The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Development of XPS Data Analysis and its Application to Gas Sensor and Catalyst Surface Studies

Mikko Aronniemi

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium K at Helsinki University of Technology (Espoo, Finland) on the 14th of August, 2007, at 13 o'clock.

Overview in PDF format (ISBN 978-951-22-8882-3)   [1512 KB]
Dissertation is also available in print (ISBN 978-951-22-8881-6)

Abstract

In many applications of surface science, the key question is the chemical composition of the surface. X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is an experimental technique which is commonly used in these studies. In this method, the sample is irradiated with x-rays of known energy, thus causing emission of electrons by the photoelectric effect. The binding energy spectrum of these photoelectrons is recorded, and by analyzing the spectrum, the elements and their chemical states can be determined within a few nanometers from the sample surface. The two main steps in the data analysis are subtracting the background intensity caused by inelastically scattered photoelectrons and resolving the overlapping contributions of different chemical species.

This work deals with development of methods for XPS data analysis. Decomposing spectra with overlapping states by curve-fitting is illustrated with a case study of iron and chromium oxides. A lineshape suitable for fitting the core level spectra is described and the differences between three background subtraction methods are investigated. When studying typical sources of uncertainty, the choice of the background turns out to be significant. Factor analysis is presented as an alternative method to determine the contributions of different chemical states in a set of XPS spectra. Aspects affecting the accuracy of the analysis results are pointed out and modifications to a commonly used analysis procedure are proposed. It is shown that in the case of two-component data, a simple scanning of a delta peak along the binding energy axis is capable of generating acceptable component spectra.

The described analysis methods are utilized in two different applications, chromium oxide catalyst and iron oxide gas sensor. In both of these, the functionality of the surface depends strongly on the chemical state of the metal ions. In the catalyst study, XPS is used to analyze the chemical states of chromium in samples subjected to oxidation and reduction treatments. In the case of the gas sensor, XPS provides insight into the sensing mechanism of an iron oxide thin film.

This thesis consists of an overview and of the following 6 publications:

  1. M. Aronniemi, J. Sainio, J. Lahtinen, Chemical state quantification of iron and chromium oxides using XPS: the effect of the background subtraction method, Surface Science 578, 108-123 (2005). © 2005 Elsevier Science. By permission.
  2. M. Aronniemi, J. Sainio, J. Lahtinen, Aspects of using the factor analysis for XPS data interpretation, Surface Science 601, 479-489 (2007). © 2007 Elsevier Science. By permission.
  3. J. Sainio, M. Aronniemi, O. Pakarinen, K. Kauraala, S. Airaksinen, O. Krause, J. Lahtinen, An XPS study of CrOx on a thin alumina film and in alumina supported catalysts, Applied Surface Science 252, 1076-1083 (2005). © 2005 Elsevier Science. By permission.
  4. M. Aronniemi, J. Lahtinen, P. Hautojärvi, Characterization of iron oxide thin films, Surface and Interface Analysis 36, 1004-1006 (2004). © 2004 John Wiley & Sons. By permission.
  5. M. Aronniemi, J. Sainio, J. Lahtinen, Characterization and gas-sensing behavior of an iron oxide thin film prepared by atomic layer deposition, Helsinki University of Technology, Publications in Engineering Physics A, Report TKK-F-A850 (2007). © 2007 by authors.
  6. M. Aronniemi, J. Sainio, J. Lahtinen, XPS study on the correlation between chemical state and oxygen-sensing properties of an iron oxide thin film, Helsinki University of Technology, Publications in Engineering Physics A, Report TKK-F-A851 (2007). Applied Surface Science, accepted for publication (2007). © 2007 by authors and © 2007 Elsevier Science. By permission.

Keywords: XPS, data analysis, factor analysis, iron oxide, chromium oxide, gas sensor, catalyst

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© 2007 Helsinki University of Technology

Last update 2011-05-26