Bubble-assisted growth of hollow palladium nanospheres with structure control allowing very thin shells for highly enhanced catalysis

Article abstract of DOI:10.1039/c4ra00371c

Palladium shells are prepared via a one-step solvothermal synthesis involving nanobubbles as soft templates although no stabilizer is present, while still allowing control over diameter, thickness, and grain size, leading to grain-monolayer thin and porous shells with very high and durable electrocatalytic activity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00371c

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Bubble-assisted growth of hollow palladium
nanospheres with structure control allowing very
Cite this: RSC Adv., 2014, 4, 13729
thin shells for highly enhanced catalysis†
Received 14th January 2014
Accepted 3rd March 2014
Shaochun Tang, Sascha Vongehr, Xiangyu Wang, Yongguang Wang
and Xiangkang Meng*
DOI: 10.1039/c4ra00371c
www.rsc.org/advances
Palladium shells are prepared via a one-step solvothermal synthesis for catalysis. The growth mechanism and the stability of the
involving nanobubbles as soft templates although no stabilizer is bubbles are discussed. We also studied the catalytic perfor-
present, while still allowing control over diameter, thickness, and grain mance by benchmarking with formic acid oxidation, comparing
size, leading to grain-monolayer thin and porous shells with very high between Pd NPs, Pd black and our Pd shell products with
and durable electrocatalytic activity.
different shell thicknesses.
Fig. 1a presents the product from a typical synthesis. The
isolated spherical particles have rough surfaces and a diameter
of d ¼ (145 ꢁ 25) nm based on the survey of 50 randomly chosen
spheres (Fig. 1b). The width of the size distribution is relatively
large, which conceivably may result from that the equilibrium
bubble size changes over time, but such is for now mere spec-
ulation. The TEM image (Fig. 1c) shows the spheres' cavities;
shell thickness h is only (16 ꢁ 4) nm.
Hollow nanostructures have attracted much interest due to
1
their high specic surface areas. Metallic, hollow nanospheres
2
3
4
are important in catalysis, biomedicine, as optical devices,
and so on. Typically, they are prepared by metal deposition onto
carbon, silica, or polymer spheres which are removed by
5
6,7
thermal treatment or etched away. Galvanic replacement of
8
silver and cobalt as sacricial templates, and co-assembly of
metal nanoparticles (NPs) with organic molecules are also
used. These require many steps or hazardous reagents. To
achieve a high catalytic activity, the grain size and shell width
should be as small as possible, but the usual techniques
The SAED pattern (inset) recorded from a shell reveals a
polycrystalline nature with diffraction rings corresponding to
facets of face-center-cubic (fcc) Pd. Magnication (Fig. 1d) and
HRTEM from a sphere's edge (Fig. 1e) show the (6 ꢁ 2) nm
single-crystalline grains; their crystallographic orientations lie
9
1
–9
6
10,11
damage especially the desired thin shells.
2
ꢀ1
in random directions. The BET area is 30 m g (Fig. 1f), about
ve times larger than when assuming only the outside surfaces
Bubble-assisted synthesis is a green method for large-scale
fabrication of hollow spheres. However, due to their high Lap-

12
lace pressure, nanobubbles dissolve too fast. There has been
only limited success with bubble-assisted preparation of
13–15
nanoshells,
and stabilizing surfactants are deemed crucial.
Strong adhesion of surfactants in pores makes their complete
removal difficult, impacting catalytic performance. Herein we
report on Pd shells from a novel one-step solvothermal
synthesis using hydrogen nanobubbles as so templates
without surfactants. Shell diameter, thickness, and grain size
2ꢀ
are controlled by concentrations (CHCOOH and PdCl
4
concentration CPd), and temperature T. The method allows
small grain, grain-monolayer thin, incomplete shells superior
Institute of Materials Engineering, National Laboratory of Solid State Microstructures
and College of Engineering and Applied Sciences, Nanjing University, Jiangsu, P. R.
China. E-mail: mengxk@nju.edu.cn; Tel: +86 25 8368 5585
†
Electronic supplementary information (ESI) available: Experimental section,
TEM images showing Pd products obtained with different CHCOOH, different
Pd, and varying r. See DOI: 10.1039/c4ra00371c
Fig. 1 SEM (a), histogram of diameters for 50 shells (b), TEM (c–d) with
SAED pattern inset in c, and HRTEM image (e). N sorption isotherm (f)
of the typical product and its pore-size distribution (inset).
2
C
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of dense smooth shells contribute. In that case, the specic therefore decreases with increasing CHCOOH. It should be noted
2
surface equals the outer surface A ¼ 4p(d/2) divided by the that at a high CHCOOH of 5.04 M, the shells are roughly grain-
3
shell's mass m ¼ V ꢂ r , with shell volume V ¼ (4p/3)[(d/2) ꢀ monolayers because the average thickness is not even twice the
Pd
3
2
ꢀ1
(
d/2 ꢀ h) ]. This estimate gives about 6 m g . An about three average grain size, as indicated by HRTEM images. Even higher
times larger BET indicates merely a rough surface, but the factor concentrations will lead to less surface coverage, which can no
of ve indicates that there is already signicant porosity (so that longer maintain a stable spherical shape, as conrmed by the
the assumption of a dense shell with bulk Pd density rPd is not debris obtained with CHCOOH ¼ 5.76 M. The BET specic surface
2
ꢀ1
valid).
area of the 5.04 M product is measured to be 87.5 m g .
With CHCOOH equal or below 0.72 M, only NP aggregates with Addition of 0.2 mmol CTAB and other conditions being the
irregular shapes are obtained (Fig. S1a†). Increasing CHCOOH to same (CHCOOH ¼ 5.04 M) shows that d is widely distributed and
1
.44 M results in shells with diameters of (85 ꢁ 13) nm, but the shells are thick (Fig. S2†). Further increasing C
can no
HCOOH
irregular and large aggregates are still observed (Fig. S1b†). At longer achieve monolayer Pd shells. This indicates that an
C
HCOOH ¼ 2.16 M, the diameter is (122 ꢁ 22) nm (Fig. 2a). A addition of surfactants into the reaction system does not facil-
CHCOOH of 2.88 M results in the already discussed sample shown itate the formation of shells but makes size control difficult.
in Fig. 1c. At 3.60 M and 4.32 M, the diameter increases to (175
Keeping CHCOOH at 2.88 M, shells do not form when CPd is
ꢁ
28) nm and (250 ꢁ 30) nm (Fig. 2b and c). Around 5.04 M, below 1.0 mM (Fig. S3a†). The typical CPd of 1.5 mM results in a
most of the resulting 360 nm diameter shells are broken and the thickness of 16 nm. At 2.5 mM, the shell thickness h increases to
surface is covered incompletely (Fig. 2d). Please notice that the (23 ꢁ 4) nm (Fig. S4a†), and at 4.0 mM, the shells are (28 ꢁ 5)
front and back halves of a shell overlap each other in the image. nm thick but irregular NP aggregates start to be observed
The uncovered areas of the shell are therefore even larger than it (Fig. S4b†). Beyond 5.0 mM, large 3D networks form (Fig. S3b†).
appears. Further increasing CHCOOH to 5.76 M results in only The diameter of the shells stays surprisingly constant when
debris of broken shells (Fig. S1c†). When CHCOOH is above 6.0 M, changing CPd. Therefore, the dependence between the total
no hollow spheres but network-like nanostructures are amount of Pd and h again proves a varying number of bubbles.
obtained (Fig. S1d†).
The thicknesses h of the shells (insets of Fig. 2) obtained with HCOOH into H
different CHCOOH of 1.44, 2.16, 2.88, 3.60, 4.32 and 5.04 M are 13, density of Pd NPs increases linearly with CPd. More Pd results in
1, 16, 12, 10, and 7 nm, respectively (all with standard devia- more bubbles, suggesting already that the bubbles grow on the
This is here due to that the Pd is actively involved in turning
2
. Since the grain size stays constant, the number
2
tions of ꢃ4 nm). Since the amount of Pd was kept constant Pd, so the growth mechanism is more complex than just having
9
throughout (CPd was not changed), one expects that h decreases NPs attach to pre-existing bubbles or micelles as usual with a
with increasing diameter. The diameter changes almost linearly surfactant facilitated synthesis. The diameter is indeed robustly
with CHCOOH (Fig. 2e). However, d increases by roughly a factor dependent only on CHCOOH, as conrmed by the product from
of three while h decreases roughly by the same factor of
three from 21 to 7 nm (we omit the 85 nm diameter shells, 360 nm again, and h thus lowered to 18 nm (Fig. S4c†).
which come together with many irregular aggregates, from this Reaction temperature T inuences crystal growth and thus
CPd ¼ 4.0 mM and CHCOOH ¼ 5.04 M the diameter is as expected
analysis). This does not reect the square law dependence determines the grain size. Fig. S4d–f† shows TEM images of Pd
between surface and diameter! The number of shells, which is shells obtained with different T while keeping other parameters
1
1
around 10 as calculated from the amount of used Pd, typical (CHCOOH ¼ 2.88 M and CPd ¼ 1.5 mM). This synthesis
series reliably obtains hollow spheres with a diameter of ꢃ145
nm. However, the average grain sizes obtained at 60, 100, 160
ꢄ
and 200 C are 4 ꢁ 2 nm (Fig. S4d†), 6 ꢁ 2 nm (Fig. S4e†), 11 ꢁ 4
nm, and 21 ꢁ 7 nm (Fig. S4f†). The XRD patterns (Fig. S4g†)
verify the products' high purity and crystallinity. The four peaks
ꢄ
ꢄ
ꢄ
ꢄ
at 39.8 , 46.3 , 67.6 and 81.9 correspond to the (111), (200),
220) and (311) of fcc Pd. The peak broadening is associated
(
with the grain size. According to the Scherrer equation, the
ꢄ
average grain sizes from different T of 60, 100, and 200 C are
about 5, 8, and 22 nm, consistent with the TEM results. The
average grain size's dependence on T is depicted in Fig. S4h.†
Table S1† summarizes reaction parameters and corresponding
sizes, demonstrating that the average d, h, and grain size are
controllable.
At an autoclave lling ratio of r ¼ 100%, bubbles cannot
form in the incompressible solvent. When r is increased from
the typical 60% to 65%, shell diameters are not uniform
(
Fig. S5a†). At 75%, no hollow spheres but networks are
Fig. 2 TEM images of the Pd products obtained with different CHCOOH
of 2.16 (a), 3.60 (b), 4.32 (c), and 5.04 M (d), and the dependence of the
diameter and shell thickness on CHCOOH (e).
obtained (Fig. S5b†). If r is decreased to 50%, only debris of
shells is obtained (see Fig. S5c–d†). Although the variation of r
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7
seemed small and the large effects thus surprising, an estima- under the hydrogen desorption peak. The specic EASA of
2
ꢀ1
tion of the pressures in the reaction system indeed tripled when the Pd-7 is calculated to be 112.7 m g , which is more than
2
ꢀ1
changing r from 50% to 70%.
ve times that of Pd NPs (21.8 m
g
), and signicantly
2
ꢀ1
The growth mechanism of the Pd shells must be discussed higher than that of the commercial Pd black (57.2 m g ) and
together with the stability of the bubbles. HCOOH reduces those of the other Pd shells (78.6 and 65.8 m g for the Pd-
Pd(II) into Pd(0) which gives a suspension of Pd NPs whose 12 and Pd-21).
2
ꢀ1
2
ꢀ
grain size depends on T. Once reduction of PdCl
a nish and many Pd grains are present, they decompose solution containing 0.25 M HCOOH. In the potential scans in
HCOOH into H and CO . The Pd ions are reduced and both positive and negative directions, the overall current
4
comes to
2 2 4
Fig. 3b shows CVs of the GCEs in a N -saturated 0.5 M H SO
2
2
exhausted by nucleation and production of Pd NPs before the density for formic acid oxidation measured on Pd-7 is the
growth into shells happens. Therefore, when considering the highest, in agreement with the ECSAs. The peak current density
ꢀ
2
bubbles as they participate in the growth, HCOOH still at the positive scan on Pd-7 is 15.95 mA cm , almost 8 times
ꢀ
2
reducing Pd ions is no longer a process that competes for that on Pd NPs (2.05 mA cm ) and almost twice that on
ꢀ2
HCOOH, thus it does not need to be considered. CO
2
commercial Pd black (8.62 mA cm ). The peak potential of the
dissolves well in water, but H not. Therefore, H will formic acid oxidation for the Pd-7 electrode is decreased by 0.05
2
2
become oversaturated. Nevertheless, homogeneous bubble V. These results conrm the expectations from the discussion of
nucleation needs extreme oversaturations, hence bubble the nanostructure. The enhancement of activity can be attrib-
11
nucleation is known to be generally heterogeneous. Hetero- uted to the hollow chamber and the richness of the interfaces
18
geneous nucleation easily starts at the rough Pd grain between small grains providing highly dispersed Pd active
surfaces where H concentration is large. In this surfactant- sites, which is why smaller grains as well as thinner shells,
2
free system, the bubbles cannot exist independently, waiting which present all grains and more of their interfaces accessibly
for the NPs to assemble on them. Since the NPs favor an to the solution (high EASA), are preferable. The catalysts have
interfacial location (that is why they assemble on bubbles at been used repetitively for more than three times without
all), the Pd never separates from the bubbles they produce. signicant loss of activity. The hollow chamber is also well
Bubbles and attached grains fuse into larger bubble/grain preserved aer successive catalytic reactions (Fig. S7a†). That
systems, so the bubble size initially varies. That H2 is the Pd catalyst retains its activity during repetitive uses is also
produced at the bubbles is again relevant, because the demonstrated by XPS analysis in Fig. S7b.† A further increase of
concentration directly at the bubble determines the diffusive the activity can be expected from an optimization of these
1
6

ow equilibrium through the bubble surface.
Formic acid oxidation was used to characterize the cata- size and thin shells possible together via the respective
lytic activity of the Pd shells in comparison with commercial synthesis conditions.
Pd black (Sigma-Aldrich, 99.8 wt%) and (5 ꢁ 2) nm Pd NPs In summary, spherical Pd shells were prepared by a one-
Fig. S6a and b†). Fig. 3a shows cyclic voltammograms (CVs) step solvothermal synthesis, where the formic acid reducer
parameters, namely nding the optimum pair of small grain
(
of glassy carbon electrodes (GCEs) modied with the different also provided nanobubbles serving as so templates. Control
Pd materials [Pd black, Pd NPs, and 21, 12, as well as 7 nm over the diameter (85 to 360 nm), shell thickness (7 to 28 nm),
thick Pd shells (“Pd-21”, “Pd-12”, and “Pd-7”)] in N -saturated and grain size (4 to 21 nm) are achieved by adjusting CHCOOH
,
2
ꢀ
1
0
.5 M H
2
SO
4
solution at a scan rate of 50 mV s . The Pd mass
C
Pd, and T. The grain-monolayer thin, porous shells with a
2
ꢀ1
loading on the GCEs is the same for all the tests, as ensured high specic surface area of 87.5 m g possess a very high
by the preparation of the GCE (see ESI†). The peak appearing activity in electrocatalysis, their hollow chambers are stable
at 0.47 V (vs. SCE) in the forward scan originates from when the product is stored long term in ethanol for example.
desorption of atomic hydrogen. The electrochemically active The shells can be used many times, showing a good potential
surface area (EASA) of the GCEs was determined by the charge for direct fuel cell applications. The method should be
extendable to platinum. The catalytic performance can be
further optimized by aiming for smaller grain sizes obtained
at lower temperatures. Changing T also shis the upper limit
on CHCOOH responsible for thin shells, therefore, the location
of the optimum product in parameter space is not a trivial
matter. The open questions concerning the bubble facilitated
growth mechanism should inspire more theoretical and
experimental work.
Acknowledgements
This work was jointly supported by the PAPD, the Natural
Science Foundation of Jiangsu Province, the National Natural
Science Foundation of China and the State Key Program for
Basic Research of China.
Fig. 3 Cyclic voltammograms of the different Pd catalysts in 0.5 M
H
2
SO
4 2 4
(a) and 0.5 M H SO + 0.25 M HCOOH (b) at a scan rate of
ꢀ1
5
0 mV s . The inset of a shows EASA values calculated from the CV
curves.
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