AgInS₂-Embedded Photocatalytic Membrane: Insights into the Excited State and Electron Transfer Dynamics

Photocatalytic reactions at semiconductor nanocrystal surfaces are useful for synthesizing value-added chemicals using sunlight. Semiconductor nanocrystals dispersed in a rigid framework, such as polymer film, can mitigate issues such as aggregation, product separation, and other challenges that are usually encountered in suspensions or slurries. Using a cation exchange technique, we successfully embedded AgInS₂ nanoparticles into a Nafion matrix, termed AgInS₂-Nafion. This was achieved through a galvanic exchange between In and Ag in In₂S₃ present within the Nafion film, enabling an adjustable Ag:In ratio for optimized photophysical properties. As in the case of colloidal suspension, the AgInS₂ particles embedded in Nafion exhibit a long absorption tail, a broad emission band with a large Stokes shift, and emission lifetimes extending into the microseconds that are characteristic of donor-acceptor pairs, DAP. Remediation of surface states with the treatment of 3-mercaptopropionic acid resulted in significant enhancement in the emission yield. Charge carrier generation through bandgap excitation as well as activation of DAP states which reside within the bandgap is probed through transient absorption spectroscopy. The photocatalytic activity of AgInS₂-Nafion was probed by using thionine as an electron acceptor. The electron transfer rate constant from excited AgInS₂ to thionine as observed from transient absorption spectroscopy was determined to be ∼6.3 × 10¹⁰ s^-1. The design of a photoactive membrane offers new ways to carry out photocatalytic processes with greater selectivity.