Polyaniline Surface Morphology During the Doping Process Using Atomic Force Microscopy
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were HPLC grade or better and no buffer solutions were used. Chemical structures for the polyaniline base and the two protonated forms are shown below in Figure 1. H
H H
H
c1 H
H
CH3
IH
H I
H
03
H CHS
Figure 1. Chemical structure of polyanilines: (top); PAn/HCl (middle); PAn/TOS (bottom).
emeraldine base
The PAn film was cut and mounted into the in-situ AFM wet cell. The morphology changes of PAn films were studied during the doping and dedoping processes. The AFM and electrochemical wet cell used in our studies were of commercially available models[15] . The AFM probing tip was attached to an insulating cantilever which had a low spring constant (-0.1-1N/m) and was deflected in response to the forces between the probe tip and the sample. The deflections of the cantilever in our AFM were monitored using a laser deflection method[16] . All AFM experiments were performed using contact mode where the force applied between the tip and sample was -10 nN. The electrical behavior was determined by fitting the samples into a quarter wavelength R-Band waveguide sample holder and measuring the scattering parameters using an HP 8510B Network Analyzer with the millimeter wave option. The scattering data were then used to calculate the complex permittivity. The details of this procedure have been described by Buckley and Dudeck[17]. RESULTS AND DISCUSSION Two different counterion species were used in this study. The physically larger tosylic counterion was compared to the smaller chloride ion. The effect on the PAn surface when doping and dedoping with these ions, as observed by in-situ AFM, along with the corresponding complex permittivity is described below. For the case of tosylic acid (TOS), AFM images were captured just after injecting the cell with TOS (pH=0.2) which involved the protonation of the polymer backbone and counterion insertion. 198
The PAn was cycled between the conducting and insulating states using TOS and ammonium hydroxide (NH4OH; pH=12), respectively. The surface morphologies for the doped and dedoped states are The change in surface shown by the AFM images in Figures 2. morphology was observed immediately for all of the PAn/TOS samples.
Figure 2. AFM images for PAn/TOS dedoped (bottom) states.
(pH=0.2) in the doped (top) and
The morphology changes were very consistent for each state; the sample described here was cycled 6 times between TOS and NH4OH; each time PAn was doped and dedoped the morphology changed in the The reproducibility of these changes in morphology can same way. be seen by the surface roughness results shown in Table 1 below.
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Table 1. Surface Analysis Sample
Roughness(AVE)
Roughness(RMS)
(RMS-AVE)
PAn/TOS
6.2 nm
8.1 nm
1.9
PAn base
5.9 nm
7.6 nm
1.7
PAn/TOS
6.9 nm
8.7 nm
1.8
PAn base
5.4 nm
7.2 nm
1.8
PAn/TOS
6.8 nm
8.6 nm
1.8
7.1 nm 1.8 5.3 nm PAn base * above data taken after 3 cycles to establish stable surface morphologies The root mean square of the surface roughness (Rms) and the mean roughness (Rm) of the TOS doped PAn were consist
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