Large-Scale Synthesis and Raman and Photoluminescence

Dec 10, 2008 - College of Electromechanical Engineering, Qingdao University of Science and Technology. , ‡. College of Chemistry and Molecular ...
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J. Phys. Chem. C 2009, 113, 91–96

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Large-Scale Synthesis and Raman and Photoluminescence Properties of Single Crystalline β-SiC Nanowires Periodically Wrapped by Amorphous SiO2 Nanospheres 2 Zhenjiang Li,† Weidong Gao,† Alan Meng,*,‡ Zaidan Geng,§ and Li Gao† Key Laboratory of Eco-chemical Engineering, Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China, College of Electromechanical Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266061, People’s Republic of China, and Zhengzhou Video-UniVersity, Zhengzhou 450001, People’s Republic of China ReceiVed: July 18, 2008; ReVised Manuscript ReceiVed: NoVember 10, 2008

Novel SiC/SiO2 chainlike nanostructures have been synthesized via a simple template/catalyst-free chemical vapor reaction approach using Si-SiO2 mixture powder and CH4 as raw materials at relatively low temperatures of 1250-1200 °C. Digital camera, stereoscope, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction, and infrared spectroscopy demonstrate that large-scale blue products have been obtained on graphite substrate; the samples are composed of chainlike nanostructures having lengths up to several tens of micrometers, diameters of 20-30 nm single crystalline β-SiC nanowires, and 80-100 nm amorphous SiO2 periodic wrapping spheres, possessing [111] preferred growth direction with a high density stacking faults and twin defects. We suppose the formation of the nanostructure is induced by two-stage VS growth mechanism, especially because the defects within SiC nanowires are the critical factors for the second-stage formation of SiO2 spheres. Some unique optical properties are observed in the room-temperature Raman spectroscopy and photoluminescence measurements of the products, which may be ascribed to crystal defects and size confinement effects. Introduction Over the past few years, there have been increasing reports on the synthesis of one-dimensional (1D) composite nanostructures because of their great interest in size- and morphologydependent properties and the emerging applications in functional nanodevices and nanocomposite materials.1-3 Much effort has been devoted to the synthesis and application of various 1D composite nanostructures. Polypyrrole/carbon nanotube composite nanowires have been prepared via a template-directed electrochemical route.4 Dye-sensitized solar cells have been fabricated based on anatase TiO2 nanoparticle/nanowire composites.5 ZnO nanowire-layered basic zinc acetate (LBZA)/ ZnO nanoparticle (NP) composite films have been synthesized using a simple wet chemical route,6 and many coaxial nanocable structures have been prepared, including Ni/C,7 Ag/C,8 etc. SiC is known to possess excellent properties of high thermal conductivity, high thermal stability, excellent mechanical strength, and chemical inertness.9,10 The unique properties make SiC a suitable candidate for various applications in nanoscale electronics, optics, and field-emission devices.11 The morphology of nanomaterials is known to have influence on their properties.12 It has been reported that SiC/SiOx composite NWs can emit stable and high-intensity blue-green or violet-blue light;13,14 doped SiC/SiO2 composite NWs may allow novel devices to be designed.15 Until now, various SiC nanostructures have been synthesized, such as SiC nanowires and SiC/BN nanocables,16,17 * To whom correspondence should be addressed. Phone: +86-53288958602. Fax: +86-532-88956118. E-mail: [email protected]. † College of Electromechanical Engineering, Qingdao University of Science and Technology. ‡ College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology. § Zhengzhou Video-University.

SiC/C NWs,18 biaxial SiC/SiOx NWs,19,20 SiC nanotubes,21,22 nanosized β-SiC powder,23 and SiC hollow nanospheres.24 However, the above nanostructures were generally overall composited (having a smooth surface), and the yields were rarely studied. We report herein the large-scale synthesis of β-SiC nanowires periodically wrapped by SiO2 nanospheres via chemical vapor reaction at 1250-1200 °C. Simple procedure, template/catalyst-free, low cost, lower temperature, and high yield are the inherent advantages of the typical approach.25-27 Synthesis, characterizations, growth mechanism, Raman spectroscopy, and photoluminescence (PL) measurements are reported in this paper. In addition, the chainlike composite nanostructures may have great application in improving the properties of the nanocomposite materials due to the stronger adhesion between the contoured surface nanowires and the matrix comparing to the smooth surface of SiC nanowires. Experimental Section Preparation of Chainlike SiC/SiO2 Composite Nanowires. Milled Si-SiO2 mixture powder with a molar ratio of 1:1 and high-purity CH4 were employed as raw materials, graphite wafer was used as substrate, and a vertical furnace was employed as heating equipment. Typical experiments were carried out as follows: Initially, a substrate, a piece of carbon cloth, and Si-SiO2 powders were orderly placed into a homemade graphite chamber. Second, the chamber was placed into the furnace; before heating, it was purged 2-3 times with high-purity Ar using a rotary vacuum air pump to reduce the oxygen to a negligible level. Third, the temperature was increased to 1250 °C from room temperature at a mean rate of 600-800 °C h-1 and kept at peak temperature for 5-10 min; during the peak temperature heating a steady flow of CH4 at 0.05-0.10 sccm from the bottom of the furnace was introduced into the reaction

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Figure 1. (a) Digital camera (DC) photograph of the macroproducts synthesized on the graphite substrate. (b) Vertical-section microscope image of the sample grown on the graphite substrate. (c) Low-magnification and (d) high-magnification SEM images of the chainlike nanostructures. The inset is a typical SEM image of several nanowires directly grown on the graphite substrate.

system. Fourthly, the flow of CH4 was stopped and the temperature was reduced to 1200 °C at a mean rate of 5-10 °C min-1 following a decreasing atmospheric pressure from 600 to 200 torr, which was controlled by the air pump. Finally, the power was switched off and the furnace was cooled to room temperature. Characterization. Several measuring and analytical techniques were employed to characterize the as-achieved products. The macroproducts were indicated by a digital camera and stereoscope; the morphology and chemical elements were characterized by using a JEOL JSM-6 field emission scanning electron microscopy (FESEM) equipped with an energydispersive X-ray spectroscopy (EDX) equipment. Further detailed structural information was obtained by using a JEM2000EX transmission electron microscope for microscopy (TEM), a JEOL-2010 high-resolution transmission electron microscope (HR-TEM), and the corresponding selected area electron diffraction (SAED). The X-ray diffraction (XRD) pattern of the products was recorded by using a Rigaku D/max2400 X-ray diffractometer at room temperature. Infrared absorbance spectrum (IR) measurement was performed in a Nicolet FT-IR spectrophotometer to further confirm the nanostructure of the products. In addition, the Raman scattering spectra were measured with a Raman spectrometer (Renishaw 2000), and the PL spectral measurements related to the products were performed in a Hitachi F-4500 fluorescence spectrophotometer using a xenon lamp at room temperature. Results and Discussion Figure 1a shows the digital camera (DC) photograph of the achieved products directly grown on rectangular graphite substrate via CVR approach under catalyst/template-free condition; it reveals explicitly that mass blue products have ac-

cumulated on the graphite substrate (3 cm × 3 cm). Representative vertical-section microscope image (Figure 1b) depicts clearly that a sample layer of about 0.2 mm has uniformly formed on the graphite substrate which has a mean thickness of 2 mm. To evaluate the average yield of the products (by using 500 mg Si-SiO2 powder per run), an electronic balance is used to weigh the samples, which shows the average yield of the product is about 200 mg; to a certain extent, we believe the yields of the products are on a relatively large scale. Figure 1c displays the low-magnification FESEM image of the nanostructures; it shows that lots of diamond chainlike nanostructures randomly distribute on the substrate; the amount of the chainlike nanostructures in the sample exceeds 90%, and it means the relatively high amount and purity quotient of the products. Figure 1d is the characteristic high-magnification FESEM image of an individual nanostructure, clearly showing the wire of 20-30 nm and sphere of 80-100 nm in diameters, the representative FESEM image (inset) is a clear appearance of several nanostructures, the ends of the nanowires marked by white circle show that the tips of the nanowires are directly connected to the graphite substrate; no catalyst particles are observed on the end of the nanowires; however, it is known that the catalyst particle attached to the end of the nanostructure is inherent with the vapor-liquid-solid (VLS) growth mechanism,28 so we generally suppose the growth of the nanostructure is controlled by VS growth mechanism. Moreover, no catalyst existing in the sample means a high purity of the products. Figure 2 depicts the EDX spectrum taken on different positions of the achieved products. Figure 2a reveals that the nanowires between adjacent spheres possess Si and C elements; Figure 2b displays the nanospheres are composed of Si, C, and O chemistry; the corresponding detailed data is shown in Table 1 of Supporting Information; on the basis of the results we could

Single Crystalline β-SiC Nanowires 2

Figure 2. (a) EDX spectrum taken from the nanowires between nanospheres. (b) EDX spectrum taken from the nanospheres.

roughly infer the chainlike nanostructures are composed of silicon carbide nanowires and silicon oxide wrapping spheres. Figure 3 displays the structural information in further detail, including TEM images, SAED patterns, and HRTEM image of the nanostructures. As is obviously shown in Figure 3a, a single nanowire periodically wrapped by nanospheres has been obtained; no catalyst particles are attached to the tip of the nanowire; the corresponding SAED pattern of the nanowire marked by white circle is shown in the inset; the sharp diffraction spots clearly indicate the crystallized structure which could be indexed to the single crystalline β-SiC; the spots marked by solid and dashed-lined parallelograms obviously demonstrate the existence of twin defects in the crystal which is a distinct feature of the products. Figure 3b depicts the biaxial nanowires periodically wrapped by spheres have also been obtained; the inset shows the typical SAED patterns (marked by rectangle and parallelogram) taken on the biaxial nanowires marked by white circle, the pots could also be indexed to crystalline β-SiC, indicating the existence of the biaxial SiC/ SiC nanowires. On the basis of the TEM images (parts a and b of Figure 3), we know that diameter of a single nanowire is 20-30 nm and that the size of the wrapping sphere is 80-100 nm; the length between two adjacent nanochans is 100-200 nm. Representative HRTEM crystal lattice image taken from the SiC nanowire is shown Figure 3c; it distinctly depicts that the spacing between the (111) lattice planes is 0.25 nm, it also indicates the crystal grows along [111] direction; furthermore, we note that the SiC nanowires typically possess high density of stacking faults and twin defects. Figure 4 is the XRD pattern of the products collected on the graphite substrate at room temperature, suggesting that β-SiC is the only crystalline phase. Besides the diffraction peaks of carbon, the major diffraction peaks shown in Figure 4 are

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Figure 3. (a) TEM image of a single nanowire periodically wrapped by nanospheres; the inset is the SAED pattern of a single nanowire. (b) TEM image of biaxial nanowires periodically wrapped by nanospheres; the inset is the SAED patterns of the biaxial nanowires. (c) Typical HRTEM image of the crystalline SiC nanowire.

Figure 4. Representative XRD patterns of the products directly grown on the graphite substrate, indicating β-SiC with stacking faults (S.F.).

assigned to the (111), (200), (220), and (311) reflections of cubic β-SiC. These values are well agreed with the known values for β-SiC (JCPDS Card No.29-1129). The low-density peak marked with S.F. is due to the stacking faults;29 it confirms the existence of defects within SiC nanowires. Furthermore, the diffraction peaks corresponding to the graphite substrate were also found. Moreover, the intensities of SiC diffraction peaks become weak and the diffraction peaks are obviously broadened, which may be relative to the inner SiC nanowires and the amorphous SiO2 wrapping nanospheres. Figure 5 displays the characteristic IR spectrum of the composite nanostructures. There are two absorption peaks at around 482 and 1102 cm-1, which correspond to Si-O

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CH4)C(gas) + 2H2(gas)

(2)

When the pressure of SiO and C vapor increase to a supersaturation condition, reactions 3 and 4 would take place to decrease the system energy to form SiC nuclei on some active positions of the graphite substrate by the vapor-solid mechanism (step I)

SiO + 2C ) SiC(solid) + CO(gas) SiO + 3CO ) SiC(solid) + 2CO2(gas)

Figure 5. IR absorbance spectrum taken from the obtained composite nanostructures.

Figure 6. Schematic illustrations for the growth of SiC/SiO2 chainlike nanostructures.

stretching vibration of amorphous SiO2, and the absorption peak at around 793 cm-1 corresponds to Si-C stretching vibration of β-SiC; the results are just in agreement with the previous reports.30,31 Together with the FESEM, EDX, TEM, HRTEM, and XRD characterizations, we could confirm the amorphous phase should be SiO2, which periodically wrap the outer of crystalline β-SiC nanowires. vapor-liquid-solids,33 The vapor-solid,32 solid-liquid-solids,34 and oxide-assisted growth35 mechanisms have been suggested for the formation of SiC nanostructures. In our present work, no metal catalyst was used, and no catalyst was obtained on the tips of the nanowires. On the basis of the experimental results, we propose a reasonable two-stage vapor-solid growth mechanism for the growth of SiC/SiO2 composite nanostructures, which are schematically shown in Figure 6. First, SiO vapor is generated by a solid-solid reaction between the milled Si-SiO2 powders as at 1250 °C, which can be expressed as

Si + SiO2 ) 2SiO(gas)

(1)

Then, a gaseous flow of CH4 is introduced into the reactive chamber, the C vapor is generated from decomposing CH4, the equation is

(3) (4)

The nuclei would preferentially grow in the [111] direction, which has the lower energy than those of others in β-SiC, because it has the highest atom density and the largest d spacing;36 since the stacking faults have a lower energy than that of the SiC during the layer by layer growth,37 their existence will be helpful in the formation of SiC nanowires in maintaining lower growth energy, so the nanowires within stacking faults will be obtained (step II), the longer nanowires can be formed along with the consecutive growth of SiC (step III); however, when the gas flow of CH4 is stopped during 1250-1200 °C, a little residual SiO vapor would preferentially nucleate and grow around the defects by a vapor-solid mechanism, as we believe the energy for nucleating SiO2 around the defects is far lower than that on the other places (step IV), and they would gradually form SiO2 spheres following the gradually reducing concentration of SiO vapor at pressure-decreasing atmosphere; therefore, the chainlike SiC/SiO2 composite nanostructures are obtained at last (step V). From the viewpoint of confirming the main carbon source contribution of CH4, we have used Si-SiO2 powder as raw materials via CVR process and no SiC nanostructures were obtained on graphite substrate in our previous work, so we confirm the CH4 plays an important role in providing carbon in the synthesis of the obtained nanostructures. As noted, we have synthesized large-area SiC nanowire arrays on anodic aluminum oxide (AAO) template via this CVR approach in the previous study,26 we generally believe the growth of SiC nanowire arrays are controlled by one-stage vapor-solid growth mechanism owning to the continuous flow of CH4, while the SiC/SiO2 composite nanostructures are controlled by two-stage vapor-solid growth mechanism due to the continuous flow of CH4 in the first stage of 1250 °C and the stop of CH4 in the second-stage of 1250-1200 °C during the CVR process. As is described above, the composite nanostructures are synthesized by two-stage vapor-solid growth processes, as we know the SiC crystal is formed via layer-by-layer stacking of Si/C atoms, yet the opposite side surfaces have a difference in polarity, on one side it is all Si atoms, on the opposite side it is all C atoms. The polarity difference will lead to bending or strain in the nanowires. Thus, twins are introduced to reverse the polarity in order to reduce the strain in the crystal which is similar to the periodic twinned SiC nanowires synthesized by Wang et al.38 It is worth noting that two SiC nanowires would probably grow side by side for some reason during the firststage vapor-solid growth process, so the biaxial nanowires will be formed, while during the second-stage growth process, the biaxial SiC nanowires are selectively wrapped by SiO2 nanospheres. Therefore, the biaxial nanowires periodically wrapped SiO2 nanospheres will be obtained at last. However, the exact growth mechanism for the SiC/SiO2 chainlike composite nanostructures should be further investigated. A typical Raman spectrum taken from SiC/SiO2 composite nanostructure is shown in Figure 7. Four peaks at around 794, 880, 950, and 1352 cm-1 are clearly observed in the Raman

Single Crystalline β-SiC Nanowires 2

J. Phys. Chem. C, Vol. 113, No. 1, 2009 95 addition, the emission peak centered at 490 nm is caused by the amorphous SiO2 nanochains;46 the emission results also show some difference comparing with the previous report26,36 because of size confinement effects and inner faults owning to the unique nanosized composite structures. Conclusions

Figure 7. Raman scattering spectra of the SiC/SiO2 composite nanostructures.

In summary, large-scale SiC/SiO2 chainlike nanostructures have been directly fabricated on graphite substrate using a mixture of Si-SiO2 and CH4 as raw materials via a catalyst/ template-free CVR approach at 1250-1200 °C. The nanostructures typically have 20-30 nm diameter single crystalline β-SiC nanowires periodically wrapped by SiO2 nanospheres of 80-100 nm in diameter; their lengths are up to several tens of micrometers and have a [111] preferential growth direction with a high density of stacking faults and twin defects. The growth of the chainlike nanostructure is controlled by two-stage vapor-solid growth mechanism. Especially, the formation of SiO2 wrapping nanospheres results from the existence of defects within SiC nanowires as high active nucleation positions. In addition, the SiC/SiO2 composite nanostructures hold novel unique optical properties in the Raman spectroscopy and PL measurements, which show the different results from previous observations of SiC materials. Acknowledgment. The work reported here was supported by the National Natural Science Foundation of China under Grant No.50572041, the Natural Science Foundation of Shandong Province under Grant No. Y2007F64, the Science Item of Shandong Province under Grant No. 2006GG2203014, and the Educational Office Foundation of Shandong Province under Grant No. J06A02. We also thank the Center of Analysis & Measurement of the Physics Research Institute of the Academy of Science of China for the help on the Raman and PL measurements.

Figure 8. Room-temperature PL spectrum of the SiC/SiO2 composite nanostructures.

spectrum. The amorphous bulge at the center of 880 cm-1, which is obviously shifted to the lower wave numbers comparing with the Raman peak of amorphous SiO2,39 shows a distinct shift of 36 cm-1 comparing with the normal SiC/SiO2 core-shell nanowires reported in our previous study.27 The peak at 1355 cm-1 corresponds to the Raman peak of carbon substrate. The peaks at 794 and 950 cm-1 could be assigned to the TO and LO phonon peaks of cubic SiC, we note that they, respectively, show a shift of 2 and 22 cm-1 comparing with the modes of bulk SiC,40,41 the unique physical properties may be caused by the size confinement effect, inherent stacking faults and inner stress from the nanostructures.42 Additionally, the asymmetry and big difference in height between the TO and LO phonon peaks comparing with that of bulk SiC could also be attributed to size confinement effects and defects. Figure 8 displays a room-temperature PL spectrum of the composite nanostructures. When excited with light from a xenon source (excitation wavelength 325 nm), the composite nanostructures have a strong peak which is centered at 410 nm, it is generally compatible to the value of 3C-SiC nanobelts43 and SiC nanoneedles.36 Compared with the previously reported luminescence from the SiC nanospheres and films, the emission peak for the composite nanowires is obviously shifted.44,45 In

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