Synthesis of amorphous TiO2 modified ZnO nanorod film with enhanced photocatalytic properties

Applied Surface Science 299 (2014) 97–104

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Synthesis of amorphous TiO2 modified ZnO nanorod film with enhanced photocatalytic properties Shanshan Xiao, Lei Zhao, Xuning Leng, Xingyou Lang, Jianshe Lian ∗ Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and Department of Materials Science and Engineering, Jilin University, Changchun 130022, China

a r t i c l e

i n f o

Article history: Received 30 October 2013 Received in revised form 27 January 2014 Accepted 30 January 2014 Available online 6 February 2014 Keywords: TiO2 modified Amorphous layer ZnO nanorod arrays Photocatalyst

a b s t r a c t Amorphous TiO2 modified ZnO nanorod films were synthesized via multi-step processes: ZnO nanorod films were prepared by a wet chemical method. Amorphous TiO2 was then anchored on the tops and sides of the nanorods through immersion in tetrabutyltitanate solution for hydrolysis. The as-prepared samples were characterized for the phase structure, chemical state and surface morphology as well as optical absorption using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and ultraviolet–visible (UV–vis) spectrophotometer. The results showed that the nanorod films were covered by amorphous TiO2 layers, and their visible light absorption ability was strengthened. The photocatalytic studies revealed that TiO2 modified films exhibited enhanced photocatalytic efficiency for decomposition of methyl orange under ultraviolet–visible excitation, which might be attributed to the increased UV–vis light absorption and the separation of the charge carrier and prolonged electron lifetime due to the interface between TiO2 and ZnO. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Since the treatment of industrial wastewater to remove organic pollutants becomes a very important aspect of environmental technology, semiconductor photocatalysts have received an enormous amount of research interest in the degradation of organic pollutants [1,2]. However, the rapid recombination of the photo-induced electrons and holes inhibits the widespread use of photocatalysts. Under unremitting efforts, several appropriate ways are found to increase photocatalytic efficiency, such as decorating semiconductor by doping, noble metal nanoparticles deposition or surface sensitization [3–6], and combining some semiconductors with different band gaps to form heterojunctions in photocatalytic systems [7–10]. Researchers have paid more attention to the coupling of two semiconductors to achieve more efficient charge separation, increase the lifetime of the charge carriers, and enhance the efficiency of the interfacial charge transfer to adsorbed dyes, owning to the two redox energy levels for the corresponding conduction and valence bands [11–14]. Among all the semiconductor combinations, the integration of zinc oxide (ZnO) with titanium dioxide (TiO2 ) is one of the most promising candidates [11]. TiO2 and ZnO

∗ Corresponding author at: Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun 130025, China. Tel.: +86 431 8509 5875; fax: +86 431 8509 5876. E-mail addresses: [email protected], [email protected] (J. Lian). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.192

have been studied intensively as practically applicable materials due to their suitable band gap energy and favorable band gap positions compared to other materials [15,16]. One-dimensional (1D) ZnO nanostructures especially nanorods with a wide band gap of 3.37 eV and large exciton binding energy of 60 meV have been one of the current attractive researches, because of a variety of potential applications, ranging from photocatalysts [17,18], gas sensors [19,20], solar cells [21,22], photovoltaics [23], to field-effect transistors [24]. The unique electrical and optical properties of nanostructure ZnO with tailored morphologies meet the versatile requirements. There are a wide range of techniques to synthesize 1D ZnO nanostructures including vapor deposition [25], electrodeposition [26], hydrothermal method [27], molecular beam epitaxy [28] and so on. Among these methods, hydrothermal method has been widely used due to its low cost and scalability. However, nano-sized materials tend to aggregate in aqueous solutions through van der Waals forces during the process of fabrication and application. To avoid the difficulties in separating and reusing, nanostructure films were an ideal substitute for powders [8]. In this paper, amorphous TiO2 modified ZnO nanorod films have been synthesized to enhance the photocatalytic activity of ZnO film. Although hierarchical ZnO/TiO2 nanostructures have been intensively investigated, amorphous TiO2 modified ZnO has been seldom reported. According to Chen et al.’s point of view [29], engineering the disorder of nanophase TiO2 at the surface could enhance optical absorption, with the additional benefit of carrier trapping. Amorphous state was defined as lots of disorders, so the

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amorphous TiO2 modification was expected to achieve the same effect. The phase structure, surface morphology and optical absorption of the as-obtained samples were investigated by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and ultraviolet–visible (UV–vis) spectrophotometer. Their photocatalytic activity was evaluated by photodegradation of methyl orange (MO) under UV–vis irradiation.

2. Experiments All chemicals of analytical grade purity were used as starting materials without further purification. Commercial glass slides with the thickness of 1 mm were cut into squares with the side length of 25 mm. After being ultrasonically cleaned in acetone, alcohol, and de-ionized water for 20 min successively and dried with a flowing nitrogen gas, they were used as substrates. TiO2 modified ZnO nanorod films were synthesized by a combination of wet chemical and hydrolysis method. First, ZnO nanorod films were prepared via the wet chemical method in our previous reports [30]. In order to obtain a better orientation, the method was improved by spincoating a ZnO seed layer before nanorod growth. Then, the tops and sides of nanorods were corroded by suspending the samples into a 90 mL solution of 0.4 wt% ammonia and 0.3 wt% cetyltrimethyl ammonium bromide (CTAB) at 50 ◦ C for 3.5 h. The samples were rinsed thoroughly with de-ionized water to remove any residual salt or amino complex and dried at 100 ◦ C for 12 h. Second, the samples were put into a hydrolysis solution at 50 ◦ C for 1 h, rinsed and dried at 100 ◦ C for 30 min, and subsequently calcinated at 450 ◦ C for 1 h. The hydrolysis solution was obtained by mixing the solution containing 17 mL tetrabutyltitanate (TBT), 12 mL ethanol and 5 mL diethanolamine and stirring them for 1 h, and then continually stirring for 15 min with adding another mixed solution of 1.7 mL de-ionized water, 34 mL ethanol and 0.13 mL hydrochloric acid (37 wt%). The solution was diluted with absolute ethanol to 0.1 M TBT contained to obtain the hydrolysis solution. Pure TiO2 film was prepared by spinning coated this solution and calcinating at 450 ◦ C for 1 h. 2 mg (the same weight as our sample) commercial P25 powder was suspended in ethanol by ultrasonic and was dripped at a substrate, and then the solvent was evaporated at 100 ◦ C for 1 h. The P25 film was gained as an industrial standard. The Crystalline structures of the TiO2 modified ZnO nanorod film were examined by X-ray diffraction (XRD, Rigaku D/max 2500PC) with a Cu K␣ line of 1.5406 A˚ and a monochronometer at 40 kV and 250 mA. The multiphonon resonant Raman spectrum was collected using a micro-Raman spectrometer (Renishaw) with a laser of 532 nm wavelength. The core levels and valance band spectra were measured by X-ray photoelectron spectra (XPS) with an ESCALAB Mk II (Vacuum Generators) spectrometer using unmonochromatized Al K␣ X-rays (240 W). The binding energies were corrected by assigning C1s peak value to 284.8 eV. The morphologies of as-prepared samples were observed on field emission scanning electron microscopy (FESEM, JSM-6700F) and transmission electron microscopy (TEM, JEM-2100F). The absorption spectra of the samples were measured by ultraviolet–visible (UV–vis) spectrophotometer (6100PC). The photocatalytic activities of the samples were characterized by the degradation of methyl orange (MO) and methylene blue (MB). The as-obtained sample (a square glass with the length of 25 mm) was upside immersed into 20 mL MO (or MB) aqueous solution with a concentration of 20 mg/L. After being thoroughly stirred in the dark for 30 min by bubbling air into the vessel in order to reach the adsorption equilibrium on the catalyst, the solution was illuminated with a 250 W high pressure mercury lamp emitting 350–450 nm UV–vis light, or a 300 W xenon lamp with an AM 1.5

Fig. 1. The XRD pattern of the as-obtained sample.

filter emitting simulated solar light. The lamp was positioned at a distance of 15 cm above the solution. The solution was continuously bubbled during the experiments. Then at a definite time interval of 20 min, 4 mL MO solution was withdrawn and its concentration was monitored using UV–vis spectrophotometer at its maximum absorption wavelength of 464 nm (or 665 nm). After testing, the withdrawn solution was put back into the vessel to continue the experiment. According to Lambert–Beer law, concentration was linear proportional to absorption. As a result, the photodegradation rate was depicted as the absorption ratios versus time intervals. The repeated tests of photocatalysts were performed to examine the reusability. 3. Results and discussion Fig. 1 shows the XRD pattern of our sample. The pattern exhibits a highly crystalline hexagonal phase of ZnO wurtzite structure, indexing JCPDS 89-0510. The highest intensity of (0 0 2) diffraction peak illustrates that the ZnO nanorods grew preferentially orienting along c-axis (perpendicular to the substrate). There are no other peaks attributed to TiO2 , since either the quantity of TiO2 on the surface of ZnO nanorods was less than to be detected or TiO2 was not in crystalline state. Raman spectroscopy was used to judge the existence of TiO2 in the modified ZnO nanorod films. The Raman spectra of ZnO nanorod film and TiO2 modified film are shown in Fig. 2. For both spectra, the intensive Raman peaks located at frequencies of 438 and 577 cm−1 are observed, which are attributed to the vibration modes E2 (H), E1 (LO) [13]. Another peak at 333 cm−1 is observed, which is explained in terms of second-order Raman scattering. All three peaks are assigned to wurtzite ZnO [31]. Another three peaks exclusive to the TiO2 modified ZnO nanorod film are observed around 202, 402 and 520 cm−1 , which are the characteristic for Eg(2) , B1g(1) and A1g + B1g(2) of TiO2 material [32]. These peaks are all right shifts compared to their standard anatase peak located at 196, 395 and 516 cm−1 [33]. Among the three weak peaks, 402 cm−1 is the dominant one. According to phonon confinement model, the crystallite size can be estimated by the shift of the main Eg Raman band [34]. The shift is 7.0 cm−1 , and the mean crystallite size is calculated to be 8 nm [35]. Also, the right shifts as well as the weak intensities may indicate that the TiO2 should not be in the abnormal crystal state. The XPS measurement was performed on bare ZnO nanorod film, pure TiO2 film and TiO2 modified sample. The Zn 2p spectrum determined from bare ZnO nanorod film was shown in Fig. 3(a), which

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displays characteristic peaks at 1021.1 and 1044.2 eV, which are assigned to Zn 2p3/2 and Zn 2p1/2 , respectively. The peak separation between them is 23.1 eV, which is well fitting to the standard reference value of ZnO. The two peaks are corresponding to chemical element state of Zn2+ of ZnO [17,36]. Fig. 3(b) shows the valence band (VB) spectrum of ZnO. The VBM (valence band maximum) position can be obtained by extrapolation the linear of the leading edges to the ground line, which is 2.61 eV. As shown in Fig. 3(c) recorded on pure TiO2 film, the peaks located at binding energy of 458.1 and 463.9 eV are attributed to Ti 2p3/2 and Ti 2p1/2 states of TiO2 , respectively, which are agree with the values of Ti4+ in TiO2 lattice [37,38]. Fig. 3(d) shows the VB spectrum of TiO2 , and a VBM value of 2.21 eV is deduced. The core levels of Zn 2p and Ti 2p spectra for TiO2 modified sample are given in Fig. 3(e) and (f). The peak positions shift slightly, while the peak shapes remain unchanged, which means Zn2+ and Ti4+ still persist in the lattice. All the peak values are summarized in Table 1. The valence band offset Fig. 2. Raman spectra of ZnO nanorod film and TiO2 modified nanorod film.

Fig. 3. XPS spectra of (a) Zn 2p for bare ZnO nanorod film, (b) VBM for ZnO, (c) Ti 2p for pure TiO2 film, (d) VBM for TiO2 , (e) and (f) Zn 2p and Ti 2p for ZnO/TiO2 heterojunction.

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Table 1 Peak position summary of XPS results. Sample

Region

Binding energy (eV)

ZnO

Zn 2p3/2 VBM Ti 2p3/2 VBM Zn 2p3/2 Ti 2p3/2

1021.1 2.61 458.1 2.21 1020.9 458.2

TiO2 TiO2 modified ZnO

(VBO, Evi ) at heterojunction interface can be calculated from the equation [39]: TiO2 ZnO TiO2 TiO2 ZnO ZnO Evi = (ETi (i) − EZn 2p (i)) − (ETi 2p − EVBM ) + (EZn 2p − EVBM ) 2p TiO2 (i) − E ZnO (i)) is the energy difference between Ti 2p where (ETi Zn 2p 2p TiO2 − E TiO2 ) is the and Zn 2p core levels at the interface of i, (ETi VBM 2p energy separation between Ti 2p and VBM of TiO2 in pure TiO2 ZnO − E ZnO ) refers to the energy separation between Zn film (EZn VBM 2p 2p and VBM of ZnO in bare ZnO nanorod film. The conduction band offset (CBO, Eci ) can be calculated from the equation [40]: Eci = EZnO /g − EgTiO2 − Evi , where the band gaps EgZnO and EgTiO2 at room temperature are 3.37 eV and 3.23 eV, respectively. Based on the data in Table 1, the VBO of the TiO2 modified sample is −0.1 eV, and the CBO is 0.24 eV. As the binding energy shift [41] between Zn 2p in bare ZnO and Zn 2p in ZnO/TiO2 heterojunction is very small, the band bending at the interface can be neglected. The morphologies of the as-prepared samples were directly observed by FESEM and TEM images. FESEM images for the top and side view of the bare ZnO nanorod film is shown in Fig. 4(a). The nanorods arrange compactly and uniformly with well-defined hexagonal facets and smooth surfaces, and their average diameter and length are about 100 nm and 1 ␮m, respectively. Fig. 4(b) exhibits the ZnO nanorod arrays after corrosion, and the corrosion made the tops concave, which would be advantageous to the subsequent TiO2 modifying. After immersion in the TBT contained solution, the surfaces of the nanorods are no longer smooth, but fluffy instead, as shown in Fig. 4(c). The nanorods are surrounded by numerous small particles. Samples exfoliated from the TiO2 modified film were observed on TEM, as shown in Fig. 5(a). It is clearly seen that the tops of the nanorods are covered by TiO2 , while some of the sides have fuzzed boundary lines, and TiO2 particles were attached. The high-resolution bright field TEM images of the top and side of a ZnO rod are shown in Fig. 5(b) and (c). Fig. 5(b) demonstrates that the nanorod is virtually a single crystalline, in which the lattice spacing of the indicated crystalline plane along the growth direction is 0.26 nm, corresponding to the interspace of the (0 0 2) plane of ZnO. This is the direct evidence confirming the c-axis growth of the nanorods. The TiO2 surrounded on the top and the side of the ZnO rod are amorphous, as shown by their disordered structure and the attached fast Fourier transform (FFT) diffuse holes taken on the TiO2 phase (the insets in Fig. 5(b) and (c)). So, the above right shifts and weak intensities of the Raman peaks of TiO2 in Fig. 2 should be explained by the amorphous state of TiO2 phase. The optical absorption behaviors of the TiO2 modified sample together with pure ZnO nanorod film were studied by UV–vis absorption spectra, shown in Fig. 6(a). The spectra display typical absorption with an acute transition in the UV region, which is due to the intrinsic band gap absorption of ZnO due to the electron transitions from the valence band to the conduction band. The band edge of the ZnO nanorod film was appeared at about 377 nm while the band edge of the TiO2 modified sample red shifted to 382 nm. The absorbance coefficient (˛) was calculated to obtain the band gap energy (Eg ). The Eg values were thus acquired by the extrapolation of the linear portion of the plot (˛h)2 versus the photon

Fig. 4. (a) FESEM images of ZnO nanorod film, and inset is the cross section view, (b) ZnO nanorod arrays after corroding, (c) TiO2 modified ZnO nanorod arrays.

energy h shown in Fig. 6(b) [42]. The slight decrease from 3.29 to 3.25 eV after TiO2 modified may be owing to the incorporation of TiO2 . The more important fact is that the absorption spectrum of the TiO2 modified sample exhibits an evident increase in visible light region in comparison with that of the ZnO nanorod film, which can be considered as the extension of the bandgap tail [29]. In our case, the tail extends from 400 nm to about 600–800 nm, covering the whole visible region. The band tail has been considered to be

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Fig. 6. (a) The absorption spectra of ZnO nanorod film and TiO2 modified sample, (b) The (˛h)2 versus h curves for band gap determination of ZnO nanorod film and TiO2 modified sample.

Fig. 5. TEM observations of (a) TiO2 modified ZnO nanorod arrays, (b) HRTEM image of the nanorod’s top, (c) HRTEM image of the nanorod’s side, the insets of (b) and (c) are the FFT images accordingly.

introduced by the similar condition to Chen et al.’s paper, which was introduced by the surface disorder of the TiO2 nanoparticles [29]. The band tail or the enhanced visible light absorption in our TiO2 modified sample should be related to the amorphous state (disorder) of TiO2 . Fig. 7(a) displays the photodegradation of MO in the time course under UV light at the existence of the ZnO nanorod film, the TiO2 modified sample and P25 film, respectively. Photodegradation was complete in 120 min for the TiO2 modified sample, whereas for the bare ZnO nanorod arrays in the same time, the photocatalytic degradation was only 68%. Obviously, the photocatalytic activity was significantly enhanced by TiO2 modifying. Compared with P25

film, whose degradation time was 100 min, the TiO2 modified ZnO nanorod sample was slightly inferior. However, the similar ability could be a breakthrough for ZnO based film. The photodegradation kinetics of MO was also studied. A good linear relationship between −Ln(C/C0 ) and irradiation time shown in Fig. 7(b) suggests that the photodegradation of MO follows the pseudo first order kinetics [43]. According to the equation of −Ln(C/C0 ) = kt, the reaction rate constant k for ZnO nanorod, TiO2 modified sample and P25 film can be calculated to be 0.012/min, 0.036/min and 0.042/min, respectively. The higher k value means a better photocatalytic activity, indicating that the ability of TiO2 modified sample is three times as that of ZnO. To examine the reusability, the same sample was repeatedly used in the degradation of MO under irradiation for 2 h. Five cycles were completed under the same condition and the results are shown in Fig. 7(c), where high photocatalytic degradation of 84–90% were repeated, i.e., the TiO2 modified ZnO nanorod film is stable as photocatalyst and can be reused. The slightly decrease in photocatalytic activity to ∼84% for the fourth and fifth recycles may be due to the adsorption of intermediate products and the shield of the active sites [8]. The influence of PH values on the photodegradation of MO for TiO2 modified sample and P25 film were shown in Fig. 7(d) and (e). The results showed the acid atmosphere was beneficial to the degradation of MO than the alkaline one. The literature reported PH variation modifies the surface charge of the photocatalysts [41]. Hence, the negative charge of the anionic MO made it more readily adsorbed at the positive sites on the photocatalyst provided by the

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Fig. 7. (a) Photocatalytic activities of MO for ZnO nanorod film, TiO2 modified sample and P25 film, (b) −Ln(C/C0 ) versus irradiation time, (c) recycle and reuse for photodegradation of MO for TiO2 modified nanorod film, (d) and (e) photodegradation of MO under different PH values for TiO2 modified nanorod film and P25 film, (f) Photodegradation of MB, (g) photodegradation of MO under simulated solar light.

acid solution. On the contrary, the alkaline solution would lead to the less absorption, which is disadvantageous to the degradation of MO. As methylene blue (MB) is a cationic dye, opposite to the anionic dye MO, their photodegradations under UV light were also

studied. In Fig. 7(f), it can be seen the TiO2 modified sample still take the advantage in degradation, but not obvious. However, the high activity means the adsorption properties of the dye on the catalyst play a less role in the photocatalytic decomposition. It is reported photogenerated holes are the key activated species for degradation

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Consequently the recombination probability of the electron hole charge carriers is markedly reduced and the charge carrier lifetime is increased. So both the enhanced visible absorption and the increased charge carrier lifetime would result in the enhancement of the photocatalytic activity of the TiO2 modified ZnO nanorod film. 4. Conclusion

Fig. 8. Schematic diagram for the band alignment structure of ZnO/TiO2 heterojunction.

of MB, different from the OH• radicals for that of MO [44]. Maybe it is the reason for the small gap between samples. The photodegradation of MO under simulated solar light was shown in Fig. 7(g). After 120 min irradiation, the decomposition for TiO2 modified sample reaches to 41%, better than that for ZnO nanorod sample, whose decomposition reaches to 36%. ZnO and TiO2 are both famous photocatalysts under UV light. The results show the photocatalytic activity under solar light was not as good as that under UV light. However, TiO2 modified sample exhibits slightly higher activity than that of P25, corresponding to the higher optical absorption. The possible mechanism of the photocatalytic properties was explored to understand the enhancement in the photodegradation. There are several reasons accounting for it, such as larger specific surface area [45] and higher UV–vis light absorption [46], as well as the special band structure formed in the TiO2 modified sample. Chen et al. [29] pointed out the introducing of disorder at the surface of TiO2 nanoparticles would enhance visible absorption, with the additional benefit of carrier trapping. It is believed that the surface amorphous TiO2 layer on ZnO would provide more severe disorder. Abundant of disorder in amorphous TiO2 would introduce midgap states, which could result in the band tail states merging with both the conduction and valence band, as testified by the enhanced visible absorption spectrum of the TiO2 modified sample shown in Fig. 6(a). A continuum extending to and overlapping with both the band edges would narrow the optical gap, and substantially enhance the visible absorption [47]. For the present TiO2 toping on ZnO, a tentative mechanism for the band configuration at the contacted interface of TiO2 and ZnO material is proposed. As the XPS analysis before, the band alignment schema was shown in Fig. 8. When ZnO nanorods are irradiated by photon with higher energy than its band gap energy, electrons are photoexcited from the valence band (VB) to the conduction band (CB) while holes are leaving in VB. The electrons transfer from CB of ZnO to CB of TiO2 due to the potential fall between them. Meanwhile, the photogenerated holes transfer from VB of TiO2 to VB of ZnO to keep thermal equilibrium state in the semiconductor system [4,6]. The electrons located on CB of TiO2 may react with O2 to form O2 •− , which subsequently form hydroxyl radicals OH• , while the highly oxidative holes located on VB of ZnO not only react with MO molecules but also oxidize H2 O/OH− to produce OH• radicals, which are of great importance in the photodegradation of MO.

In summary, amorphous TiO2 modified ZnO nanorod films have been synthesized by a combination of wet chemical and hydrolysis method. A thin layer of amorphous TiO2 was found dispersing on the tops and sides of ZnO nanorods. The TiO2 modified ZnO nanorod film exhibited superior photocatalytic activity in photodegradation of MO to that of the ZnO nanorod arrays. The enhanced photocatalytic activity can be primarily ascribed to enlarged specific surface area and the formation of TiO2 /ZnO heterojunction which is favorable for improving the light absorption capability and lowering the recombination rate of photo-induced electron and hole pairs due to the unique charge transfer process at the contact interface. It is expected this work could provide a promising platform for fabrication highly efficient ZnO based photocatalyst in environmental remediation. Acknowledgements This work was supported by the Foundation of National Key Basic Research and Development Program (No. 2010CB631001) and the Program for Changjiang Scholars and Innovative Research Team in University. References [1] A. Kubacka, M. Fernández-García, G. Colón, Advanced nanoarchitectures for solar photocatalytic applications, Chem. Rev. 112 (2011) 1555–1614. [2] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911–921. [3] Y. Lu, Y. Lin, T. Xie, L. Chen, S. Yi, D. Wang, Effect of photogenerated charge transfer on the photocatalysis in high-performance hybrid Pt-Co:ZnO nanostructure photocatalyst, ACS Appl. Mater. Interfaces 5 (2013) 4017–4020. [4] P. Zhang, C. Shao, X. Li, M. Zhang, X. Zhang, Y. Sun, Y. Liu, In situ assembly of well-dispersed au nanoparticles on TiO2 /ZnO nanofibers: a three-way synergistic heterostructure with enhanced photocatalytic activity, J. Hazard. Mater. 237–238 (2012) 331–338. [5] F.X. Xiao, Self-assembly preparation of gold nanoparticles-TiO2 nanotube arrays binary hybrid nanocomposites for photocatalytic applications, J. Mater. Chem. 22 (2012) 7819–7830. [6] G. Yang, Z. Yan, T. Xiao, Preparation and characterization of SnO2 /ZnO/TiO2 composite semiconductor with enhanced photocatalytic activity, Appl. Surf. Sci. 258 (2012) 8704–8712. [7] M.J. Zhou, Y. Hu, Y. Liu, W.L. Yang, H.S. Qian, Microwave-assisted route to fabricate coaxial ZnO/C/CdS nanocables with enhanced visible light-driven photocatalytic activity, CrystEngComm 14 (2012) 7686–7693. [8] H. Zhu, R. Jiang, Y. Fu, Y. Guan, J. Yao, L. Xiao, G. Zeng, Effective photocatalytic decolorization of methyl orange utilizing TiO2 /ZnO/chitosan nanocomposite films under simulated solar irradiation, Desalination 286 (2012) 41–48. [9] Z. Zhang, F. Xiao, Y. Guo, S. Wang, Y. Liu, One-pot self-assembled threedimensional TiO2 -graphene hydrogel with improved adsorption capacities and photocatalytic and electrochemical activities, ACS Appl. Mater. Interfaces 5 (2013) 2227–2233. [10] G. Yang, B. Yang, T. Xiao, Z. Yan, One-step solvothermal synthesis of hierarchically porous nanostructured CdS/TiO2 heterojunction with higher visible light photocatalytic activity, Appl. Surf. Sci. 283 (2013) 402–410. [11] L. Lin, Y. Yang, L. Men, X. Wang, D. He, Y. Chai, B. Zhao, S. Ghoshroy, Q. Tang, A highly efficient TiO2 @ZnO n–p–n heterojunction nanorod photocatalyst, Nanoscale 5 (2013) 588–593. [12] K. Li, B. Chai, T. Peng, J. Mao, L. Zan, Preparation of AgIn5S8/TiO2 heterojunction nanocomposite and its enhanced photocatalytic H2 production property under visible light, ACS Catal. 3 (2013) 170–177. [13] M.T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, L. Servant, M.M. Müller, H.-J. Kleebe, J. Ziegler, W. Jaegermann, Nanostructured SnO2 –ZnO heterojunction photocatalysts showing enhanced photocatalytic activity for the degradation of organic dyes, Inorg. Chem. 51 (2012) 7764–7773. [14] J. Shi, On the synergetic catalytic effect in heterogeneous nanocomposite catalysts, Chem. Rev. 113 (2013) 2139–2181.

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