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Enhanced photocatalytic activity of ZnO thin films deriving from a porous structure
Materials Letters 150 (2015) 1–4
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced photocatalytic activity of ZnO thin films deriving from a porous structure Gaige Zheng, Wenjuan Shang, Linhua Xu n, Shan Guo, Zihao Zhou School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2014 Accepted 1 March 2015 Available online 9 March 2015
ZnO thin films are prepared by sol–gel method. Their surface and sectional morphologies have been observed by scanning electron microscopy (SEM). It is found that the Sr-doping leads to a porous structure in ZnO thin films and this kind of porous structure distributes throughout the whole film rather than only distribute on the film surface. Furthermore, the variation of Sr-doping concentration can tailor the pore density in the films. The formation of the porous structure probably results from the lattice distortion due to the large difference between the ion radiuses of Sr2 þ and Zn2 þ . The mapping of the samples shows that the Sr element is uniformly distributed in the whole thin films. The photocatalytic experiments show that the photocatalytic activity of Sr-doped ZnO thin films has been enhanced compared with that of undoped ZnO thin films. This enhancement should be attributed to the increase of specific surface area deriving from the porous structure. & 2015 Elsevier B.V. All rights reserved.
Keywords: Porous materials Sol–gel preparation Thin films Nanocrystalline materials Semiconductors
1. Introduction Nowadays, the treatment of a large amount of industrial sewage and domestic wasterwater has been a daunting task for human. Conventional techniques for wasterwater purification cannot effectively remove all the hazardous substances such as some organic matter, high valence cations and so on. Therefore, it is urgent to develop some new techniques for wasterwater purification. Recently, a new technique, namely, semiconductor photocatalytic technique, attracts wide attention. The semiconductor materials which can be used as photocatalysts include TiO2, ZnO, SnO2, etc. As for TiO2 materials, lots of photocatalytic studies have been done on them [1,2]. However, the photocatalytic studies of ZnO materials are relatively few. Many results show that ZnO materials can be as efficient as TiO2 materials in photocatalytic degradation of some organic substances and even in some cases ZnO has a higher photocatalytic activity than TiO2 [3,4]. Therefore, the photocatalytic studies on ZnO materials are also extensively carried out [5–12] recently. The main factors affecting ZnO photocatalytic performance are: (1) the specific surface area; (2) the absorption of ZnO materials for irradiation light; and (3) the separation efficiency of electron– hole pairs. Owing to the wide bandgap of 3.37 eV at room temperature, ZnO materials mainly absorb ultraviolet light. That is to say, the photocatalytic performance of ZnO is based on its UV
n
Corresponding author. Tel./fax: þ 86 25 58731031. E-mail address: [email protected] (L. Xu).
http://dx.doi.org/10.1016/j.matlet.2015.03.001 0167-577X/& 2015 Elsevier B.V. All rights reserved.
absorption. In recent years, some researchers try to change the bandgaps of ZnO materials in order to increase the absorption for incident light but with little success. Many results show that the morphology of ZnO thin films plays a decisive role for their photocatalytic performance [5,6], so enhancement of specific surface area of the films through adjusting the preparation conditions or annealing conditions is a feasible method for improving photocatalytic activity. Because the porous ZnO thin films usually possess higher specific surface area and stronger absorption ability for incident light, they have aroused researchers’ interest in recent years. However, as for those ZnO thin films with many pores on the surface, their photocatalytic activity will usually degrade after used for several times [9]. Therefore, it can be expected that if the pores are not only distributed on the film surface but also throughout the whole film, these special porous ZnO films probably still have good photocatalytic activity after used for several times. In a previous work [13], we prepared Sr-doped ZnO thin films and mainly studied the optical properties of the samples. It was found that the incorporation of Sr affected the photoluminescence of ZnO. In this work, the morphology and the photocatalytic performance of the Sr-doped ZnO thin films was deeply investigated. It is found that the Sr-doping leads to a porous structure in the films. This kind of porous structure is distributed throughout the whole film unlike those which are distributed only on the film surface reported by others. Furthermore, we have also found that the variation of Sr-doping concentration can tailor the pore density and size. The photocatalytic results show that these porous
2
G. Zheng et al. / Materials Letters 150 (2015) 1–4
films possess excellent photocatalytic activity for degradation of Rhodamine B and have a good repeatability.
2. Experiments The preparation process of Sr-doped ZnO thin films was described in detail elsewhere [13]. In short, the Sr-doped ZnO thin films were deposited by sol–gel method. Strontium chloride was used as Sr dopant source. The molar ratio of Sr/Zn was 0, 1, 3, 5, and 7%, respectively. The ZnO sols were aged at room temperature for 24 h; and then ZnO thin films were prepared by spin-coating these ZnO sols on glass substrates. The spin-coating time was 30 s. In the beginning 10 s, the spin-speed was 1200 rpm; in the latter 20 s, the spin-speed was 3000 rpm. At last, the samples were annealed at 500 1C in air atmosphere for 1 h.
The surface and cross-section morphologies were observed by a scanning electron microscope (S4800, Hitachi) operated at 4 and 8 kV. The crystal phase was identified by an X-ray diffractometer (Bruker D8 Advance) with Cu Kα radiation at a wavelength of 0.15406 nm. The mapping of the film was used to observe the distribution of Sr element. The absorbance was recorded by a spectrophotometer (TU-1901, Purkinje General Instrument Co., Ltd.) in the wavelength range of 400–600 nm. In order to examine the photocatalytic performance of this kind of porous ZnO thin films, the photocatalytic activity has been investigated using Rhodamine B (RB) as a degradation target with irradiation by a mercury lamp. The absorbance at 554 nm of the RB solution was utilized to obtain the degradation efficiency (η) by the following formula:
η¼
C0 C A0 A 100% ¼ 100% C0 A0
Fig. 1. Surface and cross-section morphology images of the ZnO thin films.
G. Zheng et al. / Materials Letters 150 (2015) 1–4
where A0 is the initial absorbance after absorption equilibrium and A is the absorbance at a reaction time of 2 h. The concentration of RB in the solution is 5 mg/L and the area of the films is 1.5 1.5 cm2.
3. Results and discussion Fig. 1 shows the surface and cross-section morphology images of Sr-doped ZnO thin films. It can be seen that the undoped ZnO thin film has a compact structure, a smooth surface and uniform grain sizes. However, the incorporation of Sr leads to many pores in ZnO thin films. With the increase of Sr contents, the pore density also increases up to 5% Sr-doping level. Afterwards, the pore density decreases with a further increase of Sr contents. From the crosssection morphology images, one can clearly see that this kind of porous structure is not only distributed on the film surface but also throughout the whole film. Furthermore, the Sr-doped ZnO thin films also have rougher surface than pure ones. As for the porous structure in doped ZnO thin films, the similar results have also been reported by others. For example, Duan et al. [14] deposited Ag-doped ZnO thin films by magnetron sputtering; they found that the Ag-doping led to many pores on the films compared with the pure ZnO thin films. Then, why does doping lead to porous structure in ZnO thin films? We think that this may be connected with the large difference of ionic radiuses between Zn and doped atoms. Whether it is silver or strontium, their ionic radiuses are far larger than that of Zn ion. This will induce lattice distortion in ZnO, which in turn leads to stress in the film. As a result, the porous structure occurs in the Sr-doped ZnO thin films. Therefore, the formation of the porous structure here should be attributed mainly to the Sr-doping. Then, why does the density of pores decrease when the Sr-doping concentration is above 5%? This is probably related to the nature of ZnO sols. Because strontium chloride is slightly soluble in anhydrous ethanol, only part of it will dissolve in ethanol when the Sr-doping level is relatively high. We found that there was some strontium chloride undissolved
3
in the 5% and 7% Sr-doped ZnO sols. Maybe just the change of the nature of ZnO sols leads to the variation of the pore density in the ZnO thin films. Fig. 2 displays the typical mapping images of 1% Sr-doped ZnO thin film. It can be seen that the Sr element has been uniformly distributed in the film. In addition, the XRD results show that the Sr-doping does not affect the wurtzite structure of ZnO and all the films are preferentially oriented along the c-axis direction. The detailed analysis about XRD is presented in Ref. [13]. Fig. 3(a) displays the absorption spectra of RB solution at different reaction time using 7% Sr-doped ZnO thin film as the photocatalyst. It is obvious that the absorbance of this solution gradually decreases with the increase of reaction time, meaning that the concentration of RB gradually decreases. It can be seen from Fig. 3(b) that Rhodamine B is also decomposed only with irradiation of UV light and no photocatalysts. However, the degradation rate is very low. Furthermore, without irradiation of UV light, ZnO has almost no photocatalytic activity. Compared with pure ZnO thin films, the Sr-doped porous ZnO thin films exhibit very good photocatalytic performance. The improvement of photocatalytic activity of porous ZnO thin films can be attributed to the following three reasons: (1) due to higher specific surface area, the porous ZnO thin films are exposed to more pollutants; (2) contaminant molecules can enter inside the film along the pore canal, so photo-generated electron–hole pairs have chance to react with pollutants as long as they diffuse to the surface of ZnO grains rather than necessarily film surface; (3) the porous structure greatly increases the scattering of incident UV light, leading to more absorption of UV light by ZnO thin films. Among the samples, 3 and 5% Sr-doped ZnO thin films exhibit better photocatalytic activity. This can be ascribed to their higher pore density. In order to examine the repeatability of photocatalytic activity of the porous films, the photocatalytic experiment is repeated 6 times under the same conditions. The degradation efficiency is almost the same, indicating that this kind of porous ZnO thin films has a good repeatability.
Fig. 2. A representative mapping of 1% Sr-doped ZnO thin film.
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G. Zheng et al. / Materials Letters 150 (2015) 1–4
Fig. 3. The absorption spectra of Rhodamine B solution at different reaction time (a) and degradation efficiency of the porous ZnO thin films (b).
4. Conclusion
References
ZnO thin films were prepared by sol–gel method. Sr-doping led to a porous structure in the films and the porous structure was not only distributed on the film surface but also throughout the film. The Sr-doping concentration could tailor the pore density. Among the samples, the 5% Sr-doped ZnO thin film showed the highest pore density. All the porous films exhibited good photocatalytic performance for degradation of RB, which should be ascribed to higher specific surface area and stronger absorption for the incident UV light. It is speculated that this kind of porous ZnO thin films also have potential applications in gas sensing and solar cells.
[1] Kumaresan L, Prabhu A, Palanichamy M, Arumugam E, Murugesan V. J Hazard Mater 2011;186:1183–92. [2] Kumaresan L, Prabhu A, Palanichamy M, Murugesan V. Mater Chem Phys 2011;126:445–52. [3] Kansal SK, Singh M, Sud D. J Hazard Mater 2007;141:581–90. [4] Kavitha R, Meghani S, Jayaram V. Mater Sci Eng B 2007;139:134–40. [5] Kaneva N, Stambolova I, Blaskov V, Dimitriev Y, Bojinova A, Dushkin C. Surf Coat Technol 2012;207:5–10. [6] Kaneva NV, Dimitrov DT, Dushkin CD. Appl Surf Sci 2011;257:8113–20. [7] Stambolova I, Blaskov V, Shipochka M, Vassilev S, Dushkin C, Dimitriev Y. Mater Chem Phys 2010;121:447–52. [8] Wang H, Dong S, Chang Y, Zhou X, Hu X. Appl Surf Sci 2012;258:4288–93. [9] Wang HJ, Sun YY, Cao Y, Yu XH, Ji XM, Yang L. Chem Eng J 2011;178:8–14. [10] Kao CY, Liao JD, Chang CW, Wang RY. Appl Surf Sci 2011;258:1813–8. [11] Nejand BA, Sanjabi S, Ahmadi V. Appl Surf Sci 2011;257:10434–42. [12] Mohamed RM, Salam MA. Mater Res Bull 2014;50:85–90. [13] Xu LH, Xiao SR, Zhang CY, Zheng GG, Su J, Zhao LL, et al. Mater Chem Phys 2014;148:720–6. [14] Duan L, Gao W, Chen R, Fu Z. Solid State Commun 2008;145:479–81.
Acknowledgements This work is supported by the Innovation and Entrepreneurship Training Program for College Students in Jiangsu Province (Grant no. 201410300083X) and partially supported by the Natural Science Foundation of Jiangsu Province (Grant no. BK20141483).
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced photocatalytic activity of ZnO thin films deriving from a porous structure Gaige Zheng, Wenjuan Shang, Linhua Xu n, Shan Guo, Zihao Zhou School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2014 Accepted 1 March 2015 Available online 9 March 2015
ZnO thin films are prepared by sol–gel method. Their surface and sectional morphologies have been observed by scanning electron microscopy (SEM). It is found that the Sr-doping leads to a porous structure in ZnO thin films and this kind of porous structure distributes throughout the whole film rather than only distribute on the film surface. Furthermore, the variation of Sr-doping concentration can tailor the pore density in the films. The formation of the porous structure probably results from the lattice distortion due to the large difference between the ion radiuses of Sr2 þ and Zn2 þ . The mapping of the samples shows that the Sr element is uniformly distributed in the whole thin films. The photocatalytic experiments show that the photocatalytic activity of Sr-doped ZnO thin films has been enhanced compared with that of undoped ZnO thin films. This enhancement should be attributed to the increase of specific surface area deriving from the porous structure. & 2015 Elsevier B.V. All rights reserved.
Keywords: Porous materials Sol–gel preparation Thin films Nanocrystalline materials Semiconductors
1. Introduction Nowadays, the treatment of a large amount of industrial sewage and domestic wasterwater has been a daunting task for human. Conventional techniques for wasterwater purification cannot effectively remove all the hazardous substances such as some organic matter, high valence cations and so on. Therefore, it is urgent to develop some new techniques for wasterwater purification. Recently, a new technique, namely, semiconductor photocatalytic technique, attracts wide attention. The semiconductor materials which can be used as photocatalysts include TiO2, ZnO, SnO2, etc. As for TiO2 materials, lots of photocatalytic studies have been done on them [1,2]. However, the photocatalytic studies of ZnO materials are relatively few. Many results show that ZnO materials can be as efficient as TiO2 materials in photocatalytic degradation of some organic substances and even in some cases ZnO has a higher photocatalytic activity than TiO2 [3,4]. Therefore, the photocatalytic studies on ZnO materials are also extensively carried out [5–12] recently. The main factors affecting ZnO photocatalytic performance are: (1) the specific surface area; (2) the absorption of ZnO materials for irradiation light; and (3) the separation efficiency of electron– hole pairs. Owing to the wide bandgap of 3.37 eV at room temperature, ZnO materials mainly absorb ultraviolet light. That is to say, the photocatalytic performance of ZnO is based on its UV
n
Corresponding author. Tel./fax: þ 86 25 58731031. E-mail address: [email protected] (L. Xu).
http://dx.doi.org/10.1016/j.matlet.2015.03.001 0167-577X/& 2015 Elsevier B.V. All rights reserved.
absorption. In recent years, some researchers try to change the bandgaps of ZnO materials in order to increase the absorption for incident light but with little success. Many results show that the morphology of ZnO thin films plays a decisive role for their photocatalytic performance [5,6], so enhancement of specific surface area of the films through adjusting the preparation conditions or annealing conditions is a feasible method for improving photocatalytic activity. Because the porous ZnO thin films usually possess higher specific surface area and stronger absorption ability for incident light, they have aroused researchers’ interest in recent years. However, as for those ZnO thin films with many pores on the surface, their photocatalytic activity will usually degrade after used for several times [9]. Therefore, it can be expected that if the pores are not only distributed on the film surface but also throughout the whole film, these special porous ZnO films probably still have good photocatalytic activity after used for several times. In a previous work [13], we prepared Sr-doped ZnO thin films and mainly studied the optical properties of the samples. It was found that the incorporation of Sr affected the photoluminescence of ZnO. In this work, the morphology and the photocatalytic performance of the Sr-doped ZnO thin films was deeply investigated. It is found that the Sr-doping leads to a porous structure in the films. This kind of porous structure is distributed throughout the whole film unlike those which are distributed only on the film surface reported by others. Furthermore, we have also found that the variation of Sr-doping concentration can tailor the pore density and size. The photocatalytic results show that these porous
2
G. Zheng et al. / Materials Letters 150 (2015) 1–4
films possess excellent photocatalytic activity for degradation of Rhodamine B and have a good repeatability.
2. Experiments The preparation process of Sr-doped ZnO thin films was described in detail elsewhere [13]. In short, the Sr-doped ZnO thin films were deposited by sol–gel method. Strontium chloride was used as Sr dopant source. The molar ratio of Sr/Zn was 0, 1, 3, 5, and 7%, respectively. The ZnO sols were aged at room temperature for 24 h; and then ZnO thin films were prepared by spin-coating these ZnO sols on glass substrates. The spin-coating time was 30 s. In the beginning 10 s, the spin-speed was 1200 rpm; in the latter 20 s, the spin-speed was 3000 rpm. At last, the samples were annealed at 500 1C in air atmosphere for 1 h.
The surface and cross-section morphologies were observed by a scanning electron microscope (S4800, Hitachi) operated at 4 and 8 kV. The crystal phase was identified by an X-ray diffractometer (Bruker D8 Advance) with Cu Kα radiation at a wavelength of 0.15406 nm. The mapping of the film was used to observe the distribution of Sr element. The absorbance was recorded by a spectrophotometer (TU-1901, Purkinje General Instrument Co., Ltd.) in the wavelength range of 400–600 nm. In order to examine the photocatalytic performance of this kind of porous ZnO thin films, the photocatalytic activity has been investigated using Rhodamine B (RB) as a degradation target with irradiation by a mercury lamp. The absorbance at 554 nm of the RB solution was utilized to obtain the degradation efficiency (η) by the following formula:
η¼
C0 C A0 A 100% ¼ 100% C0 A0
Fig. 1. Surface and cross-section morphology images of the ZnO thin films.
G. Zheng et al. / Materials Letters 150 (2015) 1–4
where A0 is the initial absorbance after absorption equilibrium and A is the absorbance at a reaction time of 2 h. The concentration of RB in the solution is 5 mg/L and the area of the films is 1.5 1.5 cm2.
3. Results and discussion Fig. 1 shows the surface and cross-section morphology images of Sr-doped ZnO thin films. It can be seen that the undoped ZnO thin film has a compact structure, a smooth surface and uniform grain sizes. However, the incorporation of Sr leads to many pores in ZnO thin films. With the increase of Sr contents, the pore density also increases up to 5% Sr-doping level. Afterwards, the pore density decreases with a further increase of Sr contents. From the crosssection morphology images, one can clearly see that this kind of porous structure is not only distributed on the film surface but also throughout the whole film. Furthermore, the Sr-doped ZnO thin films also have rougher surface than pure ones. As for the porous structure in doped ZnO thin films, the similar results have also been reported by others. For example, Duan et al. [14] deposited Ag-doped ZnO thin films by magnetron sputtering; they found that the Ag-doping led to many pores on the films compared with the pure ZnO thin films. Then, why does doping lead to porous structure in ZnO thin films? We think that this may be connected with the large difference of ionic radiuses between Zn and doped atoms. Whether it is silver or strontium, their ionic radiuses are far larger than that of Zn ion. This will induce lattice distortion in ZnO, which in turn leads to stress in the film. As a result, the porous structure occurs in the Sr-doped ZnO thin films. Therefore, the formation of the porous structure here should be attributed mainly to the Sr-doping. Then, why does the density of pores decrease when the Sr-doping concentration is above 5%? This is probably related to the nature of ZnO sols. Because strontium chloride is slightly soluble in anhydrous ethanol, only part of it will dissolve in ethanol when the Sr-doping level is relatively high. We found that there was some strontium chloride undissolved
3
in the 5% and 7% Sr-doped ZnO sols. Maybe just the change of the nature of ZnO sols leads to the variation of the pore density in the ZnO thin films. Fig. 2 displays the typical mapping images of 1% Sr-doped ZnO thin film. It can be seen that the Sr element has been uniformly distributed in the film. In addition, the XRD results show that the Sr-doping does not affect the wurtzite structure of ZnO and all the films are preferentially oriented along the c-axis direction. The detailed analysis about XRD is presented in Ref. [13]. Fig. 3(a) displays the absorption spectra of RB solution at different reaction time using 7% Sr-doped ZnO thin film as the photocatalyst. It is obvious that the absorbance of this solution gradually decreases with the increase of reaction time, meaning that the concentration of RB gradually decreases. It can be seen from Fig. 3(b) that Rhodamine B is also decomposed only with irradiation of UV light and no photocatalysts. However, the degradation rate is very low. Furthermore, without irradiation of UV light, ZnO has almost no photocatalytic activity. Compared with pure ZnO thin films, the Sr-doped porous ZnO thin films exhibit very good photocatalytic performance. The improvement of photocatalytic activity of porous ZnO thin films can be attributed to the following three reasons: (1) due to higher specific surface area, the porous ZnO thin films are exposed to more pollutants; (2) contaminant molecules can enter inside the film along the pore canal, so photo-generated electron–hole pairs have chance to react with pollutants as long as they diffuse to the surface of ZnO grains rather than necessarily film surface; (3) the porous structure greatly increases the scattering of incident UV light, leading to more absorption of UV light by ZnO thin films. Among the samples, 3 and 5% Sr-doped ZnO thin films exhibit better photocatalytic activity. This can be ascribed to their higher pore density. In order to examine the repeatability of photocatalytic activity of the porous films, the photocatalytic experiment is repeated 6 times under the same conditions. The degradation efficiency is almost the same, indicating that this kind of porous ZnO thin films has a good repeatability.
Fig. 2. A representative mapping of 1% Sr-doped ZnO thin film.
4
G. Zheng et al. / Materials Letters 150 (2015) 1–4
Fig. 3. The absorption spectra of Rhodamine B solution at different reaction time (a) and degradation efficiency of the porous ZnO thin films (b).
4. Conclusion
References
ZnO thin films were prepared by sol–gel method. Sr-doping led to a porous structure in the films and the porous structure was not only distributed on the film surface but also throughout the film. The Sr-doping concentration could tailor the pore density. Among the samples, the 5% Sr-doped ZnO thin film showed the highest pore density. All the porous films exhibited good photocatalytic performance for degradation of RB, which should be ascribed to higher specific surface area and stronger absorption for the incident UV light. It is speculated that this kind of porous ZnO thin films also have potential applications in gas sensing and solar cells.
[1] Kumaresan L, Prabhu A, Palanichamy M, Arumugam E, Murugesan V. J Hazard Mater 2011;186:1183–92. [2] Kumaresan L, Prabhu A, Palanichamy M, Murugesan V. Mater Chem Phys 2011;126:445–52. [3] Kansal SK, Singh M, Sud D. J Hazard Mater 2007;141:581–90. [4] Kavitha R, Meghani S, Jayaram V. Mater Sci Eng B 2007;139:134–40. [5] Kaneva N, Stambolova I, Blaskov V, Dimitriev Y, Bojinova A, Dushkin C. Surf Coat Technol 2012;207:5–10. [6] Kaneva NV, Dimitrov DT, Dushkin CD. Appl Surf Sci 2011;257:8113–20. [7] Stambolova I, Blaskov V, Shipochka M, Vassilev S, Dushkin C, Dimitriev Y. Mater Chem Phys 2010;121:447–52. [8] Wang H, Dong S, Chang Y, Zhou X, Hu X. Appl Surf Sci 2012;258:4288–93. [9] Wang HJ, Sun YY, Cao Y, Yu XH, Ji XM, Yang L. Chem Eng J 2011;178:8–14. [10] Kao CY, Liao JD, Chang CW, Wang RY. Appl Surf Sci 2011;258:1813–8. [11] Nejand BA, Sanjabi S, Ahmadi V. Appl Surf Sci 2011;257:10434–42. [12] Mohamed RM, Salam MA. Mater Res Bull 2014;50:85–90. [13] Xu LH, Xiao SR, Zhang CY, Zheng GG, Su J, Zhao LL, et al. Mater Chem Phys 2014;148:720–6. [14] Duan L, Gao W, Chen R, Fu Z. Solid State Commun 2008;145:479–81.
Acknowledgements This work is supported by the Innovation and Entrepreneurship Training Program for College Students in Jiangsu Province (Grant no. 201410300083X) and partially supported by the Natural Science Foundation of Jiangsu Province (Grant no. BK20141483).