Angewandte
Communications
Chemie
containing THH nanocrystals in the presence of full-spectrum
irradiation. A temperature increase of only 28C was observed
over 15 min of irradiation. Figure S10 shows there is no
significant increase in the hydrogen production rate at 278C
compared to that at 258C, thus photothermal effect has no
significant contribution to enhance the dehydrogenation
reaction.
A schematic diagram for the dehydrogenation mechanism
of AB in the presence of metal nanocatalyst is presented in
Figure S11. The enhanced hydrogen production rate in the
presence of light by Au–Pd core–shell THH nanocatalyst can
be explained by considering the localized surface plasmon
resonance effect. Recently, Yamashita et al. have shown that
surface charge separation generated by the collective oscil-
lations of free electrons on Ag nanoparticles are responsible
for the activation of AB, resulting in enhanced hydrogen
production in the presence of light.[25] Similarly, Majima et al.
showed that Pd-tipped Au nanorods produces a large extent
of surface charge heterogeneity on the co-exposed Pd and Au
surfaces in the presence of light, and hence an improved
hydrogen production rate from formic acid in the presence of
light.[30] To determine the region of electric field amplification
upon SPR excitation as a function of incident polarization
direction and the role of surface charge heterogeneity on the
dehydrogenation of AB, 3D-FDTD simulations were per-
formed and shown in Figure S12. It clearly indicates that the
near-field resonances are distributed along the edges or
corners of nanocrystals with cubic, octahedral, and THH
shapes. According to the Mie theory, it is easy to imagine that
the density of surface hot electrons along the edges or corners
is larger than a uniform area.[38] XPS spectra and XRD
patterns have shown that core–shell nanocrystals have
modified surface electronic properties. When these core–
shell nanocrystals are exposed to light, much more efficient
electron transfer takes place from the Au core to the Pd
shell,[29–31] and the generated hot electrons get more concen-
trated along the edges and corners of the well-defined
nanocrystals, making the uniform area more electron-defi-
cient which produces enhanced surface charge heterogeneity.
THH nanocrystals contain the highest number of edges and
corners followed by cubes and octahedra, so it is expected that
they will produce a higher degree of surface charge hetero-
geneity in the presence of light. The protonic hydrogens of
NH3 in AB prefer to attach to the corner and edge palladium
atoms, which are more electron-rich in the presence of light.
Similarly, the hydridic hydrogens of BH3 part prefer the
planar areas. Thus surface charge heterogeneity on the
nanocrystal surface in the presence of light favors enhanced
binding of AB which effectively reduces the activation barrier
Figure 3. Time-dependent UV/Vis absorption spectra for the reduction
of 2-amino-5-nitrophenol by ammonia borane in the presence of Au–
Pd core–shell THH nanocrystals a) with light and b) without light at
258C. c) ln[C0/Ct] versus time plot for the reaction with and without
light irradiation.
Figure 3 shows the reduction of 2-amino-5-nitrophenol by
AB in the presence of Au–Pd core–shell THH nanocrystals
with and without light irradiation. Figure S13 confirms
selective formation of 2,5-diaminophenol. At 258C, the
reduction rate constants in light and dark are 0.3495 and
0.1309 minÀ1, respectively (Figure 3c), indicating an enhance-
ment of 2.67-fold due to the light energy. Figure S14 confirms
that in the absence of Pd surface no nitro reduction takes
place. In the presence of light, local hydride concentration on
the nanocrystal surface are much higher compared to dark,
which catalyze efficient electron transfer from the donor HÀ
to the acceptor 2-amino-5-nitrophenol, resulting in enhanced
reduction rate. Au–Pd THH particles were also deposited on
activated carbon to prepare recyclable catalyst. Figure S15
gives volume of hydrogen produced with respect to time in
the presence of light for 3 cycles of the reactions. Consistently
good hydrogen production has been demonstrated. TEM
images of the Au-Pd THH/C catalyst before and after the
reactions show their good reusability.
In conclusion, we have synthesized Au–Pd bimetallic
core–shell nanocrystals with THH, cubic, and octahedral
shapes and compared their plasmonic photocatalytic activity
for dehydrogenation reaction of AB with comparable-sized
Pd and Au cubes and octahedra. The Au–Pd THH particles
displayed exceptionally high reactivity with the highest TOF
value and three-fold rate enhancement in the presence of
light, which has been well explained using XRD, XPS, UV/Vis
spectra, and 3D-FDTD simulations. A plasmon-enhanced
nitro reduction reaction was recorded. This study shows how
thoughtful choice of facet and incorporation of a plasmonic
core in a nanocrystal can modify the particle’s surface
À
and weakens the B N bond in AB. Furthermore, when these
hot electrons revert back to their original state, they release
their excess energy which can also be transferred to the
À
surface-attached AB molecules, thus weakening the B N
bond. H2O molecules subsequently attack on these weakened
À
“B N” bonds to generate BH3 intermediates, which then
undergo hydrolysis to release hydrogen gas. Thus activation of
AB takes place more efficiently in the presence of light
resulting in enhanced hydrogen production.
Angew. Chem. Int. Ed. 2016, 55, 7222 –7226
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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