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J. Phys. Chem. C 2008, 112, 11722–11728
Microspheric Organization of Multilayered ZnO Nanosheets with Hierarchically Porous Structures Zhou Xingfu,*,† Hu Zhaolin,† Fan Yiqun,† Chen Su,† Ding Weiping,‡ and Xu Nanping† State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China, and Laboratory of Mesoscopic Chemistry and School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: March 26, 2008; ReVised Manuscript ReceiVed: June 1, 2008
Novel superstructures of multilayered ZnO nanosheets with hierarchically porous structures are successfully synthesized from a hydrothermal preparation and thermal decomposition of a layered precursor of zinc hydroxide carbonate (denoted as ZnHC). Nanosheet-based ZnHC microspheres are self-assembled by the hydrothermal process by using zinc nitrate hexahydrate and urea as the starting materials. The corresponding microspheric organizations of multilayered ZnO nanosheets with hierarchically porous structures are obtained by the thermal decomposition ZnHC precursor at 573 K. SEM images show that the average diameter of ZnO assembled microspheres is about 15 µm, and the length of a ZnO nanosheet building block which is made up of thin mutilayered sheets is around ∼7 µm. Studies also show well-crystallized pores with hierarchically distributed pore sizes are embedded in the multilayered ZnO nanosheets building blocks. A plausible dissociation-deposition mechanism using in situ formed Zn(OH)2 nuclei as the “sacrificial” templates for the microsphere assembly is also proposed. Introduction The self-assembly of nanobuilding blocks into the microscale hierarchical structures, inspiring and simulating the natural phenomena, has been a hotspot in the field of materials.1 Specially, hierarchically intricate self-assembled ZnO superstructures with the controlled size, morphology based on onedimensional (1D) and 2D nanobuilding units have been extensively investigated2,3 due to their outstanding electronic and optical properties, and the potential wide-ranging applications for nanolasers, solar cells, and other functional devices.4–8 Welldefine ZnO 1D and 2D nanostructures with various configurations such as nanowires,9,10 nanorods,12,13 nanobelts,14,15 nanorings,16,17 nanotubes,18–20 nanosheets,21 and other novel nanostructures22,23 have been fabricated. In fact, successful preparation of these “manipulated” 1D and 2D building units has made the further assemble into advanced 3-D superstructures possible. Recently, self-assembly of ZnO micro- and nanospheres based on the above-mentioned 1D and 2D nanobuilding blocks has been documented by different methods.24 Among these approaches, the wet chemical method has been widely employed, because of its advantages of simple operation and mild reaction condition. Now the solution method combined with the template technique is proved to be an extremely useful approach and easily implemented for the synthesis of spherical structures, in which the template is either removed by posttreatment or “sacrificed” during the reaction process. For instance, micro- and submicroscale hollow ZnO dandelions with nanorods and nanoplatelets were synthesized via a Kirkendall process by Zeng,25 in which Zn powders were used as a hard template. Jiang et al.26 obtained the hierarchically size- and * To whom correspondence should be addressed. E-mail: zhouxf@ njut.edu.cn. † Nanjing University of Technology. ‡ Nanjing University.
morphology-controlled ZnO microspheres by using ZnO nanowires as the building units through a similar method. Furthermore, polymer is usually used as a soft template directing the microsphere assembly of the ZnO nanostructures. Our groups27 have reported the microspheric organization of ZnO nanorods and nanocones directed by the poly(ethylene glycols) (PEG) with different molecular weights. Mo et al.28 have given an example of ZnO nanorod-based microhemisphere assemblies obtained in the presence of a long-chain Ploy (sodium 4-styrenesulfonate) (PSS). Layered metal hydroxide salts (LMHs), a family of lamellar compounds, have attracted increasing attention on account of their specific structure and potential applications for ionexchange, catalysts, absorption, separation, and composite materials.29,30 LMHs consist of two parts: positively charged brucite-like host layers, and exchangeable anions and water molecules in the interlayer. It is reported that different metal oxides (MOs) could be massively obtained by thermaldecomposition of the corresponding LMHs compounds.31,32 Zinc hydroxide salts, such as zinc hydroxide carbonate and zinc hydroxide acetate, could be used as a precursor to prepare nanostructured ZnO. For example, Zhang33 has found that the zinc hydroxide chloride single-crystal sheets can be converted to highly oriented polycrystalline ZnO sheets by calcinations. Song34 has prepared hierarchical ZnO nanostructures by thermal treatment of layered zinc hydroxide acetate. Compared with the existing reports on the spherical assembly of micronanostructured ZnO superstructures, to the best of our knowledge, the studies about the microspheric self-assembly of mutilayered ZnO nanosheets with hierarchically mesoporous structures using a layered precursor of zinc hydroxide carbonate (denoted as ZnHC) are still less reported. In this paper, we present here a simple route to large-scale fabrication of a novel superstructure of multilayered ZnO nanosheets with hierarchically mesoporous structures from a
10.1021/jp802619j CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
Organization of Multilayered ZnO Nanosheets hydrothermally prepared ZnHC precursor. Microspheric assembly of layered ZnHC nanosheets is synthesized in situ by the hydrothermal method, and conveniently converted into the corresponding ZnO nanosheets with hierarchically mesoporous structures followed by a calcination process without deformation of the microstructures. Experimental Section All chemicals are analytical-grade regents without further purification. In a typical experiment, 0.002 mol of Zn(NO3)2 · 6H2O, 0.01 mol of urea, and an amount of cetyltrimethlammonium bromide (CTAB) were dissolved in a mixed solution of 5-10 mL of 1-butanol and 50 mL of deionized water. The mixed solution was further magnetically stirred for 2 h. The solution was then transferred into a Teflon-lined autoclave of 80-mL capacity. The autoclave was heated to 373-423 K for 8-24 h. After being cooled to room temperature, the white powders were collected and washed several times with distilled water and ethanol to remove the impurities and then dried at 353 K for 8 h. Postannealing of the obtained hydrothermal products was carried out at 573 K for 5 h in a temperature-programmed Muffle furnace. XRD measurements were performed on a Bruker-D8Advance X-ray diffractometer, with graphite monochromatized highintensity Cu KR radiation at 40 KV and 30 mA. Scanning electron microscopy (SEM) pictures were recorded on a FEI Quanta-200 instrument. The high-resolution transmission electron microscopy (HRTEM) observations and energy-dispersive X-ray analysis (EDX) were performed on a JEOL JEM2010UHR instrument at an acceleration voltage of 200 KV. Renishaw confocal Raman spectroscopy (Invia, λ ) 514 nm) was used to detect the ZnO superstructures. Infrared spectra (Bruker Tensor27, Germany) were used for detection of the precursor and ZnO. Thermal analysis (TG/DSC, STA409, Netzsch, Germany) was performed at a heating rate of 5 deg/ min under a nitrogen atmosphere. Brunauer-Emmett-Teller (BET, Omnisorp100cx, Coulter, USA) measurement was used to give insight into the porous structure and distribution of the samples. The photoluminescence (PL) spectra were measured with an excitation wavelength of 328 nm at room temperature. Results and Discussions 1. Morphology and Structures. SEM images in parts A and B of Figure 1 show that the monodisperse and uniform flowerlike microspheres are constructed by the ZnO nanosheets. The diameter of microspheric assembly of the ZnO nanosheets is around ∼15 µm. The building blocks, i.e., ZnO nanosheets with a length of ∼7 µm and a thickness of several nanometers, are concisely aligned out of the microspherical assembly in a dispersive mode (as shown in Figure 1C). The surface of ZnO nanosheets is smooth, while the fringes of ZnO nanosheets are serrate. The 2-D anisotropic ZnO nanosheets were organized into 3-D isotropic microspheres, which are different from some previous studies mainly focused on nanorods as the nanobuilding blocks. Figure 1D is the typical XRD pattern of the ZnO obtained from the thermal decomposition of hydrothermally prepared ZnHC. The ZnO phase is indeed well crystallized. All of the peaks can be indexed to hexagonal ZnO with a wurtzite structure, and the measured lattice constants of c and a of this hexagonal phase are 5.21 and 3.25 Å (P63mc, JCPDS Card No. 36-1451), respectively. No other diffraction peaks were detected, indicating that no impurity exists and the precursor have completely transformed into the ZnO phase.
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11723 Considering the details of morphology and structure of the as-prepared ZnO nanosheets, the high-resolution transmission electron microscopy is employed. The samples are dispersed in anhydrous ethanol and experienced a 10 min supersonic pretreatment in a pulverizer with a power of 200 to 800 W. The ZnO nanosheets with the disordered pore structures, which detached from the microsphere assembly with strong supersonic treatment, are clearly shown in Figure 2. It is apparent that the size of the mesopores is hierarchically distributed in a range of 2-50 nm from the TEM observations (also see the Supporting Information). Figure 2A clearly shows the porous structures of the ZnO nanosheets. Furthermore, the red circle marked area shows the pores with an approximate size of 50 nm are embedded in the ZnO nanosheet. Figure 2B shows the magnified TEM image obtained from the marked fringe of the ZnO nanosheet in Figure 2 A. The mesopores with irregular shape and an estimated size of ∼15 nm are clearly observed in the area of the red circle. A further magnified image is shown in Figure 2C, which detects lots of mesopores with smaller diameters of approximate 5 nm in the ZnO nanosheet. It is interesting that the ZnO nanosheet building block is not a single layer but mutilayered through the observations from Figure 2D (also see the Supporting Information, Figure S2.). The red arrowheads in Figure 2D indicate the visible boundary of the different layers of the ZnO nanosheet building block, and the boundary of the different layers was outlined by the red dashes, respectively. The green arrowhead indicates two partially overlapped mesopores embedded in two different layers, and it is also evident that the ZnO nanosheet building block is multilayered. The HRTEM image (inset in Figure 2D) shows an absolutely penetrated pore. The orderly and clear lattice fringes, parallel to each other, show that the pores are wellcrystallized, and the interplanar distance between adjacent lattice planes is 0.256 nm, corresponding to the d-spacing value of (001) planes. Although the ZnO nanosheet building block is a single crystal, it is interesting that the ZnO nanosheet seems flexible from the SEM observations attributing to their thinness. The flexibility of the ZnO nanosheet maybe favors its great potential application in piezoelectric properties. Energy dispersive X-ray (EDX) spectroscopy shows that microscale spherical assemblies of ZnO nanosheets are elementally composed of zinc and oxygen, and that no other elements are detected. To give further insight into the porous structure and pore size distribution of the as-obtained products, the Brunauer-EmmettTeller (BET) measurement was performed to examine the characteristic of the pores. As shown in Figure 3, the nitrogen adsorption-desorption isotherm belongs to type IV, revealing the existence of abundant mesoporous structures in the architectures. From the corresponding pore diameter distribution curves (inset in Figure 3), we find that the size of mesopores is not uniform, but hierarchically distributed in the range of 2-50 nm. This fits well with the TEM results. The pore sizes are mainly distributed around the estimated sizes of 5, 15, and 50 nm. Note that the peak around 3 nm is too small, which is derived from consumption of CTAB,35 and we attribute the larger size of the mesopores mainly arising from the explosive release of CO2 in the confined space of the smaller mesopores during the thermal process of ZnHC precursor.36 It is reasonable that the explosive release of CO2 in the confined space and the collapse of interconnected mesopores enlarge and change the smaller mesopores into the irregularlly shaped larger mesopores. The specific surface area of the as-synthesized ZnO is found to be approximately 30.33 m2 g-1, which is larger than that of commercial ZnO powders (ca. 4-5 m2 g-1). The specific surface
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Figure 1. (A and B) SEM overviews of the microspheric assembly of ZnO nanosheets obtained by thermal decomposition of ZnHC at 573 K. (C) SEM image of an individual microspheric assembly of ZnO nanosheets. (D) XRD patterns of the obtained ZnO products.
area is basically identical with that of the reported ZnO mesocrystal,37 while Polarz et al.38 have obtained the ordered mesoporous ZnO with a high surface area of up to 200 m2 g-1 based on mesoporous carbons matrix. The surface area is not greatly improved as expected in our experiments, which is obviously related to the fact that the ZnO nanosheet building block is a special lamellar structure, and pores embedded in the nanoscale thin layer do not have too much inner surface area. The as-synthesized organized microspheres of the ZnO nanosheets were further investigated with use of the confocal Raman spectrum. As we can see form Figure 4A, the remarkable Raman mode at 438 cm-1, which is the characteristic band of the wurtzite ZnO, is attributed to the ZnO nonpolar optical phonons E2-(high) vibration mode. The peak at 331, 380 cm-1 can be attributed to the 3E2H-E2L mode of ZnO and the A1(TO) model, respectively. In addition, the low band centered at 580 cm-1 is a superposition of the A1(LO) mode at 574 cm-1 and
the E1(LO) mode at 583 cm-1 in which Zn atoms and O atoms have the same vibration direction, respectively, to the neighbor lattices of the wurtzite ZnO.39 Obviously, the results reveal that the as-synthesized ZnO are the wurtzite structure. The room temperature photoluminescence (PL) spectra of the as-obtained microspherical assembly were recorded under UV excitations with an excitation wavelength of 328 nm. As shown in Figure 4B, the plot displays an intense UV emission at about 390 nm and a relatively low blue-green light emission around 450 nm, similar to those reported by Song.34 They are attributed to the excitonic transitions with a band gap of 3.24 eV and a photoexcited vacancy with a specific defect such as oxygen vacancies, respectively. No other peaks are observed at longer wavelengths. 2. Growth Mechanisms. To obtain the details on evolving from the precursor of ZnHC to ZnO and the correlation between heat treatment and structural change of the products, the precursor ZnHC is minutely characterized by multiplicate
Organization of Multilayered ZnO Nanosheets
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11725
Figure 2. (A) Typical TEM image of an individual ZnO porous nanosheet. (B and C) Magnified TEM images obtained from the white dashmarked fringe of ZnO nanosheet. (D) HRTEM image of ZnO nanosheet.
Figure 3. Nitrogen adsorption/desorption isotherm and BarrettJoyner-Halenda (BJH) pore size distribution plot (inset) of the assynthesized ZnO microspheres.
measurements. Parts A and B of Figure 5 show the SEM images of the precursor of hydrothermally prepared ZnHC prior to further heat treatment, which displays ZnHC nanosheets-based
assembly with a typical diameter of ∼15 µm. Moreover, from the SEM investigation, we can see that both the morphology and the size of the assembled ZnHC microsphere are almost the same as that of the ZnO porous nanosheets-based microsphere, implying that heat treatment has little influence on the morphology of the final products. Thermal treatment only induces ZnHC into ZnO and forms mesoporous structures in ZnO nanosheets. Figure 5C shows the XRD pattern of the hydrothermally synthetic precursor, which indexes very well to Zn4(CO3)(OH)6 · H2O (JCPDS Card No. 11-0287). Zn4(CO3)(OH)6 · H2O is the typical metal hydroxide salt (MHS), which is a kind of lamellar structure compound. We can also see from Figure 5B, that it is distinct that the building blocks of the microspherical structure have the characteristics of a sheet-like body. Consequently, it is reasonable that LMHS compounds can be converted to the corresponding multilayered metal oxides (LMO) without microstructural deformation.
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Figure 4. (A) Raman spectra of the as-synthesized microspheric assembled porous ZnO nanosheets. (B) Photoluminescence spectra of the assynthesized ZnO microspheres.
Figure 5. (A) SEM overview and (B) an individual as-synthesized ZnHC precursor. (C) XRD patterns of the ZnHC precursor. (D) TG-DSC plot of the precursor.
Thermal properties of the as-synthesized precursor ZnHC were evaluated by Thermogravimetic analysis and differential
scanning calorimeter (TG-DSC). Test conditions were started from 0 to 1273 K at a rate of 5 deg/min in a flowing nitrogen
Organization of Multilayered ZnO Nanosheets
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11727 of carbonate and organic groups are greatly weakened, even vanishing in the range 1500-800 cm-1, indicating complete decomposition of ZnHC and formation of ZnO nanosheets assembled microspheres. Urea plays an important role during the solution reaction process. It is used as a dual-role agent, not only as a provider of carbonate but also as a slow-released pH adjusting agent. The reaction in the hydrothermal process could be simply formulated as follows:
CO(NH2)2 + 3H2O f 2NH3 · H2O + CO2
(2)
H2O + CO2 f 2NH4+ + CO32-
(3)
Zn2 + 2OH- f Zn(OH)2
(4)
NH3 · 2NH3 · Figure 6. FTIR spectra of (A) the precursor and (B) ZnO obtained by calcination at 523 K.
(1)
H2O f NH4+ + OH-
4Zn2+ + 6OH- + CO32- + H2O f Zn4(CO3)(OH)6 · H2O (5)
Figure 7. Schematic illustration of the formation process of the microspheric assembly.
environment. It is well-known that Zn4(CO3)(OH)6 · H2O is converted to zinc oxide by release of H2O and CO2 with the elevating temperature, as described in the following equation.
Zn4(CO3)(OH)6 · H2O f 4ZnO + 4H2O (16.3 mass loss %) + CO2 (10.0 mass loss %) The TG-DSC curves of the precursor are shown in Figure 5D. It can be seen that the DSC plot presents the sole endothermic peak at 538 K, which corresponds to the decomposition of hydroxyl and carbonate of the precursor. The weight loss occurs in a narrow temperature range from 473 to 573 K, and the total weight loss is about 26%, which is very close to the expected theoretical value of 26.3%. The curves of TG are slightly increased after around 723 K, maybe the oxygen vacancies of ZnO derived in the thermal process are filled with ambient nitrogen,40 and this experimental phenomenon further confirms the existence of the photoexcited vacancy that led to green PL emission. Figure 6 shows the FT-IR spectrum of the ZnHC precursor and microspheric assembly of ZnO nanosheets. Figure 6A is the FT-IR spectrum of the ZnHC precursor. The majority of infrared absorption peaks are located in the middle and lower frequency zones. The hydroxyl groups and water molecules are confirmed by the broad bands detected at high frequency at around 3376 cm-1, where the broadening of this band is due to hydrogen bond formation between water molecules and CO32in the interlayer. The strong peaks at 1509, 1386, 835, and 709 cm-1 correspond to the vibration and bending modes of CO32-.41 The peaks at 2925, 2853, and 1045 cm-1 belong to -CH3 and -CH2- vibrations, which originate from the surfactant of CTAB. An obscured shoulder peak appearing at 1634 cm-1 arises from the bending mode δH2O of the water molecule; it becomes remarkable after thermal treatment as shown in Figure 6B. IR spectra of ZnO nanosheets show that absorption peaks
Initially, the aqueous solution is a neutral environment, and then slowly turns to weak alkaline with increasing hydroxyl, owing to hydrolysis of the urea. Zinc salt solution reacts with hydroxyl to form Zn(OH)2. As an amphoteric compound, zinc hydroxide coordinated by OH-, can give birth to soluble species such as [Zn(OH)4]2-, hence “zinc hydroxide” is more stable in weak basic condition.31,33,42 Zinc cations and carbonates in the aqueous solution lead to the formation of ZnHC precursor. Cetyltrimethlammonium bromide (CTAB) is used in our experiment as a shape-controlled agent, absorbing on the surface of Zn(OH)2 nuclei to control their further agglomeration and keep their size uniformity. The comparative experiment is performed in the absence of CTAB, and the results show that the as-synthesized products exhibit a great difference in size, and some are even broken. Summing up the above results and discussion, the most plausible mechanism of the dissociation-deposition that directs the formation of microspheric assembly of ZnO nanosheets from ZnHC precursor is proposed and schematically illustrated in Figure 7. This process is somewhat similar to the mechanism reported by Xu et al.43 First, when the solution concentration reaches the degree of supersaturation in basic condition, solid particles, that is Zn(OH)2 precipitate, are shaped through nucleation and growth. Meanwhile, these units also acted as the “sacrificial” templates for directing the nanosheets growth in the followed steps (Figure 7a). We consider that the dissociation of Zn(OH)2 occurring at the solution-solid interface is a homeostasis process, and zinc cations contributed by the dissociation of Zn(OH)2 are able to react with hydroxyls, carbonates, and waters in the solution, then generate the sheetlike ZnHC precursor, perpendicularly against the spherical surface of the Zn(OH)2 nuclei, and form the upright-standing quasiarray (Figure 7b). Influenced by the concentration gradient, the sheet-like ZnHC crystals grow vertically from inside to outside. The reason that the peculiar structure can maintain stabilization is that, on the one hand, the orderly arrangement of nanosheets is favorable to minimize the systemic surface energy via reducing exposed areas.44 On the other hand, Hosono et al.31 reported that the surface of the layered zinc hydroxide carbonate is hydrophobic, whereas the lateral sides are hydrophilic. In this case, the Zn(OH)2 nuclei are distinctly a hydrophilic substrate, therefore ZnHC nanosheets only grow vertically on the surface of Zn(OH)2 nuclei. As the reaction progresses, more Zn(OH)2 dissociates and more ZnHC nanosheets are formed and steadily arrayed on the surface of Zn(OH)2
11728 J. Phys. Chem. C, Vol. 112, No. 31, 2008 nuclei, until the center Zn(OH)2 nuclei completely dissociate and eventually form a flower-like microspherical morphology (Figure 7d). Pure ZnO with the wurtzite structure is obtained by calcinations of precursor ZnHC without deformation in structures. The thermal decomposition of ZnHC and CTAB and the release of gas in confined space during the thermal process lead to formation of the hierarchically porous structure (Figure 7e). We believe this kind of ZnO superstructure will find its potential applications in catalytic, optical, electronic, optoelectronic, ultrasensitive gas sensing, and piezoelectric properties on their collective sensing. Conclusions In summary, we here reported a facile and cheap route to prepare flower-like microspheric assembly of multilayered ZnO nanosheets with hierarchically mesoporous structures by a hydrothermal and postcalcination approach. The sheet-like zinc hydroxide carbonates obtained by the hydrothermal method were used as the precursor, and converted to ZnO crystals after thermal-decomposition treatment without the morphology deformation. The micorspheres of ZnO nanosheets had a diameter of around ∼15 µm, and the hierarchically mesoporous structures in the ZnO nanosheets building blocks were observed. The spherical structure was formed in the hydrothermal process, and postannealing treatment of ZnHC precursors merely induced the formation of mesoporous ZnO. Studies showed hierarchical mesopores with well-crystallized structures formed in the multilayered ZnO nanosheets building blocks of the microspheres. On the basis of the experiment, we proposed a plausible dissociation-deposition mechanism using in situ formation of Zn(OH)2 nuclei as the “sacrificial” templates for the microspherical assembly of multilayered porous ZnO nanosheets. Acknowledgment. This research was financially supported by the National Science Foundation of China (No: 20636020) Supporting Information Available: SEM and TEM images of the multilayered porous ZnO nanosheets. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Colfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (2) Liu, B.; Yu, S. H.; Zhang, F.; Li, L. J.; Zhang, Q.; Ren, L.; Jiang, K. J. Phys. Chem. B 2004, 104, 4338. (3) Sounart, T. L.; Liu, J.; Voigt, J. A.; Hsu, J. W. P.; Spoerke, E. D.; Tian, Z.; Jiang, Y. B. AdV. Funct. Mater. 2006, 16, 335. (4) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (5) Yang, J. L.; An, S. J.; Park, W. I.; Yi, G. C.; Choi, W. AdV. Mater. 2004, 16, 1661. (6) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (7) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (8) Tammy, P. C.; Zhang, Q. F.; Glen, E. F.; Cao, G. Z. AdV. Mater. 2007, 19, 2588. (9) Wu, J. J.; Liu, S. C. AdV. Mater. 2002, 14, 215.
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