The Preparation of Clean Metal Surfaces

In the study of work-function changes of metals caused by the presence of adsorbates, the most troublesome aspect is that of obtaining a sufficiently well-defined surface. There is considerable experimental evidence based on 

The results of Hickmott (33), however, have caused these criteria for the cleanliness of evaporated metal films to be accepted with some reserve. In this instance a W film was prepared by evaporation from a W filament, commencing at a pressure of 10 mm. Hg, in an all-glass system which had been subjected to a rigorous outgassing in accordance with ultra high- 

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As already pointed out, the preparation of clean metal surfaces is an indispensable prerequisite for the study of surface reactions. The term clean , however, needs further definition. As it is commonly used in surface science, it means that the concentration of any contaminants on a clean surface is below or near the limits of detectability in Auger spectroscopy. Of course, this... 

More than with continuous improvement in the preparation of clean surfaces in electrochemistry, the measurement of 0 is rather casual in surface science at present. In particular, work functions are mostly measured for d-metals rather than for sp-metals, which are more common in electrochemical double-layer studies. As a consequence, compilations of work function values report data for sp-metals that are 20 to 30 years old.63,856,857 This does not imply that the data are unreliable, but imparts to the situation a sense of frustration related to the immobility in one of the variables. 

Preparing a clean surface is often a prerequisite for surface-science studies. UHV-based methods of sample preparation and characterization are established, and these may be exploited for studies of surfaces immersed in solution by interfacing an electrochemical cell with an UHV chamber. Samples can then be transferred from UHV and immersed into electrolyte solution under a purified-Ar atmosphere. However, even under these clean conditions, some metals oxidize or get contaminated prior to immersion. Other techniques for the preparation of clean surfaces that do not require UHV techniques are available for some metals. For example, flame annealing and quenching have been successfully used, but this procedure is probably limited to Au, Pt, Rh, Pd, Ir, and Ag substrates. In this technique, substrates are annealed in an oxygen flame and quenched in pure water. 

Tompkins (1978) concentrates on the fundamental and experimental aspects of the chemisorption of gases on metals. The book covers techniques for the preparation and maintenance of clean metal surfaces, the basic principles of the adsorption process, thermal accommodation and molecular beam scattering, desorption phenomena, adsorption isotherms, heats of chemisorption, thermodynamics of chemisorption, statistical thermodynamics of adsorption, electronic theory of metals, electronic theory of metal surfaces, perturbation of surface electronic properties by chemisorption, low energy electron diffraction (LEED), infra-red spectroscopy of chemisorbed molecules, field emmission microscopy, field ion microscopy, mobility of species, electron impact auger spectroscopy. X-ray and ultra-violet photoelectron spectroscopy, ion neutralization spectroscopy, electron energy loss spectroscopy, appearance potential spectroscopy, electronic properties of adsorbed layers. 

In this chapter, we differentiate between polycrystalline substrates and singlecrystalline substrates of a well-defined orientation. An additional distinction is made between thin metal films on a support material and bulk substrates such as disks or wires. The use of a thin metal film-as opposed to bulk material - gives rise to a number of additional parameters associated with the preparation process, which determine the properties of the resulting surface and the adsorbed SAM these will be discussed in depth. The process of adsorption of molecules on a substrate is sensitive to the presence of impurities on the surface. Therefore, cleaning pretreatment of surfaces will be addressed, as it plays an important role in the preparation procedure, as will be annealing procedures that bring about healing of surface defects and are crucial in the preparation of (atomically) flat surfaces. 

Hexafluorozirconic acid is used ia metal finishing and cleaning of metal surfaces, whereas the fluorozirconates are used in the manufacture of abrasive grinding wheels, in aluminum metallurgy, ceramics industry, glass manufacturing, in electrolytic cells, in the preparation of fluxes, and as a fire retardant (see Abrasives Metal surface treati nts). 

The fdr studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52-55° rotation about the molecular axis. This tilt is a result of the chains reestabUshing VDW contact in an assembly with - 0.5 nm S—S distance, larger than the distance of - 0.46 nm, usually quoted for perpendicular alkyl chains in a close-packed layer. On the other hand, thiolate monolayers on Ag(lll) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(lll), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(lll) surface, the alkyl chains are practically perpendicular to the surface. 

Swedish standard SIS 055900 contains two pictorial standards for manual cleaning, St2 and St3. Both require the removal of loose millscale, surface rust and foreign matter. The second and higher standard describes the prepared and dusted surface as having a pronounced metallic sheen. The St2 preparation is described as a faint metallic sheen . Both are expected to correspond with their respective coloured prints in the standard. These relate to four grades of new unpainted steel ... 

The practice of corrosion inhibition requires that the inhibitive species should have easy access to the metal surface. Surfaces should therefore be clean and not contaminated by oil, grease, corrosion products, water hardness scales, etc. Furthermore, care should be taken to avoid the presence of deposited solid particles, e.g. stones, swarf, building materials, etc. This ideal state of affairs is often difficult to achieve but there are many cases where less than adequate consideration has been given to the preparation of systems to receive inhibitive treatment. Acid treatments, notably with 3-5% citric acid, with or without associated detergent washes, are often recommended and adopted for cleaning systems prior to inhibition. However, it is not always appreciated that these treatments will not remove particulate material particularly when, as is often the case, the material is insoluble in acids. 

Virtually all subsequent surface science-related studies on Fe oxide films have been performed using the Pt(l 1 1) surface as a metallic substrate. The established preparation procedure for well-ordered Fe oxide films on Pt(l 11) involves PVDof Fe in 1-2 M L quantities onto clean Pt(l 11), followed by annealing in oxygen at elevated temperature this process can be repeated until oxide layers of the desired thickness have been formed. The preparation of Fe oxides on Pt(l 1 1) and the morphology of the resulting films as a function of the preparation parameters as well as the properties of Fe oxides in relation to catalysis have been comprehensively reviewed by Weiss and Ranke [106]. 

Phosphoric acid is used in many ways that include foods and beverages. It is also used in cleaning metal surfaces, electroplating, fertilizer production, preparing flame-proofing compounds, and other processes in the chemical industry, making it one of the most important of the chemicals of commerce. 

Ultrasound activation of metals. Since its discovery as an activation technique some fifty years ago, sonication has been demonstrated to be of general use for the depassivation of different metals. In this case, the activation consists in cleaning the surface of the metal, making possible better contact between the reagents. Probably the most important applications of ultrasonic irradiation are the direct preparation of organometallic reagents and different metal-assisted reactions, for instance in aqueous media . 

The preparation of a Grigncird reagent begins with magnesium metal and dry ether (in most cases, either diethyl ether or THF, tetrahydrofuran). The ether cleans the surface of the metal and takes the reagent into solution for reaction. (If either the ether or the reaction vessel contains moisture, the yield is poor.) The magnesium then reacts with either an alkyl halide or an aryl halide. The ease of reactivity decreases in the order R1 > RBr > RCl. Iodides may react too rapidly, but chlorides may react too slowly. Thus bromides are usually the best. The general reaction is... 

It must be mentioned that when precautions have been taken to get a clean Pt surface, no peroxide is produced above 0.1 V (Fig. 5). However, the production of H2O2 is observed when care in the preparation of the electrode or purification of the solution has been insufficient [59]. At electrode potentials lower than 0.2 V, when species in relation with hydrogen evolution are adsorbed, peroxide is produced in a parallel way to the 4e reaction. The catalytic site density diminishes, so that it is tempting to hold this responsible for the decrease in bond breaking because bridging an O2 molecule between two adjacent metal atoms (Scheme 5) becomes more difficult when other species... 

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