Strong acid doping for the preparation of conductive polyaniline nanoflowers, nanotubes, and nanofibers

Abstract

High-pressure CO2-supported aniline polymerization was thoroughly investigated with a focus on the effect of acid doping on the morphology of the prepared polyaniline (PANI). The polymerization was conducted using HCl at three different concentrations and H2SO4 as the doping agents. Self-assembled HCl-doped PANI nanoflowers (PANNFL) were synthesized for the first time by high-pressure CO2-supported polymerization in the presence of 2 M HCl. Furthermore, HCl-doped PANI nanotubes (HCl-PANNTs) and nanofibers (HCL-PANNFs), as well as sulfuric acid-doped PANI nanofibers (SUA-PANNFs), were successfully prepared by changing either the acidity of the reaction or the doping agent. With 0.1 M HCl doping, circular and rectangular cross-sectional HCl-PANNTs with a wall thickness of 40 nm were prepared. Upon increasing the acid concentration to 2 M HCl, the morphology of the PANI structures transformed from nanotubes to nanoflowers while retaining the same wall thickness. Further increasing the HCl concentration resulted in a mixture of nanosheet structures, and the amount of nanoflowers decreased. Additionally, 1.1 M H2SO4 doping produced nanofibers (SUA-PANNF) with an average fiber diameter of 45 nm. All the prepared PANIs had partially crystallized structure in the emeraldine salt form. Additionally, the crystallinities of the prepared HCl-PANI structures were higher than that of the SUA-PANI structure. It was clearly observed that the type of acid dopant and the acid concentration affected the polymerization yield (PY) and could control the electrical conductivity (EC). The highest EC, i.e., 3.7 S/cm, was exhibited by PANI prepared with 5 M HCl.