New Luminescent Materials Based on Aluminosilicate and Gallium Silicate Inorganic Polymers

Abstract

This thesis describes the development of new luminescent materials using aluminosilicate and gallium silicate inorganic polymers as hosts for rare earth ions. The newly developed phosphors were investigated using XRD, SEM/EDS, solid state NMR, ion beam analysis techniques and photoluminescence spectroscopy.

Sm³⁺ and Eu³⁺ activator ions were incorporated into the inorganic polymer hosts by ion exchange. The Sm³⁺- and Eu³⁺-exchanged potassium aluminosilicate and gallium silicate inorganic polymers exhibited the characteristic structural features of geopolymers, being X-ray amorphous and consisting of tetrahedrally coordinated aluminium or gallium and silicon. EDS analysis indicated that the rare earth ions were for the most part distributed throughout the materials, partially replacing K⁺ in the structure as expected. The phosphors exhibited the characteristic photoluminescence of the 4f-4f transitions of Sm³⁺ and Eu³⁺ ions together with a broad host-related emission, which overwhelmed the weaker activator emission in the case of the Sm³⁺ phosphors. The relative intensity of the magnetic dipole and forced electric dipole transitions of Eu³⁺ was the same in the two types of host, suggesting that the sites occupied by the activator ions in the two inorganic polymer hosts are similar. The rare earth doping concentration was varied by using different concentrations of the exchange solution, and ion exchange was found to be more effective in the gallium silicate inorganic polymer. Concentration quenching of the photoluminescence was only observed at higher concentrations in the Eu³⁺-exchanged gallium silicate inorganic polymers; for the other activator/host combinations the limiting factor appeared to be the extent of ion exchange that could be achieved.

The performance of the phosphors was effectively improved by heat treatment in air. The photoluminescence intensity increased with increasing heating temperature, the most dramatic increase occurring between 800 and 1000 °C. The best-performing phosphors were obtained by heating the gallium silicate inorganic polymer phosphors at 1000 °C and the aluminosilicate inorganic polymers at 1200 °C; at higher temperatures the sample pellets melted and could not be retrieved intact. At these temperatures KAlSi₂O₆ was formed in the Sm³⁺- and Eu³⁺- exchanged and unexchanged aluminosilicate inorganic polymers, KGaSi₂O₆ was formed in the unexchanged gallium silicate inorganic polymer, and β-Ga₂O₃ was formed in the Sm³⁺- and Eu³⁺- exchanged gallium silicate inorganic polymers, but a significant content of the inorganic polymer remained in all samples. The enhancement of the Sm³⁺ and Eu³⁺ photoluminescence by heating was attributed to changes in the nature of the host and in the sites occupied by the rare earth ions as water is removed from the inorganic polymer, unreacted silica is incorporated, and crystalline phases begin to form. Changes in the relative intensity of hypersensitive 4f-4f emission and excitation spectra of the Eu³⁺-exchanged inorganic polymers with heating temperature reflected the changes in the activator sites and suggested that the activators continue to occupy similar sites in the aluminosilicate and gallium silicate hosts, most likely in the remaining inorganic polymer phase. The optimum heated phosphors were prepared with lower ion exchange solution concentrations compared with the optimum unheated phosphors. In the heated gallium silicate hosts the increase in intensity with doping concentration was limited by the formation of Sm₄Si₃(SiO₄)O₁₀ and Eu₄Si₃(SiO₄)O₁₀ when exchange solutions of concentration >0.005 M were used, while solutions of 0.03 M or higher caused the aluminosilicate samples to melt when heated at 1200 °C.

Eu²⁺ phosphors, obtained by heating Eu³⁺-exchanged aluminosilicate inorganic polymers in 5% H₂, 95% N₂, gave rise to a broad emission band in the blue region with a wide excitation window from ~240-400 nm. Eu³⁺ reduction appeared to occur at 400 °C, but the best phosphors were obtained by heating at 1000 °C. The position of the emission band could be varied by changing the concentration of the ion exchange solution in the range 0.001-0.03 M, with increasing emission wavelength and more effective excitation at longer wavelengths with higher Eu contents. The excitation and emission spectra of the aluminosilicate and gallium silicate Eu²⁺ phosphors were very similar, but reduction seemed to be more effective in the aluminosilicate inorganic polymer host and substantially more intense Eu²⁺ emission was produced compared with the gallium silicate inorganic polymer.

The chemosynthetic aluminosilicate and gallium silicate inorganic polymers used as hosts exhibited emission bands in the UV-blue and green regions without the incorporation of rare earth activators. The green emission originated from the silica fume starting material, whereas the UV-blue emission, which appeared to arise from more than one type of luminescent species or site, was also exhibited by clay-based geopolymers prepared using different starting materials. The green emission became more intense after heating in air, but after heating at temperature > 400 °C the intensity was diminished and the band could no longer be distinguished from the broad band centred at shorter wavelength. For the UV-blue emission, changes in the features of the excitation and emission spectra with reducing temperature suggest changes in the contributing luminescence centres.

The development of these luminescent materials is of interest as a demonstration of a new type of host material with good chemical and thermal stability, and also represents another advanced application exploiting the versatility of inorganic polymers.