Evidencing early pyrochlore formation in rare-earth doped TiO2 nanocrystals: Structure sensing via VIS and NIR Er3+ light emission

Abstract

Er3+ doping of TiO2 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 Er3+ light emission and location within the TiO2 structures is still incomplete. Er3+ photoluminescence emission both in the visible (upconverted) and infrared photoluminescence is used for the first time to probe the ions location within the different TiO2 structures. The results show that Er3+ ions in the as-prepared xerogels are not embedded in the anatase crystallites, and only upon thermal treatment Er3+ diffusion is induced into crystal interstitial positions to form a solid solution. At higher temperatures rutile is formed inducing Er3+ segregation and giving rise to the formation of pyrochlore (Er2Ti2O7), as shown by a distinct emission in the infrared spectrum due to the Er3+ 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 Er3+ concentrations (1 mol %). Increasing Er3+ concentration promotes its enrichment at the nanocrystallites surface accomplished by the anatase-to-rutile phase transformation, suggesting that Er3+ ions control the TiO2 nanocrystals surface properties.