Bi2WO6 core-shell heterostructure as carriers transfer channel for enhancing photocatalytic activity

Materials Research Bulletin 85 (2017) 140–146

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Constructing two-dimension MoS2/Bi2WO6 core-shell heterostructure as carriers transfer channel for enhancing photocatalytic activity Jun Zhanga,* , Lihai Huanga , Haoyun Jina , Yunlei Suna , Xinming Maa , Erpan Zhanga , Hongbo Wangc , Zhe Konga , Junhua Xia , Zhenguo Jia,b,** a b c

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, People’s Republic of China State Key Lab of Silicon Materials, Zhejiang University, Hangzhou, 310018, People’s Republic of China College of Automation, Hangzhou Dianzi University, Hangzhou, 310018, People’s Republic of China

A R T I C L E I N F O

Article history: Received 19 July 2016 Received in revised form 13 September 2016 Accepted 13 September 2016 Available online 14 September 2016 Keywords: Bi2WO6 MoS2 Heterostructure Photocatalysis

A B S T R A C T

A MoS2/Bi2WO6 core-hell heterostructure constructed by two-dimension MoS2 and BiWO6 nanoparticles was obtained though a simple hydrothermal reaction and the photocatalytic activity was compared. The HRTEM images show that the two-dimension MoS2 grown on the surface of Bi2WO6 nanoparticles to construct a core-shell heterostructure. In the photocatalytic degradation of dye, MoS2/Bi2WO6 composite with optimal Mo:Bi ratio demonstrated an obvious enhancement of photocatalytic activity as much as 82% than that of pristine Bi2WO6 nanoparticles. The result confirms this MoS2/Bi2WO6 core-hell heterostructure is an ideal transfer channel for photo-induced carriers. In this way, photo-induced electrons produced in MoS2 will flow to Bi2WO6; thus, photo-induced holes produced in Bi2WO6 will transfer to MoS2. This carriers' transfer will improve the separation efficiency and enhance the photocatalytic activity of composites. This two-dimension MoS2 composite is finally confirmed an effective approach to improve the photocatalytic activity of semiconductors. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Photocatalysis is a photoelectrochemistry reaction to transform solar energy to electrochemical or chemical energy which can be applied in pollution degradation and hydrogen production. In this energy conversion, the use efficiency of solar relies on the separation rate of photo-induced electrons and holes. In fact, main photo-induced carriers get recombination in the photocatalysts and only few electrons and holes can move to the surface and participate in redox reaction. To solve this problem, pristine photocatalysts always composited with other semiconductors to construct heterostructure [1]. Semiconductor heterostructure is an important strategy to improve the electrical properties such as separation efficiency of carriers. In this heterostructure, the energy band structures of component semiconductors are beneficial to the transferring of electrons and holes through the interface between the heterostructure results in a longer carrier lifetime.

* Corresponding author. ** Corresponding author at: College of Materials and Environmental Engineering, Hangzhou 310018, People’s Republic of China. E-mail addresses: [email protected] (J. Zhang), [email protected] (Z. Ji). http://dx.doi.org/10.1016/j.materresbull.2016.09.013 0025-5408/ã 2016 Elsevier Ltd. All rights reserved.

In this photocatalysts, TiO2 is the most important photocatalyst for its high photocatalytic ability, cheap, stability, and nontoxicity [2]. However, the wide bandgap (3.0–3.2 eV) of TiO2 results in a low harvest efficiency of solar (only about 4%). Recently, as a photocatalyst with visible light absorption, Bi2WO6 has attracted wide attention [3–14]. However, the quantum separation efficiency of Bi2WO6 is much low. So Bi2WO6 was always composited with other semiconductors to enhance separation rate of photo-induced electrons and holes such as TiO2 [15–27], or others [28–37]. In this works, Bi2WO6 was rarely combined with two-dimensional materials such as sulfide. Two-dimensional material MoS2 has a high specific surface area and narrow band gap which shows potential in composites. Recently, MoS2/TiO2 composites are developed to exhibit excellent enhancement for lithium-ion batteries and photocatalytic hydrogen production [38–51]. In this research, MoS2 nanosheets were growth on the surface of TiO2 to construct a MoS2/TiO2 heterostructure. As the conduction band of MoS2 is under that of anatase TiO2, photo-induced electrons in TiO2 will transfer into MoS2 for prolonging the lifetime. Furthermore, compared with zero-dimension nanoparticles, two-dimension MoS2 can act as a network to extend the transmission distance and life of carriers. Even compared with two-dimension graphene, MoS2 also can product photocatalytic electron-holes to inject other

J. Zhang et al. / Materials Research Bulletin 85 (2017) 140–146

semiconductor results in a synergistic effect. So that twodimension MoS2 is an ideal comparing substrate. However, MoS2/Bi2WO6 composites have not been reported. It is still a challenge to obtain MoS2/Bi2WO6 heterostructure. In previous work, we demonstrates a strategy to employing specific exposed {001} facets of anatase TiO2 as substrate to construct excellent heterostructure with Bi2WO6 [27]. This structure brings a 35% enhancement of photocatalytic activity. Here, we develop a new strategy to obtain MoS2/Bi2WO6 heterostructure. In this work, we will demonstrate a synthesis of MoS2/Bi2WO6 core-shell heterostructure. This MoS2/Bi2WO6 heterostructure show an obvious enhancement of photocatalytic activity as 82% than that of pristine Bi2WO6 nanoparticles. This MoS2/Bi2WO6 core-shell heterostructure is confirmed to be an ideal carrier's transfer channel to improve the separation efficiency of photo-carriers and enhance photocatalytic activity. 2. Experimental section 2.1. Synthesis of samples All chemicals used in this experiment were of analytical-grade without further treatment. The Bi2WO6 (BWO) nanoparticles were prepared via a solvothermal method. A typical process was performed as following: approximately 0.97 g Bi(NO3)3
5H2O and 0.33 g Na2WO4
2H2O were dissolved in 60 ml of ethylene alcohol, after being stirred for 2 h, the transparent solution was added into a 100 ml Teflon-lined autoclave up to 60% of the total volume. Then the autoclave was sealed in a stainless steel tank and heated at 180  C for 24 h, in which the heating rate is 6  C/min. Subsequently, the reactor was cooled to room temperature naturally, the products were washed by de-ionized water and alcohol in turn and then the BWO nanoparticles were obtained after being dried at 102  C in air. MoS2/Bi2WO6 heterostructures were fabricated via a further hydrothermal route. In this process, relative Na2MoO4
2H2O and thiourea were dissolved in 60 ml deionized water, then 150 mg BWO was mixed with the Na2MoO4
2H2O and thiourea solution in a 100 ml Teflon-lined autoclave. Then the mixtures were kept at 200  C for 24 h. The precipitates were separated from the suspension by centrifugation (10,000 rpm, 15 min). To wash the powder, the products were further suspended and centrifuged in absolute ethanol for three times, followed by drying at 102  C for 12 h. Then the obtained powders were calcined at 300  C for 4 h. Samples obtained from different Mo:Bi mole ratios were respectively labeled as BMS1 (0.62 mg Na2MoO4
2H2O and 1.24 mg thiourea, Mo:Bi = 1%), BMS2 (1.55 mg Na2MoO4
2H2O and 3.1 mg thiourea, Mo:Bi = 2.5%), BMS3 (3.1 mg Na2MoO4
2H2O and 6.2 mg thiourea, Mo:Bi = 5%) and BMS4 (4.65 mg Na2MoO4
2H2O and 9.3 mg thiourea, Mo:Bi = 7.5%). Another hydrothermal route (300 mg Na2MoO4
2H2O and 600 mg thiourea were dissolved in 60 ml deionized water) was executed to obtained pure MoS2 (MS).

The C 1s peak at 284.8 eV of the adventitious carbon was referenced to rectify the binding energies. UV–vis diffuse reflectance spectroscopy (DRS) was performed using a UV–vis spectrophotometer (UV-3600, Shimadzu, Tokyo, Japan) with a multipurpose large sample compartment (MPC-3100, Shimadzu, Tokyo, Japan). BaSO4 was used as a reflectance standard in the DRS measurement. The Brunauer Emmett Teller (BET) specific surface areas were obtained using a nitrogen adsorption apparatus (JW-BK, Beijing, China). Fluorescence (FL) emission spectra were detected with a fluorescence spectrophotometer (RF-530TPC, Shimadzu, Japan) using a 340 nm line from a xenon lamp. 2.3. Photocatalytic experiments To measure the photocatalytic activity of the obtained samples, methylene blue (MB) aqueous solutions were degraded under UV– vis irradiation in the presence of a photocatalyst, and the decomposition rates were examined. In the degradation, a 250 W high-pressure mercury lamp was employed as a light source in the experiment. The lamp was placed 8 cm above the liquid surface. Approximately 20 mg of photocatalyst were added into 100 ml of 1 10 5 M MB aqueous solution. The mixed solution was stirred continuously in the dark for 60 min prior to photodegradation to reach adsorption-desorption equilibrium. The residual concentration of the dye was measured. Then, 3 ml of solution were extracted every 20 min during the photocatalytic process to test the residual concentrations of MB. Dye concentration was evaluated by measuring the change in maximum absorbance through UV–vis spectrometry (UV-3600, Shimadzu, Tokyo, Japan). The absorbance peak at around 664 nm was selected, and the residual concentration was obtained by evaluating the intensity ratio between the residual and original MB solutions. 3. Results and discussion 3.1. Crystal structure and morphology The XRD patterns of all samples are shown in Fig. 1. From the results, we can compare the crystal structures and phases of BWO, BMS1, BMS 2, BMS3, BMS4, and MS. An end-centered crystal structure can be identified in the patterns of BWO and composites (JCPDS data file No. 73-2020). All the peak intensities of BWO in composites showed obvious enhanced after second hydrothermal route, which could be ascribed to the re-crystallization in higher temperature. Though comparing the full width at half maximum of the (113) peaks of BWO, we can calculate the crystallite sizes of pristine BWO and BWO in the composites which are listed in Table 1. The crystallite sizes of composites enlarged obviously which resulted in decrease of the high specific surface area of

2.2. Characterizations An X-ray diffraction (XRD) (TD-3500, Dandong Tongda Instrument, Dandong, China) was empolyed to measure the crystal structures and phases of the samples with Cu-Ka radiation at a scan rate of 0.02 s 1. The accelerating voltage and the applied current were set as 30 kV and 20 mA, respectively. Morphologies and microstructures of the samples were characterized by a highresolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S-Twin, FEI, Hillsboro, USA). X-ray photoelectron spectroscopy (XPS) measurements were made on a Kratos AXIS Ultra DLD (Kratos, Japan) spectrometer with a charge neutralizer to gain information on the chemical binding energy of the photocatalysts.

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Fig. 1. XRD patterns of BWO, BMS1, BMS2, BMS3, BMS4 and MS.

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Table 1 Structural information of the samples prepared with different experimental conditions. Samples

Mo:Bia

SBETb (m3/g)

Crystallite sizec (nm)

Bandgap (eV)

kd

BWO BMS1 BMS2 BMS3 BMS4 MS

– 1% 2.5% 5% 7.5% –

72 54 52 55 60 –

11.1 16.3 16.3 18.1 15.5 –

2.92 2.43 2.30 2.30 2.29 –

0.00584 0.00639 0.00817 0.01063 0.00606 0.00659

a b c d

The mole ratio. SBET is the specific surface area. Which are calculated from the (113) peaks of BWO. The degradation rate of MB under UV–vis light.

composites. The sample obtained by hydrothermal route using 300 mg Na2MoO4
2H2O and 600 mg thiourea as precursors could be classified as MoS2 (JCPDS data file No. 87–2416), and no other peak was found. Three MoS2 peaks were found at approximately 14.4 , 33.5 , and 58.4 which could be ascribed to the (002), (101), and (110) facets of MoS2. However, no MoS2 diffraction peaks can be found in the diffractogram of composites, which could be assigned to the small amount of MoS2 and high dispersion on the BWO surface. To obtain the micro-feature and structure of composite, HRTEM was performed. Fig. 2 demonstrates the HRTEM images of BMS3. The low-resolution image in Fig. 2A and B shows that the Bi2WO6 nanoparticles with a size of 15–20 nm are covered with MoS2 nanosheets. In the high-resolution TEM images, the few-layered MoS2 nanosheets grown along the [002] index on the {113} facets

of Bi2WO6 can be found to be transparent and flexible (Fig. 2C). More important, this MoS2/Bi2WO6 heterostructure showed a faceto-face contact and formed a core-hell structure (Figs. 2D and 3). This tight face-to-face contact between MoS2 and TiO2 can ensure favorable carriers transmission between the two semiconductors. 3.2. XPS spectra and UV–vis DRS The BMS3 was analyzed by XPS to study the surface chemical states. Four elements Bi, W, O and Mo can be distinguished in the XPS survey spectrum shown in Fig. 3. Fig. 3A shown a local XPS spectrum of Bi, and there are two peaks at the binding energies of 164.5 and 159.2 eV can be assigned to Bi 4f5/2 and 4f7/2 respectively, which mean Bi3+ in BWO. The binding energies of 37.6 and 35.5 eV in Fig. 4B are W 4f5/2 and 4f7/2 respectively in the form of BWO. A

Fig. 2. HRTEM images of BMS3 (A-B) low resolution, (C-D) high resolution.

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Fig. 3. XPS spectra of (A) Bi 4f, (B) W 4f, (C) O 1s and (D) Mo 3d for BMS3.

peak at 530.2 eV is O 1s (Fig. 4C). The Mo 3d spectrum is shown in Fig. 4D, in which the binding energies of Mo 3d3/2 and 3d5/2 are located at 232.7 and 229.4 eV, respectively, corresponding to Mo4+.

As the peaks of S 1s at 162.0 and 161.2 eV for S 2P1/2 and S 2P3/2 were covered by Bi 4f peaks, the spectrogram of S are not shown here. The XPS results confirm the exiting of MoS2. UV-vis DRS was executed to obtain the change in the band gap of MoS2/Bi2WO6. Fig. 4A illustrates the UV–vis DRS spectra of all samples. In the UV region, the absorption intensities of the composites show some decrease. This can be ascribed to the lower absorption intensity of surface MoS2. Moreover, compared with pure BWO, the MoS2/Bi2WO6 composites show more obvious changes in the absorption of visible light. After composited, the absorbance intensities were elevated in the overall visible light region and that increased according to the enhancement of MoS2 composites. But the elevation was not obvious when the ratio of Mo exceeded 5%. Moreover, obvious red shifts of the spectrum onset can be found. It seems the surface composite MoS2 will bonding with the surface atoms of BWO and change the energy binding of composites. The precise band gaps were calculated as shown in Fig. 4B and the calculated results are listed in Table 1. The calculated band gaps of BWO, BMS1, BMS2, BMS3 and BMS4 were 2.92, 2.43, 2.30, 2.30 and 2.29 eV, respectively, which can confirm the continuous red shift. It seems when the ratio of Mo exceeded 5%, the energy banding of composites have little change according to the increase of MoS2. It can be understood as: initial MoS2 covered the surface of BWO and formed bonding with BWO molecules which can influence the energy binding of composites; and overmuch MoS2 covered the surface of initial MoS2 which had little influence on the energy binding. MS showed similar absorption intensity in all the UV and visible light region without obvious spectrum onset. 3.3. Photocatalytic activity

Fig. 4. (A) UV–vis diffuse reflectance spectra of BWO, BMS1, BMS2, BMS3, BMS4 and MS; (B) the plots of [ahv]1/2 vs. photon energy of BWO, BMS1, BMS2, BMS3 and BMS4. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

To simulate pollution treatment in waste water, we chose to degrade dye as MB in aqueous solution to investigate the photocatalytic activity of the obtained samples. MB molecules

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are very stable and the concentration did not reduce even expose in UV light at 60 min which is shown as the black dots in Fig. 5A. Then the MB aqueous solutions were mixed with photocatalysts. As photocatalysts will adsorb some MB molecules on the surface, before degradation, the adsorption concentration of MB should be deducted by stirring and put the mixtures in the dark for 60 min (Fig. 5A). Though calculating and comparing the adsorption ability, we can know the surface activity of photocatalysts. The photocatalysts show a diverse adsorption abilities order as follows: MS > BMS4 > BMS3 > BWO > BMS1 > BMS2. The composites BMS1 and BMS2 showed weaker adsorption abilities than BWO due to the increase of crystallite sizes of BWO and the decrease of specific surface area. The composition with MoS2 can obviously enhance the adsorption ability. The adsorption abilities of BMS3 and BMS4 were enhanced along with the increase in MoS2 because the large specific surface area of MS. Pure MS showed strong adsorption ability for MB. This result confirms the synergistic effect between MoS2 and Bi2WO6. MoS2 can disperse more uniformly on the surface of Bi2WO6 as an obvious core-shell structure. When illuminated under UV–vis light, the blue color of MB aqueous solution continually declined which confirmed the MB molecules were degraded by photocatalysts. The photocatalytic activity of composites can be calculated by recording in the absorbance intensity of MB at fixed interval. The degradation curves of all samples (BWO, BMS1, BMS2, BMS3, BMS4 and MS) are shown in Fig. 5A. As the degradation curves contained the adsorption content, the degradation rates can not be obtained only by comparing the residual concentration of MB. When the initial concentration of dyes is low, the degradation rate of could be ascribed as a pseudo-first-order kinetics reaction according to a Langmuir Hinshelwood model: ln(C0/C) = kt, where k is the apparent first-order rate constant. Here, the dots at 0 min (illumination beginning) were chosen as original points to exclude

Fig. 5. (A) Comparison of photocatalytic activities of BWO, BMS1, BMS2, BMS3, BMS4 and MS for methylene blue decomposition under UV–vis irradiation; (B) relative apparent first order rate constants k. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. A schematic diagram of carries exchange in the MoS2/Bi2WO6 heterostructure.

the influence of adsorption. The k has been calculated and displayed in Fig. 5B and Table 1. After composited, BMS2 and BMS3 show enhanced photocatalytic activity than pristine BWO and MS. With optimal composited ratio, BMS3 demonstrates the highest photocatalytic activity as much as 82% than that of BWO which confirms the enhancement by MoS2 composite. As the photocatalytic activity of pristine BWO and MS are both lower than BMS2 and BMS3. Hence, the enhancement of photocatalytic activity of composites should be ascribed to the enhanced separation efficiency of photo-induced carriers by the MoS2/ Bi2WO6 heterostructure. The energy band structure of the MoS2/ Bi2WO6 heterostructure is studied and demonstrated in Fig. 6. As the conduction band of MoS2 is higher than that of Bi2WO6, the photo-induced electrons produced in MoS2 will flow to Bi2WO6. Thus, photo-induced holes produced in Bi2WO6 will transfer to MoS2 because the valence band of MoS2 is higher than that of Bi2WO6. Moreover, the two-dimensional structure of MoS2 can increase the area of heterostructure lead to a longer transfer distance and life of carriers. So this MoS2/Bi2WO6 core-hell heterostructure constructed by two-dimension MoS2 and BiWO6 nanoparticles is an ideal structure. The composite MoS2 will increase the absorption of visible light. Compared single semiconductor, this carrier exchange via the heterostructure can effectively reduce the recombination and extend the spreading distance and separation time, thereby enhancing photocatalytic activity. The separation efficiency of photo-induced carriers can be investigated by the Fluorescence (FL) emission spectra, since the FL emission results from the recombination of free charge carriers [52]. Fig. 7 presents the comparison of FL emission spectra of pure BWO and MoS2/Bi2WO6 photocatalysts excited at 340 nm. A strong emission peak at approximately 469 nm could be assigned to the band gap transition of Bi2WO6, which shows obvious blue shift compared with the value of 425 nm obtained from BWO (Fig. 4B). The blue

Fig. 7. Fluorescence spectra of BWO, BMS1, BMS2, BMS3 and BMS4.

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Acknowledgments This work was supported by the Chinese National Natural Science Foundation (No. E021401 and 61072015), Zhejiang Provincial Natural Science Foundation of China (No. LY16E020007 and LY13F040006), the Science and Technology Project of Zhejiang Province (No.2015C37037). References

Fig. 8. (A) Cycling degradation curves and (B) the apparent first order rate constant k vs. times for BMS3.

shift was caused by a re-absorption mechanism. The intensities of emission peaks of composites can be effectively quenched in the presence of MoS2 due to the efficient interfacial photo-induced carriers transfer between MoS2 and Bi2WO6. However, exceed surface covering of MoS2 nano-layers will hinder light absorption of BWO and weaken photocatalytic activity. Thus, only a suitable composite ratio range can ensure the enhancement of photocatalytic activity. In the recycle degradation of MB as Fig. 8A shown, the activity of BMS3 remains about 86% after 5 times degradations (Fig. 8B) indicate its high stability. 4. Conclusion In this work, Bi2WO6 nanoparticles were composited with two-dimension MoS2 though a hydrothermal reaction and the photocatalytic activity was compared. The HRTEM images show that the two-dimension MoS2 grown on the surface of Bi2WO6 nanoparticles to construct a core-shell heterostructure. The photocatalytic degradation tests show that MoS2/Bi2WO6 composite with optimal ratio demonstrate an obvious enhancement of photocatalytic activity as 82% than that of pristine Bi2WO6 nanoparticles. The result confirms this MoS2/Bi2WO6 heterostructure is an ideal carriers' transfer channel. In this way, photoinduced electrons produced in MoS2 will flow to Bi2WO6 because the conduction band of MoS2 is higher than that of Bi2WO6; thus, photo-induced holes produced in Bi2WO6 will transfer to MoS2 because the valence band of MoS2 is higher than that of Bi2WO6. This carriers' transmission will improve the separation efficiency and enhance the photocatalytic activity of composites. This two-dimension MoS2 composite is finally confirmed an effective approach to improve the photocatalytic activity of semiconductors.

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