The ability of Keggin-type phosphododecamolybdates (PMo 12 O 40 3-) to undergo chemisorption and to form anionic monolayers on platinum surfaces is explored here to produce stable colloidal solutions of polyoxometalate-protected platinum nanoparticles (size, 5-10 nm, as determined by transmission electron microscopy and scanning tunneling microscopy). By dip-coating in the above solution, the particles can readily be assembled on carbon electrode substrates. The layer-by-layer method, which involves alternate exposures to the solutions of PMo 12 O 40 3--stabilized Pt nanoparticles and anilinium cations, has been utilized to grow in a controlled manner hybrid network films in which the negatively charged layers of polyoxometalate-protected Pt nanoparticles are linked, or electrostatically attracted, by ultrathin positively charged polyaniline layers. The phosphomolybdate-decorated Pt nanoparticles (immobilized within ultrathin polyaniline film) are attractive for electrocatalysis: they show promising reactivity toward the electroreduction of oxygen.

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Cobalt(lI) hexacyanoferrate(lll,Il), a system analogous to prussian blue, is a unique electrochromic material: its color is not only dependent on the oxidation potential, but also on the nature of the countercations sorbed from electrolyte during reduction. The electrodeposition of cobalt hexacyanoferrate thin films, their voltammetric behavior and spectroelectrochemical identity are reported here in potassium and sodium electrolytes. The oxidized film is purple brown in both electrolytes, but following reduction, the system turns olive-brown in 1 M KCI and becomes green in 1 M NaCl.Much attention has been devoted lately to the preparation and characterization of thin films of metal hexacyanoferrates because of their interesting alkali-metal cation storage capabilities and possible applications in batteries, sensors, and electrochromic display devices. A number of reports emphasize the stability and long-term reversibility of electrochromic redox reactions using prussian blue, KFe"[F&(CN)6], and its metal (Fe10) substituted analogues.These structures are fairly open, since they typically feature cubic, or almost cubic, rigid frameworks with interstices spacious enough to easily accommodate the compensating countercations (particularly hydrated-K) which are necessary for charge balance during redox reactions.While many prussian blue analogs are known, the spectroelectrochemical properties of only few metal hexacyanoferrates have been well established and documented. Recently, the synthesis and electrochemical characterization of thin films of cobalt(ll) hexacyanoferrate(l 11,11) have been reported.125 These films show reversible electrochemical behavior, and they are stable during prolonged potential cycling in supporting electrolytes containing potassium or sodium. We demonstrate in this paper that cobalt hexacyanoferrate is electrochromic and its spectroelectrochromic responses are different in electrolytes containing potassium and sodium ions. Also, simple solution tests show that X2Co°[Fe"(CN)6] precipitates have different colors depending on a choice of the X alkali metal cation. We think that this concept may be of importance to the construction of multicolor display devices. ExperimentalElectrochemical measurements were done with Bioanalytical Systems Model 100 W analyzer. Quartz disks (diam, 22 mm), covered with Sn-doped indium oxide (resistance, 20 1 /L1), or Au-covered foil (gold thickness, ca. 50 nm) were applied as optically transparent electrodes for spectroelectrochemistry. Visible spectra were taken using Hewlett Packard 8452 diode array spectrophotometer.All chemicals were reagent grade and were used as-obtained from suppliers. Experiments were carried out at room temperature. All potentials are expressed vs. the saturated calomel electrode (SCE) reference. The electrode substrates were pretreated as described earlier.16 The potassium or sodium containing cobalt hexacyanoferrate films were grown by potential cycling (at 100 mV s) for 10 mm from 0.85 to 0 V in freshly prepared modification solutions...