Seminar"Theoretical X-ray absorption spectra - a database approach towards understanding amorphous materials"

Title"Theoretical X-ray absorption spectra - a database approach towards understanding
amorphous materials"

Speaker: Fabian Weber, Institute of Methods for Material Development, Helmholtz-Zentrum Berlin f¨ur Materialien und Energie GmbH, Germany

Abstract:

The requirements for photocatalytic water splitting reactions are the ability to absorb light and
to transform this energy in a well-separable electron-hole pair of a suitable potential energy
to induce the oxidation and reduction reactions, respectively. Most of these requirements can
already be investigated by looking at the electronic structure of a material, i.e. its spectroscopic
behaviour. Grapheneoxide quantum dots (GO-QDs) present a resource-efficient material which
have proven [1] to meet these requirements and that can be readily produced by applying strong
oxidizing reagents to graphite. Due to the harsh reaction conditions, however, the functionalization
with oxygen on the QD surface becomes randomized [2].
Until now, the importance of specific functionalization patterns on the GO-QD surface for its
catalytic activity has not been investigated. Since there exists no clear repetitive pattern in
the GO-QD structure, it is not easy to probe the photocatalytic activity experimentally in a
systematic way. Since X-ray absorption (XA) spectroscopy is a standard technique to identify
neighbouring functionalities in a very selective way, we developed a time-dependent density
functional theory (TDDFT) approach to probe the constitution of single specific atoms in
model systems in a meaningful way. Since these localized carbon K-edge XA spectra have
shown to be specific for up to about 3 chemical bonds, we may create a database of model
systems of relatively small size to generate surrounding specific XA spectra of several unique
functionalization patterns. In this talk we show how systematic comparison of this database of
several hundreds of different surrounding-specific XA spectra with experimental data leads to
a straightforward and systematically improvable method to gain insights on structural features
of amorphous materials.
References:
[1] Yeh, Cihl`aˇr, Chang, Cheng, Teng, Mater. Today, 16, 78 (2013)
[2] Dreyer, Park, Bielawski, Ruoff, Chem. Soc. Rev., 39, 228 (2010)
[3] Zunger, J. Phys. C., 7, 76 (1974)