Influence of ozone on N-doped multi-walled carbon nanotubes

Figures & data

Figure 1. SEM micrographs taken for untreated (a) and ozone-treated N-MWCNT films (b, c, d). The SEM images were obtained with an accelerating voltage of 20 kV and magnification factors of 1600× (a), 500 × (b), 2000× (c) and 6000× (d).

Figure 2. Raman spectra recorded for either untreated (a) or ozone-treated N-MWCNT film for 60 s (b) and 180 s (c). The symbols D and G represent the Raman D- and G-bands, respectively.

Figure 3. Variation of the intensity ratio of G- and D-bands (I _G/I _D) with the ozonolysis time of N-MWCNT film.

Figure 4. Representative CVs displaying the effect of ozonolysis time on electrochemical performance of N-MWCNT film. The CVs were recorded at v = 0.05 V s⁻¹ in the presence of [Fe(CN)₆]^3−/4− (1.0 × 10⁻³mol L⁻¹) in KCl (0.1 mol L⁻¹) on either untreated (dotted line) or ozone-treated N-MWCNT film for 60 s (dashed line), 120 s (dotted-dashed line) and 180 s (solid line).

Figure 5. Representative EIS spectra illustrating the effect of ozonolysis time on impedance behaviour of N-MWCNT film. The EIS spectra were recorded in the presence of [Fe(CN)₆]^3−/4− (1.0 × 10⁻³mol L⁻¹) in KCl (0.1 mol L⁻¹) either on untreated (filled square) or ozone-treated N-MWCNT film for 60 s (filled circle), 120 s (filled upward triangle) and 180 s (filled downward triangle). For better readability, the solution ohmic resistance (∼10 Ω) was subtracted from the real part of the impedance data. The EIS spectra were recorded at the half-wave potential of [Fe(CN)₆]^3−/4− (E _1/2 ≈ +0.21 V versus Ag/AgCl).

Table 1. Raman D-bands, G-bands and intensity ratios of G- and D-bands (I _G/I _D) for the untreated and ozone-treated N-MWCNT for different time periods.

	Ozone treatment time (s)
	0	60	120	180
v (D) (cm^−1)	1289	1289	1288	1288
v (G) (cm^−1)	1597	1597	1598	1598
I G/I D	1.25	0.73	0.62	0.56

Table 2. Anodic (E _p ^ox) and cathodic (E _p ^red) peak potentials,^a peak potential separations (ΔE _p), anodic peak current densities (), cathodic and anodic peak current ratios (/), half-wave potentials (E _1/2),^a active surface areas (A), heterogeneous electron transfer rate constants (k _s),^b charge transfer coefficients (a),^c double layer capacitances (C _dl),^d charge transfer resistances (R _ct)^d and Warburg parameters (σ)^de for [Fe(CN)₆]^3−/4− (1.0 × 10⁻³mol L⁻¹) on untreated and ozone-treated N-MWCNT film for various time periods.

	Ozone treatment time (s)
	0	60	120	180
E p ^ox (V)	0.252	0.307	0.313	0.320
E p ^red (V)	0.164	0.114	0.108	0.094
ΔE p (V)	0.088	0.193	0.205	0.226
(10^−3 A cm^2)	0.150	0.160	0.160	0.160
/	1.04	1.01	1.05	1.07
E 1/2 (V)	0.208	0.210	0.210	0.207
A (cm^2)	1.71	4.18	4.36	4.44
k s (10^−3 cm s^−1)	5.56	0.87	0.75	0.60
A	0.50	0.50	0.50	0.50
C dl (10^−3 F)	0.09	0.09	0.03	0.03
R ct (Ω)	42.7	44.6	69.8	84.4
σ (Ω s^−1/2)	267	250	215	204
Notes: ^aAll potentials are reported versus Ag/AgCl (KCl sat.) reference electrode.
^bThe ks values were determined according to the relation: ψ = (Do /DR ) ^a ^/2 ks (nπFvDo/RT)^−1/2, where ψ is a kinetic parameter, a the charge transfer coefficient, D o, D R the diffusion coefficients of the oxidized and reduced species, respectively (D o ≈ D R) and n the number of the electrons involved in the reaction (n = 1) 46.
^cThe a values were determined graphically according to the following relation: Ep = E^o -(RT/anaF) ln(anaF/RTk s)-(RT/anaF)lnv 44,45.
^dThe EIS parameters were determined by using the electrical circuit model (R s + (C dl/(R ct + Z w))) (software Thales, version 4.15).
^eThe Warburg impedance ZW is connected with the Warburg parameter σ according to the relation: Z w = σ/(iω)^1/2, where ω is the angular frequency 33.

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