Synthesis of Boron Carbide Core− Shell Nanorods and a Qualitative

J. Phys. Chem. C 2009, 113, 1657–1661

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Synthesis of Boron Carbide Core-Shell Nanorods and a Qualitative Model To Explain Formation of Rough Shell Nanorods Mohammad Jazirehpour* and Ali Alizadeh Materials Science and Engineering Department, MUT UniVersity, Tehran 16765-3454, Iran ReceiVed: October 26, 2008; ReVised Manuscript ReceiVed: NoVember 28, 2008

Boron carbide nanorods with different morphologies via a simple carbothermal chemical vapor deposition process have been synthesized on a graphite substrate using Co-B catalyst nanoparticles. Densely applied Co-B nanoparticles resulted in formation of a large number of nanorods with a biphasic structure which was composed of a rough amorphous shell on a uniform crystalline core. When Co-B nanoparticles were sporadically applied on the substrate, lone nanorods were synthesized which had smooth surfaces. Samples were characterized by X-ray diffraction, photoluminescence spectroscopy, scanning electron microscopy, and transmission electron microscopy. A qualitative model is also proposed to explain the formation of rough shell nanorods. Introduction Boron carbide is a refractory p-type semiconductor with outstanding properties such as low density (2.5 g cm-3), small thermal extension coefficient (5.73 × 10-6 K-1), extreme hardness (about 30 GPa), high Young’s modulus, good chemical resistance, attractive high-temperature thermoelectric properties, and a high neutron absorption cross section.1-4 Also it has potential applications in ceramic or metal matrix composites as a reinforcing phase, wear-resistant ceramics, lightweight body armor,5,6 neutron absorbent shields, solid-state neutron detectors, high temperature thermoelectric energy converters,7,8 and field emission devices.9 Previously, boron carbide nanostructures including nanowires, nanorods,nanonecklaces,andnanospringshavebeensynthesized.10-12 Pender et al. synthesized aligned boron carbide nanowires by pyrolyzing a single source molecular precursor in porous alumina templates.13 Zhang et al. fabricated boron carbide nanowires by a plasma-enhanced chemical vapor deposition (PECVD) method.10,12,14 Moreover, carbon nanotubes were used as templates to synthesize boron carbide nanorods.15-18 An electrostatic spinning method has also been developed to synthesize boron carbide from a specific polymeric precursor.19 Bando et al. synthesized high-purity boron carbide nanowires via a catalyst-free carbothermal route.20 Carlsson et al. developed a carbothermal vapor-liquid-solid (VLS) process to synthesize boron carbide elongated nanostructures.21 Despite these methods, improvement of more convenient and facile methods for largescale synthesis of boron carbide one-dimensional nanostructures is still needed. In this paper, we report a simple carbothermal chemical vapor deposition process to synthesize a large number of boron carbide nanorods on a graphite substrate in the presence of Co-B nanoparticles as catalyst. As-synthesized products have been characterized. Both lone nanorods with smooth surfaces and densely grown nanorods with rough surface morphology have been synthesized when catalyst nanoparticles are applied on the substrate with different distribution densities. No relation between the planar density of applied catalyst nanoparticles on the growth substrate and the surface morphology of nanorods * Corresponding author. E-mail: [email protected].

has been previously reported; in this paper such a relation is presented. Also, we propose a qualitative model to explain the formation of rough surface morphology on the nanorods. We believe that the proposed model could improve understanding of a new aspect of the VLS growth mechanism. Rough surface nanorods may be applicable as an enhanced reinforcement phase in different composite materials due to improved interlocking with the matrix phase, while isolated higher quality nanorods with smooth surface morphology may be more interesting in field emission, electronic, and thermoelectric devices. Experimental Section Starting materials including boron oxide (99.99% purity), activated carbon, and sodium chloride were used in a molar ratio of B2O3:C:NaCl ) 9:5:0.2. Starting materials were mixed adequately in a planetary mill and loaded in a graphite crucible. Co-B nanoparticles were prepared via a mechanical alloying process. Cobalt and boron in the atomic ratio of Co63B37 were milled in a planetary mill for 25 h under argon atmosphere. The powder was then suspended in ethanol and subsequently was sonicated for 1 h.The solution was filtered to separate coarse particles and then left to stand for 4 h. This was done to settle larger particles. Polished graphite substrates were cleaned by hydrochloric acid and then washed by deionized water. A dropper was then used to apply the solution on the substrate. It was then left to evaporate ethanol gradually. Rapid evaporation of ethanol causes nanoparticles to agglomerate. Graphite substrate was placed in the crucible as shown in Figure 1. Afterward, the crucible was placed at the center of a tube furnace, before the evacuation process. The furnace was heated and kept at the maximum set temperature (1200-1400 °C in different examinations) for 90 min. During the process a constant flow of argon gas (∼300 mL min-1) was fed into the tube. After the process, the furnace was naturally cooled to room temperature. In another examination a catalyst containing solution was extremely diluted and applied on the growth substrate; other conditions were similar to those in previous samples. This was done to investigate how nanorod morphologies change with the distribution density of applied catalyst nanoparticles on the

10.1021/jp809470u CCC: $40.75  2009 American Chemical Society Published on Web 01/08/2009

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Figure 1. Schematic diagram of the crucible setup and location of the growth substrate.

Figure 3. SEM images of rough surface boron carbide nanorods. (a) SEM micrograph of boron carbide nanorods synthesized at 1400 °C for 90 min with catalyst nanoparticles applied densely on the growth substrate; (b) higher magnification SEM micrograph, showing that nanorods are the only products.

Figure 2. Typical XRD pattern of synthesized nanorods at 1400 °C.

substrate. Samples were characterized by X-ray diffraction (XRD, Philips X’pert MPD). Synthesis products were first coated with Au and then observed using scanning electron microscopy (SEM, JSM-6301) equipped with energy-dispersive X-ray spectroscopy (EDS). Nanorods were mounted on carboncoated copper grids, and samples were further characterized using a Philips CM200 transmission electron microscope (TEM). Photoluminescence (PL) spectroscopy was carried out by a HITACHI F-4500 spectroscopy system using a 530 nm excitation light at room temperature. Results and Discussion Figure 2 shows a typical XRD pattern of samples synthesized at 1400 °C. The acute peaks are mainly attributed to B4C (PDF No. 75-0424, a ) 0.5600 nm, b ) 1.0286 nm),22 except that two peaks match the substrate material (graphite). Some discordant features compared to standard powder diffraction patterns could be attributed to the small size of the nanostructures as well as crystalline defects such as stacking faults and twins.23 Also, the boron to carbon content ratio can influence the XRD pattern.19 Boron carbide typically contains a high concentration of twins.1,24,25 Decrease in synthesis temperature intensifies the density of twins.23 The presence of twins can considerably change XRD patterns by introducing peak broadenings, peak shifts, and the disappearance of some peaks and appearance of others,23 but no change in the relative intensities of peaks is

expected. Broadenings at 2Θ values of about 23° and 37° are observable which could be attributed to twin effects. The peak at 2Θ of about 38° is somewhat higher than the one at about 35° compared to standard patterns. It has been shown24 that relative peak heights depend on nanostructure size. Surface morphologies of nanorods in our investigations differ depending on whether catalyst nanoparticles were dispersed densely on the substrate or not. Figure 3 shows the nanorods synthesized using Co-B nanoparticles which were densely dispersed in an ethanol solution and applied on the substrate. Figure 3 shows the general morphologies of these nanorods. The nanorods are about 30-120 nm in diameter and several micrometers in length. These nanorods have rough surface morphologies; however, they are almost straight along their axes. In order to investigate the nanorod growth mechanism, compositions of different points of nanorods were measured using EDS examination under high-resolution SEM. Typical EDS spectra obtained from a nanorod tip and the rod itself are respectively shown in Figure 4b and 4c. In all EDS spectra, Au element coming from Au coating was detected. The EDS spectra revealed that the droplet at the tip of the nanorod contains B, C, and Co. In contrast, the nanorod was composed of B and C only. Figure 5 shows a TEM image with further details of a typical nanorod with rough surface morphology. The nanorod has a core-shell structure. The core is almost uniform in diameter and is sheathed by an amorphous shell with a diverse thickness (5-30 nm) along the nanorod. The bottom inset in Figure 5 demonstrates that the shell is completely amorphous. Parallel striations along the core can be attributed to twins which were formed at the low temperature (1400 °C) of the synthesis process. The presence of twins in boron carbide has been

Synthesis of Boron Carbide Core-Shell Nanorods

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Figure 6. SEM image of a typical lone nanorod which was synthesized when ultradiluted catalyst containing solution was applied on the growth substrate (1400 °C for 90 min). The insets show higher magnification images from the tip and middle of the nanorod.

Figure 4. SEM image of boron carbide nanorods and typical EDS spectra obtained from the nanorod tip and the nanorod itself. Au is introduced to EDS spectra due to Au coating. (a) SEM image of boron carbide nanorods; (b) EDS spectrum of point A; (c) EDS spectrum of point B.

Figure 7. TEM image of a typical lone nanorod with uniform core-shell structure. The bottom inset shows the electron diffraction pattern for the selected region of the amorphous shell.

Figure 5. Typical TEM image of a boron carbide nanorod with a rough shell which was synthesized at 1400 °C for 90 min. The top inset shows the same nanorod with enhanced image contrast, where the core is more clearly visible. The core is uniform along the diameter; striations along the length are due to twinned structure. The bottom inset shows the electron diffraction pattern for the selected region which demonstrates the shell is completely amorphous.

previously confirmed even with low-resolution transmission electron microscopy.26-29 The effect of this twinned structure on the XRD patterns was mentioned above. Figure 6 shows a SEM image of an almost long individual nanorod. This nanorod was formed on the graphite substrate which was coated by very sparsely dispersed catalyst nanoparticles contained in ethanol solution. Its length is about 10 µm, and the diameter is about 100 nm. It is completely straight along the whole length and its surface seems to be smooth in the SEM image. The rod itself is a little thicker than the tip. The EDS spectra obtained from the rod tip and the rod itself are identical to Figure 4b,c. Figure 7 shows a typical TEM image of a lone nanorod similar to that one in Figure 6, revealing a uniform core-shell structure. According to the synthesis conditions, vapor-solid (VS)20 and vapor-liquid-solid (VLS)10,21,30,31 growth mechanisms can

be responsible, but since catalyst nanoparticles were applied in the process, the vapor-liquid-solid mechanism was more probable. If a VS mechanism were responsible, when catalyst nanoparticles were applied sporadically using an amply thinned solution, more nanorods should be synthesized densely on the substrate while synthesized nanorods are completely isolated and sparse. Anyway, EDS examinations indicated catalyst material at the tips of the nanorods, confirming the occurrence of the VLS mechanism. In the vapor-liquid-solid (VLS) growth mechanism, a metal catalyst acts as a preferred location for condensation of reactant vapors. In our experiments, reactant vapors are generated via the carbon thermal reduction of boron oxide above 1200 °C and the overall reaction21 is

2B2O3(l) + 7C(s) f B4C(s) + 6CO(g)

(1)

Catalyst nanoparticle is initially melted, and then it absorbs atoms from the vapor phase containing boron carbide constituents. The growth begins after boron carbide species become supersaturated in a molten catalyst droplet. Catalyst droplet is located at the growth front, and precipitation of boron carbide nanorod occurs on this nanocatalyst. During the process NaCl in the starting mixture decomposes and releases chlorine into the environment. Chlorine is beneficial in the formation of

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Figure 8. Schematic illustration of the proposed qualitative model. Ta is the favored temperature of amorphous phase formation, and Tc is the favored for the crystalline phase. Formation of rough surface nanorods at 1400 °C could be assumed as a cyclic process: (a) at first the catalyst nanoparticle is melted and absorbs gas feedstock from surrounding vapors, which results in a local lack within the surrounding space. (b) The nanorod grows and the concentration of feedstock within the catalyst is decreased; simultaneously the global temperature of the catalyst is increased. (c) Heat is transported to surrounding space and diffusion of gas species is accelerated; therefore the local lack of the gas feedstock around the catalyst droplet is compensated. (d) The catalyst is saturated again and growth is continued.

adequate vapor feedstock; it has also a great influence on the yield of boron carbide nanostructures.21 Besides, chlorine function in the formation of boron carbide gaseous constituents, it reacts with the catalyst droplet which causes a graduate decrease in droplet size and an identical decrease in the diameter of the nanorod. When the droplet size becomes smaller, the rate of catalyst evaporation becomes faster and the decrease of the nanorod diameter becomes more sensible. It seems that the reason is the increase of the surface to volume ratio. It is observable in Figure 6 (right inset) that the nanorod tip is thinner. It is well-known that the growth temperature can influence the crystal growth and morphological characteristics.32,33 Because of radiative losses, conductive heat transfer with surrounding vapor and conduction along the as-forming nanorod catalyst droplet cannot be assumed as an adiabatic system. At least there is no testimony to postulate the existence of any temperature uniformity in the catalyst droplet. Anyway, it has been shown that the formation of biphasic nanowires has been derived from the existence of temperature gradients within the catalyst droplet. In the case of SiC core-shell nanowires, Zhang et al.34 have proposed that the temperature near the surface of the catalyst is lower than in the core and that reduced temperature favors the formation of amorphous material, as opposed to the higher temperature of the core, which favors crystalline growth. We believe that while a large number of boron carbide nanorods are growing densely in relative low temperature ranges (1200-1400 °C) the reciprocal effects of adjacent growing nanorods cannot be neglected. Hence a simplified qualitative model is presented which describes the formation of rough shell morphology on the nanorods. In the current model, reciprocal effects of adjacent nanorods are supposed as the concentration profile near the growing nanorod. A thermal gradient within the catalyst droplet is also assumed due to radiative losses, conductive heat transfer with the surrounding vapor, and conduction along the as-forming nanorod. According to the proposed model (Figure 8) during the elementary stage, the nanorod begins to grow and the catalyst droplet is lifted off from the substrate. The catalyst droplet absorbs nanorod constituents from the surrounding vapor which causes a temporary local decrease in density of nanorod constituents around the catalyst. Also, adjacent growing nanorods have similar effects on their neighborhood. Therefore, it takes a while for nanorod constituents to compensate for the

lack by diffusion. The density of adjacent growing nanorods, growth temperature, and vapor pressure of gas constituents seem to be effective in the formation of the lack zone and the time needed to compensate for the lack. As the nanorod grows, the density of nanorod constituents within the catalyst decreases gradually (Figure 8, stages b and c). This should apparently reduce the synthesized volume of both amorphous and crystalline phases of the nanorod. There is another parameter which is effective here. Formation of a solid boron carbide nanorod is accompanied by a heat release at the catalyst-nanorod interface, which results in a global temperature increase within the catalyst droplet (Figure 8, stage b). The effect of the temperature gradient in the catalyst droplet was mentioned above. Increase in the catalyst temperature and decrease in the density of nanorod constituents both taper the amorphous phase, while an increase in the temperature of the catalyst droplet favors the formation of crystalline phase. Consequently, the crystalline phase does not exhibit a sensible change in thickness while tapering becomes noticeable in the amorphous phase. The heat released due to the formation of boron carbide nanorod is transferred into the surrounding space, which accelerates the diffusion of atoms. The local lack zone is compensated, so the catalyst is saturated again (Figure 8, stage c). Subsequently the temperature gradient and the concentration profile within the catalyst return to the previous state and the cycle is repeated once again according to the proposed model. During the growth of a lone nanorod, fluctuations in concentration within adjacent space of the nanorod could be disregarded. Accordingly, no bumpiness could be observable on surfaces of sparsely grown nanorods. Figure 6 shows a typical sparsely grown nanorod. The surface of the nanorod is smooth, whereas for the nanorods in Figures 3 and 4, all synthesis conditions are similar except the compactness of applied catalyst nanoparticles on the substrate; this confirms the mentioned model. Figure 9 shows the room temperature photoluminescence spectrum of rough surface boron carbide nanorods under a 530 nm light excitation source. It exhibits an acute maximum peak at 1.905 eV and two weaker peaks at 1.884 and 1.873 eV. Previously, photoluminescence properties of bulk boron carbide were investigated and two maximum peaks were observed in the PL spectra at 1.563 and 1.572 eV. The mentioned peaks are attributed to the indirect allowed recombination of free excitions.35 It can be considered that the changes in the electronic

Synthesis of Boron Carbide Core-Shell Nanorods

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Figure 9. Photoluminescence spectrum of boron carbide rough surface nanorods.

structure of semiconductors are caused by the quantum size effect36,37 which leads to blue shift of the current PL peaks of boron carbide nanorods. Conclusion In conclusion, we synthesized boron carbide biphasic nanorods with two different surface morphologies via a convenient catalyst assisted carbothermal chemical vapor deposition process. Photoluminescence properties of rough shell boron carbide nanorods were also investigated, and an acute maximum peak at 1.905 eV and two weaker peaks at 1.884 and 1.873 eV were observed. The growth mechanism of the nanorods was investigated by scanning and transmission microscopies. Reciprocal effects of adjacent growing nanorods besides the temperature gradient within catalyst droplets were supposed to induce a concentration profile within the surrounding space of every nanorod. Accordingly, a model was proposed to describe the formation of biphasic structure and rough shell morphology. Boron carbide nanorods might be applicable in field emission devices and thermoelectric energy converters. Specifically, rough surface nanorods might get significant potential application in composite materials as an enhanced reinforcement phase due to improved interlocking with the matrix or as a viable candidate for catalyst substrates. Straight lone nanorods are particularly interesting because of their potential application in thermoelectric devices and other electronic applications. Furthermore, rough shell nanorods with regular bumpiness may be important as building blocks in MEMS systems. Results of this investigation may present a beneficial concept to get better control of the growth of one-dimensional nanostructures via a VLS growth mechanism.

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