Si nanostructure and hematite/Si nanowire application for Photoelectrochemical Water Splitting

Abstract

As fossil fuels are being exhausted and their burning contributes to the global warming, it is urgently needed to develop clean and renewable sources of energy. Harnessing renewable energy sources to provide future sources of energy for the planet is an area in which intense research efforts are underway. Photoelectrochemical (PEC) water splitting represents a promising and environmentally benign method for solar hydrogen generation, and has been studied for decades. More importantly, water splitting is a clean reaction that would not produce undesired carbon-based byproducts. Nanostructures of silicon (Si), which remains the most important material for current semiconductor industry, are well-documented as promising building blocks for devices in the fields of nanoelectronics, opto-electronics, energy conversion, and energy storage, as well as bio- and chemical sensors. Characteristic parameters, such as crystalline orientation, crystalline quality, strain, orientation relative to the substrate and size affect the properties of Si nanostructures and are thus important for their application in PEC Hematite (α-Fe2O3) has emerged as a promising photo-electrode material due to its favorable optical band gap, extraordinary chemical stability in oxidative environment and abundance, and low cost. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties, such as short excited-state lifetime, poor absorbtivity, poor oxygen evolution reaction kinetics and short hole diffusion length. To improve the PEC performance of hematite photoanodes, nanostructures and doping during hematite material growth have been commonly used to enhance facile post-growth doping of hematite and its impact on the electrical and surface properties and thus the PEC performance of hematite have been rarely investigated. Hematite (α-Fe2O3) was grown on vertically aligned Si nanowires (NWs) using deposition to form a dual-absorber system. Compared with hematite-only photoelectrodes, those with Si NWs exhibited a photocurrent turn-on potential as low as 0.6V vs RHE. This result represents one of the lowest turn-on potentials observed for hematite-based PEC water splitting systems. It addresses a critical challenge of using hematite for PEC water splitting, namely, the fact that the band-edge positions are too positive for positive for high-efficiency water splitting.