Preparation and Characterization of ZnO Hollow Spheres and ZnO

products obtained show well photocatalytic activities and ZnO-carbon composite “dandelions” have the highest photocatalytic efficiency in all prod...
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6706

J. Phys. Chem. C 2007, 111, 6706-6712

Preparation and Characterization of ZnO Hollow Spheres and ZnO-Carbon Composite Materials Using Colloidal Carbon Spheres as Templates Xi Wang,†,‡ Peng Hu,†,‡ Yuan Fangli,*,† and Lingjie Yu†,‡ State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: January 16, 2007; In Final Form: March 5, 2007

In this paper, a general strategy for synthesis of carbon-based core/ZnO-shell-type composite and hollow materials is reported. These materials include colloidal carbon spheres@ZnO nanospheres, ZnO-carbon composite “dandelions”, ZnO-carbon branched “dandelions”, Au-ZnO-carbon composite “dandelions”, and size-tunable hollow ZnO spheres. As-synthesized composite “dandelions” exhibited a color change, which originated from the surface plasmon resonance of gold nanoparticles, showing their optical application. All products obtained show well photocatalytic activities and ZnO-carbon composite “dandelions” have the highest photocatalytic efficiency in all products.

Introduction Colloidal micro- and nanospheres have been of continuous research interest, since their intrinsic properties can be well tuned by changing parameters such as diameter, chemical composition, bulk structure, and crystallinity. Among them, colloidal carbon spheres (CCSs) have been paid considerable attention due to their potential applications as reinforcement materials for rubber, supports for catalysts and lubricating materials,1 templates, and anodes in second lithium ion batteries.2 The success of these applications strongly depends on the availability of colloidal spheres with tightly controlled size and surface properties.3-5 Nowadays, the design and controlled fabrication of nanomaterials with functional properties are required to meet the everincreasing demands (structural and compositional complexity etc.).6 Although there has already been much progress in the synthesis and assembly of nanoscale materials such as nanoparticles, nanotubes, nanowires, nanorods, and nanobelts,7 effective strategies to produce tailored nanostructured materials are still required. Surface coating (or surface modification) has been recognized as one of the most intriguing methods to build tailored nanomaterials.6,8 Coatings can alter the charge, functionality, and reactivity of the surface, and enhance a material’s thermal, mechanical, or chemical stability. Typically, surface coating involves tailoring the surface properties of the nanomaterials, often accomplished by coating them on the surface of a preferred material. For instance, the immobilization of transition metal or their oxide nanoparticles on preferred spheres could allow them to retain high catalytic or biocatalytic activity on recycling, enhance their pH and temperature stability, and enable easy separation from reaction media for reuse.9-11 In our studies, ZnO nanoparticles and nanorods have been chosen as the coating materials for the following two reasons: (a) ZnO is an important multifunctional semiconductor with unique electrical, optoelectronic, and luminescent properties, and many important practical applications;12 and (b) ZnO nanomaterials * Corresponding author. Phone: +86-10-82627058. Fax: +86-1062561822. E-mail: [email protected]. † Institute of Process Engineering. ‡ Graduate University of the Chinese Academy of Sciences.

such as nanoparticles, nanowires,12 nanorods,13-15 nanotubes,16-17 and nanobelts18-20 represent a broad class of nanoscale building blocks that have been used to assemble functional devices such as lasers,21 photodetectors,22 field emitters,23-24 and gas sensors.25 It is expected that the surface coating of CCSs with a preferred ZnO material enables us to construct carbon-based core/ZnO-shell-type composites with novel optical, photocatalytic and electrical properties. In this paper, CCSs@ZnO nanoparticles, ZnO-carbon composite dandelions, and AuZnO-carbon composite dandelions are successfully achieved by one-pot hydrothermal reaction. Another important goal for material scientists has been tailoring the structure to obtain particular morphologies for more and more advanced applications.26-35 In particular, hollow structures show a lower density, higher surface area, and distinct optical property, which have resulted in various applications in drug delivery, plasmonics and catalysis.36-38 As an important wide-band gap semiconductor with Eg ) 3.37 eV, ZnO has also been of great research interest as a result of its unique applications in many fields.18,39-41 For these applications, the morphology control of ZnO structures is critical to tune their chemical and physical properties to the appropriate ones. Recently, many research efforts have been made to prepare ZnO with a hollow structure.42-45 For example, hollow ZnO spheres have been synthesized by ethanol droplets as soft templates or by employing the vapor deposition method. However, until now, it still remains a great challenge to develop facile methods for creating size- and surface-tunable hollow spherical structures of semiconductor materials. In this paper, size- and surfacetunable hollow ZnO spheres were synthesized by using CCSs as templates. Wide band gap semiconductors are widely used as effective photocatalysts for the degradation of organic materials.46-47 By consideration that photochemical reactions mainly take place on the surface of the catalyst, ZnO materials with different morphologies are believed to have different photocatalytic properties. In this paper, photocatalytic abilities of the synthesized ZnO hollow spheres and ZnO-carbon composite materials were characterized.

10.1021/jp070382w CCC: $37.00 © 2007 American Chemical Society Published on Web 04/14/2007

ZnO Hollow Spheres and ZnO-Carbon Materials

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SCHEME 1: Schematic Illustration of the Synthesis of ZnO Products: (A) Hollow Spheres, (B) ZnO-Carbon Composite Dandelions and Au-ZnO-carbon Composite Dandelions, and (C) CCSs@ZnO Nanoparticles

Experimental All the analytical chemicals were purchased from Beijing Chemical Reagents Company and used without further purification. Synthesis of ZnO Hollow Spheres. The colloidal carbon spheres (CCSs) were prepared according to the literature.48 Assynthesized CCSs were dispersed in a 50 mL of Zn(AC)2 (0.5 M), and then ultrasonicated for 40 min. The suspension obtained was aged for 24h, and then followed with three cycles of centrifugationwash/redispersion in water. The oven-dried sample was heated to 480-550 °C in a furnace in air for 1 h, and then naturally cooled to room temperature. Synthesis of ZnO-Carbon Composite “Dandelions”. Asprepared carbon spheres were dispersed in a 50 mL solution of Zn(AC)2 (0.5 M), and then ultrasonicated for 40 min. The suspension obtained was aged for 24 h, and then followed with three cycles of centrifugation/wash/redispersion in water. The composite carbon spheres obtained were added in the mixed solution,49 which included 50 mL of ethanol, 10 mL of deionized water, 10 mL of ethylenediamine, and 6 mL of alkali solution of zinc. (The alkali solution of zinc was prepared by dissolving 5.5 g of Zn(AC)2 and 20 g of NaOH in deionized water to form a 50 mL of solution.) Then above mixture were sealed in a 100 mL Teflon-lined autoclave and maintained at 180 °C for 624 h. Synthesis of Au-ZnO-Carbon Composite Spheres. A 0.2 g sample of ZnO-carbon composite dandelions synthesized by above method was dispersed in 50 mL of deionized water; then a mixture with 0.2 g of sodium citrate, 3 mL of 0.02 M HAuCl4, and 0.8 g of ascorbic acid was added into above solution with vigorous stirring at room temperature for 10 min. Then the mixtures were ultrasonicated for 10 min, aged for 1 h, and subjected to four centrifugation/water wash/redispersion cycles to remove Au ions. Synthesis of CCSs@ZnO Nanoparticles. As-prepared carbon spheres were dispersed in a 50 mL solution of Zn(AC)2 (0.5M), and then ultrasonicated for 40 min. The suspension obtained was aged for 24 h, and then followed with three cycles of centrifugation/wash/redispersion in water. The composite carbon spheres were added in the mixed solution included 60 mL of ethanol and 3 g of Zn(AC)2 and 5 mL of ethanolamine. Then above mixture were sealed into a 100 mL Teflon-lined autoclave and maintained at 180 °C for 10 h. Methylene blue (MB) decomposition testings were carried out to study photocatalytic ability of as-synthesized products. On the basis of the Beer-Lambert law,50 the methylene blue aqueous solution with 10-5 M is linearly proportional to the intensity of the measured spectrum. In the present study, the MB (5.3 × 10-5 M) solutions containing 0.17 wt % of well-

dispersed as-synthesized products were illuminated employing a 6 W xenon UV-lamp. Photocatalytic decomposition of the MB solutions was characterized by a UV-vis spectrometer (HITACHI U-3010). The wavelengths that are prevailingly absorbed by MB are 665, 292, and 246 nm, whereas the maximum peak occurs at 665 nm. The extent of decomposition was calculated from the integrated area of the peak at 665 nm. The phases of the products were characterized by an X-ray diffractometer (Philips X’Pert PRO MPD) through the 2θ-range from 10° to 90° at a scan rate of 0.02° s-1 using Cu KR radiation and operated at 40 kV and 30 mA. The morphology of the particles was investigated by both field-emission scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, Hitachi H-800). The detailed morphology and structural characterization were investigated by high-resolution transmission electron microscope (HRTEM) and selected-area electron diffraction (SAED) in the same transmission electron microscope (TEM, JEOL JEM-2010). The specific surface area of the resultant products was determined by a nitrogen gas adsorption instrument based on the BET method. Results and Discussion The surface of synthesized carbon spheres has a distribution of OH and CdO groups,51 which are formed from non- or just partially dehydrated carbohydrates and easy to modify with heteroatom. Elemental analysis reveals the products are mainly composed of carbon; the remaining mass can be attributed to the oxygen and hydrogen atoms in the hydrophilic shell. The remarkable property of CCSs is intense absorption to metal ions due to negative charge on their surface. Because of their intense absorption to metal ions, CCSs should have a more pronounced application in biochemistry, diagnostics, and drug delivery, and especially as templates for hybrid core/shell structures or hollow/porous materials. Here, size-tunable hollow ZnO nanospheres, ZnO-carbon composite “dandelions”, Au-ZnO-carbon composite “dandelions”, and CCSs@ZnO nanoparticles were successfully synthesized using the CCSs as templates. The strategy used to obtain above products is shown schematically in Scheme 1, which can be divided into two steps, that is the adsorption of zinc ions and the successive growth of ZnO. In detail, Scheme 1A shows the synthesis of hollow ZnO spheres: After the adsorption of zinc ions from solution into surface layer, the carbon cores were removed by the calcination of composite spheres in air, which results in ZnO hollow spheres. Scheme 1B shows the synthesis of ZnO-carbon composite “dandelions” structure: after the adsorption of zinc ions from solution into surface layer, ZnO nanorods grown on the surface of CCSs under hydrothermal

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Figure 1. SEM images of as-synthesized ZnO hollow products with different diameters: (A) ∼200 nm, 550 °C; (B) ∼500 nm, 550 °C; (C) ∼900 nm, 550 °C; (D) ∼1.2 µm, 480 °C; (E) ∼1 µm, 500 °C.

conditions, which results in ZnO-carbon composite “dandelions” structures; Au nanoparticles can be loaded on the surface of the above ZnO nanorods. Scheme 1C shows the synthesis of CCSs@ZnO nanoparticles: after the adsorption of zinc ions from solution into surface layer, ZnO nanosparticles can be coated on the surface of CCSs under hydrothermal conditions.52 The size of synthesized ZnO hollow spheres can be controlled by the diameter of starting templates. Figure 1A-C reveals the typical SEM images of hollow spheres synthesized using carbon templates with different size, from which we can conclude that the size of final products could be closely controlled in the range from 200 nm to 1.5 µm with an almost constant shrinkage ratio about 40% by alternating the size of carbon templates from 350 nm to 4 µm. When the carbon spheres with a diameter of 350 nm were used as templates, the hollow spherical products with a diameter of 200 nm were obtained, as shown in Figure 1A. XRD analysis, as shown in Figure S1(Supporting Information), showed that the products were ZnO. However, the size of ZnO products increases to 500 nm, while 800 nm of carbon templates was used (shown in Figure 1B). It can be observed in the image inserted in Figure 1B that the partially broken particle clearly shows a hollow interior, indicating the formation of hollow ZnO spheres. When the size of templates increased to 1.5 µm, ZnO spheres with a diameter of 900 nm were dominant in the final products, as shown in Figure 1C. The hollow structure could be evidenced by the cracked particle in the inset image of Figure 1C. In the typical synthesis process, calcination temperature plays key roles in the surface morphology of hollow products. That is, the shell of ZnO hollow spheres gradually evolves from rough to smooth as the temperature increases. Figure 1C-E shows the typical SEM images of ZnO hollow spheres obtained at

Wang et al.

Figure 2. SEM images of as-synthesized productes: (A) CCSs, (B) CCSs@ZnO nanoparticles, and (C) detailed structure of individual CCSs@ZnO nanoparticles.

different temperature with the same templates (1.5 µm). As shown in Figure 1D, as-synthesized ZnO spheres were composed of small nanocrystals, and their surface seemed to be rough when calcination temperature was 480 °C. With the temperature increasing, such as 500 °C, the surface of corresponding products became smooth, as shown in Figure 1E. Smoother surface was obtained while the temperature arrived at 550 °C, as shown in Figure 1C. In addition, the shrinkage ratio varies with the calcination temperature. In our experiments, the shrinkage ratio is ∼25%, ∼30%, and 40% while the temperature changed from 480 and 500 to 550 °C, respectively. The carbon templates, which were selected to synthesize CCSs@ZnO nanoparticles and ZnO-carbon composite “dandelions”, are showed in Figure 2A, and their diameter is about 2.5 µm. Their detailed surface morphology shown in Figure S2 (see Supporting Information) indicates that their surface is smooth. Figure 2B shows the general morphology of synthesized CCSs@ZnO nanoparticles. From the fine structure of the individual particle shown in Figure 2C, one can see that the surface of products was aggregated from small nanoparticles. Figure S3 (Supporting Information) showed the XRD spectrum, which indicates that the products were composed of ZnO and carbon. That is, ZnO nanoparticles were successfully coated on the surface of CCSs. Figure 3 shows the morphology of as-synthesized “dandelions”, from which one can see that ZnO nanorods grow on the surface of carbon spheres. As shown in Figure 3A-C, the ZnOcarbon composite dandelions comprise numerous one-dimensional nanorods with their c-axes (SAED) pointing toward the center of dandelions. The diameter and length of these nanorods

ZnO Hollow Spheres and ZnO-Carbon Materials

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Figure 3. SEM images of ZnO-carbon dandelions obtained at different reation time: (A) 6, (B) 10, (C) 15, and (D) 24 h (inset image: scale bar, 300 nm). (E) HRTEM and SAED(inset) images.

are in the ranges of 50-300 nm and 0.5-5 µm, respectively, depending on the synthetic conditions. For example, the reaction time was selected as variable. When the time is 6 h, the thickness of ZnO shell (equal to the nanorod length) in Figure 3Ais estimated to be about 0.5-1 µm. In comparison, as the reation time is arrived at 10 and 15 h, the thickness of ZnO shell, as shown in Figure 3B,C, is estimated to be about 2-3 and 3-5 µm, respectively. As the time was increasing to 24 h, the secondary direction growth was occurred, as shown in Figure

3D, and the branched structures orthogonally grew along the c-axis. From the magnified image inserted in Figure 3D, one can see that the diameter and length of these nanorods synthesized by the secondary growth are in the ranges of 2050 and 100-500 nm, respectively. The synthesis of branchstructured ZnO by vapor method has been reported by many researchers,53-54 but the synthesis in solution has seldom been reported. The high-resolution TEM (HRTEM) image in Figure 3E shows a clean and perfect structure without dislocation and

6710 J. Phys. Chem. C, Vol. 111, No. 18, 2007 stacking faults observed. The lattice spacing of adjacent planes is 0.52 nm. The inset SAED pattern was taken from the same nanorod, as shown in the inset image of Figure 3E. It, together with the HRTEM, confirms that the synthesized ZnO nanorods are single crystalline and grow along the [0001] direction.55-56 As shown in Figure S4(Supporting Information), high yields of ZnO dandelions had been achieved in this work. In addition to the control of the above morphology, composition of the prepared ZnO-carbon dandelions could also be tailored. The individual ZnO-carbon dandelion and the detailed morphology assembled by ZnO nanorods are shown in panels A and B, respectively, of Figure 4. In comparison, Figure 4C shows such a synthetic flexibility in fabricating Au-ZnOcarbon nanocomposites where Au nanoparticles (5-30 nm) have been evenly deposited onto the surfaces of nanorods. Interestingly, the tips of nanorods are also decorated with Au nanoparticles as shown in Figure 4C. The chemical compositions of the nanoparticles were analyzed by energy dispersive X-ray spectroscopy (EDX). The results shown in Figure 4D illustrate that only the characteristic peaks of Au, Zn, O, and C were detected. This, together with the XRD analysis of ZnO-carbon dandelions in Figure S5(Supporting Information), indicated the deposition of Au nanoparticles on ZnO-carbon dandelions. The resultant Au-ZnO-carbon dandelions show a distinct color change (inset image in Figure 5). The UV-vis spectra of synthesized ZnO-carbon dandelions and Au-ZnO-carbon dandelions were characterized using a UV-vis spectrometer (HITACHI U-3010). The products were dispersed in ethanol solution. Consistent with this, a surface plasma resonance peak of Au is observed at ca. 530 nm (Figure 5), apart from the UV absorption of ZnO at 373 nm (not shown).49,57-58 One of the structural features of these dandelion-like materials is that oneor two-dimensional building blocks (anisotropic) can be selectively aligned on the surface of CCSs to form highly symmetrical three-dimensional conformations (isotropic), which may promise us new types of applications. For example, this class of metal-oxides and metal-semiconductor composites has potentially applications such as storage of light-generated electrons,59 three-dimensional lasing,21 and new ways of photocatalysis.60-61 The formation of CCSs@ZnO nanoparticles and ZnO-carbon composite dandelions can be attributed to the following aspects. First, the concentration of zinc ions near or on the surface of CCSs is higher than that of solution due to the absorption of metal ions. As a result, the concentration of zinc ions near or on the surface of CCSs first arrives at supersaturation. Second, the carbon spheres might act as heterogeneous nucleation sites to induce heterogeneous nucleation on their surface owing to the lower critical nucleation concentration needed for heterogeneous nucleation than that for homogeneous nucleation. Finally, the mechanism of forming ZnO nanoparticles and nanorods is the same as bulk ZnO nanocrystals.43,52 Figure 6 shows degradation of the MB solution under the UV irradiation as a function of time for various as-synthesized products. Moreover, we also analyzed the photocatalytic degradation of MB by ZnO nanorods (Figure S6, synthesized in our laboratory62) and Degussa P25 titania. The mechanism of photocatalytic degradation of the MB solution for ZnO nanoparticles is well elucidated in the literature.63 From the data in Figure 6, we can conclude that the decomposition rates for all samples are high within the first 10 min. After irradiated for 10 min, the decomposition rate of MB for P25 titania is 46.8%; in comparison, this value is 43.3%, 45.4%, 58.1%, and 90.7% for ZnO nanorods, CCSs@ZnO nanoparticles, ZnO hollow spheres

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Figure 4. (A) SEM images of ZnO-carbon composite dandelions. (B) Detailed structures assembled by ZnO nanorods. (C) Magnified SEM image of Au nanoparticles loading on the surface of ZnO nanorods (dandelions). (D) EDX pattern of Au nanoparticles loading on the surface of ZnO nanorods (dandelions).

and dandelions, respectively. As MB concentration decreases, probability of MB molecules reacting with ZnO nanoparticles decreases and consequently decomposition rate decreases. When the irradiated time is increased to 20 min, the decomposition rate of MB for P25 titania is 59.5%; it is 56.7%, 58.8%, 73.7%, and 100% for ZnO nanorods, CCSs@ZnO nanoparticls, ZnO

ZnO Hollow Spheres and ZnO-Carbon Materials

J. Phys. Chem. C, Vol. 111, No. 18, 2007 6711 area, the sample has the lower MB decomposition rate, which conforms that the morphology of particles plays more important role on their photocatalytic ability. In our studies, MB decomposition rate of ZnO dandelions is higher than other products with higher BET surface area (such as, ZnO hollow spheres and ZnO nanorods); even though possessing the lowest BET surface area, CCSs@ZnO nanoparticls has the higher MB decomposition rate than ZnO nanorods. These results further indicate that the morphologies of as-synthesized products considerably affect their photocatalytic activity. Conclusions

Figure 5. UV-vis spectra of ZnO and Au-ZnO-carbon dandelion suspensions (in water). Color change upon the Au deposition (inset image): (a) white (ZnO-carbon dandelions), (b) purple (Au-ZnOcarbon composite dandelions).

In summary, using colloidal carbon spheres (CCSs) as templates, size-tunable ZnO hollow spheres, CCSs@ZnO nanoparticles, ZnO-carbon composite dandelions, ZnO-carbon branched dandelions, and Au-ZnO-carbon dandelions were synthesized. The obtained hybrid particles of Au-ZnO-carbon dandelions exhibited a color change, which originated from the surface plasma resonance of gold nanoparticles, showing their optical application. Photocatalytic results indicate that both the BET surface area and the morphlogies of synthesized products can affect their photocatalytic efficiency. ZnO-carbon composite dandelions have the excellent photocatalytic properties. It is expected that these novel carbon-based core/ ZnO-shelltype composites and size-tunable hollow ZnO spheres have more great applications in many fields. The proposed facile strategy for the preparation of core/ shell or hollow structures can be extended to other materials with similar structures. Supporting Information Available: Detailed morphology and structure analysis of synthesized products. Figure S1 shows the XRD pattern of synthesized ZnO hollow spheres. Figure S2 shows the detailed surface structure of colloidal carbon spheres. Figure S3 reveals the XRD of products shown in Figure 2B of the manuscript. Figure S4 shows the morphology of synthesized ZnO-carbon composite dandelions with high yields. Figure S5 shows the XRD pattern of ZnO-carbon composite dandelions. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Photocatalytic activities of synthesized products.

TABLE 1: Specific Surface Area (BET) and Chemical Analysis of As-Synthesized Products specimen

ZnO dandelions

ZnO hollow spheres

CCSs@ZnO nanoparticls

ZnO nanorods

BET (m2 g-1)

8.19

18.7

5.89

12

hollow spheres, and dandelions, respectively. The overall decomposition rates associated with individual specimens decrease in the order: ZnO dandelions > ZnO hollow spheres > P25 Titania > CCSs@ZnO nanoparticls > ZnO nanorods. That is, the photocatalytic performance of the ZnO dandelions is much better than commercial Degussa P25 titania and others’. It just took 10 min to decompose 90.7% of the MB solution for ZnO dandelions; however, it took 50 min for P25 Titania to reach the same value. In general, the catalytic activity of a photocatalyst should be closely related to its BET surface area, morphology and so on.63-66 The dependence of the photocatalytic activity of as-synthesized products seems to be a synthetic result that reflects these factors. On one hand, the products with higher BET surface area should have higher photocatalytic activity, because the enhancement in the BET surface area might increase the reactant adsorption. As shown in Table 1, the orders of the photocatalytic efficiency, ZnO hollow spheres > CCSs@ZnO nanoparticls and ZnO hollow spheres > ZnO nanorods, are according with the above conclusion. On the other hand, note that even though possessing the higher BET surface

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