TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment

Metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells. However, their performance is often limited by poor charge carrier transport. We show that this problem can be addressed by using separate materials for light absorption and carrier transport. Here, we report a Ta:TiO2|BiVO4 nanowire photoanode, in which BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. Electrochemical and spectroscopic measurements provide experimental evidence for the type II band alignment necessary for favorable electron transfer from BiVO4 to TiO2. The host-guest nanowire architecture presented here allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent density of 2.1 mA/cm(2) at 1.23 V vs RHE.

A mechanism for the hole-mediated water photooxidation on TiO2 (1 0 1) surfaces

The mechanism of water photooxidation on TiO2 surfaces is still controversial. Here we report a first-principles density functional study based on a hybrid functional method in which an adsorbed water molecule is found to directly interact with a self-trapped hole at a bridging oxygen site and to transform into an OH(•) radical species through a concerted proton/hole transfer. This study analyzes both the thermodynamics and kinetics of this step of the reaction, which is generally considered to be the rate determining one. The fate of the OH(•) radical is then investigated in terms of its reactivity with different surface species, with a second OH(•) radical, or with a second water molecule coming from the environment. We find that OH(•) radicals can either acquire a hydrogen from surrounding water molecules or, if they meet, couple to form hydrogen peroxide with highly associated energy gain.

Photoelectrochemical water splitting using WO3 photoanodes: the substrate and temperature roles

The influence of a substrate on the performance of WO3 photoanodes is assessed as a function of temperature. Two samples were studied: WO3 deposited on a FTO glass and anodized on a tungsten foil. Current-voltage curves and electrochemical impedance spectroscopy measurements were used to characterize these samples between 25 °C and 65 °C. The photocurrent density increased with temperature for both samples and the onset potential shifted to lower potentials. However, for WO3/FTO, a negative shift of the dark current onset was also observed. The intrinsic resistivity of this substrate limits the photocurrent plateau potential range. On the other hand, this behavior was not observed for WO3/metal. Therefore, the earlier dark current onset observed for WO3/FTO was assigned to the FTO layer. The optimal operating temperatures observed were 45 °C and 55 °C for WO3/FTO and WO3/metal, respectively. For higher temperatures, the bulk electron-hole recombination phenomenon greatly affects the overall performance of WO3 photoanodes. The stability behavior was then studied at these temperatures over 72 h. For WO3/FTO, a crystalline-to-amorphous phase transformation occurred during the stability test, which may justify the current decrease observed after the aging period. The WO3/metal remained stable, maintaining its morphology and good crystallinity. Interestingly, the preferential orientation of the aged crystals was shifted to the (-222) and (222) planes, suggesting that this was responsible for its better and more stable performance. These findings provide crucial information for allowing further developments on the preparation of WO3 photoanodes, envisaging their commercial application in PEC water splitting cells.

Recent Advances in Visible-Light-Driven Photoelectrochemical Water Splitting: Catalyst Nanostructures and Reaction Systems

Photoelectrochemical (PEC) water splitting using solar energy has attracted great attention for generation of renewable hydrogen with less carbon footprint, while there are enormous challenges that still remain for improving solar energy water splitting efficiency, due to limited light harvesting, energy loss associated to fast recombination of photogenerated charge carriers, as well as electrode degradation. This overview focuses on the recent development about catalyst nanomaterials and nanostructures in different PEC water splitting systems. As photoanode, Au nanoparticle-decorated TiO₂ nanowire electrodes exhibited enhanced photoactivity in both the UV and the visible regions due to surface plasmon resonance of Au and showed the largest photocurrent generation of up to 710 nm. Pt/CdS/CGSe electrodes were developed as photocathode. With the role of p-n heterojunction, the photoelectrode showed high stability and evolved hydrogen continuously for more than 10 days. Further, in the Z-scheme system (Bi₂S₃/TNA as photoanode and Pt/SiPVC as photocathode at the same time), a self-bias (open-circuit voltage V _oc = 0.766 V) was formed between two photoelectrodes, which could facilitate photogenerated charge transfers and enhance the photoelectrochemical performance, and which might provide new hints for PEC water splitting. Meanwhile, the existing problems and prospective solutions have also been reviewed.

Enhanced photoelectrochemical performance of electrodeposited hematite films decorated with nanostructured NiMnOx

In the present work, we report a novel nickel-manganese oxide (NiMnO_x) decorated hematite (α-Fe₂O₃) photoanode for efficient water splitting in a photoelectrochemical (PEC) cell.

Highly efficient continuous-flow oxidative coupling of amines using promising nanoscale CeO2–M/SiO2 (M = MoO3 and WO3) solid acid catalysts

Nanoscale CeO₂–MoO₃/SiO₂ solid acid shows an outstanding catalytic performance in the oxidative coupling of amines under industrially-favourable conditions.

Heterojunction Cr2O3/CuO:Ni photocathodes for enhanced photoelectrochemical performance

Heterojunction p-type photoelectrodes consisting of chromium oxide (Cr₂O₃) and copper oxide (CuO)-doped nickel (Ni) were prepared using aerosol-assisted chemical vapour deposition (AACVD) and spin-coating.

Construction of inorganic–organic 2D/2D WO3/g-C3N4 nanosheet arrays toward efficient photoelectrochemical splitting of natural seawater

Inorganic–organic 2D/2D WO₃/g-C₃N₄ nanosheet arrays are fabricated and proved to be an efficient and stable photoanode for photoelectrochemical splitting of natural seawater.

Iron and nickel co-doped cobalt hydroxide nanosheets with enhanced activity for oxygen evolution reaction

Herein, we have successfully synthesized iron and nickel co-doped cobalt hydroxide (Co–Ni–Fe₅₁₁) nanosheets for the OER.

A three-dimensional interconnected hierarchical FeOOH/TiO₂/ZnO nanostructural photoanode for enhancing the performance of photoelectrochemical water oxidation

A novel ZnO/TiO2/FeOOH hierarchical nanostructure has been synthesized by a low temperature chemical bath deposition method. The integrated three-dimensional (3D) nanostructure consists of one-dimensional (1D) ZnO/TiO2 core-shell nanowire arrays and two-dimensional (2D) interconnected FeOOH nanosheets. By applying such a hierarchical nanostructure as a photoanode for photoelectrochemical water reaction, higher photostability and photocurrent density are gained compared with the reported ZnO based nanostructures. It is concluded that the giant enhancement of the properties is because, in the process of photoelectrochemical reaction, electron-hole separation and transfer are enhanced efficiently through the ZnO/TiO2 heterojunction, and in the meanwhile, terminal interconnected FeOOH nanosheets play both the roles of a surface catalyst and a protective layer effectively to accelerate water splitting reaction and enhance photostability. Based on such an environmentally friendly hierarchical nanostructure, photoelectrochemical water splitting and other similar reactions could be performed effectively and economically.

Discovery of Overcoating Metal Oxides on Photoelectrode for Water Splitting by Automated Screening

We applied an automated semiconductor synthesis and screen system to discover overcoating film materials and optimize coating conditions on the BiVO4/WO3 composite photoelectrode to enhance stability and photocurrent. Thirteen metallic elements for overcoating oxides were examined with various coating amounts. The stability of the BiVO4/WO3 photoelectrode in a highly concentrated carbonate electrolyte aqueous solution was significantly improved by overcoating with Ta2O5 film, which was amorphous and porous when calcined at 550 °C. The photocurrent for the water oxidation reaction was only minimally inhibited by the presence of the Ta2O5 film on the BiVO4/WO3 photoelectrode.

Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy

The conversion, storage, and utilization of renewable energy have all become more important than ever before as a response to ever-growing energy and environment concerns. The performance of energy-related technologies strongly relies on the structure and property of the material used. The earth-abundant family of tungsten oxides (WOx ≤3 ) receives considerable attention in photocatalysis, electrochemistry, and phototherapy due to their highly tunable structures and unique physicochemical properties. Great breakthroughs have been made in enhancing the optical absorption, charge separation, redox capability, and electrical conductivity of WOx ≤3 through control of the composition, crystal structure, morphology, and construction of composite structures with other materials, which significantly promotes the efficiency of processes and devices based on this material. Herein, the properties and synthesis of WOx ≤3 family are reviewed, and then their energy-related applications are highlighted, including solar-light-driven water splitting, CO2 reduction, and pollutant removal, electrochromism, supercapacitors, lithium batteries, solar and fuel cells, non-volatile memory devices, gas sensors, and cancer therapy, from the aspect of function-oriented structure design and control.

Highly Efficient Photoelectrochemical Hydrogen Generation Using Zn(x)Bi2S(3+x) Sensitized Platelike WO₃ Photoelectrodes

Zn(x)Bi2S(3+x) sensitized platelike WO3 photoelectrodes on FTO substrates were for the first time prepared via a sequential ionic layer adsorption reaction (SILAR) process. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet visible spectrometry (UV-vis), and Raman spectra. The results show that the ZnxBi2S3+x quantum dots (QDs) are uniformly coated on the entire surface of WO3 plates, forming a WO3/Zn(x)Bi2S(3+x) core/shell structure. The Zn(x)Bi2S(3+x)/WO3 films show a superior ability to capture visible light. High-efficiency photoelectrochemical (PEC) hydrogen generation is demonstrated using the prepared electrodes as photoanodes in a typical three-electrode electrochemical cell. Compared to the Bi2S3/WO3 photoelectrodes, the Zn(x)Bi2S(3+x)/WO3 photoelectrodes exhibit good photostability and excellent PEC activity, and the photocurrent density is up to 7.0 mA cm(-2) at -0.1 V versus Ag/AgCl under visible light illumination. Investigation of the electron transport properties of the photoelectrodes shows that the introduction of ZnS enhances the photoelectrons' transport rate in the photoelectrode. The high PEC activity demonstrates the potential of the Zn(x)Bi2S(3+x)/WO3 film as an efficient photoelectrode for hydrogen generation.

Tuning of the crystal engineering and photoelectrochemical properties of crystalline tungsten oxide for optoelectronic device applications

WO₃crystals with {002} or {111} facets primarily exposed, WO₃films with dominant orientations, doping and heterostructuring are highlighted.

Enhancing photoelectrochemical activity of CdS quantum dots sensitized WO3 photoelectrodes by Mn doping

Mn-doped CdS quantum dots sensitized WO₃ photoelectrodes were successfully synthesized by a combination of hydrothermal and chemical bath deposition (CBD) methods.

Au nanoparticle decorated WO3 photoelectrode for enhanced photoelectrochemical properties

WO₃–Au photoanode exhibited about 4 times enhancement of photocurrent density compared to WO₃ photoanode because WO₃–Au possess the higher light absorption and lower transport resistance due to surface plasmonic resonance of Au nanoparticles.

Exposed facet and crystal phase tuning of hierarchical tungsten oxide nanostructures and their enhanced visible-light-driven photocatalytic performance

Tungsten oxide hierarchical nanostructures controllably assembled with one dimensional nanostructures which exhibit different exposed facets and crystal phases were synthesized via a facile hydrothermal reaction assisted by urea.

Hydrothermal grown nanoporous iron based titanate, Fe₂TiO₅ for light driven water splitting

We report the synthesis of iron based titanate (Fe2TiO5) thin films using a simple low cost hydrothermal technique. We show that this Fe2TiO5 works well as a photoanode for the photoelectrochemical splitting of water due to favorable band energetic. Further characterization of thin films including band positions with respect to water redox levels has been investigated. We conclude that Fe2TiO5 is a promising material comparable to hematite for constructing PEC cells.

WO₃ nanoflakes for enhanced photoelectrochemical conversion

We developed a postgrowth modification method of two-dimensional WO3 nanoflakes by a simultaneous solution etching and reducing process in a weakly acidic condition. The obtained dual etched and reduced WO3 nanoflakes have a much rougher surface, in which oxygen vacancies are created during the simultaneous etching/reducing process for optimized photoelectrochemical performance. The obtained photoanodes show an enhanced photocurrent density of ∼1.10 mA/cm(2) at 1.0 V vs Ag/AgCl (∼1.23 V vs reversible hydrogen electrode), compared to 0.62 mA/cm(2) of pristine WO3 nanoflakes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that this improved performance of dual etched and reduced WO3 nanoflakes is attributed to the increase of charge carrier density as a result of the synergetic effect of etching and reducing.

Tungsten oxide nanorod growth by pulsed laser deposition: influence of substrate and process conditions

Tungsten oxide nanorods (NRs) have been grown on W, Ta and Cu substrates following 193 nm pulsed laser ablation of a WO3 target in a low background pressure of oxygen. The deposited materials were analysed by scanning and (high resolution) transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), X-ray diffraction, Raman and X-ray photoemission spectroscopy, and tested for field emission. In each case, HRTEM analysis shows NR growth along the [100] direction, and clear stacking faults running along this direction (which are also revealed by streaking in the SAED pattern perpendicular to the growth axis). The NR composition in each case is thus determined as sub-stoichiometric WO(3-δ), but the NR morphologies are very different. NRs grown on W or Ta are short (hundreds of nm in length) and have a uniform cross-section, whereas those grown on a Cu substrate are typically an order of magnitude larger, tapered, and display a branched, dendritic microstructure. Only these latter NRs give significant field emission.

Nanostructured tungsten trioxide thin films synthesized for photoelectrocatalytic water oxidation: a review

The recent developments of nanostructured WO3 thin films synthesized through the electrochemical route of electrochemical anodization and cathodic electrodeposition for the application in photoelectrochemical (PEC) water splitting are reviewed. The key fundamental reaction mechanisms of electrochemical anodization and cathodic electrodeposition methods for synthesizing nanostructured WO3 thin films are explained. In addition, the effects of metal oxide precursors, electrode substrates, applied potentials and current densities, and annealing temperatures on size, composition, and thickness of the electrochemically synthesized nanostructured WO3 thin films are elucidated in detail. Finally, a summary is given for the general evaluation practices used to calculate the energy conversion efficiency of nanostructured WO3 thin films and a recommendation is provided to standardize the presentation of research results in the field to allow for easy comparison of reported PEC efficiencies in the near future.

Branched WO3 nanosheet array with layered C3 N4 heterojunctions and CoOx nanoparticles as a flexible photoanode for efficient photoelectrochemical water oxidation

A hybrid WO3 /C3 N4 /CoOx system exhibits excellent photoelectrochemical activity for water oxidation. The system comprises a novel three-dimensionally branched WO3 nanosheet array coated with a layer of C3 N4 heterojunctions that are further decorated with CoOx nanoparticles. The photoelectrochemical activity arises from the effective light harvesting due to the 3D structure and "window effect," the excellent charge separation and transport in the heterojunction, and the fast interfacial charge collection and surface reactions due to the large surface area.

Synthesis of WO3 nanoblades by the dealloying of glancing angle deposited W-Fe nanocolumnar thin films

Glancing angle co-deposition of well-separated W-Fe nanocolumns was carried out using a W oblique angle sputter source and a Fe confocal incidence source. As-deposited nanocolumns with an overall composition of W64.6Fe35.4 (at.%) exhibited an average column width w nc of 77 ± 15 nm with predominant growth in the β-W phase. With the aim of synthesizing highly porous nanostructures, the as-deposited precursor W-Fe nanocolumnar thin films were immersed in aqueous HNO3 solution for various dealloying durations (t d ). Formation of nanoflake-, nanocactus-, and nanoblade-like structures were observed during the dealloying treatment, as a result of selective dissolution of Fe from the W-Fe precursor films and simultaneous oxidation of W adatoms. By increasing the dealloying duration, the Fe concentration within the film reduced drastically and the film thickness increased by about three times in comparison to the as-deposited film. The dealloyed film exhibited an overall composition of W95.6Fe4.4, where the effective surface area of the film increased substantially. It was found that W adatom diffusion and subsequent rearrangement are crucially important in determining the resultant thin film morphology. The morphological development, corresponding compositions and crystallographic properties of different nanostructures were found to be significantly dependent on the dealloying duration. For optimized processing parameters, the selective dissolution process led to formation of single crystal monoclinic WO3 nanoblades, with growth along [002] and [020] axes.

Photoelectrochemical water oxidation on photoanodes fabricated with hexagonal nanoflower and nanoblock WO3

Hexagonal nanoflower WO3 arrays have been prepared by using RCOO(-) as the structure directing agent in the microwave-assisted hydrothermal synthesis process. The photoelectrochemical performance of the synthesized hexagonal flower-like WO3 electrode was enhanced compared with the block-like WO3 film.

Mn-modified Bi2Ti2O7 photocatalysts: bandgap engineered multifunctional photocatalysts for hydrogen generation

The applicability of pyrochlore bismuth titanate as a photocatalyst amenable to additional element inclusion resulting in a bandgap engineered composite oxide nanostructure (BECON) offers significant potential for multifunctional photo-driven applications.

A facile two-step method for fabrication of plate-like WO3 photoanode under mild conditions

Fabrication of photoelectrodes on a large-scale, with low-cost and high efficiency is a challenge for their practical application in photoelectrochemical (PEC) water splitting. In this work, a typical plate-like WO₃ photoanode was fabricated with chemical etching of the as-prepared mixed tungsten–metal oxides (W–M–O, M = Cu, Zn or Al) by a reactive magnetron co-sputtering technique, which results in a greatly enhanced PEC performance for water oxidation in comparison with that obtained from a conventional magnetron sputtering method. The current approach is applicable for the fabrication of some other semiconductor photoelectrodes and is promising for the scaling up of applications for highly efficient solar energy conversion systems.

Rational and scalable fabrication of high-quality WO3/CdS core/shell nanowire arrays for photoanodes toward enhanced charge separation and transport under visible light

High-quality one-dimensional WO3/CdS core/shell nanowire arrays used as photoanodes in photoelectrochemical (PEC) cells were for the first time prepared via a rational, two-step chemical vapor deposition process. The narrow band-gap CdS shell was homogeneously coated on the entire surface of as-grown WO3 core nanowire arrays, forming coaxial heterostructures. The one-dimensional core/shell heterostructure facilitates the photogenerated electron-hole pair separation and the electron transfer from CdS to WO3 nanowires under visible light illumination. Moreover, the core nanowire arrays provide a direct pathway for the electron transport. The present results imply that the WO3/CdS core/shell heterostructure nanowire arrays may be useful in the design of nanostructure photoanodes toward highly efficient PEC cells.

Efficient plasma route to nanostructure materials: case study on the use of m-WO3 for solar water splitting

One of the main challenges in developing highly efficient nanostructured photoelectrodes is to achieve good control over the desired morphology and good electrical conductivity. We present an efficient plasma-processing technique to form porous structures in tungsten substrates. After an optimized two-step annealling procedure, the mesoporous tungsten transforms into photoactive monoclinic WO3. The excellent control over the feature size and good contact between the crystallites obtained with the plasma technique offers an exciting new synthesis route for nanostructured materials for use in processes such as solar water splitting.