Electron Spectroscopy: Applications for Chemical Analysis - Journal of

Waters Symposium: Electron Spectroscopy for Chemical Analysis edited by

Waters Symposium

John P. Baltrus National Energy Technology Laboratory U.S. Department of Energy Pittsburgh, PA 15236

Electron Spectroscopy: Applications for Chemical Analysis

David M. Hercules Department of Chemistry, Vanderbilt University, Nashville, TN 37235; [email protected]

This review is an elaboration of a paper presented by the author at the 2002 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, held in New Orleans, Louisiana, March 17–22, 2002. An annual event at Pittcon is the James L. Waters Symposium, established to provide an historical account of the development and commercialization of scientific instrumentation that has had significant impact on analytical chemistry and applied spectroscopy. The 2002 Waters Symposium dealt with X-ray photoelectron spectroscopy (XPS, ESCA), a technique that has had a dramatic impact on the analytical chemistry of surfaces. I was invited to review the early development of the technique from the perspective of use for chemical analysis. Thus I will focus on early applications relevant to analytical problems. It is important to focus for a moment on terminology. In the early days of electron spectroscopy a number of acronyms were proposed for the technique, only two of which gained fairly wide use: XPS and ESCA. The rationale for the former is straightforward; the latter stands for Electron Spectroscopy for Chemical Analysis (1). Over time, XPS seems to have gained wide acceptance and so that will be the acronym used here. The value of XPS for analytical chemistry is derived from its many applications to surface analysis. Early applications of XPS demonstrated its potential for studying surface films, heterogeneous catalysts, corrosion, and polymers, and its ability to study surface-modification reactions and electrochemical processes. Of particular interest is how fundamental studies of XPS and development of instrumentation worked together to make the technique ultimately valuable for surface analysis. In many ways, the development of XPS is a classic study of how the development of technology allows one to study the science, and how feedback from the science is essential to improve the technology. The present article will focus primarily on the period from 1964–1977. This is the time during which chemical shifts in XPS became widely recognized and development of the technique occurred for a variety of chemical problems. It was during this period that XPS developed into a tool used primarily for surface analysis and also during which time other surface-analysis techniques were developed (initially quite independently). At the end of this period, the importance of doing surface analysis in the multitechnique mode was recognized, rather than using the individual surface methods alone. The realization was that seldom is a major surface-analysis problem solved by only one technique; several are usually required. The discussion will be presented by topical area rather than strictly chronologically. The idea will be to identify early articles in each general area of application. Often there will www.JCE.DivCHED.org

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be several articles from different laboratories appearing at about the same time (within a year or so) as is frequently the case in the development of science. Selection of specific areas of application will reflect the author’s personal bias and I apologize in advance for any inadvertent omissions. Historical Perspective The development of XPS as an important analytical technique was the outgrowth of research by Kai Siegbahn and his group at the University of Uppsala, Sweden, for which he received the Nobel Prize in Physics in 1981. In his Nobel Lecture (2) he provides insight into the circuitous pathway he took from high-energy physics to low-energy electrons. Discovery of the chemical shift in electron-binding energies, correlated with chemical state, was first observed in 1958 (3, 4) but its potential value was not fully appreciated until six years later (5–7). It was this discovery that provided the impetus for development of XPS as a chemical analytical tool. It also distinguished XPS, early on, from other types of surface spectroscopy that were developing at the same time because they could provide little or no readily interpreted chemical information, in which XPS had the potential for being very rich. Prior to the discovery of chemical shifts, Siegbahn’s work had focused primarily on the development of instrumentation, particularly for providing high spectral resolution, and these developments were key to providing the value that XPS attained for chemical analysis. XPS is a good example of the important role that development of instrumentation plays in the advancement of science. When viewing an historical event, even in recent history, it is important to reflect it against the backdrop of the times in which it occurred. In the case of XPS, it is particularly important to look at where the practice of chemistry stood in the mid 1960s. Spectroscopic tools commonly available for structure determination were what we would currently term as antiquated: mass spectrometry had unit mass resolution and 100 MHz NMR was considered to be high field. Infrared and UV spectrophotometry were well developed; the former was relied on heavily and the latter had only limited value. Heteronuclear NMR was known for certain elements (e.g., 19F, 31P) but was not in common use; widespread use of 13C NMR was for the future (8). XPS arrived on the scene having the capability of giving spectra for all elements except H and He and offering the possibility of electron binding energy shifts correlated with chemical structure. Thus, early work in the chemical community was dominated by the general idea of developing XPS as a structural tool, applicable to a wide range of elements. This scenario lasted for several years, but was beset by two major problems. First, the magnitude of XPS chemical shifts

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Journal of Chemical Education

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Waters Symposium: Electron Spectroscopy for Chemical Analysis

relative to line widths was considerably smaller than for proton NMR and varied significantly among elements. For most elements the ability to resolve small chemical shifts was severely restricted. Correlations between chemical shifts and partial atomic charge were complicated both by experimental factors and the level of sophistication necessary for good charge calculations. Second, XPS signals were dominated by the chemical state of an element on the sample surface, which could well be different from the bulk. The use of XPS as a structural tool in chemistry became moot as heteronuclear NMR developed and came into wide use. The potential of XPS for surface analysis was appreciated early on, but did not become the dominant theme in the field because of reasons mentioned above. However, the early work on measuring chemical shifts and correlating them with partial atomic charge were extremely valuable, because it is exactly these effects that make the technique valuable for surface analysis. Prophetic of the way in which XPS developed were the introductory remarks of Siegbahn at the first International Conference on Electron Spectroscopy, held in Asilomar, California in 1971 (9). Speaking about science and the rather capricious way science proceeds, it has been said that science could be compared to a car provided with a gas pedal and a brake but no steering wheel. Electron spectroscopy is one very good example to illustrate the progress of science according to this statement...If electron spectroscopy did only probe the atomic structure I don’t think that it would become so interesting today.

XPS was noticed by the chemical community because of the discovery of chemical shifts, but its major contribution was its ability to measure species on surfaces. This transition was far from immediate. It is interesting to note that the first demonstration of the potential for XPS as a surface and analytical tool predated the work of Siegbahn’s group on chemical shifts and at the time he was developing the double-focusing magnetic analyzer, once again demonstrating the importance of development in instrumentation. Three articles were published in the early 1950s by Earl J. Serfass and Ralph Steinhardt at Lehigh University that clearly documented the analytical potential of XPS (10–12). The intent of the articles is indicated by their titles: X-Ray Photoelectron Spectrometer for Chemical Analysis (1951); Surface Analysis with the X-Ray Photoelectron Spectrometer (1953); and X-Ray Photoelectron Spectrometer with Electrostatic Deflection (1955). Steinhardt’s first instrument used a conventional (Mo) X-ray tube, a permanent magnet (iron) electron energy analyzer and a GM counter as a detector (10). They recorded spectra from a number of metals (Cu, Zn, Ag, Au) and were able to identify the components (Cu, Zn) in brass. They accurately analyzed the composition of an Au–Ag alloy (1:4). In their second article (11) Steinhardt et al. demonstrated the reduction in intensity from a gold foil by the deposition of barium stearate layers, showing that it followed an exponential curve. They also showed that the intensity from a Pt foil increased with surface roughness and that Pt black showed significantly higher XPS intensities than roughened Pt foils. The main problem that Steinhardt et al. encountered was poor spectral resolution that stemmed from the intrinsic line 1752


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width of the Mo X-rays and the poor resolution of their electron energy analyzer. In the third article (12) Steinhardt describes an electrostatic analyzer with improved resolution. However, even this device gave broad peaks and high background (owing to inelastically scattered electrons); the best spectrum that they could achieve (at a resolution of ∆E兾E = 0.003) from a gold foil barely resolved the spacing of the Mo Kα X-ray doublet. A Personal Note My own involvement with XPS began in the summer of 1964 when I first read articles from the Uppsala group on XPS chemical shifts in sulfur. I became aware of these articles by following my own advice given to analytical chemistry graduate students, If you want to discover the next major instrumental development for analytical chemistry read the physics literature. Spectroscopic techniques are almost invariably first described by physicists, and chemists develop them for use in chemical analysis.

That summer I wrote a letter to Kai Siegbahn asking about the availability of plans that I could use to construct a magnetic electron spectrometer. He responded with two pieces of information that were to prove of significant value. First, that Stig Hagstrom from his laboratory was spending a sabbatical at MIT (where I was at the time) and that I should contact him for what turned out to be valuable discussions for planning installation of an instrument. Second, that the Uppsala group had no detailed drawings for a spectrometer (they were continually updating and modifying instruments), but that he would consider constructing an instrument for me. I then approached John Pomeroy of the Atomic Energy Commission (AEC was supporting my work) about purchasing an instrument and, with the usual delays with Washington red tape, the instrument was funded. I first met with Kai Siegbahn and Carl Nordling in Boston in June of 1965 to discuss construction of the instrument and a visit to Uppsala, which occurred in the spring of 1966. After the exchange of many letters and drawings with Carl Nordling about details, the instrument arrived at MIT about a year later; this was the start of our program in XPS. When one establishes a research program in an area in which one is neither an expert nor has formal training, reliance on the help and good will of others is imperative. This is the way in which I came to XPS. I was trained as an analytical chemist and did my thesis research in optical spectroscopy (luminescence). The research program that I had developed up to 1964 was involved with various aspects of fluorescence, phosphorescence, chemiluminescence, and photochemistry. Although we worked on development of instrumentation for this research, nothing had prepared me (or my group) for an entré into XPS. Thus, I owe a particular debt of gratitude to Kai Siegbahn and Carl Nordling who produced the essential piece of hardware as well as contributing more than they probably realize to my education. On another note, I am probably unique in that I have strong personal connections to both (the late) Earl Serfass and Kai Siegbahn. The relationship with Siegbahn as mentor is obvious from the above. Earl Serfass was the Chair of


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the Department of Chemistry at Lehigh University when I arrived at that institution in the fall of 1957, fresh out of graduate school, for my first academic appointment. I regard Earl Serfass as one of my mentors in the development of my career in academe. He was a talented individual both scientifically and administratively. I had heard about the XPS program at Lehigh when I arrived, but by that time Earl had turned his interests elsewhere. Only after our program in XPS was in full swing did I recall some discussions I had with him about this rather unusual method that he had developed. There are others who have played a significant role in development of our program in XPS. First of all, I can never thank Stig Hagstrom enough for the time and effort he took to help with the design of our instrumental setup for the magnetic spectrometer. There were three people who worked (sometimes around the clock) on getting the spectrometer set up and functioning. My first two students, John Jack and William Swartz worked above and beyond the call of duty. The project would have probably never gotten off the ground but for the help of an electrical engineer, Tony Waraksa, who came to MIT in the evenings after his regular job. Tony solved many problems and built important circuitry. Two other people from the Uppsala group were helpful, Ragnar Nordberg and Anders Fahlman. Similarly, Royal Albridge from the Physics Department at Vanderbilt University helped us with computerization of our instrument. There are three people who played long-standing and very significant roles in development of the scientific aspects of my program. Jim Carver came to the group with considerable knowledge of XPS (trained by Tom Carlson) and was valuable in helping to decide on the early directions of the

program. Later, Andy Proctor and Marwan Houalla provided intellectual leadership in data processing and surface spectroscopy of catalysis, respectively. Finally, it is important to acknowledge financial support that I have received over the course of the years. Initially the program was funded by the Atomic Energy Commission and later by the Department of Energy. I have been fortunate to enjoy long standing support from the National Science Foundation. Other federal agencies from which I have received support are the EPA, the Army Research Office, and DARPA. Other organizations that have provided financial support are NATO, the Petroleum Research Fund, the John Simon Guggenheim Foundation, and the Alexander von Humboldt Stiftung. I have also been fortunate to receive industrial support from Bayer, DuPont, Exxon, British Petroleum, LeyboldHeraeus Instrumentation Laboratory, the American Iron and Steel Institute, Alcoa, PPG, and IBM.

Figure 1. Photoelectron spectra compared for Na2S2O3 and elemental sulfur (reproduced from ref 6).

Figure 2. Photoelectron spectrum of sodium azide (reproduced from ref 1).

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Early Events As is always the case in science, when a new, important discovery comes on the scene, there is a flurry of activity designed to inform the community about its origin, its unsolved problems, and possible applications. Such was certainly the case for XPS, taking the form of conferences, symposia, general articles, books, and other forms of communication. Another technique, ultraviolet photoelectron spectroscopy (UPS) arrived on the scene about the same time as XPS and the two were often treated together early on. They later diverged in application, UPS for gas phase molecules and XPS for solids and surfaces.


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The first international conference on electron spectroscopy was organized by David Shirley, then of the University of California, Berkeley at Asilomar, CA in September of 1971 (9). Subsequent conferences were a NATO summer school in 1972 (13), two discussions of the Faraday Society in 1972 and 1975 (14, 15), and a second international conference in Namur, Belgium organized by Ron Caudano and Jacques Verbist in 1974 (16). A particularly significant conference was organized by Kai Siegbahn in Uppsala, Sweden in 1977 as part of the 500th anniversary of Uppsala University (17). The first Gordon Research Conference on Electron Spectroscopy was organized by the author at Brewster Academy in Wolfeboro, NH in July of 1974. The first broadly-based and general book on XPS (and Auger) was published by Tom Carlson of Oak Ridge National Laboratory in 1975 (18). Two years later, David Briggs of ICI published a handbook of UPS and XPS (19) that has gone through several iterations and upgrades. In the period of 1975–1978, several monographs appeared, reflecting the shift of XPS into the area of surface analysis: Characterization of Solid Surfaces, edited by P. Kane and G. Larrabee (20); Methods of Surface Analysis edited by A. Czanderna (21); Characterization of Metal and Polymer Surfaces, edited by L.-H. Lee (22); Electron Spectroscopy for Surface Analysis, edited by H. Ibach (23); and Quantitative Surface Analysis of Materials (24), edited by N. McIntyre. In 1977 Brundle and Baker began a series of monographs titled, Electron Spectroscopy: Theory, Techniques and Applications (25). The Journal of Electron Spectroscopy and Related Phenomena, Tom Carlson and Dick Brundle editors, was first published in 1972–1973, and a widely circulated newsletter was provided early on (random periodicity) by Gheorghes Mateescu of Case-Western Reserve

University. The journal, Analytical Chemistry, added both XPS (26) and UPS (27) to its fundamental review issue in 1972. An interesting and somewhat humorous account of the problems encountered operating a magnetic spectrometer in an urban environment have been chronicled by William Swartz (28) along with information about the early history of XPS. Chemical Shifts

Early Chemical Shift Studies Most of the research on XPS chemical shifts, immediately following their initial discovery (5–7), was performed by Kai Siegbahn’s group in Uppsala. Their work in this period is summarized in two volumes, published in 1967 (1) and 1969 (29). The focus of this early research was to identify factors that were important for understanding the XPS phenomenon generally and chemical shifts specifically. The Uppsala group dealt with both organic and inorganic compounds; these will be summarized separately. The compound that led initially to the discovery of chemical shifts was sodium thiosulfate and an early spectrum of this compound (1) is shown in Figure 1. The split between the +6 and ᎑2 oxidation states in thiosulfate is about 6 eV and is easily seen. Similarly, a shift of about 4.5 eV was measured for sodium azide with the peak intensities showing the requisite 2:1 ratio, as shown in Figure 2. The initial interpretation was that the shift in thiosulfate is related to formal oxidation state, but this is not as clear in the case of the azide ion. Although the early XPS articles emphasized correlation with oxidation state, the Uppsala group quickly recognized that chemical shifts were related to partial atomic charge rather than simply the oxidation state (1). They began de-

Figure 3. Correlation between C 1s chemical shifts and charges obtained from CNDO calculations (reproduced from ref 30).

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velopment of what was to become the charge-potential model for correlation of charge with XPS chemical shifts. The initial efforts of the Uppsala group focused on charges derived from Pauling electronegativities, but soon it became clear that more sophisticated methods such as MO–LCAO and CNDO calculations were necessary, particularly for molecular species. In their second book (29) Siegbahn et al. demonstrated the value of using CNDO methods to calculate partial charges for correlation with XPS chemical shifts. The second important factor for chemical shifts that the Uppsala group identified was the importance of charges in the vicinity of the photoionized atom. In crystalline solids this amounts to a lattice potential and in molecules to a molecular potential. Thus it became clear that to use calculations effectively, it would be necessary to calculate not only the charge on the atom of interest, but on its neighbors, sometimes at a significant distance. The equation established was, ∆Eb = kq + V + l

(1)

where ∆Eb is a change in XPS binding energy, q is the difference in charge between that atom of interest and a reference atom, V is the molecular or lattice potential, and k and l are constants from a least-squares fit. Correlations were published for a number of elements using different methods of charge calculation. An example is shown in Figure 3 (30) in which C 1s binding energies were correlated with CNDO charges for an extensive set of carbon compounds, including saturated and unsaturated hydrocarbons, and oxygenated compounds. Calculating these potentials posed a serious

limitation in the late 1960s. Computers were slower and methods of calculation were not developed to the level of sophistication known today. The Uppsala group also studied a variety of organic compounds in this early period. A number of impressive spectra were obtained such as the one for ethyl triflouroacetate shown in Figure 4. Conveniently the peaks line up under the appropriate carbon atoms in the structure. The most extensive sets of compounds studied in this period were sulfur and nitrogen compounds (1), ca. 40 nitrogen and 50 sulfur compounds. Later they obtained data for chemical shifts of carbon and oxygen atoms in organic compounds (29). The one effect, which can be important for correlating XPS binding energies with partial atomic charge and atomic structure, not addressed in detail by the Uppsala group early on, is that of final-state effects, commonly referred to as relaxation. Evaluation of this factor for XPS came from the group of David Shirley (31, 32), which pointed out its importance, particularly extra-atomic relaxation effects. Relaxation effects arise from changes in electron distribution during the ionization process, and can be of different magnitudes for two atoms of the same element in different chemical environments. For example, final-state effects contribute significantly to “anomalous” chemical shifts such as Pb to PbO of ca. 1.5 eV, and Pb to PbO2 of ca. 0.7 eV. Generally, relaxation effects are small when chemical shifts are large, but relaxation can become very important when binding energy changes are small. A particularly interesting example is for the N 1s binding energies of alkyl amines as shown in Figure 5 (33). The open circles show the predicted chemical shifts by including relaxation energies. Computations not including relaxation predict shifts in the wrong direction as noted by the points marked as “x”.

Elemental Chemical Shifts Research on XPS chemical shifts was conducted outside of Uppsala, commencing about 1968. Generally it occurred at places where an XPS instrument was extant (all spectrometers at this time were homebuilt) and accessible to a group of chemists. The first such major program developed at Berkeley, a collaboration between W. Jolly, J. Hollander, and D. Hendrickson. This group studied extensive series of com-

Figure 4. Photoelectron spectrum of ethyl trifluoroacetate (reproduced from ref 1).

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Figure 5. Correlation between measured chemical shifts and binding energies calculated with (o) and without (x) relaxation energies (reproduced from ref 33).


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pounds: nitrogen-56 (34, 35); boron-25 (36); chromium-17 (36); and phosphorus-53 (37). They established correlations with a variety of MO calculations. About 1970 a second group published a series of articles, a collaboration between R. Albridge and J. Van Wazer of Vanderbilt University. They published data for 16 silicon compounds (38) and 17 phosphorus compounds (39). Additionally, they investigated the effect of substituting S for O in phosphorus compounds (40). Tom Carlson at the Oak Ridge National Laboratory began publishing chemical applications of XPS in 1970. He collaborated with a group at the University of Tennessee to work on chemical shifts: bromine and arsenic compounds (41) and group IIIA, VB, and VIB compounds (42). Additionally, Carlson investigated multicomponent structure in spectra of transition-metal compounds, primarily to correlate with delectron structure (43–45). Although most of the early work by our own group dealt with organic compounds, in this period we published chemical shifts for Se and Te (46).

Coordination Compounds Another natural area of interest was the application of XPS to study bonding in coordination compounds. A flurry of activity in this area began in 1970–1972 and continued for some years thereafter. The first article came from the Vanderbilt group who studied triphenylphosphine complexes of transition metals (47); subsequently they studied Pt and Pd compounds (48). However, the bulk of the early work came from the other side of the Atlantic. In 1971 Michael Barber’s group at Manchester published four articles and David Clark’s group at Durham published three. Barber’s group studied transition metal carbonyls (49, 50), phosphines (51) and ferrocenes (52). Clark’s group studied phosphine complexes of Pt and Pd (53) and Cr, Fe, Co, and Ni complexes of several ligands (54, 55). Both of these groups continued with extensive studies of coordination compounds as well as branching out into other applications of XPS (vide infra). David Briggs (a coworker of Clark; ref 53) continued an independent XPS program covering diverse topics. Jolly’s group at Berkeley studied the N 1s binding energies of an extensive set of transition-metal complexes of nitrosyls (56) and of azide, dinitrogen, and nitride complexes (57). Other early XPS studies of coordination compounds were Pt and Pd complexes by Riggs at Dupont (58), metal-halide complexes by Biloen at Shell, Amsterdam (59) and the hexachlorometallates of the platinum metals by our group (60). Organic Compounds As stated above, there has been a large volume of work done on the XPS of organic compounds. The goal here will be to review some of the early work, to approximately 1972. As with inorganic compounds, organic XPS was performed with the goal of correlating binding energy shifts with charge calculations and to determine the relationship between XPS and molecular structure. Extensive early work was performed by Siegbahn’s group, primarily on sulfur- and nitrogen-containing compounds; this work has been summarized (1). The Siegbahn group was fortunate to have as a member Bernt Lindberg, a chemist from Pharmacia. His particular interest was the influence of sulfur groups on organic XPS (e.g., 61). He was coauthor on many of the organic XPS articles from the Uppsala group. In addition, he authored a 1756


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definitive review on the XPS of sulfur compounds (62). The Uppsala group also published extensively on the XPS chemical shifts of carbon in organic compounds under the leadership of Ulrik Gelius (1, 29). They published a definitive work showing good correlation between XPS binding energies and MO calculations (30). All of this provided a valuable data base for others to pursue organic XPS and its applications. Other research groups began studying organic XPS at this time. Of major interest were compounds containing nitrogen, phosphorus, or sulfur. The first work on nitrogen compounds outside of Uppsala was done by the Berkeley group, which studied 19 compounds and found a linear correlation with CNDO calculations (34). Clark’s group studied the N 1s level in adenine (63) and six-member nitrogen heterocycles and the effect of perhalogenation on binding energies (64). Our research group’s first scientific communication reported a study of the effect of structure on the XPS binding energies of quaternary nitrogen compounds, both aliphatic and aromatic (65). This was followed by a study of monoprotonated nitrogen bases including amino-substituted aromatics and aromatic heterocycles (66). Another early article from our group dealt with the effect of structure on the XPS binding energies of quaternary phosphonium compounds (67). The binding energies showed a narrow range and could not be correlated with substituent group electronegativities. A follow-up study investigated some more complex phosphine iminium salts (68). Nine cyclic phosphazines were studied by Green and Sherwood and were interpreted in terms of charge distributions (69). Clark’s group reported a study of thiathiophenes, using XPS to distinguish between symmetrical and unsymmetrical structures (70). Studies of C 1s binding energies for series of organic compounds began to appear around 1970. Thomas studied gas-phase spectra for a series of hydrocarbons and compared them with calculations (71). At about the same time Mateescu published the first in a series of articles on the C 1s binding energies of carbonium ions (72). Certainly the most prolific group publishing on the C 1s spectra of organic compounds was that of David Clark, whose work dealt initially with aromatic hydrocarbons (73, 74). Table 1. Group Chemical Shifts and Electronegativities for Carbon Group

∆E/eV

Electronegativity

⫺CH2OH

᎑0.42

2.0

⫺CH2OCH2C

᎑0.23

2.1

⫺CH2C

᎑0.06

2.3

⫺C(O)OH

᎑0.15

2.3

⫺H

0.01

2.6

⫺NH2

0.25

2.9

⫺CF2

0.59

3.1

⫺Br

0.89

3.3

⫺OCH2

1.46

3.5

⫺C1

1.56

3.6

⫺OC(O)C

1.91

3.7

⫺F

2.79

4.0

NOTE: Data taken from ref 30; the electronegativities are in Pauling units.


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Interpreting Chemical Shifts Although correlations of XPS chemical shifts with charges calculated by electronegativity or MO methods were helpful for understanding their relationship to chemical structure, the converse was found not to be useful on a day-today basis. Knowing a binding energy may give an idea of the charge on an atom, but it does not necessarily define a particular structure. Also, at this time (loc. cit.) the complexity and accuracy of charge calculations did not allow their use as a practical laboratory tool. Therefore a number of different approaches were tried either to simplify charge calculations, or to provide a simple correlation with structure (or both). Also, there was interest in determining how XPS chemical shifts related to those measured by other methods. One very clever approach to the structure correlation problem was the equivalent-cores method developed by Jolly

and his group, which allowed XPS chemical shifts to be calculated from thermochemical data (75, 76). This approach avoided the necessity for calculating charge by MO methods. Removal of a core electron is the equivalent to increasing the nuclear charge on the ionized atom by one. Thus, one can model, for example, the photoionization of methane by using thermochemical data for NH4+, similarly, BH4− can be modeled by CH4. This approach also has the advantage that it compensates directly for the molecular or lattice potential. Good correlations were obtained using the equivalent cores method for nitrogen, boron, and carbon compounds. Several groups experimented with modified electronegativity approaches trying to overcome some of the problems inherent in the use of Pauling electronegativities. One such approach was the electronegativity equalization procedure

Figure 6. Comparison of charge correlations for organosilicon compounds: A–Pauling electronegativity, B–Jolly and Perry methods, C– modified Sanderson method, and D–CNDO calculations (adapted from ref 79).

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introduced by Jolly and Perry (77). The method used orbital electronegativities that involves the assumption that in a bond, electrons flow between atoms in the bonding orbitals until energy is minimized. Although the calculations by this method are more complicated than simple Pauling calculations, energy minimization is obtained and a better final partial atomic charge results. Measured and calculated binding energies were compared for carbon, nitrogen, oxygen, and fluorine, yielding standard deviations in the binding energies typically of 0.5 eV. An approach initiated by the Uppsala group as an alternative to the use of electronegativities and MO calculations was that of group shifts (64). The logic is that the chemical shift for a given atom is the net effect of all of the functional groups attached to that atom. For a carbon atom bonded to groups P, Q, R, and S, the chemical shift would be the sum of ∆E(C − P) + ∆E(C − Q) ... and so forth, which can be derived from least-squares fitting of binding energy data. Group electronegativities can be calculated from experimental XPS data. An example of group shifts for carbon is shown in Table 1. Good results were obtained for carbon, nitrogen, phosphorus, and arsenic compounds. Our group developed a different empirical method based on a modification of Sanderson electronegativities (78), in a way combining the Jolly and Uppsala approaches. The Sanderson method involved charge normalization between an atom and a molecule based on elemental electronegativities. Our modification was to introduce group electronegativities into the calculation and to calculate partial charge for an atom from its surrounding groups. We compared different methods for XPS binding energy–charge correlations: the modified Sanderson method (MSM), the method of Jolly and Perry, Pauling electronegativity, CNDO calculations and group shifts (Figure 6; ref 79). All charge calculation methods gave good correlations when corrected for molecular potential; such was not necessary using MSM. Subsequently MSM was applied to chemical shifts for 14 elements and gave performance comparable to the CNDO and Jolly methods. Performance was least successful for N, O, Cl, and Br (80). A natural question for chemists is whether the XPS binding energy shifts correlate with other spectroscopic methods, for example NMR. Three early studies addressed the issue with NMR. Our group found a moderately good correlation between XPS and 31P shifts in quaternary phosphonium compounds (67). Clark’s group correlated XPS and 13C NMR shifts for alkyl halides (81), and Mateescu found a poor correlation in general for carbon compounds, but a good correlation for alkyl halides (82). Gelius et al. (83) and Zeroka (84) established why XPS–NMR correlations should not be expected generally. Later, Lindberg et al. (85) found an interesting correlation between S 2p XPS shifts and 19F NMR shifts in 1,2-dithiole derivatives. Lindberg also critically reviewed this area (86). For a related method, Clark’s group found a correlation between the Cl 2p XPS binding energies and the 35Cl NQR frequencies in square-planar Pt complexes (87). An area for which there was a more sound theoretical underpinning was correlation between XPS and Mössbauer chemical shifts. Barber et al. investigated Sn compounds (88) and the Manchester group looked at Fe(II) low-spin complexes and ferrocenes (89, 90). Definite correlations were ob1758


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served in all cases. Holsboer and Beck combined XPS and Mössbauer chemical shifts in a study of Au and Ir complexes (91). However, a major disadvantage for this method is the small number of elements having common isotopes that give reasonable Mössbauer spectra.

Auger Chemical Shifts Photoexcited Auger electron lines are commonly observed in XPS spectra. It was known from the early days that Auger lines showed chemical shifts as well as photoelectron lines, again the first chemical shift was measured for the sulfur atoms in thiosulfate (92) as shown in Figure 7. The Auger chemical shift for sulfur was approximately that observed for the photoelectron lines, ca. 5 eV; essentially that predicted by first-order theory. This was a somewhat unfortunate situation because it gave rise to the expectation that Auger shifts would parallel those of core electrons. It had been noticed that distributions within Auger patterns varied with chemical bonding (1), but again the differences were not particularly impressive. In 1972 Wagner published a article detailing the prominence of Auger lines in XPS spectra and noted chemical effects on shapes, intensities, and locations of Auger lines (93). A year later Wagner and Biloen published a article that described rather large Auger chemical shifts they had observed while studying the oxidation of metal surfaces (94). In addition to an extended study of Mg, they compared photoelectron and Auger shifts for Zn, Ga, Ge, As, Cd, In, and Sn. In the following year Castle and Epler detailed LMM Auger chemical shifts for a number of elements and compared them to photoemission lines for the same elements (95). So it became clear that Auger lines in XPS spectra provided an addi-

Figure 7. First Auger spectrum recorded for Na2S2O3 (reproduced from ref 92).


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tional valuable handle for chemical state determination. Some comparisons reported are summarized in Table 2. The value of Auger lines in XPS spectra was enhanced tremendously when Wagner introduced the concept of the Auger parameter (96). The Auger parameter was defined as the difference in the kinetic energies between Auger and photoelectron lines for an element. The Auger parameter has the advantage that it measures an energy difference within a spectrum, so the effects of sample charging are minimal. Thus it allows one to distinguish between two states of an element having identical photoelectron energies but different Auger energies. Wagner developed this concept using two-dimensional plots, an example of which is shown in Figure 8 (97). The boxes represent the ranges of energies for the compound listed. For example, the XPS photoelectron binding energies of ZnO, ZnS, and ZnF2 are all about 1022.4 eV, but the Auger energies range from 986–988 eV. Thus, one can distinguish between them using the Auger parameter. Quantitative Analysis Three characteristics that make XPS so valuable for surface studies are: (i) the capability for quantitative analysis (RSD ±5%); (ii) its absolute sensitivity; and (iii) its ability to distinguish different oxidation states of the same element. This section will deal with the early development of XPS for quantitative analysis and for measuring trace quantities (presumably quantitatively). It was recognized early on that XPS

Figure 8. Wagner plot of chemical shifts of Auger and photoelectrons for zinc (reproduced from ref 97).

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Table 2. Comparison of Chemical Shifts for Photoelectron and Auger Spectra Element

∆E (Auger)

∆E (Photoelectron)

Mg → Mg (+2)

6.2 (KLL)

1.2 (2s)

Fe → Fe (+3)

0.2 (LMM)

4.8 (2p3/2)

Cu → Cu (+1)

2.8 (LMM)

0.4 (2p3/2)

Cu → Cu (+2)

0.8 (KMM)

1.2 (2p3/2)

Zn → Ζn (+2)

3.5 (KMM)

0.3 (2p3/2)

Ge → Ge (+4)

8.0 (LMM)

4.0 (3d)

As → As (+5)

6.4 (LMM)

3.6 (3d)

Cd → Cd (+2)

5.5 (MNN)

0.9 (4d)

Sn → Sn (+2)

3.9 (MNN)

1.5 (4d)

NOTE: Data are taken from various sources.

had limited capacity for quantitative measurements using absolute intensities; the use of a standard or some comparable measuring protocol was essential. Quantitative analysis using XPS was first reported by Steinhardt and Serfass who measured the composition of an 80:20 Ag:Au alloy to 4% absolute accuracy (10). The Uppsala group recognized the potential of XPS for both quantitative and trace analysis (1), for example, the detection of Co in vitamin B12 (< 1% atomic) and nanogram quantities of iodine. They also determined C:Cl:S ratios for organic compounds and analyzed brass samples. Quantitative XPS will be considered in three parts: trace analysis, general quantification, and approximate composition.

Trace Analysis As stated above, the ability of XPS to detect small numbers of atoms or molecules gives it the capability for trace analysis. A practical problem, however, is to get the small number of atoms or molecules onto a probe having an area of about 1–10 mm2. The first published approach was that of Brinen and McClure who electrochemically deposited metals onto an Hg-coated sample probe (98, 99). They were able to detect Pb at the ppb level. About the same time our group developed chelate-coated fiberglass disks to extract metals from solution (100). Similarly we observed ppb detection levels for Pb, Tl, and Hg using a dithizone-coated glass (Figure 9). Czuha and Riggs used an acrylic acid-grafted polypropylene ion exchange resin to extract trace metals from solution, achieving ppm detection and linear concentration relationships (101). Later our group developed a method for As determination combining hydride volatilization and XPS detection (102). It was possible to simultaneously detect As, Se, Sn, and Sb at 100 ppb from a 1-mL sample with an S兾N > 10. Two other early studies helped to define the limits for trace-element detection. Brundle studied the adsorption of Hg on Au and reported detection of less than 0.2% of a monolayer (103), essentially defining the limit of detection for XPS in trace analysis. Our group performed model calculations for adsorption of metal ions at a chelated surface and compared them with data for the Pb-dithizone system (104). This helped to define the effect of pH and other variables on trace analyses using XPS.


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General Quantification XPS, while sensitive to small numbers of atoms or molecules, is not a particularly sensitive bulk technique, being able to measure percentages of about 1% (absolute) for light elements and 0.1% for heavy elements. Kramer and Klein reported what is probably the first calibration curve for an XPS analysis, studying mixtures of ferro- and ferricyanide in frozen aqueous solutions. Relative intensities were linear with concentration in the range 0.1–1 M (105). Our group established the first calibration for a mixed-oxide system: MoO2 and MoO3; as shown in Figure 10. It was possible to perform bulk analyses of mixed oxides with a RSD of ±2% (106). The next step to expand XPS bulk analysis was introduction of the use of an internal standard. A linear calibration curve was established using known mixtures of calcium carbonate and silica and was applied successfully to analysis of glass (107). Another study was the effect of the matrix on calibration curves, using different lead salts (108). Even with an internal standard, calibration curves for lead halides showed major differences in slope, making it impossible to measure lead salt mixtures. If the salts were converted chemically into one lead species (e.g., PbSO4) the matrix effect disappeared. Finally, we applied XPS to bulk analysis of fluoridated dental enamel (109). On the basis of the XPS analyses of different layers, exposed by Ar-ion etching, it was possible to determine which of a number of possible reactions had occurred in a given enamel layer during the fluoridation process.

Approximate Composition When one obtains an XPS spectrum that has peaks corresponding to several different elements, it is important to be able to calculate the approximate composition of the sample from the XPS data. To accomplish this task it is necessary to convert the intensities of the element peaks into relative concentrations. However, the XPS response produced by a given number of atoms varies significantly across the periodic table. This problem was dealt with by Wagner who defined and measured atomic sensitivity factors, initially for 42 elements (110). Sensitivity factors were determined relative to F (defined as 1.00) by measuring series of (preferably) binary compounds. For example, in NaF the Na intensity is 2.51 times that of F, so the sensitivity factor for Na is 2.51. Then by measuring the Na兾Br intensity ratio in NaBr the sensitivity factor for Br could be determined as 0.59. By continuing this process one can (hypothetically) obtain sensitivity factors for all of the elements. A select set of sensitivity factors is shown in Table 3 (97). Sensitivity factors are instrument dependent—no two types of XPS spectrometers will have exactly the same set. However, for a given instrument sensitivity factors are usually quite dependable and will give an approximate analysis of the composition of a sample to within about ±10% relative. Because XPS is a surface technique, surface contaminants can limit the applicability of sensitivity factors for use in bulk analysis. Wagner has addressed some other limitations (111) of using sensitivity factors, but despite the problems, use of atomic sensitivity factors for approximate analyses of surface composition is a highly valuable and much used tool. Surface Analysis As XPS matured as a spectroscopic technique, research emphasis changed from defining potential use as a method for chemical identification to its place as a core method for the chemical analysis of surfaces. The importance of the surface region for XPS spectra was known early on (1) but by the mid-1970s the surface analysis possibilities provided by XPS became a major emerging theme. Further, it was becoming recognized that XPS would not be the only method in

Figure 9. XPS spectra obtained from dithizone-coated glass: A– from 100 mL of a 10 ppb Pb solution, B–broad scan obtained by dipping the glass into a solution containing 0.1 ppm each of Bi, Pb, Tl, and Hg (reproduced from refs 16 and 100).

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Figure 10. Calibration curve for mixtures of MoO2 and MoO3 (reproduced from ref 106).


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the surface analyst’s arsenal, but one of several. At this time the thought process in surface analysis was changing from single-method studies to multimethod investigations. The purpose of the present section is to chronicle some of the early XPS research in areas for which it would assume significance. The development of XPS as a surface analytical method was aided by the enormous volume of information available owing to earlier work defining chemical shifts. Surface analysis had major impact on two technologies of great importance to the chemical industry: catalysts and polymers. Most synthetic chemicals are made by catalytic reactions, and polymers represent the major class of materials synthesized.

Catalysis Two works appeared in 1970, indicating the potential of XPS as a tool for catalyst research. Wolberg et al. studied the nature of the dispersed Cu phase on Cu–alumina catalysts, demonstrating the value of XPS for such studies (112). Delgass et al. published an article reviewing XPS as a tool for catalysis research and presenting some important early results. They considered effects like activation, aging, and poi-

soning of catalysts as well as indicating the change in chemical character of “oxides” on a support (113). A year later Barber et al. studied Mo–alumina catalysts, noting the changes that occurred in the Mo XPS spectra when the catalyst was calcined (114). In a subsequent article they determined that when Mo(CO)6 is condensed on alumina and activated, the Mo no longer exists as the hexacarbonyl (115). In that same year Brinen and Melera correlated the metal:oxide ratio of Rh on charcoal catalysts with their catalytic activity (116). Subsequently Brinen et al. studied Rh dispersion as a function of reduction for the same catalyst system (117). All of the early articles referenced above involved studies on essentially individual (or a few) catalysts to illustrate important points for the use of XPS. In 1973 Biloen and Pott reported a series of reduction measurements on W–alumina catalysts and showed that calcination leads to the formation of a nonreducible tungsten species on alumina, but not on silica (118). Our group published articles later on both the Co–Mo– and Ni–W–alumina systems (Figure 11). We were able to follow the reduction process for Mo as a function of temperature and to identify specific Mo oxidation states on

Table 3. Atomic Sensitivity Factors (ASA) for Selected Elements Z

Element

05

B

Line 1s

ASF (Area) 0.088

06

C

1s

0.205

07

N

1s

0.38

08

O

1s

0.63

09

F

1s

1.00

11

Na

1s

12

Mg

1s

2.51 ᎑3.65

13

Al

2p

0.11

14

Si

2p

0.17

15

P

2p

0.25

16

S

2p

0.35

17

Cl

2p

0.48

19

K

2p3/2

0.55

20

Ca

2p3/2

0.71

22

Ti

2p3/2

1.1

23

V

2p3/2

1.4

24

Cr

2p3/2

1.7

25

Mn

2p3/2

2.1

26

Fe

2pa

3.8

27

Co

2pa

4.5

28

Ni

2pa

5.4

29

Cu

2p3/2

4.3

30

Zn

2p3/2

5.3

31

Ga

2p3/2

6.9

32

Ge

2p3/2

9.2

----

----

3d

33

As

2p3/2

0.3 ᎑9.1

----

----

3d

0.38

34

Se

3d

0.48

35

Br

3d

0.59

NOTE: Data taken from ref 97.

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Figure 11. Mo 3d spectra obtained from a Co–Mo–Al2O3 catalyst as a function of reduction time with H2 at 500 ⬚C: A–0.0min, B– 15 min, C–50 min, D–60 min, E–120 min, F–240 min (reproduced from ref 120).


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the catalyst surface. We also studied the effects of sulfiding these catalysts (119, 120). A similar study was performed for the Ni–W system; we were able to monitor reduction of Ni in the presence of (unreducible) W and the sulfiding of both, also as a function of time (121). Of particular importance in these studies was the introduction of a sealable probe that allowed transport from a catalytic reactor to the XPS spectrometer without exposure to air.

Polymers XPS has made a dramatic impact on the field of polymer surface chemistry (Figure 12). Polymers are a natural target for XPS studies, they are flat, generally stable and can readily be produced as thin films. Thus it is quite astounding that activity in this field was essentially absent prior to 1971. The first two articles describing XPS of polymers both dealt with fluoropolymers, a natural choice. Clark and Kilcast studied both core levels and the valence band of PTFE (122) and Riggs described XPS of PTFE, poly(vinyl fluoride), poly(vinylidene fluoride), and poly(trifluoroethylene) (123). Riggs followed his initial work with a study of the surface treatment of PTFE and a fluoro-copolymer, correlating contact angle measurements with XPS results (124). Clark’s follow up to his initial study was an extensive and extended series of articles detailing with many aspects of polymer surface chemistry for a wide variety of polymers. His second article dealt with XPS of nitroso rubbers and focused on measuring the core energies and correlating them with CNDO calculations (125). The next article measured core binding energies in a series of fluoropolymers (similar to those studied by Riggs) and correlated them with CNDO calculations, also noting that XPS can be used to determine copolymer composition (126). This was then followed by determination of the composition and molecular structure of copolymers, comparing XPS results with those from conventional analytical methods (127). Many more articles on polymer XPS followed from the Clark group. In this same period our group had become interested in the bonding of polymers to metals. XPS was used to show the presence of a thin layer of polymer remaining on Al after they had been bonded and the polymer had been stripped from the Al. Thus bond failure was attributed to cohesive failure within the polymer (128). About this time David Briggs began to publish work on polymers. The first article from his group concerned an XPS study of the chromic acid etching of polyolefin films (129). This was followed by a study of the effect of melting polyethylene on an Al surface (130). Both oxidation and unsaturation were induced by the melting process. Briggs continued research on polymer surfaces, employing both XPS and SIMS, and published a number of important contributions. Adsorption and Surface Reactions Early work in this area included chemisorption studies, use of XPS to monitor reactions occurring at surfaces, and development of methodology to make XPS more surface sensitive. The angular distribution of photoelectrons from a single crystal was first observed by Siegbahn’s group (131). Not long afterward similar results were observed by Fadley et al. also noting strong periodicity owing to diffraction ef-

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Figure 12. XPS spectra of (top) poly(vinyl chloride) and (bottom) poly(i-propyl acrylate) (redrawn from Vol. 2, p 13 of ref 22).

fects (132). Fadley then elaborated on this idea to enhance the surface sensitivity of XPS. His group observed that angular effects are sensitive to the surface profile, specifically measuring the change in Al2p(oxide)兾Al2p(metal) intensity ratio oxide layers on an Al substrate (133). They then went on to note that grazing x-ray incidence angles significantly enhanced signals from surface species (134). Thus XPS angular distribution was born as a method for studying surface profiles, and it has been used extensively as a method for enhancing signals from surface layers (Figure 13). Another important development was that XPS could be used to study chemisorption of gases on metals. Brundle et al. studied chemisorption of CO on W and Mo films at 293 K, noting that it was possible to distinguish weakly and strongly bound states (135). The CO–Mo system was subsequently studied in greater detail at 80 K, two states were revealed in a ratio of about 4:1 (136). They concluded that CO adsorbs as molecules at low temperature and dissociates on warming. About the same time, Madey and Yates published a study of the chemisorption of CO and O2 on polycrystalline W (137). They studied the spectral features of both the adsorbed layer and the W substrate. In a subsequent study of the core levels of CO adsorbed on a W ribbon, they were able to identify interconversion between states of CO as a function of temperature (138). These articles represent the first two in an extensive series from that group dealing with


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Figure 13. Angular distributions from a gold foil having carbon overlayer: (top) Au 4f intensity, (center) C 1s intensity, (bottom) ratio of C 1s/Au 4f relative to the ratio measured at 90⬚ (reproduced from p 752 of ref16).

the use of XPS to study adsorbed species on highly ordered surfaces. As the surface capabilities of XPS became evident, a number of workers began to use the technique to study the chemical effects of surface treatments and reactions. One of the earliest was the work of Millard who studied the plasma and corona discharge treatment of wool (139). He monitored binding energy changes for S, N, C, and O and observed that only S showed a significant change, probably being oxidized to sulfate. Later work from this group characterized plasma-polymerized fluorocarbon films deposited on other polymers and wool (140). Carbon groups containing 1, 2, and 3 fluorine atoms were detected; the extent of film coverage was measured. Our group investigated the attachment of organic functionalities to fiberglass surfaces (107). The extent of amine coating by reaction with organosilanes was measured using the glass Si 2s line as an internal standard. Additionally it was possible to monitor reaction of benzenesulfonyl chloride with the surface-attached amine by XPS. We also studied plasma deposition of a thin polymer layer on an acrylic polymer that was used for corneal contact lenses, using deposition by an acetylene兾nitrogen兾water RF glow-discharge method (141). XPS showed that the nitrogen-containing species in the deposited film did not contain N⫺O bonds and was probably an amide.

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Electrochemistry and Corrosion Application of XPS to studies of electrochemistry and corrosion was another “natural” for the technique. Winograd began his extensive career in surface analysis by studying the Pt–O interaction on Pt electrodes by comparison with chemisorbed O on Pt (142). The presence of PtO on the electrode surface was established. A follow-up study of Pd electrodes observed PdO, PdO2, and Pd hydroxides (143). It was also possible to estimate the oxide film thickness, which varied with applied potential. Later the Winograd group extended their work to studies of underpotential deposition of Ag and Cu on Pt electrodes (144). About the same time, Castle had initiated studies of Cu– Ni alloy oxidation, which would lead into his work on applying XPS to study corrosion (145). His group studied interdiffusion in Cu–Ni alloys showing that surface enrichment of Ni is diffusion controlled (146). At this time they also published their first article on changes in the surface composition of Al–brass by electrochemical polarization in seawater, using this system as a model for corrosion of condenser tubes in marine environments (147). About the time Castle was beginning his work, Hulett published a article describing the composition of thin films formed when Cu–Ni alloys are electrochemically oxidized (148). The surface layer is primarily Ni when passivation occurs. Sherwood and Dickinson also began studies using XPS to monitor electrochemical reactions. They first studied anodic oxidation of Au and Pt electrodes, detecting oxides on the electrode surface (149). They correlated the quantities of oxide and film thickness measured by XPS with electrochemical results. Next they studied film formation on Au electrodes when polarized in acidic chromate solutions (150). They were able to show that the major component of the film was Cr(+3). They then began to study dissolution and passivation of Ni electrodes (151). They could obtain film compositions in the active and various passive regions, and developed an electrochemical cell that allowed sample transfer to the spectrometer with negligible air exposure. Atmospheric Particulates There was considerable interest in using XPS to study the composition of atmospheric particulates. Two studies appeared in 1971. Hulett et al. studied the chemical nature of sulfur oxides adsorbed on fly ash and coal smoke particles (152). It appeared that coal smoke particles contained sulfide while fly ash particles showed only oxidized sulfur species. In the same year Araktingi et al. published a report using broad range XPS scans to identify N, Pb, and S in air samples collected on filter article (153). They could identify the nature of Pb and S compounds in the samples; a total of ten elements were detected. The following year, Novakov et al. published the first in an extensive series of articles describing the chemical states of primarily N and S species in smog particles collected in Pasadena, CA (154). Lead was also detected. The particles were collected in a cascade impactor allowing particles to be both size and time segregated. The chemical states of sulfur and nitrogen were clearly identified. The sulfur content was highest at night with SO2 presumed to be the major species, which was then oxidized during the day. Nitrogen occurred


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Figure 14. Nitrogen XPS spectra for atmospheric particulate samples: I–nitrate, II–ammonium, III–amines, IV–pyridine (reproduced from ref 154).

as ammonium, nitrate, amino-, and pryidino- functionalities. An example of these studies is shown in Figure 14. Epilogue The purpose of this account has been to chronicle the development of XPS, ultimately as an effective method for surface analysis. Subsequent to 1977 almost all efforts to utilize XPS have been focused on that application. They have been highly successful. XPS is able to examine surface layers having thicknesses from sub-monolayers to ca. 50-nm thick layers. It provides element-specific information, and can frequently provide information about the oxidation states of an element. XPS also can often distinguish between organic functionalities involving the same element(s). Detection limits, stated as a fraction of the surface layer are generally about 0.01 monolayer. XPS has imaging capabilities with spatial resolution on the order of 10 µm. Unlike many surface methods, XPS data can be obtained for both conductors and insulators. A variety of samples can be handled effectively, which is one of the major reasons for the popularity of the technique. XPS is not the most surface-sensitive technique; lowenergy ion scattering is sensitive to only the top atomic layer. Also, XPS spectra do not have the greatest information content; secondary-ion mass spectrometry (SIMS) has much more. XPS does not have the best spatial resolution for imaging; Auger spectroscopy and SIMS do better than 0.1 µm. The value of XPS for surface analysis is that it can perform all of these measurements well and provide quantification, 1764


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making XPS a method that is applicable to a wide variety of materials problems. Because of this general applicability, it is quite likely that XPS will remain a major surface analytical technique for some time to come. Literature Cited 1. Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.-E.; Lindgren, I.; Lindberg, B. ESCA Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy; Presented to the Royal Society of Sciences of Uppsala, December 3, 1965; Amqvist & Wiksells: Uppsala, 1967. 2. Siegbahn, K. Electron Spectroscopy for Atoms, Molecules and Condensed Matter; Nobel Lecture, December 8, 1981. 3. Sokolowski, E.; Nordling, C.; Siegbahn, K. Phys. Rev. 1958, 110, 776. 4. Nordling, C.; Sokolowski, E.; Siegbahn, K. Arkiv Fysik 1958, 13, 483–500. 5. Nordling, C.; Hagstrom, S.; Siegbahn, K. Z. Physik 1964, 178, 433–438. 6. Hagstrom, S.; Nordling, C.; Siegbahn, K. Z. Physik 1964, 178, 439–444. 7. Hagstrom, S.; Nordling, C.; Siegbahn, K. Phys. Letters 1964, 9, 235–236. 8. Silverstein, R. M.; Bassler, G. C. Spectrometric Identification of Organic Compounds, 2nd ed.; John Wiley & Sons, Inc.: New York, 1967. 9. Siegbahn, K. Perspectives and Problems in Electron Spectroscopy. In Electron Spectroscopy; Shirley, D. A., Ed.; Proceedings of an International Conference Held at Asilomar, Pacific Grove, CA, Sep-


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10. 11. 12. 13.

14.

15.

16.

17. 18. 19. 20. 21.

22.

23. 24.

25.

26. 27. 28. 29.

30. 31. 32. 33. 34. 35. 36. 37. 38.

tember 7–10, 1971; North-Holland Publishing Company: Amsterdam, 1972. Steinhardt, R. G., Jr.; Serfass, E. J. Anal. Chem. 1951, 23, 1585– 1590. Steinhardt, R. G., Jr.; Serfass, E. J. Anal. Chem. 1953, 25, 697– 700. Steinhardt, R. G., Jr.; Granados, F. A. D.; Post, G. I. Anal. Chem. 1955, 27, 1046–1050. Electron Emission Spectroscopy; Dekeyser, W., Fiermans, L., Vanderkelen, G., Vennick, J., Eds.; Reidel: Dordrecht,The Netherlands, 1973. The Photoelectron Spectroscopy of Molecules. Faraday Disc. Chem. Soc. No. 54; Contributions from Meeting Held in Brighton, 1972; The Faraday Division Chemical Society: London, Electron Spectroscopy of Solids and Surfaces. Faraday Disc. Chem. Soc., No. 60; The Faraday Division Chemical Society: London, 1975. Electron Spectroscopy; Proceedings of the International Conference on Electron Spectroscopy held in Namur, April 16–19, 1974; Caudano, R.; Verbist, J., Eds.; Elsevier: Amsterdam, 1974. Physica Scripta 1977, 16, No. 5-6. Carlson, T. A. Photoelectron and Auger Spectroscopy; Plenum Press: New York, 1975. Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden: London, 1977. Characterization of Solid Surfaces; Kane, Philip F., Larrabee, Graydon B., Eds.; Plenum Press: New York, 1974. Methods of Surface Analysis. In Methods and Phenomena: Their Applications in Science and Technology; Czanderna, A. W., Ed.; Elsevier: Amsterdam, 1975. Characterization of Metal and Polymer Surfaces; Lee, Lieng-Huang, Ed.; Academic Press, Inc.: New York, 1977; Vol. 1 Metal Surfaces, Vol. 2, Polymer Surfaces. Electron Spectroscopy for Surface Analysis; Ibach, H., Ed.; SpringerVerlag: Berlin, 1977. Quantitative Surface Analysis of Materials; McIntyre, N. S., Ed.; A Symposium Sponsored by ASTM Committee E-42 on Surface Analysis; Cleveland, Ohio, March 2–3, 1977; American Society for Testing and Materials: Philadelphia, 1978. Electron Spectroscopy: Theory, Techniques and Applications; Brundle, C. R., Baker, A. D., Eds.; Academic Press: London, 1977; Vol. 1, p 459. Hercules, D. M. Anal. Chem. 1972, 44, 106R. Betteridge, D. Anal. Chem. 1972, 44, 100R. A History of Analytical Chemistry; Laitinen, H. A., Ewing, G. W., Eds.; Maple Press: York, PA, 1977; pp 198–204. Siegbahn, K.; Nordling, C.; Johansson, G.; Hedman, J.; Heden, P. F.; Hamrin, K.; Gelius, U.; Bergmark, T.; Werme, L. O.; Manne, R.; Baer, Y. ESCA Applied to Free Molecules; North-Holland Publishing Company: Amsterdam, 1969. Gelius, U.; Heden, P. F.; Hedman, J.; Linberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70. Ley, L.; Kowalczyk, S. P.; McFeely, F. R.; Pollack, R. A.; Shirley, D. A. Phys. Rev. B 1973, 8, 2392. Davis, D. W.; Shirley, D. A. J. Electron Spectros. 1974, 3, 137. Davis, D. W.; Banna, M. S.; Shirley, D. A. J. Chem. Phys. 1974, 60, 237. Hollander, J. M.; Hendrickson, D. N.; Jolly, W. L. J. Chem. Phys. 1968, 49, 3315–3316. Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Inorg. Chem. 1969, 8, 2642–2647. Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Inorg. Chem. 1970, 9, 612–615. Pelavin, M.; Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. J. Phys. Chem. 1970, 74, 1116–1121. Nordberg, R.; Brecht, H.; Albridge, R. G.; Fahlman, A.; Van Wazer, J. R. Inorg. Chem. 1970, 9, 2469–2474.

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39. Morgan, W. E.; Stec, W. J.; Albridge, R. G.; Van Wazer, J. R. Inorg. Chem. 1971, 10, 926–930. 40. Stec, W. J.; Modderman, W. E.; Albridge, R. G.; Van Wazer, J. R. J. Phys. Chem. 1971, 75, 3975–3979. 41. Hulett, L. D., Jr.; Carlson, T. A. Appl. Spectrosc. 1971, 25, 33–36. 42. McGuire, G. E.; Schweitzer, G. K.; Carlson, T. A. Inorg. Chem. 1973, 12, 2450–2453. 43. Carter, W. J.; Schweitzer, G. K.; Carlson, Thomas A. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 827–835. 44. Carlson, T. A.; Carver, J. C.; Saethre, L. J.; Garcia Santibanez, F.; Vernon, G. A. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 247– 258. 45. Carlson, T. A.; Carver, J. C.; Verson, G. A. J. Chem. Phys. 1975, 62, 932–935. 46. Swartz, W. E., Jr.; Wynne, K. J.; Hercules, D. M. Anal. Chem. 1971, 43, 1884. 47. Blackburn, J. R.; Nordberg, R.; Stevie, F.; Albridge, R. G.; Jones, M. M. Inorg. Chem. 1970, 9, 2374–2376. 48. Moddeman, W. E.; Blackburn, J. R.; Kumar, G.; Morgan, K. A.; Albridge, R. G.; Jones, M. M. Inorg. Chem. 1972, 11, 1715–1717. 49. Hillier, I. H.; Barber, M.; Connor, J. A.; Saunders, V. R. J. Chem. Soc. D 1971, 13, 682. 50. Barber, M.; Connor, J. A.; Hiller, I. H. Chem. Phys. Lett. 1971, 9, 570–571. 51. Hillier, I. H.; Barber, M.; Connor, J. A.; Guest, M. F.; Saunders, V. R. J. Chem. Soc. D 1971, 16, 943–944. 52. Cowan, D. O.; Park, J.; Barber, M.; Swift, P. J. Chem. Soc. D 1971, 22, 1444–1446. 53. Clark, D. T.; Adams, D. B.; Briggs, D. J. Chem. Soc. D 1971, 12, 602–604. 54. Clark, D. T.; Adams, D. B. Chem. Phys. Lett. 1971, 10, 121–126. 55. Clark, D. T.; Adams, D. B. J. Chem. Soc. D 1971, 14, 740–741. 56. Finn, P.; Jolly, W. L. Inorg. Chem. 1972, 11, 893–895. 57. Finn, P.; Jolly, W. L. Inorg. Chem. 1972, 11, 1434–1435. 58. Riggs, W. M. Anal. Chem. 1972, 44, 830–832. 59. Biloen, P.; Prins, R. Chem. Phys. Lett. 1972, 16, 611–613. 60. Cox, L. E.; Hercules, D. M. J. Electron Spectrosc. Relat. Phenom. 1972–1973, 1, 193. 61. Lindberg, B. J. Acta Chem. Scand. 1970, 24, 2242–2244. 62. Lindberg, B. J. Int. J. Sulfur Chem. 1972, 7, 33–53. 63. Barber, M.; Clark, D. T. J. Chem. Soc. D 1970, 1, 23–24. 64. Clark, D. T.; Chambers, R. D.; Kilcast, D.; Musgrave, W. K. R. J. Chem. Soc. 1972, 68, 309–319. 65. Jack, J. J.; Hercules, D. M. Anal. Chem. 1971, 43, 729. 66. Cox, L. E.; Jack, J. J.; Hercules, D. M. J. Am. Chem. Soc. 1972, 94, 6575. 67. Swartz, W. E., Jr.; Hercules, D. M. Anal. Chem. 1971, 43, 1066. 68. Swartz, W. E., Jr.; Ruff, J. K.; Hercules, D. M. J. Am. Chem. Soc. 1972, 94, 5227. 69. Green, B.; Ridley, D. C.; Sherwood, P. M. A. J. Chem. Soc. 1973, 10, 1042–1044. 70. Clark, D. T.; Kilcast, D.; Reid, D. H. J. Chem. Soc. D 1971, 12, 638–639. 71. Thomas, T. D. J. Chem. Phys. 1970, 52, 1373–1380. 72. Olah, G. A.; Mateescu, G. D.; Wilson, L. A.; Gross, M. H. J. Am. Chem. Soc. 1970, 92, 7231-7232. 73. Clark, D. T.; Kilcast, D.; Musgrave, W. K. R. J. Chem. Soc. D 1971, 10, 516–518. 74. Clark, D. T.; Adams, D. B.; Kilcast, D. Chem. Phys. Lett. 1972, 13, 439–445. 75. Jolly, W. L.; Hendrickson, D. N. J. Am. Chem. Soc. 1970, 92, 1863. 76. Jolly, W. L. J. Am. Chem. Soc. 1970, 92, 3260. 77. Jolly, W. L.; Perry, W. B. J. Am. Chem. Soc. 1973, 95, 5442. 78. Carver, J. C.; Gray, R. C.; Hercules, D. M. J. Am. Chem. Soc. 1974, 96, 6851. 79. Gray, R. C.; Carver, J. C.; Hercules, D. M. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 343.


•


1765

Waters Symposium: Electron Spectroscopy for Chemical Analysis 80. Gray, R. C.; Hercules, D. M. J. Electron Spectrosc. Relat. Phenom. 1977, 12, 37–53. 81. Clark, D. T.; Kilcast, D. J. Chem. Soc. A. 1971, 3286. 82. Mateescu, G. D.; Riemenschneider, J. L. In Electron Spectroscopy; Shirley, D. A., Ed.; North-Holland: Amsterdam, 1972; p 661. 83. Gelius, U.; Johansson, G.; Siegbahn, H.; Allan, C. J.; Allison, D. A.; Allison, J.; Siegbahn, K. J. Electron Spectrosc. 1972–1973, 1, 285. 84. Zeroka, D. Chem. Phys. Lett. 1972, 14, 471. 85. Lindberg, B. J.; Pinel, R.; Mollier, Y. Tetrahedron 1974, 30, 2537– 2540. 86. Lindberg, B. J. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 149– 166. 87. Clark, D. T.; Briggs, D.; Adams, D. B. J. Chem. Soc., Dalton Trans. 1973, 2, 168–172. 88. Barber, M.; Swift, P.; Cunningham, D.; Frazer, M. J. J. Chem. Soc. D 1970, 20, 1338. 89. Adams, I.; Thomas, J. M.; Bancroft, G. M.; Butler, K. D.; Barber, M. J. C. S. Chem. Comm. 1972, 751. 90. Cowan, D. O.; Park, J.; Barber, M.; Swift, P. J. Chem. Soc. D 1971, 22, 1444–1446. 91. Holsboer, F.; Beck W. Z. Naturforsch. 1972, 27b, 884. 92. Fahlman, A.; Hamrin, K.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Letters 1966, 20, 159–160. 93. Wagner, C. D. Anal. Chem. 1972, 44, 967–973. 94. Wagner, C. D.; Biloen, P. Surface Sci. 1973, 35, 82–95. 95. Castle, J. E.; Epler, D. Proc. Roy. Soc. London, Ser. A 1974, 339, 49–72. 96. Wagner, C. D. Anal. Chem. 1975, 47, 1201–1203 97. Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-Ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corporation: Eden Prairie, MN, 1979. 98. Brinen, J. S.; McClure, J. E. Anal. Lett. 1972, 5, 737–743. 99. Brinen, Jacob S.; McClure, James E. J. Electron Spectrosc. 1974, 4, 243–248. 100. Hercules, D. M.; Cox, L. E.; Onisick, S.; Nichols, G. D; Carver, J. C. Anal. Chem. 1973, 45, 1973. 101. Czuha, M., Jr.; Riggs, W. M. Anal. Chem. 1975, 47, 1836–1838. 102. Carvalho, M. B.; Hercules, D. M. Anal. Chem. 1978, 50, 2030. 103. Brundle, C. R.; Roberts, M. W. Chem. Phys. Lett. 1973, 18, 380– 381. 104. Phillips, L. V.; Hercules, D. M. In Quantitative Surface Analysis of Materials; McIntyre, N. S., Ed.; A Symposium Sponsored by ASTM Committee E-42 on Surface Analysis, Cleveland, Ohio, March 2– 3, 1977; American Society for Testing and Materials: Philadelphia, 1978; p 47. 105. Kramer, L. N., Klein, M. P. J. Chem. Phys. 1969, 51, 3620–3621. 106. Swartz, W. E., Jr.; Hercules, D. M. Anal. Chem. 1971, 43, 1774. 107. Nichols, G. D.; Hercules, D. M. Applied Spectrosc. 1974, 28, 219. 108. Wyatt, D. M.; Carver, J. C.; Hercules, D. M. Anal. Chem. 1975, 47, 1297. 109. Hercules, D. M.; Craig, N. L. J. Dental Research 1976, 55, 829– 835. 110. Wagner, C. D. Anal. Chem. 1972, 44, 1050–1053. 111. Wagner, C. D. Anal. Chem. 1977, 49, 1282–1290. 112. Wolberg, A.; Ogilvie, J. L.; Roth, J. F. J. Catal. 1970, 19, 86–89. 113. Delgass, W. N.; Hughes, T. R.; Fadley, C. S. Catal. Rev. 1970, 4, 179–219. 114. Miller, A. W.; Atkinson, W.; Barber, M.; Swift, P. J. Catal. 1971, 22, 140–142. 115. Whan, D. A.; Barber, M.; P. Swift J. Chem. Soc., Chem. Comm. 1972, 198. 116. Brinen, J. S.; Melera, A. J. Phys. Chem. 1972, 76, 2525–2526. 117. Brinen, J. S.; Schmitt, J. L.; Doughman, W. R.; Achorn, P. J.; Siegel, L. A.; Delgass, W. N. J. Catal. 1975, 40, 295–300. 118. Biloen, P.; Pott, G. T. J. Catal. 1973, 30, 169–174. 119. Patterson, T. A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M.

1766


•

Spectrosc. Lett. 1976, 9, 65–72. 120. Patterson, T.A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J. Phys. Chem. 1976, 80, 1700. 121. Ng, K. T.; Hercules, D. M. J. Phys. Chem. 1976, 80, 2094. 122. Clark, D. T.; Kilcast, D. Nature, Phys. Sci. 1971, 233, 77–79. 123. Ginnard, C. R.; Riggs, W. M. Anal. Chem. 1972, 44, 1310–1312. 124. Riggs, W. M.; Dwight, D. W. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 447–460. 125. Clark, D. T.; Kilcast, D.; Feast, W. J.; Musgrave, M. K. R. J. Polym. Sci., Part A-1 1972, 10, 1637–1654. 126. Clark, D. T.; Feast, W. J.; Kilcast, D.; Musgrave, W. K. R. J. Polym. Chem. Ed. 1973, 11, 389–411. 127. Clark, D. T.; Feast, W. J.; Ritchie, I.; Musgrave, W. K. R.; Modena, M.; Ragazzini, M. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 1049– 1063. 128. Wyatt, D. M.; Gray, R. C.; Carver, J. C.; Hercules, D. M.; Masters, L. W. Applied Spectrosc. 1974, 28, 439. 129. Briggs, D.; Brewis, D. M.; Konieczko, M. B. J. Mater. Sci. 1976, 11, 1270–1277. 130. Briggs, D.; Brewis, D. M.; Konieczko, M. B. J. Mater. Sci. 1977, 12, 429–433. 131. Siegbahn, K.; Gelius, U.; Siegbahn, H.; Olson, E. Phys. Scr. 1970, 1, 272–276. 132. Fadley, C. S.; Bergstrom, S. A. L. Phys. Lett. A 1971, 35, 375– 376. 133. Baird, R. J.; Fadley, C. S.; Kawamoto, S.; Mehta, M. Chem. Phys. Lett. 1975, 34, 49–54. 134. Mehta, M.; Fadley, C. S. Phys. Lett. A 1975, 55A, 59–61. 135. Atkinson, S. J.; Brundle, C. R.; Roberts, M. W. J. Electron Spectrosc. Relat. Phenom. 1973, 2, 105–109. 136. Atkinson, S. J., Brundle, C. R.; Roberts, M. W. Chem. Phys. Lett. 1974, 24, 175–178. 137. Madey, T. E.; Yates, J. T., Jr.; Erickson, N. E. Chem. Phys. Lett. 1973, 19, 487–492. 138. Yates, J. T., Jr.; Madey, T. E.; Erickson, N. E. Surface Sci. 1974, 43, 257–274. 139. Millard, M. M. Anal. Chem. 1972, 44, 828–829. 140. Millard, M. M.; Pavlath, A. E. J. Macromol. Sci., Chem. 1976, A10, 579–597. 141. Yasuda, H.; Bumgarner, M. O.; Marsh, H. C.; Yamanashi, B. S.; Devito, D. P.; Wolbarsht, M. L.; Reed, J. W.; Bessler, M.; Landers, M. B., III; Hercules, D. M.; Carver, J. J. Biomed. Mater. Res. 1975, 9, 629–643. 142. Kim, K. S.; Winograd, N.; Davis, R. E. J. Am. Chem. Soc. 1971, 93, 6296–6297. 143. Kim, K. S.; Gossman, A. F.; Winograd, Nicholas Anal. Chem. 1974, 46, 197–200. 144. Hammond, J. S.; Winograd, N. J. Electrochem. Soc. 1977, 124, 826–833. 145. Castle, J. E. Nature, Phys. Sci. 1971, 234, 93–95. 146. Castle, J. E.; Nasserian-Riabi, M. Corros. Sci. 1975, 15, 537–543. 147. Castle, J. E.; Epler, D. C. Surf. Sci. 1975, 53, 286–296. 148. Hulett, L. D.; Bacarella, A. L.; LiDonnici, L.; Griess, J. C. J. Electron Spectrosc. Relat. Phenom. 1972, 1, 169–178. 149. Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1975, 71, 298–311. 150. Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1976, 72, 686–705. 151. Dickinson, T.; Povey, A. F.; Sherwood, P.M. A. J. Chem. Soc., Faraday Trans. 1 1977, 73, 327–343. 152. Hulett, L. D.; Carlson, T. A.; Fish, B. R.; Durham, J. L. In Determination Air Quality, Proceedings ACS Symposium, 1971; Mamantov, G., Ed.; Plenum: New York, 1972. 153. Araktingi, Y. E.; Bhacca, N. S.; Proctor, W. G.; Robinson, J. W. Spectroscop. Letters 1971, 4, 365. 154. Novakov, T.; Mueller, P. K.; Alcocer, A. E.; Otvos, J. W. J. Colloid Interface Sci. 1972, 39, 225–234.


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