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

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 ²P_3/2(1) → ²P_1/2 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.