Preparation of hybrid WO3–TiO2 nanotube photoelectrodes using anodization and wet impregnation: Improved water-splitting hydrogen generation performance

Hybrid tungsten trioxide-titanium dioxide (WO₃–TiO₂) nanotube photoelectrodes were prepared using simple anodization and wet impregnation. These hybrid nanotube photoelectrodes significantly enhanced their photoelectrochemical (PEC) water-splitting performances compared with pure TiO₂ nanotube photoelectrodes. This study aims to determine the optimum soaking time in ammonium paratungstate used as the tungsten precursor for incorporating WO₃ species into TiO₂ nanotube photoelectrodes. A low content of WO₃ species successfully diffused into the TiO₂ lattice and formed W–O–Ti bonds, which significantly promoted effective charge separation by trapping photo-induced electrons from TiO₂. Thus, the photocurrent density, photoconversion efficiency, STH efficiency, and H₂ generation of the resultant hybrid nanotubes were increased. However, excess WO₃ species in the TiO₂ nanotubes resulted in poor PEC water-splitting performance. This behavior was attributed to the large agglomerates of WO₃ species were covered on the surface nanotubes that formed undesirable layers. Consequently, these undesirable layers would act as recombination sites for photo-induced electrons and holes.     

TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact

In traditional solar cells, metal-semiconductor contacts used to extract photogenerated carriers are very important. In dye-sensitized solar cells (DSSC) not much attention has been given to contact between the TiO₂ and the transparent conducting glass (TCO), which is used instead of a metal contact to extract electrons. TiO₂ layers obtained by microwave-activated chemical-bath deposition (MW-CBD) are proposed to improve TiO₂ contact to conducting glass. Spectra of incident photon to current conversion efficiency (IPCE) are obtained for two-photoelectrode TiO₂ photoelectrochemical cells. IPCE spectra show higher values when TiO₂ double layer photoelectrodes are used. In these, the first layer or contacting layer is made by MW-CBD. Best results are obtained for double layer photoelectrodes on FTO (SnO₂:F) as conducting oxide substrate. Modeling of IPCE spectra reveals the importance of electrical contact and electron extraction rate at the TiO₂/TCO interface.     

Photoelectrochemical application of mesoporous TiO2/WO3 nanohoneycomb prepared by sol–gel method

A mesoporous TiO₂/WO₃ nanohoneycomb at a molar ratio of 3:1 was prepared by sol–gel method for photoelectrochemical splitting of water. In order to create a highly porous structure, the composite TiO₂/WO₃ with a block copolymer internal template was deposited on the substrate covered with polystyrene (PS) nanospheres. A mesoporous TiO₂/WO₃ composite nanohoneycomb was obtained after removing the PS spheres and copolymer by thermal treatment. It exhibited a lower band gap energy than TiO₂ so that the optical absorption edge was shifted toward the visible light region. It also showed a better photoelectrochemical efficiency of water splitting and higher production of hydrogen due to lower energy gap, higher reactive surface area, and better charge separation efficiency.     

Iron oxide (n-Fe2O3) nanowire films and carbon modified (CM)-n-Fe2O3 thin films for hydrogen production by photosplitting of water

Iron oxide n-Fe₂O₃ nanowire photoelectrodes were synthesized by thermal oxidation of Fe metal sheet (Alfa Co. 0.25 mm thick) in an electric oven then tested for their photoactivity. The photoresponse of the n-Fe₂O₃ nanowires was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen, which is proportional to photocurrent density, J_p. The optimized electric oven-made n-Fe₂O₃ nanowire photoelectrodes showed photocurrent densities of 1.46 mA cm⁻² at measured potential of 0.1 V/SCE at illumination intensity of 100 mW cm⁻² from a Solar simulator with a global AM 1.5 filter. For the optimized carbon modified (CM)-n-TiO₂ synthesized by thermal flame oxidation the photocurrent density for water splitting was found to increase by two fold to 3.0 mA cm⁻² measured at the same measured potential and the illumination intensity. The carbon modified (CM)-n-Fe₂O₃ electrode showed a shift of the open circuit potential by −100 mV/SCE compared to undoped n-Fe₂O₃ nanowires. A maximum photoconversion efficiency of 2.3% at applied potential of 0.5 V/E_aoc was found for CM-n-Fe₂O₃ compared to 1.69% for n-Fe₂O₃ nanowires at higher applied potential of 0.7 V/E_aoc. These CM-n- Fe₂O₃ and n- Fe₂O₃ nanowires thin films were characterized using photocurrent density measurements under monochromatic light illumination, UV-Vis spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO₃/TiO₂ nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO₂ and WO₃ layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO₃ microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO₂ nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO₃ and TiO₂ films show thickness and microstructure dependence. Among individual WO₃ layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO₂ nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO₃-wedge type layer with a layer of TiO₂ nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO₃ layers. The highest photocurrent that is measured in the combinatorial library of porous WO₃/TiO₂ films is as high as 0.11 mA/cm². Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

Photocatalytic oxidation on nanostructured chalcogenide modified titanium dioxide

Surface mesoscopic titanium dioxide (P25) films deposited onto conducting glass plates (SnO₂:F) were modified by colloidal Ru_xSe_y nanoparticles (2 nm diameter). A decrease of the photocurrent was found upon modification of TiO₂ films. However, interfacial electron transfer kinetics to oxygen was favored. The increase of the catalyst surface concentration onto TiO₂, shifts the onset of the photocurrent under UV-illumination, to 0.6 V/RHE in presence of oxygen dissolved in the electrolyte. Concomitant to this, the cathodic current becomes important and shifts to more positive potentials. This phenomenon allows the system to work catalytically under open circuit conditions. On non-modified TiO₂, the application of a 0.3 V/RHE potential leads to an enhancement of the photooxidation of formic acid. Photocurrent images revealed a non-homogeneous distribution of the catalyst on the titanium dioxide films.     

The influence of surface morphology of TiO2 coating on the performance of dye-sensitized solar cells

The optimization of solar energy conversion efficiency of dye-sensitized solar cells (DSSCs) was investigated by the tuning of TiO₂ photoelectrode's surface morphology. Double-layered TiO₂ photoelectrodes with four different structures were designed by the coating of TiO₂ suspension, incorporated with low and high molecular weight poly(ethylene glycol) as a binder. Among these four systems, P2P1, where P1 and P2 correspond to the molecular weight of 20,000 and 200,000, respectively, showed the highest efficiency under the conditions of identical film thickness and constant irradiation. This can be explained by the larger pore size and higher surface area of P2P1 TiO₂ electrode than the other materials as revealed by scanning electron microscopic (SEM) and Brunauer–Emmett–Teller (BET) analyses. Electrochemical Impedance Spectroscopy (EIS) analysis shows that P2P1 formulation displayed a smaller resistance than the others at the TiO₂/electrolyte interface. The best efficiency (η) of 9.04% with the short-circuit photocurrent density (J_sc) and open-circuit voltage (V_oc) of 18.9 mA/cm² and 0.74 V, respectively, was obtained for a solar cell by introducing the light-scattering particles to the TiO₂ nanoparticles matrix coated on FTO electrode having the sheet resistivity of 8 Ω/sq.     

Oxidative photoelectrochemical technology with Ti/TiO2 anodes

Rutile and anatase TiO₂ films have been grown on Ti plates by thermal (500–800°C) and anodic oxidation followed by thermal annealing (400–500°C), respectively. The photoelectrochemical efficiency of these photoanodes, evaluated by current density measurements in the photooxidation of 4-methoxybenzyl alcohol in deaerated CH₃CN, has been determined. The photocurrent efficiency increases with the thickness of the TiO₂ rutile film up to 1 μm (the most efficient thickness). At the wavelengths furnished by the irradiation apparatus similar thicknesses of anatase and rutile films show nearly the same efficiencies. Anodic bias produces similar relative increases of current intensity in both crystalline forms.     

Does carbon doping of TiO2 allow water splitting in visible light? Comments on “Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting”

Reports of water splitting by carbon-doped titanium dioxide (TiO₂) photoelectrodes under visible illumination are critically examined. Xu et al. [Sol. Energy Mater. Sol. Cells 91 (2007) 938] recently reported significant incident photon conversion efficiencies (IPCEs) at visible wavelengths for carbon-doped TiO₂ in thin film and nanotube form. Evidence is given here that these results were due to an artefact in the measurements. Further, it is pointed out that the mechanism proposed for water splitting under visible illumination is unphysical, and the photocurrents presented are shown to be grossly inconsistent with the IPCE data. Other workers have also measured non-zero IPCEs at visible wavelengths for carbon-doped TiO₂, but have not presented this as evidence of water splitting. In other cases, carbon doping was performed in a reducing atmosphere, and measured visible activity is most likely a result of oxygen vacancies. It is concluded that there is no convincing evidence in the literature of water splitting under visible light in carbon-doped TiO₂.     

Single crystalline TiO2 nanorods with enhanced visible light activity for solar hydrogen generation

Single crystalline TiO₂ nanorods and polycrystalline nanotubes were fabricated with same length to investigate the effects of their nanostructures on photocatalytic properties for splitting water. In order to enhance the visible light absorbance, TiO₂ nanorods and nanotubes were sensitized with semiconductor nanoparticles such as CdS, CdSe, and CdS/CdSe, and compared in viewpoint of solar hydrogen generation. It was observed that single-crystalline nanorods showed superior photocatalytic properties to polycrystalline nanotubes, and also the potential level of the nanorods with rutile phase was measured as lower than that of the nanotubes with mixture of anatase and rutile. Further improvement of photo-conversion efficiency was obtained by subsequent heat treatments of the sensitized photoelectrodes. It turns out that the improvement is attributed to the improved crystallinity and the increased size of the nanoparticles during the post-annealing treatments. It was demonstrated that TiO₂ nanorods with lower potential level and a single crystalline phase on FTO glass were advantageous for effective charge injection from the sensitized nanoparticles and transport without recombination lost at grain boundaries.     

Basic concepts of photoelectrochemical solar energy conversion systems

Abstract

The development of novel oxides, nanocomposites and architectures is in demand for the direct conversion of solar energy into other forms of energy, such as chemical and electrical energy. Especially, those oxides and devices, which can be used for photolysis of water (hydrogen and oxygen evolution) and for water purification, are of interest. So far, semiconducting oxides such as TiO₂, Fe₂O₃, WO₃, SrTiO₃, tantalates and niobates are the only class of materials which have shown high stability as photoelectrodes towards corrosion in the rate limiting step in the oxygen evolution reaction (OER) under illumination at the electrode/electrolyte interface during photolysis of water. Oxides have to be developed to be highly conductive and have a bandgap, which can be achieved by tailoring the defect chemistry of the oxides or by formation of a suited mixed oxide phase. Of special interest is the preparation of highly conductive p- and n-type TiO₂ and WO₃ as well as their alloys as corrosion stable and photoelectrocatalytically active electrodes. Bandgap reduction of pure TiO₂ involved the formations of solid solutions of TiO₂–FeO and TiO₂–Fe₂O₃, which were reported to have a bandgap of ～2·2 eV.¹ Besides TiO₂, WO₃ also has a superior stability as a photoelectrode material in the OER. Alloying with FeO also leads to lowering of the bandgap. Alternatively, ternary oxides of the systems Ni–Co–O and Ni–Fe–O are known for their high catalytic activity in the OER. They are considered as potential cocatalysts in the process of water oxidation. The materials can also be used in hybrid photoelectrodes consisting of a photovoltaic structure to absorb the sunlight with a corrosion stable and catalytically active window layer, which is in contact with the electrolyte.     

Cycle test and degradation analysis of WO3/PC+LiClO4/CeO2·TiO2 electrochromic device

We fabricated an electrochromic full cell device adopting WO₃ as a working electrode, and 1 M LiClO₄ in PC with 3% water addition as an electrolyte and CeO₂·TiO₂ with various thicknesses as an ion storage layer. CeO₂·TiO₂ with less than 100 nm shows large charge density but the long-term cyclability is not good due to lithium ion diffusion into ITO thin film. Therefore, the thickness of CeO₂·TiO₂ ion storage layer should be coated at more than 200 mm/min. Long-term cycle test results show that CeO₂·TiO₂ ion storage layer with more than 150 nm thickness and two time coating enhance the long-term stability. SIMS analysis results show that the degradation is due to the remaining lithium ion in the working electrode, WO₃.     

Direct synthesis of nanocrystalline titanium dioxide/carbon composite and its catalytic effect on NaAlH4 for hydrogen storage

Nanocrystalline titanium dioxide/carbon composite (TiO₂/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)₄) and resol as precursors. The prepared TiO₂/C composite was mainly in the anatase structure with an average particle size under 20 nm, which was then introduced in NaAlH₄ as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO₂/C and TiO₂ can obviously improve the kinetics of NaAlH₄, while NaAlH₄ with 3 mol% TiO₂/C exhibits better cycling stability than NaAlH₄ with 3 mol%TiO₂. The hydrogen storage capacity of NaAlH₄ with TiO₂/C remains stable after 5th cycle, and about 94% of initial hydrogen is released, while the capacity of NaAlH₄ with TiO₂ decreases continuously during cycling, and only 88% of initial hydrogen is released after 10th cycle. Furthermore, NaAlH₄ with 3 mol%TiO₂/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload 4.16 and 1.63wt% hydrogen at 50 and 30 bar hydrogen pressures, respectively.     

Preparation of Pt supported on WO3-C with enhanced catalytic activity by microwave-pyrolysis method

The WO₃-C hybrid materials are prepared by intermittently microwave-pyrolysis using ammonium tungstate as the precursor, and then Pt nano-particles are deposited by microwave-assited polyol process on WO₃-C. The TEM images show the dispersion of ∼10 nm WO₃ particles size supported on carbon and ∼3 nm Pt metal crystallites supported on WO₃-C. XRD results illustrate that WO₃ presented as monoclinic phase and the content of WO₃ in WO₃/C and Pt/WO₃-C catalysts is further characterized by EDAX. Furthermore, XPS characterizations indicate that the interaction between Pt and WO₃ is dramatically enhanced after heat treatment at 200 °C. The activities of Pt/WO₃-C for the electrochemical oxidation of methanol are compared with Pt/C in acid solution by cyclic voltammetry, CO-stripping and chronoaperometry. Pt/WO₃-C catalyst calcined at 200 °C exhibits the highest activity per electrochemical active surface area for methanol oxidation and is 60 mV more negative for CO electro-oxidation than that of Pt/C and Pt/WO₃-C without heat treatment. The great enhancement of electrochemical performance may be due to the improvement of the synergistic effect between Pt and WO₃ in Pt/WO₃-C catalyst after heat treatment.     

Mix-solvent-thermal method for the synthesis of anatase nanocrystalline titanium dioxide used in dye-sensitized solar cell

Nanocrystalline TiO₂ with almost pure anatase form has been synthesized through the Mix-solvent-thermal method (MST) by using TiCl₄ as the starting material. The mean size of the synthesized TiO₂ is 10 nm with narrow distribution. High-performance dye-sensitized solar cell with nanocrystalline TiO₂ electrode formed from MST was achieved. Its I_sc and V_oc values reached 21.62 mA/cm² and 727.9 mV, respectively, and the photovoltaic conversion efficiency reached 9.13%, i.e. 7.5% higher than those of solar cells with TiO₂ made from the traditional precursor of titanium alkoxides. To our knowledge they are the highest values obtained from the solar cells with nanocrystalline TiO₂ electrode formed from the hydrolysis of TiCl₄.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Sol–gel derived nanocrystalline CeO2–TiO2 coatings for electrochromic windows

Mixed CeO₂–TiO₂ coatings synthesized by sol–gel spin coating process using mixed organic–inorganic Ti(OC₃H₇)₄ and CeCl₃·7H₂O precursors with different Ce/Ti mole ratios were investigated by a wide range of characterization techniques. The attempts were directed towards achieving coatings with high transparency in the visible region and good electrochemical properties. Elucidation of the structural and optical features of the films yielded information on the aspects relevant to their usage in transmissive electrochromic devices. The films have been found to exhibit properties for counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion, high enough for practical uses. The high optical modulation and fastest switching for WO₃ film in the device configuration with the Ce/Ti (1:1) film is interpreted in terms of conducive microstructural changes induced by addition of TiO₂ in an amount equivalent to CeO₂.     

Fabrication of porous titanium dioxide fibers and their photocatalytic activity for hydrogen evolution

TiO₂ semiconductor is one of the important photocatalysts for solar light conversion. The challenge is how to improve their efficiency. Creation of porous structures on/in the fibers could favor them a higher surface area as compared to the conventional solid counterparts, which thus could make the achievement for the desired high efficiency. In present work, we report the fabrication of porous TiO₂ fibers with high purity via electrospinning of butyl titanate (TBOT) and polyvinylpyrrolidone (PVP) combined with the subsequent calcination in air. It is found that the TBOT content in the spinning solution plays a profound effect on the growth of the fibers, enabling the synthesis of porous TiO₂ fibers with tunable structures and high purity. The photocatalytic activity for hydrogen evolution of the as-fabricated TiO₂ nanostrcutres has been investigated, suggesting that porous TiO₂ nanomaterials with a high purity and well-defined one-dimensional fiber shape could be an excellent candidate to be utilized as the photocatalyst for hydrogen evolution.     

Electrochemical evaluation of rutile TiO2 nanoparticles as negative electrode for Li-ion batteries

Nanosized rutile TiO₂ has been prepared by sol–gel chemistry from a glycerol-modified titanium precursor in the presence of an anionic surfactant. The sample has been characterized by X-ray diffraction, nitrogen sorption, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and electrochemical tests. Nanosized rutile TiO₂ has been electrochemically investigated using two potential windows: 1.2–3 V and 1–3 V. It exhibits excellent high rates capabilities and good cycling stability.