Synthesis and Magnetic Properties of Pure Cubic CoO Nanocrystals

The TEM images of 3.8 nm (estimated with the Scherrer equation) are illustrated in Figure 2. The strong ring patterns from selected area electron diff...
2 downloads 0 Views 3MB Size
Download PDF
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3353–3358

Articles Synthesis and Magnetic Properties of Pure Cubic CoO Nanocrystals and Nanoaggregates I. Panagiotopoulos,*,† V. Alexandrakis,‡ G. Basina,‡ S. Pal,§ H. Srikanth,§ D. Niarchos,‡ G. Hadjipanayis,| and V. Tzitzios*,‡ Department of Materials Science and Engineering, UniVersity of Ioannina, 45 110, Ioannina, Greece, Institute of Materials Science, N.C.S.R. “Demokritos” Agia ParaskeVi 15310, Athens, Greece, Department of Physics, UniVersity of South Florida, Tampa, Florida 33620, and Department of Physics & Astronomy, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed June 19, 2008; ReVised Manuscript ReceiVed June 16, 2009

ABSTRACT: Cubic (fcc) CoO nanoparticles and nanoaggregates in the sub-15 nm range have been prepared by the decomposition of Co(NO3)2 in polyethylene-glycol (PEG-600) at 250 °C. The particles have been characterized by X-ray diffraction, transmission electron microscopy, and cognate techniques. The particles are very stable in nonpolar organic solvents because of the organic coating that occurs in situ. Anomalous magnetic properties, such as ferromagnetic features and shifted magnetic hysteresis loops, are observed indicating the presence of both rotatable and fixed uncompensated surface spins with high local anisotropy. Introduction Colloidal nanocrystals of transition metal oxides have attracted great attention in the past decade because of their potential applications in many technological fields.1,2 The dependence of physical properties on the size, shape, and structural phases makes nanocrystals promising building blocks for materials with designed functionalities. In particular, CoO nanoparticles are technologically important owing to their potential applications based on magnetic, catalytic, and gassensing properties.3-6 Although there are a few reports in the literature on the preparation of CoO in bulk form, this material is difficult to obtain in pure form by simple methods in the sub-15 nm range, often being contaminated with Co3O4 or Co metal phases. While bulk CoO is antiferromagnetic, with the susceptibility showing some direction dependence, there have been conflicting reports on the magnetic properties of the nanoparticles.7 CoO nanoparticles in the 10-80 nm range have been prepared by heating the gel precursor obtained by drying a solution of Co(NO3)2 and poly(vinyl alcohol) at 225 °C in a H2 atmosphere.8 Ghosh et al.9 synthesized fcc CoO nanoparticles in the 4.5-18 nm range by the decomposition of Co(II)cupferronate in decalin at 270 °C under solvothermal conditions. It is interesting that * To whom correspondence should be addressed. (V.T.) Phone: +302106503321; fax: +302106519430; e-mail: [email protected]; (I. P.) Phone: +3026510 97182; e-mail: [email protected]. † University of Ioannina. ‡ Institute of Materials Science, N.C.S.R. “Demokritos”. § University of South Florida. | University of Delaware.

nanoparticles with sizes less than 16 nm show ferromagnetic features below 10 K. Risbud et al.10 reported the synthesis of nearly pure wurtzite CoO involving the decomposition of Co(acac)2 in refluxing benzyl ether. Magnetic measurements indicate ferromagnetic behavior which arises from the second fcc-Co phase and not from wurtzite CoO. Monodispersed 3 nm fcc CoO nanoparticles have been synthesized by the treatment of dicobalt octacarbonyl with 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) in THF at room temperature, in the presence of oleic acid.11 In addition, 4.5-33 nm monodisperse CoO nanocrystals were synthesized by the decomposition of Co(acac)2 in oleylamine.12,13 In the present work, we report the use of commercially available, environmentally friendly and inexpensive poly ethyleneglycol-600 (PEG-600) as the medium for the synthesis of pure fcc CoO nanocrystals and 3-D nanostructures with different particle sizes in the sub-15 nm range by using a very simple chemical procedure that involves the decomposition of Co(NO3)2. Experimental Section Synthetic Procedure. In a typical experimental procedure fcc CoO nanoparticles were synthesized by thermolytic decomposition of Co(NO3)2 in commercial PEG-600 at 250 °C. The preparation of nanoparticles was carried out in a 100 mL spherical flask. The flask was charged with 20 mL of PEG-600, 3 mmol of oleic acid, 3 mmol of oleyl amine, and the temperature was raised to 150 °C. To the hot mixture 0.5-2 mmol of the cobalt precursor was added under vigorous stirring. The reaction mixture increased to 250 °C and remained at this temperature for 60 min. After the mixture was cooled to room temperature, the dark solution of fcc CoO nanoparticles was precipitated

10.1021/cg8006487 CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009

3354

Crystal Growth & Design, Vol. 9, No. 8, 2009

Figure 1. XRD patterns of 3.8 nm (a), 9.2 nm (b), and 11.7 nm (c) fcc CoO nanocrystals. by the addition of ethanol. The precipitate was isolated by centrifugation and washed with ethanol several times in order to remove the excess of the PEG and capping agents. Finally, the washed nanoparticles were dispersed in ethanol and dried at room temperature by spreading on a glass plate. Characterization. The crystal structure of the as-prepared CoO nanoparticles was characterized by X-ray powder diffraction (XRD) (Siemens D 500 Cu Ka radiation) and energy dispersive X-ray spectroscopy (EDX). The morphology was investigated by using transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), and scanning electron microscopy (SEM). The as-prepared nanoparticles were also characterized using Fourier transform infrared (FT-IR) spectroscopy. Magnetic hysteresis curves were measured using a commercial Physical Properties Measurement System (PPMS) from Quantum Design. High field measurements were performed in a 33 T magnet with Bitter coil in the High Field Magnet laboratory (HFML) in Radboud University Nijmegen.

Results and Discussion Structural and Morphological Characterization. The XRD patterns of the CoO samples provided in Figure 1 clearly show pure nanocrystalline cubic phase. The diffraction peaks of the cubic phase are well matched with those of the corresponding bulk CoO (Fm3m, R ) 4.261 Å), while peaks are slightly broadened with decreasing nanocrystal size. The calculated lattice parameters based on (200) peak is 4.265, 4.269, and 4.263 Å for the 3.8, 9, and 11.7 nm CoO particles, respectively, values that are in very good agreement with the standard value of bulk fcc CoO. Size control of cubic CoO nanocrystals was also achieved by variations in the reaction conditions. From the XRD patterns, there is no evidence for the presence of other oxide phases such as hcp CoO or Co3O4. Also although the PEGs are well-known mild reducing agents, in the case of Co(NO3)2 there are no metallic Co traces. It is worth mentioning that if the reaction takes place only in PEG with Co(acac)2 as precursor and lower surfactant concentration leads mainly to the formation of metallic fcc Co nanoparticles as we can see from the XRD pattern (Supporting Information, Figure 1S). The TEM images of 3.8 nm (estimated with the Scherrer equation) are illustrated in Figure 2. The strong ring patterns from selected area electron diffraction (SAED) shown in the inset of Figure 2 can be well indexed to the fcc CoO structure. The particles have a nearly spherical shape and diameter between 3-5 nm, a value that is in good agreement with those estimated from the Scherrer equation. The single crystallinity of the particles was established by the high-resolution TEM (HR-TEM) images. The calculated lattice spacing from the

Panagiotopoulos et al.

image is 2.44 Å and corresponds to the interplanar separation between (111) lattice planes. In Figure 3, the TEM, HR-TEM, and dark field images of 11.7 nm CoO particles are represented. The particles reveal a uniform-sized cubic shape (9-14 nm), which coincides with the calculated value from Scherrer equation (11.7 nm). A very important issue is that the particles are organized in larger structures that reveal uniformity in the size as well as the shape. Although this is a usual phenomenon for metal nanoparticles,14-16 it is the first time that is observed in CoO nanostructures. These aggregates were symmetrically grown, in almost spherical shapes with a narrow size distribution between 60 to 80 nm. Another important issue revealed here concerns the driving force of this phenomenon. The fact that it was not observed in the other samples, covered by the same capping agents, shows that the driving force does not originate from the organic molecules but probably from the concentration effect of the inorganic Co precursor. The aggregate formation may also be due to van der Waals interactions which are stronger when the particle size is increasing. The synthesis of the smaller particles (3.8 and 9.2 nm) was carried out with 25 and 60 mM Co(NO3)2 concentrations, respectively, while in the case of 11.7 nm particles, which organized in 3-D structures, the Co2+ concentration was 100 mM. Finally, the CoO particles are highly crystalline as seen in the HRTEM and bright field images. The size and the uniformity of the 3-D CoO nanoagreggates were confirmed by SEM images (Figure 4). As can be seen, the 3-D nanoagreggates have uniform spherical shape and narrow size distribution with about 60 nm average diameter. The as-prepared capped CoO nanoparticles are highly soluble (25 mg/mL) in nonpolar solvents such as toluene, hexane, and chloroform producing organosols that are stable for months without signs of decomposition or precipitation. We must emphasize that both capping molecules are necessary for achieving highly soluble nanoparticles. Thus, the fact that capping with only oleic acid or only oleyl amine yields nanoparticles poorly soluble in organic solvents suggests that during synthesis both oleic acid and oleyl amine become chemically bonded to the surface atoms of the nanoparticles. This grafting is clearly testified by IR spectroscopy. Figure 5 shows the FT-IR spectrum of capped CoO nanoparticles. The spectra show strong absorptions at 2924 and 2852 cm-1 due to the CH2 stretching of the aliphatic chains, whereas a weak peak at 3005 cm-1 is characteristic of the cis -HC)CH- arrangement in the oleic acid and oleyl amine. The peak at 1626 cm-1 is due to the ν(CdC) stretching mode. The absorption at 1457 cm-1 is due to the CH2 scissor. The very broad peak in the 3430 cm-1 region is due to the ν(N-H) stretching mode, which suggest that oleyl amine is adsorbed on the surface of CoO nanocrystals. The existence of two bands at 1554 cm-1 (asym) and 1412 cm-1 (sym) indicates unidentate attachment of the carboxylate anion to the surface of the nanocrystals. Unidentate binding is further reinforced from the difference ∆ν ) νas - νs (1554-1412 ) 142 cm-1). These results strongly indicate that the surface of the CoO nanocrystals are modified with both oleic acid and oleyl amine molecules. Magnetic Properties. Magnetic Properties of 3.8 nm fcc CoO Nanoparticles. Antiferromagnetic fine particles are known to exhibit anomalous magnetic properties, such as large moments, coercivities, thermo-remnant magnetization, nonsaturation at high fields, and hysteresis with shifted loops.17 These phenomena can be classified under the category of “weak ferromagnetism” and are typically explained as a result of incomplete canceling out of the magnetic moments of the two

Properties of Pure Cubic CoO Nanocrystals

Crystal Growth & Design, Vol. 9, No. 8, 2009 3355

Figure 2. TEM and HR-TEM images of 3.8 nm fcc CoO nanocrystals (scale bar is 5 nm).

Figure 3. TEM images of the hierarchical 3-D CoO nanostructures (a, b), HR-TEM image (c) and dark field image of 11.7 nm fcc CoO nanocrystals.

sublattices due to their finite size. In fact, the reduced coordination of surface spins could lead to a reconstruction of the

magnetic order throughout the particle.18,19 In this case, a clear distinction between surface and bulk spin contributions to the

3356

Crystal Growth & Design, Vol. 9, No. 8, 2009

Panagiotopoulos et al.

Figure 4. SEM images of fcc CoO nanocrystals organized in 3D spherical structure (11.7 nm estimated from the Scherrer equation).

Figure 5. FT-IR spectra of 11.7 nm fcc CoO nanocrystals.

Figure 6. Hysteresis loops for 3.8 nm fcc CoO nanoparticles measured at 5 K after field cooling (FC) in 50 kOe and zero field cooling (ZFC).

total magnetic moment is difficult to make. However, it is convenient to talk in terms of uncompensated surface spins which are often divided into free and pinned when discussing the magnetic properties of such particles or their ability to cause intrinsic exchange bias between the ferromagnetic phases.20,21 It should be noted that relatively weak exchange bias has been reported in some nanoparticles and ascribed to the coupling between disordered surface spins forming a spin-glass-like shell with the ordered core spins.22 In Figure 6, the magnetization curves measured at 5 K after field cooling in 50 kOe and zero field cooling are compared. In both curves, the high field linear part has a slope of 50 µemu/g kOe which is compatible with the susceptibility of the AF phase. The curves show practically no hysteresis, but the FC curve is displaced along the magnetization axis by 0.42 emu/g indicating the presence of fixed uncompensated surface spins. Within the

Stoner-Wohlfarth model, the hysteresis loop is symmetric since the magnetocrystalline anisotropy has inversion symmetry. A shifted loop with no hysteresis would result if the applied field is not sufficient to reverse the particle moment (we end up with what is termed a minor hysteresis loop). Thus, the anisotropy field of the pinned moments must be well above the applied field of 50 kOe. The simultaneous existence of a rotatable ferromagnetic component gives a slight S-shape to the magnetization curves. If the high field slope is subtracted, the rotatable ferromagnetic component is found to be the same (0.58 emu/g) in both curves. This indicates that the pinned moments (which manifest as loop displacement in the case of the FC loop) in the case of the ZFC are randomly distributed and remain fixed within the field strengths used in the measurement adding no contribution.

Properties of Pure Cubic CoO Nanocrystals

Crystal Growth & Design, Vol. 9, No. 8, 2009 3357

Figure 7. Low field part of hysteresis loops measured up to different values of maximum field (from 50 to 300 kOe) at 5 K after zero field cooling (ZFC). Lines are connecting points. The inset shows the dependence of the coercivity on the maximum field used. The line is a linear fit with a slope of 0.0024 ( 0.0002.

Figure 9. Magnetization as a function of temperature measured during warming up in a field of 1 kOe after field cooling in 50 kOe from 300, for different Co oxide samples. The inset shows a magnified region of the two lower curves.

Figure 8. Magnetization as a function of temperature measured during warming up in a field of 1 kOe after field cooling in 50 kOe from 300, 100, and 50 K. Note that the y-axis scale is logarithmic to enhance minor contributions.

Figure 10. Inverse susceptibility as a function of temperature for different Co oxide samples.

An estimation of the magnitude of the fields required to overcome the frozen-in random anisotropy of the ZFC state can be obtained by measuring the dependence of the coercivity (after ZFC) on the maximum field used to trace the hysteresis curve. A high field (33 T) magnet with Bitter coil was used for this type of experiment and the results are presented in Figure 7. The HC varies slowly as a function of the maximum applied field. It shows a linear dependence with a slope of 0.0024 up to 300 kOe. If we assume that this trend holds in higher fields, 2000 kOe would be required to reach the 5 kOe value observed in the displaced FC loop. This indicates the existence of extremely high anisotropy fields in the sample. The temperature dependence of the loop displacement can be derived by measuring the remanent magnetization as a function of temperature. Measurements were done during warming up under a field of 1 kOe after FC from 300 K, 100 and 50 K. Because of the large anisotropies in this type of sample, it is helpful to monitor the loop characteristics as a function of the maximum applied field. This way minor loop effects originating from the applied field limitations may be estimated.23 In the FC from the 300 K curve, the magnetization disappears around 175 K, which is well below the Ne´el temperature of the bulk antiferromagnetic CoO (291 K), as typically occurs in fine AF particles. The low magnetization values above 175 K when plotted as H/M vs T show a linear dependence and can be assigned solely to the Curie susceptibility of the AF phase, thus excluding the presence of any trace metallic phase (Figure 8). An estimate of the existing blocking temperature TB distribution can be obtained by field cooling from in-between temperatures. By field cooling from a temperature T* only the particles of

the sample with TB < T* will align contributing to the remanent magnetization. When field cooled from below 100 K, the remanent magnetization and loop displacement, respectively, are reduced. This indicates that only the higher TB components are those that contribute to the loop displacement. Magnetic Properties of Larger CoO Nanoparticles and 3-D Structures. In Figure 9, the thermoremanent magnetization as a function of temperature measured during warming up in a field of 1 kOe after field cooling in 50 kOe from 300 K, for different CoO samples is shown. The larger diameter particles and 3-D structure show only low TB components that correspond to loose surface spins. Accordingly, their FC loops show no displacement. For the 11.7 nm (3D) particles, a kink in the M vs T curve at 230 K that can be attributed to the Ne´el temperature is observed (inset of Figure 9). One could also argue that the magnetization upturn below 20 K is not an effect of loose uncompensated surface spins but could be attributed to the presence of a small Co3O4 secondary phase that is undetected within the XRD resolution. The bulk critical temperature of Co3O4 is 40 K, but a substantial decrease due to size or superparamagnetic effects cannot be excluded. As an example, in Figure 9 the magnetization curves of 12 nm Co3O4 particles obtained by heat-treating 8 nm CoO particles at 250 °C in air, are also plotted. They have a Ne´el temperature of 21 K. In 20 nm Co3O4 particles, this transition is found at 28 K.24 However, the curves of the Co3O4 nanoparticles are different. This difference becomes more apparent when the data are plotted versus T (Figure 10). There the AF transitions appear sharp and are followed by intervals of linear versus T dependence in contrast to the surface spin freezing ones. Furthermore, since the preparation method is the same for all the CoO samples a different surface oxidation state for the larger particles is not probable. We therefore tend to attribute the low temperature

3358

Crystal Growth & Design, Vol. 9, No. 8, 2009

upturn of the magnetization of the larger CoO nanoparticles to lose surface spins in accordance with the lack of FC loop displacement in all samples in which this effect is dominant. The larger values observed for the smaller size of 3.8 nm are in agreement with the surface origin of the weak ferromagnetism effects. This includes both increased uncompensated moment and surface anisotropy contributions. The blocking temperature of 175 K implies a quite large surface anisotropy value KS ) 1.14 erg/cm2,25 if we assume that the core of the particle retains its bulk anisotropy value of KV ) 3 × 106 erg/cm3.26 In the absence of the surface contribution the blocking temperature would be only 25 K. Summarizing, pure fcc CoO nanoparticles and sponge like 3-D nanostructures have been prepared by a very simple chemical route from the decomposition of Co(NO3)2 in PEG600 at 250 °C. The size of the particles and the morphology can be controlled by variations in the reaction conditions. In all particles, weak ferromagnetism phenomena are observed. In 3.8 nm particles, higher blocking temperatures and large FC loop displacements are obtained. The 11.7 nm particles are organized in larger almost spherical 3-D structures characterized by remarkable size and shape uniformity. This sponge-like organization as well as the narrow size distribution makes them attractive tools in nanotechnology and especially in homogeneous catalysis. Acknowledgment. The authors from USF (H.S. and S.P.) thank the DoD-USAMRMC for supporting this work through grant No. W81XWH-07-1-0708. Supporting Information Available: XRD pattern of Co nanoparticles prepared changing the reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Kahn, M. L.; Monge, M.; Snoeck, E.; Maisonnat, A.; Chaudret, B. Small 2005, 1, 221. (2) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H.-J.; Park, J.-H.; Bae, C. J.; Park, J.-G.; Hyeon, T. AdV. Mater. 2005, 17, 429.

Panagiotopoulos et al. (3) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogues, J. Nature 2003, 423, 850. (4) Gravelle, P. C.; Teichner, S. AdV. Catal. 1969, 20, 167. (5) Morup, S.; Madsen, D. E.; Frandsen, C.; Bahl, C. R. H.; Hansen, M. F. J. Phys.: Condens. Matter 2007, 19, 213203. (6) Lin, H. K.; Chiu, H. C.; Tsai, H. C.; Chien, S. H.; Wang, C. B. Catal. Lett. 2003, 88, 169. (7) Koshizaki, N.; Yasumoto, K.; Sasaki, T. Sensors Actuators B 2000, 66, 122. (8) Zhang, L.; Xue, D.; Gao, C. J. Magn. Magn. Mater. 2003, 267, 111. (9) Ghosh, M.; Sampathkumaran, E. V.; Rao, C. N. R. Chem. Mater. 2005, 17, 2348. (10) Risbud, A. S.; Snedeker, L. P.; Elcombe, M. M.; Cheetham, A. K.; Seshadri, R. Chem. Mater. 2005, 17, 834. (11) Lagunas, A.; Payeras, A. M.; Jimenoa, C.; Pericas, M. A. Chem. Commun. 2006, 1307. (12) Seo, W. S.; Shim, J. H.; Oh, S. J.; Lee, E. K.; Hur, N. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 6188. (13) Zhang, H.-T.; Chen, X.-H. Nanotechnology 2005, 16, 2288. (14) Pelzer, K.; Vidoni, O.; Philippot, K.; Chaudret, B.; Colliere, V. AdV. Funct. Mater. 2003, 13, 118. (15) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (16) Schmid, G.; Lehnert, A.; Malm, J.-O.; Bovin, J.-O. Angew. Chem., Int. Ed. 1991, 30, 874. (17) Morrish, A. H. In The Physical Principles of Magnetism; John Wiley: New York, 1965. (18) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Phys. ReV. Lett. 1997, 79, 1393. (19) Flipse, C. F. J.; Rouwelaar, C. B.; de Groot, F. M. F. Eur. Phys. J. D 1999, 9, 479. (20) Salazar-Alvarez, G. J. Am. Chem. Soc. 2007, 129, 9102. (21) Ohldag, H.; Scholl, A.; Nolting, F.; Arenholz, E.; Maat, S.; Young, A. T.; Carey, M.; Stohr, J. Phys. ReV. Lett. 2003, 91, 17203. (22) Nogues, J.; Sort, J.; Langlais, V.; Skumryev, V.; Surinach, S.; Munoz, J. S.; Baro, M. D. Phys. Rep. 2005, 422, 65. (23) Bodker, F.; Mørup, S.; Linderoth, S. Phys. ReV. Lett. 1994, 72, 282. (24) Mollard, P.; de Bergevin, F.; Germi, P.; Figlarz, M.; Fievet-Vincent, F. Phys. ReV. B. 1973, 8, 1265. (25) Roy, S.; Fitzsimmons, M. R.; Park, S.; Dorn, M.; Petracic, O.; Roshchin, I. V.; Li, Z.-P.; Batlle, X.; Morales, R.; Misra, A.; Zhang, X.; Chesnel, K.; Kortright, J. B.; Sinha, S. K.; Schuller, I. K. Phys. ReV. Lett. 2005, 95, 047201. (26) Makhlouf, S. A. J. Magn. Magn. Mater. 2002, 246, 184.

CG8006487