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 behaviour of Nb2O5 thin films with different morphologies obtained by spray pyrolysis

Two different procedures to stabilize the precursor NbCl₅ have been applied to obtain Nb₂O₅ thin films by spray pyrolysis. Depending on the procedure used, determined by the way in which the precursor solution was injected into the air stream of the spray nozzle, niobium oxide thin films with different surface morphologies can be obtained. The structural properties of the Nb₂O₅ thin films depend on the post-annealing temperature because as-deposited films are amorphous, independently of the synthesis procedure used. The electrochromic behaviour has been estimated for all films, where monochromatic colouration efficiency (at 660 nm) of 25.5 cm²/C and a cathodic charge density close to 24 mC/cm² were found to give the best results to date for niobium oxide thin films obtained by spray pyrolysis.     

Optical and photoelectrical properties of Bi2S3 thin films obtained by spray pyrolysis technique

Polycrystalline films of Bi₂S₃ compound have been prepared, at substrate temperature of 570 K, from Bismuth chloride (BiCl₃) and Thiourea (CS (NH₂)₂) solutions. The structural characterisation has been carried out by the X-ray diffraction analysis (XRD). Study of optical properties shows that Bi₂S₃ compound has an allowed indirect transition at 1.43 eV energy. The photoelectrical parameters (diffusion length and lifetime) have been determined from the study of the photoconductivity.     

Nanostructured SrTiO3 thin films sensitized by Cu2O for photoelectrochemical hydrogen generation

We prepared nanostructured thin films of pristine SrTiO₃, Cu₂O and SrTiO₃/Cu₂O heterojunction with varying the thickness of Cu₂O. SrTiO₃ and Cu₂O thin films were deposited on ITO (Sn:In₂O₃) glass substrate using sol–gel spin-coating technique and spray pyrolysis method respectively. Samples were characterized using XRD (X-ray diffractometry), SEM (Scanning electron microscopy), and UV–Visible absorption spectroscopy. Nanostructured thin films of pristine SrTiO₃, Cu₂O and SrTiO₃/Cu₂O heterojunction systems were used as photoelectrode in the Photoelectrochemical (PEC) cell for water splitting reaction. Maximum photocurrent density value of 2.44 mA cm⁻² at 0.95 V/SCE were observed for SrTiO₃/Cu₂O heterojunction photoelectrode with 454 nm thickness, which was approximately 34 times higher than pristine SrTiO₃ thin film. Increased photocurrent density observed for the heterojunction can be attributed to the improved conductivity and better separation of the photogenerated charge carriers at the SrTiO₃/Cu₂O interface.     

Photoresponse of n-TiO2 thin film and nanowire electrodes

Thin film and nanowire electrodes of n-type titanium oxide (n-TiO₂) were fabricated and their photoresponses towards water-splitting reaction were studied. A more than twofold increase in maximum photoconversion efficiency was observed when a single-layer thin film of n-TiO₂ was replaced by nanowires. Highest photoconversion efficiency was observed at an applied potential of 0.61 V vs. E_aoc where the electrode potential at open circuit, E_aoc was found to be −1.0 V SCE⁻¹ at illumination intensity of 40 mW cm⁻² in 5.0 M KOH solution. The maximum photoconversion efficiency was approximately of another twofold increase for water splitting at nanowire electrodes when methanol was used as a sacrifacial hole scavenger (depolarizing agent) in the electrolyte. Also, this maximum photoconversion efficiency was found at a lower applied potential of 0.41 V vs. E_aoc in presence of methanol. The band-gap energy of the n-TiO₂ films and nanowires annealed at 750°C was found to be 2.98 eV, which indicates their rutile structure.     

Preparation and photoelectrochemical properties of functional carbon nanotubes and Ti co-doped Fe2O3 thin films

Functional carbon nanotubes (CNTs) were incorporated into Ti-doped Fe₂O₃ thin films by a facile, one-step co-electrodeposition method. The films were characterized by X-ray diffraction, scanning electron microscopy, UV–visible absorption, and X-ray photoelectron spectroscopy. The introduction of CNTs results in a better absorption in visible region and greatly enhances the photoelectrochemical properties of the Ti–Fe₂O₃ films. The improved photoelectrochemical properties of the CNTs and Ti co-doped Fe₂O₃ films are due to the charge equilibration which interplays between the Ti–Fe₂O₃ and CNTs. The effect of CNTs to mediate fast charge transfer and to retard charge recombination rate in the composites is also demonstrated by kinetics analysis and electrochemical impedance spectroscopy. The influence of different groups-modified CNTs and different content of CNTs was also studied. The highest photocurrent is 4.5 mA/cm² at 1.23 V (vs. RHE) obtained by incorporating 0.10 mg/mL amino-group modified CNTs in the Ti–Fe₂O₃ film. The amino-functionalized CNTs doped film exhibits the highest photoelectric response compared with those doped by the pristine and acid-treated CNTs under the same conditions, which can be ascribed to the better hydrophilicity and dispersibility of the amino-functionalized CNTs.     

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.     

Structural and optical properties of In4Se3 thin films obtained by flash evaporation

Structural identification shows that the important thin films of composition In₄Se₃ can be obtained by the flash evaporation technique from a critical ratio of metal/calgonite of 1.247 in the material from which the films are prepared. At a substrate temperature of 450 K, the thin films are polycrystalline. The structural characterization by X-ray diffraction shows that the films are orthorhombic with a texture in which the b-axis is parallel to the substrate. The IR studies of the films confirm the results obtained by the X-ray diffraction. The spectral absorption study in the range of 0.8–1.8 eV indicates a direct optical gap at 1.24 eV. Calculations show that the allowed indirect transitions are not negligible and are contributed to the absorption spectra, the indirect gap is found to be at 1.17 eV.     

Determination of optical properties of amorphous Ta2O5 films deposited by spin- and dip-coating methods

The optical properties of amorphous Ta₂O₅ films prepared by the sol-gel dip- and spin-coating deposition technique and dried at 60°C have been investigated. Refractive index, extinction coefficient and optical energy gap have been calculated from optical transmission measurements using the Swanepoel method. The films of similar thickness deposited by the two methods were compared. It is shown that the optical properties are dependent on the deposition methods. The energy band gap of the Ta₂O₅ films is 3.75 ± 0.12 eV and is independent of the coating methods.     

CuInTe2 and In-rich telluride chalcopyrites thin films obtained by electrodeposition techniques

In order to obtain single-phase thin films of the system Cu–In–Te with optoelectronic properties adequate for solar cells, electrodeposition techniques were used on substrates of molybdenum supported by glass. Different annealings in Te atmosphere have been done that affect the Te concentration and In/Cu atomic ratio. Single chalcopyrite phase appears in two ranges of composition where the In/Cu atomic ratio varies between 0.21–0.76 and 0.90–3.46, respectively. Morphology, cell lattice parameters and electrical resistance for single-phase samples depend strongly on the composition in the annealed samples. The cell parameters ranges are a=6.141–6.183 Å and c=12.201–12.375 Å.     

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.     

TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact

In traditional solar cells, metal-semiconductor contacts used to extract photogenerated carriers are very important. In dye-sensitized solar cells (DSSC) not much attention has been given to contact between the TiO₂ and the transparent conducting glass (TCO), which is used instead of a metal contact to extract electrons. TiO₂ layers obtained by microwave-activated chemical-bath deposition (MW-CBD) are proposed to improve TiO₂ contact to conducting glass. Spectra of incident photon to current conversion efficiency (IPCE) are obtained for two-photoelectrode TiO₂ photoelectrochemical cells. IPCE spectra show higher values when TiO₂ double layer photoelectrodes are used. In these, the first layer or contacting layer is made by MW-CBD. Best results are obtained for double layer photoelectrodes on FTO (SnO₂:F) as conducting oxide substrate. Modeling of IPCE spectra reveals the importance of electrical contact and electron extraction rate at the TiO₂/TCO interface.     

Electrical properties of CuInSe2 films prepared by evaporation of Cu2Se and In2Se3 compounds

We prepared CuInSe₂ films by evaporating In₂Se₃ and Cu₂Se compounds instead of elemental sources. The resulting CuInSe₂ film grown at 680°C had a smooth and dense microstructure with the grain size of 2 3 μm. But the CuInSe₂ films were Cu-rich, with a low resistivity of about 0.1 Ω cm. So we conducted H₂ post annealing to control the electrical resistivity and composition of CuInSe₂ films. In a H₂ atmosphere, the resistivity increased to about 100 Ω cm by annealing at 350°C for 1 h. The resistivity decreased again when the annealing temperature was above 350°C. This resistivity change might be related to the contents of Cu, In, Se atoms and the valency states of Cu and In ions in the films. We discussed the reason of resistivity change caused by H₂ post annealing in this paper.     

Efficient indium tin oxide/Cr-doped-TiO2 multilayer thin films for H2 production by photocatalytic water-splitting

Cr-doped-TiO₂ thin films, with three different Cr concentrations (2, 5.5, and 9 at.%), have been synthesized by radio-frequency magnetron sputtering in order to sensitize TiO₂ in visible light. UV–visible spectra showed that maximum narrowing (2.1 eV) of the TiO₂ band gap is obtained for the highest Cr concentration. However, negligible photocurrent was measured with Indium Tin Oxide (ITO)/Cr-doped-TiO₂ (9 at.%) single bilayer sample due to the increased recombination rate of the photo-generated charges on the defects associated to the Cr³⁺ ions. To lower the charge recombination rate in the Cr-doped-TiO₂, multilayer films with different numbers of ITO/Cr-doped-TiO₂ (9 at.%) bilayers (namely, 3-, 4-, 5-, 6- and 7-bilayers) were deposited by keeping the total thickness of TiO₂ constant in each multilayer film. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Cr-doped-TiO₂. The enhanced photocurrent is attributed to: 1) higher absorption of visible light by Cr-doped-TiO₂, 2) number of space charge layers in form of ITO/TiO₂ interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space charge layer by decreasing the Cr-doped-TiO₂ layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by studying the photocurrent kinetic curve. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water-splitting: we obtained indeed 24.4 μmol/h of H₂ production rate, a value about two times higher than that of pure TiO₂ (12.5 μmol/h).     

Sol-gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol-gel dip-coating technique can be used to fabricate a SiO₂/TiO₂ bilayer film with self-cleaning and antireflection properties. The bottom SiO₂ layer acts as an antireflection coating due to its lower refractive index; the top TiO₂ layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO₂/TiO₂ bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO₂ nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO₂ nanoparticles. After illuminated by ultraviolet light, the SiO₂/TiO₂ bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.     

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.     

Electrical conduction properties of ZnPc/TiO2 thin films

In this paper we have reported the electrical properties of Al/ZnPc–TiO₂/Al thin film device. The composite layers were prepared by the vacuum evaporation of ZnPc and sol–gel-coated TiO₂. The current–voltage characteristics are found to be Ohmic and non-Ohmic at low field and high field, respectively. The charge transport conduction appears to be Poole-Frankel type of conduction. The total number of interface states, which is responsible for the dispersion of capacitance, is found to be increasing with increase in bias voltage.     

Investigation of photoelectrochemical characteristics of n-type Cu2O films

n-Cu₂O photoelectrodes are obtained by immersing a copper plate in a CuSO₄(10⁻³ M) and HCl (10⁻³ M) solutions. Samples are characterized with XRD and SEM measurements. It is found that Cu²⁺ ions and the copper substrate are essential to obtain n-type photoresponses from Cu₂O. Photocurrent action spectra are investigated with various Cu₂O amounts formed on the copper substrate, made from two different methods. Photoluminescence (PL) measurements of the samples prepared by the two methods are examined. Time development of the photocurrent is investigated in KI(10⁻² M)+I₂ (10⁻⁴ M) and trisodium citrate solutions.     

Enhancement of photoelectrochemical response by Au modified in TiO2 nanorods

We report on the design and synthesis of a novel Au/TiO₂/Au heterostructure and its implementation as a photoanode for photoelectrochemical (PEC) application. The Au/TiO₂/Au heterostructure was produced by assembling Au nanoparticles and TiO₂ nanorods (NRs) onto FTO substrate, followed by electrodepositing Au nanoparticles on the TiO₂NRs. Field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize the prepared photoanodes. Compared to the system involving Au nanoparticles directly linked to TiO₂, this Au/TiO₂/Au heterostructure exhibits significant improved photoresponse as a photoanode, as demonstrated good performance in PECs. This study illustrates the importance of pre-deposited Au underlayers in influencing PEC properties of hybrid assembled nanostructures. As the Au/TiO₂NRs/Au photoanodes are easily fabricated and highly stable, Au/TiO₂NRs/Au can serve as a good substitution for TiO₂ in a variety of solar energy driven applications including PEC water splitting, photocatalysis, and solar cells.