Facile Synthesis and Enhanced Photocatalytic Performance of Flower

Dec 16, 2009 - The flower-like ZnO sample shows an enhanced photocatalytic performance compared with the other nanostructured ZnO powders of nanoparti...
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Facile Synthesis and Enhanced Photocatalytic Performance of Flower-like ZnO Hierarchical Microstructures Benxia Li* and Yanfen Wang College of Science and Engineering of Materials, Anhui UniVersity of Science and Technology, Huainan, Anhui 232001, People’s Republic of China ReceiVed: October 2, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009

ZnO hierarchical microstructures with uniform flower-like morphology were prepared on a large scale through a template- and surfactant-free low-temperature (80 °C) aqueous solution route. The product was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Brunauer-EmmettTeller N2 adsorption-desorption analyses. The flower-like ZnO microstructures are assembled by many interleaving nanosheets which have the uniform thickness of about 10 nm and a well-crystalline structure with dominant surfaces as {21j1j0} planes. Control experiments revealed that the formation of the flower-like ZnO was based on the fast nucleation-growth kinetics. The flower-like ZnO sample shows an enhanced photocatalytic performance compared with the other nanostructured ZnO powders of nanoparticles, nanosheets, and nanorods, which can be attributed to the special structural feature with an open and porous nanostructured surface layer that significantly facilitates the diffusion and mass transportation of RhB molecules and oxygen species in photochemical reaction of RhB degradation. 1. Introduction Facile approaches to the hierarchical micro-/nanoarchitectures with controlled morphology, orientation, and surface represent a significant challenge in the field of nanoscaled science, because these parameters determine the optical, electronic, and catalytic responses of materials.1 Currently, self-assembly of nanoscaled building blocks into complex structures has been a research hotspot. Material scientists have paid more and more attention to the organization of complex micro-/nanoarchitectures, especially three-dimensional (3D) hierarchical architectures which are assembled by nanoscaled building blocks such as nanoplates, nanorods, nanoparticles, and so on.2 Such hierarchical architectures combining the features of nanoscale building blocks will show unique properties different from those of the monomorphological structures. Thus, developing facile and environment-friendly routes to produce 3D hierarchical micro-/ nanostructures is very important to nanoscience and synthetic chemistry. Zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor, has stimulated great research interest due to its unique optical and electrical properties that are useful for nanolasers,3 piezoelectric nanogenerators,4 solar cells,5 gas sensors,6 photocatalyst,7 and so on. Considerable effort has been devoted to fabricating various ZnO nanostructures, including nanowires,8 nanorods,9 nanobelts,10 nanotubes,11 nanodisks,12 and so on, because of their size- or morphology-dependent properties or device performances. Recently, semiconductor photocatalysts on a nanometer scale have become more and more attractive due to their different physical and chemical properties from bulk materials.13 As one of the most important semiconductor photocatalysts, ZnO has attracted considerable interests because of its high photosensitivity and stability.7,8b Due to the fact that a photocatalytic reaction occurs at the interface between catalyst and organic pollutants,14 the photocatalytic activity of ZnO is * To whom correspondence should be addressed. Tel.: 86-554-6668649. E-mail: [email protected].

strongly dependent on the growth manner of the crystal, and it has been demonstrated that fine-tuning of face orientation could result in optimization of the photocatalytic activity of nanostructured semiconductors.15 As a polar crystal, ZnO is constructed from a number of positively charged (0001) planes rich in Zn2+ ions, alternating with negatively charged (0001) planes rich in O2- ions, stacked along the c axis.16 Normally, ZnO nuclei tend to aggregate along the c axis resulting in a onedimensional (1D) nanostructure (nanowires, nanobelts, nanorods, and nanotubes) due to electrostatic interaction.17 Although recent reports have demonstrated that the ZnO nanoplates can be generated by using polymers and surfactants as growth modifiers to suppress crystal growth along the [0001] axis,18 there still remains a significant challenge to directly prepare twodimensional (2D) ZnO nanoplates or nanosheets as well as to fabricate 3D hierarchical and complex ZnO structures assembled by these nanosheets. Such 3D hierarchical structures combining the features of nanoscaled building blocks not only show unique properties different from those of the monomorphological structures, but also may provide more opportunity for the surface photochemical reaction to realize region-dependent surface reactivity, due to their high specific areas and porous nanostructured surface layers.19 Therefore, to develop convenient synthetic strategies to synthesize the hierarchical ZnO microstructures assembled by nanosheets with a larger population of unconventional planes is desirable and significative for exploring the photocatalytic property of ZnO material. Because of its easily controllable condition and relatively cheap equipments, the lowtemperature synthesis in an aqueous solution is highly desirable and represents an environmentally kind and user-friendly approach, which may be considered to be a relatively green chemical alternative of practical significance. However, various organic additives are usually involved in the synthesis of the hierarchical ZnO microstructures.19a,20 With the existence of organic additives, there is considerable chance to absorb organic

10.1021/jp909478q  2010 American Chemical Society Published on Web 12/16/2009

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Figure 1. FESEM images and XRD pattern of the ZnO product prepared by the solution reaction with 2.0 mmol of ZnCl2 and 10.0 mmol of NaOH at 80 °C for 24 h: (a) Panoramic FESEM image; (b, c) magnified FESEM images; (d) XRD pattern.

molecules on the surface of ZnO and thus the efficient surface area decreases, resulting in the decrease in the photodegradation efficiency. On the basis of the above considerations, this work developed a facile and environment-friendly low-temperature (80 °C) aqueous solution route to fabricate flower-like ZnO microstructures on a large scale, without using any organic solvent or surfactant. The as-prepared flower-like ZnO microstructures are built by many interlaced nanosheets which have uniform thickness of 10 nm and well-crystalline structures with the dominant surface being the {21j1j0} plane. Such ZnO structures show a high surface-to-volume ratio and stability against aggregation. As expected, the flower-like ZnO presented a strong morphology-induced enhancement of photocatalytic performance and exhibit the significantly improved photocatalytic efficiency in the photodegradation of rhodamine B (RhB) compared with the monomorphological ZnO powders of nanoparticles, nanosheets, and nanorods. This work not only gives insight into understanding the growth behavior of complex ZnO nanostructures in a solution-phase synthetic system but also sheds some light on the improvement of the photocatalytic performance by designing the assembly of materials. 2. Experimental Section Sample Synthesis. All reagents were of analytical grade, purchased from Shanghai Chemical Co. and used without further purification. In a typical procedure to synthesize flower-like ZnO microstructures, zinc chloride (ZnCl2, 2.0 mmol, 0.2726 g) and sodium hydroxide (NaOH, 10.0 mmol, 0.4000 g) were dissolved in 30 mL of distilled water under stirring. The mixed solution was sealed in a glass bottle (60 mL), kept static at 80 °C for 24 h, and then cooled to room temperature naturally. The final white precipitate was separated by centrifuge, washed with distilled water and absolute alcohol several times to remove the possible residues, and then dried at 80 °C for 12 h.

Characterization. The X-ray diffraction patterns (XRD) were recorded on a Japan Rigaku D/max-χA X-ray diffractometer equipped with graphite-monochromatized highintensity Cu KR radiation (λ ) 1.54178 Å); the field emission scanning electron microscopy (FESEM) was performed on JEOL JSM-6700F; transmission electron microscopy (TEM) images associated with select area electron diffraction (SAED) and high-resolution TEM (HRTEM) images were performed on JEOL-2010 TEM with an acceleration voltage of 200 kV. Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption was measured using a Micromeritics ASAP 2020 V3.01 H analyzer. The UV-vis absorption spectra were recorded on a DUV-3700 DUV-vis-near-IR recording spectrophotometer from Shimadzu Corp. Photocatalytic Activity Measurements. The photocatalytic activity was investigated using an RhB aqueous solution as a probe and a Pyrex beaker (250 mL) as the photoreactor vessel. The reaction system containing 100 mL of RhB solution with an initial concentration of 5.0 × 10-5 M and 30 mg of ZnO samples was magnetically stirred in the dark for 1 h to reach adsorption equilibrium. The solution was then exposed to UV irradiation from a 200 W high-pressure Hg lamp (the strongest emission at 365 nm) at room temperature. Solutions were collected every 20 min to measure the RhB degradation by UV-vis spectra on Shimadzu UV2550 spectrophotometer. 3. Results and Discussion 3.1. Morphology and Structure. Figure 1 shows the FESEM images and XRD pattern of the as-prepared ZnO product. A panoramic morphology of the product is presented in Figure 1a, indicating the high yield and uniformity. A magnified FESEM image showing the close observation of the nanostructures is presented in Figure 1b. It reveals that the detailed morphology of ZnO product is well-defined flower-like three-dimension (3D) microstructures with

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Figure 2. (a) TEM image of a flower-like ZnO microstructure; (b) TEM image of a typical nanosheet from the flower-like ZnO microstructure (inset: SEAD pattern corresponding to the marked frame area, recorded along the [21j1j0] zone); (c) the corresponding HRTEM image.

diameters in the range of 1-2 µm, assembled by many densely arranged nanosheets as “petals”. A close-up view of the nanosheets-built flower-like microstructures in Figure 1c reveals that the nanosheets are about 10 nm in thickness, and they alternately connect with each other to form a network-like surface of the “flower”. Many pores with different sizes are engendered in the 3D microstructures, which may improve the chemical properties or serve as transport paths for small molecules. Figure 1d shows XRD pattern of the product, in which the diffraction of standard hexagonal (wurtzite) ZnO powders is presented (JCPDS No. 36-1451). Obviously, all the diffraction peaks can be indexed to the wurtzite ZnO. Further information about the ZnO product was obtained from TEM and HRTEM images associated with SAED pattern in Figure 2. Figure 2a shows a typical TEM image of a flowerlike ZnO microstructure, confirming the hierarchical 3D structure with diameter around 1.5 µm is constructed by numerous nanosheets. Figure 2b presents a TEM image of a typical isolated ZnO nanosheet obtained by ultrasonic dispersion of the asprepared ZnO sample in ethanol. The corresponding SAED pattern (the inset in Figure 2b) indicates the single crystalline nature of the nanosheet. The HRTEM image shown in Figure 2c exhibits well-resolved two-dimensional lattice fringes with the spacings of 5.2 and 2.8 Å, which are in good agreement with the interplanar spacings of {0001} and {0110} planes, respectively. It indicates that the nanosheet has a well-crystalline structure with {21j1j0} planes as the sheet surface. Full nitrogen sorption isotherms of the 3D hierarchical ZnO architecture were measured to gain the information about the specific surface area and the pore sizes. As shown in Figure 3, the nitrogen sorption isotherms present a reverse “S” shape, which is identified as type IV and characteristic of mesoporous materials.21 The specific surface area was calculated to be 25.1617 m2/g by the BET equation.22 The corresponding Barrett-Joyner-Halenda (BJH) analyses (the inset in Figure 3) exhibit that most of the pores fall into the size range from 2 to 75 nm, and the cumulative pore volume is 0.171516 cm3/g from BJH adsorption. These pores presumably arise from the spaces among the nanosheets within hierarchical ZnO architectures. Although the relatively low surface area for ZnO mesoporous material results from its high density, the present surface area data of the flower-like ZnO is much higher than that of the reported nanostructured ZnO.23 The high surface area and mesoporous structure of the hierarchical ZnO microstructures provide the possibility for the efficient diffusion and transportation of the degradable organic molecules and hydroxyl radicals in photochemical reaction, which will lead to the enhanced photocatalytic performance of ZnO material.

Figure 3. Typical N2 gas adsorption-desorption isotherm of a flowerlike ZnO sample. The inset is the corresponding pore-size distribution.

3.2. Formation of the Flower-like ZnO Microstructures. To understand the formation of the flower-like ZnO hierarchical microstructures, the time-dependent morphological evolution was examined by FESEM. Figure 4 shows the FESEM images of the samples obtained at different reaction stages. The 3 h reaction resulted in the rudiment of flowerlike ZnO which consisted of some incompact nanosheets (Figure 4a), and it was noticed that some tiny nanoparticles appeared on the surfaces of these nanosheets, which might provide the growing points for other nanosheets of next stage. After 6 h reaction, more and larger nanosheets grew out and the shapes of the flower-like ZnO microstructures were further developed (Figure 4b). As shown in Figure 4c, the sample obtained after 9 h reaction is composed of well-shaped flower-like 3D microstructures with larger sizes which are assembled by numerous interleaving nanosheets. With an extension of the reaction time, the morphology and size of ZnO remained nearly unchanged, as shown in Figure 1. In addition, the reaction system was studied by monitoring the influences of the concentrations of Zn2+ and OH- on the morphology of ZnO as recorded by FESEM images in Figure 5. The ZnO sample obtained from the aqueous solution containing 2 mmol of ZnCl2 and 2.5 mmol of NaOH, with other experimental conditions unchanged, consists of conical-like and irregular nanoparticles with a rough surface (Figure 5a). Whereas, in the aqueous solution containing 1 mmol of ZnCl2 and 10 mmol of NaOH, nanosheets with diameters in the range of 300 nm to 1.0 µm were obtained (Figure 5b), and these nanosheets presented nonuniform morphology and sizes. These results indicate that the concentrations of Zn2+ and OH- are the key parameters for the formation of well-shaped flowerlike ZnO microstructures. Moreover, the choice of zinc source

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Figure 4. FESEM images of the samples obtained after different reaction times at 80 °C in the solution reaction with 2.0 mmol of ZnCl2 and 10.0 mmol of NaOH: (a) 3, (b) 6, and (c) 9 h.

Figure 5. FESEM images of the products synthesized from the reaction solution with different amounts of ZnCl2 and NaOH at 80 °C for 24 h: (a) 2.0 mmol of ZnCl2 and 2.5 mmol of NaOH; (b) 1.0 mmol of ZnCl2 and 10.0 mmol of NaOH. (c, d) FESEM image of the product prepared with zinc foil instead of ZnCl2 at 80 °C for 24 h (10.0 mmol of NaOH).

Figure 6. Schematic illustration of (a) the formation process of flower-like ZnO microstructures and (b, c) the growth models for (b) nanorods and (c) nanosheets.

in the reaction system also shows an obvious influence on the formation of the flower-like ZnO nanostructures. Zinc foil has been used instead of ZnCl2 as the starting material while keeping all other conditions unchanged, and hexagonal ZnO nanorods with uniform diameters about 300 nm (Figure 5c,d) were obtained. Regarding the formation of flower-like ZnO, the concentrations of Zn2+ and OH- must have played key roles, since no template, organic additive, or surfactant existed in the reaction

system. Therefore, we propose the following explanation for the formation of the 3D flower-like ZnO nanostructures, and the schematic illustration of the formation process and mechanism are presented in Figure 6. It is well-known that shape control of the crystals can be achieved by manipulating the growth kinetics.24 In the present case, the contributing growthdriving force for ZnO crystals is the concentration of Zn2+ (or ZnO22-) ions. In the reaction solution containing 2 mmol of ZnCl2 and 10 mmol of NaOH, the high reactants’ concentration

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led to the burst of initial homogeneous nucleation, and the supersaturated ZnO nuclei would aggregate together. As the reaction proceeded, the concentration of the ZnO22- monomer became lower, and some active sites on the surface of the initially formed ZnO aggregations would grow along the oriented direction as the chemical environment constantly provides reactants. The preferential growth along the [0001] and [011j0] directions within the {21j1j0} plane formed nanosheets on the surface of the initially formed ZnO aggregation. Subsequently, more and more nanosheets with a {21j1j0}-planar surface interlaced and overlapped with each other into a multilayer and network structure, and the hierarchical ZnO flower-like microstructures were shaped. To understand the reason why [0001] and [011j0] are the fastest growth directions, we start from the crystal structure of ZnO and its present growth condition. The structure of ZnO single crystal can be described as a number of alternating planes composed of coordinated O2and Zn2+ ions. The oppositely charged ions are made of the positively charged Zn-(0001) and negatively charged O-(0001) polar surfaces. In the usually reported growth modes with the assistance of catalysts or substrates,25 hexagonal rods elongated along the c-axis have been synthesized as Figure 6b due to the intrinsic anisotropy in its growth rate ν with ν[0001] > > ν[011j0] > ν[0001j].26 In the present case, a very high concentration of OH- presented in the aqueous solution (the molar ratio of OHto Zn2+ is 5). Following the decrease in the concentration of ZnO22- monomer due to the initial fast nucleation of ZnO, the absorption of OH- on the positively charged Zn-(0001) plane would dominate in the competition with ZnO22- growth units. Therefore, the superfluous OH- ions stabilized the surface charge and the structure of Zn-(0001) surfaces to some extent, allowing the fast growth along [011j0], leading to the formation of ZnO nanosheets with a {21j1j0}-planar surface. As a result, the formation of nanosheets with a high proportion of {21j1j0} planes is surely due to a suppression of crystal growth along the [0001] axis with a relative enhancement of crystal growth along the [011j0 ]direction, as illustrated in Figure 6c. Obviously, the formation of such a hierarchical microstructure should depend on the added amounts of Zn2+ and OH- ions into the reaction solution. When the amount of OH- was reduced to 2.5 mmol and that of Zn2+ remained unchanged, a high concentration of Zn2+ ions led to a fast nucleation and congregation of ZnO, resulting in the conicallike and irregular ZnO nanoparticles. The subsequent growth for ZnO nanosheets would be difficult because of the low concentration of OH- in the reaction solution. Contrarily, when the concentration of Zn2+ was decreased, a low growth rate would result in quasi-equilibrium growth of ZnO. Thus, the initial nucleation could not form ZnO aggregations. Meanwhile, the superfluous OH- ions stabilized the surface charge and the structure of Zn-(0001) plane, which generated the scattered ZnO nanosheets. The formation of flower-like ZnO can only occur in the aqueous solution containing a certain amount of ZnCl2 and NaOH, which provides a kinetically favorable condition for ZnO hierarchical growth. While zinc foil was used instead of ZnCl2 as the starting material, the Zn foil would act as Zn source as well as a substrate which has tiny Zn clusters to result in catalyzed growth of ZnO along the active direction of the [0001] axis, and the array of hexagonal ZnO nanorods was obtained. 3.3. Photocatalytic Activity. It is well-known that ZnO has been used as a semiconductor photocatalyst for the photocatalytic degradation of organic pollutants in aqueous solution.7 Obviously, the as-prepared flower-like ZnO micro-

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Figure 7. UV-vis absorption spectrum of flower-like ZnO. (Inset: The band gap (Eg) of the ZnO sample is estimated to be about 3.23 eV from the absorption edge.)

structures with open and porous nanostructured surface layers as well as the uniform and thin nanosheets should show a higher photocatalytic activity. In our study, the photocatalytic performance was evaluated by photodegradation of the wellknown organic azo-dye N,N,N′,N′-tetraethylated rhodamine (RhB), a typical pollutant in the textile industry, under ultraviolet irradiation. The UV-vis absorption spectrum of flower-like ZnO is shown in Figure 7. The ZnO sample showed a strong absorption in the ultraviolet region near the visible-light region. The energy of the band gap of the flowerlike ZnO photocatalyst could be obtained from the plots of (Rhν)2 versus photon energy (hν), as shown in the Figure 7 inset, and the value estimated from the intercept of the tangents to the plots was 3.23 eV, which was consistent with that of the reported nanostructured ZnO.27 Figure 8a shows the UV-vis absorption spectrum of the aqueous solution of RhB (initial concentration, 5.0 × 10-5 mol/L; 100 mL) with 30 mg of flower-like ZnO powder as photocatalyst and exposure to ultraviolet light for various durations. The characteristic absorption of RhB at 553 nm decreases rapidly with extension of the exposure time, and completely disappears after about 100 min. Further exposure leads to no absorption peak in the whole spectrum, indicating the total decomposition of RhB. To demonstrate the morphologyinduced enhancement of the photocatalytic performance of the flower-like ZnO, a further experiment was performed using other nanostructured ZnO powders (nanoparticles shown in Figure 5a, nanosheets shown in Figure 5b, and nanorods scraped off from the Zn foil in Figure 5c,d) as photocatalyst for the photodegradation of RhB under the same condition. The results of the RhB degradation in a series of experimental conditions are summarized in Figure 8b. Without any catalyst, only a slow decrease in the concentration of RhB was detected under UV irradiation. The addition of catalysts leads to obvious degradation of RhB, and the photocatalytic activity depends on the morphology of the ZnO samples. The activity increases in turn for the nanostructured ZnO powders: nanoparticles, nanorods, nanosheets, and flower-like microstructures. After 100 min of irradiation, the RhB in aqueous solution could be almost completely eliminated by the flower-like ZnO sample. The photocatalytic superiority of the flower-like ZnO over the other nanostructured ZnO can be attributed to their special structural features. In terms of the previously proposed mechanism for the photocatalytic degradation of organic dye,14,28

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Figure 8. (a) Changes of UV-vis spectra RhB solution as a function of irradiation time; (b) RhB concentration changes over photocatalyst-free solution, ZnO nanoparticles in Figure 5a, ZnO nanosheets in Figure 5b, nanorods scraped off from the Zn foil in Figure 5c,d, and flower-like ZnO microstructures.

which occurs by an indirect pathway involving hydroxyl radicals as the oxidizing intermediate as follows:

ZnO + hν f ZnO(eCB- + hVB+)

(1)

H2O + hVB+ f H+ + •OH

(2)

dye + •OH f oxidation products

(3)

-

The conduction-band electrons (eCB ) and valence-band holes (hVB+) are generated on the surfaces of ZnO nanostructures when they are illuminated by UV light with energy greater than the band gap energy. Holes can react with water adhering to the surfaces of ZnO nanostructures to form highly reactive hydroxyl radicals (OH•) which have a powerful oxidation ability to degrade organic dye. First, the net-like arrangement of nanosheets in the 3D flower-like ZnO microstructures would effectively prevent aggregation and thus maintain a large active surface area. The surface photochemical reaction of RhB degradation occurs in the sequence of dispersion, diffusion, adsorption, surface reaction, and final desorption, and each of these elementary processes affects the surface reaction rate. The large surface area and capacious interspaces in the flower-like ZnO nanostructures offered more opportunity for the diffusion and mass transportation of RhB molecules and hydroxyl radicals in photochemical reaction of RhB degradation. Second, compared with the synthesis of ZnO nanostructures by the assistance of organic additives,18,20 the present reaction system is responsible for the enhancement of photodegradation efficiency due to the “efficient surface” of ZnO. With the existence of organic additives, there is considerable chance to absorb some organic molecules on the surface of ZnO and thus the efficient surface areas decrease, resulting in the decrease in the photodegradation efficiency. The ZnO product obtained in our surfactant-free reaction avoided such unwanted absorption of organic additives, which will enhance the photocatalytic activity to some degree. In addition, the nanosheets in the flower-like ZnO microstructures have very small thickness of 10 nm which is close to the regime where quantum size effect is prominent. The sizequantized nanosheets would promote the charge transfer in the materials, and the increase in the charge-transfer rates drastically reduces the direct recombination of the photogenerated electron/ hole pairs,7c which is essential to enhance the photocatalytic efficiency in the degradation of organic pollutants. As stated above, a good photocatalytic performance of flower-like ZnO is obtained. Obviously, the other nanostructured ZnO photocatalysts, i.e., nanoparticles, nanorods, and nanosheets, lack the above-mentioned structural advantage, and thus present rela-

tively inferior photocatalytic activity in the photodegradation of RhB. As for the superiority in the photocatalytic efficiency of ZnO nanorods than that of the ZnO nanoparticles, it is due to the photocatalytic activity of ZnO being also strongly dependent on the surface orientation of the nanocrystals which could result in orientation-dependent charge-transfer processes.15,29 Although the hexagonal ZnO nanorods (Figure 5d) have a lower surface-to-volume ratio than ZnO nanoparticles (Figure 5a), its regular surface orientation with larger emergences of {101j0} and {0001} surfaces would be propitious to the separation of the photoinduced electrons and holes. Therefore, the ZnO nanorods displayed relatively higher photocatalytic efficiency than ZnO nanoparticles with irregular growth manner. 4. Conclusion In conclusion, a simple and efficient low-temperature (80 °C) aqueous solution route, without templates or surfactants, was demonstrated to synthesize 3D flower-like ZnO hierarchical microstructures on a large scale and with good uniformity. The as-prepared flower-like ZnO microstructures were built by many interlaced nanosheets with dominant surfaces as {21j1j0} planes. According to the experimental results, a growth model based on the fast nucleation-growth kinetics was discussed. The flower-like ZnO displayed an enhanced photocatalytic performance compared with the other naostructured ZnO powders of nanoparticles, nanosheets, and nanorods. The enhanced photocatalytic performance was attributed to the “efficient surface” of ZnO from the surfactant-free synthetic system as well as the special structural features which can significantly facilitate RhBmolecules diffusion and mass transportation in photochemical reaction of RhB degradation. The as-synthesized flower-like ZnO hierarchical microstructures are also expected to be useful for other application such as gas sensing. Moreover, this work further hints that this facile and economical approach can be extended to synthesize other metal-oxide materials with a novel morphology. Acknowledgment. We thank Dr. Yang Xu of the University of Science and Technology of China for his help in the sample characterization and good suggestions in the results’ analysis. This work was supported by the introduced doctor’s startup fund from the Anhui University of Science and Technology (Grant No. 2008YB0205). References and Notes (1) (a) Yin, L. W.; Bando, Y.; Zhan, J. H.; Li, M. S.; Golberg, D. AdV. Mater. 2005, 17, 1972. (b) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin,

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