Solid-liquid interfaces, scanning

Barker A L, Gonsalves M, Maepherson J V, Slevin C J and Unwin P R 1999 Scanning electrochemical microscopy beyond the solid/liquid interface Anal. Chim. Acta 385 223... 

In contrast to many other surface analytical techniques, like e. g. scanning electron microscopy, AFM does not require vacuum. Therefore, it can be operated under ambient conditions which enables direct observation of processes at solid-gas and solid-liquid interfaces. The latter can be accomplished by means of a liquid cell which is schematically shown in Fig. 5.6. The cell is formed by the sample at the bottom, a glass cover - holding the cantilever - at the top, and a silicone o-ring seal between. Studies with such a liquid cell can also be performed under potential control which opens up valuable opportunities for electrochemistry [5.11, 5.12]. Moreover, imaging under liquids opens up the possibility to protect sensitive surfaces by in-situ preparation and imaging under an inert fluid [5.13]. 

The latter report demonstrated the unique ability of this technique to resolve surface structure as well as surface composition at the electrified solid-liquid interfaces. In particular, STM has become an important tool for ex situ and in situ characterization of surfaces at the atomic level, in spite its significant limitations regarding surface composition characterization for bimetallic systems, such as the lack of contrast for different elements and the scanned surface area being too small to be representative for the entire surface. To avoid these limitations, STM has been mostly used as a complementary tool in surface characterization. 

Scanning tunneling (STM) was invented a decade ago by Binnig and Rohrer [72], and was first applied to the solid-liquid interface by Sonnenfeld and Hansma in 1986 [73]. Since then, there have been numerous applications of STM to in situ electrochemical experiments [74-76]. Because the STM method is based on tunneling currents between the surface and an extremely small probe tip, the sample must be reasonably conductive. Hence, STM is particularly suited to investigations of redox and conducting polymer-modified electrodes [76,77],... 

Since the first report on a copper(II) Pc adlayer on Cu(100) [178], several studies describing the formation of Pc adlayers in air, in ultra-high vacuum (UHV), or at the solid-liquid interface have been reported [179-183], most of them involving the use of scanning tunneling microscopy (STM), a widely used technique for studying the organization of Pc derivatives on surfaces. 

Barker, A. L., Gonsalves, M., MacPherson, 1. V., Slevin, C. 1. and Unwin, P. R. (1999), Scanning electrochemical microscopy Beyond the solid/liquid interface. Anal. Chim. Acta, 385(1-3) 223-240. 

According to Sec. 3, the characterization of interface states at semiconductor electrodes is a key question since these states influence the behavior of the interface [76, 77]. The many different techniques of characterization have been reviewed [35, 76, 77] for the solid/liquid interface. True STS has not been applied until now. In first attempts to derive energy information the sample voltage was scanned to avoid problems with the electrochemical current at the tip extremity. The possibility of scanning the tip bias is discussed later. 

Sowerby, S.J., and Petersen, G.B. (1997) Scanning tunneling microscopy of uracil monolayers self-assembled at the solid/ liquid interface. Journal of... 

For the investigation of adsorption/desorption kinetics and surface diffusion rates, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed scanning electrochemical induced desorption (SECMID) (1), but historically this represents the first use of SECM in an equilibrium perturbation mode of operation. Later developments of this mode are highlighted towards the end of Sec. II.C. The principles of SECMID are illustrated schematically in Figure 2, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface, although the technique should be applicable to any solid/liquid interface, provided that the adsorbate of interest can be detected amperometrically. 

Recently, the structure of the solid/liquid interface has been studied with a wide range of in-situ structural techniques. In particular, scanned probe microscopes [1-5] and synchrotron-based methods [6-9] have yielded a wealth of structural information. The ultimate goal of this work is an understanding of the structure and reactivity of the electrode surface at the atomic level. One of the most extensively studied processes is metal underpotential deposition (UPD) [10], which involves the formation of one or more metal monolayers at a potential positive of the reversible Nemst potential for bulk deposition. 

Gewirtli A A and Siegentlialer H (eds) 1995 Nanoscale Probes of the Solid/Liquid Interface (NATO ASI Series 288) (London Kluwer) A survey of applications of scanning probes to electrochemical problems. 

Schindler, W, M, Hugelmann, and P, Hugelmann, In situ scanning probe spectroscopy at nanoscale solid/liquid interfaces. Electrochimica Acta, 2005, 50(15) pp, 3077-3083... 

Nanocell is the smallest electrochemical cell developed by Sugimura and Nakagiri [11] and further developed and utilized for ENT by BloeB et al. [10]. The nanocell consists of two electrodes distance between electrodes is generally maintained in the order of less than 1 nm. In between two electrodes, absorbed water film acts as an electrolyte whose volume is maintained by vapor pressure and ranges from 10 to 10 cm. Double layer capacitance is not formed across the solid liquid interface in the nanocell due to the much smaller inter-electrode gap and hence, generated hydrogen ion and hydroxyl ion recombine immediately. Nanotip of microtool such as tip of scanning probe microscope (SPM) or AFM tip is most suitable for the formation of electrochemical nanoceU. 

In contrast to the above example, in situ formation of ultrathin films at high pressures is difficult to follow with IRRAS because of interfering absorption by the ambient atmosphere [338-341] to remove this obstacle, as in the in situ spectroscopy of the solid-liquid interface, the PM method can be applied [342-344] (Section 4.7). As an alternative, s- and jo-polarized spectra are measured successively or by taking alternatively a few scans with each polarization until the desired SNR is obtained and the resulting spectrum is represented in Asp units [Eq. (7.1)]. 

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