Nano-wall like visible-light active carbon modified n-TiO2 thin films for efficient photoelectrochemical oxygen separation from water

Efficient photoelectrochemical oxygen separation from water was demonstrated using a nano-wall like carbon modified n-type titanium oxide (CM-n-TiO₂) electrode during water splitting reaction. The CM-n-TiO₂ electrode was synthesized by flame-oxidation of Ti metal sample. The combustion products of natural gas flame acted as the carbon source. The oxygen separation rate during water splitting was evaluated in terms of anodic photocurrent density, J_p, under solar simulated light illumination of 1 sun. Upon incorporation of carbon within the titanium oxide, the photocurrent density was enhanced to 4.97 mA cm⁻² at CM-n-TiO₂ electrode compared to 0.66 mA cm⁻² at regular of n-TiO₂ both at the same measured potential of - 0.6 V/SCE. Such a multiple-fold increase in photocurrent density at CM-n-TiO₂ thin film electrode was attributed to its enhanced absorption in the UV region, red-shift to visible region due to carbon incorporation and as well as due to pronounced nano-wall like surface morphology generated under the harsh conditions of flame oxidation. CM-n-TiO₂ photoelectrodes were characterized in terms of photocurrent measurements under white light and as well as under monochromatic light illuminations, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), the valence band X-ray photoelectron spectroscopy (XPS) and the AC-impedance measurements.     

Iron oxide (n-Fe2O3) nanowire films and carbon modified (CM)-n-Fe2O3 thin films for hydrogen production by photosplitting of water

Iron oxide n-Fe₂O₃ nanowire photoelectrodes were synthesized by thermal oxidation of Fe metal sheet (Alfa Co. 0.25 mm thick) in an electric oven then tested for their photoactivity. The photoresponse of the n-Fe₂O₃ nanowires was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen, which is proportional to photocurrent density, J_p. The optimized electric oven-made n-Fe₂O₃ nanowire photoelectrodes showed photocurrent densities of 1.46 mA cm⁻² at measured potential of 0.1 V/SCE at illumination intensity of 100 mW cm⁻² from a Solar simulator with a global AM 1.5 filter. For the optimized carbon modified (CM)-n-TiO₂ synthesized by thermal flame oxidation the photocurrent density for water splitting was found to increase by two fold to 3.0 mA cm⁻² measured at the same measured potential and the illumination intensity. The carbon modified (CM)-n-Fe₂O₃ electrode showed a shift of the open circuit potential by −100 mV/SCE compared to undoped n-Fe₂O₃ nanowires. A maximum photoconversion efficiency of 2.3% at applied potential of 0.5 V/E_aoc was found for CM-n-Fe₂O₃ compared to 1.69% for n-Fe₂O₃ nanowires at higher applied potential of 0.7 V/E_aoc. These CM-n- Fe₂O₃ and n- Fe₂O₃ nanowires thin films were characterized using photocurrent density measurements under monochromatic light illumination, UV-Vis spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Copper oxide nanoparticle made by flame spray pyrolysis for photoelectrochemical water splitting - Part II. Photoelectrochemical study

A scalable method for hydrogen generation by splitting water via a photoelectrochemical cell was studied. Flame spray pyrolysis and spin coating processing methods were used for preparing copper oxide nanoparticles and copper oxide photocathodes. Copper oxide p-type semiconductor nanoparticles made by flame spray pyrolysis were spin coated on conducting ITO substrates and served as photocathodes for photoelectrochemical splitting of water. The film thickness was controlled by the concentration of the CuO suspension solution and numbers of layer deposited on the substrate. As sintering temperature increased to 600 °C, crystalline diameter increased from 28 nm (before sintering) to 110 nm and the bandgaps decreased from 1.68 eV to 1.44 eV. A 387 nm thickness CuO film with bandgap 1.44 eV was demonstrated to have 1.48% total conversion efficiency and 0.91% photon-to-hydrogen generation efficiency. The net photocurrent density (photocurrent - dark current) was measured to be 1.20 mA/cm² at applied voltage of −0.55 V vs. Ag/AgCl in 1 M KOH electrolyte with 1 sun (AM1.5G) illumination. Based on the Mott-Schottky plot, the carrier density was estimated to be 1.5 × 10²¹ cm⁻³ and the flatband potential to be 0.23 V vs. Ag/AgCl. Furthermore, the valence band edge and conduction band levels were found to lie at −5.00 eV and −3.56 eV respect to the vacuum respectively.     

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.     

Influence of grain size on photoelectrocatalytic performance of CuBi2O4 photocathodes

CuBi₂O₄ is an ideal photocathode material owing to its suitable optical bandgap (～1.8 eV), positive onset potential (～1 V vs. RHE), and high theoretical photocurrent density (19.7–29 mA/cm²). However, its relatively poor efficiency in transporting carriers hinders it from achieving high photoelectrocatalytic performance. In this study, we propose the preparation of phase-pure large-grain CuBi₂O₄ thin film photocathodes through solvent pre-annealing and two-step annealing, with the aim of improving the carrier transport efficiency. The maximum grain size of CuBi₂O₄ reached an astonishing 1 μm at the optimal ethanol vapor concentration. Through time-resolved photoluminescence, we discovered that after treating CuBi₂O₄ with the proposed technique, the carrier lifetime improved by more than one order of magnitude. This improvement was achieved because the large grain size reduced the inhibition of carrier transport through grain boundaries. Therefore, the photocurrent density of large-grained CuBi₂O₄ reached 0.27 mA/cm², which is 27 times that of a direct annealing treatment. Finally, we used atomic layer deposition to load a ZnO protective layer and Pt catalyst onto the surface of CuBi₂O₄ photocathodes, and the photocurrent density of CuBi₂O₄/ZnO/Pt was further increased to 0.46 mA/cm² without using an electron scavenger (0.4 V vs. RHE).     

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.     

Photoconductivity and defect levels in ZnxCd1−xSe with (x=0.5, 0.55) crystals

Gold surface barriers on Zn_xCd_1−xSe alloys have been investigated for composition with x=(0.5, 0.55). The electrical characteristics were studied as a function of air annealing. The common feature of all the Schottky devices was the reduction of reverse bias leakage current after heating in air. Typical measurements of capacitance as a function of bias voltage can provide information on the charge density and diffusion potential. The barrier height was found to increase after air annealing at 200°C for 2 min. The spectral response of the photocurrent and photocapacitance associated with these device layers enable a donor level at 0.13 eV and acceptor levels at 1.08, 1.3 and 1.45 eV below the bottom of the conduction band to be identified. The results are discussed in terms of the effects of oxygen absorption.     

Electrochemical nano-patterning of brass for stable and visible light-induced photoelectrochemical water splitting

A novel propitious nano-patterned brass oxide nanowires were fabricated via controlled anodization of α-brass in aqueous electrolytes at room temperature. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS) techniques were used to investigate the morphology, structure, and composition of the fabricated materials. The morphology of the resulted structures was found to depend on the concentration of the sodium bicarbonate used for anodization as well as the post treatment. The XRD analysis confirmed the existence of both ZnO and CuO. The XPS results suggest the formation of CuZnO nanowires. The fabricated nanowires showed exceptional optical activity with an absorption wavelength extending to 800 nm, corresponding to a bandgap energy of 1.7 eV. This bandgap energy was also confirmed via DFT calculations. The fabricated nanostructures were used to split water photoelectrochemically under AM 1.5 illumination. They showed very promising results towards visible light water splitting with a photocurrent of 1.88 mA/cm² at −0.5 V versus Ag/AgCl, an incident photon-to-current efficiency (IPCE) of ∼15% at 400 nm, and a production of ∼875 μmol of H₂ gas upon illumination for 5 h. The obtained photocurrent is at least five times higher than that reported for ZnO and TiO₂. The transient photocurrent measurements showed the fabricated electrode to be photostable under the operating conditions.     

Photoelectrochemical characterisation of indium nitride and tin nitride in aqueous solution

Indium nitride (InN) and tin nitride (SnN_x) films were produced with reactive d.c. magnetron sputtering technique. The thin film semiconductors were optically and photoelectrochemically characterised and the energetic positions of the two semiconductors’ band edges were determined with respect to the normal hydrogen electrode. The sputtered InN thin film showed an indirect bandgap of 1.4 eV and a direct bandgap of 1.8 eV. The optical spectra of SnN_x indicated a bandgap energy of approximately 1.4 eV. All nitride films showed n-type photoresponse in KI (aq) electrolyte at an irradiation intensity of 1000 W/m². The photoelectrochemical characterisation indicated that InN and SnN_x with a bias of about 400 mV or less can be used for photo-oxidation of water.     

Band gap reduction of (Mo+N) co-doped TiO2 nanotube arrays with a significant enhancement in visible light photo-conversion: A combination of experimental and theoretical study

The present research aimed to evaluate the effects of co-doping TiO₂ nanotube aligned arrays (TNAs) with molybdenum and nitrogen on photocatalytic activity/performance under visible light irradiation. The surface morphology, electronic and optical properties of the pure and modified TNAs based on experimental characterization and theoretical calculations are reported. Both, pure and doped/modified TNAs were synthesized using a single step/low cost anodization method. Titanium sheets were immersed in ethylene glycol-based electrolytes containing NH₄F and NH₄F + K₂MoO₄ to fabricate highly ordered TNAs and Mo-doped TNAs, respectively. Mo–N-doped TNAs were fabricated by a thermal annealing process of Mo-doped samples in nitrogen environment (N₂-gas flow rate of 400 cc/min) for 2hr at 520 °C. Physical/chemical characteristics, structural and photo-electrochemical/electronic properties of the photo-electrodes were observed using several techniques including, field emission scanning electron microscopy (FESEM), Energy Dispersive X-Ray Spectroscopy (EDX), XRD, X-ray photoelectron spectroscopy (XPS), Raman and UV–Vis spectroscopy. We further used a full potential density functional theory (DFT) method to estimate the morphological and electronic structure of the synthesized photo-anodes and also observed a good agreement between theoretical calculations and characterization results. The characterization techniques confirm that Mo and N atoms have been incorporated into the lattice of anodized TNAs and molybdenum atoms partially substituted titanium atoms in the structure of TNAs. UV–Vis DRS spectroscopy experiments and theoretical results reveal that (Mo + N) co-doping creates a positive synergic effect on the band structure of TNAs which can enhance photo-conversion activity, compared to the single Mo/N-doped TNAs samples. In presence of sacrificial agent/electrolyte (aqueous solution of Na₂S/Na₂SO₃) and visible light irradiation, average photocurrent density of the co-doped TNAs photo-anode is 14 times greater than that of the undoped TNAs.     

Photoluminescence studies of CdS thin films annealed in CdCl2 atmosphere

We have grown CdS films by the Close Spaced Vapor Transport technique under specific conditions: substrate temperature (T_s): 450 °C, source temperature (T_so): 725 °C, argon pressure in the chamber (P_Ar): 100, 200 and 500 mT, deposition time (t_d): 100 s. The films were studied by measuring the luminescence properties at different temperatures in the range 10–300 K. The room-temperature PL spectrum of the as-grown CdS films showed a very broad band centered at 2.26 eV and a shoulder in the low-energy side at 1.80 eV. After CdCl₂ thermal annealing at 300 K, the spectrum showed better PL characteristics: a strong band in the low-energy side at 1.67 eV and a band in the high-energy side at 2.47 eV. The analysis at lower temperatures showed that the high-energy band becomes most intense and shifts to higher energies reaching a value of 2.54 eV, very close to the energy band gap at 10 K. The low-energy band becomes broader and centered around 1.9 eV. Analysis of the PL intensity as a function of temperature in an Arrhenius representation, allows applying a theoretical model for the quenching of the PL intensity.     

Structural,optical and electrochemical transient photoresponse properties of ZnO/Ga2O3 nanocomposites prepared by two-step CVD method

The influence of ZnO layer thickness on the structural, optical and electrochemical transient photoresponse properties of Ga₂O₃/ZnO heterostructured nanocomposites is evaluated. The increasing thickness of ZnO layer from 0 to 7.868 μm on the nanocomposites show an increase in the concentration of photogenerated electron-hole pairs, improve carrier separation and transportation to the electrolyte. This results to an enhanced photocurrent density from 164 to 833 μA/cm² at 1 V vs Ag/AgCl electrode. The unhybridized β-Ga₂O₃ film and the ZnO/Ga₂O₃ nanocomposites exhibit nanoblock-like and nanoplate-like morphologies, respectively. Structural studies reveal hexagonal ZnO and β-Ga₂O₃ crystalline phases for the nanocomposites. Optical bandgap of the reference β-Ga₂O₃ film is found to be 4.79 eV whereas, the nanocomposites exhibit two bandgaps: 3.30–3.25 eV belonging to the crystalline hexagonal ZnO phase, and 4.87–4.83 eV traced to the β-Ga₂O₃ phase. Photoluminescence spectroscopy exhibits blue-green emissions for the unhybridized film and ultraviolet-green emissions for the nanocomposites.     

Photoactivity of MWCNTs modified α-Fe2O3 photoelectrode towards efficient solar water splitting

MWCNTs (Multiwalled Carbon Nanotubes) modified α-Fe₂O₃ (hematite) photoelectrodes have been investigated for their possible application in hydrogen generation via photoelectrochemical (PEC) splitting of water. Enhanced photoresponse seen in comparison to the pristine α-Fe₂O₃ films is credited to the effective charge facilitation and charge separation provided by MWCNT conducting support. 0.2 wt% MWCNTs modified α-Fe₂O₃ thin film exhibited the maximum photocurrent density of 2.8 mA/cm² at 0.75 V/SCE. Measured values of flat band potential, donor density, resistance, Applied bias photon-to-current efficiency (ABPE) and Incident-photon-to current-conversion efficiency (IPCE) support the observed enhancement in photocurrent.     

Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting

Highly efficient water splitting electrode based on uniform platinum (Pt) nanoparticles on self-organized TiO₂ nanotube arrays (TNTAs) was prepared by a combination of multi-step electrochemical anodization with facile photoreduction process. Uniform platinum (Pt) nanoparticles with an average diameter of 8 nm are distributed homogeneously on nanoporous top layer and underneath TiO₂ nanotube wall. In comparison to pristine TNTAs, Pt@TNTAs show substantially enhanced photocurrent density and the incident photon-to-current conversion efficiency (IPCE) in the entire wavelength window. The maximum photocurrent density and IPCE from the optimized Pt@TNTAs photoelectrode (Pt, ～1.57 wt%) were about 24.2 mA cm⁻² and 87.9% at 350 nm, which is much higher than that of the pure nanotubes sample (16.3 mA cm⁻² and 67.3%). The resultant Pt@TNTAs architecture exhibited significantly enhanced photoelectrochemical activities for solar water splitting with hydrogen evolution rate up to 495 μmol h⁻¹ cm⁻² in 2 M Na₂CO₃ + 0.5 M ethylene glycol under the optimal external bias of −0.3 V_SCE.     

ZnBi38O60/Bi2O3 photocathode for hydrogen production from water splitting

Hydrogen production from water splitting into photoelectrochemical cells is a promising alternative for reducing the use of fossil fuels. Here, we synthesize by spray pyrolysis a porous ZnBi₃₈O₆₀/γ-Bi₂O₃ film with a surface area of 744 m² g⁻¹ for use as a photocathode in water-splitting cells. The film of ZnBi₃₈O₆₀ with 3 wt% Bi₂O₃ has 2.3 eV bandgap energy and a conduction band energy of −2.14 V vs. RHE at pH 6.99, which is thermodynamically suitable for reducing H⁺ to H₂. Under illumination, the film produces a current density of −1.55 mA cm⁻² at 0 V vs. RHE with an onset potential of 0.84 V vs. RHE. HC-STH efficiency is 0.09% at 0.17 V vs. RHE and IPCE at 0 V vs. RHE is 3.8% at 480 nm. Under continuous operation, the ZnBi₃₈O₆₀/γ-Bi₂O₃ film shows a stable photocurrent of −0.4 mA cm⁻² at 0 V vs. RHE for 1800 s with 100% Faradaic efficiency.     

Effect of Mo-doping in SnO2 thin film photoanodes for water oxidation

New semiconducting metal oxides of various compositions are of great interest for efficient solar water oxidation. In this report, Mo-doped SnO₂ (Mo:SnO₂) thin films deposited by reactive magnetron co-sputtering in the Ar and O₂ gas environment are studied. The Sn to Mo ratio in the films can be controlled by changing the O₂ partial pressure and the deposition power of the Sn and Mo targets. Increasing the Mo concentration in the film leads to the increase in the oxygen vacancy density, which limits the maximum achievable photocurrent density. The thin films exhibit a direct band gap of 2.7 eV, the maximum achievable photocurrent density of 0.6 mA cm⁻² at 0 V_RHE and the onset potential of 0.14 V_RHE. The incident photon to current transfer (IPCE) efficiency of 22% is shown at a 450 nm wavelength. The initial performance of the Mo:SnO₂ thin films is evaluated for solar water oxidation.     

Heat treatment effects on the optical properties of Sol–gel Ta2O5 thin films

In this work optical properties of Ta₂O₅ thin films with respect to heat treatment temperature were investigated. Ta₂O₅ thin films were prepared by sol–gel process using dip-coated method with a constant speed of 107 mm/min. Optical properties have been calculated from optical transmission measurements as a function of heat treatment temperature. The refractive indices and absorption coefficients were affected by heat treatment. The refractive index at λ=550 nm increased from 1.84 to 2.04 and absorption coefficient increased from 241 to 5668 cm⁻¹ when heat treatment temperature increased from 100°C to 500°C. The thickness of the film decreased from 272 to 190 nm and their optical band gap decreased from 3.68±0.09 eV to 3.51±0.08 eV for the film heated from 100°C to 500°C.     

Microwave-assisted low temperature fabrication of nanostructured α-Fe2O3 electrodes for solar-driven hydrogen generation

It is demonstrated for the first time that significant enhancement of photoelectrochemical performance could be achieved by using microwave-assisted annealing for the fabrication of α-Fe₂O₃ thin films. The process can also lead to significant energy savings (>60% when compared with conventional methods). Different types of Fe thin films were oxidized using both microwave and conventional heating techniques. The photoelectrochemical performance of electrodeposited, undoped and Si-doped iron oxide samples showed that microwave-annealing resulted in superior structural and performance enhancements. The photocurrent densities obtained from microwave annealed samples are among the highest values reported for α-Fe₂O₃ photoelectrodes fabricated at low temperatures and short times; the highest photocurrent density at 0.55 V vs. V_Ag/AgCl, before the dark current onset, was 450 μA cm⁻² for the Si-doped films annealed at 270 °C for 15 min using microwave irradiation (and 180 μA cm⁻² at 0.23 V vs. V_Ag/AgCl) while conventional annealing at the same temperature resulted in samples with negligible (3 μA cm⁻²) photoactivity. In contrast, a 450 °C/15 min conventional heat treatment only resulted in a film with 25% lower photocurrent density than that of the microwave annealed sample. The improved performance is attributed to the lower processing temperatures and rapidity of the microwave method that help to retain the nanostructure of the thin films whilst restricting the grain growth to a minimum. The lower processing temperature requirements of the microwave process can also open up the possibility of fabricating hematite thin films on conducting, flexible, plastic electronic substrates.     

Highly stable CdS-modified short TiO2 nanotube array electrode for efficient visible-light hydrogen generation

A highly stable photoelectrocatalytic electrode made of CdS-modified short, robust, and highly-ordered TiO₂ nanotube array for efficient visible-light hydrogen generation was prepared via sonoelectrochemical anodization and sonoelectrochemical deposition method. The short nanotube electrode possesses excellent charge separation and transfer properties, while the sonoelectrochemical deposition method improves the combination between CdS and TiO₂ nanotubes, as well as the dispersion of CdS nanoparticles. Different characterization techniques were used to study the nanocomposite electrode. UV-vis absorption and photoelectrochemical measurements proved that the CdS coating extends the visible spectrum absorption and the solar spectrum-induced photocurrent response. Comparing the photoactivity of the CdS/TiO₂ electrode obtained using sonoelectrochemical deposition method with others that synthesized using plain electrochemical deposition, the current density of the former electrode is ∼1.2 times higher that of the latter when biased at 0.5 V. A ∼7-fold enhancement in photocurrent response is obtained using the sonoelectrochemically fabricated CdS/TiO₂ electrode in comparison with the pure TiO₂ nanotube electrode. Under AM1.5 illumination the composite photoelectrode generate hydrogen at a rate of 30.3 μmol h⁻¹ cm⁻², nearly 13 times higher than that of pure titania nanotube electrode. Recycle experiments demonstrated the excellent stability and reliability of CdS/TiO₂ electrode prepared by sonoelectrochemical deposition. This composite electrode, with its strong mechanical stability and excellent combination of CdS and TiO₂ nanotubes, offers promising applications in visible-light-driven renewable energy generation.     

Improvement of TiO2 nanotubes for photoelectrochemical water splitting: Review

Photoelectrochemical (PEC) water splitting is an ideal method to produce clean hydrogen. Developing photoelectrodes that fulfill the PEC water-splitting criteria has become the greatest challenge for commercialization of this technology. Titanium dioxide, the first material used for this application, remain appealing due to its one-dimensional nanotube structure. However, the bandgap of TiO₂ nanotubes, ~3.0 eV, is relatively wide, leading to problems such as limited utilization of light energy and easy recombination of the photogenerated products, i.e., electrons and holes. Several approaches have been developed to overcome this problem, including (i) modification of surface morphology to enhance the active catalytic area, (ii) band structure modification to reduce photogenerated charge recombination, and (iii) surface sensitization to improve light absorption ability. This review reports the improvements achieved by all of these approaches for TiO₂ nanotubes, including the basic principles of the photocatalytic water-splitting process and the preparation and polymorphs of TiO₂ nanotubes. This review also discusses combinations of several methods that enable high photocurrent density with fabulous stability.