Carbon Core−Shell ... - ACS Publications

ARTICLE pubs.acs.org/JPCC

Worm-Like Palladium/Carbon Core
Shell Nanocomposites: One-Step Hydrothermal Reduction
Carbonization Synthesis and Electrocatalytic Activity Wenjun Kang,† Haibo Li,†,‡ Yan Yan,† Peipei Xiao,† Lingling Zhu,† Kaibin Tang,† Yongchun Zhu,*,† and Yitai Qian*,† † ‡

Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Department of Chemistry, Liaocheng University, Liaocheng, Shandong 252059, P. R. China ABSTRACT: Worm-like palladium/carbon (Pd/C) core
shell nanocomposites have been hydrothermally prepared starting from PdCl2 and R-lactose monohydrate (R-LM) in the presence of polyacrylamide (PAM) at 200 °C. The thickness of carbonaceous shells varied from 5 to 45 nm with increasing temperature from 140 to 200 °C. When the dose of PAM or PdCl2 was increased, spherical Pd/C core
shell nanocomposites were obtained. Time-dependent experiments confirmed that formation of Pd/C core
shell nanocomposites underwent an entrapment
reduction
carbonization process. Such a route has also been extended to synthesize spherical Ag/C core
shell composites. A cyclic voltammetry (CV) study reveals that the as-prepared Pd/C core
shell nanocomposites exhibit electrocatalytic activity toward oxidation of ascorbic acid (AA).

1. INTRODUCTION Metal/carbon (M/C) core
shell structure nanocomposites, which combine the advantages of both core and shell materials,1,2 have attracted much attention for their potential applications in in vivo bioimaging,3 fuel cell,4 lithium-ion battery,5
7 and catalytic synthesis.8,9 For example, Pd/C core
shell nanocomposites have been applied as catalyst for selective hydrogenation of phenol to cyclohexanol in the aqueous phase8 and aerobic oxidation of alcohols.9 The study of core
shell M/C started from the LaC2/C nanocomposites, which were first prepared by arc discharge.10 Subsequently, Fe/C, Co/C, and Ni/C nanocomposites were also synthesized by the similar method.11,12 Afterward, other synthesis approaches including ion-beam sputtering,13 high-temperature annealing,14 pyrolysis of organometallic precursors,15,16 and catalytic chemical vapor deposition17 were used to prepare magnetic M/C composites. However, the synthesis temperature for these methods is usually higher than 250 °C. Nowadays, the one-step solution-based hydrothermal synthesis route is highly desirable for its mild reaction conditions, in which the reduction and coating of metals can be simultaneously completed below 200 °C. Such a synthesis route has wide applications in the fabrication of polyhedral Cu/C,18 cablelike Se (or Te)/C,19
22 and spherical Ag (or Au)/C23,24 composites. For the synthesis of cable-like M/C, some organic polymers are usually introduced to the reaction system. Previous studies have reported that Cu/C25 and Ag/C26,27 nanocables were prepared with the assistance of poly(vinyl alcohol) (PVA) or poly(ethylene glycol) (PEG). More recently, our group has developed a PAM-assisted hydrothermal route to synthesize Cu-embedded carbonaceous r 2011 American Chemical Society

matrices, which can convert to Cu/C core
shell composites following a further thermal treatment process at 600 °C.28 In this paper, we extend such an approach to the synthesis of Pd/C core
shell nanocomposites through a one-step hydrothermal process at 200 °C. We investigate the influence of reaction temperature and the dose of PAM or PdCl2 on the morphology of products, and an entrapment
reduction
carbonization growth mechanism is proposed to interpret formation of products on the basis of time-dependent experiment results. A cyclic voltammetry (CV) study reveals that the as-prepared Pd/C core
shell nanocomposites exhibit electrocatalytic activity toward oxidation of AA.

2. EXPERIMENTAL SECTION Materials. Palladium chloride (PdCl2, AR), polyacrylamide (PAM, MW g 3 000 000), R-lactose monohydrate (R-LM, AR), and ascorbic acid (AA, AR) were purchased from Shanghai Chemical Reagent Co. (China). All reagents were used without further purification. Synthesis of Worm-Like Pd/C Core
Shell Nanocomposites. In a typical procedure, 0.2 g of PAM was dissolved in 35 mL of distilled water; then 9 mg (5  10
5 mol) of PdCl2 and 0.5 g of RLM were subsequently added into the above-mentioned solution under vigorous stirring. After being sufficiently stirred, the solution was transferred and sealed in a 50 mL Teflon-lined autoclave. The Received: December 9, 2010 Revised: January 29, 2011 Published: March 23, 2011 6250

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Figure 1. XRD pattern of the worm-like Pd/C core
shell nanocomposites prepared at 200 °C for 6 h (nPdCl2 = 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g).

autoclave was maintained at 200 °C for 6 h in an oven and then allowed to cool to room temperature naturally. Finally, the precipitates were collected and washed with distilled water and absolute ethanol several times. It is worth noting that bare Pd nanoparticles without carbon encapsulation can be prepared when keeping the reaction time within 1 h, and carbon spheres (CSs) can be prepared in the absence of PdCl2. Preparation of Pd/C-Modified Glassy Carbon Electrode (Pd/C/GCE). The GCE (3 mm in diameter) was first polished successively with 0.3 and 0.05 μm alumina slurry and then sonicated in ethanol and distilled water for 1 min. To prepare a Pd/C/GCE, a certain amount of worm-like Pd/C nanocomposites was mixed with 0.5 mL of chitosan acetic acid solution (1%, w/w) by sonication. Then 2 μL of Pd/C-chitosan suspension was dropped onto the surface of the GCE. After solvent evaporation, the Pd/C was stuck onto the surface of GCE. Characterization. X-ray powder diffraction (XRD) was carried out on a Philips X’pert X-ray diffractometer with Cu KR radiation (λ = 1.5418 Å). The microstructures of as-prepared products were observed on a H-7650 (Hitachi) transmission electron microscope (TEM) at an acceleration voltage of 100 kV. A Raman spectrum study was performed on a JY LABRAM-HR confocal laser micro-Raman spectrometer with a 514.5 nm Arþ laser as the excitation source. The Fourier transform infrared (FTIR) spectrum was measured on a Bruker Vector-22 FTIR spectrometer from 4000 to 500 cm
1 at room temperature. X-ray photoelectron spectra (XPS) were obtained on an ESCALab MKII X-ray photoelectron spectrometer using Mg KR radiation as the exciting source. CVs were studied using a CHI660D electrochemical workstation (Shanghai, China), and a conventional three-electrode system was employed with the Pd/C/GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire as the counter electrode.

3. RESULTS AND DISCUSSION Phase and Morphology of Worm-Like Pd/C Core
Shell Nanocomposites. Figure 1 shows a typical XRD pattern of the

as-prepared products. The three peaks with strong diffraction can be indexed as (111), (200), and (220) diffraction planes of the face-centered-cubic (fcc) metallic Pd, which are consistent with the literature value (JCPDS Card No. 05-0681). On the basis of

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Figure 2. Raman spectrum of the worm-like Pd/C core
shell nanocomposites prepared at 200 °C for 6 h (nPdCl2= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g).

Figure 3. XPS spectrum of the worm-like Pd/C core
shell nanocomposites prepared at 200 °C for 6 h (nPdCl2= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g).

the calculation of the Scherrer formula,29,30 the average diameter of Pd nanoparticles is 15 nm. There is a broad peak observed at 2θ = 20
26°, corresponding to the noncrystalline carbon phase of products. The as-prepared products were further characterized by the Raman spectrum. As shown in Figure 2, there are two Raman characteristic peaks located at 1375 and 1585 cm
1. The peak at 1375 cm
1, named the D band, is associated with the A1g vibration of disordered carbon atoms. While the peak at 1585 cm
1, named the G band, corresponds to the in-plane E2g vibration of graphitic carbon atoms.31 The ratio of ID/IG is 3:1, implying a highly disordered graphitic structure for carbon shells. These are also consistent with the results of the XRD pattern. XPS is a good tool for surface component analysis. The XPS of products is shown in Figure 3. The peaks located at 286.2 and 532.8 eV correspond to the C1s and O1s binding energies, respectively. There are no peaks for the Pd3d3/2 or Pd3d5/2 binding energy detected, suggesting that metallic Pd nanoparticles are completely encapsulated in carbonaceous shells. Quantitative analysis reveals that the molar ratio of C/O is 3:1. The high percent of element O may come from the rich hydrophilic groups in carbonaceous shells. As exhibited in Figure 4, the FTIR spectrum of products shows that there exist a large amount of functional groups on carbonaceous shells. The bands at 1697 and 1614 cm
1 arise from the CdO and CdC stretching vibrations, 6251

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The Journal of Physical Chemistry C respectively, and the band at 2923 cm
1 can be assigned to the C
H stretching vibration. The bands at 1000
1300 cm
1 correspond to the C
OH stretching and O
H bending vibrations. It seems reasonable to explain the high stability of products in aqueous solution for the existence of hydrophilic groups.19 Figure 5a and 5b shows the low-magnification TEM images of products, which reveal the high yield of Pd/C core
shell nanocomposites. From Figure 5c, it can be seen that the product is worm-like structure with a diameter of about 100 nm and a length of 100
500 nm. The dark/light contrast suggests the Pd/C core
shell structure of nanocomposites. The encapsulated Pd nanoparticles are 10
15 nm in diameter, consistent with the calculation of the Scherrer formula, and arranged into a 1D chain-like structure. The metallic Pd component is further confirmed by the corresponding electron diffraction

Figure 4. FTIR spectrum of the worm-like Pd/C core
shell nanocomposites prepared at 200 °C for 6 h (nPdCl2= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g).

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(ED) pattern in Figure 5d, and its diffraction rings can be indexed as (111), (200), (220), and (311) diffraction planes of fcc metallic Pd. Influence of Reaction Conditions on the Morphology of Pd/C Core
Shell Nanocomposites. Several factors are found to influence the morphology of Pd/C core
shell nanocomposites: reaction temperature, dose of PAM, and concentration of Pd2þ. According to the previous study, carbonization of carbohydrates is a dehydration and cross-linking process,24,32 which is greatly affected by reaction temperature. As shown in Figures 6a
c and 3d, the thickness of carbonaceous shells varies from 5 to 45 nm with the reaction temperature (140
200 °C) increased, while no obvious average size change is found for encapsulated Pd nanoparticles. The above results indicate that the higher the reaction temperature, the faster the carbonization rate. These results are also in good agreement with Yu’s conclusion in the synthesis of Te/C nanocables.32 Figure 7 shows the TEM images of Pd/C core
shell nanocomposites prepared at 200 °C with different doses of PAM. In the absence of PAM, Pd/C core
shell nanocomposites still can be obtained; however, the Pd nanoparticles with larger size (50
100 nm) are highly aggregated (Figure 7a). As 0.1 g of PAM is added, the products are a branched one-dimensional structure with a high aspect ratio (Figure 7b). When the amount of PAM reaches 0.3 or 0.4 g, spherical Pd/C core
shell nanocomposites encapsulating mono- or multi-Pd cores are prepared (Figure 7c and 7d). Apart from PAM, the microstructure of Pd/C core
shell nanocomposites can also be tuned by varying the dose of PdCl2. From Figures 8a, 8b, and 5d, it can be noticed that the morphology of products gradually converts to a spherical structure with the increased amount of PdCl2. The above phenomenon may be a consequence of the interaction between Pd2þ cations and acylamide groups. A previous study has

Figure 5. (a, b) Low- and (c, d) high-magnification TEM images of the worm-like Pd/C core
shell nanocomposites prepared at 200 °C for 6 h (nPdCl2= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g). Inset of d shows the corresponding ED pattern of Pd nanoparticles. Scale bars for a and b = 500 nm, and scale bars for c and d = 200 nm. 6252

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Figure 6. TEM images of the Pd/C core
shell nanocomposites prepared at different reaction temperatures: (a) 140, (b) 160, and (c) 180 °C (nPdCl2= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g). Scale bars = 150 nm.

Figure 7. TEM images of the Pd/C core
shell nanocomposites prepared at 200 °C with different dosage of PAM: (a) 0, (b) 0.1, (c) 0.3, and (d) 0.4 g (nPdCl2= 5  10
5 mol and mR-LM = 0.5 g). Scale bars for a and c = 400 nm, and scale bars for b and d = 100 nm.

Figure 8. TEM images of the Pd/C core
shell nanocomposites prepared at 200 °C with different dosage of PdCl2: (a) 10  10
5 and (b) 15  10
5 mol (mPAM = 0.2 g and mR-LM = 0.5 g). Scale bars = 300 nm.

reported that metal cations can serve as weak cross-linkers to interconnect polymer chains.33 It is also known that PAM is a linear polymer with rich acylamide (
CONH2) groups, and the intermolecular H bonds (NH 3 3 3 OC) may lead to formation of highly cross-linked PAM clusters.34,35 Thus, the amount of Pd2þ

or PAM added in the reaction system would affect the crosslinked structure of PAM clusters and further influence the final morphology of Pd/C nanocomposites. Time-Dependent Experiments. The evolution of Pd/C core
shell nanocomposites is studied by time-dependent 6253

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Figure 9. TEM images of intermediate products obtained at different reaction stages: (a) 1, (b) 2, and (c) 3 h (nPdCl2= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g). Scale bars for a and b = 50 nm, and scale bar for c = 200 nm.

Scheme 1. Schematic Illustration of the Growth Mechanism of Pd/C Core
Shell Nanocomposites

experiments. When PdCl2 is added to a solution of PAM, some brown flocculations appear in solution, rising from the coordination interaction between PAM and Pd2þ. After reacting for 1 h at 200 °C, Pd nanoparticles with a size of 10 nm are found in products (Figure 9a), whose component is further confirmed by XRD pattern (data not shown). When the reaction time was prolonged to 2 h, the quasi-one-dimensional Pd/C core
shell structure is observed and the thickness of the carbonaceous shell is ∼6 nm (Figure 9b). Upon further prolonging the reaction time to 3 h, worm-like Pd/C nanocomposites exhibit more obvious core
shell structure; meanwhile, the thickness of the carbonaceous shell increases to ∼30 nm (Figure 9c). On further extending the reaction time to 6 h, the thickness of the carbonaceous shells can reach 45 nm. Growth Mechanism of Pd/C Core
Shell Nanocomposites. PAM is believed to be responsible for the formation of Pd/C core
shell nanocomposites. Sodium polyacrylate (NaPA) is similar with PAM in molecular structure only with
COONa instead of
CONH2; however, no Pd/C nanocomposites with core
shell structure are obtained when an equal amount of NaPA is used (data not shown). It can be explained that the strong interaction between
COO
and Pd2þ initiates serious precipitation of PA
from solution, which inhibits the following carbonization coating on Pd nanoparticles. The above phenomenon was also observed in NaPA solution containing Ca2þ or Agþ cations.36,37 Parallel experiments are also conducted with PEG or PVA instead of PAM, and it is found that bare Pd nanoparticles are separated from carbonaceous products (data not shown). It suggests that neither PEG nor PVA can facilitate formation of Pd/C core
shell nanocomposites, although they have been proved to be good stabilizers or capping agents for preparing metallic Pd nanoparticles.38,39

On the basis of the above analysis, dual-functional roles of PAM in the reaction system should be pointed out: (i) it acts as a coordinating agent to entrap Pd2þ and kinetically control the reduction rate of Pd2þ cations; (ii) it also serves as a scaffold to bind Pd nanoparticles and avail formation of the core
shell structure. In fact, formation of Pd/C core
shell nanocomposites undergoes an entrapment
reduction
carbonization process, which is similar with our previous study.28 The schematic illustration in Scheme 1 depicts the growth process. First, cross-linked PAM clusters with rich
CONH2 entrap the Pd2þ ions in solution by a coordination effect (step a). Then the chelated Pd2þ ions are gradually reduced to metallic Pd by RLM (step b). Finally, carbonization coating occurs on the surfaces of Pd nanoparticles fixed by PAM clusters (step c), and Pd/C core
shell nanocomposites are obtained. Such a growth process can also be supported by the above timedependent experiment results. Synthesis of Spherical Ag/C Core
Shell Nanocomposites. Such a route can be extended to fabricate spherical Ag/C core
shell nanocomposites. Figure 10a shows a typical XRD pattern of products, which can be indexed to the fcc metallic Ag (JCPDS No. 89-3722). As shown in Figure 10 b and 10c, the dark/light contrast can be clearly observed for spherical Ag/C core
shell nanocomposites. The typical diameter of Ag cores is about 90 nm, and the thickness of carbonaceous shells is 70 nm. We notice that most carbonaceous spheres contain a mono-Ag core in the center, and few of them encapsulate more than one Ag core. Electrocatalytic Activity of Worm-Like Pd/C Core
Shell Nanocomposites. In previous studies, it has been reported that the M/C core
shell composites can translate into carbon 6254

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Figure 10. (a) XRD pattern and (b, c) TEM images of the spherical Ag/C core
shell nanocomposites prepared at 200 °C for 6 h (nAgNO3= 5  10
5 mol, mPAM = 0.2 g, mR-LM = 0.5 g). Scale bars = 300 nm.

Figure 11. (a) Cyclic voltammograms of Pd/C/GCE (A, D), Pd/GCE (B), and CS/GCE (C) in the presence (A, B, C) and absence (D) of 10 mM AA in 0.01 M PBS (pH = 7.4). (b) Cyclic voltammograms of Pd/C/GCE in 0.01 M PBS containing 0.5, 2.5, 5.0, 7.5, and 10 mM AA from bottom to top. (Inset) Calibration curve corresponding to amperometric response. Scan rate = 0.05 V s
1.

materials or corresponding semiconductor/C core
shell composites via a wet chemical route.19,40,41 It implies that the carbon shells formed under the hydrothermal carbonization conditions are porous, which can allow penetration of molecules/ions. Besides, Pd/C core
shell composites have also been applied as catalysts for selective partial hydrogenation of hydroxyl aromatic,8 which can further confirm the porous character of carbon shells. It is the above research results that inspired us to investigate the elelctrocatalytic activity of Pd nanoparticles encapsulated in carbon shells. AA is an important antioxidant;42 here, it is applied as a model to investigate the electrocatalytic activity of worm-like Pd/C core
shell nanocomposites. As shown in Figure 11a for the Pd/C/GCE there is an obvious oxidation peak (curve A) centered at 0.76 V toward 10 mM AA in 0.01 M phosphate buffer solution (PBS) (pH = 7.4) compared with AA absence from solution (curve D). Its position is close to the case of bare metallic Pd-modified GCE (Pd/GCE) (curve B), which suggests that the metallic Pd nanoparticles encapsulated in the carbon shell exhibit similar electrocatalytic activity with bare metallic Pd nanoparticles. Besides, the CS-modified GCE (CS/ GCE) is also applied as a working electrode to evaluate its electrocatalytic activity toward AA. From curve C in Figure 11a it can be found that the oxidation peak is located at 0.98 V, suggesting a lower electrocatalytic activity compared with Pd/ C/GCE. Above experiment results imply that the metallic Pd encapsulated in carbon shells is responsible for the electrocatalytic activity. Besides, we also investigated the current response of Pd/C/GCE toward different concentrations of AA according to previous studies.43 As shown in Figure 11b, the peak current increases with the increase of the AA

concentration, and a linear relationship between the amperometric responses and the concentrations of AA is observed (inset of Figure 11b).

4. CONCLUSIONS In summary, worm-like Pd/C core
shell nanocomposites were prepared via a hydrothermal process using PdCl2, PAM, and R-LM. By varying reaction conditions, the carbonaceous shell thickness and morphology of Pd/C nanocomposites can be conveniently tuned. Polymer PAM, serving as both a coordinating agent and a structure scaffold, is believed to play an important role in formation of the core
shell structure. Time-dependent experiments confirm that formation of Pd/C core
shell nanocomposites undergoes an entrapment
reduction
carbonization process. Such a route has also been applied to fabricate a Ag/C spherical core
shell structure. A CV study reveals that the as-prepared Pd/C core
shell nanocomposites exhibit electrocatalytic activity toward oxidation of AA. ’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86-551-360-1589. Fax: þ86-551-360-7402. E-mail: [email protected] (Y.T.Q.), [email protected] (Y.C.Z.).

’ ACKNOWLEDGMENT This work was financially supported by the National Nature Science Fund of China (No. 91022033), the 973 Project of China (No. 2011CB935900), and the Doctor Foundation of Liaocheng University (No. 31805). 6255

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