The Electrified Solid-Liquid Interface

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

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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... 

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. 

STM and AFM Studies of the Electrified Solid-Liquid Interface Monolayers, Multilayers, and Organic Transformations... 

In-situ vibrational spectroscopy has long been used to study the electrified solid/ liquid interface. By using the information given by peak position, width, and lifetime, vibrational spectroscopy can provide the chemical identity of the adsorbate, an estimation of surface coverage, and the orientation and even dynamics of molecules at the electrode. Three different types of vibrational spectroscopy are relevant to the solid/liquid interface. The first two of these, Raman and infrared spectroscopy, are thoroughly discussed in this book. A third technique successfully used to probe the Uquid/soUd electrochemical interface is vibrational sum frequency generation (SFG). SFG was developed as a surface probe some 20 years ago [1], and its use was extended to the electrochemical interface by Tadjeddine over a decade ago [2]. Several reviews examining the use of SFG in non-electrochemical environments exist [3-11]. Tadjeddine wrote two reviews on the application of SFG to electrochemical problems [12, 13). This chapter updates the Tadjeddine work and focuses on the promise and problems of state-of-the-art electrochemical SFG. 

I 5 Sum Frequency Generation Studies of the Electrified Solid/liquid Interface... 

Rossmeisl, J., E. Skulason, M.E. Bjorketun, V. Tripkovic, and J.K. Nprskov, Modeling the electrified solid-liquid interface. Chemical Physics Letters, 2008. 466(1-3) p. 68-71. 

Rossmeisl J, Skulason E, Bjorketun ME, Tripkovic V, Norskov JK. Modeling the electrified solid-liquid interface. Chem. Phys. Lett. 2008 466 68-71. 

Figure 2b. The two distinct models were finally combined into a new model by Stem in 1924 who recognized that the electrified solid-liquid interface comprised both the fixed Helmholtz layer and the diffuse one of Gouy and Chapman, Figure 2c (4-7).

Figure 4. Contemporary model of the electrified solid-liquid interface taking into account the electrode structure, specifically adsorbed anions in the inner layer, structure of the solvent (here water) molecules at the electrode surface, hydrated ions in the diffuse layer and the interfacial electron transfer, 1995 (Provided by and reproduced with permission of K. Itaya).

In recent years, much attention was given to the role of neutral species and specifically adsorbed anions on the structure of the electrochemical interface, the electric field distribution in their vicinity and their role in electrocatalysis. EC STM became a key experimental technique in providing insight into the structure of the adsorbed neutral species or the specifically adsorbed anions copresent with underpotential deposited metallic layers 52-54) and in supporting data on the anion surface coverage based on chronocoulometry experiments (55-57). These structural results derived fi om various experimental approaches have led to a contemporary model of the electrified solid-liquid interface which is presented in Figure 4. 

At noble metals, the growth of submonolayer and monolayer oxides can be studied in detail by application of electrochemical techniques such as cyclic-voltammetry, CV 11-20) and such measurements allow precise determination of the oxide reduction charge densities. Complementary X-Ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), infra-red (IR) or elUpsommetry experiments lead to elucidation of the oxidation state of the metal cation within the oxide and estimation of the thickness of one oxide monolayer 12,21-23), Coupling of electrochemical and surface-science techniques results in meaningful characterization of the electrified solid/liquid interface and in assessment of the relation between the mechanism and kinetics of the anodic process under scrutiny and the chemical and electronic structure of the electrode s surface 21-23). 

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