We have investigated morphological changes in the galvanostatic deposition of polyaniline. From the chronopotentiometric curve, we have identified three distinct regions related to the different growth process. At first, after the nucleation process and initial polymerization on the bare ITO surface, 2-D horizontal growth occurs predominantly and 1-D vertical growth occurs at the same time. And then, the dominant 1-D vertical growth process on the first layer extends the fibrous structure, which provides significantly improved charge carrier mobility. Finally, there is an only 1-D vertical growth process, resulting in an interconnected network of a fibrous structure. After reaching a stationary potential for the remaining deposition time, further growth produces a fibrous structure with some aggregate, despite the competition between the polymerization and degradation process. The aggregation-induced structural disorder reduces the charge carrier mobility, which is closely related to poor conductivity/high sheet resistance.

정전류 증착 방법에 의한 폴리아닐린 성장 과정에서 모폴로지 변화를 확인하였다. Chronopotentiometry 곡선에서 서로 다른 성장 과정과 관련된 세 가지 영역을 확인하였다. 초기에는, 핵 생성과 ITO 표면에서의 초기 중합후 2-D 수평 성장이 주도적이었고, 1-D 수직 성장도 동시에 일어났다. 그 다음에, 첫 번째 층 위에서 1-D 수직 성장이 지배적으로 일어나서 섬유상 구조가 확장되었고, 결과적으로 전하 캐리어 이동도가 향상되었다. 마지막에는, 1-D 수직 성장만 일어났으므로 섬유상 구조의 상호 연결된 네트워크를 형성하였다. 중합과 분해 과정간의 경쟁에도 불구하고, 고정 전위에 도달한 후에도 고분자 성장이 진행되었으며, 약간의 응집체가 포함된 섬유상 구조가 생성되었다. 응집체에 의한 구조적 무질서는 전하 캐리어 이동도를 감소시켰으며, 이는 낮은 전도성과 높은 면저항에 밀접한 관련이 있었다.

Keywords: polyaniline, galvanostatic, electrodeposition, morphology, growth process

Conducting polymers (CPs) are polymeric materials that display high electrical conductivity, good electrochemical activity, unique optical properties, and biocompatibility.¹ The greatest advantage of CPs is that their chemical, electrical, and physical properties can be tailored to the specific needs of a given application. ² CPs can be polymerized by both chemical and electrochemical methods. Chemical methods are suitable when bulk quantities of the polymers are necessary, and these methods have so far been dominantly applied for commercial applications. Nevertheless, the most widely used technique for applications such as polymer-based electronics and electrochemical devices is electrochemical anodic oxidation. Electrochemical procedures have several advantages; they avoid usage of oxidants since CPs are obtained at the anode upon application of the positive potential, leading to increased purity. Also, because the polymer is deposited on the electrode, further electrochemical characterization is facilitated. In addition, electrochemical methods are faster than chemical methods for the polymerization of CPs (a few minutes versus a few hours).^1,3 Polyaniline (PAni) is unique among CPs because of its simple non-redox doping/dedoping chemistry based on acid/base reactions. PAni is known as a mixed oxidation state polymer composed of reduced benzenoid (amine N) units and oxidized quinoid (imine N) units. It exists mainly in three welldefined oxidation states; namely, the fully reduced (leucoemeraldine), the half oxidized (emeraldine), and the fully oxidized (pernigraniline) states, with a virtually infinite number of possible oxidation states existing in between these.^4,5 Protonation of the polymer, also known as doping, is the process by which PAni becomes electrically conductive. Insulating emeraldine base (EB) can be protonated to give an emeraldine salt (ES), also called the radical cation (EB + HCl → ES⁺ + Cl-), and the polymer formed by radical cations and incorporated counterions can present a metallic character.^5,6 Since the electrochemical properties depend on the polymer morphology, it is of interest to observe the morphological changes related to the growth process of CPs in the electrodeposition.^4,7-9 However, there have been only few systematic studies on the galvanostatic deposition of PAni.¹⁰ In our previous study, the morphological changes related to the growth process of PAni was visualized in both two-dimensional (2-D) and three-dimensional (3-D) images in the potentiostatic deposition.¹¹ Because current control is simple and can cause the reaction to proceed at a constant rate, the galvanostatic technique allows control of the film thickness by adjusting the deposition time (t). In this study, morphological changes related to the growth process of PAni in the galvanostatic deposition and their electrochemical properties were investigated based on the results from the chronopotentiometric (CP) curve, Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) absorption spectroscopy, scanning electron microscopy (SEM), and four-point probe electrical measurements.