Preparation and magnetic behavior of carbon-encapsulated cobalt ...

906 chemical engineering research and design 8 6 (2008) 904–908Fig. 2 – TEM images of the M@Cs samples: (a) and (b) Co@Cs; (c) and (d) Ni@Cs.sition of these tiny spots on the surface of the samples. Moredata is needed to clarify this, and the work is in progress now.Fig. 2 shows that the typical TEM images of the Co@Cs andNi@Cs materials, showing that the Co@Cs materials with adiameter in a narrow range of 30–50 nm are spherical or quasisphericalin shape, and exhibit well-constructed core/shellstructure. The HRTEM examination (Fig. 2b) reveals that thecarbon shells of Co@Cs are graphitic sheets, which is similarto the carbon shell structure of the Fe@Cs samples obtainedfrom starch in our previous study (Qiu et al., 2006a, submittedfor publication, in press). But this is not the case for the Ni@Csmaterials (Fig. 2c and d). The diameter of the Ni@Cs samplesvaries in a range of 200–500 nm, which is much bigger than thatof the Co@Cs materials. The HRTEM images (Fig. 2d) show thatthe outside carbon layers of the Ni@Cs materials are amorphouscarbon. This implies that the metal precursor specieshave a substantial influence on the diameter and carbon shellstructure of the M@Cs samples, which is further confirmed bythe XRD results discussed below.The XRD patterns of the as-prepared Co@Cs and Ni@Csmaterials are shown in Fig. 3, in which the peak at 25.6 ◦ inthe case of Co@Cs and Ni@Cs materials is due to the (0 0 2)diffraction of graphite layers. Nevertheless, the (0 0 2) peak ofthe Co@Cs materials is narrow and sharp in comparison to the(0 0 2) peak of Ni@Cs samples that is broader and lower, implyingthe Co@Cs materials have a high degree of graphitization.This is in agreement with the TEM results discussed above,i.e. the shell layer in the Co@Cs samples is made of graphiticsheets, while the shell layer in the Ni@Cs samples is madeof amorphous carbon. For the Co@Cs material, the diffractionpeaks at 44.2 ◦ , 51.5 ◦ and 75.9 ◦ can be attributed to (1 1 1), (2 0 0)and (2 2 0) planes of fcc-Co, respectively (Sun et al., 1999). Whilein the case of the Ni@Cs material, the diffraction peaks at 44.4 ◦ ,51.8 ◦ and 76.3 ◦ are due to (1 1 1), (2 0 0) and (2 2 0) planes of fcc-Ni, respectively. It is also found that the relative intensity of themetal peaks varies depending on the different metal precursorsused. The fcc-Ni peak is stronger than the fcc-Co peak,implying that the diameter of nickel nanoparticles is biggerthan that of cobalt nanoparticles. With the Scherrer equation,the size of metal cores in the M@Cs samples can be calculatedfrom the XRD dada, i.e. on the basis of the (200) diffractionpeaks, the (1 1 1) and (2 0 0) diffraction peaks of fcc-Co or fcc-Ni. The calculated size of fcc-Co core in the Co@Cs samplesranges from 20 to 35 nm. While the calculated size of fcc-Nicore in the Ni@Cs samples is in a range of 30–50 nm. Here itshould be noted that there is a discrepancy between the XRDresults and the TEM results in terms of the size or diameterof the M@Cs. The reason behind this phenomenon may beFig. 3 – XRD patterns of the Co@Cs and Ni@Cs samples.

chemical engineering research and design 8 6 (2008) 904–908 907Fig. 4 – Magnetic hysteresis curves of (a) Co@Cs and (b) Ni@Cs samples measured at 25 ◦ C, and (c) photos of Co@Cs materialsin alcohol under the effect of a magnet bar outside the bottle, showing the M@Cs are magnetically separable.because the XRD technique gives rise to the average informationof the samples while the TEM examination may partlyreveal the information of the diameter of the M@Cs particles,some of which may not be loaded onto the copper grid andmay not be observed by the TEM.The VSM magnetic measurement results for the assynthesizedCo@Cs and Ni@Cs materials are shown in Fig. 4.The hysteresis loops of the as-made M@Cs materials revealthe intrinsic magnetic behavior, indicated by the saturationmagnetization (Ms), the remanent magnetization (Mr) andthe coercive field (Hc) for the M@Cs samples. The ratio ofremanence to saturation magnetization (Mr/Ms) of the Co@Csand Ni@Cs samples is 0.131 and 0.109, respectively, indicatingthat the M@Cs samples made from the starch and nitrate aresuperparamagnetic at room temperature (Sajitha et al., 2004).The magnetic performance of the M@Cs samples was demonstratedin a liquid phase by placing a magnet bar near the glassbottle, showing clearly that the black powder, i.e. the M@Csmaterials could move under the magnetic force (see Fig. 4c).This makes the M@Cs materials ideal adsorbents and catalystsupports that are magnetically separable.The results presented here clearly show that the biomaterial,core-derived starch, can be used as carbon source forproducing Co@Cs or Ni@Cs samples. Efforts have been madeto figure out the possible mechanism involved in the formationprocess of the M@Cs materials. It is believed thatthe formation of metal core/carbon shell structure would berelated to the helical structure of the starch precursor. Theamylose starch is known to be capable of forming helical structuresof variable pocket sizes according to the size of the hostbody. This helical structure would encapsulate metal clustersin the system, providing the possibility of forming thecore/shell structure with spherical or quasi-spherical morphology,evidenced by the previous studies (Qiu et al., 2006a,submitted for publication, in press; Sarma et al., 2004). It isknown that the thermal decomposition process of starch issimilar to that of cellulose, a kind of amylose (Bacon andTang, 1964a,b; Qiu et al., 2006a; Sarma and Chattopadhyay,2004). During the preparation process, a series of reactionswould occur in the starch/metal carbonization process, suchas desorption of physically adsorbed water, chain scissionsand breaking of C O and C C bonds within ring units ofthe starch, yielding highly active species such as chain-likecarbon molecules that would further polymerize to form thegraphite-like layer as the carbon shell structure observed inthe M@Cs materials. This process is like a catalytic graphiticprocess because of the presence of iron-group metals. Thestarch/metal composites undergo a stage in which they aretransformed to metal–carbon fused phase, and as the reactionscontinue, the supersaturated carbon would be formedand precipitated on the surface of the nanoparticles, resultingin the carbon-encapsulated metal nanoparticles (Tsai et al.,2000; Herring et al., 2003).4. ConclusionsThe M@Cs materials with a unique core/shell structure havebeen successfully prepared from the starch and the metalnitrate by carbonization in flowing hydrogen. This providesa new approach to synthesis of carbon-encapsulated metalnanoparticles using the cheap biomaterials as carbon source.The SEM examination reveals that the M@Cs materials areellipsoidal in shape with the diameter being in a range of10–50 m. The XRD examination reveals that the Co@Cs materialsare made of the fcc-Co core and the graphitic shellstructure, with the metal core diameter being in a range of20–35 nm; while in the case of the Ni@Cs materials, the fcc-Nicore and the amorphous carbon shell are observed, with themetal core size varying from 30 to 50 nm. The starch used inthe present work has dual functions for the formation of M@Csmaterials that are superparamagnetic at room temperature:carbon precursor and stabilizer inhibiting the agglomerationof metal nanoparticles.