Preparation, Characterization, and Catalytic Application

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FeO@MoS Core-Shell Composites: Preparation, Characterization and Catalytic Application Tianran Lin, Jing Wang, Liangqia Guo, and FengFu Fu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02516 • Publication Date (Web): 27 May 2015 Downloaded from http://pubs.acs.org on May 29, 2015

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The Journal of Physical Chemistry

Fe3O4@MoS2 Core-Shell Composites: Preparation, Characterization and Catalytic Application Tianran Lin, Jing Wang, Liangqia Guo,* and Fengfu Fu

Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection Technology for Food Safety, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China

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KEYWORDS: MoS2, Fe3O4, 4-nitrophenol, 4-aminophenol

ABSTRACT: Molybdenum disulfide (MoS2) has received tremendous attention due to the earth-abundant composition and high catalytic activity. However, the catalytic activity of MoS2 except electro- and photocatalytic has seldom been explored. Herein, Fe3O4@MoS2 core-shell composites were prepared for the first time by in situ growth of MoS2 nanosheets on the surfaces of Fe3O4 nanoparticles under different temperature and the catalytic performance of the resulting composites were evaluated by using the catalytic reduction of 4-nitrophenol to 4-aminophenol. FE-SEM, TEM, XRD, and XPS analyses verified the core-shell structure with MoS2 nanosheets of defect-rich and oxygen-incorporation on the surfaces of Fe3O4 nanoparticles. Fe3O4@MoS2 composites were found to exhibit a high catalytic activity for the reduction of 4-nitrophenol with the highest activity factor k=3773 min-1 g-1. A plausible catalytic mechanism for the reduction of 4-nitrophenol was also proposed. This study presented an inexpensive, reusable, fast and high efficient catalyst for the reduction of 4-nitrophenol without noble metals.


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INTRODUCTION Transition metal dichalcogenides (TMDCs) with lamellar structures have attracted growing attention because some of them are semiconductors with sizable bandgaps and are naturally abundant. 1-4 As a prototypical TMDCs material, the bulk MoS2 crystal is an indirect band gap semiconductor with an energy gap of 1.29 eV, which is composed of covalently bonded S-Mo-S sheets that are held together by weak van der Waals interactions in hexagonally packed structures. However, single-layer MoS2 is a direct gap semiconductor with a bandgap of 1.8 eV.5 The transition from indirect bandgap to direct bandgap results in significant enhancement in photoluminescent quantum yield.6-9 MoS2 displays many intriguing physical and chemical properties with a wide range of applications including lubrication,10 optoelectronic devices,11-12 transistors,13-15 sodium and lithium ion batteries,16-19 supercapacitors,20 sensors.21-25 Two-dimensional (2D) MoS2 nanosheets also have showed potential applications in biomedical region such as NIR photothermal therapy,26-28 drug delivery,29 antibacterial activity,30 enzyme mimetic,31 etc. Owing to the semiconducting property with bandgaps ranging from the visible to the near-infrared as well as the catalytic activity of its edge sites, MoS2 was found to be promising electro-catalysts32-37 and photocatalysts.38 Considerable efforts have been dedicated to improving and optimizing the catalytic activity of various MoS2 materials such as oxygen-incorporated MoS2 nanosheets,39 defect-rich MoS2 nanosheets,40 metallic MoS2 nanosheets,41,42 and hybrid with other nanomaterials.43-48 To date, although electro- and photo-catalytic 3 ACS Paragon Plus Environment


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activities of MoS2 have been studied extensively, the other catalytic activity of MoS2 has less been explored.31 4-Nitrophenol (4-NP) is one of the common organic pollutants in industrial and agricultural wastewaters and 4-aminophenol (4-AP) is very useful and important in many applications including analgesic and antipyretic drugs, photographic development, corrosion inhibition, anticorrosion lubrication, etc. The conversion of 4-NP to 4-AP has been rigorously investigated for the degradation of 4-NP and the efficient production of 4-AP. To date, most of the existing investigations on this reaction are based on noble-metal catalysts such as Au,49-52 Pd,49, 50, 52,53 Pt,49, 50, 52, 54 Ag.55,56,57 Recently, MoS2 nanosheets was prepared by chemical exfoliation using n-butyllithium solution and performed as catalysts for a number of model reduction reactions.58 However, it was difficult to recycle MoS2 nanosheets from solution. And chemical exfoliation was time-consuming due to the limited diffusion rate of intercalation compound such as n-butyllithium. From the viewpoint of practical applications in the degradation of environmental pollutions, it is highly necessary to explore eco-friendly, low cost and highly active as well as recyclable catalysts. Herein, we report for the first time the preparation of Fe3O4@MoS2 core-shell composites by a “bottom-up” hydrothermal method for use in the catalytic reduction of 4-NP to 4-AP with sodium borohydride (Figure 1). Fe3O4@MoS2 core-shell composites were prepared by in situ growth of MoS2 nanosheets on the surfaces of Fe3O4 nanoparticles (NPs). Fe3O4@MoS2 core-shell composites were found to exhibit


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high catalytic activity and excellent reusability for the reduction of 4-NP without the use of noble metals. RESULTS AND DISCUSSIONS Characterization of Fe3O4@MoS2 core-shell composites. The Fe3O4 NPs were prepared via a hydrothermal method. As verified by the FE-SEM (Figure S1(A, B)) and TEM (Figure S1(C, D)) images, Fe3O4 NPs have a mean diameter of about 200 nm. Fe3O4 NPs were employed as catalyst supports due to their rapid, high efficient and cost-effective magnetic separation ability. Fe3O4@MoS2 core-shell composites were prepared by in situ growth of MoS2 nanosheets on the surfaces of Fe3O4 NP using (NH4)6Mo7O24 and excess thiourea as precursors under different hydrothermal temperature (160 °C, 180 °C, 200 °C) (Figure 1). As the increasing of synthesis temperature, more nanosheets with sharp edges are wrapped on the surfaces of Fe3O4 NPs and the crystallinity as well as the order of MoS2 nanosheets also increases. Small nanosheets with obscure edges are patched on the surface of Fe3O4 NPs (Figure S1(E-H)), indicating the formation of MoS2 nanosheets with low crystallization under low-temperature conditions (160 °C). Under the synthesis temperature of 180 °C MoS2 nanosheets with both obscure edges and sharp edges (Figure 2 (A-D) are wrapped on the surfaces of Fe3O4 NPs, indicating the crystallinity of MoS2 nanosheets is still insufficient.40 Flower-like Fe3O4@MoS2 core-shell composites with obvious ripples and corrugations on the surface can be observed under the synthesis temperature of 200 °C (Figure S1(I-L)), revealing the formation of high crystalline and ultrathin MoS2 nanosheets. Interestingly, accompanied by the decreased synthesis 5 ACS Paragon Plus Environment


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temperature, the fringes of the curled edges become more discontinued, suggesting the increased number of defects. TEM images (Figure 2(C, D) and Figure S1(G, H, K, L)) verify that the as-prepared composites are composed of a compact core and a lower density shell of about 10 nm to 150 nm thickness. The XRD patterns of Fe3O4 NPs and Fe3O4@MoS2 core-shell composites synthesized at different temperture are shown in Figure S2. The diffraction peaks of Fe3O4 NPs at 2θ values of 18.3, 30.1, 35.5, 43.1, 57.0, and 62.6° could be indexed to the (111), (220), (311), (400), (511), and (440) planes of the magnetite structure (JCPDS card 19-629), respectively.59 Two new weak diffraction peaks emerge at the low-angle region and the peak at 32° corresponding to the (100) plane of the pristine 2H-MoS2 in the XRD pattern of Fe3O4@MoS2 core-shell composites.39,40 All the diffraction peaks of MoS2 are broadened with decreased synthesis temperature, suggesting the declined crystallinity of MoS2 nanosheets. Taking the composites synthesized at 180 °C as examples, the chemical composition and microstructure of as-prepared composites were further characterized by elemental mapping, HRTEM, XPS techniques. The high-angle annular dark-field scanning TEM (HAADF-STEM) image and corresponding elemental mapping images (Figure S3) confrmed that Fe and O atoms were homogeneously distributed in the core and Mo and S atoms were homogeneously distributed in the shell, which corroborated the core-shell structure of the as-prepared composites. It is noteworthy to mention that oxygen was also incorporated in the MoS2 shell. Many dislocations and distortions can be observed from the HRTEM images (Figure S4), which suggests a novel defect-rich structure. 6 ACS Paragon Plus Environment

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The disordered atomic arrangement causes the cracking of the basal planes and thus results in the formation of additional edges, which maight significantly increase the exposure of active edge sites. The XPS Mo 3d core-shell spectrum was shown in Figure S5A, the component at 226.0 eV corresponds to S 2s of MoS2. The two main intense Mo 3d5/2 (228.8 eV) and Mo 3d3/2 (232.1 eV) components are characteristic of MoS2,39, 43 while the high binding energy peak of Mo 3d (235.2 eV) corresponds to MoO3 or MoO42-, which may be inherited from the molybdate precursor due to the low synthesis temperature resulting in insufficient reaction process.39 In XPS S 2p core-level spectrum (Figure S5B), the main doublet located at binding energies of 161.6 and 162.9 eV corresponds to the S 2p3/2 and S 2p1/2 components of MoS2.39, 43 The MoSx stoichiometry in the composites is MoS2.05 determined by the difference between the binding energies of Mo 3d5/2 and S 2p3/2,60 suggesting the existence of unsaturated sulfur atoms and the disorderd structure on the edges.39 Therefore, the growth of oxygen-incorporated MoS2 nanosheets with insufficient crystallinity and defect-rich structure on the surfaces of Fe3O4 NPs due to low synthesis temperature and excess thiourea during the synthesis process39,40,43 could be corroborated for Fe3O4@MoS2 core-shell composites (180 °C) from the combined analysis of FE-SEM, TEM, XRD, and XPS spectroscopy. The masses of MoS2 nanosheets and Fe3O4 NPs were determined by ICP-MS and the content of MoS2 nanosheets in the composites (180 °C) was calculated to about 43.7%. Magnetic characterization using a magnetometer at 300 K (Figure S6A) indicates that the Fe3O4 NPs and Fe3O4@MoS2 core-shell composites both exhibit 7 ACS Paragon Plus Environment


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superparamagnetic behavior and little hysteresis, remanence, and coercivity due to the fact that the particles are composed of ultrafine magnetite nanocrystals. The magnetization saturation values were measured to be 32.282 and 15.237 emu g-1 for Fe3O4 NPs and Fe3O4@MoS2 core-shell composites, respectively. The saturation magnetization of Fe3O4@MoS2 core-shell composites is about one half lower than that of Fe3O4 NPs. The decrease of saturated magnetization is undoubtedly related to the surfaces of Fe3O4 covered by MoS2 nanosheets. The Fe3O4@MoS2 core-shell composites can be dispersed in water by vigorous shaking, resulting in a black-colored suspension. Very fast aggregation of the composites from their homogeneous dispersion can be observed in the presence of an external magnetic field, while re-dispersion occurs quickly with a slight shaking once the magnetic field is removed (Figure S6B). These results show that Fe3O4@MoS2 core-shell composites possess excellent magnetic responsivity and re-dispersibility, which is important in terms of their practical manipulation. Application of Fe3O4@MoS2 core-shell composites for catalytic reduction of 4-NP. The catalytic performance of Fe3O4@MoS2 core-shell composites was evaluated by catalytic reduction of 4-NP with an excess amount of NaBH4 (Figure 1). The 4-NP solution exhibits a bright yellow color with an absorbance at 400 nm after immediate addition of freshly prepared NaBH4 solution. It can be ascribed to the formation of 4-nitrophenolate ions in alkaline condition which is caused by the addition of NaBH4 solution.49, 51, 54, 55 No change in the absorption is determined (Figure S7) and the mixtures remain yellow color (Figure S8 (A,C)) even after 8 ACS Paragon Plus Environment

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standing for 24 h, indicating that the reduction does not proceed without catalyst or at the present of Fe3O4 NPs. However, after addition of MoS2 nanosheets or Fe3O4@MoS2 core-shell composites, the absorption peaks at 400 nm gradually decrease with a concomitant increase in the peak intensity at about 295 nm (Figure S9 and Figure S10) and the observation of a fading and ultimate bleaching of the bright yellow color of reaction mixture within 4 min (Figure S8 (B) and Figure S9 (B)), revealing the reduction of 4-NP to 4-AP49, 51, 52 and the catalytic activity of MoS2 nanosheets. As shown from the XPS (Figure S5 (C, D)), the chemical state and composition of MoS2 nanosheets on the surfaces of Fe3O4 NPs are almost not changed after the catalytic reaction, which further confirms the intrinsic catalytic property of MoS2 nanosheets. The selectivity for the reduction reaction was also examined by mass spectrometry. The selectivity for 4-AP is very high, since almost all initial 4-NP is transformed to 4-AP (Figure S11). It was worthwhile to note that the reduction reaction started immediately after the addition of the composites and there was no induction time required. This might be advantageous for ease of use in real technological applications. Because NaBH4 is in great excess in the reaction, its concentration can be regarded as being constant throughout the reaction. Therefore, pseudo-first-order kinetics can be applied with respect to 4-NP. Figure 3A shows the plot of ln[C(t)/C(0)] against reaction time (t), where C(t) and C(0) are the concentrations of 4-NP at time t and 0, respectively. The linear fit between ln[C(t)/C(0)] and reaction time (t) with a coefficient of determination very close to unity supports the assumption of 9 ACS Paragon Plus Environment


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pseudo-first-order kinetics. The apparent rate constants (kapp) calculated from the slope are 0.240 min-1, 0.732 min-1, and 0.546 min-1 for the Fe3O4@MoS2 core-shell composites prepared under 160 °C, 180 °C and 200 °C, respectively. The Fe3O4@MoS2 core-shell composites prepared under 180 °C showed the highest catalytic efficiency, which was comparable with that of ce-MoS2 nanosheets.58 The highest catalytic activity may benefit from the defect-rich structure and the incorporation of oxygen in MoS2 nanosheets due to the low synthesis temperature resulting in insufficient reaction process. The number of active sites and conductivity are two crucial factors to affect the catalytic activity of MoS2 nanosheets.32-48 For the Fe3O4@MoS2 core-shell composites prepared under 180 °C the defect-rich structure can offer abundant unsaturated sulfur atoms to generate active sites, while the oxygen incorporation can effectively reduce the bandgap of MoS2 catalyst and lead to the enhancement of the intrinsic conductivity.39 Therefore, the Fe3O4@MoS2 core-shell composites prepared under 180℃ show the highest catalytic activity in the reduction of 4-NP. To compare our result with literature values, the ratio of apparent rate constant kapp to the total mass of the catalyst (k=kapp/m, m is the mass of MoS2 nanosheets calculated by ICP-MS) was calculated. The highest activity factor for MoS2 nanosheets is k=3773 min-1 g-1, which is much larger than those reported previously for several noble-metal NPs (Table S1). Recyclability of catalysts is important for their practical application. An advantage of the Fe3O4@MoS2 core-shell composites was that they could be easily separated by using an external magnet. As shown in Figure 3B, the composites could 10 ACS Paragon Plus Environment

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be successfully recycled and reused for at least fifteen times with a stable conversion of more than 95%, suggesting an excellent stability and long life. In addition, the growth of MoS2 nanosheets on the surfaces of Fe3O4 NPs avoids the aggregation of MoS2 nanosheets. Undoubtedly, Fe3O4@MoS2 core-shell composites exhibited a better performance than the ce-MoS2 nanosheets58 from the aspects of recycling operation and stability. Additionally, the temperature-dependent rate constant (k) of this catalytic reduction of 4-NP was further investigated at five different temperatures (25, 35, 45, 55, and 65 °C) and the corresponding rate constants were evaluated. It is clearly shown in Figure 4A that the value of k increases with the increasing temperature, indicating that the rate of the reduction reaction is accelerated with the increasing of temperature. Moreover, a good linear relationship was observed between lnk and 1000/T (Figure 4B). The apparent activation energy (Ea) can be calculated from the Arrhenius equation (Equation 1).54 lnk = lnA − Ea/RT

(1)

where lnA is the intercept of the line and R is the gas constant. The calculated Ea value is approximately 22.36 kJ mol-1, which is smaller than those of other metallic NPs.54 A small Ea value confirms that the MoS2 nanosheets exhibit a considerably high catalytic activity. Although the reaction between 4-NP and NaBH4 is thermodynamically favored, in which electron transfer from BH4- (-1.33 V vs. NHE) to 4-nitrophenol (-0.76 V vs. NHE) is mediated by surface metal atoms,61 the reaction only proceeds effectively in 11 ACS Paragon Plus Environment


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the presence of a catalyst. The catalytic mechanism for 4-NP reduction relies on electrons from the BH4- donor to the acceptor 4-NP.61 It has been reported that BH4adsorption is a fast and reversible process at the surfaces of catalyst, and the coadsorption of 4-nitrophenolate ions overcomes the kinetic barrier for the reaction and initiates an interfacial electron transfer reaction.61 To verify the adsorption of BH4- on the surfaces of MoS2 nanosheets EDTA was added and mixed with the catalyst before the reduction reaction (Figure S12). The catalytic reduction of 4-NP was decreased with the increase of EDTA, which indicated that EDTA competed with BH4- to form complexes with Mo4+ on the edges and resulted in the decrease of the adsorption of BH4- on the surfaces of MoS2 nanosheets. Unsaturated sulfur atoms on the edge of MoS2 have been reported as active sites for hydrogen evolution catalysis.39, 40 Therefore, a plausible explanation for the reduction of 4-NP (Figure 5) is that Mo4+ on the edges of MoS2 nanosheets acts as the trap centers that adsorb negative BH4- and unsaturated sulfur atoms as active sites that capture 4-NP molecules on a neighboring site after addition of 4-NP. After electron transfer to MoS2, the hydrogen atom forms from the adsorbed BH4-, and attacks 4-NP molecules. Meanwhile, the uptake of electrons by the adsorbed 4-NP molecules at the active sites leads to the reduction of 4-NP into the 4-AP products. The neutral 4-AP molecules desorb from the surfaces of MoS2 nanosheets and create a free surface for the next catalytic cycle. Simultaneously, the BH4- ions help to release the trapped oxygen atoms from the active sites, resulting in the formation of borate ions.61 The defect-rich structure of MoS2 nanosheets provides abundant active sites and trap centers at the 12 ACS Paragon Plus Environment

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edge sites and the oxygen incorporation enhances the intrinsic conductivity to facilitate the electron transfer from BH4- to 4-NP, resulting in the high catalytic activity. CONCLUSIONS In summary, Fe3O4@MoS2 core-shell composites were prepared for the first time by in situ growth of MoS2 nanosheets on the surfaces of Fe3O4 NPs under different temperature. The catalytic property of Fe3O4@MoS2 core-shell composites was investigated in a typical model reaction based on the reduction of 4-NP with sodium borohydride to 4-AP. The as-prepared composites show excellent catalytic performance in reduction of 4-NP at room temperature with easy reusability and good stability. The high catalytic activity may benefit from the defect-rich structure and the incorporation of oxygen in MoS2 nanosheets. The strategy reported here provides the development of noble-metal-free catalysts with characters of low cost, reusability, fastness, and high efficiency, which may be able to be found widespread usages in a number of industry applications.

EXPERIMENTAL SECTION Materials:

Ferric

chloride

hexahydrate

(FeCl3·6H2O),

sodium

acetate

(CH3COONa·3H2O), ethylene glycol, thiourea (CN2H4S) and sodium borohydride (NaBH4)

were

bought

from

Sinopharm

chemical

reagent

Co.,

Ltd.

(NH4)6Mo7O24·4H2O was bought from Aladdin. 4-nitrophenol (4-NP) was bought



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from Acros. 4-aminophenol (4-AP) was bought from J&K Scientific. All materials were analytical reagents. Synthesis of Fe3O4@MoS2 core-shell composites: Fe3O4@MoS2 core-shell composites were prepared by a two-step hydrothermal method. Firstly, Fe3O4 NPs were synthesized according to a previous report of Deng et al

56

with a slightly

modification. Typically, FeCl3·6H2O (2.15 g) and sodium acetate (0.49 g) were dissolved in ethylene glycol (18 mL). Then, the mixture was transferred to a 25 mL teflon-lined stainless-steel autoclave and sealed to heat at 200 °C for 8 h. After cooling to room temperature, the black product was washed several times with ethanol and then dried in vacuum at 60 °C for 12 h. Secondly, (NH4)6Mo7O24·4H2O (0.35 g, 0.28 mmol) and thiourea (0.76 g, 10 mmol) were dissolved in 10 mL distilled water by ultrasound to form a homogeneous solution. Then, 20 mg Fe3O4 NPs was added into the solution and dispersed by ultrasound for another 10 min. The mixture was transferred into a 25 mL teflon-lined stainless steel autoclave and maintained at different temperatures (160 °C, 180 °C, 200 °C) for 10 h. Thereafter, the reaction system was allowed to cool down to room temperature naturally. The final product was collected by magnetic separation, washed with water, and dispersed in 10 mL water for the next catalytic reaction or dried at 60 °C under vacuum for characterization. MoS2 nanosheets were prepared by the similar procedure without addition of Fe3O4 NPs. Catalytic reduction of 4-NP: 100 μL of aqueous 4-NP solution (1 mmol L-1) was added to 1 mL of as-prepared Fe3O4@MoS2 core-shell composites (0.25 mg mL-1). 14 ACS Paragon Plus Environment

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Subsequently, the above solution was mixed with 100 μL of freshly prepared NaBH4 solution (100 mmol L-1). The reaction was carried out at room temperature with slightly shaking. The reaction progress was monitored by UV-vis spectroscopy at a certain time interval. After the whole reduction process was completed, Fe3O4@MoS2 core-shell composites were separated from the mixture with an external magnet and washed with deionized water 3 times before the next catalytic cycle. Unless otherwise specified,

Fe3O4@MoS2 core-shell composites which were prepared at 180 ℃

were used to perform the catalytic reaction. Characterization: The structure and morphology of the as-prepared composites were investigated by TEM (Hitachi H-7500), FE-SEM (Hitachi SU-8020), HRTEM and FE-TEM (FEI Tecnai G2 F20 STWIN). All samples for FE-SEM and TEM measurements were deposited on a carbon film supported by copper grids. Adsorption spectra and kinetic measurements were carried out on a Lamda 750 UV-Vis-NIR spectrophotometer (PE, USA). Powder X-ray diffraction patterns were collected on a PANalytical X’Pert Pro diffractometer with Cu Ka radiation. The mass of MoS2 was determined by a Thermo Scientific XSERIES 2 ICP-MS. Magnetization curve was measured on a BKT-4500 vibrating sample magnetometer.



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CN2H4S (NH4)6 Mo7O24·4H2O Fe3O4

Fe3O4@MoS2

Hydrothermal

4-NP

S N

4-AP

Figure 1. Schematic representation of the synthesis of Fe3O4@MoS2 core-shell composites and the reduction of 4-nitrophenol.


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A

B

C

D

Figure 2. SEM images (A, B) and TEM images (C, D) of Fe3O4@MoS2 core-shell composites (180℃). All scale bars in the top right corner of each image represent 200 nm.


0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0

A

160 °C 180 °C 200 °C 0

50

100 150 200 250 300 Time (s)

Conversion Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ln [C(t)/C(0)]


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B

100 80 60 40 20 0

3

6 9 12 Run number

15

Figure 3. (A) Plot of ln[C(t)/C(0)] against the reaction time using Fe3O4@MoS2 core-shell composites as the catalyst and (B) the reusability of the Fe3O4@MoS2 core-shell composites (180 °C).


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25 °C 35 °C 45 °C 55 °C 65 °C

0.8 0.6

A

0.4 0.2 0.0

0

80

-3.0 -3.2 -3.4 -3.6 -3.8 -4.0

B

ln k

1.0 C(t)/C(0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


160 240 320 400 Time (s)

2.9

3.0

3.1

3.2

1000/T (K-1)

3.3

3.4

Figure 4. (A) Plots of C(t)/C(0) versus reaction time for the reduction of 4-NP over Fe3O4@MoS2 core-shell composites at different temperature. The inset in (A) shows the plot of ln(Ct/C0) versus reaction time. (B) Arrhenius plot of lnk vs (1000/T).



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Figure 5. Illustration of possible reduction process of 4-NP on the surfaces of Fe3O4@MoS2 core-shell composites


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ASSOCIATED CONTENT

Supporting

Information.

TEM,

SEM,

XRD,

XPS,

magnetic

property,

time-dependent absorption spectra and color of 4-NP solution, Mass spectrometry, Comparison of reaction rate constant. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author [email protected]

Funding Sources The National Natural Science Foundation of China (21177023)，the program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116) and the Science Foundation of Fujian Province of China (2013J01044).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors gratefully acknowledge NSFC (21177023), the program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116) and Science Foundation of Fujian Province of China (2013J01044) for financial support.



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TABLE OF CONTENTS GRAPHIC

e

N

H

B

Fe3O4 O Mo4+ Mo6+ S

4-nitrophenol

4-aminophenol


Preparation, Characterization, and Catalytic Application

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