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.     

Nanostructured hematite for photoelectrochemical generation of hydrogen

With the advent of nanotechnology there has been a resurge of interest in α$\alpha$-Fe₂O₃, as suitable candidate for photoelectrochemical (PEC) splitting of water to generate hydrogen. This paper describes the enhanced PEC behaviour of nanostructured α$\alpha$-Fe₂O₃ thin films modified by various techniques. Nanostructured thin film/pellets of α$\alpha$-Fe₂O₃ prepared by various techniques and various dopants were investigated for their photoelectrochemical response. Thin films prepared by spray pyrolysis having particle size of 20–30 nm exhibited better photoresponse as compared to the films prepared by sol–gel methods, which further improved on doping with Zn. These films were further modified by (i) depositing Zn dots on the surface of α$\alpha$-Fe₂O₃ films using thermal evaporation method and (ii) irradiating it with 170 MeV Au¹³⁺${\mathrm{Au}}^{13+}$ ions. When used as electrode in photoelectrochemical cell, a significant increase in the photoresponse of these modified films were observed, details of which are discussed.     

Unique photoelectrochemical behavior of TiO2 nanorods wrapped with novel titanium Oxy-Nitride (TiOxNy) nanoparticles

In this work, we developed novel titanium oxynitride (TiO_xN_y) nanoparticles with diameter of 25 ± 2 nm and crystalline size of ～15 nm on hydrothermally grown one-dimensional (1D) TiO₂ nanorod (TNR) arrays. Herein, the TiO_xN_y nanoparticles were synthesized by facile nitridation using TiO₂ powder at 100% NH₃ gas atmosphere. Titanium oxynitride composed of potentially energetic metal-nitrogen bonds (TiN), compared to the weaker TiO bond, becomes chemically stable in the alkaline environment, and is considered as a suitable material for photoelectrochemical (PEC) system. The PEC performance of TiO_xN_y decorated TNR (abbreviated as TiO_xN_y @TNR) films was evaluated in 0.1 M KOH solution under solar illumination condition, and achieved the potentially high photocurrent density (J) of 2.1 mA/cm² at 1.23 V versus reversible hydrogen electrode (RHE) (abbreviated as V_RHE) in the TiO_xN_y@TNR arrays, in comparison with the poor photoresponse (0.7 mA/cm² at 1.23 V_RHE) of the pristine TNR arrays. A nearly three-fold enhancement was attained in the TiO_xN_y decorated TNR arrays, attributed to the high visible light absorption and fast carrier separation, due to the hybridization with the visible active TiO_xN_y nanoparticles in the cascading band alignment between the TiO_xN_y and TNR materials. Furthermore, the introduction of TiO_xN_y layer on the TNR surface quite reduces the interfacial resistance in the solid-liquid interface region, and further, the TiO_xN_y layer contributes to the passivation of the surface states (e.g., defect, trap sites etc.) where the charge recombination reaction frequently happens, leading to the improvement of PEC performance.     

Preparation and photocatalytic activities of two new Zn-doped SrTiO3 and BaTiO3 photocatalysts for hydrogen production from water without cocatalysts loading

Sr_2/3Zn_1/3TiO₃ (1) and Ba_5/6Zn_1/6TiO₃ (2) with perovskite structure have been prepared by a facile sol–gel method, among which the A sites (Sr and Ba for 1 and 2, respectively) were partially replaced by zinc ions. The photocatalysts were characterized by powder X-ray diffraction, UV–vis diffuse reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results of photocatalysis experiment showed that both 1 and 2 exhibited good activity for water reduction to form H₂ without cocatalysts loading under ultraviolet light irradiation. Compared with pure SrTiO₃ and BaTiO₃, 1 and 2 showed remarkable improvement in H₂ production efficiency, respectively. And the photocatalytic mechanisms of them were investigated by the theoretical method. This study demonstrated that the substitutional doping of appropriate ions could change the crystal and band structures and significantly promote electron transport in catalysts, which result in their good photocatalytic activities.     

Chemical bath deposition synthesis of TiO2/Cu2O core/shell nanowire arrays with enhanced photoelectrochemical water splitting for H2 evolution and photostability

In this study, we have developed a facile chemical bath deposition (CBD) method to grow p-type Cu₂O nanoparticles on n-type TiO₂ nanowire arrays (TiO₂ NWAs) to fabricate TiO₂/Cu₂O core/shell heterojunction nanowire arrays (TiO₂/Cu₂O core/shell NWAs). When used as photoelectrode, the fabricated TiO₂/Cu₂O core/shell NWAs show improved photoelectrochemical (PEC) water splitting activity to pure TiO₂ NWAs. The effects of the CBD cycle times on the PEC activities have been studied. The TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode prepared by cycling 5 times in the CBD process achieves the highest photocurrent of 2.5 mA cm^?2, which is 2.5 times higher than that of pure TiO₂ NWAs. In addition, the H₂ generation rate of this photoelectrode reaches to 32 μmol h^?1 cm^?2, 1.7 times higher than that of pure TiO₂ NWAs. Furthermore, the TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode shows excellent photostability and achieves a stable photocurrent of over 2.3 mA cm^?2 during long light illumination time of 5 h. The enhanced photocatalytic activity of TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode is attributed to the synergistic actions of TiO₂ and Cu₂O for improving visible light harvesting, and efficient transfer and separation of photogenerated electrons and holes.     

Electrophoretic behavior of solvothermal synthesized anion replaced Cu2ZnSn(SxSe1-x)4 films for photoelectrochemical water splitting

Quaternary Cu₂ZnSn(S_xSe_1-x)₄ (CZT(S_xSe_1-x)₄) compounds have drawn a great deal of attention for being used in the fabrication of optoelectronic devices such as solar cells, photocatalysts, and photoelectrochemical water splitting. However, one major challenge facing the utilization of this material is to reduce the production cost of synthesis and fabrication of high quality CZT(S_xSe_1-x)₄ films. In the present study, a facile and beneficial solvothermal route has been reported for synthesis of CZT(S_xSe_1-x)₄ compounds. The process of electrophoretic deposition (EPD) of synthesized CZT(S_xSe_1-x)₄ nanoparticles, is systematically compared with each other in order to obtain high quality films with appropriate porosity. The XRD patterns, EDS and Raman spectra confirm the formation of CZT(S_xSe_1-x)₄ phases with no trace of impurities and appreciable crystallinity and also with near stoichiometry composition in all the samples. The obtained particle size for CZTS, CZTSSe and CZTSe samples was in the range of 50–100 nm and also for some agglomerate particles was in the range of 500 nm to 2 μm. Based on the obtained results for thin films prepared using EPD in the present study, the best EPD parameters for each CZTS, CZTSe and CZTSSe samples with 120 V and 5 min as applied voltage and deposition time were reported as the best samples. The obtained photocurrent-potential and current-time curves of CZT(S_xSe_1-x)₄ thin film samples demonstrate that the photocurrents of each CZTS, CZTSe, CZTSSe thin films, are different in the range of ?2.1 to ?6 mA/cm² and also the CZTS and CZTSe samples show a detectable current under the exposure of sunlight that can have an appropriate stability for 3000 s but the CZTSSe sample showed a stable photocurrent just for 2000 s. According to the mentioned results in this study, the CZTS and CZTSe samples can potentially be suitable candidates for further applications.     

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.     

Heterostructured thin LaFeO3/g-C3N4 films for efficient photoelectrochemical hydrogen evolution

The deposition of LaFeO₃ at the surface of a graphitic carbon nitride (g-C₃N₄) film via magnetron sputtering followed by oxidation for photoelectrochemical (PEC) water splitting is reported. The LaFeO₃/g-C₃N₄ film was investigated by various characterization techniques including SEM, XRD, Raman spectroscopy, XPS and photo-electrochemical measurements. Our results show that the hydrogen production rate of a g-C₃N₄ film covered by a LaFeO₃ film, exhibiting both a thickness of ca. 50 nm, is of 10.8 μmol h⁻¹ cm⁻² under visible light irradiation. This value is ca. 70% higher than that measured for pure LaFeO₃ and g-C₃N₄ films and confirms the effective separation of electron-hole pairs at the interface of LaFeO₃/g-C₃N₄ films. Moreover, the LaFeO₃/g-C₃N₄ films were demonstrated to be stable and retained a high activity (ca. 70%) after the third reuse.     

Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting

TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO₂ film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H₂ generation rate is obtained for long period of time by both the TiO₂ films because of the separated evolution of H₂ and O₂ gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H₂ generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO₂ film by the energetic target ions during deposition in pure Ar gas pressure.     

Nanostructured Ni:BiVO4 photoanode in photoelectrochemical water splitting for hydrogen generation

Photoelectrochemical water splitting using bismuth vanadate (BiVO₄) is drawing attention but on account of presence of high charge recombination and poor water oxidation kinetics its performance is restricted. Present study attempts to understand the role of dopant Ni on BiVO₄ in a) reducing the charge recombination and b) to improve water oxidation kinetics. Ni doped BiVO₄ thin films are prepared via electrodeposition method and photoelectrochemical properties are investigated in 0.1 M phosphate buffer solution with and without sodium sulfite hole scavenger. Photocurrent density of 1.36 mA/cm² at 1.23 V vs. RHE has been obtained using 1.5% Ni doped BiVO₄. This sample also offered lower flat band potential, high open circuit potential and applied bias photon-to-current conversion efficiency. Addition of hole scavenger significantly increases the photoelectrochemical performance. Ni as a dopant therefore can play an important role in not only suppressing the electron-hole pair recombination but also in offering significantly enhanced photoelectrochemical response.     

Hydrogen production over metal-loaded mesoporous-assembled SrTiO3 nanocrystal photocatalysts: Effects of metal type and loading

Mesoporous-assembled SrTiO₃ photocatalysts with different loaded metal co-catalysts (Au,Pt, Ag, Ni, Ce, and Fe) synthesized by the single-step sol–gel method with the aid of a structure-directing surfactant were tested for the photocatalytic activity of hydrogen production from a methanol aqueous solution under both UV and visible light irradiation. The Au, Pt, Ag, and Ni loadings had a positive effect on the photocatalytic activity enhancement, whereas the Ce and Fe loadings did not. The best loaded metal was found to be Au due to its electrochemical properties compatible with the SrTiO₃-based photocatalyst and its visible light harvesting enhancement. A 1 wt.% Au-loaded SrTiO₃ photocatalyst exhibited the highest photocatalytic hydrogen production activity with a hydrogen production rate of 337 and 200 μmol h⁻¹ g_cat⁻¹ under UV and visible light irradiation, respectively. The hydrogen diffusivity from the liquid phase to the gas phase also significantly affected the photocatalytic hydrogen production efficiency. An increase in the hydrogen diffusability led to an increase in the photocatalytic hydrogen production efficiency.     

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.     

Electrodeposited p-type Cu2O as photocatalyst for H2 evolution from water reduction in the presence of WO3

Different p-type Cu₂O powders were prepared from electrodeposition and subjected to analysis of their photocatalytic activity in water reduction. The electrodeposited Cu₂O powders were obtained by scraping the deposited films off the substrate. Under illumination the Cu₂O powders alone were not able to catalyze H₂ generation from water reduction. However, these Cu₂O powders exhibited photocatalytic activity in H₂ generation when they were coupled with n-type WO₃ in suspensions. The coupling was made to avoid back reactions of the photo-induced charges. The electrodeposited Cu₂O powders showed higher photocatalytic activity than a commercially available Cu₂O powder. The suspension containing electrodeposited Cu₂O with a strong [1 1 1] orientation gave a larger amount of H₂ evolution than that containing Cu₂O with a [1 1 0] orientation. Appropriate crystalline-texture tuning, as well as charge delocalization promotion, is looked to as the key issue for efficient H₂ generation from water reduction over p-type Cu₂O photocatalysts.     

TiO2-rutile/anatase homojunction with enhanced charge separation for photoelectrochemical water splitting

The mismatched interfaces of heterojunction usually have lots of defects, deriving in recombination of generated electron-hole pairs. On the other hand, homojunction interfaces are considered to be beneficial to the separation of charge carriers due to the similar characteristics in two sides of homojunction. TiO₂ have rutile and anatase two typical photoactive phases in nature. In this work, TiO₂-rutile/anatase (TiO₂-R/A) homojunction photoanode is fabricated by in situ growth of anatase TiO₂ on TiO₂-R surface. By contrast with TiO₂-rutile/rutile (TiO₂-R/R) photoanode, TiO₂-R/A displays higher photocurrent density (1.70 mA cm^?2 at 0.6 V vs. SCE). Deep insight into the mechanism suggests that TiO₂-R/A homojunction has intense band bending and enhanced surface area, which facilitate the charge separation and transmission. This study offers some novel insights to design and fabricate semiconductors photoanodes for highly efficient photocatalytic reactions.     

Structural, optical and luminescent characteristics of sprayed fluorine-doped In2O3 thin films for solar cells

Fluorine-doped indium oxide thin films, F-In₂O₃, prepared by the spray pyrolysis technique on glass substrates have been studied using cathodoluminescence spectroscopy, X-ray diffraction and spectrophotometry. These films, deposited at the optimal substrate temperature (T_s=450 °C), crystallize in a cubic structure with a preferential orientation along the (4 0 0) direction. For this temperature, the electrical resistivity is in the order of 6×10⁻³ Ω cm and the average optical transmission in the visible region is larger than 95%. At room temperature, the cathodoluminescence spectra of F-In₂O₃ present two emission peaks: blue indirect band gap peak at 410 nm and a red emission at 650 nm.     

Absorption coefficient of bulk and thin film Cu2O

The optical absorption coefficient of thin film and bulk Cu₂O at room temperature is obtained from an accurate analysis of their transmittance and reflectance spectra. These absorption spectra are modeled, together with the low temperature data reported in the literature, using an analytical expression to assess and quantify the role of the different absorption mechanisms. The results suggest that direct forbidden transitions and indirect transitions play an almost equally relevant role. A table of the optical constants of Cu₂O single crystal is given for reference.     

A novel photoelectrochemical cell with self-organized TiO2 nanotubes as photoanodes for hydrogen generation

A photoelectrochemical (PEC) cell with an innovative design for hydrogen generation via photoelectrocatalytic water splitting is proposed and investigated. It consisted of a TiO₂ nanotube photoanode, a Pt/C cathode and a commercial asbestos diaphragm. The PEC could generate hydrogen under ultraviolet (UV) light-excitation with applied bias in KOH solution. The Ti mesh was used as the substrate to synthesize the self-organized TiO₂ nanotubular array layers. The effect of the morphology of the nanotubular array layers on the photovoltaic performances was investigated. When TiO₂ photocatalyst was irradiated with UV-excitation, it prompted the water splitting under applied bias (0.6 V vs. Normal Hydrogen Electrode, NHE.). Photocurrent generation of 0.58 mA/cm² under UV-light irradiation showed good performance on hydrogen production.     

Role of precursor solution in controlling the opto-electronic properties of spray pyrolysed Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄, a potential candidate for application as absorber layer in thin film solar cells, were successfully deposited on soda lime glass substrates using spray pyrolysis and the effect of variation of precursor on the structural and opto-electronic properties was investigated. We used stannous as well as stannic chloride as precursors of tin in the spray solution. All the films exhibited kesterite structure with preferential orientation along the (1 1 2) direction. But crystallinity and grain size were better for stannic chloride based films. Also they possessed a direct band gap of 1.5 eV and the absorption coefficient was >10⁴ cm⁻¹. Carrier concentration and mobility could be enhanced and the resistivity reduced by two orders by using stannic chloride in spray solution. Junction trials were performed with CZTS films prepared using stannic chloride precursor as the absorber layer and indium sulfide as the buffer layer. XPS depth profiling of the junction was done. Formation of CZTS could be confirmed and also information about the junction interface could be obtained from the XPS results. We obtained an open-circuit voltage of 380 mV and short-circuit current density of 2.4 mA/cm².     

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.     

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.