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

The route for commercial photoelectrochemical water splitting: a review of large-area devices and key upscaling challenges

Green-hydrogen is considered a "key player" in the energy market for the upcoming decades. Among currently available hydrogen (H2) production processes, photoelectrochemical (PEC) water splitting has one of the lowest environmental impacts. However, it still presents prohibitively high production costs compared to more mature technologies, such as steam methane reforming. Therefore, the competitiveness of PEC water splitting must rely on its environmental and functional advantages, which are strongly linked to the reactor design, to the intrinsic properties of its components, and to their successful upscaling. This review gives special attention to the engineering aspects and categorizes PEC devices into four main types, according to the configuration of electrodes and strategies for gas separation: wired back-to-back, wireless back-to-back, wired side-by-side, and wired separated electrode membrane-free. Independently of the device architecture, the use of concentrated sunlight was found to be mandatory for achieving competitive green-H2 production. Additionally, feasible strategies for upscaling the key components of PEC devices, especially photoelectrodes, are urgently needed. In a pragmatic context, the way to move forward is to accept that PEC devices will operate close to their thermodynamic limits at large-scale, which requires a solid convergence between academics and industry. Research efforts must be redirected to: (i) build and demonstrate modular devices with a low-cost and highly recyclable embodiment; (ii) optimize thermal and power management; (iii) reduce ohmic losses; (iv) enhance the chemical stability towards a thousand hours; (v) couple solar concentrators with PEC devices; (vi) boost PEC-H2 production through the use of organic compounds; and (vii) reach consensual standardized methods for evaluating PEC devices, at both environmental and techno-economic levels. If these targets are not met in the next few years, the feasibility of PEC-H2 production and its acceptance by industry and by the general public will be seriously compromised.

One-Step Dry Coating of Hybrid ZnO-WO3 Nanosheet Photoanodes for Photoelectrochemical Water Splitting with Composition-Dependent Performance

In this study, the potential of zinc oxide (ZnO), tungsten oxide (WO3), and their composites (ZnO-WO3) as photoanodes for photoelectrochemical (PEC) water splitting was investigated. ZnO-WO3 nanocomposites (NCs) were deposited on fluorine-doped tin oxide substrates at room temperature using a one-step dry coating process, the nanoparticle deposition system, with no post-processes. Different compositions of ZnO-WO3 NCs were optimized to enhance the kinetics of the PEC water-splitting reaction. Surface morphology analysis revealed the transformation of microsized particle nanosheets (NS) powder into nanosized particle nanosheets (NS) across all photoanodes. The optical characteristics of ZnO-WO3 photoanodes were scrutinized using diffuse reflectance and photoluminescence emission spectroscopy. Of all the hybrid photoanodes tested, the photoanode containing 10 wt.% WO3 exhibited the lowest bandgap of 3.20 eV and the lowest emission intensity, indicating an enhanced separation of photogenerated carriers and solar energy capture. The photoelectrochemical results showed a 10% increase in the photocurrent with increasing WO3 content in ZnO-WO3 NCs, which is attributed to improved charge transfer kinetics and carrier segregation. The maximum photocurrent for a NC, i.e., 10 wt.% WO3, was recorded at 0.133 mA·cm-2 at 1.23V vs. a reversible hydrogen electrode (RHE). The observed improvement in photocurrent was nearly 22 times higher than pure WO3 nanosheets and 7.3 times more than that of pure ZnO nanosheets, indicating the composition-dependence of PEC performance, where the synergy requirement strongly relies on utilizing the optimal ZnO-WO3 ratio in the hybrid NCs.

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.

Temperature-Controlled Transformation of WO3 Nanowires into Active Facets-Exposed Hexagonal Prisms toward Efficient Visible-Light-Driven Water Oxidation

A unique transformation of WO₃ nanowires (NW-WO₃) into hexagonal prisms (HP-WO₃) was demonstrated by tuning the temperature of the (N₂H₄)WO₃ precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO₃ nanocrystals (ca. <100 nm width, 3-5 μm length) with anisotropic growth of monoclinic WO₃ crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO₃ nanocrystals (ca. 500-1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO₃ nanocrystals with a single-crystalline character. The HP-WO₃ electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO₃ electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO₃ was 3.1-fold higher than 15% for the NW-WO₃ electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO₃ layer with the unidirectional nanowire structure is more efficient than that through the HP-WO₃ layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO₃ electrode is more efficient than the NW-WO₃ electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO₃ electrode. The efficient water oxidation reaction at the surface for the HP-WO₃ electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO₃ to suppress the electron-hole recombination at the surface.

Controllable Synthesis of N2-Intercalated WO3 Nanorod Photoanode Harvesting a Wide Range of Visible Light for Photoelectrochemical Water Oxidation

A highly efficient visible-light-driven photoanode, N2-intercalated tungsten trioxide (WO3) nanorod, has been controllably synthesized by using the dual role of hydrazine (N2H4), which functioned simultaneously as a structure directing agent and as a nitrogen source for N2 intercalation. The SEM results indicated that the controllable formation of WO3 nanorod by changing the amount of N2H4. The β values of lattice parameters of the monoclinic phase and the lattice volume changed significantly with the nW: nN2H4 ratio. This is consistent with the addition of N2H4 dependence of the N content, clarifying the intercalation of N2 in the WO3 lattice. The UV-visible diffuse reflectance spectra (DRS) of N2-intercalated exhibited a significant redshift in the absorption edge with new shoulders appearing at 470–600 nm, which became more intense as the nW:nN2H4 ratio increased from 1:1.2 and then decreased up to 1:5 through the maximum at 1:2.5. This addition of N2H4 dependence is consistent with the case of the N contents. This suggests that N2 intercalating into the WO3 lattice is responsible for the considerable red shift in the absorption edge, with a new shoulder appearing at 470−600 nm owing to formation of an intra-bandgap above the VB edges and a dopant energy level below the CB of WO3. The N2 intercalated WO3 photoanode generated a photoanodic current under visible light irradiation below 530 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO3 doing so below 470 nm. The high incident photon-to-current conversion efficiency (IPCE) of the WO3-2.5 photoanode is due to efficient electron transport through the WO3 nanorod film.

In vitro analysis of the cytotoxic effect of two different sizes ITER-like tungsten nanoparticles on human dermal fibroblasts

Background

Based on the current configuration of the International Thermonuclear Experimental Reactor, tungsten (W) was chosen as the armour material. Nevertheless, during operation, the expected power and temperature of plasma can trigger the formation of W dust in the plasma chamber. According to the scenario for a Loss Of Vacuum Accident (LOVA), in the case of confinement failure dust is released, which can lead to occupational or accidental exposure.

Methods

For a first evidence of potential risks, fusion devices relevant W dust has been produced on purpose, using a magnetron sputtering gas aggregation source. We aimed to assess the in vitro cytotoxicity of synthesized tungsten nanoparticles (W-NPs) with diameters of 30 and 100 nm, on human BJ fibroblasts. That was systematically investigated using different cytotoxic endpoints (metabolic activity, cellular ATP, AK release and caspase-3/7 activity) and by direct observation with optical and scanning electron microscopy.

Results

Increasing concentrations of W-NPs of both sizes induced cell viability decrease, but the effect was significantly higher for large W-NPs, starting from 200 μg/mL. In direct correlation with the effect on the cell membrane integrity, high concentrations of large W-NPs appear to increase AK release in the first 24 h of treatment. On the other hand, activation of the cellular caspase 3/7 was found significantly increased after 16 h of treatment solely for low concentrations of small W-NPs. SEM images revealed an increased tendency of agglomeration of small W-NPs in liquid medium, but no major differences in cells development and morphology were observed after treatment. An apparent internalization of nanoparticles under the cell membrane was also identified.

Conclusion

These results provide evidence for different toxicological outputs identified as mechanistic responses of BJ fibroblasts to different sizes of W-NPs, indicating also that small W-NPs (30 nm) display lower cytotoxicity compared to larger ones (100 nm).

Photocatalytic degradation of dissolved organic matter in landfill leachate by heterostructural ZnO-rGO composite catalysts

The dissolved organic matter (DOM) in landfill pollutes not only the landfill and surroundings, but also the environment far away from the landfill by infiltrating into the soil and/or flowing on the ground surface. Developing an efficient photocatalyst to degrade DOM is an interesting topic. Herein, the catalysts composed of ZnO and reduced graphene oxide (ZnO-rGO) with different morphologies were fabricated with a two-step hydrothermal method. The phase composite and microstructure were analyzed, and the degradation efficiency of the DOM under ultraviolet light was investigated. Three kinds of ZnO-rGO composite catalysts with different morphologies were successfully synthesized, and rGO was coated on the ZnO surface to form heterostructural composite catalysts. The catalyst powders have similar Raman and FT-IR spectra, but have different specific surface areas and band gaps. The degradation efficiency of DOM by ZnO-rGO composites is higher than that of pure ZnO powder. Compared to pure ZnO, ZnO-rGO composite catalysts contain more oxygen vacancies and a narrower band gap, and the heterostructure is beneficial for accelerating electron separation, inhibiting electron recombination.

Synergistic role of hydrogen treatment and heterojunction in H-WO3-x/TiO2-x NT/Ti foil-based photoanodes for photoelectrochemical wastewater detoxification and antibacterial activity

The process of photoelectrochemical wastewater detoxification is limited by significant charge recombination, which is difficult to suppress with efficient single-material photoanodes. We demonstrated the effectiveness of hydrogen treatment in evaluating charge separation properties in WO_3-x/TiO_2-x NT/Ti foil heterojunction photoanodes. The influence of varying hydrogen annealing (200-400 °C) on the structural and photoelectrochemical properties of WO₃/TiO₂ NS/NT heterojunction is studied systematically. Additionally, after hydrogen treatment of pristine WO₃/TiO₂ NT/Ti foil photoanodes, substoichiometric H-WO_3-x/TiO_2-x NT-300 achieved the 1.21 mA/cm² photocurrent density, which is 8.06 and 3.27 times than TiO₂ NT and WO₃/TiO₂ NT. The hydrogen-treated H-WO_3-x/TiO_2-x NT-300 electrode exhibits 3 times greater bulk efficiencies than the WO₃/TiO₂ NT electrode due to the production of oxygen vacancies at the interface. Additionally, optimum H-WO_3-x/TiO_2-x NS/NT-300 photoanode exhibited 93.8% E. coli and 99.8% BPA decomposition efficiencies. The present work shows the effectiveness of microwave-assisted H-WO_3-x/TiO_2-x NT heterojunction photoanodes for organic decomposition and antibacterial activity in a neutral environment without surface-loaded co-catalysts.

Controlled synthesis of tungsten trioxide with globular clusters constructed of nanoplates by rapid breakdown anodization

Herein, electrochemical synthesis of tungsten trioxide (WO₃) with globular clusters constructed of nanoplates is demonstrated. Under a breakdown anodization potential of 25 V at 50 °C, tungsten foil anode was efficiently electro-oxidized into WO₃nanoplates-aggragated globular clusters powder, rather than a thin film structure as conventional anodization occurs. The WO₃globular clusters were characterized by SEM, TEM, and XRD. Effects of electrolyte composition on the breakdown anodization of the W substrate has been discussed. It is suggested that the growth of the WO₃nanoplates is initiated by localized anodic dielectric breakdown, and followed by an effective crystal growth in the electrolyte at high breakdown field.

A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials Perspective

Exactly 50 years ago, the first article on electrochromism was published. Today electrochromic materials are highly popular in various devices. Interest in nanostructured electrochromic and nanocomposite organic/inorganic nanostructured electrochromic materials has increased in the last decade. These materials can enhance the electrochemical and electrochromic properties of devices related to them. This article describes electrochromic materials, proposes their classification and systematization for organic inorganic and nanostructured electrochromic materials, identifies their advantages and shortcomings, analyzes current tendencies in the development of nanomaterials used in electrochromic coatings (films) and their practical use in various optical devices for protection from light radiation, in particular, their use as light filters and light modulators for optoelectronic devices, as well as methods for their preparation. The modern technologies of “Smart Windows”, which are based on chromogenic materials and liquid crystals, are analyzed, and their advantages and disadvantages are also given. Various types of chromogenic materials are presented, examples of which include photochromic, thermochromic and gasochromic materials, as well as the main physical effects affecting changes in their optical properties. Additionally, this study describes electrochromic technologies based on WO₃ films prepared by different methods, such as electrochemical deposition, magnetron sputtering, spray pyrolysis, sol–gel, etc. An example of an electrochromic “Smart Window” based on WO₃ is shown in the article. A modern analysis of electrochromic devices based on nanostructured materials used in various applications is presented. The paper discusses the causes of internal and external size effects in the process of modifying WO₃ electrochromic films using nanomaterials, in particular, GO/rGO nanomaterials.

Novel Method for Fabricating Visible-Light Phototransistors Based on a Homojunction-Porous IGZO Thin Film Using Mechano-Chemical Treatment

A homojunction-structured oxide phototransistor based on a mechano-chemically treated indium-gallium-zinc oxide (IGZO) absorption layer is reported. Through this novel and facile mechano-chemical treatment, mechanical removal of the cellophane adhesive tape induces reactive radicals and organic compounds on the sputtered IGZO film surface. Surface modification, following the mechano-chemical treatment, caused porous sites in the solution-processed IGZO film, which can give rise to a homojunction-porous IGZO (HPI) layer and generate sub-gap states from oxygen-related defects. These intentionally generated sub-gap states played a key role in photoelectron generation under illumination with relatively long-wavelength visible light despite the wide band gap of IGZO (>3.0 eV). Compared with conventional IGZO phototransistors, our HPI phototransistor displayed outstanding optoelectronic characteristics and sensitivity; we measured a threshold voltage (V_th) shift from 3.64 to -6.27 V and an on/off current ratio shift from 4.21 × 10¹⁰ to 4.92 × 10² under illumination with a 532 nm green light of 10 mW/mm² intensity and calculated a photosensitivity of 1.16 × 10⁸. The remarkable optoelectronic characteristics and high optical transparency suggest optical sensor applications.

Simple solvothermal approach to highly nanostructured hematite thin films

In this work, we present a solvothermal method for the synthesis of hematite thin films on fluorine-doped tin oxide substrates. This simple method uses a precursor solution of iron(III) 2,4-pentanedionate dissolved in ethanol with a microliter-scale amount of water and yields hematite ∼500 nm thick films after annealing. The synthesized films were characterized using an array of methods, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance, and powder X-ray diffraction. Incorporating water into the precursor solution provides nucleation sites for the reaction and results show that by altering the amount of water used in the synthesis, it is possible to generate nanocrystalline films of different morphologies, nanocrystal size distributions, and surface areas. This synthetic procedure therefore provides control over the films’ physical and electrochemical characteristics. Doping of hematite thin films is also possible using this synthesis, as exemplified by doping with tin by adding tin(II) 2,4-pentanedionate to the precursor solution. To demonstrate utility, we build prototype photoelectrochemical cells using the synthesized hematite as the photoanode.

Hybrid Nanostructures Obtained by Transport and Condensation of Tungsten Oxide Vapours onto CNW Templates

We present hybrid nanomaterial architectures, consisting of carbon nanowalls (CNW) templates decorated with tungsten oxide nanoparticles, synthesized using a mechanism based on tungsten oxide sublimation, vapor transport, followed by vapor condensation, in the absence or presence of plasma. The key steps in the decoration mechanism are the sublimation of tungsten oxides, when are exposed in vacuum at high temperature (800 °C), and their redeposition on colder surfaces (400-600 °C). The morphology and chemical composition of the hybrid architectures, as obtained from Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy, are discussed with respect to substrate nature and the physical conditions of synthesis. We pointed out that the decoration process is strongly dependent on the temperature of the CNW templates and plasma presence. Thus, the decoration process performed with plasma was effective for a wider range of template temperatures, in contrast with the decoration process performed without plasma. The results are useful for applications using the sensing and photochemical properties of tungsten oxides, and have also relevance for fusion technology, tungsten walls erosion and material redeposition being widely observed in fusion machines.

Highly efficient visible-light photocatalytic ethane oxidation into ethyl hydroperoxide as a radical reservoir

Photocatalytic ethane conversion into value-added chemicals is a great challenge especially under visible light irradiation. The production of ethyl hydroperoxide (CH₃CH₂OOH), which is a promising radical reservoir for regulating the oxidative stress in cells, is even more challenging due to its facile decomposition. Here, we demonstrated a design of a highly efficient visible-light-responsive photocatalyst, Au/WO₃, for ethane oxidation into CH₃CH₂OOH, achieving an impressive yield of 1887 μmol g_cat ^-1 in two hours under visible light irradiation at room temperature for the first time. Furthermore, thermal energy was introduced into the photocatalytic system to increase the driving force for ethane oxidation, enhancing CH₃CH₂OOH production by six times to 11 233 μmol g_cat ^-1 at 100 °C and achieving a significant apparent quantum efficiency of 17.9% at 450 nm. In addition, trapping active species and isotope-labeling reactants revealed the reaction pathway. These findings pave the way for scalable ethane conversion into CH₃CH₂OOH as a potential anticancer drug.

Acid-Resistant BiVO4 Photoanodes: Insolubility Control by Solvents and Weak W Diffusion in the Lattice

We have revealed for the first time that BiVO₄ photoanodes can be used even in strong acid media by mixing organic solvents into the electrolyte and depositing multilayers with a WO₃ bottom layer. In general, the BiVO₄ photoanodes are photocorrosive, especially in acid solutions. However, this shortcoming has been overcome using a combination of the two aforementioned modifications. We deduced that the contribution of each mixing organic solvent for the anti-photocorrosion of BiVO₄ in sulfuric acid solutions can be evaluated on the basis of a new empirical indicator that incorporates molecular density, the Hansen solubility parameter, and molecular polarizability. Acetone and tert-butyl alcohol were especially promising solvents for stabilizing BiVO₄ in acid media. We confirmed that the mixed organic solvents stabilized surface-emergent Bi oxide species as a passivation layer, which was generated via multilayering with a WO₃ bottom layer. During heat treatment in the fabrication process, W weakly diffused into the BiVO₄ layer and a Bi oxide layer was formed on the outermost surface because of the Bi segregation that arose from the charge compensation between W⁶⁺ and V⁵⁺ in the BiVO₄ lattice. The surface Bi oxide layer, which was protected by the mixed organic solvents, steadily served as a passivation layer for anti-photocorrosion of the underlying BiVO₄ layer. We have confirmed that the BiVO₄/WO₃ photoanodes in acetone-mixed aqueous sulfuric acid solution reliably functioned for a photoelectrochemical reaction under simulated sunlight illumination, and photoelectrochemical production of S₂O₈^2- ions was confirmed under light irradiation at λ > 480 nm. These results suggest that the BiVO₄-based photoanodes have significant potential for use in acid media in conjunction with very straightforward modifications.

In situ observation of charge transfer and crystal field formation via high energy resolution X-ray spectroscopy during temperature programmed oxidation

Herein, it has been demonstrated how resonant X-ray emission spectroscopy can be employed to study the charge transfer dynamics in real-time during the temperature-induced oxidation of metallic tungsten. Application of high energy resolution schemes allowed distinguishing charge transfer to separate orbitals resulting from crystal field splitting. Based on the time-resolved studies, it was possible to determine the corresponding charge transfer rates. From the experimental data, we determined that the electron transfer during the thermal oxidation of the metal dominates in the temperature range of 470-570 °C, reaching a maximum of 0.036 electrons per °C.

Improving Photoelectrochemical Properties of Anodic WO3 Layers by Optimizing Electrosynthesis Conditions

Although anodic tungsten oxide has attracted increasing attention in recent years, there is still a lack of detailed studies on the photoelectrochemical (PEC) properties of such kind of materials grown in different electrolytes under various sets of conditions. In addition, the morphology of photoanode is not a single factor responsible for its PEC performance. Therefore, the attempt was to correlate different anodizing conditions (especially electrolyte composition) with the surface morphology, oxide thickness, semiconducting, and photoelectrochemical properties of anodized oxide layers. As expected, the surface morphology of WO₃ depends strongly on anodizing conditions. Annealing of as-synthesized tungsten oxide layers at 500 °C for 2 h leads to obtaining a monoclinic WO₃ phase in all cases. From the Mott-Schottky analysis, it has been confirmed that all as prepared anodic oxide samples are n-type semiconductors. Band gap energy values estimated from incident photon−to−current efficiency (IPCE) measurements neither differ significantly for as−synthesized WO₃ layers nor depend on anodizing conditions such as electrolyte composition, time and applied potential. Although the estimated band gaps are similar, photoelectrochemical properties are different because of many different reasons, including the layer morphology (homogeneity, porosity, pore size, active surface area), oxide layer thickness, and semiconducting properties of the material, which depend on the electrolyte composition used for anodization.

Comparative study of the around-Fermi electronic structure of 5d metals and metal-oxides by means of high-resolution X-ray emission and absorption spectroscopies

The composition of occupied and unoccupied electronic states in the vicinity of Fermi energies is vital for all materials and relates to their physical, chemical and mechanical properties. This work demonstrates how the combination of resonant and non-resonant X-ray emission spectroscopies supplemented with theoretical modelling allows for quantitative analysis of electronic states in 5d transition metal and metal-oxide materials. Application of X-rays provides element selectivity that, in combination with the penetrating properties of hard X-rays, allows determination of the composition of electronic states under working conditions, i.e. non-vacuum environment. Tungsten metal and tungsten oxide are evaluated to show the capability to simultaneously assess composition of around-band-gap electronic states as well as the character and magnitude of the crystal field splitting.

FeCoSe2 Nanoparticles Embedded in g-C3N4: A Highly Active and Stable bifunctional electrocatalyst for overall water splitting

To investigate cost affordable and robust HER and OER catalysts with significant low overpotentials, we have successfully embedded FeCoSe2 spheres on smooth surfaces of graphitic carbon nitride that demonstrated high stability and electrocatalytic activity for H2 production. We systematically analyzed the composition and morphology of FexCo1-xSe2/g-C3N4 and attributed the remarkable electrochemical performance of the catalyst to its unique structure. Fe0.2Co0.8Se2/g-C3N4 showed a superior HER activity, with quite low overpotential value (83 mV at -20 mA cm-2 in 0.5 M H2SO4) and a current density of -3.24, -7.84, -14.80, -30.12 mA cm-2 at 0 V (vs RHE) in Dulbecco's Phosphate-Buffered Saline (DPBS), artificial sea water (ASW), 0.5 M H2SO4 and 1 M KOH, respectively. To the best of our knowledge, these are the highest reported current densities at this low potential value, showing intrinsic catalytic activity of the synthesized material. Also, the catalyst was found to deliver a high and stable current density of -1000 mA cm-2 at an overpotential of just 317 mV. Moreover, the synthesized catalyst delivered a constant current density of -30 mA cm-2 for 24 h without any noticeable change in potential at -0.2 V. These attributes confer our synthesized catalyst to be used for renewable fuel production and applications.

Review on Recent Progress in the Development of Tungsten Oxide Based Electrodes for Electrochemical Energy Storage

Current progress in the advancement of energy-storage devices is the most important factor that will allow the scientific community to develop resources to meet the global energy demands of the 21st century. Nanostructured materials can be used as effective electrodes for energy-storage devices because they offer various promising features, including high surface-to-volume ratios, exceptional charge-transport features, and good physicochemical properties. Until now, the successful research frontrunners have focused on the preparation of positive electrode materials for energy-storage applications; nevertheless, the electrochemical performance of negative electrodes is less frequently reported. This review mainly focuses on the current progress in the development of tungsten oxide-based electrodes for energy-storage applications, primarily supercapacitors (SCs) and batteries. Tungsten is found in various stoichiometric and nonstoichiometric oxides. Among the different tungsten oxide materials, tungsten trioxide (WO₃ ) has been intensively investigated as an electrode material for different applications because of its excellent charge-transport features, unique physicochemical properties, and good resistance to corrosion. Various WO₃ composites, such as WO₃ /carbon, WO₃ /polymers, WO₃ /metal oxides, and tungsten-based binary metal oxides, have been used for application in SCs and batteries. However, pristine WO₃ suffers from a relatively low specific surface area and low energy density. Therefore, it is crucial to thoroughly summarize recent progress in utilizing WO₃ -based materials from various perspectives to enhance their performance. Herein, the potential- and pH-dependent behavior of tungsten in aqueous media is discussed. Recent progress in the advancement of nanostructured WO₃ and tungsten oxide-based composites, along with related charge-storage mechanisms and their electrochemical performances in SCs and batteries, is systematically summarized. Finally, remarks are made on future research challenges and the prospect of using tungsten oxide-based materials to further upgrade energy-storage devices.

Nanoporous BiVO4 nanoflake array photoanode for efficient photoelectrochemical water splitting

A high-quality nanoporous BiVO₄ nanoflake array photoanode was prepared by using an in situ transformation approach, which exhibited an excellent photoelectrochemical activity.

Tungsten oxide-based visible light-driven photocatalysts: crystal and electronic structures and strategies for photocatalytic efficiency enhancement

Photocatalysis (PC) technology has received global attention due to its high potential of addressing both environmental and energy issues using only solar light as energy input.

Charge Separation, Band-Bending, and Recombination in WO3 Photoanodes

In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO₃ photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.

Study of the Thermal Annealing on Structural and Morphological Properties of High-Porosity A-WO3 Films Synthesized by HFCVD

High-porosity nanostructured amorphous tungsten OXIDE (a-WO₃) films were synthesized by a Hot Filament Chemical Vapor Deposition technique (HFCVD) and then transformed into a crystalline WO₃ by simple thermal annealing. The a-WO₃ films were annealed at 100, 300, and 500 °C for 10 min in an air environment. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and UV–vis spectroscopy. Results revealed that the a-WO₃ films were highly porous, composed of cauliflower-like structures made of nanoparticles with average sizes of 12 nm. It was shown that the effect of annealing on the morphology of the a-WO₃ films leads to a sintering process. However, the morphology is conserved. It was found that at annealing temperatures of 100 °C, the a-WO₃ films are of an amorphous nature, while at 300 °C, the films crystallize in the monoclinic phase of WO₃. The calculated bandgap for the a-WO₃ was 3.09 eV, and 2.53 eV for the film annealed at 500 °C. Finally, the results show that porous WO₃ films preserve the morphology and maintain the porosity, even after the annealing at 500 °C.

All-Solution-Processed WO3/BiVO4 Core-Shell Nanorod Arrays for Highly Stable Photoanodes

Tungsten oxide (WO₃) and bismuth vanadate (BiVO₄) are one of the most attractive combinations to construct an efficient heterojunction for photoelectrochemical (PEC) applications. Here, we report an all-solution-processed WO₃/BiVO₄ heteronanostructure photoanode with highly enhanced photoactivity and stability for sustainable energy production. The vertically aligned WO₃ nanorods were synthesized on a fluorine-doped tin oxide/glass substrate by the hydrothermal method without a seed layer and BiVO₄ was deposited by pulsed electrodeposition for conformal coating. Owing to the long diffusion lengths of charge carriers in the WO₃ nanorods, the ability to absorb the wider range of wavelengths, and appropriate band-edge positions of the WO₃/BiVO₄ heterojunction for spontaneous PEC reaction, the optimum WO₃/BiVO₄ photoanode has a photocurrent density of 4.15 mA/cm² at 1.23 V versus RHE and an incident-photon-to-current efficiency of 75.9% at 430 nm under front illumination, which are a double and quadruple those of pristine WO₃ nanorod arrays, respectively. Our work suggests an environment-friendly and low-cost all-solution process route to synthesize high-quality photoelectrodes.

Pd@H yWO3- x Nanowires Efficiently Catalyze the CO2 Heterogeneous Reduction Reaction with a Pronounced Light Effect

The design of photocatalysts able to reduce CO₂ to value-added chemicals and fuels could enable a closed carbon circular economy. A common theme running through the design of photocatalysts for CO₂ reduction is the utilization of semiconductor materials with high-energy conduction bands able to generate highly reducing electrons. Far less explored in this respect are low-energy conduction band materials such as WO₃. Specifically, we focus attention on the use of Pd nanocrystal decorated WO₃ nanowires as a heretofore-unexplored photocatalyst for the hydrogenation of CO₂. Powder X-ray diffraction, thermogravimetric analysis, ultraviolet-visible-near infrared, and in situ X-ray photoelectron spectroscopy analytical techniques elucidate the hydrogen tungsten bronze, H _yWO_{3- x}, as the catalytically active species formed via the H₂ spillover effect by Pd. The existence in H _yWO_{3- x} of Brønsted acid hydroxyls OH, W(V) sites, and oxygen vacancies (V_O) facilitate CO₂ capture and reduction reactions. Under solar irradiation, CO₂ reduction attains CO production rates as high as 3.0 mmol g_cat^-1 hr^-1 with a selectivity exceeding 99%. A combination of reaction kinetic studies and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements provide a valuable insight into thermochemical compared to photochemical surface reaction pathways, considered responsible for the hydrogenation of CO₂ by Pd@H _yWO_{3- x}.

Semiconducting Synthetic Melanin-Based Organic/Inorganic Hybrid Photoanodes for Solar Water Oxidation

We report the development of semiconducting melanin-based organic/inorganic hybrid photoanodes for solar water oxidation. Synthetic melanin thin-film incorporating polyoxometalate (POM) water oxidation catalysts (WOCs) are readily deposited on the surface of various n-type inorganic semiconductors (e.g., Fe₂ O₃ , BiVO₄ , and TiO₂ ) by electropolymerization. The deposition of melanin and POM hybrid (MP) thin-film results in the remarkably improved performance of an underlying semiconductor photoanode for solar water oxidation with a significantly increased photocurrent density and decreased onset potential for water oxidation through the formation of a melanin-based p-n heterojunction structure. We believe that this study can provide insights into the design and fabrication of various melanin-based optoelectronic devices by utilizing its unique physicochemical properties.

Fabrication of hollow boron-doped diamond nanostructure via electrochemical corrosion of a tungsten oxide template

In the study, a hollow boron-doped diamond (BDD) nanostructure electrode is fabricated to increase the reactive surface area for electrochemical applications. Tungsten oxide nanorods are deposited on the silicon substrate as a template by the hot filament chemical vapor deposition (HFCVD) method. The template is coated with a 100 nm BDD layer deposited by HFCVD to form a core-shell nanostructure. The WO _x core is finally electrochemically dissolved to form hollow BDD nanostructure. The fabricated hollow BDD nanostructure electrode is investigated via scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The specific surface areas of the electrodes were analyzed and compared by using Brunauer-Emmett-Teller method. Furthermore, cyclic voltammetry and chronocoulometry are used to investigate the electrochemical characteristics and the reactive surface area of the as-prepared hollow BDD nanostructure electrode. A hollow BDD nanostructure electrode exhibits a reactive area that is 15 times that of a planar BDD thin electrode.

Spinel Structural Disorder Influences Solar-Water-Splitting Performance of ZnFe2 O4 Nanorod Photoanodes

Zinc spinel ferrite, ZnFe2 O4 (ZFO), is an emerging photoanode material for photoelectrochemical (PEC) solar fuel production. However, a lack of fundamental insight into the factors limiting the photocurrent has prevented substantial advance in its performance. Herein, it is found that ZFO nanorod array photoelectrodes with varying crystallinity exhibit vastly different PEC properties. Using a sacrificial hole scavenger (H2 O2 ), spatially defined carrier generation, and electrochemical impedance spectroscopy, it is shown that ZFO with a relatively poor crystallinity but a higher spinel inversion degree (due to cation disorder) exhibits superior photogenerated charge separation efficiency and improved majority charge carrier transport compared to ZFO with higher crystallinity and a lower inversion degree. Conversely, the latter condition leads to better charge injection efficiency. Optimization of these factors, and the addition of a nickel-iron oxide cocatalyst overlayer, leads to a new benchmark solar photocurrent for ZFO of 1.0 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE) and 1.7 mA cm-2 at 1.6 V versus RHE. Importantly, the observed correlation between the cation disorder and the PEC performance represents a new insight into the factors important to the PEC performance of the spinel ferrites and suggests a path to further improvement.

BSA-caged metal clusters to exfoliate MoS2 nanosheets towards their hybridized functionalization

Herein, we develop a facile exfoliation and in situ functionalization strategy to produce hybridized Au/MoS2 nanostructures comprised of size-controlled gold nanoparticles (Au NPs) and ultrathin MoS2 nanosheets by using bovine serum albumin (BSA)-caged Au25 clusters as both exfoliating and functionalizing agents. As revealed, BSA molecules are strongly adsorbed on MoS2via their hydrophobic interaction, and this drives the expansion of the BSA molecules that initially protect Au25 cores at pH 4, leading to the effective exfoliation of MoS2 nanosheets together with the epitaxial growth of Au25 cores into 5 nm-sized Au NPs on MoS2 nanosheets due to their reduced surface protection. Upon the addition of H2O2, the resulting Au NPs can further grow to achieve a controlled size from 5 to 30 nm with an increase of the reaction time. It is demonstrated that the hybridized Au/MoS2 nanosheets exhibit a better performance in the photocatalytic degradation of substrates compared to the individual components or their mixture. Moreover, the hybridized Ag/MoS2, Au/WO3 and Au/graphene nanosheets are further produced by the usage of BSA-caged Ag and Au clusters, respectively. Overall, this work reports the first utilization of protein-caged metal clusters for the exfoliation and hybridized functionalization of 2D materials, and this brings more opportunities to exploit unusual properties of hybridized 2D materials for novel applications.

Photoelectrochemical Performance for Water Oxidation Improved by Molecular Nickel Porphyrin-Integrated WO3 /TiO2 Photoanode

A WO₃ /TiO₂ heterojunction photoanode was prepared by in situ growth of WO₃ on a mesoporous TiO₂ electrode. The photoinduced charge-transfer properties and chargeseparation improvement in this kind of type-II heterojunction were characterized by transient surface photovoltage spectra. By using sulfite oxidation as a hole scavenger, we demonstrated that 72 % of the photo-generated holes are reaching the surface of the photoanode, but the efficiency of hole injection (η_ox ) into the electrolyte was only 48 %. For the first time, a Ni^II meso-tetra(4-carboxyphenyl)porphyrin (NiTCPP) was incorporated as a water oxidation catalyst into the WO₃ /TiO₂ heterojunction photoanode, which promoted the value of η_ox to 81 %. The maximum applied bias photon-to-current efficiency for the WO₃ /TiO₂ /NiTCPP photoanode was determined to be 0.2 % at 1.01 V vs. the reversible hydrogen electrode (RHE), under which condition a Faradic efficiency of 89 % for water oxidation was achieved (averaged over 1 h of photolysis).