ZnO Nanowires Co-Using Solar and

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Enhanced H2 Production of TiO2/ZnO Nanowires Co-Using Solar and Mechanical Energy through Piezo-Photocatalytic Effect Zijian Wang, Tianchen Hu, Haoxuan He, Yongming Fu, Xia Zhang, Jing Sun, Li-Li Xing, Baodan Liu, Yan Zhang, and Xinyu Xue ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01480 • Publication Date (Web): 30 Jun 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced H2 Production of TiO2/ZnO Nanowires Co-Using Solar and Mechanical Energy through Piezo-Photocatalytic Effect Zijian Wangb,∇, Tianchen Hub,∇, Haoxuan Heb, Yongming Fub, Xia Zhangb, Jing Sun*,b , Lili Xinga,b, Baodan Liuc, Yan Zhang*,a, and Xinyu Xue*,a,b a

School of Physics, University of Electronic Science and Technology of China, Chengdu 610054,

China b

College of Sciences, Northeastern University, Shenyang 110819, China

c

Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research

(IMR), Chinese Academy of Sciences (CAS), Shenyang 110016, China *Corresponding Authors *E-mail: [email protected] (X. Xue). *E-mail: [email protected] (Y. Zhang). *E-mail: [email protected] (J. Sun). KEYWORDS: photocatalytic H2 production, TiO2/ZnO nanowire, piezoelectric effect, piezophotocatalytic effect, heterostructure

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ABSTRACT

TiO2/ZnO nanowire arrays on stainless steel mesh for piezo-photocatalytic H2 production are synthesized via a two-step hydrothermal route. The photocatalytic H2 production efficiency under solar illumination can be enhanced by introducing mechanical vibration energy. As coutilizing the solar and mechanical energy (ultrasonic irradiation), the nanowires show high H2 production. The nanowire arrays also show excellent recyclability and stability. Additionally, this mesh-based structure can be retrieved easily from aqueous solution, which can meet the practical application demand. The working mechanism can be attributed to the piezophotocatalytic effect, in which the piezoelectric field of bent ZnO nanowires and the built-in electric field of TiO2/ZnO heterostructures can efficiently separate the photo-generated electrons and holes for effectively producing H2. Present results provide a new strategy for developing photocatalytic H2 production techniques.

INTRODUCTION The industrial activities and life styles in modern society lead to the huge daily energy consumption. And the combustion of fossil fuels causes serious environmental contamination problems. It is necessary to acquire clean renewable energy for the sustainable development of human beings. H2 is reckoned to be an ideal and clean fuel for the future, and it can be obtained via decomposing water, which has large stock on earth. Solar energy is considered as renewable energy for H2 production.1-3 Many researchers have revealed that semiconducting metal-oxide nanostructures as photocatalysts can efficiently decompose H2O into H2 and O2 under UV or solar irradiation.4-7 Among various metal-oxide nanostructures, ZnO and TiO2 nanostructures are demonstrated to be excellent photocatalysts for H2 production with low cost, high photocatalytic


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efficiency and good environmental friendliness.7-11 In a typical photocatalytic H2 production process, photo-generated electrons (e-)/holes (h+) can move to the surface interface between the nanostructure and reaction solution, and these photo-generated carriers can react with water and produce H2 and O2.12, 13 However, the photocatalytic efficiency of the nanostructures is restrained by the high recombination rate of photo-generated carriers.14, 15 Recently, some efforts have been devoted to lower down the recombination rate of photogenerated electron-hole pairs for enhancing the photocatalytic efficiency of metal-oxide nanostructures. One method is to construct heterostructures between two metal-oxide nanostructures, such as TiO2/ZnO nanowires,16-19 and the heterojunctions at the interface between the two components can establish a built-in electric field to separate the photo-generated electrons/holes (lowering down the recombination rate). ZnO nanowires are different from other metal-oxide nanostructured photocatalysts, which have piezoelectric properties.8,

20, 21

Under

applied deformation, a piezoelectric field can be created along or across ZnO nanowires. If this piezoelectric field can be introduced into the photocatalytic process, the recombination rate of photo-generated carriers may be further lowered down through driving the electrons and holes migrating along the opposite directions. In this paper, TiO2/ZnO nanowire arrays on stainless steel mesh for piezo-photocatalytic H2 production are presented. The photocatalytic H2 production efficiency under solar illumination can be enhanced by introducing mechanical vibration energy (based on the piezoelectric properties of ZnO nanowires). The nanowire arrays exhibit excellent recyclability and stability. The working mechanism can be attributed to the piezo-photocatalytic effect, in which the piezoelectric field of bent ZnO nanowires and the built-in electric field of TiO2/ZnO heterostructures can efficiently separate the photo-generated carriers (lowering down the


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recombination rate) for effectively producing H2. The piezo-photocatalytic H2 production mechanism of co-utilizing solar and mechanical energy may have potential in applications of eco-friendly H2 fuel production.

EXPERIMENTAL SECTION Raw Materials. All chemicals were analytic pure and used without further purification. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), tetrabutyl titanate (C16H36O4Ti), diethanolamine (C4H11NO2), absolute ethanol, absolute methanol and ammonia solution were purchased from Sinopharm Chemical Reagent Co. Ltd. TiO2 nanopowder (P25) was purchased from Evonik Industries AG. Stainless steel mesh (300 mesh) was purchased from Shenyang Merged Albert Metal Ltd. Synthesis of TiO2/ZnO nanowires. Stainless steel mesh was used as the substrate, and TiO2/ZnO nanowire arrays were synthesized on this substrate via a simple two-step hydrothermal route. Firstly, ZnO nanowires were grown on the substrate via a hydrothermal method. Prior to synthesis, the pre-cut stainless steel mesh (5.0×9.0 cm in area) was cleaned in deionized water/ethanol, and dried at 60°C. 4.33g of Zn(NO3)2·6H2O was dissolved in 266 mL of deionized water in a beaker, and kept stirring for 5 min at room temperature. 13.3 mL of ammonia solution was added into the above solution under continuous stirring. The pre-cleaned stainless steel mesh substrate was immersed into the solution. The beaker was sealed and kept at 83°C for 24 h. After reaction, the mesh substrate was washed with deionized water/ethanol, and dried at 60°C overnight. Secondly, TiO2 shell was deposited on ZnO nanowires by a hydrothermal method. 18 mL of C16H36O4Ti and 6 mL of C4H11NO2 were dissolved into 140 mL of ethanol under vigorous


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magnetic stirring. 2 mL of deionized water was dissolved in 20 ml of ethanol under stirring, which was then added into the above solution. The mixed solution was continuously stirred for 5 h. Then the mesh substrate with ZnO nanowire arrays was immersed in the solution for 4 min and dried at 60°C. This process was repeated for 8 times to increase the amount of TiO2 precursor on ZnO nanowires. Finally, the treated mesh substrate was annealed at 500°C in air for 1 h and cooled to room temperature naturally. The substrate was weighted before and after the growth of nanowires, and the weight of TiO2/ZnO nanoarrays on the substrate was calculated by the subtraction. The mass density of TiO2/ZnO nanowire arrays on the mesh substrate was ~5 mg·cm−2. Characterization. Scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, JEOL JEM-2100) were used to characterize the microstructures of TiO2/ZnO nanowire arrays. An X-ray diffraction (XRD, Dmax 2550V, Cu K radiation) diffractometer was used to characterize the crystal phase of TiO2/ZnO nanowires. Roomtemperature UV-vis absorption spectrum and photoluminescence spectra of the nanowires were obtained

with

an

UV-vis-NIR

spectrometer

(Hitachi

U-3900)

and

a

fluorescence

spectrophotometer (Hitachi F-7000). The two spectra were measured in the wavelength range of 200-800 nm and 300-700 nm, respectively. Measurement Methods. Piezo-photocatalytic H2 production activity of TiO2/ZnO nanowires was evaluated by measuring the amount of H2 production. The photocatalytic H2 production was conducted using a photocatalytic reaction system (Perfectlight Labsolar-ⅢAG). Photocatalytic reactions were conducted in the glass reaction vessel of the system with 150 mL of methanol aqueous solution (20 vol.%). The mesh sample (5.0×9.0 cm, loading ~220 mg of TiO2/ZnO nanowires) was placed horizontally in the reaction vessel. In the test, methanol was chosen as


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sacrificial reagent which scavenges photo-generated holes. A xenon lamp (Perfectlight PLSSXE300CUV, 50W, 200-2500nm) was used as the light source, and the mechanical vibration was simulated and provided by an ultrasonic cleaner (KH-50A, 50W). Before reaction, the system was thoroughly vacuumized. The H2 product was analyzed using a gas chromatograph (Techcomp GC7900) with N2 as carrier gas.

Figure 1. (a) Synthesis process of TiO2/ZnO nanowires on stainless steel mesh substrate. (b) Images of bare stainless steel mesh substrate and the substrate with TiO2/ZnO nanowire arrays. (c) Piezo-photocatalytic H2 production efficiency measurement by measuring the H2 production of mesh-based TiO2/ZnO nanowire arrays in a glass reaction vessel under solar and ultrasonic irradiation. (d) Pragmatic application of energy harvesting system with mesh-based TiO2/ZnO nanowire arrays for future practical applications RESULTS AND DISCUSSION


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Experimental Design. Figure 1 shows the experimental design. As shown in Figure 1a, the TiO2/ZnO nanowire arrays on the stainless steel mesh substrate can be easily synthesized via a two-step hydrothermal route. The mesh-based substrate is fully covered with nanowires and has good flexibility, as shown in Figure 1b. The weight of TiO2/ZnO-coated stainless steel mesh substrate before and after the flexibility test is 1.2606 and 1.2604 g, respectively, which shows no obvious weight loss. Compared with other nanopowder or nanowire photocatalysts, the meshsupported TiO2/ZnO nanowire arrays can be extracted easily from water in practical application. As shown in Figure 1c, under both solar and ultrasonic irradiation, the piezo-photocatalytic H2 production process of TiO2/ZnO nanowires is performed in a glass reaction vessel with methanol aqueous solution. Figure 1d illustrates a potential application of mesh-based TiO2/ZnO nanowire arrays co-utilizing solar and mechanical vibration energy for H2 production. In practical application, the tide or other stream can provide enough mechanical vibration energy. The water flow (with a speed higher than 0.9 m/s) and wind current can easily generate 50 W of power.22-25 TiO2/ZnO nanowire arrays can be easily synthesized via a two-step hydrothermal route with common and inexpensive raw materials. Thus, this system for the H2 production is feasible and low-cost.


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Figure 2. (a) XRD patterns and (b) EDS spectrum of bare ZnO and TiO2/ZnO nanowire arrays on stainless steel mesh substrate. Characterization of Nanowire Arrays. Figure 2 shows the XRD patterns and EDS spectrum of bare ZnO and TiO2/ZnO nanowire arrays. As shown in Figure 2a, the diffraction peaks around 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 72.6°, 77.0° and 81.4° can be indexed to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202) and (104) of ZnO crystal (JCPDS File No. 36-1451), respectively. The peaks around 25.3° can be indexed to (101) of TiO2 crystal (JCPDS File No. 21-1272). And other peaks around 43.6°, 50.8°, and 74.7° correspond to stainless steel mesh substrate. No other clear sharp diffraction peaks are coincident with the impurity of the sample, revealing that the product is TiO2/ZnO composite on stainless steel mesh with high crystalline purity. The EDS spectrum of bare ZnO and TiO2/ZnO nanowire arrays are shown in Figure 2b. Ti, Zn, O and Fe elements can be observed, and no other elements of impurity can be observed. These results further confirm the phase purity of the product.


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Figure 3. Morphology of bare ZnO and TiO2/ZnO nanowire arrays. SEM images with different view angles and magnifications of (a-f) bare ZnO nanowire arrays and (g-l) TiO2/ZnO nanowire arrays on stainless steel mesh substrate. (m, n) TEM images of TiO2/ZnO nanowire showing the interface between ZnO and TiO2.


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The SEM and TEM images of TiO2/ZnO nanowire are shown in Figure 3. Figure 3a-f are SEM images of bare ZnO nanowire arrays with different magnifications, showing the morphology. The nanowire arrays are vertically and uniformly aligned on the surface of mesh substrate, as shown in figure 3a-d. The cross section of ZnO nanowires is in hexagonal shape. Figure 3e and f are the side-view SEM images of bare ZnO nanowire arrays. These images illustrate that the length and diameter of ZnO nanowires are about 9 µm and 600 nm, respectively. The ZnO nanowires have clean surface and sharp edges, revealing that the nanowires are of good crystalline. It can be further confirmed that ZnO nanowire arrays are vertically aligned on the mesh substrate. Figure 3g-l are SEM images of TiO2/ZnO nanowires on mesh substrate. As shown in Figure 3g-j, the ZnO nanowire arrays after coating TiO2 nanoparticles are still vertically and uniformly aligned on the substrate. The nanowire density on the mesh substrate does not change. Side-view SEM images of TiO2/ZnO nanowire arrays (Figure 3k and l) clearly show the detail of core-shell heterostructures. The edge of the nanowires become rough, and the surface of ZnO nanowires is covered with a thin and uniform layer of TiO2 nanoparticles. This heterostructured nanowire is of large surface area, which can enhance the photocatalytic efficiency due to the large surfacesolution contact area. TEM images of TiO2/ZnO nanowires are shown in Figure 3m and n. As shown in Figure 3m, ZnO nanowire is covered by dispersed TiO2 nanoparticles, which confirms the quasi core-shell structure. High resolution TEM (HRTEM) image of one single TiO2/ZnO nanowire is shown in Figure 3n. The image is taken from the interface region between TiO2 and ZnO. The lattice fringes of 0.260 nm can correspond to (0002) plane of ZnO. The lattice fringes of 0.167 and


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0.189 nm can correspond to the (213) and (200) planes of anatase crystal structure of TiO2. These results further confirm the quasi core-shell heterostructure of TiO2/ZnO nanowires.

Figure 4. (a) Diffused reflectance UV spectrum of mesh-based TiO2/ZnO and ZnO nanowire arrays collected in absorbance mode. (b) Tauc’s plot of TiO2/ZnO and ZnO nanowire arrays. The UV-vis absorption spectra of TiO2/ZnO and ZnO nanowire arrays are shown in Figure 4a. Both TiO2/ZnO and ZnO exhibit intense absorption bands with a steep edge at about 390 nm, revealing that the band gaps of the two nanowire arrays are about 3.2 eV. And the two spectra shows strong absorption in UV region (λ

ZnO Nanowires Co-Using Solar and

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