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Preparation, characterization and photocatalytic activity of N-doped NaTaO3 nanocubes
Powder Technology 214 (2011) 155–160
Contents lists available at SciVerse ScienceDirect
Powder Technology 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 / p ow t e c
Preparation, characterization and photocatalytic activity of N-doped NaTaO3 nanocubes Yi-Xin Zhao, Da-Rui Liu, Fang-Fei Li, Dian-Fan Yang ⁎, Yin-Shan Jiang ⁎ Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, 5988 People's Avenue, Changchun, 130025, P.R. China
a r t i c l e
i n f o
Article history: Received 2 January 2011 Received in revised form 18 July 2011 Accepted 5 August 2011 Available online 16 August 2011 Keyword: Nanocubes NaTaO3 Methylene Blue Photocatalytic activity
a b s t r a c t The N-doped NaTaO3 catalysts with cubic morphology had been successfully synthesized by one-step hydrothermal method, using Ta2O5 and NH3·H2O as the raw materials for the first time. The as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectra. The results showed that the synthesis parameters such as reaction temperature and reaction time played important roles in the formation of Ndoped NaTaO3 nanocubes. As observed by SEM images, when the reaction time reached 12 h, most of the products exhibited the square morphology with the size of 200–500 nm, which can be easily removed by filtration after photocatalytic reaction. The photocatalytic activity of N-doped NaTaO3 was tested by Methylene Blue degradation process under ultraviolet and visible light irradiation, respectively. The results showed that both under the ultraviolet and visible light irradiation, N-doped NaTaO3 displayed much higher photocatalytic activity than that of pure NaTaO3. In addition, NaTaO3-xNx composites exhibited excellent stability in the visible-light photocatalytic process. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, photocatalysis method has played an important role in the environmental procedures such as air purification, water disinfection and purification [1–6]. Among the various semiconductors applied, titanium dioxide (TiO2) has been employed in photocatalytic method to decompose many organic pollutants under ultraviolet light irradiation. However, the photocatalytic activity of titania in visible light is very low due to its wide band gap (3.0– 3.2 eV), which prevents the efficient absorption of sunlight [7–9]. Therefore, it is of great significance to develop the novel photocatalysts that can be used in both UV and visible light irradiation to enhance the photocatalysis efficiency. At present, the perovskite-type alkali tantalate, NaTaO3 has been widely studied because of its outstanding performance in photocatalytic split of water into H2 and O2 under `UV light irradiation [10,11]. However, the valence band of NaTaO3 predominantly consists of O2p orbitals whose potential energy levels are located at a deep position of about 3 V versus NHE. Due to this fact, it is not active under visible-light irradiation [12]. It is widely recognized that that doping nonmetal element is an efficient method for narrowing the band gap energy of semiconductor oxide and shifts the threshold wavelength to visible light region, because the related impurity states are supposed to be close to the maximum
⁎ Corresponding authors. Tel.: + 86 0431 85094856. E-mail address: [email protected] (Y.-S. Jiang). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.08.006
valence band. [13–15]. Recently, the band gap of TiO2 has been narrowed successfully by doping with nonmetal ions, replacing lattice oxygen with B, C, N, or S dopants [16–20]. As we all know, there are very few investigations about the effects of nonmetal elements doping on the visible-light photocatalytic activity of NaTaO3. Moreover, there is no report to systematically investigate the effects of N doping on the photocatalytic behavior of NaTaO3 under UV and visible light irradiation. In the previous work, we had synthesized N-doped NaTaO3 compounds by an improved solid state reaction method [21]. The results showed that NaTaO3-xNx catalysts display higher photocatalytic activity than that of pure NaTaO3 under UV-light irradiation. Although, the absorption edge shifts to a longer wavelength, the shift is slight. So the photocatalyst does not have visible light response. It is highly probable that preparation method and conditions largely affect the photocatalytic activity, because the solid state reaction method has many disadvantages such as inhomogeneous product and lack of control over crystallinity and particle sizes, which results in a change in the photocatalytic activity of catalyst. Since excellent photocatalyst always requires homogeneous and high-purity, an alternative synthesis procedure is needed [22,23]. In the recent study, we have successfully synthesized N-doped NaTaO3 nanocubes by one-step hydrothermal process for the first time [24]. The results showed that NaTaO3-xNx exhibits much higher photocatalytic activity for Methyl Orange than that of pure NaTaO3 under visible light irradiation. Whereas, the influence of synthesis parameters such as hydrothermal time, temperature on the crystal structure and photocatalytic activity was not investigated.
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In the present work, we mainly investigated the effect of the reaction time on the morphology and photocatalytic activity of samples. Based on the SEM result, a possible mechanism of the cubic structure forming of NaTaO3-xNx was proposed. In addition, the photocatalytic activity of NaTaO3-xNx catalyst obtained at the different temperature was tested on the degradation Methylene Blue under UV and visible light irradiation, respectively. 2. Experimental 2.1. Preparation of the N-doped NaTaO3 compounds All of the reagents were analytical grade and used without further purification. NaTaO3 and N-doped NaTaO3 were prepared by one-step hydrothermal method. In a typical synthesis procedure, 0.441 g of Ta2O5, 1.2 g of NaOH and 200 μL of NH3·H2O were added into a Teflonline stainless steel autoclave (50 mL capacity) that filled with the mixture to 80% of the total volume. The autoclave was sealed and put into a preheated oven to perform hydrothermal treatment at 160, 180 and 200 °C for a designed period of time. After hydrothermal processing, the resulting precipitates were collected by centrifugation and washed with deionized water for several times, and dried at 80 °C for 2 h before further characterization and photocatalytic reaction. 2.2. Characterization X-ray diffraction (XRD) measurements were carried out with a Bruker D 8 Advabce Powder X-ray diffractometer with a Cu Kα radiation (λ = 1.518 Å) radiation source. Diffraction patterns were collected from 10 ~ 80° at a speed of 4°/min. The morphology and microstructure were observed with scanning electronic microscope (SEM, Philps XL-30). UV–vis DRS was recored using a Hitachi U-3010 spectrometer (Japan). BaSO4 was the reference sample and the spectra were recorded in the range of 200–700 nm. XPS analysis was measured on a PHI 5300 ESCA instrument using an Al Ka X-ray source at a power of 250 W. 2.3. Photocatalytic test The photocatalytic activity of the samples was evaluated by the degradation of MB under UV (UVC) and visible light irradiation (λ N 400 nm), respectively. The UV and visible light were obtained by a 250 W high pressure mercury lamp (Philips) with a cutoff filter. In each experiment, 100 mg of photocatalyst was added into 100 mL of 20 mg/L MB aqueous solution. Before irradiation, the suspension
Fig. 1. XRD patterns of samples prepared at different synthesis temperatures for 12 h: (a) NaTaO3 200 °C, (b) N-doped NaTaO3, 160 °C, (c) N-doped NaTaO3,180 °C, (d) Ndoped NaTaO3, 200 °C.
Fig. 2. XRD patterns of the as-prepared samples at 180 °C for different reaction times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h.
was magnetically stirred in the dark for 60 min to ensure the adsorption/desorption equilibrium between the photocatalyst and MB. At given time intervals, 3 mL of aliquots were sampled, and centrifugated to remove the particles. The filtrates were analyzed by recording the variations of the absorption band maximum (553 nm) in the UV–vis spectrum of MB using a Hitachi U-3010 spectrophotometer.
3. Results and discussion 3.1. XRD analysis To determine the hydrothermal temperature for N-doped NaTaO3 to be well-crystallized, the precursor suspension was heated for 12 h at 160, 180 and 200 °C, respectively. The XRD patterns of samples are shown in Fig. 1. It is clearly seen that the XRD pattern of sample obtained at 160 °C by hydrothermal process remains the peaks of Ta2O5, implying that NaTaO3 did not completely form. All diffraction peaks of NaTaO3 crystal appeared when the temperature was higher than 180 °C, which could be easily indexed as a pure perovskite structure according to the standard card (JCPDS card 74–2478) [25]. And no other nitrogen-containing compounds or other tantalum polymorphs were detected by XRD. The results suggested that the replacement of O with N atom in NaTaO3 did not result in significant structural changes. This could be understood that the concentration of the doped N atom might be insufficient to cause a structural rebuilding, and similar phenomena were also reported in our previous
Fig. 3. XPS spectra of N-doped NaTaO3 sample obtained at 160, 180 and 200 °C for 12 h.
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C and Ta are confirmed to exist in the sample by the five peaks at binding energies of 1074, 530, 397, 282 and 26 eV, which are related to Na1s, O1s, N1s, C1s and Ta4f, respectively. The N1s peaks in XPS spectra of samples obtained at different temperatures are displayed in the inset of Fig. 4. The binding energy of N 1 s for all the samples is about 397 eV. This peak is characteristic of N 3− generally that corresponds to Ta\N bonds, which is in agreement with other reports [26]. The mass content of N for N-doped NaTaO3 obtained at 160, 180 and 200 °C is 0.026, 0.049 and 0.051 at.%, respectively. Considering the results of XRD and XPS analysis, the sample was described as NaTaO3-xNx. Fig. 4. The schematic diagram of N doped into NaTaO3 during one-step hydrothermal process.
report [21]. Moreover, the intensity of the diffraction peaks became stronger with the increase of temperature. Compared to the solid state reaction, the hydrothermal method is obviously a low-cost, energy saving and easy way to obtain well-crystalline NaTaO3 at a lower temperature. In order to investigate the influence of the reaction time on the structure and morphology of the resultant products, the experiments were performed at 180 °C for 3, 6, 9 and 12 h, respectively. Fig. 2 exhibits the XRD patterns of the samples. As can be seen, the diffraction patterns of the resultant products were mainly attributed to Ta2O5 when the reaction time was less than 3 h. Until 6 h of the hydrothermal reaction, the feeble NaTaO3 crystalline phase appeared. A well crystalline phase was formed from the poorly crystallized one in the subsequent period when the reaction time reached 9 h. Moreover, the intensity of the diffraction peaks became stronger as the reaction time increased. 3.2. X-ray photoelectron spectroscopy XPS analysis is used to determine the chemical states and content of each element possibly existing in the samples obtained at 160, 180 and 200 °C, and the result are presented in Fig. 3. Elements of Na, O, N,
3.3. Morphology of N-doped NaTaO3 photocatalysts The schematic diagram of N doped into NaTaO3 during one-step hydrothermal process is shown in Fig. 4. At the initial hydrothermal reaction, Ta2O5 reacted with Na + ions in the solution to form a continuous NaTaO3 layer. With the reaction time increasing, Na + reacted with Ta2O5 through the NaTaO3 layers until Ta2O5 complete response. During the formation process of the NaTaO3, N 3− may be substituted for partial O 2− of NaTaO3 to form NaTaO3-xNx. The Ndoped NaTaO3 photocatalysts are obtained by the following reactions: þ
þ
−
Na þ Ta2 O5 þ NH4 þ OH →NaTaO3x Nx þ H2 O Fig. 5 shows the representative SEM images of the samples obtained from different reaction time. As exhibited in Fig. 5, after 3 h of hydrothermal reaction, the sample comprised irregular particles, which are ascribed to the reactant of Ta2O5 powder. With the time prolonged to 6 h, NaTaO3-xNx nanocubes started to emerge, but there were many small irregular particles coexisted on the surface of the NaTaO3-xNx cubes. With the time increasing, more nanocubes formed and grew further at the cost of the smaller particles, suggesting that the nanocubes grow at the consummation of the small particles due to the difference in solubility between the large particles and the small
Fig. 5. SEM images of the as-prepared N-doped NaTaO3 samples for different reaction times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h.
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particles. When the reaction time reached 12 h, most of the products exhibited the square morphology with the size of 200–500 nm, and the average size was about 300 nm as shown in Fig. 5d. NaTaO3 belongs to ABO3 perovskite-type structure in which TaO6 octrahedra are connected by corner-sharing, which have the habit to form the cubic morphology [27]. 3.4. UV–vis diffuse reflectance property Fig. 6 shows the typical diffuse reflection spectra for the undoped and N-doped NaTaO3 samples. For the undoped NaTaO3 sample, the absorption spectrum was cut off at ~315 nm, from which the band gap was estimated to be 3.94 eV. For the doped sample, an add-on shoulder was imposed onto the cutoff edge of the absorption spectrum, which extended the absorption from 370 to 500 nm. Moreover, the absorbance intensity for N-doped sample at 220–245 nm is stronger than that of undoped sample, due to the combination between N and Ta. The extension of the light absorption from the UV to the visible range arises from the contributions of both the doped nitrogen atoms and the oxygen vacancies in the lattice, because the interstitial nitrogen atoms induced the local states near the valence band edge and the oxygen vacancies give rise to the local states below the conduction edge. Excitation from such local states to the conduction band is consistent with the “add-on shoulder” on the absorption edge of the UV–visible spectrum [28]. The band gap energy (Eg) values for the samples were calculated from the UV–vis spectra using the equation Eg (eV) = 1240/λg (nm) [29]. The Eg values of N-doped NaTaO3 (2.48 eV) is much smaller than that of pure NaTaO3 (3.94 eV), which indicated that N-doped NaTaO3 can be sensitive to visible light irradiation.
peaks of MB solution were decreased rapidly. The dispersions discolored completely after irradiation for about 40 min. The degradation curve of MB by NaTaO3 and N-doped NaTaO3 prepared at different temperature under visible light irradiation was presented in Fig. 8a. A trend in the photocatalytic has been observed in the following order: N- NaTaO 3 -180 °C N N- NaTaO 3-200 °C N NaTaO3-180 °C. Fig. 8b shows the UV–vis spectral changes of MB solution mediated by N-doped NaTaO3 prepared at 180 °C for 12 h under visible light irradiation. With the increase in the illumination time, the absorption peak of 600 and 660 nm decreased gradually, and after only 5 h of irradiation, it almost disappeared. On the basis of the results, it is observed that both under the ultraviolet and visible light, the N-doped NaTaO3 exhibited higher photocatalytic activity than that of pure NaTaO3. The reason is that in the case of the right nitrogen, the newly formed intra-band gap states were found to be close enough to the conduction band edge (shallow surface states) to induce electronic coupling, which may prevent charge recombination. So the photocatalytic activity of samples was enhanced [30]. 3.6. Photochemical stability of the catalysts To evaluate the photochemical stability of the catalysts, the repeated experiments for the photodegradation of MB were performed by NaTaO2.951N0.049 under visible light irradiation, and the results are shown in Fig. 9. After each photocatalytic reaction, the sample was separated by centrifugation, and then reused for the next experiment. Then the resultant sample obtained in the fourth reaction cycle was
3.5. Photocatalytic activity The photocatalytic degradation of MB was performed in order to investigate the photocatalytic activity of NaTaO3 and N-doped NaTaO3 under UV and visible light, respectively. Fig. 7a exhibits the degradation of MB in the presence of NaTaO3 and N-doped NaTaO3 obtained at different temperature under UV light irradiation. As shown in the Fig. 7a, the sample obtained at 160 °C had the lowest photocatalytic activity. The possible reason was that the sample obtained at 160 °C was not pure NaTaO3, which is accord with the result of XRD. It is clear that both of the N-doped NaTaO3 obtained at 180 and 200 °C displayed higher photocatalytic activity than that of pure NaTaO3. The UV–vis spectral changes during the photocatalytic degradation of MB in the aqueous are shown in Fig. 7b. With the light irradiating, both the 600 and 660 nm absorption
Fig. 6. UV–vis diffuse reflectance spectra of pure NaTaO3 and N-doped NaTaO3.
Fig. 7. The UV photocatalytic of as-prepared samples at different synthesis temperatures for 12 h (a) and UV–vis spectra of the MB solution in the presence of NaTaO2.951N0.049 under UV irradiation in different photocatalytic reaction stages (b).
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disappear, followed by activity decrease. But after being calcined at 500 °C, the catalyst displays the similar photocatalytic activity as that of the sample used for the first time, respectively. During calcination process, the surface-covered organics were removed, and the NaTaO2.951N0.049 sample recovered its photocatalytic activity. All of these indicated that NaTaO3-xNx catalysts had not only high activity, but also good stability in the photocatalytic process. 4. Conclusions In this work, N-doped NaTaO3 catalysts with cubic morphology were successfully synthesized by one-step hydrothermal method. The synthesis parameters such as reaction temperature and reaction time played important roles in the formation of N-doped NaTaO3 nanocubes. The photocatalytic activity of N-doped NaTaO3 was tested by the MB degradation process under ultraviolet and visible light irradiation, respectively. The results showed that both under the ultraviolet and visible light, N-doped NaTaO3 displayed higher photocatalytic activity than that of pure NaTaO3. In addition, NaTaO3-xNx exhibited excellent stability in the visible-light photocatalytic process. Acknowledgement We acknowledge the financial supports of National Natural Science Foundation of China (Grants no. 50574043, 40772028 and 41072025). References
Fig. 8. The visible light photocatalytic of samples obtained at different synthesis temperatures for 12 h (a) and UV–vis spectra of the MB solution in the presence of NaTaO2.951N0.049 under visible light irradiation in different photocatalytic reaction stages (b).
calcined at 500 °C to release the surface adsorbed organics and prepare for the fifth reaction cycle. As shown in Fig. 9, the reused catalysts do not show apparent change on photocatalytic activity after the third reaction cycle, which exhibits the excellent photochemical stability. In the fourth cycle, the photocatalytic activity of NaTaO2.951N0.049 decreased rapidly. A possible explanation is that the surface of the catalysts should be contaminated by the reactant and other substances (CO2, O2 absorbed on the surface) more or less, which may make some of the active places
Fig. 9. The photocatalytic cycles of NaTaO2.951N0.049 (C0 = 20 mg/L, catalyst dose 100 mg/100 mL, pH = 4.4).
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Contents lists available at SciVerse ScienceDirect
Powder Technology 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 / p ow t e c
Preparation, characterization and photocatalytic activity of N-doped NaTaO3 nanocubes Yi-Xin Zhao, Da-Rui Liu, Fang-Fei Li, Dian-Fan Yang ⁎, Yin-Shan Jiang ⁎ Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, 5988 People's Avenue, Changchun, 130025, P.R. China
a r t i c l e
i n f o
Article history: Received 2 January 2011 Received in revised form 18 July 2011 Accepted 5 August 2011 Available online 16 August 2011 Keyword: Nanocubes NaTaO3 Methylene Blue Photocatalytic activity
a b s t r a c t The N-doped NaTaO3 catalysts with cubic morphology had been successfully synthesized by one-step hydrothermal method, using Ta2O5 and NH3·H2O as the raw materials for the first time. The as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectra. The results showed that the synthesis parameters such as reaction temperature and reaction time played important roles in the formation of Ndoped NaTaO3 nanocubes. As observed by SEM images, when the reaction time reached 12 h, most of the products exhibited the square morphology with the size of 200–500 nm, which can be easily removed by filtration after photocatalytic reaction. The photocatalytic activity of N-doped NaTaO3 was tested by Methylene Blue degradation process under ultraviolet and visible light irradiation, respectively. The results showed that both under the ultraviolet and visible light irradiation, N-doped NaTaO3 displayed much higher photocatalytic activity than that of pure NaTaO3. In addition, NaTaO3-xNx composites exhibited excellent stability in the visible-light photocatalytic process. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, photocatalysis method has played an important role in the environmental procedures such as air purification, water disinfection and purification [1–6]. Among the various semiconductors applied, titanium dioxide (TiO2) has been employed in photocatalytic method to decompose many organic pollutants under ultraviolet light irradiation. However, the photocatalytic activity of titania in visible light is very low due to its wide band gap (3.0– 3.2 eV), which prevents the efficient absorption of sunlight [7–9]. Therefore, it is of great significance to develop the novel photocatalysts that can be used in both UV and visible light irradiation to enhance the photocatalysis efficiency. At present, the perovskite-type alkali tantalate, NaTaO3 has been widely studied because of its outstanding performance in photocatalytic split of water into H2 and O2 under `UV light irradiation [10,11]. However, the valence band of NaTaO3 predominantly consists of O2p orbitals whose potential energy levels are located at a deep position of about 3 V versus NHE. Due to this fact, it is not active under visible-light irradiation [12]. It is widely recognized that that doping nonmetal element is an efficient method for narrowing the band gap energy of semiconductor oxide and shifts the threshold wavelength to visible light region, because the related impurity states are supposed to be close to the maximum
⁎ Corresponding authors. Tel.: + 86 0431 85094856. E-mail address: [email protected] (Y.-S. Jiang). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.08.006
valence band. [13–15]. Recently, the band gap of TiO2 has been narrowed successfully by doping with nonmetal ions, replacing lattice oxygen with B, C, N, or S dopants [16–20]. As we all know, there are very few investigations about the effects of nonmetal elements doping on the visible-light photocatalytic activity of NaTaO3. Moreover, there is no report to systematically investigate the effects of N doping on the photocatalytic behavior of NaTaO3 under UV and visible light irradiation. In the previous work, we had synthesized N-doped NaTaO3 compounds by an improved solid state reaction method [21]. The results showed that NaTaO3-xNx catalysts display higher photocatalytic activity than that of pure NaTaO3 under UV-light irradiation. Although, the absorption edge shifts to a longer wavelength, the shift is slight. So the photocatalyst does not have visible light response. It is highly probable that preparation method and conditions largely affect the photocatalytic activity, because the solid state reaction method has many disadvantages such as inhomogeneous product and lack of control over crystallinity and particle sizes, which results in a change in the photocatalytic activity of catalyst. Since excellent photocatalyst always requires homogeneous and high-purity, an alternative synthesis procedure is needed [22,23]. In the recent study, we have successfully synthesized N-doped NaTaO3 nanocubes by one-step hydrothermal process for the first time [24]. The results showed that NaTaO3-xNx exhibits much higher photocatalytic activity for Methyl Orange than that of pure NaTaO3 under visible light irradiation. Whereas, the influence of synthesis parameters such as hydrothermal time, temperature on the crystal structure and photocatalytic activity was not investigated.
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In the present work, we mainly investigated the effect of the reaction time on the morphology and photocatalytic activity of samples. Based on the SEM result, a possible mechanism of the cubic structure forming of NaTaO3-xNx was proposed. In addition, the photocatalytic activity of NaTaO3-xNx catalyst obtained at the different temperature was tested on the degradation Methylene Blue under UV and visible light irradiation, respectively. 2. Experimental 2.1. Preparation of the N-doped NaTaO3 compounds All of the reagents were analytical grade and used without further purification. NaTaO3 and N-doped NaTaO3 were prepared by one-step hydrothermal method. In a typical synthesis procedure, 0.441 g of Ta2O5, 1.2 g of NaOH and 200 μL of NH3·H2O were added into a Teflonline stainless steel autoclave (50 mL capacity) that filled with the mixture to 80% of the total volume. The autoclave was sealed and put into a preheated oven to perform hydrothermal treatment at 160, 180 and 200 °C for a designed period of time. After hydrothermal processing, the resulting precipitates were collected by centrifugation and washed with deionized water for several times, and dried at 80 °C for 2 h before further characterization and photocatalytic reaction. 2.2. Characterization X-ray diffraction (XRD) measurements were carried out with a Bruker D 8 Advabce Powder X-ray diffractometer with a Cu Kα radiation (λ = 1.518 Å) radiation source. Diffraction patterns were collected from 10 ~ 80° at a speed of 4°/min. The morphology and microstructure were observed with scanning electronic microscope (SEM, Philps XL-30). UV–vis DRS was recored using a Hitachi U-3010 spectrometer (Japan). BaSO4 was the reference sample and the spectra were recorded in the range of 200–700 nm. XPS analysis was measured on a PHI 5300 ESCA instrument using an Al Ka X-ray source at a power of 250 W. 2.3. Photocatalytic test The photocatalytic activity of the samples was evaluated by the degradation of MB under UV (UVC) and visible light irradiation (λ N 400 nm), respectively. The UV and visible light were obtained by a 250 W high pressure mercury lamp (Philips) with a cutoff filter. In each experiment, 100 mg of photocatalyst was added into 100 mL of 20 mg/L MB aqueous solution. Before irradiation, the suspension
Fig. 1. XRD patterns of samples prepared at different synthesis temperatures for 12 h: (a) NaTaO3 200 °C, (b) N-doped NaTaO3, 160 °C, (c) N-doped NaTaO3,180 °C, (d) Ndoped NaTaO3, 200 °C.
Fig. 2. XRD patterns of the as-prepared samples at 180 °C for different reaction times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h.
was magnetically stirred in the dark for 60 min to ensure the adsorption/desorption equilibrium between the photocatalyst and MB. At given time intervals, 3 mL of aliquots were sampled, and centrifugated to remove the particles. The filtrates were analyzed by recording the variations of the absorption band maximum (553 nm) in the UV–vis spectrum of MB using a Hitachi U-3010 spectrophotometer.
3. Results and discussion 3.1. XRD analysis To determine the hydrothermal temperature for N-doped NaTaO3 to be well-crystallized, the precursor suspension was heated for 12 h at 160, 180 and 200 °C, respectively. The XRD patterns of samples are shown in Fig. 1. It is clearly seen that the XRD pattern of sample obtained at 160 °C by hydrothermal process remains the peaks of Ta2O5, implying that NaTaO3 did not completely form. All diffraction peaks of NaTaO3 crystal appeared when the temperature was higher than 180 °C, which could be easily indexed as a pure perovskite structure according to the standard card (JCPDS card 74–2478) [25]. And no other nitrogen-containing compounds or other tantalum polymorphs were detected by XRD. The results suggested that the replacement of O with N atom in NaTaO3 did not result in significant structural changes. This could be understood that the concentration of the doped N atom might be insufficient to cause a structural rebuilding, and similar phenomena were also reported in our previous
Fig. 3. XPS spectra of N-doped NaTaO3 sample obtained at 160, 180 and 200 °C for 12 h.
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C and Ta are confirmed to exist in the sample by the five peaks at binding energies of 1074, 530, 397, 282 and 26 eV, which are related to Na1s, O1s, N1s, C1s and Ta4f, respectively. The N1s peaks in XPS spectra of samples obtained at different temperatures are displayed in the inset of Fig. 4. The binding energy of N 1 s for all the samples is about 397 eV. This peak is characteristic of N 3− generally that corresponds to Ta\N bonds, which is in agreement with other reports [26]. The mass content of N for N-doped NaTaO3 obtained at 160, 180 and 200 °C is 0.026, 0.049 and 0.051 at.%, respectively. Considering the results of XRD and XPS analysis, the sample was described as NaTaO3-xNx. Fig. 4. The schematic diagram of N doped into NaTaO3 during one-step hydrothermal process.
report [21]. Moreover, the intensity of the diffraction peaks became stronger with the increase of temperature. Compared to the solid state reaction, the hydrothermal method is obviously a low-cost, energy saving and easy way to obtain well-crystalline NaTaO3 at a lower temperature. In order to investigate the influence of the reaction time on the structure and morphology of the resultant products, the experiments were performed at 180 °C for 3, 6, 9 and 12 h, respectively. Fig. 2 exhibits the XRD patterns of the samples. As can be seen, the diffraction patterns of the resultant products were mainly attributed to Ta2O5 when the reaction time was less than 3 h. Until 6 h of the hydrothermal reaction, the feeble NaTaO3 crystalline phase appeared. A well crystalline phase was formed from the poorly crystallized one in the subsequent period when the reaction time reached 9 h. Moreover, the intensity of the diffraction peaks became stronger as the reaction time increased. 3.2. X-ray photoelectron spectroscopy XPS analysis is used to determine the chemical states and content of each element possibly existing in the samples obtained at 160, 180 and 200 °C, and the result are presented in Fig. 3. Elements of Na, O, N,
3.3. Morphology of N-doped NaTaO3 photocatalysts The schematic diagram of N doped into NaTaO3 during one-step hydrothermal process is shown in Fig. 4. At the initial hydrothermal reaction, Ta2O5 reacted with Na + ions in the solution to form a continuous NaTaO3 layer. With the reaction time increasing, Na + reacted with Ta2O5 through the NaTaO3 layers until Ta2O5 complete response. During the formation process of the NaTaO3, N 3− may be substituted for partial O 2− of NaTaO3 to form NaTaO3-xNx. The Ndoped NaTaO3 photocatalysts are obtained by the following reactions: þ
þ
−
Na þ Ta2 O5 þ NH4 þ OH →NaTaO3x Nx þ H2 O Fig. 5 shows the representative SEM images of the samples obtained from different reaction time. As exhibited in Fig. 5, after 3 h of hydrothermal reaction, the sample comprised irregular particles, which are ascribed to the reactant of Ta2O5 powder. With the time prolonged to 6 h, NaTaO3-xNx nanocubes started to emerge, but there were many small irregular particles coexisted on the surface of the NaTaO3-xNx cubes. With the time increasing, more nanocubes formed and grew further at the cost of the smaller particles, suggesting that the nanocubes grow at the consummation of the small particles due to the difference in solubility between the large particles and the small
Fig. 5. SEM images of the as-prepared N-doped NaTaO3 samples for different reaction times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h.
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particles. When the reaction time reached 12 h, most of the products exhibited the square morphology with the size of 200–500 nm, and the average size was about 300 nm as shown in Fig. 5d. NaTaO3 belongs to ABO3 perovskite-type structure in which TaO6 octrahedra are connected by corner-sharing, which have the habit to form the cubic morphology [27]. 3.4. UV–vis diffuse reflectance property Fig. 6 shows the typical diffuse reflection spectra for the undoped and N-doped NaTaO3 samples. For the undoped NaTaO3 sample, the absorption spectrum was cut off at ~315 nm, from which the band gap was estimated to be 3.94 eV. For the doped sample, an add-on shoulder was imposed onto the cutoff edge of the absorption spectrum, which extended the absorption from 370 to 500 nm. Moreover, the absorbance intensity for N-doped sample at 220–245 nm is stronger than that of undoped sample, due to the combination between N and Ta. The extension of the light absorption from the UV to the visible range arises from the contributions of both the doped nitrogen atoms and the oxygen vacancies in the lattice, because the interstitial nitrogen atoms induced the local states near the valence band edge and the oxygen vacancies give rise to the local states below the conduction edge. Excitation from such local states to the conduction band is consistent with the “add-on shoulder” on the absorption edge of the UV–visible spectrum [28]. The band gap energy (Eg) values for the samples were calculated from the UV–vis spectra using the equation Eg (eV) = 1240/λg (nm) [29]. The Eg values of N-doped NaTaO3 (2.48 eV) is much smaller than that of pure NaTaO3 (3.94 eV), which indicated that N-doped NaTaO3 can be sensitive to visible light irradiation.
peaks of MB solution were decreased rapidly. The dispersions discolored completely after irradiation for about 40 min. The degradation curve of MB by NaTaO3 and N-doped NaTaO3 prepared at different temperature under visible light irradiation was presented in Fig. 8a. A trend in the photocatalytic has been observed in the following order: N- NaTaO 3 -180 °C N N- NaTaO 3-200 °C N NaTaO3-180 °C. Fig. 8b shows the UV–vis spectral changes of MB solution mediated by N-doped NaTaO3 prepared at 180 °C for 12 h under visible light irradiation. With the increase in the illumination time, the absorption peak of 600 and 660 nm decreased gradually, and after only 5 h of irradiation, it almost disappeared. On the basis of the results, it is observed that both under the ultraviolet and visible light, the N-doped NaTaO3 exhibited higher photocatalytic activity than that of pure NaTaO3. The reason is that in the case of the right nitrogen, the newly formed intra-band gap states were found to be close enough to the conduction band edge (shallow surface states) to induce electronic coupling, which may prevent charge recombination. So the photocatalytic activity of samples was enhanced [30]. 3.6. Photochemical stability of the catalysts To evaluate the photochemical stability of the catalysts, the repeated experiments for the photodegradation of MB were performed by NaTaO2.951N0.049 under visible light irradiation, and the results are shown in Fig. 9. After each photocatalytic reaction, the sample was separated by centrifugation, and then reused for the next experiment. Then the resultant sample obtained in the fourth reaction cycle was
3.5. Photocatalytic activity The photocatalytic degradation of MB was performed in order to investigate the photocatalytic activity of NaTaO3 and N-doped NaTaO3 under UV and visible light, respectively. Fig. 7a exhibits the degradation of MB in the presence of NaTaO3 and N-doped NaTaO3 obtained at different temperature under UV light irradiation. As shown in the Fig. 7a, the sample obtained at 160 °C had the lowest photocatalytic activity. The possible reason was that the sample obtained at 160 °C was not pure NaTaO3, which is accord with the result of XRD. It is clear that both of the N-doped NaTaO3 obtained at 180 and 200 °C displayed higher photocatalytic activity than that of pure NaTaO3. The UV–vis spectral changes during the photocatalytic degradation of MB in the aqueous are shown in Fig. 7b. With the light irradiating, both the 600 and 660 nm absorption
Fig. 6. UV–vis diffuse reflectance spectra of pure NaTaO3 and N-doped NaTaO3.
Fig. 7. The UV photocatalytic of as-prepared samples at different synthesis temperatures for 12 h (a) and UV–vis spectra of the MB solution in the presence of NaTaO2.951N0.049 under UV irradiation in different photocatalytic reaction stages (b).
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disappear, followed by activity decrease. But after being calcined at 500 °C, the catalyst displays the similar photocatalytic activity as that of the sample used for the first time, respectively. During calcination process, the surface-covered organics were removed, and the NaTaO2.951N0.049 sample recovered its photocatalytic activity. All of these indicated that NaTaO3-xNx catalysts had not only high activity, but also good stability in the photocatalytic process. 4. Conclusions In this work, N-doped NaTaO3 catalysts with cubic morphology were successfully synthesized by one-step hydrothermal method. The synthesis parameters such as reaction temperature and reaction time played important roles in the formation of N-doped NaTaO3 nanocubes. The photocatalytic activity of N-doped NaTaO3 was tested by the MB degradation process under ultraviolet and visible light irradiation, respectively. The results showed that both under the ultraviolet and visible light, N-doped NaTaO3 displayed higher photocatalytic activity than that of pure NaTaO3. In addition, NaTaO3-xNx exhibited excellent stability in the visible-light photocatalytic process. Acknowledgement We acknowledge the financial supports of National Natural Science Foundation of China (Grants no. 50574043, 40772028 and 41072025). References
Fig. 8. The visible light photocatalytic of samples obtained at different synthesis temperatures for 12 h (a) and UV–vis spectra of the MB solution in the presence of NaTaO2.951N0.049 under visible light irradiation in different photocatalytic reaction stages (b).
calcined at 500 °C to release the surface adsorbed organics and prepare for the fifth reaction cycle. As shown in Fig. 9, the reused catalysts do not show apparent change on photocatalytic activity after the third reaction cycle, which exhibits the excellent photochemical stability. In the fourth cycle, the photocatalytic activity of NaTaO2.951N0.049 decreased rapidly. A possible explanation is that the surface of the catalysts should be contaminated by the reactant and other substances (CO2, O2 absorbed on the surface) more or less, which may make some of the active places
Fig. 9. The photocatalytic cycles of NaTaO2.951N0.049 (C0 = 20 mg/L, catalyst dose 100 mg/100 mL, pH = 4.4).
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