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Photocatalytic degradation of organic dyes using ZnO nanorods supported by stainless steel wire mesh deposited by one-step method
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Photocatalytic degradation of organic dyes using ZnO nanorods supported by stainless steel wire mesh deposited by one-step method
T
Linhua Xua,b,*, Fenglin Xiana,b, Shixin Peia,b, Yunguang Zhua,c a
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing, 210044, China c Physics Experiment Center, Nanjing University of Information Science & Technology, Nanjing, 210044, China b
A R T IC LE I N F O
ABS TRA CT
Keywords: Photocatalyst ZnO nanorods One-step method Photoluminescence Stainless steel wire mesh
In this work, a sol-gel method was used to deposit Al-doped ZnO coatings on stainless steel wire meshes. The effect of coating thickness on the morphology evolution and the photoluminescence, photocatalytic properties were investigated. Field emission scanning electron microscope (FESEM) observed that when the Al-doped ZnO coating was thin, it was composed of many nanoparticles. However, with the increase of the coating thickness, some sparse nanorods appeared on the ZnO coating surface. When the coating thickness was increased to six layers, the areal density of nanorods was largely increased. X-ray diffraction (XRD) patterns revealed that the ZnO coatings crystallized into a wurtzite structure, and the nanorods were preferentially oriented along the c-axis direction. Photoluminescence spectra showed that after ZnO nanorods were formed, the UV emission of ZnO coatings was significantly enhanced, which meant that the nanorods had better crystal quality than the nanoparticles. The photocatalytic performance tests for these samples showed that the sample formed by six layers of ZnO sol had the best photocatalytic activity due to its high crystalline quality and the large specific surface area. These ZnO coatings loaded on stainless steel wire meshes exhibited good photocatalytic stability.
1. Introduction Since the beginning of the 21st century, the two major problems of environmental pollution and energy shortage faced by mankind have become increasingly serious. While solar energy is an inexhaustible source of clean energy, how to effectively use solar energy has naturally caused widespread concern throughout the world. So far, solar energy has been widely used by means of solar water heaters and solar cells. In addition, the use of solar energy to treat sewage is also attracting more and more attention [1–6]. For the use of solar energy to treat sewage, the key factor is photocatalyst. Semiconductor photocatalysts can absorb photon energy and then generate photoinduced holes and electrons which react with water molecules and oxygen to in turn produce hydroxyl radicals and superoxide radicals, which can degrade organic pollutants into carbon dioxide, water and harmless inorganic small molecules. Semiconductor photocatalysts have been studied in the form of unsupported powders and thin films or nanostructures supported by substrates. As unsupported powder photocatalysts are not easy to separate from water after use, but also easy to cause secondary ⁎ Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing, 210044, China. E-mail address: [email protected] (L. Xu).
https://doi.org/10.1016/j.ijleo.2019.164036 Received 4 November 2019; Accepted 10 December 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 1. The photograph of stainless steel wire mesh floating on water.
pollution, so the photocatalysts loaded on the substrates will have a wider application. Since glass is cheap and easy to obtain, photocatalysts supported by glass have been extensively studied. In addition to glass, stainless steel wire mesh can also be used as a photocatalyst support. The materials loaded on stainless steel wire meshes can be applied in different fields, such as photocatalytic hydrogen production [7], potassium ion batteries [8], supercapacitors [9], dye-sensitized solar cells [10], oil-water separation [11]. As a support of photocatalyst, unlike glass which must be immersed in liquid, stainless steel wire mesh can not only submerge in water, but also float on water (as shown in Fig. 1), which is more conducive to absorbing sunlight. Therefore, the study on photocatalysts loaded on stainless steel wire meshes has important practical significance for sewage treatment. ZnO nanomaterials, especially the ZnO nanorods, have been widely investigated as photocatalysts. Due to the low cost and high production efficiency, the chemical solution methods, especially the hydrothermal method, have been widely used to grow ZnO nanorods for photocatalytic application. However, the hydrothermal method for growth of ZnO nanorods on substrates requires two steps [12–14]: first, the seed layer is deposited on the substrate, and then the ZnO nanorods are grown on the seed layer. In this study, we have obtained ZnO nanorods by one-step process. The ZnO nanorods on stainless steel wire meshes exhibit higher photocatalytic activity than ZnO coatings deposited on stainless steel wire mesh and glass substrate. The details are described below. 2. Experiments The Al-doped ZnO coatings were deposited on stainless steel wire meshes using a wet chemical method, namely the sol-gel method. The raw materials for preparing the ZnO sol were: zinc acetate (the precursor of Zn), aluminum chloride (the doping source of Al), ethanol (solvent), and monoethanolamine (stabilizer). The molar ratio of Al3+ to Zn2+ was 0.04 in the sol; the concentration of zinc acetate in the ethanol was 0.3 mol/L. The ZnO sol was coated on the thoroughly cleaned stainless steel wire meshes by a dipcoating method. The Al-doped ZnO coating was formed after a heat treatment. In order to analyze the effect of different thickness on the morphology and photocatalytic activity of the ZnO coating, 2, 4, 6 and 8 layers of Al-doped ZnO sol were coated on four stainless steel wire meshes (the obtained samples were labeled A, B, C and D correspondingly). The samples were preheated at 300 ℃ for 3 min and annealed at 450 ℃ for 30 min. In order to compare the photocatalytic activity of Al-doped and undoped ZnO coatings, one undoped ZnO coating was also prepared under the same conditions. The crystal structure of the ZnO coatings was analyzed by X-ray diffraction (Bruker D8 Advance); the morphology of the coatings was observed by field emission scanning electron microscopy (S-4800); the composition of the coating surface was analyzed by X-ray photoelectron spectroscopy (XPS). The crystal quality of the samples was determined by photoluminescence. The photocatalytic activity of the samples was tested under the irradiation of a mercury lamp (125 W) with methylene blue as a simulated organic pollutant. The photocatalytic efficiency of the samples was evaluated by measuring the change in the concentration of the methylene blue solution by an ultraviolet-visible spectrophotometer (TU-1901). The initial concentration of the methylene blue solution was 5 mg/L, and the size of the stainless steel wire mesh used in the photocatalytic reaction was 1.5 cm × 1.5 cm. 3. Results and discussion 3.1. Crystal structure, morphology and composition of Al-doped ZnO coatings Fig. 2 shows the images of the stainless steel wire mesh (a1–a3) and Al-doped ZnO coatings (b1-e3) at different resolutions observed by a field emission scanning electron microscope. As can be seen from Fig. 2 (a1–a3), there is no ZnO coating composed of nanoparticles on stainless steel wire mesh before the ZnO sol is coated. However, after two layers of Al-doped ZnO sol are coated and annealed, the polycrystalline ZnO coating is formed, which is made up of many nanoparticles. It should be pointed out that the 2
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 2. SEM images of stainless steel wire mesh (a1–a3), sample A (b1–b3), sample B (c1–c3), sample C (d1–d3) and sample D (e1–e3).
surface morphology of the undoped ZnO coating on the stainless steel wire mesh is similar to that of Fig. 2 (b1), but its particle size is bigger (Al-doping leads to the decrease of the grain size of ZnO, which has been found in many studies [15]). SEM images of pure ZnO coatings are not given here. Some ZnO nanorods are observed on the coating surface when four layers of Al-doped ZnO sol are coated and annealed, but the number of nanorods is very small. However, when the coating thickness is further raised, the areal density of nanorods is also greatly increased. Compared with the particle-based ZnO coatings, the specific surface area of the nanorod-based ZnO coatings is greatly improved due to the appearance of these nanorods. The nanorods are conducive to charge carrier transport, which is favorable to the improvement of photocatalytic performance. When the number of sol layer reaches 8, the diameter of ZnO nanorods greatly increases and the top of the nanorods is no longer flat but becomes sharp. The whole nanorod looks like a pencil. In addition, it can be also seen that cracks appear at those locations where two stainless steel wires intersect, which may be mainly produced by the stress in the coating due to different thermal expansion coefficients between the coating and substrate. The thicker the ZnO coating is, the more pronounced the crack is. The occurrence of the cracks is unfavorable for the stability of the
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Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 3. XRD patterns of the samples (The diffraction peaks indicated by the inverted triangle sign are the ones of stainless steel).
photocatalyst. The formation of ZnO nanorods on Al-doped ZnO thin films surface prepared on glass substrates has been observed and explained by others [16]. Fig. 3 shows the XRD patterns of Al-doped ZnO coatings with different thickness. In addition to the XRD peaks of stainless steel (indicated by inverted triangular symbols), the (100), (002) and (101) diffraction peaks of ZnO can also be observed, which means that the ZnO coatings have crystallized and the wurtzite structure has been formed. With the rise of coating thickness, the (002) peak intensity increases gradually, which suggests that the orientation of ZnO nanorods is preferentially along c-axis direction. In order to determine the composition of the coatings, the XPS spectra were measured for sample B. The results are shown in Fig. 4. Fig. 4(a) displays the full-scan spectrum, while Fig. 4(b)–(d) gives the high-resolution photoelectron spectra of Zn 2p, Al 2p and O 1s, respectively. These spectra show that there are no other impurities in the ZnO coating except intentionally doped Al.
Fig. 4. Typical XPS spectra of sample B: (a) survey spectrum, (b) high-resolution spectrum of Zn 2p, (c) high-resolution spectrum of Al 2p, (d) highresolution spectrum of O 1s. 4
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 5. Photoluminescence spectra of the samples.
3.2. Photoluminescence and photocatalytic properties of ZnO coatings supported by stainless steel wire meshes Photoluminescence spectra can be used to check the crystalline quality of ZnO materials. Zinc oxide usually exhibits two luminescent regions [17–20], i.e., a UV emission peak and a visible emission band. The UV emission (∼380 nm) is caused by free exciton recombination in ZnO materials, while visible emissions originate from defect level related electron transition. Generally speaking, the higher the crystalline quality of ZnO, the stronger its UV emission; and the lower the crystalline quality of ZnO, the more defects it contains, the weaker its UV emission. High crystalline quality of ZnO is beneficial to the improvement of photocatalytic performance, because photogenerated electrons and holes are easier to migrate to grain surface to participate in redox reaction. Fig. 5 shows the photoluminescence spectra of the samples excited by 325 nm UV light. It can be seen that with the rise of coating thickness, the UV emission intensity increases gradually. Especially for sample C, its UV emission intensity is greatly enhanced, which should be mainly attributed to the formation of high quality ZnO nanorods. The above results indicate that the ZnO nanorods have high crystalline quality. Before photocatalytic reaction, the samples were placed in methylene blue aqueous solution for 1 h in the dark to achieve adsorption-desorption equilibrium for dye molecules on the photocatalyst surface. Then photocatalytic reaction was carried out under the mercury lamp irradiation. In this process, the heat brought by the light source was eliminated by circulating water flow. The photocatalytic reaction time was 100 min. After the photocatalytic reaction, some methylene blue solutions were taken out to test the absorbance and calculate the degradation efficiency (the formula used is shown in reference [21]. Fig. 6(a) displays the timedependent curves of the absorbance of methylene blue solution when sample C is used as a photocatalyst. With the increase of reaction time, the absorbance decreased gradually, indicating that methylene blue was decomposed gradually. Fig. 6(b) shows the photocatalytic degradation efficiency of different samples. It can be seen that sample C shows the highest degradation efficiency. This is mainly attributed to the following factors: (1) it has the highest specific surface area; (2) it possesses high crystalline quality; (3) the photogenerated electrons and holes in the ZnO nanorods are easier to migrate to the surface to participate in the redox reaction. For sample D, the diameter of nanorods has been greatly increased, which reduces the areal density of nanorods (meanwhile decreases the specific surface area of the ZnO coating); and on the other hand, it will take longer time for photogenerated electrons and holes to migrate to the surface of nanorods. These factors may lead to the degradation of photocatalytic performance for sample D. Compared with undoped ZnO coatings deposited on stainless steel wire mesh and glass substrate, Al-doped ZnO coatings show higher photocatalytic activity, which should be attributed to the higher specific surface area owing to the formation of nanorods. In order to check the stability of ZnO photocatalysts loaded on stainless steel wire meshes, sample C was used for photocatalytic degradation of MB several times under the same conditions. The results are shown in Fig. 6(c). After four rounds of photocatalytic reaction, the degradation efficiency of the sample do not decrease largely, which indicates that the sample has good reusability. The slight decrease in photocatalytic activity of the sample may be due to the photocorrosion of ZnO [22], which could be overcome by surface modification [23–25].
4. Conclusion In this work, the morphology, photoluminescence and photocatalytic properties of Al-doped ZnO coatings supported by stainless steel wire meshes were studied. SEM images showed that when the ZnO coating exceeded a certain thickness, the ZnO nanorods were formed on the coating surface. The formation of ZnO nanorods not only enhanced the specific surface area of the ZnO coating, but also facilitated the rapid migration of photogenerated electrons and holes to the surface to participate in redox reaction. Photocatalytic experiments showed that the sample formed by six layer of Al-doped ZnO sol exhibited the best photocatalytic performance with high stability. The results show that the stainless steel wire mesh is a good support for ZnO photocatalysts which have a potential application in wastewater treatment. 5
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 6. (a) the absorbance curves of methylene blue solution with sample C as the photocatalyst, (b) degradation efficiency of different samples (E is the sample coated with 6 layers of pure ZnO sol on stainless steel wire mesh, F is the sample coated with 6 layers of pure ZnO sol on a glass substrate), (c) the recycling ability of sample C for MB degradation.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions in China (Grant no. 16KJB430021, 17KJB430022) and the Natural Science Foundation of Jiangsu Province in China (Grant no. BK 20180784). References [1] M.K. Singh, M.S. Mehata, Phase-dependent optical and photocatalytic performance of synthesized titanium dioxide (TiO2) nanoparticles, Optik 193 (2019) 163011. [2] B.G. Park, Photoluminescence of Eu3+-doped CaTiO3 perovskites and their photocatalytic properties with a metal ion loading, Chem. Phys. Lett. 722 (2019) 44. [3] S.J. Hazarika, D. Mohanta, Excitation dependent light emission and enhanced photocatalytic response of WS2/C-dot hybrid nanoscale systems, J. Lumin. 206 (2019) 530. [4] M. Sun, P. Guo, M. Wang, F. Ren, The effect of pH on the photocatalytic performance of BiVO4 for phenol mine sewage degradation under visible light, Optik 179 (2019) 672. [5] C. Yu, Z. Tong, S. Li, Y. Yin, Enhancing the photocatalytic activity of ZnO by using tourmaline, Mater. Lett. 240 (2019) 161. [6] S. Park, H.J. Park, K. Yoo, et al., Scratch test for immobilization of photocatalytic ZnO nanopowders synthesized by solution combustion method, J. Phys. Chem. Solids 69 (2008) 1495. [7] C.J. Chang, Z. Lee, C.F. Wang, Photocatalytic hydrogen production by stainless steel@ZnS core-shell wire mesh photocatalyst from saltwater, Int. J. Hydrogen Energy 39 (2014) 20754. [8] G. Suo, D. Li, L. Feng, X. Hou, Y. Yang, W. Wang, SnO2 nanosheets grown on stainless steel mesh as a binder free anode for potassium ion batteries, J. Electroanal. Chem. 833 (2019) 113. [9] S.N. Khatavkar, S.D. Sartale, α-Fe2O3 thin film on stainless steel mesh: a flexible electrode for supercapacitor, Mater. Chem. Phys. 225 (2019) 284. [10] Z. Li, G. Liu, Y. Zhang, Y. Zhou, Y. Yang, Porous nanosheet-based hierarchical zinc oxide aggregations grown on compacted stainless steel meshes: enhanced flexible dye-sensitized solar cells and photocatalytic activity, Mater. Res. Bull. 80 (2016) 191. [11] C. Cao, J. Cheng, Fabrication of superhydrophobic copper stearate@Fe3O4 coating on stainless steel meshes by dip-coating for oil/water separation, Surf. Coat. Technol. 349 (2018) 296. [12] J.M. Downing, M.P. Ryan, M.A. McLachlan, Hydrothermal growth of ZnO nanorods: the role of KCl in controlling rod morphology, Thin Solid Films 539 (2013) 18.
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[13] Y. Cheng, J. Wang, P.G. Jönsson, Z. Zhao, Improvement and optimization of the growth quality of upright ZnO rod arrays by the response surface methodology, Appl. Surf. Sci. 351 (2015) 451. [14] F. Xian, G. Zheng, L. Xu, W. Kuang, S. Pei, Z. Cao, J. Li, M. Lai, Temperature and excitation power dependence of photoluminescence of ZnO nanorods synthesized by pattern assisted hydrothermal method, J. Alloys Compd. 710 (2017) 695. [15] H. Zhou, D. Yi, Z. Yu, L. Xiao, J. Li, Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties, Thin Solid Films 515 (2007) 6909. [16] N. Huang, C. Sun, M. Zhu, B. Zhang, J. Gong, X. Jiang, Microstructure evolution of zinc oxide films derived from dip-coating sol–gel technique: formation of nanorods through orientation attachment, Nanotechnology 22 (2011) 265612. [17] P. Che, J. Meng, L. Guo, Oriented growth and luminescence of ZnO:Eu films prepared by sol–gel process, J. Lumin. 122–123 (2007) 168. [18] J. Wang, S. Yu, H. Zhang, Effect of surfactants on photoluminescence properties of ZnO synthesized by hydrothermal method, Optik 180 (2019) 20. [19] N. Tu, H.V. Bui, D.Q. Trung, A.T. Duong, D.M. Thuy, D.H. Nguyen, K.T. Nguyen, P.T. Huy, Surface oxygen vacancies of ZnO: a facile fabrication method and their contribution to the photoluminescence, J. Alloys Compd. 791 (2019) 722. [20] E. Musavi, M. Khanlary, Z. Khakpour, Red-orange photoluminescence emission of sol-gel dip-coated prepared ZnO and ZnO:Al nano-crystalline films, J. Lumin. 216 (2019) 116696. [21] G. Zheng, W. Shang, L. Xu, S. Guo, Z. Zhou, Enhanced photocatalytic activity of ZnO thin films deriving from a porous structure, Mater. Lett. 150 (2015) 1. [22] T.T. Vu, L. Río, T. Valdés-Solís, G. Marbán, Fabrication of wire mesh-supported ZnO photocatalysts protected against photocorrosion, Appl. Catal. B 140-141 (2013) 189. [23] L. Xu, J. Su, G. Zheng, L. Zhang, Enhanced photocatalytic performance of porous ZnO thin films by CuO nanoparticles surface modification, Mater. Sci. Eng. B 248 (2019) 114405. [24] A. Sáenz-Trevizo, P. Amézaga-Madrid, P. Pizá-Ruiz, W. Antúnez-Flores, C. Ornelas-Gutiérrez, M. Miki-Yoshida, Efficient and durable ZnO core-shell structures for photocatalytic applications inaqueous media, Mater. Sci. Semicond. Process. 45 (2016) 57. [25] S. Lam, J. Sin, A.Z. Abdullah, A.R. Mohamed, Transition metal oxide loaded ZnO nanorods: preparation, characterization and their UV–vis photocatalytic activities, Sep. Purif. Technol. 132 (2014) 378.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Photocatalytic degradation of organic dyes using ZnO nanorods supported by stainless steel wire mesh deposited by one-step method
T
Linhua Xua,b,*, Fenglin Xiana,b, Shixin Peia,b, Yunguang Zhua,c a
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing, 210044, China c Physics Experiment Center, Nanjing University of Information Science & Technology, Nanjing, 210044, China b
A R T IC LE I N F O
ABS TRA CT
Keywords: Photocatalyst ZnO nanorods One-step method Photoluminescence Stainless steel wire mesh
In this work, a sol-gel method was used to deposit Al-doped ZnO coatings on stainless steel wire meshes. The effect of coating thickness on the morphology evolution and the photoluminescence, photocatalytic properties were investigated. Field emission scanning electron microscope (FESEM) observed that when the Al-doped ZnO coating was thin, it was composed of many nanoparticles. However, with the increase of the coating thickness, some sparse nanorods appeared on the ZnO coating surface. When the coating thickness was increased to six layers, the areal density of nanorods was largely increased. X-ray diffraction (XRD) patterns revealed that the ZnO coatings crystallized into a wurtzite structure, and the nanorods were preferentially oriented along the c-axis direction. Photoluminescence spectra showed that after ZnO nanorods were formed, the UV emission of ZnO coatings was significantly enhanced, which meant that the nanorods had better crystal quality than the nanoparticles. The photocatalytic performance tests for these samples showed that the sample formed by six layers of ZnO sol had the best photocatalytic activity due to its high crystalline quality and the large specific surface area. These ZnO coatings loaded on stainless steel wire meshes exhibited good photocatalytic stability.
1. Introduction Since the beginning of the 21st century, the two major problems of environmental pollution and energy shortage faced by mankind have become increasingly serious. While solar energy is an inexhaustible source of clean energy, how to effectively use solar energy has naturally caused widespread concern throughout the world. So far, solar energy has been widely used by means of solar water heaters and solar cells. In addition, the use of solar energy to treat sewage is also attracting more and more attention [1–6]. For the use of solar energy to treat sewage, the key factor is photocatalyst. Semiconductor photocatalysts can absorb photon energy and then generate photoinduced holes and electrons which react with water molecules and oxygen to in turn produce hydroxyl radicals and superoxide radicals, which can degrade organic pollutants into carbon dioxide, water and harmless inorganic small molecules. Semiconductor photocatalysts have been studied in the form of unsupported powders and thin films or nanostructures supported by substrates. As unsupported powder photocatalysts are not easy to separate from water after use, but also easy to cause secondary ⁎ Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing, 210044, China. E-mail address: [email protected] (L. Xu).
https://doi.org/10.1016/j.ijleo.2019.164036 Received 4 November 2019; Accepted 10 December 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 1. The photograph of stainless steel wire mesh floating on water.
pollution, so the photocatalysts loaded on the substrates will have a wider application. Since glass is cheap and easy to obtain, photocatalysts supported by glass have been extensively studied. In addition to glass, stainless steel wire mesh can also be used as a photocatalyst support. The materials loaded on stainless steel wire meshes can be applied in different fields, such as photocatalytic hydrogen production [7], potassium ion batteries [8], supercapacitors [9], dye-sensitized solar cells [10], oil-water separation [11]. As a support of photocatalyst, unlike glass which must be immersed in liquid, stainless steel wire mesh can not only submerge in water, but also float on water (as shown in Fig. 1), which is more conducive to absorbing sunlight. Therefore, the study on photocatalysts loaded on stainless steel wire meshes has important practical significance for sewage treatment. ZnO nanomaterials, especially the ZnO nanorods, have been widely investigated as photocatalysts. Due to the low cost and high production efficiency, the chemical solution methods, especially the hydrothermal method, have been widely used to grow ZnO nanorods for photocatalytic application. However, the hydrothermal method for growth of ZnO nanorods on substrates requires two steps [12–14]: first, the seed layer is deposited on the substrate, and then the ZnO nanorods are grown on the seed layer. In this study, we have obtained ZnO nanorods by one-step process. The ZnO nanorods on stainless steel wire meshes exhibit higher photocatalytic activity than ZnO coatings deposited on stainless steel wire mesh and glass substrate. The details are described below. 2. Experiments The Al-doped ZnO coatings were deposited on stainless steel wire meshes using a wet chemical method, namely the sol-gel method. The raw materials for preparing the ZnO sol were: zinc acetate (the precursor of Zn), aluminum chloride (the doping source of Al), ethanol (solvent), and monoethanolamine (stabilizer). The molar ratio of Al3+ to Zn2+ was 0.04 in the sol; the concentration of zinc acetate in the ethanol was 0.3 mol/L. The ZnO sol was coated on the thoroughly cleaned stainless steel wire meshes by a dipcoating method. The Al-doped ZnO coating was formed after a heat treatment. In order to analyze the effect of different thickness on the morphology and photocatalytic activity of the ZnO coating, 2, 4, 6 and 8 layers of Al-doped ZnO sol were coated on four stainless steel wire meshes (the obtained samples were labeled A, B, C and D correspondingly). The samples were preheated at 300 ℃ for 3 min and annealed at 450 ℃ for 30 min. In order to compare the photocatalytic activity of Al-doped and undoped ZnO coatings, one undoped ZnO coating was also prepared under the same conditions. The crystal structure of the ZnO coatings was analyzed by X-ray diffraction (Bruker D8 Advance); the morphology of the coatings was observed by field emission scanning electron microscopy (S-4800); the composition of the coating surface was analyzed by X-ray photoelectron spectroscopy (XPS). The crystal quality of the samples was determined by photoluminescence. The photocatalytic activity of the samples was tested under the irradiation of a mercury lamp (125 W) with methylene blue as a simulated organic pollutant. The photocatalytic efficiency of the samples was evaluated by measuring the change in the concentration of the methylene blue solution by an ultraviolet-visible spectrophotometer (TU-1901). The initial concentration of the methylene blue solution was 5 mg/L, and the size of the stainless steel wire mesh used in the photocatalytic reaction was 1.5 cm × 1.5 cm. 3. Results and discussion 3.1. Crystal structure, morphology and composition of Al-doped ZnO coatings Fig. 2 shows the images of the stainless steel wire mesh (a1–a3) and Al-doped ZnO coatings (b1-e3) at different resolutions observed by a field emission scanning electron microscope. As can be seen from Fig. 2 (a1–a3), there is no ZnO coating composed of nanoparticles on stainless steel wire mesh before the ZnO sol is coated. However, after two layers of Al-doped ZnO sol are coated and annealed, the polycrystalline ZnO coating is formed, which is made up of many nanoparticles. It should be pointed out that the 2
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 2. SEM images of stainless steel wire mesh (a1–a3), sample A (b1–b3), sample B (c1–c3), sample C (d1–d3) and sample D (e1–e3).
surface morphology of the undoped ZnO coating on the stainless steel wire mesh is similar to that of Fig. 2 (b1), but its particle size is bigger (Al-doping leads to the decrease of the grain size of ZnO, which has been found in many studies [15]). SEM images of pure ZnO coatings are not given here. Some ZnO nanorods are observed on the coating surface when four layers of Al-doped ZnO sol are coated and annealed, but the number of nanorods is very small. However, when the coating thickness is further raised, the areal density of nanorods is also greatly increased. Compared with the particle-based ZnO coatings, the specific surface area of the nanorod-based ZnO coatings is greatly improved due to the appearance of these nanorods. The nanorods are conducive to charge carrier transport, which is favorable to the improvement of photocatalytic performance. When the number of sol layer reaches 8, the diameter of ZnO nanorods greatly increases and the top of the nanorods is no longer flat but becomes sharp. The whole nanorod looks like a pencil. In addition, it can be also seen that cracks appear at those locations where two stainless steel wires intersect, which may be mainly produced by the stress in the coating due to different thermal expansion coefficients between the coating and substrate. The thicker the ZnO coating is, the more pronounced the crack is. The occurrence of the cracks is unfavorable for the stability of the
3
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
L. Xu, et al.
Fig. 3. XRD patterns of the samples (The diffraction peaks indicated by the inverted triangle sign are the ones of stainless steel).
photocatalyst. The formation of ZnO nanorods on Al-doped ZnO thin films surface prepared on glass substrates has been observed and explained by others [16]. Fig. 3 shows the XRD patterns of Al-doped ZnO coatings with different thickness. In addition to the XRD peaks of stainless steel (indicated by inverted triangular symbols), the (100), (002) and (101) diffraction peaks of ZnO can also be observed, which means that the ZnO coatings have crystallized and the wurtzite structure has been formed. With the rise of coating thickness, the (002) peak intensity increases gradually, which suggests that the orientation of ZnO nanorods is preferentially along c-axis direction. In order to determine the composition of the coatings, the XPS spectra were measured for sample B. The results are shown in Fig. 4. Fig. 4(a) displays the full-scan spectrum, while Fig. 4(b)–(d) gives the high-resolution photoelectron spectra of Zn 2p, Al 2p and O 1s, respectively. These spectra show that there are no other impurities in the ZnO coating except intentionally doped Al.
Fig. 4. Typical XPS spectra of sample B: (a) survey spectrum, (b) high-resolution spectrum of Zn 2p, (c) high-resolution spectrum of Al 2p, (d) highresolution spectrum of O 1s. 4
Optik - International Journal for Light and Electron Optics 203 (2020) 164036
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Fig. 5. Photoluminescence spectra of the samples.
3.2. Photoluminescence and photocatalytic properties of ZnO coatings supported by stainless steel wire meshes Photoluminescence spectra can be used to check the crystalline quality of ZnO materials. Zinc oxide usually exhibits two luminescent regions [17–20], i.e., a UV emission peak and a visible emission band. The UV emission (∼380 nm) is caused by free exciton recombination in ZnO materials, while visible emissions originate from defect level related electron transition. Generally speaking, the higher the crystalline quality of ZnO, the stronger its UV emission; and the lower the crystalline quality of ZnO, the more defects it contains, the weaker its UV emission. High crystalline quality of ZnO is beneficial to the improvement of photocatalytic performance, because photogenerated electrons and holes are easier to migrate to grain surface to participate in redox reaction. Fig. 5 shows the photoluminescence spectra of the samples excited by 325 nm UV light. It can be seen that with the rise of coating thickness, the UV emission intensity increases gradually. Especially for sample C, its UV emission intensity is greatly enhanced, which should be mainly attributed to the formation of high quality ZnO nanorods. The above results indicate that the ZnO nanorods have high crystalline quality. Before photocatalytic reaction, the samples were placed in methylene blue aqueous solution for 1 h in the dark to achieve adsorption-desorption equilibrium for dye molecules on the photocatalyst surface. Then photocatalytic reaction was carried out under the mercury lamp irradiation. In this process, the heat brought by the light source was eliminated by circulating water flow. The photocatalytic reaction time was 100 min. After the photocatalytic reaction, some methylene blue solutions were taken out to test the absorbance and calculate the degradation efficiency (the formula used is shown in reference [21]. Fig. 6(a) displays the timedependent curves of the absorbance of methylene blue solution when sample C is used as a photocatalyst. With the increase of reaction time, the absorbance decreased gradually, indicating that methylene blue was decomposed gradually. Fig. 6(b) shows the photocatalytic degradation efficiency of different samples. It can be seen that sample C shows the highest degradation efficiency. This is mainly attributed to the following factors: (1) it has the highest specific surface area; (2) it possesses high crystalline quality; (3) the photogenerated electrons and holes in the ZnO nanorods are easier to migrate to the surface to participate in the redox reaction. For sample D, the diameter of nanorods has been greatly increased, which reduces the areal density of nanorods (meanwhile decreases the specific surface area of the ZnO coating); and on the other hand, it will take longer time for photogenerated electrons and holes to migrate to the surface of nanorods. These factors may lead to the degradation of photocatalytic performance for sample D. Compared with undoped ZnO coatings deposited on stainless steel wire mesh and glass substrate, Al-doped ZnO coatings show higher photocatalytic activity, which should be attributed to the higher specific surface area owing to the formation of nanorods. In order to check the stability of ZnO photocatalysts loaded on stainless steel wire meshes, sample C was used for photocatalytic degradation of MB several times under the same conditions. The results are shown in Fig. 6(c). After four rounds of photocatalytic reaction, the degradation efficiency of the sample do not decrease largely, which indicates that the sample has good reusability. The slight decrease in photocatalytic activity of the sample may be due to the photocorrosion of ZnO [22], which could be overcome by surface modification [23–25].
4. Conclusion In this work, the morphology, photoluminescence and photocatalytic properties of Al-doped ZnO coatings supported by stainless steel wire meshes were studied. SEM images showed that when the ZnO coating exceeded a certain thickness, the ZnO nanorods were formed on the coating surface. The formation of ZnO nanorods not only enhanced the specific surface area of the ZnO coating, but also facilitated the rapid migration of photogenerated electrons and holes to the surface to participate in redox reaction. Photocatalytic experiments showed that the sample formed by six layer of Al-doped ZnO sol exhibited the best photocatalytic performance with high stability. The results show that the stainless steel wire mesh is a good support for ZnO photocatalysts which have a potential application in wastewater treatment. 5
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Fig. 6. (a) the absorbance curves of methylene blue solution with sample C as the photocatalyst, (b) degradation efficiency of different samples (E is the sample coated with 6 layers of pure ZnO sol on stainless steel wire mesh, F is the sample coated with 6 layers of pure ZnO sol on a glass substrate), (c) the recycling ability of sample C for MB degradation.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions in China (Grant no. 16KJB430021, 17KJB430022) and the Natural Science Foundation of Jiangsu Province in China (Grant no. BK 20180784). References [1] M.K. Singh, M.S. Mehata, Phase-dependent optical and photocatalytic performance of synthesized titanium dioxide (TiO2) nanoparticles, Optik 193 (2019) 163011. [2] B.G. Park, Photoluminescence of Eu3+-doped CaTiO3 perovskites and their photocatalytic properties with a metal ion loading, Chem. Phys. Lett. 722 (2019) 44. [3] S.J. Hazarika, D. Mohanta, Excitation dependent light emission and enhanced photocatalytic response of WS2/C-dot hybrid nanoscale systems, J. Lumin. 206 (2019) 530. [4] M. Sun, P. Guo, M. Wang, F. Ren, The effect of pH on the photocatalytic performance of BiVO4 for phenol mine sewage degradation under visible light, Optik 179 (2019) 672. [5] C. Yu, Z. Tong, S. Li, Y. Yin, Enhancing the photocatalytic activity of ZnO by using tourmaline, Mater. Lett. 240 (2019) 161. [6] S. Park, H.J. Park, K. Yoo, et al., Scratch test for immobilization of photocatalytic ZnO nanopowders synthesized by solution combustion method, J. Phys. Chem. Solids 69 (2008) 1495. [7] C.J. Chang, Z. Lee, C.F. Wang, Photocatalytic hydrogen production by stainless steel@ZnS core-shell wire mesh photocatalyst from saltwater, Int. J. Hydrogen Energy 39 (2014) 20754. [8] G. Suo, D. Li, L. Feng, X. Hou, Y. Yang, W. Wang, SnO2 nanosheets grown on stainless steel mesh as a binder free anode for potassium ion batteries, J. Electroanal. Chem. 833 (2019) 113. [9] S.N. Khatavkar, S.D. Sartale, α-Fe2O3 thin film on stainless steel mesh: a flexible electrode for supercapacitor, Mater. Chem. Phys. 225 (2019) 284. [10] Z. Li, G. Liu, Y. Zhang, Y. Zhou, Y. Yang, Porous nanosheet-based hierarchical zinc oxide aggregations grown on compacted stainless steel meshes: enhanced flexible dye-sensitized solar cells and photocatalytic activity, Mater. Res. Bull. 80 (2016) 191. [11] C. Cao, J. Cheng, Fabrication of superhydrophobic copper stearate@Fe3O4 coating on stainless steel meshes by dip-coating for oil/water separation, Surf. Coat. Technol. 349 (2018) 296. [12] J.M. Downing, M.P. Ryan, M.A. McLachlan, Hydrothermal growth of ZnO nanorods: the role of KCl in controlling rod morphology, Thin Solid Films 539 (2013) 18.
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