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Hierarchical Growth of Co Nanoflowers Composed of Nanorods in Polyol Qiying Liu, Xiaohui Guo, Yong Li, and Wenjie Shen* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ReceiVed: September 14, 2008; ReVised Manuscript ReceiVed: January 04, 2009

Hierarchical Co nanoflowers composed of nanorods were fabricated through a simple solvothermal synthesis in polyol using Ru as the heterogeneous nucleation agent and hexadecylamine as the structure-directing agent. The solid cobalt alkoxide that is produced in the primary stage mediates the growth rate of the Co nanoflowers, which follow a hierarchical growth mode. The core is initially formed, followed by anisotropic growth into a wheat fringe that further grows to nanoflower. The sizes of the nanoflowers and the petals can be simply adjusted by varying the concentration of hexadecylamine. Typically, well-defined nanoflowers of about 500 nm having petals with lengths of about 500 nm and diameters of 50 nm were obtained. The Co nanoflowers showed quite promising catalytic performance in the hydrogenolysis of glycerol to propylene glycol, demonstrating a potential application in heterogeneous catalysis. 1. Introduction Nanomaterials with hierarchical structures have been of great interest because of their novel properties, differing from those of their bulk or discrete counterparts.1 The assembly of hierarchical structures usually occurs through interactions among presynthesized building blocks, including van der Waals, magnetic, and electrostatic forces, to decrease the system energy by self-organization and to obtain the specific geometric configurations.2 However, this route often results in hierarchical nanostructures that are prone to collapse when the systems are subjected to heating and ultrasonic conditions, which makes use of the materials for potential applications in nanodevices difficult.3 Comparatively, the one-pot synthesis of a hierarchical material that combines the formation of the building blocks and their in situ assembly is highly desired for obtaining robust nanostructures. Using organic surfactants, hierarchical spongelike Rh nanoparticles,4 Au dendrimers and combs, and Au-Pt flowers5 have been successfully synthesized. The surfactants act as the structure-directing agents to stabilize the metal nanocrystals and/or to regulate anisotropic growth into hierarchical structures by strongly absorbing onto the facets of the growing nucleus. Co nanomaterials have diverse applications in catalysis, highdensity information storage, and magnetic separation.6 Because of their shape-dependent physical and chemical properties, sustainable efforts have been made to control the shapes of the Co nanomaterials, and several approaches have been developed to synthesize Co nanomaterials with multidimensional structures. Zero-dimensional nanoparticles;7 one-dimensional nanorods, nanowires, and nanobelts;8 two-dimensional nanodisks and nanoplatelets;9 and multidimensional hollow spheres, bowls, and snowflakes10 have been fabricated mainly by solvothermal syntheses in the presence of surfactants. However, the synthesis of Co nanoflowers with ordered hierarchical structures has rarely been reported because of the complexity of the growth mechanism.11-13 Co flowers are usually constructed as a large core surrounded by platelets as petals driven by the self* Corresponding author. Tel.: +86-411-84379085. Fax: +86-41184694447. E-mail: [email protected].

assembly process. For example, Co flowers with sizes of 1-5 µm assembled from crispate flakes of about 1 µm were synthesized using N2H4 as the reducing agent and 2-hydroxy4-(1-methylheptyl) benzophenone oxime as the complexing agent.11 Co flowers with a size of about 5 µm and platelets with an edge of 1 µm and a thickness of 50-100 nm were also synthesized by the same approach but using dimethylglyoxime as complexing agent.12 During the synthesis, the platelets formed initially and then aggregated, branched, and fused into the flowery architectures. Co flowers have also been produced by using N2H4 as the reducing agent and ethylenediamine tetraacetic acid sodium as the complexing agent in ethanol/water solution.13 The flowers had a size of about 10 µm, and the plates had a size of about 5 µm and a thickness of about 200 nm. These Co flowers showed significantly enhanced magnetic properties when compared to bulk Co particles and Co nanowires. However, the synthetic route still requires complexing agents, strong basic solutions, and long synthesis periods. The large building blocks in the micrometer range also make the flower sizes up to several micrometers. Therefore, controlling hierarchical structures made up of small building blocks (especially on the nanoscale) is of great importance, but is still an open challenge. Recently, solvothermal synthesis in the presence of a trace amount of noble metals as nucleation agents has been shown to be effective for producing transition-metal nanostructures at low temperatures. For example, CoNi bimetallic nanowires with lengths of 100-500 nm and diameters of 5 nm were synthesized using Ru as the nucleation agent in basic polyol.14 However, this approach might not be efficient for preparing Co nanowires, because only spherical nanoparticles15 or nonuniform urchins16 have been obtained. Here, we report the synthesis of Co nanoflowers composed of nanorods through a hierarchical growth mode in polyol using Ru as the heterogeneous nucleation agent and fatty amine as the structure-directing agent. The Co nanoflowers could be made to be less than 500 nm with petals (nanorods) having a length of 500 nm and a diameter of 50 nm by adjusting the synthetic parameters. Herein, the role of the fatty amine and the formation mechanism of the flowers are extensively discussed. The Co nanoflowers exhibited a quite

10.1021/jp8081744 CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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promising catalytic performance in the hydrogenolysis of glycerol to propylene glycol. 2. Experiment Section 2.1. Materials Preparation. Certain amounts of cobalt acetate, fatty amine, and ruthenium chloride were initially dissolved in 1,2-propanediol, and the mixture was transferred to an autoclave, heated gradually to 170 °C, and kept at that temperature for a certain period. In a typical synthesis, 0.75 g of Co(OAc)2 · 4H2O was dissolved in 70 mL of 1,2-propanediol, and then a mixture of 3.62 g of hexadecylamine and 0.02 g of RuCl3 · xH2O (35 wt % Ru) dissolved in 5 mL of 1,2propanediolwas added. The mixed solution was transferred into a Teflon-lined autoclave (100 mL), which was gradually heated to 170 °C and maintained at this temperature for 10 h. The solid obtained was centrifuged and thoroughly washed with petroleum ether and ethanol. The resulting solid was then dried at 50 °C for 5 h under vacuum. For comparison, the concentration and type of the fatty amine were also examined. 2.2. Characterization. X-ray powder diffraction (XRD) patterns of the samples were recorded with a D/Max-2500/PC diffractometer (Rigaku, Tokyo, Japan) operated at 40 kV and 30-100 mA, using nickel-filtered Cu KR (λ ) 0.154 18 nm) radiation. Field-emission scanning electron microscopy (FESEM) images were recorded using a Philips Fei Quanta 200F instrument operated at 20 kV. The samples were placed on a conductive carbon tape adhered to an aluminum sample holder. Transmission electron microscopy (TEM) images were obtained using a Philips Tecnai G2 Spirit microscope operated at 120 kV. The samples were ultrasonically dispersed in ethanol, and drops of the suspension were placed on a carbon-enhanced copper grid and then dried in air. Thermogravimetric (TG) analysis was performed with a Pyris Diamond (Perkin-Elmer) from room temperature to 725 °C at a rate of 5 °C/min under nitrogen flow. 2.3. Catalytic Performance. Hydrogenolysis of glycerol was conducted in a 100-mL autoclave. For each experiment, 0.05 g of Co nanomaterial was added to 40 g of 10 wt % aqueous glycerol solution. The reaction system was heated to 220 °C under stirring and kept at this temperature for 7 h under a hydrogen pressure of 5.2 MPa. The products were analyzed on a gas chromatograph equipped with a flame-ionization detector and a Carbowax 20 M capillary column (25 m in length and 0.2 mm in diameter). 3. Results and Discussion In the current solvothermal synthesis, 1,2-propanediol is used as a solvent for cobalt acetate and a reducing agent for Co2+. It initially dehydrates to produce propionaldehyde, and the aldehyde then reacts with Co2+ in solution to form a Co nucleus.17,18 Ru acts as the extrinsic seed to facilitate the reduction of Co2+. Ru3+ can be easily reduced to metallic ruthenium by 1,2propanediol at temperatures as low as 140 °C,19 and thus, the current temperature (170 °C) does not alter its reduction kinetics. 3.1. Co Nanoflowers. Figure 1 shows an XRD pattern of the Co materials obtained under the typical conditions. The diffraction peaks at 2θ ) 41.5°, 44.4°, 47.4°, 62.5°, and 75.8° are assigned to the (100), (002), (101), (102), and (110) planes, respectively, of the hexagonal-close-packed Co phase (hcp, JCPDS card no. 5-727). Figure 2 shows SEM and TEM images of the Co material. Apparently, this material has a hollow spherical structure with a size of 20 µm, and it is assembled by small flowers having sizes of 0.5-1 µm. The magnified image indicates that the

Figure 1. XRD pattern of the Co nanoflowers.

Figure 2. (a,b) SEM and (c,d) TEM images of the Co nanoflowers.

flowers are composed of nanorods as petals radiating from the core in all directions. The core is about 200 nm in diameter, and the lengths of the petals are less than 300 nm. The diameters of the petals are in the range of 20-50 nm, with thin tips and thick roots. The maintenance of this flowery structure under ultrasonic treatment and electron beam irradiation suggests that the hierarchical structure is very stable. These flowers assembled from nanorods are robust, unlike the previously reported nanoflowers that are simply fabricated from aggregated blocks through magnetic interactions.11 3.2. Shape Evolution. The chemical composition and shape evolution of the Co intermediates during the synthesis of Co nanoflowers were monitored by XRD measurements (Figure 3) and TEM observations (Figure 4). When the synthesis was conducted for 1.5 h, the XRD pattern of the solid product, which presented as spheres with sizes of 1-2 µm, showed the typical diffractions of cobalt alkoxide (2θ ) 9.8° and 16.0°) (Figure 4a,b). Probably, this alkoxide precursor is produced by the reaction of propanediol with the Co2+ species in the solution, as confirmed by previous reports.17,18 When the synthesis was lengthened to 2.5 h, the diffraction peaks of the cobalt alkoxide decreased significantly, and weak diffractions assigned to a metallic Co phase appeared, suggesting that the reduction of

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Figure 3. XRD patterns of the Co intermediates during the synthesis of Co nanoflowers: (a) 1.5, (b) 2.5, (c) 3, (d) 5, and (e) 10 h. The experiments were conducted at 170 °C with 0.2 mol · L-1 hexadecylamine.

Co2+ in the solution occurred. In addition to the cobalt alkoxide precursor (Figure 4c), Co nanoparticles were also formed with a size range of 150-300 nm, and rodlike protrudings having a diameter of 15 nm radiated from the Co surface (Figure 4d). When the synthesis was extended to 3 h, the diffraction peaks of Co further intensified, and peaks of the (102) and (110) planes of Co appeared. This Co material is flowery with a size of less than 1 µm, and the size of the petals is 300-400 nm (Figure 4e). Each petal is composed of principal axes with surrounding salients of about 30 nm, showing a wheat-fringe-like shape (Figure 4f). At 5 h, the diffraction peaks of the cobalt alkoxide precursor disappeared almost completely, and the diffraction peaks of Co intensified greatly, indicating the end of Co2+ reduction. The flowers became uniform with a size of about 500 nm, and most of the petals grew into nanorods with a length of 300 nm and a diameter of 30 nm (Figure 4g,h). At 10 h, the diffraction peaks of Co were further strengthened, and almost all of the petals grew into regular nanorods, making the hyperstructure more uniform (Figure 4i,j). Figure 5 compares the thermal stabilities of the cobalt alkoxide precursor and the Co nanoflowers. Two distinct weight losses were observed at 389 °C (35%) and 452 °C (14%) for the precursor, mainly representing the decomposition of the cobalt alkoxide. Comparatively, the Co nanoflowers presented only a slight weight loss of 1.6% at 310 °C, assigned to the decomposition of hexadecylamine covering the surface.20 This small amount of fatty amine plays a crucial role in maintaining the morphology and chemical stability of Co flowers through strong absorption onto the Co surface, preventing it from oxidation.21 3.3. Hexadecylamine Concentration. The influence of the hexadecylamine concentration on the morphologies of the Co materials was further investigated. As shown in Figure 6, when there was no fatty amine, the Co materials presented a chainlike structure assembled by spheres with a size of about 500 nm. When the concentration of hexadecylamine was 0.02 mol · L-1, the Co materials showed an urchinlike shape having a size of 1-2.5 µm with rods protruding from the large core. The rods had a length of about 700 nm and a diameter of about 80 nm. As the concentration of hexadecylamine increased to 0.1 mol · L-1, the urchins vanished, but a cloudy structure formed. The cloud consisted of flowers with sizes of 0.5-1 µm. The flowers were mainly composed of petals having sharp tips and wide roots with lengths of about 500 nm and diameters of

Figure 4. TEM images of the Co intermediates during the synthesis of Co nanoflowers: (a,b) 1.5, (c,d) 2.5, (e,f) 3, (g,h) 5, and (i,j) 10 h. Insets in c and e indicate the cobalt alkoxide precursor. The experiments were conducted at 170 °C with 0.2 mol · L-1 hexadecylamine.

10-60 nm. The petals radiated from the core in all directions, which made the flowers have blooming appearance. As the concentration of hexadecylamine was further increased to 0.2 mol · L-1, the size of the flowers decreased to about 500 nm, and the length of the petals decreased to less than 500 nm, with diameters of 10-50 nm. At 0.4 mol · L-1, the nonuniform Co nanostructures were mainly composed of aggregated nanoparticles of 50 nm and nanorods with lengths of about 200 nm and diameters of 50 nm. Figure 7 compares the XRD patterns of the Co nanostructures synthesized at varying concentrations of hexadecylamine. With

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Figure 5. TG profiles of (a) the cobalt alkoxide precursor and (b) the Co nanoflowers. The samples were heated from room temperature to 725 °C at the rate of 5 °C/min under nitrogen flow.

increasing concentration of hexadecylamine, the intensities of the diffraction peaks of the hcp Co phase decreased and broadened. This is due to the size effect of the declining building blocks, which is further confirmed by the estimated mean crystalline sizes based on the characteristic diffraction planes of the Co materials (Table 1). The large size along the [002] direction, to some extent, reflects the anisotropic nature of the petals. It is obvious that the concentration of hexadecylamine significantly influences the size of the hierarchical Co nanostructure as well as the building block. When the concentration of hexadecylamine increases, the sizes of the Co nanoflowers and the subunits decrease, indicating coating-dependent growth kinetics in which the size of the hierarchical structure strongly depends on the size of the building block. Capping of the surfactant could direct the anisotropic growth of the crystals and also retard the growth rate by decreasing the surface energy of the facets, but simultaneously diminish the aggregation between the flowers as a result of the spatial repulsion of the surfactant, thus reducing the size of the flowers and promoting monodispersity. However, higher concentrations of hexadecylamine would cause deterioration of the flowers because of a heavy coating on the Co crystals, yielding nanoparticles and nanorods.22 Under the present conditions, an amine concentration of 0.2 mol · L-1 seems to be suitable for the synthesis of welldefined Co nanoflowers. 3.4. Formation Mechanism. The cobalt alkoxide precursor, which is initially produced by the combination of Co2+ in the solution with the solvent, vanishes gradually with the reduction of Co2+ in the solution. This solid intermediate, acting as a Co2+ reservoir, equilibrates the concentration of Co2+ between the solution and the solid and thus controls the nucleation and growth rate of the Co nanocrystals through a dynamic reduction process.17,18 Because this type of cobalt alkoxide precursor was also produced in the mixture of polyol and aqueous sodium hydroxide solution,15,16 it is highly possible that hexadecylamine here promotes the formation of the solid precursor because of its basic nature. The fabrication of Co nanoflowers might involve three stages. As illustrated in Figure 4, a core of 150-300 nm with short nanorods spherically protruding from its surface is formed at an early stage during the synthesis. The short rods continuously grow into longer ones, and the longer rods then act as the substrate for the latitudinal growth of the salients along their side planes, resulting in secondary growth of the

Figure 6. TEM and SEM images of the Co nanomaterials synthesized at varying concentrations of hexadecylamine: (a,b) 0, (c,d) 0.02, (e,f) 0.1, (g,h) 0.2, and (i,j) 0.4 mol · L-1.

core to form the wheat-fringe-like structure, which is constructed by the principal axis (large nanorods) and the surrounding salients on the side planes of the nanorods. Similarly, the salients also provide suitable sites for the tertiary growth of the fringes to the final nanorods. These rods protrude as petals from the core, forming the flowery shape. Therefore, the formation of Co flowers presents an interesting hierarchical growth mode, which was previously used to describe the formation of ZnS combs23 and CdSe saws,24 involving the initial formation of the primary nanobelts and the subsequent growth of the rods/teeth from the edges of the belts.

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Figure 7. XRD patterns of the Co nanomaterials synthesized at varying concentrations of hexadecylamine: (a) 0.02, (b) 0.1, (c) 0.2, and (d) 0.4 mol · L-1.

TABLE 1: Mean Crystal Sizes along the [100], [002], and [101] Directions (Lm) for the Co Materials

Figure 8. SEM images of the flower-like Co nanostructures synthesized using (a) dodecylamine, (b) tetradecylamine, (c) hexadecylamine, and (d) octadecylamine as the surfactant.

Lm (nm) samplea a b c d a

[100]

[002]

[101]

33 28 26 22

37 31 36 29

31 24 25 21

Sample labels correspond to those in Figure 7.

Fatty amine is widely used to synthesize metal nanostructures because of its compatible chemical properties such as its basic nature and its affinity to metal crystals.9,25 The amine preferentially coordinates along the specific facet of a growing metal nanocrystal to obtain diverse nanostructures. Here, hexadecylamine might also act as a structure-directing agent in the formation of the Co nanoflowers, in addition to promoting the formation of the cobalt alkoxide precursor in the initial stage. It selectively absorbs onto the surface of the growing Co crystals and induces anisotropic growth along a special direction to form the primary nanorods and, subsequently, the secondary and tertiary growing processes of the flowers. This structure-directing role is further confirmed by the fact that only spherical Co nanoparticles were produced when no amine was used (Figure 6a,b). 3.5. Fatty Amine. Figure 8 shows the influence of the alkyl chain length of the fatty amine on the shape of the Co nanoflowers. When dodecylamine was used, the product appeared as flat flowers (0.5-2 µm) that were composed of rods with lengths of 200-1000 nm and diameters of about 50 nm. These nanorods radiated from the core in one plane as petals, but some of them (especially the short rods) protruded radiantly from the same plane as pistils. As the alkyl carbon number was increased to 14 (tetradecylamine), the sizes of the flowers decreased to 0.5-1 µm. The lengths and the diameters of the petals also decreased to about 500 and 30 nm, respectively. The petals had a smooth surface with sharp tips. When the carbon number was further increased to 16 (hexadecylamine), uniform Co flowers with a size of about 500 nm were obtained. The length of the petals was about 500 nm, and the diameter increased to about 50 nm. However, when octadecylamine with a carbon number of 18 was used, the flowers had a size of about

500 nm and consisted of nanorods with a length of about 500 nm and a diameter of 30 nm. These nanorods had a hexagonal cross section with an edge of 15 nm. Apparently, the alkyl chain length of the fatty amine regulates the size of the petal and thus determines the size of the hyperstructure. Because these amines have very similar basicities, their effects in promoting the formation of the cobalt alkoxide precursor should be very similar. Therefore, the structure-directing role should be attributed to the variation of the carbon chain length. It seems that the absorption of the fatty amine on the Co nanocrystal is a dynamic process and the growth rate of the crystal decreases with increasing carbon chain length in the amine. Fatty amine with a longer carbon chain has been previously shown to provide a low exchange frequency between the chains absorbed on the crystal surface and those in solution.26 As a result, increasing the carbon number of the amine could lead to relatively slower growth and thus decrease the sizes of the flowers and the petals. 3.6. Catalytic Performance. The Co nanoflowers were used to catalyze the hydrogenolysis of glycerol to propylene glycol, which is one of the most promising approaches for the effective utilization of glycerol, the major byproduct in biodiesel production by transesterification of vegetable oil or animal fat. Noble metals such as Ru, Rh, and Pt are usually used to catalyze this reaction, but the yield of propylene glycol is no more than 20%, even when conducted at high temperature and hydrogen pressure.27 Using Co nanoflowers as the catalyst, the glycerol conversion was 49%, and the selectivity of propylene glycol (1,2-/1,3-propanediol) was 58%, giving a propylene glycol yield of 28% under these very mild conditions. Comparatively, conventional Co nanoparticles (Figure 6a,b) exhibited a glycerol conversion of 20% and a propylene glycol selectivity of 54%, and the yield of propylene glycol was only 10.8%. The quite promising catalytic properties of the Co nanoflowers are probably associated with the unique morphology, especially the primary nanorods, which usually tend to preferentially expose more reactive planes than conventional nanoparticles.28 This interesting catalytic performance clearly demonstrates the potential application of Co nanoflowers in heterogeneous catalysis.

Hierarchical Growth of Co Nanoflowers 4. Conclusions Co nanoflowers constructed from nanorods were synthesized through a facile solvothermal route using Ru as a heterogeneous nucleation agent. Cobalt alkoxide, which is produced in the initial stage in the presence of the basic amine, mediates the nucleation and growth rate of the Co nanocrystals to form a flowery structure through a dynamic reduction process. Hexadecylamine also acts as the structure-directing agent and regulates the growth of the flowery nanostructure through its strong absorption onto the Co nanocrystals, leading to a hierarchical growth mode. Both the concentration and the type of the fatty amine markedly influence the size of the nanoflowers by controlling the growth rate of the primary nanorods. With proper selection of the concentration or the alkyl chain length of the fatty amine, the length and diameter of the petals (nanorods) can be easily controlled to about 500 and 50 nm, respectively, thus producing the nanoflowers with a size of about 500 nm. The Co nanoflowers show a quite promising catalytic performance in hydrogenolysis of glycerol, indicating their potential application in heterogeneous catalysis. References and Notes ¨ lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (1) (a) CO (b) Huang, Y.; Lieber, C. M. Pure Appl. Chem. 2004, 76, 2051. (c) Murray, C. B.; Sun, S. H.; Doyle, H.; Betley, T. Mater. Res. Soc. Bull. 2001, 26, 985. (2) (a) Min, Y. J.; Akbulut, M.; Kristiansen, K.; Golan, Y.; Israelachvili, J. Nat. Mater. 2008, 7, 527. (b) Tripp, S. L.; Pusztay, S. V.; Ribbe, A. E.; Wei, A. J. Am. Chem. Soc. 2002, 124, 7914. (3) (a) Hou, Y. L.; Kondoh, H.; Ohta, T. Chem. Mater. 2005, 17, 3994. (b) Wang, X.; Yuang, F. L.; Hu, P.; Yu, L. J.; Bai, L. Y. J. Phys. Chem. C 2008, 112, 8773. (4) Ewers, T. D.; Sra, A. K.; Norris, B. C.; Cable, R. E.; Cheng, C. H.; Shantz, D. F.; Schaak, R. E. Chem. Mater. 2005, 17, 514. (5) (a) Pang, S. F.; Kondo, T.; Kawai, T. Chem. Mater. 2005, 17, 3636. (b) Zhao, N. N.; Wei, Y.; Sun, N. J.; Chen, Q.; Bai, J. W.; Zhou, L. P.; Qin, Y.; Li, M. X.; Qi, L. M. Langmuir 2008, 24, 991. (c) Qian, L.; Yang, X. R. J. Phys. Chem. B 2006, 110, 16672. (6) (a) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. H. Science 2000, 290, 1131. (b) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X. D.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956. (c) Jun, Y. W.; Choi, J. S.; Cheon, J. Chem. Commun. 2007, 1203. (7) (a) Sun, S. H.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (b) Wang, Z. L.; Dai, Z. R.; Sun, S. H. AdV. Mater. 2000, 12, 1944. (8) (a) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M. C.; Casanove, M. J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286. (b) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.;

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