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High Piezo-Photocatalytic Efficiency of CuS/ZnO Nanowires CoUsing Solar and Mechanical Energy for Degrading Organic Dye Deyi Hong, Weili Zang, Xiao Guo, Yongming Fu, Haoxuan He, Jing Sun, Li-Li Xing, Baodan Liu, and Xinyu Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05252 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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ACS Applied Materials & Interfaces

High Piezo-Photocatalytic Efficiency of CuS/ZnO Nanowires Co-Using Solar and Mechanical Energy for Degrading Organic Dye Deyi Hong,†,∇ Weili Zang, †,∇ Xiao Guo,† Yongming Fu,† Haoxuan He,† Jing Sun,† ,* Lili Xing,† Baodan Liu, ‡,* Xinyu Xue†,* †

College of Sciences, Northeastern University, Shenyang 110004, China

‡

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

(IMR), Chinese Academy of Sciences (CAS), Shenyang 110016, China ABSTRACT: High piezo-photocatalytic efficiency of degrading organic pollutants has been realized from CuS/ZnO nanowires co-using solar and mechanical energy. CuS/ZnO heterostructured nanowire arrays are compactly/vertically aligned on stainless steel mesh by a simple two-step wet-chemical method. The mesh-supported nanocomposites can facilitate an efficient light harvesting due to the large surface area, and can also be easily removed from the treated solution. Under both solar and ultrasonic irradiation, CuS/ZnO nanowires can rapidly degrade methylene blue (MB) in aqueous solution, and the recyclability is investigated. In this process, the ultrasonic assistance can greatly enhance the photocatalytic activity. Such a performance can be attributed to the coupling of the build-in electric field of heterostructures and the piezoelectric field of ZnO nanowires. The build-in electric field of the heterostructure can

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effectively separate the photo-generated electrons/holes and facilitate the carrier transportation. The CuS component can improve the visible light utilization. The piezoelectric field created by ZnO nanowires can further separate the photo-generated electrons/holes through driving them to migrate along opposite directions. The present results demonstrate a new water-pollution solution in green technologies for the environmental remediation at the industrial level. KEYWORDS: photocatalytic activity, piezoelectric effect, heterostructure, CuS/ZnO nanowire, piezo-photocatalytic process 1. INTRODUCTION In recent years, energy shortage and environmental pollution are worldwide crises that many researchers are trying to resolve. The wastewater treatment technique, such as photocatalytic degradation, is one of the most important solutions to environmental pollution.1-3 The nanostructured semiconducting photocatalyst can use solar energy to degrade organic pollutants into nontoxic molecules in aqueous solution, showing significant potentials for purifying polluted-water as a green technique.4-5 For example, ZnO nanostructure has been recognized as an excellent photocatalyst for the degradation of environmental pollutants. This is mainly due to its interesting properties for practical applications, such as a direct and wide band gap (∼3.37 eV, UV wavelength), extremely large exciton binding energy (∼60 meV), abundant surface defects and oxygen vacancies (green light absorption), and other remarkable optical and piezoelectric properties. Additionally, ZnO is nontoxic, abundant in reserves, and easy to obtain, which make it an ideal candidate for practical applications on environmental treatments.6-9 In a typical photocatalytic process, the photo-generated electrons/holes migrate to the surface of ZnO nanostructures to participate in the oxidative/reductive processes on the surface, degrading organic pollutants into nontoxic inorganic molecules.10-11 However, in order to meet the practical


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application requirements, more efforts need to be made to enhance the photocatalytic efficiency of ZnO, such as enhancing the utilization of visible light and lowering down the recombination rate of photo-generated electrons and holes. Nowadays, some research effort has been made on developing ZnO-based nanocomposites to obtain high visible light utilization rate and high photocatalytic efficiency, such as synthesizing heterostructured nanomateirals.12-15 The heterojunctions at the interfaces can induce energy band bending and establish build-in electric field around the surface of the two components. The build-in electric field can separate the photo-generated electrons/holes, facilitate the carrier transportation and lower down the recombination rate. At the same time, the adsorption rate of visible light can be enhanced by the surface modification of narrow-band-gap components, such as CuS, CuO and CdS.16-19 On the other hand, introducing piezoelectric effect of ZnO nanostructures into the photocatalytic process has been demonstrated as an effective way for enhancing the photocatalytic efficiency.20-21 Our previous work has demonstrated that the piezoelectric field created by ZnO nanowires can separate the photo-generated electrons/holes through driving them to migrate along opposite directions, lowering down the recombination rate.20 In this paper, both the build-in electric field and piezoelectric field are introduced into the photocatalytic process of CuS/ZnO nanowires, and high piezo-photocatalytic efficiency of cousing solar and mechanical energy has been realized. CuS/ZnO heterostructured nanowire arrays are vertically aligned on stainless steel mesh by a simple two-step wet-chemical method. Under both solar and ultrasonic irradiation, CuS/ZnO nanowires can rapidly degrade methylene blue (MB) in aqueous solution, and the recyclability is investigated. The ultrasonic assistance can greatly enhance the photocatalytic activity. This new water-pollution solution could have



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potential applications in green technologies for the environmental remediation at the industrial level. 2. EXPERIMENTAL SECTION 2.1. Materials. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), hexamethylenetetramine (HMTA, C6H12N4), copper sulfate pentahydrate (CuSO4·5H2O), sodium sulfide monohydrate (Na2S·9H2O), methylene blue (C16H18ClN3S) and absolute ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification. The stainless steel mesh as substrate was purchased from a supermarket. 2.2 Synthesis of CuS/ZnO Nanowire Arrays. ZnO nanowire arrays were synthesized on stainless steel mesh by a seed-assisted hydrothermal method.22 Before growth, a ZnO seed layer was deposited on the stainless steel mesh. 44 mg of zinc acetate dihydrate was dissolved in 200 mL of ethanol and stirred for 30 min to yield seeding suspension. A piece of pre-cleaned stainless steel mesh (400 mesh, 6.0 × 6.0 cm in area) was dipped into the solution for 4 min, and dried at 60℃. The mesh was annealed in air at 350℃ for 20 min to yield a compact ZnO seed layer on the surface. The ZnO nanowire arrays were then grown on the as-seeded stainless steel mesh via a hydrothermal route. 1.40 g of HMTA and 2.97 g of zinc acetate dihydrate was dissolved in 200 mL of distilled water and stirred for 30 min. The mesh substrate was immersed in the solution. The beaker was sealed and maintained at 80℃ for 12 h. After cooled to room temperature, the mesh substrate was removed from the solution, rinsed with distilled water and ethanol, and dried in air at 60℃. The ZnO nanowire arrays on the mesh substrate were coated with CuS nanoparticles via a successive ionic layer adsorption and reaction (SILAR) method.23 The mesh substrate was immersed into 10 mM sodium sulfide aqueous solution for 30 s and subsequently dipped in 5


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mM copper sulfate aqueous solution for 30 s. The mesh substrate was rinsed with distilled water for 1 min. This procedure was repeated for three times to achieve a uniform CuS nanoparticle layer on the surface of ZnO. Finally, the products were dried in air at 60℃. The weight of CuS/ZnO nanowires on the mesh substrate (6.0 × 6.0 cm) was about 100 mg. 2.3. Characterization. X-ray diffraction (XRD, D/max 2550 V, CuKα Radiation) was used to characterize the crystal phase of CuS/ZnO nanowire arrays on stainless steel mesh. Scanning electron microscope (SEM, JEOL JSM-6700F) and transmission electron microscope (TEM, JEOL JEM-2010) were used to determine their morphologies and microstructures. UV-vis diffuse reflectance spectra were measured at room temperature in the 200-800 nm wavelength range using an UV−vis-NIR spectrometer. Photoluminescence (PL) measurements on CuS/ZnO nanowire arrays were performed with a He-Cd cw laser at room temperature. The specific surface area (SBET) was calculated using the Brunauer−Emmett−Teller (BET) equation between 0.05 and 0.30 relative pressure (P/P0). The surface elements and their electronic states of the sample was analyzed using X-ray photoelectron spectroscopy (XPS). 2.4. Piezo-Photocatalytic Activity Measurement. To evaluate the piezo-photocatalytic activity of CuS/ZnO nanowires, MB was chosen as the probe molecule. The MB degradation by the piezo-photocatalytic activity of CuS/ZnO nanowires on stainless steel mesh was investigated in aqueous solution under simulated solar and ultrasonic irradiation. The mesh sample (6.0 × 6.0 cm, and the weight of the nanowires was ~100 mg) was immersed into 50 mL of MB aqueous solution (5 mg/L). Before irradiation, the mixture containing the photocatalyst and MB was stirred in the dark for 30 min to reach adsorption-desorption equilibrium between the MB solution and the photocatalyst. The solar light was simulated with a xenon lamp (500 W, 2001100 nm), that contains ~6% of UV. And the irradiance of natural solar (600-1000 W/m2) is



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about 10 times larger than that of xenon lamp (~85 W/m2 for ~18 cm distance between the photocatalyst and xenon lamp in the test). The UV light of the xenon lamp can participate in the photocatalytic process. In real sunlight, almost none wavelength below 350 nm is present. Thus the solar light was roughly simulated with the xenon lamp. The ultrasonic irradiation was provided by an ultrasonic probe (200 W). During the experiment, the solution was kept homogeneous by slight stirring, and an electric fan was used to avoid the temperature rise. 2 mL of the tested MB solution was taken out every 4 min and analyzed by a UV-vis spectrometer (Hitachi U-3010). The MB concentration was obtained at λ= 664 nm (the maximum absorption peak of MB).

Figure 1. (a) Synthesis procedure of CuS/ZnO nanowire arrays on stainless steel mesh. (b) Optical image of the mesh substrate before and after growing the nanowire arrays. (c) The degradation of MB solution by the piezo-photocatalytic activity of CuS/ZnO nanowires under both solar and ultrasonic irradiation. (d) Practical application of CuS/ZnO nanowires arrays on


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stainless steel mesh co-using solar and mechanical energy in the nature for water-pollution treatment. 3. RESULTS AND DISCUSSION 3.1. Experimental Design. The design of this work is illustrated in Figure 1. CuS/ZnO nanowire arrays are vertically aligned on stainless steel mesh by a simple two-step wet-chemical method (Figure 1a). As shown Figure 1b, the mesh substrate is flexible and the nanowires are grown on the whole mesh. The deformation can be easily applied on the mesh, and the piezoelectric field can be created along the nanowires. The mesh substrate can facilitate the light harvesting due to the large surface area, and can also be easily removed from the treated solution, suitable for practical operation. The degradation of MB by the piezo-photocatalytic activity of CuS/ZnO nanowire arrays is performed in aqueous solution under both solar and ultrasonic irradiation, as shown in Figure 1c. The inset shows the color change of the MB solution. Figure 1d simply shows the practical application of CuS/ZnO nanowire arrays on stainless steel mesh co-using solar and mechanical energy in the nature for water-pollution treatment. All the chemicals for synthesizing ZnO and CuS are inexpensive and common (Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, C6H12N4, CuSO4·5H2O, Na2S·9H2O). The stainless steel mesh as substrate can be purchased from supermarket. The synthesis of ZnO is carried out at low temperature (80ºC). The SILAR method for coating CuS is conducted at room temperature. In natural river, even though the energy conversion efficiency of vibration is low, it is very common that the water flow with the speed of 0.2 m3/s and 1-metre water level difference can generate the power of 200 W.24-25 The solar energy is unexhausted. Thus the material system and the piezo-photocatalytic process for environmental remediation are economic for industrial production.



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Figure 2. (a) XRD patterns of bare ZnO and CuS/ZnO nanowires on stainless steel mesh. (b) EDS spectra of bare ZnO and CuS/ZnO nanowires on stainless steel mesh. 3.2. Structure, morphology and optical properties. Figure 2a shows XRD patterns of bare ZnO and CuS/ZnO nanowires on stainless steel mesh. The sharp diffraction peaks indicate good crystalline quality. The diffraction peaks around 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, and 68.0° can be indexed to (100), (002), (101), (102), (110), (103), and (112) of ZnO crystal (JCPDS 36-1451), respectively. The diffraction peaks around 29.3°, 31.8°, 47.9° and 67.3° can be indexed to (102), (103), (110) and (118) of CuS crystal (JCPDS 06-0464), respectively. The CuS (103) peak is very close to the ZnO (100) peak and thus they are overlapped around 31.8° in the pattern. The diffraction peaks around 43.6°, 50.8° and 74.7° can be indexed to stainless steel. No other peaks for impurity can be observed, suggesting that the compositions of the sample are ZnO, CuS and stainless steel. Figure 2b is energy dispersive X-ray spectra (EDS) of bare ZnO and CuS/ZnO nanowires on stainless steel mesh, showing the element components. Four elements (O, Zn, S and Cu) can be observed in CuS/ZnO nanowire on the mesh, further confirming the CuS and ZnO components. The Fe peak arises from the stainless steel mesh.


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Figure 3. (a)-(f) SEM images of bare ZnO nanowire arrays on stainless steel mesh with different magnifications and view angles. (g)-(l) SEM images of CuS/ZnO nanowire arrays on stainless steel mesh with different magnifications and view angles. (m) & (n) TEM images of CuS/ZnO nanowire. 9 - Environment ACS Paragon -Plus


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Figure 3a-f are SEM images of bare ZnO nanowire arrays on stainless steel mesh with different magnifications and view angles. It can be seen that the whole surface of the stainless steel mesh (the diameter is 25 µm) are compactly covered with the nanowire arrays, and the nanowire arrays are vertically and uniformly aligned on the mesh. Figure 3d and 3e are highmagnification SEM images of bare ZnO nanowires on the top view, showing that the crosssectional shape of ZnO nanowires is hexagonal. The uniformity of the nanowire arrays is high, and the average diameter is about 100 nm. Figure 3f is SEM image of bare ZnO nanowire arrays on the side view, further confirming that the nanowire arrays are vertically aligned on the surface of the mesh. The average length of the nanowires is about 4 µm. Figure 3g-l are SEM images of CuS/ZnO nanowire arrays on stainless steel mesh with different magnifications and view angles. It can be seen that the whole surface of the mesh is still compactly covered with the nanowire arrays, and the nanowire arrays are still vertically/uniformly aligned on the mesh. During the coating process, the general morphology of the nanowires does not change, and the nanowires do not fall off. Figure 3j and 3k are highmagnification SEM images of CuS/ZnO nanowires on the top view, showing more detailed structural characteristics of the core-shell heteroarchitectures. The whole surface of all ZnO nanowires is uniformly coated with CuS nanoparticles, and the average diameter of the composite nanowires is about 150 nm. The CuS layer enlarges the diameter of the nanowire. Figure 3l is SEM image of CuS/ZnO nanowire arrays on the side view, further confirming that the nanowire arrays are still vertically aligned on the mesh. The surface area of CuS/ZnO heterostructured nanowires as determined from BET analysis is ~14 m2/g (Figure S1). The large surface area of the nanostructured photocatalysts can promote a large amount of surface active adsorption sites and photocatalytic reaction centers, leading to high photocatalytic efficiency.

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Figure 3m is TEM image of one single CuS/ZnO nanowire, further confirming the core-shell structure. The CuS nanoparticles are very small and the whole surface of all ZnO nanowires is uniformly coated with CuS nanoparticles. The CuS layer is about 15 nm in thickness. Figure 3n is high-resolution TEM image of one single CuS/ZnO nanowire (tip region). The lattice spacing of 0.26 and 0.31 nm correspond to ZnO (002) and CuS (102) planes, respectively, and it can also be concluded that ZnO nanowires grow along the [001] direction (c-axis).

Figure 4. (a) UV-vis absorption spectrum of CuS/ZnO heterostructured nanowires on stainless steel mesh; (b) Direct band gap model of CuS/ZnO heterostructured nanowires on stainless steel mesh. Figure 4a shows the UV-vis diffuse reflectance spectrum of CuS/ZnO heterostructured nanowire arrays on stainless steel mesh. As expected, CuS/ZnO heterostructured nanowires clearly exhibit a broad and strong absorption in the visible light range (400-500 nm) as well as in the near-IR region (700-800 nm), which imply that the heterostructured nanowires might display good photocatalytic behavior under visible light irradiation. The additional absorption in the range of 700-800 nm is due to the d-d transition of Cu2+.26 The band gap energy (Eg) of CuS/ZnO can be calculated using the Tauc’s relation αhν / Ahν E ,27-29 where A is a constant



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and the exponent n depends on the type of transition, n=1/2 and 2 for direct and indirect transitions, respectively. To find the optical band gap of the sample we have studied the variation of (αhν)2 versus hν for direct band gap, as shown in Figure 4b. The band gap value is calculated to be about 2.39 eV. The narrow band gap of the heterostructured nanowires is attributed to the incorporation of CuS with the absorption range of 200-600 nm.30-31

Figure 5. Room-temperature PL spectrum of CuS/ZnO heterostructured nanowire arrays on stainless steel mesh. The inset shows the magnified view of the peaks at 406 and 420 nm. Figure 5 shows the room-temperature PL spectrum of CuS/ZnO heterostructured nanowire arrays on stainless steel mesh. The sample displays an ultraviolet (UV) emission peak with maximal intensity at around 388 nm (~3.2 eV), which is attributed to near band-edge (NBE) emission of ZnO. The strong violet peak located at 406 nm (3.05 eV) is ascribed to the energy difference between the bottom of the conduction band and energy level of zinc vacancy. The defect peak at 420 nm (2.95 eV) is due to oxygen and zinc vacancies, and also attributed to


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interstitial oxygen.32 The sample shows a broad emission peak in the range of 500-600 nm. This broad emission peak has been widely reported for ZnO nanostructures, which can be attributed to electron transition from zinc interstitial or oxygen vacancy to the top of the valence band.33,34 From the emission range of 500-580 nm, multi-peaks can be observed, which can probably be ascribed to the band-edge emission of CuS, the blue-shift of the band-edge mission of small CuS nanoparticles (quantum confinement effect), and complex defects of CuS nanostructures.35,36



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Figure 6. XPS patterns of CuS/ZnO heterostructured nanowire arrays on stainless steel mesh. (a) XPS survey spectrum; (b) C 1s region XPS spectrum; (c) Zn 2p region XPS spectrum; (d) O 1s region XPS spectrum; (e) Cu 2p region XPS spectrum; (f) S 2p region XPS spectrum of CuS/ZnO heterostructured nanowire arrays on stainless steel mesh.


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The electronic state and the chemical composition of CuS/ZnO heterostructured nanowires can be further provided by XPS measurements. The wide scan XPS spectra of CuS/ZnO nanowires (Figure 6) clearly indicate that the sample is composed of Cu, S, Zn, O and C, and no peaks of other elements can be observed. The high-resolution XPS spectra of C 1s, Zn 2p, O 1s, Cu 2p, and S 2p are shown in Figure 6b-f, respectively. The weak peaks of C 1s (284.8 eV) come from CO2 adsorbed on the surface of the sample and adventitious hydrocarbon from the XPS instrument itself. As shown in Figure 6, the two strong peaks at 1023.7 and 1046.8 eV are assigned to the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively, suggesting the existence of Zn2+. The peak of 531.9 eV is ascribed to O 1s. The binding energies of Cu 2p1/2 and Cu 2p3/2 are observed at 952.0 and 932.1 eV, respectively. In addition, these peaks are accompanied by weak shake-up satellite peaks at approximately 943 eV, which indicates the presence of Cu2+.31, 37 The peaks at 168.9 and 162.2 eV are ascribed to S 2p1/2 and S 2p3/2, respectively. Toupance et al. have demonstrated that energy band strongly depends on the morphology, the size and exposed facets of the nanomaterials. The exact energy band alignment of CuS/ZnO heterostructured nanowires can be determined by analyzing XPS, UV-vis and PL results.28, 38-39 The valence band (VB) offset ∆EV of CuS/ZnO heterostructures can be calculated according to the equation ( ∆E ∙ ∙ ∙ ∙ ∆E! ), where ∆E! ∙"

∙" #$%$&'(%&)%&$ is the energy difference between Zn 2p and Cu 2p core levels (CLs) in CuS/ZnO heterostructures; ∙ and ∙ are the binding energies in bulk ZnO and CuS, respectively; ∙ and ∙ are the VB maxima in bulk ZnO and CuS, respectively. And the VB spectra of pure CuS and ZnO could be tested using Ultraviolet Photoelectron Spectroscopy (UPS) measurement for providing the values of VB maxima in bulk ZnO and CuS. In future



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work, the VB spectra (UPS analysis) of individual components need to be measured for confirming the exact band alignment at CuS/ZnO junction.

Figure 7. Piezo-photocatalytic degradation of MB by CuS/ZnO nanowires on stainless steel mesh under different experimental conditions: (a) UV-vis spectra of MB solution upon piezophotodegradation catalyzed by CuS/ZnO nanowires on the mesh under simulated solar and


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ultrasonic irradiation (200 W). The inset is optical image of MB solutions upon degradation at different time. (b) UV-vis spectra of MB solution upon photodegradation catalyzed by CuS/ZnO nanowires on the mesh under only simulated solar irradiation (without ultrasonic irradiation). The inset is optical image of MB solutions upon degradation at different time. (c) Degradation profiles of MB as a function of irradiation time for bare ZnO and CuS/ZnO nanowires on the mesh under different conditions. (d) Photocatalytic degradation kinetic curves of MB solution catalyzed by bare ZnO and CuS/ZnO nanowires on stainless steel mesh under different experimental conditions (the self-degradation of MB is also investigated). 3.3. Piezo-Photocatalytic Performance. Figure 7a and b show the piezo-photocatalytic and photocatalytic activity of CuS/ZnO nanowire arrays on stainless steel mesh, respectively. The corresponding optical images of the MB solution with different operation time are inserted below. It can be seen that the ultrasonic irradiation can greatly improve the photocatalytic efficiency. Under both solar and ultrasonic irradiation, the piezo-photocatalytic efficiency is extremely high. MB can be completely degraded within 20 min, as shown in Figure 7a. The ultrasonic power is 200 W, and the solar light power is 500 W. Under only solar irradiation, MB can be degraded by merely 63% within 20 min, as shown in Figure 7b. For comparison, the photodegradation ability of bare ZnO and CuS/ZnO nanowires under different irradiation conditions (only solar irradiation, only ultrasonic irradiation, both solar and ultrasonic irradiation) have been investigated, as shown in Figure 7c. The percentage of degradation is defined as C/C0 (C is test concentration, C0 is initial concentration). The selfdegradation of MB under both solar and ultrasonic irradiation can be neglected, demonstrating that the degradation of MB arises from the catalytic effect. Under only ultrasonic irradiation, the degradation of MB by both bare ZnO and CuS/ZnO nanowires is very limited, confirming that



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the photo-generated electrons/holes play the key role for degrading MB, and mechanical vibration cannot generate extra electrons/holes. For bare ZnO nanowires, photocatalytic activity under only solar irradiation is very low due to the low utilization rate of visible light. The piezophotocatalytic activity of bare ZnO nanowires under both solar and ultrasonic irradiation is slightly higher due to the separation of photo-generated electrons/holes by the piezoelectric field. For CuS/ZnO nanowires, the photocatalytic activity under only solar irradiation is high, and after 20 min the concentration of MB decreases to 37%. For CuS/ZnO nanowires, under both solar and ultrasonic irradiation, the piezo-photocatalytic activity is extremely high. Within 20 min almost all MB is degraded. In order to better understand the photocatalytic efficiency of CuS/ZnO nanowires, the photocatalytic degradation kinetics of MB is investigated. Figure 7d illustrates the photocatalytic degradation kinetics of CuS/ZnO and bare ZnO nanowires under different experimental conditions (the self-degradation of MB is also investigated). According to Langmuir-Hinshelwood model, the photocatalytic degradation processes can be fit to pseudo first order kinetics (lnC/C- kt), where k is the photocatalytic reaction apparent rate constant; C0 is the initial concentration of MB solution after equilibrium adsorption; C is the concentration during the reaction and t is the time of piezo-photodegradation.40-42 It can be seen that the k values for CuS/ZnO nanowires (under both solar and ultrasonic irradiation), CuS/ZnO nanowires (under only solar irradiation), CuS/ZnO nanowires (under only ultrasonic irradiation), ZnO nanowires (under both solar and ultrasonic irradiation), ZnO nanowires (under only solar irradiation) and ZnO nanowires (under only ultrasonic irradiation) are 0.18236, 0.04635, 0.00115, 0.00643, 0.00213 and 0.00044 min-1, respectively. Obviously, the k value of CuS/ZnO nanowires under both solar and ultrasonic irradiation is far higher than those of the others,


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indicating that piezoelectric field can effectively enhance the photocatalytic activity of CuS/ZnO nanowires. Compared with TiO2 powders (P25) and other previously reported photocatalysts (Table S1),1117, 48-51

CuS/ZnO nanowire arrays on stainless steel mesh co-using solar and mechanical energy

have very high piezo-photocatalytic activity, showing potential applications for the environment improvement at the industrial level. As shown in Figure S2, the ultrasonic irradiation cannot enhance the photocatalytic efficiency of TiO2 because TiO2 is not a piezoelectric material, further confirming the piezo-photocatalytic process of ZnO-based nanocomposites. It has been reported that the isopotential point (pHiep) of ZnO surface is about 9.0 and pHiep of CuS surface is less than 3.0.43-46 It can be seen from the SEM and TEM images of CuS/ZnO nanowires that the whole surface of ZnO nanowires are uniformly coated with CuS nanoparticles and thus the pHiep of CuS/ZnO nanowires is mainly dominated by CuS surface (less than 3.0). The pH value of the solution after the introduction of CuS/ZnO nanowires into the MB solution is 6.9 (measured by a portable pH meter) which is larger than the pHiep of CuS/ZnO nanowires. Therefore, the surface of CuS/ZnO nanowires possesses many negative surface charges. It has been reported that MB is a kind of monovalent cation dye, thus it can be easily absorbed on the surface of CuS/ZnO nanowires through electrostatic interaction.47 It can be seen from Figure S3 that 7% of MB is absorbed on the surface after 30-min adsorption equilibrium in dark, confirming the above discussion. During the photocatalytic process, the adsorption of MB on the surface of CuS/ZnO nanowires can promote the degradation of MB. The mesh number of stainless steel mesh has great influence on the growing morphologies of CuS/ZnO nanowire arrays and the piezo-photocatalytic activity. Figure 8a-d show the morphology of CuS/ZnO nanowire arrays grown on 50 mesh stainless steel mesh, and Figure 8e-



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h show the morphology of the nanowires grown on 400 mesh stainless steel mesh. CuS/ZnO nanowire arrays are uniformly grown on the two stainless steel mesh substrates. It can be clearly seen that CuS/ZnO nanowire arrays grown on 400 mesh stainless steel mesh have high growth density than the nanowire arrays on 50 mesh stainless steel mesh. The diameters of CuS/ZnO nanowires on 50 and 400 mesh stainless steel mesh are ~500 and ~150 nm, respectively. Figure 8i shows the piezo-photocatalytic activity of CuS/ZnO nanowire arrays on different mesh numbers of stainless steel mesh. It can be seen that the piezo-photocatalytic performance increases with increasing mesh number of the stainless steel mesh substrate due to the decreasing diameter of CuS/ZnO nanowires.


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Figure 8. (a)-(d) SEM images of CuS/ZnO nanowire arrays on stainless steel mesh (50 mesh) with different magnifications. (e)-(h) SEM images of CuS/ZnO nanowire arrays on stainless steel mesh (400 mesh) with different magnifications. (i) Piezo-photocatalytic degradation of MB by CuS/ZnO nanowires grown on different mesh numbers of stainless steel mesh under simulated solar and ultrasonic irradiation (200 W).

Figure 9. Degradation profiles of MB solution catalyzed by CuS/ZnO nanowires on stainless steel mesh under different ultrasonic irradiation. The solar irradiation keeps at 500 W. Figure 9 shows the photodegradation profiles of MB solution catalyzed by CuS/ZnO nanowire arrays on stainless steel mesh under different ultrasonic irradiation powers (0, 100, and 200 W). The solar light power in the tests keeps constant (500 W). It can be seen that the piezophotocatalytic activity increases with increasing power of ultrasonic irradiation. Higher ultrasonic power can result in larger deformation on the nanowires, generating stronger piezoelectric field.


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Figure 10. (a) The recyclability of the piezo-photocatalytic activity of CuS/ZnO nanowire arrays on stainless steel mesh for degrading MB solution (the ultrasonic irradiation power is 200 W, and the solar light power is 500 W). (b)-(d) SEM images of CuS/ZnO nanowire arrays on stainless steel mesh with different magnifications and view angles after the 6th piezo-photocatalytic measurement. (e) & (f) TEM and HRTEM images of one single CuS/ZnO nanowire after the 6th piezo-photocatalytic measurement. (g) XRD pattern and (h) EDS spectrum of CuS/ZnO nanowires on stainless steel mesh after the 6th piezo-photocatalytic measurement. Figure 10a shows the recyclability of the piezo-photocatalytic activity of CuS/ZnO nanowire arrays on stainless steel mesh for degrading MB solution (the ultrasonic irradiation power is 200 W, and the solar light power is 500 W). The mesh-based material can be easily removed from the treated water, which can facilitate the practical application. For the 1st, 2nd, 3rd, 4th, 5th and 6th cycle, within 20 min, the degradation of MB is 99%, 96%, 92%, 89%, 88% and 84%, respectively. It should be noted that the degradation efficiency of CuS/ZnO nanowires slightly decreases after several degradation cycles because of the loss of the material and the probable photocorrosion of CuS nanoparticles. Figure 10b-d are SEM images of CuS/ZnO nanowire arrays after the 6th piezo-photocatalytic experiment. The general morphology of the composite nanowires does not change; CuS nanoparticles are still uniformly coated on ZnO nanowires; and CuS/ZnO nanowires are uniformly and vertically aligned on the stainless steel mesh. Figure 10e and Figure 10f are TEM images of CuS/ZnO nanowire after piezo-photocatalytic experiment. It can be seen that the amount of CuS nanoparticles after piezo-photocatalytic experiment is a little smaller than that before the measurement. Figure 10g shows XRD pattern of CuS/ZnO nanowires on stainless steel mesh after piezo-photocatalytic experiment. The composite nanowires are still composed of CuS and ZnO, and the crystal quality does not change. And there


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are no other clear sharp peaks coincident with the impurities. Figure 10h shows EDS spectrum of CuS/ZnO nanowire arrays after piezo-photocatalytic experiment. It can be seen that no new elements take place after the piezo-photocatalytic measurement.

Figure 11. (a) Schematic illustration showing the piezo-photocatalytic process of CuS/ZnO nanowires on stainless steel mesh under both solar and ultrasonic irradiation. (b) Schematic illustration showing the energy band diagram of CuS/ZnO heterostructure under both solar and ultrasonic irradiation. 3.4. Mechanism of Piezo-Photocatalytic Process. The working mechanism for the piezophotocatalytic process of CuS/ZnO nanowires on stainless steel mesh under both solar and ultrasonic irradiation is shown in Figure 11. The high piezo-photocatalytic activity of CuS/ZnO nanowires on the mesh can be attributed to the coupling of the build-in electric field of heterostructures and the piezoelectric field of ZnO nanowires (Figure 11a). The energy band structure of CuS/ZnO nanowires under both solar and ultrasonic irradiation is shown in Figure 11b.



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It has been demonstrated by many literatures that ZnO and CuS are n-type and p-type semiconductors, respectively.52-53 The electron affinity, work function and band gap of ZnO are 4.02, 5.30 and 3.2 eV, respectively. The electron affinity, work function and band gap of CuS are -3.89, 100 MPa).21,

59

As the material system is under ultrasonic

irradiation (Figure 11b), a pressure of water exerting can lead to the bending of ZnO nanowires. A piezoelectric field can be created crossing ZnO nanowires with negative piezo-potential on the compressive strain region and positive piezo-potential on the tensile strain region.60-61 The piezoelectric field inside the composite nanowires can drive the photo-generated electrons/holes to migrate along opposite directions. The photo-generated electrons migrate to the positive piezo-potential surface, and the holes migrate to the negative piezo-potential surface.62 This


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piezoelectric field is a driving force for promoting the separation and transportation of the photogenerated electron-hole pairs, avoiding charge recombination and enhancing the photocatalytic activity.63 The photo-generated holes can rapidly move to the surface of the nanowires due to the short transportation path of nanostructures and the electric fields, directly reacting with MB or water to produce highly-reactive hydroxyl radicals. The photo-generated electrons on the surface of nanowires can react with dissolved oxygen to yield superoxide radicals and subsequently produce. The hydroxyl radicals can effectively degrade MB due to their high oxidability. Many literatures have demonstrated that the degradation of MB by photocatalysts can produce inorganic ions (SO42−, NO3−, Cl−) and CO2 (or HCOOH).49,

58, 64-67

The series of chemical

reactions for the piezo-photocatalytic degradation of MB can be expressed as follows: 68-69 789:;/?@8 9>9BCD8B E89>?F

CuS/ZnO + ℎ6 GHHHHHHHHHHHHHHHHHHHHHI CuShJ + ZnOe@

(1)

OH @ + hJ → • OH

(2)

O + e@ → • O@

(3)

H J +• O@ → HO •

(4)

2HO • → H O + O

(5)

@ • O@ + H O → • OH + OH + O

(6)

• OH + MB → inorganic ions + H O + CO HCOOH

(7)

4. CONCLUSION



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In summary, CuS/ZnO heterostructured nanowire arrays are vertically and compactly aligned on stainless steel mesh by a simple two-step wet-chemical method. The composite nanowires exhibit extremely high piezo-photocatalytic activity for degrading MB under both solar and ultrasonic irradiation. For 100 mg of CuS/ZnO nanowires, under both solar (500 W xenon lamp) and ultrasonic irradiation (200 W), almost all of MB in aqueous solution (5 mg/L, 50 mL) can be completely degraded within 20 min. Such a high performance can be attributed to the coupling between the build-in electric field of heterostructures and the piezoelectric field of ZnO nanowires. The present results demonstrate a new water-pollution solution in green technologies for the environmental remediation at the industrial level. For practical application, the recyclability of CuS/ZnO nanowires as piezo-photocatalyst needs to be further enhanced, and the exact band alignment at CuS/ZnO junction needs to be characterized in future work. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Comparison of photocatalytic activity between previous results and this work, BET data, MB degradation curves by commercial TiO2 powders (P25) under UV and ultrasonic irradiation, and UV-vis spectra of MB solution vigorously stirred at room temperature and kept in the dark for 30 min to ensure an adsorption/desorption equilibrium. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]


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* E-mail: [email protected] Author Contributions ∇These

authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51102041 and 11104025), the Fundamental Research Funds for the Central Universities (N120205001 and N140505004), and Program for New Century Excellent Talents in University (NCET-13-0112). REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W., Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. (2) Cao, S. W.; Low, J. X.; Yu, J. G.; Jaroniec, M., Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater. 2015, 27, 2150-2176. (3) Zhou, H. L.; Qu, Y. Q.; Zeid, T.; Duan, X. F., Towards Highly Efficient Photocatalysts Using Semiconductor Nanoarchitectures. Energy Environ. Sci. 2012, 5, 6732-6743. (4) Xu, H.; Ouyang, S. X.; Liu, L. Q.; Reunchan, P.; Umezawa, N.; Ye, J. H., Recent Advances in TiO2-Based Photocatalysis. J. Mater. Chem. A 2014, 2, 12642-12661.



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High Piezo-Photocatalytic Efficiency of CuS/ZnO Nanowires Co-Using Solar and Mechanical Energy for Degrading Organic Dye


ZnO Nanowires Using

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