STM and TS investigations of silver polyoxometalate monolayers: model compounds and potential multifunctional oxidation catalysts

Different silver–heteropolyanion (Ag–HPA) salts have been studied after deposition of their aqueous suspensions on highly oriented pyrolytic graphite (HOPG), using scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). The images of Ag3PMo12O40 and Ag3PW12O40 achieved in air clearly demonstrate the formation of well-ordered, self-assembled, two-dimensional arrays. The lattice periodicities obtained from these images were 12–13 Å, indicating the deposition of the intact molecules. In addition, well-ordered arrays of much smaller particles (7–9 Å) could be found when the pH of the solution was increased (pH>2), clearly indicating the fragmentation of the parent Keggin structures. Tunneling spectra of Ag3PMo12O40 show characteristic negative differential resistance (NDR) behavior. The substitution of W for Mo (Ag3PW12O40) resulted in a shift of the NDR peak value from −0.62 V to −0.78 V, in very good agreement with the lesser reducibility of the tungsten salts and the correlation previously demonstrated between NDR voltage and Keggin redox properties. The exchange of small amount of cesium (Ag:Cs=9:1) in the silver phosphomolybdate salt did not appreciably alter the packing in the ordered AgCs(9:1)3PMo12O40 monolayers, but did produce a measurable shift of the NDR values to −0.68 V, showing a potential change of the redox properties of these species. In this work we show that the redox environment of silver can be controlled with high definition by the heteropolyanion monolayer matrix.