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.     

H2 sensors based on WO3 thin films activated by platinum nanoparticles synthesized by electroless process

Platinum (Pt) nanoparticles were successfully synthesized on tungsten oxide (WO₃) thin films by electroless process without any further post-treatment. The prepared Pt nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and field-emission scanning electron microscopy. Gas sensors based on the Pt–WO₃ films were found to provide repeatable and significant responses to ppm-level H₂. The size of Pt nanoparticles increases with the deposition time and has improved the sensing characteristics of the sensors. The work in this paper paves a facile way to the fabrication of Pt nanoparticles on metal oxide surface at a low temperature (68 °C).     

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.     

The dependence of the chemical potential of WO3 films on hydrogen insertion

Knowledge of the chemical potential of electrochromic films coloured by hydrogen is important for matching the elements of an electrochromic device, for understanding the colouring mechanism and for obtaining information about the microscopic structure of the film. The dependence of the chemical potential on the hydrogen concentration was measured electrochemically for tungsten oxide films of different crystallinity and water content. A new method for determining the chemical potential by catalytic coloration by hydrogen gas is introduced. It revealed that the increase in electromotive force with increasing crystallinity is due only to different binding energies of the protons. We expect the protons to be located in the centres of hexagons, which are created by WO₆ octahedra. According to our model, amorphous sputtered films show a hexagonal structure which is similar to that of evaporated films, but the hexagons are connected, leading to more hexagon centre sites, which increases the electromotive force.     

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.     

Hydrogen gas sensor based on mesoporous In2O3 with fast response/recovery and ppb level detection limit

Hydrogen gas sensors were fabricated using mesoporous In₂O₃ synthesized using hydrothermal reaction and calcination processes. Their best performance for the hydrogen detection was found at a working temperature of 260 °C with a high response of 18.0 toward 500 ppm hydrogen, fast response/recovery times (e.g. 1.7 s/1.5 s for 500 ppm hydrogen), and a low detection limit down to 10 ppb. Using air as the carrier gas, the mesoporous In₂O₃ sensors exhibited good reversibility and repeatability towards hydrogen gas. They also showed a good selectivity for hydrogen compared to other commonly investigated gases including NH₃, CO, ethyl alcohol, ethyl acetate, styrene, CH₂Cl₂ and formaldehyde. In addition, the sensors showed good long-term stability. The good sensing performance of these hydrogen sensors is attributed to the formation of mesoporous structures, large specific surface areas and numerous chemisorbed oxygen ions on the surfaces of the mesoporous In₂O₃.     

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.     

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.     

High performance catalyst for electrochemical hydrogen evolution reaction based on SiO2/WO3−x nanofacets

The electrochemical hydrogen evolution reaction (HER) was studied over silica/tungsten oxide nanofacets (SiO₂/WO_3−x) that was prepared by calcinations of electrospun polyacrylonitrile nanofibers containing silicotungstic acid under air atmosphere. It was found that the Keggin structure of precursor (H₄SiW₁₂O₄₀.29H₂O) was decomposed and transferred to crystalline monoclinic WO₃ after calcinations at 500 °C. The morphology of prepared catalyst after pyrolysis, observed by FE-SEM, was nanocrystals deposited on joined nanoparticles fiber. The size of nanocrystal increases with increasing annealing time. In addition, increasing annealing time also enhances interaction between SiO₂ and WO_3−x. The synthesized catalyst was employed as an electrocatalyst for HER. It was found that the catalyst annealed at 500 °C for 5 h showed 6.6 times higher HER activity than the bulk WO₃ and exhibits excellent electrochemical stability over 100 cycles.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Highly selective hydrogen sensing of Pt-loaded WO3 synthesized by hydrothermal/impregnation methods

Unloaded and 0.25–1.0 wt% Pt-loaded WO₃ nanoparticles were synthesized by hydrothermal method using sodium tungstate dihydrate and sodium chloride as precursors in an acidic condition and impregnated using platinum acetylacetonate. Pt-loaded WO₃ films on an Al₂O₃ substrate with interdigitated Au electrodes were prepared by spin-coating technique. The response of WO₃ sensors with different Pt-loading concentrations was tested towards 0.01–1.0 vol% of H₂ in air as a function of operating temperature (200–350 °C). The 1.0 wt% Pt-loaded WO₃ sensing film showed the highest response of ∼2.16 × 10⁴ to 1.0 vol% H₂ at 250 °C. Therefore, an operating temperature of 250 °C was optimal for H₂ detection. The responses of 1.0 wt% Pt-loaded WO₃ sensing film to other flammable gases, including C₂H₅OH, C₂H₄ and CO, were considerably less, demonstrating Pt-loaded WO₃ sensing film to be highly selective to H₂.     

Resistive-type hydrogen gas sensor based on TiO2: A review

Owing to its high energy density and environmentally friendly nature, hydrogen has already been regarded as the ultimate energy of the 21st century and gained significant attention from the worldwide researchers. Meanwhile, there are increasing concerns about its safe use, storage and transport as, despite being colorless and odorless, after certain concentration level it becomes flammable and explosive in air. Therefore, it is imperative to develop H₂ sensors for real-time monitoring of the H₂ leakage for an early warning. This paper firstly introduces the general hydrogen gas sensing mechanism of TiO₂-based hydrogen sensors. Then we summarize and comment on the current hydrogen gas sensor based on various TiO₂ materials, which include pristine TiO₂, metal-assisted TiO₂, organic-TiO₂ composites, carbon-TiO₂ composites, MOX-TiO₂ composites and novel sensor concept with effective top-bottom electrode configuration. Finally, we briefly discuss the obstacles that TiO₂-based H₂ sensors have to overcome in the progress of the systematically practical application, possible solutions, and future research perspectives that can be focused in this area.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

High performance hydrogen sensor based on Pd/TiO2 composite film

Hydrogen sensors with fast response and recovery rate based on nanoporous palladium (Pd) and titanium dioxide (TiO₂) composite films supported by anodic aluminum oxide (AAO) template have been demonstrated. Nanoporous TiO₂ film was sprayed on the porous AAO templates, followed by Pd film deposited on TiO₂ layer by DC magnetron sputtering. We have researched the detection performance of the hydrogen sensors depending on different thickness of TiO₂ layer from 6 to 30 nm with keeping the thickness of Pd as 30 nm. The results have demonstrated the sensors with 10 nm thickness of TiO₂ achieve the best performance with a response/recovery time as short as 4/8s at 0.8% and 0.4% hydrogen concentration, respectively. The sensors exhibited very good performance under hydrogen concentrations from 0.4% to 1.8%.     

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.     

Size effects of WO3 nanocrystals for photooxidation of water in particulate suspension and photoelectrochemical film systems

Monoclinic WO₃ nanocrystals were synthesized by a hydrothermal reaction and post calcination. Their particle sizes were varied from 30 nm to 500 nm by changing calcination temperature from 500 °C to 800 °C. Photooxidation of water was studied in particulate suspension (PS) system and photoelectrochemical (PEC) film system. For PS system, WO₃ nanocrystals were suspended in 50 mM AgNO₃ solution to measure O₂ evolution rate. For PEC system, WO₃ films were fabricated by doctor blade method using synthesized nanocrystals. Photocurrent density was measured at AM 1.5 G (1 sun) solar condition in 0.5 M H₂SO₄. In PS system, the sample calcined at the highest temperature generated the largest amount of oxygen, whereas in PEC system the sample calcined at 600 °C showed the maximum photocurrent. The two systems also showed opposite response to deposition of the Pt co-catalyst. These different behaviors were attributed to different mechanisms of charge separation in the two systems.     

Numerical approach to evaluate performance of porous SiC5/4O3/2 as potential high temperature hydrogen gas sensor

Porous silicon oxycarbide (SiCO) is a novel class of nano-porous material with superior gas sensing performance. In this work, the amorphous porous structure of SiC_5/4O_3/2 is successfully reproduced by simulating the experimental etching process, and the gas sensing performance of porous SiCO at high temperature is investigated. The calculation results show porous SiC_5/4O_3/2 exhibits a much higher sensitivity towards H₂ than CO, NO₂ and acetone at 773 K. Compared with the other three gases, H₂ absorbed system show shorter adsorption distance and more obvious increasing in density of states around Fermi level. Therefore, porous SiC_5/4O_3/2 shows a highly selective sensitivity toward H₂ at high temperature. Moreover, our results show the Si–C/O units are the major sensing sites of H₂ at high temperature, and the large diffusion coefficient of H₂ in SiC_5/4O_3/2 is related to the fast response of porous SiCO gas sensor.     

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.     

Micro-contacted self-assembled tungsten oxide nanorods for hydrogen gas sensing

Electron beam lithography was used to fabricate platinum μ-contacts over tungsten oxide nanorods formed on a mica substrate. This made possible the measurement of sensorial response of these self-assembled tungsten oxide nanorods to hydrogen gas for the first time. The nanorods were prepared by thermal evaporation from an oxide source. Consequently, two types of conductometric sensors were assembled: a) percolating network of nanorods and b) set of individually contacted WO₃ nanorods. The preparation procedures are described in detail and the comparison of response of both types of assemblies is given. The first sensorial measurements revealed a good response of the b) type of sensor and the minimum repeatedly detected concentration of H₂ was 50 ppm.