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Growth of SiC nanowires using oil palm empty fruit bunch fibres infiltrated with tetraethyl orthosilicate
Physica E 44 (2012) 2041–2049
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Growth of SiC nanowires using oil palm empty fruit bunch fibres infiltrated with tetraethyl orthosilicate Y.L. Chiew, K.Y. Cheong n Energy Efficiency & Sustainable Semiconductor Research Group, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
H I G H L I G H T S c
c
c
c
Ability to grow SiC nanowires by pyrolysing TEOS-infiltrated oil palm fibre waste. Observation of lower yield of SiC nanowires at higher TEOS concentration. A decreasing trend in diameter with increasing pyrolysis temperature. Nanowire growth attributed to combination of solid-state and vapour– solid reactions.
G R A P H I C A L
A B S T R A C T
SiC nanowires formed by pyrolyzing TEOS-infiltrated oil palm empty fruit bunch fibres.
a r t i c l e i n f o
abstract
Article history: Received 7 May 2012 Received in revised form 10 June 2012 Accepted 11 June 2012 Available online 29 June 2012
SiC nanowires were produced by pyrolyzing oil palm empty fruit bunch fibres infiltrated with tetraethyl orthosilicate. The effects of the concentration of TEOS (10%, 50% and 100%) and the pyrolysis temperatures (1250 1C, 1300 1C, 1350 1C and 1400 1C) were studied. An increase in TEOS concentration led to an increment in silica content. However, when TEOS was infiltrated into the fibres, the small sizes of the lumens in the oil palm fibres and low fluidity of TEOS resulted in lower amount of silica deposited onto the surface when the concentration was increased to 100%. This in turn resulted in a lower yield of SiC nanowires at higher TEOS concentration. When pyrolysis temperature was raised, there was a decrease in diameter but the lengths of nanowires reached tens of mm. The growth of the nanowires was attributed to the combination of solid-state reaction and vapour–solid growth mechanisms. & 2012 Elsevier B.V. All rights reserved.
Keywords: SiC Nanowires Oil palm empty fruit bunch fibres Tetraethyl orthosilicate
1. Introduction Silicon carbide (SiC) nanowires have been attracting attentions due to their excellent thermal, mechanical, chemical and electronic properties. Their excellent properties made them applicable in various fields from acting as reinforcements in ceramic matrix composite to areas of nanoelectronics, nanooptics and nanosensors [1–3]. Various methods have been developed in synthesising SiC nanowires, including chemical vapour deposition (CVD) [4–7], carbothermal reduction [8–11], laser ablation [12], chemical n
Corresponding author. Tel.: þ60 4599 5259; fax: þ 60 4594 1011. E-mail address: [email protected] (K.Y. Cheong).
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vapour growth (CVG) [13] and thermal evaporation [3,14–16]. Carbothermal reduction is a common method in producing SiC nanowires and the usage of agricultural wastes as source materials is a potential route with wastes such as rice husks, bamboo, sugarcane leaf with rice straw, bean–curd refuse and cotton fibre, as reviewed in previous article [17]. Although various waste materials had been used in the synthesis of SiC nanowires, oil palm fibres had not been used. Oil palm empty fruit bunch fibres was chosen as a carbon source in the production of SiC nanowires due to their high carbon content and low ash content [18]. The amorphous nature of carbon present and the porous structure of the fibres also make the carbon highly reactive. In addition, these low cost wastes are abundant in Malaysia with annual production of 20 million tonnes per year [19].
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Although the oil palm fibres contain inherent SiO2, it was insufficient for SiC formation reactions to take place. This was validated by a control experiment performed on oil palm fibres alone pyrolyzed at 1300 1C for 1 h that showed no nanostructures were formed. This shows that an external source of SiO2 is necessary and tetraethyl orthosilicate (TEOS) is a suitable choice of silicon source. In previous study, rice husk ash was used as a silicon source [20] though it was found that the yield of nanowires was significantly lower and a higher growth temperature was needed to initiate the growth, as compared to those produced using TEOS. TEOS is a chemical compound with the formula Si(OC2H5)4 that normally exist in liquid form and could be converted into SiO2 easily at elevated temperatures above 600 1C [21]. TEOS is also a cheap source of Si. The use of TEOS as silicon source in the synthesis of SiC nanowires had been reported previously though the carbon source differed, with Jute fibres [22], bamboo slices [23] and polyvinylpyrrolidone (PVP) [24]. It had also been reported that TEOS was used as both silicon and carbon source in the formation of SiC nanowires [25]. In these studies, the nanowires formed were of 3C–SiC and had large diameters (40–200 nm) but shorter lengths (10–20 mm) [23,25], as well as additional processing was required in converting TEOS into SiO2 by hydrolysis and alcohol condensation [22,24]. In this work, 3C–SiC nanowires were grown by pyrolyzing acid-treated oil palm empty fruit bunch fibres (TOP) infiltrated with TEOS in a horizontal tube at temperatures of 1250–1400 1C for 1 h using different concentrations of TEOS (10%, 50% and 100%) under argon atmosphere. The effects of TEOS concentration and pyrolysis temperatures on the growth of nanowires were studied. The growth mechanism of the nanowires was also discussed.
2. Materials and methods Oil palm empty fruit bunch fibres in the present study were obtained from local mill in Nibong Tebal, Malaysia. The oil palm fibres were first sieved and water-washed to remove dirt. They were next subjected to acid leaching using 1 M hydrochloric acid (HCl) heated at 100 1C for 1 h to remove metallic impurities and to weaken the carbohydrate bondings in the fibres [26]. X-ray fluorescence (XRF, Casio) and CHNS Analyser (Perkin Elmer 2400) were performed on the raw oil palm fibres and the acid-treated oil palm fibres to determine the chemical compositions. The reduction of impurities after acid treatment could be proven by the (XRF) results in Table 1. The acid-treated oil palm fibres were then designated as TOP. The TOP fibres were then cut into lengths of approximately 1 cm before being infiltrated with TEOS. The different concentrations of TEOS solution (10%, 50% and 100%) were prepared by diluting the TEOS with ethanol (95%). The samples after TEOS Table 1 Chemical compositions of OP, TOP, 0.1TEOS, 0.5TEOS and 1.0TEOS. Composition
OP
TOP
TOP-0.1TEOS TOP-0.5TEOS TOP-1.0TEOS
SiO2 MgO Al2O3 P2O5 SO3 K2O CaO MnO Fe2O3 LOI Of which is C
0.653 0.076 0.050 0.037 0.080 0.167 0.202 0.013 0.407 98.045 47.11
1.335 0.004 0.047 0.021 0.171 0.059 0.007 – 0.037 98.253 45.56
4.773 0.311 0.026 0.003 0.063 0.003 0.395 0.002 0.365 94.050 45.56
4.941 0.143 0.023 0.003 0.053 0.004 0.192 0.001 0.170 94.457 45.56
3.829 0.058 0.017 0.005 0.056 0.003 0.073 – 0.065 95.880 45.56
infiltration were assigned as 0.1TEOS, 0.5TEOS and 1.0TEOS, according their respective TEOS concentration. During the infiltration process, TOP fibres were immersed in the TEOS solutions and evacuated in a vacuum chamber by a rotary pump. The infiltration process was performed in a vacuum condition in order to remove the air inside the fibres as much as possible and to ensure the impregnation of TEOS into the pores of the individual fibres by capillary action [22]. After infiltration process, the fibres were dried in an oven at 70 1C for 24 h. The dried TEOS-filled TOP fibres were subsequently placed in an alumina crucible that would be closed with an alumina lid during pyrolysis process. Pyrolysis was then performed in a horizontal tube furnace (Model STF 15/180/3216P1, Carbolite Furnace Ltd., UK) at temperatures of 1250–1400 1C for 1 h under argon flow of 50 sccm and heating rate of 10 1C/min from room temperature to the pyrolysis temperature. After pyrolysis, the excess carbon in the remaining pyrolysis product was removed by burning in a muffle furnace at 700 1C for 3 h, followed by removal of excess SiO2 using 40% hydrofluoric acid (HF). Finally, the products were collected and characterised. A fieldemission scanning electron microscope (FESEM, LEO GEMINI) equipped with an energy dispersive X-ray spectrometer (EDS, ZEISS SUPRATM 35VP) was used to examine the samples after pyrolysis. An energy-filtered transmission electron microscope (EFTEM, ZEISS LIBRA 120) equipped with electron spectroscopic imaging (ESI) was used to examine the sample after the decarbonisation and HF leaching processes. The samples for EFTEM were dispersed in absolute ethanol and the suspension was dripped on a copper grid covered with holey carbon film. The samples were also tested by Fourier Transform Infrared Spectrophotometer (Perkin-Elmer System 2000) under transmission mode. The samples were prepared using KBr pellet technique. Approximately 0.1 mg samples were ground with 50 mg KBr in an agate mortar and compacted using a hydraulic press.
3. Results and discussion 3.1. Effect of TEOS concentration Typical FESEM micrographs of the TOP fibres before and after TEOS infiltration at different concentrations are presented in Fig. 1. They showed that TEOS was found in the lumens of the TOP fibres after infiltration and the amount increased as the TEOS concentration was increased. The chemical composition in Table 1, as obtained using XRF and CHNS Analyser, showed that there was an increase in SiO2 content when TEOS concentration was increased from 10% to 50% but the SiO2 content decreased when the concentration was further increased to 100%. This observation might be attributed to the better fluidity at lower concentration since TEOS was diluted by ethanol, allowing it to be infiltrated into the fibres easily [22]. Thus, the fibres treated with 50% TEOS might have higher SiO2 content due to the dilution process, making infiltration of TEOS into the fibres easier. As for the low SiO2 content in the sample treated with 100% TEOS, it might be because of the low fluidity of the viscous TEOS solution, reducing the ability of the TEOS to penetrate the lumens of the fibres due to low capillary action. Therefore most of the TEOS molecules were just deposited onto the surfaces of TOP, instead of inside the lumens, as proven by the FESEM images. These TEOS molecules on the TOP surface could be easily removed during transfer and this might led to lower SiO2 content. After pyrolysis, the TOP fibres appeared charred and coated with blue layers. After decarbonisation, whitish depositions were observed instead, as shown in Fig. 2. Maximum yields were found at lower concentrations of TEOS and only small specks of black
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Fig. 1. FESEM micrographs of TOP fibres (a) before TEOS infiltration and after TEOS infiltration with (b) 10% TEOS, (c) 50% TEOS and (d) 100% TEOS.
Fig. 2. Appearance of the pyrolyzed TEOS-filled TOP samples with (a)–(c) showing the samples after pyrolysis and (d)–(f) showing the samples after decarbonisation for 0.1TEOS, 0.5TEOS and 1.0TEOS, respectively.
particles were observed in the samples after decarbonisation. FESEM micrographs in Fig. 3 showed that the blue layers on TOP fibre surfaces were nanowires. With increasing concentration, the yield of nanowires on the TOP fibre surfaces was decreased. From the higher magnification micrographs [Fig. 3(d)–(f)], the nanowires were found to be a combination of curled and straight nanowires. EDS results showed that these nanowires were consisted of Si, C and O.
Fig. 4 shows the FTIR spectra of the TEOS-filled TOP fibres after each processing under different TEOS concentration and the species associated with the adsorption bands are shown in Table 2 [27–31]. The disappearance of peaks associated with CH, CH2, CH3, Si(CH)3 and C(CH)3 after pyrolysis indicated decomposition of the cellulose and lignine components, transforming from aliphatic compounds into aromatic compounds of CQC and CQO [31]. The strong Si–O peak (1200 1000 cm 1) in the
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Fig. 3. FESEM micrographs of pyrolyzed TEOS-filled TOP samples at different TEOS concentrations of (a) 0.1TEOS, (b) 0.5TEOS and (c) 1.0TEOS at low magnification and (d)–(f) along with the insets show higher magnification micrographs of the nanowires at concentrations of 0.1TEOS, 0.5TEOS and 1.0TEOS, respectively.
Fig. 4. FTIR spectra of (a) TOP and after each process of (b) pyrolysis, (c) decarbonisation and (d) HF treatment under different TEOS concentrations.
spectra was a result of superposition of Si–O and C–OH bonding [28,29]. After decarbonisation, peaks associated with Si–O and Si–C became more prominent due to removal of most organic
compounds. However, small peaks of CQO were still detected because of the black carbon particles encapsulated by the melted SiO2 during SiO2 surface melting [32]. The SiO2 melt protected the
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Table 2 Absorption bands in FTIR spectra and their assignments. Regions (cm 1)
Assignment of surface functional groups
Ref.
3800–3100
Valent vibrations of O–H bond in water molecules bonded by hydrogen bonds OH groups present in cellulose, hemicellulose and lignine CQO carbonyl groups CQC groups Deformation vibrations of the water molecules (d–H2O) CQO (aromatic) groups C–H, CH2, CH3, Si(CH)3 and C(CH)3 groups Unsaturated C–C and C–O skeletal vibrations Si–O stretching Vibrations of C–OH bond Si–C–O bonding Si–O vibrations Si–C stretching Free SiO2 (O–Si–O bending vibrations)
[27,29,31]
1750–1700 1650–1500 1640 1500–1415 1500–1200 1360–1310 1200–1000 933 898–825 825–782 486–464
[29–31] [29–31] [29] [31] [27,31] [31] [28,29,31] [33] [31] [31] [28,31]
Fig. 5. EFTEM images of the nanowires formed in (a) 0.1TEOS, (b) 0.5TEOS and (c) 1.0TEOS. (d) Shows the composition of nanowires determined by ESI and (e) shows the diameter distribution of the nanowires at different TEOS concentrations.
carbon particles from oxidation during decarbonisation by preventing direct contact between carbon and oxygen. This could be validated by the observation of small specks of black particles in the samples after decarbonisation. After HF leaching, the Si–C peaks (825–782 cm 1) became stronger while the Si–O peaks (1200–1000 cm 1, 486–464 cm 1) weakened due to removal of excess of SiO2 but not completely removed due to incomplete reaction. An additional peak at 933 cm 1 had been found to associated with Si–C–O bonding and this bonding was normally
found in SiC with larger grain size [33]. When the TEOS concentration was increased, there was less effect on the Si–O peaks since the SiO2 content in the three samples were almost similar. However, the Si–C peak was the strongest in the sample treated with 10% TEOS, suggesting a high SiC yield. The Si–C peaks then became broader with increasing TEOS concentration, indicating a wide range of diameters in the SiC nanowires [33]. EFTEM images in Fig. 5(a)–(c) show that the nanowires consisted of single layer structures with presence of stacking
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faults and microtwins after HF leaching. The nanowires observed in the sample treated with 10% TEOS and 50% TEOS had smooth sidewalls, but the nanowires produced from the sample treated with 100% TEOS had rough sidewalls. The nanowires had average diameters between 25 and 35 nm with lengths up to tens of mm. The diameter distribution of nanowires in Fig. 5(e) showed an increase in the average diameter and the range of diameters were wider in the sample treated with 100% TEOS, which was in accordance with FTIR results showing a wide Si–C band. The ESI results [Fig. 5(d)] showed that the nanowires consisted of Si and C only. A representative XRD spectrum of the nanowires formed after HF treatment in Fig. 6 showed diffraction peaks of Moissanite 3C– SiC (ICDD 01-073-1665) and a small diffraction peak of Moissanite 2H–SiC (ICDD 01-089-2213), indicating that the SiC nanowires were consisted of two polytypes with the main polytype as cubic 3C–SiC and a side-product of hexagonal 2H–SiC. Thus, combining the results from FTIR, ESI and XRD, it could be concluded that the nanowires were of SiC. 3.2. Effect of pyrolysis temperature To study the effect of pyrolysis temperature on the growth of SiC nanowires using TEOS-filled TOP fibres, the TEOS concentration was fixed at 10% since it was able to produce maximum yield. After pyrolysis, the amount of blue depositions on the charred fibres increased with increasing temperature from 1250 1C to 1350 1C but the blue deposition decreased in amount at 1400 1C [Fig. 7(a)–(d)]. After decarbonisation, the blue depositions changed to whitish depositions with decreasing amount of black particles. It was also observed that maximum yield was obtained at 1350 1C after decarbonisation [Fig. 7(e)–(h)]. FESEM micrographs of the pyrolyzed samples at different temperatures are shown in Fig. 8. The lower magnification micrographs show that the TOP fibres were covered with nanowires and were consisted of Si and C based on EDS results. The nanowire density on the fibres increased with temperature and became thicker. From the higher magnification images [Fig. 8(e)–(h)], it could also be observed that the diameters of the nanowires reduced with increasing temperature. Fig. 9 shows the FTIR spectra of the TEOS-filled TOP fibres after each processing at different pyrolysis temperatures. The FTIR spectra showed similar absorption bands as listed in Table 2. When the pyrolysis temperature was increased, the peaks associated with Si–O (1200–1000 cm 1) decreased in intensity because the increase in temperature provided the extra energy for higher reaction rate between SiO and CO. The SiC peaks were also observed to become broader but decreased in intensity with increasing temperature. The SiC peaks were more prominent at temperatures of 1300–1350 1C, indicating a high yield of SiC in
Fig. 7. Appearance of the pyrolyzed TEOS-filled TOP samples fixed at 0.1TEOS with (a)–(d) showing the samples after pyrolysis and (e)–(h) showing the samples after decarbonisation at pyrolysis temperatures of 1250 1C, 1300 1C, 1350 1C and 1400 1C, respectively.
these samples. The broad band at 1400 1C might be a result of the transformation of nanowires into particles that were larger in size, as proven by the detection of SiC particles in EFTEM results. The EFTEM images in Fig. 10(a)–(c) showed the presence of nanowires only at temperatures from 1250–1350 1C but at 1400 1C, particles started to appear aside from nanowires and ESI showed that the nanowires along with particles in micron size consisted of Si and C [Fig. 10(e)]. So, SiC particles were also produced at higher temperature. The formation of particles was reported to be a result of coagulative recrystallisation of nanowires due to higher reaction rate in producing SiC at higher temperatures [34,35]. According to the diameter distribution in Fig. 10(f), a decrease in the diameters of nanowires was observed as opposed to the common trend of an increase in diameter with increasing temperature [36,37]. The decrease in diameter might be attributed to the higher surface diffusion rate of the adsorbed species on the sidewalls to the tips that were more energetically favourable [38]. The increase in the diffusion rate was made possible by the extra energy provided by higher temperature. This tendency contributed to thinner nanowires with increasing length, as noticed in the study here. 3.3. Growth mechanism
Fig. 6. A representative XRD spectrum of the nanowires after HF treatment.
To determine how the nanowires formed, the origin of the nanowire was observed. Fig. 11(a) shows the FESEM micrographs of an individual nanowire growing from a cluster that acted as a nucleus and the EDS results in Fig. 11(b) showed that the cluster consisted of Si, C and O only without any metallic elements. The lack of metallic impurities in the nanowires helped to rule out the
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Fig. 8. FESEM micrographs of pyrolyzed TEOS-filled TOP samples fixed at 0.1TEOS at different pyrolysis temperatures of (a) 1250 1C, (b) 1300 1C, (c) 1350 1C and (d) 1400 1C. Insets show the higher magnification micrographs of the pyrolyzed TEOS-filled TOP samples.
Fig. 9. FTIR spectra of (a) TOP and the samples after the process of (b) pyrolysis, (c) decarbonisation and (d) HF treatment under different pyrolysis temperatures fixed at 0.1TEOS.
vapour–liquid–solid (VLS) mechanism, which is normally observed with metallic droplets at the tips or bases of nanowires [1,39]. Therefore, taking into consideration of the results and the free energies, the following mechanism was proposed. The free energies (G) were calculated at atmospheric pressure and at a temperature (T) of 1300 1C using the equation GT ¼HT T ST, where H and S represent enthalpy and entropy, respectively. The properties were obtained from NIST-JANAF thermochemical tables [40].
When the TEOS-filled TOP fibres were pyrolyzed, the organic compounds inside TOP decomposed into amorphous carbon. As the temperature was raised beyond 600 1C, TEOS started to decompose according to Eq. (1). SiðOC2 H5 Þ4 ðlÞ-SiO2 ðsÞ þ 2OðC 2 H5 Þ2 ðlÞ
ð1Þ
The thermal decomposition of TEOS was basically a rearrangement process, converting TEOS into SiO2 and diethylether [21].
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Fig. 10. EFTEM images of the nanostructures formed under different pyrolysis temperatures of (a) 1250 1C, (b) 1300 1C, (c) 1350 1C and (d) 1400 1C. (e) Shows the composition of the nanowires and particles using ESI and (f) shows the diameter distribution of the nanostructures formed under different pyrolysis temperatures.
Fig. 11. (a) FESEM micrograph showing the origin of an individual nanowire growing from a cluster and the corresponding EDS results of the cluster at the base of nanowire in (b).
The volatile diethylether was burned off while the SiO2 deposited on the TOP surface. The intimate contact between amorphous carbon and SiO2, either from inherent SiO2 particles or SiO2 from TEOS, allowed the direct reaction between both components, even though it was not thermodynamically favourable in terms of free energy. It had been reported that the solid state reaction between SiO2 and C predominated at temperatures 1300–1900 1C when the SiO2 and C were in close contact [41,42]. The reactions
between the two components are shown as follow. SiO2 ðsÞ þ 3CðsÞ-SiCðsÞ þ 2COðgÞ, SiO2 ðsÞ þ CðsÞ-SiOðgÞ þCOðgÞ,
DG ¼ 70:16 kJ=mol
ð2Þ
DG ¼ 147:865 kJ=mol
ð3Þ
The SiC species, formed from the solid-state reaction in Eq. (2), would form SiC clusters on the TOP surface. The formation of SiC nanowires were then further promoted by gaseous reactions
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through vapour–solid (VS) growth involving the SiO and CO gaseous species produced from Eq. (3). The SiO species could either react with the amorphous C on the TOP surface or the CO species to produce SiC, as shown in Eqs. (4)–(6). Eq. (6) showed that SiO2 was formed along with SiC during the reaction between SiO and C, which was commonly observed as amorphous layer surrounding SiC in nanowires produced using carbothermal reduction of SiO2 [43–46]. However, no oxide layers were observed in the nanowires through EFTEM in this study due to HF leaching that removed the oxide layers while removing excess SiO2 in the fibres. SiOðgÞ þ2CðsÞ-SiCðsÞ þ COðgÞ,
DG ¼ 77:71 kJ=mol
2SiOðgÞ þ3CðsÞ-2SiCðsÞ þ CO2 ðgÞ, 3SiOðgÞ þCOðgÞ-SiCðsÞ þ 2SiO2 ðsÞ,
DG ¼ 51:77 kJ=mol DG ¼ 1273:44 kJ=mol
ð4Þ ð5Þ ð6Þ
The SiC formed were adsorbed into the SiC clusters, which later acted as nuclei for the growth of SiC nanowires. Once SiC supersaturated in the clusters, SiC nanowires precipitated out and grew in length as the reactants were adsorbed onto the tips of nanowires. A continuous supply of CO was replaced by the reaction as follow. CO2 ðgÞ þ CðsÞ-2COðgÞ,
DG ¼ 103:648 kJ=mol
ð7Þ
4. Conclusion This study shows the possibility of growing SiC nanowires using oil palm empty fruit bunch fibres as carbon source and TEOS as silicon source. The SiC nanowires were mainly consisted of cubic 3C–SiC with small amount of 2H–SiC structure. An increase in TEOS concentration from 10% to 50% caused a slight increase in silica content but further increase in TEOS concentration to 100% caused a drop in silica content due to lower fluidity of the viscous solution and lower capillary action. This in turn resulted in lower yield of SiC nanowires at higher TEOS concentration due to lack of reactant species. When pyrolysis temperature was increased, the nanowires were found increasing in amount but the diameters decreased due to higher surface diffusion rate that led to desorption of adsorbed atoms at sidewalls to the tips of nanowires. The growth of nanowires was formed from the precipitation of SiC nanowires from clusters through solid-state reactions and vapour–solid growth.
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Physica E journal homepage: www.elsevier.com/locate/physe
Growth of SiC nanowires using oil palm empty fruit bunch fibres infiltrated with tetraethyl orthosilicate Y.L. Chiew, K.Y. Cheong n Energy Efficiency & Sustainable Semiconductor Research Group, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
H I G H L I G H T S c
c
c
c
Ability to grow SiC nanowires by pyrolysing TEOS-infiltrated oil palm fibre waste. Observation of lower yield of SiC nanowires at higher TEOS concentration. A decreasing trend in diameter with increasing pyrolysis temperature. Nanowire growth attributed to combination of solid-state and vapour– solid reactions.
G R A P H I C A L
A B S T R A C T
SiC nanowires formed by pyrolyzing TEOS-infiltrated oil palm empty fruit bunch fibres.
a r t i c l e i n f o
abstract
Article history: Received 7 May 2012 Received in revised form 10 June 2012 Accepted 11 June 2012 Available online 29 June 2012
SiC nanowires were produced by pyrolyzing oil palm empty fruit bunch fibres infiltrated with tetraethyl orthosilicate. The effects of the concentration of TEOS (10%, 50% and 100%) and the pyrolysis temperatures (1250 1C, 1300 1C, 1350 1C and 1400 1C) were studied. An increase in TEOS concentration led to an increment in silica content. However, when TEOS was infiltrated into the fibres, the small sizes of the lumens in the oil palm fibres and low fluidity of TEOS resulted in lower amount of silica deposited onto the surface when the concentration was increased to 100%. This in turn resulted in a lower yield of SiC nanowires at higher TEOS concentration. When pyrolysis temperature was raised, there was a decrease in diameter but the lengths of nanowires reached tens of mm. The growth of the nanowires was attributed to the combination of solid-state reaction and vapour–solid growth mechanisms. & 2012 Elsevier B.V. All rights reserved.
Keywords: SiC Nanowires Oil palm empty fruit bunch fibres Tetraethyl orthosilicate
1. Introduction Silicon carbide (SiC) nanowires have been attracting attentions due to their excellent thermal, mechanical, chemical and electronic properties. Their excellent properties made them applicable in various fields from acting as reinforcements in ceramic matrix composite to areas of nanoelectronics, nanooptics and nanosensors [1–3]. Various methods have been developed in synthesising SiC nanowires, including chemical vapour deposition (CVD) [4–7], carbothermal reduction [8–11], laser ablation [12], chemical n
Corresponding author. Tel.: þ60 4599 5259; fax: þ 60 4594 1011. E-mail address: [email protected] (K.Y. Cheong).
1386-9477/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2012.06.008
vapour growth (CVG) [13] and thermal evaporation [3,14–16]. Carbothermal reduction is a common method in producing SiC nanowires and the usage of agricultural wastes as source materials is a potential route with wastes such as rice husks, bamboo, sugarcane leaf with rice straw, bean–curd refuse and cotton fibre, as reviewed in previous article [17]. Although various waste materials had been used in the synthesis of SiC nanowires, oil palm fibres had not been used. Oil palm empty fruit bunch fibres was chosen as a carbon source in the production of SiC nanowires due to their high carbon content and low ash content [18]. The amorphous nature of carbon present and the porous structure of the fibres also make the carbon highly reactive. In addition, these low cost wastes are abundant in Malaysia with annual production of 20 million tonnes per year [19].
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Although the oil palm fibres contain inherent SiO2, it was insufficient for SiC formation reactions to take place. This was validated by a control experiment performed on oil palm fibres alone pyrolyzed at 1300 1C for 1 h that showed no nanostructures were formed. This shows that an external source of SiO2 is necessary and tetraethyl orthosilicate (TEOS) is a suitable choice of silicon source. In previous study, rice husk ash was used as a silicon source [20] though it was found that the yield of nanowires was significantly lower and a higher growth temperature was needed to initiate the growth, as compared to those produced using TEOS. TEOS is a chemical compound with the formula Si(OC2H5)4 that normally exist in liquid form and could be converted into SiO2 easily at elevated temperatures above 600 1C [21]. TEOS is also a cheap source of Si. The use of TEOS as silicon source in the synthesis of SiC nanowires had been reported previously though the carbon source differed, with Jute fibres [22], bamboo slices [23] and polyvinylpyrrolidone (PVP) [24]. It had also been reported that TEOS was used as both silicon and carbon source in the formation of SiC nanowires [25]. In these studies, the nanowires formed were of 3C–SiC and had large diameters (40–200 nm) but shorter lengths (10–20 mm) [23,25], as well as additional processing was required in converting TEOS into SiO2 by hydrolysis and alcohol condensation [22,24]. In this work, 3C–SiC nanowires were grown by pyrolyzing acid-treated oil palm empty fruit bunch fibres (TOP) infiltrated with TEOS in a horizontal tube at temperatures of 1250–1400 1C for 1 h using different concentrations of TEOS (10%, 50% and 100%) under argon atmosphere. The effects of TEOS concentration and pyrolysis temperatures on the growth of nanowires were studied. The growth mechanism of the nanowires was also discussed.
2. Materials and methods Oil palm empty fruit bunch fibres in the present study were obtained from local mill in Nibong Tebal, Malaysia. The oil palm fibres were first sieved and water-washed to remove dirt. They were next subjected to acid leaching using 1 M hydrochloric acid (HCl) heated at 100 1C for 1 h to remove metallic impurities and to weaken the carbohydrate bondings in the fibres [26]. X-ray fluorescence (XRF, Casio) and CHNS Analyser (Perkin Elmer 2400) were performed on the raw oil palm fibres and the acid-treated oil palm fibres to determine the chemical compositions. The reduction of impurities after acid treatment could be proven by the (XRF) results in Table 1. The acid-treated oil palm fibres were then designated as TOP. The TOP fibres were then cut into lengths of approximately 1 cm before being infiltrated with TEOS. The different concentrations of TEOS solution (10%, 50% and 100%) were prepared by diluting the TEOS with ethanol (95%). The samples after TEOS Table 1 Chemical compositions of OP, TOP, 0.1TEOS, 0.5TEOS and 1.0TEOS. Composition
OP
TOP
TOP-0.1TEOS TOP-0.5TEOS TOP-1.0TEOS
SiO2 MgO Al2O3 P2O5 SO3 K2O CaO MnO Fe2O3 LOI Of which is C
0.653 0.076 0.050 0.037 0.080 0.167 0.202 0.013 0.407 98.045 47.11
1.335 0.004 0.047 0.021 0.171 0.059 0.007 – 0.037 98.253 45.56
4.773 0.311 0.026 0.003 0.063 0.003 0.395 0.002 0.365 94.050 45.56
4.941 0.143 0.023 0.003 0.053 0.004 0.192 0.001 0.170 94.457 45.56
3.829 0.058 0.017 0.005 0.056 0.003 0.073 – 0.065 95.880 45.56
infiltration were assigned as 0.1TEOS, 0.5TEOS and 1.0TEOS, according their respective TEOS concentration. During the infiltration process, TOP fibres were immersed in the TEOS solutions and evacuated in a vacuum chamber by a rotary pump. The infiltration process was performed in a vacuum condition in order to remove the air inside the fibres as much as possible and to ensure the impregnation of TEOS into the pores of the individual fibres by capillary action [22]. After infiltration process, the fibres were dried in an oven at 70 1C for 24 h. The dried TEOS-filled TOP fibres were subsequently placed in an alumina crucible that would be closed with an alumina lid during pyrolysis process. Pyrolysis was then performed in a horizontal tube furnace (Model STF 15/180/3216P1, Carbolite Furnace Ltd., UK) at temperatures of 1250–1400 1C for 1 h under argon flow of 50 sccm and heating rate of 10 1C/min from room temperature to the pyrolysis temperature. After pyrolysis, the excess carbon in the remaining pyrolysis product was removed by burning in a muffle furnace at 700 1C for 3 h, followed by removal of excess SiO2 using 40% hydrofluoric acid (HF). Finally, the products were collected and characterised. A fieldemission scanning electron microscope (FESEM, LEO GEMINI) equipped with an energy dispersive X-ray spectrometer (EDS, ZEISS SUPRATM 35VP) was used to examine the samples after pyrolysis. An energy-filtered transmission electron microscope (EFTEM, ZEISS LIBRA 120) equipped with electron spectroscopic imaging (ESI) was used to examine the sample after the decarbonisation and HF leaching processes. The samples for EFTEM were dispersed in absolute ethanol and the suspension was dripped on a copper grid covered with holey carbon film. The samples were also tested by Fourier Transform Infrared Spectrophotometer (Perkin-Elmer System 2000) under transmission mode. The samples were prepared using KBr pellet technique. Approximately 0.1 mg samples were ground with 50 mg KBr in an agate mortar and compacted using a hydraulic press.
3. Results and discussion 3.1. Effect of TEOS concentration Typical FESEM micrographs of the TOP fibres before and after TEOS infiltration at different concentrations are presented in Fig. 1. They showed that TEOS was found in the lumens of the TOP fibres after infiltration and the amount increased as the TEOS concentration was increased. The chemical composition in Table 1, as obtained using XRF and CHNS Analyser, showed that there was an increase in SiO2 content when TEOS concentration was increased from 10% to 50% but the SiO2 content decreased when the concentration was further increased to 100%. This observation might be attributed to the better fluidity at lower concentration since TEOS was diluted by ethanol, allowing it to be infiltrated into the fibres easily [22]. Thus, the fibres treated with 50% TEOS might have higher SiO2 content due to the dilution process, making infiltration of TEOS into the fibres easier. As for the low SiO2 content in the sample treated with 100% TEOS, it might be because of the low fluidity of the viscous TEOS solution, reducing the ability of the TEOS to penetrate the lumens of the fibres due to low capillary action. Therefore most of the TEOS molecules were just deposited onto the surfaces of TOP, instead of inside the lumens, as proven by the FESEM images. These TEOS molecules on the TOP surface could be easily removed during transfer and this might led to lower SiO2 content. After pyrolysis, the TOP fibres appeared charred and coated with blue layers. After decarbonisation, whitish depositions were observed instead, as shown in Fig. 2. Maximum yields were found at lower concentrations of TEOS and only small specks of black
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Fig. 1. FESEM micrographs of TOP fibres (a) before TEOS infiltration and after TEOS infiltration with (b) 10% TEOS, (c) 50% TEOS and (d) 100% TEOS.
Fig. 2. Appearance of the pyrolyzed TEOS-filled TOP samples with (a)–(c) showing the samples after pyrolysis and (d)–(f) showing the samples after decarbonisation for 0.1TEOS, 0.5TEOS and 1.0TEOS, respectively.
particles were observed in the samples after decarbonisation. FESEM micrographs in Fig. 3 showed that the blue layers on TOP fibre surfaces were nanowires. With increasing concentration, the yield of nanowires on the TOP fibre surfaces was decreased. From the higher magnification micrographs [Fig. 3(d)–(f)], the nanowires were found to be a combination of curled and straight nanowires. EDS results showed that these nanowires were consisted of Si, C and O.
Fig. 4 shows the FTIR spectra of the TEOS-filled TOP fibres after each processing under different TEOS concentration and the species associated with the adsorption bands are shown in Table 2 [27–31]. The disappearance of peaks associated with CH, CH2, CH3, Si(CH)3 and C(CH)3 after pyrolysis indicated decomposition of the cellulose and lignine components, transforming from aliphatic compounds into aromatic compounds of CQC and CQO [31]. The strong Si–O peak (1200 1000 cm 1) in the
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Fig. 3. FESEM micrographs of pyrolyzed TEOS-filled TOP samples at different TEOS concentrations of (a) 0.1TEOS, (b) 0.5TEOS and (c) 1.0TEOS at low magnification and (d)–(f) along with the insets show higher magnification micrographs of the nanowires at concentrations of 0.1TEOS, 0.5TEOS and 1.0TEOS, respectively.
Fig. 4. FTIR spectra of (a) TOP and after each process of (b) pyrolysis, (c) decarbonisation and (d) HF treatment under different TEOS concentrations.
spectra was a result of superposition of Si–O and C–OH bonding [28,29]. After decarbonisation, peaks associated with Si–O and Si–C became more prominent due to removal of most organic
compounds. However, small peaks of CQO were still detected because of the black carbon particles encapsulated by the melted SiO2 during SiO2 surface melting [32]. The SiO2 melt protected the
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Table 2 Absorption bands in FTIR spectra and their assignments. Regions (cm 1)
Assignment of surface functional groups
Ref.
3800–3100
Valent vibrations of O–H bond in water molecules bonded by hydrogen bonds OH groups present in cellulose, hemicellulose and lignine CQO carbonyl groups CQC groups Deformation vibrations of the water molecules (d–H2O) CQO (aromatic) groups C–H, CH2, CH3, Si(CH)3 and C(CH)3 groups Unsaturated C–C and C–O skeletal vibrations Si–O stretching Vibrations of C–OH bond Si–C–O bonding Si–O vibrations Si–C stretching Free SiO2 (O–Si–O bending vibrations)
[27,29,31]
1750–1700 1650–1500 1640 1500–1415 1500–1200 1360–1310 1200–1000 933 898–825 825–782 486–464
[29–31] [29–31] [29] [31] [27,31] [31] [28,29,31] [33] [31] [31] [28,31]
Fig. 5. EFTEM images of the nanowires formed in (a) 0.1TEOS, (b) 0.5TEOS and (c) 1.0TEOS. (d) Shows the composition of nanowires determined by ESI and (e) shows the diameter distribution of the nanowires at different TEOS concentrations.
carbon particles from oxidation during decarbonisation by preventing direct contact between carbon and oxygen. This could be validated by the observation of small specks of black particles in the samples after decarbonisation. After HF leaching, the Si–C peaks (825–782 cm 1) became stronger while the Si–O peaks (1200–1000 cm 1, 486–464 cm 1) weakened due to removal of excess of SiO2 but not completely removed due to incomplete reaction. An additional peak at 933 cm 1 had been found to associated with Si–C–O bonding and this bonding was normally
found in SiC with larger grain size [33]. When the TEOS concentration was increased, there was less effect on the Si–O peaks since the SiO2 content in the three samples were almost similar. However, the Si–C peak was the strongest in the sample treated with 10% TEOS, suggesting a high SiC yield. The Si–C peaks then became broader with increasing TEOS concentration, indicating a wide range of diameters in the SiC nanowires [33]. EFTEM images in Fig. 5(a)–(c) show that the nanowires consisted of single layer structures with presence of stacking
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faults and microtwins after HF leaching. The nanowires observed in the sample treated with 10% TEOS and 50% TEOS had smooth sidewalls, but the nanowires produced from the sample treated with 100% TEOS had rough sidewalls. The nanowires had average diameters between 25 and 35 nm with lengths up to tens of mm. The diameter distribution of nanowires in Fig. 5(e) showed an increase in the average diameter and the range of diameters were wider in the sample treated with 100% TEOS, which was in accordance with FTIR results showing a wide Si–C band. The ESI results [Fig. 5(d)] showed that the nanowires consisted of Si and C only. A representative XRD spectrum of the nanowires formed after HF treatment in Fig. 6 showed diffraction peaks of Moissanite 3C– SiC (ICDD 01-073-1665) and a small diffraction peak of Moissanite 2H–SiC (ICDD 01-089-2213), indicating that the SiC nanowires were consisted of two polytypes with the main polytype as cubic 3C–SiC and a side-product of hexagonal 2H–SiC. Thus, combining the results from FTIR, ESI and XRD, it could be concluded that the nanowires were of SiC. 3.2. Effect of pyrolysis temperature To study the effect of pyrolysis temperature on the growth of SiC nanowires using TEOS-filled TOP fibres, the TEOS concentration was fixed at 10% since it was able to produce maximum yield. After pyrolysis, the amount of blue depositions on the charred fibres increased with increasing temperature from 1250 1C to 1350 1C but the blue deposition decreased in amount at 1400 1C [Fig. 7(a)–(d)]. After decarbonisation, the blue depositions changed to whitish depositions with decreasing amount of black particles. It was also observed that maximum yield was obtained at 1350 1C after decarbonisation [Fig. 7(e)–(h)]. FESEM micrographs of the pyrolyzed samples at different temperatures are shown in Fig. 8. The lower magnification micrographs show that the TOP fibres were covered with nanowires and were consisted of Si and C based on EDS results. The nanowire density on the fibres increased with temperature and became thicker. From the higher magnification images [Fig. 8(e)–(h)], it could also be observed that the diameters of the nanowires reduced with increasing temperature. Fig. 9 shows the FTIR spectra of the TEOS-filled TOP fibres after each processing at different pyrolysis temperatures. The FTIR spectra showed similar absorption bands as listed in Table 2. When the pyrolysis temperature was increased, the peaks associated with Si–O (1200–1000 cm 1) decreased in intensity because the increase in temperature provided the extra energy for higher reaction rate between SiO and CO. The SiC peaks were also observed to become broader but decreased in intensity with increasing temperature. The SiC peaks were more prominent at temperatures of 1300–1350 1C, indicating a high yield of SiC in
Fig. 7. Appearance of the pyrolyzed TEOS-filled TOP samples fixed at 0.1TEOS with (a)–(d) showing the samples after pyrolysis and (e)–(h) showing the samples after decarbonisation at pyrolysis temperatures of 1250 1C, 1300 1C, 1350 1C and 1400 1C, respectively.
these samples. The broad band at 1400 1C might be a result of the transformation of nanowires into particles that were larger in size, as proven by the detection of SiC particles in EFTEM results. The EFTEM images in Fig. 10(a)–(c) showed the presence of nanowires only at temperatures from 1250–1350 1C but at 1400 1C, particles started to appear aside from nanowires and ESI showed that the nanowires along with particles in micron size consisted of Si and C [Fig. 10(e)]. So, SiC particles were also produced at higher temperature. The formation of particles was reported to be a result of coagulative recrystallisation of nanowires due to higher reaction rate in producing SiC at higher temperatures [34,35]. According to the diameter distribution in Fig. 10(f), a decrease in the diameters of nanowires was observed as opposed to the common trend of an increase in diameter with increasing temperature [36,37]. The decrease in diameter might be attributed to the higher surface diffusion rate of the adsorbed species on the sidewalls to the tips that were more energetically favourable [38]. The increase in the diffusion rate was made possible by the extra energy provided by higher temperature. This tendency contributed to thinner nanowires with increasing length, as noticed in the study here. 3.3. Growth mechanism
Fig. 6. A representative XRD spectrum of the nanowires after HF treatment.
To determine how the nanowires formed, the origin of the nanowire was observed. Fig. 11(a) shows the FESEM micrographs of an individual nanowire growing from a cluster that acted as a nucleus and the EDS results in Fig. 11(b) showed that the cluster consisted of Si, C and O only without any metallic elements. The lack of metallic impurities in the nanowires helped to rule out the
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Fig. 8. FESEM micrographs of pyrolyzed TEOS-filled TOP samples fixed at 0.1TEOS at different pyrolysis temperatures of (a) 1250 1C, (b) 1300 1C, (c) 1350 1C and (d) 1400 1C. Insets show the higher magnification micrographs of the pyrolyzed TEOS-filled TOP samples.
Fig. 9. FTIR spectra of (a) TOP and the samples after the process of (b) pyrolysis, (c) decarbonisation and (d) HF treatment under different pyrolysis temperatures fixed at 0.1TEOS.
vapour–liquid–solid (VLS) mechanism, which is normally observed with metallic droplets at the tips or bases of nanowires [1,39]. Therefore, taking into consideration of the results and the free energies, the following mechanism was proposed. The free energies (G) were calculated at atmospheric pressure and at a temperature (T) of 1300 1C using the equation GT ¼HT T ST, where H and S represent enthalpy and entropy, respectively. The properties were obtained from NIST-JANAF thermochemical tables [40].
When the TEOS-filled TOP fibres were pyrolyzed, the organic compounds inside TOP decomposed into amorphous carbon. As the temperature was raised beyond 600 1C, TEOS started to decompose according to Eq. (1). SiðOC2 H5 Þ4 ðlÞ-SiO2 ðsÞ þ 2OðC 2 H5 Þ2 ðlÞ
ð1Þ
The thermal decomposition of TEOS was basically a rearrangement process, converting TEOS into SiO2 and diethylether [21].
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Fig. 10. EFTEM images of the nanostructures formed under different pyrolysis temperatures of (a) 1250 1C, (b) 1300 1C, (c) 1350 1C and (d) 1400 1C. (e) Shows the composition of the nanowires and particles using ESI and (f) shows the diameter distribution of the nanostructures formed under different pyrolysis temperatures.
Fig. 11. (a) FESEM micrograph showing the origin of an individual nanowire growing from a cluster and the corresponding EDS results of the cluster at the base of nanowire in (b).
The volatile diethylether was burned off while the SiO2 deposited on the TOP surface. The intimate contact between amorphous carbon and SiO2, either from inherent SiO2 particles or SiO2 from TEOS, allowed the direct reaction between both components, even though it was not thermodynamically favourable in terms of free energy. It had been reported that the solid state reaction between SiO2 and C predominated at temperatures 1300–1900 1C when the SiO2 and C were in close contact [41,42]. The reactions
between the two components are shown as follow. SiO2 ðsÞ þ 3CðsÞ-SiCðsÞ þ 2COðgÞ, SiO2 ðsÞ þ CðsÞ-SiOðgÞ þCOðgÞ,
DG ¼ 70:16 kJ=mol
ð2Þ
DG ¼ 147:865 kJ=mol
ð3Þ
The SiC species, formed from the solid-state reaction in Eq. (2), would form SiC clusters on the TOP surface. The formation of SiC nanowires were then further promoted by gaseous reactions
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through vapour–solid (VS) growth involving the SiO and CO gaseous species produced from Eq. (3). The SiO species could either react with the amorphous C on the TOP surface or the CO species to produce SiC, as shown in Eqs. (4)–(6). Eq. (6) showed that SiO2 was formed along with SiC during the reaction between SiO and C, which was commonly observed as amorphous layer surrounding SiC in nanowires produced using carbothermal reduction of SiO2 [43–46]. However, no oxide layers were observed in the nanowires through EFTEM in this study due to HF leaching that removed the oxide layers while removing excess SiO2 in the fibres. SiOðgÞ þ2CðsÞ-SiCðsÞ þ COðgÞ,
DG ¼ 77:71 kJ=mol
2SiOðgÞ þ3CðsÞ-2SiCðsÞ þ CO2 ðgÞ, 3SiOðgÞ þCOðgÞ-SiCðsÞ þ 2SiO2 ðsÞ,
DG ¼ 51:77 kJ=mol DG ¼ 1273:44 kJ=mol
ð4Þ ð5Þ ð6Þ
The SiC formed were adsorbed into the SiC clusters, which later acted as nuclei for the growth of SiC nanowires. Once SiC supersaturated in the clusters, SiC nanowires precipitated out and grew in length as the reactants were adsorbed onto the tips of nanowires. A continuous supply of CO was replaced by the reaction as follow. CO2 ðgÞ þ CðsÞ-2COðgÞ,
DG ¼ 103:648 kJ=mol
ð7Þ
4. Conclusion This study shows the possibility of growing SiC nanowires using oil palm empty fruit bunch fibres as carbon source and TEOS as silicon source. The SiC nanowires were mainly consisted of cubic 3C–SiC with small amount of 2H–SiC structure. An increase in TEOS concentration from 10% to 50% caused a slight increase in silica content but further increase in TEOS concentration to 100% caused a drop in silica content due to lower fluidity of the viscous solution and lower capillary action. This in turn resulted in lower yield of SiC nanowires at higher TEOS concentration due to lack of reactant species. When pyrolysis temperature was increased, the nanowires were found increasing in amount but the diameters decreased due to higher surface diffusion rate that led to desorption of adsorbed atoms at sidewalls to the tips of nanowires. The growth of nanowires was formed from the precipitation of SiC nanowires from clusters through solid-state reactions and vapour–solid growth.
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