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 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.

Keywords: STEM; deep eutectic solvent (DES); green synthesis; metastable; morphology; semiconductor.

Figure 1

The crystal structures of the tetragonal, orthorhombic, and hexagonal polymorphs of AgInS₂. Atoms are color-coded: In—purple, Ag—gray, S—yellow, and the site with mixed In/Ag occupancy is denoted as a half-purple and half-grey sphere. The unit cell is shown with a black line. The orientation is chosen to highlight similarities in the structures of polymorphs.

Figure 2

PXRD patterns of the 1-step synthesis of water (blue shaded region) and DES-synthesized (orange shaded region) AgInS₂ with varying concentrations of metal precursors. Diffraction peaks for Ag₂S impurity are denoted with an asterisk (*). The theoretical PXRD patterns include ICSD 51617, representing the I$\overline{4}$2d; ICSD 76276, representing the P6₃mc; and ICSD 605408, representing the Pna2₁ polymorph.

Figure 3

SEM images of the 1-step-synthesized AgInS₂ made with varying metal precursor concentrations in water: (a) 0.1 mmol, (b) 0.5 mmol, and (c) 1 mmol; and in DES: (d) 0.5 mmol, (e) 1 mmol, and (f) 3 mmol.

Figure 4

PXRD patterns of the 2-step synthesis of water (blue shaded region) and DES-synthesized AgInS₂ (orange shaded region), with varying concentrations of metal precursors. Diffraction peaks for the Ag₂S impurity are denoted with an asterisk (*). The theoretical PXRD patterns include ICSD 51617, representing the I$\overline{4}$2d; ICSD 76276, representing the P6₃mc; and ICSD 605408, representing the Pna2₁ polymorph.

Figure 5

SEM images of the 2-step-synthesized AgInS₂ made with varying metal precursor concentrations in water: (a) 0.1 mmol, (b) 0.5 mmol, and (c) 1 mmol; and in DES: (d) 1 mmol, (e) 3 mmol, and (f) 5 mmol.

Figure 6

High-resolution HAADF–STEM image of AgInS₂ crystallite prepared via 2-step heating profile, 0.5 mmol metal concentration, and DES. The arrows indicate the phase boundary between orthorhombic and tetragonal (in blue) polymorphs.

Figure 7

(Top and Middle, Left) High-resolution HAADF–STEM images and corresponding electron diffraction (ED) patterns of the two main zones of the (pseudo)-wurtzite AgInS₂ structure: top panel, [001] and middle panel, [010]_ort/[100]_hex. High-intensity diffraction spots can be indexed using hexagonal wurtzite structure (P6₃mc, yellow). All of the diffraction spots, including those with lower intensity can be indexed using the orthorhombic wurtzite-like structure (Pna2₁ space group). Top and Middle, Right: Magnified high-resolution HAADF–STEM images of the two main zones and simulated images based on orthorhombic wurtzite-like structure (Pna2₁ space group) as inserts. Bottom: Images of the EDX-STEM elemental mapping for Ag L(red), In L (green), S K (blue), and an overlapping color image.

Figure 8

HT–PXRD data of 0.5 mmol the 2-step DES-synthesized AgInS₂ upon heating and cooling.

Figure 9

Tauc plots for AgInS₂ obtained via (a) 1-step and (b) 2-step synthesis; (c) the PXRD pattern of high-temperature Pna2₁ orthorhombic AgInS₂ polymorph, synthesized via a high-temperature route; and (d) Tauc plot of high-temperature Pna2₁ orthorhombic AgInS₂ polymorph, synthesized via high-temperature route.