Investigation of the Influence of Organometallic Precursors on the

Jennifer N. Duggan and Christopher B. Roberts. The Journal of ... Zhenguo Chen , Jie Liu , Andrew J. Evans , Laura Alberch , and Alexander Wei. Chemis...
1 downloads 0 Views 374KB Size
Download PDF
10320

J. Phys. Chem. C 2007, 111, 10320-10328

Investigation of the Influence of Organometallic Precursors on the Formation of Cobalt Nanoparticles Rohini M. de Silva,† Vadim Palshin,† Frank R. Fronczek,‡ Josef Hormes,† and Challa S. S. R. Kumar*,† Centre for AdVanced Microstructures and DeVices, Louisiana State UniVersity, 6980 Jefferson Highway, Baton Rouge, Louisiana 70806, and Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed: January 20, 2007; In Final Form: April 26, 2007

An investigation of the influence of organometallic precursors on the formation of cobalt nanoparticles is presented. The cobalt nanoparticles were obtained by decomposing two different organometallic cobalt complexes, [(Co2(µ-HCtCH)(CO)6] and [Co2(CO)8], under identical experimental conditions. Time-dependent FT-IR analysis revealed different decomposition routes, rates, and reaction intermediates leading to the formation of cobalt nanoparticles. Spectroscopic data in addition to X-ray crystallography confirmed the structures of reaction intermediates from [(Co2(µ-HCtCH)(CO)6]. The crystal structure, particle size, size distribution, and magnetic properties of the cobalt nanoparticles from the two precursors were analyzed using synchrotron radiation based X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry, respectively, and showed significant differences.

1. Introduction A great deal of research continues to be focused on nanometer-sized materials in general and cobalt-based magnetic materials in particular due to their potential applications in the field of electronics,1 high-density data storage media,2 catalysis,3 and biomedical sciences.4 Numerous physical and chemical methods such as sputtering,5 chemical vapor deposition,6 reverse micelle synthesis,7 mechanical milling,8 solution-phase metal salt reduction,9-13 and decomposition of neutral organometallic precursors14 have already been well-established to produce cobalt nanoparticles (NPs). Of these, high-temperature wetchemical reactions, as compared to other techniques, are particularly attractive as they are known to offer better control over size, size distribution, shape, and crystal structure of cobalt nanoparticles. For example, high-temperature reduction of salts such as CoCl2,10 CoI2,11 Co(CH3COO)2,9,12 and Co(acac)313 using lithium and sodium compounds in the presence of stabilizing agents is known to provide controlled particle sizes and avoid agglomeration of cobalt nanoparticles. The thermal decomposition of dicobalt octacarbonyl (DCO) under inert atmospheric conditions in the presence of surfactants is known to produce cobalt NPs of controlled size, shape, and crystal structure.9-10,14-17 The effect of reaction conditions such as temperature,18 time of addition of reagents,16 solvents,19 utilization of surfactants,9-10,20 polymeric stabilizers,21 and type of reaction vessels (for example, utilization of microreactors)22 for control of cobalt nanoparticle formation is well-documented. It is also reported that the particle size and size distribution are affected by the actual position of injection of the precursor into the reaction vessel.16 Surfactants, in particular, have the ability to control not only the particle size but also the shape and crystal structure.14b,17,23 The decomposition of DCO in the presence of † ‡

Centre for Advanced Microstructures and Devices. Department of Chemistry.

trioctylphosphine oxide (TOPO) resulted in obtaining -cobalt nanoparticles.24 However, in the absence of TOPO, fcc cobalt nanoparticles were obtained.24 The synthesis of -cobalt nanoparticles by the thermal decomposition of DCO has been achieved using oleic acid and triphenyl phosphine25 or a mixture of surfactants composed of oleic acid (OA), lauric acid, and trioctyl phosphine (TOP).20 The synthesis of multiply twinned fcc cobalt nanoparticles was accomplished by thermal decomposition of DCO in the presence of OA and tributyl phosphine.26 The -cobalt and fcc-cobalt phases require annealing at 300500 °C to convert into the hcp phase.5b,10,27 Alivisatos et al.16a have reported direct synthesis of hcp Co nanoparticles eliminating the need for annealing at high temperatures. Chaudret’s group synthesized hcp Co nanoparticles by thermolysis of [Co(η3-C8H13)(η4-C8H12)].28 In order to obtain high quality singlecrystal cobalt NPs, careful selection of surfactants is essential,14b in addition to manipulation of particle growth kinetics accomplished by adjusting the capping agents and their ratios.15a,16 It is believed that the capping molecules bind to crystal faces in a selective manner slowing down the growth of the cappingagent-bound crystal face relative to the other faces resulting in obtaining highly anisotropic crystal shapes such as discs,16 rods,23,28 and wires29 of cobalt NPs. While the effect of surfactants, reaction conditions, and different types of reactors on the formation of Co nanoparticles has been investigated, the influence of different precursors has not been examined systematically. Such an investigation assumes more importance as it is well-established that properties of cobalt nanoparticles are extremely sensitive to the presence of even minute surface impurities.7b,30,31 Recently, Gugliotti et al.32 revealed the importance of the ligands bound to the metal center of organometallic precursors in controlling the particle shape and size of Pd and Pt nanoparticles. They observed that changing the ligand dibenzylideneacetone (DBA) bound to Pd and Pt metals with triphenyphosphine (PPh3) resulted in particles

10.1021/jp070499k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007

Influence of Co Nanoparticle Precursors

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10321

Figure 1. Schematic representation of the decomposition of two organometallic cobalt complexes under identical reaction conditions.

with different sizes and shapes under identical reaction conditions. However, to the best of our knowledge similar studies to understand the effects of the nature of precursor on the formation of cobalt NPs have not been undertaken. It is also to be noted that DCO is the only organometallic cobalt complex that has been extensively investigated as a precursor even though there are other organometallic cobalt complexes that have been reported as precursors for cobalt nanoparticles. For example, Chaudret et al. used the thermolysis of [Co(η3-C8H13)(η4C8H12)]21c,30 in the presence of hydrogen to obtain different sizes and shapes of cobalt NPs. Similarly, sonication of the complex [Co(CO)3(NO)] in decane solution in the presence of oleic acid led to the formation of amorphous cobalt nanoparticles.33 In a recent publication, Lagunas et al. have reported a mechanistic investigation of the effect of additives (trioctylphosphine oxide [TOPO] and oleic acid) on the kinetics and rate of the thermal decomposition of DCO and identified reaction intermediates.35a However, no comparative studies on the influence of different precursors, for example with reference to DCO, on the formation of cobalt nanoparticles have been reported. In order to initiate investigations in this direction, we have identified two organometallic cobalt complexes, alkynebridged dicobalthexacarbonyl [(Co2(µ-HCtCH)(CO)6] and the well-known dicobalt octacarbonyl. The two precursors were decomposed under identical reaction conditions, and the mechanism of their decomposition was monitored (Figure 1). The cobalt nanoparticles obtained were characterized, and their properties were compared. The results obtained from these studies are given below. 2. Experimental Section The alkyne-bridged dicobalt hexacarbonyl compound [(Co2(µ-HCtCH)(CO)6] (ADH) was prepared using the procedure reported in the literature.34 Dicobalt octacarbonyl [Co2(CO)8] was purchased from Alfa Aesar. Cobalt nanoparticles starting from [Co2(CO)8] or [(Co2(µ-HCtCH)(CO)6] were synthesized by modifying the known method and under inert atmospheric conditions.3b In brief, solution containing oleic acid in dioctyl ether was degassed for 30 min under nitrogen atmosphere. The surfactant solution is then heated to 90 °C. A dioctyl ether

solution of [Co2(CO)8] or [(Co2(µ-HCtCH)(CO)6] was quickly added, and the temperature was increased to 240 °C (it took ∼25 min to reach this temperature). The reaction mixture was kept at this temperature for 30 min, after which the reaction was allowed to cool to room temperature. A black precipitate was obtained on addition of ethanol. The precipitated Co nanoparticles were isolated, washed with ethanol, dried, and analyzed. The FT-IR spectra were obtained, using a Nexus 670 FT-IR spectrometer in transmission mode, for neat samples at different reaction times by removing aliquots of approximately 1 mL of the reaction mixture and after subtracting the solvent spectrum of dioctyl ether. The transmission electron microscopy (TEM) analysis was carried out using Hitachi H-7600 with a 125 kV accelerating voltage. TEM samples were prepared by dropping hexane solution of cobalt nanoparticles onto a carbon coated copper grids and evaporating the solvent. Particle sizes, size distributions, and standard deviations were calculated manually by measuring about 100 particles in each TEM image. Electron diffraction patterns of cobalt nanoparticles, prepared from precursors ADH and DCO, were obtained using JEOL 2010 (200 kV accelerating voltage) and Hitachi 7000 (100 kV accelerating voltage). Cobalt K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were carried out at the Double-Crystal Monochromator (DCM) beamline at the 1.3 GeV electron energy storage ring synchrotron radiation facility of the Centre for Advanced Microstructures & Devices (CAMD) of Louisiana State University and as described previously.22,31 The molecular structure of the intermediate [Co3(CO)9CCH3] was determined using diffraction data collected at 150 K using graphite monochromated Mo KR radiation on a Nonius Kappa CCD diffractometer. The 1H NMR spectra and ESI mass spectra were recorded on Bruker DPX 250 and QSTAR XL quadrapole TOF spectrometers, respectively. The amount of cobalt in the cobalt nanoparticles was determined using inductively coupled plasma (ICP) optical emission spectrometry, performed using a Spectro Ciros CCD spectrometer.

10322 J. Phys. Chem. C, Vol. 111, No. 28, 2007

Figure 2. FT-IR spectra of the reaction mixture containing DCO and oleic acid in dioctyl ether medium taken after 1, 5, 10, and 15 min of the reaction.

de Silva et al.

Figure 4. Molecular structure of the intermediate, [Co3(CO)9CCH3], including the atom numbering scheme.

Figure 3. FT-IR spectra of the mixture of ADH and oleic acid in dioctyl ether medium taken after 1, 5, 7, 10, 15, and 20 min of reaction time.

3. Results and Discussion While the mechanistic investigations on the decomposition of DCO reported earlier were carried out in o-dichlorobenzene,35a we found dioctyl ether to be a more suitable solvent for decomposition of DCO and ADH as both the precursors are readily soluble in it. In situ FT-IR is a very useful tool in mechanisticinvestigationoftheformationofcobaltnanoparticles,35a and the decomposition process for the two precursors was monitored using FT-IR by focusing on the disappearance of the carbonyl peak in the IR spectrum. FT-IR spectra of the samples taken at regular intervals during the decomposition of DCO and the ADH are given in the Figures 2 and 3, respectively. FT-IR spectra for ADH and DCO in dioctyl ether at room temperature are also shown in the figures for comparison. Three strong absorption bands at 2022, 2041, and 2069 cm-1 and a weak band at 1854 cm-1 with a shoulder at 1867 cm-1 in Figure 2 are characteristic of the terminal and bridging CO bonds, respectively, in DCO.35b The first sample removed 1 min after the addition of Co2(CO)8, still at 90 °C, shows two strong bands at 2054 and 2063 cm-1 and a medium absorption band at 1864 cm-1 corresponding to the terminal and bridging

Figure 5. (a) IR spectra of the complex [(Co2(µ-HCtCH)(CO)6] (orange) and the mixture of complexes [Co3(CO)9CCH3] and [Co3(CO)9CCOOCH3] (purple). (b) Comparison of the IR spectrum of the mixture of complexes [Co3(CO)9CCH3] and [Co3(CO)9CCOOCH3] (green) with the reaction mixture removed at 7 min (red).

CO bonds, respectively, in Co4(CO)12.35c It is apparent from the IR data that the decomposition of DCO into Co4(CO)12 is a facile process and completes in less than a minute. As the

Influence of Co Nanoparticle Precursors

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10323

Figure 6. TEM micrograph of cobalt nanoparticles obtained from (a) [(Co2(µ-HCtCH)(CO)6] and (b) [Co2(CO)8]. The size distributions are shown in the parts c and d, respectively.

reaction proceeds further, no additional changes of these peaks were observed except for reduction in their intensity due to the consumption of Co4(CO)12. Additional spectra were recorded (but not shown) in between the time intervals given in the figure in order to ensure no additional peaks are appearing within that time frame. After 10 min, we can barely see any peaks related to CO, and therefore, it can be concluded that the signals due to soluble cobalt carbonyl species do not exist after this time. Similar results for the decomposition DCO were also reported by other researchers.35,36 In the case of ADH, decomposition under identical conditions monitored by FT-IR at regular intervals shows no formation of the intermediate, Co4(CO)12 (Figure 3). However, the color of the reaction mixture changed from an initial orange color to a deep purple color within 5 min (see Figure S1 in Supporting Information), unlike in the DCO case where no such color change was observed. The compound(s) responsible for this color was investigated by isolation using column chromatography. It was found that a mixture of tricyclic organocobalt complexes, [Co3(CO)9CCOOCH3] and [Co3(CO)9CCH3], having the same color and identical Rf values in the chromatographic column as reported in the literature,37 are responsible for this color. It was difficult to separate the two compounds due to similarities in Rf values. However, the compound [Co3(CO)9CCH3] crystallized from the mixture at low temperature and was characterized using single X-ray crystallographic analysis.38 The molecular structure of this complex is shown in Figure 4 together with the atomic numbering scheme. Crystallographic experimental details, atomic coordinates, bond lengths, angles and other parameters are given in the Supporting Information (S4 and Tables S1-S3). As both of these complexes contain similar C3V symmetry for the Co3(CO)9 fragment, they both exhibit identical carbonyl stretching frequencies. However, the enhanced intensity band observed for the purified mixture at 1740 cm-1 (Figure 5a) can be attributed to the ester CO group attached to the carbon of

[Co3(CO)9CCOOCH3]. The presence of the complex [Co3(CO)9CCOOCH3] was also further confirmed by 1H NMR and ESIMS spectrometries (see Figures S2 and S3 in the Supporting Information). As can be seen from Figure 5a, the IR bands at 2102(w), 2051(vs), 2036(s), and 2017(w) cm-1 of the intermediate tricobalt complexes are very close to the original ADH complex where the bands appear at 2097(w), 2057(s), and 2030(vs) cm-1. However, the sample removed at 7 min clearly shows the presence of only these complexes (Figure 5b). After that, no additional peaks corresponding to any other carbonyl complexes were observed apart from the decrease in the concentration of the two compounds and changes in color of the solution from purple to brown/black after about 17 min (Figure S1). The IR spectrum of the sample removed at 20 min shows that no soluble cobalt carbonyl species are present beyond this time. In the case of DCO, all the signals due to CO groups bonded to cobalt disappeared shortly after 10 min even at 160 °C, whereas, for the ADH, it took about 17-20 min for completion of decomposition. The first conclusion of the FT-IR investigation is that there are differences in decomposition rates and pathways for the two precursors leading to the formation of cobalt nanoparticles. Different decomposition rates, depending on the thermal stability of precursors/intermediates, lead to different concentrations of initial Co nuclei formed. Usually metal-CO bond strength in pure metal carbonyl clusters is weaker than that of the substituted metal carbonyl clusters,39 and hence, there is a higher rate of decomposition of DCO. This higher decomposition rate of DCO leads to a larger number of nuclei leading to smaller size particles while in the case of ADH a slower decomposition rate has given rise to a smaller number of nuclei followed by particle growth leading to bigger particles. This argument is supported by the TEM analysis. The TEM photographs show the mean sizes of the nanoparticles obtained from ADH and DCO at the end of the reaction (30 min) (Figure 6). The particle size was found to be 6.2 nm (σ ) 10%) and

10324 J. Phys. Chem. C, Vol. 111, No. 28, 2007

de Silva et al.

Figure 7. Co K-edge normalized XANES spectra of cobalt nanoparticles obtained at the end of the reaction (30 min).

Figure 8. Magnitude of the non-phase-corrected Fourier transform of χ(k)k0, ∆k ) 2.1-12.5 Å-1, of Co nanoparticles from ADH and DCO. Also shown are theoretical FT EXAFS spectra generated by IFEFFIT.

3.1 nm (σ ) 27%) for Co nanoparticles obtained from ADH and DCO, respectively. Therefore, even though the reaction was carried out under identical conditions, cobalt nanoparticles obtained from the precursor ADH are bigger and more monodispersed than those obtained from the precursor DCO. However, the morphology (not a perfect sphere) of the particles appears to be similar in both cases. In order to further understand these influences on the formation of cobalt nanoparticle and in turn on their physical properties, a detailed synchrotron radiation-based X-ray absorption spectroscopy of the cobalt nanoparticles was carried out. To prevent oxidation during the sample preparation, samples were prepared in a nitrogen-filled glove box and loaded into the beam line in a sealed container with no exposure to air. The phase structure of the freshly prepared Co nanoparticles was analyzed by comparing their spectral features to those of the theoretical spectra of the fcc, hcp, bcc, and  phases of cobalt. Figure 7 shows the spectra of Co nanoparticles obtained from ADH and DCO precursors. The theoretical hcp, fcc, bcc, and  cobalt spectra calculated using FEFF8 code are given in Figure

S4 in Supporting Information.40 The most dramatic differences between the theoretical spectra can be observed in the white line region; i.e., the first strong peak above the absorption edge (approximately 7725-7735 eV for Co). The  and bcc phases have only one peak in the middle of the white line (about 7730 eV) while the hcp and fcc structures have two well-resolved peaks (around 7727 and 7734 eV); the difference between the fcc and hcp phase structure is the relative intensity of these two peaks. The reliability of these calculations is clearly visible when comparing the theoretical and experimental spectra of hcp-Co. Good agreement with the experiment is achieved with respect to all spectral features: the characteristic pre-edge structure and the double-peak white line where the second peak amplitude are higher than that of the first peak. However, the two peaks in the white line are not as clearly resolved as in the simulated spectra and we can only observe “shoulders” at their respective energies. This is particularly true for the nanoparticle samples. The shape of the white line of the spectrum of Co nanoparticles from ADH shows more resemblance to the theoretical spectrum of fcc-Co, with the maximum of intensity around 7727 eV, as opposed to 7733 eV in hcp spectra. While exact crystal structure based solely on comparison with calculated XANES spectra cannot be assigned, we can conclude that both nanoparticle spectra match either hcp or fcc model, possibly containing a mixture of these two phases, which is a common situation for Co. The Fourier transform (FT) EXAFS fingerprinting and fitting discussed below provide additional information for positive identification of the crystal structures. The FT EXAFS spectra of both samples show observable features at distances as large as 6 Å (Figure 8) indicating the presence of short- to medium-range order beyond the first coordination shell. As in XANES, positions and relative amplitudes of the peaks provide a distinct fingerprint for each phase, Figures 8 and S7. Again, the spectra of the cobalt nanoparticles show the most resemblance to those of the hcp and fcc structures. The patterns produced by hcp and fcc phases show many similarities with the first coordination shells being nearly identical; however, there are sufficient differences in the higher shells to attempt to differentiate between these two phases. Upon visual inspection of the 3-5 Å area in the FT spectra, the spectrum of ADH Co nanoparticles has more

Influence of Co Nanoparticle Precursors resemblance to the fcc phase fingerprint, and the DCO spectrum exhibits a more hcp-like shape. The first peak in the FT EXAFS spectra appears around 2.1 Å (all distances in the FT EXAFS spectra are not phase-shift corrected and are shifted to lower values compared to the true bond lengths). The first coordination shell peaks in both samples do not seem to have a low-R peak or shoulder below 2 Å typically produced by the presence of a light back scatterer, such as O or C, coordinated to the absorbing atom. Although it is logical to expect such coordination at the surface of the cobalt nanoparticles, it was not detectable by XAS in these samples. As a first step, fitting was performed over the 1.6-2.9 Å range which corresponds to the first Co‚‚‚Co coordination shell. As expected, both hcp and fcc models produce excellent fits in this range, with nearly identical parameters. In order to obtain more detailed structural information, fitting of higher coordination shells was performed over the range 1.6-5.2 Å using both hcp and fcc models. In all the fits, the degeneracy values for all scattering paths in a fit were modified by the same factor, and one common value for ∆r, as well as the Debye-Waller factor (or mean square displacement) σ2, was used for all scattering paths. This allowed preservation of the distinct fingerprint of each phase and a study of the effect of different models on the quality of the fit. The EXAFS fitting results are summarized in Table S4. Only fcc and hcp models are reported, since bcc and  cobalt models failed to produce acceptable fits. In order to establish which model produces the best fit, we compared the goodness of fit parameters, reduced χ-square χ2v, and the R-factor, as well as examined the fits visually. If ν ) Nind - Nvarys is the number of degrees of freedom in the fit (number of independent points minus the number of variables), then one standard deviation in the χ2v value is x2/ν. The difference between the χ2v values of two models, (χ2v1 - χ2v2)/χ2v2, needs to be greater than 2x2/ν for it to be statistically significant, and for one model to be considered better than the other. In our study, ν ) 12 for both samples. The above criterion is satisfied for the Co nanoparticles from DCO, in combination with the R-factors and visual inspection of the fits providing proof of the predominantly hcp character of this sample. For the Co nanoparticles from ADH, the situation is somewhat more complicated. The χ2v factors are essentially identical, and the R-factor of the fcc model is only slightly lower. However, upon the visual examination of the fits it becomes clear that the fcc model resulted in a better match for both peak positions and amplitudes, especially in the 3.5-5 Å range which includes the third and fourth coordination shells (Figure 9a). The assignment is also confirmed from electron diffraction analysis (see Figure S5 in Supporting Information). The opposite was observed for the Co nanoparticles prepared from DCO (Figure 9b), where the hcp model resulted in much better fit quality parameters, and from the visual inspection of the fits, the hcp model is clearly better than fcc at reproducing the Co nanoparticle spectrum. However, the assignment could not be confirmed from the electron diffraction pattern of the Co nanoparticles prepared from DCO.35a It would have been very helpful to have XRD analysis. However, the signal-to-noise ratio was very small when attempts were made to obtain XRD patterns using Cu KR source. Overall, it can be concluded, based on the synchrotron radiation based XAS analysis, that the Co NPs obtained from ADH and DCO have different crystal arrangements. In addition to the differences in the crystal structure, cobalt nanoparticles obtained from the two different precursors were also found to have different magnetic properties. The temper-

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10325

Figure 9. (a) Fits of the FT EXAFS of Co nanoparticles from ADH. (b) Fits of the FT EXAFS of Co nanoparticles from DCO. Fitting was done with R-Co and β-Co models over ∆R ) 1.6-5.2 Å range, k-range ) 2.1-2.5 Å-1, k-weights ) 1, 2, 3.

ature dependence of magnetization was measured in an applied magnetic field of 100 Oe between 2 and 300 K using zerofield-cooling (ZFC) and field-cooling procedures, and the results are shown in Figure 10. Comparison of the ZFC/FC curves of Co nanoparticles from ADH (Figure 10a) with Co nanoparticles from DCO (Figure 10b) reveals a sharp increase in the magnetic moment below 15 K for both ZFC and FC curves in the case of Co nanoparticles from DCO, unlike in the case of Co nanoparticles from ADH. The origin of such an increase has been attributed to a thin oxide shell on the cobalt nanoparticles.23b This observation was also supported from XAS studies (Figure S6 in the Supporting Information) where Co nanoparticles from DCO were found to be less oxidatively stable compared to those from ADH. The ZFC/FC curves also evidence a typical superparamagnetic behavior with blocking temperatures TB ) 170 and 30 K for the cobalt nanoparticles prepared from ADH and DCO, respectively. Such superparamagnetic behavior at room temperature is also supported by the data from the hysteresis curves at 300 K. Both the Co nanoparticles, from

10326 J. Phys. Chem. C, Vol. 111, No. 28, 2007

de Silva et al.

Figure 10. Temperature dependence of magnetization measured after zero-field cooling (ZFC) and field cooling (FC) at 100 Oe for Co nanoparticles obtained from (a) [(Co2(µ-HCtCH)(CO)6] and (b) [Co2(CO)8]. Hysteresis loops of cobalt nanoparticles obtained from (c) [(Co2(µ-HCtCH)(CO)6] and (d) [Co2(CO)8] at 10 and 300 K. Insets in parts c and d show a magnified view of the loops at 10 K.

ADH (Figure 10c) and DCO (Figure 10d), do not show any hysteresis at room temperature. However, both types of particles show hysteresis at 10 K indicating ferromagnetic behavior at lower temperatures. The magnified portion of the hysteresis loops obtained at 10 K, given in insets in Figure 10a,b, supports the ferromagnetic behavior of both the particles at 10 K. In addition, the Co nanoparticles from ADH show a larger coercive field of 1577 Oe compared to the Co nanoparticles from DCO with only 533 Oe at 10 K. The differences in crystal phases and magnetic properties of cobalt nanoparticles formed from the two different precursors can also be attributed to the variations in surfactant binding modes, as previously reported. 16 In order to confirm this, cobalt nanoparticles obtained from ADH and DCO were thoroughly washed with ethanol there by ensuring that only chemically bound oleic acid (OA) or any other organic intermediates will remain on the surface of the particles. FT-IR spectra of these particles are shown in Figure 11. As seen from the figure, no bands are found at 1710 cm-1 for both cobalt nanoparticles obtained from DCO and ADH, evidencing the absence of pure oleic acid and the presence of

oleate in the samples. Both spectra also show the presence of two new bands at 1550 and 1409 cm-1 corresponding to asymmetric and symmetric stretching, respectively, for the COO- group. The splitting for the absorption bands of valency vibrations νas(OCO) and νs(OCO) (∆ν ) νas - νs)41 in the IR spectra of cobalt nanoparticles of DCO and ADH precursors have values around 145, suggesting a bridging bidentate where two oxygen atoms in the carboxylate are coordinated symmetrically to the Co atoms as previously identified.12 However, further differences in the νs(OCO) band of the oleic acid bound to Co nanoparticles from ADH split into a doublet (1409 and 1380 cm-1) suggest the presence of two nonequivalent carboxylate groups such as chelating bidentate and bridging bidentate architectures.42 In addition, variation in the intensities of the doublet (1050 and 1022 cm-1) in the CO stretching band (1020-1100 cm-1) region also supports this argument (see Figure S8 in the Supporting Information). These differences in the nature of the binding of oleic acid could be responsible for variations in the oxidative stability of the two types of cobalt nanoparticles, as it is known that the nature of the stabilizing agent can affect their oxidative stability.43 In order to investigate

Influence of Co Nanoparticle Precursors

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10327

Figure 11. FT-IR spectra of Co nanoparticles obtained from DCO and ADH. Only the 2000-800 cm-1 range is shown to identify difference in the bonding mode.

this possibility, Co nanoparticles obtained from ADH and DCO were exposed to atmospheric conditions, and their air stability was monitored using Co K-edge XANES. As can be seen from the XANES spectra of samples exposed to air for a period 1 month (see Figure S6 given in the Supporting Information), Co nanoparticles formed from ADH are relatively more stable against oxidation in comparison with Co NPs obtained from DCO. 4. Conclusion The investigations reported here demonstrate that the nature of precursors strongly influences the formation of cobalt nanoparticles synthesized wet-chemically. The precursors influence not only the formation of cobalt nanoparticles but also their physical properties such as particle size, crystal structure, magnetic properties, and oxidative stability. The cobalt nanoparticles obtained from ADH are bigger in size with low polydispersity, have fcc structure, and are stable against aerial oxidation. In comparison, those obtained from DCO are smaller in size with high polydispersity, have hcp structure, and are relatively less stable against aerial oxidation. Similarly, the cobalt nanoparticles obtained from ADH have significantly better magnetic properties (higher blocking temperature and coercivity) than those obtained from DCO. It is difficult to pinpoint, on the basis of the data we have generated so far, how exactly the precursors are influencing the formation of cobalt nanoparticles and in turn their properties. However, two possible scenarios can be envisaged. First, the presence of reaction intermediates from the two precursors interacting with the surfaces of nuclei could be influencing the formation of cobalt nanoparticles differently. Second, nucleation and growth kinetics arising due to different decomposition rates of precursors and the reaction intermediates could also be responsible. The differences in particle size, size distribution, and crystal structure can be explained on the basis of the variations in nucleation and growth rates. However, the differences in the oxidative stability can only be explained on the basis of the changes in surface coatings arising either from precursors/intermediates during the cobalt nanoparticle formation or from variations in binding modes of the surfactants on the final particle. Further

work is in progress in order to investigate which of the two scenarios are predominant and how they influence the properties of cobalt nanoparticles. It is anticipated that investigations in this direction will lead to elucidation of a comprehensive relationship between the structures of organometallic cobalt precursors and the properties of cobalt nanoparticles obtained. Acknowledgment. This work was financially supported by a grant from DARPA (Grant HR0011-04-C-0068). Authors gratefully acknowledge the support of Dr. Nalin De Silva. Authors also thank Dr. A. Maverick and Dr. E. Nesterov for use of their laboratory facilities and Dr. Larry Henry, Southern University A&M College, Baton Rouge, for assistance with SQUID measurements. Dr. Orhan Kizilkaya’s support with FTIR instrumentation is gratefully acknowledged. We are thankful to anonymous reviewers for their critical comments and suggestions. Supporting Information Available: Color changes observed during the decomposition of [(Co2(µ-HCtCH)(CO)6], 1H NMR spectrum of the mixture of intermediates, ESI-MS spectrum of the mixture of intermediates, crystallographic data file for Co3(CO)9CCH3, including experimental details for the crystallographic analysis and tables of selected geometrical parameters, FEFF8 calculated XANES spectra for different crystal structures of cobalt, EXAFS fitting results, electron diffraction spectra, Co K-edge XANES investigation of the oxidative stability Co NPs obtained using the two precursors ADH & DCO, non-phase-corrected FT EXAFS, and expanded region of FT-IR of the final Co NPs from ADH and DCO. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Heath, J. R. Acc. Chem. Res. 1999, 32, 388. (2) Charap, S. H.; Lu, P. L.; He, Y. IEEE Trans. Magn. 1997, 33, 978. (3) (a) Klingelho¨fer, S.; Heitz, W.; Greiner, A.; Oestreich, S.; Foster, S.; Antonietti, M. J. Am. Chem. Soc. 1997, 119, 10116. (b) Kim, S. W.; Son, S. U.; Lee, S. S.; Hyeon, T.; Chung, Y. K. Chem. Commun. 2000, 2212.

10328 J. Phys. Chem. C, Vol. 111, No. 28, 2007 (4) (a) Bao, Y.; Krishnan, K. M. J. Magn. Magn. Mater. 2005, 293, 15. (b) Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536. (5) (a) Kitakami, O.; Sato, H.; Shimada, Y.; Sato, F.; Tanaka, M. Phys. ReV. B 1997, 21, 13849. (b) Sato, H.; Kitkami, O.; Sakurai, T.; Shimada, Y.; Otani, Y.; Fukamichi, K. J. Appl. Phys. 1997, 81, 1858. (6) Billas, I. M. L.; Chaˆtelain, A.; de Heer, W. A. J. Magn. Magn. Mater. 1997, 168, 64. (7) (a) Petit, C.; Pilen, M. P. J. Magn. Magn. Mater. 1997, 166, 82. (b) Connolly, J.; Pierre, T. G.; Rutnakornpituk, M.; Riffle, J. S. J. Phys. D: Appl. Phys. 2004, 37, 2475. (c) Chen, J. P.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 76, 6316. (8) Huang, J. Y.; Wu, Y. K.; Ye, H. Q. Acta Mater. 1996, 44, 1201. (9) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47. (10) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (11) Pelecky, D. L. L.; Bonder, M.; Martin, T.; Kirkpatrick, E. M.; Liu, Y.; Zhang, X. Q.; Kim, S. H.; Rieke, R. D. Chem. Mater. 1998, 10, 3732. (12) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383. (13) Cha, S. I.; Chan, B. M.; Kim, K. T.; Hong, S. H. J. Mater. Res. 2005, 20, 2148. (14) (a) Masala, O.; Seshadri, R. Annu. ReV. Mater. Res. 2004, 34, 41. (b) Green, M. Chem. Commun. 2005, 3002 and references therein. (15) (a) Puntes, V. F.; Krishnan, M. K.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (16) (a) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (b) Wuhn, M.; Weckesser, J.; Woll, Ch. Langmuir 2001, 17, 7605. (17) Hyeon, T. Chem. Commun. 2003, 927. (18) Holzwarth, A.; Lou, J.; Hatton, A. T.; Laibinis, P. E. Ind. Eng. Chem. Res. 1998, 37, 2701. (19) Leslie-Pelecky, D. L.; Zhang, X. Q.; Rieke, R. D. J. Appl. Phys. 1996, 79, 5312. (20) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Appl. Phys. Lett. 2001, 78, 2187. (21) (a) Abes, J. I.; Cohen, R. E.; Ross, C. A. Chem. Mater. 2003, 15, 1131 and references therein. (b) Tadd, E. H.; Bradley, J.; Tannenbaum, R. Langmuir 2002, 18, 2378. (c) Wostek-Wojciechowska, D.; Jeszka, J. K.; Amiens, C.; Chaudret, B.; Lecante, P. J. Colloid Interface Sci. 2005, 287, 107. (22) Song, Y.; Hartwig, M.; Henry, L. L.; Saw, C. K.; Doomes, E. E.; Palshin, V.; Kumar, C. S. S. R. Chem. Mater. 2006, 18, 2817. (23) (a) Shukla, N.; Svedberg, E. B.; Ell, J.; Roy, A. J. Mater. Lett. 2006, 60, 1950. (b) Bao, Y.; Beerman, M.; Pakhomov, A. B.; Krishnan, K. M. J. Phys. Chem B 2005, 109, 7220. (24) Dinega, D. P.; Bawendi, M. G. Angew., Chem. Int. Ed. 1999, 38, 1788. (25) Yang, H. T.; Shen, C. M.; Su, Y. K.; Yang, T. Z.; Gao, H. J.; Wang, Y. G. Appl. Phys. Lett. 2003, 82, 4729. (26) Wang, Z. L.; Dai, Z.; Sun, S. AdV. Mater. 2000, 12, 1944. (27) Petit, C.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 15309. (28) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M. C.; Casanove, M. J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286.

de Silva et al. (29) Cha, S. I.; Mo, C. B.; Kim, K. T.; Hong, S. H. J. Mater. Res. 2005, 20, 2148. (30) Respaud, M.; Broto, J. M.; Rakoto, H.; Fert, A. R.; Thomas, L.; Barbara, B.; Verelst, M.; Snoeck, E.; Lecante, P.; Mosset, A.; Osuna, J.; Ely, T. O.; Amiens, C.; Chaudret, B. Phys. ReV. B 1998, 57, 2925. (31) Hormes, J.; Modrow, H.; Bonnemann, H.; Kumar, C. S. S. R. J. Appl. Phys. 2005, 97 (10, Pt. 3), 10R102/1-10R102/6. (32) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. J. Am. Chem. Soc. 2005, 127, 17814. (33) Shafi, K. V. P. M.; Gedanken, A.; Prozorov, R. AdV. Mater. 1998, 10, 590. (34) Sternburg, H. W.; Greenfield, H.; Wotiz, J. H.; Friedel, R. A.; Markby, R.; Wender, I. J. Am. Chem. Soc. 1954, 76, 1457. (35) (a) Lagunas, A.; Jimeno, C.; Font, D.; Sola, L.; Pericas, M. A. Langmuir 2006, 22 (8), 3823. (b) Tannenbaum, R. Inorg. Chim. Acta 1994, 227, 233. (c) Tannenbaum, R.; Bor, G. J. Organomet. Chem. 1999, 586, 18. (d) Diana, F. S.; Lee, S-H.; Petroff, P. M.; Edward, J. K. Nano Lett 2003, 3, 891. (36) Rutnakornpituk, M.; Thompson, M. S.; Harris, L. A.; Farmer, K. E.; Esker, A. R.; Riffle, J. S.; Connolly, J.; St. Pierre, T. G. Polymer 2002, 43, 2337. (37) (a) Sutton, P. W.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89, 261. (b) Markby, R.; Wender, I.; Friedel, R. A.; Cotton, F. A.; Sternburg, S. W. J. Am. Chem. Soc. 1958, 80, 6529. (38) Suitable crystals of [Co3(CO)9CCH3] for the X-ray analysis were grown as dark purple plates from a saturated hexane solution of a mixture of [Co3(CO)9CCH3] and [Co3(CO)9CCOOCH3] at 4 °C. The structure was determined using diffraction data collected at 150 K using graphite monochromated Mo KR radiation on a Nonius KappaCCD diffractometer. Crystal data follow: triclinic, space group P1h, a ) 7.7450(10) Å, b ) 8.7295(11) Å, c )12.1758(15) Å, R ) 86.927(8)°, β ) 82.201(6)°, γ ) 67.662(7)°, V ) 754.39(16) Å3, Z ) 2, µ(Mo KR) ) 3.30 mm-1, 25541 reflections collected with θ < 36.3°, 7061 unique, Rint ) 0.027; R ) 0.040, wR2 ) 0.113, refined on F2. Co-Co distances are in the range 2.4669(4)-2.4770(5) Å. Crystals are destroyed by an apparent phase change at lower temperatures. (39) (a) Housecroft, C. E.; Wade, K.; Smith, B. C. J. Chem. Soc., Chem. Commun. 1978, 765. (b) Smith, D. W. J. Chem. Soc., Chem. Commun. 1976, 834. (40) (a) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. ReV. B 1998, 58, 7565. (b) Modrow, H.; Bucher, S.; Rehr, J. J.; Ankudinov, A. Phys. ReV. B 2003, 67, 035123. (c) Gilbert, B.; Frazer, B. H.; Belz, A.; Conrad, P. G.; Nealson, K. H.; Haskel, D.; Lang, J. C.; de Stasi, G. J. Phys. Chem. A 2003, 107, 2839. (d) Hallmeier, K. H.; Uhlig, L.; Szargan, R. J. J. Electron Spectrosc. Relat. Phenom. 2002, 122, 91. (e) Reich, A.; Pantho¨fer, M.; Modrow, H.; Wedig, U.; Jansen, M. J. Am. Chem. Soc. 2004, 126, 14428. (41) (a) Simon-Kutscher, J.; Gericke, A.; Huhnerfuss, H. Langmuir 1996, 12, 1027. (b) Ren, Y.; Iimura, K-I, Kato, T. Langmuir 2001, 17, 2688. (42) Ptaszynski, B.; Zwolinska, A. J. Therm. Anal. Calorim. 2004, 75 (1), 301. (43) Bonnemann, H.; Brijoux, W.; Brinkmann, R.; Matoussevitch, N.; Waldofner, N.; Palina, N.; Modrow, H. Inorg. Chim. Acta 2003, 350, 617624.