Controlled Synthesis of Ultralong Carbon Nanotubes with Perfect

Feb 10, 2017 - Abstract Image. Conspectus. Carbon nanotubes (CNTs) have drawn intensive research interest in the past 25 years due to their excellent ...
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Controlled Synthesis of Ultralong Carbon Nanotubes with Perfect Structures and Extraordinary Properties Rufan Zhang,†,§ Yingying Zhang,‡ and Fei Wei*,† †

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, and Department of Chemistry and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China

‡

CONSPECTUS: Carbon nanotubes (CNTs) have drawn intensive research interest in the past 25 years due to their excellent properties and wide applications. Ultralong CNTs refers to the horizontally aligned CNT arrays which are usually grown on flat substrates, parallel with each other with large intertube distances. They usually have perfect structures, excellent properties, and lengths up to centimeters, even decimeters. Ultralong CNTs are promising candidates as building blocks for transparent displays, nanoelectronics, superstrong tethers, aeronautics and aerospace materials, etc. The controlled synthesis of ultralong CNTs with perfect structures is the key to fully exploit the extraordinary properties of CNTs. CNTs are typical one-dimensional single-crystal nanomaterials. It has always been a great challenge how to grow macroscale single-crystals with no defects. Thus, the synthesis of ultralong CNTs with no defect is of significant importance from both fundamental and industrial aspects. In this Account, we focus on our progress on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. A deep understanding of the CNT growth mechanism is the first step for the controlled synthesis of ultralong CNTs with high quality. We first introduce the growth mechanism for ultralong CNTs and the main factor affecting their structures. We then discuss the strategies to control the defects in the as-grown ultralong CNTs. With these approaches, ultralong high-quality CNTs with different structures can be obtained. By completely eliminating the factors which may induce defects in the CNT walls, ultralong CNTs with perfect structures can be obtained. Their chiral indices keep unchanged for several centimeters long along the axial direction of the CNTs. The defect-free structures render the ultralong CNTs with excellent electrical, mechanical and thermal properties. The as-grown ultralong CNTs exhibit superhigh mechanical strength (>100 GPa) and their breaking strain (>17.5%) reach the theoretical limits. They also show excellent electrical and thermal properties. In addition, centimeters long CNTs showed macroscale interwall superlubricious properties due to their defect-free structures. Ultralong, defect-free CNTs with controlled structures are highly desirable for many high-end applications. We hope that this Account will shed light on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. Moreover, the growth mechanism and controlled synthesis of ultralong CNTs with perfect structures also offers a good model for other one-dimensional nanomaterials. properties. Ultralong CNTs refer to those grow on flat substrates, parallel with each other with large intertube distances. They usually have centimeters and even decimeters lengths, better structures, low defect densities, and excellent properties. Chemical vapor deposition (CVD) is the mainstream for synthesizing ultralong CNTs. Compared with the base-growth mode in which the CNTs keep staying on the substrates during the whole growth,7,8 the tip-growth mode directed by gas-flow is more favorable for growing ultralong CNTs with centimeter-scale length, fewer defects, and better properties.9−11 In the past two decades, intensive studies were conducted on ultralong CNTs.2,12 However, it has always been a great challenge to synthesize ultralong, defect-free CNTs with macroscale lengths, desirable structures, homogeneous chiralities, and extraordinary properties, etc. The periodic makeup of CNTs suggest that their growth is just like the growth of

1. INTRODUCTION Carbon nanotubes (CNTs) have attracted intensive research interest since 1991. CNTs are a kind of Dirac material, which refers to the materials that have Dirac nodes in the spectrum.1 This unique property of Dirac materials renders CNTs with outstanding mechanical, electrical, and thermal properties and great potential for numerous applications, e.g., superstrong fibers, nanoelectronics, ultrafast photonics, sensors, etc.2,3 For instance, CNTs are able to carry current densities up to 109 A cm−2, which is 1000 times greater than that of noble metals. Semiconducting single-walled CNTs (SWCNTs), with both electron and hole mobility as high as 79 000 cm2 V−1 s−1,4 are regarded as the best candidate for next-generation field-effect transistors (FETs).5 The thermal conductivity of SWCNTs is as high as 6600 W/(m·K),6 three times higher than that of diamond. However, the structural defect in practically obtained CNTs makes their properties much lower than theoretical predictions. Among all types of CNTs, ultralong CNTs are the only one that can own theoretically perfect structures and © 2017 American Chemical Society

Received: August 27, 2016 Published: February 10, 2017 179

DOI: 10.1021/acs.accounts.6b00430 Acc. Chem. Res. 2017, 50, 179−189

Article

Accounts of Chemical Research

Figure 1. (a) Schematic drawing of the tip-growth mechanism of ultralong CNTs. (b) AFM image of the oriented long CNTs with a particle on the tip. (c) Illustration of tip-growth of ultralong CNTs based on Schulz−Flory distribution. (d) Theoretical percentage distribution of ultralong CNTs with different lengths and different catalyst activity probabilities. (e) Theoretical density distribution of ultralong CNTs with different lengths and different catalyst activity probabilities. (a) Reprinted with permission from ref 11. Copyright 2007 American Chemical Society. (b) Reprinted with permission from ref 21. Copyright 2004 American Chemical Society. (c−e) Reprinted with permission from ref 20. Copyright 2013 American Chemical Society.

crystals13 It has always been a great challenge to grow large single-crystals with perfect structures. Therefore, the growth of ultralong, defect-free CNTs with perfect structures is of special significance for the study of not only CNTs but also other nanomaterials. In this Account, we review our progress on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. We hope this Review will shed light on the future development of ultralong CNTs with perfect structures and extraordinary properties.

that, in the catalyst region, not all the CNTs are able to grow continuously and eventually become ultralong ones.15−17 For the CNT growth directed by the gas flow, the number of ultralong CNTs always decreases along the length direction (Figure 1c). It is widely recognized that the catalyst plays a key role in the growth of ultralong CNTs. According to the screwlike dislocation theory,13 ultralong CNTs can be viewed as a kind of linear carbon polymers, the growth of which obeys Schulz−Flory (SF) distribution. SF distribution is a mathematical formula which expresses the percentage of linear polymers with different lengths after a polymerization process.18,19 During the growth of ultralong CNTs, it can be regarded as a probability event that if the catalysts can keep active enough during the whole CNT growth process. We define the probability that a catalyst nanoparticle can keep active enough to support a CNT growth when adding a unit length as α (Figure 1c).20 According to the SF distribution, the number density of CNTs (dL) at the distance L from the starting position on the substrate is described as dL = α(L−1) and the CNT percentage (PL) with length L is described as PL = α(L−1)(1 − α). The theoretical percentage and areal density distribution of ultralong CNTs with different lengths and different α is shown in Figure 1d and e. As seen in Figure 1e, no

2. GROWTH MECHANISM OF ULTRALONG CNTs As mentioned above, the tip-growth mode is more favorable than the base-growth for synthesizing ultralong CNTs with perfect structures due to the fact that they can get rid of the influence of substrates.9−11,14 During the CNT growth process, there is a temperature difference between the gas flow and the substrate, which produces a thermal buoyancy vertical to the substrate (Figure 1a). Because of the thermal buoyancy, some CNTs will lift off from the catalyst region and then keep floating in the gas flow in CVD process, during which the catalysts keep on the tip of the CNTs (Figure 1b). The tipgrowth usually results in ultralong CNTs with centimeters lengths or even longer. However, it has been widely observed 180

DOI: 10.1021/acs.accounts.6b00430 Acc. Chem. Res. 2017, 50, 179−189

Article

Accounts of Chemical Research

Figure 2. (a) SWCNT−catalyst particle interface, where a circular tube’s open end is attached to a step edge on the catalyst particle. (b) A fraction of the SWCNT-catalyst step (a step along the (211) direction on the (111) surface of the fcc crystal) interface is modeled as an interface of the graphene-stepped metal surface. (c−e) Healing of the pentagon (p defect), heptagon (h defect), and pentagon-heptagon pair (5|7) and the corresponding geometries (original defect formations, transition states and products after healing) involved. (f) Energy barrier (Ea) (black real line with symbols) and the reaction energy (Er) (red dashed line with symbols) for the p, h, and 5|7 defects healing on stepped Fe(111)/Co(111)/ Ni(111) surfaces. (g−i) Relationship between catalyst activity probability (α) and different factors. (g) Relationship between α and growth temperature. (h) Relationship between α and water concentration. (i) Relationship between α and H2/CH4 ratio. (j) Relationship between α and gas velocity. (a−f) Reprinted with permission from ref 31. Copyright 2012 American Physical Society. (g−j) Reprinted with permission from ref 20. Copyright 2013 American Chemical Society.

matter for what kinds of α, it is an irreversible trend that, with the increase of CNT lengths, their number density will decrease irreversibly. A low α means that the growth of CNTs is easy to be terminated, leading to a low percentage of long CNTs. In order to increase the number density and length of ultralong CNTs, it is crucial to improve α as high as possible. The SF distribution fits well with most experimental observations and successfully interprets the growth mechanism of ultralong CNT arrays. The catalyst activity probability also influences the CNT structures. As stated above, a small α indicates that there are many unfavorable factors affecting the CNT growth. Thus, the

probability of producing ultralong CNTs with structural defects becomes higher. In contrary, a large α indicates the growing conditions are favorable for CNTs to continuously grow into longer ones with fewer defects.

3. DEFECT CONTROL STRATEGIES FOR ULTRALONG CNTs Defects in CNTs such as vacancies, dopant, pentagons, or heptagons can affect their properties drastically.22−27 Just a few pairs of topological defects can result in a dramatic decrease of the tensile strength of the CNTs.28 The breaking strain of a 181

DOI: 10.1021/acs.accounts.6b00430 Acc. Chem. Res. 2017, 50, 179−189

Article

Accounts of Chemical Research

Figure 3. (a) (left) Photo of the 100 mm long Si substrate on which individual TWCNTs were grown; (middle) SEM images of the end segments of a 100 mm long TWCNT; (right) end of TWCNT with a catalyst particle. (b)) SEM images of the region where the growth started (0 to 40 mm on the Si substrate) showing horizontally aligned, straight TWCNTs. (c) TEM image of a portion of the TWCNT (enclosed by the red rectangle in (b)) transferred to TEM grids. (d) HRTEM image showing a triple-walled structure of the as-grown tube. (e, f) Structure distribution of CNTs grown at different temperatures: (e) distribution of numbers of walls of centimeter long CNTs; and (f) distribution of outer diameters of centimeterlong CNTs. (g) Optical picture of the 20 cm long Si substrate. (h) Optical picture of the substrate connected by Si (dark substrates) and SiO2 (white substrates) substrates. (i, j) TEM pictures of a DWCNT and a TWCNT. (k) Raman spectra of a DWCNT. (l) SEM image of 550 mm long CNTs. (m) Number of CNTs at different length on the substrate. Inset: TEM images of as-grown CNTs. (n) Raman spectrum of as-grown CNTs. (o) Mechanical properties of as-grown CNTs. (a−f) Reprinted with permission from ref 35. Copyright 2010 Wiley-VCH. (g-k) Reprinted with permission from ref 16. Copyright 2010 American Chemical Society. (l−o) Reprinted with permission from ref 20. Copyright 2013 American Chemical Society.

efficiency for healing defects (Figure 2f).31 As mentioned above, the tip-growth is more favorable than the base-growth for producing ultralong CNTs as they can get rid of the influence of substrates. Temperature is a key factor affecting the activity of metal catalysts (Figure 2g). It is critical to keep the temperature stable and identical during the CNT growth. Another important factor affecting the catalyst activity is the feedstock. The most widely employed carbon sources for growing ultralong CNTs are methane and ethanol.2,12,33 When choosing these carbon sources, one should also take the purity of these feedstocks into consideration. In addition, it was confirmed that the growth of ultrlaong CNTs could be greatly improved by adding a trace amount of water vapor into the feedstock.16 The concentration of water vapor in the feedstock significantly affects the catalyst activity (Figure 2h). Besides, the H2/CH4 ratio also influences the catalyst activity (Figure 2i).20 Researchers also found that the gas velocity could remarkably affect the floating state of growing CNTs as it can change the Richardson number and Renault number of the gas flow (Figure 2j).34 In order to obtain ultralong CNTs with high

CNT with topological defects will be greatly reduced by more than 50%.28 Therefore, it is of significant importance to control the defect formation during the CNT growth. The efficient defect healing is crucial for obtaining high-quality CNTs. It is found that the ring reconstruction plays a significant role in healing the topological defects.29−31 With the assistance of a metal catalyst, topological defects which are generated at the CNT-catalyst interface when adding C atoms are able to be healed efficiently (Figure 2a−e).31 Moreover, as for the defects which are not promptly healed during CNT nucleation, they still can be vanished by moving to the CNT end where the catalyst can efficiently heal them.32 In order to synthesize ultralong CNTs with perfect structures, there are several prerequisites needed to be satisfied, e.g., highly active catalysts with long lifetime, a stable environment for the steady nucleation and adding of carbon atoms, proper temperature, proper substrates, etc. It has been found that among the various catalysts which are most frequently employed, Fe is the best one for synthesizing ultralong CNTs with high qualities as it has the highest 182

DOI: 10.1021/acs.accounts.6b00430 Acc. Chem. Res. 2017, 50, 179−189

Article

Accounts of Chemical Research

Figure 4. (a) Illustration of a 100 mm long TWCNT and HRTEM images on three positions that are 25, 60, and 70 mm away from the growthstarting point. (b) Electron diffraction patterns recorded on the TWCNT at these three positions, and the chiral indices assigned to all three shells at each location. (c) Current flow through individual TWCNTs measured in air and nitrogen. The arrows (labeled 1, 2, and 3) indicate shell breakdowns occurred during the failure. Inset, illustration of the current-carrying capability of each shell in the TWCNT. (d, e) Raman spectra of a TWCNT and DWCNT. (a−c) Reprinted with permission from ref 35. Copyright 2010 Wiley-VCH. (d) Reprinted with permission from ref 20. Copyright 2013 American Chemical Society. (e) Reprinted with permission from ref 36. Copyright 2013 Nature Publishing Group.

precisely controlling the temperature at 1000 °C, we were able to improve the selectivity of TWCNTs to be as high as 90%. However, by changing the temperature with a small increase, e.g., 20−40 °C, we could get CNTs with different structures which contain high selectivity of SWCNTs (>70%) and DWCNTs (>50%) (Figure 3e). The diameter of CNTs increased from