Highly Conductive Doped Hybrid Carbon NanotubeâGraphene Wires

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Functional Nanostructured Materials (including low-D carbon)

Highly conductive doped hybrid carbon nanotube-graphene wires Sandra Lepak-Kuc, Karolina Zofia Milowska, Slawomir Boncel, Miros#aw Szybowicz, Anna Dychalska, Iwona Jozwik, Krzysztof K. Koziol, Malgorzata Jakubowska, and Agnieszka Lekawa-Raus ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08198 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 9, 2019

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ACS Applied Materials & Interfaces

Highly conductive doped hybrid carbon nanotube-graphene wires †

Sandra Lepak-Kuc,

Szybowicz,

§

∗,‡

Karolina Z. Milowska,

§

Anna Dychalska,

k,⊥

Iwona Jozwik,

Jakubowska,

†

¶

Slawomir Boncel,

Miroslaw

#

Krzysztof K. Koziol,

and Agnieszka Lekawa-Raus

Malgorzata

∗,†

†Department of Mechatronics, Warsaw University of Technology, Poland ‡TCM Group, Cavendish Laboratory, University of Cambridge, United Kingdom ¶Faculty of Chemistry, Silesian University of Technology, Poland §Faculty of Technical Physics, Poznan University of Technology, Poland kInstitute of Electronic Materials Technology, Poland ⊥National Centre for Nuclear Research/NOMATEN Centre of Excellence, Poland #Enhanced Composites & Structures Centre, Craneld University, United Kingdom E-mail: [email protected]; [email protected]

Abstract The following paper explores the nature of electronic transport in a hybrid carbon nanotube graphene conductive network. These networks may have a tremendous impact on the future formation of new electrical conductors, batteries and supercapacitors as well as many other electronic and electrical applications. The experiments described show that the deposition of graphene nanoakes within a carbon nanotube network improves both its electrical conductivity and its current carrying capacity. They also show that the eectiveness of doping is enhanced. To explain the eects observed in the hybrid carbon nanotube graphene conductive network, a theoretical model was 1

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developed. The theory explains that graphenes are not merely eective conductive llers of the carbon nanotube networks but also eective bridges able to introduce additional states at the Fermi level of carbon nanotubes.

Keywords carbon nanotube-graphene hybrid materials, carbon nanotubes, graphene akes,carbon nanotube bres, carbon nanotube wires, halogen doping, density functional theory, conductance, current

1

Introduction

Pure carbon nanotube bres, that is thread-like macroscopic assemblies of aligned carbon nanotubes (CNTs), are extremely promising materials for many elds of science, especially those of electronics and electrical engineering. 1,2 Hence there has been, signicant interest in this eld over recent years. 36 Although the electrical performance of CNT bres has yet to achieve the theoretical potential, i.e. absolute electrical conductivity matching up to individual CNTs or at least better than copper wires, the richness of the ideas published is highly interesting not only in the context of applications but also purely scientically. 3,7 One such idea is hybridization of CNT bres with graphene. Graphene is a close relative of CNTs being formed by the same single-atom layer of sp2 hybridized carbon atom orbitals that form the CNTs walls, yet in graphene it remains in the planar form instead of forming a tubule. In theory, such a structure should exhibit the values of electrical conductivity obtained for CNTs, while also remaining very light-weight. 8 Moreover, its 2D structure and 2D isotropic electrical conductivity could make it a good bridging material when placed in a network of 1D CNTs forming a bre. Finally and importantly, the conductivity of graphenes can be modied by similar types of dopants to those which are most eective in the case of CNTs, such as strongly oxidizing 2


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acids or halogens. 912 This common doping opportunity does not exist for example in case of conductive metals (copper, aluminium, silver) which have been also often used as hybridizing agents for modication of the electrical performance of CNT bres. 13,14 Although the graphenes nanoakes currently available in the marketplace dier from the ideal case of a one-atom thick graphene lattice and may possess many layers, dierent dimensions, defects and shapes, they have been extensively studied as hybridizing agents for CNTs due to the existing applications of these materials in such areas as supercapacitors, batteries and solar cells. 1521 However, to the best of our knowledge, among the vast literature on the formation of CNT- graphene hybrids there have been only three reports on hybridization of dry spun CNT bres with graphene. These works include deposition of reduced graphene oxide in array spun CNT bres and co-synthesis of graphenes on CNT bres produced via oating catalyst chemical vapour deposition (CVD) method. 21 The latter method was reported to result in the bres of 105 S/m electrical conductivity which is in the range of conductivities measured for pure CVD spun CNT bres. 21 However, the eect of the addition of graphene on the conductivity was not estimated. In case of array spun bres Sun et al. has reported approx. 20% increase after deposition and reduction of graphene oxide, 17 while Foroughi et al. reported 400% increase upon electrodeposition of chemically converted graphene. 18 Although the latter result seems quite impressive, the nal conductivity amounted to only 9x104 S/m, as the initial conductivity array spun bres was lower than for CVD spun bres. These considerably divergent results currently make it rather dicult to draw any denitive conclusions regarding the eect of graphene on the electric transport in the CNT bres. This is mainly due to the use of dierent methods of hybridization, varying base bres and dierent graphenes including the use of reduced graphene oxides, which are well-known insulators before reduction. 3



The judgement of the actual impact of graphene presence on electrical conductivity of CNT bres is also blurred by the fact that addition of graphenes simply increases the amount of conductive material in normally porous structures of CNT bres. This issue is not taken into account when calculating standard electrical conductivity (based on conductance, surface area and length of the sample). Finally, to the best of our knowledge, the doping of the hybrid CNT bre-graphene materials has not been studied. Therefore, this paper presents an in-depth study on hybridization of one type of CNT bre produced via oating catalyst method with graphene akes of various sizes and thicknesses applied by a range of techniques. The hybrid CNT-graphene (CNTs+G) bres were further doped and characterised electrically, as well as using microscopy techniques and Raman spectroscopy. Finally, so as to fully understand the physical mechanisms governing the highly interesting trends observed in the electrical performance of CNTs+G hybrid materials upon doping, a theoretical model was constructed utilising rst principles calculations of structural, electronic and transport properties.

2

Materials and methods

2.1 CNT bres The CNT bres used in this study were prepared via the oating catalyst chemical vapor deposition (CVD) method, according to the procedures described earlier. 2224 The feedstock used for their synthesis comprised methane, thiophene and ferrocene. The material was extracted from the reactor in the non-condensed form and wound for an hour at a speed of 15 m/min on a rotating spindle to obtain non-transparent CNT lms. The lm samples were cut to 1 cm wide and 3-7 cm long rectangles, in accordance with the CNTs alignment. Depending on the hybrid formation method, the bres were formed by mechanical the rolling of the lms taking place before or after the purication and application of the graphene ake 4


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steps. For each experiment a minimum of 5 samples were used.

2.2 Graphene akes The graphene akes used for hybridization included from Cambridge Nanosystems Ltd., UK,

GNP:

G3:

GamGraph GR3 graphene akes

Graphene Nano Plates from CheapTubes

Inc., USA, M25: XGnP-M-25 from XG sciences Inc., USA. These graphene akes have dierent sizes and thicknesses. Such a selection was made to verify if the parameters presented by each type of ake aect the hybridization eciency or electrical performance of hybrid bres. According to the respective manufacturers datasheets, the GNP akes have a thicknesses in the range of 8-15 nm and diameters of approximately 1-2 µm, 25 M25 akes, have larger in-plane dimension of approx. 25 µm, while their thickness is slightly smaller 6-8 nm. 26 Finally, G3 akes are characterized by both a smaller number of layers, and thus a lower thickness of 1-5 nm, and much smaller diameters of 250-550 nm, 27 which according to the ISO/TS 80004-13:2017 norm may be the only one understood as "graphene" from all the three materials considered. A graphene suspension for the spray coating technique was prepared by using a 1 hourlong ultrasonication (Utrasonix Proclean 2.0 M ultrasonic bath, nominal power 60 W) of respective graphenes in acetone with the small addition of surfactant (AKM-0531 from NOF Corporation, Japan 2 wt.% in relation to graphene). 0.1 Wt.% suspensions were used for all types of graphenes. Graphene suspensions for inltration method were prepared by suspending akes in various suspension media with a small surfactant addition (AKM-0531). The suspensions were sonicated for one hour in an ultrasonic bath. Hybridization was based on soaking CNT bres in such graphene suspensions for 24 hours.

5



2.3 Chemical compounds for purication and doping A list of chemical compounds used for purication and doping treatments included: acetone (C3 H6 O), chloroform (CHCl3 ), hydrogen peroxide (H2 O2 (60% aq)), sulfuric acid (H2 SO4 (98% aq)), perchloric acid (HClO4 (70% aq)), ICl, Br2 , I2 , Cl2 (obtained via the reaction of KMnO4 with conc. HClaq , F4TCNQ (2,3,5,6-tetrauoro-7,7,8,8-tetracyanoquinodimethane). The above-mentioned compounds were purchased from Linegal Chemicals (C3 H6 O, CHCl3 , H2 O2 , HCl, H2 SO4 , HClO4 , KMnO4 ), Sigma-Aldrich (ICl), TriMen Chemicals, (F4TCNQ);

Chem Point. Poland (I2 ) and "REKO" PR-W (Br2 ). Our previous extensive experimental work, 24 exploring the inuence of various doping methods and compounds on the electrical performance of pure CNT bres, showed that the eect of the doping of CVD spun bres is reinforced after a well-optimized purication procedure. All of the lm/bre samples in this work were puried according to the procedures elaborated earlier before hybridization with graphene. These included: annealing at 400° C (laboratory mue furnace Czylok FCF 7 SHM), 10 minutes long ultrasonication (Utrasonix Proclean 2.0 M ultrasonic bath) and 24 hour long soaking in acetone and soaking in conc. HCl (aq) for 24 hours. The doping of the CNTs+G hybrid materials with acids was performed by soaking for 24 hours. The F4TCNQ was diluted in chloroform (0.4mmol/l) according to the procedure described in the previous report. 28 The diluted F4TCNQ as well as liquid ICl and Br2 were applied in the form of droplets of approx. 0.25-0.5 mm volume, produced using a glass pipette. Depending on the bre length, 5-10 droplets were applied along the samples placed in a petri dish. I2 and Cl2 were introduced in their vapor form. The I2 vapours were obtained by sublimation of 100 g of I2 powder at 185 o C, while the Cl2 vapours were obtained in the reaction of 90 g of KMnO4 with 374 ml of 37% HCl. In both cases the vapours were produced in a sealed 2 litre desiccator jar. Thus, the concentration of the gases amounted to approx. 0,05 g/cm3 . The bres dedicated for doping, were mounted on the lid of a desiccator and left there overnight, to obtain "adsorption equilibrium. 6


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2.4 Methods 2.4.1 Basic physical properties measurements Diameter measurements

were performed under the digital optical microscope Keyence

VHX-900F.

Mass measurements were carried out using Radwag Ultra-Microbalance UYA 2.4Y. 2.4.2 Electrical measurements The samples for electrical testing were connected using commercial silver conductive paste, Electrolube. Electrical resistance was measured using the two-point probe method and True RMS multimeter UNI-T UT804. The current carrying capacity setup comprised: A DC power supply TTi QL564P, DC Keithley 2000 multimeter in a voltmeter mode and IMPAC IPE 140 pyrometer, all synchronised via a custom LabView software programme enabling the measurement control and data recording.

2.4.3 Microscopy The CNT specimens were investigated using the scanning electron microscope (Auriga CrossBeam Workstation, Carl Zeiss) equipped with the In-Lens secondary electron detector (true SE1) and the Energy selective Backscattered electron detector (EsB, low-loss BSE). The energy of primary electrons in the scanning beam was carefully adjusted so as to reveal the morphology of the CNT bundles and simultaneously distinguish dierent material phases present within the bundles, based on the compositional contrast at high resolution.

2.4.4 Raman spectroscopy The nonpolarized Raman spectra were recorded in range of 1003600cm−1 , in the back scattering geometry by inVia Renishaw micro-Raman system. The excitation light of 488 nm was provided by tuneable Ar ion laser. The long working distance microscope objective

7



Leica50x LWD with numerical aperture (NA) 0.5 enabled the focussing of the laser beam to a diameter approximately 2µm on the sample surface. The Raman surface mapping measurements were taken with an area of 40x40 µm using a motorized stage. Both, data collection and deconvolution of the obtained spectra by the curve tting method were performed by Renishaw WiRE 3.4 software. For the curve tting procedure a mix of Lorentz and Gauss functions were used.

3

Computations

3.1 Structural and electronic property calculations

Figure 1: The atomistic cross-sectional and side views of fully optimized doped CNTs+G hybrid systems. Supercells marked by grey lines contain CNT bre composed of two semiconducting and one metallic CNT coupled to the graphene ake. CNTs+G hybrid system is doped by 3 Br2 groups per supercell. Br atoms are depicted in red, while carbon atoms are shown in grey. (a) Graphene ake G1 is placed above semiconducting CNTs. (b) Graphene ake G2 is rotated by 90 degrees connecting neighbouring CNT bres rather than CNTs in one bre. The structural and electronic properties of hybrid CNTs+G systems were studied in the 8


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framework of the spin-polarized density functional theory (DFT), 29,30 as implemented in the SIESTA. 31 The computations were carried out employing Generalized Gradient Approximation (GGA) in Perdew-Burke-Ernzerhof (PBE) 32 parameterization for exchange-correlation functional and double-ζ numerical basis set of orbitals localized on atoms with polarization functions (DZP). Long-range interactions were included in the total bonding energy as proposed by Grimme. 33 The Brillouin zone was sampled in 7x6x3 Monkhorst and Pack scheme, whereas the kinetic energy cuto for real-space integration was set at 350 Ry for the structural calculations and increased up to 500 Ry for electronic property calculations. The self-consistent eld (SCF) cycle was iterated until and the density matrix was changed by less than 10−5 . The hybrid models schematically presented in Figure 1 were fully optimized until a maximum force converged to lower than 0.001 eV/Å and the total energy changed by less than 10−5 eV per atom.

3.2 Electron transport calculations

Figure 2: The atomistic side view of the model used for the transport calculations of doped CNT-graphene hybrid systems. The semi-innite electrodes consisting of perfect silver are highlighted in blue. ee denotes the extended electrode region, int is the interface region and transport is the transport region. The C atoms are depicted in dark grey while Ag, O, Cl and H are shown in light grey, light red, green and white respectively. The electronic coherent transport has been studied employing the non-equilibrium Green's function (NEGF) technique within the Keldysh formalism as implemented in the Tran9



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SIESTA. 34 The computed structures were treated as two-probe systems with the central scattering region sandwiched between fully relaxed semi-innite source (left) and drain (right) silver electrode regions as presented in Figure 2. The complex energy contour was always set to value below the lowest energy in the energy spectrum of each system. The numbers of points along the arc part and on the line of the contour were set to 16 and 10, respectively, whereas the number of Fermi poles to 16. The Brillouin zone of the two-probe system was sampled using a 3x2x51 Monkhorst-Pack scheme. For accuracy of transmission spectra, the energy window has been in the range of (-2, 2) eV within 201 points. For the current calculations, we have used the default value of small nite complex part of the real energy contour (106 Ry) and increased the number of points on the close-to-real axis part of the contour in the voltage bias window to 10. Other computational details are the same as given in the previous subsection. More details of the method for calculating the transport properties are provided in the Supporting Information.

4

Results and discussion

4.1 Experiment 4.1.1 Hybridization of CNT bre with graphene akes Initially the most eective methods for the introduction of graphene akes into the CNT bres were examined. For a preliminary baseline test, manual rubbing of all three graphene akes into the CNT lm structure was performed. In this procedure 100 mg of each graphene type was rubbed into 10 samples of CNT lm until the maximum amount of particles were attached to each sample. The weight of individual bers after rubbing of graphene akes increased by approximately 70%, which gives a CNT-GNP ration of approx. 3:2. To assess the eect of the deposition of the graphene akes on the electrical properties of the bres, two parameters were considered which were absolute conductivity: σ =

10


L R·S

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(S/m), and specic conductivity σ 0 =

L2 R·m

(S·m2 /kg) (L - length, R - electrical resistance, S

- cross-sectional area, m - mass). The latter one is particularly important as it takes into account the weight of the material - it shows whether the conductivity truly increases, e.g as a result of eective low-resistance bridging of CNT network or whether eect observed is just an increased amount of conductive material integrated into the bres. Taking into account the highly porous nature of CNT bres, the latter scenario is highly probable. Hybridization with graphene is unlikely to cause any detectable change in the crosssectional area of the bres, and thus a deceptive improvement in absolute conductivity. For this reason, in all further tests both conductivities are always considered. The changes in conductivities obtained for hybrid bres with graphenes rubbed into them were not higher than ∆σ = 60%, ∆σ 0 = -3%, where the change was calculated as the dierence between the conductivity values of the obtained hybrids and the pure CNT bres. Whatever the increase in absolute conductivity, the lack of improvement in specic conductivity indicates that graphene introduced in such form is not eective in the intrinsic enhancement of the electrical properties of the bre network (e.g. by increasing the connectivity), but it is rather a conductive ller of the porous bre structure. This eect may be worsened by the fact that rubbing does not allow for a uniform distribution of graphene akes over the bre. Taking account of these considerations rubbing was discounted and to avoid these issues, a spray coating technique used in further testing. To prepare for this approach the graphene akes were suspended in a high vapour pressure organic solvent (acetone) and sprayed uniformly over the surface of the CNT lm, which was further rolled mechanically to form a biscrolled CNT bre. 35 This approach, as well as producing a better distribution of graphene akes, also appears to allow the akes to be attached more rmly to the CNT network. Acetone, also in a spray form, is well-known to cause the densication of CNT assemblies due to the capillary forces exerted on the nanotubes upon the evaporation of the solvent. 36 It may be expected that the same eect could take place in the case of graphene. 11



Figure 3 presents the changes in absolute and specic conductivities of the hybrid bres obtained using the dierent types of graphene. It is interesting to notice that a clear variation may be observed between the graphenes. GNP were characterised by the highest values of

∆σ 0 (maximum observed was 14% increase) and the highest increase in ∆σ -(maximum observed 104%).

Figure 3: Percentage change of absolute and specic conductivity after hybridization of CNT bres with three dierent graphene akes via spray coating. Whiskers represent maximum and minimum obtained for a given data series. The reason for the observed dierences may be partly inferred from the scanning electron microscope (SEM) images presented in Figure 4. It is easy to see that G3 graphene akes are signicantly smaller in diameter than the other graphenes (see also Figure S6 in the Supporting Information). However, this material looks crumpled, which is probably the reason for the poor bridging between CNT bundles and the addition of substantial weight causing a clearly negative ∆σ 0 . It is most likely the G3 graphenes could be attened by further processing and a choice of suspension media. Unfortunately, the search for such procedures was beyond the scope of this work. The larger akes of M25 and GNPs lay at on the surface, yet the extremely large platelets of M25 did not seem to be distributing evenly and may have diculty in attaching rmly to CNTs in all areas of the network. The possibility of the M25 being an intrinsically poorer conductor than the GNP, may also not be disregarded. However, as mentioned above, acetone is also a good densifying agent for the pure CNT 12


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Figure 4: SEM images of (a) G3; (b) M25 and (c) GNP graphene akes hybridized with CNT lm via spray coating technique. lms and this may play an important role in the conductivity changes; therefore the lms were also sprayed with acetone to determine the baseline for the conductivities comparison. The changes observed with pure acetone spray amounted to ∆σ = 61% and ∆σ 0 = 36%. By subtracting the eect of the acetone, we were able to ascertain that the spray coating method gives little absolute conductivity increase and a decrease in specic conductivities. The results indicate that the production of biscrolled hybrid bres via spray coating may have one disadvantage in that the graphenes are deposited on the surface of lm structure only, and thus cannot bridge the nanotubes/bundles placed deeper in the bre structure. Their distribution inside of the bre could also make better use of the available graphene material and avoid potential stacks of graphene that add considerably to the weight but are used poorly as conductors. For this reason, another approach was latterly investigated. In this hybridization method, graphene akes were introduced into the whole volume of the CNT lm by inltration using acetone, chloroform and H2O2 (60% aq). The GNP akes were chosen for these further tests as this material gave the best results in the spray-coating method. The absolute and specic electrical conductivity of so-prepared hybrids is presented in Figure 5. So as to subtract the eect of the suspension media, the lm samples were also treated with pure acetone, chloroform and hydrogen peroxide which gave ∆σ of 61%, 37% and 87% 13



Figure 5: Percentage change in absolute (a) and specic (b) conductivity after hybridization CNT bre with GNP graphene akes via soaking. and ∆σ 0 of 36%, 35% and 43%, respectively. This means the conductivity increase obtained purely by the graphene addition (calculated as the dierence between the results obtained via soaking in dierent media with and without graphene akes) amounts to ∆σ =65%,

∆σ 0 =17% for acetone, ∆σ =68%, ∆σ 0 =13% for chloroform and ∆σ =71%, ∆σ 0 =24% for H2 O2 base. This in turn indicates that inltration is a more eective method than the spray coating and that graphene is capable of enhancing both electrical conductivities of CNT bres. However, the improvement obtained is not signicant. It was observed that the best results were obtained for H2 O2 . This outcome could be potentially related to doping via oxidation of carbon materials with H2 O2 . However, as we

14


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have shown in our previous work, the use of H2 O2 of high concentration (60%) did result in a signicant swelling of CNT lms, which enabled much more ecient penetration of dopants at the later step. 24 This may suggest that such swelling of CNT lms may allow for better distribution of graphene akes within the volume of the bre structure. For that reason, these suspension media were used for the further tests, involving doping of CNT-graphene hybrid bres.

4.1.2 Doping of CNT-graphene hybrids

Figure 6: Conductivity changes after doping of CNT-GNP hybrid bre. Whiskers represent maximum and minimum values obtained in the experiment series. The hybrid CNT-GNP bres prepared via inltration method using H2 O2 , were further doped with strong oxidizing agents (selected based on earlier work 24 ) and strong p-dopant F4TCNQ 37 (Figure 6). The procedure of hybridization and further doping was preceded by a three-step purication process, dened previously, 24 , which was earlier found to be an additional factor improving absolute and specic conductivity. As shown in Table 1, the purication procedure led to a slight decrease of the 'graphene eect' as the dierence between the absolute and specic conductivities increase obtained after purication and H2 O2 treatment of CNT bre and for the analogous bre hybridized with graphene is 66% for ∆σ and 2% for ∆σ 0 . This fact indicates that the graphene is still capable of enhancing the electrical transport in the bre. However, the eect is not particularly high in comparison to 15



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the p-dopants presented in Figure 6, which signicantly increase both absolute conductivity and specic conductivity of the doped hybrids. Out of all the chosen dopants the best results were obtained for HClO4 and Br2 . Br2 treatment gave the highest absolute conductivity, while HClO4 the best specic conductivity. However, the most interesting result is that the addition of graphene akes causes a signicant increase in doping eciency in terms of improvement of electrical properties. The analysis of the data in Table 1 shows that although the percentage increase in conductivity achieved due to hybridization with graphene is only 66% for absolute conductivity and 2% for specic conductivity at the stage of hybridization, the analogous increases associated with doping of the hybrids are signicantly higher. For HClO4 doping the dierence between the conductivities increase associated with graphene presence amounts to: 223% and 85%, for absolute and specic conductivities respectively and for bromine doping: 344% and 104% absolute and specic conductivities respectively . This indicates that it is not the simple addition of benets obtained separately from hybridization and doping, but graphene akes interact with the introduced dopants and increase the eectiveness of doping, or from graphene perspective doping reinforces/boosts their eect on the bre conductivity. The latter graphene interpretation is very well visible in the data presented in Figure 7. The doping of hybrids prepared by the use of dierent graphenes shows that the trend observed in the spray application of graphenes i.e. that GNPs produce the best results (Figure 3) is still present.The doping makes it only more pronounced. However, as mentioned Table 1: Comparison of percentage conductivity increases after doping of CNT bre and hybrid CNT- GNP bre in relation to the conductivity of as-made material.

Doping of CNT bre H2 O2 after purication HClO4 after H2 O2 0 % ∆σ % ∆σ % ∆σ % ∆σ 0 113 136 704 295 Doping of CNT-graphene hybrid H2 O2 with GNP after purication HClO4 after H2 O2 % ∆σ % ∆σ 0 % ∆σ % ∆σ 0 179 138 927 380 16


Br2 after H2 O2 % ∆σ % ∆σ 0 601 156 Br2 after H2 O2 % ∆σ % ∆σ 0 945 290

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above it needs to be remembered that potential attening of G3 could also change the distribution of these results, and the results presented in Figure 7 refer only to G3 in crumpled form.

Figure 7: Percentage conductivity change after HClO4 (a) and Br2 (b) doping on CNTgraphene hybrid bres prepared using dierent graphenes. Whiskers represent values obtained in the experiment series.

4.2 Current carrying capacity To further test the inuence of the graphene presence on the electrical properties of the bres, a current carrying capacity (CCC) test was also conducted. The experiment (Figure 9) measured the maximum DC current that the bres can withstand upon its incremental increase from zero with a step of 0.01 A per second. 17



Figure 8: Comparison of absolute (a) and specic (b) conductivity values achieved for doped CNT bers, and doped hybrid CNT-graphene bers (hybridization with GNP by soaking).

Figure 9: Current carrying capacity measurements comparing samples at dierent preparation steps: as made, puried, hybridized with GNP and hybrid doped with Br2 and HClO4 . As the previous paper 24 has shown, the failure point is very much dependent on the purication and doping state of the bre, therefore samples at dierent stages of preparation were also tested for comparison. Consistent with the previous results, the purication and 18


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H2 O2 addition increased the current at failure, while bres doped with HClO4 showed the highest maximum current. Comparing hybridized bre (CNT bre puried and hybridized with GNP using H2 O2 suspension) with a pure CNT bre puried and treated with H2 O2 , there is an increase in the transmitted current for the hybrid bre, of approximately 16%. For a CNT bre puried and treated with H2 O2 without the addition of graphene akes the current that can be carried is 0.57 A and in the case of puried CNT bre hybridized with GNP using H2 O2 the value of current is 0.66 A. However, doping of hybridized bres with Br2 and HClO4 causes a small decrease in the maximum current compared to doped only CNT bres. These results indicate that in case of pure CNT bres the additional of graphene akes in the CNT network oers alternative pathways for the current ow at high temperatures and high electric elds, thus resulting in a higher maximum current. In the case of a doped material the abundance of charge carriers mitigates this eect, while a higher packing density of the material causes poorer heat removal eciency and thus quicker failure of the conductor. Finally, it is worth emphasising that the non-relative values of absolute and specic conductivity presented in Figure 8 are higher than for most doped non-hybridized CVD spun bres, 24,3841 non-doped array spun hybrid CNT bres and lms 17,18,21,42 as well as doped hybrid CNT lms. 43

4.3 Raman Spectroscopy Finally, to understand better the phenomena taking place upon the introduction of graphene and the doping of hybrids, Raman spectroscopy features were analysed for all three graphene akes used as well as raw and puried CNT bres, hybrid CNT-graphene bres and doped hybrids. The crystalline graphene is characterized by two features visible in Raman spectra, which are the G band that comes from in-plane C-C vibrations and 2D (also called G') originating from the second-order two-phonon process. The G band is centred at approximately 1580 19



cm−1 . For single-layer graphene the position of the G-band is 1587 cm−1 and it shows a down shift upon an increase in the layers number. 44 Another band that may be observed is the D band caused by disorder in graphene structure. 45 D and 2D bands are dispersive depending on the laser excitation energy. In this study a laser wavelength of 488 nm (energy

∼2.54 eV) was used. For this excitation energy the Raman spectra features should appear at approximately 1350 cm−1 for D-band, and 2700 cm−1 for 2D-band. 46 The 2D band is also useful in the determination of the number of layers of graphene as its up-shift is related to higher number of layers. Finally, the ratio of IG /ID may be also analysed to verify the graphene structure quality. 47 Here, the higher IG /ID ratio is, the less disordered is the structure. Comparison of the Raman spectra of the three types of graphene akes used in this research clearly shows shifts in both G and 2D bands positions (Figure 10 (a) and (b)). All types of graphene show a clear down-shift in G-band suggesting a multi-layer structure. However, only in case of GNP and M25 it is correlated with up-shifts of 2D bands. The G3 shows a down-shift in 2D band. However, taking into account its crumpled structure visible in the SEM images and the very small ratio of IG /ID (Figure 10 (c)), it may well be expected to nd deviations from standard Raman features of graphenes. Another important observation to mention is that the M25 shows the smallest downshift of the G-band of all graphenes which suggests a small number of layers. However, it simultaneously has the highest up-shift of 2D band among all studied graphenes. The GNP seems to be more consistent in this respect as it shows a signicant G-band down-shift and simultaneous 2D band up-shift. Such a result potentially may be explained by the smaller number of defects present in the latter material. This would also explain the previously observed eect, where the M25 akes although larger than GNP, were not producing the best conductivities in hybrid bres. Further studies were performed on as-made bres, puried bres and bres hybridized with all types of graphenes. Comparison of the band shifts between the hybrid bres and 20


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Figure 10: (a) G-band position (b) 2D-band position (c) and IG /ID ratio for dierent graphene akes. (d) G-band position, (e) 2D-band position and (f) IG /ID ratio for raw CNT bre and hybrid CNT-graphene bres with dierent graphene akes used; (g) G-band position, (h) 2D-band position and (i) IG /ID ratio for raw CNT bre, puried CNT bre and hybrid CNT-GNP bres and CNT-GNP hybrid bres doped with HClO4 and Br2 dopants used. The error bars in (a), (b), (d), (e), (g) and (h) originate from the spectral resolution of the Raman spectrometer. In (c), (f) and (i) the error bars are related to the standard deviation of measurements of the integral intensity of Raman bands. pure CNT bres (Figure 10 (d)-(f)) revealed an analogy to the positions of bands and IG /ID ratio for three dierent graphene akes (Figure 10 (a)-(c)). This implies that we observe a clear superposition of Raman activities of CNTs and graphenes, and that CNT 21



bres may be treated as a background for the graphene Raman activity. Finally, Figure 10 (g)-(i) present band shifts for as-made bres, puried bres, bres hybridized with GNP and the same hybrids doped. It is visible that upon oxidation of the structure the G and 2D band up-shifts are observed which is in agreement with the results presented for doped (non-hybridized) CNT bres. 24 All of the above-presented experimental results show that well deposited graphenes may play an important role in the electrical transport of CNT networks. It is clearly visible that in the undoped state the observable increase in the absolute conductivity is not correlated with the specic conductivity, which shows little improvement. This could indicate that the graphene only plays the role of conductive ller in the porous structure of the bre. However, examining the signicant dierence between the doping eectiveness in case of pure and hybridized bres, which also varies with the type of graphene, as well as the increase in the maximum current of the undoped hybrids, we observe that the role of graphene is not negligible. It is important to note that to clearly notice its eect it is necessary to either dope the structure or increase the electric eld. It is possible that in case of pure hybrids that the coupling between the CNTs is insucient, which may stem from the dimensions of the graphene akes and their thickness. To better understand the above observed phenomena and the transport processes involved, some rst principles calculations on various models of CNT-graphene systems, were also performed.

4.4 Modelling 4.4.1 Structural and electronic properties In order to explore the inuence of doping on the CNT-graphene materials presented above, we have performed rst principles calculations on two dierent CNT-graphene hybrid systems that are shown schematically in Figure 1. 22


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Each hybrid system contains a model of a CNT bre composed of three single-walled CNTs, one metallic (5,5) and two semiconducting (8,0), and a graphene ake with mixture of zig-zag and armchair edges. This arrangement was chosen to model the practical hybrid bre where statistically one third of nanotubes is metallic and two thirds are semiconducting, while the graphene akes may have both zig-zag and armchair edges. The calculations were performed using a rectangular supercell with dimensions 20.5 Å, 23.4 Å and 17.4 Å along x ˆ,

yˆ and zˆ directions. Due to the 3D periodic boundary conditions, our model can be viewed as a grid of innitely long parallel CNT bres. Therefore, we have considered two dierent orientations of graphene akes: above two semiconducting CNTs in one CNT bre (Figure 1 (a)) and between neighbouring CNT bres (Figure 1 (b)). Models containing such hybrid systems are denoted CNT+G1 and CNT+G2 , respectively. As doping agents we have chosen Br2 and HClO4 , which were placed around the CNT bre in a form of three separate groups per each supercell. This corresponds to 8.0 wt.% and 5.2 wt.% of Br2 and HClO4 groups in the hybrid systems, respectively. To reproduce the experimental conditions, the geometry optimization of the doped hybrid system was performed in three steps. It the rst step, we have fully optimized the structure of pure CNT bre and lattice dimension along zˆ direction. After adding the graphene ake, system was again optimized but the distances between CNTs were kept xed, as was the size of the supercell. Finally, dopant molecules were introduced into the supercell and the system was optimized one more time, still keeping the distances between CNTs and the supercell lattice parameters xed. As can be seen in Figure 1, the orientation of the dopants, especially Br2 , strongly depends on the orientation of the sp2 bonds in the carbon network. The interactions between CNTs, graphene ake and dopants induce the structural changes in all components of the system. The most pronounced are the deformations of graphene akes which are also a consequence of the size of used G models and lack of edge saturation. As in the previous work, 48 we also observe small elongations of the bonds in the dopants compared to the isolated case. However, the most important is the alteration of fragile sp2 CNT network. The coecient of CNT radius variation (CV), which 23



is a standardized measure of the change in nanotube shape, varies between only 0.0086 and 0.0436 (see Table S1 in the Supporting Information) and is higher than in case of halogen doping 48 but smaller than can be observed after covalent functionalization of single-walled CNTs. 49 It is expected that these eects will be reduced when multi-walled CNTs are used, 50 as it is done in the experiments. To fully understand the character and origin of the interactions between the CNTs, graphene akes and dopants, it is necessary to analyse the electronic properties of the doped CNTs+G hybrid systems. Figure 11 presents their projected density of states (PDOS). Comparison of the pure CNT bre and the hybrid CNTs+G systems (Figure 11 (a)) immediately reveals that addition of graphene ake into the system signicantly increases the total number of available electronic states at the Fermi level. Clearly, such modication of CNT bres introduces new states that can contribute to the electron injection and can enable the opening of new conduction channels. G1 introduces more states than G2 . This might suggest better electrical performance of CNTs+G1 system over CNTs+G2 system, however one should remember that the number of conduction channels constitutes only to the upper limit of the conductance and the further analysis of the transport properties is required. Further analysis (see Figure S3 (b) in the Supporting Information) have shown that the number of new states at the Fermi level depends on the concentration of graphene akes. The higher concentration of akes results in the higher density of states at the Fermi level. Importantly, changing the mutual orientation of graphene ake and CNTs does not aect signicantly obtained results. As can be seen in Figure S3 (a) in the Supporting Information, CNTs+G1 hybrid systems, regardless of the position of G1 ake, have a higher number of available electronic states at the Fermi level than pure CNTs bre. The presence of graphene ake above semiconducting CNTs aects the system more than placing it above metallic and semiconducting nanotubes. This indicates that graphene akes induce the metallization of semiconducting CNTs in a similar way to the metal matrix in CNT-metal composites. 14,51 To explain the experimental trends, showing that Br2 and HClO4 doping enhances the 24


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Figure 11: (a) The computed density of states of pristine CNT bre and both pristine CNTs+G hybrid systems. (b)-(g) The projected density of states (PDOS) of all modelled systems. (d)-(g) Insets show the cross-sectional visualisations of fully optimized doped hybrid systems. The C atoms are depicted in grey while O, Br, Cl and H are shown in light red, dark red, green and white respectively. electrical properties of the composite, it is rst necessary to analyse the ground-state electronic properties of all doped CNTs+G systems studied. As can be seen in Figure 11 (b) and (c), both the dopants improve the number of available states at the Fermi level in case 25



of CNTs+G2 system, whereas in case of CNTs+G1 system only the presence of HClO4 gives more states at the Fermi level comparing to the undoped system. It is also apparent, that the interaction between the hybrid CNTs+G systems and the both dopant molecules induces changes in electronic properties similar to that found from covalent bonding. 52,53 As can be seen in Figure 11 (d) and (g), they introduce the additional impurity bands. The number of impurity bands depends on the number of dopants (see Figure S3 (c) in the Supporting Information). In case of Br2 , the main contribution to the impurity bands near the Fermi level comes from the halogen compounds. In case of HClO4 , the rst impurity band is localized deeply below the Fermi level and the main contribution is coming from oxygen atoms. The superior performance of HClO4 over Br2 dopant which was visible in Figure 11 (b) and (c) (see insets) can be explained by the small contribution of carbon atoms from the CNT backbone which directly contribute to the states at the Fermi level.

4.4.2 Transport properties The transport calculations have been performed on the device models shown atomistically in Figure 2. To reproduce the experimental conditions, the central part of each model used in the structural and electronic property analysis was sandwiched between two semi-innite silver electrodes. Ag belongs to the weakly interacting metals with CNT, 54 which interaction strength depends on the distance between Ag and C atoms. 55 Importantly, the separation distance between the CNT and the metal contacts is one of the dominant factors aecting the conductance of the system. 14 Therefore, the interface regions containing Ag layers and CNT ends were fully optimized. This geometry optimization has slightly changed the geometry of both materials (cf. CV of hybrid models and CV of device models in Table S1 in the Supporting Information). The charge transfer occurring at the interface between these two dierent materials produces band bending which enables the CNT's valence band edge to align with the Fermi level of the electrode. 51,56 Unfortunately, due to poor coupling with the Ag electrodes, the transmission of metallic CNT is decreased with the respect to the 26


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intrinsic conductance of pure CNTs. 54 On the other hand, Ag contacts reduce the band gap of the semiconducting CNTs by the breaking of the mirror symmetry of the nanotube wave functions induced by the localized perturbation of the electrodes. 54 Note that metal contacts also introduce the so-called metal-induced gap states. 57,58 This is particularly important in the ballistic transport regime, where the transmission coecient at the Fermi level is correlated with the number of bands. Having described the inuence of Ag electrodes on the electronic transport properties of pure CNTs, we can now focus on the hybrid systems. The conductance of CNT junctions depends on the crossing angle between tubes, with signicant conductance peaks corresponding to maximum intertube coupling occurring only for highest inter-tube lattice matching, and on the overlap length. 59 At comparable contact length, the conductance of armchair/zigzag metallic junction is even more than one order of magnitude lower than that of an armchair/armchair contact. 7 Here, CNT bre is modelled by three CNTs of two dierent chiralities, including semiconducting CNTs, and the overlap length is equal to the transport region length as shown in Figure 2. Consequently, the current-voltage characteristic of the device containing CNT bre is non-linear, 14,50 due to inter-CNT coupling that lowers transmission at the Fermi level (see black line in Figure 12 (a)). The current transmitted through the system when U=1V is approximately 1.5 times higher than the current transmitted through an isolated CNTs sandwiched between Cu electrodes 48 and becomes nearly a factor of three greater than the current owing through dense arrangement of vertically aligned CNTs in Cu-carpet. 14,50 Further analysis of Figure 12 ((a) and (b)) and Figure S5 (a) in the Supporting Information show a very interesting result that the addition of graphene akes to such CNT system enhances the current and its conductance. Although, the increase in conductance is dependent on orientation of graphene and applied voltage, it is clearly visible in the whole calculated range. The increase reached a maximum of 110% (compared to the pure CNTs bre model) at 1.5 V for G2 and approximately 60% at 0.5 V for G1 . Such results corroborate 27



Figure 12: (a), (c) The computed current-voltage characteristics of device models. (b),(d) The computed conductance change with applied voltage. ∆G is calculated as (Gdoped Gpure ))/(Gpure )x100%, where Gdoped and Gpure are the conductance of undoped and doped hybrid device models (see Supporting Information for details). ∆Go is calculated with respect to the device model containing only CNTs. well with the experimental observations (see analysis of Table 1). However, the calculations of specic conductance (see Supporting Information for details) show that the division of conductance by mass decreases this parameter by no more than 5.3% for G1 and 3.9% for G2 (see Figure S2 (c)). This indicates that the very large drop observed in undoped samples upon hybridization in the experimental data (see for example analysis of Table 1 (66% (∆σ ) - 2% (∆σ 0 ) = 64%) may be indeed related to poor coupling of large graphene akes with individual CNTs in the hierarchical bre network. This would further conrm that well deposited graphene is not only a conductive ller of the porous CNT network. The increase in the current transmitted through the system observed both in theoretical and experimental results (see Figure 9), may also further conrm this conclusion. Note that the concentration of graphene akes in carbon nanotube network also aects the transport properties of the hybrid system and has to be carefully adjusted. Comparison

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between CNTs+G1 hybrid systems containing dierent amount of G1 akes (see Figure S4 (b) in the Supporting Information) shows that the higher concentration of akes does not have to correspond with the higher transmission coecients. Further calculations concerned the doping of the above hybrid systems. However, since the model with G1 graphene arrangement better represents the experimental situation (Figure 4), the computations mainly focused on the hybrid systems containing G1 . The calculated current-voltage characteristics and the changes of the conductance of CNTs+G systems doped with Br2 and HClO4 compared to the I(U) characteristics of the pristine CNTs+G hybrid model are depicted in Figure 12 (c) and (d), respectively. Consistent with the experimental results, our DFT calculations indicate that both types of doping can improve the electrical performance of the hybrid system. The transmission coecients at the Fermi level under zero bias voltage applied to all doped CNTs+G1 systems are higher than that of the undoped CNTs+G1 systems. The conductance dierence increases until U=1.25 V for doping with Br2 or 1.5 V for doping with HClO4 when it starts to decrease again (see Figure 12 (d)). The highest increase in the conductance (almost 12%) can be observed for Br2 doping when U=1.25 V (see Figure 12 (d)), while for HClO4 the maximum increase amounts to 7.2% at U=1.5 V. The maximum change in the specic conductance (see Supporting Information for details) calculated for these systems is reaching almost 10.1% for Br2 and over 6% for HClO4 (see Figure S2 (b) in the Supporting Information).

4.5 Comparing and contrasting experimental and calculated results The latter results show much smaller percentage changes than the experimental ones. This maybe however, quite well understood taking into account the complexity of the real system. The factors which may inuence the doping eciency may be the dopant concentration, the presence of many chiralities of nanotubes, dierent wall numbers, varying number of layers in case of graphenes, defects of hexagonal structures, dangling bonds, collapsed nanotubes, impurities, presence of catalysts etc. Models are, by necessity, much simpler than systems 29



experimentally observed, but nevertheless the rst principle calculations of such systems reproduce the experimental trends and provide useful fundamental understanding at the atomic level unavailable from the experiments. Our calculations clearly show that the presence of graphene in CNT network may improve the conductance of the network due to an increase in the density of interface states contributing to the transmission and that doping may further enhance the overall conductance of the CNTs+G hybrid systems due to the appearance of additional states.

5

Conclusions

In this paper we have looked at the electrical performance of doped hybridized CNT-graphene networks, both experimentally and theoretically. The experimental part of this investigated the possible methods of hybridization of CVD spun CNT bres with a range of commercially available graphene nanoplatelets. It has been found that the most eective hybridization technique in terms of the enhancement of the electrical transport properties, is inltration. Although, the electrical properties of the hybrids may be also aected by the intrinsic characteristics of the graphene akes, purity of the bre and the solvent used. The initial general analysis showed that the increase in absolute conductivity which may be attributed to the presence of pure graphenes does not go beyond 100% while the increase in specic conductivity is at least four times lower. These together could indicate that the graphene is only a conductive ller of the porous structure of the CNT bres. However, further results showed that presence of the graphene increased the maximum current owing through the pure bres, and also the eectiveness of the doping process signicantly increased in the presence of the grapheme akes. This indicated that the role of the graphene is denitely signicant in the electrical transport observed. The results of DFT calculations performed for a model comprising one armchair and two

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zig-zag semiconducting nanotubes were in good agreement with the above conclusions. They showed that the addition of graphene introduced additional states at the Fermi level, which increased current and conductance of the system, while doping further boosted the conductance of the hybrid system. Thus, independently of the extreme complexity of the experimental system under consideration, both experimental and theoretical results indicated an important role for the graphene in the electrical transport of these CNT networks. Finally, it is important to mention that such results are important not only from scientic perspective but also in future applications research as the clear boosting of the eectiveness of doping may be of paramount importance for researchers targeting the improvement of the maximum electrical conductivity of CNT bres, e.g. for electrical wiring related applications.

Acknowledgement All authors would like to acknowledge the help of Je Patmore (University of Cambridge, UK), who checked the manuscript for clarity and accessibility. A.L.-R., S.L.-K. and S.B. would like to thank National Centre for Research and Development in Poland for funding of this research under Lider VI grant scheme (agreement number LIDER/220/L-6/14/NCBR/2015). K.Z.M. gratefully acknowledge Interdisciplinary Centre for Mathematical and Computational Modelling at University of Warsaw (Grant No. G47-5) for providing computer facilities. S.B. greatly acknowledges nancial support from Silesian University of Technology Rector's Professorial Grant No. 04/020/RGP18/0072 and Rector's Pro-Quality Grant No. 04/020/RGJ19/0085. M.Sz. and A.D. would like to acknowledge the Ministry of Science and Higher Education for Project No. 06/65/SBAD/1952 realized at Faculty of Technical Physics, Poznan University of Technology.

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Supporting Information Available The Supporting Information are available free of charge. Details of the calculations, additional models, and additional electron microscope image of graphene

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Highly Conductive Doped Hybrid Carbon NanotubeâGraphene Wires

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