Dual Oxygen and Tungsten Vacancies on a WO3Photoanode for Enhanced Water Oxidation

On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.

Boosting Reactive Oxygen Species Generation Using Inter-Facet Edge Rich WO3 Arrays for Photoelectrochemical Conversion

Water oxidation reaction leaves room to be improved in the development of various solar fuel productions, because of the kinetically sluggish 4-electron transfer process of oxygen evolution reaction. In this work, we realize reactive oxygen species (ROS), H₂ O₂ and OH⋅, formations by water oxidation with total Faraday efficiencies of more than 90 % by using inter-facet edge (IFE) rich WO₃ arrays in an electrolyte containing CO₃ ^2- . Our results demonstrate that the IFE favors the adsorption of CO₃ ^2- while reducing the adsorption energy of OH⋅, as well as suppresses surface hole accumulation by direct 1-electron and indirect 2-electron transfer pathways. Finally, we present selective oxidation of benzyl alcohol by in situ using the formed OH⋅, which delivers a benzaldehyde production rate of ≈768 μmol h^-1 with near 100 % selectivity. This work offers a promising approach to tune or control the oxidation reaction in an aqueous solar fuel system towards high efficiency and value-added product.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

The synergistic effect of surface and bulk O vacancies in a WO3 photoanode to advance carrier separation and light harvesting for photoelectrochemical water splitting

It is critical to fabricate a photoanode with the virtues of high carrier separation efficiency and light harvesting to reduce the recombination of carriers and enhance the utilization of solar energy in photoelectrochemical (PEC) water splitting. In this work, WO₃ nanoflake photoanodes with surface and bulk O vacancies (D-WO_3-x) were fabricated via a hydrothermal method and H₂WO₄ etching to reveal the respective roles and collaborative effect of O vacancies in the surface and bulk. The surface O vacancies leave abundant active sites to reduce the redox barrier. Furthermore, the bulk O vacancies act as electron trap centers for heightening carrier separation efficiency. More importantly, the surface and bulk O vacancies in D-WO_3-x reduce the band gap so that the resistance to electron jumping is reduced and light harvesting is increased. As expected, the photocurrent density of D-WO_3-x is 0.98 mA cm^-2 at 1.23 V vs. RHE, which is 5 times that of pristine WO₃. Moreover, the carrier separation efficiencies in the surface and bulk are 2.38 and 2.26 times that of WO₃. This work provides a promising method for the development of high-performance photoanodes via introducing surface and bulk O vacancies in semiconductors.

Morphology Engineering of BiVO4 with CoOx Derived from Cobalt-containing Polyoxometalate as Co-catalyst for Oxygen Evolution

Bismuth vanadate (BiVO₄ ) as a metal oxidation semiconductor has stimulated extensive attention in the photocatalytic water splitting field. However, the poor transport ability and easy recombination of charge carriers limit photocatalytic water oxidation activity of pure BiVO₄ . Herein, the photocatalytic activity of BiVO₄ is enhanced via adjusting its morphology and combination co-catalyst. First, the Cu-BiVO₄ was synthesized by copper doping to control the growth of {110} facet of BiVO₄ , which is regarded for the separation of photo-generated charge carriers. Then the CoO_x in-situ generated from K₆ [SiCo^II (H₂ O)W₁₁ O₃₉ ] ⋅ 16H₂ O was photo-deposited on Cu-BiVO₄ surface as co-catalyst to speed up reaction kinetics. Cu-BiVO₄ @CoO_x hybrid catalyst shows highest photocatalytic activity and best stability among all the prepared catalysts. Oxygen evolution is about 34.6 μmol in pH 4 acetic acid buffer under 420 nm LED irradiation, which is nearly 20 times higher than that of pure BiVO₄ . Apparent quantum efficiency (AQE) in 1 h and O₂ yield are 1.83% and 23.1%, respectively. O₂ evolution amount nearly maintains the original value even after 5 cycles.

Photo-driven Oxygen Vacancies Extends Charge Carrier Lifetime for Efficient Solar Water Splitting

A photocharge/discharge strategy is proposed to initiate the WO₃ photoelectrode and suppress the main charge recombination, which remarkably improves the photoelectrochemical (PEC) performance. The photocharged WO₃ surrounded by a 8-10 nm overlayer and oxygen vacancies could be operated more than 25 cycles with 50 h durability without significant decay on PEC activity. A photocharged WO₃ /CuO photoanode exhibits an outstanding photocurrent of 3.2 mA cm^-2 at 1.23 V_RHE with a low onset potential of 0.6 V_RHE , which is one of the best performances of p-n heterojunction structure. Using nonadiabatic molecular dynamics combined with time-domain DFT, we clarify the prolonged charge carrier lifetime of photocharged WO₃ , as well as how electronic systems of photocharged WO₃ /CuO semiconductors enable the effective photoinduced electrons transfer from WO₃ into CuO. This work provides a feasible route to address excessive defects existed in photoelectrodes without causing extra recombination.

Oxygen Vacancy-Enhanced Photoelectrochemical Water Splitting of WO3/NiFe-Layered Double Hydroxide Photoanodes

Photoelectrochemical (PEC) water splitting serves as one of the promising approaches for producing clean and renewable energy, and their solar-hydrogen energy conversion efficiency depends on the interfacial charge separation and carrier mobility. Herein, we report an effective strategy to promote the PEC performance by fabricating a WO₃ photoanode rich in oxygen vacancies (Ov) modified by NiFe-based layered double hydroxide (LDH). When WO₃-Ov/NiFe-LDH is used as a photoanode, the maximum photocurrent density at 1.8 V versus RHE has been significantly enhanced to 2.58 mA·cm^-2, which is 4.3 times higher than that of WO₃. In addition, analogues were studied in controlled experiments without Ov, which further demonstrated that the synergistic effect of NiFe-LDH and Ov resulted in increased carrier concentration and driving force. According to electrical impedance spectroscopy, X-ray photoelectron spectroscopy, and Mott-Schottky analysis, the built-in electronic field in WO₃ homojunction, along with the accelerated hole capture by the NiFe-LDH cocatalyst contributes to the improved charge separation and transport in the WO₃-Ov/NiFe-LDH electrode. This work proposes an efficient and valuable strategy for designing the structure of WO₃-based photoelectrodes.

Atomic Sandwiched p-n Homojunctions

Semiconductor p-n junctions have been explored and applied in photoelectrochemical (PEC) water splitting, but serious carrier recombination and sluggish oxygen evolution reaction (OER) dynamics have demanded further progress in p-n junction photoelectrode design. Here, via a controllable NH₃ treatment, we construct sandwiched p-n homojunctions in three-unit-cells n-type SnS₂ (n-SnS₂ ) nanosheet arrays using nitrogen (N) as acceptor dopants. The optimal N-doped n-SnS₂ (pnp-SnS₂ ) with such unique structure achieves a record photocurrent density of 3.28 mA cm^-2 , which is 21 times as high as that of n-SnS₂ and the highest value among all the SnS₂ photoanodes reported so far. Moreover, the stoichiometric O₂ and H₂ evolution from water was achieved with Faradaic efficiencies close to 100 %. The superior performance could be attributed to the facilitated electron-hole separation/transfer, accelerated surface OER kinetics, prolonged carrier lifetime, and improved structural stability.

Tailoring the Surface Properties of Bi2O2NCN by in Situ Activation for Augmented Photoelectrochemical Water Oxidation on WO3 and CuWO4 Heterojunction Photoanodes

Bismuth(III) oxide-carbodiimide (Bi₂O₂NCN) has been recently discovered as a novel mixed-anion semiconductor, which is structurally related to bismuth oxides and oxysulfides. Given the structural versatility of these layered structures, we investigated the unexplored photochemical properties of the target compound for photoelectrochemical (PEC) water oxidation. Although Bi₂O₂NCN does not generate a noticeable photocurrent as a single photoabsorber, the fabrication of heterojunctions with the WO₃ thin film electrode shows an upsurge of current density from 0.9 to 1.1 mA cm^–2 at 1.23 V vs reversible hydrogen electrode (RHE) under 1 sun (AM 1.5G) illumination in phosphate electrolyte (pH 7.0). Mechanistic analysis and structural analysis using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM EDX) indicate that Bi₂O₂NCN transforms during operating conditions in situ to a core–shell structure Bi₂O₂NCN/BiPO₄. When compared to WO₃/BiPO₄, the in situ electrolyte-activated WO₃/Bi₂O₂NCN photoanode shows a higher photocurrent density due to superior charge separation across the oxide/oxide-carbodiimide interface layer. Changing the electrolyte from phosphate to sulfate results in a lower photocurrent and shows that the electrolyte determines the surface chemistry and mediates the PEC activity of the metal oxide-carbodiimide. A similar trend could be observed for CuWO₄ thin film photoanodes. These results show the potential of metal oxide-carbodiimides as relatively novel representatives of mixed-anion compounds and shed light on the importance of the control over the surface chemistry to enable the in situ activation.

Phosphate electrolyte activates the metal oxide−Bi₂O₂NCN heterojunction.

Photodesorption mechanism of water on WO3(001) - a combined embedded cluster, computational intelligence and wave packet approach

In this work we investigate the mechanism of photodesorption of water from a WO3(001) surface by theoretical calculations, applying an embedded cluster model. Using the CASSCF method, we have calculated both the ground state as well as the energetically preferred charge-transfer state in three degrees of freedom of the water molecule on the surface. The calculated potential energy surfaces were afterwards fitted with a neural network optimized by a genetic algorithm. A final quantum dynamic wave packet study provided insight into the photodesorption mechanism.

Tin and Oxygen-Vacancy Co-doping into Hematite Photoanode for Improved Photoelectrochemical Performances

Hematite (α-Fe₂O₃) material is regarded as a promising candidate for solar-driven water splitting because of the low cost, chemical stability, and appropriate bandgap; however, the corresponding system performances are limited by the poor electrical conductivity, short diffusion length of minority carrier, and sluggish oxygen evolution reaction. Here, we introduce the in situ Sn doping into the nanoworm-like α-Fe₂O₃ film with ultrasonic spray pyrolysis method. We show that the current density at 1.23 V vs. RHE (J_ph@1.23V) under one-sun illumination can be improved from 10 to 130 μA/cm² after optimizing the Sn dopant density. Moreover, J_ph@1.23V can be further enhanced 25-folds compared to the untreated counterpart via the post-rapid thermal process (RTP), which is used to introduce the defect doping of oxygen vacancy. Photoelectrochemical impedance spectrum and Mott-Schottky analysis indicate that the performance improvement can be ascribed to the increased carrier density and the decreased resistances for the charge trapping on the surface states and the surface charge transferring into the electrolyte. X-ray photoelectron spectrum and X-ray diffraction confirm the existence of Sn and oxygen vacancy, and the potential influences of varying levels of Sn doping and oxygen vacancy are discussed. Our work points out one universal approach to efficiently improve the photoelectrochemical performances of the metal oxide semiconductors.

Broadening the Scope of Biocatalysis in Sustainable Organic Synthesis

This Review is aimed at synthetic organic chemists who may be familiar with organometallic catalysis but have no experience with biocatalysis, and seeks to provide an answer to the perennial question: if it is so attractive, why wasn't it extensively used in the past? The development of biocatalysis in industrial organic synthesis is traced from the middle of the last century. Advances in molecular biology in the last two decades, in particular genome sequencing, gene synthesis and directed evolution of proteins, have enabled remarkable improvements in scope and substantially reduced biocatalyst development times and cost contributions. Additionally, improvements in biocatalyst recovery and reuse have been facilitated by developments in enzyme immobilization technologies. Biocatalysis has become eminently competitive with chemocatalysis and the biocatalytic production of important pharmaceutical intermediates, such as enantiopure alcohols and amines, has become mainstream organic synthesis. The synthetic space of biocatalysis has significantly expanded and is currently being extended even further to include new-to-nature biocatalytic reactions.

Structure-Activity Relationship of Defective Metal-Based Photocatalysts for Water Splitting: Experimental and Theoretical Perspectives

Photocatalytic water splitting is promising for hydrogen energy production using solar energy and developing highly efficient photocatalysts is challenging. Defect engineering is proved to be a very useful strategy to promote the photocatalytic performance of metal-based photocatalysts, however, the vital role of defects is still ambiguous. This work comprehensively reviews point defective metal-based photocatalysts for water splitting, focusing on understanding the defects' disorder effect on optical adsorption, charge separation and migration, and surface reaction. The controllable synthesis and tuning strategies of defective structure to improve the photocatalytic performance are summarized, then the characterization techniques and density functional theory calculations are discussed to unveil the defect structure, and analyze the defects induced electronic structure change of catalysts and its ultimate effect on the photocatalytic activity at the molecular level. Finally, the challenge in developing more efficient defective metal-based photocatalysts is outlined. This work may help further the understanding of the fundamental role of defect structure in the photocatalytic reaction process and guide the rational design and fabrication of highly efficient and low-cost photocatalysts.

Photonic crystal enhanced laser desorption and ionization substrate for detection of stress biomarkers under atmospheric pressure

Enhanced efficiency for generating molecular ions is essential for high-throughput and sensitive detection using mass spectrometry in clinical diagnostics and biomarker discovery. In this study, we developed a novel strategy to promote laser desorption and ionization by using photonic crystals as substrates. The WO₃-TiO₂ inverse opal photonic crystal, with a coupling stop band and laser wavelength, significantly enhanced the efficiency of laser desorption and ionization owing to the slow light effect and the porous structure of the inverse opal, which increased the interaction between the laser and WO₃-TiO₂. Furthermore, stress biomarkers were conveniently measured under atmospheric pressure by using WO₃-TiO₂ inverse opal as an enhanced substrate to evaluate the impact of chronic unpredictable mild stress. The universal and highly sensitive substrate has promised for application in the highly sensitive detection and quantification of biomarkers.

Conformal BiVO4-Layer/WO3-Nanoplate-Array Heterojunction Photoanode Modified with Cobalt Phosphate Cocatalyst for Significantly Enhanced Photoelectrochemical Performances

Constructing semiconductor heterojunctions via surface/interface engineering is an effective way to enhance the charge carrier separation/transport ability and thus the photoelectrochemical (PEC) properties of a photoelectrode. Herein, we report a conformal BiVO₄-layer/WO₃-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) as oxygen evolution cocatalyst (OEC) for significant enhancement in PEC performances. The BiVO₄/WO₃ nanocomposite is fabricated by coating a thin conformal BiVO₄ layer on the surface of presynthesized WO₃ nanoplate arrays (NPAs) via stepwise spin-coating, and the decoration of Co-Pi OEC is realized by photoassisted electrodeposition method. The optimized Co-Pi@BiVO₄/WO₃ heterojunction photoanode shows a maximum photocurrent of 1.8 mA/cm² at 1.23 V vs RHE in a phosphate buffer electrolyte under an AM1.5G solar simulator, which is 5 and 12 times higher than those of bare WO₃ and BiVO₄ photoanode, respectively. Measurements of UV-vis absorption spectra, electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra reveal that the enhanced PEC performances can be attributed to the increased charge carrier separation/transport benefited from the type II nature of BiVO₄/WO₃ heterojunction and the promoted water oxidation kinetics and photostability owing to the decoration of Co-Pi cocatalyst.

WO3/BiVO4 Type-II Heterojunction Arrays Decorated with Oxygen-Deficient ZnO Passivation Layer: A Highly Efficient and Stable Photoanode

In the present work, we report a ternary WO₃/BiVO₄/ZnO photoanode with boosted PEC efficiency and stability toward highly efficient water splitting. The type-II WO₃/BiVO₄ heterojunction arrays are firstly prepared by hydrothermal growth of WO₃ nanoplate arrays onto the substrates of fluorine-doped tin oxide (FTO)-coated glass, followed by spin-coating of BiVO₄ layers onto the WO₃ nanoplate surfaces. After that, thin ZnO layers are further introduced onto the WO₃/BiVO₄ heterojunction arrays via atomic layer deposition (ALD), leading to the construction of ternary WO₃/BiVO₄/ZnO photoanodes. It is verified that the ZnO thin layer in the WO₃/BiVO₄/ZnO photoanode contains abundant oxygen vacancies, which could act as an effective passivation layer to enhance the charge separation and surface water oxidation kinetics of photogenerated carriers. The as-prepared WO₃/BiVO₄/ZnO photoanode produces a photocurrent of 2.96 mA cm^-2 under simulated sunlight with an incident photon-to-current conversion efficiency (IPCE) of ∼72.8% at 380 nm at a potential of 1.23 V versus RHE without cocatalysts, both of which are comparable to the state-of-the-art WO₃/BiVO₄ counterparts. Moreover, the photocurrent of the WO₃/BiVO₄/ZnO photoanode shows only 9% decay after 6 h, suggesting its high photoelectrochemical (PEC) stability.

Visible-Light-Responsive Photoanodes for Highly Active, Stable Water Oxidation

Solar energy is a natural and effectively permanent resource and so the conversion of solar radiation into chemical or electrical energy is an attractive, although challenging, prospect. Photo-electrochemical (PEC) water splitting is a key aspect of producing hydrogen from solar power. However, practical water oxidation over photoanodes (in combination with water reduction at a photocathode) in PEC cells is currently difficult to achieve because of the large overpotentials in the reaction kinetics and the inefficient photoactivity of the semiconductors. The development of semiconductors that allow high solar-to-hydrogen conversion efficiencies and the utilization of these materials in photoanodes will be a necessary aspect of achieving efficient, stable water oxidation. This Review discusses advances in water oxidation activity over photoanodes of n-type visible-light-responsive (oxy)nitrides and oxides.

Conductive Tungsten Oxide Nanosheets for Highly Efficient Hydrogen Evolution

Exploring efficient and economical electrocatalysts for hydrogen evolution reaction is of great significance for water splitting on an industrial scale. Tungsten oxide, WO₃, has been long expected to be a promising non-precious-metal electrocatalyst for hydrogen production. However, the poor intrinsic activity of this material hampers its development. Herein, we design a highly efficient hydrogen evolution electrocatalyst via introducing oxygen vacancies into WO₃ nanosheets. Our first-principles calculations demonstrate that the gap states introduced by O vacancies make WO₃ act as a degenerate semiconductor with high conductivity and desirable hydrogen adsorption free energy. Experimentally, we prepared WO₃ nanosheets rich in oxygen vacancies via a liquid exfoliation, which indeed exhibits the typical character of a degenerate semiconductor. When evaluated by hydrogen evolution, the nanosheets display superior performance with a small overpotential of 38 mV at 10 mA cm^-2 and a low Tafel slope of 38 mV dec^-1. This work opens an effective route to develop conductive tungsten oxide as a potential alternative to the state-of-the-art platinum for hydrogen evolution.

Three-Dimensional WO3 Nanoplate/Bi2S3 Nanorod Heterojunction as a Highly Efficient Photoanode for Improved Photoelectrochemical Water Splitting

The rational design of semiconductor photoanodes with sufficient light absorption, efficient photogenerated carrier separation, and fast charge transport is crucial for photoelectrochemical (PEC) water splitting. Incorporating a small-band-gap semiconductor to a large-band-gap material with matched energy band position is a promising route to improve the light harvesting and charge transport. Herein, we report the fabrication of a three-dimensional heterojunction with uniform Bi₂S₃ nanorods on WO₃ nanoplates by hydrothermal process and chemical bath deposition. The seed layer strategy was used to assist the growth of Bi₂S₃ nanorods for perfect interface contact between WO₃ and Bi₂S₃. The as-prepared WO₃/Bi₂S₃ composite exhibited a much enhanced photocurrent (5.95 mA/cm² at 0.9 V vs reversible hydrogen electrode), which is 35 and 1.4 times higher than those of pristine WO₃ and WO₃/Bi₂S₃ composite without a seed layer, respectively. In addition, higher incident photon-to-current conversion efficiency (68.8%) and photoconversion efficiency (1.70%) were achieved. The enhancement mechanism was investigated in detail, and the sufficient light absorption, efficient charge transport, and high carrier density simultaneously contribute to the improved PEC activity. These findings will open up new opportunities to develop other highly efficient heterostructures as photoelectrodes for PEC applications.

Elaborate Manipulation for Sub-10 nm Hollow Catalyst Sensitized Heterogeneous Oxide Nanofibers for Room Temperature Chemical Sensors

Room-temperature (RT) operation sensors are constantly in increasing demand because of their low power consumption, simple operation, and long lifetime. However, critical challenges such as low sensing performance, vulnerability under highly humid state, and poor recyclability hinder their commercialization. In this work, sub-10 nm hollow, bimetallic Pt-Ag nanoparticles (NPs) were successfully formed by galvanic replacement reaction in bioinspired hollow protein templates and sensitized on the multidimensional SnO₂-WO₃ heterojunction nanofibers (HNFs). Formation of hollow, bimetallic NPs resulted in the double-side catalytic effect, rendering both surface and inner side chemical reactions. Subsequently, SnO₂-WO₃ HNFs were synthesized by incorporating 2D WO₃ nanosheets (NSs) with 0D SnO₂ sphere by c-axis growth inhibition effect and fluid dynamics of liquid Sn during calcination. Hierarchically assembled HNFs effectively modulate surface depletion layer of 2D WO₃ NSs by electron transfers from WO₃ to SnO₂ stemming from creation of heterojunction. Careful combination of bimetallic catalyst NPs with HNFs provided an extreme recyclability under exhaled breath (95 RH%) with outstanding H₂S sensitivity. Such sensing platform clearly distinguished between the breath of healthy people and simulated halitosis patients.

Large-Scale Tunable 3D Self-Supporting WO3 Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting

Hydrogen production from water based on photoelectrochemical (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO₃ micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidation. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO₃ nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent density of 1.2 mA cm^-2 at 1.0 V vs Ag/AgCl (1.23 V vs RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidation conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective, and controllable for large-scale fabrication, which should provide a new and general route that how the properties of conventional metal oxides can be improved via hierarchical 3D micro-nano configurations.

Covalent Surface Modification of Gallium Arsenide Photocathodes for Water Splitting in Highly Acidic Electrolyte

Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water-oxidation and proton-reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak interactions and quickly react to desorb the molecule under conditions relevant to sustained photoelectrolysis. Here, we show that covalent attachment of fluorinated, aromatic molecules to p-GaAs(1 0 0) surfaces can be employed to tune the photocurrent onset potentials of p-GaAs(1 0 0) photocathodes and reduce the external energy required for water splitting. Results indicate that initial photocurrent onset potentials can be shifted by nearly 150 mV in pH -0.5 electrolyte under 1 Sun (1000 W m^-2 ) illumination resulting from the covalently bound surface dipole. Though X-ray photoelectron spectroscopy analysis reveals that the covalent molecular dipole attachment is not robust under extended 50 h photoelectrolysis, the modified surface delays arsenic oxide formation that results in a p-GaAs(1 0 0) photoelectrode operating at a sustained photocurrent density of -20.5 mA cm^-2 within -0.5 V of the reversible hydrogen electrode.

Electrospun tungsten trioxide nanofibers decorated with palladium oxide nanoparticles exhibiting enhanced photocatalytic activity

Tungsten trioxide (WO₃) based nanofibers have many advantages as photocatalysts due to its band gap which fits with readily accessible light sources.