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

The exploration of highly efficient oxygen evolution cocatalysts is believed to be an efficient strategy to enhance the photoelectrochemical (PEC) water splitting of hematite (alpha-Fe2O3) photoanode. Herein, exfoliated CoAl layered double hydroxides (CoAl-LDHs) were successfully decorated on alpha-Fe2O3 films by using layer-by-layer assembly method with the assistance of common anions (An-), such as NO3-, CO32-, SO42- and PO43-. Results indicated An--CoAl-LDHs/alpha-Fe2O3 photoanodes exhibited strong anion-dependent performance for PEC water oxidation. The NO3-, CO32-, SO(4)(2-)and PO43- engineered CoAl-LDHs/alpha-Fe2O3 displayed the photocurrent density at 1.23 V in 1 mol L-1 KOH of 1.90, 2.36, 3.19 and 4.30mAcm(-2), which were ca. 2.5, 3.1, 4.2 and 5.6 times that of bare alpha-Fe2O3 (0.76mAcm(-2)) respectively. Experimental studies indicated tetrahedral PO43- and SO42- were much more feasible for the assembly of CoAl-LDHs than that of triangular NO3- and CO32-, 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 alpha-Fe2O3 and the charge transfer between CoAl-LDHs slabs. Mott-Schottky tests suggested p-n heterojunctions were formed at the interface of alpha-Fe2O3 and CoAl-LDHs, which could inhibit the surface charge recombination and further increase charge separation. As a result, PO43--CoAl-LDHs/alpha-Fe2O3 displays the best performance for PEC water oxidation. The strategy of engineering exfoliated CoAl-LDHs with anions on alpha-Fe2O3 in this work will stimulate us to fabricate highly efficient photoanodes with various anions and LDHs materials.