Influence of boric acid/Sr2+ ratio on the structure and luminescence properties (colour tuning) of nano-sized, complex strontium borates doped with Sm2+ and Sm3+ ions

Abstract

Several complex strontium borate matrices (Sr3(B2O6), Sr2(B2O5), SrB2O4, SrB4O7, SrB6O10 and SrB8O11) doped with Sm2+/3+ ions were prepared using a modified Pechini method, with increasing Sr2+/boric acid ratio and a two-step annealing process. The Sm3+ ions were transformed into Sm2+ ones without additional reducing agents during the syntheses performed in the air atmosphere. The obtained crystal structures were investigated using powder X-ray diffraction (XRD) method. The use of 300% molar excess of boric acid during the synthesis process (Sr:B ratio 1:3), resulted in the formation of pure SrB2O4: Sm2+ nanoparticles, whose morphology was investigated with the use of transmission electron microscopy (TEM). Luminescence properties of the Sm2+ and Sm3+ doped compounds, i.e. excitation and emission spectra, including luminescence decay curves and determined lifetimes, were investigated. The obtained doped strontium borates show strong emission from Sm2+ and Sm3+ ions, in the range from 550 to 780 nm, after UV–Vis excitation. Depending on the λex used four characteristic bands of the Sm3+ ions, i.e. 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2, 4G5/2 → 6H11/2, and a series of split, very narrow bands typical of the Sm2+ ions, i.e. 5D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3, were clearly observed in the spectra. Chromaticity coordinates (CIE diagram) were calculated for different Sr:B ratios, showing a colour tuneable luminescence of the products synthesized.