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silver@silver chloride film samples with enhanced photocatalytic performance and self-cleaning ability
Accepted Manuscript Preparation of Bismuth Stannate/Silver@Silver Chloride Film Samples with Enhanced Photocatalytic Performance and Self-cleaning Ability Xiaojuan Zhao, Xiang Lv, Hongda Cui, Tianhe Wang PII: DOI: Reference:
S0021-9797(17)30809-3 http://dx.doi.org/10.1016/j.jcis.2017.07.041 YJCIS 22570
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
25 May 2017 14 July 2017 14 July 2017
Please cite this article as: X. Zhao, X. Lv, H. Cui, T. Wang, Preparation of Bismuth Stannate/Silver@Silver Chloride Film Samples with Enhanced Photocatalytic Performance and Self-cleaning Ability, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.07.041
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Preparation of Bismuth Stannate/Silver@Silver Chloride Film Samples with Enhanced Photocatalytic Performance and Self-cleaning Ability Xiaojuan Zhao, Xiang Lv, Hongda Cui, Tianhe Wang* Chemicobiology and Functional Materials Institute, Nanjing University of Science and Technology, Nanjing, 210094, PR. China
The corresponding author e-mail address: [email protected] Tel.: +86 25 84315042
Abstract We report a novel technique to fabricate bismuth stannate/silver@silver chloride (Bi2Sn2O7/Ag@AgCl) films on conventional glass substrates. The film exhibited a remarkable self-cleaning capability against organic dyes under visible light. Porous Bi2Sn2O7 (BSO) film was first sintered on a glass substrate, followed by implantation of AgCl in it and photo-induction to produce Ag@AgCl. The degradation of organic dyes and photoelectrochemical studies indicate that, compared with BSO film, Bi2Sn2O7/Ag@AgCl film had a much improved photocatalytic ability, probably due to the enhanced electron transfer efficiency and synergistic effect of visible light absorption of the two semiconductors. The possible mechanism of this marked improvement was investigated and interpreted in terms of electrons and holes separation efficiency and charge circulation routes at the interfaces within the Bi2Sn2O7/Ag@AgCl composite film. The film provided in this study may well have practical applications due to its simplicity of preparation, excellent photocatalytic
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ability and reasonable stability. Keywords: Bi2Sn2O7/Ag@AgCl; film; photocatalysis; self-cleaning; mechanism. 1. Introduction Water pollution caused by industrialization and urbanization has become a critical issue and brought serious threats to living environments [1, 2]. Photocatalysis has received extensive interests because of its potential applications in converting solar energy efficiently [3-5]. Under the sunlight irradiation, photocatalysts can split water into H2 and O2, convert CO2 into fuels and chemicals, and remove organic compounds in an environmentally friendly manner [6-8]. TiO2 is the most studied photocatalyst [9] due to its easy availability, chemical stability and low price. However, TiO2 can utilize no more than 5% of total solar energy because of its large band gap (3.2 eV), and this is the main reason preventing its practical applications [10]. Thus, there is an urgent need to develop novel visible-light responsive photocatalysts. Recently, Bi-containing compounds, including BiVO4 [11], Bi2WO6 [12] and Bi2Sn2O7 [13], have attracted enormous attention for their excellent photocatalysis applications. Among these, Bi2Sn2O7, with a narrow band gap of 2.73 eV, has been widely studied due to its unique properties of ionic conductivity and efficient visible-light-driven photocatalysis [14-16]. Nevertheless, the photocatalytic activity of pure BSO is relatively low and there will be little prospects in real applications. A potential solution is to prolong electron-hole pairs separation and enhance charge transfer efficiency by combining two or more semiconductors with appropriate band gap energy levels [17, 18]. The surface plasmon resonance of Ag@AgCl as
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photocatalyst has been well studied due to the enhanced absorption in the visible light region. The “@” in Ag@AgCl means that Ag nanoparticles are anchored on AgCl surface which can greatly extend the photo-response of AgCl in the whole visible light region by localized surface plasmon resonance effects (LSPRs) and also can enhance the stability of AgCl under visible light irradiation [19]. According to relevant research, when Ag@AgCl is combined with another semiconductor, strong interaction took place at interface and this subsequently enhances the separation of charge thus improving photocatalysis [20-22]. In this study, we report, for the first time, a novel technique to fabricate stable BSO film and Bi2Sn2O7/Ag@AgCl film. The crystalline phases, morphology, chemical composition, and photocatalytic activity of the as-prepared film were investigated in detail. Possible mechanism of photocatalysis enhancement for as-prepared Bi2Sn2O7/Ag@AgCl film is analyzed and discussed. A comprehensive comparison between BSO film and Bi2Sn2O7/Ag@AgCl film photocatalysts was made, which provided the basis for exploring and discussing the enhancement mechanism of photocatalytic activity. 2. Experimental section 2.1 Materials Bismuth nitrate pentahydrate [Bi (NO3)3·5H2O, 99%], potassium stannate trihydrate [K2SnO3·3H2O, 95%], ammonia solution [NH3· H2O, 26%], hydrochloric acid [HCl,
36.0%], silver nitrate [AgNO3, 99.8%], acid red 18 (AR-18, 99%) and methyl orange (MO, 96%) were purchased from Aladdin, Shanghai. All reagents were analytically
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pure and used as received without further purification. Deionized water was used through out the experiments. Conventional silicate glass was used as the substrate. 2.2 Preparation of Bi2Sn2O7 powders The Bi2Sn2O7 powder was synthesized by hydrothermal method. In a typical procedure, 4.38 g of Bi (NO3)3·5H2O and 2.70 g of K2SnO3·3H2O were mixed thoroughly in 90 ml deionized water. Under vigorous stirring, the pH value of the mixture was adjusted to 12 by dropwise addition of ammonia solution. Then the mixture was stirred for 1 h continuously to promote the precipitate reaction. The mixture was then transferred into a 200 ml Teflon-lined stainless steel autoclave, in which water was added to ca. 80% of the total volume. After heating the autoclave at 180
for 24 h, the autoclave was cooled down to room temperature naturally. The
product was collected, washed, and vacuum dried at 60
.
2.3 Preparation of photocatalyst film 2.3.1 Preparation of Bi2Sn2O7 film 15 × 20 mm conventional silicate glass substrates were thoroughly cleaned with detergent followed by sequential rinsing with distilled water and finally drying in an oven. 0.2 g as prepared Bi2Sn2O7 powder, Triton-X 100 (0.08 mL), acetic acid (0.06 mL) and absolute ethanol (1.0 mL) were grinding-mixed in agate pestle and mortar, the grinding process lasted for about 10 minutes. After that, the solution was sonicated for 30 min to obtain uniformly dispersed Bi2Sn2O7 slurry. To prepare film on glass substrate, the slurry was then dropped on a clean glass substrate and spin-coated with 1000 rpm that lasted about 30 s. Afterwards it was dried at 60
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for
10 min. Then, the Bi2Sn2O7 film was calcined at 250
in air in a muffle furnace for 1
h for Triton-X elimination. 2.3.1 Preparation of Bi2Sn2O7/Ag@AgCl film The as prepared Bi2Sn2O7 film was immersed in the AgNO3 solution (0.01 M) under vacuum for 10 mins followed by washing with deionized water in order to remove the residual solution. Next, the sample was soaked in HCl solution (0.01 M) for 10 min under vacuum and washed with deionized water subsequently. Such immersion cycle was repeated for three times to complete AgCl implantation in the Bi2Sn2O7 film. Next, the Bi2Sn2O7/AgCl film was irradiated under 300 W Xe arc lamp for a period of time, allowing the preparation of Bi2Sn2O7/Ag@AgCl film to be finally completed. The as-prepared composite films were denoted as Bi2Sn2O7/Ag@AgCl-n, where “n” represents 10, 20 and 30 mins of photoreduction, respectively. 2.4 Characterization The crystalline phases were investigated with a Brucker D8 advanced X-ray diffraction (2 = 20°~80°) equipped with Cu Ka radiation source ( = 1.5406 Å). Scanning electron microscopy (SEM, HITACHI S-4800, Japan) equipped with an energy-dispersive X-ray spectroscope (EDX) was used to observe the surface morphology, actual thickness and chemical compositions of the prepared films. Atomic force microscopy (AFM) was also employed to characterize the surface and particle morphology of the sample. The specific surface area and average pore diameter of the sample were measured by Brunauer-Emmett-Teller (BET) equation based on the N2 adsorption-desorption isotherm equipped with an automated surface
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area and porosity analyser (Micromeritics ASAP2020 USA). The chemical composition of prepared films was determined by X-ray photoelectron spectroscopy (XPS) on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg K (h = 1253.6 eV) source. The binding energy of the adventitious carbon (C1s) line at 284.6 eV was used for calibration. UV-vis diffuse reflectance spectra (DRS) was recorded on a UV-vis spectrophotometer (Shimadzu UV-3100, Japan). 2.5 Photocatalytic test The photocatalytic performance of the as-prepared samples was evaluated by the degradation of AR-18 and MO aqueous solution under visible irradiation using a 300 W Xe arc lamp with a 420 nm cut off filter. Photocatalytic reactions were performed in a glass container equipped with a cooling water circulator placed on the magnetic churn dasher, and the distance of light source and solution surface is 20 cm. In each experiment, a single sample of the prepared film was placed in AR-18 or MO aqueous solution (5 mg/L, 20 mL) in the reactor. Prior to irradiation, the reaction system was magnetically stirred for 30 min in the dark to achieve adsorption/desorption equilibrium. Then 1 mL solution was taken out in the time before Xe lamp was turn on. During the irradiation, 1 mL solution in photocatalytic reaction was very carefully taken out at every 15 min (AR-18) or 30 min (MO) interval and the concentration was analyzed by a UV-vis spectrometer. After the analysis, this 1 mL solution was carefully returned to the original system. All reported data in this paper were the average values of three experimental replicates and the error bar represents the standard error of mean.
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2.6 Photoelectrochemical measurement Photoelectrochemical measurements were carried out on a CHI660E workstation in a standard three-electrode configuration with 1 M Na2SO4 solution as the electrolyte. A platinum wire and a saturated calomel electrode were used as the counter and reference electrodes. The working electrode was prepared by forming film on FTO glass (15 × 20 mm) in the same way as mentioned in the section of 2.3. The electrochemical impedance spectroscopies (EIS) was performed at the open circuit potential over the frequency range from 0.1 Hz to 105 Hz. The magnitude of the modulation signal was 5 mV. The photocurrent measurement was carried out under discontinuous illumination of 300W Xe arc lamp with a 420 nm cut off filter via four on-off cycles with interval time of 30 s. 3. Results and discussion 3.1 Formation of film The appearance of the as-prepared film samples are as given in Figure 1. Apparently, the films look smooth and uniform. From Figure 1a and 1b, it can be seen that after the deposition of AgCl, the appearance of the film virtually showed no obvious change. However, the film changed to dark brown as a result of visible light irradiation, indicating the formation of Ag in the film (Figure 1c).
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Figure 1. Digital photographs of the (a) BSO film, (b) Bi2 Sn2O7/AgCl heterostructure film (without visible light irradiation) and (c) Bi2Sn2O7/Ag@AgCl-20 film (after visible light irradiation).
The fabrication process of Bi2Sn2O7/Ag@AgCl film is illustrated in Scheme 1. And the mechanism of Bi2Sn2O7/Ag@AgCl formation can be described as follows. BSO film (Figure 1a) is porous, by soaking in the AgNO3 solution under vacuum, the porous structure of BSO film was impregnated with AgNO3. When this film was further impregnated in NaCl solution, Na+ and Cl- entered the porous structure, encountering Ag+ and (NO)3- that were already there. This allowed stable crystals of AgCl to be precipitated within the porous structure of BSO film to form Bi2Sn2O7/AgCl, as shown in Figure 1b. Next when this film is subjected to irradiation in visible light, tiny crystals of Ag were produced on the surface of AgCl crystals by photo-reduction, which was accompanied by the darkening of the film, as clearly shown in Figure 1c. )"
Scheme 1. Schematic illustrating the fabrication process and mechanism of Bi2Sn2O7/Ag@AgCl film
3.2 Characterization The crystal phases of the as-obtained films (scraped down from the glass substrates) were characterized by XRD pattern. Figure 2 shows the XRD spectra of the as-prepared films. The diffraction of the BSO film matched well with the JCPDS card No. 87-0284 with the major peaks at 2 = 28.7°, 33.3°, 47.8° and 56.7°, which represents the formation of a cubic structured phase with a=b=c=10.66 Å. The main characteristic diffraction peaks for the Bi2Sn2O7/Ag@AgCl films are sharp and well identified, indicating the AgCl crystals are well formed [23]. The peaks located at 28.6°, 33.3°, 46.1°, 54.8°, 57.3° and 77.1°, which can be indexed to the (111), (200), (220), (311), (222) and (420) planes of the dimensional face-centered cubic phase of AgCl (JCPDS 31-1238) [24–26]. Additionally, the weak peak at 38.2° suggests that there
were
small
quantity
of
Ag
phase
(JCPDS
No.04-0783)
in
the
Bi2Sn2O7/Ag@AgCl films, which may suggest that small amount of Ag was formed by the photoinduction at the external surfaces of AgCl. From these analysis, it is reasonable to conclude that Ag/AgCl was implanted in the Bi2Sn2O7, thus allowing *"
Bi2Sn2O7/Ag@AgCl film to be successfully synthesized.
Figure 2. XRD patterns of as-prepared films.
The morphologies and microstructures of BSO film and Bi2Sn2O7/Ag@AgCl-20 film are revealed by SEM photos presented in Figure 3. The BSO film consists of well dispersed and approximately spherical particles with a smooth surface and exhibit a narrow range of particle size distribution with the average particle size around 50 nm. Among these small BSO crystals are fine gaps or channels that form the porous structure of the film. Such porous structure enables functional species to be implanted in it. For the Bi2Sn2O7/Ag@AgCl-20 film, it can be clearly seen in Figure 3c and 3d that some nanoparticals with a diameter of about 400 nm were deposited on the BSO and the surfaces of BSO film became rough as a result. Also, Ag@AgCl nanoparticles were cube-like, some what different from that of BSO crystals. Figure S1 displays the EDX spectra of Bi2Sn2O7/Ag@AgCl-20 film, which only shows O, Sn, Bi, Ag and Cl elements. The element mapping images are also shown in the inset of Figure S1. These results further confirmed the coexistence of BSO and Ag@AgCl without contamination by other chemical species.
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Figure 3. SEM images of BSO film (a and b) and Bi2Sn2O7/Ag@AgCl-20 film (c and d).
Atomic force microscopy (AFM) is also a useful technique to determine the surface and particle morphology of the sample. Figure 4 shows the 2D (a) and 3D (b) AFM images of BSO films. And it shows that bulges and dents on the surface are nearly to the same degree, which suggests a reasonably homogeneous surface. This is in accordance with the SEM. The cross-sectional SEM image (Figure 4c) suggests that the thickness of the as-prepared BSO film is around 1.3 m.
Figure 4. AFM micrograph: 2D image (a), 3D image (b) and cross-sectional SEM image of BSO film (c).
The nitrogen adsorption−desorption measurement is employed to investigate the !!"
porous structure of the as-prepared Bi2Sn2O7/Ag@AgCl-20 film, as shown in Figure S2. N2 adsorption-desorption isotherm can be classified as a type IV isotherm with a hysteresis loop, exhibiting the presence of a mesoporous structure [27, 28]. The BET specific surface area of Bi2Sn2O7/Ag@AgCl-20 was measured to be 69.9 m2 g-1, with averaged pore size and specific volume being 8 nm and 0.39 cm3 g-1, respectively. And the pore-size distribution plots calculated using the BJH equation from the absorption branch of the isotherm are shown in the inset of Figure S2, it is evident that the sample shows an average pore diameter of about 8 nm with a slightly broad distribution of pore size. The results indicate that Bi2Sn2O7/Ag@AgCl-20 sample is a porous material. The chemical components of the as-prepared BSO film and Bi2Sn2O7/Ag@AgCl-20 film were investigated by XPS measurements, as displayed in Figure 5. The survey XPS spectrum (Figure 5a) confirms the existence of elements Bi, Sn, and O in BSO film and elements of O, Bi, Sn, Ag, Cl in the Bi2Sn2O7/Ag@AgCl-20 film. The Bi 4f fine XPS spectrum of the as-prepared films presented in Figure 5c shows that the binding energies of Bi 4f7/2 and Bi 4f5/2 for BSO film occur at 157.7 eV and 163 eV, respectively, which are indicative of Bi3+ [29-31]. XPS signals of Sn 3d for BSO film (Figure 5d) were observed at binding energies of 485.1 eV (Sn 3d5/2) and 493.6 eV (Sn 3d3/2), indicating the presence of Sn4+ [32]. The peaks corresponding to Bi 4f and Sn 3d in the XPS spectra of the Bi2Sn2O7/Ag@AgCl-20 film slightly shift toward higher binding energies as compared to BSO, indicating that there is a strong interaction between BSO and Ag@AgCl in the film. The interaction may affect the
!#"
formation of electron transfer channel, which may improve the photoinduced electron-hole pairs separation rate during the photocatalytic process [33]. Figure 5e illustrates the Ag 3d level spectrum. The Ag 3d5/2 and Ag 3d3/2 spin-orbital photo-electrons appear at binding energies of 366.9 eV and 373 eV from AgCl, respectively. Meanwhile, the other two peaks centered at 367.6 eV and 373.8 eV analyzed by the XPS peak fitting program correspond to the metallic Ag [34-36]. This finding indicates that Ag+ ions are successfully reduced to metallic Ag through visible irradiation. The Cl 2p XPS spectrum in Figure 5f displays double peaks at 197 eV and 198.6 eV, which could be assigned to the binding energies of Cl 2p 3/2 and Cl 2p1/2, respectively [37].
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Figure 5. XPS survey spectrum (a) and XPS spectra of O (b), Bi (c), Sn (d), Ag (e) and Cl (f) of the Bi2Sn2O7/Ag@AgCl-20 (BAA-20) film.
Light absorption ability of photocatalysts is very important for their application in degradation of dye pollutants. The properties of as-prepared samples were investigated by the UV–vis DRS. Figure 6 shows the UV–vis spectra of the substrate of silicate glass and as-prepared films. It is clear that the silicate glass has no visible light absorption, and all the photocatalyst films have absorption in the visible range. In contrast to the BSO, Bi2Sn2O7/Ag@AgCl films have stronger absorption in visible light range, suggesting that it is capable of consuming the visible light energy more !%"
efficiently for the degradation of dye pollutants. The inset of Figure 6 shows that the Bi2Sn2O7/Ag@AgCl-20 film has the strongest absorption, suggesting that it can utilize the visible light most effectively and this is one of the most important properties for improving photocatalytic activity.
Figure 6. UV–vis diffuse reflectance spectra of glass and as-prepared films.
3.3 Photocatalytic activities The photodegradation of AR-18 and MO were investigated respectively in the presence of different films under visible light irradiation. The results are displayed in Figure 7. Figure 7a and 7c show the photodegradation rates of dye, where C0 is the concentration of dye before irradiation and C is the concentration after irradiation for a fixed interval of time. For comparison, the same photodegradation experiments of AR-18 and MO without catalyst were also carried out, showing no obvious degradation after irradiated for 75 min (AR-18) and 150 min (MO), respectively. From the degradation results, 42.88% of AR-18 and 31% of MO were degraded by BSO film. In contrast, the Bi2Sn2O7/Ag@AgCl-10 film and Bi2Sn2O7/Ag@AgCl-30 film degraded 67.3% and 86.3% for AR-18 after irradiation for 75min, and 68% and
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89.1% for MO after irradiation for 150 min. Meanwhile, the Bi2Sn2O7/Ag@AgCl-20 sample exhibited the highest degradation rate of about 97.9% for AR-18 and 95% for MO, respectively. As shown in Figure 7b and 7d, the peak intensity of AR-18 and MO decreased rapidly with irradiation time at the presence of Bi2Sn2O7/Ag@AgCl-20 film. According to these results, BSO modified by Ag@AgCl had significantly improved photocatalytic activity over the pure BSO film. Furthermore, the TOC tests were measured to investigate the mineralization of MO and AR-18 in the presence of Bi2Sn2O7/Ag@AgCl-20 film sample, and the results are displayed in Figure S3. The results show that the mineralization yield reaches a value of 73% for the degradation of AR-18 after 75 min of irradiation, which is 69% for MO after 150 min of irradiation. From the TOC changes, it can be confirmed that most of the AR-18 and MO molecules were decomposed into H2O and CO2 in the photocatalysis process.
Figure 7. Photocatalytic degradation rates of different catalysts on the degradation of AR-18 (a)
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and MO (c) under visible-light irradiation; UV−vis spectra changes in the AR-18 (b) and MO (d) degradation using Bi2Sn2O7/Ag@AgCl-20 (BAA-20) film.
Figure 8a and 8c show the relationship between ln(C/C0) and irradiation time (t) for AR-18 and MO. The experimental data was fitted by first-order kinetics as expressed by the equation: ln (C0/C) = kappt [38, 39]. Here kapp stands for an apparent first-order kinetic constant (min-1) and t represents the irradiation time. The plot shows a liner correlation between ln(C/C0) and t. The photodegrading rate (k) of the different samples for AR-18 and MO are displayed in Figure 8b and 8d. Apparently, the Bi2Sn2O7/Ag@AgCl-20 film exhibited the highest photodegrading efficiency with kapp = 0.04735 min-1 for AR-18 and kapp = 0.01924 min-1 for MO, which is 7.25 times and 9.14 times higher than that of BSO film, respectively.
Figure 8. Plot of ln(C/C0) vs irradiation time (t) and the photodegrading rate (k) of the different samples for the degradation of AR-18 (a, b) and MO (c, d) under visible light irradiation.
We have further investigated the photodegradation of AR-18 and MO over different !("
photocatalysts by analyzing the photocatalytic graphs fitted with calculated curves with kinetic constants (Figure S4). It is apparent that the photodegradation of AR-18 and MO coincides well with the fitted curve. This suggests that our early analysis on the results earlier should be reasonably accurate. Besides, the photodegradation of the refractory pollutant of phenol in the presence of Bi2Sn2O7/Ag@AgCl-20 film was also investigated under visible irradiation and the result (Figure S5) shows that the as-prepared film exhibits high photocatalytic activity in degrading the phenol (5 mg/L, 20 mL). 3.4 Photoelectrochemical properties To analyze the enhanced photocatalytic activity, the charge separation and transport was investigated by EIS, and the results are displayed in Figure 9. Figure 9a shows the EIS of the as-prepared films. Apparently, the Bi2Sn2O7/Ag@AgCl films have smaller radius than that of the pure Bi2Sn2O7 film, suggesting a more efficient interfacial charge transfer and higher charge separation for Bi2Sn2O7/Ag@AgCl films. A significant result of this work is that the Bi2 Sn2O7/Ag@AgCl-20 film have the smallest radius, which indicates that it has the highest charge separation. Figure 9b displays the EIS of the Bi2Sn2O7/Ag@AgCl-20 film under dark and visible light irradiated conditions. It can be found that the radius is largely reduced when irradiated under visible light, indicating a large number of carriers generated on the Bi2Sn2O7/Ag@AgCl-20 film under visible light irradiation.
!)"
Figure 9. (a) EIS of Bi2Sn2 O7 film and Bi2Sn2O7/Ag@AgCl (BAA) films; (b) effect of visible light irradiation on the EIS at Bi2Sn2O7/Ag@AgCl-20 film
Additionally, photocurrent is well known for analyzing the separation efficiency of photogenerated electron-hole pairs in a photocatalyic system. A higher photocurrent represents a higher degree of charge separation which is conducive to the photocatalysis process [40, 41]. The current response of BSO film and Bi2Sn2O7/Ag@AgCl-20 film is shown in Figure S6. Clearly, the current shows abrupt changes with light on and off for both the samples. It decreased to zero when the light is turned off and rapidly recovered as soon as the light is turned on, which are caused by the movements of photogenerated electrons. It is interesting to note that compared to BSO film, Bi2Sn2O7/Ag@AgCl-20 film shows much higher photocurrent response which may be attributed to the decreasing recombination of photogenerated electron-hole pairs in consequence of the heterojunction between BSO and Ag@AgCl. As this result is in accordance with the results of organic dyes degradation, it is reasonable to assume that the Bi2Sn2O7/Ag@AgCl heterojunction was effectively constructed within the film, enhancing photo-induced charge carrier separation and interfacial charge transfer rate in photocatalytic process. 3.5 Self-cleaning Ability
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Figure 10 illustrates the test of self-cleaning ability of the as-prepared Bi2Sn2O7/Ag@AgCl-20 film under visible light irradiation. In this experiment, 0.1 mL (10 mg/L) AR-18 was dropped directly onto the Bi2Sn2O7/Ag@AgCl-20 film (Figure 10a). Figure 10b shows that the dye was stable and remained there intact on the film when it was placed in dark where it was dried naturally. However, when it was irradiated under 300 W Xe Lamp for 10 min (during this process it was also dried), the dye was degraded and disappeared (Figure 10c). When Figure 13b and Figure 13c are compared, the conclusion can be drawn that the Bi2Sn2O7/Ag@AgCl film has a strong self-cleaning ability under visible light against, at least, AR-18. This result implies potential industrial applications in many aspects. A foreseeable example would be that when a large panel of low cost silicate glass coated with this Bi2Sn2O7/Ag@AgCl film is placed in the organically contaminated water that is continuously flowing through the film under natural light, it is possible to remove the organic pollutant as the contaminated water flowing continuously through the film.
Figure 10. (a) AR-18 (0.1 mL, 10 mg/L) dropped on the Bi2Sn2O7 /Ag@AgCl-20 film without any treatment; (b) dried naturally in the dark; (c) irradiated in visible light (300 W Xe Lamp) for 10 min.
3.6 Reusability #+"
We have also studied the recycling property of the as-prepared samples as it is a vital concern for the practical application. The reusability of the Bi2Sn2O7/Ag@AgCl-20 film was evaluated by cycled degradation of AR-18 with identical conditions for six times. Before each experiment, the film sample was washed by deionized water and dried at room temperature. The results are shown in Figure 11a , the sample maintained the original photocatalytic activity after 6 cycling runs and there was almost no obvious decrease for the photodegradation rate of AR-18, and the photodegradation rate is 97.9%, 96.5%, 95.8%, 97.5%, 96.2%, 96.5% orderly. The crystal structure and morphology of the sample used for the tests were examined. From Figure 11b, one can see that XRD pattern of the used sample still shows the sharp and strong diffraction peaks and is almost identical to that of the as-prepared fresh sample, indicating that the crystal structure of photocatalyst did not change after the recycled photocatalytic reaction. The SEM image (Figure 11c) shows that the sample still maintains the original morphology, and the cubic morphology of Ag@AgCl nanoparticles distributed on the BSO surface remain intact after six cycles. These results suggest that, at least for the tested cycles, the prepared film samples are stable as photocatalyst to photodegradate dyes under visible light irradiation.
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Figure 11. Cycling runs for photocatalytic degradation of AR-18 over the Bi2Sn2O7/Ag@AgCl-20 film under visible light irradiation (a); XRD patterns (b) and SEM image (c) of Bi2Sn2 O7/Ag@AgCl-20 film used after six cycling experiments.
4 Photocatalytic mechanisms In order to understand the photocatalytic mechanism, the reactive species of the as-prepared photocatalyst in the photodegradation of AR-18 system were investigated by adding a series of scavengers into the reaction. In this trapping experiments, 2 mM iso-propyl alcohol (IPA), 1, 4-benzoquinone (BQ) and ammonium oxalate were used as scavengers of hydroxyl radicals (•OH), super oxide anion radical (•O2−) and holes (h+) [42–44], respectively. Figure S7 illustrates the photocatalytic degradation of AR-18 on the Bi2Sn2O7/Ag@AgCl-20 in the presence of the selected scavengers. It can be clearly seen that the degradation rate of AR-18 is 99.75% without any scavenger. In contrast, the degradation rate has a little decrease and reaches 84.8% in the presence of BQ, indicating that the •O2− is not the main reactive species in this photocatalysis system. However, the addition of IPA causes a remarkable decelerated for the degradation of AR-18 with the photodegradation rate decreasing to 40%. ##"
Besides, the addition of ammonium oxalate greatly reduces the photodegradation rate and only 22.8% AR-18 was photodegradated. Based on these results, it can be concluded that both the •OH and h+ are the dominant reactive species in this photocatalytic system and the role of •O2− is also important but secondary. It is known that the energy band gap of Bi2Sn2O7 is 2.73 eV. For a semiconductor, ECB can be determined by the empirical equation: ECB=X−Ee−0.5Eg [45], where X is the electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), Eg is the band gap energy of the semiconductor, and ECB is the conduction band potential. Accordingly, the ECB of the BSO is evaluated at 0.22 eV, and correspondingly, the valence band potential (EVB) was calculated at about 2.95 eV. Similarly, the ECB and EVB of AgCl are calculated to be −0.06 and +3.2 eV, respectively. On the basis of the band gap structures of Bi2Sn2O7/Ag@AgCl and the trapping experiments results, a schematic mechanism for the degradation of AR-18 with the as-prepared photocatalysts is proposed and illustrated in Scheme 2. It is found that AgCl cannot be excited by visible light due to its large band gap. Whereas BSO and Ag nanoparticles can absorb visible-light photons to produce photogenerated electrons and holes as irradiated under visible light. The active electrons on the Ag nanoparticles quickly transport to the CB of AgCl and react with the absorbed O2 on the surface to form H2O2, which can further produce •OH radicals responsible for the degradation of the pollutants [46, 47]. The holes remain on the Ag nanoparticles. Meanwhile, the photogenerated electrons of BSO migrate to the Ag nanoparticles to recombine with the hole in reserve. In addition, the photoexcited
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holes accumulate in the VB of BSO, on the one hand, they can directly oxidize the dye molecule absorbed on the surface; on the other hand, a large amount of holes transfer to the surface of the AgCl particles because the surface was negatively charged. Subsequently, these holes combine with Cl- to form Cl0, which is also a reactive radical specie capable of degrading the dyes and then become reduced to Clso that the system remains balanced and stable [48-51]. Apparently, comparing with pure BSO film, the migration of electrons and holes in BSO, Ag and AgCl can substantially decrease the recombination of charges and thus promote photocatalytic performance. Besides, the BSO film which was embedded with enormous number of Ag@AgCl nanocrystals, could provide more adsorption sites for the pollutant. Furthermore, the adsorbed pollutant can also migrate to the decomposition centers located on the catalyst surface
Scheme 2. Proposed mechanism
5 Conclusions The pure BSO film and Bi2Sn2O7/Ag@AgCl film on conventional silicate glass were successfully prepared by a facile but novel method. The obtained films were uniform #%"
and exhibited excellent visible light catalytic performance for degradation of organic dyes, with a number of favorable characteristics including stability, reusability and self-cleaning
ability
under
visible
light
irradiation.
In
comparison,
Bi2Sn2O7/Ag@AgCl had much improved photocatalytic capability over the single phased Bi2Sn2O7 film. The possible mechanism of this improvement was investigated and interpreted in terms of electrons and holes separation efficiency and charge circulation routes at the interfaces within the Bi2Sn2O7/Ag@AgCl film. Although there have been several reports to develop Bi2Sn2O7 powder composite catalysts with improved visible light photocatalytic performance for degrading organic pollutants [13, 16, 52], Bi2Sn2O7 film and Bi2Sn2O7/Ag@AgCl film catalyst has not been reported
previously
to
the
best
of
our
knowledge.
Furthermore,
the
Bi2Sn2O7/Ag@AgCl film exhibited a remarkable self-cleaning ability under visible light irradiation leading the ways to various potential applications. A foreseeable example would be that when a large panel of low cost silicate glass coated with this Bi2Sn2O7/Ag@AgCl film is placed in the organically contaminated water that is continuously flowing through the film under natural light, it is possible to remove the organic pollutant as the contaminated water flowing continuously through the film. The study also provides a practical technique to form a functional film on conventional glass and may also offer a strategy for preparations of other functional films.
Acknowledgments The authors would like to thank the Scientific Research Foundation of Nanjing
#&"
University of Science and Technology (AE89909) for financial support.
References [1] V.J. Jones, N.L. Rose, A.E. Self, N. Solovieva, H. Yang, Evidence of global pollution and recent environmental change in Kamchatka, Glob. Planet. Change. 134 (2015) 82-90. [2] H. Hettige, M. Mani, D. Wheeler, Industrial pollution in economic development: the environmental Kuznets curve revisited, J. Dev. Econ. 62 (2000) 445-476. [3] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B-Environ. 49 (2004) 1-14. [4] K.Yu, S.G. Yang, C. Liu, H.Z. Chen, H. Li, C. Sun, S.A. Boyd, Degradation of organic dyes via bismuth silver oxide initiated direct oxidation coupled with sodium bismuth ate based visible light photocatalysis, Environ. Sci. Technol. 46 (2012) 7318-7326. [5] C. Domínguez, J. García, M.A. Pedraz, A. Torres, M.A. Galán, Photocatalytic oxidation of organic pollutants in water, Catal. Today. 40 (1998) 85-101. [6] W. Ong, L. Tan, Y.H. Ng, S. Yong, S. Chai, Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (2016) 7159-7329. [7] X. Liu, J. Iocozzia, Y. Wang, X. Cui, Y. Chen, S. Zhao, Z. Li Z. Lin, Noble metal–metal oxide nanohybrids with tailored nanostructures for efficient solar energy
#'"
conversion, photocatalysis and environmental remediation, Energy Environ. Sci. 10 (2017) 402. [8] K. Maeda, K. Domen, Development of novel photocatalyst and cocatalyst materials for water splitting under visible light, Bull. Chem. Soc. Jpn. 89 (2016) 627-648. [9] G.L. Puma, A. Bono, D. Krishnaiah, J.G. Collin, Preparation of titanium dioxide photocatalyst loaded onto activated carbon support using chemical vapor deposition: A review paper, J. Hazard. Mater. 157 (2008) 209-219. [10] S.Y. Guo, S. Han, H.F. Mao, S.M. Dong, C.C. Wu, L.C. Jia, Structurally controlled ZnO/TiO2 heterostructures as efficient photocatalysts for hydrogen generation
from
water
without
noble
metals:
the
role
of
microporous
amorphous/crystalline composite structure, J. Power Sources 245 (2014) 979–985. [11] A. Kudo, K. Omori, H. Kato, A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperature and its photocatalytic and photophysical properties, J. Am. Chem. Soc. 121 (1999) 11459-11467. [12] J. Ren, W. Wang, S. Sun, L. Zhang, J. Chang, Enhanced photocatalytic activity of Bi2WO6 loaded with Ag nanoparticles under visible light irradiation, Appl. Catal. B-Environ. 92(2009) 50-55. [13] W. Xu, J. Fang, Y. Chen, S. Lu, G. Zhou, X. Zhu, Z. Fang, Novel heterostructured Bi2S3/Bi2Sn2O7 with highly visible light photocatalytic activity for the removal of rhodamine B, Mater. Chem. Phys. 154 (2015) 30-37.
#("
[14] J.J. Wu, F.Q. Huang, X.J. Lu, P. Chen, D.Y. Wan, F.F. Xu, Improved visible-light photocatalysis of nano-Bi2Sn2O7 with dispersed s-bands. J. Mater. Chem. 21 (2011) 3872–3876. [15] H. Liu, Z. Jin, Y. Su, Y. Wang, Visible light-driven Bi2Sn2O7/reduced graphene oxide nanocomposite for efficient photocatalytic degradation of organic contaminants, Sep. Purif. Technol. 142 (2015) 25-32. [16] Y. Xing, W. Que, X. Yin, X. Liu, H.M. Asif Javed, Y. Yang, L. Kong, Fabrication of Bi2Sn2O7-ZnO heterostructures with enhanced photocatalytic activity, RSC. Adv. 5 (2015) 27576-27583. [17] M. Agrawal, S. Gupta, A. Pich, N.E. Zafeiropoulos, M. Stamm, A Facile Approach to Fabrication of ZnO-TiO2 Hollow Spheres, Chem. Mater. 21 (2009) 5343-5348. [18] S. Li, S. Hu, W. Jiang, Y. Liu, J. Liu, Z. Wang, Facile synthesis of flower-like Ag3 VO4/Bi2WO6 heterojunction with enhanced visible-light photocatalytic activity, J. Colloid. Interf. Sci. 501 (2017) 156–163 [19] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911-921. [20] R. Qiao, M. Mao, E. Hu, Y. Zhong, J. Ning, Y. Hu, Facile Formation of Mesoporous
BiVO4/Ag/AgCl
Heterostructured
Microspheres
with
Enhanced
Visible-Light Photoactivity, Inorg. Chem. 54 (2015) 9033-9039. [21] L. Liu, J. Deng, T. Niu, G. Zheng, P. Zhang, Y. Jin, Z. Jiao, X. Sun, One-step synthesis of Ag/AgCl/GO composite: A photocatalyst of extraordinary photoactivity
#)"
and stability, J. Colloid. Interf. Sci. 493 (2017) 281-287. [22] D. Xu, W. Shi, C. Song, M. Chen, S. Yang, W. Fan, B. Chen, In-situ synthesis and
enhanced
photocatalytic
activity
of
visible-light-driven
plasmonic
Ag/AgCl/NaTaO3 nanocubes photocatalysts, Appl. Catal. B-Environ. 191 (2016) 228-234. [23] L. Ai, C. Zhang, J. Jiang. Hierarchical porous AgCl@Ag hollow architectures: Self-templating synthesis and highly enhanced visible light photocatalytic activity, Appl. Catal. B-Environ. 142 (2013) 744-751. [24] T.S. Dhas, V.G. Kumar, V. Karthick, K.J. Angel, K.. Govindaraju, Facile synthesis of silver chloride nanoparticles using marine alga and its antibacterial efficacy, Spectrochim. ACTA. A. 120 (2014) 416-420. [25] M.R.H. Siddiqui, S.F Adil, K. Nour, M.E. Assa, A. Al-Warthan, Ionic liquid behavior and high thermal stability of silver chloride nanoparticles: synthesis and characterization, Arab. J. Chem. 6 (2013) 435-438. [26] B. Naik, V. Desai, M. Kowshik, V.S. Prasadc, G.F. Fernandod, N.N. Ghosha, Synthesis of Ag/AgCl-mesoporous silica nanocomposites using a simple aqueous solution-based chemical method and a study of their antibacterial activity on E. coli, Particuology, 9 (2011) 243-247. [27] X. Zhou, Z. Hu, Y. Fan, S. Chen, W. Ding, N. Xu, Microspheric organization of multilayered ZnO nanosheets with hierarchically porous structures, J. Phys. Chem. C. 112 (2008) 11722-11728. [28] M. Kruk, M. Jaroniec, Gas adsorption characterization of ordered
#*"
organic-inorganic nanocomposite materials, Chem. Mater. 13 (2001) 3169-3183.. [29] H. Liu, W. Cao, Y. Su, Y. Wang, X. Wang, Synthesis, characterization and photocatalytic performance of novel visible-light-induced Ag/BiOI, Appl. Catal. B-Environ. 111 (2012) 271-279. [30] H. Liu, Y. Su, Z. Chen, Z. Jin, Y. Wang, Graphene sheets grafted three-dimensional BiOBr0.2I0.8 microspheres with excellent photocatalytic activity under visible light, J. Hazard. Mater. 266 (2014) 75-83. [31] H. Liu, W. Cao, Y. Su, Z. Chen, Y. Wang, Bismuth oxyiodide–graphene nanocomposites with high visible light photocatalytic activity, J. Colloid. Interf. Sci. 398 (2013) 161-167. [32] C.D. Wagner, Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer, 1979. [33] B. Jiang, C. Tian, Q. Pan, Z. Jiang, J. Wang, W. Yan, H. Fu, Enhanced photocatalytic activity and electron transfer mechanisms of graphene/TiO2 with exposed {001} facets, J. Phys. Chem. C. 115 (2011) 23718-23725. [34] Z. Feng, J. Yu, D. Sun, T. Wang, Visible-light-driven photocatalysts Ag/AgCl dispersed on mesoporous Al2O3 with enhanced photocatalytic performance, J. Colloid. Interf. Sci. 480 (2016) 184-190. [35] P. Wang, B. Huang, Z. Lou, X. Zhang, X. Qin, Y. Dai, Z. Zheng, X. Wang, Synthesis of highly efficient Ag@AgCl plasmonic photocatalysts with various structures, Chem. Eur. J. 16 (2010) 538−544. [36] R. Dong, B. Tian, C. Zeng, T. Li, T. Wang, J. Zhang, Ecofriendly synthesis and
$+"
photocatalytic activity of uniform cubic Ag@AgCl plasmonic photocatalyst, J. Phys. Chem. C. 117 (2013) 213−220. [37] L. Dong, D Liang, R. Gong, In Situ Photoactivated AgCl/Ag nanocomposites with enhanced visible light photocatalytic and antibacterial activity, Eur. J. Inorg. Chem. 2012, 3200−3208. [38] S. Han, L. Hu, N. Gao, A.A.Al-Ghamdi, X. Fang, Efficient self-assembly synthesis of uniform CdS spherical nanoparticles-Au nanoparticles hybrids with enhanced photoactivity, Adv. Funct. Mater. 24 (2014) 3725-3733. [39] J. Krýsa, G. Waldner, H. Mšt’ánková, J. Jirkovsky´, G. Grabner, Photocatalytic degradation of model organic pollutants on an immobilized particulate TiO2 layer: Roles of adsorption processes and mechanistic complexity, Appl. Catal. B-Environ. 64 (2006) 290-301. [40] J. Jiang, X. Zhang, Sun P, L. Zhang, ZnO/BiOI heterostructures: photoinduced charge-transfer property and enhanced visible-light photocatalytic activity, J. Phys. Chem. C. 115 (2011) 20555-20564. [41] Q. Xiang, J. Yu, M. Jaroniec, Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites, J. Phys. Chem. C. 115 (2011) 7355-7363. [42] Y. Chen, S. Yang, K. Wang, L. Lou, Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of acid orange 7, J.Photochem. Photobiol. A 172 (2005) 47–54. [43] S.H. Yoon, J.H. Lee, Oxidation mechanism of As(III) in the UV/TiO2
$!"
system:evidence for a direct hole oxidation mechanism, Environ. Sci. Technol. 39 (2005) 9695–9701. [44] Y. Li, J. Wang, H. Yao, L. Dang, Z. Li, Efficient decomposition of organic compounds and reaction mechanism with BiOI photocatalyst under visible light irradiation, J. Mol. Catal. A-Chem. 334 (2011) 116–122. [45]
M.A.
Butler;
D.S.
Ginley,
Prediction
of
flatband
potentials
at
semiconductor-electrolyte interfaces from atomic electronegativities, J. Electrochem. Soc. 125 (1978) 228−232. [46] W. Li, D. Li, Y. Lin, P. Wang, W. Chen, Xi. Fu, Y. Shao, Evidence for the active species involved in the photodegradation process of methyl orange on TiO2, J. Phys. Chem. C. 116 (2012) 3552-3560. [47] X. Zhang, X. Quan, S. Chen, H. Yua, Constructing graphene/InNbO4 composite with excellent adsorptivity and charge separation performance for enhanced visible-light-driven photocatalytic ability, Appl. Catal. B-Environ. 105 (2011) 237-242. [48] Y. Xu, H. Xu, H. Li, J. Xia, C. Liu, L. Liu, Enhanced photocatalytic activity of new photocatalyst Ag/AgCl/ZnO, J. Alloy. Compd. 509 (2011) 3286-3292. [49] G. Begum, J. Manna, RK. Rana, Controlled orientation in a bio-Inspired assembly
of
Ag/AgCl/ZnO
nanostructures
enables
enhancement
in
visible-light-induced photocatalytic performance, Chem–Eur. J. 18(2012) 6847-6853. [50] F. Chen, H. Liu, S. Bagwasi, X. Shen, J. Zhang, Photocatalytic study of BiOCl for degradation of organic pollutants under UV irradiation, J. Photoch. Photobio. A.
$#"
215 (2010) 76-80. [51] M.A. Gondal, X.F. Chang, Z.H. Yamani, UV-light induced photocatalytic decolorization of Rhodamine 6G molecules over BiOCl from aqueous solution, Chem. Eng. J. 165 (2010) 250-257. [52] W. Xu, J. Fang, X. Zhu, Z, Fang, C. Cen, Fabricaion of improved novel p-n junction BiOI/Bi2Sn2O7 nanocomposite for visible light driven photocatalysis, Mater. Res. Bull. 72 (2015) 229-234.
Graphic Abstract "
"
$$"
S0021-9797(17)30809-3 http://dx.doi.org/10.1016/j.jcis.2017.07.041 YJCIS 22570
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
25 May 2017 14 July 2017 14 July 2017
Please cite this article as: X. Zhao, X. Lv, H. Cui, T. Wang, Preparation of Bismuth Stannate/Silver@Silver Chloride Film Samples with Enhanced Photocatalytic Performance and Self-cleaning Ability, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.07.041
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.
Preparation of Bismuth Stannate/Silver@Silver Chloride Film Samples with Enhanced Photocatalytic Performance and Self-cleaning Ability Xiaojuan Zhao, Xiang Lv, Hongda Cui, Tianhe Wang* Chemicobiology and Functional Materials Institute, Nanjing University of Science and Technology, Nanjing, 210094, PR. China
The corresponding author e-mail address: [email protected] Tel.: +86 25 84315042
Abstract We report a novel technique to fabricate bismuth stannate/silver@silver chloride (Bi2Sn2O7/Ag@AgCl) films on conventional glass substrates. The film exhibited a remarkable self-cleaning capability against organic dyes under visible light. Porous Bi2Sn2O7 (BSO) film was first sintered on a glass substrate, followed by implantation of AgCl in it and photo-induction to produce Ag@AgCl. The degradation of organic dyes and photoelectrochemical studies indicate that, compared with BSO film, Bi2Sn2O7/Ag@AgCl film had a much improved photocatalytic ability, probably due to the enhanced electron transfer efficiency and synergistic effect of visible light absorption of the two semiconductors. The possible mechanism of this marked improvement was investigated and interpreted in terms of electrons and holes separation efficiency and charge circulation routes at the interfaces within the Bi2Sn2O7/Ag@AgCl composite film. The film provided in this study may well have practical applications due to its simplicity of preparation, excellent photocatalytic
!"
ability and reasonable stability. Keywords: Bi2Sn2O7/Ag@AgCl; film; photocatalysis; self-cleaning; mechanism. 1. Introduction Water pollution caused by industrialization and urbanization has become a critical issue and brought serious threats to living environments [1, 2]. Photocatalysis has received extensive interests because of its potential applications in converting solar energy efficiently [3-5]. Under the sunlight irradiation, photocatalysts can split water into H2 and O2, convert CO2 into fuels and chemicals, and remove organic compounds in an environmentally friendly manner [6-8]. TiO2 is the most studied photocatalyst [9] due to its easy availability, chemical stability and low price. However, TiO2 can utilize no more than 5% of total solar energy because of its large band gap (3.2 eV), and this is the main reason preventing its practical applications [10]. Thus, there is an urgent need to develop novel visible-light responsive photocatalysts. Recently, Bi-containing compounds, including BiVO4 [11], Bi2WO6 [12] and Bi2Sn2O7 [13], have attracted enormous attention for their excellent photocatalysis applications. Among these, Bi2Sn2O7, with a narrow band gap of 2.73 eV, has been widely studied due to its unique properties of ionic conductivity and efficient visible-light-driven photocatalysis [14-16]. Nevertheless, the photocatalytic activity of pure BSO is relatively low and there will be little prospects in real applications. A potential solution is to prolong electron-hole pairs separation and enhance charge transfer efficiency by combining two or more semiconductors with appropriate band gap energy levels [17, 18]. The surface plasmon resonance of Ag@AgCl as
#"
photocatalyst has been well studied due to the enhanced absorption in the visible light region. The “@” in Ag@AgCl means that Ag nanoparticles are anchored on AgCl surface which can greatly extend the photo-response of AgCl in the whole visible light region by localized surface plasmon resonance effects (LSPRs) and also can enhance the stability of AgCl under visible light irradiation [19]. According to relevant research, when Ag@AgCl is combined with another semiconductor, strong interaction took place at interface and this subsequently enhances the separation of charge thus improving photocatalysis [20-22]. In this study, we report, for the first time, a novel technique to fabricate stable BSO film and Bi2Sn2O7/Ag@AgCl film. The crystalline phases, morphology, chemical composition, and photocatalytic activity of the as-prepared film were investigated in detail. Possible mechanism of photocatalysis enhancement for as-prepared Bi2Sn2O7/Ag@AgCl film is analyzed and discussed. A comprehensive comparison between BSO film and Bi2Sn2O7/Ag@AgCl film photocatalysts was made, which provided the basis for exploring and discussing the enhancement mechanism of photocatalytic activity. 2. Experimental section 2.1 Materials Bismuth nitrate pentahydrate [Bi (NO3)3·5H2O, 99%], potassium stannate trihydrate [K2SnO3·3H2O, 95%], ammonia solution [NH3· H2O, 26%], hydrochloric acid [HCl,
36.0%], silver nitrate [AgNO3, 99.8%], acid red 18 (AR-18, 99%) and methyl orange (MO, 96%) were purchased from Aladdin, Shanghai. All reagents were analytically
$"
pure and used as received without further purification. Deionized water was used through out the experiments. Conventional silicate glass was used as the substrate. 2.2 Preparation of Bi2Sn2O7 powders The Bi2Sn2O7 powder was synthesized by hydrothermal method. In a typical procedure, 4.38 g of Bi (NO3)3·5H2O and 2.70 g of K2SnO3·3H2O were mixed thoroughly in 90 ml deionized water. Under vigorous stirring, the pH value of the mixture was adjusted to 12 by dropwise addition of ammonia solution. Then the mixture was stirred for 1 h continuously to promote the precipitate reaction. The mixture was then transferred into a 200 ml Teflon-lined stainless steel autoclave, in which water was added to ca. 80% of the total volume. After heating the autoclave at 180
for 24 h, the autoclave was cooled down to room temperature naturally. The
product was collected, washed, and vacuum dried at 60
.
2.3 Preparation of photocatalyst film 2.3.1 Preparation of Bi2Sn2O7 film 15 × 20 mm conventional silicate glass substrates were thoroughly cleaned with detergent followed by sequential rinsing with distilled water and finally drying in an oven. 0.2 g as prepared Bi2Sn2O7 powder, Triton-X 100 (0.08 mL), acetic acid (0.06 mL) and absolute ethanol (1.0 mL) were grinding-mixed in agate pestle and mortar, the grinding process lasted for about 10 minutes. After that, the solution was sonicated for 30 min to obtain uniformly dispersed Bi2Sn2O7 slurry. To prepare film on glass substrate, the slurry was then dropped on a clean glass substrate and spin-coated with 1000 rpm that lasted about 30 s. Afterwards it was dried at 60
%"
for
10 min. Then, the Bi2Sn2O7 film was calcined at 250
in air in a muffle furnace for 1
h for Triton-X elimination. 2.3.1 Preparation of Bi2Sn2O7/Ag@AgCl film The as prepared Bi2Sn2O7 film was immersed in the AgNO3 solution (0.01 M) under vacuum for 10 mins followed by washing with deionized water in order to remove the residual solution. Next, the sample was soaked in HCl solution (0.01 M) for 10 min under vacuum and washed with deionized water subsequently. Such immersion cycle was repeated for three times to complete AgCl implantation in the Bi2Sn2O7 film. Next, the Bi2Sn2O7/AgCl film was irradiated under 300 W Xe arc lamp for a period of time, allowing the preparation of Bi2Sn2O7/Ag@AgCl film to be finally completed. The as-prepared composite films were denoted as Bi2Sn2O7/Ag@AgCl-n, where “n” represents 10, 20 and 30 mins of photoreduction, respectively. 2.4 Characterization The crystalline phases were investigated with a Brucker D8 advanced X-ray diffraction (2 = 20°~80°) equipped with Cu Ka radiation source ( = 1.5406 Å). Scanning electron microscopy (SEM, HITACHI S-4800, Japan) equipped with an energy-dispersive X-ray spectroscope (EDX) was used to observe the surface morphology, actual thickness and chemical compositions of the prepared films. Atomic force microscopy (AFM) was also employed to characterize the surface and particle morphology of the sample. The specific surface area and average pore diameter of the sample were measured by Brunauer-Emmett-Teller (BET) equation based on the N2 adsorption-desorption isotherm equipped with an automated surface
&"
area and porosity analyser (Micromeritics ASAP2020 USA). The chemical composition of prepared films was determined by X-ray photoelectron spectroscopy (XPS) on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg K (h = 1253.6 eV) source. The binding energy of the adventitious carbon (C1s) line at 284.6 eV was used for calibration. UV-vis diffuse reflectance spectra (DRS) was recorded on a UV-vis spectrophotometer (Shimadzu UV-3100, Japan). 2.5 Photocatalytic test The photocatalytic performance of the as-prepared samples was evaluated by the degradation of AR-18 and MO aqueous solution under visible irradiation using a 300 W Xe arc lamp with a 420 nm cut off filter. Photocatalytic reactions were performed in a glass container equipped with a cooling water circulator placed on the magnetic churn dasher, and the distance of light source and solution surface is 20 cm. In each experiment, a single sample of the prepared film was placed in AR-18 or MO aqueous solution (5 mg/L, 20 mL) in the reactor. Prior to irradiation, the reaction system was magnetically stirred for 30 min in the dark to achieve adsorption/desorption equilibrium. Then 1 mL solution was taken out in the time before Xe lamp was turn on. During the irradiation, 1 mL solution in photocatalytic reaction was very carefully taken out at every 15 min (AR-18) or 30 min (MO) interval and the concentration was analyzed by a UV-vis spectrometer. After the analysis, this 1 mL solution was carefully returned to the original system. All reported data in this paper were the average values of three experimental replicates and the error bar represents the standard error of mean.
'"
2.6 Photoelectrochemical measurement Photoelectrochemical measurements were carried out on a CHI660E workstation in a standard three-electrode configuration with 1 M Na2SO4 solution as the electrolyte. A platinum wire and a saturated calomel electrode were used as the counter and reference electrodes. The working electrode was prepared by forming film on FTO glass (15 × 20 mm) in the same way as mentioned in the section of 2.3. The electrochemical impedance spectroscopies (EIS) was performed at the open circuit potential over the frequency range from 0.1 Hz to 105 Hz. The magnitude of the modulation signal was 5 mV. The photocurrent measurement was carried out under discontinuous illumination of 300W Xe arc lamp with a 420 nm cut off filter via four on-off cycles with interval time of 30 s. 3. Results and discussion 3.1 Formation of film The appearance of the as-prepared film samples are as given in Figure 1. Apparently, the films look smooth and uniform. From Figure 1a and 1b, it can be seen that after the deposition of AgCl, the appearance of the film virtually showed no obvious change. However, the film changed to dark brown as a result of visible light irradiation, indicating the formation of Ag in the film (Figure 1c).
("
Figure 1. Digital photographs of the (a) BSO film, (b) Bi2 Sn2O7/AgCl heterostructure film (without visible light irradiation) and (c) Bi2Sn2O7/Ag@AgCl-20 film (after visible light irradiation).
The fabrication process of Bi2Sn2O7/Ag@AgCl film is illustrated in Scheme 1. And the mechanism of Bi2Sn2O7/Ag@AgCl formation can be described as follows. BSO film (Figure 1a) is porous, by soaking in the AgNO3 solution under vacuum, the porous structure of BSO film was impregnated with AgNO3. When this film was further impregnated in NaCl solution, Na+ and Cl- entered the porous structure, encountering Ag+ and (NO)3- that were already there. This allowed stable crystals of AgCl to be precipitated within the porous structure of BSO film to form Bi2Sn2O7/AgCl, as shown in Figure 1b. Next when this film is subjected to irradiation in visible light, tiny crystals of Ag were produced on the surface of AgCl crystals by photo-reduction, which was accompanied by the darkening of the film, as clearly shown in Figure 1c. )"
Scheme 1. Schematic illustrating the fabrication process and mechanism of Bi2Sn2O7/Ag@AgCl film
3.2 Characterization The crystal phases of the as-obtained films (scraped down from the glass substrates) were characterized by XRD pattern. Figure 2 shows the XRD spectra of the as-prepared films. The diffraction of the BSO film matched well with the JCPDS card No. 87-0284 with the major peaks at 2 = 28.7°, 33.3°, 47.8° and 56.7°, which represents the formation of a cubic structured phase with a=b=c=10.66 Å. The main characteristic diffraction peaks for the Bi2Sn2O7/Ag@AgCl films are sharp and well identified, indicating the AgCl crystals are well formed [23]. The peaks located at 28.6°, 33.3°, 46.1°, 54.8°, 57.3° and 77.1°, which can be indexed to the (111), (200), (220), (311), (222) and (420) planes of the dimensional face-centered cubic phase of AgCl (JCPDS 31-1238) [24–26]. Additionally, the weak peak at 38.2° suggests that there
were
small
quantity
of
Ag
phase
(JCPDS
No.04-0783)
in
the
Bi2Sn2O7/Ag@AgCl films, which may suggest that small amount of Ag was formed by the photoinduction at the external surfaces of AgCl. From these analysis, it is reasonable to conclude that Ag/AgCl was implanted in the Bi2Sn2O7, thus allowing *"
Bi2Sn2O7/Ag@AgCl film to be successfully synthesized.
Figure 2. XRD patterns of as-prepared films.
The morphologies and microstructures of BSO film and Bi2Sn2O7/Ag@AgCl-20 film are revealed by SEM photos presented in Figure 3. The BSO film consists of well dispersed and approximately spherical particles with a smooth surface and exhibit a narrow range of particle size distribution with the average particle size around 50 nm. Among these small BSO crystals are fine gaps or channels that form the porous structure of the film. Such porous structure enables functional species to be implanted in it. For the Bi2Sn2O7/Ag@AgCl-20 film, it can be clearly seen in Figure 3c and 3d that some nanoparticals with a diameter of about 400 nm were deposited on the BSO and the surfaces of BSO film became rough as a result. Also, Ag@AgCl nanoparticles were cube-like, some what different from that of BSO crystals. Figure S1 displays the EDX spectra of Bi2Sn2O7/Ag@AgCl-20 film, which only shows O, Sn, Bi, Ag and Cl elements. The element mapping images are also shown in the inset of Figure S1. These results further confirmed the coexistence of BSO and Ag@AgCl without contamination by other chemical species.
!+"
Figure 3. SEM images of BSO film (a and b) and Bi2Sn2O7/Ag@AgCl-20 film (c and d).
Atomic force microscopy (AFM) is also a useful technique to determine the surface and particle morphology of the sample. Figure 4 shows the 2D (a) and 3D (b) AFM images of BSO films. And it shows that bulges and dents on the surface are nearly to the same degree, which suggests a reasonably homogeneous surface. This is in accordance with the SEM. The cross-sectional SEM image (Figure 4c) suggests that the thickness of the as-prepared BSO film is around 1.3 m.
Figure 4. AFM micrograph: 2D image (a), 3D image (b) and cross-sectional SEM image of BSO film (c).
The nitrogen adsorption−desorption measurement is employed to investigate the !!"
porous structure of the as-prepared Bi2Sn2O7/Ag@AgCl-20 film, as shown in Figure S2. N2 adsorption-desorption isotherm can be classified as a type IV isotherm with a hysteresis loop, exhibiting the presence of a mesoporous structure [27, 28]. The BET specific surface area of Bi2Sn2O7/Ag@AgCl-20 was measured to be 69.9 m2 g-1, with averaged pore size and specific volume being 8 nm and 0.39 cm3 g-1, respectively. And the pore-size distribution plots calculated using the BJH equation from the absorption branch of the isotherm are shown in the inset of Figure S2, it is evident that the sample shows an average pore diameter of about 8 nm with a slightly broad distribution of pore size. The results indicate that Bi2Sn2O7/Ag@AgCl-20 sample is a porous material. The chemical components of the as-prepared BSO film and Bi2Sn2O7/Ag@AgCl-20 film were investigated by XPS measurements, as displayed in Figure 5. The survey XPS spectrum (Figure 5a) confirms the existence of elements Bi, Sn, and O in BSO film and elements of O, Bi, Sn, Ag, Cl in the Bi2Sn2O7/Ag@AgCl-20 film. The Bi 4f fine XPS spectrum of the as-prepared films presented in Figure 5c shows that the binding energies of Bi 4f7/2 and Bi 4f5/2 for BSO film occur at 157.7 eV and 163 eV, respectively, which are indicative of Bi3+ [29-31]. XPS signals of Sn 3d for BSO film (Figure 5d) were observed at binding energies of 485.1 eV (Sn 3d5/2) and 493.6 eV (Sn 3d3/2), indicating the presence of Sn4+ [32]. The peaks corresponding to Bi 4f and Sn 3d in the XPS spectra of the Bi2Sn2O7/Ag@AgCl-20 film slightly shift toward higher binding energies as compared to BSO, indicating that there is a strong interaction between BSO and Ag@AgCl in the film. The interaction may affect the
!#"
formation of electron transfer channel, which may improve the photoinduced electron-hole pairs separation rate during the photocatalytic process [33]. Figure 5e illustrates the Ag 3d level spectrum. The Ag 3d5/2 and Ag 3d3/2 spin-orbital photo-electrons appear at binding energies of 366.9 eV and 373 eV from AgCl, respectively. Meanwhile, the other two peaks centered at 367.6 eV and 373.8 eV analyzed by the XPS peak fitting program correspond to the metallic Ag [34-36]. This finding indicates that Ag+ ions are successfully reduced to metallic Ag through visible irradiation. The Cl 2p XPS spectrum in Figure 5f displays double peaks at 197 eV and 198.6 eV, which could be assigned to the binding energies of Cl 2p 3/2 and Cl 2p1/2, respectively [37].
!$"
Figure 5. XPS survey spectrum (a) and XPS spectra of O (b), Bi (c), Sn (d), Ag (e) and Cl (f) of the Bi2Sn2O7/Ag@AgCl-20 (BAA-20) film.
Light absorption ability of photocatalysts is very important for their application in degradation of dye pollutants. The properties of as-prepared samples were investigated by the UV–vis DRS. Figure 6 shows the UV–vis spectra of the substrate of silicate glass and as-prepared films. It is clear that the silicate glass has no visible light absorption, and all the photocatalyst films have absorption in the visible range. In contrast to the BSO, Bi2Sn2O7/Ag@AgCl films have stronger absorption in visible light range, suggesting that it is capable of consuming the visible light energy more !%"
efficiently for the degradation of dye pollutants. The inset of Figure 6 shows that the Bi2Sn2O7/Ag@AgCl-20 film has the strongest absorption, suggesting that it can utilize the visible light most effectively and this is one of the most important properties for improving photocatalytic activity.
Figure 6. UV–vis diffuse reflectance spectra of glass and as-prepared films.
3.3 Photocatalytic activities The photodegradation of AR-18 and MO were investigated respectively in the presence of different films under visible light irradiation. The results are displayed in Figure 7. Figure 7a and 7c show the photodegradation rates of dye, where C0 is the concentration of dye before irradiation and C is the concentration after irradiation for a fixed interval of time. For comparison, the same photodegradation experiments of AR-18 and MO without catalyst were also carried out, showing no obvious degradation after irradiated for 75 min (AR-18) and 150 min (MO), respectively. From the degradation results, 42.88% of AR-18 and 31% of MO were degraded by BSO film. In contrast, the Bi2Sn2O7/Ag@AgCl-10 film and Bi2Sn2O7/Ag@AgCl-30 film degraded 67.3% and 86.3% for AR-18 after irradiation for 75min, and 68% and
!&"
89.1% for MO after irradiation for 150 min. Meanwhile, the Bi2Sn2O7/Ag@AgCl-20 sample exhibited the highest degradation rate of about 97.9% for AR-18 and 95% for MO, respectively. As shown in Figure 7b and 7d, the peak intensity of AR-18 and MO decreased rapidly with irradiation time at the presence of Bi2Sn2O7/Ag@AgCl-20 film. According to these results, BSO modified by Ag@AgCl had significantly improved photocatalytic activity over the pure BSO film. Furthermore, the TOC tests were measured to investigate the mineralization of MO and AR-18 in the presence of Bi2Sn2O7/Ag@AgCl-20 film sample, and the results are displayed in Figure S3. The results show that the mineralization yield reaches a value of 73% for the degradation of AR-18 after 75 min of irradiation, which is 69% for MO after 150 min of irradiation. From the TOC changes, it can be confirmed that most of the AR-18 and MO molecules were decomposed into H2O and CO2 in the photocatalysis process.
Figure 7. Photocatalytic degradation rates of different catalysts on the degradation of AR-18 (a)
!'"
and MO (c) under visible-light irradiation; UV−vis spectra changes in the AR-18 (b) and MO (d) degradation using Bi2Sn2O7/Ag@AgCl-20 (BAA-20) film.
Figure 8a and 8c show the relationship between ln(C/C0) and irradiation time (t) for AR-18 and MO. The experimental data was fitted by first-order kinetics as expressed by the equation: ln (C0/C) = kappt [38, 39]. Here kapp stands for an apparent first-order kinetic constant (min-1) and t represents the irradiation time. The plot shows a liner correlation between ln(C/C0) and t. The photodegrading rate (k) of the different samples for AR-18 and MO are displayed in Figure 8b and 8d. Apparently, the Bi2Sn2O7/Ag@AgCl-20 film exhibited the highest photodegrading efficiency with kapp = 0.04735 min-1 for AR-18 and kapp = 0.01924 min-1 for MO, which is 7.25 times and 9.14 times higher than that of BSO film, respectively.
Figure 8. Plot of ln(C/C0) vs irradiation time (t) and the photodegrading rate (k) of the different samples for the degradation of AR-18 (a, b) and MO (c, d) under visible light irradiation.
We have further investigated the photodegradation of AR-18 and MO over different !("
photocatalysts by analyzing the photocatalytic graphs fitted with calculated curves with kinetic constants (Figure S4). It is apparent that the photodegradation of AR-18 and MO coincides well with the fitted curve. This suggests that our early analysis on the results earlier should be reasonably accurate. Besides, the photodegradation of the refractory pollutant of phenol in the presence of Bi2Sn2O7/Ag@AgCl-20 film was also investigated under visible irradiation and the result (Figure S5) shows that the as-prepared film exhibits high photocatalytic activity in degrading the phenol (5 mg/L, 20 mL). 3.4 Photoelectrochemical properties To analyze the enhanced photocatalytic activity, the charge separation and transport was investigated by EIS, and the results are displayed in Figure 9. Figure 9a shows the EIS of the as-prepared films. Apparently, the Bi2Sn2O7/Ag@AgCl films have smaller radius than that of the pure Bi2Sn2O7 film, suggesting a more efficient interfacial charge transfer and higher charge separation for Bi2Sn2O7/Ag@AgCl films. A significant result of this work is that the Bi2 Sn2O7/Ag@AgCl-20 film have the smallest radius, which indicates that it has the highest charge separation. Figure 9b displays the EIS of the Bi2Sn2O7/Ag@AgCl-20 film under dark and visible light irradiated conditions. It can be found that the radius is largely reduced when irradiated under visible light, indicating a large number of carriers generated on the Bi2Sn2O7/Ag@AgCl-20 film under visible light irradiation.
!)"
Figure 9. (a) EIS of Bi2Sn2 O7 film and Bi2Sn2O7/Ag@AgCl (BAA) films; (b) effect of visible light irradiation on the EIS at Bi2Sn2O7/Ag@AgCl-20 film
Additionally, photocurrent is well known for analyzing the separation efficiency of photogenerated electron-hole pairs in a photocatalyic system. A higher photocurrent represents a higher degree of charge separation which is conducive to the photocatalysis process [40, 41]. The current response of BSO film and Bi2Sn2O7/Ag@AgCl-20 film is shown in Figure S6. Clearly, the current shows abrupt changes with light on and off for both the samples. It decreased to zero when the light is turned off and rapidly recovered as soon as the light is turned on, which are caused by the movements of photogenerated electrons. It is interesting to note that compared to BSO film, Bi2Sn2O7/Ag@AgCl-20 film shows much higher photocurrent response which may be attributed to the decreasing recombination of photogenerated electron-hole pairs in consequence of the heterojunction between BSO and Ag@AgCl. As this result is in accordance with the results of organic dyes degradation, it is reasonable to assume that the Bi2Sn2O7/Ag@AgCl heterojunction was effectively constructed within the film, enhancing photo-induced charge carrier separation and interfacial charge transfer rate in photocatalytic process. 3.5 Self-cleaning Ability
!*"
Figure 10 illustrates the test of self-cleaning ability of the as-prepared Bi2Sn2O7/Ag@AgCl-20 film under visible light irradiation. In this experiment, 0.1 mL (10 mg/L) AR-18 was dropped directly onto the Bi2Sn2O7/Ag@AgCl-20 film (Figure 10a). Figure 10b shows that the dye was stable and remained there intact on the film when it was placed in dark where it was dried naturally. However, when it was irradiated under 300 W Xe Lamp for 10 min (during this process it was also dried), the dye was degraded and disappeared (Figure 10c). When Figure 13b and Figure 13c are compared, the conclusion can be drawn that the Bi2Sn2O7/Ag@AgCl film has a strong self-cleaning ability under visible light against, at least, AR-18. This result implies potential industrial applications in many aspects. A foreseeable example would be that when a large panel of low cost silicate glass coated with this Bi2Sn2O7/Ag@AgCl film is placed in the organically contaminated water that is continuously flowing through the film under natural light, it is possible to remove the organic pollutant as the contaminated water flowing continuously through the film.
Figure 10. (a) AR-18 (0.1 mL, 10 mg/L) dropped on the Bi2Sn2O7 /Ag@AgCl-20 film without any treatment; (b) dried naturally in the dark; (c) irradiated in visible light (300 W Xe Lamp) for 10 min.
3.6 Reusability #+"
We have also studied the recycling property of the as-prepared samples as it is a vital concern for the practical application. The reusability of the Bi2Sn2O7/Ag@AgCl-20 film was evaluated by cycled degradation of AR-18 with identical conditions for six times. Before each experiment, the film sample was washed by deionized water and dried at room temperature. The results are shown in Figure 11a , the sample maintained the original photocatalytic activity after 6 cycling runs and there was almost no obvious decrease for the photodegradation rate of AR-18, and the photodegradation rate is 97.9%, 96.5%, 95.8%, 97.5%, 96.2%, 96.5% orderly. The crystal structure and morphology of the sample used for the tests were examined. From Figure 11b, one can see that XRD pattern of the used sample still shows the sharp and strong diffraction peaks and is almost identical to that of the as-prepared fresh sample, indicating that the crystal structure of photocatalyst did not change after the recycled photocatalytic reaction. The SEM image (Figure 11c) shows that the sample still maintains the original morphology, and the cubic morphology of Ag@AgCl nanoparticles distributed on the BSO surface remain intact after six cycles. These results suggest that, at least for the tested cycles, the prepared film samples are stable as photocatalyst to photodegradate dyes under visible light irradiation.
#!"
Figure 11. Cycling runs for photocatalytic degradation of AR-18 over the Bi2Sn2O7/Ag@AgCl-20 film under visible light irradiation (a); XRD patterns (b) and SEM image (c) of Bi2Sn2 O7/Ag@AgCl-20 film used after six cycling experiments.
4 Photocatalytic mechanisms In order to understand the photocatalytic mechanism, the reactive species of the as-prepared photocatalyst in the photodegradation of AR-18 system were investigated by adding a series of scavengers into the reaction. In this trapping experiments, 2 mM iso-propyl alcohol (IPA), 1, 4-benzoquinone (BQ) and ammonium oxalate were used as scavengers of hydroxyl radicals (•OH), super oxide anion radical (•O2−) and holes (h+) [42–44], respectively. Figure S7 illustrates the photocatalytic degradation of AR-18 on the Bi2Sn2O7/Ag@AgCl-20 in the presence of the selected scavengers. It can be clearly seen that the degradation rate of AR-18 is 99.75% without any scavenger. In contrast, the degradation rate has a little decrease and reaches 84.8% in the presence of BQ, indicating that the •O2− is not the main reactive species in this photocatalysis system. However, the addition of IPA causes a remarkable decelerated for the degradation of AR-18 with the photodegradation rate decreasing to 40%. ##"
Besides, the addition of ammonium oxalate greatly reduces the photodegradation rate and only 22.8% AR-18 was photodegradated. Based on these results, it can be concluded that both the •OH and h+ are the dominant reactive species in this photocatalytic system and the role of •O2− is also important but secondary. It is known that the energy band gap of Bi2Sn2O7 is 2.73 eV. For a semiconductor, ECB can be determined by the empirical equation: ECB=X−Ee−0.5Eg [45], where X is the electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), Eg is the band gap energy of the semiconductor, and ECB is the conduction band potential. Accordingly, the ECB of the BSO is evaluated at 0.22 eV, and correspondingly, the valence band potential (EVB) was calculated at about 2.95 eV. Similarly, the ECB and EVB of AgCl are calculated to be −0.06 and +3.2 eV, respectively. On the basis of the band gap structures of Bi2Sn2O7/Ag@AgCl and the trapping experiments results, a schematic mechanism for the degradation of AR-18 with the as-prepared photocatalysts is proposed and illustrated in Scheme 2. It is found that AgCl cannot be excited by visible light due to its large band gap. Whereas BSO and Ag nanoparticles can absorb visible-light photons to produce photogenerated electrons and holes as irradiated under visible light. The active electrons on the Ag nanoparticles quickly transport to the CB of AgCl and react with the absorbed O2 on the surface to form H2O2, which can further produce •OH radicals responsible for the degradation of the pollutants [46, 47]. The holes remain on the Ag nanoparticles. Meanwhile, the photogenerated electrons of BSO migrate to the Ag nanoparticles to recombine with the hole in reserve. In addition, the photoexcited
#$"
holes accumulate in the VB of BSO, on the one hand, they can directly oxidize the dye molecule absorbed on the surface; on the other hand, a large amount of holes transfer to the surface of the AgCl particles because the surface was negatively charged. Subsequently, these holes combine with Cl- to form Cl0, which is also a reactive radical specie capable of degrading the dyes and then become reduced to Clso that the system remains balanced and stable [48-51]. Apparently, comparing with pure BSO film, the migration of electrons and holes in BSO, Ag and AgCl can substantially decrease the recombination of charges and thus promote photocatalytic performance. Besides, the BSO film which was embedded with enormous number of Ag@AgCl nanocrystals, could provide more adsorption sites for the pollutant. Furthermore, the adsorbed pollutant can also migrate to the decomposition centers located on the catalyst surface
Scheme 2. Proposed mechanism
5 Conclusions The pure BSO film and Bi2Sn2O7/Ag@AgCl film on conventional silicate glass were successfully prepared by a facile but novel method. The obtained films were uniform #%"
and exhibited excellent visible light catalytic performance for degradation of organic dyes, with a number of favorable characteristics including stability, reusability and self-cleaning
ability
under
visible
light
irradiation.
In
comparison,
Bi2Sn2O7/Ag@AgCl had much improved photocatalytic capability over the single phased Bi2Sn2O7 film. The possible mechanism of this improvement was investigated and interpreted in terms of electrons and holes separation efficiency and charge circulation routes at the interfaces within the Bi2Sn2O7/Ag@AgCl film. Although there have been several reports to develop Bi2Sn2O7 powder composite catalysts with improved visible light photocatalytic performance for degrading organic pollutants [13, 16, 52], Bi2Sn2O7 film and Bi2Sn2O7/Ag@AgCl film catalyst has not been reported
previously
to
the
best
of
our
knowledge.
Furthermore,
the
Bi2Sn2O7/Ag@AgCl film exhibited a remarkable self-cleaning ability under visible light irradiation leading the ways to various potential applications. A foreseeable example would be that when a large panel of low cost silicate glass coated with this Bi2Sn2O7/Ag@AgCl film is placed in the organically contaminated water that is continuously flowing through the film under natural light, it is possible to remove the organic pollutant as the contaminated water flowing continuously through the film. The study also provides a practical technique to form a functional film on conventional glass and may also offer a strategy for preparations of other functional films.
Acknowledgments The authors would like to thank the Scientific Research Foundation of Nanjing
#&"
University of Science and Technology (AE89909) for financial support.
References [1] V.J. Jones, N.L. Rose, A.E. Self, N. Solovieva, H. Yang, Evidence of global pollution and recent environmental change in Kamchatka, Glob. Planet. Change. 134 (2015) 82-90. [2] H. Hettige, M. Mani, D. Wheeler, Industrial pollution in economic development: the environmental Kuznets curve revisited, J. Dev. Econ. 62 (2000) 445-476. [3] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B-Environ. 49 (2004) 1-14. [4] K.Yu, S.G. Yang, C. Liu, H.Z. Chen, H. Li, C. Sun, S.A. Boyd, Degradation of organic dyes via bismuth silver oxide initiated direct oxidation coupled with sodium bismuth ate based visible light photocatalysis, Environ. Sci. Technol. 46 (2012) 7318-7326. [5] C. Domínguez, J. García, M.A. Pedraz, A. Torres, M.A. Galán, Photocatalytic oxidation of organic pollutants in water, Catal. Today. 40 (1998) 85-101. [6] W. Ong, L. Tan, Y.H. Ng, S. Yong, S. Chai, Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (2016) 7159-7329. [7] X. Liu, J. Iocozzia, Y. Wang, X. Cui, Y. Chen, S. Zhao, Z. Li Z. Lin, Noble metal–metal oxide nanohybrids with tailored nanostructures for efficient solar energy
#'"
conversion, photocatalysis and environmental remediation, Energy Environ. Sci. 10 (2017) 402. [8] K. Maeda, K. Domen, Development of novel photocatalyst and cocatalyst materials for water splitting under visible light, Bull. Chem. Soc. Jpn. 89 (2016) 627-648. [9] G.L. Puma, A. Bono, D. Krishnaiah, J.G. Collin, Preparation of titanium dioxide photocatalyst loaded onto activated carbon support using chemical vapor deposition: A review paper, J. Hazard. Mater. 157 (2008) 209-219. [10] S.Y. Guo, S. Han, H.F. Mao, S.M. Dong, C.C. Wu, L.C. Jia, Structurally controlled ZnO/TiO2 heterostructures as efficient photocatalysts for hydrogen generation
from
water
without
noble
metals:
the
role
of
microporous
amorphous/crystalline composite structure, J. Power Sources 245 (2014) 979–985. [11] A. Kudo, K. Omori, H. Kato, A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperature and its photocatalytic and photophysical properties, J. Am. Chem. Soc. 121 (1999) 11459-11467. [12] J. Ren, W. Wang, S. Sun, L. Zhang, J. Chang, Enhanced photocatalytic activity of Bi2WO6 loaded with Ag nanoparticles under visible light irradiation, Appl. Catal. B-Environ. 92(2009) 50-55. [13] W. Xu, J. Fang, Y. Chen, S. Lu, G. Zhou, X. Zhu, Z. Fang, Novel heterostructured Bi2S3/Bi2Sn2O7 with highly visible light photocatalytic activity for the removal of rhodamine B, Mater. Chem. Phys. 154 (2015) 30-37.
#("
[14] J.J. Wu, F.Q. Huang, X.J. Lu, P. Chen, D.Y. Wan, F.F. Xu, Improved visible-light photocatalysis of nano-Bi2Sn2O7 with dispersed s-bands. J. Mater. Chem. 21 (2011) 3872–3876. [15] H. Liu, Z. Jin, Y. Su, Y. Wang, Visible light-driven Bi2Sn2O7/reduced graphene oxide nanocomposite for efficient photocatalytic degradation of organic contaminants, Sep. Purif. Technol. 142 (2015) 25-32. [16] Y. Xing, W. Que, X. Yin, X. Liu, H.M. Asif Javed, Y. Yang, L. Kong, Fabrication of Bi2Sn2O7-ZnO heterostructures with enhanced photocatalytic activity, RSC. Adv. 5 (2015) 27576-27583. [17] M. Agrawal, S. Gupta, A. Pich, N.E. Zafeiropoulos, M. Stamm, A Facile Approach to Fabrication of ZnO-TiO2 Hollow Spheres, Chem. Mater. 21 (2009) 5343-5348. [18] S. Li, S. Hu, W. Jiang, Y. Liu, J. Liu, Z. Wang, Facile synthesis of flower-like Ag3 VO4/Bi2WO6 heterojunction with enhanced visible-light photocatalytic activity, J. Colloid. Interf. Sci. 501 (2017) 156–163 [19] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911-921. [20] R. Qiao, M. Mao, E. Hu, Y. Zhong, J. Ning, Y. Hu, Facile Formation of Mesoporous
BiVO4/Ag/AgCl
Heterostructured
Microspheres
with
Enhanced
Visible-Light Photoactivity, Inorg. Chem. 54 (2015) 9033-9039. [21] L. Liu, J. Deng, T. Niu, G. Zheng, P. Zhang, Y. Jin, Z. Jiao, X. Sun, One-step synthesis of Ag/AgCl/GO composite: A photocatalyst of extraordinary photoactivity
#)"
and stability, J. Colloid. Interf. Sci. 493 (2017) 281-287. [22] D. Xu, W. Shi, C. Song, M. Chen, S. Yang, W. Fan, B. Chen, In-situ synthesis and
enhanced
photocatalytic
activity
of
visible-light-driven
plasmonic
Ag/AgCl/NaTaO3 nanocubes photocatalysts, Appl. Catal. B-Environ. 191 (2016) 228-234. [23] L. Ai, C. Zhang, J. Jiang. Hierarchical porous AgCl@Ag hollow architectures: Self-templating synthesis and highly enhanced visible light photocatalytic activity, Appl. Catal. B-Environ. 142 (2013) 744-751. [24] T.S. Dhas, V.G. Kumar, V. Karthick, K.J. Angel, K.. Govindaraju, Facile synthesis of silver chloride nanoparticles using marine alga and its antibacterial efficacy, Spectrochim. ACTA. A. 120 (2014) 416-420. [25] M.R.H. Siddiqui, S.F Adil, K. Nour, M.E. Assa, A. Al-Warthan, Ionic liquid behavior and high thermal stability of silver chloride nanoparticles: synthesis and characterization, Arab. J. Chem. 6 (2013) 435-438. [26] B. Naik, V. Desai, M. Kowshik, V.S. Prasadc, G.F. Fernandod, N.N. Ghosha, Synthesis of Ag/AgCl-mesoporous silica nanocomposites using a simple aqueous solution-based chemical method and a study of their antibacterial activity on E. coli, Particuology, 9 (2011) 243-247. [27] X. Zhou, Z. Hu, Y. Fan, S. Chen, W. Ding, N. Xu, Microspheric organization of multilayered ZnO nanosheets with hierarchically porous structures, J. Phys. Chem. C. 112 (2008) 11722-11728. [28] M. Kruk, M. Jaroniec, Gas adsorption characterization of ordered
#*"
organic-inorganic nanocomposite materials, Chem. Mater. 13 (2001) 3169-3183.. [29] H. Liu, W. Cao, Y. Su, Y. Wang, X. Wang, Synthesis, characterization and photocatalytic performance of novel visible-light-induced Ag/BiOI, Appl. Catal. B-Environ. 111 (2012) 271-279. [30] H. Liu, Y. Su, Z. Chen, Z. Jin, Y. Wang, Graphene sheets grafted three-dimensional BiOBr0.2I0.8 microspheres with excellent photocatalytic activity under visible light, J. Hazard. Mater. 266 (2014) 75-83. [31] H. Liu, W. Cao, Y. Su, Z. Chen, Y. Wang, Bismuth oxyiodide–graphene nanocomposites with high visible light photocatalytic activity, J. Colloid. Interf. Sci. 398 (2013) 161-167. [32] C.D. Wagner, Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer, 1979. [33] B. Jiang, C. Tian, Q. Pan, Z. Jiang, J. Wang, W. Yan, H. Fu, Enhanced photocatalytic activity and electron transfer mechanisms of graphene/TiO2 with exposed {001} facets, J. Phys. Chem. C. 115 (2011) 23718-23725. [34] Z. Feng, J. Yu, D. Sun, T. Wang, Visible-light-driven photocatalysts Ag/AgCl dispersed on mesoporous Al2O3 with enhanced photocatalytic performance, J. Colloid. Interf. Sci. 480 (2016) 184-190. [35] P. Wang, B. Huang, Z. Lou, X. Zhang, X. Qin, Y. Dai, Z. Zheng, X. Wang, Synthesis of highly efficient Ag@AgCl plasmonic photocatalysts with various structures, Chem. Eur. J. 16 (2010) 538−544. [36] R. Dong, B. Tian, C. Zeng, T. Li, T. Wang, J. Zhang, Ecofriendly synthesis and
$+"
photocatalytic activity of uniform cubic Ag@AgCl plasmonic photocatalyst, J. Phys. Chem. C. 117 (2013) 213−220. [37] L. Dong, D Liang, R. Gong, In Situ Photoactivated AgCl/Ag nanocomposites with enhanced visible light photocatalytic and antibacterial activity, Eur. J. Inorg. Chem. 2012, 3200−3208. [38] S. Han, L. Hu, N. Gao, A.A.Al-Ghamdi, X. Fang, Efficient self-assembly synthesis of uniform CdS spherical nanoparticles-Au nanoparticles hybrids with enhanced photoactivity, Adv. Funct. Mater. 24 (2014) 3725-3733. [39] J. Krýsa, G. Waldner, H. Mšt’ánková, J. Jirkovsky´, G. Grabner, Photocatalytic degradation of model organic pollutants on an immobilized particulate TiO2 layer: Roles of adsorption processes and mechanistic complexity, Appl. Catal. B-Environ. 64 (2006) 290-301. [40] J. Jiang, X. Zhang, Sun P, L. Zhang, ZnO/BiOI heterostructures: photoinduced charge-transfer property and enhanced visible-light photocatalytic activity, J. Phys. Chem. C. 115 (2011) 20555-20564. [41] Q. Xiang, J. Yu, M. Jaroniec, Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites, J. Phys. Chem. C. 115 (2011) 7355-7363. [42] Y. Chen, S. Yang, K. Wang, L. Lou, Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of acid orange 7, J.Photochem. Photobiol. A 172 (2005) 47–54. [43] S.H. Yoon, J.H. Lee, Oxidation mechanism of As(III) in the UV/TiO2
$!"
system:evidence for a direct hole oxidation mechanism, Environ. Sci. Technol. 39 (2005) 9695–9701. [44] Y. Li, J. Wang, H. Yao, L. Dang, Z. Li, Efficient decomposition of organic compounds and reaction mechanism with BiOI photocatalyst under visible light irradiation, J. Mol. Catal. A-Chem. 334 (2011) 116–122. [45]
M.A.
Butler;
D.S.
Ginley,
Prediction
of
flatband
potentials
at
semiconductor-electrolyte interfaces from atomic electronegativities, J. Electrochem. Soc. 125 (1978) 228−232. [46] W. Li, D. Li, Y. Lin, P. Wang, W. Chen, Xi. Fu, Y. Shao, Evidence for the active species involved in the photodegradation process of methyl orange on TiO2, J. Phys. Chem. C. 116 (2012) 3552-3560. [47] X. Zhang, X. Quan, S. Chen, H. Yua, Constructing graphene/InNbO4 composite with excellent adsorptivity and charge separation performance for enhanced visible-light-driven photocatalytic ability, Appl. Catal. B-Environ. 105 (2011) 237-242. [48] Y. Xu, H. Xu, H. Li, J. Xia, C. Liu, L. Liu, Enhanced photocatalytic activity of new photocatalyst Ag/AgCl/ZnO, J. Alloy. Compd. 509 (2011) 3286-3292. [49] G. Begum, J. Manna, RK. Rana, Controlled orientation in a bio-Inspired assembly
of
Ag/AgCl/ZnO
nanostructures
enables
enhancement
in
visible-light-induced photocatalytic performance, Chem–Eur. J. 18(2012) 6847-6853. [50] F. Chen, H. Liu, S. Bagwasi, X. Shen, J. Zhang, Photocatalytic study of BiOCl for degradation of organic pollutants under UV irradiation, J. Photoch. Photobio. A.
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215 (2010) 76-80. [51] M.A. Gondal, X.F. Chang, Z.H. Yamani, UV-light induced photocatalytic decolorization of Rhodamine 6G molecules over BiOCl from aqueous solution, Chem. Eng. J. 165 (2010) 250-257. [52] W. Xu, J. Fang, X. Zhu, Z, Fang, C. Cen, Fabricaion of improved novel p-n junction BiOI/Bi2Sn2O7 nanocomposite for visible light driven photocatalysis, Mater. Res. Bull. 72 (2015) 229-234.
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