EC-STM

The chemisorption of benzene on a Pd(l 11) Clavilier-bead facet yielded two stable stmctures, as depicted by the EC-STM images in Fig. 8, that depended upon the applied potential within the 

Stractural models of the Pd(l 1 l)-c(2V3x3)-iect-C6H6 adlattice are proposed in Figs. 9(A) and 9(B). In Fig. 9(A), each benzene molecule is spread out over four metal-surface atoms but centered on two-fold bridging sites this helps rationalize the dumb-bell shape of the aromatic molecules. To account for the pseudo-triangular shape of the molecular image, the model in Fig. 9(B) is 

The structural model of the Pd(lll)-(3x3)-C6H6 adlattice is shown in Fig. 9(C), where each benzene molecule is situated at a three-fold hollow site. The image for such a model would be three spots arranged as an equilateral triangle, an expectation borne out by the results shown by the EC-STM image in Fig. 8(B). It is important to note that there ate two unoccupied three-fold hollow sites inside the molecular unit cell these ate of sufficient size to hold at least one water molecule. The existence of coadsorbed water would account for the extra (smaller and less bright) spots observed in Fig. 8(B). 

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We have found new CO-tolerant catalysts by alloying Pt with a second, nonprecious, metal (Pt-Fe, Pt-Co, Pt-Ni, etc.) [Fujino, 1996 Watanabe et al., 1999 Igarashi et al., 2001]. In this section, we demonstrate the properties of these new alloy catalysts together with Pt-Ru alloy, based on voltammetric measurements, electrochemical quartz crystal microbalance (EQCM), electrochemical scanning tunneling microscopy (EC-STM), in situ Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). 

BASIL CIS CV CVD DSSC ECALE EC-STM EDX, EDS, EDAX EIS EMF EQCM FAB MS FFG-NMR Biphasic Acid Scavenging Utilizing Ionic Liquids Copper-indium-selenide Cyclic Voltammetry Chemical Vapor Deposition Dye Sensitized Solar Cell Electrochemical Atomic Layer Epitaxy Electrochemical in situ scanning tunnelling microscopy Energy Dispersive X-ray analysis Electrochemical Impedance Spectroscopy Electromotive Force Electrochemical Quarz Crystal Microbalance Fast atom bombardment mass spectroscopy Fixed Field Gradient Nuclear Magnetic Resonance... 

A detailed review and discussion of EC-STM studies can be found in Chapter 9. 

The cleanliness and single crystallinity of electrode surfaces are not assumed even if the preparative steps outlined above are followed. The verification or identification of initial, intermediate, and final interfacial stmctures and compositions is an essential ingredient in our studies. The interfacial characterization methods employed to date have been conveniently classified in terms of whether they are conducted under reaction conditions (in situ) or outside the electrochemical cell (ex situ). In situ methods here consisted of cychc voltammetry (CV), EC-STM and DBMS. Ex situ methods included LEED, AES, and HREELS. 

Consequently, STM quickly became a pillar among the many powerful techitiques employed in surface science. While such advances may tempt a few to regard EC-STM as the elixir of the myriad problems in interfacial electrochemical science, the enthusiasm has to be tempered by the realization that tmmeling microscopy is unable to probe other fundamental issues such as surface energetics, composition, and electronic structure EC-STM will always require additional surface characterization techniques if a more complete understanding of complex heterogeneous processes is desired. 

Figure 4. EC-STM configuration. (A) Electt onic (bipo-tentiostat) circuiby. (B) Four-elecb ode experimental set-up.

EC-STM was carried out with a Nanoscope E microscope (Veeco Metrology, Santa Barbara, CA) equipped with a custom-built Kel-F electrochemical cell. The tunneling tips were prepared by electrochemically etching a tungsten wire, 0.25-mm in diameter, in 1 M KOH at 15 VAC. The attaimnent of atomically sharp STM tips may be confirmed with a microscope of at least 1000-fold magnification. The choice tips were then coated with transpa-... 

Figure 7. High-resolution EC-STM images of a Pd(l 11) facet on a Clavilier bead. Bias voltage 60 mV tunneling current 20 nA. (A) In an environment of high-purity argon. (B) In 0.01 M H2SO4 at potentials in the double-layer region.

Unfortnnately, no distinct LEED patterns could be generated from the adlayer of benzene chemisorbed on a Pd(lll) single-crystal electrode hence, meaningful results were obtained only from the HREELS experiments. Figure 10(A) shows the HREEL spectmm of benzene on Pd(lll) formed and emersed at 0.5 V based upon the above EC-STM results, a Pd(lll)-c(2V3x3)-rect-CeHe adlayer ( CgUg = 0.17) was assumed to be present on the surface. Except for the peak at 1717 cm, which is due to adventitious CO, all the peaks, when compared to published vibrational spectra of unadsorbed and adsorbed " benzene, are attributable to chemisorbed starting material. Unique to the surface-immobilized aromatic are the peaks labeled (a), 265 cm, and (b), 515 cm", which arise from direct metal-adsorbate (Pd-C) chemical bonds. Peaks (c) and (d) are out-of-plane C-H bends, y(C-H) peaks (e) and (i) are in-plane stretches, v(C-H), whereas peaks (f) and (g) are both inplane bends, 5(C-H). 

Such indications are in consonance with the EC-STM results in that ... 

Figure 16 shows unfiltered high-resolution EC-STM images of the ordered Q adlattice acquired at 0.5 V. In Fig. 16(A), a molecular array of hexagonal symmetry in registry with the Pd(l 11)...

Figure 16. Unfiltered high-resolution EC-STM image of a (3x3) ben-zoquinone adlayer at 0.5 V (RHE). (A) Normal view. (B) Enlarged view. Bias voltage 100 mV tunneling current 20 nA.

Two stmctural models for the (3x3)-Q adlattice, each consistent with the present results, are depicted in Fig. 17. In the first, the O atoms occupy atop sites in the second, the O atoms are located at two-fold bridge sites. Unfortunately, the EC-STM images obtained in this study are unable to differentiate between the two possible models. In both stmctnres, the center of the quinonoid ring is located on a two-fold site. 

The anodic oxidation of chemisorbed Q also appears to follow the above reaction pathway. Evidence is provided by HREELS spectra (Fig. 22) obtained when the potentials are made progressively more positive. It can be seen that the spectral features for unimpaired Q persist, but with diminished intensities, at anodic-oxidation potentials. The new peaks above 3000 cm are due to the formation of hydrated surface oxides. Evidently, a small fraction of chemisorbed Q is also able to resist anodic oxidation Unfortunately, no acceptable EC-STM images could be obtained due... 

It bears repetition that, based upon the EC-STM image in Fig. 21, the hydrogenative (or oxidative) attack of the adsorbed Q by the surface hydrides (or hydroxyls) occurs at the periphery of the ordered organic adlattices. 

Endres F, Schweizer A (2000) The electrodeposition of copper on Au(lll) and on HOPG from the 66/34 mol% aluminium chloride/l-butyl-3-methylimidazolium chloride room temperature molten salt an EC-STM study. Phys Chem Chem Phys 2 5455-5460... 

By STM, most limitations of the TEM method may be overcome. This holds, in particular, for the electrochemical STM (EC-STM) technique that allows a real-time in situ study of electrodissolution processes at a lateral resolution at the nanometer scale or better, with the substrate and the tip controlled potentiostatically or gal-vanostatically during imaging (see Chapter 3.1 in Volume 3). Moreover, atomic height steps and topographic changes in the subnanometer range can be resolved [138). On the other hand, chemical information is... 

Au—Ag alloys Oppenheim and coworkers monitored corrosion of Ag—Au (111) surfaces in 0.1 M HCIO4 by EC-STM with monolayer depth resolution. By choosing low-Ag content alloys with compositions well below the parting limit, the preferential dissolution of Ag was confined to the first few atomic layers. Under these... 

Cu—Au alloys Chen and coworkers studied Cu dissolution from gold-rich, disordered AU3CU films with 111 surfaces in 0.6 M NaCl 4- 0.01 M HCI by EC-STM. In... 

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