One-step pyrolytic synthesis of ZnO nanorods with enhanced photocatalytic activity and high photostability under visible light and UV light irradiation

Accepted Manuscript One-step pyrolytic synthesis of ZnO nanorods with enhanced photocatalytic activity and high photostability under visible light and UV light irradiation Ni Huang, Jinxia Shu, Zhonghua Wang, Ming Chen, Chunguang Ren, Wei Zhang PII:

S0925-8388(15)30435-7

DOI:

10.1016/j.jallcom.2015.07.039

Reference:

JALCOM 34728

To appear in:

Journal of Alloys and Compounds

Received Date: 4 April 2015 Revised Date:

22 June 2015

Accepted Date: 4 July 2015

Please cite this article as: N. Huang, J. Shu, Z. Wang, M. Chen, C. Ren, W. Zhang , One-step pyrolytic synthesis of ZnO nanorods with enhanced photocatalytic activity and high photostability under visible light and UV light irradiation, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.07.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphical Abstract

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0.6 ZnO nanorods ZnO nanospheres ZnO nanosheets

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ACCEPTED MANUSCRIPT One-step pyrolytic synthesis of ZnO nanorods with enhanced photocatalytic activity and high photostability under visible light and UV light irradiation Ni Huang a, Jinxia Shu a, Zhonghua Wang a,*, Ming Chen a, Chunguang Ren b, Wei Zhang c,* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of

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a

Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, P.R. China b

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama,

Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key

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c

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Ikoma, Nara 630-0192, Japan

Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou Industrial Park, Suzhou

*Corresponding authors.

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215123, P.R. China

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Tel: (+86) 817-2568081, Fax: (+86) 817-2445233.

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E-mail: [email protected] (Z. Wang), [email protected] (W. Zhang).

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ACCEPTED MANUSCRIPT Abstract Zinc oxide (ZnO) nanostructures with different morphologies, including nanorods, nanospheres and nanosheets, were prepared by a simple, one-step method via the pyrolysis of zinc acetate, zinc oxalate and

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zinc nitrate, respectively. The as-prepared ZnO nanostructures were characterized by X-ray powder

diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-visible diffuse reflectance spectroscopy (DRS). The

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photocatalytic activities of the ZnO nanostructures were evaluated by the photodegradation of two

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typical organic dyes, rhodamine B (RhB) and methyl orange (MO). It was found that the ZnO nanorods exhibited the highest photocatalytic activity among the three ZnO nanostructures under both visible light and UV-visible light irradiation. Furthermore, the ZnO nanorods photocatalyst also showed excellent photostability and reusability under visible and UV-visible light irradiation. In

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addition, mechanism studies by using active species trapping experiments suggested that hydroxyl radicals (•OH), photoinduced holes (h+) and superoxide anion radicals (•O2−) were involved in the photocatalytic process. The •O2− played a major role under visible light irradiation, whereas the •OH

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was the main active species under UV light irradiation. A possible mechanism for the charge

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separation and organic dye pollutants degradation was proposed.

Key words: ZnO nanostructures; photocatalytic activity; photostability; organic pollutants; environmental remediation.

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ACCEPTED MANUSCRIPT 1. Introduction Zinc oxide (ZnO), a representative II−VI semiconductor with a wide direct band gap of ~3.3 eV and a large excitation binding energy of 60 meV [1], has received considerable attention over the past

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few years due to its low cost and wide applications in the fields of optics [2], optoelectronics [2], gas sensors [3-4], solar cells [5-6], piezoelectronics and photocatalysis [7-8]. As a photocatalyst, ZnO is believed to be an alternative to titanium dioxide (TiO2) [7,9-10], the most extensively investigated

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photocatalyst [11-12], for the band gap and photocatalytic mechanism of ZnO are similar to those of

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TiO2. Moreover, it has been reported that ZnO exhibits better photocatalytic performances than TiO2 (Degussa P25) for the photocatalytic degradation of some organic pollutants [13-15]. The photocatalytic performance of ZnO photocatalyst is significantly influenced by its morphology [16-17], particle size [16], crystal orientation [17], crystallinity [16], color [18] and

applications,

various

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surface defect.[19] With the aim to improve the photocatalytic performance of ZnO for practical kinds

of

synthetic

approaches,

including

aqueous

method

[20],

hydrothermal/solvothermal growth [16,21-22], solution combustion synthesis [23], sol−gel method

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[24-25], chemical vapor deposition [26], and ultrasonic assisted method [27], have been developed

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for the preparation of ZnO particles with different sizes and morphologies, such as nanocubes [27-28], nanoflowers [21,24] and nanourchins [22]. Apart from synthetic methods, the morphologies and photocatalytic performances of ZnO nanostructures are also dramatically affected by the use of different zinc salts [20,29]. These findings potentially open an avenue for the preparation of efficient ZnO photocatalyst for photocatalytic degradation of organic pollutants. However, some of these synthetic methods involve complex procedures, sophisticated equipments, rigorous experimental conditions or long processing time. In addition, some of the synthetic methods cannot reach the

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ACCEPTED MANUSCRIPT standard for mass production or the obtained ZnO products exhibit very low photocatalytic activity. Therefore, it is worthy of developing a simple and cost-effective method to prepare ZnO photocatalyst with high potocatalytic activity and good photostability.

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Herein, a simple and economical one-step method for the preparation of ZnO nanostructures with different morphologies was reported. ZnO nanorods, nanospheres and nanosheets were prepared by the pyrolysis of zinc acetate, zinc oxalate and zinc nitrate as zinc sources, respectively. The

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photocatalytic activities of the obtained ZnO nanostructures were evaluated by the photodegradation

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of a cationic dye rhodamine B (RhB) and an anionic dye methyl orange (MO). It was found that the ZnO nanorods using zinc acetate as precursor exhibited the best photocatalytic performance for the photodegradation of RhB and MO. Moreover, the ZnO nanorods also exhibited remarkably high photostability and long lifetime under both visible light and UV light irradiation, which was proved

2.1 materials

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2. Experimental section

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by the retainment of the photocatalytic activity after at least 10-cycle repeated use.

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Zinc acetate (Zn(CH3COO)2·2H2O), zinc nitrate (Zn(NO3)2·6H2O), ammonium oxalate ((NH4)2C2O4), rhodamine B (RhB) and methyl orange (MO) were purchased from Kelong Chemical Reagent Company (Chengdu, China). Zinc oxalate (ZnC2O4·2H2O) was prepared by the precipitation reaction of zinc nitrate with ammonium oxalate in aqueous solution. Deionized water was used in all experiments.

2.2 Preparation of ZnO nanostructures

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ACCEPTED MANUSCRIPT All the ZnO nanostructures were prepared via a simple pyrolysis method under ambient pressure in air without the use of any templates or surfactants. The ZnO nanorods were prepared by the pyrolysis of zinc acetate as precursor. In a typical synthesis, 2.0 g of Zn(CH3COO)2·2H2O was put into a 30 mL

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crucible with a cover, and then the crucible was put into a programmable muffle furnace and calcined at 550 °C for 2 h with heating rate of 1 °C min-1. Finally the pyrolyzed products were cooled down to room temperature naturally. The ZnO nanospheres and nanosheets were prepared with the same

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procedure as stated above using zinc oxalate and zinc nitrate as precursors, respectively.

2.3 Characterization

X-ray powder diffraction (XRD) was collected on a Rigaku Dmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) was taken

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with a field emission Hitachi S4800 scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were measured on an FEI Tecnai F20 microscope with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy

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(XPS) was performed on an XPS spectrometer (Kratos XSAM800). UV-Vis diffuse reflectance

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spectroscopy (DRS) was recorded on a Shimadzu UV-3600 spectrophotometer equipped with diffuse reflectance accessories using BaSO4 as reference.

2.4 Evaluation of photocatalytic activity The photocatalytic activities of the ZnO nanostrutures were evaluated by the degradation of two typical organic dyes, rhodamine B (RhB) and methyl orange (MO), under visible light and UV-visible light irradiation at ambient temperature in air with magnetic stirring. The visible light was

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ACCEPTED MANUSCRIPT provided by a 70 W metal halide lamp equipped with an ultraviolet cutoff filter to provide visible light with λ ≥ 400 nm. The UV-visible light was also provided by the metal halide lamp without use of the cutoff filter. For the degradation of RhB under visible light irradiation, 50 mg of ZnO powder

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was added to 50 mL of RhB aqueous solution (10 mg L-1) at room temperature. Prior to visible light (λ ≥ 400 nm) irradiation, the mixture of ZnO and RhB solution was firstly sonicated for 5 min and then magnetically stirred for 60 min in the dark to obtain an adsorption-desorption equilibrium. At

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certain time intervals, about 2.5 mL sample was taken out from the reaction system and centrifuged

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to remove the photocatalyst powders for analysis. The absorbance of the remnant dye was measured using a Shimadzu UV-2550 UV-Vis spectrophotometer (Japan) or a 723N visible spectrophotometer (Shanghai, China). The relative concentration (C/C0) of the RhB solution was calculated by the relative absorbance (A/A0) at 554 nm according to Beer–Lambert law. A0 and A are the absorbance of

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the RhB solution at the beginning time (t = 0) of visible light irradiation and at time t, respectively. C0 and C are the concentrations of RhB at the beginning of visible light irradiation and at time t, respectively.

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The photodegradation of RhB under UV-visible light irradiation was similar to that under visible

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light irradiation except for the light source. The photodegradation of MO (10 mg L-1) was similar to that of RhB except that the detection wavelength was 464 nm.

3. Results and discussion

3.1 Characterization of the ZnO nanostructures Figure 1 shows the SEM images of the ZnO products prepared via the pyrolysis of zinc acetate, zinc oxalate and zinc nitrate, respectively. The ZnO sample prepared from zinc acetate displayed

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ACCEPTED MANUSCRIPT rod-like nanostructure with length in the range of several micrometers and diameter in the range of 30−100 nm (Figure 1a and b). For the zinc oxalate precursor, the appearance of the ZnO sample showed puffed rice candy, in which sphere-like ZnO nanoparticles were connected with each other

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(Figure 1c). High magnification SEM image revealed that the diameter of the spheres was in the range of 30−150 nm (Figure 1d). In the case of ZnO sample prepared from zinc nitrate, sheet-like

several micrometers were observed (Figure 1e and f).

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structures with thickness less than 50 nm and diameter in the range of several hundred nanometers to

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The morphology and crystalline nature of the ZnO nanostructures were further revealed by TEM observations. The ZnO nanorods showed uniform morphology with diameter about 50 nm (Figure 2a), which was in accordance with the SEM observations. The lattice fringes with an interplanar distance of 0.26 nm could be assigned to the (002) planes of hexagonal ZnO (Figure 2b), indicating

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that the growth direction of the ZnO nanorods was along the (001) direction. The TEM image of the sphere-like ZnO sample is shown in Figure 2c. The average diameter of the ZnO nanospheres was about 100 nm, agreeing well with the SEM results. The distance between the adjacent lattice

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fringes was 0.28 nm (Figure 2d), corresponding well to the interplanar spacing of ZnO (100) planes,

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suggesting that the sphere-like ZnO was formed mainly with (001) planes exposed. The TEM image of the ZnO nanosheets looked like some broken pieces (Figure 2e). This observation was not in agreement well with the SEM result, which should be caused by the ultrasonication during the sample preparation process for TEM measurement. The HRTEM image of the ZnO nanosheets showed that the ZnO sample was composed of ZnO polycrystallites (Figure 2f). The observed lattice distances (0.28 and 0.26 nm) were in agreement with the interplanar spacings of ZnO (100) and (002)

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ACCEPTED MANUSCRIPT planes. As in the case of the ZnO nanorods, the lattice orientations were almost the same through the observed structures, indicating the better crystallinity of the ZnO nanorods sample. Figure 3 shows the XRD patterns of the as-prepared ZnO nanostructures. All characteristic peaks

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were well matched with the standard JCPDS card of ZnO (JCPDS No. 36-1451) with hexagonal wurtzite crystal structure (a = b = 3.250 Å, and c = 5.207 Å). The distinguishable diffraction peaks observed at 2θ values of 31.74°, 34.44°, 36.26°, 47.56°, 56.52°, 62.86°, 66.38°, 67.92° and 69.08°

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corresponded well to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes

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of hexagonal ZnO (JCPDS No. 36-1451), indicating the crystalline nature of the ZnO samples. In addition, it could be seen from the XRD patterns that compared with the ZnO nanosheets, the ZnO nanorods and nanospheres exhibited sharper diffraction peaks, indicating the good crystalline quality of the ZnO nanorods and nanospheres. The relatively low intensity of the ZnO nanosheets sample

observations.

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prepared from zinc nitrate might be due to its low crystallinity, which was consistent with the TEM

The X-ray photoelectron spectroscopy (XPS) measurements were performed to know the chemical

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states of Zn and O species in the ZnO nanostructures. All XPS spectra were adjusted with respect to

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the C1s binding energy of 284.8 eV. As shown in Figure 4a, 4c and 4e, the binding energies of Zn 2p3/2 and Zn 2p1/2 were observed at ~1021.8 and ~1044.8 eV, respectively, indicating that Zn existed mainly in Zn2+ chemical state on the sample surfaces [30]. The O 1s XPS spectra of the as-prepared ZnO nanostructures were shown in Figure 4b, 4d and 4f, and each of them can be fitted with two peaks. The O 1s peak at ~530.6 eV corresponded to the oxygen atoms that coordinated with Zn atoms in ZnO [30-31]. The peak at 531.9 eV for the ZnO nanosheets (Figure 4f) could be ascribed to the O−H bonds of surface-adsorbed water [32]. The high intensity of the O−H peak indicated the

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ACCEPTED MANUSCRIPT high content of hydroxyl groups on the surface of ZnO nanosheets, which might be related to the destruction of Zn−O−Zn and the formation of Zn−OH on the surface of ZnO nanosheets [32]. The higher binding energy peak (~532.5 eV) for the ZnO nanorods and nanospheres (Figure 4b and 4d)

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was approximately 2 eV above the oxide peak of 530.6 eV. According to previous reports, the peak at ~532.5 eV can be assigned to chemisorbed oxygen of the surface hydroxyls [33-35] or chemisorbed O2 in the oxygen deficient regions of ZnO [36]. The surface hydroxyls, chemisorbed O2 and oxygen

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vacancies can play important roles in the photocatalytic process [33,37-38].

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The optical properties of the as-prepared ZnO nanostructures were investigated by UV-visible diffuse reflectance spectroscopy (DRS). Figure 5a shows the DRS spectra and the corresponding photographs of the ZnO samples. All the DRS spectra of the samples had an analogical shape, which exhibited strong absorption in the ultraviolet region due to the electron transition from the valence

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band to the conduction band of ZnO under UV light irradiation. In addition, all the ZnO samples also had weak absorption in the visible region of 400−800 nm, corresponding to the colors of gray, pastel yellow and pale yellow of the ZnO nanorods, nanospheres and nanosheets, respectively. The optical

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bandgap (Eg) can be estimated from the plot of (αhv)2 versus hv [39], where α and hv are the

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absorption coefficient and the photo energy. The Eg values of the ZnO nanostructures were estimated by extrapolating the linear portion of the curves to zero (Figure 5b). The estimated Eg values for the ZnO nanorods, nanospheres and nanosheets were 3.20, 3.19 and 3.10 eV, respectively (Figure 5b). These values were close to the bang gap of ZnO reported previously [37,39]. It was noted that the ZnO nanorods sample with gray color showed better visible light (400−800 nm) response than the ZnO nanospheres and nanosheets, although the bandgap value of the ZnO nanorods (3.20 eV) was a

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ACCEPTED MANUSCRIPT little bit larger than that of the latter two ZnO nanostructures, indicating that the ZnO nanorods may have better photocatalytic performance under visible light irradiation.

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3.2 Photocatalytic performances of the ZnO nanostructures Two typical organic dyes, a cationic dye rhodamine B (RhB) and an anionic dye methyl orange (MO), were selected as model pollutants to evaluate the photocatalytic activities of the as-prepared

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ZnO nanostructures under both visible light and UV-visible light irradiation. Figure 6a shows the

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absorption spectrum change of RhB solution under visible light irradiation in the presence of the as-prepared ZnO nanorods. The characteristic absorption peak of RhB at the wavelength of ~554 nm decreased with increasing irradiation time. After 100 min of visible light irradiation, the peak at ~554 nm almost completely disappeared and the RhB solution became colorless (Inset in Figure 6a).

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However, the RhB solutions were still colorful after 100 min visible light irradiation in the presence of the as-prepared ZnO nanospheres and nonosheets (Insets in Figures 6b and c), although the peak intensities at 554 nm decreased to some degree (Figures 6b and c). For comparison, the photolysis of

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RhB in the absence of ZnO photocatalyst was also carried out. When there was no ZnO photocatalyst

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and only with visible light irradiation, the RhB dye was stable and no obvious degradation was observed (Figure 6d), suggesting that the photoinduced self-sensitized photolysis of RhB without ZnO was negligible. These results indicated preliminarily that the ZnO nanorods possessed higher photocatalytic activity than the ZnO nanospheres and nanosheets under visible light irradiation. To further confirm the enhanced photocatalytic performance of the ZnO nanorods, the photodegradation of MO under visible light irradiation and the photodegradation of RhB and MO under UV-visible light irradiation were also performed. For comparison, photolysis experiments

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ACCEPTED MANUSCRIPT without ZnO photocatalyst were carried out under the same conditions. The time-dependent absorption spectra of MO solution under visible light irradiation, RhB and MO solutions under UV-visible light irradiation are shown in Figures S1, S2 and S3, respectively. And the variations of

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relative concentration (C/C0) versus irradiation time are depicted in Figure 7. It can be seen that the photocatalytic activity of the ZnO nanrods was higher than that of the ZnO nanospheres and nanosheets for the photodegradation of RhB and MO under both visible light and UV-visible light

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irradiation. The photocatalytic activity followed the order of ZnO nanorods > ZnO nanospheres >

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ZnO nanosheets. Moreover, the absorption spectra of RhB and MO solutions were almost unchanged under both visible light and UV light irradiation in the absence of ZnO photocatalyst, indicating that the photolysis of RhB and MO was negligible under our experimental conditions and the photocatalysis was indeed the cause for the degradation of RhB and MO dyes.

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To quantitatively compare the photocatalytic activities of the as-prepared ZnO nanostructures, we performed the plots of ln(C/C0) versus irradiation time (Figure S4), assuming that the degradation reaction of dye molecules over the ZnO photocatalysts followed pseudo-first-order kinetics (-dC/dt =

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kC). The rate constants estimated from Figure S4 are summarized in Table S1 and compared in

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Figure 8. Obviously, the ZnO nanorods exhibited the highest rate constant for the photodegradation of RhB and MO under both visible light and UV-visible light irradiation (Figure 8).

3.3 Photochemical stability

As an applicable photocatalyst for practical applications, photochemical stability is another important aspect besides photocatalytic activity. For examining the photochemical stability of the as-prepared ZnO nanostructures, we have focused our study on the ZnO nanorods photocatalyst

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ACCEPTED MANUSCRIPT because it showed the best photocatalytic performance among the three ZnO nanostructures studied. Figure 9 shows the cyclability of the ZnO nanorods for the photodegradation of RhB and MO under both visible light and UV-visible light irradiation, respectively. Under visible light irradiation, the

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photocatalytic activity of the ZnO nanorods was not significantly changed within 10 cycles of RhB degradation or 12 cycles of MO degradation (Figures. 9a and b), indicating the good stability and reusability of the as-prepared ZnO nanorods. The photocatalytic activity of the ZnO nanorods

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showed a slight decrease after the 7th cycle of MO degradation, this might be caused by the loss of a

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small amount of ZnO photocatalyst during the recovery process, since the decrease of photocatalytic activity was not observed in the following 5 cycles (from 8th to 12th). To further confirm the photostability of the ZnO nanorods as a photocatalyst, photodegradation of RhB and MO dyes under UV-visible light irradiation was carried out. The ZnO nanorods also exhibited good photochemical

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stability and reusability under UV-visible light irradiation (Figures 9c and d). These results suggested that the as-prepared ZnO nanorods photocatalyst possessed high photochemical stability not only

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under visible light but also under UV light irradiation.

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3.4 Photocatalytic mechanism

The excellent photocatalytic activity and photostability of the as-prepared ZnO nonorods photocatalyst motivated us to probe the main active species during the photocatalytic process for an in-depth insight into the underlying photocatalytic mechanism. In general, the active species in the photooxidation of dyes mainly include hydroxyl radicals (•OH), superoxide anion radicals (•O2−) and photogenerated holes (h+) in the valence bond [40-41]. These primary active species can be detected through the trapping experiments for radicals and holes by using some scavengers. In this study,

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ACCEPTED MANUSCRIPT isopropyl alcohol (IPA) was used as •OH scavenger [42-43], benzoquinone (BQ) was added as •O2− scavenger [44], and ammonium oxalate (AO) was introduced as h+ scavenger [43,45]. Under visible light irradiation, the degradation efficiency of RhB decreased significantly from about 99% to 14%

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when BQ as •O2− scavenger agent was added into the photocatalytic reaction system, while the degradation efficiency reached 68% and 78% when IPA and AO were added as the trapping agents for •OH and h+, respectively (Figures 10a and b). This indicated that •O2− played dominant role in the

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photodegaradation of RhB under visible light irradiation. Under UV-visible light irradiation, the

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degradation efficiency of RhB was only decreased to 45% after addition of BQ, while dramatically decreased to 17% when IPA as •OH trapping agent was added to the photocatalytic system (Figures 10c and d,), indicating that •OH was the key factor in the photodegradation of RhB under UV light irradiation. The effects of IPA, BQ and AO on the degradation efficiency of MO under visible and

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UV-visible light irradiation were similar to those of RhB (Figure S5). These results indicated that compared with visible light irradiation, more •OH radicals were produced from •O2− under UV light irradiation.

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It has been reported that some dyes such as RhB can be excited by visible light [46-47]. The

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excited dye molecules (denoted as dye*) can inject electrons to the conduction band (CB) of a semiconductor such as TiO2 with dye* being converted to the cation radicals (denoted as dye•+). In turn, the injected electrons on the CB react with O2 that adsorbed on the semiconductor surface to produce reactive oxygen species (ROS) [46]. The dye•+ reacts with ROS to yield intermediate products or other radical species and may ultimately lead to the mineralization of dye molecules. Based on the above results and discussion, we proposed a putative mechanism for the photodegradation of RhB under visible light and UV light radiation (Figure 11). Under visible light

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ACCEPTED MANUSCRIPT irradiation, RhB was mainly degraded by the dye self-photosensitization. First, RhB was excited to excited state (RhB*) by visible light, then the RhB* injected electrons to the conduction band (CB) of ZnO semiconductor accompanied by the conversion of RhB* to the cation radicals, RhB•+ (Eqs. 1−2).

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Then, the injected electrons on the CB band of ZnO was captured by oxygen molecules that adsorbed on the surface of ZnO to generate reactive oxygen species (ROS), such as •O2− and •OH radicals (Eqs. 3−5). The RhB•+ cation radicals were attacked easily by the ROS and finally completed the

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degradation of RhB aqueous solution (Eqs. 6−7). Compared with RhB, MO dye is more difficult to

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be excited by visible light and more difficult to be degraded under visible light irradiation. This was coincident with the degradation efficiency results that RhB could be almost completely degraded by the ZnO nanorods with about 60 min under visible light irradiation, while more than 180 min was needed to complete MO degradation. Dye + hv (visible) → Dye*

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(1)

Dye* + ZnO → Dye•+ + ZnO (e−)

(2)

e− + O2 → •O2−

(3)

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•O2− + 2H+ + e− → H2O2

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H2O2 + e− → •OH + OH−

(4) (5)

Dye + •O2− → Oxidation product

(6)

Dye + •OH→ Oxidation product

(7)

Under UV light irradiation, the electrons in the ZnO valence band (VB) were firstly excited to the CB of ZnO, while the photogenerated holes (h+) were retained in the VB of ZnO (Eq. 8). Then water molecules and hydroxide anions that adsorbed on the surface of ZnO were oxidized to •OH radicals by the photogenerated h+ (Eqs. 9−10). Finally, organic molecules were attacked by the •OH radicals

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ACCEPTED MANUSCRIPT (Eq. 7) or directly oxidized by the photogenerated h+ (Eq. 11). The excited electrons in the CB of ZnO were captured by preadsorbed oxygen molecules to form reactive oxygen species and consequently completed the oxidation of organic dyes (Eqs. 3−7). Compared with visible light

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irradiation, more holes and electrons could be produced under UV light irradiation and consequently more •OH radicals could be generated. This was in agreement with the experimental results that the degradation of RhB and MO under UV-visible light irradiation was much faster than that under

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visible light irradiation and •OH radicals played more important roles under UV light irradiation than

ZnO + hv (UV) → h+ + e−

(8)

H2O + h+ → •OH + H+

(9)

OH− + h+ → •OH

(10)

(11)

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Dye + h+ → Oxidation product

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under visible light irradiation.

It is well know that the morphology, crystallinity and surface property of a photocatalyst are key issues for heterocatalysis, for the reaction occurs on the surface of the catalyst. The photocatalytic

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activity of a semiconductor photocatalyst will be enhanced with more active site, easier mass transportation, higher light-harvesting efficiency, faster electron capture and lower electron-hole pair

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recombination [19]. According to the UV-visible DRS results, the estimated bandgap of ZnO nanorods (3.20 eV) was very close to that of ZnO nanospheres (3.19 eV), the photocatalytic performance of ZnO nanorods was much better than that of ZnO nanospheres. The bandgap of ZnO nanosheets (3.10 eV) is smaller than that of ZnO nanorods and nanospheres, but the photocatalytic activity of the ZnO nanosheets is much lower than that of the latter two ZnO samples. These results indicate that the photocatalytic activity of ZnO semiconductor is not only related to bandgap (adsorption edge) but also significantly influenced by other factors, such as oxygen vacancy, crystallinity and crystal growth directions, which will be discussed in detail latter. The XPS spectra indicated that chemisorbed oxygen might exist on the surfaces of the ZnO nanorods and nanospheres in the oxygen deficient regions due to oxygen vacancies. The oxygen 15

ACCEPTED MANUSCRIPT vacancies could serve as photoinduced electron traps for the fast capture of photoexcited electrons, adsorption sites for the charge transfers to adsorbed species as well as hindrance for the prevention of electron-hole pair recombination [19,23,48-50]. In this case, excited electrons could be rapidly captured by oxygen molecules that adsorbed on ZnO surface to produce more reactive oxygen

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species, like •O2− and •OH radicals, consequently leading to the enhancement of photocatalytic performance. Therefore, the chemisorbed oxygen and the oxygen vacancies on the ZnO surfaces were important factors for enhancement of the photocatalytic activities of the ZnO nanorods and nanospheres.

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The crystallinity and the oxygen deficiency of ZnO nanospheres were similar to that of ZnO

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nanorods, but the photocatalytic performance of the ZnO nanospheres was much lower than that of the ZnO nanorods. The different photocatalytic performance of the two ZnO samples may be caused by the growth direction and exposed facets of the two ZnO samples. The growth direction of the ZnO nanorods was along the (001) direction, while the sphere-like ZnO was formed mainly with (001)

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planes exposed. It has been reported recently that the good charge separation between polar

{001} surfaces can effectively reduce the probability of recombination of photogenerated electrons and holes, and thus the polar {001} facets are highly reactive facets for degradation

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of organic pollutants [51]. Therefore, the better photocatalytic performance of the ZnO

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nanorods should be due to the good charge separation between the polar {001} facets. Indeed, photocatalysis is a complex process [19,52], it is not only associated with light harvesting, photoinduced electron-hole pairs separation, transportation and recombination, electron capture, molecule diffusion kinetics and adsorption thermodynamics, but also related to the particle size, particle morphology, crystal plane orientation and crystallinity of the semiconductor material. Therefore, it was possible that the enhanced photocatalytic performance of the ZnO nanorods

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ACCEPTED MANUSCRIPT photocatalyst was attributed to its good crystallinity, oxygen deficiency, uniform crystal orientation, and 1D rod-like structure with (001) growth direction.

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4. Conclusions ZnO nanostructures with various morphologies, including nanorods, nanospheres and nanosheets, were successfully prepared by a simple pyrolysis method using zinc acetate, zinc oxalate and zinc

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nitrate as precursors, respectively. The photocatalytic performances of as prepared ZnO

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nanostructures were evaluated by degrading typical cationic dye RhB and anionic dye MO under visible light and UV-visible light irradiations. It was found that the ZnO nanorods prepared from zinc acetate exhibited the highest photocatalytic activity among the three ZnO nanostructures studied in this work. The enhanced photocatalytic performance of the ZnO nanorods photocatalyst may be

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attributed to its good crystallinity, oxygen vacancy, uniform crystal orientation, and 1D rod-like structure with (001) growth direction. Furthermore, the ZnO nanorods photocatalyst also displayed good photostability under both visible light and UV light irradiation. The simple synthetic method,

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enhanced photocatalytic activity and high photochemical stability of the ZnO nanorods are beneficial

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for the photodegradation of organic pollutants in environmental remediation.

Acknowledgements

This work was supported by the Innovation Team Project of the Education Department of Sichuan Province (15TD0018), and the Open Project of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province of China (No. CSPC2011-7-2).

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ACCEPTED MANUSCRIPT Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://

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Figure Captions

Figure 1. Low magnification and high magnification SEM images of the as-prepared ZnO nanorods (a and b), nanospheres (c and d), and nanosheets (e and f). Figure 2. TEM and HRTEM images of the as-prepared ZnO nanorods (a and b), nanospheres (c and d), and nanosheets (e and f). 20

ACCEPTED MANUSCRIPT Figure 3. XRD patterns of the as-prepared ZnO nanostructures. The standard XRD pattern of wurtzite crystal structure of ZnO (JCPDS 36-1451) is also shown for comparison. Figure 4. Zn 2p and O1s XPS spectra of the as-prepared ZnO nanorods (a and b), nanospheres (c and

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d), and nanosheets (e and f). Figure 5. UV-visible diffuse reflectance spectra (a) and the energy band gap (b) of the as-prepared ZnO nanostructures. The inset in figure (a) shows the color of the as-prepared ZnO nanostructures.

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Figure 6. Time-dependent absorption spectra of RhB solutions under visible light irradiation in the

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presence of ZnO nanorods (a), nanospheres (b), nanosheets (c), and in the absence of ZnO photocatalyst (d). Inset shows the corresponding photograph of color change of RhB solutions. Figure 7. Photocatalytic degradation kinetics of RhB and MO under visible light (a and b) and UV-visible light (c and d) irradiation.

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Figure 8. Comparison of the degradation rate constants of RhB and MO over the as-prepared ZnO nanostructures under visible light (a) and UV-visible light (b) irradiation. Figure 9. Cycling kinetic curves of the as-prepared ZnO nanorods photocatalyst for the

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and d) irradiation.

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photodegradation of RhB and MO dye solutions under visible light (a and b) and UV-visible light (c

Figure 10. Effects of some scavengers on the photocatalytic degradation kinetics and the degradation efficiencies of RhB solution in the presence of the as-prepared ZnO nanorods under visible light (a and b) and UV-visible light (c and d) irradiation. (IPA: 10 mM; BQ: 0.2 mM; AO: 10 mM). Figure 11. Schematic diagram of electron–hole pairs separation and possible degradation mechanism of RhB on the ZnO photocatalyst under visible light and UV-visible light irradiation.

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Highlights ► ZnO nanostructures with different morphologies were prepared by a one-step method.

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► ZnO nanorods show enhanced photocatalytic activity and high photostability.

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► ZnO nanorods are beneficial for practical application in environmental remediation.

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Supporting Information

One-step pyrolytic synthesis of ZnO nanorods with enhanced photocatalytic activity and

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high photostability under visible light and UV light irradiation Ni Huang a, Jinxia Shu a, Zhonghua Wang a,*, Ming Chen a, Chunguang Ren b, Wei Zhang c,* a

Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of

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Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, P.R.

b

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China

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5

Takayama, Ikoma, Nara 630-0192, Japan c

Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key

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Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou Industrial Park,

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Suzhou 215123, P.R. China

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*Corresponding authors.

Tel: (+86) 817-2568081, Fax: (+86) 817-2445233. E-mail: [email protected] (Z. Wang), [email protected] (W. Zhang).

S1

ACCEPTED MANUSCRIPT Table S1. Pseudo-first-order degradation rate constants of RhB and MO under visible light and UV-visible light irradiation. Sample

Visible light

UV-visible light

RhB (min )

MO (min )

RhB (min−1)

MO (min−1)

ZnO nanorods

0.0586

0.0158

0.416

0.201

ZnO nanospheres

0.0122

0.00310

0.0742

ZnO nanosheets

0.00316

0.000670

0.00757

RI PT

−1

0.0593

0.00213

AC C

EP

TE D

M AN U

SC

−1

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ACCEPTED MANUSCRIPT

0.8 (b) 0 min 60 min 120 min 180 min 240 min

0.6

0.4

270

0.2

0.0 200

0 min 60 min 120 min 180 min 240 min

0.6

0.4

0.2

0.0 300

400

500

600

700

800

200

300

Wavelength (nm)

500

600

700

800

SC

0.8 (d)

0.4

0.2

0.0

Absorbance (a.u.)

0 min 60 min 120 min 180 min 240 min

0.6

0.6

M AN U

Absorbance (a.u.)

400

Wavelength (nm)

0.8 (c)

200

RI PT

464

Absorbance (a.u.)

Absorbance (a.u.)

0.8 (a)

0 min 60 min 120 min 180 min 240 min

0.4

0.2

0.0

300

400

500

600

800

TE D

Wavelength (nm)

700

200

300

400

500

600

700

800

Wavelength (nm)

Figure S1. Time-dependent absorption spectra of MO solution in the presence of the ZnO nanorods (a), nanospheres (b), nanosheets (c), and in the absence of ZnO photocatalyst (d) under

AC C

EP

visible light (λ ≥ 400 nm ) irradiation.

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1.6

1.6

(b)

0 min 2 min 4 min 6 min 8 min

1.2

0 min 4 min 8 min 12 min 16 min 20 min 24 min

1.2

0.8

0.4

0.8

0.4

0.0

0.0 200

300

400

500

600

700

200

800

300

1.6 Absorbance (a.u.)

1.2

0.8

0.4

0.0 200

300

400

500

600

700

800

TE D

Wavelength (nm)

500

600

700

800

700

800

SC

(d)

0 min 8 min 16 min 24 min 32 min 40 min 48 min

1.2

0 min 8 min 16 min 24 min 32 min 40 min 48 min

M AN U

Absorbance (a.u.)

(c)

400

Wavelength (nm)

Wavelength (nm)

1.6

RI PT

554

Absorbance (a.u.)

Absorbance (a.u.)

(a)

0.8 0.4

0.0 200

300

400

500

600

Wavelength (nm)

Figure S2. Time-dependent absorption spectra of RhB solution in the presence of the ZnO nanorods (a), nanospheres (b), nanosheets (c), and in the absence of ZnO photocatalyst (d) under

AC C

EP

UV-visible light irradiation.

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0.8 (b) 0 min 5 min 10 min 15 min 20 min

0.6

0.4

270

0.2

0.0 200

0 min 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min

0.6

0.4

0.2

0.0

300

400

500

600

700

200

800

300

500

600

700

800

SC

0.8 (d)

0.8 (c)

0.4

0.2

0.0

Absorbance (a.u.)

0 min 10 min 20 min 30 min 40 min 50 min 60 min

0.6

0.6

M AN U

Absorbance (a.u.)

400

Wavelength (nm)

Wavelength (nm)

200

RI PT

464 Absorbance (a.u.)

Absorbance (a.u.)

0.8 (a)

0 min 10 min 20 min 30 min 40 min 50 min 60 min

0.4

0.2

0.0

300

400

500

600

700

800

TE D

Wavelength (nm)

200

300

400

500

600

700

800

Wavelength (nm)

Figure S3. Time-dependent absorption spectra of MO solution in the presence of the ZnO nanorods (a), nanospheres (b), nanosheets (c), and in the absence of ZnO photocatalyst (d) under

AC C

EP

UV-visible light irradiation.

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(a)

(b) 0

0 -1

-1

-3 -4 ZnO nanorods ZnO nanospheres ZnO Nanosheets Blank

-5 -6

-2 -3 ZnO nanorods ZnO nanospheres ZnO nanosheets Blank

-4 -5

-7 0

20

40

60

80

0

100

40

(d) 0 -1 ln(C/C0)

M AN U

ln (C/C0)

-1

-2

ZnO nanorods ZnO nanospheres ZnO nanosheets Blank

10

20

120

160

200

240

30

40

-2 -3

ZnO nanorods ZnO nanospheres ZnO nanosheets Blank

-4 -5

50

0

TE D

Time (min)

SC

(c) 0

0

80

Time (min)

Time (min)

-3

RI PT

ln(C/C0)

ln(C/C0)

-2

10

20

30

40

50

60

Time (min)

Figure S4. Kinetic linear simulation analysis of RhB and MO under visible light (a and b) and

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UV-visible light irradiation (c and d) for the determination of rate constants.

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100 Degradation efficiency (%)

C/C0

0.8 0.6 0.4 No scavenger IPA BQ AO

0.2 0.0

(b) No scavenger

80 AO

60 IPA

40 BQ

20 0

0

30

60

90

120

150

180

Scavenger

Time (min)

(c) Degradation efficiency (%)

0.6

No scavenger IPA BQ AO

0.2 0.0

80 60

No scavenger

M AN U

C/C0

0.8

100

SC

(d)

1.0

0.4

RI PT

(a) 1.0

40

AO BQ

IPA

20 0

0

5

10

15

Time (min)

20

Scavenger

TE D

Figure S5. Effects of some scavengers on the photocatalytic degradation kinetics and degradation efficiencies of MO solution in the presence of the ZnO nanorods under visible light

AC C

EP

(a and b) and UV-visible light (c and d) irradiation. (IPA: 10 mM; BQ: 0.2 mM; AO: 10 mM).

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