DOI: 10.1021/cg901200u
Hierarchical ZnO Nanorod-Assembled Hollow Superstructures for Catalytic and Photoluminescence Applications
2010, Vol. 10 40–43
Jingzhou Yin,†,§ Qingyi Lu,*,† Zhinan Yu,† Jianjun Wang,‡ Huan Pang,† and Feng Gao*,‡ †
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China, ‡Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P. R. China, and §School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai’an 223300, P. R. China Received September 30, 2009; Revised Manuscript Received November 13, 2009
ABSTRACT: In this study, hierarchically complex hollow cage-like superstructures assembled by ZnO nanorods have been successfully constructed with water-soluble biopolymer sodium carboxymethyl cellulose as crystal growth modifiers. The number of the hollow cage could be adjusted from single-cage, double-cage, multi-cage to connected-cage. A possible formation mechanism of the hollow superstructures has also been proposed. The catalytic study shows that these ZnO superstructures have good abilities to enhance propellant combustion of ammonium perchlorate (an important oxidizer used in solid rocket propellants), by decreasing its decomposition temperature to as low as 285 °C. Photoluminescence studies reveal that the increase in the cage number leads to an increase in the relative photoluminescence intensity around 500 to 700 nm, which might be attributed to the increase in radiative defects at the interface of the components of the ZnO hollow structure with the growth in cage number. Self-assembled structures with highly specific morphology could bring brand-new properties which have stimulated great interest in many fields.1-3 Although highly organized building blocks have been synthesized by using various methods, the controlled organization from rod-like building blocks has been confirmed to be a difficult task due to their anisotropy, but it is of importance not only in understanding the concept of self-assembly with artificial building blocks but also for its great application potential.4 The hollow space of self-assembled cages and capsules provides an isolated micro environment which has promising applications in catalysis, drug delivery system, microreactors, and chemical storage.5,6 However, so far the organization of primary building units into hollow structures remains a challenge for research activities on “crystal tectonics”, especially for the assembly of nanorods into hollow structures.7 Very few literature reports have been published concerning the construction of hollow morphology of nanorods except that Wan et al. reported the synthesis of self-assembled vanadium pentoxide (V2O5) hollow microspheres from nanorods.8 ZnO, an environmentally friendly oxide semiconductor, has become one of the most important functional materials with a wide range of envisaged applications.9-11 The hollow structure from ZnO nanorods has been expected to have exciting applications, but its synthesis still is a challenge to chemists and materials scientists. Although many studies have synthesized ZnO hollow structures using different methods, these hollow structures are usually from ZnO nanoparticles12,13 and/or have one cage spherical morphology to lower the surface energy.14 Synthesizing new 3D hollow structures from ZnO nanorods is highly desirable for complex structure design in crystallology and to distinguish properties in material engineering. In this study, by exploiting the assembling function of watersoluble biopolymer sodium carboxymethyl cellulose (CMC) as crystal growth modifiers, hierarchically complex hollow superstructures assembled by ZnO nanorods have been successfully constructed. The number of the hollow cage could be adjusted from single-cage, double-cage, multi-cage to connected-cage and a possible formation mechanism of the hollow superstructures has also been proposed. The catalytic and photoluminescence studies of these different hollow ZnO superstructures have been *To whom correspondence should be addressed. E-mail:
[email protected]. cn (Q.L.);
[email protected] (F.G.). pubs.acs.org/crystal
Published on Web 12/14/2009
performed to explore distinguished properties and to reveal the effect of the structure on the materials’ properties. With the addition of single-cage ZnO nanorod-superstructures, ammonium perchlorate (AP, an important oxidizer used in solid rocket propellants) could be catalyzed to decompose at a temperature as low as 285 °C, which is lower than the decomposition temperatures of AP catalyzed by nanoparticles reported in recent literature reports.15-18 The hierarchically complex ZnO nanorod superstructures were synthesized with the assistance of CMC under hydrothermal conditions. In a typical synthesis, 0.44 g of ZnAc2 3 2H2O (analytically pure) was dissolved in 8 mL of deionized water with magnetic stirring, and then 2 mL of ammonia solution (25%, analytically pure) was added. After that, 10 mL of 2.8 g/L CMC (300-800 mPa 3 S, chemically pure) aqueous solution was added dropwise into the solution containing zinc acetate and ammonia. After 5 min of stirring, the mixture was transferred to and sealed in a 50 mL Teflon-lined autoclave, kept at 120 °C for 2 h, and finally cooled to room temperature. The precipitate was collected by centrifugation (4000 rpm, 3 min), washed alternately with deionized water and ethanol, and dried in air under ambient conditions. X-ray diffraction (XRD) characterization confirmed that the product is pure wurtzite ZnO (JCPDS 36-1451) with high crystallinity (Supporting Information, Figure SI1). Scanning electronic microscopy (SEM) images of the sample prepared with the typical procedure are shown in Figure 1. SEM image (Figure 1a) with low magnification displays that the sample has a double-headed structure with a microscale size. The SEM images (Figure 1b-d) with high magnifications reveal that the two-headed structures are assembled by many short ZnO nanorods from center to tops. The length of these nanorods are in the range of 500-1000 nm and their diameters are calculated to be in the range of 50-100 nm based on a top-view of the end of the nanorods. Moreover, several broken superstructures confirm that these kinds of structures are hollow inside and these nanorods are arranged to form curved structures, which outline two hollow cages of the superstructure. This complex morphology can be synthesized easily and with very high yield. Cage structures have been known very useful and have wide potential applications in many fields. By the method presented here, the number of the hollow cage could be adjusted, which might bring different properties and wider applications. When we increased the reaction time from 2 to 24 h or even more, a cracked r 2009 American Chemical Society
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Figure 1. SEM images of the double-cage nanorod-assembled superstructures.
phenomenon at the center (the connecting place of two cages) was observed. SEM images (Figure 2a,b) show that the samples prepared for 24 h have single-cage structures which are also hollow inside and assembled by short nanorods. Figure 2c,d shows SEM images of the sample prepared with 1 mmol of ZnAc2 3 2H2O, 1 mL of 25% ammonia solution, and 10 mL of sodium CMC solution at 140 °C for 3 h. This sample possesses the same hollow and rod-assembled structure but visibly has several cages grown from one center. Figure SI2a,b (Supporting Information) shows SEM images of the sample prepared with 1 mmol of ZnAc2 3 2H2O, 1.5 mL of ammonia solution, and 5 mL of sodium CMC solution at 140 °C for 2 h. It could be seen that this sample also has hollow cages assembled by ZnO nanorods, but these cages are adjacently connected. Interestingly, with the increase in the ammonia amount (to 2.4 mL) and reaction temperature (to 160 °C), many nanorod arrays could be observed on the top surfaces of nanorods of hollow ZnO superstructures. Figure 2e,f clearly shows that these nanorods are very fine and short and grown from the out-surface of nanorods which are the building blocks for the hollow superstructures. This phenomenon might be attributed to the secondary growth of ZnO nanorods.19 Nevertheless, low ammonia concentration at high reaction temperature is harmful for the hollow structure. SEM images shown in Figure SI2c,d, Supporting Information confirm that the sample prepared with 1.6 mL of ammonia solution at 200 °C for 12 h does not have the hollow structure though the doublehead nanorod-assembled superstructures remain. As known, the CMC molecules contain a large number of carboxymethyl groups, such that the hydroxyl groups of the backbone cannot get close enough to form hydrogen bonds to each other, resulting in that water can slip in between the CMC molecules and hydrate them.20 So the solvent can be divided into numerous “channels” by the CMC molecules in the reaction system. The addition of CMC is very important for the hierarchical ZnO nanorod-assembled hollow superstructures. During the process, CMC not only serves as a soft template to confine the growth of ZnO to form the ZnO nanorod, but also serves as
assembling agent to construct the nanorods into the hollow superstructures. Without the addition of the CMC, only microrods with an average diameter of about 1.5 μm can be obtained, as the SEM image in Figure SI3, Supporting Information shows. As literature reported, in pure solutions the most stable morphology is hexagonal, with the crystal elongated along the c-axis.21 In this work, hexagonal plates or tubes could also be found in the ZnOnanorod superstructures as shown in Figure SI4a,b, Supporting Information. On the basis of these SEM images, a possible growth mechanism of the double-cage superstructure is proposed, as shown in Scheme 1. During the experiment, hexagonal ZnO microplates or microrods might form first, and then the nanorods grow in the “channels” on the polar top face (0001) and the polar bottom face (0001) of the hexagonal ZnO microplate or microrod. As time goes on, numerous ZnO nanorods form, and in order to lower the surface energy these nanorods array to form the cages like “broccoli”. At the most important step (step C) for the formation of hollow structure, the existence of a large amount of NH3 3 H2O and a gas-liquid equilibrium in the autoclave is the key factor, which makes the bundled nanorods erode from the center of the broccoli-like structure.14 During this process, the competition between the NH3 3 H2O erosion and the ZnO growth is the key factor to form the hollow cavity of the superstructure. At low reaction temperature such as 120 °C and high NH3 3 H2O concentration, the low growth rate of ZnO and the high erosion speed result in the formation of the hollow superstructures (Figures 1 and 2). But at high reaction temperature and low NH3 3 H2O concentration, the growth rate is very fast and the erosion function of NH3 3 H2O is weak, leading to the solid superstructure (Figure SI2c,d, Supporting Information). It can be confirmed by the experimental fact that without the addition of NH3 3 H2O, only solid ZnO structures could be obtained (SEM images are shown in Figure SI4c,d, Supporting Information). When the reaction time is prolonged (step D), the shell of the hollow structure becomes thin, and the connection of the two cages fractures, leading to the formation of single-caged ZnO superstructures.
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Figure 2. (a, b) SEM images of the single-cage nanorod-assembled superstructures; (c, d) SEM images of the multicage nanorod-assembled superstructures; (e, f) SEM images of the sample prepared with high ammonia amount (2.4 mL) at 160 °C for 2 h.
Scheme 1. Possible Growth Mechanism of ZnO Nanorod-Assembled Superstructures
AP is an important oxidizer used in solid rocket propellants with a usual decomposition temperature of around 450 °C. Many metal and metal oxide nanomaterials have been used as catalysts to decrease the decomposition temperature of AP; however, so far it is quite difficult to make AP decompose at a temperature lower than 300 °C. Here, we studied the catalytic properties of the ZnO nanorod-based hollow superstructures on the decomposition of AP and found that the addition of ZnO superstructures can dramatically decrease the AP decomposition temperature. Figure 3 shows the differential scanning calorimetry (DSC) curves of both pure AP and the mixtures of AP with the single-, double-,
multi-, and connected-cage ZnO superstructures at a 2% mass basis. All the curves show an endothermic peak at about 250 °C, which is due to the crystal transformation of AP from orthorhombic to cubic phase. From Figure 3a, there is an exothermic peak at above 450 °C corresponding to the thermal decomposition of AP. After the addition of the ZnO hollow superstructures, the exothermic peaks have shifted to lower temperature at about 300 °C for all kinds of hollow ZnO superstructures, separately. In particular, for the single-cage ZnO nanorod-assembled superstructure, AP could be decomposed at as low as 285 °C, which is lower than the decomposition temperature of AP catalyzed with ZnO cones and other additive such as iron oxide, cobalt oxide, and metal Ni recently reported in the literature.15-18 The decomposition temperatures of AP with double-, multi-, and connectedcage ZnO superstructures are about 310 °C, 297 °C, and 305 °C, respectively. These results should be related to their structures. From the SEM images shown in the Figures 1, 2 and SI2, the double-cage ZnO superstructures are perfect with a few broken cages, while the multi- and connected-cage ZnO superstructures have more broken cages, leading to larger contacting surface areas with AP and lower decomposition temperatures. And for
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nanorods. Our results emphasize that with the control of the hollow structure from single-cage to double-, multi-, and connected-cage, the properties of ZnO superstructures can be made favorable for new and promising applications. The relative peak intensity of the excitonic transitions PL peak is connected to the number of cages for the ZnO hollow superstructures. Although all the superstructures with different cage structures have good catalysis properties, the single-cage ZnO nanorodassembled superstructures show enhanced activity over other multicage superstructures for the catalysis decomposition of AP. Further work on tailoring of the additive structure on the nanoscale may lead to safer and more efficient solid rocket propulsion or the ability to meet application-specific propellant requirements.
Figure 3. DSC curves of pure AP (a) and the mixture of AP with the single- (b), double- (c), multi- (d), and connected-cage (e) ZnO superstructures.
Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant Nos. 20671049, 20721002, and 50772047), the National Basic Research Program of China (Grant No. 2007CB925102), the Natural Science Foundation of Jiangsu Province (Grant No. BK2007129), and Program for New Century Excellent Talents in University. Supporting Information Available: XRD pattern and SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 4. Room-temperature photoluminescence spectra of the single-, double-, multi-, and connected-cage superstructures: a: single-, b: double-, c: multi-, and d: connected-cage ZnO.
the single-cage ZnO hollow superstructures the open cages would lead to the largest contacting surface area with AP, resulting in the lowest decomposition temperature of AP. Room-temperature photoluminescence (PL) properties of the obtained ZnO superstructures at an excitation wavelength of 325 nm were studied and the spectra are shown in Figure 4. The peak around 385 nm is attributed to the excitonic transitions. The strong emission peak ranging from 500 to 700 nm corresponds to the deep level of trap-state emission of ZnO, which is known to be related to the radiative defects at the interface of the components of ZnO hollow structure, and the existence of the different defects such as the singly ionized oxygen vacancy and surface defects would lead to the peak splitting.14,22 It could be deduced from the figure that with the increase in the cage number, the relative intensity of the peak around 500 to 700 nm increases, which might be attributed to the increase of radiative defects at the interface of the components of ZnO hollow structure with the growth of cage number. In summary, we report a simple reaction for the construction of different kinds of cage-like superstructures assembled by ZnO
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