Mg-Catalyzed Autoclave Synthesis of Aligned Silicon Carbide

can reinforce effectively the strength and toughness of ceramic composites.12 ..... performances should find applications in fabrications of light...
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J. Phys. Chem. B 2006, 110, 14172-14178

Mg-Catalyzed Autoclave Synthesis of Aligned Silicon Carbide Nanostructures Guangcheng Xi, Yankuan Liu, Xiaoyan Liu, Xiaoqing Wang, and Yitai Qian* Hefei National Laboratory for Physical Science at Microscale, Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui 230026, P.R. China ReceiVed: March 21, 2006; In Final Form: May 21, 2006

In this article, a novel magnesium-catalyzed co-reduction route was developed for the large-scale synthesis of aligned β-SiC one-dimensional (1D) nanostructures at relative lower temperature (600 °C). By carefully controlling the reagent concentrations, we could synthesize β-SiC rodlike and needlelike nanostructures. The possible growth mechanism of the as-synthesized β-SiC 1D nanostructures has been investigated. The structure and morphology of the as-synthesized β-SiC nanostructures are characterized using X-ray diffraction, Fourier transform infrared absorption, and scanning and transmission electron microscopes. Raman and photoluminescence properties are also investigated at room temperature. The as-synthesized β-SiC nanostructures exhibit strong shape-dependent field emission properties. Corresponding to their shapes, the as-synthesized nanorods and nanoneedles display the turn-on fields of 12, 8.4, and 1.8 V/µm, respectively.

1. Introduction The fabrication of one-dimensional (1D), nanoscale building blocks such as nanowires, nanorods, nanotubes, and nanobelts, as well hierarchical nanostructures, has been researched intensively because of their potential applications in various fields.1 SiC is an outstanding wide-gap semiconducting material for high temperature and high power electronic applications due to its excellent properties, such as high mechanical strength, high thermal stability, and high thermal conductivity.2 As a result of the large band gap of SiC (2.39 eV for 3C (β)-SiC, 3.02 eV for 6H-SiC, 3.26 eV for 4H-SiC, and 3.33 eV for 2H-SiC at room temperature),3 it processes a very high breakdown field (2.12 MV/cm for 3C-SiC, 2.2 MV/cm for 4H-SiC, and 2.5 MV/cm for 6H-SiC), typically 8-10 times higher than Si (0.25 MV/cm). In addition, the prominent thermal conductivity of SiC (3.2 W cm-1 K-1 for 3C-SiC, 3.7 W cm-1 K-1 for 4H-SiC, and 4.9 W cm-1 K-1 for 6H-SiC) is typically 2-3 times higher than that of silicon (1.5 W cm-1 K-1), which allows SiC to remove heat more efficiently.4 Therefore, the synthesis and physical properties of SiC have been widely investigated. Recently, 1D SiC nanostructures such as nanorods,5 nanowires,6 nanotubes,7 and nanobelts,8 as well some complicated nanostructures,9 have been synthesized and attracted a lot of research interest due to their shape-induced unique electrical and optical properties. For example, SiC nanocrystals have been demonstrated to be good blue luminescence material10 and SiC nanowires and nanorods are considered to be excellent candidates for field emission devices.5f,6d,h Wong et al. showed that SiC nanowires and nanorods have yield strengths that can be more than 50 GPa, a value that is far larger than the corresponding values for microscale SiC whiskers and fibers.11 Yang et al. found that SiC nanowires can reinforce effectively the strength and toughness of ceramic composites.12 Conventionally, high-temperature (above 1000 °C) physical thermal evaporation methods are used to synthesize SiC nanocrystals, which increases the industrial cost. Therefore, * Corresponding author. Phone: 86-551-3603204. Fax: 86-551-3607402. E-mail: [email protected].

finding a low-temperature synthetic route to SiC nanocrystals is an effective approach to reduce the industrial cost. Here, a novel Mg-catalyzed chemical co-reduction route is developed for the large-scale synthesis of aligned β-SiC nanorods and nanoneedles at relative lower temperature (600 °C). The assynthesized nanostructures are highly crystalline and have uniform crystal lattice growth orientation ([111]). In this method, different from conventional VLS mechanism, metal Mg is used not only as catalyst but also as reducing agent. By carefully controlling the reaction conditions, we could synthesize β-SiC nanorods and nanoneedles, respectively. At room temperature, strong blue emissions at 462, 459, and 442 nm were found in the as-synthesized nanostructures. Furthermore, the as-synthesized nanorods and nanoneedles exhibit shape-dependent field emission properties with turn-on fields of 12, 8.4, and 1.8 V/µm, respectively. The results suggest that aligned β-SiC nanorods and nanoneedles can be synthesized at relative lower temperature by the Mg-catalyzed chemical co-reduction route and their optical and electronic properties can be effectively adjusted by shape control. 2. Experimental Section All the reagents used in the experiments were analytical, purchased from Shanghai Chemical Reagents Company, and used without further purification. In a typical process to prepare sample I, 1.4 mL of SiCl4 and 0.3 mL of 2-ethoxyethanol (HOCH2CH2OCH2CH3) were added into a 15-mL stainless steel autoclave, and then three pieces of Mg ribbons (80-mm long, 3-mm wide, and 0.2-mm thick) were put into the autoclave. All of the above manipulations were performed in a glovebox with flowing nitrogen gas. The autoclave was tightly sealed and heated in an electric stove with an increasing speed of 10 °C/ min and maintained at 600 °C for 1 h and then cooled to room temperature naturally. The ribbons were then collected from the autoclave, rinsed with distilled water and absolute alcohol, and vacuum-dried at 50 °C for 3 h for SEM characterization. The gray product grown at the surface of the ribbons was scraped and further washed with diluted hydrochloric acid for X-ray powder diffraction (XRD), Fourier transform infrared

10.1021/jp0617468 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/30/2006

Autoclave Synthesis of Aligned SiC Nanostructures

Figure 1. XRD patterns of the as-synthesized samples: (a) sample I, (b) sample II, and (c) sample III.

absorption (FTIR), transmission electron microscopy (TEM), high-resolution transmission electron microscope (HRTEM), Raman, photoluminescence (PL), and field emission characterizations. For samples II and III, a similar process was used except that the amounts of SiCl4 and 2-ethoxyethanol were changed. Synthesis of sample II: SiCl4 2.8 mL and 2-ethoxyethanol 0.6 mL. Synthesis of sample III: SiCl4 1 mL and 2-ethoxyethanol 0.21 mL. XRD patterns of the products were recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu KR1 radiation (λ ) 1.54178 Å). FTIR was recorded from a Magna IR-750FT spectrometer. The Raman spectra were produced at room temperature with a LABRAMHR confocal laser micro-Raman spectrometer. The emission scanning electron microscope (SEM) image of the products was examined by a field emission SEM (JEOL-6300F). The TEM images, selected area electron diffraction (SAED) patterns, and HRTEM images were recorded on a JEOL 2010 microscope. PL spectrum measurement was performed in a Fluorolog-3TAU fluorescence spectrophotometer with a Xe lamp at room temperature. Field emission measurement was carried out in a vacuum chamber with a pressure better than 3 × 10-7 Pa at room temperature under a two-parallel-plate configuration. The as-synthesized products were attached to a stainless steel plate using conducting glue as a cathode. Another parallel stainless steel plate served as the anode at a fixed distance of 200 µm during all the measurements. A voltage with a sweep step of 50 V was applied between the anode and cathode to supply an electric field. The emission current was monitored by a Keithley 485 picoammeter. 3. Results and Discussion Powder XRD, FTIR, Morphology, Crystalline Orientation, and Raman Spectrum. The phase composition and phase structure of the as-synthesized products were examined by XRD and FTIR. Figure 1a shows a typical XRD pattern of the asprepared sample I. All the peaks can be indexed as β-SiC with lattice constant of a ) 4.342 Å, which is close to the value of β-SiC (a ) 4.359 Å) (JCPDS, NO. 29-1129). The strong and sharp peaks indicated that the samples were well-crystallized. Figure 1b,c shows the XRD pattern of samples II and III, which displays that they are also β-SiC and have good crystallinity. The low intensity peaks marked with SF in Figure 1a,c might be attributed to stacking faults.13 The FTIR spectra for the as-

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Figure 2. FTIR spectra of the as-synthesized samples: (a) sample I, (b) sample II, and (c) sample III.

synthesized samples are shown in Figure 2. The obvious absorption peak at about 814 cm-1 can be indexed as the TO phonons of β-SiC, which is consistent with earlier reports,14 while the weaker absorption peak at about 1640 cm-1, which corresponds to the H-O-H bending vibration [&(OH2)], could be due to the absorption of H2O in these samples. The XRD patterns and FTIR spectra indicate that crystalline β-SiC products were obtained through the present synthetic route. A typical low-magnification SEM image of sample I is shown in Figure 3a. It can be seen that sample I is composed of large numbers of nanorods, which are straight and aligned on the Mg ribbon substrate with a high uniformity across the entire substrate, indicating that this technique can be scaled up for large-area production. Cross-sectional SEM images, such as that shown in the Figure 3a inset, have further revealed that these nanorods exhibit near-rectangular cross sections. Imaging of many nanorods from sample I shows that the average width of a typical β-SiC nanorod is 50-75 nm with lengths reaching 2-3 µm. When more amounts of raw materials were added into the reaction system (SiCl4 2.8 mL and HOCH2CH2OCH2CH3 0.6 mL), aligned β-SiC nanorods (sample II) can be also obtained (see Figure 3b). Different from the nanorod contains in sample I, these nanorods contained in sample II have a compressed cross section (see the inset in Figure 3b). Generally, the nanorods are 80-nm wide, 30-nm thick, and 2-µm long. If only 1 mL of SiCl4 and 0.21 mL of HOCH2CH2OCH2CH3 are added into the reaction mixture, the finally products (sample III) are needlelike β-SiC nanostructures (Figure 3c). A highmagnification SEM image (inset in Figure 3c) shows that the needlelike nanostructures have sharp tips, suggesting a promising configuration for field emission applications. This experimental phenomenon demonstrated that the shape-controlled synthesis of β-SiC 1D nanostructures could be realized by adjusting the concentrations of the raw materials. The nanorods and nanoneedles were dispersed on TEM grids to additional structural analyses. Figure 4a shows the TEM image of the β-SiC nanorods from sample I. It clearly shows that the nanorods have uniform width on the whole length, and the surfaces of the nanorods are very smooth. The HRTEM image of a single β-SiC nanorod (Figure 4b) exhibits clearly fringes perpendicular to the nanorod axis. The fringe spacing measures 0.25 nm, which concurs well with the interplanar spacing of (111) and alludes to the nanorod growth direction along [111]. This is consistent with the SAED pattern in the inset of Figure 4a. Silicon dioxide lays are not found on the

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Figure 4. (a) Typical TEM image of sample I (inset, corresponding SAED pattern). (b) HRTEM image of an individual β-SiC nanorod. (c) TEM image and SAED pattern of a single β-SiC nanorod (sample II). (d) HRTEM image of the β-SiC nanorod. (e) TEM image and SAED pattern of a single β-SiC nanoneedle. (f) HRTEM image taken from the tip of the nanoneedle.

Figure 3. Representative SEM images of the as-synthesized β-SiC samples: (a) sample I, (b) sample II, and (c) sample III.

edge of the nanorod, which is generally unavoidable when SiO2 is used as silicon source in CVD or high-temperature evaporation methods. More nanorods were checked by HRTEM, and a small quantity of staking faults and microtwins was found in the HRTEM images, which is consistent with the results of the XRD. Figure 4c displays the TEM image and SAED pattern recorded from a single β-SiC nanorod (sample II). It also clearly shows that the width of the nanorod is uniform and the surface is smooth. Figure 4d corresponds to the HRTEM image of the nanorod, which reveals that the nanorod has single-crystalline structure and grows along the [111] direction and there are no silicon dioxide lays on the edge of the nanorod. Figure 4e shows the TEM image of the nanoneedles. The trunk section of the nanoneedle is an average 40 nm in diameter, and the tip is only

about 8 nm in diameter. The HRTEM image shows that the growth direction of the nanoneedles is [111] and the tip of the nanoneedles is highly crystalline (Figure 4f). Raman scattering is a useful tool for the characterization of nanosized materials and a qualitative probe of the presence of lattice defects in solids (e.g., the crystalline quality can be judged from the band shapes and the selection rules). The Raman spectrum of as-synthesized sample I (Figure 5a) shows the presence of two sharp peaks at 795 and 973 cm-1, which correspond to the TO and LO phonons at the Γ point of β-SiC, respectively. Figure 5b,c shows the Raman spectrum of samples II and III, which also shows two sharp peaks at about 795 and 973 cm-1. The low intensity peaks marked with SF in the Raman spectra might be attributed to stacking faults in the nanocrystals. 15 The sharp peaks confirm that the as-synthesized β-SiC nanostructures are very crystalline. Furthermore, the absence of the folded modes that are related to the SiC polytypes (natural superlattices due to various stacking orders of Si-C bilayers) in the Raman spectra is consistent with the X-ray data and suggests that the as-synthesized products are only β-SiC.16 At the same time, these results are consistent with the HRTEM image observations.

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Figure 6. SEM images of the surface of the Mg ribbon recorded at various times: (a) before reaction, (b) after 15-min reaction, (c) after 30-min reaction, and (d) after 1-h reaction.

Figure 5. Raman spectra of the as-synthesized β-SiC samples: (a) sample I, (b) sample II, and (c) sample III.

Growth Process and Mechanism. To substantially understand the growth mechanism of the β-SiC 1D nanostructures under the present synthetic route, we have systematically surveyed the growth process by analyzing the β-SiC nanorods (sample I) at different growth stages. Figure 6 gives the electron microscopy images of four samples that were taken from the different stages of the reaction. Figure 6a displays a SEM image of the Mg ribbon before reaction, which shows that the surface of the Mg ribbon was considerably smooth. After reaction for 15 min, many small protuberance arrays were found growing upright from the surface of the Mg ribbon (Figure 6b). After a 30-min reaction, the small protuberances were transformed into short nanorods (Figure 6c). Finally, as the reaction proceeded long enough (1 h), nanowires were obtained (Figure 6d). It was noted that the end of many of the nanowires that were not washed with acid solution often attached solid nanoparticles (Figure 7a). Nanobeam EDS analyses indicate that the nanoparticles were composed of Si, C, O, Cl, and Mg (Figure 7b), whereas EDS from the trunk of the nanorods shows only C

Figure 7. (a) TEM image of an individual nanorod with catalyst particle. (b) EDS spectrum taken from the catalyst particle. (c) EDS spectrum taken from the trunk of the nanorod.

and Si (Figure 7c). Note that Cu peaks in Figure 7b,c are due to the TEM grid. On the other hand, the XRD pattern of the nanorods that are not washed with acid solution can be indexed as a mixture of MgO, MgCl2, and SiC. From the EDS and XRD analyses, the chemical equations contained in the reaction process can be reasonably formulated as follows:

HOCH2CH2OCH2CH3 (g) + Mg (l) f C (s) + MgO (s) + H2 (g) SiCl4 (g) + Mg (l) f Si (s) + MgCl2 (s) C + Si f SiC

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Figure 8. Schematic illustration of the growth process of the aligned 1D nanostructures.

On the basis of the above experimental observation, we believe that the present β-SiC nanostructures were produced by an Mg-catalyzed chemical co-reduction growth mechanism. That is to say, during the formation process of the β-SiC 1D nanostructures, metal Mg acted not only as catalyst but also as reducing agent. At the present experimental temperature (600 °C), SiCl4 and 2-ethoxyethanol will partly transform from liquid state to vapor state (the boiling points of SiCl4 and 2-ethoxyethanol are 56 and 150 °C, respectively) and form a supercritical fluid, which has been demonstrated to be an excellent medium for synthesis of various nanostructures.17 Though the present reaction temperature (600 °C) is lower than the melting point of Mg (648 °C), the surface of the Mg ribbon should be a melting state because of the generation of the heat of reaction, and many nanoscale Mg drops will appear on the surface of the ribbon. When the supercritical SiCl4 and 2-ethoxyethanol fluid contact the Mg drops (acting as reducing agent and liquid catalyst), they will be reduced to Si and C atoms. These newly reduced atoms will dissolve into the Mg drops. The fresh C atoms and Si atoms have higher activity than commercial graphite and silicon, which make the formation of SiC nanoclusters at lower temperature (600 °C). At the same time, the fast transport of the SiC species in the supercritical fluid might be responsible for the growth of the highly crystalline SiC 1D nanostructures.18 It has been noted that the final morphologies of the products are not determined by the different ratios of raw materials, but different amounts of raw materials, which suggests that the total concentration of reaction mixture strongly affects the growth process of the SiC nanocrystals.18a On the other hand, the total pressure in the reaction system will change following the change of the amounts of raw materials, which certainly affects the properties of the supercritical fluid. Furthermore, the property change of the supercritical fluid might affect the growth kinetics of the SiC nanocrystals, such as growth rate and diffusion rate. Figure 8 shows the schematic illustration of the growth process of the aligned 1D nanostructures. However, as it is difficult to monitor the reaction once the system is sealed, much work needs to be done to investigate the kinetics of the reaction. Room-Temperature PL Spectra of the As-Synthesized Products. The room-temperature PL spectra of the obtained β-SiC samples are shown in Figure 9. These spectra exhibit different features depending on their morphological variation. Curve (a) shows the PL spectrum of sample I under excitation at 367 nm. It clearly displays a strong blue light emission at

Xi et al.

Figure 9. PL spectra of the as-synthesized β-SiC samples: (a) sample I, (b) sample II, and (c) sample III.

462 nm (2.69 eV). Compared with the band gap of bulk β-SiC, 2.39 eV (at room temperature), the PL peak energy of the β-SiC nanorods is considerable blue-shifted. Curve (b) shows the PL spectrum of sample II, which displays a light emission at 459 nm. Curve (c) shows the PL spectrum of the nanoneedles, which displays a light emission at 442 nm. Compared with the nanorods (samples I and II), the emission wavelength of the nanoneedles is more blue-shifted. Recently, various emission wavelengths from β-SiC nanostructures have been reported,6f,j,7a,19 indicating that the luminescence characteristic depends strongly on the β-SiC nanostructure, which is affected by the synthetic variables such as the materials used, reaction temperature, and the synthetic route of reaction. Therefore, we tend to think the different optical performances of the as-synthesized samples result from their different sizes and shapes.20 Field Emission Properties of the As-Synthesized Products. The results of field emission measurements are summarized in Figure 10. It is can be seen from Figure 10a that the emission current density from sample I is 1.1 mA/cm2 at the field of 20 V/µm, and the turn-on field, which is defined as the macroscopic field required to produce a current density of 10 µA/cm2, is about 12 V/µm. As can be seen (Figure 10b), the emission current density from sample II is 1.8 mA/cm2 at the field of 22 V/µm, and the turn-on field is about 8.4 V/µm. Figure 10c shows that the emission current density of the nanoneedles is as high as 2.09 mA/ cm2 at the field of 3 V/µm, and the turn-on field is only about 1.6 V/µm. The behavior of the emission current versus the applied voltage was analyzed using the FowlerNordheim (F-N) equation, ln(J/E2) ) -(Bφ3/2/β)E-1 + ln(ARβ2/ φ), where J is the current density, E is the applied field (V/d), φ is the work function, B and A are constants, R is the effective emission area, and β is the F-N enhancement factor.21 The slopes of the corresponding F-N plot shown in the insets of Figure 10 exhibit approximate linear dependence, revealing that the emission currents of the samples are caused by the conventional field emission mechanism. Compared with the as-synthesized β-SiC nanorods, the field emission performance of the nanoneedles is obviously more excellent (the turn-on field of the nanoneedles is only 1.6 V/µm). Studies have found that the structures with a sharp tip generally have better field emission properties,22 which leads us to attribute the different field emission performances among the as-synthesized samples to their different shapes. From sample I through sample II to sample III, their tips become more and more sharp, and

Autoclave Synthesis of Aligned SiC Nanostructures

J. Phys. Chem. B, Vol. 110, No. 29, 2006 14177 that they have very high mechanical stability, high aspect ratio, high melting point, high thermal conductivity, and long-term stability to harsh environments. Because 0.1 mA/cm2 can produce enough brightness (>1000 cd/m2) under practical display operating conditions, so the as-synthesized β-SiC nanorods and nanoneedles can be used as appropriate lightemission materials for flat-panel displays.24 4. Conclusions In summary, we have demonstrated a novel Mg-catalyzed chemical co-reduction route for the controlled synthesis of aligned and single-crystalline β-SiC nanorods and nanoneedles under supercritical fluid conditions. The as-synthesized products display shape-dependent optical and field emission properties. This route provides an efficient and relatively mild method to fabricate uniform semiconductor 1D nanostructures with obvious advantages over the traditional high-temperature approach, and could be applicable to other semiconductor systems. These materials with interesting optical and field emission performances should find applications in fabrications of light emitting diodes and flat panel displays. Acknowledgment. We acknowledge the financial support from the 973 Climbing Project Foundation of China. References and Notes

Figure 10. J-E plots of field emission taken from the as-synthesized β-SiC samples: (a) sample I, (b) sample II, and (c) sample III. Insets: the corresponding F-N plot showing approximate linear dependence.

therefore, the field emission properties of the nanoneedles are most excellent. In addition, the turn-on field of the assynthesized nanoneedles is far lower than that of many β-SiC 1D nanostructures reported by others, which demonstrates that the field emission is considerably enhanced due to the very sharp tips of the as-synthesized β-SiC nanoneedles. Compared to many other 1D nanomaterials (such as ZnO nanowires, carbon nanotubes, AlN nanoneedles, GaN nanowires, GaAs nanowires, SnO2 nanorods, and NbS2 nanowires), the turn-on field of the as-synthesized β-SiC nanoneedles is also lower.23 More importantly, as field emission materials, the as-synthesized β-SiC 1D nanostructures have the advantage over other nanomaterials in

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