Band gap tuning to improve the photoanodic activity of ZnInxSy for photoelectrochemical water oxidation

Abstract

Photoelectrochemical (PEC) water splitting being a greener and ecofriendly pathway has become a renowned technique to generate hydrogen (H₂). To attain remarkable photoconversion efficiency, it is highly required to develop efficient photoelectrodes for PEC water splitting. For this, ternary metal chalcogenide ZnIn_xS_y (x = 1.6, 2, 2.2, and 3) is synthesized as an efficient photoanode for PEC water splitting. Tuning of morphology helps to improve the PEC performance through enhanced light absorption and charge transportation. Similarly, elemental doping is a very fruitful strategy to modulate the band structure. Here, a facile hydrothermal approach is developed to synthesize thin sheets of ZnIn_xS_y (x = 1.6, 2, 2.2, and 3) followed by calcination. Through controlling the calcination time and the indium content, the band structure and morphology of ZnIn_xS_y are modulated. The observed results indicate that ZnIn_2.2S_y has the optimum and appropriate amount of indium content and oxygen doping. ZnIn_2.2S_y can generate a maximum photocurrent density of 4.83 mA cm⁻² at ‘0.7767’ vs. RHE. Furthermore, with the help of Mott–Schottky analysis the carrier density is calculated. The calculated carrier density of ZnIn_2.2S_y is 7.886 × 10²¹ cm⁻³, which is 2.37, 1.77, and 3.69-fold higher compared to ZnIn_xS_y (x = 1.6, 2, and 3). Photoconversion efficiency (η) is direct evidence to legitimize the superiority of ZnIn_2.2S_y; it shows a maximum efficiency of 2.744% at potential 0.507 V vs. RHE. ZnIn_2.2S_y shows high stability, i.e., it can generate nearly unaltered photocurrent density for 1000 seconds. The determined band alignment of ZnIn_2.2S_y indicates the more negative shift of valence band energy compared to others, which promotes easy oxidation of H₂O to O₂.