J. Phys. Chem. C 2010, 114, 7541–7547
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Influence of Oxygen Flow Rate on the Morphology and Magnetism of SnO2 Nanostructures Li Zhang, Shihui Ge,* Yalu Zuo, Bangmin Zhang, and Li Xi Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou UniVersity, Lanzhou 730000, China
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ReceiVed: July 12, 2009; ReVised Manuscript ReceiVed: March 16, 2010
Room-temperature ferromagnetism has been observed in undoped SnO2 nanostructures with various morphologies synthesized by a chemical vapor deposition (CVD) method. The magnetization depends on the oxygen flow rate during the CVD synthesis and decreases monotonically with the increase of the oxygen flow rate. X-ray diffraction and X-ray photoelectron spectroscopy measurements show that all the samples possess a typical rutile structure, and no other impurity phases are observed. Photoluminescence and X-ray photoelectron spectroscopies were employed to evidence the presence of oxygen vacancies in these samples and reveal that the oxygen vacancies contribute to the ferromagnetism. To further test the oxygen-vacancyrelated ferromagnetism, post-thermal annealing in different ambiances was performed. The results confirm that the oxygen vacancies, not Sn interstitials, play a crucial role in inducing the ferromagnetism in undoped SnO2 nanostructures. I. Introduction SnO2 is an n-type semiconducting oxide with a wide band gap (Eg ) 3.6 eV at 300 K) and well-known for its potential applications in dye-based solar cells,1 semiconductors,2 photoconductors,3 and gas sensors.4–6 Recently, the ferromagnetic properties obtained by doping 3d transition-metal ions into SnO2 also attracted wide attention due to its potential applications in spintronics.7,8 Although numbers of results have been reported, the origin of ferromagnetism (FM) in transition-metal-doped SnO2 has not been fully understood. When different groups try to give various explanations for the FM, an unexpected FM was found in the undoped HfO2 thin films.9,10 This result challenges the understanding of FM for researchers because neither Hf4+ nor O2- are magnetic ions. Following this, FM was observed in undoped wide band gap semiconductor nanoparticles and thin films, such as TiO2, ZnO, SnO2, In2O3, CeO2, and so on.11–14 These reports created great excitement in this new phenomenon, also known as d0 magnetism.15 In these systems, it is known that oxygen vacancies (VO) behave as an n-type dopant, and the existing vacancies might be the reason for the magnetic order. Experimentally, VO might easily be generated during various growth processes. However, VO are difficult to be measured. Moreover, the reports on room-temperature (RT) FM in pure SnO2 are still insufficient. The study on the RT FM in SnO2 films and particles confirmed that the source of the FM can be VO.13,31 However, the study on the RT FM in ZnO thin films confirmed that the source of the FM cannot be VO, but defects on Zn sites.14 Considering that both Sn interstitials (Sni) and VO exist in the nanostructure of SnO2 due to their surprisingly low formation energies,16 it is necessary to take into account whether Sni may contribute to FM. So far, there is no report mentioning the influence of Sni on FM. Hence, the understanding of FM origination in SnO2 is still in urgent demand. From this point, the controlled introduction of intrinsic defects into SnO2 by annealing in an appropriate atmosphere may explore * To whom correspondence should be addressed. Tel: 86-0931-8914177. Fax: 86-0931-8914160. E-mail: gesh@ lzu.edu.cn.
the defects’ influence on the FM of SnO2. In this paper, we studied the effect by systematically changing VO concentration in our samples. This was achieved by changing the oxygen flow rate during the sample synthesis by the CVD method and annealing the as-deposited samples. We present the evidence from photoluminescence (PL) and X-ray photoelectron spectroscopies (XPS) for the crucial dependence of observed FM on the presence of VO. II. Experimental Section SnO2 was synthesized by the CVD method. The source material, the high-purity (99.9%) Sn powder, was loaded in a ceramic boat, which was located at the center of a quartz tube. The n-type Si (100) substrates coated with 5 nm Au layers serving as the catalyst were positioned in the boat at an appropriate distance away from the source material. The quartz tube was evacuated to a pressure of less than 2 Pa and then heated to 900 °C. The stable argon flow with a rate of 100 sccm (sccm denotes standard cubic centimeters per minute at STP) was introduced into the quartz tube as the carrier gas during the heating and cooling process. At the synthesis temperature, oxygen was introduced and the flux rate of argon was modulated to 30 sccm; the process took 30 min, and the white material was formed on the surface of the substrates. In our work, five kinds of samples were prepared under different oxygen flow rates of 0.5, 2, 5, 20, and 30 sccm and were denoted as O1, O2, O3, O4, and O5, respectively. The morphology and structure of the synthesized products were characterized by scanning electron microscopy (SEM) (Hitachi S-4800) and X-ray diffraction (XRD) (Philips X’ Pert model) with Cu KR radiation. Further microstructural characterization of the samples was performed using high-resolution transmission electron microscopy (HRTEM) (JEOL, JEM-2010 at 200 kV) imaging and selected area electron diffraction (SAED) pattern analysis. The magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM) (LakeShore 7304 model) at room temperature (RT). In performing the magnetic experiment, each sample was scraped off from the Si substrate and then put into a capsule, which was settled
10.1021/jp9065604 2010 American Chemical Society Published on Web 04/07/2010
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Zhang et al.
Figure 1. Low-magnification SEM images of SnO2 for samples O1 (a), O2 (b), O3 (c), and O4 (d). The insets of (a-c) show the corresponding high-magnification SEM images for O1, O2, and O3, respectively.
in the sample holder of the VSM. Microphotoluminescence (PL) measurements were carried out at RT using a He-Cd laser with a wavelength of 325 nm and an output power of 15W as the excitation source. XPS measurements were obtained using a VG Scientific ESCALAB-210 spectrometer. Monochromatic Mg KR X-rays (1253.6 eV) were employed. Survey spectra from 0 to 1100 eV binding energy were collected at a 100 eV pass energy and a dwell time of 100 ms per point. High-resolution XPS spectra were collected at 20 eV pass energy. The operating pressure of the spectrometer was typically 9.5× 10-8 Pa. III. Results and Discussion In our experiments, oxygen was the only oxidizer in the synthesis. Therefore, the quantity of oxygen into the reaction system would affect the deposition of tin oxide. Figure 1 shows the SEM images of SnO2 materials synthesized at different flow rates of oxygen. At the lower flow rate of 0.5 sccm, it is found that the long and straight SnO2 nanowires are direction-free on the substrate with a high density (as shown in Figure 1a). The higher-magnification SEM image in the inset of Figure 1a shows that the SnO2 nanowires are uniform with diameters around 50 nm and lengths greater than 20 µm. At the flow rate of 2 sccm, a representative SEM image (Figure 1b) of several SnO2 1D nanostructures reveals that their geometrical shape is beltlike, which is distinct from those of nanowires and nanotubes. Their typical widths and thicknesses are in the ranges of 100-150 and 10-30 nm, respectively, as shown in the inset of Figure 1b. When the oxygen flow rate is increased to 5 sccm, the main as-grown product is the irregular shaped SnO2 structures, including sheets and nanowires (as seen in Figure 1c). The magnified SEM image (the inset of Figure 1c) depicts that some structures consist of two or three nanowires, linked together at the end of winglike sheets. The sheets are in the range of 300 nm to 2 µm in width and 30-250 nm in thickness. The diameter of the wires varies from 30 to 300 nm, while each individual nanowire has a uniform diameter and clean surface, and their length reaches 15 µm. When the rate increases to 20 sccm, sheetlike nanostructures with the erose morphology are synthesized (Figure 1d). Few rods are still found in the product
with the diameter of around 500 nm. At the flow rate of 30 sccm, the morphology of the products is similar to that grown under the flow rate of 20 sccm but larger in size. All these facts suggest that the oxygen flow rate has great influence on the morphology of SnO2. The nanostructures of the SnO2 nanowires were further characterized with HRTEM. Figure 2A shows a HRTEM image of a single nanowire with a diameter of 50 nm. The HRTEM image shows that the nanowire is structurally uniform, and the clear lattice fringes illustrate that the nanowire is singlecrystalline. The interplanar spacing is about 0.335 nm, which corresponds to the {110} plane of the rutile crystalline SnO2, implying that the nanowires grow along 〈110〉 direction. The inset of Figure 2A shows that the SAED pattern consists of the intense diffraction spots and can be indexed on a tetragonal cell with lattice parameters of a ) 4.738 Å and c ) 3.188 Å, indicating that the nanowire is a single-crystal rutile SnO2. The HRTEM image (Figure 2B) observed at the side of a nanobelt and the corresponding fast Fourier transform pattern (FFT, the inset of Figure 2B) confirm the single-crystal nature of the nanobelts. The spacing between adjacent lattice planes is 0.264 nm, corresponding to the (101) planes of rutile SnO2. When the HRTEM observation is combined with FFT analysis, it is suggested that the nanobelts prefer to grow along their [101] direction. Figure 2C shows a typical transmission electron microscope (TEM) bright-field image of an individual winglike sheet for sample O3. The sheets are about 300 nm in width. A ripple-like contrast observed in the TEM image is due to the strain resulting from the bending of the nanowire. For understanding the growth orientation of this winglike sheet, the HRTEM images and the corresponding fast Fourier transform pattern (inset) of the junction region (marked “1”) and two arms region (marked “2” and “3”) are shown in Figure 2, panels 1-3, respectively. Through the combining of the HRTEM observation with FFT analysis, it can be concluded that the arms have the different growth orientation. For the region of “2”, the nanowires prefer to grow along the [200] direction, which is the same as the growth of SnO2 nanobelts synthesized with the chemical vapor deposition method.17 However, the spacing of 0.264 nm
Influence of Oxygen Flow Rate on SnO2 Nanostructures
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Figure 2. (A) HRTEM image and corresponding SAED pattern (in the inset) of sample O1. (B) HRTEM image of sample O2; the corresponding FFT pattern is shown in the inset. (C) TEM image of an individual winglike sheet for sample O3. HRTEM images and the corresponding FFT patterns (the insets) of (1-3) showing the junction region and two arms region of sample O3, respectively.
corresponds to the distance between (101) planes, indicating 〈101〉 as the growth direction for the region “3”. To explore the influence of oxygen flow rate on nanostructures, XRD is used to study the crystalline structure and the crystalline alignment. Figure 3 shows XRD patterns of the O1, O2, O3, O4, and O5 samples. For each sample, all observed peaks can be indexed with the rutile-type tetragonal structure of SnO2 (International Centre for Diffraction Data card no. 411445). Under the limit of instrument sensitivity, no characteristic peaks of impurities, such as other forms of tin oxides or Sn, were detected. It is worth noting that, as the oxygen flow rate is lower than 2 sccm, the SnO2 phase has the (110) preferred orientation, while the (101) and (200) peaks of SnO2 increase with the flow rate of oxygen and become the preferred orientation under the higher flow rate of oxygen. This change may be related with the morphology of SnO2 varied with the
Figure 3. XRD patterns of samples O1, O2, O3, O4, and O5.
oxygen flow rate (as reflected in Figure 2). It is necessary to point out that any difference in lattice parameters upon growing at different flow rates of oxygen is hardly seen from the XRD
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Figure 4. Magnetic hysteresis loops of samples O1, O2, O3, O4, and O5 measured at RT, where the contributions from the diamagnetism of the Si substrate and the PM of the nanowires were deducted.
data because there is almost no shift in positions of peaks. Oxygen content in the samples is assumed to change, but, in fact, its effects on the lattice parameters are tiny and hard to be seen. Figure 4 presents the magnetic hysteresis (M-H) curves of the samples grown at the different oxygen flow rates measured at RT with the maximum applied magnetic field of 8 kOe, where the contributions from the paramagnetism (PM) of the samples were deducted. It is observed that samples O1, O2, O3, and O4 are clearly ferromagnetic at RT and their loops have small coercivities in the range from 90 to 130 Oe, whereas sample O5 shows nonmagnetic behavior. It is worth noting that the magnetism of the samples is very sensitive to the oxygen flow rate. A large special magnetization of 0.081 emu/g is observed for O1, much larger than that of SnO2 nanoparticles (
