3.1. Modeling the Process of Forming SWCNTs and Graphene Sheets Hybrids
Tubes of different chiralities, the most frequently synthesized ones, were selected for the study. As is known, most of synthesized tubes are semiconductors. It is also known that the least amount of non-chiral tubes is synthesized from the total number. Therefore, for the study, we chose nanotubes of following types: (6,5), (6,3), (7,5), (12,6), (8,3), (8,4), (7,6), (12,8), (14,4), (10,6), (8,6), (9,4), (11,10) and (16,0) [97
]. Series of calculations were carried out for indicated tubes of the same length of ~4–5 nm. To ensure that length of nanotubes was approximately the same, a corresponding different number of unit cells was taken for different nanotubes. Investigations were also carried out for the same nanotubes with 2–10 times greater length. Figure 2
shows absorption plots for all the listed nanotubes of 4–5 nm length. For thin tubes of subnanometer diameter, absorption curves are shown in Figure 2
a; for tubes with diameter 1 nm and more, curves are shown in Figure 2
b. All tubes show the same tendency towards maximum absorption in the UV region. In particular, in the wavelength range 200–300 nm, the largest number of absorption peaks is observed, regardless of chirality type of nanotube. Particularly prominent are wavelengths in the region of 266 nm. This region contains intensity peaks for the largest amount of SWCNTs of various chiralities. In this case, the most pronounced peaks at this wavelength are observed in semiconductor tubes. The intensity peak for the tube (11,10) is especially noticeable, which is explained not only by semiconductor type of conductivity, but also by the largest diameter of this particular tube in comparison with others, at 1.45 Å. The larger the surface, the more energy it will absorb from incident electromagnetic wave. Fermi energy for different types of tubes is 5.1 ± 0.3 eV. As noted above, tubes of different lengths were considered, and it was found that an increase in tube length did not qualitatively affect the absorption spectrum. That is, short and long SWCNTs have the largest peak absorption intensity in the wavelength region of about ~266 nm. Experimental absorption spectra for SWCNT (OCSiAl Ltd.) were given in [98
], in Figure 1
. Absorption spectra of MWCNT (NanoTechCenter Ltd. (Taunit)) were given in [99
], in Figure 1
. It can be seen from the absorption spectra that absorption decreases when going from UV to visible and IR ranges. In visible and near-IR ranges, there are also sloping regions of increase in absorption, the amplitude of which is not comparable with amplitude in UV region.
Next, we simulated the interaction of nanotubes with graphene sheets under laser irradiation with a wavelength of 266 nm. Previously, authors carried out series of numerical experiments, which resulted in formation of covalent bonds between nanotubes. It was shown that covalent bonds are formed primarily in the regions of defects in nanotubes. In this paper, we investigate nanowelding of defect-free tubes with graphene sheets. Various mutual positions of nanotube and sheet are considered. In all cases, graphene sheet was located at a distance of 2.8–3.0 Å. The results are presented in Figure 3
for thin tube (6,3), with diameter 0.6 nm, and in Figure 4
for tube (14,4), with diameter 1.3 nm. Figure 3
a shows an atomistic model of nanotube and graphene sheet when graphene edge is located along the tube. The results of modeling and the process of covalent bond formation are shown. Under laser irradiation, redistribution of electron charge density occurs. As a result, charge transfer occurs: from nanotube, one electron is transferred to graphene. Figure 3
a shows that the edge graphene atoms have an excess charge (blue), and nanotube atoms in the immediate vicinity of graphene have a lack of charge (yellow). As early as 120 fs after the onset of irradiation, first covalent bonds appear; after 400 fs, contact between nanotube and graphene is completely formed. The same figure shows non-hexagonal elements in nanowelding region. Pentagon is marked in red, heptagons in blue and octagons in green. Figure 3
b also shows the process of nanotube–graphene contact formation when graphene sheet is placed not along the nanotube, but end-to-end with it. In this case, bonds are formed faster, and the first covalent bonds are formed already after 100 fs. As in the previous version, already in first moments, a redistribution of charge is observed and charge transfer from nanotube to graphene occurs again, although to a lesser degree in this case. As a result, four stable bonds were formed in 400 fs, and heptagon, pentagon, and hexagon appear in the contact region. In all cases, bond length in contact region is ~1.54–1.57 Å, which corresponds to C–C bond length in the case of sp3 hybridization of electron clouds.
Similar results were obtained for a nanotube with diameter twice as large. From the data presented in Figure 4
, it can be seen that atoms of nanotube located next to graphene have a lack of charge. It can be said that these atoms and graphene atoms interact electrostatically. The nanotube loses its charge again, as in the case of the thin tube (6,3). However, unlike the thin nanotube, bonds are formed more slowly. Stable bonds appear not in 400 fs, but in 750 fs. Non-hexagonal elements are also formed in contact area.
3.2. Structural Features of the Created Carbon Nanomaterials
Structure change of films of SWCNT and MWCNT networks after laser irradiation compared to initial CNT films is shown in Figure 5
. SEM images show that SWCNT and MWCNT are presented as isolated nanotubes and their bundles. This is especially pronounced for SWCNT (Figure 5
a,c,e,g,i). Initially, the energy density of laser radiation was determined, which, on the one hand, ensures formation of nanotube networks due to laser nanowelding, and, on the other hand, does not initiate the process of carbon nanostructure sublimation. SEM images of SWCNT films show that after laser irradiation with energy density 0.14 J/cm2
c), networks began to form compared to initial nanotubes (Figure 5
a). Laser irradiation with energy density 0.3 J/cm2
led to formation of a branched SWCNT network. Laser energy is converted into the energy of atoms and promotes breaking of C–C bonds inside nanotubes with further formation of new bonds between atoms of neighboring CNTs. Laser energy density 0.5 J/cm2
g) and especially 0.8 J/cm2
i) ensured structural disruption. Such structural disturbances, most likely, are not associated with formation of vacancy defects, but are more similar to the appearance of condensation products—nanosized inclusions on the surface of nanotubes. Thus, appearance of such nanoinclusions can be associated with sublimation of defective nanotubes with subsequent condensation of amorphous carbon on nanotubes that have not been irradiated by laser [77
A similar situation developed for MWCNT film, but threshold radiation energy density was higher than for SWCNT. The MWCNT network with the highest number of bonds between nanotubes was obtained at the energy density of 0.5 J/cm2
h). It can be seen from SEM images that with an increase of energy density from 0.3 to 0.5 J/cm2
, MWCNTs were bent more due to change in defectiveness [80
]. This contributed to formation of more bonds between nanotubes. At 0.81 J/cm2
, the appearance of nanoinclusions was observed on MWCNT as on SWCNT. Conclusions about the optimal energy density for the formation of carbon nanotube networks cannot be made only on the basis of SEM images. The main quality criteria for the formed networks of nanomaterials are an increase in values of their electrical conductivity and hardness. However, based on SEM images, it is possible to determine the value of threshold irradiation energy density at which defect regions of nanotubes do not sublimate with their subsequent condensation, as in Figure 5
g,i,j. As a result, energy density values 0.3 and 0.5 J/cm2
were chosen to irradiate SWCNT/rGO and MWCNT/rGO hybrids films, respectively.
When exposed to laser irradiation on SWCNT/rGO and MWCNT/rGO films with selected values of energy density, the effect of CNT binding to rGO sheets was obtained (Figure 6
). It can be seen on SEM images that binding of SWCNTs and MWCNTs, as well as their bundles with rGO, occurred mainly in regions of defects on nanotubes’ lateral surface with graphene sheet’s edge region (Figure 6
c,d). Nanotubes also bonded to each other. Figure 6
a,b show that nanotubes and their bundles acted as connecting bridges between graphene sheets.
shows the Raman spectra of SWCNT and MWCNT (Figure 7
a,b) samples and SWCNT/RGO and MWCNT/RGO hybrids (Figure 7
c,d) before and after exposure to laser radiation with selected energy densities. The presented graphs reproduce the main characteristic modes of the samples: RBM (0–300 cm−1
), D (1300–1400 cm−1
), G (~1580 cm−1
) and 2D (2500–2900 cm−1
) for SWCNT, D (1300–1400 cm−1
), G (1580–1600 cm−1
) and 2D (~2700 cm−1
) for MWCNT. RBM mode is typical for SWCNT samples. It is characteristic of the nanotube cylindrical geometry and is caused by uniform radial displacement of atoms. In this regard, it slightly depends on the atomic structure and is insensitive to small deviations of the nanotube surface from the ideal cylinder. The D mode originates from a double resonance Raman scattering process [95
]. The D mode is observed for the presence of defects in the graphite structure and its intensity is proportional to the amount of disorder (crystallite boundary) in the sample [100
]. It is known that the G mode, corresponding to the in-plane optical phonon modes, is characteristic of all sp2 carbon materials, and the defectiveness of the carbon structure is estimated by the ID
The values of the characteristic modes for each sample are presented in Table 2
. The data in the table were obtained by processing Raman spectra. Table 2
shows the frequency values for the main modes that characterize carbon nanomaterials (RBM, G, D, 2D), as well as the ratio of D and G modes’ intensities—ID
. For the initial SWCNT, a small value of the ID
ratio characterizes a relatively low defectiveness. Films from SWCNT networks after laser irradiation with an energy density of 0.3 J/cm2
received an increase in the ID
parameter by 0.008 (28%), which corresponds to an increase in the defectiveness of the nanostructure. Defectiveness of SWCNT nanostructures is mainly associated with the formation of vacancy defects with the breaking of C–C bonds and the formation of new bonds on the lateral surface of nanotubes. Along with an increase in defectiveness, broadening of the 2D mode is observed. The low-frequency mode G–
does not disappear with an increase in the intensity of mode D. This characterizes the appearance of defects in the structure that are not directly caused by graphitization of the sample. Such defects are probably caused by the formation of joints at the sites of defects on the nanotubes’ walls, which suggest a distortion of the original symmetric carbon structure. After exposure to the laser, SWCNTs did not undergo significant damage, which is confirmed by the preservation of the spectrum shape and the absence of additional high-frequency modes, such as D′ band intensity (around 1620 cm−1
), which is often observed in defective graphene samples [100
The ratio of the ID/IG intensities of the initial MWCNT is greater than 1, which characterizes a higher defect rate in comparison with SWCNT. The presence of bands of the initial MWCNT in the spectral range from 100 to 1000 cm−1 refers to the contribution from the catalyst remaining after synthesis. For the initial MWCNT and MWCNT after laser exposure, a decrease in ID/IG by 0.095 was obtained, which is associated with partial annealing of amorphous carbon from the surface of the outer walls of the nanotubes. This is confirmed by an increase in the intensity of the 2D mode. Moreover, after laser exposure, MWCNT is characterized by an increase in the D + G mode of about 2940 cm−1, which is a defect-induced two-phonon process. SEM images confirm that the defectiveness for MWCNT after laser exposure on one side is reduced by annealing the surface of the outer walls of the MWCNT. On the other side, the spectrum characterizes the presence of various defect types in the structure associated with the appearance of curvatures and the formation of bonds between CNTs at the sites of defects.
The spectra of SWCNT/rGO hybrids after exposure to laser radiation in comparison with the same without laser irradiation demonstrate a number of changes. First, the number of maxima in the region of the RBM mode has decreased. This can be explained by the fact that tubes of smaller diameter have less heat capacity and are much less resistant to heating from laser radiation, as a result of which they are destroyed faster. In addition, the heating of SWCNT/rGO hybrids is not uniform, and the presence of defects additionally creates uneven heating areas. Thus, as a result, larger diameter tubes prevail in the sample [101
]. Secondly, the value of the ID
parameter has sharply increased by 4.4 times. The shift of the G mode higher in frequency characterizes the change in the length of C=C bonds, probably due to the emergence of new C–C bonds between the tubes and rGO [102
The MWCNT/rGO spectrum after laser irradiation with an energy density of 0.5 J/cm2 is characterized by a significant shift in the G mode, broadening of the D mode, and an increase in the ID/IG ratio by 0.031 (~3%), which may be ascribed to the increment in sp2 domains caused by attaching carbon nanotubes to rGO; it could also be associated with decreasing of the outer diameter of nanotubes. This is consistent with a change in the shape of the spectra in the 2000–4000 cm−1 region, in particular, with changes in the 2D and D + G modes. It is also worth noting that the SWCNT and SWCNT/rGO samples are characterized by resistance to laser radiation with an energy density of 0.3 J/cm2, and the MWCNT and MWCNT/rGO samples with a value of 0.5 J/cm2, since the shape of the spectra was preserved relative to the untreated samples.
Presented data demonstrate that laser irradiation with certain energy density leads to formation of defects (change in ID/IG) in the structure of nanotubes for all samples without amorphization of the structure. The change in ID/IG is mainly due to the welding of carbon nanotubes to each other and to graphene sheets. In this regard, it can be assumed that electrical conductivity of the obtained networks of nanotubes can slightly decrease due to the appearance of vacancy defects and can greatly increase due to the appearance of new contacts between nanomaterials.
3.4. Electrical Conductivity of Nanomaterials
When quantity and quality of connections between nanotubes changes, as well as between nanotubes and graphene sheets, their electrical conductivity changes. Therefore, electrical conductivity of films based on investigated carbon nanomaterials was calculated. Surface resistance of films made of SWCNT, MWCNT, and their hybrids with rGO was measured before and after laser irradiation with energy densities of 0.14 J/cm2
, 0.3 J/cm2
, 0.5 J/cm2
, 0.8 J/cm2
. Then, based on the values of surface resistance, specific electrical conductivity of films was calculated taking into account their thickness. Electrical conductivity measurement results are shown in Table 3
Thickness of the films made of carbon nanomaterials was ~500 ± 100 nm. Laser had a diffraction length (Rayleigh length) of ~1 mm. Thus, laser irradiation efficiency on the film had the same energy density throughout the entire thickness of the film. Therefore, it can be assumed that electrical conductivity of the formed films from nanotubes and their hybrids with rGO are the same on the surface and inside the films.
According to electrical conductivity measurement results, it can be seen that for all samples there is dependence of electrical conductivity on the laser energy density. Irradiation with energy density 0.14 J/cm2
led to an insignificant increase in electrical conductivity of films based on SWCNT and MWCNT. This result is in good agreement with obtained SEM images. Laser irradiation initiates formation of contacts between nanotubes; this helps to improve transport of electrons. Further increase of energy density to 0.3 J/cm2
leads to an even greater increase in conductivity values for films based on SWCNT and MWCNT networks, because even more connections between CNTs are being formed. However, irradiation of SWCNT films with energy density 0.5 J/cm2
leads to a decrease in electrical conductivity; irradiation of MWCNT films leads to an increase in electrical conductivity. Further increase in energy density to 0.8 J/cm2
in both samples causes structural destruction of nanotubes and, as a consequence, deterioration of conductive properties of the films. This effect has been demonstrated in SEM images. Electrical conductivity of hybrid SWCNT/rGO and MWCNT/rGO nanostructures has dependence on laser energy density similar to that of SWCNT and MWCNT films. SWCNT/rGO and MWCNT/rGO hybrids have conductivity peaks as a result of laser irradiation with energy density 0.3 J/cm2
and 0.5 J/cm2
, respectively. Initial and maximum increase in electrical conductivity as a result of laser irradiation for all films is shown in Figure 9
Initial SWCNT and MWCNT films exhibit electrical conductivity of 3.61 kS/m and 14.32 kS/m, respectively. The higher electrical conductivity of MWCNT can be explained by the presence of multiple walls, which contribute to the most efficient electron transport. Laser irradiation leads to an increase in electrical conductivity up to 11.51 kS/m for SWCNT and 18.43 kS/m for MWCNT. This corresponds to an increase of 3.2 and 1.3 times, respectively, which indicates formation of new conducting networks with percolation nodes. Raman spectra analysis confirms these findings. Increase in CNT defectiveness indicates formation of C–C bonds between nanotubes.
Hybrid nanostructures SWCNT/rGO and MWCNT/rGO before laser irradiation had conductivities of 5.91 kS/m and 16.32 kS/m, respectively. After laser irradiation, electrical conductivity increased to 10.34 kS/m for SWCNT/rGO, and to 22.60 kS/m for MWCNT/rGO. This corresponds to an increase of 1.8 and 1.4 times, respectively. The highest electrical conductivity of 22.60 kS/m was demonstrated by MWCNT/rGO hybrid film after laser irradiation. This speaks about the effective formation of links between MWCNT and MWCNT with rGO. Electrical conductivity of hybrid MWCNT/rGO nanostructure after laser irradiation exceeds the electrical conductivity of MWCNT film after irradiation with the same laser energy density. Under laser irradiation, nonuniform absorption of radiation by nanotubes and graphene oxide sheets occurs, followed by redistribution of electron charge density. As a result, there is a transfer of charge from nanotubes to graphene sheets. Increase in electrical conductivity can be explained by the appearance of different types of percolation nodes. They appear due to complementary topological and electrophysical features of two nanocarbon modifications (due to formation of bonds between MWCNTs and rGO, as well as formation of bonds only between MWCNTs). As a result, the number of different types of paths for electron transport in the network increases. Improved conductive properties of CNT hybrids and graphene sheets are due to unsaturated π-bonds of CNT atoms in defect regions, which lead to formation of stronger bonds with graphene atoms.
It can be assumed that addition of graphene sheets leads to formation of less densely packed film from SWCNT/rGO than from SWCNT only. Morphology of SWCNT/rGO film becomes more friable. The probability of contact formation between SWCNTs is reduced compared to densely packed SWCNT only film. Consequently, electrophysical and mechanical properties of SWCNT/rGO hybrid film deteriorate. A possible reason for this effect is the significant difference in geometric dimensions of SWCNT and rGO.
We compared obtained value of electrical conductivity to results for CNT/graphene hybrid structures reported by other researchers. Based on the analysis, it was found that obtained electrical conductivity of hybrid nanostructures SWCNT/rGO ~22.6 kS/m exceeds the values obtained earlier. Fan et al. obtained the value of hybrid structures’ conductivity of 180.1 S/m [49
]. Tian et al. in their work obtained the value of hybrid structures’ conductivity of 53.8 S/m [105
]. In work of Zhu et al. comparable values of electrical conductivity (11.7 kS/m) were obtained for hybrid structures, which besides graphene and nanotubes contained metal nanoparticles [106