The magnetic property of carbon-doped TiO2
Introduction
Recently, TiO2-based diluted magnetic semiconductors (DMSs) have been studied for room-temperature ferromagnetism (RT FM) [1]. However, different and even contradictory results were reported over same type of samples either prepared by different methods or fabricated through different procedures. There are controversies on the magnetic properties (FM, paramagnetism (PM), superparamagnetism (SPM)) for TiO2 doped with transition metal (TM) [2], [3], [4]. The confusion is that the TM dopant can segregate to form precipitates or clusters that are ferromagnetic, making the origin of the ferromagnetic property uncertain. To avoid such uncertainty, researchers started to work on DMSs generated using dopants of nonmagnetic elements. The idea is that since a nonmagnetic dopant (e.g. carbon) should make no contribution to FM, the magnetic property induced by doping TiO2 with the nonmagnetic dopant can be considered as purely intrinsic. According to the results of theoretical and experimental investigations, thin films of Cu-doped TiO2 are considered to be RT DMSs [5], [6], [7], [8]. In a number of systems related to carbon, there was the detection of FM [9], [10], [11], [12], [13]. Li et al. conducted first-principles density functional theory calculations and reported that with the oxygen atoms partially substituted by carbon atoms, carbon-doping may induce FM in rutile TiO2 [14]. On the other hand, Yang et al. reported FM in carbon-doped anatase TiO2 with magnetic moment per carbon of about [15]. To verify the issue further, we investigated the structure and magnetic properties of carbon-doped TiO2 fabricated by solid state reaction process.
Section snippets
Experimental
The carbon-doped TiO2 samples with a nominal carbon concentration of 10 mol% were prepared according to a process of solid-state reaction. Fine powders of high-purity carbon and TiO2 with prescribed molar ratio were ground for 3 h to generate a homogeneous mixture. The mixture was pressed into a slice and sintered at 1000 °C via a rapid thermal process (70 °C/s) under a continuous flow of N2 for 30 min, and then rapidly cooled down to RT in ambient environment. The as-obtained sample was in the
Results and discussion
The typical XRD patterns of samples A and C are shown in Fig. 1. The peaks labeled with () are due to TiO2 of anatase structure, and the others due to TiO2 of rutile structure. It is apparent that during sintering at 1000 °C, there was transformation of anatase to rutile structure. Since no peaks other than those of TiO2 are observed, we deduce that there is partial substitution of lattice oxygen by carbon as well as the presence of interstitial carbon atoms in the TiO2 lattices.
Raman
Conclusion
In summary, the samples of carbon-doped TiO2 with nominal carbon concentration of 10 mol% sintered either in argon or in nitrogen are ferromagnetic at RT. In the case of sintering in N2, there is partial substitution of oxygen by carbon as well as by nitrogen. With nitrogen incorporation, there is decline in concentration of D-bond (disordered) and G-bond (graphitic) defects, causing magnetization (per carbon atom) to decrease. With increase of carbon concentration, the magnetic moment per
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No. 10674059), the National High Technology Research and Development Program of China (Grant No. 2007AA021805), the National Key Project for Basic Research (Grant Nos. 2010CB923402 and 2005CB623605), People's Republic of China, and the Scientific Research Foundation of Graduate School of Nanjing University.
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