A Facile Route to the Preparation of Highly Uniform ZnO@TiO2 Core-Shell Nanorod Arrays with Enhanced Photocatalytic Properties
Design and synthesis of ZnO@TiO2 core-shell nanorod arrays as promising photocatalysts have been widely reported. However, it remains a challenge to develop a low-temperature, low-cost, and environmentally friendly method to prepare ZnO@TiO2 core-shell nanorod arrays over a large area for future device applications. Here, a facile, green, and efficient route is designed to prepare the ZnO@TiO2 nanorod arrays with a highly uniform core-shell structure over a large area on Zn wafer via a vapor-thermal method at relatively low temperature. The growth mechanism is proposed as a layer-by-layer assembly. The photocatalytic decomposition reaction of methylene blue (MB) reveals that the ZnO@TiO2 core-shell nanorod arrays have excellent photocatalytic activities when compared with the performance of the ZnO nanorod arrays. The improved photocatalytic activity could be attributed to the core-shell structure, which can effectively reduce the recombination rate of electron-hole pairs, significantly increase the optical absorption range, and offer a high density of surface active catalytic sites for the decomposition of organic pollutants. In addition, it is very easy to separate or recover ZnO@TiO2 core-shell nanorod array catalysts when they are used in water purification processes.
In recent years, there has been increasing interest in the design and synthesis of ZnO@TiO2 core-shell nanorod arrays to improve the quantum efficiency of Dye-Sensitized Solar Cells (DSSC), highly transparent self-cleaning coating for LEDs, and photocatalysts for the decomposition of organic pollutants in waste water [1–10]. This is mainly attributed that the large binding energy of ZnO and the high reactivity of TiO2 can significantly increase the process of electron and hole transfer between the corresponding conduction and valence bands. Thus, compared with photoanode materials or single metal oxide catalysts [11–18], a better separation of photogenerated carriers can be obtained in ZnO@TiO2 core-shell structures.
Until now, several types of ZnO@TiO2 core-shell nanorod arrays are fabricated by different methods. Typically, Wang et al. have prepared ZnO/TiO2 core-shell nanorod arrays by a plasma sputtering decoration route . Heterostructure ZnO/TiO2 core-brush nanostructures are synthesized on glass substrates by a combination of aqueous solution growth and magnetron sputtering method [6, 20]. Greene et al. have reported that the TiO2 shells are grown on the ZnO nanorod arrays in a traveling-wave atomic layer deposition (ALD) system using TiCl4 and water at 300°C with a process pressure of 300–500 mTorr . All the ZnO@TiO2 core-shell nanorod arrays in the above-mentioned reports are obtained under severe conditions (including low pressure, relatively high temperature, and anaerobic conditions); and special equipment is required. Thus, it remains a challenge to develop a low-temperature, low-cost, and environmentally friendly method to prepare ZnO@TiO2 core-shell nanorod arrays over a large area for future device applications.
Herein, a facile, green, and efficient route is proposed to achieve a decoration of ZnO nanorod arrays with TiO2 nanoparticle via a vapor-thermal method at relatively low temperature. And the morphology of the ZnO@TiO2 nanorod arrays obtained heterostructure shows a core-shell arrangement with a highly uniform over a large area on Zn wafer. A 60 mL stainless steel autoclave with a Teflon liner is used to achieve the preparation process, which is simpler equipment than magnetron sputtering system and ALD system that had been widely used previously. The growth mechanism of the ZnO@TiO2 nanorod arrays is discussed. The high UV light photocatalytic efficiency of the product is achieved via heterojunction effect. And the advantages of ZnO@TiO2 heterostructures have the following features: (a) the ZnO@TiO2 core-shell structure heterojunction can improve electron-charge separation efficiency. So, the proposed route to prepare the ZnO@TiO2 core-shell nanorod arrays represents a significant advance for the degradation of organic pollutants in water; (b) compared with some nanoscale catalyst particles, the ZnO@TiO2 film with a core-shell structure has unique advantages for practical applications from the aspect of long life, low-cost, and high degradation efficiency. Particularly, when TiO2 nanoparticles are coated on the surface of ZnO nanorod arrays, it is very convenient to separate or recover them in water purification processes; (c) ZnO is sensitive to both acidic and basic solutions (chemical corrosion). However, when the outer TiO2 layers are formed on the surface of ZnO nanorod arrays, they can serve as protective layers in solutions.
2.1. Preparation of Highly Uniform ZnO@TiO2 Core-Shell Nanorod Arrays
All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The reagents were of analytical grade and used without further purification. The ZnO@TiO2 nanorod arrays on Zn wafer were prepared by following two steps: firstly, a piece of zinc wafer (1 cm × 1 cm, 99.99%) pretreated by sonication in ethanol and deionized water and dried in air was placed in a sealed bottle containing of zinc nitrate hydrate (0.1 M) and hexamethylenetetramine (0.1 M) for three hours at 75°C. The ZnO nanorod arrays were obtained. The samples were rinsed with deionized water for several times and dried at 60°C for several hours before being used . Secondly, the ZnO@TiO2 nanorod arrays on Zn wafer were prepared by the vapor-thermal method using tetrabutyl titanate (TBOT) as the titanium source. In a typical procedure, the piece of zinc wafer with ZnO nanorod arrays (1 cm × 1 cm) was placed 10 mL beaker with the mixture of ethanol and TBOT. Then, the beaker was placed into a 60 mL stainless steel autoclave with a Teflon liner. The free space between the Teflon liner and the beaker was filled with distilled water. After sealing, the autoclave was heated to 150°C for 6 h. At the end of the reaction, the autoclave was cooled naturally to room temperature, and the products were collected and washed for three cycles using deionized water and ethanol, respectively. And then, they were dried at 50°C in a vacuum oven for 3 h.
The surface morphologies of the products were examined by Field emission scanning electron microscope (FESEM, Quanta 200 FEG). Transmission electron microscopy (TEM), high resolution transmission electron microscope (HRTEM) images, and selected area energy dispersive X-ray spectrum (EDS) were obtained on a JEOL-2012 TEM coupled with an EDS detector, operated at an acceleration voltage of 200 kV. The UV-Vis spectra were measured by a UV/Vis/NIR spectrophotometer (Hitachi U-4100).
2.3. Photocatalytic Properties
In our experiments, photocatalytic decomposition of MB under UV light illumination has been conducted at room temperature. Both the ZnO nanorod arrays and ZnO@TiO2 nanorod arrays grown on Zn wafer with size 1 cm × 1 cm were soaked into two identical bottles containing methylene blue (MB, 10 mL, 1 × 10−5 M). Before UV light illumination (λ: 254 nm, output power: 8 W, the irradiance of the UV light source is 20 μW/cm2), they were stored in the dark for 30 min to ensure the establishment of an adsorption equilibrium for MB at ambient temperature. In addition, in order to ensure that both the samples could receive the same amount of UV illumination, they were placed 5 cm away from the UV light sources in our experiments. As a comparison, one bottle containing blank MB solution was also carried out under the same condition. The concentrations of MB in the above three bottles at given time intervals were determined using a UV/Vis/NIR spectrophotometer (Hitachi U-4100).
3. Results and Discussion
Figures 1(a) and 1(b) show scanning electron microscope (SEM) images of the ZnO nanorod arrays grown on Zn wafer. The low-magnification SEM image (Figure 1(a)) shows a large area uniform film-like structure deposited on the wafer. From the high-magnification SEM image (Figure 1(b)), it is clear that high density ZnO nanorod arrays are grown vertically on the ZnO wafer. These nanorods have a nearly uniform diameter of 180–200 nm. The inset in Figure 1(b) is the TEM image of a single ZnO nanorod with length about 1.4 μm. From the image, it is confirmed that the diameter of ZnO nanorod is about 190 nm. Figures 1(c) and 1(d) show SEM images of the ZnO@TiO2 nanorod arrays grown on the surface of the Zn wafer. The low-magnification image (Figure 1(c)) also displays a large scale uniform film-like structure grown on the ZnO wafer. Figure 1(d) shows the high-magnification SEM image of ZnO@TiO2 nanorod arrays, from which it displays that high density ZnO@TiO2 nanorod arrays with the diameter of about 230–260 nm are grown vertically on the surface of the ZnO wafer. From the high-magnification images (Figures 1(b) and 1(c)), it is clear that the diameter of ZnO@TiO2 nanorod is obviously larger than that of ZnO nanorod.
The high-magnification SEM image shown in Figure 2(a) further observes the detailed structures of ZnO@TiO2 nanorod arrays. It seems that the ZnO@TiO2 nanorods have core-shell structures (the black and white arrows indicate the shell structure and the core structure, resp.). In addition, the open tip of ZnO@TiO2 nanorod shown in Figure 2(a) reveals the top of ZnO nanorod structure (indicated by the black dotted circle). Figure 2(b) shows that the inner edge of the TiO2 shell and the outer edge of ZnO core have hexagonal structures. The thickness of the TiO2 shell is about 40–60 nm. The TEM image shown in Figure 2(c) clearly confirms that the ZnO@TiO2 nanorod has core-shell structure. The TiO2 shell thickness of 50 nm is consistent with the result of the observation from Figure 2(b).
Figure 3 shows the TEM image and the corresponding energy dispersive spectroscopic (EDS) analysis of ZnO@TiO2 nanorod. In the top right corner of Figure 3(a), the lattice fringe spacing of 0.525 nm observed from the HRTEM image corresponded to the (001) plane of hexagonal ZnO phase (JCPDS card number 00-005-0664, space group: P63mc (186)) with -axial growth direction. The hexagonal ZnO phase is identical with the reported ones . The lattice fringe spacing of 0.357 nm observed from the HRTEM image corresponded to the (101) plane of anatase TiO2 phase in the bottom right corner of Figure 3(a) (JCPDS card number 00-021-1272, space group: I41/amd (141)). This result is consistent with that reported previously . In order to further confirm the presence of both TiO2 and ZnO components in the ZnO@TiO2 nanorods, electron mapping image analysis is used to analyze the sample (Figures 3(a)–3(d)). The electron mapping images can be obtained via visualizing the inelastically scattered electrons in the energy loss windows for elemental Zn, Ti, and O. The different color areas displayed in Figures 3(a)–3(d) indicate Zn-, Ti-, and O-enriched areas of the sample, respectively, which indicate the presence of both ZnO and TiO2 in the ZnO@TiO2 nanorods. The images also show that the ZnO@TiO2 nanorod has very good uniform core-shell structures. The EDS analysis (Figure 3(e)) of the ZnO@TiO2 nanorods indicates the existence of Zn, Ti, and O elements (copper signals are present from the copper grid support).
The growth mechanism of the ZnO@TiO2 nanorod arrays can be described as follows. During the experiment, the ethanol evaporates firstly when the reaction system is heated. It is mainly due to the fact that the ethanol has a lower boiling point than water. And then, the water vapor promotes the hydrolysis of TBOT dissolved in ethanol. It is supposed that there are many nucleation sites on the surface of the ZnO nanorod. During the initial stage, TiO2 nanoparticles formed by hydrolysis of TBOT are deposited on the nucleation site of the ZnO surface. With the passage of reaction time (Figure 4(b)), the as-formed TiO2 nanoparticles subsequently serve as nucleation sites for further deposition of TiO2 nanoparticles (Figure 4(c)), which lead to layer-by-layer assembly.
Figure 5(a) shows the typical diffuse reflectance spectra (DRS) for ZnO nanorod arrays and ZnO@TiO2 nanorod arrays at room temperature. As shown in curve 1 of Figure 5(a), ZnO nanorod arrays present a continuous wide absorption band in the UV light region. When TiO2 shell is formed on the surface of ZnO nanorod arrays (ZnO@TiO2 nanorod arrays), the increase in peak intensity can be observed obviously (as shown in curve 2 of Figure 5(a)). In the UV region, from 200 to 320 nm, the peak intensity in continuous UV absorption band is stronger than that of ZnO nanorod arrays. The changes of UV peak intensity may be affected by scattering, particle size, aggregates, and so on . In addition, as shown in Figure 5(a), a red shift of the band gap absorption edge can also be observed in the nm region. To determine the band gaps of the ZnO@TiO2 nanorod heterostructures, we replotted the spectra shown in Figure 5(a) in the form shown in the inset image of Figure 5(a) obtained by the application of the Kubelka-Munch algorithm . From the inset image of Figure 5(a), the estimated band gap of the ZnO@TiO2 is 2.95 eV, which is smaller than that of ZnO 3.01 eV. This may be due to the formation of a ZnO@TiO2 heterojunction slowing the electron-hole recombination and hence reducing the level of emission. Thus, the photocatalytic ability of the ZnO@TiO2 heterostructures is improved, which is demonstrated by the following photocatalytic experiments.
As shown in Figure 5(b), the MB decomposition rates of the samples are compared over a period of time. The MB decomposition rate in the bottle containing blank MB solution with catalysts is only about 12% after 200 min of photocatalytic reaction. However, 60% and 86% of MB concentration are decomposed by the ZnO nanorod arrays and ZnO@TiO2 nanorod arrays grown on Zn wafer, respectively. The ZnO@TiO2 nanorod arrays grown on Zn wafer show superior photocatalytic activity to the ZnO nanorod arrays grown on Zn wafer. This can be attributed to the formation of the P-N core-shell heterojunction between ZnO and TiO2. The P-N core-shell heterojunction can enhance the charge separation effect under UV illumination. Thus, the composite nanostructures can further promote the photocatalytic activity [6, 11, 25, 26]. In addition, the comparison with the photocatalytic activity of the conventional benchmark (TiO2 P25 Evonik) has also been investigated. From Figure 5(b), it is clear that the photocatalytic degradation capacity of TiO2 (P25 Evonik) particles is higher than that of the ZnO@TiO2 films with a core-shell structure. However, compared with TiO2 (P25 Evonik) particles, the ZnO@TiO2 films with a core-shell structure have unique advantages for practical applications. It is very convenient to separate or recover them in water purification processes when TiO2 nanoparticles are coated on the surface of ZnO nanorod arrays.
The photocatalytic durability of the ZnO@TiO2 nanorod arrays is also tested. Figure 5(c) displays the cyclic photodegradation of MB using ZnO@TiO2 nanorod arrays. After fifteen cycles, the ZnO@TiO2 nanorod arrays still maintain high photocatalytic ability. The core-shell structures of the ZnO@TiO2 nanorod arrays are expected to have a long service life. Actually, in our sample, ZnO is sensitive to both acidic and basic solutions (chemical corrosion). However, when the outer TiO2 layers are formed on the surface of ZnO nanorod arrays, they can serve as protective layers in solutions. Furthermore, the photocorrosion of ZnO under UV illumination can also be prevented by TiO2 layers. Photocorrosion is a very important shortcoming for photocatalysts such as CdS and ZnO but not for TiO2 [27, 28]. Thus, these uniform core-shell structures of ZnO@TiO2 nanorod arrays can obviously improve the efficiency and stability of the photocatalyst.
The photocatalytic mechanism of ZnO@TiO2 nanorod arrays for MB is displayed in Figure 5(d). Under UV illumination, both ZnO and TiO2 can absorb the photons. Thus, the electron (e-) and hole (h+) are obtained. The produced e- transfers from the conduction band (CB) ZnO to TiO2, while h+ transfers from the valence band (VB) of TiO2 to ZnO. As a result, an efficient separation of photogenerated electron and hole pairs at the core-shell heterojunction interface of ZnO and TiO2 is reached . These electron and hole pairs improve the occurrence of redox processes; electrons reduce dissolved O2 to , while h+ forms . Thus, MB molecules will be easily decomposed into CO2 and H2O by the strong oxidizer-reducer through some oxidation-reduction reactions.
In summary, we have reported a facile, green, efficient method for the preparation of ZnO@TiO2 core-shell nanorod arrays grown on Zn wafer. The product has highly uniform structures over a large area. It is shown that the product has excellent photocatalytic activities for the degradation of MB under UV light illumination when compared with the performance of the ZnO nanorod arrays grown on Zn wafer. This is because the ZnO@TiO2 core-shell structure heterojunction can improve electron-charge separation efficiency. So, the proposed route to prepare the ZnO@TiO2 core-shell nanorod arrays represents a significant advance for the degradation of organic pollutants in water. In addition, compared with some nanoscale catalyst particles, the ZnO@TiO2 film with a core-shell structure has unique advantages for practical applications from the aspect of long life low-cost and high degradation efficiency. When TiO2 nanoparticles are coated on the surface of ZnO nanorod arrays, it is very convenient to separate or recover them in water purification processes. Recycling of photocatalyst in nanometer scale from the environmental system is an important focus in practical applications . So, our proposed route to prepare ZnO@TiO2 core-shell structure catalysts represents a very important advance for environmental remediation applications.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was supported by the National Natural Science Foundation of China (51302102 and 11504120), the Natural Science Foundation of Anhui Province (1708085ME96), the Key Natural Science Research Project for Colleges and Universities of Anhui Province (KJ2016A638 and KJ2016SD51), the Huaibei Scientific Talent Development Scheme (20140305 and 20140318), and the Huaibei Normal University Youth Research Project (2014xq017).
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