In-situ STM

Experimentally, different structure- and surface-sensitive techniques such as in situ scanning tunnelling microscopy (STM), in situ X-ray diffraction (XRD), transition electron microscopy (TEM), and in situ infrared (IR) spectroscopy have been... 

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). 

Figure 22. (a) High resolution STM in situ image recorded on Ir (111) electrode covered with HSO4 anions showing ( 3x 7) structure, (b) structural model for HSO4 anions and chains of IliO ions or H2O molecules. Reprinted from Ref. 215, Copyright (2001) with permission from Elsevier. 

Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active... 

The importance of low pressures has already been stressed as a criterion for surface science studies. However, it is also a limitation because real-world phenomena do not occur in a controlled vacuum. Instead, they occur at atmospheric pressures or higher, often at elevated temperatures, and in conditions of humidity or even contamination. Hence, a major tlmist in surface science has been to modify existmg techniques and equipment to pemiit detailed surface analysis under conditions that are less than ideal. The scamiing tunnelling microscope (STM) is a recent addition to the surface science arsenal and has the capability of providing atomic-scale infomiation at ambient pressures and elevated temperatures. Incredible insight into the nature of surface reactions has been achieved by means of the STM and other in situ teclmiques. 

Surface science studies of corrosion phenomena are excellent examples of in situ characterization of surface reactions. In particular, the investigation of corrosion reactions with STM is promising because not only can it be used to study solid-gas interfaces, but also solid-liquid interfaces. 

The characterization of surfaces undergoing corrosion phenomena at liquid-solid and gas-solid interfaces remains a challenging task. The use of STM for in situ studies of corrosion reactions will continue to shape the atomic-level understanding of such surface reactions. 

Two notable in situ teelmiques are at the forefront of the surfaee seienee of eatalysis STM and SFG. STM is used to investigate surfaee stmetnres while SFG is used to investigate surfaee reaetion intemiediates. The... 

SECM is a scaiming-probe teclmiqiie introduced by Bard et aJ in 1989 [49, and M ] based on previous studies by the same group on in situ STM [ ] and simultaneous work by Engstrom et aJ [53 and M], who were the first to show that an amperometric microelectrode could be used as a local probe to map the concentration profile of a larger active electrode. SECM may be envisaged as a chemical microscope based on faradic current changes as a microelectrode is moved across a surface of a sample. It has proved iisefiil for... 

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

Figure C2.10.2. Cyclic voltammogram of Cu(l 11)/10 mM HCl and in situ measured STM micrographs revealing tire bare Cu(l 1 l)surface (-1.05 V, left) and tire (V3 x A/3)R30°-Cladsorbate superstmcture (-0.6 V, right) (from [39]).

The presented examples clearly demonstrate tliat a combination of several different teclmiques is urgently recommended for a complete characterization of tire chemical composition and tire atomic stmcture of electrode surfaces and a reliable interiDretation of tire related results. Stmcture sensitive metliods should be combined witli spectroscopic and electrochemical teclmiques. Besides in situ techniques such as SXS, XAS and STM or AFM, ex situ vacuum teclmiques have proven tlieir significance for tlie investigation of tlie electrode/electrolyte interface. 

Yamada T, Ogaki K, Okubo S and itaya K 1996 Continuous variation of iodine adiattices on Ag(111) eiectrodes In situ STM and ex situ LEED studies 1996 Surf. Sc/. 369 321-35... 

Carnal D, Oden P I, Muller U, Schmidt E and Siegenthaler FI 1995 In situ STM investigation of T1 and Pb underpotential deposition on chemically polished Ag(111) electrodes E/ecfroc/r/m. Acta 40 1223-35... 

Batina N, Will T and Kolb D M 1992 Study of the initial stages of copper deposition by in situ STM Faraday Discuss. 94 93-106... 

Several methods have been employed to study chemical reactions theoretically. Mean-field modeling using ordinary differential equations (ODE) is a widely used method [8]. Further extensions of the ODE framework to include diffusional terms are very useful and, e.g., have allowed one to describe spatio-temporal patterns in diffusion-reaction systems [9]. However, these methods are essentially limited because they always consider average environments of reactants and adsorption sites, ignoring stochastic fluctuations and correlations that naturally emerge in actual systems e.g., very recently by means of in situ STM measurements it has been demon-... 

Germanium In situ STM studies on Ge electrodeposition on gold from an ionic liquid have quite recently been started at our institute [59, 60]. In these studies we used dry [BMIM][PF<3] as a solvent and dissolved Gel4 at estimated concentrations of 0.1-1 mmol 1 the substrate being Au(lll). This ionic liquid has, in its dry state, an electrochemical window of a little more than 4 V on gold, and the bulk deposition of Ge started several hundreds of mV positive from the solvent decomposition. Furthermore, distinct underpotential phenomena were observed. Some insight into the nanoscale processes at the electrode surface is given in Section 6.2.2.3. 

The main technique employed for in situ electrochemical studies on the nanometer scale is the Scanning Tunneling Microscope (STM), invented in 1982 by Binnig and Rohrer [62] and combined a little later with a potentiostat to allow electrochemical experiments [63]. The principle of its operation is remarkably simple, a typical simplified circuit being shown in Figure 6.2-2. 

In the reductive regime, a strong, apparently irreversible, reduction peak is observed, located at -1510 mV vs. the quasi reference electrode used in this system. With in situ STM, a certain influence of the tip on the electrodeposition process was observed. The tip was therefore retracted, the electrode potential was set to -2000 mV, and after two hours the tip was reapproached. The surface topography that we obtained is presented in Figure 6.2-14. 

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