Hydrogen induced interface engineering in Fe2O3-TiO2 heterostructures for efficient charge separation for solar-driven water oxidation in photoelectrochemical cells

Semiconductor heterostructure junctions are known to improve the water oxidation performance in photoelectrochemical (PEC) cells. Depending on the semiconductor materials involved, different kinds of junctions can appear, for instance, type II band alignment where the conduction and valence bands of the semiconductor materials are staggered with respect to each other. This band alignment allows for a charge separation of the photogenerated electron-hole pairs, where the holes will go from low-to-high valance band levels and vice versa for the electrons. For this reason, interface engineering has attracted intensive attention in recent years. In this work, a simplified model of the Fe2O3-TiO2 heterostructure was investigated via first-principles calculations. The results show that Fe2O3-TiO2 produces a type I band alignment in the heterojunction, which is detrimental to the water oxidation reaction. However, the results also show that interstitial hydrogens are energetically allowed in TiO2 and that they introduce states above the valance band, which can assist in the transfer of holes through the TiO2 layer. In response, well-defined planar Fe2O3-TiO2 heterostructures were manufactured, and measurements confirm the formation of a type I band alignment in the case of Fe2O3-TiO2, with very low photocurrent density as a result. However, once TiO2 was subjected to hydrogen treatment, there was a nine times higher photocurrent density at 1.50 V vs. the reversible hydrogen electrode under 1 sun illumination as compared to the original heterostructured photoanode. Via optical absorption, XPS analysis, and (photo)electrochemical measurements, it is clear that hydrogen treated TiO2 results in a type II band alignment in the Fe2O3-H:TiO2 heterostructure. This work is an example of how hydrogen doping in TiO2 can tailor the band alignment in TiO2-Fe2O3 heterostructures. As such, it provides valuable insights for the further development of similar material combinations. This journal i

BiVO₄/TiO₂ core-shell heterostructure: wide range optical absorption and enhanced photoelectrochemical and photocatalytic performance

In the present study, pristine BiVO4, TiO2 and BiVO4/TiO2 core-shell heterostructured nanoparticles are prepared by hydrothermal methods and studied for structural, morphological, optical, photoelectrochemical water splitting and photocatalytic degradation of methylene blue as an organic pollutant. Both pristine BiVO4 and TiO2 exhibit poor PEC and PC performance under visible light illumination. However, an enhanced PEC and PC activity in BiVO4/TiO2 core-shell heterostructure is observed due to high solar energy absorption and superior charge separation properties in core-shell nanoparticles. The photoelectrode prepared using BiVO4, TiO2 core-shell nanoparticles exhibit a photocathode behavior and produced cathodic photocurrent, however, the pristine BiVO4 and TiO2 photoelectrodes act as photoanode and produced anodic photocurrent. This behavior of change in current direction is also observe in the Mott-Schottky analysis where the BiVO4, TiO2 core-shell nanoparticles photoelectrode exhibits the positive slow showing p-type semiconducting behavior. The change in cathodic photoresponse in core-shell nanoparticles in comparison to anodic photoresponse of BiVO4 and TiO2 nanoparticles is explained in terms of the variations in the work function values. These results highlight the advantages of core-shell nanoparticle of suitable materials for photocatalytic and photoelectrochemical applications

Improvement in the structural, optical, electronic and photoelectrochemical properties of hydrogen treated bismuth vanadate thin films

In the present study structural, optical and electronic properties of bismuth vanadate (BiVO4) thin films prepared by rf-sputtering technique were modified by post-hydrogen treatment to improve the photoelectrochemical (PEC) performance for water oxidation. X-ray diffraction and Raman analysis do not reveal any major structural changes but show increase in crystallite size and creation of defect states, however, optical absorption studies shows changes in band gap energy values due to the creation of inter-band states on hydrogen treatment. X-ray photoelectron spectroscopy studies show that the hydrogen treatment reduces surface Bi4+ considerably and increases the density of hydroxyl groups on the BiVO4 surface. The combined effect of these changes manifests in terms of enhanced photocurrent density of 3.31 mA/cm2 (at applied potential 1.0 V versus Ag/AgCl), which is about nine time higher than the pristine BiVO4 and reduced photocurrent onset potential

Synthesis of MoS₂-TiO₂ nanocomposite for enhanced photocatalytic and photoelectrochemical performance under visible light irradiation

In this work, we have prepared MoS2 nanoflakes modified TiO2 nanoparticles (MoS2-TiO2 nanocomposite) with varying concentration of MoS2 (2.5–10 wt.%) by a two-step hydrothermal synthesis method involving specific preparation conditions for the TiO2 nanoparticles and MoS2 nanoflakes. The prepared samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS) techniques. The photocatalytic activity of the pristine TiO2 nanoparticles and MoS2-TiO2 nanocomposite samples were evaluated by examining the photocatalytic degradation of Rhodamine B (RhB). The photoelectrochemical activity of these samples were measured by performing solar water splitting experiments under visible light irradiation. It was observed that the MoS2-TiO2 nanocomposite with 7.5 wt.% MoS2 exhibits highest photocatalytic and photoelectrochemical activity as it has the optimum amount of MoS2 nanoflakes which probably minimizes the recombination of photogenerated charge carriers as compared to other concentrations of MoS2 in MoS2-TiO2 nanocomposite and pristine TiO2 nanoparticles. In addition, a rather high photocatalytic reaction rate constant was observed for MoS2-TiO2 nanocomposite with 7.5 wt.% MoS2 nanoflakes

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

Nanostructured alpha-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 (PEG) 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. PEG 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 similar to 1098 mu A/cm(2) 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. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (α-Fe2O3) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO2) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 °C. The samples annealed at 500 °C show an in situ diffusion of indium from the ITO substrate to the surface of α-Fe2O3, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 °C enhances the optical absorption, increases the electrode–electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed by ex situ diffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. The in situ In diffused α-Fe2O3 photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 μA cm−2 at 1.23 VRHE, compared to 37 μA cm−2 for the photoanode prepared at 350 °C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 μA cm−2 at 1.23 VRHE and the photocurrent onset potential decreases to 0.93 VRHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.Peer reviewe

Plasma-chemical reduction of iron oxide photoanodes for efficient solar hydrogen production

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe3O4:alpha-Fe2O3) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm(2) at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 VRHE (untreated) to 1.28 VRHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Favourable band edge alignment and increased visible light absorption in β-MoO3/α-MoO3 oxide heterojunction for enhanced photoelectrochemical performance

Optimum band gap values, favourable band edge positions and stability in the electrolyte are critical parameters required for a semiconductor to have efficient photoelectrode properties. The present investigation carried out on the phase pure α & β MoO3\ua0thin film shows that the low bandgap β-MoO3\ua0possesses a mis-alignment with the water oxidation potential, while a more suitable band alignment is observed for the comparatively large bandgap α-MoO3. Both experimental and DFT calculations show that the valence edge of the orthorhombic (α-MoO3) phase is located at a higher energy (0.9 eV higher in VB-XPS and 1 eV higher in the DOS plots) than the monoclinic (β-MoO3) phase, while the conduction edge value is roughly at the same energy level (−2.5 eV) in both polymorphs. Based on the above investigations, an all oxide heterojunction comprising of β-MoO3/α-MoO3\ua0is found to be suitable for improved PEC performance due to favourable energy band diagram and increased visible light absorption in β-MoO3. Significantly higher cathodic photocurrent is observed for the β-MoO3/α-MoO3\ua0(1.6 mA/cm2\ua0at applied bias of −0.3VRHE\ua0under simulated 1 sun irradiation) as compared to the very low anodic response in β-MoO3\ua0(∼1.0 nA/cm2) and α-MoO3\ua0(32 μA/cm2)

Improved Water Oxidation Performance of Ultra-thin Planar Hematite Photoanode: Synergistic Effect of In/Sn doping and an Overlayer of Metal Oxyhydroxides

Hematite is a promising photoanode candidate with many favorable material properties, such as stability and suitable band-gap. However, there are some severe challenges, including high losses due to charge recombination and slow oxidation kinetics, which can be addressed by doping and addition of co-catalysts. Here, the effects of temperature driven diffusion of substrate impurities (doping) and subsequent surface modification by metal oxy-hydroxides (co-catalysts) have been studied for enhanced water-oxidation performance in photoelectrochemical (PEC) measurements. Diffusion of indium and tin from the indium-doped tin oxide (ITO) substrate into planar films of α-Fe2O3 photoanodes results in a photocurrent density (Jph) of 0.09 mA/cm2, corresponding to an approximate 9-fold enhancement over the control pristine α-Fe2O3 (0.01 mA/cm2) at 1.23 VRHE. A thin amorphous FeOOH coating over the In/Sn co-doped α-Fe2O3 photoanode improves the water oxidation performance further, with a 211 % enhancement in Jph at 1.23 VRHE and a 0.21 V cathodic shift in onset potential. Thin layers of NiOOH and FeNiOOH co-catalysts exhibit 100 and 155 % enhancement in Jph, respectively. Characterization and electrochemical measurements reveal that the enhanced performance is a result of reduced bulk recombination by temperature driven In/Sn substrate impurity doping and improved surface oxidation kinetics by the metal oxy-hydroxide overlayer. Especially deposition of FeOOH onto In/Sn co-doped α-Fe2O3 significantly reduces resistance at the semiconductor/electrolyte interface, leading to the shift in onset potential. Further, the results indicate that all the samples exhibit a quantitative correlation between the cathodic shift in photocurrent onset potential (Vonset) and flat band potential (Vfb)

Visible-light-driven photoelectrochemical and photocatalytic performance of NaNbO₃/Ag₂S core-shell heterostructures

Herein, we report the fabrication of visible-light-active NaNbO₃/Ag₂S staggered-gap core–shell semiconductor heterostructures with excellent photoelectrochemical activity toward water splitting, and the degradation of a model pollutant (methylene blue) was also monitored. The heterostructures show a pronounced photocurrent density of approximately 2.44 mA cm⁻² at 0.9 V versus Ag/AgCl in 0.5 m Na₂SO₄ and exhibit a positive shift in onset potential by approximately 1.1 V. The high photoactivity is attributed to the efficient photoinduced interfacial charge transfer (IFCT). The core–shell design alleviates the challenges associated with the electron–hole paths across semiconductor junctions and at the electrolyte–semiconductor interface. These properties demonstrate that NaNbO₃/Ag₂S core–shell heterostructures show promising visible-light photoactivity and are also efficient, stable, and recyclable photocatalysts