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Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification
Journal Pre-proof Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification Nguyen Dang Phu, Luc Huy Hoang, Pham Van Hai, Tran Quang Huy, Xiang-Bai Chen, Wu Ching Chou PII:
S0925-8388(20)30277-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.153914
Reference:
JALCOM 153914
To appear in:
Journal of Alloys and Compounds
Received Date: 24 July 2019 Revised Date:
23 December 2019
Accepted Date: 17 January 2020
Please cite this article as: N.D. Phu, L.H. Hoang, P. Van Hai, T.Q. Huy, X.-B. Chen, W.C. Chou, Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153914. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
AUTHOR CONTRIBUTIONS Luc Huy Hoang: Conceptualization, Supervision, Methodology, Writing - Original Draft,
Writing - Review & Editing. Nguyen Dang Phu: Investigation, Visualization, Data Curation, Formal analysis. Xiang-Bai Chen: Visualization, Writing - Review & Editing. Pham Van Hai: Data Curation, Formal analysis. Wu Ching Chou: Formal analysis, Resources. Tran Quang Huy: Formal analysis, Resources.
Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification Nguyen Dang Phu1, Luc Huy Hoang1*, Pham Van Hai1, Tran Quang Huy2 Xiang-Bai Chen3*and Wu Ching Chou4 1
2
Faculty of Physics, Hanoi National University of Education, 136 Xuanthuy, CauGiay, Hanoi, Vietnam
Phenikaa University Nano Institute (PHENA), Phenikaa University, Yen Nghia Ward, Ha Dong, Hanoi, Vietnam 3
4
Hubei Key Laboratory of Optical Information and Pattern Recognition, Wuhan Institute of Technology, Wuhan 430205, China
Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan
Abstract Bi2WO6 nanoparticles doped with Ag, and modified with Ag nanoparticles, were successfully prepared using a two-step microwave assisted and hydrothermal synthesis. With Ag doping, the Bi ions were substituted by the dopantAg ions in the host lattice structure of Bi2WO6. With Ag nanoparticles modification, the Ag nanoparticles grew on the surface of the Bi2WO6 nanoparticles without changing the host lattice structure. The photocatalytic activities of the samples were studied by visible light irradiation of rhodamine B. Both the Ag doping and the Ag nanoparticles modification enhanced the photocatalytic activity of the Bi2WO6 nanoparticles. Moreover, Ag nanoparticles modification produced a more than two times higher enhancement of the photocatalytic activity than the Ag doping method. Furthermore, a mechanism of the significant photocatalytic activity enhancement by Ag nanoparticles modification is proposed.
Keywords: Bi2WO6 nanoparticles, photocatalytic activity.
Ag
doping,
Ag
nanoparticles
*Corresponding authors, Email: [email protected], [email protected].
modification,
1. Introduction In recent years, visible-light-driven semiconductor photocatalysts have attracted much attention [1-8]. Among them, Bi2WO6 is a promising photocatalyst which has outstanding stability, efficient electron transport properties, and potential photocatalytic activity under visible light irradiation [4-7,9]. However, the photocatalytic activity of pure Bi2WO6 nanoparticles is usually low due to the relatively fast electron-hole recombination [6,7,10]. To improve the photocatalytic activity, coupling Bi2WO6 with another semiconductor to form hetero-structured nanomaterials [11,12] and doping Bi2WO6 nanoparticles with metal or nonmetal elements [13-17] are two common approaches. In recent years it has been demonstrated that Ag modified semiconductors are promising with regards to enhanced photocatalytic activity [18-21]. For Bi2WO6, significant enhancement of photocatalytic activity has been achieved by Ag nanoparticles modification [22-25], which can achieve efficient separation of electron-hole pairs and significant enhancement of visible light absorption. Also, Ag nanoparticles have shown high antibacterial activity [26]. When preparing Ag modified Bi2WO6, Ag may be used as a dopant in the host lattice structure of Bi2WO6 [15,22], or loaded as nanoparticles on the surface of Bi2WO6 [23-25, 27]. However, a systematic comparison of photocatalytic enhancement by Ag doping, and by Ag nanoparticles modification, has not been undertaken, and the mechanism of the photocatalytic activity by Ag nanoparticles modification, has not been fully understood. These studies would be helpful for providing directions to achieve high photocatalytic activity nanoparticles – not only Bi2WO6, but also other semiconductor nanoparticles. In this work, both Ag-doped, and Ag nanoparticles modified Bi2WO6 nanoparticles were prepared by a two-step microwave assisted and hydrothermal method. The photocatalytic properties of the Ag-doped, and Ag nanoparticles modified samples were investigated and systematically compared. Our results showed that Ag nanoparticles modification of Bi2WO6 nanoparticles can achieve a more than two times higher photocatalytic activity than the Ag doping method. This significant improvement of catalytic activity can be correlated with the increase of surface area, and the enhancement of surface plasmon resonance, induced by the Ag nanoparticles. In addition, the active species in the photocatalytic process were investigated, which showed that holes are the main active species of Ag-doped Bi2WO6, and both holes and electrons are the active species of Ag nanoparticles modified Bi2WO6. Furthermore, the photocatalytic activity mechanism of Ag nanoparticles modified Bi2WO6 is proposed.
2. Experimental The reagents Na2WO4.2H2O, Bi(NO3)3.5H2O, and AgNO3 were supplied by Merck and used as received. The solution of Ag nanoparticles, with an average particle size of 20 nm, was prepared with the same process as Ref. 26. The two-step microwave-assisted and hydrothermal synthesis was performed with the same process as Ref. 10. For Ag-doped Bi2WO6 samples, different contents of Ag doping were produced with the molar ratio of the reagents AgNO3 and Bi(NO3)3.5H2O being 0, 1.5, 2.5, 5.0, 7.5, and 10.0%. For Ag nanoparticles modified Bi2WO6 samples, different content of Ag nanoparticles loading were obtained with the molar ratios of Ag and Bi2WO6 being 0, 5, 7.5, and 10.0 %. The morphology, crystal structure, chemical composition, optical properties, and photocatalytic activity of the samples were investigated by scanning electron microscopy, (SEM), transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy, N2 adsorption-desorption, ultraviolet-visible (UV–VIS) diffuse reflectance, photoluminescence, and visible light irradiation of rhodamine B, with the same instruments and procedures as in our previous publications [7,10,28]. The Brunauer-Emmett-Teller (BET) experiments for determining the nanoparticles surface areas were performed using a Tristart 3000 Micromeritics. The energy-dispersive X-ray spectroscopy (EDXS) experiments for determining the real amount of Ag in the prepared samples were performed using a JEOL JSM-7600F. To determine the active species in the photocatalytic process, trapping experiments were performed. Isopropyl alcohol (IPA) and potassium iodide (KI) were used as scavengers for photogenerated electrons, and holes, respectively.
3. Results and Discussion Fig. 1 shows the XRD patterns of both the Bi2WO6 samples doped with Ag, and the samples modified with Ag nanoparticles. For the Ag-doped Bi2WO6 samples, all the diffraction peaks can be well indexed to the orthorhombic Bi2WO6 (JCPDS Card No. 390256), and no diffraction peak of the secondary phase is observed, as shown in Fig. 1(a). Therefore, Ag doping does not change the crystal structure of the host Bi2WO6 lattice. Fig. 1(b) presents the enlarged diffraction peak (131) as a function of Ag doping. The peak position systematically shifts to a lower angle with increasing the content of Ag doping, indicating that Ag doping increases the lattice parameter of the host Bi2WO6 crystal. The increase of lattice parameter can be correlated with the replacement of Bi3+ ions (0.96-1.12 Å) with the larger Ag+ ions (1.00-1.28 Å) [22]. To estimate the real amount of Ag in the prepared samples, EDXS experiments were performed. The results, which are presented in
Table 1, show that about half of the initial Ag was successfully incorporated into the host Bi2WO6 crystal. For Ag nanoparticles modified Bi2WO6 samples, the peak positions of the host Bi2WO6 crystal matrix have a negligible shift with increasing the content of Ag nanoparticles, as shown in Fig. 1(c), and more clearly observed in Fig. 1(d). The negligible influence of the host Bi2WO6 crystal with Ag nanoparticles modification indicates that the Ag nanoparticles are loaded on the surface of the host Bi2WO6. The morphologies of the pure Bi2WO6, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6 samples, were investigated by SEM and TEM. Fig. 2a shows the SEM image of pure Bi2WO6 nanoparticles, which are ensembles of plate-like nanostructures. Both Ag doping, and Ag nanoparticles modification have a weak influence on the morphology of the host Bi2WO6 nanoparticles, as shown in Figs. 2b and 2c. BET experiments showed that the specific surface area of the host pure Bi2WO6, 7.5% Agdoped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6 was 15.2, 15.5, and 19.2 m2/g, respectively. The surface area of Ag nanoparticles modified Bi2WO6 was about 30% larger than that of the host pure Bi2WO6 or Ag-doped Bi2WO6. The higher surface area with the Ag nanoparticles modification indicates that this would be helpful to improve the photocatalytic activity of Bi2WO6 nanoparticles. Also, the higher surface area suggests that the Ag nanoparticles would be loaded on the surface of the host Bi2WO6 nanoparticles. Fig. 2d presents the TEM image of the 7.5% Ag-doped Bi2WO6 sample. A lattice interplanar spacing of 0.27 nm was observed, which corresponds to the (020) planes of the orthorhombic Bi2WO6, and no imperfect point was observed. This further confirms that the doped Ag is well incorporated in the host Bi2WO6 lattice. Fig. 2e presents the TEM image of the 7.5% Ag nanoparticles modified Bi2WO6 sample. A lattice interplanar spacing of 0.35 nm and 0.31 nm was clearly observed, corresponding to the (101) planes of Ag, and the (131) planes of Bi2WO6, respectively. This further confirms that the Ag nanoparticles, with a diameter about 10 nm, are distributed randomly and in close contact with the surface of the host Bi2WO6 nanoparticles. Fig. 3 depicts the UV-VIS diffuse reflection spectra (DRS) of the pure Bi2WO6, Ag-doped Bi2WO6, and the Ag nanoparticles modified Bi2WO6 samples. As shown in Fig. 3a, the host pure Bi2WO6 sample shows a strong absorption band from the UV region to visible light, shorter than 450 nm, which is correlated to the intrinsic bandgap transition. The absorption band of Ag-doped Bi2WO6 shows a weak redshift with increasing Ag dopant concentration, indicating a decrease of band gap due to the Ag doping (inset of Fig. 3a). For Ag nanoparticles modified Bi2WO6, the band gap is about same as that of pure
Bi2WO6 (Fig. 3b), indicating that the Ag loading is not incorporated into the host lattice Bi2WO6. In addition, Ag nanoparticles modification produces a new broad absorption band above 500 nm, which can be attributed to the surface plasmon resonance (SPR) band of Ag nanoparticles [23-25]. The SPR of Ag nanoparticles would also be helpful to improve the photocatalytic activity of Bi2WO6 nanoparticles. Fig. 4 shows the XPS spectra of Bi2WO6 samples doped with 7.5% Ag and modified with 7.5% Ag nanoparticles. The binding energies of the Bi, W, and O elements of the Ag-doped, and Ag-modified samples, can be estimated from the XPS spectra in Fig. 4 (a), (b), and (c), respectively. The obtained values, listed in Table 2, are consistent with the reported results of pure Bi2WO6 [28]. With Ag doping, two weak peaks at 373.6 and 367.6 eV are observed (Fig. 4d lower spectrum), which can be assigned to Ag 3d3/2 and Ag 3d5/2 of Ag+ [29,30], indicating the substitution of Ag+ into Bi3+ in the host lattice of Bi2WO6. With Ag nanoparticles modification, two strong peaks at 368.2 and 374.2 eV are observed (Fig. 4d upper spectrum), which can be assigned to Ag 3d3/2 and Ag 3d5/2 of metallic silver [29-31], indicating that Ag nanoparticles are loaded on the surface of the host Bi2WO6. The photocatalytic degradation of RhB molecules was used to evaluate the photocatalytic activity of the samples under visible light irradiation. Fig. 5 shows the photocatalytic activity of pure Bi2WO6, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6 samples. As can be seen in Fig. 5a, above 50% of RhB can be degraded with pure Bi2WO6 in 120 mins. All the Ag-doped samples showed much better photocatalytic activity than that of pure Bi2WO6. The Bi2WO6 doped with 7.5% Ag, which degraded 94% of RhB in 120 mins, had the highest photocatalytic activity among the Ag-doped samples. It is very interesting to see that Ag nanoparticles modification is very promising to achieve excellent photocatalytic activity, being significantly more efficient for the degradation of RhB than Ag doping. The Bi2WO6 modified with 7.5% Ag nanoparticles, which degraded 92% of RhB in just 30 mins, showed the highest photocatalytic activity amongst all the samples. The degradation efficiencies of the pure Bi2WO6, 7.5% Ag-doped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6, are presented in Fig. 5b. It can be seen that Ag nanoparticles modification can achieve a photocatalytic activity of 4.2 and 2.4 times higher than that of pure Bi2WO6, and Ag-doped Bi2WO6, respectively. Fig. 6 presents the PL spectra of pure Bi2WO6, 7.5% Ag-doped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6. The pure Bi2WO6 sample has a broad emission peak at 544 nm, which originates from the transition of Bi6s and O2p hybrid orbit (VB) to W5d orbit
(CB) in the WO62- complex [32]. The PL intensity of 7.5% Ag-doped Bi2WO6 is much lower than that of pure Bi2WO6, indicating Ag doping can significantly slow the electronhole recombination rate. The DRS result showed that the absorption band of Bi2WO6 is weakly affected by Ag doping. Therefore, the high catalytic activity of Ag-doped Bi2WO6 can be mainly correlated with the slow electron-hole recombination rate due to Ag doping. Fig. 6 indicates that the electron-hole recombination rate of Ag nanoparticles modified Bi2WO6 is only slightly higher than that of Ag-doped Bi2WO6, while the catalytic activity of Ag nanoparticles modified Bi2WO6 is more than two times higher than that of Ag-doped Bi2WO6. This significant improvement of catalytic activity can be correlated with the increase of surface area and the enhancement of surface plasmon resonance induced by Ag nanoparticles. The surface area of Ag nanoparticles modified Bi2WO6 is about 30% larger than that of Ag-doped Bi2WO6. Therefore, the surface plasmon resonance enhancement plays the dominant role for improving the photocatalytic activity. Trapping experiments were performed to determine the active species in the photocatalytic process of 7.5% Ag-doped Bi2WO6 and 7.5 % Ag nanoparticles modified Bi2WO6 samples. The results are shown in Fig. 7. For 7.5% Ag-doped Bi2WO6, the photocatalytic activity is almost extinguished with the addition of potassium iodide (KI), whereas the addition of isopropyl alcohol (IPA) has a negligible effect. This indicates that holes are the main active species in Ag-doped Bi2WO6. For 7.5% Ag nanoparticles modified Bi2WO6, the photocatalytic activity is significantly decreased upon the addition of either IPA or KI, with KI inducing a much more dramatic decrease than IPA. Therefore, both holes and electrons are the active species in Ag nanoparticles modified Bi2WO6, and the holes play a much more important role for the degradation of RhB. According to the above results, a photocatalytic degradation mechanism is proposed in Fig. 8. For Ag-doped Bi2WO6, the doping produces defect levels which can trap the photogenerated electrons, thus lowering the electron-hole recombination rate [33]. Therefore, the photocatalytic activity is enhanced, and the holes are the main active species. For Ag nanoparticles modified Bi2WO6 with light irradiation shorter than 450 nm, the photogenerated electrons transfer from Bi2WO6 to Ag nanoparticles, thus lowering the electron-hole recombination rate [34,35]. The transferred electrons on the Ag nanoparticles are the active species, and the holes left on Bi2WO6 are also active species. This charge transfer is less efficient for lowering the electron-hole recombination rate than defect trapping, since the PL intensity of the Ag nanoparticles modified Bi2WO6 is higher than that of the Ag-doped Bi2WO6, while the Ag nanoparticles modification can achieve a
photocatalytic activity 2.4 times higher than that of Ag-doped Bi2WO6. This indicates that surface plasmon resonance induced electrons, and/or holes, on the Ag nanoparticles are important for enhancing the photocatalytic activity. With light irradiation above 500 nm, plasmon hot electrons are generated by the localized surface plasmon resonance oscillations of the Ag nanoparticles. These hot electrons could be useful for enhancing the photocatalytic activity. However, the trapping experiments indicated that the holes play a much more important role for the degradation of RhB. We propose that the hot electrons interact strongly with phonons in Bi2WO6 and dissipate their energy quickly. Thus, these hot electrons cannot contribute to the enhancement of photocatalytic activity. Only the holes left on the Ag nanoparticles can contribute to the enhanced photocatalytic activity. Therefore, for Ag nanoparticles modified Bi2WO6, both holes and electrons are active species, and there are significantly more hole active species than electron active species.
Conclusions In summary, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6 samples were successfully synthesized using a two-step microwave assisted and hydrothermal synthesis. For Ag-doped Bi2WO6, the Ag ions substituted the Bi ions without changing the crystal structure of the host Bi2WO6 lattice. For Ag nanoparticles modified Bi2WO6, the Ag nanoparticles grew on the surface of host Bi2WO6 nanoparticles. Both Ag doping and Ag nanoparticles modification can significantly enhance the photocatalytic activity for the degradation of RhB. The Ag nanoparticles modification is much more promising for enhancing the photocatalytic activity than the Ag doping method, which is mainly correlated with the enhanced surface plasmon resonance induced by the Ag nanoparticles. In addition, the photocatalytic degradation mechanism is explained for the Ag-doped and Ag nanoparticles modified Bi2WO6 samples.
Acknowledgments The authors would like to thank the Vietnam’s National Foundation for Science and Technology Development (NAFOSTED), grant 103.02-2016.21 for financial support. X. B. Chen acknowledges the support by the National Natural Science Foundation of China (Grant No. 11574241).
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Table 1. The amount of Ag estimated by EDXS experiments in the Ag doped Bi2WO6. Samples
Bi2WO6
Doped with 1.5% Ag
Doped with 2.5% Ag
Doped with 5% Ag
Doped with 7.5% Ag
Doped with 10 % Ag
EDXS results: Ag:Bi (%)
0
0
0.5%
1.24%
2.33%
4.53%
Table 2. The binding energies of Bi , W, and O of pure Bi2WO6 [10], Ag doped Bi2WO6, and Ag nanoparticles modified Bi2WO6. Bi
W
O 530.7
Ag
Bi2WO6
159.2 – 164.6
35.4 – 37.5
Ag-doped
159.2 – 164.6
35.6 – 37.8
530.0 - 531.3
Ag-modified
159.2 – 164.6
35.1 – 37.3
528.6 - 530.0 368.2 – 374.2 – 531.5
367.6 – 373.6
Figure Captions Figure 1. (a) XRD patterns of Ag-doped Bi2WO6 samples. (b) Enlarged diffraction peak (131) of (a). (c) XRD patterns of Ag nanoparticles modified Bi2WO6 samples. (d) Enlarged diffraction peak (131) of (c). Figure 2. (a) SEM images of pure Bi2WO6, (b) Ag-doped Bi2WO6, and (c) Ag nanoparticles modified Bi2WO6. (d) TEM images of 7.5% Ag-doped Bi2WO6, and (e) 7.5% Ag nanoparticles modified Bi2WO6. Figure 3. (a) DRS spectra of Ag-doped Bi2WO6, and (b) Ag nanoparticles modified Bi2WO6. Figure 4. XPS spectra of Bi, W, O, and Ag of Bi2WO6 samples doped with 7.5% Ag and modified with 7.5% Ag nanoparticles. Figure 5. (a) Time dependent absorbance change of RhB (554 nm peak) in the presence of pure Bi2WO6, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6. (b) Time dependence of ln(At/Ao) for the photodegradation of RhB by pure Bi2WO6, 7.5% Agdoped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6. Figure 6. PL spectra of pure Bi2WO6, 7.5% Ag-doped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6. Figure 7. Degradation efficiency of RhB by 7.5% Ag-doped Bi2WO6 and 7.5 % Ag nanoparticles modified Bi2WO6, before and after adding IPA and KI. Figure 8. Schematic diagram of the separation and transfer of photogenerated charges in the Ag-doped and Ag nanoparticles modified Bi2WO6.
Highlights •
Ag doped and Ag nanoparticles modified Bi2WO6 samples were synthesized.
•
The Ag nanoparticles modification is much more promising for enhancing photocatalytic activity than Ag doping method.
•
Both holes and electrons are the active species in the Ag nanoparticles modified Bi2WO6.
•
Surface plasmon resonance enhancement is crucial for improving photocatalytic activity.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30277-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.153914
Reference:
JALCOM 153914
To appear in:
Journal of Alloys and Compounds
Received Date: 24 July 2019 Revised Date:
23 December 2019
Accepted Date: 17 January 2020
Please cite this article as: N.D. Phu, L.H. Hoang, P. Van Hai, T.Q. Huy, X.-B. Chen, W.C. Chou, Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153914. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
AUTHOR CONTRIBUTIONS Luc Huy Hoang: Conceptualization, Supervision, Methodology, Writing - Original Draft,
Writing - Review & Editing. Nguyen Dang Phu: Investigation, Visualization, Data Curation, Formal analysis. Xiang-Bai Chen: Visualization, Writing - Review & Editing. Pham Van Hai: Data Curation, Formal analysis. Wu Ching Chou: Formal analysis, Resources. Tran Quang Huy: Formal analysis, Resources.
Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification Nguyen Dang Phu1, Luc Huy Hoang1*, Pham Van Hai1, Tran Quang Huy2 Xiang-Bai Chen3*and Wu Ching Chou4 1
2
Faculty of Physics, Hanoi National University of Education, 136 Xuanthuy, CauGiay, Hanoi, Vietnam
Phenikaa University Nano Institute (PHENA), Phenikaa University, Yen Nghia Ward, Ha Dong, Hanoi, Vietnam 3
4
Hubei Key Laboratory of Optical Information and Pattern Recognition, Wuhan Institute of Technology, Wuhan 430205, China
Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan
Abstract Bi2WO6 nanoparticles doped with Ag, and modified with Ag nanoparticles, were successfully prepared using a two-step microwave assisted and hydrothermal synthesis. With Ag doping, the Bi ions were substituted by the dopantAg ions in the host lattice structure of Bi2WO6. With Ag nanoparticles modification, the Ag nanoparticles grew on the surface of the Bi2WO6 nanoparticles without changing the host lattice structure. The photocatalytic activities of the samples were studied by visible light irradiation of rhodamine B. Both the Ag doping and the Ag nanoparticles modification enhanced the photocatalytic activity of the Bi2WO6 nanoparticles. Moreover, Ag nanoparticles modification produced a more than two times higher enhancement of the photocatalytic activity than the Ag doping method. Furthermore, a mechanism of the significant photocatalytic activity enhancement by Ag nanoparticles modification is proposed.
Keywords: Bi2WO6 nanoparticles, photocatalytic activity.
Ag
doping,
Ag
nanoparticles
*Corresponding authors, Email: [email protected], [email protected].
modification,
1. Introduction In recent years, visible-light-driven semiconductor photocatalysts have attracted much attention [1-8]. Among them, Bi2WO6 is a promising photocatalyst which has outstanding stability, efficient electron transport properties, and potential photocatalytic activity under visible light irradiation [4-7,9]. However, the photocatalytic activity of pure Bi2WO6 nanoparticles is usually low due to the relatively fast electron-hole recombination [6,7,10]. To improve the photocatalytic activity, coupling Bi2WO6 with another semiconductor to form hetero-structured nanomaterials [11,12] and doping Bi2WO6 nanoparticles with metal or nonmetal elements [13-17] are two common approaches. In recent years it has been demonstrated that Ag modified semiconductors are promising with regards to enhanced photocatalytic activity [18-21]. For Bi2WO6, significant enhancement of photocatalytic activity has been achieved by Ag nanoparticles modification [22-25], which can achieve efficient separation of electron-hole pairs and significant enhancement of visible light absorption. Also, Ag nanoparticles have shown high antibacterial activity [26]. When preparing Ag modified Bi2WO6, Ag may be used as a dopant in the host lattice structure of Bi2WO6 [15,22], or loaded as nanoparticles on the surface of Bi2WO6 [23-25, 27]. However, a systematic comparison of photocatalytic enhancement by Ag doping, and by Ag nanoparticles modification, has not been undertaken, and the mechanism of the photocatalytic activity by Ag nanoparticles modification, has not been fully understood. These studies would be helpful for providing directions to achieve high photocatalytic activity nanoparticles – not only Bi2WO6, but also other semiconductor nanoparticles. In this work, both Ag-doped, and Ag nanoparticles modified Bi2WO6 nanoparticles were prepared by a two-step microwave assisted and hydrothermal method. The photocatalytic properties of the Ag-doped, and Ag nanoparticles modified samples were investigated and systematically compared. Our results showed that Ag nanoparticles modification of Bi2WO6 nanoparticles can achieve a more than two times higher photocatalytic activity than the Ag doping method. This significant improvement of catalytic activity can be correlated with the increase of surface area, and the enhancement of surface plasmon resonance, induced by the Ag nanoparticles. In addition, the active species in the photocatalytic process were investigated, which showed that holes are the main active species of Ag-doped Bi2WO6, and both holes and electrons are the active species of Ag nanoparticles modified Bi2WO6. Furthermore, the photocatalytic activity mechanism of Ag nanoparticles modified Bi2WO6 is proposed.
2. Experimental The reagents Na2WO4.2H2O, Bi(NO3)3.5H2O, and AgNO3 were supplied by Merck and used as received. The solution of Ag nanoparticles, with an average particle size of 20 nm, was prepared with the same process as Ref. 26. The two-step microwave-assisted and hydrothermal synthesis was performed with the same process as Ref. 10. For Ag-doped Bi2WO6 samples, different contents of Ag doping were produced with the molar ratio of the reagents AgNO3 and Bi(NO3)3.5H2O being 0, 1.5, 2.5, 5.0, 7.5, and 10.0%. For Ag nanoparticles modified Bi2WO6 samples, different content of Ag nanoparticles loading were obtained with the molar ratios of Ag and Bi2WO6 being 0, 5, 7.5, and 10.0 %. The morphology, crystal structure, chemical composition, optical properties, and photocatalytic activity of the samples were investigated by scanning electron microscopy, (SEM), transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy, N2 adsorption-desorption, ultraviolet-visible (UV–VIS) diffuse reflectance, photoluminescence, and visible light irradiation of rhodamine B, with the same instruments and procedures as in our previous publications [7,10,28]. The Brunauer-Emmett-Teller (BET) experiments for determining the nanoparticles surface areas were performed using a Tristart 3000 Micromeritics. The energy-dispersive X-ray spectroscopy (EDXS) experiments for determining the real amount of Ag in the prepared samples were performed using a JEOL JSM-7600F. To determine the active species in the photocatalytic process, trapping experiments were performed. Isopropyl alcohol (IPA) and potassium iodide (KI) were used as scavengers for photogenerated electrons, and holes, respectively.
3. Results and Discussion Fig. 1 shows the XRD patterns of both the Bi2WO6 samples doped with Ag, and the samples modified with Ag nanoparticles. For the Ag-doped Bi2WO6 samples, all the diffraction peaks can be well indexed to the orthorhombic Bi2WO6 (JCPDS Card No. 390256), and no diffraction peak of the secondary phase is observed, as shown in Fig. 1(a). Therefore, Ag doping does not change the crystal structure of the host Bi2WO6 lattice. Fig. 1(b) presents the enlarged diffraction peak (131) as a function of Ag doping. The peak position systematically shifts to a lower angle with increasing the content of Ag doping, indicating that Ag doping increases the lattice parameter of the host Bi2WO6 crystal. The increase of lattice parameter can be correlated with the replacement of Bi3+ ions (0.96-1.12 Å) with the larger Ag+ ions (1.00-1.28 Å) [22]. To estimate the real amount of Ag in the prepared samples, EDXS experiments were performed. The results, which are presented in
Table 1, show that about half of the initial Ag was successfully incorporated into the host Bi2WO6 crystal. For Ag nanoparticles modified Bi2WO6 samples, the peak positions of the host Bi2WO6 crystal matrix have a negligible shift with increasing the content of Ag nanoparticles, as shown in Fig. 1(c), and more clearly observed in Fig. 1(d). The negligible influence of the host Bi2WO6 crystal with Ag nanoparticles modification indicates that the Ag nanoparticles are loaded on the surface of the host Bi2WO6. The morphologies of the pure Bi2WO6, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6 samples, were investigated by SEM and TEM. Fig. 2a shows the SEM image of pure Bi2WO6 nanoparticles, which are ensembles of plate-like nanostructures. Both Ag doping, and Ag nanoparticles modification have a weak influence on the morphology of the host Bi2WO6 nanoparticles, as shown in Figs. 2b and 2c. BET experiments showed that the specific surface area of the host pure Bi2WO6, 7.5% Agdoped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6 was 15.2, 15.5, and 19.2 m2/g, respectively. The surface area of Ag nanoparticles modified Bi2WO6 was about 30% larger than that of the host pure Bi2WO6 or Ag-doped Bi2WO6. The higher surface area with the Ag nanoparticles modification indicates that this would be helpful to improve the photocatalytic activity of Bi2WO6 nanoparticles. Also, the higher surface area suggests that the Ag nanoparticles would be loaded on the surface of the host Bi2WO6 nanoparticles. Fig. 2d presents the TEM image of the 7.5% Ag-doped Bi2WO6 sample. A lattice interplanar spacing of 0.27 nm was observed, which corresponds to the (020) planes of the orthorhombic Bi2WO6, and no imperfect point was observed. This further confirms that the doped Ag is well incorporated in the host Bi2WO6 lattice. Fig. 2e presents the TEM image of the 7.5% Ag nanoparticles modified Bi2WO6 sample. A lattice interplanar spacing of 0.35 nm and 0.31 nm was clearly observed, corresponding to the (101) planes of Ag, and the (131) planes of Bi2WO6, respectively. This further confirms that the Ag nanoparticles, with a diameter about 10 nm, are distributed randomly and in close contact with the surface of the host Bi2WO6 nanoparticles. Fig. 3 depicts the UV-VIS diffuse reflection spectra (DRS) of the pure Bi2WO6, Ag-doped Bi2WO6, and the Ag nanoparticles modified Bi2WO6 samples. As shown in Fig. 3a, the host pure Bi2WO6 sample shows a strong absorption band from the UV region to visible light, shorter than 450 nm, which is correlated to the intrinsic bandgap transition. The absorption band of Ag-doped Bi2WO6 shows a weak redshift with increasing Ag dopant concentration, indicating a decrease of band gap due to the Ag doping (inset of Fig. 3a). For Ag nanoparticles modified Bi2WO6, the band gap is about same as that of pure
Bi2WO6 (Fig. 3b), indicating that the Ag loading is not incorporated into the host lattice Bi2WO6. In addition, Ag nanoparticles modification produces a new broad absorption band above 500 nm, which can be attributed to the surface plasmon resonance (SPR) band of Ag nanoparticles [23-25]. The SPR of Ag nanoparticles would also be helpful to improve the photocatalytic activity of Bi2WO6 nanoparticles. Fig. 4 shows the XPS spectra of Bi2WO6 samples doped with 7.5% Ag and modified with 7.5% Ag nanoparticles. The binding energies of the Bi, W, and O elements of the Ag-doped, and Ag-modified samples, can be estimated from the XPS spectra in Fig. 4 (a), (b), and (c), respectively. The obtained values, listed in Table 2, are consistent with the reported results of pure Bi2WO6 [28]. With Ag doping, two weak peaks at 373.6 and 367.6 eV are observed (Fig. 4d lower spectrum), which can be assigned to Ag 3d3/2 and Ag 3d5/2 of Ag+ [29,30], indicating the substitution of Ag+ into Bi3+ in the host lattice of Bi2WO6. With Ag nanoparticles modification, two strong peaks at 368.2 and 374.2 eV are observed (Fig. 4d upper spectrum), which can be assigned to Ag 3d3/2 and Ag 3d5/2 of metallic silver [29-31], indicating that Ag nanoparticles are loaded on the surface of the host Bi2WO6. The photocatalytic degradation of RhB molecules was used to evaluate the photocatalytic activity of the samples under visible light irradiation. Fig. 5 shows the photocatalytic activity of pure Bi2WO6, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6 samples. As can be seen in Fig. 5a, above 50% of RhB can be degraded with pure Bi2WO6 in 120 mins. All the Ag-doped samples showed much better photocatalytic activity than that of pure Bi2WO6. The Bi2WO6 doped with 7.5% Ag, which degraded 94% of RhB in 120 mins, had the highest photocatalytic activity among the Ag-doped samples. It is very interesting to see that Ag nanoparticles modification is very promising to achieve excellent photocatalytic activity, being significantly more efficient for the degradation of RhB than Ag doping. The Bi2WO6 modified with 7.5% Ag nanoparticles, which degraded 92% of RhB in just 30 mins, showed the highest photocatalytic activity amongst all the samples. The degradation efficiencies of the pure Bi2WO6, 7.5% Ag-doped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6, are presented in Fig. 5b. It can be seen that Ag nanoparticles modification can achieve a photocatalytic activity of 4.2 and 2.4 times higher than that of pure Bi2WO6, and Ag-doped Bi2WO6, respectively. Fig. 6 presents the PL spectra of pure Bi2WO6, 7.5% Ag-doped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6. The pure Bi2WO6 sample has a broad emission peak at 544 nm, which originates from the transition of Bi6s and O2p hybrid orbit (VB) to W5d orbit
(CB) in the WO62- complex [32]. The PL intensity of 7.5% Ag-doped Bi2WO6 is much lower than that of pure Bi2WO6, indicating Ag doping can significantly slow the electronhole recombination rate. The DRS result showed that the absorption band of Bi2WO6 is weakly affected by Ag doping. Therefore, the high catalytic activity of Ag-doped Bi2WO6 can be mainly correlated with the slow electron-hole recombination rate due to Ag doping. Fig. 6 indicates that the electron-hole recombination rate of Ag nanoparticles modified Bi2WO6 is only slightly higher than that of Ag-doped Bi2WO6, while the catalytic activity of Ag nanoparticles modified Bi2WO6 is more than two times higher than that of Ag-doped Bi2WO6. This significant improvement of catalytic activity can be correlated with the increase of surface area and the enhancement of surface plasmon resonance induced by Ag nanoparticles. The surface area of Ag nanoparticles modified Bi2WO6 is about 30% larger than that of Ag-doped Bi2WO6. Therefore, the surface plasmon resonance enhancement plays the dominant role for improving the photocatalytic activity. Trapping experiments were performed to determine the active species in the photocatalytic process of 7.5% Ag-doped Bi2WO6 and 7.5 % Ag nanoparticles modified Bi2WO6 samples. The results are shown in Fig. 7. For 7.5% Ag-doped Bi2WO6, the photocatalytic activity is almost extinguished with the addition of potassium iodide (KI), whereas the addition of isopropyl alcohol (IPA) has a negligible effect. This indicates that holes are the main active species in Ag-doped Bi2WO6. For 7.5% Ag nanoparticles modified Bi2WO6, the photocatalytic activity is significantly decreased upon the addition of either IPA or KI, with KI inducing a much more dramatic decrease than IPA. Therefore, both holes and electrons are the active species in Ag nanoparticles modified Bi2WO6, and the holes play a much more important role for the degradation of RhB. According to the above results, a photocatalytic degradation mechanism is proposed in Fig. 8. For Ag-doped Bi2WO6, the doping produces defect levels which can trap the photogenerated electrons, thus lowering the electron-hole recombination rate [33]. Therefore, the photocatalytic activity is enhanced, and the holes are the main active species. For Ag nanoparticles modified Bi2WO6 with light irradiation shorter than 450 nm, the photogenerated electrons transfer from Bi2WO6 to Ag nanoparticles, thus lowering the electron-hole recombination rate [34,35]. The transferred electrons on the Ag nanoparticles are the active species, and the holes left on Bi2WO6 are also active species. This charge transfer is less efficient for lowering the electron-hole recombination rate than defect trapping, since the PL intensity of the Ag nanoparticles modified Bi2WO6 is higher than that of the Ag-doped Bi2WO6, while the Ag nanoparticles modification can achieve a
photocatalytic activity 2.4 times higher than that of Ag-doped Bi2WO6. This indicates that surface plasmon resonance induced electrons, and/or holes, on the Ag nanoparticles are important for enhancing the photocatalytic activity. With light irradiation above 500 nm, plasmon hot electrons are generated by the localized surface plasmon resonance oscillations of the Ag nanoparticles. These hot electrons could be useful for enhancing the photocatalytic activity. However, the trapping experiments indicated that the holes play a much more important role for the degradation of RhB. We propose that the hot electrons interact strongly with phonons in Bi2WO6 and dissipate their energy quickly. Thus, these hot electrons cannot contribute to the enhancement of photocatalytic activity. Only the holes left on the Ag nanoparticles can contribute to the enhanced photocatalytic activity. Therefore, for Ag nanoparticles modified Bi2WO6, both holes and electrons are active species, and there are significantly more hole active species than electron active species.
Conclusions In summary, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6 samples were successfully synthesized using a two-step microwave assisted and hydrothermal synthesis. For Ag-doped Bi2WO6, the Ag ions substituted the Bi ions without changing the crystal structure of the host Bi2WO6 lattice. For Ag nanoparticles modified Bi2WO6, the Ag nanoparticles grew on the surface of host Bi2WO6 nanoparticles. Both Ag doping and Ag nanoparticles modification can significantly enhance the photocatalytic activity for the degradation of RhB. The Ag nanoparticles modification is much more promising for enhancing the photocatalytic activity than the Ag doping method, which is mainly correlated with the enhanced surface plasmon resonance induced by the Ag nanoparticles. In addition, the photocatalytic degradation mechanism is explained for the Ag-doped and Ag nanoparticles modified Bi2WO6 samples.
Acknowledgments The authors would like to thank the Vietnam’s National Foundation for Science and Technology Development (NAFOSTED), grant 103.02-2016.21 for financial support. X. B. Chen acknowledges the support by the National Natural Science Foundation of China (Grant No. 11574241).
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Table 1. The amount of Ag estimated by EDXS experiments in the Ag doped Bi2WO6. Samples
Bi2WO6
Doped with 1.5% Ag
Doped with 2.5% Ag
Doped with 5% Ag
Doped with 7.5% Ag
Doped with 10 % Ag
EDXS results: Ag:Bi (%)
0
0
0.5%
1.24%
2.33%
4.53%
Table 2. The binding energies of Bi , W, and O of pure Bi2WO6 [10], Ag doped Bi2WO6, and Ag nanoparticles modified Bi2WO6. Bi
W
O 530.7
Ag
Bi2WO6
159.2 – 164.6
35.4 – 37.5
Ag-doped
159.2 – 164.6
35.6 – 37.8
530.0 - 531.3
Ag-modified
159.2 – 164.6
35.1 – 37.3
528.6 - 530.0 368.2 – 374.2 – 531.5
367.6 – 373.6
Figure Captions Figure 1. (a) XRD patterns of Ag-doped Bi2WO6 samples. (b) Enlarged diffraction peak (131) of (a). (c) XRD patterns of Ag nanoparticles modified Bi2WO6 samples. (d) Enlarged diffraction peak (131) of (c). Figure 2. (a) SEM images of pure Bi2WO6, (b) Ag-doped Bi2WO6, and (c) Ag nanoparticles modified Bi2WO6. (d) TEM images of 7.5% Ag-doped Bi2WO6, and (e) 7.5% Ag nanoparticles modified Bi2WO6. Figure 3. (a) DRS spectra of Ag-doped Bi2WO6, and (b) Ag nanoparticles modified Bi2WO6. Figure 4. XPS spectra of Bi, W, O, and Ag of Bi2WO6 samples doped with 7.5% Ag and modified with 7.5% Ag nanoparticles. Figure 5. (a) Time dependent absorbance change of RhB (554 nm peak) in the presence of pure Bi2WO6, Ag-doped Bi2WO6, and Ag nanoparticles modified Bi2WO6. (b) Time dependence of ln(At/Ao) for the photodegradation of RhB by pure Bi2WO6, 7.5% Agdoped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6. Figure 6. PL spectra of pure Bi2WO6, 7.5% Ag-doped Bi2WO6, and 7.5% Ag nanoparticles modified Bi2WO6. Figure 7. Degradation efficiency of RhB by 7.5% Ag-doped Bi2WO6 and 7.5 % Ag nanoparticles modified Bi2WO6, before and after adding IPA and KI. Figure 8. Schematic diagram of the separation and transfer of photogenerated charges in the Ag-doped and Ag nanoparticles modified Bi2WO6.
Highlights •
Ag doped and Ag nanoparticles modified Bi2WO6 samples were synthesized.
•
The Ag nanoparticles modification is much more promising for enhancing photocatalytic activity than Ag doping method.
•
Both holes and electrons are the active species in the Ag nanoparticles modified Bi2WO6.
•
Surface plasmon resonance enhancement is crucial for improving photocatalytic activity.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: