Crystal Growth & Design - ACS Publications - American Chemical


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CRYSTAL GROWTH & DESIGN

A New Route toward ZnO Hollow Spheres by a Base-Erosion Mechanism Zhitao Chen and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China

2008 VOL. 8, NO. 2 460–464

ReceiVed March 23, 2007; ReVised Manuscript ReceiVed August 17, 2007

ABSTRACT: In this paper, thermal treatment of Zn(NH3)42+ precursor in ethanol solvent leading to the formation of the ZnO hollow spheres is reported. It was demonstrated that the pH value of the initial mixture and the volume ratio of the ethanol with the solution plays the key function in the formation of hollow spheres. Transmission electron microscopy and scanning electron microscopy images showed that hollow spheres of ∼600 nm in diameter were built by ZnO nanorods. On the basis of experimental results, a possible formation mechanism in the growth processes is discussed.

1. Introduction Extensive studies have been devoted to the controlled synthesis of nanocrystals in the past few decades. The arrangement of micro- and nanostructured building blocks into hierarchical structure attracts great interest to researchers. Generally speaking, by precisely controlling the nucleation and growth, and following by self-assembly, novel architectures composed of various surface can be achieved. Thus, a better understanding of the growth mechanisms by which morphologies and spatial organizations of anisotropic nanostructure can be systematically synthesized, and new synthetic strategies are two of the key issues in the development of hierarchical nanomaterials. Inside them, hollow sphere structures have recently attracted a great deal of attention because of their wide variety of potential applications, including the delivery of drugs, catalysis, chemical storage, microcapsule reactors,1–4 as building blocks in the fabrication of photonic band gap crystals,5,6 and so on. Various methodologies have been developed to achieve this special nanostructure, including a spray-drying method,7 sol–gel process,8,9 emulsion strategies,10,11 self-assembly techniques,12,13 Ostward ripening,14 and templating against colloid particles.15–18 The most-applied method is the templating of larger colloidal particles, including hard and soft templates, such as polystyrene beads, silica spheres, carbon particles, mulsions, micelles, and gas bubbles.19–22 As we well know, ZnO is an attractive semiconductor with a direct gap of 3.37 eV, which attracts much attention in tailoring its size and shape to optimize its physical properties. So to synthesize the hollow structure of ZnO by a simple and efficient way is meaningful and challenging. Few research studies on this field have been done. Li and co-workers fabricated ZnO hollow spheres with diameters of 300–600 nm using a coordination polymer as reactant.23 Zeng and co-workers reported an oxidation–reduction method to synthesize ZnO hollow spheres using zinc powder as reagent.24 Yan and co-workers fabricated ZnO hollow spheres by a solution-based method using zinc hydroxide carbonate as precursor.25 These methods are fascinating. However, to some extent, they also possess some disadvantages, such as nonuniform size in product and complicated operations in experimental procedures. Therefore, it is highly desirable to develop a simple synthesis method, which can * To whom correspondence should be addressed. Tel.: +86 21 52412718; fax: +86 21 52413122; E-mail: [email protected].

control the diameter of the product and just require mild conditions, such as low temperature, few cleaning steps, etc. In this work, we report a novel and simple method of conversion of Zn(NH3)42+ to ZnO hollow spheres under hydrothermal conditions. By adjusting the reaction condition, we can get high morphological yield of hollow spheres, even to 100%. Moreover, we investigated the growth process of hollow spheres. The control experimental results disclose that the hollow spheres are formed by aggregation of ZnO nanorods in the prophase and the eroded effect of OH- in the latter step. We also discuss the possible mechanism of these hollow sphere nanostructures in detail.

2. Experimental Procedures 2.1. Synthesis. All of the reagents (analytical grade purity) were purchased from SCRC Chemical Co. and used without any further purification. Two starting aqueous solutions, Zn(Ac)2 (0.2 M), NH3 · H2O (4 M), were prepared. The ammonia was dropped into the zinc acetate solution to adjust the pH value of mixture to 11.6 under vigorous stirring. In a typical experimental procedure, 7 mL of this mixture was mixed with 70 mL of anhydrous ethanol, and the mixture was transferred into a 80 mL Teflon-lined autoclave under constant stirring. Then, the autoclave was maintained at 180 °C for 12 h and cooled down to room temperature naturally. The white precipitate was collected and rinsed with distilled water and absolute ethanol several times. Finally, the products were obtained by centrifugation and drying in a vacuum at 60 °C. The reaction time and pH value of mixture was changed to investigate the influential factors of the reaction conditions to the morphology of products. 2.2. Characterization. The crystalline structure of the samples was analyzed on a D/max 2550V diffractometer using Cu KR radiation (λ ) 1.5406 Å) and the scan step was 0.02° in 2θ. A field emission scanning electron microscope (FE-SEM, JSM-6700F, operating at 10 keV) was used to examine the morphology of the samples. The roomtemperature photoluminescence spectra of the samples were recorded on a Perkin Elmer LS55 photoluminescence spectrometer (Perkin-Elmer Co., Shelton, CT) with a Xe lamp. The size, morphology, and structure of ZnO crystallites were probed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) on a FE-TEM (JEM2100F; accelerating voltage: 200 kV).

3. Results and Discussion The as-prepared ZnO hollow spheres were structurally characterized by X-ray diffraction, indicating a wurtzite structure with high crystallinity. A typical XRD pattern of the ZnO hollow spheres is illustrated in Figure 1 with all diffraction peaks well

10.1021/cg070277b CCC: $40.75  2008 American Chemical Society Published on Web 01/08/2008

ZnO Hollow Spheres by a Base-Erosion Mechanism

Figure 1. The XRD pattern of ZnO hollow spheres prepared at 180 °C for 12 h.

indexed to hexagonal phase ZnO with JCPDS card No. 79-2205. No characteristic peaks were observed for other impurities such as Zn(OH)2. The morphologies of the ZnO hollow spheres obtained under typical conditions were examined using TEM analysis. As shown in Figure 2a, the as-obtained ZnO morphology is spheres with diameters ranging from 0.4 to 1 µm, and the insides of these spheres are hollow, which indicates that the prepared product is composed of sub-micrometer hollow spheres. Careful TEM observation shows that the shell thickness is about 100 nm, and the surface of the hollow spheres are constructed by many nanorods with diameters ranging from 80 to 150 nm, as seen in a magnified TEM image of Figure 2b,c. The hollow spheres could have application in a microchemical reactor owing to inside space with an inner diameter of 200 nm. As shown in the FE-SEM image in Figure 2d, the panoramic morphology of the as-obtained ZnO is spherical with diameters ranging from 0.5 to 1 µm, which corresponds to the TEM results. From the magnified SEM images, we can see that the surfaces of spheres are constructed by nanorods as seen in Figure 2e, which is also confirmed by TEM observations. Interestingly, these nanorods connect together from the same center to form a sphere structure. Moreover, the hollow nature of the spheres is shown by another typical SEM image (Figure 2f), which provided an illumination to see the opened structure. These observations indicate that the high yield of hollow ZnO spheres can be obtained under the present experimental conditions. The results of many control experiments show that this approach has excellent reproducibility, and the hollow spheres morphology are well retained as even the products were stored in air for several weeks. Moreover, we also investigated the mass yield of the ultimate product. In the initial stage of the experiment, the amount of Zn2+ is 0.75 mmol. At different reaction times, the mass of ultimate product is 0.056, 0.050, 0.042, 0.030, 0.024 g, corresponding to a reaction time of 3, 6, 9, 12, 18 h, respectively. The mass yield is determined to 92.2, 82.3, 69.1, 49.4, 39.5%, respectively. The composition of product was confirmed by using energy dispersive spectrometry (EDS) analysis. In the EDS spectrum of a typical product, the peaks of O and Zn are obviously observed without any other peaks (the Cu peaks in the spectrum are due to background from the copper TEM grid), which confirm that the hollow sphere is pure ZnO. To understand the formation mechanism of the ZnO hollow spheres, time-dependent experiments were carried out at 180 °C, and the products were analyzed by TEM. From Figure 3,

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the morphological transformations of ZnO nanorods to hollow spheres under varying reaction times can be observed clearly. From the TEM micrographs of Figure 3a-e, it is found that by varying the hydrothermal reaction time, five different evolution stages could be clearly observed. When the reaction time is 3 h, we find that the main morphology of the product is nanorods with a 200–400 nm diameter and their large flower-like aggregates. The inset of Figure 3a-1 and 3a-2 show these two typical morphologies, respectively. As shown in Figure 3b, the multiped-like aggregations of nanorods are the main yield, and the morphology of hollow spheres take shape, when the reaction is prolonged to 6 h. By prolonging the reaction time to 9 h, we find that the hollow spheres become the main morphology of product, as shown in Figure 3c. In particular, the inset in Figure 3c-1 shows a typical hollow multiped aggregation. It demonstrates that the hollow multiped aggregation is only the intermediate product. When the reaction time is 12 h, the hollow sphere is the absolute morphology of product. However, the diameter of the product is not uniform and varies from 0.5 to 1 µm, as shown in Figure 3d. When the time is changed to 18 h, we find that the diameter of hollow spheres is uniform at about 700 nm. From the entire Figure 3, we can conclude that when the reaction time exceeds 12 h, the hollow sphere is the absolute morphology of the product. Moreover, further prolonging the reaction time makes the diameter of spheres more uniform in the function of the Ostward ripening mechanism.26 The XRD patterns of the products prepared at different reaction times are shown in Figure 4. It is found that by prolonging the reaction time, the crystallization of ZnO hollow spheres is improved with different trends. From 3 to 6 h, the crystallinity of the ZnO is improved clearly. And then, from 6 h to 9 h, 18 h, the crystallinity of the ZnO is improved slightly. Compared with the standard diffraction pattern, only the pattern of product prepared at 3 h has the deviation of peak intensity of the (100) plane. It implies that the (100) direction is the preferred growth orientation of ZnO nanorods at the first stage of product formation. At the latter stage this tendency becomes weak. The photoluminescence (PL) spectrum of the ZnO hollow spheres was measured on a fluorescence spectrophotometer using a Xe lamp with an excitation wavelength of 325 nm at room temperature. The powder accessory of machine is used to measure powder samples. In the experiments, the samples are abraded as homogeneous as possible to avoid surface structure effects. Forty milligram powders were added onto the center of the silica window, and the cap was screwed to keep the samples held tightly. To avoid this inaccuracy, we kept the mass of different samples constant and other physical conditions fixed in the process of measurement. As shown in Figure 5, a strong ultraviolet emission at ∼390 nm and three weak emitting bands (a blue emission band at ∼420 nm, a blue-green band at ∼485 nm, and a green band at 530 nm) could be observed in all products. The strong UV emission corresponds to the near band edge emission of the wide band gap of ZnO due to the recombination of excitonic centers and as a result of the quantum confinement effect.27–29 The green band emission corresponds to the singly ionized oxygen vacancy in ZnO and results from the recombination of a photogenerate hole with the single ionized charge state of this defect. The weak green emission also implies that there are few surface defects in the ZnO hollow spheres. Nonetheless, the mechanism of the blue emission is unclear. From Figure 5, we can also see that the longer the reaction time is, the stronger the PL intensity of product is. That is attributed to the higher crystallinity and fewer defects of product when the reaction time is longer.

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Figure 2. Morphology of the as-prepared product. (a) TEM image of the samples. (b, c) Typical magnified TEM images of hollow spheres. (d, e) SEM image of the samples. (f) Typical magnified SEM image of a hollow sphere. (g) The EDS spectrum of hollow spheres.

In the preparation, when the pH value of Zn(Ac)2 solution is adjusted to 11.6 with ammonia, Zn(NH3)42+ complexes are formed. The chemical reaction at this stage can be expressed as Zn(CH3COO)2 (aq) + 2NH3·H2O (aq) f Zn(OH)2 (s) + 2NH4CH3COO (aq)

(1)

Zn(OH)2 (s) + 4NH3·H2O (aq) f Zn(NH3)2+ 4 + 2OH (aq) (2)

Thermal treatment of the Zn(NH3)42+ solution in ethanol leads to the formation of hollow ZnO spheres according to equation: Zn(NH3)2+ 4 + 2OH f ZnO (s) + 4NH3 (aq) + H2O

(3) On the basis of these observations, a mechanism that combines aggregation at the initial stage with base-erosion at the last stage to form ZnO hollow spheres is proposed, with reference to the preparation of multiped-like ZnO30 and tubular ZnO.31 In the reactions, the ZnO nanorods are first formed by the initial nucleation, which is attributed to the anisotropy of ZnO.32 As illustrated in Figure 6, after the formation of nanorods with different scales, at step A, the size of the nanorods tends to be uniform at the erosion effect of extra base as reported.25 At step B, the individual nanorods begin to aggregate to bundled ZnO nanorods arranged in a three-dimensional array, which have been reported in many research reports.30,33 Along with the

increase of reaction time, this tendency is completed and the uniform bundled ZnO nanorods are shaped, as shown in step C. At the most important step (step D) for formation of hollow spheres, the existence of larger amounts of NH3 and a gas–liquid equilibrium in the autoclave is the key factor, which makes the bundled nanorods erode from the center point to the outside in succession. At the center of the spheres, the local zinc concentration is highest, which makes the center of spheres have the fastest erosion velocity. In the experiment, when we reduce the amount of ammonia to some extent, the hollow spheres cannot be obtained. When the heating time is prolonged, as shown in step E, the thick shell of the hollow spheres becomes thin. To investigate the effect of solvent and the amount of ammonia, we do the control experiments that vary the kind of solvent and the additional amount of ammonia, which corresponds to the pH value of the initial mixture. When the ethanol is not present, the dominant morphology of the product is nanorods with a diameter of about 0.4 µm, which corresponds to the result reported by other researchers.34 We attempted to change the ethanol to propanol or acetone, and the result shows that no hollow spheres can be obtained and the morphology of product is mainly nanorods and spheres. The exotic environment can remarkably influence the growth of nanocrystals. And the kinetics of nucleation and growth as well as processes such as coarsening and aggregation are expected to be strongly dependent on the properties of the solvent.35 Water is a dipolar, amphiprotic solvent with a high dielectric constant, and most inorganic compounds are readily dissolved. In contrast, ethanol

ZnO Hollow Spheres by a Base-Erosion Mechanism

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Figure 3. TEM images of comparison experiments with different reaction times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h, and (e) 18 h. The inset is the high magnification of the corresponding image.

Figure 4. The XRD pattern of products at different reaction times: 3 h, 6 h, 9 h, 18 h, respectively.

possesses a low dielectric constant, weak polarity, and low surface tension, and most organic compounds are readily dissolved.36 In our work, the ethanol/water solvent with low polarity and surface tension is in favor of the formation of hollow spheres. Moreover, we also varied the amount of ammonia in the Zn(NH3)42+ precursor to investigate its effect. At conditions of different ammonia amounts, we obtained different kinds of morphological results, such as flower-like, leaves-like, rods, particles, etc. So the appropriate amount of ammonia is the key factor to form ZnO hollow spheres. In the earlier research of Yan et al.,31 they have successfully synthesized tubular ZnO at a quantitative amount of ammonia, but the yield is not high enough and it is accompanied by nanoparticles. Moreover, we also studied the influence of the zinc concentration. However, the morphology change derived from the increase of the zinc concentration is similar to that resulting from the reduction of the ammonia concentration. In a word, the morphology is mainly dependent on the concentra-

Figure 5. The room temperature photoluminescent spectrum of hollow spheres of ZnO prepared at different reaction times. (A) 3 h, (B) 6 h, (C) 9 h, (D) 12 h, (E) 18 h.

tion ratio of zinc and ammonia. Meanwhile, we varied the reaction temperature in the experiments, and the results verify that the temperature is also a important factor. We did the control experiments at 100, 140, 160, 200 °C, respectively. When tempereture was below 150 °C, we obtained the aggregation of ZnO nanoparticles and nanorods. At 200 °C, the size of spheres becomes bigger and the shell becomes thinner. All the experimental results indicate that at the process of reaction to form hollow spheres, the erosion function of excessive ammonia to ZnO nanorods operates at all times, which is the base-erosion mechanism we proposed.

4. Conclusions In summary, a novel and simple approach has been successfully developed to synthesize ZnO hollow spheres with an outer

464 Crystal Growth & Design, Vol. 8, No. 2, 2008

Chen and Gao (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Figure 6. Schematic representation of the formation mechanism of ZnO hollow spheres.

diameter of 600 nm and inner diameter of 100 nm. The ZnO hollow spheres prepared exhibited strong room-temperature photoluminescent properties with UV emission at 390 nm. So it is expected that the ZnO hollow spheres prepared by our method have potential applications in catalysis, microchemical reactors, or optics. Control experiments demonstrate that the pH value of initial mixture and the volume ratio of the ethanol with the solution play important functions in the formation of hollow spheres. On the basis of the results of the control experiments, we put forward a mechanism for the formation of hollow spheres. Because of the simple reaction process, this method can also be used to synthesize hollow spheres of other metal oxides whose metal cations can form complex ions with ammonia. Acknowledgment. We thank the support from the National Key Project of Fundamental Research (Grant No. 2005CB623605), National Nature Science Foundation of China (No. 50572116), and Shanghai Nanotechnology Promotion Center (Nos. 0552nm045 and 0652nm022). Special thanks are given to Professor J. W. Feng for TEM observations and Dr. Q. H. Zhang for helpful discussions on the research work.

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