CN112441574B - Method for controllable growth of metallic single-walled carbon nanotube through substrate design - Google Patents
Method for controllable growth of metallic single-walled carbon nanotube through substrate design Download PDFInfo
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Abstract
The invention relates to the field of controllable preparation of metallic single-walled carbon nanotubes, in particular to a method for controllable growth of metallic single-walled carbon nanotubes by substrate design. Preparing metal oxide nanoclusters with uniform size by using spinel as a substrate and adopting a block copolymer self-assembly method; the size, structure and high-temperature stability of the catalyst nano-particles are regulated and controlled by utilizing the solid solution and pinning effects of the spinel substrate on the catalyst nano-particles, and the selective growth of the metallic single-walled carbon nano-tubes is realized by combining the influence of the spinel substrate on the growth rates of the single-walled carbon nano-tubes with different conductive properties. The diameter of the prepared metallic single-walled carbon nanotube is 1.1 +/-0.2 nm, and the content of the prepared metallic single-walled carbon nanotube is 75-85%. The invention realizes the direct controllable growth of the metallic single-walled carbon nanotube with narrow diameter distribution through the design and selection of the substrate, and lays a material foundation for promoting the application of the metallic single-walled carbon nanotube.
Description
Technical Field
The invention relates to the field of controllable preparation of metallic single-walled carbon nanotubes, in particular to a method for controllable growth of metallic single-walled carbon nanotubes by substrate design.
Background
Single-walled carbon nanotubes may be characterized as metallic or semiconducting due to differences in their chiral angles and diameters. The metallic single-walled carbon nanotube has quantum transport effect and can be used for flexible electrode materials and interconnection wires of future nanoelectronic devices. However, the single-walled carbon nanotube samples that are typically prepared are a mixture of metallic and semiconducting carbon tubes. How to obtain the high-purity metallic single-walled carbon nanotubes is the key to promote the practical application of the nanotubes. Metallic single-walled carbon nanotubes typically have only about 1/3% in the sample and are more chemically active than semiconducting single-walled carbon nanotubes, making selective preparation of metallic single-walled carbon nanotubes more difficult. Currently, representative work for selectively preparing metallic single-walled carbon nanotubes is as follows: (1) controlling the surface morphology of the catalyst during nucleation to preferentially grow metallic single-walled carbon nanotubes (ref: Harutyunyan A.R.; Cheng, G., Sumanasekera, G.U.et. al. science,2009,326,116); (2) preparing metallic enriched single-walled carbon nanotubes by using a hydrogen selective etchant and using small-diameter semiconductor carbon nanotubes (document II: Hou, P.X.; Li, W.S.; Liu C.et al.ACS Nano 2013,8, 7156); (3) growing metallic carbon nanotubes by controlling the specific crystal face of the catalyst to be matched with the structure of the single-walled carbon nanotubes (three documents: Yang, F.; Wang, X.; Li, Y.et al.Nature 2014,510,7506); (4) metallic carbon nanotubes were prepared by controlling the symmetry matching of the catalyst and carbon nanotubes (four: Zhang, s.c.; Kang, l.x.; Zhang, j.et al. nature 2017,543,7644). (5) The size and oxygen content of the high-melting-point non-metal oxide catalyst nanoparticles are controlled to realize the direct growth of the narrow-diameter distribution and metallic single-walled carbon nanotubes (fifth document: Zhang, L.L.; Sun, D.M.; Liu, C.et al.advanced Materials,2017,29, 32).
However, there are still many problems with the current preparation of metallic single-walled carbon nanotubes: (1) the catalyst is on the surface of the silicon substrate with small interaction with the catalyst, so that the problem of poor high-temperature thermal stability exists, and the diameter uniformity of the carbon nano tube is poor; (2) the difficulty in regulating and controlling the crystal face structure and symmetry of the catalyst is high, so that the repeatability is poor; (3) the mechanism of controlled growth of metallic single-walled carbon nanotubes remains unclear; (4) the thermodynamic and kinetic factors that the substrate directly affects the growth of carbon nanotubes are not well defined.
Therefore, the main problems facing today are: on the basis of understanding the controllable growth mechanism, the difference between nucleation and growth of metallic and semiconductor single-walled carbon nanotubes is enlarged, and a simple and convenient method for growing the metallic single-walled carbon nanotubes with narrow diameter distribution in a controllable manner is developed.
Disclosure of Invention
The invention aims to provide a simple and controllable method for controllably growing the metallic single-walled carbon nanotube through substrate design, which realizes the direct controllable growth of the metallic single-walled carbon nanotube with narrow diameter distribution through the substrate design and selection and lays a material foundation for promoting the application of the metallic single-walled carbon nanotube.
The technical scheme of the invention is as follows:
a method for controllable growth of metallic single-walled carbon nanotubes by substrate design selects magnesium aluminate spinel single crystal which generates lattice thermal vibration at high temperature and takes phonons as main heat conduction mode as a substrate, and controls the size, structure and high-temperature stability of a catalyst by utilizing the strong interaction of the substrate and the catalyst; the energy supply required by the growth of the carbon nano tube is regulated and controlled by utilizing the energy fluctuation caused by the lattice thermal vibration of the substrate at high temperature, the maximization of the growth rate difference of the metallic single-walled carbon nano tube and the semiconductor single-walled carbon nano tube is realized, and the metallic single-walled carbon nano tube is selectively grown.
According to the method for designing the controllable growth of the metallic single-walled carbon nanotube through the substrate, the substrate needs to be respectively subjected to heat treatment in an oxidation atmosphere and a reduction atmosphere, the crystallinity of the substrate is improved, and atomic steps suitable for the growth of the carbon nanotube are obtained; and carrying out oxygen plasma treatment on the substrate to improve the wettability of the catalyst precursor solution and the surface of the substrate.
The method for controllably growing the metallic single-walled carbon nanotube through substrate design is characterized in that a catalyst is a mixture of chemically adsorbed cations of a block copolymer micelle prepared by self-assembly of a block copolymer, the mixture is uniformly covered on the surface of a substrate through spin coating to form a film, and the block copolymer on the surface is removed through oxygen plasma treatment to obtain the monodisperse metal oxide nanocluster.
The method for growing the controllable metallic single-walled carbon nanotube through the substrate design comprises the following steps of sequentially carrying out high-temperature oxidation heat treatment on a metal oxide nanocluster at 400-600 ℃ for 1-5 min in an air atmosphere and reduction heat treatment at 700-850 ℃ for 1-6 min in a hydrogen/argon mixed atmosphere to obtain metal nanoparticles with the diameter distribution of 1-3 nm; by regulating and controlling the reduction temperature of heat treatment, the solid solution of the catalyst and the substrate is effectively realized, the high-temperature thermal stability of the catalyst is improved, the metal nanoparticles are pinned on the surface of the spinel, and the thermal stability of the metal nanoparticle catalyst is improved.
The method for controllably growing the metallic single-walled carbon nanotube through the substrate design comprises the steps of enabling the hydrogen flow rate to be 5-10 sccm and enabling the argon flow rate to be 50-100 sccm in a hydrogen/argon mixed atmosphere.
The method for controllably growing the metallic single-walled carbon nanotube through the substrate design enlarges the difference of energy required by the growth of the carbon nanotubes with different conductive properties by regulating and controlling the lattice thermal vibration and phonon thermal conductivity of the substrate under the condition of the chemical vapor deposition of the critical nucleation growth, so that the average length of the grown metallic carbon nanotube is far longer than that of the semiconductor carbon nanotube, thereby realizing the controllable growth of the metallic single-walled carbon nanotube.
The method for controllable growth of the metallic single-walled carbon nanotube through the substrate design comprises the following chemical vapor deposition conditions: the growth temperature is 755-775 ℃, the carbon source flow is 5-20 sccm, the hydrogen flow is 0.5-5 sccm, the argon carrier flow is 40-100 sccm, the total gas flow is 50-125 sccm, and the growth time is 5-15 min.
The method for controllably growing the metallic single-walled carbon nanotube through the substrate design has the advantages that the length of the grown metallic single-walled carbon nanotube is 2-10 mu m, the diameter of the grown metallic single-walled carbon nanotube is concentrated on 1.1 +/-0.2 nm, and the quantity content of the metallic carbon nanotube is 75-85%.
The design idea of the invention is as follows:
the invention provides a method for preparing a catalyst, which is characterized in that spinel with the melting point of 2130 ℃, the structure of face-centered cubic Fd3m and the magnesium-aluminum structure with lattice thermal vibration and cation exchange at high temperature is selected as a substrate for growing a carbon nano tube, and the strong interaction of solid solution and pinning of the spinel single crystal substrate and catalyst nano particles is utilized to effectively avoid the high-temperature agglomeration of the catalyst, so that the size, the structure and the high-temperature stability of the catalyst are controlled; meanwhile, energy supply required by the growth of the carbon nano tube is regulated and controlled by utilizing energy fluctuation caused by lattice thermal vibration of the spinel substrate at high temperature, the maximization of the difference of the growth rates of the metallic single-walled carbon nano tube and the semiconductor single-walled carbon nano tube is realized, the growth rates of the single-walled carbon nano tubes with different conductive properties are further influenced, the growth of the metallic single-walled carbon nano tube is regulated and controlled under the condition of critical nucleation growth, and finally the metallic single-walled carbon nano tube with narrow diameter distribution and high quality is obtained.
The invention has the advantages and beneficial effects that:
(1) according to the invention, through the strong interaction between the substrate and the catalyst, the control on the size, structure, chemical state and high-temperature thermal stability of the nano particles is realized, and the problem of high-temperature agglomeration of the catalyst is solved;
(2) according to the invention, through the high-temperature lattice thermal vibration of the substrate, the fluctuation of energy is generated on the surface of the substrate, and the energy supply required by the growth of the carbon nano tube is influenced, so that the growth rate difference of the metallic single-walled carbon nano tube and the semiconductor single-walled carbon nano tube is increased;
(3) the invention directly grows the high-purity metallic enriched single-walled carbon nanotube by using a simple and universal method and taking cobalt for efficiently growing the carbon nanotube as a catalyst;
(4) the invention realizes the diameter control of the metallic single-walled carbon nanotube;
(5) the invention clarifies the main factors influencing the growth of the metallic single-walled carbon nanotube and provides a new idea for regulating the growth of the carbon nanotube.
Drawings
FIG. 1 is a flow chart of self-assembly preparation of catalyst nanoparticles from block copolymers.
FIG. 2 is a schematic diagram of the principle of growing narrow diameter distribution metallic single-walled carbon nanotubes on a cobalt-spinel substrate structure.
Fig. 3 (110) atomic force microscope photomicrograph (a) of the surface of the spinel substrate after heat treatment and its atomic step height statistics (b).
FIG. 4 shows Raman characteristic peaks of (110) plane spinel substrate subjected to different heat treatments, at 400-500 ℃ for 2-4 h, at 600-700 ℃ for 2-4 h, and at 800-900 ℃ for 2-4 h.
Fig. 5. on the (110) face of the spinel: (a) a transmission electron microscope photo of the morphology of the catalyst nanoparticles, (b) the particle size statistical distribution of the nanoparticles, and (c) a scanning transmission electron imaging photo of the nanoparticles.
FIG. 6. growth temperature 765 deg.C, of carbon nanotubes on spinel (110) face: (a) scanning an electron microscope picture; (b) a transmission electron microscope photograph; (c) diameter histogram, Diameter on abscissa represents Diameter (nm) and Counts on ordinate represents Counts.
FIG. 7 growth temperature 765 deg.C, Raman characterization of carbon nanotubes on spinel (110) face: (a) a breath mode excited by laser with the wavelength of 633 nm; (b) a breathing mode excited by laser with a wavelength of 532 nm; (c)785nm wavelength laser-excited breathing mode; (d) d, G mode excited by 633nm laser.
FIG. 8 shows the chirality and growth rate statistics of carbon nanotubes grown on the spinel (110) face at a growth temperature 765 deg.C: (a) scanning the surface of the Raman laser with the wavelength of 633 nm; (b) raman laser surface scanning with wavelength of 532 nm; (c) and (5) counting the corresponding growth lengths of the carbon nanotubes with different conductive properties.
FIG. 9 shows the morphology and structure of cobalt catalyst particles prepared on the surface of a silicon wafer. (a) Atomic force microscopy of nanoparticles; (b) transmission electron micrographs of nanoparticles; (c) a particle size distribution histogram of transmission electron microscopy statistics; (d) scanning electron microscope picture of single-walled carbon nanotube grown on the surface of silicon wafer.
FIG. 10. radial breathing mode of multi-wavelength Raman spectrum of single-walled carbon nanotube grown on the surface of silicon wafer: (a) laser with wavelength of 532 nm; (b) laser with wavelength of 633 nm; (c)785nm wavelength laser; (d) d, G mode excited by 633nm laser.
FIG. 11 scanning electron micrographs of single-walled carbon nanotubes grown on the spinel (110) face at a growth temperature of 825 ℃ and corresponding multi-wavelength Raman spectroscopy radial breathing mode: (a) scanning an electron microscope picture; (b) laser with wavelength of 532 nm; (c) laser with wavelength of 633 nm.
Detailed Description
In the specific implementation process, spinel is used as a substrate, and a block copolymer self-composing method is adopted to assemble and prepare the metal oxide nanoclusters with small and uniform size; the size, structure and high-temperature stability of the catalyst nano-particles are regulated and controlled by utilizing the solid solution and pinning effects of the spinel substrate on the catalyst nano-particles, and the selective growth of the metallic single-walled carbon nano-tubes is realized by adopting a chemical vapor deposition method in combination with the influence of the spinel substrate on the growth rates of the single-walled carbon nano-tubes with different conductive properties.
As shown in fig. 2, the controllable growth mechanism of the metallic single-walled carbon nanotube is that the spinel substrate can effectively inhibit catalyst agglomeration, high-temperature oswald ripening, high-temperature melting of nanoparticles and the like due to the strong interaction between the atomic steps on the surface of the substrate and the exposed atoms on the surface of the substrate and the catalyst cobalt, so that the size of the carbon cap is stabilized at the nucleation stage, and the diameter of the grown single-walled carbon nanotube is controlled. According to the ensemble theory of statistical thermodynamics, the substrate can be regarded as an ideal heat source, and can be idealized into a regular ensemble by exchanging energy with the nanoparticles, the molecules containing the carbon-hydrogen elements and the carbon nanotubes. Due to the lattice thermal vibration of the substrate and the phonon heat conduction on the surface of the substrate, energy fluctuation exists, and the energy fluctuation of a regular ensemble is shown in formula (1). The amplitude of energy fluctuation is inversely proportional to the number of particles, so that the critical nucleation growth conditions of low temperature, low carbon source, low hydrogen and low flow rate are adopted, the energy fluctuation effect is obvious, the energy supply difference required by nucleation growth of different chiral carbon nanotubes is favorably expanded, and the carbon nanotubes with specific chirality overcome the potential barrier rate and are firstly nucleated to form carbon caps;
wherein the content of the first and second substances,
the mean value of the system energy in all possible microscopic states, unit J; e is the energy of the system in a certain state, and the unit is J; e and
relative deviation of
Mean of squares of deviations
Called energy fluctuation, ratio of after-evolution to average energy
Is the relative fluctuation of energy; n is the total number of particles in the system and is in mol.
In the aspect of dynamics, on the basis of obtaining the carbon cap with uniform size, the maximum difference of the growth rate of the metallic and semiconductor single-walled carbon nanotubes is realized by combining the substrate lattice thermal vibration and the difference of growth energy and carbon source supply. So that carbon atoms are slowly diffused and assembled on the surface of the catalyst under the critical condition, and finally, the length difference between the metallic carbon nanotube and the semiconductor carbon nanotube is realized through the control of the growth time. Because the generation, the transmission and the absorption of energy are quantized, the energy disturbance of the carbon nano tube under the critical conditions of nucleation and growth is beneficial to expanding the difference of the growth rates of different chiral single-walled carbon nano tubes, so that the carbon nano tube with a specific structure preferentially grows.
The method comprises the following specific preparation steps:
(1) pretreatment of the spinel substrate:
ultrasonically cleaning the (110) magnesium aluminate spinel single crystal substrate in ethanol for 5-10 min, drying the substrate by a nitrogen gun, placing the substrate in a closed ceramic boat, and placing the boat in a muffle furnace for heat treatment at 400-500 ℃ for 30 min-4 h to obtain an atomic step with higher crystallinity and suitable for the growth of the carbon nano tube. The atomic force representation and the step height of the atomic steps are shown in figure 3, and the step average height of the (110) surface is 0.8-1 nm, the surface roughness and the step fluctuation are relatively small, so that the method is suitable for completing the atomic scale self-assembly growth of the carbon nano tube. The atomic steps formed on the surface of the spinel also reflect the thermal vibration of the crystal lattice at high temperature. FIG. 4 shows Raman spectra of (110) plane substrates after heat treatment at 400-500 deg.C, 600-700 deg.C, 800-900 deg.C for 2-4 h, respectively, with Raman characteristic vibration mode Fd3m ═ A 1g +E g +T 1g +3T 2g +2A 2u +2E u +4T 1u +2T 2u The summary is as follows: heat treatment below 500 deg.c to obtain substrate with Raman characteristic peak Eg 398cm -1 The crystal lattice is relatively sharp, and the degree of order, crystallinity and symmetry of the crystal lattice are high; the Eg half-height width is obviously widened and the symmetry is obviously reduced along with the increase of the heat treatment temperature, which shows that the thermal vibration of crystal lattices of the substrate is intensified and AlO is generated along with the increase of the heat treatment temperature 4 Characteristic of (A) a breathing pattern 1g ＝717cm -1 ) This is enhanced with increasing temperature, which is attributable to the cation exchange and disordered transition of magnesium ions in the regular tetrahedral interstitial spaces with aluminum ions in the regular octahedral interstitial spaces.
(2) Preparation of cobalt metal catalyst on spinel substrate: as shown in FIG. 1, the spinel substrate with the exposed (110) surface is placed in a chamber with a power of 17-32W and a vacuum degree of 0.5-0.8 Torr for an oxygen plasma treatment for 3-5 min; by nitrogen-to-nitrogenDimethyl acetamide (DMF) is used as a solvent to prepare a PS block copolymer with the concentration of 0.005-0.015 wt% 2033 -b-P4VP 133 And the concentration of CoCl is 0.05-0.1 mM 2 ·6H 2 Heating and stirring the mixed solution of O in an oil bath at the temperature of 80-95 ℃ for 1-2 h to form a uniform mixture of the block copolymer micelle chemisorbed cations; spin-coating the mixture on the surface of a spinel substrate subjected to hydrophilic treatment at 1500-2000 rpm, treating the surface of the spinel substrate for 3-5 min by using oxygen plasma, and removing a block copolymer on the surface to obtain a metal oxide nanocluster; placing the substrate with the metal oxide nanoclusters dispersed on the surface in a quartz boat, pushing the quartz boat into a tube furnace, carrying out heat treatment for 1-5 min in air at 400-600 ℃, and then cooling to room temperature; introducing 500-800 sccm argon gas into the tubular furnace for 4min, and then switching to 50-100 sccm Ar and 5-10 sccm H 2 And pushing the substrate into a constant temperature area of 700-850 ℃ to reduce for 1-6 min to prepare the metal nanoparticles.
(3) Growing metallic single-walled carbon nanotubes on the spinel substrate: taking cobalt particles loaded on the crystal face of the spinel (110) prepared in the step (2) as a catalyst, loading 5-20 sccm argon into ethanol (in an ice-water bath at 0 ℃) as a carbon source at the temperature of 755-775 ℃, and taking 0.5-5 sccm H 2 And (3) as an etching body for reducing the nano particles and regulating the growth rate of the carbon nano tubes, introducing 40-100 sccm of argon gas to regulate the flow rate of gas and the concentration of a carbon source and hydrogen gas, keeping the total flow of the gas at 50-125 sccm, and growing the carbon nano tubes for 5-15 min.
The present invention will be explained in further detail below by way of examples and figures.
Example 1
In the embodiment, the spinel containing the (110) crystal face treated in the step (1) is subjected to heat treatment for 3 hours at 450 ℃ in air.
Loading cobalt particles on the surface of spinel (110) in the step (2), and carrying out oxidation and reduction treatment under the oxidation condition of 450 ℃ for 3 min; the reduction conditions were 765 deg.C, 90sccmAr +4sccmH 2 And 3 min. Characterizing the morphology of the cobalt particles prepared in the step (2) by using a transmission electron microscope (figure 5a), and finding that the particles are small and uniform in size; counted under a transmission electron microscopeThe diameter distribution of the cobalt particles (FIG. 5b) shows that the particle size is mainly concentrated in the range of 1-3 nm; the scanning transmission electron image of fig. 5c further demonstrates the compositional uniformity of the cobalt particles. The X-ray photoelectron spectroscopy and secondary ion mass spectroscopy sputtering are respectively carried out on the surface of the substrate, and the results show that the cobalt element is dissolved into the spinel substrate and presents a distribution with a gradient decreasing with the depth, the cobalt 2p3/2 peak position (778eV) on the surface is in a reduction state, and the cobalt 2p3/2 peak position (775eV) inside the substrate is in an oxidation state (+3 valence and +2 valence). The results show that the substrate and the catalyst form a bond, strong pinning and confinement effects are generated, and the catalyst atom dissolution and cation exchange are realized when the substrate undergoes disordered transition of magnesium ions and aluminum ions. The high-resolution transmission electron microscope photo of the cobalt nanoparticles shows good crystallinity, the cobalt nanoparticles are of a typical face-centered cubic structure, the lattice fringe spacing of 50 particles is counted to be concentrated at 0.2-0.22 nm, and the substrate has the effect of remarkably stabilizing the catalyst structure.
And (3) growing the single-walled carbon nanotube at 765 ℃ for 5min, wherein the flow of argon loaded with ethanol is 15sccm, the flow of hydrogen is 0.5sccm, the flow of argon carrier gas is 80 sccm. And (3) utilizing a scanning electron microscope to represent the morphology of the single-walled carbon nanotube grown in the step (3) (figure 6a), and finding that the carbon nanotube grows directionally and has the length of about 2-5 mu m. The transmission electron microscope (fig. 6b) characterization shows that the wall of the single-walled carbon nanotube is straight and clear, which indicates that the single-walled carbon nanotube has good crystallinity. The diameters of 150 carbon nanotubes are randomly counted under a transmission electron microscope (fig. 6c), and the diameters are distributed in a very narrow range, mainly centered at 0.9-1.3 nm. Respiratory mode of multiwavelength (532nm,633nm,785nm) laser raman spectroscopy as shown in fig. 7(a-c), most of the excited single-walled carbon nanotubes lie in the metallic carbon tube region according to Katarula plots, and the G mode (fig. 7d) is a typical metallic BWF type peak. According to the Katarula plots diagram and the number of the peak positions of the breathing modes in the corresponding interval, the content of the metallic single-walled carbon nano-tube is estimated to be about 85 percent. As a result of scanning the surface of the carbon nanotubes grown in step (3) with laser beams having wavelengths of 633nm and 532nm, as shown in FIG. 8, it can be seen that the length of the metallic carbon nanotubes is long (5 to 10 μm) and the length of the semiconducting carbon nanotubes is short (1 to 6 μm).
Example 2:
in this example, in step (1), the heat treatment temperature and time in air were 500 ℃ and 2 hours, respectively, as in step (1) of the example.
In step (2), the oxidation temperature and time are 600 ℃ and 1min respectively, the reduction temperature and time are 850 ℃ and 1min respectively, and the reduction atmosphere is 100sccmAr +3sccmH 2 . And (3) representing the size and the morphology of the Co particles by using a transmission electron microscope, and finding that the particle size is small and uniform and is mainly concentrated at 1.5-3 nm. The lattice fringe spacing of 50 particles is counted to be concentrated in the range of 0.2-0.22 nm, and the substrate has the effect of remarkably stabilizing the catalyst structure.
Step (3) the same as step (3) of the example, the growth temperature and time of the single-walled carbon nanotube were 775 ℃ and 7min, respectively, the flow rate of argon loaded with ethanol was 17sccm, the flow rate of hydrogen was 1sccm, and the flow rate of argon carrier gas was 90 sccm. The scanning electron microscope result shows that the single-walled carbon nanotube grows directionally and has the length of 3-7 mu m; the transmission electron microscope representation shows that the wall of the single-walled carbon nanotube is straight and clear, which shows that the single-walled carbon nanotube has good crystallinity; the diameters of 150 carbon nanotubes are randomly counted under a transmission electron microscope, and the diameters are mainly concentrated in 1.0-1.3 nm; the respiratory mode of the Raman spectrum of the multi-wavelength (532nm,633nm and 785nm) laser shows that most of the excited single-walled carbon nanotubes are positioned in the interval of metallic carbon tubes, and the content of the metallic single-walled carbon nanotubes is estimated to be about 78 percent according to the Katarula plots diagram.
Example 3:
in this example, in step (1), the heat treatment temperature and time in air were 400 ℃ and 3 hours, respectively, as in step (1) of the example.
In step (2), the oxidation temperature and time were 400 ℃ and 5min, the reduction temperature and time were 700 ℃ and 5min, respectively, and the reduction atmosphere was 80sccmAr +5sccmH, as in step (2) of the example 2 . The size and the morphology of Co particles are represented by a transmission electron microscope, and the particles are small and uniform in size and mainly concentrated at 1.0-2.5 nm. The lattice fringe spacing of 50 particles is counted to be concentrated in the range of 0.2-0.22 nm, and the substrate has the effect of remarkably stabilizing the catalyst structure.
Step (3) is the same as step (3) of the embodiment, the growth temperature and time of the single-walled carbon nanotube are 765 ℃ and 10min respectively, the flow of argon loaded with ethanol is 20sccm, the flow of hydrogen is 3sccm, and the flow of argon carrier gas is 100 sccm. The scanning electron microscope result shows that the single-walled carbon nanotube grows directionally and has the length of 4-8 mu m; the transmission electron microscope representation shows that the wall of the single-walled carbon nanotube is straight and clear, which shows that the single-walled carbon nanotube has good crystallinity; the diameters of 150 carbon nanotubes are randomly counted under a transmission electron microscope, and the diameters are mainly concentrated in 0.9-1.3 nm; the respiratory mode of the Raman spectrum of the multi-wavelength (532nm,633nm and 785nm) laser shows that most of the excited single-walled carbon nanotubes are positioned in the interval of metallic carbon tubes, and the content of the metallic single-walled carbon nanotubes is estimated to be about 80 percent according to the Katarula plots diagram.
Comparative example 1: thermodynamic and kinetic condition control-preparation of catalyst on the surface of silicon substrate and growth of single-wall carbon nanotube.
The morphology and the size of the obtained cobalt particles of the catalyst are shown in figures 9a-b, and the particle size distribution is not uniform by using a silicon wafer of amorphous silicon oxide with the melting point of 1410 ℃ and the surface oxide layer thickness of 300nm as a substrate and adopting the completely same catalyst preparation and treatment method of the embodiment (1). The diameter distribution of 150 Co particles is counted under a transmission electron microscope (figure 9c), and the diameter distribution of the Co particles is found to be within the range of 1.5-5.5 nm, which is larger than the average size and wider than the diameter distribution range of the particles prepared in the example (1), and shows that the interaction between the silicon substrate and the cobalt catalyst is weaker.
Co nanoparticles prepared from the above silicon substrate were used as a catalyst, and single-walled carbon nanotubes were prepared under the same chemical vapor deposition conditions as in step (3) of example (1). Fig. 9d shows that a long and dense carbon nanotube network grows on the surface of the silicon wafer, and the silicon wafer has no control effect on the orientation and growth rate of the carbon nanotubes. The conductive property of the carbon nanotubes was analyzed by raman spectroscopy (fig. 10), and it was found that the diameter distribution of the carbon nanotubes was wide, the content of the metallic carbon nanotubes was about 35%, and the selectivity of the non-conductive property thereof was seen from model D, G. The key role of the spinel substrate in controlling the growth of metallic carbon nanotubes is verified.
By respectively carrying out heat treatment on the substrate of the silicon wafer for 2-4 h at 500 ℃, 700 ℃ and 900 ℃, the half-height width of the Raman characteristic peak of the silicon wafer is not obviously changed, and the symmetry is very strong, which shows that the thermal vibration of the crystal lattice of the substrate of the silicon wafer is weaker than that of spinel, and the influence of the thermal vibration of the crystal lattice of the substrate of the silicon wafer on the growth of the carbon nano tube is weaker.
Comparative example 2: dynamic condition control-temperature
Preparing and treating Co nanoparticles by using spinel as a substrate and adopting the same steps as those in the embodiment (1); using this as a catalyst, the same chemical vapor deposition conditions as in example (1) were used except that a higher growth temperature (825 ℃ C.) was used to grow single-walled carbon nanotubes. The scanning electron micrograph shows (fig. 11a) that the density and the length of the carbon nano tube are both obviously improved, which shows that the growth efficiency and the growth rate of the carbon tube both change along with the change of the temperature. The multi-wavelength raman spectrum (fig. 11b-c) shows a broadening of the distribution range of the diameter and conductivity properties of carbon nanotubes. The comparative examples show that under the low-temperature critical nucleation growth condition, lattice thermal vibration and energy fluctuation of the substrate are beneficial to differentiated supply of energy, so that the growth rate difference of the metallic carbon nanotube and the semiconductor carbon nanotube is maximized; at high temperature, the temperature of the reaction system is enough to supply energy for the growth of the carbon nanotubes with different properties, and the effect of the substrate on the growth control of the carbon nanotubes is weakened.
The above examples and comparative examples show that compared with a common silicon substrate, spinel can effectively dissolve and pin metal nanoparticles, solve the problems of catalyst agglomeration and Oswald ripening, and improve the high-temperature thermal stability of the catalyst; under the critical nucleation growth condition of low temperature and low carbon source, energy difference supply of metal and semiconductor carbon nano tubes is realized due to violent lattice thermal vibration of spinel, and single-walled carbon nano tubes with the length of 5-10 mu m, narrow diameter distribution (0.9-1.3 nm) and 75-85% of metal quantity content are grown and are more than 2 times of that of a common sample; the method is simple and easy, and has strong applicability; the main factors influencing the growth of the metallic carbon nanotube and the growth mechanism thereof are clarified, and a novel method and thought are provided for the controllable growth of the carbon nanotube. The present invention is not limited to the above-mentioned embodiments and comparative examples, and various modifications and improvements of the single crystal substrate made by those skilled in the art in the light of the present invention shall be protected by the following claims.
Claims (4)
1. A method for controllable growth of metallic single-walled carbon nanotubes by substrate design is characterized in that magnesia-alumina spinel single crystals which generate lattice thermal vibration at high temperature and take phonons as a main heat conduction mode are selected as a substrate, and the size, the structure and the high-temperature stability of a catalyst are controlled by utilizing the strong interaction of the substrate and the catalyst; regulating and controlling energy supply required by the growth of the carbon nano tube by utilizing energy fluctuation caused by lattice thermal vibration of the substrate at high temperature, realizing maximization of growth rate difference of the metallic single-walled carbon nano tube and the semiconductor single-walled carbon nano tube, and selectively growing the metallic single-walled carbon nano tube;
the substrate needs to be respectively subjected to heat treatment in an oxidation atmosphere and a reduction atmosphere, so that the crystallinity of the substrate is improved, and an atomic step suitable for the growth of the carbon nano tube is obtained; performing oxygen plasma treatment on the substrate to improve the wettability of the catalyst precursor solution and the surface of the substrate;
the catalyst is a mixture of block copolymer micelle chemisorption cations prepared by self-assembly of block copolymers, the mixture is uniformly covered on the surface of a substrate by spin coating to form a film, and the block copolymers on the surface are removed by oxygen plasma treatment to obtain monodisperse metal oxide nanoclusters;
sequentially carrying out high-temperature oxidation heat treatment on the metal oxide nanoclusters at 400-600 ℃ for 1-5 min in an air atmosphere and reduction heat treatment at 700-850 ℃ for 1-6 min in a hydrogen/argon mixed atmosphere to obtain metal nanoparticles with the diameter distribution of 1-3 nm; by regulating and controlling the reduction temperature of heat treatment, the solid solution of the catalyst and the substrate is effectively realized, the high-temperature thermal stability of the catalyst is improved, the metal nanoparticles are pinned on the surface of spinel, and the thermal stability of the metal nanoparticle catalyst is improved;
the length of the grown metallic single-walled carbon nanotube is 2-10 mu m, the diameter of the metallic single-walled carbon nanotube is 1.1 +/-0.2 nm, and the quantity content of the metallic carbon nanotube is 75-85%.
2. The method for controlled growth of metallic single-walled carbon nanotubes through substrate design according to claim 1, wherein the hydrogen flow rate is 5 to 10sccm and the argon flow rate is 50 to 100sccm in a hydrogen/argon mixed atmosphere.
3. The method for controlled growth of metallic single-walled carbon nanotubes through substrate design according to claim 1, wherein under the condition of chemical vapor deposition of critical nucleation growth, the difference of energy required for growth of carbon nanotubes with different conductive properties is enlarged by regulating and controlling lattice thermal vibration and phonon thermal conductivity of the substrate, so that the average length of the grown metallic carbon nanotubes is much longer than that of semiconducting carbon nanotubes, thereby realizing the controlled growth of the metallic single-walled carbon nanotubes.
4. The method for controlled growth of metallic single-walled carbon nanotubes by substrate design according to claim 3, wherein the chemical vapor deposition conditions are: the growth temperature is 755-775 ℃, the carbon source flow is 5-20 sccm, the hydrogen flow is 0.5-5 sccm, the argon carrier flow is 40-100 sccm, the total gas flow is 50-125 sccm, and the growth time is 5-15 min.
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