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 dep osition of their aqueous suspensions on highly oriented pyrolytic graphite (HOPG), using scanning tunneling microscopy (STM) and tunneling spectroscop y (TS). The images of Ag3PMo12O40 and Ag3PW12O40 achieved in air clearly de monstrate the formation of well-ordered, self-assembled, two-dimensional ar rays. The lattice periodicities obtained from these images were 12-13 Angst rom, indicating the deposition of the intact molecules. In addition, well-o rdered arrays of much smaller particles (7-9 Angstrom) could be found when the pH of the solution was increased (pH > 2). clearly indicating the fragm entation of the parent Keggin structures. Tunneling spectra of Ag3PMo12O40 show characteristic negative differential resistance (NDR) behavior. The su bstitution of W for Mo (Ag3PW12O40) resulted in a shift of the NDR peak val ue from -0.62 V to -0.78 V, in very good agreement with the lesser reducibi lity of the tungsten salts and the correlation previously demonstrated betw een NDR voltage and Keggin redox properties. The exchange of small amount o f cesium (Ag:Cs = 9:1) in the silver phosphomolybdate salt did not apprecia bly alter the packing in the ordered AgCs(9:1)(3)PMo12O40 monolayers, but d id produce a measurable shift of the NDR values to -0.68 V, showing a poten tial 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. (C) 2000 Elsevier Science B.V. Al l rights reserved.