XPS Investigations

X-ray photoelectron spectroscopy (XPS), which is synonymous with ESCA (Electron Spectroscopy for Chemical Analysis), is one of the most powerful surface science techniques as it allows not only for qualitative and quantitative analysis of surfaces (more precisely of the top 3-5 monolayers at a surface) but also provides additional information on the chemical environment of species via the observed core level electron shifts. The basic principle is shown schematically in Fig. 5.34. 

This is called electrochemical shift and simply stems from the fact that the Fermi level of the reference electrode is not equal to that of the working electrode and thus to the Fermi level of the detector. Furthermore if one changes UWr via a potentiostat the core level electron binding energies of species associated with the reference electrode will shift according to Eq. (5.66), i.e. the XPS analyzer acts also as a (very expensive) voltmeter. 

When doing in situ XPS in solid state electrochemistry one must be aware of the following experimental realities 6,56 62 

The working electrode, assuming it is the electrode under observation, should preferably be grounded. If the reference electrode is grounded instead, one should be constantly aware of the above electrochemical shifts. 

The sample temperature should be sufficiently high to ensure sufficient conductivity of the solid electrolyte and thus avoid charging of the solid electrolyte. This means temperatures above 300°C for YSZ and above 100°Cfor p -Al203. 

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Flecht D and Strehblow FI-FI 1997 XPS investigations of the electrochemical double layer on silver in alkaline chloride solutions J. Electroanal. Chem. 440 211-17... 

Babu PK, Lewera A, Chung JH, Hunger R, Jaegermann W, Alonso-Vante N, Wieckowski A, Oldfield E (2007) Selenium becomes metallic in Ru-Se fuel cell catalysts An EC-NMR and XPS investigation. J Am Chem Soc 129 15140-15141... 

An XPS Investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M /g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

Iron and Stainless Steel. The purpose of XPS investigations on typical corrosion systems like iron or stainless steel, is the determination of the composition of the passive surface layer, if possible, as a function of depth. As a consequence of the technical and economic relevance of corrosion reactions, XPS investigations on corrosion systems are numerous. With respect to the application of XPS, there is no difference between corrosion systems and any other electrochemical surface reaction like oxide formation on noble metals. Therefore, in this paragraph only a few recent typical results of such studies, using XPS, will be mentioned. For a detailed collection of XPS corrosion studies the reader is referred to references [43,104], A review of aqueous corrosion studies, using XPS, was given by McIntyre for the elements O, Cr, Mn, Fe, Co, Ni, Cu and Mo [105], The book edited by M. Froment [111] gives an impression of the research achieved on passivity of metals up to 1983. 

XPS investigations of the composition of the anodically grown passive layer on Ti electrodes were performed by Armstrong and Quinn [123, 124], The formation of a suboxide layer between the underlying Ti metal substrate and the stoichiometric Ti02 on top was demonstrated using XPS, AES and electrochemical techniques. 

Another example from Liu s team in this field concerns the selective hydrogenation of citronellal to citronellol by using a Ru/PVP colloid obtained by NaBH4 reduction method [112]. This colloid contains relatively small particles with a narrow size distribution (1.3 to 1.8 nm by TEM), whereas the metallic state of Ru was confirmed by XPS investigation. This colloid exhibited a selectivity to citronellol of 95.2% with a yield of 84.2% (total conversion 88.4%), which represented a good result for a monometallic catalyst. 

An AFM and XPS Investigation of the Selective Flocculation of Kaolinite from a Mineral Mixture... 

XPS also provides evidence that, at its endpoint, the reaction of metal ions in LB films with H2S is not stoichiometric as depicted in Eq. (4). For example, XPS analysis for a number of MBe films (M = Cd or Hg) to H2S for 1 h gave an average S M ratio of 0.76 instead of 1 as predicted by Eq. (4). Both QCM and UV/visible absorbance measurements indicate that 1 h of H2S exposure is more than enough for the reaction to reach its endpoint. In another XPS investigation of films of calixerenes containing Cd2+ ions, S Cd ratios of 0.84 0.1, on average, were obtained (37). 

An XPS investigation of these films was carried out [69]. The pH was more accurately measured to be between 2.2 and 2.5. Also it was noted that, although higher concentrations of acetic acid minimized the codeposition of hydroxide, above 0.1 M acetic acid, the films were not homogeneous and poorly adherent. From the XPS spectra, it was concluded the films were of the composition In(OH)S, with small variations in the S OH ratio. Sulphate, probably as surface oxidized In-S, was also present. 

An XPS investigation of the adsorption of aminosilanes onto metal substrates... 

Morkovich, V. Z. 1996. Synthesis and XPS investigation of superdense lithium-graphite intercalation compound, LiC2. Synth. Metals 80 243-247. 

As another example, oxide films on a vapor-deposited Ag substrate are presented [116]. Detailed XPS investigations show the development of Ag20 already 0.15 V below its equilibrium potential of E = 0.35 V [ 115]. Fig.50a presents the k-weighted Fourier Transform of the reflectivity-EXAFS, FT(ARk). of a 2.5 nm thick oxide film formed in 1 M NaOH at E = 0.40 V at an angle 0 = 0.09° relative to the surface. 

The structure of the passive layer on Ni has been studied very carefully with STM [139] by the group of P. Marcus and with XRD by the group of R.J. Behm [140], Ni surfaces usually have at room temperature a thin layer of an air-formed film that consists of 0.6 nm thick inner oxide, and 0.3 nm thick outer hydroxide, parts, according to XPS investigations of several authors this was discussed in detail in Section 5.1.5. Potentiodynamic cathodic polarization in 0.1 M Na2S04 solution of pH 3 yields a small reduction peak at E = 0.1 V with a charge of ca. 1000 pC cm-2... 

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