Infrared Absorption of Zn_0.5Cd_0.5S Photocatalyst Bandgap-Excited Under an Aqueous Environment

Abstract

The adoption of infrared and near-infrared spectrometry techniques for mechanistic and kinetic studies on the photocatalytic water splitting reaction has been steadily increasing over the years. Herein, a transition metal sulfide photocatalyst, Zn 0.5 Cd 0.5 S (ZCS), which has been proven to be an efficient hydrogen (H 2 ) evolution photocatalyst, was investigated through attenuated total reflection FTIR. Electronic absorption in the wavenumber region of 500−3000 cm −1 was detected, while the photocatalyst was irradiated by a UV light source in a pure water environment. The source of absorption was further characterized by employing scavenging solutions, and it was found that the contribution from photogenerated electrons and holes coincidentally appeared in the same wavenumber window. This is likely to be unprecedented as prior studies generally were not able to observe electronic absorption by holes, or the hole absorption was present close to the visible wavenumber window instead. Nevertheless, the shape of absorption peak associated with holes was noticeably distinctive from that of electrons when their normalized spectra were compared, allowing easy identification of the nature of the detected absorption. These observations suggested that electron and hole trap states were present in ZCS, and it is proposed that the trap states could be innately present due to the formation of physical defects during chemical synthesis, or they were instantaneously created upon photoexcitation where polarons were manifested by the disruption of ionic equilibrium in the valence band and conduction band through injection of foreign photogenerated electrons or holes.