Hydrothermal synthesis, structural characterization and photocatalytic properties of β-Ag2MoO4 microcrystals: Correlation between experimental and theoretical data

Abstract

In this paper, we report about hydrothermal synthesis, structural characterization and photocatalytic properties of beta-silver molybdate (β-Ag2MoO4) microcrystals obtained at different temperatures (100, 120, 140 and 160 °C) for 2 h. These crystals were characterized structurally using X-ray diffraction (XRD), X-ray fluorescence, Rietveld refinement, micro-Raman (m-Raman) and Fourier-transform infrared (FT-IR) spectroscopies. Experimental and theoretical band gap values were correlated by ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy and periodic first-principles calculations in the framework of density functional theory (DFT) with the B3LYP-D3 hybrid functional. The crystals morphology was observed through field-emission scanning electron microscopy (FE-SEM) images. The photocatalytic properties of these crystals were investigated for degradation of rhodamine B (RhB) dye under UV-light. XRD patterns and Rietveld refinement data indicate that all crystals exhibit a spinel-type cubic structure with space group (Fd3′m) formed by tetrahedral [MoO4] clusters and distorted octahedral [AgO6] clusters. m-Raman spectra exhibited five Raman-active modes in a range from 50 to 1000 cm−1, while FT-IR spectra have three infrared active modes in a range from 400 to 1100 cm−1. The experimental results from Raman and IR modes are in reasonable agreement with theoretically calculated results. Experimental UV–Vis spectra indicate a decrease in optical band gap (Egap = 3.35 eV to 3.32 eV) with the temperature rise. The calculated band structure revealed an indirect optical band gap (Egap) of approximately 3.94 eV. Moreover, theoretical calculations based on density of states and electron density maps were employed to understand the polarization phenomenon induced by structural defects in the β-Ag2MoO4 microcrystals. FE-SEM images revealed that the increase of processing temperatures promotes a change in shape of microcrystals from potatoes-like to coral-like. Finally, photocatalytic measures to degradation of the RhB dye resulted in the best catalytic performance for β-Ag2MoO4 microcrystals synthesized at temperatures of 120 and 140 °C, corresponding to 97.3% and 96.8% in the photodegradation of RhB dye under UV–light up to 2 h. The stability of the β-Ag2MoO4 was investigated by reusing, resulting in f 97.2, 93.9 and 78.8% degradation of the RhB dye for the first, second and third cycle, respectively. Keywords: β-Ag2MoO4 microcrystals, Theoretical calculations, Band gap, Coral-like, Photocatalytic properties