Fabrication and photocatalytic activities of ZnO arrays with different nanostructures

Applied Surface Science 263 (2012) 704–711

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Fabrication and photocatalytic activities of ZnO arrays with different nanostructures Fazhe Sun a , Xueliang Qiao a,∗ , Fatang Tan a , Wei Wang a , Xiaolin Qiu b a b

State Key Laboratory of Plastic Forming Simulation and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, Hubei, PR China Nanomaterials Research Center, Nanchang Institute of Technology, Nanchang, 330013, Jiangxi, PR China

a r t i c l e

i n f o

Article history: Received 8 February 2012 Received in revised form 21 September 2012 Accepted 21 September 2012 Available online 3 October 2012 Keywords: Crystal growth Nanocrystalline materials Hydrothermal method Photocatalytic activity

a b s t r a c t Large-scale ZnO arrays with a series of morphologies, including nest-like, tower-like, and flower-like samples, have been successfully synthesized by a simple hydrothermal method. The morphologies of the obtained ZnO arrays can be conveniently tailored by changing seeding conditions. The samples were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. Their PL spectra depend on their morphologies and defects density. The morphology-dependent photocatalytic performances were studied by analyzing the degradation of methylene blue (MB) in aqueous solution. The nest-like ZnO arrays exhibited higher photocatalytic activity than tower-like and flower-like ZnO arrays. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, controlling the morphology and size of nanomaterials has attracted intensive attention since they have great effects on electrical, optical, and catalytic properties [1–3]. Many works have been devoted to controllable synthesis of nanomaterials with various morphologies to elucidate the structure–property relationship [4–6]. ZnO is a semiconductor with exceptional electronic and photonic properties as well as great thermal stability and oxidation resistance [7]. It has been extensively investigated owing to its potential applications in photocatalysis [8,9], light emitting diodes [10], nano-generators [11], solar cells [12–14], ultraviolet sensors [15,16] and field-effect transistors [17]. In the field of photocatalysis, ZnO is a noteworthy candidate for the photocatalytic degradation of environmental pollutants, since it has similar band gap energy (3.37 eV) compared with TiO2 (3.20 eV). What is more, there are many reports of ZnO exhibiting better activity than TiO2 [18–20]. Navrotsky et al. [19] reported that hierarchically assembled porous ZnO nanoparticles were more effective in the photodegradation of phenol than TiO2 nanoparticles. In Ye et al.’s work [20], ZnO nanoplatelets showed superior activity to

∗ Corresponding author. Tel.: +86 2787541540; fax: +86 2787541540. E-mail address: [email protected] (X. Qiao). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.144

commercial Degussa P25 TiO2 particles in the photodegradation of eosin B. More and more investigations have been carried out for photodegradating different organic pollutants in aqueous solution using ZnO nano-powders as photocatalysts. However, it is very difficult to separate and recover ZnO powders from the treated solution. Thus, there has been much interest in developing various supported ZnO nanostructures (ZnO arrays, for example) for photocatalytic processes. Several methods have been developed for the fabrication of ZnO arrays, such as chemical vapor deposition [21,22], thermal evaporation method [23], pulsed laser deposition [24], electrochemical deposition [25,26], template-assisted growth [27] and chemical solution deposition [28,29]. Solution chemical methods are usually simpler and less expensive than physical methods. However, ZnO obtained by solution chemical methods usually has low crystallinity, which results in the low photocatalytic activity [30]. Hydrothermal method is a cheap and environmentally friendly way to high quality of ZnO crystals, especially at a low growth temperature (80–95 ◦ C) [31]. So far, various ZnO arrays such as flower-like [32], nanowire [33], nanorod [34,35], and nanotube [36] have been synthesized by hydrothermal method, but the development of method for effectively tuning arrays structure and its relationship with photocatalytic activity is rather limited. In this study, we synthesized large-scale ZnO arrays with different nanostructures by hydrothermal method. By changing seeding conditions, nest-like, tower-like, and flower-like ZnO arrays have been

F. Sun et al. / Applied Surface Science 263 (2012) 704–711

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Fig. 1. FESEM images of ZnO arrays with different morphologies: (a, b) nest-like; (c, d) tower-like; (e, f) flower-like.

prepared. The influence of arrays structure on photocatalytic properties of ZnO arrays was studied. The possible growth mechanisms of ZnO arrays were also discussed.

2. Experimental 2.1. Materials Zinc acetate [Zn(CH3 COO)2 ·2H2 O] for this study was obtained from Sinopharm Chemical Reagent Co. Ltd. The zinc nitrate [Zn(NO3 )2 ·6H2 O] and hexamethylenetetramine [HMT; (CH2 )6 N4 ] were purchased from Tianjin Chemical Reagent Co. Ltd. All chemicals were of analytical grade and were used without further purification. The glass substrates (microscope glass slides, Hubei Xinying Noble Metal Co. Ltd.) were successively cleaned with acetone, anhydrous ethanol and distilled water in an ultrasonic bath, then dried in the air.

2.2. Synthesis of ZnO arrays The synthesis process of ZnO arrays can be divided into two steps: (1) the formation of initial ZnO seed layer [37]. Firstly, 10 mM zinc acetate ethanol solution was prepared. Then, 1 ␮L of the above solution was spin-coated on a 2 cm × 2 cm glass substrate at a speed of 3200 rpm for 30 s, and this process was repeated five additional times. After that, the substrate was annealed in an oven at 300 ◦ C for 2.5 h to form a ZnO seed layer. (2) ZnO hydrothermal growth process [32,34]. Firstly, the precursor solution was prepared by mixing 0.1 M HMT with 0.1 M Zn(NO3 )2 ·6H2 O aqueous solution. The hydrothermal growth was carried out at 90 ◦ C for 6 h in a Teflon-lined stainless steel autoclave by immersing the modified substrate in the precursor solution. After 6 h, the growth solution was cooled down to room temperature naturally. The substrate was pulled out and thoroughly washed with deionized water to remove any residual salt or amino complex on the surface and allowed to dry at 60 ◦ C in an oven. Various nanostructures, including nest-like, tower-like, and flower-like ZnO arrays were obtained by varying Step (1).

2.3. Characterization The morphologies of the samples were characterized by using FESEM (Sirion 200, FEI Company, Holland) at 20 kV. The phase structures were analyzed by a Philips X’Pert PRO diffractometer ˚ at room temperature. The accelwith Cu K␣ radiation ( = l.5418 A) erating voltage was set at 40 kV with 40 mA emission current. Room-temperature PL spectra were recorded using a fluorescence spectrometer (Jasco, Japan). The excitation source was a Xe lamp and the excitation wavelength was 325 nm. 2.4. Photocatalytic measurements The photocatalytic activities of as-prepared ZnO arrays were evaluated by the degradation of MB solution. The photocatalytic degradation was performed in a 50 mL cylindrical quartz reactor with water cooling. The ZnO arrays film with an area of 4 cm2 was placed in 15 mL of aqueous MB solution (initial concentration: 10 mg/L). A 300 W high-pressure mercury lamp (max = 365 nm, Shanghai Bilon Experiment Equipment Company, China) was used as the UV light source. The film was kept at a 10 cm distance from the light source. Prior to irradiation, the MB solution with ZnO arrays film inside was left in a dark environment for 30 min to achieve adsorption/desorption equilibrium. During the photoreaction, the concentration of MB solution was determined by measuring the absorbance periodically every 30 min using a UV–vis spectrophotometer. 3. Results and discussion 3.1. Morphologies and structures Fig. 1 shows the FESEM images of ZnO arrays with different nanostructures. The morphologies and the corresponding seeding conditions are summarized in Table 1. From Fig. 1 and Table 1, it is found that initial seed layer has great effects on the morphologies of the ZnO arrays. When the seed layer is made by dipping the substrate into 10 mM zinc acetate ethanol solution for 12 h, nest-like ZnO arrays can be obtained after hydrothermal process. Nest-like ZnO arrays (Fig. 1a, b) are assembled with nanosheets, which are about 80 nm in thickness. The nanosheets are intercrossed or piled

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Table 1 Seeding conditions and their corresponding morphologies. Sample

Seeding conditions

A B C

Dip for 12 h Spin-coating Spin-coating

Zinc acetate (mM) 10 10 5

Morphology Nest-like Tower-like Flower-like

with each other, which is due to the long range electrostatic interactions among the polar charges of the {0 0 0 1} planes [38]. The TEM image in Fig. 2a confirms that the diameter of the nanosheets is about 350 nm. If the seed layer is made by spin-coating 10 mM zinc acetate ethanol solution on the substrate, tower-like ZnO arrays will be obtained (Fig. 1c, d). The towers have a regular hexagonal cross-section, consistent with the symmetry characteristic of ZnO along the [0 0 0 1] direction, indicating that they are likely to grow along the c-axis. The diameter of individual tower decreases gradually from the bottom to the top (Fig. 2b). By spin-coating 5 mM zinc acetate ethanol solution on the substrate to form the seed layer, flower-like ZnO arrays can be obtained after hydrothermal growth (Fig. 1e). Figs. 1f and 2c show that an individual flower is composed of many straight and uniform nanorods, which radiate from the center of the crystals. In addition, low-magnification FESEM images (Fig. 3) reveal that nest-like, tower-like, and flower-like ZnO arrays crystals are widespread on the glass substrate, implying that this hydrothermal method has wide application in the fabrication of large scale arrayed crystals. The phases of the samples were analyzed using XRD. As shown in Fig. 4, all the obtained ZnO crystals are of wurtzite structure, and the diffraction peaks match with the standard data of hexagonal structural ZnO with lattice constants of a = 0.3249 nm and c = 0.5206 nm (JCPDS card no. 36-1451). The sharp diffraction peaks indicate the good crystalline of the prepared crystals. The (0 0 2) diffraction peak of pattern b in Fig. 4 is greatly enhanced relative to the usual [1 0 1] maximum reflection, indicating that (0 0 1) is the relatively preferred growth direction of the tower-like structure. It is noticeable that the ratios of relative XRD intensities of (1 0 0)/(1 0 1) are different between the pattern a and pattern c in Fig. 4, corresponding to the nest-like and flower-like structure. Such a difference is attributed to the different degrees of preferred growth orientation along the c-axis of the hexagonal phase. 3.2. Possible growth mechanisms Fig. 5 depicts the possible growth models of ZnO arrays with different nanostructures. The initial ZnO seed layer plays a critical role on the subsequent growth of the ZnO arrays in hydrothermal process. After 10 mM zinc acetate ethanol solution was spin-coated on the glass substrate, layered hydroxide zinc acetate [LHZA, Zn5 (OH)8 (CH3 COO)2 ·2H2 O] film was then transformed into ZnO seed layer by heating at 300 ◦ C for 2.5 h. The seed layer consisted of closely packed hexagonal ZnO crystallite, which acted as hexagonal ZnO tower bases. In the hydrothermal growth process, the possible chemical reactions in the aqueous solution can be described as follows [39]: (CH2 )6 N4 + 6H2 O → 6HCHO + 4NH3 +

NH3 + H2 O → NH4 + OH

−

(1) (2)

−

(3)

Zn(OH)2 → ZnO + H2 O

(4)

2+

Zn

+ 2OH → Zn(OH)2

The most stable crystal of ZnO is wurtzite structure consisting of polar {0 0 0 1} planes and nonpolar {1 0 0 0} planes with

Fig. 2. TEM images of ZnO arrays with different morphologies: (a) nest-like; (b) tower-like; (c) flower-like.

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Fig. 4. The XRD patterns of ZnO arrays with different morphologies: (a) nest-like; (b) tower-like; (c) flower-like.

When the glass substrate was dipped into10 mM zinc acetate ethanol solution for 12 h, much more LHZA nuclei were formed on the glass and these LHZA nuclei were liable to aggregate together, which gave birth to the formation of numerous oriented LHZA nanoclusters. In order to minimize the total surface energy, the LHZA nanoclusters on the glass would be aggregated into the hemisphere-like morphology. When LHZA was decomposed into ZnO seed layer, the aggregated nanoclusters would rearrange themselves and orientedly attach along the c-axis to each other to decrease the energy of the system due to the intrinsic anisotropic character of hexagonal ZnO [43]. In hydrothermal process, ZnO nucleation is favored on ZnO seed layer and micro-hemispheres composed of nanosheets are formed via an Oswald’s ripening process, in which an oriented attachment progress plays a critical role [44–46]. In the subsequent stage, the continuing growth of the micro-hemispheres will lead to their cramming together to form nest-like ZnO film. 3.3. Optical properties

Fig. 3. Large-scale FESEM images of ZnO arrays with different morphologies: (a) nest-like; (b) tower-like; (c) flower-like.

C6v symmetry. Due to the anisotropic crystal property, the c-axis is the most preferred growth orientation, and the velocities of growth in different directions under hydrothermal condition are V[0 0 0 1] > V[0 1 1 0] > V[1 0 0 0] [40]. The initial seed layer can serve as heterogeneous nucleation sites and initiate a layer-by-layer growth along the c-axis (Fig. 6), which results in the formation of tower-like arrays. By decreasing zinc acetate ethanol solution from 10 mM to 5 mM, fewer LHZA nuclei were formed, which led to fewer ZnO seeds in the seed layer. In hydrothermal process, these ZnO seeds have more free space to grow up [41]. Some active sites can generate around circumference of ZnO seeds, so that ZnO will preferentially grow on the active sites to form radiating nuclei [42]. Then, such radiating structure leads to the formation of petals with c-axis. The sequential feeding of reactants enhanced the further growth of petals in the flower-like ZnO nanostructures.

Fig. 7 shows the room temperature PL spectra of ZnO arrays with various morphologies. It is found that the spectra of all the samples similarly show three emission bands. The UV emission at around 390–400 nm is attributed to the near band edge (NBE) emission of ZnO, which results from the excitonic transitions between the electrons in the conduction bands and the holes in the valence bands. The blue-green emission at about 470 nm and the yellow emission at about 590 nm are probably ascribed to surface deep traps or intrinsic defects such as Zn interstitials and oxygen vacancies. Because the reaction takes place in the sealed Teflon-lined autoclave and the samples were formed in a poor oxygen environment. Consequently some O2− vacancies might appear in the samples. In contrast to tower-like ZnO and nest-like ZnO, the UV emission peak position of flower-like ZnO has a blue-shift of about 5 nm, which is attributed to the difference in morphology, since towerlike ZnO and nest-like ZnO are both composed of nanosheets while flower-like ZnO consists of nanorods. The ratio of intensity of deep level emission (DLE) to that of NBE reflects the extent of defect and the ratios for nest-like, flower-like, and tower-like structures are 0.0787, 0.1501 and 0.2891, respectively. The lower ratio of intensity of DLE to that of NBE for nest-like ZnO might be caused by higher crystallization quality and lower defect density, which indicates that the optical properties of ZnO are very sensitive to the morphology and defect densities.

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Fig. 5. Schematic growth models of ZnO arrays with different morphologies.

Fig. 6. Schematic illustration of layer-by-layer growth.

3.4. Photocatalytic performance As a semiconductor photocatalyst, ZnO has been used for the photocatalytic degradation of organic pollutants in aqueous solution. Herein, methylene blue (MB) dye was selected as model compound to evaluate the photocatalytic activities of ZnO arrays. Fig. 8 represents the variation of MB concentration as a function of UV irradiation time over different nanostructured ZnO arrays, where C0 and C indicate the initial concentration and the real-time concentration of MB, respectively. In the absence of photocatalyst under UV light, the photodegradation of MB can be negligible. The

addition of catalysts results in obvious degradation of MB, and the photocatalytic activity increases in the order of tower-like, flowerlike, and nest-like ZnO arrays. MB degradation percent by nest-like ZnO arrays after 4 h is of about 86%, whereas values of about 74% and 68% can be found after 4 h for flower-like and tower-like ZnO arrays, respectively. Fig. 9 depicts the time-dependent absorbance spectra of MB solution in the presence of nest-like ZnO arrays. MB gives rise to a large absorption peak at 664 nm. As the illumination time is increased, this peak is reduced in intensity. For a better understanding of the photocatalytic efficiency of ZnO arrays, the kinetic analysis of MB degradation is discussed in

Fig. 7. PL spectra of ZnO arrays with different morphologies: (a) tower-like ZnO; (b) flower-like ZnO; (c) nest-like ZnO.

Fig. 8. Photodegradation of MB versus UV irradiation time in the presence of different catalysts.

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Table 2 Degradation parameter of MB by different samples.

Fig. 9. The time-dependent absorption spectra of MB solution in the presence of nest-like ZnO arrays.

the following. It is generally assumed that reaction kinetics can be described in terms of Langmuir–Hinshelwood model [47]: −

dC kKC = 1 + KC dt

(5)

where (−dC/dt) is the degradation rate of MB, k is the photocatalysis rate constant, K is the equilibrium adsorption coefficient and C is the concentration of MB. At low MB concentrations, the term KC (Eq. (5)) in the denominator can be neglected with respect to unity, and Eq. (5) can be described as first-order kinetics model: ln

C0 = kKt = kapp t C

(6)

where C0 is the initial MB concentration and kapp is the apparent rate constant. The apparent rate constant can be chosen as the basic kinetic parameter for different photocatalysts, enables one to determine a photocatalytic activity independent of the previous adsorption period in the dark and the concentration of MB remaining in the solution [48]. Fig. 10 plots the kinetic curves for MB degradation, and a good linear relationship was observed between ln(C0 /C) and the reaction time (R2 > 0.99). The apparent rate constants (kapp ) of MB, obtained from the slopes, are listed in Table 2. From the data we can see that kapp of nest-like ZnO arrays is nearly 1.7 times as that of tower-like ZnO arrays. It can be concluded that different ZnO nanostructures resulted in different photocatalytic properties. Similar results were also reported about

Fig. 10. The corresponding kinetic analysis associated with a first-order reaction for MB.

Samples

MB degradation percent (%)

kapp (h−1 )

R2

Nest-like Tower-like Flower-like

86 68 74

0.49277 0.29051 0.33756

0.99796 0.99895 0.99732

TiO2 crystals. Huang et al. [49] reported that the TiO2 thin film with colloidal crystal structure exhibited enhanced photocatalytic activity for photodegradation of methyl orange under simulated solar light, in comparison to plain TiO2 particle thin film. Xiao and Zhang [50] investigated photocatalytic properties of nano TiO2 with different crystalline structure, and they concluded that TiO2 nanocrystals with 67% anatase and 33% rutile showed the highest photocatalytic activity for photodegradation of methyl orange. Sun et al. [51] prepared TiO2 nanotubes on the base of irregular TiO2 particles, and they found that such a tube-like structure had an excellent photocatalytic activity for photodegradation of 2,4-dichlorophenol. When ZnO arrays are irradiated by the UV light of photon energy higher than or equal to the band gap, electrons in the valence band can be excited to the conduction band with simultaneous generation of the same amount of holes in valence band. The electrons can be scavenged by O2 dissolved in the MB solution to form superoxide anion radicals. The superoxide anion radicals can react with H2 O to form H2 O2 , which could further yield reactive hydroxyl radicals (• OH). The holes can also react with H2 O to produce reactive hydroxyl radicals (• OH). The formed radicals can react with organic compounds and decompose them to CO2 , H2 O and other minerals [52]. The reactions are summarized as below [53]: ZnO + h → e− + h+

(7)

e− + O2 → • O2 − •O − 2 +

+ H2 O → H2 O2 →

h + H2 O → H • OH

(8)

+

2• OH

+ • OH

+ dye → intermediates → CO2 + H2 O + minerals

(9) (10) (11)

Due to the fact that a photocatalytic reaction occurs at the interface between catalyst surfaces and organic pollutants, the photocatalytic activity of the catalyst is usually dependent on the surface area of the crystal faces. Tachikawa et al. [54] reported a single-molecule, single-particle fluorescence approach to elucidate the inherent photocatalytic activity of exposed surfaces of anatase TiO2 , and they demonstrated that {1 0 1} facets of the crystal showed a higher photocatalytic reduction activity than the {0 0 1} facets. Jang et al. [55] explored the relationship between surface orientation of ZnO crystals and their photocatalytic efficiency, and they found that an increase of polar Zn (0 0 0 1) faces resulted in a significant enhancement of photocatalytic activity, whereas the area of nonpolar {0 1 1¯ 0} planes had negligible influence on the formation ion of H2 O2 . According to Eq. (9), the more H2 O2 formed, the more • OH radicals generated, which could play a crucial role in MB degradation. It is therefore concluded that the polar Zn (0 0 0 1) plane was the most active site for MB degradation. In our work, the surface areas of Zn (0 0 0 1) faces corresponding to nest-like ZnO, flower-like, and tower-like nanostructures were calculated to be 2.23, 0.09 and 0.08 m2 /g, respectively. It is well known that photocatalytic activity increases with increasing surface activation sites where MB containing solution can be in close contact with the ZnO arrays. The nest-like structure with larger surface area of polar Zn (0 0 0 1) faces can provide more surface activation sites for photocatalytic reaction. Meanwhile, nest-like structure assembled with intercrossed thin sheets can be more easily penetrated by the UV light source and thus absorb more photons, which will induce

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more electron–hole pairs to be generated and enhance the photocatalytic activity [56]. Therefore, nest-like ZnO arrays achieved higher photocatalytic activity than flower-like and tower-like ZnO arrays. 4. Conclusions In conclusion, ZnO arrays with a series of morphologies, including nest-like, tower-like, and flower-like samples, have been successfully fabricated through a simple low temperature hydrothermal method. The initial seed layer had an important effect on the morphologies of ZnO arrays. The morphology of ZnO crystals had significant influence on their optical and photocatalytic performances. Compared to tower-like and flower-like samples, nest-like ZnO arrays exhibited better photocatalytic performance for the degradation of MB solution. Acknowledgements This work was supported by the National Basic Research Program of China (Grant No. 2009CB939705) and the Fundamental Research Funds for the Central Universities (Grant No. HUST: 2012QN042). We appreciate Analytical and Testing Center, Huazhong University of Science and Technology, P.R. China, for obtaining XRD, TEM, and FESEM images. References [1] X. Hu, J.C. Yu, Continuous aspect-ratio tuning and fine shape control of monodisperse ␣-Fe2 O3 Nanocrystals by a programmed microwave-hydrothermal method, Advanced Functional Materials 18 (2008) 880–887. [2] N. Shpaisman, U. Givan, F. Patolsky, Electrochemical synthesis of morphologycontrolled segmented CdSe nanowires, ACS Nano 4 (2010) 1901–1906. [3] M. Gusatti, C.E.M. Campos, J.A. Rosario, D.A.R. Souza, N.C. Kuhnen, H.G. Riella, The rapid preparation of ZnO nanorods via low-temperatures solochemical method, Journal of Nanoscience and Nanotechnology 11 (2011) 5187–5192. [4] W.J. Liu, X.Q. Meng, Y. Zheng, W. Xia, Synthesis and photoluminescence properties of ZnO nanorods and nanotubes, Applied Surface Science 257 (2010) 677–679. [5] J. Yu, B. Huang, X. Qin, X. Zhang, Z. Wang, H. Liu, Hydrothermal synthesis and characterization of ZnO films with different nanostructures, Applied Surface Science 257 (2011) 5563–5565. [6] N. Sutradhar, A. Sinhamahapatra, S.K. Pahari, P. Pal, H.C. Bajaj, I. Mukhopadhyay, A.B. Panda, Controlled synthesis of different morphologies of MgO and their use as solid base catalysts, The Journal of Physical Chemistry C 115 (2011) 12308–12316. [7] D.P. Singh, Synthesis and growth of ZnO nanowires, Science of Advanced Materials 2 (2010) 245–272. [8] T.M. Milao, V.R. de Mendonc¸a, V.D. Araújo, W. Avansi, C. Ribeiro, E. Longo, M.I. Bernardi, Microwave hydrothermal synthesis and photocatalytic performance of ZnO and M:ZnO nanostructures (M = V, Fe Co), Science of Advanced Materials 4 (2012) 54–60. [9] Z. He, W. Que, Enhanced photocatalytic activity of N-Cetyl-N,N,N-Trimethyl ammonium bromide-assisted solvothermal grown fluff-like ZnO nanoparticles, Journal of Nanoengineering and Nanomanufacturing 2 (2012) 17–21. [10] C.-H. Chen, S.-J. Chang, S.-P. Chang, M.-J. Li, I.C. Chen, T.-J. Hsueh, A.-D. Hsu, C.-L. Hsu, Fabrication of a white-light-emitting diode by doping gallium into ZnO nanowire on a p-GaN substrate, The Journal of Physical Chemistry C 114 (2010) 12422–12426. [11] Z.L. Wang, J. Song, Piezoelectric nanogenerators based on zinc oxide nanowire arrays, Science 312 (2006) 242–246. [12] M.S. Akhtar, S. Ameen, S.A. Ansari, O.B. Yang, Synthesis, Characterization of ZnO nanorods and balls nanomaterials for dye sensitized solar cells, Journal of Nanoengineering and Nanomanufacturing 1 (2011) 71–76. [13] Q.F. Zhang, C.S. Dandeneau, X.Y. Zhou, G.Z. Cao, ZnO nanostructures for dyesensitized solar cells, Advanced Materials 21 (2009) 4087–4108. [14] S. Zhang, C.I. Pelligra, G. Keskar, J. Jiang, P.W. Majewski, A.D. Taylor, S. IsmailBeigi, L.D. Pfefferle, C.O. Osuji, Directed self-assembly of hybrid oxide/polymer core/shell nanowires with transport optimized morphology for photovoltaics, Advanced Materials 24 (2012) 82–87. [15] W. Wu, S. Bai, N. Cui, F. Ma, Z. Wei, Y. Qin, E. Xie, Increasing UV photon response of ZnO sensor with nanowires array, Science of Advanced Materials 2 (2010) 402–406. [16] J. Liu, W. Wu, S. Bai, Y. Qin, Synthesis of high crystallinity ZnO nanowire array on polymer substrate and flexible fiber-based sensor, ACS Applied Materials and Interfaces 3 (2011) 4197–4200.

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