Enhancement of the Solar Water Splitting Efficiency Mediated by Surface Segregation in Ti-Doped Hematite Nanorods

Band engineering is employed thoroughly and targets technologicallyscalable photoanodes for solar water splitting applications. Complexand costly recipes are necessary, often for average performances.Here, we report simple photoanode growth and thermal annealing witheffective band engineering results. By comparing Ti-doped hematitephotoanodes annealed under nitrogen to photoanodes annealed in air,we found a strongly enhanced photocurrent of more than 200% in thefirst case. Using electrochemical impedance spectroscopy and synchrotronX-ray spectromicroscopy, we demonstrate that oxidized surface statesand increased density of charge carriers are responsible for the enhancedphotoelectrochemical (PEC) activity. Surface states are found to berelated to the formation of pseudo-brookite clusters by surface Tisegregation. Spectro-ptychography is used for the first time at theTi L-3 absorption edge to isolate Ti chemical coordinationarising from pseudo-brookite cluster contribution. Correlated withelectron microscopy investigation and density functional theory calculations,the synchrotron spectromicroscopy data unambiguously prove the originof enhanced PEC activity of N-2-annealed Ti-doped hematitenanorods. Finally, we present here a handy and cheap surface engineeringmethod beyond the known oxygen vacancy doping, allowing a net gainin the PEC activity for the hematite-based photoanodes.