Synthesis of Multiwalled Carbon Nanotubes on Different Cobalt Nanoparticle-Based Substrates

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors demonstrated a study to determine the optimal experimental conditions for the orientation of carbon nanotubes. The observed results showed that the optimal temperature for the Co/SiO₂/Si catalyst is 800 °C, while for the Co/C catalyst it is 450 °C. The growth of CNTs occurs from the top in the Co/C system, while for the Co/SiO₂/Si catalyst, the growth is from the bottom-up, which makes the latter more suitable for the synthesis of CNTs intended for nanoelectronic devices. The authors confirm the obtained results using TEM and SEM. Despite the fact that the aim of the work is to determine the optimal conditions for the synthesis of CNTs intended for nanoelectronic devices, nothing is reported about the properties of the obtained carbon product and its purity. The presence of amorphous carbon should have a significant effect on the electrophysical properties of CNTs. However, this is not discussed in the work. In addition, the manuscript is structured as an experimental report, and not a meaningful scientific paper. Nevertheless, manuscript can be recommended for publication in Inorganics after addressing some major issues that can be addressed to improve it:

# 1 Introduction section: The authors report that carbon nanotubes have a diameter of 0.7 to 10 nm and cite the work of Ajayan, P.M.; Ebbesen, T.W. Nanometre-size tubes of carbon; Rep. Prog. Phys. 1997,60, 1025. The diameter of nanotubes can reach 100 nm. In the current manuscript, the diameter of CNTs grown at 800 C is 47 nm. The relevant information should be corrected and it is recommended to cite more recent papers, since there was significantly less information on CNTs in 1997.

# 2 Results section: The catalysts abbreviation discussed in sections 2.1 and 2.2 are introduced only in the section caption, and are not given in the text. This is confusing. When describing the results in the relevant sections, Co/SiO₂/Si and Co/C should be used so that the reader clearly understands which catalyst is being discussed.

#3 Results. Section 2.1.1.: The authors report that amorphous carbon spheres formed at 350 mbar acetylene pressure have similar chemical inertness, thermal conductivity, and mechanical strength characteristics to diamond. However, no information is provided on how they investigated this.

#4 Results. Section 2.1.2.: The authors investigate the initial temperature of acetylene at the reactor inlet at 50 mbar acetylene pressure. Why 50 mbar? If 50 mbar was chosen as the optimal pressure, this should have been stated in Section 2.1.1., with the corresponding SEM images of the carbon product, and an appropriate conclusion.

#5 Results. Section 2.1.2.: It is reported that the diameters of Co and CNT nanoparticles were measured using the Feret's method. What is this method? An explanation and appropriate reference should be provided.

#6 Results. Section2.2, Line 292: Should be replaced in this paper with in this section.

#7 Results. Section 2.2.1.: The Co film thickness in the studied Co/C sample is not specified.

#8 Results. Section 2.2.2.: The effect of synthesis temperature is investigated on a Co/C sample with a Co film thickness of 20 Å. Why 20 Å was chosen? In the case of Co/SiO₂/Si, a thickness of 30 Å was chosen.

#9 In this paper, the authors investigated two different catalytic systems in order to determine the conditions for the preparation of CNTs intended for nanoelectronic devices. The conditions are determined, but nothing is reported about the properties of the obtained carbon product. In addition, the presence of amorphous carbon should have a significant effect on the electrophysical properties of CNTs. It is necessary to study the purity and electrophysical properties of CNTs obtained under optimal experimental conditions on Co/SiO₂/Si and Co/C catalysts.

Author Response

Dear Reviewer,
thank you very much for the time spent to improve our article. 
We appreciate your suggestions. We hope that in its current form the manuscript will find your approval. The answers to your suggestions are reported below and the changes are reported in red in the manuscript.
Kind regards
The Authors

Answers:
1) Introduction section: The authors report that carbon nanotubes have a diameter of 0.7 to 10 nm and cite the work of Ajayan, P.M.; Ebbesen, T.W. Nanometre-size tubes of carbon; Rep. Prog. Phys. 1997,60, 1025. The diameter of nanotubes can reach 100 nm. In the current manuscript, the diameter of CNTs grown at 800 C is 47 nm. The relevant information should be corrected and it is recommended to cite more recent papers, since there was significantly less information on CNTs in 1997.
Reply 1. The following sentence has been added to the introduction “They have a diameter that can reach up to 100 nm [3].” and the reference has been replaced with a more recent one. In introduction other references have been added [6][7[8].
[3]. Shoukat, R., Khan, M.I. Carbon nanotubes: a review on properties, synthesis methods and applications in micro and nanotechnology. Microsyst Technol. 2021, 27, 4183–4192. https://doi.org/10.1007/s00542-021-05211-6.
[6] Yahyazadeh, A.; Nanda, S.; Dalai, A.K. Carbon Nanotubes: A Review of Synthesis Methods and Applications. Reactions 2024, 5, 429-451. https://doi.org/10.3390/reactions5030022
[7]Tao, Z.; Zhao, Y.; Wang, Y.; Zhang, G. Recent Advances in Carbon Nanotube Technology: Bridging the Gap from Fundamental Science to Wide Applications. C 2024, 10, 69. https://doi.org/10.3390/c10030069
[8]Shooshtari, M.; Salehi A.; Vollebregt, S. Effect of Humidity on Gas Sensing Performance of Carbon Nanotube Gas Sensors Operated at Room Temperature. in IEEE Sensors Journal. 2021, 21,5, 5763-5770, doi: 10.1109/JSEN.2020.3038647.

2) Results section: The catalysts abbreviation discussed in sections 2.1 and 2.2 are introduced only in the section caption, and are not given in the text. This is confusing. When describing the results in the relevant sections, Co/SiO₂/Si and Co/C should be used so that the reader clearly understands which catalyst is being discussed.
Reply 2 The titles of subparagraphs 2.1 and 2.2 have been integrated.

3) Results. Section 2.1.1.: The authors report that amorphous carbon spheres formed at 350 mbar acetylene pressure have similar chemical inertness, thermal conductivity, and mechanical strength characteristics to diamond. However, no information is provided on how they investigated this.
Reply 3. We have referred to previous studies reported in literature by reporting the references. To explain better we have added the following parts. "... as reported in previous literature studies". We have also added other parts: " High pressure significantly affects the formation of carbon spheres rather than carbon nanotubes. This can be attributed to the higher thermodynamic stability of carbon spheres. In high-pressure environments, spherical structures are expected to be more stable than the open cylindrical structures of nanotubes.
4) Results. Section 2.1.2.: The authors investigate the initial temperature of acetylene at the reactor inlet at 50 mbar acetylene pressure. Why 50 mbar? If 50 mbar was chosen as the optimal pressure, this should have been stated in Section 2.1.1., with the corresponding SEM images of the carbon product, and an appropriate conclusion.
Reply 4. The following sentece has been added in paragraph 3:"The operational conditions selected and adopted in this research are the result of preliminary investigations and careful evaluations. We sought a balance between the optimal conditions to achieve our objectives and the experimental availability of our laboratory. 
5) Results. Section 2.1.2.: It is reported that the diameters of Co and CNT nanoparticles were measured using the Feret's method. What is this method? An explanation and appropriate reference should be provided.
Reply 5. This sentences has been added in paragraph 2.1.3: The Feret method, also known as the Feret diameter, is commonly used to analyze particle size. It is often applied in microscopy to measure the size of a three-dimensional object projected onto a two-dimensional plane. In the case of cobalt (Co) nanoparticles and carbon nanotubes (CNTs), the Feret diameter is measured as the distance between two parallel tangents touching the perimeter of the particle's projected area. This method is useful for characterizing the size and distribution of particles in a sample."
6) Results. Section2.2, Line 292: Should be replaced in this paper with in this section.
Reply 6. The sentence "In this work similar experiments were carried out to those reported on Co/SiO2/Si samples." was posted in section 3.1
7) Results. Section 2.2.1.: The Co film thickness in the studied Co/C sample is not specified.
Reply 7 In the figure caption .. "on a Co/C sample with 20 Å of Co deposit" has been added
8) Results. Section 2.2.2.: The effect of synthesis temperature is investigated on a Co/C sample with a Co film thickness of 20 Å. Why 20 Å was chosen? In the case of Co/SiO₂/Si, a thickness of 30 Å was chosen.
Reply 8.The following sentence has been added to the paragraph 3.1: The choice of the Co film thickness of 20 Å for the Co/C sample and 30 Å for the Co/SiO₂/Si sample was derived from preliminary investigations and considerations that evaluated the conditions for an adequate interaction with the substrate.
9) In this paper, the authors investigated two different catalytic systems in order to determine the conditions for the preparation of CNTs intended for nanoelectronic devices. The conditions are determined, but nothing is reported about the properties of the obtained carbon product. In addition, the presence of amorphous carbon should have a significant effect on the electrophysical properties of CNTs. It is necessary to study the purity and electrophysical properties of CNTs obtained under optimal experimental conditions on Co/SiO₂/Si and Co/C catalysts
Reply 9. Although the properties of the carbon product obtained have not been reported in detail, it is important to emphasize that the main objective was to identify the most effective synthesis conditions. The characterization of the electrophysical properties and purity of the obtained CNTs will be the subject of future studies, in which advanced analytical techniques, such as Raman spectroscopy and transmission electron microscopy (TEM), will be used to evaluate the influence of amorphous carbon and other impurities on the performance of the CNTs.
To better explain this, the following sentence was added to the conclusions: 'This study represents a gradual approach that has allowed the optimization of the synthesis conditions. A more in-depth characterization of the obtained products, such as purity and electrophysical properties, is expected to be the subject of future studies.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This paper investigates the synthesis of carbon nanotubes (CNTs) using two different catalytic supports, carbon grids and Si/SiO2 substrates, with cobalt nanoparticles as the catalyst. The process involves physical vapor deposition (PVD) of cobalt, followed by a catalytic chemical vapor deposition (CCVD) technique using acetylene, with a detailed description of the experimental conditions and characterization methods including TEM and SEM. In the following sections, I will outline my concerns and provide constructive comments to enhance the quality of the article:

 

1. The introduction provides a comprehensive background on carbon nanotubes (CNTs) but could benefit from better organization. The paragraph jumps between different topics, such as the history of CNTs, their properties, synthesis methods, and applications.
2. Some sentences could be more consistent. For instance, when describing the applications of CNTs, it would be helpful to list the types of applications in a more organized way (e.g., sensing, catalysis, reinforcement materials, etc.).
3. Are there specific properties of cobalt that make it more suitable for the CCVD process?
4. The manuscript could provide a more detailed explanation of the properties of CNTs (e.g., electrical, thermal, mechanical) and how these relate to their applications, particularly in nanoelectronics and sensors. Improve your literature review by adding new references about CNT application such as DOI: 10.1109/JSEN.2020.3038647.
5. The introduction could benefit from a clearer statement on why studying cobalt nanoparticle-based catalysts for CNT synthesis is important.
6. Put a dot end of line 92.
7. Provide more insight into why this high pressure produces carbon spheres rather than carbon nanotubes.
8. Explain more about the factors (e.g., catalyst deactivation, carbon saturation, etc.) that cause the morphology to shift to pyrolytic carbon at higher temperatures.
9. A comparison of CNT diameters, lengths, or other structural properties could support the observations about the effect of synthetic parameters.

Author Response

Dear Reviewer,
thank you very much for the time spent to improve our article. 
We appreciate your suggestions. We hope that in its current form the manuscript will find your approval. The answers to your suggestions are reported below and the changes are reported in red in the manuscript.
Kind regards
The Authors
ANSWERS:
1).The introduction provides a comprehensive background on carbon nanotubes (CNTs) but could benefit from better organization. The paragraph jumps between different topics, such as the history of CNTs, their properties, synthesis methods, and applications.
Reply 1. The introduction has been revised and better organized.  New parts have been added (see R2; R3; R4)
2). Some sentences could be more consistent. For instance, when describing the applications of CNTs, it would be helpful to list the types of applications in a more organized way (e.g., sensing, catalysis, reinforcement materials, etc.).
Reply 2 The following sentence has been added “Thanks to their particular characteristics and properties, carbon nanotubes find application in various sectors such as sensing, reinforcement and adsorption materials, catalysis.”
3). Are there specific properties of cobalt that make it more suitable for the CCVD process?
Reply 3 "The following statement was included in the manuscript: "The choice of cobalt was dictated by its properties. In fact, cobalt possesses certain characteristics that make it particularly suitable for the catalyst-assisted chemical vapor deposition (CCVD) process. It has high resistance to corrosion and oxidation and exhibits good thermal stability, meaning it can maintain its physical and chemical properties even at high temperatures. Furthermore, as a ferromagnetic metal, it can potentially respond to external magnetic fields, allowing the manipulation of the direction and density of nanotube growth.
4). The manuscript could provide a more detailed explanation of the properties of CNTs (e.g., electrical, thermal, mechanical) and how these relate to their applications, particularly in nanoelectronics and sensors. Improve your literature review by adding new references about CNT application such as DOI: 10.1109/JSEN.2020.3038647.
Reply 4. The following sentence was included in the manuscript  introduction: "CNTs have excellent electrical properties, including high electron mobility and low resistance. These attributes make them ideal for applications in nanoelectronics, such as high-speed transistors, interconnects, and storage devices. CNTs can also act as semiconductors or metallic conductors, depending on their structure and arrangement. CNTs exhibit extraordinary thermal conductivity, which can exceed that of the best conductive materials currently available. This makes them useful for heat dissipation in miniaturized electronic devices, improving the efficiency and lifetime of electronic components. Their ability to conduct heat is particularly advantageous for preventing overheating in integrated circuits and sensors. CNTs are extremely sensitive to changes in their surrounding environment, making them excellent sensor materials. They can detect gases, biomolecules, radiation, and temperature changes with high accuracy and sensitivity. Their high specific surface area and ability to respond rapidly to environmental changes make them ideal for applications in chemical, biological, an environmental sensors [8]."
[8] M. Shooshtari, A. Salehi and S. Vollebregt, "Effect of Humidity on Gas Sensing Performance of Carbon Nanotube Gas Sensors Operated at Room Temperature," in IEEE Sensors Journal, vol. 21, no. 5, pp. 5763-5770, 1 March1, 2021, doi: 10.1109/JSEN.2020.3038647.
5). The introduction could benefit from a clearer statement on why studying cobalt nanoparticle-based catalysts for CNT synthesis is important.
Reply 5. See R3
6). Put a dot end of line 92.
Reply 6. The point was added in the manuscript
7). Provide more insight into why this high pressure produces carbon spheres rather than carbon nanotubes.
Reply 7. The following sentence has been added to paragraph 2.1.1.: High pressure significantly affects the formation of carbon spheres rather than carbon nanotubes. This can be attributed to the higher thermodynamic stability of carbon spheres. In high-pressure environments, spherical structures are expected to be more stable than the open cylindrical structures of nanotubes.
8). Explain more about the factors (e.g., catalyst deactivation, carbon saturation, etc.) that cause the morphology to shift to pyrolytic carbon at higher temperatures.
Reply 8. The following sentence has been added to paragraph 2.1.3: At high temperatures, catalysts can undergo structural modifications or sintering, reducing their effectiveness. This can lead to the formation of pyrolytic carbon, as the active sites of the catalyst are no longer available for the orderly growth of carbon nanotubes. Additionally, at high temperatures, the amount of available carbon can exceed the catalyst's capacity to manage it efficiently. This can result in the formation of pyrolytic carbon deposits, which cover the active sites of the catalyst and inhibit the growth of carbon nanotubes. Furthermore, at high temperatures, pyrolytic structures can be more thermodynamically stable than carbon nanotubes. This means that, in the presence of high temperatures, pyrolytic carbon can become the preferred form of carbon atom aggregation.
9). A comparison of CNT diameters, lengths, or other structural properties could support the observations about the effect of synthetic parameters.
Reply 9. The following sentence has been added in conclusion “A systematic comparison of the diameters, lengths, or other structural properties of carbon nanotubes (CNTs) could provide a solid basis for supporting observations about the effect of synthetic parameters. For example, it is well-known that the size of CNTs can significantly influence their electrical, thermal, and mechanical properties. Evaluating how variations in synthesis parameters, such as temperature, growth time, or catalyst type, affect these dimensions and other structural characteristics would allow for a better understanding of the relationships between synthesis conditions and the final properties of CNTs. Future studies could therefore focus on these comparisons to further optimize synthesis conditions and enhance the performance of CNTs in their specific application fields.”

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The aim of this paper was to determine the optimal conditions for obtaining CNTs intended for nanoelectronic devices. In order to have grounds for choosing the optimal conditions, it is necessary to carefully analyze the obtained carbon products. However, this work does not report anything about the properties of the obtained carbon product and its purity. The presence of amorphous carbon should have a significant effect on the electrophysical properties of CNTs, but this is not discussed in the work. In my opinion, the authors responded to the main criticism of the paper in an extremely superficial manner and referred to the fact that this issue will be investigated in future works. I recommend that the authors analyze the properties and quality of carbon products directly in this work so that the study looks holistic.

Author Response

Dear Reviewer,
We sincerely thank you for your detailed comments and constructive observations regarding our manuscript. We deeply appreciate the time and attention you dedicated to reviewing our work, as well as your valuable suggestions for improving it further.
We recognize that a more in-depth analysis of the obtained carbon nanotubes, including their purity and the presence of amorphous carbon, would represent a significant contribution to the completeness of the study. However, due to certain technical and time constraints associated with the current phase of research, we were unable to include such data in this manuscript. We fully agree that the presence of amorphous carbon could significantly affect the electrophysical properties of CNTs, and we are aware of the importance of this aspect.
Nonetheless, we would like to emphasize that the primary aim of our study was to identify the optimal experimental conditions for achieving aligned carbon nanotube growth. For this reason, we have chosen to focus our efforts on this fundamental aspect during this initial phase, postponing the analysis of purity and electrophysical properties of CNTs to future studies, which we intend to carry out using the samples obtained in this work as a starting point.
Additionally, we apologize if our previous response was not sufficiently comprehensive, and we intend to improve the manuscript by integrating the following contents to better specify the scope of our work:
•    In the abstract: "The primary aim of this research was to identify the optimal experimental conditions for obtaining aligned carbon nanotubes, temporarily leaving aside aspects such as the purity of carbon nanotubes, which is nonetheless crucial for potential applications in the field of nanoelectronics."
•    In the introduction: "The presence of amorphous carbon in samples of carbon nanotubes (CNTs) can significantly influence their electrophysical properties. Amorphous carbon, which often deposits as a by-product during CNT synthesis, may introduce impurities that alter such properties. Specifically, amorphous carbon, being less structurally organized, can act as a barrier to electronic transport along CNTs, causing electron scattering or reducing the continuity of conductive paths. In nanoelectronic devices, amorphous carbon may form insulating or semiconducting layers around CNTs, negatively affecting CNT-electrode interfaces. Additionally, amorphous carbon can accumulate unwanted surface charges, altering the electrostatic properties of the system and negatively impacting device performance. Minimizing or eliminating amorphous carbon through purification methods or optimized synthesis techniques is fundamental to fully leveraging the electrophysical potential of CNTs. While we are fully aware of the crucial role of amorphous carbon and its influence on the electrophysical properties of carbon nanotubes (CNTs), in this research, we have focused, as a first phase, on identifying the optimal experimental conditions for obtaining aligned carbon nanotubes. The detailed analysis of amorphous carbon and its impact will be addressed in future studies, using the samples obtained in this work as the basis for further investigations."
We hope that these integrations will address your observations and enhance the overall value of our study. 
Best regards, 
The Authors

 

Reviewer 2 Report

Comments and Suggestions for Authors

N/A

Author Response

Dear Reviewer II,

We sincerely thank you for the time and effort you dedicated to improving our work. 
Best regards
The authors

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Accept