In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 515

13.5.3 X-Ray Spectroscopic Techniques
X-ray spectroscopic techniques (XRD, XAS, XRF, and XRR) offer a variety
of utilities for studying functional materials related to crystal structure, fine
structure for nearest-neighbor chemical bonding, and other relevant physical
and chemical information from atomic to micrometer scale. X-ray spectrome-
try has been reviewed by Szaloki et al. (2000). For in situ investigations, XRD
has been used to study crystal structure changes and phase transformations in
topotactic electrochemical reactions in battery electrodes quite commonly to
date (e.g., Yang, Sun, McBreen 2000; Dubarry et al. 2008). Russell and Rose
(2004) provided a comprehensive review about XAS for the study of fuel cell
catalysts is a good example of using in situ investigations to reveal the catalyst
behavior in a working condition. The behavior of the catalyst can be studied as
a function of particle size, composition, and morphology in porous structure.
Analysis of the x-ray absorption near-edge structure (XANES) can provide
the information of the oxidation state of the catalyst center, and the extended
x-ray absorption fine structure (EXAFS) can reveal the short-range crystal
structure information of the reaction site. This type of information can be
valuable to the understanding of the reaction pathway and mechanism. These
techniques are, however, not designed to study the porous structure regarding
pore size, porosity, and pore distribution.
X-ray scattering at glancing angles known as grazing incidence x-ray reflec-
tometry (GIXR) is a powerful tool for thin film analysis (Stoev and Sakurai
1997). Similar in principle to XRD and ellipsometry, x-ray reflectometry
(XRR) is a nondestructive and noninvasive technique commonly for thin film
thickness determination between a few to several hundreds nm with a precision
of about 1-3 A. XRR takes the advantage of surface reflectivity changes due
to film coverage or roughness by measuring the intensity of X-rays reflected
from a surface as a function of the AOI. Thin films on a surface can give rise
to oscillations of the x-ray intensity with the AOI. This technique is therefore
useful for the determination of density and roughness of films and multilayers
with a high precision. Holy, Pietsch, and Baumbach (Holy et al. 1999) provide
an excellent discussion of the theoretical basis of the x-ray scattering measure-
ments and model simulation. Recent developments in the three-dimensional
micro XRF analysis, which employ two aligned confocal x-ray optics, pro-
vides interesting opportunities for three-dimensional imaging and quantita-
tive analysis of a variety of samples, ranging from art pieces to geological and
biological species (Malzer 2006). The visualization of major, minor, and trace
constituents in these samples offers a powerful tool for nondestructive, in situ
studies quantitatively (Janssens et al. 2004; Vincze et al. 2004). Illustrated
in Figure 13.16 is an example by Briscoe et al. (2007) using a bending mica
method to enable a proof-of-principle XRR measurements on three very dif-
ferent systems: (a) a Cr-Au nanofilm thermally evaporated in high vacuum
on oxygen plasma-cleaned mica, (b) a surface-grown zwitterionic polymer
brush made from poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC)