BiVO4 Hetero-Nanoflowers

Sep 10, 2015 - The generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof (PBE) was used to deal with the exchange and co...
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Hydrothermal Synthesis of Novel MoS2/BiVO4 Hetero-Nanoflowers with Enhanced Photocatalytic Activity and a Mechanism Investigation Honglin Li, Ke Yu,* Xiang Lei, Bangjun Guo, Hao Fu, and Ziqiang Zhu Key Laboratory of Polar Materials and Devices (Ministry of Education of China), Department of Electronic Engineering, East China Normal University, Shanghai 200241, China ABSTRACT: The flower-like MoS2/BiVO4 composite with heterojunction has been successfully fabricated by a two-step approach. A possible formation mechanism of this heterostructure was investigated. The calculated valence band offset (VBO) and conduction band offset (CBO) of MoS2/BiVO4 heterojunction showed that the VBO and CBO of MoS2/BiVO4 are 1.4 and 0.3 eV, respectively, implying the formation of well-defined staggered type II band alignment. The photodegradation of methylene blue (MB) was adopted to assess the photocatalytic ability of the pristine MoS2 and BiVO4 as well as MoS2/BiVO4 composites. It exhibited that the MoS2/BiVO4 composite structures performed much better than that of the pristine MoS2 and BiVO4, which was due to the staggered band alignment formed between the two structures. Besides, the corresponding mechanism of enhanced photocatalysis regarding the separation of the photogenerated electron−hole pairs for the heterojunction has also been investigated by the first-principles calculation.

1. INTRODUCTION In recent years, 2-dimensional (2D) materials, hexagonal graphene, BN sheet, and MXenes, for example, have attracted great attention for their special performances, which are mainly derived from ultrathin thickness characteristic and the relevant quantum effects.1 As a member of the layered transition metal dichalcogenides, MoS2 is consisted of the weakly coupled S− Mo−S atoms sandwich layers.2 Different MoS2 structures such as nanosheet and nanoribbon have been successfully synthesized, and their unique physical properties distinguish from other materials and have been applied to many applications such as catalytic, photovoltaic, and lubricant.3 MoS2 and analogous materials, such as MoS2, WS2, and SnS2, have recently aroused great interest due to their high theoretical capacities and become of a high interest in several other energy storage and conversion applications due to its price efficiency and performance competitiveness.4 Utilization in fuel cells and biomass conversion/hydro-deoxygenation are two such examples.5,6 However, the photocatalytic property of MoS2 has not been widely used in industry for the nature of rapid recombination of photoinduced electrons (e−) and holes (h+). Generally, bismuth is stable and free of toxicity. The widespread usage of bismuth-based materials has attracted a lot of attention in recent years. For example, the ternary BiVO4 compound has been extensively studied in the field of catalysis.7,8 In general, there exists three crystal lattice structures of BiVO4: tetragonal zircon/scheelite and monoclinic scheelite. Since the monoclinic BiVO4 (m-BiVO4) has a smaller bandgap (2.4 eV) compared with the other two structures, it shows a better photodegradation performance of organic compounds as well as water splitting for oxygen evolution. However, the low © 2015 American Chemical Society

mobilities of photogenerated electrons and holes and the corresponding poor separation characteristic limit the photocatalytic activity of m-BiVO4-based catalysts. It will be helpful if a more effective photocatalytic system is intentionally designed to promote the charges separation of m-BiVO4. The previous studies have proven that the heterostructured composites are in favor of the improving of performances, such as photoluminous, catalytic, and sensing properties for the formation of different new band alignment forms, surface states change, and so on. To couple m-BiVO4 with an another semiconductor to produce new form of composite is an effective way to separate the photogenerated charges. For these heterojunctions, the carriers cross the junction due to the mismatch of electronic structure for the both layers and the barrier heights altered in the process of carriers’ transport.9 Su et al.10 have been successfully synthesized macroporous V2O5/BiVO4 composites under the coexistence of carbon spheres in colloidal state. It was proved that the formation of V2O5/BiVO4 heterojunction in the composite made a critical difference to the separation of photogenerated charges. Hong et al.11 fabricated WO3/BiVO4 heterojunction electrodes in the form of layer-by-layer on a conducting glass. The four layers of WO3 covered by a single layer of BiVO4 demonstrated enhanced photoactivity by 74% and 730% relative to bare WO3 and BiVO4, respectively. Liu et al.12 prepared BiVO4/cobalt phthalocyanine hierarchical nanostructures. The photocatalytic activities of the BiVO4/ CoPc photocatalysts showed a significantly enhanced photoReceived: July 13, 2015 Revised: September 10, 2015 Published: September 10, 2015 22681

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

Article

The Journal of Physical Chemistry C

After being cooled down to room temperature naturally, the resultants were collected and washed by deionized water and ethanol several times to ensure the impurities were cleaned away. Finally, the synthesized products were dried at 60 °C for 6 h. The obtained products were labeled as MBV1, MBV3, MBV5, and MBV7. 2.3. Sample Characterization. Field emission scanning electron microscopy (FESEM, JEOL-JSM-6700F, operating accelerating voltage of 20 kV) was used to analyze the morphology and structure of the synthesized nanostructures, Xray diffraction (XRD) measurements were conducted using Cu Kα radiation (λ = 1.5418 Å) (Bruker D8 Advance diffractometer), and transmission electron microscopy (TEM, JEOL-JEM-2100) at the accelerating voltage of 200 kV. X-ray photoelectron spectrometry (XPS) measurements were conducted with an ESCALAB 250Xi instrument using monochromatic Al Kα radiation. All the above measurements were performed at room temperature. Photoluminescence (PL) spectra of the samples were obtained using an Edinburgh Analytical Instrument PLS920 system under the excitation light at 400 nm.17 2.4. Photocatalytic Measurements. In this part, the photocatalysis capacities of different samples were evaluated by the photodegradation of MB at room temperature under light irradiation. The highest optical absorption of MB at 664 nm was used as monitor wavelength of photodegradation. First, 0.1 g of as-prepared samples was added to 100 mL of MB aqueous solution (40 mg L−1). In order to establish the adsorption/ desorption equilibrium, the mixed liquor was stirred for 1 h in the dark before photodegradation reaction. The reactor was situated in a glass container and cooled by flowing water to eliminate the possible thermal influence. The visible-light source was a 500 W Xe lamp with cutoff filter (≥400 nm), which was placed 5 cm away from the liquid surface of the MB suspension. The absorption surveys were conducted by a UNICO 2802 spectrophotometer at an interval of 20 min for 120 min. The mineralization degree of the corresponding solutions was analyzed by TOC (total organic carbon) analyzer (AnalytikJena, Multi N/C 2100S). Photocurrent was measured by an electrochemical analyzer (CHI 660D, China) in a threeelectrode system. The active area of working electrodes was about 1 cm2, which could be prepared by the following procedures: 0.5 g of as-prepared sample was ground with 0.2 g of poly(ethylene glycol) and 3 mL of ethanol to make into slurry; then a 1 cm × 1 cm In-doped SnO2-coated glass (ITO glass) electrode was coated by the above slurry and was dried in an oven and calcined at 300 °C for 30 min under a flowing of pure argon. A Pt wire (purity 99.99%) and Ag/AgCl (saturated KCl) acted as the counter electrode and the reference electrode, respectively; 0.1 M Na2SO4 solution was used as the electrolyte. The simulated sunlight irradiation was provided by a 500 W xenon lamp.18 2.5. Computational Details. In this work, we performed a comprehensive first-principles calculation to investigate the electronic properties of this heterogeneous structure to give the theoretical basis of the promoted photocatalytic activity. All calculations were performed using density functional theory (DFT), as implemented in the Vienna ab initio Simulation Package (VASP). The generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof (PBE) was used to deal with the exchange and correlation potentials. A 5 × 5 × 1 Monkhorst−Pack k-point sampling for the Brillouin zone k-point mesh and a 450 eV cutoff energy were used for

catalytic performance in comparison with pristine BiVO4 nanofibers, and the heterojunction could greatly increase the separation of charges and extend the corresponding carriers’ lifetime of the composites.13 In this paper, we used a two-step hydrothermal method and successfully synthesized binary MoS2/BiVO4 heterojunction composites based on MoS2 nanoflowers, which is free from the usage of additives or surfactants for the first time. The related preparation methods were simple and controllable. We also calculated the different energy band positions of MoS2 and BiVO4 that are in favor of the separation of the photoinduced carriers for the formation of well-defined type II stagger band alignment, which is the foundation of the effective carriers separation processes.14 The photocatalytic abilities of pristine BiVO4/MoS2 and the MoS2/BiVO4 hetero-nanoflower structures were evaluated by the degradation of MB. The results exhibited that the synthesized MoS2/BiVO4 hetero-nanoflowers possessed a prominent improved photocatalytic performance in comparison with pristine BiVO4 and MoS2. We further systematically explored the corresponding electronic structures and charge transfer mechanisms between BiVO4 and MoS2 via the first-principles calculation. These results may be helpful for the design and application of MoS2/BiVO4 composite nanostructure for using in photocatalytic materials.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MoS2 Nanoflowers and Peanut BiVO4. As for the synthesis of pristine MoS2 nanoflower, 1.2 g of sodium molybdate dihydrate and 1.6 g of NH2CSNH2 were served as Mo and S source, respectively. The above solutes together with 0.6 g of oxalic acid were dissolved in 80 mL of deionized water to adjust the pH value to an acid environment. The mixed liquor was magnetically stirred for about 30 min and then transferred to a 100 mL Teflon-lined stainless-steel autoclave. Finally, the autoclave was sealed and heated at a drying oven in 180 °C for 24 h. After being cooled down to room temperature naturally, the black resultants were generated and attached on the inner wall of linear. These black products were cleaned by a ultrasonic cleaning with ethanol and distilled water alternately for several times to remove impurities and then dried in a vacuum at 50 °C for 12 h to obtain black MoS2 powders.15 In a typical synthesis of m-BiVO4 according to previously reported procedure,16 0.002 mol of bismuth nitrate and 0.002 mol of NH4VO3 were dissolved in 40 mL of ethylene glycol and hot water, respectively. Then, the above NH4VO3 solution was slowly added into the Bi(NO3)3 solution under vigorous stirring for 30 min and then transferred to a 100 mL autoclave and heated at 100 °C for 12 h. The yellowish resultants obtained from the reaction were precipitated naturally and washed with pure ethanol and distilled water several times. 2.2. Synthesis of MoS2/BiVO4 Hetero-Nanoflowers. The MoS2/BiVO4 hetero-nanoflowers were also synthesized through the hydrothermal method. 0.1 g of MoS2 as fabricated above and 0.1, 0.3, 0.5, and 0.7 mmol of Bi(NO3)3 were dissolved in 40 mL of ethylene glycol; the same amount of NH4VO3 with the corresponding Bi(NO3)3 was dissolved in 40 mL of hot water. Both were stirred well to ensure the solutes completely dispersed, and then the above NH4VO3 solution was slowly added into the MoS2/Bi(NO3)3 mixed solution under vigorous stirring. After magnetic stirring the above mixed liquor for 30 min, the suspensions were transferred into a autoclave with Teflon-lined and heated at 100 °C for 12 h. 22682

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

Article

The Journal of Physical Chemistry C

Figure 1. (a, b) Low- and high-magnification SEM images of the pristine MoS2 nanoflowers. (c) Schematic diagram of pristine MoS2 nanoflower with petals. (d) Low-magnification SEM image of the pristine BiVO4. Inset shows the appearance of the fabricated products. (e, f) Two typical microstructures of the pristine BiVO4. (g) SEM image of MBV5 hetero-nanoflower; the upper inset shows the microstructure of MBV7, and the lower presents the synthesized MBV5 powders. (h) High magnification of a MBV5 sphere. (i) Schematic diagram of hetero-nanoflower. (j) Highresolution TEM image of the MBV5. Inset is the intensity signal along the red line. (k) EDX mapping images of Mo, Bi, and V.

hierarchical heteroarchitecture. MoS2 nanoflowers and BiVO4 nanoparticles are clearly showed from Figures 1g−j, and it is apparent that the MoS2 petals are inserted by BiVO4 for the MBV5 hetero-nanoflowers. In general, the formation of pristine MoS2 nanoflowers associates with the nucleation in certain reaction conditions of amorphous primary nanoparticles.19 Numerous amorphous MoS2 structures take shape in the solution during the primary reaction period, in which the added NH2CSNH2 acts as S resource and reductant. These primary structures can spontaneously and freely aggregate into spheres and then curl to structured petals gradually in the surface for the layered nature of MoS2 when the temperature exceeds a certain level. As for the formation mechanism of MoS2/BiVO4 heterostructure, we consider the formation of BiVO4 is greatly affected by the existence of MoS2, since the morphology of pristine BiVO4 is quite different from the composite. The ethylene glycol has strong chelating ability, and thus it can dissolve some inorganic materials to form an uniform solution. It can coordinate with Bi(NO 3 ) 3 to generate EG−Bi 3+ complexes and then nucleate with VO3+ to grow into the primary BiVO4 nanoparticles and gather into 3D aggregates uniformly distributed around MoS2 in the corresponding solvothermal processes. The BiVO4 nanostructures aggregated in the corresponding EG−water system are kinetically slow and exhibit not only the insertion but also a potential (defect) nucleation and internal growth of BiVO4 as shown in Figure 1i. It will then generate the confined growth of BiVO4 finally. One other thing to note is that when 0.7 mmol of Bi(NO3)3/ NH4VO3 involved in the related reactions, the morphology of the composite turns into spherical structure as revealed in the upper inset of Figure 1g. This kind of morphology neither exists in the pristine MoS2 nor in BiVO4 and clearly belongs to the MoS2/BiVO4 heterostructure when excessive BiVO4 is incorporated which belongs to the MBV7 sample. The purity and crystalline phase of the hydrothermally synthesized samples are further characterized by XRD patterns. Figure 2 shows the corresponding spectra of the different samples. The black curve of the pattern shows the XRD

the calculations. All initiating structures have been fully relaxed until the convergence criteria of energy and force were less than 10−5 eV and 0.01 eV Å−1, respectively.

3. RESULTS AND DISCUSSION The morphology and microscopic structure of the as-prepared pristine MoS2 nanoflowers are characterized by SEM, the corresponding images being shown in Figures 1a,b. As presented in Figure 1a, every MoS2 nanoflower structure shows an average diameter of 0.5−1 μm. The surfaces of these nanoflowers possess massive petals, which are freely and closely aggregated as shown in Figure 1b. It can be seen that the petals grown on the surface of MoS2 flowers are disorderly intersected together and point toward a common center of sphere to form the spherical structure. As can be seen from Figures 1d−f, the surface of the synthesized pristine BiVO4 is relatively smooth and has two typical peanut/olive-like structures (2 μm in length and 1 μm in width). Figure 1g shows the morphology of the synthesized MBV5 heterostructure nanoflower characterized by SEM. It can be seen that some BiVO4 attached on the petals of MoS2 nanoflowers. Figure 1h exhibits the high-magnification image of the heterostructured nanoflower; it is revealed that BiVO4 tightly inserts into the petals of MoS2. Comparing Figure 1b with Figure 1h, it is shown that the groove depth of the adjacent petals clearly decreased, and this phenomenon can be attributed to the generated BiVO4 inserts into groove as indicated in Figure 1i. A further characterization should be conducted based on TEM and XRD surveys. The HRTEM image of Figure 1j provides the perfect illustration of the coexistence of hexagonal MoS2 and monoclinic BiVO4. The fringes of d = 0.62 nm is in agreement with the (002) plane and also shows that the fabricated petals grow in a high density way. The inserted BiVO4 exhibits lattice fringers of 0.3187 nm, which can be indexed to (103) plane of m-BiVO4. It is apparent that heterojunction formed between MoS2 and BiVO4 in the MBV5. From the elemental mapping of Figure 1k for MBV5, it can be seen that the extensive distribution of Mo, Bi, and V. In brief, SEM and TEM analyses of the heterostructure prove the formation of the 3D 22683

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

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The Journal of Physical Chemistry C

spectrum of the as-prepared MoS2 nanoflower. The observed diffraction peaks at 2θ = 14.2°, 33.2°, and 58.8° can be well attributed to the (002), (100), and (110) planes of hexagonal MoS2 (JCPDS #37-1492), respectively. No other impurity peak is observed in the spectrum, implying a high purity of the fabricated MoS2 structure. All the diffraction peaks can match well with the hexagonal phase of MoS2, but the weak diffraction of (002) plane implies the low crystallinity, indicating the petal of MoS2 nanoflowers constituted by a few layers of nanosheets. Besides, the slight (002) peak shift compared with standard hexagonal phase MoS2 is ascribed to the lattice distortion for the synthesized MoS2 nanoflowers. The blue curve can be well indexed to the monoclinic BiVO4 (JCPDS #14-0688). This indicates that the employed ethylene glycol assisted solvothemal process can produce a well-crystallized high-purity product. Besides, no other impurities diffraction peaks such as Bi2O3 and V2O5 can be detected in these patterns, indicating this product is pure m-BiVO4 phase. The MoS2/BiVO4 heterostructures pattern are labeled as MBV1−7 with various Bi(NO3)3/NH4VO3 millimoles. It is observed that the different characteristic peaks appeared in each of the four XRD patterns index to hexagonal MoS2 and monoclinic BiVO4 simultaneously, indicating the composite structures consisted by hexagonal MoS2 and monoclinic BiVO4. Both MoS2 and BiVO4 with an obvious crystallization have been formed in each sample. We also note that as the amount

Figure 2. XRD patterns of the pristine MoS2/BiVO4 and MBV1 to MBV7.

Figure 3. XPS patterns of MBV5. (a) Typical overall XPS, (b) V 2p spectra, (c) Mo 3d and S 2s spectra, and (d) Bi 4f and S 2p spectra. Experimental data points are fitted by Voigt (mixed Lorentzian−Gaussian) line shapes after the application of a Shirley background. 22684

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

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The Journal of Physical Chemistry C

comparison with the other heterojunction photocatalysts, Yang et al.26 fabricated the Cu2O−ZnO heterostructure by magnetron sputtering. The VBO/CBO of 2.91/1.71 eV and 2.52/1.32 eV were obtained for Cu2O/ZnO and ZnO/Cu2O, respectively. Huang et al.27 successively deposited thin Cu2O layers on TiO2 and conducted band offset analyses. It was exhibited that the VBO and CBO were estimated to be 2.13 ± 0.1 and 0.73 ± 0.1 eV, respectively. Moreover, Khanchandani et al.28 designed ZnO/CdS and ZnO/Ag2S core−shell nanostructures with the same shell thickness. The MB degradation results indicated that ZnO/Ag2S core−shell nanostructures exhibited 40- and 2-fold apparent rate constant enhancement in comparison with pristine ZnO and ZnO/CdS core−shell heterostructure, respectively. They considered that the improved photocatalytic ability was originated from a smaller CBO between ZnO and Ag2S, which would facilitate the charge separation at the interface. In this work, the calculated values of VBO/CBO are smaller than the above photocatalysis heterojunction, implying this MoS2/BiVO4 heterostructure may also available for the photocatalysis usage.29 With a major absorption peak at 664 nm, MB is a common contaminant in industrial wastewater and often used to evaluate the photocatalysis ability of the synthesized samples. Figures 4a and 4b show the variation of absorption extracted form MB solution in the presence of pristine MoS2 and MBV5 composite, respectively. The continually reduced absorption indicates the gradually decreased concentration of MB with

of Bi(NO3)3/NH4VO3 increases from 0.1 to 0.7 mmol, the diffraction peaks (121) of BiVO4 are gradually intensified and the full widths of their diffraction peaks decrease meanwhile, implying the increase of BiVO4 crystalline size and the improvement of the corresponding crystallinity. This phenomenon also can be revealed from Figure 1h and the inset of Figure 1g. It is shown that the BiVO4 first fills in the groove of MoS2 and then grows on the surface of MoS2. Figure 1i shows the initiation step of the corresponding insertion and internal growth processes. The elements constitution of the MBV5 sample is further analyzed by XPS measurement. Figure 3 presents a typical overall XPS spectrum and a series of high-resolution XPS spectra of V 2p, Mo 3d, Bi 4f, and S 2p. As shown in the overall XPS spectrum of the MBV5, Mo, Bi, V, O, and C can be detected on the surface. The observed peak of C 1s at 284.6 eV is originated from the signal of carbon in the instrument, and it is inherent. The peaks at binding energies of 524.9 (V 2p1/2) and 517.3 eV (V 2p3/2) are the split signals of V 2p, assignable to the surface V5+ species for the spin−orbit splitting. This implies that V ions in MBV5 are presented as V5+ (Figure 3b). The three main peaks of Mo 3d3/2 (232.8 eV), Mo 3d5/2 (229.6 eV), and S 2s (226.7 eV) are typical characteristics of MoS2 (Figure 3c). As shown in Figure 3d, five peaks can be decomposed from 154 to 168 eV. The peak at 165.3 eV can be attributed to the 2p state of S in MoS2.20 The peaks for S 2p1/2 and 2p3/2 at 163.7 and 162.4 eV attribute to the coexistence of bridging S2− or apical S22−, which derive from the unsaturated S atoms.21 The peaks located at 164.3 and 159.1 eV are ascribed to the Bi 4f5/2 and Bi 4f7/2, respectively, confirming the bismuth species in the composite is Bi3+ cations. One thing to note is that no peak around 161.2 and 166.5 eV can be obtained, implying the state of Bi4+ in the composite is inexistent. This can ensure the purity of the synthesized MBV5 and has not been oxidized to Bi4+ in air. It is widely believed that the photoinduced carriers can transfer to the surface of catalyst to react with the adsorbed reactants; meanwhile, the band alignment of heterojunction influences the migration direction of the photoinduced charge carriers. The valence band (VB) positions of MoS2/BiVO4 can be calculated by the equation EVB = X − Ee + 0.5Eg at the point of zero charge, in which EVB is the VB edge potential, X is the electronegativity of the corresponding semiconductor obtained form the geometric mean of the electronegativity for the component atoms (5.32 eV for MoS2 and 6.16 eV for BiVO4, respectively),22,23 Ee is the free electrons energy based on the hydrogen scale (∼4.5 eV), and Eg is the band gap of the 2 semiconductor.13 We define the ΔEV as VBO and ΔEV = EMoS VB BiVO4 − EVB ; thus, the CBO can be calculated via the formula ΔEc 2 4 = (EMoS + ΔEv − EBiVO ). The VBO value is determined to be g g 1.4 eV by plugging these experimental values into the above 2 formula. Utilizing the recently reported band gap values (EMoS g BiVO4 24,25 = 1.3 eV, Eg = 2.4 eV), the ΔEc is computed to be ∼0.3 eV. According to the above results, a well-defined type II stagger band alignment is formed for the MoS 2/BiVO4 heterostructure. Generally, a staggered band alignment at the interface will lead to the electrons or/and holes trapped in the spike, incurring the reduced surface photochemical activity. The accurate calculation of the band offsets for MoS2/BiVO4 indicates that this nanoflower structured composite shows staggered type II band alignment and can guarantee an effective carriers separation in MoS2-based photoelectric device. In

Figure 4. (a, b) Absorption spectra of MB solution after 120 min irradiation at room temperature in the presence of pristine MoS2 and MBV5 heteronanostructure, respectively. (c) Normalized decrease concentration C/C0 of the MB solution containing different catalysts. (d) Logarithm (ln(C0/C)) of the normalized concentrations vs irradiation time. Dashed lines denote the data fitting curves of t against ln(C0/C). (e) Remaining TOC for the MB dye solutions containing different catalysts. (f) Photocatalytic degradation of MB after four recycling runs in the presence of the MBV5 sample under irradiation. 22685

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

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The Journal of Physical Chemistry C

irradiation, which is lower than that of the other catalysts and broadly in line with the above photocatalytic abilities. This implies most of the MB molecules can be mineralized through a photocatalytic process for the synthesized MBV5 composite. Except for photocatalytic performances, an another important aspect of photocatalyst for the practical application is stability. So the cycle runs are conducted to evaluate the stability of the samples under irradiation. In this procedure, all processes and parameters remain the same. As indicated in Figure 4f, MBV5 heteronanostructure used for degradation has a slight decline after four cycles of catalyzing: ∼89.5% of the original MB is degraded after four runs, while the maximum degradation is 94.2% for the first run. This experiment confirms the stability of the synthesized MBV5 heteronanostructure photocatalysts during photocatalytic process. To explain the above results, the photocurrent responses measurements were conducted for different photoelectrodes prepared with various catalysts (MBV5, pristine BiVO4, and MoS2). As shown in Figure 5a, all three electrodes under

increasing of the reaction time. As irradiation time increased, the MB solution containing MBV5 heteronanostructure presents a much faster peak-descending tendency than that of pristine MoS2 nanoflower. It is obvious that the photocatalytic activity of the MBV5 composite is much better than that of the pristine MoS2 nanoflowers. Figure 4c presents the dependency of the degradation rate within the irradiation time of 120 min. A degradation of MB solution without photocatalyst is also conducted, and almost no photocatalytic decolorization of MB solution is observed after irradiation for 120 min as indicated by the black line. This explains the stability of MB under long time irradiation. Accordingly, catalyst plays a key role in the process of degradation under irradiation. Before irradiation, the mixed solution of catalysts and MB were stirred in dark to establish the adsorption/desorption equilibrium on the surface of catalysts. After irradiation for 120 min, C/C0 significantly reduce to 54.1% and 44.2% for pristine MoS2 and BiVO4, respectively. Photoexcited electrons from valence band of MoS2 transfer to conduction band and electron−hole pairs generated. The photoexcited electrons react with the oxygen in the water to produce radical anions, and valence band holes were captured by water molecules to form hydroxyl radicals to photo-oxidize organic molecules. Daage30 considered that the massive edge sites of MoS2 were the active sites and proposed a “rim-edge” mode. Atomically, every S and Mo atoms coordinate two Mo atoms on the S-edge and four S atoms on the Mo-edge, respectively; these edge sites have strong interaction with MB molecules. Thus, the wrinkled surfaces with abound defects or faults of MoS2 nanoflower can provide abundant of edge structures that acted as active sites for the photodegradation of MB. Another viewpoint is that the unsaturated sulfur atoms on the surface of MoS2 materials have some certain chemical activities,21 which may also participate in the corresponding catalytic processes and facile the degradation of MB dye solution. These unsaturated sulfur atoms can exist in the form of bridging S2− or apical S22−, and this has been verified by the above XPS analysis. The degradation C/C0 of pristine structures and composites are shown in Figure 4c. After 120 min irradiation of the six samples under the same conditions, the degradation curves prove that MBV5 heterostructure possesses the optimal photocatalytic performance than other composite structures. To quantitatively characterize the reaction kinetics of the MB degradation, the experimental data were fitted by the Langmuir−Hinshelwood model, as expressed by ln(C0/Ct) = kt, where C0 and C are the concentrations of pollutant in solution at time t0 and t, respectively, and k denotes the apparent rate constant. As can be seen from Figure 4d, all fitted curves of the irradiation time (t) against ln(C0/C) are nearly linear. It should be noted that the deviation from a straight line for MBV5 sample is often related to some Langmuir-type adsorption/reaction models, while we consider that the pseudofirst-order kinetics play a leading role. For the different photocatalysts, the rate constant of pristine MoS2 is 0.006 min−1, and a higher value of 0.021 min−1 is obtained for the MBV5 composite. In the actual applications, the relevant degradation processes will produce some kinds of toxic intermediate products, and thus the mineralization degree is equally important for the photodegradation of organic pollutants.31 Figure 4e shows the variation of TOC during the photodegradation of MB after 120 min to investigate the mineralization abilities of the six catalysts. The TOC/TOC0 value of MBV5 decreased to ca. 29.2% after 120 min of

Figure 5. (a) Comparison of photocurrent responses of MBV5, pristine BiVO4, and MoS2 electrodes in Na2SO4 electrolyte solution under simulated sunlight illumination. (b) Room temperature PL spectra of pristine BiVO4 and MBV5 composite with a 400 nm laser radiation source.

intermittent irradiation in Na2SO4 solution demonstrate apparent photocurrent responses. The fast and uniform responses to each light on/off interval of the three photoelectrodes indicate the good reproducibility of the corresponding samples. One thing to note is that the pristine MoS2 exhibits a relatively low photocurrent, implying the low quantum efficiency of MoS 2 . In contrast, the MBV5 heterostructure shows a much elevated photocurrent intensity 22686

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

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The Journal of Physical Chemistry C

Figure 6. (a, e) Partial charge density distributions of the α bands for MoS2 and BiVO4, respectively. (b, d) Band structures of MoS2 and BiVO4, respectively. (c) Density of states (DOS) of MoS2 and BiVO4. (f, g) Partial charge density distributions of the β bands for MoS2 and BiVO4, respectively.

charge density pattern shows that the highest VB of MoS2 (β) mainly originates from the 4d orbital of the Mo atoms, and S′s p orbitals have minor contributions. The lowest CB (α) is also mainly contributed by the 4d orbital of the Mo atoms. As for BiVO4 shown in Figure 6g, the highest valence band mainly originates from the p orbital of the O atoms, and the lowest conduction band is attributed to the d orbital of the V atoms. One thing to note here is that the correlation between velocity of electron v and the energy of electron E can be expressed as v = ΔkE(k)/ℏ. This means that the more flat the energy band, the more localized the charge carriers. Therefore, both the mobilities of electrons and holes are relatively low for the mBiVO4 as indicated in Figure 6d, which will inevitably restrain the separation efficiency of photogenerated carriers and weaken its photocatalytic activity. Accordingly, even from the viewpoint of a perfect bulk crystal of m-BiVO4, there still exists large space for improving its photocatalytic performance. Since the CB and the VB of BiVO4 lie below the corresponding energy bands of MoS2, the photoexcited electrons/holes can easily cross the interface and transfer to the CB of BiVO4 and VB of MoS2 when the MoS2/BiVO4 heteronanostructure irradiated by light; thus, the photogenerated electrons and holes can be separated efficiently at the interface and improve the photocatalytic ability of the corresponding composite for the reduced recombination rate. After the photogenerated electrons and holes separated, the surface hydroxyl groups (or H2O) capture the holes (h+) at the surface of catalyst and generate •OH radicals.36 Besides, water molecular also can react with superoxide radical anions (•O2−) that originated from the reaction between dissolved O2 and electrons to generate hydroperoxyl radicals (HO2•). Finally, powerful oxidizing agents of oxidol (H2O2) and hydroxyl radicals (OH•) can effectively decompose the MB molecules.

than that of pristine MoS2 and BiVO4. It is generally believed the photocurrent is mainly originated from the separation of photogenerated charges within the photoelectrode, in which electrons are transported to the back contact while holes are taken up by the hole acceptors in the electrolyte. Therefore, when the composite photoanode is irradiated by light, photoinduced electrons from the excited BiVO4 are probably transferred to ITO while holes are transferred to MoS2, resulting in the separation of the photogenerated charges as much as possible. A higher photocurrent response of the MBV5 suggests that heterostructure do have a positive effect on the restrain of recombination for photogenerated charges.32,33 The extended lifetime of the photogenerated charges can be further verified by the PL spectra, which is useful to elucidate the transfer and recombination processes of the photogenerated charges for semiconductors. It should be noted a fact that multilayered MoS2 has no photoluminescence property for the nature of indirect bandgap, and only monolayer MoS2 can exhibit this characteristic; therefore, the PL spectrum of MoS2 should not be included along with BiVO4 and MBV5 since it will lead to misunderstanding. The comparison between pristine BiVO4 and MBV5 is enough to illustrate the issue here. As presented in Figure 5b, the PL emission wavelength of pristine BiVO4 is centered at 540 nm under an excitation wavelength of 400 nm, which is ascribed to the recombination of holes and electrons in the VB and CB, respectively. It is obvious that the PL intensity of MBV5 composite demonstrates a considerable quenched fluorescence after composites with MoS2 and thus can offer an enhanced photocatalytic activity. The fluorescence quenching mainly attributes to the sufficient interfacial contact between MoS2 and BiVO4. Similar phenomena can also be observed in the previous reported nanocomposites.34,35 The lower PL emission intensity of MBV5 in comparison with pristine BiVO4 suggests that the recombination of the photogenerated charge carriers is efficiently restrained, and the lifetimes of photogenerated charge carriers get prolonged. The result is well in accordance with the above photocatalysis and photocurrent response analyses. Based on the above experiments, the enhanced photodegradation ability of MoS2/BiVO4 heteronanostructure can be attributed to the specific charge-transfer mechanism. For the relatively narrow band gap of MoS2, MB can be catalyzed by visible light of MoS2. When the MoS2/BiVO4 heteronanostructure formed, it will lead to MoS2 and BiVO4 have the same Fermi energy level EF at the interface. Thus, a staggered type II band alignment and a built-in electric field formed near the interface. Also, the built-in electric field facilitates the separation of photogenerated carriers. As shown in Figure 6f, the partial

4. CONCLUSIONS In summary, novel MoS2/BiVO4 hetero-nanoflowers were successfully synthesized via a two-step hydrothermal method. The type II band alignment of VBO/CBO of about 1.4 and 0.3 eV were obtained. The band alignment parameters of MoS2/ BiVO4 heterojunction are believed to facilitate the design of MoS2 and BiVO4-based photocatalyst. The relatively small band offsets are significant for this heterojunction as neither photogenerated electrons nor holes are trapped, and both kinds of carriers can freely migrate to the shell and drive photocatalysis reactions. It is also found that the photocatalysts performance of MoS2/BiVO4 heterogeneous composites were much better than that of pristine nanoflowers MoS2 and peanut/olive-like BiVO4 through photodegradation of MB and 22687

DOI: 10.1021/acs.jpcc.5b06729 J. Phys. Chem. C 2015, 119, 22681−22689

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that the 0.5 mmol of NH4VO3/Bi(NO3)3 incorporated composite exhibited the optimal photocatalysis ability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the NSF of China (Grants 61274014, 61474043, 61425004, and 61574055), Innovation Research Project of Shanghai Education Commission (Grant 13zz033), and Project of Key Laboratory of Polar Materials and Devices (Grant KFKT20140003).

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