Facile Fabrication of Bi2WO6/Ag2S Heterostructure with Enhanced Visible-Light-Driven Photocatalytic Performances
© Tang et al. 2016
Received: 9 January 2016
Accepted: 17 February 2016
Published: 8 March 2016
In this report, a novel photocatalyst based on Bi2WO6/Ag2S heterostructures was prepared by a 3-mercaptopropionic acid (MPA)-assisted route at room temperature. Compared to bare Bi2WO6 and Ag2S nanoparticles, the as-formed Bi2WO6/Ag2S heterostructures exhibit enhanced photocatalytic activity for the degradation of rhodamine B (Rh B) under visible-light irradiation. This kind of enhancement in the photocatalytic activity is considered to be the synergistic effects of both the effective electron-hole separation and expansion of the light-absorption range. The pH of the solution is of vital importance to the photocatalytic activity of the as-formed Bi2WO6/Ag2S heterostructures. Under low pH value, the photosensitization process is suppressed, while under higher pH value, the photosensitization process is favored. The mechanism of the photocatalytic process was proposed by the active-species-trapping experiments, indicating that the photogenerated holes (h+) play a crucial role in the degradation of Rh B under visible light. The enhanced photocatalytic performance of this heterostructure makes it a promising material for the treatment of dye-containing wastewater.
With the widespread application of visible-light-driven photocatalysts, Bi2WO6 has attracted more and more attention because of its unique crystal structure and physicochemical properties [1–3]. As the simplest member of the Aurivillius oxide family, Bi2WO6 exhibits good photocatalytic performance under visible-light irradiation. Therefore, many efforts have been devoted for the preparation of Bi2WO6-based photocatalysts, such as the solid-state method , microwave-solvothermal method , ultrasonic synthetic method , and hydrothermal reactions . However, Bi2WO6 can only respond to visible light with the wavelength shorter than 450 nm , which accounts for only a small part of the solar light. Meanwhile, the rapid recombination of the photo-induced electron-hole pairs also greatly decreases the photocatalytic activity of Bi2WO6, which prevents it from further large-scale applications . To broaden the light-absorption range and promote the separation of photogenerated carriers of Bi2WO6, two main methods are employed. The first one is based on the element doping of Bi2WO6 (such as B, Gd, Ag, N, Ce, and F codoping) [10–14]. The other method is based on the formation of heterostructures between Bi2WO6 and other kind of materials, such as g-C3N4 , C60 , graphene , metals [18, 19], and various semiconductors.
The combination of Bi2WO6 with other semiconductors has been proved to be an effective method for the preparation of the photocatalysts with enhanced photocatalytic performances. On the one hand, the coupling of Bi2WO6 with other semiconductors will broaden the light-absorption range via the formation of intermediate energy levels. On the other hand, the recombination rate of photo-induced charge carriers will be decreased because of the charge transfer on the interfaces of heterostructures. As a result, a variety of heterostructures based on Bi2WO6 have been successfully prepared, which exhibit enhanced photocatalytic activities under visible light. For instance, Yang and coworkers reported the preparation of a BiOCl-Bi2WO6 heterojunction with a chemically bonded interface. The decomposition rate constant for rhodamine B is about 2 times faster than that for pure BiOCl (0.029 min−1) and 1.5 times faster than that for Bi2WO6 (0.041 min−1) . Zhang and coworkers have also succeeded in the preparation of a novel Bi2S3/Bi2WO6 composite photocatalyst using hydrothermal method. The apparent rate constant is calculated to be 0.0062 min−1 for the Bi2S3/Bi2WO6 composite, which is 6.2 times higher than the corresponding value of bare Bi2WO6 (0.001 min−1) . Other kinds of heterostructures such as Bi2WO6/α-Fe2O3 , Bi2WO6/TiO2 , Bi2WO6/BiOBr , Bi2WO6/BiIO4 , and Bi2WO6/BiVO4  have also been successfully synthesized, all of which exhibit enhanced photocatalytic activities as compared to bare Bi2WO6. However, developing new heterostructures based on Bi2WO6 is still a big challenge for the chemists, especially by a simple and economic method.
As a semiconductor with narrow bandgap (1.0 eV), Ag2S has been widely used in various fields such as photoconductors, photovoltaic cells, IR detectors [26, 27], photography , and luminescent devices . Because of its narrow bandgap, Ag2S can absorb light with the wavelength lower than 1000 nm, which covers the whole visible-light region. Meanwhile, the conduction band (CB) and valence band (VB) position of Ag2S is higher than the corresponding values of Bi2WO6, which can form the type-II heterostructures when coupling with Bi2WO6. These two fascinating characteristics make the Bi2WO6/Ag2S heterostructure a good candidate for the photodegradation of organic dyes. However, few reports are concerned on the fabrication and photocatalytic activity of Bi2WO6/Ag2S heterostructure. In this report, the Bi2WO6/Ag2S heterostructures were successfully prepared by a surface functionalization method using 3-mercaptopropionic acid (MPA) as the surface-functionalizing agent. The as-formed Bi2WO6/Ag2S heterostructures exhibit enhanced photocatalytic activity as compared to bare Bi2WO6 and Ag2S. Accordingly, a rational model is proposed to illustrate the key roles of Ag2S in the photocatalytic process and the corresponding photocatalytic mechanism of the as-formed heterostructure is also proposed.
All the reagents are commercially available and used without further treatments.
Synthesis of Flower-Like Bi2WO6
The flower-like Bi2WO6 were synthesized by a hydrothermal method as we have previously reported . In a typical process, 2 mmol Bi(NO3)3·5H2O and 1 mmol Na2WO4·2H2O were added to 22.5 mL of deionized water under magnetic stirring, respectively. Then, the two solutions were mixed and stirred for another 30 min. The resulting white suspension was then transferred into a 50-mL Teflon-lined autoclave and heated at 200 °C for 12 h. After cooling to room temperature naturally, the precipitates were collected by centrifugation, washed with deionized water and ethanol, and then dried at 60 °C for 6 h in vacuum.
Synthesis of Bi2WO6/Ag2S Heterostructures
Bi2WO6/Ag2S heterostructures were prepared by a surface functionalization route which employs MPA as the surface-functionalizing agent . In a typical process, 1 g of Bi2WO6 was dispersed in 40 mL of deionized water to form a slurry under magnetic stirring. Then, 20 μL of MPA was added into the above suspension, followed by vigorous stirring for 4 h to ensure the complete surface functionalization of Bi2WO6. In the next step, 0.05 g of AgNO3 was added to the above reaction mixture and the suspension was stirred for another 2 h at room temperature. At last, 0.03 g of Na2S·9H2O was added dropwise to the abovementioned system. The resulting suspensions were stirred at room temperature for another 1 h. The molar ratio between elements Ag and S was 2:1. Finally, the product was separated by centrifugation, washed with ethanol and water for several times, and dried under vacuum at 60 °C to obtain the Bi2WO6/Ag2S heterostructures. The resulting powder was collected for further characterization.
Synthesis of Ag2S Nanoparticles
For comparison purpose, Ag2S nanoparticles were also synthesized by a simple precipitation method. In a typical process, 1.0 g of AgNO3 was dissolved in 40 mL of deionized water to form a transparent solution. Then, 0.71 g of Na2S·9H2O was added to the above solution and stirred for another 1 h at room temperature. The resulting products were separated by centrifugation, washed with deionized water and absolute alcohol for 3 times, and then dried at 60 °C for 12 h in vacuum.
X-ray diffraction (XRD) patterns were monitored by a Philips X’Pert Pro Super diffractometer using Cu Kα radiation (λ = 1.5416 Å). The scanning rate of 0.05°s−1 was applied to record the patterns in the 2θ range of 10°–70°. The scanning electron microscope (SEM) characterizations were performed on the S-4800 (Hitachi) field emission scanning electron microscope (FESEM) equipped with a GENESIS4000 energy-dispersive X-ray spectroscope. The transmission electron microscope (TEM) analyses were performed using a Hitachi H-7650 transmission microscope at an accelerating voltage of 100 kV, and the high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL-2010 TEM at an acceleration voltage of 200 kV. The Brunauer-Emmett-Teller (BET) tests were determined via a Quantachrome autosorb IQ-C nitrogen-adsorption apparatus. All the as-prepared samples were degassed at 150 °C for 4 h prior to nitrogen-adsorption measurements. X-ray photoelectron spectroscopy (XPS) analysis was performed on Thermo ESCALAB 250 system with a monochromatic Al Kα source with 1486.6 eV of energy and 150 W of power. The light-absorption properties were measured using a UV-vis diffuse reflectance spectrophotometer (DRS) (Shimadzu, UV-2550) by using BaSO4 as the background at room temperature and were converted from reflection to absorbance by the Kubelka-Munk method. The photoluminescence (PL) spectra of the samples are recorded with the F-7000 FL spectrophotometer.
The photocatalytic activities of the photocatalysts were evaluated by the degradation of rhodamine B (Rh B) using a 500-W Xe lamp with a 400-nm cutoff filter. The working distance from the Xe lamp to the beaker is 20 cm. In this process, 80 mg of the photocatalyst was added into 80 mL Rh B solution (10−5 mol L−1). Temperature of the beaker containing the dispersion of Rh B and the samples was maintained at 20 °C by using circulating water during the whole process. Prior to irradiation, the mixture was magnetically stirred in the dark for 30 min to ensure the adsorption/desorption equilibrium between the photocatalyst and Rh B. Then, light was turned on, and optical power is maintained at 25.8 mW cm2. At given time intervals, 6 mL of the suspension was taken out and centrifuged at 9000 rpm to remove the residual photocatalyst powders for analysis. The clear solution was analyzed through UV-vis spectrophotometer (Agilent Cary 5000E) by recording the variations of the absorption band at 553 nm.
For the detection of active species generated in the photocatalytic reaction, active-species-trapping experiments were carried out. In this process, various kinds of scavengers such as benzoquinone (BQ) (a quencher of O2 ·−), ammonium oxalate (AO) (a quencher of h+), AgNO3 (a quencher of e −), and t-BuOH (a quencher of ·OH) were employed. And the amount of scavenger was fixed to be 10 mM except BQ, which was 1 mM. The whole process was similar to the photocatalytic experiments mentioned above.
For the preparation of the photoelectrode, the ITO (Indium tin oxide) glass was firstly cut into 3 cm (length) × 0.7 cm (width) slices, washed with water and 1 M NaOH solution several times before use. In the next step, 10 mg of the photocatalysts was dispersed in 1 mL of absolute ethanol by ultrasonication. Then, 10 μL of the resulting dispersion was drop-casted onto the ITO slice by a pipette and the area was fixed to be 0.7 cm2. The as-prepared photoelectrodes were dried in the air for further investigation. The photoelectrochemical characteristics were measured using a CHI 660B (Chen Hua Instruments, Shanghai, China) electrochemical working station with a standard three-electrode configuration under visible light provided by a 500-W Xe lamp. The as-prepared photoelectrode, Pt wire, and Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. During the electrochemical test, 0.1 M phosphate-buffered saline (pH = 7.4) was used as the electrolyte, and the optical power density on the ITO electrode was determined to be 70 mW cm2.
Results and Discussion
Characterizations of the Bi2WO6/Ag2S Heterostructures
In general, the BET surface area of the photocatalyst can greatly affect its photocatalytic performance, and a high BET surface area is usually beneficial for the improvement of photocatalytic activity [36, 37]. The BET-specific surface areas of Bi2WO6 and Bi2WO6/Ag2S were determined by nitrogen-adsorption BET method. The specific surface areas (Additional file 1: Figure S3) of pure Bi2WO6 and Bi2WO6/Ag2S were determined to be about 13.6 and 11.0 m2 g−1, respectively. Compared with bare Bi2WO6, the surface area of the as-formed Bi2WO6/Ag2S heterostructure is a little smaller. The decrease in surface area may result from the surface coverage of Ag2S, and similar experimental results have also been observed in the plasmonic nanocomposite photocatalysts Ag/AgX-CNTs (X = Cl, Br, I) . In this report, the specific surface area of Ag/AgCl-CNTs, Ag/AgBr-CNTs, and Ag/AgI-CNTs are 50.3, 20.8, and 18.4 m2 g−1, respectively, which are also a little smaller than the surface areas of CNTs (59.2 m2 g−1).
Optical Property of the As-Formed Bi2WO6/Ag2S Heterostructure
Calculated values of bandgap, conduction band (CB) and valence band (VB) of samples Bi2WO6, Ag2S, and Bi2WO6/Ag2S
Conduction band (eV)
Valence band (eV)
Photocatalytic Performance of the Bi2WO6/Ag2S Heterostructure
Photocatalytic efficiency and rate constant of the Rh B decomposition process in the presence of Bi2WO6, Ag2S, and Bi2WO6/Ag2S
Photocatalytic efficiency (%)
Rate constant (min−1)
1.318 × 10−2
1.390 × 10−3
1.860 × 10−4
2.079 × 10−2
Influence of pH on the Photocatalytic Activity of the As-Formed Bi2WO6/Ag2S Heterostructure
To study the influence of pH on the photocatalytic activity of Bi2WO6/Ag2S heterostructure, a series of photocatalytic experiments were carried out under different pH values. In this process, aqueous Rh B solution was adjusted to different initial pH values (2, 3, 4, 5, 6, 7, and 8) using diluted nitrate acid or sodium hydroxide, while keeping other conditions constant. According to the previous reports, the influence of pH mainly takes effect in two aspects. On the one hand, the pH of the solution will influence the adsorption of dye molecules via changing the surface charges of the photocatalysts . The variation in the adsorption of dye molecules will inevitably influence the photocatalytic efficiencies because the photocatalytic process mainly takes place on the surfaces of the photocatalysts. It has been reported that the adsorption of organic pollutants on the surface of the photocatalyst is a prerequisite for efficient photocatalytic degradation because the photocatalytic reaction usually takes place on the surface of the photocatalyst. Usually, strong adsorption benefits the photocatalytic degradation [52, 53]. On the other hand, the pH of the solution also exerts tremendous influence on the molecular structure of dyes, which will determine the attaching modes of Rh B molecules to the surfaces of the photocatalysts . To be specific, the Rh B molecules can attach to the surfaces of the photocatalysts by the carboxylic group or the amino group, and the attaching modes are greatly influenced by the pH of the solution. If Rh B molecules attach to the surfaces of the photocatalysts with the amino group, the photosensitization process will be unfavored. \If Rh B molecules attach to the surfaces of the photocatalysts via the amino group, the photosentization process will be favored, which could be judged by the blue shifts of absorption peaks during the photocatalytic process. So, the influence of pH during the photocatalytic process will be the synergistic effects of the two effects mention above. According to the experimental result, both the photodegradation efficiency and the photodegradation rate of Rh B show monotonous decrease when the pH of the solution increases from 2 to 8 (Fig. 7a, b). About 98 % of the Rh B molecules can be degraded after irradiation for about 90 min when the pH of the solution is 2, whereas about 83 % of the Rh B molecules can be degraded when the pH of the solution is 8. The degradation rate constant is about 0.051 min−1 when pH = 2, while the corresponding value decreases to 0.018 min−1 when the pH of the solution increases to 8.
Besides the influence on the adsorption amount of Rh B molecules, the adsorption modes of the Rh B will also be greatly influenced when the pH of the solution changes. According to the previous report, the carboxylic group of will be protonated when the pH is below 3.22  and the carboxylic group will change to its protonated states. Under this situation, the Rh B molecules mainly attached to the surfaces of Bi2WO6/Ag2S heterostructures via the carboxylic group. The benzene ring linked to the carboxylic group is twisted against the chromophoric group, making the electron injection through the carboxylic group impossible. And this effect will greatly suppress the photosensitization process. This hypothesis can be verified by the corresponding spectra of Rh B during the photocatalytic process when the pH of the solution is 2 (Additional file 1: Figure S6a). The blue shift of the absorption band is about 9 nm when the pH of the solution is 2, indicating that the photosensitization process is suppressed. When the pH increased, the Rh B molecules will attach to the surfaces of the heterostructure via the amino groups, which will enable the injection of electrons from Rh B molecules to the photocatalyst. And this kind of attaching mode will favor for the photosensitization process, which has been verified by the spectra of Rh B during the photocatalytic process under different pH values. As it is shown in Additional file 1: Figure S6b–g, the blue shifts of Rh B increase from 13 to 22 nm when the pH of the solution increases from 3 to 8, indicating that the photosensitization process is favored under high pH values.
Mechanism of the Photocatalytic Process
In this paper, a novel photocatalyst based on Bi2WO6/Ag2S heterostructures was prepared by a facile surface functionalization method. The as-prepared Bi2WO6/Ag2S heterostructure displays enhanced photocatalytic activity for the degradation of Rh B under visible-light irradiation compared to its individual components. The decomposition rate for Rh B in the presence of Bi2WO6/Ag2S is about 1.6 times higher than the corresponding value when Bi2WO6 is used as the photocatalyst. The PL and photocurrent measurement were applied to verify the effective separation of electron-hole pairs, which indicate that the enhanced separation of the charge carriers is the main reason for the enhanced photocatalytic activity of the heterostructure. The pH of the solution exerts tremendous influence on the photocatalytic activity of the as-formed heterostructures. According to the active-species-trapping experiments, the photogenerated holes (h+) are determined to be the main reactive species for Bi2WO6/Ag2S in this photocatalytic process.
This work was supported by the National Natural Science Foundation of China (Grant No. 21373106), National Basic Research Program of China (Grant No. 2011CBA00701), and Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.
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