Anion engineering of exfoliated CoAl layered double hydroxides on hematite photoanode toward highly efficient photoelectrochemical water splitting

Abstract The exploration of highly efficient oxygen evolution cocatalysts is believed to be an efficient strategy to enhance the photoelectrochemical (PEC) water splitting of hematite (α-Fe 2 O 3 ) photoanode. Herein, exfoliated CoAl layered double hydroxides (CoAl-LDHs) were successfully decorated on α-Fe 2 O 3 films by using layer-by-layer assembly method with the assistance of common anions (A n− ), such as NO 3 − , CO 3 2− , SO 4 2− and PO 4 3− . Results indicated A n− -CoAl-LDHs/α-Fe 2 O 3 photoanodes exhibited strong anion-dependent performance for PEC water oxidation. The NO 3 − , CO 3 2− , SO 4 2− and PO 4 3− engineered CoAl-LDHs/α-Fe 2 O 3 displayed the photocurrent density at 1.23 V in 1 mol L −1 KOH of 1.90, 2.36, 3.19 and 4.30 mA cm −2 , which were ca. 2.5, 3.1, 4.2 and 5.6 times that of bare α-Fe 2 O 3 (0.76 mA cm −2 ) respectively. Experimental studies indicated tetrahedral PO 4 3− and SO 4 2− were much more feasible for the assembly of CoAl-LDHs than that of triangular NO 3 − and CO 3 2− , thus caused higher electrochemical surface area (ECSA). The anion with higher charge number and the tetrahedral configuration were more beneficial for the charge separation in bulk α-Fe 2 O 3 and the charge transfer between CoAl-LDHs slabs. Mott-Schottky tests suggested p-n heterojunctions were formed at the interface of α-Fe 2 O 3 and CoAl-LDHs, which could inhibit the surface charge recombination and further increase charge separation. As a result, PO 4 3− -CoAl-LDHs/α-Fe 2 O 3 displays the best performance for PEC water oxidation. The strategy of engineering exfoliated CoAl-LDHs with anions on α-Fe 2 O 3 in this work will stimulate us to fabricate highly efficient photoanodes with various anions and LDHs materials.