DOI: 10.1021/cg901030e
Dissolution-Recrystallization Induced Hierarchical Structure in ZnO: Bunched Roselike and Core-Shell-like Particles
2010, Vol. 10 626–631
Xiulan Hu,* Yoshitake Masuda, Tatsuki Ohji, and Kazumi Kato Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST ), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, Japan Received August 26, 2009; Revised Manuscript Received November 26, 2009
ABSTRACT: Zn(OH)2/ZnO nanohybrid particles were fabricated by the dissolution-recrystallization route in hot distilled water. Hierarchical structured ZnO particles were easily obtained by the further dehydration reaction without morphology deformation. X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy clarified the formation mechansim. Thermodynamically less stable polar planes (0001) of ZnO rods tended to dissolve quickly; the thermodynamically stable Zn(OH)2 first grows on ZnO surfaces by heterogeneous nucleation to minimize their surface energy. The morphologies, bunched roselike and core-shell-like structures, are controllable by adjusting the immersion period. Further dissolution-recrystallization resulted in the formation of core-shell morphology. As-fabricated hierarchical structured products will have potential applications in optoelectronic devices such as dye-sensitive solar cells and gas sensor due to its high porous structure and strongly adsorption property.
Introduction Zinc oxide (ZnO), as an important functional oxide, is a direct wide band gap (3.37 eV) semiconducting and piezoelectric material having many useful properties, such as optical absorption and emission, electromechanical or vice versa conversion, photocatalysis, and sensitivity to gases.1-4 It is well-known that the shape and size of nanocrystals can control their widely changing optoelectronic and chemical properties. Various methods generally were adopted to prepare ZnO particles such as the sol-gel method,5 the microemulsion method,1 thermal decomposition,6 hydrothermal synthesis,7 chemical bath deposition,8,9 and vapor phase transport.10 Different synthesis methods gave ZnO particles of various sizes and shapes, such as sheetlike and platelike,11,12 roselike,13 starlike and needlelike,14,15 flower, disk, and dumbbell-like,16 flower-like made up of thin nanosheets,17 tetraneedle-like,18 tower-like and tubelike,9 and ellipsoidal.19 Controlling the size and morphology of ZnO particles is a matter of considerable importance for the manufacture of microcrystalline particles, because for many applications both the size and shape of the particles determine the usefulness of the product.15 Recently hierarchical structured materials, which possess dual or multiple morphologies and structures, are greatly valuable for performance enhancement and further realizing potential applications. ZnO nanosheets produced in a wafer scale on polycrystalline Al substrates show higher efficiency in photodegrading organic dyes than ZnO nanorods.20 Hierarchical structured ZnO (joint sheetlike nanostructure, single sheetlike nanostructure, thornlike nanostructure), which were fabricated assisted by a sodium dodecyl sulfate (SDS) surfactant and adjusting the reaction temperature via a wet-chemical route, exhibit highly sensitive gas sensing characteristics combined with excellent optical properties.21 In the present study, we addressed a solutionbased approach to fabricate hierarchical structured ZnO *Corresponding author. E-mail:
[email protected] or whoxiulan@ gmail.com. pubs.acs.org/crystal
Published on Web 12/17/2009
(bunched roselike and core-shell-like) by a simple dissolution-recrystallization route without using any surfactant or catalyst. These morphologies are controllable by adjusting the dissolution-recrystallization periods in the hot water. Experimental Section The starting materials for preparing ZnO rods were zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O, 99%), hexamethylenetetramine (HMT, C6H12N4, 99%), and polyethylenimine (PEI, (C2H5N)n, branched mean molecular weight of 600, 99%). All chemicals (Wako Pure Chemical Industries, Ltd., Japan) were used as received without further purification. Slide glass (Matsunami Micro Slide Glass) was used as the loading substrate of ZnO products. Scheme 1 shows the schematic processes for fabricating hierarchical structured ZnO particles. A 200 mL aqueous solution of zinc nitrate hexahydrate, hexamethylenetetramine, and polyethylenimine with molar ratio of 5:5:1 was prepared for deposition of ZnO. The initial total concentration of Zn2þ was fixed at 0.1 M. The reaction
Figure 1. XRD pattern of as-deposited ZnO on the glass substrate. r 2009 American Chemical Society
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Figure 2. FE-SEM images of products versus the immersion periods: (a) 0; (b) 10; (c) 20; (d) 25; (e) 50; (f ) 60 days.
Scheme 1. Schematic Processes for Fabricating Hierarchical Structured Products
system has been reported for fabrication of a ZnO nanowhiskers film in elsewhere.22,23 After 30 min stirring, the solution was heated to 88 °C in a thermostatically regulated oil bath. Thus the clean glass substrates were tilted, immersed in the preheated solution, and kept at 88 °C for 1 h. The glass substrate loaded with ZnO precipitates was drawn from the reaction solution to wash using distilled water several times and then tilted and immersed in distilled water (60 mL) in a glass bottle at room temperature. Thus the closed bottle was kept in an oven at 65 °C for a desired time without stirring. Finally, the glass substrate was taken out of the water, rinsed with distilled water, and air-dried at 65 °C in oven or dehydration reaction at 150-350 °C for 2 h for characterization. The products were characterized using X-ray diffraction (XRD; RINT-2100 V, Rigaku) with Cu KR radiation (40 kV, 30 mA) at a
scan rate of 2°/min, field emission scanning electron microscopy (FE-SEM; JSM-6335FM, JEOL Ltd.) with an accelerating voltage of 5 kV and emission current of 12 μA, and transmission electron microscopy (TEM; Hitachi HF-2000, Hitachi Kyowa Engineering Co. Ltd.). The chemical compositions were analyzed by FE-SEM (FE-SEM; S-4300, Hitachi) in conjunction with energy-dispersive X-ray (EDX) measurements. Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method using krypton adsorption isotherms in the range of 0.1 < P/P0 < 0.3.
Results and Discussion Morphology and Structure of ZnO Particles. Figure 1 shows the XRD pattern of as-deposited products on the
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Figure 3. FE-SEM images of products prepared by 85 days immersion: (a, b) a large scale area; (c, d) magnified views of the surrounding surface and top surface, respectively; (e, f ) inner views of the core-shell structured ZnO.
glass substrate in the aqueous solution at 88 °C for 1 h. The broaden peak at about 2θ ≈ 25° is derived from the glass substrate. The other diffraction peaks are in good agreement with the JCPDS card (36-1451) for a typical wurtzite-type ZnO crystal (hexagonal, P63mc). The FE-SEM image was shown in the Figure 2a. The individual ZnO rod seems to grow homocentrically like the flower shape. Figure 2 shows the morphology changes of ZnO particles versus immersion periods by FE-SEM. The amount of nanosheet phase increased as a function of the immersion period. At a short time of 10 days, a smaller amount of nanosheet phase was easily observed at the top of the ZnO rods in Figure 2b. When the immersion time increased to 20 days, a large amount of nanosheet phase was easily observed at the top of the rods. After 25 days, many more nanosheets were formed, becoming like a bunched rose. The surrounding surfaces along the long direction of the ZnO rods were still observed in Figure 2c,d. In the case of 50 days, nanosheets were also observed on the surrounding surfaces of ZnO rods in Figure 2e. The ZnO rods were covered completely with nanosheets when the immersion time
increased to 60 days as shown in Figure 2f. It was wellknown that the ZnO rod was structured with the (0001) crystal face at the top and (1010) crystal face in the surrounding planes. Thus these results clarified that the nanosheet phase first grows on the top of the (0001) crystal face at an initial stage. Figure 3 shows the FE-SEM images of the products for a longer immersion time of 85 days. Comparison to results of 60 days in Figure 2f, many more nanosheets covered the whole surface of ZnO rods as shown in Figure 3a,b. The magnified views of the surrounding and top surfaces as shown in Figure 3c,d indicate the morphology of the nanosheets. The inner views in Figure 3e,f show that the core-shell structure consisted of destroyed rod as the core (marked region with dashed line) and nanosheets as the shell. These results clarified that formation of the nanosheets is ascribed to the dissolution of predeposited ZnO rods. The detailed formation mechanism will be discussed later. Moreover, the BET specific surface area of the core-shell structured ZnO product obtained from 60 days treatment of 25.11 m2/g (Figure 4) is higher than that of hierarchical
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structured ZnO derived from a sodium dodecyl sulfate (SDS) surfactant induced wet-chemical route (9.860 m2/g).21 The result indicates the enhanced adsorption properties of the core-shell structured ZnO products. It was noted that the BET specific surface area of the core-shell ZnO product may become larger with increasing immersion time due to further dissolution of ZnO rods and concurrent growth of nanosheets. The detailed comparisons are being studied in order to indicate the ability to control the core-shell structure and surface properties. It was noted that the nanosheet product was found to grow on the surface of a glass substrate by heterogeneous nucleation and growth. To further confirm the chemical composition of the nanosheet product, EDX analysis was carried out. The nanosheet products grown on the surface of a beaker were collected on a Si substrate for characterization. Figure 5a shows the FE-SEM image. The EDX spectrum shown in
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Figure 5b identifies the elements Zn and O in the nanosheet product, in addition to elements Si and Al in the Si substrate. Figure 6 is a typical TEM image of the as-obtained product at 65 °C for 25 days without sintering treatment. Nanosheets were formed on the top, together with a few on the surrounding surface, of the ZnO rod. Insets are the selected area electron diffraction (SAED) of the circled areas. The SAED pattern of the nanosheet part confirmed that the nanosheet is a single crystal and originated from Zn(OH)2. The SAED pattern of the rod part confirmed that the rod is still a single crystal with hexagonal structure and originated from ZnO. Thus these results confirmed that the as-grown nanosheet product is zinc hydroxide. Growth Mechansm of the Zn(OH)2 Nanosheet. As is wellknown, the thermodynamic free energies (ΔGf°) are -318.3 and -553.6 kJ 3 mol-1 for ZnO(s) and Zn(OH)2(s), respectively at 25 °C. Thus Zn(OH)2 is readily formed as a thermodynamically stable material at an initial stages during the ZnO synthesis process. At elevated temperatures, the precursor Zn(OH)2 transforms to ZnO, depending on the degree of supersaturation and the chemical potential of OH-, by the following reaction:24-27 Zn2þ þ 2OH - T ZnðOHÞ2 ðsÞ T ZnOðsÞ þ H2 O
Figure 4. BET surface area of core-shell structured ZnO (65 °C, 60 days).
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The solubility of ZnO is almost the same as that of Zn(OH)2 at 25 °C from the phase stability diagrams of ZnO(s)-H2O and Zn(OH)2(s)-H2O as a function of pH.28 As has been reported, the pH of solution (at a particular temperature) is the primary factor determining the stable hydrolytic state of a solubilized metal ion aquo complex shown as follows: (i) positively charged metal ion hydroxocomplexes are the stable forms in acidic solutions; (ii) negatively charged metal ion hydroxocomplexes are the stable forms in alkaline solutions; and (iii) uncharged hydroxocomplexes tend to be the thermodynamically stable forms in neutral pH solutions.29 In the present study, the as-deposited ZnO particles were kept in neutral pH water, thus thermodynamically stable and uncharged hydroxocomplexes, Zn(OH)2, inclined to form as
Figure 5. FE-SEM image (a) and EDX spectrum (b) of nanosheet product loaded on the Si substrate.
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Figure 6. A typical TEM image of the as-obtained product at 65 °C for 25 days without sintering treatment. Insets are the selected area electron diffraction (SAED) of the circled areas.
the stable products by the dissolution-recrystallization. In general, wurtzite crystal ZnO was a polar crystal, having a hexagonal unit cell with nonpolar (1010) prismatic faces capped by polar (0001) and (0001) basal planes.30,31 Polar faces with surface dipoles are thermodynamically less stable than nonpolar faces, often undergoing rearrangement to minimize their surface energy. Recently, the reported fabrications of ZnO microtubes and nanosheets were related to the preferential chemical dissolution of the metastable (0001)-Zn faces.32,33 Thus the stable Zn(OH)2 phase readily grew on the polar planes of ZnO rod by heterogeneous nucleation due to ZnO dissolution. Further dissolution of ZnO caused a greater amount of Zn(OH)2 nanosheet formation. These results are in good agreement with the previously reported solubility behavior of ZnO in aqueous solutions.29,34,35 Crystallization to Hierarchial Structure in ZnO. As was reported, Zn(OH)2 was produced in industry as a precursor of ZnO by the further dehydration reaction. In this study, ZnO was produced from the Zn(OH)2 nanosheet at 150350 °C for 2 h by a solid phase transformation process without morphology deformation. Thus hierarchial structured ZnO nanosheet/ZnO rod particles were readily produced from the nanostructured Zn(OH)2/ZnO hybrid particles. These results clarified that the dissolutionrecrystallization route allows the design of hierarchial structured ZnO nanosheet/ZnO rod particles according to the desired applications. Conclusion Hierarchical structured ZnO particles (typically a bunched roselike and core-shell-like morphology) were fabricated in hot distilled water at 65 °C by the dissolution-recrystallization and subsequent dehydration treatment without
morphology deformation. The dissolution of ZnO induced the formation of thermodynamically stable Zn(OH)2 on its surfaces by heterogeneous nucleation and growth in a neutral pH solution. A prior dissolution of the thermodynamically less stable (0001) polar planes of ZnO rods resulted in the initial growth of thermodynamiclly stable Zn(OH)2 phase on its polar planes to minimize their surface energy. The hierarchical morphologies are controllable as a function of immersion periods. Further dissolution of ZnO promoted the growth of lower thermodynamic free energy Zn(OH)2 and resulted in the various hierarchical structures. As-fabricated hierarchical structured products will have potential application in dye-sensitive solar cells and gas sensors due to their highly porous structure and strong adsorption property.
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