Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.

Size controlled nanographene oxides (NGOs; <50 nm) were prepared by a two-step oxidation process and NGOs were self-assembled with TiO 2 nanoparticles to form the core/shell structure. Nanosized GO-coated TiO 2 nanoparticles (NGOTs) were then reduced by a photocatalytic process under UV irradiation to obtain graphene-coated TiO 2 . This is clearly different from the typical graphene/TiO 2 composite with the particles-on-a-sheet geometry and is the first study on the core/ shell structure of its kind. The physicochemical properties of NGOs and the reduced NGOTs (r-NGOTs) were characterized by various analytical and spectroscopic methods (AFM, FT-IR, XPS, TEM, EELS, etc.). The photocatalytic and photoelectrochemical activities of r-NGOT were compared with a composite of r-GO/TiO 2 that has TiO 2 nanoparticles loaded on a larger graphene sheet (r-LGOT). The photocatalytic production of hydrogen was measured in the aqueous suspension of the composite photocatalyst under UV irradiation (λ > 320 nm), and the photoelectrochemical behaviors were characterized using the electrode coated with the composite photocatalyst. The rates of H 2 production and photocurrent generation were higher with r-NGOT than r-LGOT, which indicates that the presence of r-GO shell on the surface of TiO 2 facilitates the interfacial electron transfer. The direct contact between r-NGO and TiO 2 is maximized in r-NGOT by retarding the charge recombination and accelerating the electron transfer. The geometry of the core/shell structure should be effective in the design of a graphene/TiO 2 composite for solar conversion applications.

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For monocrystalline TiO2 electrodes, capacitive currents are observed at potentials that are negative enough to induce the filling of conduction band states. Nanoparticulate electrodes exhibit, apart from these currents, an additional pair of capacitive peaks at more positive potentials, which can be attributed to charge traps in the band gap. We have taken advantage of the well-defined morphology and crystal structure of three different types of rutile electrodes to investigate the nature of these band gap states. In particular, nanostructured films composed of oriented wires, films of randomly distributed nanoparticles, and smooth single crystals have been used. The analysis of the cyclic voltammetry response reveals a strong dependence of the trap state concentration on the morphological structure of the films. On the basis of results concerning the surface modification of the electrodes, we propose a model with a location of these band gap states at grain boundaries. We report, furthermore, on a new procedure to prepare hierarchically organized nanostructures by direct deposition of nanowires onto nanoparticulate films in aqueous solutions at low temperature. From a practical point of view, this procedure allows for a systematic tuning of the inner surface area and the porosity of the original samples.

Titanium improves water oxidation yields over hematite photoanodes, tailoring its surface state density (kinetics) and hematite-pseudobrookite heterojunctions (energetics).

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Tungsten trioxide (WO 3 ) is being investigated as one of the most promising materials for water oxidation using solar light. Its inherent surface-related drawbacks (e.g., fast charge recombination caused by surface defect sites, the formation of surface peroxo-species, etc.) are nowadays being progressively overcome by different methods, such as surface passivation and the deposition of co-catalysts. Among them, the role of surface passivation is still poorly understood. Herein, transparent WO 3 (electrodeposited) and Al 2 O 3 /WO 3 (prepared by atomic layer deposition, ALD) thin film electrodes were employed to investigate the role of an alumina overlayer by using both photoelectrochemical and laser flash photolysis measurements. Films with a 5 nm-alumina overlayer (30 ALD cycles) showed an optimum photoelectrochemical performance, portraying a 3-fold photocurrent and Faradaic efficiency enhancement under voltage biases. Moreover, IPCE measurements revealed that alumina effect was only significant with an applied potential ca. 1 V (vs. Ag/AgCl), matching the thermodynamic potential for water oxidation at pH 1 (0.97 V vs. Ag/AgCl).According to the investigation of electron accumulation through optical absorption measurements, the alumina overlayer dominantly decreased the number of electron trapping sites on the WO 3 surface, eventually facilitating photoelectron transfer to the external circuit in the presence of a positive bias. In addition, the laser flash photolysis measurements of WO 3 and Al 2 O 3 /WO 3 thin films clearly showed that the electron trapping decreased in the presence of the alumina overlayer whereas the hole trapping relatively increased with alumina, facilitating water photooxidation and rendering a more sluggish recombination process. These results provide a physical insight into the passivation process that could be used as a guideline for further development of efficient photoanodes in artificial photosynthesis. Broader contextWater oxidation as a supply of electrons and protons is currently considered a major problem in realizing articial photosynthesis and water splitting for solar energy conversion and storage. Tungsten trioxide (WO 3 ) is being investigated as one of the most promising materials for water photooxidation. Its inherent surface-related drawbacks (e.g., fast charge recombination caused by surface defect sites, the formation of surface peroxo-species, etc.) are being progressively overcome nowadays by means of different methods, such as surface passivation and the deposition of co-catalysts. Among these, the role of surface passivation is not well understood. In this study, a systematic investigation was tried to nd out the role of an Al 2 O 3 overlayer. Al 2 O 3 /WO 3 (overcoated by atomic layer deposition) showed marked enhancements in both the photocurrent (ca. 3 times) and the Faradaic efficiency (ca. 3 times) for O 2 photogeneration in the presence of an applied bias. We suggest that the alumina overlayer not only suppresses the formation of surface peroxo-species b...

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