Hybrid molecular films of gemini amphiphiles and Keggin-type polyoxometalates: effect of the spacer length on the electrochemical properties

Abstract

New organic–inorganic hybrid molecular films, consisting of a Keggin-type polyoxometalate PW₁₂ and a series of gemini amphiphiles with various lengths of the flexible spacers, have been fabricated using the Langmuir–Blodgett (LB) technique and the electrochemical properties of the hybrid films were investigated. When the positively charged gemini amphiphiles were spread on the aqueous subphase containing PW₁₂, hybrid monolayers were formed in situ at the air/water interface through electrostatic interaction. The hybrid monolayers could be subsequently transferred onto solid substrates and the resulted multilayer films were characterized by UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Marked distinctions between the compression isotherms and different topographies of the transferred films were observed depending on the spacer length of the gemini amphiphiles. The electrochemical properties of the hybrid films were investigated by cyclic voltammetry (CV) and the second reduction peaks were found to shift gradually to negative with increasing spacer length. When the hybrid films were deposited onto glassy carbon electrode surfaces, the modified electrodes exhibited high electrocatalytic response to the reduction of NO₂⁻. The effect of the spacer length of the gemini amphiphiles on the electrocatalytic efficiency of the hybrid films was also observed, and the greatest efficiency was achieved for the hybrid film composed of PW₁₂ and the gemini with two methylenes as spacer.