High yield synthesis of multi-walled carbon nanotubed from CaCO3 supported iron (III) nitrate catalyst

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 Preface and target of the work Chapter Introduction to carbon nanotubes material 1.1 Brief history of carbon canotubes 1.2 Geometry of carbon nanotubes 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 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 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 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 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 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 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 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 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 shows an overview of carbon nanotubes material, the CNTs synthesizing methods and ability in industrial applications Chapter lists the experimental process for synthesis of carbon nanotubes This chapter also introduces investigation methods mainly used during this thesis Chapter 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 Chapter 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 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] 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 = and 30o, and chiral nanotubes correspond to < 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) According to Ref [12] the formation of bamboo-shaped MWCNTs involves to following parameters: (i) the presence of nitrogen or heavy gases; (ii) keeping an active and clean top surface of the catalyst particles; (iii) prolonging carbon bulk diffusion of the catalysts; (iv) larger catalyst size; and (v) preventing atmospheric reactions to form a passivation layer on the catalyst surfaces In our case, the continuously supplying nitrogen in the whole process is satisfied conditions (i), (ii) and (iii) The bamboo-shaped MWCNT encapsulating the big Fe particle in Fig 3.6 confirms the condition (v) 3.2 Effect of growth temperature It is known that in a CVD process, parameters such as catalyst, gas composition, and temperature are the factors establishing their products The gas composition of N2, H2, C2H2 with various ratios were investigated in this work In this thesis the experimental gas flow rate ratio C2H2 : H2 : N2 = 50 : 100 : 300 sccm mentioned above is the most select As stated in section 3.1 the most favorable catalyst is CaCO3 + Fe(NO3)3.9H2O mixture containing 5% wt Fe The sequential studies for the most advantageous temperature are revealed in this section 3.2.1 CNTs performance The dependence of CNTs performance (diameter, carbon deposit according to Eq (3.1), CNTs yield in terms of Eq (3.5) defined below) on temperature was surveyed About grams catalyst was used for each CVD process CNTs yield was defined by the equation (3.5): Yc(%) = 100mf/ms (3.5) where mf is the weight of purified CNTs, ms is the weight of carbon in C2H2 flow and can be calculated by ms = Flow rate (l/min) x Time (min) † 22.4 (l/mol) x 24 (g/mol) Table catalogues the performance of the CNTs grown at 700oC, 800oC, 900oC 53 Table Dependence of CNTs performance on temperature Temperature Diameters Carbon deposit CNTs yield (oC) (nm) Hc(%) Yc(%) 700 30 – 100 28.6 61.92 800 15 – 90 30 78.61 900 10 – 80 33 47.22 As the growth temperature increases, the metal catalyst surface becomes rougher [19] Thus, they have smaller active regions nucleating smaller nanotubes The SEM counted diameters of CNTs in table also gives the decrease when increasing the growth temperature The slightly raise in carbon deposit is due to an increase in atomic mobility induced at the higher temperature leading the metal catalyst become „liquid-like‟ as suggested by Teo et al [75] The significant reduction in CNTs yield at 900oC is probably the high rate of the formation of iron carbide limiting CNTs growth At 700oC, the dispersion of isolated Fe nanoparticles is not high because of low thermodynamic gases and little CO2 gas from CaCO3 support This explain why the CNTs yield in this case is lower than the case at 800oC which is high enough to form isolated Fe nanoparticles well as mentioned above 3.2.2 Structural characteristics The purified CNTs grown at different temperatures were characterized by Raman spectroscopy, X-ray diffraction and Thermogravimetric analysis The graphitic crystallinity in each purified CNTs sample can be qualitatively evaluated from its Raman spectrum The graphitic crystal phase of the purified CNTs can be identified by X-ray diffraction patterns In addition, the absence of the catalyst peak 54 partially indicates the purity of the CNTs samples Thermogravimetric analysis (TGA) not only shows the thermal stability of the CNTs but also evaluates qualitatively the CNTs purity Raman spectroscopy Raman spectra of the CNTs grown at different temperatures are shown in Fig 3.11 These spectra have well known D and G bands at around 1330 cm-1 and around 1580 cm-1, respectively And the D‟ called band appears as a shoulder on the G band at around 1615 cm-1 and no second order scattering peak at 1740 cm-1 suggest that our CNTs are multiwall [26, 40, 62] The D band corresponds to the defect-induced Raman band and is known as the defect mode associated with the disordered of graphite or diamond-like carbon In MWCNTs, this band may be attributed to defects in curved graphene sheets, tube ends and finite size crystalline domains of the tubes together with the contribution of impurities as amorphous carbon [4, 46, 72] The G band represents the Ramanallowed E2g mode of graphite and is related to the vibration of C–C bonded in a two-dimensional hexagonal lattice of a graphite layer The D‟ band is also associated with the presence of defects in the lattice and originates from a double resonance process involving q ~ 2k phonons close to the Brillouin zone center [64, 73] The relative intensity ratio between D and G bands ID/IG has inverse relation to the in-plane graphite crystallite size [80] So by resolving G band from D‟ band the ID/IG and ID‟/IG were used to evaluate the level of defects in our MWCNTs The ID/IG and ID‟/IG ratios of the purified MWCNTs decrease when the growth temperature increase Indicating the higher temperature grown MWCNTs has more crystalline perfection than the lower one This also shows D and D‟ band has the same characteristics 55 Fig 3.11: Raman spectra of the purified MWCNTs grown at 700oC, 800oC, 900oC Xray diffraction The XRD patterns of the purified MWCNTs grown at 700oC, 800oC, 900oC are shown in Fig 3.12 It can be seen that the diffraction graphitic peak C (002) mainly dominates the XRD pattern in three samples (a, b, c) The C (002) peak is around 26o indicating the crystalline phase for CNTs Apart from of these the XRD have the cluster of iron (III) carbide Fe3C peaks and no CaCO3 or CaO peak From the graphitic (002) peak the calculated average interlayer distances are 3.397 Å (pattern a), 3.385 Å (pattern b) and 3.385 Å (pattern c), which are slightly higher than the interlayer distance (d = 3.354 Å) in graphite Moreover, the graphitic (002) peak sharpens as increasing the growth temperature This implies the crystallinity of this phase is enhanced 56 Fig 3.12: Xray diffraction patterns of CNTs synthesized at (a) 700oC, (b) 800oC and (c) 900oC Thermogravimetric analysis Fig 3.13 and Fig 3.14 show TGA and DrTGA curves of the purified MWCNTs grown at different temperatures, respectively From the TGA curves we can see that the 800oC-MWCNTs (grown at 800oC) has the highest purity, 97.9% as compared with 700oC-MWCNTs (84.6%) and 900oC-MWCNTs (88.5%) The discrepancy of purity between these MWCNTs samples can be probably explained by the effect of growth temperature At 700oC, there is little amount of CO2 gas released from CaCO3 to spread out Fe nanoparticles (mentioned in section 3.1.2) and the thermodynamics of gases is low These make Fe nanoparticles be large in diameters and not well scattered in dispersion As a result, 700oC-MWCNTs grew over large Fe nanoparticles and then encapsulated them Because Fe has specific mass heavier than that of C, the mass ratio Fe/MWCNTs in 700oC-sample is high, leading to high amount of impurities For 800oC and 900oC-MWCNTs, higher 57 growth temperature resulting to more amount of CO2 gas and higher thermodynamics of gases that make Fe nanoparticles scatter well and reduce sizes The reason that 900oC-MWCNTs sample has more amount of impurities compared with those in 800oC-MWCNTs is possibly the higher formation rate of iron carbide Consequently, iron carbide nanoparticles limit CNTs growth that results in the higher mass ratio of iron carbide to MWCNTs Iron carbide, mainly contributing to impurities, is difficult to be dissolved by dilute HCl acid in the purification process The 700oC-MWCNTs has single exothermal peak at 620oC like that at 682oC of 900oC-MWCNTs (see Fig 3.14) This indicates there is only single phase existing in 700oC and 900oC-MWCNTs The thermal stability in 900oC-MWCNTs (starts combusting at 525oC with maximum loss at 682oC) is higher than that (starts combusting at 440oC with maximum loss at 620oC) of 700oC-MWCNTs The 800oC-MWCNTs sample is similar to the composition of the 700oC and the 900oC ones It has two combustion regions, the first with the maximum loss peak at 585oC and the second with the one at 668oC Fig 3.13: TGA curves of the purified MWCNTs grown at different temperatures 58 Fig 3.14: DrTGA curves of the purified MWCNTs grown at different temperatures As mentioned in section 3.1.2 by TEM study, it is affirmed again that 800oC MWCNTs have two geometries (hollow and bamboo shape) and each corresponds to each region of weight loss The combustion region with maximum weight loss at 585oC can be deduced to be contribution of bamboo MWCNTs because that of hollow 800oC-MWCNTs has to be at 668oC higher than that of 700oC-MWCNTs at 620oC It is concluded that MWCNTs grown at higher temperature has the higher crystallinity This also agrees with the above Raman and X-ray results MWCNTs grown at 800oC have highest purity (97.9%) The bamboo MWCNTs has more defects and less graphitic crystallinity than the hollow one 3.3 Optimal procedure for large-scale synthesis of MWCNTs This final section lists the most favorable conditions derived from section 3.1 and 3.2 to produce large-scale production of MWCNTs For temperature parameter, 59 800oC is the choice because it gives the highest CNTs yield (78.61%), lowest impurity (2.1%) and acceptable graphitic crystallinity The gas flow ratio in CVD reaction is C2H2 : H2 : N2 = 50 : 100 : 300 sccm The composition of catalyst is the CaCO3 + Fe(NO3)3.9H2O mixture containing 5% wt Fe The purification uses 10% HCl acid, paper membranes, distilled water, heating furnace However, all discussed results of MWCNTs are produced from approximately grams catalyst (salt mixtures) Thus, the effect of catalyst weights needs investigating to gain the optimal CVD process for economic solution This is evaluated by the catalyst weight efficiency determined as the ratio of purified MWCNTs weight to catalyst weight Table 3.3 shows the results of catalyst weight efficiency obtained from various catalyst weights From table 3.3 we can see that the approximate 3.1 grams catalyst has the highest yield corresponding to 1.2 grams purified MWCNTs This amount is high as compared with the maximum amount of 1.6 gram of carbon in the feedstock (corresponding to completely decomposing 50 sccm C2H2 flow in 30 minutes) In summary, we have synthesized MWCNTs with yield of 78.61%, a small impurity of 2.1% and diameters ranging from 15 to 90 nm The synthesis procedure is illustrated in Fig 3.15 Table 3.3 Catalyst weight efficiency Weight of catalyst (g) 1.06 1.48 1.94 3.11 4.12 6.05 Catalyst weight efficiency (%) 25.4 28.3 32.0 38.6 29.6 20.1 60 Fe(NO3)3.9H2O CaCO3 Mixture of CaCO3 and Fe(NO3)3.9H2O Grinding 3.1 grams of CaCO3 + Fe(NO3)3.9H2O mixture containing 5% wt Fe 800oC, N2/ H2 Formation of Fe nanoparticles on the support C2H2/H2/N2 for 30 minutes CNTs growing over supported Fe nanoparticles Separation of CNTs by using 10% HCl, filter, distilled water 1.2 gram CNTs with purity of 97.9% Fig 3.15: Schematic procedure of the optimal MWCNTs synthesis 61 Conclusion In this thesis, we have developed a simple technique for growing CNTs using CaCO3 supported iron (III) nitrate catalyst It was confirmed that by using this technique the 97.9% purity, 15 – 90 nm diameter MWCNTs could be obtained in a single CVD and purification process With 3.1 grams of CaCO3 and Fe(NO3)3.9H2O mixture containing 5% wt Fe we can synthesize 1.2 grams of MWCNTs for 30 minutes decomposing 50 sccm C2H2 flow (corresponding to 78% CNTs yield) This is very promising for large scale production of MWCNTs The catalyst support (CaCO3, CaO) in the CVD product was completely removed in the purification process The purity was evaluated by TGA, X-ray diffraction The formation of Fe nanoparticles essential to CNTs growth on the support was investigated by SEM study and EDX analysis By using growth temperature at 800oC, we exploit the self-gas release of CaCO3 support through 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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... the relative efficiency of catalysts in terms of carbon deposit and density of CNTs The iron weight content of 1%, 3%, 5%, 7%, 10%, 15%, 20% in CaCO3 + Fe(NO3)3.9H2O and in CaCO3 + FeCl3.6H2O are... 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,