Effect of Electrosynthesis Potential on Nucleation, Growth, Adhesion, and Electronic Properties of Polypyrrole Thin Films on Fluorine-Doped Tin Oxide (FTO)

Abstract

Polypyrrole (PPy) is one of the most attractive conducting polymers for thin film applications due to its good electrical conductivity, stability, optical properties, and biocompatibility. Among the technologies in which PPy has gained prominence are optoelectronics and solar energy conversion, where transparent electrodes such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) are frequently used. However, FTO substrates have the notable advantage that their components are widely available in nature, unlike those of ITO. Recognizing the importance that the FTO/polypyrrole system has gained in various applications, here, we studied for the first time the nucleation and growth mechanism of electro-synthesized PPy on FTO. Additionally, the effect of the synthesis potential (0.9, 1.0, 1.1, and 1.2 V vs. Ag/AgCl) on the homogeneity, adhesion, conductivity, and HOMO energy levels of PPy films was determined. From current-time transients and scanning electron microscopy, it was found that films synthesized at 0.9 and 1.0 V exhibit 3D growth with progressive nucleation (as well as lower homogeneity and higher adhesion to FTO). In contrast, films synthesized at 1.1 and 1.2 V follow 2D growth with instantaneous nucleation. It was also evident that increasing the polymerization potential leads to polymers with lower conductivity and more negative HOMO levels (versus vacuum). These findings are relevant to encourage the use of electro-synthesized PPy in thin film applications that require a high control of material properties.

Keywords: adhesion; electronic properties; electrosynthesis; nucleation and growth; polypyrrole; thin films.

Figure 1

Current–time transients for the electropolymerization of pyrrole on FTO substrates at different anodic potentials. Synthesis solution (0.25 M pyrrole and 0.5 M LiClO₄ in acetonitrile +2% w/w H₂O). The inset shows a magnification of the curve at 1.0 V at a very early time in the electrosynthesis.

Figure 2

Dimensionless plots of maximum currents (Figure 1) and theoretical curves for instantaneous and for progressive nucleation with (A) 3D growth and (B) 2D growth during the electrosynthesis of polypyrrole.

Figure 3

SEM images of PPy synthesized on FTO with electric charge control (mC/cm²). (a–c): polymerization at 0.9 V vs. Ag/AgCl; (d–f): polymerization at 1.2 V vs. Ag/AgCl.

Figure 4

ASTM D 3359 tape adhesion test for PPy films on FTO substrates. The photos show the coatings remaining on the substrate after peeling off the tape. The films were synthesized at different potentials: (a) 0.9 V, (b) 1.0 V, (c) 1.1 V, and (d) 1.2 V. The electric charge supplied was 21 mC/cm² in all cases.

Figure 5

Photos of polypyrrole films synthesized at: (a) 0.9 V, (b) 1.0 V, (c) 1.1 V, and (d) 1.2 V (vs. Ag/AgCl). Synthesis solution (0.25 M pyrrole and 0.5 M LiClO₄ in acetonitrile +2% w/w H₂O). In all cases, the electric charge supplied was 10 mC/cm². The complete photo shows PPy on FTO synthesized at 1.2 V.

Figure 6

UV-Vis spectra of PPy deposited on FTO under control of electric charge and potential during the synthesis.

Figure 7

Cyclic voltammograms of polypyrrole films synthesized at 0.9 V, 1.0 V, 1.1 V, and 1.2 V on FTO substrates. Potential window for (A) oxidation and (B) reduction of PPy. In all cases, the electric charge supplied during polymerization was 42 mC/cm². Electrolyte: 0.1 M NBu₄PF₆ in acetonitrile, scan rate of 20 mV/s.