High yield synthesis of multi-walled carbon nanotubed from caco3 supported iron (iii) nitrate catalyst

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VIETNAM NATIONAL UNIVERSITY HANOI COLLEGE OF TECHNOLOGY Nguyen Duc Dung HIGH YIELD SYNTHESIS OF MULTI-WALLED CARBON NANOTUBES FROM CaCO3 SUPPORTED IRON (III) NITRATE CATALYST MASTER THESIS Hanoi - 2006 VIETNAM NATIONAL UNIVERSITY HANOI COLLEGE OF TECHNOLOGY Nguyen Duc Dung HIGH YIELD SYNTHESIS OF MULTI-WALLED CARBON NANOTUBES FROM CaCO3 SUPPORTED IRON (III) NITRATE CATALYST Speciality: Nano Materials and Devices MASTER THESIS Advisor: Dr. Phan Ngoc Minh Hanoi - 2006 Content Abbreviations 3 Preface and target of the work 4 Chapter 1. Introduction to carbon nanotubes material 7 1.1. Brief history of carbon canotubes 7 1.2. Geometry of carbon nanotubes 8 1.3. Syntheses of carbon canotubes 13 1.3.1. Arc discharge 13 1.3.2. Laser ablation 14 1.3.3. Chemical vapor deposition 15 1.4. Growth mechanism of carbon nanotubes 19 1.5. Purification 20 1.5.1. Oxidization 21 1.5.2. Acid treatment 21 1.5.3. Micro filtration 21 1.6. Physical properties 22 1.6.1. Electronic properties 22 1.6.2. Mechanical properties 25 1.7. Application of carbon nanotubes 27 1.7.1. Energy storage 27 1.7.2. Composite materials 29 Chapter 2. Experimental and investigation methods 31 2.1. Experimental 31 2.1.1. Description of the CVD system for growing carbon nanotubes 31 2.1.2 Synthesis of carbon nanotubes 31 2.2. Investigation methods 35 2.2.1. Electron microscope 35 1 2.2.2. Raman spectroscopy of carbon nanotubes 38 2.2.3. Xray diffraction of carbon nanotubes 41 2.2.4. Thermogravimetric analysis 42 Chapter 3. Results and discussion 43 3.1. Catalytic Fe nanoparticles in the CNTs growth process 43 3.1.1. Effect of supported iron salts on the CVD products 43 3.1.2. Formation of catalytic Fe nanoparticles nucleating CNTs 47 3.2. Effect of growth temperature 53 3.2.1. CNTs perfomance 53 3.2.2. Structural characteristics of CNTs 54 3.3. Optimal procedure for large-scale synthesis of MWCNTs 59 Conclusion 62 References 63 2 Abbreviations CCVD Catalytic Chemical Vapor Deposition CFs Carbon Fibers CNTs Carbon Nanotubes CVD Chemical Vapor Deposition DrTGA Differential Thermo-Gravimetric Ananlysis ECDL Electro-Chemical Double Layer EDX Energy Dispersive X-ray spectroscopy FTIR Fourier Transform Infrared HRTEM High Resolution Transmission Electron microscope MWCNTs Multi-Walled Carbon Nanotubes PECVD Plasma Enhanced Chemical Vapor Deposition SCCM Standard Cubic Centimeters per Minute SEM Scanning Electron Microscope STEM Scanning Transmission Electron Microscope STM Scanning Tunneling Microscope SWCNTs Single-Walled Carbon Nanotubes TEM Transmission Electron microscope TGA Thermo-Gravimetric Analysis XRD X-Ray Diffraction 3 Preface and target of the work Carbon nanotubes were identified for the first time in 1991 by Sumio Iijima at the NEC Research Laboratory. By using high resolution transmission electron microscope (HRTEM) he clearly observed the tiny tubes called multi-walled carbon nanotubes (MWCNTs) in the soot made from by-product obtained in the synthesis of fullerenes. The MWCNTs comprise carbon atoms arranged in a graphitic structure rolled up to form concentric cylinders [38]. Two years later, single-walled carbon nanotubes (SWCNTs) were synthesized by adding metal particles to the carbon electrodes [9, 36]. Their small diameter (of the order of a nanometer) and their long length (of the order of microns) lead to aspect ratios so large that the carbon nanotubes possibly reach to ideal one-dimensional (1D) systems. Depending on the chirality of their atomic structure, they can be excellent metals or semiconductors with a band gap that is inversely proportional to their diameter. Theoretical and experimental results have shown extremely high elastic modulus, greater than 1 TPa and strengths 10100 times higher than strongest steel [77]. In addition to exceptional mechanical properties, they also possess superior thermal properties: thermally stable up to 2800oC in vacuum, thermal conductivity about twice as high as diamond [16]. The above properties make carbon nanotubes (CNTs) the object of widespread studies in both basic science and technology. They can be applied in many fields: fabrication of nano sized electronic devices, energy storage equipments, field emission display, nano probes, nano composites,... There are many methods (mentioned in detail in section 1.3) for synthesizing carbon nanotubes having different performance from diverse material sources. The arc discharge method relates to connecting two graphite rods to a power supply, placing them millimeters apart, and vaporizing carbon by a hot plasma. Its product can be SWCNTs and MWCNTs with few structural defects. Tubes tend to be short with random sizes and directions. This method can produce large scale production 4 of CNTs but its typical yield of about 30% is not high. Laser ablation method was firstly used in 1996 by Smalley at al. using intense laser pulses blasting graphite to form primarily SWCNTs. The diameters of SWCNTs can be controlled in a large range by varying the reaction temperature. Although the yield of laser ablation method can reach to 70%, it has never been candidate for large-scale production because of requiring expensive lasers and the limitation of a laser spot area. Emerging as the best method for industrial production of CNTs is chemical vapor deposition (CVD). Carbon feedstocks are hydrocarbons in gaseous and liquid phases, alcohol, etc., decomposed at 600-1200oC into carbon atoms recombining to nanotubes over metal nanoparticles. Carbon nanotubes produced by CVD having the yield probably up to 100% are usually long MWCNTs with quite high defects. Thus, the investigation of suitable technologies to synthesize large-scale production of carbon nanotubes with high yield and purity to reduce cost satisfying for industrial demands is an opening solution until now. The most common and optimal method for large-scale production of CNTs is catalytic chemical vapor deposition (CCVD) (discussed in section 1.3.3). In the CCVD process, catalyst supports are the essential ingredients such as, MgO, Al2O3, SiO2, CaCO3 etc., due to their high surface area for CCVD reaction. The choice of CaCO3 as catalyst support was reported in Ref. [18]. The advantages of this technique are:  CaCO3 support is easily dissolved in a dilute acid, thus the CNTs purification is a one-step procedure, simple and harmless to CNTs structure.  CaCO3 and Fe salts from which catalysts synthesized are available in market and low cost.  This is the simplest CVD method for large scale production of CNTs. By supplying catalysts and collecting CVD product continuously, the production yield is significantly increased. With the aim of large scale and low cost production and the idea using CaCO3 support, this thesis investigates the technological aspects that relate to synthesis of 5 carbon nanotubes. We develop a simple method for making catalyst only by grinding CaCO3 and Fe salts, therefore, neglect the impregnating and drying steps, that reduce stages in CNTs synthesis. The addition of H2 gas in CVD process is believed not only to form Fe nanoparticles enhancing catalytic activity but also to improve the CNTs yield. By varying growth temperature, another role of CaCO3 as the factor contributing to the formation of Fe nanoparticles necessary to the CNTs growth is studied in this thesis. Furthermore, Fe salt radicals are found significant to the creation of Fe nanoparticles on the support (CaCO3 or CaO). At last, the more dilute acid (HCl 10%) is used for purification process. The arrangement of the thesis: In addition to the “Preface and target of the work” and “Conclusion” parts the thesis is organized into three chapters as follow: Chapter 1 shows an overview of carbon nanotubes material, the CNTs synthesizing methods and ability in industrial applications. Chapter 2 lists the experimental process for synthesis of carbon nanotubes. This chapter also introduces investigation methods mainly used during this thesis. Chapter 3 indicates the effect of Fe content in catalysts. The formation of Fe nanoparticles necessary to CNTs growth is studied. The structural characteristics of the CNTs depend on the growth temperature are characterized. The optimal chemical vapor deposition process is established for the aim of large-scale production of carbon nanotubes. It is confirmed that by using the presented technique we can produce 97.9 % purity, 78.6 % yield CNTs with mass of 50 grams/day. 6 Chapter 1. Introduction to carbon nanotubes material 1.1. Brief history of carbon nanotubes In 1970‟s and 1980‟s, small diameter carbon filaments were produced through the synthesis of carbon fibers by the decomposition of hydrocarbons at high temperature in the presence of transition metal catalyst nanoparticles [57, 78]. However, there was not any detailed systematic study on such small filaments until the observation of carbon nanotubes by Iijima in 1991 [38]. These tubes (called multiwall carbon nanotubes) contained at least two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm with both closed ends. A new class of carbon nanotubes with only single layer was discovered in 1993 [9, 36]. These single-walled nanotubes with diameters typically in the range 12 nm are generally narrower than the multiwalled nanotubes, and tend to be curved rather than straight. Since these pioneering works, the study of carbon nanotubes has developed rapidly. Fig 1.1: Multi-walled CNTs observed in 1991 [38]. 7 1.2. Geometry of carbon nanotubes The structure of carbon nanotubes has been characterized by High Resolution Transmission Electron Microscope (HRTEM) and Scanning Tunneling Microscope (STM). These techniques directly confirmed that the carbon nanotubes are cylinders derived from the honeycomb lattice representing a single atomic layer of crystalline graphite (a graphen sheet). Most important structures are single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). A SWCNT is considered as a cylinder with only one wrapped graphene sheet. Multi walled carbon nanotubes (MWCNTs) are similar to a set of concentric SWNTs. The structure of a single walled carbon nanotube is explained in terms of its 1D unit cell, defined by the vectors Ch and T in Fig. 1.2a [20]. The circumference of any carbon nanotube is expressed in terms of the chiral vector Ch = nâ1 + mâ2 which connects two crystallographically equivalent sites on a 2D graphene sheet (see Fig. 1.2a). The construction in Fig. 1.2a depends uniquely on the pair of integers (n, m) which specify the chiral vector. Fig. 1.2a shows the chiral angle θ between the chiral vector and the “zigzag” direction (θ = 0) and the unit vectors â1 and â2 of the hexagonal honeycomb lattice of the graphene sheet. Three distinct types of carbon nanotube structures can be generated by rolling up the graphene sheet into a cylinder as discribe below and shown in Fig. 1.3. The zigzag and armchair nanotubes, respectively, correspond to chiral angles of θ = 0 and 30o, and chiral nanotubes correspond to 0 < θ < 30o. The intersection of the  vector OB (which is normal to Ch) with the first lattice point determines the fundamental one dimension (1D) translation vector T. The unit cell of the 1D lattice is the rectangle defined by the vectors Ch and T (Fig. 1.2a). 8 (a) (b) Fig 1.2: (a) The chiral vector OA or Ch = nâ1 + mâ2 is defined on the honeycomb lattice of carbon atoms by unit vector â1 and â2 and the chiral angle θ with respect to the zigzag axis. Along the zigzag axis θ = 0o. Also shown are the lattice vector OB = T of the 1D nanotube unit cell and the rotation angle ψ and the translation τ which constitute the basic symmetry operation R = ( ψ/τ) for the carbon nanotube. The diagram is constructed for (n, m) = (4, 2). (b) Possible vectors specified by the pair of integers (n, m) for general carbon nanotubes, including zigzag, armchair, and chiral nanotubes. Below each pair of integers (n, m) is listed the number of distinct caps that can be joined continuously to the carbon nanotube denoted by (n, m). The encircled dots denote metallic nanotubes while the small dots are for semiconducting nanotubes. 9 Fig. 1.3: Schematic models for single-wall carbon nanotubes with the nanotube axis normal to the chiral vector which, inturn, is along: (a) the θ = 30o direction [an armchair (n, n) nanotube], (b) the θ = 0o direction [a zigzag (n, 0) nanotube] and (c) a general θ direction with 0 < θ < 30o [a chiral (n, m) nanotube]. The actual nanotubes shown here correspond to (n, m) values of : (a) (5, 5), (b) (9, 0), and (c) (10, 5). The cylinder connecting the two hemispherical caps of the carbon nanotube (see Fig. 1.3) is formed by superimposing the two ends of the vector Ch and the    cylinder joint is made along the two lines OB and AB ' in Fig. 1.2a. The lines OB  and AB ' are both perpendicular to the vector Ch at each end of Ch. In the (n, m) notation for Ch = nâ1 + mâ2, the vectors (n, 0) or (0, m) denote zigzag nanotubes and the vectors (n, n) denote armchair nanotubes. All other vectors (n, m) correspond to chiral nanotubes. The nanotube diameter dt is given by dt  3aC C (m2  ma  m2 )1/ 2 /   Ch /  (1.1) where Ch is the length of Ch, aC-C is the C – C bond length (1.42 Å). The chiral angle θ is given by 10   tan 1  3n /  2m  n  (1.2) From (1.2) it follows that θ = 30o for the (n, n) armchair nanotube and that the (n, 0) zigzag nanotube would have θ =60o. From Fig. 1.2a it follows that if we limit θ to be between 0 < θ < 30o, then by symmetry θ = 0 for a zigzag nanotube. Both armchair and zigzag nanotubes have a mirror plane and thus are considered as achiral. Differences in the nanotube diameter dt and chiral angle θ give rise to differences in the properties of various carbon nanotubes. The symmetry vector R = (ψ/τ) of the symmetry group for the nanotubes is indicated in Fig. 1.2a, where both the translation unit or pitch τ and the rotation angle ψ are shown. The number of hexagons, N, per unit cell of a chiral nanotube, specified by integers (n, m) is given by N = 2(m2 + mn + n2)/dR (1.3) where dR = d if n – m is not a multiple of 3d or dR = 3d, if n – m is a multiple of 3d and d is defined as the largest common divisor of (n, m). Each hexagon in the honeycomb lattice (Fig. 1.2a) contains two carbon atoms. The unit cell area of the carbon nanotube is N times larger than that for a graphen layer and consequently the unit cell area for the nanotube in reciprocal space is corresponding 1/N times smaller. Table 1.1 [21] provides a summary of relation useful for describing the structure of single wall nanotubes. Fig. 1.2b indicates the nanotubes that are semiconducting and those that are metallic, and shows the number of distinct fullerene caps that can be used to close the end of an (n, m) nanotube, such that the fullerene cap satisfies the isolated pentagon rule. 11 Table 1.1 Structural parameters for carbon nanotubes Symbol Name aC-C C – C distance a Length of unit vector a1, a2 Unit vectors b1, b2 Reciprocal vectors lattice Chiral vector Length of Ch dt Diameter θ Chiral angle d dR Gcd (n, m)b) Gcd (2n + m, 2m +n)b) Translational vector T Length of T N Number of hexagons in the nanotube cell R Symmetry vector τ Pitch of R ψ Rotation angle of R M Number of T in N.R Value 1.421 Å 2.456 Å 3a CC a1  Ch L T Formula a  3 a  3   , a2    2   1  2  1   1   1  2π  2π    b1  3  , b2   3   a  a     1   1  Ch = n.a1 + m.a2 x, y coordinate x, y coordinate (0 ≤ |n| ≤ m) L  3a CC (n 2  nm  m 2 ) dt  L π   Ch .a1 2n  m cos      Ch . a1 2 n 2  nm  m 2 dR  3d if (nm) 3 d if (nm) 3 T  t 1 .t 1  t 2 .t 2 , t1  2.n  m 2.m  n , t2   dR dR T 3L dR N 2 ( n 2  nm  m 2 ) dR gcd(t1, t2) = 1b) R = p.a1+q.a2, gcd(p, q) = t1.p - t2.q=1 (0 ≤ mp - nq ≤ N) 1b) (mp  nq)T  N radians 2  N N.R = M.Ch + M.T 12 a) In this table n, m, t1, t2, p, q are integers and d, dR, N and M are integer functions of these integers b) gcd (n, m) denotes the greatest common divisor of the two integers n and m. 1.3. Syntheses of carbon nanotubes 1.3.1. Arc discharge The arc discharge was the first available method for the production of both MWCNTs and SWCNTs. This technique has been in use for a long time for the production of carbon fibers. It is a worthy note that carbon nanotubes were probably observed before 1991 but not recognized. In the arc-discharge technique, two graphite rods were used as the anode and cathode, and placed inside a growth chamber filled with helium atmosphere. When a high current is passed through the opposing graphite anode and cathode, helium gas plasma evaporates carbon atoms in the anode, which deposits to form carbon nanotubes on the cathode. Fig. 1.4: An arc discharge apparatus produced the first CNTs [78]. 13 To produce isolated SWCNT catalysts such as Co, Ni, Fe, Y are used. Mixed catalysts such as Fe/Ni, Co/Ni and Co/Pt are used to grow bundles of SWNTs. For the synthesis of MWCNTs no catalyst is necessary and with the use of halide (potassium chloride) as a promoter in hydrogen atmosphere large scale production would be expected [34]. The nanotubes are found in the inner region of the cathode deposit and they are surrounded by a hard shell consisting of nanoparticles, fullerenes and amorphous carbon [1, 37]. 1.3.2. Laser ablation An efficient route for synthesis of CNTs with a narrow diameter distribution is a laser ablation of a graphite target. In the laser ablation technique, an intense laser beam is utilized to vaporize a graphite target doped with metal catalysts in a tube furnace at temperatures near 1200oC, while an inert gas jet passing through the deposition chamber carries nanotubes onto a metal collector. Fig. 1.5: Laser ablation system for growing CNTs [87]. 14 In the early report [30], the laser beam scanned across the target surface under computer control to maintain the smooth, uniform face for vaporization. The target was supported by graphite poles in a 1 inch quartz tube evacuated to 10 mTorr and then filled with 500 Torr argon flowing at 50 sccm. The flow tube was mounted in a high temperature furnace with a maximum temperature of 1200 oC. The soot produced by the laser vaporization was swept by the flowing Ar gas from the high temperature zone, and deposit on a water-cooled copper collector positioned downstream, just outside the furnace. MWCNTs are produced if the target is made of pure graphite [33] but in case of the target composed of graphite and metal [76] SWCNTs are synthesized. 1.3.3. Chemical vapor deposition (CVD) Chemical vapor deposition (CVD) is a process whereby a solid material is deposited from a vapor by a chemical reaction occurring on or in a surrounding area of a normally heated substrate surface. The solid material is obtained as a coating, a powder, or as single crystals. By varying the experimental conditions: substrate material, substrate temperature, composition of the reaction gas mixture, total pressure gas flows, etc., materials with different properties can be grown. In the CVD synthesis of CNTs different hydrocarbons such as acetylene (C2H2), methane (CH4), benzene (C6H6), etc. and also carbon monoxide (CO) are decomposed over different metals (Fe, Co, Ni…) at temperatures between 400oC and 1200°C. There are some other carbon allotropes except CNTs in a CVD product such as fibers [31, 53], planar nano-graphenes [63], amorphous, etc. depending on the CVD conditions. This is why technological research on synthesizing CNTs is necessary beside basic research. It can be classified CVD methods for synthesis of CNTs according to techniques making catalyst as described below: CVD method on flat substrates For the CVD synthesis of CNTs on flat substrates, a quartz tube is often 15 horizontally placed in a thermal or electrical furnace. The substrate (e.g. silicon, aluminium, graphite…) is coated with a metal catalyst. The metal catalyst on the substrate is derived from coating metal oxalate, nitrate solution [39, 54] or evaporating the metal film [91]. The substrate and the metal are then exposed to an atmosphere containing a hydrocarbon gas and a carrier gas (Ar, N2) for a certain time to grow CNTs. An aided CVD method for flat substrates is plasma enhanced chemical vapor depostion (PECVD). In the PECVD method, carbon nanotubes are also deposited on a metal coated flat substrate but the substrate is located inside a plasma. Usually one of the following plasmas is used: RF- (radio frequency), MW-(microwave) or a DC-(direct current) plasma. Often a methane (CH4) and hydrogen (H2) gas mixture with a ratio of 1% CH4 to 99% H2 is used at a total pressure between 1 and 40 mbar and a temperature of 900 °C [43]. Fig. 1.6: Schematic diagram of PECVD system. 16 CVD method on catalytic substrates Catalytic substrates are usually 3d metallic substrates such as Ni containing alloy, stainless steel grid and plate, etc. Metallic substrate surface prior to CVD reaction is important because of conditions for forming nanoparticles. The oxidation-reduction treatment is oxidation of the metallic surface by airflow at elevated temperatures and then by flow of hydrogen for reduction of the metal oxide [81]. The other for treating the surface is introducing hydrogen flow in the whole CVD process [32]. Oxidation in air can result in the fragment of the metallic surface to form fine nanoparticle structures. The reduction by hydrogen is necessary for forming metal nanoparticles enhancing catalytic activity. The two treatments also create granular structures increasing the catalytic surface area. Most of CVD products in cases using bulk metallic substrates were MWCNTs [32, 49, 71, 81]. Floating catalytic CVD Floating catalytic CVD is a process where un-supported catalysts are formed in situ by injecting volatile organometallic molecules, such as metallocenes or iron pentacarbonyl, into a reactor with a carbon feedstock [2, 13]. The schematic diagrams of two conventional apparatuses for floating catalyst method are shown in Fig. 1.7 [74]. The carbon feedstock can be in liquid or gaseous phase. The furnace has two heating zone, one with the sublimation temperature of precursor (90-110oC for ferrocen), the other with temperature of CVD reaction (900-1100oC) [74]. The precursor (ferrocen) vapor is carried into the reactor by argon flux that includes a hydrocarbon flow (Fig. 1.7b) or the vapor of ethanol that is generated by bubbling Ar gas via ethanol liquid (Fig. 1.7a). This method allows a three-dimensional dispersion of carbon feedstock with the catalytic particles so that the ability for continuous production of high purity CNTs with low cost is preferable for industrial applications [13, 50]. 17 (a) (b) Fig. 1.7: Schematic diagram of floating CVD system with (a) gaseous and (b) liquid carbon feedstock. Catalytic CVD (CCVD) This is the most promising method for synthesis of large-scale CNTs with low cost because of the ability to control the reaction parameters [52, 58]. By this process, a variety of liquid, solid, or gaseous carbon sources are introduced into the reaction zone over a catalyst at a controlled temperature [52, 58]. Most of catalysts employed are related to Fe, Co, Ni and other transition metals (or combination of them) dispersed on different supports such as SiO2, Al2O3, MgO or zeolites [11, 17, 52, 70]. For synthesizing these supported catalysts, the impregnant method is usually used. Metal salts or mixture of metal salts are dissolved in distilled water, ethanol or other suitable solvents to form solution which is then added the apposite support amount and evaporated. Fig. 1.8 [41] shows a simple scheme for a CCVD system. According to the scheme, a catalyst is sprayed on the quartz boat and is placed in the furnace center. After a CCVD process, CNTs deposited on the quartz boat are taken out. 18
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