# The First Stages of Chemical and Electrochemical Aniline Oxidation--Spectroscopic Comparative Study
## 1 Introduction  
Polyaniline (PANI) is one of the most studied conducting polymers. It has a wide range of applications, including wastewater processing [1, 2], catalysis [2], sensing [3, 4, 5, 6], biomedical [5, 7, 8], optical [9] and electrochemical [9, 10, 11, 12, 13] applications. Many of these areas require PANI in the form of a thin film. PANI and aniline oligomer (AO) thin films can be prepared by in-situ chemical [14, 15, 16] or electrochemical [15, 16] polymerization. In both cases the type of formed product depends upon the initial pH [17, 18]: In an acidic medium, the linear PANI chain is formed with aniline units coupled exclusively in the _para_ positions [16, 17, 19, 20, 21, 22]. On the other hand, in neutral or basic media, _ortho_-coupling becomes possible in addition to _para_-coupling, which leads to a formation of a mixture of various AO structures [19, 20, 22, 23]. The first stages of aniline oxidation influence the morphology of the obtained PANI powders and films [24]. At low pH, the granular morphology of PANI is observed on PANI powder samples [24, 17, 20], while the films display the brush-like ordering of the PANI chains [15]. The chemical oxidation of PANI in a neutral medium or a medium of moderate acidity leads to the formation of nanotubes and nanorods. There are several theories explaining the mechanism of the linear structure formation [24, 25, 17, 20]. In basic media, hollow microspheres of AOs are formed [24]. However, in this case, the morphology is caused more likely by the poor solubility of aniline in the basic medium than by the properties of AOs. Specific AO species based on the \(N\)-phenylphenazinium cation were believed to be responsible for the tubular morphology formation and film adherence to the substrates [26, 27, 28, 17, 20]. However, aniline dimer was proven to be the adsorbing species in electrochemical aniline oxidation at anypH [18]. The PANI film obtained by the in-situ chemical oxidation of aniline at very low pH also seems to contain no \(N\)-phenylphenazinium cation structures. However, as only finished film was studied in this case, Raman spectra without surface-enhanced Raman scattering (SERS) enhancement brings restricted information about the first products [29]. These findings lead to the question whether the \(N\)-phenylphenazinium cation-based AOs play any role in film formation within in-situ chemical aniline oxidation. If the absence of the \(N\)-phenylphenazinium cation in the PANI films prepared by in-situ chemical polymerization is proven, the in-situ chemical PANI film preparation must then be equal to the electrochemical method from the PANI-chain linearity point of view. Beside this, there are other relevant factors in the chemical and electrochemical preparation of PANI films that influence the applicability of these methods: (1) Electrochemical PANI film preparation requires a conducting substrate that serves as a working electrode in the polymerization cell. The in-situ chemical polymerization of aniline operates at any surface, including e.g., glass, textiles, the solution-air interface, etc. [16; 14]. (2) In the case of the electrochemical method, PANI is only formed at the electrode, while during the chemical method, PANI also forms as a precipitate in the bulk solution, increasing reactant consumption for a unit of surface area of formed PANI film [14]. Although chemically prepared AOs have already been studied by various methods [27; 28; 30], a SERS study of the reaction kinetics has not been performed yet. The presence of \(N\)-phenylphenazinium cation-based AOs during chemical oxidation has been proven only by its detection in fully formed film [28]. In the case of the electrochemical oxidation of aniline, only studies in low pH and dimerization are available [31; 32; 15; 22; 33]. Studies of electrochemical aniline oxidation at neutral pH are also limited simply to the detection of \(N\)-phenylphenazinium cation [18; 34].