New phosphors based on the reduction of Eu(III) to Eu(II) in ion-exchanged aluminosilicate and gallium silicate inorganic polymers

Abstract

Novel blue-emitting phosphors based on Eu2+ ions dispersed within chemosynthetic aluminosilicate and gallium silicate inorganic polymer (geopolymer) hosts were prepared by heat treatment of Eu3+-exchanged inorganic polymers in reducing conditions and investigated using photoluminescence spectroscopy, 29Si, 27Al and 71Ga MAS NMR spectroscopy, XRD, and XPS. Eu2+ photoluminescence could be distinguished from the photoluminescence of the inorganic polymer host after heating at ≥ 400°C in the case of the aluminosilicate inorganic polymer and ≥ 600°C in the case of the gallium silicate inorganic polymer and became more intense as the reduction temperature was increased up to 1000°C. The gallium silicate-based phosphors heated at ≥ 900°C contained small amounts of crystalline KGaSi2O6 or β-Ga2O3, but the X-ray amorphous structure of the inorganic polymer was retained. After heating at 1000°C, the Eu2+ photoluminescence intensity was substantially more intense for the aluminosilicate phosphor, but the shape of the excitation and emission spectra of the aluminosilicate and gallium silicate phosphors were similar, with a broad emission band centred at 440–475nm when excited at 240–400nm. This band shifted to longer wavelengths as the ion exchange solution concentration was increased from 0.001M to 0.03M.