SYNTHESIS OF MULTI-WALLED CARBON NANOTUBES BY CVD METHOD ON SUPPORTED CATALYSTS

SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 27 SYNTHESIS OF MULTI-WALLED CARBON NANOTUBES BY CVD METHOD ON SUPPORTED CATALYSTS A.A. Rakhimov¹, M.M. Adilov2, I.Kh. Khudoyqulov3, Kh.B. Ashurov4 Institute of Ion-Plasma and Laser Technologies, Uzbekistan Academy of Sciences, 33 Durmon Yuli Street, 100125, Tashkent, Uzbekistan1,2,3,4 https://doi.org/10.5281/zenodo.17606473 Abstract. In this work, multi-walled carbon nanotubes (MWCNTs) were synthesized by the CVD method using a thin Ni catalyst layer in the vapor phase. A silicon substrate was used as the base material, on which a 40 nm thick TiO2 buffer layer was deposited. AFM image analysis of the Ni thin film treated in an H₂ plasma environment at 700 °C revealed that nanoparticles with an average diameter of 15 nm were formed when the film thickness was 3.7 nm, whereas the diameter increased to 38.9 nm when the film thickness reached 17.6 nm. SEM morphological analysis of the synthesized CNTs showed the formation of nanotubes with diameters ranging from 12 to 25 nm and lengths of several micrometers. EDS analysis confirmed that the composition mainly consisted of carbon. Raman spectroscopic analysis indicated that the nanotubes were of high quality and confirmed the formation of multi-walled carbon nanotubes. Keywords: Electron beam evaporation, catalyst nanoparticles, multi walled carbon nanotubes, thin film, chemical vapor deposition. Introduction Carbon nanotubes (CNTs) have been extensively studied by researchers due to their remarkable physical properties, such as high resistance to mechanical stress [1], excellent electrical and thermal conductivity [2], chemical stability [3], and lightweight nature. Owing to these unique characteristics, CNTs have found applications in various fields, including energy storage systems [4], transistors [5], composite materials [6], and biomedicine [7]. However, these application areas require the synthesis of nanotubes with specific properties tailored to their intended use. Meeting these requirements involves the synthesis of high-quality nanotubes with controllable size and morphological characteristics. There are several methods for synthesizing carbon nanotubes (CNTs), including laser ablation, arc discharge, and chemical vapor deposition (CVD). Among these methods, the CVD technique is considered the standard approach for CNT synthesis due to its advantages such as controllable reaction conditions, direct growth on substrate materials, high product quality, and large-scale synthesis capability. Typically, the synthesis of CNTs by the CVD method involves the following stages: deposition of a thin catalyst layer on the substrate surface, formation of catalytically active nanoparticles from the thin film, and synthesis of nanotubes at high temperatures using a carbon source in the presence of the catalyst. Transition metals such as Ni, Co, and Fe are widely used as catalyst particles. Among these, Ni nanoparticles exhibit high efficiency in CNT synthesis via the CVD process [8]. Moreover, oxide buffer layers grown on the substrate surface significantly affect the CNT growth process, as they limit the diffusion of nanoparticles into the substrate at high temperatures, thereby improving the quality and growth rate of CNTs [9]. However, different oxide buffer layers exhibit different effects during the CVD process. In particular, an Al₂O₃ buffer layer provides higher efficiency in CNT synthesis compared to other oxide layers [10–11]. Therefore, the choice of buffer layer plays a crucial role in CNT SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 28 synthesis. In this work, the objective was to evaluate the effect of Ni catalyst layer thickness and thermal treatment in a plasma environment on nanoparticle formation, as well as to assess their catalytic activity in CNT synthesis. Experiment A silicon (111) single-crystal wafer was used as the substrate for vapor-phase deposition of the nickel catalyst thin film. The wafer was n-type, with a diameter of 7.5 cm and a thickness of 378 µm. To remove various organic compounds from the wafer surface, 325 ml of distilled water was first poured into a glass container, then 27% ammonium hydroxide (NH4OH) was added and the solution was heated on a hot plate up to 70 °C. The hot solution was then removed from the plate and 65 ml of hydrogen peroxide (H2O2) was added. Bubbles formed in the solution within two minutes, making it ready for use. The substrate samples were immersed in this solution for 15 minutes. Afterwards, the samples were rinsed several times in a container with deionized water. To remove the oxide layer from the substrate, 20 ml of 49% hydrofluoric acid (HF) was added to 480 ml of water in a plastic container, and the wafers were immersed in this solution for two minutes. The silicon wafers were then rinsed several times in deionized water and dried under a nitrogen atmosphere. Next, a 50 nm thick TiO2 thin film was grown on the Si substrate by atomic layer deposition (ALD) using an SI PEALD system (SENTECH Instruments GmbH). The deposition conditions were described in detail in our previous work [12]. An electron-beam evaporation method was used to deposit the nickel thin film on the buffer layer surface. The Ni evaporation process was carried out under a vacuum of 10-6 Torr, with an emission current of 50 mA and a substrate-totarget distance of 20 cm for 30 seconds. Ellipsometric measurements revealed that the thickness of the Ni film was 9 nm. The conversion of the deposited nickel thin film into catalyst nanoparticles and the subsequent growth of carbon nanotubes were performed using a chemical vapor deposition (CVD) system. Initially, the catalyst particles were activated at 700 °C for 20 minutes in a mixed gas atmosphere of H2 and Ar at a 1:3 ratio. After activation, acetylene (C2H2) gas was introduced into the chamber at a flow rate of 200 sccm to initiate CNT synthesis. The synthesis process was maintained for 30 minutes. The topography of the Ni catalyst nanoparticles was analyzed using Atomic Force Microscopy (NT-MDT Solver Next), while the morphology of the CNTs was evaluated through Scanning Electron Microscopy (SEM) using a ZEISS Sigma 500 – TSTVN system. SEM images were obtained at an accelerating voltage of 5 kV and a working distance of 6.5 mm. Additionally, the structural analysis of the CNTs was performed using Transmission Electron Microscopy (TEM) (HRTEM JEM-3010 URP, JEOL). The quality of the nanotubes was examined with a Renishaw RM 100 (In-Via Raman spectrometer) analytical system using a laser excitation wavelength of 785 nm (1.58 eV) and a laser spot diameter of 2 µm. The thicknesses of the catalyst and buffer layers were measured using a Spectroscopic Ellipsometer (model SER850). Result and discussion Fragmentation of the Ni Thin Film into Nanoparticles. Our investigation initially focused on studying the formation process of Ni nanoparticles under the influence of H2 plasma at high temperatures. AFM analysis of Ni thin films deposited from the vapor phase onto TiO2 buffer layers at room temperature (without plasma treatment) showed that the films were uniformly formed (homogeneous) (not shown in this paper). Subsequently, the thin films were thermally treated in H₂ plasma at 700 °C for 30 minutes. The thicknesses of the obtained catalyst layers were 3.7 nm, 9.2 nm, and 17.6 nm, respectively. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 29 Figure 1 shows the two-dimensional AFM images of catalyst layers with different thicknesses (Fig. 1 (a–c)) and the corresponding one-dimensional surface height profiles (Fig. 1 (d–f)). As can be seen from the images, exposure to the plasma environment led to the fargmentation and rearrangement of the Ni thin films into Ni nanoparticles. In Fig. 1 (a), at a film thickness of 3.7 nm, the particles exhibit a spherical shape and are uniformly distributed across the surface, with an average particle size of 15 nm. The height profile (Fig. 1 (d)) shows that the surface roughness is very low, with an average value of Ra = 1.2 nm. When the Ni catalyst layer thickness increased to 9.2 nm, both the average particle diameter and surface roughness increased correspondingly to 28.2 nm and Ra = 3.9 nm (Fig. 1 (b, e)). At a film thickness of 17.6 nm, the nanoparticles were observed to be evenly distributed over the surface but had coalesced to form larger aggregates. For this case, the average nanoparticle diameter and surface roughness increased to 38.9 nm and Ra = 12.7 nm, respectively. The observed increase in Ni cluster size with increasing film thickness is consistent with previously reported results by other research groups [13–14]. The decomposition of the Ni film into nanoparticles and the variation in their size under thermal treatment in a plasma environment can be explained by several factors. Initially, the as-deposited thin film is in a metastable state, and its rupture may occur along grain boundaries or due to thickness fluctuations and voids within the film. In metal/oxide systems, there exists a driving force for dewetting, which acts to minimize the total free energy. This force is associated with a reduction in the interfacial area between the film and the substrate, leading to the agglomeration of the film into three-dimensional (3D) islands [15]. Additionally, subsurface diffusion and Ostwald ripening processes also occur during these transformations. That is, the film breaks up into nanoparticles, which may diffuse into the underlying buffer layer, causing smaller particles to shrink. As the temperature increases, smaller nanoparticles tend to evaporate and diffuse toward larger ones, resulting in a more uniform size distribution of nanoparticles on the surface [16]. An increase in film thickness, however, slows down the diffusion process and promotes the formation of larger particles. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 30 Figure 1. AFM images (a–c) of Ni thin films with different thicknesses (3.7 nm, 9.2 nm, and 17.6 nm) grown on TiO2 buffer layers after thermal treatment in H2 plasma environment, and the corresponding surface height profiles (d–f). Morphology and EDS Analysis of CNTs Synthesized on Ni Nanoparticles. The morphology of the synthesized CNTs was evaluated using scanning electron microscopy (SEM). Figure 2. Surface (a) and cross-sectional (b) SEM images of the CNTs. Figures 2 (a, b) show the surface and cross-sectional SEM images of CNTs grown on the Ni/TiO2/Si sample. From both the surface and cross-sectional views, it can be observed that randomly oriented CNT bundles were formed. It was also determined that the length of the CNTs was several micrometers. It was also noted that the nanostructure quality was relatively low, with the presence of a considerable amount of amorphous carbon. However, the nanotubes were densely packed, and ImageJ analysis showed that their diameters ranged from 12 nm to 25 nm. Elemental Analysis of the Nanotubes. Figure 3 shows the energy-dispersive X-ray spectroscopy (EDS) spectrum of the nanotubes. The EDS spectrum of the synthesized CNTs revealed the presence of C, Ti, O, Si, and Ni elements in the composition. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 31 Figure 3. EDS spectrum of the CNT sample grown on the Ni/TiO2/Si surface. As seen from the spectrum, carbon (C) and silicon (Si) are the main elements in the composition, while oxygen (O), titanium (Ti), and nickel (Ni) are present in smaller mass fractions of 0.73%, 0.85%, and 0.24%, respectively. The presence of aluminum (Al) was also detected, which can be attributed to impurities present in the CVD chamber during the synthesis process. Table 1 presents the elemental composition of the sample in percentage values. Table 1. Elemental composition and mass percentages of the CNT sample synthesized on the Ni/TiO2/Si surface. Element Line Mass (%) Atom (%) C K 78.2 88.65 Si K 19.35 9.39 O K 0.73 0.63 Ni K 0.24 0.34 Ti K 0.85 0.24 Al K 0.63 0.75 Total 100.00 100.00 Raman analysis of CNTs. Raman spectroscopy was performed to evaluate the quality and structure of the synthesized CNTs (Figure 4). The spectrum shows a G band at 1571 cm-1, which corresponds to the sp2-hybridized carbon atoms in the nanotube walls, and a D band at 1339 cm-1, associated with structural defects. Additionally, a G’ band (second-order Raman scattering) was observed at 2687 cm-1. However, no RBM (radial breathing mode) peak characteristic of singlewalled CNTs was detected, indicating that the nanotubes are multi-walled carbon nanotubes (MWCNTs). The G’ peak provides a more precise assessment of the quality or purity of MWCNTs, as in the case of carbon nanotubes, the resonant Raman intensity in the G’ range increases sharply [17]. In the spectrum, the low intensity of the G peak indicates a low mass fraction of CNTs in the sample. This is because the G’ band arises from a two-phonon process, and when the sample is more disordered — that is, when more impurities are present — the coupling effect required for this two-phonon process does not occur, resulting in a decrease in intensity [18]. The obtained Raman spectrum analysis supports the results from the SEM image analysis. Typically, the ratio of D to G peak intensities characterizes the quality of the nanotubes, where a lower ratio indicates SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 32 a higher degree of crystallinity. From the spectral data, the intensity ratio (ID/IG=0.5) was determined, indicating that the nanotubes have few defects and possess high structural quality. Figure 4. Raman spectrum of the synthesized CNTs. Conclusion Carbon nanotubes (CNTs) were synthesized by the CVD method based on a Ni thin film grown from the vapor phase. The Ni thin film was activated as a catalyst for nanotube synthesis under high-temperature plasma treatment. AFM images showed that thermal treatment of the thin film in the plasma environment led to the formation of uniformly distributed nanoparticles across the surface. With increasing film thickness, both the average particle diameter and surface roughness were observed to increase correspondingly. Morphological SEM analysis of the CNTs synthesized at 700 0C using C2H2 precursor on Ni catalyst particles demonstrated the formation of high-quality nanotubes along with traces of amorphous carbon. 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