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Photocatalytic photodegradation of xanthate over Zn1−xMnxO under visible light irradiation
Journal of Alloys and Compounds 479 (2009) L4–L7
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Photocatalytic photodegradation of xanthate over Zn1−x Mnx O under visible light irradiation Qi Xiao ∗ , Linli Ouyang School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 8 April 2008 Received in revised form 11 December 2008 Accepted 15 December 2008 Available online 25 December 2008 Keywords: Mn-doped ZnO Visible light Photocatalytic activity
a b s t r a c t Mn-doped ZnO photocatalysts were prepared by precipitation method, and characterized by powder Xray diffraction, UV–vis diffuses reflectance spectroscopy, and photoluminescence spectra. Mn-doped ZnO photocatalysts showed high photocatalytic activities for xanthate photodegradation at pH 7 under visible light irradiation. Zn0.95 Mn0.05 O exhibited the highest photocatalytic photodegradation efficiency, which was attributed to the low recombination of the photogenerated electrons and holes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Photocatalytic photodegradation of organic pollutants in water and air using semiconductive particles, such as TiO2 and ZnO, has attracted extensive attention in the past two decades [1–4]. While TiO2 is widely employed as a photocatalyst, ZnO is a suitable alternative to TiO2 as it has a similar band gap energy (3.2 eV) [4], larger quantum efficiency than TiO2 and higher photocatalytic efficiencies have been reported [2,5–8]. It has also suggested that ZnO is a low cost alternative photocatalyst to TiO2 for photodegradation of organics in aqueous solutions [9]. However, ZnO has a wide band gap of about 3.2 eV and can only absorb UV light with < 387 nm [10]. Therefore, ZnO-based materials capable of visible light photocatalysis are required for solar-energy applications. Many efforts have been made to prepare TiO2 photocatalysts that can drive the photodegradation reaction under visible light, including transitional metal ions doping, e.g. Co, Ni or Mn [11], or N doping [12]. Recently, it was reported that Mn-doped ZnO was an active photocatalyst for oxygen production from water under visible light irradiation [11]. However, little work has been reported on the photocatalytic photodegradation of organic contaminants by Mn-doped ZnO. In this work, the Mn-doped ZnO photocatalyst was synthesized and the photocatalytic photodegradation of potassium ethyl xanthate (KEX) was carried out under visible light irradiation.
All chemicals (analytical grade reagents) were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. Zn1−x Mnx O (x = 0, 0.025, 0.05, 0.075) was prepared by precipitation method. The analytical grade zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O), manganese acetate tetrahydrate (Mn(CH3 COO)2 ·4H2 O), oxalate acid (H2 C2 O4 ·2H2 O) and ammonia (25%) were used as raw materials. In a typical experiment, the H2 C2 O4 ·2H2 O aqueous solution was added to the solution containing Zn(NO3 )2 ·6H2 O and Mn(CH3 COO)2 ·4H2 O under stirring. After adjusting the pH value with ammonia to 7, the mixture solution was heated at 40 ◦ C for 1 h. The precipitate was separated from the solution by filtration, washed several times with distilled water and absolute ethanol, then dried in air at 80 ◦ C for 4 h, and a series of Zn1−x Mnx O powders were obtained by thermal decomposition of the precursors at 500 ◦ C for 2 h in air. The processes of the oxalate precursors thermal decomposition were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC) with a NETZSCH STA 449C instrument in air at a heating rate of 10 ◦ C min−1 in alumina sample holders with alumina as a reference sample. The crystalline structure of the samples was determined by a D/max-␥A diffractometer (Cu K␣ radiation, = 0.154056 nm) studies. The diffuse reflectance spectra (DRS) of the photocatalyst sample in the wavelength range of 300–900 nm were obtained using a UV–vis scanning spectrophotometer (Shimadzu UV-3101), and were converted from reflectance to absorbance by the Kubelka–Munk method. The photoluminescence (PL) spectra of the samples were recorded with a Fluorescence Spectrophotometer F-4500. For a typical photocatalytic experiment, 100 mg of the photocatalyst was added to 100 mL of the 5 mg/L potassium ethyl xanthate aqueous solution. The photocatalysts were dispersed under ultrasonic vibration for 10 min. Prior to light irradiation, the reactor was left in the dark for at least 30 min until an adsorption–desorption equilibrium was finally established. A 100-W tungsten lamp with a UV cut-off filter fixed at a distance of 150 mm above the surface solution was used as visible light source. During irradiation, about 5 mL suspension was continually taken from the reaction cell at given time intervals for MB concentration analysis by the UV–vis spectrometer (Shimadzu UV-3101). The UV spectrometer was set to measure absorbance at 301 nm, where xanthates in aqueous solution have an absorbance maximum (shown in Fig. 1).
∗ Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815. E-mail address: [email protected] (Q. Xiao). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.085
Q. Xiao, L. Ouyang / Journal of Alloys and Compounds 479 (2009) L4–L7
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Fig. 1. UV–vis absorption spectra of xanthate.
3. Results and discussion 3.1. Characterization of the as-prepared samples The TG/DSC curves of the oxalate precursors were shown in Fig. 2. It was found that there were two stages for the thermal decomposition of the oxalate precursor below about 420 ◦ C. Crystalloid water was lost at the first step, and Zn1−x Mnx O formed at the second step. Two endothermic peaks were observed in the DSC curve of the zinc oxalate precursor (shown in Fig. 2(a)). However, there were an endothermic peak and an exothermic peak for the DSC curves of the Mn-containing precursors (shown in Fig. 2(b)–(d)), which might be due to the different thermal decomposition mechanism when manganese was added into the oxalate precursor. Further studies on the thermal decomposition mechanism of zinc oxalate precursor are in progress. XRD patterns of the prepared Zn1−x Mnx O samples were shown in Fig. 3. The diffraction peaks of each sample were well in agreement with JCPDS card of ZnO (JCPDS 36-1451). No traces of impurity peaks other than ZnO were observed. These results indicated that the as-synthesized samples were single phases with a hexagonal wurtzite structure. Table 1 showed that the lattice constants of Zn1−x Mnx O were slightly larger than those of pure ZnO, because the ionic radius of Mn(II) (0.066 nm) was larger than that of Zn(II)
Fig. 2. The TG/DSC curves of the oxalate precursors.
Fig. 3. XRD patterns of Zn1−x Mnx O photocatalysts with different Mn contents. (a) x = 0, (b) x = 0.025, (c) x = 0.05 and (d) x = 0.075. Table 1 The lattice constants calculated from the XRD data of Zn1−x Mnx O. Zn1−x Mnx O
a (nm)
c (nm)
V (×103 nm3 )
x=0 x = 2.5 x = 5.0 x = 7.5
0.32439 0.32467 0.32584 0.32482
0.51966 0.52094 0.52168 0.52092
47.357 47.556 47.967 47.598
(0.060 nm). The expansion of the lattice constants of Zn1−x Mnx O indicated that manganese was really doped into the ZnO structure. 3.2. Optical absorption properties of the as-prepared samples The UV–vis absorption spectra of Zn1−x Mnx O photocatalysts with different Mn contents were shown in Fig. 4. It was found that while pure ZnO had no absorption in the visible region (>400 nm), Mn-doped ZnO samples showed very broad absorption band covering the entire visible region. This broad absorption band was assigned as d–d transition from ground state 6 A1 to the excited states 4 T2 , 4 A1 , 4 E of Mn2+ ion, which was due to crystal field split of 4 G state of free Mn2+ ion in the wide band gap ZnO [13]. In addition, visible absorption for Mn-doped ZnO samples increased with the increase of Mn ion content.
Fig. 4. UV–vis absorption spectra of Zn1−x Mnx O photocatalysts with different Mn contents.
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Fig. 5. PL spectrum of Mn-doped ZnO with the excitation wavelength of 325 nm.
Fig. 7. The ln (C0 /C) vs. time curves of xanthate photocatalytic photodegradation using Zn1−x Mnx O photocatalysts (photocatalysts loading, 1 g/L; initial xanthate concentration, 10 mg/L; pH = 7).
3.3. Photoluminescence spectra of the as-prepared samples The photoluminescence spectra have been widely used to investigate the efficiency of charge carrier trapping, immigration and transfer, and to understand the fate of electron/hole pairs in semiconductor particles [14]. Fig. 5 showed the PL spectrum of Mn-doped ZnO with the excitation wavelength of 325 nm. All Mn-doped ZnO samples exhibited a strong PL peaks at about 470 nm, which could be attributed to surface oxygen vacancies and defects of ZnO nanoparticles. It has been reported that the stronger the excitonic PL spectrum, the higher the content of surface oxygen vacancy and defect [15]. In addition, the order of PL intensity for Mn-doped ZnO samples was as follows: 5.0 > 7.5 > 2.5 mol%, which indicated that the content of surface oxygen vacancy arrived at the highest degree when Mn2+ content was 5.0 mol%. 3.4. Photocatalytic activity of the as-prepared samples Fig. 6 showed the photocatalytic photodegradation of xanthate on Zn1−x Mnx O photocatalysts with different Mn contents at pH = 7 under visible light irradiation. It was found that pure ZnO had little ability to mineralize xanthate under visible light irradiation. All of the Mn-doped ZnO photocatalysts exhibited higher
photocatalytic activity than that of pure ZnO under visible light irradiation. The order of photocatalytic activity of Mn-doped ZnO at 90 min was as follows: 5.0 ≥ 7.5 ≥ 2.5 mol%, which suggested that there was an optimum doping content of Mn2+ ions in ZnO. The Zn0.95 Mn0.05 O photocatalyst exhibited the highest photocatalytic photodegradation efficiency among all of the Mn-doped ZnO samples. Fig. 7 showed the ln (C0 /C) versus time curves of the xanthate photocatalytic photodegradation using Zn1−x Mnx O photocatalysts, were C0 and C was the concentration of the primal and remaining MB, respectively. It was found that all curves were linear, revealing that the kinetic data of the xanthate photocatalytic photodegradation fit well to the first-order reaction kinetic model. The apparent first-order reaction rate constant values calculated from the slopes of those curves were shown in Table 2. It was demonstrated that the apparent rate constant values of the xanthate photocatalytic photodegradation using Zn1−x Mnx O photocatalysts were as follows: 5.0 ≥ 7.5 ≥ 2.5 mol%, and the Zn0.95 Mn0.05 O photocatalyst was about eight orders magnitude larger than that using the undoped ZnO photocatalyst. According to Figs. 5 and 7, it should be pointed that the order of photocatalytic activity was the same as that of PL intensity, namely, the stronger the PL intensity, the higher the photocatalytic activity. During the process of PL, oxygen vacancies and defects could bind photoinduced electrons to form free or binding exactions so that PL signal could easily occur, and the larger the content of oxygen vacancies or defects, the stronger the PL intensity. But, during the process of photocatalytic reactions, oxygen vacancies and defects could became the centers to capture photoinduced electrons so that the recombination of photoinduced electrons and holes could be effectively inhibited. Moreover, oxygen vacancies could promote the adsorption of O2 , and there was
Table 2 Photocatalytic photodegradation of rate constants of xanthate using Zn1−x Mnx O photocatalysts.
Fig. 6. Photocatalytic photodegradation kinetics of xanthate using Zn1−x Mnx O photocatalysts (photocatalysts loading, 1 g/L; initial xanthate concentration, 10 mg/L; pH = 11.5).
Samples
Rate constant (min−1 )
ZnO Zn0.975 Mn0.025 O Zn0.95 Mn0.05 O Zn0.925 Mn0.075 O
0 1.43E−3 2.6E−3 1.52E−3
Q. Xiao, L. Ouyang / Journal of Alloys and Compounds 479 (2009) L4–L7
strong interaction between the photoinduced electrons bound by oxygen vacancies and adsorbed O2 . This indicated that the binding for photoinduced electrons of oxygen vacancies could make for the capture for photoinduced electrons of adsorbed O2 , and • O2 free group was produced at the same time. Thus, oxygen vacancies and defects were in favor of photocatalytic reactions in that O2 was active to promote the oxidation of organic substances [16]. The above results demonstrated that stronger the PL intensity, higher the photocatalytic activity. Therefore, in this study 5.0 mol% was the most suitable content of Mn2+ in the ZnO, at which the recombination of photoinduced electrons and holes could be effectively inhibited and thereby the highest photocatalytic activity was formed. 4. Conclusion Mn-doped ZnO photocatalysts were prepared by precipitation method, and their photocatalytic activity was evaluated by measuring degradation rates of xanthate under visible light. The results of DRS showed that Mn-doped ZnO samples showed very broad absorption band covering the entire visible region (400–900 nm), which increased with the increase of Mn ion content. The prepared Mn-doped ZnO photocatalysts showed high photocatalytic activities for xanthate photodegradation at pH 7 under visible light irradiation. It was found that the Zn0.95 Mn0.05 O photocatalyst exhibited the highest photocatalytic photodegradation efficiency, which was attributed to the low recombination of the photogenerated electrons and holes.
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Acknowledgement This work was supported by the Provincial Excellent Ph.D. Thesis Research Program of Hunan (No. 2004-141). References [1] R.W. Matthews, Water Res. 25 (1991) 1169. [2] A. Sharma, P. Rao, R.P. Mathur, S.C. Ameta, J. Photochem. Photobiol. A: Chem. 86 (1995) 197. [3] C.A.K. Gouveˆa, F. Wypych, S.G. Moraes, N. Duraˇın, P. Peralta-Zamora, Chemosphere 40 (2000) 427. [4] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65. [5] A.A. Khodja, T. Sehili, J.F. Pilichowski, P. Boule, J. Photochem. Photobiol. A: Chem. 141 (2001) 231. [6] I. Poulios, I. Tsachpinis, J. Chem. Technol. Biotechnol. 74 (1999) 349. [7] K. Gouvea, F. Wypych, S.G. Moraes, N. Duran, N. Nagata, P. Peralta-Zamora, Chemosphere 40 (2000) 433. [8] S. Dindar, J. Icli, Photochem. Photobiol. A: Chem. 140 (2001) 263. [9] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A: Chem. 162 (2004) 317. [10] N. Serpone, D. Lawless, J. Disdier, J.-M. Herrmann, Langmuir 10 (1994) 643. [11] S. Ekambaram, Y. Iikubo, A. Kudo, J. Alloys Compd. 433 (2007) 237. [12] H.-F. Lin, S.-C. Liao, S.-W. Hung, J. Photochem. Photobiol. A: Chem. 174 (2005) 82. [13] T. Mizokaw, T. Nambu, A. Fujimori, T. Fukumura, M. Kawasaki, Phys. Rev. B 65 (2002) 085209. [14] H. Yamashita, Y. Ichihashi, S.G. Zhang, Y. Matrumura, Y. Souma, T. Tatsumi, M. Anpo, Appl. Surf. Sci. 121/122 (1997) 305. [15] J. Liqiang, Q. Yichun, W. Baiqi, L. Shudan, J. Baojiang, Y. Libin, F. Wei, F. Honggang, S. Jiazhong, Sol. Energy Mater. Sol. Cells 90 (2006) 1773. [16] H. Liu, S. Cheng, M. Wu, H. Wu, J. Zhang, W. Li, C. Cao, J. Phys. Chem. A 104 (2000) 7016.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Photocatalytic photodegradation of xanthate over Zn1−x Mnx O under visible light irradiation Qi Xiao ∗ , Linli Ouyang School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 8 April 2008 Received in revised form 11 December 2008 Accepted 15 December 2008 Available online 25 December 2008 Keywords: Mn-doped ZnO Visible light Photocatalytic activity
a b s t r a c t Mn-doped ZnO photocatalysts were prepared by precipitation method, and characterized by powder Xray diffraction, UV–vis diffuses reflectance spectroscopy, and photoluminescence spectra. Mn-doped ZnO photocatalysts showed high photocatalytic activities for xanthate photodegradation at pH 7 under visible light irradiation. Zn0.95 Mn0.05 O exhibited the highest photocatalytic photodegradation efficiency, which was attributed to the low recombination of the photogenerated electrons and holes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Photocatalytic photodegradation of organic pollutants in water and air using semiconductive particles, such as TiO2 and ZnO, has attracted extensive attention in the past two decades [1–4]. While TiO2 is widely employed as a photocatalyst, ZnO is a suitable alternative to TiO2 as it has a similar band gap energy (3.2 eV) [4], larger quantum efficiency than TiO2 and higher photocatalytic efficiencies have been reported [2,5–8]. It has also suggested that ZnO is a low cost alternative photocatalyst to TiO2 for photodegradation of organics in aqueous solutions [9]. However, ZnO has a wide band gap of about 3.2 eV and can only absorb UV light with < 387 nm [10]. Therefore, ZnO-based materials capable of visible light photocatalysis are required for solar-energy applications. Many efforts have been made to prepare TiO2 photocatalysts that can drive the photodegradation reaction under visible light, including transitional metal ions doping, e.g. Co, Ni or Mn [11], or N doping [12]. Recently, it was reported that Mn-doped ZnO was an active photocatalyst for oxygen production from water under visible light irradiation [11]. However, little work has been reported on the photocatalytic photodegradation of organic contaminants by Mn-doped ZnO. In this work, the Mn-doped ZnO photocatalyst was synthesized and the photocatalytic photodegradation of potassium ethyl xanthate (KEX) was carried out under visible light irradiation.
All chemicals (analytical grade reagents) were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. Zn1−x Mnx O (x = 0, 0.025, 0.05, 0.075) was prepared by precipitation method. The analytical grade zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O), manganese acetate tetrahydrate (Mn(CH3 COO)2 ·4H2 O), oxalate acid (H2 C2 O4 ·2H2 O) and ammonia (25%) were used as raw materials. In a typical experiment, the H2 C2 O4 ·2H2 O aqueous solution was added to the solution containing Zn(NO3 )2 ·6H2 O and Mn(CH3 COO)2 ·4H2 O under stirring. After adjusting the pH value with ammonia to 7, the mixture solution was heated at 40 ◦ C for 1 h. The precipitate was separated from the solution by filtration, washed several times with distilled water and absolute ethanol, then dried in air at 80 ◦ C for 4 h, and a series of Zn1−x Mnx O powders were obtained by thermal decomposition of the precursors at 500 ◦ C for 2 h in air. The processes of the oxalate precursors thermal decomposition were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC) with a NETZSCH STA 449C instrument in air at a heating rate of 10 ◦ C min−1 in alumina sample holders with alumina as a reference sample. The crystalline structure of the samples was determined by a D/max-␥A diffractometer (Cu K␣ radiation, = 0.154056 nm) studies. The diffuse reflectance spectra (DRS) of the photocatalyst sample in the wavelength range of 300–900 nm were obtained using a UV–vis scanning spectrophotometer (Shimadzu UV-3101), and were converted from reflectance to absorbance by the Kubelka–Munk method. The photoluminescence (PL) spectra of the samples were recorded with a Fluorescence Spectrophotometer F-4500. For a typical photocatalytic experiment, 100 mg of the photocatalyst was added to 100 mL of the 5 mg/L potassium ethyl xanthate aqueous solution. The photocatalysts were dispersed under ultrasonic vibration for 10 min. Prior to light irradiation, the reactor was left in the dark for at least 30 min until an adsorption–desorption equilibrium was finally established. A 100-W tungsten lamp with a UV cut-off filter fixed at a distance of 150 mm above the surface solution was used as visible light source. During irradiation, about 5 mL suspension was continually taken from the reaction cell at given time intervals for MB concentration analysis by the UV–vis spectrometer (Shimadzu UV-3101). The UV spectrometer was set to measure absorbance at 301 nm, where xanthates in aqueous solution have an absorbance maximum (shown in Fig. 1).
∗ Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815. E-mail address: [email protected] (Q. Xiao). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.085
Q. Xiao, L. Ouyang / Journal of Alloys and Compounds 479 (2009) L4–L7
L5
Fig. 1. UV–vis absorption spectra of xanthate.
3. Results and discussion 3.1. Characterization of the as-prepared samples The TG/DSC curves of the oxalate precursors were shown in Fig. 2. It was found that there were two stages for the thermal decomposition of the oxalate precursor below about 420 ◦ C. Crystalloid water was lost at the first step, and Zn1−x Mnx O formed at the second step. Two endothermic peaks were observed in the DSC curve of the zinc oxalate precursor (shown in Fig. 2(a)). However, there were an endothermic peak and an exothermic peak for the DSC curves of the Mn-containing precursors (shown in Fig. 2(b)–(d)), which might be due to the different thermal decomposition mechanism when manganese was added into the oxalate precursor. Further studies on the thermal decomposition mechanism of zinc oxalate precursor are in progress. XRD patterns of the prepared Zn1−x Mnx O samples were shown in Fig. 3. The diffraction peaks of each sample were well in agreement with JCPDS card of ZnO (JCPDS 36-1451). No traces of impurity peaks other than ZnO were observed. These results indicated that the as-synthesized samples were single phases with a hexagonal wurtzite structure. Table 1 showed that the lattice constants of Zn1−x Mnx O were slightly larger than those of pure ZnO, because the ionic radius of Mn(II) (0.066 nm) was larger than that of Zn(II)
Fig. 2. The TG/DSC curves of the oxalate precursors.
Fig. 3. XRD patterns of Zn1−x Mnx O photocatalysts with different Mn contents. (a) x = 0, (b) x = 0.025, (c) x = 0.05 and (d) x = 0.075. Table 1 The lattice constants calculated from the XRD data of Zn1−x Mnx O. Zn1−x Mnx O
a (nm)
c (nm)
V (×103 nm3 )
x=0 x = 2.5 x = 5.0 x = 7.5
0.32439 0.32467 0.32584 0.32482
0.51966 0.52094 0.52168 0.52092
47.357 47.556 47.967 47.598
(0.060 nm). The expansion of the lattice constants of Zn1−x Mnx O indicated that manganese was really doped into the ZnO structure. 3.2. Optical absorption properties of the as-prepared samples The UV–vis absorption spectra of Zn1−x Mnx O photocatalysts with different Mn contents were shown in Fig. 4. It was found that while pure ZnO had no absorption in the visible region (>400 nm), Mn-doped ZnO samples showed very broad absorption band covering the entire visible region. This broad absorption band was assigned as d–d transition from ground state 6 A1 to the excited states 4 T2 , 4 A1 , 4 E of Mn2+ ion, which was due to crystal field split of 4 G state of free Mn2+ ion in the wide band gap ZnO [13]. In addition, visible absorption for Mn-doped ZnO samples increased with the increase of Mn ion content.
Fig. 4. UV–vis absorption spectra of Zn1−x Mnx O photocatalysts with different Mn contents.
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Fig. 5. PL spectrum of Mn-doped ZnO with the excitation wavelength of 325 nm.
Fig. 7. The ln (C0 /C) vs. time curves of xanthate photocatalytic photodegradation using Zn1−x Mnx O photocatalysts (photocatalysts loading, 1 g/L; initial xanthate concentration, 10 mg/L; pH = 7).
3.3. Photoluminescence spectra of the as-prepared samples The photoluminescence spectra have been widely used to investigate the efficiency of charge carrier trapping, immigration and transfer, and to understand the fate of electron/hole pairs in semiconductor particles [14]. Fig. 5 showed the PL spectrum of Mn-doped ZnO with the excitation wavelength of 325 nm. All Mn-doped ZnO samples exhibited a strong PL peaks at about 470 nm, which could be attributed to surface oxygen vacancies and defects of ZnO nanoparticles. It has been reported that the stronger the excitonic PL spectrum, the higher the content of surface oxygen vacancy and defect [15]. In addition, the order of PL intensity for Mn-doped ZnO samples was as follows: 5.0 > 7.5 > 2.5 mol%, which indicated that the content of surface oxygen vacancy arrived at the highest degree when Mn2+ content was 5.0 mol%. 3.4. Photocatalytic activity of the as-prepared samples Fig. 6 showed the photocatalytic photodegradation of xanthate on Zn1−x Mnx O photocatalysts with different Mn contents at pH = 7 under visible light irradiation. It was found that pure ZnO had little ability to mineralize xanthate under visible light irradiation. All of the Mn-doped ZnO photocatalysts exhibited higher
photocatalytic activity than that of pure ZnO under visible light irradiation. The order of photocatalytic activity of Mn-doped ZnO at 90 min was as follows: 5.0 ≥ 7.5 ≥ 2.5 mol%, which suggested that there was an optimum doping content of Mn2+ ions in ZnO. The Zn0.95 Mn0.05 O photocatalyst exhibited the highest photocatalytic photodegradation efficiency among all of the Mn-doped ZnO samples. Fig. 7 showed the ln (C0 /C) versus time curves of the xanthate photocatalytic photodegradation using Zn1−x Mnx O photocatalysts, were C0 and C was the concentration of the primal and remaining MB, respectively. It was found that all curves were linear, revealing that the kinetic data of the xanthate photocatalytic photodegradation fit well to the first-order reaction kinetic model. The apparent first-order reaction rate constant values calculated from the slopes of those curves were shown in Table 2. It was demonstrated that the apparent rate constant values of the xanthate photocatalytic photodegradation using Zn1−x Mnx O photocatalysts were as follows: 5.0 ≥ 7.5 ≥ 2.5 mol%, and the Zn0.95 Mn0.05 O photocatalyst was about eight orders magnitude larger than that using the undoped ZnO photocatalyst. According to Figs. 5 and 7, it should be pointed that the order of photocatalytic activity was the same as that of PL intensity, namely, the stronger the PL intensity, the higher the photocatalytic activity. During the process of PL, oxygen vacancies and defects could bind photoinduced electrons to form free or binding exactions so that PL signal could easily occur, and the larger the content of oxygen vacancies or defects, the stronger the PL intensity. But, during the process of photocatalytic reactions, oxygen vacancies and defects could became the centers to capture photoinduced electrons so that the recombination of photoinduced electrons and holes could be effectively inhibited. Moreover, oxygen vacancies could promote the adsorption of O2 , and there was
Table 2 Photocatalytic photodegradation of rate constants of xanthate using Zn1−x Mnx O photocatalysts.
Fig. 6. Photocatalytic photodegradation kinetics of xanthate using Zn1−x Mnx O photocatalysts (photocatalysts loading, 1 g/L; initial xanthate concentration, 10 mg/L; pH = 11.5).
Samples
Rate constant (min−1 )
ZnO Zn0.975 Mn0.025 O Zn0.95 Mn0.05 O Zn0.925 Mn0.075 O
0 1.43E−3 2.6E−3 1.52E−3
Q. Xiao, L. Ouyang / Journal of Alloys and Compounds 479 (2009) L4–L7
strong interaction between the photoinduced electrons bound by oxygen vacancies and adsorbed O2 . This indicated that the binding for photoinduced electrons of oxygen vacancies could make for the capture for photoinduced electrons of adsorbed O2 , and • O2 free group was produced at the same time. Thus, oxygen vacancies and defects were in favor of photocatalytic reactions in that O2 was active to promote the oxidation of organic substances [16]. The above results demonstrated that stronger the PL intensity, higher the photocatalytic activity. Therefore, in this study 5.0 mol% was the most suitable content of Mn2+ in the ZnO, at which the recombination of photoinduced electrons and holes could be effectively inhibited and thereby the highest photocatalytic activity was formed. 4. Conclusion Mn-doped ZnO photocatalysts were prepared by precipitation method, and their photocatalytic activity was evaluated by measuring degradation rates of xanthate under visible light. The results of DRS showed that Mn-doped ZnO samples showed very broad absorption band covering the entire visible region (400–900 nm), which increased with the increase of Mn ion content. The prepared Mn-doped ZnO photocatalysts showed high photocatalytic activities for xanthate photodegradation at pH 7 under visible light irradiation. It was found that the Zn0.95 Mn0.05 O photocatalyst exhibited the highest photocatalytic photodegradation efficiency, which was attributed to the low recombination of the photogenerated electrons and holes.
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