Chapter 1 Introduction
3.3 Results and Discussion
3.3.8 Electrochemical impedance
by any particle in suspension and is measured by the difference of potential at the slipping plane and the bulk suspension medium. Particles with higher magnitude of zeta potential, either positive or negative, have the tendency to repel each other such that they do not coagulate or flocculate. On the other hand, the closer the magni- tude of zeta potential to zero, the less dominant the repulsive force is. Hence, they have a higher tendency to coagulate or flocculate. Since the magnitude of the zeta potential gives an indication of the stability of the dispersion, it is expected that the zeta potential of a dispersion of hydrophilic dry oxidized CNT in DI water will be quite high. In contrast, a dispersion of superhydrophobic vacuum annealed CNT in DI water is expected to exhibit a near zero zeta potential.
As expected, the zeta potential of a dispersion of superhydrophobic CNT with an average O/C ratio of 7% in DI water is almost zero. In agreement with the qualitative observation of the dispersion, a near zero zeta potential means that the dispersion is highly unstable and the superhydrophobic CNT coagulate rapidly in DI water. The value of zeta potential of CNT dispersion decreases monotonically as the increase of O/C ratio (Figure 3.28.c). An increase of O/C ratio by 1% results in an decrease of zeta potential by 10mV. For the dispersion of hydrophilic CNT with an average O/C ratio of 11%, its zeta potential is measured to be -36mV, which in theory should exhibit a moderate stability. Indeed, as observed qualitatively, these hydrophilic CNT exhibit a better dispersibility, although sedimentation of CNT is still observable on the bottom of the dispersion. Dispersion of highly oxidized CNT with an average O/C ratio of 15.5% exhibits a zeta potential of -45mV, which in theory indicates a good dispersion stability. Indeed, qualitative observation confirms that the dispersion of highly oxidized CNT is stable even for more than two months of settling time.
low electrochemical capacitance in aqueous solution and the presence of carboxyl groups may increase such capacitance by a factor of three (Kim et al., 2005). In agreement with that study, the impedance of CNT arrays was found to be highly dependent on their wettability. In other words, the impedance of the CNT arrays in aqueous electrolytes can be varied by manipulating the surface concentration of oxygenated functional groups via dry oxidation and vacuum annealing treatments.
The Bode impedance plot of the electrochemical impedance spectroscopy (EIS) data in 1M NaCl aqueous electrolyte shows that the superhydrophobic CNT arrays yield much higher impedance than the hydrophilic counterpart. The difference in impedance can be observed clearly at considerably low frequency off <1 kHz, where the effect of double layer capacitance becomes increasingly dominant. In fact, the frequency at which the transition from pure resistive behavior to a capacitive-resistive behavior decreases with the increase of wettability of the array. Such a transition can be observed from the absolute impedance of the CNT arrays, where a pure resistive behavior is indicated by a constant absolute impedance over a large frequency range, and a capacitive-resistive behavior is indicated by an exponential increase of absolute impedance with the exponential decrease of frequency (Figure 3.29.a). This transition can also be seen from the change in phase angle of the impedance (Figure 3.29.b), where a pure resistive behavior is indicated by a constant phase angle over a large frequency range, and a capacitive-resistive behavior is indicated by an exponential decrease of phase angle with the exponential decrease of frequency. Based on these two plots, it is obvious that the transition for the superhydrophobic CNT arrays occurs at a frequency of about 1.5 kHz, while the transition for the highly hydrophilic CNT arrays occurs at a much lower frequency of about 45 Hz.
The change in transition frequency as the change in wettability of CNT arrays can also be observed from the Nyquist impedance plot of the EIS data in the high frequency regime (Figure 3.30.c). The Nyquist plot of the superhydrophobic CNT ar- rays shows a transition from pure resistive behavior to a capacitive-resistive behavior at a frequency of 108.6 kHz. This transition frequency is shifted down as the wetta- bility of CNT arrays improves. In fact, the Nyquist plot of the as-grown CNT arrays
Figure 3.29. Bode plot of CNT arrays with various wettability in DI water. Plot of magnitude of impedance (a) and phase angle (b) as a function of the applied alternating electric field frequency.
shows a transition at a frequency of 27 kHz. Similarly, the Nyquist plot of the mildly hydrophilic and highly hydrophilic CNT arrays shows a transition at a frequency of 841.4 Hz and 209.8 Hz respectively. This behavior suggests that the transition fre- quency decreases rapidly with the decrease of static contact angle of CNT arrays in the high static contact angle regime, and it decreases slowly in the low static contact angle regime.
The absolute impedance of the superhydrophobic CNT arrays at a very low fre- quency is measured to be about three orders of magnitude higher than that of the hydrophilic CNT arrays at the same frequency. Indeed, at a frequency of about 13 mHz, the absolute impedance of the superhydrophobic CNT arrays is about 160Ω, while that of the highly hyrophilic CNT arrays is about 650Ω. The extremely large discrepancy of absolute impedance at lower frequency suggests that the double layer capacitance of superhydrophobic CNT arrays is much smaller than that of hydrophilic CNT arrays. The double layer capacitance can be approximated by fitting the Nyquist diagram with the Randles equivalent electrical circuit for both low-frequency regime (Figure 3.30.a and Figure 3.30.b), and high frequency regime (Figure 3.30.c). The Randles equivalent electrical circuit consist of an electrolyte resistance (Rs) in series with the parallel combination of the double-layer capacitance (Cdl) and an impedance of a faradaic reaction. The impedance of a faradaic reaction itself consist of a charge transfer resistance (Rct) and a specific electrochemical element of diffusion (W). In short, Randles equivalent electrical circuit can be represented as ((Rct+W)/Cdl)+Rs) circuit. Since Rs and W are a function of the electrolyte, their values are constant for exactly the same electrolyte.
The fit of ((Rct+W)/Cdl)+Rs) circuit to the Nyquist diagram of CNT arrays with different wettability reveals a monotonic increase of double layer capacitance and monotonic decrease of charge transfer resistance as the increase of O/C ratio of the CNT arrays. For superhydrophobic CNT arrays with an average O/C ratio of 6.5%, the double layer capacitance is found to be extremely low at 2×10−5 F (Figure 3.31.a), while the charge transfer resistance is extremely high at 2×105Ω (Figure 3.31.b). For highly hydrophobic CNT arrays with an average O/C ratio of
Figure 3.30. Nyquist plot of CNT arrays with various wettability in DI water. Plot of imaginary part versus real part of impedance for hydrophobic CNT arrays (a) and hydrophilic CNT arrays (b) at low-frequency range. Plot of the same at high- frequency range (c).
Figure 3.31. Double layer capacitance (a) and charge transfer resistance (b) of CNTs with various wetting properties in DI water as a function of their oxygen/carbon atomic ratio.
8%, the double layer capacitance is found to be an order of magnitude higher at 1.5×10−4 F, while the charge transfer resistance decreases to 6×104Ω. Further increase in O/C ratios improves both double layer capacitance and charge transfer resistance by several orders of magnitude. For highly hydrophilic CNT arrays with an average O/C ratio of 15.5%, the double layer capacitance is found at 1.5×10−2 F, while the charge transfer resistance is found at 1×103Ω. The effect of oxygenated groups on the double layer capacitance of CNT arrays will be discussed in detail in Chapter 6.
This finding is expected because of the presence of a thin film of air on the inter- face between the bulk surface of the superhydrophobic CNT array and the aqueous electrolyte, which inhibits electrons transfer from the arrays and blocks protons in the electrolyte to approach the surface of the array (Kim et al., 2005). In addition, this thin air film significantly reduces the effective contact area between the surface of the CNT and the electrolyte molecules, which results in a much smaller effective area of Helmholtz layer and a dramatic decrease of double layer capacitance. Thus, the double layer capacitance of the superhydrophobic CNT arrays is measured more than two orders of magnitude smaller than that of the hydrophilic CNT arrays. Similarly, the charge transfer resistance of the superhydrophobic CNT arrays is measured more than two orders of magnitude larger than that of the hydrophilic CNT arrays.