2924
J. Phys. Chem. C 2007, 111, 2924-2928
Structural, Optical, and Magnetic Properties of Co-doped SnO2 Powders Synthesized by the Coprecipitation Technique A. Bouaine and N. Brihi Laboratoire d’Etude des Mate´ riaux (LEM), UniVersite´ de Jijel 18000, Jijel, Algeria
G. Schmerber, C. Ulhaq-Bouillet, S. Colis,* and A. Dinia Institut de Physique et Chimie des Mate´ riaux de Strasbourg (IPCMS), UMR 7504 ULP-CNRS (ULP-ECPM), 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France ReceiVed: October 20, 2006; In Final Form: December 5, 2006
We have used the coprecipitation technique to synthesize polycrystalline Co-doped SnO2 diluted magnetic semiconductors with Co concentrations of 0, 0.5, and 2.0%. X-ray diffraction patterns showed for all samples the expected SnO2 tetragonal structure with no additional peaks corresponding to parasitic phases. Transmission electron microscopy (TEM) did not indicate the presence of magnetic parasitic phases and confirmed that Co ions are uniformly distributed inside the samples. Optical absorption measurements showed an energy band gap which decreases when increasing the Co concentration. Raman spectroscopy shows an intensity loss of classical cassiterite SnO2 vibration lines, which is an indication of significant structural modifications and disorder of the SnO2 lattice. Magnetization measurements revealed a mixture of paramagnetic and antiferromagnetic behavior for Co-doped SnO2 with no sign of ferromagnetism.
I. Introduction In recent years, the interest for the physical properties of diluted magnetic semiconductors (DMSs) has significantly increased due to their potential technological applications in the field of optoelectronics, magnetoelectronics, and microwave devices. Since the work of Matsumoto et al.,1 doping semiconducting oxides with transition metals (TM) in order to induce room-temperature ferromagnetism has become one of the main challenges for many research groups. Many studies report on high-temperature ferromagnetism in oxide materials such as TMdoped anatase (TiO2),1-3 zinc oxide (ZnO),4-9 and tin dioxide (SnO2)10-20 with TM ) Co, Ni, Cr, Mn, V, and Fe. Among these oxides, tin dioxide (SnO2) presents special properties, such as transparency or remarkable chemical and thermal stabilities, with direct applications for photodetectors, catalysts for oxidation and hydrogenation, solar cells, semiconducting gas sensors, liquid crystal displays, protective coatings, and starting materials for indium-tin oxide films used as transparent conducting electrodes.21-24 SnO2 has the rutile-type tetragonal structure belonging to the P42/mnm space group. The lattice parameters are a ) b ) 4.7382 Å and c ) 3.1871 Å, and the band energy-gap is in the ultraviolet range between 3.5 and 3.8 eV as estimated from experimental results and theoretical calculations.10,13,16,25 Many works reported ferromagnetic properties of TM-doped SnO2 thin films. Ogale et al.16 reported room-temperature ferromagnetism in pulsed laser deposited SnO2:Co (5 and 27%) thin films. More recently, Fitzgerald et al.10 found ferromagnetism in Co-doped SnO2 thin films for Co contents ranging from 0.1 to 15%. Nevertheless, ferromagnetism is much more difficult to find in polycrystalline samples. Punnoose et al.12 * To whom correspondence should be addressed. E-mail: colis@ ipcms.u-strasbg.fr.
detected room-temperature ferromagnetism in Co-doped SnO2 powders prepared in the 350-600 °C range only in samples containing less than 1% of Co. For higher concentrations, ferromagnetism vanished and the samples showed a paramagnetic behavior. Moreover, the formation of a very small fraction of metastable SnO2 orthorhombic phase and the presence of Co3O4 was also evidenced when the Co concentration exceeded 8%. These controversial observations raise questions about the intrinsic nature of ferromagnetism in the Co-doped SnO2 system. This is of particular importance since recent studies on thin films DMS suggested that ferromagnetism could have an extrinsic origin.26,27 In this work, we have synthesized undoped and Co-doped SnO2 powders with two Co doping concentrations of 0.5 and 2.0 at. % using the coprecipitation technique. For both doped samples, we do not observe ferromagnetism but only a mixture of paramagnetism and antiferromagnetism, which is in contradiction with earlier publications.10-20 II. Experimental Procedures Undoped and Co-doped SnO2 powders were synthesized by coprecipitation technique. SnCl2‚2H2O and CoCl2‚6H2O in the proportion 50:1 were first dissolved in 100 mL of deoxygenated distilled water. The salts were then precipitated at 80 °C using 100 mL of NH4OH and kept at this temperature for several hours. The precipitate was filtered, thoroughly washed, and dried at 40 °C. Annealing treatments of 1 h in air were performed at 400 and 750 °C to obtain the final Co-doped SnO2 powder. The cationic composition of the powder was checked by means of energy dispersive X-ray spectroscopy (EDS). The crystalline quality and the grain size of the samples was evaluated using X-ray diffraction (XRD) measurements. The study was carried out using a Siemens D-5000 diffractometer using a Cu KR1 radiation (λ ) 1.54056 Å) in the θ - 2θ geometry.
10.1021/jp066897p CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007
Properties of Co-Doped SnO2 Powders
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2925
Figure 1. Room-temperature X-ray diffraction of SnO2:Co (0, 0.5, and 2.0%) powders prepared at 400 and 750 °C for 1 h. The continuous lines and the circles represent the experimental data and the Rietveld fit, respectively. The indicated peak indexation corresponds to the rutile structure of SnO2.
TABLE 1: Summary of Grain Size Values of SnO2:Co (0, 0.5, and 2.0%) Powders Prepared at 400 and 750 °C for 1 h, as Calculated from the (211) Peak of the X-ray Spectra
Figure 2. High-resolution TEM images (a and b) and electron diffraction patterns (c and d) recorded on SnO2:Co (0.5%) and SnO2: Co (2.0%) powders prepared at 750 °C. The white arcs from panels c and d are a guide for the eyes for the electron diffraction patterns corresponding to the SnO2 structure. The red arcs correspond to small traces of orthorhombic SnO phase (space group Pm21n).
grain size (nm) sample
T ) 400 °C
T ) 750 °C
SnO2 SnO2:Co 0.5% SnO2:Co 2.0%
85.6 21.9 27.9
59.7 30.7 40.7
The crystalline structure, the Co ion distribution of the powders and the presence of parasitic phases were evaluated by transmission electron microscopy (TEM) using a TOPCON 002B microscope with a point to point resolution of 1.8 Å and coupled with an EDS analyzer. In addition to TEM, Raman and UV-vis-NIR spectroscopies were used to check the insertion of the Co ions in the SnO2 lattice. The Raman spectroscopy measurements were obtained in back-scattering geometry with a Renishaw Spectra-Physics 164 spectrometer. The excitation was provided by an argon laser operating at a wavelength of 514.5 nm. The Raman shifts were corrected by using a Si reference spectrum. The optical measurements were taken with a conventional UV-vis-NIR Perkin-Elmer spectrometer at room-temperature. The magnetic properties of the Co-doped SnO2 powders were carried out with a Superconducting Quantum Interference Device (SQUID) magnetometer. III. Results and Discussion Figure 1 shows the room-temperature X-ray diffraction spectra for SnO2:Co (0, 0.5, and 2.0%) prepared at 400 and 750 °C for 1 h. The Rietveld profile analysis of the spectra revealed the expected rutile structure of SnO2 with a P42/mnm symmetry. No additional phases such as the SnO2 orthorhombic
phase, metallic Co, or other SnO or CoO-based phases are observed. The lattice parameters of SnO2 were estimated at a ) 4.7446(6) Å and c ) 3.1914(1) Å. These parameters decrease by less than 0.15% when 2% of Co are inserted in the SnO2 matrix. This small variation is consistent with the smaller radius of the Co2+ ion (0.58 Å) with respect to the Sn4+ radius (0.69 Å),13 and with the small Co concentration used for doping. The crystallite size was estimated using the Scherrer formula and their values are reported in Table 1. It is interesting to note that for a given calcination temperature, the grain size decreases strongly when Co is inserted in the SnO2 matrix, due to the different sizes of the ionic radius and valences between the Co and Sn ions. In order to check the presence of small grains corresponding to eventual parasitic phases that could not be detected by X-ray diffraction, transmission electron microscopy (TEM) highresolution images and electron diffraction patterns were recorded for the SnO2:Co (0.5%) and SnO2:Co (2.0%) samples prepared at 750 °C (Figure 2). Both samples consist of aggregates of uniform crystallites with an average size in good agreement with that estimated from the XRD data. Although many regions of the two samples were observed, no trace of magnetic parasitic phases or clustering phenomena could be detected. However, small traces of SnO orthorhombic phase (space group Pm21n) were evidenced, probably due to deviations from the local O stoichiometry. This can be easily explained by the redox effect observed on oxides heated at high temperatures. Nevertheless, since Co ions are uniformly distributed inside the sample, as shown by local energy dispersive X-ray spectroscopy (EDS)
2926 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Figure 3. Room-temperature optical absorption spectra for SnO2:Co (0, 0.5, and 2.0%) powders prepared at 400 and 750 °C. The inset shows the variation of the energy band gap as a function of the Co concentration.
observations, and since in a crystalline grain the atomic planes are continuous, with a uniform and free of distortions contrast, we can fairly assume that most of Co2+ ions substitute Sn4+ in the SnO2 matrix. In order to confirm that Co2+ has been substituted for Sn4+, optical measurements have been performed at room-temperature by UV-vis-NIR spectroscopy. Figure 3 shows the optical absorption spectra for SnO2:Co (0, 0.5 and 2.0%) prepared at 400 and 750 °C. The spectrum of undoped SnO2 shows a sharp absorption edge at 337 nm (3.68 eV) that corresponds to the theoretical band gap Eg of SnO2. An interesting result is the red shift of the Eg edge from 3.68 to 3.45 eV when inserting 2% of Co in the SnO2 matrix (inset of Figure 3). This decrease in the energy band gap was already observed in Co and Mn doped SnO2,16,18 as well as in other transition metal doped oxides.28,29 This observation can be explained on the basis of the sp-d exchange interactions between the band electrons and the localized d electrons of the Co2+ ions substituting Sn4+ ions. The s-d and p-d exchange interactions give rise to a negative and positive corrections to the conduction-band and valenceband edges, respectively, leading to a band gap narrowing. Figure 3 shows no traces of Co3O4 that should give a characteristic absorption line at 1050 nm.13 The only “peaks” that are observed (e.g., at 380 and 870 nm) are due to the filter change noise and have therefore no physical meaning. In order to confirm the substitution of Sn4+ by Co2+ ions, Raman spectroscopy measurements have been also performed. The SnO2 has a unit cell that consists of two tin and four oxygen atoms. The 6 unit cell atoms give a total of 18 branches for the vibration modes in the first Brillouin’s zone. The mechanical representation of the normal vibration modes at the center of the Brillouin’s zone is given by31,32
Γ ) Γ1+ (A1g) + Γ2+ (A2g) + Γ3+ (B1g) + Γ4+ (B2g) + Γ5(Eg) + 2Γ1- (A2u) + 2Γ4- (B1u) + 4Γ5+ (Eu) (1) Figure 4 shows the room-temperature Raman spectra of SnO2: Co (0, 0.5, and 2.0%) powders. The most intense Raman peak at 634 cm-1 can be attributed to the A1g mode. The Raman bands at 476 and 776 cm-1 are the vibration modes Eg and B2g, respectively. The A1g and B2g modes are nondegenerate and vibrate in the plane perpendicular to the c axis, whereas the doubly degenerated Eg mode vibrates in the direction of the c axis.31,32
Bouaine et al
Figure 4. Room-temperature Raman spectra for SnO2:Co (0, 0.5, and 2.0%) powders prepared at 400 and 750 °C.
The addition of 0.5 and 2.0% of Co induce a broadening and an intensity reduction of the principal peaks at 476, 634, and 776 cm-1, indicating that microstructural transformations occurred. However, as in the case of optical measurements, no additional peaks typical of the incorporation of Co in the SnO2 matrix are observed. This loss in the intensity of the Raman peaks could be related to the changes of the grain size of the doped samples with respect to the undoped ones, since the lattice parameters showed only limited variations with the Co concentrations. Other authors13 had already similar results, but observed two additional peaks at 300 and 692 cm-1 that were attributed to substitution of Sn4+ with Co2+ in the SnO2 matrix or to the Co3O4 parasitic phase. As Co2+ substitutes for Sn4+ and since the mass of Co is lower than the mass of Sn, an additional vibration mode of Co is expected at higher frequencies.14 The position of this Co vibration mode can be estimated using the mass defect equation
ω ) ωM
x11 -- f
(2)
where ωM is the maximum of the TO phonons (here 634 cm-1), ) 1 - (MCo/MSn) ) 0.5, and f is the relative value of optical and acoustical phonon density of states which can be taken equal to the value of the TiO2 system, i.e., 0.72. From this equation, a Co vibration mode is expected at 718 cm-1. It is interesting to note that in our case, although no peak is observed at 300 cm-1, a peak at 692 cm-1 is evidenced solely in the pure SnO2 sample. This peak cannot therefore be attributed neither to parasitic phases nor to the insertion of Co in the SnO2 lattice. Similar remarks can be made on the evolution of the Raman spectra with the calcination temperature. The intensity of the peaks in the powders prepared at 400 °C is systematically smaller than the ones observed in the powders prepared at 750 °C. As the peak intensity is directly correlated with the insertion of Co in the SnO2 matrix, this is an important indication on the influence of temperature on the quality of the transition metal doping of magnetic oxides. Figure 5 shows the magnetization curves measured at 5 K for SnO2:Co (0, 0.5, and 2.0%) powders prepared at 750 °C, compared to pure SnO2 which presents as expected a diamagnetic behavior with a negative magnetic susceptibility. The Codoped powders have a magnetization loop typical for paramagnetic systems with no indication of ferromagnetism, the coercive field and the remnant magnetization being very close to zero.
Properties of Co-Doped SnO2 Powders
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2927 is quite surprising in our case since the substitution of Sn4+ ions by Co2+ ions is expected to give rise to a natural p type conduction. IV. Conclusion In conclusion, we have successfully synthesized SnO2:Co (0, 0.5, and 2.0%) diluted magnetic semiconductor powders using the coprecipitation method. The rutile structure of SnO2 was confirmed by the X-ray diffraction and TEM studies, and no presence of metallic cobalt or other magnetic parasitic phases could be detected. Nevertheless, the magnetization measurements have shown no evidence of ferromagnetism for the lowdoped samples due to the absence of free carriers.
Figure 5. Low-temperature magnetization curves of SnO2:Co (0, 0.5, and 2.0%) powders prepared at 750 °C. The inset reports the variation of the inverse susceptibility with the temperature for the powder containing 2.0% of Co. The continuous line is a linear fit of the variation in the high-temperature regime.
Moreover, it is important to note that the value of the magnetization at 5 T (M5T) of the 2.0% Co-doped SnO2 (1.48 emu/g) and the 0.5% Co-doped SnO2 (0.79 emu/g) samples are in a ratio of about 2 although a ratio of 4 is expected considering the Co concentrations. This is equivalent with the reduction of the magnetic moment per Co ion when increasing the Co concentration. Such deviation from a linear proportionality between M5T and the Co concentration constitutes an indication of the existence of antiferromagnetic interactions between the Co ions through O atoms. Indeed, fitting the magnetization curves using a Brillouin function by keeping constant the total angular momentum J at 3/2 as expected for Co2+ has led to Co concentrations two times smaller than the ones corresponding to the real values estimated by EDS. Such conclusions, indicating a mixture of paramagnetic and antiferromagnetic interactions, have already been drawn for other transition metal doped semiconductors prepared by the coprecipitation technique.30 Additional information on the magnetic behavior of our samples can be obtained from the variation of the inverse molar susceptibility 1/χ with temperature T (inset of Figure 5). This variation is consistent with the existence of a mixture of paramagnetic and antiferromagnetic interactions. The existence of antiferromagnetic interactions is evidenced as well in the χT vs T variation (not reported here) which decreases monotonically with the decreasing temperature. When the Co concentration is 2.0% the inverse susceptibility can be well fitted using a CurieWeiss law
1 T-θ ) χ C
(3)
where C the Curie constant, θ is the Curie-Weiss temperature, and T is the temperature. In order to obtain θ, which is a measurement of the strength and nature of the magnetic interactions, we have extrapolated the fitted straight lines of 1/χ recorded in the high-temperature regime. θ is negative (about -140 K) confirming the existence of antiferromagnetic interaction between the magnetic ions. As suggested in the case of other magnetic oxides, the absence of ferromagnetism can be due to the absence of free carriers which mediate the interaction between magnetic ions. The absence of free carriers
Acknowledgment. The authors thank G. Pourroy for fruitful discussions. This work was supported by the CMEP (No 04MDU608) contract between ULP-CNRS (IPCMS) and the Universite´ F. Abbas of Se´tif, Algeria. References and Notes (1) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Science 2001, 291, 854. (2) Punnoose, A.; Seehra, M. S.; Park, W. K.; Moodera, J. S. J. Appl. Phys. 2003, 93, 7867. (3) Manivannan, A.; Seehra, M. S.; Majumder, S. B.; Katiyar, R. S. Appl. Phys. Lett. 2003, 83, 111. (4) Rao, C. N. R.; Deepak, F. L. J. Mater. Chem. 2005, 15, 573. (5) Ueda, K.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 2001, 79, 988. (6) Kane, M. H.; Shalini, K.; Summers, C. J.; Varatharajan, R.; Nause, J.; Vestal, C. R.; Zhang, Z. J.; Ferguson, I. T. J. Appl. Phys. 2005, 97, 023906. (7) Dinia, A.; Schmerber, G.; Me´ny, C.; Pierron-Bohnes, V.; Beaurepaire, E. J. Appl. Phys. 2005, 97, 123908. (8) Ndilimabaka, H.; Colis, S.; Schmerber, G.; Mu¨ller, D.; Grob, J. J.; Gravier, L.; Jan, C.; Beaurepaire, E.; Dinia, A. Chem. Phys. Lett. 2006, 421, 184. (9) Sharma, P.; Gupta, A.; Rao, K. V.; Owens, F. J.; Sharma, R.; Ahuja, R.; Osorio, Guillen, J. M.; Johansson, B.; Gehring, G. A. Nat. Mater. 2003, 2, 673. (10) Fitzgerald, C. B., Venkatesan, M.; Dorneles, L. S.; Gunning, R.; Stamenov, P.; Coey, J. M. D.; Stampe, P. A.; Kennedy, R. J.; Moreira, E. C.; Sias, U. S. Phys. ReV. B 2006, 74, 115307. (11) Coey, J. M. D.; Douvalis, A. P.; Fitzgerald, C. B.; Venkatesan, M. Appl. Phys. Lett. 2004, 84, 1332. (12) Punnoose, A.; Hays, J.; Gopal, V.; Shutthanandan, V. Appl. Phys. Lett. 2004, 85, 1559. (13) Hays, J.; Punnoose, A.; Baldner, R.; Engelhard, M. H.; Peloquin, J.; Reddy, K. M. Phys. ReV. B 2005, 72, 075203. (14) Mathew, X.; Hays, J.; Mejia-Garcia, C.; Contreras-Puente, G.; Punnoose, A. J. Appl. Phys. 2006, 99, 08M101. (15) Punnoose, A.; Engelhard, M. H.; Hays, J. Solid State Comm. 2006, 139, 434. (16) Ogale, S. B.; Choudhary, R. J.; Buban, J. P.; Lofland, S. E.; Shinde, S. R.; Kale, S. N.; Kulkarni, V. N.; Higgins, J.; Lanci, C.; Simpson, J. R.; Browning, N. D.; Das, Sarma, S.; Drew, H. D.; Greene, R. L.; Venkatesan, T. Phys. ReV. Lett. 2003, 91, 077205. (17) Kimura, H.; Fukumura, T.; Kawasaki, M.; Inaba, K.; Hasegawa, T.; Koinuma, H. Appl. Phys. Lett. 2002, 80, 94. (18) Park, Y. R.; Kim, K. J. J. Appl. Phys. 2003, 94, 6401. (19) Hong, N. H.; Ruyter, A.; Prellier, W.; Sakai, J.; Huong, N. T. J. Phys: Condens. Matter 2005, 17, 6533. (20) Hong, N. H.; Sakai, J.; Huong, N. T.; Poirot, N.; Ruyter, A. Phys. ReV. B 2005, 72, 045336. (21) Tadeev, A. V.; Delabouglise, G.; Labeau, M. Thin Solid Films 1999, 337, 163. (22) Zang, G. Z.; Wang, J. F.; Chen, H. C.; Su, W. B.; Wang, C. M.; Qi, P. J. Phys. D: Appl. Phys 2005, 38, 1072. (23) Mohagheghi, M. M. B.; Saremi, M. S. Semicond. Sci. Technol. 2004, 19, 764. (24) Punnoose, A.; Reddy, K. M.; Hays, J.; Thurber, A.; Engelhard, M. H. Appl. Phys. Lett. 2006, 89, 112509.
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