CHARACTERIZATION OF SURFACE FUNCTIONAL GROUPS ON CARBON MATERIALS WITH X-RAY ABSORPTION NEAR EDGE STRUCTURE (XANES) SPECTROSCOPY

Open Access
Author:
Kim, Kyungsoo
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Yongsheng Chen, Thesis Advisor
Keywords:
  • functional group
  • surface
  • carbon
  • XANES
  • characterization
Abstract:
Carbon materials have many applications due to their bulk or surface properties, and for the latter, surface functional groups play a decisive role. Many characterization techniques have been used to identify and quantify surface functional groups, but none is completely satisfactory so far especially when it comes to quantification. X-ray absorption near edge structure (XANES) spectroscopy has several advantages over the commonly used techniques, e.g., element specific, high sensitivity, characteristic spectra for different functional groups, and straightforward quantitative analysis. For oxygen functional groups on carbon materials, oxygen K-edge (543.1 eV) XANES is preferred over carbon K-edge (284.2 eV) since it minimizes the strong interference from the carbon substrate. Oxygen K-edge XANES was attempted over 13 years ago, but it was only partially successful with qualitative analysis due to spectrum distortion from charging effect because of poor sample conductivity. In this work, we collect oxygen K-edge spectra in both fluorescence yield (FY) and total electron yield (TEY) modes. Although relatively noisier, spectra collected in FY mode do not suffer from charging effect, which is very important for quantitative analysis by linear combination fitting. It is shown that an oxygen K-edge spectrum of oxygen containing functional group mainly consists of a pre-edge peak and a whiteline (the first intense post-edge peak). The XANES spectra are unique in the position and/or the intensity of the pre-edge peak and the whiteline. Thus, identification of different functional groups is possible by fingerprinting. By measuring known functional groups in reference compounds, it is determined that oxygen functional groups on carbon materials can be grouped into three categories. Each category consists of a few functional groups, which have very similar XANES spectra, while significant difference exists between categories. Carboxyl-type groups include carboxyl, ester, and anhydride groups, and the features are a strong pre-edge peak at about 531 eV and a broad whiteline at 539 eV. Carbonyl-type groups include carbonyl and aldehyde groups, and the features are a strong pre-edge peak at below 530 eV and a broad whiteline at 539 eV. Hydroxyl-type groups include hydroxyl, phenol, and ether groups, and the features are no or very weak pre-edge peak at about 531 eV and a broad whiteline at 536-538 eV. Four activated carbons that underwent different treatments are analyzed by XANES. The relative total oxygen contents can be calculated from the edge step of their XANES spectra, and the identity and relative abundance of different functional groups are determined by fitting of a sample XANES spectrum to a linear combination of spectra of the reference compounds. It is concluded that oxygen K-edge XANES spectroscopy is a reliable characterization technique for the identification and quantification of surface functional groups on carbon materials