DOI: 10.1021/cg901216g
2009, Vol. 9 5014–5016
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Surface Polarity Determination of ZnO Spherical Particles Synthesized via Solvothermal Route Kenji Matsumoto,†,‡ Noriko Saito,*,† Toshitsugu Mitate, Junichi Hojo,§ Miki Inada,§ and Hajime Haneda†,‡ National Institute for Materials Science, 1-1 Namiki, Tsukuba 3050044, Japan, ‡Department of Applied Science for Electronics & Materials, Kyushu University, 6-1 Kasuga-kouen, Kasuga, Fukuoka, 8168580, Japan, NTT Advanced Technology Corp., 3-1 Morinosato Wakamiya, Atsugi, 2430124, Japan, and § Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 8190395, Japan )
†
Received October 4, 2009; Revised Manuscript Received October 15, 2009
ABSTRACT: ZnO monodispersed microspheres exhibiting only a c(þ)-plane on the surface were successfully synthesized by a novel soft-chemical solvothermal method with ethylene glycol (EG) and hexamethylenetetramine. The concentration of EG plays a critical role in the formation of these spherical particles. The spherical particles are dense and have hierarchical structure, where small crystallites are radially aligned along the c-axis. The polarity was determined using convergent-beam electron diffraction, and we established that the whole surface of the microspheres is a c(þ)-plane. Control of the shape and orientation of zinc oxide (ZnO) nanostructures is one of the most interesting topics in modern materials science.1-3 A wide variety of ZnO morphology (e.g., nanowires, rods, plates, stars, flowers, rings, and spheres) are observed for precipitates and thin films. ZnO nanostructures are promising candidates for optoelectronic applications such as in light-emitting devices,4 piezoelectric nanogenerators,5 dye-sensitized solar cells,6 and catalysts.7-10 ZnO has a wurtzite-type structure and shows spontaneous electrical polarization along its c-axis. The positive polar face (0001), c(þ)-plane, and the negative polar face (0001-), c(-)plane, are rich in zinc and oxygen atoms, respectively. Various properties of ZnO depend on its polarity: surface electronic structure,11 chemical stability,12 and catalytic properties.8-10 Photocorrosion of ZnO under UV light irradiation, which often results in decreased photocatalytic activity in aqueous solutions, also depends on the crystal face: The c(-)-plane is more soluble than the c(þ)-plane.9 ZnO powders exposing a higher proportion of the c(þ) plane showed higher photocatalytic activity.10 ZnO spherical particles covered with the c(þ)-plane are expected to be resistant to photocorrosion and excellent photocatalysts. As far as we know, however, there are no reports of such powders where the orientation and polarity are clarified. Many processes to synthesize ZnO spherical particles have been reported such as the solvothermal method with polyol media. Sub-micrometer spherical ZnO particles have been synthesized in diethylene glycol at 160-180 °C, and the obtained spheres were porous with small primary particles (about 10 nm). 13,14 ZnO microspheres composed of sheets have also been prepared in ethylene glycol (EG) and water at 200 °C.15 In this study, we investigated a novel solvothermal synthesis of ZnO microspheres using EG and hexamethylenetetramine (HMT). HMT is often employed to produce ZnO nanostructures in aqueous solutions as a homogeneous precipitation method.2,16 As the temperature increases, HMT decomposes to formaldehyde and ammonia, which acts as a base and induces ZnO precipitation in aqueous solutions. Zinc acetate anhydride (Wako Pure Chemical Industries) (2.202 g) and hexamethylenetetramine (HMT, Wako) (1.682 g) were each dissolved in 20 mL of mixed solution of ethylene glycol (Nacalai Tesque, Inc.) and water. The mixed solution was placed in a Teflon-lined stainless
steel cylindrical chamber of 50 mL capacity and was then heated in an oven at 95 °C for 12 h. After the sample was cooled to room temperature, the precipitates were separated by centrifugation. Washing with ethanol and ultrasonication was repeated three times. The resultant powders were dried at room temperature in a vacuum. We investigated the structures of the resultant particles and tried to determine their orientation and polarity. We succeeded in fabricating spherical ZnO powder in the 87.5 and 95 vol%-EG solvents at 95 °C (Figure 1). The particle size distribution of the powders prepared in 95 vol%-EG deducted from measuring 500 particles on a scanning electron microscopy (SEM) photograph is shown in Figure 2. The mean particle size and the standard deviation were 3.1 and 1.0 μm, respectively. The
*Corresponding author. Tel: þ81-29-860-4665; fax: þ81-29-855-1196; e-mail:
[email protected].
Figure 1. SEM photographs of the ZnO particles prepared in (a) 0, (b) 87.5, and (c) 95 vol%-EG solutions.
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Published on Web 10/27/2009
r 2009 American Chemical Society
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Figure 2. Particle size distribution for the ZnO microspheres prepared in 95 vol%-EG solution. Table 1. EG Concentration Dependence on the Phase, Shape, and Coherent Lengths along the a- and c-Directions, which Were Estimated from 100 and 002 XRD Line Broadening, Respectively EG (vol%) 0 25 50 75 87.5 95 98
phase
shape
ZnO þ unknown ZnO þ unknown ZnO þ unknown ZnO ZnO ZnO none
hexagonal hexagonal hexagonal irregular sphere sphere
coherent length (a-axis/c-axis) (nm)
Figure 3. SEM photographs of (a) spheres, (b) circular cones, and (c) wedge-shaped parts of the ZnO particles prepared in 87.5 vol%-EG solution and corresponding illustrations.
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particle size distribution of the powders prepared in 87.5 vol%EG was broader. We confirmed that the powders consist of a ZnO single phase by X-ray powder diffraction (XRD; X’Pert PRO MPD, PANalytical). ZnO spheres were obtained at lower reaction temperatures than the solvothermal process reported previously (at 160-200 °C).13-15 This is an advantage of combining the synthesis with homogeneous precipitation as hydrolysis is promoted by the decomposition of HMT. In 0-50 vol%-EG solutions, the precipitates were found to be hexagonally shaped ZnO particles, and they consisted of ZnO and an unknown phase. In the 98 vol%-EG solvent, no precipitation occurred (Table 1). The coherent lengths along the a- and c-axis were estimated using Sherrer formula from the peak width of the XRD diffractograms for the samples that were prepared in the different EG solutions (Table 1). ZnO powder (Kojundo Chemical Laboratory, Co. Ltd.) annealed at 850 °C for 6 h was used as a standard. With an increase in EG concentration, the coherent length decreased and the length ratio for the a- and c-axis changed slightly. EG promotes the formation of spherical polycrystalline particles and interrupts the growth of each individual crystallite. This phenomenon is similar to that observed for other compounds synthesized by the solvothermal process in EG/water. The morphology of Fe2O3 changed from octahedron particles to spheres and the crystallinity decreased when the ratio of EG to water incereased.17 For the doughnut-shaped ZnO particles, the primary particle size decreased as the EG content increased, and this is due to the capping effect of EG which restricts crystal growth.15 SEM (HITACHI, S-5000) photographs of portions of the resultant powder that was prepared in the 87.5 vol%-EG solution and corresponding illustrations of (a) the spherical particle, (b) the circular cone, and (c) wedge-shaped particles are shown in Figure 3. These are probably fragments of the ZnO spherical particles that were produced by ultrasonic treatment. A hierarchal structure for the spherical particles is present. The first stage consists of small triangular pyramid-like crystallites of 30-100 nm in size, and this is comparable to the XRD results. The second stage consists of wedge-shaped particles as shown in Figure 3c and the final stage consists of microspheres. The cross-section of a spherical particle was observed by transmission electron microscopy (TEM; JEOL, JEM-2100F)
Figure 4. TEM cross-sectional view of the particle and SAED patterns of a ZnO particle prepared in the 87.5 vol%-EG solution.
Figure 5. (a) TEM photograph of and (b) experimental and (c) simulated CBED patterns for the wedge-shape fragment of a ZnO particle prepared in the 87.5 vol%-EG solution.
with field emission gun, operated at 200 kV (Figure 4). The selector aperture was about 160 nm in diameter. The specimens were prepared by using a focused ion beam (FIB; Seiko Instruments Inc., SMI3200SE). The spherical particles are almost dense and a radial contrast is seen, which implies that the sphere is made of wedge-shape parts. Selected area electron diffraction (SAED) patterns show that the crystallites are radially aligned along the caxis. Convergent-beam electron diffraction (CBED) is known to give reliable data for the determination of the polarity of ZnO nanomaterials.18,19 The CBED pattern for a wedge-shaped
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fragment is shown in Figure 5. The incident azimuth was along the m-axis, . Simulated CBED patterns were calculated under the assumption that the base of the wedge is the c(þ)-plane and this pattern was similar to the experimental patterns.20,21 This shows that the surface of the ZnO spherical particles corresponds to the c(þ)-plane. It is the first evidence of polycrystalline particles exhibiting only a specific crystal plane. Particles with a specific crystal face are expected to have unique properties. The only solution is to be found in the spheres where the wedge crystallites are arranged radially. Synthesis of these types of structure with other materials and the investigation of their photocatalytic properties are interesting future challenges. In summary, hierarchically nanostructured, monodispersed, spherical ZnO powders were synthesized at 95 °C by a novel softchemical solvothermal method with EG and HMT. The concentration of EG plays a critical role in the formation of these spherical particles. The nanostructure of the particles was investigated using XRD, SEM, and TEM. The spherical particles are dense and consist of small crystallites which are radially aligned along the c-axis. Additionally, we succeeded in determining the polarity of the particles using CBED, and we established that the whole surface of the spherical particles is a c(þ)-plane. Acknowledgment. This study was supported in part by a Grant in Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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