Suppressed charge recombination in hematite photoanode via protonation and annealing
Hematite as promising photoanode for solar water splitting suffers from severe bulk and surface charge recombination. This work describes that a protonation-annealing treatment can effectively suppress both bulk and surface charge recombination in hematite. Protons/electrons are electrochemically incorporated into hematite under 0.2 VRHE followed by annealing at 120 oC. The photocurrent density increases from ~0.9 mA cm-2 to 1.8 mA cm-2 at 1.23 VRHE under 1 sun, and further to 2.7 mA cm-2 after loading cobalt phosphate, stabilizing at round 2.4 mA cm-2. A cathodic shift of the onset potential of photocurrent is also observed. H2O2 oxidation, impedance spectroscopy and Mott-Schottky measurements show that the protonation suppresses bulk recombination and enhances donor density, but introducing more surface recombination. The annealing reduces surface recombination, while preserving relatively high bulk charge separation efficiency. Different from previous reports on the electrochemically reduced hematite, this work demonstrates that the performance improvement should be ascribed to the proton incorporation instead of the formation of Fe3O4 or metal Fe. This facile treatment by protonation and annealing could be applied in other semiconductors to promote the development of high performing photoelectrodes.