Microwave-Assisted Solution Synthesis of Metastable Intergrowth of AgInS 2 Polymorphs.

The intergrowth of stable and metastable AgInS 2 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. AgInS 2 cannot be synthesized from crystalline binary Ag 2 S or In 2 S 3 via this route. The solution synthesis reported here results in the intergrowth of the thermodynamically stable polymorph (space group I 4¯2 d , chalcopyrite structure) and the high-temperature polymorph (space group Pna 2 1 , wurtzite-like structure) 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 unambiguously established that the long-thought hexagonal wurtzite polymorph has pseudo-hexagonal symmetry and is best described with the orthorhombic unit cell. The solution-synthesized AgInS 2 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 I 4¯2 d (1.86 eV) and orthorhombic Pna 2 1 (1.98 eV) polymorphs.