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Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity
Applied Catalysis B: Environmental 62 (2006) 144–149 www.elsevier.com/locate/apcatb
Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity O.A. Fouad *, A.A. Ismail, Z.I. Zaki, R.M. Mohamed Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt Received 17 April 2005; received in revised form 4 July 2005; accepted 19 July 2005 Available online 12 September 2005
Abstract Thin zinc oxide (ZnO) films have been grown on silicon substrates by thermal physical vapor deposition approach. X-ray diffraction (XRD) analyses reveal that the deposited films are polycrystalline ZnO phase. Atomic force microscopy images (AFM) show needle-like shape highly oriented ZnO crystals. Thin film thickness ranges from 10 to 80 nm. X-ray photoelectron spectroscopy (XPS) results declare that the films compose mainly of Zn and O. Nevertheless, Si is not detected in the films and consequently no possibility of any silicide formation as is confirmed by XRD analysis. Photocatalytic decomposition of azo-reactive dye on ZnO films is tested. The results show that the dye decomposition efficiency increases with decreasing pH. Maximum photodecomposition, 99.6% is obtained at pH 2 with 10 mg/l dye concentration. # 2005 Elsevier B.V. All rights reserved. Keywords: ZnO thin films; Vapor deposition; Azo-reactive dye; Photocatalytic activity
1. Introduction Highly controlled functional semiconductor materials find wide scope of applications in many fields such as photocatalysis. Illumination of semiconductor with photon of energy greater than its bandgap energy enhances the oxidizing power of organic pollutants present at or near its surface. The photocatalysis gives the advantages of degrading pollutants using only oxygen as an oxidant, low costs and resulting in safe and environmentally friendly products [1–5]. The main drawback in semiconductor photocatalysis is the relatively low value of the overall energy efficiency. This is mainly because of the very short mean free path of electrons results from fast recombination of photoinduced electron–hole pairs at/or near the surface. This leads to low quantum efficiency of the photocatalytic processes and termination of the photodegradation reaction at the reaction conditions [1,5,6]. One of the methods of enhancing the * Corresponding author. Tel.: +20 2 5010642; fax: +20 2 5010639. E-mail address: [email protected] (O.A. Fouad). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.07.006
efficiencies of photocatalysts is the use of nano-sized semiconductor crystallites instead of bulk materials. Nanosized semiconductors have found widespread applications as new functional materials because of their characteristic surface properties [1,7]. Semiconductor zinc oxide (ZnO) has been a subject of many research groups nowadays. Its availability in bulk, single crystal form and its high exciton binding energy (60 meV) are of great importance for its application to UV light emitters, wave device, piezoelectric transducers, gas sensing and solar cells. As a wide bandgap (3.27 eV at room temperature) semiconductor, ZnO is a candidate host for blue to UV optoelectronics [1,2]. In addition, ZnO has been used as environmental photocatalyst for water purification with the aid of artificial light source [3,4,6,8– 10]. Many techniques have been used for deposition of high quality ZnO films such as chemical vapor deposition (CVD), pulsed laser deposition (PLD), molecular beam epitaxy (MBE), anodizing, sputtering and thermal evaporation [1,2,11–16]. Despite of its obvious economic and simplicity, thermal oxidation deposition technique has found very little
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attention by research groups [17–19]. Moreover, up to the best of our knowledge, there are no photocatalytic tests have been performed on polycrystalline ZnO films deposited by this technique at a relatively low temperature. In this paper we present the growth and photocatalytic activity of ZnO thin films prepared by thermal physical vapor deposition approach. The properties of the asdeposited films, crystal structure, chemical composition and surface morphology are explored. The photocatalytic activity of the deposited films on an azo-reactive dye are tested and its results are also presented.
2. Experimental 2.1. Growth of ZnO thin films ZnO thin films of 10–80 nm size are deposited on silicon substrates. Zinc targets (5 mm 5 mm and purity 99.5%) are used as a source of zinc vapor for zinc oxide deposition. The silicon substrate (1 cm 1 cm) is hold on a ceramic holder and inserted inside the deposition chamber which then evacuated using a combinational rotary and oil diffusion vacuum pumps to a base pressure of 1 105 Torr. Torr. The total pressure of the chamber during deposition is controlled at 5 103 Torr. The target and substrate are heated by a heating coil about 5 mm apart from both of them. The deposition and oxidation temperatures range from 350 to 650 8C. The total deposition time is varied from 10 to 30 min. The chamber gas environment during deposition consisted of hydrogen, argon and oxygen gases. Mass flow controllers and needle valves are installed to control the flow of the gases at 20, 15 and 5 sccm, respectively. 2.2. Characterization of ZnO films Phase identification of the deposited zinc oxide films is carried out by X-ray diffraction (XRD; Rigaku Rint700 and Bruker axs D8, Germany). The XRD machine is operated at 40 kV and 30 mA. The u–2u mode is employed and Cu Ka ˚ is used. Chemical composition of radiation of l = 1.5405 A the films is determined by X-ray photoelectron spectroscopy (XPS; Kratos Axi-HS) using Mg Ka radiation (1253.6 eV) as X-ray source. Morphologies of the deposited films are investigated by scanning electron microscopy using a focused ion beam system (FIB; Hitachi FB-2000A) and atomic force microscopy (AFM; SPI 3800N probe station).
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of C.I. Reactive Black 5, RB5 (Remazol Black B 133X) dye solution. C.I. Reactive Black 5 is selected, as a model for the photocatalytic degradation experiments because it is nonvolatile and is a common contaminant in the industrial wastewaters. A black light-blue florescent bulb (F18WBLB) is horizontally positioned above the reactor to supply UV illumination. The wavelength of the light passing through a reaction suspension is 365 nm. The experiments are performed by immersing the film into the reactor with 100 ml dyes (10–100 mg/l). The reaction is carried out isothermally at 25 8C and pH 2–10 for 3 h. Changing the original pH of the solution is carried out by addition of either NaOH or HCl solutions. At the end of reaction time, the UV power is turned off and the zinc oxide film is taken out of the solution which is then analyzed. The extent of photocatalytic degradation is evaluated by measuring the absorbance of the solution samples with a UV–vis spectrophotometer (UV– vis; Cecil7200) at lmax = 525 nm. The decomposition efficiency of dye is estimated by applying the following equation: dye decomposition efficiency ð%Þ ¼ ½ðC0 CÞ=C0 100 where C0 and C are the initial and residual dye contents in solution, respectively.
3. Results and discussion 3.1. Crystallinity, composition and morphology of ZnO film XRD pattern of the deposited zinc oxide thin film at 350 8C and oxidized at 550 8C, at 10 min total deposition time, and at 20, 15 and 5 sccm flowrates of H2, Ar and O2, respectively, is presented in Fig. 1. The XRD pattern of deposited film at lower, 500 8C, and higher oxidation temperature, 650 8C, are also shown there for comparison. We ascribed the diffraction peaks from the zinc oxide film to
2.3. Photocatalytic activity test All experiments are carried out in a 250 ml Pyrex beaker in which the dye solution is introduced. The beaker including the dye solution is put on a magnetic stirrer and the solution is stirred by means of a magnetic rod. Silicon substrates, on which zinc oxide films are deposited, are hanged in the middle of the beaker and fixed inside the bulk
Fig. 1. XRD patterns of zinc oxide films deposited at 350 8C and oxidized at: (a) 500 8C, (b) 550 8C and (c) 650 8C.
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the (0 0 2), (1 0 1), (1 0 2) and (1 1 0) diffraction planes at 2u = 34.42, 36.29, 47.53 and 56.578, respectively (JCPDS# 79-2205). On the other hand we ascribed the diffraction peaks from zinc film to the (0 0 2), (1 0 0), (1 0 1) and (1 0 2) diffraction planes at 2u = 36.28, 38.94, 43.23 and 54.338, respectively (JCPDS# 4-831). It is obvious that the deposited films are polycrystalline. It has been reported that trace amount of zinc metal is also detected in the film prepared by evaporating a mixture of 70% Zn and 30% Se at wet oxidation condition (10% O2 + 5% H2 + 85% Ar) at 100 8C [20]. From Fig. 1 it is also clear that peak intensities are fairly weak. This is probably due to roughness and/or coarsening of the film or due to the film thickness. Fig. 2 shows XPS spectra of the as-deposited zinc oxide film (Fig. 2a) and after sputtering by Ar+ ion for 0.5 min (Fig. 2b). The peaks from Zn and O of ZnO film are detected at 1023 eV (Zn 2p) and 532 eV (O 1s), respectively. Also, carbon is adsorbed on the film surface as the C 1s peak is observed at 285 eV. Carbon contaminates only the film surface and did not show significant concentration in the bulk of the film. This observation is supported by the XRD analyses, which did not confirm the formation of any carbide phase in the film. After sputtering the film by Ar+ ion for 0.5 min, the intensity of carbon peak decreased significantly. Whereas the intensity of oxygen and zinc peaks increased (Fig. 2b). This might be due to the removal of the surface organic carbon layer that contaminated the film surface. No significant chemical shift is observed in peak positions upon sputtering. This indicates that the composition of the film surface is more likely the same as its bulk. More detail investigation of the XPS peaks for each element in the asdeposited film and after sputtering is illustrated in Fig. 3. SEM image of the zinc oxide film deposited at 350 8C and oxidized at 550 8C is shown in Fig. 4. It illustrates that the film surface is homogeneous and uniform. It is clear that the deposited film composes mainly of spherical-like shape grains. SEM investigations of samples oxidized at higher temperatures show that the films compose of larger grains.
Fig. 2. XPS spectra of zinc oxide films: (a) as-deposited and (b) after Ar+ ion sputtering.
More detailed investigation of the film surface and its structure is obtained from AFM images in Fig. 5. It reveals the growth of highly oriented needle-like shape ZnO crystals in the range of 50–80 nm. Fig. 5a and b shows the film image (2D and 3D views) at lower magnification whereas Fig. 5c and d shows the film image (2D and 3D views) at higher magnification. 3.2. Photocatalytic activity tests of ZnO films In the beginning, photodecomposition experiment of dye is carried out under the irradiation with UV light without addition of catalyst as a blank test to evaluate the contribution of direct photolysis in the total photocatalytic decomposition process. No significant photodecomposition efficiency is noticed. To promote the photocatalytic decomposition of the dye, three ZnO film samples named ZF, ZD, and ZU have been tested. For all of these three samples, zinc is deposited at 350 8C and oxidized at 500, 550 and 650 8C, respectively. The influence of the initial dye concentration on the photocatalytic decomposition reaction together with the photocatalyst loading are important factors to evaluate the whole process. The effect of initial dye concentration on the photocatalytic decomposition efficiency using the three zinc oxide films is investigated in the range 10–100 mg/l dye concentration. We studied their effects on dye photodecomposition at pH 6.7 for 3 h. The results in Fig. 6 show that the extent of dye decomposition is not more than 23% at various dye concentrations regardless the difference in zinc oxide film samples. One can say that in the case of 100 mg/l during the same period 23 mg/l of the dye has been decomposed, while at 10 mg/l only 2.3 mg/l. So, at the same other conditions, at 100 mg/l dye concentration it is decomposed of an order faster than at 10 mg/l. This result is in agreement to what have been observed when ZnO particles are used [8–10]. One of the important parameters governing the rate of reaction taking place on zinc oxide particle is the pH of solution. This is because of the amphoteric property (acid– base property) of ZnO semiconductor which influences the surface-charge property of the photocatalyst. In addition, wastewater from textile industries usually have a wide range of pH values. Further, pH plays an important role in generation of hydroxyl radicals. The OH radicals are extremely strong nonselective oxidant (E0 = +3.06 V). This property leads to partial or complete mineralization of several organic pollutants [9]. Fig. 7 presents the dye decomposition using the three ZnO films at dye solution concentration 10 mg/l and different pH values 2, 6 and 10. The photocatalytic decomposition of the dye increased using ZnO films in the following order ZD > ZU > ZF in the tested pH range. This can be attributed to the difference in oxidation temperature for the three ZnO films. At pH 2, for ZF sample the oxidation temperature is 500 8C, the dye
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Fig. 3. Narrow range XPS spectra of Zn in zinc oxide films: (a) as-deposited and (b) after Ar+ ion sputtering.
photocatalytic decomposition is 60%. This is might be due to incomplete ZnO growth at such temperature, whereas for sample (ZD) prepared at 550 8C, dye photodecomposition is 99.6% due to complete growth of ZnO film as shown in the XRD patterns (Fig. 1). Meanwhile, for ZU sample which prepared via oxidation at temperature 650 8C, dye photocatalytic decomposition is about 90%. This can be attributed to the decrease in specific surface area as a result of decrease in percentage of atoms on the semiconductor surface due to
Fig. 4. SEM image of zinc oxide film surface deposited at 350 8C and oxidized at 550 8C.
agglomeration of ZnO grains at such higher temperature which accompanied by change in particle surface, including sintering among particles and surface defects [21]. It is also observed from Fig. 7 that in acidic solution photocatalytic decomposition efficiency of the dye is more than that in alkaline solution. In addition, acidic and alkaline solutions are more favored than nearly neutral solution. Maximum dye photocatalytic decomposition 99.6% is achieved at pH 2. Whereas at pH 10, photocatalytic decomposition of dye is higher than that at pH 6 and a photodecomposition efficiency of about 60% is achieved for all films. The lowest dye photocatalytic decomposition efficiency (10–25%) for all the thin films is observed at pH 6. This is might be due to photocorrosion of ZnO particles. It was reported that photodecomposition of ZnO took place in acidic solutions and its photocorrosion induced by selfoxidation is complete at pH lower than 4 [8]. The interpretation of pH effects on the photocatalytic decomposition of process of the dye is a difficult task. This is because of the contribution of three possible reaction mechanisms to the dye decomposition. These mechanisms are: dye degradation by hydroxyl radical attack, direct oxidation by positive hole, and direct reduction by the electron in the conduction band [22]. The importance of each mechanism has been found to be depend on the catalyst nature and pH [22]. In the present work, it can be suggested that two mechanisms governed the photocatalytic decomposition of RB5 dye on ZnO films. The first mechanism can be implied at pH 2–6 (acidic solution) and the reaction process is presented by protonation of the dye which indeed affected the adsorption characteristics of the dye and its redox activity. The zero-point charge (zpc) for ZnO is 9.0. Based
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Fig. 5. AFM profile of zinc oxide film deposited at 350 8C and oxidized at 550 8C (a and b) low magnification and (c and d) high magnification.
on the zpc value, ZnO surface is positively charged below pH 9 based on its zpc. At low pH value, electrostatic interactions between the positive photocatalyst surface and dye anions (which generated from the dissociation of sodium salt of the dye molecule, [dye–Na] ! [dye] + Na+) in aqueous solution may be lead to strong adsorption of the dye on the catalyst surface. Sakthivel et al. reported the same observation for Acid Brown 14, AB14, dye on ZnO catalyst [9]. The second mechanism can be implied at high pH value (alkaline solution) and the reaction process is presented by the hydroxyl radicals (OH) attack, which are favored at the investigated high pH value, 10. The presence of large quantities of OH ions on the particle surfaces as well as in the reaction medium favors the formation of OH radicals. The reason why the photocatalytic decomposition of the RB5 dye is lower in alkaline
than in acidic solution can be attributed to its chemical structure, Fig. 8. RB5 has sulfuric groups in its structure, which are negatively charged in alkaline solutions, therefore, in the alkaline solution dye may not be adsorbed onto the surface of the photocatalyst effectively. This result is also found for acid red 14, AR14, by Daneshvar et al. [8]. On the other hand at such pH value the hydroxyl radicals are so rapidly scavenged that they do not have the opportunity to react effectively with the dye [8,9]. When the crystallite dimension of a semiconductor particle falls down to a nanometer size, the rate constant of charge transfer increases. As a result of this, the photoefficiencies for systems in which the rate-limiting step is charge transfer increased. Size reduction also leads to major changes in the effective redox potential of the photogenerated charge carriers. An increase in redox potential
Fig. 6. Effect of initial dye concentration on the photocatalytic decomposition efficiency.
Fig. 7. Effect of dye solution pH on the photocatalytic decomposition efficiency.
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Fig. 8. Chemical structure of C.I. Reactive Black 5 dye.
means there is more driving force behind the photocatalytic reaction and increased rates will be observed. The reaction is thought to be proceeded via reaction of the dye with hydroxyl ions onto catalyst surface followed by reaction with holes existing in excited semiconductor catalyst due to UV illumination. The overall process can be represented by a group of equations. Upon exposure to UV irradiation, ZnO is photoexcited and an electron–hole pair is formed as follows: þ ZnO þ hn ! ZnOðe CB þ hVB Þ
(2)
The formed OH are oxidized further by the hole to produce OH radicals: hþ VB þ OH ! OH
(3)
The OH radicals lead to partial or complete dye decomposition as follows: OH þ C:I: RB5 dye ! decomposed dye
(4)
On the other hand, in acidic solution ZnO reacted with the photogenerated holes and undergoes self-oxidation as follows: 2þ ZnO þ 2hþ þ O ðactive oxygenÞ VB ! Zn
evaporation deposition has been investigated. XPS and XRD analyses show that the produced film are polycrystalline ZnO and composes mainly of Zn and O. AFM images illustrate that the films are thin and ranges in thickness from 10 to 80 nm. The results show that the deposited and oxidized film at temperature 550 8C (ZD) gave the highest photodecomposition efficiency. Maximum photocatalytic decomposition (99.6%) of the dye is achieved at pH 2 with dye concentration 10 mg/l and at the irradiation time of 3 h. The presented results in this paper indicate that UV/ZnO thin film is an effective combination for photocatalytic degradation of C.I. Reactive Black 5 dye. UV and ZnO have negligible effect on catalytic degradation of the dye when they are used individually.
(1)
þ where e CB is the electron in the conduction band and hVB is þ the hole in the valence band. ZnOðeCB þ hVB Þ is photoexcited ZnO with the formation of electron in the conduction band and hole in the valence band. Due to the amphoteric property of ZnO semiconductor, water molecules in alkaline solution adsorbed on its excited surface and decomposed by oxidative potential of the hole according to the following equation: þ hþ VB þ H2 O ! H þ OH
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(5)
In this case the dye decomposed by the action of the generated active oxygen. As the mobility of active oxygen is might be higher than that of OH radical, the photocatalytic decomposition efficiency of the dye in acidic solution is higher than that in alkaline solution.
4. Conclusion Photocatalytic decomposition of C.I. Reactive Black 5 dye using ZnO thin films photocatalyst prepared by thermal
References [1] J. Liqianga, S. Xiaojunb, S. Jingc, C. Weiminb, X. Zilic, D. Yaoguoc, F. Hongganga, Sol. Energy Mater. Sol. C 79 (2003) 133. [2] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct. 34 (2003) 3. [3] S. Chakrabarti, B.K. Dutta, J. Hazard. Mater. B 112 (2004) 269. [4] S. Rabindranathan, S. Devipriya, S. Yesodharan, J. Hazard. Mater. B 102 (2003) 217. [5] D. Chen, A.K. Ray, Chem. Eng. Sci. 56 (2001) 1561. [6] K. Tennakone, J. Bandara, Appl. Catal. A: Chem. 208 (2001) 335. [7] B. Pal, M. Sharon, Mater. Chem. Phys. 76 (2002) 82. [8] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A 162 (2004) 317. [9] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. C 77 (2003) 65. [10] A. Akyola, H.C. Yatmaza, M. Bayramoglu, Appl. Catal. B: Environ. 54 (2004) 19. [11] S.-S. Lin, J.-L. Huang, Surf. Coat. Technol. 185 (2004) 222. [12] Ya.I. Alivov, A.V. Chernykh, M.V. Chukichev, R.Y. Korotkov, Thin Solid Films 473 (2005) 241. [13] A.Kh. Abduev, B.M. Ataev, A.M. Bagamadova, Inorg. Mater. 11 (1987) 1928. [14] S. Choopun, H. Tabata, T. Kawai, J. Cryst. Growth 274 (2005) 167. [15] Y. Chen, D.M. Bagnall, H. Koh, K. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [16] Y. Yamaguchi, M. Yamazaki, S. Yoshihara, T. Shirakashi, J. Electroanal. Chem. 442 (1998) 1. [17] S.J. Chen, Y.C. Liu, J.G. Ma, D.X. Zhao, Z.Z. Zhi, Y.M. Lu, J.Y. Zhang, D.Z. Shen, X.W. Fan, J. Cryst. Growth 240 (2002) 467. [18] R.K. Gupta, N. Sridhar, M. Katiar, Mater. Sci. Semicond. Process. 5 (2002) 11. [19] Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, H.H. Hng, J. Appl. Phys. 94 (2003) 354. [20] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [21] D. Li, H. Haneda, Chemosphere 51 (2003) 129. [22] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. B: Environ. 89 (2002) 303.
Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity O.A. Fouad *, A.A. Ismail, Z.I. Zaki, R.M. Mohamed Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt Received 17 April 2005; received in revised form 4 July 2005; accepted 19 July 2005 Available online 12 September 2005
Abstract Thin zinc oxide (ZnO) films have been grown on silicon substrates by thermal physical vapor deposition approach. X-ray diffraction (XRD) analyses reveal that the deposited films are polycrystalline ZnO phase. Atomic force microscopy images (AFM) show needle-like shape highly oriented ZnO crystals. Thin film thickness ranges from 10 to 80 nm. X-ray photoelectron spectroscopy (XPS) results declare that the films compose mainly of Zn and O. Nevertheless, Si is not detected in the films and consequently no possibility of any silicide formation as is confirmed by XRD analysis. Photocatalytic decomposition of azo-reactive dye on ZnO films is tested. The results show that the dye decomposition efficiency increases with decreasing pH. Maximum photodecomposition, 99.6% is obtained at pH 2 with 10 mg/l dye concentration. # 2005 Elsevier B.V. All rights reserved. Keywords: ZnO thin films; Vapor deposition; Azo-reactive dye; Photocatalytic activity
1. Introduction Highly controlled functional semiconductor materials find wide scope of applications in many fields such as photocatalysis. Illumination of semiconductor with photon of energy greater than its bandgap energy enhances the oxidizing power of organic pollutants present at or near its surface. The photocatalysis gives the advantages of degrading pollutants using only oxygen as an oxidant, low costs and resulting in safe and environmentally friendly products [1–5]. The main drawback in semiconductor photocatalysis is the relatively low value of the overall energy efficiency. This is mainly because of the very short mean free path of electrons results from fast recombination of photoinduced electron–hole pairs at/or near the surface. This leads to low quantum efficiency of the photocatalytic processes and termination of the photodegradation reaction at the reaction conditions [1,5,6]. One of the methods of enhancing the * Corresponding author. Tel.: +20 2 5010642; fax: +20 2 5010639. E-mail address: [email protected] (O.A. Fouad). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.07.006
efficiencies of photocatalysts is the use of nano-sized semiconductor crystallites instead of bulk materials. Nanosized semiconductors have found widespread applications as new functional materials because of their characteristic surface properties [1,7]. Semiconductor zinc oxide (ZnO) has been a subject of many research groups nowadays. Its availability in bulk, single crystal form and its high exciton binding energy (60 meV) are of great importance for its application to UV light emitters, wave device, piezoelectric transducers, gas sensing and solar cells. As a wide bandgap (3.27 eV at room temperature) semiconductor, ZnO is a candidate host for blue to UV optoelectronics [1,2]. In addition, ZnO has been used as environmental photocatalyst for water purification with the aid of artificial light source [3,4,6,8– 10]. Many techniques have been used for deposition of high quality ZnO films such as chemical vapor deposition (CVD), pulsed laser deposition (PLD), molecular beam epitaxy (MBE), anodizing, sputtering and thermal evaporation [1,2,11–16]. Despite of its obvious economic and simplicity, thermal oxidation deposition technique has found very little
O.A. Fouad et al. / Applied Catalysis B: Environmental 62 (2006) 144–149
attention by research groups [17–19]. Moreover, up to the best of our knowledge, there are no photocatalytic tests have been performed on polycrystalline ZnO films deposited by this technique at a relatively low temperature. In this paper we present the growth and photocatalytic activity of ZnO thin films prepared by thermal physical vapor deposition approach. The properties of the asdeposited films, crystal structure, chemical composition and surface morphology are explored. The photocatalytic activity of the deposited films on an azo-reactive dye are tested and its results are also presented.
2. Experimental 2.1. Growth of ZnO thin films ZnO thin films of 10–80 nm size are deposited on silicon substrates. Zinc targets (5 mm 5 mm and purity 99.5%) are used as a source of zinc vapor for zinc oxide deposition. The silicon substrate (1 cm 1 cm) is hold on a ceramic holder and inserted inside the deposition chamber which then evacuated using a combinational rotary and oil diffusion vacuum pumps to a base pressure of 1 105 Torr. Torr. The total pressure of the chamber during deposition is controlled at 5 103 Torr. The target and substrate are heated by a heating coil about 5 mm apart from both of them. The deposition and oxidation temperatures range from 350 to 650 8C. The total deposition time is varied from 10 to 30 min. The chamber gas environment during deposition consisted of hydrogen, argon and oxygen gases. Mass flow controllers and needle valves are installed to control the flow of the gases at 20, 15 and 5 sccm, respectively. 2.2. Characterization of ZnO films Phase identification of the deposited zinc oxide films is carried out by X-ray diffraction (XRD; Rigaku Rint700 and Bruker axs D8, Germany). The XRD machine is operated at 40 kV and 30 mA. The u–2u mode is employed and Cu Ka ˚ is used. Chemical composition of radiation of l = 1.5405 A the films is determined by X-ray photoelectron spectroscopy (XPS; Kratos Axi-HS) using Mg Ka radiation (1253.6 eV) as X-ray source. Morphologies of the deposited films are investigated by scanning electron microscopy using a focused ion beam system (FIB; Hitachi FB-2000A) and atomic force microscopy (AFM; SPI 3800N probe station).
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of C.I. Reactive Black 5, RB5 (Remazol Black B 133X) dye solution. C.I. Reactive Black 5 is selected, as a model for the photocatalytic degradation experiments because it is nonvolatile and is a common contaminant in the industrial wastewaters. A black light-blue florescent bulb (F18WBLB) is horizontally positioned above the reactor to supply UV illumination. The wavelength of the light passing through a reaction suspension is 365 nm. The experiments are performed by immersing the film into the reactor with 100 ml dyes (10–100 mg/l). The reaction is carried out isothermally at 25 8C and pH 2–10 for 3 h. Changing the original pH of the solution is carried out by addition of either NaOH or HCl solutions. At the end of reaction time, the UV power is turned off and the zinc oxide film is taken out of the solution which is then analyzed. The extent of photocatalytic degradation is evaluated by measuring the absorbance of the solution samples with a UV–vis spectrophotometer (UV– vis; Cecil7200) at lmax = 525 nm. The decomposition efficiency of dye is estimated by applying the following equation: dye decomposition efficiency ð%Þ ¼ ½ðC0 CÞ=C0 100 where C0 and C are the initial and residual dye contents in solution, respectively.
3. Results and discussion 3.1. Crystallinity, composition and morphology of ZnO film XRD pattern of the deposited zinc oxide thin film at 350 8C and oxidized at 550 8C, at 10 min total deposition time, and at 20, 15 and 5 sccm flowrates of H2, Ar and O2, respectively, is presented in Fig. 1. The XRD pattern of deposited film at lower, 500 8C, and higher oxidation temperature, 650 8C, are also shown there for comparison. We ascribed the diffraction peaks from the zinc oxide film to
2.3. Photocatalytic activity test All experiments are carried out in a 250 ml Pyrex beaker in which the dye solution is introduced. The beaker including the dye solution is put on a magnetic stirrer and the solution is stirred by means of a magnetic rod. Silicon substrates, on which zinc oxide films are deposited, are hanged in the middle of the beaker and fixed inside the bulk
Fig. 1. XRD patterns of zinc oxide films deposited at 350 8C and oxidized at: (a) 500 8C, (b) 550 8C and (c) 650 8C.
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the (0 0 2), (1 0 1), (1 0 2) and (1 1 0) diffraction planes at 2u = 34.42, 36.29, 47.53 and 56.578, respectively (JCPDS# 79-2205). On the other hand we ascribed the diffraction peaks from zinc film to the (0 0 2), (1 0 0), (1 0 1) and (1 0 2) diffraction planes at 2u = 36.28, 38.94, 43.23 and 54.338, respectively (JCPDS# 4-831). It is obvious that the deposited films are polycrystalline. It has been reported that trace amount of zinc metal is also detected in the film prepared by evaporating a mixture of 70% Zn and 30% Se at wet oxidation condition (10% O2 + 5% H2 + 85% Ar) at 100 8C [20]. From Fig. 1 it is also clear that peak intensities are fairly weak. This is probably due to roughness and/or coarsening of the film or due to the film thickness. Fig. 2 shows XPS spectra of the as-deposited zinc oxide film (Fig. 2a) and after sputtering by Ar+ ion for 0.5 min (Fig. 2b). The peaks from Zn and O of ZnO film are detected at 1023 eV (Zn 2p) and 532 eV (O 1s), respectively. Also, carbon is adsorbed on the film surface as the C 1s peak is observed at 285 eV. Carbon contaminates only the film surface and did not show significant concentration in the bulk of the film. This observation is supported by the XRD analyses, which did not confirm the formation of any carbide phase in the film. After sputtering the film by Ar+ ion for 0.5 min, the intensity of carbon peak decreased significantly. Whereas the intensity of oxygen and zinc peaks increased (Fig. 2b). This might be due to the removal of the surface organic carbon layer that contaminated the film surface. No significant chemical shift is observed in peak positions upon sputtering. This indicates that the composition of the film surface is more likely the same as its bulk. More detail investigation of the XPS peaks for each element in the asdeposited film and after sputtering is illustrated in Fig. 3. SEM image of the zinc oxide film deposited at 350 8C and oxidized at 550 8C is shown in Fig. 4. It illustrates that the film surface is homogeneous and uniform. It is clear that the deposited film composes mainly of spherical-like shape grains. SEM investigations of samples oxidized at higher temperatures show that the films compose of larger grains.
Fig. 2. XPS spectra of zinc oxide films: (a) as-deposited and (b) after Ar+ ion sputtering.
More detailed investigation of the film surface and its structure is obtained from AFM images in Fig. 5. It reveals the growth of highly oriented needle-like shape ZnO crystals in the range of 50–80 nm. Fig. 5a and b shows the film image (2D and 3D views) at lower magnification whereas Fig. 5c and d shows the film image (2D and 3D views) at higher magnification. 3.2. Photocatalytic activity tests of ZnO films In the beginning, photodecomposition experiment of dye is carried out under the irradiation with UV light without addition of catalyst as a blank test to evaluate the contribution of direct photolysis in the total photocatalytic decomposition process. No significant photodecomposition efficiency is noticed. To promote the photocatalytic decomposition of the dye, three ZnO film samples named ZF, ZD, and ZU have been tested. For all of these three samples, zinc is deposited at 350 8C and oxidized at 500, 550 and 650 8C, respectively. The influence of the initial dye concentration on the photocatalytic decomposition reaction together with the photocatalyst loading are important factors to evaluate the whole process. The effect of initial dye concentration on the photocatalytic decomposition efficiency using the three zinc oxide films is investigated in the range 10–100 mg/l dye concentration. We studied their effects on dye photodecomposition at pH 6.7 for 3 h. The results in Fig. 6 show that the extent of dye decomposition is not more than 23% at various dye concentrations regardless the difference in zinc oxide film samples. One can say that in the case of 100 mg/l during the same period 23 mg/l of the dye has been decomposed, while at 10 mg/l only 2.3 mg/l. So, at the same other conditions, at 100 mg/l dye concentration it is decomposed of an order faster than at 10 mg/l. This result is in agreement to what have been observed when ZnO particles are used [8–10]. One of the important parameters governing the rate of reaction taking place on zinc oxide particle is the pH of solution. This is because of the amphoteric property (acid– base property) of ZnO semiconductor which influences the surface-charge property of the photocatalyst. In addition, wastewater from textile industries usually have a wide range of pH values. Further, pH plays an important role in generation of hydroxyl radicals. The OH radicals are extremely strong nonselective oxidant (E0 = +3.06 V). This property leads to partial or complete mineralization of several organic pollutants [9]. Fig. 7 presents the dye decomposition using the three ZnO films at dye solution concentration 10 mg/l and different pH values 2, 6 and 10. The photocatalytic decomposition of the dye increased using ZnO films in the following order ZD > ZU > ZF in the tested pH range. This can be attributed to the difference in oxidation temperature for the three ZnO films. At pH 2, for ZF sample the oxidation temperature is 500 8C, the dye
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Fig. 3. Narrow range XPS spectra of Zn in zinc oxide films: (a) as-deposited and (b) after Ar+ ion sputtering.
photocatalytic decomposition is 60%. This is might be due to incomplete ZnO growth at such temperature, whereas for sample (ZD) prepared at 550 8C, dye photodecomposition is 99.6% due to complete growth of ZnO film as shown in the XRD patterns (Fig. 1). Meanwhile, for ZU sample which prepared via oxidation at temperature 650 8C, dye photocatalytic decomposition is about 90%. This can be attributed to the decrease in specific surface area as a result of decrease in percentage of atoms on the semiconductor surface due to
Fig. 4. SEM image of zinc oxide film surface deposited at 350 8C and oxidized at 550 8C.
agglomeration of ZnO grains at such higher temperature which accompanied by change in particle surface, including sintering among particles and surface defects [21]. It is also observed from Fig. 7 that in acidic solution photocatalytic decomposition efficiency of the dye is more than that in alkaline solution. In addition, acidic and alkaline solutions are more favored than nearly neutral solution. Maximum dye photocatalytic decomposition 99.6% is achieved at pH 2. Whereas at pH 10, photocatalytic decomposition of dye is higher than that at pH 6 and a photodecomposition efficiency of about 60% is achieved for all films. The lowest dye photocatalytic decomposition efficiency (10–25%) for all the thin films is observed at pH 6. This is might be due to photocorrosion of ZnO particles. It was reported that photodecomposition of ZnO took place in acidic solutions and its photocorrosion induced by selfoxidation is complete at pH lower than 4 [8]. The interpretation of pH effects on the photocatalytic decomposition of process of the dye is a difficult task. This is because of the contribution of three possible reaction mechanisms to the dye decomposition. These mechanisms are: dye degradation by hydroxyl radical attack, direct oxidation by positive hole, and direct reduction by the electron in the conduction band [22]. The importance of each mechanism has been found to be depend on the catalyst nature and pH [22]. In the present work, it can be suggested that two mechanisms governed the photocatalytic decomposition of RB5 dye on ZnO films. The first mechanism can be implied at pH 2–6 (acidic solution) and the reaction process is presented by protonation of the dye which indeed affected the adsorption characteristics of the dye and its redox activity. The zero-point charge (zpc) for ZnO is 9.0. Based
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Fig. 5. AFM profile of zinc oxide film deposited at 350 8C and oxidized at 550 8C (a and b) low magnification and (c and d) high magnification.
on the zpc value, ZnO surface is positively charged below pH 9 based on its zpc. At low pH value, electrostatic interactions between the positive photocatalyst surface and dye anions (which generated from the dissociation of sodium salt of the dye molecule, [dye–Na] ! [dye] + Na+) in aqueous solution may be lead to strong adsorption of the dye on the catalyst surface. Sakthivel et al. reported the same observation for Acid Brown 14, AB14, dye on ZnO catalyst [9]. The second mechanism can be implied at high pH value (alkaline solution) and the reaction process is presented by the hydroxyl radicals (OH) attack, which are favored at the investigated high pH value, 10. The presence of large quantities of OH ions on the particle surfaces as well as in the reaction medium favors the formation of OH radicals. The reason why the photocatalytic decomposition of the RB5 dye is lower in alkaline
than in acidic solution can be attributed to its chemical structure, Fig. 8. RB5 has sulfuric groups in its structure, which are negatively charged in alkaline solutions, therefore, in the alkaline solution dye may not be adsorbed onto the surface of the photocatalyst effectively. This result is also found for acid red 14, AR14, by Daneshvar et al. [8]. On the other hand at such pH value the hydroxyl radicals are so rapidly scavenged that they do not have the opportunity to react effectively with the dye [8,9]. When the crystallite dimension of a semiconductor particle falls down to a nanometer size, the rate constant of charge transfer increases. As a result of this, the photoefficiencies for systems in which the rate-limiting step is charge transfer increased. Size reduction also leads to major changes in the effective redox potential of the photogenerated charge carriers. An increase in redox potential
Fig. 6. Effect of initial dye concentration on the photocatalytic decomposition efficiency.
Fig. 7. Effect of dye solution pH on the photocatalytic decomposition efficiency.
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Fig. 8. Chemical structure of C.I. Reactive Black 5 dye.
means there is more driving force behind the photocatalytic reaction and increased rates will be observed. The reaction is thought to be proceeded via reaction of the dye with hydroxyl ions onto catalyst surface followed by reaction with holes existing in excited semiconductor catalyst due to UV illumination. The overall process can be represented by a group of equations. Upon exposure to UV irradiation, ZnO is photoexcited and an electron–hole pair is formed as follows: þ ZnO þ hn ! ZnOðe CB þ hVB Þ
(2)
The formed OH are oxidized further by the hole to produce OH radicals: hþ VB þ OH ! OH
(3)
The OH radicals lead to partial or complete dye decomposition as follows: OH þ C:I: RB5 dye ! decomposed dye
(4)
On the other hand, in acidic solution ZnO reacted with the photogenerated holes and undergoes self-oxidation as follows: 2þ ZnO þ 2hþ þ O ðactive oxygenÞ VB ! Zn
evaporation deposition has been investigated. XPS and XRD analyses show that the produced film are polycrystalline ZnO and composes mainly of Zn and O. AFM images illustrate that the films are thin and ranges in thickness from 10 to 80 nm. The results show that the deposited and oxidized film at temperature 550 8C (ZD) gave the highest photodecomposition efficiency. Maximum photocatalytic decomposition (99.6%) of the dye is achieved at pH 2 with dye concentration 10 mg/l and at the irradiation time of 3 h. The presented results in this paper indicate that UV/ZnO thin film is an effective combination for photocatalytic degradation of C.I. Reactive Black 5 dye. UV and ZnO have negligible effect on catalytic degradation of the dye when they are used individually.
(1)
þ where e CB is the electron in the conduction band and hVB is þ the hole in the valence band. ZnOðeCB þ hVB Þ is photoexcited ZnO with the formation of electron in the conduction band and hole in the valence band. Due to the amphoteric property of ZnO semiconductor, water molecules in alkaline solution adsorbed on its excited surface and decomposed by oxidative potential of the hole according to the following equation: þ hþ VB þ H2 O ! H þ OH
149
(5)
In this case the dye decomposed by the action of the generated active oxygen. As the mobility of active oxygen is might be higher than that of OH radical, the photocatalytic decomposition efficiency of the dye in acidic solution is higher than that in alkaline solution.
4. Conclusion Photocatalytic decomposition of C.I. Reactive Black 5 dye using ZnO thin films photocatalyst prepared by thermal
References [1] J. Liqianga, S. Xiaojunb, S. Jingc, C. Weiminb, X. Zilic, D. Yaoguoc, F. Hongganga, Sol. Energy Mater. Sol. C 79 (2003) 133. [2] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct. 34 (2003) 3. [3] S. Chakrabarti, B.K. Dutta, J. Hazard. Mater. B 112 (2004) 269. [4] S. Rabindranathan, S. Devipriya, S. Yesodharan, J. Hazard. Mater. B 102 (2003) 217. [5] D. Chen, A.K. Ray, Chem. Eng. Sci. 56 (2001) 1561. [6] K. Tennakone, J. Bandara, Appl. Catal. A: Chem. 208 (2001) 335. [7] B. Pal, M. Sharon, Mater. Chem. Phys. 76 (2002) 82. [8] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A 162 (2004) 317. [9] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. C 77 (2003) 65. [10] A. Akyola, H.C. Yatmaza, M. Bayramoglu, Appl. Catal. B: Environ. 54 (2004) 19. [11] S.-S. Lin, J.-L. Huang, Surf. Coat. Technol. 185 (2004) 222. [12] Ya.I. Alivov, A.V. Chernykh, M.V. Chukichev, R.Y. Korotkov, Thin Solid Films 473 (2005) 241. [13] A.Kh. Abduev, B.M. Ataev, A.M. Bagamadova, Inorg. Mater. 11 (1987) 1928. [14] S. Choopun, H. Tabata, T. Kawai, J. Cryst. Growth 274 (2005) 167. [15] Y. Chen, D.M. Bagnall, H. Koh, K. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [16] Y. Yamaguchi, M. Yamazaki, S. Yoshihara, T. Shirakashi, J. Electroanal. Chem. 442 (1998) 1. [17] S.J. Chen, Y.C. Liu, J.G. Ma, D.X. Zhao, Z.Z. Zhi, Y.M. Lu, J.Y. Zhang, D.Z. Shen, X.W. Fan, J. Cryst. Growth 240 (2002) 467. [18] R.K. Gupta, N. Sridhar, M. Katiar, Mater. Sci. Semicond. Process. 5 (2002) 11. [19] Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, H.H. Hng, J. Appl. Phys. 94 (2003) 354. [20] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [21] D. Li, H. Haneda, Chemosphere 51 (2003) 129. [22] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. B: Environ. 89 (2002) 303.