Photoelectrochemical performance and ultrafast dynamics of photogenerated electrons and holes in highly titanium-doped hematite

Abstract

Elemental doping of hematite has been widely performed to improve its mobility, electrical conductivity as well as to suppress electron–hole recombination in photoelectrochemical applications. When hematite is doped with high titanium concentrations, above 5%, pseudobrookite layers may be formed as overlayers leading to improved photocurrent while further doping beyond 15% could lead to the formation of a titania overlayer which has an effect of suppressing photocurrent. In this study, we observed that doping hematite with titanium improves photocurrent, reaching a maximum of 1.83 mA cm⁻² at a titanium concentration of 15%, the highest achieved photocurrent with spin coating method. Further titanium incorporation to 20% resulted in a decrease of the photocurrent. XRD measurements shows that a Fe₂TiO₅ layer formed at 15% Ti concentration which resulted in the observed increase in photocurrent while a reduction in photocurrent at 20% Ti concentration could have resulted from the formation of a TiO₂ layer. Analysis of the transient absorption spectroscopy data was achieved using a four-component sequential analysis scheme in the Glotaran software. We observed major doping concentration dependent lifetimes in the τ₃ and τ₄ values where the 15% doped samples had the slowest recombination rates. We also observed a blueshift in the spectra with increasing doping concentration, suggesting the occurance of the Burstein–Moss effect. This work shows that doping hematite with titanium leads to structural changes of the photoanodes at Ti concentrations of over 10%, in addition to the well documented conductivity enhancement.