Facile Synthesis of Heterostructured WS2/Bi2MoO6 as High-Performance Visible-Light-Driven Photocatalysts
© The Author(s). 2017
Received: 29 March 2017
Accepted: 19 May 2017
Published: 30 May 2017
In this paper, novel WS2/Bi2MoO6 heterostructured photocatalysts were successfully fabricated via a facile solvothermal growth method using pre-exfoliated layered WS2 nanoslices as a substrate. The structure, morphology, and optical properties of the as-prepared WS2/Bi2MoO6 samples were characterized by XRD, XPS, SEM, TEM (HRTEM), and UV-vis diffuse reflectance spectra (DRS). Results confirmed the existence of an excellent nanojunction interface between layered WS2 nanoslices and Bi2MoO6 nanoflakes. Under visible light (>420 nm), the WS2/Bi2MoO6 composites exhibit significantly enhanced photocatalytic activity compared with pure Bi2MoO6 toward the decomposition of rhodamine B (RhB). Meanwhile, the active species trapping experiments indicated that holes (h+) were the main active species during the photocatalytic reaction. The enhanced photocatalytic performance can be ascribed to the effective light harvesting, fast photogenerated electron–hole pairs separation, and excellent charge carrier transport of the WS2/Bi2MoO6 heterostructures. Moreover, the prepared WS2/Bi2MoO6 composites also show good structural and activity stability in repeatability experiments.
The photocatalysis is widely regarded as one of the most promising environmental remediation technique due to the clean energy utilization method [1, 2]. Generally, some accepted that high-efficient photocatalysts with wide forbidden gap, such as TiO2 and ZnO, can only utilize ultraviolet light irradiation . As to practical application, photocatalysis strategy will be a huge boost once a photocatalyst can favorably absorb the abundant solar energy in visible region. For this purpose, many attempts to probe visible-light photocatalyst for sufficient solar energy utilization by using the narrow band semiconductor [4–6]. Despite the single-phase photocatalyst can be excited smoothly by visible light, it still manifests low energy conversion efficiency due to poor charge separation efficiency resulting from rapid recombination of photo-induced electrons and holes . It is widely accepted that the heterostructure can improve the separation probability of light-induced charge because the contact interfacial region of heterojunction will provide an internal electric field to restrain the recombination probability, thus resulting in an efficient photocatalytic performance. In general, the designed heterostructure will adopt at least one narrow band semiconductor to harvest more visual-light energy and then to generate more photo-induced charges [8, 9].
As a novel photocatalyst, Bi2MoO6 has received attention in the field of visual-light-driven photocatalysis because it possesses distinct sandwiched layered structure [10, 11]. As previously mentioned, the pure Bi2MoO6 is not suitable for the utilization as an efficient visible-light photocatalyst due to the high recombination probability of photogenerated charge carrier. Therefore, some effective strategies to meet this challenge by using the architecture of proper hybrid nanostructure and especially the introduction of two dimensional (2D) nanosheets have been proved as an effectual approach to strengthen interfacial charge transfer between two components in the process of photocatalytic reaction. Obviously, it is anticipated that the heterostructure between Bi2MoO6 and 2D layered material will increase photocatalytic efficiency by visual-light irradiation .
Layered transition metal dichalcogenides (TMDs) are widely regarded as a kind of promising loading material because of their analogous graphene reticular structure [12, 13]. Especially, monolayer and few layers of TMDs have important application for catalysis and energy storage due to their distinct electronic properties and high specific surface areas [14, 15]. For example, monolayered and few-layer MoS2 have recently paid the attention of the scientific community in photocatalysis research, which ascribes the lack of interlayer coupling and the absence of inversion symmetry resulting in the photoelectric property that differ markedly from those of the bulk [14, 16, 17]. From the material design perspective for an efficient visible-light-driven sensitized heterojunctional photocatalyst, the primary concern is that the hybrid narrow band gaps (1.1–1.7 eV) can closely match the solar spectrum . In fact, the typical 2D layered semiconductors, such as MoS2 or g-C3N4, have received significant attention to explore potential photocatalysis applications, which lead to TMD nanosheet which is often utilized as a supporter to establish the heterostructured composite photocatalysts via different energy band hybrid strategies [19, 20]. For instance, the hierarchical MoS2/Bi2MoO6 composites exhibited an efficient performance for photocatalytic oxidation of rhodamine B under visible-light irradiation . However, the mono- or few-layer heterostructured architecture of WS2/Bi2MoO6 as a visual-light photocatalyst has not been reported.
Herein, we demonstrated a facile strategy to fabricate heterostructured WS2/Bi2MoO6 composite via a facile solvothermal growth method using pre-exfoliated layered WS2 nanoslices as a supporter. The WS2/Bi2MoO6 exhibits excellent photocatalytic activity towards the degradation of rhodamine B (RhB) under visible-light (λ > 420 nm) irradiation. According to the microstructure characterization analysis of XRD, XPS, SEM, and TEM, the possible photocatalytic mechanism of the few-layer WS2/Bi2MoO6 composite was also elucidated. It is believed that the formation of junctions between Bi2MoO6 and WS2 can allow the prompt migration of photogenerated charge and reduce the self-agglomeration. It is postulated that the excellent photocatalytic activity of WS2/Bi2MoO6 should be ascribed to its high migration efficiency of photo-induced carriers and the interfacial electronic interaction. These results also probably provide a valuable perspective to insight into the design of other heterostructured photocatalysts.
Preparation of the Few-Layer WS2 Nanoslices
The liquid exfoliation of layered commercial WS2 was accomplished following the modified report method . Briefly, 50 mg commercial WS2 powder (purchased from Aladdin Industrial Corporation) was added to 20 mL of ethanol/water with EtOH volume fractions of 40% added as dispersion solvent. The sealed flask was sonicated for 10 h, and then the dispersion was centrifuged at 3000 rpm for 20 min to remove aggregations. Finally, the supernatant was collected to obtain few-layer WS2 nanoslices. To determine the concentrations of 2D nanosheets in the supernatant, we estimated the mass remaining in the supernatant by measuring the UV-vis absorption spectrum at fixed wavelength of 630 nm. The calculation result by virtue of Lambert–Beer Law indicated that the exfoliated WS2 dispersion concentration was about 0.265 ± 0.02 mg/ml.
Synthesis of Hierarchical WS2/Bi2MoO6 Composites
The WS2/Bi2MoO6 samples were synthesized using a facile solvothermal method. Typically, 2 mmol of Bi(NO3)3·5H2O was added to 10 mL of ethylene glycol solution containing dissolved Na2MoO4·2H2O with the Bi/Mo molar ratio of 2:1 under magnetic stirring. An appropriate amount of exfoliated WS2 nanoslices was dispersed into 20 mL ethanol and ultrasonicated at room temperature for 45 min. Then, it was slowly added into the above solution, followed by stirring for 10 min to form a homogeneous phase. The resulting solution was transferred into a 50-mL Teflon-lined stainless steel autoclave and kept at 160 °C for 10 h. Subsequently, the autoclave was cooled to room temperature gradually. Finally, the precipitate was centrifuged and washed with ethanol and deionized water several times and dried in a vacuum oven at 80 °C for 6 h. According to this method, WS2/Bi2MoO6 composites with different WS2 mass ratios (1, 3, 5, and 7 wt%) were synthesized. For comparison, the blank Bi2MoO6 was prepared in the absence of WS2 using the same experimental conditions.
Characterization of Photocatalysts
Structure and morphology of the sample was investigated by scanning electron microscopy (SEM; JEOL JSM-6701F, Japan), transmission electron microscopy (TEM; JEOL 2100, Japan), high-resolution transmission electron microscopy (HRTEM; JEOL 2100, Japan), and powder X-ray diffraction (XRD; Bruker D8 Advance using Cu-Kα radiation source, λ = 1.5406 Å, USA). The ultraviolet-visible diffuse reflectance spectra (DRS) of samples were performed at room temperature in the range of 200–800 nm on a UV-vis spectrophotometer (Cary 500 Scan Spectrophotometers,Varian, USA) equipped with an integrating sphere attachment. The electronic states of surface elements of the catalysts were identified using X-ray photoelectron spectroscopy (XPS; Shimadzu Corporation, Japan, Al-Kα X-ray source).
Measurement of Photocatalytic Activity
where C is RhB concentration at reaction time t, C 0 is the adsorption/desorption equilibrium concentration of RhB at the starting reaction time, and A and A 0 are the corresponding absorbance values.
In addition, to identify the active species generated during photocatalytic reactivity, various scavengers were added into the solution of RhB, including 2 mM isopropanol (IPA, a quencher of ·OH), 2 mM disodium ethylenediamine tetraacetic acid (EDTA; a quencher of h+), and 2 mM p-benzoquinone (BQ; a ·O2 − scavenger), and 40 mL/min N2 (an electron quencher). The comparative trials of photocatalytic degradation were performed under the same reaction conditions as those mentioned above.
Results and Discussion
Micostructure and Morphology Analysis
Electronic Structure and Spectrum Analysis
where α, hv, A, and E g are absorption coefficient, photon energy, proportionality constant, and bandgap, respectively. The value of n is determined by the type of transition (direct (n = 1) or indirect (n = 4)) [27, 28]. A plot of (ahv)2 versus (hv) is converted according the UV-Vis-DRS. As shown in Fig. 5b, the E g values of pure WS2 and Bi2MoO6 have been estimated to be 1.47 and 2.72 eV, respectively.
In addition, the pseudo-first-order kinetics model was used to fit the experimental data of the photocatalytic degradation of the RhB solution, and the results are given in Fig. 6b. The rate constant k is 0.0280 min−1 for the hierarchical WS2/Bi2MoO6 (5 wt%) composites, which is 3.8 and 7.1 times greater than those of mechanically mixed WS2 and Bi2MoO6 and pure Bi2MoO6, respectively. These results indicated that RhB could be degraded more efficiently by the hierarchical WS2/Bi2MoO6 composite photocatalyst.
Possible Photocatalytic Mechanism
To explain the enhanced photocatalytic performance, conduction band (CB) and valence band (VB) of WS2 and Bi2MoO6 potentials should be calculated. For a semiconductor, the bottom CB and top VB can be estimated by the empirical formula : E CB = X − E 0 − 0.5E g and E VB = E CB + E g, where E CB (E VB) is the CB (VB) edge potential; X is the electronegativity of the semiconductor; E 0 is the energy of free electrons of the hydrogenscale (~4.5 eV vs NHE); and E g is the band gap energy of the semiconductor obtained from the UV-visible diffuse reflectance absorption. The X values for WS2 and Bi2MoO6 are calculated to be 5.66 and 5.55 eV, respectively [28, 32, 33]. Thus, E CB and E VB values of WS2 are determined to be +0.43 and +1.9 eV and Bi2MoO6 are −0.31 and +2.41 eV, respectively.
In summary, a novel WS2/Bi2MoO6 heterostructured photocatalysts were successfully fabricated via a facile solvothermal growth method using pre-exfoliated layered WS2 nanoslices as a substrate. The hierarchical WS2/Bi2MoO6 exhibits excellent photocatalytic activity towards the degradation of rhodamine B (RhB) under visible-light irradiation. Based on the results of a series of structure and performance tests, it is believed that there formed a tight nanojunction interface between layered WS2 nanoslices and Bi2MoO6 nanoflakes, which make the photo-induced electrons be easily transferred to the WS2 substrate. As a result, the recombination of charges was decreased and the lifetime of holes was prolonged. Therefore, the hierarchical WS2/Bi2MoO6 composites exhibit much higher visible-light-driven photocatalytic activity than the pure Bi2MoO6. Furthermore, the WS2/Bi2MoO6 composites are very stable under visible-light irradiation and cycling photocatalytic tests. Thus, the as-prepared WS2/Bi2MoO6 photocatalyst has potential application for pollutant abatement.
The authors genuinely appreciate the financial support of this work by the National Natural Science Foundation of China (No. 51568068, No. 51568067).
JG carried out the sample preparation and experimental measurements and drafted the manuscript. CL and FW conceived the work, supervised the experiments, and revised the manuscript. LJ and KD helped to analyze the characterization results. TL supervised all of the study and provided financial support. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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