2D MoS2 p-n heterojunction immobilizing

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In-situ growth of NiFe2O4/2D MoS2 p-n heterojunction immobilizing palladium nanoparticles for enhanced visible-light photocatalytic activities Wenzhi Fu, Xiaowei Xu, Wenbin Wang, Mingxin Ye, and Jianfeng Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01299 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Sustainable Chemistry & Engineering

In-situ growth of NiFe2O4/2D MoS2 p-n heterojunction immobilizing palladium nanoparticles for enhanced visible-light photocatalytic activities Wenzhi Fu, Xiaowei Xu, Wenbin Wang, Jianfeng Shen*, Mingxin Ye*

Institute of Special Materials and Technology, Fudan University, Shanghai, 200433, P. R. China

Corresponding Author: *[email protected], [email protected].

1

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Abstract Solar energy is considered as a green and abundant energy for catalytic reactions. In this work, a magnetically recoverable NiFe2O4/2D MoS2-Pd nanocomposite is successfully synthesized via a simple one-pot hydrothermal method. The intimate interfacial contact between NiFe2O4 nanocubes and corrugated MoS2 nanosheets forms the NiFe2O4/2D MoS2 p-n heterojunction, while plasmonic Pd nanoparticles are uniformly immobilized on the surface of it. Dye degradation and Suzuki-Miyaura coupling reaction are employed to evaluate the photocatalytic activity of the NiFe2O4/2D MoS2-Pd nanocomposite. Significantly, both dye degradation and Suzuki-Miyaura coupling reaction can be efficiently performed in a short time under mild conditions. In comparison, the physically mixed NiFe2O4+2D MoS2 heterojunction immobilizing palladium nanoparticles shows poor photocatalytic activity. Photocatalytic results demonstrate that the in-situ formation of NiFe2O4/2D MoS2 p-n heterojunction greatly improves the visible-light absorption and facilitates the transferring of photogenerated electrons and holes. Moreover, Pd nanoparticles as the electron reservoirs can further suppress the electron-hole recombination and enhance the photocatalytic activity. The construction of semiconductive p-n heterojunction to immobilize metal nanocatalysts will be an inspiration for other useful photocatalytic applications.

Key words: photocatalyst, magnetically recoverable, p-n heterojunction, RhB degradation, Suzuki-Miyaura reaction 2


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Introduction Solar energy is considered as a clean and abundant source. It is of great importance to develop friendly and efficient photocatalysts for solar energy conversion.1-2 Traditional pure semiconductors such as TiO2, ZrO2 and CdS have been widely investigated due to their particular optical properties,3-7 but relatively narrow sunlight absorption range and rapid electron-hole recombination lead to low solar energy utilization. It is well known that constructing semiconductor heterojunction is an efficient way to improve photocatalytic performance.8-11 The formation of semiconductor heterojunction not only extends the sunlight absorption range, but also accelerates photogenerated electrons and holes transferring. Among various semiconductor heterojunctions, p-n heterojunction can further enhance the performance of photocatalyst because of the big difference in work function between p-type and n-type semiconductors.12-15 MoS2 nanosheet, as a rising star in two dimensional (2D) graphene analogues, has been widely studied in many fields including electrocatalysts,16 energy storage,17 sensors18 and drug delivery19 in recent years. As we know, 2D MoS2 is a p-type semiconductor and has a direct band gap with a value of 1.9 eV due to quantum confinement effect, which shows great potential in photocatalytic applications.20 Besides, 2D MoS2 owns large surface area and high thermal stability, which is suitable to be used as a matrix to dominate other nanomaterials.21-23 NiFe2O4, as a potential n-type semiconductor, is recently attracting tremendous attentions because

3



of its good light absorption ability and stable physical and chemical properties.24-26 In addition, the incorporation of NiFe2O4 provides photocatalysts good magnetic separability and catalytic recyclability. Furthermore, the introduction of noble metal nanoparticles (Au, Ag and Pd) to p-n heterojunction photocatalyst can greatly improve photocatalytic performance due to the formation of plasmonic heterojunction photocatalyst. On the one hand, the effect of localized surface plasmon resonance (LSPR) between noble metal and heterojunction can enlarge the light absorption range. On the other hand, noble metal nanoparticles can work as the electron reservoirs and accelerate the transferring of photogenerated electrons to substrates. Xu et al. prepared plasmonic p-n heterojunction photocatalyst Ag/ Ag2S/ BiVO4 with high photocatalytic activity on reduction of Cr6+.27 Zhang et al. reported plasmonic BiOCl/PANI/Pd photocatalyst that showed excellent activity on dye degradation.28 However, the preparation of these ternary plasmonic photocatalysts requires a multi-step process and relatively harsh reaction conditions. In this work, we design and synthesize a ternary plasmonic NiFe2O4/2D MoS2-Pd nanocomposite via a one-pot green and facile hydrothermal method. NiFe2O4/2D MoS2 p-n heterojunction is formed by in-situ hydrothermal process, while Pd nanoparticles are uniformly immobilized on the surface of NiFe2O4/2D MoS2 heterojunction. The as-obtained NiFe2O4/2D MoS2-Pd nanocomposite shows great visible light absorption ability and presents excellent photocatalytic activity in RhB degradation and Suzuki-Miyaura coupling reaction under mild conditions. 4


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Importantly, Pd nanoparticles as the electron reservoirs not only further enhance the visible-light absorption efficiency, but also improve the photocatalytic activity due to LSPR effect. As a comparison, the physical mixture NiFe2O4+2D MoS2-Pd presents poor photocatalytic activity. Moreover, the NiFe2O4/2D MoS2-Pd catalyst can be conveniently separated for recycling via an external magnet and the catalytic activity do not meet significant loss. Lastly, the possible photocatalytic mechanism is proposed.

Experimental Section Preparation of LixMoS2. The preparation of LixMoS2 is according to a previous report.29 Briefly, inside a glove box, 1.2 g of MoS2 bulk power and 60 mL of 0.5M n-BuLi solution in hexane were added into a 100 mL stainless-steel autoclave. The autoclave was heated at 100 oC for 4 h. After cooling down to room temperature, the LixMoS2 was filtered and washed with hexane (3×30 mL) to remove excess n-butyllithium. Then, the black LixMoS2 power was dried in vacuum at 55 oC overnight. Preparation of catalysts. To prepare NiFe2O4/2D MoS2-Pd, 60 mg of LixMoS2 power was dispersed in 50 mL of H2O under ultrasonic for 2 h to form a uniform solution. Then, NiCl2.6H2O (94.8 mg, 0.4 mmol), FeCl3 (129.8 mg, 0.8 mmol) and K2PdCl4 (32.6 mg, 0.1 mmol) were added to the MoS2 solution. After continued stirring for 3 h, 40 mL of ethanol was added. Then, 5M NaOH solution was added dropwisely until the solution pH reached 11. The mixture was transferred into a 150 5



mL stainless-steel autoclave and heated in an oven at 180 oC for 20 h. The black residue was collected and washed with water and ethanol for 3 times. The final NiFe2O4/2D MoS2-Pd was dried in vacuum at 65 oC for 12 h. The preparation of NiFe2O4, NiFe2O4-Pd, MoS2-Pd and NiFe2O4/2D MoS2 were same to above conditions except for the addition of the corresponding materials. To prepare physical mixture NiFe2O4+2D MoS2-Pd, the hydrothermal obtained NiFe2O4 was added to MoS2 solution, followed by ultrasonic for 2 h to form a suspension. K2PdCl4 was then added, and other conditions were same to above. Photocatalytic RhB degradation. The dye degradation experiments were conducted under visible-light irradiation by a 300 W Xe lamp with an optical filter (λ>400 nm) at an environmental temperature of 30 oC. 50 mg of NiFe2O4/2D MoS2-Pd catalyst was added to 50 mL of RhB solution (20 mg/L). The suspension was stirring for 60 min in dark to obtain the adsorption-desorption equilibrium between catalyst and dye molecules. After turning on Xe lamp, the concentration of RhB solution was determined by an UV-vis spectrophotometer at regular intervals. The photocatalyst was separated by using a magnet and used to evaluate catalytic recycling ability after washing with water and ethanol. Photocatalytic Suzuki-Miyaura reactions. Typically, aryl halide (0.5 mmol), phenyl boronic acid (0.75 mmol), K2CO3 (1 mmol), NiFe2O4/2D MoS2-Pd catalyst (3 mg), H2O (2.5 mL) and C2H5OH (2.5 mL) were added to a 25 mL Schlenk tube with a stir bar. The reaction was performed under the irradiation of a 300 W Xe lamp with an optical filter (λ>400 nm) in N2 atmosphere. The distance from the lamp to tube was 6


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25 cm and the reaction temperature was kept at 30 oC. After the reaction, the crude product was purified by flash silica-gel chromatography to obtain the desired compound. For the recycling applications, the used catalyst was separated from reaction system with a magnet and washed with water and ethanol. Then, the dried catalyst was used in next run. Characterization. The morphologies of the samples were imaged from scanning electron microscopy (SEM, Philips XL30FEG) and transmission electron microscopy (TEM, JEM-2100F handled at 200 kV). The crystal structures of the samples were determined by X-ray diffraction (XRD, D/max-γB diffractometer with Cu Kα radiation). Raman spectra were collected from a multi-channel confocal micro spectrometer (Dilor LABRAM-1B with 514 nm laser excitation). The UV-vis diffuse reflectance measurements were conducted on an UV-vis spectrophotometer (Shimadzu, UV-3600). The magnetization curves were recorded on a Quantum Design MPMS XL-7 at 300 K. The binding energies of elements were obtained from X-ray photoelectron spectroscopy (XPS, XR 5 VG using a monochromatic Mg X-ray source). Photoluminescence (PL) spectra were recorded on a Cary Eclipse spectrophotometer.

Transient

photocurrent

responses

were

tested

on

an

electrochemical workstation (PG 302N) with a conventional three-electrode system. The samples were load on indium tin oxide (ITO) electrode. 0.5 M Na2SO4 aqueous solution was used as electrolyte. 1H and

13

C NMR spectra were measured on a 500

MHz NMR spectrometer (Bruker Advance III HD).

7



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Results and Discussion Formation of NiFe2O4/2D MoS2-Pd nanocomposite. The photocatalyst NiFe2O4/2D MoS2-Pd was synthesized through a one-pot hydrothermal process as outlined in Scheme 1. Ultrathin MoS2 nanosheets as the composite matrix were obtained by ultrasonic exfoliation of LixMoS2 in water. As we know, these chemically exfoliated MoS2 nanosheets possess a large number of negative charges which are uniformly distributed on the surface.30 Therefore, the negative charged MoS2 nanosheets could not only disperse stably in aqueous solution, but also adsorb other metal cations such as Fe3+, Ni2+, Pd2+. In the hydrothermal process, on the one hand, Fe(OH)3 was transferred to β-FeOOH and then reacted with Ni(OH)2 to form NiFe2O4. 2D MoS2 nanosheets could be used as the stabilizers or structure-directing agents, and NiFe2O4 nanocubes formed successfully by in-situ growth on the surface of MoS2 nanosheets during the hydrothermal process. The formation of NiFe2O4 nanocubes could be attributed to the following reactions:31 Fe3+ + 3OH- → Fe(OH)3

(1)

Ni2+ + 2OH- → Ni(OH)2

(2)

Fe(OH)3 → β-FeOOH + H2O

(3)

2β-FeOOH + Ni(OH)2 →NiFe2O4 + 2H2O

(4)

On the other hand, Pd2+ ions were reduced to Pd nanoparticles with ethanol worked as a reducing agent.32 As a result, the ternary plasmonic NiFe2O4/2D MoS2-Pd hybrid structure was formed by in-situ growth of NiFe2O4 nanocubes and Pd nanoparticles on MoS2 nanosheets. 8


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Scheme 1. The synthesis route of ternary NiFe2O4/2D MoS2-Pd hybrid structure.

Characterization of NiFe2O4/2D MoS2-Pd nanocomposite. The surface morphologies of the NiFe2O4/2D MoS2-Pd nanocomposite were imaged by SEM. From Figure 1a and b, we can clearly see that MoS2 nanosheets are separated into curled paper-like morphology without any stack. The NiFe2O4 nanocubes are homogeneously intercalated into MoS2 layers, which prevent the MoS2 nanosheets from restacking. These ultrathin MoS2 nanosheets possess large surface to wrap the in-situ formed NiFe2O4 nanocubes, which is able to acquire a large amount of NiFe2O4/2D MoS2 heterojunction and thus improve the catalytic performance in the experiments. The ultrasmall Pd nanoparticles are uniformly distributed on the surface of NiFe2O4/2D MoS2 heterojunction. The uniformity of Pd nanoparticles increases interfacial interaction between Pd and NiFe2O4/2D MoS2 heterojunction and provides more active centers which can improve the photocatalytic activities. In general, the hydrothermal prepared MoS2 can easily restack to form clusters. However, the in-situ formation of NiFe2O4 nanocubes and Pd nanoparticles can greatly prevent the MoS2 nanosheets from restacking, which is impossible to form clusters. SEM images of MoS2-Pd, NiFe2O4-Pd and NiFe2O4/2D MoS2 are also shown in Figure S1-3. Compared to the in-situ formation of NiFe2O4/2D MoS2-Pd nanocomposite, the surface morphologies of the physical mixture NiFe2O4+2D MoS2-Pd present serious 9



aggregation (Figure S4). According to EDX elements analysis (Figure 1c), the nanocomposites contain Mo, S, Ni, Fe, O and Pd elements, with 7.44 wt% of Pd loading. Furthermore, the EDX-mapping spectroscopy (Figure 1d-j) shows that all elements have a uniform distribution on the surface of the as-prepared NiFe2O4/2D MoS2-Pd catalyst.

Figure 1. (a, b) SEM images, (c) EDX and (d-j) EDX-mapping of NiFe2O4/2D MoS2-Pd

Further confirmation of ternary NiFe2O4/2D MoS2-Pd hybrid structure was achieved by TEM and HRTEM images. As we can see in the TEM images (Figure 2a, b), NiFe2O4 nanocubes and Pd nanoparticles are firmly wrapped in the corrugated MoS2 nanosheets, which reveal the strong interfacial interaction. The HRTEM images (Figure 2c, d) indicate that Pd nanoparticles are not only homogeneously anchored on the surface of NiFe2O4 nanocubes and MoS2 nanosheets, but also embedded in the interface of NiFe2O4/2D MoS2 heterojunction, which also clearly demonstrate the 10


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tight interfacial contact between different components. The average diameter of Pd nanoparticles is around 4 nm. The lattice fringe spaces of 0.23 nm and 0.29 nm are assigned to (111) plane of Pd and (220) plane of NiFe2O4, respectively. Besides, the lattice fringe spaces of 0.62 nm is consistent to (002) crystal plane of MoS2.

Figure 2. (a, b) TEM and (c, d) HRTEM images of NiFe2O4/2D MoS2-Pd.

The XRD patterns of as-prepared samples are shown in Figure 3a. For MoS2-Pd, the diffraction peaks at 14.4o, 32.7o, 49.8o, 58.4o are corresponding to (002), (100), (105) and (110) planes of MoS2, respectively. Compared to bulk MoS2, the (002) peak of MoS2-Pd and NiFe2O4/2D MoS2-Pd is very week because of the complete exfoliation of MoS2 nanosheets. In addition, the obvious peaks at 2θ values of 30.1o, 35.3o, 43.0o, 53.5o, 56.3o and 62.4o are assigned to (220), (311), (400), (422), (511) and (440) crystal planes of NiFe2O4. As for NiFe2O4/2D MoS2-Pd nanocomposite, the diffraction peak at 39.6o can be attributed to (111) plane of crystalline Pd. The Raman spectra (Figure 3b) also illustrate the NiFe2O4/2D MoS2 heterojunction structure in 11



NiFe2O4/2D MoS2-Pd nanocomposite. The Raman peaks at 383 and 402 cm-1 are assigned to E12g and A1g mode of the hexagonal MoS2. Besides, the appearance of peaks at 470 and 681 cm-1 corresponds to the T22g and Ag vibration modes of trevorite NiFe2O4. The light absorption abilities of catalysts are shown in the UV-vis diffuse reflectance spectra (Figure 3c). NiFe2O4, NiFe2O4+2D MoS2-Pd and MoS2-Pd exhibit good absorption in the visible light region from 400 to 800 nm. In addition, the band gap (Eg) is calculated to be 1.7 eV via the formula:33 (αhν)2 = A(hν– Eg). The corresponding Kubelka-Munk transformed reflectance spectrum is inserted in Figure 3c. Compared to NiFe2O4, MoS2-Pd and physically mixed catalyst, NiFe2O4/2D MoS2-Pd exhibits stronger light absorption performance due to the synergistic effect between different components. The magnetization curves of NiFe2O4, MoS2-Pd and NiFe2O4/2D MoS2-Pd are displayed in Figure 3d. We can see that MoS2-Pd has no magnetism, while the saturation magnetization values of NiFe2O4, NiFe2O4/2D MoS2-Pd and NiFe2O4+2D MoS2-Pd are tested to be 58, 34 and 32 emu g−1 at room temperature, respectively.

12


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Figure 3. (a) XRD, (b) Raman, (c) UV-vis diffuse reflectance spectra and (d) Magnetization curves of NiFe2O4, MoS2-Pd, NiFe2O4+2D MoS2-Pd and NiFe2O4/2D MoS2-Pd.

X-ray photoelectron spectroscopy (XPS) was also employed to analyze the chemical valence and composition of the as-prepared NiFe2O4/2D MoS2-Pd. The survey spectrum (Figure 4a) shows the presence of Mo, S, Pd, Ni, Fe and O elements, which is consistent with the result of EDX elements analysis. Mo 3d high-resolution XPS spectrum is displayed in Figure 4b, with two major peaks at 229.0 and 232.3 eV for Mo 3d5/2 and Mo 3d3/2, indicating that MoS2 2H-phase is predominant due to the phase reversion from 1T-phase during hydrothermal process.34 Furthermore, 2H-phase peaks at 162.2 and 163.5 eV for S 2p XPS spectrum also reveal the MoS2 phase transition (Figure 4c). As expected, another peak located at 160.3 eV is attributed to rich-electron sulfur which is generated from electron transfer of butyllithium during the intercalation.35 In addition, the XPS spectrum of Pd 3d (Figure 13



4d) shows two peaks at 335.5 and 340.8 eV, corresponding to Pd 3d5/2 and Pd 3d3/2, respectively. Importantly, only metal Pd can exhibit the plasmonic effect in this catalyst. We cannot find the peaks of Pd oxide in the XPS spectrum. During the hydrothermal process to prepare our catalyst, C2H5OH was employed as the reducing agent, and Pd2+ ions were reduced to metal Pd nanoparticles under high temperature and pressure. Two peaks present at 856.1 and 862.4 eV arise from Ni 2p3/2 and Ni 2p1/2 (Figure 4e). The binding energies located at 711.2 and 725.0 eV belong to Fe2p3/2 and Fe 2p1/2, respectively (Figure 4f). Thus, the composition of the NiFe2O4/2D MoS2-Pd is further confirmed by XPS analysis.

Figure 4. (a) XPS survey spectrum of NiFe2O4/2D MoS2-Pd, XPS spectra of (b) Mo 3d, (c) S 2p, (d) Pd 3d, (e) Ni 2p and (f) Fe 2p, respectively.

Catalytic properties of NiFe2O4/2D MoS2-Pd for RhB degradation. RhB as a typical artificial dye is widely used as color additive on dyeing industry. However, RhB can lead to serious environmental pollution and health problems.36 Photodegradation of RhB is a hopeful strategy to decrease the damage. Figure 5a reveals the decomposition performance of RhB solution in the presence of 14


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NiFe2O4/2D MoS2-Pd. The adsorption of characteristic peak is 554 nm. The peak intensity decreases obviously with the extending of irradiation time. Figure 5b illustrates the photodegration efficiency (C/C0) of different catalysts. C and C0 represent the instantaneous and initial concentration of RhB solution respectively. When no catalyst was added, little change of RhB solution was detected, which means RhB solution is quite stable. NiFe2O4 and NiFe2O4-Pd showed poor photodegration abilities with the efficiencies of 9.1% and 14.2%, respectively. To our delight, the catalysts NiFe2O4/2D MoS2 and NiFe2O4/2D MoS2-Pd exhibited excellent photodegration efficiency under 30 min of visible light irradiation. The decomposition percentages of RhB solution were 82.2% and 95.3%. In comparison, the photocatalytic efficiency decreased to 59.2% when the photocatalyst was replaced by physical mixture NiFe2O4+2D MoS2-Pd, which demonstrates the crucial role of the contact NiFe2O4/2D MoS2 heterojunction. It is worth noting that a considerable concentration of RhB solution decreased before the visible light irradiation, which is attributed to the strong RhB adsorption of NiFe2O4/2D MoS2 and NiFe2O4/2D MoS2-Pd. The above results indicate that the incorporation of MoS2 improves the RhB adsorption ability which is benefit to the subsequent photodegration, while the in-situ formation of NiFe2O4/2D MoS2 heterojunction can greatly improve the light absorption efficiency. Besides, the combination of plasmonic Pd nanoparticles can further promote the decomposition of RhB solution due to facilitating the separation of photogenerated electron-hole pairs and accelerating the transferring of energetic electrons to substrates.37 Figure 5c shows the recycling degradation of RhB solution 15



over NiFe2O4/2D MoS2-Pd. We can see that there is no significant activity loss after three repeated uses, which demonstrates that NiFe2O4/2D MoS2-Pd has good stability for dye degradation.

Figure 5. (a) Time-dependant absorption spectra of RhB in the presence of NiFe2O4/2D MoS2-Pd. (b) Photodegradable efficiency of RhB over different catalysts. (c) Cycling degradation tests of RhB over NiFe2O4/2D MoS2-Pd.

Catalytic properties of NiFe2O4/2D MoS2-Pd for Suzuki-Miyaura coupling reaction. Despite conventional homogeneous palladium catalysts have achieved relatively high catalytic activity, their disadvantages such as toxic ligands, high reaction temperature and difficult recycling, have promoted the development toward exploring more stable, cost-effective and environmentally friendly heterogeneous catalysts for Suzuki-Miyaura coupling reaction.38-43 We took the coupling of iodobenzene with phenylboronic acid as a template reaction to explore the photocatalytic activity of NiFe2O4/2D MoS2-Pd (Table 1). We began our study by investigating the influence of several commonly used solvents such as DMF, C2H5OH and H2O. DMF was proved to be a disappoint choice (Table 1, entry 1). Low yields were obtained when reactions were performed in C2H5OH and H2O (Table 1, entry 2, 3). To our delight, the mixture solvent could significantly improve the photocatalytic reactivity. DMF-H2O (1:1) as the solvent could improve the yield up to 71% (Table 1,

16


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entry 4). We could afford the biphenyl in 97% yield when C2H5OH-H2O (1:1) worked as the solvent (Table 1, entry 5). Without the protection of N2 atmosphere, the yield of corresponding product decreased to 80% (Table 1, entry 6). As expected, we could hardly obtain the desired product when the visible light was removed (Table 1, entry 7). In addition, the reaction could proceed well at 80 oC in dark, with a yield of 98% (Table 1, entry 8). Table 1. Screening of Suzuki-Miyaura coupling reactiona

a

Entry

Solvent

Light

Yield (%)b

1

DMF

+

trace

2

C2H5OH

+

7

3

H2 O

+

18

4

DMF-H2O (1:1)

+

71

5

C2H5OH-H2O (1:1)

+

97

6c

C2H5OH-H2O (1:1)

+

80

7

C2H5OH-H2O (1:1)

−

trace

8d

C2H5OH-H2O (1:1)

−

98

Reactions conditions: iodobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), K2CO3 (1 mmol),

NiFe2O4/2D MoS2-Pd (3 mg), solvent (5 mL), 300 W Xe lamp (λ > 400 nm), N2 atmosphere, 30 o

C, 1 h. bYields were determined by 1H NMR spectroscopy with 1,3,5-trimethoxybenzene as an

internal standard. cAir atmosphere. dReaction temperature: 80 oC, 1 h.

After getting the optimized reaction conditions, the substrate scope of photocatalytic Suzuki-Miyaura reaction was investigated. As listed in Table 2, various aryl halides and arylboronic acids reacted smoothly to obtain the corresponding products. Arylboronic acids bearing the substitutes such as fluoride and methoxyl 17



were converted to biaryl compounds with excellent yields (Table 2, entry 1, 2). Aryl iodides containing electron-rich groups and electron-deficient groups reacted efficiently with phenylboronic acid to afford desired products in good yields (Table 2, entry 3-7). As a comparison, the steric hindrance has a negative effect on the coupling reaction (Table 2, entry 5). However, aryl bromides as the substrates showed lower reactivity compared to their iodine counterparts (Table 2, entry 8-10). Table 2. Substrate scope of photocatalytic Suzuki-Miyaura reactiona

a

Reactions conditions: aryl halide (0.5 mmol), arylboronic acid(0.6 mmol), K2CO3 (1 mmol),

NiFe2O4/2D MoS2-Pd (3 mg), C2H5OH-H2O (1:1) (5 mL), 300 W Xe lamp (λ> 400 nm), N2 atmosphere, 30 oC. bIsolated yields. 18


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Recyclability and stability are two important factors for heterogeneous catalyst from the perspective of cost and environmental protection. Herein, we investigated the recyclability and stability of NiFe2O4/2D MoS2-Pd catalyst for photocatalytic Suzuki-Miyaura coupling reaction. After each cycle, the used catalyst could be separated easily from the reaction system using an external magnet, and tested in next run after washing and drying. As we can see in Figure 6, the recycled catalyst could be used five times without significant loss in catalytic activity, which demonstrates the good catalyst stability. Leaching issues exist in many heterogeneous systems, and the leached Pd particles in the solution lead to the loss of Pd content of the catalyst. According to EDX elements analysis (Figure S5), Pd content of the catalyst decreases from 7.44 wt% to 6.94 wt% after five repeated uses. In addition, TEM images (Figure S6) of recovered catalyst after five times show no obvious aggregation of Pd nanoparticles or change in morphology of NiFe2O4/2D MoS2-Pd catalyst.

Figure 6. Recycling ability of NiFe2O4/2D MoS2-Pd for photocatalytic Suzuki coupling reaction.

Several control reactions were performed to investigate the photocatalytic mechanism of NiFe2O4/2D MoS2-Pd (Table 3). As an electron-trapping agent, P-benzoquinone was used to capture the photogenerated electrons on the surface of 19



catalyst. The yield of desired biphenyl decreased to 42% (Table 3, entry 1). Meanwhile, when Et3N was employed as a photogenerated holes scavenger, the conversion to corresponding compound also declined to 70% (Table 3, entry 2). Thus, the trapping experiments indicate the important roles of photogenerated electrons and holes in the Suzuki-Miyaura coupling reaction. The existence of O2 could lead to the lower yield of product (Table 1, entry 6). The reason is that O2 can combine with photogenerated electrons to form ·O2−, which can not only consume the energetic electrons on catalyst surface, but also decompose the adsorbed substrates. Compared to the standard catalyst NiFe2O4/2D MoS2-Pd (Table 3, entry 3), no product was obtained in the absence the Pd nanoparticles (Table 3, entry 4). As another control experiment, just trace yield was observed when the catalyst was replaced by MoS2-Pd (Table 3, entry 5). On the one hand, the light absorption efficiency decreases significantly compared to NiFe2O4/2D MoS2-Pd. On the other hand, the poor conductivity of MoS2-Pd can lead to rapid recombination of photogenerated electrons and holes. Furthermore, the yield of 57% was obtained using NiFe2O4-Pd as the photocatalyst (Table 3, entry 6).

As another comparison, the catalyst NiFe2O4+2D

MoS2-Pd formed by physical mixture of NiFe2O4 and 2D MoS2 showed very poor photocatalytic activity (Table 3, entry 7). It can be concluded that the in-situ growth of heterojunction between NiFe2O4 and MoS2 can significantly improve photocatalytic activity. Besides, the intimate interfacial contact between NiFe2O4 and Pd formed by in-situ hydrothermal process also plays an important role in Suzuki coupling reaction.

20


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Table 3. Control experiments for photocatalytic Suzuki coupling reactiona

a

Entry

Catalyst

Additive

Yield (%)b

1

NiFe2O4/2D MoS2-Pd

benzoquinone

42

2

NiFe2O4/2D MoS2-Pd

Et3N

70

3

NiFe2O4/2D MoS2-Pd

−

97

4

NiFe2O4/2D MoS2

−

—

5

2D MoS2-Pd

−

trace

6

NiFe2O4-Pd

−

57

7

NiFe2O4+2D MoS2-Pd

−

11

Reactions conditions: iodobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), K2CO3 (1 mmol),

catalyst (3 mg), additive (0.5 mmol), C2H5OH-H2O (1:1) (5 mL), 300 W Xe lamp (λ > 400 nm), N2 atmosphere, 30

o

C.

b

Yields were determined by

1

H NMR spectroscopy with

1,3,5-trimethoxybenzene as an internal standard.

Photoluminescence (PL) test as an effective tool can evaluate the separation efficiency of photogenerated electrons and holes of catalyst. The PL spectra of NiFe2O4, NiFe2O4/2D MoS2 and NiFe2O4/2D MoS2-Pd with an excitation wavelength of 410 nm are shown in Figure 7a. For NiFe2O4, a strong PL intensity is observed due to low separation efficiency of photogenerated electrons and holes. The PL intensity is reduced significantly after combining with MoS2, because the formation of NiFe2O4/2D MoS2 heterojunction can efficiently promote the transferring of photo-induced carriers. Compared to NiFe2O4/2D MoS2, NiFe2O4/2D MoS2-Pd presents stronger PL quenching signal, which indicates Pd nanoparticles can trap photogenerated

electrons

on

catalyst

and

suppress

21


the

recombination

of


photogenerated electrons and holes. Furthermore, the transient photocurrent responses of these samples were collected to reflect the separation efficiency of photogenerated charge carriers under visible-light irradiation. As shown in Figure 7b, NiFe2O4/2D MoS2-Pd presents the highest photocurrent intensity, which also confirms the best separation efficiency of photogenerated electrons and holes.

Figure 7. (a) PL spectra and (b) transient photocurrent response of NiFe2O4, NiFe2O4/2D MoS2 and NiFe2O4/2D MoS2-Pd.

Proposed mechanism for photocatalytic applications. On the basis of the results of the photocatalytic control experiments, PL analysis and photocurrent responses, it can be concluded that the construction of NiFe2O4/2D MoS2 heterojunction and introduction of Pd nanoparticles can significantly suppress the recombination of photogenerated electron-hole pairs and improve the photocatalytic performance on RhB degradation and Suzuki-Miyaura reaction. A possible photocatalytic mechanism is proposed in Figure 8. For Suzuki-Miyaura reaction, on the one hand, charge separations begin to form electron-hole pairs within NiFe2O4 nanocubes and MoS2 nanosheets under irradiation of visible light. Due to the intimate interfacial contact between Pd and NiFe2O4/2D MoS2 heterojunction, Pd nanoparticles can trap the electrons located on the conduction band of NiFe2O4, and then these energetic 22


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electrons transfer to the conduction band of MoS2. At the same time, the valence band holes on MoS2 migrate to the valence band of NiFe2O4. The above process of charge carriers transferring realizes a high separation efficiency of electron-hole pairs. On the other hand, Pd nanoparticles as the electron reservoirs can trap the energetic electrons due to the LSPR effect.44 These energetic Pd nanoparticles as the active center can attack C-I bond of the adsorbed iodobenzene molecule, and form aryl-Pd complex subsequently. Meanwhile, phenylboronic acid combines with OH− to produce negative B(OH)3-, which next suffers C-B bond cleavage to generate the biaryl-Pd complex with the assist of photoexcited holes. Finally, the desired biphenyl product is obtained via a step of reductive elimination.45 For RhB degradation, the photogenerated electrons and holes react with O2 and H2O respectively, which can obtain the active ·O2− and ·OH species. Consequently, these active ·O2− and ·OH species can oxidize RhB into harmless products such as CO2 and H2O.

Figure 8. Suggested mechanism for photocatalytic applications by NiFe2O4/2D MoS2-Pd.

23



Conclusions In summary, a ternary plasmonic NiFe2O4/2D MoS2-Pd photocatalyst is designed, and each component contributes to promote the efficiency of the photocatalytic RhB degradation and Suzuki-Miyaura coupling reaction due to the synergistic effect. The p-n heterojunction of NiFe2O4/2D MoS2 in catalyst not only enlarges the absorption of visible light, but also accelerates the transferring of photogenerated electrons and holes. Pd nanoparticles immobilized on the surface of NiFe2O4/2D MoS2 heterojunction trap the moving energetic electrons and enrich the electron density of Pd catalytic center, which can significantly facilitate the degradation of RhB and activation of aryl halides. Compared to the in-situ growth of NiFe2O4/2D MoS2 heterojunction, the physical mixture of NiFe2O4 and 2D MoS2 presents bad performance on photocatalytic applications. Meanwhile, the good magnetic properties help to the rapid and easy separation of catalyst. To the end, this work can provide an inspiration of constructing other novel semiconductor heterojunction materials to immobilize metal nanocatalysts for more photocatalytic applications.

Supporting information SEM images of other catalysts, EDX spectrum and TEM images of reused catalyst, and NMR data of biphenyl products.

24


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For Table of Contents Use Only

NiFe2O4/2D MoS2-Pd presents good visible-light photocatalytic activities on RhB degradation and Suzuki-Miyaura coupling reaction with high sustainability.

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2D MoS2 p-n heterojunction immobilizing

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