Determining the influence mechanism of defects on remarkable room temperature ferromagnetic behavior for C-doped titanium dioxide aided by carboxymethyl cellulose as C source

Abstract

C-doped titanium dioxide nanoparticles (C–TiO2) were synthesized through a two-step sol-gel process aided by carboxymethyl cellulose (CMC) as C source, following by annealed at different temperature under vacuum. Lattice defects and room temperature ferromagnetism (RTFM) influenced by C doping and annealing temperature are investigated in detail. Correlation between the variation tendency of defects and that of saturation magnetization (Ms) of C–TiO2 is determined, and influence mechanism of defects on RTFM is clarified. Results reveal that influenced by the increase of annealing temperature, C–TiO2 transforms from anatase to rutile phase, followed by some brookite phase and a small amount of elemental carbon precipitate. Three types of defects are introduced in C–TiO2 lattice, namely, substitutional C atoms at titanium sites (CTi), interstitial C defects (Ci) and oxygen vacancies (VOs). Content of CTi shows a gradual increase with increase in annealing temperature from 200 °C to 500 °C, after which a decrement is observed, and that of Ci shows an opposite tendency, while VOs content increases monotonically with increased annealing temperature. Influenced by annealing temperature, Ms shows a similar variation tendency to that of CTi. Joint effect of CTi and VOs promotes the formation of RTFM in C–TiO2, and CTi defects play a major role by inducing ferromagnetic coupling interaction between C 2p and O 2p states. CMC is a more suitable C source than citric acid to induce remarkable room temperature ferromagnetic behavior in C–TiO2 nanoparticles.