Effect of synthesis conditions on the growth of various ZnO nanostructures and corresponding morphology-dependent photocatalytic activities

Accepted Manuscript Effect of synthesis conditions on the growth of various ZnO nanostructures and corresponding morphology-dependent photocatalytic activities Zao Yi, Jiangshan Luo, Xin Ye, Yougen Yi, Jin Huang, Yong Yi, Tao Duan, Weibin Zhang, Yongjian Tang PII:

S0749-6036(16)30552-3

DOI:

10.1016/j.spmi.2016.10.049

Reference:

YSPMI 4594

To appear in:

Superlattices and Microstructures

Received Date: 25 July 2016 Revised Date:

19 October 2016

Accepted Date: 19 October 2016

Please cite this article as: Z. Yi, J. Luo, X. Ye, Y. Yi, J. Huang, Y. Yi, T. Duan, W. Zhang, Y. Tang, Effect of synthesis conditions on the growth of various ZnO nanostructures and corresponding morphology-dependent photocatalytic activities, Superlattices and Microstructures (2016), doi: 10.1016/ j.spmi.2016.10.049. 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.

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Effect of synthesis conditions on the growth of various ZnO nanostructures and corresponding morphology-dependent photocatalytic activities

Zhang5, Yongjian Tang1,2,3 1

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Zao Yi1,2, Jiangshan Luo3, Xin Ye3, Yougen Yi4*, Jin Huang3∗, Yong Yi1,2*, Tao Duan1,2*, Weibin

Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang 621900, China

Co-Innovation Center for Energetic Materials, Southwest University of Science and Technology,

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2

Mianyang 621900, China

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China 4

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College of Physics and Electronics, Central South University, Changsha 410083, China Department of Physics, Dongguk University, Seoul, 100715, Korea

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∗ Correspondence should be addressed to Yougen Yi, Jin Huang, Yong Yi, Tao Duan. Tel: 86-0816-2480830; Fax: 86-0816-2480830 E-mail

address:

[email protected];

[email protected];

[email protected] 1

[email protected];

ACCEPTED MANUSCRIPT Abstract: Well-aligned ZnO nanorod arrays have been prepared by using seed-assisted hydrothermal method, in which the c-axis oriented ZnO films prepared by a soft-chemical were used as the seed layers on quartz glass. The epitaxial growth mechanisms and corresponding morphology effect of samples are

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discussed. The results show that the morphology of ZnO nanorod arrays is closely related to the hydrothermal temperature, pH value and initial concentration of Zn source in precursor solution. The as-prepared samples are characterized by X-ray diffraction (XRD), scanning electron microscopy

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(SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), Raman spectroscopy,

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photoluminescence (PL) spectra and UV-vis spectra. The well-aligned ZnO nanorod arrays exhibit improved photocatalytic degradation with Rhodamine 6G (R6G) solution under UV radiation, according with the UV-vis spectra investigation and photocatalytic improvement speculation. Noteworthy, the photocatalytic properties of well-aligned ZnO nanorod arrays have obviously improved comparing with

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other ZnO samples without proper alignment. The oxygen vacancies suppressed recombination process

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of photon induced electrons and holes, as the oxygen vacancy trapped the charge carriers.

Keywords: ZnO nanorod arrays; Hydrothermal method; Morphology control; Photocatalytic activity; Optical properties

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ACCEPTED MANUSCRIPT 1. Introduction Highly oriented ZnO nanorod arrays on a solid substrate have been widely concerned in the area of nano-electronics and nano-optoelectronic devices because of their excellent performance [1-4].

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Especially, the discovery of photoluminescence laser at room temperature made ZnO nanorod-arrays quickly become an international hotspot of ultraviolet semiconductor laser device material [5]. About the preparation of ZnO nanorod arrays, Park [6] and Kim [7] et al got the ZnO nanoneedle arrays on the

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silicon or on the ZnO nanoparticles coated silicon substrate by metal-organic chemical vapor deposition

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(MOCVD). Zhu [8], Zhang [9] and Bunyod [10] et al prepared the ZnO nanorod arrays on the silicon substrate or TiO2 film by chemical vapor deposition (CVD). These MOCVD and CVD methods required expensive equipments and complex operations. And the reaction conditions of methods should be carried out at a lower pressure (less than 0.005 MPa) and at a higher temperature (greater than 400 °C).

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The key of ZnO nanorod array preparation with the template method is the complex process of template preparation [11]. When removing the template, the newly-made ZnO nanorod arrays could be contaminated easily. The originally scattered nanorods tend to be bonded together, destroying the

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original nanostructures. Therefore, it is very importance to explore a simple, inexpensive and effective

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method to prepare ZnO nanorod arrays. Hydrothermal method is an effective method for large-scale preparation of ZnO nanorod arrays,

which can be easily operated in mild reaction conditions without producing any pollution [12-16]. In 2003, Vayssieres et al reported that using Zn(NO3)2·6H2O and C6H12N4 as precursor, the highly oriented ZnO crystal rod arrays were successfully prepared at 95 °C by the hydrothermal method [14]. Son et al [15] developed a two-step low temperature hydrothermal method, systematically investigated the effect of ZnO thin film on the growth of ZnO nanorods. Yang et al have shown that periodic individual ZnO

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ACCEPTED MANUSCRIPT nanorod arrays were grown on indium tin oxide (ITO) conductive substrates buffered with a ZnO seed layer by hydrothermal method [16]. Now, controlling the orientation, morphology, growth density, diameter distribution and aspect ratio of arrayed ZnO nanorods is still the most challenging issue in

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hydrothermal deposition of well-aligned ZnO nanorods [17, 18]. However, detailed studies on these issues up to now are inadequate. So, it is necessary to explore systematically the effect of preparing conditions on the growth of well-aligned ZnO nanorods in solution.

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Over the past few years, tremendous effort has been made to control the shape of ZnO nanocrystals.

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There are some literatures reported the preparation (solution processes) and photocatalytic properties of ZnO based photocatalysts with different nanostructures, such as nanoflower [19, 20], nanorod-array films [21, 22], wire meshes[23], and pore-array films [24].

Herein, a facile, surfactant-free and environmental-friendly solution route was developed to

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synthesize well-aligned ZnO nanorod arrays on quartz glass substrate. The effect of the morphology of ZnO nanorod arrays is discussed. This method can be accepted in moderate condition, no additive and template agent, and can be operated easily without any pollution to the environment. We demonstrate

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that these preparation conditions, including the reaction solution pH, hydrothermal temperature and the

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concentration of the precursors, play important role in the morphological and the alignment orientation of ZnO nanorod arrays. The mechanisms of the morphology of ZnO nanorod arrays effect for photocatalyst were discussed. The optical properties of as-prepared ZnO samples, such as photoluminescence, UV-vis spectra and Raman spectra were investigated. Moreover, the morphology dependent photocatalytic activity of as-prepared ZnO samples were investigated, and the degradation of Rhodamine 6G (R6G) of ZnO nanorod arrays are performed and compared with other ZnO nanostructures. This simple synthetic route could provide a good starting point for the research of

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ACCEPTED MANUSCRIPT morphology construction and shape-dependent catalytic properties of other materials. 2. Materials and Methods 2.1 Chemicals

Deionized water was used throughout the experiments. 2.2 Preparation of the samples

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2.2.1 Preparation of the seed layer

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All chemicals (analytical grade) were provided by Aldrich, and used without further purification.

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The quartz glass substrate (7.5 × 2.5-cm) was cleaned by deionized water and absolute alcohol, and then dried in the oven. Preparation of ZnO seed liquid was as following: At first, 1.0 g Zn (CH3COO)2·2H2O was dissolved in anhydrous ethanol solution (50 ml). Then, the homogeneous solution obtained by magnetic stirring for 30 minutes at 60 °C. ZnO gel films were coated on a clean

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substrate by dipping method. The immersion time of immersion seed solution was 1min, and the drying process was 80 °C. Repeat the above operation 3 times. Finally, to obtain seed film, the coated film was subjected to an isothermal treatment for 1 hour where the temperature was maintained at 550 °C for 1 h.

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2.2.2 Hydrothermal deposition

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In a typical processing, different amounts of ZnC12 were added into 60 mL of deionized water to get different concentration aqueous solution (0.005 M, 0.03 M and 0.1 M). And the pH value of solution was adjusted via ammonia solution (pH =8, 9, 10 respectively). Then, the solution was divided into three parts (every 30 mL), which were transferred into three autoclaves (50 mL). Then the ZnO seed layer coated quartz substrates were directly immersed into the growth solution in Teflon lined autoclave. The autoclaves were held at different reaction temperature, as 80 °C, 90 °C and 100 °C, respectively. The reaction vessels were placed in the oven for 70 minutes before the substrates were removed from them.

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ACCEPTED MANUSCRIPT At last, the ZnO nanorod arrays substrates were cleaned by deionized water and dried in air at room temperature. 2.3 Characterization

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The morphology and crystal structure of these samples were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The detailed morphology of the sample was researched using transmission electron microscopy (TEM). For TEM preparation, the nanrods were

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removed from the glass substrate and dispersed into alcohol, before being placed in a carbon film TEM

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grid. UV-vis absorption spectra analysis was performed on a Shimadzu UV-3101 PC spectrophotometer with film samples at room temperature. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) of the sample was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All the as-prepared samples were degassed at 100 °C prior to nitrogen adsorption

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measurements. Raman spectra were measured at room temperature using a LabRAM XploRA Raman spectrometer (Horiba Jobin Yvon, French). The wavelength of laser was 532 nm and the spot size was about 3 nm2. Photoluminescence measurements were carried out by Cary Eclipse (Varian, America).

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2.4 Photocatalytic tests

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In order to estimate the photocatalytic activity of ZnO nanostructures, in our work, we have studied the degradation of R6G that was used as a dye pollutant. The weight of nanostructured ZnO photocatalyst grown on the quartz glass substrate (7.5 × 2.5-cm) is about 0.015g by measuring the weight before and after the growth of ZnO. Firstly, the sample film was fixed onto R6G aqueous solution (50 mL, 10 ppm) for the photocatalytic study. The estimation of the photocatalytic activity was achieved by a photochemical reactor. These ZnO samples were immersed into R6G aqueous solutions, followed by a subsequent irradiation by a UV lamp (35 W, 4.0 mW/cm2). In order to analyze the

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ACCEPTED MANUSCRIPT degradation process of reaction solutions, the absorption spectra of the R6G solutions were recorded every 20 min. 3. Results and Discussion

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3.1 Morphology analysis 3.1.1 Reaction mechanism analysis

The typical ZnO nanorod arrays were synthesized in the conditions as followed: the initial Zn source

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concentration is 0.03 M, reaction solution pH value is 9, reaction temperature is 90 °C and the growth

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time is for 6 hours. As shown in Fig.1, the prepared ZnO nanorod arrays are directly characterized under SEM, in order to study their morphologies. As shown in the Fig.1 A, large area uniform vertically aligned ZnO nanorod arrays were observed on the glass substrate. The large coverage and uniform area reaching more than 0.04 cm2 has been obtained. As the ZnO seeds were large area uniform vertically

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aligned, the ZnO nanorod arrays with the same morphology would grow from the ZnO seeds via induced hydrothermal method. The higher magnification of ZnO nanorod arrays were shown in Fig.1 B and Fig.1 C. It can be observed that the ZnO nanorods have grown into a structure of cylindrical prim

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with a rod diameter is about 150 nm. Fig.1 D illustrates the cross section scan image corresponding to

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the ZnO nanorod arrays in Fig.1 A-C, shows that the nanorods grow directionally perpendicular to the quartz substrate in the state of arrays, and almost all nanorods align in the same orientation, growing discretely in an order. Near the glass part, we can see some ZnO nanoparticles which are prepared by dip coating. As the crystal seed grows in array ZnO nanoparticles can not only reduce the lattice mismatch between hydrothermal synthesizd ZnO layer and the glass plate, also known as the buffer layer, but also is good for the directional growth of ZnO nanorods. In the picture the nanorod length is about 2 µm. The optical images of quartz glass substrate, ZnO seeds and ZnO nanorod arrays prepared in a standard

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ACCEPTED MANUSCRIPT experiment (Sample 1-2 #) is shown (Supporting Information, Figure S1). Compared the quartz glass substrate, there is no change in the appearance of the ZnO seeds layer. ZnO nanorod arrays sample is milky white.

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The TEM images of ZnO nanorod with low and high magnification are shown in Fig.2 A and B respectively. From the Fig.2 A, we find the diameter of the ZnO nanorod is about 150 nm. The Fig.2 B is a typical high resolution TEM image, showing the lattice fringes of the ZnO nanorod. From the figure, we

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observe the little defects of either line or plane type. In addition, we observe the clear crystal lattice {002}

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of ZnO with inter planar distance of 0.26 nm, which indicates the preferred growth direction of ZnO along crystalline [002] direction.

ZnO is a polar crystal whose positive polar plane is rich in Zn and the negative polar plane is rich in O. Several growth mechanisms [25] have been proposed for aqueous chemical solution deposition. The most

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important growth path for a single crystal is the so-called Ostwald ripening process [25]. This is a spontaneous process that occurs because larger crystals are more energetically favored than smaller crystals. In this case, kinetics and nucleate nucleation favored tiny crystallites nucleate first in

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supersaturated medium and are followed by the growth of larger particles (thermodynamically favored)

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due to the energy difference between large and smaller particles of higher solubility based on the Gibbs-Thomson law [26]. In the condition of ammonia, the mechanism of the formation of ZnO is usually accepted as follows:

NH 3 + H 2O ↔ NH 3· H 2O ↔ NH 4 + + OH −

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Zn 2+ + 4 NH 3 → [ Zn( NH 3 )4 ]2 +

(2)

Zn 2+ + OH − → [ Zn(OH )n ]( n − 2 )−

(3)

[ Zn( NH 3 )4 ]2 + + OH − → [ Zn(OH ) n ]( n − 2) − + NH 3

(4)

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ACCEPTED MANUSCRIPT [ Zn(OH ) n ]( n − 2) − → ZnO + H 2O + OH − Here, n=2 or 4. For supersaturated

Zn2+/NH3·H2O solution system,

(5) [Zn(NH3)4]2+ and

[Zn(OH)n](n−2)− complexes may be formed while mixing the solution at room temperature, as shown in

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Eqs.(2) and (3). At higher temperature, Zn2+ with enough high energy may disengage from [Zn(NH3)4]2+ to compose [Zn(OH)n](n−2)−. Meanwhile, ZnO starts to nucleate heterogeneously on the interface between substrate and solution by the dehydration of [Zn(OH)n](n−2)−, as shown in

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Eqs. (4) and (5). Subsequently, ZnO hexagonal rods begin to grow up along c-axis preference on seed

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surfaces with the best structural fit for solution species. The reason for c-axis orientation growth is that [1000] basal plane of hexagonal rod is polar and has relative high surface energy. 3.1.2 Effect of react temperature on morphology

Temperature plays an important role in crystal growth [27]. Note that all of the results mentioned

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above are carried out at a fixed temperature of 90 °C. Here, we will explore the effect of temperature on the morphology of the final products. Fig.3 shows the products that growth time for 6 hours, initial Zn source concentration is 0.03 M, reaction solution pH is 9, that is, the higher the temperature, the bigger

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the growth rate. As shown in Fig.3 A, in the water bath temperature is 70 °C, it can be seen clearly that

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on the seed layer grow the ZnO nanorod arrays, the end face shape of each nanorod is roughly needle pointed and the rod diameter is smaller. As shown in Fig.3 B, in the water bath temperature is 110 °C, the end face shape of nanorods is very irregular and the nanorod size is also not uniform. Because the water bath reaction process of the formation of the ZnO nanorods is an endothermic reaction, when the bath temperature is too low (such as 70 °C), the reaction speed is very slow, resulting in smaller size of ZnO nanorods. When the bath temperature is increased, the reaction rate is significantly improved. Obviously, in the same reaction time, with the increasing temperature, the lateral size and length of

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ACCEPTED MANUSCRIPT nanorods will increase. On the basis of the above experimental results, the optimal temperature for the formation of the ZnO nanorod arrays is 90 °C. 3.1.3 Effect of pH on morphology

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As we know, changing the pH of the solution can affect the morphology of nanoparticles significantly [28]. In order to study the effect of the pH of the reaction solution on the formation of ZnO nanorod arrays, in this part, the pH of the solution is changed, and other reaction conditions are kept

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constant as used for preparing others ZnO nanoparticles. The growth time is for 6 h, initial Zn source

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concentration is 0.03 M, reaction temperature is 90 °C. Fig.4 shows the SEM images of these samples obtained with different pH of the solutions. When pH value of the reaction solution is 8, as shown in Fig.4 A, the generated film is very uneven, and the diameter of the ZnO rod is very small. When the pH value of the reaction solution is 10, as shown in Fig.4 B, the morphology of ZnO crystal is like the

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flower, the dumbbell, or the cluster consisting of a plurality of flower-like crystal rods. When the pH value of the reaction solution is adjusted, ammonia plays two roles in the formation of well-aligned ZnO nanorod arrays. One is that NH3·H2O reacts with zinc chloride to generate precursor [Zn(NH3)4]2+, and

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the precursor can produce ZnO crystal by dehydration under hydrothermal conditions. The other one is

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that NH3·H2O will generate NH4+ and OH- by adjusting the pH value of the reaction solution. These NH4+ may be adsorbed on the surface of the nucleus of ZnO, and influence the nucleation of interfacial energy that causes nucleation aggregation. According to the above experimental results, in our experiment, the optimal pH for the formation of the ZnO nanorod arrays is 9. 3.1.4 Effect of zinc concentration on morphology In our experiment, the concentration of ZnC12 is found to play an important role in the formation of the ZnO nanorod arrays. In order to study the influence of the concentration of ZnC12 on the formation

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ACCEPTED MANUSCRIPT of the ZnO nanorod arrays, the concentration of ZnC12 in the reaction solution is changed, and other conditions were kept constant as used for preparing other samples. The growth time is 6 h, reaction solution pH is 9, reaction temperature is 90 °C. Fig.5 shows the SEM of ZnO nanorod arrays obtained

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from different concentrations of precursor solution. From Fig.5 A, it can be seen when the concentration of Zn source in the precursor solution is 0.005 M, end face of the ZnO nanorod is nearly circular, the diameter is less than 100 nm and the rods are different in height. When the concentration of Zn source in

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the precursor solution is 0.1 M, end face of the ZnO nanorods is a polygon, and the gap between the rod

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and the rod is very small, some even fuse into one (as shown in the Fig.5 B ). When the concentration of Zn source in the precursor solution is low, the driving force of the chemical reaction is low, leading to the differences between the growth rates of ZnO of each crystal with smaller face. Therefore, as shown in Fig.5 A, the end face of the ZnO nanorod is not a typical hexagon and the rod diameter is very small.

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With increasing of the concentration of precursor solution, the difference between ZnO crystal growth rates will increase with the increase of the reaction driving force, so the end face of ZnO nanorod gradually appears themselves as regular hexagons. When the concentration of precursor solution is too

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high, the end face of the ZnO nanorod becomes irregular graphics and the gap between rods is too small.

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On the other hand, when the concentration of precursor solution is low, some large grains which are based on the ZnO seed crystal thin films will grow nanorods preferentially, they become the main consumption of Zn in the source solution in the subsequent growth. However, the growth of nanorods, which takes the small grain as the nucleation center, is suppressed due to the lack of the Zn source. So, in this case, the growth of ZnO nanorods is extremely different in length. Obviously, if the concentration of Zn source in the precursor solution is high enough, it can ensure that the growth of ZnO nanorods can get enough Zn source. Thus after a period of time, the length of the ZnO nanorods is consistent.

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ACCEPTED MANUSCRIPT According to the above experimental results, in our experiment, the optimal concentration of ZnC12 for the formation of the ZnO nanorod arrays is 0.03 M. 3.2 Structure of the ZnO nanorod arrays

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Fig.6 B shows the XRD pattern of the ZnO samples obtained with different reaction temperature: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#) and (c) 110 °C (Sample 1-3#). As shown in the Fig.6 A, the diffraction peaks are corresponding to crystal planes (100), (002), (101), (102), (110), (103), (112),

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(201), (004). The mapping data of the product of diffraction peaks of XRD is consistent with the PDF

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card for 79-0208, which belongs to the six corners of spiauterite ZnO, the space group is P63mc, the lattice constant a=3.624 nm, c=5.219 nm [29]. The peak of diffraction is sharp and narrow, and Zn and other impurities diffraction peaks are not observed, which show that the purity of the product is very high. In the Fig.6 B-a, the diffraction intensity of (002) crystal face is highest, which explains that most

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of ZnO nanorods grow along the (002) crystal face, and the strong diffraction peak suggests that the crystal phase of ZnO nanorod prepared by hydrothermal method is good. Fig.6 B shows the N2 adsorption-desorption isotherms of the ZnO samples obtained with different

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reaction temperature: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#) and (c) 110 °C (Sample 1-3#).

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Here, in this paper, these quartz glass substrates have been removed. It was found that the BET surface area the Sample 1-2# (ZnO nanorod arrays, see Fig.6 B-a, 40.3 m2/g) was larger than that of Sample 1-1# (see Fig.6 B-b, 26.5 m2/g) and Sample 1-3# (see Fig.6 B-c, 21.7 m2/g), indicating the synthesized ZnO nanorod arrays have good BET surface area. Figure S2 shows the N2 adsorption-desorption BET isotherms: (A) ZnO samples obtained with different pH: (a) pH=9 (Sample 1-2#), (b) pH=8 (Sample 2-1#), (c) pH=10 (Sample 2-3#). (B) ZnO samples obtained with different concentrations of Zn source: (a) 0.03 M (Sample 1-2#), (b) 0.005M (Sample 3-1#), (c) 0.1M (Sample 3-3#). It was found that the

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ACCEPTED MANUSCRIPT BET surface area the Sample 1-2# have good BET surface area. 3.3 Spectra analysis ZnO crystallizes are characterized by a hexagonal wurtzite structure, which belongs to the space

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group P63 mc. The group theory predicts six sets of phonon modes: 1 A, 2 B, 1 E and 2 E. Among them, the B mode is not Raman active [30]. Fig.7 A shows Raman spectra of the ZnO samples obtained with different reaction temperatures: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#). As can be seen, at

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329, 386, 436 and 580 cm-1, four peaks appear. The most intense peak at 436 cm-1 is from the high

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frequency vibration mode E2H of optical phonon, which is the most typical Raman active peaks of wurtzite ZnO. A very weak peak at 329 cm-1 is belonging to the multi phonon scattering superposition process (E2H-E2L). The peak at 386 cm-1 is corresponding to the A1(TO) optical mode of ZnO. The appearance of the E1(LO) peak is associated with the formation of defects such as oxygen vacancy, zinc

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interstitial, or their complexes [31, 32]. It can be seen that the E1(LO) peak of the Sample 1-2# (ZnO nanorod arrays, see Fig.7 A-a ) is stronger than that of the Sample 1-1# (see Fig.7 A-b), indicating that the defect of the former is higher than that of the latter.

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Fig.7 B shows photoluminescence spectra of the ZnO samples obtained at different reaction

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temperatures: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#), (c) 110 °C (Sample 1-3#). The PL measurements were carried out at room temperature with an excitation wavelength of 325 nm. As we known, the PL spectrum of ZnO products consisted of two emission peaks, one is the weak emission peak appeared in the UV-region (λ=380 nm), and another one is the strong emission peak appeared in the visible region (λ=500-650 nm). It was indicated that the UV emission (λ=380 nm) was attributed to the recombination of the band edge exciton. A broad green-yellow peak around 500-650 nm was originated from defect state luminescence which was associated with electron acceptors or defects such

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ACCEPTED MANUSCRIPT as Zn vacancy or O interstitials in ZnO, and it may also be associated with oxygen adsorbed in ZnO grain boundaries [33, 34], which can bring about a strong visible light emission. Moreover, it was found that the PL intensity of the Sample 1-2# (ZnO nanorod arrays, seen from Fig.7 B-a) was stronger than

ZnO nanorod arrays showed good optical properties.

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that of Sample 1-1# and Sample 1-3# (seen from Fig.7 B-b and Fig.7 B-c), indicating the synthesized

Fig.7 C shows UV-vis spectra of the ZnO samples obtained with different reaction temperatures: (a)

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90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#), (c) 110 °C (Sample 1-3#). They show similar broad and

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strong absorption with a maximum at about 390 nm, which is characteristic of ZnO wide band semiconductor material. Moreover, it was found that the UV-vis absorption intensity of the Sample 1-2# (ZnO nanorod arrays, seen from Fig.C-a) was stronger than that of Sample 1-1# and Sample 1-3# (see Fig.7 C-b and Fig.7 C-c), indicating the synthesized ZnO nanorod arrays have good optical properties.

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3.4 Photocatalytic activities of the ZnO samples

ZnO nanoparticles have been used as a photo catalyst for the photo degradation of some water pollutants, photo reduction of some halogenated benzene derivatives, photo reduction of some toxic

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metal ions and photo catalytic water splitting [35]. In order to know the relationship between the

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nanostructure and photo catalytic property of ZnO nanoparticles, through the degradation of R6G solutions under UV illumination, we have researched the photo catalytic activities of prepared ZnO with different morphologies. In order to investigate its photodegradation characteristic, the visible absorption band (~526 nm) of the R6G has been monitored in its absorption spectra during the photo catalytic reaction experiment. As shown the kinetics of R6G photodegradation (Fig.8), the photocatalytic activities for R6G degradation are obviously different in different ZnO samples. The Fig.8 A shows the kinetics of R6G photodegradation by different ZnO samples obtained with different reaction

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ACCEPTED MANUSCRIPT temperatures: (b) 90 °C (Sample 1-2#), (c) 70 °C (Sample 1-1#) and (d) 110 °C (Sample 1-3#). The SEM of these samples is shown in the Fig.3. The Fig.8 B shows the kinetics of R6G photodegradation by different ZnO samples that obtained with different pH: (b) pH=9 (Sample 1-2#), (e) pH=8 (Sample

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2-1#), (f) pH=10 (Sample 2-3#). The SEM of these samples is as shown in the Fig.4. The Fig.8 C shows the kinetics of R6G photodegradation by different ZnO samples that obtained with different concentrations of Zn source: (b) 0.03 M (Sample 1-2#), (g) 0.005M (Sample 3-1#), (h) 0.1M (Sample

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3-3#). As shown these curves, the ZnO nanorod arrays (sample 1-2#) have shown the best catalytic

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efficiency. Here, to justify the commercial potential of the ZnO nanorod arrays (sample 1-2#), the photocatalytic activity of Degussa-P25 were measured under similar test conditions and compared with the obtained results, as shown the Figure S3. Furthermore, in order to test the stability and repeatability of ZnO nanorod arrays (sample 1-2#), the photocatalytic degradation of R6G was repeated for five

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cycles under the same conditions. As shown in Figure S4, only a slight decrease in the photocatalytic activity (of about 6%) was observed with recycled photocatalyst after five reaction cycles, which indicates the excellent stability and repeatability that is important for its practical application.

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Here, in our experiment, what causes the change in the photo catalytic activity? In order to answer

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this question, we have to comprehend the mechanism of semiconductor photo catalysis. Once the semiconductor nanoparticles are irradiated by incident light with energy higher or equal to the band gap, the electrons (e-) in the valence band (VB) could be excited to the conduction band (CB), at the same time, and leave holes (h+) in the VB. The electronic acceptors, like adsorbed O2 could trap photo-excited electrons, so as to form superoxide radical anion (O2-). At the same time, these electron donors like OHor organic pollutants could trap the photo-induced holes, so as to further oxidize organic pollutants [36, 37]. In order to explain the effect of oxygen defects on photo catalysis, we have proposed photocatalytic

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ACCEPTED MANUSCRIPT mechanisms for the photo degradation of R6G, as shown in Fig.9. The electrons can be trapped by the lattices of oxygen deficiencies (Vo••) and Zn interstitials (Zni•), so as to maintain the charge neutrality [38].

V0• → V0•• + e '

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(6)

Zni• → Zni•• + e '

V0•• + O0 + H 2O(g) → OH 0• + OH 0•

(7) (8)

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Here, Vo•• indicates two positively charged vacancies and a perfect lattice on oxygen site. Once the

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incident light with energy is higher or equal to the band gap of the material, electron-hole pairs will be generated. Oxygen vacancies on the surface of ZnO nanorod are active sites for the dissociative and chemisorptions of water. As shown in the Eq.8, two hydroxyl groups could be generated by the dissociation of one water molecule. Besides the Eq.8, the defect chemical reactions could be

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accompanied by a chain reaction, as shown in the Eqs.9-15. Because of an oxidation process, extra oxygen would accommodate into the lattice of these ZnO nanorods, and become interstitial ions [39]. As shown in Eq.10 and Fig.7, these holes can diffuse to the surface of ZnO nanorods. At the same time,

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these holes can react with OH species, so as to form OH• radicals. As shown in the Eq.11, these electrons

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can react with oxygen to produce oxygen ions (i.e., O2-, O-). Then, these oxygen ions will react with water molecules (Eq.12), so as to form the hydroperoxyl radicals (HO2•) with hydroxyl ions (OH-).

1 O2 → Oi'' + 2h• 2

(9)

OH − + h + → OH •

(10)

O2 + e − → O2 −

(11)

O2 − + H 2O(g) → HO2 • + OH −

(12)

2HO2• → H 2O2 + O2

(13)

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(14)

H 2O2 + e − → OH • + OH −

(15)

From the Eqs.13 and 14, there are two reaction routes may further dissociate the HO2•. Via

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combination with O2, these HO2• can dissociate into hydrogen peroxide (H2O2), as shown in the Eq.13. At the same time, these hydroperoxyl radicals may react with water molecules and electrons, so as to form the H2O2 and OH-, as shown in the Eq.14. As shown in the Eq.15, the prepared H2O2 will further

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oxidizing agents for decomposition of the R6G solutions.

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react with electron carriers, so as to form OH• and OH-. Importantly, these HO2• and OH- are powerful

According to these photo catalysis results, the difference in photo catalytic activity among ZnO samples may be related to several factors. Firstly, the surface adsorption ability can affect the photo catalytic activity. Second, the type and concentration of oxygen defects on the surface ZnO samples can

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affect the photo catalytic activity. Generally speaking, as the active centers, oxygen defects in ZnO samples can capture photo induced electrons. Importantly, the recombination time of holes and electrons can be effectively prolonged [40]. Therefore, in our experiment, the higher photo-catalytic activity of the

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4. Conclusions

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ZnO nanorod arrays may result from the larger content of oxygen vacancy on the surface.

In summary, the well-aligned ZnO nanorod arrays have been successfully prepared using

low-temperature hydrothermal method in this paper. The control experiments have been explored for a more thorough understanding of the growth mechanism. The typical well-aligned ZnO nanorod arrays were synthesized as following: the initial Zn source concentration was 0.03 M, reaction solution pH was 9, the reaction temperature was 90 °C and the growth time was 6 hours. The Raman spectra, UV-vis spectra and PL spectra indicated the as-prepared samples have good optical properties. Moreover, the ZnO

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degradation of organic pollutants. Acknowledgment

The work is supported by the National Natural Science Foundation of China (No. 51606158;

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11375159; 61405180), State Key Laboratory of Ultra-Precision Machining Technology foundation of

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CAEP (ZZ14010), the National Key Scientific Instrument and Equipment Development Project of China (No. 2014YQ090709), Science and Technology Development Foundation of Chinese Academy of Engineering Physics (2013A0302016), Research Fund for the Doctoral Program of Southwest University of Science and Technology (No. 14zx7144).

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Appendix A. Supplementary data

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ACCEPTED MANUSCRIPT Figure captions: Fig. 1 SEM images of the ZnO nanorod arrays prepared in a standard experiment (Sample 1-2#). Fig. 2 The TEM image of ZnO nanorod: (A) Low magnification; (B) High magnification. (Sample 1-2#)

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Fig. 3 SEM images of ZnO samples obtained with different reaction temperature: (A) 70 °C (Sample 1-1#), (B) 110 °C (Sample 1-3#). The growth time is 6 hours, initial Zn source concentration is 0.03 M, reaction solution pH is 9.

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Fig. 4 SEM images of ZnO samples obtained with different pH: (A) pH=8 (Sample 2-1#); (B) pH=10 (Sample 2-3#). The growth time is 6 hours, initial Zn source concentration is 0.03 M, reaction

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temperature is 90 °C.

Fig. 5 SEM images of ZnO samples obtained with different concentrations of Zn source: (A) 0.005M (Sample 3-1#); (B) 0.1M (Sample 3-3#). The growth time is 6 hours, reaction solution pH is 9, reaction temperature is 90 °C.

Fig. 6 Structure of the ZnO nanorod arrays: (A) XRD pattern: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#), (c) 110 °C (Sample 1-3#); (B) N2 adsorption-desorption BET isotherms: (a) 90 °C

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(Sample 1-2#), (b) 70 °C (Sample 1-1#), (c) 110 °C (Sample 1-3#). Fig. 7 Spectra analysis of the ZnO samples obtained with different reaction temperature: (A) Raman spectra analysis: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#); (B) Photoluminescence

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spectra analysis: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#), (c) 110 °C (Sample 1-3#); (C) UV-vis spectra analysis: (a) 90 °C (Sample 1-2#), (b) 70 °C (Sample 1-1#), (c) 110 °C (Sample

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1-3#).

Fig. 8 Kinetics of R6G photodegradation by different samples. (A) ZnO samples obtained with different reaction temperature: (b) 90 °C (Sample 1-2#), (c) 70 °C (Sample 1-1#), (d) 110 °C (Sample 1-3#). (B) ZnO samples obtained with different pH: (b) pH=9 (Sample 1-2#), (e) pH=8 (Sample 2-1#), (f) pH=10 (Sample 2-3#). (C) ZnO samples obtained with different concentrations of Zn source: (b) 0.03 M (Sample 1-2#), (g) 0.005M (Sample 3-1#), (h) 0.1M (Sample 3-3#). The photocatalytic efficiency is represented by the absorbance at ~526 nm normalized to that of R6G prior to the reaction (C/C0) versus reaction time. Fig. 9 Photocatalytic mechanism of the as-synthesized ZnO nanophotocatalysts. 23

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Fig. 10

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ACCEPTED MANUSCRIPT Highlights > Well-aligned ZnO nanorod arrays were prepared by seed-assisted hydrothermal method. > The epitaxial growth mechanism and the effect of the morphology are discussed.

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> ZnO nanorod arrays exhibited improved ability on the photocatalytic.