Optical and structural properties of hematite for photoelectrochemical water splitting

Abstract

The continuous energy and climate change issues has motivated the urgent need to explore and develop green and renewable energy technologies, worldwide. Photoelectrochemical (PEC) water splitting offers great potential to convert water into solar fuels using sunlight. Hematite is a prospective photoanode material for solar water oxidation half reaction because of its favorable optical energy band gap of 2.1 eV. However, its practical performance is greatly limited by low efficiencies, mainly due to rapid recombination of the photogenerated electron-hole pairs. To address the problem of high charge recombination, many research efforts have been adopted such as morphology control, doping and heterostructuring. Doping remains one of the effective strategies to suppress charge recombination through increasing conductivity and charge mobility. Among several investigated dopants, titanium substitution of Fe has been shown to effectively enhance the performance of hematite. In this study, we investigated Ti-doped hematite thin films prepared by a simple and cost effective chemical solution methods, varying the Ti concentration from 0.1 at% to 20 at%. A thin film photoanode produced by spin coating showed a significantly enhanced photoelectrochemical performance with increased photocurrent density and reduced current onset potentials compared to pristine hematite. We also determined the effect of electrochemical oxidation (anodization) on microstructural properties at the surface of pristine hematite photoanode under PEC environment conditions. Anodization is considered an operational step in the functionality of a photoelectrochemical cell and may have influence of functionality of the photoanode in a PEC cell. Hematite thin film were anodized at 500 and 700 mV versus Ag/AgCl in KOH under illumination, for various anodization times. XRD diffractometry revealed an increase in the average crystallite size upon anodization. Microscopic surface scanning analyses also revealed an increase in the average particle size at the surface upon anodization. The increased particle size, particularly at longer exposure times, could contribute to poor photoelectrochemical effect due to increased minority carrier diffusion distances. This dissertation is one of the first steps into an attempt to investigate the effect of anodization on the microstructural properties of hematite in a PEC cell, for solar energy conversion applications.