SiO2 Nanowires: Simple Gram-Scale Production and

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Ultra-long SiC/SiO2 nanowires: simple gram-scale production and their effective blue-violet photo-luminescence, microwave absorption properties Meng Zhang, Jian Zhao, Zhenjiang Li, Shiqi Ding, Yaqi Wang, Guanhao Qiu, Alan Meng, and Qingdang Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03908 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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ACS Sustainable Chemistry & Engineering

Ultra-long SiC/SiO2 nanowires: simple gram-scale production and their effective blue-violet photoluminescence, microwave absorption properties Meng Zhanga, Jian Zhaob,‡, Zhenjiang Lib,*, Shiqi Dinga, Yaqi Wangc, Guanhao Qiua, Alan Mengc,* and Qingdang Lib a School of Electromechanical Engineering, Qingdao University of Science and Technology, Qingdao, Shandong province, 266061, China. b College of Sino-German Science and Technology, Qingdao University of Science and Technology, Qingdao 266061, P.R.China. c State Key Laboratory Base of Eco-chemical Engineering, School of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, Shandong province, 266042, China.

Corresponding Authors * E-mail address: [email protected] (Z. J. Li) Tel.: +86(532) 8895 9055 * E-mail address: [email protected] (A. L. Meng)

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KEYWORDS： Ultra-long nanowires, Silicon Carbide, Gram-scale, Properties

ABSTRACT: In the present work, high-quality ultra-long SiC/SiO2 nanowires have been prepared via simple Ni-assisted chemical vapor deposition (CVD) approach. Systematic characterization results reveal that plenty of nanowires, which consists of single-crystalline cubic SiC nanowire cores and uniform amorphous SiO2 shells, entangle with each other and construct a macroscopic thin film with the length of approximately 17 cm. Remarkably, the yield of the assynthesized SiC/SiO2 nanowires achieve to gram-scale (2.164 g) under the assistance of the ingenious homemade reactor, exceeding the yield through traditional techniques. Furthermore, the ultra-long SiC/SiO2 nanowires exhibit an blue-violet luminescence with significant blue-shift and good microwave absorption (MA) properties in the frequency range of 2~18 GHz, indicating their potential application in optical-electronic devices and electromagnetic absorption fields.

INTRODUCTION By virtue of the large surface areas and quantum confinement effect, a variety of onedimensional (1D) inorganic nanostructures, including nanotubes,1,2 nanowires,3,4 nanobelts,5 and nanorods,6 have achieved more outstanding performances than bulk counterparts in photonic, nanoelectronic, information storage, energy conversion, catalysis, and biosensor fields.7-15 As one of the most significant semiconductors, SiC 1D nanostructures have stimulated extensive interests from scientists in the past few decades owing to their low density, good mechanical strength, excellent thermal and chemical stability, high critical break-down field and electron saturation velocity as well as other intriguing electrical properties.16-24 Among these nanostructures, SiC/SiO2 nanowire (also known as nanocable), which consists of SiC nanowire


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core and uniform amorphous SiO2 shell, constructs a nanoscale semiconductor-insulator heterojunction, which enables the synergistic combination of the merits stemmed from individual component, resulting in a series of novel mechanical, optical, field emission and microwave absorption properties better than SiC and SiO2 themselves.25-27 Recently, both top-down and bottom-up strategy present their own advantages to fabricate SiC 1D nanostructures. Compared to the bottom-up method, the top-down takes full advantage of the compatibility with the conventional fabrication process for micro-electro-mechanical system devices. In order to improve the performance of bare SiC 1D nanomaterials, SiC/SiO2 nanowires with heterogeneous junction structure have been obtained according to top-down strategy.28-31 For example, as a promising composite reinforcement material, SiC/SiO2 nanowires played a significant role in enhancing the strength and thermal conductivity of the fluororubber.32 Ryu et al. reported that the field emission current of the SiC nanowires with an appropriate thickness SiO2 coating (~10nm) is higher than that of pure SiC nanowires.33 Li and collaborators demonstrated that the photoluminescent intensity of the SiC nanowires can be effectively improved by an optimized thickness amorphous SiO2 coatings.34 Compared with common SiC/SiO2 nanowire with several microns length, long products are beneficial to achieve better mechanical, thermodynamic, and electric properties.35 To date, various strategies have been developed to obtain ultra-long SiC/SiO2 nanowires. Bechelany and co-workers synthesized SiC/SiO2 coaxial nanocables with the length of several hundreds of nanometers and the coating thickness in the range of 2~10nm.36 Ultra-long SiC/SiO2 nanocables were produced by Cai and collaborators through an organic precursor method using dimethyl siloxane as raw materials, which exhibits good photo-luminescence properties.37-39 By a simple arc-discharge method, Yao et al. fabricated large-scale SiC/SiOx nanocables, whose diameter


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could be controlled by adjusting the relevant technological parameter, and the corresponding photo-luminescence pattern displayed an obvious blue-shift.40 In our previous work, we also developed an iron-catalysis chemical vapor deposition (CVD) way to obtain SiC/SiO2 coaxial nanowires at lower temperature using C3H6 as carbon source.41 Although some important results have been obtained, however, it is easily found that the yields of the long SiC/SiO2 nanowires synthesized through most of these methods are milligram-scale, which largely limits their further practical application. Therefore, larger yield, especially gram-scale high-quality production of ultra-long SiC/SiO2 nanowires is still a critical restriction issue and significant objective of pursuing for some scientists. As one of the good candidate photo-luminescent (PL) materials and most promising microwave absorbers, SiC 1D nanostructure have widely captured researchers attentions depended on their intrinsic advantages, such as lower specific gravity, high thermal stability, excellent chemical inertia, and strong-absorption, wide-frequency range.42-44 In addition, some works have pointed out that SiO2 shell layer plays positive role in improving the PL and microwave absorption (MA) performances of SiC 1D nanostrctures.45,46Therefore, it is significant to conduct the research on the large-scale synthesis and PL, MA properties of ultralong SiC/SiO2 nanowires. In the present investigation, the gram-scale (~2.164 g) high-quality ultra-long SiC/SiO2 nanowires have been produced in a homemade reactor via Ni-assisted CVD approach. In order to elaborate the growth process of the products, a vapor-solid-liquid (VLS) mechanism was proposed. Moreover, for exploring their feasible application domain, the PL properties excited by a 325 nm wavelength beam and the MA properties in the frequency range of 2~18 GHz have also been systemically investigated. The effective characterization results


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demonstrated that the as-synthesized SiC/SiO2 nanowires appear extensive potential applications as blue-violet light emitting diodes (LED) and electromagnetic shielding absorbers. EXPERIMENTAL SECTION Materials. Si and SiO2 powders were mixed together (with molar ratio Si: SiO2=1.5: 1) by ball milling and used as composite silicon source. High purity CH4 gas was chosen as carbon source, and a graphite wafer (approximately 18 cm×12 cm) coated with Ni(NO3)2 was employed as substrate. Synthesis of ultra-long SiC/SiO2 nanowires. Firstly, the as-milled Si-SiO2 mixture powders and the dried graphite substrate were successively placed in a homemade reactor box located in the center of the vacuum atmosphere furnace. Before heating, the furnace chamber was evacuated by a rotary pump for 30 min to exhaust air. Secondly, the system was heated up to 1100 oC with 20 oC/min and remained 5 min. Then, the furnace was further heated up to 1270 oC with 5.5 oC/min for 120 min. Meanwhile, the CH4 gas was injected into the furnace from the bottom of the furnace at 150 standard cubic centimeter per minute (sccm) for 120 min. After finishing the heat preservation stage, the switch was turned off, and the furnace was cooled to room temperature gradually. Finally, thin film-like products with light-blue color were obtained on the graphite substrate. Characterization. The morphology, micro-structure and phase composition of the assynthesized SiC/SiO2 nanowires were characterized by scanning electron microscope (SEM, JEOL JSM-6) equipped with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM, JEM-2100), X-ray diffraction (XRD, Rigaku, Tokyo, Japan, D/max-2400 Xray diffract meter with Cu Ka radiation) and infrared absorbance spectrum (IR, Perkin-Elmer 983 FT-IR). Photo-luminescence spectrum was performed on a Hitachi F-4600 fluorescence


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spectrophotometer at ambient temperature. In order to characterize the MA properties, SiC/SiO2 nanowires were uniformly mixed with paraffin with the mass proportion of 3:7, and compressed to be a standard torus with the outer diameter of 7 mm, inner diameter of 3 mm and thickness of 3 mm. A vector network analyzer (Agilent, N5230A) was employed to record the corresponding electromagnetic parameters in the frequency of 2~18GHz. RESULTS AND DISCUSSION Figure 1a~1c display the digital camera photographs of the as-synthesized SiC/SiO2 nanowires, which is macroscopical film-like products deposited evenly on the surface of the substrate. The weight of the nanowires is greater than 2 gram (~2.164 g) after scraping from the substrate, and the length of the nanowire-constructed film achieves approximately 17 cm. As indicated in Figure 1d, a piece of nanowire-constructed film is breezily wrapped around a glass rod, indicating the excellent flexibility of the SiC/SiO2 nanowires. The phase composition of the assynthesized SiC/SiO2 nanowires was characterized by XRD, and the corresponding pattern was displayed in Figure 1e. The major diffraction peaks at 35.58º, 41.38º, 60.10º and 71.80º have been assigned to (111), (200), (220) and (311) planes of the cubic SiC (JCPSD Card No. 291129), respectively.47,48 The low-intensity peak appeared at 33.49º is usually attributed to stacking faults (marked with “SF”) within the SiC. The high intensity and narrow shape of the diffraction peaks suggest that the as-synthesized SiC/SiO2 nanowires possess high-quality and good crystallinity. Figure 1f shows the representative FT-IR spectrum of the products. The presence of two absorption peaks at 802 cm-1 and 949 cm-1 correspond to Si-C stretching vibration of SiC core, and the two absorption peaks at 483 cm-1 and 1104 cm-1 are attributed to the stretching vibration of Si-O bonds within the amorphous SiO2 shell.49-51


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In order to obtain the accurate morphology information, SEM was introduced to study the SiC/SiO2 nanowires, and the corresponding results have been exhibited in Figure 2a and 2b. Figure 2a is the low-magnification SEM image of the as-synthesized SiC/SiO2 nanowires, which shows numerous of ultra-long nanowires (at least several hundred microns length, as indicated by the yellow arrows) desultorily entangle each other to construct a film. Figure 2b reveals that the diameters of the SiC/SiO2 nanowires are in the range of 60~85 nm, and the length distribution of the as-synthesized SiC/SiO2 nanowires is in the range of 480 µm to 1230 µm. EDX spectrum (inset in Figure 2b) demonstrates that the nanowires are composed of Si, C and O elements, which infers that the nanowires are composed of SiC nanowire core and amorphous SiO2 shell. TEM was employed to collect further information of the micro-structure of the products, and typical images have been presented in Figure 2c and 2d. The results demonstrate that the diameter of the SiC nanowire core with dark contrast is about 40 nm and the thickness of the amorphous SiO2 shell with bright contrast is about 20 nm. HRTEM image in the inset of Figure 2d reveals that the lattice fringe spacing between two adjacent atom layers is 0.25 nm along the growth direction of the nanowire, in good agreement with (111) plane of cubic SiC. The corresponding SAED pattern of an isolated SiC/SiO2 nanowire captured from the red circle area is also displayed, and the diffraction spots are indexed to cubic SiC phase too. Noticeably, symbolic Ni catalyst particles have not been observed in the perspective of the SEM and TEM images and EDX pattern, it can be speculated that the growth process of the SiC/SiO2 nanowires may be controlled by vapor-liquid-solid (VLS) mechanism, which is common for the growth of SiC 1D nanostructures. The structure schematic of the homemade reactor, the location arrangement of the raw material together with the graphite substrate, and the gas flow direction were presented in Figure


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3. Figure 3a and 3b are the fractal and assembly vertical cross section of the whole reactor, respectively. In the preparation stage before reaction, the milled mixed powders were laid into three annular grooves of the gas distributor covered by the substrate with catalyst. Figure 3c shows the gas flow path (indicated by the blue arrow). From our own perspective, the homemade reactor played an positive role in synthesizing the ultra-long SiC/SiO2 nanowires, whose unique advantage and primary function were summarized as follows: The injected high purity CH4 gas could easily fill the whole reactor through the tiny holes evenly distributed on the gas distributor at the reaction temperature, which guaranteed the dispersal uniformity of the airflow field in the reactor space as well as intensive mixing with Si active component. According to the reaction condition and the characterization results, a typical VLS mechanism was proposed to elaborate the growth process of the SiC/SiO2 nanowires, as indicated in Figure 3d. Firstly, plenteous activate carbon molecules generated from the continuous methane gas when the temperature reached to 1270 oC. CH4 (g) = C (g) +2H2 (g)

(1)

Secondly, Ni(NO3)2 was decomposed into nanoscale NiO and further reduced into tiny nickel nano-droplets, which evenly distributed on the surface of the graphite substrate. 4Ni(NO3)2 (s) = 4NiO (s) + 8NO2 (g) + 2O2 (g)

(2)

NiO (l) + H2 (g) = Ni (l) + H2O (g)

(3)

During this process, the milled Si-SiO2 mixed powders easily reacted to form gaseous SiO through the following reaction.52 Si (s) + SiO2 (s) =2SiO (g)

(4)


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Subsequently, more and more gaseous activate C and SiO molecules were produced, and filled the space of the reactor quickly. Then, SiO and C molecules were absorbed by Ni nanodroplets to form Ni-Si-O-C alloy system (Figure 3d I). With increase of the concentration of the components reached over-saturation within the alloy system, the SiO and C reacted with each other following the reaction (4) and (5) to obtain SiC nucleus. According to the crystal growth principle, the produced SiO2 preferentially wrapped outside of the SiC nucleus. SiO (g) + 2C (g) = SiC (s) + CO (g)

(5)

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

(6)

With the reactions progressing, the SiC and SiO2 nucleus preferentially grew along the [111] axial direction under the confined effect of the catalyst (Figure 3d II). Finally, ultra-long SiC/SiO2 nanowires were collected after the reaction. Furthermore, it could be reasonably concluded that the nanowires became bent influenced by the slightly perturbation of airflow (Figure 3d III). From our own perspective, the homemade reactor played an positive role in producing the ultra-long SiC/SiO2 nanowires, whose unique advantage and primary function were summarized as follows: The injected high purity CH4 gas could easily fill the whole reactor through the tiny holes evenly distributed on the gas distributor at the reaction temperature, which guaranteed the dispersal uniformity of the airflow field in the reactor space as well as intensive mixing with Si active component. Through the ages, SiC-based bulk material with weak emission at room temperature is not regarded as a desirable luminescent material. However, the emission intensity can be significantly enhanced through overcoming the inherent deficiencies of the bulk counterpart when the size of SiC diminishes to several or tens of nanometers (SiC 1D nanostructures).34,53,54 Figure 4 depicts the PL spectra recorded from the as-synthesized SiC/SiO2 nanowires with an


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excitation wavelength of 325nm (Eex=3.82 eV) at ambient temperature, and a broad spectral band range from 405 nm to 495 nm with the maximum peak located at approximately 440 nm was presented. Compared with the blue-green luminescence from the bulk SiC (2.39 eV), the blueviolet luminescence (Eg=1240/440=2.81 eV) of the SiC/SiO2 nanostructures was apparently blue-shifted. Moreover, the spectrum could be deconvolved into three peaks at 423, 441, and 455 nm respectively according to the Gauss-fit. As we all know, there were many surface or defect states in SiC nanostructures, which exerted important effects on the luminescence intensity and position of the as-synthesized SiC/SiO2 nanowires. The emission peak located at 423 nm was attributed to attributed to SiC near-band-edge emission as well as limited quantum confinement effect.

55-57

Cao et al reported that the luminescence peak centred at 440 nm was stemmed from

some defect states recombination emission at the interface between SiC and SiO2, such as 2-fold coordinated silicon lone-pair centers (-O-Si-O-). 58 Similarly, the peak at 441 nm in Figure 4 was also ascribed to the structure defect existing in interface regain. The peak centered at 455 nm might be caused by oxygen vacancy defects in the SiO2 coating. The emission performance demonstrate that the SiC/SiO2 NWs can serve as nanodevices wide application potential in optoelectronics. The MA performances of the SiC/SiO2 nanowires at various thickness can be calculated according to the following equations: R L (dB) = 20 log

Z in =

Zin − 1 Zin + 1

[

µr tanh j(2πdf ) ε r µ r c εr

(7)

] (8)

where Zin is the input impedance of the absorber, d is the matching thickness, f is the frequency of the microwaves, c is the velocity of light in vacuum, respectively.


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Figure 5 displays the MA characterization results of the as-synthesized SiC/SiO2 nanowires with filler loading of 30 wt% in the frequency range of 2~18 GHz. In Figure 5a, it can be clearly observed that the thickness of the SiC/SiO2 nanowires has an obvious influence on the MA performance, and the minimum reflection loss (RL) values gradually migrate toward the lower frequencies with the increase of the matching thicknesses, revealing that the absorption frequency can be adjusted by controlling the thickness of the absorber. The RL below -10 dB (90% microwave absorption) can be achieved in the frequency range of 3~18 GHz when the abosrber thickness is in the range of 1.5 to 5.5 mm. Especially, when the thickness is 1.5mm, the absorber shows excellent MA properties with the minimum RL is up to -19.13 dB at 15.68 GHz, and the absorption bandwidth below -10dB is larger than 4.4 GHz (from 13.6 to 18 GHz). Additionally, the maximum RL of the abosrber is -24.11 dB at 11.12 GHz when the thickness increase to 2.0 mm. Both of these results suggest that the SiC/SiO2 nanowires possess excellent performance particularly in the high frequency region. A corresponding three-dimensional (3D) map of the MA versus different thickness in the frequency range of 2~18GHz was indicated in Figure 5b. Figure 5b shows the corresponding three-dimensional (3D) mapping of calculated theoretical RLs of the SiC/SiO2 nanowires at various thicknesses (1.5~5.5 mm) in the frequency range of 2~18GHz with the absorber loading of 30 wt%. It can be found that the microwave absorbing ability of SiC/SiO2 nanowires at different frequencies can be easily adjusted by controlling the thickness of absorbers, and the absorption belt move toward lower frequency with the increase of absorber thickness. Furthermore, the available microwave absorption with RL below -10 dB (yellow region) gradually decrease from the initial isolate islands to narrow belt with the increase of matching thicknesses, revealing that the SiC/SiO2 nanowires have better microwave absorption performance in high frequency region. The good MA properties of the as-


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synthesized SiC/SiO2 nanowires suggest their potential application as a promising absorber with strong-absorption, thin-thickness, light-weight, and broad absorption band. In order to further illuminate the probable MA mechanism of the as-synthesized SiC/SiO2 nanowires, the relative complex permittivity (εr=ε′-jε″) and complex permeability (µr=µ′-jµ″) have been evaluated, as shown in Figure 6a and 6b. In Figure 6a, the value of the real part of the complex permittivity (ε′) is in the range of 8.82~17.47 and the value of the imaginary part of the complex permittivity (ε′′) evidently decreased from 8.06 to 4.59 in the 2~10.96 GHz, and then slightly increase to 6.10 in the frequency range of 10.96~18 GHz. Figure 6b presents the frequency dependence of the real part (µ′) and imaginary part (µ′′) of the permeability of the SiC/SiO2 nanowires. It is clearly seen that the two curves almost hold constant around 0 and 1.1 with slight fluctuation in the frequency range of 2~18 GHz, respectively. Both the dielectric loss tangent (tanδε =ε′′/ε′) and the magnetic loss tangent (tanδµ=µ′′/µ′) display multiple variation tendencies in the frequency range of 2~18 GHz, as shown in Figure 6c and 6d. Especially, the values of tanδε are much higher than that of tanδµ, which verifies that the as-synthesized SiC/SiO2 nanowire is a dielectric loss type MA material, and the main factor of the performance can be attributed to the dielectric loss. CONCLUSIONS In this work, for improving the production of ultra-long SiC/SiO2 nanowires, a catalyst-assisted CVD approach has been developed using the homemade reactor, and gram-scale high quality products have been synthesized. Systematic characterization results show that the growth process of the as-synthesized SiC/SiO2 nanowires follows VLS mechanism. The photo-luminescence pattern of the ultra-long SiC/SiO2 nanowires shows strong blue-violet luminescence with obvious blue-shift, implying their availability in the optical nanodevices field. In addition, the


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MA measurements of the nanowires demonstrate an excellent MA performance in the range of 2~18 GHz with the broadest absorption bandwidth (

SiO2 Nanowires: Simple Gram-Scale Production and

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