Catalyst-Free Synthesis of Multiwalled Carbon Nanotubes via

Article pubs.acs.org/IECR

Catalyst-Free Synthesis of Multiwalled Carbon Nanotubes via Microwave-Induced Processing of Biomass Kaiqi Shi,† Jiefeng Yan,† Edward Lester,‡ and Tao Wu*,† †

Municipal Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, Ningbo, 315100, China ‡ Department of Chemical and Environmental Engineering, The University of Nottingham, Nottingham NG7 2RD, U.K. ABSTRACT: Carbon nanotubes (CNTs) have been the focus of research in the past two decades due to their fascinating properties and significant potential for a range of applications, from electronics to high performance polymers. In this research, multiwalled CNTs with a diameter of 50 nm and a wall thickness around 5 nm were successfully prepared via microwave-induced pyrolysis of gumwood at 500 °C. The mechanism for the growth of such CNTs is under microwave irradiation. Volatiles were released from the biomass and left behind char particles. These char particles then acted as substrates, mineral matter in char particles (originating from biomass) acted as the catalyst, and the volatiles released acted as the carbon source gas. The volatiles were then undergoing thermal and/or catalytic cracking on the surface of char, forming the amorphous carbon nanospheres; the carbon nanospheres then subsequently self-assembled into multiwalled CNTs under the effects of microwave irradiation.

1. INTRODUCTION

reactors, all of which complicate the development of large-scale production and utilization of CNT products. In this paper, we report how MWCNTs can be synthesized at a lower temperature of 500 °C from biomass without the use of any external catalyst, substrate, or source gases. The new mechanism of CNT synthesis via the microwave-induced pyrolysis (MWP) of lignocellulosic biomass is also proposed.

Carbon nanotubes (CNTs) have some highly significant properties, such as relative strength and conductivity, which make them fascinating materials with significant potential in a range of applications. Much effort has been made to control and scale up the synthesis of carbon nanomaterials in the past few decades.1,2 Arc discharge, chemical vapor deposition, and catalytic decomposition of carbon feedstock are the three major approaches for the synthesis of CNTs.3 However, in recent years, microwave irradiation, creating a high-energy-density plasma, has also been shown to enhance the preparation process of nanomaterials such as carbon nanofibers,4 carbon nanowalls,5 graphene flakes,6 and CNTs.7,8 Metallic elements, such as Fe, Co, Ni, and their alloys, tend to be used as catalysts for the synthesis of CNTs via high temperature catalytic decomposition of hydrocarbons under conventional heating.9 Normally, substrates (such as Si, activated carbon, and Al2O3) are necessary for the conventional preparation of CNTs10 using carbon source gases, such as CH4, C2H4, and C2H2, to effectively form a growing CNT deposit on the substrates.11 Furthermore, catalyst-free self-assembling multiwalled CNTs (MWCNTs) have been produced on substrates of carbon black and iron coated silica.12 Compared with single-walled carbon tubes (SWCNTs), MWCNTs have multiwalled structures and more defects resulting in their unique application as electrode materials, additives for composites, functionalized materials, etc. Much research has focused on the use of vegetable oils, fibrous plant tissues, and seeds as cost-effective plant-derived precursors for the synthesis of CNTs.3,13 Carbonized iron-rich biomass, such as mushrooms, seaweed, black fungus, and black sesame seed, have also been tried as precursors for CNT preparation.14 Despite these efforts, the synthesis of CNTs using renewable material sources is still a complicated process of some challenges in terms of the selection of precursors, catalysts, substrates, and source gases simultaneously in certain © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. In this study, gumwood obtained from Huzhou (Zhejiang Province, China) was used as the feedstock. The sample was air-dried and ground to sub-212 μm following standard milling procedures.15 Silicon carbide (SiC; Sino Reagent, product code 20035660, purity >98.5%) was added as a microwave receptor (to increase the rate of heating during processing), with a particle above 212 μm to allow easy separation from the product after pyrolysis. Dichloromethane (CH2Cl2; Sigma, product code 650463, CHROMASOLV Plus grade) was used as a solvent for the collection of pyrolytic biooil. 2.2. Microwave Synthesis Methodology. Microwaveinduced pyrolysis of gumwood was carried out in a 2.45 GHz multimode microwave cavity (Nanjing Jiequan Microwave Co., Ltd., China, shown in Figure 1). The temperature-controlled mode was used with a fixed microwave power input of 300 W. The pyrolysis temperature was set up as 500 °C and maintained for 30 min. The power was switched off automatically when the temperature exceeded 500 °C. About 5.0 g of biomass was mixed with SiC with a mass ratio to biomass of 20:1. A nitrogen purge was at a flow rate of 100 mL·min−1 to maintain an oxygen-free atmosphere. CNTs were grown on the surface of Received: Revised: Accepted: Published: 15012

August 1, 2014 September 2, 2014 September 10, 2014 September 10, 2014 dx.doi.org/10.1021/ie503076n | Ind. Eng. Chem. Res. 2014, 53, 15012−15019

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determined by subtracting the weight of ash from the amount of solid that remained after burning. 2.5. Characterization of Pyrolytic Byproducts. The surface areas of the chars produced via conventional pyrolysis and via microwave-induced pyrolysis of biomass were measured using an ASAP-2000 (Micrometrics Corp., USA). The thermal stability of the pyrolytic solid products was characterized using thermogravimetric analysis with an air purge at a flow rate of 20 mL·min−1. The heating ramp was from 105 to 1200 °C with a heating rate of 20 °C·min−1. Furthermore, the pyrolytic liquid product, bio-oil, was analyzed using a gas chromatograph−mass spectrometer (GC−MS; Agilent 7890−5975C, USA) with an HP-5ms capillary column to determine the contents of phenol, naphthalene, benzoic acid, benzene, alkene, cedrol, ester, glucose, and their derivatives in bio-oil. The pyrolytic gas product, biogas, was analyzed using a gas chromatograph (GC; Agilent 6890, USA) to find out the contents of light hydrocarbons such as hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4).

Figure 1. Schematic diagram of microwave-induced processing of gumwood.

pyrolytic solid product. Solid, liquid, and gas products were collected separately for further characterization. Meanwhile, conventional pyrolysis (CP) was also carried out in a vertical tube furnace at the same temperature and the same N2 flow rate in order to compare products with those generated from microwave-induced processing of gumwood. 2.3. Biomass Characterization. Proximate analysis of gumwood was carried out using a thermogravimetric analyzer (TGA; NETZSCH STA449F3, Germany), following the procedures described elsewhere.14 Carbon, hydrogen, nitrogen, and sulfur contents of the biomass samples were determined using a CHNS/O element analyzer (PerkinElmer 2400, USA). The cellulose, hemicellulose, and lignin contents of gumwood were determined via acid detergent fiber (ADF), neutral detergent fiber (NDF), and acid detergent lignin (ADL) methods, respectively.16 Samples of gumwood ash were prepared with a low temperature plasma asher (PR300, Yamato, Japan), and the elemental composition was characterized using an energy dispersive X-ray spectrometer (EDS; Oxford Instruments, U.K.). 2.4. Characterization of CNTs. The morphology and microstructure of the solid pyrolytic products, biochars including CNTs, were characterized using a scanning electron microscope (SEM; Sigma VP, Zeiss, Germany, 5 keV) and a high-resolution transmission electron microscope (HRTEM; JEM-2100F, JEOL, Japan, 200 kV). The solid samples were also heated in an SEM sample cavity on a heating stage (Kammrath & Weiss GmbH, Germany) to reveal the morphological change of CNTs upon heating to 1200 °C with a heating rate of 50 °C· min−1. The sample for TEM analysis was obtained by drying droplets of the sample (suspended in ethanol) onto a Cu grid coated with a carbon film, which was then dried prior to imaging. The composition of CNTs was characterized using EDS attached to the SEM and TEM. The crystallinity of the CNT sample was analyzed using X-ray diffraction (XRD; D8 Advance A25, Bruker, Germany) with a Cu X-ray tube (λ 1.5406 Å) and LynxEye detector. The 2θ was ranged from 10 to 100° with a step distance of 0.01°. Raman spectroscopy (Renishaw, inVia-reflex, U.K.) equipped with a 532 nm laser diode was used to identify any MWCNTs on the surface of pyrolytic chars. To determine the yield of CNTs, microwave-induced pyrolytic char, a mixture of amorphous char mixed with CNTs, was burned in air at a temperature of 555 °C for 3 h. At such a temperature all amorphous char was burned. CNTs remained unchanged and were mixed with ash originated from biomass. Since the amount of ash in char was determined via proximate analysis of biomass, the yield of CNTs was therefore

3. RESULTS AND DISCUSSION 3.1. Characterization of Gumwood. Table 1 shows the results of proximate, ultimate, and lignocellulosic analyses of Table 1. Proximate, Ultimate, and Lignocellulosic Analyses of Gumwood proximate analysis (wt %) moisture volatiles fixed carbon ash ultimate analysisa,b (wt %) carbon hydrogen oxygenc nitrogen sulfur lignocellulosic analysisa (wt %) cellulose hemicellulose lignin other a

2.1 86.0 11.8 0.1 47.1 6.3 43.5 2.1 1.0 48.9 28.4 14.3 8.4

Dry basis. bAsh free basis. cBy difference.

gumwood. Gumwood is a biomass with a high volatiles content (86 wt %) and low ash content (0.1 wt % dry basis). The carbon and oxygen contents are almost the same, around 45 wt %. The total percentage of cellulose and hemicellulose is 77 wt %, and can be decomposed easily at a temperature between 300 and 350 °C.17 3.2. Characterization of CNTs. Figure 2a shows an SEM image of a pyrolytic char from conventional heating. After pyrolysis, some original pores, fibers, and carbon structure can be observed but no CNTs are present on the surface of the conventional pyrolyzed char. Parts b, c, and d of Figure 2 are the SEM images of microwave-induced pyrolytic chars, which are very different from conventional pyrolytic chars in terms of morphology and microstructure. In Figure 2b, there are a number of nanowire materials grown on the surface of the pyrolytic char particle. Figure 2c,d is at a higher magnification for the same fibers. It is clear from Figure 2c that these fibers have a diameter between 50 and 100 nm and a length ranging 15013

dx.doi.org/10.1021/ie503076n | Ind. Eng. Chem. Res. 2014, 53, 15012−15019

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The XRD pattern obtained from microwave-induced pyrolytic char with CNTs is presented in Figure 4. It is well-

Figure 2. SEM images of pyrolytic char produced via (a) conventional pyrolysis at 500 °C for 30 min and (b−d) microwave-induced processing at 500 °C for 30 min, where SiC was used as microwave energy absorber. (c) Close-up view of carbon nanofiber materials. (d) Higher magnification of a region in (c).

Figure 4. X-ray diffraction pattern for microwave-induced pyrolytic char with CNTs.

known that pure CNTs exhibit two typical peaks at 2θ = 26 and 43°, corresponding to the graphite (002) and (100) reflections.22 In Figure 4, there are two major diffraction peaks at 2θ = 23.1 and 43.2°, which are attributed to graphitelike (002) and (100) structures, respectively.23 According to Bragg’s law (2d sin θ = λ), the d-spacing for CNTs is calculated to be 0.385 nm (2θ = 23.1°), which is larger than that of pure graphite (0.335 nm). This value matches well with the average d-spacing measured from HRTEM shown in Figure 3b. In addition, the full width at half-maximum of the (002) peak is seen to be wide, which indicates poor crystallinity and some disordered structures in the CNTs compared with pure CNTs.22 Furthermore, the Raman spectrum (Figure 5) of the sample also shows the typical features of MWCNTs.20,24 The spectrum

from 600 to 1600 nm, while in Figure 2d it is evident that these fibers are hollow. Furthermore, Figure 3 shows the TEM images of microwaveinduced pyrolytic char, which illustrates more detailed

Figure 3. (a) TEM image of CNTs grown on microwave pyrolytic char. (b) HRTEM image of the wall of CNT formed on char. (c) SAED pattern of the wall of CNT.

structures of these CNTs. As shown in Figure 3a, the MWCNTs are formed on the amorphous pyrolytic char particle. The diameter of the CNTs is approximately 50 nm. There is no filling material inside CNTs. HRTEM imaging (Figure 3b) clearly shows the nanotubes have a multiwalled structure with around 15 layers of carbon wall with a wall thickness of 5−7 nm. The d-spacing of the walls is about 0.38 nm, which is slightly larger than those of pure graphite (0.34 nm)18 and CNTs (0.36 nm) prepared on bamboo charcoal at 1300 °C described by Zhu and co-workers.13 This increase in dspacing is probably due to the curvature of the graphene which leads to an increase of the repulsive force. The size effect is more intense when the diameter is smaller than 10 nm.19 The selected area electron diffraction (SAED) pattern of the CNT in Figure 3c reveals that the tube had a graphitic layer with both ordered and disordered orientation. The SAED pattern in Figure 3c exhibits diffuse diffraction rings, which indicates some orientation of a 002 plane together with some defects20 as well as some amorphous carbon structuring.21 The surface area of microwave-induced pyrolytic char with CNTs is 5.19 m2·g−1, which is approximate 5 times higher than that of the conventional pyrolytic char. The existence of CNTs contributes to the increase in surface area of the microwave-induced pyrolytic char.

Figure 5. Raman spectrum of microwave-induced pyrolytic char with CNTs.

demonstrates the two most prominent peaks, which are the socalled G (graphitic) band at a frequency of 1588 cm−1 and D (disordered induced) band at a frequency of 1346 cm−1. The ID/IG intensity ratio is 0.93, indicating a poor crystalline quality of the microwave-induced pyrolytic char. The disorder in the sample is mainly due to the presence of amorphous carbon in the char24 and defects in the MWCNTs.6 This is in good agreement with observations from Figure 3. 15014

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Figure 6 shows the TGA results from the pyrolytic chars derived from biomass from both the microwave-induced and

Table 2. Distribution of Products Produced via MicrowaveInduced and Conventional Pyrolysis

biochar yield (wt %) bio-oil yield (wt %) bio-oil composition (wt %) phenol and its derivatives naphthalene and its derivatives benzoic acid benzene and its derivatives alkene cedrol ester glucose and its derivatives biogas yielda (wt %) biogas composition (vol %) H2 CO CO2 CH4 C2H6 C2H4 CxHy

Figure 6. Thermogravimetric profiles for both conventional and microwave treated pyrolytic chars.

conventional processing methods. There are two peak temperatures of microwave-induced pyrolytic char, which are 555 and 618 °C, corresponding to the oxidation of amorphous char and CNTs, respectively.25 The peak temperatures of microwave-induced pyrolytic char are significantly higher than that of conventional pyrolytic char (483 °C). The ignition and burnout temperatures for the microwave pyrolytic char are 435 and 630 °C, respectively, both of which are also higher than the conventional pyrolytic char values. The microwave-induced pyrolytic char was covered with CNTs, which have a higher thermal stability than the char substrate itself. Therefore, the microwave-induced pyrolytic char is more thermally stable than conventional pyrolytic char during the combustion process due to the formation of CNTs. In this study, microwave-induced pyrolytic char is burned in air at a temperature of 555 °C for 3 h to remove all amorphous char. The percent of the residue is 14.70%, which is higher than the residue of 8.92% (mainly ash) in Figure 6. Thus, the CNT yield was calculated by difference, which was 5.78% (WCNTs/ Wchar%) based on the weight of microwave-induced pyrolytic char. Although the CNT yield is lower than those of previously reported works,26−29 this approach is of great importance due to the sustainable nature of the raw materials. The approach is also simple and cheap. In addition, the byproduct of microwave-induced processing of biomass, syngas, could be applied as an energy source. Therefore, low-cost CNTs prepared via the microwave-induced method are of some potential for application in real industry after a certain purification process.30−33 3.3. Roles of Pyrolytic Products on CNT Growth. It is clear from Table 2 that microwave-induced pyrolysis of gumwood has a different product distribution compared with conventional pyrolysis. The yield of microwave pyrolytic char is 18.22 wt %, which acts as the substrate for CNT synthesis.13 More bio-oil and biogas were produced during microwaveinduced pyrolysis, i.e., 8.52 and 73.26 wt %, respectively. Due to the volumetric heating (and therefore higher heating rate) induced during microwave-induced pyrolysis, the inner temperature of gumwood particles is probably higher than that under conventional heating conditions, 34 leading to a faster decomposition of the biomass. Therefore, it is reasonable to observe that the yields of pyrolytic bio-oil and biogas via

a

microwave-induced pyrolysis

conventional pyrolysis

18.22 8.52

29.68 5.73

25.23

38.80

10.05

1.82

2.62 4.13

1.29 0.96

11.55 1.25 9.03 0.54

5.32 1.31 3.2 1.22

73.26

64.59

13.21 49.31 22.41 13.48 0.52 0.99 0.09

4.92 38.68 41.83 11.41 0.84 1.42 0.88

By difference.

microwave irradiation are higher than those produced during conventional heating. These hydrocarbons may potentially function as carbon sources during CNT growth. It can be seen from Table 2 that microwave-induced pyrolysis of gumwood generates more H2, CO, and CH4 and less CO2 than those produced during pyrolysis with conventional heating. This demonstrates a significant advantage as a result of microwave irradiation on product yields as well as the morphology of pyrolytic chars. The initial moisture content of the gumwood in these experiments was low (2.1 wt %), meaning that the potential heating rate during microwave treatment (through the absorbance in the moisture phase) was low. To facilitate microwave-induced pyrolysis of gumwood, SiC (a good microwave absorber) was used as a microwave receptor to increase the heating rate of the biomass particles (via conductive heat transfer) to achieve a higher heating intensity and uniform heating of samples.35 It can be seen that CH4 is one of the major biogas products in this study, which is widely used as a carbon source gas for CNT synthesis.7,10 The CH4 produced via microwave-induced pyrolysis of biomass is most likely therefore to be the carbon precursor during CNT synthesis. The GC−MS analysis of pyrolytic bio-oil illustrates that biooil produced via microwave-induced pyrolysis of gumwood has fewer components than conventional pyrolytic bio-oil. Table 2 lists the major compounds in bio-oil. By comparing with conventional pyrolytic bio-oil, bio-oil produced via microwaveinduced pyrolysis contains less phenol and its derivatives, which are approximately 25.23 wt %. However, more naphthalene, benzene, alkene, and their derivatives are found in bio-oil produced via microwave-induced pyrolysis than in bio-oil via conventional pyrolysis. It is reported that the pyrolysis of toluene during microwave plasma enhanced chemical vapor 15015

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Table 3. EDS Results of Gumwood Ash and MWCNTs Gumwood Ash element content (wt %)

C 9.77

O 38.11

element content (wt %) element content (wt %)

Na 1.16

Mg 3.01

C 98.08 C 92.49

Al 0.54 MWCNTs

Si 1.81

O 1.62 Carbon Nanospheres O 4.59

Si 0.04

P 1.56

S 2.18

Si 0.02

Cl 15.17 Ca 0.06

Ca 1.76

K 14.22

Ca 12.45 Cu 0.21

Cu 0.95

P 0.18

Figure 7. SEM and HRTEM images showing the various stages of CNT growth. (a) and (d) are the SEM and HRTEM images of carbon nanospheres. SEM image (b) and TEM image (e) show the self-assembled carbon nanospheres, and (c) and (f) are the SEM and HRTEM images of the CNTs.

deposition also generates CNTs.22 Catalytic pyrolysis of benzene can also be applied to prepare CNTs.36 Therefore, it is likely that naphthalene, which has a molecular structure similar to those of toluene and benzene, could decompose to form CNTs. It is evident from Table 2 that microwave-induced pyrolysis produced more naphthalene and benzene based derivatives. It is therefore logical that the microwave-induced processing of gumwood produced more favorable conditions for the formation of CNTs compared to conventional pyrolysis. 3.4. Impacts of Minerals on the Growth of CNTs. Ash sample of gumwood was analyzed using EDS. As listed in Table 3, the gumwood ash consists of high levels of C, O, Cl, K, and Ca but lower levels of Na, Mg, Al, Si, P, and S. The prepared CNTs not only contain carbon, but also have elements such as O, Si, and Ca. The copper was predominantly from the copper grid used during TEM analysis. No metal element was found in the CNTs, meaning there were no metallic components playing a catalytic role in the growth of the CNTs. Based on the report that carbon can be dissolved into silicates to produce CNTs,13 it is therefore reasonable to assume that Si and Ca may play a key role in CNT formation during microwave treatment. 3.5. Mechanism of the Growth of CNTs. As mentioned in section 3.2, both crystalline and amorphous phases are present in the char particles prepared via microwave-induced processing. Although the pyrolysis temperature, reaction time, and carrier gas conditions were the same, the chars produced via microwave-induced pyrolysis are very different from those produced via conventional pyrolysis in terms of morphology and microstructure. Therefore, it is believed that the formation of CNTs under microwave-induced pyrolysis can be attributed

to the unique effect of microwave radiation on the thermochemical processing of biomass samples, which was also reported by others.37 In microwave-induced pyrolysis, gumwood is heated to the target temperature where it starts to decompose, generating volatiles (bio-oil and biogas). The minerals in gumwood, at the surface of the particle, can act as catalytic sites where the released volatiles decompose to form carbon nanospheres on the surface of pyrolytic char, as shown in the SEM image of Figure 7a. Figure 7d is an HRTEM image of a carbon nanosphere. It is clear that this carbon nanosphere is amorphous without any graphitic structure and contains C, O, Ca, Si, and P elements (Table 3), which is a composition similar to that of CNTs. In the microwave-induced process, it is speculated that localized heating results in “hot spots” being generated resulting in very high temperatures38 on the surface, i.e., much higher than the bulk temperature (500 °C) that is measured. These hot spots are actually microplasmas which are confined to a tiny region of the space lasting for just a fraction of a second.4 There are some similarities (in terms of morphology) with the amorphous carbon nanoparticles prepared via microwave plasma chemical vapor deposition.39 These carbon nanospheres are highly graphitized with a higher dielectric loss tangent (0.11−0.29) than that of initial biomass (