Exploring the mechanism of room temperature ferromagnetism in C-doped TiO2 nanoclusters by tuning the defects by different annealing temperature using citric acid as C source

Revealing the mechanism of room temperature ferromagnetism (RTFM) in carbon-doped TiO2 (C–TiO2) is critical to accelerate the development of TiO2-based spintronic devices. Herein, C–TiO2 nanoclusters were prepared by a sol-gel process using citric acid as C source, and the defects and RTFM of the samples were tuned by vacuum annealing at different temperature, and then the mechanism of RTFM induced by C doping is studied. Results revealed that with the increase of annealing temperature, residual organic precursor was gradually removed, and the phase transformed from pure anatase to rutile and brookite, followed by the precipitating of elemental C at 600 °C and 700 °C. Due to the doping of C, defects including substitutional C atoms for Ti sites (CTi), interstitial C atoms (Ci) and oxygen vacancy were introduced into the lattice. The concentration of CTi increases first till 500 °C, and then decreases gradually with the annealing temperature, while that of Ci exhibits opposite trend, and the oxygen vacancy concentration raises continuously. Influenced by the annealing temperature, RTFM of the samples exhibits similar trend to that of CTi, and the saturation magnetization (Ms) reaches the maximum at 500 °C. The RTFM is synergistically induced by CTi and oxygen vacancy, and ferromagnetic p-p coupling interaction trigged by CTi plays a more critical role. The results provide a significant reference for tuning the defects and RTFM in C–TiO2.