Symposium EE14 : Titanium Oxides---From Fundamental Understanding to Applications

Nanostructured TiO2 has been recognized as key elements in diverse functional applications and devices including energy conversion and storate, photocatalysis, biomedical theranostics, etc. In this talk, I overview our recent efforts and accomplishments to exploit hybrid TiO2 nanomaterials for electrodes in energy conversion devices, solar fuel generation via water splitting, and environmental remediation. Thin films of carbon-TiO2 were prepared by direct calcination of UV-stabilized diblock copolymer inverse micell templates and integrated into the interface between the FTO and photoanode as well as between the photoanode and electrolytes in dye-sensitized solar cells. We interprete the markedly enhanced cell performance in terms of the decrease in barrier resistance. Doped-TiO2 via carbonization of polymer-TiO2 precursors was shown to exhibit visible light activity in photocatalytic water splitting and pollutant degration. To this end, we developed inverse opal nanostuctures consisting of metal oxides or multimetallic oxides decorated with carbon moieties. Enhanced hotocatalysis mediated by Fenton-photon reaction and hydrogen peroxides will be introduced along with electron paramagnetic resonance analysis. Importantly, we will propose a novel analytical tool to investigate the photocatalytic environmental remediation process based on in-situ surface plasmon resonance spectroscopy.

The common picture of excess electrons - originating from photoexcitation/ photo-injection, intrinsic defects or dopants - in TiO₂ is based on the polaron model, where the electron becomes localized by inducing a lattice distortion that stabilizes the localized state. The location and energies of the trapping sites are of fundamental importance in essentially all applications of this material, but are still rather controversial. We present first principles molecular dynamics simulations of the localization and dynamics of excess electrons on bare TiO₂ anatase surfaces and anatase-water interfaces. Our results provide insights into the effects of different surface terminations, hydroxyls, impurities, and the interaction between localized carriers and water at the interface.

The conduction band (CB) engineering of TiO₂ is of fundamental implication for photocatalysis, solar cells and solar fuel generation. The position of CB edge controls the driving force for photocatalytic hydrogen evolution from water, potential of dye-sensitized solar cell (DSC) and recombination blocking in perovskite solar cells. The staggered alignment in mixed anatase/rutile phases is assumed to enhance photocatalytic activity, but it is unclear whether the CB of rutile or that of anatase is higher. XPS and most DFT simulations support the former, but the flat-band potential measurements provide just opposite results. The controversy can be explained by taking into account the adsorption of OH^– and H⁺ from the electrolyte solution on the electrode surface. XPS indicates that the CB edge of (001)-anatase is upshifted by 0.1 eV referenced to (101)-anatase in agreement with the DFT calculation and with the electrochemical flatband potentials (upshift of CB by 60 meV).

About one billion people worldwide do not have access to safe drinking water. The World Health Organization (WHO) has estimated that 80 percent of illnesses in the developing world are the results from waterborne diseases. Effective and low-cost water disinfection methods are urgently needed. Chlorination, ozonation, and UV irradiation are common methods for water disinfection. While these methods can control the proliferation of waterborne infectious agents, they are not perfect technologies. Chlorination and ozonation often generate toxic disinfection byproducts, and UV has limited penetration power. TiO₂-based photocatalysis represents a promising alternative to the traditional techniques for water disinfection. It works by generating powerful but short-lived hydroxyl radicals upon irradiation. These radicals can inhibit the growth of bacteria. In addition to the advances in basic research, practical photocatalytic disinfection products will also be introduced in this presentation.

References
W.J. Wang, G.C. Huang, J.C. Yu, P.K. Wong, "Advances in Photocatalytic Disinfection of Bacteria: Development of Photocatalysts and Mechanisms," Journal of Environmental Sciences 2015, 34, 232-247.
W.J. Wang, J.C. Yu, P.K. Wong, "Photocatalysts for Solar-Induced Water Disinfection: New Developments and Opportunities," Chapter 2 in Photocatalytic Materials & Surfaces for Environmental Cleanup-II, Ed.: Rajesh J. Tayade, pp. 63-89, Trans Tech Publications Inc., Switzerland, 2013.
D.Q. Zhang, G.S. Li, J.C. Yu, "Inorganic Materials for Photocatalytic Water Disinfection," Journal of Materials Chemistry 2010, 20, 4529-4536.
T.Y. Leung, C.Y. Chan, C. Hu, J.C. Yu, P.K. Wong, "Photocatalytic Disinfection of Marine Bacteria Using Fluorescent Light," Water Research 2008, 42, 4827-4837.
J.C. Yu, W.K. Ho, J.G. Yu, H.Y. Yip, P.K. Wong, J.C. Zhao, "Efficient Visible-Light-Induced Photocatalytic Disinfection on Sulfur-Doped Nanocrystalline Titania," Environmental Science and Technology 2005, 39, 1175-1179.

White titanium dioxide (TiO₂) has a large bandgap energy (3.03 eV for rutile (R-TiO₂) and 3.20 eV for anatase (A-TiO₂)); thus, it can only absorb UV light. As a consequence, the theoretical limit of hydrogen production by white TiO₂ is less than 5% under air mass (AM) 1.5 global solar illumination. In this presentation, I will report a various chemical reduction methods to maximize the efficiency of solar water splitting in TiO₂ nanomaterials on a transparent conducting oxide (TCO) substrate. The controlled reduction leads to oxygen vacancies in the TiO₂ arrays, which subsequently induce a cathodic on-set potential shift and sharp photocurrent evolution. Moreover, controlled morphologies of TiO₂ is also very effective for Li ion storage, which will be applicable for anode materials of Li-ion batteries.

Flame synthesis, a $15 billion/year industry is the most widely used method for the global production of TiO₂. Industry leaders include, Cristal Global, DuPont, and Evonik. A key challenge in industry is that flame reactors are usually built for a single product and the possibility of modifying the reactor’s operational parameters is limited. It is, therefore, important to develop flame reactors that will allow for a wide range of growth parameters and that can be easily scaled, specifically for the production of advanced materials, particularly next-generation TiO₂. The most commonly used flame types for material synthesis include premixed, normal diffusion, inverse diffusion, and co-flow flames. The use of multiple diffusion flames offers several key advantages, such as uniform temperature and chemical species profiles and many of the limitations related to premixed flames such as flashback and flame speed are avoided. Using the multiple-diffusion burner in a single-step, we have demonstrated the growth of carbon-coated titanium dioxide (TiO₂) nanoparticles, TiO₂ doped/coated with different metals (vanadium, silicon, and iron), and SiO₂ doped with carbon.

In lab scale experiments, laminar flames remain the dominant approach for the flame synthesis of nanomaterials. However in industry, turbulent flames are commonly utilized as it enables larger-scale production of nanomaterials. Recently we investigated the use of a novel turbulent burner for the flame synthesis of nanoparticles. It involves the use of an axisymmetric curved-wall jet (CWJ) burner, where the Coanda effect enables flame stability. Key advantages of this method include the enhancement of turbulence due to the radially inward velocity component of the jet. The vaporized precursor mixes with the gases delivered in the axisymmetric curved-wall jet burner. The combined effects enable enhanced mixing of the precursor, which is important in gas-phase synthesis. Results suggest we can effectively control the growth of anatase and rutile particles based on the operating conditions of the flame. Such a setup can be used to burn low cost chemical precursors due to the high temperature of the turbulent flame. It is possible this setup can provide new pathways for scalable synthesis of advanced TiO₂ nanoparticles.

High dielectric constant materials are critical for applications in nanoscale microelectronics, and as capacitors for energy storage and memory devices. Both Al₂O₃ and TiO₂ have been extensively investigated as high-k materials to replace SiO₂ as a gate and for high-capacitance capacitors for electronics. The dielectric constants of Al₂O₃ and TiO₂ are approximately 7 and 80, respectively. Our previous studies showed that amorphous Ti_xAl_1-xO_y films exhibit dielectric constant of ~30, with 4.8 eV bandgap. Here we report that giant dielectric constants (> 800) can be achieved with Al₂O₃/TiO_x nano-laminates, synthesized by atomic layer deposition (ALD) and with sub-layer thickness ≤ 1 nm, for frequencies up to 10⁶ Hz. The high dielectric constant is attributed to Maxwell-Wagner (M-W) relaxations, resulting from electrical heterogeneity of the multilayers. Al oxidation is favored over Ti-oxide since the Gibbs free energy for Al oxidation is more negative than for Ti. It appears that the Ti-oxide sub-layers are not stoichiometric TiO₂, but semiconducting TiO_x. The difference in electrical conductivities of the TiO_x and Al₂O₃ layers results in surface charge accumulation at the interfaces. The surface charges relax with AC field and cause M-W relaxation. An interface layer inserted at the interface between top electrode and Al₂O₃/TiO_x nanolaminate is critical to yield high-k up to about 1 MHz with low dielectric losses (~0.02) and low leakage current (~ 10^-9 A/cm²). We used ALD to produce large area capacitors via conformal coating of large area ridge arrays fabricated on Si surfaces. These capacitors can yield ≥ 10 µF/cm² capacitance.
We also report the feasibility of controlling the dielectric properties - high dielectric constant (k) and substantially extended relaxation frequency of thin film nanolaminates (NLs) with various sublayer thicknesses, uniquely realized by ALD process. For 150 nm thick TiO_x/Al₂O₃ NLs with sub-nanometer thick sublayers, few Angstrom change in sublayer thickness dramatically increases relaxation cut-off frequency by more than 3 orders of magnitude (from kHz to MHz) with high dielectric constant (> 800 ).
The nano-laminates are also explored for applications such as energy storage embedded capacitors in a Si microchip implantable in the human retina to restore sight to people blinded by genetically-induced degeneration of photoreceptors, for supercapacitors integrated with ferroelectric-based high-efficiency photovoltaic devices for energy generation/storage systems, and for high-k gate oxide with low leakage current and losses for next generation DRAMs and nanoscale CMOS devices.

TiO₂ is touted as a material capable of use in photo-catalytic environmental remediation strategies. However it exhibits efficient recombination processes and exhibits a bandgap congruent with ultraviolet light which represents ~5-6% of the solar spectrum. Engineering of TiO₂’s bandgap by doping with a variety of cations and anions has allowed visible light, which represents a far larger proportion of solar irradiation, to be utilised whilst reducing the recombination of charge carriers.

However, serious long term testing of dopant stability under irradiation has yet to feature within the literature. We present long term testing using XPS and photo-catalytic tests to demonstrate the lability of dopants in doped TiO₂’s which have been previously characterised. Depending on the dopant, such as phosphorus or tantalum, and its location (interstitial versus substitutional) some exhibit better stabilities and relevance for environmental applications than others. We discuss the issues and consequences of these findings for the field, with big potential impacts in solar cells, H₂ production and photo-catalysis in general. We go on to stress the importance of long term testing within the field of TiO₂ and photo-catalysis in general so that decisions regarding its use for environmental applications are made using clear design rules.

Metal halide perovskite solar cells have seen an incredible surge in efficiency to over 20 % in recent years. In the first incarnation, the perovskite was used as a sensitizer on a mesoporous TiO₂ scaffold, and functioned in a manner similar to the dye sensitized solar cell. The technology rapidly transferred to a solid state system which offered advantages in terms of stability. The current state of the art consists of a both planar and mesostructured architectures the vast majority of which continue to employ TiO₂ as the electron selective contacts.

This talk will focus on recent work from our group investigating the role of TiO₂ as the electron selective contact in solar cells using the commonly employed CH₃NH₃PbI₃ semiconductor. Our results suggest that while TiO₂ is currently used in the state of the art architectures, TiO₂ may not be the ideal material in terms of its effects on absolute efficiency, hysteresis phenomena, and perhaps most importantly, device stability. The origins of hysteresis in perovskite solar cells will be discussed through an analysis of device behavior with varying selective contacts at different temperatures and scan rates. The role of band alignment plays a large role in both absolute efficiency and the extent to which hysteresis can be manifested in devices, and can be improved by changing contact materials. We find that fullerenes form an improved contact with the perovskite, allowing us to obtain improved performances with minimal hysteresis. This is quantified by means of photoluminescence quenching experiments with varying biases and contact layers. These demonstrate clearly that electron transfer to planar TiO₂ layers is hindered by an energetic barrier whose height can be temporarily tuned by poling induced ion migration, resulting in strong hysteretic behavior.

On an equally important note, we address the impact of TiO₂ layers on the stability of perovskite solar cells. We focus particularly on the effect of UV light exposure on the stability of the devices. We have found that the complex interaction of adsorbed oxygen, UV light, and TiO₂ surface states leads to the formation of deep trap sites which result in a rapid reduction in performance of TiO₂ containing solar cells. This effect can be remedied by altering the recombination mechanisms of the solar cells or simply by replacing the TiO₂ with fullerenes, which do not suffer from such UV activated states. It appears that while TiO₂ can be a useful material to make high performance perovskite solar cells, it may not be the ideal material for reaching the efficiencies and stability ultimately required of the technology.

In this work, we focus on developing various novel nanostructured TiO2 for enhancing photoelectrocatalytic activity in applications of clean environments and energy. An ultrasonication-assisted sequential chemical bath deposition (S-CBD) method was developed to modify anatase TiO2NTAs with Cu2O nanoparticles to form p-n heterojunction photoelectrodes for enhancing visible light photocurrent and photocatalytic performance through manipulating the size and content of deposited Cu2O nanoparticles. It was shown that a small amount of Cu2O nanoparticles significantly improved the photocatalytic activity. The as-prepared heterojunction photoelectrodes obtained from ultrasonication-assisted (S-CBD) method for 4 min possessed the highest photocurrent and photocatalytic degradation rate of Rhodamine B under both UV and visible illumination. When applied a 0.5 V bias potential, photocatalytic electrodes exhibited the superior photoelectrocatalytic activitydue to the synergistic effect from the photocatalysis and electrochemistry. The ZnFe2O4-decoratedanatase TiO2 NTA electrodes was constructed via a hydrothermal technique. The size and loading amount of ZnFe2O4 nanoparticles were adjusted by different hydrothermal process. It was displayed that the photocurrent and photocatalytic efficiency of TiO2 NTA electrode after loading of ZnFe2O4nanoparticles under visible light irradiation were incredibly increased. Among the photocatalystssynthesized for different times, the electrode obtained from the hydrothermal reaction for 6 h presented the optimal photocatalytic performance in degrading Acid Orange II under visible light illumination. Electrochemical measurements (i.e., electrochemical impedance spectra and Mott-Schottky test) revealed that the modification of ZnFe2O4 nanoparticles promoted the charge carrier transfer at the interface of the photocatalyst and the electrolyte, which efficiently inhibited the recombination of photogenerated charge carriers. The SrTiO3-modified rutile TiO2 nanorod array electrodes were fabricated via the three consecutive facile hydrothermal techniques. In the first two hydrothermal reactions, rutile TiO2 NRAs were synthesized and etched to increase the specific surface area of the electrodes. During the subsequent hydrothermal treatment, rutile TiO2 NRAs served as both the structure-directing scaffold and Ti source and rutile TiO2 at the surface layer of nanorods was converted into SrTiO3. The conversion content of rutile TiO2 to SrTiO3 can be adjusted through tuning the hydrothermal time. It was demonstrated that after partially turning into SrTiO3, the prepared photocatalytic electrodes showed not only boosted UV light response but also enhanced photocatalytic degradation rate of methylene blue under UV illumination. The heterojunction electrodes obtained from the hydrothermal reaction for 12 h possessed the best photocatalytic performance. It was found from the electrochemical impedance measurements that the SrTiO₃ modification suppressed the recombination of electron-hole pairs and improved the carrier mobility at the interface of electrode and electrolyte.

Here, we would like to introduce our recent work on the green TiO₂ photocatalysts, including the discovery, properties, and photocatalytic performances in photocatalytic hydrogen generation and pollutant removal.

The development of new color switching systems that can reversibly change color in response to external stimuli, such as light or heat, has attracted a great deal of attentions for their important applications in sensing devices, display and signage technologies, rewritable media, and security features. Here we discuss a new color switching system based on reversible redox reaction that could be initiated by photocatalytic response of TiO₂ nanocrystals. With the assistance of TiO₂ nanocrystal-based photocatalysts, UV light irradiation can rapidly reduce the imaging materials and result in obvious color change, while the recoloration can be achieved by re-oxidizing the system with the assistance of visible light irradiation or heating. The excellent performance of the new color switching system promises their potential applications as attractive rewritable media to meet our society’s increasing needs for sustainability and environmental conservation.

One-dimensional nanoarrays such as nanorods, nanowires, or nanotubes have attracted significant interest for photovoltaic applications. However, the power conversion efficiency of dye-sensitized solar cells (DSSCs) based on one-dimensional nanoarray photoelectrode is still lower than the commercial nanoparticle due to the lower surface area for the dye loading. Hence, the current challenge is to synthesize the nanoarray photoelectrode possessing larger internal surface area for larger amount of dye loading. Here, we report the facile hydrothermal fabrication of hierarchical and hyperbranched anatase TiO₂ nanowire array for efficient DSSCs and quantum dot-sensitized solar cells (QDSSCs). For example, ultra-long (47 um in length) anatase TiO₂ nanowire array was prepared via a multi-step hydrothermal reaction which leading to an impressive power conversion efficiency of 9.40 %. Furthermore, a series of TiO₂ nanowire array-nanosheet (or nanorod) branch-tiny nanorod hyperbranched structures have been also successfully prepared via the simple hydrothermal process which exhibit more than 11% photovoltaic performance in DSSCs. The higher efficiency for such hierarchical TiO₂ nanoarrays can be mainly attributed to the fast electron transport and slow electron recombination, efficient dye loading and superior light scattering ability, which was investigated by intensity modulate photocurrent spectroscopy (IMPS), intensity modulate photovoltage spectroscopy (IMVS), electrochemical impedance spectroscopy (EIS), and UV-vis diffused spectroscopy etc. Such novel TiO₂ nanowire arrays are anticipated to exhibit the significant applications in photovoltaic, photocatalysis, water splitting, CO₂ conversion, optoelectronics, energy conversion and storage.

Keywords: Solar cells, TiO₂, nanowire, photovoltaic performance

References
[1] Wu, W. Q.; Feng, H. L.; Rao, H. S.; Xu, Y. F.; Kuang, D. B.;^* Su, C. Y., Nat. Commun, 2014, 5, 3968,
[2] Wu, W. Q.; Xu, Y. F.; Rao, H. S.; Kuang, D. B.;^* Su, C. Y., J. Am. Chem. Soc, 2014, 136 (17), 6437–6445
[3] Wu, W. Q.; Xu, Y. F.; Rao, H. S.; Feng, H. L.; Su, C. Y.; Kuang, D. B.,^* Angew. Chem, 2014, 126 (19), 4916-4921; Angew. Chem. Int. Ed, 2014, 53(19), 4816-4821
[4] Wu, W. Q.; Xu, Y. F. ; Su, C. Y.; Kuang, D. B. ^* Energy Environ. Sci., 2014, 7(2), 644-649.
[5] Wu, W. Q.; Lei, B. X.; Rao, H. S.; Xu, Y. F.; Wang, Y. F.; Su, C. Y.; Kuang, D. B. ^* Sci. Rep. 2013, 3,1352.
[6] Liao, J. Y.; Lei, B. X.; Chen, H. Y.; Kuang, D. B.; ^* Su, C. Y. Energy Environ. Sci. 2012, 5, 5750.

Photoelectrochemical (PEC) splitting of water represents a promising strategy for the low cost and environmentally friendly production of solar fuel. A key challenge in PEC water splitting is to fabricate nanostructured photoelectrodes with desirable architectures and properties. In this work, heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls were designed and fabricated as a new type of photoanodes for PEC water splitting. The unique heterostructures with a hierarchical architecture were readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two-dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated transfer of photogenerated electrons from rutile to anatase. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the FTO substrate showed significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the TiO₂ NR@NB array photoelectrode yielded a photocurrent density of 1.24 mA/cm² at 1.23 V with respect to the reversible hydrogen electrode (RHE), which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate. This work may open new avenues towards building complex semiconductor nanostructures with desirable architectures for efficient harvesting and utilization of solar energy.

The trapping and mobility of electrons in nanocrystalline oxide materials underpins a diverse range of applications in areas such as solar energy generation, catalysis, gas sensing and nanoelectronics. However, directly probing the properties of electrons in such complex nanocrystalline systems is extremely challenging. Here, we provide insight into these important issues through first principles based modeling of the interaction of electrons with surfaces and grain boundaries in rutile TiO2. We show that different surface orientations exhibit markedly different electron affinities: some preferring to trap electrons with others repelling electrons. The equilibrium nanocrystal morphology exposes both electron trapping and electron repelling facets and therefore is predicted to posses highly anisotropic electron trapping properties [1]. Interfaces between nanoparticles (grain boundaries) are associated with high concentrations of strong electron trapping sites which hamper electron transport between grains. However, we show how this effect is partially ameliorated at high current densities (>0.01 mA/cm2) as a result of a highly nonlinear trap filling effect [2]. We discuss how with atomistic insight into the electron trapping properties of nanocrystalline materials one can suggest ways to improve the performance of materials for applications, for example by designing optimal nanocrystal morphologies.

[1] S. Wallace and K. P. McKenna, Journal of Physical Chemistry C 119, 1913 (2015)

[2] S. Wallace and K. P. McKenna, Advanced Materials Interfaces 1, 1400078 (2014)

The use of TiO₂ photoelectrodes (or TiO₂ suspensions) to produce hydrogen from various electrolytes (with or without sacrificial agents) has been highly investigated over the past decades and still remains the most investigated semiconductive material. [1] The advantage of anodic TiO₂ nanotubes (NTs) is not only that they provide a one dimensional charge-transfer path, but also they are back-contacted on metal substrate that can be used as electrodes directly. [2] Pathways to create noble-metal-free H₂ evolution activity on TiO₂ NTs are not only of considerable scientific but also of high economic interest. [3-4] Here we introduce an efficient way to introduce a sub-surface configuration of a broad range of lattice defects (namely vacancy/interstitial pairs) into any crystalline material using high energy ion implantation. [5]
In this study, we report the creation of sub-surface configuration of defects containing TiO₂ NTs that strongly acts as photo co-catalysts. These new class of catalysts are obtained by low-dose nitrogen ion-implantation that can be tuned to obtain a highly defined ion and defect distribution within the TiO₂ NTs substrate. A particular advantage of creating these defect centers by N-implantation is that the active centers can be placed as a coherent layer at a defined sub-surface location. The catalysts present considerable photocatalytic activity for hydrogen evolution under solar light illumination in the absence of noble metals and external bias. Thus these results provide a concept for a new generation of TiO₂ based photocatalysts.


References
[1] A. Fujishima and K. Honda, Nature, 1972, 238, 37-38.
[2] X. Zhou, N. T. Nguyen, S. Özkan, P. Schmuki, Electrochem. Comm., 2014(46,) 157–162.
[3] X. Zhou, N. Liu and P. Schmuki, Electrochem. Commun., 2014 (49), 60–64.
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[5] X. Zhou, V. Häublein, N. Liu, N. T. Nguyen, E. M. Zolnhofer, H. Tsuchiya, M. S. Killian, K. Meyer, L. Frey, and P. Schmuki, 2015, submitted.

We report that the valence and conduction band energies of TiO₂ can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an absolute energy reference, using an accurate multiscale quantum-chemical approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO₂ as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO₂ as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochemical water splitting through polymorph engineering of TiO₂.

Nanoscale conformal layers of TiO₂ can be coated on the inner pore walls of porous silicon films by room-temperature infiltration of a sol-gel precursor consisting of titanium t-butoxide and triethanolamine in ethanol. The composite films can act as label-free optical interference sensors for chemicals (VOCs) and biological agents, and the photoactivity of the TiO₂-coating provides a self-cleaning function when irradiated with UV light. Selectivity for biological agents is achieved by physical adsorption of protein A, followed by the specific binding of rabbit anti-sheep immunoglobulin (IgG) and then specific capture of sheep IgG. The specificity of the protein A, rabbit anti-sheep IgG construct on the sensor is confirmed by tests with non-binding chicken IgG. The sensitivity is 8210±170 nm/refractive index unit (RIU).

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 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 based on TiO₂. Several families of TiO₂ based materials with visible light response have been synthesized and their photocatalytic properties have been characterized. These materials include self-doped TiO₂ with Ti³⁺ introduced by hydrothermal and combustion methods as well as post-synthetic treatments by different reducing agents. The research also shows that different Ti precursors can also have a great impact in the materials’ photocatalytic properties. A systematic study of the structural defect, Ti³⁺ concentrations and their impact to the materials photocatalytic properties will be discussed. The materials with different morphologies and facets have been obtained and will be presented.

Thin-film semiconducting anatase TiO₂ finds several applications, including photocatalysis, wherein control over carrier concentration and other material properties would be useful. Literature tends to pursue relationships between specific synthesis conditions and film morphology with little attention given to carrier concentration. Meanwhile, attempts to significantly vary carrier concertation have centered heavily around the use of dopants such as nitrogen. Both bodies put little focus on annealing and other post processing conditions.

The present work employs atomic layer deposition to fabricate thin amorphous films of TiO₂, which are then annealed into polycrystalline anatase. Variations in the temperature and growth rate for deposition, as well as in the ramp rate and final temperature employed during the annealing, were used to determine how carrier concentration can be affected without the use of dopants. Effects of annealing in oxygen scavenging atmospheres (H₂ and Cl₂), as well as super band gap photostimulation, are reported as well.

These variations propagate into changes in one or more of the final carrier concentration, crystallite size and bulk density. Many of these surprising findings can be interpreted in terms of medium range atomic order existing in the initial amorphous films. The findings further demonstrate that medium range order may be a physical property capable of being manipulated in metal oxide systems

Improving the photocatalytic (PC) properties of titania has been the focus of this research. In particular, increasing titania’s PC activity in the visible region of the spectrum is crucial for increasing its widespread use as a catalyst. Two recent methods have been shown to increase titania’s PC activity. First, codoping anatase titania nanoparticles with transition metals and nitrogen increases PC activity in the visible region, however, the usual route for making these nanoparticles, the hydrothermal method, creates particles with widely-varying sizes. This variation in size is thought to hinder charge transport. To address this, the second method creates uniformly-sized (also known as monodisperse) anatase titania nanoparticles. These particles are produced by a two-step method in which monodisperse titania nanospheres are produced via a sol gel route, and these particles are then used a template for growth of mesoporous, anatase nanospheres. The question naturally arises as to whether combining of these two techniques to create monodisperse, codoped titania would exhibit even higher activity. Here we describe the process for making monodisperse, codoped, mesoporous titania nanoparticles and compare their photocatalytic performance with those having a large variance in size.

Titanium oxide is an interesting material for microelectronic, opto-electrical, and photochemical applications due to its stability, high-K dielectric properties, and high refractive index. Novel applications include passivation layers for production of solar fuels [1], resistance switching material in non-volatile memory ReRAM [2] and as tunnel dielectrics for MIS contacts [3]. These applications require precise control of thickness, composition, and crystallinity and low temperature processes are desired. In this presentation we compare results obtained from ALD deposited titanium oxide films using a variety of precursors and a wide range of process temperature on 300 mm native-oxide silicon wafers with an ASM Pulsar^® 3000 cross-flow ALD reactor.

Titanium precursors studied include titanium isopropoxide (TTIP), titanium methoxide (TMOT), or titanium tetrachloride (TiCl₄) with oxygen sources of water (H₂O), ozone (O₃), or TTIP [4]. Deposition temperatures from 50°C to 300°C were studied, with the specific range depending on the precursor used. Films were characterized for thickness and refractive index by ellipsometry. The effect of precursor and temperature on film composition was analyzed by SIMS and ERD. Density and crystallinity was also characterized by XRR and XRD.

A wide range of growth rates were observed depending on temperature and precursors used. Growth at temperatures as low as 50°C was observed for TiCl₄ and water. Ozone provides higher growth rate than water. The highest growth rate was achieved using the metal alkoxide TTIP as the oxygen source with TiCl₄ [4]. Refractive index and density of the films increased with temperature. Slight changes in O/Ti ratio was observed depending on precursors used. TiCl4-based processes can result in Cl contamination at lower temperatures, but higher temperatures reduce Cl to negligible levels. Films deposited at 300°C have crystalline anatase form. Amorphous films can be deposited at 200°C, but readily crystallize upon subsequent anneal at 400°C. High quality films are deposited with high RI and density with multiple precursors in the temperature range of 250-300°C.

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[3] J.Y.J. Lin, A.M. Roy, K.C. Saraswat, IEEE Electron Device Lett., 33, 11, p. 1541-1543 (2012)
[4] M. Ritala, K. Kukli, A. Rahtu, P. Räisänen, M. Leskelä, T. Sajavaara, J. Keinonen, Science, 288, 5464, p. 319-321 (2000)

Large-band gap metal oxide TiO₂ have suitable band positions for photoelectrochemical cells (PECs) for solar-driven water splitting, but uses only UV light region in the solar spectrum which represent only about 5 % of the energy. On the other hand, α-Fe₂O₃ with suitable bandgaps for efficient absorption in the solar spectrum require an external bias to drive hydrogen generation at the cathode due to the conduction band of α-Fe₂O₃ below the H₂ evolution potential and have short carrier diffusion lengths. Synthesizing the metal oxide nanomaterials which have both suitable band position to drive reaction and visible light absorbed band gap is one of the major challenge in PECs for water splitting field. Hetero structure of α-Fe₂O₃ and TiO₂ offer a potential solution to improve this problem. However, the inherent low electrical conductivity resulting in the high electron-hole pair recombination rate and short carrier diffusion length of α-Fe₂O₃ limit its practical use. Here we report a novel hierarchical heterostructure of α-Fe₂O₃ ultrathin nanoflakes branched on TiO₂ nanotube strategy for PEC water splitting. On the basis of the detailed experimental results and associated theoretical analysis, we demonstrate that suitable morphological control of α-Fe₂O₃ and TiO₂ plays an important role in enhancing the photoelectrochemical water splitting performance.

Photoelectrochemical (PEC) water splitting can be employed for the production of hydrogen by exploiting solar radiation; however, the development of innovative photoanode and cathode materials and architectures still represents a key challenge for the realization of an efficient and competitive PEC cell. In this context, this study focuses on quasi-1D hydrogen-treated titanium oxide nanostructures as photoanodes.
Titanium dioxide is one of the most studied materials as photoanode for water splitting, nonetheless it presents some limitations that are currently addressed following two main strategies, i.e., first, doping or sensitization, to shift the absorption to the visible; second, controlling the material structure and morphology at the nanoscale, to enhance light harvesting and quantum efficiency. A recent innovative approach towards the first goal consists in the hydrogenation or reduction of titanium dioxide, leading to the so-called black titania, which exhibits remarkable efficiency for the water splitting reaction, although being not fully understood [1]. On the other hand, quasi-1D hierarchical nanostructures have been considered because of their advantages, such as large surface area, effective light scattering, mesoporous structure and anisotropic morphology.

In this work an explorative combined approach, i.e. extension of the photoresponse to the visible range as well as optimization of morphology and structure, has been investigated. Amorphous hierarchical or quasi-1D TiO₂ nanostructures were synthetized by Pulsed Laser Deposition (PLD) in an O₂ background atmosphere [2], and different strategies were explored to achieve hydrogenation or reduction, namely changing the deposition background gas (from pure O₂ to Ar/O₂ or Ar/H₂ mixtures), or substituting/combining the air annealing process at 500°C (necessary to induce crystallization to the anatase phase) with Ar/H₂ annealing. SEM, Raman spectroscopy and UV-vis-IR spectroscopy were employed to investigate and understand the material morphology, structure and optical properties. A photoluminescence background and a tail absorption towards the visible region emerged for hydrogen-treated samples.

Photocurrent measurements under solar simulator illumination showed a noteworthy increase of photocatalytic response for the Ar/O₂ deposited samples followed by a double air+Ar/H₂ thermal treatment with respect to the O₂ deposited/air annealed stoichiometric TiO₂ samples. It is suggested that this effect could be ascribed to an optimized morphology characterized by large surface area, good electron transport, reduced recombinations, combined with oxygen vacancy-related tail states in the bandgap. Further investigations for a deeper understanding of the physical/chemical mechanisms involved in the hydrogenation process at the atomic scale are currently being carried out.

[1] Chen, X. et al. Chem. Soc. Rev. 2015, 44 (7), 1861–1885.
[2] Matarrese, R. et al. Chem. Eng. Trans. 2014, 41, 313–318.

Abstract
Among various metal oxides, TiO₂ has attracted much attention due to its potential to be used in wide range of applications in the field of light harvesting, photocatalysis, gas sensing, and so on. However, in most cases its performance is limited due to high bandgap (~3-3.2 eV), even by fabricating nanostructures, and thus it requires bandgap engineering. In this scenario, ion beam irradiation is known to be a promising technique for tuning bandgap through introduction of defects, especially vacancies and/or interstitials in TiO₂ matrix. Here we will show how the optical bandgap of self-assembled TiO₂ nanorods on chemically etched Si pyramids can be tuned by 50 keV Ar⁺-ions with fluences in the range of 5×10¹⁴ to 1×10¹⁷ ions/cm² at room temperature. In particular, we will demonstrate the appearance of degenerate states in TiO₂ matrix at a critical fluence of 1×10¹⁷ ions/cm² using ultraviolet-visible spectroscopy. This intriguing phenomenon will be discussed in the light of Burstein-Moss effect due to the gradual increase of oxygen vacancies in TiO₂ as established by transmission electron microscopy and further supported by x-ray photoelectron spectroscopy.

Titanium dioxide, TiO₂, is a very versatile and cheap material that can be used in a lot of different applications like (photo)catalysis, sensing and self cleaning coatings.¹ Many of these applications take advantage of a high specific surface area and a high degree of crystallinity. As there is always the trade-off between these two parameters, obtaining both in the same material is not straightforward. By using microwave irradiation we were able to achieve this goal. A highly mesoporous titania material with a specific surface area above 330 m²/g,which is among the highest numbers presented in literature, was obtained by adding an additional microwave irradiation step during a normal evaporation-induced self-assembly (EISA) synthesis. The degree of crystallinity which is often neglected in literature, was determined using Rietveld refinement and an increased degree of crystallinity with more than 10 % was achieved. The isoelectric point, particle size and surface groups remained the same after the addition of amicrowave irradiation step to the synthesis, thus retaining the material properties that are crucial for applications.

One step further in obtaining outstanding materials is the addition of noble metal nanoparticles on mesoporous titania. This kind of materials has a lot of applications in the (photo)catalyst sector. Many synthesis routes are already developped to prepare these kind of materials, but little is known about the oxidation state of the metal ions present in these materials even though it can be an important factor when one studies the reaction mechanisms of a reaction. Therefore we performed a study of the oxidation state of gold atoms of Au/TiO₂ materials reduced in four different ways, using X-ray adsorption spectroscopy (XAS).² We found that different particle sizes, oxidation states and interactions of the gold with the titania support are obtained when one uses different reduction methods.

When one uses these composite materials in photocatalytic remediation reactions or alcohol oxidation reactions, one can observe differeneces between these samples even though the gold loading stays the same.

1. M. Arin, P. Lommens, et al., J. Eur. Ceram. Soc., 2011, 31, 1067-1074.
2. M. Meire, P. Tack, et al., Spectrochim. Acta B, 2015, 110, 45-50.

As a typical intercalation material, various nanostructured TiO₂ has been extensively studied as an alternative and safer anode material for use in high power density Li-ion batteries. However, the studies have been mainly dedicated to the development of powder type electrode materials and relatively little attention has been paid to studies of other electrode architectures. While composite electrodes containing a mixture of the TiO₂ particles, binders and conductive additives still are commonly used, such electrodes often yield poor material utilization, undefined material/component arrangements and a lot of complex interfaces. Binder- and additive-free nanostructured oxide electrodes can, on the other hand, provide a pristine model electrode system for careful evaluation of the battery performance of various electrodes.[1] In the present work, we demonstrate that highly ordered, free-standing anodic TiO₂ nanotube array electrodes, can be either be directly used as high energy and power density electrodes for Li-ion microbatteries or as model monolithic electrodes for electrode engineering and Na-ion battery studies. By using anatase TiO₂ nanotube electrodes, an areal capacity of 0.37 mAh cm^-2 (i.e., 40 mAh g^-1) at a rate of 10C (using a (dis-)charge current density of 9 mA cm^-2), and 1 mAh cm^-2 (i.e., 91 mAh g^-1) at a rate of C/5, can be achieved. [2] In addition, well-defined monolithic TiO₂ nanotube electrodes with fine-tuned nanotube size gradients (e.g., tube length, diameter and wall thickness) can also be manufactured using a bipolar electrochemistry approach. [3] The gradient nanotube electrodes can provide excellent rate performance, with capacities of 0.16 mAh cm^-2 or 169 mAh g^-1 at a rate of C/5 to of 0.04 mAh cm^-2 and 42 mAh g^-1 at a rate of 50C. A careful comparison of the Li-ion and Na-ion battery performances of anatase TiO₂ nanotube and amorphous nanotube electrodes will also be presented.[4] Refs: [1] W. Wei, et al., J. Mater. Chem. A 2013, 1, 8160; [2] W. Wei, et al., submitted, 2015 ; [3] W. Wei, et al., Electrochim. Acta 2015, 176, 1393; [4] W. Wei, et al., in preparation, 2015

Titanium dioxide is well known for its ability to absorb and resist radiation, especially in the form of ultraviolet light. However, it is a soft material and is not suitable to apply as a protective barrier by itself. The addition of zirconia to titania in order to form a nanocomposite film is being investigated for use as a protective coating on steel in a range of industrial applications. Zirconia, usually stabilized by yttria, is desirable in protective coatings as it is chemically inert, but it can still be fractured in stressful conditions; preliminary work has shown that zirconia is further stabilized when blended with other oxides. The purpose of this work is to investigate a range of TiO₂/ZrO₂ nanocomposite films in order to understand the effect of composition changes on their performance under mechanical, chemical, or radiological stress and how they can be combined in order to simultaneously enhance their beneficial properties and while mitigating the individual component’s shortcomings. Physical vapor deposition and co-sputtering of TiO₂ and ZrO₂ were the primary deposition techniques used for this study. Initial single layer coatings were fabricated and characterized to fully understand the properties associated with the individual component thin films. Following monolayer deposition, a range of TiO₂/ZrO₂ compositions was created and characterized. Before making measurements of the functional properties and mechanical, chemical, and radiological performance of the as deposited coatings, they were characterized for composition, microstructure, and morphology and these properties were correlated to coating composition so that performance could be further correlated back to deposition. The single component titania and zirconia coatings performed as expected with respect to hardness, corrosion, and radiation attenuation and in context of what is widely seen in literature for these materials. Composite performance varied as expected across the range of component concentrations and showed alternating enhancement and mitigating effects, leading to the conclusion that individual composite formulations may find use depending on application needs.

Abstract:
Functional oxides are very promising materials for next-generation electronics, as they can undergo transitions from insulator to semiconductor and further to metal, as the charge carrier density increases. Nanocrystalline titanium dioxide (TiO₂) is one of the most investigated oxide materials that already found applications in sensing, electrochromics, photovoltaic, and photocatalytic devices. [1]
We fabricated, using an unconventional patterning process, polycristalline TiO₂ electrolyte gated (EG) transistors making use of high surface area activated carbon (AC), as a gate electrode, and the ionic liquid (IL) 1-Butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI], as the gating media.
To explore the effect of the double layer capacitance on device performance we investigated bottom-contact top-gated transistor where we varied the area of the active layer in contact with electrolyte and the area of overlap of source/drain electrodes with gate. The operating voltage of the devices is lower than 2 V and the ON/OFF ratio as high as 10⁴. To shed light on the doping mechanisms of such transistors we performed electrical measurements, cyclic voltammetry and electrochemical impedance spectroscopy. The electron mobilities were in the range of 0.05 cm²/Vs - 1 cm²/Vs and correlate with double layer capacitance.
We believe that these simple architecture devices working at low voltages are promising for low cost and large area electronics.
[1] C. Wei and C. Chang, 2011, Materials Transactions, 52 (3), 554- 559.

Titanium dioxide (TiO₂) is a well-known photocatalytic material for oxidative degradation of various organic contaminants and water splitting. TiO₂ has outstanding photocatalytic properties along with good chemical stability, nontoxic nature and low production cost. The rutile phase of TiO₂ is thermodynamically stable and it is able to absorb violet light along with a proportion of visible light but the fast recombination rate prevents its photocatalytic performance. So, many methods have been studied to modify the properties of rutile TiO₂ to enhance photocatlytic performance. Among them, the creating defects on the TiO₂ structures and incorporation of Ti³⁺ show good performance due to the decrease of the recombination rate.
In this study, we demonstrate a facile synthesis method to modify rutile TiO₂ properties via microwave irradiation with hydrogen peroxide (H₂O₂). The original and modified rutile TiO₂ samples will be discussed using by X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. The surface modifications will be reported using Transmission electron microscopy. Finally, the photocatalytic performance using degradation of methylene blue and disinfection of anti-bacterial under visible light will be highlighted.

Visible light photocatalytic activity of doped TiO2 has been widely investigated for the application of solar water splitting; and nitrogen has been intensively studied as a dopant where it has been introduced into the TiO2 lattice in either the pre-synthesis or post-synthesis step. My work focuses on the spatial distribution of nitrogen dopants in the TiO2 lattice. It involves doping single crystals of TiO2 Rutile, which are depth profiled through using X-ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry and Low Energy Ion Scattering. These advanced characterisation techniques, in conjunction with each other, are used to identify the chemical states of dopants and the spatial distribution of these dopants.The distribution of dopant ions (and therefore photoactivity) can be highly dependent on the method used for nitrogen incorporation and associated synthesis parameters. The effect of the TiO2 single crystal orientation on the concentration of nitrogen dopant incorporated using solid-state diffusion was investigated. The TiO2 facets in order of their increasing nitrogen dopant ion incorporation in the top ~ 2-3 nm are (110) > (001) > (100). While interstitially doped Nitrogen species predominates the near-surface region, only substitutionally doped Nitrogen species is seen below the near-surface region. These results were compared against the samples doped using Ammonolysis. In addition, it was found that employing a reduction step prior to the doping step resulted in greater nitrogen ion incorporation. Effect on TiO2 surface topography upon nitrogen doping was also investigated. The solid-state diffusion of dopants into the single crystals is favoured by low temperatures as higher temperatures results in surface topographical effects such as roughening. This was investigated further by employing lower temperatures and longer annealing times. The Atomic Force Microscopy images show smoothening of the (100) TiO2 surface upon nitrogen doping, which was not observed with (110) and (001) facets. This work involving a detailed investigation of the structure of TiO2 is key in designing new and improved photocatalytic materials.

Titanium dioxide and doped titanium dioxide offer many potential uses for various photocatalytic processes, such as water splitting, antimicrobial coatings and pollutant degradation. Previous studies suggest that antimony doping could improve the photocatalytic activity of TiO₂ though there has been minimal investigation into the structural composition of Sb doped TiO₂.

The work to be presented focuses on the effect of synthesis method and amount of dopant on the integration of Sb into the TiO₂ lattice. Doped TiO₂ has been achieved through solid-state reaction of TiO₂ and Sb_xO_y powders both in air and under vacuum. Interestingly, no doping can be achieved under vacuum- all Sb leaches out of the starting mixture- whereas, at certain temperatures, Sb is successfully doped into TiO₂ powders heated in air.

Diffusion of the dopant into the lattice has been studied by doping single crystals and analysis with X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and Secondary Ion Mass Spectrometry (SIMS). Spatial distribution of the dopant ions and their behaviour upon secondary thermal treatment has been studied, to further understand the stability of these materials. It appears that Sb does not deeply penetrate into the lattice and tends to segregate substantially to the surface of the material.

Lattice parameter changes indicate that the solubility limit of Sb in TiO₂ has been determined as 17%, above which secondary phases of Sb_xO_y are consistently observed in XRD. Furthermore, the implication of Sb surface segregation and the effect of this on other dopants when used in combination with Sb is interesting. When used as a codopant, it appears that Sb can restrict diffusion of other dopant ions into the TiO₂ lattice and could therefore be used as a method to control the distribution of other ions.

Memristors are described as the future of non-volatile devices for novel computing because of their fast operation and multistep storage capabilities. Memristors, where a few nanometers thin transition metal oxide film is sandwiched between two metal electrodes, are operated on the principle of dynamic resistive switching (RS) due to the electromigration of oxygen vacancies. Recent studies for the Pt/TiO₂/Pt memristor devices reveal 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. Electroforming process in TiO₂ based memristors are generally dominated by oxygen vacancies evolution through Joule heating and therefore can be destructive in nature and also uncontrollable. It can damage the device or even can destroy it completely. Therefore it’s necessary to fabricate a device with the electroforming voltage as low as possible to avoid Joule heating effect and to achieve an electric field dominating resistive switching in TiO₂ based memristors while maintaining the high endurance and stable electronic behavior.
Here, we exploit the switching behavior of a low bias electroforming memristor device based on Pt/TiO_x/Pt, fabricated at room temperature, using a combination of maskless photolithography and high vacuum magnetron sputtering. In this case, TiO_x is an oxygen deficient phase of titanium oxide with a high density of in-built vacancies available for electromigration. The physical characterization of the TiO_x film is performed using SEM, AFM, XRD and SIMS, and the electronic behavior (I-V characteristics) of the memristor devices are analyzed using a semiconductor analyzer. Before electrically tested, the device stays in its virgin state exhibiting a Schottkey behavior. An electroforming voltage as small as ±1.5 V is required to switch the device to LRS from HRS state where the device changes its behavior from Schottkey to Ohmic. Once the irreversible electroforming step is over, the device exhibits a stable ON and OFF switching under a low sweep voltage (±1.0 V) with remarkable ON/OFF ratio over 260 at 0.5 V. Junction size dependency is observed for four different junctions (5×5, 10×10, 20×20 and 50×50 µm²). High retention capacities are observed for 10⁵ s while high endurance and fast switching are achieved up to 150 switching cycles.
This study describes a promising process for fabricating efficient nanostructured metal oxide based memristors to improve the performance. Next step towards improvement will be incorporation of new hybrid nanostructured materials with appropriate structural modification, which will lead to a high-performance memristor device with even higher endurance and lower energy consumption.

Carbon coated P-25 TiO₂ nanoparticles were utilized for synthesis of titanium carbide (TiC) and titanium diboride (TiB₂) nanoparticles via carbothermal reduction process. The synthesis procedure was optimized for narrow size distribution of the product. As synthesized TiC and TiB₂ nanoparticles were coated onto surface of commercial 304, 430, and P91 steel substrates with HVOF thermal spray method to form a dense coating layer as a cost efficient method to improve high temperature corrosion resistance of the steels. Coated steel samples were tested for corrosion resistant properties at 550°C to 750°C in simulated flue gas conditions and in air. Crystallinity of the nanoparticles were examined with X-Ray Diffration (XRD). Surface and structural morphology of the coated steels were studied using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS). Improved corrosion resistance including retarded oxidation on surface of steel and lower oxidation penetration were observed for the TiC and TiB₂ coated steel substrates both under oxygen and sulfur attack.

The synthesis of titanium carbide nanofibers using a carbothermal process was investigated. Titanium oxide nanofibers were fabricated by electrospinning method from a sol containing a mineral acid for peptizing, followed by calcination at temperatures ranging from 250 - 800°C. Carbon was coated on the nanofibers by cracking of propylene (C₃H₆). Carbon coated titania fibers were reacted at 1400°C and 1500°C for 1 or 2 hours under argon atmosphere to form titanium carbide nanofibers. The titanium carbide nanofibers were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), and X-Ray diffraction (XRD). The average diameter of the TiO₂ fibers and TiC fibers averaged at 150 nm and 350nm respectively. Due to the high theoretical conductivity, chemically and electrochemically stability in strong acid and high potential environment, the synthesized TiC fibers show great potential for fuel cell applications such as support of bipolar plates and catalyst support. The electrical and thermal properties of the synthesized TiC will be reported and compared with the theoretical values.

Scattering effects of crystalline mesoporous titanium dioxide (TiO₂) scatters have been widely utilized in various optoelectronic devices. Various structures of mesoporous TiO₂ scatters would induce complex interactions between photons and scatters to show specific optical properties especially in the cases of more than one kind of scatters embedded in the scattering medium. At the longer wavelength regime, the multiple-scattering effects become important and change with the inter-particle spacing and scatter size. Until now, with different analysis techniques, to clarify the scattering effect of various scatters is always a work of necessity. In previous researches, most analyses of the scattering effects are based on Mie scattering model which is limited to a single spherical scatter with single-scattering effect. In this work, we utilize a fiber based low-coherence interferometry to analyze the samples of mesoporous TiO₂ scatters. The setup is based on Michelson interferometer with a broadband low-coherence super-luminance diode source (853nm). This optical system provides advantages of high axial resolution, and deeper probing range. Furthermore, the depth signals have been fitted by the extend Huygens-Fresnel model with further consideration of multiple-scattering effects. Here, a series of mesoporous TiO₂ scatters with average sizes of 20, 150, 300, 500 nm have been prepared on the glass substrate with sol-gel process. These samples were measured by UV-Vis spectrophotometer at the wavelength regime of 300-1100 nm. For the mesoporous TiO₂ scatters size of 20 nm, the results of normalized diffused-reflectance spectrum show the highest reflectance peak at 400 nm and decrease to 40% at 800 nm. By increasing the scatter size up to 300 nm, the resul show the highest reflectance peak at 450nm and decrease to 70% at 800 nm. In comparison, the red-shift of reflectance peak is resulted from the Mie scattering effect. On the other hand, for the bigger scatters, higher reflectance intensities at the longer wavelength regime are shown and may be caused by enhancement of multiple-scattering effects. Simultaneously, the multiple-scattering effects had also been quantified and confirmed for each samples by low-coherence interferometry. After fitting algorithm based on EHF model, the percentage sharing of single- and multiple-scattering effects had been studied. By increasing the sizes of mesoporous TiO₂ scatters from 20, 150, 300, to 500 nm, the percentages of multiple-scattering effects are enhanced from 5%, 10%, to 40%, and then decreased to 20%. The scattering coefficient could also be extracted from 13.5±0.6, 16.5±0.6, to 20.2±0.6 mm^-1, and then decreased to 18.5±0.6 mm^-1. In this work, the size-dependent multiple-scattering effects had been first analyzed by low-coherence interferometry. The mesoporous TiO₂ scatters with size of 300 nm have the largest reflectance intensity due to the larger contribution percentage of multiple-scattering effects.

Titanium dioxide (TiO₂) has been attracting extensive attention because of its importance in several technological applications including catalysis, nano-electronic devices, and sensors. A well-understood and atomically flat TiO₂ surface is prerequisite for investigating the atomistic details of catalysis on this surface, as well as for the growth of ultra-thin rutile transition metal oxide heterostructures. Among the low-index surfaces, rutile TiO₂(001) is less intensively studied because of its morphological instability due to its high surface energy; the standard approach of sputtering and annealing usually introduces faceting. Here we demonstrate a facile method to create atomically flat, non-faceted TiO₂(001) surfaces. A step-terrace surface morphology is achieved through buffered hydrofluoric acid (BHF) etching and subsequent annealing at 810 °C in air or in flowing O₂.Low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) show sharp (1x1) diffraction spots accompanied by predominant c(7√2 X 5√2)R45⁰ reconstruction with two perpendicular domains for the as-prepared substrate. The reconstructed surface persists even after O₂ plasma treatment at 600 °C, which is higher than the traditional substrate temperature used for epitaxial films growth. Periodic stripes, corresponding to c(7√2 X 5√2)R45⁰ reconstruction, are observed by scanning tunneling microscopy (STM). Furthermore, a homoepitaxial TiO₂(001) film of 6 monolayers is successfully grown via pulsed laser deposition (PLD). These results show that relatively stable and atomically flat TiO₂(001) surfaces can be obtained, which could serve as an ideal template for subsequent rutile transition metal oxide heterostructure growth and the investigation of the catalytic properties of TiO₂(001).

Titanium Nitride is known for its corrosion resistance, wear-resistance, high hardness, and extremely high melting temperatures, making it an optimal material system for applications at elevated temperatures. However, most high temperature applications also occur in oxygen rich environments, favoring the formation of oxide scales which lack the desirable properties of the base material. While there has been extensive experimental and computational work done to characterize the initial steps of oxidation little has been done to characterize the nature of fully formed oxide scales on TiN. This work uses Density Functional Theory (DFT) calculations to analyze the energetics and stability of TiN/TiO2 interfaces. Specifically, the work of adhesion and migration energy barriers for oxygen diffusion through the interface are calculated for multiple possible interface geometries and terminations.

Protonated titanate nanotubes were chosen as precursor in hydrogenation process. Owing to the high capacity for molecular hydrogen storage of naotubes, TiO₂ nanotubes can be hydrogenated through thermal treatment under N₂ and H₂ mixed flow at lower temperature. A series of hydrogenated TiO₂ naotubes and nanobelts were synthesized and characterized by XRD, UV-Vis, TEM, EPR and XPS. The results showed that the hydrogenated TiO₂ nanotubes possess tiny and uniform diameters of 8-10 nm and the walls thickness of 2-3 nm, and were mainly anatase. The anatase TiO₂ nanotubes transformed to TiO₂-B nanobelts at the higher hydrothermal temperature. The light absorption of hydrogenated TiO₂ nanotubes was expanded to visible light. However, air-TiO₂ and hydrogenated TiO₂ nanobelts only absorbed ultraviolet light. According to XPS and EPR analysis, hydrogenated TiO₂ nanotubes displayed stable core-shell structures, in which the surface was mainly stoichiometric TiO₂ and the core was non-stoichiometric TiO₂ with Ti³⁺ and oxygen vacancies. The adsorption and photocatalytic performance were evaluated by removal rate of phenol. Based on pseudo-first order kinetic model, the degradation rate constant was obtained with the regression analysis. The highest degradation rate constant of hydrogenated TiO₂ nanotubes was 5.2 times higher than air-TiO₂. In comparison, the degradation rate constants of hydrogenated TiO₂ nanobelts were much lower than air-TiO₂. The results showed that the precursor with nanotubes structure can be hydrogenated easily in lower temperature comparing with nanobelts, resulting in the photocatalytic activity of hydrogenated TiO₂ nanotubes enhanced drastically.

Mesoporous titanium dioxide (titania) thin films with 2D Hexagonal Close Packed (HCP) cylindrical nanopores have been synthesized by the evaporation-induced self-assembly (EISA) technique with Pluronic surfactant F127 as template. To provide orthogonal alignment, surface modification of substrates with crosslinked F127 has been used. GISAXS studies show that aging at 4 °C is indeed necessary for ordered mesostructure development and that aging at this temperature helps to provide orthogonal orientation of the cylindrical nanopores which forms directly by a disorder-order transition. F127 provides pores with 8-9 nm diameter, which is precisely the structure expected to provide short carrier diffusion length and high hole conductivity required for efficient bulk heterojunction solar cells. Anatase titania is a n-type semiconductor with the valence and conduction bands located at +3.1 and -0.1 eV relative to the Fermi level, thus giving a band gap of +3.2 eV. Therefore, titania readily absorbs UV light with a wavelength below 387 nm. Because of this, these titania films can be used as a window layer with a p-type semiconductor incorporated into the pores and at the top surface of the device to synthesize a photovoltaic cell. The pores provide opportunities to increase the surface area for contact between the two semiconductors, to align organic semiconductors, and to induce quantum confinement effects. Using these films as window layer and CdTe as absorber, a power conversion efficiency of 5.53% has been obtained which is 3 times higher than that obtained using planar titania films and is attributed to high surface area of nanoporous TiO₂ and reduced recombination at the interface. The charge carrier concentration is higher in case of nanoporous TiO₂ as compared to planar titania. These films are also used as negative electrodes in Li-ion batteries where their high surface area and thin walls allow for rapid Li ion diffusion and efficient insertion without capacity fading. Capacity as high as 300 mAh/g has been achieved after 50 cycles which is pretty close to theoretical capacity of titania (330 mAh/g). These films has been used to develop nafion-titania composite membranes which can be used both as separator and electrolyte in solid state batteries. Interfacial modification between nafion and titania has been achieved using silane chemistry and been studied using FTIR and impedance spectroscopy.

A novel methodology has been recently developed for the fabrication of structured and functionalized composite polymeric micro- and nano-spheres. This unique approach utilizes a thermally-induced fluid instability occurring at the interfaces within multimaterial fibers by which the internal structuring and composition of the fibers are imparted upon the generated spheres. Applications of such spheres span a variety of industries and fields including pharmaceutical therapies, imaging and biomedical diagnostics, paints and coatings, and cosmetics. Moreover, the process can be applied to a wide range of thermally compatible materials and additives, potentially producing spheres with previously unattainable compositions, structuring, and geometric allocation of functionalities. We present here one such realization in the form of heterogeneous composite microspheres composed of a polycycloolefin matrix integrated with a random distribution of TiO₂ nanoparticles. In homogeneous microspheres of the same size (tens to hundreds of microns) composed of a dielectric, one expects, upon irradiation, that the microsphere exhibits the characteristic Mie scattering. However, due to the large index contrast, high transparency, and randomized, discrete nature of the nanoparticle distribution in the polymer matrix, irradiation of the composite microspheres produced speckle patterns indicative of diffusive optical scattering. This behavior was observed by tightly focusing a 632-nm-wavelength laser beam onto a solitary composite microsphere suspended in-fiber and imaging the scattered field via transmission microscopy. It is readily determined from the speckle pattern that the multiply-scattered waves exiting the microsphere vary randomly in amplitude and phase independent of the incident polarization, thus exhibiting behavior beyond the known Mie scattering typical of a homogenous microsphere.

Bulk doping and use of different substrates for deposition can induce significant variation in the surface electronic structures of the film. The variation in the surface defect structure is expected due to mismatch between the lattice parameters of titanium oxide and substrate under deposition. Also, in case of doped titanium oxide, the mismatch came due to the differences in the ionic radii, electro-negativity and valence states between dopant and the host ions that can induce different bulk as well as surface active defects. Titanium oxide is very well known photo-catalyst thereby expected to be an excellent gas sensor as both are exclusively surface active phenomena.^[1,2] We attempted here to enhance the H₂S sensitivity by using titanium oxide films on engineering the surface defect structure.

The titanium oxide thin films were prepared using Pulsed Laser Deposition (PLD) technique. The target pellet used for the ablation was defect-free phase-pure stoichiometric TiO₂. Recently, we observed that mere change in ablation energy of laser pulse from 200 mJ to 500 mJ can induce substantial variation in surface defect chemistry of the film.^[3] The different substrates such as Si, LAO, STO, c-Al₂O₃, polycrystalline Al₂O₃, soda-lime glass etc. were used for the deposition to get variation in the defect-richness in the film surface. Furthermore, thin films of titanium oxide doped by different elements such as Cu, Zn, Al, Na, Fe etc. were deposited on LAO substrate and have attempted towards enhancement in H₂S gas sensitivity by modification in the film surface activity. The characterization techniques such as XPS, UPS, synchrotron XAS, EXAFS, SPM, GIXRD etc. were employed to reveal the surface electronic structure responsible for H₂S gas response. Gold pads of 1mm apart were made on these films using thermal evaporation technique which were used to make ohmic contacts required in testing H₂S gas response using a laboratory made gas sensing set-up.

It was observed that films deposited on Si and LAO were found to be highly sensitive to H₂S than any other gas at relatively lower sensor operating temperature (100^oC) with faster response (few seconds) and quicker recovery (~4 min) at 50 ppm of H₂S. Also, Cu-doped titanium oxide film exhibited extra-ordinary H₂S response than any other doped titanium oxide systems. The H₂S sensitivity observed is the best in the literature for pure titanium oxide film on Si (001) and Cu-doped titanium oxide film on LAO and it is SR % ~ 1,12,000 and SR % ~ 1,36,000 respectively at 50 ppm of H₂S. The same films were found be considerably sensitive to H₂S at sub-ppm level. XPS, UPS and synchrotron XAS measurements show that film having extra-ordinary H₂S response has richness in surface defects. The valence states of titanium in film were +4 and 0. The film composition concluded to be Ti/TiO_x and AFM imaging showed increasing surface roughness with vertically aligned short nano-wires on highly H₂S sensitive films. It is inferred that the enhanced H₂S sensitivity is due to the excessive over layers of chemi-sorbed oxygen on the film surface. These data will be discussed and presented.
One of the authors TCJ would like to acknowledge DST, Govt. Of India for INSPIRE Faculty Award.

References:
(1) Chem. Rev., 2007, 107, 2891;
(2) J. Phys. Chem. C 2008, 112, 14595;
(3) RSC Adv., 2015, 5, 93081

We demonstrate that is possible to finely tune the electrical and optical properties of TiO₂ and Ta-doped TiO₂ (TaTO) thin films by engineering their structure and morphology at the nanoscale. Donor doped TiO₂-based Transparent Conducting Oxides (TCO) [1] are receiving increasing attention because of the particular properties of TiO₂, such as photoactivity or chemical stability in reducing atmospheres; the use of TiO₂ as an electron selective layer or photoanode in electrochemical or perovskite solar cells motivates the goal of controlling functional properties in these TiO₂-based materials.
TiO₂ and TaTO films were deposited at room T via pulsed laser deposition, followed by thermal annealing in a reducing atmosphere, which is necessary to obtain conducting polycrystalline anatase. The best functional properties (resistivity ~5×10^-4 Ωcm, mean transmittance in the visible >80% for a 150 nm film) were obtained when depositing at 1 Pa O₂ followed by annealing in vacuum at 550°C [2]. X-ray diffraction, Raman spectroscopy and positron annihilation spectroscopy have been employed to examine the microstructure and defect chemistry in terms of deposition and annealing atmospheres, thus opening the possibility to finely tune the functional properties (e.g. carrier density) for both doped and undoped TiO₂.
We also demonstrate the possibility to crystallize the films exploiting an ultra-fast thermal treatment at ambient pressure in N₂, yielding virtually the same conductivity as a conventional anneal carried out in vacuum (for doped and undoped TiO₂). By monitoring the crystallization threshold via in-situ electrical measurements we were able to investigate the effects of O incorporation on the electrical properties. This process, which is industrially scalable, reduces the annealing time from about 3 hours to a few minutes and allows the electrical properties to be uncoupled from the influence of the annealing environment.
Finally, by increasing the O₂ pressure during the deposition process we are able to obtain hierarchically nanostructured mesoporous layers, which could effectively act as large surface area photoanodes with tunable functional properties [3]. This opens the way to the realization of an all-TiO₂ transparent electrode/selective layer/photoanode with a reduced number of interfaces and thus of recombination centers, which could be beneficial for electron transport in real devices.

1. Y. Furubayashi et al.. Appl. Phys. Lett. 86, 252101 (2005)
2. P. Mazzolini et al., J. Phys. Chem C 119, 6988 (2015)
3. L. Passoni et al., ACS Nano 7, 10023 (2013)

Interest in the integration of TiO₂ within microfluidic devices has grown considerably since the first demonstration of a TiO₂-based photocatalytic microreactor nearly a decade ago. Examples of device applications reported since then include those for water purification, water-splitting, and photochemistry. The photocatalytic properties of TiO₂ have also been exploited as a novel means for achieving microfluid control, and site-specific nanoparticle synthesis in situ. In many of these applications, miniaturization affords opportunity for significantly enhancing performance relative to conventional bulk reactors, due to increased surface area, reduced diffusion length, and greater uniformity of irradiation. However, low volumetric throughput remains a critical limitation in many applications, as does the difficulty associated with integrating TiO₂ uniformly within complex microfluidic device geometries.

As we have reported earlier, growth of nanoporous TiO₂ (NPT) within Ti-based microfluidic devices may provide a new means for addressing these issues. In this approach, NPT is grown in situ, directly from the Ti channel surfaces, using a peroxide-based oxidation process. The advantages of this approach include: a) conformal coverage of complex geometries that would be difficult, if not impossible to coat uniformly using conventional sol-gel or thin-film deposition routes; b) reticulated, open-framework porosity that provides higher surface area than dense films, and greater fluidic accessibility than nanoparticle-based films; and c) potential for fabrication of robust, large-area photocatalytic devices that can provide volumetric throughputs approaching those required for commercial feasibility in many applications. However, while this approach shows promise, our current reliance upon NPT growth conditions originally developed for other applications suggests potential for improvement via optimization of growth conditions specifically for use in photocatalytic microreactors.

Herein, we report our recent efforts to explore this potential through use of a Taguchi-based experimental design. Given the relatively large parameter space involved, the use of design of experiments techniques is needed to ensure efficient optimization. Using an orthogonal array composed of three parameters (oxidation time, temperature, and concentration) with three levels, an experimental design of nine trials was developed, with the reaction rate constant for methylene blue degradation serving as the quality reporter. Our studies helped identify the growth parameters that most strongly influence photocatalytic performance, as well as the optimal combination thereof. Furthermore, our studies also showed that optimally-grown NPT provided performance approaching that of Degussa P25 TiO₂ nanoparticle films with comparable surface area. Collectively, these results represent important steps towards our goal of developing robust, high-performance photocatalytic microreactors.

Synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications.
It is the nanotubular TiO₂ that has received the highest attention after porous Al₂O₃, motivated by its range of applications, including photo-catalysis, solar cells and biomedical uses. However, the TiO₂ nanotube potential for a range of advanced devices, in particular when considering all possible shapes and geometries, has not at all been exploited. One of the major issues to extend the functional range of nanotubes is to coat homogenously tube interiors by a secondary material, potentially until the complete tube filling.
The presentation will focus in detail on the filling, decoration or coating of the nanotubes by various means. The deposited materials influence strongly optical, electrical and mechanical properties of nanotubes. Experimental details and some very recent results will be presented and discussed.

Dye sensitized solar cell (DSSC) has attracted considerable attention and is being considered as a promising candidate for environment friendly photovoltaic technologies. A typical DSSC utilizes fluorine doped tin oxide (FTO) glass as the transparent conductive substrate. However if the heavy, fragile glass could be replaced by plastic conductive substrate, it enables to meet the requirements of light-weight and flexibility. The flexible DSSC modules can be tailored for specific applications while satisfying the special shape and surfaces needs. In addition, the industrial large-scale production of flexible DSSC module will become possible by roll-to-roll processing, which will result in low cost module production. As the plastic conductive substrate (ITO/PEN) cannot withstand high temperature process, herein, a novel binder-free TiO₂ paste is developed that enables the fabrication of flexible TiO₂ photo-anode at room temperature. Moreover, in order to realize the fully printable flexible module, low-temperature, non-Pt printable materials for flexible counter electrodes were employed. Cold isostatic press (CIP) technique was utilized to not only improve the film quality but also increase the TiO₂ film thickness. We will present the DSSC evolution from laboratory cell to glass module and finally to flexible module. The efficiency loss from small cell to photovoltaic module was studied and analyzed. Based on the employment of binder-free TiO₂ paste, flexible DSSC module with size of 100 mm × 100 mm was prepared and optimized. With the treatment of cold isostatic press (CIP) technique and augment of film thickness, conversion efficiency of 3.27% was achieved for flexible DSSC module. The practical application for mobile phone charging under indoor light was also demonstrated by flexible DSSC module (100 mm × 100 mm) with W type series connection

Excitonic solar cells including dye-sensitized solar cells, quantum dot-sensitized solar cells, bulk heterojunction organic photovoltaics, are built upon nanostructures of various functional materials. Nanostructures are essential for the high power conversion efficiency, for example, in dye-sensitized solar cells and quantum dot-sensitized solar cells, mesoporous TiO₂ or ZnO photoanodes made of nanoparticles offer large specific surface area for loading a large amount of dyes or quantum dots so as to capture a sufficient fraction of photons. However, the large surface area of such nanostructures also provide easy pathways for charge recombination, and surface defects and connections between adjacent nanoparticles may retard effective charge injection and charge transport, leading to a loss of power conversion efficiency. Surface facets and chemistry may also affect the conformal coating and adhesion of dye molecules and polymer layers. In this presentation, I will present and discuss our recent work on the design and control of (1) nanostructures and surface chemistry of photoanodes for dye- or quantum dot-sensitized solar cells and (2) nonstoichiometric composition, doping, and allignment of quantum dots in quantum dot-sensitized solar cells, (3) introduction of semiconductor passivation layers to minimize the charge recombination, and (4) incorporation of plasmonic nanocrystals to improve the light absorption. Our research has demonstrated that the power conversion efficiency can be significantly enhanced with excellent device stability when both nanostructures and interface chemistry are properly controlled.

TiO2 as an important semiconducting material play a key role in a variety of photoelectrochemical (PEC) energy conversion systems including PEC water splitting and PEC solar cells. In the past few years, we have been focusing the following two aspects of TiO₂ based materials; 1) exfoliation of layered titanate to prepare single TiO₂ nanosheets and their re-assembled composites for photocatalysis and photoanodes in dye (quantum dots) sensitised solar cells; and 2) the design of nanosized TiO₂ as compact layers or scaffold layers for new generation perovskite solar cells, which all exhibited excellent performance for solar fuels and solar electricity generation.

Titanium dioxide (TiO₂) is one very important metal oxide semiconductor having a wide range of applications such as solar cells, photocatalysis, lithium storage, and sensor. The interaction between the surface/interface of TiO₂ and ions/molecules is an essential and basic process controlling the performances. Modulating the surface-interface structures of TiO₂ to steer such interaction represents a long-lasting active research topic. In this talk, two aspects in modulating surface-interface structures of TiO₂ for photocatalytic solar energy conversion and lithium energy storage will be introduced. One is tailoring the exposure of facets of TiO₂ crystals to decrease the barrier of electron transport across the interface between TiO₂ and metal contact. It was found that the electric conductivity along the [001] direction is one order of magnitude higher than that along the [010] direction. On the basis of this finding, an efficient anode containing TiO₂(001)-W interfaces for lithium storage was constructed. The other is constructing crystalline-amorphous core-shell structure with Ti³⁺ contained interface region. With the assist of such core-shell structure, the photocatalytic hydrogen evolution of rutile TiO₂ was activated. The reason for this activation is attributed to the promoted charge separation of photogenerated electrons and holes in the bulk as a result of hole storing capability of the interface region and regulated abundant major oxidizing and minor reducing reaction sites on the photocatalyst surface. These results are representative of using crystal facet engineering and amorphization to tailor surface-interface structures of rutile TiO₂. It is anticipated that such structure design and construction could be extendable to other metal oxides based functional materials.

Titanium dioxide is the most studied photocatalytic material because of its abundance, non-toxicity, stability and high activity. Nevertheless, electron-hole recombination is the biggest problem limiting its efficiency. Several approaches have been previously explored to improve charge separation, including structural modification, doping, or by formation of heterostructures. Here, we clearly show the power of anisotropy in boosting the photocatalytic activity; specifically that H₂ production in uniform, 1-dimensional brookite nanorods is highly enhanced by engineering their length. By using complimentary characterization techniques to probe excited electron and holes, we demonstrate that the high rates are due to their anisotropic structure, which favors electron-hole separation and efficient carrier utilization. Quantum efficiency for hydrogen production from ethanol, glycerol, and glucose as high as 65, 35, and 6%, respectively, demonstrate the promise and generality of this approach for improving the photoactivity of semiconducting nanostructures.

Titanium dioxide is one of the most important semiconductor-based photocatalysts and had been widely studied for environmental applications. Most previous studies focused on the photocatalytic oxidation of various environmental pollutants, while the studies on the photocatalytic reduction of environmental pollutants were relatively limited. For the photocatalytic reduction of environmental pollutants, they react with photogenerated electrons. Thus, in order to enhance the photocatalytic reduction activity of TiO₂, proper material design and modification are needed to efficiently separate photogenerated electron/hole pairs and achieve their oriented migrations to enrich the presence of photogenerated electrons on the surface of TiO₂.

Crystal facet engineering is becoming an important approach to obtain photocatalysts with higher activity. It was found that the photocatalytic activity of anatase TiO₂ could be modulated by incorporating different exposed crystal facets. Because there are slight surface energy differences of the valence and conduction bands between different facets, photogenerated electrons and holes could be driven to different facets. So the separation of photogenerated electrons and holes could be more effective and oriented in space. Noble metal modification is another effective way to initiate the oriented migration of photogenerated electrons due to their higher work functions compared with that of TiO₂. Noble metal nanoparticles deposited on the surface of TiO₂ could serve as trapping centers for photogenerated electrons and subsequently as active centers for photocatalytic reactions. Thus, it would be desirable to selectively deposit noble metal nanoparticles onto {101} facets of anatase TiO₂ nanocrystals with co-exposed {001} and {101} facets, which could combine the advantages from both the crystal facet effect and noble metal modification to initiate the oriented migration of photogenerated electrons and further enrich their presence onto TiO₂ surface for the enhanced photocatalytic reduction activity.

Herein, we reported a newly developed process to selectively deposit silver nanoparticles onto {101} facets of anatase TiO₂ nanocrystals with co-exposed {001}/{101} facets through a modified photo-deposition process. The synergistic effect of crystal facet engineering and the selective deposition of silver nanoparticles on {101} facets largely enhanced the separation of photogenerated carriers and enriched the presence of photogenerated electrons onto the surface of TiO₂ to a net negative-charged surface, which largely enhanced its photocatalytic reduction performance. These TiO₂ nanocrystal with co-exposed {001}/{101} facets and silver modification on {101} facets demonstrated a superior photocatalytic reduction activity on aqueous nitrate under a simulated solar illumination. The removal ratio of aqueous nitrate reached ~ 95%, and most of the reduction product was the desirable N₂ with the selectivity over 90%.

A new synthetic approach to produce thin-walled titania nanotubes, with wall thickness less than the charge carrier diffusion length, is presented. The calculated crystallite size for the thin-walled nanotubes is as small as 7 nm, leading to the creation of a large number of under coordinated Ti atoms on the grain surface and consequently high dark current. The thin-walled nanotubes showed almost double the photocurrent of the thick-walled nanotubes upon their use in water splitting arrangement. The IPCE and electrochemical impedance measurements confirmed the superiority of the thin-walled nanotubes over their thick-walled counterparts. The Mott–Schottky analysis
revealed a flat band potential of 0.7 V versus SCE with a charge carrier density of 5 x 10¹⁸ cm³. Our work confirmed the
importance of using ultra thin-walled titania nanotubes as photoanodes for efficient solar water splitting.

Strontium titanate is one of the best n-type oxide thermoelectrics with ZT (thermoelectric figure of merit) of approximately 0.38 at high temperature (~1000K). Performance is limited by modest electrical conductivity and high thermal conductivity. Incorporating graphene into the ceramic microstructure offers a route to address both the deficiencies of strontium titanate. Composites of Lanthanum doped Strontium Titanate (LSTO) with graphene have been prepared with the aim of developing materials with a wide operating temperature range, down to room temperature. The base oxides were prepared by both chemical and mixed oxide routes. The LSTO composites incorporated one wt% or less of graphene and were sintered under an argon/hydrogen atmosphere. The resulting materials were highly reduced and exhibited a multiphase structure with nano-sized grains. The thermal conductivity of the nanocomposites generally decreased with increasing concentration of graphene, whilst the incorporation of graphene significantly improved both the electrical conductivity and power factor. These characteristics, together with a moderate Seebeck coefficient, meant that a high power factor of ~2500 μWm^-1K^-2 was reached at room temperature at a loading of 0.6 wt% graphene. The highest thermoelectric figure of merit (ZT) was also achieved when 0.6 wt% graphene was added (ZT = 0.42 at room temperature and 0.36 at 750 °C), with >281% enhancement to that of pure LSTO. A preliminary 7 couple device was produced using bismuth strontium cobalt oxide (BSCO)/graphene-LSTO pucks which confirmed thermoelectric properties with a device Seebeck coefficient of ~1500 μV/K and an open voltage of 600 mV when the hot side was raised to 420 °C.

Many efforts have been devoted to improve the photocatalytic performances of titanium dioxide (TiO₂) by doping it with non-metal elements [1]. In comparison, few studies have been conducted on the doping of TiO₂ for solar cell applications. However, the charge transport in the TiO₂ photoanode is a critical aspect to optimize the dye-sensitized solar cells (DSSCs) efficiently. By the TiO₂ doping, the number of charge carrier increases which leads to a higher conductivity of the film and finally a better charge transport. Indeed, as the crystalline structure and the microstructure, the conductivity of the TiO₂ thin films is a key parameter to develop DSSCs.
In a previous work [2], we shown that nanostructured TiO₂ thin films can be synthesized by reactive magnetron sputtering combined with Glancing Angle Deposition (GLAD). GLAD enables the growth of columnar thin films with unique nanostructures and can be extremely porous (up to 100 m²/g for TiO₂ [3]). The tuning of the experimental conditions has allows a fine control of the chemical composition, the crystalline structure and the microstructure of the films analyzed by XPS, XRD and SEM, respectively. Pure anatase films which is the preferred polymorph as electron acceptor in DSSCs [4] has been obtain with well define columnar structure.
p-type doping was investigated by introducting nitrogen in the discharge, respectively. The optical (transmittance) and electrical (type, density and mobility of charge carrier) properties of doped thin films were characterized by spectrophotometry and Hall Effect, respectively. Preliminary data reveal that the doping element concentration and the substitutional or interstitial incorporation can be controlled by the experimental conditions. In order to increase the substitutional nitrogen part in the film, the oxygen flux in the discharge has to be low, meaning before the poisoning of the target occurs (for 10 % of O₂ flow). Moreover, the nitrogen flux has to be high (15%), ideally after the transition Ti/TiN. Nevertheless, the transmittance rapidly decreases (T = 5% for 10% of N) with the nitrogen content in the film.

[1] M. V. Dozzi et al, J. Photochem. Photobiol. C Photochem. Rev., vol. 14, pp. 13–28, Mar. 2013.
[2] J. Dervaux et al., Vacuum, vol. 114, pp. 213–220, Nov. 2014.
[3] D. W. Flaherty et al., J. Phys. Chem. C, vol. 111, pp. 4765–4773, 2007.
[4] G. Li et al., Dalton Transactions 45 (2009) 10078-10085.

Since the seminal Nature paper by O’Regan and Grätzel in 1991 [1] the highest efficiencies of dye-sensitized solar cells (DSC) have been achieved using the iodide/tri-iodide redox system. A disadvantage of this mediator is the large internal losses caused by the fact that it is a two-electron redox couple. In 2010 we made a breakthrough by using one-electron transfer redox systems such as cobalt-complexes, in combination with a new generation of organic dyes, which efficiently prevents recombination losses [2]. This discovery was quickly embraced by Grätzel and co-workers, and the new world record for DSC is at present 13.0% by using a Co-complex redox couple and a porphyrin [3]. Our focus now is to develop high efficiency DSC utilizing different colors such as blue, green, yellow and red aiming for aesthetically attractive applications in for example building integration.
Besides liquid DSC we develop solid-state DSC (ssDSC). In one configuration, we prepare a conducting polymer by in situ photopolymerization of the monomers in a photoelectrochemical cell. ssDSCs based on an organic dye, D35, gives together with PEDOT or PEDOP as hole transporting material (HTM) efficiencies up to 7%. Recently [4], we showed that copper phenanthroline complexes in the solid phase can act as an efficient HTM. We prepared ssDSCs with the organic dye LEG4 and copper(I/II)-phenantroline as redox system and achieved power conversion efficiencies of more than 8%, with open-circuit potentials of more than 1.0 V.
The phenomenal breakthrough of the so called perovskite solar cells (PSC) originates from the ideas of replacing the dye layer adsorbed on the mesoporous oxide surface with an ultrathin inorganic perovskite layer and replacing the liquid electrolyte with a solid-state hole conductor [5, 6]. We will report on our latest work on optimizing the solar cell efficiency that at present is above 20% in our laboratories. We have developed new hole conductor materials that reach efficiencies similar to the conventional hole conductor spiro-OMeTAD but with the advantage of being more easily synthesized.

References
[1] B. O’Regan, M. Grätzel, Nature, 353 (1991) 737.
[2] Feldt et al., J. Am. Chem. Soc., 132 (2010) 16714.
[3] Mathew et al. Nature Chemistry 6, 242–247 (2014)
[4] Freitag et al., Energy & Envir. Sci., DOI: 10.1039/C5EE1204J.
[5] Kim et al. Sci Rep-Uk, 2 (2012) 591.
[6] Lee et al. Science, 338 (2012) 643.

In many important energy systems, such as energy conversion and energy storage, advanced materials and fundamental interfacial phenomena play crucial roles in device performance and functionality due to the complexity of the material architecture, chemistry, and interactions among the constituents within. Complexes of Ti−O have been the most interesting model compounds in the demonstration of energy research and applications, such as in the first artificial photosynthesis or water splitting experiment in the 1970s, dye-moleculesensitized solar cells, quantum-dot-sensitized solar cells, and photocatalysis for purification of air- and waterborne pollutants. Nowadays, functionalized systems are becoming more and more complex to achieve emerging properties that cannot be counted for from individual constituents.

Understanding, and ultimately controlling, charge propagation in the interfaces of devices with heterostructure architectures calls for powerful in situ/operando characterization tools (e.g., optical and X-ray spectroscopy and theoretical simulations) that combined provide unique information on the electronic structure, band gap, and band levels. This together with the ability to tailor the intrinsic properties of functional semiconductors at reduced scale and manipulate the morphological properties not only holds great promise for advancement in the field of alternative energy science and technology but also offers great insight into the fundamental physics and chemistry at the atomistic/molecular level. We will present some of the most recent state-of-the-art advancement in the research area of materials science and alternative energy with a focus on the electronic and optical properties of nanostructured TiO₂.

Small metal oxide nanoparticles, for example of TiO₂, of a couple of nanometers in size find use in many applications aimed at harvesting sun light, including dye-synthesized solar cells and photocatalytic water splitting. While large nanoparticles are expected to behave essentially like the bulk, bar potential shifts in the optical gap due to quantum confinement, the surface to volume ratio of these smaller nanoparticles is such that every atom is near the surface. As a result the atoms in these nanoparticles are less constrained than they would be in the bulk or even on bulk surfaces. Using (Time-Dependent) Density Functional Theory calculations on nanoparticles of a range of materials, besides TiO₂ [1,2] also ZnS [3,4] and MgO [5], we explore the effect of this atomic scale flexibility on the strength of (self-)trapping of both excitons and charge carriers in such small nanoparticles and show that they are much stronger trapped than in the bulk. We will furthermore show there is likely a landscape of different trapped states, varying in degree of localisation, and that the trapping of an excited state or charge carrier can be imagined as it trickling down a cascade, becoming ever more localised and trapped at every step. We will finish our contribution with a discussion on how we envisage this influences the use of TiO₂ nanoparticles when converting sun light into energy.

[1] Berardo, E. et. al. J. Chem. Theo. Chem., 10, 5538, 2014.
[2] Berardo, E.; Zwijnenburg, M.A. J. Phys. Chem. C., 119, 13384, 2015.
[3] Zwijnenburg, M.A. Nanoscale, 4, 3711, 2012
[4] Zwijnenburg, M.A. Phys. Chem. Chem. Phys., 15, 11119, 2013.
[5] Wobbe, M.C.C.; Zwijnenburg, M.A. manuscript in preparation

Organic sensitizers are a key component of dye-sensitized solar cells (DSCs), which dramatically influence the device power conversion efficiency. Understanding the structure-performance relationship for these dyes is crucial for rationally designing high efficiency DSC devices. The exploration of organic building blocks capable of accessing the NIR spectral region is crucial for practically improving DSCs. Given the plethora of available options, judicious selection of organic building blocks via physical organic principles is critical to efficiently evaluating synthetic targets. Proaromatic building blocks promote charge transfer upon light absorption through introduction of aromaticity in the excited-state. This concept offers a unique platform for accessing relatively low molecular weight small molecules with substantial absorption breadths. Computational insights into these proaromatic materials will be discussed. Incorporation of proaromatic building blocks into donor, bridge and acceptor building blocks for D-π-A dyes designed for DSCs will each be addressed. The performance of these materials in devices, which demonstrate IPCE spectrum reaching 800 nm from concise organic dyes available in as few as 4 synthetic steps, will be discussed.

Photoelectrochemical (PEC) splitting of water into hydrogen and oxygen by the direct use of sunlight is an ideal, renewable method of hydrogen production.
TiO₂ is the most widely investigated material for PEC applications. Among the different designs, the one-dimensional ordered nanotube architecture offers an excellent electrical channel for charge transfer so that effective charge separation of the photoinduced electron-holes pairs can be achieved.
Besides the photoelectrode, the PEC cell structure is also an important issue. The majority of PEC cell studies utilize aqueous electrolyte as both the electrolyte and the photoanode reactants. Only few studies have attempted to separate the two half reactions compartments with a polymeric conducting membrane which acts as the electrolyte. In most of these cases acidic and alkaline solutions are used as anode reactants, but little work has been done with gaseous feedstock for the anode. Solid-state electrolytes in conjunction with gaseous reactants have potential advantages over liquid ones such as operation at higher temperatures and pressures (for improving the electrode kinetics), direct production of compressed H₂, compact and robust design besides the separation of the products at the electrode compartments.
A novel designed PEC cell is proposed here and is successfully utilized for H₂ production. The core of the cell is a membrane electrode assembly, utilizing a polymeric proton conductor, which serves both as a compact reactor for water splitting and as gas separator. The design was inspired by PEM electrolysis technology and was modified appropriately for allowing illumination. The proposed design is also equipped with a third compartment which enables, for the first time, the use of a hydrogen reference electrode. Another innovative element of this study concerns the photoanode which was fabricated by anodization of Ti-felt for the development of TiO₂ nanotube arrays.
The performance of the PEC cell was evaluated for water splitting with two different carrier gases He and Air, by supplying the photoanode with a gaseous stream made by bubbling He or Air through a H₂O-containing saturator. Even though the performance is lower with Air, this operation is of great interest since it mimics conditions where water molecules can be captured from ambient air.
Furthemore, it was found that the addition of ethanol in the H₂O/He stream can affect significantly the performance by (a) leading to an increase in the photocurrent and (b) developing an electromotive force which is sufficiently high enough to enable bias-free PEC operation.
Finally, the use of a reliable reference electrode in the novel reactor design allowed the de-convolution of the cell potential to each electrode, thus facilitating direct comparison with studies in aqueous phase electrolytes.

In the past decade, anodically formed self-organized TiO2 nanotube layers attracted considerable scientific interest (see e.g. reviews [1,2]). This is mainly due to the fact that these layers combine the defined nanotubular geometry with a broad range of functional features inherent to TiO2. Particularly in photoelectrochemical water splitting nanotubular layers have shown remarkable results. The presentation will first give an overview of the most promising features of classic nanotube layers for photocatalysis and then focus on TiO2 nanotube (NT) arrays, converted by a high pressure H2 treatment to anatase-like "black titania"[3,4]. This modification shows a remarkable high open-circuit photocatalytic hydrogen production rate without the presence of any cocatalyst. Other attempts to form black titania, namely using classic reduction treatments (e.g., atmospheric pressure H2/Ar annealing) do not show this surprising effect. We will discuss the main difference caused by the high H2 pressure annealing and the origin of this outstanding activity of black titania.


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Synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The nanoporous Al₂O₃ – most established member of this group of materials – has been prepared by anodic oxidation of Al under suitable electrochemical conditions two decades ago into perfectly ordered, honeycomb-like porous structures (1). Owing to the flexibility of the pore diameter/length and the relative ease of the Al₂O₃ dissolution, its porous membranes have been since than widely used as template material of the choice for a range of materials (2-4).
It is the TiO₂ that has received the highest attention after Al₂O₃ motivated by its range of applications, including photocatalysis, water splitting, solar cells and biomedical uses. Very significant research efforts have led to reproducible synthesis of self-organized TiO₂ nanotube layers by means of anodic oxidation, during which the starting Ti substrate is converted into highly-ordered nanotubular layer by anodization in suitable electrolytes (5-7). Although advancements in the anodic synthesis of self-organized TiO₂ nanotube layers have been presented over past years (8), as shown in Fig. 1a, the degree of ordering has not reached so far the level known from porous alumina (1). Numerous factors influence the ordering and the homogeneity of the TiO₂ nanotube layers.
In the presentation, we aim to demonstrate significant advancements in the ordering of anodic TiO₂ nanotubes compared to known state-of-art. Utilizing AFM, SEM and optical measurements, we will show how to obtain via tailored anodization protocols a very decent degree of uniformity and homogeneity of the nanotube layers. Moreover, based on SEM, EBSD and TEM measurements, we will demonstrate how the Ti grain structure influences the lateral uniformity of the nanotube layers (9).


References
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Titania nanotubes were prepared by anodic oxidation. The titania surfaces were partially coated with zinc by reacting zinc acetate with the nanotubes and then annealed [1]. An annealed nanotube cluster was placed carefully on a Si wafer using a tweezer. Secondary electron images were acquired before and after the SIMS analysis with a SEM. Specific areas with various orientations (vertical and horizontal orientations) of the nanotubes were selected for NanoSIMS 50L analyses. The NanoSIMS 50L at the Arizona State University is the state-of-art analytical instrument made by Ametek Cameca, France and is capable of doing in situ isotopic analysis of surfaces at high spatial resolution (~25nm) [2]. In the NanoSIMS, the surface of the nanotubes was sputtered with a focused Cs⁺ (<50nm at 16keV) primary ion beam and secondary ions of 16O^-, 48Ti16O^-, and 65Zn16O^- were imaged simultaneously on several different areas on the wafer. The oxides of the Zn and Ti were measured because Cs⁺ beam size is a lot smaller than the O^- beam, and the electron affinities for the oxides are higher than the metals. Images were acquired with 60 ms dwell time at ~20x20 µm², 256X256 pixels and 5 planes, upto a depth of 1 to 2 nm. Much smaller areas of ~500x500 nm² were evaluated for their elemental composition with the WinImage software by choosing regions of interest. The average ZnO/TiO ~1.8%, confirming the actual content of Zn used during synthesis of the nanotubes. Qualitatively, the TiO/ZnO ratio increased with increasing depth implying that ZnO concentration was decreasing as we probed into the nanotubes.

[1] S. Banerjee, S Mohapatra, M. Misra & I. B. Mishra, Nanotechnology, 20 (2009) 057502
[2] Peter Williams and Maitrayee Bose (2014) A Redesigned Cesium Source for Cameca SIMS Instruments, 26th Annual Workshop on Secondary Ion Mass Spectrometry and Related Techniques.

To develop the next generation highly scalable resistive switching memory devices, the efficacy of 50 keV Ar⁺-ion irradiation to fabricate self-organized TiO₂ nano-channel-based structures at a threshold ion fluence of 5×10¹⁶ ions/cm² will be demonstrated. Given the structural modification of the TiO₂ layer with increasing ion fluence by x-ray diffraction, we will discuss the formation of nano-channels between voids by transmission electron microscopy (TEM). Moreover, we will show the evolution of near surface oxygen-deficient region by in-situ energy-filtered TEM at a fluence of 5×10¹⁶ ions/cm², in consistence with our x-ray photoelectron spectroscopy results. We will also analyze the origin of local resistive switching behavior by conductive atomic force microscopy (c-AFM), and verify its practical application by measuring current-voltage (I-V) characteristics after fabricating two-terminal Pt/TiO_x/TiO₂/Pt devices. We will discuss these experimental results in the light of oxygen vacancy migration through the self-organized nano-channels in TiO₂ layers.

Photocatalytic degradation is one of the most popular routes applied in the wastewater treatments. But the most used nanoparticle photocatalysts have a serious drawback that the particles are not easy to precipitate and
recover from water, which seriously hinder its applications.In our study, titania/carbon composite prepared by simple methods with titania nanoparticles as precursors. The obtained composite can precipitate from the solution easily, which is the major reason that it can be used in actual applications. We found that the titania particles, as catalysts, could enhance the reaction between carbon and oxygen during the calcinations process, and as a template, would determine the size and position of the pores on the carbon substrate simultaneously. Due to the unique porous structure, the titania/carbon composite shows considerable adsorption and high photocatalytic activity. Combining the advantages of titania and carbon, the composite can be also applied in many other fields, such as water splitting for hydrogen and lithium ion batteries.

Quantum Cascade Lasers (QCL) are fundamentally different from conventional lasers in that their wavelengths can be programmed over an unprecedented range by choosing the thickness of the materials in the active region. In QCL, the optical transition does not occur across the bandgap as it does for double heterostructure lasers but rather occurs between discrete electronic states within the conduction band. These states arise from the quantization of electron motion in the active region’s nanometer-thick layers called quantum wells. The active region-injector stages of the QC laser give rise to an energy staircase in which photons are emitted at each of the steps.
Ti_1-xSn_xO₂ was first fabricated to act as either the well or the barrier, together with TiO₂, in the active region of the QCL such as not to introduce additional electrons to the active region. Ti_1-xSn_xO₂ films were grown by pulsed laser deposition on rutile TiO₂ (110) substrates with estimated values of x ranging from 0.005 to 0.15. Energy-dispersive X-ray spectroscopy has indicated the presence of Sn in the Ti_1-xSn_xO₂ films. The concentration values are measured by Rutherford backscattering (RBS) and X-ray Fluorescence spectroscopy. In addition, X-ray spectroscopy shows that the valence state of Sn is 4+ according to the binding energy in the samples and the surface of the samples are Sn richer as compared to bulk. RBS indicates high crystallinity of the films and the substitution of Sn⁴⁺ at the site of the Ti⁴⁺ ion.
Layer-by-layer growth mode has been achieved as verified by RHEED intensity oscillations, with roughness values (rms) ranging from 0.1nm to 1.5nm as observed by AFM. Lattice constants along the c-axis are calculated from the XRD results using Bragg’s Law and is shown to increase with increase in Sn concentration. Lattice constant might increase due to 2 reasons: 1) Substitution of Sn ion into the Ti lattice site. Sn⁴⁺ has ionic radius of 71pm and Ti⁴⁺ has ionic radius of 68pm; 2) Lattice strain in the films induced by lattice mismatch. The atoms in the Ti_1-xSn_xO₂ films are strained as proven in the reciprocal space mapping of the samples. The interference fringes in the 2θ-ω scan shows that the films are single crystal and the thickness of the films can be calculated from the fringes.

We report here the discovery of a giant resonant exciton (at about 6 eV) in pure (001) anatase Ta_xTi_1-xO₂. Upon Ta addition, the exciton showed a significant enhancement measured using high energy reflectivity (4.5-35 eV) and spectroscopic ellipsometry (0.5-5.6 eV), followed by significant spectral weight transfer at high energies. The fact that this is an exciton was further confirmed by the absence of a corresponding feature in the loss function ruling out plasmonic excitation. This giant exciton has been confirmed by many body perturbation theory calculations which show that strong electron correlation with the resultant anomalous spectral weight transfer is necessary to observe this phenomenon. Scientifically this is a dramatic demonstration of the strong electron correlation in TiO₂ and the observation of a strong 6 eV exciton opens the door for the exciting possibility of solid state light emitters in the deep UV region.

The interfaces between water and titanium dioxide (TiO₂) are associated with photocatalysis, gas sensor and so on. In order to understand the mechanism of the photocatalytic reaction, it is necessary to examine the interaction between the TiO₂ surface and water under the wet condition. Although first-principles calculation based on density functional theory (DFT) is the effective method for the study of electronic states, it is difficult to deal with liquid such as water. Therefore, we tried to determine the interaction of the water-TiO₂ by 3D-RISM-SCF method [1]. This method is the hybrid method that is treating the water with the 3D-RISM method [1] and treating TiO₂ with first-principles calculation. The 3D-RISM method is one of techniques for handling liquid with a solution theory based on statistical mechanics. Calculations have been carried out using Advance/PHASE. This code uses the generalized gradient approximation to describe the exchange-correlation effects, the ultra-soft pseudopotential method, and the plane-wave basis set. Because of the strongly correlated nature of the d-electrons in TiO₂, It is corrected by using the DFT + U method that describes the electron correlation effects by adding a Hubbard-U term, representing an on-site Coulomb repulsion, in the energy functional. We calculated the rutile TiO₂(1 1 0)-water interfaces because the rutile TiO₂(1 1 0) surface is the most widely studied model of the TiO₂ surface.
We examined the rutile TiO₂(1 1 0)-water interfaces in the following steps;
(1) Determining the distribution function of water using 3D-RISM-SCF method by treating all of the water molecules with RISM method.
(2) Placing the water molecules near the rutile TiO₂(1 1 0) surface in accordance with the distribution function.
(3) Examination the effects of water on the interaction between the rutile TiO₂(1 1 0) surfaces and water by 3D-RISM-SCF method.
In the rutile TiO₂(1 1 0) surface, there are two types of Ti; one is six-fold Ti (Ti⁶) and the other is five-fold Ti (Ti⁵), and two types of O; one is bridging O (O^B) and the other is in-plane O (O^P). The H atoms in water distribute near O^B and the O atoms in water do near Ti⁵. Because water molecules are polarized, a H atom is interacted with O^B that is negatively charged and an O atom is interacted with Ti⁵ that is positively charged and are unsaturated. These results are consistent with the first-principles molecular dynamics simulations [3]. In the presentation, We will also discuss the influence of the water on the water dissociation and on the formation energy of the oxygen-defects in the rutile TiO₂(1 1 0) surfaces.

[1] H. Sato, A. Kovalenko, F. Hirata, J. Chem. Phys. 112, (2000) 9463.
[2] A. Kovalenko, F. Hirata, Chem. Phys. Lett. 290 (1998) 237.
[3] L.-M. Liu, C. Zhang, G. Thornton, A. Michaelides, Phys. Rev. B 82 (2010) 161415(R).

A novel type of hierarchical nanocomposites consisted of MoS₂ nanosheets coating on the self-ordered TiO₂ nanotube arrays is successfully prepared by a facile combination of anodization and hydrothermal methods. The MoS₂ nanosheets are uniformly decorated on the tube top-surface and the intertubular voids with film appearance changing from brown to black color. Anatase TiO₂ nanotube arrays (NTAs) with clean top surface and the appropriate amount of MoS₂ precursors are the keys to grow perfect compositing TiO₂@MoS₂ hybrids with significantly enhanced photoactivity. The photocatalytic rate constant was found to be approximately 2.6 times higher than that of the pristine TiO₂ NTAs. Furthermore, the photocurrent response of the TiO₂@MoS₂ nanocomposites is remarkably improved by about two folds. These results revealed that the decoration of MoS₂ nanosheets onto TiO₂ NTAs system not only increases the light absorption in the visible range, but also facilitates faster charge transfer from MoS₂ nanosheets to TiO₂ NTAs, leading to a higher separation efficiency of the photo-generated electron-hole pairs, thus improve the photocatalytic activity and the photocurrent response.

Carbon nanotube based field emission devices have attracted widespread attention in the past decades in light of potential applications in displays, X-ray sources and microwave devices. However, there is conventionally a trade-off between low emission threshold and high emission stability, which induces the impediment of commercial applications of carbon nanotube based field emission devices. To improve the comprehensive field emission properties of carbon nanotube based field emission devices, we introduced a deposition of TiO₂ gel on the surface of screen-printed CNT films by a simple wet process to form composite films as an alternative choice for electron emitters. The composite field emission devices exhibit outstanding field emission characteristics, including ultralow turn on field of 0.383V μm^-1 and threshold field of 0.657V µm^-1 corresponding with a very high field enhancement factor of 20000, exceptional current stability, and excellent emission uniformity. The improved field emission properties are attributed to the enhanced edge effect simultaneously with the reduced screening effect, and the lowered work function of the composite cathode. In addition, the highly stable electron emission is found due to the presence of titanium dioxide nanoparticles on the carbon nanotubes, which prohibits the cathode from the influence of ions and free radical created in the emission process as well as residual oxygen gas in the device. The high-performance solution-processed composite cathode demonstrates great potential application in vacuum electronic devices.

The traditional method of formation of nanoporous titanium oxide is electrochemical anodizing by direct current. The search of ways of nanoporous titanium oxide structure control is important for practical use. It is known that the value of anodizing voltage determines the structure parameters (pore diameter, their length, etc.) which define their physicochemical properties (catalytic activity, etc.).
The paper presents the results of pilot research of nanoporous titanium oxide synthesis by the method of electrochemical anodizing by mixed AC-DC current. As electrolyte the standard electrolyte consisting of 0.3% NH₄F (mass.), 0.5% H₂O (mass.) and ethylene glycol is used. The voltage constant component (DC) varied within 20-80 V, the variable component (AC) had a sinusoidal form; the amplitude varied within 1-10 V, the frequency - within 1–100 kHz. Electrode system geometry was plane-plane.
The data on change of surface morphology, pore structure parameters (diameter, distance between pores, length, etc.) at various parameters of a variable component of anodizing voltage were obtained.
It is shown that at the voltage variable component the amplitude of which exceeds 10-20% of the constant component value, the change of coating structure parameters is observed: packing density increases, the tubes cross-section becomes more round, the tubes are separated relative to each other, closer to the coating surface. The dependences of packing density on the AC-DC parameters of voltage and frequency are received. It is established that at the increase of variable component amplitude the length growth rate of porous structure increases. Experimental dependences of pore growth rate at various amplitude and frequency of anodizing voltage variable component, and also time dependences of anode current constant component are received.
The obtained results are of great importance for the synthesis of new types of catalytically active coatings on the basis of nanoporous titanium oxide.

The paper presents the results of mathematical modeling of the process of barrier electrically insulated oxide coatings on the basis of titanium oxide with the use of AC high frequency component (1-100 MHz) on the constant anodizing voltage. The optimum conditions under which the formation of a thick oxide layer is provided are defined.
The mathematical model of oxide coatings formation is based on the equations describing ion flows through an oxide matrix (the directed and diffusive component), considers oxide film heating caused by dielectric losses and possibility of the voltage form change in the course of coating formation. With the help of the model it is shown that with use of alternating voltage application on the main anodizing voltage it is possible to receive continuous barrier coatings, 1.5 - 2 times thicker, than the coatings obtained by traditional technology.
The experimental results of formation of TiO₂ electrically insulated oxide coatings are presented.

Anatase TiO₂ polycrystalline thin films with strong <001> texture and {001} facets have great potential for different photoelectrochemical applications, such as photovoltaics, water splitting, CO₂ reduction, and photocatalysis. To realize a practical opto-electronic or photovoltaic device based on polycrystalline TiO₂ thin films, it is critical to understand the photocarrier generation and collection as a function of photon energy. Here we present our experimental results on the photocurrent response of microscopic photodetectors based on anatase TiO₂ thin films with strong <001> texture. We have observed a strong and narrow peak in photocurrent spectrum at 355 nm (~3.5 eV) with FWHM ~ 40 nm at room temperature. The photocurrent intensity depends linearly on the bias voltage between the electrodes. We attributed this strong photoluminescence peak to the exciton formation in TiO₂ nanoparticles. These results demonstrate that anatase TiO₂ nanomaterials strong <001> texture and {001} facets have great potential as efficient photoactive materials in opto-electronic devices ranging from photovoltaic devices to UV photodetectors.

Metal nanoparticles and titanium dioxide have been of great interest due to their distinctive features such as photocatalytic, optical, magnetic, and electrical properties. The silver-doped titanium dioxide nanostructures have a variety of applications, such as disinfecting and cleaning materials as a consequence of their photocatalytic properties. In this paper, we report the synthesis of Ag-TiO₂ nanoparticles focusing on Green Chemistry synthesis using orange and banana peel extract as a precursor, where they act as reducing agents of Ag-TiO₂ nanoparticles. Green synthesis of nanoparticles has been achieved using plant extracts and an environment-friendly reducing and capping agent. The obtained Ag-TiO₂ nanoparticles have been characterized by X-ray Diffraction (XRD), Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and UV-Vis Spectroscopy. The photocatalytic properties and activity of these materials were studied on the degradation of the methylene blue dye by heterogeneous photocatalysis with the irradiation of UV light of 365 nm of wavelength.

Thin films of TiO₂/SnO₂:F multilayers were prepared by spray pyrolysis technique on glass substrate. The samples were prepared using titanium(IV) propoxide 98%, ammonium fluoride and tin(II) chloride dehydrate extra pure as precursor materials. Thus, a TiO₂ thickness dependence optical study of TiO₂/SnO₂:F/glass system is presented. The optical property was characterized by UV-Visible transmittance spectroscopy. For all the samples, the average transmissions in the visible wavelength region (400-800 nm) were between 60 and 85%. The optical parameters, such as refractive index n₁ and extinction coefficient n₂, of TiO₂ and SnO₂:F in the TiO₂/SnO₂:F thin films structure, determined by fitting the measured optical transmittance (T) spectra, are presented and analyzed. Among the various classical dispersion relations for the dielectric function, the Drude model combined with the Lorentzian oscillators was used to get a good fit of T in the measured spectral range. Results on related parameters such as high frequency dielectric constant, plasma frequency, film thickness and band gap are discussed.

A facile process was developed to fabricate two dimensional titania nanosheets (t-NS) by using graphene oxide (GO) as template. In-situ growth of anatase TiO₂ nanoparticles on GO suspension was achieved during a solvothermal process without further calcination. Stoichiometric amounts of acetic acid were added to introduce esterification with butanol to release sufficient water for the hydrolysis of titanium butoxide. A following thermal treatment is applied to remove GO, and simultaneously to increase the crystalline size of anatase. Electron paramagnetic resonance (EPR) spectra were utilized to in situ identify the concentrations and types of photo-induced radicals by mixing nitroxide spin trap, 3-aminoproxyl (AP) or 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO), with t-NS suspensions under ambient conditions. The reaction of AP with UV-induced radicals from photoactive t-NS leads to the decay in AP concentration. In contrast, specific radicals produced from the irradiated suspension are identified by characteristic EPR signals of DMPO spin adducts. Moreover, the presence of stable radicals, probably attributing to the defects, on t-NS was confirmed by comparing the difference in the intensity of initial EPR signal from AP/t-NS suspension before irradiation. XPS spectra confirmed the presence of Ti-O-C bonding, which benefits the charge transfer and reduces the recombination of photoinduced carriers. Preliminary results suggested that the r-NS with an optimum Ti to C ratio exhibited weaker photoluminescence and higher activity on CO₂ reduction. A gas chromatograph equipped with a pulsed discharge helium ionization detector was applied to identify the products (carbon monoxide and methanol) of photocatalytic reduction of CO₂ with the presence of water vapor. The kinetics and mechanism of photocatalytic reduction of CO₂ over t-NS with adjustable characteristics are in progress in order to enhance the conversion of CO₂ by using solar energy.

The anatase titania (TiO2) polymorph is well known for its photocatalytic activity in the degradation of dyes and industrial pollutants. However, titania possesses a relatively large band gap (3.2 eV) and consequently, its photocatalytic activity is restricted only to the UV region of the spectrum, thereby limiting drastically its potential applications as a photocatalyst. An efficient way to solve this problem consists of engineering the band gap of titania upon chemical doping with anions and cations. The photocatalytic performance of semiconductor oxides is also improved by increasing the surface area, generally by nanostructuring and/or fabricating materials with different aspect ratio. Based upon the foregoing, we have designed a simple, yet inexpensive and highly versatile approach to fabricate titania nanofibers or polymeric nanocomposites as support for titania particles. To this end, we used polyaniline (PANI) since it is a good electrical conductor, presents a high absorption of visible light and is a good hole acceptor and electron donor, it can be used for photocatalytic applications.
We present here our preliminary results towards the fabrication and characterization of transition metal-doped titania fibers, as well as composite nanofibers consisting of titania fibers embedded into a polymer matrix. Solutions of a titanium alkoxide precursor, polyvinyl pyrrolidone with different concentrations of transition metal (Mn) ions were prepared and titania fibers were subsequently fabricated from these solution via electrospinning. The resulting fibers have been annealed in order to remove the organic phase, yielding metal doped titania nanofibers with different concentrations of the doping metal. Following a similar procedure, nanofiber composites were fabricated by using PANI as the polymer matrix. The structure of the pristine and polymer-oxide composite nanofibers was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and energy dispersive X-ray spectroscopy (EDX). The electron microscopy results indicated that the addition of metal ions yields nanofibers with a higher uniformity. EDX results confirmed the successful doping of the titania fiber with transition metal ions, whereas XRD and Raman spectroscopy have shown that titania fibers are stabilized in the anatase form. Diffuse reflectance measurements allowed for the determination of the band gap of samples. The photocatalytic activity of titania fibers and polymer-titania fibers were compared.

TiO₂ foam-like films were grown by water-based Streaming Process for Electrodeless Electrochemical Deposition (SPEED). The morphology of the ~1 micron thick films consists of a tangled ropy structure with strands of ~200 nm diameter and open pores of 0.1 to 3 micron dimensions. Such films are advantageous for perovskite solar cells with CH₃NH₃PbI₃ absorber. Such solar cells based on spin-coated mesoporous TiO₂ scaffold, CH₃NH₃PbI₃ light-absorption layer, and a 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-bifluorene (spiro-OMeTAD) hole transport layer have been reported with power-conversion efficiency as high as 15% and with 20% considered achievable. SPEED provides a more scalable and manufacturing friendly process, and it gives opportunity to replace the poor quality and poor electron conducting mesoporous TiO₂ layer, and the less robust organic spiro-OMeTAD and poor hole transport layer, with more robust foam-like TiO₂ film, and durable, excellent inorganic hole transport layer CuS. Our proposed structure comprises "glass / SnO₂:F / TiO₂ film / CH₃NH₃PbI₃ / CuS / Mo". The electron-hole pairs generated in the perovskite CH₃NH₃PbI₃ for 300-800 nm wavelength light will be separated by (1) the injection of electrons into the embedded TiO₂ wires that transport these electrons to the SnO₂:F electrode; and (2) the injection of holes into the inorganic CuS hole transport layer to the Mo electrode to form a phtotovoltaic cell. SPEED is an aqueous process that deposits self-assembled nanomaterial inorganic thin films over large areas, without a vacuum. Water-soluble compounds with complexing agents grow films by heterogeneous reaction on the substrate, with little wasteful homogeneous reaction. Hydrophilic substrates bind hydroxyl ions (OH^–), which are attachment sites for nucleation with density exceeding 10¹² per square centimeter. The substrate is heated to provide at least the reaction activation energy. The SPEED version used here for TiO₂ growth is vapor phase SPEED (VPSPEED). The water based precursor, nebulized into 10 to 20 μm droplets, impinges on the substrate, which is at temperatures of 300 - 400 C, giving growth rate exceeding 200 nm per minute. Reaction byproducts are decomposed and volatilized. The choice of ligand and growth temperature gives self-assembled nanoporous/microporous TiO₂ film growth with no template. Lateral film resistivity is in the range 20 – 200 kOhm-cm, increasing with growth temperature and with post-growth annealing; however, the film resistivity in the vertical growth direction is much lower. XRD confirms presence of TiO₂ crystals, and spectroscopy shows high transmission below the expected 3.2 eV TiO₂ bandgap. VPSPEED will also be used to grow the CH₃NH₃PbI3 absorber layer using organic solvent instead of water. Our proposed CuS hole-transport layer and SnO₂:F transparent conducting oxide have already been grown by SPEED.

A facile electrochemical method was used to modify anatase TiO2 with Na+ or H+ under a cathodic bias in an ethylene glycol electrolyte. TiO2 nanotube arrays (TNTAs) were fabricated by anodization method for the benefit of growing TiO2 directly on the Ti substrate. The obtained perpendicular TNTAs were served as charge collectors in the electrochemical doping process and following application tests. The electrochemical performance of the TiO2 samples was studied by cyclic voltammograms (CV) and galvanostatic charge/discharge (GCD) measurements. The doped TNTAs yield a specific capacitance of 2.63 mF cm-2 at a discharge current density of 50 uA cm-2, which is 40 times higher than the non-dope TiO2. Importantly, the doped TNTAs also show remarkable rate capability with nearly 80% areal capacitance retained when the current densities increase from 50 to 200 uA cm-2. The areal capacitance also shows little drop after 2000 cycles at a scan rate of 100 mV s-1. Furthermore, the photocatalytic degradation of Rhodamine (RB) shows that the doped TiO2 has an improved photocatalytic efficiency.

Defectbased material engineering has found a niche in bandgap modification, photocatalysis and energy storage. Using a process called surface mechanical attrition treatment (SMAT)—normally reserved for deforming the surfaces of bulk material we propose that this process can also be applied to delicate surfaces, allowing for the creation of strain and defect based effects through stress concentration, vacancy creation, and/or dopant injection. Furthermore, with the great diversity of processing parameters of SMAT, one can deliver tailored treatment specific to the surface being modified as well as fo the functionality being desired.
In this work, we illustrate this concept by implementing SMAT in the modification of titanium dioxide (TiO2) nanotube array,leading to improved photocatalytic efficiency under solar radiation. We also suggest that with the further inclusion of surface dopants due to SMAT—when combined with the strain induced by said process—also yield a synergistic effect in light-harvesting in the modified TiO2, which has led to additional long-term operational stability on top of improved photocatalytic behavior being observed. Finally, with the enhanced defect creation via SMAT, the capacitance of the material has been enhanced from its original performance, demonstrating the value of using SMAT as a novel step in the functionalization process, as well as an effective post treatment on already functionalized surfaces.

Recently there have been intensive efforts in exploring innovative solar cell structures with high performance and profitable manufacturing costs. One alternative is the use of carbon nanotubes (CNT) in photoelectrochemical cells, as an effective strategy to increase device efficiency. To this end, this work aims to use carbon nanotubes functionalized metal dioxide nanoparticles (TiO₂) using mechanical-thermal process, characterizing the chemical structural properties, and optical properties of the resulting composites. In a first step, a sintering process is applied, varying temperatures, with the goal of introducing vacancies on the outer layer of carbon nanotubes, thereby improving the functionalization by OH or amino groups which will be at a later stage wet-chemistry, in order to control the electronic and optical gap of CNT's. The functionalized CNT's are subjected to a mechanical process adding TiO₂ nanometer level, generate a complex nanomaterial. Characterization was carried out by DRX, Micro Raman and UV-Vis spectroscopy. After a 5 hours of annealing to 1000 °c in a cylindrical oven the mwcnt show us a shift in the RBM comparing with mwcnt pristine and previously treated with mechanical alloying mesaured with Raman. The integration of TiO₂ nanopaticles to the system given us a bond appearing in the UV-Vis spectra at 592 nm.

A facile technique was adopted to synthesize beautiful lilac bush resembling TiO₂/SnO₂ microflowers aggregates for photodegradation of Congo Red (CR). The TiO₂/SnO₂ microflowers in the 2-3 µm range with high surface area (80 m²/g), under optimized conditions of catalyst dosage (0.3 g/L), dye concentration (100 ppm) and pH value is 10, exhibit excellent photocatalytic activity under visible light, whereby, 98.3% of the CR aqueous solution was degraded in 40 min of illumination time and also shows good recyclable photocatalytic activities. Further, the gas sensing properties of the as-synthesized material were evaluated towards detection of a variety of volatile organic compounds, such as acetone, methanol, benzene, ammonia, toluene, diethyl ether, and ethanol. The prepared microflowers were found to be highly sensitive and selective towards ethanol at relatively low working temperature of 240 °C. Finally, the high surface area induced by unique morphology causes faster ionic and electronic transportation and improved photocatalytic and sensing performance was observed.
Keywords: Hydrothermal synthesis, TiO₂/SnO₂, Catalysis, Gas Sensing.

There have been many works analyzing thermionic currents and chemicurrents generated on various electrolyte-free metal/semiconductor nanostructures. More recently, the chemicurrent phenomenon was reported for mesoporous Pt/semiconductor systems adept at converting surface-released chemical energy into a stationary electrical signal at room temperature conditions. The present work points out the existence of an entire class of such surface-driven functional nanosystems. Here, the reaction current generation of Pt/TiO2 and Pt/ZrO2 systems were studied at room temperature under exposure to oxyhydrogen environments for mesoporous zirconia; this nanostructure was capable of the continuous oxidation of hydrogen, producing a long-standing stationary current. Synthesis parameters during the anodization process were manipulated to control sample pore density and the average pore diameter. Deposited via wide-angle PVD sputtering, the Pt phase forms an electrically continuous topographical nanomesh layer, and thus the Pt/TiO2/gas interface is regulated through the manipulation of titania and zirconia porosity. We observed reaction current enhancements with increasing porosity due to the lengthening of the Pt/oxide. The most porous sample was significantly more sensitive to initial hydrogen additions, pointing toward the spillover of positive ionic charge across the Pt/oxide interface as the origin of the observed electromotive force.

Nanostructured TiO₂ had been identified as a good candidate to replace conventional nanoparticles as photo anode in Dye Sensitive Solar Cells (DSSCs). Indeed, ordered and porous columnar TiO₂ would provide large surface area for dye absorption ton enhance light trapping [1]. Moreover enhancing the electron transfer these structures should improve the cell performances.
In this work, nanostructured TiO₂ thin films are synthesized by reactive magnetron sputtering combined with Glancing Angle Deposition (GLAD), in order to control the chemical composition, the crystalline structure and the microstructure of the films. GLAD enables the growth of columnar thin films with unique microstructures which can be controlled by modifying the experimental conditions as shown in our precedent work [2]. These films are extremely porous (up to 100 m2/g for TiO₂ [3]). The chemical composition, the crystalline structure and the morphology of the films were analyzed by XPS, XRD and SEM, respectively. Pure anatase which is the preferred polymorph as electron acceptors in DSSCs [4] has been obtain with well define columnar structure.
The aim of this work is to go further in the understanding of the dye grafting on the TiO₂ nanostructures. First, the impregnation efficiency (dye absorption) on the TiO₂ thin film was evaluated by UV-visible spectrophotometry regarding the microstructure (slanted column, zigzag, pillar,…). The nanostructured thin films synthesized by GLAD reveal, after grafting, a high absorption around 5 times more than a conventional sputtered thin film. The data also reveal that a successive grafting from a large to a small size of dye maximizes the amount of dye molecules grafted onto the TiO₂ films. Indeed, the smaller dyes are able graft the free sites which are not available by the larger one. This consecutive grafting leads to larger photon absorption rate in a broad wavelength range (from UV to 700 nm) which should enhance the DSSC efficiency. Second, the nature of the interaction between the TiO₂ surface and the carboxylic group of the dye molecule are investigated by a detailed study of the valence band performed by high-energy resolution Ultraviolet Photoemission Spectroscopy (UPS) using synchrotron facilities. A changing of the electronic states at the dye/TiO₂ interface, in particular by forming chemisorption-induced gap states will highlight the nature of the interaction between the dye molecule and the TiO₂ surface. Preliminary analyses carried out by XPS have shown that the grafting of the dye modifies the TiO₂ valence band; namely the density of electronic states increased near the Fermi level.

[1] H-Y Yang et al., Thin Solid Films 518 (2009) 1590–1594.
[2] J. Dervaux et al., Vacuum, vol. 114, pp. 213–220, Nov. 2014.
[3] D. W. Flaherty et al., J. Phys. Chem. C, vol. 111, pp. 4765–4773, 2007.
[4] G. Li et al., Dalton Transactions 45 (2009) 10078-10085.

Mesoporous titanium oxide (TiO₂) films have garnered intense research interest in various solar energy conversion technologies due to their large surface area resulting in enhancement of the solar energy conversion efficiency especially in dye-sensitized solar cells (DSSC) and related systems. However, due to their small dimensions and presence of large concentration of localized trapped electrons, the popular Mott–Schottky plot or the photocurrent onset potential failed to give accurate information about their conduction band edge. Herein we describe a new simple spectroelectrochemical method which simultaneously produces the conduction band (CB) energy and the extinction coefficient of the free conduction band electrons in mesoporous TiO₂ electrodes. A reductive potential was applied to the mesoporous TiO₂ photoanode film to raise its Fermi level and the corresponding absorbance was measured throughout the UV-Vis region. A blue shift of the band gap absorbance was observed while raising the Fermi level which was a result of filling the lowest energy states in the conduction band, similar to the Burstein-Moss shift. The density of electrons in the CB was calculated from the magnitude of this bandgap shift and the conduction band was determined. Additionally, analysis of the absorbance spectra implies that the delocalized CB electrons have the major contribution to the absorption features in the visible range. Comparisons of the density of CB electrons with the absorption spectra allowed us to determine the free electron extinction coefficient. Moreover, an upward shift of the CB edge was observed which was attributed to the presence of electroactive surface states in these high surface area semiconductor films resulting potential drop across the Helmholtz layer. A quantitative analysis of this band edge shift allowed us to accurately determine electron distribution in the film. The trap states were found to be distributed on the surface as well as in the bulk and with increasing negative potential bulk traps dominate the overall trap concentration.

Titanium dioxide (TiO₂) is a prominent photocatalytic material due to its favorable properties, including stability against corrosion and proper band alignment relative to the water redox potential. In this work, we use first-principles density functional theory to provide atomistic insights into the atomic-scale mechanism of the water splitting oxygen evolution reaction in pure and doped two-dimensional TiO₂ nanosheets. These nanosheets exhibit a lepidocrocite structure and recent experimental work has demonstrated that while the pure nanosheets exhibit slow and sluggish oxygen evolution kinetics, the introduction of isolated rhodium dopants can increase the oxygen evolution rate by a factor of 10. Our first-principles analysis reveals water dissociation pathways and corresponding Gibbs free energy profiles for both doped and undoped lepidocrocite TiO2 nanosheets. We find that the nanosheet surface, unlike the rutile 110 surface which is known to be a good photocatalytic material with five-fold coordinated Ti atoms, is less reactive since the surface Ti atoms are fully six-fold coordinated. In the undoped sheets, our simulations illustrate that water absorption and the first deprotonation are rate-limiting steps due to significant energy barriers in the free energy profiles. The presence of Rh dopant atoms reduces the energy cost for dissociative adsorption of water molecules from 1.3 eV in the undoped system to 0.9 eV when Rh dopant atoms are present. Other differences in the OER Gibbs free energy landscape are also present and will be discussed. Analysis of the electronic structure and density of states reveals the reasons for the different behavior with and without dopants present in lepidocrocite TiO₂. Also it shows how the presence of the dopants helps with rate limiting steps, and suggests design rules for identification of more effective dopant species. These results will provide new insights into the design of advanced photocatalysts that overcome the bottleneck steps of the water dissociation pathway.

We present an investigation of the vibrational properties of Nb- and Ta-doped TiO₂ films by means of Raman spectroscopy. Transparent conducting TiO₂ can be obtained by donor doping of the anatase titanium oxide structure, e.g. by substitutional Nb or Ta doping [1,2], and is gaining increasing attention for the development of transparent electrodes with novel functionalities. Usually, regardless the deposition technique, a crystallization process is necessary to achieve large mobility values. However the annealing environment appears to be crucial; in fact, using oxygen atmosphere results in highly insulating samples, regardless of the presence of Ta or Nb as a doping element, while for reducing atmosphere, typically high vacuum or H₂ at temperatures above 500°C, a conduction electron density of the order of 10²⁰-10²¹ cm^-3 is obtained. In addition, the electrical and optical properties are also strongly sensitive to the deposition atmosphere and an optimal oxygen pressure is in general found depending on the amount of extrinsic doping. These facts reveal that the defect chemistry of donor doped TiO₂, similarly to other Transparent Conducting Oxides, is not trivial and involves a strict interplay between extrinsic dopant atoms, oxygen vacancies and ‘electron killer’ defects such as Ti vacancies and O interstitials. In this complex framework Raman spectroscopy can provide an invaluable contribution towards understanding the material structure and its influence on the functional properties, in particular electrical conductivity, as already reported for Al-doped ZnO [3].
Here we present a systematic investigation of Nb- and Ta-doped TiO₂ films obtained by Pulsed Laser Deposition in different atmospheres and annealing conditions, and for different doping levels, using Raman spectroscopy. Correlations between structure, crystallinity, shift and width of Raman peaks and functional properties (electrical conductivity and optical transparency) are shown and discussed. In particular, a clear correlation between the shift of the E_g(1) anatase vibrational mode from its reference position at 144 cm^-1 and the electron carrier density is found, while the B_1g(1) mode related to Ti-Ti vibrations is significantly affected by the doping level. Complementary information obtained by XRD provides insight into the structural properties of donor-doped TiO₂ films.

1. T. Hitosugi et al., Jpn. J. Appl. Phys. 44, L1063 (2005)
2. P. Mazzolini et al., J. Phys. Chem. C 119, 6988 (2015)
3. V. Russo et al., J. Appl. Phys. 115, 073508 (2014)

Modifying nanoscale interconnects between TiO₂ nanoparticles comprising anatase TiO₂ aerogel networks dramatically enhances UV photocatalytic H₂ generation, while only modestly impacting the activity of TiO₂ aerogels for photocatalytic oxidative degradation of dichloroacetic acid (DCA). We synthesized a series of TiO₂ aerogels with different mechanical strengths by varying the conditions of the sol–gel synthesis. The strongest aerogels were ~8× more active for photocatalytic H₂ generation than the standard materials. In contrast, photocatalytic DCA degradation rates increased only ~30% upon strengthening the aerogels. The aerogels were also cast as films and analyzed using dynamic photoelectrochemical methods. Intensity-modulated photovoltage spectroscopy (IMVS) revealed that excited-state electron lifetimes were ~30–50% higher in the strongest TiO₂ aerogels than in the standard gels; intensity-modulated photocurrent spectroscopy (IMPS) revealed that electron transport is ~30–50% slower in the stronger materials.

The modest improvement in DCA degradation in strengthened TiO₂ aerogels correlates well to the increased electron lifetimes, while the more substantial improvement of photocatalytic H₂ generation rates suggests an increased concentration of photocatalytic active sites (for reductive water splitting) in the strengthened materials. The photocatalytic and photoelectrochemical results considered together suggest that the active sites act as electron traps, which increase electron lifetimes, but also slow electron transport. It is probable that the active sites/trapping sites arise at aerogel interparticle contacts, or “necks”. Time-resolved spectroscopic experiments are show that the dynamics of the trapping/detrapping processes in the different materials are identical at picosecond time scales, verifying that the changes in dynamics that control the photocatalytic performance of the aerogels occurs on the longer photoelectrochemical times scales (on the order of milliseconds) observed using the IMPS and IMVS techniques.

This work is supported by the Office of Naval Research.

Formic acid adsorption on rutile TiO₂(110) single crystals exposed to different oxygen partial pressures have been investigated with infrared reflection-absorption spectroscopy (IRRAS) employing p- and s-polarized light incident along the [001] crystal direction at a temperatures between 273K and 303K. IRRAS spectra, prior to oxygen exposure, shows that formic acid dissociates upon adsorption and binds to the surface as bridging formate species with the symmetric ν_s(OCO) and asymmetric ν_as(OCO) peak at 1360 and 1531 cm^-1, respectively. A new asymmetric ν_as(OCO) band at 1517 cm^-1 develops in the presence of O₂, which is correlated with the O₂ partial pressure, whereas no changes is seen in the symmetric ν_s(OCO) band. At low O₂ pressures (<1 * 10^-7 mbar), this band is not detectable within 3 h of O₂ exposure. At higher O₂ pressures, 5 * 10^-7 mbar, the band starts to develop, and at 1 * 10^-6 mbar, a pronounced band develops after 30 min exposure. The O₂ induced band at 1517 cm^-1 does not disappear as the O₂ gas is removed, suggesting that a stable chemical modification of adsorbed formate molecules. No changes were observed in the s-polarized IRRAS spectra with O₂ exposure, showing that there is no rotation of the formate molecules in the plane of the surface. The appearance of the new ν_as(OCO) band are consistent with a model where O₂ dissociated to form O adatoms bonded to the Ti_5c atoms, which interact with bridging formate molecules bonded along the [001] direction. Our results provide new insight into the interaction between O₂ and TiO₂ surface at elevated pressures relevant for practical applications.

Niobium-doped TiO₂ (TNO) films can be used as transparent electrodes for various applications, i.e., photovoltaics [1], light emitting diodes [2], or flat displays [3]. We have prepared TNO films by reactive magnetron sputtering from a metallic titanium-niobium alloy target (3.2 at% Nb) in argon/oxygen gas mixture onto unheated substrates. The stoichiometry of as-deposited films was exactly controlled by tuning the magnetron discharge voltage adjusting the sputtering power, while the oxygen flow and the total sputtering pressure were held constant. The as-deposited films were amorphous, as shown by Raman spectroscopy and X-ray diffraction, and exhibited a high resistivity in the range of 10 to 10⁵ W cm, depending on the oxygen content. A subsequent annealing in vacuum at 400^○C for 10 min led to crystallisation into the anatase phase of TiO₂ accompanied by an increase of the optical transparency and the electrical conductivity. These polycrystalline TiO₂:Nb films on borosilicate glass show a quite low resistivity (≈10^-3 W cm), a high carrier mobility (≈8 cm² V^-1 s^-1) and a high optical transparency (about 87 % at 550 nm wavelength). In this study, the relationships between the deposition parameters of the niobium-doped TiO₂ films and their electrical (Hall- and conductivity measurements) and optical properties (UV/VIS and IR spectrometry) were investigated in detail.

[1] H. Natsuhara, K. Matsumoto, N. Yoshida, T. Itoh, S. Nonomura, M. Fukawa and K. Sato, Solar Energy Materials and Solar Cells 90 (17), 2867-2880 (2006).
[2] J. Kasai, T. Hitosugi, M. Moriyama, K. Goshonoo, N. L. H. Hoang, S. Nakao, N. Yamada and T. Hasegawa, Journal of applied physics 107 (5), 053110 (2010).
[3] Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, Y. Hirose, T. Shimada and T. Hasegawa, Applied Physics Letters 86 (25), 2101 (2005).

Indium oxide (InO_x)-based thin-film transistors (TFTs) have been widely investigated. However, InO_x material is more sensitive to the oxygen pressure during sputtering deposition because the bond dissociation energy between In and oxygen is small value (346 kJ/mol). Here, we pay attention of TiO₂ material as a novel channel material of TFT because the bond-dissociation energy of Ti-O is high (667 kJ/mol). We also found that oxygen vacancy (V_o) was created into anatase TiO₂ (anatase-TiO₂) by oxidation of trimethylaluminium (TMA). In this study, we examined electrical properties of polycrystalline anatase-TiO₂ films by changing supply condition of TMA precursor and discuss about the mechanism of V_o formation into TiO₂ film.
Anatase-TiO₂ films were prepared as follows. TiO₂ films were deposited by atomic layer deposition at 200 °C using Ti[N(CH₃)₂]₄ precursor and water gas. As-grown TiO₂ film had amorphous structure and the anatase-TiO₂ films were fabricated by annealing at 500 °C in O₂. To examine electrical properties of the TiO₂ film, TMA precursor was only supplied in pulse time of 0.1 sec under a vacuum. The supply temperature and number of TMA supply cycle were varied at 25 ~ 300 °C and 0 ~ 50, respectively. The resistivity and Hall mobility of anatase-TiO₂ films were examined by Hall-effect measurement.
The initial anatase-TiO₂ film had insulating property. The TiO₂ films became from insulator to conductor as TMA supply temperature increases over 150 °C. Furthermore, the anatase-TiO₂ film exhibited relatively high Hall mobilities of 1 ~ 8 cm²/Vs. Next, we examined TMA supply cycle dependence of resistivity for the anatase-TiO₂ film. The film maintained to be insulating up to 10 cycles and changed low resistivity over 15 cycles. We found that the electrical properties of the anatase-TiO₂ film changed drastically in TMA supply conditions at 150 °C and 15 cycles.
Here, we consider about the change of electrical property in the anatase-TiO₂ film by supplying TMA. The change is thought to relate to the electron generated by V_o formation. This V_o formation in anatase-TiO₂ film involves a two-step mechanism. The first step is that TMA precursor adsorbs on the surface of TiO₂ film. Next, oxygen is removed from TiO₂ by the reaction with TMA and oxygen of TiO₂ and results in V_o formation. To confirm reaction of TMA and oxygen of TiO₂, we evaluated XPS Al_2p spectra of anatase-TiO₂ films as a function of the number of TMA supply cycle. The Al_2p peak intensity of anatase-TiO₂ films were saturated as the number of TMA supply cycle increased over 10 cycles. The Al₂O₃ thickness estimated from the peak area of Al_2p was about 0.3 nm (1 mono-layer of Al₂O₃). This indicates that no oxidation of TMA occur after covering Al₂O₃ layer on the surface of TiO₂ film. We conclude that the anatase-TiO₂ fabricated by the oxidation of TMA has one of candidate materials as a channel of oxide TFT.
Part of this research is supported by CREST, JST.

Photoelectrochemical sensing represents a unique means for chemical and biological detection, with foci of optimizing semiconductor composition and electronic structures, surface functionalization layers, and chemical detection methods. Here, we have briefly discussed our recent developments of TiO₂ nanowire-based photoelectrochemical sensing, with particular emphasis of three main detection mechanisms and corresponding examples. We have also demonstrated the use of the photoelectrochemical sensing of real-time molecular reaction kinetic measurement, as well as direct interfacing of living cells and probing of cellular functions.

Hydrogen production from photoelectrochemical (PEC) water splitting is a promising pathway for solar energy conversion. To achieve efficient and durable solar to hydrogen conversion, PEC photoelectrodes desirably need the following characteristics: effective and broad band light absorption, rapid charge separation, and superior stability. N-type semiconductor oxides, such as titanium dioxide (TiO₂), hematite (Fe₂O₃), bismuth vanadate (BiVO₄) and tungsten trioxide (WO₃), are popular candidates for constructing robust photoanodes due to their excellent chemical stability. The main drawbacks of these oxides are the limited light absorption and poor charge separation efficiency due to their large band gap and high trapping density. There are two predominant strategies to enhance the separation of electron-hole pairs in photoanodes: reducing the crystal size to the scale of the hole diffusion length; and increasing the carrier conductivity by morphology and crystallography control. Nevertheless, both strategies are restricted by the limit of synthesis procedures.

Permanent electrical polarization induced by ionic displacement (e.g., ferroelectric and piezoelectric potential) has shown great promises in engineering the interfacial band structure and manipulating the charge transfer property of heterostructures. PEC water splitting is an electrochemical system driven by similar energy discontinuity at the electrode/electrolyte interface. One can expect positive (maybe significant) performance gain when ferroelectric polarization is appropriately introduced to such a system. Here, we report a development of ferroelectric-enhanced PEC photoanode on the basis of TiO₂/barium titanate (BTO) core/shell nanowire (NW) arrays. Through a one-step hydrothermal process, a uniform, epitaxial, and spontaneously poled BTO layer was created on single crystalline TiO₂ NWs. Compared to pristine TiO₂ NWs, the 5 nm BTO-coated TiO₂ NWs achieved 67% photocurrent density enhancement. By numerically calculating the potential distribution across the TiO₂/BTO/electrolyte heterojunctions and systematically investigating the light absorption, charge injection and separation properties of TiO₂ and TiO₂/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. The ferroelectric polarization could be switched by external electric field poling and yielded PEC performance gain or loss based on the direction of the polarization. This study evidences that the piezotronic effect (ferroelectric or piezoelectric potential-induced band structure engineering) holds great promises in improving the performance of PEC photoelectrodes in addition to chemistry and structure optimization.

Titanium dioxide (TiO₂) has been studied as a suitable material for use in n-type metal-insulator-semiconductor (MIS) contacts due to its low conduction band offset with silicon. TiO₂ serves to de-pin the Fermi level between the metal and semiconductor, but it also introduces an additional tunneling resistance which is detrimental to creating low resistance contacts. It is known that oxygen deficient TiO₂ is n-doped, which lowers the resistivity of the material. In this work, we demonstrate a method to control film stoichiometry and resistivity in TiO₂ deposited using atomic layer deposition (ALD) through the use of rapid thermal annealing (RTA). We show that we can reduce the resistivity by more than two orders of magnitude by creating a highly oxygen deficient film.
TiO₂ was grown using ALD with alternating pulses of tetrakis(dimethylamido)titanium (TDMAT) and water. The film was deposited on a silicon substrate covered in a thick layer of insulating SiO₂, which restricts conduction to the TiO₂ film. The substrate temperature was kept constant at 150°C throughout the deposition. Following film deposition, RTA was performed to alter the oxygen content in TiO₂. The time was set to 5 minutes while the anneal temperature was varied from 300°C to 450°C. Oxygen (O₂), nitrogen (N₂), and forming gas (Ar/H₂) were used as the different annealing environments. The TiO₂ film thickness, resistivity, and stoichiometry were determined using ellipsometry, four point probe measurement, and x-ray photoelectron spectroscopy (XPS), respectively.
The as-deposited film had a thickness of approximately 35 nm with an average resistivity of 4 Ω-cm. Annealing in oxygen at temperatures as low as 300°C showed a significant increase in resistivity. Annealing in both nitrogen and forming gas at 300°C showed no appreciable change in resistivity as compared to the as-deposited sample. However, nitrogen and forming gas anneals at 450°C resulted in resistivities of approximately 3x10^-2 Ω-cm, a reduction of more than two orders of magnitude.
XPS analysis indicated that the as-deposited film is stoichiometric TiO₂. The minimal number of oxygen vacancies in the film results in a highly resistive oxide. However, the samples annealed in nitrogen and forming gas at 450°C demonstrated sub-stoichiometric oxides, as indicated by the presence of peaks corresponding to the Ti⁴⁺ and Ti³⁺ oxidation states in the XPS spectra. Comparison of the relative areas of these peaks showed that the films from both annealing conditions result in TiO_2-x, with x~0.1. During the annealing process, oxygen in the film is reduced, resulting in oxygen vacancy defect states. These defects serve to dope the film n-type, which correlates with the large reduction in resistivity.

A number of previous studies have shown that meshes and porous structures can be used to separate oil/water mixtures through careful control of surface energy and preferential wettability. However, these structured surfaces are prone to fouling by oil and dirt. The photocatalytic and hydrophilic nature of titania coatings can be exploited to ensure preferential wetting of water over oil under ultraviolet (UV) irradiation and this provides a mechanism for recovery from fouling. In this work, we studied the photoinduced cleaning ability of nanoporous titania coatings in oil-water environments for fouling recovery in oil-water separation applications.

Titania nanoporous surfaces were prepared by depositing TiO₂ nanoparticles onto flat substrates using the Layer-by-Layer (LBL) self assembly technique, and then contaminated with oil to simulate typical fouling conditions experienced in oil-water separation applications. The resulting hydrophobic surfaces were irradiated with UV light in an oil-water environment to photocatalytically decompose the organic pollutant oil and restore hydrophilicity. The kinetics of this conversion from hydrophobicity to hydrophilicity were studied in situ under various UV intensities using goniometric measurements. We observed that the rate of recovery under UV irradiation was much faster than the initial rate of fouling.

A simple adsorption-photocatalysis model (LuCY – Langmuir-Hinshelwood Cassie-Baxter Young) was developed to relate two surface phenomena - photocatalysis and wetting. In this way, we could study photocatalysis rates using wetting data for the first time. We also demonstrated that UV irradiation can be used for in situ removal of oil fouling from a mesh coated with TiO₂ nanostructures. Our kinetics model can be applied to find the rate of removal of oil from the surface, and using this model we can optimally control the fouling remediation in membrane recovery applications.