Applied Physics A
DOI: 10.1007/s00339-005-3407-x
Materials Science & Processing
c.e. allmond
v.p. oleshko
j.m. howe
j.m. fitz-gerald,u
Non-aggregated Pd nanoparticles
deposited onto catalytic supports
Department of Material Science and Engineering, University of Virginia,
116 Engineers Way, Charlottesville, VA 22904, USA
Received: 4 June 2005/Accepted: 23 August 2005
Published online: 7 December 2005 • © Springer-Verlag 2005
ABSTRACT Nanostructured powders have shown great promise for a variety of applications including chemical gas sensors, high surface area supports for catalysis,
tribology, chemical mechanical polishing, and optoelectronics. In this report, highly
dispersed Pd nanoparticles with a narrow size distribution, and mean diameter of 2 ±
0.2 nm, were deposited at room temperature onto amorphous carbon and oxide supports (TiO2 , Al2 O3 ) by pulsed-laser ablation of a Pd sputtering target. Depositions
were performed in Ar at a back-fill pressure of 3 mTorr after reaching a base pressure
of 10−7 Torr. Populations of uniformly dispersed particles with an interparticle spacing of 3 to 10 nm were observed by high-resolution transmission electron microscopy
with little evidence of nanoparticle aggregation. The chemical compositions of individual nanoparticles were confirmed by high spatial resolution energy-dispersive
X-ray spectroscopy.
PACS 81.15.Fg;
1
68.35.Dv; 68.37.Lp; 81.07.Wx; 81.16.Mk
Introduction
Current research in several
areas of application such as catalysis
shows that it is critical to control particle size, particle size distribution, ionic
state, and support interactions [1–4]. It
is well established that controlling the
size of metal particles governs the number of related active sites on structuresensitive supported metallic catalysts.
For example, in the Pd–mica system the
rate of catalytic CO decomposition was
enhanced five fold by reducing the Pd
particle size from 5 nm to 2 nm [5].
When developing nanoparticle coatings, two main challenges exist in terms
of synthesizing nanoparticles with controlled properties (size, distribution)
that attach to the support substrate
without secondary effects (agglomeration, surface contamination). There
are a number of methods presently utilized in the synthesis of both oxide
and non-oxide raw nanoparticles, including spray pyrolysis [6, 7], plasma
synthesis [8, 9], precipitation out of either metal–organic or aqueous salt solutions [10, 11], plasma arc synthesis [12],
laser vaporization, and gas-phase condensation [13]. While some of these
processes are capable of yielding true
nanoparticles (< 10 nm), others, such
as spray pyrolysis, produce particles
in the 20- to 50-nm size range. Furthermore, maintaining a narrow particle
size distribution, an important factor
for specific applications such as catalysis, is not possible with a majority
of these techniques. With respect to
catalysis, most metal-particle oxidesupported heterogeneous nanocatalysts
are prepared by chemical methods on
support matrices. These include the
sol–gel process, cross linking, ultraviolet reaction, bacterial superstructures,
and template-directed and reduction
methods [14, 15].
u Fax: +1 434 982-5660, E-mail: jfitz@virginia.edu
As an alternative to chemical techniques, laser synthesis has emerged as
a viable method for the production of
nanoparticles and bulk powders [16–
20]. In addition, several classes of engineered particulate coatings have been
developed using laser ablation in combination with fluidized beds [21–24].
The growth of noble nanoparticles including Pd and Pt by laser techniques
has been performed via laser ablation at
high pressures into background carrier
gases [25] and through photolysis [26,
27]. Recent work by Arrii et al. [1] and
Li et al. [13] has illustrated the use of
laser ablation in a high-pressure gas in
combination with ion beam selection
processes to deposit Au nanoparticles
on catalytic supports. Here we present
results from direct pulsed-laser deposition (PLD) of Pd nanoparticles onto dissimilar oxide and carbon supports, comparing their respective particle sizes,
compositions, and spatial distributions
as revealed by high-resolution transmission electron microscopy (HRTEM) and
field-emission analytical electron microscopy (FE-AEM) employing high
spatial resolution energy-dispersive
X-ray spectroscopy (EDXS) for
compositional analysis of individual
nanoparticles.
2
Experimental
The experimental process of
laser ablation has been described previously [28]. A polished Pd sputtering target (Lesker, Inc., 99.95% pure)
was ablated with a KrF excimer laser
(λ = 248 nm; focused to a fluence of
2.5 J/cm2 , 25-ns pulse duration, 100
consecutive pulses at 5 Hz) inside a stainless steel high-vacuum chamber (base
Rapid communication
Appl. Phys. A 82, 675–678 (2006)
676
Applied Physics A – Materials Science & Processing
pressure of 3 × 10−7 Torr) maintained at
a static pressure of 3 mTorr of 99.995%
Ar. Pd nanoparticles were deposited
onto 15- to 20-nm-thick TEM grids
(lacey carbon, 300 Cu mesh, Ted Pella),
50- to 75-nm oxide supports of TiO2
(rutile and anatase polymorphs, Degussa P-25), and Al2 O3 (α and γ phases,
Mager Scientific AP-312) positioned
on the TEM grids at a stand-off distance of 12 cm. Phase compositions of
the supports were confirmed by highangle X-ray diffraction analyses. The
Pd nanoparticles were examined using
bright-field (BF) and dark-field (DF)
TEM, in selected-area electron diffraction (SAED) and HRTEM modes. Imaging was performed with a JEOL 4000EX
TEM having a spherical aberration coefficient Cs = 1.0 mm and 0.18-nm point
resolution at 400 kV with a LaB6 filament, and in a JEOL 2010F AEM
equipped with a Schottky field-emission
gun operating at 200 kV, with Cs =
1.0 mm and 0.23-nm point resolution.
EDXS analyses were performed in the
JEOL 2010F FE-AEM, which had an
Oxford Instruments Pentafet ultra-thinwindow EDX detector, having 136-eV
resolution (FWHM) at Mn K α 5.898 keV
and nanoprobe capabilities with probe
diameter ranging from 2.4 nm down
to 0.5 nm.
3
Results
Figures 1 to 3 show HRTEM
images of Pd nanoparticles deposited
FIGURE 1 Pd nanoparticles deposited onto a
continuous carbon film in 3 mTorr of Ar. The
inset displays the SAED pattern of the Pd
nanoparticles
FIGURE 2 Pd nanoparticles deposited onto
50-nm Al2 O3 supports in 3 mTorr of Ar. The inset displays the SAED pattern associated with the
nanocrystalline α and γ phases of Al2 O3
FIGURE 3 Pd nanoparticles deposited onto
50-nm TiO2 supports in 3 mTorr Ar. The SAED
pattern shown in the inset displays point Bragg
reflections and discrete rings corresponding to
anatase and rutile phases of TiO2
onto continuous amorphous carbon films
and Al2 O3 and TiO2 supports, respectively. Some of the metal particles shown
in Fig. 1 revealed (111) lattice fringes
barely visible against the amorphous
support. For the Pd particles deposited
onto nanocrystalline oxide supports,
the dominant fringe contrast from the
supports could interfere with the particle contrast, resulting in significant
variations of particle visibility with
the defocus of the electron beam. Although small Pd particles are difficult
to observe, some of them were clearly
identified both in the plan-view and
profile-view images shown by arrows in
Figs. 2 and 3. The inset in Fig. 1 shows
a SAED pattern of the Pd nanoparticles, exhibiting a broadened diffuse ring
corresponding to the most probable interatomic spacing of 0.23 nm. Only this
diffuse ring is observed due to the small
size of the crystallites (below 5 nm)
that leads to significant broadening of
the diffraction rings. Similar insets in
Figs. 2 and 3 display many discrete
diffraction rings and spots, mainly corresponding to the crystalline supports
rather than the Pd nanoparticles.
Figure 4 shows the size distributions of Pd nanoparticles deposited onto
the amorphous carbon film and Al2 03
and Ti02 supports. The Pd nanoparticles displayed a mean diameter of approximately 2 ± 0.2 nm with standard
deviations of 0.23 to 0.56 nm, which
appeared to be independent of the support system. All of the distributions
FIGURE 4 Size distributions taken from Pd nanoparticles deposited onto (a) lacey carbon films
and (b) Al2 O3 and (c) TiO2 carrier supports in 3 mTorr of Ar with mean diameters of 2.1 nm, 2.0 nm,
and 2.3 nm, respectively
ALLMOND et al.
Non-aggregated Pd nanoparticles deposited onto catalytic supports
EDX spectra of a 5-nm Pd nanoparticle corresponding to 2.0 × 10−20 g or one atomic column for a 0.5-nm probe and 1.6 × 10−19 g or 17 atomic columns for a 1.6-nm probe, respectively. Au
and Ag represent impurities in the starting Pd sputtering target
FIGURE 5
appear roughly log-normal and multimodal, comparable to thermally evaporated metal particles [13]. Elemental
analysis of individual Pd nanoparticles
on an amorphous carbon film was performed by EDXS, as shown in Fig. 5.
Two probe sizes were utilized to determine the compositions of the particles:
0.5-nm and 1.6-nm diameter. The former corresponds to one atomic column,
while the latter represents 17 atomic
columns. The EDXS spectra revealed
an admixture of both Au and Ag impurities in the individual particles. There
are known impurities in the target material, although their relatively high levels
in some particles indicate that segregation or some other effect may be
present.
4
Discussion
While it is known that nanoparticles can form at all pressures during
laser ablation, extensive studies have focused on the formation and growth of
nanoparticles at pressures > 100 mTorr
at both room and elevated temperatures.
The formation of nanoparticles from the
laser-generated plume arises due to interactions both within the dense plasma
at near vacuum and with a controlled
back-fill gas environment at higher pressures [16, 17, 20, 21, 25, 29–32]. The
effects have been shown to produce particles ranging from 1 nm to several microns in size with broad distributions
for pressures ranging from 175 mTorr
to 100 Torr [17, 25]. It has also been
shown that temperature significantly affects the growth of nanoparticles in the
presence of a heated substrate by controlling the movement of the ablated
plume via a temperature gradient [17].
This effectively serves to limit the time
available for nanoparticle formation and
deposition on the substrate. In this set
of experiments where no thermal source
is present, the enhanced nanoparticle
formation and transfer at this lower
pressure is expected, enabling deposition onto the particulate supports at
large distances. This effect is shown at
a stand-off distance of 12 cm, which is
not unreasonable considering effects of
forward plume penetration into background gases as a function of the combined system molecular weight and
pressure [16]. In this case, we suggest
that during consecutive pulses, nanoparticles first form at the initial plume expansion front, freely expanding through
the Ar, and uniformly depositing on the
support materials, rather than formation
via nucleation and cluster growth on
the supports. The measured interparticle
spacing was on the order of 3 to 10 nm,
with no detectable Pd in the gap regions shown by EDXS. It can be argued
that cluster-atom mobility is sufficient
to allow nucleation and growth on this
scale, but because nearly identical sizes
and distributions were observed on three
support surfaces with markedly differ-
677
ent diffusion coefficients and surface
characteristics may lend further evidence to nanoparticle formation within
the vapor.
In terms of the scale properties, the
Pd nanoparticles produced by direct ablation (i.e. without mass selection or
gas carrier) onto the three supports displayed sizes and surface morphologies
comparable to those observed when
chemical processing methods are utilized [15, 27, 33, 34]. The statistical results, displayed in Fig. 4, exhibit no
significant size deviations between the
Pd nanoparticles deposited onto the
amorphous carbon grids or Al2 O3 or
TiO2 supports. This implies that the
surface chemistry and morphology of
the support does not significantly affect
the Pd particle size or surface densities under the selected ablation conditions. In addition, the particles were
uniformly distributed over the support
surfaces, regardless of the support material. TEM observation did reveal, however, local areas on the supports where
the Pd particles did not cover the entire surface. This is most likely due to
the geometry of the target relative to the
support and/or shadowing from other
supports.
Observations of Pd nanoparticles
on the Al2 O3 and TiO2 supports (examples can be observed by arrows in
Fig. 2) show that the Pd nanoparticles
wet the substrate, i.e. the contact angle
θ between the Pd–oxide interface and
the Pd nanoparticle surface is in the
range of 50◦ to 75◦ . According to the
Young–Dupre relation [35], this corresponds to high values of the thermodynamic work of adhesion, Wad = γm (1 +
cos θ) ≥ 2.0 J m−2 , where γm is the surface energy of the metal, and is typical
for high-melting-point metals such as Pt
and Pd. Since the Pd nanoparticles presumably are spherical and vary in size
from 1 to 4 nm in diameter, the fact that
they wet the Al2 O3 and TiO2 surfaces
indicates strong attraction between the
Pd nanoparticle and the support. This
further suggests that the particles have
sufficient mobility to rearrange when
landing on the support. The contact
angles for 10 different particles were
similar, but with substantial variations
depending on the exact surface morphology, e.g. faceted or rough, with similar
observations observed for Pd particles
on TiO2 supports.
678
5
Applied Physics A – Materials Science & Processing
Summary
We have reported Pd nanoparticles synthesized onto three dissimilar supports in a low-pressure Ar atmosphere. The mean size of the nanoparticles was 2 nm, regardless of the support material. TEM was used in brightand dark-field modes to quantify size
distributions and EDXS confirmed the
elemental composition of individual
Pd nanoparticles. The growth of this
system on markedly different supports
suggests that nanoparticles may have
formed during flight, adhering to the
supports on impact, likely due to Van
Der Waals and stronger metal-support
interactions in this size regime.
ACKNOWLEDGEMENTS This research was partly funded by Philip Morris USA,
under contract No. 119 105. VPO and JMH gratefully acknowledge the support by the Director,
Office of Science, Division of Materials Science
& Engineering, US Department of Energy, under
contract DE–FG02–01ER45918.
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