Sum frequency generation and catalytic reaction studies of the
removal of the organic capping agents from Pt nanoparticles
by UV-ozone treatment
Cesar Aliaga,
†
Jeong Y. Park,
†
Yusuke Yamada,
† #
Hyun Sook Lee,
†
Chia-Kuang
Tsung,
†
Peidong Yang,
†
and Gabor Somorjai.
†
*
† Department of Chemistry, University of California, Berkeley, CA 94720 and Materials
Sciences Division and Chemical Sciences Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720
Abstract
We report the structure of the organic capping layers of platinum colloid nanoparticles
and their removal by UV-ozone exposure. Sum frequency generation vibrational
spectroscopy (SFGVS) studies identify the carbon-hydrogen stretching modes on
poly(vinylpyrrolidone) (PVP), and tetradecyl tributylammonium bromide (TTAB) capped
platinum nanoparticles. We found that the UV-ozone treatment technique effectively
removes the capping layer, based on several analytical measurements including SFGVS,
X-ray photoelectron spectroscopy, and Diffuse Reflectance Infrared Fourier Transform
Spectroscopy (DRIFTS). The overall shape of the nanoparticles was preserved after the
removal of capping layers, as confirmed by transmission electron microscopy (TEM).
SFGVS of ethylene hydrogenation on the clean platinum nanoparticles demonstrates the
existence of ethylidyne and di--bonded species, indicating the similarity between single
crystal and nanoparticle systems.
TITLE RUNNING HEAD: Sum Frequency Generation and Catalytic Reaction Studies of
the removal of the organic capping agents from Pt nanoparticles by UV ozone treatment.
* Author to whom correspondence should be addressed. E-mail:somorjai@berkeley.edu
# Current Address: Research Institute of Ubiquitous Energy Devices, National Institute of
Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka
563-8577 Japan.
1
I. Introduction.
Industrial heterogeneous catalysts are usually composed of nanometer sized
metallic clusters capped with diverse organic compounds, and supported by porous
materials. Traditionally, fundamental catalysis studies are conducted on metal single
crystals, which provide extensive information of the molecular processes at the surface,
under optimal and well-controlled conditions. However, most of the industrial catalysts
are highly dispersed nanoparticles and catalytic processes occur at elevated temperatures
and high pressures.
1 ,2
Therefore, it is essential to perform in-situ studies of model
systems that have complexities associated with real catalytic conditions.
3
The evolution
of the science of nanomaterials has permitted the use of nanoparticles as model systems,
4
and the capability of controlling their size, shape, and composition by colloid chemistry
brings new opportunities to develop novel nanocatalysts.
5 ,6
Colloidal nanoparticles synthesis involves the reduction of metal ions in solution,
and since the surface energy of finely divided matter is high, freshly precipitated metallic
particles naturally tend to aggregate. In order to overcome that problem, freshly prepared
nanoparticles are usually precipitated in a solution that contains an organic compound,
which will adhere to their surface and keep the particles suspended.
7
After transfer to a
solid support, this capping layer remains at the surface of the particles. It is known,
however, that at the molecular scale, the organic capping layer contains open spaces that
permit the reactant and product molecules to adsorb and desorb from the surface of the
catalytically active metal. Earlier studies indicate that catalytic activity highly depends on
the nature of the interaction between the molecules and the metal sites
8,9
. However, the
2
role of the capping agent in the catalytic activity and selectivity is poorly understood in
spite of its importance in heterogeneous catalysis as an emerging model system.
Sum frequency generation vibrational spectroscopy (SFGVS) is an important tool
in the basic understanding of catalysis at the molecular level since it is able to identify
reaction intermediates under catalytic conditions. Several SFGVS studies on catalytic
reactions, including CO oxidation over colloid nanoparticles, were carried out earlier.
The vibrational signature of CO does not overlap with the capping agent vibrational
modes, facilitating the interpretation of the resulting spectra.
10-12
Other reactions, such as
pyridine hydrogenation, were also investigated, although only outside of the methyl-
methylene stretch vibrational region.
13
Reactions more relevant to the industrial processes
of hydrogenation, cracking, etc, have been performed on platinum single crystals and
probed by SFGVS, but have not been carried out on nanoparticles due to the
abovementioned difficulty.
14,15
The approach to investigate the role of capping agents involves the removal of
capping layers with UV/ozone (O
3
) treatment. The UV/ozone oxidation has been used for
decades as a means of cleaning slightly soiled surfaces for a variety of applications. The
method typically uses ultraviolet light that includes the wavelengths of 185 and 257 nm,
where the former generates ozone upon interacting with molecular oxygen. The UV/
ozone oxidation process involves the simultaneous action of ozone and ultraviolet light,
which are responsible for the oxidation of the carbon containing compounds into carbon
dioxide and water.
16
Ultraviolet radiation can also be used for polymer surface
modification with applications in photolithography, microfluidics, and bioengineering,
and its surface effects were studied using SFGVS,
17,18
which was used as well to
3
investigate the effect of ultraviolet radiation and ozone on typical organic substances
present in the upper atmosphere.
19
Earlier results of removal of the capping agent using
the same method were furthermore reported for gold nanospheres with satisfactory
results, as well as for bimetallic PtFe nanoparticles.
20,21
In this work, the application of UV/ozone oxidation is used to remove the capping
agent so that SFGVS catalysis studies at the surface of nanoparticles catalysts can be
carried out. Previous attempts using methods such as plasma cleaning, hydrogen
treatment, or heat treatment under different gaseous atmospheres proved inefficient,
because of the irreversible modification of the shape and distribution of the nanoparticles.
We monitored the removal of the capping agent from the surface of platinum nanocubes
with various analytical methods including SFGVS, XPS, and DRIFTS. The shape and
spatial distribution of the nanoparticles are examined with SEM and TEM before and
after UV/ozone treatment. In addition ethylene hydrogenation at room temperature and
atmospheric pressure is performed at the surface of the catalytic nanocubes and the
reaction intermediates are probed with SFGVS.
II. Experimental procedures
II.a. Synthesis of TTAB or PVP capped nanoparticles.
The nanoparticles we used in this study consist of 10 nm platinum cubes capped
with tetradecyl trimethylammonium bromide (TTAB), or polivynil pyrrolidone (PVP).
TTAB coated platinum nanocubes were synthesized according to literature methods.
6
4
Aqueous solutions of tetradecyltrimethylammonium bromide (400mM, 2.5mL) and
dipotassium tetrachloroplatinate(II) (10mM, 1mL) were added to water (5.9mL) in a
reaction vial at room temperature. The solution was mixed with a vortex mixer for 30 sec
and let still until needle shape crystals formed. The mixture was then heated at 50°C in
oil bath under magnetic stirring until the crystals dissolved. An ice cooled aqueous
solution of sodium borohydride (500mM, 0.6mL) was added rapidly to the solution. The
hydrogen gas formed was released via a needle spearing the septum rubber capping the
reaction vial. The needle was removed after 15min of reaction. The reaction mixture
was then magnetically stirred for more than 7 hours at 50°C. The resulting brown
solution was centrifuged at 3000rpm for 30 min to remove the larger Pt nanoparticles.
The supernatant solution was separated and centrifuged again at 12000 rpm for 10 min.
The precipitate was collected and re-dispersed in deionized water.
The synthetic procedure of PVP capped Pt nanoparticles is described elsewhere.
22
Briefly, 0.1 mmol ammonium hexachloroplatinate (IV), 1.5 mmol
trimethyl(tetradecyl)ammonium bromide, and 2 mmol poly(vinylpyrrolidone) were added
to 20 ml ethylene glycol in a 50 ml three-necked flask at room temperature. The stock
solution was heated to 80 °C in a Glas-Col electromantle (60 W; 50 ml) with a Cole-
Parmer temperature controller (Diqi-sense®), and was evacuated at this temperature for
20 min to remove water and oxygen under magnetic stirring. The flask was then heated to
180 °C at 10 °C / min, and maintained at this temperature for 1 h under Ar. When the
reaction was complete, an excess of acetone was added at room temperature to form a
cloudy black suspension. This suspension was separated by centrifugation at 4200 rpm
for 10 min, and the black product was collected by discarding the colorless supernatant.
5
