Critical role and modification of surface states in hematite films for enhancing oxygen evolution activity

Hematite films deposited by plasma-enhanced chemical vapor deposition of iron pentacarbonyl [Fe(CO) 5 ] in an oxygen plasma were modified by postdeposition (i) oxygen plasma treatment and (ii) short annealing treatments to reduce the defects and to modify the (sub)surface states and consequently the photoelectrochemical properties. The oxygen plasma treatment resulted in the increase of particle size and augmented surface roughening by densification of grains. Moreover, it induced saturated surface states with reactive oxygen species (O − , OH − ), evident in the X-ray photoelectron spectroscopy (XPS). Under standard illumination (1.5 AM; 100 mW/cm 2 ; 150 W xenon lamp), when compared to the pristine hematite coating (0.696 mA/cm 2 at 1.23 V versus RHE and 0.74 V onset ) the oxygen plasma-treated films showed severe deterioration in photocurrent density of 0.035 mA/cm 2 and an anodic shift in the onset potential (1.10 V onset ) due to oxygen rich surface. In a second set of experiments, the oxygen plasma-treated hematite films were briefly annealed (10 min at 750 °C) and the signals of Fe 2 p and O 1 s recovered to higher binding energies, indicating the formation of oxygen vacancies. In addition, a superior photocurrent density value of max. 1.306 mA/cm 2 at 1.23 V versus RHE to that of the pristine hematite photoanode with 0.74 V onset was obtained. Transient absorption spectroscopy further elucidated that the oxygen plasma-induced electron trap states acting as recombination centers that are unfavorable for photoelectrochemical activity. The alteration in Fe:O stoichiometry and thus photocurrent density are corroborated by determination of water oxidation rates in annealed (7.1 s −1 ) and oxygen plasma treated (2.5 s −1 ) samples.