Growth of SiC Whiskers onto Carbonizing Coir ... - ACS Publications

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Growth of SiC whiskers onto carbonizing coir fibers by using silicon slurry waste Yue Cao, Daoping Xiang, Hui Li, Rong Ren, and Zhiheng Xing ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02558 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Sustainable Chemistry & Engineering

Growth of SiC whiskers onto carbonizing coir fibers by using silicon slurry waste Yue Cao, Daoping Xiang *, Hui Li, Rong Ren, Zhiheng Xing State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, No. 58 People's Road, Meilan District, Haikou 570228, China.

*Corresponding author: Daoping Xiang, E-mail: [email protected] Tel /Fax: +86 0898 66279161

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Abstract: In order to reduce environmental pollution and waste of resources, it is very important to recycle valuable materials in silicon slurry waste (SSW). We report a novel method for the rapid preparation of SiC nano whiskers by spark plasma (SP) assisted thermal treatment using SSW as the silicon source. In the preparation process, coir fibers were used as the carbon source and whisker growth substrate. At 1100–1300 °C, carbon-rich tadpole-like 3C–SiC whiskers with Fe catalyst caps were prepared by the vapor–liquid–solid growth mechanism. At 1400–1600 °C, carbon-rich, stick-like 3C–SiC whiskers without Fe catalyst caps with aspect ratios of about 40, and diameters of about 50 nm were prepared by the vapor–solid growth mechanism. The SiC whiskers grew along the [111] direction on the (111) plane at different temperatures. At the optimum temperature of 1500 °C, the silicon in SSW reacted completely, and SiC whiskers with good morphology were prepared. Furthermore, the photoluminescence (PL) spectra of SiC whiskers showed strong blue-violet emission at 450 nm. Accordingly, this study provides an environmentally friendly method for preparing SiC whiskers.

Keywords: SiC whiskers, silicon slurry waste (SSW), coir fibers, spark plasma (SP), Luminescence.

Introduction Such SiC whiskers have shown exceptional properties, such as chemical inertness, large band gaps, and good thermal conductivity, thus they are used extensively in various 2


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applications.1–3 Due to the nature of single crystals, the strength of SiC whiskers is almost equal to their theoretical values, which has allowed them to be widely used in the field of structure application.4 In recent years, studies on the preparation of SiC whiskers through vapor–liquid–solid (VLS) and vapor–solid (VS) mechanisms have received much attention.5, 6 The main methods include carbothermal reduction of SiO2,7 direct reaction of silicon and carbon, sublimation recrystallization of silicon carbide, electrospinning,8 reaction of siliconcontaining compounds with carbon nanotubes,9, 10 chemical vapor deposition,11 pyrolysis of organosilicon compounds and reaction between silicon and hydrocarbon.12, 13 However, the above-mentioned methods are time consuming and/or costly. In the past 20 years, the photovoltaic industry has developed rapidly because of the need to replace fossil fuel energy with cleaner alternatives.14 Although solar cells provide pollutionfree energy while operating, substantial amounts of solid and liquid wastes called silicon slurry waste (SSW) are produced during the multi-wire slicing process of cutting silicon ingots into wafers.15 Such SSW is mainly composed of polyethylene glycol (PEG) cutting fluid, silicon carbide abrasives, high-purity silicon powders, small amounts of iron from the cutting wire, and water.16 Untreated SSW not only increases water chemical oxygen demand and biochemical oxygen demand but also wastes useful ingredients.17 Therefore, it is beneficial to recycle SSW. Particles Si, SiC and Fe form a stable suspension in PEG, and the Si/SiC powder particles possess overlapping size ranges and similar physicochemical 3



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properties. This makes it difficult to separate and recycle Si and SiC powders from SSW.18 A promising approach is to recycle Si and SiC powders together and use them to prepare highadded-value materials such as SiC whiskers. However, there are no existing reports on the preparation of SiC whiskers using SSW as the silicon source. In this study, raw material powders of Si, SiC and Fe were obtained by diluting and filtering SSW. In the raw material powders, Si was used as the silicon source, Fe as the catalyst, and coir fiber as the carbon source and whisker growth substrate. Finally, SiC whiskers were prepared rapidly in a spark plasma system.

Experimental Section Raw material pre-treatment The SSW used in this study was acquired from Hainan Yingli New Energy Co., Ltd. (Hainan, China). Its compositions mainly are Si, SiC, PEG, small amounts of water and Fe filings. Excess ethanol was added to the SSW and stirred for 1 h at room temperature. After filtration, a filter cake and filtrate were obtained. The filter cake was dried and ground to obtain the raw material powders. The PEG cutting fluid and ethanol were recovered via twostep distillation at distillation temperatures of 82 °C and 103 °C, respectively. The distillate obtained at 82 °C was the recovered ethanol. When the distillate was no longer produced at 103 °C, the remaining liquid was the recycled PEG. In this study, coir fibers from Hainan Province, China, were selected as the carbon source and whiskers growth substrates. 4


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Synthesis of SiC whiskers Raw material powders were placed at the bottom of a crucible graphite mould, and then the coir fibers were placed above the powders. After the raw materials were loaded, the mould was placed in a spark plasma system (LABOX-3010K, Japan) and heated to 1100–1600 °C under an argon atmosphere at a heating rate of 100 °C/min for 10 min. Subsequently, SiC whiskers and residual powders were obtained. The experimental flowchart is shown in Figure S1. Characterisation Some of the above-mentioned raw material powders were added with a strong alkali solution and then stirred and filtered to remove silicon. The silicon content in the raw material powders was determined on the basis of the mass change of the powders before and after alkali washing. The carbonization rate of the coir fibers was analyzed and determined using a thermogravimetric analyzer (NETZSCH STA449F5). The coir fibers were heated to 900 °C under an argon atmosphere at a heating rate of 10 °C/min. The phase and microstructure of the raw material powders, resultant whiskers and residual powders were analyzed by X-ray diffraction (XRD; Shimadzu XRD-6100) and scanning electron microscopy (SEM; Hitachi S-4800N). Infrared (IR) spectra were obtained in the wavenumber range of 4000–400 cm-1 via Fourier transform infrared spectroscopy (FT-IR; Bruker TENSOR27) using a KBr wafer. The particle sizes of the raw material and residual 5



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powders were measured using a Malvin laser particle sizer (Malvern, 2000). The fiber/whisker products were also analyzed by Raman spectroscopy (Renishaw RM 2000) using a He–Ne laser at 523 nm. The fine structures of the SiC whiskers were studied by fieldemission transmission electron microscopy (TEM; JEOL JEM-2100F), and the elements were analyzed using energy dispersive spectroscopy (EDS; Oxford). Photoluminescence (PL) spectra were observed in a fluorescence spectrophotometer (HORIBA iHR550) with a Xe lamp as the light source.

Results and Discussion Characterization of raw materials The XRD pattern and size distribution of the raw material powders obtained by pre-treating SSW are displayed in Figures S2a and S2b, respectively. Clearly, the raw material powders were mixtures of Si (JCPDS Card No. 75-0590), SiC and Fe. Amongst these components, SiC presented crystal types of α-SiC (JCPDS Card No. 74-1302) and β-SiC (JCPDS Card No. 750254). The raw material powders were mainly composed of two different-sized particles, namely, a small particle with a maximum volume fraction of 1.9 µm, and a large particle with a maximum volume fraction of 6.6 µm. Meanwhile, SEM images showed that aggregates measuring within 10 µm had formed in the raw material powders (Figure S3). The thermogravimetric (TG) curve and SEM morphology of the coir fibers are shown in Figure S4a. The coir fiber diameter was less than 200 µm, and some punctate protuberances 6


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were uniformly distributed on the fiber surface. The TG curve indicates that the fiber carbonization process mainly involved drying (AB), pyrolysis (BC), pyrocondensation polymerization (CD) and carbonization completion (DE) stages. The drying stage below 250 °C caused 10% weight loss by cellulose dehydration.19 By contrast, at the pyrolysis stage of 250–350 °C, small molecular volatile components formed and caused approximately 55% large mass loss due to bond cleavage in the –OH, −COOH and branched chain aliphatic side chains.20, 21 In the temperature range of 350–800 °C coinciding with the pyrocondensation polymerization stage, the fibers weight continued to decline due to the cleavage of C–H bonds on the aromatic rings into free radicals, as well as concurrent dehydrogenation and polycondensation reactions.21 Above 800 °C, the coir fibers were completely carbonized, and the final carbon residue rate was about 20%. The XRD pattern of the carbon fibers after carbonization at 1100 °C is shown in Figure S4b (carbon fiber SEM image in the inset). The XRD pattern revealed two broad peaks at around 24° and 44°, which indicated the amorphous nature of the fiber substrate.22 Meanwhile, the corresponding SEM image showed that the coir fibers retained their original basic structure, even after carbonization. Cellulose decomposition in coir can provide sufficient CO, which favors the formation of SiO.4 Simultaneously, CO can continue to react with SiO or Si vapor to synthesize SiC. Therefore, coir fiber pyrolysis contributed to SiC whisker growth. The carbonized fibers were composed of turbostratic stacking graphite structure and had proper asperities, which 7



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facilitated the formation of SiC whiskers.23, 24 Therefore, the coir fibers not only served as the growth substrates for the SiC whiskers, but also provided a carbon source during whiskers growth. This experiment used ethanol as a diluent to separate PEG and raw materials, which is more environmentally friendly than the acetone diluents used in earlier literature.25–28 At the same time, ethanol and PEG were separated by two-step cryogenic distillation (Figures S5 and S6). Phase and particle size of the residual powders The XRD patterns of the residual powders after heat treatment at different temperatures are shown in Figure S7. At temperatures below 1200 °C, the residual powders contained large amounts of Si and SiC, and small amounts of FeSi and FeSi2. The diffraction peaks of each phase did not obviously change with increasing temperature. When the temperature reached 1300–1400 °C, the diffraction peaks of Si decreased noticeably, indicating that most of the Si had been transformed from solid to vapor. Above 1500 °C, the Si diffraction peaks disappeared completely. Thus, all the Si particles changed from solid to vapor at 1500 °C. The residual powders were composed of SiC and a small amount of FeSi. The disappearance of the FeSi2 diffraction peaks could be related to the following reaction:29 FeSi → FeSi + Si (1) Given that Eq. (2) indicates the intensity ratio between the two phase XRD diffraction 8


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peaks of the residual powders, Eq. (3) was deduced to calculate the mass fraction of the two phases as follows:

=

×

= ×

(2) (3)

where I is the diffraction peak intensity in the XRD pattern, and K refers to the K value of the samples when Al2O3 is used as a reference (RIR). In this study, the mass ratios of Si/α-SiC and Si/β-SiC (XSi/Xα-SiC and XSi/Xβ-SiC) were calculated using the K value method’s Eq. (3) (Supporting Information S3.2).30, 31 In Figure S8, the variation trend curves of XSi (111)/Xα-SiC (101)

and XSi

(111)/Xβ-SiC (311)

were obtained in accordance with Eq. (3) during elevated

temperature. The variation trend curves revealed that Xj/Xi was greater at 1100 °C and 1200 °C than at other temperatures. Xj/Xi decreased continuously at 1300–1450 °C and reached zero beyond 1500 °C. Hence, most of the Si powders in the raw material powders were translated into Si vapor at 1300–1450 °C, then were completely lost beyond 1500 °C. Figure S9 shows the particle size distribution curve of the residual powders that were heat treated at 1500 °C. The curves revealed that the residual powders were mainly composed of 10 µm particles. Compared with Figure S2b, we noted that the Si powders with a particle size of ~1.9 µm disappeared almost entirely after heat treatment. Therefore, to ensure that Si and SiC were completely separated, the heat treatment temperature should not be less than 1500 °C. These residual powders can be recovered as silicon carbide abrasives. 9



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Phase and microstructure of SiC whiskers XRD analysis The XRD patterns of the whiskers and fiber substrates at different temperatures are shown in Figure 1. Below 1250 °C, two broad peaks were observed at around 24° and 44°, which indicate the amorphous nature of the fiber substrate.22 Meanwhile, the weak β-SiC (JCPDS Card No. 75-0254) diffraction peak at about 35° (blue box) suggests that a small amount of SiC was synthesized on the fiber surface at 1200 °C and 1250 °C. At 1300–1500 °C, the characteristic diffraction peaks of β-SiC (JCPDS Card No. 75-0254) appeared in the XRD patterns, and the diffraction peak intensity increased with temperatures. This result implies that SiC was synthesized at an increasing rate as the temperature increased from 1300– 1500 °C. The intensity of the β-SiC diffraction peaks did not obviously change with rising temperatures beyond 1500 °C, indicating complete product synthesis at this temperature. The local XRD patterns of the SiC products above 1400 °C (Figure S10) also revealed a small diffraction peak at 33.6°, thereby indicating the presence of stacking faults (SF) on the (111) plane.8

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Figure 1. XRD patterns of whiskers and fiber substrate at different preparation temperatures. Raman analysis Figure 2 shows the Raman spectrum of the whiskers and fiber substrates prepared at 1500 °C. The 789 and 945 cm−1 readings were the optical phonon (TO) mode and longitudinal phonon (LO) mode, respectively, and they correspond to the zone–center TO mode at 796 cm−1 and zone–center LO mode at 972 cm−1.7 The TO and LO modes presented obvious ‘red shifts’ of 7 and 27 cm−1, respectively, which were mainly related to the material size effect and structural defects.32 Nanowhiskers were present between the macroscopic and atomic levels and produced a red shift of less than 10 cm−1.33 In addition, the two shoulder peaks observed between the TO and LO modes (860 and 900 cm−1) indicated the presence of numerous defects.34 The above analysis of Figure S8 suggests that the structural defects of the sample were related to SFs on the (111) plane. The actual red shift of the LO mode was large, which shows that the red shift induced by SFs was higher than that of the size effect in onedimensional SiC products.33 11



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Figure 2. Raman spectrum of the whiskers and fiber substrates prepared at 1500 °C. SEM analysis Figure 3 shows SEM images of the whiskers and fiber substrates at different temperatures. Bent whiskers were distributed across the fiber surface at 1100 °C (Figure 3a). Whisker diameters were less than 50 nm, and the catalyst capped the tops of the tadpole-like whiskers. The caps were liquid catalyst globules formed by the melting of small solid catalyst particles.4 The presence of the catalyst caps indicated whisker growth through the VLS mechanism (Figure S11a). A large amount of CO was produced during coir fiber pyrolysis, and the CO and Si powders reacted according to Eq. (4) to generate SiO gas and C.35, 36 Under conditions of CO and SiO vapor saturation, as shown in Eq. (5), the liquid catalyst globules absorbed Si and C from SiO and CO vapor until supersaturation and separation in the form of SiC whiskers.5, 6, 37 The CO2 produced during whisker preparation could react with Si and C as per Eqs. (6) and (7) to generate SiO and CO, respectively, and provided silicon and carbon sources for SiC whisker growth. At this point, the amount of SiC whiskers produced was low, 12


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and the morphology was irregular due to the low SiO vapor pressure present at 1100 °C.6

Si(s) + CO(g) → SiO(g) + C(s) ∆H ° = 7.487 kJ mol( (4) SiO(g) + 3CO(g) → SiC(whiskers) + 2CO (g) ∆H ° = −412.731 kJ mol( (5) Si(s) + CO (g) → SiO(g) + CO(g) ∆H ° = 174.099 kJ mol( (6) C(s) + CO (g) → 2CO(g) ∆H ° = 166.612 kJ mol( (7) At 1200–1300 °C (Figures 3b and 3c), the catalyst caps on the whiskers enlarged as the temperature increased, resulting in concurrent whisker diameter expansion and needle-like shape formation. Thus, temperature affected the catalytic droplet size, which controlled the morphology of the SiC nanostructures.5, 38 A small amount of Si powder also began to change into Si vapor as the temperature increased. The Si vapor reacted with CO in the catalyst caps and generated SiC whiskers according to Eq. (8).

Si(g) + 2CO(g) → SiC(whiskers) + CO ∆H ° = −682.546 kJ mol( (8) At 1400–1600 °C (Figures 3d–3f), the SiC whisker lengths increased obviously, whereas their diameters only changed slightly, to about 50 nm. The aspect ratios of the SiC whiskers were approximately 40. The catalyst caps were completely obliterated, indicating SiC whisker growth through the VS mechanism (Figure S11b). Si vapor was initially produced from Si powders and then deposited on active carbon atoms as per Eq. (9) to form SiC nuclei at high temperature. Subsequently, the aggregates grew into SiC whiskers as Si vapor was deposited onto the SiC nuclei continuously. The corresponding reaction is shown in Eq. (10).39 13



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According to thermodynamic analysis (Supporting Information S4.3),40, 41, 42 these reactions are feasible. Si(g) + C(s) → SiC(nuclei) ∆H4 ° = −515.888 kJ mol( (9) Si(g) + C(s) → SiC(whiskers) ∆H4 ° = −515.888 kJ mol( (10)

Figure 3. SEM images of the SiC whiskers at different preparation temperatures: (a) 1100 °C, (b) 1200 °C, (c) 1300 °C, (d) 1400 °C, (e) 1500 °C and (f) 1600 °C. TEM analysis Figure 4 shows the TEM and high-resolution TEM (HR-TEM) images, as well as the EDS spectra, of the SiC whiskers prepared at 1200 °C. The catalyst cap on the tadpole-like whisker in Figure 4a corresponds to Figure 3b. The EDS analysis revealed that the catalyst cap was 14


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composed of C, Si, Fe and O elements with the atomic percentages of 42.83%, 32.74%, 17.26% and 7.17%, respectively. This result indirectly confirms that CO and SiO react in the Fe catalyst globule to synthesize SiC and undergo downward deposition. The EDS analysis also showed that the whisker was composed of C, Si, Fe and O elements with atomic percentages of 77.25%, 17.35%, 0.02% and 5.38%, respectively. Fe was only used as catalyst, as confirmed by the low Fe content in the whisker. The presence of oxygen atoms in the SiC whiskers indicats SiO2 existed there at this preparation temperature. The oxygen atoms also suggest the occurrence of a side reaction according to Eq. (11):4 3SiO(g) + CO(g) → SiC(s) + 2SiO (s) (11) The atomic percentage of C was much higher than that of Si in the whiskers. This result implies that the obtained whiskers were carbon-rich SiC whiskers with great potential for use in nanodevices and field emitters.43 Meanwhile, the HR-TEM image of the SiC whisker prepared at 1200 °C (Figure 4b) shows that their lattice spacing was 0.25 nm, which is equivalent to the inter-planar distance between the 3C–SiC (111) planes. This illustrates that the whiskers grew on the 3C–SiC (111) plane due to the low surface energy caused by the high atomic density and large ‘d’ spacing.4,

44

With the continuous supply of gaseous Si

sources and gaseous C sources, the SiC whiskers continued to grow along the [111] direction.

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Figure 4. SiC whiskers prepared at 1200 °C. (a) TEM image (inset: EDS spectrum) and (b) HR-TEM image. Figure 5 shows TEM and HR-TEM images, selected-area electron diffraction (SAED) patterns, and an EDS spectrum of the SiC whiskers prepared at 1500 °C. The TEM micrographs (Figures S13a–d) show that the whiskers were long and straight, whereas some presented a striped morphology. This finding signifies the presence of SFs and microtwins in the whiskers.4, 6 The appearance of high-density SFs is due to fluctuations in the kinetics of the growth conditions, which helped to preserve the low growth energy of the whiskers. 45 A magnified view of Figure S13a, illustrating a whisker without striped morphology, is shown in Figure 5a. The EDS analysis showed that the whisker was composed of C, Si and O elements at atomic percentages of 62.55%, 37.36% and 0.09%, respectively. The oxygen atomic percentage was low; thus, the purity of the SiC whiskers increased with temperatures. Given that the C/Si atomic ratio in the whiskers was 1.67, they remained as carbon-rich SiC whiskers at 1500 °C. The SAED pattern in the inset of Figure 5a reveals that the whisker without stripes was mainly composed of single-crystal cubic 3C–SiC structure with a 16


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zone axis. Meanwhile, a magnified view of Figure S13b, illustrating a whisker with striped morphology, is shown in Figure 5b. The SAED pattern in the inset of Figure 5b reveals that the striped whisker mainly consisted of single-crystal cubic 3C–SiC structure with a zone axis. Figure 5c shows the HR-TEM image of a SiC whisker with no striped morphology (Figure 5a). The SiC whisker possessed a lattice fringe spacing of 0.25 nm, equivalent to the inter-planar distance between the 3C–SiC (111) planes. This finding suggests that the whiskers grew in the [111] direction.

Figure 5. SiC whiskers prepared at 1500 °C. (a) TEM image (insets: EDS spectrum and SAED pattern), (b) TEM image (inset: SAED pattern) and (c) HR-TEM image. PL analysis The PL emission spectra were employed to appraise the application of the synthesized SiC 17



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whiskers in optical components. The PL spectra in Figure 6 were obtained at a 325 nm excitation wavelength at room temperature. They display strong blue emission centralities at 450 nm and obvious blue-shift compared to existing studies on bulk SiC (519 nm).46 The main causes of the PL spectra blue-shift are the size confinement effect and high-density defects, including SFs of SiC whiskers.47

Figure 6. PL emission spectra of the SiC whiskers at different temperatures (inset: optical image of the SiC whiskers prepared at 1500 °C).

Conclusion In this study, SSW was used as the silicon source, and coir fibers were used as the whisker growth substrate and carbon source. The SiC whiskers were prepared by spark plasmaassisted heat treatment. Whisker growth evaluation revealed the occurrence of two different SiC whisker growth mechanisms that ensued successively with rising temperature. At the low temperature range of 1100–1300 °C, SiC whiskers grew by the VLS mechanism. Obvious catalyst caps were found on the whiskers, and the whiskers gradually changed from bent to 18


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straight as the temperature increased. At the high temperature range of 1400–1600 °C, the SiC whiskers grew by the VS mechanism, and catalyst caps were absent. The EDS and XRD analysis showed that the whiskers were carbon-rich 3C–SiC whiskers. Meanwhile, HR-TEM analysis revealed that the SiC whiskers constantly grew along the [111] direction on the (111) plane at different temperatures despite proceeding through two different mechanisms. In the residual powder XRD patterns, the characteristic diffraction peak of Si disappeared completely at 1500 °C. Thus, this temperature is considered to be the optimum temperature for preparation of SiC whiskers from SSW. Moreover, the PL characterization of the SiC whiskers displayed intensive blue-violet excitation around 450 nm, indicating promising application in nano-optical devices. This study not only opens new perspectives for recycling SSW through an environmentally friendly method, but also provides a new way for utilizing biomass fibers, although our results come from laboratory studies.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51464010) and the Natural Science Foundation of Hainan Province (Grant No. 20165207).

Associated Content Supporting Information. More studies and characterization of raw materials, residual powders, recycled PEG and SiC whiskers. The following files are available free of charge.

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For Table of Contents Use Only Synopsis

Industrial waste (silicon slurry waste) and agricultural waste (coir fibers) used to prepare SiC nano whiskers.

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Growth of SiC Whiskers onto Carbonizing Coir ... - ACS Publications

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