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© Applied Science Innovations Pvt. Ltd., India Carbon – Sci. Tech. 1 (2008) 46 - 56

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REVIEW ARTICLE Received : 02/04/2008, Accepted : 06/05/2008. ----------------------------------------------------------------------------------------------------------------------------- Semiconducting, Magnetic or Superconducting Nanoparticles Encapsulated in Carbon Shells by RAPET Method Vilas G. Pol (*), Swati V. Pol and Aharon Gedanken Center for Advanced Materials and Nanotechnology, Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel. An efficient, solvent-free, environmentally friendly, RAPET (Reactions under Autogenic Pressure at Elevated Temperature) synthetic approach is discussed for the fabrication of core-shell nanostructures. The semiconducting, magnetic or superconducting nanoparticles are encapsulated in a carbon shell. RAPET is a one-step, thermal decomposition reaction of chemical compound (s) followed by the formation of core-shell nanoparticles in a closed stainless steel reactor. The representative examples are discussed, where a variety of nanomaterials are trapped in situ in a carbon shell that offers fascinating properties. ----------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction : In recent years, considerable efforts has been devoted to the design and controlled fabrication of nanostructured materials with functional properties [1]. The interest in nanoscale materials stems from the fact that their properties (optical, electrical, mechanical, chemical, etc.) are a function of their size, composition, and structural order. Over the last decade there have been immense efforts to fabricate core-shell colloidal materials with tailored structural, optical, and surface properties [2]. They can be utilized as model systems to investigate factors governing colloidal interactions and stabilization [3], and to gain valuable information on the properties of concentrated dispersions [4]. Particle coating is carried out for a myriad of reasons [5]. For example, the shell can alter the charge, functionality, and reactivity of the surface, and can enhance the stability, dispersibility of the colloidal core, and sometimes can avoid the spreading of the nanoparticles (NPs) in the atmosphere. Magnetic, optical, or catalytic functions may be readily imparted to the dispersed colloidal matter depending on the properties of the coating. Encasing colloids in a shell of a different composition may also protect the core from extraneous chemical and physical changes [6]. Core-shell nanostructures [NSs] often exhibit improved physical and chemical properties over their single-component counterparts, and hence are potentially useful in a broader range of applications. Therefore, methods to ‘engineer’ such materials with controlled precision have long been sought. This review provides an overview of our recently developed RAPET approach to synthesize core-shell NSs of semiconducting, magnetic and superconducting NPs encapsulated in a carbon shell. Semiconducting NPs have been widely investigated because of the miniaturization of electronic components and their integration to accommodate a huge number of them in small volumes. This has enabled us to have very compact

digital [7] watches, calculators, computers, laptops etc. For II-VI and III-V semiconductors [8], the change in the band gaps with particle size is also demonstrated. Semiconductor NPs exhibit size-dependent properties, when their size is comparable to the size of Bohr diameter for exciton [9]. This can be exploited to increase fluorescence efficiency or to increase the internal magnetic field strength in doped semiconductors. Magnetic NPs are of great interest for researchers from a wide range of disciplines, including in magnetic fluids [10], catalysis [11], biotechnology / biomedicine [12], magnetic resonance imaging [13], data storage [14], and environmental remediation [15]. While a number of suitable methods have been developed for the synthesis of magnetic NPs of various different compositions, the successful application of such magnetic NPs in the areas listed above is highly dependent on the stability of the particles and their properties under a range of different conditions. In most of the envisaged applications, the particles perform best when the size of the NPs is below a critical value that is dependent on the material, but is typically around 10 – 20 nm. Each NP then becomes a single magnetic domain and shows superparamagnetic [16] behavior when the temperature is above the so-called blocking temperature. Such individual NPs have a large constant magnetic moment and behave like a giant paramagnetic atom with a fast response to applied magnetic fields with negligible remanence (residual magnetism) and coercivity (the field required to bring the magnetization to zero). Although to date most studies have focused on the development of polymer or silica protective coatings, carbon-protected magnetic NPs have recently received more attention because carbon-based materials have a higher chemical and thermal stability, inertness as well as biocompatibility [16]. Moreover, carbon-coated NPs are usually in the metallic state, and thus have a higher magnetic moment than the

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corresponding oxides. Johnson et al. described a simple method to prepare carbon-coated magnetic Fe - NPs by the direct pyrolysis of iron stearate at 900 °C under an argon

[17] atmosphere and are stable up to 400 0C under air. The salt- synthesized 11 nm carbon-coated cobalt NPs are also highly stable [18] under air. Alivisatos and co-workers reported on the synthesis of magnetic nanodisks [19] by the thermal decomposition of a cobalt carbonyl precursor. The hot-electron phenomena in low-dimensional superconducting systems are of fundamental importance for high energy resolution bolometers [20]. Photon absorption in a superconducting detector creates an avalanche electron charge, 2 or 3 orders of magnitude higher than that in a semiconductor for the same photon energy. This result in an enhanced resolution in energy-resolving devices, such as superconducting tunnel junctions [21], and extends the range of detectable energies [22]. On the other hand, the coherence of electron beams (e-beams) is of great interest for both practical and academic reasons since a highly coherent e-beam greatly improves the spatial resolution in electron optical systems [23]. In particular, the development of coherent e-beams with a high intensity will contribute to the further implementation of electron holography in practice [24]. In the pursuit of developing an ultracoherent e-beam, studies have shown that the e-beam from a superconducting tip is monochromatic [25] and that e-beams emitted from different sites of a carbon nanotube can interfere with each other coherently [26]. Superconducting nanowires and nanorods from lead [27] and tin [28] have already been synthesized, but are not suitable for handling since they are sensitive to oxidation. If such nanostructures can be protected and stabilized, they would be ideal for studying the fundamental physical and chemical properties that are essential prior to their implementation as nano building blocks in nanoscale devices such as electron field emission sources or superconducting devices. There are various ways to synthesize semiconducting, magnetic or superconducting NPs. However, each technique has its own advantages and disadvantages. Additionally, most of the techniques require a high energy input and are not suitable for mass production because of the low yield and the complicated process control. Therefore, the research community is seeking methods for producing stable NPs and further experimental research on their synthesis is needed. Considering the advantages and disadvantages of the known methods and the requirements of today’s nanotechnology, the RAPET [Reaction under Autogenic Pressure at Elevated Temperature], approach for the synthesis of nanosized core-shell nanostructures is promising. 2. Experimental System for RAPET : The fundamental instrumentation required for RAPET reactions includes a reactor made by using stainless steel [SS] Swagelok parts, a tubular heating furnace and an inert glove box system. The fabrication of various NSs is carried out as follows. The chemical precursors are introduced into a 3-5 ml closed stainless steel [SS] reactor, which assembled from a threaded union and caps. Generally, precursors are introduced into the reactor at room temperature under a nitrogen-filled glove box to avoid

oxygen interference. The filled SS reactor is closed tightly and then placed inside an iron pipe in the middle of the tube furnace for optimal heating. The temperature is raised at a rate of 5-30 oC min. to the desired/required temperature and held for an optimized time. The chemical dissociation and transformation reaction takes place under the autogenic pressure of the precursor at a certain temperature, followed by gradual cooling of the reactor to room temperature, and then opened. Scheme 1 represents the overview of RAPET approach employed for the synthesis of core-shell nanostructures.

Scheme (1) : An overview of the RAPET process.

The systematic compositional, morphological,

structural characterization of the obtained products, and an understanding of related properties is a key role of the RAPET invention. The considerable advantages of the RAPET method for the synthesis of a variety of fascinating NSs are : 1) A simple, non-aqueous, template-, surfactant-, and solvent-free method for the production of pure crystalline products, without further processing. 2) Due to their closed system and the fact that they are reproducible, possess easy cyclability, are efficient and non-firing. 3) The RAPET method is also applicable to highly reactive, hazardous or carcinogeneous materials (e.g. carbonyls), as it is user friendly. 4) RAPET fabricates a wide range of nanomaterials such as metals, oxides, carbides, sulfides, phosphides, carbonaceous materials or their core-shell morphologies. In the following section, representative examples are given for the encapsulation of semiconducting, magnetic or superconducting NPs in a carbon shell forming stable core-shell NSs. The convention for core-shell NSs is symbolized as “core@shell”.

3. Results and Discussion : 3.1 Carbon-coated titania nanocrystals : Titania (TiO2) is an n-type semiconductor [band gap of 3.2 eV], a typical photocatalyst for the detoxification of air and water, and is attracting much attention from both fundamental and practical viewpoints [29]. TiO2 has also been demonstrated as a promising electron-transport material for dye-sensitized solar cells [30].

The thermal decomposition of Ti[OCH(CH3)2]4 at 700 oC in a closed SS reactor produced NPs of TiO2 surrounded by carbon as shown in Figure (1), and the surrounding carbon can not be separated from anatase titania [31]. The presence of carbon layers on the surfaces of TiO2 NPs is found to be beneficial to their photocatalytic activity, suppressing the phase transformation of anatase to the rutile


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structure, and preventing sintering and crystal growth even at high temperatures [32]. The diameter of core TiO2 NPs is in the range of approximately 50 nm and has a tendency to form a bulk of approximately 100 NPs surrounded by carbon [Figure (1a)]. The pictures at higher magnification show that the semi-graphitic carbon layers cover [Figure (1b)] TiO2 NP. The measured surface area for the TiO2@carbon NSs is 16 m2/g. Using an analogous RAPET technique, Shan et al. synthesized TiO2@C NSs and demonstrated high-performance electrocatalyst [33] and photochemical activities [34].

(a)

(b) Figure (1) : Transmission electron micrographs of (a) anatase TiO2@carbon, (b) high resolution TEM of an anatase TiO2@carbon particle.

3.2 ZnSe nanocrystals in situ coated with carbon shell : Sulfides or selenides are wide-band-gap II-VI semiconducting materials, possessing applications in the fields of light-emitting devices, solar cells, sensors, and optical recording materials [35]. Researchers have focused on the fabrication of semiconducting nanocrystallites encapsulated by graphite layers, forming core-shell NSs due to their unusual properties [36]. Carbon-encapsulated ZnSe NPs are synthesized by the one-step thermal decomposition of diethyl zinc (DEZ) in the presence of Se powders [37] in a closed reactor at 700

oC/30 min. under their autogenic pressure in an inert atmosphere. Since DEZ is highly reactive with selenium, care should be taken to close the SS reactor immediately after the addition of the reactants to avoid the formation of non-stoichiometric ZnSe particles. Around 4 nm, semi-graphitic layers uniformly cover the surfaces of the ZnSe cores [Figure (2a)].

Figure (2) : (a) TEM image of ZnSe@C NSs, (b) Raman spectrum of ZnSe@C NSs focusing on the range of ZnSe bands, (c) Raman spectrum of ZnSe@C NSs focusing on the range of carbon bands, and d) BET surface area analysis of ZnSe@C NSs.

In the Raman spectrum of ZnSe@Carbon NSs, the obtained dominant [Figure (2b)] Raman bands at 205 and 252 cm−1 are attributed to the transverse optic [TO] and


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longitudinal optic [LO] phonon [38] modes of ZnSe, respectively. Figure (2c) depicts a graphitic band (G-peak) at 1600 cm-1, while disordered carbon (D-peak) is observed at 1340 cm-1. Here, the intensity of the graphitic peak [37] is larger than the disordered peak, in accordance with the HR-TEM image. The detected semi-graphitic layers are not well ordered because the growth temperature (700 oC) is not high enough to improve the local order of the formed layers. The N2 adsorption-desorption isotherm for the ZnSe@C NSs is presented in Figure (2d). The measured BET surface area of ZnSe@C NSs is 15 m2/g and a total pore volume of 0.000294 cm3/g is recorded. The maximum (29.45 cm3/g) nitrogen gas adsorption is recorded at 0.994 relative pressures (757.54 mm Hg) at a liquid nitrogen temperature (77 oK). Therefore, at room temperature, (298 oK), the amount of adsorbed nitrogen on the ZnSe@C NSs is 113.97 cm3/g. The following reaction is proposed for the formation of ZnSe and carbon as the solid products [37].

C2H5-Zn - C2H5 + 1.2 Se → ZnSe (s) + C (s) + CxHy ↑ + xH2Se ↑

In this typical synthesis, an excess amount of Se is provided due to the obvious formation of gases byproducts. Recently, Geng et al. reported on the enhanced blue emission [39] and the quenching of the orange emission of carbon-encapsulated nanocrystalline ZnSe NPs. 3.3 Encapsulation of SnS nanorods in a carbon shell : SnS@carbon NSs is an important net-like anode material for lithium ion batteries [40], as well as it is an excellent solid [41] lubricants [Hüner et al. U.S. patent, 2001]. Core-shell 1-dimensional [D] NSs are even more promising, since they facilitate electron transport. The carbon-encapsulated SnS nanorods [42] are prepared by the thermal decomposition of tetramethyl tin in the presence of S powder in a closed SS reactor at 700 oC / 40 min, avoiding the generally used highly toxic H2S gas. The X-ray diffraction pattern of the as-prepared SnS@C NSs is presented in Figure (3a). The reflection lines can be readily indexed to those of the Herzenbergite orthorhombic phase (Pbnm 62 space group) of SnS with lattice constants a = 4.32, b = 11.19 and c = 3.98 Å (Powder Diffraction File, PDF No. 39 - 354). The XRD pattern confirms the formed pure orthorhombic phase of SnS, although no reflection lines are obtained for the coated carbon due to its semi-crystalline nature, as shown in the TEM image. The SEM image in Figure (3b) demonstrates the overall morphology of SnS@C NSs. Rod-like particles with diameters of 100 to 200 nm and lengths of more than micrometer size are observed. Figure (3c) presents the HR-TEM image of a single nanorod at its edge; the cores of SnS are uniformly covered with approximately 12 nm thick semi-graphitic carbon layers. Our recent report on SnS nanoflakes synthesized via microwave-superheating phenomena showed a stable reversible capacity [> 600 mAh/g] in Li-ion batteries [43].

(b)

(c) Figure (3) : (a) XRD pattern of SnS@carbon, (b) SEM of SnS@carbon, and (c) TEM of SnS@carbon. 3.4 Stabilization of metastable FCC cobalt and the tetragonal phase of zirconia by a carbon shell : Zirconia (ZrO2) is a well-known structural ceramic material which exhibits a tetragonal-to-monoclinic martenstic phase transformation. This phase transformation is of technological importance because it contributes to the toughening of the ceramics [44], higher ionic conductivity [45], and catalyst / catalyst support for various gas-phase reactions [46]. Hence, synthesizing ZrO2 particles with a metastable tetragonal crystal structure is important. Doping ZrO2 with trivalent impurities has been a traditional approach for metastable tetragonal phase stabilization [47]. The metastable face-centered cubic (FCC) phase of cobalt


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was also previously detected by Liu [48] et al. inside carbon nanotubes.

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Figure (4) : (a) LR-TEM of carbon-coated Co and ZrO2 NSs, (b) HR-TEM of a carbon-coated cobalt NP.

The employed RAPET process uses a single precursor without an external catalyst, and without a stabilizing agent to fabricate metastable FCC cobalt and tetragonal ZrO2 phases [49]. The thermal dissociation of CoZr2(acac)2(OiPr)8 at 700 oC resulted in two main products, namely, carbon-coated cobalt (CCC) and carbon-coated zirconia (CCZ). In both cases, Co and ZrO2 NPs formed the cores and are homogeneously embedded in a carbon shell (Figure 4a). The diameter of these cores is ~12 nm and the outer carbon shell is ~4 nm [Figure (4b)]. The stabilization of the Co or ZrO2 cores is due to the carbon shell, which does not allow a change in the metastable phases. The dissociation of CoZr2(acac)2(OPr)8 at such a high temperature as 700 oC leads primarily to the formation of highly volatile homometallic acetylacetonato-alkoxides of zirconium and of cobalt, analogous to the dissociation caused by the electron beam impact in the mass-spectrometric experiment. Further destruction in the gas phase results in uniform multi-center nucleation, and the

rapid simultaneous growth of highly crystalline, almost spherical particles of the high-energy phases of ZrO2 and of metallic Co (due to reduction by the organic residues). Lowering of the gas phase concentration of metal-organic precursor results subsequently in the catalytic thermal decomposition of the purely organic components on the surface of NPs and produces core-shell NSs. The reaction mechanism for the particle growth here supposedly involves the reactions of ether and β-elimination, typical for transition metal-organic decomposition [50]. The magnetic properties were investigated by magnetic susceptibility measurements for the mixture of CCC and CCZ , and show a ferromagnetic [48] behavior. The Ms and coercivity are 18 emu g-1 and ~70 Oe, respectively. Using the RAPET technique, and without doping any trivalent impurities, spherical ZrO2 particles exhibiting a metastable tetragonal crystal structure at room temperature have been obtained. Our interpretation is based on high-resolution TEM and selected area energy dispersive X-ray analysis, a ~4 nm carbon shell on the surface of ZrO2, and Co nanocrystallites, is responsible for stabilizing the high-temperature metastable tetragonal phase of ZrO2 and FCC phase of Co, respectively, at room temperature [49]. 3.5 Encapsulating super-paramagnetic Fe3O4 NPs in a carbon shell : The thermal decomposition [700 °C] of Iron (II) acetyl acetonate, Fe(C5H7O2)2, in a closed SS reactor in an inert atmosphere yielded Fe3O4 NPs encapsulated in a carbon [IONEC] shell. The TEM of IONEC depicts dark spherical NPs surrounded by a faint material forming core-shell NSs [Figure (5a)]. These dark NPs are identified by EDS measurements as the Fe3O4 cores homogeneously embedded in a carbon shell. Due to the surrounding carbon shell, the Fe3O4 core magnetic particles are not agglomerated. The diameter of these Fe3O4 NPs ranges between 15 to 50 nm. The high resolution TEM image [Figure (5b)] reveals a 15 nm Fe3O4 crystalline core, surrounded by a ~10 nm carbon shell. The probable mechanism for the encapsulation of Fe3O4 nanoparticles in a carbon shell is reported elsewhere [51].

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Figure (5a)


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Figure (5) : (a) TEM of IONEC, (b) HR-TEM of IONEC, (c) Magnetization vs. magnetic field curves for an IONEC sample.

A room temperature magnetic susceptibility measurement of IONEC samples was conducted by employing a vibrating sample magnetometer (VSM). About 30 mg of IONEC is inserted into a gelatin capsule, placing the cotton wool on top of the sample, and then sealing the capsule. This prevents NPs from any movement caused by the vibration of the VSM sample holder rod, or by a variation of the applied magnetic field during the measurements. The IONEC illustrates the maximum magnetization of 31 emu/g [Figure (5c)]. Taking into account that IONEC contains 63.53 % of Fe3O4 particles, this gives a value of 48.8 emu/g, assuming 100% of Fe3O4 particles. The characteristic behavior of super-paramagnetism is demonstrated in the magnetization curve, which does not show hysteresis. It does not saturate, even at 16 kOe, which is due to the small size, 15 nm, of the Fe3O4 crystalline core (shown in TEM measurements). The magnetization of ferromagnetic materials is very sensitive to the microstructure of nanomaterials. If a specimen consists of small particles, its total magnetization decreases with the decreasing particle size due to the increased dispersion in the exchange integral [52], and, finally, reaches the super-paramagnetic state, when each particle acts as a big ‘spin’ with suppressed exchange interaction between the particles, which is more hindered because of the presence of a carbon shell. Nanomagnetic particles are

expected to exhibit reduced magnetization due to the large percentage of surface spins with disordered magnetization orientation [53]. The microwave-synthesized pure Fe3O4 nanoparticles (8–9 nm) depict the Ms of ~65 emu/g [54]. Our measured Ms value for Fe3O4@C is smaller, which is probably because of the coated carbon. The obtained smaller magnetization indicates that there is no magnetic proximity effect between the magnetic Fe3O4 NPs and carbon. This is consistent with recent experiments [55].

3.6 Graphitic nanocapsules encaging cobalt crystals : The thermal dissociation of cobalt naphthenate in a

RAPET system at 700 oC produced graphitic nanocapsules filled with cobalt crystals (GNFC) in an inert atmosphere.

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Figure (6) : (a) TEM image of graphitic nanocapsules encaging cobalt crystals, (b) graphitic carbon coated on a cobalt surface. The core-shell morphologies are well distinguished in the TEM image of GNFC shown in Figure (6a). Around 6-12 nm, a carbon shell encapsulates the core of dark cobalt particles. The graphitic nature of the surrounding carbon is demonstrated in Figure (6b). The interlayer spacings of the coated graphitic carbon are ~3.5 nm. The calculated element percentages of C, H, O, Co in the reactant cobalt naphthenate are 65, 5, 65, 14.49%, respectively. The measured element percentages of carbon and hydrogen in the GNFC product are 79 % carbon, hydrogen 0.4 % and


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0.2% oxygen. The particles exhibit typical ferromagnetic curves with a Ms of 22 emu/g, remanance magnetization of 2.6 emu/g and a coercivity of 151 G. The measured BET surface area of the GNFC sample prepared at 700 oC is 138 m2/g. Similarly, the thermal decomposition of ferrocene at 750 oC in an inert atmosphere in a closed SS reactor led to the formation of Fe nanoparticles encapsulated in carbon nanotubes as the RAPET product. Additionally, the RAPET of Ni acetylacetonate at 700 oC for 3 hours resulted in the encapsulation of ferromagnetic Ni nanospheres into the onion-structured [56] graphitic layers that possess interesting magnetic properties. The measured magnetoresistance (MR) of Ni nanospheres encapsulated in a fullerene-like carbon shows a large negative [57] MR of the order of 10%. The proposed mechanism for the formation of the Ni-C core-shell system is reported based on the segregation and the surface flux [58] formed in the Ni and carbon particles during reaction under autogenic pressure at elevated temperature. The measured Brunauer-Emmett-Teller (BET) surface area of the NCCS sample is 75.6 m2/g. 3.7 In-situ filling of ferromagnetic FeS NPs into carbon shells : The NPs of FeS have attracted much scientific and technological interest in environmental remediation [59]. Many efforts have been made to study their magnetic properties and crystal structure [60]. Nano-sized FeS particles are known to be a reactive medium that displays a higher reactivity than zero-valent iron in the reductive dechlorination of chlorinated hydrocarbons [61]. The ferromagnetic FeS NPs are covered in situ in a carbon shell by the one-step thermolysis of Fe(CO)5 in the presence of S powder in a closed reactor at 700 oC/30 min. under its autogenic pressure in an inert atmosphere [62]. The 1D fiber-like morphology of a FeS@C sample is shown in Figure (7a). The fibers are around 80 nm in diameter with lengths of several microns for the FeS@C sample. The TEM analysis of a single FeS@C fiber is shown in Figure (7b). The fiber is ~ 50 nm in diameter with a dark core [~20 nm] of the FeS material, which is completely covered by a carbon shell [15 nm] to form a 1D structure. The nitrogen gas adsorption on the surface of FeS@C NSs is determined by BET surface area analysis, while the ferromagnetic nature is understood using a VSM experiment. From a N2 adsorption-desorption isotherm of FeS@C NSs, the measured surface area is 14.9 m2/g with a pore volume of 0.027 cm3/g. Employing the BM [Brinker and Mukherji] formula [63] (Eq. 1), and using surface area [SA] and mode pore radius [MPR] data, the pore volume was calculated for FeS@C NSs. PVBET = (SA . MPR) / 2 ---------------------(Eq. 1)

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Figure (7) : (a) SEM image of a FeS@C NSs, (b) TEM image of the tip of a FeS@C fiber, (c) BET surface area curve of a FeS@C NSs [filled square: adsorption, hollow: desorption] and (d) magnetization vs. magnetic field curve for a FeSe@C NSs.


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The measured pore radius for FeS@C NSs is 35 Å. Therefore, the calculated BET pore volumes are 0.0261 cm3/g and 0.0208 cm3/g, respectively. These calculated values of pore volumes are in close agreement with the measured values for the 1D ferromagnetic core-shell fibers. The room temperature M-H curve of a FeS@C NS shown in Figure (7c), exhibits an Ms of 8 emu/g. Surprisingly, our FeS@C fibers showed ferromagnetic behavior, although the FeS (Troilite) does not possess ferromagnetic properties. However, the hexagonal phase of the current FeS product suggests its similarity not to the Troilite phase, but rather to the hexagonal structures of pyrrhotite Fe(1-x)S (x = 0 to 0.2), which is a weakly-ferromagnetic material. A coercivity of 300 Oe with a remanent magnetization of 3 emu/g is measured for FeS@C. Considering that the FeS@C NSs contains 71% of FeS (29 % C), this gives a value of 11.3 emu/g for 100% FeS. Wang et al. also demonstrated magnetic hysteresis with ferromagnetic behavior [64] for the as-prepared FeS nanowires.

3.8 Encapsulating superconducting MgCNi3 in a carbon nanoflask :

Yoshida was the beginner to measure the critical temperature of a superconductive material encapsulated in carbon nanotubes [65]. Recently, Jankovicˇ et al. synthesized high yield, template free, one-pot, free-standing tin nanowires [66] fully protected by carbon, and hence totally inert in air. We reported on a chemical method that enables the encapsulation of MgCNi3, which is a non toxic perovskite superconductor. This material has an unconventional superconductivity [67] with a critical temperature of 8K. MgCNi3 was encapsulated inside carbon nanoflasks (CNFs), a unique form of carbon structure that has a bulbous base connected to a narrower tubular part [68]. Such NSs are prepared by thermally decomposing Ni(CO)4 at 900-1000 oC in the presence of Mg granules that have a surface covered with Mg(OH) [68]. The as-prepared product was further treated with dil. HCl to obtain pure MgCNi3@CNFs. Using the same principle, Rana et al. also reported on CNFs encapsulating a MgCxCo3 ternary [69]

phase. The TEM image of the MgCNi3@CNFs product

obtained after HCl treatment represents a typical CNF structure. The globular part of the CNFs is partially filled with encapsulated MgCNi3 material [Figure (8a)]. The tubes emerging from these globular structures are also observed in this picture. The tubes are usually connected to the globular part through a neck that is an integral part of the flask on the TEM grid. The atomic force micrograph [AFM] of CNFs is shown in Figure (8b). The flask is about 250 nm in diameter and 30 nm high. As a test for mechanical stability, we used AFM manipulation to move the flasks on a Si substrate. The flasks maintained their original geometry without a noticeable change in the overall shape. The starred peaks are graphitic peaks and the non-starred peaks are from MgCNi3 NPs. The diffraction peaks match very well with the diffraction in the PDF file number 41-0903, thus indicating a composition of MgCNi3. The AFM image of a single MgCNi3 encapsulated in CNFs on a Si substrate is presented in Figure 8c.

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Figure (8) : (a) TEM image of MgCNi3 encapsulated in CNFs, (b) Powder XRD pattern of MgCNi3 encapsulated in CNFs, (c) AFM image of a single MgCNi3 encapsulated in CNFs on a Si substrate (non contact mode). and (d) A magnetic susceptibility measurement χ(T) plot for MgCNi3 encapsulated in CNFs exhibiting the superconducting transition. Inset: The magnetization hysteresis loop measured at 5K for MgCNi3, encapsulated in CNFs.


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To check how the size and shape affect the superconducting critical temperature, the direct current (DC) magnetic susceptibility measurements for MgCNi3 encapsulated in CNFs were performed using a commercial superconducting quantum interface device magnetometer. Figure (8d) shows the χ(T) plot (magnetic susceptibility (M/H) versus T) measured under an applied field of 5.8 Oe in the temperature range of 5-100k. The analogous onset on the superconducting state at the Tc value of the present and that reported in the literature [70] for bulk MgCNi3 samples is noticed. Above Tc, the χ(T) plot is linear and does not follow the Curie-Weiss law. We also measured the isothermal field dependence of the magnetization at 5K [Inset Figure (8d)]. The virgin low field region is almost linear and the Meissner fraction is deduced as 48 %, indicating a bulk superconducting matrix. The fact that the critical temperature of the MgCNi3 encapsulated in CNFs is almost equal to that of bulk MgCNi3 indicates that the encapsulated superconductor is highly ordered and the size of the superconducting wire exceeds the Anderson limit [71].

In conclusion, the RAPET (Reactions under Autogenic Pressure at Elevated Temperature) synthetic approach is demonstrated for the encapsulation of semiconducting, magnetic or superconducting NPs in-situ in a carbon shell forming core-shell NSs. Representative examples are demonstrated, where various core-shell NSs offer captivating [air-stabilization, phase stabilization and super-paramagnetic] properties.

4. Summary and Perspectives :

In the literature, there are various ways to synthesize semiconducting, magnetic and superconducting NPs, with their own advantages and disadvantages. Here, we have demonstrated the RAPET approach for the synthesis of nanosized core-shell nanostructures. Syntheses of various NPs often require the use of toxic and/or expensive precursors, and the reaction is often performed in an organic phase at high temperature at high dilution. In the case of silica-coated NPs, it is difficult or impossible to achieve a fully dense and nonporous silica coating, and it is thus difficult to maintain high stability of these NPs under harsh conditions, especially in basic environments. There is still a need to explore novel synthetic methods to ensure the stability of various NPs at high temperatures and under acidic and basic conditions. Carbon-coated NPs are remarkably stable under harsh conditions, and are conventionally prepared by arc-discharge or laser ablation - where extremely high temperatures are required. These processes are often connected with a very poor yield and sometimes incomplete coating of the produced NPs. On the contrary, our RAPET obtained NSs are always completely coated by carbon with high yield. However, the generation of a carbon coating on individual dispersed NPs and the control of the shell thickness still remain unresolved problems. Our future work is focused on the development of the high capacity reactor and optimization of reaction parameters for the mass fabrication of well dispersed semiconducting, magnetic and superconducting NPs coated with carbon forming core-shell NSs.

Acknowledgements : S.V.P. and V.G.P. are thankful to Bar-Ilan University for their doctoral and postdoctoral fellowships, respectively. All the authors are thankful to Prof. Z. Malik, Prof. A. Zaban, Prof. D. Aurbach, Prof. Y. Koltypin, Prof. I. Felner, Dr. Y. Grinblat, Dr. Yafa, Mrs. Louise Braverman, Mrs. Aya Shoham and Mr. Aharon Nagan, all from Israel. We also appreciate the help of research collaborators, Prof. S. Asai (Japan), J. M. Calderon Moreno (Spain) and Prof. V.G. Kessalar (Sweden).

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Dr. Vilas G. Pol : Dr. Vilas received M.Sc. and M.Phil. degrees in chemistry from University of Pune, India. He achieved Ph.D. from Nanocenter of Bar-Ilan University, Israel under Prof. Aharon Gedanken. After first postdoc with Prof. Arie Zaban in dye sensitized solar cells he joined to Dr. P. Thiyagarajan at Argonne National Laboratory,


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USA. He has published more than 60 research articles in refereed international scientific journals and his articles are cited for more than 700 times (including by Nobel Laureates). Vilas has written 2 journal reviews, 2 book chapters and filed 3 US patents. He has presented his research work in 15 international and 12 national conferences. Vilas is a recipient of Intel Student Prize, Best Student Award, as well as 'Gold Medal' of DRDO Sports Board, India. Recently, Vilas became a Directors postdoctoral fellow at Argonne National Laboratory, USA. He is pursuing his scientific career in the synthesis of nanomaterials for their application in energy storage and energy conversion systems.

Dr. Swati Vilas Pol : Dr. Swati obtained M.Sc. degree in physical chemistry from University of Pune, India. She completed her Ph.D. under the supervision of Prof. Aharon Gedanken from Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Israel. She has published more than 30 scientific articles, and filed 1 US patent. Swati is a ‘Meritorious student’ in Maharashtra Talent Search Examination and Bharati University Scholar in competitive exams for English & Mathematics. Now a day, Swati is pursuing her postdoctoral research work at Argonne National Laboratory, USA.

Prof. Aharon Gedanken : Prof. Aharon obtained his M. Sc. from Bar-Ilanh University, and his Ph. D. degrees from Tel Aviv University, Israel. After Postdoc. at USC in Los Angeles, he returned to Bar-Ilan in 1975 as a senior faculty. He has published more than 475 scientific papers in the peer reviewed journals. Professor Gedanken also filed more than 15 US patents. Prof. Gedanken is a group leader of 20 researchers which includes students, postdocs and scientists.

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