Carbon Nanotube Fiber Pretreatments for Electrodeposition of Copper

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Introduction
e exceptional properties of carbon nanotubes (CNTs), such as stiffness, low density, high mechanical strength, and excellent electrical and thermal conductivity, have attracted researchers to utilize them in various structures. While individual nanotubes exhibit extraordinary properties, they are difficult to transfer to macroscopic CNT materials due to the existence of unoptimal contact junctions between nanotubes, warranting the research for composite structures. One particular class of such composites is based on carbon nanotubes surrounded by a copper matrix [1][2][3]. Low-density CNT composites with significantly increased conductivity, current-carrying capacity, and mechanical properties could revolutionize many different industry sectors with applications in electrical wiring, microelectronics, and power transfer [4][5][6]. Such composites can be formed by utilizing carbon nanotube wires, such as fibers or films, as the base material [7][8][9][10].
An important issue of producing CNT material on a large scale is that the manufacturing techniques generate many impurities and unwanted CNT types creating an inhomogeneous mixture of components. Undesirable impurities include, for example, soot, amorphous carbon, metal catalysts (such as Fe), carbon nanoparticles, and other unwanted particles. As made, CNT structures are typically improved by procedures that clean the CNT sidewalls of unwanted impurities and add additional reactivity by modification and introduction of new functionalities, such as oxygen-containing functional groups [11,12]. Other methods include densifying the CNT material, either by organic solvents [13] or mechanical application [14], effectively bridging the distance between individual nanotubes to decrease contact resistances. Doping with halogen gases and organic and ionic species have also been explored to alter the carrier density, and thus electrical resistance and activity of CNT material [15]. Modi cation or puri cation of CNT base material is routinely applied before producing composite structures [16][17][18][19].
Previously, Xu et al. [7] reported using anodization as a pretreatment method for producing functionalized CNT-Cu composites. e method combined continuous ber spinning from CNT array, ber anodization, and nally copper deposition. e anodized CNT surfaces were shown to exhibit stronger bonding with the deposited copper compared to unanodized surfaces due to the e ect of functional groups. CNT ber coated with 1 μm copper cladding after anodization resulted in a strength of about 800 MPa, whereas without anodization, the corresponding value was 611 MPa. Additionally, the conductivity of anodized CNT composite was shown to be almost four times higher. However, the e ect of anodization on the electrochemical polarization of the CNT ber was not described.
Electrochemical impedance spectroscopy (EIS) has not been conventionally applied for CNT research. However, it allows for the investigation of CNT material electrochemical properties. Properties, which a ect the electrochemical reactivity, such as electrical resistivity, polarization resistance, and surface area can be compared by EIS investigation. It is important to compare these properties as pretreatment modi cation of CNT ber structures is likely to a ect many properties simultaneously. e aim of this study was to investigate pretreatment methods, which can be used to positively a ect the electrochemical activity of CNT material before electrochemical deposition of copper. Carbon nanotube-copper composites were also produced, and their microstructure and speci c conductivity were described.

CNT Materials.
Di erent lengths (10-350 mm) of CNT bers and yarns were used. Diameter of bers was 10 µm, while yarns were 150-200 µm in diameter. CNT material consists mainly of multiwalled carbon nanotubes (MWCNTs) with a small percentage of double-walled (DWCNTs) and single-walled carbon nanotubes (SWCNTs). Yarn samples consist of a plethora of bers rolled together and treated with acetone to densify and bundle them together to form yarns [13]. e CNT samples were produced by dry spinning directly from a CVD reactor. Pristine CNT materials were unfunctionalized and contained approximately 15% of carbonaceous impurities and catalyst metals [12].
Macroscopic objects made of CNTs have very low weight (a ber sample 2 cm in length weighs less than 10 µg), and so it is a challenging sample material to work with, easily oating or breaking. e tests were conducted by using sample holders, and the schematics are presented in Figure 1. e PVC sample holder (a) was made by gluing a copper wire to the plastic holder, and the glass-plate sample holder (b) was made with conductive Foil Shielding Tape no. 1183 (resistance 0.005 Ω) manufactured by 3 M on the glass plate. e electrical contact to the ber in both sample holders was made with conductive silver paste (42469, Alfa Aesar). Table 1 shows the employed bath compositions. Copper sulfate bath was used to electrodeposit bers with copper. Watts bath is typically used to electrodeposit nickel, but here it was used without applied current or voltage, that is, as an immersion bath for Ni and B doping. Anodizing bath was used to electrooxidize the CNT material by anodization. Boric acid bath contains equal concentration of boric acid compared to Watts bath, in order to observe changes in CNT material by just boric acid application method. All chemicals used were of pro-analysis grade. Solutions were made in distilled water.

Electrochemical Cell Setup.
Electrochemical tests were carried out using ACM Instruments Gill AC potentiostat with Gill AC Sequencer software. A standard three-electrode cell was used ( Figure 2). e reference electrode was a calomel electrode (SCE, Radiometer 401) placed close to the CNT sample using a protective Schott tube with Pt frit (Schott B281). e anode was a copper electrode made of 0.5 mm thick sheet. In anodization, the counter electrode was an inert platinum electrode.

EIS.
EIS tests were carried out in copper sulfate bath before and after di erent treatments in order to observe changes in CNT materials. e behavior of single CNT bers were tested as CNT ber material is inhomogeneous and by using this method the change in properties could be reliably measured. First, the EIS measurement of a single CNT ber was conducted in copper sulfate solution. en, the sample was rinsed with copious amounts of water to remove electrolyte from the surface. e sample was then immersed in the studied bath or heat treated for designated time. Afterwards, the sample was lightly rinsed with deionized water by dipping into deionized water. EIS measurement was then performed again in copper sulfate solution. us, the e ect of di erent treatments on the properties of a single CNT ber in copper sulfate electrolyte could be compared pre-and posttreatments. Electrochemical impedance spectroscopy results were presented as Bode plots. Polarization resistance, solution resistance, and capacitance were calculated with the relative change of results. Results can be compared as EIS measurements do not disturb the sample composition or properties. Solution resistance (R Ω ) was determined at high frequency (100,000 Hz). Polarization resistance (R p ) was determined from the di erence between impedance at high (100,000 Hz) and low frequency (0.01 Hz). Capacitance (C dl ) was calculated using the following equation [20]: where ω max is the angular speed corresponding to phase angle maximum, R p is the polarization resistance, and R Ω is the solution resistance.

CNT Material Pretreatment Methods.
Heat treatment, Watts nickel bath, anodization, and boric acid treatment were investigated as potential CNT ber activation methods. Treatment methods and parameters are listed in Table 2. e heat treatment (treatment no. 1) was carried out in furnace under oxygen ow, samples 2-6 were immersed completely in the selected solutions. In the anodization test (no. 6), CNT ber sample was immersed in 10 wt.% H 2 SO 4 solution and anodized at 2500 mV versus SCE for 50 s.

Copper Deposition.
Carbon nanotube bers were electrodeposited with copper in order to estimate the kinetics of the reaction. e electrical resistance of samples was measured by four-point electrical resistance measurement (Fluke 8846a Multimeter). Copper cladding growth rates were determined as a function of current, sample length, deposition time, and copper cladding length.

E ect of CNT Pretreatment
Methods. CNT materials behave as inhomogeneous resistive substrates rather than metallic substrates and exhibited signi cant terminal e ect. e analysis by Alkire and Varjian [21] and Matlosz et al. [22] shows that the electrochemical copper deposition overpotentials change along electrically resistive wires. Closest to the current feed point, the activation polarization and ohmic resistance polarization are the strongest. As the distance along the wire increases, the e ect of substrate resistance becomes larger causing the activation and electrolyte ohmic polarization to decrease. After a certain distance, the driving force of the deposition, that is, the activation overpotential, becomes too small to support deposition. is e ect is decreased during electrodeposition, when the growing metal on the surface of the substrate provides additional conductivity and thus the deposition can progress further. us, the copper growth on CNT bers progresses further from close to the electrical contact as the deposition is continued. Improving the conductivity and activity of the substrate through pretreatments is expected to increase this deposition speed. e e ect of di erent pretreatment methods on these surface properties were studied using electrochemical impedance spectroscopy (EIS). Table  3 shows the relative changes in the CNT ber polarization resistance, solution resistance, and capacitance based on EIS results after di erent treatments. e following reasoning was used to analyze the results: e polarization resistance is a measure of the CNT surface that supports charge-transfer reaction for copper deposition.
e ohmic resistance describes sample resistivity, but it includes both the sample resistance and the uncompensated solution resistance. e uncompensated resistance was not  Relative resistance change ∆R p < 1 indicates improved CNT surface activity, and relative ohmic resistance ∆R Ω < 1 indicates improved conductivity, whereas values above 1 for ∆R p and ∆R Ω indicate impaired properties. If relative value of C dl was <1, the active area was reduced and if C dl was >1, the active area had increased. e typical double layer capacitance of metals and CNT in electrolyte is about the same, about 10-50 mF/cm 2 , and the capacitance value can be used as a measure of electrochemically active area [23,24]. As the polarization resistance (Ω·cm 2 ) and capacitance (F/cm 2 ) have opposite relation to surface area, the relative changes can be used to estimate if the changes result from surface activation or surface area changes.
From Table 3, it can be observed that the heat treatment strongly a ects both the polarization resistance and solution resistance of the sample (sample 1). is indicates the positive e ect of heat treatment on CNT surface activity and conductivity. is result is attributed to the small amount of functional groups generated during mild heat treatment at low temperatures and the removal of less-conductive amorphous carbon particles. e decrease in capacitance indicates decreased active area, possibly due to removal of large impurity particles. e decrease in both polarization resistance and capacitance indicates that while the total surface area was decreased, the surface was more active due to the decrease in resistance and addition of oxygen-containing functional groups. e purpose of the immersion in the Watts bath was to dope the CNT material surface with Ni and B. e e ect of Watts bath treatment was shown to activate CNT surface signi cantly (samples 2, 3 and 4), similarly to heat treatment.
e Bode plot of EIS before and after Watts bath immersion for ber sample 3 is shown in Figure 3. After 60-minute immersion in Ni-and B-containing bath, the ohmic resistance decreased to 16% of the original value indicating improved conductivity of CNTs as noted previously for CNTs and graphene [25][26][27]. e decrease in polarization resistance indicates activation of the CNT ber surface by providing more electrochemically active sites. e capacitance decreased, indicating a decrease of active surface area. e immersion time also had an e ect; during longer times, the ber was doped more heavily, leading to more pronounced decrease in ohmic resistance and polarization resistance. is result is further con rmed by rinsing the sample with copious amounts of water after immersion in Watts bath (sample 4), which then produced much smaller e ect. is result stems from the removal of activity and conductivity increasing dopants. e improved electrochemical response of CNT bers immersed in Watts bath before electrodeposition is also shown later by galvanostatic copper deposition tests (Figure 7). EIS for sample 5 before and after boric acid immersion is shown in Figure 4. is test was conducted to observe the changes made by B doping only. In our case, the resistivity of CNT ber was only slightly decreased after B doping due to the small immersion time and low concentration of boric acid. However, the impedance and phase angle were reduced and polarization resistance decreased signi cantly after boric acid treatment. Analogous to our work, CNTs doped with boron have shown increased electrochemical activity for oxygen reduction reaction [28]. Based on the pretreatment results by Watts bath and boric acid, it can be concluded that both Ni and B doping showed an improvement in increasing the electrochemical reactivity of CNT material. Doping with both elements simultaneously and for longer times (until 72 h) gave the best results.

Advances in Materials Science and Engineering
Anodization at 2.5 V versus SCE is a highly oxidizing procedure that strongly a ects the surface structure of nanotube material, sample 6. Strong anodization introduced oxygen-containing functional groups at the CNT material surface, which then reduced the charge-transfer resistance of the CNT ber [16]. At the same time, the sample resistance increased as the anodization process was so severe that the mean free path of electrons was decreased due to deterioration of nanotube sidewalls [29]. Finally, the active surface area of the ber increased considerably likely due to the decrease in hydrophobicity by large additions of functional groups.

Electrochemical Deposition for CNT-Cu
Production. CNT-Cu composite samples were produced by electrodeposition on CNT yarns in copper sulfate electrolyte ( Figure 5). Samples were produced up to 30 centimeters in length with a typical diameter of approximately 150 µm. Density of produced composite samples varied from 3.0 to 6.5 g/cm 3 , increasing with the amount of copper deposited. Samples in Figures 5(b) and 5(c) were fractured by bending to observe the composite wire insides. e surface of yarns was covered by a continuous copper cladding, while copper also penetrated inside the CNT material ( Figure 5(c)). e speci c electrical conductivities of produced composite samples are shown in Figure 6. Speci c conductivities normalized by the sample weight were used as the crosssectional area of CNT material and resulting composite are never uniform. e speci c conductivity of pristine CNT yarns is also shown for comparison (data point at 100 wt.% CNT). e speci c conductivity of CNT-Cu composites strongly increases with the amount of copper. e conductivity is increased approximately tenfold when comparing 5 wt.% CNTs to the pure CNT yarn. e results are in line with the previously published values of CNT-Cu wires [7,9,10] e speci c conductivity reported here decreases less as the weight percentage of CNTs increases compared with thinner bers [10].
is result is attributed to the di culty of depositing continuous copper layers inside thick and dense yarn matrices. us, in comparison, more copper is deposited as a continuous cladding on top of the yarn surface leading to an improvement in the electrical conductivity.
Finally, galvanostatic deposition tests were performed on carbon nanotube bers. Due to the previously mentioned terminal e ect causing the localized plating phenomenon on CNT bers, the plating speed could be determined. Deposition speed rate is calculated from optical microscopy inspection of deposit length after deposition test. First, CNT bers were deposited at varying applied current to observe the plating kinetics of nonmodi ed materials. en, the ber samples were pretreated by immersion in Watts bath for 60 minutes before copper deposition. is test was conducted twice to con rm the e ect of Watts bath. e plating rate results are shown in Figure 7. It was observed that the highest coating rates were for samples immersed in Watts bath, con rming the results from EIS measurements. At the same applied potential, the deposition speed was approximately ve times higher for Watts bath-pretreated samples. e deposition speed was higher at over 10 times less applied current, exhibiting the powerful e ect of Ni and B doping provided by the Watts bath immersion. Similar copper deposition enhancement has been shown for oxygenfunctionalized CNT materials elsewhere [16].
In conclusion, we studied improving the electrochemical deposition of copper on CNT bers by various treatments. ese treatments changed the conductivity, polarization resistance, and active surface area of the samples. e copper deposition rate was enhanced considerably by Ni and B doping from Watts bath without the need for harsh treatments, such as oxidation, that are known to deteriorate the nanotube properties [29]. Previously, similar improvements in enhancing the copper deposition rate on CNT macrostructures have been attained by utilizing harsh oxidation as a pretreatment method [7,16]. In this case, we used a "soft" doping method, which had a positive e ect on the CNT material conductivity. e speci c conductivity of thick CNT-Cu yarn composites (90% of Cu at ca. 5 wt.% CNTs) decreased less than previously shown [10] thinner CNT-Cu bers (50% of Cu at ca. 5 wt.% CNTs), likely due to less penetration of copper inside the thick CNT matrix.

Conclusion
Carbon nanotube (CNT) bers were pretreated by various solutions and oxidative treatments. ese treatments were shown to change the electrochemical response of CNT bers for copper deposition. Surface properties of CNT bers were characterized by electrochemical impedance spectroscopy (EIS) to observe changes in polarization resistance, ohmic resistance, and capacitance. e deposition rates of pristine and Watts bath-immersed samples were compared by galvanostatic copper deposition and it was found that the deposition rate increased drastically with pretreatment in Watts Z before immersion Z a er immersion  before immersion  a er immersion bath as predicted by EIS measurements. e specific electrical conductivity of carbon nanotube-copper composite samples with various CNT wt.% were reported. e highest specific conductivity was found to be at 87 wt.% of copper at 4.7 wt.% of CNTs. e results show that the pretreatments can be used to tune the electrochemical response of CNT fibers for copper deposition. Simple immersion in Watts bath before deposition was shown to increase the deposition rate on CNT fibers by approximately fivefold. e study demonstrated that with careful optimization, it is possible to obtain CNT-Cu composites at a high rate. As indicated by the literature, such materials (Cu reinforced with nanocarbon) could show improved electrical, thermal, or mechanical properties as compared with pure copper. Further experiments are under way to determine it.

Conflicts of Interest
e authors declare that they have no conflicts of interest.