Study of solid surfaces

The interface between a solid and its vapor (or an inert gas) is discussed in this chapter from an essentially phenomenological point of view. We are interested in surface energies and free energies and in how they may be measured or estimated theoretically. The study of solid surfaces at the molecular level, through the methods of spectroscopy and diffraction, is taken up in Chapter VIII. 

There is an important difference between the two techniques in that photons, produced by XRF, can pass through a relatively large thickness of a solid sample, typically 4000 nm, whereas electrons can penetrate only about 2 nm. This means that AES is more useful in the study of solid surfaces, whereas XRF gives information referring more to the bulk of a solid or liquid. 

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

With the advances in AFM, especially the optical beam deflection method in the repulsive-force regime, the AFM study of solid surfaces under an electrolyte becomes practical. Atomic resolution with AFM at a liquid-solid interface has been routinely achieved (Manne et al., 1990, 1991). A typical fluid cell for the AFM study of electrochemistry is shown in Fig. 15.10. The top of the cell is made of glass to allow light to go in and out. 

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

Canter, K.F. (1986). Slow positron optics. In Positron Studies of Solids, Surfaces and Atoms, eds. A.P. Mills Jr., W.S. Crane and K.F. Canter (World Scientific) pp. 102-120. 

Infrared spectroscopy has been a common tool for the study of solid surfaces (3). As in any surface spectroscopy, the number of adsorbed molecules and the surface area of the solid determines the sensitivity needed for IR studies. For low area surfaces, reflection techniques have been used to measure IR spectra of adsorbed monolayers on metal surfaces (7). However, for nonmetallic surfaces such as mica, the low reflectivity of mica makes reflection techniques less suitable for IR measurements. At the same time, the biaxial properties of mica, the parallel nature of the surfaces, and the absorbance of the mica itself present difficulties in IR spectroscopy (8). 

The third problem also concerns the choice of whether to leave out certain material. In a book of this size it is not possible to cover all branches of spectroscopy. Such decisions are difficult ones but I have chosen not to include spin resonance spectroscopy (NMR and ESR), nuclear quadrupole resonance spectroscopy (NQR), and Mossbauer spectroscopy. The exclusion of these areas, which have been well covered in other texts, has been caused, I suppose, by the inclusion, in Chapter 8, of photoelectron spectroscopy (ultraviolet and X-ray), Auger electron spectroscopy, and extended X-ray absorption fine structure, including applications to studies of solid surfaces, and, in Chapter 9, the theory and some examples of lasers and some of their uses in spectroscopy. Most of the material in these two chapters will not be found in comparable texts but is of very great importance in spectroscopy today. 

A P Mills, Jr, W S Crane K F Canter, Positron Studies of Solids, Surfaces... 

This review deals with the applications of photolurainescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis, After a short introduction, the review presents the basic principles of photolumines-cence spectrosajpy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following coordination chemistry of ions at the surface of alkaline-earth and zirconium oxides, energy and electron transfer processes, photoluminesccncc and local structure of grafted vanadium oxide, and photoluniinescence of various oxide-... 

This review covers adsorption, catalysis, and photocatalysis that can be investigated and understood by photoluminescence spectroscopy. Most of the results discussed in this review have been obtained by photoluminescence techniques, but other, complementary techniques, are also discussed to emphasize the originality and potential value of photoluminescence spectroscopy, particularly with regard to anion coordination chemistry, excited states, and reaction dynamics. The latter field is of utmost importance in chemistry (35). Additional applications of photoluminescence spectroscopy to the study of solid surfaces are reviewed in the books Photochemistry on Solid Surfaces"(. 6) and Surface Photochemistry (37). 

AFM microscopy is more suited for the morphological study of solid surfaces, such as metallic plates with variable roughness therefore, it is extremely helpful when used in combination with the SERS investigation. In this way, the SERS... 

Since electrons cannot pass without energy losses due to collisions through a few cm of gas at atmospheric pressure it is necessary to evacuate the electron path from the sample to the detector a rule of thumb says that the product of pressure times path length should not exceed 1 cm torr. Multiplicative electron detectors like channeltrons require vacua of 10 " to 10" torr or better, dependent on the type, but the study of solid surfaces requires much lower pressures, say 10" torr, in order to give the experimenter sufficient time to obtain spectra of the pure surface before it becomes contaminated by the gases in the sample compartment. 

Experimental studies of solid surfaces and the chemical reactions that occur on them require very low gas pressures to avoid surface contamination. High-vacuum apparatus for... 

Generally, the study of solid surfaces is dependent on understanding not only the reactivity of the surface but also the underlying structures that determine that reactivity. Understanding the effects of different morphologies may lead to a process for enhancement of a given morphology and hence to improved reaction selectivities and product yields. 

It is often wondered whether SERS is a useful tool in the study of solid surfaces in general or, perhaps, it is only a unique property of silver and its family. In the latter case the utility of SERS would be greatly diminished. From the theoretical point of view it is also extremely interesting to know which solid substrates support SERS. This may be an important clue in elucidating the enhancement mechanism itself. 

M.J. Tricker, J.M. Thomas, and A.P. Winterbottom. Conversion Electron Mossbauer Spectroscopy for the Study of Solid Surfaces. Surf. Sci. 45 601 (1974). 

Ion microprobe analyzers are more sophisticated (and more expensive) instruments based on a beam of primary ions focused to a diameter of 2(X) nm to 1 pm. This beam can be moved across a surface (rastered) for about, 3(K) pm in both the. v and y directions, A. microscope is provided lo permit visual adjustment of the beam position. Mass analysis is performed with a double-focusing spectrometer. In some instruments, the primary ion beam passes through an additional low-resolution mass spectrometer so that only a single type of priniilry ion bombards the sample. The ion mi-croprobc veisioii of SIMS permits detailed studies of solid surfaces. 

Lascr-microprt)be mass speciromelers arc used for the study of solid surfaces. Ablation of the surface is accomplished with a high-power, pulsed laser, usually a Nd-YAG laser. After frequency quadrupling, the Nd-YA(i laser can produce 266-nm radiation focused to a spot as small as 0.5 pm. The power density of the radiation within this spot can be as high as 10 to 10" W/cm On ablation of the surface a small fraction of the atoms arc ionized. The ions produced are accel eraied and then analyzed, usually by limc-of-nighi mass spectrometry. In some cases laser microprobes have been combined with quadrupole ion traps and with Fourier transform mas speciromelers. Laser-microprobe tandem mass speclromeiry is also reeeiv-... 

Electron diffraction, because of the low penetration, cannot easily be used to investigate crystai structure. It is, however, employed to measure bond lengths and angles of molecules in gases. Moreover, it is extensively used in the study of solid surfaces and absorption. The main techniques are low-energy electron diffraction (LEED) in which the electron beam is reflected onto a fluorescent screen, and high-energy electron dif-... 

The underlying reason for the differences of the conductivity mechanisms and emission properties on the surfaces of the different materials lies in the differences in their electronic band structure. The band structure model of solids has been successful in explaining many solid-state properties, and we may apply it with confidence in studies of solid surfaces. There are many excellent textbooks on the subject of solid-state physics giving detailed descriptions of the band theory of solids, and a description is not presented here. In the following section a basic understanding of electron bands is assumed. 

The study of solid surfaces has provided another area of application in... 

Material Flow Tracers are used in studies of solid surfaces and the flow of materials. Metal atoms hundreds of layers deep within a solid have been shown to exchange with metal ions from the surrounding solution within a matter of minutes. Chemists and engineers use tracers to study material movement in semiconductor chips, paint, and metal plating, in detergent action, and in the process of corrosion, to mention just a few of many applications. 

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