Nitrogen-Doping Effect on Electrochemical Activity

The ET behavior of CNx NTs are explored using a potassium ferrocyanide redox

probe (5 mM K4Fe(CN)6 in 1M KCl). A typical CV curve of CNx NTs

microelectrode in this redox couple system is shown in Figure 4.4(a). The

well-defined peaks obtained in the forward and reverse scans are due to the

Fe³⁺/Fe²⁺ redox couple. The reversible redox reaction of the CNx NT

microelectrodes is further evidenced by the linear Ipa and Ipc vs υ^1/2 plots shown in

Figure 4.4(b), where Ipa、Ipc and υ are the corresponding peak current densities of the

cathodic and anodic reactions and the scan rate, respectively. These data indicate

that the whole reactions are limited by semi-infinite linear diffusion of the reactants

to the electrode surface. Moreover, the effective surface area of CNx NT arrays can

CNx NTs surface to the geometrical electrode surface area, as a function of N2 flow

rate is depicted in the inset of Figure 4.4(b). It is noted that the roughness factor of

CNx NT microelectrode prepared with N2 flow rate of 40 sccm is ~33, showing

significant enhancement in the effective surface area. Clearly, the N-doping induced

electrochemically active sites on the surface of CNx NT microelectrodes prepared

with N2 flow rate of 40 sccm are optimized.

The peak-to-peak separation ( Ep△ ) of potassium ferrocyanide redox probe is

strongly dependent on the ET rate, namely, the reactivity of electrode materials to

the electrolyte. In Figure 4.4(c), Ep△ of CNx NTs with N2 flow rate of 40 sccm is

around 59 mV, reflecting excellent ET reactions. This is also related to the reduced

internal resistance of the CNx NT structures, which was determined by EC

impedance (EIS) in 1M KCl solution containing 5 mM K4Fe(CN)6 at an AC

frequency varying from 0.1 to 100 kHz as shown in Figure 4.5. From the point

intersecting with the real axis in the range of high frequency, the internal resistance

of the electrode is obtained. As shown in the inset of Figure 4.5, the arrayed CNx

NTs microelectrode prepared with N2 flow rate of 40 sccm shows the lowest

resistance of all, which is in good agreement with the above mentioned

measurements. Moreover, the Nyquist complex plane plot of the CNx NT

capability with selective N dopant.

The enhanced ET kinetics observed at CNx NTs surface may in part be attributed

to higher electronegativity of the CNx NTs surface. The attractive interaction

between the C-N dipoles present at the surface may attract the negatively charged

members of the Fe(CN)63-/4- and accelerates the redox reactions. In fact, the rate

constant for ET from the reactant to the electrode can be expressed as

kox = ∫ dє (1 - f(є,T))wox(є),

where wox is the rate of ET from an occupied level of the reactant to an empty

level of the electrode.[72] This ET rates are given in terms of the density of

available states, the electron-resonance width and the strength of the coupling to the

phonon bath.[73,74] The activation energy for the reaction decreases with increasing

electronic interaction width. The structure of the NTs, as well as their local density

of states, might be responsible for the increase of the electronic-energy interaction

width. In our case, arrayed CNx NT microelectrode prepared with N2 flow rate of 40

sccm has higher local-density of states (including surface defects induced states

from XPS results), and promotes the enhanced ET kinetics.

The unique structures thus created in the in-situ N doping process can be verified

by selective EC deposition technique, which is a direct and quantitative measure of

deposition of metal at the chemical reactive sites such as the defect sites on CNx

NTs. Figures 6 shows the comparative TEM images of the CNx NTs prepared with

various N2 flow rate during growth and followed by 0.5M H2SO4 & 0.0025M

H2PtCl6 mixture solutions. In Figure 4.6(a) most of the Pt particles aggregate on the

top of the electrode to form large agglomerates, indicating lack of nucleation sites on

the side surface of CNx NT prepared with no N2 flow during growth. Figure 4.6(b)

shows more uniform and higher density of Pt NPs nucleating on the side surface of

CNxNTs prepared with 40 sccm N2 flow rate during growth. However, excessive N2

flow rate may cause adverse effect. Figure 4.6(c) shows Pt agglomerates of tens of

nanometers scattered throughout the CNx NTs, which can be attributed to low

nucleation density at this growth condition (120 sccm N2 flow rate). Figure 4.6(b)

shows clear indication of the nearly monodisperse Pt NPs of 2-5 nm on the side

surface of CNx NTs having optimal defect density created under the 40 sccm N2

flow rate during growth. Furthermore, a typical CV curve measured in 1 M

CH3OH/1 M H2SO4 solution for the arrayed Pt NP-CNx NTs hybrid nanocomposites

was shown in Figure 4.7. Remarkably, a significantly enhanced electrocatalytic

activity is yielded for 40 sccm nominal N2 flow rate mainly due to keep higher

accessible surface area of discrete Pt NPs finely dispersed on CNx NT. In sharp

the pretty good electrocatalytic activity can be obtained from Pt NP/CNx NT

nanocomposites with the N2 flow rate of 40 sccm dopant, indicating that this arrayed

cable-like nanocomposites can be the most promising candidate for useful catalytic

applications in the μFCDs.