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.     

Yb-doped WO3 photocatalysts for water oxidation with visible light

Yb-doped WO₃ photocatalysts were prepared by co-sputtering WO₃ and Yb, followed by annealing in air for water oxidation with visible light. All the obtained photocatalysts were monoclinic with sputtering power of Yb up to 10 W and displayed no optical absorption red shift. In photoelectrochemical (PEC) studies, the photocurrent densities were improved with up to 0.34 at.% Yb in WO_3, with the highest photocurrent of 1.3 mA/cm² (1.2 V vs. Ag/AgCl) achieved with <0.1 at.% Yb. Electrochemical impedance spectroscopy (EIS) measurements showed that optimized Yb doping reduced charge transfer resistance and increased donor density of WO₃ photocatalyst. The improvement in photocurrent density was attributed to enhanced conductive carrier path, increased oxygen vacancies and 4f¹³ orbital configuration due to Yb³⁺ substitution of W⁶⁺.     

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.     

Enhancing photoelectrochemical activity of nanocrystalline WO3 electrodes by surface tuning with Fe(Ⅲ)

Tungsten oxide (WO₃) photoelectrodes with the surface tuned by Fe(Ⅲ) for photoelectrochemical water splitting were successfully synthesized. Nanostructured WO₃ films were prepared using doctor blade method, then a facile and economical deposition-annealing process was employed to fabricate Fe(Ⅲ) modified WO₃ films. The resulting composite's structural and optical properties were analyzed by SEM, EDX, XRD, UV–Vis spectrometry and XPS. The photoelectrochemical properties were evaluated by photocurrent density under 500 W Xe lamp with an intensity of 100 mW/cm². The Fe(Ⅲ) modified WO₃ electrode exhibited a larger photocurrent than the pure WO₃ electrode. Significantly, the optimized Fe(Ⅲ) modified WO₃ film achieved the maximum photocurrent density of 1.18 mA/cm² at 0.8 V vs. Ag/AgCl in the 0.2 M Na₂SO₄. The enhanced photocurrent was attributed to the extension of the light response and the electron hole separation at the interface Fe(Ⅲ)/WO₃ which was confirmed by Mott–Schottky and electrochemical impedance spectroscopy.     

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.     

High-throughput characterization of Ag–V–O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting

Ag–V–O thin-film materials libraries, with both composition (Ag_22-77V_23-78O_x) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV₆O₁₅, Ag₂V₄O₁₁, AgVO₃, and Ag₄V₂O₇ were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm²) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag₃₂V₆₈O_x) which confirmed the presence of Ag₂V₄O₁₁ as the dominating phase along with the minor phases AgV₆O₁₅ and AgVO₃. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting.     

Incorporation of oxidized zeolitic imidazolate framework 67 as effective Co-catalyst on W-doped BiVO4 for catalyzing photoelectrochemical water oxidation

Bismuth vanadate (BiVO₄) with suitable conduction/valence band edges and band gap has been largely applied on photoelectrochemical catalytic water oxidation as clean energy technology. To solve the short charge diffusion length and poor water oxidation kinetics of BiVO₄, it is useful to dope heteroatoms and decorate co-catalysts for improving its catalytic ability and electricity. Oxidized zeolitic imidazolate framework 67 (ZIF67) with polyhedron structure and possible composition of cobalt oxide is highly attractive as co-catalyst of BiVO₄. In this study, it is the first time to decorate oxidized ZIF67 (O67) as co-catalyst on W-doped BiVO₄ (WBVO) using the drop casting technique for catalyzing photoelectrochemical water oxidation. Different amounts of ZIF67 are decorated on WBVO using the drop-casting technique. The O67 is merged into WBVO nanorod arrays after converting ZIF67 into O67 using the annealing process. The highest photocurrent density of 2.08 mA/cm² at 1.23 V versus reversible hydrogen electrode (V_RHE) is obtained for the optimal WBVO/O67 electrode in the electrolyte without hole scavenger, while the WBVO electrode only shows the photocurrent density of 1.20 mA/cm² at the same condition. The decoration of O67 can increase light absorption, generate active sites, and reduce charge recombination. The photocurrent retention higher than 92% is achieved for the WBVO/O67 electrode under continuous illumination for 5000 s. The greatly enhanced photoelectrochemical catalytic ability of WBVO/O67 implies the feasibility of utilizing oxidized ZIF67 as a co-catalyst to accelerating water oxidation.     

Photoelectrochemical generation of hydrogen using 100 Mev Si ion irradiated electrodeposited iron oxide thin films

Thin films of hematite, synthesized by electrodeposition were irradiated by 100 Mev Si⁸⁺ swift heavy ions (SHI) at various fluences. The grain size, surface morphology, optical absorption edge, capacitance measurements and photoelectrochemical response of these films were studied and analyzed before and after exposure to irradiation. Irradiated samples exhibited significantly better photoelectrochemical response than the unirradiated α-Fe₂O₃ sample. Photocurrent density was observed to increase at lower ion fluence and the film irradiated at a fluence of 5 × 10¹² ions/cm² exhibited maximum photocurrent density. Crystallinity of the films and particle size were also observed to increase after irradiation up to this fluence. The irradiation caused a decrease in the resistivity, increase in donor density and increase in flatband potential for the sample irradiated at fluence 5 × 10¹² ions/cm², which may be responsible for its better photoelectrochemical properties. Rate of hydrogen as measured for best performing electrode was 2.4 mL/h-cm².     

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

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

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.     

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.     

Efficient photocurrent generation through a self-assembled monolayer of C60-mercaptophenylanthrylacetylene

A novel conjugated molecule, C₆₀-mercaptophenylanthrylacetylene (C₆₀-MPAA) has been self-assembled on a gold surface to form a highly ordered monolayer. The electrochemical properties of this molecule in solution or in a self-assembled monolayer (SAM) were investigated using cyclic voltammetry. A stable cathodic photocurrent was observed from a photoelectrochemical cell using the C₆₀-MPAA SAM modified gold as the working electrode in the presence of methyl viologen (MV²⁺) as electron carrier. The quantum yield (14%) of the cell was obtained at a bias of −100 mV versus Ag/AgCl under the illumination of monochromatic light at 400 nm. The dependence of photocurrent on the potential bias applied to the self-assembled monolayer modified Au electrode indicated that electron flow from C₆₀-MPAA to the counter electrode through the electron carriers.     

A heterostructured catalyst composed of poly-2,6-diaminopyridine and TiO2 microspheres used as a photoanode for efficient water splitting

Photoelectrocatalytic (PEC) water splitting provides an alternative to direct solar-to-fuel production. In this study, a novel heterostructure formed between a conjugated polymer [poly-2,6-diaminopyridine (PDAP)] and three-dimensional TiO₂ microspheres was grown in situ on a Ti substrate (PDAP-3DTiO₂MSs/Ti) and used as photoanode for water oxidation in alkaline media under AM 1.5G illumination. The PDAP-3DTiO₂MSs/Ti can produce applied bias photon-to-current efficiency of 0.85% at 0.44 V vs. Pt and a photocurrent density of 1.56 mA cm⁻² at 1.23 V vs. RHE. Moreover, PDAP-3DTiO₂MSs/Ti displays impressive photoelectrochemical stability with 93% of its initial photocurrent being retained after 4 h of reaction. Based on physical-chemical characterization and photo-/electro-chemical measurements, the superior PEC water splitting performance of PDAP-3DTiO₂MSs/Ti should benefit from the coexistence of Ti³⁺ and Ti⁴⁺ in 3DTiO₂MSs, the light harvest capability of PDAP and the type II heterojunction formed between 3DTiO₂MSs and PDAP, which result in the enhanced generation and separation of photocarriers.     

Manipulating the charge separation via piezoelectric field and heterojunction to enhance the photoelectrochemical water splitting ability of Bi2WO6/BiOBr photoanode

The unsatisfactory separation efficiency of photogenerated charge is one of the significant problems restricting the application of photoelectrochemical water splitting. Herein, we successfully prepared a Bi₂WO₆/BiOBr nanoplate arrays structure with the heterostructure for efficient piezoelectric photoelectric water splitting. A charge transfer path is formed by constructing a heterojunction to accelerate the spatial separation of photogenerated charges. The Bi₂WO₆/BiOBr shows the better photoelectrochemical performance with high photocurrent density of 0.068 mA/cm² at 1.23 V vs. RHE which is 1.8 times higher than simple Bi₂WO₆. In addition, after the introduction of piezoelectric polarization, the carrier separation efficiency is further enhanced under the synergistic action of the piezoelectric polarization and the heterojunction, and the photocurrent of Bi₂WO₆/BiOBr photoanode is increased to 0.088 mA/cm² at 1.23 V vs. RHE. By comparing the photoelectrocatalytic performance of the samples before and after the introduction of piezoelectric field, it further explains the role of piezoelectric built-in electric field in promoting carrier separation. This work provides a new method to improve the carrier separation efficiency by combining heterojunction and piezoelectric polarization.     

Size effects of zeolitic imidazolate framework 67 polyhedrons as Co-catalyst of W-doped BiVO4 electrodes for photoelectrochemically catalytic water oxidation

Co-catalyst decoration and heteroatom doping techniques are largely applied to enhance photoelectrochemical catalytic ability of BiVO₄. Zeolitic imidazolate framework 67 (ZIF67) with cobalt center and high specific area is promising as co-catalyst of BiVO₄. In this study, it is the first time to synthesize different sizes of ZIF67 polyhedrons as co-catalyst of W-doped BiVO₄ (WBVO) using electrodeposition and hydrothermal techniques to catalyze water oxidation. Decorating ZIF67 on WBVO (WBVO/ZIF67) can enhance light absorption intensity, create active sites, and suppress recombination to improve water oxidation kinetics. The highest photocurrent density of 2.30 mA/cm² at 1.23 V versus reversible hydrogen electrode (V_RHE) is obtained for WBVO/ZIF67 electrode with the smallest size of ZIF67 in electrolyte without hole scavenger, while the WBVO electrode only presents the photocurrent density of 0.9 mA/cm² at the same condition. The smallest size of ZIF67 can provide largest contacts with WBVO and electrolyte and create efficient charge transfer paths and numerous active sites. The photocurrent retention higher than 80% is achieved for WBVO/ZIF67 electrode under continuous illumination for 5000 s. The highly improved photoelectrochemical catalytic ability of WBVO/ZIF67 confirms the feasibility of applying ZIF67 as co-catalyst and indicates the significance of sizes of ZIF67 co-catalyst on catalytic ability.     

Facile preparation of BiVO4/FeVO4 heterostructure for efficient water-splitting applications

In this paper, a BiVO₄/FeVO₄ heterostructure photoanode was synthesized by electrospray technique, and its photoelectrochemical water oxidation performance was investigated. The maximum photocurrent density of 0.4 mA cm⁻² at 1.23 V_RHE was 6 times higher than that of pristine BiVO₄ films (0.06 mA cm⁻²). Through the analysis of the electrochemical impedance spectroscopy (EIS) results, the improvement of photoelectrochemical performance could be attributed to the formation of heterostructure at the two-phase interface, which led to the effective separation of electron-hole pairs. This work offers a new effective strategy to construct semiconductor nanocomposites for efficient photoelectrochemical water oxidation.     

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.     

Photoelectrochemical properties of hematite films grown by plasma enhanced chemical vapor deposition

Nanostructured α-Fe₂O₃ thin films were grown by plasma-enhanced chemical vapor deposition (PE-CVD) using iron pentacarbonyl (Fe(CO)₅) as precursor. Influence of the plasma parameters on photoelectrochemical (PEC) properties of the resulting hematite thin films toward solar oxidation of water was investigated under one sun illumination in a basic (1 M NaOH) electrolyte. PEC data analyzed in conjunction with the data obtained by scanning electron microscopy, X-ray diffraction and Mott–Schottky analysis showed 100 W plasma power to be an optimal RF-power value for achieving a high photocurrent density of ∼1098 μA/cm² at 0.9 V/SCE external applied potential. The donor density, flat band potential, grain size and porosity of the films were observed to be highly affected by RF-power, which in turn resulted in enhanced photoresponse.     

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.     

Plasmonic nickel nanoparticles decorated on to LaFeO3 photocathode for enhanced solar hydrogen generation

Plasmonic Ni nanoparticles were incorporated into LaFeO₃ photocathode (LFO-Ni) to excite the surface plasmon resonances (SPR) for enhanced light harvesting for enhancing the photoelectrochemical (PEC) hydrogen evolution reaction. The nanostructured LFO photocathode was prepared by spray pyrolysis method and Ni nanoparticles were incorporated on to the photocathode by spin coating technique. The LFO-Ni photocathode demonstrated strong optical absorption and higher current density where the untreated LFO film exhibited a maximum photocurrent of 0.036 mA/cm² at 0.6 V vs RHE, and when incorporating 2.84 mmol Ni nanoparticles the photocurrent density reached a maximum of 0.066 mA/cm² at 0.6 V vs RHE due to the SPR effect. This subsequently led to enhanced hydrogen production, where more than double (2.64 times) the amount of hydrogen was generated compared to the untreated LFO photocathode. Ni nanoparticles were modelled using Finite Difference Time Domain (FDTD) analysis and the results showed optimal particle size in the range of 70–100 nm for Surface Plasmon Resonance (SPR) enhancement.