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Selective-syntheses, characterizations and photocatalytic activities of nanocrystalline ZnTa2O6 photocatalysts
Materials Letters 65 (2011) 1598–1600
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Selective-syntheses, characterizations and photocatalytic activities of nanocrystalline ZnTa2O6 photocatalysts Zhengxin Ding, Weiming Wu, Shijing Liang, Huarong Zheng, Ling Wu ⁎ State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2010 Accepted 3 March 2011 Available online 13 March 2011 Keywords: Sol–gel method ZnTa2O6 Nanocrystalline Photocatalysis
a b s t r a c t Nanocrystalline ZnTa2O6 photocatalysts with different crystal structures were prepared via a simple and facile sol–gel method in a temperature range of 650–950 °C. The absorption edges and particle sizes of the samples were located at about 285 nm (corresponding a band gap of 4.35 eV) and ranged from 25 to 150 nm, respectively. The photocatalytic activities of the samples were tested by the degradation of methyl orange under UV light irradiation. The results indicated that the crystal structure of ZnTa2O6 was a main factor for the different photocatalytic activities of the ZnTa2O6 samples. Moreover, the effects of crystallinities and surface areas of the obtained samples on the catalytic activities were also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tantalate photocatalysts, such as Sr2M2O7 (M = Nb and Ta) [1] and Bi3TaO7 [2], show excellent performances in environment and energy fields. ZnTa2O6, as a member of tantalates, should be a promising photocatalyst. However, little work has been done to investigate the photocatalytic activity of ZnTa2O6. Furthermore, the synthesis of ZnTa2O6 in previous work proceeds via a conventional solid state reaction (SSR) [3–6]. This method provides almost no control over the particle sizes and surface areas of ZnTa2O6 particles. Development of new synthetic methods to better gain control over the particle features of nano-sized ZnTa2O6 can potentially enable a deeper understanding of its properties. Recently, Zhang et al. have reported that ZnTa2O6 nanoparticles can be obtained by a citrate sol–gel technique at a low temperature (900 °C) [7]. Herein, a modified sol–gel method was developed to selectively prepare nanocrystalline ZnTa2O6 powders with different crystal structures. The photocatalytic activities of the ZnTa2O6 samples were investigated by the degradation of methyl orange (MO) under UV light. Furthermore, the effects of the crystal structures, surface areas and crystallinities of the samples on the photocatalytic activities were discussed in detail.
into a Teflon-lined autoclave (30 mL), and then kept in 160 °C for 4 h to obtain a colorless solution. Ta2O5·nH2O was collected by dropping ammonia solution into this solution until the pH value of the solution was adjusted to ~9. The Ta2O5·nH2O precipitate was separated from the solution by centrifugation, and was washed with deionized water several times. Ta2O5·nH2O and citric acid (A.R., 4.20 g, 20 mmol) was dissolved in a H2O2 solution (A.R., 30 mL) by stirring. The complex solution, Zn(NO3)2·6H2O (A.R., 1.49 g, 5 mmol) and citric acid (2.10 g, 10 mol) were mixed well by stirring. An ammonia solution was added
2. Experimental Nanocrystalline ZnTa2O6 photocatalysts were prepared by a sol–gel method. Ta2O5 (C.R., 2.21 g, 5 mmol) and HF (A.R., 8 mL) were added ⁎ Corresponding author. Fax: +86 591 83779105. E-mail address: [email protected] (L. Wu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.028
Fig. 1. XRD patterns of the ZnTa2O6 samples prepared by the sol–gel method.
Z. Ding et al. / Materials Letters 65 (2011) 1598–1600
1599
(BET) surface areas of the samples were measured with an ASAP2020M apparatus (Micromeritics Instrument Co.). Transmission electron microscopy (TEM) images were measured by a FEI Tencai 20 microscope at an accelerating voltage of 200 kV. In catalytic test, three 4 W ultraviolet lamps (λ= 254 nm, Philips Co.) were used as the illumination source. An 80 mg catalyst was suspended in an MO solution (160 mL, 20 mg/L) in a quartz tube (200 mL). Before the test, the suspension was stirred in the dark for 1 h to ensure the establishment of absorption/desorption equilibrium between the catalyst and MO. A 3 mL suspension solution was taken at 20 min intervals during the catalytic reaction and was centrifuged. The MO concentrations during the degradation process were analyzed by measuring the absorbance at 464 nm with a Cary 50 UV–vis spectrophotometer (Varian Co.). The whole photocatalytic process was carried out under O2 bubbling with the flow rate of 30 mL/min.
Fig. 2. Diffuse reflectance spectra of the as-prepared samples.
to this solution until the pH value was up to ca. 6.5 to obtain a precursor solution. After the water was evaporated, the solution turned into a resin-like gel. The gel was presintered at 300 °C for 2 h and then calcinated at 650, 750, 850, and 950 °C for 4 h, respectively. The samples were denoted as ZT-650, ZT-750, ZT-850 and ZT-950, corresponding to the products obtained at 650, 750, 850 and 950 °C, respectively. X-ray diffraction (XRD) patterns of the as-prepared samples were collected on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained on a Cary 500 UV–vis–NIR spectrophotometer. Brunauer–Emmett–Teller
3. Results and discussion XRD patterns of the ZnTa2O6 samples prepared by the sol–gel method are shown in Fig. 1. As shown in Fig. 1, ZnTa2O6 with different crystal structures can be obtained at a temperatures range from 650 to 950 °C. The XRD pattern of ZT-650 agrees well with the literature (JCPDS Card No. 049-0746), which is a low temperature modification of ZnTa2O6. With increasing the calcination temperature, ZnTa2O6 (JCPDS Card No. 039-1484) appears in the as-prepared samples. The proportion of ZnTa2O6 (JCPDS Card No. 039-1484) increases with the increase of the calcination temperature. Finally, pure orthorhombic ZnTa2O6, a stable form at high temperatures, can be obtained after being calcined above 950 °C.
Fig. 3. TEM images of the ZnTa2O6 samples: (a) ZT-650, (b) ZT-750, (c) ZT-850 and (d) ZT-950.
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Z. Ding et al. / Materials Letters 65 (2011) 1598–1600
Fig. 4. Concentration changes of MO at 464 nm as a function of irradiation time during the degradation process.
Diffuse reflectance spectra of the as-prepared samples are shown in Fig. 2. The wavelength of the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangential line drawn on the absorption spectra. It is found that the absorption of ZnTa2O6 is located at ca. 285 nm, corresponding to a band gap of ~4.35 eV. ZT-650 shows a broad band between 300 and 600 nm due to the incomplete combustion of organics at the calcination temperature of 650 °C. For ZnTa2O6, the filled band formed by Zn2+ seems more positively than that of O2− because Zn2+ is hardly to become trivalent [8]. Therefore, the valence band of ZnTa2O6 is mainly composed of O 2p orbital hybridized with Zn 3d orbital, while orbital of Ta 5d in Ta5+ contributes to the formation of conduction band [1]. TEM images of the ZnTa2O6 samples are shown in Fig. 3. With increasing the calcination temperature, the crystallinity of ZnTa2O6 is enhanced. The sizes of ZnTa2O6 particles obtained at different temperatures (650–850 °C) range from 25 to 75 nm, while the average particle size of ZT-950 is about 150 nm. Furthermore, the surface areas are 18, 16, 7 and 2 m2/g for ZT-650, ZT-750, ZT-850 and ZT-950, respectively. This result can be ascribed to the increase of the particle size of the sample as mentioned above. The photocatalytic activities of the as-prepared samples were investigated by the degradation of MO under UV light irradiation. The concentration changes of MO at 464 nm as a function of irradiation time during the degradation process are shown in Fig. 4. ZT-750 and ZT-850 can degrade MO completely within 80 min under UV light, while the concentration of MO has no significant change without the catalyst. In general, photocatalysts which contain different crystal structures can show excellent performances in environment and energy fields due to the mixed crystal effect, such as Degussa P25. Different crystal structures in photocatalysts particles can enhance the separation of photo-exited holes and electrons, and consequently improve the catalytic activities of
photocatalysts. Therefore, the catalytic activities of ZT-750 and ZT-850 are much higher than those of ZT-650 and ZT-950 because ZT-750 and ZT-850 contain two kinds of crystal structures, whereas ZT-650 and ZT950 does not. Furthermore, it is known that photocatalysts consisted of different crystal structures with various proportions should show different catalytic activities. However, ZT-750 and ZT-850 show similar photocatalytic activities for the MO degradation when they contain two kinds of crystal structures with different proportions. This can be ascribed to the effects of the surface areas and crystallinities of the as-prepared samples. A photocatalyst with a large surface area can be obtained at low temperatures as mentioned above. The large surface area will improve the catalytic activity of the photocatalyst because it is important to the absorption of organic contaminants. However, a high crystallinity, which is obtained at high temperatures (see Fig. 3), can also enhance the photocatalytic activity because the defect concentration goes down. As a result, ZT-750 and ZT-850 show similar activities for the photodegradation of MO. Therefore, the photocatalytic activity of ZnTa2O6 prepared by the sol–gel method is determined by the balance among crystal structure, surface area and crystallinity in the calcination temperature range of 750–850 °C. 4. Conclusions Nanocrystalline ZnTa2O6 photocatalysts with different crystal structures can be selectively prepared via a simple and facile sol–gel method at a temperature range of 650–950 °C. The absorption edge of ZnTa2O6 was located at ~285 nm, corresponding to a band gap of about 4.35 eV. ZnTa2O6 samples showed excellent performances in the photodegradation of methyl orange under UV light irradiation. The photocatalytic activities of ZnTa2O6 samples with different crystal structures are higher than those of ZnTa2O6 samples with a single crystal structure due to the mixed crystal effect. It was also found that the photocatalytic activity of ZnTa2O6 was determined by the balance among crystal structure, surface area and crystallinity in a calcination temperature range from 750 to 850 °C. Acknowledgements The work was supported by NNSFC (20777011 and U1033603), 973 Program (2007CB613306) and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818). References [1] Kudo A, Kato H, Nakagawa S. J Phys Chem B 2000;104:571–5. [2] Zhang G, Li Ming, Yu S, Zhang S, Huang B, Yu J. J Colloid Interface Sci 2010;345: 467–73. [3] Lee HJ, Kim IT, Hong KS. Jpn J Appl Phys 1997;36:L1318–20. [4] Waburg M, Muller-Buschbaum H. Z Anorg Allg Chem 1984;508:55–60. [5] Ferrari CR, Hernandes AC. J Eur Ceram Soc 2002;22:2101–5. [6] Zhang YC, Yue ZX, Qi X, Li B, Gui ZL, Li LT. Mater Lett 2004;5:1392–5. [7] Zhang YC, Fu BJ, Wang X. J Alloy Compd 2009;478:498–500. [8] Kudo A, Nakagawa S, Kato H. Chem Lett 1999;28:1197–9.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Selective-syntheses, characterizations and photocatalytic activities of nanocrystalline ZnTa2O6 photocatalysts Zhengxin Ding, Weiming Wu, Shijing Liang, Huarong Zheng, Ling Wu ⁎ State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2010 Accepted 3 March 2011 Available online 13 March 2011 Keywords: Sol–gel method ZnTa2O6 Nanocrystalline Photocatalysis
a b s t r a c t Nanocrystalline ZnTa2O6 photocatalysts with different crystal structures were prepared via a simple and facile sol–gel method in a temperature range of 650–950 °C. The absorption edges and particle sizes of the samples were located at about 285 nm (corresponding a band gap of 4.35 eV) and ranged from 25 to 150 nm, respectively. The photocatalytic activities of the samples were tested by the degradation of methyl orange under UV light irradiation. The results indicated that the crystal structure of ZnTa2O6 was a main factor for the different photocatalytic activities of the ZnTa2O6 samples. Moreover, the effects of crystallinities and surface areas of the obtained samples on the catalytic activities were also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tantalate photocatalysts, such as Sr2M2O7 (M = Nb and Ta) [1] and Bi3TaO7 [2], show excellent performances in environment and energy fields. ZnTa2O6, as a member of tantalates, should be a promising photocatalyst. However, little work has been done to investigate the photocatalytic activity of ZnTa2O6. Furthermore, the synthesis of ZnTa2O6 in previous work proceeds via a conventional solid state reaction (SSR) [3–6]. This method provides almost no control over the particle sizes and surface areas of ZnTa2O6 particles. Development of new synthetic methods to better gain control over the particle features of nano-sized ZnTa2O6 can potentially enable a deeper understanding of its properties. Recently, Zhang et al. have reported that ZnTa2O6 nanoparticles can be obtained by a citrate sol–gel technique at a low temperature (900 °C) [7]. Herein, a modified sol–gel method was developed to selectively prepare nanocrystalline ZnTa2O6 powders with different crystal structures. The photocatalytic activities of the ZnTa2O6 samples were investigated by the degradation of methyl orange (MO) under UV light. Furthermore, the effects of the crystal structures, surface areas and crystallinities of the samples on the photocatalytic activities were discussed in detail.
into a Teflon-lined autoclave (30 mL), and then kept in 160 °C for 4 h to obtain a colorless solution. Ta2O5·nH2O was collected by dropping ammonia solution into this solution until the pH value of the solution was adjusted to ~9. The Ta2O5·nH2O precipitate was separated from the solution by centrifugation, and was washed with deionized water several times. Ta2O5·nH2O and citric acid (A.R., 4.20 g, 20 mmol) was dissolved in a H2O2 solution (A.R., 30 mL) by stirring. The complex solution, Zn(NO3)2·6H2O (A.R., 1.49 g, 5 mmol) and citric acid (2.10 g, 10 mol) were mixed well by stirring. An ammonia solution was added
2. Experimental Nanocrystalline ZnTa2O6 photocatalysts were prepared by a sol–gel method. Ta2O5 (C.R., 2.21 g, 5 mmol) and HF (A.R., 8 mL) were added ⁎ Corresponding author. Fax: +86 591 83779105. E-mail address: [email protected] (L. Wu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.028
Fig. 1. XRD patterns of the ZnTa2O6 samples prepared by the sol–gel method.
Z. Ding et al. / Materials Letters 65 (2011) 1598–1600
1599
(BET) surface areas of the samples were measured with an ASAP2020M apparatus (Micromeritics Instrument Co.). Transmission electron microscopy (TEM) images were measured by a FEI Tencai 20 microscope at an accelerating voltage of 200 kV. In catalytic test, three 4 W ultraviolet lamps (λ= 254 nm, Philips Co.) were used as the illumination source. An 80 mg catalyst was suspended in an MO solution (160 mL, 20 mg/L) in a quartz tube (200 mL). Before the test, the suspension was stirred in the dark for 1 h to ensure the establishment of absorption/desorption equilibrium between the catalyst and MO. A 3 mL suspension solution was taken at 20 min intervals during the catalytic reaction and was centrifuged. The MO concentrations during the degradation process were analyzed by measuring the absorbance at 464 nm with a Cary 50 UV–vis spectrophotometer (Varian Co.). The whole photocatalytic process was carried out under O2 bubbling with the flow rate of 30 mL/min.
Fig. 2. Diffuse reflectance spectra of the as-prepared samples.
to this solution until the pH value was up to ca. 6.5 to obtain a precursor solution. After the water was evaporated, the solution turned into a resin-like gel. The gel was presintered at 300 °C for 2 h and then calcinated at 650, 750, 850, and 950 °C for 4 h, respectively. The samples were denoted as ZT-650, ZT-750, ZT-850 and ZT-950, corresponding to the products obtained at 650, 750, 850 and 950 °C, respectively. X-ray diffraction (XRD) patterns of the as-prepared samples were collected on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained on a Cary 500 UV–vis–NIR spectrophotometer. Brunauer–Emmett–Teller
3. Results and discussion XRD patterns of the ZnTa2O6 samples prepared by the sol–gel method are shown in Fig. 1. As shown in Fig. 1, ZnTa2O6 with different crystal structures can be obtained at a temperatures range from 650 to 950 °C. The XRD pattern of ZT-650 agrees well with the literature (JCPDS Card No. 049-0746), which is a low temperature modification of ZnTa2O6. With increasing the calcination temperature, ZnTa2O6 (JCPDS Card No. 039-1484) appears in the as-prepared samples. The proportion of ZnTa2O6 (JCPDS Card No. 039-1484) increases with the increase of the calcination temperature. Finally, pure orthorhombic ZnTa2O6, a stable form at high temperatures, can be obtained after being calcined above 950 °C.
Fig. 3. TEM images of the ZnTa2O6 samples: (a) ZT-650, (b) ZT-750, (c) ZT-850 and (d) ZT-950.
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Z. Ding et al. / Materials Letters 65 (2011) 1598–1600
Fig. 4. Concentration changes of MO at 464 nm as a function of irradiation time during the degradation process.
Diffuse reflectance spectra of the as-prepared samples are shown in Fig. 2. The wavelength of the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangential line drawn on the absorption spectra. It is found that the absorption of ZnTa2O6 is located at ca. 285 nm, corresponding to a band gap of ~4.35 eV. ZT-650 shows a broad band between 300 and 600 nm due to the incomplete combustion of organics at the calcination temperature of 650 °C. For ZnTa2O6, the filled band formed by Zn2+ seems more positively than that of O2− because Zn2+ is hardly to become trivalent [8]. Therefore, the valence band of ZnTa2O6 is mainly composed of O 2p orbital hybridized with Zn 3d orbital, while orbital of Ta 5d in Ta5+ contributes to the formation of conduction band [1]. TEM images of the ZnTa2O6 samples are shown in Fig. 3. With increasing the calcination temperature, the crystallinity of ZnTa2O6 is enhanced. The sizes of ZnTa2O6 particles obtained at different temperatures (650–850 °C) range from 25 to 75 nm, while the average particle size of ZT-950 is about 150 nm. Furthermore, the surface areas are 18, 16, 7 and 2 m2/g for ZT-650, ZT-750, ZT-850 and ZT-950, respectively. This result can be ascribed to the increase of the particle size of the sample as mentioned above. The photocatalytic activities of the as-prepared samples were investigated by the degradation of MO under UV light irradiation. The concentration changes of MO at 464 nm as a function of irradiation time during the degradation process are shown in Fig. 4. ZT-750 and ZT-850 can degrade MO completely within 80 min under UV light, while the concentration of MO has no significant change without the catalyst. In general, photocatalysts which contain different crystal structures can show excellent performances in environment and energy fields due to the mixed crystal effect, such as Degussa P25. Different crystal structures in photocatalysts particles can enhance the separation of photo-exited holes and electrons, and consequently improve the catalytic activities of
photocatalysts. Therefore, the catalytic activities of ZT-750 and ZT-850 are much higher than those of ZT-650 and ZT-950 because ZT-750 and ZT-850 contain two kinds of crystal structures, whereas ZT-650 and ZT950 does not. Furthermore, it is known that photocatalysts consisted of different crystal structures with various proportions should show different catalytic activities. However, ZT-750 and ZT-850 show similar photocatalytic activities for the MO degradation when they contain two kinds of crystal structures with different proportions. This can be ascribed to the effects of the surface areas and crystallinities of the as-prepared samples. A photocatalyst with a large surface area can be obtained at low temperatures as mentioned above. The large surface area will improve the catalytic activity of the photocatalyst because it is important to the absorption of organic contaminants. However, a high crystallinity, which is obtained at high temperatures (see Fig. 3), can also enhance the photocatalytic activity because the defect concentration goes down. As a result, ZT-750 and ZT-850 show similar activities for the photodegradation of MO. Therefore, the photocatalytic activity of ZnTa2O6 prepared by the sol–gel method is determined by the balance among crystal structure, surface area and crystallinity in the calcination temperature range of 750–850 °C. 4. Conclusions Nanocrystalline ZnTa2O6 photocatalysts with different crystal structures can be selectively prepared via a simple and facile sol–gel method at a temperature range of 650–950 °C. The absorption edge of ZnTa2O6 was located at ~285 nm, corresponding to a band gap of about 4.35 eV. ZnTa2O6 samples showed excellent performances in the photodegradation of methyl orange under UV light irradiation. The photocatalytic activities of ZnTa2O6 samples with different crystal structures are higher than those of ZnTa2O6 samples with a single crystal structure due to the mixed crystal effect. It was also found that the photocatalytic activity of ZnTa2O6 was determined by the balance among crystal structure, surface area and crystallinity in a calcination temperature range from 750 to 850 °C. Acknowledgements The work was supported by NNSFC (20777011 and U1033603), 973 Program (2007CB613306) and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818). References [1] Kudo A, Kato H, Nakagawa S. J Phys Chem B 2000;104:571–5. [2] Zhang G, Li Ming, Yu S, Zhang S, Huang B, Yu J. J Colloid Interface Sci 2010;345: 467–73. [3] Lee HJ, Kim IT, Hong KS. Jpn J Appl Phys 1997;36:L1318–20. [4] Waburg M, Muller-Buschbaum H. Z Anorg Allg Chem 1984;508:55–60. [5] Ferrari CR, Hernandes AC. J Eur Ceram Soc 2002;22:2101–5. [6] Zhang YC, Yue ZX, Qi X, Li B, Gui ZL, Li LT. Mater Lett 2004;5:1392–5. [7] Zhang YC, Fu BJ, Wang X. J Alloy Compd 2009;478:498–500. [8] Kudo A, Nakagawa S, Kato H. Chem Lett 1999;28:1197–9.