Molecular Adsorbates

The results shown above indicate a rich interplay between adsorbates and the electrode surface. Early efforts directed at imaging organic molecules on surfaces were hampered by ill-defined surface chemistry and agglomeration of adsorbates. However, within the past two years, several groups, many of which are represented in this volume, have reported images of a variety of adsorbates on electrode surfaces. Because the diversity of organic and inorganic adsorbates is so large and because molecular transformations are crucial to devices as important as sensors and fuel cells, these studies will continue to have importance for some time.

We used the STM to monitor the electrooxidation of phenoxide to oligophenol on Au(lll) in alkaline solutions [16]. Prior to oxidation, phenol associates as phenoxide to Au(l 11) in a (V3 x V3)R30° structure with the molecule oriented end-on through the O atom (Fig. 5(a), 5(b)). After oxidation, a disordered, close-packed array of oxidation products consisting of monomers, dimers, trimers, and a few higher oligomers is observed (Fig. 5(c)). The oxidation products are oriented with the ring roughly parallel to the electrode surface. Complementary IR studies conducted prior to oxidation confirmed the nonparallel orientation of the ring relative to the electrode surface and showed that the orientation of the molecule does not change with potential through the double-layer region of the voltammetry. These results provide structural insight into one of the transformations that organic molecules can undergo on electrode surfaces.

We have also examined adsorption of inorganic molecules on electrode surfaces [17]. Figure 6 shows the c(5 x 3V3) structure formed by the self-assembly of a-dodecatungstosilicate, a-Si^O^4". These molecules spontaneously form adherent, passivating, monolayer-thick, ordered molecular arrays on Ag(lll) surfaces upon immersion of the surfaces into an aqueous solution of the acid. These monolayers represent the first example of inorganic monolayer self-assembly and provide a way to derivatize surfaces with inorganic functionality. Polyoxometalates are known to function not only as superacids in cases like a-^SiW^O« but also as ion exchangers, corrosion inhibitors, electron transfer reagents, catalysts, and photochemical oxidants. Since they can accommodate a wide range of organic, organometallic, and inorganic functional groups, we anticipate widespread interest in these and other classes of inorganic molecules as self-assembled monolayers.

Fig. 5. (a) 4.5nm x 4.5nm STM image of the phenoxide structure on Au(lll) at +50 mV vs. NHE. Spacing is 0.5 ± 0.02 nm and corresponds to a (a/3 x V3)R30° overlayer. 7tip = 2 nA, £t,p = 28 mV. (b) Model of the phenoxide overlayer showing the underlying Au(lll) lattice. Open circles represent Au, while filled ovals represent phenoxide. (c) 10 nm x 20 nm STM image of oligomers on Au(lll) at +340 mV vs. NHE, following the electrooxidation of phenoxide by sweeping potential to +600 mV. Molecule width is 0.72+0.1 nm and length varies from 0.8 to 2.5 nm. /„p = 2 nA, £tip = 355 mV.

Fig. 5. (a) 4.5nm x 4.5nm STM image of the phenoxide structure on Au(lll) at +50 mV vs. NHE. Spacing is 0.5 ± 0.02 nm and corresponds to a (a/3 x V3)R30° overlayer. 7tip = 2 nA, £t,p = 28 mV. (b) Model of the phenoxide overlayer showing the underlying Au(lll) lattice. Open circles represent Au, while filled ovals represent phenoxide. (c) 10 nm x 20 nm STM image of oligomers on Au(lll) at +340 mV vs. NHE, following the electrooxidation of phenoxide by sweeping potential to +600 mV. Molecule width is 0.72+0.1 nm and length varies from 0.8 to 2.5 nm. /„p = 2 nA, £tip = 355 mV.

Fig. 6. 28 nm x 28 nm STM image of a monolayer of a-SiWi2O404" on an Ag( 111) surface in 0.1 M HC104. /up = 2.7 nA, £t,p = 100 mV.

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