Intensive-visible-light-responsive ANbO2N (A = Sr, Ba) synthesized from layered perovskite A5Nb4O15 for enhanced photoelectrochemical water splitting
Ramaraj Sukanya, Jeongsuk Seo
Abstract
Nb-based, perovskite-type oxynitrides ANb(O,N)3 (A = Sr, Ba) have been significantly attractive semiconductor materials for photoelectrochemical (PEC) water splitting due to their absorption abilities of intensive-visible-light. However, PEC activities of these perovskites are relatively low due to reduction of Nb species during nitridation leading to the generation of anion defects and impurity phases. Herein, we propose nitridation of A-rich, layered perovskite A5Nb4O15 as starting oxides for general synthesis of photoactive ANbO2N. These layered perovskite A5Nb4O15 were completely transformed to single phase perovskite-type ANbO2N by nitridation. Wavelength onsets of light absorption were observed at 700 nm for SrNbO2N and at 740 nm for BaNbO2N. According to X-ray photoelectron spectroscopy results, excess Lewis base A species significantly suppressed the generation of reduced species such as Nb4+ and Nb3+ during nitridation, although it led to an amorphous surface of the as-prepared oxynitride. Subsequent annealing in Ar largely enhanced surface crystallinity of ANbO2N. As a result, Co(OH)x/ANbO2N/FTO photoanodes, prepared by loading Co(OH)x electrocatalyst, showed high photocurrent density of 1.6 (A = Sr) and 2.4 mA cm−2 (A = Ba) at 1.23 VRHE under AM 1.5 G simulated sunlight. These results demonstrate that A-rich layered perovskite A5Nb4O15 are effective starting precursors for preparing low-defective ANbO2N, thus improving PEC water splitting activity.