Self-Assembly of Clewlike ZnO Superstructures in the Presence of

J. Phys. Chem. C 2007, 111, 9729-9733

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Self-Assembly of Clewlike ZnO Superstructures in the Presence of Copolymer Peng Bai,†,‡ Pingping Wu,†,§ Zifeng Yan,‡ Jinkai Zhou,† and X. S Zhao*,†,§ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, and Nanoscience and Nanotechnology InitiatiVe, National UniVersity of Singapore, Singapore 117576, and State Key Laboratory for HeaVy Oil Processing, PetroChina Key Laboratory of Catalysis, China UniVersity of Petroleum, Dongying 257061, People’s Republic of China ReceiVed: February 16, 2007; In Final Form: May 9, 2007

Clewlike ZnO superstructures were synthesized using a copolymer-controlled self-assembly method in the presence of urea. The structural properties were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction techniques. It was observed that the concentrations of the copolymer and urea are the key parameters determining the superstructure morphology. Experimental data also showed that the copolymer has played a dual function in the self-assembly process; namely, one is to control the oriented attachment of the nanoparticles, and the other is to stabilize the superstructures. Room-temperature photoluminescence data showed interesting optical properties of the ZnO superstructures.

Introduction Because of its wide direct band gap (3.37 eV), high excitation binding energy (60 meV), and piezoelectric properties, zinc oxide (ZnO) is an important semiconductor for a variety of applications, ranging from piezoelectric transducers1 to varistors,2 photocatalysts,3 photovoltaic devices,4 optical waveguides,5 surface acoustic wave devices,6 phosphors,7 UV-light emitters,8 and solar cells.9 In addition, ZnO exhibits sensitivities to many gaseous species and thus is suitable for sensing applications.10 Moreover, ZnO is biocompatible for biomedical applications.11 Particle size and morphology can critically determine the properties of ZnO crystals. Thus, a great deal of research effort has been devoted to the rational control over the shape, size, spatial orientation, and morphology of ZnO structures. Various ZnO nanostructures, including nanobelts,12-14 nanowires,15 nanotubes,16 nanorods,17 nanoribbons,18 nanopins,19 nanosheets,20 nanohelixes,21 nanocables,22 and nanorings,23 have been reported over the past few years. The synthesis methods for these nanostructures include thermal evaporation,15,18,24 vapor-liquidsolid growth,25 laser ablation,26 template approaches,27-28 and solution-based self-assembly.17,29 Among these demonstrated methods, solution-based self-assembly offers a number of advantages over the other methods, such as mild synthesis conditions and low cost. For instance, by using a proper agent, the nucleation and growth rates of ZnO superstructures can be manipulated.30 While the synthesis of one-dimensional (1D) ZnO nanostructures has been widely explored, three-dimensional (3D) ZnO superstructures have rarely been reported due to the difficulties in the rational control over the assembly of the primary particles into 3D superstructures.30-32 Gao and co-workers30 described * To whom correspondence should be addressed. Phone: 65-65164727. Fax: 65-67791936. E-mail: [email protected]. † Department of Chemical and Biomolecular Engineering, National University of Singapore. ‡ China University of Petroleum. § Nanoscience and Nanotechnology Initiative, National University of Singapore.

the synthesis of ZnO-based hollow microspheres using a biopolymer-assisted assembly method. Mo et al.31 reported the self-assembly of ZnO nanorods and nanosheets into hollow microhemispheres and microspheres through the hydrothermal thermolysis of a zinc ethylenediamine-derived complex precursor in the presence of a solution-soluble polymer. According to Liu et al.,32 hierarchical ZnO superstructures with ringlike nanosheets standing on spindlelike rods were synthesized using Zn5(OH)8Cl2‚H2O as the precursor. In general, the abovementioned methods usually involve a special polymer as the morphology-controlling agent and/or a complicated synthesis process of a unique precursor. Therefore, simple and effective approaches by using ordinary inorganic salts in the presence of common organic additives are strongly desirable for the fabrication of 3D ZnO superstructures. Herein, we describe a mild, low-cost, and environmentally benign solution-based self-assembly approach to the synthesis of 3D clewlike ZnO superstructures. To the best of our knowledge, such complex superstructures have not been reported, and the complex structures display interesting physical and chemical properties. In addition, the formation mechanism of the ZnO superstructures will be discussed. Experimental Section Materials Synthesis. In a typical synthesis, 2.32 g of (PEO)20(PPO)70(PEO)20 (P123, Aldrich, typical Mn ) 5800) was dissolved in 65.0 mL of deionized water to form a clear solution, to which 5.95 g of Zn(NO3)2‚6H2O (Merck, 98.5%) was then added. After the salt was totally dissolved, 7.2 g of urea (ACS reagent, Sigma-Aldrich, 99.0-100.5%) was added. The final mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 100 °C in an oven for 24 h. The white precipitate was filtered off, washed with deionized water, and dried at 60 °C for 24 h. The solid was calcined in air at 500 °C for 2 h with a heating rate of 1 °C/ min. The effect of the synthesis recipe on the solid morphology was investigated by changing the amount of one composition matter while maintaining the rest to be constant.

10.1021/jp0713256 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007

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Figure 1. XRD patterns of (a) Zn5(CO3)2(OH)6 and (b) ZnO obtained after calcination of Zn5(CO3)2(OH)6 at 500 °C for 2 h.

Characterization. The crystalline phases of the solid samples were characterized using the X-ray diffraction (XRD-6000, Shimadzu, Japan) technique with a Cu KR radiation of wavelength λ ) 0.15418 nm. The microscopic features of the solids were observed with a JEOL JSM-5600LV scanning electron microscope operated at 15 kV, a field-emission scanning electron microscope (JSM- 6700F, JEOL Japan) operated at 5 kV, a transmission electron microscope (JEM 2010 from JEOL) operated at 200 kV, and a field-emission transmission electron microscope (JEM 2010F, JEOL, Japan) operated at 200 kV. UV-vis spectra were measured on a UV-vis-NIR scanning spectrophotometer (Shimudzu, UV-3101 PC) with an ISR-3100 integrating sphere attachment and BaSO4 as an internal reference. Photoluminescence (PL) spectra were collected on a Perkin-Elmer luminescence spectrometer (LS-50B) using a solid accessory. The sample was evacuated at 473 K for 2 h before the PL measurement. Results and Discussion Figure 1a shows the X-ray diffraction (XRD) patterns of a ZnO sample before and after calcination. According to JCPDS card no. 19-1458, the sample before calcination is a monoclinic zinc carbonate hydroxide (ZCH) phase [Zn5(CO3)2(OH)6]. Its space group is C2/m with lattice parameters of a ) 13.58 Å, b ) 6.28 Å, c ) 5.41 Å, and β ) 95.6°. After calcination at 500 °C, the ZCH phase was converted to a pure hexagonal wurtzite phase according to JCPDS card no. 36-1451, with space group P63mc and lattice parameters of a ) 3.25 Å and c ) 5.21 Å.17 The formation of the ZCH phase is believed to proceed along the following reactions in the hydrothermal system:9

CO(NH2)2 + H2O f CO2v + 2NH3

(1)

5Zn(NO3)2 + 10NH3 + 2CO2 + 8H2O f Zn5(CO3)2 (OH)6V + 10NH4NO3 (2) CO(NH2)2 + 3H2O + CO2 f 2NH4HCO3

(3)

Combining the above reactions yields an overall reaction of

5Zn(NO3)2 + 8CO(NH2)2 + 22H2O f Zn5(CO3)2(OH)6V + 10NH4NO3 + 6NH4HCO3 The scanning electron microscopy (SEM) images shown in Figure 2 reveal an interesting morphology of the ZCH phase, which consists of numerous thin nanostrips forming a clewlike sphere. While ZnO nanosheets have been synthesized in the presence of urea,9 such a complex 3D ZnO superstructure has not been described in the literature. The thickness of the

Figure 2. SEM images of Zn5(CO3)2(OH)6 under different magnifications (a-c) and high-magnification FESEM images of Zn5CO2(OH)6 nanostrips (d).

Figure 3. (a) SEM top and side views (inset; the white dashed arrow indicates the cavity axis) of an as-synthesized clew and (b) selectedarea electron diffraction pattern taken from a single nanostrip, confirmed to be the [1,0,1] zone.

nanostrips ranges from 30 to 50 nm, and the diameters of the clews range from 10 to 20 µm. A close examination of Figure 2d reveals that the nanostrips wrap each other in a clew-twisting way to form the 3D architecture. Figure 3a shows the transmission electron microscopy (TEM) image of a single clew with the central cavity standing upward. The white center of the image suggests that the central cavity may penetrate through the whole clew with openings on both sides. After observations of over hundreds of spheres with SEM, all clews were observed to have an opening window. The sideviewing image (Figure 3a, inset) shows that the clewlike sphere has a very symmetric structure along the cavity axis. The selected-area electron diffraction (SAED) pattern shown in Figure 3b indicates that the nanostrips are a single-crystalline phase that preferentially grows perpendicular to the [1,0,1] direction. The SEM and SAED data suggest that if we compare the clews to Earth, the nanostrips near the two polar regions align around the central cavity with the [1,0,1] direction approximately perpendicular to the cavity axis. With latitude decreasing, the angle between the [1,0,1] zone axis and the cavity axis decreases. In the equator region, the nanostrips align perpendicular to the cavity axis and the [1,0,1] direction is parallel to the cavity axis. After calcination at 500 °C for 2 h, the ZnO phase displays the same clewlike morphology as that of the ZCH phase (see Figure 4a). The magnified field-emission SEM (FESEM) images shown in Figure 4b reveal that the ZnO superstructures are made up of thin ZnO nanostrips with a thickness of about 50 nm, which consist of irregular ZnO primary nanoparticles with small bridges. The ZnO nanoparticle sizes range from 50 to 100 nm, while the bridges are generally thinner than 30 nm. This unique

Self-Assembly of Clewlike ZnO Superstructures

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Figure 4. (a) SEM image of a 3D ZnO superstructure obtained from calcination of a Zn5CO2(OH)6 precursor and (b) FESEM image of ZnO nanostrips aligned parallel to the electron beam incidence direction (inset: nanopartricles constituting the ZnO nanostrips).

Figure 6. SEM images of the samples prepared with different urea contents, molar ratio urea/Zn(NO3)2 ) 1/1 (a), 2/1 (b), 7.5/1 (c), and 15/1 (d).

Figure 5. (a) Nanoholes in the nanostrips and low-magnification TEM image of several nanostrips overlapped together (inset). (b) Magnified HRTEM images of the part indicated by the white box in (a). (c) (1,0,1,0) lattice fringe. (d) SAED pattern taken from a single ZnO nanostrip, confirmed to be the [1,-2,1,0] zone. (e) Stacking fault observed between two sets of lattices.

nanostructure can cause the nanostrips to be easily demolished under ultrasonic sonication. When the sample was treated in an ultrasonic bath for 20 min, the nanostrips became highly dispersed nanoparticles as can be seen from the inset of Figure 4b, which has also been observed previously.33 This property is highly favorable for ZnO to be applied in photocatalysis, in which a good dispersion of photocatalyst nanoparticles is desired for the efficient degradation of organic compounds. The structural characteristics of the ZnO nanostrips were further investigated by using high-resolution TEM (HRTEM) and SAED techniques. The HRTEM images shown in Figure 5a indicate that the ZnO nanostrips contain nanoholes with diameters ranging from 30 to 100 nm, causing the nanostrips to exhibit a porous motif. Several lattice fringes with different spacings and orientations are seen from the ZnO nanoparticles. Figure 5b shows the (0,0,0,2) lattice fringe of spacing 2.6 Å, and no edge dislocations on the pore surface can be seen. Figure 5c shows the (1,0,-1,0) fringe of spacing 2.8 Å. In the same nanoparticle with one lattice orientation, no dislocations or stacking faults can be observed, indicating a high quality of the crystal structure of the ZnO nanoparticles. However, on the borders of two different lattice sets, there exists a stacking fault as can be seen from Figure 5e, the formation of which may be associated with release of the lattice strain due to the bending between two nanoparticles.34 The stacking faults can also be

applied to explain the easy demolishment of ZnO nanostrips by ultrasonic sonication. The SAED pattern shown in Figure 5d taken from a single nanostrip indicates that ZnO nanostrips preferably grow along the direction perpendicular to the [1,2,1,0] zone direction. With decreasing latitude, the angle between the [1,-2,1,0] zone axis and the cavity axis decreases. In the equator region, the nanostrips align perpendicular to the cavity axis with the [1,-2,0,1] direction parallel to the cavity axis. To understand the key parameters determining the morphology, the effect of synthesis recipes on the superstructure morphology was investigated by using the SEM technique. It was experimentally found that the urea and P123 contents are the crucial factors influencing the morphology. Figure 6 shows that the clewlike superstructure can only be obtained in a narrow urea/Zn(NO3)2 molar ratio range. When the urea/Zn(NO3)2 molar ratio was 1, nanoflakes instead of clewlike spheres were formed. When the urea/Zn(NO3)2 molar ratio was increased to 2, some spheres were seen and the diameters of the spheres were relatively smaller (less than 10 µm) compared with that of the sample shown in Figure 2, which was prepared using a molar ratio of urea/Zn(NO3)2 ) 6. When the urea/Zn(NO3)2 molar ratio was above 7.5, very large cylindrical rods were produced. The diameters of the rods are in the range of 80-100 µm, and the length is over 150 µm. The reason that the clewlike superstructure can only be obtained in a narrow urea/Zn(NO3)2 molar ratio is due to the fact that the urea concentration determines the precipitation speed of the Zn species. When the urea/Zn(NO3)2 molar ratio was below 2, the precipitation speed was so slow that only some nanoflakes could be obtained under our synthesis conditions. When the urea/Zn(NO3)2 molar ratio was above 7.5, the precipitation speed was so high that numerous nanoflakes may have formed initially, which are thermodynamically unstable and then would have been converted to the larger cylindrical rods through the dissolutionreprecipitation process. According to our experimental data, triblock copolymer P123 also played an important role in controlling the superstructure morphology. Figure 7a shows the SEM image of the sample prepared without P123. Irregular-shaped particles with small disordered particles on their surfaces are seen, indicating that no regular clewlike spheres were formed without the presence of P123. The samples prepared with low P123 concentrations (P123/Zn(NO3)2 ) 0.005-0.01) are a mixture of clewlike

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Figure 7. SEM images of samples prepared with different P123 contents, molar ratio P123/Zn(NO3)2 ) 0 (a), 0.005 (b), 0.01 (c), and 0.04(d).

spheres and large rods with triangle-cone-shaped heads. Since some clewlike spheres were embedded in the rods, it is quite clear that the rods grew at the expense of the smaller clews. The insets of Figure 3b,c reveal that some disordered particles are on the surface of the spheres, resulting in an irregular arrangement of the nanostrips in comparison with that of the sample prepared with a P123/Zn(NO3)2 molar ratio of 0.02 (see Figure 2). As the P123 concentration was increased, the surface of the spheres became more and more smooth and regular. However, when the P123/Zn(NO3)2 molar ratio was further increased to 0.04, neither clews nor rods were obtained. Only some nanoflakes without well-defined morphology were formed. The above experimental results suggest that P123 may have dual functions in controlling the superstructure morphology. One is to control the oriented attachment of the nanoparticles into 3D clewlike superstructures. The other is to stabilize the 3D superstructure. However, when the P123/Zn(NO3)2molar ratio was low, say less than 0.02, the role of P123 as a morphologycontrolling agent may still have been operative, but may not have completely stabilized the superstructure by lowering its surface free energy because of incomplete coverage of P123 on the crystal surface.35 As a consequence, the clewlike structure further grew into large rods via an Ostwald ripening mechanism.36 Further increasing the P123/Zn(NO3)2 molar ratio to 0.04 caused an increase in the solution viscosity due to the micellization behavior of the triblock copolymer.37,38 The high solution viscosity reduced the mobility of the nanoparticles and hindered the self-assembly process. To examine the morphological evolution of the ZnO superstructures during the hydrothermal treatment, kinetic products were collected and their morphologies were characterized using the SEM technique. As can be seen from Figure 8a, the sample obtained after hydrothermal treatment for 4 h exhibits an orangelike morphology with six segments. The segments are composed of small nanoflakes with a gap between two segments. The sample displays a loosely structured morphology because of the central cavity and the segment gaps. With the hydrothermal treatment going on, the sample obtained after 8 h shows a more compact structure (see Figure 8b) due to the continuous shrinkage of the central cavity and segment gaps caused by the coagulation of the nanoflakes and the surface-smoothing effect.39 The sample obtained after 17 h exhibits a well-developed clewlike superstructure as is seen from Figure 8c. With further prolongation of hydrothermal treatment for 72 h, only giant rods

Bai et al.

Figure 8. SEM images of the samples after different times of hydrothermal treatment: (a) 4 h (the inset is a side view), (b) 8 h (the inset is a side view), (c) 17 h, and (d) 48 h.

Figure 9. (a) UV-vis spectrum and (b) PL spectrum (excited at 250 nm; the inset is the excitation spectrum) of the ZnO superstructures.

with a diameter of about 80 µm were produced (see Figure 8d), implying that the clewlike superstructure is a metastable phase during the morphological evolution process and the giant rod is the thermodynamically stable phase. Thus, it can be concluded that the clewlike superstructure is a kinetic product. UV-vis and room-temperature photoluminescence spectra are shown in Figure 9. The UV-vis spectrum shown in Figure 9a displays a well-defined absorption band at about 359 nm with a blue shift of about 14 nm in comparison with that of bulk ZnO (at about 373 nm). Here, a quantum size effect is unlikely because the dimension of the nanoparticles constituting the ZnO superstructures ranges from 50 to 100 nm, much larger than 10 nm, the Bohr radius of ZnO. The blue shift is likely associated with the very thin bridges between the nanoparticles, some of which have a diameter of less than 10 nm. The PL spectrum shown in Figure 9b exhibits an intense and sharp bandedge emission centered at about 396 nm, showing that the material can be potentially used as a high-performance ultraviolet emitter.40 Some weak deep-level or trap-state emissions are also seen in the visible spectral region, indicating a small amount of structural defects, such as oxygen vacancies and

Self-Assembly of Clewlike ZnO Superstructures impurities, exist in the ZnO nanocrystals.40 The excitation spectrum of the ZnO superstructure shown in the inset of Figure 9b displays an intense peak at about 250 nm, which was used as the excitation wavelength for the PL measurement in this work. As for the formation mechanism of the ZnO superstructures, an oriented attachment may underlie the current synthesis, in which the complex 3D superstructure may be constructed through the self-assembly process 0D f 2D f 3D.39,41 First, at the early stage, numerous ZCH primary colloidal particles are engendered due to the homogeneous pH increase throughout the solution.42 Then the ZCH primary particles prefer growth perpendicular to the [1,0,1] direction due to the selective adsorption of P123 on the (1,0,1) faces, giving birth to ZCH nanostrips.43,44 As the reaction further progresses, the ZCH nanostrips gradually evolve into clewlike superstructures though oriented attachment. The driving force during such a nanostrip aggregation process should be the reduction of the surface free energy of the superstructure owing to the selective adsorption of P123 on some specific crystallographic faces of ZCH crystals. Since urea also played an important role in controlling the superstructure morphology as mentioned above, it is worth pointing out that the production of such a complex clewlike superstructure should be the outcome of cooperative functions of P123 and urea. Conclusions In summary, we have demonstrated a copolymer-controlled solution-based self-assembly approach to the fabrication of unique 3D clewlike ZnO superstructures. The concentrations of copolymer and urea are the key parameters in controlling the superstructure morphology. The copolymer may have dual functions in this self-assembly process: to control the oriented attachment of the nanoparticles and stabilize the superstructures. The PL measurement confirms the superior optical properties of ZnO superstructures. Considering the mild synthesis conditions, environmental benign characteristics, low cost, and easy operation of our one-pot synthesis route, the prepared material exhibits great potential in various practical applications, such as UV-light emitters, photocatalysts, solar cells, and gas sensors. Acknowledgment. P.B. is thankful for financial support from the National University of Singapore and China University of Petroleum. References and Notes (1) Bai, X. D.; Gao, P. X.; Wang, Z. L.; Wang, E. G. Appl. Phys. Lett. 2003, 82, 4806. (2) Raghu, N.; Kutty, T. R. N. Appl. Phys. Lett. 1992, 60, 100. (3) Ramakrishna, G.; Ghosh, H. N. Langmuir 2003, 19, 3006. (4) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 5585.

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