Composition-Dependent Optical Band Bowing, Vibrational, and Photochemical Behavior of Aqueous Glutathione-Capped (Cu, Ag)–In–S Quantum Dots

A mild aqueous synthesis of colloidal 2-4 nm (Cu,Ag)-In-S (CAIS) quantum dots (QDs) stabilized by surface metal complexes with glutathione was introduced. Linear variations of the interplanar distances as well as of the characteristic Ag(Cu)-S-related Raman vibrational frequencies of CAIS QDs with increasing copper content show such QDs to be solid solutions rather than a mixture of AIS and CIS phases. At the same time, the band gaps and the energies of photoluminescence (PL) band maxima of CAIS QDs show non-monotonous changes decreasing from AIS to CAIS QDs (50 molar% Cu) and then increasing back for Cu-richer CAIS compositions and pure CIS. This behavior was interpreted as a result of the band bowing phenomenon. The bowing parameters of CAIS QDs determined from both absorption spectra (1.10 eV) and PL spectra (0.38 eV) are close to the range typically reported for ternary bulk MI-MIII-S compounds with the MI sites occupied by a mixture of copper and silver cations. The PL intensity of CAIS QDs was found to decrease during PL registration due to the photochemical decomposition of QDs, the efficiency of this process increasing with increasing copper content. A similar trend was found in the photocatalytic reduction of methylviologen cations by hydrosulfide anions in the presence of CAIS. The initial rate of this reaction increased monotonously from AIS to CAIS to CIS QDs, with the activity of the CAIS QDs (50 molar% Cu) and pure CIS QDs being respectively 1.5 and 2.7 times higher than the photoactivity of pure AIS QDs. This trend is compliant with a strong decrease in the PL emission efficiency observed from AIS to CAIS to CIS QDs. Similar optical and photochemical properties were revealed for core/shell CAIS/ZnS QDs. The band-bowing effect and photochemical activity of mixed CAIS (CAIS/ZnS) QDs open good perspectives for light-conversion applications in the photon energy range down to 1.8 eV.