A Facile Route to the Preparation of Highly Uniform ZnO @ TiO 2 Core-Shell Nanorod Arrays with Enhanced Photocatalytic Properties

Design and synthesis of ZnO@TiO 2 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 friendlymethod to prepare ZnO@TiO 2 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@TiO 2 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@TiO 2 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@TiO 2 core-shell nanorod array catalysts when they are used in water purification processes.


Introduction
In recent years, there has been increasing interest in the design and synthesis of ZnO@TiO 2 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][2][3][4][5][6][7][8][9][10].This is mainly attributed that the large binding energy of ZnO and the high reactivity of TiO 2 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][12][13][14][15][16][17][18], a better separation of photogenerated carriers can be obtained in ZnO@TiO 2 core-shell structures.
Until now, several types of ZnO@TiO 2 core-shell nanorod arrays are fabricated by different methods.Typically, Wang et al. have prepared ZnO/TiO 2 core-shell nanorod arrays by a plasma sputtering decoration route [19].Heterostructure ZnO/TiO 2 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 TiO 2 shells are grown on the ZnO nanorod arrays in a traveling-wave atomic layer deposition (ALD) system using TiCl 4 and water at 300 ∘ C with a process pressure of 300-500 mTorr [21].All the ZnO@TiO 2 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@TiO 2 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 TiO 2 nanoparticle via a vapor-thermal method at relatively low Journal of Chemistry temperature.And the morphology of the ZnO@TiO 2 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@TiO 2 nanorod arrays is discussed.The high UV light photocatalytic efficiency of the product is achieved via heterojunction effect.And the advantages of ZnO@TiO 2 heterostructures have the following features: (a) the ZnO@TiO 2 core-shell structure heterojunction can improve electron-charge separation efficiency.So, the proposed route to prepare the ZnO@TiO 2 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@TiO 2 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 TiO 2 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 TiO 2 layers are formed on the surface of ZnO nanorod arrays, they can serve as protective layers in solutions.

Experimental
2.1.Preparation of Highly Uniform ZnO@TiO 2 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@TiO 2 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 [22].Secondly, the ZnO@TiO 2 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.

Characterization.
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).

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@TiO 2 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 (: 254 nm, output power: 8 W, the irradiance of the UV light source is 20 W/cm 2 ), 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).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@TiO 2 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@TiO 2 nanorod arrays, from which it displays that high density ZnO@TiO 2 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@TiO 2 nanorod is obviously larger than that of ZnO nanorod.

Results and Discussion
The high-magnification SEM image shown in Figure 2(a) further observes the detailed structures of ZnO@TiO 2 nanorod arrays.It seems that the ZnO@TiO 2 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@TiO 2 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  Figure 3 shows the TEM image and the corresponding energy dispersive spectroscopic (EDS) analysis of ZnO@TiO 2 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 [22].The lattice fringe spacing of 0.357 nm observed from the HRTEM image corresponded to the (101) plane of anatase TiO 2 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 [23].In order to further confirm the presence of both TiO The growth mechanism of the ZnO@TiO 2 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, TiO 2 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 TiO 2 nanoparticles subsequently serve as nucleation sites for further deposition of TiO 2 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@TiO 2 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 TiO 2 shell is formed on the surface of ZnO nanorod arrays (ZnO@TiO 2 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 [24].In addition, as shown in Figure 5(a), a red shift of the band gap absorption edge can also be observed in the  > 320 nm region.To determine the band gaps of the ZnO@TiO 2 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 [14].From the inset image of Figure 5(a), the estimated band gap of the ZnO@TiO 2 is 2.95 eV, which is smaller than that of ZnO 3.01 eV.This may be due to the formation of a ZnO@TiO 2 heterojunction slowing the electron-hole recombination and hence reducing the level of emission.Thus, the photocatalytic ability of the ZnO@TiO 2 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@TiO 2 nanorod arrays grown on Zn wafer, respectively.The ZnO@TiO 2 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 TiO 2 .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 (TiO 2 P25 Evonik) has also been investigated.From Figure 5(b), it is clear that the photocatalytic degradation capacity of TiO 2 (P25 Evonik) particles is higher than that of the ZnO@TiO 2 films with a core-shell structure.However, compared with TiO 2 (P25 Evonik) particles, the ZnO@TiO 2 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 TiO 2 nanoparticles are coated on the surface of ZnO nanorod arrays.
The photocatalytic durability of the ZnO@TiO 2 nanorod arrays is also tested.Figure 5(c) displays the cyclic photodegradation of MB using ZnO@TiO 2 nanorod arrays.After fifteen cycles, the ZnO@TiO 2 nanorod arrays still maintain high photocatalytic ability.The core-shell structures of the ZnO@TiO 2 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 TiO 2 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 TiO 2 layers.Photocorrosion is a very important shortcoming for photocatalysts such as CdS and ZnO but not for TiO 2 [27,28].Thus, these uniform core-shell structures of ZnO@TiO 2 nanorod arrays can obviously improve the efficiency and stability of the photocatalyst.
The photocatalytic mechanism of ZnO@TiO 2 nanorod arrays for MB is displayed in Figure 5(d).Under UV illumination, both ZnO and TiO 2 can absorb the photons.Thus, the electron (e − ) and hole (h + ) are obtained.The produced e − transfers from the conduction band (CB) ZnO to TiO 2 , while h + transfers from the valence band (VB) of TiO 2 to ZnO.As a result, an efficient separation of photogenerated electron and hole pairs at the core-shell heterojunction interface of ZnO and TiO 2 is reached [29].These electron and hole pairs improve the occurrence of redox processes; electrons reduce dissolved O 2 to • O 2 − , while h + forms HO • .Thus, MB molecules will be easily decomposed into CO 2 and H 2 O by the strong oxidizer-reducer through some oxidation-reduction reactions.

Conclusion
In summary, we have reported a facile, green, efficient method for the preparation of ZnO@TiO 2 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@TiO 2 core-shell structure heterojunction can improve electron-charge separation efficiency.So, the proposed route to prepare the ZnO@TiO 2 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@TiO 2 film with a coreshell structure has unique advantages for practical applications from the aspect of long life low-cost and high degradation efficiency.When TiO 2 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 [30].So, our proposed route to prepare ZnO@TiO 2 core-shell structure catalysts represents a very important advance for environmental remediation applications.

Figures 1 (
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 m.From the image, it is confirmed that the diameter of ZnO nanorod is about 190 nm.Figures1(c) and 1(d) show SEM images of the ZnO@TiO 2 nanorod arrays grown on the surface of the Zn wafer.The low-magnification image (Figure1(c)) also displays a large scale uniform film-like structure grown on the ZnO wafer.Figure1(d)shows the high-magnification SEM image of ZnO@TiO 2 nanorod arrays, from which it displays that high density ZnO@TiO 2 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 (Figures1(b) and 1(c)), it is clear that the diameter of ZnO@TiO 2 nanorod is obviously larger than that of ZnO nanorod.The high-magnification SEM image shown in Figure2(a) further observes the detailed structures of ZnO@TiO 2 nanorod arrays.It seems that the ZnO@TiO 2 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@TiO 2 nanorod shown in Figure2(a) reveals the top of ZnO nanorod structure (indicated by the black dotted circle).Figure2(b) shows that the inner edge of the
2 and ZnO components in the ZnO@TiO 2 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 TiO 2 in the ZnO@TiO 2 nanorods.The images also show that the ZnO@TiO 2 nanorod has very good uniform core-shell structures.The EDS analysis (Figure 3(e)) of the ZnO@TiO 2 nanorods indicates the existence of Zn, Ti, and O elements (copper signals are present from the copper grid support).