Microwave-Assisted Solution Synthesis of Metastable Intergrowth of AgInS₂ Polymorphs

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 I4¯2d, chalcopyrite structure) and the high-temperature polymorph (space group Pna2₁, 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 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 I4¯2d (1.86 eV) and orthorhombic Pna2₁ (1.98 eV) polymorphs.