Synthesis of SiC@SiO2 Nanocables via a Catalyst-free Carbothermal Reduction Method

TIAN Zhao-Bo, CHEN Ke-Xin, SUN Si-Yuan, ZHANG Jie, CUI Wei, LIU Guang-Hua

Synthesis of SiC@SiO2Nanocablesa Catalyst-free Carbothermal Reduction Method

TIAN Zhao-Bo, CHEN Ke-Xin, SUN Si-Yuan, ZHANG Jie, CUI Wei, LIU Guang-Hua

(State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China)

SiC@SiO2nanocables (NC), as a new functional nanocomposite, have captured widespread attention due to their excellent performances and widely application prospects. Therefore, it is significant to develop a kind of effective, economical and environmental method to prepare SiC@SiO2NC. Herein, a catalyst free carbothermal reduction method was developed to synthesize SiC@SiO2NC fast and efficently, through heating the mixture of silicon powder and silica sol at 1500 ℃ in Ar. The NC is composed of single-crystal-SiC core and amorphous SiO2sheath, with the length of hundreds of micrometers and the diameter of 60–80 nm. And the size of the core-shell can be adjusted by the holding time. The formation of the NC is explained based on the experimental data and the vapor-solid (VS) mechanism. The experiment results can also enrich the mechanism, and offer inspiration for their industrial-scale production.

SiC; nanocables; carbothermal reduction; catalyst-free

In recent years, nanocables, a newly type of one- di­mensional (1D) nanostructures, have already attracted the attention of many reasearchers, as their applications could be further enhanced by fabricating the core with sheath of different materials. The ‘‘nanocables’’ is firstly referred to the coaxial structures of several layers of nanotubes and clusters[1-2]. Among the 1D nanomaterials sheathed with an outer layer, SiC NC with different coating materials, such as SiC/SiO2[3-4], SiC/Al2O3[5], SiC/C[1, 6]and SiC/BN became particularly significant, since they showed outstanding excellent field emission characteristics[3], photoluminescence performance[5], and enhanced electrical properties[7].

It is well known that SiC@SiO2core-shell NC exhibits excellent super hydrophilic properties and mechanical strength[8-9], holding great promise for application as self-cleaning materials and functional ceramic reinforcements, respectively. To date, various strategies have been developed to obtain SiC/SiO2NC[10]. Ryu,[11]has synthesized the core-shell SiC@SiO2nanowires by using NiO catalyst. Cai and collaborators[12]produced SiC@SiO2NC through an organic precursor method using dimethylsiloxane as raw material. By arc-discharge method, Yao,[13]fabricated large-scale SiC@SiOnanocables. Li,[14]also developed an iron-catalysis chemical vapor deposition (CVD) method to obtain SiC@SiO2coaxial nanowires using C3H6as carbon source. However, usage of the metal catalyst, such as NiO[11]which is difficult to be removed from the product, is not matching the rule of convenience; the application of the expensive raw material and the complex process is not conforming with the principle of economization; and usage of the polluting gas, CH4or C3H6, is not according with the law of environment protection. Therefore, it is significant to develop an effective, economical and environmental method to synthesize SiC@SiO2NC.

In this study, we developed a modified method to synthesize long SiC@SiO2NC. In the process, the mixture of silicon powder (500 nm) and silica sol, coupled with carbon felt (CF) were heated at 1500 ℃ in Ar without any catalyst. The growth mechanism of SiC@SiO2NC was discussed, and the formed structure and optical properties were characterized.

1 Experimental

Si powders (500 nm, Jinan Yinfeng Co., China), and silica sol (prepared according to our former work[15]) were mixed in an agate pot by ball milling for 60 min. After drying at 80 ℃in oven for 2 h, the powder mixtures were transferred into the corundum crucible, covered with carbon felt (TZ-307, Taek. Co., Korea). Then the whole “sandwich” was put in the center of horizontal tube furnace. After replacing with high purity Ar, the furnace was heated to 1500 ℃, and kept for 4–6 h. Then the furnace was cooled down to room temperature. During the whole process, flowing Ar of 10 sccm was introduced from the furnace.

The crystalline phase of the products was determined by X-ray diffraction analysis (XRD, D8 Advance, Bruker Co., Germany). The microstructure was observed using scanning electron microscope (SEM; Melin, Zeiss Co., Germany) at the accelerating voltage of 15.0 kV. The morphology and the diffraction pattern were detected by high-resolution transmission electron microscopy (TEM; JEM-2100F, JEOL Co., Japan), equipped with energy dispersive spectrum (EDS), operated at the accelerating voltage of 200 kV. Raman spectra were excited using the 633 nm excitation source from He-Ne laser and collected by a micro-Raman spectrometer at room temperature.

2 Results and discussion

It is obvious that large quantity of long straight and curved nanowires are synthesized, as shown in Fig.1(a). A typical FESEM image further reveals that the diameter of the nanowires is uniformly ranged from 60 to 80 nm (inset in Fig. 1(a)) with smooth surface and the tens of micrometers in length, implying a large aspect ratio (Fig. 1(b)). For the hundreds of wires, no spherical caps are observed at the tips of the wires, which reveals that the wires form by the vapour-solid (VS) mechanism.

X-ray diffraction is conducted to investigate the phase and structure of the as-synthesized products. As shown in Fig. 2, the product is identified as the-SiC structure. The strong intensities and narrow widths of the peaks indicate that the nanowires are crystalline. The major diffraction peaks at 2=36°, 41°, 60° and 72°, are attributed to the (111), (200), (220), and (311) respectively, agreed well with-SiC (PDF 29-1129)[14]. The low intensity peak at 2=33° (marked “SF”) is typically observed in XRD pattern of-SiC, ascribed to stacking faults within the crystals[16]. Meanwhile, there is amorphous background and swell in 2=23° of the XRD pattern, which is marked as amorphous SiO2[14, 17].

To further characterize the structure in detail, TEM image and selected area electron diffraction (SAED) are tested. Fig. 3(a) is the typical TEM image of the core-shell NC with the diameter of 80 nm, obtained by being kept in 1500 ℃for 4 h. It clearly reveals that the NC possesses a darker inner core of about 17 nm in diameter and a lighter outer shell with the thickness of nearly 30 nm .There are some slashes in the core, which may be stacking faults[18], and this is in agreement with the XRD result (Fig. 2). After being kept in 1500 ℃for 6 h, the diameter of inner core of the NC grows up to 40 nm and the thickness of outer shell reduces to nearly 20 nm, as shown in Fig. 3(b). Therefore, we can infer that through controlling the holding time in 1500 ℃, diameter of the inner core can be regulated. Fig. 3(c) shows the edge of core SiC and shell SiO2. It is obvious that the core SiC shows regular arrangement of atoms, indicating high crystallinity, while the shell SiO2presents atoms are disorder, manifesting the amorphous structure. Fig. 3(d) reveals the interspace of the lattice fringes is 0.25 nm, corresponding to the-spacing of the (111) plane in-SiC, as indicated in the insert SAED pattern. It is generally accepted that the SiC nanowires should grow preferentially along the [111] direction to maintain the lowest growing energy, as the surface energy of (111) plane is much lower than those of the other planes[11, 19].

Fig. 1 SEM images of core-shell SiC@SiO2 NC (a) Low-magnification image, with inset showing distribution of the SiC@SiO2 NC diameters; (b) High-magnification image, with inset showing the corresponding image of a single NC

Fig. 2 Typical XRD pattern obtained from the core-shell SiC@SiO2 NC

Furthermore, we investigated the mapping mode of the EDS to indicate the distribution of the elements to insure the core and sheath. As shown in Fig. 4(c), the element of O mainly distributes in the sheath, while the core is “dark”, manifesting the lack of O element. It is obvious that the element of C mainly enriches in the core, shown in Fig. 4(d). In Fig. 4(b), the core is brighter than the sheath, manifesting the higher content of Si element in the core, as the molar percentage of Si in SiC 50%, while 33.3% in SiO2.

Fig. 3 TEM images of the core-shell SiC@SiO2 NC kept in 1500 ℃for 4 (a) and 6 h (b), the corresponding SAED (c), and HRTEM image (d)

Fig. 4 Mapping mode of the EDS of the elements, Si/C/O in the NC

A typical Raman spectrum of-SiC@SiO2core-shell NC is shown in Fig. 5. Two peaks at around 794 and 916 cm–1are observed in the Raman spectrum. The center of 916 cm–1is corresponding to the Raman peak of amorphous SiO2[14], and the peak at 794 cm–1is assigned to the TO phonon at the G point of cubic SiC[20]. We note that the TO (G) phonon line shows a low number shift of 2 cm–1compared with the TO phonon modes of bulk SiC (796 cm–1for TO and 980 cm–1for LO)[21-22]. The reason for this exception may originate from the confinement effect, stacking faults and inner stress from the hetero structure of core-shell SiC NC[23-24]. In addition, we have not found the LO phonon of-SiC in the Raman spectrum. Maybe the LO phonon shifts to the low number, andis coincidentally eclipsed by the amorphous bulge of SiO2.

Based on the characterization of the morphologies and crystal structures, the mechanism for the formation of the SiC@SiO2core-shell NC is governed by the VS process[17, 19]. Fig. 6 shows the schematic illustration of the growth and morphology evolution of the SiC-SiO2nanocables. Firstly, in the Si/silica sol mixtures, Si reactes with SiO2to form SiO vapour at 1300 according to reaction (1)[25]. The as-formed SiO vapour reacts with C and CO vapour in accordance with reaction (2-4)[25-26]. In reaction (2), SiC nuclei firstly generates at the active sites of the CF, as no spherical caps at the tips of the wires shown in Fig. 1. Subsequently, the formed SiC nuclei acts as seeds to grow SiC nanostructures[27-28]. At the early stage, SiC grows on the surface of the substrate, which can maintain the low energy consumption of the growth system[26]. In this part, the nanostructures present a highly defective structure as indicated in Fig. 3(b), which could also be helpful in reducing energy during the growth of SiC nanostructures.

Fig. 5 Raman scattering spectrum of SiC@SiO2 core-shell NC

Fig. 6 Schematic illustration of the growth and morphology evolution of the SiC-SiO2 NC

Once the nanostructures protruded from the CF, stable growth conditions can be easily achieved. In this case, the SiC nanostructures begin to grow along the specific direction [111], certified by the HRTEM and SAED images in Fig. 3(d). And the growth of the SiC@SiO2NC is no longer controlled by reaction (2), as the growth frontier is far away from the solid carbon source[29]. Meanwhile, reactions (3-4) are responsible for further growth of the SiC nanostructures, as all reactants are gases. Thus, the diameter grows with the holding time in 1500 ℃increasing, as shown in Fig. 3(c). During the cooling process, SiO2formed by reaction (4) deposits on the surface of SiC nanostructures firstly and then gradually condense into solid, forming SiC-SiO2core-shell structures[11].

SiO2(s) + Si(s) = 2SiO(g) (1)

SiO(g) + 2C(s) = SiC(s) + CO(g) (2)

SiO(g) + 3CO(g) = SiC(s) + 2CO2(g) (3)

3SiO(g) + CO(g) = SiC(s) + 2SiO2(s) (4)

3 Conclutions

In this study, we developed a modified catalyst-free carbothermal reduction method to synthesize long SiC@SiO2NC, which is effective, economical and environmental. This may offers inspiration for their industrial- scale production. As the diameter of inner core can be regulated by controlling the holding time in 1500 ℃,it also enriches the VSmechanism which is employed to interpret the NC formation. What’s more, the enriched mechanism could provide great prospect in possible applications to synthesize other long NC.

[1] YAN LI-WEN, HONG CHANG-QING, SUN BO-QIAN,growth of core-sheath heterostructural SiC nanowire arrays on carbon fibers and enhanced electromagnetic wave absorption performance., 2017, 9(7): 6320–6331.

[2] WU REN-BING, ZHOU KUN, YUE CHEE-YOON,. Recent progress in synthesis, properties and potential applications of SiC nanomaterials., 2015, 72: 1–60.

[3] ZHONG BO, SONG LIANG, HUANG XIAO-XIAO,. Novel coaxial SiC-SiO2-BN nanocable: large-scale synthesis, formation mechanism and photoluminescence property., 2011, 21(38): 14432–14440.

[4] WU REN-BING, ZHA BAI-LIN, WANG LIU-YING,. Core-shell SiC/SiO2heterostructures in nanowires., 2012, 209(3): 553–558.

[5] CUI H, GONG L, SUN Y,. Direct synthesis of novel SiC@Al2O3core-shell epitaxial nanowires and field emission characteristics., 2011, 13(5): 1416–1421.

[6] WANG XIANG-YU, ZHAI HUA-ZHANG, CAO CHUAN-BAO,. One-step synthesis of orientation accumulation SiC-C coaxial nanocables at low temperature., 2009, 19(19): 2958–2962.

[7] LI YU-BAO, DOROZHKIN PAVEL-S, BANDO YOSHIO,. Controllable modification of SiC nanowires encapsulated in BN nanotubes., 2005, 17(5): 545–549.

[8] WU REN-BING, ZHOU KUN, YANG ZHI-HONG,. Molten- salt-mediated synthesis of SiC nanowires for microwave absorption applications., 2013, 15(3): 570–576.

[9] QIAO MING-TAO, LEI XING-FENG, MA YONG,. Application of yolk-shell Fe3O4@N-doped carbon nanochains as highly effective microwave-absorption material., 2018, 11(3): 1500–1519.

[10] WU REN-BING, YANG ZHI-HONG, FU MAO-SEN,.growth of SiC nanowire arrays on carbon fibers and their microwave absorption properties., 2016, 687: 833–838.

[11] RYU YONGHWAN, TAK YOUNGYO, YONG KIJUNG,. Direct growth of core-shell SiC-SiO2nanowires and field emission characteristics., 2005, 16(7): S370–S374.

[12] CAI K F, LEI Q, ZHANG L C,. Ultra long SiC/SiO2core-shell nanocables from organic precursor., 2005, 5(11): 1925–1928.

[13] LIU XUE-MIN, YAO KE-FU. Large-scale synthesis and photoluminescence properties of SiC/SiOnanocables., 2005, 16(12): 2932–2935.

[14] MENG ALAN, LI ZHEN-JIANG, ZHANG JIN-LI,. Synthesis and Raman scattering of-SiC/SiO2core-shell nanowires., 2007, 308(2): 263–268.

[15] SUN SI-YUAN, GE YI-YAO, TIAN ZHAO-BO,. A simple method to ameliorate hierarchical porous structures of SiO2xerogels through adjusting water contents., 2017, 28(10): 2496–2502.

[16] YE HAI-HUI. SiC nanowires synthesized from electrospun nanofiber templates., 2005, 17 (12): 1531–1535.

[17] LI ZHEN-JIANG, ZHAO JIAN, ZHANG MENG,. SiC nanowires with thickness-controlled SiO2shells: fabrication, mechanism, reaction kinetics and photoluminescence properties., 2015, 7(4): 462–472.

[18] TATEYUMA KYUSHU, NOMA HIROAKI, ADACHI YOSHIO,. Prediction of stacking faults in-silicon carbide: X-ray and NMR Studies., 1997, 9(3): 766–772.

[19] LI Z J, YU H Y, SONG G Y,. Ten-gram scale SiC@SiO2nanowires: high-yield synthesis towards industrialization,growth mechanism and their peculiar photoluminescence and electromagnetic wave absorption properties., 2017, 19(5): 3948–3954.

[20] LI ZHEN-JIANG, GAO WEI-DONG, MENG ALIAN,. Large-scale synthesis and raman and photoluminescence properties of single crystalline-SiC nanowires periodically wrapped by amorphous SiO2nanospheres., 2008, 113(1): 91–96.

[21] ZHANG XIAO-DONG, HUANG XIAO-XIAO, WEN GUANG-WU,. Novel SiOC nanocomposites for high-yield preparation of ultra-large-scale SiC nanowires., 2010, 21(38): 385601–1–8.

[22] GLINKA Y D, JARONIEC M. Spontaneous and stimulated Raman scattering from surface phonon modes in aggregated SiO2nanoparticles., 1997, 101(44): 8832–8835.

[23] MENG G W, ZHANG L D, QIN Y,. Synthesis of-SiC nanowires with SiO2wrappers., 1999, 12(5): 1003–1006.

[24] SHI WEN-SHENG, ZHENG YU-FENG, PENG HONG-YING,. Laser ablation synthesis and optical characterization of silicon carbide nanowires., 2000, 83(12): 3228–3230.

[25] XU GUO-SHENG, YAMAKAMI TOMOHIKO, YAMAGUCHI TOMOHIRO,. Surface modification of carbon nanofibers with SiC by heating different SiO vapor sources in argon atmosphere., 2014, 122(1429): 822–828.

[26] ZHANG MENG, ZHAO JIAN, LI ZHEN-JIANG,. Ultralong SiC/SiO2nanowires: simple gram-scale production and their effective blue-violet photoluminescence and microwave absorption properties., 2018, 6(3): 3596–3603.

[27] DAI JI-XIANG, SHA JIA-JUN, ZHANG ZHAO-FU,. Synthesis of novel hierarchical SiC-SiO2heterostructuresa catalyst free method., 2017, 19(43): 6540–6546.

[28] BECHELANY MIKHAEL, BRIOUDE ARNAUD, STADELMANN PIERRE,. Very long SiC-based coaxial nanocables with tunable chemical composition., 2007, 17(16): 3251–3257.

[29] WANG C S, ZHANG J L, MENG A L,. Large-scale synthesis of-SiC/SiOcoaxial nanocables by chemical vapor reaction approach., 2007, 39(1): 128–132.

无催化剂碳热还原法制备SiC@SiO2纳米电缆

田兆波, 陈克新, 孙思源, 张杰, 崔巍, 刘光华

(清华大学新型陶瓷与精细工艺国家重点实验室, 北京 100084)

SiC@SiO2纳米电缆作为一种新型的功能性纳米复合材料, 以其优异的性能和广泛的应用前景受到了广泛关注。因此，开发一种有效、经济、方便, SiC@SiO2纳米电缆的制备方法具有重要意义。本研究采用无催化剂的碳热还原法在1500 ℃的Ar气氛下, 通过加热硅粉和硅溶胶混合物从而快速高效地制备了SiC@SiO2纳米电缆。该核壳的纳米电缆是由单晶-SiC核心和无定形SiO2壳组成, 其长度达几百微米, 直径为60~80 nm, 而且通过调节保温时间可以调控核壳的尺寸。结合实验数据并依据气–固(VS)机理解释了SiC@SiO2纳米电缆的形成过程, 同时也进一步丰富了该生长机制, 为其工业化生产提供了参考。

碳化硅; 纳米电缆; 碳热还原; 无催化剂

TQ174

A

date:2019-01-14;

Modified date: 2019-05-08

TIAN Zhao-Bo(1988–), male, candidate of PhD. E-mail: tzb15@mails.tsinghua.edu.cn

Corresponding author:LIU Guang-Hua, professor. E-mail: liuguanghua@mail.tsinghua.edu.cn

1000-324X(2019)11-1217-05

10.15541/jim20190028