Impact of TiO2/B2O3 ratio on the structure and optical properties of Na2O–CoCl2–TiO2–B2O3 glass

Abstract

In this study, borate glass doped with constant cobalt chloride (CoCl2) content and different ratios of TiO2/B2O3 were prepared using the melt-quenching technique. The influence of the TiO2/B2O3 ratio on structure, optical, magnetic, electronic transitions, and linear and nonlinear optical features was studied. Structurally, XRD confirmed the amorphous structure, while EDX confirmed the incorporation of titanium cation. Meanwhile, the density increases, and the molar volume reduces with further ratios of TiO2/B2O3 to the glass matrix. Also, FTIR spectra confirmed the decreased presence of non-bridging oxygens (NBO), and different structural units such as BO3 and BO4. Optical studies showed the formation of absorption bands at about 301–327 nm, 375 nm, and 434 nm with TiO2/B2O3 ratios. Deconvolution procedures are necessary to detach the three electronic transitions and the glass absorption edge. The three electronic transitions can be explained in terms of titanium cations (Ti) oxidation states where Ti3+ cations display absorption bands in the wavelength range 440–480 nm. In contrast, Ti4+ cations display no absorption bands in the visible range but only show ultraviolet absorption bands. According to the electronic transition associated with Co cations are; the band at ∼497–500 nm is associated with 4T1g (F) → 2T1g (H) octahedral electronic transitions of Co2+ cations, while the band positioned at 577–578 nm is associated with 4A2 (4F)→4T1(4P) tetrahedral electronic transitions of Co2+ cations. The band is positioned at 648–650 nm and is associated with 5T2g→ 5Eg octahedral electronic transitions of Co3+ cations. On the other hand, with increasing TiO2/B2O3 ratio, optical band gaps decrease from 3.51 eV to 3.15 eV, and the samples color transformed from light blue to dark blue. Furthermore, the addition of TiO2/B2O3 ratio increases the linear and nonlinear refractive indices. ESR data indicate that Co2+ ions exist in both octahedral sites, with g ⁓ 4.10, and tetrahedral sites, with g ⁓ 2.40. Finally, we concluded the significant influence of the TiO2/B2O3 ratio on the structure, optical and magnetic properties, and linear and nonlinear optical parameters of cobalt borate glass. These results recommend the present glass samples for future applications in various photonic devices within the visible range.