Surface-sensitive methods

Measuring the uptake of a gas by a surface as a function of the dose to which the surface is exposed is the most straightforward way to determine a sticking coefficient. In such experiments, great care should be taken to ensure that gas and surface are in thermal equilibrium. In addition, we need to determine the coverage, either by surface sensitive methods (XPS, AES, IR) or by thermal desorption and ensure that adsorption is not accompanied by desorption. 

The Volta potential is defined as the difference between the electrostatic outer potentials of two condensed phases in equilibrium. The measurement of this and related quantities is performed using a system of voltaic cells. This technique, which in some applications is called the surface potential method, is one of the oldest but still frequently used experimental methods for studying phenomena at electrified solid and hquid surfaces and interfaces. The difficulty with the method, which in fact is common to most electrochemical methods, is lack of molecular specificity. However, combined with modem surface-sensitive methods such as spectroscopy, it can provide important physicochemical information. Even without such complementary molecular information, the voltaic cell method is still the source of much basic electrochemical data. 

Corrosion processes can be very complex and, as the above examples show, surface analytical techniques can often provide unique information important for the understanding of these processes and to the solution of corrosion problems. By their basic nature, surface sensitive methods excel at examining thin layers at surfaces and interfaces that are difficult to detect and analyze by other methods but which can have a large influence in corrosion. The higher spatial resolution surface techniques are particularly useful for analysis of small area corrosion problems such as pitting and corrosion of electronic components and integrated circuits. 

Adsorption of polymers on inorganic substrates has been subject of numerous investigations because of the simple procedure for application [12-15], In this work the influence of the amino group content and pH of the PVFA-co-PVAm solution on the amount of adsorbed polymer on various metal surfaces has been studied. Metal substrate samples of different size and shape (particles, sheets) have been used which limits the application of special surface sensitive methods such as ellipsometry. 

Ideally, a model catalyst should be prepared and characterized under clean conditions in order to avoid contamination. The catalyst should then be exposed to impurity-free reactive gases and a surface reaction should be studied by surface-sensitive methods at pressures approaching those of a technological process. These requirements can be met by ultrahigh vacuum (UHV)-grown nanoparticle model catalysts and in situ spectroscopy. 

Study of the modification of solid surfaces requires, preferably, surface sensitive methods. Spectroscopic techniques, for example X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy are excellent tools for gathering information on the chemical surface composition and the kind and number of functional surface groups. The fact that the carbon and nitrogen containing organic phase is only introduced during the adsorption procedure and locally fixed on the outside of the particles allows the use of established methods for polymer and solid-state characterization, particularly NMR and solid-state NMR spectroscopy (e.g. 13C CP MAS NMR). 

Increasing use is being made of Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, FTIR-ATR, as a surface sensitive method which is capable of giving quantitative... 

Although the electronic conductivity of an interphase that is present on an electrode can be related to various optoelectronic properties that are also measurable with spectroscopic techniques, the direct measurement of surface conductivities is not a spectroelectrochemical method. It is nevertheless a surface sensitive method that provides results closely related to those of other methods discussed in this book. Data on the electrosorption of alcohols on gold electrodes [45] or the electrode potential dependent conductivity of intrinsically conducting polymers [46] have been obtained with in situ surface conductivity measurements. Figure 4.4 shows the electrical resistance of a poly(2-propylaniline) film measured in situ under experimental conditions suppressing any influence of solution phase conduction. The influence of... 

A general feature of practically all spectroscopic and surface sensitive methods described in the following chapters should be kept in mind Compared to many electrochemical methods, in many cases these techniques provide information on a... 

Indium phosphide has been a successful material in the preparation of solid-state, photovoltaic, and photoelectrocatalytic electrochemical solar cells [237-240]. Photovoltaic soUd-state solar cells reach single-junction efHciencies above 24% [237]. When used as a photocathode in photoelectrochemical solar energy conversion, the material has shown excellent stability [239], related to the unique surface chemistry of the polar InP(lll) A-face that exposes In atoms only [240]. The photoelectrochemical conditioning of single-crystalline p-type InP with the aim of preparing efficient and stable photoelectrochemical solar cells for photovoltaic and photoelectrocatalytic operation is described in the following and the induced surface transformations are analyzed employing a variety of surface-sensitive methods. 

Surface plasmon resonance is a surface-sensitive method for chemical sensing based on RI changes on the surface of a metal film. Variations in light intensity reflected from the back of the film are... 

An alternative route suggested in the literature [114, 176-178] to overcome these limitations is the emersion of the elec-hode from the elechochemical cell and hansferring it into an ulhahigh vacuum (UHV) chamber, where studies using surface-sensitive methods can be carried out. 

It can be seen from these examples that cells and tissues are very sensitive to the composition of the materials surface. Several physicochemical techniques for analysing the composition of materials surface have been described. However, attention is drawn to the fact that the surface analysed by most of these methods is not the surface analysed by living cells, as cells recognise only the outermost layer of a hydrated material. Analysing this ultimate hydrated layer by a physicochemical method is a real challenge. Indeed, the most surface-sensitive methods such as electron spectroscopy for chemical analysis (ESCA also known as X-ray photoelectron spectroscopy, XPS) and secondary-ion mass spectrometry (SIMS) can analyse respectively a few layers at once or one layer after the other, but in strictly dry conditions. Performing an ESCA analysis at a very low temperature in order to keep water frozen has been described, but this is currently far from a routine method. Conversely, analysis of hydrated surfaces by ATR-IR is usual, but this method determines the composition of many layers in addition to the ultimate layer, as it analyses a depth of more than 1 pm. 

This chapter describes fundamental aspects of the electronic structure of organic semiconductors (small molecules and polymers), and their interfaces, and the method to bridge the electronic structure and electrical property more directly using ultraviolet photoemission spectroscopy (UPS). Penning ionization electron spectroscopy (PIES), which is the most surface-sensitive method, is also introduced for study of electronic states at outermost surfaces of solids, which are responsible for charge exchange through the interfaces between different materials when they get contact to form a hybrid system. 

The first task in any experimental study of low temperature corrosion is to prepare a metal surface free of oxide and adsorbed species. For this the sample must be placed in an ultra- high vacuum (UHV) chamber. One then eliminates the natural oxide layer and adsorbed impurities by ion sputtering. Because ion bombardment disturbs the uppermost atomic layers of the metal (Chapter 3), a thermal treatment is sometimes applied in order to reestablish the original surface structure. Once a clean metal surface is available one introduces a known amount of oxygen into the UHV chamber by setting its partial pressure, and one follows the evolution of the reaction using a suitable method. Surface sensitive methods for the study of adsorption and thin film growth include surface analysis by AES, XPS, SIMS, optical methods, in particular ellipsometry, or mass sensitive methods such as the quartz microbalance. 

Electron spin resonance (ESR) spectroscopy is not applied as a surface-sensitive method. Based on its sensitivity, however, it is possible to detect covalently bound molecules at the polymer surface if these molecules are labeled with a spin marker such as 4-amino-TEMPO. The application of atomic force microscopy (AFM) in comparison to scanning electron microscopy (SEM) delivers information about surface properties as far as molecular dimensions. Another advantage of AFM compared with SEM is that the sample is investigated in the original state (no sputtering) [73]. The full characterization of the surface of a... 

Other surface-sensitive methods can and have been used... 

XPS is a well established surface sensitive method for the analysis of solid materials, allowing the determination of surface composition and the assessment of oxidation state and chemical enviromnent of the different species. In fact, core level XPS is highly sensitive to the chemical environment, i.e. the binding energy may be strongly dependent on the oxidation state of the investigated ion, as well as on the neighbouring atoms. 

Unlike the homogeneous bulk, the interface is unique in the break of symmetry, which may induce order or disorder. Numerous techniques have been used to acquire structure and composition information at the interface of ILs, many of which require UHV. The negligible vapor pressure of ILs makes those techniques possible to interrogate their air-liquid interface. Other surface-sensitive methods with ambient conditions including NR, SFG, AFM, and tensiometry are also applicable to measure air-liquid, solid-liquid, and liquid-liquid interfaces. 

Many experimental methods for studying H and H induced effects on metals are known. Most of them are used for surface and adsorption studies in general, a few of them, however, are specific to H on metals. The direct detection of H on metals by some surface sensitive methods is rather difficult because of the low atomic number of H. Auger electron emission e.g. does not work principally, photoemission has extremely low cross section and X-ray scattering is very weak even at grazing incidence. Thus, in many cases, one does not analyze H directly but the H induced variations of the substrate properties. (See also Malinowski, 1983). 

Low energy noble gas ion scattering, a surface sensitive method, is generally incapable of probing surface hydrogen directly. However, if pulsed rare gas ion beams at grazing incident angle are used to bombard the surface. 

TmTe is a material very difficult to be prepared stoichiometrically, which is a fact with all rare earth tellurides. Whenever it is nonstoichiometric it is extremely sensitive to oxidation and trivalent Tm203 layers form at the surface. They can be observed with surface sensitive methods like photoemission or XA (e.g., Kaindl et al. 1983). The problems connected with the material preparation have been discussed by Kaldis and Fritzler (1982) and Ott and Hulliger (1983). 

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