Abstract
Undoped and Nd3+-doped xerogels (Nd3+-dopant content in the range 0.5–10 mol%) were obtained from colloidal sols in turn prepared by a “green sol-gel route” in aqueous media. The as-synthesized xerogels were characterized by X-ray diffractometry (XRD), high resolution electron microscopy (HREM) together with selected area electron diffractometry (SAED), and Raman spectroscopy (RS), finding that they crystallize as tiny nanocrystals of anatase with traces of brookite and rutile. The thermal stability of the as-synthesized xerogels was next studied in-situ by X-ray thermo-diffractometry (XRTD) as well as ex-situ by XRD, HREM/SAED, and RS on xerogels calcined at different temperatures (400–900 °C), concluding that Nd3+ doping results in the formation of substitutional solid solutions with enlarged unit-cell volume that (i) stabilizes the anatase phase retarding the onset of its transformation to rutile to ever higher temperatures and (ii) provides the anatase nanocrystals with superior resistance to crystal growth. It was also found that Nd4Ti9O24 eventually precipitates at high temperatures when the Nd3+-dopant content is equal to or higher than 3 mol%, which leaves the still-existing anatase nanocrystals without stabilizer cations therefore causing their immediate transformation to rutile. Finally, the photoluminescence (PL) behaviour of the Nd3+-doped TiO2 xerogels with both anatase and rutile crystal structures was investigated and critically compared, observing that the PL emission (i) increases with increasing crystallinity of the host nanocrystals, (ii) decreases above a certain optimal Nd3+-dopant content due to concentration quenching, and (iii) is much greater for rutile host matrices. In this scenario, the 3 mol% Nd3+-doped rutile xerogel maximized the PL emission, emitting at both ∼915 and 925 nm due to the existence of two distinct local crystal fields for the Nd3+ cations.
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