Soft Matter and Functional Materials - Helmholtz-Zentrum Berlin

Research on Soft Matter ________________________________________________________
Polymers and nanocomposites
In-situ Growth of Catalytic Active Au-Pt Bimetallic Nanorods in Thermo-Responsive
Core-Shell Microgels
We demonstrate that bimetallic Au-Pt
nanorods (NRs) can be grown in situ
into thermosensitive core-shell microgel
particles by a novel two-step approach.
In the first step, Au NRs with an average
width of 6.6±0.3 nm and length of
34.5±5.2 nm (aspect ratio 5.2±0.6) were
homogeneously embedded into the
shell of PNIPA networks. The volume
transition of the microgel network leads
to a strong red shift of the longitudinal
plasmon band of the Au NRs. In the
second step, platinum was preferentially
deposited onto the tips of Au NRs to
form dumbbell-shaped bimetallic
nanoparticles. The novel synthesis forms bimetallic Au-Pt NRs immobilized in microgels
without impeding their colloidal stability. Quantitative analysis of the catalytic activity for the
reduction of 4-nitrophenol indicates that bimetallic Au-Pt NRs show highly enhanced catalytic
activity, which is due to the synergistic effect of bimetallic nanoparticles. The catalytic activity
of immobilized Au-Pt NRs can be modulated by the volume transition of thermosensitive
microgels. This demonstrates that core-shell microgels are capable of serving as “smart
nanoreactors” for the catalytic active bimetallic nanoparticles with controlled morphology and
high colloidal stability. 1
Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles
Immobilized in Spherical Polyelectrolyte Brushes
We present a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the
presence of metal nanoparticles. The nanoparticles are embedded in spherical
polyelectrolyte brushes, which consist of a polystyrene core onto which a dense layer of
cationic polyelectrolyte brushes are grafted. The average size of the nanoparticles is
approximately 2 nm. The kinetic data obtained by monitoring the reduction of 4-nitrophenol
by UV/vis-spectroscopy could be explained in terms of the Langmuir-Hinshelwood model:
The borohydride ions transfer a surface-hydrogen species in a reversible manner to the
surface. Concomitantly, 4-nitrophenol is adsorbed and the rate-determining step consists of
the reduction of nitrophenol by the surface-hydrogen species. The apparent reaction rate can
therefore be related to the total surface S of the nanoparticles, to the kinetic constant k
related to the rate-determining step, and to the adsorption constants K(Nip) and K(BH4) of
nitrophenol and of borohydride, respectively. In all cases, an induction time t(0) was
observed of the order of minutes. The reciprocal induction time can be treated as a reaction
rate that is directly related to the kinetics of the surface reaction because there is a linear
1 Y. Lu, J. Yuan, F. Polzer, M. Drechsler, J. Preussner, ACS Nano 2010, 4, 7078-7086.
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