Microstructural characteristics and photoluminescence performance of nanograined thermally treated CeO 2 -TiO 2 xerogels
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Nanostructured thermally treated xerogels have been synthesized using a sol-gel process involving cerium (Ce) chloride heptahydrate and titanium (Ti) propoxide mixed in different Ce:Ti molar ratios. Structural features of the xerogels have been correlated with their photoluminescence (PL) response. The crystallite sizes in the samples lie in the nanorange. The x-ray diffraction and transmission electron microscopy results have confirmed the coexistence of CeO2 and TiO2 nanocrystallites in these xerogels. In general, a decrease in the CeO2 crystallite size and an increase in the TiO2 crystallite size are observed in the xerogels as a function of Ti content. Scanning electron microscopy results have evidenced the evolution of ordered structure in the xerogels as a function of TiO2 content. Although both of the phases (CeO2 and TiO2) have exhibited PL in ultraviolet and visible regions, the major luminescence contribution has been made by the CeO2 phase. The largest sized CeO2 crystallites in 1:1 thermally treated xerogel have led to its highest PL response. PL emission in the xerogels is assigned to their nanocrystalline nature and oxygen vacancy-related defects.
I. INTRODUCTION
Luminescent properties of the rare earth oxides along with other semiconductor and insulator oxides are widely used to investigate the active doping sites. The luminescence signals from these materials are determined by the electronic structure of the rare earth and are almost independent of the host matrix. The width and the relative intensity of luminescent emission bands is affected by the nature of the lattice structure in such a way that some signals vary broadly. This feature can be used to tune the emission of the material depending on its practical applications. Thus, it is important to continue research on characteristics of rare earths luminescence in different compositions. Photoluminescence (PL) has been widely used to investigate the structure and properties of active sites on the surface of supported (or bulk) metal oxides, zeolites, etc.1 Such techniques are highly sensitive and nondestructive. In the sol-gel process, using alkoxide compounds, hydrolysis and condensation reactions are crucial for obtaining a gel. Through hydroxylation-condensation reactions, oxopolymers from transition metal alkoxides (TMA) can be grown into an oxide network.2,3 However,
the normal course of the reaction for TMA dissolved in a solvent leads to precipitation of the polymers. Control of the reactivity of TMA is necessary to obtain sols and gels. In titanium-based systems, this control may be achieved through the addition of complexing agents.4–6 However, the simplest way to control hydrolysis and condensation reactions is to add inorganic acids such as HCl or salts such as ceric ammonium nitrate and cerium chloride.7–11 We have to understand and control these reactions to control the preparation conditions of the gel. To our knowledge, PL characteristics of sol-gel– derived CeO2-TiO2 xerogels have never been reported before. Moreover, the micr
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