Revealing the Role of TiO₂ Surface Treatment of Hematite Nanorods Photoanodes for Solar Water Splitting

Ultrathin TiO₂ is deposited on conventional hydrothermal grown hematite nanorod arrays by atomic layer deposition (ALD). Significant photoelectrochemical water oxidation performance improvement is observed when the ALD TiO₂-treated samples are annealed at 650 °C or higher temperatures. The electrochemical impedance spectroscopy (EIS) study shows a surface trap-mediated charge transfer process exists at the hematite-electrolyte interface. Thus, one possible reason for the improvement could be the increased surface states at the hematite surface, which leads to better charge separation, less electron-hole recombination, and hence, greater improvement of photocurrent. Our Raman study shows the increase in surface defects on the ALD TiO₂-coated hematite sample after being annealed at 650 °C or higher temperatures. A photocurrent of 1.9 mA cm^-2 at 1.23 V (vs RHE) with a maximum of 2.5 mA cm^-2 at 1.8 V (vs RHE) in 1 M NaOH under AM 1.5 simulated solar illumination is achieved in optimized deposition and annealing conditions.