Photoactive Tungsten-Oxide Nanomaterials for Water-Splitting

Surface Engineering of Anodic WO3 Layers by In Situ Doping for Light-Assisted Water Splitting

This study presents a novel approach to fabricating anodic Co-F-WO3 layers via a single-step electrochemical synthesis, utilizing cobalt fluoride as a dopant source in the electrolyte. The proposed in situ doping technique capitalizes on the high electronegativity of fluorine, ensuring the stability of CoF2 throughout the synthesis process. The nanoporous layer formation, resulting from anodic oxide dissolution in the presence of fluoride ions, is expected to facilitate the effective incorporation of cobalt compounds into the film. The research explores the impact of dopant concentration in the electrolyte, conducting a comprehensive characterization of the resulting materials, including morphology, composition, optical, electrochemical, and photoelectrochemical properties. The successful doping of WO3 was confirmed by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, photoluminescence measurements, X-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis. Optical studies reveal lower absorption in Co-doped materials, with a slight shift in band gap energies. Photoelectrochemical (PEC) analysis demonstrates improved PEC activity for Co-doped layers, with the observed shift in photocurrent onset potential attributed to both cobalt and fluoride ions catalytic effects. The study includes an in-depth discussion of the observed phenomena and their implications for applications in solar water splitting, emphasizing the potential of the anodic Co-F-WO3 layers as efficient photoelectrodes. In addition, the research presents a comprehensive exploration of the electrochemical synthesis and characterization of anodic Co-F-WO3, emphasizing their photocatalytic properties for the oxygen evolution reaction (OER). It was found that Co-doped WO3 materials exhibited higher PEC activity, with a maximum 5-fold enhancement compared to pristine materials. Furthermore, the studies demonstrated that these photoanodes can be effectively reused for PEC water-splitting experiments.

The Emergence of High-Performance Conjugated Polymer/Inorganic Semiconductor Hybrid Photoelectrodes for Solar-Driven Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) energy conversion holds great potential in converting solar energy into storable and transportable chemicals or fuels, providing a viable route toward a carbon-neutral society. Conjugated polymers are rapidly emerging as a new class of materials for PEC water splitting. They exhibit many intriguing properties including tunable electronic structures through molecular engineering, excellent light harvesting capability with high absorption coefficients, and facile fabrication of large-area thin films via solution processing. Recent advances have indicated that integrating rationally designed conjugated polymers with inorganic semiconductors is a promising strategy for fabricating efficient and stable hybrid photoelectrodes for high-efficiency PEC water splitting. This review introduces the history of developing conjugated polymers for PEC water splitting. Notable examples of utilizing conjugated polymers to broaden the light absorption range, improve stability, and enhance the charge separation efficiency of hybrid photoelectrodes are highlighted. Furthermore, key challenges and future research opportunities for further improvements are also presented. This review provides an up-to-date overview of fabricating stable and high-efficiency PEC devices by integrating conjugated polymers with state-of-the-art semiconductors and would have significant implications for the broad solar-to-chemical energy conversion research.

Anodic Oxidation of Tungsten under Illumination-Multi-Method Characterization and Modeling at the Molecular Level

Tungsten oxide has received considerable attention as photo-anode in photo-assisted water splitting due to its considerable advantages such as significant light absorption in the visible region, good catalytic properties, and stability in acidic and oxidative conditions. The present paper is a first step in a detailed study of the mechanism of porous WO3 growth via anodic oxidation. In-situ electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS) during oxidation of W illuminated with UV and visible light are employed to study the ionic and electronic processes in slightly acidic sulfate-fluoride electrolytes and a range of potentials 4-10 V. The respective responses are discussed in terms of the influence of fluoride addition on ionic and electronic process rates. A kinetic model is proposed and parameterized via regression of experimental data to the EIS and IMPS transfer functions.

Tungsten contamination, behavior and remediation in complex environmental settings

Tungsten (W) is a rare element and present in the earth's crust mainly as iron, aluminium, and calcium minerals including wolframite and scheelite. This review aims to offer an overview on the current knowledge on W pollution in complex environmental settlings, including terrestrial and aquatic ecosystems, linking to its natural and anthropogenic sources, behavior in soil and water, environmental and human health hazards, and remediation strategies. Tungsten is used in many alloys mainly as wafers, which have wide industrial applications, such as incandescent light bulb filaments, X-ray tubes, arc welding electrodes, radiation shielding, and industrial catalysts. The rigidity and high density of W enable it to be suitable for defence applications replacing lead. In soil, W metal is oxidised to the tungstate anion and occurs in oxidation states from - 2 to + 6, with the most prevalent oxidation state of + 6. However, recently, people have been alerted to the risk posed by W alloys and its particulates, which can cause cancer and have other detrimental health effects in animals and humans. The population is subject to W pollution in the workplace by breathing, ingestion, and dermal contact. Remediation of W-polluted soil and aquatic environments can be accomplished via stabilization or solubilization. Stabilization of W in soil and groundwater using immobilizing agents inhibits the bioavailability of W, thereby preventing the contaminant from reaching the food chain, while solubilization of W in soil involving mobilizing materials accelerates the elimination of W via soil washing and root absorption. Future research opportunities covering risk-based remediation of W pollution in these complex settings are presented.

The Piezoresponse in WO3 Thin Films Due to N2-Filled Nanovoids Enrichment by Atom Probe Tomography

Tungsten trioxide (WO3) is a versatile n-type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d33 = 35 pmV-1 on WO3 thin films ~200 nm deposited using RF-sputtering onto alumina (Al2O3) substrate with post-deposit annealing treatment of 400 °C in a 3% H2/N2-forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO3 with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N2-filled nanovoids that acts as strain centers producing a local deformation of the WO3 lattice into a non-centrosymmetric structure, which is related to piezoresponse observations.

Multiple roles for LaFeO3 in enhancing the Photoelectrochemical performance of WO3

For photoelectrochemical (PEC) water splitting, constructing heterojunctions and loading co-catalysts are effective means to realizing sufficient light absorption, effective photogenerated carrier separation and fast charge transport. However, during implementation, the PEC performance of the catalyst is affected by both parasitic light absorption and reflection and the change in energy band structure due to the creation of new interfaces. Herein, in order to minimize the effect of recombination of photogenerated electron-hole pairs on the catalyst PEC performance due to the nascent interface arising from the co-catalyst compounding, WO3 and Ni/Co co-doped LaFeO3 (LFO) are constructed as heterojunctions, in which NiCo-LFO acts both as a part of the heterojunction to enhance photogenerated carrier separation and a co-catalyst to enhance the conductivity and modulate the surface state density at the catalyst-electrolyte interface. The current density of NiCo-LFO/WO3 reaches 3.92 mA cm-2, which is more than 7 times that of LFO/WO3. This work provides a reference for the efficient water splitting of B-site doped, especially the co-doped perovskite oxide as multifunctional roles integrated with conventional photoelectrodes.

Investigation of Photoelectrochemical Performance under the Piezoelectric Effect Based on Different Zinc Oxide Morphologies

Recently, the piezoelectric effect has been widely used in photoelectrochemical (PEC) water splitting, and the morphology of the piezoelectric material is a critical factor affecting the piezo-photoelectrochemical water splitting performance. Herein, we explored the mechanism of the piezo-photoelectrochemical performance of zinc oxide (ZnO) that is affected by the morphology. Firstly, three different ZnO nanostructures (nanosheets, nanorods, and nanospheres) were synthesized by the electrodeposition, hydrothermal, and sol-gel methods, respectively. Then, the measurements of PEC water splitting performance under the piezoelectric effect revealed a 3-fold increase for the ZnO nanosheets, a 1.4-fold increase for the nanorods, and a 1.2-fold increase for the nanospheres compared to no piezoelectric effect. Finally, finite element simulation showed that nanosheets generated the highest piezoelectric potential (0.6 V), followed by nanorods (0.2 V), and nanospheres the lowest (0.04 V). Thus, among the three morphologies, the ZnO nanosheets exhibited a great improvement in PEC performance under the piezoelectric effect. The great improvement is due to the non-axial vertical homogeneous growth of the ZnO nanosheets, subjecting them to the highest effective deformation stress, which enables the ZnO nanosheets to produce the highest piezoelectric potential to accelerate the carrier separation and limit the recombination of photoelectrons and holes. This work serves as a guide for developing various photoelectrodes that are used in piezo-photoelectrochemical water splitting.

Preparation and Photocatalytic/Photoelectrochemical Investigation of 2D ZnO/CdS Nanocomposites

Properties of heterotructured semiconductors based on ZnO/CdS nanosheets are investigated for their possible application in photocatalytic and photoelectrochemical reactions. Semiconductor material is the main active coating of photoanodes, which triggers the half-reaction of water oxidation and reduction, which entails the purifying or splitting of water. This article explains nanocomposite assembly by convenient and simple methods. The study of the physicochemical properties of semiconductor layers is carried out using electron microscopy, X-ray diffractometry, and UV-visible spectroscopy. Studies of electrochemical properties are carried out by potential static methods in electrochemical cells.

Highly effective and stable MWCNT/WO3 nanocatalyst for ammonia gas sensing, photodegradation of ciprofloxacin and peroxidase mimic activity

The present study discusses the ammonia (NH₃) sensing characteristics, photocatalytic degradation of emerging pollutants, and peroxidase mimic activity of multifunctional multi-walled carbon nanotube-tungsten oxide nanocomposite (MWCNT/WO₃) prepared by conventional solvothermal method. The prepared MWCNT/WO₃ nanocomposites were characterized by various analytical techniques like XRD, Raman, XPS, N₂ adsorption, FESEM with elemental analysis and diffuse reflection spectroscopy. The prepared 1% MWCNT/WO₃ nanocomposite showed better gas sensing performance for the NH₃ vapors at 10-100 ppm than the pristine WO₃ and the response and recover time of about 13 and 15s towards 20 ppm of ammonia (NH₃) was achieved. The photocatalytic activity of MWCNT/WO₃ towards organic dyes such as Rhodamine-B (Rh.B) methylene blue (MB) and pharmaceutical compound ciprofloxacin (CIP) were studied and achieved above 90% degradation at 160 min for CIP and 60 min for MB and Rho-B respectively. The radicle scavenging activity for MWCNT/WO₃ nanocomposite showed the predominant formation of hydroxyl (OH^•) and superoxide radicle (^•O^2-). Further, the MWCNT/WO₃ nanocomposite showed peroxidase mimic activity and exhibit the limit of detection (LOD) of about 321 nM. From the overall analysis, MWCNT/WO₃ hybrid seems to have potential characteristics that can be explored for multiple functional applications.

Electrochemical Synthesis-Dependent Photoelectrochemical Properties of Tungsten Oxide Powders

A rapid, facile, and environmentally benign strategy to electrochemical oxidation of metallic tungsten under pulse alternating current in an aqueous electrolyte solution was reported. Particle size, morphology, and electronic structure of the obtained WO3 nanopowders showed strong dependence on electrolyte composition (nitric, sulfuric, and oxalic acid). The use of oxalic acid as an electrolyte provides a gram-scale synthesis of WO3 nanopowders with tungsten electrochemical oxidation rate of up to 0.31 g·cm−2·h−1 that is much higher compared to the strong acids. The materials were examined as photoanodes in photoelectrochemical reforming of organic substances under solar light. WO3 synthesized in oxalic acid is shown to exhibit excellent activity towards the photoelectrochemical reforming of glucose and ethylene glycol, with photocurrents that are nearly equal to those achieved in the presence of simple alcohol such as ethanol. This work demonstrates the promise of pulse alternating current electrosynthesis in oxalic acid as an efficient and sustainable method to produce WO3 nanopowders for photoelectrochemical applications.

Mechanistic Insights into WO3 Sensing and Related Perspectives

Tungsten trioxide (WO₃) is taking on an increasing level of importance as an active material for chemoresistive sensors. However, many different issues have to be considered when trying to understand the sensing properties of WO₃ in order to rationally design sensing devices. In this review, several key points are critically summarized. After a quick review of the sensing results, showing the most timely trends, the complex system of crystallographic WO₃ phase transitions is considered, with reference to the phases possibly involved in gas sensing. Appropriate attention is given to related investigations of first principles, since they have been shown to be a solid support for understanding the physical properties of crucially important systems. Then, the surface properties of WO₃ are considered from both an experimental and first principles point of view, with reference to the paramount importance of oxygen vacancies. Finally, the few investigations of the sensing mechanisms of WO₃ are discussed, showing a promising convergence between the proposed hypotheses and several experimental and theoretical studies presented in the previous sections.

Boosting photoresposive ability of WO3-Bi2O3nanocomposite rods via annealing-induced intrinsic precipitation of nanosized Bi particles

In this study, Bi-particle-functionalized tungsten trioxide-bismuth oxide (WO₃-Bi₂O₃) composite nanorods were prepared by integrating sputtering and hydrothermal syntheses with an appropriate postannealing procedure to induce Bi particle precipitation. Unlike other routes in which metal particle decoration is achieved externally, in this study, photoresponsive one-dimensional WO₃-Bi₂O₃composite nanorods were decorated with Bi particles by using the internal precipitation method. Structural analysis revealed that the Bi-metal-particle-functionalized WO₃-Bi₂O₃composite nanorods with particle size ranging from 5 to 10 nm were formed through hydrogen gas annealing at an optimal annealing temperature of 350 °C. Compared with the pristine WO₃nanorod template, the Bi-WO₃-Bi₂O₃composite nanorods exhibited higher photoresponsive performance, substantial photogenerated charge transfer ability, and efficient separation of photogenerated electron-hole pairs. The study results indicated that the Bi-WO₃-Bi₂O₃composite nanorods had superior decontamination ability and excellent stability toward RhB dye as compared with pristine WO₃. Moreover, the photogenerated charge separation and migration efficiencies of the WO₃-Bi₂O₃nanorods could be tuned through appropriate reduction of the surface oxide layer; this is a promising approach to designing WO₃-Bi₂O₃nanorods with high photoactive performance.

Optimizing crystal characterization of WO3–ZnO composites for boosting photoactive performance via manipulating crystal formation conditions

Controlling crystal formation conditions enables the manipulation of crystal quality and photoactive performance of WO₃–ZnO nanorods.