Microwave-Assisted Solution Synthesis of Metastable Intergrowth of AgInS2 Polymorphs

Abstract

The intergrowth of stable and metastable AgInS2 polymorphs was synthesized using a microwave-assisted synthesis. The samples were synthesized in water and in a deep eutectic solvent (DES) consisting of choline chloride and thiourea. An increase in the metal precursor concentration improved the crystallinity of the synthesized samples and affected the particle size. AgInS2 cannot be synthesized from crystalline binary Ag2S or In2S3 via this route. The solution synthesis reported here results in the intergrowth of the thermodynamically stable polymorph (space group I42d, chal-copyrite structure) and the high-temperature polymorph (space group Pna21, wurtzite-like struc-ture) that is metastable at room temperature. A scanning transmission microscopy (STEM) study revealed the intergrowth of tetragonal and orthorhombic polymorphs in a single particle and un-ambiguously established that the long-thought hexagonal wurtzite polymorph has pseudo-hexag-onal symmetry and is best described with the orthorhombic unit cell. The solution-synthesized AgInS2 polymorphs intergrowth has slightly lower bandgap values in the range of 1.73 eV–1.91 eV compared to the previously reported values for tetragonal I42d (1.86 eV) and orthorhombic Pna21 (1.98 eV) polymorphs.