Research group 19. Current research topics
The subjects of our current studies are metal oxides doped with nanoparticles of
other metals, which are used as supports for catalysts. The changes in the electron state of
oxide-support and in the doped metal-interactive chemical species induced by various
preparation method, thermal processing and the impact of the selected reactive gases are
examined using XPS and AES.
Ceria oxide doped by nanosize particles of gold and one of the metals: Co, Sn, Mn, Fe, is a
model system of investigations. The method of preparation of such catalysts significantly
affects the electron state of both oxide support and doped metals. An important prerequisite
is to obtain highly dispersed nanoparticles of metals whose interaction with the oxide
support leads to the formation of catalytically active systems. Particularly important is the
Au doping. Unlike traditional metal supported catalysts, metallic gold nanoparticles are not
active in the catalytic reaction; for the catalytic activity the nonmetallic, positively charged
gold nanoparticles (described as Auare responsible.They are are strongly bound with
CeO2. The presence of such nanoparticles on the catalyst surface shows the XPS spectrum
of Au 4f , recorded on PHI 5000 VersaProbe spectrometer for Au/CeO2 sample doped with
Mn.
Intensity [a.u.]
Au
Au
Au
92
90
88
0
+
+3
86
84
82
Binding Energy [eV]
Methods of preparation significantly affect the electron state of gold species supported on
CeO2 support; in addition to the desired states of Au. The negatively charged states of
gold species Au can be generated as a result of electron transfer from the ceria support, or
as a result of the agglomeration the metallic Au crystallites. However, the latter process
does not generate the catalytic activity. In the model system Au-CeO2, the active site
location of Au nanoparticles in ceria support is formed by oxygen vacancies, lattice sites
with a deficit of oxygen, in which the cerium atoms are at a lower oxidation state (Ce+3)
compared to the basic structure of the crystal lattice of CeO2 (Ce+4).
Model systems are also studied to determine the selected parameters of electron transport,
particularly the mean free path by EPES. This parameter is very important in the practical
quantitative analysis in XPS and AES spectroscopy. These studies are intended to explain
the course of basic catalytic processes, selection of the optimal reaction conditions and the
selection of the optimal catalyst support for the chosen reaction. Systems of metal
nanoparticles on oxide supports are known as catalyst systems, with an exceptionally wide
range of application in environmental processes: low-temperature gas combustion
(removal of hydrocarbons, oxides of nitrogen) and low temperature water gas conversion
reaction as a source of pure hydrogen for energy (fuel cells, clean motor fuel) and also
photocatalytic removal of pollution from surface waters.
One should mention that the laboratory is very well equipped in the software
packets that make possible extensive processing of the recorded spectra (identification of
elements, identification of chemical state, quantitative analysis, concentration profiles,
etc.). Some of the software packets are currently developed as a result of projects realized
by the research group. As an example, one can mention the software packets EPES and
MULTI. Program EPES realizes, in a user-friendly way, calculations associated with
applications of elastic peak spectroscopy. Main application is the determination of the
inelastic mean free path of signal electrons. Software packet MULTI implements theory
that determines the surface composition from measured intensities of all XPS peaks visible
in the spectra. In the research group, theoretical studies are realized that are devoted to
modeling electron transport in condensed matter. Two aspects of theory are currently
addressed: calculations of elastic electron backscattering probability for solid surfaces, and
the problem of the sampling depth of electron spectroscopies.
The studies of modified Ti surfaces for biomedical purposes have been
concentrated on observations of morphology and identifying their physicochemical
properties. A promising approach to meet the above requirements it to create modern type
of composite coatings on Ti substrate, namely Ca-P/Ag/TiO2 composite coatings, with
well-defined microstructure, chemical and phase composition, controlled porosity and
surface topography. Preliminary studies have shown that TiO2 nanotubes fabricated via
anodization technique have ordered structure and their growth is perpendicular to Ti
substrate. The specific surface morphology of TiO2 nanotubes facilitates the formation of
calcium phosphate (Ca-P) layers under physiological conditions. Additional loading of the
resulted coatings with Ag nanoparticles with diameter of 2 – 50 nm using sputter
deposition technique is expected to provide antiseptic properties. Composite layers on Ti
consisting of bioactive ceramic coating and Ag nanoparticles should have a positive impact
on the osteoblasts activity and prevent bacterial adhesion to the implant surface. Moreover,
nanoporous oxide layers (TiO2, Al2O3) decorated with nanoparticles of Ag, Au or Cu can
be used as model substrates for SERS investigations. Such systems may be particularly
active substrates capable of increasing the cross sections for Raman scattering of adsorbed
organic molecules (e.g. pyridine) to a degree much higher than is possible on the
electrochemically roughened surfaces of SERS active metals. SERS spectra of molecules
adsorbed on the surface of new nano-structured materials will allow to understand
electromagnetic (depending on the morphology) and chemical (the effect of CT (charge
transfer) transition associated with a partial charge between adsorbate and adsorbent)
effects. SERS investigation using substrates with variable nanoporous morphology and
different sizes of metal nanoparticles, is of particular interest since the reduction in size of
material through the nanoscale regime (< 100 nm) can lead to a new chemical n\and
physical properties clearly different from the bulk counterparts of those materials.
Using simple methods of chemical synthesis of oxides based on rare-earth in aqueous
solutions of organic acids, combined with heat treatment can lead to create a new
generation of nanomaterials (oxygen ions conductors based on CeO2). These types of
materials that can be used as solid electrolyte for intermediate-temperature solid oxide fuel
cells (SOFC) or oxygen pumps, are showing a higher ionic conductivity in the temperature
range 500C - 700C than stabilized zirconia (YSZ) used so far. Low working temperature
of electrolytes based on CeO2 and stability in a reducing atmosphere (in contrast to
materials based on Bi2O3) can allow for their wider use.
TiO2 nanotubes with silver nanoparticles afte immersion in Hanks’ solution, 6 h.
Ważniejsze publikacje:
1. D. Andreeva, M. Kantcheva, I. Ivanov, L. Ilieva, J.W. Sobczak, W.
Lisowski
Gold supported on ceria doped by Me3+ (Me3+=Al and Sm) for water gas
shift reaction: Influence of dopant and preparation method, Catalysis Today,
158, 69-77 (2010)
2. Jablonski A.
“Quantification of surface sensitive electron spectroscopies ”. Surface
Sci. 603, 1342-1352 (2009).
3. A.Roguska, M.Pisarek, M.Andrzejczuk, M.Dolata, M.Lewandowska,
M.Janik-Czachor
“Characterization of a calcium phosphate -TiO2 nanotube composite layer
for biomedical applications”. Materials Science and Engineering C, 31
(2011) 906-914.