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Bi12O17Cl2 nanosheets: Enhanced plasmonic visible light photocatalysis and in situ DRIFTS investigation
Accepted Manuscript Full Length Article Ag/AgCl nanoparticles assembled on BiOCl/Bi12O17Cl2 nanosheets: Enhanced plasmonic visible light photocatalysis and in situ DRIFTS investigation Wendong Zhang, Xing'an Dong, Yi Liang, Yanjuan Sun, Fan Dong PII: DOI: Reference:
S0169-4332(18)31487-9 https://doi.org/10.1016/j.apsusc.2018.05.171 APSUSC 39447
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
4 December 2017 24 March 2018 21 May 2018
Please cite this article as: W. Zhang, X. Dong, Y. Liang, Y. Sun, F. Dong, Ag/AgCl nanoparticles assembled on BiOCl/Bi12O17Cl2 nanosheets: Enhanced plasmonic visible light photocatalysis and in situ DRIFTS investigation, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.05.171
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Ag/AgCl nanoparticles assembled on BiOCl/Bi12O17Cl2 nanosheets: Enhanced plasmonic visible light photocatalysis and in situ DRIFTS investigation Wendong Zhanga‡, Xing’an Dongb‡, Yi Lianga, Yanjuan Sunb,*, Fan Dongb,c a
Chongqing Key Laboratory of Inorganic Functional Materials, Department of
Scientific Research Management, Chongqing Normal University, Chongqing, 401331, China, b
Chongqing Key Laboratory of Catalysis and New Environmental Materials, College
of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China, c
School of Materials Science and Engineering, Southwest Petroleum University,
Sichuan, 610500, China.
To whom correspondence should be addressed. E-mail: [email protected] (Yanjuan Sun); [email protected] (Fan Dong). Tel/Fax: + 23-62769785-605. ‡ These authors contributed equally to this work. 1
Abstract: The Ag/AgCl@BiOCl/Bi12O17Cl2 (Ag/AgCl@BOC) plasmonic composites have been successfully synthesized by anchoring Ag/AgCl nanoparticles on the surfaces of BiOCl/Bi12O17Cl2 nanosheets via a deposition-precipitation strategy at room temperature. The XRD, XPS, SME, TEM, UV-vis DRS, PL, Photocurrent, EIS, BET-BJH, and ESR were applied to explore the intrinsic microstructure and physicochemical properties. Furthermore, the in situ diffuse reflectance infrared Fourier transform spectroscopy is used to investigate the adsorption and photocatalytic reaction mechanism during the NOx oxidation process. The optimized 1:2 Ag/AgCl@BOC composites not only exhibited excellent photocatalytic performance (49.5%) but also displayed high photochemical stability for removal of NO at the indoor air level under visible-light irradiation. Based on the DMPO-ESR spin trapping, the active species generated from Ag/AgCl@BOC were •O2- radicals and •OH radicals under visible light. The results demonstrate that the synergetic effect of surface plasmon resonance of the Ag/AgCl nanoparticles and the effective carrier separation ability result in the improvement of photocatalytic efficiency. The present work can provide a facile strategy to the design of high and stable performance bismuth-based plasmonic photocatalysts for environmental purification. Keywords: Ag/AgCl nanoparticles; BiOCl/Bi12O17Cl2 nanosheets; Plasmonic photocatalysis; In situ DRIFTS
2
1. Introduction Smog pollution has become one of the major environmental challenges in China and other rapidly industrializing countries, which causes a serious impact to the sustainable development of modern society [1-3]. Up to now, it has been difficult to find a high efficiency strategy for the removal of atmospheric pollutants, such as nitrogen oxides (NOx) [4], volatile organic pollutants (VOCs) [5], tropospheric ozone (O3) [6], and so on. Therefore, it is highly desirable to explore an eco-friendly and cost-effective technology for smog purification. Semiconductor photocatalysis technology for air purification and wastewater cleaning has attracted extensive attention because they can eliminate most environmental contaminants [7,8]. However, two factors are the main challenges for exploiting high performance photocatalysts [9-12]. On the one hand, traditional semiconductor photocatalysts (TiO2, WO3, Bi2O3, etc) can only utilize the ultraviolet spectrum (about 4% of the solar light). Meanwhile, the ultraviolet energy is far less than visible light energy (about 48% of the solar light). Hence, the development of visible light driven photocatalysts with efficient utilization of solar light is essential. On the other hand, a majority of photocatalysts exhibit poor separation ability of photo-induced electron-hole pairs limiting their quantum efficiency. Therefore, an excellent photocatalyst should possess high quantum efficiency for practical application. Recently, Bi12O17Cl2 photocatalyst has been gained special attention due to its unique microstructure and optical property, which make it potential application for 3
environmental purification [13]. However, Bi12O17Cl2 possesses high recombination rate of photo-induced electron-hole pairs, resulting in the poor visible light photocatalytic
activity.
More
recently,
several
kinds
of
Bi12O17Cl2-based
heterojunctions with other semiconductor photocatalysts have been successfully designed, which demonstrates enhanced visible light photocatalytic performance. Nan et al. developed a facile solvothermal-calcining method to synthesize novel three-dimensional flower-like Bi12O17Cl2/β-Bi2 O3 composites, which displayed excellent
photocatalytic
performance
under
visible
light
irradiation
for
4-tert-butylphenol degradation [14]. Chen et al. fabricated Pt@Bi12O17Cl2 composites with high visible light photocatalytic activities in methyl orange (MO) and phenol degradation [15]. Huang et al. synthesized BiOI@Bi12O17Cl2 p-n junction to enhance visible light photocatalytic activity for the degradation of 2,4-dichlorophenol (2,4-DCP), rhodamine B (RhB), phenol, bisphenol A (BPA) and tetracycline hydrochloride [16]. Very recently, novel BiOCl/Bi12O17Cl2 composites were synthesized by a facile one-step method at room temperature, which showed a good visible light photocatalytic activity for the removal of NO at ppb level [17]. Nevertheless, it is still necessary to find an effective strategy to further improve the visible light photocatalytic performance of BiOCl/Bi12O17Cl2 composites for large-scale application. As is well-known, the surface plasmon resonance (SPR) of metallic Ag-based nanoparticles can be introduced to semiconductor photocatalysts via constructing nanocomposites, which could significantly enhance absorption in the visible light region and could effectively promote the separation/transfer of the 4
photo-induced electron-hole pairs. Successful examples include Ag/AgCl-based nanocomposites of Ag/AgCl/TiO2 [18], Ag/AgCl/BiVO4 [19], Ag/AgCl/Bi20TiO4 [20], Ag/AgCl@g-C3N4 [21], Ag/AgCl/BiOCl [22], Ag/AgCl/WO3 [23], etc. However, to the best of our knowledge, there is little report about the coupling Ag/AgCl nanoparticles and BiOCl/Bi12O17Cl2 nanosheets for visible light photocatalytic removal of NOx at indoor air level. Herein,
novel
Ag/AgCl@BiOCl/Bi12O17Cl2
visible-light-driven
plasmonic
photocatalyst has been successfully synthesized by a facile method at room temperature (see supporting information in detail). Moreover, the in situ molecular reaction mechanism and intermediates during the photocatalytic NOx oxidation process were elucidated for the first time. The present work might provide a new route to design a series of high efficient and stable BiOCl/Bi12O17Cl2-based visible light driven photocatalysts.
2. Results and discussion 2.1. Visible light photocatalytic activity and stability In order to explore the potential performance for indoor air purification, the removal of NO was applied to evaluate the photocatalytic activity and stability. As shown in Fig.1a, the removal ratio of pure BOC was 33.9%. As Ag/AgCl nanoparticles was deposited onto the surface of BOC, the photocatalytic removal ratio was increased to 50.0% for 1:5 Ag/AgCl@BOC, 49.5% for 1:2 Ag/AgCl@BOC, 51.4% for 1:1 Ag/AgCl@BOC, respectively. In addition, 1:2 Ag/AgCl@BOC shows 5
excellent photocatalytic stability in contrast to 1:1 Ag/AgCl@BOC and 1:5 Ag/AgCl@BOC samples. As shown in Fig. 1b, 1:2 Ag/AgCl@BOC sample does not show significant loss of photocatalytic activity after five cycling runs for the removal of NO under visible light irradiation, indicating that 1:2 Ag/AgCl@BOC sample has good stability during the photocatalytic removal of pollutants and also potential application in environmental purification. The results exhibit that the as-obtained Ag/AgCl@BiOCl/Bi12O17Cl2 plasmonic photocatalyst showed superior photocatalytic activities and photochemical stabilities in removing NOx under visible light irradiation. 2.2. Phase structure The crystal structure of BOC and various Ag/AgCl@BOC samples were characterized by XRD. Fig.2 shows that the typical diffraction peaks of BiOCl and Bi12O17Cl2 can be indexed to the tetragonal BiOCl (JCPDS #06-0249) and tetragonal Bi12O17Cl2 (JCPDS #37-0702) [17], respectively. In addition, all the peaks of AgCl can be assigned to the cubic phase of AgCl (JCPDS #31-1238). When the content of AgCl was increased, the intensity of diffraction peaks of AgCl was gradually strengthened correspondingly in the composites. The result implies that the composites were successfully synthesized via a chemical deposition strategy at room temperature. Interestingly, it is noteworthy to mention that the peak intensity of composites are increased with increasing AgCl content, demonstrating that a high AgCl content is favorable for crystallinity of BiOCl/Bi12O17Cl2 composites. The metallic Ag has not been detected by XRD, which can be attributed to the low 6
concentration and high dispersion of Ag nanoparticles on the composites surfaces.
2.3. XPS The XPS survey spectrum (Fig. S5) illstrates that the 1:2 Ag/AgCl@BOC is composed of Ag, Cl, Bi and O elements. As shown in Fig.3a, the two strong peaks at 159.4 and 164.7 eV are attributed to Bi 4f7/2 and Bi 4f5/2, respectively, which are the characteristic of Bi3+ species [24]. The peaks with binding energies located at 530.4 and 531.5 eV correspond to the O1s XPS spectra (Fig.3b), which can be indexed to the characteristics of Bi-O in BiOCl and Bi12O17Cl2, respectively [15,16,22]. Two peaks centered at 198.0 and 199.7 eV are consistent with the peaks of Cl 2p3/2 and Cl 2p1/2 (Fig.3c) [22,25], respectively. Typical peaks of Ag3d consist of two peaks at approximately 367.8 and 373.8 eV (Fig.3d), which can be ascribed to Ag 3d5/2 and Ag 3d3/2 [26,27], respectively. Moreover, the Ag 3d5/2 and Ag 3d3/2 peaks can be further divided into four different peaks [27,28]. The peaks at 368.5 and 374.7 eV can be attributed to metallic Ag0. Similarly, the peaks at 367.8 and 373.8 eV can be ascribed to Ag+. Based on the above mentioned, it could be confirmed the coexistence of Ag and AgCl in the Ag/AgCl@BOC sample.
2.4. Morphology The morphology and microstructures of the BOC and 1:2 Ag/AgCl@BOC photocatalysts were investigated by SEM and TEM. As shown in Fig.4, the BOC sample displayed irregular morphology with a typical sheet-like structure. However, 7
the 1:2 Ag/AgCl@BOC samples not only have the similar morphology, but also possess much more compacted microstructure among nanosheets in contrast to BOC sample (Fig. 5a, 5b). The TEM images (Fig.5c, 5d) reveal that Ag/AgCl nanosheets anchored onto the surface of BOC. The high-resolution TEM (HRTEM) was applied to further confirm the intimate coupling between Ag/AgCl and BiOCl/Bi12O17Cl2, and the images are shown in Fig.5e and 5f. The HRTEM images show clearly the interplanar distance of the lattice is 0.28 nm for AgCl, and 0.236 nm for metallic Ag, corresponding to the (200) plane of the AgCl, and (111) plane of the Ag, respectively. In addition, the X-ray spectrum (EDS) further confirms that the composition of Ag/AgCl@BOC only exist Ag (Fig. 5g), Bi (Fig. 5h), Cl (Fig. 5i) and O (Fig. 5j) elements. The result indicates that the nanojunction is successfully constructed at the interfaces of those nanosheets with good crystallinity.
2.5. Optical properties and charge separation Fig.6a shows the UV-vis diffuse reflectance spectra of BOC and various Ag/AgCl@BOC samples. All the samples exhibit a strong absorption in the visible light region. Furthermore, the Ag/AgCl@BOC samples show broad and strong absorption in the 400-800 nm region of visible light, which can be attributed to the strong surface plasmon resonance of Ag/AgCl nanoparticles [20-22]. The fact indicates that the introduction of Ag/AgCl nanoparticles onto the surface of BiOCl/Bi12O17Cl2 composites benefit the usage of visible light. As can be seen from Fig.6b, the PL intensity of 1:2 Ag/AgCl@BOC is significantly lower than pure BOC, 8
which displays that the Ag/AgCl nanoparticles deposited on BiOCl/Bi12O17Cl2 can conspicuously inhibit the recombination of the photo-excited electron-hole pairs due to the Ag serves as the electron traps [29]. Therefore, it is expected to improve the visible light photocatalytical activity. Photocurrent generation was applied to evaluate the electronic interaction between Ag/AgCl nanoparticles and BOC nanosheets. It can be seen that the photocurrent (Fig.6c) of the 1:2 Ag/AgCl@BOC electrode is significantly higher than that of the pure BOC nanosheet electrode, which can be ascribed to the enhanced visible light absorption and improved photo-generated electron-hole pair separation by the plasmonic Ag nanoparticles [29]. EIS Nyquist plots were carried out to investigate the interface charge separation and transfer efficiency. It was interesting to find that the arc radius on the EIS Nyquist plot (Fig.6d) of 1:2 Ag/AgCl@BOC was smaller than that of the BOC nanosheets under visible light irradiation, indicating that the 1:2 Ag/AgCl@BOC has much more efficient separation and transfer of photo-generated electron-hole pairs among the interface of the Ag/AgCl nanoparticles and BOC nanosheets.
2.6. BET-BJH The BET surface areas and corresponding pore size distribution of BOC and various Ag/AgCl@BOC samples were studied by nitrogen adsorption-desorption measurement. As shown in Fig.7a, the adsorption-desorption isotherms of all the samples can be indexed to type IV with a typical H3 hysteresis loop according to the Brunauer-Deming-Deming-Teller classification, which demonstrates the presence of 9
mesopores [30]. The H3 hysteresis loop could result from the aggregation of nanosheets with slit-like pores, which is in accordance with the SEM and TEM results. Fig.7b shows the corresponding pore distribution curves. All the samples contain mesopores and macropores. The BET surface area and total pore volume are 29.7 m2/g and 0.158 cm3/g for BOC, 26.5 m2/g and 0.125 cm3/g for 1:5 Ag/AgCl@BOC, 23.1 m2/g and 0.124 cm3/g for 1:2 Ag/AgCl@BOC, and 17.0 m2/g and 0.105 cm3/g for 1:1 Ag/AgCl@BOC (Table 1), respectively, indicating that the BET surface areas and total pore volume tend to decrease with increasing AgCl content.
2.7. ESR The electron spin resonance (ESR) technique can be further applied to investigating the active species. As can be seen from Fig.8, superoxide (•O2 -) radicals and hydroxyl (•OH) radicals can be significantly detected by the ESR under visible light irradiation and the product process shows in Equ. 1-3 [17,31,32]. With irradiation time, the intensity of these peaks all show an enhanced trend, which indicates that •O2- and •OH are continuously produced during the light irradiation. The fact suggests that both •O2- radicals and •OH radicals are the main active species during the photocatalytic reaction process. e− + O2 → •O2−
(1)
•O2−+ 2H+ + e− → H2O2
(2)
H2O2 + e− → •OH + OH−
(3)
10
2.8. In situ DRIFTS investigation To reveal the mechanism of photocatalytic NO oxidation with 1:2 Ag/AgCl@BOC, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) is applied to monitor of the time-dependent reaction process under simulated visible light irradiation. Moreover, the adsorption spectra in the dark and photocatalytic reaction spectra under visible light irradiation are presented in this paper. For adsorption process (Fig.8a), the peak at 1061 cm-1 can be assigned to NO, which is the result of physical adsorption. In addition, there is also chemical adsorption, the NO2and N coordinated nitro are detected at 1042 and 1171 cm-1, respectively. Moreover, the weak peaks in the range of 930 to 1030 cm-1 can be assigned to NO3-, which could be further oxidation of minority NO2 - by O2. However, no adsorption peaks can be detected in the range of 800 to 900 cm-1 (Fig.S6). For photocatalytic reaction process (Fig.8b), the bands at 1372 and 1457 cm-1 are assigned to NO2, and these bands increases rapidly when the light turned on and then the reaction equilibrium is reached during the whole irradiation process [33-35]. It demonstrates that the NO molecule was oxidized to NO2 adsorbed on the surface of photocatalysts (Equ. 4) and then reached the dynamic equilibrium rapidly. The bands at 810, 839 and 1555 cm-1 can be assigned to NO2-, which is the product of the adsorbed NO2 receiving an electron (Equ. 5) [36,37]. And then these NO2- would transform to more stable bridging NO2 - species at 1058 and 1180 cm-1 [37,38]. Moreover, the products of monodentate NO3- and bridging NO3 - at 1010 and 1260 cm-1 can be detected, which are oxidized by •O2 - and •OH (Equ. 6 and 7) [35-38], respectively. These increased 11
peaks indicates that the continuous photocatalysis reaction happens, which agrees with the activity result of 1:2 Ag/AgCl@BOC in Fig.1a. The result indicates that the NO is firstly oxidized to NO2, and then NO2 is converted to stable final products bridging NO2 - and bridging NO3-, respectively. Compared with the reaction process of BiOCl/Bi12O17Cl2 sample [17], the most obvious difference is that •OH is involved in the reaction. •OH is regarded to possess greater oxidation capacity than •O2 -. So the reaction activity is considerably promoted. 2NO + O2 → 2NO2
(4)
NO2 + e− → NO2−
(5)
2NO2− + •O2- → 2NO3− + e−
(6)
NO2− + 2•OH → NO3− + H2O
(7)
2.9 Mechanisms of activity enhancement and reaction pathway A possible photocatalytic oxidation mechanism is proposed, based on the band gap structures of BiOCl, Bi12O17Cl2 and AgCl, and it’s illustrated in Scheme 1. The band gap of BiOCl and AgCl is 3.2 and 3.25 eV, respectively. Hence, BiOCl and AgCl can not be excited under visible light irradiation due to their large band gaps. Based on our previous reports, the well-matched band structure between BiOCl and Bi12O17Cl2 is favorable for the separation and transfer of photo-induced electrons and holes. The CB potential of Bi12O17Cl2 (-0.34 eV) is more negative than the reduction potential of oxygen E0 (O2/•O2-) (-0.046 eV) [17], and therefore the O2 could be reduced by the photo-induced electrons in the CB of Bi12O17Cl2 to generated •O2- active species. 12
Simultaneously, the photo-induced electrons can transfer to the low energy states in Ag nanoparticles and could be excited to the higher states as hot electons by the SPR effect, which leads to the formation of a schottky barrier at the interface of Bi12O17Cl2 and Ag nanoparticles that reduce the recombination rate of photo-induced electron-hole pairs. In addition, Ag nanoparticles can produce plasmonic-induced electron-hole pairs after the Ag/AgCl nanoplates were irradiated by visible light, and the plasmonic-induced electrons can be further injected into the CB of AgCl, which leads to the reaction with molecular oxygen to form •O2- and •OH active species. However, the plasmonic-induced holes are transported from the Ag nanoparticles to the surface of AgCl, which can oxidize Cl- (AgCl) to •Cl active radical species. Therefore, the major active species during the photocatalytic NO oxidation are •O2-, •OH and •Cl.
3. Conclusion In summary, benefiting from the unique SPR effect of metallic Ag nanoparticles, the
as-obtained
Ag/AgCl@BiOCl/Bi12O17Cl2
composites
exhibit
excellent
photocatalytic activities and photochemical stabilities for the removal of NOx under visible light irradiation. Furthermore, the adsorption and photocatalytic reaction mechanism of NOx is proposed for the first time. This work could provide a new perspective for the design and fabrication of high performance and stable BiOCl/Bi12O17Cl2-based photocatalysts via a facile method at room temperature.
13
Acknowledgements This research is financially supported by the National Natural Science Foundation of China (Grant No. 51708078, 21576034), Chongqing Postdoctoral Science Foundation funded project (No. Xm2016027), the Scientific and Technological Research Program of Chongqing Education Commission (KJ1600307), the Innovative Research Team of Chongqing (CXTDG201602014, CXTDX201601016).
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19
Figure captions Fig. 1 (a) Photocatalytic removal of NO over the as-obtained samples, (b) cycling runs of the 1:2 Ag/AgCl@BOC composite in air under visible light irradiation (λ ≥ 420 nm). Fig. 2 The XRD patterns of BOC and various Ag/AgCl@BOC samples. Fig. 3 XPS spectra of (a) Bi4f, (b) O1s, (c) Cl2p and (d) Ag3d for the 1:2 Ag/AgCl@BOC samples. Fig. 4 SEM images of BOC sample. Fig. 5 SEM (a, b) and TEM (c, d, e, f) images of 1:2 Ag/AgCl@BOC. EDS elemental mapping (g-j) of the same region, indicating the spatial distribution of Ag (g), Bi (h), Cl (i) and O (j), respectively. Fig. 6 UV-vis diffuse reflectance spectra (a) for various Ag/AgCl@BOC samples, PL spectra (b) for the BOC and 1:2 Ag/AgCl@BOC samples, photocurrent generation (c) and Nyquist plots (d) for the BOC and 1:2 Ag/AgCl@BOC samples under visible light irradiation (λ ≥ 420 nm, [Na2SO4] = 0.5 M). Fig. 7 BET-BJH of BOC and various Ag/AgCl@BOC samples. Fig. 8 (a) DMPO spin-trapping ESR spectra of Ag/AgCl@BOC in methanol dispersion for DMPO-•O2-. (b) DMPO spin-trapping ESR spectra of Ag/AgCl@BOC in aqueous dispersion for DMPO-•OH. Fig. 9 The in situ DRIFTS spectrum of the adsorption and photocatalytic degradation of NO on the surface of 1:2 Ag/AgCl@BOC. 20
Fig. 10 The proposed schematic mechanism for the photocatalytic reation on the surface of Ag/AgCl@BOC.
21
Fig. 1
22
Fig. 2
23
Fig. 3
24
Fig. 4
(a)
(b)
(c)
(d)
25
Fig. 5
(a)
(b)
(c)
(d)
(e)
(f) AgCl, d=0.28nm (200)
Ag, d=0.236nm (111)
(g)
(h)
26
(i)
(j)
27
Fig. 6
28
Fig. 7
29
Fig. 8
30
Fig. 9
Fig. 10 31
32
Table 1 The BET surface areas (SBET), total pore volume (Vp), NO removal ratio (η) of as-obtained samples. Sample
SBET (m2/g)
Vp (cm3/g)
η (%)
BOC
29.7
0.158
37.2
1:1 Ag/AgCl@BOC
17.0
0.105
51.4
1:2 Ag/AgCl@BOC
23.1
0.124
49.5
1:5 Ag/AgCl@BOC
26.5
0.125
50.0
33
Highlights ● Novel Ag/AgCl@BiOCl/Bi12O17Cl2 composites were constructed. ● The plasmonic composites exhibit enhanced visible light photocatalytic activity. ● The in situ DRIFT was employed to investigate the photocatalysis. ●The composites exhibit typical conversion pathways of photocatalytic NO oxidation.
34
Graghical Abstract Novel Ag/AgCl@BiOCl/Bi12O17Cl2 composites with typical conversion pathways of visible light photocatalytic NO oxidation were constructed by a facile method at room temperature.
35
S0169-4332(18)31487-9 https://doi.org/10.1016/j.apsusc.2018.05.171 APSUSC 39447
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
4 December 2017 24 March 2018 21 May 2018
Please cite this article as: W. Zhang, X. Dong, Y. Liang, Y. Sun, F. Dong, Ag/AgCl nanoparticles assembled on BiOCl/Bi12O17Cl2 nanosheets: Enhanced plasmonic visible light photocatalysis and in situ DRIFTS investigation, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.05.171
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Ag/AgCl nanoparticles assembled on BiOCl/Bi12O17Cl2 nanosheets: Enhanced plasmonic visible light photocatalysis and in situ DRIFTS investigation Wendong Zhanga‡, Xing’an Dongb‡, Yi Lianga, Yanjuan Sunb,*, Fan Dongb,c a
Chongqing Key Laboratory of Inorganic Functional Materials, Department of
Scientific Research Management, Chongqing Normal University, Chongqing, 401331, China, b
Chongqing Key Laboratory of Catalysis and New Environmental Materials, College
of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China, c
School of Materials Science and Engineering, Southwest Petroleum University,
Sichuan, 610500, China.
To whom correspondence should be addressed. E-mail: [email protected] (Yanjuan Sun); [email protected] (Fan Dong). Tel/Fax: + 23-62769785-605. ‡ These authors contributed equally to this work. 1
Abstract: The Ag/AgCl@BiOCl/Bi12O17Cl2 (Ag/AgCl@BOC) plasmonic composites have been successfully synthesized by anchoring Ag/AgCl nanoparticles on the surfaces of BiOCl/Bi12O17Cl2 nanosheets via a deposition-precipitation strategy at room temperature. The XRD, XPS, SME, TEM, UV-vis DRS, PL, Photocurrent, EIS, BET-BJH, and ESR were applied to explore the intrinsic microstructure and physicochemical properties. Furthermore, the in situ diffuse reflectance infrared Fourier transform spectroscopy is used to investigate the adsorption and photocatalytic reaction mechanism during the NOx oxidation process. The optimized 1:2 Ag/AgCl@BOC composites not only exhibited excellent photocatalytic performance (49.5%) but also displayed high photochemical stability for removal of NO at the indoor air level under visible-light irradiation. Based on the DMPO-ESR spin trapping, the active species generated from Ag/AgCl@BOC were •O2- radicals and •OH radicals under visible light. The results demonstrate that the synergetic effect of surface plasmon resonance of the Ag/AgCl nanoparticles and the effective carrier separation ability result in the improvement of photocatalytic efficiency. The present work can provide a facile strategy to the design of high and stable performance bismuth-based plasmonic photocatalysts for environmental purification. Keywords: Ag/AgCl nanoparticles; BiOCl/Bi12O17Cl2 nanosheets; Plasmonic photocatalysis; In situ DRIFTS
2
1. Introduction Smog pollution has become one of the major environmental challenges in China and other rapidly industrializing countries, which causes a serious impact to the sustainable development of modern society [1-3]. Up to now, it has been difficult to find a high efficiency strategy for the removal of atmospheric pollutants, such as nitrogen oxides (NOx) [4], volatile organic pollutants (VOCs) [5], tropospheric ozone (O3) [6], and so on. Therefore, it is highly desirable to explore an eco-friendly and cost-effective technology for smog purification. Semiconductor photocatalysis technology for air purification and wastewater cleaning has attracted extensive attention because they can eliminate most environmental contaminants [7,8]. However, two factors are the main challenges for exploiting high performance photocatalysts [9-12]. On the one hand, traditional semiconductor photocatalysts (TiO2, WO3, Bi2O3, etc) can only utilize the ultraviolet spectrum (about 4% of the solar light). Meanwhile, the ultraviolet energy is far less than visible light energy (about 48% of the solar light). Hence, the development of visible light driven photocatalysts with efficient utilization of solar light is essential. On the other hand, a majority of photocatalysts exhibit poor separation ability of photo-induced electron-hole pairs limiting their quantum efficiency. Therefore, an excellent photocatalyst should possess high quantum efficiency for practical application. Recently, Bi12O17Cl2 photocatalyst has been gained special attention due to its unique microstructure and optical property, which make it potential application for 3
environmental purification [13]. However, Bi12O17Cl2 possesses high recombination rate of photo-induced electron-hole pairs, resulting in the poor visible light photocatalytic
activity.
More
recently,
several
kinds
of
Bi12O17Cl2-based
heterojunctions with other semiconductor photocatalysts have been successfully designed, which demonstrates enhanced visible light photocatalytic performance. Nan et al. developed a facile solvothermal-calcining method to synthesize novel three-dimensional flower-like Bi12O17Cl2/β-Bi2 O3 composites, which displayed excellent
photocatalytic
performance
under
visible
light
irradiation
for
4-tert-butylphenol degradation [14]. Chen et al. fabricated Pt@Bi12O17Cl2 composites with high visible light photocatalytic activities in methyl orange (MO) and phenol degradation [15]. Huang et al. synthesized BiOI@Bi12O17Cl2 p-n junction to enhance visible light photocatalytic activity for the degradation of 2,4-dichlorophenol (2,4-DCP), rhodamine B (RhB), phenol, bisphenol A (BPA) and tetracycline hydrochloride [16]. Very recently, novel BiOCl/Bi12O17Cl2 composites were synthesized by a facile one-step method at room temperature, which showed a good visible light photocatalytic activity for the removal of NO at ppb level [17]. Nevertheless, it is still necessary to find an effective strategy to further improve the visible light photocatalytic performance of BiOCl/Bi12O17Cl2 composites for large-scale application. As is well-known, the surface plasmon resonance (SPR) of metallic Ag-based nanoparticles can be introduced to semiconductor photocatalysts via constructing nanocomposites, which could significantly enhance absorption in the visible light region and could effectively promote the separation/transfer of the 4
photo-induced electron-hole pairs. Successful examples include Ag/AgCl-based nanocomposites of Ag/AgCl/TiO2 [18], Ag/AgCl/BiVO4 [19], Ag/AgCl/Bi20TiO4 [20], Ag/AgCl@g-C3N4 [21], Ag/AgCl/BiOCl [22], Ag/AgCl/WO3 [23], etc. However, to the best of our knowledge, there is little report about the coupling Ag/AgCl nanoparticles and BiOCl/Bi12O17Cl2 nanosheets for visible light photocatalytic removal of NOx at indoor air level. Herein,
novel
Ag/AgCl@BiOCl/Bi12O17Cl2
visible-light-driven
plasmonic
photocatalyst has been successfully synthesized by a facile method at room temperature (see supporting information in detail). Moreover, the in situ molecular reaction mechanism and intermediates during the photocatalytic NOx oxidation process were elucidated for the first time. The present work might provide a new route to design a series of high efficient and stable BiOCl/Bi12O17Cl2-based visible light driven photocatalysts.
2. Results and discussion 2.1. Visible light photocatalytic activity and stability In order to explore the potential performance for indoor air purification, the removal of NO was applied to evaluate the photocatalytic activity and stability. As shown in Fig.1a, the removal ratio of pure BOC was 33.9%. As Ag/AgCl nanoparticles was deposited onto the surface of BOC, the photocatalytic removal ratio was increased to 50.0% for 1:5 Ag/AgCl@BOC, 49.5% for 1:2 Ag/AgCl@BOC, 51.4% for 1:1 Ag/AgCl@BOC, respectively. In addition, 1:2 Ag/AgCl@BOC shows 5
excellent photocatalytic stability in contrast to 1:1 Ag/AgCl@BOC and 1:5 Ag/AgCl@BOC samples. As shown in Fig. 1b, 1:2 Ag/AgCl@BOC sample does not show significant loss of photocatalytic activity after five cycling runs for the removal of NO under visible light irradiation, indicating that 1:2 Ag/AgCl@BOC sample has good stability during the photocatalytic removal of pollutants and also potential application in environmental purification. The results exhibit that the as-obtained Ag/AgCl@BiOCl/Bi12O17Cl2 plasmonic photocatalyst showed superior photocatalytic activities and photochemical stabilities in removing NOx under visible light irradiation. 2.2. Phase structure The crystal structure of BOC and various Ag/AgCl@BOC samples were characterized by XRD. Fig.2 shows that the typical diffraction peaks of BiOCl and Bi12O17Cl2 can be indexed to the tetragonal BiOCl (JCPDS #06-0249) and tetragonal Bi12O17Cl2 (JCPDS #37-0702) [17], respectively. In addition, all the peaks of AgCl can be assigned to the cubic phase of AgCl (JCPDS #31-1238). When the content of AgCl was increased, the intensity of diffraction peaks of AgCl was gradually strengthened correspondingly in the composites. The result implies that the composites were successfully synthesized via a chemical deposition strategy at room temperature. Interestingly, it is noteworthy to mention that the peak intensity of composites are increased with increasing AgCl content, demonstrating that a high AgCl content is favorable for crystallinity of BiOCl/Bi12O17Cl2 composites. The metallic Ag has not been detected by XRD, which can be attributed to the low 6
concentration and high dispersion of Ag nanoparticles on the composites surfaces.
2.3. XPS The XPS survey spectrum (Fig. S5) illstrates that the 1:2 Ag/AgCl@BOC is composed of Ag, Cl, Bi and O elements. As shown in Fig.3a, the two strong peaks at 159.4 and 164.7 eV are attributed to Bi 4f7/2 and Bi 4f5/2, respectively, which are the characteristic of Bi3+ species [24]. The peaks with binding energies located at 530.4 and 531.5 eV correspond to the O1s XPS spectra (Fig.3b), which can be indexed to the characteristics of Bi-O in BiOCl and Bi12O17Cl2, respectively [15,16,22]. Two peaks centered at 198.0 and 199.7 eV are consistent with the peaks of Cl 2p3/2 and Cl 2p1/2 (Fig.3c) [22,25], respectively. Typical peaks of Ag3d consist of two peaks at approximately 367.8 and 373.8 eV (Fig.3d), which can be ascribed to Ag 3d5/2 and Ag 3d3/2 [26,27], respectively. Moreover, the Ag 3d5/2 and Ag 3d3/2 peaks can be further divided into four different peaks [27,28]. The peaks at 368.5 and 374.7 eV can be attributed to metallic Ag0. Similarly, the peaks at 367.8 and 373.8 eV can be ascribed to Ag+. Based on the above mentioned, it could be confirmed the coexistence of Ag and AgCl in the Ag/AgCl@BOC sample.
2.4. Morphology The morphology and microstructures of the BOC and 1:2 Ag/AgCl@BOC photocatalysts were investigated by SEM and TEM. As shown in Fig.4, the BOC sample displayed irregular morphology with a typical sheet-like structure. However, 7
the 1:2 Ag/AgCl@BOC samples not only have the similar morphology, but also possess much more compacted microstructure among nanosheets in contrast to BOC sample (Fig. 5a, 5b). The TEM images (Fig.5c, 5d) reveal that Ag/AgCl nanosheets anchored onto the surface of BOC. The high-resolution TEM (HRTEM) was applied to further confirm the intimate coupling between Ag/AgCl and BiOCl/Bi12O17Cl2, and the images are shown in Fig.5e and 5f. The HRTEM images show clearly the interplanar distance of the lattice is 0.28 nm for AgCl, and 0.236 nm for metallic Ag, corresponding to the (200) plane of the AgCl, and (111) plane of the Ag, respectively. In addition, the X-ray spectrum (EDS) further confirms that the composition of Ag/AgCl@BOC only exist Ag (Fig. 5g), Bi (Fig. 5h), Cl (Fig. 5i) and O (Fig. 5j) elements. The result indicates that the nanojunction is successfully constructed at the interfaces of those nanosheets with good crystallinity.
2.5. Optical properties and charge separation Fig.6a shows the UV-vis diffuse reflectance spectra of BOC and various Ag/AgCl@BOC samples. All the samples exhibit a strong absorption in the visible light region. Furthermore, the Ag/AgCl@BOC samples show broad and strong absorption in the 400-800 nm region of visible light, which can be attributed to the strong surface plasmon resonance of Ag/AgCl nanoparticles [20-22]. The fact indicates that the introduction of Ag/AgCl nanoparticles onto the surface of BiOCl/Bi12O17Cl2 composites benefit the usage of visible light. As can be seen from Fig.6b, the PL intensity of 1:2 Ag/AgCl@BOC is significantly lower than pure BOC, 8
which displays that the Ag/AgCl nanoparticles deposited on BiOCl/Bi12O17Cl2 can conspicuously inhibit the recombination of the photo-excited electron-hole pairs due to the Ag serves as the electron traps [29]. Therefore, it is expected to improve the visible light photocatalytical activity. Photocurrent generation was applied to evaluate the electronic interaction between Ag/AgCl nanoparticles and BOC nanosheets. It can be seen that the photocurrent (Fig.6c) of the 1:2 Ag/AgCl@BOC electrode is significantly higher than that of the pure BOC nanosheet electrode, which can be ascribed to the enhanced visible light absorption and improved photo-generated electron-hole pair separation by the plasmonic Ag nanoparticles [29]. EIS Nyquist plots were carried out to investigate the interface charge separation and transfer efficiency. It was interesting to find that the arc radius on the EIS Nyquist plot (Fig.6d) of 1:2 Ag/AgCl@BOC was smaller than that of the BOC nanosheets under visible light irradiation, indicating that the 1:2 Ag/AgCl@BOC has much more efficient separation and transfer of photo-generated electron-hole pairs among the interface of the Ag/AgCl nanoparticles and BOC nanosheets.
2.6. BET-BJH The BET surface areas and corresponding pore size distribution of BOC and various Ag/AgCl@BOC samples were studied by nitrogen adsorption-desorption measurement. As shown in Fig.7a, the adsorption-desorption isotherms of all the samples can be indexed to type IV with a typical H3 hysteresis loop according to the Brunauer-Deming-Deming-Teller classification, which demonstrates the presence of 9
mesopores [30]. The H3 hysteresis loop could result from the aggregation of nanosheets with slit-like pores, which is in accordance with the SEM and TEM results. Fig.7b shows the corresponding pore distribution curves. All the samples contain mesopores and macropores. The BET surface area and total pore volume are 29.7 m2/g and 0.158 cm3/g for BOC, 26.5 m2/g and 0.125 cm3/g for 1:5 Ag/AgCl@BOC, 23.1 m2/g and 0.124 cm3/g for 1:2 Ag/AgCl@BOC, and 17.0 m2/g and 0.105 cm3/g for 1:1 Ag/AgCl@BOC (Table 1), respectively, indicating that the BET surface areas and total pore volume tend to decrease with increasing AgCl content.
2.7. ESR The electron spin resonance (ESR) technique can be further applied to investigating the active species. As can be seen from Fig.8, superoxide (•O2 -) radicals and hydroxyl (•OH) radicals can be significantly detected by the ESR under visible light irradiation and the product process shows in Equ. 1-3 [17,31,32]. With irradiation time, the intensity of these peaks all show an enhanced trend, which indicates that •O2- and •OH are continuously produced during the light irradiation. The fact suggests that both •O2- radicals and •OH radicals are the main active species during the photocatalytic reaction process. e− + O2 → •O2−
(1)
•O2−+ 2H+ + e− → H2O2
(2)
H2O2 + e− → •OH + OH−
(3)
10
2.8. In situ DRIFTS investigation To reveal the mechanism of photocatalytic NO oxidation with 1:2 Ag/AgCl@BOC, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) is applied to monitor of the time-dependent reaction process under simulated visible light irradiation. Moreover, the adsorption spectra in the dark and photocatalytic reaction spectra under visible light irradiation are presented in this paper. For adsorption process (Fig.8a), the peak at 1061 cm-1 can be assigned to NO, which is the result of physical adsorption. In addition, there is also chemical adsorption, the NO2and N coordinated nitro are detected at 1042 and 1171 cm-1, respectively. Moreover, the weak peaks in the range of 930 to 1030 cm-1 can be assigned to NO3-, which could be further oxidation of minority NO2 - by O2. However, no adsorption peaks can be detected in the range of 800 to 900 cm-1 (Fig.S6). For photocatalytic reaction process (Fig.8b), the bands at 1372 and 1457 cm-1 are assigned to NO2, and these bands increases rapidly when the light turned on and then the reaction equilibrium is reached during the whole irradiation process [33-35]. It demonstrates that the NO molecule was oxidized to NO2 adsorbed on the surface of photocatalysts (Equ. 4) and then reached the dynamic equilibrium rapidly. The bands at 810, 839 and 1555 cm-1 can be assigned to NO2-, which is the product of the adsorbed NO2 receiving an electron (Equ. 5) [36,37]. And then these NO2- would transform to more stable bridging NO2 - species at 1058 and 1180 cm-1 [37,38]. Moreover, the products of monodentate NO3- and bridging NO3 - at 1010 and 1260 cm-1 can be detected, which are oxidized by •O2 - and •OH (Equ. 6 and 7) [35-38], respectively. These increased 11
peaks indicates that the continuous photocatalysis reaction happens, which agrees with the activity result of 1:2 Ag/AgCl@BOC in Fig.1a. The result indicates that the NO is firstly oxidized to NO2, and then NO2 is converted to stable final products bridging NO2 - and bridging NO3-, respectively. Compared with the reaction process of BiOCl/Bi12O17Cl2 sample [17], the most obvious difference is that •OH is involved in the reaction. •OH is regarded to possess greater oxidation capacity than •O2 -. So the reaction activity is considerably promoted. 2NO + O2 → 2NO2
(4)
NO2 + e− → NO2−
(5)
2NO2− + •O2- → 2NO3− + e−
(6)
NO2− + 2•OH → NO3− + H2O
(7)
2.9 Mechanisms of activity enhancement and reaction pathway A possible photocatalytic oxidation mechanism is proposed, based on the band gap structures of BiOCl, Bi12O17Cl2 and AgCl, and it’s illustrated in Scheme 1. The band gap of BiOCl and AgCl is 3.2 and 3.25 eV, respectively. Hence, BiOCl and AgCl can not be excited under visible light irradiation due to their large band gaps. Based on our previous reports, the well-matched band structure between BiOCl and Bi12O17Cl2 is favorable for the separation and transfer of photo-induced electrons and holes. The CB potential of Bi12O17Cl2 (-0.34 eV) is more negative than the reduction potential of oxygen E0 (O2/•O2-) (-0.046 eV) [17], and therefore the O2 could be reduced by the photo-induced electrons in the CB of Bi12O17Cl2 to generated •O2- active species. 12
Simultaneously, the photo-induced electrons can transfer to the low energy states in Ag nanoparticles and could be excited to the higher states as hot electons by the SPR effect, which leads to the formation of a schottky barrier at the interface of Bi12O17Cl2 and Ag nanoparticles that reduce the recombination rate of photo-induced electron-hole pairs. In addition, Ag nanoparticles can produce plasmonic-induced electron-hole pairs after the Ag/AgCl nanoplates were irradiated by visible light, and the plasmonic-induced electrons can be further injected into the CB of AgCl, which leads to the reaction with molecular oxygen to form •O2- and •OH active species. However, the plasmonic-induced holes are transported from the Ag nanoparticles to the surface of AgCl, which can oxidize Cl- (AgCl) to •Cl active radical species. Therefore, the major active species during the photocatalytic NO oxidation are •O2-, •OH and •Cl.
3. Conclusion In summary, benefiting from the unique SPR effect of metallic Ag nanoparticles, the
as-obtained
Ag/AgCl@BiOCl/Bi12O17Cl2
composites
exhibit
excellent
photocatalytic activities and photochemical stabilities for the removal of NOx under visible light irradiation. Furthermore, the adsorption and photocatalytic reaction mechanism of NOx is proposed for the first time. This work could provide a new perspective for the design and fabrication of high performance and stable BiOCl/Bi12O17Cl2-based photocatalysts via a facile method at room temperature.
13
Acknowledgements This research is financially supported by the National Natural Science Foundation of China (Grant No. 51708078, 21576034), Chongqing Postdoctoral Science Foundation funded project (No. Xm2016027), the Scientific and Technological Research Program of Chongqing Education Commission (KJ1600307), the Innovative Research Team of Chongqing (CXTDG201602014, CXTDX201601016).
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Figure captions Fig. 1 (a) Photocatalytic removal of NO over the as-obtained samples, (b) cycling runs of the 1:2 Ag/AgCl@BOC composite in air under visible light irradiation (λ ≥ 420 nm). Fig. 2 The XRD patterns of BOC and various Ag/AgCl@BOC samples. Fig. 3 XPS spectra of (a) Bi4f, (b) O1s, (c) Cl2p and (d) Ag3d for the 1:2 Ag/AgCl@BOC samples. Fig. 4 SEM images of BOC sample. Fig. 5 SEM (a, b) and TEM (c, d, e, f) images of 1:2 Ag/AgCl@BOC. EDS elemental mapping (g-j) of the same region, indicating the spatial distribution of Ag (g), Bi (h), Cl (i) and O (j), respectively. Fig. 6 UV-vis diffuse reflectance spectra (a) for various Ag/AgCl@BOC samples, PL spectra (b) for the BOC and 1:2 Ag/AgCl@BOC samples, photocurrent generation (c) and Nyquist plots (d) for the BOC and 1:2 Ag/AgCl@BOC samples under visible light irradiation (λ ≥ 420 nm, [Na2SO4] = 0.5 M). Fig. 7 BET-BJH of BOC and various Ag/AgCl@BOC samples. Fig. 8 (a) DMPO spin-trapping ESR spectra of Ag/AgCl@BOC in methanol dispersion for DMPO-•O2-. (b) DMPO spin-trapping ESR spectra of Ag/AgCl@BOC in aqueous dispersion for DMPO-•OH. Fig. 9 The in situ DRIFTS spectrum of the adsorption and photocatalytic degradation of NO on the surface of 1:2 Ag/AgCl@BOC. 20
Fig. 10 The proposed schematic mechanism for the photocatalytic reation on the surface of Ag/AgCl@BOC.
21
Fig. 1
22
Fig. 2
23
Fig. 3
24
Fig. 4
(a)
(b)
(c)
(d)
25
Fig. 5
(a)
(b)
(c)
(d)
(e)
(f) AgCl, d=0.28nm (200)
Ag, d=0.236nm (111)
(g)
(h)
26
(i)
(j)
27
Fig. 6
28
Fig. 7
29
Fig. 8
30
Fig. 9
Fig. 10 31
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Table 1 The BET surface areas (SBET), total pore volume (Vp), NO removal ratio (η) of as-obtained samples. Sample
SBET (m2/g)
Vp (cm3/g)
η (%)
BOC
29.7
0.158
37.2
1:1 Ag/AgCl@BOC
17.0
0.105
51.4
1:2 Ag/AgCl@BOC
23.1
0.124
49.5
1:5 Ag/AgCl@BOC
26.5
0.125
50.0
33
Highlights ● Novel Ag/AgCl@BiOCl/Bi12O17Cl2 composites were constructed. ● The plasmonic composites exhibit enhanced visible light photocatalytic activity. ● The in situ DRIFT was employed to investigate the photocatalysis. ●The composites exhibit typical conversion pathways of photocatalytic NO oxidation.
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Graghical Abstract Novel Ag/AgCl@BiOCl/Bi12O17Cl2 composites with typical conversion pathways of visible light photocatalytic NO oxidation were constructed by a facile method at room temperature.
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