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Hydrothermal synthesis of 3D hierarchical flower-like MoSe2 microspheres and their adsorption performances for methyl orange
Accepted Manuscript Title: Hydrothermal Synthesis of 3D Hierarchical Flower-like MoSe2 microspheres and their adsorption performances for methyl orange Author: Hua Tang Hong Huang Xiaoshuai Wang Kongqiang Wu Guogang Tang Changsheng Li PII: DOI: Reference:
S0169-4332(16)30836-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.086 APSUSC 33087
To appear in:
APSUSC
Received date: Revised date: Accepted date:
4-2-2016 6-4-2016 12-4-2016
Please cite this article as: Hua Tang, Hong Huang, Xiaoshuai Wang, Kongqiang Wu, Guogang Tang, Changsheng Li, Hydrothermal Synthesis of 3D Hierarchical Flowerlike MoSe2 microspheres and their adsorption performances for methyl orange, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrothermal Synthesis of 3D Hierarchical Flower-like MoSe2 microspheres and their adsorption performances for methyl orange
Hua Tanga*, Hong Huanga, Xiaoshuai Wanga, Kongqiang Wua, Guogang Tanga, Changsheng Lia
a
School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013,
P.R.China
*Corresponding author. Tel: +86 511 8879 0268, E-mail: [email protected]
1
Graphical abstract
2
Highlights
3D hierarchical flower-like MoSe2 microspheres have been fabricated via a hydrothermal method.
A possible evolution process of 3D hierarchical flower-like MoSe2 microspheres was discussed.
Flower-like MoSe2 microspheres exhibit excellent adsorption properties for dye methyl orange removal from aqueous solution.
3
Abstract In this paper, we report a facile and versatile modified hydrothermal method for synthesis of three-dimensional (3D) hierarchical flower-like MoSe2 microspheres using selenium powders and sodium molybdate as raw materials. The as-prepared MoSe2 was investigated for application as an adsorbent for the removal of dye contaminants from water. Power X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron
microscopy
(TEM),
X-ray
photoelectron
spectroscope
(XPS)
and
N2
adsorption-desorption analysis were carried out to study the microstructure of the as-synthesized product. A possible growth mechanism of MoSe2 flower-like microspheres was preliminarily proposed on the basis of observation of a time-dependent morphology evolution process. Moreover, the MoSe2 sample exhibited good adsorption properties, with maximum adsorption capacity of 36.91 mg/g for methyl orange. The adsorption process of methyl orange on 3D hierarchical flower-like MoSe2 microspheres was systematically investigated, which was found to obey the pseudo-second-order rate equation and Langmuir adsorption model.
Keywords: Hydrothermal synthesis, MoSe2, Hierarchical microspheres, Crystal growth, Adsorption
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1. Introduction Graphene with extraordinary physical and chemical properties has ignited extensive research in nanoelectronics, supercapacitors, fuel-cells, batteries, photovoltaics, catalysis, gas sorption, separation and storage, and sensing after Novoselov and Geim discovered it in 2004 [1-10]. Since then graphene-like layered materials have gained broad attention in materials science, condensed matter physics and chemistry due to their unique properties and great potential [11-16]. Layered transition metal sulfides MX2 (M:Mo, W; X:S, Se) have an analogous structure to graphene, which are composed of three atom layers: a M layer sandwiched between two X layers, and the triple layers are stacked and held together by weak van der Waals interactions. As we know, transition metal dichalcogenides bulk materials have good physical and chemical properties, but numerous recent studies showed that their electronic, physical and chemical properties were modified or improved when the compounds were on the nanoscale [17-28]. MoSe2, similar to MoS2, has been an upstart of transition metal dichalcogenides materials family which interest has been renewed with the discovery that a novel sandwich layer can be isolated and used for a wide variety of potential including photovoltaics, catalysis, electrode in high-energy density batteries, lubricant and others potential purpose. Traditionally, MoSe2 nanomaterial was synthesized by solid-state reaction or Chemical Vapor Deposition (CVD) between a stoichiometric amount of elemental molybdenum and selenium in a sealed evacuated tube at a temperature of at least 900°C for several days. However the above methods either involve a high temperature procedure or a complicated manipulation. More importantly, the morphology and size of the samples are difficult to control. Furthermore, the environmental regulations must also be taken into consideration in developing new methods and techniques. Therefore, it is still a challenge to develop a simple and effective solution synthetic method to prepare MoSe2 nanomaterials with complex morphology at a lower temperature.
Recently, Harpeness et al. [21] described the fabrication of MoSe2 nanorods with lengths ranging from 45 to 55 nm via a microwave-assisted polyol method between Mo(CO)6 and Se powder. Shi et al. [22] reported a chemical solution reaction approach for the fabrication of MoSe2 nano-flakes with diameters of about 100–300 nm. Compared with the above common chemical 5
solution reaction approach, hydrothermal synthesis is considered as an effective way to prepare nanomaterials due to it’s merits including mild synthetic conditions, simple manipulation and good crystallization of the products. In 2001, Chen et al. [23] have been successfully synthesized MoSe2 nanocrystalline by a facile hydrothermal method using Na2Se2O3 and Na2MoO4 as raw materials in aqueous solution at 150°C. Very recently, Fan and the co-workers [29] reported novel flower-like MoSe2 nanostructures were synthesized by a facile hydrothermal method. Motivated by previous reports mentioned above, we further develop a simple and effective modified hydrothermal approach for the synthesis of 3D flower-like MoSe2 microspheres using selenium powder and sodium molybdate as raw materials. Subsequently, the as-prepared MoSe2 nanoflowers were fully characterized utilizing various technologies. Furthermore, a possible formation mechanism was discussed in detail on the basis of time-dependent morphological evolution process. Experimental study of their performance as adsorbents for dye pollutants from water showed that they are very promising materials for wastewater treatment.
2. Experimental 2.1. Synthesis of MoSe2 nanoflowers All chemicals used in this work were of analytic purity and used without further purification. In a typical procedure, 1.645 g Na2MoO4·2H2O, 1.5492 g Se and 0.2595 g NaBH4 were dissolved in 50 mL mixture of distilled water and absolute ethanol (volume ratio 1:1) under violent stirring. The resulted solution was transferred into a 100 mL Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 200℃ for 48 h, and then cooled to room temperature naturally. A black precipitate was collected. After being washed with absolute ethanol and distilled water, the final product was dried in a vacuum box at 60℃ for 8 h.
2.2. Characterization of MoSe2 samples The X-ray diffraction patterns were recorded using a D8 advance (Bruker-AXS) diffractometer with Cu Kα radiation (λ=0.1546 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using an ultra-high vacuum VG ESCALAB 250XI electron spectrometer. The morphologies and structure of the samples were characterized by scanning electron microscopy (SEM, JEOL JXA-840A) and transmission electron microscopy (TEM) with 6
a Japan JEM-100CX Ⅱ transmission electron microscope. Nitrogen adsorption-desorption isotherms were measured in an ASAP2020 HD88 surface area and porosity analyzer (Micromeritics, USA). The BET surface area was determined by a multipoint BET method using the adsorption data. Desorption isotherm was used to determine the pore size distribution via the Barret–Joyner–Halender (BJH) method. Pore volume and average pore size were estimated at a relative pressure of 0.994 (P/P0).
2.3. Adsorption experiments Adsorption isotherm experiments were performed by adding a specified amount of adsorbent (20 mg) to a series of 250 mL beakers with dye solutions (100 mL, 5-25 mg/L). Then, the beakers were sealed and placed in a magnetic stirring apparatus at room temperature for enough time to reach equilibrium. At certain time intervals, the aqueous samples were taken and centrifuged. Then the concentration of dye in the supernatant solution was measured based on the maximum adsorption wavelength (463 nm) using a UV/Vis spectrophotometer (Shimadzu UV/Vis 2550 Spectrophotometer, Japan). The amount of adsorption at time t, qt (mg/g) was calculated by the equation (1):
(1)
Where C0 and Ct (mg/L) are the concentrations of dye initially and at specified time t, respectively, V is the volume of the dye solution (L) and W is the mass of adsorbent used (g).
3. Results and Discussions 3.1. Synthesis and characterization of flower-like MoSe2 microspheres The crystalline structure and phase purity of flower-like MoSe2 microspheres were confirmed by XRD. As shown in Fig. 1a, all observed diffraction peaks can be systematically indexed to those of the hexagonal phase of MoSe2 with lattice parameters of a0 =b0= 3.292Å and c0 =19.392Å, which are in good agreement with the values of standard card (JCPDS No. 20-0757). No peaks from other impurities are detected in the XRD pattern, indicating that the sample is highly 7
crystalline. Energy-dispersive X-ray spectrometer (EDS) results are shown in Fig. 1b, which reveals that the microspheres consist of element Mo and Se; no other elements are observed. Furthermore, the quantification of the peaks shows that the atom ratio of Mo to Se is about 1:2.09, which is very close to the stoichiometry of MoSe2.
Chemical compositions on the surface and valence states of the flower-like MoSe2 microspheres were further investigated by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 2a shows the XPS survey spectrum of MoSe2 microspheres, in which the peaks derived from Mo, Se, C and O elements were detected. Generally, a small amount of O may be due to surface adsorption of oxygen, and the C 1s peak located at 284.6 eV mainly results from the contamination of environment [30]. Fig. 2b shows the spectrum of Mo 3d and the binding energies at 229 eV and 232.1 eV belong to Mo 3d5/2 and 3d3/2 spin orbit peaks of MoSe2, confirming the existence of the Mo IV state. Fig. 2c shows a high-resolution XPS spectrum of Se 3d. The peak is split into two well-defined peaks at 54.5 eV and 55.4 eV, corresponding to the Se 3d5/2 and Se 3d3/2 peaks, further illustrating that the product is MoSe2.
The size and morphology of MoSe2 sample were identified by SEM, TEM and HRTEM. Fig. 3a shows that the as-prepared MoSe2 samples are actually uniform flower-like microspheres with mean diameter about 1.0 μm. Fig. 3b offers a clear view of the surface morphology, which reveals that the obtained MoSe2 microspheres are composed of numerous nanosheets with 20 nm in thickness. Fig. 3c shows a typical TEM image of flower-like MoSe2 microspheres, which is consistent with the above SEM results. The result also reveals that the flower-like MoSe2 is solid interior, and possesses core-corona architecture. More details for MoSe2 structure are illustrated in Fig. 3d. It shows clear lattice fringes, and the lattice spacing is ca. 0.28 nm, which is consistent with the (100) plane of the hexagonal MoSe2 phase. The obvious lamellar structures with an interlayer spacing of 0.65 nm are also observed, indicating the nature of the layered structure.
The specific surface area and porous structure of the prepared samples were determined by using nitrogen adsorption-desorption isotherm (Fig. 4). The isotherm curves of all the samples exhibit a type-IV shape, in accordance to the International Union of Pure and Applied Chemistry 8
(IUPAC) classification, and distinct hysteresis loops, which show the presence of mesopores (2-50 nm) [31]. Textural parameters extracted from nitrogen adsorption–desorption isotherms are summarized in Table 3. The BET surface area of flower-like MoSe2 microspheres were 25 m2/g, much larger than that of MoSe2 plates obtained by solid-state reaction (1.1 m2/g). The relatively large surface area of the as-prepared flower-like MoSe2 microspheres can provide more active sites for adsorption of dye molecules, which would be beneficial for the fast adsorption and transfer of adsorbate inside the hierarchically porous structure of this sample.
3.2. The growth mechanism of MoSe2 microspheres To date, crystal growth mechanisms of inorganic nanomaterials in a hydrothermal process are so complicated that the research of the crystallisation mechanism remains a challenge. In general, oriented aggregation, self-assembly, Ostwald ripening etc. was adopted to account for the growth process of 3D self-assembled structures [22-23, 29]. Recently, many novel techniques have been developed for synthesis of MoSe2 self-assembled structures and their electronical performances; however few studies have focused on the formation mechanism of MoSe2 hierarchical assemble nanostructures. In order to shed light on the morphological evolution, MoSe2 nanostructures harvested at different growth stage were carefully examined by TEM observation. When the reaction time was reached 1 h (Fig. 5a), many MoSe2 nanoparticles with the diameter of 5～10 nm were obtained. With the increase of reaction time to 6 h, sheet-like structures were formed, coexisting with many nanoparticles. By further prolonging the reaction time to 12 h, no nanoparticles existed and many nanosheets assembled to form flowerlike structures as shown in Fig. 5c. Upon gradual assembly of MoSe2 nanosheets, perfect crystalline flowerlike microspheres were obtained when reaction time reached 48 h. Therefore, based on the above experimental results, a possible formation mechanism of MoSe2 nanostructures has been elucidated (Fig. 6). In the first stage, Se could be easily reduced to NaHSe by H2O with the help of NaBH4 [32], meanwhile MoO42- could be readily reduced to Mo4+, then Mo4+ and Se2- further react with each other to form MoSe2 nanocrystals in hydrothermal system through a fast nucleation process. Based on above analysis, the above reactions can be expressed as follows:
2Se + 4BH4- +7H2O → 2HSe- + B4O72- +14H2 9
4MoO42- + BH4- +10H2O → 4Mo4+ + BO2- + 24OHHSe- + OH- → H2O + Se2Mo4+ + 2Se2- → MoSe2
Moreover, these MoSe2 nanocrystals further grew to larger particles via Ostwald ripening process. Subsequently, with the aging process continued, partial MoSe2 nanoparticles started to grow into sheet-like nanostructures through oriented aggregation. Finally, these sheet-like nanostructures gradually evolved into hierarchical flower-like MoSe2 microspheres through the self-assemble process. Based on above discussion and analysis, we believe that the growing process is consistent with the previous reports of a three-stage growth process [22-23, 29, 33], which involves a fast nucleation of amorphous primary particles; slow oriented aggregation of nanosheets and self-assembly of 3D hierarchical nanostructures. However, determining the exact nature of the growth mechanism is not yet fully understood, and will require further theoretical and experimental work.
3.3. Adsorption property Generally, if the material has porous hierarchical structures, it will possess more available active adsorption sites, efficient transport pathways, and may exhibit good adsorption performance [34]. As inspired by previous reports on layered materials that could be applied as adsobents to remove organic waste from water [35-37], the as-synthesized porous hierarchical MoSe2 was used to adsorb methyl orange (MO).
Fig. 7a shows the time profile of methyl orange adsorption at different initial concentrations with 0.02 g of the as-synthesized MoSe2. The adsorption rates were extraordinarily fast in the first 20 min under all concentrations. Then after 20 min a long period of slower uptake was followed, and finally adsorbed amount reached its equilibrium. The adsorption nearly finished within 60 min, indicating the relatively fast adsorption rate of MO on the MoSe2 in water.
Adsorption is a physicochemical process that involves mass transfer of a solute from liquid phase to the adsorbent's surface [38]. Kinetic study provided important information about the 10
mechanism of MO adsorption onto MoSe2, which was necessary to depict the adsorption rate of adsorbent and control the residual time of the whole adsorption process. The adsorption kinetics of MO onto MoSe2 was investigated with the help of two kinetic models, namely the Lagergren pseudo-first-order and pseudo-second-order model. The pseudo-first-order kinetic model is expressed by the following equation:
(2)
Where k1 is the rate constant of preudo-first-order adsorption (1/min), qt and qe are the amount adsorbed at time t and equilibrium (mg/g). According to the adsorption kinetic parameters reported in table 1, the values of correlation (r2) are relatively low for most of the adsorption data at different initial concentrations, and the experimental amount adsorbed at equilibrium qe (exp) do not agree well with the calculated ones. This shows that the adsorption process may not be the correct fit to the first-order rate equation. Another kinetic model is pseudo-second-order model, which is expressed as:
(2)
Where k2 is the rate constant of preudo-second-order adsorption (g/mg•min), qt and qe are the amount of MO absorbed at time t and at equilibrium (mg/g). k2 and qe can be obtained from the slope and intercept of plots of t/qt vs. t, which is shown in Fig. 7b. The adsorption kinetic parameters are reported in table 1. The results reveals that all the experimental data can fit well to the pseudo-second-order model. Clearly, at different initial concentrations, all correlation coefficient (r2) values are greater than 0.99 from the table 1, showing a good linear relationship between t/qt and t. In addition, the experimental amount of MO adsorption at equilibrium (qe (exp)) are very close to the calculated amount (qe (cal)). On the base of above analysis, we can draw a conclusion that the pseudo-second-order kinetic model is highly applicable to the adsorption 11
process of MO onto MoSe2.
For solid-liquid system, the equilibrium of adsorption is one of the important physico-chemical aspects in the description of adsorption behavior. The capacity of the adsorption isotherm plays an important role in the determination of the maximum capacity of adsorption. In order to adapt for the considered system, two appropriate isotherm models (Langmuir and Freundlich adsorption model) that can reproduce experimental results have been considered in the present study. The linear forms of Langmuir and Freundlich isotherms are described as equation (3) and (4), respectively:
(3)
(4)
The plots of Langmuir and Freundlich linear equations are displayed in Fig. 8. All the parameters of two equations are calculated in table 2. As we can see, the maximum adsorption capacity of MO for MoSe2 (qm) is 36.91 mg per gram of MoSe2. Moreover, two models could express the adsorption process due to the linear correlation coefficients r2 are all greater than 0.9. However, the experimental data could be better fit with the Langmuir equation with the linear correlation coefficient (r2=0.9975) being greater than that of the Freundlich equation (r2=0.9899). So it is more suitable to describe the adsorption process by Langmuir model, in other words, Langmuir model is more applicable for adsorption of MO onto MoSe2. As reported in literatures [39], 1/n is a constant relating the adsorption intensity. If 0.1<1/n<0.5, the adsorption is quite favorable; if 0.5<1/n<1, there will be some trouble in the adsorption process; if 1/n>1, it is very hard to adsorb. In this study, 1/n is found to be 0.2441, which confirms that it is easy to adsorb MO from solution for as-prepared MoSe2 microspheres.
For comparison, we investigated the removal capacity of solid-state synthesized MoSe2 12
powder. The removal capacity of the solid-state synthesized MoSe2 was found to be 15.6 mg/g, which is much lower than the removal capacity of flower-like MoSe2 microspheres (36.91 mg/g). This suggests that the as-prepared flower-like MoSe2 microspheres have good potential as new and efficient adsorbent materials for organic wastewater treatment applications. Table 3 summarizes the specific surface areas and the adsorption capacities of MO for various adsorbents. To the best of our knowledge, the as-obtained products have the higher adsorption capacity for MO. The higher adsorption activity of flower-like MoSe2 microspheres can be attributed to the following reason. First, the surface area is exclusive factor to determine the adsorption capability of the adsorbents. It is obviously that flower-like MoSe2 microspheres have much larger surface area than solid-state synthesized MoSe2 powder. The relatively large surface area can provide more active sites for adsorption of dye molecules, which would be beneficial for the fast adsorption and transfer of adsorbate inside the hierarchically porous structure of the sample. On the other hand, in previous studies, π-π stacking interactions as a dominant driven force
has been used to explain the mechanism of aromatic adsorbate to layered MoS2 or grapheme surface [47-49]. Considering aromatic rings in the molecular structure of the MO are present, we proposed a mechanism of π-π stacking interaction between aromatic compound of MO (π electron acceptor) and π electron-rich regions on the surface of MoSe2 probably are likely the reason for the enhance adsorption activities of flower-like MoSe2 microspheres.
4. Conclusion In summary, novel 3D hierarchical flower-like MoSe2 microspheres with mean diameter about 1μm were successfully synthesized via a facile hydrothermal approach. The possible grown mechanism is discussed based on the time-dependent experiments, which indicates flowerlike MoSe2
product
is
formed
via
a
three-stage
growth
process
(nucleation–oriented
aggregation–self-assembly growth). Moreover, the flowerlike MoSe2 sample possess good adsorption performance for organic dye MO, with the maximum adsorption capacity of 36.91 mg/g. The results demonstrate that the adsorption process fit the pseudo-second-order rate equation and Langmuir adsorption model. Considering the novel hierarchical structure obtained by a simple synthetic process and their good adsorption abilities, the study here not only represents an 13
attractive path to large-scale synthesis of other transition metal dichalcogenides with complex structure, but also makes a contribution to the study of transition metal dichalcogenides as potential adsorbents applied in wastewater treatment.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (51302112) and the Jiangsu Industry-University-Research Joint innovation Foundation (BY2013065-05, BY2013065-06).
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References [1] X.C. Song, Y.F. Zheng, G. Han, H.Y. Yin, G.S. Cao, Ultrathin MoS2-coated carbon nanospheres as highly efficient electrocatalyts for hydrogen evolution reaction, Chem. J. Chinese U. 26 (2005) 617-619. [2] G.K. Wang, X. Sun, F.Y. Lu, H.F. Sun, M.P. Yu, W.L. Jiang, C.S. Liu, J. Lian, Flexible pillared graphene-paper electrodes for high-performance electrochemical supercapacitors, Small 8 (2012) 452-459. [3] L. Xiao, J. Damien, J.Y. Luo, H.D. Jang, J.X. Huang, Z. He, Crumpled graphene particles for microbial fuel cell electrodes, J. Power Sources 208 (2012) 187-192. [4] C.S. Zhao, H.P. Gao, C.M. Chen, H. Wu, Reduction of graphene oxide in Li-ion batteries, J. Mater. Chem. A 3 (2015) 18360-18364. [5] K.F. Wu, H.M. Zhu, T.Q. Lian, Ultrafast exciton dynamics and light-driven H2 evolution in colloidal semiconductor aanorods and Pt-tipped nanorods, Acc. Chem. Res. 48 (2015) 851-859. [6] Q.J. Xiang, J.G. Yu, Graphene-based photocatalysts for hydrogen generation, J. Phys. Chem. Lett. 4 (2013) 753-759. [7] S. Gadipelli, Z.X. Guo, Graphene-based materials: Synthesis and gas sorption, storage and separation, Prog. Mater. Sci. 69 (2015) 1-60. [8] M.S. Xu, Y. Gao, X. Yang, H.Z. Chen, Unique synthesis of graphene-based materials for clean energy and biological sensing applications, Chinese Sci. Bull. 57 (2012) 3000-3009. [9] R. Singh, D. Kumar, C.C. Tripathi, Graphene: Potential material for nanoelectronics applications, Indian J. Pure Ap. Phy. 53 (2015) 501-513. [10] J.G. Yu, M. Jaroniec, Preface: Graphene and C3N4-based photocatalysts, Appl. Surf. Sci. 358 (2015) 1. [11] D. Li, R.B. Kaner, Materials science-Graphene-based materials, Science 320 (2008) 1170-1171. [12] Y.R. Yao, W.Z. Huang, H. Zhou, Y.F. Zheng, X.C. Song, Self-assembly of dandelion-like Fe3O4@C@BiOCl magnetic nanocomposites with excellent solar-driven photocatalytic properties, J. Nanopart. Res. 16 (2014) 1-9. [13] Q.J. Xiang, J.G. Yu, M. Jaroniec, Enhanced photocatalytic H2 production activity of 15
graphene-modified titania nanosheets, Nanoscale 3 (2011) 3670-3678. [14] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197-200. [15] K.F. Wu, Q.Y. Li, Y.Y. Jia, J.R. McBride, Z.X. Xie, T.Q. Lian, Efficient and ultrafast formation
of
long-lived
charge-transfer
exciton
state
in
atomically
thin
cadmium
selenide/cadmium telluride type-II heteronanosheets, ACS Nano 9 (2015) 961-968. [16]
X.C. Song,
E. Yang, G. Liu, Y. Zhang, Z.S. Liu, H.F. Chen, Y. Wang,
Preparation
and
photocatalytic activity of Mo-doped WO3 nanowires, J. Nanopart. Res. 12 (2010) 2813-2819. [17] Q.J. Xiang, J.G. Yu, Graphene-based photocatalysts for hydrogen generation, J. Am. Chem. Soc. 136 (2014) 8839-8842. [18] Y.D. Ma, Y. Dai, C.W. Niu, L. Yu, B.B. Huang, First-principles study of the graphene@MoSe2 heterobilayers, J. Phys. Chem. C 115 (2011) 20237-20241. [19] H.P. Li, L. Chen, Y. Zhang, X.R. Ji, S. Chen, H.J. Song, C.S. Li, H. Tang, Synthesis of MoSe2/reduced graphene oxide composites with improved tribological properties for oil-based additives, Cryst. Res. Technol. 49 (2014) 204-211. [20] Q. Li, B.D. Guo, J.G. Yu, J.R. Ran, B.H. Zhang, H.J. Yan, J.R. Gong, Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets, J. Am. Chem. Soc. 133 (2011) 10878-10884. [21] R. Harpeness, A. Gedanken, A.M. Weiss, M.A. Slifkin, Microwave-assisted synthesis of nanosized MoSe2, J. Mater. Chem. 13 (2003) 2603-2606. [22] H.Q. Shi, X.D. Zhou, Y.S. Lin, X. Fu, Synthesis of MoSe2 nano-flakes modified with dithiophosphinic acid extractant at low temperature, Mater. Lett. 62 (2008) 3649-3651. [23] X.H. Chen, R. Fan, Low-temperature hydrothermal synthesis of transition metal dichalcogenides, Chem. Mater. 32 (2001) 802-805. [24] J. Zhou, H. Xiao, B.W. Zhou, F.F. Huang, S.B. Zhou, W. Xiao, D.H. Wang, Hierarchical MoS2-rGO nanosheets with high MoS2 loading with enhanced electro-catalytic performance, Appl. Surf. Sci. 358 (2015) 152-158. [25] Y. Ding, Y.F. Zhou, W.Y. Nie, P.P. Chen, MoS2-GO nanocomposites synthesized via a hydrothermal hydrogel method for solar light photocatalytic degradation of methylene blue, Appl. 16
Surf. Sci. 357 (2015) 1606-1612. [26] L.K. Putri, W.J. Ong, W.S. Chang, S.P. Chai, Heteroatom doped graphene in photocatalysis: A review, Appl. Surf. Sci. 358 (2015) 2-14. [27] Q. Li, X. Li, S. Wageh, A.A. Al-Ghamdi, J.G. Yu, CdS/graphene nanocomposite photocatalysts, Adv. Energy Mater. 5 (2015) 1500010. [28] J.X. Low, S.W. Cao, J.G. Yu, S. Wageh, Two-dimensional layered composite photocatalysts, Chem. Commun. 50 (2014) 10768-10777. [29] C. Fan, Z.M. Wei, S.X. Yang, J.B. Li, Synthesis of MoSe2 flower-like nanostructures and their photo-responsive properties, RSC Adv. 4 (2014) 775. [30] W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107-112. [31] R.X. Chen, J.G. Yu, W. Xiao, Hierarchically porous MnO2 microspheres with enhanced adsorption performance, J. Mater. Chem. A 1 (2013) 11682. [32] D.L. Klayman, T.C. Griffin, Reaction of selenium with sodium borohydride in protic solvents. A facile method for the introduction of selenium into organic molecules, J. Am. Chem. Soc. 95 (1973) 197-199. [33] G.G. Tang, Y.J. Wang, W. Chen, H. Tang, C.S. Li, Hydrothermal synthesis and characterization of novel flowerlike MoS2 hollow microspheres, Mater. Lett. 100 (2013) 15-18. [34] P. Tian, X.Y. Han, G.L. Ning, H.X. Fang, J.W. Ye, W.T. Gong, Y. Lin, Synthesis of porous hierarchical MgO and its superb adsorption properties, ACS Appl. Mater. Interfaces 5 (2013) 12411-12418. [35] B.C. Yang, Y.Z. Guo, S.R. Zhang, T. Wen, C.M. Zhao, Synthesis of graphene by microwave irradiation for dye adsorption, RSC Adv. 4 (2014) 64771-64780. [36] H.J. Song, S.S. You, X.H. Jia, Synthesis of fungus-like MoS2 nanosheets with ultrafast adsorption capacities toward organic dyes, Appl. Phys. A 121 (2015) 541-548. [37] X.P. Zhang, D. Liu, L. Yang, L.M. Zhou, T.Y. You, Self-assembled three-dimensional graphene-based materials for dye adsorption and catalysis, J. Mater. Chem. A 3 (2015) 10031-10037. [38] L.H. Jiang, Y.G. Liu, G.M. Zeng, F.Y. Xiao, X.J. Hu, X. Hu, H. Wang, T.T. Li, L. Zhou, X.F. 17
Tan, Removal of 17β-estradiol by few-layered graphene oxide nanosheets from aqueous solutions: External influence and adsorption mechanism, Chem. Eng. J. 284 (2016) 93-102. [39] L.C. Zhou, X.G. Meng, J.W. Fu, Y.C. Yang, P. Yang, C. Mi, Highly efficient adsorption of chlorophenols onto chemically modified chitosan, Appl. Surf. Sci. 292 (2014) 735-741. [40] Y. Iida, T. Kozuka, T. Tuziuti, K. Yasui, Sonochemically enhanced adsorption and degradation of methyl orange with activated aluminas, Ultrasonics 42 (2004) 635-639. [41] B.C. Zhu, P.F. Xia, W.K. Ho, J.G. Yu, Isoelectric point and adsorption activity of porous g-C3N4, Appl. Surf. Sci. 344 (2015) 188-195. [42] D.M. Chen, J. Chen, X.L. Luan, H.P. Ji, Z.G. Xia, Characterization of anion-cationic surfactants modified montmorillonite and its application for the removal of methyl orange, Chem. Eng. J. 171 (2011) 1150-1158. [43] A. Shiue, C.M. Ma, R.T. Ruan, C.T. Chang, Adsorption kinetics and isotherms for the removal methyl orange from wastewaters using copper oxide catalyst prepared by the waste printed circuit boards, Sust. Environ. Res. 22 (2012) 209-215. [44] Y.J. Yao, B. He, F.F. Xu, X.F. Chen, Equilibrium and kinetic studies of methyl orange adsorption on multiwalled carbon nanotubes, Chem. Eng. J. 170 (2011) 82-89. [45] T.K. Saha, N.C. Bhoumik, S. Karmaker, M.G. Ahmed, H. Ichikawa, Y. Fukumori, Adsorption of methyl orange onto chitosan from aqueous solution, J. Water Res. Prot. 2 (2010) 898-906. [46] G. Annadurai, R.S. Juang, D.J. Lee, Use of cellulose-based wastes for adsorption of dyes from aqueous solutions, J. Hazard. Mater. 92 (2002) 263-274. [47] Y. Gao, Y. Li, L. Zhang, H. Huang, J.J. Hua, S.M. Shah, X.G. Su, Adsorption and removal of tetracycline antibiotics from aqueous solution by graphene oxide, J. Colloid Interf. Sci. 368 (2012) 540-546. [48] T. Wu, X. Cai, S.Z. Tan, H.Y. Li, J.S. Liu, W.D. Yang, Adsorption characteristics of acrylonitrile, p-toluenesulfonic acid, 1-naphthalenesulfonic acid and methyl blue on graphene in aqueous solutions, Chem. Eng. J. 173 (2011) 144-149. [49] Y.H. Chao, W.S. Zhu, X.Y. Wu, F.F. Hou, S.H. Xun, P.W. Wu, H.Y. Ji, H. Xu, H.M. Li, Application of graphene-like layered molybdenum disulfide and its excellent adsorption behavior for doxycycline antibiotic, Chem. Eng. J. 243 (2014) 60-67.
18
Figure captions Fig. 1 XRD pattern (a) and EDS (b) of flower-like MoSe2 microspheres Fig. 2 XPS spectra of MoSe2 sample: survey XPS spectrum (a), high resolution XPS spectra of Mo 3d (b), Se 3d (c) Fig. 3 SEM (a, b), TEM (c) and HRTEM (d) images of flower-like MoSe2 microspheres Fig. 4 Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of MoSe2 samples Fig. 5 TEM images of MoSe2 samples obtained under different reaction times (a:1h, b:6h, c:12h, d:24h) Fig. 6 Schematic illustration of flower-like MoSe2 microspheres Fig. 7 Time profiles (a) and pseudo-second-order adsorption kinetic plots (b) of MO onto MoSe2 at different initial dye concentrations Fig. 8 Langmuir (a) and Freundlich (b) plots for the adsorption of MO onto MoSe2 Fig. 9 Comparison of adsorption capacities of MO onto MoSe2 samples Table 1 Pseudo-first-order and pseudo-second-order adsorption kinetic constants for MO adsorption on MoSe2 Table 2 Parameters of the Langmuir and Freundlich equations Table 3 Comparison of the maximum sorption capacities of MO with other materials
19
Fig.1 XRD pattern (a) and EDS (b) of flower-like MoSe2 microspheres
Fig. 2 XPS spectra of MoSe2 sample: survey XPS spectrum (a), high resolution XPS spectra of Mo 3d (b), Se 3d (c)
20
Fig.3 SEM (a, b), TEM (c) and HRTEM (d) images of flower-like MoSe2 microspheres
Fig. 4 Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of MoSe2 samples
21
Fig.5 TEM images of MoSe2 samples obtained under different reaction times (a:1h, b:6h, c:12h, d:48h)
Fig.6 Schematic illustration of flower-like MoSe2 microspheres
22
Fig. 7 Time profiles (a) and pseudo-second-order adsorption kinetic plots (b) of MO onto MoSe2 at different initial dye concentrations
Fig. 8 Langmuir (a) and Freundlich (b) plots for the adsorption of MO onto MoSe2
23
Fig. 9 Comparison of adsorption capacities of MO onto MoSe2 samples
24
Table. 1 Pseudo-first-order and pseudo-second-order adsorption kinetic constants for MO adsorption on MoSe2 C0
qe (exp)
(mg/L)
(mg/g)
Pseudo-first-order k1
qe (cal)
(1/min)
(mg/g)
Pseudo-second-order r2
k2
qe (cal)
(g/mg•min)
(mg/g)
r2
5
18.26
1.08*10-2
9.93
0.8092
1.26*10-2
19.39
0.9989
10
25.54
2.06*10-2
9.56
0.74501
7.77*10-3
25.83
0.9972
15
29.74
2.38*10-2
9.77
0.89268
1.36*10-2
29.83
0.9980
20
33.03
2.58*10-2
10.23
0.77677
4.82*10-3
34.46
0.9992
25
33.73
2.71*10-2
11.02
0.9238
6.85*10-3
34.52
0.9999
Table. 2 Parameters of the Langmuir and Freundlich equations Langmuir
Freundlich
KL (L/mg)
r2
qm (mg/g)
0.5527
0.9975
36.91
KF (L/mg)
r2
1/n
17.1591
0.9899
0.2441
25
Table. 3 Comparison of the maximum sorption capacities of MO with other materials Adsorbents
SBET (m2/g)
qm (mg/g)
References
Flower-like MoSe2 microspheres
25
36.91
This work
Solid-state synthesized MoSe2
1.1
15.6
This work
Activated alumina
—
9.8
40
Porous g-C3N4
40
2.51
41
Na-montmorillonite
—
24.0
42
Copper oxide
—
1.2
43
Multiwalled carbon nanotubes
160
52.86
44
Chitosan
—
34.83
45
Banana peel
—
21
46
26
S0169-4332(16)30836-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.086 APSUSC 33087
To appear in:
APSUSC
Received date: Revised date: Accepted date:
4-2-2016 6-4-2016 12-4-2016
Please cite this article as: Hua Tang, Hong Huang, Xiaoshuai Wang, Kongqiang Wu, Guogang Tang, Changsheng Li, Hydrothermal Synthesis of 3D Hierarchical Flowerlike MoSe2 microspheres and their adsorption performances for methyl orange, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrothermal Synthesis of 3D Hierarchical Flower-like MoSe2 microspheres and their adsorption performances for methyl orange
Hua Tanga*, Hong Huanga, Xiaoshuai Wanga, Kongqiang Wua, Guogang Tanga, Changsheng Lia
a
School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013,
P.R.China
*Corresponding author. Tel: +86 511 8879 0268, E-mail: [email protected]
1
Graphical abstract
2
Highlights
3D hierarchical flower-like MoSe2 microspheres have been fabricated via a hydrothermal method.
A possible evolution process of 3D hierarchical flower-like MoSe2 microspheres was discussed.
Flower-like MoSe2 microspheres exhibit excellent adsorption properties for dye methyl orange removal from aqueous solution.
3
Abstract In this paper, we report a facile and versatile modified hydrothermal method for synthesis of three-dimensional (3D) hierarchical flower-like MoSe2 microspheres using selenium powders and sodium molybdate as raw materials. The as-prepared MoSe2 was investigated for application as an adsorbent for the removal of dye contaminants from water. Power X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron
microscopy
(TEM),
X-ray
photoelectron
spectroscope
(XPS)
and
N2
adsorption-desorption analysis were carried out to study the microstructure of the as-synthesized product. A possible growth mechanism of MoSe2 flower-like microspheres was preliminarily proposed on the basis of observation of a time-dependent morphology evolution process. Moreover, the MoSe2 sample exhibited good adsorption properties, with maximum adsorption capacity of 36.91 mg/g for methyl orange. The adsorption process of methyl orange on 3D hierarchical flower-like MoSe2 microspheres was systematically investigated, which was found to obey the pseudo-second-order rate equation and Langmuir adsorption model.
Keywords: Hydrothermal synthesis, MoSe2, Hierarchical microspheres, Crystal growth, Adsorption
4
1. Introduction Graphene with extraordinary physical and chemical properties has ignited extensive research in nanoelectronics, supercapacitors, fuel-cells, batteries, photovoltaics, catalysis, gas sorption, separation and storage, and sensing after Novoselov and Geim discovered it in 2004 [1-10]. Since then graphene-like layered materials have gained broad attention in materials science, condensed matter physics and chemistry due to their unique properties and great potential [11-16]. Layered transition metal sulfides MX2 (M:Mo, W; X:S, Se) have an analogous structure to graphene, which are composed of three atom layers: a M layer sandwiched between two X layers, and the triple layers are stacked and held together by weak van der Waals interactions. As we know, transition metal dichalcogenides bulk materials have good physical and chemical properties, but numerous recent studies showed that their electronic, physical and chemical properties were modified or improved when the compounds were on the nanoscale [17-28]. MoSe2, similar to MoS2, has been an upstart of transition metal dichalcogenides materials family which interest has been renewed with the discovery that a novel sandwich layer can be isolated and used for a wide variety of potential including photovoltaics, catalysis, electrode in high-energy density batteries, lubricant and others potential purpose. Traditionally, MoSe2 nanomaterial was synthesized by solid-state reaction or Chemical Vapor Deposition (CVD) between a stoichiometric amount of elemental molybdenum and selenium in a sealed evacuated tube at a temperature of at least 900°C for several days. However the above methods either involve a high temperature procedure or a complicated manipulation. More importantly, the morphology and size of the samples are difficult to control. Furthermore, the environmental regulations must also be taken into consideration in developing new methods and techniques. Therefore, it is still a challenge to develop a simple and effective solution synthetic method to prepare MoSe2 nanomaterials with complex morphology at a lower temperature.
Recently, Harpeness et al. [21] described the fabrication of MoSe2 nanorods with lengths ranging from 45 to 55 nm via a microwave-assisted polyol method between Mo(CO)6 and Se powder. Shi et al. [22] reported a chemical solution reaction approach for the fabrication of MoSe2 nano-flakes with diameters of about 100–300 nm. Compared with the above common chemical 5
solution reaction approach, hydrothermal synthesis is considered as an effective way to prepare nanomaterials due to it’s merits including mild synthetic conditions, simple manipulation and good crystallization of the products. In 2001, Chen et al. [23] have been successfully synthesized MoSe2 nanocrystalline by a facile hydrothermal method using Na2Se2O3 and Na2MoO4 as raw materials in aqueous solution at 150°C. Very recently, Fan and the co-workers [29] reported novel flower-like MoSe2 nanostructures were synthesized by a facile hydrothermal method. Motivated by previous reports mentioned above, we further develop a simple and effective modified hydrothermal approach for the synthesis of 3D flower-like MoSe2 microspheres using selenium powder and sodium molybdate as raw materials. Subsequently, the as-prepared MoSe2 nanoflowers were fully characterized utilizing various technologies. Furthermore, a possible formation mechanism was discussed in detail on the basis of time-dependent morphological evolution process. Experimental study of their performance as adsorbents for dye pollutants from water showed that they are very promising materials for wastewater treatment.
2. Experimental 2.1. Synthesis of MoSe2 nanoflowers All chemicals used in this work were of analytic purity and used without further purification. In a typical procedure, 1.645 g Na2MoO4·2H2O, 1.5492 g Se and 0.2595 g NaBH4 were dissolved in 50 mL mixture of distilled water and absolute ethanol (volume ratio 1:1) under violent stirring. The resulted solution was transferred into a 100 mL Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 200℃ for 48 h, and then cooled to room temperature naturally. A black precipitate was collected. After being washed with absolute ethanol and distilled water, the final product was dried in a vacuum box at 60℃ for 8 h.
2.2. Characterization of MoSe2 samples The X-ray diffraction patterns were recorded using a D8 advance (Bruker-AXS) diffractometer with Cu Kα radiation (λ=0.1546 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using an ultra-high vacuum VG ESCALAB 250XI electron spectrometer. The morphologies and structure of the samples were characterized by scanning electron microscopy (SEM, JEOL JXA-840A) and transmission electron microscopy (TEM) with 6
a Japan JEM-100CX Ⅱ transmission electron microscope. Nitrogen adsorption-desorption isotherms were measured in an ASAP2020 HD88 surface area and porosity analyzer (Micromeritics, USA). The BET surface area was determined by a multipoint BET method using the adsorption data. Desorption isotherm was used to determine the pore size distribution via the Barret–Joyner–Halender (BJH) method. Pore volume and average pore size were estimated at a relative pressure of 0.994 (P/P0).
2.3. Adsorption experiments Adsorption isotherm experiments were performed by adding a specified amount of adsorbent (20 mg) to a series of 250 mL beakers with dye solutions (100 mL, 5-25 mg/L). Then, the beakers were sealed and placed in a magnetic stirring apparatus at room temperature for enough time to reach equilibrium. At certain time intervals, the aqueous samples were taken and centrifuged. Then the concentration of dye in the supernatant solution was measured based on the maximum adsorption wavelength (463 nm) using a UV/Vis spectrophotometer (Shimadzu UV/Vis 2550 Spectrophotometer, Japan). The amount of adsorption at time t, qt (mg/g) was calculated by the equation (1):
(1)
Where C0 and Ct (mg/L) are the concentrations of dye initially and at specified time t, respectively, V is the volume of the dye solution (L) and W is the mass of adsorbent used (g).
3. Results and Discussions 3.1. Synthesis and characterization of flower-like MoSe2 microspheres The crystalline structure and phase purity of flower-like MoSe2 microspheres were confirmed by XRD. As shown in Fig. 1a, all observed diffraction peaks can be systematically indexed to those of the hexagonal phase of MoSe2 with lattice parameters of a0 =b0= 3.292Å and c0 =19.392Å, which are in good agreement with the values of standard card (JCPDS No. 20-0757). No peaks from other impurities are detected in the XRD pattern, indicating that the sample is highly 7
crystalline. Energy-dispersive X-ray spectrometer (EDS) results are shown in Fig. 1b, which reveals that the microspheres consist of element Mo and Se; no other elements are observed. Furthermore, the quantification of the peaks shows that the atom ratio of Mo to Se is about 1:2.09, which is very close to the stoichiometry of MoSe2.
Chemical compositions on the surface and valence states of the flower-like MoSe2 microspheres were further investigated by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 2a shows the XPS survey spectrum of MoSe2 microspheres, in which the peaks derived from Mo, Se, C and O elements were detected. Generally, a small amount of O may be due to surface adsorption of oxygen, and the C 1s peak located at 284.6 eV mainly results from the contamination of environment [30]. Fig. 2b shows the spectrum of Mo 3d and the binding energies at 229 eV and 232.1 eV belong to Mo 3d5/2 and 3d3/2 spin orbit peaks of MoSe2, confirming the existence of the Mo IV state. Fig. 2c shows a high-resolution XPS spectrum of Se 3d. The peak is split into two well-defined peaks at 54.5 eV and 55.4 eV, corresponding to the Se 3d5/2 and Se 3d3/2 peaks, further illustrating that the product is MoSe2.
The size and morphology of MoSe2 sample were identified by SEM, TEM and HRTEM. Fig. 3a shows that the as-prepared MoSe2 samples are actually uniform flower-like microspheres with mean diameter about 1.0 μm. Fig. 3b offers a clear view of the surface morphology, which reveals that the obtained MoSe2 microspheres are composed of numerous nanosheets with 20 nm in thickness. Fig. 3c shows a typical TEM image of flower-like MoSe2 microspheres, which is consistent with the above SEM results. The result also reveals that the flower-like MoSe2 is solid interior, and possesses core-corona architecture. More details for MoSe2 structure are illustrated in Fig. 3d. It shows clear lattice fringes, and the lattice spacing is ca. 0.28 nm, which is consistent with the (100) plane of the hexagonal MoSe2 phase. The obvious lamellar structures with an interlayer spacing of 0.65 nm are also observed, indicating the nature of the layered structure.
The specific surface area and porous structure of the prepared samples were determined by using nitrogen adsorption-desorption isotherm (Fig. 4). The isotherm curves of all the samples exhibit a type-IV shape, in accordance to the International Union of Pure and Applied Chemistry 8
(IUPAC) classification, and distinct hysteresis loops, which show the presence of mesopores (2-50 nm) [31]. Textural parameters extracted from nitrogen adsorption–desorption isotherms are summarized in Table 3. The BET surface area of flower-like MoSe2 microspheres were 25 m2/g, much larger than that of MoSe2 plates obtained by solid-state reaction (1.1 m2/g). The relatively large surface area of the as-prepared flower-like MoSe2 microspheres can provide more active sites for adsorption of dye molecules, which would be beneficial for the fast adsorption and transfer of adsorbate inside the hierarchically porous structure of this sample.
3.2. The growth mechanism of MoSe2 microspheres To date, crystal growth mechanisms of inorganic nanomaterials in a hydrothermal process are so complicated that the research of the crystallisation mechanism remains a challenge. In general, oriented aggregation, self-assembly, Ostwald ripening etc. was adopted to account for the growth process of 3D self-assembled structures [22-23, 29]. Recently, many novel techniques have been developed for synthesis of MoSe2 self-assembled structures and their electronical performances; however few studies have focused on the formation mechanism of MoSe2 hierarchical assemble nanostructures. In order to shed light on the morphological evolution, MoSe2 nanostructures harvested at different growth stage were carefully examined by TEM observation. When the reaction time was reached 1 h (Fig. 5a), many MoSe2 nanoparticles with the diameter of 5～10 nm were obtained. With the increase of reaction time to 6 h, sheet-like structures were formed, coexisting with many nanoparticles. By further prolonging the reaction time to 12 h, no nanoparticles existed and many nanosheets assembled to form flowerlike structures as shown in Fig. 5c. Upon gradual assembly of MoSe2 nanosheets, perfect crystalline flowerlike microspheres were obtained when reaction time reached 48 h. Therefore, based on the above experimental results, a possible formation mechanism of MoSe2 nanostructures has been elucidated (Fig. 6). In the first stage, Se could be easily reduced to NaHSe by H2O with the help of NaBH4 [32], meanwhile MoO42- could be readily reduced to Mo4+, then Mo4+ and Se2- further react with each other to form MoSe2 nanocrystals in hydrothermal system through a fast nucleation process. Based on above analysis, the above reactions can be expressed as follows:
2Se + 4BH4- +7H2O → 2HSe- + B4O72- +14H2 9
4MoO42- + BH4- +10H2O → 4Mo4+ + BO2- + 24OHHSe- + OH- → H2O + Se2Mo4+ + 2Se2- → MoSe2
Moreover, these MoSe2 nanocrystals further grew to larger particles via Ostwald ripening process. Subsequently, with the aging process continued, partial MoSe2 nanoparticles started to grow into sheet-like nanostructures through oriented aggregation. Finally, these sheet-like nanostructures gradually evolved into hierarchical flower-like MoSe2 microspheres through the self-assemble process. Based on above discussion and analysis, we believe that the growing process is consistent with the previous reports of a three-stage growth process [22-23, 29, 33], which involves a fast nucleation of amorphous primary particles; slow oriented aggregation of nanosheets and self-assembly of 3D hierarchical nanostructures. However, determining the exact nature of the growth mechanism is not yet fully understood, and will require further theoretical and experimental work.
3.3. Adsorption property Generally, if the material has porous hierarchical structures, it will possess more available active adsorption sites, efficient transport pathways, and may exhibit good adsorption performance [34]. As inspired by previous reports on layered materials that could be applied as adsobents to remove organic waste from water [35-37], the as-synthesized porous hierarchical MoSe2 was used to adsorb methyl orange (MO).
Fig. 7a shows the time profile of methyl orange adsorption at different initial concentrations with 0.02 g of the as-synthesized MoSe2. The adsorption rates were extraordinarily fast in the first 20 min under all concentrations. Then after 20 min a long period of slower uptake was followed, and finally adsorbed amount reached its equilibrium. The adsorption nearly finished within 60 min, indicating the relatively fast adsorption rate of MO on the MoSe2 in water.
Adsorption is a physicochemical process that involves mass transfer of a solute from liquid phase to the adsorbent's surface [38]. Kinetic study provided important information about the 10
mechanism of MO adsorption onto MoSe2, which was necessary to depict the adsorption rate of adsorbent and control the residual time of the whole adsorption process. The adsorption kinetics of MO onto MoSe2 was investigated with the help of two kinetic models, namely the Lagergren pseudo-first-order and pseudo-second-order model. The pseudo-first-order kinetic model is expressed by the following equation:
(2)
Where k1 is the rate constant of preudo-first-order adsorption (1/min), qt and qe are the amount adsorbed at time t and equilibrium (mg/g). According to the adsorption kinetic parameters reported in table 1, the values of correlation (r2) are relatively low for most of the adsorption data at different initial concentrations, and the experimental amount adsorbed at equilibrium qe (exp) do not agree well with the calculated ones. This shows that the adsorption process may not be the correct fit to the first-order rate equation. Another kinetic model is pseudo-second-order model, which is expressed as:
(2)
Where k2 is the rate constant of preudo-second-order adsorption (g/mg•min), qt and qe are the amount of MO absorbed at time t and at equilibrium (mg/g). k2 and qe can be obtained from the slope and intercept of plots of t/qt vs. t, which is shown in Fig. 7b. The adsorption kinetic parameters are reported in table 1. The results reveals that all the experimental data can fit well to the pseudo-second-order model. Clearly, at different initial concentrations, all correlation coefficient (r2) values are greater than 0.99 from the table 1, showing a good linear relationship between t/qt and t. In addition, the experimental amount of MO adsorption at equilibrium (qe (exp)) are very close to the calculated amount (qe (cal)). On the base of above analysis, we can draw a conclusion that the pseudo-second-order kinetic model is highly applicable to the adsorption 11
process of MO onto MoSe2.
For solid-liquid system, the equilibrium of adsorption is one of the important physico-chemical aspects in the description of adsorption behavior. The capacity of the adsorption isotherm plays an important role in the determination of the maximum capacity of adsorption. In order to adapt for the considered system, two appropriate isotherm models (Langmuir and Freundlich adsorption model) that can reproduce experimental results have been considered in the present study. The linear forms of Langmuir and Freundlich isotherms are described as equation (3) and (4), respectively:
(3)
(4)
The plots of Langmuir and Freundlich linear equations are displayed in Fig. 8. All the parameters of two equations are calculated in table 2. As we can see, the maximum adsorption capacity of MO for MoSe2 (qm) is 36.91 mg per gram of MoSe2. Moreover, two models could express the adsorption process due to the linear correlation coefficients r2 are all greater than 0.9. However, the experimental data could be better fit with the Langmuir equation with the linear correlation coefficient (r2=0.9975) being greater than that of the Freundlich equation (r2=0.9899). So it is more suitable to describe the adsorption process by Langmuir model, in other words, Langmuir model is more applicable for adsorption of MO onto MoSe2. As reported in literatures [39], 1/n is a constant relating the adsorption intensity. If 0.1<1/n<0.5, the adsorption is quite favorable; if 0.5<1/n<1, there will be some trouble in the adsorption process; if 1/n>1, it is very hard to adsorb. In this study, 1/n is found to be 0.2441, which confirms that it is easy to adsorb MO from solution for as-prepared MoSe2 microspheres.
For comparison, we investigated the removal capacity of solid-state synthesized MoSe2 12
powder. The removal capacity of the solid-state synthesized MoSe2 was found to be 15.6 mg/g, which is much lower than the removal capacity of flower-like MoSe2 microspheres (36.91 mg/g). This suggests that the as-prepared flower-like MoSe2 microspheres have good potential as new and efficient adsorbent materials for organic wastewater treatment applications. Table 3 summarizes the specific surface areas and the adsorption capacities of MO for various adsorbents. To the best of our knowledge, the as-obtained products have the higher adsorption capacity for MO. The higher adsorption activity of flower-like MoSe2 microspheres can be attributed to the following reason. First, the surface area is exclusive factor to determine the adsorption capability of the adsorbents. It is obviously that flower-like MoSe2 microspheres have much larger surface area than solid-state synthesized MoSe2 powder. The relatively large surface area can provide more active sites for adsorption of dye molecules, which would be beneficial for the fast adsorption and transfer of adsorbate inside the hierarchically porous structure of the sample. On the other hand, in previous studies, π-π stacking interactions as a dominant driven force
has been used to explain the mechanism of aromatic adsorbate to layered MoS2 or grapheme surface [47-49]. Considering aromatic rings in the molecular structure of the MO are present, we proposed a mechanism of π-π stacking interaction between aromatic compound of MO (π electron acceptor) and π electron-rich regions on the surface of MoSe2 probably are likely the reason for the enhance adsorption activities of flower-like MoSe2 microspheres.
4. Conclusion In summary, novel 3D hierarchical flower-like MoSe2 microspheres with mean diameter about 1μm were successfully synthesized via a facile hydrothermal approach. The possible grown mechanism is discussed based on the time-dependent experiments, which indicates flowerlike MoSe2
product
is
formed
via
a
three-stage
growth
process
(nucleation–oriented
aggregation–self-assembly growth). Moreover, the flowerlike MoSe2 sample possess good adsorption performance for organic dye MO, with the maximum adsorption capacity of 36.91 mg/g. The results demonstrate that the adsorption process fit the pseudo-second-order rate equation and Langmuir adsorption model. Considering the novel hierarchical structure obtained by a simple synthetic process and their good adsorption abilities, the study here not only represents an 13
attractive path to large-scale synthesis of other transition metal dichalcogenides with complex structure, but also makes a contribution to the study of transition metal dichalcogenides as potential adsorbents applied in wastewater treatment.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (51302112) and the Jiangsu Industry-University-Research Joint innovation Foundation (BY2013065-05, BY2013065-06).
14
References [1] X.C. Song, Y.F. Zheng, G. Han, H.Y. Yin, G.S. Cao, Ultrathin MoS2-coated carbon nanospheres as highly efficient electrocatalyts for hydrogen evolution reaction, Chem. J. Chinese U. 26 (2005) 617-619. [2] G.K. Wang, X. Sun, F.Y. Lu, H.F. Sun, M.P. Yu, W.L. Jiang, C.S. Liu, J. Lian, Flexible pillared graphene-paper electrodes for high-performance electrochemical supercapacitors, Small 8 (2012) 452-459. [3] L. Xiao, J. Damien, J.Y. Luo, H.D. Jang, J.X. Huang, Z. He, Crumpled graphene particles for microbial fuel cell electrodes, J. Power Sources 208 (2012) 187-192. [4] C.S. Zhao, H.P. Gao, C.M. Chen, H. Wu, Reduction of graphene oxide in Li-ion batteries, J. Mater. Chem. A 3 (2015) 18360-18364. [5] K.F. Wu, H.M. Zhu, T.Q. Lian, Ultrafast exciton dynamics and light-driven H2 evolution in colloidal semiconductor aanorods and Pt-tipped nanorods, Acc. Chem. Res. 48 (2015) 851-859. [6] Q.J. Xiang, J.G. Yu, Graphene-based photocatalysts for hydrogen generation, J. Phys. Chem. Lett. 4 (2013) 753-759. [7] S. Gadipelli, Z.X. Guo, Graphene-based materials: Synthesis and gas sorption, storage and separation, Prog. Mater. Sci. 69 (2015) 1-60. [8] M.S. Xu, Y. Gao, X. Yang, H.Z. Chen, Unique synthesis of graphene-based materials for clean energy and biological sensing applications, Chinese Sci. Bull. 57 (2012) 3000-3009. [9] R. Singh, D. Kumar, C.C. Tripathi, Graphene: Potential material for nanoelectronics applications, Indian J. Pure Ap. Phy. 53 (2015) 501-513. [10] J.G. Yu, M. Jaroniec, Preface: Graphene and C3N4-based photocatalysts, Appl. Surf. Sci. 358 (2015) 1. [11] D. Li, R.B. Kaner, Materials science-Graphene-based materials, Science 320 (2008) 1170-1171. [12] Y.R. Yao, W.Z. Huang, H. Zhou, Y.F. Zheng, X.C. Song, Self-assembly of dandelion-like Fe3O4@C@BiOCl magnetic nanocomposites with excellent solar-driven photocatalytic properties, J. Nanopart. Res. 16 (2014) 1-9. [13] Q.J. Xiang, J.G. Yu, M. Jaroniec, Enhanced photocatalytic H2 production activity of 15
graphene-modified titania nanosheets, Nanoscale 3 (2011) 3670-3678. [14] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197-200. [15] K.F. Wu, Q.Y. Li, Y.Y. Jia, J.R. McBride, Z.X. Xie, T.Q. Lian, Efficient and ultrafast formation
of
long-lived
charge-transfer
exciton
state
in
atomically
thin
cadmium
selenide/cadmium telluride type-II heteronanosheets, ACS Nano 9 (2015) 961-968. [16]
X.C. Song,
E. Yang, G. Liu, Y. Zhang, Z.S. Liu, H.F. Chen, Y. Wang,
Preparation
and
photocatalytic activity of Mo-doped WO3 nanowires, J. Nanopart. Res. 12 (2010) 2813-2819. [17] Q.J. Xiang, J.G. Yu, Graphene-based photocatalysts for hydrogen generation, J. Am. Chem. Soc. 136 (2014) 8839-8842. [18] Y.D. Ma, Y. Dai, C.W. Niu, L. Yu, B.B. Huang, First-principles study of the graphene@MoSe2 heterobilayers, J. Phys. Chem. C 115 (2011) 20237-20241. [19] H.P. Li, L. Chen, Y. Zhang, X.R. Ji, S. Chen, H.J. Song, C.S. Li, H. Tang, Synthesis of MoSe2/reduced graphene oxide composites with improved tribological properties for oil-based additives, Cryst. Res. Technol. 49 (2014) 204-211. [20] Q. Li, B.D. Guo, J.G. Yu, J.R. Ran, B.H. Zhang, H.J. Yan, J.R. Gong, Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets, J. Am. Chem. Soc. 133 (2011) 10878-10884. [21] R. Harpeness, A. Gedanken, A.M. Weiss, M.A. Slifkin, Microwave-assisted synthesis of nanosized MoSe2, J. Mater. Chem. 13 (2003) 2603-2606. [22] H.Q. Shi, X.D. Zhou, Y.S. Lin, X. Fu, Synthesis of MoSe2 nano-flakes modified with dithiophosphinic acid extractant at low temperature, Mater. Lett. 62 (2008) 3649-3651. [23] X.H. Chen, R. Fan, Low-temperature hydrothermal synthesis of transition metal dichalcogenides, Chem. Mater. 32 (2001) 802-805. [24] J. Zhou, H. Xiao, B.W. Zhou, F.F. Huang, S.B. Zhou, W. Xiao, D.H. Wang, Hierarchical MoS2-rGO nanosheets with high MoS2 loading with enhanced electro-catalytic performance, Appl. Surf. Sci. 358 (2015) 152-158. [25] Y. Ding, Y.F. Zhou, W.Y. Nie, P.P. Chen, MoS2-GO nanocomposites synthesized via a hydrothermal hydrogel method for solar light photocatalytic degradation of methylene blue, Appl. 16
Surf. Sci. 357 (2015) 1606-1612. [26] L.K. Putri, W.J. Ong, W.S. Chang, S.P. Chai, Heteroatom doped graphene in photocatalysis: A review, Appl. Surf. Sci. 358 (2015) 2-14. [27] Q. Li, X. Li, S. Wageh, A.A. Al-Ghamdi, J.G. Yu, CdS/graphene nanocomposite photocatalysts, Adv. Energy Mater. 5 (2015) 1500010. [28] J.X. Low, S.W. Cao, J.G. Yu, S. Wageh, Two-dimensional layered composite photocatalysts, Chem. Commun. 50 (2014) 10768-10777. [29] C. Fan, Z.M. Wei, S.X. Yang, J.B. Li, Synthesis of MoSe2 flower-like nanostructures and their photo-responsive properties, RSC Adv. 4 (2014) 775. [30] W. Yu, X.J. Liu, L.K. Pan, J.L. Li, J.Y. Liu, J. Zhang, P. Li, C. Chen, Z. Sun, Enhanced visible light photocatalytic degradation of methylene blue by F-doped TiO2, Appl. Surf. Sci. 319 (2014) 107-112. [31] R.X. Chen, J.G. Yu, W. Xiao, Hierarchically porous MnO2 microspheres with enhanced adsorption performance, J. Mater. Chem. A 1 (2013) 11682. [32] D.L. Klayman, T.C. Griffin, Reaction of selenium with sodium borohydride in protic solvents. A facile method for the introduction of selenium into organic molecules, J. Am. Chem. Soc. 95 (1973) 197-199. [33] G.G. Tang, Y.J. Wang, W. Chen, H. Tang, C.S. Li, Hydrothermal synthesis and characterization of novel flowerlike MoS2 hollow microspheres, Mater. Lett. 100 (2013) 15-18. [34] P. Tian, X.Y. Han, G.L. Ning, H.X. Fang, J.W. Ye, W.T. Gong, Y. Lin, Synthesis of porous hierarchical MgO and its superb adsorption properties, ACS Appl. Mater. Interfaces 5 (2013) 12411-12418. [35] B.C. Yang, Y.Z. Guo, S.R. Zhang, T. Wen, C.M. Zhao, Synthesis of graphene by microwave irradiation for dye adsorption, RSC Adv. 4 (2014) 64771-64780. [36] H.J. Song, S.S. You, X.H. Jia, Synthesis of fungus-like MoS2 nanosheets with ultrafast adsorption capacities toward organic dyes, Appl. Phys. A 121 (2015) 541-548. [37] X.P. Zhang, D. Liu, L. Yang, L.M. Zhou, T.Y. You, Self-assembled three-dimensional graphene-based materials for dye adsorption and catalysis, J. Mater. Chem. A 3 (2015) 10031-10037. [38] L.H. Jiang, Y.G. Liu, G.M. Zeng, F.Y. Xiao, X.J. Hu, X. Hu, H. Wang, T.T. Li, L. Zhou, X.F. 17
Tan, Removal of 17β-estradiol by few-layered graphene oxide nanosheets from aqueous solutions: External influence and adsorption mechanism, Chem. Eng. J. 284 (2016) 93-102. [39] L.C. Zhou, X.G. Meng, J.W. Fu, Y.C. Yang, P. Yang, C. Mi, Highly efficient adsorption of chlorophenols onto chemically modified chitosan, Appl. Surf. Sci. 292 (2014) 735-741. [40] Y. Iida, T. Kozuka, T. Tuziuti, K. Yasui, Sonochemically enhanced adsorption and degradation of methyl orange with activated aluminas, Ultrasonics 42 (2004) 635-639. [41] B.C. Zhu, P.F. Xia, W.K. Ho, J.G. Yu, Isoelectric point and adsorption activity of porous g-C3N4, Appl. Surf. Sci. 344 (2015) 188-195. [42] D.M. Chen, J. Chen, X.L. Luan, H.P. Ji, Z.G. Xia, Characterization of anion-cationic surfactants modified montmorillonite and its application for the removal of methyl orange, Chem. Eng. J. 171 (2011) 1150-1158. [43] A. Shiue, C.M. Ma, R.T. Ruan, C.T. Chang, Adsorption kinetics and isotherms for the removal methyl orange from wastewaters using copper oxide catalyst prepared by the waste printed circuit boards, Sust. Environ. Res. 22 (2012) 209-215. [44] Y.J. Yao, B. He, F.F. Xu, X.F. Chen, Equilibrium and kinetic studies of methyl orange adsorption on multiwalled carbon nanotubes, Chem. Eng. J. 170 (2011) 82-89. [45] T.K. Saha, N.C. Bhoumik, S. Karmaker, M.G. Ahmed, H. Ichikawa, Y. Fukumori, Adsorption of methyl orange onto chitosan from aqueous solution, J. Water Res. Prot. 2 (2010) 898-906. [46] G. Annadurai, R.S. Juang, D.J. Lee, Use of cellulose-based wastes for adsorption of dyes from aqueous solutions, J. Hazard. Mater. 92 (2002) 263-274. [47] Y. Gao, Y. Li, L. Zhang, H. Huang, J.J. Hua, S.M. Shah, X.G. Su, Adsorption and removal of tetracycline antibiotics from aqueous solution by graphene oxide, J. Colloid Interf. Sci. 368 (2012) 540-546. [48] T. Wu, X. Cai, S.Z. Tan, H.Y. Li, J.S. Liu, W.D. Yang, Adsorption characteristics of acrylonitrile, p-toluenesulfonic acid, 1-naphthalenesulfonic acid and methyl blue on graphene in aqueous solutions, Chem. Eng. J. 173 (2011) 144-149. [49] Y.H. Chao, W.S. Zhu, X.Y. Wu, F.F. Hou, S.H. Xun, P.W. Wu, H.Y. Ji, H. Xu, H.M. Li, Application of graphene-like layered molybdenum disulfide and its excellent adsorption behavior for doxycycline antibiotic, Chem. Eng. J. 243 (2014) 60-67.
18
Figure captions Fig. 1 XRD pattern (a) and EDS (b) of flower-like MoSe2 microspheres Fig. 2 XPS spectra of MoSe2 sample: survey XPS spectrum (a), high resolution XPS spectra of Mo 3d (b), Se 3d (c) Fig. 3 SEM (a, b), TEM (c) and HRTEM (d) images of flower-like MoSe2 microspheres Fig. 4 Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of MoSe2 samples Fig. 5 TEM images of MoSe2 samples obtained under different reaction times (a:1h, b:6h, c:12h, d:24h) Fig. 6 Schematic illustration of flower-like MoSe2 microspheres Fig. 7 Time profiles (a) and pseudo-second-order adsorption kinetic plots (b) of MO onto MoSe2 at different initial dye concentrations Fig. 8 Langmuir (a) and Freundlich (b) plots for the adsorption of MO onto MoSe2 Fig. 9 Comparison of adsorption capacities of MO onto MoSe2 samples Table 1 Pseudo-first-order and pseudo-second-order adsorption kinetic constants for MO adsorption on MoSe2 Table 2 Parameters of the Langmuir and Freundlich equations Table 3 Comparison of the maximum sorption capacities of MO with other materials
19
Fig.1 XRD pattern (a) and EDS (b) of flower-like MoSe2 microspheres
Fig. 2 XPS spectra of MoSe2 sample: survey XPS spectrum (a), high resolution XPS spectra of Mo 3d (b), Se 3d (c)
20
Fig.3 SEM (a, b), TEM (c) and HRTEM (d) images of flower-like MoSe2 microspheres
Fig. 4 Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of MoSe2 samples
21
Fig.5 TEM images of MoSe2 samples obtained under different reaction times (a:1h, b:6h, c:12h, d:48h)
Fig.6 Schematic illustration of flower-like MoSe2 microspheres
22
Fig. 7 Time profiles (a) and pseudo-second-order adsorption kinetic plots (b) of MO onto MoSe2 at different initial dye concentrations
Fig. 8 Langmuir (a) and Freundlich (b) plots for the adsorption of MO onto MoSe2
23
Fig. 9 Comparison of adsorption capacities of MO onto MoSe2 samples
24
Table. 1 Pseudo-first-order and pseudo-second-order adsorption kinetic constants for MO adsorption on MoSe2 C0
qe (exp)
(mg/L)
(mg/g)
Pseudo-first-order k1
qe (cal)
(1/min)
(mg/g)
Pseudo-second-order r2
k2
qe (cal)
(g/mg•min)
(mg/g)
r2
5
18.26
1.08*10-2
9.93
0.8092
1.26*10-2
19.39
0.9989
10
25.54
2.06*10-2
9.56
0.74501
7.77*10-3
25.83
0.9972
15
29.74
2.38*10-2
9.77
0.89268
1.36*10-2
29.83
0.9980
20
33.03
2.58*10-2
10.23
0.77677
4.82*10-3
34.46
0.9992
25
33.73
2.71*10-2
11.02
0.9238
6.85*10-3
34.52
0.9999
Table. 2 Parameters of the Langmuir and Freundlich equations Langmuir
Freundlich
KL (L/mg)
r2
qm (mg/g)
0.5527
0.9975
36.91
KF (L/mg)
r2
1/n
17.1591
0.9899
0.2441
25
Table. 3 Comparison of the maximum sorption capacities of MO with other materials Adsorbents
SBET (m2/g)
qm (mg/g)
References
Flower-like MoSe2 microspheres
25
36.91
This work
Solid-state synthesized MoSe2
1.1
15.6
This work
Activated alumina
—
9.8
40
Porous g-C3N4
40
2.51
41
Na-montmorillonite
—
24.0
42
Copper oxide
—
1.2
43
Multiwalled carbon nanotubes
160
52.86
44
Chitosan
—
34.83
45
Banana peel
—
21
46
26