As pure oxides and mixed oxide systems play an ever-increasing role in a variety of research fields ranging from catalysis over electrochemical applications to microelectronics, the present contribution aims at introducing a straightforward concept for the easy and reproducible preparation of well-defined and well-structured thin film model systems both for pure and mixed oxide systems. Exploiting the special structural and surface properties of vacuum-cleaved NaCl (001) growth templates, the concept is exempli-fied for the formation of nano-spheres (Ga2O3), nano-pyramids (In2O3), plates and needles (V2O5) and den-dritic structures (Ga2O3-WO3). Careful tuning of the preparation conditions (substrate temperature, depo-sition rate, oxygen partial pressure or post-annealing temperature) allows the formation of special particle morphologies at much lower substrate temperatures (less than 400°C) than previously and usually applied.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3504

Lorenz, H.

Penner, S.

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PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 1, 01NTF01(4pp) (2013)   2304-1862/2013/2(1)01NTF01(4) 01NTF01-1  2013 Sumy State University From Pure Oxides to Mixed Oxides: Model Systems for Structural and Catalytic Studies  S. Penner, H. Lorenz  University of Innsbruck, 52a, Innrain, 6020 Innsbruck, Austria  (Received 02 February 2013; published online 29 August 2013)  As pure oxides and mixed oxide systems play an ever-increasing role in a variety of research fields ranging from catalysis over electrochemical applications to microelectronics, the present contribution aims at introducing a straightforward concept for the easy and reproducible preparation of well-defined and well-structured thin film model systems both for pure and mixed oxide systems. Exploiting the special structural and surface properties of vacuum-cleaved NaCl (001) growth templates, the concept is exempli-fied for the formation of nano-spheres (Ga2O3), nano-pyramids (In2O3), plates and needles (V2O5) and den-dritic structures (Ga2O3-WO3). Careful tuning of the preparation conditions (substrate temperature, depo-sition rate, oxygen partial pressure or post-annealing temperature) allows the formation of special particle morphologies at much lower substrate temperatures (less than 400°C) than previously and usually applied.   Keywords: Model systems, Electron microscopy, Thin film, Epitaxy, Heterogeneous catalysis, Oxide, Mixed oxide.    PACS numbers: 81.15. – z, 68.55.  – a, 81.15.Aa   1. INTRODUCTION  Metal oxides, both pure and as mixed oxide sys-tems, are of exceptional importance in a variety of research areas, basically encompassing the fields of chemistry, physics and materials science. In turn, they are utilized in catalysis, as sensors, as corrosion protective coatings, in electrochemical applications or in microelectronics, to name just a few. Regarding heterogeneous catalysis, for example, oxides are wide-ly used either as a catalyst itself or as support for other oxides, metal particles or functional groups. This at the same time shows the usual challenge faced, when dealing with oxide systems: while for example the catalysis on metal particles is well-understood, oxides are significantly more complex because of their structural and chemical flexibility. This might involve different oxidation states, poly-morphism, surface termination, hydroxylation, doping or the general role of surface and bulk defects [1]. Similar considerations also hold for other research areas. The complexity is evidently multiplied, if mixed oxide systems are discussed. The latter recently got more attention due to their promising behavior in various research areas, such as sensor technology or (photo-) catalysis [2]. Obviously, the need for specially prepared model systems exhibiting well-defined parti-cle shapes and structures that can be studied by the full artillery of state-of-the-art structural and spectro-scopic characterization methods (e.g. electron micros-copy techniques) is even more imperative. One partic-ular promising pathway to prepare such model sys-tems is the use of the well-defined cleavage facet of NaCl(001) to structurally and energetically facilitate the growth of nanoparticles. It has already proven its capabilities in mediating the growth of well-shaped metal nanoparticles (Pt, Rh, Pd, Ir or Re) [3]. The present contribution extends these studies to oxidic systems and highlights the possibilities in not only modeling particular particle shapes, morphologies and structures, but also in establishing structure-property and structure-activity relationships. 2. EXPERIMENTAL  In order to prepare oxide systems that are well-suited for characterization by high-resolution electron microscopy (HRTEM), selected area electron diffraction (SAED), atomic force microscopy (AFM) or X-ray photo-electron spectroscopy (XPS), as well as for kinetic stud-ies by gas chromatography (GC), pure oxide and mixed oxide nanoparticles (and films) were vacuum-grown on a crystalline soluble substrate (i.e. NaCl(001), 0.25 cm2) via oxide or metal evaporation in a low background pressure (~10-2 Pa) of oxygen at substrate temperatures between 300 and 623 K. For some oxides, post-annealing treatments (oxidative or reductive) had to be performed. Following this routine, a variety of oxide films have been prepared, including Ga2O3, bcc-In2O3, cubic VO, monoclinic V2O5, ZnO, SiO2, TiO2, fcc-CeO2, WO3, cubic SnO2, SnO, tetragonal PdO, GeO2, bcc-Y2O3, tetragonal ZrO2, cubic Cu2O, cubic CuO, cubic NiO, as well as Ga2O3-WO3. Film thicknesses of ~25 nm are usually obtained to achieve self-supporting samples. Mixed oxide systems were prepared in a similar fash-ion, by either co-deposition or consecutive deposition of the respective oxides. In addition, the same films were prepared in parallel on freshly grown polycrystalline NaCl(001) films (up to 100 cm2) to obtain films suitable for micro-kinetic measurements. A strengthening layer of 500-1000 nm SiO2 was added to all the latter films to improve the mechanical stability. All prepared films were in turn floated into distilled water, dried and either mounted on gold grids for electron microscopy or mounted on quartz wool inside the micro-reactor [1].  3. RESULTS AND DISCUSSION  3.1 Ga2O3  Gallium oxides and Ga-containing materials are catalytically active in a variety of reactions, including hydrocarbon isomerization, dehydrogenation or metha-nol steam reforming/methanol synthesis. Various methods have been proposed as access routes to Ga-containing systems, including sputtering, laser deposi- S. PENNER, H. LORENZ PROC. NAP 2, 01NTF01 (2013)   01NTF01-2 tion, chemical vapor deposition or oxidation of GaAs or GaN surfaces [1]. For our proposed routine, Ga2O3 powder was thermally evaporated onto different NaCl surfaces between 300 and 580 K at varying deposition rates (~ 0.3-10Å s-1) and O2 background pressures. Uniformly sized Ga2O3 spheres with diameters between 20-25 nm were obtained at substrate temperatures T > 530 K. The formation of Ga2O3 spheres was explained by a growth mechanism based on highly mobile quasi-liquid sub-stoichiometric oxides of gallium, including an equilibrium between desorption of these gallium oxides and fast re-oxidation to Ga2O3 (Figure 1a). The sizes of these spheres was found to be highly depend on the oxygen partial pressure, leading to larger spheres at lower O2-pressures (Figure 1b). The size control occurs via (surface) re-oxidation of gallia spheres, which suppresses coalescence by Ostwald ripening. The XPS spectra reveal a stoichiometry close to Ga2O3, at least in the surface-near regions (Figure 1c). AFM to-pography confirms the formation of the sphere-like aggregates (Figure 1d), whose exact shape and size is even more evident in the phase image. Catalytic testing in methanol steam reforming reveals Ga2O3 being a highly active catalyst, but without pronounced selectiv-ity towards CO2 (Figure 1e) [4].  3.2 In2O3  In2O3-containing catalysts, particular in combina-tions with other oxides, such as Al2O3, TiO2 or zeolithes, have shown high activity and selectivity in deNOx reactions and for ethanol and methanol steam reforming. For the latter two cases, CO formation was almost completely suppressed down to the ppm-level. As for Ga2O3, several methods for the preparation of In2O3 nanoparticles and thin films exist: evaporation, sputter-ing, laser deposition, spray pyrolysis, thermal oxidation of pre-deposited In and electrochemical deposition [1]. Especially the preparation of nanostructures (nanocrys-tals and wires) has received increased attention due to the potential use as catalyst systems [5]. However, one of the main drawbacks is related to the very high substrate temperatures, usually being in the range of 1000-1200 K [5]. As for the In2O3 system, after deposition at 580 K in 10-2 Pa O2, the TEM images show particles with square, triangular and quasi-rhombohedral outlines (Figure 1a), giving rise to an almost single-crystalline SAED pattern of bcc-In2O3 (Figure 1b). This is also confirmed by the HRTEM images (Figure 1c). Cubic NaCl acts as a “struc-tural mediator” for the growth of crystalline In2O3, as the crystallographic matching between the lattice planes of both structures is almost perfect. Regarding catalytic behavior in methanol steam reforming, In2O3 particles turn out to be promising CO2-selective catalysts (Figure 2d) [6].  3.3 V2O3 and V2O5-based compounds  VOx- and V2O5-based systems are widely used in a variety of industrially important chemical processes such     Fig. 1 – Characterization of the thin film Ga2O3 catalyst. (a) TEM image of spheres, obtained at substrate temperatures of 580 K in 10-2 Pa O2. (b) Same film, but without additional oxygen partial pressure. the insets show the respective SAED patterns. (c) Ga 2p XPS spectra obtained in the as-grown state and after different annealing treatments. (d) AFM topography (left) and phase signal (right) of the film shown in (a). (e) Temperature-programmed methanol steam reforming reaction on the Ga2O3 thin film (adapted from [1]).    Fig. 2 – Characterization of the epitaxial thin film In2O3 cata-lyst. (a) TEM image of pyramidal-shaped particles, obtained at substrate temperatures of 580 K in 10-2 Pa O2. (b) SAED pat-tern with dark-field image of a single particle shown as inset. (c) HRTEM image of a [001]-oriented particle with FFT as inset. (d) Temperature-programmed methanol steam reform-ing reaction on the In2O3 particles (adapted from [1]).  FROM PURE OXIDES TO MIXED OXIDES: MODEL SYSTEMS… PROC. NAP 2, 01NTF01 (2013)   01NTF01-3 as the oxidation of SO2 to SO3 during the production of sulphuric acid, the oxidation of benzene to maleic anhy-dride or the selective reduction of NOx with NH3. For this purpose, vanadium oxides are almost exclusively supported on other oxide systems, including SiO2, Al2O3, TiO2 or ZrO2. Although previously assumed to only act as a structural stabilizer, the structural and energetic situation at the interface is considerably more complex, also due to the redox behavior of VOx [1]. Vanadium oxides were grown by reactive deposition of V metal in 10-2 Pa O2 at 600 K. This typically yields well-ordered V2O3 thin films, which can be converted to V2O5 by post-oxidation at 673 K. Upon oxidation at 723 K and above, elongated V2O5 needles form (Figure 3b), with typical diameters between 20 and 30 nm and average lengths between 20 and 300 nm. V2O5 crystals with near-square outlines and oblong particles are also frequently ob-served (Figure 3a). SAED patterns and HRTEM images reveal the exclusive presence of V2O5 (Figure 3c and d). Subsequently, these nanocrystals can serve as a conven-ient tool for reduction treatments as access to other nano-particulate vanadium oxides and as nanostruc-tured support for Pd or PdO particles [7].    Fig. 3 – Characterization of the epitaxial thin film V2O5 cata-lyst. (a) TEM image of elongated V2O5 particles, obtained at substrate temperatures of 580 K in 10-2 Pa O2 followed by post-annealing at 773 K. (b) Three separated V2O5 needles. (c) SAED pattern. (d) HRTEM image with atomically-resolved (200) fring-es running parallel to the particle edges of single V2O5 needles (adapted from [1]).  3.4 Mixed Ga2O3-WO3 systems  Considered as the most ambitious goal of the pre-sented studies, the thin film model system preparation routine is successfully taken to a new complexity level in establishing a new pathway to model mixed-oxide sys-tems. Exemplarily discussed for binary Ga2O3/WO3, it is shown that this preparation routine also offers exciting new possibilities to access a full range of oxide combina-tions relevant in a variety of research areas including catalysis, sensor technology and materials chemistry. Similarly to the pure oxide systems, careful tuning of the experimental conditions yields a nano-architectured mixed oxide system. Ga2O3/WO3 itself is an interesting system due to its catalytic action in methanol synthesis, where the oxide-oxide interface is assumed to govern the catalytic activity and selectivity [8]. The film morphology and structure are observed to strongly differ from those of the pure oxides and upon co-deposition of both oxides also leads to a branched network of Ga-enriched den-drites within a W-enriched binary oxide matrix (Figure 4 a-b). The Ga-enriched dendrites are easily seen in high-angle annular dark-field (HAADF) images, due to the pronounced differences of Ga2O3 and WO3 in the average atom number Z (Figure 4c, with the associated line pro-file of the energy-dispersive X-ray (EDX) spectra shown in 4d). Mediated by the GaOx/NaCl(001) contact area, the formation rather appears to be a surface-related phenomenon than being an intrinsic property of the mixed-oxide bulk [9].    Fig. 4 – Characterization of the mixed Ga2O3-WO3 thin film deposited at 600 K in 10-2 Pa O2. (a-b) TEM images of the Ga-enriched dendrites, with the amorphous SAED pattern shown as inset (c) HAADF image of the Ga dendrites with EDX line profile. (d) Ga L-edge and W M-edge intensity along the line shown in (c) (adapted from [9]).  4. CONCLUSION  Model systems for transition and noble metal oxide systems, either as thin films or nanoparticles, were prepared by vacuum deposition of oxides or oxidation of metals. These systems are well suited for atomic scale characterization by surface-specific methods, bulk de-termination methods and catalytic tests. The exploita-tion of the special surface and structural properties of NaCl, either influencing the growth of the oxide species by structural similarities or by energetic properties (i.e. wetting phenomena) allows overcoming the general structural, morphological and compositional flexibility of oxides. These usually lead to significant challenges in their characterization, being intimately connected to their peculiar, but sometimes exceptional structural, catalytic and materials properties. As shown in Figure 5, the chosen preparation pathway is rather easy and has the tremendous advantage of in-parallel prepara-tion of samples for structural and spectroscopic charac-terization as well as for determination of its materials properties (sensor, catalytic or other physicochemical), which makes the establishment of structure-activity and structure-property relationships straightforward.  S. PENNER, H. LORENZ PROC. NAP 2, 01NTF01 (2013)   01NTF01-4   Fig. 5 – Scheme of the preparation concept of epitaxially grown oxide model catalysts for structural, spectroscopic and catalytic characterization, including a scheme of the recirculated micro-reactor setup in the right lower corner.  ACKNOWLEDGEMENTS  The authors thank the Austrian Science Foundation (FWF) for financial support under the projects P20892-N19 and F4503-N16, performed within the Forschung-splattform Materials- and Nanoscience.   REFERENCES  1. Model Systems in Catalysis: Single Crystals to Enzyme Mimics (Ed. R. Rioux) (Springer, Heidelberg), Chapter 17. 2. I. Wachs, Catal. Today 100, 79 (2005). 3. G. Rupprechter, K. Hayek, H. Hofmeister, J. Catal. 173, 409 (1998). 4. S. Penner, B. Klötzer, B. Jenewein, X. Liu, E. Bertel,  F. Klauser, Thin Solid Films 516, 4742 (2008)  5. Y. Hao, G. Meng, C. Ye, C. J. Lee, S. J. An, G. C. Yi, Cryst. Growth Des. 5, 1617 (2005). 6. H. Lorenz, M. Stöger-Pollach, S. Schwarz, K. Pfaller, B. Klötzer, J. Bernardi, S. Penner, J. Phys. Chem. C. 112, 918 (2008). 7. S. Penner, B. Jenewein, B. Klötzer, PCCP 9, 2428 (2007) 8. S. H. Taylor, J.S.J. Hargreaves, G. J. Hutchings, R. W. Joyner, C. W. Lembacher, Catal. Today 42, 217 (1998). 9. S. Penner, M. Stöger-Pollach, F. Klauser, H. Lorenz, B. Klötzer, X. Liu, B. Jenewein, E. Bertel, Mater. Chem. Phys.  116, 175 (2009).  

From Pure Oxides to Mixed Oxides: Model Systems for Structural and Catalytic Studies

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 1, 01NTF01(4pp) (2013) 
 
 
2304-1862/2013/2(1)01NTF01(4) 01NTF01-1  2013 Sumy State University 
From Pure Oxides to Mixed Oxides: Model Systems for Structural and Catalytic Studies 
 
S. Penner, H. Lorenz 
 
University of Innsbruck, 52a, Innrain, 6020 Innsbruck, Austria 
 
(Received 02 February 2013; published online 29 August 2013) 
 
As pure oxides and mixed oxide systems play an ever-increasing role in a variety of research fields 
ranging from catalysis over electrochemical applications to microelectronics, the present contribution aims 
at introducing a straightforward concept for the easy and reproducible preparation of well-defined and 
well-structured thin film model systems both for pure and mixed oxide systems. Exploiting the special 
structural and surface properties of vacuum-cleaved NaCl (001) growth templates, the concept is exempli-
fied for the formation of nano-spheres (Ga2O3), nano-pyramids (In2O3), plates and needles (V2O5) and den-
dritic structures (Ga2O3-WO3). Careful tuning of the preparation conditions (substrate temperature, depo-
sition rate, oxygen partial pressure or post-annealing temperature) allows the formation of special particle 
morphologies at much lower substrate temperatures (less than 400°C) than previously and usually applied.  
 
Keywords: Model systems, Electron microscopy, Thin film, Epitaxy, Heterogeneous catalysis, Oxide, 
Mixed oxide. 
 
  PACS numbers: 81.15. – z, 68.55.  – a, 81.15.Aa 
 
 
1. INTRODUCTION 
 
Metal oxides, both pure and as mixed oxide sys-
tems, are of exceptional importance in a variety of 
research areas, basically encompassing the fields of 
chemistry, physics and materials science. In turn, 
they are utilized in catalysis, as sensors, as corrosion 
protective coatings, in electrochemical applications or 
in microelectronics, to name just a few. Regarding 
heterogeneous catalysis, for example, oxides are wide-
ly used either as a catalyst itself or as support for 
other oxides, metal particles or functional groups. 
This at the same time shows the usual challenge 
faced, when dealing with oxide systems: while for 
example the catalysis on metal particles is well-
understood, oxides are significantly more complex 
because of their structural and chemical flexibility. 
This might involve different oxidation states, poly-
morphism, surface termination, hydroxylation, doping 
or the general role of surface and bulk defects [1]. 
Similar considerations also hold for other research 
areas. The complexity is evidently multiplied, if mixed 
oxide systems are discussed. The latter recently got 
more attention due to their promising behavior in 
various research areas, such as sensor technology or 
(photo-) catalysis [2]. Obviously, the need for specially 
prepared model systems exhibiting well-defined parti-
cle shapes and structures that can be studied by the 
full artillery of state-of-the-art structural and spectro-
scopic characterization methods (e.g. electron micros-
copy techniques) is even more imperative. One partic-
ular promising pathway to prepare such model sys-
tems is the use of the well-defined cleavage facet of 
NaCl(001) to structurally and energetically facilitate 
the growth of nanoparticles. It has already proven its 
capabilities in mediating the growth of well-shaped 
metal nanoparticles (Pt, Rh, Pd, Ir or Re) [3]. The 
present contribution extends these studies to oxidic 
systems and highlights the possibilities in not only 
modeling particular particle shapes, morphologies and 
structures, but also in establishing structure-property 
and structure-activity relationships. 
2. EXPERIMENTAL 
 
In order to prepare oxide systems that are well-
suited for characterization by high-resolution electron 
microscopy (HRTEM), selected area electron diffraction 
(SAED), atomic force microscopy (AFM) or X-ray photo-
electron spectroscopy (XPS), as well as for kinetic stud-
ies by gas chromatography (GC), pure oxide and mixed 
oxide nanoparticles (and films) were vacuum-grown on 
a crystalline soluble substrate (i.e. NaCl(001), 0.25 cm2) 
via oxide or metal evaporation in a low background 
pressure (~10-2 Pa) of oxygen at substrate temperatures 
between 300 and 623 K. For some oxides, post-
annealing treatments (oxidative or reductive) had to be 
performed. Following this routine, a variety of oxide 
films have been prepared, including Ga2O3, bcc-In2O3, 
cubic VO, monoclinic V2O5, ZnO, SiO2, TiO2, fcc-CeO2, 
WO3, cubic SnO2, SnO, tetragonal PdO, GeO2, bcc-Y2O3, 
tetragonal ZrO2, cubic Cu2O, cubic CuO, cubic NiO, as 
well as Ga2O3-WO3. Film thicknesses of ~25 nm are 
usually obtained to achieve self-supporting samples. 
Mixed oxide systems were prepared in a similar fash-
ion, by either co-deposition or consecutive deposition of 
the respective oxides. In addition, the same films were 
prepared in parallel on freshly grown polycrystalline 
NaCl(001) films (up to 100 cm2) to obtain films suitable 
for micro-kinetic measurements. A strengthening layer 
of 500-1000 nm SiO2 was added to all the latter films to 
improve the mechanical stability. All prepared films 
were in turn floated into distilled water, dried and 
either mounted on gold grids for electron microscopy or 
mounted on quartz wool inside the micro-reactor [1]. 
 
3. RESULTS AND DISCUSSION 
 
3.1 Ga2O3 
 
Gallium oxides and Ga-containing materials are 
catalytically active in a variety of reactions, including 
hydrocarbon isomerization, dehydrogenation or metha-
nol steam reforming/methanol synthesis. Various 
methods have been proposed as access routes to Ga-
containing systems, including sputtering, laser deposi-
 S. PENNER, H. LORENZ PROC. NAP 2, 01NTF01 (2013) 
 
 
01NTF01-2 
tion, chemical vapor deposition or oxidation of GaAs or 
GaN surfaces [1]. For our proposed routine, Ga2O3 
powder was thermally evaporated onto different NaCl 
surfaces between 300 and 580 K at varying deposition 
rates (~ 0.3-10Å s-1) and O2 background pressures. 
Uniformly sized Ga2O3 spheres with diameters between 
20-25 nm were obtained at substrate temperatures T > 
530 K. The formation of Ga2O3 spheres was explained 
by a growth mechanism based on highly mobile quasi-
liquid sub-stoichiometric oxides of gallium, including 
an equilibrium between desorption of these gallium 
oxides and fast re-oxidation to Ga2O3 (Figure 1a). The 
sizes of these spheres was found to be highly depend on 
the oxygen partial pressure, leading to larger spheres 
at lower O2-pressures (Figure 1b). The size control 
occurs via (surface) re-oxidation of gallia spheres, 
which suppresses coalescence by Ostwald ripening. The 
XPS spectra reveal a stoichiometry close to Ga2O3, at 
least in the surface-near regions (Figure 1c). AFM to-
pography confirms the formation of the sphere-like 
aggregates (Figure 1d), whose exact shape and size is 
even more evident in the phase image. Catalytic testing 
in methanol steam reforming reveals Ga2O3 being a 
highly active catalyst, but without pronounced selectiv-
ity towards CO2 (Figure 1e) [4]. 
 
3.2 In2O3 
 
In2O3-containing catalysts, particular in combina-
tions with other oxides, such as Al2O3, TiO2 or 
zeolithes, have shown high activity and selectivity in 
deNOx reactions and for ethanol and methanol steam 
reforming. For the latter two cases, CO formation was 
almost completely suppressed down to the ppm-level. As 
for Ga2O3, several methods for the preparation of In2O3 
nanoparticles and thin films exist: evaporation, sputter-
ing, laser deposition, spray pyrolysis, thermal oxidation 
of pre-deposited In and electrochemical deposition [1]. 
Especially the preparation of nanostructures (nanocrys-
tals and wires) has received increased attention due to 
the potential use as catalyst systems [5]. However, one of 
the main drawbacks is related to the very high substrate 
temperatures, usually being in the range of 1000-1200 K 
[5]. As for the In2O3 system, after deposition at 580 K in 
10-2 Pa O2, the TEM images show particles with square, 
triangular and quasi-rhombohedral outlines (Figure 1a), 
giving rise to an almost single-crystalline SAED pattern 
of bcc-In2O3 (Figure 1b). This is also confirmed by the 
HRTEM images (Figure 1c). Cubic NaCl acts as a “struc-
tural mediator” for the growth of crystalline In2O3, as the 
crystallographic matching between the lattice planes of 
both structures is almost perfect. Regarding catalytic 
behavior in methanol steam reforming, In2O3 particles 
turn out to be promising CO2-selective catalysts (Figure 
2d) [6]. 
 
3.3 V2O3 and V2O5-based compounds 
 
VOx- and V2O5-based systems are widely used in a 
variety of industrially important chemical processes such  
 
 
 
Fig. 1 – Characterization of the thin film Ga2O3 catalyst. (a) 
TEM image of spheres, obtained at substrate temperatures of 
580 K in 10-2 Pa O2. (b) Same film, but without additional 
oxygen partial pressure. the insets show the respective SAED 
patterns. (c) Ga 2p XPS spectra obtained in the as-grown state 
and after different annealing treatments. (d) AFM topography 
(left) and phase signal (right) of the film shown in (a). (e) 
Temperature-programmed methanol steam reforming reaction 
on the Ga2O3 thin film (adapted from [1]). 
 
 
 
Fig. 2 – Characterization of the epitaxial thin film In2O3 cata-
lyst. (a) TEM image of pyramidal-shaped particles, obtained at 
substrate temperatures of 580 K in 10-2 Pa O2. (b) SAED pat-
tern with dark-field image of a single particle shown as inset. 
(c) HRTEM image of a [001]-oriented particle with FFT as 
inset. (d) Temperature-programmed methanol steam reform-
ing reaction on the In2O3 particles (adapted from [1]). 
 FROM PURE OXIDES TO MIXED OXIDES: MODEL SYSTEMS… PROC. NAP 2, 01NTF01 (2013) 
 
 
01NTF01-3 
as the oxidation of SO2 to SO3 during the production of 
sulphuric acid, the oxidation of benzene to maleic anhy-
dride or the selective reduction of NOx with NH3. For 
this purpose, vanadium oxides are almost exclusively 
supported on other oxide systems, including SiO2, Al2O3, 
TiO2 or ZrO2. Although previously assumed to only act 
as a structural stabilizer, the structural and energetic 
situation at the interface is considerably more complex, 
also due to the redox behavior of VOx [1]. Vanadium 
oxides were grown by reactive deposition of V metal in 
10-2 Pa O2 at 600 K. This typically yields well-ordered 
V2O3 thin films, which can be converted to V2O5 by post-
oxidation at 673 K. Upon oxidation at 723 K and above, 
elongated V2O5 needles form (Figure 3b), with typical 
diameters between 20 and 30 nm and average lengths 
between 20 and 300 nm. V2O5 crystals with near-square 
outlines and oblong particles are also frequently ob-
served (Figure 3a). SAED patterns and HRTEM images 
reveal the exclusive presence of V2O5 (Figure 3c and d). 
Subsequently, these nanocrystals can serve as a conven-
ient tool for reduction treatments as access to other 
nano-particulate vanadium oxides and as nanostruc-
tured support for Pd or PdO particles [7]. 
 
 
 
Fig. 3 – Characterization of the epitaxial thin film V2O5 cata-
lyst. (a) TEM image of elongated V2O5 particles, obtained at 
substrate temperatures of 580 K in 10-2 Pa O2 followed by post-
annealing at 773 K. (b) Three separated V2O5 needles. (c) SAED 
pattern. (d) HRTEM image with atomically-resolved (200) fring-
es running parallel to the particle edges of single V2O5 needles 
(adapted from [1]). 
 
3.4 Mixed Ga2O3-WO3 systems 
 
Considered as the most ambitious goal of the pre-
sented studies, the thin film model system preparation 
routine is successfully taken to a new complexity level in 
establishing a new pathway to model mixed-oxide sys-
tems. Exemplarily discussed for binary Ga2O3/WO3, it is 
shown that this preparation routine also offers exciting 
new possibilities to access a full range of oxide combina-
tions relevant in a variety of research areas including 
catalysis, sensor technology and materials chemistry. 
Similarly to the pure oxide systems, careful tuning of the 
experimental conditions yields a nano-architectured 
mixed oxide system. Ga2O3/WO3 itself is an interesting 
system due to its catalytic action in methanol synthesis, 
where the oxide-oxide interface is assumed to govern the 
catalytic activity and selectivity [8]. The film morphology 
and structure are observed to strongly differ from those 
of the pure oxides and upon co-deposition of both oxides 
also leads to a branched network of Ga-enriched den-
drites within a W-enriched binary oxide matrix (Figure 4 
a-b). The Ga-enriched dendrites are easily seen in high-
angle annular dark-field (HAADF) images, due to the 
pronounced differences of Ga2O3 and WO3 in the average 
atom number Z (Figure 4c, with the associated line pro-
file of the energy-dispersive X-ray (EDX) spectra shown 
in 4d). Mediated by the GaOx/NaCl(001) contact area, 
the formation rather appears to be a surface-related 
phenomenon than being an intrinsic property of the 
mixed-oxide bulk [9]. 
 
 
 
Fig. 4 – Characterization of the mixed Ga2O3-WO3 thin film 
deposited at 600 K in 10-2 Pa O2. (a-b) TEM images of the Ga-
enriched dendrites, with the amorphous SAED pattern shown 
as inset (c) HAADF image of the Ga dendrites with EDX line 
profile. (d) Ga L-edge and W M-edge intensity along the line 
shown in (c) (adapted from [9]). 
 
4. CONCLUSION 
 
Model systems for transition and noble metal oxide 
systems, either as thin films or nanoparticles, were 
prepared by vacuum deposition of oxides or oxidation of 
metals. These systems are well suited for atomic scale 
characterization by surface-specific methods, bulk de-
termination methods and catalytic tests. The exploita-
tion of the special surface and structural properties of 
NaCl, either influencing the growth of the oxide species 
by structural similarities or by energetic properties (i.e. 
wetting phenomena) allows overcoming the general 
structural, morphological and compositional flexibility 
of oxides. These usually lead to significant challenges 
in their characterization, being intimately connected to 
their peculiar, but sometimes exceptional structural, 
catalytic and materials properties. As shown in Figure 
5, the chosen preparation pathway is rather easy and 
has the tremendous advantage of in-parallel prepara-
tion of samples for structural and spectroscopic charac-
terization as well as for determination of its materials 
properties (sensor, catalytic or other physicochemical), 
which makes the establishment of structure-activity 
and structure-property relationships straightforward. 
 S. PENNER, H. LORENZ PROC. NAP 2, 01NTF01 (2013) 
 
 
01NTF01-4 
 
 
Fig. 5 – Scheme of the preparation concept of epitaxially grown oxide model catalysts for structural, spectroscopic and catalytic 
characterization, including a scheme of the recirculated micro-reactor setup in the right lower corner. 
 
ACKNOWLEDGEMENTS 
 
The authors thank the Austrian Science Foundation 
(FWF) for financial support under the projects P20892-
N19 and F4503-N16, performed within the Forschung-
splattform Materials- and Nanoscience. 
 
 
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From Pure Oxides to Mixed Oxides: Model Systems for Structural and Catalytic Studies

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