# Nanostructural Formation of Pd-Co Bimetallic Complex on HOPG Surfaces: XPS and AFM Studies

Authors: Lisandra Arroyo-Ramírez; Rubenier Montano-Serrano; Raphael G. Raptis; Carlos R. Cabrera
Journal: Research Letters in Nanotechnology
 (2009)

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## Abstract

A new single source approach was developed to synthesize Pd-Co nanoparticles using a bimetallic compound,[Et3NH]2[CoPd2(μ-4-I-3,5-Me2pz)4Cl4](CoPd2), as a molecular precursor to obtain dispersed catalyst on highly ordered pyrolytic graphite (HOPG) surface, in view of preparing oxygen reduction catalysts for low temperature fuel cells. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques were employed to characterize the nanostructure formations and to determine the composition and morphology of the complex on the HOPG. Results of high resolution XPS analysis (HR-XPS) revealed the binding energies corresponding to the atomic constituents of the precursor. When the precursor solution was placed on the surface of the HOPG, the bimetallic complex assumes a tubular structure and it appears that the surface of the HOPG offers a ground for the self-organization of nanostructural formations.


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## Body

## 1. Introduction

Nanoarchitecture is an emerging area that fuels the interests of
scientists and engineers alike, largely due to the novel material properties
that can be engineered and tuned at molecular levels. These nanomaterials are
mainly used for heterogeneous catalysis. Platinum is used extensively as a
catalyst in both anodes and cathodes of low-temperature polymer electrolyte
fuel cells. Low-temperature fuel cells are considered alternate power sources
for portable and transportation applications [1]. In spite of its excellent
electrocatalytic activity, the use of monometallic platinum is disadvantageous
due to its intermediate poisoning and high cost. Sophisticated bimetallic, cost-effective
Pd-Co catalysts were recently proposed for oxygen reduction reaction in low-temperature
fuel cells, especially for direct methanol fuel cell applications due to their
methanol tolerance ability and appreciable oxygen reduction efficiency [2].When bimetallic catalysts are prepared by
using multiple precursors, it is difficult to attain homogeneous catalytic
system characterized by a uniform particles size distribution. In order to achieve
such objective, in this paper, a single organometallic precursor characterized
by a well-defined Pd-Co ratio has been used. This approach would allow the
formation of nanostructures and nanoparticles with defined size and composition
distribution.This paper reports on the characterization of a CoPd2 precursor
and deposition for the nanoarchitectural formation on highly ordered pyrolytic
graphite surface through surfaces analysis techniques such as X-ray
photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).


## 2. Experimental

The ionic precursor of [Et3NH]2 [CoPd2(μ-4-I-3,5-Me2pz)4Cl4]  (CoPd2) was synthesized following
the procedure from [3]. An exfoliated piece of highly ordered pyrolytic
graphite (HOPG) spi-2 grade was used as the substrate. Approximately 100 μL of CoPd2 precursor (0.5 mM)
in dichloromethane solution was deposited on HOPG surface and allowed to dry. This
sample was placed under vacuum for one hour and analyzed by surface analytical
techniques. The reduction process was performed
in a closed  tube furnace by increasing the temperature to over 600°C under an H2 stream. The samples were cool-off in an
H2 atmosphere to ~63°C and they were purged with N2 until
they reached room temperature  before opening the stainless steel system.Our scanning probe  microscope was a Nanoscope
IIIa-Multimode atomic force microscope (AFM) from Digital Instruments, with a
scanning probe microscope controller equipped with He-Ne laser (638.2 nm) and
scanner type E. A standard Si3N4 cantilever was used for
contact mode imaging, and the scan rate was 1 Hz.X-ray photoelectron spectroscopy (XPS)
was used to determine the composition of the precursors. A PHI 5600ci
spectrometer with an Al Kα
monochromatic X-ray source at 15 kV and 350.0 W was used to obtain a survey and
multiplex XPS spectra. Spectrum was recorded at a take-off angle of 45° and
pass energy of 187.8 eV for the survey and 58.7 eV for the high-energy
resolution studies. The binding energies were corrected using the carbon (C1s)
contamination peak at 284.5 eV.


## 3. Results and Discussion

Surface analytical techniques were
employed to study the nanoarchitectural formation of a new heterobimetallic
compound on the surface of HOPG. The molecular complex had Pd(II) and Co(II)
metals with chloride and dimethyliodopyrazolate ligands. The chemical composition
of the [Et3NH]2 [CoPd2(μ-4-I-3,5-Me2pz)4Cl4]  (CoPd2) complex
and the modified HOPG surface were determined by XPS analysis.
The XPS survey spectrum for  the CoPd2 precursor showed the peaks
attributed to C, N, I, Cl, Pd, and Co atoms (see Figure 1).Figure 1
XPS survey spectra of the CoPd2 precursor.The high-resolution XPS (HR-XPS) was used
to identify the metals present in the precursor. The observed HR-XPS binding
energy (BE) peaks are summarized in Table1.Table 1
Binding energy (BE) values obtained from
HR-XPS studies for the free standing CoPd2 precursor and CoPd2 precursor
on HOPG surface.
ElementsBinding energy (eV)Free precursorPrecursor on
HOPGPd
(3d5/2)337.9338.4339.5339.6Co
(2p3/2)781.3781.9787.0787.0I
(3d5/2)621.9621.3Cl
(2p)197.3198.4198.1199.7N
(1s)399.1399.7400.1400.6401.5401.7402.6402.7C
(1s)284.5284.5285.3285.4286.1286.3Figure2 shows the high-resolution XPS
spectrum corresponding to palladium (3d) and cobalt (2p) binding energy regions
for the CoPd2 precursor. In the Pd 3d5/2 region, the
HR-XPS spectrum shows two peaks with binding energy values of 337.9 and 339.5 eV for the Pd-Cl and Pd-N, respectively. These values are in agreement with the binding energy values of 337.8 and 337.9 eV for
PdCl2 and 339.0 eV for K2Pd(NO2)4 compounds [4, 5].HR-XPS spectra corresponding to Pd 3d
and Co 2p binding energy regions of the CoPd2 precursor.
(a)(b)In the Co region, the peak with BE of
781.3 eV is attributed to cobalt at precursor and a shakeup satellite peak at
787.0 eV [6]. The binding energy values for Co 2p3/2, in the precursor,
were similar to those reported in literature for other cobalt compounds with
Cl-Co-N bonds [4]. The other binding energy values were as follows: I in
4-I-pz, 661.9 eV; and Cl in Cl-Co and Cl-Pd, 197.3 eV and 198.1 eV,
respectively. The binding energy obtained for nitrogen in the pyrazolate was
found at different regions such as 339.1, 400.1, 401.5, and 402.6 eV, and this
has been due to the different chemical environments prevailing in the
precursor. The C–C, C–I, and C–N bonds in the pyrazolate can be ascertained
from corresponding binding energy values such as 284.5, 285.3, and 286.1 eV,
respectively.The XPS spectrum for the unmodified HOPG
surface was used as a reference. A clean HOPG survey spectrum showed only the
typical C(1s) and O(1s) peaks. On the other hand, the XPS survey spectrum
confirmed the modification of the HOPG surface with the complex CoPd2.
Typical element peaks for Pd(3d), Co(2p), I(3d), N(1s), Cl(2p), C(1s), O(1s),
and Si(2p) were present in this spectrum. This has been attributed to the
adsorption of CoPd2 precursor on the HOPG surface. The Si (2p) and O
(1s) impurities might have originated from the glassware used in the
preparation of the sample.It is known that the deposition is highly
influenced by the substrate surface characteristics.
Also, the shape and size of the deposit depends on the substrate
employed. Highly ordered pyrolytic graphite is a nonpolar, highly pure carbon,
with a smooth surface and basal plane featuring a few defects. The HOPG surface
image was obtained using an AFM to study the morphological change which would
arise as a result of the modification process. AFM images of HOPG modified with
a CoPd2 precursor showed the formation of organized circular
nanostructures of different sizes (Figure 3). These nanostructures on HOPG
exhibited a tube-like shape in micro- to nanosize regime diameter and height.
The AFM section analysis has been used for size determination of the nanostructures.AFM images top and
3D view (a) 10μm
× 10 μm
× 380 nm and (b) 10 μm
× 10 μm
× 160 nm for deposited of CoPd2 precursor on HOPG surfaces.
(a)(b)In Figure3(a), the rings exhibit
diameters which range from 156 nm to 2.03 μm
and heights between 25 and 385 nm. Figure 3(b)  shows rings with diameters ranging from 234 nm to
1.86 μm
and heights between 14 and 200 nm. Their inner diameters span from 30 to 898 nm
and wall thickness between 117 and 391 nm. Some structures are volcano-like
shapes with the top end of the wall smaller in diameter than the bottom.Other samples examined by AFM and
scanning electron microscopy (SEM) showed diameters ranging from 150 nm to 8.1μm and the heights between 10 and 400 nm
with a maximum frequency at 60 nm. Similar results were observed with palladium
precursors [7, 8]. The interaction between precursor-solvent-substrate and the
evaporation process play an important role in the rings structure formation [8, 9]. Gómez-Segura et al. reported the
self-assembly of Mn12 microrings in the HOPG surfaces [9]. CoPd2 precursor is a nonpolar complex allowing the interaction with the HOPG surface.
The organic ligands ((μ-4-I-3,
5-Me2pz)4Cl4) in the precursor prevent the random
particles dispersion, and hence the solution  drops evaporate to form the same
type of structures. Different structural size formations can be seen when the
samples are prepared in different days, perhaps due to the humidity change on a given day. It is assumed
that the structural difference causes this change. Development of
palladium-cobalt nanostructures with defined sizes can be controlled by the
characteristics of the support surface and the environment used in the
deposition process. The process of circular structure formation is not fully
understood and a study with humidity control is underway.The CoPd2 precursor was thermally reduced under hydrogen atmosphere, a procedure which
removes the organic ligands. The
rings formation tendency is preserved after the
reduction process. This nanostructures show different size distributions on the
surface. Moreover, in the images of the AFM, it is possible to observe the rings
formed by bimetallic particles (see Figure 4).Figure 4
AFM images top and 3D view (10μm
× 10 μm
× 100 nm) after thermal reduction of CoPd2 precursor on HOPG
surface.In the survey spectrum after
the reduction process, the corresponding peaks of pyrazolate ligands disappear while the Pd and Co peaks shift their
binding energy. The CoPd2 precursor reduced has a BE at 335.0 eV,
335.9 eV, and 337.4 eV for Pd 3d5/2, which shows the presence of three different
species. The literature reports binding energy values of 335.1 eV for Pd metallic
[4], 335.7 eV for PdCo alloy [10, 11], and 337.9 eV for PdO2 and
PdCl2 [4, 5, 12]. For the Co region, the peak at 781.5 eV is higher
than the cobalt metallic [5] and can be attributed to cobalt bimetallic because
they are similar to reported value (781.9 eV) at literature for PtCo bimetallic
[13]. On the other hand, the BE at 787.3 eV is for the shakeup satellite of
cobalt. The binding energy shifts could be
caused by the Pd-Co alloy formation. We can observe similar results with the
precursor reduced on HOPG surface. For the Pd 3d5/2 region,
the BE is of 335.3 eV, 336.1 eV, and 338.1 eV. The binding
energies of 781.3 eV, 783.8 eV, and 788.2 eV correspond to Co region. Also, the
binding energy displacement in the Pd and Co regions shows the change of
oxidation state and demonstrates
that the modified HOPG surface changes after the reduction process.HOPG provides the surface for the
nanostructures preparation and it can be used as an electrode for electrochemical
analysis. In addition, these nanostructures with different size distribution in
the HOPG surfaces may lead to its development for diverse applications.Our future work would be focused on
optimizing the conditions (e.g., relative humidity) for the nanostructures
formation at HOPG surfaces. We are also studying different precursors and
surfaces to see if they exhibit a similar behavior. These precursors have the
same ligands but with different metal combinations like Pd-Ni, Pd-Cu, Pt-Co,
among others. These combinations open the possibility of straightforward
development of nanomaterials for various applications. Furthermore, the
obtained novel metallic nanostructures and nanoparticles from CoPd2 precursor will be evaluated for oxygen reduction reaction in fuel cell cathode
applications.


## 4. Conclusion

We prepared a promising bimetallic
nanocatalyst by chemical method from single precursor. The XPS spectrum of the
[Et3NH]2[CoPd2(μ-4-I-3,5-Me2pz)4Cl4]  precursor and modified HOPG surface confirmed the interaction between the precursor and the substrate. HR-XPS allowed the
analysis of different species present in the precursor. The AFM image showed
circular structures in the range of micro- to nanometer with the precursor’s
deposition at HOPG surface. Hence, this study is vitally important in the
aspects of developing nanostructures and nanoparticles in well-defined support
surfaces. This precursor is a promising new material for obtaining Pd-Co nanorings
with self-formation at HOPG surfaces.



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*Source: 971423-2009-04-01.xml*