23August2019

Nano-Micro Letters

Synthesis and Thermal Behavior of Metallic Cobalt Micro and Nanostructures

Marlene González Montiel1, P. Santiago-Jacinto2, J. A. I. Díaz Góngora1 E. Reguera1, Geonel Rodríguez-Gattorno1,3,*

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Nano-Micro Letters, , Volume 3, Issue 1, pp 12-19

Publication Date (Web): April 7, 2011 (Article)

DOI:10.3786/nml.v3i1.p12-19

*Corresponding author. E-mail:geonelr@mda.cinvestav.mx

 

Abstract

 


Figure 1 Representative XRD patterns for: a) all the as-prepared Co nanoparticles from borohydride reduction; b) using hydrazine as reducer, d) sample showed in a) after annealing at 500°C; c) and e) Column bar plots correspond to hcp Co- JCPDS # 01-1278 and fcc Co-JCPDS # 15-0806, respectively.

In this contribution, a comparative study of metallic cobalt micro and nanoparticles obtained in solution by four different chemical routes is reported. Classic routes such as borohydride reduction in aqueous media and the so-called poly-ol methodology were used to obtain the cobalt nanostructures to be studied. Using CTAB as surfactant, cobalt hollow nanostructures were obtained. The use of strong reducing agents, like sodium borohydride, favors the formation of quasi-monodispersed nanoparticles of about 2 nm size but accompanied of impurities; for hydrazine (a mid reducer), nanoparticles of larger size are obtained which organize in spherical microagglomerates. Valuable information on the particles thermal stability and on nature of the species anchored at their surface was obtained recording the thermogravimetric curves. The samples to be studied were characterized from UV-vis, IR, X-ray diffraction, and electron microscopy images (scanning and transmission).


 

Keywords

Metallic Cobalt; Cobalt nanoparticles; Magnetic nanoparticles; Hollow magnetic microparticles

 

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Introduction

 

The preparation of cobalt nanoparticles has been widely studied related to their promising applications in various areas, among them: recording media [1], magnetic sensors [2], magnetic memories [3], magnetic fluids [4], magnetic composites [5] and catalysis [6]. All these applications demand high purity grade products in order to avoid alteration in their magnetization or response stability [7]. Nowadays most of the nanostructured materials are obtained in phase solution because it is simple, reliable and economic route allowing the tailoring of material properties through the control of mean particle sizes and particle sizes distribution. However, due to instability of nanostructured materials as result of its excess of surface energy, the use of capping agents has become a very common way to stabilize nanoparticles in solution phase. Hence, it is not surprising that products in nanoparticles synthesis are frequently highly unpurified by capping agents and solvent molecules and the exhaustive routines of purification do not necessarily resolve this problem.
In the later years several synthesis of metallic cobalt nanoparticles have been reported and most of them mention that the nanoparticles oxidize in solution or in contact with air, however systematic studies on this topic is still absent. Therefore emerge the requirement to investigate quantitatively and qualitatively the oxidation phenomena in this type of particles in order to increase the efficiency of a variety of processes in different research areas, especially for nanostructured materials which practically are deficient in information related to the stabilization studies towards the oxidation processes. On the other hand, thermal analysis is a simple qualitative and quantitative analytical tool allowing the study of decomposition and oxidation processes, specially the thermogravimetric analysis which allows characterizing the oxidation of many metals under accelerated conditions. Few works focused on the chemical quantification of unpurified products that results from the strong the interactions at the interface: nanoparticle-media during synthesis procedures. Therefore, in the present work the oxidation thermal profiles of metallic cobalt nano and micro particles obtained in colloidal dispersion is explored. The micro and nano particles under studied were obtained using classical methods such as the reduction in aqueous media [8,9] and the so-called polyol method [10-13]. Results show that strong reducer such as borohydride contaminates the products not necessarily forming borides, as has been reported previously [8,9]. It was found that when CTAB is used as template the system forms homogeneous microstructures distributions of secondary hollow particles with spherical morphology that organize in elongated mesoscopic systems. The powders produced were characterized by means of UV-Vis and IR spectroscopies, X-ray diffraction (XRD), Scanning and Transmission Electron Microscopies and High resolution Thermogravimetric (TG) Analyses.

 

Experimental part

 

Synthesis procedures

 

Borohydride reduction in aqueous media

 

The use of a strong reducer, for instance NaBH4, is well documented in literature as a fast and easy method to obtain metallic nanoparticles [14]. The reduction of the starting salt CoCl2·6H2O (Aldrich ≥99%) was carried out at 25°C in aqueous media. First, 5mmol of CoCl2·6H2O (Aldrich, ≥99%) and 5mmol of Na3Cit·2H2O (Aldrich ≥ 99%) were dissolved into deionized water (30 ml) under vigorous stirring. After 10 min. stirring, NaBH4 (Aldrich ≥96%) 0.0189g dissolved in 20 ml of deionized water were introduced drop by drop. The molar ratio of NaBH4:CoCl2 was purposely made larger than two in orders to ensure complete reduction of CoCl2 [15]. The solution color changes from pale pink into black after approximately 10 min. Eventually, the proper amount of diethyl ether (8ml) was then added under not anaerobic conditions in order to minimize the formation of boron-cobalt compounds [8,9].

Reduction method in aqueous media via vesicles

 

First, 0.0729g of CoCl2·6H2O were dissolved in a mixture of 25 ml deionized water and 5ml methanol (Fermont 99.9%) under constant stirring. Then 17µL of oleic acid (Aldrich 90%) were added in order to allow the vesicles formation [16]. Immediately NaOH (J.T. Baker 99.7%) is added to the solution until it reaches a pH value of 8 (value near the pKa of the oleic acid) [17], it will help to the vesicles stabilization. Subsequently, 0.2502 g of Na3Cit·3H2O were added to the solution and the mixture was sonicated for 15 minutes. Afterward, 0.0189g of NaBH4 dissolved in 20 ml of deionized water were introduced, drop by drop, and this mixture was then sonicated for 5 minutes. The dispersion color changes from pale milky pink into milky black.


Reduction method using hydrazine (poly-ol process)

 

CoCl2·6H2O (0.0362 g) and CTAB (0.05466 g, Aldrich 99%) were mixed and sonicated in 30 ml of ethylene glycol (Aldrich 99.8%) during 40 minutes. Then, the appropriate amount of monohydrated hydrazine (Aldrich 98%) was added to the solution as soft reducing agent. The solution is then heated until reach the boiling point of the ethylene glycol (197 oC) and is held at this temperature for 70 minutes. After the reaction was completed, the solution is cooled to room temperature. For comparison purposes, sodium citrate (Na3Cit·3H2O) was used also as capping agent instead CTAB, under same conditions described above and just changing the heating time to 140 minutes. This constitutes the 4 methodology (sample identification: ‟Poly-ol -Citˮ).

In all cases the products were separated by centrifugation and special attention was taken in washing procedure. The settled solid was washed with ethanol, methanol, diethyl ether and deionized water several times to thoroughly remove residual contaminations, and finally the particles were dried at ambient conditions for 9 hrs.

 

Characterization

UV–Vis spectra were recorded on a Jazz spectrophotometer from Ocean Optics in the wavelengths interval of 200 to 800 nm. XRD powder patterns of the as-prepared samples were obtained using Bruker D8 Advance Diffractometer and CuKα radiation, with angular step of 0.025°. The morphology of the as-prepared Co nanoparticles was initially characterized by an Scanning Electron Microscopy instrument SEM JEOL JEM6390 LV operating at an accelerating voltage interval of 15- 20 kV and equipped with energy dispersive X-ray spectroscopy (EDS) capabilities and a High resolution SEM (HRSEM Nova 600 NanoLab, dual beam). The samples with smaller particle sizes were analyzed by Transmission Electron Microscopy (TEM JEOL JEM 1010) and High Resolution TEM (HRTEM JEOL 2010F), using an accelerating voltage of 60 kV and 200kV, respectively. The thermogravimetric studies were performed in a High Resolution TGA Q5000 IR at heating rates of 10 °C/min under a continuous air flow (21% O2). IR spectra were recorded using the KBr pressed disk technique and Spectrum One spectrophometer (from Perkin Elemer).

 

162

 

Results and Discussions

 

The obtained colloids were first analyzed from UV-Vis spectra. Colloidal dispersion of metals exhibits absorption bands in the UV-Vis range as result of intraband or interband transitions. Intraband transitions in metals are due to excitation of electron plasma resonances which in the case of metallic cobalt, is expected to occur in the 120 to 162 nm range [18]. Due to superposition of Fermi level with empty 3s bands, metallic cobalt also display interband transitions which superpose to intraband transitions. This behavior makes difficult to analyze the electronic absorption spectra of metallic cobalt colloids as function of particle size. In all cases the UV-Vis spectra of as prepared colloids (see Figure 1 in on line supplementary material) show that prevails the light scattering component along all the wavelengths range, suggesting that colloids mainly consist of particles or agglomerates having sizes in the order or bigger than the incident wavelengths [19,20]. The sample prepared from borohydride reduction in aqueous solution shows the least contribution from scattering and a progressive increase of the absorbance at smaller wavelengths. In this case the optical response suggests the presence of smaller particles/agglomerate sizes and it is in agreement with theoretical spectrum expected for metallic cobalt nanoparticles [21].

 

Figure 1 Representative XRD patterns for: a) all the as-prepared Co nanoparticles from borohydride reduction; b) using hydrazine as reducer, d) sample showed in a) after annealing at 500°C; c) and e) Column bar plots correspond to hcp Co- JCPDS # 01-1278 and fcc Co-JCPDS # 15-0806, respectively.

Figure 1 Representative XRD patterns for: a) all the as-prepared Co nanoparticles from borohydride reduction; b) using hydrazine as reducer, d) sample showed in a) after annealing at 500°C; c) and e) Column bar plots correspond to hcp Co- JCPDS # 01-1278 and fcc Co-JCPDS # 15-0806, respectively.

 

XRD pattern of the as-prepared particles obtained from borohydride reduction does not show reflections (see representative example in Figure 1a) while samples obtained by reduction with hydrazine crystallizes as hcp-metallic cobalt (see Figure 1b). At this point, it’s important to note that borohydride is stronger reducing agent than hydrazine; therefore the synthesis with the hydrazine needs to be carried at higher temperature which probable favors the growths of metallic cobalt nanoparticles. The samples that appear to be amorphous (aqueous reduction and vesicles), crystallize in FCC structure after an annealing at 500°C as shows the diffraction pattern in Figure 1d (as comparison Figure 1e shows expected reflections for this phase JCPDS #15-0806). The FCC crystal structure is the stable phase for metallic cobalt at high temperature (> 420 oC ) although reports on synthesis of FCC and e-Co metallic cobalt in solution phase at low temperature have been previously reported [22-24].

 

TEM and SEM characterizations

Only in two of the samples was possible to distinguish the primary particles (i.e.: aqueous reduction and vesicles) by means of HRTEM, in the other two samples (Poly-ol-CTAB and Poly-ol-Cit ) was impossible to observed details of samples in HRTEM due to the large size of the agglomerate, hence these two samples were studied by means of SEM were secondary particle size is well defined but primary particle can be only roughly estimated by local contrast (apparent primary particle size). Figure 2 illustrates the HR-TEM micrographs of the products obtained via aqueous reduction. High Angle Annular Dark Field (HAADF) image (Figure 2a) shows roughly spherical secondary particles with mean diameters between 15 to 30 nm; at higher resolutions the primary particles can be distinguished, having a mean diameter of 2.4 nm. The local and global Fast Fourier Transforms of images show a unique reflection which apparently corresponds to the {100} planes family from FCC metallic cobalt (JCPDS # 15-0806). This is in agreement with XRD characterization of the sample (aqueous reduction, after annealing), suggesting that FCC crystal structure seems to be favored for very small particles.

 

Figure 2 a) High Angle Annular Dark Field (HAADF) image from a zone of the obtained dispersion using sodium citrate as capping agent and sodium borohydride as reducing agent in aqueous media, b) selected HRTEM micrograph of primary of particles. The inset in the upper part shows a FFT of selected particle showing the {100} family plane reflection that corresponds with FCC metallic Co (JCPDS 15-0806).

Figure 2 a) High Angle Annular Dark Field (HAADF) image from a zone of the obtained dispersion using sodium citrate as capping agent and sodium borohydride as reducing agent in aqueous media, b) selected HRTEM micrograph of primary of particles. The inset in the upper part shows a FFT of selected particle showing the {100} family plane reflection that corresponds with FCC metallic Co (JCPDS 15-0806).

 

On the other hand, vesicles sample which was also obtained from borohydride reduction consists in poly-disperse particles with sizes ranging from 8 to 40 nm encapsulated in vesicle-like globules as shown in Figure 3a. According to the local and global FFT of micrographs at high resolution, the particles appear to be amorphous in agreement with XRD results of the as prepared samples. In general particles seems to be coated by an organized phase having an apparent interplanar distance of 27 Å (Figure 3b and 3c), a value that is in agreement with the reported {100} family planes of dodecanoic acid (JCPDS # 38-1976) indicating a multilamellar organization of the oleic acid around the agglomerate of metallic particles and suggesting the formation of a vesicular system.

For the cases where ethylene glycol was used as solvent in the presence of hydrazine as reducer, the products of both methodologies (using Sodium Citrate or CTAB as stabilizers) tend to be formed by spherical micro-agglomerates of metal particles (see Figures 4a and 4c).

However, the study of these samples in SEM shows that in both samples the secondary particle sizes oscillate between 0.5 and 1.5 µm, while the apparent primary particle sizes are between 20 to 45 nm for CTAB system and from 40 to 75nm for sodium citrate. The increasing in temperature and the use of softer reducer could be the reason why the mean particle sizes is higher in these cases. Both parameters probably favor the superposition of nucleation and growth processes which don’t allow the obtaining of small particles. Additionally, for the sample obtained in the presence of CTAB, the agglomerates organize to form hollow-elongated mesoscopic system (see Figure 4b). This is very interesting since nanoengineered colloidal particles with hollow morphology may potentially find applications as delivery systems especially those which are magnetic because they can be directed by application of an external magnetic field [25-27]. The obtaining of hollow microstructures is probably related to the formation of CTAB micelles in ethylene glycol [28]. The micelles acts as template and deposition center during nucleation and growth of metallic cobalt nanoparticles.

 

Figure 3 a) low resolution TEM, b) HRTEM and c) HAADF images of metallic cobalt obtained from vesicles. Inset in (c) corresponds to the FFT of the selected area in this image.

Figure 3 a) low resolution TEM, b) HRTEM and c) HAADF images of metallic cobalt obtained from vesicles. Inset in (c) corresponds to the FFT of the selected area in this image.

Figure 4 a) SEM images of sample obtained using CTAB as capping agent and hydrazine as reducing agent in ethylene glycol; b) Zoom in on selected particles observed in SEM micrographs that shows hollow morphology; c) sample obtained under similar conditions but using sodium citrate as capping agent.

Figure 4 a) SEM images of sample obtained using CTAB as capping agent and hydrazine as reducing agent in ethylene glycol; b) Zoom in on selected particles observed in SEM micrographs that shows hollow morphology; c) sample obtained under similar conditions but using sodium citrate as capping agent.

 


Thermogravimetric profiles

 

Pure metallic bulk cobalt starts to oxidize in air at approximately 350 oC [29]. However, cobalt nanoparticles obtained via physical methods, with mean particle size in the range of 20-57 nm, starts to oxidize in air at temperatures as low as 150 oC [30,31]. In Figure 5a the thermogravimetric curves of the four as prepared samples are shown and in the Table 2 the weight gain/loss for each sample and a proposal of the processes involved are summarized. Two general behaviors can be clearly distinguished depending on reducing agent used. Although all samples show a weight loss in the temperature ranges from 30 to 350 oC (related to dehydration and decomposition of the organic phase) the less unpurified samples are those obtained from hydrazine reduction. It is almost certainly that weight gain due to oxidation of metallic cobalt is partially overlapped with weight loss resulted from decomposition processes of organic phases. It is worth mentioning the fact that any attempt to measure the samples under inert atmosphere complicates the further interpretation of the thermal profiles because of residual carbonaceous material that result from incomplete combustion of the organic phase.

 

The unique sample showing almost no overlapping of decomposition and oxidation processes was that obtained from hydrazine reduction in the presence of citrate. This sample shows the largest experimental weight gain of 31.26 % and the expected theoretical value for pure metallic cobalt is 36.19 % according to:

 

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Figure 5 a) Thermogravimetric plots of the four as-prepared Co nanoparticles; b) Thermogravimetric curve of the sodium borohydride; c) Thermogravimetric curves of sodium citrate, CTAB and oleic acid (major probable impurities on samples).

Figure 5 a) Thermogravimetric plots of the four as-prepared Co nanoparticles; b) Thermogravimetric curve of the sodium borohydride; c) Thermogravimetric curves of sodium citrate, CTAB and oleic acid (major probable impurities on samples).

 

The differences between experimental and theoretical weight gain are usually assumed as resulting from metal surface oxidation [30]. Intriguingly, no traces of cobalt oxide were observed in XRD patterns which is probably related to low cristallinity of this compound. The existence of cobalt oxide (CoO) in metallic cobalt samples implies that its oxidation to Co3O4 contributes with a fraction to the total oxygen uptake along the thermal profiles according to:

1622

especially in the range from 400 to 900 oC [32]. An interesting feature of the thermal profiles in samples obtained from hydrazine reduction is the existence of a well defined weight loss at 905 oC which undoubtedly corresponds to the process represented by [33]:

1624

for this process the expected theoretical weight loss is 6.644%, while experimental value is 6.636 %. The quantitativeness of this process allows estimating the global cobalt content by straightforward calculations taking into account the relation between reactions (1) and (3) independently of the overlapping between organic phase decomposition and oxidation processes. However, contribution to oxygen uptake from CoO oxidation is unknown and can be only estimated if sample it´s assumed to consist in a mixture of metallic cobalt and cobalt (II) oxide and both oxidize to the same product (i.e.: Co3O4, equation 1 and 2) without superposition of oxidation and decomposition of organic phase. For example, cobalt obtained in ethylene glycol in the presence of sodium citrate gives an approximately metallic phase content of 81.5 %, 17.2 % of cobalt oxide and 1.3 % of organic phase. Obviously this is approximately because carbonaceous residues are not taken into account.

 

In samples where borohydride was used as reducer, it can be observed a two step oxidation process and not Co3O4 decomposition is observed. By comparison with the thermogravimetric curve for pure sodium borohydride (see Figure 5b) the first step appears to correspond with the oxidation of borohydride residues to borate, according to:

1625

 

This process implies the simultaneous decomposition and oxidation of this reactant. In order to confirm the presence of borates after thermogravimetric study, XRD patterns of the samples were recorded, and the formation of Co3(BO3)2 was corroborated for sample obtained in vesicular system (see Figure 2 in online supplementary material). On the other hand, for the sample obtained via aqueous reduction in the presence of citrate, the product consists in cobalt pyroborate (Co2B2O5; see Figure 3 in online supplementary material) suggesting another mechanism for reaction-oxidation of the metallic phase with borates residues. These results suggest that borate product from decomposition of borohydride residues reacts with metallic cobalt and oxygen to give cobalt borates. Other possibility that cannot be discarded is the simultaneous formation of metallic cobalt and cobalt borides composites as have been reported by Glavee et. al [8,9], in this case a two step oxidation process is also expected. Additionally, it is noted that organic phase content is higher for ‟vesiclesˮ sample and the IR spectrum corroborates the presence of oleic acid (see Figure 4 in online supplementary material). Bands observed at 1607 and 1498 cm-1are characteristic of the asymmetric νas(COO-) and the symmetric νs (COO-) stretchings of oleic acid where the interaction between the COO- group and the Co atom is covalent through bridging bidentate bound [34].

 

1623

 

Conclusion

 

The synthesis of metallic cobalt in aqueous media from borohydride reduction offers a convenient route to obtain metallic cobalt with smaller particle size; however it leads to lesser purity in products than when hydrazine is used as reducer agent. On the other hand the synthesis achieved by the poly-ol process gave bigger particle sizes which agglomerates in secondary microparticles with spherical shapes. In presence of CTAB hollow elongate mesoscopic microstructures can be obtained while in presence of sodium citrate the secondary microparticles are solid and not organized. According to thermogravimetric profiles, the most pure system is obtained when it is combined a reducer such as hydrazine in presence of sodium citrate as surface modifier.

 

Acknowledgements

 

This work was partially supported by the Consejo Nacional de Ciencia y Tecnolog´ıa (CONACYT, M´exico) under Grant SEP-CONACyT 2009 No. 129048.

 

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References

[1] M. N. Baibich, F. Leccabue and V. Sagredo, World Scientific, Singapore, 69 (1996).

[2] M. Vopálenský, P. Ripka, J. Kubík and M Tondra, Sens. Actuators. A 110, 254 (2004). http://dx.doi.org/10.1016/j.sna.2003.09.036

[3] R. P. Cowburn, Mater. Today 6, 32 (2003). http://dx.doi.org/10.1016/S1369-7021(03)00730-2

[4] C. Scherer and A. M. Figueiredo Neto, Braz. J. Phys. 35, 718 (2005). http://dx.doi.org/10.1590/S0103-97332005000400018

[5] Halit S.Gokturk, Thomas J. Fiske and Dilhan M. Kalyon, J. Appl. Phys. 73, 5598 (1993). http://dx.doi.org/10.1063/1.353663

[6] G. Schmid, B. Cornils and W. A. Herrmann, Wiley-VCH: Weinheim, Germany, 2, 636 (1996).

[7] Y. Xu, M. Mahmood, Z. Li, E. Dervishi, S. Trigwell, V. P. Zharov, N. Ali, V. Saini, A. R. Biris, D. Lupu, D. Boldor and A. S. Biris, Nanotechnology 19, 435102 (2008). http://dx.doi.org/10.1088/0957-4484/19/43/435102

[8] George N. Glavee, Kenneth J. Klabunde, Christopher M. Sorensen and George C. Hadjapanayis, Langmuir 8, 771 (1992). http://dx.doi.org/10.1021/la00039a008

[9] G. N. Glavee, K. J. Klabunde, C. M. Sorensen and G. C. Hadjipanayis, Langmuir 9, 162 (1993). http://dx.doi.org/10.1021/la00025a034

[10] G. Viau, F. Fiévet-Vincent and F. Fiévet, Solid State Ionics 84, 259 (1996). http://dx.doi.org/10.1016/0167-2738(96)00005-7

[11] L. Poul, N. Jouini and F. Fiévet, Chem. Mater. 12, 3123 (2000). http://dx.doi.org/10.1021/cm991179j

[12] F. Fiévet, M. Figlarz and J. P. Lagier, U. S. Patent 4539041 (1985).

[13] F. Fiévet, J. P. Lagier and B. Blin, Solid State Ionics 198, 32 (1989).

[14] L. C. Brian, V. L. Kolesnichenko and C. J. O´Connor, Chem. Rev. 104, 3893 (2004).

[15] A. M. L. Jackelen, M. Jungbauer and G. N. Glavee, Langmuir 15, 2322 (1999). http://dx.doi.org/10.1021/la9807311

[16] M. L. Rogerson, B. H. Robinson, S. Bucak and P. Walde, Colloids Surf. B 48, 24 (2006). http://dx.doi.org/10.1016/j.colsurfb.2006.01.001

[17] C. L. Apel, M. N. Mautner and D. W. Deamer, Biochim. Biophys. Acta. Gen. Subj. 1559, 1 (2002).

[18] L. Ward and E. D. Palik, Elsevier Science, 2,435 (1991).

[19] C. F. Bohren and D. R. Huffman, Wiley, New York, (1983).

[20] H. C. Van de Hulst, Dover Publications, New York (1981).

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Citation Information

Marlene Gonz´alez Montiel, P. Santiago-Jacinto, J. A. I. D´ıaz G´ongora, E. Reguera and Geonel Rodr´ıguez-Gattorno, Synthesis and Thermal Behavior of Metallic Cobalt Micro and Nanostructures. Nano-Micro Lett. 3 (1), 12-19 (2011). http://dx.doi.org/10.3786/nml.v3i1.p12-19

History

Received 5 Jan 2011; accepted 9 Mar 2011; published online 7 April 2011.

 


Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    Synthesis and Thermal Behavior of Metallic Cobalt Micro and Nanostructures

  • Author: Marlene González Montiel,P. Santiago-Jacinto,P. Santiago-Jacinto,E. Reguera,Geonel Rodríguez-Gattorno
  • Year: 2011
  • Volume: 3
  • Issue: 1
  • Journal Name: Nano-Micro Letters
  • Publisher: OPEN ACCESS HOUSE SCIENCE & TECHNOLOGY
  • ISSN: 2150-5551
  • URL: http://dx.doi.org/10.5101/nml.v3i1.p12-19
  • Abstract:

        In this contribution, a comparative study of metallic cobalt micro and nanoparticles obtained in solution by four different chemical routes is reported. Classic routes such as borohydride reduction in aqueous media and the so-called poly-ol methodology were used to obtain the cobalt nanostructures to be studied. Using CTAB as surfactant, cobalt hollow nanostructures were obtained. The use of strong reducing agents, like sodium borohydride, favors the formation of quasi-monodispersed nanoparticles of about 2 nm size but accompanied of impurities; for hydrazine (a mid reducer), nanoparticles of larger size are obtained which organize in spherical microagglomerates. Valuable information on the particles thermal stability and on nature of the species anchored at their surface was obtained recording the thermogravimetric curves. The samples to be studied were characterized from UV-vis, IR, X-ray diffraction, and electron microscopy images (scanning and transmission).

  • Publish Date: Thursday, 07 April 2011
  • Start Page: 12
  • Endpage: 19
  • DOI: 10.5101/nml.v3i1.p12-19