Controllable Tuning Various Ratios of ZnO Polar Facets by Crystal

Apr 10, 2014 - In this paper, Bi2WO6 was, for the first time, utilized as crystal seeds to fine control the ratio of ZnO polar facets under a mild con...
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Controllable Tuning Various Ratios of ZnO Polar Facets by Crystal Seed-Assisted Growth and Their Photocatalytic Activity Mianli Huang,† Yu Yan,‡ Wenhui Feng,† Sunxian Weng,† Zuyang Zheng,† Xianzhi Fu,† and Ping Liu*,† †

State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fujian 350002, China College of Materials Science and Engineering, Fuzhou University, Fujian 350002, China

‡

S Supporting Information *

ABSTRACT: In this paper, Bi2WO6 was, for the first time, utilized as crystal seeds to fine control the ratio of ZnO polar facets under a mild condition. The structure and photocatalytic properties of the ZnO were investigated by powder X-ray diffraction, Raman scattering measurements, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, total organic C analyzer, Brunauer−Emmett−Teller surface areas, and electrochemical workstation. It was found that ZnO architectures were gradually changed from nanorods to nanosheets after adding a certain amount of Bi2WO6 crystal seeds, which led to the change of the ratio of ZnO polar facets. The formation mechanism for this ZnO and the effect of ZnO polar facets on the photocatalytic activity are explored in detail. We find that the nucleation and growth rates of ZnO are controlled by adjusting the quantity of the crystal seeds. Besides, photocatalytic activity of the samples tuned by Bi2WO6 crystal seeds was compared. The ratio of ZnO polar facets is the main factor influencing the photocatalytic activity. The results clearly demonstrate that the sample with a higher proportion of Zn-terminated (0001) and Oterminated (0001̅) polar facets is beneficial to enhance photocatalytic activity.

1. INTRODUCTION

energies will gradually diminish with crystal growth, which results from the minimization of surface energy.14 On the basis of the above analysis, the high energy (0001) and (0001̅) polar facets are generally energetically unfavorable unless the surface charges are compensated by capping agents. The crystal growth habits can be modified by selective adsorption of additives on polar facets. Often, surfactants are very effective additives, which are introduced into reaction systems to influence the final crystal growth habits or moderate the growth rates.15 Some modifiers,16 such as CTAB, polymers, and citrate ions, have been added to fabricate exceptional shapes. However, those organic additives and modifiers are hard to be removed or recycle. Therefore, researchers are trying to use an inorganic or organic-free method to expose ZnO polar facets. Xu et al.17 demonstrated a high-temperature molten salt route to synthesize metal oxides with polar surfaces. Chang et al.18 reported a nonhydrolytic aminolysis synthesis route for controlling the exposed facets. Recently, Boppella et al.19 reported the formation of unique oriented ZnO structures by controlling the zinc source and the concentration of reactants. However, they are not very environmentally friendly in the technological condition and it is difficult to obtain various ratios of polar facets. Therefore, it is still challenging to

Anisotropy is a basic property of single crystals, and the various facets for a crystal may exhibit different chemical and physical properties.1−3 Usually, the photocatalytic reaction occurs at the interface between the reactant and the catalyst, and the photocatalytic activity is strongly dependent on the surface morphology and electronic structure.4−7 As a result, it is quite important to explore the relationship between crystal surface orientation and photocatalytic activity. Zinc oxide (ZnO) has aroused an explosion of interests in the past decade for the merits of synthesis and unique catalytic, photochemical, and optoelectronic properties.8,9 ZnO is a wideband-gap (3.37 eV) semiconductor with 60 meV excition binding energy, which is often believed to be an alternative photocatalyst material to TiO2, since they have similar band gaps and similar photocatalytic mechanisms. ZnO has a typical wurtzite structure with spontaneous electrical polarization along its c axis. Various properties of ZnO, such as catalytic properties, chemical stability, and surface electronic structure, depend on its polarity.10−12 Obviously, photocatalytic activity of ZnO crystals is closely associated with high energy polar facets.13 In such case, it is extremely significant to tune the morphology to obtain different polar-to-nonpolar facets ratios, and discuss the relationship between facets and photocatalysis systematically. Generally, the positively Zn2+-terminated (0001) and negatively O2−-terminated (0001)̅ polar surfaces have high surface energies. However, the polar surfaces with high surface © 2014 American Chemical Society

Received: November 8, 2013 Revised: March 23, 2014 Published: April 10, 2014 2179

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Photocatalysis Measurement. To characterize the photocatalytic activity of the samples, measurements on photodegradation of Rhodamine B (RhB) were carried out under ultraviolet light irradiation at room temperature. For each experiment, 0.08 g of sample was put in a quartz tube containing 80 mL of 1 × 10−5 M RhB solution. The tube was illuminated by four surrounding wideband lamps (4 W, Philips TL/05) with a predominant wavelength at 365 nm. Prior to irradiation, the solution was sonicated for 15 min and then stirred in the dark for 60 min to establish adsorption and desorption equilibrium. Then, the lights were turned on to initiate the photocatalytic reaction. About 4 mL of solution was collected per 20 min, and then centrifuged to remove the photocatalysts. The concentrations of the remnant RhB were monitored by Varian Cary 50 UV−vis spectroscopy. Photoelectrochemical Measurements. The photocurrent was measured by the conventional three-electrode electrochemical cell in the BAS Epsilon Electrochemical System. A 10 mg portion of photocatalyst powders was dispersed into 0.1 mL of dimethylformamide under sonication for 1 h to get a slurry, which was spread onto the conductive surface of indium−tin oxide (ITO) glasses over 0.25 cm2. Then, the ITO glasses were used as the working electrode for photoelectrochemical measurements, which was immersed in a Na2SO4 (0.2 mol L−1) electrolyte solution. A 300 W xenon lamp (Beijin Changtuo, CHF-XM300) with 365 nm was utilized as the light source.

control the ratio of ZnO polar facets by a simple inorganic method. Generally, by changing the growth rate of different crystal facets, we can obtain different ratios of ZnO polar facets. A seed crystal is usually used to facilitate the growth of a particular crystal. It gives us a new clue to expose certain active facets by adding a certain amount of crystal seeds. Considering that the relationship between ZnO polar facets and photocatalytic activity is still controversial,8,18−21 the successful synthesis of ZnO crystals with various ratios of polar facets is favorable to assess an explicit relationship between the facet orientation and the photocatalytic activity.22 In this paper, we have carried out systematic research on the synthesis of nanoscale ZnO crystals exposing various ratios of polar facets via exercising control on the amount of Bi2WO6 crystal seeds. The formation mechanism and the relationship between the ratio of polar facets and photocatalytic activity have also been discussed in detail. It is expected that this relatively facile and environmentally friendly approach can inspire the preparation of other metal oxide nanocatalysts with preferential exposure of highly active facets via the assistance of crystal seeds.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION The XRD pattern of the precursor prepared at the first step is shown in Figure 1a. All the diffraction peaks are well-indexed to

Materials. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), sodium tungstate dihydrate (Na2WO4·2H2O), and hexamethylenetetramine (HMTA) were purchased from Sinopharm Chemical Reagent Company. RhB and other chemical reagents were of analytical grade without further purification. Deionized water was used throughout this study. Synthesis of ZnO/Bi2WO6. The ZnO composites with different ratios of polar facets were synthesized via a two-step chemical bath deposition process. First, the snowlike Bi2WO6 composed of plates with different directions was prepared via a hydrothermal method reported by Zhuo et al.23 Second, the ZnO/Bi2WO6 composites were synthesized by a simple reflux condensation procedure without using any surfactants. Typically, a certain quantity of the Bi2WO6 was dispersed in the solution. The solution consisted of Zn(NO3)2·6H2O and hexamethylenetetramine (HMTA) with equal molar ratios dissolved into 50 mL of deionized water to form a 5 × 10−3 M solution. After ultrasonication for 30 min, the bottle was put into an oil tank and maintained at 90 °C for 24 h. Finally, the products were washed with deionized water and ethanol several times to remove the possible residues and then dried at 80 °C for 24 h for further characterization. The obtained materials were denoted as Z-BW0, ZBW1, Z-BW5, Z-BW8, and Z-BW10, where Z and BW represented ZnO and different mass ratios of Bi2WO6, respectively. Materials Structure Characterization. The phase structures of the samples were characterized via a Bruker D8 Advance X-ray diffractometer (Cu−Kα target, λ = 0.15406 nm; voltage: 40 kV; current: 40 mA; step width: 0.02° 2θ s−1). Raman scattering measurements (Renishaw Company) were recorded with confocal microscopy using a 532 nm laser, focused on the 50× microscope objective lens at room temperature, and the spectrum acquisition time was 5 s. The chemical composition and the valence state were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 photoelectron spectrometer). The field emission scanning electron microscope (FESEM, FEI Nova NanoSEM 230) and a 200 kV transmission electron microscope (TEM, Tecnai G2F20 S-TWIN, FEI Company) were utilized to study the morphologies and microstructures of the samples. The UV−vis diffuse reflectance spectroscopy (Varian Cary500 spectrophotometer) was applied to examine the optical properties. The total organic C was measured using an automated total organic C analyzer (TOC-Vcph, Shimadzu, Japan). A micrometrics ASAP 2020 analyzer was used to record the nitrogen adsorption and desorption isotherms at 77 K after the samples were degassed at 100 °C under a vacuum condition for 4 h. The BAS Epsilon workstation was used to measure light or dark short-circuit photocurrent response.

Figure 1. (a) XRD pattern and (b) SEM image of Bi2WO6..

the orthorhombic Bi2WO6 crystal phase (JCPDS No. 39-0256, with lattice constants of a = 5.457 nm, b = 16.435 nm, and c = 5.438 nm). No peaks of other phases are detected in pure Bi2WO6, which indicates that the Bi2WO6 is the only phase in the product. Figure 1b is the SEM micrograph of the Bi2WO6 crystal seeds, which clearly displays that they are nanosheets, shown as reported by Zhuo et al.,23 and exhibit a special morphology with a cross and porous structure. The as-prepared products of ZnO/Bi2WO6 nanocomposites are characterized by SEM, shown in Figure 2a−f. It can be seen from Figure 2a, without the crystal seeds Bi2WO6, the shape of the pure ZnO is a nanorod with a low percentage of the (0001) and (0001̅) polar facets. Growth rate and crystal dimensions are controlled by the deposition conditions. SEM images of pure ZnO products with different deposition times and deposition temperatures are shown in Figure S2 (Supporting Information). When crystals grow in the presence of Bi2WO6, as shown in Figure 2b−f, the shape of the crystal is altered. Although the crystals are still hexagonally prismatic, their axial lengths are reduced, and they show flattened or platy morphologies. When the mass fraction of Bi2WO6 is reaching 8%, the shape of the 2180

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Figure 3. (A) XRD patterns of the products prepared corresponding to different Bi2WO6 mass fractions: (a) 0, (b) 5%, and (c) 10%. (B) Plot of the (100)/(002) intensity ratio vs Bi2WO6 mass fraction. Figure 2. SEM images of ZnO products in the presence of different mass fractions of Bi2WO6: (a) 0, (b) 1%, (c) 3%, (d) 5%, (e) 8%, and (f) 10%. Insets: the corresponding cross sections of SEM images.

peak (shown in Figure 3A, pattern c). Meanwhile, the morphology of ZnO changes from nanorod to nanosheet with (001) preferential orientation. These results indicate that the amount of Bi2WO6 has a vital influence on ZnO preferential growth orientation. As shown in Figure 3B, the intensity ratio of (100)/(002) evidently increases with the increasing of the mass fraction of Bi2WO6. It was found earlier that the changing relative intensity of the (100) and (002) peaks in the XRD patterns corresponds to a change in crystal shape.8 A small (100)/(002) ratio is indicative of the formation of rods oriented along the c axis. Conversely, a very large (100)/(002) ratio demonstrates shortening along the c axis.24 The degree of crystallinity of ZnO is much higher than that of Bi2WO6. Because of the treatment of noise reduction, X-ray diffraction peaks of Bi2WO6 are not very obvious.25 Therefore, the Raman spectrum investigation is utilized for the as-prepared samples. Figure 4 shows Raman spectra for the synthesized samples in the range of 50−1000 cm−1. Raman-active modes for wurtzite ZnO are 2E2 + E1 + A1. Raman spectra of Z-BW0,

sample changes from nanorod to an ultrathin nanosheet, exposing the largest population of unconventional (0001) and (0001)̅ facets. This change in shape is in accordance with the reduced preferable axial growth relative to equatorial growth. However, increasing the mass fraction of Bi2WO6 to 10% (Figure 2f) will not yield the further increase of the width across the hexagonal facet, and the axial length gradually increases. The dimensions of the width, the length, and the aspect ratio of the sample adding different mass fractions of Bi2WO6 are shown in Figure S3 (Supporting Information). Rodlike samples have a large aspect ratio. However, sheetlike crystals have a small aspect ratio, exposing a larger ratio of polar facets than rodlike samples. The detected results of XRD analysis of pure ZnO, pure Bi2WO6, and the sample of Z-BW8 with the largest ratio of polar facet are shown in Figure S1a (Supporting Information). It confirms the existence of Bi2WO6 in the final products. Consistent with the XRD result, the EDS result (Figure S1b, Supporting Information) also demonstrates that the elements of Zn, Bi, W, and O are contained in the sample. The crystal structures of ZnO are characterized to further investigate the orientation growth of the ZnO. Figure 3A represents the XRD patterns of the samples fabricated with different Bi2WO6 mass fractions. X-ray diffraction (XRD) data confirm that all of the samples are crystalline and have the hexagonal wurtzite structure of bulk ZnO with lattice parameters matching those in the literature (JCPDS, 361451).18 Three characteristic (100), (002), and (101) peaks appear on the patterns for various samples. Without Bi2WO6, the nanorods show the normal c-axis preferential orientation (Figure 3A, pattern a). While increasing the Bi2WO6 mass fraction to 5%, a slight increase of (100) intensity is observed (see Figure 3A, pattern b). By further increasing the amount of Bi2WO6 to 10%, the XRD pattern exhibits a very strong (100)

Figure 4. Raman spectra of the products prepared corresponding to the different Bi2WO6 mass fractions: Z-BW8, Z-BW5, Z-BW3, Z-BW1, and Z-BW0. 2181

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assigned to Bi3+ in the crystal, as shown in Figure 5c.36 W 4f contains two peaks with binding energies around 37.5 and 35.2 eV in Figure 5d, which indicates a W6+ oxidation state of Bi2WO6.37 As shown in Figure 5e, the O 1s region can be fitted into three peaks. The peaks at 529.66 and 531.04 eV correspond to the crystal lattice oxygen, which are consistent to different chemical environments of oxygen element in [Bi2O2]2+ and [WO4]2− layers.36 According to the literature,38 the lower energy peak located at 529.5−531.4 eV is associated with the O−Zn bond. The peak at 531.77 eV can be assigned to surface absorbed oxygen.39 To explore how the ZnO couples with the Bi2WO 6 nanosheets, TEM and HRTEM analyses on the ZnO/ Bi2WO6 composites are conducted, as presented in Figure 6a,b. The interplanar spacing is 0.28 nm, which corresponds to

the pure ZnO, has two high intensity peaks at 99.7 and 441.02 cm−1, which are associated with the low and high E2 modes of nonpolar optical phonons, respectively, typical of the wurtzite phase.26 There are two weaker peaks at 332.7 and 383.11 cm−1 assigned to the E2H−E2L multiphonon and A1T modes, respectively. A broad 578.4 cm−1 peak corresponds to the E1L mode.27,28 Compared with Z-BW0, the peaks of the samples adding Bi2WO6 crystal seeds in the range of 600−1000 cm−1 are assigned to the stretching of the W−O bands, according to a report by Crane et al.29 In more detail, the intensity of the peak at 715 cm−1 is interpreted as an antisymmetric bridging mode, associated with the tungstate chain.30 The bands at 790 and 820 cm−1 are assigned to antisymmetric and symmetric Ag modes of terminal O−W−O, respectively. The mode at 310 cm−1 is associated with simultaneous movements of Bi3+ and WO66− involving translational modes.31 The lattice dynamics calculation predicts that the band near 147 cm−1 corresponds to the A1g mode of the tetragonal phase, involving movement of Bi3+ ions perpendicular to the layers.32,33 In order to investigate chemical composition and the valence states of the as-obtained Z-BW8 nanosheet, the X-ray photoelectron spectroscopy (XPS) analysis is conducted. The XPS spectrum of the sample Z−BW8 illustrates that the sample contains Zn, Bi, W, and O elements and a trace amount of carbon (Figure 5a). The XPS peak for C element is on account of the adventitious hydrocarbon from the XPS instrument itself.34 Figure 5b−e shows the high-resolution spectra of the Zn 2p, Bi 4f, W 4f, and O 1s regions, respectively. The Zn 2p (Figure 5b) are observed at about 1045 and 1022 eV, respectively, as reference.35 The peaks at 164 and 159.2 eV corresponding to Bi 4f5/2 and Bi 4f7/2, respectively, can be

Figure 6. (a) Corresponding high-resolution TEM images of the circular area in (b). (b) Low-magnification TEM image of the ZnO/ Bi2WO6 nanostructure.

the (101̅0) plane of a hexagonal ZnO phase. The preferred growth direction of ZnO is along (101̅0), which leads to the formation of hexagonal ZnO nanosheets with a high degree of exposure of (0001) facets.4 In contrast, most reported synthetic procedures of wurtzite ZnO nanomaterial are hexagonal rods elongated along the c axis. The Zn-terminated (0001) plane has the highest energy among all the facets. One-dimensional growth along the c axis leads to a preference of exposing low energy {101̅0} planes. HRTEM observation on the interface between ZnO and Bi2WO6 reveals that the ZnO nanosheets grow epitaxially on the side surfaces of the Bi2WO6 nanoplates. The (101̅0) planes of ZnO stack parallel to the (200) ones of Bi2WO6, forming the ZnO (101̅0)//Bi2WO6 (200), implying that the ZnO nanosheets epitaxially grow along the normal direction of (200) Bi2WO6 planes. A necessary condition for potent heterogeneous nucleation is the existence of low energy coherent interfaces, which are determined by the strain of lattice misfits at the interface. The strain energy between the nucleus and the substrate is greatly minimized if the mismatch of the interatomic spacing is below 10%. The interplanar misfit should be less than 6%.40 According to the above analysis, it is suggested that different

Figure 5. XPS spectra of the sample Z-BW8 nanosheet: (a) The survey spectra, and the high-resolution XPS spectra of the sample (b) Zn 2p, (c) Bi 4f, (d) W 4f, and (e) O 1s. 2182

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preferential growth orientations of the ZnO on distinct crystallographic surfaces of the Bi2WO6 are driven by the least interfacial lattice mismatches.41 The lattice spacing of 0.28 nm for the ZnO and 0.27 nm for the Bi2WO6 are in good agreement with (1010̅ ) and (200) interplanar spacing of nanosheets ZnO and Bi2WO6, respectively. The interplanar mismatch between ZnO (101̅0) and Bi2WO6 (200) therein is 2.5% [(0.28−0.27)/0.28 ≈ 2.5%]. Such small lattice misfit is beneficial to the thermal stability of the nanoheterostructures.42 Then, the formation mechanism of the ZnO architecture is discussed. Zn(NO3)2 and HMTA may be the most commonly utilized chemical agents to synthesize ZnO nanowires in the existing literature.43,44 HMTA is a nonionic cyclic tertiary amine acting as a bidentate Lewis base. It readily hydrolyzes in water and gradually produces NH3 and HCHO, as shown in eqs 1−5.45 Moreover, except for the inherent fast growth along the direction of wurtzite ZnO polar facets, attachment of HMTA to the nonpolar surfaces also promotes the anisotropic growth in the [0001] direction, forming ZnO nanorods or nanowires.46 HMTA + 6H 2O ⇄ 4NH3 + 6HCHO

(1)

NH3 + H 2O ⇄ NH4 + + OH−

(2)

+

Figure 7. Schematic crystal structures of (a) Bi2WO6 and (b) ZnO.

8A. Compared with the Z-BW0, the absorption edges of samples adding Bi2WO6 crystal seeds provide a significant red

+

Zn 2 + 4NH3 ⇄ [Zn(NH3)4 ]2

(3)

+

Zn 2 + 2OH− ⇄ Zn(OH)2

(4)

Zn(OH)2 ⇄ ZnO + H 2O

(5)

While interestingly, with the assistance of Bi2WO6 crystal seeds, the ZnO architecture gradually changes from ZnO nanorods to nanosheets. The mechanisms involved in seedmediated growth of ZnO/Bi2WO6 nanostructures are investigated. Why can seed crystals facilitate crystal nucleation? The classical nucleation theory indicates that, in order to grow, crystallites of the stable state have to exceed a critical nucleus.47 Crystallization will proceed spontaneously if seed crystals with a larger size than the critical nucleus are added to the metastable liquid phase. Only if the seed particles exceed a well critical minimum size, the crystal seeds can be effective crystallization promoters, which can act as crystallization catalysts.48 Besides, electrostatic effects between ZnO and Bi2WO6 are another reason for the shape change of ZnO. Bi2WO6 belongs to the orthorhombic layered structure, space group Pca21, which includes a number of alternating perovskite-like (WO4)n2− layers and (Bi2O2)n2+ layers, constructing along the c axis.49 As a result, the oppositely charged layers may lead to the formation of plates with polar charges on their top and bottom surfaces.36 The schematic crystal structure of Bi2WO6 is presented in Figure 7a. ZnO is a kind of typical polar crystal with the positive polar facet Zn-(0001), and the negative polar facet O-(0001̅), respectively, as presented in Figure 7b. Becase of electrostatic effects, ZnO polar facets can connect to the Bi2WO6 nanoplates. As a result, this altered the deposition rate on the ZnO (0001) facet and suppressed the growth rate of the thermodynamically preferred facet.50 Finally, the ZnO nanosheets were formed. UV−vis diffuse reflectance spectra (DRS) are used to determine the optical properties of the samples. The roomtemperature UV−vis diffuse reflectance spectra of the Z-BW0, Z-BW1, Z-BW5, Z-BW8, and Z-BW10 are displayed in Figure

Figure 8. (A) UV−vis diffuse reflectance spectra of Z-BW0, Z-BW1, Z-BW5, Z-BW8, and Z-BW10. Inset: the corresponding plots of (αhν)1/2 vs hν (α = absorption coefficient) used to calculate the bandgap energy. (B) The pseudo-first-order kinetics for the photocatalytic degradation of RhB under visible light irradiation.

shift. The band gap of the Z-BW8 is estimated to be 3.02 eV based on the UV−visible diffuse reflectance spectrum (inset of Figure 8A), lower than that of the raw ZnO (3.11 eV). The photocatalytic decomposition of Rhodamine B (RhB) under UV irradiation (365 nm) was studied as a probe reaction using the Z-BW0, Z-BW1, Z-BW5, Z-BW8, and Z-BW10 powders. To evaluate the efficiency of photodegradation of RhB by the photocatalysts quantitatively, the pseudo-first-order model51 is chosen −ln(Ct /C0) = kt

(6)

where C0 and Ct are the concentrations of RhB in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. Figure 8B shows the pseudo-first-order rate constants (k) of the samples decreasing in the order: Z-BW0 > Z-BW8 > 2183

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Z-BW5 > Z-BW10 > Z-BW1. The corresponding pseudo-firstorder rate constants are displayed in Table 1. Table 1. Kinetic Parameters for the Photocatalytic Decomposition of RhB and the BET Surface Area of the Samples sample

K (min−1)

R2

BET surf. area (m2 g−1)

k′ (g min−1 m−2)

Z-BW0 Z-BW1 Z-BW5 Z-BW8 Z-BW10

0.00724 0.00282 0.00453 0.00603 0.00423

0.9984 0.9943 0.9996 0.9997 0.9971

5.79 3.68 3.65 4.44 4.15

0.00125 0.00077 0.00124 0.00136 0.00102

Figure 10. A comparison of the reaction rates for degradation of RhB over pure ZnO and pure Bi2WO6 under ultraviolet light irradiation.

The decolorization could not reveal the mineralization of the organic species. The total organic carbon (TOC) reflects the general concentration of organics in solution, which has been widely used to evaluate the degree of degradation or mineralization of organic species. The TOC removal is shown in Figure S4 (Supporting Information). Partial mineralization of the organic matter was confirmed by total organic carbon (TOC) reduction. The TOC changes during photocatalytic degradation are in agreement with the trend of decolorization. In order to evaluate the factors that influence ZnO photocatalysis, the relationship between photocatalytic activity and surface area or facet orientation is examined.52 First, the influence of surface area is investigated, shown in Table 1. The N2 adsorption−desorption isotherms and the pore size distributions of the products are provided in Figure S5 (Supporting Information). The isotherms are of type IV with H3 hysteresis loops.53 The inset pore size distributions data indicate that there are almost no pore structures in the materials.54 The comparison of the reaction rate constant for decomposition of RhB under ultraviolet light irradiation before and after normalization with surface areas is shown in Figure 9.

changes from nanorod to nanosheet, exposing more and more polar facets. Further increasing the mass fraction of Bi2WO6 to 10%, the ratio of polar facets will not further increase. As a result, the degradation rate constant gradually reduces. Therefore, facet orientation is the main factor influencing the photocatalytic activity.55 The results clearly demonstrate that the modified samples with a high proportion of Zn-terminated (0001) and O-terminated (0001̅) polar facets are beneficial to enhance photocatalytic activity.19 The variations of photoelectric response are distinguished through the transient photocurrent responses.56 Figure 11

Figure 11. Transient photocurrent responses of Z-BW1, Z-BW5, ZBW8, and Z-BW10 under the irradiation of 365 nm UV light.

shows the transient photocurrent responses of different samples under intermittent 365 nm UV light irradiation. It can be seen that the order of the photocurrent is consistent with that of the pseudo-first-order rate constants (k). Photocurrrent can be enhanced with a higher ratio of ZnO polar facets, and as long as the light is turned off, the photocurrent quickly decreases. The enhanced photocurrent suggests a longer lifetime of the photogenerated charge carriers or a more efficient separation of the photoexcited electron and hole pairs. For the reason that the photocurrent is formed mainly by the diffusion of the photogenerated electrons to the ITO, and at the same time, the photoinduced holes are captured by the hole acceptors in the electrolyte.56

Figure 9. Reaction rate constants for degradation of RhB over Z-BW0, Z-BW1, Z-BW5, Z-BW8, and Z-BW10 under ultraviolet light irradiation before and after normalization with surface areas.

The degradation rate constant of Z-BW8 (k′4 = 1.36 × 10−3 g min−1 m−2) is higher than that of the raw Z-BW0 (k′0 = 1.25 × 10−3 g min−1 m−2) after normalization with surface areas. The photocatalytic activity of the pure ZnO and Bi2WO6 is also compared in Figure 10. The photocatalytic activity of the pure Bi2WO6 (K = 0.00521) is worse than that of the pure ZnO (K = 0.00724). However, with the increasing of the Bi2WO6 mass ratio, the degradation rate constants for the samples adding Bi2WO6 gradually increase. For the reason that increases the mass ratio of Bi2WO6, the shape of the samples gradually

4. CONCLUSION In summary, ZnO nanostructures with different morphologies and percentages of polar facets are synthesized successfully by 2184

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chemical bath deposition. An appreciable control of the fraction of polar facets is achieved by simply adding a certain amount of Bi2WO6 crystal seeds. The experimental results demonstrate that the introduction of Bi2WO6 crystal seeds has a significant effect on the growth properties of ZnO crystals. Without the crystal seeds Bi2WO6, the shape of raw ZnO is nanorods with a low percentage of the (0001) and (0001̅) facets. With the increasing of the mass ratio of Bi2WO6 crystal seeds, the ZnO architecture gradually changes from nanorods to nanosheets, exposing a larger ratio of polar facets. The ZnO nanosheets with a higher proportion of Zn-terminated (0001) and Oterminated (0001̅) polar facets exhibit enhanced photocatalytic activities in the photocatalytic decomposition of RhB solution, which possibly provides a clue for exploring the facet orientation and the photocatalytic activity. Furthermore, this facile, controllable, and low-cost synthetic method does not need any templates or organic additives. It is expected that this successful synthetic approach can be extended to the preparation of other metal oxide nanocatalysts with unique morphologies yielding exotic physical and chemical properties.

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ASSOCIATED CONTENT

S Supporting Information *

XRD and EDS patterns, SEM images, crystal aspect ratio, TOC removal rate, and BET results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-591-83779239. Tel: +86-591-83779239. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21173046, 21273035) and the National Basic Research Program of China (2013CB632405, 2014CB239303).

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