Electrochemical growth and characterization of polyoxometalate-containing monolayers and multilayers on alkanethiol monolayers self-assembled on goldelectrodes

A general strategy has been developed for fabrication of ultrathin monolaye r and multilayer composite films composed of nearly all kinds of polyoxomet alates (POMs), including isopolyanions (IPAs), and heteropolyanions (HPAs). It involves stepwise adsorption between the anionic POMs and a cationic po lymer on alkanethiol (cysteamine and 3-mercaptopropionic acid) self-assembl ed monolayers (SAMs) based on electrostatic interaction. Here a Keggin-type HPA SiMo11VO405- was chosen as a main representative to elucidate, in deta il, the fabrication and characterization of the as-prepared composite films . A novel electrochemical growth method we developed for film formation inv olves cyclic potential sweeps over a suitable potential range in modifier s olutions. It was comparatively studied with a commonly used method of immer sion growth, i.e., alternately dipping a substrate into modifier solutions. Growth processes and structural characteristics of the composite films are characterized in detail by cyclic voltammetry, UV-vis spectroscopy (UV-vis ), X-ray photoelectron spectroscopy (XPS), micro-Fourier transform infrared reflection-absorption spectroscopy (FTIR-RA), and electrochemical quartz c rystal microbalance (EQCM). The electrochemical growth is proven to be more advantageous than the immersion growth. The composite films exhibit well-d efined surface waves characteristic of the HPAs' redox reactions. In additi on, the composite films by the electrochemical growth show a uniform struct ure and an excellent stability. Ion motions accompanying the redox processe s of SiMo11VO405- in multilayer films are examined by in situ time-resolved EQCM and some results are first reported. The strategy used here has been successfully popularized to IPAs as well as other HPAs no matter what struc ture and composition they have.