The relevance of Cr defects and photoelectrochemical water oxidation activity of monoclinic PbCrO4 films

Abstract

The presence of Cr defects in monoclinic PbCrO₄ is closely related to its solar water oxidation activity, which has been overlooked in the preparation of PbCrO₄ films and remains unexplored in research on PbCrO₄ photoanodes. Herein, monoclinic PbCrO₄ films with few Cr defects (PbCrO₄-F_VCr) were prepared on FTO substrate by drop-coating Pb²⁺/Cr³⁺ precursor solution to reduce the loss of Cr during thermal treatment. Relative to the monoclinic PbCrO₄ films with rich Cr defects (PbCrO₄-R_VCr), higher solar water oxidation activity was achieved using the PbCrO₄-F_VCr films as photoanodes. At 1.23 V vs. RHE, a higher water oxidation photocurrent of 1.13 mA cm⁻² was produced on the PbCrO₄-F_VCr film photoanodes, which is twice that of the PbCrO₄-R_VCr film photoanodes (0.55 mA cm⁻²). Meanwhile, the PbCrO₄-F_VCr film photoanodes had faster water oxidation kinetics than PbCrO₄-R_VCr film photoanodes. The water oxidation rate constant (k_O2) on the PbCrO₄-F_VCr film photoanodes was 40.6 s⁻¹, while a lower k_O2 of 8.2 s⁻¹ was observed on the PbCrO₄-R_VCr film photoanodes. Experimental and theoretical analyses jointly revealed that PbCrO₄ films with fewer Cr defects have higher solar water oxidation activity for the following reasons: (i) the presence of Cr defects can result in the formation of deep energy levels in PbCrO₄, which are unfavorable for carrier transfer in the bulk of PbCrO₄ films; (ii) the presence of Cr defects leads to the formation of unsaturated O dangling bonds on PbCrO₄, which act as detrimental traps that hinder carrier separation at the surface of PbCrO₄ films; (iii) Cr has a half-filled 3d⁵ orbital to provide active sites for reaction, so the presence of Cr defects weakens the catalytic activity of PbCrO₄ for water oxidation, and the dehydrogenation of *OH into *O becomes the rate-limiting step, which requires a high energy barrier of 1.88 eV. The present work provides insights into monoclinic PbCrO₄ film photoanodes in terms of preparation conditions, Cr defects, water oxidation activity and reaction mechanism.