Growth, structure and annealing behaviour of epitaxial ZrO2 films on Pt(1 1 1)

Article abstract of DOI:10.1016/S0039-6028(03)00187-0

For future investigations of catalytic reactions on sulfated ZrO₂ surfaces by means of scanning tunneling microscopy (STM), ZrO₂ films are prepared on Pt(1 1 1) and characterized by STM and low-energy electron diffraction (LEED). The films are prepared by vapor deposition of Zr in an O₂ atmosphere followed by annealing (also in an O₂ atmosphere). Conditions are searched where well-ordered, continuous and smooth ZrO₂ films are formed. Depositing the films according to literature data at room temperature [Surf. Sci. 237 (1990) 166] yields films with hillock morphology which decay during annealing and loose continuity. The desired film perfection is attained, however, if the films are deposited at 470 K and postannealed at 950 K. Continuous films are obtained displaying large terraces and a clear p(1 × 1) ZrO₂(1 1 1) LEED pattern. A splitting of the LEED spots reveals that the films are slightly rotated with respect to Pt(1 1 1). However, annealing the films at temperatures > 1000 K again yields discontinuous films which indicates the metastable character of the ZrO₂ films on Pt(1 1 1) substrate.

Full text of DOI:10.1016/S0039-6028(03)00187-0

Surface Science 532–535 (2003) 420–424
www.elsevier.com/locate/susc
Growth, structure and annealing behaviour of epitaxial ZrO2
films on Pt(1 1 1)
*
Klaus Meinel, Karl-Michael Schindler, Henning Neddermeyer
Fachbereich Physik, Martin-Luther-Universit€at Halle-Wittenberg, D-06099 Halle, Germany
Abstract
For future investigations of catalytic reactions on sulfated ZrO2 surfaces by means of scanning tunneling microscopy
(STM), ZrO2 films are prepared on Pt(1 1 1) and characterized by STM and low-energy electron diffraction (LEED).
The films are prepared by vapor deposition of Zr in an O2 atmosphere followed by annealing (also in an O2 atmo-
sphere). Conditions are searched where well-ordered, continuous and smooth ZrO2 films are formed. Depositing the
films according to literature data at room temperature [Surf. Sci. 237 (1990) 166] yields films with hillock morphology
which decay during annealing and loose continuity. The desired film perfection is attained, however, if the films are
deposited at 470 Kand postannealed at 950 K. Continuous films are obtained displaying large terraces and a clear
p(1 ꢀ 1) ZrO2(1 1 1) LEED pattern. A splitting of the LEED spots reveals that the films are slightly rotated with respect
to Pt(1 1 1). However, annealing the films at temperatures >1000 Kagain yields discontinuous films which indicates the
metastable character of the ZrO2 films on Pt(1 1 1) substrate.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Zirconium; Scanning tunneling microscopy; Surface structure, morphology, roughness, and topography; Growth; Epitaxy;
Low energy electron diffraction (LEED)
1. Introduction
investigations of the ZrO2 surface during reaction
are very promising for elucidating details of the
reaction mechanism [2]. For such studies, it is
suitable to prepare the insulating ZrO2 as a thin
film on a conducting substrate before sulfation for
preventing charging effects [3,4]. The films, how-
ever, should display a well-ordered structure and
an almost two-dimensional (2D) morphology
characterized by large and smooth terraces for
having clear conditions for further investigations.
In our studies, thin ZrO2 films are prepared on a
Pt(1 1 1) substrate and characterized by STM and
LEED. The preparation follows a procedure de-
scribed by Maurice et al. [5] who reported on
the formation of ZrO2(1 1 1) films on Pt(1 1 1)
substrate displaying face centered cubic (fcc) C1
Sulfated ZrO2 catalysts are extremely active in
the low temperature isomerization of n-alkanes [1].
However, the mechanism of the reaction, in par-
ticular the specificity and role of surface active
centers, are unknown up to now. A deeper un-
derstanding of the activation of alkanes on the
ZrO2 surface is of crucial relevance as alkanes are
the chemical raw materials of the future. STM
* Corresponding author. Tel.: +49-345-55-25560; fax: +49-
345-55-27160.
E-mail address: neddermeyer@physik.uni-halle.de (H. Ned-
dermeyer).
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00187-0

K. Meinel et al. / Surface Science 532–535 (2003) 420–424
421
(calcium fluoride like) structure. The films showed
a (8/3 ꢀ 8/3) periodicity with regard to the Pt(1 1 1)
substrate as deduced from LEED which was as-
signed to a p(2 ꢀ 2) reconstruction induced by
missing O atoms. From the results of integral
techniques of surface analysis, the authors de-
duced a 2D film morphology of the ZrO2 films.
The films have been prepared by evaporating Zr
onto Pt(1 1 1) at room temperature in an O2 at-
mosphere of about 10ꢁ6 mbar and by postan-
nealing (also in an O2 atmosphere of about 10ꢁ6
mbar) at temperatures between 900 and 1100 K.
However, we found that the films prepared ac-
cording to the same procedure described in [5]
display a hillock or even a discontinuous mor-
phology as revealed by STM. We have therefore
increased the temperature of deposition for ame-
liorating the film quality. Similar as in previous
studies of the growth of other oxide films [4], best
conditions for preparing high-quality films were
found for deposition temperatures around 470 K.
After annealing, such films display the desired
well-ordered structure and an almost 2D film
morphology. However, annealing the films at
temperatures above 1000 Kinduces the beginning
of film decay which demonstrates the metastable
character of the ZrO2 films.
pressure about 1 ꢀ 10ꢁ6 mbar) from the ending of
a Zr wire (purity 0.9995) heated by electron
bombardment. The evaporation rate was mea-
sured by means of a quartz microbalance and by
STM measurements of the volume of deposited
submonolayers of clean Zr (UHV deposition) and
ZrO2, respectively. The evaporation rate was re-
stricted to values between 0.1 and 0.2 monolayers
(ML)/min due to the low vapor pressure of Zr.
After deposition of the film, the sample was post-
annealed at temperatures between 470 and 1150 K
(also in an O2 atmosphere of 1 ꢀ 10ꢁ6 mbar) and
finally characterized by LEED and STM. In the
LEED experiments, the usual electron energy was
66 eV. The STM studies were performed in the
constant current mode with tunneling currents
between 0.1 and 1 nA and positive sample biases
between 0.1 and 4 V.
3. Results and discussion
In a first series of experiments, we prepared the
films according to the procedure described by
Maurice et al. [5], i.e. the Zr was deposited at room
temperature and the sample postannealed up to
temperatures where ZrO2 LEED spots appear. Fig.
1(a) shows the STM image of a 4 ML thick ZrO2
film taken after deposition. A high density of small
hillocks is revealed which is obviously induced by
multilayer growth where several layer levels are
simultaneously developed. Annealing the film at
temperature above 900 Kinduces film smoothing.
However, the hillock morphology remains even if
the film is annealed up to 1000 Kas demonstrated
by the STM image of Fig. 1(b). About four dif-
ferent layer levels are identified formed by islands
and terraces, the width of which does not exceed 10
nm. Increasing the postannealing temperature to
about 1050 Kdoes not yield the desired comple-
tion of the film smoothing. Instead, the film gets a
discontinuous character. First, deep holes are
formed ranging down to the substrate as shown by
the STM image of Fig. 1(c) (see arrow). Continu-
ation of the heating completely destroys film con-
nection and induces the formation of three
dimensional aggregations of ZrO2 sitting on the
free lying Pt(1 1 1) substrate (Fig. 1(d)).
2. Experiments
The experiments have been performed in an
UHV chamber (base pressure 10ꢁ10 mbar), hous-
ing a room temperature STM and a high-resolu-
tion LEED system for spot profile analysis
(SPALEED). In addition, the chamber is equipped
with facilities for sample heating, Arþ ion bom-
bardment, defined gas inlet and Zr evaporation.
The Pt(1 1 1) substrate (miscut < 0.1°) was cleaned
in-situ by cycles of Arþ ion bombardment, UHV
heating at temperatures of about 1300 Kand
heating in an O2 atmosphere at 800 Kas described
elsewhere [6]. Temperature was measured by
means of a pyrometer. The final surface showed a
bright LEED pattern and STM revealed clean and
large terraces (width > 100 nm) separated by
monoatomic steps. For preparing the ZrO2 layers,
Zr was evaporated in an O2 atmosphere (O2

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K. Meinel et al. / Surface Science 532–535 (2003) 420–424
Fig. 1. STM images of 4 ML thick ZrO2 films on Pt(1 1 1) deposited at room temperature taken after deposition (a) and after pos-
tannealing at 1000 K(b) (annealing time 20 s) and 1050 K(c,d; annealing time 20 and 60 s, respectively). Image size 75 nm ꢀ 150 nm.
In another series of experiments, the films (mean
thickness also 4 ML) have been deposited at a
temperature of 470 Kwhere in our group best
conditions were found for a 2D film formation for
ZrO2(1 1 1) LEED pattern. It is observed together
with the p(1 ꢀ 1) LEED pattern of the Pt(1 1 1)
substrate which is yet visible at film thicknesses
below 5 ML. The reciprocal unit vectors (shortest
distance between two spots) of the p(1 ꢀ 1) Pt
LEED pattern correspond to a 0.28 nm distance
between neighboring Pt atoms in real space. In
contrast, the ZrO2 LEED pattern indicates a (1 1 1)
lattice with a mesh size of about 0.36 nm, which
other oxides [4]. The 470 Kdeposition of ZrO
2
films also yields a hillock morphology induced by
multilayer growth as shown by the STM image
of Fig. 2(a). The films, however, display already
immediately after deposition a weak p(1 ꢀ 1)
Fig. 2. STM images and LEED patterns of 4 ML thick ZrO2 films on Pt(1 1 1) deposited at 470 Ktaken after deposition (a) and after
postannealing at 950 K(b,c; annealing time 10 and 20 min, respectively) and 1020 K(annealing time 20 min). Image size 150 nm ꢀ 150
nm. LEED energy 66 eV. In the LEED patterns the unit cells of the p(1 ꢀ 1) and p(2 ꢀ 2) structures are indicated.

K. Meinel et al. / Surface Science 532–535 (2003) 420–424
423
Fig. 3. STM images of continuous and smooth ZrO2 film on Pt(1 1 1). Mean thickness 4 ML. 470 Kdeposition, postannealing at 950
K. Image size 100 nm ꢀ 100 nm (a), 20 nm ꢀ 20 nm (b), and 8 nm ꢀ 8 nm (c). For explanation see text.
corresponds to the (1 1 1) planes of fcc C1 ZrO2
having a calcium fluoride like structure [2,7]. For
such a film, postannealing at temperatures around
950 Kresults in a morphology which is nearly 2D
as shown by the STM images of Fig. 2(b) and (c).
2D ZrO2 islands are observed residing on large
terraces. The island size is increased with annealing
time (compare Fig. 2(b) and (c)). Simultaneously,
the LEED spots of the p(1 ꢀ 1) ZrO2 become
sharper. No indications of a p(2 ꢀ 2) superstruc-
ture are found. The LEED spots are partly splitted
by ꢂ5.5° (Fig. 2(c)) which indicates the presence of
domains slightly rotated with respect to the
Pt(1 1 1) substrate. This slight film rotation may
ameliorate film/substrate accommodation as ob-
served for other systems [8]. However, the prepa-
ration of perfect ZrO2 films presupposes that the
temperature does not essentially exceeds 1000 K.
Fig. 2(d) shows the STM image of a 4 ML thick
ZrO2 film, which was annealed at 1020 Kimme-
diately after the 470 Kdeposition. Obviously, the
film quality is reduced at such a high annealing
temperature. Again a hillock morphology is ob-
served where at least four levels are developed. In
addition, small holes are formed ranging down to
the substrate (see arrow in Fig. 2(d)), which can be
interpreted as an indicative of beginning film de-
cay. In the LEED pattern, weak spots of a p(2 ꢀ 2)
structure are perceivable, similar as described by
Maurice et al. [5].
found different values (i.e. 0.14, 0.24, 0.32 and 0.45
nm). For some of the islands, an atomic resolution
of the p(1 ꢀ 1) structure was achieved (e.g. in Fig.
3(c) where island i of Fig. 3(b) is imaged at high
resolution). These differences of island height and
imaging of the surface structure of the islands,
respectively, may indicate that different island
types are present on the surface. It is quite possible
that different surface terminations are established
on top of the different islands. In the fcc C1
structure of ZrO2 (calcium fluoride like), the (1 1 1)
plains contain either O atoms or Zr atoms. The
stacking sequence is two O planes for one Zr
plane. Hence, three different terminations (one Zr
and two different O terminations) are in principle
possible. Recent calculations of the phase diagram
of surface termination of fcc C1 ZrO2(1 1 1) sug-
gest the coexistence of different terminations for
low O vapor pressures, partly induced by H [9].
4. Conclusions
In our endeavors of preparing well-ordered
epitaxial ZrO2 layers on Pt(1 1 1), we obtained best
results for a 470 Kdeposition of Zr in an O at-
2
mosphere followed by 950 Kannealing (also in O 2
atmosphere). The films display large terraces cov-
ered by 2D islands. Bright LEED spots evidence
the excellent ordering of the films. According to
the LEED pattern the films are formed by stacks
of (1 1 1) planes of fcc C1 ZrO2 without displaying
any superstructure. Indications are found for dif-
ferent terminations of the islands and terraces of
In Fig. 3, STM images are displayed showing
the surface of smooth ZrO2 films at higher reso-
lution. Measuring the apparent heights of the 2D
islands and of the terrace steps, respectively, we

424
K. Meinel et al. / Surface Science 532–535 (2003) 420–424
the film, respectively. The films are slightly rotated
which is probably induced by film /substrate in-
teraction. Hence, films are now available which are
very suitable for future STM investigations of
catalytic reactions on sulfated ZrO2 surfaces. Due
to the assumed lateral differences in surface termi-
nation, one has to expect also lateral variations of
the catalytic activity. However, we have to empha-
size that well-ordered ZrO2 films on Pt(1 1 1) display
a metastable character and become discontinuous
if the preparation temperature is increased above
1000 K.
Forschergruppe ‘‘Oxidic interfaces’’ at the Martin-
€
Luther-Universitat Halle-Wittenberg are gratefully
acknowledged.
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This work was supported by the Deutsche
Forschungsgemeinschaft through the priority pro-
gram 1091 ‘‘From the ideal to real systems: bridg-
ing the pressure and material gap in heterogeneous
catalysis’’. Helpful discussions with members of the
[9] A. Eichler, private communication.