Hematite photoanodes with size-controlled nanoparticles for enhanced photoelectrochemical water oxidation

The synthesis of hematite photoanodes with size-controlled nanoparticles is challenging due to the difficulty in finding the appropriate morphology-directing agents. In this paper, Ba²⁺ and Sr²⁺ ions were successfully employed to synthesize hematite photoanodes with size-controlled nanoparticles using a facile chemical bath deposition method. The synthesized electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectrometry (ICP-OES), (photo)electrochemical impedance spectroscopy ((P)EIS), X-ray photoelectron spectroscopy (XPS) and intensity modulated photocurrent spectroscopy (IMPS) measurements. It was found that the average diameter of hematite nanoparticles is decreased by a factor of 13% and 35% by the addition of Ba²⁺ and Sr²⁺ ions into the chemical bath, respectively. In agreement with these results, the electroactive surface area of Ba- and Sr-modified hematite electrodes increased by 2.4 and 3.2 times, respectively, in comparison with that of bare hematite. The photoelectrochemical measurements under the standard illumination conditions revealed that the generated photocurrent at 1.23 V vs. RHE on Ba- and Sr-modified hematite photoanodes is 2.6 and 3 times higher than that of bare hematite, respectively. Moreover, the photocurrent onset potential for water oxidation on Ba- and Sr-modified hematite photoanodes was shifted cathodically by about 150 and 220 mV, respectively. Based on these results, a good correlation between the photocurrent and the electroactive surface area was observed at high bias potential (i.e., 1.23 V vs. RHE) evincing that the photocurrent enhancement can be readily attributed to the enhanced electroactive surface area. The cathodic shift in the photocurrent onset potential of water oxidation was elucidated, however, by the higher density of surface states and the higher rate constant of charge transfer as proven by the PEIS and IMPS measurements, respectively.