Ag–In–Zn–S Quaternary Nanocrystals Prepared from InCl₂ Precursor: Photophysical and Spectroscopic Properties and Application as Visible Light Photocatalysts of Aromatic Aldehyde Photoreduction

It has been demonstrated that nonstoichiometric quaternary Ag–In–Zn–S nanocrystals prepared from InCl2 exhibit composition-dependent selectivity in industrially important processes of aromatic aldehydes such as furfural reductions, which have never been reported for this type of nanomaterials. Two sets of nanocrystals were prepared exhibiting red (731 nm) and green (528 nm) photoluminescence, abbreviated as R and G, respectively. These hydrophobic R and G nanocrystals could be easily transformed into hydrophilic ones through the exchange of their primary ligands for 11-mercaptoundecanoic acid (MUA) ligands, yielding R-MUA and G-MUA nanoparticles. All studied nanocrystals (hydrophobic and hydrophilic) exhibited long photoluminescence lifetimes, implying their potentially promising photocatalytic properties. The reductions of aldehydes were carried out at room temperature either in a nonpolar solvent (C6D6) or in water (D2O) using green, blue, and ultraviolet light-emitting diodes (LEDs) as a source of radiation. p-Toluenethiol and isopropanol were tested as reducing agents, and cesium acetate (AcOCs), triethylamine (Et3N), and potassium tert-butoxide (tert-BuOK) were tested as bases. In the reaction of furfural reduction, hydrophobic R nanocatalysts emitting red light (Eg = 2.0 eV) selectively yielded furfuryl alcohol, independently of the applied reducer/base couple. Green light-emitting G nanocatalysts (Eg = 3.2 eV) in the same test reaction selectively yielded deoxyfuroin as a product of pinacol coupling. Photocatalytic reduction of furfural in aqueous (D2O) media with p-toluenethiol reducer resulted in the selective formation of furfuryl alcohol with conversion degrees increasing with an increasing strength of the applied base. In the absence of a base, hydrofuroin was selectively obtained.