Article pubs.acs.org/JPCC
Electrochemical Charge Storage Properties of Vertically Aligned Carbon Nanotube Films: Effects of Thermal Oxidation Billyde Brown,*,†,∥ Charles B. Parker,† Brian R. Stoner,†,§ Warren M. Grill,‡ and Jeffrey T. Glass† †
Department of Electrical and Computer Engineering and ‡Departments of Biomedical Engineering, Neurobiology, and Surgery, Duke University, Durham, North Carolina 27708, United States § RTI International, Center for Materials & Electronic Technologies, RTP, North Carolina 27709, United States ABSTRACT: Vertically aligned carbon nanotube (VACNT) films were synthesized and investigated in vitro for their potential use as a neural stimulation electrode. Materials and electrochemical (EC) characterization (cyclic voltammetry, electrochemical impedance spectroscopy, and high-rate potential transient measurements) were performed before and after flash oxidation in O2 at various temperatures and over a wide frequency range. The results showed distinct EC behavior within three ranges of treatment temperature. Oxidative thermal treatments that did not visibly etch the VACNT film caused a significant improvement in electrode performance compared to the as-deposited electrode. Surprisingly, flash oxidation within a narrow temperature range (400 < T ≤ 450 °C) selectively increased capacitance/charge injection at high frequencies (102−104 Hz). A phenomenological model is proposed to explain the temperaturedependent behavior and indicates the importance of modifying a porous coating to increase the charging rate rather than maximizing the total charge accumulated at long times for high-rate charge storage applications.
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INTRODUCTION Controlled thermal oxidation in air can provide a simple method to remove disordered carbon from and affect charge storage properties of carbon nanotubes (CNTs).1,2 For example, Li et al.3 showed that the capacitance determined by cyclic voltammetry (CV) more than doubled when vertically aligned carbon nanotube (VACNT) films were treated at 450 °C in air for 10 h. Further, although thermal oxidation may not directly remove residual catalyst,4−6 it can expose carbonencapsulated metal for subsequent removal treatments. Hightemperature oxidation in air was also determined to be a very simple and successful strategy for purification of arc-dischargederived multiwalled carbon nanotubes (MWCNTs), which are metal-free CNTs with few wall defects.6 However, for other CNT materials, such as chemical vapor deposition (CVD)grown single-walled carbon nanotubes (SWCNTs) and MWCNTs that contain a larger concentration of inherent wall defects and metal impurities, achieving selective etching of disordered carbon as opposed to directly etching the CNTs is more difficult.6 Flash oxidation at 550 °C for 15 or 30 min in air induced the same or greater chemical change in CNT films as occurred at 530 °C for 5 h, but without the unacceptably large weight loss.7 The flash oxidation method can thus remove a-C impurities or create reactive sites on CNTs while minimizing weight loss (≤10%), and for this reason, flash oxidation was employed in the present study. According to Pandolfo and Hollenkamp,8 covalent attachment of hydrophilic oxygen-based functionalities can significantly increase the double-layer capacitance of CNT electrodes by reducing hydrophobicity, and increase pseudocapacitance by enabling reversible Faradaic reactions at the functional groups © 2012 American Chemical Society
themselves. However, physically adsorbed molecular oxygen or oxygen complexes can also increase the rate of capacitor selfdischarge or leakage and contribute to capacitor instability, which results in increased equivalent series resistance (ESR) and deterioration of capacitance. Thermal treatment at 400 °C for 1 h of a VACNT microelectrode array increased the capacitance from 5.4 to 1600 μF/cm2 and the high-rate (1 ms pulse) charge-injection limit from 0.02 to 1−1.6 mC/cm2.9 Although other surface modification methods were employed, including noncovalent techniques, the thermal oxidation treatment resulted in the greatest charge storage capacity and charge-injection limits. These improvements were attributed to increases in the accessible surface area resulting from increased hydrophilicity by forming surface oxides at the tips and defect sites of the VACNTs and removal of a-C between neighboring VACNTs. However, the mechanical stability of the array was degraded after heat treatment and some VACNTs detached from the substrate during electrochemical measurements. In summary, the motivation to modify VACNT films using gas-phase oxidative treatment for the improvement of charge storage properties in the present study was four-fold: (i) Appropriate thermal treatments in oxidative environments are capable of purifying carbon nanotubes (CNTs) by eliminating carbonaceous impurities6,7 that could act as blocking sites that prevent electrolyte penetration. (ii) Oxidative treatments can facilitate the attachment of hydrophilic oxygenated functional Received: May 7, 2012 Revised: August 16, 2012 Published: August 20, 2012 19526
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spectra were collected at three different random locations on the top surface of the cleaved 1 cm2 CNT-coated silicon substrates, and the data were averaged. To prevent material changes due to heating, the laser exposure time was limited to 450 °C), the CSCc increased relative to Region II. Growth, treatment, and EC characterization of VACNT films as a function of temperature were repeated multiple times, and the resulting trends were similar, including the drop at 425 °C. Charge Storage at High Frequencies. Potential transient measurements were performed on thermally treated samples using cathodic-first, symmetric, and biphasic rectangular current pulses. The fast capacitance values (i.e., charge-injection limits normalized to the voltage excursion) were plotted versus treatment temperature in Figure 3. As with CSCc, the data were assigned to three different temperature ranges or regions. In Region I, the fast capacitance increased with increasing
in each plot. At 500 °C, the CNTs were completely oxidized, as evidenced by the absence of black CNT deposits after removal from the furnace. Only a reddish-brown color, presumably from the residual iron catalyst nanoparticles, was observed. All thermal treatments in oxygen at temperatures below 500 °C significantly increased the electrode’s “slow” capacitance relative to the as-deposited CNT film. The anodic and cathodic charging currents increased with increasing temperature except for a notable outlier at the 425 °C treatment temperature where there is a marked decrease in the capacitance. All of the CV curves are fairly featureless, indicating capacitive character, except for the film treated at 475 °C, which has noticeable redox peaks. For comparison to the thermally treated CNT samples, an Fe-catalyst-coated substrate that was only pretreated by the dewetting process10 (no CNT growth) and consisted of a 2D array of iron nanoparticles is shown in Figure 1f. The electrochemical reactions observed at ∼0 and −0.5 V vs 19528
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(negative) region of the current pulse determines the fast capacitance values (Cfast) plotted in Figure 3, since the anodic pulse is only for charge-balancing purposes. In this case, the specific capacitance for the cathodic phase was more than twice the value of the capacitance for the anodic phase (1982 vs 789 μF/cm2, respectively), whereas in Region I, the fast capacitance for the cathodic and anodic phases are nearly equivalent. This is particularly interesting given the similar behavior noted for the CV curves after thermal treatments described in the previous section and also reported by Phely-Bobin et al.16 Thus, a significant increase in charge injection could be achieved for neural stimulation by cathodically pulsing VACNT electrodes oxidized in the appropriate temperature range. Regardless of the asymmetry of the transient, the interpulse potential stabilized at levels within electrolysis limits and the amount of charge remained balanced during pulsing. Finally, in Region III, the form of the voltage transient changed once again. At the beginning of the current pulse, the potential changed rapidly and then saturated near the end of the current pulse. The steep slope at the beginning of the voltage transient is indicative of charging behavior with a very low capacitance, and the remainder of the voltage transient is indicative of irreversible Faradaic behavior since the voltage values saturate as charge carriers begin leaving the interface. Overall, Faradaic behavior clearly dominates in Region III, whereas capacitive behavior dominates in Regions I and II. Impedance Spectroscopy. Similar to the data presented in previous sections, the impedance data showed markedly different characteristics in the three temperature regions. In Figure 5, representative Bode plots of impedance magnitude
Figure 3. Fast capacitance obtained from PTMs for VACNT films treated in O2 at various temperatures. Temperature regions are labeled using roman numerals. Lines are intended to indicate qualitative trends.
treatment temperature, in Region II, the fast capacitance increased substantially and reached a maximum value, and in Region III, the fast capacitance decreased substantially. Additional information could be discerned by analyzing the characteristics of the voltage transients after removing the iR drop. The form of the voltage transients changed in each of the temperature regions, as shown in Figure 4. In Region I, the
Figure 5. Representative impedance (top) and phase (bottom) Bode plots for CNT films treated in O2 at the different temperature regions.
Figure 4. Representative voltage transients after removing the voltage drop across the series resistance, showing distinct characteristics for VACNT films thermally treated in O2 in their respective temperature regions.
and phase are shown for each of the three regions. The impedance magnitude increased with treatment temperature at frequencies > 10 Hz. This included high frequencies (∼1 kHz) where the phase angle was at a minimum, indicating an increased ESR. Thus, the thermal treatments caused an increase in the resistance of the VACNT electrode in Regions II and III. The low-frequency impedance (|Z| at 0.1 Hz) decreased as a function of treatment temperature in Region I, but then increased in Region II, and finally decreased again in Region III, as also shown in Figure 5. This was unlike the monotonic decrease in low-frequency impedance observed for VACNT electrodes of increasing thickness.10 The magnitude of the
voltage transients were highly symmetric, almost a perfect sawtooth curve, as shown in Figure 4 (top panel), in agreement with the PTM results for as-deposited and thermally treated VACNTs (400 °C for 1 h).9 In Region II, the voltage transient was linear with time, indicative of ideal capacitive character, similar to Region I. However, the curve exhibited significant asymmetry as seen by the different slopes of the voltage during cathodic and anodic current pulses, respectively. The negative slope of the cathodic 19529
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phase angle also decreased (toward 0°) with increasing treatment temperature (from 200 to 400 °C) within Region I (not shown), suggesting that the barrier to charge transfer is reduced by the thermal treatment. However, in Region II, the magnitude of the phase angle at low frequencies is greater than that in either Region I or III, with the lowest barrier to charge transfer occurring in Region III. EIS Circuit Modeling. The circuit model used to fit the EIS data for the VACNT electrodes is shown in Figure 6 and also
Figure 6. Circuit model used to fit impedance data for VACNT electrodes.
described in our previous report.10 Rs is the ESR that represents the combined resistance of the solution, the working electrode, contact resistance between the working electrode and current collector, and resistance of ions moving through small pores (pore resistance).8 L is a parasitic inductance that is usually attributed to the wiring of the circuit17 and is insignificant except at high frequencies.18 In Regions I and II, two overlapping depressed semicircles could be distinguished in the Nyquist plots (not shown), indicating that two capacitive components over slightly different frequency ranges could be deconvolved by circuit modeling. In Region III, the high- and low-frequency capacitive components were well-separated. Frequencies above 10 kHz had negligible impact on capacitance values and thus were not included in the model. The model contains constant phase elements (CPE) instead of capacitors to model more accurately surface charge inhomogeneities influenced by a porous electrode morphology. The CPEhigh component is due to double-layer capacitance near the VACNT tips or the easily accessible external surface of the VACNT electrode at frequencies greater than the knee frequency,19 and the CPElow component is due to the double-layer capacitance at the internal surface within the pores, which is only accessible at frequencies lower than the knee frequency.19 In addition, any pseudocapacitance from surface-bound redox reactions at the catalyst residue or reactive surface functional groups would likely contribute to the CPElow component. The surface functional groups are expected to bind to defect sites on initial exposure to oxygen when removing VACNTs from the reaction chamber following the deposition process.8 CPEhigh and CPElow were converted to true capacitance values designated as Chigh and Clow.10 Rint is an internal resistance that reflects a Faradaic resistance to charge transfer across the double layer and/or the internal pore resistance, which restricts double-layer charging deep in the film.19 Given that oxidative thermal treatments may strongly affect O-based functionalities that are known to alter the charge-transfer capability of carbon materials, in this case, Rint was essentially a charge-transfer resistance. Rleak is a Faradaic leakage resistance that varies inversely with leakage current due to irreversible Faradaic reactions and allows for the unrecoverable loss of charge from the interface. The trends in the capacitive components are presented in Figure 7a. In Region I, there was a substantial monotonic increase in both the Chigh and the Clow components. In Region
Figure 7. Equivalent circuit modeling results showing (a) Chigh, Clow, and (b) Rint values after fitting to impedance data for VACNT films treated in O2 at various temperatures. Temperature regions are labeled with roman numerals. Lines are intended to indicate qualitative trends.
II, the Chigh component continued to increase, reaching a maximum, while the Clow component simultaneously decreased. In fact, the Chigh component had a greater magnitude than the Clow component, which was novel behavior exclusive to this temperature range. In Region III, the Chigh component decreased dramatically while the Clow component increased dramatically. The trends observed in these EIS measurements are in agreement with the data obtained from PTM and CV measurements presented previously (Figures 2 and 3, respectively). The internal resistance, Rint, was plotted as a function of treatment temperature in Figure 7b. Cfast, Chigh, and Rint all had similar trends as a function of treatment temperature, although the data were obtained using two different measurement techniques (PTM and EIS, respectively). According to the circuit model (Figure 6), it is plausible that Chigh would increase as Rint increased, since an increase in the charge-transfer resistance could reduce the amount of capacitance or charge stored at lower frequencies and increase the capacitance or charge injected at higher frequencies. The latter could be true because of less leakage current due to Faradaic reactions across the interface. This may reflect the ability of thermal oxidation treatments to eliminate sources of double-layer leakage current, such as from oxygenated groups bound to carbonaceous impurities near the CNT tips. Total Capacitance. Differential capacitance (Cdiff) versus frequency curves were determined by fitting a series RLC circuit to measured impedance data after subtracting the parallel leakage resistance (Rleak), as estimated from fitting the circuit model in Figure 6. The capacitance value at 0.1 Hz, as determined by EIS, was plotted as a function of treatment temperature in Figure 8, and the Cfast values were included in the plot for comparison. The “slow” capacitance has a trend similar to the CSCc and the Clow circuit component. It is interesting that the maximum capacitance achieved at high frequencies coincides with a dip in the low-frequency capacitance in Region II. 19530
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Figure 8. “Slow” capacitance (Cdiff at 0.1 Hz) and “fast” capacitance obtained using EIS-derived capacitance vs frequency curves and PTMs, respectively, for VACNT films treated in O2 at various temperatures. Temperature regions are labeled with roman numerals. Lines are intended to indicate qualitative trends. Figure 9. High-magnification cross-sectional SEM images showing chemical/morphological changes occurring at catalyst nanoparticles, the Si substrate surface, and base of VACNT films after oxidative thermal treatment at (a) 25, (b) 200, (c) 350, (d) 400, (e) 425, and (f) 475 °C.
Materials Characterization Results: Effect on Morphology. The average film thickness of the oxidized samples was within the expected variation found after CVD growth for control (untreated) films (12 ± 2 μm), except for those treated at 400, 425, and 475 °C. Surprisingly, CNTs treated at 400 and 425 °C showed an increase in average CNT length of up to ∼20% from 12 to 14.4 and 14.75 μm, respectively. The heat treatments may have removed a-C and kink defects as well as caused thermal expansion of the nanotubes, leading to better vertical alignment and increased lengths. The average VACNT length decreased by 4.8 μm (from 12.2 to 7.4 μm) after thermal oxidation at 475 °C. The VACNT−air interface of the treated film had increased roughness compared to the as-deposited film, also indicating that etching had occurred. Planar SEM images (not shown) indicated that the thermal treatment at 475 °C also noticeably increased the defect density and decreased the VACNT density, especially near the edges of the sample. At regions where the density had decreased significantly, catalyst nanoparticles could be clearly observed. In fact, the color of the VACNT film was slightly reddish-brown to the unaided eye, likely due to light reflecting off the Fe nanoparticles. These observations suggest that, during EC measurements, the Fe catalyst residue was exposed to solution. High-resolution SEM images in Figure 9a−f show that a sequence of chemical and morphological changes occurred to the catalyst nanoparticles, substrate, and base of the VACNTs, as the treatment temperature was increased. The as-deposited sample (Figure 9a) had a relatively clean interface between the VACNTs and silicon substrate, and the iron catalyst particles were encapsulated in the CNT walls at the base of the VACNTs. These particles were conically shaped with the tip end pointing away from the substrate, which is consistent with the direction of the growth front and the base-growth mechanism. The image of the sample treated at 200 °C was similar to that of the as-deposited sample except that the edges of the particles began to protrude outside of the carbon walls at the bottom of the CNTs (Figure 9b). Most of the particle was still encapsulated in the CNT and retained a conical shape. At 350 °C, the catalyst particles protruded out of the base of the CNTs and appeared to react with the substrate (Figure 9c).
They lost their conical shape and became more spherical while increasing in size. The CNT walls at the base of the film were damaged after treatment at 400 °C (Figure 9d). Iron is known to catalyze the low-temperature oxidation of carbon,6 and it is likely that defects were created at the catalyst particle−CNT interface at the base of the CNTs. In fact, previous experiments have shown that the oxidation temperature of carbon black can be reduced by ∼100 °C by adding 1−2% iron.20 It appears that particles of debris from the ruptured CNT walls are scattered on the outer walls of some nanotubes. At 425 °C (Figure 9e), damage at the base of CNTs continued and was accompanied by diameter thinning. Some of the VACNTs detached from the catalyst particles. After the 475 °C treatment, a majority of the VACNTs were completely detached from the catalyst nanoparticles, and these CNTs dispersed into solution during the in vitro EC measurements. The control sample consisting of a 2D array of Fe nanoparticles (i.e., without CNTs) subjected to thermal oxidation gave further insight on the phenomena observed in Figure 9. XPS measurements revealed that the concentration of surface oxides on the Fe nanoparticles increased significantly after oxidation. The iron oxides were Fe2O3 and/or Fe3O4, which are thermodynamically favorable phases and have similar binding energies of ∼711 eV for Fe 2p3/2 and ∼530 eV for O 1s.12 Thermal oxidation is known to expand iron particles since the oxidation products have a much lower density than the reactants (7.86 and 5.18 g/cm 3 for Fe and Fe 2 O 3 , respectively).4 The present work is consistent with reports that this expansion is capable of breaking the carbon shells of encapsulating CNT walls.4 Effect on Defect/Disorder Density and O-Based Functionalities. Raman spectroscopy was used to determine changes in the defect/disorder density of the VACNT films following thermal treatments. The Raman ID/IG ratio was plotted as a function of treatment temperature in Figure 10a. Similar to the electrochemical results, the data could be 19531
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to removal of physisorbed or weakly chemisorbed water molecules and/or loosely bound oxygen-containing carbon impurities (a-C). For higher temperatures (Region III), the total carbon− oxygen binding increased substantially, indicating the onset of graphite oxidation. The similarity in the trends of the Raman ID/IG ratio and the XPS C−O concentration are explained by the preferential binding of O-based functional groups to a-C and graphitic defect sites. The removal of a-C at lower treatment temperatures corresponds with a decrease in O-based functional groups, whereas creation of new defects in the CNT structure at higher treatment temperatures corresponds to an increase in the surface concentration of O-based functional groups. Effect on Hydrophobicity. Contact angle measurements were also performed on thermally treated VACNT films. The as-deposited VACNT film exhibited superhydrophobicity, as reported and discussed previously,10 and the contact angle remained at 155° for thermal treatment in Regions I and II. Although the as-deposited sample had a larger percentage of hydrophilic oxygenated functional groups, they were associated with a-C impurities concentrated at VACNT tips, which are believed to restrict wetting by physically blocking pores. Removal of the blocking layer and the associated oxygenated functional groups was observed in Regions I and II; however, no improvement in wetting was observed. This indicated that the hydrophobicity was due primarily to the intrinsic surface properties of VACNTs, such as secondary roughness.23 At 475 °C (Region III), the contact angle decreased to 130°, indicating that the surface was still hydrophobic; however, a small improvement in wettability was attributed to an increase in Obased functionalities on VACNTs. A similar result was reported for chemical functionalization of VACNTs by refluxing in a hot concentrated acid mixture (1:1 H2SO4/HNO3).24 Thus, the thermal treatments alone did not cause a clear hydrophobic-tohydrophilic conversion. Instead, electrochemical activation was primarily responsible for imparting a hydrophilic surface, as shown in our previous report,10 and was performed prior to all electrochemical measurements.
Figure 10. Effect of treatment temperature on (a) Raman ID/IG ratio and (b) total C−O content as determined by XPS. Lines and arrows are intended to indicate qualitative trends.
categorized into three temperature regions. In Region I (t ≤ 400 °C), the ID/IG ratio decreased with increasing treatment temperature, indicating selective removal of amorphous or defective carbons. In Region II, the ID/IG ratio reached a minimum and did not change substantially. In Region III, the ID/IG ratio increased substantially, indicating the creation of new defects via oxidation of the CNT graphitic structure. At 500 °C, the VACNT film was completely oxidized and transformed into the gaseous reaction products, supporting the conclusion that graphite oxidation is the dominant process in Region III. The deconvoluted XPS spectra of the C 1s peak for the CNTs in this study (not shown) consisted of six spectral components labeled as C−C1, C−C2, C−O1, C−O2, C1 plasmon, and C2 plasmon. The total percentage of oxygenated functional groups designated by the sum of C−O1 and C−O2 components relative to the total C 1s percentage is plotted as a function of treatment temperature in Figure 10b. The nearsurface concentration of oxygen bound to carbon is highest for the as-deposited VACNT sample. This is believed to be due primarily to physisorbed water molecules.7,21,22 It may also be an indirect result of a considerable amount of a-C present near the CNT tips. The a-C is itself highly defective carbon, which may preferentially physisorb and/or chemisorb oxygen functional groups upon exposure to air directly after CVD growth. Chemisorption of O-based functionalities is possible even at ambient temperatures,8 and physisorbed H2O and chemisorption of OH and O molecules have been detected by XPS for both as-deposited a-C and graphitic surfaces.21,22 A monotonic decrease in carbon-bound oxygen occurred with increasing temperature until a minimum value was reached at 425 °C. Oxidative thermal treatment at 200 °C is not likely to cause oxidation of graphite, but can effectively remove physisorbed water and organic impurities.7 Similarly, a thermal treatment at 300 °C for 20 min dramatically reduced the surface oxygen concentration content of amorphous carbon nitrides, and this change was attributed to removal of physisorbed water and weakly bound −OH groups.21 Thus, the decrease in oxygen content in Region I is believed to be due
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MECHANISTIC DISCUSSION USING A PHENOMENOLOGICAL MODEL A phenomenological model was developed to illustrate the preceding results and is shown in Figure 11. This builds upon the model showing that electrochemical activation imparted hydrophilicity to the CNT surface and reduced the concentration of a-C.10 The combined effects of both the activation and thermal treatments are qualitatively described by the expanded model. The first row of schematics (T = 25 °C) in Figure 11 depicts the as-deposited VACNT film. This film had the highest concentration of a-C and oxygenated functional groups, as revealed by the Raman and XPS results (Figure 10), respectively. Regardless of the relatively high concentration of hydrophilic functional groups, electrolyte access to the internal surface of the porous film is still physically blocked to some extent by a surface layer of relatively dense a-C impurities.10 Thus, the extent of electrochemically active surface area and capacitance were limited, especially in the low-frequency band. The second row of schematics (T ≤ 400 °C), corresponding to thermal treatments in Region I, has the same physical structure as the as-deposited VACNT film (no damage to the graphitic CNT framework); however, the combined EC 19532
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resistance, Rint (Figure 7). This is expected to decrease redox reactions that occur at a-C and associated O-functional groups that may act as sources of leakage current across the double layer and contribute to pseudocapacitance at lower frequencies. Thus, the charge storage contribution from pseudocapacitance is reduced while the contribution due to double-layer capacitance at high frequency is increased (due to a more easily accessible VACNT film) with removal of a-C and Ofunctional groups and a decrease in pore resistance. This is in agreement with the increase in Chigh and decrease in Clow in the EIS modeling (Figure 7). The decrease in Clow is substantial and consistent with the above factors, which also caused the CSCc and the total electrode differential capacitance at low frequencies (Cdiff at 0.1 Hz) to decrease precipitously in Region II (Figures 2 and 8, respectively). It is important to note that the entire series of experiments (VACNT growth, oxidation, and characterization) was repeated multiple times with new sample sets and similar trends were obtained. This provided confidence that the decrease in capacitance in Region II was not an anomalous result. However, the EC behavior observed in this “transition” region was unexpected, especially the PTM results, which showed surprisingly high charge-injection limits for cathodic stimulation due primarily to the asymmetric voltage transients. The asymmetric, but linear shape, of the potential transients is not well-understood, but these asymmetric transients somewhat mirror the asymmetry in the charging currents obtained by CV of thermally treated CNTs in this study and for self-assembled SWCNT electrodes.16 Finally, in Region III, the thermal treatment caused oxidation of the VACNT film. A majority of the CNTs were detached from the catalyst nanoparticles and the substrate (Figure 9), causing a significant increase in the high-frequency electrode impedance (Figure 5). This oxidation was enhanced by iron, which is known to catalyze carbon oxidation.5−7 Furthermore, the total electrochemically active surface area and corresponding double-layer capacitance dropped substantially in Region III (Figures 7 and 3). An increased coverage of oxygen-containing functional groups, especially acidic oxides (COOR groups), also occurred in Region III (Figure 10b). These functional groups likely attached to newly created defect sites on the VACNTs during the higher-temperature thermal treatment via the oxidation of graphite. In addition, the oxidized iron nanoparticles were exposed to solution, as clearly identified by the CV in Figure 1e−f. Redox reactions that occurred at the oxygenated functional groups in addition to the exposed oxidized iron particles substantially increased the pseudocapacitance. The increased double-layer leakage current and pseudocapacitance in Region III is consistent with the decreased Rint and increased Clow (Figure 7). Therefore, the charging behavior of the electrode changed from a primarily double-layer mechanism to a primarily Faradaic mechanism, and this is supported by the shape of the CV (Figure 1e−f) and potential transient (Figure 4) for the VACNT film treated in Region III.
Figure 11. Phenomenological model of the effects of thermal oxidation on the material and electrochemical properties of VACNT films. The shaded regions illustrate the penetration of electrolyte ions that form the double layer. The small circular shading within the VACNT film represents a-C impurities. Defects are indicated by a red slash, and O-based functional groups are labeled. Unencapsulated catalyst residue is indicated by an array of semicircles.
activation and thermal treatment are more effective at removing a-C and associated functional groups than the activation procedure alone. This opens up additional physical space in the VACNT film, thereby increasing the rate of ionic access to regions of the internal surface.3 This facilitated charge storage at the electrode/electrolyte interface, as indicated by the increased slow capacitance (Cdiff at 0.1 Hz, Figure 8), and consistent with the increased Chigh and Clow in the EIS modeling (Figure 7). The third row of schematics in Figure 11 (400 < T ≤ 450 °C), corresponding to thermal treatments in Region II, shows that the concentration of both a-C and oxygenated functional groups is at a minimum and the bases of the VACNTs are selectively oxidized due to the catalysis of low-temperature carbon oxidation by the presence of iron nanoparticles.4,5,7 During this process, some of the VACNTs detached from the oxidized nanoparticles (Figure 9e), causing a decreased density of electrochemically active nanotubes in the film and an increase in the high-frequency electrode impedance (Figure 5). Nonetheless, the decreased CNT density and low a-C concentration created an even greater physical space for the electrolyte ions to penetrate more rapidly the internal volume of the VACNT film, thereby creating a maximum in both Cfast (Figure 3) and Chigh in Region II (Figure 7). The low concentration of a-C impurities and oxygenated functional groups also increased the barrier to charge transfer, as indicated by the increase in the potential-dependent Faradaic reaction
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CONCLUSIONS The best thermal oxidation treatment for a VACNT neural stimulation electrode was at temperatures of 400 < T ≤ 450 °C. In this region, there was only a small decrease in active surface area and the electrolyte was not significantly exposed to base catalyst particles. In addition, there was a minimal concentration of a-C impurities and O-functional groups. Both of these 19533
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factors allowed minimal double-layer leakage, a reduction of the pore resistance, and an increase in the rate of double-layer charging, thereby greatly improving performance at high frequencies. The modification of VACNT properties to impart a selective improvement at high frequencies (Region II) was a novel result and a key factor for minimizing the disparity between the CSCc and the high-rate charge-injection capacity (analogous to the “slow” and “fast” capacitance, respectively). This was accomplished by minimizing Faradaic leakage sources, blocking impurities, and hydrophobicity, while balancing a loss in electroactive surface area or film density with increased pore size. This approach should be considered when attempting to modify porous capacitive charge injection materials for neural stimulation because the traditional approach of increasing CSCc at low frequencies to increase the proportional fraction of charge injection at high frequencies is not the most efficient. For high-rate application (e.g., neural stimulation), better performance can be achieved by modifying a highly porous coating with the objective of increasing the charging rate rather than maximizing the total charge storage achieved at long times. By increasing the charging rate, the high-rate charge-injection limit was substantially improved at the expense of a lower total CSCc. The phenomenological model provides an integrated view of the effects of activation and thermal oxidation on the properties of VACNT films. The relationships between the as-deposited films, thermal treatments, and changes in material and electrochemical properties were fully characterized. Ultimately, oxidative thermal treatments that did not visibly etch the nanotubes caused a substantial improvement in electrode performance compared to the as-deposited film. However, the results were complicated by the role of catalyst residue inherent within the VACNT film, which accelerated the effects of flash oxidation treatments. This emphasizes the need for purification using vacuum-annealing techniques7,25 or the use of oxidationresistant noble metal catalysts for VACNT growth,26 which would enable better control of the effects of oxidative treatments and feasibility for use in biomedical applications. Nevertheless, the proposed mechanisms offer a frame of reference for future investigations of oxidative thermal treatments for surface modification of VACNT electrodes for high-rate charge-storage applications, including neural stimulation and pulsed-power ultracapacitors.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ∥
Luna Innovations Incorporated, 521 Bridge St., Danville, VA 24541. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We thank Dr. Scott Wolter and Evonne Yi for help with XPS measurements and the Duke Shared Materials and Instrumentation Facility (SMiF) for the use of analytical tools. This work was partially supported by AFOSR Award FA9550-06-1-0230 and NSF Award ECS-04-28540. 19534
dx.doi.org/10.1021/jp304419a | J. Phys. Chem. C 2012, 116, 19526−19534
