Investigations on the origin of ferromagnetism of Cu doped anatase TiO2 nanotubes

The role of oxygen defects and dopants of Cu and N on ferromagnetism is discussed in undoped, N-doped, Cu-doped and Cu/N-codoped anatase TiO2 nanotube-arrays fabricated by two-step anodization method. X-ray photoelectron spectroscopy shows the presence of Cu2+ and/or Cu+ in the distorted octahedral coordination of TiO2. The UV-vis absorption spectra and photoluminescence spectra reveal the presence of oxygen defect related the peaks of absorption and emission in Cu-doped and Cu/N codoped-TiO2. Room temperature ferromagnetism is observed in all the TiO2 nanotubes, The saturation magnetizations for those specimens decreases in the order of Cu-doped, undoped, Cq/N-codoped and N-doped samples, which are in consistent with the results of the first-principles calculations based on density-functional theory. It is speculated that both oxygen vacancies and Cu d-states are involved in the ferromagnetism. Spin polarization occurs by the formation of bound magnetic polaron between electrons in Cu+/Cu2+ d-states and the unpaired spins in oxygen vacancies. (C) 2016 Elsevier Ltd. All rights reserved.