Porous zinc oxide films: Controlled synthesis, cytotoxicity and photocatalytic activity

Chemical Engineering Journal 178 (2011) 8–14

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Porous zinc oxide films: Controlled synthesis, cytotoxicity and photocatalytic activity Hua-Jie Wang ∗ , Yuan-Yuan Sun, Ying Cao ∗ , Xue-Hong Yu, Xiao-Min Ji, Lin Yang College of Chemistry and Environmental Science, Henan Normal University, 46# East of Construction Road, Xinxiang 453007, PR China

a r t i c l e

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Article history: Received 3 August 2011 Received in revised form 19 September 2011 Accepted 20 September 2011 Keywords: Porous zinc oxide Thin films Anodic oxidation technique Cytotoxicity Photocatalyst

a b s t r a c t The anodic oxidation technique was successfully carried out to fabricate porous ZnO films. Energy dispersive X-ray and scanning electron microscope indicated that the anodic oxidation time was the key factor to control surface pore density, which varied from 0.2 ± 0.01% to 59.8 ± 5.4%. The cytotoxicity study indicated that porous ZnO films displayed the pore density-dependent cytotoxicity on the metabolism and spread of NIH 3T3 fibroblasts. The photocatalytic activity of porous ZnO films was investigated by employing methyl orange as the model compound and it was pore density-dependent. In view of the relatively facile separation property and subsequently the decrease of toxic side effects to the environment of film-form ZnO products compared to powder products, this study suggested the superior potential of the former as the photocatalyst. © 2011 Elsevier B.V. All rights reserved.

1. Introduction With the rapid development of textile industry and extensive applications of pesticides and herbicides, one of major thrusts in the environmental control is to develop a low-cost and effective method that can decompose the dye and agrichemical contaminations [1–4]. Recently, the applications of nano-oxide semiconductor compounds as photocatalysts in air and water purification have attracted increasing attention [5–7]. Although considerable advances have already been made, lots of literatures mainly focus on nano-titanium dioxide (TiO2 ) [8–12]. For example, Arbuj et al. [9] prepared TiO2 powders in nano-scale by the high-temperature calcinations ranging from 400 ◦ C to 900 ◦ C. They found that the TiO2 sample obtained by the 750 ◦ C calcinations had the highest photocatalytic activity. Gu et al. [10] prepared TiO2 nano-tubes decorated with nano-scale TiO2 hollow spheres. Due to the higher surface area, the hybrid material in photocatalytic efficiency was superior to the simple TiO2 nano-tubes. However, the costs and difficulties in separating the TiO2 nano-powders from the environment after degradation limit their applications and drive the development of the film-form structure [13]. Shang et al. [14] prepared porous and flexible membranes composed of TiO2 nanowires decorated with heteronanostructure of Ag particle. They found that Ag/titanate nanowire membranes not only had a higher

∗ Corresponding authors. Tel.: +86 373 3325058; fax: +86 373 3328507. E-mail address: [email protected] (H.-J. Wang). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.09.088

photocatalytic activity than titanate membranes but also exhibited a high stability as a recyclable photocatalyst. ZnO has a wide direct band-gap of 3.37 eV at room temperature but a relative lower cost compared with the frequently used TiO2 that has a wide direct band-gap of 3.0 eV or 3.2 eV in the rutile or anatase crystalline phase [4,15,16]. The film-form ZnO photocatalyst has the potential to reach a higher efficiency at a lower cost [17–19]. However, by comparison with ZnO nano-powders, the film-form structure has smaller surface area and subsequently a lower photocatalytic activity. Therefore, the film surface treatment is currently a focus of attention, linked to the improvement of photocatalytic properties of ZnO films. Notably, many groups have documented that the control over nano or micro-scale structures on ZnO film surfaces is an effective method to enhance the photocatalytic activity and different techniques have been developed to fabricate porous ZnO films, such as sol–gel method, hydrothermal synthesis, and template technique [20–22]. For example, Wang et al. fabricated porous nano-sheet-based ZnO films on Zn foil via hydrothermal synthesis [20]. They found that the high surface area of the nano-structured ZnO films and large percentage of the exposed [0 0 1] facet were the main reasons for their enhanced photocatalytic acitivity in the degradation of Rhodamine B under UV light irradiation. Pal and Sharon [21] fabricated porous ZnO thin films on glass substrate by a sol–gel process. They considered that the porous structure with a high surface had a higher adsorption affinity towards the reactant molecules filled in the pores and could play a role in enhancing the photocatalytic activity. Herein, we introduced the fabrication of porous ZnO films with different pore densities by using the anodic oxidation technique.

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This study particularly focused on understanding the influence of pore densities on the photocatalytic activity of porous ZnO films. In addition, we also studied the cytotoxicity of porous ZnO films by metabolic and morphological methods in attempts to investigate their possible toxic side effects to the environment and further displayed the superiority of the film-form ZnO products as photocatalysts. 2. Materials and methods 2.1. Controllable fabrication of porous ZnO films ZnO films with different pore densities were fabricated by the anodic oxidation of zinc sheets for different oxidation time. The detailed experimental conditions were shown in Table 1. In brief, zinc sheets (thickness: 270 ␮m, purity: 99.99%) were cleaned in an ultrasonic bath of alcohol for 30 min, and followed by an immersion treatment with 10% of NaOH solution for 10 min. After being further rinsed with distilled water, zinc sheets were electropolished in a phosphoric acid solution containing 20 mg/mL of agar to obtain the flat, smooth and shiny surfaces. The electropolished zinc sheet was set as the anode and kryptol was chosen as the cathode. A constant voltage of 15 V was used throughout the anodic oxidation process and experimental temperature was kept at 30 ◦ C. The oxidation time was 2, 5, 10, 15, 90 and 120 min, respectively. In order to present the data clearly, porous ZnO films were abbreviated as ZnO2min , ZnO5min , ZnO10min , ZnO15min , ZnO90min and ZnO120min , respectively, according to the corresponding anodic oxidation time. 2.2. Characterization The morphologies of porous ZnO films were observed by scanning electron microscope (SEM, JSM6390LV, JEOL, Japan). The pore size and pore density were analyzed by using the ImageJ software (http://rsb.info.nih.gov/ij). The composition of film surface was analyzed by an Oxford energy dispersive X-ray (EDX) detector. The room temperature photoluminescence (PL) spectra of porous ZnO films were measured with 325 nm excitation by a JASCO FP-6500 fluorescence spectrometer (JASCO, Tokyo, Japan). 2.3. Toxic side effects assessment of ZnO films 2.3.1. Cells seeding NIH 3T3 fibroblasts were used as a model to study toxic side effects of porous ZnO films. NIH 3T3 cells were routinely cultured in tissue culture flasks with RPMI1640 medium, containing 10% total bovine serum and incubated at 37 ◦ C in a humidified atmosphere with 95% air and 5% CO2 . The culture medium was refreshed every two days. When cells became almost confluent after 5 days, they were released by treatment with 0.25% trypsin. Before cell seeding, porous ZnO films were placed into 96-well tissue culture plate and sterilized by UV for 20 min. Then the ZnO films were equilibrated with the prewarmed medium (37 ◦ C) for 2 h. After removing the

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medium from the wells, cells were counted to 104 cells/cm2 and 200 ␮L of the cells suspension were poured onto each ZnO film. 2.3.2. Cell morphology and distribution After 48 h of cell culture, the attached cells on porous ZnO films were stained with acridine orange fluorescent dye in PBS (pH 7.2) for 5 min. The morphologies and distribution of attached cells were analyzed by using a fluorescence microscope (Axioskop 40, ZEISS, Germany). 2.3.3. Cell viability The metabolic method was performed to analyze cell viability by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay (MTT) based on succinic dehydrogenase activity at optical density (OD) 490 nm, and 630 nm was chosen as the reference wavelength. Briefly, the cells on porous ZnO films were washed with PBS carefully, 20 ␮L of MTT was added in 180 ␮L of culture medium, and the cell culture was continued for 4 h. Then, the solution was removed and the cells were washed twice with PBS. Dimethyl sulfoxide (200 ␮L) was added and OD values were read on a microplate reader (Multiskan MK3, Thermo Labsystems, USA). 2.4. Bovine serum albumin adsorption on porous ZnO films Bovine serum albumin (BSA, purity ≥98%, Mw = 68,000) was used to determine the adsorption abilities of porous ZnO films. 200 ␮L of PBS solution (0.1 mol/L and pH 7.2) containing 500 ␮g/mL of protein was pipetted onto porous ZnO film in a 96-well plate. The adsorption study was conducted in a 37 ◦ C incubator for 30 min. Porous ZnO films were then rinsed carefully with PBS buffer to remove the nonadherent protein. The quantitative measurement of the adsorbed protein on porous ZnO films was made by the BCA method. Briefly, an aqueous solution (200 ␮L of 1%) of sodium dodecyl sulfate was added to desorb the proteins on the membrane surfaces. The plate was then left shaking for 30 min at room temperature. The desorption fraction was quantified with the micro BCA protein assay reagent kit. 2.5. Photocatalytic activity of ZnO films with different pore densities Methyl orange (MO) was chosen as the model to evaluate the photocatalytic activity of porous ZnO films. Briefly, 1.0 cm2 of ZnO film was immersed into a small beaker filled with 20 mL of MO aqueous solution (5 ppm). After illumination with 300 W highly pressure mercury vapor lamp, the kinetic photodecomposition process was monitored by measuring MO concentrations at the regular intervals with a TU-1900 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China) at 464 nm. A blank test without photocatalyst upon with a 300 W highly pressure mercury vapor lamp illumination was chosen as the control. Besides, in order to assure the adsorption content of MO on porous ZnO films, another control experiment without illumination (in the dark) in the presence of porous ZnO films was carried out

Table 1 Experimental conditions for ZnO films preparation by anodic oxidation technique and the corresponding surface pore density and pore size (n = 4, mean ± SD). Voltage (V) Electrolyte Temperature (◦ C) Oxidation time (min) Pore size (␮m) Pore density (%)

15

2 0.83 ± 0.12 0.2 ± 0.01

5 1.08 ± 0.24 0.6 ± 0.07

3% of Phosphoric acid in ethanol (v/v) 30 10 15 2.11 ± 0.34 3.28 ± 0.51 1.3 ± 0.1 7.6 ± 0.4

90 6.52 ± 1.2 29.5 ± 3.2

120 8.30 ± 1.47 59.8 ± 5.4

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Fig. 1. SEM images of zinc oxide films with different pore densities prepared by the anodic oxidation technique using different oxidation time. A: 2 min; B: 5 min; C: 10 min; D: 15 min; E: 90 min; F: 120 min.

and the adsorption kinetics of MO on porous ZnO films in 180 min was determined. 2.6. Statistical analysis The number of independent replica was listed individually for each experiment. Where applicable, all data were mean ± SD. The analysis of data for cell proliferation was performed by one-way factorial analysis of variances (ANOVA) and multiple comparisons (Fisher’s method as Post-Hoc test, p < 0.05). 3. Results and discussion 3.1. Synthesis of ZnO films with the controlled pore densities The anodic oxidation technique can create porous metal oxide films and the pore size and number are dependent on the applied voltage and treatment time. In the present, the applied voltage and current density values were fixed and only the oxidation time was changed. The variation in pore size and pore density of ZnO films were compared and shown in Fig. 1 and Table 1. It can be seen that the porous microstructure is formed on ZnO films. Pore densities and pore sizes vary with the prolongation of anodic oxidation time. After 2 min of anodic oxidation treatment, few pores can be observed on the ZnO2min film surface, and the pore density is only 0.2 ± 0.01% (Fig. 1A1 and Table 1). 5 min later, the pores with 1.08 ± 0.24 ␮m in diameter can be observed but the pore density is still not more than 1.0% (Fig. 1A2 and Table 1). With the further prolongation of anodic oxidation time ranging from 10 min to 120 min, pore sizes and pore densities gradually increase and vary from 2.11 ± 0.34 ␮m to 8.3 ± 1.47 ␮m and from 1.3 ± 0.1% to 59.8 ± 5.4%, respectively (Fig. 1A3 –A6 and Table 1). The composition of the anodic oxidation product was analyzed by EDX. Zinc and oxygen signals appear on the zinc sheet surfaces after 2 min of anodic oxidation treatment but the atomic ratio of zinc and oxygen is about 13.3:1, indicating the formation of a thin ZnO film on zinc sheet surface (Fig. 2). With the prolongation of anodic oxidation time in 120 min, zinc content on zinc sheet surface decreases from 72.4% to 62.8% and oxygen content increases from 5.44% to 10.32% (Fig. 2). During the process of anodic oxidation, Zn metal content is gradually decreased with dissolving Zn

ion by anodic oxidation and O content is relatively increased with dissolving Zn metal and oxidizing Zn metal sheet to ZnO [23]. The optical properties of porous ZnO films were characterized by the PL spectrum measurement at room temperature under a photo excitation of 325 nm. As shown in Fig. 3, all porous ZnO films have similar PL spectra. Both emission peaks centered at 398 nm and 470 nm can be observed. The UV emission results from a near band edge emission (NBE) of wide band gap of ZnO and may originate from the recombination of the free excitons through an exciton–exciton collision process. The green band emission can be due to the radiative recombination of a photo-generated hole with an electron that belongs to a singly ionized oxygen vacancy in the surface and sub-surface lattices of ZnO films [24,25]. However, a shift of the UV emission peak could be found by comparison with the reported work [24,26]. For example, Xu et al. reported that the UV emission peak of ZnO plates, ZnO rods, flower-like ZnO bundles, and ZnO nanowire arrays appeared at 375, 385, 399 and

Fig. 2. The content changes of zinc and oxygen on zinc sheet surface with the prolongation of anodic oxidation time analyzed by EDX.

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1000

150min

ZnO 90min ZnO 15min ZnO 10min ZnO 5min ZnO 2min ZnO

800

Intensity

398 nm 600

470 nm

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concentration of native defects. Moreover, there was only a thinner ZnO layer on the surface of Zn sheet and it might be another possible reason for a broad and weak UV emission peak. Additionally, we also could observe that the peak intensities of porous ZnO films gradually increased with increase of the surface pore density. Although the detailed mechanism is still unclear, it suggests that we could easily monitor the anodic oxidation process by PL analysis.

400

3.2. Cytotoxicity of ZnO films

200

0 400

450

500

550

Wavelength (nm) Fig. 3. The room temperature fluorescence spectra of porous ZnO films prepared by the anodic oxidation technique using different oxidation time periods. The excitation wavelength was 325 nm.

392 nm, respectively. They considered that the shift of UV emission peaks was due to different native defect and free carrier concentrations in various ZnO products [24]. In addition, the UV emission peak of porous ZnO films is not strong or sharp. According to the work of Xu et al. [24] and Wang et al. [26], we deduced that the weak and broad peak of porous ZnO films might result from a high

The safety of ZnO products is currently a focus of attention, linked to the widely applications of ZnO products in cosmetics, sunscreens and other dermatological pharmaceutics [27,28]. Unfortunately, some recent literatures have reported the toxicity of ZnO nano-particles [29–31]. For example, Sharma et al. [32] found that ZnO nano-particles even at low concentrations could induce a genotoxic potential in human epidermal cells. Here, the cytotoxicity of porous ZnO films was evaluated by cell morphological and metabolic methods. Fig. 4 shows a series of images of NIH 3T3 cells adhered onto porous ZnO films for 48 h after a fluorescent staining. It is clear that NIH 3T3 cells on the glass matrix develop a characteristic polygonal morphology and spread completely. An almost confluent monolayer cells are formed (Fig. 4A). However, porous ZnO films exhibit a significant inhibition effect on the spread and proliferation of NIH 3T3 cells as shown in Fig. 4B–G. All cells contract and form a round shape, suggesting a lower metabolic activity.

Fig. 4. Morphologies and distributions of NIH 3T3 cells cultured on ZnO films with different pore densities for 48 h after being stained with acridine orange fluorescent dye in PBS for 5 min. (A): Glass; (B): ZnO2min ; (C): ZnO5min ; (D): ZnO10min ; (E): ZnO15min ; (F): ZnO90min and (F): ZnO120min .

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Fig. 5. Metabolic activity of NIH 3T3 cells cultured on ZnO films with different pore densities determined by MTT method based on succinic dehydrogenase activity. Cells were seeded at 104 cells/cm2 and allowed to proliferate for 48 h. The data were expressed as means ± SD (n = 3).

Fig. 7. Photodegradation kinetics of methyl orange in the presence of ZnO films with different pore densities (A), decoloration of methyl orange after 180 min of irradiation, and adsorption kinetics of methyl orange (MO) on porous ZnO films (C).

Consequently, the toxic side effects of porous ZnO products have to be considered when they are applied in the environment. Especially for ZnO powders, the separation from the environment could not be accomplished completely. In view of the toxic side effects to the environment of the ZnO products and difficulties in separating the nano-powder ZnO form the environment after applications, the film-form ZnO products might be more suitable to be as photocatalysts. 3.3. Photocatalytic activity comparison of ZnO films with different pore densities The photocatalytic activity of ZnO films with different pore densities was assessed from photodegradation kinetics of MO. As shown in Fig. 7A, in the absence of porous ZnO films, MO decomposes only 17.1% in 180 min under the irradiation and the rate of reaction is 0.09. However, with the application of porous ZnO films, the rate of reaction is significantly accelerated. Generally, the higher the pore density of ZnO films is, the better the photodegradation efficiency is. Moreover, the rate of reaction of porous ZnO films ranges from 0.14 to 1.05 with the increase of pore density varying from 1.3 ± 0.1% to 59.8 ± 5.4%. With the time prolongation of irradiation exposure, the MO concentration gradually decreases. 28.4% of MO can be decomposed in 180 min under the effect of the ZnO2min film. As for ZnO5min , ZnO10min and ZnO15min films, the Normalised concentration (Ct/C0)

The cell number in each porous ZnO film group is significantly decreased compared with that in control group. Fig. 5 shows the viabilities of NIH 3T3 cells on porous ZnO films after 48 h of cell culture measured by the MTT method. It can be seen that the viability of NIH 3T3 cells on ZnO films was significantly inhibited by comparison with that on glass control. However, the inhibition effect of porous ZnO films gradually decreases with the increase of the pore density of the ZnO film surface. The activities of NIH 3T3 cells on ZnO films vary from 14.4 ± 1.9% to 33.4 ± 4.0% when pore densities increase from 0.2% to 59.8%. Collectively the morphological and metabolic data, porous ZnO films not only could inhibit the spread and proliferation of NIH 3T3 cells, but also they could inhibit cells metabolic viability. However, a relative increase of cell growth on porous ZnO films with the pore densities was also observed. It has been documented that surface physical structures could elicit specific cellular responses and direct cell growth [33–36]. As one of the typical surface characteristics, many groups, including ours, have previously demonstrated that a higher specific surface area of material surface can supply much more sites to protein adsorption in seconds, and then cells sequentially detect the protein, adhere to the surface and grow on it [37–40]. Therefore, porous ZnO films with higher pore densities might be able to adsorb more protein that stimulated the growth of NIH 3T3 cells. In order to verify this hypothesis, we determined protein adsorption on different porous ZnO films. As shown in Fig. 6, the protein adsorption content on porous ZnO films is pore density-dependent and increases with the increase of pore densities. Therefore, although ZnO films are toxic to NIH 3T3 cells, the viability of NIH 3T3 cells still increases as the pore size increases.

100 80

10th 8th

60

6th 5th

40

4th 3rd

20

2nd 1st

0 0

10

20

30

40

50

Irradiation time (min) Fig. 6. BSA adsorption on porous ZnO films with different pore densities in 30 min.

Fig. 8. Cycling degradation curves for ZnO120min .

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Fig. 9. SEM images of ZnO120min after 2 (A), 4 (B), 6 (C) and 10 (D) times of cycling degradation tests, respectively.

photodegradation of MO gets to 49.3%, 66.5% and 90.2% after 180 min of irradiation, respectively. It is notable that the solution becomes completely transparent under the effect of the ZnO90min film after 120 min of irradiation (Fig. 7B). Furthermore, the ZnO120min film has the strongest photocatalytic activity and it can promote a completely decolorization of MO in 90 min. In order to understand the possible reason that porous ZnO films with higher pore density had the higher photoactivity, a series of control experiments about the adsorption tests of MO on porous ZnO films have been supplemented. As shown in Fig. 7C, the adsorbed MO content increased with the prolongation of immersion time and with the increase of pore density on porous ZnO films. 180 min later, the adsorbed MO content was 3.3%, 3.3%, 4.6%, 7.5%, 19.9% and 21.0%, respectively. That was to say, a higher specific surface area of porous ZnO films corresponded to a higher MO adsorption content, which further enhanced the photocatalytic activity of ZnO films. As for the ZnO120min film, we also studied its photostability at the photocatalytic degradation of MO. As shown in Fig. 8, the ZnO120min film can decompose 94.8% of MO in 60 min at the first cycle. After three recycles for photodegradation of MO, the ZnO120min film appears a loss of activity but the photodegradation of MO still can get to 75% in 60 min. Five recycles later, photodegradation activity of the ZnO120min film is half of the initial state and about 50% of MO can be decomposed in 60 min. However, after ten recycles, the photodegradation efficiency is greatly reduced and only 26.2% of MO can be decomposed in 60 min. Taken together, the ZnO films with higher pore density and subsequently larger surface areas can provide more active sites and photocatalytic reaction centers for the adsorption of MO like that of macro/mesoporous titanate [41]. Furthermore, ZnO films with higher pore density are more advantageous for diffusion and exchange of reactant and product molecules on the surface of ZnO. Therefore, it could be observed that the ZnO120min film had the best photocatalytic activity (Fig. 7) [42]. However, our study also

indicated that a deactivation of the ZnO120min film during a repeatable utilization occurred (Fig. 8). According to the report of Daneshvar et al. [43], ZnO could undergo photocorrosion through self oxidation and it could be explained by Eq. (1). Consequently, it is not surprising that the photocatalytic activity gradually weakened during the repeatable utilization. ZnO+2h+ → Zn2+ +

1 O2 2

(1)

In order to verify whether porous ZnO films were photocorroded after cycling degradation test, SEM observation was performed to investigate the surface structure of the reused ZnO120min films. SEM images of ZnO120min film after 2, 4, 6 and 10 times of cycling degradation tests are shown in Fig. 9. It can be seen that the porous structure still exists after 6 times of cycling degradation test, however, the pores gradually become flat. 10 times later, the porous structure is destroyed completely, essentially due to photocorrosion. Therefore, how to enhance the stability study of porous ZnO films will be further studied in our future work. 4. Conclusions In summary, we successfully fabricated the ZnO films with controlled pore densities by the anodic oxidation technique at 30 ◦ C. It was demonstrated that the anodic oxidation time could cause the differences in the pore size and the pore density. Moreover, ZnO films exhibited a pore density-dependent photocatalytic activity to the degradation of MO. In view of the toxicity of nano-powder ZnO and difficulties in separating the nano-powders photocatalysts from the environment after degradation, this study hinted that the film-form ZnO products might have the superior potential to nano-powder products as photocatalysts. However, the detailed mechanisms about the difference of photocatalytic activity among different porous zinc oxide films and about the stability of the repeatable utilization will be studied in our future work.

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Conflict of interest The authors have no conflicts of interest. Acknowledgements This work was financially supported by the National Science Foundation of China (31000774 and 20971039) and National Key Basic Research and Development Program of China (2009CB626610), the Innovation Scientists and Technicians Troop Construction Projects of Henna Province (Grant No. 114200510004) and Program for Changjiang Scholars and Innovative Research Team in University (IRT1061). References [1] H.A.J.L. Mourao, A.R. Malagutti, C. Ribeiro, Synthesis of TiO2 -coated CoFe2 O4 photocatalysts applied to the photodegradation of atrazine and Rhodamine B in water, Appl. Catal. A-Gen. 382 (2010) 284–292. [2] D. Yu, R. Cai, Z. Liu, Studies on the photodegradation of Rhodamine dyes on nanometer-sized zinc oxide, Spectrochim. Acta A 60 (7) (2004) 1617–1624. [3] X. Li, L. Zen, P. Liu, H. Wang, A. Shui, Photocatalytic degradation of methyl orange on ZnO in aqueous, Key Eng. Mater. 336-338 (2007) 1983–1985. [4] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2 , Sol. Energy Mater. Sol. Cells 77 (2003) 65–82. [5] Y. Wang, X. Li, N. Wang, X. Quan, Y. Chen, Controllable synthesis of ZnO nanoflowers and their morphology-dependent photocatalytic activities, Sep. Purif. Technol. 62 (2008) 727–732. [6] Y.J. Jang, C. Simer, T. Ohm, Comparison of zinc oxide nanoparticles and its nano-crystalline particles on the photocatalytic degradation of methylene blue, Mater. Res. Bull. 41 (2006) 67–77. [7] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C 1 (2000) 1–21. [8] Q.Q. Cheng, Y. Cao, L. Yang, P.P. Zhang, K. Wang, H.J. Wang, Synthesis and photocatalytic activity of titania microspheres with hierarchical structures, Mater. Res. Bull. 46 (3) (2011) 372–377. [9] S.S. Arbuj, R.R. Hawaldar, U.P. Mulik, B.N. Wani, D.P. Amalnerkar, S.B. Waghmode, Preparation, characterization and photocatalytic activity of TiO2 towards methylene blue degradation, Mater. Sci. Eng. B 168 (2010) 90–94. [10] Y. Gu, X. Liu, T. Niu, J. Huang, Titania nanotube/hollow sphere hybrid material: dual-template synthesis and photocatalytic property, Mater. Res. Bull. 45 (2010) 536–541. [11] T.J. Kemp, R.A. McIntyre, Transition metal-doped titanium (IV) dioxide: characterisation and influence on photodegradation of poly(vinyl chloride), Polym. Degrad. Stab. 91 (2006) 165–194. [12] Z. Li, W. Shen, W. He, X. Zu, Effect of Fe-doped TiO2 nanoparticle derived from modified hydrothermal process on the photocatalytic degradation performance on methylene blue, J. Hazard. Mater. 155 (2008) 590–594. [13] Y. Xu, M. Shen, Fabrication of anatase-type TiO2 films by reactive pulsed laser deposition for photocatalyst application, J. Mater. Process. Technol. 202 (2008) 301–306. [14] L. Shang, B. Li, W. Dong, B. Chen, C. Li, W. Tang, G. Wang, J. Wu, Y. Ying, Heteronanostructure of Ag particle on titanate nanowire membrane with enhanced photocatalytic properties and bactericidal activities, J. Hazard. Mater. 178 (2010) 1109–1114. [15] G.T. Delgado, C.I.Z. Romero, S.A.M. Hernandez, R.C. Perez, O.Z. Angel, Optical and structural properties of the sol-gel-prepared ZnO thin films and their effect on the photocatalytic activity, Sol. Energy Mater. Sol. Cells 93 (2009) 55–59. [16] M.R. Golobostanfard, R. Ebrahimifard, H. Abdizadeh, Synthesis of TiO2 /ZnO core/shell type nanocomposite via sol-gel method, Key Eng. Mater. 471–472 (2011) 993–998. [17] F. Peng, S.H. Chen, L. Zhang, H.J. Wang, X.Z. Yong, Preparation of visible-light response nano-sized ZnO film and its photocatalytic degradation to methyl orange, Acta Phys.-Chim. Sin. 21 (9) (2005) 944–948. [18] J. Lv, W. Gong, K. Huang, J. Zhu, F. Meng, X. Song, Z. Sun, Effect of annealing temperature on photocatalytic activity of ZnO thin films prepared by sol-gel method, Superlattices Microstruct. (2011), doi:10.1016/j.spmi.2011.05.003. [19] J. Xie, H. Wang, M. Duan, Controlled growth of self-assembled ZnO thin films and characterization of their photocatalytic properties, Acta Phys.-Chim. Sin. 27 (1) (2011) 193–198.

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