Ferromagnetism of Co-doped TiO2(B) nanotubes

X. W. Wang, X. P. Gao, G. R. Li, L. Gao, T. Y. Yan, H. Y. Zhu
2007 Applied Physics Letters  
The Co-doped titanate nanotubes, synthesized via a hydrothermal reaction, are calcined at 300, 400, and 500°C for 2 h in an argon atmosphere to yield Co-doped TiO 2 ͑B͒ nanotubes and anatase nanotubes with a dark gray color. It is shown that all calcined titania nanotubes have a stronger absorption in visible region, attributed to the formation of oxygen vacancies. The saturation magnetization of all Co-doped titania nanotubes is stronger than that of as-prepared Co-doped titanate nanotubes. In
more » ... anate nanotubes. In particular, Co-doped TiO 2 ͑B͒ nanotubes calcined at 300°C exhibit the strongest ferromagnetism due to the existence of oxygen vacancies, as confirmed further by electron paramagnetic resonance spectra. The titanium oxides with one-dimensional ͑1D͒ nanostructures, such as nanotubes, nanorods, and nanowires, are of particular significance because their unique morphology and microstructure bring several important features. The titania nanotubes have been found to be very attractive due to their superior physicochemical properties as active materials for photocatalysis, 1,2 photoelectrochemistry, 3-5 lithium storage. 6-11 Following the discovery by Matsumoto et al. in 2001 of room temperature ferromagnetism in Co-doped TiO 2 anatase film, 12 more attentions have been focused on Codoped TiO 2 film, nanocrystals, and nanorods. 13, 14 Recently, Co/ Fe-doped titanate nanotubes and Co-doped anatase nanotubes with room temperature ferromagnetism have been reported for potential applications in spintronics. [15] [16] [17] [18] [19] It is believed that the appearance of oxygen vacancies near Co 2+ sites in Co-doped TiO 2 has important influence on the room temperature ferromagnetism. 18 Among titania polymorphs ͓including anatase, rutile, TiO 2 ͑B͒, and brookite͔, monoclinic TiO 2 ͑B͒ as a n-type semiconductor has a relatively open tunnel structure and a low density as compared with other titania polymorphs. TiO 2 ͑B͒ nanotubes have shown excellent electrochemical lithium storage and photocatalytic dehydrogenation of ethanol. 20-23 Abundant surface states or oxygen vacancies were reported to exist in TiO 2 ͑B͒ with 1D nanostructure. 24 Co-doped TiO 2 ͑B͒ nanotubes would have an improved ferromagnetism due to the metastable phase and more structure defects. In this work, Co-doped titanate nanotubes were prepared through a hydrothermal reaction and converted into Codoped TiO 2 ͑B͒ nanotubes after the calcination. An appropriate amount of cobalt nitrate Co͑NO 3 ͒ 2 ·6H 2 O ͑8 at. % ͒ was fully dissolved in a small amount of water ͑7 ml͒, and TiO 2 ͑anatase͒ powders were added in the solution and dispersed ultrasonically for 1 h. After that 11 M NaOH aqueous solution ͑43 ml͒ was transferred to the solution and mixed sufficiently under an ultrasonic treatment. The hydrothermal reaction was conducted at 130°C for 48 h in a Teflon autoclave ͑50 ml/ 65 ml͒. The products were thoroughly washed with distilled water and 0.1 M HCl to a pH value of about 7 and dried at 100°C. A ceramic boat with the asprepared sample was located in the center of a tubular furnace. Then argon gas was introduced to the above furnace at a flow rate of 100 SCCM ͑SCCM denotes cubic centimeters per minute at STP͒ for 1 h prior to the calcination. The asprepared sample was heated to 300, 400, and 500°C at a rate of 10°C / min and kept for 2 h in argon atmosphere. The microstructure of the samples was characterized using x-ray diffraction ͑XRD͒ ͑Rigaku D/max-2500͒, and transmission electron microscopy ͑TEM͒ ͑FEI Tecnai 20͒. Elemental analysis of cobalt was conducted on a Thermo Jarrell-Ash model inductively coupled plasma emission spectrometer ͑ICPES͒ ͓9000͑N+M͔͒. N 2 adsorption data were measured using NOVA 2000e ͑Quantachrome͒ instrument and the specific surface area was calculated by the Brvnaver-Emmett-Teller ͑BET͒ equation. The optical property was investigated by UV-visible spectrophotometer ͑Varian Cary-100͒. The magnetic property was measured using a superconducting interference device ͑MPMS-XL7, Quantum Design͒ at 300 K. Electron paramagnetic resonance ͑EPR͒ spectra were measured on a Bruker EMX-6/1 EPR spectrometer at room temperature. XRD patterns of as prepared and calcined samples are given in Fig. 1 . All diffraction peaks of the as-prepared sample can be assigned to layered protonated titanate ͑Ref. 25͒ with poor crystallinity. 8, 26 It is also shown that the interlayer distance of nanotubes is reduced gradually with further increasing calcination temperature. After the calcination at 500°C, the anatase phase seems to coexist with the metastable monoclinic TiO 2 ͑B͒ ͑Ref. 27͒, which is usually formed during the dehydration of layered titanate nanotubes at low temperature. When the as-prepared sample was calcined at 300°C, only TiO 2 ͑B͒ phase with a poor crystallinity is obtained, in good agreement with the previous results. 22, 23, 28 In particular, the anatase phase is hardly formed after calcination at 400°C in argon atmosphere, which usually coexists with TiO 2 ͑B͒ after calcination in air. 22,29 Therefore, the argon atmosphere used during calcination is important to prohibit the phase transition from TiO 2 ͑B͒ to anatase. However, no diffraction peaks of cobalt species can be detected in the XRD patterns. a͒ Electronic
doi:10.1063/1.2789734 fatcat:peswlffqgbdkbgvjq6ez7vuleu