Nickel surface analysis techniques

UHV surface analysis, apparatus designs, 36 4-14 see also Ultrahigh vacuum surface analysis mechanisms, 32 313, 319-320 Modified Raney nickel catalyst defined, 32 215-217 hydrogenation, 32 224-229 Modifying technique of catalysts, 32 262-264 Modulated-beam mass spectrometry, in detection of surface-generated gas-phase radicals, 35 148-149 MojFe S CpjfCOlj, 38 352 Molar integral entropy of adsorption, 38 158, 160-161... 

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

The analysis of the thermograms recorded during the interaction of the successive doses of the different reactants in the sequence may also yield very relevant informations. Through the use of different techniques, it has been shown, for instance, that the different steps of the mechanism of the CO oxidation, at room temperature, at the surface of pure [TNJiO (200)3 19, 82) or lithium-doped 54) nickel oxide, may be written ... 

Electrography — Electrography, introduced independently by A. Glazunov and H. Fritz, is an obsolete technique for the direct electrochemical analysis of solid materials. The principle is that a solid specimen is pressed on a paper which is soaked with an electrolyte solution. By anodic oxidation of the surface of the solid specimen the reaction products (e.g., nickel(II) ions) react with a reagent in the paper (e.g., dimethylglyoxime) to give colored reaction product (red in case of nickel(II) and dimethylglyoxime). This produces a print that clearly shows the distribution of the reactive element (nickel, in our example) on the surface of the specimen. 

The first column materials employed in the developmental stage of the technique were fabricated from plastic materials (Tygon and nylon) and metal (aluminum, nickel, copper, stainless steel, and gold). Plastic capillaries, which are thermoplastic in nature, had temperature limitations, whereas metallic capillary columns had the disadvantage of catalytic activity. Rugged, flexible stainless-steel columns rapidly became state-of-the-art, and were widely used for many applications, mainly for petroleum analyses. The reactive metallic surface proved to be unfavorable in the analysis of polar and catalytically sensitive species. In addition. 

While the spatial resolution of AES, XPS and SIMS continues to improve, atomic scale analysis can only be obtained by transmission electron microscopy (TEM), combined with energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS). EDX detects X-rays characteristic of the elements present and EELS probes electrons which lose energy due to their interaction with the specimen. The energy losses are characteristic of both the elements present and their chemistry. Reflection high-energy electron diffraction (RHEED) provides information on surface slmcture and crystallinity. Further details of the principles of AES, XPS, SIMS and other techniques can be found in a recent publication [1]. This chapter includes the use of AES, XPS, SIMS, RHEED and TEM to study the composition of oxides on nickel, chromia and alumina formers, silicon, gallium arsenide, indium phosphide and indium aluminum phosphide. Details of the instrumentation can be found in previous reviews [2-4]. 

RHEED, in combination with X-ray emission analysis (XRE), has been used to study the initial stages of oxidation of nickel at low temperatures (e.g. room temperature to 200°C) [5,6]. The RHEED/XRE facility developed at NRC yields structural information comparable to the LEED (low energy electron diffraction) technique and elemental and chemical information about the surface similar to AES. Eor nickel single crystals the rates of oxidation vary considerably with substrate orientation, but three stages of oxidation, viz. chemisorption, nucleation and lateral growth, and oxide thickening, are always observed. 

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