Seedless Growth of Palladium Nanocrystals with Tunable Structures: From Tetrahedra to Nanosheets

Article abstract of DOI:10.1021/acs.nanolett.5b04019

Despite the great success that has been accomplished on the controlled synthesis of Pd nanocrystals with various sizes and morphologies, an efficient approach to systematic production of well-defined Pd nanocrystals without seed-mediated approaches remains a significant challenge. In this work, we have developed an efficient synthetic method to directly produce Pd nanocrystals with a highly controllable feature. Three distinct Pd nanocrystals, namely, Pd nanosheets, Pd concave tetrahedra, and Pd tetrahedra, have been selectively prepared by simply introducing a small amount of ascorbic acid (AA) and/or water without the other synthesis conditions changed. We found that the combined use of AA and water is of importance for the successful production of the unique Pd nanosheets. Detailed catalytic investigations showed that all the obtained Pd nanocrystals exhibit higher activity in the formic acid electrooxidation and styrene hydrogenation with respect to the Pd black, and their activities are highly shape-dependent with Pd nanosheets demonstrating a higher activity than both the Pd concave tetrahedra and Pd tetrahedra, which is likely due to the simple yet important feature of ultrathin thickness of Pd nanosheets. The present work highlights the importance of structures in tuning the related properties of metallic nanocrystals.

Full text of DOI:10.1021/acs.nanolett.5b04019

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Communication
Seedless Growth of Palladium Nanocrystals with
Tunable Structures: From Tetrahedra to Nanosheets
Ying Zhang, Mingsong Wang, Enbo Zhu, Yuebing Zheng, Yu Huang, and Xiaoqing Huang
Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04019 • Publication Date (Web): 21 Oct 2015
Downloaded from http://pubs.acs.org on October 22, 2015
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Seedless Growth of Palladium Nanocrystals with Tunable
Structures: From Tetrahedra to Nanosheets
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Ying Zhang,† Mingsong Wang,‡ Enbo Zhu,§ Yuebing Zheng,‡ Yu Huang*§ and Xiaoqing Huang*†
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu,
215123, China.
‡Department of Mechanical Engineering, Materials Science and Engineering Program, and Texas
Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States.
§Department of Materials Science and Engineering, University of California, Los Angeles, California
90095, United States.
*Address correspondence to: hxq006@suda.edu.cn; yhuang@seas.ucla.edu.
Abstract: Despite great success has been accomplished on the controlled synthesis of Pd nanocrystals
with various sizes and morphologies, an efficient approach to systematic production of well-defined Pd
nanocrystals without seed-mediated approaches remains a significant challenge. In this work, we have
developed an efficient synthetic method to directly produce Pd nanocrystals with highly controllable
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feature. Three distinct Pd nanocrystals, namely, Pd nanosheets, Pd concave tetrahedra, and Pd tetrahedra
have been selectively prepared by simply introducing a small amount of ascorbic acid (AA) and/or water
without the other synthesis condition changed. We found that the combined use of AA and water is of
importance for the successful production of the unique Pd nanosheets. Detailed catalytic investigations
showed that all the obtained Pd nanocrystals exhibit higher activity in the formic acid electrooxidation
and styrene hydrogenation with respect to the Pd black, and their activities are highly shape-dependent
with Pd nanosheets demonstrating a higher activity than both the Pd concave tetrahedra and Pd
tetrahedra, which is likely due to the simple yet important feature of ultrathin thickness of Pd nanosheets.
The present work highlights the importance of structures in tuning the related properties of metallic
nanocrystals.
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Keywords: Palladium, Nanosheet, Tetrahedron, Hydrogenation, Electrooxidation
The synthesis of well-defined Pd nanocrystals has continually attracted extensive research attention
in multiple fields of fundamental science and technology mainly due to their promising properties and
various potential applications, including highly active catalysts in various organic reactions,
electrocatalysts in fuel cell reactions, and hydrogen storage/sensing.1-10 All of these broad applications
require precise control over Pd nanocrystals to enhance their properties.11 In general, the properties of Pd
nanocrystals are mainly determined by two factors: 1) the intrinsic nature of active sites, which are
dictated by the exposed facets of the Pd nanocrystals; 2) the accessible active sites, which are determined
by specific surface area of the Pd nanocrystals.12-15 As a consequent, many researchers pay great
attention to the two aforementioned aspects when designing and preparing various Pd nanocrystals.1-3,11
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Studying the well-defined Pd nanocrystal system is important for answering fundamental questions that
could allow the preparation of Pd nanocrystals with improved performance.11
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Towards improving the properties of Pd nanocrystals, much progress has been achieved on the
control of Pd nanocrystals over the past decades.1-3,11 Pd nanocrystals with various morphologies,
including nanosphere, nanocube, octahedron, tetrahedron, nanorod/nanowire, nanosheet and so forth,
have been successfully created by various strategies.16-30 For example, the tetrahexahedral Pd
nanocrystals show much high ethanol electrooxidation activity than commercial Pd black owing to the
presence of highly active surface structure.18 Because of the ultrathin nature, the Pd nanosheets exhibit a
well-defined surface plasmon resonance peak in the near-infrared region and highly improved surface
area.24 Although the previous results have contributed much to the preparation of Pd nanocrystals with
much improved properties, it should be pointed out that these syntheses were conducted under very
different conditions, which are not suitable for the shape dependent property studies. Furthermore, it
should be noted that the majority of previous reported Pd nanocrystals lack systematic structure
tunability, which are however important to the rational design of high-performance Pd nanocrystals with
potentially practical applications. In fact, the synthesis of the Pd nanocrystals with systematic tuned
structures can be achieved through the seed-mediated strategy.31-34 The seed mediated strategy, i.e. Pd
seed syntheses and the following growth on the Pd seeds, however, are neither cost-effective nor
efficient. To scale up their production, an effective method has yet to be exploited. Therefore, the
development of effective wet-chemical approach for the direct preparation of Pd nanocrystals with
systematic tunable structure is highly desirable, but remains a tremendous challenge.29-30
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Herein, we demonstrate an effective wet-chemical method for the production of well-defined Pd
nanocrystals in a highly controllable manner. Three distinct nanostructures, namely, unique Pd
nanosheets, Pd concave tetrahedra and Pd tetrahedra have been selectively prepared. To the best of our
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knowledge, this is the first time that Pd nanosheets have been prepared without the use of toxic carbon
monooxide or metal carbonyls.24,27 Furthermore, unlike many previous work on the systematic control of
Pd nanocrystals that largely relied on the seed-mediated approaches, our current tunable control of Pd
nanocrystals have been achieved directly under similar growth conditions except for introducing various
small molecules such as AA and/or H2O. Therefore, one can readily image that our systematic tuning of
Pd nanocrystals with well-defined structures provides an ideal platform for investigating the effect of
structures on their related properties and catalytic activities. We found that the obtained Pd nanocrystals
exhibit highly shape-dependent catalytic activities in the formic acid electrooxidation and styrene
hydrogenation with Pd nanosheets showing higher activities than those of the Pd concave tetrahedra, Pd
tetrahedra and also Pd black, which is likely due to the simple yet important feature of ultrathin thickness
of the Pd nanosheets.
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Scheme 1. Schematic illustration of the growth conditions to the synthesis of well-defined Pd nanocrystals with
controlled manner.
The well-defined Pd nanocrystals were made by reduction of palladium (II) acetylacetonate
(Pd(acac)2) by using N, N-dimethylformamide (DMF) as both solvent and reducing agent, which
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contained polyvinylpyrrolidone (PVP) as surfactant and iron(II) acetylacetonate (Fe(acac)2) as shape-
directing agent, in the juridical presence of AA and/or water for the selective creation of Pd nanosheets,
Pd concave tetrahedra and Pd tetrahedra (Scheme 1, Figure S1). The typical characterizations of the
obtained Pd tetrahedra are shown in Figure 1 and Figure S2. The representative transmission electron
microscopy (TEM) (Figure 1a) and high-angle annular dark-field scanning TEM (HAADF-STEM)
images (Figure 1b) show that the majority of the products have tetrahedral shape (Figure 1c). The
tetrahedra are monodisperse, with an average edge length of 30±4 nm. The powder X-ray diffraction
(PXRD) pattern of the nanocrystals shows a face-centered-cubic (fcc) structure with the peak positions
matching well with the metallic Pd diffractions (Figure S1), confirming the formation of Pd
nanocrystals. The lattice spacing are measured to be 0.23 nm, corresponding to the (111) plane of fcc Pd
nanocrystals (Figure S2). When a small amount of AA was introduced with the other synthesis
condition the same to that of Pd tetrahedra, we produced Pd concave tetrahedra of 40±5 nm in edge
length. The successful synthesis of Pd nanocrystals was confirmed by the PXRD, as shown in Figure S1.
Figure 1d-e are the typical TEM and HAADF-STEM images of the Pd concave tetrahedra, which
clearly demonstrate the high-quality of the products with concave feature (Figure 1f). The lattice
spacing along the edge of an octahedron shown in the HRTEM image is 0.14 nm (Figure S2), close to
that of the (220) plane of Pd nanocrystals. As confirmed by inductively coupled plasma atomic emission
spectroscopy (ICP-AES), no Fe was detected in these two Pd polyhedra despite Fe(acac)2 was used in
the synthesis.
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Figure 1. Representative TEM image (a) and HAADF-STEM image (b) of the Pd tetrahedra. (c) TEM image and
corresponding geometric model of an individual Pd tetrahedron. Representative TEM image (d) and HAADF-STEM
image (e) of the Pd concave tetrahedra. (f) TEM image and corresponding geometric model of an individual Pd
concave tetrahedron.
Significantly, when we introduced a small amount of water with other growth parameters identical to
the synthesis of Pd concave tetrahedra, unique Pd nanosheets were generated (Figure 2 and Figure S3).
The representative TEM and HAADF-STEM images are shown in Figure 2a-d and Figure S3, which
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clearly showed that the product consisted of sheet-like structures with a typical yield approaching 87%.
The majority of the byproduct is Pd concave tetrahedra. The unique Pd nanosheets had an average edge
length of 124 nm and a thickness of only about 2.3 nm (Figure 1e). Interestingly, each nanosheet was
corolla-like with multibranched subunits. This striking feature is clearly apparent in the high-
magnification TEM and HAADF-STEM images (Figure 2b, 2d). Both selected area electron diffraction
(SAED) and HRTEM measurements (Figure 2f, 2g) on individual nanosheet indicated the single-
crystalline nature of the corolla-like Pd nanosheet. The SAED pattern showed a series of diffraction
spots with six-fold rotational symmetry. The HRTEM image showed lattice fringes with interplanar
spacing of 0.24 nm, corresponding to 1/3(422) fringes of fcc Pd (Figure 2g). These results suggest that
the obtained Pd nanosheet has basal (111) plane and stacking faults parallel to the basal (111) plane.24,30
The fcc structure of the Pd nanosheets is also supported by their PXRD pattern (Figure S1).
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The achievement of creating unique Pd nanosheets without the use of any toxic carbon monooxide
or metal carbonyl is the most striking feature of the synthesis shown herein.24,27 To gain better
understanding of the growth mechanism for the synthesis of Pd nanosheets, Pd concave tetrahedra and
also the Pd tetrahedra, we designed sets of experiments to investigate how the reaction kinetics and/or
etching affect the proceeding of reactions and thus the final different shapes. The structure evolution
process for Pd nanosheets, Pd concave tetrahedra and also the Pd tetrahedra were summarized in Figure
3, Figure S4, and Figure S5, respectively. Specifically, the products collected at 15 min were
hexagonal Pd nanosheets with an average edge length of 49 nm. Noticeably, the nanosheets collected at
30 min were slightly etched at their edges. The average size of this nanosheet was 93 nm. The
nanosheets increased to 122, 123, 122, and 124 nm in size when the reaction time was prolonged to 60,
120, 480, and 600 min, respectively. During the growth of the nanosheets, their etched feature also
became more and more prominent. After 480 min, no further change in the size or the morphology of the
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nanosheets was observed (Figure 3). Therefore, these results imply that the etched sites likely resulting
from iron species oxidation at the early stage of the synthesis served as the growth sites for forming the
corolla-like nanosheets.35 For the Pd concave tetrahedra, small Pd concave tetrahedra were already
formed at 15 min. The size of the Pd concave tetrahedra increased with prolonged reaction time. It was
observed that the concave feature of the nanocrystals was maintained throughout the whole growth
process. These results imply that the concave feature was developed at the very beginning and
concomitant with the growth process (Figure S4). Similar growth process was found in the case of Pd
tetrahedra, where small Pd tetrahedra were already formed at the early stage and the size of the Pd
tetrahedra enlarged with increasing reaction time (Figure S5).
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Figure 2. (a, b) TEM images and (c, d) HAADF-STEM images of the Pd nanosheets. (e) TEM image of a single Pd
nanosheet attached on a carbon nanotube. (f) SAED pattern of a single Pd nanosheet. Bottom-right inset shows the
corresponding TEM image. (g) HRTEM image on the edge of a Pd nanosheet.
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The critical role of H2O in the growth of the unique Pd nanosheets is quite unexpected. To try to
understand the critical role of H2O in controlling the synthesis of Pd nanosheets, we have conducted
additional experiments to thoroughly investigate the impacts of different amount of H2O on the synthesis
of Pd nanosheets. As illustrated by TEM analyses, only Pd concave tetrahedra were obtained in the
absence of any H2O (Figure S6). The population of Pd tetrahedra decreases and the population of Pd
nanosheets increases with increasing the amount of H2O to 0.5% water in DMF. The Pd nanosheets with
high yield were obtained at 1% water in DMF and onward (1.5% water in DMF) (Figure S6). Based on
the controlled experiments, we infer that the specific and appropriate amount of H2O is essential to grow
the Pd nanosheets, possibly due to the optimal kinetic growth of Pd nanosheets corresponding to the
specific H2O concentration. In many previous wet chemical synthesis, the introduction of specific
additives has been proved to be an effective strategy to control metal nanocrystals.3 Further deeper
understanding of such controlled synthesis are likely due to the ability to precise control of the reaction
kinetics by introduction of additives.26,30 In our present synthesis of Pd nanosheets, we visually observed
that the addition of H2O could retard the reduction, as the reaction mixture containing H2O changed
more slowly than that of reaction without H2O. We therefore measured the formation of Pd nanocrystals
at different synthesis time to qualitatively calculate the reduction kinetics responsible for the formation
of Pd nanosheets, Pd concave tetrahedra, and Pd tetrahedra. The percent conversions of Pd(acac)2 as a
function of reaction time for the standard syntheses of Pd nanosheets, Pd concave tetrahedra, and Pd
tetrahedra are shown in Figure S7. At t = 15 min, the conversions were 2.8%, 10.7% and 21.0% for the
cases of Pd nanosheets, Pd concave tetrahedra and Pd tetrahedra, respectively. At t = 120 min, the
corresponding conversions became 35.6%, 67.5% and 75.5%, respectively. On the basis of the above
studies, it can be concluded that the reduction rate of the precursor is in the order of Pd tetrahedra > Pd
concave tetrahedra > Pd nanosheets. Our results suggest that distinct Pd nanocrystals from Pd nanosheets
to Pd tetrahedra have been obtained by simply introducing AA and/or water without all other parameters
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changed. Because the type of additives can affect the reduction kinetics, the initial reduction rate of a
synthesis is responsible for the formation of seeds with different structures. In our synthesis, while a fast
reduction rate is responsible for the growth of single crystal Pd tetrahedra, a slow reduction rate is
beneficial for the growth of Pd nanosheets with stacking faults, which is consistent with the recent
published results, where Pd nanoplates were the major products when the reduction rate was reduced in
the low synthesis temperature.30 Therefore, by controlling the reduction rate of a synthesis, we could
obtain Pd nanocrystals with distinctive structures.
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To achieve better control of the growth of Pd nanosheets, we also thoroughly investigated the
impacts of the other synthesis reagents on the growth of Pd nanosheets. As showed in Figure S8, the
reaction in the absence of AA cannot produce Pd nanosheets. The formation of Pd nanosheets was
observed once AA was introduced into the synthesis, even though the quality of the Pd nanosheets is
dependent on the amount of AA added (Figure S8). As showed in Figure S9, when AA are replaced
with citric acid, the product containing the mixture of polyhedra are obtained, indicating that the AA is
also an important shape-directing agent in the growth of Pd nanosheets. To understand why nanosheets
could not form when AA was replaced by citric acid, we further analyzed the intermediates collected at
different stages when citric acid was used. We observed that the reaction mixture containing citric acid
was unchanged and no separable particles were obtained from the reaction until 15 min. This
phenomenon indicates that the citric acid greatly retarded the reduction of Pd(acac)2. Pd nanocrystals
collected at 30 min were irregular nanoparticles with different shapes. The average size of the
nanocrystals increased when the reaction time was prolonged (Figure S10). Because the type of
additives can affect the coordinating environment and thus the reduction kinetics, the initial reduction
rate of a synthesis is responsible for the formation of seeds with different structures during the
nucleation. Since citric acid is a very strong tridentate agent, it is reasonable to think that the citric acid
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will heavily affect the nucleation and thus the formation of Pd nanosheets. In addition to the critical role
of AA, the use of Fe(acac)2 are also important for the growth of Pd nanosheets. As confirmed by the
SEM-EDX, the presence of Fe species was also detected in the final product even after severe wash
(Figure S11). It seems that high-quality of Pd nanosheets can only be prepared in a certain amount of
Fe(acac)2, as shown in Figure S12. In the synthesis of Pd nanosheets, we have also studied the effect of
the Pd precursor concentrations on the synthesis of the Pd nanosheets. It turns out that the high
concentration of the Pd precursors is not beneficial for the production of Pd nanosheets with high purity
(Figure S13).
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Figure 3. TEM images of Pd nanosheet intermediates produced in (a) 15, (b) 30, (c) 60, (d) 120, (e) 480, and (f) 600
min reactions.
Although great success has been achieved on the controlled preparation of Pd nanocrystals with
different sizes and morphologies, a robust approach to systematic production of well-defined
nanostructures without seeds remains a significant challenge. The realization of production of Pd
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nanocrystals with highly controlled fashion is highly beneficial for studying shape-dependent properties.
Nanostructured Pd nanocrystals are active catalyst for various organic reactions and have remarkable
performance in electrooxidation of small organic molecules and hydrogenation reactions.1,36 To this end,
we first chose the formic acid electrooxidation as a model reaction to investigate the catalytic
performance of as-made Pd nanocrystals. The commercial Pd black was chosen as reference for
comparison (Figure S14). Figure 4a shows the cyclic voltammetry (CV) curves of Pd nanosheets, Pd
concave tetrahedra, Pd tetrahedra and Pd black recorded at room temperature in 0.1M HClO4 solution at
a sweep rate of 50 mVs-1. According to the CV curves shown in Figure 4a, the electrochemical surface
areas (ECSAs) were calculated as 35.6 m2g-1 for Pd nanosheets, 18.3 m2g-1 for Pd concave tetrahedra,
16.7 m2g-1 for Pd tetrahedra and 15.5 m2g-1 for Pd black. Figure 4b shows the CV curves of those Pd
nanomaterials recorded at room temperature in 0.1M HClO4 + 0.2 M HCOOH solution at a sweep rate
of 50 mVs-1. The current density was normalized over the Pd loading to give the mass activity (Figure
4b). We find that the mass activities follow the order of Pd nanosheets > Pd concave tetrahedra > Pd
tetrahedra > Pd black. The mass activity of Pd nanosheets is measured to be 634.3 mAmg-1Pd, which is
1.38 times, 2.67 times and 3.51 times higher than those of the Pd concave tetrahedra, Pd tetrahedra and
Pd black, respectively (Figure 4b). Furthermore, the durability of all the Pd catalysts were tested by
repeating the CV sweeps for 200 cycles (Figure 4c). After 200 sweeping cycles, 54% of the initial
catalytic activity was still maintained for the Pd nanosheets (28.3% for Pd concave tetrahedra and 33.7%
for Pd tetrahedra), as compared with the loss of 96% for Pd black, highlighting a better catalytic stability
of the as-made Pd nanocrystals.
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To investigate the catalytic differences of the prepared Pd nanocrystals as hydrogenation catalysts,
the hydrogenation of styrene was selected and carried out at room temperature. In a typical
hydrogenation of styrene, styrene was readily converted into ethylbenzene over different Pd nanocrystals
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by using hydrogen gas as the hydrogen source and ethanol as the solvent. Figure 4d shows the plots of
conversion of styrene against time after the addition of the Pd catalysts with the same amount of Pd. All
the catalysts are effective catalysts for this hydrogenation reaction. However, the conversion efficiencies
are different from each other. At t = 30 min, the conversions of styrene were 88%, 82%, 63%, and 38%
for the Pd nanosheets, the Pd concave tetrahedra, the Pd tetrahedra, and the Pd black, respectively.
Again, the activity of the Pd nanosheets is better than that of Pd concave tetrahedra, Pd tetrahedra, Pd
black and the reason behind this could be attributed to the simple yet important ultrathin feature and thus
high surface area of the Pd nanosheets.
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Figure 4. (a) Cyclic voltammograms and (b) formic acid oxidation curves recorded at room temperature of different Pd
catalysts. (c) Variation of normalized peak current densities of electrooxidation in the positive-going potential sweep
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during potential cycling. Potential was continuously scanned at 50 mVs-1 in 0.1 M HClO4 + 0.2 M formic acid. (d)
Conversion of styrene during hydrogenation reactions catalyzed by different Pd catalysts
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To summarize, we have presented an efficient approach that can realize the production of well-
defined Pd nanocrystals in a highly controllable fashion. Pd nanosheets, Pd concave tetrahedra and Pd
tetrahedra have been selectively obtained with high yield. To the best of our knowledge, it is the first
example that unique Pd nanosheets have been obtained without the use of any toxic and/or explosive
carbon monooxide or metal carbonyls. These well-defined Pd nanocrystals herein represent an ideal
platform for investigating the shape dependent properties and we have demonstrated that their properties
are highly dependent on their morphologies. It has been showed that the unique Pd nanosheets can serve
as highly efficient catalysts for both styrene hydrogenation and formic acid electrooxidation, where they
outperformed the other two Pd polyhedra and commercial Pd black. These enhanced properties of Pd
nanosheets can be attributed to their ultrathin feature that provides increased active sites. We expect
these newly generated Pd nanocrystals with unique and tunable structure will enable various
applications, such as fuel cell reactions, advanced catalysis and beyond.
ASSOCIATED CONTENT
Supporting Information. Experimental details and data. Figure S1-14. This material is available free of
charge via the Internet at http://pubs.acs.org.
Corresponding Author
hxq006@suda.edu.cn; yhuang@seas.ucla.edu.
ACKNOWLEDGMENT
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This work was financially supported by the start-up funding from Soochow University, Young
Thousand Talented Program, Ralph E. Powe Junior Faculty Enhancement Award. E.Z and Y.H
acknowledge support from the National Science Foundation (NSF) through award DMR-1437263.
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The authors declare no competing financial interest.
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