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Mg modifications over BaTaO 2 N reduce Ta4+ defects concentration, contributing to much improved photocatalytic activities for water oxidation reactions. • Mg modifications strengthen Ta-O/N bonds in BaTaO 2 N. • Ta4+ defects concentration decreases in Mg modified BaTaO 2 N. • Photocatalytic performance is much improved after Mg modifications. • Band edges are positively shifted in Mg modified BaTaO 2 N. • Mg contributes to improved interactions between BaTaO 2 N and CoO x. BaTaO 2 N owns a promising band gap of 1.8 eV and suitable band edge positions but generally exhibits poor photocatalytic activity for water splitting reactions. In this work, we have modified BaTaO 2 N by introducing Mg to the B site of perovskite lattice to form a new compound, i.e. BaTa 0.95 Mg 0.05 O 2+x N 1−y. This modification results in an alteration to a number of important parameters such as unit cell size, nitrogen content, optical absorption and microstructures etc. More importantly, Mg modifications are responsible for stronger Ta-O/N bonds, lower Ta4+ defects concentration and positively shift of band edge positions, which in turn contribute to high photocatalytic activities for water oxidation reactions. Apparent quantum efficiency as high as 2.59% has been realized at 420 ± 20 nm, being the highest for BaTaO 2 N reported to date under similar conditions. Further photoelectrochemical analysis confirms better charge utilizations in Mg modified BaTaO 2 N. [ABSTRACT FROM AUTHOR]

Although BaNbO2N has visible light absorption as far as 740 nm, its photocatalytic activity is generally very poor under ordinary conditions, being incommensurate to its light absorption properties. In this work, we introduce Ca into the B site of BaNbO2N to form a complex perovskite BaNb1−x/3Cax/3O2+yN1−y (0 ≤ x, y ≤ 1). The presence of Ca in BaNbO2N has great impact on a number of important properties such as band gap value, nitrogen content, surface hydrophilicity, and defects levels. In particular, defects such as Nb4+ species are effectively suppressed in Ca‐modified BaNbO2N. More importantly, photocatalytic activity of BaNbO2N for water oxidation reactions is substantially improved after Ca modifications. Compared with pristine BaNbO2N, more than twofold enhancement in photocatalytic oxygen evolution is observed for BaNb0.8Ca0.2O2+yN1−y (x = 0.6). Such improvements probably stem from the suppression of Nb4+ species in the structure as well as enhanced surface hydrophilicity after Ca modification. The facile approach by incorporating alkaline earth cations into crystal structure can be well extended to other perovskite oxynitrides whose photocatalytic activity is subject to those defective species. [ABSTRACT FROM AUTHOR]

Despite a strong visible light absorbance as far as 660 nm, BaTaO 2 N 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 BaTaO 2 N by introducing Ca to the structure, i.e. BaCa x/3 Ta 1-x/3 O 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 Ta 4+ 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 BaTaO 2 N. An apparent quantum efficiency as high as ∼ 2.1% at 420 ± 20 nm has been achieved and stands as the highest AQE for BaTaO 2 N reported to date. Photoelectrochemical (PEC) analysis reveals a much higher photocurrent in Ca modified BaTaO 2 N and identifies the role of cocatalyst CoO z 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 BaTaO 2 N and functions of cocatalyst CoO z in collecting and storing photo-generated holes. The optimal loading point for cocatalyst CoO z corresponds to the one that gives the highest capacity for hole storage. [ABSTRACT FROM AUTHOR]

Introducing Mg into the B site of CaNbO 2 N helps to decrease defects levels, enhance charge mobility and separation, positively shift valence band maximum, all of which contributes to improved photocatalytic activity for water oxidation reactions. • CaNbO 2 N can be modified by Mg with slight unit cell shrinkage. • Band gap, nitrogen content, defects level can be tuned by Mg modifications. • Nb4+ defects levels in CaNbO 2 N are suppressed after Mg modifications. • Mg modifications improve photocatalytic activity of CaNbO 2 N. • Charge mobility and separation is improved by Mg modifications. In spite of a strong visible light absorbance as far as 600 nm, CaNbO 2 N generally exhibits poor photocatalytic activity under normal conditions because of various structural defects and poor charge mobility. In this work, we have synthesized a series of Mg-modified CaNbO 2 N, i.e. CaNb 1-x Mg x O 2+y N 1-y (0 ≤ x ≤ 0.2), and performed a detailed investigation on their crystal structures, optical absorption and other physicochemical properties. Our results show that there is a slight shrinkage of the unit cell and a blue-shift of absorption edges upon Mg incorporation into CaNbO 2 N. The nitrogen contents as well as defects levels can be effectively tuned by altering the content of Mg. More strikingly, photocatalytic oxygen productions are much improved after Mg modifications under visible light irradiation (λ ≥ 420 nm). An average oxygen production rate as much as ∼126.8 umol h−1 and an apparent quantum efficiency as high as ∼3.4 % at 420 ± 20 nm is achieved for CaNb 0.9 Mg 0.1 O 2+y N 1-y (x = 0.1). These improvements probably stem from a substantial decrease of Nb4+ defects in CaNbO 2 N as well as slight positive shift of valence band maximum (VBM) after Mg modifications. Meanwhile, photoelectrochemical analysis suggests charge migration is somewhat enhanced in response to Mg modifications. [ABSTRACT FROM AUTHOR]