PNNL-13501 - Pacific Northwest National Laboratory

Anatase (001)
Anatase (101)
Figure 1. RHEED pattern obtained at 15 keV for an anatase
(001) film (top) and for an anatase (101) film (bottom)
studies will be performed to better understand the
interaction water). A comparison between experiment
and single-scattering calculations gave good agreement
for a model with a bulk-like anatase termination. These
are the first measurements, with atomic specificity, that
indicate anatase is stable under ultrahigh vacuum
conditions. Furthermore, a direct comparison was made
between rutile and anatase using x-ray photoelectron
spectroscopy and it was found that the core-level binding
energies were identical for the polymorphs for Ti 2p and
O 1s emission.
Mass-Spectroscopy of Recoiled Ions
On further processing of the anatase (001) thin films, it
was found that the surface undergoes a (1x4)
reconstruction, as opposed to the expected (1x1) bulk
termination. The low-energy electron diffraction pattern
is shown in Figure 2 with the unit cells for a (1x1)
termination and the two domain (1x4) termination
indicated. This reconstruction was found to be very stable
and has not previously been reported. We have used
angle-resolved mass spectroscopy of recoiled ions
(AR-MSRI), a technique that has recently been developed
in the EMSL, to better understand the surface
reconstruction, and the influence it may have on the
photocatalytic hydrogen production from water. A
comparison between our experimental data with
theoretical simulations for over 30 reconstructions results
in a best fit for a model which microfacets to expose
lower energy {013} surface planes. It is clearly evident
336 FY 2000 Laboratory Directed Research and Development Annual Report
70 eV
(1x1)
Two-Domain
(1x4)
Figure 2. LEED pattern obtained for an anatase (001) film
grown on SrTiO 3 (001). Note the presence of a two-domain
(1x4) reconstruction.
that to fully understand the interaction of water with
anatase surfaces it is essential to experimentally
determine the detailed atomic coordinates of the surface.
For example, prior theoretical studies on the interaction of
water with anatase used a bulk terminated (1x1) anatase
surface which apparently is not stable. We have also
performed the AR-MSRI experiments on the anatase
(101) surface. Preliminary results suggest that there are
surface relaxations in the very top layers. Further analysis
of the data is currently under way. Total energy
minimization calculations are currently being performed
by the group from PU to complement our studies.
Synchrotron Radiation Studies
With the group at Lawrence Berkeley National
Laboratory, we have performed experiments to measure
the electronic structure of anatase. We have measured the
x-ray absorption spectra, x-ray photoelectron diffraction
data, and high-energy resolution x-ray photoelectron
spectra for both anatase and rutile surfaces. These
experiments were performed at the Advanced Light
Source in Berkeley, California, on beamline 9.3.2. The
data confirm that the surface of anatase is fully oxidized,
and the x-ray photoelectron diffraction data are currently
being analyzed.
Scanning Tunneling Microscopy
Recently, we have performed scanning tunneling
microscopy on the anatase (101) surface in collaboration
with the group at Tulane University. The surface has
several different types of defects. This work has also