Electrodeposition of layered manganese oxide nanocomposites intercalated with strong and weak polyelectrolytes

Multilayered manganese oxide nanocomposites intercalated with strong (poly(diallyldimethylammonium) chloride, PDDA) and weak (poly(allylamine hydrochloride), PAH) polyelectrolytes can be produced on polycrystalline platinum electrode in a thin film form by a simple, one-step electrochemical route. The process involves a potentiostatic oxidation of aqueous Mn2+ ions at around +1.0 V (vs Ag/AgCl) in the presence of polyelectrolytes. Fully charged PDDA polycations are accommodated tightly in the interlayer space by electrostatic interaction with negative charges on the manganese oxide layers, leading to an interlayer distance of 0.97 nm. The layered film prepared with PAH has a larger polymer content (PAH/Mn molar ratio of 0.98) than that (PDDA/Mn molar ratio of 0.43) made with PDDA because of the smaller charging degree of PAH, exhibiting a larger interlayer distance (1.19 nm). The interlayer PAH contains neutral (-NH2) and positively charged (-NH3+) amine groups, and the -NH3+ groups are associated with Cl- (to generate -NH3+Cl- ion pairs) as well as the negatively charged manganese oxide layers. Both polyelectrolytes once incorporated were not ion exchanged with small cations in solution. The layered structure of PDDA/MnOx was collapsed during the reduction process in a KCl electrolyte solution, accompanying an expansion of the interlayer as a result of incorporation of K+ ions for charge neutrality. On the contrary, the layered PAR/MnOx film showed a good electrochemical response due to the redox reaction of Mn3+/Mn4+ couple with no change in the structure. X-ray photoelectron spectroscopy revealed that, in this case, excess negative charges generated on the manganese oxide layers upon reduction can be balanced by the protons being released from the -NH3+Cl- sites in the interlayer PAH; the Cl- anions becoming unnecessary are inevitably excluded from the interlayer, and vice versa upon oxidation.