Sự hình thành sợ nano-cacbon trên các sợi cacbon bằng phương pháp lắng đọng thể hơi hóa học. KL 95new KL 69 qxd 1 INTRODUCTION Synthesis of carbon nanofibers (CNFs) has been attracting great attention; promising new class of materials for their applications, such as catalyst supports, field e[.]
24 Journal of Science and Technology of Cơng trình nghiên cứu Sự hình thành sợi nano-cacbon sợi cacbon phương pháp lắng đọng thể hóa học sử dụng xúc tác sắt Carbon nanofibers grown on carbon fibers by chemical vapor deposition using the iron catalyst HOÀNG VĂN VƯƠNG1,*, NGUYỄN VĂN ĐỨC1 Viện Khoa học Kỹ thuật Vật liệu, Trường Đại học Bách khoa Hà Nội, Số Đại Cồ Việt, Hà Nội *Email: vuong.hoangvan@hust.edu.vn Ngày nhận bài: 2/3/2021, Ngày duyệt đăng:12/4/2021 ABSTRACT Carbon nanofibers (CNFs) were synthesized by the catalytic chemical vapour deposition (CCVD) process using iron catalyst below the iron-carbon eutetic temperature (1147 oC) CNFs were formed on carbon fibers as substrates at the growth temperatures of 750, 800, and 850 oC The effect of processing temperature on the formation and the morphology of CNFs was studied Moreover, the effect of composition of gase mixture (C2H4, H2, and Ar) on the morphology of CNFs was also performed The morphology and microstructure of as-obtained CNFs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) Results revealed that temperature and gas composition play an important role in forming of CNFs Keywords: Carbon nanofibers, catalytic CVD, carbon fibers TÓM TẮT Sợi nano-cacbon tổng hợp trình lắng đọng pha hóa học xúc tác sử dụng chất xúc tác sắt nhiệt độ eutetic sắt-cacbon (1147 oC) Sợi hình thành sợi cacbon làm chất nhiệt độ 750, 800 850 oC Ảnh hưởng nhiệt độ xử lý đến hình thành hình thái sợi nghiên cứu Hơn nữa, ảnh hưởng thành phần hỗn hợp khí (C2H4, H2 Ar) đến hình thái CNFs thực Hình thái cấu trúc vi mơ sợi nano-cacbon khảo sát kính hiển vi điện tử quét, kính hiển vi điện tử truyền qua nhiễu xạ điện tử Kết cho thấy nhiệt độ thành phần khí đóng vai trị quan trọng đến số lượng hình thái sợi cacbon Từ khóa: sợi nano-cacbon, lắng đọng pha hóa học, xúc tác, sợi cacbon INTRODUCTION Synthesis of carbon nanofibers (CNFs) has been attracting great attention; promising new class of materials for their applications, such as catalyst supports, field emission, memory devices and fillers for polymer composites In the 1990s, many reports showed that carbon nanotubes (CNTs) were formed during arc-charge synthesis CNFs and CNTs are grown by the diffusion of carbon through a metal catalyst and subsequently precipitated as graphitic filaments [19] Depending on the fabrications, structure types of CNFs was generated for their different properties and applications The recent studies indicated that three distinct structural types of filaments have been identified based on the angle of the _ DOI: 10.52923/vmfs.jstm.42021.95.04 graphenic layers with respect to the filament axis [5, 7], namely stacked, herringbone (or cupstacked) [8], and nanotular [9] To synthesis CNFs, many synthesis methodologies and routes have been developed Most commonly, carbon nanofilaments are produced by evaporating solid carbon in an arc discharge, by laser beams or by catalytic chemical vapor deposition (CCVD) of carbon-containing gases [9-11] In the CCVD method, the commonly used metal catalyst are Ni, Pt, Co, Mo, Mn and Fe or metal mixtures Ni-Fe, Mo-Fe, Co-Fe compounds with carbon to form the active phases This metal catalyst is coated on various subtrates such as Si, carbon fibers, or glass fibers In the early 1990s, the most accepted mechaSố 95 tháng 4/2021 TAP CHI KHOA HOC-CONG NGHE KIM LOAI Journal of Science and Technology of Cơng trình nghiên cứu nism was postulated by Baker et al, who explained the growth of carbon filaments by catalytic decomposition of the carbon feedstock and bulk diffusion of carbon [12] The hydrocarbon gases decompose on the front-exposed surfaces of metal particle to release hydrogen and carbon, which dissolve in the metal particle The dissolved carbon diffuses through the particle and is precipitated to form the body of the carbon filament [5, 13] Rodriquez et al [5, 14] indicated the ability to control and tailor the structure of nanofibers (stacked or herringbone) Hou et al indicated that the small iron particles were highly mobile at growth temperatures [15] Under certain condition of gas composition, temperature, and catalyst composition, the high mobility and reactivity of the metal atoms, the catalyst nanoparticles undergo surface reconstruction to form unique geometrical shapes which drive the formation of nanofibers [5, 14, 16-17] In this paper, we show how to prepare CNFs based Fe catalyst on carbon fibers as substrates via catalytic growth by the CCVD method at growth temperatures below 1000 oC The presence of iron on carbon fibers is easy to form iron carbides The active iron catalyst is in “fluctuating solid state” of “iron carbide”, the carbon diffusion is volumetric to form CNFs during the growth temperature The catalyst-surface functionalization plays a very important role in growing CNFs on the substrates The catalyst was formed by the floating catalyst method, using solution of Fe(NO3)3 EXPERIMENTAL Catalyst preparation In the general, carbon nanofibers growth by the CCVD method require metal catalyst nanoparticles, a carbon feedstock (e.g, hydrocarbon), and 25 heat This work presents carbon nanofibers procedured by the floating catalyst method using iron as catalyst [18, 19] The floating catalyst method is commonly used for the bulk/mass production of filaments that has nonplanar geometries In this method, a liquid solution containing the catalyst in salt form is applied to the substrates The main advantage of using this technique is the purification [19] By this way, mixtures of different metal salts have been used as catalysts for nanofiber growth Soluble salt used for our experiment is iron nitrates Fe(NO3)3.9H2O [2022] The carbon fibers (Hankuk Carbon Co., Ltd) were used as the substrate The substrates were first repeatedly cleaned in isopropanal and deionized water and then dried By immersion of the substrate into the salt solution of 0.003 M of Fe(NO3)3 for a certain time period of about min., a catalyst layer was formed on surface of the carbon fiber After that, carbon fibers were dried by calcinations for 10 at 50 oC Synthesis of carbon nanofibers CNFs were grown by CVD using a horizontal CVD system The catalyst coated carbon fibers were first placed on the top of the alumina boat at the center And then the boat was inserted at the center of the tube furnace The CVD system was evacuated to low vacuum (102 torr) and then purged with high purity argon for several times This was done to ensure that no carrier gas was introduced into the reaction tube and acted for protection of material from heat transfer to outside The CVD system was heated from room temperature to growth temperature The CNFs growing was carried out at different temperatures of 750, 800, and 850 oC for a 30 deposition When the growth temperature is reached, the carbon Figure FE-SEM images of CNFs synthesized on carbon fibers at different temperatures of 850 oC (a), 800 oC (b), and 750 oC (c) using the C2H4/H2/Ar reactant TAP CHI KHOA HOC-CONG NGHE KIM LOAI Số 95 tháng 4/2021 26 Công trình nghiên cứu Journal of Science and Technology of feedstock is introduced In this experiment, mixure gases of C2H4(30 sccm)/H2(100 sccm)/Ar (100 sccm) and C2H4(30 sccm)/H2(100 sccm) were employed as the reactant Characterization techniques The as-synthesized CNFs were characterized via field emission scanning electron microscopy (FE-SEM, 4800, Hitachi, Japan) and 300 kV in-situ transmission electron microscope (In-Situ TEM, JEM-3011 HR, JEOL, Japan) RESULTS AND DISCUSSION The growth temperature plays an important role in the successful growth of CNFs by the catalytic chemical vapour deposition Effect of temperature on the formation of the Fe particles on the substrates and diffusion of carbon through a metal catalyst and its subsequent precipitation as graphitic filaments control the morphologies of CNFs grown on the carbon fibers The higher temperature, the easier is formation of the larger Fe particles and the easier is diffusion of carbon through Fe catalyst particles and it’s easier to form CNFs of larger diameter We found that relatively non-uniform growth of carbon fibers (CFs) could be observed at the growth temperature when the deposition temperature is greater than 750 oC As shown in Figure 1, CFs were successfully grown on the carbon fibers at temperatures of 750, 800, and 850 oC Figure shows the mean diameters of CFs grown at different temperatures An increase of CFs’ diameter was found from 750 to 850 oC The Figure The mean diameter of CFs grown with different temperature growing temperature increased, the CFs diameter increased and its quantity deceased Due to high temperature, merging of the Fe catalyst particles could increase size and decrease number of particles Ignocio et al [19] reported that this increase in diameter was caused by the growth of iron particles, which corresponds to the diameters of CFs synthesized The diameter of CFs is in the range of 40 nm to 460 nm The FE-SEM image of the CFs synthesized at 850 oC revealed that the CFs were formed in the largest diameter, but the shortest length Herein, the growth rate of CNFs was low when synthesized between 750 and 800 oC The uniform diameter of CNFs synthesized at 800 oC was of 100 nm and much smaller than that of CNFs obtained at 850 oC To study the effect of the gase flow rates on the morphologies of CNFs, the Figure FE-SEM images of CNFs synthesized on carbon fibers at 800 oC with different gase systems (a) C2H4(30 sccm)/H2(100 sccm)/Ar(100 sccm) and (b) C2H4(30 sccm)/H2(100 sccm) Số 95 tháng 4/2021 TAP CHI KHOA HOC-CONG NGHE KIM LOAI Journal of Science and Technology of Cơng trình nghiên cứu 27 Figure TEM images of CNFs synthesised at 800 oC on carbon fiber with gase systems and flow raetes of C2H4: 30 sccm / H2:100 sccm at (a) low magnification and (b) high magnification Figure (a) TEM image and (b) electron diffraction pattern of a single carbon fiber growth temperature of 800 oC and the concentration of iron catalyst solution of 0.003 M Fe(NO3)3 were maintained Gas composition and flow rate were H2(100 sccm)/Ar(100 sccm)/C2H4(30 sccm) and H2 (100 sccm)/C2H4(30 sccm) sition using the iron catalyst prepaired by floating catalyst method Different morphologies of CFs were synthesized by controlling the growth condition It suggests that the growth temperature and gas composition play an important role in the mophology, diameter and quantity of CFs The growing temperature increased the CFs diameter and decreased its quantity The CNFs synthesized at temperature 750 and 800 oC were uniform and had diameter less than 100 nm Without Ar gas, the obtained CNFs had larger diameter, shorter length, and less density Figure is a TEM image showing clusters of CNFs on the surface of carbon fiber A higher magnification image clearly shows the hollow core structure The average diameter of the hollow core calculated from the TEM image is about 18 nm Figure shows the selected area electron diffraction pattern from the CNFs grown on carbon fibers at 800 oC with C2H4/H2 reactant The measurement shows that the diffraction rings confirm the presence of polycrystalline carbon fibers CONCLUSION Carbon nanofibers successfully grown on carbon fibers with the catalytic chemical vapour depo- TAP CHI KHOA HOC-CONG NGHE KIM LOAI Số 95 tháng 4/2021 ACKNOWLEDGEMENT This research is supported by the Hanoi University of Science and Technology (HUST), under project number T2018-TT-205 28 Cơng trình nghiên cứu Journal of Science and Technology of REFERENCES R T K Baker, M A Baker, P S Harris, F S Feates, and R J Waite; Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene, J Catal., Vol 26, 1972, pp 51-62 R T K Baker, P S Harris, R B Thomas, and R J Waite; Formation of filamentous carbon from iron, cobalt and chromium catalyzed decomposition of acetylene, J Catal., Vol 30, 1973, pp 86-95 R 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