Lithium ion batteries are playing indispensible role in our modern life full of portable electronics and in constant move. However, the advancement of lithium ion battery has lagged way behind electronics, in spite of extensive research efforts across research venues and industries allover the world. Search for materials with better electrical energy storage have covered all facets including different chemistry, microstructures, nanocoatings, and nanocomposites to address various issues of lithium ion batteries. The charge and discharge in lithium ion batteries proceed through lithium ion inetercalation and deintercalation, which comprises (1) electrochemical reaction at the interface between solid electrode and electrolyte, (2) solid state diffusion of mass and charge in electrodes, and (3) lattice expansion and contraction. Nanostructures offer some obvious advantages: large surface area for effective electrochemical reaction at the interface, small dimension for easy mass and charge transport and for accommodating volume change. Nanostructured materials also possess high surface energy, and are away from thermodynamic equilibrium. In this presentation, I will discuss the impacts of the surface defects and the nove nanostructures of TiO₂ and Li₄Ti₅O₁₂ on the lithium-ion intercalation and electrical energy storage properties when such materials are used as electrodes in lithium ion batteries.

Nanosized titania has emerged as an important material for the photocatlytic degradation of organic and biological contaminants in water, air, and on surfaces. Unfortunately, the self-cleaning functionality of TiO₂ is highly dependent on the presence of UV or visible light. In this research project, we have created TiO₂ hybrid materials that display enhanced photocatlytic activity and potent chlorine chemistry to destroy organic and biological contaminants under both light and dark conditions. These TiO₂ hybrid materials were biomimetically synthesized, utilizing peptides/polymers to initiate and mediate the precipitation of titania and subsequently devitrified under hydrothermal conditions. This low-temperature processing route yielded highly crystalline nanomaterials of enhanced photocatlytic activity whilst preserving the organic constituents of the initially precipitated hybrid material. The preservation of the nitrogen-rich organic materials within these hybrids is significant as these molecules are available for subsequent chemical reactions, including chlorination to form halamine compounds. The presence of these oxidizing halamine groups provides for a second (illumination-independent) route to the destruction of potentially harmful biological or chemical species.

We here report a robust colloidal synthesis method to construct TiO2 nanocrystals with complex structures which composed of an arbitrary TiO2 nanocrystal “core” and anatase TiO2 nanowhisker “antennas”. In the synthesis, TiO2 nanocrystal “cores” with various shapes are first produced by different methods. By using these shape-controlled TiO2 nanocrystals as seeds, secondary structure of TiO2 can be grown onto the seed nanocrystals through a sol-gel pyrolysis reaction and form “core-antenna” type of nanocrystals. Characterizations of structural evolution during the growth of TiO2 “antennas” clearly revealed these TiO2 “antennas” are grown onto the TiO2 nanocrystal “core” through epitaxial growth along a certain direction. By systematically tuning the shape of the “core” and the morphology of the “antennas” via control of synthesis conditions, we are able to produce TiO2 nanocrystals with complex three-dimensional structural configurations in a highly predictable manner.

Results from the successful growth of high quality KH₂PO₄ (KDP) crystals with incorporated TiO₂ anatase nanoparticles and the characterization of these crystals using several complementary methods are presented. Visual observations, transmission and scanning electron microscopy have shown that the anatase nanoparticles were captured mainly by the pyramidal growth sector and, to a considerably lesser extent, by the prismatic growth sector. Energy dispersive x-ray analysis was able to confirm that the growth layer stacks contain the TiO₂ particles. Fourier transformation infrared spectra have clearly shown the presence of an absorption band at about 800 cm^-1 in both KDP:TiO₂ and TiO₂, and the disappearance of the band, associated with hydroxyl OH^- groups on the TiO₂ surface in KDP:TiO₂. Significant variation in the imaginary and real parts of the cubic nonlinear optical susceptibilities and refractive index changes at continuous wave excitation were found in prism and pyramid parts of pure KDP and KDP:TiO₂ samples. Deciphering complicated electron paramagnetic resonance spectra (EPR) in KDP:TiO₂ and comparison with published data permitted the identification of paramagnetic defects along with their associated g-factors and zero-field splitting parameters (in some cases for the first time). It was found that the dominant lines belong to four different centers Fe_A³⁺, Fe_B³⁺, Cr_R³⁺ and Cr_GB³⁺. From analysis of line intensities it was concluded that the concentration of intrinsic defects like potassium and hydrogen vacancies in KDP:TiO₂ is comparatively small, that the concentration of non-controlled impurities in nominally pure KDP samples is several times larger than in KDP:TiO₂, and that the concentration of non-controlled impurities in the prismatic part of the KDP:TiO₂ boule is approximately twice as large as in the pyramid part. The study allowed the nature and distribution of macroscopic and microscopic defects in the KDP:TiO₂ crystals to be clarified. The relationship between these defects and the distribution of TiO₂ nanoparticles, and the influence of incorporated nanoparticles on the nonlinear optical properties of composite crystals in comparison with pure crystals were also elucidated.

Co-doping with Cu and Mo is shown to have a synergistic effect on the photocatalytic activity of TiO₂. The enhancement in activity is observed only if the synthesis route results in TiO₂ in which (Cu, Mo) co-dopants are forced into the TiO₂ lattice. Using x-ray photoelectron spectroscopy, Cu and Mo are shown to be present in the +2 and +6 oxidation states respectively. A systematic study of the ternary system shows that TiO₂ containing 6 mole % CuO and 1.5 mole % MoO₃ is the most active ternary composition. Ab-initio calculations show that co-doping of TiO₂ using (Mo, Cu) introduces levels above valence band, and below conduction band, resulting in a significant reduction in the band gap (~0.8 eV). Co-doping however also introduces deep defect states, which can have a deleterious impact on photoactivity. This helps rationalize the narrow compositional window over which the enhancement in photocatalytic activity is observed.

The development of sustainable strategies for the formation of well-defined nanoporous titania materials with high surface area is of great interest for applications including photocatalysis, photovoltaics and electrochemical energy storage. We will discuss the use of nanocrystalline cellulose (N#1057;#1057;) as a novel shape-persistent biotemplating agent enabling the efficient synthesis and tailoring of mesoporous titania materials, including thin films. For the latter, our approach involves the introduction of purified cellulose nanocrystals to titania precursor solutions, followed by direct film deposition on substrates and template combustion. The resulting titania networks are highly porous anatase morphologies having well-defined, narrow pore size distributions.^[1]
Key advantages of NCC templates compared to the frequently used ‘soft templates&’ such as surfactant micelles include shape persistence, high temperature stability and tunable dimensions of the pore system. For example, significant changes of NCC-templated titania surface area, pore size, pore anisotropy and dimensions of titania crystallites can be achieved with different titania-to-template ratios in the precursor solution. Moreover, we show a significant tunability of porosity due to post-treatments at high humidity prior to calcination. The resulting biotemplated titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol, outperforming surfactant-templated mesoporous titania films. The NCC replicated titania networks were also successfully employed as anodes in dye-sensitized solar cells. Remarkably, we can significantly enhance the photocatalytic activity of the titania thin films by introducing into the sol-gel-based biotemplated scaffold pre-formed ultrasmall anatase nanoparticles. The gradual addition of pre-formed nanoparticles leads to a continuous increase in the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the shape-persistent nature of the NCC template. We will show that our biotemplating approach can be generalized for the fabrication of crystalline mesoporous thin films of other oxides, thus offering promising prospects for the sustainable generation of porous oxides for diverse applications.
[1] Ivanova, A.; Fattakhova-Rohlfing, D.; Kayaalp, B. E.; Rathouský, J.; Bein, T. J. Am. Chem. Soc.2014, 136, 5930-5937

In this presentation, I will discuss our recent efforts on crafting nanostructured TiO₂ (TiO₂ nanorods, nanotubes and nanoflowers, upconversion/semiconductor Na_xGdF_yO_z:Yb/Er@TiO₂ hollow spheres, and plasmonic/semiconductor Au/TiO₂ core/shell nanoparticles with precisely controlled dimensions) for solar energy conversion into electricity (dye-sensitized solar cells and perovskite solar cells) and chemical energy (photocatalysis) with enhanced performance.

Titanium dioxide (TiO₂) in the anatase phase is a desirable material for application in high-power Li-ion battery negative electrodes as most of commonly used liquid electrolytes do not decompose in its potential frame. Therefore no solid electrolyte interphase (SEI) forms on the surface of TiO₂, which is of special importance if the surface area is enlarged by nanostructuring, compared to a dense film. We present an efficient approach for fabricating nanoporous TiO₂ nanocolumns. Nanostructuring provides shorter diffusion paths for diffusion-controlled reactions and increased porosity for catalytic processes as compared to dense thin films. In addition, only fabrication techniques that could potentially be expanded into larger scale production sizes, such as sputtering, wet etching and annealing, are used in this study. Glancing angle sputter deposition (GLAD) is used to synthesize bundled nanocolumnar thin films of various multi-component Ti-based precursor thin films containing sacrificial metals (Fe, Cu, Ag). Upon subsequent selective dealloying of these precursor thin films, nanoporous TiO₂ nanocolumns with 7:1 aspect ratio are obtained. They show an increased intercolumnar spacing in comparison to pure Ti, non-dealloyed columnar thin films. Combinatorial deposition and dealloying processes are used to identify optimum precursor compositions and dealloying conditions (i.e. minimize residual sacrificial metal content and nanocolumn diameter down to 20 nm). The resulting crystalline phases are examined using XRD. Sub-stoichiometric Ti oxide phases are found after dealloying. These are further oxidized in order to maximize the content of the anatase phase using wet thermal annealing in a reaction tube. The obtained transition metal doped TiO₂ nanocolumnar films are promising as photoanode material in photoelectrochemical solar-water splitting. Transition metal doping can alter the bandgap of porous TiO₂ structures. In spite of short carrier diffusion lengths in TiO₂, nanorod-like structures exhibit efficient routes for holes to diffuse across the radius of a nanorod. Thus, the dealloying based approach presents a scalable route for fabrication of multi-functional TiO₂ nanostructures.

The goal in biomaterial surface modification is to possess desired recognition and specificity through modifying its surface condition like topological structure. Here we develop a unique strategy of high-energy electropulsing treatment (EPT) for surface modification of basal textured Ti-6Al-4V alloy strips with the surface nano-topographic oxide coating. Novel TiO₂ nanopores topological structure formed on the materials surface under EPT results in better biocompatibility with more active fibroblast bio-reaction with higher cells viability, better physiological morphology and stronger adhesion binding than that on the ordinary TiO₂ film and matrix alloy, which is a consequence of the fact that fixation of cell membrane receptors and ligands are strengthened to facilitate gene expression by accurate manipulating surface chemical components, surface energy and specific surface area under EPT. The key role of forming TiO₂ nanopores structure solely with EPT is the selective effect of the electropulses going through the textured specimen, which builds a significant bride connecting the interior structure and surface in the materials science. The positive contributions of EPT in the thermodynamics and kinetics of TiO₂ nanopores topology are attributed to the reduction of nucleation energy barrier and acceleration of atomic diffusion. Thus, the highly-efficient and energy-saving approach for fabricating nanopore structures titanium dioxide on the biomaterial surfaces can be exploited for various tissue engineering applications and surgical implants fields due to the low temperature and ultrafast procedure.

TiO₂ nanopowders obtained using different methods with the mean size of 5, 15, 30 and 40 nm have been investigated by Raman spectroscopy in wide spectral range. Nano-size of TiO₂ crystals lead to a shift and broadening of the first-order Raman lines through a relaxation of the q = 0 selection rule and effects on to the position, width and asymmetry of a Raman bands. The details of the evolution of the 144 cm^-1 Raman line shape on the size and distributions of the nanopowders are presented and discussed in frame of confined phonons model. Analysis of Raman spectra shows that structural characteristics of nanopowders may be different even size of the nanopowders is the same. Structural features of the material depend on preparation methods/conditions and can be extracted from Raman spectra of the material.

TiO₂ has been the workhorse of n-type charge collection materials for dye-sensitized solar cells for the last 20 years and mesoporous TiO₂ has facilitated a rich landscape for broad PV materials investigations. As an example, in order to create a high surface area porous TiO₂ which also exhibits extremely high crystallinity, we have developed a route to synthesise mesoporous single crystals of anataze TiO₂ and integrate these effectively into dye-sensitized solar cells. These materials may also be useful in energy storage devices, where similar demands for high accessible surface and conductivity are made. However, the photovoltaics research landscape has changed considerably in the last few years. The early demonstration of organic-inorganic halide perovskite solar cells followed device architectures employing mesoporous TiO₂, but in 2012 this paradigm was irreversible shifted. We made an unexpected discovery that CH₃NH₃PbI_3-xCl_x could be processed at low temperatures to form highly crystalline films in thin film solar cell architectures, or within the pores of an insulating scaffold. These devices exhibited remarkably good photovoltaic properties which has opened many different device architectures and materials choice for the p and n-type charge collection layers. Here, I will show examples of the different embodiments of the perovskite solar cells in which we can incorporate low temperature processed TiO₂, and compare these critically with alternative materials. I will finish with the question; is there still a role for TiO₂ within this perovskite solar technology?

TiO₂ crystal has been actively used in both photocatalysis and lithium ion battery areas due to its various merits including low-cost, nontoxicity, high-efficiency, and easy availability. Photocatalysis and lithium storage processes are both surface-sensitive, so it is highly desirable to control the surface of TiO₂ crystal. In the past years, faceted TiO₂ has experienced an explosive growth with the purpose of controllable exposure of different facets, particularly high-energy facets.^[1,2] The recent interest has switched to revealing the relationship between facet property and performance of faceted crystals. Recently, we have explored the facet dependent electrical conductivity and lithium storage capability of faceted TiO₂.^[3] Their strong relationship is established. Besides this, the integration of faceting and mesopores was realized in single crystal rutile TiO₂, which leads to an order magnitude better photoelectrochemical water splitting activity than the solid reference sample.^[4] On the other hand, the large bandgap of around 3.0 eV and low electrical conductivity has intrinsically limited the application of TiO₂ in both fields. To overcome these bottlenecks, we developed a gradient B/N doping strategy to narrow the bandgap of anatase TiO₂ to 1.94 eV so that a high absorbance within the whole visible light range is obtained.^[5] In addition, TiO₂ nanoparticles with a homogeneous N/S doping gives an improved high-rate lithium storage capability.^[6] Furthermore, a rutile TiO₂ film with an amount of 15 at% oxygen vacancies supported on a Ti substrate were developed to give a great improvement in photoelectrochemical water splitting.^[7]
Reference
[1] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, G. Q. Lu, Nature 2008, 453, 638.
[2] G. Liu, H. G. Yang, G. Q. Lu, H. M. Cheng, and et al. Chemical Reviews 2014, DOI:10.1021/cr400621z
[3] G. Liu, L. C. Yin, J. Pan, F. Li, L. Wen, C. Zhen, H. M. Cheng, submitted.
[4] W. Jiao, Y. P. Xie, R. Z. Chen, C. Zhen, G. Liu,* X. L. Ma, H. M. Cheng, Chem Commun 2013, 49, 11770.
[5] G. Liu, L.-C. Yin, J. Q. Wang, P. Niu, C. Zhen, Y. P. Xie, H.-M. Cheng, Energy Environ Sci 2012, 5, 9603.
[6] W. Jiao, N. Li, L. Z. Wang, G. Liu, L. Wen, F. Li, H. M. Cheng, Chem Commun 2013, 49, 3461.
[7] C. Zhen, L. Z. Wang, L. Liu, G. Liu, G. Q. Lu, H. M. Cheng, Chem Commun 2013, 49, 6191.

The solar energy is recognized as one of the most valuable renewable energies for the future. The technologies based on solar energy have changed and are changing our society and have aroused important interest. However the development of photocatalysis process reaches a bottleneck effect, the low process efficiency due to the very low light absorption efficiency. Huge research effort has been devoted to finding new solution to improve the light absorption efficiency. Chemical methods such as doping and dye sensitizer have been used to extend the light absorption zone and to increase light absorption, respectively and have generated very promising effect. Quantum dots have also been used to increasing light absorption efficiency. What&’s the next?
Slow photon effect, which is until now less explored in spite of its discovery since long time, shows its high power ^[1-8]. The present work from experimental and theoretical point of view revealed that the photocatalytic activity can be dramatically enhanced by utilizing slow photons located at the Photonic Band Gap edges with energies close to the electronic bandgap of the semiconductor ^[3-8]. This structure effect combining with chemical effects and quantum dots will give a new breakthrough to the field of photocatalysis.
Keywords: Photocatalysis, Slow Photons, Photonic Crystals,
References:
1) Y. Li, Z. Y. Fu, B. L. Su, Adv. Funct. Mater., 2012, 22, 4634.
2) Q. Dai, L. Y. Shi, Y. G. Luo, J. L. Blin, B. L. Su, J. Photochem. Photobio. A, 2002, 148, 295
3) M. Wu, Y. Li, Z. Deng, B.L. Su, ChemSusChem, 2011, 4, 1481.
4) Y. Li, F. Piret, T. Léonard, B. L. Su, J. Colloid Interf. Sci., 2010, 348, 43.
5) M. Wu, A. M. Zheng, F. Deng, B. L. Su, Appl. Catal. B. Envron., 2013, 138-139, 219.
6) M. Wu, J. Liu, C. Wang, S. Huang, Z. Deng, Y. Li, B. L. Su, Appl. Catal. B, 2014, 150-151, 411-420.
7) M. Wu J. Liu, Z. Deng, Y. Li, O. Deparis, B. L. Su, J. Mater. Chem. A, 2013,1, 15491.
8) J. Liu, Y. Li, H. W. Huang, C. Wang, M. Wu, L. H. Chen, B. L. Su, J. Mater. Chem., A, 2014, 2, 505

Titania nanosheets obtained via exfoliation of layered titanates are one of the important 2D materials, which function as photoelectrodes and photocatalysts. Recently, we have found that a monolayer film of the titania nanosheet surfaces, the thickness of which is less than 1 nm, yields a highly hydrophilic state upon UV light irradiation [1]. In the present study, we have examined the dependence of hydrophilic conversion rate on the UV light intensity as well as the number of layers of titania nanosheets. We have also investigated the enhancement of the hydrophilic conversion rate of the titania nanosheet surfaces by combining them with other kinds of 2D materials such as the reduced graphene oxide (rGO).
We found that the hydrophilic conversion rate for a monolayer film of titania nanosheets was proportionally enhanced as the UV light intensity increased. This result suggests that the hydrophilic conversion proceeds on the monolayer film of titania nanosheets under light-limited conditions even with a high concentration of photoexcited carriers. In contrast, the hydrophilic conversion rate for a 10-layer film of titania nanosheet surfaces was proportional to the square root of the light intensity, suggesting that the recombination processes are dominant in the multilayer films of titania nanosheets.
The differences in the hydrophilic conversion behavior may be explained by the difference of a diffusion path length. In the case of the monolayer film, the diffusion process of photoexcited carriers to surfaces are not required because the thickness of titania nanosheets is only 1 nm, and hence the hydrophilic conversion reaction may occur on the nanosheets where the photoexcited carriers are generated. In the case of 10-layer film, on the other hand, the diffusion path length of the electrons and holes produced within the film is larger than that in the case of the monolayer film because they migrate to the topmost layer by hopping from one to another to contribute to the hydrophilic conversion. In the diffusion processes, the recombination probability of electrons and holes should increase as the diffusion path length increases, resulting in the domination of recombination processes.
We also examined the hydrophilic conversion properties of heteroassembled films consisting of titania nanosheets and rGO. We found that the insertion of a monolayer film of rGO underneath a monolayer film of titania nanosheets enhanced the hydrophilic conversion rate of the titania nanosheet surfaces. This is probably because rGO works as an efficient electron transfer mediator. Since rGO is intimately contacted with titania nanosheets, the excited electrons in titania nanosheets are efficiently injected into rGO, and are further transferred to molecular oxygen in the air, which results in the efficient utilization of holes to the hydrophilic conversion reaction.
[1] N. Sakai, T. Sasaki, et al., J. Phys. Chem. B, 110, 6198 (2006).

Nanoparticle characteristics such as particle size and size distribution, degree and type of agglomeration or crystallinity determine the properties of materials and devices in which they are incorporated such as heterogeneous catalysts or dye solar cells. Chemical vapor synthesis (CVS) is a gas phase process which enables the scalable production of nanoparticles with small size, narrow size distribution, low degree of agglomeration and high crystallinity respectively low defect density [1]. Titania is the archetypical model material of the CVS process as synthesis is possible over a wide range of process parameters and particle characteristics are readily available by experimental and theoretical methods. Process parameters, especially the time-temperature-profile in CVS, control the nanoparticle characteristics [2]. We discuss a new method to find paths to optimized titania nanoparticle characteristics in CVS. As example, we predict optimal time-temperature profiles for a minimized degree of agglomeration integrating a simple model to describe particle formation and growth into an Monte Carlo optimization algorithm.
[1] M. Winterer, Nanocrystalline Ceramics - Synthesis and Structure, Springer 2002
[2] R. Djenadic and M. Winterer, chapter 2, Chemical Vapor Synthesis of Nanocrystalline Oxides, in A. Lorke, M. Winterer, R. Schmechel, and C. Schulz, Nanoparticles from the Gas Phase, Springer 201

This talk will present highlights of the latest results of studies directed at developing oxygen-deficient titanium dioxide nanostructures as electrode materials, which show significantly enhanced performance in applications for energy conversion and charge storage. The enhanced photoelectrochemical performance is attributed to the enhanced separation of photoinduced carriers as a result of improved carrier density by controlled creation of oxygen vacancies. Oxygen-deficient titanium dioxide with increased electrical conductivity also substantially improved its performance as an electrode for electrochemical capacitors.

We have recently developed novel visible-light photocatalysts consisting of metal complexes and semiconductors for sunlight-driven chemical processes named as “solar chemical transformations”. The first topic is the metal oxide cluster-TiO₂ system (MOs/TiO₂) for the solar environmental purification. Molecular scale oxides of 3d metals can be formed on the TiO₂ surface using the acetylacetonato (acac) complex as a precursor by the chemisorption-calcination cycle technique (Angew. Chem. Int. Ed. 2011, 50, 3501). The valence band maximum of TiO₂ is finely tuned by controlling the loading amount precisely. The unique physicochemical and electronic properties of MOs/TiO₂ give rise to the outstanding photo- and thermo-catalytic activities for the decomposition of model organic pollutants. Combination of experiments and first principles density functional theory simulations shows that Co₂O₃/TiO₂ can be a prototype of the solar environmental catalyst with high levels of photo(UV and visible)- and thermo-catalytic activities. The second topic is the Cu(acac)₂-BiVO₄ system for the solar oxidative organic synthesis. Among the 3d metal-acac complexes, Cu(acac)₂ is chemisorbed on the monoclinic sheelite (ms)-BiVO₄ surface to yield an O₂-bridged binuclear complex (OBBC/ms-BiVO₄). Multi-electron reduction of O₂ is induced by the visible-light irradiation of the OBBC/ms-BiVO₄ in the same manner as a blue Cu enzyme in the bio-system. The drastic enhancement of the O₂ reduction renders ms-BiVO₄ to work as a good visible-light photocatalyst without any sacrificial reagents. As a model reaction, we show that this bio-mimetic hybrid photocatalyst exhibits a high level of activity for the aerobic oxidations of amines to aldehydes in aqueous solution and imines in THF solution at 25 0C with selectivity > 99% under visible-light irradiation.

Hollow TiO₂ nanostructures are promising building blocks for electrode scaffolds and catalyst carriers in energy-related systems. By introducing TiCl₄ vapor pulses to ZnO nanowire templates, we obtained TiO₂ tubular nanostructures with well-preserved dimensions and morphology. This process involved the cation exchange reaction between TiCl₄ vapor and ZnO solid and the diffusion of reactants and products in their vapor or solid phases, which was likely a manifestation of the Kirkendall effect. The characteristic morphologies and the evolution phenomena of the hollow nanostructures from this vapor-solid system were in a good agreement with the Kirkendall effect discovered in solution systems. Complex hollow TiO₂ nanostructures were successfully acquired by replicating various ZnO nano-morphologies, suggesting that this unique cation exchange process could also be a versatile tool for nanostructure replication in vapor-solid growth systems. The evolution of TiO₂ nanotubes from ZnO NW scaffolds was seamlessly integrated with TiO₂ NR branch growth, and thus realized a pure TiO₂-phased 3D NW architecture. Due to the significantly enlarged surface area and the trace amount of Zn left in the TiO₂ crystals, such 3D TiO₂ nanoforests demonstrated enhanced photoelectrochemical performance particularly under AM (air mass) 1.5G illumination, offering a new route for hierarchical functional nanomaterial assembly and application.

Optimization of the photocatalytic properties of TiO₂, has been of interest for the last 40 years [1], due to its importance in clean energy and water remediation systems [2]. Rutile TiO₂ has a lower band-gap and better stability than the anatase phase. Nonetheless, anatase is widely studied for photocatalysis due to its higher photo-activity compared to rutile TiO₂. The rutile phase, could, however, represent a better alternative for use in long-term industrial photocatalytic reactions as it is more robust under aggressive pH and temperature conditions. Surface nanopatterning may increase the photon-harvesting properties and increase the surface area of rutile TiO₂ and thus its photo-,activity. Patterning processes may often induce damages of the crystalline structure or vary the stoichiometry, which influence the reactivity of TiO₂ surfaces. The exact role of such modifications should be quantified separately and balanced carefully to properly evaluate the effect on photo-activity.
In this contribution we present results on the enhanced photo-activity of crystalline nanospikes created on (001) rutile TiO₂ single crystals and thermally grown polycrystalline rutile TiO₂ surfaces. The method for generating such nanospikes is based on high energy ion beam irradiation and a subsequent chemical HF etching [3], which previously has been employed for modifying the TiO₂ response to biomolecules [4] and cells. Although the created nanospikes basically preserved the rutile structure, they contain minor damage. We have experimentally studied the structural, compositional, and optical properties of the nanostructures and relate this to observed changes in the efficiency of UV photo-degradation of organic molecules [5]. In addition, we performed a complementary set of lower energy ion implantation experiments to compare the effects of different surface damage. The obtained results shows: i) nanospikes act as effective antireflective surface layer increasing the U.V. photo-catalytic yield of rutile TiO₂, ii) not all the surface of the nanospikes are accessible for organic compounds aqueous solutions, and iii) the photo-activity of both single crystal and polycrystalline rutile TiO₂ are slightly reduced after conventional ion implantation. The present work highlights the complexity involved in understanding the photo-activity of TiO₂ nanostructures, where many factors - e.g. optics, reactivity and effective exposed surface - have to be taken into account when comparing the efficiency of TiO₂ nanostructures with flat surfaces.
[1] Fujishima, A.; Zhang, X.; Tryk, D.A. Surf. Sci. Rep. 2008, 63, 515-582.
[2] Qu, X.; Brame,J.; Li, Q.; Alvarez, P. J. J. Acc. Chem. Res. 2013, 46, 834-843.
[3] R. Sanz et al Nano Lett. 2006, 6, 1065-1068.
[4] J. V. Pérez-Giroacute;n et al. Nuclear Inst.and Methods in Physics Res.B 2014, 379, 67-74.
[5] ISO 10678:2010(E). International Organization for Standardization (2010). Switzerland.

TiO₂ thin-films as-deposited by Atomic Layer Deposition (ALD) at temperatures < 100 °C are in general amorphous and exhibit poor optical properties due to impurities from the organic precursors compared to their crystalline counterparts. When heated, crystal phase transformations take place (amorphous → anatase → rutile) and the accompanying volume reduction as well as the impurity combustion contribute to the mechanical instability of the film. However, low temperature ALD is the method of choice, if temperature-sensitive templates have to be conformally covered. ALD is a sequential deposition process based on self-limiting surface reactions of gaseous precursors. We utilize this technique to fabricate inverse opaline structures from periodic arrays of polymer spheres (diameter 300 - 900 nm).
It is known that the formation and stabilization of a polymorphic form of TiO₂ is strongly dependent on the preparation conditions, but also on the cation content present in TiO₂ . Minimizing the structural damage caused by high thermal treatments can be achieved through thermal phase stabilization by doping the thin films with different types of cations (e.g. Al or Nb). Since Al₂O₃ ALD and TiO₂ ALD processes are compatible from the point of view of operating temperature window, oxidant and growth rates, the combination of both ALD processes can introduce Al cations as a dopant in the TiO2 films. By properly cycling both processes simultaneously, the doping level can be accurately controlled.
We have combined the ALD processes for TiO₂ and Al₂O₃ at temperatures below 100 °C using different TiO₂:Al₂O₃ pulse ratios to obtain Al₂O₃-doped TiO₂ films with various atomic ratios Al / [Al + Ti] of up to 10 %. The film thicknesses could be exactly adjusted by the number of ALD (super-) cycles. The low deposition temperature allowed for the simultaneous infiltration of a self-assembled regular array of monodisperse polystyrene spheres in the same reactor, without affecting the direct opal structure. After removal of the polystyrene scaffold, the remaining inverse opal structure acts as a reflector in a wavelength range tunable by the size of the original spheres.
Both thin-films and inverse opal structures were subject to different thermal treatments in order to quantify a range of temperatures, in which such a system can operate without structural failure. The doping level affects the crystal phase transition temperatures as well as the refractive index (i.e. the optical properties) of the structures. X-Ray Diffractometry and UV/VIS spectroscopy were used to investigate the samples before and after thermal treatment. In this presentation, the effect of doping on the temperature stability is correlated to the resulting optical properties of the structures and an ideal doping level is proposed.
We gratefully acknowledge financial support from the German Research Foundation (DFG) via SFB 986 "M^3", projects C2, C3 and C4.

Adsorption of large organic molecules at solid surfaces allows the fabrication of two-dimensional networks which are interesting as models for applications ranging from organic electronics to sensors and catalysis. Metal surfaces are tipically adopted as a support, as they are easy to prepare and are well suited to scanning tunneling microscopy investigations. Organic self-assemblies at insulator surfaces are less studied, even though some of these systems, e.g. those composed of organic macrocycles and metal oxides, have been proposed as candidates for dye-sensitized solar cells. In this talk I will discuss the results of some recent theoretical and experimental investigations of prototypical systems composed by organic macrocycles, such as the phthalocyanine and the tetraphenylporphyrin free bases, adsorbed on the TiO₂(110) surface. I will show that organic molecules undergo chemical transformations also involving the exchange of atoms between the support and the overlayer. Thus, the TiO₂ surface plays an active role in determining the nature of adsorbed species, and cannot be simply considered as a plain worktable where organic molecules are assembled.
This work is in collaboration with M. Casarin and D. Forrer (CNR-IENI and University of Padova) and D. Cossaro, L. Floreano, and G. Lovat (CNR-IOM).

A series of photoelectrochemical TiO2 sensors have been developed for the determination of organic compounds under UV illumination1-5. The analytical principle is based on exhaustive and indiscriminate degradation of organic compounds at the electrode surface due to the strong oxidation power of photoholes. Via the manipulation of the bandgap of the TiO2 electrodes, the sensitivity of the TiO2 sensors has been significantly enhanced, and most importantly, selective degradation of organic compounds can be achieved. The light absorption properties of TiO2 electrode can be tailored via incorporation of gold nanoparticles and hydrogenation processes that extend the absorption of light to visible light range via inducing plasmonic effect and mid-gap energy levels, respectively. A photoelectrochemical electrode that is fabricated by hydrogenating a Au-decorated TiO2 film. The Au nanoparticles decorated TiO2 is further treated with a hydrogenated process in order to improve its PEC performance under the illumination with visible and UV irradiation. The high selectivity of PEC degrading glucose is illustrated under the visible light with a perfect linear relationship from 0 to 1.0mM glucose solution. Additionally, the universal photoelectrochemical organic detector of hydrogenated Au nanoparticles decorated TiO2 electrode can be delivered under UV irradiation by indiscriminatingly photooxidating a various kinds of organic compounds from 0 to 18 meq organic compoundsolution.

CO oxidation has been used as a prototypical reaction to demonstrate the potential of gold nanoparticles in promoting heterogeneous catalysis around room temperature. Here we report on a unique gold/titania-based catalyst that displays a second active regime at much lower temperatures, as low as 120 K. This new catalytic regime follows a mechanism different than that seen at higher temperatures, involving at least three titania-adsorbed CO species and a synergy between the CO and O uptakes on the surface. New titanate sites, formed by the treatment of Au/TiO-amorphous catalyst with NaOH, are believed to be responsible for the opening of this new reaction channel.

Titanium oxide (TiO₂) is commercially very important as a white pigment because of its maximum light scattering with minimal absorption. The physical and chemical characteristics of TiO₂ can be controlled by its particle size and morphology. The TiO₂ nanoparticle size and the high surface area have attracted substantial interests due to their unusual optical, electrical, and catalytic properties [1, 2]. Out of the three crystalline phases of TiO₂ namely rutile, anatase, and brookite, the rutile phase is the most thermodynamically stable, while the anatase phase exhibits the highest photocatalytic and antifogging activity [3].
TiO₂ films have recently been put to use for its antifouling and antifogging capabilities [4, 5] because of the oleophilic and hydrophilic nature of the films when exposed to ultraviolet light. The hydrophilic behavior means that the contact angle of water tends to 0°, preventing fog formation on the surface. The dual oleophilic and hydrophilic behavior of the titania surfaces allows built up dirt and grime to be washed off by flowing water, such as rain.
In this work Reactive Spray Deposition Technology (RSDT) has been explored as a single step flame based direct deposition process for synthesizing 5-10 nm of Si doped TiO₂ particles of metastable anatase phase directly on glass and mirror. Synthesis of the metastable anatase phase can be achieved by the rapid heating and cooling produced in the flame by air quenching the particles once they exit the flame at 700 #730;C. The short residence time in the flame and rapid air quench prevents the particles to shift to the thermodynamically stable rutile phase. Tuning the ratio of the TiO₂ and Si precursor to maintain Si doping value of 1-10 wt% in TiO₂ leads to the precipitation of Si in the grain boundary of TiO₂ which prevents sintering of the particles at high temperature.
Contact angle measurements of the films revealed the drop of contact angle from 25° to 0° in less than 1 s proving super hydrophilicity of the films. Antifogging property was also confirmed in a real time scenario by bringing the film coated glass in contact with water vapor. To further probe the properties and the remarkable antifogging behavior, additional investigations were done using high resolution Transmission Electron Microscopy (HRTEM), Brunauer-Emmett-Teller (BET) surface area, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and 3-D Optical Scanning Interferometer Profilometer.
Authors would like to acknowledge support from the University of Connecticut, School of Engineering for funding this work.
References:
[1] Rhee, C. H. et.al, 2005, Chem. Lett., 34, pp. 660-661.
[2] Yu, J. C. et.al, 2001, Chem. Commun. Cambridge, 2001, pp. 1942-1944.
[3] Ovenstone, J. et.al, 1999, Chem. Mater., 11, pp. 2770-2774.
[4] Zhang, X. et.al, 2005, Carbon, 43, pp. 1700-1708.
[5] Wagner, C. et.al, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Weinheim, p. 38.

Water/solid interfaces play a key role in photocatalysis. The activity of the photocatalysts is strongly related to the structure and electronic properties of the interface, but understanding of solvation effects is still limited because of their complexity. In this talk, I will discuss some recent results on the structure of interfacial water and the reactivity of TiO₂ surfaces in water environment through large-scale ab initio molecular dynamics (AIMD) and solvation model simulations. The results show that the solvation can affect not only the stable adsorption configurations, but also the reaction mechanism for some systems.

Nanostructured metal oxides (MO), such as TiO₂, are promising for light energy harvesting into electricity or chemical fuel such as hydrogen. Due to their large bandgap and thereby weak visible absorption, various strategies have been developed to enhance visible light absorption, including dye or quantum dot (QD) sensitization. In the meantime, charge transport in nanostructures is often limited due to a high density of trap states. Rational introduction of bandgap states, e.g. oxygen vacancies through hydrogen or other treatment of MO nanostructures, can substantially improve charge transport. In addition, plasmonic metal nanostructures can be used to enhance light absorption of sensitizers such as QDs. Therefore, by using a multi-pronged approach that combines QD sensitization for efficient charge injection, chemical treatment of MO for enhancing charge transport, and plasmonic enhancement of light absorption of QDs, we can achieve synergistic improvement in photoelectrochemical performance. Systematic characterizations using a variety of techniques, including ultrafast lasers, were carried out to gain insight into the underlying fundamental mechanism. The approach developed is general and can be potentially useful for many different applications.

Titanium oxide (TiO₂) is a versatile semiconductor material, which is used in diverse fields of application such as opto-electronics, semiconductor devices or photo-catalytic water splitting. A critical issue in all technological applications is the thermodynamic stability of the materials system under realistic (i.e. operating) conditions which can drive the system far away from standard equilibrium conditions.
Utilising the insight gained from our recently developed unified approach, which connects and “translates” concepts from semiconductor defect chemistry and electrochemistry [1], we construct TiO₂ defect stability phase diagrams considering the whole range of admissible chemical potential conditions [2]. As input we use defect energies for rutile TiO₂ obtained by density-functional calculations with the HSE exchange-correlation functional [3]. These diagrams reveal a narrow region of TiO₂ stability against point defect formation and enable us to draw conclusions about the stability of TiO₂ in electrochemical environment, discuss doping strategies, and identify optimal operating conditions for TiO₂ to be used as a photo-catalyst for water-splitting.
[1] M. Todorova and J. Neugebauer, Phys. Rev. Applied. 1, 014001 (2014).
[2] M. Todorova and J. Neugebauer, Surf. Sci. (2014), DOI: 10.1016/j.susc.2014.07.023
[3] A. Janotti, J.B. Varley, P. Rinke, N. Umezawa, G. Kresse, and C.G. Van de Walle, Phys. Rev. B 81, 085212 (2010).

Solar driven hydrogen production by semiconductor photocatalytic water splitting is a promising way to provide the clean and renewable energy in the near future. In order to achieve the highest efficiency of H₂ evolution, photocatalysts with well-defined active facets have been vastly pursued. However, the active facets are usually high energy surface and tend to be passivated by the surface reconstructions. Here we demonstrated an H₂ plasma etching method to reactivate such facets by introducing the meta-stable reactive sites and local defects on the surfaces. It has remarkably improved the surface structure determined quantum efficiency (QE) of the photocatalysts, by which the amazing H₂ evolution rate of the {100} facet dominant anatase TiO₂ nanobelts under a standard solar simulator irradiation (69.7 mmol g^-1 h^-1) were obtained. Our results revealed a possible solution to the surface passivation induced catalyst performance decay. It brings the new opportunity to achieve the most active photocatalyst for the future H₂ economy.

A myriad of articles, patents and reviews have appeared about applications of heterogeneous catalysis using TiO₂ as a photocatalyst for environmental remediation due to its favorable properties. Chemically inertness and stability, cost effectiveness and abundance are only a few to mention. The synthesis of TiO₂ by wet chemical methods such as dispersion coating and sol-gel results in poor mechanical stability. On the other hand materials deposited by physical vapor deposition (PVD) methods (and more specifically sputtering) have improved mechanical stability and adherence to substrates which is ideal for commercialization. The chemical composition and the properties of the reactively sputtered TiO₂ are highly dependent on the deposition conditions. In this study we synthesize TiO₂ by DC reactive magnetron sputtering and optimize the process conditions. We found the optimum oxygen partial pressure for deposition by monitoring the optical properties of the thin films using spectroscopic ellipsometry. The effect of post deposition annealing on the crystal structure of TiO₂ thin films was analyzed by x-ray diffractometer (XRD). We analyze the microstructure of the thin films by high resolution scanning electron microscopy (HRSEM). In-situ AFM analysis was performed on a patterned TiO₂ thin film in presence and absence of UV light irradiation. Finally the photocatalytic activity of the deposited TiO₂ thin films is systematically investigated in a microreactor and analyzed by a numerical model. The thin films before post deposition annealing were amorphous. After heat treatment the peak corresponding to anatase appeared in XRD pattern. In addition the interaction force between the AFM tip-silica and TiO₂ surface changes dramatically upon UV irradiation showing the surface charge difference caused by electron-hole formation in the TiO₂ thin films. The organic compound model was photcatalytically degraded within only 20 seconds inside the microreactor.

Synthesis of TiO₂ nanostructures by thermal evaporation and chemical vapour deposition method is limited due to low vapor pressure and high melting point of titanium. To avoid catalyst poisoning at high temperature a thicker dielectric and /or insulating support layer (50-200nm) are often required for the catalyst-assisted vapor-liquid-solid growth of TiO₂ nanowires on Si. Growth of a TiO₂ nanostructure by alternative vapor deposition method on a conductive substrate with a thinner support layer (<50nm) is therefore extremely important for future technological applications.
Here, we report the successful synthesis of 1D TiO₂ nanostructures by using pulsed laser deposition on Si substrate with the aid of a thinner SiO₂ support layer (< 50 nm) and a size-controlled gold nanoisland catalysts. The thickness of the SiO₂ layer on the substrate and the growth temperature are found to play an important role in controlling the morphology of resulting TiO₂ nanostructures, including nanowires, nanobelts, and nanoflakes. In particular, the growth at 700°C on thicker (29 ± 1 nm) and thinner (3 ± 2 nm) SiO₂ layers produces nanobelt and uniform nanowire arrays, respectively, while deposition at relatively lower and higher temperatures produces stacking-fault and tapered nanowires, respectively. Interestingly, a 2D nanoflake-like structure is observed on a H-terminated Si substrate. Photoelectrochemical measurement under simulated sunlight illumination shows that the photocurrent density depends on the morphology and SiO₂ layer thickness. For a typical SiO₂ support layer, the photocurrent increases with the increase in surface roughness, while it decreases with the increase in SiO₂ layer thickness regardless of surface type.
This study therefore illustrates several unique controls provided by catalyst-assisted pulsed laser deposition method for the synthesis of 1D TiO₂ nanostructures. The pulsed laser deposition method also offers the inherit advantages of easy doping and homo- and hetero-structure growth. These advantages should enable us to further improve the photoelectrochemical performance by doping and/or sensitization and to easily extend our approach to developing other wide-band gap semiconductor based photocatalysts.

Solid-state dye-sensitized solar cells (ss-DSSCs) have record power conversion efficiencies at 8.5%.[1] By using a solid-state organic hole-conductor, ss-DSSCs avoid the problems of liquid electrolyte stability faced by the standard DSSC design. However, recombination at the TiO₂/hole-conductor interface is much higher than that with the liquid electrolyte. There have also been efforts to replace the organic dyes with inorganic absorber materials; most recently, the success of devices with CH₃NH₃PbI₃ perovskite absorbers, now at 17.9% efficiencies, have drawn attention to solid-state inorganic-absorber devices.[2]
In this work, we replaced the dye in ss-DSSCs with semiconductor quantum dots (QDs) as the active light absorber, creating solid-state quantum dot-sensitized solar cells (ss-QDSSCs). QDs show favorable absorption properties due to their size-dependent band gap, in addition to higher molar extinction coefficients than commonly-used dyes. Increased absorption is critical as the optimal active layer thickness, i.e. the thickness of the nanostructured TiO₂ film sensitized with the absorber, is limited by the higher rates of recombination in solid-state devices. However, using QDs as the light-absorbing material introduces a new problem, that it is difficult to achieve high QD loading in the nanostructured TiO₂ film. The lower QD coverage of the TiO₂ surface can lead to further increases in internal recombination.
We have increased the QD loading in ss-QDSSCs by adjusting pH of the QD precursor solutions with triethanolamine (TEA) during deposition via successive ion layer adsorption and reaction (SILAR). This increased QD loading was confirmed by transmission electron microscopy. We observed a doubling of efficiency with the increased QD loading, to an efficiency of 0.65%. UV-vis spectroscopy and external quantum efficiency (EQE) measurements showed that the increased efficiency is partly due to increased absorption from the higher number of QDs. Transient photovoltage measurements of the electron lifetimes in the devices showed that the higher QD loading also decreased the recombination at the TiO₂/hole-conductor interface. To determine whether the decreased recombination is due to higher QD coverage of the TiO₂ surface, or passivation of the TiO₂ surface by TEA, we compared devices QDs grown with TEA in the SILAR solution and devices with a post-QD-growth treatment of TEA. Higher electron lifetimes were only observed in the devices with TEA in the SILAR precursor solutions, indicating that the decreased recombination is likely due to the increased QD coverage of the TiO₂ surface. In addition to TEA, we investigated multiple bases to adjust the pH, to explore the robustness of this approach to increase QD loading.
[1] I. Chung et al., Nature Lett., 485, (2012): 486.
[2] S. Ryu et al., Energy Environ. Sci., 7 (2014): 2614.

TiO₂ has been considered as a promising anode material for lithium ion batteries due to its high safety, low production cost, environmental benignity, non-toxicity and good chemical stability. However, the poor rate capability of TiO₂ originating from its intrinsically low lithium ion diffusivity and poor electronic conductivity prevents its practical application in lithium ion batteries. Reduction of TiO₂ particles to nanosize is able to improve its rate capability, because of the reduced path of lithium ion diffusion and electron transport. However, nanoparticles have low packing density resulting in a decreased volumetric capacity, which also limits its practical application as an anode material. Micron-sized TiO₂ particles that consist of carbon coated small nanoparticles materials are a better choice for the electrode materials due to their larger packing density, preservation of advantages of nanosized particles and the good electronic conductivity resulting from the coated carbon. In addition, the mesopores formed between the primary nanoparticles facilitate sufficient contact between the electrode materials and the electrolyte.
In this work, we developed a template method for the synthesis of mesoporous TiO₂ nanofibers with lengths in micrometer range constructed of nanoparticles with about 7.5 nm which are coated with a thin layer of N doped carbon. The nanofibers are flat owing to the nanobelt-like templates, with the length about 1 to 2 µm, width about 200-300 nm and thickness 80-100 nm. Electrochemical characterizations reveal that the composite exhibits excellent electrochemical performance in terms of specific capacity, rate performance, cycling stability. Specifically, at 10 C, after 500 cycles, a high capacity of 119 mA h g^-1 is still retained. The outstanding electrochemical performance of the composite makes it a promising candidate for practical application in lithium ion batteries.

In this work we performed a systematic study on the effect of temperature and residence time on TiO₂ nanoparticle synthesis by utilizing a microfluidic reactor. Microreactors provide a robust platform for studying material synthesis as temperature, residence time and concentration of reagents can be controlled precisely unlike in batch-wise synthesis methods. This microreactor was designed to produce monodisperse nanoparticles by separating nucleation and growth processes. Therefore it has nucleation and growth zones that are thermally isolated from each other and they can be kept at different temperatures while the rest of the reactor is cooled to quench the reactions outside of these zones. The microreactor was made out of a silicon substrate bonded to a Pyrex top layer. Heating of the zones were achieved by small ceramic heaters that were attached to the reactor surface and temperatures were measured by using thermocouples together with an infrared camera. Cooling of the rest of the chip was done with a cooling channel attached to the surface of the reactor. Ability of quenching the reactions provides precise residence times for nucleation and growth. In the microreactor a droplet-based flow was used where particles were synthesized inside droplets that were carried in an immiscible outer fluid (or carrier fluid). This provides uniform residence time for each sample. As droplets pass through the heated areas they were first nucleated at the nucleation zone and later grown in the growth zone that was kept at a lower temperature than the nucleation zone. The reaction time in these zones was controlled by the fluid flow rate was determined by syringe pumps. Samples were collected after each zone to observe the nucleation and growth stage of particles. TiO2 nanoparticles were synthesized in the microreactor under several different temperature and residence time conditions to investigate the effects of these parameters. The hydrolysis reactions responsible for the formation of TiO2 proceed via the formation of hydroxo-bridged oligomers that grow, dehydrate, and gradually increase in crystallinity. Unless there is a significant driving force, such as high pH, hydrolysis reactions are slow at ambient temperature. The microreactor affords the opportunity to observe the impact of temperature and time on particle characteristics. Nucleation temperature was varied between 35-200#730;C while growth temperature was varied between 70-100#730;C. Residence time in the growth zone was changed between 100-240 seconds. It was observed that after the nucleation step, nanoparticles form spherical clusters to reduce their energy and after the growth zone individual nanoparticles were formed. Nanoparticles in the sizes of 15-42 nm in diameter with a standard deviation less than 10% were observed. This is the first study that focuses on the formation of TiO₂ nanoparticles where the reaction conditions are varied systematically in a well controlled environment.

Titanium based materials have been intensively investigated and considered as good potential electrode materials for pseudocapacitors in reason of their exceptional properties, such as high thermal chemical stability, ready availability and low cost. However, the particle size is a key parameter for good electrochemical performance of metallic oxides. In this sense herein, we report a successful strategy to prepare TiO₂ nanoparticles with a size as small as 6 nm via sol-gel route under reflux. The influences of solvent nature, reflux time duration, drying temperature and time are reported. It will be shown that drying at 100^oC for 2 hours plays an important role in converting the as-formed amorphous TiO₂ matrix into a crystalline phase. These results unambiguously challenge the widely existing idea that high temperature annealing is required for attaining high-quality crystalline semiconducting oxide nanoparticles. They also reveal an excellent reproducibility of the TiO₂ nanoparticles synthesis, demonstrating applicability as an inexpensive, facile, and highly scalable route. The microstructure, chemical composition and crystalline phase of the synthesized TiO₂ nanoparticles are characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD).
The electrochemical performance of the electrodes made of these TiO₂ nanoparticles are determined by cyclic voltammetry and galvanostatic charge-discharge measurements. The influence of morphology, crystalline order, particle size and particle dispersion on the capacitance were emphasized and discussed in detail. The comparison of electrochemical properties of these materials shows that the structures of electrodes have great influence on their performances. The present work demonstrates a synthesis route of TiO₂ nanoparticles providing promising materials for electrodes in high energy density supercapacitors.

Self-organizing formation of nanocrystals in pores of mesoporous silica has advantages of control of the size and alignment. For the purpose of investigation of the size effect on properties of nanoparticles and the development of functional devices, the formation processes of thin film of mesoporous silica SBA-15, the one-dimensional pores of which are spontaneously arranged in two-dimensional hexagonal-system, and the syntheses of oxide nanoparticles in the pores of SBA-15 film have been developed. However, quantitative understanding of the optical property of titanium dioxide (TiO₂) nanoparticles confined in the pores is insufficient. In this research, we synthesized TiO₂ nanoparticles in the pores of SBA-15 thin films and investigated the optical properties (absorption coefficient, optical transition, and bandgap) for discussion about the characteristics of TiO₂ nanoparticle confined in the pore system.
Mesoporous silica SBA-15 thin films were formed with spin-coating of a chemical solution and through thermal processes. The solution consisted of Ethylene oxide/propylene oxide/ethylene oxide (EO20PO70EO20) triblock copolymer (P123) and tetraethylorthosilicate (TEOS). After the coated samples were dried and fired, the samples were soaked in a TiO₂ precursor solution for 24 hours to penetrate the solution into the pores of SBA-15. Soon after the samples were picked up from the solution, the precursor solution remaining on the surface of film was successfully removed by ethanol and ultrapure water. Finally, calcination of the samples was carried out in a furnace at 600°C. The SBA-15 film structure and pore size were evaluated from the analysis of X-ray diffraction (XRD) and X-ray-reflectivity (XRR), and the optical absorption spectrum was measured by UV-Vis spectrometer.
It was confirmed that the cylindrical pores of SBA-15 are parallel to the substrate surface and aligned in hexagonal system. The pore size was well controlled with the ratio of P123 to TEOS. The formation of TiO₂ nanoparticles in the pores was confirmed by XRR method and also X-ray photoelectron spectroscopy indicated that the TiO₂ nanoparticles included a small amount of defects. The optical absorption edge of TiO₂ nanoparticles synthesized in the pores, the cylindrical diameter of which is 2.4-5.4 nm, shifted to higher energy side as compared with bulk TiO₂. In the case of 5.4 nm size, the optical absorption increased rapidly with decrease in wavelength from 320nm. This change is caused by interband transition absorption. Tauc plot of the absorption spectrum showed that the optical transition is indirect and the optical bandgap is about 3.8 eV. These results cannot be explained only by the formation of quantum levels in the nanoparticle and therefore it suggests that the energy band dispersion is modulated due to the downsizing of TiO₂.

The developments of the flexible and wearable electronic devices have stimulated great interest to explore the flexible and lightweight energy storage devices with high energy and power densities. Among the promising energy storage device, lithium-ion batteries (LIB) have attracted great attention as a favorable power source due to their high energy density, low self-discharge and high operating voltage but short cycle life. The fundamental component for assembling flexible LIBs (FLIBs) is to develop suitable and high-performance flexible electrode materials. The realization of low-cost, high working voltage electrode material with good cyclic stability is highly desirable. Particularly, TiO₂ have been widely explored as attractive anode material for LIB due to its abundance, low cost, environmentally benign material with a very low volume change (3-4%), relatively high lithium insertion/extraction voltage at about 1.7 V, which is vital for large-scale energy storage. Its nanocomposites have been proved to display attractive lithium storage performance.
Recently, we present TiO₂ core shell nanocomposites as suitable anode material for FLIB. The TiO₂@TiN core-shell nanowires are grown on a flexible substrate to exploit its potential application in FLIB. Firstly, the free standing TiO₂ core nanowire (with diameter of 300 nm) was synthesized via hydrothermal reaction, and the TiN shell, with thickness of about 2.5 nm, was coated on the surface of the TiO₂ nanowires by thermal treatment in both N₂ and NH₃ atmosphere. The fabricated free-standing flexible TiO₂@TiN core-shell nanowires shows superior lithium storage performance, with exhibits excellent rate capability, delivering a specific capacity of 136 mAh/g at a current density of 30 C (1 C = 335 mA/g). It also shows remarkable long-term cycle stability and 84% capacity retention of the maximum specific capacity after 650 cycles, at a current density of 10 C. With such excellent performance, a full FLIB device was assembled employing the TiO₂@TiN anode and a commercial LiCoO₂ cathode, in order to demonstrate its potential application. The flexible TiO₂@TiN//LiCoO₂ device, at a current density of 335 mA/g, shows a specific capacity of 227 mAh/g for 30 cycles. This device also exhibited excellent flexibility, and showed 3% capacity loss, after cycling at 90^o bending position. The device also displayed a higher energy and power densities of 444 Wh/kg and 671 W/kg, respectively. The fabricated flexible TiO₂@TiN//LiCoO₂ device was used to power blue LED at the flat and bending positions.

Oxide semiconductors have attracted widely attention in the past decades, due to the potential applications in the areas such as photocatalyst, solar cell and optoelectronic, etc. However, its applications are still limited because of its inherent defects involving wide band gap and results in low light utilization efficiency. In order to overcome these disadvantages, many processes have been proposed in the last several years involving compounds, metal and nonmetal doping, surface modifications. Generally, coupling with oxide semiconductor is a simple and efficient method to improve photocatalytic efficiency. That is to say, a heterojunction is introduced between different oxide conductors for enhancing utilization of solar energy and also reducing the recombination ratio of photo induced electron-hole pairs simultaneously. Up to now, many photocatalytic composites have been prepared by using variant methods, such as metal-organic vapor-phase epitaxy, hydrothermal process, chemical vapor deposition (CVD), and microwave assisted method, etc.
In this paper, we introduce a novel physical route for preparing heterojunction composites via high temperature treatments. The general process is as follows: 1) preparation of the substrate; 2) pulse plating a metal layer on the substrate; 3) thermally treating the composite and “in situ” to obtain the metal oxide on substrate composite. Four kinds of heterojunction composites were prepared base according to this route, including ZnO/TiO₂ heterojunction fibres, ZnO/graphene composite, ZnO/TiO2 vertical- nanoneedle-on-film heterojunction, and porous micro-nano-structure NiO/ZnO heterojunction. Comparing with the other heterojunction from regular chemical routes, the present process provided a tight contact and combination between the components, which eventually led to a heterojunction between the two kinds of components. The most important was that a full lattice coherent or partial lattice coherent heterojunction was formed between the components in the elevated conditions through short atom diffusion at the interface, and the formation mechanism was different from the chemical route. This kind of heterojunction exhibited great improvement for separation efficiency of photo-generate electron-hole pairs. Experimental results of ultraviolet-visible light catalysis demonstrated that the photocatalytic activity of the heterojunction composite had been greatly improved. For example, the photocatalytic efficiency of the ZnO/TiO₂ vertical- nanoneedle-on-film heterojunction composites has been improved for 2 times higher than that of pure TiO₂ and ZnO, and the ZnO/TiO₂ composite fibres also increased the photocatalytic efficiency more than 2 times than that of the pristine TiO₂ fibres.

Three-dimensionally ordered macroporous ceramic materials have been intensively investigated over the past years. Their highly interconnected porosity in the form of inverse opals is attractive for many applications, such as solid oxide fuel cells, porous membranes, and catalysts. Additionally, the ordered arrangement of the pores results in periodic modulation of the dielectric function, imparting the ceramic with the functionality of a photonic crystal, which can control and manipulate the flow of electromagnetic radiation. Titania inverse opals have been widely studied due to their high refractive index and catalytic activity. However, in high temperature environments (>1000 °C), these structures can exhibit significant undesired microstructural changes (phase changes, grain growth, shrinkage) that result in a loss of the desired properties. These are particularly challenging for the titania system, which exhibits an anatase-to-rutile phase transformation in parallel with rapid grain growth at temperatures as low as ~600 °C.
We present our study and highlight several interesting observations about the high-temperature behaviour of titania inverse opals and their structural, optical, and phase properties. Titania inverse opals were prepared from templates of vertically self-assembled polystyrene monospheres by atomic layer deposition infiltration and subsequent calcination. Resistance to grain growth and a shift in the phase transformation of anatase to rutile to higher temperatures was observed, with vacuum annealed structures demonstrating dramatic stability up to 1300 °C. These intrinsic effects were achieved without additional doping or surface passivation. In addition to being technologically enabling, the results lead to a better understanding of the titania system and its phase transformation mechanism.

Mercury (Hg) released from coal-fired power plants are one of the major pollutants that affects human health and ecosystem. In an effort to enhance Hg removal from the coal-fired power plant flue gases, several studies focus on searching for new material with cost-effective and high Hg capture ability. To facilitate these novel Hg support designs, a deep understanding about adsorption interaction between Hg and its adsorbent is very important. Hence, the aim of this study is to provide synergetic effects of the metal nanoclusters (Pd, Au and Ag) supported on titanium dioxide (TiO₂) toward Hg adsorption by using density functional theory (DFT) calculations. The results reveal that the Hg strongly adsorbs on the metal/TiO₂ composites but weakly binds to either metal cluster or TiO₂ (110) surface. Based on Mulliken charge analysis, charge density difference and density of states (DOS), the electronic interaction of Hg and the metal/TiO₂ composite is mainly dominated by electron transfer from Hg to both deposited metal and entire TiO₂ support via the metal-oxygen interface. In addition, the amount of these electron transfer has been found as a key factor for the Hg binding strength. For that reason, it can be stated that the Hg behaves like electron donor when it is adsorbed over the support. Thus, we strongly suggest that an efficient substrate for Hg removal can be designed by tailoring its electrophilicity.

In this study, 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, particularly regarding 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 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. Analysis from XPS shows a defluorination of the films (C/F ratio >1) from the deposition process, with the final film being a mixture of PVDF and polyvinyl fluoride (PVD). Other parameters such as the dielectric constant and the breakdown voltage are also given.

In the present work, TiO₂/Graphene composite materials were prepared and their photocatalytic activity ion NO oxidation was investigated. Initially, natural graphite was oxidized to graphite oxide (GtO) via Hummers method. The prepared GtO was exfoliated to graphene oxide (GO) in water solutions using ultrasonication. The graphite oxidation was monitored by XRD analysis and RAMAN spectroscopy. The concentration of GtO in the water solutions was evaluated employing UV-Vis spectroscopy. Pollutants
The synthesis of TiO₂ and its coupling with GO was performed in autoclave applying hydrothermal treatment at 180^oC. Titanium isopropoxide was used as precursor. It was found that during the hydrothermal treatment the GO is partially reduced, its functional groups are removed and the delocalization of electrons is restored. The TiO₂ crystallization in the form of anatase was revealed by the XRD analysis. The crystallite size calculated using Sherrer formula was found to be ~10 nm.
The activity of TiO₂/Graphene composites in photocatalytic NO and acetaldehyde oxidation was determined following standard ISO / DIS 22197-1: 2007 and ISO 22197-2:2011 procedures respectively. It was established that the presence of reduced graphene oxide improved the photocatalytic activity of TiO₂ under both UV and Vis irradiation. The highest photocatalytic efficiency was demonstrated by the composites containing low GO concentrations namely 0.1 % under UV and 0.01 % under visible light irradiation. Finally the composite TiO₂/Graphene materials exhibited excellent De-NO_x ability. This outcome was related to the synergetic effect of TiO₂ and Graphene nanosheets.
Keywords: Graphene, Photocatalysis, TiO₂, Anatase, Photoactivity, NOx, Acetaldehyde

In the present work synthesis of anatase nanocrystals with dominant {001} either {101} or {010} facets in the micro and nanoscale and their application in air pollutants oxidation is presented. As a titanium precursor titanium tetrafluoride (TiF₄) or titanium isopropoxide [Ti(C₃H₇O)₄] was used and added in water or ethanol. The produced crystal morphologies were studied according to their structure and photocatalytic activity. Doping with Mg and Mn ions and photodeposition of silver nanoparticles on {101} facets of the nanocrystals was examined. The structure of the nanocrystals was investigated using XRD, SEM, TEM, Raman, BET N₂ adsorption/desorption. Their energy gap was determined with DRS measurements. The photocatalytic activity was evaluated by oxidizing nitrogen oxide (NO) gas as well as decomposing the organic acetaldehyde gas (CH₃CHO).
It was demonstrated that the oxidation of NO is higher when dominated are {101} crystal facets whereas the decomposition of acetaldehyde is enhanced when the {001} crystal facets are dominated in the anatase nanocrystals. As a consequence all the anatase crystals that had dominant {101} facets exhibited a high photonic efficiency regarding the NO oxidation whereas those crystals with dominant {001} facets showed a high photonic efficiency for the decomposition of the acetaldehyde gas. Doping the anatase crystals with magnesium ions or/and photodeposing silver nanoparticles on the {101} facets of the anatase crystals enhanced significantly the photocatalytic activity of the samples under UVA irradiation in comparison to the pure anatase. The nanocystals doped with manganese ions exhibited photocatalytic activity under visible light.
Keywords: Morphosynthesis bull; TiO₂ bull; Photocatalysis bull;Photodeposition bull; Facets Engineering
[1] S.W. Liu, JG Yu, J. Am. Chem. Soc., 2010, 132, 11914-11916.
[2] M.V. Sofianou , C. Trapalis, et al., Environ. Sci. Pollut. R,2012, 19, 3719-3726.
[3] M.V. Sofianou , et al. Appl. Catal. B-Environ,2013, 142-143, 761-768.

Bacterium-related environmental issues like the drinking water problem due to the presence of microorganisms in water are gaining intensive worldwide concerns. Apart from its great potential for degrading toxic compounds, titania was evidenced to have antibacterial properties. Yet use of nanosized titania particles usually suffers from difficulties in removal after photocatalytic elimination of bacteria. Here we report nanocrystalline TiO₂ hollow microspheres prepared by suspension plasma-spraying nanosized titania particles. Anatase particles in the size of ~10nm in diameter were used as the starting feedstock. Results showed that the as-sprayed microspheres with the size of ~50mm in diameter have interior cavities with a mesoporous shell of ~2mm in thickness. The crucial variables affecting the balling efficiency and the characteristics of the hollow spheres were investigated. Formation mechanism of the spheres was also analyzed. In addition, the ability of the hollow titania spheres to kill E. coli bacteria by photocatalysis was examined and elucidated.

Cobalt mediators have been known for their slow mass transport property. They show about three times lower bulk diffusion coefficient compared to I₃^minus; /I^minus; mediator system, and even worse for the case within a mesoporous TiO₂ film, namely, ten times slower than I₃^minus; /I^minus; system. The contrasting characteristics are due to the relatively large molecular size of the cobalt redox mediators, and their limited solubility compared to I₃^minus; /I^minus; system. To cope with mass transport issues of cobalt mediators. Mono-dispersed microspheres of highly exposed (001)-facets anatase TiO₂ were synthesized by a facile hydrothermal route in this study. Each of these microspheres, having a pore size of ~30 nm and a surface area of 132 m²/g, consists of nanosheets with a side length of 30-50 nm and a thickness of 3-5 nm. The TiO₂ thin film served as the semiconductor layer for a dye-sensitized solar cell (DSSC). We demonstrate that, owing to the large pores inside the microspheres, an extremely high porous TiO₂ film is formed, thus offering a facile freeway for electrolyte diffusion. Meanwhile, microspheres adsorb extremely high quantity of dye molecules per unit surface area (dye surface concentration), ascribing to the nature of highly exposed (001)-facets nanosheets. Further, the pore size in a microsphere can be varied, thus allowing for independently tuning their mass transporting properties. Moreover, ultra-long electron lifetimes further increase their open-circuit voltage. By optimizing the film thickness, a cell efficiency of 11.4% was obtained, which is 30% higher than that obtained with a standard photoanode having 4 µm scattering layer (PT-501A, Ya Chung industrial Co., Ltd., Taipei, Taiwan) on top of 4 µm transparent layer (Ti-Nanoxide HT/SP, Solaronix) (8.97%). We demonstrate that the anatase TiO₂ nanosheet assemblies provide long-sought-after material solutions in mass transfer limited cobalt-based DSSCs.

The excellent physical and chemical properties of titanium oxide have made it an useful material in versatile applications, such as the photocatalytic water splitting medium, the anode material coated on silicon nanowires in lithium batteries, and resistive switching layer in resistive random access memories (ReRAM). In these applications, the most routine and reliable preparation method is the atomic layer deposition (ALD). The conformity and uniformity of the titanium oxide thin films prepared by ALD makes this process an ideal tool to deposit titanium oxide thin films on the surfaces of complicated three-dimensional nanostructures. The crystallinity of the titanium oxide thin film is often an important factor, which determines the performance of applications; for example, the amorphous titanium oxide thin film shows lower leakage current than the crystalline phase. However, it is noted that there exists a critical thickness, beyond which the amorphous phase transforms into crystalline thin film; such transformation sometimes leads to delineation of the as-deposited films. The critical thickness is also a function of the deposition temperature. In this study, we deposit TiO₂ thin films at 250°C with different thicknesses on Si(100) substrates. It is found that the critical thickness is around 12 nm - below that, the TiO₂ thin film stays amorphous; otherwise, the anatase TiO₂ phase is formed. For evaluating the residual stress between these TiO₂ thin films and Si substrates, the Si substrates coated with TiO₂ are thinned down to about 100 nm, thin enough for the residual stress in the TiO₂ film to bend the Si substrates. Convergent beam electron diffraction (CBED) patterns and the bending contours of the Si substrates are recorded in transmission electron microscopy (TEM) to determine the thickness and curvature of the substrates, which are further used to calculate the residue stress using the Stoney equation. We find that the residual stress at the interface is tensile and the stress in the amorphous TiO₂ layer is several times higher than those of thicker crystalline TiO₂ layers. It is suggested that the stress in the amorphous TiO₂ layer is built up after cooling from the ALD process temperature, due to the mismatch of thermal expansion with the Si substrates. The transformation of thicker TiO₂ layers from amorphous to crystalline state is driven by the relaxation of a larger thermal stress.

Combining photocatalytic properties with a high specific surface area, mesoporous titania can be used in a wide range of applications such as water treatment, air purification and water splitting. Conventionally, soft template synthesis routes comparable to silica syntheses (EISA, hydrothermal syntheses, etc.) are used. These syntheses are time consuming processes, including long aging times and inefficient heating procedures while still only moderate degrees of crystallinity are obtained. [1] Obtaining a material with both a high porosity and a high crystallinity poses a challenge as crystal growth leads to collapsing of the pores. Microwave irradiation can be the solution for these problems. Microwaves directly couple to the solvent molecules used during synthesis, making it possible to create very fast heating, avoiding slow heat transfer through different materials (air-metal-solution) typical for hydrothermal routes. In TiO₂ nanoparticle synthesis, replacing a hydrothermal synthesis by microwave irradiation not only reduced the synthesis time but also an increase in crystallinity was observed [2], therefore it is also possible that microwave irradiation improves the degree of crystallinity in mesoporous titania, solving both problems of conventional solution syntheses.
We found that the use of microwave irradiation makes it indeed possible to enhance the degree of crystallinity of mesoporous titania, while avoiding the loss of specific surface area. Other properties like particle size, band gap energy and isoelectric point also remained unaffected by the microwave treatment. Against expectations the photocatalytic activity of the microwave irradiated samples, exhibiting an increased crystallinity, was lower compared to the non treated samples. We are investigating this reduction using EPR and other techniques to understand the underlying effects of microwave irradiation.
[1] Ismail, A. A. and Bahnemann, D. W., J. Mater.Chem. 32 (2011) 11686.
[2] Dufour, F., Cassaignon, S., Durupthy, O., Colbeau-Justin, C. and Chaneac, C., Eur. J. Inorg. Chem. 16 (2012) 2707.

Mixed-phase samples of rutile and anatase TiO₂ have been experimentally shown to have synergic effect on the photocatalytic activity of water-splitting reactions under sunlight. This extraordinary behavior is generally believed to be mainly attributed to the electric field built-in at the interface between anatase and rutile phases of TiO₂, which can effectively enhance the photo-excited charge carrier separation to increase the generated photocurrents. Nevertheless, there remain some debates regarding the band lineups between the anatase and rutile phases of TiO₂ in terms of different experimental viewpoints.
In this study, we have employed first-principles density function theory calculations to investigate the band alignment between the rutile and anatase phases of TiO₂. At variance with the previous theoretical studies on this material system, we indeed constructed two realistic heterojunction model structures based on the experiments, anatase (112) / rutile (100) and anatase (110) / rutile (011), respectively, to deduce their theoretical band alignments. These heterojunction interface structures were generated using ab initio molecular dynamics simulated annealing method so that they are much more reliable than those previously generated using classical force field modeling. Our generated structure models reveal that the fivefold- and sixfold-coordinated Ti atoms are the most predominant bonding configurations in the interface region rather than the fourfold-coordinated Ti atoms as suggested by a recent tight-binding molecular dynamic study.
Our calculations show that the offset values of VBM/CBM are 0.409 ± 0.12eV / 0.239 ± 0.12eV for anatase (112) / rutile (100) interface, and 0.588 ± 0.03eV / 0.418 ± 0.07eV for anatase (110)/ rutile (011) interface, respectively, which are consistent with the most recent experimental measurements. These results indicate the direction of the photo-excited electron transport should be majorly from the rutile side to the anatase phase of TiO₂. Furthermore, we also applied other theoretical methods, such as the branch point energy lineup and vacuum level lineup, to predict the band alignments of these two phases without constructing the realistic interface structure models. As compared with those calculated using the generated heterojunction models, we obtained satisfactory results for the trend predicted using these indirect theoretical approaches though the relative magnitudes of the band offsets appear to be slightly different. Nevertheless, they can provide a much easier way to acquire useful preliminary guess for the band alignments of the relatively more complicated oxide material systems.

The development of new technologies for energy generation and storage has been the focus of extensive research for the past decades. However, the availability of some of these technologies has been limited due to cost and technical effectiveness. Proton exchange membrane fuel cells (PEMFCs) are among these technologies in which the necessity for more cost effective and highly selective membranes still provides an important research field. In our research laboratory we work with polymer nanocomposite membranes aiming for selective transport properties at a low cost.
In this study sulfonated block copolymers with at least one elastomeric block were functionalized by the incorporation of titanium dioxide (TiO₂) nanoparticles (as received, amine, and sulfonic acid functionalized). The properties of these materials were extensively characterized in order to determine their suitability for PEM applications. Chemical, thermal, mechanical, and morphological characterization techniques allowed us to understand how these nanoparticles affect the performance of the membranes. Changes in the transition temperature and energy required for these transitions were observed after the different nanoparticles were incorporated. Although no major changes were observed on the degradation temperature upon nanoparticle incorporation, upon sulfonation the degradation temperature increased by as much as 50°C due to the additional phase segregation. The incorporation of nanoparticles, both functionalized and unfunctionalized, significantly enhanced the selectivity of the membranes for some membranes, but not for others due to the different transport mechanism for protons and methanol and the changes in the nanostructure of the resulting polymer-nanocomposite membranes. Small angle X-ray scattering (SAXS) results showed that the incorporation of nanoparticles did not alter the ionic interstitial distance, suggesting that they were incorporated in the elastomeric domains. The mechanical characterization results showed that the elasticity of the membranes was not altered, but the yield stress was highly reduced. Additional materials characterization results were used to describe the resulting nanostructure and their influence in the transport mechanism of protons and methanol for direct methanol fuel cell (DMFC) applications.

Titanium oxide is one go-to material for a host of technological applications in advanced fields of research. However, the suitability of TiO₂ in most of these applications is strongly dependent on the ability to control not only size, structure of defects and doping, but also on its crystallographic phase [1], as anatase is a metastable phase which transforms irreversibly into rutile with different properties and applicability. The anatase to rutile transition (ART) is not totally understood so far, and several issues still require to be addressed.
In this work anatase TiO₂ nanoparticles doped with Al or Fe have been synthesized via a modified Pechini method which allows to reach high control in size and composition. Microstructural analysis confirms the good crystallinity of the doped anatase nanoparticles with average sizes around 5 nm and dopant cationic concentrations up to 30 %. The influence of Al and Fe doping in the structural and luminescent properties of anatase TiO₂ nanoparticles has been analyzed, with special attention focused on the effect of doping in the ART induced by thermal annealing or locally laser irradiation. Thermo-diffraction measurements indicate that the thermally-driven phase transition can be either promoted or inhibited by Fe or Al doping, respectively. In this case anatase phase has been stabilized up to temperatures above 900 °C by appropriate Al doping [2], which spreads the applicability of this metastable phase. In addition, advantage has been taken of the controlled ART by laser irradiation, and rutile or mixed anatase/rutile phases have been locally generated with micrometric resolution, thus performing micropatterning based on titania polymorphs [3]. Achieving a suitable control of the anatase to rutile phase transformation and micropatterning assures significant progress in challenging microdevices design. The results of this study provide relevant new insights that can help to explain the ART process, such as the influence of dopants (Al, Fe) in the kinetic of the ART, the key role played by the Ti-O bonding at the surface, the relevance of Ti³⁺ and oxygen deficiency, or the formation of rutile nucleation points at twinned regions.
The structural, compositional and optical characterization of the samples have been carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM), conventional and high resolution transmission electron microscopy (TEM, HRTEM), selected area electron diffraction (SAED), x-ray energy dispersive spectroscopy (EDS), photoluminescence (PL), Raman spectroscopy, and x-ray absorption spectroscopy (XAS).
[1] G. C. Vásquez, M.A. Peche-Herrero, D. Maestre, A. Cremades, J. Ramírez-Castellanos, J.M. González-Calbet, J. Piqueras. J. Phys.Chem. C, 117, 1941, (2013)
[2] OEPM Patent P201400722, (2014)
[3] OEPM Patent P201400759, (2014)

Biomaterials are widely used for dental prostheses, orthopedic devices, cardiac pacemakers and catheters [1]. A search of the Sciencedirect® data base with the key words “metallic implants” produced 23,625 results, a testimony both of the importance of this topic, research efforts but also of the many problems that are yet to be solved. Generally Ti and its alloys are favored as bio implants due to their surface characteristics, which promote biocompatibility [2]. However, the surface of titanium maybe contaminated during casting or surface structuring due to its highly reactive nature, which in turn lessen the biocompatibility at the tissue/bio implant interface [3-4]. The surface structuring techniques such as sand blasting and chemical etching tend to cause contamination on the implant surface, while the high temperature plasma coating or laser texturing result in increased cost [5].
Ti has also several disadvantages including a modest bone bonding ability leading to a higher rate of bone formation, a slow dissolution in the body fluids, which is mostly critical for TiAl4V6 alloys since V present cytotoxicity, a high friction coefficient and low wear resistance, which could results in particles debris, causing inflammation and poor bone-integration. After many years of practice it has been realized that dissolution or metallic particle released from the prosthesis could cause serious health problems. It is obvious that most of these problems are originating or connected with the surface of prosthesis and implants.
The main objective of the research proposal is to engineer a smart Ti-based prosthesis surface by chemical mechanical polishing process aiming at significantly reducing the out-diffusion of Ti and other metallic impurities from prosthesis in contact with body fluids and tissue and simultaneously enhancing the surface mechanical, chemical and biological properties. CMP technique was used for the growth of a thicker and denser self-protective native oxide on Ti and Ti alloys samples while simultaneously possessing a controlled surface roughness. It was demonstrated that the Ti based dental implants with self-protective surfaces minimize chemical and bacterial reactivity, Ti ion dissolution while promoting their biocompatibility through surface patterning. The studied self-protective oxide films can also be utilized for many additional applications including bio-sensors.
1.Shirkhanzadeh, M., Azadegan, M., Liu, G. Q., Mater. Lett., 24 ,7-12, (1995).
2.Shibata K, Kamegai A. Titanium in dentistry: Biocompatibility of titanium. Quintessence, Tokyo, pp.35-41,(1988).
3.Miyakawa O, Okawa S, Kobayashi M, Uematsu K. Surface contamination of titanium by abrading treatment. Dent in Japan; 34#129;F90-96, (1998).
4.Akhter R, Okawa S, Nakano S, Kobayashi M, Miyakawa O. Surface composition and structure of titanium polishedwith aqueous slurry of ferric oxide. Dent Mater J; 19: 10-21, (2000).

Highly organized titania nanotube arrays have been used in photoelectrocatalytic applications as water splitting, wastewater treatment and other catalytic processes. The high efficiencies reported have been attributed to the material aspect ratio, surface area and charge carrier mobility. Those characteristics depend of the morphology of the titania nanotube arrays, which are easily tunable when the material is produced by anodizing through changing the anodization conditions. Herein the influence of nanotube length, diameter and wall roughness was studied as independent variables to model the hydrogen evolution in a photoelectrocatalytic cell. Control over morphology was achieved adjusting water content of the anodizing solution, duration and voltage profile. Morphology was characterized by scanning electron microscopy. In the voltage range of 60 to 100 V, it was found that diameter is dependent on the water content of the ethylene glycol based solution rather than on the voltage. Length was controlled adjusting the duration of the anodizing process. Roughness was tuned applying an alternating voltage profile instead of keeping it constant. The voltage profile used consisted of two steps at two different voltages alternated during the anodizing until the desired length was achieved. High voltage steps determined diameter and length of the nanotubes, while each step at low voltage formed a ring on the outside of the nanotubes wall. Using these results, experiments were conducted to determine the effect of the morphology on the hydrogen evolution during photoelectrocatalytic water splitting.

Ruthenium incorporated titanium oxides (Ru_x/TiO₂) are synthesized by one step hydrothermal method using Ti(SO₄)₂ and RuCl₃ as the sources of Titanium and Ruthenium, respectively. XRD, TEM, EDS mapping analyses confirm that ruthenium atoms are well dispersed in the anatase phase of TiO₂ and the size of crystallite of Ru_x/TiO₂ (raquo;17 nm) is smaller than that of pure TiO₂ (raquo;45 nm). Our solid catalysts exhibit high reactivity and selectivity without formation of any byproduct for oxidation of benzyl alcohol into benzaldehyde using oxidant. Under N₂ condition not using any oxidant, benzaldehyde are produced with high selectivity (ge;93%) for the oxidation of benzyl alcohol. When the catalysts are applied the oxidation of benzyl amine, three oxidative compounds (benzonitrile, benzylidiene benzylamine, benzaldehyde) are synthesized. Under the no oxidant condition (O₂-free), particularly, the oxidative self-coupling product that is benzylidiene benzylamine is produced as a major product (selectivity, le;81%). The crystallites aggregate each other to change into crystal phase of rutile with the size of approximately bigger than 140 nm by after annealing Ru_0.07/TiO₂ at 700 ^oC for 6 hr. The annealed sample appears the formation of RuO₂ crystallites which are embedded in TiO₂ with phase segregation, and results in the decrease of catalytic activity for organic oxidation reactions.

Hierarchical cactus-like rutile TiO₂/carbon-fiber cloth is synthesized by a two-step hydrothermal reaction. Firstly, the rutile TiO₂ microrods along [001] direction with 300-500 nm width and about 3 mu;m length were grown on carbon-fiber cloth. TiO₂ branches also along [001] direction are further fabricated in the second step. The branches are 3-10 nm in diameter and ~120 nm in length. The density of branches can be adjusted by varying the concentration of HCl. Moreover, the Li⁺ and Na⁺ intercalation properties of these cactus-like TiO₂ materials were investigated by cyclic voltammetry (CV) and chronoamperometry. It exhibits good Li⁺ and Na⁺ storage abilities. Especially, the Na⁺ intercalated/extracted density of cactus-like TiO₂ is 241.6/199.1 mC cm^-2. The branches provide numerous large channels which induce good ion intercalation/extraction performance. Therefore, the cactus-like rutile TiO₂/carbon-fiber cloth can be a promising material for Li⁺ and Na⁺ storage.

TiO₂ nanotube arrays with highly ordered porous structure have been attracting much attention as photocatalysis, membrane, gas sensor, photoelectrode in dye-sensitized solar cells, interferometric biosensing chip and so on. TiO₂ nanotube arrays were stable in acid and easy to make using anodic oxidation. From these structural and chemical advantages, TiO₂ nanotube arrays were used as a porous sensing chip in an interferometric biosensor. The interferometric biosensor used a white light and optical interference, which is occurred in nanoporous TiO₂ and backscattered. Interference spectrum was converted to optical thickness by fast Fourier transform algorithm. To improve a sensing performance, TiO₂ nanotube arrays having various structures were fabricated and optimized through the control of anodizing factors such as voltage, time, and water contents in electrolyte. In TiO₂ nanostructure which has the diameter of ~140nm and length of ~4.5mu;m, the best sensing performance were observed. The optimized structure was obtained at 60V, ~60minutes, and ~10% of water in organic electrolyte. To increase the optical interference in porous layer, Pt thin film was additionally deposited on TiO₂ nanotube arrays. The thickness of porous layer was measured by field emission scanning electron microscope and spectroscopic liquid infiltration method. The optical thicknesses on the drugs including ibuprofen and chlorhexidine were monitored in real time. In ibuprofen of 5%, optical thickness change of ~10 nm was observed for a day. TiO₂ nanotube arrays as an interferometric biosensing chip were very useful to detect the drug elution.

TiO₂ nanoparticles have been usually used as a photoelectrode in dye-sensitized solar cells. The performance of electronic charge collection in TiO₂ photoelectrode is one of issues to increase the power conversion efficiency in dye-sensitized solar cells. To increase the electronic charge collection in photoelectrode, TiO₂ nanostructures of mixed dimensionality was used. The nanostructures included TiO₂ nanoparticles, TiO₂ nanofibers, and carbon nanotubes. TiO₂ nanostructures of mixed dimensionality showed a better charge collection thana conventional TiO₂ nanoparticledue to one-dimensional structure of TiO₂ nanofibers and carbon nanotubes without decreasing specific surface area. The one-dimensional structure permitted a faster charge percolation and slower recombination of electrons. TiO₂ nanofibers and carbon nanotubes were fabricated by electrospinning method.Electrospun TiO₂nanofibers and TiO₂ nanoparticles were observed as an anatase phase by X-ray diffraction pattern.Diameters of TiO₂ nanoparticles, TiO₂ nanofibers, and carbon nanotubes were ~30nm, ~500nm, and ~10nm, respectively. Various manufacturing conditions forTiO₂ nanostructures of mixed dimensionality were discussed to obtain the best power conversion efficiency. Compared to conventional dye-sensitized solar cells with TiO₂ nanoparticles, dye-sensitized solar cells with TiO₂ nanostructures of mixed dimensionality showed a higher short circuit current density of 4.35mA/cm², a higher power conversion efficiency of 2.35 %.Electrochemical impedance spectroscopy was observed to understand an electron transfer and life time in photoelectrode.TiO₂ nanostructures of mixed dimensionality were very efficient to improve the performance of dye-sensitized solar cells.

Memristors are the non-volatile passive devices completing the group of the other three, i.e., resistors, inductors and capacitors and are the best suitable replacements for the well-known volatile flash memories. Along with resistance random access memory (RRAM) memristors based on transition metal oxides that operate on the principle of dynamic resistive switching provide high performance with low power consumption, which meets the great demand in the ultra-large-scale electronic systems. Recent studies on the Pt/TiO₂/Pt memristor devices have shown that the switching from high resistance state (HRS) to low resistance state (LRS) and vice versa is associated with the electroforming process at high applied bias. As electroforming process is destructive in nature and inherently uncontrollable, it could damage and even completely destroy the device. It is therefore a great challenge to fabricate a non-electroforming memristor device with high endurance and stable electronic behavior.
Here, we exploit the current-voltage behavior of a low bias non-electroforming memristor device based on a Pt/TiO_x/Pt layered structure, fabricated at room temperature using a combination of maskless photolithography and high vacuum magnetron sputtering. The TiO_x film is characterized using SEM, AFM, XRD and UV-Vis spectroscopy, and the IV behavior of the memristor devices are determined using a semiconductor analyzer. Unlike previously reported devices, no high bias is required for switching the device from high-resistance to low-resistance state. We also observe a remarkably low switching voltage (-1.0 to 1.0 V) for a 5×5 µm² junction. The device remains in its virgin (high-resistance or off) state for first few sweep cycles (less than 3 cycles) and then it turns on (i.e. switching to the low-resistance state), with a remarkably high on-to-off ratio. We also achieve high endurance and fast switching for up to 100 switching cycles for these devices.
This study illustrates a low-cost, simple process for fabricating high-performance memristor devices based on nanostructured metal oxides by using an efficient physical vapour deposition approach. Further improvement will be made by incorporating new hybrid nanostructured materials with appropriate structural modifications, which will lead to next-generation memristor devices with even higher endurance and lower energy consumption.

Single crystalline porous anatase TiO₂ nanostructures on titanium metal thin film were fabricated by annealing treatment of related titanates which were prepared by facile hydrothermal method in different concentrated NaOH solutions. The nanowires with 50 nm diameter and 5 mm in length are obtained in 1M NaOH solution, whereas, the nanosheets with 300 nm in width and 8mu;m in length were obtained in 3 M NaOH solution. Both the as-prepared anatase TiO₂ nanostuctures acted as binder free anodes in Li-ion batteries. The nanosheets exhibits a high reversible discharge capacitance of 212 mA h g^minus;1 at a rate of 1 C after 100 discharge-charge cycles and also shows a high stability when the discharge-charge rate was up to 20 C ( 95 mA h g^-1).On the other hand, the nanowires shows 142 mA h g^minus;1 at a rate of 1 C after 100 cycles and 49 mA h g^minus;1 at a rate of 20 C. During charge-discharge, porous nanostructures provide short lithium ion diffusion path and growing directly on conductive substrate ensures fast electron transport. This result reveals the great potential of using TiO₂ nanowires directly deposited on titanium metal thin film as an anode material in Li-ion batteries.

This work shows the application of metal ion-implantation to realize efficient second-generation TiO₂ photocatalyst. High fluence Fe⁺ ions were implanted into thin TiO₂ film and subsequently annealed up to 550 °C. The ion-implantation process modified the TiO₂ pure film, locally lowering its band-gap energy from 3.2 eV to a minimum value of 1.6 eV, making the material sensitive to visible light. The measured optical band-gap of 1.6 eV was associated to the presence of energy levels in the energy band structure of the titanium dioxide, due to the implantation-induced defects. An accurate structural characterization was performed by Rutherford backscattering spectrometry, transmission electron microscopy, Raman spectroscopy, X-ray diffraction and UV/VIS spectroscopy. The synthesized materials revealed a remarkable photocatalytic efficiency in the degradation of organic compounds in water under visible light irradiation, without the help of any thermal treatments. The photocatalytic activity has been correlated to the amount of defects induced by the ion-implantation process, clarifying the physical mechanism acting. These results can be fruitfully applied for environmental applications of TiO₂.

Creation of high efficiency and safe air purification systems is the important task caused by their wide use in living quarters, medical institutions, industrial areas. The most effective cleaning systems are the ozone based ones which is formed as the result of the corona or barrier discharge. The main disadvantage of these purification systems is high concentration of ozone in discharge air.
The paper concentrates on the study of catalytically active coatings on the basis of titanium dioxide for effective destruction of ozone inside air purification systems.
It is shown that use of catalytically active coatings of collecting electrodes on the basis of titanium dioxide and manganese oxide allows to decrease significantly (20-50%) the ozone concentration at the filter exit.
As the results of the researches the following requirements have been determined:
the requirements to the lateral electric resistance of catalytic coatings;
the requirements to the technological modes of catalytically active coatings formation;
the requirements to the structure and chemical composition of coatings providing high efficiency of purification and usability.

Titanium dioxide (TiO₂) films were grown on Silicon (Si) and fluorine-doped tin oxide coated glass (FTO-glass) substrates by atomic layer deposition (ALD). TiCl₄ and H₂O were employed as precursors, and substrate temperatures were arranged from 100 to 500^oC. TiO₂ films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The crystal growth behaviors of ALD-TiO₂ films were carefully investigated. Especially, an interesting phenomenon had been found that is a high-temperature crystal phase of rutile TiO₂ film can be grown at low temperature of 300 ^oC. Moreover, the TiO₂ film is a single rutile-phase, which is an unusual phenomenon because a mixture phase of anatase/rutile is usually produced in phase-transformation of anatase-to-rutile.
The TiO₂ films grown on n-type Si substrates by ALD showed that the growth temperature should be above 400 ^oC, thus a little rutile-phase can be obtained in TiO₂ films. Besides, the TiO₂ films grown at temperatures between 400 and 500 ^oC have a two-phase mixed crystalline structure, containing both anatase and little rutile phases. However, for ALD-TiO₂ films grown on FTO-glass substrates, the rutile phase early appeared at temperature of 300 ^oC, and the crystal phase of TiO₂ film has completely transferred from anatase to rutile phase via only 50 ^oC (from 250 to 300 ^oC) increase, moreover, no anatase/rutile mixture phase has been detected. This interesting phenomenon implicated that the TiO₂ films grown on FTO-glass substrates have different crystal-growth behaviors to the films grown on Si substrates by ALD. The interesting phenomenon is attributed to epitaxial growth of TiO₂ film grown on FTO.
According to the systematically analyzed results, we concluded that the ALD-TiO₂ films grown on Si substrates have three kinds of growth behaviors: amorphous deposition, space-limited crystal growth, and conventional crystal growth grown that depended on substrate temperatures varied from 100 to 500 ^oC. Moreover, there are amorphous deposition, space-limited crystal growth, and epitaxial crystal growth of ALD-TiO₂ films grown on FTO-glass substrates.
This work was supported by the Ministry of Science and Technology, Taiwan. (Contract No. MOST 102-2218-E-153-001, and MOST 103-2221-E-153-001)

Complex three-dimensional (3D) hierarchical nanostructures based on well-defined low-dimensional nanobranches of different sizes and specific exposed facets are highly desirable to obtain tunable physicochemical properties. Herein, a facile, one-step hydrothermal method is employed to construct self-branching anatase TiO₂ (SBAT) 3D hierarchical nanostructures. By simply controlling the reaction time and weight ratio of F127/TBAH, SBAT nanorods can be obtained with a large percentage of exposed {010} facets. Based on X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis, a growth mechanism is proposed for the formation of such self-branching 3D nanostructures, which involves the formation of the L-shaped step edges on the [103] surfaces and the alignment of the crystal facets (103) of anatase nanocrystals with the (103) face on the tips of the main anatase TiO₂ nanorods. The dye-sensitized solar cell assembled with the SBAT nanorods exhibits an outstanding power conversion efficiency of 7.17%, which is superior to that of the devices based on the 1D anatase TiO₂ nanorods and P25 TiO₂. This high performance can be attributed to the high dye-uptake density, large size and unique self-branching 3D hierarchical nanostructures built from 1D nanobranches growing epitaxially from the main rod.

In this study, a hierarchical anatase TiO₂ nanotube (HTNT) has been synthesized via an electrospinning method followed by a hydrothermal process. It is found that the dense nanoparticles were grown on the HTNT. The materials with different morphologies were characterized by the SEM, XRD, TEM and BET measurements. The results indicated that the surface morphologies and surface area were varied with different structures. It was found that the HTNT has pure anatase crystalline phase, high dye loading, high light scattering ability, fast electron transport, and great charge collection efficiency. For application as the photoanode material of DSSC, the device fabricated with HTNT showed 32.5% higher power conversion efficiency than that with P25 TiO₂ nanoparticles. Besides, the significant improvement in DSSC efficiency was confirmed by the investigation of its intensity modulated photocurrent spectroscopy (IMPS), intensity-modulated photovoltage spectroscopy (IMVS) and electrochemical impedance spectroscopy (EIS) measurements. In all, the HTNT prepared in this study exhibited large improvement in performance of DSSC and can be a potential photoanode material for DSSC.

Titanium dioxide (TiO₂) thin films have been widely applied in sensors, specifically the ones to be used in biomedical fields, due to their large applicability [1]. It presents a large response as a function of pH, so it has been very used as the sensitive part of chemical sensors. Moreover, the surface of a TiO₂ thin film can be functionalized, by enzyme immobilization for example, allowing its application in different kinds of sensors. The focus of this study was the development, characterization and usage of TiO₂ thin films as the sensitive layer of an extended gate field effect transistor device used as urea sensor. The TiO₂ was deposited by three different deposition techniques (casting, dip-coating, and spin-coating). Due to their capability of sensing the ionic variation on the surface, the films were firstly applied as a pH sensor in the pH range from 2 to 12. The films optimized for each deposition technique had their surface functionalized with urease enzymes. Then, they were tested in solutions with different urea concentrations. The reaction between urease and urea leads to a more alkaline solution, due to the formation of OH^- ions. The regular urea concentration in human blood is about 10 to 40 mg/dL [2]. Dysfunction is characterized for concentrations from 40 mg/dL up to about 180 mg/dL. So it is extremely important that a fast and sensitive device detects diseases in medical fields. Each deposition technique leads to thin films with different ion sensing range. The casting deposition techniques lead to films that presented a good response for the largest range of urea concentration. They can differentiate the changes in pH for an urea range from 5 to 200 mg/dL. For dip-coating fabricated films, the sensor works in a urea range concentration from 20 up to 100 mg/dL only. For spin-coated TiO₂ thin films, the sensor works only for the highest concentrations, in a range from 70 to 140 mg/dL. The comparison between the ions sensing behavior for films fabricated using each deposition technique will be discussed, as well as a comparison with literature data. This work was supported by CNPq, CAPES and FAPESP Brazilian agencies.
References:
[1] Yang, H-Y., et. al., Sensors and Actuators B, vol. 205, pp. 322-328 (2014).
[2] Hamilton, A., and Breslin, C. B., Electroch. Acta, vol .145, pp. 19-26 (2014).

Titanium oxide nanomaterials have been synthesized in various structures and morphologies. Among them, titanium oxide nanosheets have attracted significant attention due to their graphene-like 2D structure [1,2]. We have shown that layered titanates undergo osmotic swelling when reacted with amine solutions. A huge expansion of interlayer separation to ~100 times the original spacing can be induced under appropriate conditions via permeation of a large volume of the solution [3]. The layers can fall apart upon applying some mechanical shear because of the weakened interaction between them. As a consequence, colloidal 2D crystals corresponding to elementary host layers are derived. The obtained nanosheet with a general composition of Ti_1-dO₂^4d- is characterized by its very high 2D anisotropy; a thickness of ~1 nm versus a lateral size of over 10 µm. The nanosheets act as a wide-gap semiconductor, showing superior photochemical and dielectric properties. Doping with magnetic elements such as Co²⁺ and Fe³⁺ is possible to produce ferromagnetic nanosheets. Because the nanosheets are obtained as monodispersed polyanions, they can be organized into various nanostructures such as nanofilms, nanocomposites, hydrogels, and so forth through solution-based processes [1, 4, 5]. Design of various functional materials, e. g., photocatalytic coating, high-k nanofilms, is possible through this approach.
[1] R. Ma and T. Sasaki, Adv. Mater. 22, (2010) 5082.
[2] L. Z. Wang and T. Sasaki, Chem. Rev. Chem. Rev. 114 (2014) 9455.
[3] F. Geng, R. Ma, A. Nakamura, K. Akatsuka, Y. Ebina, Y. Yamauchi, N. Miyamoto, and T. Sasaki, Nat. Commun. 4 (2013) 1632.
[4] M. Osada and T. Sasaki, Adv. Mater. 24 (2012) 210.
[5] M. Liu, Y. Ishida, Y. Ebina, T. Sasaki, and K. Aida, Nat. Commun. 4 (2013) 3029.

The rational design and controlled synthesis of nanostructured materials with tailored morphologies and architectures has attracted considerable attention because the properties of nanomaterials are largely dependent on the shape and structure of the primary building blocks and the way how the building blocks are assembled. As one of the most important semiconductor materials, titanium oxide (TiO₂) has shown promising applications in a variety of energy-related fields ranging from solar energy harvesting to electrochemical energy storage. However, it remains a challenge to realize the controllable and feasible synthesis of TiO₂ nanomaterials with desirable architectures for specific applications. This presentation summarizes our recent work on the architectural control of TiO₂ nanomaterials with special attention paid to their applications in photocatalysis, antireflection, and lithium ion batteries (LIBs).
Firstly, uniform anatase TiO₂ nanocuboids enclosed by active {100} and {001} facets with controllable aspect ratios were solvothermally synthesized through hydrolysis of titanium tetraisopropoxide using acetic acid as the solvent and the ionic liquid [bmim][BF₄] as the capping agent. The produced anatase nanocuboids exhibited considerably enhanced photocatalytic activity owing to the wholly exposed active facets. Secondly, unique spindle-shaped nanoporous anatase TiO₂ mesocrystals with a single-crystal-like structure were successfully fabricated on a large scale through mesoscale assembly in the tetrabutyl titanate-acetic acid system without any additives under solvothermal conditions. It was revealed that as a coordinating solvent, acetic acid played multiple key roles during the nonhydrolytic processing. The obtained nanoporous anatase mesocrystals exhibited improved performance as anode materials for LIBs, which may be largely attributed to the intrinsic single-crystal-like nature as well as the high porosity. Thirdly, mesocrystalline rutile TiO₂ nanorod arrays grown on Ti substrates were synthesized by controlling the hydrolysis of tetrabutyl titanate using concentrated hydrochloric acid. The nanoarrays exhibited excellent broadband and quasi-omnidirectional antireflection properties owing to an optimized graded refractive index profile, and hence may find potential applications as antireflective coatings for enhanced light harvesting. Lastly, hierarchical structures consisting of TiO₂ nanorod arrays grown uniformly on the inner surfaces of TiO₂ nanobowl arrays were fabricated using nanosphere lithography at the gas-liquid interface followed by solution-phase growth. The resultant TiO₂ architectures showed improved catalytic activity for photoelectrochemical water splitting owing to effective charge separation and collection as well as enhanced light harvesting.

There is great interest in improving TiO₂&’s photocatalytic activity in the visible portion of electromagnetic spectrum. Recent work has shown that co-doping mesoporous TiO₂ microparticles with a transition metal and nitrogen, hereby designated as (M,N) TiO₂, significantly increases its visible light absorption.[1] However, the hydrothermal method used to produce the microparticles creates a wide distribution in the size of the microparticles, which could affect the absorption properties. Recently, it has become possible to produce monodisperse, mesoporous TiO₂ microparticles with engineered sizes using a hybrid sol-gel/hydrothermal technique.[2] Further, it has also been shown that the size of monodisperse TiO₂ microparticles affects the the photocatalytic activity.
This study investigated whether using mondodisperse (M,N) TiO₂ microparticles would further increase visible-light absorption for (M,N)TiO₂. The first-row transition metals chosen for this study - Cr, Mn, Fe, Co, Ni, and Cu - have been characterized in the earlier (M,N) TiO₂ UV-vis study, which was used as a baseline. The doping levels of the transition metals samples were set at the 2.5 percent level previously shown to be optimum for photocatalytic activity.
[1] J. E. Mathis, Z. Bi, C. A. Bridges, M. K. Kidder and M. P. Paranthaman, MRS Proceedings, 2013, dx.doi.org/10.1557/opl.2013.666 1547.
[2] D. Chen, L. Cao, F. Huang, P. Imperia, Y-B Cheng, and R. A. Caruso, J. Am. Chem. Soc., 2010, 132, 4438-4444.

Metal oxide heterojunctions are widely used in energy harvesting devices such as solar cells and photoelectrochemical cells [1]. An important requirement common to many device structures is the control of carrier transport across interfaces. Interface band alignment modulation by inserting electrostatic dipoles is an effective technique for such purpose but the formation of interface states has often limited the tunable range [2]. This partly owes to the materials used for the dipole layer, namely organic molecules or covalent semiconductors with short-range directional bonds readily forming localized interface states. By exploiting the strong ionic character of metal oxides, the band alignments are expected to be tunable in a wide range with minimal generation of interface states [3].
Here we demonstrate this approach by tuning the Schottky barrier height (SBH) in a model system, Pt/anatase TiO₂ Schottky junctions, using an ultrathin epitaxial layer of the perovskite LaAlO₃ as the dipole layer. LaAlO₃/TiO₂ (001) heterojunctions were grown on LaAlO₃ (001) substrates by pulsed laser deposition under conditions in which the oxygen vacancies in the TiO₂ are well controlled [4]. Polycrystalline Pt Schottky electrodes were deposited on top by e-beam evaporation and SBH measurements were conducted by capacitance-voltage (CV) and internal photoemission (IPE) techniques. Both CV and IPE measurements revealed a linearly decreasing built-in potential over 0.4 V by inserting 1 unit cell of LaAlO₃ at the Pt/TiO₂ interface. These results demonstrate that polar surfaces of metal oxides can be utilized to control band alignments even at interfaces between materials with different crystal structures, opening up possibilities to control band alignments at functional devices for energy applications.
[1] M. R. Hoffmann et al., Chem. Rev. 95, 69 (2001).
[2] C. Tengstedt et al., Appl. Phys. Lett. 88, 053502 (2006).
[3] S. Kurtin et al., Phys. Rev. Lett. 22, 1433 (1969).
[4] T. Tachikawa et al., Appl. Phys. Lett. 101, 022104 (2012).

TiO2 has been studied widely as a good photocatalyst for photocatalytic environmental pollution removal, solar hydrogen generation, photocatalytic water splitting, etc. However, it is not a good microwave absorbing material due to its poor absorbing performance in the microwave region due to the lack of efficient dipole rotations or magentic resonance. Here, we would like to show that hydrogenated TiO2 nanoparticles can show excellent microwave absorbing performance, without the assistance of the traditional dipole rotation or magentic resonance mechanism.

O 1s x-ray absorption spectroscopy and first principle calculation have been used to probe the size effect on the orbital character and hybridization of the conduction band of anatase TiO₂ nanocrystals over two orders of magnitude in average diameter (2-200 nm). The appearance and predominance of unoccupied delocalized states derived from the hybridization of antibonding O 2p and Ti 4s rather than 3d is observed when the nanoparticle size approaches the exciton radius. These results provide an experimental evidence of quantum size effect on unoccupied states in anatase TiO₂ nanocrystals. Concomitantly, in-situ characterization of the lateral displacement, complex impedance spectroscopy, and electrical resistivity measurements of TiO₂ quantum dots as a function of temperature within a controlled environment and as a function of oxygen partial pressure have been carried out and will be presented.

Reactive gases (e.g., O₂ ) and moisture cause food to perish, create electrical instability in microelectronics, and reduce the life span of electrodes. Thin films designed to prevent gas or water molecules from permeating through a polymer film remains a major challenge for food, medicine and electronics. Up to recently, metal oxide films deposited on polymer have been used to enhance the gas barrier ability of its polymer film. Previous reports about preparation of metal oxide film (Al₂O₃, TiO₂, ZrO₂ et al.) are focused on vacuum processing such as atomic layer deposition (ALD), chemical vapor deposition (CVD). However, in order to large area, cheap and short-time processing, study of solution processed barrier film is essential. One strategy of fabrication in solution processing barrier film is sol-gel method which has been used to prepare a variety of metal oxide thin films owing to its versatility.
In the present work, single layers metal oxide film are deposited on polymeric substrate by using sol-gel method to investigate the gas barrier property of these thin film. 86 nm-thick TiO₂ film coated on polyethylene naphthalate substrate (PEN) exhibits 0.0387 g#8729;m^-2#8729;day^-1 water vapor transmittance rate (WVTR) in condition of 25#8451;, 90% relative humidity.

Threats of chemical and biological warfare agents (CBWA) represent a serious global concern and require rapid and efficient neutralization methods. We present a highly effective micromotor strategy for photocatalytic degradation of CBWA based on light-activated TiO2/Au/Mg microspheres that propel autonomously in natural water and obviate the need for external fuel, decontaminating reagent, or mechanical agitation. The activated TiO2/Au/Mg micromotors generate highly reactive oxygen species responsible for the efficient destruction of the cell membranes of the anthrax simulant Bacillus globigii spore, as well as rapid and complete in situ mineralization of the highly persistent organophosphate nerve agents into nonharmful products. The water-driven propulsion of the TiO2/Au/Mg micromotors facilitates efficient fluid transport and dispersion of the photogenerated reactive oxidative species and their interaction with the CBWA. Coupling of the photocatalytic surface of the micromotors and their autonomous water-driven propulsion thus leads to a reagent-free operation which holds a considerable promise for diverse “green” defense and environmental applications.
[Ref]: Jinxing Li, Virendra V. Singh, Sirilak Sattayasamitsathit, Jahir Orozco, Kevin Kaufmann, Renfeng Dong, Wei Gao, Beatriz Jurado-Sanchez, Yuri Fedorak, Joseph Wang*, "Water-Driven Micromotors for Rapid PhotocatalyticDegradation of Biological and Chemical Warfare Agents", ACS Nano, 2014, DOI: 10.1021/nn505029k.

While the crystal TiO₂, including rutile, anatase, and brookite, have been extensively studied for their electronic structures, defect levels and polaron formations, the studies for the amorphous polymorph are relatively rare. However, it is the amorphousness that sometimes plays crucial roles in particular applications. In actual technical applications, amorphous TiO₂ (a-TiO₂) has been widely used, either as an active photocatalyst, substrate, or protection layer. For instance, the amorphous titanium dioxide has drawn attention recently due to the finding on its promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The electronic structures, optical properties, carrier dynamics and defect properties of such a system are obviously functions of the structural characterization. The presence of defects can play critical role in the materials&’ application, while the formation energies of such defects, and their electronic structures can be very different between bulk and amorphous structures. It is thus necessary to understand the role of amorphousness in TiO₂, in addition to its crystal counterparts. In this work, we use the molecular dynamics method (MD) to construct the atomic structure of a model amorphous TiO_2. The electronic structure of the model system is then studied by means of density functional theory (DFT). The doping of different impurity elements, including C, N and F is investigated to examine the mid-gap states and the mechanism of the defect-state conduction in a-TiO₂. The formation energy of defects was found to reduce significantly upon the amorphization, therefore the charge transfer via these defect channel could be of great interest for applications of this material in solar energy conversion.

Nanostructured anatase TiO₂ has is a promising material for gas sensing and photocatalysis. In order to modify its catalytic properties, the lanthanide (Ln) ions Eu³⁺, Gd³⁺, Nd³⁺ and Yb³⁺ were precipitated on the surface of TiO₂ nanoparticles (NPs) by hydrothermal treatment. Results from Raman spectroscopy and x-ray diffraction (XRD) measurements showed that the anatase structure of the TiO₂ nanoparticles was preserved after hydrothermal treatment. A volume weighted mean size of ~ 13 nm was estimated from the Rietveld refinement of the XRD data. Scanning electron microscopy showed a heterogeneous distribution in size and a nanocrystallite morphology of the TiO₂ NPs and energy-dispersive x-ray spectroscopy confirmed the presence of the Ln-ion surface doping after hydrothermal treatment. An increase in photoluminescence (PL) was observed for the Ln-surface-doped TiO₂ NPs when measurements were made in forming gas (5% H₂ + 95% Ar) at 520 °C. In contrast, the measurements at room temperature did not show any noticeable difference in forming gas and air environment. We infer from these results that the modification of oxygen-vacancies and hole-defects by hydrothermal treatment are responsible for the noticeable features observed in the PL spectra.
Keywords: titanium dioxide, gas sensing, photocatalysts, hydrothermal process, nanomaterials, photoluminescence, energy.

Hierarchically ordered macro-mesoporous titania films have been produced through a confinement self-assembly method within the regular voids of a colloidal crystal with three-dimensional periodicity. Furthermore, graphene as an excellent electron-accepting and electron-transporting material has been incorporated into the hierarchically ordered macro-mesoporous titania frameworks by in situ reduction of graphene oxide added in the self-assembly system. Incorporation of interconnected macropores in mesoporous films improves the mass transport through the film, reduces the length of the mesopore channel, and increases the accessible surface area of the thin film, whereas the introduction of graphene effectively suppresses the charge recombination. Therefore, the significant enhancement of photocatalytic activity for degrading the methyl blue has been achieved. Similar to graphene, graphdiyne is also a two dimensional planar structure material which has several properties to improve the photocatalytic performance of TiO₂, including a large surface area and the high electron mobility. In the present work, we also prepared nanocomposites of P25-graphdiyne with different addition ratios of graphdiyne to titania and used these composite products as photocatalyst in the degradation of methylene blue. In all photodegradation reactions, P25-0.6 wt % GD showed significantly higher activity compared to the bare P25, P25-CNTs and P25-GR, particularly when compared under visible light conditions. As a result, we can expect that graphdiyne will become a superb competitor among the different types of 2D carbon materials in the applications of photocatalysis and photovoltaics.

The typical photoanode in dye- and quantum dot- sensitized solar cells is 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, by favoring charge transport and collection, while reducing charge recombination.
We systematically investigated several TiO₂-based photoanodes for dye- and quantum dot-sensitized solar cells: (i) TiO₂ nanoparticles / ZnO nanowires [1]; (ii) Multiwall carbon nanotubes (MWCNTs) / TiO₂ nanoparticles [2]; (iii) Graphene / TiO₂ nanoparticles [3]; (iv) TiO₂ nanotubes [4].
Both dye molecules and semiconducting quantum dots were applied as light harvesters. Possible tailoring of the structure and morphology of the photoanodes, and their implication in improving the functional properties of these kinds of excitonic solar cells will be discussed in detail.
As an instance, the investigation of the 2D graphene / TiO₂ system showed that optimum concentration is far below the percolation threshold, indicating that the increased photoconversion efficiency does not rely on the formation of an interconnected network, as inferred by prior investigations, but rather, to increased charge injection from TiO₂ to the front electrode.
References:
[1] A. Vomiero, et al., Appl. Phys. Lett. 95 (2009) 193104.
[2] K.T. Dembele et al., J. Phys. Chem, C 117 (2013) 14510.
[3] K.T. Dembele et al., J. Mater. Chem. A, in press
[4] A. Vomiero, et al., Energy Environ. Sci. 4 (2011) 3408.

Thin films of Titanium Dioxide (TiO₂) have been utilized for a wide range of applications such as photocatalysts, chemical sensors, transparent electrodes, anti-reflecting and self-cleaning coatings, and solar cells. Obtaining crystalline films of TiO₂ has, however, proved to be more challenging, and often heating to a temperature of 500-600°C is required to produce a crystalline phase, usually anatase, which converts into the rutile phase at higher temperatures>1000°C. The few methods that have achieved crystallinity at low temperatures have done so by high pressure heating or UV illumination and have mostly been successful in producing thick films (>200nm). In many applications, such as gas sensors, where a thin film coating has to be applied to a thermally unstable substrate such as a polymer, however, the production of a crystalline thin film at lower temperatures is desirable. Again for certain chemical sensors where the sensing property or enhancement of signal is a distance dependent effect, such as those using Surface Plasmon Resonance or Surface Enhanced Raman spectroscopy (SERS), thickness of thin films in the range of hundreds of nanometers is unacceptable. Here, we report the low temperature preparation of ultra-thin films (5-10nm) crystalline TiO₂ in the anatase form on gold substrates using layer-by-layer epitaxial growth based on self-assembly from solution phase. Amorphous titania gel films have been produced by a Surface Sol Gel Process using a Titanium (IV) Butoxide precursor, originally developed by Kunitake and coworkers^1-2. In contrast to conventional sol-gel processes, this method is capable of producing ultra-thin films with molecular scale control that conform to underlying nano-structured surfaces. We demonstrate that the atomic regularity in the amorphous films so formed can be utilized to form well-ordered crystalline films of anatase TiO₂ by annealing the gel film in air at temperatures as low as 350°C. The crystallinity and the effects of annealing temperature on film structure have been well-characterized using Glancing Incidence X-Ray Diffraction and Raman Spectroscopy. The surface chemical changes on annealing have been studied by X-Ray Photoelectron Spectroscopy. Finally, the ability of the surface sol-gel process to conform to underlying nanostructures has been utilized along with the benefits of low-temperature processing to form ultra-thin nanocrystalline anatase films on fabricated arrays of gold nanostructures on fused silica. The capacity of the film synthesis procedure to form unique chemical sensors and SERS substrates for monitoring of molecular adsorption from gas and liquid phase and heterogeneous reaction dynamics on crystalline anatase surfaces using optical techniques, such as Localized Surface Plasmon Resonance sensing and Raman Spectroscopy is demonstrated.
References: 1) L. Bao et. al. Analytical Chemistry 76 (2004) 4531
2)I. Ichinose, H. Senzu, T. Kunitake, Chemistry of Materials 9 (1997) 1296

Our ability to dope TiO₂ has motivated extensive studies for its applications ranging from photocatalysis, solar cells, and sensors. So far two major drawbacks holding back its wide-ranging application are: large intrinsic bandgap (3.0 to 3.2 eV) and low quantum photoefficiency due to the high recombination rate of photogenerated electronminus;hole pairs. Low quantum photoefficiency is mainly attributed to oxygen vacancy. In this work we show that, both these two issues can be addressed by doping with cations.
An extensive assessment of the physicochemical factors that control the behavior of dopant-related electronic structure of Anatase TiO₂ has been performed using high-throughput first-principles computations. We selectively dope cations that show strong +2, +3 and +4 oxidation states in stoichiometric and sub-stoichiometric TiO₂. We find that +2 and +3 are easier to dope in sub-stoichiometric TiO₂ compared to stoichiometric. For one oxygen vacancy in bulk TiO₂, substitutional doping of one +2 cation and two +3 cations removes defect energy levels due to oxygen vacancy. Further, smaller radius cations like B and Al tend to increase bandgap while larger radius cations like La tends to decrease bandgap significantly without creating new defect states. Our results are in agreement with available experimental data (with no violations thus far), and points to an attractive computational route to dopant selection in TiO₂ to tune their bandgap as well as in other materials.

TiO₂ nanomaterials offer unique solid-state platforms on which diverse optoelectronic, thermal, electrochemical and catalytic properties coexist, holding considerable potential for low-cost and renewable energy technologies. Colloidal routes stand out among the most powerful synthetic approaches to create nanocrystals with programmable crystal structure, geometry and composition through judicious control of thermodynamic conditions and kinetically controlled growth processes in liquid media. Further advances in these techniques have recently enabled access to increasingly sophisticate hybrid nanocrystal architectures that combine epitaxially interconnected sections of different materials into free-standing, easily processable multifunctional nanoheterostructures.
In this lecture, we will illustrate progress made by our research group in the wet-chemical development of complex TiO₂-based nanostructures, including TiO₂ nanocrystals with size-tunable linear and branched shapes in the anatase, brookite, and bronze phases and multicomponent nanocrystal heterostructures based on epitaxially joint sections of different TiO₂ polymorphs and/or other metal or metal-oxide materials. The relevant formation mechanisms, chemical-physical properties and some applications will be illustrated. Criteria for the rational engineering of future breeds of TiO₂-based nanomaterials with refined structural complexity and increased functionality will be suggested.

The critical challenge for future chemical sensing methodologies is to complement high sensitivity with identification. Optical techniques, such as sensing adsorbed molecules via optical micro-cavity resonance shifts, can provide this high sensitivity due to their strong field-localization at the surface. However, most optical transduction methods simply report the buildup of material on the surface and in solution, providing sensitivity without specificity. Although we can infer chemical information using a targeted binding approach, identification of unknown chemistries remains elusive. We propose an alternative approach that monitors Raman scattering of nearby molecules using the evanescent field at the surface of a waveguide. In this presentation, we report our progress toward this goal. In the first part, we present our fabrication work toward developing an optical microchip from titanium dioxide (TiO₂). We choose TiO₂ for its high refractive index (n > 2.5) that strongly localizes evanescent fields close to the waveguide&’s surface. Meanwhile, TiO₂&’s visible-wavelength compatibility increases Raman scattering and its indirect bandgap limits parasitic fluorescence. In the second part, we will discuss our theoretical and experimental Raman scattering results, compare geometries, and identify the most sensitive configuration for the Raman detection. Lastly, we will discuss the potential impact of this approach.

This work addresses the critical variables limiting the performance of TiO₂ for solar-driven heterogeneous photocatalysis, namely: (1) inefficient absorption of visible light and (2) short lifetimes of photogenerated electron-hole pairs. We have previously demonstrated that nanostructured titania aerogels with incorporated ~5-nm gold nanoparticles feature (a) Au surface plasmon resonance (SPR)-sensitized photocatalytic activity that spans a broad portion of the visible spectrum; and (b) enhanced electron-hole lifetimes courtesy of the 3D interconnected oxide network. In this study, we explicitly examine the strong effects of the interfacial arrangement between the metal nanoparticles and mesoporous oxide network on both sensitization efficiency and photocatalytic activity. Additionally, we determine the effects of structural modifications made to the titania aerogel networks on electron transport and trapping within the nanoscale TiO₂ network, and how these carrier dynamics influence different classes of heterogeneous photochemical reactions, depending on whether they are driven by electron or hole transfer to the reactants or intermediates.

Solar reflective coatings on title roofs, walls or vehicles as a type of cool materials have received increasing attention with the aim of cutting down air-conditioning energy. Rutile TiO₂ has been identified as an effective pigment because it possesses a high near infrared (NIR) reflectance. In contrast to irregular solid TiO₂ submicron particles, this work explores Zn-doped TiO₂ hollow spheres (~1µm) as an innovative NIR-reflective pigment because both hollow contour and doped titania lattice, in principle, benefit reflecting and scattering NIR rays. The synthesis uses a one-pot facile solvothermal route in which the TiO₂ hollow structure is realized by utilizing glycine as coordination template. At the same time, a Zn²⁺- complex is included in the solvothermal synthesis system to implement the Zn-doping to TiO₂ lattice. Subsequently, in the first post-synthesis calcination at 500 ^oC, porous hollow spheres with Zn-doped anatase bead wall are obtained. Indeed, they exhibit clearly improved solar-reflectance profiles compared with undoped-TiO₂ hollow spheres and Degusa-P25^Ograve; (anatase TiO₂), respectively, when employed as pigment in a polymer coating. More appealingly, the ligand used to form chelating Zn²⁺ complex significantly impacts solar reflectance of the pigment obtained, which is attributed to various dispersions of Zn in anatase lattice according to XRD and SEM characterizations. The Zn-doped anatase TiO₂ hollow spheres are then converted to their rutile counterparts through calcination at 1000 ^oC. As expected, despite largely increases in the solar-reflectance profiles, the impact of doping on solar reflectivity as prior observed remains basically unchanged. This investigation unveils the contributions of hollow chamber and doped TiO₂ lattice to the efficacy of rejecting solar lights.

We demonstrate the fabrication of high-quality one-dimensional inorganic nanofibers of multiple morphologies consisting of various semiconducting metal oxides (SMO) in a simple, cost-effective, and scalable technique. A novel Gas-Jet Fibers spinning (GJF) technique is used to make precursor fibers of single and bi-component systems of titanium dioxide (TiO₂) and tin doped indium oxide (ITO). Creation of specific arrangements of inorganic components in the same nanofibers so as to obtain morphologies (such as core-shell ITO-TiO₂ or side-by-side ITO-TiO₂) that exhibit interesting photocatalytic properties is shown in this work. The mesoporous organization of the nanocrystals, the heterojunction surface area between two SMOs (titanium dioxide and tin doped indium oxide) and crystal structure of titanium dioxide are unique attributes of the materials used in this study.
The overall dimensions of the inorganic fibers such as single component TiO₂ and bi-component TiO₂-ITO are noted to be in the range of 50-200 and 80-300 nm, respectively, depending upon precursor concentration and GJF spinning conditions. Photocatalytic oxidation of ethanol on TiO₂ based single or bi-component mesoporous nanofibrous surfaces having core-shell and side-by-side morphologies is carried out to understand the degradation reaction pathway and to compare the rate of degradation with the help of in situ infrared (IR) spectroscopy. In particular, the photocatalytic activity of core-shell ITO-TiO₂ and mesoporous TiO₂ nanofibers is found to be higher than that of commercial TiO₂ (Degussa or P25) photocatalyst. This observation could be attributed to the formation of a heterojunction between ITO and TiO₂, as well as the inherent advantages of a one-dimensional fibrous structure.

Layered titanates such as H_0.8Ti_1.2Fe_0.8O₄ exhibit reversible swelling up to 100-fold in aqueous solution in the presence of amine bases with potential applications in nanocomposites, photonic crystals, and sensors. The origin of the amine-induced swelling has not been well understood and chemically specific differences in swelling behavior by the introduction of alternative stoichiometry of the nanosheet as well as major differences in swelling through the use of various amines have been experimentally observed. This contribution introduces atomistic models and molecular dynamics simulations up to the 100 nm length scale that reproduce known properties of such titanates such as the x-ray structure, ionic interlayer profiles, and surface energies. The simulations provide first insight into the distribution of the protonated amines in the swollen aqueous interlayer as a function of the extent of ion exchange and water content for the examples of protonated dimethylaminoethanol (DMAEH+), tetramethylammonium ions (TMA), and tetrabutylammonium ions (TBA) as swelling agents. Energetic preferences upon reversible exfoliation and their relation to the expected interlayer expansion will be described. The molecular models also allowed testing of various hypotheses of the real entity of the nanosheets in combination with specific experimental observations (X-Ray, TEM), illustrated for proton-defective titanate nanosheets such as H_1.07Ti_1.73O₄.

The current dependency on non-renewable and polluting fossil fuels is responsible for many social, environmental, and health problems such as air pollution, greenhouse effects and climate changes. It is therefore essential that alternative means of generating energy and fuels be developed. Of all alternatives, conversion of solar energy to chemical fuels or electricity is one of the most attractive routes because of the unlimited supply and easy accessibility of sunlight. In this talk, I will focus on our efforts in the synthesis and characterization of visible-light responsive photocatalytic materials. Several families of materials with visible light response have been synthesized and their photocatalytic properties have been characterized. We have not only made progress on the development of new photocatalytic materials with novel chemical compositions but also succeeded in improving the band structures and morphologies of well-known photocatalytic materials for water reduction or oxidation.

Simple glycols are employed as models in the mechanistic studies of catalytic deoxygenation reactions on a prototypical reducible oxide surface, rutile TiO₂(110). Glycols contain functional groups present on more complex oxygenates such as sugars and cellulose and as such are used to identify the role individual moieties play in the deoxygenation of biomass. At low coverages scanning tunneling microscopy is employed to follow a complete sequence of elemental steps in the reaction of ethylene glycol (EG) and 1,3-propylene (PG) glycol on TiO₂(110). The use of EG and 1,3-PG allows us to compare and contrast the chemistries of two functionally similar molecules with different steric constraints and attain information about how molecular geometry may influence the observed chemical reactivity. In our temperature-dependent studies we observe reversible O-H dissociation/reformation dynamics of titanium-row-bound glycol molecules at temperatures as low as 125 K. We follow glycol diffusion to and dissociation in bridging oxygen vacancy sites and find that they dissociate via both C-O and O-H bond cleavage above ~230 K. Our high temperature studies further reveal the formation of a new intermediate above ~400 K. The experimental results are further corroborated by density functional theory calculations. Temperature programmed desorption studies demonstrate that alkenes and aldehydes are the major carbon-containing products formed from EG and 1,3-PG. While alkenes are observed from the lowest coverages, aldehydes form primarily at high coverages. The increased separation of two OH groups in 1,3-PG leads to stronger binding on Ti rows and results in additional reaction products, ethylene and formaldehyde, formed via C-C bond cleavage. The carbon containing products are accompanied by water and hydrogen formation. Hydrogen production from glycols is in sharp contrast to alcohols, from which only water is observed. Experiments with OD-labelled glycols show that hydrogen originates exclusively from the glycol hydroxyl groups. Increasing the steric hindrance around the OD groups by methyl side groups is found to inhibit and eventually eliminate hydrogen formation. Disordering the surface and/or scavenging the surface charge are shown to completely suppress hydrogen formation. These results suggest that hydrogen formation results from proximal glycol pairs, and that this redox reaction is driven by defect electrons of TiO₂(110). Combined, our results provide an unprecedented level of quantitative, molecular-level detail on this hitherto poorly understood chemical reaction.

Due to their unique chemical and physical properties titanium dioxide nanoparticles (TiO₂) have received a great deal of attention in wide range of fields like photocatalysis, dye-sensitized solar cells and gas-sensing. The unique properties are originating not only from the crystal phase and particle size but also from the particles morphology. Moreover changing shape is more flexible and provides more variable electronic states in comparison to simple changes in size.
In this contribution we are presenting a partial transition-metal-cationic substitution as a flexible approach to tune the TiO₂ nanoparticles morphology during non-hydrolytic sol-gel synthesis. In addition to influencing the reaction kinetics in TiO₂ nanoparticles synthesis giving rise to various nanoparticles shapes the incorporation of these transition metals (Sb³⁺, Cr³⁺, Nb⁵⁺ and combinations Nb⁵⁺/Cr³⁺, Nb⁵⁺/Sr³⁺,) into the TiO₂ core structure and/or its surface structure modification extends light response into visible range and also influences its photocatalytic activity.
The results will be presented and discussed based on data obtained from STEM, XPS and UV-Vis characterisation. In addition, the experimental data will be compared with theoretical calculations, with special emphasis on the effect of chemical substitutions on the TiO₂ surface electronic properties.

Discussions of the effect of particle size often focus on changes in the absorption on-set and fluorescence wavelength but other electronic and related properties of nanoparticles are also likely to change upon reduction of the particle size. Indeed, there is, for instance, experimental evidence for an effect of particle size and shape on the photocatalytic activity of TiO₂ nanoparticles [1,2]. The emphasis on optical properties is probably simply the result of the fact that they are relatively easy to measure compared with other properties, especially in the presence of a solvent. Theory does not have this constraint and also has the added advantage that it can complement experiment by providing an unrivalled microscopic picture of the processes underlying the measured/calculated properties.
In our contribution we will present result from our recent DFT/TD-DFT calculations on the photocatalytic properties of 1-2 nm hydroxylated rutile TiO₂ nanoparticles [3]. These calculations include a description of the effect of adsorbed water as well as bulk water surrounding the particles and build-on extensive benchmarking work by to validate our computational approach [4-5]. We will focus on discussing the predicted chemical potentials of free electrons in the conduction band, free holes in the valence band and excitons (electron-hole pairs) in these particles, and compare these potentials to those associated with the proton reduction and water oxidation reactions. We will also touch upon the predicted exciton lifetimes and the localisation of the different electronic species in the particle. Based on these results, we will show that small particles, which are essentially all surface, are very flexible and that hence trapping of excitons and charge carriers is energetically strongly favoured and leads to large changes in the particle photocatalytic properties.
[1] Cho, C. H.; Han, M. H.; Kim, D. H.; Kim, D. K. Mater. Chem. Phys.2005, 92, 104.
[2] Cernuto, G.; Masciocchi, N.; Cervellino, A.; Colonna, G. M.; Guagliardi, A. Journal of the American Chemical Society2011, 133, 3114.
[3] Berardo, E.; Zwijnenburg M.A., manuscript in preparation.
[4] Berardo E., Hu H.S., Shevlin S., Woodley S.M., Kowalski K., Zwijnenburg, M.A. J. Chem. Theory Comput. 2014, 10, 1189.
[5] Berardo E., Hu H.S., Shevlin S., Woodley S.M., Kowalski K., Zwijnenburg, M.A. submitted to J. Chem. Theory Comput. 2014

TiO₂ is a prototypical metal oxide used in photocatalysis [1], photoelectrochemical (Grätzel) solar cells [2], and transparent conducting oxides [3]. Industrially two forms of TiO₂ are used, rutile and anatase. The behavior of charge carriers is of key importance in virtually all applications of these materials. When excess electrons are added to the conduction band of an oxide, the electron-phonon interaction may result in electron trapping - the formation of either localized (small) or delocalized (large) polarons [4].
We used a combination of STM, STS and DFT+U to investigate the nature of electron polarons in rutile and anatase [5,6]. The excess electrons in rutile can localize at any lattice Ti atom, forming a small polaron. The polarons in rutile can easily hop to neighboring sites. Electrons in a perfect anatase lattice prefer delocalized (band-like) solutions, while electron localization is only possible at defects. Delocalized electrons were observed in Nb-doped anatase in vicinity of subsurface Nb dopants. The consequences of different electron behavior in these materials are illustrated on several examples - electrical conductivity, adsorption of simple molecules, and band alignment of rutile/anatase.
The work was supported by ERC Advanced Research Grant ‘Oxide Surfaces&’
[1] M. A. Henderson, Surf. Sci. Rep. 66, 185 (2011)
[2] M. Grtzel, Nature 2001, 414, 338 - 344
[3] S. X. Zhang, et al., J. Appl. Physics 102, 013701 (2007)
[4] I. G. Austin and N. F. Mott, Adv. Phys. 50, 757 (2001)
[5] M. Setvin et al., PRL 113, 086402 (2014)
[6] M. Setvin et al., Angew. Chem. Int. Ed. 53, 4714 (2014)

Quantum dot (QD) sensitization of metal oxides such as TiO₂ is a powerful method to expand light absorption into the visible region for improving the efficiency of photoelectrochemical (PEC) hydrogen generation. The key parameter influencing the PEC performance of such systems is the interfacial electron transfer (ET) process between the QDs and TiO₂, which depends on the distance as well as the energy barrier between them, as determined by the linker molecules between them. To understand key factors affecting the ET in QD-sensitized TiO₂ for PEC hydrogen generation, arene substituted (ortho, meta and para) bifunctional molecular linkers of the HS-[C₆H₄]-COOH type are designed and employed to link CdSe QDs to TiO₂. Interestingly, the CdSe-TiO₂ sample with a para substituted linker (representing the longest distance between CdSe and TiO₂) shows the best PEC performance as compared to ortho and meta substituted linkers. Ultrafast transient absorption spectroscopy of CdSe-linker-TiO₂ films enables determination of the interfacial photoinduced ET rate constants (k_et). The k_et results are consistent with PEC measurements in that the CdSe-TiO₂ sample with a para substituted linker shows the highest k_et. In order to further understand the ET mechanism, we performed density functional theory (DFT) calculations. The results of the lowest unoccupied molecular orbitals (LUMOs) reveal that the para substituted linker should have the lowest energy barrier and thus most effective for ET, consistent with experimental results. This study provides important new insight into the mechanism of interfacial ET that is important in the design of QD-sensitized metal oxide nanostructures for solar energy conversion applications.

High-performance Li-ion batteries with ultrafast charging and discharging rates is in great needs for portable electronic devices and electrical vehicles applications. A rationally designed 3-D nanostructured electrodes is essential for this goal by selecting materials with good intrinsic ion and electron transport kinetics, providing short diffusion length and large surface area to facilitate the ion intercalation, offering an ordered porous geometry to smooth ion migration and provide space for electrode material expansion, and presenting an interconnected network for structural stability and minimized electron transport resistance. TiO₂(B), with its layered structure and low volume expansion, has the intrinsic properties as electrode material for high-performance Li-ion battery. With perpendicularly oriented graphene (POG) grown encircling the struts of nickel foam as template, here we report layered TiO₂(B) nanosheets are assembled along POG sheets to construct a hierarchical 3D nanostructured electrodes. Electrochemical measurements reveal that such a nanostructured electrode possess extremely stable high-rate capability. It exhibits a large capacity of 320 mAh/g at 0.3C, 250 mAh/g at 2C, and 100mAh/g at the extremely fast 103C. After 2000 charge/discharge cycling at a current density of 2A/g (12C), it maintains 89% of its initial capacity, and when coming back to a lower current density of 0.5 A/g (2C) after the cycling stress, it exhibits negligible capacity lose. These data indicate that such a structure can be developed as a very stable and high-rate anode for high-performance Li-ion batteries. The detail synthesis, material structure and properties, and electrochemical study will be reported.

We recently demonstrated several approaches for increasing the photocatalytic performance of anatase-TiO₂ nanocrystals in sunlight for water treatment. These methods are based on decorating a graphene substrate with the TiO₂ nanocrystals, and then pursuing different strategies to enhance electron hole separation and photoexcitation. In this talk we report on first-principles calculations that elucidate the mechanisms of enhanced photoactivity. We also use a modified Wulff construction to examine the role of graphene as a morphactant for anatase.

Metal oxide heterojunctions have been proposed as an interesting alternative approach to modify the gas-sensing performances of single oxides, by properly coupling the features of each component. It is anyway necessary to define what is the “coupling”, and hence which features should be coupled. A useful hint comes from heterogeneous catalysis, where in systems like TiO₂-V₂O₅ and TiO₂-WO₃ the impregnation or grafting of TiO₂ with the other oxide gives rise to well-known and efficient catalysts for several oxidation reactions, making such systems potential candidates for reducing gas-sensing applications. The active sites and the reason for the catalytic efficiency in such composite oxides are still matter of debate, and obviously depend on the preparation procedure. In any case, the surface modification of anatase TiO₂ by the other metal oxide species seems the relevant feature. This concept of coupling seemed interesting to be transferred to gas-sensing field. It is a surface-chemistry related application and it was hypothesized that the presence of additional active surface sites could work as an efficient electronic antenna toward the otherwise poorly sensing TiO₂. What initially was an hypothesis found a confirmation in the remarkably improved sensing properties of TiO₂-V₂O₅ with respect to pure TiO₂. On this basis, we further explored the synthesis of TiO₂-WO₃ composites. They are of particular interest in photoelectrochemical devices but there are only very few studies in gas-sensing field. Moreover, the different reactivity of the W precursor with respect to V was expected to provide a broader range of materials architectures. TiO₂ anatase nanocrystals were prepared by coupling sol-gel and solvothermal synthesis. Before the crystallization step in oleic acid at 250 °C, the W chloroalkoxide precursor was added. Two concentrations were explored, ranging from 0.16 to 0.64 nominal atomic W : Ti ratio. After crossing several characterization techniques, it was concluded that for 0.16 [W] the surface of TiO₂ heat-treated at 500 °C was covered by W oxide species forming a dense layer. For 0.64 [W], an heterojunction was formed, where surface modified TiO₂ nanocrystals were mixed with WO₃ nanocrystals. The mean size of the anatase nanocrystals ranged around 8-10 nm for the investigated systems. Hence, different sensing properties could be expected with respect to the pure TiO₂ host system, depending on the W concentration. As a sample gaseous analyte, acetone was chosen, for testing the oxidation properties of the prepared materials towards organic species. Even addition of the lowest W concentration boosted the sensor response to values comparable to those of pure WO₃, ranging over 2-3 orders of magnitude of conductance variation. The surface modification by W oxides will be compared with the case of heterojunctions (high [W]) showing that more complex phenomena beyond doping should be invoked, like cross-talk between the different nanograins.