Effect of Si Doping and Porosity on Hematite's (α-Fe2O3) Photoelectrochemical Water Oxidation Performance

The photoelectrochemical water oxidation performance under simulated solar irradiation of hematite (alpha-Fe2O3) films synthesized by coevaporation of pure Si and Fe in an oxygen ambient, a process known as reactive ballistic deposition, is studied as a function of Si doping level and film porosity, ranging from dense films to nanocolumnar films. It is found that Si segregates to the hematite surface, does not improve the bulk conductivity, and lowers the optical absorption coefficient. Nevertheless, the photoelectrochemical performance of Si-doped, porous films is significantly improved relative to undoped, porous films. However, the improvement relative to dense, undoped films is marginal. It is concluded that Si acts to passivate the hematite surface and aids charge transfer to the solution. Additionally, from incident photon conversion efficiency measurements it is found that Si doping and porosity have little effect on the normalized spectral response of 100 nm thick hematite films.