Synthesis and physical characterization of ZnW1-xInxO4 nanostructures for hydrogen production

Abstract

ZnW_1-xIn_xO₄ (x = 0, 0.02, 0.05, 0.07, 0.1) samples were synthesized utilizing the hydrothermal procedure. Synchrotron x-ray diffraction patterns were analyzed applying Rietveld refinement methodology to find out the variation in unit cell parameters, crystallite dimension, atomic positional coordinates, and occupancy values upon doping with different In-concentration (x). Upon doping with In, analysis manifested that both Ni- and W-octahedrons exhibited increased distortion that would engender a polarization moment. Analysis of the measured Raman spectrum confirmed the distortion occurred in the WO₆ octahedra by In-doping, where the vibrational modes were shifted or split. The chemical composition and the oxidation states of elements present were investigated using XPS and EDS analyses. The E_g values of the ZnW_1-xIn_xO₄ samples are 3.93, 3.9, 3.93, 3.94, and 4.04 eV for x = 0, 0.02, 0.05, 0.07, 0.1 respectively. Photoluminescence (PL) intensity was almost completely quenched upon doping with indium which signifies optimal charge separation conducive to enhanced photocatalytic performance. The current ZnW_1-xIn_xO₄ samples were tested as effective catalysts for hydrogen production via sodium borohydride (NaBH₄) hydrolysis and methanolysis. Samples with indium content 7% or 5% demonstrated the highest generation rates of 6332 mL min^-1g^-1 for methanol and 385 mL min^-1g^-1 for water, respectively. The activation energy (E_a) of the hydrolysis of NaBH₄ over ZnW_0.95In_0.05O₄ as a catalyst was determined. The hydrogen production rate increased with increasing temperature. The activated energy was calculated to be 54.87 kJ/mol.