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater., 24(2), 229-251, (2012).
[12] S. Ouyang, H. Tong, N. Umezawa, J. Cao, P. Li, Y. Bi, Y. Zhang, J. Ye, J. Am. Chem. Soc., 134, 1974minus;1977 (2012).
[13] G. Xi, S. Ouyang, P. Li, J.Ye, et al., Angew Chem Int. Ed., 51, 2395 -2399(2012).

Thin TiO₂ coatings can be used to create transparent, photocatalytically active, self-cleaning, antifogging, and superhydrophilic surfaces and therefore have a very high industrial relevance. For high scale industrial applications, it is important to develop cheap, fast and environmentally friendly deposition techniques. Functionality in this kind of TiO₂ layers is only obtained after crystallization of the material at temperatures above 400 °C. This means that most state-of-the-art deposition approaches such as sol-gel deposition or chemical vapour deposition do not allow deposition on heat-sensitive substrates such as polymers or wood for example. Therefore, it is important to investigate in innovative deposition processes that can significantly reduce the minimal conversion temperature.
In this research, we developed different, preferably water-based, Ti⁴⁺ precursor solutions and studied their conversion into crystalline titania nanoparticles by microwave-assisted solvothermal treatment. The use of microwaves allows to optimize the production and energy efficiency of the synthetic process. Once stable suspensions of the anatase nanoparticles (max 50 nm diameter) are obtained, these are optimized for use in ink-jet printing devices. This allows very efficient deposition of transparent titania coatings at reduced temperatures down to 150 °C. Weathering/durability tests and extensive characterization of the photocatalytic activity of these layers were performed to analyze the performance and processing limits for this kind of titania coatings.
Research funded by EU project EFECTS (FP7-NMP-2007-SMALL-1 grant n°205854).

Nanostructured TiO2 has shown promise as a dielectric material for high energy density capacitors because of its high dielectric constant and high dielectric breakdown strength. Strategies to improve the breakdown strength include doping with transition metal ions. Mn-doped TiO2 followed by an appropriate thermal treatment exhibit a significantl increase of the grain boundary resistivity by lowering the dielectric loss. Electrical measurements along with electron paramagnetic resonance of Mn-doped nanoscopic TiO2 demonstrate that sintering at ~900°C produces optimal electrical properties that are correlated with a non-uniform distribution of dopant ions, which are concentrated at the grain boundaries. Effect of various dopants including vanadium and the sintering atmosphere on the impedance analysis of nanostructured TiO2 dielectrics will be discussed and correlated with the results of electron paramagnetic resonance.

TiO2 is one of the most important materials for photocatalysis, photovoltaic cell, water splitting and bio & gas senor applications. Although nanoparticles, such as P25, possess high efficiency for photocatalysis or other applications, the nanosized morphology of them causes high cost for separation from reaction system after processing, difficulty for operation in preparation of device, or failure for recycle application. More over, it is difficult to further improve photocatalysis properties or broaden the absorption wavelength region by using assembling heterostructure on the nanoparticles. TiO2 nanobelts provide such a opportunity for getting high performance photocatalyst materials. Although TiO2 nanobelts also have some characteristics of nanosized materials, the very large surface of the nanobelts provide an enough operation area for assembling nanoheterostructures on it and form TiO2 nanobelt surface heterostructures. TiO2 nanobelt surface heterostructures keep the basic properties of TiO2 nanoparticle, and the heterostructure would bestow the nanobelt two important functions: 1. inhibiting re-combination of photo-produced hole and electron; 2. broadening the absorption wave length region from ultraviolet to visible wavelength. Most importantly, free standing catalysis membrane can be obtained by assembling the nanobelt using a paper-making method. TiO2 nanobelt surface heterostructures -based free standing membrane can be used for high performance continuous photocatalysis system by using our patented instrument.
This report illustrates the design, preparation, and photocatalysis properties of TiO2 nanobelt surface heterostructures. Continuous photocatalysis water treatment system and process was demonstrated in this report. Other applications of su TiO2 nanobelt surface heterostructures, such as, gas sensor and biosensor are also reviewed.

Metal substrates have important advantages relative to transparent conducting oxide-coated glass (TCO/glass) for dye-sensitized solar cells (DSCs). The high conductivity of metal substrates is an essential characteristic for the construction of large-area (~100 cm2) single module DSCs. However, the opacity of metal electrodes requires architectures that are different from those of traditional DSCs based on TCO/glass. The strategies that we have investigated for metal-substrate cells include: (1) back-illumination of an opaque anode, which consists of a metal foil overlaid with a gradient film of TiO2 nanoparticles; (2) a semi-transparent metal mesh cathode; (3) transparent electrolyte. The anode was formed by coating the foil with multiple layers of TiO2 nanoparticle pastes, each having a different amount of scattering nanoparticles. The gradient of scatterers was reversed relative to layers deposited on TCO/glass of conventional DSCs. The optical properties, dye loading, and electrical contact of the films were investigated, and an optimized cell was constructed with high conversion efficiency of 8.9% under one sun illumination. The cathode consisted of a Ti metal mesh with 90% light transmission, upon which Pt nanoparticles were electrodeposited. We report optimized conditions for the electrodeposition of Pt on the Ti metal substrate. Finally, we report the use and optimization of a transparent sulfur/iodide electrolyte in place of the traditional iodide/tri-iodide redox shuttle.

Ultrahydrophilic nano-crystalline transparent films of ceramic oxides, such as anatase phase of TiO₂ and pure cubic ZrO₂ (diamond simulant), have been produced by ion beam assisted deposition (IBAD) processes. The IBAD combines an electron beam evaporation system with an ion beam bombardment in a high vacuum environment with a base pressure of 10-8 torr. Ion-enhanced processes employ energetic ionic bombardment to “stitch” ceramic films to the substrate during deposition resulting in more adherent coatings than those produced without ion enhancement. The crystal size, thin film morphology, crystallinity, and phases (anatase versus rutile for TiO₂ and cubic versus monoclinic for ZrO₂) were controlled by optimizing ion beam conditions (varying the ion-to-atom arrival ratio) with the energy from 0~1500 eV, ion currents (0~500 mu;A/cm2), and well-controlled deposition temperatures. Source materials were 99.9% pure rutile TiO₂ and 99.7% pure monoclinic phase of zirconia from Alfa Assar respectively. Various mixtures of oxygen, nitrogen, and argon gases are used as source materials to generate energetic ions to produce these coatings with differential grain sizes (4 to 70 nm) that affect the wettability, roughness, and the mechanical and optical properties of the coating.
Nanostructurally stabilized ZrO₂ films systematically show 0-10° contact angle depending on the deposition conditions. On the other hand, TiO₂ shows contact angle between 0° and 70° for as-synthesized film. However, for some deposition conditions, they maintain small contact angle after long periods of time in ambient conditions without UV irradiation, i.e. 14° after one year. We attribute these small, apparent contact angles to the large true surface area and explain it in terms of Wenzel&’s model. We have compared the growth of a bona fide mesenchymal stem cell (MSC) cell line OMA-AD on the nano-structured coatings with orthopaedic materials. Our results have shown that biocompatibility of these ultrahydrophilic nanoengineered coatings are superior to commonly used orthopaedic materials including titanium and even hydroxyapatite (HA) (1). Mechanisms responsible for the stabilization of various phases in produced nanostructured films will be discussed.
1. Namavar, F; Rubenstein, A; Sabirianov, R. MRS Proceedings / Volume 1418 / DOI:10.1557/opl.2012.

We report on a new record rate for the sunlight-driven conversion of CO2 into light hydrocarbons using semiconductor photocatalysts. We achieved this advance by developing a new approach to prepare periodically modulated double-walled anatase TiO2 nanotube (PMTiNT) arrays loaded with bimetallic catalysts. The PMTiNTs synthesized by electrochemical anodization consisted of TiO2 nanotubes whose diameters were periodically varied along the tube-axis using a programmed sequence of current pulses [1]. The PMTiNTs were decorated with a series of bimetallic coatings. The best performance was obtained using a photodeposited Cu-Pt coating. An overall hydrocarbon production (CH4, C2H4, and C2H6) rate of 3.7 mLg-1h-1 or 610 nmolcm-2hr-1 was obtained under AM 1.5G illumination from a solar simulator by optimizing the combination of bimetallic components and the PMTiNT support, which is at least a 4-fold improvement in photocatalytic CO2 conversion rates over prior reports [2]. For specific geometries, the PMTiNTs behave as 1-D photonic crystals with a stop-band at visible wavelengths as indicated by our band-structure calculations. The resulting simulated reflectance spectra of the PMTiNTs agreed well with the observed experimental values. Despite the wide band-gap of anatase TiO2 (3.2 eV), quantum yield measurements of CO2 photoreduction demonstrated photoreduction by visible light as well as ultraviolet radiation, which we attribute to the synergistic effect of Interfacial Charge Transfer (IFCT) involving Cu [3], and light trapping by the Cu-Pt coated PMTiNTs [4].
1. Zhang, X. et al. Photocatalytic Conversion of Diluted CO2 into Light Hydrocarbons Using Periodically Modulated Multiwalled Nanotube Arrays. Angewandte Chemie, doi:10.1002/anie.201205619 (2012).
2. Wang, W.-N. et al. Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc., doi:10.1021/ja304075b (2012).
3. Nakajima, A. et al. Preparation and properties of Cu-grafted transparent TiO2-nanosheet thin films. Mater. Lett. 63, 1699-1701 (2009).
4. Ouchani, N., Bria, D., Djafari-Rouhani, B. & Nougaoui, A. Defect modes in one-dimensional anisotropic photonic crystal. Journal of Applied Physics 106, 113107-113108 (2009).

Photocatalysis reaction possesses a great potential for environmental remediation and fuel production [1]. Nitrogen-doped TiO2 is a well-known visible-light sensitive photocatalyst where deep impurity states associated with substitutional nitrogen at oxygen sites (N_O) are believed to be the source of the red shift in photo-absorption edge. However, such a deep level should trap hole carriers, degrading oxidation process. The contradiction between the deep N_O level and rather a high oxidation power of N-doped TiO2 has been an unsolved puzzle. Here, we propose a new interpretation of the effect of nitrogen doping based on advanced electronic structure calculations [2]. The N_O strongly binds with a titanium atom at an interstitial site, forming a defect-impurity band, which consists of bonding and anti-bonding states of nitrogen p and titanium d and narrows the band gap. Such a newly formed band, which is connected to the valence band maximum of the host TiO2, becomes the migration path of photo-induced hole carriers, assisting carrier transfer to the surface. This clearly explains the photocatalytic activity of N-doped TiO2 for the both aspects, i.e., the visible-light absorption and oxidation reaction. Intrinsic properties of several native defects in anatase TiO2 are also discussed.
This work was supported by the Japan Science and Technology Agency (JST) Precursory Research for Embryonic Science and Technology (PRESTO) program.
[1] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24, 229 (2012).
[2] Naoto Umezawa and Jinhua Ye, Phys. Chem. Chem. Phys., 14, 5924 (2012).

Combining efficient greenhouse gas capture from stationary sources, such as power plants, with catalytic or photocatalytic reforming provides an avenue for realizing carbon-neutral hydrocarbon fuel cycles that reduce human impact on the global climate system. Solid-supported CO₂ capture materials could have important potential advantages over liquid alkanolamine solutions that are currently used in industry. CO₂ capture on solid surfaces can substantially reduce the large energy cost associated with the thermal removal of CO₂ from liquid solutions, which is primarily due to the large heat capacity of water. Solid capture media may be more readily engineered to optimize the reactions involved in binding and releasing CO₂, and to reduce the susceptibility to degradation due to SO_x and NO_x present in the flue gas of power plants using fossil fuels. The interfacial chemistry between TiO₂ and alkanolamines, in addition to the relatively low cost of titanium dioxide, could make these material combinations potentially viable for large-scale CO₂ capture.
We have studied titania-supported 3-amino-1-propanol (3AP) as a potential CO₂ capture material using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and reactive force field (ReaxFF) computational methods. ReaxFF calculations show that in the optimized structure the hydroxyl group of 3AP forms donor-acceptor bonds with 5-fold coordinated surface Ti atoms, while the amine group is unbonded and remains available for capturing CO₂. The appearance of a C(1s) peak at 286.1 eV and a N(1s) peak at 399.4 eV, along with simultaneously decreasing intensities of Ti(2p) peaks of the substrate, demonstrate the successful adsorption of 3AP onto rutile TiO₂(110) under ultrahigh vacuum (UHV) conditions at 300 K. Exposure of the 3AP/TiO₂(110) to 0.05 Torr of CO₂ gives rise to additional C(1s) peaks at binding energies of 289.5 eV and 294.3 eV, attributed to captured CO₂ and gas phase CO₂ respectively. The peak attributed to captured CO₂ is consistent with a carbamate (-NHCOO) species on the surface. The adsorbed CO₂ peak continues to increase with time during the exposure to 0.05 Torr of CO₂ up to approximately 1 hour. We will discuss the mechanism and energy budget of reversible CO₂ capture on adsorbed 3-amino-1-propanol on titania, as well as the broader implications of our studies on this model system on solid-state CO₂ capture.

CeO2/TiO2 nanobelt heterostructures were synthesized via a cost-effective hydrothermal method. The as-prepared nanocomposites consisted of CeO2 nanoparticles assembled on the rough surface of TiO2 nanobelts. In comparison with P25 TiO2 colloids, surface-coarsened TiO2 nanobelts, and CeO2 nanoparticles, the CeO2/TiO2 nanobelt heterostructures exhibited a markedly enhanced photocatalytic activity in the degradation of organic pollutants such as methyl orange (MO) under either UV or visible light irradiation. The enhanced photocatalytic performance is attributed to a novel capture-photodegradation-release degradation mechanism. During the photocatalytic process, MO molecules were captured by CeO2 nanoparticles, and degraded by the photogenerated free radicals, and then released to the solution. With its high degradation efficiency, broad active light wavelength, and good stability, the CeO2/TiO2 nanobelt heterostructures represent a new effective photocatalyst that is low-cost, recyclable, and will have wide applications in photodegradation of various organic pollutants. The new capture-photodegradation-release degradation mechanism for improving photocatalysis properties is of importance in the rational design and synthesis of new photocatalysts.