PdCo/Pd-Hexacyanocobaltate Hybrid Nanoflowers: Cyanogel-Bridged One-Pot Synthesis and Their Enhanced Catalytic Performance
PdCo/Pd-Hexacyanocobaltate Hybrid Nanoflowers: Cyanogel- Bridged One-Pot Synthesis and Their Enhanced Catalytic Performance
Zhen-Yuan Liu 0
Geng-Tao Fu 0 1
Lu Zhang 2
Xiao-Yu Yang 0
Zhen-Qi Liu 0
Dong-Mei Sun 0
Lin Xu 0
Ya-Wen Tang 0
0 Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University , Nanjing 210023 , PR China
1 Materials Science and Engineering Program & Texas Materials Institute, the University of Texas at Austin , Austin, Texas 78712 , United States
2 Department of Applied Chemistry, Graduate School of Engineering , Hiroshima
OPEN Elaborate architectural manipulation of nanohybrids with multi-components into controllable 3D hierarchical structures is of great significance for both fundamental scientific interest and realization of various functionalities, yet remains a great challenge because different materials with distinct physical/ chemical properties could hardly be incorporated simultaneously into the synthesis process. Here, we develop a novel one-pot cyanogel-bridged synthetic approach for the generation of 3D flower-like metal/Prussian blue analogue nanohybrids, namely PdCo/Pd-hexacyanocobaltate for the first time. The judicious introduction of polyethylene glycol (PEG) and the formation of cyanogel are prerequisite for the successful fabrication of such fascinating hierarchical nanostructures. Due to the unique 3D hierarchical structure and the synergistic effect between hybrid components, the as-prepared hybrid nanoflowers exhibit a remarkable catalytic activity and durability toward the reduction of Rhodamine B (RhB) by NaBH4. We expect that the obtained hybrid nanoflowers may hold great promises in water remediation field and beyond. Furthermore, the facile synthetic strategy presented here for synthesizing functional hybrid materials can be extendable for the synthesis of various functional hybrid nanomaterials owing to its versatility and feasibility.
Rational hybridization and nanostructure engineering allow for achieving optimized or diversified material
functionalities and thus have attracted increasing research interests in nanochemistry community1?10. Hybrid
nanostructures with multi-components in one nanoscale entity could not only possess combined properties from the
individual component, but also be capable of demonstrating new synergistic effects, which are induced by the
nanoscale interactions and inaccessible from the isolated components or their physical mixtures. Therefore, a
great number of nanocomposites have been synthesized and hold promising applications in various fields,
including catalysis11?13, energy conversion and storage14?17, optoelectronic devices18?20, etc. Generally, the exceptional
synergistic functionalities of the hybrid nanostructures are not only determined by the nature of each
constituent component, but also more sensitively dependent upon the geometrical arrangement of the building units.
Specifically, elaborate architectural manipulation of low dimensional (0D, 1D and 2D) primary building blocks
into controllable 3D hierarchical structures is of great significance for both fundamental scientific interest and
technological applications, and also provides a promising approach toward the future realization of functional
nanodevices5,10. Owing to their unique structures, 3D hierarchical structures could possess the advantages of
the pristine building blocks, and more importantly, also may exhibit even new physicochemical characteristics
induced by coupling or ensemble effects, in comparison with their 1D or 2D counterparts21?25. Hitherto, despite
considerable achievements have been made in such interesting field, it still remains a great challenge to develop a
facile and controllable route for the construction of hierarchical architectures. Especially, it is extremely difficult
to integrate multi-components into a hybrid hierarchical nanostructure based on the protocols established before,
because different materials with distinct physical/chemical properties could hardly be incorporated
simultaneously into the synthesis process. Therefore, it is highly desirable to develop a straightforward synthetic approach
to generate nanohybrids with hierarchical architectures.
Cyanogel, pioneered by Bocarsly, is a kind of coordination polymer obtained from the reaction of
aqueous solutions of a tetrachlorometalate ([RCl4]2?, R = Pd, Pt, Ir, Sn) and a transition metal
cyanometalate ([M(CN)n]2?/3?, n = 4, 6; M = Co, Fe, Ru, Os, Ni, Cr), as illustrated in Equation (
) in Supplementary
Information26?29. By taking advantages of the structural features of cyanogels, such as 3D characteristic backbones
and uniform distribution of the two kinds of metal ions, we have developed a versatile cyanogel-based approach
for the synthesis of various 3D noble metal-based nanostructures with improved catalytic performances30?34. Our
previous results demonstrate that the cyanogel-based approach has the capacity to address some of the challenges
in controlled construction of hybrid nanomaterials.
Herein, for the first time, we extend the capability of one-pot cyanogel-based hydrothermal approach to
achieve 3D flower-like metal/Prussian blue analogue nanohybrids, namely PdCo/Pd-hexacyanocobaltate (PdCo/
PdHCC), constructed by numerous radial 2D ultrathin nanosheets, by using K2PdCl4/K3Co(CN)6-PEG hybrid
cyanogel as the reaction precursor (Fig.?1). Control experiments indicate that the elaborate co-existence of
cyanogel and PEG is crucial for the generation of such interesting hierarchical architecture. Remarkably, due
to the unique 3D hierarchical structure and the synergistic effect between hybrid components, the as-prepared
PdCo/PdHCC hybrid nanoflowers exhibit an excellent catalytic activity and durability toward the reduction of
Rhodamine B (RhB) by NaBH4, as compared with the Pd and PdHCC nanoparticles.
Results and Discussion
Physicochemical characterization of PdCo/PdHCC hybrid nanoflowers. For a standard synthesis
of PdCo/PdHCC hybrid nanoflowers, yellowish jelly-like K2PdCl4/K3Co(CN)6-PEG hybrid cyanogel was firstly
generated by mixing K2PdCl4-PEG solution and K3Co(CN)6-PEG solution. Upon a hydrothermal treatment, the
hybrid cyanogel could be readily converted to 3D nanostructures owing to its intrinsic 3D characteristic
backbones and the structural-directing effect of PEG. Simultaneously, PdCo alloy nanoparticles could be in-situ
generated thanks to the weak reducing ability of PEG. Thus, the as-synthesized hybrid cyanogel could be evolved to
uniform 3D flower-like PdCo/PdHCC nanohybrids after a hydrothermal treatment (see Experimental section
X-ray diffraction (XRD) pattern in Fig.?2a indicates that both face-centered cubic (fcc)-phased PdCo alloy
and Prussian blue analogue, Pd-hexacyanocobaltate, coexist in the obtained product35. Figure S1
schematically illustrates the possible crystal structure of PdHCC. Analogous to Prussian blue, it has a three-dimensional
cyano-bridged bimetallic basic unit with alternating Pd(II) and Co(III) located in a fcc lattice36?38. Fourier
transform infrared (FTIR) analysis (Fig.?2b) shows the characteristic stretching peaks of C?N around 2170 cm?1 and
the absorption peak of Pd-CN-Co at 452 cm?1, confirming the successful formation of Prussian blue analogue39,40.
The thermal stability of the product was investigated by thermogravimetry analysis (TGA) under air atmosphere.
As displayed in Figure S2, the weight loss from room temperature to ~115 ?C is caused by the loss of free water41.
The weight loss in the temperature range of 195?240?C can be assigned to the removal of coordinating water for
Prussian blue42. When the temperature is increased above 245 ?C, the PdHCC species begin to thermally
decompose in air40.
A panoramic scanning electron microscopy (SEM) image shown in Fig.?3a demonstrates that the product is
almost entirely composed of uniform nanoflowers with diameter of 320 ? 20 nm. No other morphologies could
be detected, indicating a high yield of these hierarchical structures. It is clearly shown that these nanoflowers are
(2?50 nm in size)43. This result can be further confirmed by corresponding pore-size distribution curve (Fig.?5b),
in which a peak centred at 34 nm can be observed. As revealed by the SEM observation, these meso-pores can
be attributed to the space between the intercrossed 2D nanosheets44. The BET surface area of the PdCo/PdHCC
nanoflowers calculated from N2 isotherms is 32.8 m2 g?1. The valance states of Pd and Co in the hybrid
nanoflowers were examined by X-ray photoelectron spectroscopy (XPS) technique, revealing that both metallic and
oxidic states of Pd and Co exist in the hybrid nanoflowers (Fig.?5c,d)45,46. These results further verify the hybrid
compositions as PdCo/Pd hexacyanocobaltate.
To develop an understanding of the mechanism behind the formation of PdCo/PdHCC nanoflowers, the
chemical fate of each involved reagent has been considered. When PEG is absent from the reaction system while the other
reaction parameters remain unchanged, although the yellowish jelly-like cyanogel could be still formed (Figure S4a),
the resulting product achieved after the hydrothermal treatment is made of the isolated palladium
hexacyanocobaltate nanoparticles with an average size of 70 nm, as confirmed by XRD and TEM images (Figure S4b?d).
When there is no K3Co(CN)6 introduced, the reduction of K2PdCl4 by PEG could only produce irregular
aggregated nanoparticles (Figure S5a). In comparison, the hydrothermal treatment of the mixture only containing
K3Co(CN)6 and PEG could generate intercrossed nanochains (Figure S5b). Collectively, all these results
unambiguously suggest that the presence of PEG and the formation of cyanogel are indispensable for the successful
formation of 3D PdCo/PdHCC hybrid nanoflowers.
Furthermore, time-dependent experiments have been carefully carried out to reveal the morphological
evolution. Figure?6 illustrates the representative TEM images of the intermediate products collected at different
reaction intervals. As shown in Fig.?6a, the sample consists of numerous flocculated agglomerates without a
discernible morphology when the hybrid cyanogel was hydrothermally treated for 1 h. As the reaction time was
prolonged to 2 h, the flocculation tended to aggregates together, forming a large number of nanoparticles (Fig.?6b).
Interestingly, some nanosheets began to germinate from the surface of nanoparticles when the reaction time was
increased to 3 h, as indicated by red arrows and inset of Fig.?6c. As a consequence of continuous growth,
development and ripening, more and more nanosheets sprouted from the surface of nanoparticles and the obtained
hierarchical architectures became ripening and plumy, accompanied by the gradual depletion of the flocculation
(Fig.?6d). Eventually, uniform well-developed 3D PdCo/PdHCC hybrid nanoflowers constructed by 2D
nanosheets were formed when the reaction time was proceeded more than 5 h (Fig.?6e).
Based on the above TEM observations, the possible formation mechanism of the 3D PdCo/PdHCC hybrid
nanoflowers could be proposed as follows. As we know, PEG is a kind of nonionic surfactant which possesses
hydrophilic -O- and hydrophobic -CH2-CH2- radicals on its long chains, and usually serves as structure-directing
agent or soft template for engineering ordered nanostructures due to its selective adsorption to inhibit
crystal growth and thus modify the morphology of nanocrystallite47,48. In this work, PdCo-based cyanogel
will be enwrapped into the coil of intertwisted PEG and form flocculated agglomerates when the precursors
are initially mixed. From the thermodynamic viewpoint, the flocculation has a tendency to self-aggregate into
nanoparticles to minimize the total surface energy when hydrothermally treated. As the reaction proceeds,
the formed nanoparticles continue to grow by combining with the remaining flocculated agglomerates and
recrystallize. Meanwhile, PEG may selectively bind to certain specific crystallographic facets49. Such a
preferential adsorption could effectively facilitate the anisotropic growth, leading to the formation of 2D
nanosheets. Therefore, with the further increase of reaction time, more and more 2D nanosheets are germinated
from the surface of nanoparticles, and the nanoparticles gradually evolve into hierarchical nanoflowers at a
later stage. Therefore, the formation of 3D PdCo/PdHCC hybrid nanoflowers can be rationally expressed as a
?nucleation-aggregation-dissolution-recrystallization? mechanism50,51. During the formation of PdHCC
nanoflowers with the assistance of PEG, the PdHCC could be partially reduced by PEG to form PdCo alloy
nanoparticles which are simultaneously dispersed on the surface of PdHCC nanoflowers. The plausible formation process
can be schematically illustrated in Fig.?7.
Catalysis for the hydrogenation of RhB. Such a hierarchical architecture and integrated multiple
compositions in nanoscale might bring out some unusual physiochemical properties. As a proof-of-concept
application of this intriguing hybrid nanostructure, the obtained 3D PdCo/PdHCC hybrid nanoflowers were employed
as a catalyst for the hydrogenation of RhB in the presence of NaBH4. The catalytic reduction of RhB is
schematically illustrated in Fig.?8a 52. The characteristic absorption peak of RhB at 554nm was selected to monitor the
catalytic reduction process. For comparison, a series of control experiments were also performed under
different conditions: (
) without NaBH4 but in the presence of PdCo/PdHCC hybrid nanoflowers, (
) without any
catalyst but in the presence of excess NaBH4, and (
) catalyzed by Pd or PdHCC nanoparticles. As shown in
Figure S6a, the physical adsorption experiment demonstrates that the PdCo/PdHCC hybrid nanoflowers have a
very weak adsorption capability toward RhB (only 2.2% in 24 h), precluding the physical adsorption of RhB by
agglomeration of PdCo nanoparticles during the reaction. Furthermore, the hierarchical nanoflowers offer a high
surface-to-volume ratio and have plenty of open meso-pores, providing more molecular accessibility, efficient
transport paths and thus improved catalytic activity toward the reduction of RhB56?58.
In summary, we have developed a novel cyanogel-bridged one-pot synthesis approach for the generation of
3D flower-like metal/Prussian blue analogue nanohybrid, namely PdCo/Pd-hexacyanocobaltate, for the first time.
The judicious introduction of PEG and the formation of cyanogel are indispensable for the successful formation
of such fascinating hierarchical nanostructures. Owing to the unique 3D hierarchical structure and the synergistic
effect between hybrid components, the as-synthesized hybrid nanoflowers exhibit an excellent catalytic activity
and durability toward the reduction of RhB by NaBH4, which indicates that the hybrid nanoflowers may hold
great promise in water remediation field and beyond, such as electrocatalysis and sensor, etc. Furthermore, the
novel method developed in this work for synthesizing functional hybrid materials with hierarchical structures can
be extended to the fabrication of various functional hybrid nanomaterials thanks to its versatility and feasibility.
Synthesis of PdCo/PdHCC hybrid nanoflowers. In a typical synthesis, 2.0 mL of 50 mM K2PdCl4
solution containing 340 mg PEG and 1.0 mL of 50 mM K3Co(CN)6 solution containing 170 mg PEG were mixed
and kept still for 2 h at 30 ?C, allowing for the formation of yellow jelly-like K2PdCl4/K3Co(CN)6-PEG hybrid
cyanogel. Subsequently, the obtained cyanogel was transferred to a 20 mL Teflon-lined stainless autoclave and
heated at 150 ?C for 6 h. After being cooled to room temperature, the black product was separated by
centrifugation, washed with 0.1 M HClO4 solution and water several times, and then dried at 40 ?C in a vacuum oven
for 12 h. The acid-wash process could ensure the removal of possible byproducts or impurities. For comparison,
the single-component Pd nanoparticles were prepared by only using K2PdCl4 as reaction precursor under the
similar experimental conditions. The PdHCC nanoparticles were also prepared using the mixture of K2PdCl4 and
K3Co(CN)6 yet without PEG as reaction precursors under the identical experimental conditions.
Characterization. The morphology and particle size of the samples were investigated using a JEOL
JEM2010 transmission electron microscopy (TEM) operated at an accelerating potential of 200 kV. Scanning
electron microscopy (SEM) images were captured on a Hitachi S-4800 scanning electron microscope, operating at
5 kV. X-ray diffraction (XRD) patterns were performed on Model D/max-rC X-ray diffractometer using Cu K?
radiation source (? = 1.5406 ?) and operating at 40 kV and 100 mA. X-ray photoelectron spectroscopy (XPS)
measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer with a monochromatic
Al K? X-ray source (1486.6 eV photons). The binding energy was calibrated with respect to C1s at 284.6eV.
The compositions of the catalysts were determined using the energy dispersive X-ray (EDX) technique. The
Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution were measured at 77 K using a
Micromeritics ASAP 2050 system. Fourier transform infrared (FTIR) spectrum was recorded with a Nicolet 520
SXFTIR spectrometer. The UV-vis spectra were recorded at room temperature on a UV3600 spectrophotometer.
Thermal analysis was performed on a Perkin Elmer thermogravimetric analyzer under air atmosphere with a
heating rate of 10 ?C min?1.
Catalytic measurements. The reduction of organic dye molecules, such as RhB, with NaBH4 was chosen as
a model reaction to evaluate the catalytic performance of the as-obtained PdCo/PdHCC nanoflowers. A NaBH4
solution (0.20 mg/mL) was freshly prepared and stored in refrigerator in the dark. The reduction of the RhB dyes
was carried out in a quartz cuvette having a path length of 1 cm. For a catalytic reaction, 2 mL of 3.33 ? 10?5 M
RhB dye solution was mixed with 0.5 mL of 0.20 mg/mL NaBH4, followed by gentle shaking. Subsequently, 0.5 mL
of 0.40 mg/mL PdCo/PdHCC nanoflower solution was added, and the progress of the reduction was monitored
spectrophotometrically using an in-situ UV-vis spectrophotometer. For comparison, the catalytic processes
catalyzed by PdHCC and monometallic Pd nanoparticles were also performed under the identical conditions.
The authors are grateful for the financial of the National Natural Science Foundation of China (21576139,
21503111, 21376122, and 21273116), United Fund of NSFC and Yunnan Province (No. U1137602), Natural
Science Foundation of Jiangsu Province (No. BK20131395), China Scholarship Council (CSC, No. 201506860013),
University Postgraduate Research and Innovation Project in Jiangsu Province (No. KYZZ15_0213), National and
Local Joint Engineering Research Center of Biomedical Functional Material, and a project funded by the Priority
Academic Program Development of Jiangsu Higher Education Institutions, National and Local Joint Engineering
Research Center of Biomedical Functional Materials.
Z.-Y.L., G.-T.F., L.Z., X.-Y.Y., Z.-Q.L. and D.-M.S. designed the experiments and performed the materials
synthesis, characterization and electrochemical measurements. L.X. and Y.-W.T. wrote the main manuscript text.
L.X. and Y.-W.T. supervised the project, and all authors participated in the review of the manuscript.
Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests. How to cite this article: Liu, Z.-Y. et al. PdCo/Pd-Hexacyanocobaltate Hybrid Nanoflowers: Cyanogel-Bridged One-Pot Synthesis and Their Enhanced Catalytic Performance. Sci. Rep. 6, 32402; doi: 10.1038/srep32402 (2016).
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