Exposure of WO3 Photoanodes to Ultraviolet Light Enhances Photoelectrochemical Water Oxidation

Ultraviolet Light-Induced Surface Changes of Tungsten Oxide in Air: Combined Scanning Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy Analysis

Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy analyses were combined to clarify the ultraviolet light-induced surface changes of WO3 in air. Identical-location STEM (IL-STEM) analysis showed that the WO3 particle surface was covered with an amorphous thin film after ultraviolet irradiation in air. X-ray photoelectron spectroscopy analysis showed that hydrocarbon decomposition and the formation of carboxyl/hydroxyl species occurred. These results suggested that the amorphous thin films consisted of photocatalytic oxidative species of hydrocarbon. The IL-STEM analysis could detect small light-induced changes. This technique will be useful for the microscopic characterization of photocatalysis or photoinduced hydrophilic conversion.

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

Hematite has been considered as a promising photoanode candidate for photoelectrochemical (PEC) water oxidation and has attracted numerous interests in the past decades. However, intrinsic drawbacks drastically lower its photocatalytic activity. Ti-based modifications including Ti-doping, Fe₂O₃/Fe₂TiO₅ heterostructures, TiO₂ passivation layers, and Ti-containing underlayers have shown great potential in enhancing the PEC conversion efficiency of hematite. Moreover, the combination of Ti-based modifications with various strategies towards more efficient hematite photoanodes has been widely investigated. Nevertheless, a corresponding comprehensive overview, especially with the most recent working mechanisms, is still lacking, limiting further improvement. In this respect, by summarizing the recent progress in Ti-modified hematite photoanodes, this review aims to demonstrate how the integration of titanium and iron atoms into hematite influences the PEC properties by tuning the carrier behaviours. It will provide more cues for the rational design of high-performance hematite photoanodes towards future practical applications.

Crystal Facet-Modulated WO3 Nanoplate Photoanode for Photoelectrochemical Glyoxal Semi-oxidation into Glyoxylic Acid

Transforming glyoxal to value-added glyoxylic acid (GA) is highly desirable but challenging due to the uncontrollable over-oxidation. In this work, we report on a first demonstration of semi-oxidation of glyoxal with high selectivity (86.5%) and activity on WO₃ nanoplate photoanode through the photoelectrochemical strategy. The optimization of reactivity was achieved via crystal facet regulation, showing a satisfactory GA production rate of 308.4 mmol m^-2 h^-2, 84.0% faradaic efficiency, and 4.3% total solar-to-glyoxylic acid efficiency on WO₃ with enriched {200} facets at 1.6 V versus RHE. WO₃ with a high {200} facet ratio exhibits more efficient electron-hole transfer kinetics, resulting in the facilitated formation of hydroxyl radicals (^•OH) and glyoxal radicals. Meanwhile, the theoretical calculation results indicate that the high selectivity and activity come from the strong adsorption ability for glyoxal and the low reaction energy for glyoxal radical generation on the (200) facets of WO₃. Moreover, the high energy demand toward oxalic acid production on WO₃ leads to the exciting semi-oxidation process.

Tuning the Electronic Structure of Monoclinic Tungsten Oxide Nanoblocks by Indium Doping for Boosted Photoelectrochemical Performance

Photoelectrochemical (PEC) water oxidation, a desirable strategy to meet future energy demands, has several bottle-necks to resolve. One of the prominent issues is the availability of charge carriers at the surface reaction site to promote water oxidation. Of the several approaches, metal dopants to enhance the carrier density of the semiconductors, is an important one. In this work, we have studied the effect of In-doping on monoclinic WO₃ nanoblocks, growing vertically over fluorine-doped tin oxide (FTO) without the aid of any seed layer. X-ray photoelectron spectroscopy (XPS) data reveals that In³⁺ ions are partially occupying the W⁶⁺ ions in In-doped WO₃ photoanode. In³⁺ ions are offering better performance by adding additional charge carriers for amplifying the expression of the number of carriers. The maximum current density value of 2.18 mA/cm² has been provided by the optimized In-doped WO₃ photoanode with 3 wt% indium doping at 1.23 V vs. RHE, which is ∼3 times higher than that of undoped monoclinic WO₃ photoanode. Mott-Schottky (MS) analysis reveals charge carrier density (N_D ) for In-doped WO₃ photoanode has been enhanced by a factor of 3. An average Faradic yield of ∼90 percent has been achieved which can serve as a model system using In³⁺ as a dopant for an inexpensive and attractive method for enhanced WO₃ based PEC water oxidation.

Photoelectrochemical Decomposition of Lignin Model Compound on a BiVO4 Photoanode

The photoelectrochemical decomposition of lignin model compounds at a BiVO₄ photoanode is demonstrated with simulated sunlight and an applied bias of 2.0 V. These prototypical lignin model compounds are photoelectrochemically converted into the corresponding aryl aldehyde and phenol derivatives in a single step with conversion of up to ≈64 % over 20 h. Control experiments suggest that vanadium sites are electrocatalytically active, which precludes the need for a redox mediator in solution. This feature of the system is corroborated by a layer of V₂ O₅ deposited on BiVO₄ serving to boost the conversion by 10 %. Our methodology capitalizes on the reactive power of sunlight to drive reactions that have only been studied previously by electrochemical or catalytic methods. The use of a BiVO₄ photoanode to drive lignin model decomposition therefore provides a new platform to extract valuable aromatic chemical feedstocks using solar energy, electricity and biomass as the only inputs.

Self-improvement of solar water oxidation for the continuously-irradiated hematite photoanode

Improving bulk- or surface-properties has been found as an effective route to regulate and enhance the photoelectrochemical (PEC) performances of some metal-oxide photoelectrodes. However, both bulk and surface self-improvement resulting from the photocharging (PC) effect is rarely reported and as a result the underlying mechanism of the PC effect is not fully understood. Here, we demonstrate that the hematite photoanode integrated with Sn doping and a TiO2 underlayer shows a substantial increase in the photocurrent density (i.e., from 0.69 to 1.12 mA cm-2 at 1.23 V relative to the standard hydrogen electrode) and a cathodic shift of the onset potential after being irradiated by a one-sun simulator for 12 h. The primary reasons for these can be categorized into two fundamental factors: (1) the enhanced bulk conductivity and the resulting decrease in carrier bulk recombination from the gradually increasing ratio of Fe2+ and Fe3+; (2) the reduced carrier surface recombination from the photogenerated passivation layer. Ultimately, both the bulk and surface electrical properties of the hematite photoanode are substantially self-improved under continuous irradiation. This work deepens the understanding of the PC effect and proves that it is a promising technique for the PEC-performance enhancement of the hematite photoanode.

Regulating effect of heterojunctions on electrocatalytic oxidation of methanol for Pt/WO3-NaTaO3 catalysts

Pt/WO₃-NaTaO₃ composite catalysts for different W/Ta molar ratios were obtained via a facile hydrothermal method.

New aspects of improving the performance of WO3 thin films for photoelectrochemical water splitting by tuning the ultrathin depletion region

In this work, we explored a facile, scalable and effective method for substantially enhancing photocurrent and incident-photon-to-current efficiency of WO₃ thin-film photoanodes by a mild reduction treatment under low oxygen pressure. Experimental data from photoelectrochemical and electrochemical impedance spectroscopies have shown that such treatment can increase the charge carrier density on WO₃ photoanode surfaces resulting in improvements in hole collection efficiency and reduction in charge recombination. Despite a much thinner layer of WO₃ (about 500 nm) compared to those in other published studies, the electrodes exhibited an ultra-high photocurrent density of 1.81 mA cm⁻² at 1.23 V vs. RHE. This current density is one of the highest ones among WO₃-based photoanodes described in literature. The proposed surface modulation approach offers an effective and scalable method to prepare high-performance thin film photoanodes for photoelectrochemical water splitting.

Surface modulation approach offers an effective and scalable method for high-performance WO₃ photoanodes.

Light induced formation of a surface heterojunction in photocharged CuWO4 photoanodes

Photocharging of CuWO₄ photoanodes enhances its water oxidation kinetics as a result of improved charge separation near the electrode/electrolyte interface post photocharging.

Photoelectrochemical oxidation of organic substrates in organic media

There is a global effort to convert sunlight into fuels by photoelectrochemically splitting water to form hydrogen fuels, but the dioxygen byproduct bears little economic value. This raises the important question of whether higher value commodities can be produced instead of dioxygen. We report here photoelectrochemistry at a BiVO₄ photoanode involving the oxidation of substrates in organic media. The use of MeCN instead of water enables a broader set of chemical transformations to be performed (e.g., alcohol oxidation and C-H activation/oxidation), while suppressing photocorrosion of BiVO₄ that otherwise occurs readily in water, and sunlight reduces the electrical energy required to drive organic transformations by 60%. These collective results demonstrate the utility of using photoelectrochemical cells to mediate organic transformations that otherwise require expensive and toxic reagents or catalysts.Photoelectrochemical water splitting is a promising method for H₂ fuel production, but the O₂ by-product generated has little economic value. Here, Berlinguette and colleagues demonstrate that BiVO₄ photoanodes immersed in organic media can instead perform valuable alcohol oxidation and C-H functionalization reactions.