Evidencing early pyrochlore formation in rare-earth doped TiO 2 nanocrystals: Structure sensing via VIS and NIR Er 3+ light emission

Abstract Er 3+ doping of TiO 2 colloidal nanocrystals enhances their performance for photo-induced applications. Such doping is known to delay the anatase to rutile transformation under thermal treatment; nonetheless relevant information on the Er 3+ light emission and location within the TiO 2 structures is still incomplete. Er 3+ photoluminescence emission both in the visible (upconverted) and infrared photoluminescence is used for the first time to probe the ions location within the different TiO 2 structures. The results show that Er 3+ ions in the as-prepared xerogels are not embedded in the anatase crystallites, and only upon thermal treatment Er 3+ diffusion is induced into crystal interstitial positions to form a solid solution. At higher temperatures rutile is formed inducing Er 3+ segregation and giving rise to the formation of pyrochlore (Er 2 Ti 2 O 7 ), as shown by a distinct emission in the infrared spectrum due to the Er 3+ ions located within the pyrochlore compound. Although pyrochlore is usually a high temperature phase, analysis of the photoluminescence allows its labeling at temperatures as low as 600–700 °C for small Er 3+ concentrations (1 mol %). Increasing Er 3+ concentration promotes its enrichment at the nanocrystallites surface accomplished by the anatase-to-rutile phase transformation, suggesting that Er 3+ ions control the TiO 2 nanocrystals surface properties.