Electrochemical charging and photochemical discharging in heterodeposited polymernanosheet assembly

Abstract

We demonstrate electrochemical charge storage and photochemical release of the charge in heterodeposited polymer nanosheets assemblies. Two kinds of redox polymer nanosheets, those containing ruthenium dipyridyl complex (p(DDA/Rubpy₃²⁺)) and ferrocene derivative (p(DDA/Fc)), respectively were heterodeposited onto an ITO electrode using the Langmuir–Blodgett technique. The heterodeposited assembly forms a clear bilayer structure in which p(DDA/Rubpy₃²⁺) is located in an inner layer and p(DDA/Fc) in an outer layer. The electrochemical property of the heterodeposited assembly was studied by cyclic voltammetry measurement and compared with their single component film. The cyclic voltammogram of p(DDA/Rubpy₃²⁺) monolayer film showed a redox peak of Rubpy₃^2+/3+ reaction around 1.1 V (vs.Ag/AgCl), and that of p(DDA/Fc) monolayer showed a redox peak of Fc^0/+1 around 0.4 V. On the other hand, the cyclic voltammogram of the bilayer film showed no oxidation current around 0.4 V, and large oxidation currents were observed from 0.9 V. In the heterodeposited assembly, the p(DDA/Fc) layer was separated from the electrode about 5 nm by the inner p(DDA/Rubpy₃²⁺) layers. Therefore the oxidation of Fc moieties occurred by mediation of the inner p(DDA/Rubpy₃²⁺) layers, which started around 0.9 V. On the other hand, reduction of Fc⁺ by scanning the potential to cathodic direction was not observed, because reduction of Fc⁺ is thermodynamically unable for Rubpy₃²⁺. As the results, the outer p(DDA/Fc) layers store charges as Fc⁺ by one potential cycle (electrochemical charging). Moreover, we demonstrate the Fc⁺ reduction by using the photoinduced electron transfer from an excited ruthenium complex to Fc⁺ (photochemical discharging). The repeatability of the charging-discharging process was about 5 times. To improve the repeatability, Prussian blue (PB) nanoparticles were used instead of the p(DDA/Fc) layers. PB nanoparticles were assembled on a solid substrate using a cationic polymer nanosheet, poly(N-dodecylacrylamide-co- N-(2-(2-(2-aminoethoxy)ethoxy)ethyl)acrylamide) (p(DDA/DONH)) as a template. A hybrid heterodeposited assembly was prepared using p(DDA/Rubpy₃²⁺) as an inner layer and p(DDA/DONH) templated PB as an outer layer. The hybrid heterodeposited assembly stored and released charges by a similar process with the p(DDA/Rubpy₃²⁺)-p(DDA/Fc) heterodeposited polymer nanosheets assembly. The repeatability of the charging-discharging process in the hybrid heterodeposited assembly was increased compared to that in the p(DDA/Rubpy₃²⁺)-p(DDA/Fc) heterodeposited assembly due to the high stability of the oxidized form of PB. The high repeatability of the hybrid heterodeposited assembly indicates that the assembly is applicable to molecular-memory and battery.