In situ' formation and voltammetric characterization of polymeric linear [(Mn(H2O)3)2(H2W12O42)]n6n- polyoxometalates in vertically-oriented mesoporous silica thin films

Overview & Synopsis

We present in this paper the 'in situ' preparation of manganese-containing polyoxotungstate [Mn(H2O)3)2(H2W12O42)]n 6n-(1) chains in vertically-oriented mesoporous silica thin films on indium-tin oxide (ITO) electrodes. Taking advantage of a soft sequential method allowing the formation of 1 inside the mesoporous matrix, we succeed in its 'in situ' formation according to two main steps: (i) a first one consisting of the immobilization of [Mn(H2O)6] 2+ in sulfonatefunctionalized silica thin films; and (ii) a second one involving the reaction of the [Mn(H2O)6] 2+ moieties with Na2WO42H2O. The electrochemical characterization of the functionalized materials obtained clearly demonstrates that the content of sulfonate groups in the starting films is a critical parameter to get the expected electroactive [Mn(H2O)3)2(H2W12O42)]n 6n-compound inside the silica nanochannels. The experimentally determined chemical composition of the films performed by X-ray Photoelectron spectroscopy (XPS) is in good agreement with the expected composition of 1 in the case of films obtained with the optimal content of sulfonate functions (i.e., 5-6.5% sulfonate with respect to silica). Energy Dispersive analysis of X-rays coupled with Transmission Electron Microscopy (TEM-EDX) carried out on the cross section of the films show that a uniform composition in terms of Mn/W ratio is obtained after the functionalization process. This confirms the formation of [Mn(H2O)3)2(H2W12O42)]n 6n- uniformly within the whole thickness of the silica film (i.e., 80 nm), explaining also the longrange charge transfer reactions occurring through such insulating silica membrane. Using sulfonated silica films as template, we develop a robust 'in situ' synthesis strategy to assemble 1D linear polyoxotungstates onto ITO electrodes, exhibiting a stable electrochemical response once confined within the silica nanochannels.