No abstract

A detailed investigation of the acid doping behavior of polyaniline has led to a robust and reproducible procedure for controlled adjustment of the redox state of dry polyaniline films. The initial step in this procedure is the casting of PANI films from formic acid. The subsequent exchange of the trapped formic acid for other primary dopants obtained from mono-and polyprotic acids (e.g., CH 3 COO -, BF 4 -, HSO 4 -, SO 4 2-, H 2 PO 4 -, and HPO 4 2-) is demonstrated. The voltammetric and the spectroscopic behavior of the PANI doped with different anions indicate that both the protons and the anions of dopant acids influence the structure and redox properties of the polymer. The redox state of PANI doped with homologous series of chloroacetic and carboxylic acids correlates with the pK a of the dopant acid. These results show that it is possible to prepare the polymer with a desired oxidation state according to the pK a of the dopant acid of a given homologous series. The exchange of the formic acid for both stronger and weaker doping acid can be repeatedly accomplished by electrochemical cycling.

The oxidative adsorption of n-alkanethiolates (C n c H2 n c +1S-) at Ag(111) in aqueous and methanolic solutions containing 0.5 M NaOH has been investigated by cyclic voltammetry and in-situ surface-enhanced Raman spectroscopy (SERS). Reversible adsorption of C n c H2 n c +1S- under active potential control in solutions containing millimolar concentrations of C n c H2 n c +1S- provides a means to control the deposition of n-alkanethiolate monolayers, and allows for the direct voltammetric measurement of the free energy of monolayer formation. Oxidative adsorption of short chain alkanethiolates (n c ≤ 6) in aqueous 0.5 M NaOH is characterized by two voltammetric waves, demonstrating that monolayer formation involves at least two energetically distinct chemical steps. The first voltammetric wave corresponds to the reversible and rapid adsorption of C n c H2 n c +1S- at submonolayer coverages. The redox potential of this wave ( = −1.19 ± 0.02 V vs Ag/AgCl) is independent of n c , suggesting that the interactions between adsorbed molecules are minimal at low surface coverages and that the energetics of adsorption are determined primarily by the strength of the Ag(111)−S bond. A second voltammetric wave is observed at more positive electrode potentials, corresponding to further adsorption of C n c H2 n c +1S- to yield a complete monolayer (Γmax ∼ 7.7 × 10-10 mol/cm2). The redox potential for the second wave, , is a function of chain length, shifting to more negative potentials with increasing n c . The dependence of on n c reflects the influence of hydrophobic interactions and intermolecular forces between the hydrocarbon chains. For n c > 6, shifts to potentials negative of , and the two voltammetric waves merge into a single wave, suggesting that the more structurally ordered monolayer is energetically favored for longer chain lengths (i.e., n c > 6). In-situ SERS is used to establish the potential-dependent adsorption isotherm of n-hexanethiolate adsorbed on roughened Ag electrodes. The potential dependence of the SERS intensities of the trans and gauche ν(C−S) stretching modes provides a means to monitor the structural ordering of the alkanethiolate monolayer during electrochemical deposition. The electrochemical data are used to separate the total adsorption free energy (ΔG ads) into the individual contributions associated with the formation of the Ag(111)−S bond (−22.8 and −16.6 kcal/mol for the low- and high-density structures, respectively) and that associated with hydrophobic interactions and intermolecular forces between hydrocarbon chains (−1.02 ± 0.04 kcal/mol per n c ). Voltammetric data and ΔG ads values are also reported for the adsorption of C n c H2 n c +1S- (2 ≤ n c ≤ 16) onto Ag(111) from basic methanolic solutions.

The formation and reactivity of sulfur adlayers on single-crystal Ag electrodes ((111), (110), and (100) orientations) in aqueous solutions (pH = 13) containing HS- are reported. Oxidative adsorption of HS- (Ag + HS- → AgSH + e-) occurs on all three low-index surfaces at potentials ranging between −0.5 and −0.7 V of the thermodynamic value for bulk Ag2S formation. Voltammetric and electrochemical quartz crystal measurements demonstrate that the resulting AgSH adlayer undergoes a second one-electron oxidation (AgSH + Ag + OH- → Ag2S + H2O + e-) at the (111) and (110) surfaces prior to bulk Ag2S formation, yielding an underpotential deposited Ag2S adlayer (surface coverages (S/Ag):  θAg(111) = 0.46 ± 0.02 and θAg(110) = 0.54 ± 0.03). In contrast, the AgSH adlayer on Ag(100) is chemically inert prior to bulk Ag2S formation. Structural models indicate that the formation of a nearly stoichiometric Ag2S adlayer (i.e., θ ∼ 0.5) is feasible on the (111) and (110) surfaces without significant reconstruction of the outermost atomic layers of the substrate, but not on the (100) surface. The results suggest that the formation of a Ag2S monolayer is allowed only when the number density of S and Ag atoms at the interface is nearly coincident with the reaction stoichiometry.