8604
J. Phys. Chem. C 2008, 112, 8604–8608
Preparation and Characterization of Room Temperature Ferromagnetic Co-Doped Anatase TiO2 Nanobelts Hongye Zhang,† Tianhao Ji,*,† Yifan Liu,‡ and Jianwang Cai‡ School of Materials Science and Technology, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100083, China, and State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Science, Beijing 100080, China ReceiVed: January 14, 2008; ReVised Manuscript ReceiVed: March 5, 2008
We herein report a preparation process of Co-doped anatase TiO2 nanobelts facilely combining ion-exchange with hydrothermal treatment. A series of TiO2 materials with different Co2+ contents were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared (FT-IR), and magnetic measurement techniques. The results show that Co2+ cations have doped into the TiO2 lattice structure and that any metallic cobalt clusters or nanoparticles could not be found. The magnetic result demonstrates that the prepared Co-doped TiO2 samples are room-temperature ferromagnetic materials, whereas with the increase of Co2+ content, the remnant magnetization tends to decrease due to the superexchange coupling interaction between Co2+ ions owing to the ununiform distribution. Introduction Recently, spintronics, combining electron spin that carries information with electron charge, is getting attention owing to the requirement of a new generation of devices.1 In spintronics, the nanomaterials of diluted magnetic semiconductors (DMS) for magnetic transition-metal ion-doped materials of II-VI, III-V, or IV-VI groups, in which ferromagnetic DMS with Curie temperatures (TC) above room temperature (RT) are given special attention partly because of their pursuable RT spin based on electronic devices, are very important.2–5 The discovery of a Co-doped anatase TiO2 thin film with a high TC by Matsumoto et al. has accelerated experimental and theoretical research work of ferromagnetic DMS.6 Until now, the investigation of RT ferromagnetic DMS still focused on doped metal oxides, mainly on the doped ZnO and TiO2 thin films or their nanoparticles, based on the origin of controversial ferromagnetic properties or the investigation of interesting physical properties.7–12 However, the preparation and properties of doped TiO2 nanowires/nanobelts with a high TC above RT are still lacking. It has been noted that nanowires/nanobelts with a high TC would be very important application materials in magneto-optic or magnetoelectric nanodevices.13 The nanowires mainly concentrated on the study on doped ZnO nanowires, whereas less magnetic investigation of doped TiO2 nanowires/nanobelts was reported, except the fabrication of an electric field effect transistor of a Co-doped TiO2 polycrystalline nanowire in Lee’s literature, in which he has not elucidated any magnetic properties.14 We herein exhibit the preparation and characterization of Co-doped TiO2 nanobelts by a novel preparation process combining ion-exchange with hydrothermal treatment. This preparation process avoids the formation of reduced metal cobalt inside/outside of the TiO2 nanobelts and ensures * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 86-10-82339503. Fax: 86-10-82339503. † Beijing University of Aeronautics and Astronautics. ‡ Chinese Academy of Science.
the doping of Co2+ cations into the TiO2 nanobelt lattice. The effect of Co dopant on the lattice structure of TiO2 nanobelts was investigated in detail by various measurement techniques, and further, magnetic measurement demonstrated the RT ferromagnetic property of Co-doped TiO2 nanobelts. Experimental Section Layered titanate nanobelts (LTO-NBs) with Na+ and H+ cations between two layers were first prepared according to Kagusa’s report.15 A typical preparation procedure of the LTONBs is as follows: A mixture of 0.4 g of anatase TiO2 nanoparticles and 25 mL of 10 M NaOH aqueous were reacted in a Teflon-lined autoclave at 180 °C for 40 h. Then, different amounts of Co2+ cations for CoCl2 · 6H2O dissolved in an aqueous solution substituted for Na+ or H+ ions in the LTONBs at 120 °C for 10 h in the autoclave by an ion-exchangeable procedure. The titanate nanobelts with Co2+ cations were further treated in distilled water in the autoclave at 160 °C for several hours to prepare Co-doped TiO2 nanobelts (NBs). The coconcentration in TiO2 NBs can be controlled by the molar ratio of added Co2+ to LTO-NBs. Finally, the doped products were washed by 0.1 M HCl and distilled water. The TiO2 products with/without Co2+ ions were identified by powder X-ray diffraction (XRD) with a Rigaku D/MAXPC2200 diffractometer equipped with monochromatic highintensity Cu KR radiation (λ ) 1.54056 Å). Scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) analysis was taken on a JEOL-JSM 5800. Transmission electron microscope (TEM) and high-resolution electron microscope (HRTEM) images were measured on a JEOL JEM-2010 and a JEOL JEM-2010F at an acceleration voltage of 200 kV, respectively. The X-ray photoelectron spectroscope (XPS) measurement was performed on a VG Scientific ESCALab220iXL. Raman spectroscopy was measured on a HR 800 with 633 nm radiation having an output power of 8 mW. Fourier transmission infrared (FTIR) spectra were recorded on an Avatar
10.1021/jp8003294 CCC: $40.75 2008 American Chemical Society Published on Web 05/10/2008
Ferromagnetic Co-Doped Anatase TiO2 Nanobelts TABLE 1: Co-to-Ti Molar Ratios and Actual Compositions of Co-Doped TiO2 NBs materials (NBs)
reaction T (°C)
reaction t (h)
Co2+/Ti4+ (molar ratio, ICP)
actual composition
TiO2 LCT MCT HCT
160 160 160 160
10 10 10 10
0 0.01 0.03 0.05
TiO2 Ti0.99Co0.01O2 Ti0.97Co0.03O2 Ti0.95Co0.05O2
360 FT-IR spectrometer. Magnetic properties were measured at room temperature with a MPMS-7. Elemental compositions (Co/Ti) were analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Perkin-Elmer, SCIEX ELAN 5000). Results and Discussion Powder XRD analysis of the three samples TiO2, LCT, and HCT prepared under the same preparation conditions in Table 1 reveals that all three patterns are attributed to the anatase TiO2 phase, in which the (101) peak is the strongest, whereby the other patterns of impurities cannot be observed (Figure 1). Neither the low-content Co2+ in the LCT nor the high-content Co2+ cations in the HCT can lead to the change of the anatase TiO2 phase. The ICP-MS analyses of the LCT and HCT in the table demonstrate the existence of the Co atoms, which may be doped into the TiO2 lattice, or formed metallic cobalt clusters, not to be detected by XRD measurement. The shift of diffraction peaks can support the doping of metal ions into the lattice. Because the ion radius of Co2+ (0.072nm) is larger than that of Ti4+ (0.061nm), after Co2+ cations are doped into the TiO2 lattice, the diffraction peaks of TiO2 should be shifted to a lower diffraction angle, in good agreement with our XRD measurement. With the increase of Co content from the LCT to HCT, the gradual shift to low diffraction angle can be observed in
Figure 1. (a) XRD patterns of the prepared TiO2 NBs, LCT, MCT, and HCT; (b) the magnified XRD patterns of the above samples at 2θ from 22.5 to 30°.
J. Phys. Chem. C, Vol. 112, No. 23, 2008 8605 Figure 1a, which is particularly clear for the (101) peak in Figure 1b, and the corresponding d101 value also increases from 0.3506 to 0.3528 nm. Therefore, we can conclude that Co2+ cations were doped into the TiO2 lattice. In addition, the formation of metal Co is hard to imagine. In the preparation process, no reductions or organic compounds have been used, and also, the doped TiO2 was treated using 0.1 M HCl. Figure 2 shows the SEM and TEM images of the Co-doped LTO-NBs and the corresponding HCT NBs. The nanobelt shape of the Co-doped LTO-NBs can be clearly observed from the inset of Figure 2a. From the low-magnified SEM and TEM images in Figure 2a and b, after the transformation of LTONBs with Co2+ cations into the HCT, the morphology of the nanobelts still remains, but the surface becomes much rougher, which results from the phase transformation from layered titanate to anatase TiO2. Wong and co-worker demonstrated that such a transformation was an in situ phase conversion process.16 On the basis of a large number of experimental results, they concluded that as-formed anatase TiO2 nanoparticles attaching to layered titanate nanowires aggregated and further fused to form anatase TiO2 nanowires; thereby, the surface of the formed TiO2 nanowires became rougher. The EDS elemental mapping image shown in the inset of Figure 2b identifies the elemental distribution of the HCT, in which the elements Co, Ti, and O can be detected. The Au peak is caused by a deposited gold conduction film on the sample. The result demonstrates the existence of the Co element in the HCT. In the TEM image of Figure 2c, the rough surface of the HCT NBs can be clearly observed, and the SAED pattern of the inset shows the (200) and (110) diffraction planes of the tetragonal structure of anatase. The HRTEM image in Figure 2d shows the lattice structure of a bit of one nanobelt in Figure 2c. Owing to the low concentration of Co2+ in the sample, we cannot find a large number of obvious defects from the lattice structure, whereas we can see the wrinkle, as shown by the arrow. We can also observe the 0.352 nm lattice spacing between (101) places, which is perpendicular to the [101] orientation of the nanobelt, in good agreement with the strongest (101) peak in the above XRD measurement. We also note that, for the two adjacent particles, the lattice structure connects them, indicating the HCT NBs are not the aggregation of nanoparticles. The XPS measurement was carried out to obtain information on the oxidation state of cobalt in TiO2 NBs. The Co 2p and Ti 2p XPS spectra of the HCT NBs are shown in Figure 3. The Co 2p spectrum in Figure 3a shows four peaks of the 2p3/2 and 2p1/2 doublet. The Co 2p3/2 and 2p1/2 binding energies at ∼780.6 and 796.3 eV correspond to the energies of the photoelectrons of Co2+.17 The Co 2p binding energies are 2 eV higher than that of metallic cobalt, indicating that there are no metallic cobalt clusters or nanoparticles in/on TiO2 NBs. The Ti 2p spectrum in Figure 3b shows two peaks at 458.4 and 464.2 eV, assigned to binding energies of Ti 2p3/2 and Ti 2p1/2, respectively. The positions of the Ti 2p peaks show slight shifts toward higher energies compared to the values of the bare TiO2, implying that the local chemical state is influenced slightly by Co2+ doping.18 Similar behavior for the shift of the Ti 2p peaks has been demonstrated due to Cr3+ doping into the TiO2 lattice in the literature.19 To investigate the uniformity of Co2+ in the TiO2 lattice, the EDS elemental mapping images of the four positions on the single Co-doped TiO2 nanobelt in the TEM image for the HCT were measured as shown in Figure 4. The Cu peaks are caused by the Cu grid and the other elements, Co, Ti, and O, can be clearly observed. The Co contents of the four positions in Figure
8606 J. Phys. Chem. C, Vol. 112, No. 23, 2008
Zhang et al.
Figure 2. SEM and TEM images of HCT NBs. (a) The SEM image of layered titanate nanobelts (LTO-NBs) with Co2+ ions prior to the formation of the HCT (scale bar: 1 µm). (b) The SEM image of the HCT (scale bar: 2 µm); the inset shows the EDX analysis of the HCT. (c) The TEM image of the HCT (scale bar: 50 nm); the inset is its SAED. (d) The high-resolution TEM image of the HCT (scale bar: 5 nm).
Figure 3. Co 2p (a) and Ti 2p (b) XPS spectra of HCT recorded at room temperature.
4a to d are 4.7, 3.0, 3.5, and 5.7 atom %, respectively, whereas, in fact, the Co content of the HCT is 5.0 atom %. Therefore, the measurement results show that, to some extent, the distribution of Co2+ cations in the nanobelt is not uniform.
Raman scattering is a very useful measurement technique to investigate the microstructural change of nanocrystalline materials. The typical Raman spectra of the TiO2, LCT, and HCT NBs are shown in Figure 5. The three peaks at around 402, 521, and 643 cm-1 in the range of 300-700 cm-1, designated by the characteristic B1g, A1g + B1g, and Eg phonon modes, respectively, are the characteristic Raman bands of the tetragonal anatase phase,20,21 in good agreement with the above XRD measurement. The anatase phase also shows a prominent characteristic mode at around the low frequencies of 150 and 202 cm-1, assigned to the Eg mode. The low-frequency modes at Eg of 150 cm-1 and those at B1g of 402 cm-1 are O-Ti-O bending-type vibrations, and the modes at Eg of 643 cm-1 and those at A1g + B1g of 521 cm-1 are the Ti-O bond stretchingtype vibrations. In comparison with the Raman spectrum of the TiO2 NBs (Figure 5a), we also find that the peaks of the LCT and HCT at Eg of 161 cm-1 or that of the HCT at Eg of 643 cm-1, compared to the peaks themselves at 521 cm-1, have larger intensity. As the Eg mode is an O-Ti-O bending vibration or Ti-O bond stretching vibration, it is sensitive to oxygen deficiency, and in particular, the O-Ti-O bending vibration has a more significant effect than the Ti-O stretching vibration.22 Therefore, as oxygen defects in the lattice structure increase with the increase of the Co content from the LCT to HCT, the Eg modes of the O-Ti-O bending vibration at 161 cm-1 and the Ti-O stretching vibration at 643 cm-1 are influenced more obviously, implying the incorporation of Co2+ cations into the TiO2 lattice. Additionally, no other vibration modes from secondary phases (e.g., metallic Co clusters or various Co-Ti oxide species) were detected in the spectra yet. If Co interstitials or small clusters had been formed, a broad peak observed at 680 cm-1 would have been observed.23
Ferromagnetic Co-Doped Anatase TiO2 Nanobelts
J. Phys. Chem. C, Vol. 112, No. 23, 2008 8607
Figure 4. EDS mapping images of the four positions on the single Co-doped TiO2 nanobelt in the TEM for the HCT.
Figure 5. Raman spectra of the TiO2 (a), LCT (b), and HCT (c) NBs.
Figure 7. Magnetic hysteresis loops measured at 300 K as a function of Co concentration. The inset shows the enlargement of the hysteresis loops near the origin.
Figure 6. FTIR spectra of the TiO2, LCT, MCT, and HCT NBs.
Figure 6 is the FTIR spectra of the four specimen TiO2, LCT, MCT, and HCT NBs in the wavenumber range of 400-1000 cm-1. The two absorption peaks at 825 cm-1 and around 508 cm-1 can be observed. The band of the former is assigned to the O-O stretching mode for TiOOH on the surface, and the peak position for the four specimen does not appear to shift;24 however, the position of the latter assigned to the Ti-O vibration mode appears as a low-wavelength shift. According to Huckel’s vibration theory K ) 4π2ν2m and E ) KR6, where K is the force constant of the chemical bond, ν is the vibration frequency, m is the reduced mass, and E is the bond energy; the larger the bond energy E, the stronger the force constant K is, and with the increase of the E and the decrease of the m, the vibration wavenumber of the chemical bond shifts higher. It is well-known that the bond energy of Co-O (1067 kJ/mol) is much larger than that of Ti-O (662 kJ/mol), whereas their reduced masses m are similar; thereby, the doping of Co2+
cations in TiO2 NBs should lead to the higher shift of the wavenumber of the Ti-O lattice vibration. However, the wavenumber of the Ti-O vibration at around 508 cm-1 tends to shift lower with the increase of doped Co2+ content in Figure 6, indicating the existence of much more defects, especially oxygen vacancies, in good agreement with the above measurement results. Because the absence of partial oxygen atoms caused by Co2+ doping in the lattice diminishes the number of Ti-O or Co-O, the average force constant K value of chemical bonds decreases, and thus, the wavenumber of the Ti-O vibration shifts lower. Figure 7 shows the magnetic hysteresis loops measured at 300 K for the three above samples LCT, MCT, and HCT NBs. Their remnant magnetizations (Mr) and coercive fields are very small. With the increase of the Co2+ cation content, the Mr decreases from 9.6 × 10-4 emu/g of the LCT to 3.5 × 10-4 emu/g of the MCT and 5.6 × 10-5 emu/g of the HCT, and the coercive fields are 61, 80, and 22 Oe, respectively. Interestingly, compared with the HCT with 5 atom % Co2+, the LCT with the lowest Co content of 1 atom % Co2+ has the highest Mr and magnetization, whereas its saturation magnetization Ms is not easily saturated. This unsaturated behavior has also been observed in Co-doped anatase TiO2 nanoparticles, but they hardly made a reasonable explanation.25 Assuming that the Co2+ cations had a uniform distribution in TiO2 nanobelts, based on Goodenough-Kanamori rules for relative-high-content M2+ ions (
