Synthesis and characterization of a photosensitive interface for hydrogen generation: Chemically modified p-type semiconducting silicon photocathodes

Spontaneous Formation of >90% Optically Transmissive, Electrochemically Active CoP Films for Photoelectrochemical Hydrogen Evolution

Earth-abundant catalysts for the hydrogen-evolution reaction require increased mass loadings, relative to Pt films, to achieve comparable activity and stability in acidic electrolytes. We report herein that spontaneous nanostructuring of opaque, electrodeposited CoP films, 40-120 nm in thickness, leads to transparent electrocatalyst films that exhibit up to 90% optical transmission in the visible spectrum. The photocurrent density under simulated sunlight at a representative n⁺p-Si(100)/CoP photocathode increases by 200% after exposure to 0.50 M H₂SO₄(aq) and remains stable for 12 h of continuous operation. Atomic force microscopy and scanning electron microscopy of the film before and after exposure to 0.50 M H₂SO₄(aq) validate an optical model for transparent CoP films as probed with spectroscopic ellipsometry.

Dynamic Photoelectrochemical Device Using an Electrolyte-Permeable NiO x/SiO2/Si Photocathode with an Open-Circuit Potential of 0.75 V

As a thermodynamic driving force obtained from sunlight, the open-circuit potential (OCP) in photoelectrochemical cells is typically limited by the photovoltage ( V_ph). In this work, we establish that the OCP can exceed the value of V_ph when an electrolyte-permeable NiO _x thin film is employed as an electrocatalyst in a Si photocathode. The built-in potential developed at the NiO _x/Si junction is adjusted in situ according to the progress of the NiO _x hydration for the hydrogen evolution reaction (HER). As a result of decoupling of the OCP from V_ph, a high OCP value of 0.75 V (vs reversible hydrogen electrode) is obtained after 1 h operation of HER in an alkaline electrolyte (pH = 14), thus outperforming the highest value (0.64 V) reported to date with conventional Si photoelectrodes. This finding might offer insight into novel photocathode designs such as those based on tandem water-splitting systems.

Photoelectrochemical water splitting at semiconductor electrodes: fundamental problems and new perspectives

Some fundamental aspects of light-driven water splitting at semiconductor electrodes are reviewed along with recent experimental and theoretical progress. The roles of thermodynamics and kinetics in defining criteria for successful water-splitting systems are examined. An overview of recent research is given that places emphasis on new electrode materials, theoretical advances and the development of semi-quantitative experimental methods to study the dynamics of light-driven water-splitting reactions. Key areas are identified that will need particular attention as the search continues for stable, efficient and cost-effective light-driven photoelectrolysis systems that exploit electron/hole separation in semiconductor/electrolyte junctions.

Silicon nanowire array/Cu2O crystalline core-shell nanosystem for solar-driven photocatalytic water splitting

P-type Cu2O nanocrystals were deposited on n-type silicon nanowire arrays (Si NWs) to form core-shell heterojunction arrays structure via a simple electroless deposition technique. Scanning electron microscopy, transmission electron microscope and x-ray diffraction were utilized to characterize the morphology and structure of the core-shell nanosystem. The reflectivity of the obtained core-shell structure measured by UV/vis spectrometry showed a comparatively low reflectivity in the visible-light region, which implied good optical absorption performance. The water splitting performance of the obtained Si NWs, planar Si/Cu2O structure and Si NW/Cu2O core-shell nanosystem were studied. Owing to the large specific surface area, heterojunctions formed between Cu2O nanocrystallites and Si NWs and the light trapping effect of the NW array structure, the photocatalytic performance of the Si NW/Cu2O core-shell nanosystem increased markedly compared with that of pure silicon NWs and a planar Si/Cu2O structure, which means excellent hydrogen production capacity under irradiation with simulated sunlight. In addition, the photocatalytic performance of the core-shell nanosystem was improved obviously after platinum nanoparticles were electrodeposited on it.

Silicon nanowires/reduced graphene oxide composites for enhanced photoelectrochemical properties

The top of silicon nanowires (SiNWs) arrays was coated with reduced graphene oxide (rGO) by the facile spin-coating method. The resulting SiNWs/rGO composite exhibits enhanced photoelectrochemical properties, with short-circuit photocurrent density more than 4 times higher than that of the pristine SiNWs and more than 600 times higher than that of planar Si/rGO composite. The trapping and recombination of photogenerated carriers at the surface state of SiNWs were reduced after the application of rGO. The results of electrochemical impedance spectroscopy measurements suggest that the reduction of trapping and recombination of photogenerated carriers as well as remarkably enhancement of photoelectrochemical properties can be attributed to the low charge transfer resistance at the SiNWs-rGO interface and rGO-electrolyte interface. The method and results shown here indicate a convenient and applicable approach to further exploitation of high activity materials for photoelectrochemical applications.

Metal on metal oxide nanowire Co-catalyzed Si photocathode for solar water splitting

We report a systematic study of Si|ZnO and Si|ZnO| metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n(+)-ZnO) and Si|ZnO(N(2)) (thin films prepared in N(2)/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.

3D branched nanowire heterojunction photoelectrodes for high-efficiency solar water splitting and H2 generation

We report the fabrication of a three dimensional branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. Specifically, we demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature. The branching nanowire heterostructures offer direct functional integration of different materials for high efficiency water photoelectrolysis and scalable photoelectrodes for clean hydrogen fuel generation.

Enhanced photoelectrochemical hydrogen production from silicon nanowire array photocathode

Herein we report that silicon nanowires (SiNWs) fabricated via metal-catalyzed electroless etching yielded a photoelectrochemical hydrogen generation performance superior to that of a planar Si, which is attributed to a lower kinetic overpotential due to a higher surface roughness, favorable shift in the flat-band potential, and light-trapping effects of the SiNW surface. The SiNW photocathode yielded a photovoltage of 0.42 V, one of the highest values ever reported for hydrogen generation on p-type Si/electrolyte interfaces.

Control of the Stability, Electron-Transfer Kinetics, and pH-Dependent Energetics of Si/H2O Interfaces through Methyl Termination of Si(111) Surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.