Photoluminescence, optical properties, thermal behavior and nano-crystallization of molybdenum-doped borobismuthate glasses

Abstract

Molybdenum-doped borobismuthate glasses, having the chemical composition of xMoO3–30B2O3–(70–x)Bi2O3, where x = 0, 2.5, 5, 7.5 and 10; in mol%, have been prepared by the melt-quenching method. The amorphous structure of these glasses was confirmed by X-ray diffraction analysis. Spectroscopic properties of the glasses were studied by means of ultraviolet–visible (UV–VIS) optical absorption spectra, Fourier transform infrared (FTIR) absorption spectra, photoluminescence (PL) excitation and emission spectra. UV–VIS spectra revealed peaks belonging to electronic transitions of Bi3+ ions as well as to d–d transitions of Mo3+ and Mo5+ ions. Electron spin resonance (ESR) spectra confirmed the presence of these valence states of Mo where signals corresponding to Mo3+ and Mo5+ paramagnetic active species were recorded. The significant decrease of intensity of these ESR signals, with increasing MoO3 content, led to the suggestion that mutual redox reactions take place between Bi and Mo converting part of Mo3+ and Mo5+ to Mo6+ ions and, in turn, resulting in reduction of part of Bi3+ ions to Bi2+. Upon UV excitation at 300 nm, PL emission spectra of the as-prepared glasses exhibited blue light emissions at 460 and 470 nm and red emissions at 600 and 620 nm. Based on the ESR results, the blue and red emissions can be attributed to the 3P1 → 1S0 transition of Bi3+ ions and the 2P3/2(1) → 2P1/2 transition of Bi2+ ions, respectively. Differential thermal analysis was used to recognize the thermal behavior of the glasses and nano-crystallization process could be performed. The produced glass–ceramics contain nano-crystallites of cubic Bi2O3 and monoclinic bismuth molybdates (Bi2MoO6 and Bi26Mo10O69) phases whose average size ranges from about 15 to 30 nm.