Activating BaTaO2N by Ca modifications and cobalt oxide for visible light photocatalytic water oxidation reactions

Despite a strong visible light absorbance as far as 660 nm, BaTaO2N generally exhibits a poor photocatalytic activity for water splitting under ordinary conditions. High levels of defects as well as photocatalytic self-decomposition are likely the major causes for this incommensuration. In this work, we have modified BaTaO2N by introducing Ca to the structure, i.e. BaCax/3Ta1-x/3O(2+y)N(1-y) (0 <= x, y <= 1). A number of important properties such as band gap, defects levels and nitrogen content can be tuned by varying the amounts of Ca in the structure. In particular, defects such as Ta4+ species can be effectively suppressed by Ca incorporation. More importantly, photocatalytic activity for water oxidation has been significantly enhanced and stability against photocatalytic self-decomposition has also been largely improved in Ca modified BaTaO2N. An apparent quantum efficiency as high as similar to 2.1% at 420 +/- 20 nm has been achieved and stands as the highest AQE for BaTaO2N reported to date. Photoelectrochemical (PEC) analysis reveals a much higher photocurrent in Ca modified BaTaO2N and identifies the role of cocatalyst CoOz in reducing charge transfer resistance for water oxidation reactions. Open-circuit voltage decay (OCVD) measurements further confirm the improved charge separation conditions in Ca modified BaTaO2N and functions of cocatalyst CoOz in collecting and storing photo-generated holes. The optimal loading point for cocatalyst CoOz corresponds to the one that gives the highest capacity for hole storage.