Materials For Photoelectrochemical Water Splitting Research Articles

Energy Transfer-Induced Photoelectrochemical Improvement from Porous Zeolitic Imidazolate Framework-Decorated BiVO4 Photoelectrodes.

BiVO4 , which is a representative photoanode material for photoelectrochemical water splitting, intrinsically restricts high conversion efficiency, owing to faster recombination, low electron mobility, and short electron diffusion length. While the photocurrent density of typical BiVO4 corresponds to only 21.3% of the maximum photocurrent density (4.68mAcm-2 ), decoration of the BiVO4 photoanode with zeolitic imidazolate framework-67 (ZIF-67) exhibits a synergetic effect to raise the overall photocatalytic ability at the BiVO4 surface region to a higher level via the energy-transfer process from BiVO4 to ZIF-67. The hybrid ZIF-67/BiVO4 photoanode follows two convenient photoelectrochemical pathways: 1) energy-transfer-induced water oxidation reaction in ZIF-67 and 2) water oxidation reaction by direct contact between the BiVO4 surface and electrolytes. Compared to the moderate photocurrent density (≈1mAcm-2 ) of single-layer BiVO4 , the proposed ZIF-67/BiVO4 photoanodes show a remarkably high photocurrent (2.25mAcm-2 ) with high stability, despite the lack of hole scavengers in the electrolyte. Furthermore, the absorbed photon-to-current efficiency of the ZIF-67/BiVO4 photoanode is ≈2.5 times greater than that of BiVO4 . This work proposes a promising solution for efficient water oxidation that overcomes the intrinsic material limitations of BiVO4 photoelectrodes by using energy transfer-induced photon recycling and the decoration of porous ZIFs.

Photoelectrochemical Water Splitting: Thermal Annealing Challenges on Hematite Nanowires

Hematite is getting great attention as an environmentally friendly material for photoelectrochemical water splitting, due to its narrow band gap (1.9–2.2 eV), nontoxicity, low cost, high stability ...

Development of a magnetic nanohybrid for multifunctional application: From immobile photocatalysis to efficient photoelectrochemical water splitting: A combined experimental and computational study

Engineering of highly efficient nanomaterials with visible-light photocatalytic activity that are easily separable and recoverable from treated water is the ultimate goal in the ongoing research in the field of photocatalysis. On the other hand, photoelectrochemical (PEC) water splitting is one of the most promising technologies for hydrogen production using solar light with the aim to build sustainable, renewable and green energy. Hematite (α-Fe2O3) is a promising material that offers both enhanced photocatalytic activity and PEC activity due to its electronic band structure, high chemical stability, great abundance, and low cost. Despite these promising properties, the application of this system is limited due to its very fast electron-hole recombination rate and low carrier mobility. Here we report the design and synthesis of a newly envisioned nanohybrid based on Fe2O3 and phthalocyanine. The nanohybrid is an essential component to reduce the charge carrier recombination rate of the system. As a result of it, the nanohybrid shows higher photocatalytic activity and acts as a better photoanode material for photoelectrochemical water splitting. The formation of the nanohybrid is established using a picosecond resolved optical technique as well as by Raman Spectroscopy. Ab-initio study on the similar modeled system has been performed to investigate the insight of various physical properties. Electron microscopy reveals a distinct change of morphology in the nanohybrid compared to pristine one. While the photocatalytic activity of the nanohybrid increased 1.5 times with respect to the pristine system, the Photoelectrochemical activity almost doubles in the hybrid system. For real-world applications, our developed nanohybrid has been deposited on an extended surface of a stainless-steel metal mesh (size 2 cm × 2 cm, pore size 150 μm × 200 μm). Such a prototype active filter containing an immobilized photocatalyst shows significant chemical filtration of MB (by a degradation process) along with a physical filtration by separating the suspended particulates from the water. DFT + U calculation results show there is a possibility of higher charge separation from PC to Fe2O3, which is the reason for the higher activity in the hybrid systems.

A Model System for Photocatalysis: Ti-Doped α-Fe2O3(11̅02) Single-Crystalline Films.

Hematite (α-Fe2O3) is one of the most investigated anode materials for photoelectrochemical water splitting. Its efficiency improves by doping with Ti, but the underlying mechanisms are not understood. One hurdle is separating the influence of doping on conductivity, surface states, and morphology, which all affect performance. To address this complexity, one needs well-defined model systems. We build such a model system by growing single-crystalline, atomically flat Ti-doped α-Fe2O3(11̅02) films by pulsed laser deposition (PLD). We characterize their surfaces, combining in situ scanning tunneling microscopy (STM) with density functional theory (DFT), and reveal how dilute Ti impurities modify the atomic-scale structure of the surface as a function of the oxygen chemical potential and Ti content. Ti preferentially substitutes subsurface Fe and causes a local restructuring of the topmost surface layers. Based on the experimental quantification of Ti-induced surface modifications and the structural model we have established, we propose a strategy that can be used to separate the effects of Ti-induced modifications to the surface atomic and electronic structures and bulk conductivity on the reactivity of Ti-doped hematite.

High-Performance Phase-Pure SnS Photocathodes for Photoelectrochemical Water Splitting Obtained via Molecular Ink-Derived Seed-Assisted Growth of Nanoplates.

Although tin monosulfide (SnS) is one of the promising earth-abundant semiconducting materials for photoelectrochemical water splitting, the performance of SnS photocathodes remains poor. Herein, we report a stepwise approach for the fabrication of highly efficient photocathodes based on SnS nanoplates via elaborate modulation of molecular solutions. It is demonstrated that phase-pure SnS nanoplates without detrimental secondary phases (such as SnS2 and Sn2S3) can be readily obtained by adjusting the amounts of Sn and S in the precursor solution. Additionally, the orientation of SnS nanoplates is controlled by implementing different types of SnS seed layers. The orientations of the SnS seed layers are changed according to the molecular shapes of the Sn-S bonds in the molecular solutions, depending on the relative nucleophilicity of the molecular moieties formed by specific thiol-amine reactions. The molecular Sn-S sheets in the seed ink was obtained by the reaction in a solvent mixture of thiogylcolic acid and ethanolamine. By contrast, the short Sn-S molecular rods result from the reaction in a solvent mixture of 2-mercaptoethanol and ethylenediamine. Interestingly, the relatively short rodlike morphology of the SnS seed induces the growth of SnS nanostructures faceted by preferred (111) and (101) planes, leading to fast charge transport. With the formation of a proper band alignment with n-type CdS and TiO2, the preferred (111)- and (101)-oriented SnS nanoplate-based photocathode exhibited a photocurrent density of -19 mA cm-2 at 0 V versus a reversible hydrogen electrode, establishing a new benchmark for SnS photocathodes.

Photoelectrochemistry of Ferrites: Theoretical Predictions vs. Experimental Results

AbstractThis paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferritesMFe2O4. These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites withM= Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange ofMand Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.

Computer modeling of semiconductor nanotubes for water splitting

One-dimensional nanostructures such as nanorods and nanotubes (NTs) are regarded as promising materials for photoelectrochemical water splitting. Modeling the electronic properties of oxidic NTs with diameters in the range of 3–30 nm in contact with liquid water is challenging owing to the fact that the systems are too large for direct ab initio molecular dynamics simulations. Here we summarize recent efforts to develop strained two-dimensional model systems for pristine and doped titania NTs. We have studied their structural, optical and photoelectrochemical properties by a number of different techniques attributable to quantum mechanical density functional theory.

Intentional Extrinsic Doping into ZnFe2O4 Nanorod Photoanode for Enhanced Photoelectrochemical Water Splitting

Zinc ferrite (ZnFe2O4) is a promising candidate photoanode material for photoelectrochemical water splitting, but its poor electronic properties deter it from achieving high performance. To improve the electronic properties, Ti4+ and Sn4+ are incorporated into Fe3+ sites of ZnFe2O4 as electron donors under annealing conditions of 550, 800 °C, or hybrid microwave annealing (HMA). The intentional external doping mainly promotes charge transport with increased charge carrier density and thereby enhances the bulk charge separation efficiency (ηbulk). Effectiveness of the doping varies with annealing conditions. Both Ti‐doped and Sn‐doped ZnFe2O4 annealed at 550 °C show the most significant improvement in ηbulk with little effect on the surface charge separation efficiency (ηsurface). Doping under high‐temperature annealing (800 °C, HMA) improves ηbulk to a lesser degree but creates a passivation layer of TiOx or SnOx, resulting in a higher ηsurface as well. Finally, loading the NiFeOx oxygen evolution catalyst (OEC) further improves ηsurface values, and especially 2% Ti‐doped ZnFe2O4 annealed at 800 °C shows near 100% ηsurface at 1.23 VRHE, and its photocurrent generation under simulated 1 sun (0.312 mA cm−2 at 1.23 VRHE) represents a ≈18 times increment from that of unmodified ZnFe2O4 annealed at 550 °C.

Photocurrent Recombination Through Surface Segregation in Al–Cr–Fe–O Photocathodes

Abstract Chemical surface segregation is a design variable in the optimization of phocathodes but has largely been investigated through surface passivation or decoration. In this study a long charge carrier lifetime material, Al–Cr–Fe–O, exhibiting strong photocurrent recombination is investigated for its atomic scale crystallographic and chemical inhomogeneity. Combined scanning transmission electron microscopy and atom probe tomography unveils that insulating Al- and Cr-rich surface layers form during processing. These are discussed to be the primary reason for experimentally observed charge carrier recombination. This study highlights the importance of processing in the design, discovery and optimization of new light absorber materials for photoelectrochemical water splitting.

Impact of lattice defects on water oxidation properties in SnNb2O6 photoanode prepared by pulsed-laser deposition method

SnNb2O6 is one of the promising photoanode materials for photoelectrochemical water splitting. We prepared the SnNb2O6 thin-film photoanode with various Sn/Nb ratios by a pulsed-laser deposition method and investigated impacts of lattice defects in the SnNb2O6 photoanode on the water oxidation property. Photoelectrochemical measurements and Raman spectroscopy analyses for the SnNb2O6 photoanodes presented that the water oxidation property can be improved by suppressing the formation of the acceptor site caused by the substitution of Sn4+ for the Nb5+ site in the crystal lattice of SnNb2O6. This study provides material’s design strategy based on defect chemistry to realize an efficient SnNb2O6 photoanode used for the photoelectrochemical cell, which can operate without an external bias.

Flame spray pyrolysis for the one-step fabrication of transition metal oxide films: Recent progress in electrochemical and photoelectrochemical water splitting

Developing large scale deposition techniques to fabricate thin porous films with suitable opto-electronic properties for water catalysis is a necessity to mitigate climate change and have a sustainable environment. In this review, flame spray pyrolysis (FSP) technique, a rapid and scalable methodology to synthesize nanostructured transitional metal oxide films with designed functionalities, is firstly introduced. Furthermore, applications in electrochemical (EC) and photoelectrochemical (PEC) water splitting for the production of hydrogen fuel is also presented. The high combustion temperature and the aggregation of flame aerosol ensure that the FSP-made films possess high crystallinity, tunable porosity and high surface areas, making this method suitable either as catalysts for EC water splitting or as efficient semiconductor materials for PEC water splitting. Finally, a perspective on the next generation FSP engineered films with potential applications in energy storage and conversion is described.

P-Type NiO modified BiVO4 photoanodes with enhanced charge separation and solar water oxidation kinetics

Bismuth vanadate is considered as a promising photoanode material for photoelectrochemical water splitting. However, the frequent bulk charge recombination and poor surface reaction kinetics restrict its performance in photoelectrochemical cell seriously. Herein, we developed a facile method to build a p-n heterojunction of NiO/BiVO4 with enhanced charge separation and surface reaction kinetics. The internal electric field of our heterojunction can improve the electron-hole separation efficiency from 51% to 65% at 0.6 VRHE. Moreover, the p-type NiO can significantly enhance the surface oxygen evolution reaction kinetics. Based on these synergistic effects, our NiO/BiVO4 photoanode achieved a high solar to hydrogen efficiency of 0.72% at 0.76 VRHE.

Gallium Phosphide photoanode coated with TiO2 and CoOx for stable photoelectrochemical water oxidation.

Gallium Phosphide (GaP) has a band gap of 2.26 eV and a valance band edge that is more negative than the water oxidation level. Hence, it may be a promising material for photoelectrochemical water splitting. However, one thing GaP has in common with other III-V semiconductors is that it corrodes in photoelectrochemical reactions. Cobalt oxide (CoOx) is a chemically stable and highly active oxygen evolution reaction co-catalyst. In this study, we protected a GaP photoanode by using a 20 nm TiO2 as a protection layer and a 2 nm cobalt oxide co-catalyst layer, which were both deposited via atomic layer deposition (ALD). A GaP photoanode that was modified by CoOx exhibited much higher photocurrent, potential, and photon-to-current efficiency than a bare GaP photoanode under AM1.5G illumination. A photoanode that was coated with both TiO2 and CoOx layers was stable for over 24 h during constant reaction in 1 M NaOH (pH 13.7) solution under one sun illumination.

Stoichiometric and non-stoichiometric tungsten doping effect in bismuth vanadate based photoactive material for photoelectrochemical water splitting

In photoelectrochemical (PEC) water splitting, BiVO4 has attracted attention due to its favorable band gap but it suffers low PEC performance due to poor conductivity. The vast majority of publications on this system has examined doping of stoichiometric composition of tungsten (W) on this system to increase bulk and interfacial conductivity while managing the contaminant generation of crystallographic defects and recombination sites. In this paper, a deep investigation was carried out to examine the effect of non-stoichiometric W doping in BiVO4 system. Stoichiometric and non-stoichiometric W-doped monoclinic BiVO4 (i.e. Bi1-(x+δ)V1-xWx+δO4; BiV1-xWx+δO4 and BiV1-yWyO4; x = 0.008; y = 0.03 and δ = 0.005) were prepared using a facile dip coating technique. The stoichiometric composition contains charge balanced Bi, V and W atoms whereas non-stoichiometric compositions contain excess Bi and excess Bi and W. The non-stoichiometric composition BiV1-xWx+δO4 has shown better photoelectrochemical water splitting performance with respect to other compositions at 1.23 V vs RHE, under one sun illumination of electrode. The XRD and XPS results shows that non-stoichiometric doping with excess Bi or with excess Bi and W can possibly create an environment where V5+ ions are substitutional replaced by W6+ ions without generating other defects. But there was no significant difference in band gap of different compositional samples observed. Further electrochemical impedance technique was used to analyze change in bulk and surface charge mobility with W-doping in BiVO4. The electrochemical impedance analysis showed the presence of low interfacial resistance, lower charge transfer resistance and high charge donor/surface state density for non-stoichiometric composition BiV1-xWx+δO4 electrode. It is evident from and cyclic voltammetry that the addition of excess Bi and W from its stoichiometric quantity efficiently suppressed the formation of hole-electron pair recombination sites. The electrochemical analytical results lead us to believe that the particular non-stoichiometric composition of BiV1-xWx+δO4 can significantly lower trap sites and enhances kinetics of charge transfer, leading to the better photoelectrochemical water splitting performance.

Partially Dissociated Water Dimers at the Water–Hematite Interface

The oxygen evolution reaction (OER) is thought to occur via a four-step mechanism with *O, *OH, and *OOH as adsorbed intermediates. Linear scaling of the *OH and **OOH adsorption energies is proposed to limit the oxides’ efficiency as OER catalysts, but the use of simple descriptors to screen candidate materials neglects potentially important water–water interactions. Here, we use a combination of temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), noncontact atomic force microscopy (nc-AFM), and density functional theory (DFT)-based computations to show that highly stable HO–H2O dimer species form at the (1102) facet of hematite; a promising anode material for photoelectrochemical water splitting. The UHV-based results are complemented by measurements following exposure to liquid water and are consistent with prior X-ray scattering results. The presence of strongly bound water agglomerates is generally not taken into account in OER reaction schemes but may play a role in det...

Niobium oxynitride prepared by thermal NH3 nitridation as a photoanode material for solar water splitting

Niobium oxynitride (NbON) is a promising photoanode material for photoelectrochemical water splitting using visible light up to ∼600 nm in wavelength. However, the NH3 nitridation method widely used for oxynitride synthesis cannot convert Nb2O5 (Nb5+) to crystalline NbON (Nb5+), due to the generation of NbOxNy (Nb2+−3+). To prevent Nb reduction during nitridation, we added O2 as an oxidant to the NH3 nitridation reaction system. Investigation of the relationship between O2 concentration and the crystal phase of the resulting samples revealed that Nb2O5 nitridation produces crystalline NbON (Nb5+) within a specific O2 concentration range. The O2 concentration required to synthesize NbON increases from 0.05% to over 0.40% upon increasing the nitridation temperature from 600 °C to 750 °C. The photoelectrochemical properties of NbON films were also investigated. Under simulated sunlight illumination, a NbON photoanode produced a photocurrent of 0.6 mA cm-2 at 1.2 V vs. RHE in H2O2 aqueous solution.

Structural, optical, and bifunctional applications: Supercapacitor and photoelectrochemical water splitting of Ni-doped ZnO nanostructures

Over the past few decades, doped ZnO structures have attracted significant attention because of their distinctive properties and a wide range of applications in catalysis and energy-storage devices. However, effective simple synthesis of doped ZnO structures for photoelectrocatalytic and supercapacitor applications still remains challenging. In this study, Ni-doped ZnO structures were synthesized at different Ni concentrations. Analysis of the obtained samples confirmed the formation of Ni-doped ZnO; 1.5 mol% Ni-doped ZnO showed enhanced water splitting activity and supercapacitor properties. The highest photocurrent density of 4.6 mA/cm2 was obtained in a 0.1 M KOH solution at an applied bias photon-to-current efficiency of 4.2%, which is almost twice that obtained with pristine ZnO (2.8%), indicating an enhanced electron-hole separation. Doped ZnO exhibits a photocurrent 1.78 times higher than pristine ZnO under light illumination. Ni-doping induces effective charge separation and transfer, efficiently diminishing the recombination rate and reducing intrinsic defects. Further, the highest specific capacity of ~96 F g−1 was observed for 1.5% Ni-doped ZnO at an applied scan rate of 10 mV s−1. The optimized sample, 1.5% Ni-doped ZnO, exhibited a high specific capacitance retention and coulombic efficiency of ~98% and ~99.2%, respectively. These results are expected to be very helpful in developing cheap and simple fabrication methods and efficient electrode materials for photoelectrochemical water splitting and supercapacitor applications.

Fabrication and enhancement in photoconductive response of $$\alpha$$ α -Fe2O3/graphene nanocomposites as anode material

We report a facile fabrication for the synthesis of graphene based α-Fe2O3 nanocomposites as anode material for photoelectrochemical water splitting. The effect of introduction of graphene as a solid-state electron material has been investigated in detail by controlling the synthesis parameters. The XRD pattern of hematite α-Fe2O3 nanoparticles were indexed to rhombohedral structure of α-Fe2O3, while the TEM images illustrate the flaky structure of graphene supported by hematite nanoparticles. The rGO/ $$\alpha$$ -Fe2O3 nanocomposites showed enhanced photocurrent of ~ 4 mA cm−2 at (1.23 V vs. RHE) under standard illumination conditions (AM 1.5 G 100 mW cm−2). The enhanced photoelectrochemical performance may be attributed to synergistic effect of graphene and $$\alpha$$ -Fe2O3 by improving the charge transport properties. The optical properties were also observed to be influenced by the coupling of rGO and α-Fe2O3 composites as witnessed in the DRS spectra.

Tailored NiOx/Ni Cocatalysts on Silicon for Highly Efficient Water Splitting Photoanodes via Pulsed Electrodeposition

Converting solar energy by photoelectrochemical water splitting has been regarded as a promising way to resolve the global energy crisis and alleviate environmental pollution. Silicon, which is earth-abundant and has a narrow band gap, is an attractive material for photoelectrochemical water splitting. However, Si-based photoelectrodes suffer from photocorrosion, which leads to instability in electrolytes and high overpotential. Herein, we have fabricated a metal–insulator–semiconductor structure of NiOx/Ni/n-Si photoanodes for highly efficient water splitting. NiOx/Ni nanoparticles, which act as well-known oxygen evolution catalysts, are deposited on the surface of silicon by facile pulsed electrodeposition. Light absorption and catalytic activity are greatly affected by the coverage of Ni nanoparticles, and the highly efficient NiOx/Ni catalyst structure is induced by simple annealing. The NiOx/Ni nanoparticles show highly enhanced charge separation and transport efficiency, which are vital factors for ...

Active and robust novel bilayer photoanode architectures for hydrogen generation via direct non-electric bias induced photo-electrochemical water splitting

Photo-electrochemical (PEC) water splitting is a promising and environmentally benign approach for generation of hydrogen using solar energy with minimum greenhouse gas emissions. The development of semiconductor materials for photoanode with superior optoelectronic properties combined with excellent photoelectrochemical activity and stability is vital for the realization of viable commercial development of PEC water splitting systems. Herein, we report for the very first time, the study of nanoscale bilayer architecture of WO3 and Nb and N co-doped SnO2 nanotubes (NTs), wherein WO3 NTs are coated with (Sn0.95Nb0.05)O2:N-600 (annealed in NH3 at 600 °C) layer of different thicknesses, as a potential semiconductor photoanode material for PEC water splitting. An excellent long term photoelectrochemical stability under illumination in the acidic electrolyte solution combined with a solar-to-hydrogen efficiency (STH) of ∼3.83% (under zero applied potential) is obtained for the bilayer NTs, which is the highest STH obtained thus far, to the best of our knowledge compared to the other well studied semiconductor materials, such as TiO2, ZnO and Fe2O3. These promising results demonstrate the excellent potential of bilayer NTs as a viable and promising photoanode in PEC water splitting.