Hematite photoanodes for solar water splitting: Directly sputtered vs. anodically oxidized sputtered Fe

Abstract

Hematite iron oxide has been extensively studied for photoelectrochemical (PEC) water splitting. Nanostructuring of hematite-based photoanodes represents an effective strategy to supress the negative impact of a short diffusion length of photoexcited holes on the PEC performance. Here we present a comparative structural and photoelectrochemical study of hematite photoanodes fabricated in the forms of two-dimensional (2D) very thin (∼25nm) nanocrystaline films and one-dimensional (1D) nanostructures including nanotubes and nanorods. Hematite films on fluorine-doped tin oxide (FTO) coated glass were prepared by two methods (i) by reactive high-power impulse magnetron sputtering (HiPIMS) and (ii) by anodic oxidation of Fe films deposited on FTO by HiPIMS. While in the first case very thin, dense, compact hematite films were deposited, the second approach yielded transparent nanotubular or nanorod hematite nanostructures. In both cases, the photoelectrochemical response was crucially influenced by the post thermal treatment at 750°C resulting in the Sn4+ diffusion from the FTO substrate and the improvement of conductivity across the FTO/Fe2O3 interface. Fe2O3 films exhibit a photocurrent onset at potential 1.1V (RHE) with almost linear increase of photocurrent with applied potential. The highest photocurrents were obtained for planar thin hematite electrodes prepared directly by HIPIMS technique (0.55mAcm−2 at 0.5V vs. Ag/AgCl). The observed minimal bias for photoelectrochemical water splitting with hematite photoanode was 1.25V. For applied potential 0.25V (vs. Ag/AgCl) and bias 1.3V, the observed photocurrent density and hydrogen production rate was 0.305mA/cm2 and 5.8μmol/h/cm2, respectively.