Controllable ZnO Architectures by Ethanolamine-Assisted

Jan 21, 2011 - The TEM image (Figure 2a) further confirms that the flowerlike structures are constructed by radial pyramidlike rod arrays from the cen...
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Controllable ZnO Architectures by Ethanolamine-Assisted Hydrothermal Reaction for Enhanced Photocatalytic Activity Xinjuan Wang,† Qinglin Zhang,† Qiang Wan,† Guozhang Dai,‡ Chunjiao Zhou,† and Bingsuo Zou*,‡,† † ‡

State Key Lab of CBSC, School of Physics, Hunan University, Changsha, 410082, China Micro-nano Technology Center and School of MSE, BIT, Beijing 100081, China

bS Supporting Information ABSTRACT: Controllable ZnO architectures with different morphologies were synthesized via an ethanolamine-assisted one-pot hydrothermal method. By adjusting reaction conditions such as the molar ratio between Zn(OAc)2 and NaOH and the volume ratio of the ethanolamine/water, the flowerlike, spindlelike, swordlike, and umbellarlike ZnO architectures of the hexagonal phase have been obtained. The as-prepared ZnO products were characterized by FE-SEM, TEM, XRD, BET, PL, and Raman spectroscopy. The ethanolamine significantly influenced the morphology of ZnO products, which is related to the competitive adsorption between ethanolamine and [Zn(OH)4]2- to ZnO nuclei. Their PL spectra depend on their morphologies and phase structures. The morphologydependent photocatalytic performances in the degradation of methylene blue under UV illumination were observed, in which the flowerlike ZnO structures exhibit highest activity for their most irregular surface Zn sites.

1. INTRODUCTION In recent years, controlling the morphology and size of inorganic materials has attracted intensive attention due to the fact they play very important roles in determining optical, electrical, and other physicochemical properties.1-4 Efforts have been devoted to controllable synthesis of inorganic materials with various morphologies for exploring its potential applications in various fields. One-dimensional (1D) nanostructures, such as nanowires, nanotubes, nanobelts, and nanorods, have been prepared and exhibit lots of special properties in optical, electronic, and mechanical properties.5-8 In addition, some more complex two- and three-dimensional (2D, 3D) architectures, such as hexagonal microplates,9 urchin-like nanospheres,10 nanoflowers,11 and hierarchical nanocolumns12 have also been successfully synthesized and exhibit unique properties different from those 1D structures due to their complicated structures. As an important functional oxide semiconductor with a direct wide band gap (3.37 eV) and a large exciton binding energy (60 meV), ZnO has attracted great interest owing to its potential applications in photocatalysis,13 solar cells,14 sensors,15 nanogenerators,16 room-temperature UV laser,17 optical waveguides,18 and so forth. Among these applications of ZnO, the photocatalysis is the most important one for environmental protection. Due to the fact that a photocatalytic reaction occurs at the interface between catalyst and organic pollutants,19 the photocatalytic performance of ZnO is strongly dependent on the growth manner of the crystal,20 r 2011 American Chemical Society

and it has been demonstrated that the morphology control could result in optimization of the photocatalytic activity of ZnO nanostructured.1 Therefore, considerable efforts have been devoted to design of effective methods to synthesize ZnO with tunable size and morphology. Various synthesis routes, such as thermal evaporation, chemical vapor deposition, carbon-thermal reduction, low-temperature oxidization, template-assisted growth, and solution-based approaches, have been developed to synthesize ZnO with various morphologies.21-25 However, most of the synthesis techniques demand vacuum, high temperature, or complicated controlling processes, which are unfavorable for low-cost and large-scale production. Therefore, it is of great importance and necessity to develop a technique to synthesize ZnO only in mild reaction conditions. Solution chemical routes have been proven to be a versatile approach for preparation of ZnO due to the convenience and simplicity in the fabrication. By adjusting the reaction conditions, we have obtained bundled nanowires grown on ZnO hexagonal disks,26 well-aligned ZnO whisker arrays,27 and ZnO flowers28 in aqueous solution. Recent studies have shown that the use of organic additives in the aqueous solution can effectively tune the shape of the product by selective adsorption and subsequent controlled removal of organic additives at interfaces.29,30 Yin et al. Received: October 8, 2010 Revised: December 18, 2010 Published: January 21, 2011 2769

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The Journal of Physical Chemistry C have synthesized hierarchical ZnO nanorod-assembled hollow superstructures for catalysis and photoluminescence applications by the water-soluble biopolymer sodium carboxymethyl cellulose (CMC)-assisted hydrothermal method.31 Raula et al. have reported a simple solution-phase method of preparing mainly flowerlike ZnO nanostructures with controllable sizes using ascorbate as a shape-directing/capping agent, and the flowerlike ZnO nanostructures show higher catalytic activity than spherical nanostructures.32 Wu et al. have developed a facile amino acid histidine assisted hydrothermal route to synthesize ZnO hierarchical architectures, including prismlike and flowerlike structures and hollow microspheres, and they exhibit different photocatalytic activities.33 These studies showed high morphology controllability of ZnO; however, most organic additives used in these methods were expensive longchain molecules, and there are few studies in which ethanolamine has been used as the surface-modifying reagent to control the fabrication of ZnO. In this paper, we report a facile and tunable synthesis of ZnO with different morphologies via an ethanolamine-assisted hydrothermal process. Herein, ethanolamine was introduced as an assembling and structure-directing agent to controllable synthesis of ZnO architectures without any other long-chain organic molecules assisting in a one-step hydrothermal process. Ethanolamine plays important roles in the formation and self-assembly of ZnO architectures, such as flowerlike, spindlelike, swordlike, and umbellarlike structures, when the composition of the reaction mixture was changed. The ZnO morphology has significant influence on the optical and catalytic performance of ZnO.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the ZnO Architectures. All reagents were analytical grade. In a typical synthesis, 0.2195 g (1 mmol) of Zn(CH3COO)2 3 2H2O was dissolved in a given amount of deionized water with magnetic stirring to form a homogeneous solution, and then a given amount of ethanolamine (EA) was added to the zinc acetate solution at room temperature and was continually stirred for 10 min. NaOH (0.32 g, 8 mmol) was dissolved in 10 mL of deionized water, and the NaOH solution was added dropwise into the above solution. After 30 min stirring, the mixture was transferred into a Teflon-lined stainless autoclave (50 mL capacity). The autoclave was sealed and maintained at 140 °C for 12 h. The system was then cooled to ambient temperature naturally. In the series of syntheses, the amount of NaOH and ethanol amine was changed at the designed molar or volume ratio. The precipitate was collected by centrifugation (3000 rpm, 5 min), washed alternately with deionized water and ethanol, and dried in air at 60 °C for 4 h. 2.2. Characterization. The morphology of the as-prepared samples was observed on a Hitachi S-4800 field-emission scanning electron microscopy (FE-SEM). The crystal structure of the products was studied using a Bruker D8 X-ray diffractometer with Cu KR irradiation at λ = 1.5406 Å. For high-resolution TEM measurements, one drop of the aqueous redispersed suspension of sample was placed on a carbon-coated copper grid and allowed to dry in air. The grid was then observed on a JEOL JEM-3010 electron microscope operated at an accelerating voltage of 300 kV. Room-temperature photoluminescence (PL) was recorded on an Alpha (WITec, Germany) near-field scanning optical microscope (NSOM) system from RHK Technology using the He-Cd laser (325 nm) as the excitation source. The Raman-scattering spectrum was collected on an Alpha (WITec, Germany) near-field scanning

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optical microscope (NSOM) system from RHK Technology using the Arþ laser (488 nm) as the excitation source at room temperature. For BET measurements, the sample was pretreated by degassing at 120 °C overnight to remove any adsorbed species. N2 physisorption was performed in a Micromeritics ASAP 2020 instrument to study the surface area of the ZnO samples. 2.3. Photocatalytic Measurements. The photocatalytic activity experiments on the obtained ZnO architectures for the decomposition of methylene blue in air were performed at ambient temperature. A Pyrex beaker (100 mL) was used as the photoreactor vessels. ZnO products as catalyst (20 mg) were added in the aqueous methylene blue solution (C16H18ClN3S 3 3H2O) (SigmaAldrich Chemical Co.; 1.0  10-5 M, 50 mL), and the solution was magnetically stirred in the dark for 1 h to reach the adsorption equilibrium of methylene blue with the catalyst and then exposed to light from a 6 W UV lamp (Shanghai Anting Electronic Instrument Factory, ZF-2) with wavelength centered at 365 nm. At given irradiation time intervals, a series of aqueous solutions in a certain volume were collected and centrifuged to remove the catalysts and were then analyzed on a TU-1901 UV-vis spectrophotometer. The concentration of methylene blue was determined by monitoring the changes in the main absorbance centered at 663 nm.

3. RESULTS AND DISCUSSION The reaction parameters including the temperature, time, and the composition of the reaction mixture play crucial roles in controlling the size and morphology of the products. In the present studies, the morphology and phase structure of the as-synthesized ZnO could be facilely tuned just by changing the composition of the reaction mixture such as zinc acetate/NaOH and EA/H2O, while the reaction temperature and time remained unchanged. 3.1. Morphologies and Structures of ZnO Architectures. Figure 1a,b shows typical field-emission scanning elec-

tron microscopy (FE-SEM) images of the sample synthesized with a Zn2þ/NaOH molar ratio of 1:2 and EA/H2O volume ratio of 1:6. The low-magnification FE-SEM image (Figure 1a) demonstrates that the typical products consist of a large quantity of well-dispersed flowerlike structures. The size of the flowerlike structure is on the scale of micrometers with a diameter of about 4-6 μm. The highmagnification FE-SEM image (Figure 1b) reveals that each flowerlike structure consisted of closely packed pyramidlike rods with lengths of 2-3 μm. The diameter of the rods gradually becomes smaller along the growth direction, leading to a pyramidlike structure with a sharp tip. The diameter varies from 600 nm at the bottom to about 70 nm at the tip. The TEM image (Figure 2a) further confirms that the flowerlike structures are constructed by radial pyramidlike rod arrays from the center to the surface of the flower. The HRTEM image and the inserted FFT ED pattern (Figure 2b) display clear lattice fringes and reveal the single crystalline nature of the sample. The measured lattice spacing is about 0.28 nm corresponding to the (010) lattice plane of wurtzite ZnO. When the Zn2þ/NaOH molar ratio was adjusted to 1:6 with the EA/H2O volume ratio of 1:6, the rods with spindlelike shapes were obtained, as shown in images c and d in Figure 1. The length of the rods was around 12 μm. The diameter of the rods varied from the bottom to the tip. The maximum diameter was about 2 μm, and the minimum was around 100 nm. Most of the rods have smooth side faces. A few spindles were joined across the basal plane to make a long rod, as shown in the upper right inset to Figure 1c. The perfectly aligned lattice planes, as shown in Figure 2d, provide strong evidence for the well-crystallized structure. The spacing 2770

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Figure 1. FE-SEM images of as-prepared ZnO samples synthesized with different compositions of the reaction mixture: (a, b) Zn2þ/NaOH 1:2, EA/ H2O 1:6; (c, d) Zn2þ/NaOH 1:6, EA/H2O 1:6; (e, f) Zn2þ/NaOH 1:12, EA/H2O 1:6; (g, h) Zn2þ/NaOH 1:12, EA/H2O 6:1.

Figure 2. TEM image and HRTEM image of as-prepared ZnO samples synthesized with different composition of the reaction mixture: (a, b) Zn2þ/ NaOH 1:2, EA/H2O 1:6; (c, d) Zn2þ/NaOH 1:6, EA/H2O 1:6; (e, f) Zn2þ/NaOH 1:12, EA/H2O 1:6. The upper right insets in (c, e) are the corresponding SEM images of the sample. The FFT ED of the corresponding HREM micrographs are shown as upper right insets in (b, d, f).

distance of 0.25 nm obtained from the HRTEM image can be ascribed to the adjacent (101h1) planes of the ZnO. The wellcrystallized nature of the sample has also been confirmed by Fourier transform electron diffraction (FFT ED, the upper-right inset in Figure 2d). Swordlike structures have been synthesized with further change of the Zn2þ/NaOH molar ratio to 1:12 maintaining the EA/H2O volume ratio of 1:6, as shown in Figure 1e,f. The length of these swordlike rods was between 30 and 70 μm, and the diameter was about 5 μm. According to Figure 1f, these swordlike rods have a sharp tip and smooth surfaces. Figure 2f shows the high-resolution electron microscopy (HREM) images of the as-obtained products. The image clearly reveals that only the fringes of (101h1) planes with a lattice spacing of about 0.25 nm can be observed, indicating that the swordlike ZnO rod is single crystal in nature.

In the case of altering the EA/H2O volume ratio from 1:6 to 6:1 and having the Zn2þ/NaOH molar ratio of 1:12, the as-obtained products were the mixture of umbellarlike and short swordlike structures, as shown in Figure 1g,h. The umbellarlike structures were composed of short swordlike rods with about 1 μm diameter and 1-2.5 μm length. Upon closer examination of the morphology in Figure 1h, it is clear that some of the rods had hollows on their surface. To investigate the influence of ethanolamine in the formation of ZnO architectures, ZnO samples were also synthesized without the participation of ethanolamine under the same conditions, including the corresponding Zn2þ/NaOH molar ratio, reaction temperature, and time. When the Zn2þ/NaOH molar ratio was 1:2, steamed corn breadlike ZnO was obtained with hollow in bottom (Figure S1 Supporting Information). The results clearly suggest that ethanolamine 2771

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Figure 3. XRD patterns of as-prepared ZnO samples synthesized with different composition of the reaction mixture: (a) Zn2þ/NaOH 1:2, EA/H2O 1:6; (b) Zn2þ/NaOH 1:6, EA/H2O 1:6; (c) Zn2þ/NaOH 1:12, EA/H2O 1:6; (d) Zn2þ/NaOH 1:12, EA/H2O 6:1.

plays a key role in the formation of the ZnO flowerlike structure. When the Zn2þ/NaOH molar ratio was changed to 1:6 and 1:12, irregular nanosheets, nanoparticles, and interconnected flowerlike structures were synthesized (Figures S2 and S3 Supporting Information), which implies that ethanolamine is also necessary for the formation of pindlelike, swordlike, and umbellarlike structures. Therefore, on the basis of the morphological study, it can be concluded that ethanolamine produces a significant effect on the size and shape of the obtained ZnO samples. The phase structure of the as-synthesized samples was further characterized using powder X-ray diffraction (XRD). Figure 3 shows the XRD patterns of the flowerlike, spindlelike, swordlike, and mixture of umbellarlike and short swordlike samples, corresponding to the samples shown in Figures 1a,c,e,g, respectively. The sharp diffraction peaks indicate the excellent crystalline nature for all the samples, and all peaks can be indexed to the hexagonal structure of ZnO (JCPDS card No. 36-1451). By comparing the peak intensities of the samples, it is noticeable that the ratios of relative XRD intensities of (100)/(101) planes varied significantly when the composition of the reaction mixture changed, indicating the different degrees of preferred growth orientation along the c-axis of the hexagonal phase.33 Raman spectroscopy is also performed to study the crystallization degree of the as-synthesized ZnO samples with different morphology. Figure 4 shows the room-temperature Raman spectra of these samples in the range of 200-800 cm-1. All the samples exhibit similar scattering patterns. On the basis of the scattering modes of the bulk ZnO, the observed obvious peaks at 383, 413, and 438 cm-1 can be assigned to A1(TO), E1(TO), and E2(high) modes, respectively.34 The relatively weak peaks at 540 and 575 cm-1 are attributed to A1(LO) and E1(LO) modes.35 It is generally accepted that the E2(high) mode originates from the lattice vibration of ZnO,36 while the E1(LO) mode is associated with defects in ZnO such as oxygen vacancy or other defect states.34 The asymmetric and broad E2 modes reflected the low crystallinity. By comparing the Raman spectra of the samples (a,b,d), it was found that the E2(high) mode of three samples becomes stronger when the Zn2þ/NaOH molar ratio decreases, which means the improvement of the crystallization of prepared ZnO samples.35

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Figure 4. Raman spectra of the as-prepared ZnO samples synthesized with different composition of the reaction mixture: (a) Zn2þ/NaOH 1:2, EA/H2O 1:6; (b) Zn2þ/NaOH 1:6, EA/H2O 1:6; (c) Zn2þ/NaOH 1:12, EA/H2O 6:1; (d) Zn2þ/NaOH 1:12, EA/H2O 1:6.

While the EA/H2O volume ratio increases from 1:6 to 6:1, the E2(high) mode of sample c becomes weaker compared with the sample d. This indicates that overmuch ethanolamine molecules may influence the degree of crystallinity, which agrees well with the macroscopic irregularity of the product in the SEM observations (Figure 1e,g). 3.2. Possible Formation Mechanism of ZnO Architectures. On the basis of these results, the formation mechanisms of the ZnO hierarchical architectures are probably as follows. Ethanolamine molecules play important roles of complexing, assembling, and structure-directing agents in the present synthetic system. Before the hydrothermal treatment, ethanolamine mostly acted as complexing molecules in the precursor solution; the Zn source was primarily in the form of [Zn(EA)m]2þ; and the remaining Zn source existed in the form of Zn(OH)2 and Zn2þ. The formation of coordinated ions [Zn(EA)m]2þ restricts the existence of free zinc ions and controls the formation of ZnO nuclei. When the precursor solution is heated, reactions can occur as follows ½ZnðEAÞm 2þ T Zn2þ þ mEA

ð1Þ

Zn2þ þ 2OH - T ZnðOHÞ2

ð2Þ

ZnðOHÞ2 f ZnOV þ H2 O

ð3Þ

ZnO þ H2 O þ 2OH - T ½ZnðOHÞ4 2 -

ð4Þ

where m is a positive integer. As the temperature increases, coordinated ions [Zn(EA)m]2þ decompose into Zn2þ ions and ethanolamine molecules at an elevated temperature, according to reaction 1. The Zn2þ ions will further form Zn(OH)2 with OH(reaction 2). Finally, ZnO is obtained by decomposition of Zn(OH)2 (reaction 3). When the concentration of ZnO has reached supersaturation, ZnO crystal nuclei form and then grow according to the growth habit of ZnO crystals. At a relatively high Zn2þ/ NaOH molar ratio of 1:2, some ethanolamine molecules, served as assembling agents and structure-directing agents, adsorb on the surface of ZnO nuclei and result in the formation of flowerlike 2772

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Scheme 1. Schematic Representation of the Growth Mechanism of ZnO Architectures

Figure 5. Room-temperature PL of the ZnO architectures under excitation with UV light at 325 nm, using a He/Cd laser at room temperature: (a) flowerlike ZnO; (b) spindlelike ZnO; (c) swordlike ZnO; (d) mixture of umbellarlike and short swordlike ZnO.

architectures. When the Zn2þ/NaOH molar ratio is changed to 1:6 and 1:12, the increasing OH- ions result in the dissolution of ZnO to some extent and yield [Zn(OH)4]2-according to reaction 4. Compared with the polar ethanolamine molecule, the negative charged [Zn(OH)4]2-complexes preferably adsorb on the surface of ZnO nuclei, which contributes to the growth of ZnO nuclei along the [0001] direction.13 Thus, anisotropic spindlelike or swordlike structures are formed finally. While the EA/H2O volume ratio is changed to 6:1, some of the ZnO nuclei are covered by the ethanolamine molecules or [Zn(OH)4]2- ions or both of them, and the synergetic effects of ethanolamine and [Zn(OH)4]2- ions result in the formation of the umbellarlike structures. A schematic diagram of the possible formation mechanism has been shown as Scheme 1. 3.3. Photoluminescence of ZnO Architectures. The photoluminescence (PL) spectrum of the as-synthesized ZnO products was measured at room temperature using a 325 nm He-Cd laser as the excitation source, as shown in Figure 5. It can be seen from the spectra that the flowerlike ZnO shows a narrow UV emission at about 382 nm and a broad blue-green emission with two peaks at 535 and 566 nm, respectively. The spindlelike ZnO presents the highest PL intensity among the as-synthesized ZnO architectures with four strong emissions at 376, 476, 490, and 506 nm. The spectrum of the swordlike ZnO shows a very strong and sharp UV emission similar to that of the flowerlike one but with a weak bluegreen emission centered around 534 nm. The PL of the product of mixed umbellarlike and short swordlike ZnO exhibits a UV emission similar to that of the swordlike structure and a broad blue-green emission band centered at 493 nm with the lowest PL intensity of all the samples. It is established that the UV emissions at 382 and 376 nm are attributed to the band gap emission of ZnO resulting from the exciton recombination,37 reflecting a dominant good ZnO crystal. The blue-green emission band between 470 and 570 nm is related to the exciton scattering by some definite defects, usually attributed to a singly charged oxygen vacancy or other defects, which originate from the recombination of a photoexcited hole with a charge state of the specific defect, such as oxygen vacancies, or it results from surface-deep traps.38 The PL peak position and the relative intensity of band-edge emission to the deep-trap emission are closely related to the morphology, crystallinity, and dimension of ZnO nanostructures.39,40 High band-edge emission indicates a high monocrystal structure, while dominant deep-trap emission reflected that a lot of defects or oxygen vacancy

or amorphous surface in ZnO crystal still influence the exciton transition. So, spindlelike ZnO and swordlike structures possess good crystal structure and a smooth crystal face, hence showing a strong and sharp UV emission band. The mixed umbellarlike and short swordlike ZnO show the weakest and broad UV and visible emission, indicating a worse inner crystal structure. The flowerlike structure also exhibits low emission, which represented that the higher number of surface defects and low crystallinity intervene in the photocarrier relaxations. The radiative process possesses a much lower percent in all the photoinduced carrier relaxations in semiconductor nanostructures, so the PL data can reflect the amount of other photophysical and photochemical processes to an extent. Therefore, the PL of these ZnO architectures can be manipulated by controlling their morphologies and phase structures, indicating the versatility and capabilities of these building blocks for application in diverse fields. 3.4. Photocatalytic Activity of ZnO Architectures. It is wellknown that ZnO has been used as a semiconductor photocatalyst for the photocatalytic degradation of organic pollutants in aqueous solution. To demonstrate the potential applicability of as-synthesized ZnO materials in these applications, the photocatalytic degradation of methylene blue (MB) was chosen as a reference, and the characteristic absorption of the methylene blue seen at 663 nm was selected for monitoring the adsorption and photocatalytic degradation process. For MB, the polar site containing N and S in the heterocycle may play an important role in the adsorption. Figure 6a shows absorption spectra of an aqueous solution of methylene blue (initial concentration: 1.0  10-5 M, 50 mL) in the presence of 20 mg of ZnO flowerlike structure under exposure to the ultraviolet light lamp (6 W) for various durations. The absorption peak corresponding to methylene blue at 663 nm diminishes sharply and even disappears completely after irradiation for about 100 min, compared to the intensity of the initial methylene blue solution. Further experiments were performed using other morphologies of ZnO under the same conditions to demonstrate the morphology-induced enhancement of the photocatalytic performance of ZnO samples. Figure 6b shows the curves of the concentration of methylene blue with UV light irradiation time over the as-prepared flowerlike, spindlelike, swordlike, and commercial ZnO materials. Without any catalyst, only a slow decrease in the concentration of methylene blue was detected under UV irradiation. The addition of catalysts leads to obvious degradation of methylene blue, and the photocatalytic activity depends on the morphology of the ZnO 2773

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Figure 6. (a) Absorption spectrum of methylene blue in the presence of flowerlike ZnO under UV light. (b) Methylene blue concentration changes over photocatalyst-free solution, flowerlike ZnO, spindlelike ZnO, swordlike ZnO, and commercial ZnO.

samples. The activity increases in turn for the ZnO materials: commercial, swordlike, spindlelike, and flowerlike structures. Here flowerlike ZnO showed the highest catalytic activity. After 100 min of irradiation, the percent of the methylene blue degraded by the flowerlike ZnO sample was up to 94%. Kim and Park41 reported that the maximum degradation of methylene blue under UV light (1 kW) irradiation by 1.5 g/L rod- and spherical-shaped ZnO nanoparticles was 98.5% and 74%, respectively. Compared to their report, our flowerlike ZnO catalyst has the advantages of being active at lower catalyst concentration (0.4 g/L) and lower UV light power (UV irradiation: 6 W). Usually, the photocatalytic activity is positively related with the surface area of catalysts.42 However, in this work, higher surface areas of the catalysts did not result in higher photocatalytic performance. The BET surface areas of the as-prepared flowerlike, spindlelike, swordlike, and commercial ZnO materials are 4.8, 16.3, 191.3, and 5.4 m2/g, respectively. Thus, there was no correlation between the BET surface areas and the photocatalytic activity for our materials, demonstrating that there are other more important surface sites that govern this catalytic activity, such as both crystallinity and specific crystal face.1 It can be seen from the Raman spectra (Figure 4) that the crystallinity of samples (a,b,d) increases in the order of flowerlike, spindlelike, and swordlike. The flowerlike ZnO has a relatively lower crystallinity and more defects. For the nanostructure prepared in solution, defects usually are located on the surface. The flower structure possesses more tip or step or kink sites than that of the spindlelike and swordlike ZnO structures, so more naked Zn atoms on the defects prolong to bind S and N atoms in the MB molecules and then photocatalyze it. The order of catalytic activities of ZnO samples with different morphology correlated well with the degree of their respective crystallinity. The defects in crystals especially on the surface can serve as active sites which play a major role for the catalytic activity.43,44 So, crystallinity is an important factor that is responsible for this catalytic activity. This fact indicates that a specific type of surface site is responsible for this catalytic process, although we cannot distinguish it accurately at the current technical level.

4. CONCLUSIONS In conclusion, a versatile one-step hydrothermal approach for ZnO architectures with controllable morphologies and phase structures was developed by using hydrothermal reaction via ethanolamine assistance. ZnO architectures with different morphologies, such as flowerlike, spindlelike, swordlike, and umbellarlike structures, have been synthesized by controlling the reaction conditions such as the molar ratio between Zn2þ and NaOH and

the volume ratio of the EA/H2O. The ethanolamine significantly influenced the morphology of ZnO products, which was related to the competitive adsorption between ethanolamine and [Zn(OH)4]2- to ZnO nuclei. The PL spectra showed a narrow and sharp UV emission and a broad blue-green emission when excited by UV light, which could be manipulated by changing the morphologies and phase structures of products. The flowerlike ZnO displayed an enhanced photocatalytic performance as compared with other ZnO architectures under mild UV-light irradiation in the methylene blue photodegradation experiments at ambient temperature, which may be related to specific site-selective photocatalytic behavior related to Zn sites. These findings could be helpful in the future development of ZnO nanomaterials with different morphologies.

’ ASSOCIATED CONTENT

bS

Supporting Information. FE-SEM images of the ZnO synthesized without ethanolamine. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: 010-68913948. Fax: 1068913938.

’ ACKNOWLEDGMENT We thank the financial support of the National Natural Science Foundation of China (Grant Nos. 90606001 and 20873039). ’ REFERENCES (1) Xu, L. P.; Hu, Y. L.; Pelligra, C.; Chen, C. H.; Jin, L.; Huang, H.; Sithambaram, S.; Aindow, M.; Joesten, R.; Suib, S. L. Chem. Mater. 2009, 21, 2875. (2) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067. (3) Xiong, Y.; Xia, Y. Adv. Mater. 2007, 19, 3385. (4) Zhang, J.; Liu, H.; Wang, Z.; Ming, N.; Li, Z.; Biris, A. S. Adv. Funct. Mater. 2007, 17, 3897. (5) Jeong, S. J.; Moon, H. S.; Shin, J.; Kim, B. H.; Shin, D. O.; Kim, J. Y.; Lee, Y. H.; Kim, J. U.; Kim, S. O. Nano Lett. 2010, 10, 3500. (6) Zhu, G.; Yang, R.; Wang, S.; Wang, Z. L. Nano Lett. 2010, 10, 3151. (7) Yuan, G. D.; Zhang, W. J.; Jie, J. S.; Fan, X.; Tang, J. X.; Shafiq, I.; Ye, Z. Z.; Lee, C. S.; Lee, S. T. Adv. Mater. 2008, 20, 168. (8) Wang, Z. L. Mater. Sci. Eng., R 2009, 64, 33. 2774

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp1096822 |J. Phys. Chem. C 2011, 115, 2769–2775