Influence of Capillary Effects on Electric Response of Well-Ordered

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Influence of Capillary Effect on Electric Response of Well-ordered Carbon Nanotube Film Quan Zhang, Peng Meng, Rui-Ting Zheng, Xiaoling Wu, and Guo-An Cheng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03250 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Influence of Capillary Effect on Electric Response of Well-ordered Carbon Nanotube Film

Quan Zhang, Peng Meng, Ruiting Zheng, Xiaoling Wu, Guoan Cheng*

College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China

Abstract Interface structure in nano-composite materials often directly influences electric, thermal and mechanical properties of functional architectures, and limits their application in many fields, besides the characteristics of their nano-building blocks. In this work, we report that the electronic transport characteristic of the well-ordered carbon nanotube film is adjusted by the structural evolution of the junction caused by effect of capillary. This mechanism can explain the resistance change and recovery in the whole immersion-evaporation process, and even the anomalous transient decrease of resistance. Meanwhile, we establish a relationship of the resistance change ratio of the film versus the interface tension between the film and the immersing liquid. Such the ability to sensitive and repeatable resistance change of carbon nanotube film could have important implications in measurement of liquid properties, liquid sensors, and solution analysis, and provide a new avenue for the new multi-functional architecture.

Introduction The unique properties of the carbon nanotube (CNT), such as low mass density, higher mechanical strength1-3 and higher electric conductance4, 5, make it have many potential applications in multi-functional sensors6-9, flexible electrodes10-13 and matrix of organic composites14-16. In the production of light-weight and ultra-strong CNT film or fiber by spinning, a volatile liquid is usually used to compact the CNT

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assemblies17. As-made CNT film or fiber is drawn through the volatile organic solvent such as ethanol. Due to the wetting effect, the solvent fills into the pores in the loose structure of the as-made assemblies. During the liquid evaporation, drastic shrinkage caused by effect of capillary force occurs according to the kinetics of liquid18. It results in the densification of CNT film or fiber, which increases the mechanical strength distinctly. Many researches compare the effect of different liquid and succeed to increase the mechanical strength of CNT assemblies. Windle and his coworkers find that the electrical behavior of CNT-based fiber is also influenced by liquid medium, including ethanol, toluene, acetone, cyclohexanone and N-Methylpyrrolidone (NMP)19-21. However, there are few reports focused on the electrical transport behavior of the CNT assemblies besides its mechanical properties during the liquid infiltration and evaporation process. Especially, the change mechanism of the electrical transport behavior in whole process is unclear. Motivated by this, we analyze the electronic transport of the well-ordered CNT film immersed by different liquid. With different wettability of the liquid, the change of electrical conductivity is different. Furthermore, the hysteresis of electrical response is also analyzed when the liquid evaporates out of the CNT film. All of these results are attributed to the electronic transport in the CNT junctions of the well-ordered film influenced by the liquid medium. This sensitive, recoverable and repeatable adjustment of characteristics of the assemblies induced by interface engineering provides new insights into the applications of nanocomposite architectures, such as fabrication of nanocomposite materials, liquid sensor and material analysis.

Experimental Section Fabrication and characteristics of well-ordered CNT film. Vertical multi-walled CNT arrays are synthesized on (1-0-0) silicon wafer by microwave plasma-enhanced chemical vapor deposition (MW-CVD) using ferric chloride ethanol solution, acetylene and hydrogen as the catalyst precursor, carbon source and carried gas, respectively. The 50 µL ferric chloride ethanol solution (0.025mol/L) is dropped on the 1.5×1.5 cm2 silicon wafer and dries naturally. The substrate is annealed for a


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few minutes at 750 ℃ under hydrogen conditions. Then, the substrate temperature is adjusted to 600 ℃ of CNT arrays, following by introducing microwave with power 300 W and acetylene. The volume ratio of hydrogen and acetylene is kept 50:5 during the growth. Au electrodes with 100 nm thickness and 10 mm width are pre-deposited on the clean organic substrate by magnetron sputtering. The distance between Au electrodes is 10 mm. PTFE film is chosen as the organic substrate due to its chemical inertness. The well-ordered CNT film is prepared by the rolling-transfer process.1, 2 Direction of rolling (namely axial direction of CNTs) is parallel to that of applied voltage. The micro-morphology of the samples is determined by scanning electron microscopy (SEM) (Hitachi S-4800). Contact angle is measured by Wilhelmy Plate method, which is suitable for the CNTs.3-6 Raman spectroscopy (LabRAM Aramis, wavelength 532nm) indicates that the CNTs don’t change during the measurement of direct current response. All of optical images are achieved by light microscope. Direct Current response measurements. Hand-made equipment is used to monitor direct current response of well-ordered CNT film in different medium. The measurement of time-resolved current is carried out by Keithley 4200-SCS under a constant voltage of 1 V. Keithley 4200-SCS hold that the fluctuation of voltage and current in this work is less than 50 µV and 10 nA, respectively. To avoid introducing the error from operation, the film is fixed on a support and the vessel is placed on a lifting platform. The CNT film is immersed into and pulled out of the liquid by raising and lowering the platform. In addition, the environmental temperature and humidity are controlled within 25±1 ℃ and 45±5 %, respectively. We choose ethanol-water with different mass fraction, pure acetone (CH3COCH3) and trichloromethane (CHCl3) as the measured liquid. This scheme can satisfy the requirements of measured liquid, including volatility, low electrical conductivity and chemical stability. Molecular dynamics (MD) simulations. We perform all MD simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS; http://lammps.sandia.gov/). (6,0) single-walled CNT, TIP3P H2O molecule and C2H5OH molecule are used to analyze the interruption of electronic transport path


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between CNTs causing by capillary effect. The molecular interaction is described by standard Leonard-Jones potential. The LJ parameters are listed in supplementary information. A close-contact interface is preformed between two CNTs with 10 nm length, in which one end of CNTs is fixed and another was moveable. Ethanol aqueous solution with different concentration is placed in a finite region and then relaxed in the NVE ensemble at 298±5 K for a sufficiently long time (10 ps) to obtain a homogeneous state before it infiltrating into the CNT junction. After that, the system evolves for 45 ps in the NVE ensemble at 298 K for data collection. The temperature fluctuation of system is always controlled in 5 K.

Results and Discussion Figure 1(a) is the schematic of the well-ordered CNT film on the PTFE substrate deposited Au electrodes. For avoiding the change of resistance from the interface between the CNTs and the electrodes, polydimethylsiloxane (PDMS) coating is deposited on the CNTs/Au contacts. Figure 1(b), a SEM image of the CNT film, shows the ordered morphology. Almost every CNT clings to its adjacent nanotubes. This junction structure is maintained by the molecular interaction and provides a large amount of electronic transport path. When the external force is stronger than the interaction between CNTs, the electronic transport paths will be broken and the volume resistance of the CNT film increases distinctly. A specific hollow tube structure of CNTs with a layer distance ~0.34 nm is shown in the inset in Figure 1(b) and the mean diameter of the CNTs is about 5 nm. A two-wire method is adopted to measure the direct current (DC) of the CNT film. The typical bias voltage is 1V. During measurement, the area covered by CNT film is completely immersed into the liquid. In this way, the error caused by the non-uniformity of droplet location is estimated.


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Figure 1 (a) Schematic of the well-ordered CNT film on the PTFE substrate deposited Au electrodes. (b) A SEM image of aligned CNT film. The scale bar is 100 nm. The inset is a high-resolution TEM image of an individual CNT. The scale bar is 5 nm

Figure 2(a) is a time-resolved resistance change ratio of the CNT film immersed in pure ethanol. It is divided into four stages, including initial state (stage I), wet state (stage II), evaporating state (stage III) and dry state (stage IV) (S4 in supporting information). The resistance of the film is calibrated to a fixed value before immersion (initial state). When the film is immersed by ethanol, its resistance increased abruptly. Then, the resistance goes up slightly (wet state). When the film is drawn out from the liquid, the resistance of the CNT film gradually recovers to the initial value (evaporating state). The period of stage III is dependent on the evaporation time of the absorbed liquid. It is noted that an abnormal temporary resistance decrease occurred at the end of evaporation process. For trichloromethane, its stage III is much shorter because of its higher evaporating rate (as shown in Figure 2(b)) and there is no temporary resistance decrease at the end of this stage. When the film is immersed by water, the resistance of the CNT film reduces slightly, which is opposite to that for ethanol or trichloromethane (as shown in Figure 2(c)). A temporary reduction of resistance also exists in the stage III of water, and its degree is larger than that for ethanol. This phenomenon implies that water molecules may be a key factor triggering the temporary resistance change. Comparing the time-resolved resistance curves of the film immersed in different liquids, the resistance change ratios are +29.92%, +60.13% and -2.20% respectively for ethanol, trichloromethane and deionized water (conductivity of ~1.6×10-6 S/m). It means that the different


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interactions occur at the interface of the CNTs and liquid molecules when the CNT film is immersed into the different liquids. Figure 2(d) shows the Raman spectroscopy of the well-ordered CNT film before and after cyclic test. The peak at 1580 cm-1 means existence of the graphitic structure in the sample. After the cyclic test, there is not obvious change of characteristics peaks at 1342 cm-1 and 1580 cm-1, including the peak positions and relative intensity ratio. It indicates that the repeating measurements have no effect on the structure of the CNTs in our work.

Figure 2 Time-resolved resistance change of the as-made CNT film for (a) ethanol, (b) trichloromethane and (c) water. Four stages are corresponding to initial state, wet state, evaporating state and dry state in turn. (d) Raman spectroscopy (using 532 nm wavelength excitation) of the CNT film before and after liquid immersion-evaporation experiment.

In order to further investigate the effect of liquid wetting on the electronic transport of well-ordered CNT film, we record the time-resolved DC response of film


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to different liquid in a complete immersion-evaporation process. The resistance of CNT film is calculated by Ohm’s Law R film + Rother =

U , in which R film is the resistance I

of the film, Rother is the external resistance of electrodes and wires, U is the applied voltage and I is the measured current. The external resistance of electrodes and wires has been deducted (S4 in the supporting information). Ethanol calibration is applied to adjust the initial resistance of CNT film before it is immersed into the measured liquid. The following measurements are performed on a same CNT film, whose initial resistance R0 is controlled in 18.8±0.2 Ω. The resistance of the film at10s and 950s after immersion is defined as the resistance of wet CNT film R (10s ) and the resistance of the dry film R ( 950s ) , respectively. The resistance change degree, rather than the absolute resistance is used to reflect the conductivity of the CNT film at different state.

Figure 3 (a) Bar graph of different parameters measured on a same CNT film. H2O = water, C2H5OH = ethanol, CH3COCH3 = acetone and CHCl3 = trichloromethane. Black is the change ratio of resistance ∆R (10s ) R0 . Red is the surface tension of different liquid at 25℃. Blue is the contact angle between liquid and the film


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measured by Wilhelmy Plate method. Pink is the relative interface tension between liquid and the film calculated by Young’s Equation. (b) Resistance change ratio of the well-ordered CNT film at wet state for versus the relative interface tension. The inset shows the plot of ln(y-y0) versus the relative interface tension, in which y is

∆R (10s ) R0 and y0 is the vertical intercept from the curve-fitting in (b). (c) Resistance change ratio of the well-ordered CNT film at dry state versus the relative interface tension. The inset shows the plot of ln(y-y0) versus the relative interface tension, in which y is ∆R ( 950s ) R0 and y0 is the vertical intercept from the curve-fitting in (c). (d) The time of peak (black square) and the maximum resistance change (blue circle) of the well-ordered CNT film as functions of the mass fraction of ethanol aqueous solution.

For statistics, all the experimental data of 5 measurements per liquid are averaged, which are listed in the supplementary information (Table S2 in the supporting information). In Figure 3(a), the order of the liquids along the horizontal axis is chosen to give a continual increase in the change ratio ∆R (10s ) R0 (black). No matter surface tension of the liquid (red) or contact angle between the liquid and the CNT film (blue), there is no obvious change law similar to that of ∆R (10s ) R0 . For instance, the surface tension of trichloromethane is ~27.14 mN/m slightly larger than that of pure ethanol ~21.82 mN/m, but the change ratio of the resistance for trichloromethane is approximately double of that for ethanol. On the contrary, the resistance of the film has a little change about -2.19% when the film is immersed by water, although the surface tension of water 72.1 mN/m is distinctly larger than that of ethanol, acetone and trichrolomethane. Likewise, the contact angle of ethanol, acetone and trichrolomethane on the surface of the CNT film is similar, smaller than that of water, but the change ratio is both large and small. The relative interface tension is calculated by Young’s Equation, in which the constant surface tension of CNTs is assumed as zero. As the relative interface tension decreases gradually, the change of


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the wet resistance increases for different measured liquid. Figure 3(b) shows the relationship between the change ratio of resistance at wet state ( ∆R (10s ) R0 ) and the relative interface tension. The relative interface tension between the CNTs and the measured liquid is calculated by the Young’s Equation

γ SL = −γ LV cos θ , in which γ LV , γ SL and θ are the surface tension of the liquid, the relative interface tension and the contact angle between the CNTs and the liquid, respectively (S5 in the supporting information). Due to the loose structure of the well-ordered CNT film, the contact angle is measured by the Wilhelmy Plate method (S6 in the supporting information). The measuring error of ∆R (10s ) R0 is less than 0.1%. For a given temperature, the change ratio gradually decreases with the increase of the relative interface tension. The curve-fitting result is

∆R (10s ) R0 = −0.17 + 0.74 × exp ( ( −γ SL − 15.53) 6.10 ) where ∆R (10s ) R0 is the change ratio of the film resistance at wet state and γ SL is the relative interface tension between the CNTs and the measured liquid. The R-square is 0.996. The inset shows the nearly linear relationship between ln(y-y0) and the relative interface tension. The high fitting degree indicates that the change ratio of the resistance induced by liquid infiltration is directly connected with the relative interface tension between the well-ordered CNT film and the immersing liquid.

The change of resistance at dry state isn’t as obvious as that at wet state. There is a small deviation between the initial resistance R0 and dry resistance R ( 950 s ) . Comparing with the wet resistance, the change ratio of dry resistance ∆R ( 950s ) / R0 shows a similar trend with different liquid medium (as shown in Figure 3(c)). The degree of fitting slightly declines. The R-square of the fitting formula is 0.982. The inset in Figure 3(c) is the plot of ln(y-y0) versus the relative interface and shows the linear relationship with R-square 0.901. Besides the measuring error, the lower fitting degree is mainly attributed to the effect of the humidity when the liquid evaporates


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out of the film. The relationship between the interface tension and ∆R ( 950s ) / R0 demonstrates that the deviation of dry resistance is relative to the capillary force during the evaporation of liquid. The data of trichloromethane (black star) deviates far from the fitting curve obviously. It has a connection with the transient over-shrinkage caused by water evaporation. Owing to high hydrophobicity of trichloromethane, the dry resistance only reflects the restorability of the CNT film to trichloromethane evaporation.

Figure 4 (a) MD simulation of efficient contact evolution of an individual CNT junction immersed by ethanol aqueous solution with different mass fraction. (b) and (c) are the simulated morphology of the CNT film model composed of several junctions at dry state and wet state (immersed by ethanol), respectively.

Under the saturated vapor pressure, the maximum resistance change of the film caused by gas absorption (including water, ethanol, acetone and trichloromethane) in our experiment is less than 0.1%. Comparing with semiconductive single-walled CNTs, electrical conductivity of multi-walled CNTs is hardly influenced by molecular absorption. Thus, the effect of molecule adsorption is ignored in this work. As known, the CNT junctions are the electronic transport paths from one CNT to another in the CNT film. Electron tunneling transport in CNT-CNT contact can be characterized by a single contact resistance, which affects the electron flow at the interfaces and further


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influences the volume resistance of the film22-24. The effective resistance of the junction increases with distance between CNTs. Meanwhile, the infiltration of the liquid molecules into the gap of CNT junctions will further weaken the ability of the film to transport electrons. Molecular dynamics (MD) simulation performed by LAMMPS shows microstructure evolution of CNT junctions immerses in liquid. The parameters for MD simulation are listed in Table S1. Figure 4(a) shows that the efficient contact evolution of an individual CNT junction immersed by the ethanol aqueous solution with different mass fraction. The initial distance between center axes of CNTs is defined as 1.5 nm. One end of CNTs is fixed during whole simulation process (the electrodes in the experiment). Another ends forms an efficient contact about 4.1 nm under intermolecular interaction after 20 ps. We measure the efficient contact of the individual CNT junction with separation distance lower than 0.34 nm. The efficient contact decreases gradually and become stable when the junction is surrounded by many ethanol molecules. In the first few picoseconds (about 10 ps), ethanol molecules are adsorbed on the surface of CNTs (S2 in the supporting information). Then, ethanol molecules infiltrates into the gap of the junction under the capillary force. As the process goes on, the efficient contact reduces to nearly zero which indicates that the conductive path is destroyed. Due to random thermal motions, the junction isn’t separated symmetrically. However, the reduction of the efficient contact is caused by the increase of the distance between CNTs. The infiltration of the liquid molecules and subsequent separation of the junctions increase the potential barrier of electron tunneling transport in CNT-CNT contacts, which lead to the increase of the overall resistance of the CNT film. The simulated results verify that the contact resistance can be tuned by liquid infiltration under effect of capillary.

In addition, the infiltration of different liquid will disrupt the electrical contacts of CNT junctions on different levels. Different from the junction separation induced by ethanol, the free ends of CNTs stick together tightly in water. It makes the efficient contact of the junction almost unchanged with time. It’s consistent with the slight


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increase of resistance, when the film is immersed in the water. The concentration of ethanol in solution influences the degree of junction damage (as shown in Figure 4(b)). In the same simulated time, the efficient contact for low concentration (30% mass fraction marked by green triangle) solution is larger than that for high concentration solution (70% mass fraction marked by blue star). A series of results indicate that the CNT junctions will be broken to different degree, when it is immersed by different liquid. It influences the contact resistance of the CNT junctions, further leading to the different change level of the volume resistance.

Figure 5 (a) Thickness distribution of CNT film at dry state and wet state. Schematic diagrams are that the microstructure of CNT junctions transforms from initial state (b) to wet state (c). The black cylinders symbolize the CNTs and the conductive paths are marked by the yellow lines.

Figure 4(b) and (c) shows the MD simulation of a CNT film composed of several CNT junctions at dry state and wet state (immersed by ethanol). The parts in the yellow dash rectangle are considered as the contact of the film and the electrode, which are fixed in whole simulation process. At the dry state, the CNT junctions in the film are in a balanced state maintained by the intermolecular interaction (Figure


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4(b)). When the film is immersed by ethanol, the ethanol molecules distance the CNTs, reduce the contact area of junctions and fill the interspace between CNTs due to the capillarity (Figure 4(b)). These behaviors increase the potential barrier of electron tunneling transport in CNT-CNT contact, and further result in the increase of the film resistance. Figure 5(a) is the thickness distribution of the film at dry state and wet state (ethanol), respectively. All of the data are measured from the optical images of the film section. The mean thickness of the film expands from 16.03±0.15 µm to19.29±1.43 µm after ethanol immersion and shrinks back to 16.23±1.22 µm after ethanol evaporation. This restorable swelling behavior is consistent with the simulation of the film composed of several CNT junctions. In addition, the integrity of the film isn’t destroyed in the simulation. The intermolecular interaction between the CNTs maintains the complete structure of the film, although the gaps increase due to the effect of capillary. Meanwhile, many complicated structures, including entanglement, buckling and local disorder coexist in a film in practice, which have been ignored in this model. These factors make the film integral during the liquid immersion.

The schematics (Figure 5(b) and (c)) reveal the evolution of the electronic transport path in an ideal well-ordered CNT film before and after liquid immersion. At initial state, electrons can easily flow through the interfaces between the CNTs from low potential side to high potential side. (The conductive paths are labeled by the yellow lines.) After the liquid molecules infiltrates into the junctions of the CNTs, the efficient contact of the junctions reduces (like A or B to G in Figure 5(c)). Some junctions even are separated (like D, E, F, I and J in Figure 5(c)). Only part of the junctions is maintained (like C to H in Figure 5(c)). No matter the reduction of the efficient contact or the separation of the CNTs, the structural damage of the junctions can directly decrease the conductivity of the CNT film. It is noted that broadening of gaps and reduction of contact area is frequent in practice, comparing with complete separation of junctions, because the film can maintain its structural integrity during the repeating immersion-evaporation process. The behavior caused by structural


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evolution is consistent with the electric response in the experiment. The expansion of well-ordered film caused by capillarity at the CNT junctions increases the contact resistance; on the contrary, the shrinkage of film with liquid evaporation will decrease the contact resistance by recovering junctions. The natural evaporation of liquid will form the capillary bridges in the CNT gaps18, 25-29. Following the evaporation of liquid, the film gradually shrinks to balanced state. Degree of the shrinkage depends on the competitive relation among the cohesion of the liquid droplets, the interface tension and the load of the CNT cantilever. In this work, the capillary force is not enough to cause plastic deformation of CNTs. Thus, the maximum load corresponding to the maximum shrinkage of the CNT junctions is dependent on the minimum between the cohesion and the interface tension .If the cohesion is larger than the interface tension, the capillary bridge will fracture at the interface; whereas it will crack at the neck of the capillary bridge.

Abnormal decrease of resistance at the end of evaporation (stage III) is not mentioned by previous reports. Based on above discussion, we consider that it originates from the over-shrinkage of the CNT film. In this work, ethanol aqueous solution, acetone and trichloromethane are used as the measured liquid medium. But the abnormal peak appeares during liquid evaporation in all measured liquid except for trichloromethane. Comparing with the time-resolved mass variation curves (S7 in the supporting information), the time of abnormal peak corresponds to the final stage of evaporation (as shown in Figure 3(d)). The previous researches show that the final stage in the ethanol aqueous solution is water evaporation 30-32. It indicates that the temporary over-shrinkage in the film is caused by evaporation of the capillary bridge comprised by water molecular. With increase of ethanol, the peak value of resistance gradually goes down and its appearing moment shifts to an earlier time accordingly. The fluctuation in the liquid with lower-concentration ethanol is triggered by the variation of absorption. Due to the non-wetting property, the CNT film can only absorb little water, which leads to a shorter evaporation time. In addition, although there is hardly any water in the pure ethanol and acetone, they also can absorb water


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molecular from the environment 30. On the contrary, we don’t observe any transient resistance decrease during trichloromethane evaporation due to its high hydrophobicity.

Summary and Conclusions In conclusion, we demonstrates that the mechanism of the electrical conductivity change of the well-ordered CNT film is determined by the structural evolution of the electronic transport network in CNT film, when it is immersed into liquid. The curve-fitting reflects that the resistance change ratio of the film at wet (or dry state) is directly relative to the interface tension between the CNTs and the liquid medium. This behavior is different from the doping effect caused by molecular adsorption in gas sensing. The effect of capillary is a main factor that causes the change of the resistance of the film. The infiltration of the liquid causes the separation of the CNT junctions, which interrupts the transport paths of electrons and expands the film. MD simulation verifies that the CNT junctions are unfolded by the capillarity of liquid in theory. On the contrary, the reformation of the junctions rebuilds the transportation paths during liquid evaporation. The simulated results indicate that the ability of the liquid to break the electronic transport paths is dependent on the interface tension between liquid and CNTs. This discovery opens new avenues for the measurement of liquid characteristics and sensing architectures from multi-functional nanomaterials. The transient reduction of the film resistance at the end of evaporation originates from the over-shrinkage of the film caused by the evaporation of the capillary bridge composed of H2O molecules. It is easily influenced by environmental humidity. Such the ability to sensitive and repeatable resistance change of CNT film with liquid immersion has many potential applications, including manufacture of CNT composites, measurement of liquid properties, liquid sensor and material detection.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.


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The simulated results and the data for MD simulation are listed in the supporting information S2. The original data are listed in S5 and S6. More results and technical details are described in the supporting information.

Author Information Corresponding author Guoan Cheng

E-mail: [email protected]

Notes The authors declare no competing financial interests.

Acknowledgements This work is supported by the Beijing Science and Technology Major Project (No.Z171100002017008), the National Basic Research Program of China (No. 2010CB832905) and the National Nature Science Foundation of China (11575025).

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176x118mm (150 x 150 DPI)


Influence of Capillary Effects on Electric Response of Well-Ordered

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