Conductive Polymer Synthesis with Single-Crystallinity via a Novel Plasma Polymerization Technique for Gas Sensor Applications

Abstract

This study proposes a new nanostructured conductive polymer synthesis method that can grow the single-crystalline high-density plasma-polymerized nanoparticle structures by enhancing the sufficient nucleation and fragmentation of the pyrrole monomer using a novel atmospheric pressure plasma jet (APPJ) technique. Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM) results show that the plasma-polymerized pyrrole (pPPy) nanoparticles have a fast deposition rate of 0.93 µm·min^-1 under a room-temperature process and have single-crystalline characteristics with porous properties. In addition, the single-crystalline high-density pPPy nanoparticle structures were successfully synthesized on the glass, plastic, and interdigitated gas sensor electrode substrates using a novel plasma polymerization technique at room temperature. To check the suitability of the active layer for the fabrication of electrochemical toxic gas sensors, the resistance variations of the pPPy nanoparticles grown on the interdigitated gas sensor electrodes were examined by doping with iodine. As a result, the proposed APPJ device could obtain the high-density and ultra-fast single-crystalline pPPy thin films for various gas sensor applications. This work will contribute to the design of highly sensitive gas sensors adopting the novel plasma-polymerized conductive polymer as new active layer.

Keywords: atmospheric pressure plasma; gas sensor; iodine doping; plasma-polymerized pyrrole; single-crystalline.

Figure 1

(a) Schematic diagram of experimental setup in this study and images of plasmas produced in the nucleation region; and (b) applied voltages and total currents of novel atmospheric pressure plasma jets (APPJs) whose insulating substrate holders (or polytetrafluoroethylene (PTFE) bottom cap) are placed outside or inside the glass tube.

Figure 2

Optical emission spectra using optical emission spectrometer (OES) measured in the nucleation region of novel APPJs, whose insulating substrate holders are placed outside or inside the glass tube.

Figure 3

Changes in top and cross-section views of scanning electron microscopy (SEM) images of plasma-polymerized pyrrole (pPPy) nanoparticle thin films prepared via proposed APPJs after a deposition of 30 min in case of an adopting jet whose insulating substrate holders are placed outside or inside the glass tube. Scale bar = 2 µm.

Figure 4

(a) Transmission electron microscopy (TEM) images of pPPy nanoparticles prepared via proposed APPJs, whose insulating substrate holder is placed inside the glass tube. High-resolution TEM images of single-crystalline pPPy nanoparticles; insets in (a) represent the selected area electron diffraction (SAED) pattern of pPPy nanoparticles; (b) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping images of C, O, and N. Scale bar = 2 nm (left) and 20 nm (right).

Figure 5

Changes in Fourier transform infrared spectroscopy (FTIR) spectra of pPPy nanofibers and nanoparticles thin film prepared using the proposed APPJs after a deposition of 60 min on plastic substrates in the outside and inside cases of Figure 1a, respectively.

Figure 6

Differences in the inside and outside cases of pPPy nanofibers and nanoparticle thin film prepared using proposed APPJs after a deposition of 60 min on glass substrates. (a) X-ray photoelectron spectroscopy (XPS) survey spectra and detailed (b) C 1s (high resolution); (c) N 1s, and (d) O 1s spectra. Insets in (a) represent the atom percent in pPPy film. The XPS data is based in 1s orbitals.

Figure 7

Changes in the resistance of pPPy nanofibers and nanoparticle thin films on substrates of interdigitated gas sensor electrodes under various iodine exposure (doping) times prepared using proposed APPJs with the insulating substrate holder placed inside the glass tube.