Nanocomposites with Hexagonal

Dec 31, 2010 - ABSTRACT: Hierarchical Co micro/nanocomposites with hexagonal plate and polyhedron shapes in micrometer scales are synthesized ...
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DOI: 10.1021/cg101254k

Synthesis of Hierarchical Co Micro/Nanocomposites with Hexagonal Plate and Polyhedron Shapes and Their Catalytic Activities in Glycerol Hydrogenolysis

2011, Vol. 11 472–479

Yue-Bin Cao,† Xing Zhang, Jun-Mei Fan, Peng Hu, Liu-Yang Bai, Hai-Bao Zhang,† Fang-Li Yuan,* and Yun-Fa Chen State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences (CAS), Beijing 100190, China. †Also at Graduate School of CAS, Beijing, China. Received September 24, 2010; Revised Manuscript Received November 16, 2010

ABSTRACT: Hierarchical Co micro/nanocomposites with hexagonal plate and polyhedron shapes in micrometer scales are synthesized through polyol reduction. Flower-surfaces Co hexagonal plates (FCHPs) show the edge lengths of the hexagonal plates about 1-2 μm in micrometer scales, and the flower-like surfaces of each individual hexagonal plate is composed of nanodiscs with diameters of about 200-250 nm and thicknesses about 20 nm. Flower-surfaces Co polyhedrons (FCPs) show the twin-hexagonal-frustum-pyramid morphology in micrometer scales with diameters of about 2-3 μm, and the flower-like surfaces of each polyhedron is also composed of nanodiscs with diameters of about 100-150 nm and thicknesses of about 2030 nm. An oxidation-dissolution and subsequent reduction mechanism is proposed to explain the formation of their flower-like surface structure. Both FCHPs and FCPs exhibit good catalytic activities in glycerol hydrogenolysis.

1. Introduction Because of the affinitive relationship between the shape and property, controlling the morphology of micro/nanostructures has been of great interest in recent materials research fields.1 A hierarchical three-dimensional (3D) micro/nanocomposite structure is composed of nanosized building blocks hierarchically while the total size is in the micrometer scale. Such a hierarchical structure with the cooperation of microstructure and nanostructure provides a novel approach to bring forth new properties. For example, hierarchical ceria micro/nanocomposite exhibits a photovoltaic response, while normal ceria does not show this response.2 Thus, the research on micro/nanostructures has rapidly extended from simple structures to the assembly of nanocrystals into ordered hierarchical structures aiming to achieve increased structure complexity and functionality.3 As an important magneticmetal material, micro/nanosized Co crystals have extensive applications in ultrahigh-density magnetic recording, sensors, and heterogeneous catalysis.4 In the last decades, micro/ nanosized Co with different morphologies such as wires, discs, cubes, polyhedrons, and hollow structures have been synthesized.5 Recently, some efforts have been focused on fabricating hierarchical Co 3D micro/nanocomposite structures due to their potential applications in catalysis and sensors.6 For instance, Shen and co-workers reported the synthesis of hierarchical Co hollow spheres and their high catalytic activity in heterogeneous hydrogenation.7 However, the formation of hierarchical 3D micro/nanocomposite structure is usually realized by aggregation of primary particles, which cause the synthesized 3D Co micro/nanocomposites present spherical shape in micrometer scales due to the low surface energy of spheres. Although some progresses had been made in the synthesis of hierarchical Co 3D micro/nanocom*Corresponding author. Phone: þ86-10-82627058. Fax: þ86-10-62561822. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 12/31/2010

posites, it is still a great challenge to develop simple and reliable methods to synthesize hierarchical Co 3D micro/ nanocomposites, especially hierarchical Co 3D micro/nanocomposites with nonspherical shapes in micrometer scales. Polyol reduction is an effective method to synthesize metal particles and control their shapes. In the past several years, various micro/nanosized metal particles with regular shapes such as Ag cubes,8 Pt wires,9 and Ni icosahedra10 have been synthesized through this method. Herein, we further extend this method to synthesize hierarchical 3D micro/nanocomposite Co with hexagonal plate and polyhedron shapes in micrometer scales. On the basis of the morphology evolution with increasing reaction time, the formation mechanism of hierarchical structure is proposed, which presents an interesting growth mode, different with the formation of other hierarchical 3D Co micro/nanocomposites. The specific structure of hierarchical 3D Co micro/nanocomposites, with their surfaces composed of interconnected nanodiscs, endows them with the advantage of high catalytic activities in glycerol hydrogenolysis. 2. Experimental Section 2.1. Materials Preparation. Synthesis of flower-surfaces Co hexagonal plates (FCHPs): CoCl6 3 6H2O (0.5 g) and NaOH (0.8 M) were loaded into a 100 mL poly tetrafluoroethylene (PTFE) lined stainless steel autoclave, which was then filled with 60 mL of ethylene glycol (EG). The autoclave was sealed and maintained at 200 °C for 8 h, and then cooled down to room temperature. The final product was filtered, rinsed with distilled water and ethanol for several times, and dried at 30 °C for 4 h. The procedure for the synthesis of flower-surfaces Co polyhedrons (FCPs) was similar to that of the FCHPs, except that EG was replaced by glycerol water cosolvent (volume ratio of glycerol/water was 1:1) and the reaction time was changed to 12 h. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using a PANalytical X’Pert PRO MPD X-ray diffractometer operated at 40 kV and 30 mA with Cu KR radiation. FESEM measurements were carried out using a field-emission r 2010 American Chemical Society

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Figure 1. (A) XRD pattern of the FCHPs. (B-D) SEM images of the FCHPs with different magnification: (B) low-magnification SEM image; (C) SEM image of a single FCHP; (D) high-magnification SEM image of the surface of a single FCHP. (E) TEM image of a single FCHP. (F) TEM image of the edge of a single FCHP. (G) Typical HRTEM image taken from a nanodisc of the FCHP. (H) SAED taken from the nanodiscs of a single FCHP. microscope (JEOL JSM-6700F) operated at an acceleration voltage of 5 kV. TEM images were taken on a Hitachi H-800 and JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. The BET surface area of the sample was determined by N2 adsorption-desorption isotherm measurements at 77 K (surface area analyzer, SA 3100). 2.3. Catalytic Performances. Hydrogenolysis of glycerol was conducted in a 250 mL autoclave. The synthesized Co particles (0.1 g) were added to a glycerol aqueous solution (80 g) with concentration of glycerol (5 wt %). The reaction system was heated at 220 °C under stirring and kept at this temperature for 8 h. The hydrogen pressure was 6.0 MPa. The products were analyzed by gas chromatography equipped with a flame-ionization detector and a Carbowax 20 M capillary column with 25 m length and 0.2 mm diameter.

3. Results and Discussion 3.1. Synthesis of FCHPs. 3.1.1. Characterization of the FCHPs. FCHPs were obtained when a mixed dispersion of cobalt chloride hexahydrate (CoCl2 3 6H2O) and sodium hydroxide (NaOH) in EG was heated at 200 °C for 8 h. XRD pattern of the product is shown in Figure 1A. The diffraction peaks can be indexed to (111) and (220) planes of the fcc structure of cobalt (JCPDS No. 15-0806). Fieldemission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution TEM

(HRTEM) images provide further insight into the structure and morphology of the FCHPs. As shown in the low magnification scanning electron microscopy (SEM) image (Figure 1B), the obtained Co show a hexagonal plate shape, with an edge length of about 1-2 μm. The typical SEM image of a single hexagonal plate is presented in Figure 1C, which indicates that the surface of the Co hexagonal plate presents a flower-like structure. The detailed morphology of the flower-like surface is shown in Figure 1D, which reveals that the flower-like surface is composed of many nanodiscs with diameters of about 200-250 nm and thicknesses of about 20 nm. TEM image of a single FCHP is shown in Figure 1E. The edge of the FCHP was brighter than the center, indicating the core-shell structure of the hexagonal plate. Figure 1F gives an enlarged TEM image of the edge of a single FCHP. It is clear that the nanodiscs only covered the outer surface of the hexagonal plate, while the interior of the hexagonal plate was dense. (Two other cases support the core-shell structure of the sample. One, the following timedependent experiments as we will discuss in Section 3.1.2 indicate the flower-like surface of the sample is evolved from a dense smooth surface sample. Another, the measured BET surface areas of the flower-surface sample as we have shown in Table 1 is about 8.2 m2/g, which is smaller than the calculated value (70.8 m2/g) for an ideal Co nanoplate with

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Table 1. The BET Surface Areas and Catalytic Performances of the Co Particles on the Hydrogenolysis of Glycerol selectivity (%)

yield (%)

catalyst

BET surface area (m2 g-1)

conversion of glycerol (%)

1,2-PDO

1,3-PDO

1,2-PDO

reaction rate (10-3 mol m-2 h-1)

FCHPs FCPs

8.2 3.4

25.8 33.5

60.2 47.6

0 0

15.5 15.9

1.7 5.4

Figure 2. (A) XRD patterns of the products obtained at different reaction times: (a) 2; (b) 4; (c) 24; (d) 48 h. (B-H) SEM images of the products obtained at different reaction times: (B) 2; (C, D) 4; (E, F) 6; (G) 24; (H) 48 h.

a thickness of 20 nm.6a) A HRTEM image taken from a nanodisc reveals the polycrystalline nature of the nanodisc (Figure 1G). The selected area electron diffraction (SAED) taken from the nanodiscs exhibits ring patterns (Figure 1H), also showing the nanodiscs on the surface of hexagonal plate are polycrystalline and the rings can be indexed respectively to the (111), (200), and (220) crystal diffractions of fcc Co.

3.1.2. Effect of Reaction Time on the Phase and Morphology of Product. The evolution of the FCHPs was investigated in detail by characterizing the products acquired at different reaction times. Figure 2A displays a series of evolutional XRD patterns of the samples obtained at different reaction times. The pattern of the pink product obtained at the reaction time 2 h presents no obvious peak (Figure 2A

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trace a), characteristic of an amorphous structure. The composition information of the product obtained from the energy dispersive detector (EDS) spectrum gives a Co/O atom ratio of about 1:2 (Figure S1A, trace a, Supporting Information). The EDS analysis, together with the pink color of the product, confirms the product may be Co(OH)2. When increasing the reaction time to 4 h, the diffraction peaks of the XRD pattern of the gray color product can be readily indexed to phase-pure fcc Co (Figure 2A, trace b). The EDS spectrum as shown in Figure S1A, trace b, Supporting Information also reveals the product was pure Co (the existence of trace O in the EDS spectrum may be ascribed to partial oxidation of Co during SEM sample preparation). The XRD pattern and EDS spectrum indicate that the amorphous Co(OH)2 has been completely decompose-reduced to Co at this time. The XRD patterns have no obvious change when the reaction time further increased to 24 h (Figure 2A, trace c). When the reaction time increased to 48 h, the diffraction peaks of Co show an increase (Figure 2A, trace d). The time-dependent product morphology of the sample is demonstrated in Figure 2B-H. In a short reaction time of 2 h, amorphous Co(OH)2 nanoparticles were obtained, as shown in Figure 2B. When reacted for 4 h, hexagonal Co plates with smooth surface were formed from the decomposition-reduction of amorphous Co(OH)2 (Figure 2C,D). The TEM image of a single hexagonal Co plate with smooth surface is shown in Figure S1B, Supporting Information, Supporting Information. As the time increases (6 h), smooth-surface Co hexagonal plates (SCHPs) were transferred to FCHPs (Figure 2E). Figure 2F shows the enlarged SEM image of the FCHPs, which indicates the nanodiscs on the surfaces of FCHPs with diameters and thicknesses around 150 and 40 nm, respectively. With increasing reaction time (8 h), the diameters of the nanodiscs grew to about 200-250 nm, but the thicknesses of the nanodiscs decreased to about 20 nm, as shown in Figure 1C,D. It is interesting that when the reaction time was extended to 24 h, the nanodiscs on the surfaces of the hexagonal plates disappeared and the flower-like surfaces became compact again, as shown in Figure 2G. The typical SEM image of a single compact-surface hexagonal plate is shown in Figure S1C, Supporting Information. After being reacted for 48 h, the nanodiscs formed again on the surfaces of hexagonal plates as shown in Figure 2H. The structure of the FCHPs has no change when the reaction time is further increased to 60 and 72 h (not shown in here). On the basis of the above results, the evolution of FCHPs was shown in Figure S2, Supporting Information. Initially, SCHPs were formed by decomposition-reduction of amorphous Co(OH)2. Then the nanodiscs appeared on the surfaces of the SCHPs and grew gradually to a larger size, resulting in the flower-like structure of their surfaces (steps 1 and 2). To understand the transformation process of SCHPs to FCHPs, the following experiments were conducted: The SCHPs obtained at a reaction time of 4 h were collected, redispersed in alkaline solution of EG with CNaOH 0.8 M and VEG 60 mL, and reacted at 200 °C for 8 h. After the reaction the SCHPs also changed to FCHPs, whose surfaces show a similar flower-like structure to the product obtained when the initial cobalt chloride solution directly reacted at 200 °C for 8 h (Figure S3, Supporting Information). This experiment indicates that the FCHPs were directly transferred from SCHPs, and the flower-like surfaces were not formed from the further reduction of residual Co(II) in solution.6b

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The fact that SCHPs changed to FCHPs in the absence of Co(II) also ruled out the self-assembly mechanism in the formation of the flower-like surfaces,6a,11 which is a common phenomenon in the formation of hierarchical spheres through self-assembly of their building blocks. The NaOH in solution was proven to play an important role in the transfer of SCHPs to FCHPs, because in the absence of NaOH, the morphology of SCHPs has no change after reacted at 200 °C for the same reaction time (8 h). On the basis of the above results, the main chemical reactions that are involved in the transfer of SCHPs to FCHPs are suggested as follows: Co þ 1=2O2 þ 2OH - þ H2 O f CoðOHÞ4 2 -

ð1Þ

CoðOHÞ4 2 - þ 2HOCH2 CH2 OH f Co þ CH3 COOCCH3 þ 4H2 O þ 2OH -

ð2Þ

At first, the surface Co(0) of SCHPs was gradually oxidized to Co(II) by the trace oxygen dissolved in solution. Then tetrahydroxo-cobalt(II) anion (Co(OH)42-) is likely to be generated under a strong basic environment, because of the stabilization of Co(OH)42-.12 It can be understood that the oxide-dissolved Co(OH)42- was more prone to accumulate near the plate surfaces. Under solvothermal conditions, the Co(OH)42- was again reduced to Co by EG, and the newly formed Co heterogeneous nucleated and grew on the surfaces of the hexagonal plates, and finally grew to the nanodiscs. Indeed, after the SCHPs were redispersed in alkaline solution of EG and reacted at 200 °C for 8 h, the Co(II) can be detected in the filtrate through adding NH4SCN as the indicator. (The color of the filtrate changed to blue because the formation of Co(SCN)42- when NH4SCN was added to the filtrate.) It further provides evidence to support our proposed mechanism. The proposed mechanism here is somewhat analogous to what has been reported for the formation of netlike framework Co constructed by nanorods, where Co nanorods with a smaller size dissolved to Co2þ and subsequently was reduced again to Co nanorods with a larger size.7 As the nanodiscs on the surfaces of hexagonal plates exhibit a polycrystalline nature, it is reasonable to deduce that the nanodiscs were formed by the nonoriented aggregation of primary particles. The nonoriented aggregation of primary particles to anisotropic disk morphology may be assisted with ethanol glycerol molecules. The size evolution of nanodiscs (diameters change from about 150 nm to 200-250 nm and thicknesses change from about 40 to 20 nm) (step 3) can be attributed to Ostwald ripening process, which has been adopted to explain the thinner and sharper of Ni7S6 nanopetals.13 In this process, the Co nanocrystals with a little size on the surface of nanodiscs dissolved and regrew on the edge of the nanodiscs, which induced the nanodiscs to grow to a larger size and become thinner. The mechanism of how FCHPs change to compact-surface ones again after being reacted for 24 h (step 4) is still not very clear. It is suggested that the coalescence between these nanodiscs driving by minimization of surface energy and subsequent Ostwald ripening may contribute to the formation of compact surfaces of the hexagonal plates, which is similar to the reported formation process of cantaloupe-like Fe2O3 and cubic Cu mesocrystals.14 In the fifth step, the compact-surfaces hexagonal plates transferred to flower-surfaces ones again

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Figure 3. SEM images of the Co particles obtained in EG with different NaOH concentrations: (A, B) 0.4 M; (C, D) 1.6 M.

through a similar mechanism as described in steps 1 and 2. Different from step 4, after a further increase in the reaction time the structure of the flower-surfaces hexagonal plates does not change, indicating the stability of the sample. The stability of the FCHPs obtained at this stage may be caused by its high crystallization, since high crystallization means less defects in crystals.15 When the density of defects is high, it has a large tendency for coalescence or dissolutionrecrystallization (Ostwald ripening),15a as in our experiments the flower-surface structure (8 h) became a compact surface (24 h). In contrast, when the density of defects is low, it is more stable and coalescence or Ostwald ripening is not likely to happen, and the flower-surface (48 h) remains unchanged when the reaction times increase. 3.1.3. Effect of NaOH Concentration on the Morphology of Products. The concentration of NaOH has a great effect on the morphology of the Co product. When the concentration of NaOH decreased to 0.4 M, no hexagonal plates and only spheres with diameters about 2 μm were obtained, as shown in Figure 3A. Local magnification reveals that the surfaces of the spheres also consist of nanodiscs (Figure 3B). When the concentration of NaOH increased to 1.6 M, chainlike Co with lengths of tens of micrometers was formed (Figure 3C). The magnified SEM image of the product indicates the chains are closely connected by spheres (with a diameter of about 2-3 μm), and the spheres also show flower-like surfaces (Figure 3D). Usually, in the synthesis of magnetic metal particles the slow reduction rates of metals are favorable for the formation of a chainlike structure. For example, Ni chains assembled by spheres can be formed in Ni(C4H2O6)2- solution because the formation of complex can slow down the reduction rate of Ni, while in the absence of C4H2O62- the reduction rate of Ni will be accelerated and dispersive Ni spheres are obtained.16 The effect of NaOH concentration on the morphologies of prepared magnetic metals had also been investigated by some researchers. In accordance with

our results, as the NaOH concentration increased the obtained magnetic metals changed from dispersive spheres to chains.16,17 However, the mechanism of dispersive spheres change to chains as the increase of NaOH concentration is still not clear. Since an increase of NaOH concentration usually accelerates the reduction rate of metals, it is hard to explain it through the reaction rate. We think the change of spheres to chains with the increase of NaOH concentration may be attributed to their different reaction routes (in particular, their different reaction intermediates). When the concentration of NaOH is high (1.6 M), soluble Co(OH)42complex was formed first, and then Co particles were formed through the reduction of Co2þ dissociated from Co(OH)42-. Driven by the magnetic dipole-dipole attractions, it is likely that the Co particles will preferentially assembly to a chainlike structure. When the concentration of NaOH is low (0.4 M), solid phase intermediate Co(OH)2 was formed first, and then it was reduced in situ gradually with increasing reaction time. In this case, after the nucleation the growth of Co particles was restricted in the solid phase and, finally, dispersive Co particles were obtained as a result of the complete consumption of solid Co(OH)2. Thus, we think the different morphologies of the Co obtained at different NaOH concentrations may be attributed to the different intermediates. 3.2. Synthesis of FCPs. 3.2.1. Characterization of the FCPs. FCPs were obtained when a mixed dispersion of cobalt chloride hexahydrate (CoCl2 3 6H2O) and sodium hydroxide (NaOH) in glycerol water cosolvent was heated at 200 °C for 12 h. The XRD pattern of the product indicates the crystalline phase of hcp Co (JCPDS No. 5-727) (Figure 4A). Figures 4B and S4A show the low-magnification SEM images of the FCPs, which reveal the prepared polyhedrons were uniform with diameters of about 2-3 μm. (The diameter is defined as the distance between the opposite apexes of the polyhedral maximum cross section.) A medium-magnification SEM image (Figure 4C) reveals the obtained polyhedral Co have regular twin-hexagonal-frustum-pyramid morphology,

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Figure 4. (A) XRD pattern of the FCPs. (B-D) SEM images of the FCPs with different magnifications: (B) low magnification; (C) medium magnification; (D) high magnification (the letters a, b, c in Figure 4D mark the different faces of the polyhedron). (E) TEM image of a single FCP. (F) HRTEM image taken from a nanodisc of the FCP.

which coincides well with the Wulff polyhedron of hcp-Co.18 The structural model of Wulff polyhedron of hcp-Co constructed by the Gibbs-Wulff relation19 is shown in Figure S4B, Supporting Information. Similar to FCHPs, a clearly flower-like structure of the polyhedrons surfaces can also be seen from Figure 4C. Figure 4D shows the high-magnification SEM image of a polyhedron surfaces, which indicates that the flower-like surfaces of the polyhedron are also composed of nanodiscs with diameters of about 100-150 nm and thicknesses of about 20-30 nm. The typical TEM image of a single FCP is shown in Figure 4E, which indicates the nanodiscs also grow on the surfaces of dense polyhedrons, forming a coreshell structure. The HRTEM taken from a nanodisc, covered on the surfaces of the polyhedrons, also shows the polycrystalline nature of the nanodiscs (Figure 4F). 3.2.2. Effect of Reaction Time on the Phase and Morphology of Products. The chemical composition and shape of the Co intermediates during the synthesis of Co polyhedrons were monitored by XRD measurements and SEM observations (Figure 5). When the synthesis was conducted for 3 h, the XRD pattern of the reddish purple product as shown in Figure 5A, trace a is in agreement with the main peaks of cobalt alkoxide.5c,20 SEM image of the sample indicates they are irregular nanoparticles (not shown here). As the reaction

time increased to 4 h, pure hcp Co was obtained (Figure 5A, trace b). At this stage, immature Co polyhedrons were formed but with a relative small size, of about 700-800 nm (Figure 5B). A high-magnification SEM image of the immature polyhedrons is shown in Figure S5A, Supporting Information. It can be seen that the immature polyhedrons present a close stacking layered structure. It indicates that the immature polyhedrons may be formed through a layerby-layer growth process, which is similar to the formation of tower-like ZnO and plate-built LaF3 cylinder.21 At this stage, except for the immature polyhedrons, there are also many nanoparticles in this product. When the reaction time was extended to 8 h, the nanoparticles disappeared and the immature polyhedrons were evolved to regular polyhedrons with the diameters increased to about 2-3 μm (Figure 5C). XRD pattern of the sample shows the intensity of the diffraction peaks have an obvious increase (Figure 5A, trace c), indicating the further crystallization of the sample from immature polyhedrons to regular polyhedrons. Figures 5D and S5B show the side view and top view of the polyhedrons, respectively, which indicate the polyhedrons present a regular twin-hexagonalfrustum-pyramid shape. It can also be seen that the surfaces of the polyhedrons were somewhat rough, which suggests the growth of immature polyhedrons to rough-surfaces regular

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Figure 5. (A) XRD patterns of the product obtained in a glycerol-water system reacted for different times: (a) 3; (b) 4; (c) 8 h. (B-D) SEM images of the product obtained at different reaction times: (B) 4; (C, D) 8 h.

polyhedrons may be through a nonclassical particle-based crystallization process.22 In addition to a nonclassical particlebased crystallization process, the classical Ostwald ripening process may also play a role in the formation of rough-surfaces regular polyhedrons. The company of Ostwald ripening can explain the fact that no obvious primary particles, but just roughness, was found on the surfaces of the regular polyhedrons.14b,23 When reacted for 12 h, the rough-surfaces polyhedrons were transferred to flower-surfaces polyhedrons, as shown in Figure 4B,C. The polyhedrons with rough surfaces obtained at 8 h (Figure 5C) were collected, redispersed in alkaline solution of glycerol water cosolvent (with CNaOH 0.8 M and glycerol/water volume ratio 1:1), and reacted at 200 °C for 12 h. After the reaction, the rough-surfaces polyhedrons can also be transferred to flower-surfaces as shown in Figure S5B, Supporting Information. Thus, we can deduce that the transfer of rough-surfaces polyhedrons to FCPs was also through the oxidation-dissolution and reduction process which is similar to the transfer of SCHPs to FCHPs as we have suggested above. When the reaction time was further increased (24 h, 36 h), the flower-surfaces structure of the polyhedrons has no obvious change. 3.3. Catalytic Activities. The catalytic activities of the synthesized Co were investigated on catalytic hydrogenolysis of glycerol to produce propanediol (1,2-propanediol (1,2PDO); 1,3-propanediol (1,3-PDO)) which is one of the most promising approaches for the effective utilization of glycerol. The noble metal catalysts have been used to catalyze this reaction, but the yield of 1,2-PDO was no more than 10% when operated at 200 °C and hydrogen pressure of 4.0 MPa (Ru/C and Pt/C)24 and at 160 °C and hydrogen pressure of 8.0 MPa (Ru/SiO2).25 The catalytic activities of the two types of Co are shown in Table 1. Using FCHPs as the catalyst, the conversion of glycerol was 25.8%, and the selectivity of 1,2-PDO was 60.2%, giving a 1,2-PDO yield of 15.5%. Compared with FCHPs, using FCPs as the catalyst, the

conversion of glycerol was higher (33.5%), but the selectivity of 1,2-PDO was lower (47.6%), giving the yield of 1,2-PDO similar to that with FCHPs as catalyst (15.9%). Obviously, the yield of 1,2-PDO presented by either FCHPs or FCPs is higher than those obtained with noble metals. They are also better than conventional spherical Co nanoparticles, which gives the yield of 1,2-PDO only 10.8%.26 The higher catalytic activities of either FCHPs or FCPs compared to Co nanoparticles may be attributed to their specific interconnected nanodisc structure of their surfaces.27 However, only 1,2-PDO was obtained with either FCHPs or FCPs as catalyst, and no 1,3-PDO was detected. The calculated average reaction rate of glycerol conversion (molecules of glycerol converted over the surface area of Co particles) over FCHps and FCPs was 1.7  10-3 and 5.4  10-3 mol m-2 h-1, respectively. The FCPs catalyst was up to about 300% more efficient per unit surface area than FCHPs, indicating the higher catalytic activity of hcp-Co than fcc-Co in glycerol hydrogenolysis. The shape and crystalline phase of FCHPs and FCPs remained unchanged after the catalytic reaction (the XRD patterns and SEM images not shown in here). The catalysts can be recycled for further catalytic reactions. After the catalyst of FCHPs was used repeatedly 3 times, the conversion of glycerol was 24.3% and the selectivity of 1,2-PDO was 62.0%. After FCPs as a catalyst was used 3 times, the conversion of glycerol and selectivity of 1,2-PDO were 33.2% and 47.1%, respectively. It can be seen that the catalytic activity of both FCHPs and FCPs have almost no decrease after being repeated 3 times. 4. Conclusions We successfully synthesized hierarchical 3D micro/nanocomposite Co with hexagonal plate and polyhedron shapes in micrometer scales through the polyol reduction method. A possible oxidation-dissolution and subsequent reduction

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mechanism for the interesting hierarchical architectures is proposed to interpret the growth process. The hierarchical Co 3D micro/nanocomposites exhibit good catalytic performances in glycerol hydrogenolysis. The as-prepared hierarchical Co 3D micro/nanocomposites should be promising materials for heterogeneous catalysis and is expected to be useful in many other application fields due to their specific structures. Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 50907072, 50974111) and the Foundation of State Key Laboratory of Multiphase Complex Systems. Supporting Information Available: Figure S1 shows the EDS spectrum of Co(OH)2 and Co (A), and TEM image of a single smooth surface hexagonal plate (B), and SEM image of a typical hexagonal Co plate with compact surface (C). Figure S2 shows the evolution process of FCHPs. Figure S3 shows the SEM image of the flower-surfaces Co plates transferred from smooth-surfaces ones. Figure S4 shows the low-magnification SEM image of the FCPs (A), and the structural model of Wulff polyhedron of hcp-Co (B). Figure S5 shows the enlarged SEM image of immature Co polyhedrons (A), and a rough-surface regular polyhedron from the top view (B), and a flower-surface Co polyhedron transferred from rough-surface regular polyhedron (C). This material is available free of charge via the Internet at http://pubs.acs.org.

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