Identification of defects in the transition metal oxide-doped glass nanocomposite xV2O5–(1-x)(0.05MoO3–0.95ZnO) using positron annihilation spectroscopy and other techniques

Abstract

A series of transition metal oxide-doped glass nanocomposites of the common nomenclature xV2O5–(1-x)(0.05MoO3–0.95ZnO) for x=0 to 0.9 have been synthesized through the melt quenching process and characterized by X-ray diffraction, Transmission electron microscopy and optical absorption spectroscopic studies. Positron annihilation lifetime and coincidence Doppler broadening measurements are also performed to identify the vacancy type defects present in the samples of the differing V2O5 concentration. The X-ray diffraction patterns showed clear diffraction peaks indicating the presence of nanocrystallites in samples of x=0, 0.1, 0.2 and 0.3 whereas significantly broadened patterns were additionally observed in the case of samples of x=0.5, 0.7 and 0.9 reflecting a dominant amorphous nature. Transmission electron microscopy images indicated the formation of nanocrystallites of varying sizes and morphologies and the high resolution transmission electron microscopy patterns showed well defined fringes, some of them showing discontinuities in samples of larger x. Further, selected area electron diffraction patterns displayed diffuse rings superposed with bright spots, indicating the formation of nanocrystallites dispersed in an amorphous matrix. The optical band gap energy estimated from the UV–Vis absorption measurements decreased with increasing content of V2O5. Positron lifetime measurements indicated the trapping of positrons in the interfacial gaps around the nanocrystallites for lower values of x (i.e., 0 to 0.3) and then in the porous defects within the amorphous matrix of the samples of higher values of x (=0.5 to 0.9). The coincidence Doppler broadening spectroscopic measurements further confirmed the presence of vacancy type defects arising from cation non-stoichiometry and the amorphous character of the largely doped nanocomposites. The studies demonstrated defect characterization as an alternative approach in the investigation of metal oxide nanocomposite formation and evolution during compositional variations and thermal treatments.