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

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, Si-29, Al-27 and Ga-71 MAS NMR spectroscopy, XRD, and XPS. Eu2+ photoluminescence could be distinguished from the photoluminescence of the inorganic polymer host after heating at >= 400 degrees C in the case of the aluminosilicate inorganic polymer and >= 600 degrees C in the case of the gallium silicate inorganic polymer and became more intense as the reduction temperature was increased up to 1000 degrees C. The gallium silicate-based phosphors heated at >= 900 degrees C contained small amounts of crystalline KGaSi2O6 or beta-Ga2O3, but the X-ray amorphous structure of the inorganic polymer was retained. After heating at 1000 degrees C, the Eu2+ photoluminescence intensity was substantially more intense for the aluminosilicate phosphor, but the shape of the excitation and emission spectra of the alumino-silicate and gallium silicate phosphors were similar, with a broad emission band centred at 440-475 nm when excited at 240-400 nm. This band shifted to longer wavelengths as the ion exchange solution concentration was increased from 0.001M to 0.03 M.