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NaTaO3 photocatalysts: Enhanced charge separation and photocatalytic properties under visible light irradiation
Catalysis Communications 84 (2016) 163–166
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Fabrication of ternary p-n heterostructures AgCl/Ag2O/NaTaO3 photocatalysts: Enhanced charge separation and photocatalytic properties under visible light irradiation Dongbo Xu a,b, Weidong Shi a,b,⁎, Songbo Yang b, Biyi Chen b, Hongye Bai b, Lisong Xiao b a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
a r t i c l e
i n f o
Article history: Received 20 March 2016 Received in revised form 9 June 2016 Accepted 26 June 2016 Available online 28 June 2016 Keywords: Ternary p-n Heterostructures AgCl/Ag2O/NaTaO3 Visible light Enhanced charge separation Photocatalytic properties
a b s t r a c t Ternary p-n heterostructures photocatalysts of AgCl/Ag2O/NaTaO3 were synthesized via a simple method and the crystal structure characterized by X-ray diffraction (XRD). The morphology of the photocatalysts were characterized by scanning electron microscopy and transmission electron microscopy. The composition of the photocatalysts was studied by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscope. The photocatalytic activity of the AgCl/Ag2O/NaTaO3 photocatalysts was evaluated using the degradation of Rhodamine B. The AgCl/Ag2O/NaTaO3 photocatalysts showed higher visible light activity than the pure NaTaO3 and Ag2O/NaTaO3 photocatalysts. Additionally, the photocatalytic mechanism was that the rapidly separation of photoinduced electrons and holes resulted the enhancement of photocatalytic activity. © 2016 Published by Elsevier B.V.
1. Introduction Semiconductor photocatalytic materials have attracted much attention for environmental protection because of the shortage of clean water [1–3]. Up to now, many tantalates photocatalysts have been developed for the photodegradation of organic pollutants, such as alkali tantalates LiTaO3, NaTaO3, and KTaO3, alkaline earth tantalates SrTa2O6, CaTa2O6, BaTa2O6 [4–8]. However, they are only photoexcited by ultraviolet light which restricted their extensive application [9]. In recent years, Ag2O as a n-type semiconductor has been reported to be a good sensitizer by compositing with p-type semiconductor photocatalysts because fabricating p-n heterjunctions have the faster separation of the photogenerated electron-hole pairs in photocatalysts [10]. For example, there are many reports about Ag2O compositing with TiO2 and Ag2O-TiO2 composite photocatalysts presented enhanced photocatalytic activity under both UV and visible light irradiation [11– 14]. AgCl is a kind of photosensitive material so that extensively used
⁎ Corresponding author at: School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China. E-mail address: [email protected] (W. Shi).
http://dx.doi.org/10.1016/j.catcom.2016.06.023 1566-7367/© 2016 Published by Elsevier B.V.
in photocatalytic area. However, as like tantalates, it is only photoexcited by ultraviolet light [15]. Therefore, here we develop a three-component p-n heterjunctions photocatalysts of AgCl/Ag2O/NaTaO3 system to improve the photocatalytic activity of NaTaO3 and AgCl via a simple synthesis process. The photocatalytic activities of the three-component AgCl/Ag2O/NaTaO3 pn heterojunction photocatalysts are evaluated by degrading Rhodamine B (RhB) under visible-light irradiation. Furthermore, the photocatalytic enhancement mechanism of AgCl/Ag2O/NaTaO3 photocatalysts has been proposed based on the relative band gap position and the integrated p-n heterostructure of these semiconductors. 2. Experimental 2.1. Synthesis of Ag2O/NaTaO3 heterogeneous All Chemical reagents were of analytic grade purity and directly used without further purification. NaTaO3 nanocubes were synthesized by the reported hydrothermal method [16]. In a typical process, 0.3 g of synthesized NaTaO3 nanocubes were added to 150 mL of AgNO3 (1.07 g) aqueous solution and stirred for 10 min at room temperature. Then excessive amounts of NaOH aqueous solution was dropwise
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added into the mixed solution under magnetic stirring. The final Ag2O/ NaTaO3 powders were filtered and washed with deionized water until the pH was 7, and then dried at 60 °C for 24 h. 2.2. Synthesis of AgCl/Ag2O/NaTaO3 heterogeneous 0.1 g Ag2O/NaTaO3 and 180 mL deionized water were mixed under stirring in an open beaker. Then 0.1 M of HCl aqueous solution was dropwise added into the mixed solution under magnetic stirring. By controlling the amount of HCl aqueous solution to be 0, 2, 2.5, 3, 3.5, and 4 mL, the obtained AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts were labeled as H-0 to H-5, respectively. The final photocatalysts were filtered and washed with deionized water until the pH was 7, and then dried at 60 °C for 24 h.
3. Results and discussion 3.1. XRD patterns analysis Fig. 1 shows the XRD patterns of the obtained photocatalysts of AgCl/ Ag2O/NaTaO3 and the pure NaTaO3 nanocubes. It indicates that the diffraction peaks of pure NaTaO3 nanocubes can be assigned to the JCPDS card No. 25-0863. According to the H-0, characteristic peaks of Ag2O are observable which are marked with “*” (JCPDS No. 41-1104). However, when the HCl aqueous solution is added, the peaks marked with “#” are assigned to the phases of AgCl (JCPDS No. 31-1238). As shown in Fig. S1 (B, C, D), when the HCl aqueous solution sequentially added, the peaks of Ag2O are decreased and the peaks of AgCl are strength. No peaks of impurities can be found in all the as-prepared photocatalysts. 3.2. XPS patterns analysis
2.3. Characterization The crystallographic structures of AgCl/Ag2O/NaTaO3 photocatalysts were investigated by X-ray diffraction (XRD, Bruker D8, Cu-Kα). Morphology and composition of photocatalysts was characterized by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) with a JEM-2100F electron microscope (JEOL, Japan) as well as the scanning electron microscopy (SEM; Hitachi, S-4800) equipped with an energy-dispersive X-ray spectroscope (EDS), respectively. X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) was used to probe the composition of the obtained photocatalysts. UV − vis absorption spectra were recorded on a UV-3100 (Shimadzu, Japan) spectrophotometer with BaSO4 as the reference standard. Solid-state photoluminescent (PL) spectra were recorded on a Photon Technology International Model Quantamaster-QM4m spectrofluorimeter equipped with a 75 W lamp and dual excitation monochromators. 2.4. Photocatalytic activity test RhB was used as model pollutant to evaluate the photocatalytic activity of the AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts. Photodegradation studies were carried out under a 250 W Xe lamp with a UV-cutoff filter (λ N 425 nm). AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts (50 mg) were added into 100 mL of RhB aqueous solution (10 mg/L). The suspensions were kept in the dark for 30 min with magnetically stirring to reach an adsorption-desorption equilibrium. At a given time interval, 5 mL of solution was taken and immediately centrifuged. Then UV−vis spectra of the centrifuged solution were recorded using a UV-3100 spectrophotometer.
Fig. 1. XRD patterns of as-prepared photocatalysts.
The composition and chemical states of the obtained photocatalysts were confirmed by XPS. Fig. 2 shows the XPS spectra of AgCl/Ag2O/ NaTaO3 heterojunction (H-3). The peaks of Ag, Ta, Cl and O can be detected in Fig. 2. The high-resolution XPS spectra of Ta 4f, Ag 3d and Cl 2p for the obtained photocatalysts are shown in Fig. S2 (A–C). The binding energies of Ag 3d3 and Ag 3d5 are 373.9 and 368.3 eV, indicating the existence of Ag+ ions in Ag2O [17–18]. The peak of Cl 2p located at 198.7 eV is ascribed to Cl in AgCl. The peak of Ta 4f located at 24.9 and 26.3 eV are ascribed to Ta in NaTaO3. The result of XPS indicates the presence of AgCl, Ag2O and NaTaO3, implying the formation of AgCl/ Ag2O/NaTaO3 heterojunction. 3.3. Morphology of AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts Fig. S3 (A-B) show the SEM images of pure NaTaO3 and the H-3 AgCl/ Ag2O/NaTaO3 photocatalysts, respectively. They are demonstrated clearly that the pure NaTaO3 is nanocubes and a uniform rough layer covered the smooth surface of NaTaO3 nanocubes. Fig. S3C shows the TEM image of AgCl/Ag2O/NaTaO3 photocatalysts, which demonstrated that there is a rough layer of Ag2O on the surface of NaTaO3. Furthermore, there are some AgCl nanoparticles on the surface of Ag2O. The above results further indicate the formation of AgCl/Ag2O/NaTaO3 ternary p-n heterojunctions. Fig. S3D displays the EDS spectroscopy of AgCl/Ag2O/NaTaO3 photocatalysts. From which, one can see that there are Ag, Cl, Ta and O elements. The HRTEM image of H-3 (Fig. 3) shows the clear lattice fringes of the Ag2O and AgCl on the surface of NaTaO3
Fig. 2. XPS spectra of AgCl/Ag2O/NaTaO3 heterojunction photocatalysts. (B-D) highresolution XPS spectra of Ag 3d, Cl 2P and Ta 4f binding energy, respectively.
D. Xu et al. / Catalysis Communications 84 (2016) 163–166
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Fig. 5. Photocatalytic activities of pure NaTaO3 and AgCl/Ag2O/NaTaO3 (H-0 to H-5) p-n heterojunction photocatalysts for the degradation of RhB under visible light. Fig. 3. HRTEM image of AgCl/Ag2O/NaTaO3 photocatalysts.
nanocubes. The interplanar spacing of 0.16 nm are consistent with the (2 2 2) crystal face of AgCl and the interplanar spacing of 0.27 nm is consistent with the (1 1 1) crystal face of Ag2O, respectively. The result of HRTEM is in good agreement with the XRD results so that it is clearly the AgCl/Ag2O/NaTaO3 ternary p-n heterojunction photocatalysts have been synthesized successfully. 3.4. Optical property of obtained photocatalysts UV − vis absorption spectra of pure NaTaO3 nanocubes, Ag2O/ NaTaO3 and AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts are shown in Fig. 4A. There is no significant absorbance in visible light region can be seen of NaTaO3 because of its wide band gap. After pAg2O/n-NaTaO3 heterojunction photocatalysts being formed, it exhibits a wide and strong light absorption range of 350–700 nm. As for the obtained AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts, the optical absorption in the visible-light region increases stronger than the Ag2O/NaTaO3 photocatalysts. Thus, AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts have a broader spectral response and enhances the utilized efficiency of visible light. PL emission measurement is used to investigate the capability of the photogenerated electron-hole pairs separation in the photocatalysts because the decrease in the recombination rate indicates a low PL intensity [19]. As shown in Fig. 4B, from the PL spectra of pure NaTaO3, Ag2O/ NaTaO3 and AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts excited at 362 nm, it can be seen that the PL intensity around 470 nm strongly decreases with the Ag2O/NaTaO3 p-n heterojunctions formed. In the ternary AgCl/Ag2O/NaTaO3 p-n heterojunction, the PL intensity
is weaker than the Ag2O/NaTaO3 which implying p-n heterojunctions can deeply suppress the recombination of electron-hole pairs and ternary AgCl/Ag2O/NaTaO3 p-n heterojunction can be beneficial for applications that efficient charge separation state to improve the photocatalytic performances. 3.5. Enhancement of photocatalytic activity The photocatalytic activity of pure NaTaO3, Ag2O/NaTaO3 and AgCl/ Ag2O/NaTaO3 p-n heterojunction photocatalysts were evaluated by photodegrading the RhB aqueous solution under visible light irradiation. As one can see in Fig. 5, the photocatalysis of RhB with the presence of pure NaTaO3 photocatalysts under visible light irradiation can be negligible. About 50% of RhB can be degraded by H-0 under visible light irradiation. However, RhB can be completely decomposed within 60 min with the presence of AgCl/Ag2O/NaTaO3 (H-3) p-n heterojunction photocatalyst under visible light irradiation. While further increasing the AgCl, the photocatalytic activity of the AgCl/Ag2O/NaTaO3 photocatalysts decreases, and about 83% and 77% of RhB can be degraded for H-4 and H-5 photocatalyst, respectively. The order of photocatalytic activity for AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts can be summarized: H-3 N H-2 N H-4 N H-5 N H-1 N H-0 N pure NaTaO3. From which, the formation of AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts is beneficial to improve the photocatalytic activity of pure NaTaO3 photocatalyst, and it reaches maximum value of photocatalytic activity at the molar ratio of Ag2O, AgCl and pure NaTaO3 presented in H-3. The reusability of the novel AgCl/Ag2O/ NaTaO3 p-n heterojunction photocatalysts is very important for the real application so that the photocatalytic degradation experiments of
Fig. 4. UV–Vis absorption (A) and PL spectra (B) of pure NaTaO3, Ag2O/NaTaO3 and AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts.
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21522603 and 21576121), the Chinese-German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), the Natural Science Foundation of Jiangsu Province, China (BK20151348), the Jiangsu Postdoctoral Sustentation Fund, China (2015M571674), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).
Appendix A. Supplementary data Fig. 6. p-n Heterojunction formation mechanism of AgCl/Ag2O/NaTaO3 photocatalysts.
RhB were repeated five times. As shown in Fig. S4, after five-cycle degradation tests, about 73% of RhB can be degraded for H-3 photocatalysts. Hence, these results strongly indicate that the photocatalysts are good for reusability under visible light irradiation. As shown in Fig. 6, a schematic illustration for p-n heterojunction formation mechanism of AgCl/Ag2O/NaTaO3 photocatalysts is proposed. The band gaps of the AgCl, Ag2O and NaTaO3 in the standard literature energy levels are shown in Fig. 6A. When the p-n heterojunctions formed between AgCl, Ag2O and NaTaO3, the Fermi level of Ag2O is moved up until an equilibrium state with the Fermi level of AgCl and NaTaO3 as shown in Fig. 6B. The Ag2O can be photoexcited by visible light to produce electrons and holes. Then the electrons transfer to the conduction band (CB) of NaTaO3 and then be trapped by O2 to form superoxide ions (•O–2) and other reactive oxygen species [20–22]. The photogenerated holes of Ag2O transfer to the AgCl surface to oxidate the Cl− to Cl0 atoms [23–25]. The Cl0 atoms were reactive radical species to oxidize RhB molecules and hence reduced to Cl− again so that the AgCl/Ag2O/NaTaO3 system remains stable [26–27]. The above results reveal that the AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts enhance the photocatalytic activity than pure NaTaO3 and H-3 has the highest photocatalytic activity. 4. Conclusions A series of AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts are successfully synthesized with pure NaTaO3 nanocubes. The photocatalytic activity of the obtained AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts is evaluated by degrading RhB under visible light irradiation, and the H-3 shows the highest photocatalytic activity. Furthermore, the photocatalytic mechanism has been proposed based on the UV–vis diffuse reflectance spectra, photoluminescent spectra and relative band position of these three semiconductors. The work would provide the higher effective photocatalysts that have potential applications in future. Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (21477050, 21301076, 21303074, 21401082,
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.06.023.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Fabrication of ternary p-n heterostructures AgCl/Ag2O/NaTaO3 photocatalysts: Enhanced charge separation and photocatalytic properties under visible light irradiation Dongbo Xu a,b, Weidong Shi a,b,⁎, Songbo Yang b, Biyi Chen b, Hongye Bai b, Lisong Xiao b a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
a r t i c l e
i n f o
Article history: Received 20 March 2016 Received in revised form 9 June 2016 Accepted 26 June 2016 Available online 28 June 2016 Keywords: Ternary p-n Heterostructures AgCl/Ag2O/NaTaO3 Visible light Enhanced charge separation Photocatalytic properties
a b s t r a c t Ternary p-n heterostructures photocatalysts of AgCl/Ag2O/NaTaO3 were synthesized via a simple method and the crystal structure characterized by X-ray diffraction (XRD). The morphology of the photocatalysts were characterized by scanning electron microscopy and transmission electron microscopy. The composition of the photocatalysts was studied by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscope. The photocatalytic activity of the AgCl/Ag2O/NaTaO3 photocatalysts was evaluated using the degradation of Rhodamine B. The AgCl/Ag2O/NaTaO3 photocatalysts showed higher visible light activity than the pure NaTaO3 and Ag2O/NaTaO3 photocatalysts. Additionally, the photocatalytic mechanism was that the rapidly separation of photoinduced electrons and holes resulted the enhancement of photocatalytic activity. © 2016 Published by Elsevier B.V.
1. Introduction Semiconductor photocatalytic materials have attracted much attention for environmental protection because of the shortage of clean water [1–3]. Up to now, many tantalates photocatalysts have been developed for the photodegradation of organic pollutants, such as alkali tantalates LiTaO3, NaTaO3, and KTaO3, alkaline earth tantalates SrTa2O6, CaTa2O6, BaTa2O6 [4–8]. However, they are only photoexcited by ultraviolet light which restricted their extensive application [9]. In recent years, Ag2O as a n-type semiconductor has been reported to be a good sensitizer by compositing with p-type semiconductor photocatalysts because fabricating p-n heterjunctions have the faster separation of the photogenerated electron-hole pairs in photocatalysts [10]. For example, there are many reports about Ag2O compositing with TiO2 and Ag2O-TiO2 composite photocatalysts presented enhanced photocatalytic activity under both UV and visible light irradiation [11– 14]. AgCl is a kind of photosensitive material so that extensively used
⁎ Corresponding author at: School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China. E-mail address: [email protected] (W. Shi).
http://dx.doi.org/10.1016/j.catcom.2016.06.023 1566-7367/© 2016 Published by Elsevier B.V.
in photocatalytic area. However, as like tantalates, it is only photoexcited by ultraviolet light [15]. Therefore, here we develop a three-component p-n heterjunctions photocatalysts of AgCl/Ag2O/NaTaO3 system to improve the photocatalytic activity of NaTaO3 and AgCl via a simple synthesis process. The photocatalytic activities of the three-component AgCl/Ag2O/NaTaO3 pn heterojunction photocatalysts are evaluated by degrading Rhodamine B (RhB) under visible-light irradiation. Furthermore, the photocatalytic enhancement mechanism of AgCl/Ag2O/NaTaO3 photocatalysts has been proposed based on the relative band gap position and the integrated p-n heterostructure of these semiconductors. 2. Experimental 2.1. Synthesis of Ag2O/NaTaO3 heterogeneous All Chemical reagents were of analytic grade purity and directly used without further purification. NaTaO3 nanocubes were synthesized by the reported hydrothermal method [16]. In a typical process, 0.3 g of synthesized NaTaO3 nanocubes were added to 150 mL of AgNO3 (1.07 g) aqueous solution and stirred for 10 min at room temperature. Then excessive amounts of NaOH aqueous solution was dropwise
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added into the mixed solution under magnetic stirring. The final Ag2O/ NaTaO3 powders were filtered and washed with deionized water until the pH was 7, and then dried at 60 °C for 24 h. 2.2. Synthesis of AgCl/Ag2O/NaTaO3 heterogeneous 0.1 g Ag2O/NaTaO3 and 180 mL deionized water were mixed under stirring in an open beaker. Then 0.1 M of HCl aqueous solution was dropwise added into the mixed solution under magnetic stirring. By controlling the amount of HCl aqueous solution to be 0, 2, 2.5, 3, 3.5, and 4 mL, the obtained AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts were labeled as H-0 to H-5, respectively. The final photocatalysts were filtered and washed with deionized water until the pH was 7, and then dried at 60 °C for 24 h.
3. Results and discussion 3.1. XRD patterns analysis Fig. 1 shows the XRD patterns of the obtained photocatalysts of AgCl/ Ag2O/NaTaO3 and the pure NaTaO3 nanocubes. It indicates that the diffraction peaks of pure NaTaO3 nanocubes can be assigned to the JCPDS card No. 25-0863. According to the H-0, characteristic peaks of Ag2O are observable which are marked with “*” (JCPDS No. 41-1104). However, when the HCl aqueous solution is added, the peaks marked with “#” are assigned to the phases of AgCl (JCPDS No. 31-1238). As shown in Fig. S1 (B, C, D), when the HCl aqueous solution sequentially added, the peaks of Ag2O are decreased and the peaks of AgCl are strength. No peaks of impurities can be found in all the as-prepared photocatalysts. 3.2. XPS patterns analysis
2.3. Characterization The crystallographic structures of AgCl/Ag2O/NaTaO3 photocatalysts were investigated by X-ray diffraction (XRD, Bruker D8, Cu-Kα). Morphology and composition of photocatalysts was characterized by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) with a JEM-2100F electron microscope (JEOL, Japan) as well as the scanning electron microscopy (SEM; Hitachi, S-4800) equipped with an energy-dispersive X-ray spectroscope (EDS), respectively. X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) was used to probe the composition of the obtained photocatalysts. UV − vis absorption spectra were recorded on a UV-3100 (Shimadzu, Japan) spectrophotometer with BaSO4 as the reference standard. Solid-state photoluminescent (PL) spectra were recorded on a Photon Technology International Model Quantamaster-QM4m spectrofluorimeter equipped with a 75 W lamp and dual excitation monochromators. 2.4. Photocatalytic activity test RhB was used as model pollutant to evaluate the photocatalytic activity of the AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts. Photodegradation studies were carried out under a 250 W Xe lamp with a UV-cutoff filter (λ N 425 nm). AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts (50 mg) were added into 100 mL of RhB aqueous solution (10 mg/L). The suspensions were kept in the dark for 30 min with magnetically stirring to reach an adsorption-desorption equilibrium. At a given time interval, 5 mL of solution was taken and immediately centrifuged. Then UV−vis spectra of the centrifuged solution were recorded using a UV-3100 spectrophotometer.
Fig. 1. XRD patterns of as-prepared photocatalysts.
The composition and chemical states of the obtained photocatalysts were confirmed by XPS. Fig. 2 shows the XPS spectra of AgCl/Ag2O/ NaTaO3 heterojunction (H-3). The peaks of Ag, Ta, Cl and O can be detected in Fig. 2. The high-resolution XPS spectra of Ta 4f, Ag 3d and Cl 2p for the obtained photocatalysts are shown in Fig. S2 (A–C). The binding energies of Ag 3d3 and Ag 3d5 are 373.9 and 368.3 eV, indicating the existence of Ag+ ions in Ag2O [17–18]. The peak of Cl 2p located at 198.7 eV is ascribed to Cl in AgCl. The peak of Ta 4f located at 24.9 and 26.3 eV are ascribed to Ta in NaTaO3. The result of XPS indicates the presence of AgCl, Ag2O and NaTaO3, implying the formation of AgCl/ Ag2O/NaTaO3 heterojunction. 3.3. Morphology of AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts Fig. S3 (A-B) show the SEM images of pure NaTaO3 and the H-3 AgCl/ Ag2O/NaTaO3 photocatalysts, respectively. They are demonstrated clearly that the pure NaTaO3 is nanocubes and a uniform rough layer covered the smooth surface of NaTaO3 nanocubes. Fig. S3C shows the TEM image of AgCl/Ag2O/NaTaO3 photocatalysts, which demonstrated that there is a rough layer of Ag2O on the surface of NaTaO3. Furthermore, there are some AgCl nanoparticles on the surface of Ag2O. The above results further indicate the formation of AgCl/Ag2O/NaTaO3 ternary p-n heterojunctions. Fig. S3D displays the EDS spectroscopy of AgCl/Ag2O/NaTaO3 photocatalysts. From which, one can see that there are Ag, Cl, Ta and O elements. The HRTEM image of H-3 (Fig. 3) shows the clear lattice fringes of the Ag2O and AgCl on the surface of NaTaO3
Fig. 2. XPS spectra of AgCl/Ag2O/NaTaO3 heterojunction photocatalysts. (B-D) highresolution XPS spectra of Ag 3d, Cl 2P and Ta 4f binding energy, respectively.
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Fig. 5. Photocatalytic activities of pure NaTaO3 and AgCl/Ag2O/NaTaO3 (H-0 to H-5) p-n heterojunction photocatalysts for the degradation of RhB under visible light. Fig. 3. HRTEM image of AgCl/Ag2O/NaTaO3 photocatalysts.
nanocubes. The interplanar spacing of 0.16 nm are consistent with the (2 2 2) crystal face of AgCl and the interplanar spacing of 0.27 nm is consistent with the (1 1 1) crystal face of Ag2O, respectively. The result of HRTEM is in good agreement with the XRD results so that it is clearly the AgCl/Ag2O/NaTaO3 ternary p-n heterojunction photocatalysts have been synthesized successfully. 3.4. Optical property of obtained photocatalysts UV − vis absorption spectra of pure NaTaO3 nanocubes, Ag2O/ NaTaO3 and AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts are shown in Fig. 4A. There is no significant absorbance in visible light region can be seen of NaTaO3 because of its wide band gap. After pAg2O/n-NaTaO3 heterojunction photocatalysts being formed, it exhibits a wide and strong light absorption range of 350–700 nm. As for the obtained AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts, the optical absorption in the visible-light region increases stronger than the Ag2O/NaTaO3 photocatalysts. Thus, AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts have a broader spectral response and enhances the utilized efficiency of visible light. PL emission measurement is used to investigate the capability of the photogenerated electron-hole pairs separation in the photocatalysts because the decrease in the recombination rate indicates a low PL intensity [19]. As shown in Fig. 4B, from the PL spectra of pure NaTaO3, Ag2O/ NaTaO3 and AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts excited at 362 nm, it can be seen that the PL intensity around 470 nm strongly decreases with the Ag2O/NaTaO3 p-n heterojunctions formed. In the ternary AgCl/Ag2O/NaTaO3 p-n heterojunction, the PL intensity
is weaker than the Ag2O/NaTaO3 which implying p-n heterojunctions can deeply suppress the recombination of electron-hole pairs and ternary AgCl/Ag2O/NaTaO3 p-n heterojunction can be beneficial for applications that efficient charge separation state to improve the photocatalytic performances. 3.5. Enhancement of photocatalytic activity The photocatalytic activity of pure NaTaO3, Ag2O/NaTaO3 and AgCl/ Ag2O/NaTaO3 p-n heterojunction photocatalysts were evaluated by photodegrading the RhB aqueous solution under visible light irradiation. As one can see in Fig. 5, the photocatalysis of RhB with the presence of pure NaTaO3 photocatalysts under visible light irradiation can be negligible. About 50% of RhB can be degraded by H-0 under visible light irradiation. However, RhB can be completely decomposed within 60 min with the presence of AgCl/Ag2O/NaTaO3 (H-3) p-n heterojunction photocatalyst under visible light irradiation. While further increasing the AgCl, the photocatalytic activity of the AgCl/Ag2O/NaTaO3 photocatalysts decreases, and about 83% and 77% of RhB can be degraded for H-4 and H-5 photocatalyst, respectively. The order of photocatalytic activity for AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts can be summarized: H-3 N H-2 N H-4 N H-5 N H-1 N H-0 N pure NaTaO3. From which, the formation of AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts is beneficial to improve the photocatalytic activity of pure NaTaO3 photocatalyst, and it reaches maximum value of photocatalytic activity at the molar ratio of Ag2O, AgCl and pure NaTaO3 presented in H-3. The reusability of the novel AgCl/Ag2O/ NaTaO3 p-n heterojunction photocatalysts is very important for the real application so that the photocatalytic degradation experiments of
Fig. 4. UV–Vis absorption (A) and PL spectra (B) of pure NaTaO3, Ag2O/NaTaO3 and AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts.
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21522603 and 21576121), the Chinese-German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), the Natural Science Foundation of Jiangsu Province, China (BK20151348), the Jiangsu Postdoctoral Sustentation Fund, China (2015M571674), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).
Appendix A. Supplementary data Fig. 6. p-n Heterojunction formation mechanism of AgCl/Ag2O/NaTaO3 photocatalysts.
RhB were repeated five times. As shown in Fig. S4, after five-cycle degradation tests, about 73% of RhB can be degraded for H-3 photocatalysts. Hence, these results strongly indicate that the photocatalysts are good for reusability under visible light irradiation. As shown in Fig. 6, a schematic illustration for p-n heterojunction formation mechanism of AgCl/Ag2O/NaTaO3 photocatalysts is proposed. The band gaps of the AgCl, Ag2O and NaTaO3 in the standard literature energy levels are shown in Fig. 6A. When the p-n heterojunctions formed between AgCl, Ag2O and NaTaO3, the Fermi level of Ag2O is moved up until an equilibrium state with the Fermi level of AgCl and NaTaO3 as shown in Fig. 6B. The Ag2O can be photoexcited by visible light to produce electrons and holes. Then the electrons transfer to the conduction band (CB) of NaTaO3 and then be trapped by O2 to form superoxide ions (•O–2) and other reactive oxygen species [20–22]. The photogenerated holes of Ag2O transfer to the AgCl surface to oxidate the Cl− to Cl0 atoms [23–25]. The Cl0 atoms were reactive radical species to oxidize RhB molecules and hence reduced to Cl− again so that the AgCl/Ag2O/NaTaO3 system remains stable [26–27]. The above results reveal that the AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts enhance the photocatalytic activity than pure NaTaO3 and H-3 has the highest photocatalytic activity. 4. Conclusions A series of AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts are successfully synthesized with pure NaTaO3 nanocubes. The photocatalytic activity of the obtained AgCl/Ag2O/NaTaO3 p-n heterojunction photocatalysts is evaluated by degrading RhB under visible light irradiation, and the H-3 shows the highest photocatalytic activity. Furthermore, the photocatalytic mechanism has been proposed based on the UV–vis diffuse reflectance spectra, photoluminescent spectra and relative band position of these three semiconductors. The work would provide the higher effective photocatalysts that have potential applications in future. Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (21477050, 21301076, 21303074, 21401082,
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.06.023.
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