X-ray Photoelectron Spectroscopy XPS

XPS is among the most frequently used techniques in catalysis. It yields information on the elemental composition, the oxidation state of the elements and, in favorable cases, on the dispersion of one phase over another [ J.W. Niemantsverdriet, Spectroscopy in Catalysis, An Introduction (2000), Wiley-VCH, Weinheim G. Ertl and J. Kiippers, Low Energy Electrons and Surface Chemistry (1985), VCH, Weinheim L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin Film Analysis (1986), North-Holland, Amsterdam]. 

Spin-orbit splittings as well as binding energies of a particular electron level increase with increasing atomic number. The intensity ratio of the peaks from a spin-orbit doublet is determined by the multiplicity of the corresponding levels, equal to 2j + 1. Hence, the intensity ratio of the j = and j = components of the Rh 3d doublet is 6 4 or 3 2. Thus, photoelectron peaks from core levels come in pairs -doublets - except for s levels, which normally give a single peak. 

XPS and AES are among the most often applied techniques in the characterization of solid surfaces [15], while UPS is a typical surface science method which is best suited to fundamental studies on single crystals. All three spectroscopies provide surface-sensitive information, however. 

XPS and UPS are based on the photoelectric effect, whereby an atom absorbs a photon of energy, hv, after which a core or valence electron with binding energy Eb is ejected with kinetic energy (Fig. 3.2)  

Ek is the kinetic energy of the photoelectron h is Planck s constant v is the frequency of the exciting radiation  

Eb is the binding energy of the photoelectron with respect to the Fermi level of the sample  

X-ray photoelectron spectroscopy works on the principle of photoelectronic effect. The investigating surface is bombarded with X-ray photons, which leads to the emission of 

As its name implies, XPS is a technique that relies on the ionization of the species of interest and that is particularly useful to investigate smface properties, and as such has been widely used to investigate actinide oxides, in particular those of uranium (given its predominant role in nuclear fuels) in different forms such as solids or thin films. There, the core levels associated with the 4/ electrons are particularly interesting, first due to their distinctive positions and the magnitude of the splitting of the 4/5/2 and 4fj/2 due to spin-orbit coupling (about 10 eV in the UO2 crystal [97]). 

it is interesting to note that approaches based on more approximate two-component relativistic Hamiltonians, in which spin-orbit coupling is introduced in a mean-field fashion [105], can reproduce quite well the results of more computationally expensive four-component calculations for the 4/ spectra in U +, thus opening the perspective of treating systems of relatively larger size. 

Due to the solid environments and the importance of long-range electrostatic effects on the states energies, embedded cluster models [5] are chosen. While the point-charge embedding has been widely used (see Bagus et al. [98] and references therein) due to its simplicity, it is not without drawbacks (such as a tendency to spuriously stabilize delocalized states, in 

Using x-ray photoelectron spectroscopy, Pena and co-workers [94] examined the factors affecting the adsorption of organophosphorus polymer stabilisers on to carbon black. 

Among all the methods of ESCA, XPS has been found to show the greatest applicability. It has been widely used for the surface characterization of materials, especially catalysts. As addressed above, XPS analysis can give sufficient information about the qualitative and quantitative elemental surface composition of a catalyst, the oxidation state of an atom, the chemical environment, and so forth. The following paragraphs give some typical analysis examples of XPS for fuel cell catalysts. 

Polymer electrolyte and direct methanol fuel cells (PEFCs and DMFCs) are the most promising power sources for applications such as electric vehicles and electronic portable devices, due to their high power density, relatively quick startup, rapid response to varying loading, and low operating temperature [131]. Pt-based catalysts are the most important electrocatalysts in these fuel cells [132]. It has been widely reported that the catalytic activity of a Pt-based catalyst for the oxygen reduction reaction (ORR) and the methanol oxidation reaction in fuel cells is highly dependent on the oxidation states of the Pt crystallites on the surface of the catalyst [133, 134]. The oxidation states of Pt and the crystallites contents can 

The Cr(2p), Co(2p), and Ni(2p) X-ray photoelectron spectra for the samples were also studied, and the oxidation states of Cr, Co, and Ni as well as their relative intensities were obtained. From these data it was found that the Pt-Co/C sample had the lowest overall oxidizing components among the binary- and ternary-alloy electrocatalysts. Surface atomic ratios for Cr Pt, Co Pt, and Ni Pt of the carbon supported electrocatalysts, obtained from their respective X-ray photoelectron spectra, are summarized in Table 10.5. The results indicate some surface enrichment of platinum metal in all the binary-alloy electrocatalysts, namely Pt-Cr/C, Pt-Co/C, and Pt-Ni/ C. However, a surface enrichment of base metals was found in the ternary-alloy electrocatalysts, as can be seen from Table 10.5. The results suggest a higher electrocatalytic activity towards the oxygen 

Another obstacle for DMFC applications is the low catalytic activity of electrodes for both the oxygen reduction reaction and the methanol oxidation reaction. It is well known that the catalytic activity of an electrocatalyst is strongly dependent on the particle dispersion of the active components. Many doping techniques have been explored to widely distribute the active components on the catalyst supports [145-148]. In addition to the synthesis method, catalyst support also plays an important role in the dispersion of active components. Carbon materials with high surface areas (e.g., Vulcan XC72 carbon black) have been widely employed as electrocatalyst supports to enhance the dispersion of metal nanoparticles and thus to increase the utilization of the precious metal eatalyst 

Values in parentheses indicate the bulk atomic ratios. 

X-ray photoelectron spectroscopy (XPS) provides an indication of the oxidation states of the metals in dusters by comparisons of their binding energies with those of standards. Typically, the determinations are not exact and need further confirmation by other methods. XPS is espedally useful for detedion of changes in oxidation states. Since the technique requires ultrahigh vacuum, instability and volatility of the samples are often complications. This technique has been used to characterize the formation of Rh(CO)2 in NaY zeolite. [85] 

A brief introduction of the underlying principles for XPS are stated in Sect. 3.2.5 along with the other used EES techniques, thus only the experimental details are stated. 

Experimental details—The measurements of XPS spectra of supported cluster materials (Sect. 5.1.2) were performed on different samples. For comparing different sizes particular samples were prepared, by deposition of size-selected clusters on silicon wafers pieces (8 x 15 mm). 

Prior to deposition the wafer pieces were cleaned in the following order in acetone, methanol and iso-propanol in an ultrasonic bath for 5 min [118]. After deposition, the samples were brought to ambient conditions and subsequently transferred to the separate XPS UHV setup in air. 

Two samples at a time were mounted onto an electrically grounded z-transfer manipulator. The samples were introduced into a separate UHV analysis chamber with a typical pressure in the region of 1 x 10 mbar, equipped with a Leybold Heraeus LHS-IO X-ray photoelectron spectrometer. Non-monochromatized MgKa (1253.6 eV) radiation was used for excitation of the electrons [119] the spectra were recorded digitally using a multichannel scalar and a PC (Collect Spectra 8.0 software). 

Post detection data treatment was performed with IGOR Pro 6.22, using Doniach-Sunjic [120] fit functions. Further details on the setup as well as measurement conditions and parameters are stated in the appendix in Sect. A. 1.5. 

Optimum surface sensitivity (X 0.5 nm) is achieved with electrons at kinetic energies in the range 50-250 eV, where almost half of the photoelectrons come from the outermost layer. 

Also known as electron spectroscopy for chemical analysis (ESCA), XPS utilizes x-rays and photoelectric phenomena to study electronic structure, compound composition, electron and chemical states, electron bonding, and surface analysis [34-36]. In practice, a sample is bombarded by a monochromatic single wavelength x-ray beam. This causes core electrons from the sample to overcome their binding energy and escape to the sample surface where they are detected [34]. 

At different beam energies and wavelengths, specific characteristic bonds will interact with the beam. The whole process must be performed in an ultrahigh vacuum environment (UHV) and can reveal detailed information about the molecular composition of a surface. The technique is often used in industry to study catalysis, polymer surface modification, corrosion, adhesion, semiconductor and dielectric materials, electronics packaging, magnetic media, and thin film coatings [36,37]. 

For ES development, XPS is usually used to examine the oxidation states of different pseudocapacitive materials. A study of this use examined the oxidation states of ruthenium oxide powders with various water contents [38,39]. XPS is also used to study electrode functionalization through elemental analysis. For example, it can be used to investigate and improve the concentrations and types of nitrogen groups created by doping graphene and CNTs by various procedures. XPS also provides chemical characterization analysis for advanced electrolytes [40,41]. All these data help researchers determine the correlation between chemical structures and the capacitive characteristics of materials. 

Only a few experiments have been reported where X-ray photoelectron spectroscopy (XPS) has been used to study solid-state reactions between salts and zeolites. XPS enables us to determine changes in the surface composition of a zeolite sample before and after it has been subjected to solid-state ion exchange. This technique is suitable to monitor, for instance, variations in the surface ratios n /nAi and n /ngi of the zeolite upon solid-state reaction as a function of temperature, the ratio salt/zeolite and the reaction time. Examples will be provided in, e.g.. Sects. 5.3.2.1 and 5.S.4.4. 

A KRATOS XSAM800 instrument with a MgKa X-ray source (1253.6 eV proton energy, no monochromator) was used to analyse clean membranes and surface deposits. The pass energy was 40 eV in fixed analyser transmission (FAT) mode. No sample preparation was required for XPS. Results of this analysis are presented in Chapter 7. 

In this case the surface of the sample is irradiated with soft X-rays. These X-rays are sufficiently energetic to cause photoemission of electrons from the core levels of atoms present on the sample surface. The photoelectrons generated are collected and passed to an electron energy analyser and detector. The measured kinetic energy (KE) of an electron is given by  

Because only electrons from the top few atomic layers will have sufficient energy to escape, the technique is very surface specific and results are normally obtained from between 5 and 10 nm depth at a detection limit of around 0.1 atom percent. The analysis area is typically 3-10 mm and all the elements with the only exception of hydrogen can be detected. 

Because of its ability to determine surface chemistry, XPS can be particularly useful in the analysis of polymer surfaces to see if oxidation has taken place and, if so, what functional groups (e.g., ester, acid, aldehyde) the oxygen is present in. 

Another application is the analysis of polymers to determine if certain surface treatments (e.g., corona discharge) have been carried out satisfactorily. 

In common with the other surface analysis techniques, it can also be used to investigate surface contamination problems and to determine the composition of the fracture surfaces in adhesion failures. 

a powerful technique for the determination of atomic concentrations at the sample surface and the acquisition of chemical bonding information. 

---

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

X-ray photoelectron spectroscopy (XPS) is among the most frequently used surface chemical characterization teclmiques. Several excellent books on XPS are available [1, 2, 3, 4, 5, 6 and 7], XPS is based on the photoelectric effect an atom absorbs a photon of energy hv from an x-ray source next, a core or valence electron with bindmg energy is ejected with kinetic energy (figure Bl.25.1) ... 

Figure 8.1 Processes occurring in (a) ultraviolet photoelectron spectroscopy (UPS), (b) X-ray photoelectron spectroscopy (XPS) and (c) Auger electron spectroscopy (AES)...

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (xps) and Auger electron spectroscopy (aes) are related techniques (19) that are initiated with the same fundamental event, the stimulated ejection of an electron from a surface. The fundamental aspects of these techniques will be discussed separately, but since the instmmental needs required to perform such methods are similar, xps and aes instmmentation will be discussed together. 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

Near edge x-ray absorption fine stmcture spectroscopy (nexafs) and x-ray photoelectron spectroscopy (xps) have been used to study SAMs of OTS, octadecyltrimethoxysilane (OTMS), CH2(CH2) ySi(OCH2)3, and (17-aminoheptadecyl)-trimethoxysilane (AHTMS), H2N(CH2) ySi(OCH3)3 (149). A number of important observations have been reported. First, the chains in OTS SAMs are practicaUy perpendicular to the substrate surface (tilt angle... 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Roughness from sputtering causes loss of depth resolution in depth profiling for Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS), and SIMS. 

X-ray photoelectron spectroscopy (XPS) is currently the most widely used surface-analytical technique, and is therefore described here in more detail than any of the other techniques. At its inception hy Sieghahn and coworkers [2.1] it was called ESCA (electron spectroscopy for chemical analysis), hut the name ESCA is now considered too general, because many surface-electron spectroscopies exist, and the name given to each one must be precise. The name ESCA is, nevertheless, still used in many places, particularly in industrial laboratories and their publications. Briefly, the reasons for the popularity of XPS are the exceptional combination of compositional and chemical information that it provides, its ease of operation, and the ready availability of commercial equipment. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

In X-ray photoelectron spectroscopy (XPS), a beam of soft X-rays with energy hv s. focused onto the surface of a solid that is held under an ultra-high vacuum, resulting in the ejection of photoelectrons from core levels of the atoms in the solid [20]. Fig. 15 shows an energy level diagram for an atom and illustrates the processes involved in X-ray-induced photoelectron emission from a solid. 

The interface properties can usually be independently measured by a number of spectroscopic and surface analysis techniques such as secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), specular neutron reflection (SNR), forward recoil spectroscopy (FRES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), infrared (IR) and several other methods. Theoretical and computer simulation methods can also be used to evaluate H t). Thus, we assume for each interface that we have the ability to measure H t) at different times and that the function is well defined in terms of microscopic properties. 

The most widely used techniques for surface analysis are Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), Raman and infrared spectroscopy, and contact angle measurement. Some of these techniques have the ability to determine the composition of the outermost atomic layers, although each technique possesses its own special advantages and disadvantages. 

---