HIGH-PRESSURE SYNTHESIS OF SOLID STATE INORGANIC LEAD-CONTAINING COMPOUNDS AND QUATERNARY CHALCOPYRITE SEMICONDUCTORS WITH IMPROVED HYDROGEN EVOLUTION

Abstract

Inorganic semiconducting materials synthesized by solid-state methods have been a focus of interest recently in multiple areas including condensed matter physics, optoelectronic and photovoltaic studies, and heterogeneous catalysis. In this thesis, we have synthesized several inorganic semiconducting metal chalcogenides/chalcogenide halides using solid state preparation methods, and investigated their structural, physical, optical and/or photocatalytic properties. A high-pressure method was adopted to prepare new phases that have not been previously reported in ambient pressure syntheses: 1) PbS2 was prepared at 4 GPa and 600 °C and was determined to crystallize in the CuAl2 structure type, (I4/mcm (#140), a = 6.1106(5) Å, c = 7.4949(6) Å, Z = 4). Its structure consists of layers of [S2]2- dimers and Pb2+ in square antiprismatic coordination - a rare structure type for metal dichalcogenides. Electronic structure calculations suggest that the material should be an indirect band gap semiconductor. The transport properties and magnetic behavior of the compound was investigated, and the Seebeck coefficient was measured on an Ag-doped sample; it is small and shows nearly linear temperature-dependent behavior from 50 to 250 K. 2) Pb4S3I2, a high-pressure phase in the PbS-PbI2 system, was prepared by the reaction of a stoichiometric ratio of Pb, S and PbI2 at 4 GPa and 600 C. Its crystal structure, determined from single crystal X-ray diffraction, is orthorhombic (space group Pnma) with a = 8.1293(6) Å, b = 15.5613(11) Å, c = 8.1820(6) Å, and Z = 4 at ambient temperature and pressure. The structure consists of sheets of distorted edge-sharing lead-centered polyhedra and saw-tooth-shaped lead-lead bonded chains. The material is predicted by theoretical calculations to be an indirect-band semiconductor, and the band gap is measured experimentally to be around 1.6 eV. 3) Pb3Se2Br2 was synthesized at 4 GPa and 700 C. Its crystal structure is determined to be body centered cubic and non-centrosymmetric (Th3P4 structure type, space group I4 ̅3d, #220) with a = 9.3200(4) Å at room temperature and ambient pressure. The Pb atoms are coordinated by eight anions in a dodecahedron, with Se and Br occupying the same crystallographic site in a disordered 1:1 ratio. Pb3Se2Br2 is a semiconductor with an indirect band gap of 1.48 eV, determined by diffuse reflectance measurements. The indirect band gap is theoretically supported by density functional theory calculations. In a second branch of the chemical investigation of semiconductors, quaternary AgxCu1-xGayIn1-yS2 (0 ≤ x ≤ 1.0, 0 ≤ y ≤ 1.0) chalcopyrite semiconductors (SCIGS) were prepared using a solid state method at ambient pressure in a search for improved photocathode materials for fuel-producing photoelectrochemical cells. The quaternary system enables enhanced control of the materials’ electronic and electrochemical properties compared to the ternary system, and allows for adjustment of the band structures and photocatalytic abilities to a finer degree than is possible in the traditional ternaries. The prepared semiconductors were tested for photochemical hydrogen evolution, and several of the quaternary AgxCu1-xGayIn1-yS2 compositions were found to show better water reduction capability than the ternaries, even when a platinum co-catalyst is present in the ternary systems.