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AgBrO3 photocatalyst with high photocatalytic activity
Materials Chemistry and Physics xxx (2016) 1e6
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity Limin Song a, *, Tongtong Li a, Shujuan Zhang b, ** a College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin, 300387, PR China b College of Science, Tianjin University of Science & Technology, Tianjin, 300457, PR China
h i g h l i g h t s Ag/AgBrO3 with higher photodegradation ability was synthesized. OH and O 2 radicals were the main active species in the oxidation of RhB. The possible reaction mechanism was discussed in details.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2015 Received in revised form 5 June 2016 Accepted 9 July 2016 Available online xxx
A new Ag/AgBrO3 photocatalyst was prepared by mixing aqueous solutions of AgNO3 and NaBrO3. The catalyst’s structure and performance were investigated with X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The UVevis absorption spectrum of Ag/ AgBrO3 exhibits a band gap of 3.97 eV. The results show that the Ag/AgBrO3 semiconductor can be excited by ultravioletevisible light. The photodegradation of Rhodamine B displayed much higher photocatalytic activity than that of N-doped TiO2 under the same experimental conditions. Moreover, OH and O 2 generated in the photocatalysis played a key role of the photodegradation of Rhodamine B. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ag/AgBrO3 semiconductor Photodegradation Rhodamine B OH radicals O2 radicals
1. Introduction Environmental pollution from industrial wastewater is getting worse along with the development of industrial societies. Photocatalytic technology is an effective method for treatment of organic wastewater [1]. Therefore, the development of new photocatalytic materials has attracted the attention of many researchers [2,3]. Many types of semiconductor photocatalysts such as metal oxides [4e6], metal sulfides [7], organic polymers [8,9], nonmetallic oxysalts [10e12], composite microspheres [13,14] and dopant candidates [15] have been successfully prepared using a variety of methods. Nonmetallic oxysalts are novel photocatalytic materials with high photocatalytic capabilities. As reported, Ag3PO4 [16],
BiPO4 [10], BiOI [17] and AgIO4 all exhibit high photocatalytic activity [18]. It was found that nonmetallic oxysalts are excellent candidates for photocatalytic materials. To date, the use of Ag/ AgBrO3 as a semiconductor photocatalytic material has not been reported. In this study, Ag/AgBrO3 particles were synthesized successfully and their ability to affect the photodegradation of Rhodamine B (RhB) under visible-light radiation was investigated. The results show that the prepared Ag/AgBrO3 is an effective photocatalyst for the aqueous dye solution of RhB. The photocatalytic process and mechanism are discussed in detail. The study of Ag/AgBrO3’s properties may result in the creation of a new photocatalytic material. 2. Experiment
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (L. Song).
2.1. Synthesis of Ag/AgBrO3 photocatalyst Ag/AgBrO3 particles were synthesized based on a precipitation
http://dx.doi.org/10.1016/j.matchemphys.2016.07.012 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Song, et al., Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.07.012
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Fig. 2. TEM image and SAED pattern of Ag/AgBrO3.
Fig. 1. X-ray diffraction patterns of Ag/AgBrO3.
process. AgNO3 (1.5 g) and NaBrO3 (1.47 g) were each dissolved in 30 mL of distilled water under vigorous stirring. The resulting transparent solutions were then mixed under vigorous stirring. The product was then washed with deionized water and absolute ethanol, and air-dried at 60 C. N-doped TiO2 (N-TiO2) was prepared by a sol-gel method. C16H36O4Ti (15 mL) was dissolved in 40 mL of ethanol under vigorous stirring. Then 10 mL of CH3COOH and 5 mL of deionized water were dissolved in 40 mL of ethanol under vigorous stirring, and then added dropwise to the C16H36O4Ti solution. Then, 5 mL of NH3$H2O (35 wt%) was added to the above mixture. The solution
was kept at room temperature for 30 min. Subsequently, a sol was produced and dried at 65 C for 12 h to remove water and alcohol. Finally, the prepared xerogel was calcined at 500 C for 3 h in a muffle furnace.
2.2. Characterization of Ag/AgBrO3 photocatalyst The crystalline structure of samples was investigated by X-ray diffraction (XRD, Rigaku D/max 2500, l ¼ 1.5406 Å, 40 kV, 40 mA). The morphology of samples was evaluated using a high-resolution transmission electron microscope (HRTEM, JEOL JEM 2100). Surface properties were investigated with X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer PHI5300). The calibration of BE was the
Fig. 3. UVevis absorption and XPS spectra of the as-synthesized Ag/AgBrO3.
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spectra of samples were run on a Cary Eclipse PL analyzer. Electron spin resonance (ESR) spectra were recorded by an electron paramagnetic resonance spectrometer (JES FA200). 2.3. Photocatalytic activity evaluation The photodegradation of RhB was examined in a homemade evacuation system. Typically, a 0.3 g sample was immersed in 100 mL of 10 mg/L aqueous RhB solution under light radiation. The light source was a 300 W Xe lamp. The concentrations of RhB in the solutions after photoirradiation were measured from the peak absorbance at 554 nm. 3. Results and discussion 3.1. Characterization of Ag/AgBrO3
Fig. 4. (A) The dependence of photocatalytic activity on time over Ag/AgBrO3 and NTiO2. (B) The relationship between ln(C/C0) and irradiation time. (C) The dependence of photocatalytic activity on the concentration of Ag/AgBrO3.
standard peak of adventitious carbon (C1s). Optical properties were recorded using an HP8453 spectrophotometer (with BaSO4 as a reference) and a Cary Eclipse photoluminescence analyzer. The total organic carbon (TOC) content of residual solutions was analyzed using a Shimadzu-Toc-Vcph. The photoluminescence (PL)
The XRD patterns for as-made Ag/AgBrO3 are displayed in Fig. 1. Clearly, the diffraction peaks can be assigned to the (200), (211), (220), (202), (301), 103), (222), (321), (213), (400), (004), (420), (402), (323), (422), and (224) lattice planes (PDF: 06-0385, I4/m[87], a ¼ b ¼ 8.609 Å, c ¼ 8.092 Å). There is no impurity peak in Fig. 1. The result suggests that the prepared sample is a pure and tetragonal phase of Ag/AgBrO3. Compared with the XRD patterns in the PDF, all peaks of Ag/AgBrO3 shift rightwards slightly in Fig. 1. The difference arises from lattice distortion because the Ag/AgBrO3 was prepared by another route. In addition, there are no characteristic diffraction peaks of Ag because Ag0 species on the surface of AgBrO3 is too little. The mean crystallite size was calculated to be about 81 nm using the Scherrer equation based on the (202) peak in Fig. 1. TEM images of Ag/AgBrO3 (Fig. 2) show that a typical Ag/AgBrO3 sample is composed of fairly large spherical particles. The size range is around 5.5e8.5 mm. The particle size is greater than that obtained by XRD, which is mainly attributed to the co-precipitation method. The enlarged TEM images with higher accelerating voltage were not made, because Ag/AgBrO3 is sensitive to the electron beam. The selected area electron diffraction (SAED) patterns of the prepared Ag/AgBrO3 are shown in Fig. 2. The multiple latent diffuse rings show that the Ag/AgBrO3 particles are an intrinsically polycrystalline phase, which is consistent with the XRD patterns in Fig. 1. The corresponding diffuse ring patterns in Fig. 2 can be assigned to the (220), (400), (422), and (602) lattice planes of Ag/ AgBrO3. The UVevis absorption spectrum of Ag/AgBrO3 is shown in Fig. 3a. The strong absorption peak at 281 nm can be mainly attributed to the d-d transition AgBrO3. The absorption edge was found to be 331 nm, suggesting that the AgBrO3 sample can only be excited by light with a wavelength of less than 331 nm. The band gap energy of AgBrO3 is estimated as 3.97 eV according to Fig. 3b. There is slight visible light absorption for Ag/AgBrO3 within 400e800 nm if we observed carefully, which can attributed to surface plasmon resonance (SPR) absorptions of Ag0 species on the surface of AgBrO3. The SPR may improve the photocatalytic activity of AgBrO3 under visible light irradiation. The valence band level of Ag/AgBrO3 is measured as 1.97 by XPS in Fig. 3c. The XPS spectrum in the Ag 3d region of Ag/AgBrO3 is shown in Fig. 3d. The binding energy at 368.38 and 374.38 eV for Ag/AgBrO3 can be assigned to Ag 3d5/2 and Ag 3d3/2 [19], respectively. These values are higher than those for AgBr given in a previous study [20]. The higher binding energy indicates that Ag/AgBrO3 is more stable than AgBr. The XPSmeasured surface atom ratio of Ag is around 20.33 at%, a higher atomic concentration than that of Br. The valence band and conduction band levels for Ag/AgBrO3 depend mainly on valence electron orbits of Ag, so the high surface content of Ag is conducive to the photocatalytic activity of Ag/AgBrO3.
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Fig. 5. The PL signal peaks of $OH radicals at 425 nm in the 5e20 mg/L RhB solutions in the presence of Ag/AgBrO3.
3.2. Photocatalytic activity To confirm the photocatalytic ability of Ag/AgBrO3, the photodegradation of 10 mg/L aqueous RhB solutions was studied under ultraviolet light irradiation (Fig. 4a). Results show that the surface adsorption of RhB by Ag/AgBrO3 is negligible. The photolysis ratio is less than 15% in the absorbance of RhB solutions irradiated without Ag/AgBrO3 in Fig. 4a, which does not severely affect the photodegradation of Ag/AgBrO3. In the case of Ag/AgBrO3, the RhB solutions degraded almost entirely and the photodegradation rate reached 96.6% within 90 min. Compared with N-TiO2, the photodegradation ratio for RhB was only 58% under the same experimental conditions, which is lower than that of the prepared Ag/ AgBrO3. The results show that the obtained Ag/AgBrO3 exhibits high photocatalytic ability. A pseudo-first-order model was employed to investigate the kinetics of RhB degradation over Ag/ AgBrO3 and N-TiO2. The photocatalytic degradation rate constants over Ag/AgBrO3 and N-TiO2 are 0.0411 and 0.0085 min1, respectively (Fig. 4b). The rate constant for Ag/AgBrO3 is 4.83 times higher than that of N-TiO2. The effect of the original RhB concentration on photocatalytic performance is shown in Fig. 4c. Clearly, photocatalytic activity is related to the original concentration of RhB. The increase of RhB concentration from 5 to 20 mg/L results in an obvious change of photocatalytic performance. In the 5 mg/L RhB solution, the RhB photodegradation rate reached 97.6% after 45 min. In the 10, 15, and 20 mg/L RhB solutions, the RhB photodegradation rates decreased gradually from 96.6% to 86.5% after 90 min of visible light irradiation. Moreover, the synthesized Ag/ AgBrO3 was still highly photocatalytic when treated with high concentration of RhB solutions. In order to confirm the reason for the above result, $OH level in the photocatalytic reaction was measured using a PL method [21]. The $OH measurements in 5, 10,
15, and 20 mg/L RhB aqueous solutions are shown in Fig. 5aed. The PL intensity of OH at 425 nm decreased with increased RhB concentration. The results indicate that a high concentration of RhB is unconducive to the production of OH. More RhB molecules were adsorbed on the Ag/AgBrO3 surface with increased RhB concentration, thus lowering the number of active sites that formed OH. Meanwhile, more RhB molecules reduced the number of photons that reached the Ag/AgBrO3 surface by increasing light absorption. Therefore, RhB solutions of higher concentrations can significantly reduce the formation of OH in the photocatalysis. Then a recycling experiment was carried out to investigate the stability of Ag/
Fig. 6. Recycling tests of degradation of 10 mg/L RhB over Ag/AgBrO3.
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Fig. 8. ESR spectra of Ag/AgBrO3 under light irradiation. (A) DMPOOH in aqueous solution. (B) DMPOO 2 in DMSO solution. Fig. 7. (A) The UVevis absorption spectra of solution in the process of the photocatalytic degradation of 10 mg/L RhB on Ag/AgBrO3. (B) TOC evolution versus irradiation time in the photocatalytic reaction.
AgBrO3 in the photocatalytic process. The results (Fig. 6) show that the RhB photodegradation rates did not change obviously after five recycles. A slight decline in photocatalytic activity might result from the loss of Ag/AgBrO3 by centrifugation. The results indicate that the prepared Ag/AgBrO3 is a stable photocatalytic material. 3.3. Photocatalytic process and mechanism The time-dependent UVevis spectra of RhB are showed in Fig. 7a. The characteristic peaks of RhB at l ¼ 258, 308, 355, and 554 nm are associated with the p/p* electronic transitions of benzene ring, the n/p* electronic transitions of O atoms in phenyl ring, the conjugated benzene ring, and band-band transitions, respectively. Those peaks diminished gradually, and the color of mixture gradually became transparent with prolonged time in the presence of Ag/AgBrO3 under UV light radiation, which indicated that molecular structure of RhB was decomposed gradually (Fig. 7a). The absorption peak disappeared completely after 90 min of irradiation, suggesting that Ag/AgBrO3 had high photocatalytic performance. To further investigate the photodegradation of RhB solutions, the time-dependent TOC contents of the RhB solutions are displayed in Fig. 7b. TOCs after 30, 60, and 90 min of irradiation are 3.94, 3.04, and 2.97 mg/L, respectively, which are all lower than
that of the original RhB solution (4.36 mg/L). Moreover, the TOC rate of the RhB solutions was lower than the photodegradation rate. The results show the RhB solution was partially decomposed to small molecules, and also partially converted into intermediate products. As is well-known, OH has a very strong oxidizing ability for the partial or complete decomposition of organic chemicals. As showed in Fig. 5, much OH was produced in the photodegradation of RhB, which suggests that OH is the main active species, further confirming that OH mainly oxidized RhB molecules. The electron spin resonance (ESR) technique was then applied to detect the active species over Ag/AgBrO3 under light irradiation [22]. Fig. 8 shows the ESR spectra obtained from solutions containing DMPO and Ag/AgBrO3 in dark and during irradiation of 2, 4 and 6 min Fig. 8A shows that there were no apparent ESR signals over Ag/ AgBrO3 without irradiation. Nevertheless, after the light irradiation, a four-line spectrum with relative intensities of 1:2:2:1 were observed, indicating the hydroxyl radicals over Ag/AgBrO3 during irradiation. Furthermore, the OH signal intensities of Ag/AgBrO3 are obviously stronger within rising time, suggesting that more and more OH radicals generated on the surface of Ag/AgBrO3. This result was corresponding to the conclusion in Fig. 5. In addition, the characteristic peaks corresponding to the DMPOO 2 adducts were observed in Fig. 8B, indicating that O 2 radicals are also produced on Ag/AgBrO3. This result implied that O 2 was also a main active species during the photocatalysis process. The possible reaction mechanism is postulated as follows according to reference [23]:
Please cite this article in press as: L. Song, et al., Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.07.012
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When the Ag/AgBrO3 photocatalyst is irradiated by light, the Ag nanoparticles (NPs) owing to the SPR effect can form the photogenerated electronhole pairs. And the electrons formed on the Ag/AgBrO3 photocatalyst will transfer to the conductor band (CB) of AgBrO3. Then the holes left on the Ag/AgBrO3 photocatalyst can combine with OH in water and oxidize it into OH [24]. On the other hand, the photoexcited electrons transferred to the CB of AgBrO3 are captured by dissolved O2 molecules in solutions to produce O 2 radicals [25]. The OH radicals and O2 radicals are strong oxidants which can induce the degradation of RhB to generate intermediate products. Ag NPs þ hv / hþ þ e Ag NPs (e) þ AgBrO3 / AgBrO3 (e) þ Ag NPs O2 þ e / O 2 OH þ hþ / OH OH (or O 2 ) þ RhB / products
4. Conclusions In this work, a new Ag/AgBrO3 photocatalytic material was synthesized by an ion exchange process. The band gap energy of Ag/AgBrO3 was calculated to be around 3.97 eV. Moreover, Ag/ AgBrO3 can be excited by UV light, and oxidize RhB molecules. The photocatalytic ability of Ag/AgBrO3 is higher than that of N-TiO2 under the same conditions. This work can provide a lead for future research into novel photocatalytic materials. Acknowledgements This work was supported by Natural Science Foundation of Tianjin of China (Grant 14JCYBJC20500) and Graduate Program of Science and Technology Innovation of Tianjin Polytechnic University (16114). References [1] A.J. Bard, Photoelectrochemistry and heterogeneous photocatalysis at semiconductors, J. Photochem. 10 (1979) 59e75. [2] C.C. Chen, J.C. Zhao, W.H. Ma, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (2010) 4206e4219. [3] G.C. Xi, J.H. Ye, Q. Ma, N. Su, H. Bai, C. Wang, In situ growth of metal particles on 3D urchin-like WO3 nanostructures, J. Am. Chem. Soc. 134 (2012) 6508e6511. [4] L.Y. Yang, S.Y. Dong, J.H. Sun, J.L. Feng, Q.H. Wu, S.P. Sun, Microwave-assisted preparation, characterization and photocatalytic properties of a dumbbellshaped ZnO photocatalyst, J. Hazard. Mater. 179 (2010) 438e443.
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Please cite this article in press as: L. Song, et al., Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.07.012
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity Limin Song a, *, Tongtong Li a, Shujuan Zhang b, ** a College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin, 300387, PR China b College of Science, Tianjin University of Science & Technology, Tianjin, 300457, PR China
h i g h l i g h t s Ag/AgBrO3 with higher photodegradation ability was synthesized. OH and O 2 radicals were the main active species in the oxidation of RhB. The possible reaction mechanism was discussed in details.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2015 Received in revised form 5 June 2016 Accepted 9 July 2016 Available online xxx
A new Ag/AgBrO3 photocatalyst was prepared by mixing aqueous solutions of AgNO3 and NaBrO3. The catalyst’s structure and performance were investigated with X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The UVevis absorption spectrum of Ag/ AgBrO3 exhibits a band gap of 3.97 eV. The results show that the Ag/AgBrO3 semiconductor can be excited by ultravioletevisible light. The photodegradation of Rhodamine B displayed much higher photocatalytic activity than that of N-doped TiO2 under the same experimental conditions. Moreover, OH and O 2 generated in the photocatalysis played a key role of the photodegradation of Rhodamine B. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ag/AgBrO3 semiconductor Photodegradation Rhodamine B OH radicals O2 radicals
1. Introduction Environmental pollution from industrial wastewater is getting worse along with the development of industrial societies. Photocatalytic technology is an effective method for treatment of organic wastewater [1]. Therefore, the development of new photocatalytic materials has attracted the attention of many researchers [2,3]. Many types of semiconductor photocatalysts such as metal oxides [4e6], metal sulfides [7], organic polymers [8,9], nonmetallic oxysalts [10e12], composite microspheres [13,14] and dopant candidates [15] have been successfully prepared using a variety of methods. Nonmetallic oxysalts are novel photocatalytic materials with high photocatalytic capabilities. As reported, Ag3PO4 [16],
BiPO4 [10], BiOI [17] and AgIO4 all exhibit high photocatalytic activity [18]. It was found that nonmetallic oxysalts are excellent candidates for photocatalytic materials. To date, the use of Ag/ AgBrO3 as a semiconductor photocatalytic material has not been reported. In this study, Ag/AgBrO3 particles were synthesized successfully and their ability to affect the photodegradation of Rhodamine B (RhB) under visible-light radiation was investigated. The results show that the prepared Ag/AgBrO3 is an effective photocatalyst for the aqueous dye solution of RhB. The photocatalytic process and mechanism are discussed in detail. The study of Ag/AgBrO3’s properties may result in the creation of a new photocatalytic material. 2. Experiment
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (L. Song).
2.1. Synthesis of Ag/AgBrO3 photocatalyst Ag/AgBrO3 particles were synthesized based on a precipitation
http://dx.doi.org/10.1016/j.matchemphys.2016.07.012 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Song, et al., Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.07.012
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Fig. 2. TEM image and SAED pattern of Ag/AgBrO3.
Fig. 1. X-ray diffraction patterns of Ag/AgBrO3.
process. AgNO3 (1.5 g) and NaBrO3 (1.47 g) were each dissolved in 30 mL of distilled water under vigorous stirring. The resulting transparent solutions were then mixed under vigorous stirring. The product was then washed with deionized water and absolute ethanol, and air-dried at 60 C. N-doped TiO2 (N-TiO2) was prepared by a sol-gel method. C16H36O4Ti (15 mL) was dissolved in 40 mL of ethanol under vigorous stirring. Then 10 mL of CH3COOH and 5 mL of deionized water were dissolved in 40 mL of ethanol under vigorous stirring, and then added dropwise to the C16H36O4Ti solution. Then, 5 mL of NH3$H2O (35 wt%) was added to the above mixture. The solution
was kept at room temperature for 30 min. Subsequently, a sol was produced and dried at 65 C for 12 h to remove water and alcohol. Finally, the prepared xerogel was calcined at 500 C for 3 h in a muffle furnace.
2.2. Characterization of Ag/AgBrO3 photocatalyst The crystalline structure of samples was investigated by X-ray diffraction (XRD, Rigaku D/max 2500, l ¼ 1.5406 Å, 40 kV, 40 mA). The morphology of samples was evaluated using a high-resolution transmission electron microscope (HRTEM, JEOL JEM 2100). Surface properties were investigated with X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer PHI5300). The calibration of BE was the
Fig. 3. UVevis absorption and XPS spectra of the as-synthesized Ag/AgBrO3.
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spectra of samples were run on a Cary Eclipse PL analyzer. Electron spin resonance (ESR) spectra were recorded by an electron paramagnetic resonance spectrometer (JES FA200). 2.3. Photocatalytic activity evaluation The photodegradation of RhB was examined in a homemade evacuation system. Typically, a 0.3 g sample was immersed in 100 mL of 10 mg/L aqueous RhB solution under light radiation. The light source was a 300 W Xe lamp. The concentrations of RhB in the solutions after photoirradiation were measured from the peak absorbance at 554 nm. 3. Results and discussion 3.1. Characterization of Ag/AgBrO3
Fig. 4. (A) The dependence of photocatalytic activity on time over Ag/AgBrO3 and NTiO2. (B) The relationship between ln(C/C0) and irradiation time. (C) The dependence of photocatalytic activity on the concentration of Ag/AgBrO3.
standard peak of adventitious carbon (C1s). Optical properties were recorded using an HP8453 spectrophotometer (with BaSO4 as a reference) and a Cary Eclipse photoluminescence analyzer. The total organic carbon (TOC) content of residual solutions was analyzed using a Shimadzu-Toc-Vcph. The photoluminescence (PL)
The XRD patterns for as-made Ag/AgBrO3 are displayed in Fig. 1. Clearly, the diffraction peaks can be assigned to the (200), (211), (220), (202), (301), 103), (222), (321), (213), (400), (004), (420), (402), (323), (422), and (224) lattice planes (PDF: 06-0385, I4/m[87], a ¼ b ¼ 8.609 Å, c ¼ 8.092 Å). There is no impurity peak in Fig. 1. The result suggests that the prepared sample is a pure and tetragonal phase of Ag/AgBrO3. Compared with the XRD patterns in the PDF, all peaks of Ag/AgBrO3 shift rightwards slightly in Fig. 1. The difference arises from lattice distortion because the Ag/AgBrO3 was prepared by another route. In addition, there are no characteristic diffraction peaks of Ag because Ag0 species on the surface of AgBrO3 is too little. The mean crystallite size was calculated to be about 81 nm using the Scherrer equation based on the (202) peak in Fig. 1. TEM images of Ag/AgBrO3 (Fig. 2) show that a typical Ag/AgBrO3 sample is composed of fairly large spherical particles. The size range is around 5.5e8.5 mm. The particle size is greater than that obtained by XRD, which is mainly attributed to the co-precipitation method. The enlarged TEM images with higher accelerating voltage were not made, because Ag/AgBrO3 is sensitive to the electron beam. The selected area electron diffraction (SAED) patterns of the prepared Ag/AgBrO3 are shown in Fig. 2. The multiple latent diffuse rings show that the Ag/AgBrO3 particles are an intrinsically polycrystalline phase, which is consistent with the XRD patterns in Fig. 1. The corresponding diffuse ring patterns in Fig. 2 can be assigned to the (220), (400), (422), and (602) lattice planes of Ag/ AgBrO3. The UVevis absorption spectrum of Ag/AgBrO3 is shown in Fig. 3a. The strong absorption peak at 281 nm can be mainly attributed to the d-d transition AgBrO3. The absorption edge was found to be 331 nm, suggesting that the AgBrO3 sample can only be excited by light with a wavelength of less than 331 nm. The band gap energy of AgBrO3 is estimated as 3.97 eV according to Fig. 3b. There is slight visible light absorption for Ag/AgBrO3 within 400e800 nm if we observed carefully, which can attributed to surface plasmon resonance (SPR) absorptions of Ag0 species on the surface of AgBrO3. The SPR may improve the photocatalytic activity of AgBrO3 under visible light irradiation. The valence band level of Ag/AgBrO3 is measured as 1.97 by XPS in Fig. 3c. The XPS spectrum in the Ag 3d region of Ag/AgBrO3 is shown in Fig. 3d. The binding energy at 368.38 and 374.38 eV for Ag/AgBrO3 can be assigned to Ag 3d5/2 and Ag 3d3/2 [19], respectively. These values are higher than those for AgBr given in a previous study [20]. The higher binding energy indicates that Ag/AgBrO3 is more stable than AgBr. The XPSmeasured surface atom ratio of Ag is around 20.33 at%, a higher atomic concentration than that of Br. The valence band and conduction band levels for Ag/AgBrO3 depend mainly on valence electron orbits of Ag, so the high surface content of Ag is conducive to the photocatalytic activity of Ag/AgBrO3.
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Fig. 5. The PL signal peaks of $OH radicals at 425 nm in the 5e20 mg/L RhB solutions in the presence of Ag/AgBrO3.
3.2. Photocatalytic activity To confirm the photocatalytic ability of Ag/AgBrO3, the photodegradation of 10 mg/L aqueous RhB solutions was studied under ultraviolet light irradiation (Fig. 4a). Results show that the surface adsorption of RhB by Ag/AgBrO3 is negligible. The photolysis ratio is less than 15% in the absorbance of RhB solutions irradiated without Ag/AgBrO3 in Fig. 4a, which does not severely affect the photodegradation of Ag/AgBrO3. In the case of Ag/AgBrO3, the RhB solutions degraded almost entirely and the photodegradation rate reached 96.6% within 90 min. Compared with N-TiO2, the photodegradation ratio for RhB was only 58% under the same experimental conditions, which is lower than that of the prepared Ag/ AgBrO3. The results show that the obtained Ag/AgBrO3 exhibits high photocatalytic ability. A pseudo-first-order model was employed to investigate the kinetics of RhB degradation over Ag/ AgBrO3 and N-TiO2. The photocatalytic degradation rate constants over Ag/AgBrO3 and N-TiO2 are 0.0411 and 0.0085 min1, respectively (Fig. 4b). The rate constant for Ag/AgBrO3 is 4.83 times higher than that of N-TiO2. The effect of the original RhB concentration on photocatalytic performance is shown in Fig. 4c. Clearly, photocatalytic activity is related to the original concentration of RhB. The increase of RhB concentration from 5 to 20 mg/L results in an obvious change of photocatalytic performance. In the 5 mg/L RhB solution, the RhB photodegradation rate reached 97.6% after 45 min. In the 10, 15, and 20 mg/L RhB solutions, the RhB photodegradation rates decreased gradually from 96.6% to 86.5% after 90 min of visible light irradiation. Moreover, the synthesized Ag/ AgBrO3 was still highly photocatalytic when treated with high concentration of RhB solutions. In order to confirm the reason for the above result, $OH level in the photocatalytic reaction was measured using a PL method [21]. The $OH measurements in 5, 10,
15, and 20 mg/L RhB aqueous solutions are shown in Fig. 5aed. The PL intensity of OH at 425 nm decreased with increased RhB concentration. The results indicate that a high concentration of RhB is unconducive to the production of OH. More RhB molecules were adsorbed on the Ag/AgBrO3 surface with increased RhB concentration, thus lowering the number of active sites that formed OH. Meanwhile, more RhB molecules reduced the number of photons that reached the Ag/AgBrO3 surface by increasing light absorption. Therefore, RhB solutions of higher concentrations can significantly reduce the formation of OH in the photocatalysis. Then a recycling experiment was carried out to investigate the stability of Ag/
Fig. 6. Recycling tests of degradation of 10 mg/L RhB over Ag/AgBrO3.
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Fig. 8. ESR spectra of Ag/AgBrO3 under light irradiation. (A) DMPOOH in aqueous solution. (B) DMPOO 2 in DMSO solution. Fig. 7. (A) The UVevis absorption spectra of solution in the process of the photocatalytic degradation of 10 mg/L RhB on Ag/AgBrO3. (B) TOC evolution versus irradiation time in the photocatalytic reaction.
AgBrO3 in the photocatalytic process. The results (Fig. 6) show that the RhB photodegradation rates did not change obviously after five recycles. A slight decline in photocatalytic activity might result from the loss of Ag/AgBrO3 by centrifugation. The results indicate that the prepared Ag/AgBrO3 is a stable photocatalytic material. 3.3. Photocatalytic process and mechanism The time-dependent UVevis spectra of RhB are showed in Fig. 7a. The characteristic peaks of RhB at l ¼ 258, 308, 355, and 554 nm are associated with the p/p* electronic transitions of benzene ring, the n/p* electronic transitions of O atoms in phenyl ring, the conjugated benzene ring, and band-band transitions, respectively. Those peaks diminished gradually, and the color of mixture gradually became transparent with prolonged time in the presence of Ag/AgBrO3 under UV light radiation, which indicated that molecular structure of RhB was decomposed gradually (Fig. 7a). The absorption peak disappeared completely after 90 min of irradiation, suggesting that Ag/AgBrO3 had high photocatalytic performance. To further investigate the photodegradation of RhB solutions, the time-dependent TOC contents of the RhB solutions are displayed in Fig. 7b. TOCs after 30, 60, and 90 min of irradiation are 3.94, 3.04, and 2.97 mg/L, respectively, which are all lower than
that of the original RhB solution (4.36 mg/L). Moreover, the TOC rate of the RhB solutions was lower than the photodegradation rate. The results show the RhB solution was partially decomposed to small molecules, and also partially converted into intermediate products. As is well-known, OH has a very strong oxidizing ability for the partial or complete decomposition of organic chemicals. As showed in Fig. 5, much OH was produced in the photodegradation of RhB, which suggests that OH is the main active species, further confirming that OH mainly oxidized RhB molecules. The electron spin resonance (ESR) technique was then applied to detect the active species over Ag/AgBrO3 under light irradiation [22]. Fig. 8 shows the ESR spectra obtained from solutions containing DMPO and Ag/AgBrO3 in dark and during irradiation of 2, 4 and 6 min Fig. 8A shows that there were no apparent ESR signals over Ag/ AgBrO3 without irradiation. Nevertheless, after the light irradiation, a four-line spectrum with relative intensities of 1:2:2:1 were observed, indicating the hydroxyl radicals over Ag/AgBrO3 during irradiation. Furthermore, the OH signal intensities of Ag/AgBrO3 are obviously stronger within rising time, suggesting that more and more OH radicals generated on the surface of Ag/AgBrO3. This result was corresponding to the conclusion in Fig. 5. In addition, the characteristic peaks corresponding to the DMPOO 2 adducts were observed in Fig. 8B, indicating that O 2 radicals are also produced on Ag/AgBrO3. This result implied that O 2 was also a main active species during the photocatalysis process. The possible reaction mechanism is postulated as follows according to reference [23]:
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When the Ag/AgBrO3 photocatalyst is irradiated by light, the Ag nanoparticles (NPs) owing to the SPR effect can form the photogenerated electronhole pairs. And the electrons formed on the Ag/AgBrO3 photocatalyst will transfer to the conductor band (CB) of AgBrO3. Then the holes left on the Ag/AgBrO3 photocatalyst can combine with OH in water and oxidize it into OH [24]. On the other hand, the photoexcited electrons transferred to the CB of AgBrO3 are captured by dissolved O2 molecules in solutions to produce O 2 radicals [25]. The OH radicals and O2 radicals are strong oxidants which can induce the degradation of RhB to generate intermediate products. Ag NPs þ hv / hþ þ e Ag NPs (e) þ AgBrO3 / AgBrO3 (e) þ Ag NPs O2 þ e / O 2 OH þ hþ / OH OH (or O 2 ) þ RhB / products
4. Conclusions In this work, a new Ag/AgBrO3 photocatalytic material was synthesized by an ion exchange process. The band gap energy of Ag/AgBrO3 was calculated to be around 3.97 eV. Moreover, Ag/ AgBrO3 can be excited by UV light, and oxidize RhB molecules. The photocatalytic ability of Ag/AgBrO3 is higher than that of N-TiO2 under the same conditions. This work can provide a lead for future research into novel photocatalytic materials. Acknowledgements This work was supported by Natural Science Foundation of Tianjin of China (Grant 14JCYBJC20500) and Graduate Program of Science and Technology Innovation of Tianjin Polytechnic University (16114). References [1] A.J. Bard, Photoelectrochemistry and heterogeneous photocatalysis at semiconductors, J. Photochem. 10 (1979) 59e75. [2] C.C. Chen, J.C. Zhao, W.H. Ma, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (2010) 4206e4219. [3] G.C. Xi, J.H. Ye, Q. Ma, N. Su, H. Bai, C. Wang, In situ growth of metal particles on 3D urchin-like WO3 nanostructures, J. Am. Chem. Soc. 134 (2012) 6508e6511. [4] L.Y. Yang, S.Y. Dong, J.H. Sun, J.L. Feng, Q.H. Wu, S.P. Sun, Microwave-assisted preparation, characterization and photocatalytic properties of a dumbbellshaped ZnO photocatalyst, J. Hazard. Mater. 179 (2010) 438e443.
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Please cite this article in press as: L. Song, et al., Synthesis and characterization of Ag/AgBrO3 photocatalyst with high photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.07.012