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 I2d, 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 I2d (1.86 eV) and orthorhombic Pna21 (1.98 eV) polymorphs.
Highlights
IntroductionDeep eutectic solvents (DESs) are emerging solvents that are analogs of ionic liquids [1]
Deep eutectic solvents (DESs) are emerging solvents that are analogs of ionic liquids [1].deep eutectic solvent (DES) are made by mixing a hydrogen bond donor and a hydrogen bond acceptor in a specific molar ratio [1,2]
The DES-assisted synthesis utilized in this work is similar to a previously reported solvothermal synthesis of binary sulfide using a DES consisting of choline chloride and thioacetamide, where thioacetamide is the hydrogen bond donor and a sulfur source [11]
Summary
Deep eutectic solvents (DESs) are emerging solvents that are analogs of ionic liquids [1]. DESs are made by mixing a hydrogen bond donor and a hydrogen bond acceptor (usually a quaternary ammonium salt) in a specific molar ratio [1,2]. The eutectic mixture of the two DES constituents has a significantly lower melting point compared to its individual components because of the hydrogen bonding occurring in the mixture; a viscous liquid is formed [1,3]. Due to the various possible combinations of hydrogen bond donors and acceptors, DESs allow for synthetic tunability and have been employed in the syntheses of sulfides [10–12]. AgInS2 is one of the two ternary sulfides reported in the pseudo-binary Ag2 S-In2 S3 system. AgInS2 is reported to have four polymorphic modifications. The most common room-temperature polymorph has a tetragonal chalcopyrite structure
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