Preparation and photoelectrochemical study of BiVO4 thin films deposited by ultrasonic spray pyrolysis

Thin films of BiVO₄ with monoclinic structure were deposited onto indium-doped tin oxide (ITO)-coated glass substrates by ultrasonic spray pyrolysis. The effects of tungsten doping and hydrogen reducing were investigated. The films were characterized with XRD, Raman spectra, SEM, UV–Vis transmittance spectra. Furthermore, the films were investigated by electrochemical and photoelectrochemical measurements with regard to splitting water for solar energy conversion. The films possessed a scheelite-monoclinic structure with good absorption to visible light. The optical band gaps were evaluated to be about 2.65 eV. The flat band potentials were estimated to be about −0.61 V vs. saturated calomel electrode (SCE) in 0.5 M Na₂SO₄ solution from Mott–Schottky plots. For non-doping samples, the incident photon to current conversion efficiency (IPCE) was relative low because of low density and activity of carriers. When treated with hydrogen reducing, the carrier density increased due to more oxygen vacancies, resulting in the increase of IPCE. In addition, substituting 1% vanadium with equal mole tungsten can increase IPCE remarkably, which achieved about 10% at 0.3 V vs. SCE potential under 400–450 nm wavelength photo irradiation.     

Iron doped nanostructured TiO2 for photoelectrochemical generation of hydrogen

This paper describes the photoelectrochemical studies on nanostructured iron doped titanium dioxide (TiO₂) thin films prepared by sol-gel spin coating method. Thin films were characterized by X-ray diffraction, Raman spectroscopy, spectral absorbance, atomic force microscopy and photoelectrochemical (PEC) measurements. XRD study shows that the films were polycrystalline with the photoactive anatase phase of TiO₂. Doping of Fe in TiO₂ resulted in a shift of absorption edge towards the visible region of solar spectrum. The observed bandgap energy decreased from 3.3 to 2.89 eV on increasing the doping concentration upto 0.2 at.% Fe. 0.2 at.% Fe doped TiO₂ exhibited the highest photocurrent density, ∼0.92 mA/cm² at zero external bias. Flatband potential and donor density determined from the Mott–Schottky plots were found to vary with doping concentration from −0.54 to −0.92 V/SCE and 1.7 × 10¹⁹ to 4.3 × 10¹⁹ cm⁻³, respectively.     

Photoelectrochemical and structural characterization of carbon-doped WO3 films prepared via spray pyrolysis

Carbon-doped tungsten trioxide (WO₃) films were produced using a spray-pyrolysis methodology, with glucose used as the carbon dopant source. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis, scanning electron microscopy, and solid-state nuclear magnetic resonance. The photoelectrochemical activity was evaluated under near UV–visible light and visible light only irradiation conditions. The presence of carbonate-type species in the C-doped sample was confirmed by XPS and SSNMR. The C-doped WO₃ electrodes exhibited photocurrent densities up to 1.6 mA/cm² in 1 M HCl electrolyte and as high as 2.6 mA/cm² with the addition of methanol as a sacrificial agent. A high contribution (∼50%) of the photocurrent density was observed from visible light. C-doped WO₃ produced approximately 50% enhanced photocurrent densities compared with the undoped WO₃ electrode synthesized using the same procedures. The photoelectrochemical performance was optimized with respect to several synthetic parameters, including dopant concentration, calcination temperature and film thickness. These results indicate the potential for further development of WO₃ photocatalysts by simple wet chemical methods, and provide useful information towards understanding the structure and enhanced photoelectrochemical properties of these materials.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

Study on photocurrent of bilayers photoanodes using different combination of WO3 and Fe2O3

Bilayer photoanodes were prepared onto glass substrates (FTO) in order to improve generated photocurrents using UV-vis light by water splitting process. A comparative study of photocatalytic was performed over the films surface using Fe₂O_3, WO₃ and mixture of bicomponents (Fe₂O₃:WO₃). Different types of films were prepared using Fe₂O_3, WO₃ and bicomponents (mixture) on FTO substrates. The films were grown by sol gel method with the PEG-300 as the structure-directing agent. The photo-generated of the samples were determined by measuring the currents and voltages under illumination of UV-vis light. The morphology, structure and related composition distribution of the films have been characterized by SEM, XRD and EDX respectively. Photocurrent measurements indicated surface roughness as the effective parameter in this study. The deposited surfaces by bicomponents or mixture are flat without any feature on the surface while the deposited surfaces by WO₃ appears rough surface as small round (egg-shaped particles) and cauliflower-like. The surface deposited by Fe₂O₃ show rough no as well as WO₃ surface. The deposited surfaces by WO₃ reveal the higher value of photocurrent measurement due to surface roughness. Indeed, the roughness can be effective in increasing contact surface area between film and electrolyte and diffuse reflection (light scattering effect). The solution (Fe₂O₃:WO₃) shows the low photocurrent value in compare to WO₃ and Fe₂O₃ hat it may be due to decomposition the compound at 450 ± 1 °C to iron-tungstate Fe₂(WO₄)₃.     

Direct synthesis of nanostructured V2O5 films using solution plasma spray approach for lithium battery applications

We demonstrate for the first time, the synthesis of vanadium pentoxide (V₂O₅) nanoparticles and nanorods in the films using a high throughput solution plasma spray deposition approach. The scalable plasma spray method enables the direct deposition of large area nanostructured films of V₂O₅ with controllable particle size and morphology. In this approach, the solution precursors (vanadium oxychloride and ammonium metavanadate) were injected externally into the plasma jet, which atomizes and pyrolyzes the precursors in-flight, resulting in the desired films on the current collectors. The microstructure analysis of the as synthesized films revealed pure nanocrystalline phase for V₂O₅ with particles in the size range of 20-50 nm. The V₂O₅ film based electrodes showed stable reversible discharge capacity in the range of 200-250 mAh g⁻¹ when cycled in the voltage window 2-4 V. We further discuss the mechanism for controlling the particle growth and morphology, and also the optimization of reversible lithium storage capacity. The nanorods of V₂O₅ formed after the anneal treatment also show reversible storage capacity indicative of the potential use of such film based electrodes for energy storage.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

TiO2 thin films - Influence of annealing temperature on structural, optical and photocatalytic properties

Nanostructured TiO₂ thin films were deposited on glass substrates by sol-gel dip coating technique. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and UV-vis transmittance spectroscopy. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phase was initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 600 °C was about 20 nm. The lattice parameters for the films annealed at 600 °C were a = 3.7862 ? and c = 9.5172 ?, which is close to the reported values of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and was found to decrease with the annealing temperature. At 550 nm the refractive index of the films annealed at 600 °C was 2.11, which is low compared to a pore free anatase TiO₂. The room temperature electrical resistivity in the dark was of the order of 4.45 × 10⁶ ohm-cm. Photocatalytic activity of the TiO₂ films were studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 400 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Fabrication of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells

Cu₂ZnSnS₄ (CZTS) absorbers were grown by sulfurization of Cu/ZnSn/Cu precursors in sulfur atmosphere. The reaction mechanism of CZTS formation from the precursor was analyzed using XRD and Raman spectroscopy. The films with a single phase CZTS were formed at 560 and 580 °C by sulfurization for 30 min. The film grown at 560 °C showed bi-layer morphology with grooved large grains on the top and dense small grains near the bottom of the film. On the other hand, the film grown at 580 °C showed large grains with grooves that are extended from surface top to bottom of the film. The solar cell fabricated with the CZTS film grown at 560 °C showed the best conversion efficiency of 4.59% for 0.44 cm² with V_oc=0.545 V, J_sc=15.44 mA/cm², and FF=54.6. We found that further improvement of the microstructure of CZTS films can increase the efficiency of CZTS solar cells.     

Preparation of high performance pure single phase VO2 nanopowder by hydrothermally reducing the V2O5 gel

The most important potential application for VO₂ is “smart windows”. Producing polymer—VO₂ composite film is a practicable alternative for PVD film for windows retrofitting. Here the main work should be concentrated on preparing functional filler—VO₂. In this paper, high quality single crystal VO₂ nanopowder and the composite film coated with this powder had been prepared by a novel and facile route. A new vanadium precursor—V₂O₅·nH₂O sol was fabricated by the dissolution reaction of V₂O₅ in H₂O₂. Adopting this sol, VO₂ powder with an average size less than 20 nm and narrower size distribution was obtained by pre-reduction and hydrothermal treatment. The as-prepared powder showed excellent optical properties in the composite film. The films reached a sufficiently high luminous transmittance (45-60%) and keeping thermochromic effect in infrared area very well—an abrupt transmittance change greater than 50% with the phase transition was observed. Its optical properties are comparable with high quality single layer VO₂ film deposited by PVD method. The as-prepared nanopowder has a phase transition temperature (τ_c) about 55.5 °C and it can be tuned down to ambient by slight tungsten doping without deteriorating the thermochromic properties, which made this powder very suitable for practical application. Based on our experiments, the τ_c reduction efficiency of tungsten was about −21.96 K/at.%w in the doping range from 0 to 2.5 at.%w.     

Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Structural, morphological and electrochemical properties of nanocrystalline V2O5 thin films deposited by means of radiofrequency magnetron sputtering

Due to easiness of preparation and high energy density, V₂O₅ nanocrystalline thin films are particularly attractive as cathode materials for all-solid-state rechargeable lithium microbatteries. However, their electrochemical performances are strictly related to the film microstructure, which, in turn, is related to the nature and parameters of the deposition technique. For this reason, the preparation of thin films with reproducible electrochemical properties is still an open problem.Here, we report on the deposition of V₂O₅ crystalline thin films by means of reactive radiofrequency (r.f.) magnetron sputtering, using vanadium metal as the target. Different deposition times and substrate temperatures were adopted. X-ray powder diffraction (XRD) and atomic force microscopy were used to investigate the structural and morphological features of the films. In particular, XRD analysis revealed that the deposition parameters affect the crystallographic orientation of the films. A h 0 0 orientation is observed in case of thin samples (about 100 nm) prepared at 300 °C, whereas a 1 1 0 preferential growth is obtained for thicker films. Films deposited at 500 °C display a 0 0 1 orientation irrespective on the deposition time.Reversible Li intercalation/deintercalation processes and high specific capacity are observed for the h 0 0-oriented V₂O₅ thinner films, with the ab plane arranged perpendicular to the substrate. In this case, the cycling behaviour is very promising, and a stable capacity higher than 300 mAh g⁻¹ was delivered in the potential range 3.8-1.5 V at 1C rate over at least 70 cycles.     

Electrodeposition and pseudocapacitive properties of tungsten oxide/polyaniline composite

Composite films of tungsten oxide (WO₃) and polyaniline (PANI) have been electrodeposited by cyclic voltammetry in a mixed solution of aniline and precursor of tungsten oxide. Surface morphology and chemical composition of WO₃/PANI composite are characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of H₂O₂ on the electrodeposition of WO₃/PANI composite film is also investigated. Cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) results show that WO₃/PANI composite film exhibit good pseudocapacitive performance over a wide potential range of −0.5 to 0.7 V vs. SCE with the specific capacitance of 168 F g⁻¹ at current density of 1.28 mA cm⁻² and energy density of 33.6 Wh kg⁻¹, which is 91% higher than that of similarly prepared PANI (17.6 Wh kg⁻¹). An asymmetric model capacitor using WO₃/PANI as negative and PANI as positive electrodes over voltage range of 1.2 V displays a specific capacitance of 48.6 F g⁻¹ and energy density of 9.72 Wh kg⁻¹ at the power density of 53 W kg⁻¹, which is two times higher than that of a symmetric capacitor modeled by using two PANI films as both positive and negative electrodes.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

Characterization of electrolytic Co3O4 thin films as anodes for lithium-ion batteries

The electrolytic deposition of Co₃O₄ thin films on stainless steel was conducted in Co(NO₃)₂ aqueous solution for anodes in lithium-ion thin film batteries. Three major electrochemical reactions during the deposition were discussed. The coated specimens and the coating films carried out at −1.0 V (saturated KCl Ag/AgCl) were subjected to annealing treatments and further characterized by XRD, TGA/DTA, FE-SEM, Raman spectroscopy, cyclic voltammetry (CV) and discharge/charge cyclic tests. The as-coated film was β-Co(OH)₂, condensed into CoO and subsequently oxidized into nano-sized Co₃O₄ particles. The nano-sized Co₃O₄, CoO, Li₂O and Co particles revealed their own characteristics different from micro-sized ones, such as more interfacial effects on chemical bonding and crystallinity. The initial maximum capacity of Co₃O₄ coated specimen was 1930 mAh g⁻¹ which much more than its theoretical value 890 mAh g⁻¹, since the nano-sized particles offered more interfacial bondings for extra sites of Li⁺ insertion. However, a large ratio of them was trapped, resulting in a great part of irreversible capacity during the first charging. Still, it revealed a capacity 500 mAh g⁻¹ after 50 discharged-charged cycles.     

The temperature behavior of smart windows under direct solar radiation

The purpose of this paper was to investigate the variation in temperature of electrochromic devices under direct solar radiation and to compare the results with double-glazed glass. The devices consisted of a V₂O₅ layer as an ion storage film and a WO₃ layer as an electrochromic layer. The V₂O₅ and WO₃ films were prepared by thermal and electron beam evaporation, respectively. The optical properties and structures of these films were investigated. Both the ion storage film and the electrochromic layer were amorphous. The optical absorption was caused by a direct-forbidden transition in V₂O₅ and by an indirect-allowed transition in WO₃. The maximum temperatures under solar radiation were measured for colored and bleached devices, double glass and air, they were found to be approximately 63, 63, 53 and 36 °C, respectively. The rates of increasing temperature to the incident power density for colored, bleached devices and double glass were 0.051, 0.049 and 0.041 °C/(W/m²), respectively.     

Study of the Li insertion into V2O5 films deposited by CVD onto various substrates

Vanadium oxide films were synthesised by chemical vapour deposition (CVD) from pure of triisopropoxyvanadium oxide (VO(OC₃H₇)₃) and oxygen as precursors. The influence of the substrate on the crystallinity of the vanadium oxide films was studied before and after annealing at 500 °C. On mica substrates, as-deposited film was composed of crystalline V₂O₅ as revealed by XRD. On Pt, Ti, stainless steel, glass and F-doped SnO₂ substrates, an annealing procedure was required to get V₂O₅. SEM investigations have clearly evidence V₂O₅ plates but the kinetics growth seems to be strongly dependent on the nature of the substrate. The insertion/extraction of Li⁺ into the host structure was investigated in 1 M LiClO₄-PC with annealed V₂O₅ films deposited on Ti, Pt and stainless steel substrates. The best electrochemical performances were obtained in the potential range 3.8–2.8 V versus Li/Li⁺ with V₂O₅ films deposited onto stainless steel substrate: the reversible capacity reaches after subsequent cycles was about 115 mAh g⁻¹ (rate C/23). In a wider potential range (between 3.8 and 2.2 V versus Li/Li⁺), V₂O₅ deposited onto Ti substrate exhibited the higher electrochemical performances (220 mAh g⁻¹ for a rate of C/23).     

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⁺.