J. Phys. Chem. C 2007, 111, 3863-3867
3863
Single-Crystal ZnO Cup Based on Hydrothermal Decomposition Route Ying-Song Fu,†,‡ Xi-Wen Du,*,† Jing Sun,† Yun-Feng Song,† and Jim Liu‡ School of Materials Science and Engineering, Tianjin UniVersity, Tianjin, 300072, People’s Republic of China, and PRRC-Asia, Motorola (China) Electronics Ltd., Tianjin, 300457, People’s Republic of China ReceiVed: December 8, 2006; In Final Form: January 18, 2007
With maleic anhydride-modified PS (m-PS) microspheres as a template (core) and zinc nitrate and diethanolamine (DEA) as the starting materials, the m-PS/ZnO core-shell structure was synthesized by a wet chemical route. ZnO clusters were synthesized through controlled hydrothermal decomposition of ZnDEA chelate at 70-90 °C and absorbed on the surface of m-PS spheres by electrostatic attraction; the preferential growth of ZnO nuclei resulted in a cup-shape shell around the m-PS core. Finally, single-crystal ZnO with a hollow and polyhedral structure was obtained after calcination at 500 °C.
Introduction
Experimental Section
The fabrication of hollow structures with controllable sizes and shapes is currently one of the fastest growing areas of materials research because of their characteristics, such as distinct optical properties, low density, high surface area, and good permeation. The resulting structures are of great technological importance for their potential applications in photonic crystals, catalysis, fillers (or pigments/coatings), and large bimolecular-release systems.1-5 A variety of chemical and physicochemical methods, including colloidal templating approaches,6,7 the emulsion/sol-gel process,8,9 emulsion/interfacial polymerization strategies,10,11 and the self-assembly process,12 have been developed to produce hollow structures composed of polymer, inorganic, and semiconductor materials. Zinc oxide (ZnO), a direct band gap (3.37 eV) semiconductor with a relatively high exciton binding energy (60 meV), promises various applications in optical, electronic, and acoustic devices. In particular, ZnO microstructures have attracted increasing attention because of their UV lasing and optoelectronic properties.13,14 Recently, porous or hollow ZnO structures have been reported based on CVD methods15-18 and thermal decomposition of hydrozincite Zn5(OH)6(CO3)2.19 However, these approaches generally require economically prohibitive temperatures of 600-1150 °C. The low-temperature hydrothermal decomposition method is another choice. The chelate Zn2+ complex can decompose under alkaline conditions by slight heating; the liberated metal ions then react with hydroxide ions to yield precursors of the zinc oxide.20 If the precursors aggregate on the surface of the polymer template, then the polymer/ZnO core-shell structure is expected to form. However, primary results showed that plain PS spheres did not gain efficient coating because of the absence of an active surface. In the present work, PS spheres modified with carboxylic acid groups acted as a template, and a hydrothermal decomposition process was adopted to synthesize cup-shape ZnO on the PS core at low temperature, and then the single-crystal, polyhedral, and hollow ZnO shells were obtained by calcining the core-shell structure.
Materials. Styrene (St) (AR, Tianjin Chemical Reagent) was purified by reduced-pressure distillation. Maleic anhydride (MA) was purified by vacuum distillation. Lauryl sodium sulfate (AR, Tianjin Chemical Reagent) and ammonium persulfate (AR, Tianjin Chemical Reagent Factory) were recrystallized twice in water; zinc nitrate (99.9%, Zn(NO3)2‚6H2O) and diethanolamine (DEA) were used as supplied. Polyvinylpyrrolidone (PVP) with an average molecular weight of 30 000 from the Shanghai Reagent Company was used without further purification. MA-Modified PS Spheres. Ten milliliters of styrene, 2 g of MA, and 100 mL of deionized water were filled into a 250 mL four-necked flask equipped with a mechanical stirrer, thermometer with a temperature controller, an N2 inlet, a condenser, and a heating mantle. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for approximately 30 min and was then heated to 90 °C with the addition of 250 mg of ammonium persulfate. The polymerization reaction proceeded until the conversion of St reached 97% (as determined gravimetric method) with mechanical stirring at 300 rpm. The as-prepared P(St-MA) latexes were transferred to the following step. Polyhedral ZnO Core/Shell Structure. One milliliter of m-PS latexes and 0.3 g of PVP were dispersed into 100 mL of deionized water by an ultrasonic process; the purpose of adding PVP is to prevent the aggregation of core particles. Then, vigorous stirring was applied during the mixing of various reacting components in order to prevent local inhomogeneity. Suitable amounts of zinc nitrate and DEA were added in the m-PS suspension to the concentration of 0.02-0.5 M for zinc nitrate and 0.02-1.0 M for DEA; the pH value was adjusted with HNO3 solution and kept lower than 9.0. The obtained mixture suspensions were placed in a bath thermostated varying between 70 and 90 °C under stirring for several hours. On termination of aging, the resulting suspensions were cooled in an ice-water bath. Particles were collected with centrifugation at 3000 rpm for 10 min, the supernatant solutions were discarded, and the particles were resuspended in deionized water in an ultrasonic bath. The process was repeated 3 times, and the purified m-PS/ZnO core-shell particles were dried at room temperature in a vacuum oven for at least 2 days.
* Corresponding author. E-mail:
[email protected]. † Tianjin University. ‡ PRRC-Asia, Motorola (China) Electronics Ltd.
10.1021/jp068461f CCC: $37.00 © 2007 American Chemical Society Published on Web 02/16/2007
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Fu et al.
Figure 1. TEM image of the maleic anhydride-modified PS spheres; the size of PS spheres is monodisperse, and their average diameter is about 110 nm.
Figure 3. TEM images of samples taken about 5 min after visible turbidity appeared. (a) Initial clusters (labeled by arrows) on the surface of m-PS spheres; (b) a massive crystal grown along the surface of m-PS sphere. The arrows indicate the fast growth directions.
Figure 2. ATR-FTIR spectra of pure PS (a), P(St-MA) or m-PS particles (b), and m-PS/ZnO core-shell particles (c).
Hollow and Polyhedral ZnO Shell. The ZnO hollow spheres were obtained by calcining the dried m-PS/ZnO nanoparticles at 500 °C for 1 h in a muffle oven under static air. The calcination temperature was determined according to thermogravimetric (TGA) analysis, and the heating rate of calcination was 10 °C/min. Characterization. The morphology of the samples was observed using an FEI Tecnai G2 F20 transmission electron microscope (TEM) with a field-emission gun operating at 200 kV. TEM samples were prepared by dropping dilute products onto 400 mesh carbon-coated copper grids and immediately evaporating the solvent. Energy dispersive spectrum (EDS) characterization was performed with an EDX system attached to TEM. The phase structure of as-obtained particles and calcined samples was characterized by a Shimadu XD-3A X-ray diffractometer (XRD) at a scanning rate of 4°/min. Fourier transform infrared (FTIR) spectra were reordered with a Nicolet 470 FTIR spectrometer over a wavenumber range from 650 to 4000 cm-1. To determine the calcined temperature for producing a hollow structure, a Perkin-Elmer 7 analyzer was employed to make a TGA measurement at a heating rate of 20 °C/min under nitrogen. Results m-PS Spheres. A TEM image of the maleic anhydride m-PS microspheres is shown in Figure 1. The m-PS microspheres obtained by emulsion polymerization are monodisperse and have a size of about 110 nm. The FTIR spectrum of the m-PS microspheres is shown in Figure 2. On the basis of the FTIR spectrum, a characteristic peak at 1698 cm-1 is attributed to carbonyl stretching of carboxyl groups besides well-defined bands of St units,21 which suggests the copolymerization of MA and St. Thereby, the polarity was created on the surface of PS spheres, which is beneficial for the sequent absorption process. m-PS/ZnO Core-Shell Structure. Although various volumes of m-PS template latexes, reagent concentration, pH value, temperature, and time were tried, the core-shell structure only
Figure 4. m-PS/ZnO core-shell structure. (a) SEM image on a ZnO shell with cup shape, marked by an arrow. (b) Low-magnification image on several core-shell particles. (c) A typical m-PS/ZnO particle; the inserted pictures are HRTEM images of the areas in the white frames. (d) Two m-PS/ZnO core-shell particles give the side view (left) and top view (right), respectively. The dotted line denotes the cup shape of ZnO.
appeared under a narrow recipe, that is, 1 mL m-PS latex, 5.95 g Zn(NO3)2‚6H2O, 2.1 g DEA, 0.3 g PVP, T ) 70-90 °C; t ) 2-4 h. While mixing the zinc salt, DEA, and m-PS suspensions at room temperature, the reaction system displayed turbidity. Figure 3a shows the TEM image of samples taken about 5 min after visible turbidity appeared. Some initial clusters with the size of 2-8 nm exist on the surface of the m-PS sphere. In Figure 3b, a massive crystal grows along the surface of the m-PS sphere. Figure 4a shows an SEM image of ZnO shell, while the m-PS sphere was lost, the ZnO shell clearly exhibits a cup shape. Figure 4b gives the TEM image of the complete core-shell particles after 4 h reaction; the core has an approximately spherical appearance, and the diameter of the cores is consistent with that of m-PS spheres, which suggests that the m-PS spheres served as templates. The shells present a dark polyhedral structure around the cores and have the thicknesses varying from 60 to 90 nm. Figure 4c shows a typical core-shell structure with a core diameter and shell thickness of about 105 and 80 nm, respectively. Figure 4d exhibits two typical core-shell particles; the left one shows a side view, and the right one shows a top view. The EDS result on the core-shell nanoparticles demonstrates the presence of Zn, O, and C (Figure 5), and the ratio of Zn
Single-Crystal ZnO Cup
Figure 5. EDS spectrum of the as-prepared product.
J. Phys. Chem. C, Vol. 111, No. 10, 2007 3865
Figure 7. TGA curve of m-PS/ZnO core-shell particles.
Figure 8. TEM image of the hollow and polyhedral ZnO after calcinations at 500 °C; the inset is the SAED pattern of a hollow ZnO particle.
Figure 6. XRD pattern of as-prepared m-PS/ZnO core-shell particles (a) and calcined nanoparticles (b).
and O determined from the spectrum is 41.83:47.52. Considering the existence of polymer and the uncertainty of the quantification, the result corresponds to the stoichiometry of ZnO. The XRD spectrum of the core-shell particles (line a in Figure 6) shows peaks consistent with the standard structure for bulk ZnO (PDF card of 36-1451); therefore, the crystal shells are wurtzite-type ZnO. The left inset in Figure 4b shows a HRTEM image of one side of the polyhedral ZnO. The lattice spacing of 0.28 nm agrees well with the spacing of the ZnO {101h0} planes. Actually, the spacing of every lattice planes parallel to the hexagonal surface is 0.28 nm; thereby, hexagonal surfaces are {101h0} planes of ZnO. The right inset in Figure 4b presents lattice fringe in the core area; the lattice spacing is same as that of the hexagonal side, only the contrast is lower than the side wall, which suggests that a thinner ZnO layer exists in the core area. Therefore, the shell structure obtained in the present work is a cup-shape, single-crystalline ZnO with a thin bottom; the cup shape can be found in the side-view image of the core-shell structure, as traced by the dot line in Figure 4c. The nanocrystals in Figure 3b also show lattice spacing (0.26 nm) consistent with that of ZnO crystals, which suggests that they are ZnO nanocrystals. Furthermore, FTIR analysis on m-PS/ZnO particles was performed. The characteristic peak of carboxyl groups at 1698 cm-1 disappeared, whereas two strong absorption peaks at 1340 and 1008 cm-1 emerged beside the absorption bands of PS (line c in Figure 2). In our previous work, the two peaks were proven to originate from carboxylic ionic complexes (R-COO-ZnO) at the interface of m-PS and ZnO.22 Hollow and Polyhedral ZnO. Figure 7 shows TGA curves of m-PS/ZnO core-shell particles. Four stages can be observed
at 25-200, 200-430, 430-500, and 500-650 °C. The first weight loss is about 1% due to the evaporation of physically adsorbed water; the second one is about 4.5%, attributed to the decomposition and combustion of m-PS spheres; the third weight loss (1.5%) is slower, caused by the combustion of remnants; and no further weight loss can be observed above 550 °C. After calcination, the thin bottom surface was broken and the hollow structure was formed, while the polyhedral shape was well-kept (Figure 8). The calcined products were proven to be wurtizte ZnO by XRD (see line b in Figure 6); meanwhile, the extent of crystallization and purity was improved. The inserted selected area electron diffraction (SAED) pattern in Figure 8 further confirms the single-crystal structure of hollow ZnO particles. Discussion The way of nucleation and growth of the ZnO shell on the surface of m-PS microspheres is essential to understanding the formation of the core-shell structure. According to the TEM image of the early stage sample in Figure 3a, some ZnO clusters exist on the surface of m-PS microspheres; thereby, the ZnO shell should originate from ZnO clusters instead of the growth units, Zn(OH)42-; otherwise, the smooth ZnO layer should be found on the surface of m-PS. In the following sections, we discuss the formation of ZnO clusters first, then the absorption ZnO clusters onto m-PS spheres, and finally the growth of ZnO crystals on the surface of m-PS spheres. Formation of ZnO Clusters. The formation of ZnO clusters with zinc nitrate and DEA as starting materials is based on the decomposition of Zn2+ chelate (complexes) under hydrothermal conditions, which is a wet chemical method.23 After the addition of DEA solution to the reaction system, Zn-DEA chelate forms first, and then Zn-DEA chelate (complexes) begins to decompose when the temperature is raised. The decomposition process follows the reaction Thermal decomposition
Zn - DEA 98 Zn2+ + 2OH -
(1)
3866 J. Phys. Chem. C, Vol. 111, No. 10, 2007
Figure 9. Schematic illustration of the growth of a ZnO single crystal on the surface of a m-PS sphere.
As a result, the liberated hydroxide ions (OH-) and metal ions (Zn2+) are released. Next, the growth unit, Zn(OH)42-, forms in an alkaline condition according to the equation24
Zn2+ + 2OH - + 2H2O ) Zn(OH)2 + H2O ) Zn(OH)42- + 2H + (2) Subsequently, in the solution with higher Zn2+ supersaturation ([Zn2+] ) 0.2 M in our experiment), growth units Zn(OH)42are bonded together through a dehydration reaction forming the , as shown in the following aggregate ZnxOy(OH)(z+2y-2x) z equation
(nucleus) + Zn(OH)42- ) ZnxOy(OH)(z+2y-2x)z (z+2y-2x+2)Znx+1Oy+1(OH)z+2 + H2O (3)
where, the subscripts x, y, and z represent the number of Zn2+, O2-, and OH- within an aggregate, respectively. As the size reaches the critical value required for the formation of ZnO clusters precipitate, which clusters, the ZnxOy(OH)(z+2y-2x)z are customarily represented as ZnO. Absorption of ZnO Clusters onto m-PS Spheres. Under the condition of the pH value lower than 9, the m-PS spheres are negatively charged due to the ionization of R-COOH located at the m-PS surface (whose isoelectronic point is about 2),25 whereas ZnO clusters with isoelectric points of 9.4 are positively charged.26 Therefore, the carboxyl group of the maleic anhydride on the surface of the m-PS template can adsorb ZnO clusters by electrostatic attraction, and then the positively charged (0001) planes of ZnO clusters attaches to the negatively charged surface of the m-PS sphere. On the contrary, the growth unit, Zn(OH)42-, is negatively charged, and the electrostatic repulsion drives the growth units away from m-PS spheres. After the absorption, the carboxyl group related peak disappeared in the FTIR spectrum, and carboxylic ionic complex (RCOO-ZnO) related peaks emerged (see line c in Figure 2), which suggests that a chemical bond formed at the interface between m-PS and ZnO. The formation of chemical bonding facilitates to form m-PS/ZnO core-shell particles. Growth of ZnO Single Crystals on the Surface of m-PS Spheres. The ZnO clusters absorbed on the surface of m-PS can act as nuclei for further growth. Figure 9 illustrates the growth of the ZnO single crystal on the surface of the m-PS sphere. On the basis of the TEM image in Figure 3a, at the beginning, there are many clusters attached on the surface of m-PS spheres; however, most of them are too small to serve as stable nuclei for the further growth (Figure 9a). Once the size of one of the clusters exceeds the critical size, it will grow quickly by incorporating growth units into the crystal lattice at the interface.27 Alternatively, the growth velocities of the ZnO crystal in different directions follow the relationship28
V > V > V > V > V (4) where is the fastest growth direction and is
Fu et al. the slowest one; however, the growth along direction is prohibited because of the obstruction of m-PS sphere, and then the nanocrystal shows quick growth along the transverse direction first, which leads to a sheet of ZnO crystal (Figure 9b). Afterward, as the width of the ZnO sheet increases, growth along the direction is permitted, which brings the appearance of a side wall (Figure 9c); further growth results in the formation of a ZnO cup (Figure 9d). Because is the slowest growth direction, the bottom layer of the ZnO cup is very thin, and a negative polar facet (0001h) is kept as the bottom surface. In general, the preferential growth and the restriction of the m-PS template determine the formation of the ZnO cup. In Figure 4c, TEM observation from side view on a core-shell particle indicates that the bottom surface is a regular facet but the side surfaces are irregular; this is consistent with the fact that the crystal growth in different crystal facets of {101h0} is accelerated or delayed by the initial clusters existing on the surface of the m-PS spheres. Conclusions In this paper, the m-PS/ZnO core-shell structure was synthesized by a wet chemical route using maleic anhydridemodified PS spheres as the template. ZnO crystals were produced by low-temperature thermal decomposition, and the shell grew up from the ZnO nuclei absorbed on the surface of m-PS spheres by electrostatic attraction and formed a cup shape around the m-PS core; single-crystalline ZnO with a hollow and polyhedral structure was obtained after calcination at 500 °C. This strategy of preparing the ZnO hollow structure can also be applied to other promising semiconductor materials. Acknowledgment. This work is partially supported by the Natural Science Foundation of China (No. 50402010), Foundation of Tianjin Municipal Science and Technology Commission (No. 043800711), and the 985 project of Tianjin University. We acknowledge the financial and program support of the Physical Realization Research CoE (PRRC-Asia) of Motorola Inc. References and Notes (1) Scharrer, M.; Wu, X.; Yamilov, A.; Cao, H.; Chang, R. P. H. Appl. Phys. Lett. 2005, 86, 151113. (2) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (3) Kidambi, S.; Dai, J. H.; Bruening, M. L. J. Am. Chem. Soc. 2004, 126, 2658. (4) Wang, Y.; Cai, L.; Xia, Y. AdV. Mater. 2005, 17, 473. (5) Caruso, F. AdV. Mater. 2001, 13, 11. (6) Giersig, M.; Liz-Marzan, L. M.; Ung, T.; Su, D. S.; Mulvaney, P.; Bunsenges, B. Phys. Chem. 1997, 101, 1617. (7) Kawahashi, N.; Matijevic, E. J. Colloid Interface Sci. 1991, 143, 103. (8) Imhof, A. Langmuir 2001, 17, 3579. (9) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 1325. (10) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507. (11) Rana, R. K.; Mastai, Y.; Gedanken, A. AdV. Mater. 2002, 14, 1414. (12) Wendland, M. S.; Zimmerman, S. C. J. Am. Chem. Soc. 1999, 121, 1389. (13) Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (14) Kind, H.; Yan, H.; Law, M.; Messer, B.; Yang, P. AdV. Mater. 2002, 14, 158. (15) Fan, H. J.; Scholz, R.; Kolb, F. M.; Zacharias, M.; Gosele, U. Solid State Commun. 2004, 130, 517. (16) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (17) Jiang, Z. Y.; Xie, Z. X.; Zhang, X. H.; Lin, S. C.; Xu, T.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. AdV. Mater. 2004, 16, 904. (18) Duan, J. X.; Huang, X. T.; Wang, E. K.; Ai, H. H. Nanotechnology 2006, 17, 1786.
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