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.     

Outstanding stability and photoelectrochemical catalytic performance of (Fe,Ni) co-doped Co3O4 photoelectrodes for solar hydrogen production

In this work, pure and (Fe, Ni) co-doped Co₃O₄ nanostructured photoelectrodes of different doping levels and thicknesses were manufactured at constant substrate temperature (450 °C) using the spray pyrolysis technique. In addition to the chemical compositions; the structural, optical, electrical, and photoelectrochemical (PEC) properties were investigated through the use of various analysis techniques. By increasing the codpoing ratio to 6%, the low energy band gap is decreased from 1.43 to 1.3 eV and the high energy bandgap is increased from 2.63 to 2.87 eV, in addition to the reduction in particle size from 30.2 to 12.0 nm. The high energy gap vanishes by increasing the codoped film's spread volume to 60 ml. X-ray photoelectron spectroscopy of 6%(Fe, Ni)-60ml Co₃O₄ confirms the existence of Ni^2+,3+ and Fe^2+,3+. Among the studied photoelectrodes, the 6%(Fe, Ni)-60ml Co₃O₄ photoelectrode displays a photocatalytic hydrogen output rate of 150 mmol/h.cm² @-1V in 0.3M Na₂SO₄ electrolyte. The photocurrent density of 6%(Fe, Ni)-60ml photoelectrode reached up to 13.6 mA/cm²@-1V with an IPCE (incident photon to current conversion efficiency) of ～42%@405 nm and STH (solar to hydrogen conversion efficiency) of ～11.37%, which are the highest values yet for Co₃O₄-based photocatalysts. The value of ABPE(applied bias photon-to-current efficiency) is 0.34%@(-0.28V and 636 nm). Interestingly, this photoelectrode shows a photogenerated current density of ?0.14 mAcm^?2 at 0 V and a PEC current onset over 0.266V. The thermodynamic parameters, corrosion parameters, PEC surface areas, Tafel slopes, and impedance spectroscopies are also being studied to confirm and classify the PEC H₂ production mechanism. The 6%(Fe, Ni)-60ml Co₃O₄ photoelectrode stability/reusability shows only a 6.6% reduction in PEC performance after ten successive runs at -1V with a corrosion rate of 1.2 nm/year. This work offered a new codoping strategy for the design of a highly active Co₃O₄ based photocatalyst for the generation of solar light-driven hydrogen.     

Photoelectrochemical performance of Cu-doped ZnIn2S4 electrodes created using chemical bath deposition

A solution growth method for the deposition of Cu-doped zinc-indium-sulfide (ZnIn₂S₄) semiconductor film electrodes is presented. The structural, optical, and photoelectrochemical (PEC) properties of samples were studied as a function of Cu content in samples. The X-ray diffraction pattern of the cubic ZnIn₂S₄ phase of an undoped sample was obtained. No Cu alloys or other binary compounds that included the Cu element were present in Cu-doped ZnIn₂S₄ samples. Images from a scanning electron microscope and atomic ratios of elements in samples obtained from the energy dispersion analysis of X-ray reveal a change in surface morphology and composition for Cu-doped ZnIn₂S₄ samples. The direct energy band gaps, indirect energy band gaps, and thicknesses of samples prepared in this study varied in the ranges 2.07-2.58 eV, 1.60-2.06 eV, and 521-879 nm, respectively. The maximum photoelectrochemical response of samples in 0.5 M K₂SO₄ aqueous solution reached 1.15 mA cm⁻² at an external potential of +1.0 V vs. an Ag/AgCl reference electrode under illumination using a 300-W Xe lamp with light intensity kept at 100 mW cm⁻². The experimental results show that Cu doping with Cu/(Cu+Zn) atomic ratio of 0.08 in samples improves the performance of the ZnIn₂S₄ photoabsorber for PEC applications.     

Fabrication of large area nanorod like structured CdS photoanode for solar H2 generation using spray pyrolysis technique

Large area nanorod like structured CdS films (9 × 9 cm²) were deposited on the FTO glass substrate using simple and economic spray pyrolysis deposition technique for photoelectrochemical (PEC) hydrogen production. With an intention of electrode scaling-up, the deposition area of photoanode was varied to evaluate its effect on the PEC hydrogen generation capability. High photocurrent of 5 mA has been achieved from the PEC active area of 37.5 cm². Its unit area (1 cm²) counterpart yielded Solar-to-Hydrogen (STH) conversion efficiency of 0.20% at a bias of 0.2 V vs Ag/AgCl using sacrificial reagents under solar simulator (AM1.5) with 80 mW/cm² irradiance. The 500 nm thick film exhibiting uniformly distributed nano-rod features yielded 3-times more photocurrent, as well as hydrogen evolution than other films. It exhibited an enhanced photo-activity as indicated by the higher IPCE values (5–9%) in the wavelength range of 450–550 nm. It exhibited superior optical properties (E_g ∼2.4 eV), and formation of high crystallinity hexagonal CdS phase with space group P63MC. The superior performance of the photoanode is attributed to the nanostructured morphology acquired under optimized spray pyrolysis conditions. Large area photoanodes showed unaltered photo-activity indicating the homogeneity in the film properties even in scaled-up version.     

Irradiation-induced modifications and PEC response - A case study of SrTiO3 thin films irradiated by 120 MeV Ag ions

This paper deals with a study on the effect of 120 MeV Ag⁹⁺ ion irradiation on photoelectrochemical properties of SrTiO₃ thin films deposited on Indium doped Tin Oxide (ITO) coated glass by sol-gel spin-coating technique. The structural evolution in the pristine and irradiated films was determined by X-ray diffraction and X-ray photoelectron spectroscopy. Surface morphology was studied by Atomic Force Microscopy (AFM) and optical measurements were done by UV-visible absorption spectroscopy. Irradiation of SrTiO₃ thin films was found to be effective in improving its photoelectrochemical properties. A noticeable decrease in the average grain diameter from 36 to 26 nm, reduction in bandgap from 3.55 to 3.43 eV and increase in roughness after irradiation contributed in enhancing photoelectrochemical activity of SrTiO₃ thin films. Thin films irradiated at fluence 3 × 10¹² ions cm⁻², when used in PEC cell exhibited enhanced photocurrent of 0.16 mA cm⁻² at zero bias conditions, which was four times higher than that of the unirradiated sample.     

Photocurrent enhancement of interlocked LB film dye layers in a p-CuSCN sensitised photoelectrochemical cell

Dye sensitised photoelectrochemical (PEC) cells based on Cu/p-CuSCN/LB films have been studied with mixed Langmuir Blodgett (LB) films as the dye layer. The effects of mixed layers were investigated in detail by observing the changes of optical absorption and photocurrent in a PEC cell configuration. Enhancements in both optical absorption and photocurrent were found when a mixture of octadecyl methylviolet–C₁₈ (M–C₁₈) and dioctadecyl rhodamine (C₁₈–R–C₁₈) were deposited using the LB technique on p-CuSCN wide band gap semiconductor. The maximum photocurrent quantum efficiency of the PEC cell reached ≈36% in KI (10⁻² M)+I₂ (10⁻⁴ M) electrolyte solution when mixed LB films were used as the dye layer. Photocurrent enhancement is believed to be the enhancement of light absorption of the dye layers due to the interlocking of M–C₁₈ between the double C₁₈ chains of rhodamine.     

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.     

Photocorrosion-less stable heterojunction photoanode for efficient visible-light driven solar hydrogen generation

In this work, a heterostructure CdS/TiO₂ nanotubes (TNT) photoelectrode is decorated with Ni nanoparticles (NPs) to enhance hydrogen generation via the photoelectrochemical method. Herein, we report a systematic study of the effect of Ni NPs heterostructure photoelectrode to improve light absorption and photoelectrochemical (PEC) performance. The fabricated photoelectrodes were evaluated for photoelectrochemical hydrogen generation under simulated sunlight. The optimized Ni/CdS/TNT photoelectrode exhibited an improved photocurrent density of 6.5 mA cm^?2 in poly-sulfide aqueous media at a low potential of 0 V. Owing to the enhanced photocurrent density, Ni NPs also played a significant role in improving the stability of the photoelectrode. The synergistic effect with semiconductor ternary junction incites the surface plasmon resonance (SPR) for light-harvesting to enhance photoelectrochemical hydrogen generation.     

Fabrication of n-Cu2O electrodes with higher energy conversion efficiency in a photoelectrochemical cell

We observed an n-type photoresponse in cuprous oxide films, which were prepared by a simple method of immersing copper plates in a HCl solution of 3 pH at 40°C temperature, when they were used in a PEC cell. This photoresponse was much higher than the previously published values for n-Cu₂O electrodes which were prepared by other methods. The photocurrent obtained was in the order of 0.3 mA/cm² when the cell was illuminated with light intensity of 50 mW/cm². (The semiconductor electrode was biased to get a zero dark current.) The power conversion efficiency of the cell was 0.01%. The maximum quantum efficiency obtained was 15%. It is hoped that these values could be improved with a better understanding of the photoelectrochemical properties of the cell.     

Enhanced photoelectrochemical performance of an α-Fe2O3 nanorods photoanode with embedded nanocavities formed by helium ions implantation

Hematite is a prospective semiconductor in photoelectrochemical (PEC) water oxidation field due to its suitable bandgap for the solar spectrum absorption. Nevertheless, the low transfer and separation efficiency of the charge carriers are restricted by its diffusion length of hole which is 2–4 nm and further reduce the PEC performance. Here, we report an innovative method, by introducing nanocavities into the α-Fe₂O₃ nanorod arrays photoanodes through helium ions implantation, to improve the charge carriers' transfer and separation efficiency and further to enhance water oxidation performance. The result indicates that, the photocurrent density of nanocavities embedded α-Fe₂O₃ photoanode (S2-A sample) reaches 1.270 mA/cm² at 1.6 V vs. RHE which is 1-fold higher than that of the pristine α-Fe₂O₃ (0.688 mA/cm²) and the photocurrent density of S2-A sample reaches 0.652 mA/cm² at 1.23 V vs. RHE. In this work, the ion implantation combined with post annealing method is found to be a valid method to improve the photoelectrochemical performance, and it also can be further used to modify the other semiconductor photoelectrodes materials.     

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 water splitting on highly smooth and ordered TiO2 nanotube arrays for hydrogen generation

To improve the photoelectrochemical (PEC) water splitting efficiency for hydrogen production, we reported the fabrication of lotus-root-shaped, highly smooth and ordered TiO₂ nanotube arrays (TiO₂ NTs) by a simple and effective two-step anodization method. The TiO₂ NTs prepared in the two-step anodization process (2-step TiO₂ NTs) showed better surface smoothness and tube orderliness than those of TiO₂ NTs prepared in one-step anodization process (1-step TiO₂ NTs). Under illumination of 100 mW/cm² (AM 1.5, simulated solar light) in 1 M KOH solution, water was oxidized on the 2-step TiO₂ NTs electrode with higher efficiency (incident-photon-to-current efficiency of 43.4% at 360 nm and photocurrent density of 0.90 mA/cm² at 1.23 V_RHE) than that on the 1-step TiO₂ NTs electrode. The effective photon-to-hydrogen conversion efficiency was found to be 0.18% and 0.49% for 1-step TiO₂ NTs and 2-step TiO₂ NTs, respectively. These results suggested that the structural smoothness and orderliness of TiO₂ NTs played an important role in improving the PEC water splitting application for hydrogen generation.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

Nanostructured Ti-doped hematite (α-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation

We present a form of hematite (α-Fe₂O₃) nanostructured architecture suitable for photoelectrochemical water oxidation that is easily synthesized by a pulsed laser deposition (PLD) method. The architecture is a column-like porous nanostructure consisting of nanoparticles 30–50 nm in size with open channels of pores between the columns. This nanostructured film is generated by controlling the kinetic energy of the ablated species during the pulsed laser deposition process. In a comparison with the nanostructured film, hematite thin film was also synthesized by PLD. All of the developed films were successfully doped with 1.0 at% of titanium. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy were used to characterize the films. To fabricate the photoelectrochemical (PEC) cell, Ti-doped hematite films were used as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and an aqueous solution of 1 M NaOH as the electrolyte. The photovoltaic characteristics of all cells were investigated under AM 1.5G sunlight illumination of 100 mW/cm². The photocurrent density was enhanced by approximately 220% using nanostructured film at 0.7 V versus Ag/AgCl compared to hematite thin film, and the highest photocurrent density of 2.1 mA/cm² at 0.7 V/Ag/AgCl was obtained from the 1.0 at% Ti-doped hematite nanostructured film. The enhanced photocurrent density is attributed to its effective charge collection due to its unique column-like architecture with a large surface area.     

Electrodeposited zirconium-doped α-Fe2O3 thin film for photoelectrochemical water splitting

Nanostructured hematite thin films were doped with zirconium successfully using electrodeposition method for their implementation as photoanode in photoelectrochemical (PEC) cell for hydrogen generation. XRD, Raman, XPS, SEM and UV-visible spectroscopy techniques were used to characterize the thin films. Highest photocurrent density of 2.1 mA/cm² at 0.6 V/SCE was observed for 2.0 at.% Zr⁴⁺ doped α-Fe₂O₃ sample with solar to hydrogen conversion efficiency of 1.43%. Flatband potential (−0.74 V/SCE) and donor density (2.6 × 10²¹ cm⁻³) were found to be maximum for the same sample. These results suggest substantial potential of hematite thin films with controlled doping of zirconium in PEC water splitting applications.     

A novel in situ synthesis of TiO2/CdS heterojunction for improving photoelectrochemical water splitting

We report a novel hydrothermal in situ synthesis route for the CdS-sensitized TiO₂ nanorod array films and their photoelectrochemical (PEC) performance are investigated in this paper. Although heterojunctions have been well recognized for enhancing the PEC performance of TiO₂ nanostructures, the specific synthesis methodology of high production quality, low preparation cost, and high controllability over the heterojunction structures is still a hot and open topic. In this work, controllable nanoscale TiO₂/CdS heterojunctions have been successfully realized through hydrothermal synthesis. The absorption spectrum is shown to be broadened from ~350 nm to ~570 nm, and the photocurrent density increases from 0.35 to 2.03 mA/cm², corresponding to photo-conversion efficiency of ~0.88%. The method is considered as an efficient and facile choice in the fabrication of TiO₂-based PEC hydrogen production functional materials.     

Enhanced photocurrent by MOFs layer on Ti-doped α-Fe2O3 for PEC water oxidation

Low photocurrent density of hematite (α-Fe₂O₃) originating from the inherent defects usually hinders its application in photoelectrochemical (PEC) water oxidation. In this paper, the synergetic effect of increase of oxygen vacancies and in-situ constructing heterojunction by coating MOFs on the α-Fe₂O₃ nanoarrays gives rise to the boosted photocurrent of α-Fe₂O₃ from 0.25 mA/cm² to 2.1 mA/cm² at 1.23 V (vs. RHE). The results showed that the appropriate energy band structure engineered by the presence of MOFs layer not only facilitated the PEC water oxidation, but also enhanced the light absorption performance. With inducing oxygen vacancies in further, the intrinsic conductivity of photoanode can be well ameliorated. The value of carrier density is improved one order higher to promote charge transfer between the interfaces and raise the carrier separation efficiency as a result.     

Photoelectrochemical water oxidation of LaTaON2 under visible-light irradiation

Lanthanum tantalum oxynitride (LaTaON₂) powders were prepared by one-step flux method. LaTaON₂ photoanodes, which are fabricated by using LaTaON₂ powders, are found to exhibit photoelectrochemical activity for overall water splitting. The photocurrent for LaTaON₂ photoelectrodes was ca. 120 μA cm⁻² at 1.5 V vs. reversible hydrogen electrode (RHE) in 1 M NaOH aqueous solutions (pH = 13.6) under AM 1.5 G simulated sunlight irradiation (100 mW cm⁻²). The photocurrent of LaTaON₂ photoelectrode from back-side illumination is much larger than that from front-side illumination, suggesting that the photoelectrochemical property is mainly limited by poor continuous electron transport in the bulk. Further efforts to ameliorate the electron transport in the bulk of LaTaON₂ photoelectrodes are expected to significantly improve their photoelectrochemical performance.     

Spray pyrolytically deposited nanoporous Ti doped hematite thin films for efficient photoelectrochemical splitting of water

Nanoporous hematite (α-Fe₂O₃) thin films doped with Ti⁴⁺ deposited by spray-pyrolysis were successfully used in photoelectrochemical splitting of water for solar hydrogen production. X-ray diffraction, field emission scanning electron microscopy, UV–visible absorption and photoelectrochemical studies have been performed on the undoped and Ti⁴⁺ doped hematite thin films. Morphology of α-Fe₂O₃ thin films was observed to be nanoporous, with increased porosity (pore size ∼12 to 20 nm) on increasing doping concentration. A significant decrease in the bandgap energy from 1.95 to 1.27 eV was found due to doping. α-Fe₂O₃ film doped with 0.02 M Ti⁴⁺ ions exhibited best solar to hydrogen conversion efficiency (photoconversion efficiency) of 1.38% at 0.5 V/SCE. Highest photocurrent densities of 0.34 mA/cm² at zero bias and 1.98 mA/cm² at 0.5 V/SCE were obtained by incorporating 0.02 M Ti⁴⁺ in α-Fe₂O₃, which are significantly larger than earlier reported values. Donor density (30.8 × 10²⁰ cm⁻³) and flatband potential (−1.01 V/SCE) obtained were also maximum for this sample. Hydrogen collected in 1 hr at Pt electrode with the best photoelectrode was 2.44 mL with 150 mW/cm² visible light source.     

Chemically grown Bi2S3 nanorod films for hydrogen evolution

Bi₂S₃ nanorod films were grown on ITO-coated glass substrates through chemical bath deposition (CBD) and annealing in a sulfur atmosphere. The as-deposited films were amorphous/nanocrystalline, with a particle size of 20 nm and a direct optical band gap of 1.87 eV. Upon annealing at 350 °C, the films exhibited a nanorod morphology with a length of 300 nm. Further increasing the temperature from 400 to 450 °C resulted in an increased diameter of nanorods. The direct optical band gap decreased from 1.68 to 1.47 eV upon increasing the annealing temperature from 350 to 400 °C. Photoelectrochemical (PEC) measurements showed that the nanorod films grown on ITO-coated glass substrates exhibited significantly increased PEC activity owing to their nanorod structures. The Bi₂S₃ nanorod films formed at 400 °C exhibited a maximum photocurrent density of 6.1 mA/cm² at 1 V, which was 2.5 times higher than that of the as-deposited films. The enhancement in the photocurrent density could be due to the effective visible-light absorption of Bi₂S₃ nanorods as a result of the increased crystallinity and decreased band gap. This study demonstrates the synthesis route involving a simple and inexpensive CBD method of Bi₂S₃ nanorod films for the optimized PEC water-splitting applications.