Branched multiphase TiO2 with enhanced photoelectrochemical water splitting activity

An efficient hierarchical structure, nano-branch containing anatase TiO₂ nanofibers and rutile nanorods, was prepared via the combination of the electrospinning and hydrothermal processes. This novel configuration of TiO₂ multiphase possessed higher surface area, roughness, and fill factors compared with each single phase component prepared in the same condition, which significantly enhanced its light absorption. Our experimental results showed that within the interface of multiphase TiO₂, the heterojunction promoted the charge separation and improved the charge transfer rate, leading to higher efficiency for photoelectrochemical water splitting. The photocurrent density of the nano-branched TiO₂ electrode could reach 0.95 mA/cm², which was almost twice as large as that of the pristine TiO₂ nanorod. Our work provides a simple and feasible routine to synthesize complex TiO₂ nanoarchitectures, which lays a foundation for improving energy storage and conversion efficiency of TiO₂-based photoelectrodes.     

Photoelectrochemical and physical properties of WO3 films obtained by the polymeric precursor method

Tungsten trioxide (WO₃) films were prepared by a solution-based method using ammonium metatungstate as the precursor and polyethylene glycol as the structure-directing agent. With the measurements of thermogravimetric and differential thermal analysis, X-ray diffraction, scanning electron microscopy, and ultraviolet and visible absorption spectroscopy, the effect of substrates and temperature on the crystal structure and crystalline formation of WO₃ was investigated. The results show that the WO₃ films were crystallized by sintering at over 400 °C, and the films prepared on fluorine–tin oxide glass substrates were distorted cubic in crystalline phase. However, a monoclinic crystal was formed by coating films on graphite and quartz glass substrates. Photoelectrochemical activity was evaluated under visible light irradiation. The WO₃ electrode calcined at 450 °C exhibited a photocurrent density of up to 2.7 mA/cm² at 1.4 V (vs. RHE) under incident 100 mW/cm² 500 W Xe lamp and donor carrier density N_D = 2.44 × 10²² cm⁻³ in 0.5 M H₂SO₄ electrolyte. The photoanode was stable up to 90 min, and the photocurrent decreased 39% with continuous gas evolution.     

Study on the WO3 dry lithiation for all-solid-state electrochromic devices

In this report, a simple WO₃ dry lithiation is proposed for fabrication of all-solid-state electrochromic devices and characterized completely by X-ray photoelectron spectroscopy and electrochemical method. Lithiation is carried out by electron-beam evaporation of metal lithium, and the lithiated films have different components and electrochromic properties with different lithiation degrees. It is found that if Li/W ratio is less than 0.25, tungsten bronze Li_xW0₃ is formed and the lithiated by wet method. Finally, a lithium-based all-solid-state electrochromic device with proper lithiation degree is fabricated using this dry method.     

WO3 pillar-type and helical-type thin film structures to be used in microbatteries

In the present study WO₃ thin films were deposited by sputtering onto ITO glass, W/ITO and Si substrates by using the glancing angle deposition (GLAD) technique, with the objective of applying these materials in electrochemical intercalation devices. The thin films microstructure and electrochemical behavior were determined through scanning electron microscopy (SEM) and cycling at constant current with potential limitation. By mainly adjusting the substrate holder speed rotation, pillar-type and helical-type structures were obtained under high and low speed rotation levels, respectively. The electrochemical results showed that the best charge capacity performance was obtained for the WO₃/W/ITO films with pillar-type structures, which are more porous.     

WO3/g-C3N4 two-dimensional composites for visible-light driven photocatalytic hydrogen production

WO₃/g-C₃N₄ two-dimensional (2D) composite photocatalysts were prepared through a simple hydrothermal method followed by a post thermal treatment. The H₂ generation activity of these photocatalysts in the visible light was evaluated. The photocatalysts were characterized by X-ray powder diffraction, Fourier transform infrared spectra, transmission electron microscopy and UV–vis diffuse reflectance spectroscopy et al. These results show that the orthorhombic-phase WO₃ nanoparticles with a grain size from 5 to 80 nm were successfully anchored on g-C₃N₄ nanosheets surface with intimate contact. Furthermore, the charge separation mechanisms of photo-generated charge carriers of the 2D WO₃/g-C₃N₄ photocatalysts were further studied by photoelectrochemical response and electrochemical impedance spectroscopy. The result shows that the 2D WO₃/g-C₃N₄ photocatalyst with 10 wt% WO₃ possesses the maximum photocatalytic performance for H₂ generation, as high as of 1853 μmol h^?1 g^?1, which is about 6.5 times higher than that of bare g-C₃N₄, indicating the fast injection of interface interaction between 2D g-C₃N₄ and WO₃. The increased photocatalytic performance of the composite photocatalyst can be attributed to the enhanced absorption of visible light, the higher photo-generated electrons and holes separation efficiency and low recombination rate of electrons and holes generated by photoexcitation.     

WO3/BiVO4 composite photoelectrode prepared by improved auto-combustion method for highly efficient water splitting

We report on the improvement in the water splitting efficiency of a WO₃/BiVO₄ composite photoelectrode by the application of an improved auto-combustion method to the preparation of porous BiVO₄ thin films. The unique feature of this preparation method is the addition of both NH₄NO₃, as a strong oxidizing agent, and an organic additive into BiVO₄ precursor solution. The local decomposition heat of the organic additive and oxidizing agent created a porous film with small, highly crystalline BiVO₄ particles. The photoelectrode has many advantages over existing ones, such as the high light-harvesting efficiency (LHE), a single BiVO₄ phase, the facile access of the holes to the photoelectrode/electrolyte interface, and the ease of water and oxygen diffusion. The maximum incident photon-to-current efficiency (IPCE) was estimated to be 64% (at 440 nm, 1.23 V vs. RHE) and the applied bias photon-tocurrent efficiency (ABPE) reached as high as 1.28%. This ABPE value is highest among all oxide semiconductor photoelectrodes reported previously, except for the case of a stacking photoelectrode system.     

Combinatorial development of nanoporous WO3 thin film photoelectrodes for solar water splitting by dealloying of binary alloys

A combinatorial materials approach is suggested for the development of nanoporous thin film oxides for photoelectrochemical solar water splitting. As a precursor for nanoporous WO₃ films, metallic nanoporous W films were synthesized by dealloying sputtered W_1−xAl_x and W_1−xFe_x (0.06 < x < 0.67) thin film materials libraries in aqueous HNO₃ solutions with different concentrations for 24 h under open circuit conditions. The variation of the etchant concentration provided different film nanostructures. The films were then transformed into nanoporous WO₃ by controlled thermal oxidation at 500 °C in air. Screening of the photoelectrochemical properties of nanoporous WO₃ films shows a strong porosity- and thickness-dependence of the photocurrent. At the same time the photocurrent density does not depend on precursor composition, because dealloying in acid solutions of certain concentration leads to formation of identical nanostructures in a broad range of precursor compositions.     

Structural and electrochemical studies of WO3 films deposited by pulsed spray pyrolysis

Transparent conductive and WO₃ electrochromic thin films were deposited by spray pyrolysis technique. The films were deposited using solutions of WCl₆ in dimethylformamide on SnO₂:F (FTO) substrates with different sheet resistances. Noticeable effects of substrate on structural, morphological and optical properties of the WO₃ films and on its electrochromic behavior are presented and discussed. Hexagonal and monoclinic WO₃ structures were obtained on amorphous glass substrates; also the monoclinic structure on polycrystalline FTO substrates was obtained. Cyclic structural changes during the colored and blanched states were found from XRD and electron diffraction result analysis: The hydrogen tungsten bronze in the tetragonal phase after the hydrogen extraction change to the original WO₃ monoclinic phase.     

Yb-doped WO3 photocatalysts for water oxidation with visible light

Yb-doped WO₃ photocatalysts were prepared by co-sputtering WO₃ and Yb, followed by annealing in air for water oxidation with visible light. All the obtained photocatalysts were monoclinic with sputtering power of Yb up to 10 W and displayed no optical absorption red shift. In photoelectrochemical (PEC) studies, the photocurrent densities were improved with up to 0.34 at.% Yb in WO_3, with the highest photocurrent of 1.3 mA/cm² (1.2 V vs. Ag/AgCl) achieved with <0.1 at.% Yb. Electrochemical impedance spectroscopy (EIS) measurements showed that optimized Yb doping reduced charge transfer resistance and increased donor density of WO₃ photocatalyst. The improvement in photocurrent density was attributed to enhanced conductive carrier path, increased oxygen vacancies and 4f¹³ orbital configuration due to Yb³⁺ substitution of W⁶⁺.     

Pt nanoparticles supported on WO3/C hybrid materials and their electrocatalytic activity for methanol electro-oxidation

A simple and novel method for the preparation of WO₃/C is presented. This method includes the adsorption and decomposition of phosphotungstic acid (PWA) on carbon. For the purpose of comparison, WO₃/C is also prepared by a conventional method using sodium tungstate as the precursor. These two WO₃/C species are denoted as WO₃/C-1 and WO₃/C-2, respectively. It is shown from transmission electron microscopy (TEM) that the WO₃ particles in WO₃/C-1 present a more even distribution and smaller particle size than those in WO₃/C-2. Pt particles dispersed on WO₃/C-1 display the characteristic diffraction peaks of Pt in the face-centered cubic phase. Cyclic voltammetry and chronoamperometry show that the Pt-WO₃/C-1 catalyst exhibits much better methanol oxidation activity than the Pt-WO₃/C-2 and Pt/C catalysts. This significant improvement in catalytic performance may be attributed to the hydrogen spillover effect and the uniform distribution of Pt and WO₃ particles.     

Hydrogen production from photo-driven electrolysis of biomass-derived oxygenates: A case study on methanol using Pt-modified WO3 thin film electrodes

In this paper, we demonstrate the feasibility of H₂ production from biomass-derived oxygenates with photoelectrochemical cells (PECs) based on the tandem cell hybrid photoelectrode configuration. As a proof of concept, we have studied the simplest oxygenate, methanol, which is photoelectrochemically oxidized at thin film tungsten oxide (WO₃) photoelectrodes. When the methanol oxidation reaction (MOR) is coupled with the hydrogen evolution reaction (HER), this process is known as methanol electrolysis. We demonstrate that catalytic modification of the WO₃ surface by the electrodeposition of Pt particles can greatly increase MOR activity at the photoanode, resulting in a significant increase in H₂ production rates from methanol electrolysis. This improvement is greatest at low overpotentials and high Pt loadings, with the demonstrated MOR current density of Pt-WO₃ being nearly four times that of the oxygen evolution reaction (OER) on WO₃ at a potential of 0.8 V vs. the Reversible Hydrogen Electrode. We also illustrate how the increase in WO₃ photocurrent and the decrease in the oxidation onset potential, compared to the OER, make it possible to use WO₃-based photoelectrodes in a simple tandem cell configuration whereby a common PV component such as a-Si can provide the remaining voltage to achieve unassisted methanol electrolysis. Results from methanol electrolysis reveal the potential to utilize a similar approach for larger biomass-derived oxygenates, which could be a promising pathway to H₂ production from renewable feedstock using photo-driven electrolysis.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.     

MEMS-microhotplate-based hydrogen gas sensor utilizing the nanostructured porous-anodic-alumina-supported WO3 active layer

Practical microsensors for fast, highly sensitive hydrogen gas detection were fabricated by combining silicon integral technology for MEMS microhotplate platform with newly developed technological, electrical, and electrolytic conditions for forming nanostructured porous-anodic-alumina-templated WO₃ layer as the sensing material. The morphology–structure–property relationship for the nanostructured sensing layer was determined by scanning electron microscopy, X-ray diffraction, and through systematically investigating the sensor performance at various H₂ concentrations (5–1000 ppm) and operating temperatures (20–350 °C). The sensors showed superior sensitivity to hydrogen gas, with the lowest detection limit ever reported for WO₃ semiconductors (5 ppm), the fast response and recovery times (2–3 min), and the best sensitivity at 150 °C, which was 100 times higher than that of a reference sensor having a smooth WO₃ active film. The technology developed enables high-volume, low-cost, and low-power sensor-on-a-chip solution for a hydrogen-based energy economy where the use of highly sensitive and low-power-consuming devices is encouraged.     

Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering

Polycrystalline WO₃ thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O₂ gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO₃, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm⁻² at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO₃ films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm.     

Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation

Hexagonal structured WO₃ films with tree-like morphology were synthesized on tungsten foils by a hydrothermal method. Each nanotree was composed of several (typically six) nanosheet-shaped “branches”. TEM examination revealed that the nanosheet was a single crystal and its long axis was oriented toward 〈0 0 1〉 direction. The WO₃ nanotree films retain the hexagonal structure after being annealed up to 400 °C for 2 h, while they have a phase transition to monoclinic structure after being annealed at 500 °C for 2 h. Electrochromic (EC) performance of the films was examined in a propylene carbonate solution of 1 M LiClO₄ using an electrochemical workstation and an UV-Vis spectrometer. Due to the large tunnels of hexagonal structure and highly porous surface morphology, a large modulation of optical reflectance of WO₃ nanotree films up to 30% and coloration efficiency of 43.6 cm² C⁻¹ at 500 nm were achieved by annealing the WO₃ nanotree films at 400 °C for 2 h.     

Modelling and development of photoelectrochemical reactor for H2 production

Photoelectrolysis of aqueous solutions, using one or more semiconducting electrodes in a photoelectrochemical reactor, is a potentially attractive process for hydrogen production because of its prospectively high energy efficiency, simplicity and potentially low cost. The design requirements and preliminary results of modelling a photoelectrochemical (PEC) reactor are described. Potential and current density distributions, due to ohmic potential losses in thin (non-photo) anodes on poorly conducting fluoride-doped tin oxide coated glass substrates, were modelled. The predicted current densities decayed rapidly from the terminals at the edges, towards the centre of a 0.1 × 0.1 m² anode, so limiting scale-up with such substrates. Spatial distributions of dissolved oxygen concentrations were also modelled, aiming to define operating conditions that would avoid forming bubbles, which reflect light specularly decreasing photon absorption efficiencies of photoelectrodes. The implications for the future optimization of the reactor are discussed.     

Fabrication and photoelectrochemical study of Ag@TiO2 nanoparticle thin film electrode

Ag@TiO₂ nanoparticle thin film was fabricated for photoelectrochemical water splitting in the visible light region. Under the irradiation of UV light, positive photocurrent was enhanced in both electrolytes of 0.1 M HNO₃ and 0.1 M NaOH owing to the excitation of photoelectrons within the TiO₂ shells. However, under the irradiation of visible light, the enhancement of positive photocurrent was observed only in 0.1 M HNO₃ because of the formation of a Schottky barrier band bending at the Ag-TiO₂ core-shell interface and the generation of photoelectrons resulted from the surface plasmon resonance of Ag cores. In 0.1 M NaOH, significant negative photocurrent was enhanced due to the influences of higher pH on the surface state and energy level of TiO₂ shells. Such a visible light-induced photoresponse enhancement and photocurrent direction switching made the Ag@TiO₂ nanoparticle thin film useful not only as a photoelectrode for water splitting but also as a photo-switch in a basic electrolyte.     

Significantly improve photoelectrochemical performance of Ti:Fe2O3 with CdSe modification and surface oxidation

Significant interest has been arisen to explore photoanodes for full optical absorption spectrums and good stability in photoelectrochemistry. Herein CdSe is used to modify Ti:Fe₂O₃ photoanode forming Ti:Fe₂O₃/CdSe heterojunction. Combining with an air annealing treatment, Ti:Fe₂O₃/CdSe exhibits a 6.5 times higher photocurrent density that of the pristine Ti:Fe₂O₃ to achieve 3.25 mA cm^?2 at 1.2 V vs. RHE. The photoelectrochemical (PEC) stability of Ti:Fe₂O₃/CdSe annealed in air shows great improvement comparable to both unannealed and annealed ones in Ar. The enhancement mechanisms for both heterojunction and annealing are explored for fundamental insights, which reveal that the surface oxide layer can significantly increase the PEC stability of Ti:Fe₂O₃/CdSe photoanode. X-ray photoelectron spectra and transmission electron microscope results further confirms the surface oxidation on CdSe layer after annealing in air.     

Irradiation effect in WO3 thin films

Tungsten oxide (WO₃) films were prepared on indium tin oxide coated glass substrates and Corning glass substrates by sol–gel deposition. The samples coated on the glass substrates have been irradiated to approximately 0.93–21.1 kGy dose using Co-60 gamma radioisotope. Co-60 radioisotope changed the color of the WO₃ films on samples after the irradiation. Their color turned to brownish color tones depending on the applied dose. Optical and structural properties of the samples are examined for both gamma irradiated and unirradiated coated samples. To compare the effect of the irradiation on the electrochromic properties, additional measurements were done with WO₃ coated on ITO substrates irradiated by gamma rays, separately. The coated films were characterized by atomic-force microscopy, NKD-analyzer and cyclic voltammograms. The influence of irradiation on the spectra of transmittance and on the surface structure has been investigated. These showed that the surface texture was changed dramatically by the irradiation. The electrochemical insertion and removal of lithium and proton ions was carried-out using 1 M LiClO₄ propylene carbonate (PC) electrolyte and 1 M KCl in aqueous solutions,respectively.     

The effect of water on the electrochromic properties of WO3 films prepared by vacuum and chemical methods

We present a comparative study on the effect of absorbed water on the properties of tungsten oxide films prepared by two different methods (e-gun evaporation, and an aqueous sol–gel technique). Scanning electron microscopy, Fourier transform infrared spectroscopy and electrochemical techniques have been used to assess the film properties. It has been found that the preparation method of the films greatly affects their water content and thus, electron gun evaporated films have less water incorporated into their structure than their sol–gel counterparts. The former are closely packed and transparent with most of their water content adsorbed on their surface, while the latter have a porous structure, being opaque, highly hydroxylated and hydrated to a lesser extent.Both types of films exhibit reversible electrochromism, with the evaporated films being stable up to 5000 coloration-bleaching cycles and the sol–gel ones gradually degrading after 1000 cycles. Irreversible Li⁺ trapping related to the presence of water and hydroxyl radicals has been envisaged as the cause of the inferior cycling stability of the sol–gel films.