Interfacial insight in multi-junction metal oxide photoanodes for water splitting applications

Photoelectrochemical (PEC) properties of nanostructured hematite (Fe2O3) thin films prepared using plasma-enhanced chemical vapor deposition (PE-CVD) were investigated against the influence of processing parameters and post-synthesis heat-treatment procedures. Annealing at high temperatures (>500 °C) was found to substantially affect the micro-structure (grain growth and densification) and electronic (interdiffusion at the film/substrate interface) concomitantly manifested in an enhancement in the PEC behavior. The Sn impurity level in hematite films was found to increase with the annealing temperature with highest values achieved in samples heat-treated at 750 °C, due to the interdiffusion and substitution of Sn(IV) species at Fe(III) sites. Sn:Fe2O3 films exhibited significantly high photocurrent density of 1.33 mA cm−2 at the water oxidation level of 1.23 V vs. RHE. The diffusion of Sn ions into iron oxide lattice altered the electronic properties of hematite films due to electron-donor behavior of the dopants that was verified by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy (SIMS) analyses. Deposition of a thin overlayer of TiO2 (10 nm) on hematite films by atomic layer deposition (ALD) was found to further improve the photocurrent density to 1.8 mA cm−2 at 1.23 V vs. RHE. Ab-initio calculations on the effect of substitutional Sn(IV) dopants in the Fe2O3 lattice on the electronic structure and the band alignment between hematite and the TiO2 over layer revealed that Sn-dopants led to the generation of localized Fe(II) centers augmenting the n-type behavior of hematite. No effect of the Sn-doping on the electrostatic potential was found on a macroscopic scale. However, the charge transfer from the Sn-doping to the Fe(II) centers would cause high electric fields on the nanometer scale and might hence play an important role in the efficient separation of electron and holes. The simulations showed that the hematite band edges are enclosed by the TiO2 band edges and therefore electron depletion at the surface-liquid interface is enhanced. This might lead to reduced recombination rates near the surface and consequently to increased photocurrents, since the Fe2O3/TiO2 interface constitutes a barrier for hole transport.