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Observations of copper deposition on functionalized carbon nanotube films
Accepted Manuscript Title: Observations of copper deposition on functionalized carbon nanotube films Authors: Pyry-Mikko Hannula, Jari Aromaa, Benjamin P. Wilson, Dawid Janas, Krzysztof Koziol, Olof Fors´en, Mari Lundstr¨om PII: DOI: Reference:
S0013-4686(17)30455-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.03.006 EA 29040
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
16-12-2016 27-2-2017 1-3-2017
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Observations of copper deposition on functionalized carbon nanotube films Manuscript for special issue devoted to conference “Chemistry and Chemical Technology 2016”
Pyry-Mikko Hannula1*, Jari Aromaa1, Benjamin P. Wilson1, Dawid Janas2, Krzysztof Koziol2, Olof Forsén1, Mari Lundström1 1 Aalto University, Department of Materials Science and Engineering, School of Chemical Technology, Vuorimiehentie 2, 02150 Espoo, Finland 2 Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom ABSTRACT This research details the spontaneous electroless and electrodeposition of copper onto carbon nanotube (CNT) film from a copper sulfate electrolyte. Inhomogeneous electrodeposition was found to occur on pristine CNT film due to differences in available active nucleation sites and hydrophobic nature of the film. In order to improve the electrochemical response of CNT film, oxidative pre-treatments such as heat treatment and anodization were investigated. These treatments were shown to increase the amount of oxygen containing defects at the surface of CNT film. Incorporation of functional groups were shown to enhance the wetting of the aqueous electrolyte and created a highly active surface suitable for homogenous electrodeposit. Cathodic polarization curves showed that the presence of functional groups decreased the required level of polarization for copper deposition. Nevertheless, after a certain level of functionalization the polarization starts to decrease due to increased resistivity of the CNT film. Oxygen grafted CNT films were also shown to exhibit enhanced adsorption and reduction of copper without applied voltage due to redox replacement reactions, which was observed to increase with enhanced levels of functionalization.
*
Corresponding author. Tel.+358504627017 e-mail. [email protected]
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Keywords: Carbon nanotube film; electrodeposition; functionalization; copper; surface activity.
1. INTRODUCTION Carbon nanotubes (CNTs) used as reinforcements in composite materials have been the subject of significant research due to their unique mechanical, thermal and electrical properties. Composites formed of carbon nanotubes and metals have shown both enhanced mechanical and electrical properties including improved strength and stiffness [1] and ampacity [2]. There are a multitude of methods related to CNT-metal composite production, such as sintering [3], molten metal infiltration [4], casting, as well as electroless [5], [6], [7] and electrodeposition [8], [9], [10], [11], [12], [13]. Electrodeposition is a commonly used CNT-metal composite production method that has been applied to both CNTs dispersed in the electrolyte [10], [12] and with deposition directly onto CNT substrates [8], [9], [11], [13]. Recently, electrodeposition of copper on planar CNT sheets has been reported with the resulting composite exhibiting increased specific conductivity, current carrying capacity and lifetime at elevated temperatures [13]. It has been noted that deposition of metal occurs preferentially at CNT surface defects [14], [15], leading to inhomogeneous nucleation on CNTs, an effect that can be attributed to the low reactivity of pristine CNT material. A commonly used approach to make CNT material more reactive is to functionalize the surface with reactive groups via methods that include esterification, use of ionic liquids or oxidation [16]. Functionalization also improves the dispersion of CNTs in liquids while grafting groups such as atomic oxygen (-O), hydroxyl (-OH) and carboxyl (-COOH) [10], [17], [18]. Such treatments have a direct effect on the interfacial bonding between CNTs and metals that play an important role in determining the mechanical and electrical properties of the resulting composite [19]. Theoretical calculations have also shown that oxygen containing functional groups promote adsorption, nucleation and electron exchange
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between CNTs and Cu [20], [21]. Nevertheless, the rationale for the use of one oxidation treatment rather than another is often not thoroughly explained and the degree of functionalization achieved is rarely related to the resultant electrochemical activity of the CNT material. Furthermore, although there have been numerous studies on the electrochemical deposition of copper on different carbon materials including carbon fiber [22], pyrolytic graphite [23], glassy carbon [24] and carbon nanotube fibers [9] the extent of the surface functionalization is rarely taken into consideration. As a consequence, the aim of this study was to observe how the reactivity of aligned carbon nanotube films can be adjusted through the use of different oxidation pre-treatments for copper deposition in copper sulfate electrolytes. The degree of functionalization was related to the formation of copper particles at the film surface after spontaneous electroless and electrodeposition of copper. 2. EXPERIMENTAL CNT film consisting of axially oriented single- (SWNT), double- (DWNT) and multi-walled (MWNT) CNTs was drawn continuously from an aerogel onto a spinning winder as has been reported earlier [25]. The film specific surface area is about 200 m2/g and the film consists >80 % of MWNTs. CNT film samples were cut with a surgical blade into rectangle shapes with dimensions of approximately 10 mm x 25 mm. Film samples were then attached with glue (Hybrid Glue, Loctite) to a rigid frame made of PVC and electrical contact to a copper sheet was realized with a silver paste (42469, Alfa Aesar) to ensure that current could be passed to the film (Fig. 1). Two different functionalization treatments were investigated, anodization and heat treatment. Anodization pre-treatments of CNT film prior to electrodeposition were performed under ambient conditions at potentials between 1.0 and 2.4 V vs. saturated calomel electrode (SCE) in 1M H2SO4 for 60 seconds with the setup shown in Fig. 1 using a platinum counter electrode (CE) instead of copper CE. Heat treatments of the CNT films were performed with a Scandia Oven K4/PDI 40 at 400 °C, 450 °C and 500 °C
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in a steady oxygen flow of 150 SCCM for 20-60 minutes prior to the attachment of the film samples to the sample holder. Electrochemical deposition, polarization measurements and anodization of carbon nanotube film were performed with an Autolab 30 potentiostat, equipped with a three-electrode system (working electrode (WE) - CNT film; counter electrode (CE) - copper sheet (platinum for anodization) and reference electrode (RE) - SCE). Electrodeposition was carried out in an electrolyte containing 0.6 M CuSO4*5H2O and 0.9 M H2SO4 in ambient conditions. Characterization of the samples was conducted with scanning electron microscopy (SEM) in secondary electron (SE) and backscattered electron (BSE) modes. SEM micrographs were taken with a Mira3 and LEO 1450 VP and EDX- analysis was performed with a LEO 1450 VP attached with Oxford Instruments INCA analyzer. Analysis of copper weight percentages after immersion tests at the film surface were conducted on thoroughly DI-water rinsed samples from areas of approximately 100 µm x 100 µm, representing the typical surface morphology i.e. where no large copper particles (over 5 µm) could be observed. This was done in order to ensure that the results would be comparable and represented typical film surfaces. Raman spectroscopy was performed with LabRAM HR UV-NIR (red excitation wavelength, λ = 633 nm) to observe changes in nanotube sidewalls i.e. the level of functionalization. Raman spectra were baseline corrected and curve fitted with the Gaussian-Lorentzian function. The reported Raman ID/IG-values are intensity height ratio averages of five measurements across the surface of a film sample. In addition to electrodeposition tests, immersion tests were carried out to eliminate the effect of applied voltage by immersing the film samples in the same electrolyte used for electrodeposition for 48 hours in ambient conditions. After immersion the samples were cleaned by rinsing with DI water for 10 minutes. In this way the difference in adsorption-reduction behavior of functionalized CNT films could be determined.
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3. RESULTS AND DISCUSSION The surface features of the CNT films were studied by SEM. Fig. 2a shows the surface of a pristine i.e. “asspun” CNT film and as can be seen the presence of extraneous carbon material leads to the formation of distinctive micron/submicron scale features of on the CNT film surface. The level of oxidation was measured by Raman Spectroscopy, which has been shown to give accurate results in terms of oxidation degree [26], [27], [28]. The Raman spectra of nanotubes exhibit two characteristic band regions, one centered at around 1350 cm-1 (D-band) and the other at 1590 cm-1 (G-band) and the intensity ratio of these bands can be used to estimate the level of functionalization i.e. the quality of CNT material [26], [27], [28], [29]. The G-band is related to the sp2 graphene structure of the carbon atoms at the nanotube sidewall, while the D-band is caused by the presence of sp3 carbon atoms either at defect sites on the nanotube wall or amorphous carbon. When functionalizing CNT material there are two processes that affect this ratio, the removal of amorphous carbon by oxidation and the grafting of oxygen containing groups onto defective sites. From the ID/IG-ratio of 0.14 it can be stated that the CNT film quality is high with a low number of defects or functional groups at the nanotube surfaces. A mild functionalization of the CNT film by heat treatment (20 minutes at 450 °C) resulted in a noticeable change at the surface; the amorphous carbon particles were largely removed, Fig. 2a. Similarly, the decontamination by removal of amorphous carbon particles from the film surface was evident with all functionalization treatments used and is in line with previous observations in the literature [26], [27], [28]. The change in Raman ID/IG-ratio by various heat treatments and anodization processes is shown in Fig. 3. Heat treatment time was 1 hour and anodization was carried out for 60 seconds at potentials measured vs. SCE. Fig. 3a shows the change in ID/IG-ratios from pristine to highly functionalized CNT film and it can be seen that the level of functionalization achieved was higher with all anodization treatments when compared to that attained with heat treatment. This is further detailed in Fig. 3b, which shows that the
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level of functionalization clearly depends on parameters used. As can be seen from the results, either an elevated heat treatment temperature (in this case, between 400 – 500 °C) or increase anodization potential - from 1.0 to 2.2 V – leads to an increased level of functionalization of nearly three times for heat treatment and eight times for anodization, respectively. The pristine CNT film composition analyzed by EDXA is shown in Table 1. As the detection limit for EDXA is approximately 0.1 wt% the results for Ca and Si should be treated with caution. Trace amounts of sulfur and sodium can be observed. The only impurity in significant amounts is iron, in the form of catalyst particles from the ferrocene. The amount of oxygen is due to adsorbed moisture from ambient conditions and functional groups from production of the CNT film. FTIR spectra of CNT film samples were recorded to observe changes in functional group composition after functionalization. Highly functionalized samples were used to obtain clear absorption bands. In Fig. 4 the FTIR spectra of pristine and anodized samples are shown. After anodization, noticeable bands appear at around 3360 cm-1, 2360 cm-1, 1720 cm-1 and 1150 cm-1. The band at 3360 cm-1 is related to a strong hydroxyl (O-H) stretch, whereas at 2360 cm-1 band can be attributed to the O-H stretch from strongly hydrogen bonded –COOH [30]. Moreover, the band at 1720 cm-1 relates to the carbonyl (C=O) stretching and the absorption in the range of 1000-1300 cm-1 relates to the various C-O bonds, such as those in hydroxyl, ether and phenol groups [31], [32]. The results in Fig. 4 clearly demonstrate the formation of functional groups, mainly carboxyl, at the CNT film surface and are in line with previous results on activated carbon fibers [32]. The spontaneous reduction of metal ions (mostly Ag+, Au3+, Pd2+ and Pt2+) on carbon material has been reported previously by a number of researchers on CNTs [33], [34], [35], carbon fibers [36] and graphene oxide [37], [38], [39] but to the authors knowledge (i) the spontaneous reduction of copper has not been reported on CNTs and (ii) the relative level of functionalization has not been related to the amount of
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material deposited on CNTs. For example, Choi et al. have detailed the spontaneous reduction of metals on SWNTs from baths containing their respective salts, resulting in the reduction of Pt2+ and Au3+ as nanoparticles on the surface of pristine SWNTs [33]. Improvements in adsorption and reduction of copper and silver at the surface of carbon fibers after oxidative treatments, such as heat treatment, treatment in nitric acid and treatment in potassium permanganate solutions have been shown previously [36], [40]. Although the role of defects or functional groups on spontaneous reduction of metals at the CNT surface has not been directly addressed it is known that even high quality SWNT contain one defect per 4 µm length [41], increasing defect density and functionalization leads to a higher possibility of adsorption in simulations [20], [42] and adsorption of Cu2+ on carbon is principally influenced by presence of functional groups [43]. Therefore it was assumed that increasing the amount of adsorption sites before immersion in copper sulfate electrolyte should lead to an increase in the number of observed copper particles. The simplified scheme of the CNT functionalization and spontaneous copper deposition steps is presented in Fig. 5. Samples (surface area of ca. 4 cm2) exhibiting varying levels of functionalization were immersed for 48 hours in the same copper sulfate bath used for electrodeposition. The introduction of functional groups causes an increase in hydrophilicity and negative charge at the CNT surface due to oxygen containing functional groups [34], Fig. 5 (B). These factors promote adsorption of copper ions. Once the CNT film is placed in the electrolyte the copper ions diffuse through the double layer and adsorb at the inner and outer Helmholtz planes aided by functional groups through electrostatic attraction and ion-exchange [34], [43], presented in Fig. 5 (C). Finally, at the film surface the spontaneous reduction of copper ions takes place, Fig. 5 (D). This spontaneous reaction can be explained by two similar mechanisms: the difference in redox potential between CNTs and Cu2+, or the by the difference in redox potential between the impurities at the film surface and Cu2+. The work function of different types of
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pristine and functionalized, MWNT have a large reported range of 4.0 – 5.1 eV [44], [45], [46], [47]. The potential scale versus the vacuum level is calculated from equation (1): Eabs= E (vs. SHE) + 4.44 V
(1)
, where 4.44 V is the absolute potential of SHE at 25 °C [48]. Therefore with respect to a reference electrode the standard potential of MWNTs correspond to about 0.4 – 0.7 V vs. SHE, while the reduction for Cu2+ to copper is 0.34 V vs. SHE, Fig. 6. Therefore MWNTs are able to support a direct redox replacement reaction where electron transfer from CNTs to copper ions causes copper reduction and carbon nanotube oxidation. Similarly, displacement reactions with metals less noble than Cu could also explain some of the observed copper particles through reactions such as (2) and (3): Cu2+ + Fe(s) = Fe2+ + Cu(s) 2Cu2+ + Si = 2Cu + Si4+
(2) (3)
, where EFe2+/Fe= -0.44V vs. SHE and ESi/Si4+= -0.14 V vs. SHE. Such displacement reactions can explain a part of the observed spontaneously reduced copper, but as the amount of Fe is about 1.3 wt% and Si 0.2 wt% the combined amount of copper if reactions (1) and (2) proceeded completely can explain at most approximately 2.2 wt% of the results. In Fig. 7 the amount of copper deposited (wt%) on CNT film surfaces with different levels of functionalization are compared. Copper particles were observed on the oxidized CNT film surface after immersion, while the amount of copper particles on the pristine and mildly oxidized CNT film was minimal (less than 0.5 wt% Cu). The degree of functionalization was determined from the functionalized samples prior to immersion in the fresh electrolyte. The amount of copper found by EDX analysis was averaged from CNT film areas that showed typical homogenous morphology, i.e. no large copper grains or
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continuous copper film were visible. It can be stated that the degree of functionalization strongly affected the amount of deposited copper. As the number of adsorption sites was increased the amount of copper ions available to be reduced at the film surface increased, Fig. 7. The spontaneously deposited copper was found to exhibit large variations in terms of deposit morphology and coverage. The level of deposits varied from small nuclei approximately 200 nm in diameter to dendritic, needlelike and continuous films micrometers in size (Fig. 8). Between large deposits were areas covered by small nuclei, where the EDX analysis were taken. The variation in deposit morphology is attributed to the nature of CNT film, where a multitude of different tube types and impurities are present and as such their ability to reduce copper varies along the film length. Similar results have been obtained by Wang et al. [38] who showed dendritic growth of silver particles on graphene oxide by redox replacement. The electrodeposition of copper can be simplified in the following steps: (1) the adsorption and reduction of copper ions from the electrolyte onto the carbon nanotube film to form adatoms, (2) surface diffusion and coalescence of adatoms on the film surface at active areas, (3) formation of stable copper atom clusters i.e. copper nuclei. The resulting surface morphology of metal deposits is controlled by the formation and movement of adatoms at the cathode surface. The incorporation of adatoms at the cathode surface is strongly affected by the binding energy between the surface and adatoms. At high binding energies the adatoms are relatively stable and easily assemble into new nuclei. If the binding energy is small the adatoms more easily diffuse along the surface to become incorporated into existing nuclei. The electrodeposition process on CNT film will be affected in all these steps due to the interaction between copper and the functional groups. The incorporation of any oxygen containing functional group serves to bind adsorbed copper atoms more strongly on the film surface as predicted by theoretical
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studies (e.g pristine CNT Eb= -0.53 eV vs. O-functionalized Eb = -0.73, OH-functionalized Eb= -1.35 and COOH-functionalized CNT Eb = -1.37 eV) while promoting electron exchange [20]. The diffusion of metal adatoms is proportional to exp(-Ea/kT), where Ea is the activation energy for diffusion [49]. The diffusion activation energy is directly dependent on the binding energy and therefore stronger binding of copper atoms by functional groups leads to decreased diffusion at the carbon nanotube film surface [50]. The incorporation of functional groups on the CNT film surface can be therefore assumed to have a similar effect as the introduction of step edges or kinks in a crystalline structure where copper atoms will be trapped providing more growth sites. The rate of nucleation is derived from the probability of formation of a critical nucleus by (4): ∆𝐺𝑐 𝐽 = 𝐾 ∙ exp( ) 𝑘𝑏 𝑇
(4)
, where K is a constant that takes into account the number of adsorption sites and the rate of attachment of atoms, Gc is critical free energy, kb is the Boltzmann constant and T absolute temperature [51]. An increase in nucleation rate can therefore be expected from increasing the amount of adsorption sites by functionalization. The nucleation law for uniform probability with time t of conversion of an active site on an electrode into nuclei is derived from equation (5): 𝑁 = 𝑁0 (1 − exp(−𝐴𝑡))
(5)
, where t is the time since the potential was applied, N is the number of nuclei, N0 is the number of active sites on the electrode surface and A nucleation rate [52]. The amount of observable nuclei is closely related to the amount of available active sites, which can be increased by functionalization. It is expected that the nucleation rate, A, is affected by functionalization by increasing the exchange current density, shown later.
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Galvanostatic deposition of copper was performed on pristine and strongly anodized (60 s at 2.2 V vs. SCE) films with 3.8 mA/cm2 cathodic current for 1800 s. The small current density was selected due to expected favoring of low nucleation rate and nuclei growth [52]. Firstly, the pristine CNT film exhibits strong hydrophobicity, which is demonstrated with a drop of electrolyte showing a contact angle of c.a. 120° (see insert in Fig. 9 (a)). After galvanostatic deposition, the pristine CNT film showed relatively large bare areas where no copper can be observed. Furthermore, no penetration of copper grains into the film structure can be observed and the grain size distribution was non-uniform, with an average grain size of 10.1 µm s.d 4.1 µm. The nucleation follows TDC island growth, i.e. Volmer-Weber model, where discrete islands grow from nucleation sites and with time these nuclei coalescence together to form continuous film [52]. After functionalization a drop of electrolyte was shown to wet the film surface completely and no contact angle could be distinguished (see insert in Fig. 9 (e)). The film morphology had also changed and seemed more “wrinkly” in comparison to the flat pristine CNT film. This can be attributed to the oxygen gas bubbles forming at the CNT film anode and influencing the surface morphology. Nevertheless, it can be seen that the nuclei density was highly affected by the functionalization. All of the film surface after anodization (60 s at 2.2 V vs. SCE) and galvanostatic deposition (3.8 mA/cm 2 cathodic current for 1800 s) was evenly covered in copper nuclei that were penetrating into the CNT bundles, attributed to the observed hydrophilicity. Average grain size was 6.5 µm s.d. 2.1 µm, indicating a higher and more even nucleation rate compared to non-functionalized surface. The nucleation mode was unchanged and followed 3D island growth, but the nuclei population density was markedly enhanced in contrast to the pristine CNT film. The increase in copper nuclei population density and decrease in nuclei size in functionalized CNT films relates to the nucleation step in the deposition process, where diffusing copper atoms become trapped by the oxygen containing functional groups, becoming nucleation centers for other diffusing copper
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atoms. In the pristine CNT film, the opposite can be observed as the lack of homogenous nuclei density suggests that surface diffusion is fast and the copper atoms have enough time to arrive at the few scarce areas with enough activity to be able to become nucleation centers. To further investigate the electrochemical affinity of copper towards CNT film, a novel approach was used: a CNT film sample was anodized by immersing two thirds of the film length in the anodizing solution. This was done in order to ensure that the difference seen in Fig. 9 was not due to the differences between individual pristine film samples, such as varying surface area and texture or nanotube quality, but purely due to the effect of enhanced levels of functional groups. Therefore, only the bottom part of the film was anodized and the upper part remained in pristine state above the anodization solution. Anodization was performed at 2.0 V vs. SCE for 60 s. Copper was then deposited over the whole film surface with a cathodic current of 10 mA for 300 s and the resulting morphology was again imaged. A vast difference between the anodized and pristine CNT sections can again be observed in Fig. 10. Samples were not subject to harsh cleaning, i.e. to ultrasonication so as not to change the surface morphology of CNT film, but were rinsed thoroughly with DI water. The pristine film shows sparse nuclei located in small groups, while the anodized film is evenly covered with a high density of nuclei. The observed difference in nuclei density between the anodized and pristine CNT film is of at least one order of magnitude. Previous studies of metal deposition on functionalized CNT material has been mostly done with CNT material with one functionalization treatment with no comparison between the strength of functionalization and effect on the deposition process [7], [10], [11], [14], [15]. The relationship between functionalization and electrochemical activity is an important issue because it is known that functionalization affects the conductivity of the CNT material unfavorably. Therefore while the increase in functional group concentration increases the electrochemical activity, at the same time the CNT
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sidewalls are deteriorated and the mean free path of electrons travelling across the CNT material is reduced leading to increased resistance and loss of driving force for electrochemical reactions. Cathodic polarization curves were recorded for a pristine and functionalized sample. The tests were carried out on a single sample in order to eliminate the influence of any possible differences between samples. In this test, the first cathodic polarization curve was recorded for a pristine sample, then the sample was anodized beginning at 1.2V vs. SCE for 60 seconds, and a new cathodic polarization curve was recorded. This cycle was repeated after each 0.2 V increase in potential up to 2.2 V vs. SCE. During the anodization step the previously deposited copper was completely removed and the film was increasingly functionalized. Table 2 shows the observed exchange current densities for CNT films with different levels of functionalization. The observed increase in exchange current density is attributed to the increase of electrochemically active area by incorporation of functional groups. The cathodic polarization curves of these tests are presented in Fig. 11 and as can be seen the cathodic polarization curves clearly shows that enhanced surface functionalization provides improved electrochemical activity of CNT films up to anodization voltage 1.8 V vs. SCE. Up to this point after each anodization step the film surface activity is seen to increase, as evidenced by the improved deposition rate at a given potential, which results from the enhanced adsorption of copper on the functional groups at the film surface. The reaction rate was shown to decrease after anodization at 2.0 V vs. SCE. This phenomenon is related to the deterioration of the nanotube sidewalls with increased levels of functionalization, that results in increased resistivity of the carbon nanotube film as the mean free path of electrons is reduced: the more a film is functionalized the greater the observed decrease in conductivity [53]. The strong increase in resistivity results in increasing polarization even while the amount of functional groups has increased. This contrasts with the result of increasing adsorbed copper by level of functionalization, which is conducted without external current. Therefore while the number of adsorption
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sites increases with increasing functionalization, the electrodeposition process is only enhanced up to a limit of about 0.75 in terms of ID/IG ratio, or an increase of about 570 % from original ID/IG ratio. 4. CONCLUSIONS In this work the electrochemical activity and reduction capacity of carbon nanotube (CNT) film was increased by functionalization using heat treatment and anodization. It was shown that increasing the degree of functionalization increases the amount of spontaneously nucleated copper particles at the film surface due to increase in sorption sites. Spontaneous deposition of copper was attributed to displacement reaction of impurities at the film surface and difference in redox potential between multiwall nanotubes and copper. Anodizing was found to be a more effective functionalizing treatment when compared to heat treatment in an oxygen-containing atmosphere over the investigated parameter range. The highly anodized CNT films were shown to be hydrophilic while the pristine CNT film is strongly hydrophobic. A functionalized CNT film after electrodeposition showed an even nucleation of copper at the film surface with copper deposits penetrating into the nanotube bundles, whilst in contrast pristine CNT films exhibit inhomogeneous and sparse copper deposits. The introduction of functional groups increases the exchange current density of the system. The improved activity of anodized CNT films was evident by the cathodic polarization curves, which showed increase in activity up to an anodizing voltage of 1.8 V vs. SCE. During anodization at 2.0 V vs. SCE, the film begins to deteriorate to such a degree that the activity decreased even with the increased degree of functionalization due to the increase in electrical resistance of CNT film. Nevertheless, the activity was still considerably higher when compared to that of a non-functionalized pristine film. Overall, the results show that functionalization, both via heat treatment and anodization, is an effective pre-treatment to activate CNT films before electrodeposition of copper as
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functionalization results in an (i) increase in hydrophilicity through the formation of functional groups and (ii) increase in the nucleation rate of copper deposits. ACKNOWLEDGEMENTS This work has been supported by the FP7 European project Ultrawire (Grant Agreement No. 609057). RawMatTERS Finland Infrastructure (RAMI) supported by Academy of Finland is greatly acknowledged. REFERENCES [1] S. Bakshi, D. Lahiri, A. Agarwal, Carbon nanotube reinforced metal matrix composites-a review, International Materials Reviews 55 (2010) 41-64.
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Fig. 1. Schematic drawing of the experimental setup consisting of (1) CNT film, (2) silver paste electrical contact, (3) glue, (4) CE-Copper sheet anode, (5) Copper conductor sheet, and (6) PVC sample holder.
Fig. 2. SEM micrograph of (a) pristine CNT film and (b) CNT film oxidized at 450 °C in air for 20 minutes.
22
23
Fig. 3. (a) Raman spectra of pristine, heat treated (500 °C, 1 hour) and anodized (2.2V vs. SCE, 60s) CNT films. (b) Raman ID/IG- ratio after (i) no pre-treatment, (ii) heat treatments (400 – 500 °C, 1 hour) and (iii) anodization treatments (at 1.0 – 2.2 V vs. SCE, t = 60 s).
24
Fig. 4. FTIR spectra of pristine and functionalized CNT films.
25
Fig. 5. Schematic for CNT functionalization and spontaneous reduction of copper: A) untreated pristine CNT, B) CNT after functionalization, C) sorption of copper ions to the Helmholtz layers and D) spontaneous reduction of copper due by CNT and/or impurities.
Fig. 6. Standard electrode potentials and equilibrium potentials for CNTs and selected metals.
Fig. 7. The amount of copper deposits at the film surface after immersion tests increases with the level of functionalization. The error bars represent one standard deviation.
26
27
28
Fig. 8. CNT film samples showing different morphologies after immersion in copper sulfate electrolyte for 48 hours (a) non-functionalized film (BSE) (b) functionalized film with sub-micron copper particles (BSE), inset (SE) (c) functionalized film showing micrometer sized particles (BSE) (d) functionalized film showing dendrites (BSE) and (e) functionalized film showing continuous film (BSE).
29
30
Fig. 9. SEM micrographs of CNT film after electrochemical deposition. (a) Pristine CNT film showing bare areas devoid of any copper nuclei, insert: contact angle measurement of electrolyte on pristine film surface, (b) morphology of pristine CNT film, (c) copper nuclei on top of pristine CNT film, (d) anodized CNT film (2.2V vs. SCE 60s) morphology, insert: wetting of electrolyte on anodized film surface, and (e) copper nuclei penetrating into anodized CNT structure.
31
Fig. 10. Electrochemical deposition at the interface of anodized/pristine CNT film: (a) upper part: pristine, lower part: anodized, (b) nucleation on pristine CNT film and (c-d) nucleation on oxidized CNT film.
32
Fig. 11. Cathodic polarization of CNT film in copper sulfate electrolyte after anodization at various voltages (1.2- 2.2 V vs. SCE for 60s) in 1M sulfuric acid.
Table 1. Chemical composition of CNT film determined by EDX- analysis. wt%
C
O
Fe
S
Si
Na
Ca
CNT film
88.48
9.20
1.27
0.40
0.16
0.34
0.16
33
S0013-4686(17)30455-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.03.006 EA 29040
To appear in:
Electrochimica Acta
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16-12-2016 27-2-2017 1-3-2017
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Observations of copper deposition on functionalized carbon nanotube films Manuscript for special issue devoted to conference “Chemistry and Chemical Technology 2016”
Pyry-Mikko Hannula1*, Jari Aromaa1, Benjamin P. Wilson1, Dawid Janas2, Krzysztof Koziol2, Olof Forsén1, Mari Lundström1 1 Aalto University, Department of Materials Science and Engineering, School of Chemical Technology, Vuorimiehentie 2, 02150 Espoo, Finland 2 Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom ABSTRACT This research details the spontaneous electroless and electrodeposition of copper onto carbon nanotube (CNT) film from a copper sulfate electrolyte. Inhomogeneous electrodeposition was found to occur on pristine CNT film due to differences in available active nucleation sites and hydrophobic nature of the film. In order to improve the electrochemical response of CNT film, oxidative pre-treatments such as heat treatment and anodization were investigated. These treatments were shown to increase the amount of oxygen containing defects at the surface of CNT film. Incorporation of functional groups were shown to enhance the wetting of the aqueous electrolyte and created a highly active surface suitable for homogenous electrodeposit. Cathodic polarization curves showed that the presence of functional groups decreased the required level of polarization for copper deposition. Nevertheless, after a certain level of functionalization the polarization starts to decrease due to increased resistivity of the CNT film. Oxygen grafted CNT films were also shown to exhibit enhanced adsorption and reduction of copper without applied voltage due to redox replacement reactions, which was observed to increase with enhanced levels of functionalization.
*
Corresponding author. Tel.+358504627017 e-mail. [email protected]
1
Keywords: Carbon nanotube film; electrodeposition; functionalization; copper; surface activity.
1. INTRODUCTION Carbon nanotubes (CNTs) used as reinforcements in composite materials have been the subject of significant research due to their unique mechanical, thermal and electrical properties. Composites formed of carbon nanotubes and metals have shown both enhanced mechanical and electrical properties including improved strength and stiffness [1] and ampacity [2]. There are a multitude of methods related to CNT-metal composite production, such as sintering [3], molten metal infiltration [4], casting, as well as electroless [5], [6], [7] and electrodeposition [8], [9], [10], [11], [12], [13]. Electrodeposition is a commonly used CNT-metal composite production method that has been applied to both CNTs dispersed in the electrolyte [10], [12] and with deposition directly onto CNT substrates [8], [9], [11], [13]. Recently, electrodeposition of copper on planar CNT sheets has been reported with the resulting composite exhibiting increased specific conductivity, current carrying capacity and lifetime at elevated temperatures [13]. It has been noted that deposition of metal occurs preferentially at CNT surface defects [14], [15], leading to inhomogeneous nucleation on CNTs, an effect that can be attributed to the low reactivity of pristine CNT material. A commonly used approach to make CNT material more reactive is to functionalize the surface with reactive groups via methods that include esterification, use of ionic liquids or oxidation [16]. Functionalization also improves the dispersion of CNTs in liquids while grafting groups such as atomic oxygen (-O), hydroxyl (-OH) and carboxyl (-COOH) [10], [17], [18]. Such treatments have a direct effect on the interfacial bonding between CNTs and metals that play an important role in determining the mechanical and electrical properties of the resulting composite [19]. Theoretical calculations have also shown that oxygen containing functional groups promote adsorption, nucleation and electron exchange
2
between CNTs and Cu [20], [21]. Nevertheless, the rationale for the use of one oxidation treatment rather than another is often not thoroughly explained and the degree of functionalization achieved is rarely related to the resultant electrochemical activity of the CNT material. Furthermore, although there have been numerous studies on the electrochemical deposition of copper on different carbon materials including carbon fiber [22], pyrolytic graphite [23], glassy carbon [24] and carbon nanotube fibers [9] the extent of the surface functionalization is rarely taken into consideration. As a consequence, the aim of this study was to observe how the reactivity of aligned carbon nanotube films can be adjusted through the use of different oxidation pre-treatments for copper deposition in copper sulfate electrolytes. The degree of functionalization was related to the formation of copper particles at the film surface after spontaneous electroless and electrodeposition of copper. 2. EXPERIMENTAL CNT film consisting of axially oriented single- (SWNT), double- (DWNT) and multi-walled (MWNT) CNTs was drawn continuously from an aerogel onto a spinning winder as has been reported earlier [25]. The film specific surface area is about 200 m2/g and the film consists >80 % of MWNTs. CNT film samples were cut with a surgical blade into rectangle shapes with dimensions of approximately 10 mm x 25 mm. Film samples were then attached with glue (Hybrid Glue, Loctite) to a rigid frame made of PVC and electrical contact to a copper sheet was realized with a silver paste (42469, Alfa Aesar) to ensure that current could be passed to the film (Fig. 1). Two different functionalization treatments were investigated, anodization and heat treatment. Anodization pre-treatments of CNT film prior to electrodeposition were performed under ambient conditions at potentials between 1.0 and 2.4 V vs. saturated calomel electrode (SCE) in 1M H2SO4 for 60 seconds with the setup shown in Fig. 1 using a platinum counter electrode (CE) instead of copper CE. Heat treatments of the CNT films were performed with a Scandia Oven K4/PDI 40 at 400 °C, 450 °C and 500 °C
3
in a steady oxygen flow of 150 SCCM for 20-60 minutes prior to the attachment of the film samples to the sample holder. Electrochemical deposition, polarization measurements and anodization of carbon nanotube film were performed with an Autolab 30 potentiostat, equipped with a three-electrode system (working electrode (WE) - CNT film; counter electrode (CE) - copper sheet (platinum for anodization) and reference electrode (RE) - SCE). Electrodeposition was carried out in an electrolyte containing 0.6 M CuSO4*5H2O and 0.9 M H2SO4 in ambient conditions. Characterization of the samples was conducted with scanning electron microscopy (SEM) in secondary electron (SE) and backscattered electron (BSE) modes. SEM micrographs were taken with a Mira3 and LEO 1450 VP and EDX- analysis was performed with a LEO 1450 VP attached with Oxford Instruments INCA analyzer. Analysis of copper weight percentages after immersion tests at the film surface were conducted on thoroughly DI-water rinsed samples from areas of approximately 100 µm x 100 µm, representing the typical surface morphology i.e. where no large copper particles (over 5 µm) could be observed. This was done in order to ensure that the results would be comparable and represented typical film surfaces. Raman spectroscopy was performed with LabRAM HR UV-NIR (red excitation wavelength, λ = 633 nm) to observe changes in nanotube sidewalls i.e. the level of functionalization. Raman spectra were baseline corrected and curve fitted with the Gaussian-Lorentzian function. The reported Raman ID/IG-values are intensity height ratio averages of five measurements across the surface of a film sample. In addition to electrodeposition tests, immersion tests were carried out to eliminate the effect of applied voltage by immersing the film samples in the same electrolyte used for electrodeposition for 48 hours in ambient conditions. After immersion the samples were cleaned by rinsing with DI water for 10 minutes. In this way the difference in adsorption-reduction behavior of functionalized CNT films could be determined.
4
3. RESULTS AND DISCUSSION The surface features of the CNT films were studied by SEM. Fig. 2a shows the surface of a pristine i.e. “asspun” CNT film and as can be seen the presence of extraneous carbon material leads to the formation of distinctive micron/submicron scale features of on the CNT film surface. The level of oxidation was measured by Raman Spectroscopy, which has been shown to give accurate results in terms of oxidation degree [26], [27], [28]. The Raman spectra of nanotubes exhibit two characteristic band regions, one centered at around 1350 cm-1 (D-band) and the other at 1590 cm-1 (G-band) and the intensity ratio of these bands can be used to estimate the level of functionalization i.e. the quality of CNT material [26], [27], [28], [29]. The G-band is related to the sp2 graphene structure of the carbon atoms at the nanotube sidewall, while the D-band is caused by the presence of sp3 carbon atoms either at defect sites on the nanotube wall or amorphous carbon. When functionalizing CNT material there are two processes that affect this ratio, the removal of amorphous carbon by oxidation and the grafting of oxygen containing groups onto defective sites. From the ID/IG-ratio of 0.14 it can be stated that the CNT film quality is high with a low number of defects or functional groups at the nanotube surfaces. A mild functionalization of the CNT film by heat treatment (20 minutes at 450 °C) resulted in a noticeable change at the surface; the amorphous carbon particles were largely removed, Fig. 2a. Similarly, the decontamination by removal of amorphous carbon particles from the film surface was evident with all functionalization treatments used and is in line with previous observations in the literature [26], [27], [28]. The change in Raman ID/IG-ratio by various heat treatments and anodization processes is shown in Fig. 3. Heat treatment time was 1 hour and anodization was carried out for 60 seconds at potentials measured vs. SCE. Fig. 3a shows the change in ID/IG-ratios from pristine to highly functionalized CNT film and it can be seen that the level of functionalization achieved was higher with all anodization treatments when compared to that attained with heat treatment. This is further detailed in Fig. 3b, which shows that the
5
level of functionalization clearly depends on parameters used. As can be seen from the results, either an elevated heat treatment temperature (in this case, between 400 – 500 °C) or increase anodization potential - from 1.0 to 2.2 V – leads to an increased level of functionalization of nearly three times for heat treatment and eight times for anodization, respectively. The pristine CNT film composition analyzed by EDXA is shown in Table 1. As the detection limit for EDXA is approximately 0.1 wt% the results for Ca and Si should be treated with caution. Trace amounts of sulfur and sodium can be observed. The only impurity in significant amounts is iron, in the form of catalyst particles from the ferrocene. The amount of oxygen is due to adsorbed moisture from ambient conditions and functional groups from production of the CNT film. FTIR spectra of CNT film samples were recorded to observe changes in functional group composition after functionalization. Highly functionalized samples were used to obtain clear absorption bands. In Fig. 4 the FTIR spectra of pristine and anodized samples are shown. After anodization, noticeable bands appear at around 3360 cm-1, 2360 cm-1, 1720 cm-1 and 1150 cm-1. The band at 3360 cm-1 is related to a strong hydroxyl (O-H) stretch, whereas at 2360 cm-1 band can be attributed to the O-H stretch from strongly hydrogen bonded –COOH [30]. Moreover, the band at 1720 cm-1 relates to the carbonyl (C=O) stretching and the absorption in the range of 1000-1300 cm-1 relates to the various C-O bonds, such as those in hydroxyl, ether and phenol groups [31], [32]. The results in Fig. 4 clearly demonstrate the formation of functional groups, mainly carboxyl, at the CNT film surface and are in line with previous results on activated carbon fibers [32]. The spontaneous reduction of metal ions (mostly Ag+, Au3+, Pd2+ and Pt2+) on carbon material has been reported previously by a number of researchers on CNTs [33], [34], [35], carbon fibers [36] and graphene oxide [37], [38], [39] but to the authors knowledge (i) the spontaneous reduction of copper has not been reported on CNTs and (ii) the relative level of functionalization has not been related to the amount of
6
material deposited on CNTs. For example, Choi et al. have detailed the spontaneous reduction of metals on SWNTs from baths containing their respective salts, resulting in the reduction of Pt2+ and Au3+ as nanoparticles on the surface of pristine SWNTs [33]. Improvements in adsorption and reduction of copper and silver at the surface of carbon fibers after oxidative treatments, such as heat treatment, treatment in nitric acid and treatment in potassium permanganate solutions have been shown previously [36], [40]. Although the role of defects or functional groups on spontaneous reduction of metals at the CNT surface has not been directly addressed it is known that even high quality SWNT contain one defect per 4 µm length [41], increasing defect density and functionalization leads to a higher possibility of adsorption in simulations [20], [42] and adsorption of Cu2+ on carbon is principally influenced by presence of functional groups [43]. Therefore it was assumed that increasing the amount of adsorption sites before immersion in copper sulfate electrolyte should lead to an increase in the number of observed copper particles. The simplified scheme of the CNT functionalization and spontaneous copper deposition steps is presented in Fig. 5. Samples (surface area of ca. 4 cm2) exhibiting varying levels of functionalization were immersed for 48 hours in the same copper sulfate bath used for electrodeposition. The introduction of functional groups causes an increase in hydrophilicity and negative charge at the CNT surface due to oxygen containing functional groups [34], Fig. 5 (B). These factors promote adsorption of copper ions. Once the CNT film is placed in the electrolyte the copper ions diffuse through the double layer and adsorb at the inner and outer Helmholtz planes aided by functional groups through electrostatic attraction and ion-exchange [34], [43], presented in Fig. 5 (C). Finally, at the film surface the spontaneous reduction of copper ions takes place, Fig. 5 (D). This spontaneous reaction can be explained by two similar mechanisms: the difference in redox potential between CNTs and Cu2+, or the by the difference in redox potential between the impurities at the film surface and Cu2+. The work function of different types of
7
pristine and functionalized, MWNT have a large reported range of 4.0 – 5.1 eV [44], [45], [46], [47]. The potential scale versus the vacuum level is calculated from equation (1): Eabs= E (vs. SHE) + 4.44 V
(1)
, where 4.44 V is the absolute potential of SHE at 25 °C [48]. Therefore with respect to a reference electrode the standard potential of MWNTs correspond to about 0.4 – 0.7 V vs. SHE, while the reduction for Cu2+ to copper is 0.34 V vs. SHE, Fig. 6. Therefore MWNTs are able to support a direct redox replacement reaction where electron transfer from CNTs to copper ions causes copper reduction and carbon nanotube oxidation. Similarly, displacement reactions with metals less noble than Cu could also explain some of the observed copper particles through reactions such as (2) and (3): Cu2+ + Fe(s) = Fe2+ + Cu(s) 2Cu2+ + Si = 2Cu + Si4+
(2) (3)
, where EFe2+/Fe= -0.44V vs. SHE and ESi/Si4+= -0.14 V vs. SHE. Such displacement reactions can explain a part of the observed spontaneously reduced copper, but as the amount of Fe is about 1.3 wt% and Si 0.2 wt% the combined amount of copper if reactions (1) and (2) proceeded completely can explain at most approximately 2.2 wt% of the results. In Fig. 7 the amount of copper deposited (wt%) on CNT film surfaces with different levels of functionalization are compared. Copper particles were observed on the oxidized CNT film surface after immersion, while the amount of copper particles on the pristine and mildly oxidized CNT film was minimal (less than 0.5 wt% Cu). The degree of functionalization was determined from the functionalized samples prior to immersion in the fresh electrolyte. The amount of copper found by EDX analysis was averaged from CNT film areas that showed typical homogenous morphology, i.e. no large copper grains or
8
continuous copper film were visible. It can be stated that the degree of functionalization strongly affected the amount of deposited copper. As the number of adsorption sites was increased the amount of copper ions available to be reduced at the film surface increased, Fig. 7. The spontaneously deposited copper was found to exhibit large variations in terms of deposit morphology and coverage. The level of deposits varied from small nuclei approximately 200 nm in diameter to dendritic, needlelike and continuous films micrometers in size (Fig. 8). Between large deposits were areas covered by small nuclei, where the EDX analysis were taken. The variation in deposit morphology is attributed to the nature of CNT film, where a multitude of different tube types and impurities are present and as such their ability to reduce copper varies along the film length. Similar results have been obtained by Wang et al. [38] who showed dendritic growth of silver particles on graphene oxide by redox replacement. The electrodeposition of copper can be simplified in the following steps: (1) the adsorption and reduction of copper ions from the electrolyte onto the carbon nanotube film to form adatoms, (2) surface diffusion and coalescence of adatoms on the film surface at active areas, (3) formation of stable copper atom clusters i.e. copper nuclei. The resulting surface morphology of metal deposits is controlled by the formation and movement of adatoms at the cathode surface. The incorporation of adatoms at the cathode surface is strongly affected by the binding energy between the surface and adatoms. At high binding energies the adatoms are relatively stable and easily assemble into new nuclei. If the binding energy is small the adatoms more easily diffuse along the surface to become incorporated into existing nuclei. The electrodeposition process on CNT film will be affected in all these steps due to the interaction between copper and the functional groups. The incorporation of any oxygen containing functional group serves to bind adsorbed copper atoms more strongly on the film surface as predicted by theoretical
9
studies (e.g pristine CNT Eb= -0.53 eV vs. O-functionalized Eb = -0.73, OH-functionalized Eb= -1.35 and COOH-functionalized CNT Eb = -1.37 eV) while promoting electron exchange [20]. The diffusion of metal adatoms is proportional to exp(-Ea/kT), where Ea is the activation energy for diffusion [49]. The diffusion activation energy is directly dependent on the binding energy and therefore stronger binding of copper atoms by functional groups leads to decreased diffusion at the carbon nanotube film surface [50]. The incorporation of functional groups on the CNT film surface can be therefore assumed to have a similar effect as the introduction of step edges or kinks in a crystalline structure where copper atoms will be trapped providing more growth sites. The rate of nucleation is derived from the probability of formation of a critical nucleus by (4): ∆𝐺𝑐 𝐽 = 𝐾 ∙ exp( ) 𝑘𝑏 𝑇
(4)
, where K is a constant that takes into account the number of adsorption sites and the rate of attachment of atoms, Gc is critical free energy, kb is the Boltzmann constant and T absolute temperature [51]. An increase in nucleation rate can therefore be expected from increasing the amount of adsorption sites by functionalization. The nucleation law for uniform probability with time t of conversion of an active site on an electrode into nuclei is derived from equation (5): 𝑁 = 𝑁0 (1 − exp(−𝐴𝑡))
(5)
, where t is the time since the potential was applied, N is the number of nuclei, N0 is the number of active sites on the electrode surface and A nucleation rate [52]. The amount of observable nuclei is closely related to the amount of available active sites, which can be increased by functionalization. It is expected that the nucleation rate, A, is affected by functionalization by increasing the exchange current density, shown later.
10
Galvanostatic deposition of copper was performed on pristine and strongly anodized (60 s at 2.2 V vs. SCE) films with 3.8 mA/cm2 cathodic current for 1800 s. The small current density was selected due to expected favoring of low nucleation rate and nuclei growth [52]. Firstly, the pristine CNT film exhibits strong hydrophobicity, which is demonstrated with a drop of electrolyte showing a contact angle of c.a. 120° (see insert in Fig. 9 (a)). After galvanostatic deposition, the pristine CNT film showed relatively large bare areas where no copper can be observed. Furthermore, no penetration of copper grains into the film structure can be observed and the grain size distribution was non-uniform, with an average grain size of 10.1 µm s.d 4.1 µm. The nucleation follows TDC island growth, i.e. Volmer-Weber model, where discrete islands grow from nucleation sites and with time these nuclei coalescence together to form continuous film [52]. After functionalization a drop of electrolyte was shown to wet the film surface completely and no contact angle could be distinguished (see insert in Fig. 9 (e)). The film morphology had also changed and seemed more “wrinkly” in comparison to the flat pristine CNT film. This can be attributed to the oxygen gas bubbles forming at the CNT film anode and influencing the surface morphology. Nevertheless, it can be seen that the nuclei density was highly affected by the functionalization. All of the film surface after anodization (60 s at 2.2 V vs. SCE) and galvanostatic deposition (3.8 mA/cm 2 cathodic current for 1800 s) was evenly covered in copper nuclei that were penetrating into the CNT bundles, attributed to the observed hydrophilicity. Average grain size was 6.5 µm s.d. 2.1 µm, indicating a higher and more even nucleation rate compared to non-functionalized surface. The nucleation mode was unchanged and followed 3D island growth, but the nuclei population density was markedly enhanced in contrast to the pristine CNT film. The increase in copper nuclei population density and decrease in nuclei size in functionalized CNT films relates to the nucleation step in the deposition process, where diffusing copper atoms become trapped by the oxygen containing functional groups, becoming nucleation centers for other diffusing copper
11
atoms. In the pristine CNT film, the opposite can be observed as the lack of homogenous nuclei density suggests that surface diffusion is fast and the copper atoms have enough time to arrive at the few scarce areas with enough activity to be able to become nucleation centers. To further investigate the electrochemical affinity of copper towards CNT film, a novel approach was used: a CNT film sample was anodized by immersing two thirds of the film length in the anodizing solution. This was done in order to ensure that the difference seen in Fig. 9 was not due to the differences between individual pristine film samples, such as varying surface area and texture or nanotube quality, but purely due to the effect of enhanced levels of functional groups. Therefore, only the bottom part of the film was anodized and the upper part remained in pristine state above the anodization solution. Anodization was performed at 2.0 V vs. SCE for 60 s. Copper was then deposited over the whole film surface with a cathodic current of 10 mA for 300 s and the resulting morphology was again imaged. A vast difference between the anodized and pristine CNT sections can again be observed in Fig. 10. Samples were not subject to harsh cleaning, i.e. to ultrasonication so as not to change the surface morphology of CNT film, but were rinsed thoroughly with DI water. The pristine film shows sparse nuclei located in small groups, while the anodized film is evenly covered with a high density of nuclei. The observed difference in nuclei density between the anodized and pristine CNT film is of at least one order of magnitude. Previous studies of metal deposition on functionalized CNT material has been mostly done with CNT material with one functionalization treatment with no comparison between the strength of functionalization and effect on the deposition process [7], [10], [11], [14], [15]. The relationship between functionalization and electrochemical activity is an important issue because it is known that functionalization affects the conductivity of the CNT material unfavorably. Therefore while the increase in functional group concentration increases the electrochemical activity, at the same time the CNT
12
sidewalls are deteriorated and the mean free path of electrons travelling across the CNT material is reduced leading to increased resistance and loss of driving force for electrochemical reactions. Cathodic polarization curves were recorded for a pristine and functionalized sample. The tests were carried out on a single sample in order to eliminate the influence of any possible differences between samples. In this test, the first cathodic polarization curve was recorded for a pristine sample, then the sample was anodized beginning at 1.2V vs. SCE for 60 seconds, and a new cathodic polarization curve was recorded. This cycle was repeated after each 0.2 V increase in potential up to 2.2 V vs. SCE. During the anodization step the previously deposited copper was completely removed and the film was increasingly functionalized. Table 2 shows the observed exchange current densities for CNT films with different levels of functionalization. The observed increase in exchange current density is attributed to the increase of electrochemically active area by incorporation of functional groups. The cathodic polarization curves of these tests are presented in Fig. 11 and as can be seen the cathodic polarization curves clearly shows that enhanced surface functionalization provides improved electrochemical activity of CNT films up to anodization voltage 1.8 V vs. SCE. Up to this point after each anodization step the film surface activity is seen to increase, as evidenced by the improved deposition rate at a given potential, which results from the enhanced adsorption of copper on the functional groups at the film surface. The reaction rate was shown to decrease after anodization at 2.0 V vs. SCE. This phenomenon is related to the deterioration of the nanotube sidewalls with increased levels of functionalization, that results in increased resistivity of the carbon nanotube film as the mean free path of electrons is reduced: the more a film is functionalized the greater the observed decrease in conductivity [53]. The strong increase in resistivity results in increasing polarization even while the amount of functional groups has increased. This contrasts with the result of increasing adsorbed copper by level of functionalization, which is conducted without external current. Therefore while the number of adsorption
13
sites increases with increasing functionalization, the electrodeposition process is only enhanced up to a limit of about 0.75 in terms of ID/IG ratio, or an increase of about 570 % from original ID/IG ratio. 4. CONCLUSIONS In this work the electrochemical activity and reduction capacity of carbon nanotube (CNT) film was increased by functionalization using heat treatment and anodization. It was shown that increasing the degree of functionalization increases the amount of spontaneously nucleated copper particles at the film surface due to increase in sorption sites. Spontaneous deposition of copper was attributed to displacement reaction of impurities at the film surface and difference in redox potential between multiwall nanotubes and copper. Anodizing was found to be a more effective functionalizing treatment when compared to heat treatment in an oxygen-containing atmosphere over the investigated parameter range. The highly anodized CNT films were shown to be hydrophilic while the pristine CNT film is strongly hydrophobic. A functionalized CNT film after electrodeposition showed an even nucleation of copper at the film surface with copper deposits penetrating into the nanotube bundles, whilst in contrast pristine CNT films exhibit inhomogeneous and sparse copper deposits. The introduction of functional groups increases the exchange current density of the system. The improved activity of anodized CNT films was evident by the cathodic polarization curves, which showed increase in activity up to an anodizing voltage of 1.8 V vs. SCE. During anodization at 2.0 V vs. SCE, the film begins to deteriorate to such a degree that the activity decreased even with the increased degree of functionalization due to the increase in electrical resistance of CNT film. Nevertheless, the activity was still considerably higher when compared to that of a non-functionalized pristine film. Overall, the results show that functionalization, both via heat treatment and anodization, is an effective pre-treatment to activate CNT films before electrodeposition of copper as
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functionalization results in an (i) increase in hydrophilicity through the formation of functional groups and (ii) increase in the nucleation rate of copper deposits. ACKNOWLEDGEMENTS This work has been supported by the FP7 European project Ultrawire (Grant Agreement No. 609057). RawMatTERS Finland Infrastructure (RAMI) supported by Academy of Finland is greatly acknowledged. REFERENCES [1] S. Bakshi, D. Lahiri, A. Agarwal, Carbon nanotube reinforced metal matrix composites-a review, International Materials Reviews 55 (2010) 41-64.
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Fig. 1. Schematic drawing of the experimental setup consisting of (1) CNT film, (2) silver paste electrical contact, (3) glue, (4) CE-Copper sheet anode, (5) Copper conductor sheet, and (6) PVC sample holder.
Fig. 2. SEM micrograph of (a) pristine CNT film and (b) CNT film oxidized at 450 °C in air for 20 minutes.
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Fig. 3. (a) Raman spectra of pristine, heat treated (500 °C, 1 hour) and anodized (2.2V vs. SCE, 60s) CNT films. (b) Raman ID/IG- ratio after (i) no pre-treatment, (ii) heat treatments (400 – 500 °C, 1 hour) and (iii) anodization treatments (at 1.0 – 2.2 V vs. SCE, t = 60 s).
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Fig. 4. FTIR spectra of pristine and functionalized CNT films.
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Fig. 5. Schematic for CNT functionalization and spontaneous reduction of copper: A) untreated pristine CNT, B) CNT after functionalization, C) sorption of copper ions to the Helmholtz layers and D) spontaneous reduction of copper due by CNT and/or impurities.
Fig. 6. Standard electrode potentials and equilibrium potentials for CNTs and selected metals.
Fig. 7. The amount of copper deposits at the film surface after immersion tests increases with the level of functionalization. The error bars represent one standard deviation.
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27
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Fig. 8. CNT film samples showing different morphologies after immersion in copper sulfate electrolyte for 48 hours (a) non-functionalized film (BSE) (b) functionalized film with sub-micron copper particles (BSE), inset (SE) (c) functionalized film showing micrometer sized particles (BSE) (d) functionalized film showing dendrites (BSE) and (e) functionalized film showing continuous film (BSE).
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Fig. 9. SEM micrographs of CNT film after electrochemical deposition. (a) Pristine CNT film showing bare areas devoid of any copper nuclei, insert: contact angle measurement of electrolyte on pristine film surface, (b) morphology of pristine CNT film, (c) copper nuclei on top of pristine CNT film, (d) anodized CNT film (2.2V vs. SCE 60s) morphology, insert: wetting of electrolyte on anodized film surface, and (e) copper nuclei penetrating into anodized CNT structure.
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Fig. 10. Electrochemical deposition at the interface of anodized/pristine CNT film: (a) upper part: pristine, lower part: anodized, (b) nucleation on pristine CNT film and (c-d) nucleation on oxidized CNT film.
32
Fig. 11. Cathodic polarization of CNT film in copper sulfate electrolyte after anodization at various voltages (1.2- 2.2 V vs. SCE for 60s) in 1M sulfuric acid.
Table 1. Chemical composition of CNT film determined by EDX- analysis. wt%
C
O
Fe
S
Si
Na
Ca
CNT film
88.48
9.20
1.27
0.40
0.16
0.34
0.16
33