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ZnO and ZnS microrods coated on glass and photocatalytic activity
Applied Surface Science 258 (2012) 4861–4865
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
ZnO and ZnS microrods coated on glass and photocatalytic activity I˙ lknur Altın a , I˙ smail Polat b , Emin Bacaksız b , Münevver Sökmen a,∗ a b
Department of Chemistry, Faculty of Science, Karadeniz Technical University, 61080 Trabzon, Turkey Department of Physics, Faculty of Science, Karadeniz Technical University, 61080 Trabzon, Turkey
a r t i c l e
i n f o
Article history: Received 14 November 2011 Received in revised form 5 January 2012 Accepted 14 January 2012 Available online 23 January 2012 Keywords: ZnO coating ZnS coating Spray pyrolysis Photocatalysis
a b s t r a c t In this study, the large-scale ZnO and ZnS rods at sub-micrometer were prepared on soda lime glass substrate using a spray pyrolysis method. The microstructure of the rods was characterized by X-ray diffractometry and scanning electron microscopy with the energy dispersive X-ray spectroscopy, and the optical properties were investigated. XRD and SEM results show that the wurtzite structure and rodlike ZnO and ZnS at micro scale were obtained. The optical band gap values were 3.22 and 3.44 eV for ZnO and ZnS microrods, respectively. The obtained samples were tested for their photocatalytic ability using them for the degradation of methylene blue (MB, 1 × 10−5 M). Results show that the dye can be degraded at quite high rate (74.0 ± 3.7% for ZnO and 65.0 ± 5% for ZnS) by both films under a 365 nm UV light after a 60 min exposure period. The materials were re-checked after the treatment and microstructures were observed mainly unchanged. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zinc sulfide (ZnS) is a promising material for the use in various application devices such as nano sized sensors, photodiode and photocatalyst for the degradation of organic dyes [1–3]. It is an important semiconductor with a higher band gap (3.72 eV for cubic zinc blend and 3.77 eV for hexagonal wurtzite), large exciton binding energy (40 meV) and a small Bohr radius (2.4 nm). Besides, ZnS is available in abundance and nontoxic similar to titanium dioxide (TiO2 ). ZnS nano materials have been used for the photocatalytic degradation of organic pollutants such as dyes, p-nitrophenol, and halogenated benzene derivatives in wastewater treatment [2 and references there in]. Similarly, zinc oxide (ZnO) is an oxide semiconductors for potential applications in the optoelectric field due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV) [4]. ZnO is a low cost material with various industrial applications in manufacturing gas sensors, Schottky diodes, solar cells, a photocatalyst, etc. [5–8]. Considering the ever-decreasing dimensions of electronic devices, producing self-assembled micro- and nano structured materials systems is becoming increasingly important for commercial applications. Since TiO2 in the anatase form has been used for photocatalyst application ZnS and ZnO are suitable alternative to TiO2 so far as band gap energies are concerned. While ZnO exhibits similar photocatalytic activity to TiO2 [9] it was better in some cases [10,11]. Similar to TiO2
∗ Corresponding author. Tel.: +90 462 3772532; fax: +90 462 3253196. E-mail address: [email protected] (M. Sökmen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.082
photocatalysis, ZnO or ZnS produce hydroxyl radicals via charged holes (h+ ) and excited electrons (e− ) which are the main reactive species for oxidative/reductive degradation of a wide range of organic pollutants [12–15]. But the main problems are its stability especially in acidic conditions and photo-corrosion. It should be kept in mind that the photocatalytic activity is strongly depend on the processing conditions of the material and stability can be increased by the method used for preparation. Photoactive ZnO or ZnS particles can be utilized in the form of thin film if they are coated on a surface. This eases the usage of semiconductor because after treatment period filtration of the photoactive nano particles is somehow difficult requiring sophisticated filtration techniques and increase the cost of the process. However, in the case of coating the active particles on a surface, the interaction between the molecule and catalyst must be quite different because of the distance between the particle and the molecules to be degraded. Therefore, this interaction should be checked for a specific pollutant and coated surface. Various coating methods have been discussed in literature and spray pyrolysis was one of these techniques that had been used for simple, cheap and convenient production for ZnO films [16]. The ZnS particles are interesting entities for catalytic activities, since its remarkable chemical stability against oxidation and hydrolysis, and they can be used for photocatalytic treatment of aggressive environments. Although many reports related to photocatalytic removal of dyes employing ZnO thin films are available in current literature [12,17–23], only limited number of studies had been published involving ZnS thin films for this purpose [24–26]. In a study carried out by Wang et al. [24] ZnS nano ribbon film had been successfully prepared by solvothermal treating of Zn foil and sulfur
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powder in hydrazine hydrate without any surfactant and post-high temperature treatment. Obtained films have high crystallinity and uniformity and were used as a photocatalyst for degradation of dye X-3B. ZnS nano ribbon film was highly effective (about 88.4% degradation after 100 min. irradiation with a UV light) for degradation of the dye under UV light irradiation. In another study, ZnS thin film was tested for the production of hydrogen gas from the solution containing hydrogen sulfide (HS− ) ions [25]. Similarly, Rincon et al. [26] determined the structural, optical and photo-electrochemical properties of ZnS thin film and had used it for hydrogen production. In this study, we report a simple route for coating ZnS and ZnO particles on glass substrates by spray pyrolysis method. We aimed to produce these materials at large scale. After coating an annealing step was carried out to obtain desired crystal structure and surface properties were evaluated. Following this photocatalytic activities of ZnS and ZnO coated materials were tested for their capacity for degradation of methylene blue (MB). Recently MB is considered as model chemical for evaluation of this kind of materials.
Fig. 1. The XRD pattern of the ZnO microrods (inset shows the SEM image).
2. Experimental
The decolorization efficiency, which is defined as removal percentage, was calculated as:
2.1. ZnO and ZnS coating
Removal% =
ZnO and ZnS thin films were obtained by spray pyrolysis at 550 and 490 ◦ C in air atmosphere, respectively. The experimental set up and the other experimental details were explained elsewhere [27]. ZnO stock solution was prepared from zinc chloride (ZnCl2 ) at 0.1 M concentration in deionized water. ZnS solution was prepared from zinc chloride (ZnCl2 ·6H2 O) at 0.05 M and thiourea [SC(NH2 )2 ] at 0.1 M in deionized water. The glass substrates were cleaned in ethanol and then dried in vacuum prior to coating. The growth was performed with a spray rate of about 5 ml/min and the growth rate was approximately 50 nm/min. During the growth, the substrates were rotated with a speed of 10 cycles per minute and thin films were annealed under a vacuum 30 min at 490 ◦ C. 2.2. Structural and surface analysis The X-ray diffraction (XRD) data of the films was taken using a Rigaku D/Max-IIIC diffractometer with CuK␣ radiation over the range 2 = 20–60◦ at room temperature. The surface morphology was studied using a JEOL JST-6400 scanning electron microscope. The voltage applied was 10 kV. Optical transmission data in the 190–1100 nm range was obtained with a Shimadzu UV-1601 spectrophotometer. 2.3. Photocatalytic activity of ZnO and ZnS thin films Photocatalytic efficiencies were tested for the removal of aqueous solution of methylene blue (MB). A set of experiments were designed to compare the photocatalytic actions of the coated and non-coated glass in the presence and absence of UV light. Talebian et al. [23] report that photocatalytic degradation of MB was pH dependent. Therefore, we set up our experiments at pH 6 which is also natural pH of MB. Photocatalytic experiments were carried out in aqueous solution of MB employing a UVA emitting lamp (365 nm, Spectroline ENF-260) at dye’s natural pH. A 5 mL portion of 1 × 10−5 M MB was placed in a quartz cell containing glass material (2 pieces, approximately 1 cm × 1 cm). Special attention was paid for contacting the dye solution and UV light with glass surface and solutions were shaken continuously. The absorption of MB at 668 nm (UV–vis Dr. Lange CADAS 200 Spectrometer) was monitored over 60 min in the presence and absence of light or glass material. At regular intervals of irradiation, aliquots of 2 mL sampled and then absorption was measured in terms of change in intensity at 668 nm.
C0 − C × 100 C0
where; C0 was the initial concentration and C was the concentration of dye after a certain treatment period (in the presence or absence of light). 3. Results and discussion 3.1. Structural and optical properties The XRD results indicate that the zinc chloride was completely decomposed to give ZnO as shown in Fig. 1. XRD pattern of the ZnO micro rods grown on the substrate a high intensity of (0 0 2) diffraction peak was detected and implied that ZnO micro rods have a typical hexagonal structure and grow along a c-axis direction perpendicular to the substrate well. Other characteristic planes for hexagonal ZnO were observed as weak peaks at (1 0 1) and (1 0 2). The c lattice parameter of the ZnO micro rods was found to be 0.519 nm. SEM surface micrograph of ZnO micro rods on the glass substrate is shown in the inset of Fig. 1. It is reveal that ZnO microrods display hexagonal shaped rods with submicron size diameter. The dimension and morphology of the rods do not show a homogenous distribution as illustrated in Fig. 1. The composition of ZnO micro rods was determined by the elemental dispersion analysis using X-ray (EDAX) measurements. The compositional analysis of the ZnO films indicated that the atomic percentage ratio of Zn:O is 42.2:57.8. As shown in Fig. 2, a typical XRD pattern of the ZnS film grown by spray pyrolysis at 490 ◦ C on a glass substrate. The diffraction pattern of this film revealed a wurtzite crystal structure ZnS (JCPDS no. 36-1450), with a strong preferred orientation along the hexagonal (0 0 2) ZnS plane direction. Other characteristic planes for hexagonal ZnS were observed as weak peaks at (1 0 0), (1 0 1), (1 1 0), (1 0 3), (1 1 2) and (0 0 4). The lattice parameter of samples, c, was calculated from the position of the ZnS (0 0 2) peak, found to be 0.6248 nm. SEM surface micrograph of ZnS micro rods on the glass substrate is shown in the inset of Fig. 2. As seen in Fig. 2, it is shown hexagonal shaped with submicron size diameter. Consistent with the XRD patterns in Fig. 2, the hexagonal rods were found to be regular and almost perpendicular to the substrate that rods preferentially grow along the (0 0 2) ZnS plane. The compositional analysis of the ZnS films indicated that the atomic percentage ratio of Zn:S:O is 56.7:37.4:5.9. A small fraction of oxygen is still present in the structure.
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Fig. 2. The XRD pattern of the ZnS microrods (inset shows the SEM image).
In order to determine the optical band-gap energy Eg from the absorption spectra measured in the range 300–1100 nm, we have used the variation of absorption coefficient (˛) with the photon energy by using the relation. (˛hv) = A(hv − Eg )
1/2
where Eg is the optical band gap of thin films and A is constant. Fig. 3 shows the plot of (˛hv)2 versus (hv) for the ZnS and ZnO thin films. The extrapolation of the linear portion of the plot onto the energy axis gives the energy band gap of the film and Eg values found to be 3.22 and 3.44 eV for ZnS and ZnO micro rods, respectively. Both of ZnS and ZnO band gap is lower than that of comparing bulk crystal. The native point defects such as oxygen vacancies, sulfur vacancies and zinc interstitials and therefore they are off stoichiometric films. The intrinsic defect levels generated by such vacancies and interstitials lead to n-type doping and they are laid approximately 0.01–0.05 eV below conduction band. Large surface area nature of the films used in this work probably help to accelerate this process. 3.2. Photocatalytic activity ZnO or ZnS coated surface might behave as a good sorbent for MB and this should be measured before photocatalytic experiments. Adsorption of MB (defined as removal percentage) in the absence of light employing ZnO and ZnS coated glass is given in Fig. 4a and the removal percentages determined after three replicates were given in Fig. 4b. It is clear that MB is greatly adsorbed by glass itself (22.7 ± 5.5%), ZnO coated glass (30.3 ± 5.2%) and ZnS coated glass (38.7 ± 1.2%) after a 60 min contact period. ZnS seems to be more attractive for the adsorption of MB molecules. However, removal/degradation of MB in the presence of 365 nm UV light is significantly higher 1.4e+10
-1 2 2
than non illuminated MB solution (Fig. 5). It should be noted that removal percentage of MB slightly increased (from 22.7% to 29.3 ± 5.5%) in the presence of non-coated glass and 365 nm UV light. This increase is due to bleaching of the dye solution by UV light (photolysis) rather than photocatalytic degradation. Both ZnO and ZnS were highly effective achieving 74.0 ± 3.7% and 65.0 ± 5% dye removal/degradation for ZnO and ZnS, respectively. These values are seem to be higher than some literature reports [23] but it was not possible to compare our results with published data because each study uses different experimental conditions and equipments for the catalytic degradation of MB or other dyes. In general, photocatalytic action of ZnO thin film is slightly
ZnO ZnS
1.2e+10
(αhν) (eV.cm )
Fig. 4. Removal percentages of MB by uncoated or coated glass surface (1 × 10−5 M and pH = 6).
1.0e+10 8.0e+9 6.0e+9 4.0e+9 2.0e+9 0.0
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
hν (eV) Fig. 3. Optical band gap energies of ZnO and ZnS coated glass.
Fig. 5. Removal percentages of MB by uncoated or coated glass surface after illumination with 365 nm light.
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Fig. 6. SEM images and XRD pattern of ZnO (a and c) and ZnS (b and d) films after 60 min photocatalytic treatment of MB.
higher than ZnS film. These removal rates are quite good since we only used one 6 W UVA lamp producing 365 nm light and its intensity was only 350 W/cm2 . The main idea was to use a light source near to visible light region. It would have been more effective if we had used a UVC light or a more powerful UVA lamp. After photocatalytic treatment we also re-checked the surface of the both films. SEM images and XRD data of the films are given in Fig. 6. Hexagonal rods of ZnO were still present and unchanged or damaged during the treatment. However, in the case of ZnS micro rods the surface seemed to be occupied by dye molecules and partial damage was observed in some areas. According to XRD data crystal structure of the materials remained unchanged. Both materials were checked for their re-useability after cleaning with water. Dye removal rates were lower for second and third cycles (62% and 47% for ZnO and 50% and 25% for ZnS, respectively). The same materials were cleaned and left under UV light for an hour than re-used for the same tests. It was clear that both materials were still effective but dye removal rates were lower.
4. Conclusion The results of this study revealed that ZnO and ZnS films have been successfully prepared by spray pyrolysis method. The microstructure of ZnO and ZnS rods were characterized by XRD patterns and SEM image. The XRD patterns of these rods exhibit excellent crystalline structure with the preferential orientation of (0 0 2). The SEM images indicate that the aligned hexagonal ZnO and ZnS rods can be grown uniformly in large scale. The optical
band gap values are 3.22 and 3.44 eV for ZnO and ZnS micro rods, respectively and crystal structures of both materials are suitable for photocatalytic degradation of methylene blue. Results show that the dye can be photocatalyticly degraded at high rate (74.0 ± 3.7% for ZnO and 65.0 ± 5% for ZnS) by both films under a 365 nm UV light for a 60 min exposure period. Microstructures are quite stable after treatment and can be used for further applications. Acknowledgement This work was financially supported by Turkish Research Council (TUBITAK, Grant Number 107T853) and authors greatly thank for this support. References [1] M.Y. Chen, C.C. Chang, Comparison of performance of integrated photodetectors based on ZnS and ZnSe metal–semiconductor–metal photodiodes, Jpn. J. Appl. Phys. 48 (2009) 112201. [2] X. Fang, T. Zhai, U.K. Gautam, L. Li, L. Wu, Y. Bando, D. Golberg, ZnS nanostructures: from synthesis to applications, Prog. Mater. Sci. 56 (2011) 175. [3] J.S. Jang, C.-J. Yu, S.H. Choi, S.M. Ji, E.S. Kim, J.S. Lee, Topotactic synthesis of mesoporous ZnS and ZnO nanoplates and their photocatalytic activity, J. Catal. 254 (2008) 144. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001) 1897. [5] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Progress toward 20% efficiency in Cu(In,Ga) Se2 polycrystalline thin-Film solar cells, Prog. Photovolt. 7 (1999) 311. [6] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Electrical spin injection in a ferromagnetic semiconductor heterostructure, Nature 402 (1999) 790. [7] S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Dilute magnetic semiconducting oxides, Semicond. Sci. Technol. 19 (2004) R59.
I˙ . Altın et al. / Applied Surface Science 258 (2012) 4861–4865 [8] S. Chakrabarti, B.K. Dutta, Phtocatalytic degradation of model textile dyes in wastewater using ZnO as semiconductor catalyst, J. Hazard. Mater. 112 (2004) 269. [9] M.D. Hernandez-Alonso, F. Fresno, S. Suarez, J.M. Coronado, Development of alternative photocatalysts to TiO2 : challenges and opportunities, Energ. Environ. Sci. 2 (2009) 1231. [10] C. Hariharan, Photocatalytic degradation of organic contaminants in water by ZnO nanoparticles: revisited, Appl. Catal. A 304 (2006) 55. [11] Y.Z. Li, W. Xie, X.L. Hu, G.F. Shen, X. Zhou, Y. Xiang, X.J. Zhao, P.F. Fang, Comparison of dye photodegradation and its coupling with light-to-electricity conversion over TiO2 and ZnO, Langmuir 26 (2010) 591. [12] M.A. Behnajady, N. Modirshahla, N. Daneshvar, M. Rabbani, Photocatalytic degradation of C.I. Acid Red 27 by immobilized ZnO on glass plates in continuous-mode, J. Hazard. Mater. 140 (2007) 257. [13] N. Daneshvar, D. Salari, M.A. Behnajady, Decomposition of anionic sodium dodecylbenzene sulfonate by UV/TiO2 and UV/H2 O2 processes a—comparison of reaction rates, Iran. J. Chem. Chem. Eng. 21 (2002) 55. [14] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69. [15] M. Pera-Titus, V.G. Molina, M.A. Banos, J. Gimenez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Appl. Catal. B: Environ. 47 (2004) 219. [16] M. Miki-Yoshida, V. Collins-Martinez, P. Amezaga-Madrid, A. AguilarElguezabal, Thin films of photocatalytic TiO2 and ZnO deposited inside a tubing by spray pyrolysis, Thin Solid Films 419 (2002) 60. [17] O.A. Fouad, A.A. Ismail, Z.I. Zaki, R.M. Mohamed, Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity, Appl. Catal. B: Environ. 62 (2006) 144.
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[18] M.H. Habibi, M.K. Sardashti, Preparation of glass plate-supported nanostructure ZnO thin film deposited by sol–gel spin-coating technique and its photocatalytic degradation tomonoazo textile dye, J. Nanomater., vol. 2008, article ID 356765, doi:10.1155/2008/356765. [19] C. Lizama, J. Freer, J. Baeza, H.D. Mansilla, Optimized photodegradation of reactive blue 19 on TiO2 and ZnO suspensions, Catal. Today 76 (2002) 235. [20] A. Akyol, M. Bayramoglu, Photocatalytic performance of ZnO coated tubular reactor, J. Hazard. Mater. 180 (2010) 466. [21] N. Kaneva, I. Stambolova, V. Blaskov, Y. Dimitriev, S. Vassilev, C. Dushkin, Photocatalytic activity of nanostructured ZnO films prepared by two different methods for the photoinitiated decolorization of malachite green, J. Alloys Compd. 500 (2010) 252. [22] A. Akyol, M. Bayramoglu, Preparation and characterization of supported ZnO photocatalyst by zincate method, J. Hazard. Mater. 175 (2010) 484. [23] N. Talebian, M.R. Nilforoushan, Comparative study of the structural, optical and photocatalytic properties of semiconductor metal oxides toward degradation of methylene blue, Thin Solid Films 518 (2010) 2210. [24] C. Wang, Y.H. Ao, P.F. Wang, S.H. Zhang, J. Qian, J. Hou, A simple method for large-scale preparation of ZnS nanoribbon film and its photocatalytic activity for dye degradation, Appl. Surf. Sci. 256 (2010) 4125. [25] K. Shinoda, H. Ohshima, Y. Sato, T. Ogawa, B. Jeyadevan, A. Kasuya, K. Tohji, Proceedings of the International Symposium on Cluster Assembled Materials, Ipap Conference Series 3, 2001, pp. 71–74. [26] M.E. Rincon, M.W. Martinez, M. Miranda-Hernandez, Nanostructured vs. polycrystalline CdS/ZnS thin films for photocatalytic applications, Thin Solid Films 425 (2003) 127. [27] E. Bacaksiz, S. Aksu, B.M. Basol, M. Altunbas¸, M. Parlak, E. Yanmaz, Structural, optical and magnetic properties of Zn1-xCoxO thin films prepared by spray pyrolysis, Thin Solid Films 516 (2008) 7899.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
ZnO and ZnS microrods coated on glass and photocatalytic activity I˙ lknur Altın a , I˙ smail Polat b , Emin Bacaksız b , Münevver Sökmen a,∗ a b
Department of Chemistry, Faculty of Science, Karadeniz Technical University, 61080 Trabzon, Turkey Department of Physics, Faculty of Science, Karadeniz Technical University, 61080 Trabzon, Turkey
a r t i c l e
i n f o
Article history: Received 14 November 2011 Received in revised form 5 January 2012 Accepted 14 January 2012 Available online 23 January 2012 Keywords: ZnO coating ZnS coating Spray pyrolysis Photocatalysis
a b s t r a c t In this study, the large-scale ZnO and ZnS rods at sub-micrometer were prepared on soda lime glass substrate using a spray pyrolysis method. The microstructure of the rods was characterized by X-ray diffractometry and scanning electron microscopy with the energy dispersive X-ray spectroscopy, and the optical properties were investigated. XRD and SEM results show that the wurtzite structure and rodlike ZnO and ZnS at micro scale were obtained. The optical band gap values were 3.22 and 3.44 eV for ZnO and ZnS microrods, respectively. The obtained samples were tested for their photocatalytic ability using them for the degradation of methylene blue (MB, 1 × 10−5 M). Results show that the dye can be degraded at quite high rate (74.0 ± 3.7% for ZnO and 65.0 ± 5% for ZnS) by both films under a 365 nm UV light after a 60 min exposure period. The materials were re-checked after the treatment and microstructures were observed mainly unchanged. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zinc sulfide (ZnS) is a promising material for the use in various application devices such as nano sized sensors, photodiode and photocatalyst for the degradation of organic dyes [1–3]. It is an important semiconductor with a higher band gap (3.72 eV for cubic zinc blend and 3.77 eV for hexagonal wurtzite), large exciton binding energy (40 meV) and a small Bohr radius (2.4 nm). Besides, ZnS is available in abundance and nontoxic similar to titanium dioxide (TiO2 ). ZnS nano materials have been used for the photocatalytic degradation of organic pollutants such as dyes, p-nitrophenol, and halogenated benzene derivatives in wastewater treatment [2 and references there in]. Similarly, zinc oxide (ZnO) is an oxide semiconductors for potential applications in the optoelectric field due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV) [4]. ZnO is a low cost material with various industrial applications in manufacturing gas sensors, Schottky diodes, solar cells, a photocatalyst, etc. [5–8]. Considering the ever-decreasing dimensions of electronic devices, producing self-assembled micro- and nano structured materials systems is becoming increasingly important for commercial applications. Since TiO2 in the anatase form has been used for photocatalyst application ZnS and ZnO are suitable alternative to TiO2 so far as band gap energies are concerned. While ZnO exhibits similar photocatalytic activity to TiO2 [9] it was better in some cases [10,11]. Similar to TiO2
∗ Corresponding author. Tel.: +90 462 3772532; fax: +90 462 3253196. E-mail address: [email protected] (M. Sökmen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.082
photocatalysis, ZnO or ZnS produce hydroxyl radicals via charged holes (h+ ) and excited electrons (e− ) which are the main reactive species for oxidative/reductive degradation of a wide range of organic pollutants [12–15]. But the main problems are its stability especially in acidic conditions and photo-corrosion. It should be kept in mind that the photocatalytic activity is strongly depend on the processing conditions of the material and stability can be increased by the method used for preparation. Photoactive ZnO or ZnS particles can be utilized in the form of thin film if they are coated on a surface. This eases the usage of semiconductor because after treatment period filtration of the photoactive nano particles is somehow difficult requiring sophisticated filtration techniques and increase the cost of the process. However, in the case of coating the active particles on a surface, the interaction between the molecule and catalyst must be quite different because of the distance between the particle and the molecules to be degraded. Therefore, this interaction should be checked for a specific pollutant and coated surface. Various coating methods have been discussed in literature and spray pyrolysis was one of these techniques that had been used for simple, cheap and convenient production for ZnO films [16]. The ZnS particles are interesting entities for catalytic activities, since its remarkable chemical stability against oxidation and hydrolysis, and they can be used for photocatalytic treatment of aggressive environments. Although many reports related to photocatalytic removal of dyes employing ZnO thin films are available in current literature [12,17–23], only limited number of studies had been published involving ZnS thin films for this purpose [24–26]. In a study carried out by Wang et al. [24] ZnS nano ribbon film had been successfully prepared by solvothermal treating of Zn foil and sulfur
4862
I˙ . Altın et al. / Applied Surface Science 258 (2012) 4861–4865
powder in hydrazine hydrate without any surfactant and post-high temperature treatment. Obtained films have high crystallinity and uniformity and were used as a photocatalyst for degradation of dye X-3B. ZnS nano ribbon film was highly effective (about 88.4% degradation after 100 min. irradiation with a UV light) for degradation of the dye under UV light irradiation. In another study, ZnS thin film was tested for the production of hydrogen gas from the solution containing hydrogen sulfide (HS− ) ions [25]. Similarly, Rincon et al. [26] determined the structural, optical and photo-electrochemical properties of ZnS thin film and had used it for hydrogen production. In this study, we report a simple route for coating ZnS and ZnO particles on glass substrates by spray pyrolysis method. We aimed to produce these materials at large scale. After coating an annealing step was carried out to obtain desired crystal structure and surface properties were evaluated. Following this photocatalytic activities of ZnS and ZnO coated materials were tested for their capacity for degradation of methylene blue (MB). Recently MB is considered as model chemical for evaluation of this kind of materials.
Fig. 1. The XRD pattern of the ZnO microrods (inset shows the SEM image).
2. Experimental
The decolorization efficiency, which is defined as removal percentage, was calculated as:
2.1. ZnO and ZnS coating
Removal% =
ZnO and ZnS thin films were obtained by spray pyrolysis at 550 and 490 ◦ C in air atmosphere, respectively. The experimental set up and the other experimental details were explained elsewhere [27]. ZnO stock solution was prepared from zinc chloride (ZnCl2 ) at 0.1 M concentration in deionized water. ZnS solution was prepared from zinc chloride (ZnCl2 ·6H2 O) at 0.05 M and thiourea [SC(NH2 )2 ] at 0.1 M in deionized water. The glass substrates were cleaned in ethanol and then dried in vacuum prior to coating. The growth was performed with a spray rate of about 5 ml/min and the growth rate was approximately 50 nm/min. During the growth, the substrates were rotated with a speed of 10 cycles per minute and thin films were annealed under a vacuum 30 min at 490 ◦ C. 2.2. Structural and surface analysis The X-ray diffraction (XRD) data of the films was taken using a Rigaku D/Max-IIIC diffractometer with CuK␣ radiation over the range 2 = 20–60◦ at room temperature. The surface morphology was studied using a JEOL JST-6400 scanning electron microscope. The voltage applied was 10 kV. Optical transmission data in the 190–1100 nm range was obtained with a Shimadzu UV-1601 spectrophotometer. 2.3. Photocatalytic activity of ZnO and ZnS thin films Photocatalytic efficiencies were tested for the removal of aqueous solution of methylene blue (MB). A set of experiments were designed to compare the photocatalytic actions of the coated and non-coated glass in the presence and absence of UV light. Talebian et al. [23] report that photocatalytic degradation of MB was pH dependent. Therefore, we set up our experiments at pH 6 which is also natural pH of MB. Photocatalytic experiments were carried out in aqueous solution of MB employing a UVA emitting lamp (365 nm, Spectroline ENF-260) at dye’s natural pH. A 5 mL portion of 1 × 10−5 M MB was placed in a quartz cell containing glass material (2 pieces, approximately 1 cm × 1 cm). Special attention was paid for contacting the dye solution and UV light with glass surface and solutions were shaken continuously. The absorption of MB at 668 nm (UV–vis Dr. Lange CADAS 200 Spectrometer) was monitored over 60 min in the presence and absence of light or glass material. At regular intervals of irradiation, aliquots of 2 mL sampled and then absorption was measured in terms of change in intensity at 668 nm.
C0 − C × 100 C0
where; C0 was the initial concentration and C was the concentration of dye after a certain treatment period (in the presence or absence of light). 3. Results and discussion 3.1. Structural and optical properties The XRD results indicate that the zinc chloride was completely decomposed to give ZnO as shown in Fig. 1. XRD pattern of the ZnO micro rods grown on the substrate a high intensity of (0 0 2) diffraction peak was detected and implied that ZnO micro rods have a typical hexagonal structure and grow along a c-axis direction perpendicular to the substrate well. Other characteristic planes for hexagonal ZnO were observed as weak peaks at (1 0 1) and (1 0 2). The c lattice parameter of the ZnO micro rods was found to be 0.519 nm. SEM surface micrograph of ZnO micro rods on the glass substrate is shown in the inset of Fig. 1. It is reveal that ZnO microrods display hexagonal shaped rods with submicron size diameter. The dimension and morphology of the rods do not show a homogenous distribution as illustrated in Fig. 1. The composition of ZnO micro rods was determined by the elemental dispersion analysis using X-ray (EDAX) measurements. The compositional analysis of the ZnO films indicated that the atomic percentage ratio of Zn:O is 42.2:57.8. As shown in Fig. 2, a typical XRD pattern of the ZnS film grown by spray pyrolysis at 490 ◦ C on a glass substrate. The diffraction pattern of this film revealed a wurtzite crystal structure ZnS (JCPDS no. 36-1450), with a strong preferred orientation along the hexagonal (0 0 2) ZnS plane direction. Other characteristic planes for hexagonal ZnS were observed as weak peaks at (1 0 0), (1 0 1), (1 1 0), (1 0 3), (1 1 2) and (0 0 4). The lattice parameter of samples, c, was calculated from the position of the ZnS (0 0 2) peak, found to be 0.6248 nm. SEM surface micrograph of ZnS micro rods on the glass substrate is shown in the inset of Fig. 2. As seen in Fig. 2, it is shown hexagonal shaped with submicron size diameter. Consistent with the XRD patterns in Fig. 2, the hexagonal rods were found to be regular and almost perpendicular to the substrate that rods preferentially grow along the (0 0 2) ZnS plane. The compositional analysis of the ZnS films indicated that the atomic percentage ratio of Zn:S:O is 56.7:37.4:5.9. A small fraction of oxygen is still present in the structure.
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Fig. 2. The XRD pattern of the ZnS microrods (inset shows the SEM image).
In order to determine the optical band-gap energy Eg from the absorption spectra measured in the range 300–1100 nm, we have used the variation of absorption coefficient (˛) with the photon energy by using the relation. (˛hv) = A(hv − Eg )
1/2
where Eg is the optical band gap of thin films and A is constant. Fig. 3 shows the plot of (˛hv)2 versus (hv) for the ZnS and ZnO thin films. The extrapolation of the linear portion of the plot onto the energy axis gives the energy band gap of the film and Eg values found to be 3.22 and 3.44 eV for ZnS and ZnO micro rods, respectively. Both of ZnS and ZnO band gap is lower than that of comparing bulk crystal. The native point defects such as oxygen vacancies, sulfur vacancies and zinc interstitials and therefore they are off stoichiometric films. The intrinsic defect levels generated by such vacancies and interstitials lead to n-type doping and they are laid approximately 0.01–0.05 eV below conduction band. Large surface area nature of the films used in this work probably help to accelerate this process. 3.2. Photocatalytic activity ZnO or ZnS coated surface might behave as a good sorbent for MB and this should be measured before photocatalytic experiments. Adsorption of MB (defined as removal percentage) in the absence of light employing ZnO and ZnS coated glass is given in Fig. 4a and the removal percentages determined after three replicates were given in Fig. 4b. It is clear that MB is greatly adsorbed by glass itself (22.7 ± 5.5%), ZnO coated glass (30.3 ± 5.2%) and ZnS coated glass (38.7 ± 1.2%) after a 60 min contact period. ZnS seems to be more attractive for the adsorption of MB molecules. However, removal/degradation of MB in the presence of 365 nm UV light is significantly higher 1.4e+10
-1 2 2
than non illuminated MB solution (Fig. 5). It should be noted that removal percentage of MB slightly increased (from 22.7% to 29.3 ± 5.5%) in the presence of non-coated glass and 365 nm UV light. This increase is due to bleaching of the dye solution by UV light (photolysis) rather than photocatalytic degradation. Both ZnO and ZnS were highly effective achieving 74.0 ± 3.7% and 65.0 ± 5% dye removal/degradation for ZnO and ZnS, respectively. These values are seem to be higher than some literature reports [23] but it was not possible to compare our results with published data because each study uses different experimental conditions and equipments for the catalytic degradation of MB or other dyes. In general, photocatalytic action of ZnO thin film is slightly
ZnO ZnS
1.2e+10
(αhν) (eV.cm )
Fig. 4. Removal percentages of MB by uncoated or coated glass surface (1 × 10−5 M and pH = 6).
1.0e+10 8.0e+9 6.0e+9 4.0e+9 2.0e+9 0.0
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
hν (eV) Fig. 3. Optical band gap energies of ZnO and ZnS coated glass.
Fig. 5. Removal percentages of MB by uncoated or coated glass surface after illumination with 365 nm light.
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Fig. 6. SEM images and XRD pattern of ZnO (a and c) and ZnS (b and d) films after 60 min photocatalytic treatment of MB.
higher than ZnS film. These removal rates are quite good since we only used one 6 W UVA lamp producing 365 nm light and its intensity was only 350 W/cm2 . The main idea was to use a light source near to visible light region. It would have been more effective if we had used a UVC light or a more powerful UVA lamp. After photocatalytic treatment we also re-checked the surface of the both films. SEM images and XRD data of the films are given in Fig. 6. Hexagonal rods of ZnO were still present and unchanged or damaged during the treatment. However, in the case of ZnS micro rods the surface seemed to be occupied by dye molecules and partial damage was observed in some areas. According to XRD data crystal structure of the materials remained unchanged. Both materials were checked for their re-useability after cleaning with water. Dye removal rates were lower for second and third cycles (62% and 47% for ZnO and 50% and 25% for ZnS, respectively). The same materials were cleaned and left under UV light for an hour than re-used for the same tests. It was clear that both materials were still effective but dye removal rates were lower.
4. Conclusion The results of this study revealed that ZnO and ZnS films have been successfully prepared by spray pyrolysis method. The microstructure of ZnO and ZnS rods were characterized by XRD patterns and SEM image. The XRD patterns of these rods exhibit excellent crystalline structure with the preferential orientation of (0 0 2). The SEM images indicate that the aligned hexagonal ZnO and ZnS rods can be grown uniformly in large scale. The optical
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