High-Throughput Screening and Surface Interrogation Studies of Au-Modified Hematite Photoanodes by Scanning Electrochemical Microscopy for Solar Water Splitting

Au-modified hematite photoanode was screened for photoelectrochemical (PEC) water oxidation by the scanning electrochemical microscopy (SECM) technique with a scanning probe of the optical fiber for visible light irradiation of the photoanode substrate. The Au-modified hematite exhibited an enhancement in the photo-current up to 3% (at. %), and the performance drop was observed with 4-10% (at. %) of Au modification. Subsequently, pristine and Au-modified hematite thin-film photoanodes were fabricated by the spin-coating method to confirm the results of SECM. The PEC response confirms that 3% (at. %) of Au is the optimum concentration to provide the best enhancement of PEC water oxidation with a similar to 6-fold increase compared to the pristine hematite sample. Direct Au oxidation, charge recombination, and strong light absorption by Au are responsible for the decrease in PEC performance when the Au percentage is above 3%. The pristine and Au-modified hematite materials were also characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Au was found to exist in the form of embedded metallic nanoparticles in the modified hematite. Mott-Schottky analysis of the bulk samples confirms an improvement in charge carrier density for the Au-modified hematite. Additionally, there was little plasmonic enhancement as evidenced by UV-vis spectroscopy, with a minimal contribution toward photoactivity. Surface interrogation SECM quantitatively probed the reactive surface states (RSSs) such as OH center dot formed on hematite and Au-modified hematite surfaces during water oxidation. The coverage of RSSs was found to increase with the substrate potential. The interrogated charge under the dark condition for the 3% Au-modified hematite sample is higher than the pristine hematite sample because of the enhanced electronic conductivity of the hematite film.