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 be overlooked in the preparation of PbCrO4 films and unrevealed in the researches of PbCrO4 photoanodes. Herein, monoclinic PbCrO4 films with few Cr defects (PbCrO4-FVCr) were prepared on FTO substrate through covering the drop-coating Pb2+/Cr3+ precursor solution to reduce the loss of Cr during thermal treating. Relative to the monoclinic PbCrO4 films with rich Cr defects (PbCrO4-RVCr), higher solar water oxidation activity was achieved on the PbCrO4-FVCr 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-FVCr film photoanodes, which is two-fold to the PbCrO4-RVCr film photoanodes (0.55 mA cm-2). Meanwhile, the PbCrO4-FVCr film photoanodes had faster water oxidation kinetics than PbCrO4-RVCr film photoanodes. The water oxidation rate constant (kO2) on the PbCrO4-FVCr film photoanodes was 40.6 s-1, while a lower kO2 of 8.2 s-1 on the PbCrO4-RVCr film photoanodes. Experimental detections and theoretical calculations jointly revealed that PbCrO4 films with fewer Cr defects being of higher solar water oxidation activity can be attributed to following reasons: (i) the presence of Cr defects can result in the formation of deep energy level in PbCrO4, which is unfavorable to carrier transfer in the bulk of PbCrO4 films; (ii) the presence of Cr defects can cause the formation of unsaturated O dangling bonds on PbCrO4, which are harmful traps for carrier separation on the surface of PbCrO4 films; (iii) Cr has a half-filled 3d5 orbital to provide active sites for reaction, the presence of Cr defects weakens the catalytic activity of PbCrO4 for water oxidation, and turns the rate-limiting step to the dehydrogenation of *OH into *O which requires a high energy barrier of 1.88 eV. The present work provides insight into monoclinic PbCrO4 film photoanodes from the relevance of preparation condition, Cr defects, water oxidation activity and reaction mechanism.