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Efficient synthesis of MoS2 nanoparticles modified TiO2 nanobelts with enhanced visible-light-driven photocatalytic activity
Accepted Manuscript Title: Efficient Synthesis of MoS2 Nanoparticles modified TiO2 Nanobelts with Enhanced Visible-light-driven Photocatalytic Activity Author: Hui Liu Ting Lv Chunkui Zhu Xing Su Zhenfeng Zhu PII: DOI: Reference:
S1381-1169(14)00446-4 http://dx.doi.org/doi:10.1016/j.molcata.2014.10.002 MOLCAA 9304
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
30-7-2014 30-9-2014 3-10-2014
Please cite this article as: H. Liu, T. Lv, C. Zhu, X. Su, Z. Zhu, Efficient Synthesis of MoS2 Nanoparticles modified TiO2 Nanobelts with Enhanced Visible-lightdriven Photocatalytic Activity, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.10.002 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.
Efficient Synthesis of MoS2 Nanoparticles modified TiO2 Nanobelts with Enhanced Visible-light-driven Photocatalytic Activity
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Hui Liu*, Ting Lv, Chunkui Zhu, Xing Su and Zhenfeng Zhu
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[*] Prof. Hui Liu, Zhenfeng Zhu, Ms. Ting Lv, Chunkui Zhu, Xing Su School of Materials Science and Engineering
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Shaanxi University of Science and Technology Xi’an 710021, P. R. China
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Fax: 86-29-86177018
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E-mail: [email protected]
Highlights
TiO2-MoS2 heterojunction structures were synthesized via a simple twostep hydrothermal method.
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The photocatalytic activity of the product on removing of Rh B was evaluated.
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The TM-4 shows the highest photocatalytic activity under visible light
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irradiation.
Abstract
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MoS2 nanoparticles modified TiO2 nanobelts (TiO2-MoS2) with tunable decoration amount of MoS2 nanparticles have been successfully synthesized via a two-step
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hydrothermal method, which involves preparation of TiO2 nanobelts and decoration
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of the MoS2 nanoparticles. The as-prepared samples were carefully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
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microscopy (TEM), energy dispersive X-ray and detector (EDX), X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic degradation of Rhodamine B (Rh B) under visible light irradiation. The photocurrent measurement was also tested to confirm the results of photocatalytic activity. It can be found that TiO2-MoS2 heterojunction
structures with MoS2 decoration amount of 40wt% (TM-4) possess the highest photocatalytic activity since it has the suitable amount of MoS2 for prohibiting the
recombination of photogenerated electrons and holes. In addition, highly apparent photocatalytic reaction rate constant of TM-4 is about 4.78 times than that of pure TiO2 nanobelts.
Keywords:
TiO2-MoS2;
Hydrothermal
method;
Heterojuction
structure;
Photocatalytic activity
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1. Introduction Over the past decades, wastewater treatment techniques have attracted extensive attention in the world due to the water pollution, especially pollution by synthetic dyes and pigments from industries severely disturb the aquatic ecosystem [1, 2]. In
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order to solve these problems, many solutions have been put into effect [3, 4]. TiO2, as one of the most promising semiconductor photocatalysts, is used for degradation of organic pollutants. However, there are two major barriers limit the widespread use of
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TiO2 in the photocatalytic process. The one is its relatively large optical band gap,
about 3.2eV, which limits its photoresponse to the visible light; the other is its rapid
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recombination of photogenerated electron-hole pairs, which results in a low quantum efficiency and poor photocatalytic activity [5]. In order to overcome these limitations,
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various strategies have been developed to improve its photoresponse in the visiblelight and suppress the recombination of photogenerated electron-hole pairs, such as doping with metal and nonmetal ions [6], modified with noble metals [7-11] and
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compositing with other semiconductors [12,13], including Bi2WO6, ZnO, WO3, CdS and MoS2. Coupling TiO2 with these semiconductors to form heterojunction structure is an effective approach to improve the photocatalytic activity of TiO2 [5].
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MoS2, as a two-dimensional structure, has a large specific surface area, strong
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absorption ability and the weak van der Waals gap, and shows many excellent properties such as intercalation, lubrication, anisotropy, chemically inertness, catalysis
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and so on [14,15]. As for its catalytic property, MoS2 has been widely used to remove
S and N from crude oil [16-18]. What’s more, it has a potential application in photocatalysis [19-23]. It is always used for combining with other semiconductors to form heterojunction structure with the purpose of improve the photocatalytic activity of photocatalyst.
Hence, in this paper, MoS2 nanoparticles modified TiO2 nanobelts composites
(TiO2-MoS2) with enhanced visible-light-driven photocatalytic activity were successfully synthesized via a simple two-step hydrothermal method. Owning to the introduction of MoS2 nanoparticles, the adsorption performance of dye molecules and photogenerated electron-hole pairs separation enhanced obviously, and the asprepared TiO2-MoS2 composites show higher photocatalytic efficiency for the photocatalytic decolorization of Rhodamine B (Rh B) aqueous solution under visible light irradiation. 3
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2. Experimental section 2.1 Synthesis of TiO2 nanobelts The TiO2 nanobelts were synthesized via a typical hydrothermal method. In detail, 0.4g of P25 (commercial TiO2) were dissolved in 70 mL of 10M NaOH aqueous
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solution under continuous stirring for 30 min; the resulting mixture was transferred into Teflon-lined autoclave with 100 mL capacity. The autoclave was maintained at 180°C for 24 h and then cooled to room temperature naturally. The obtained powder
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were washed with 0.2M HCl aqueous solution to pH=7, Then washed with deionized water and absolute ethanol several times and then dried at 80°C for 12 h. Finally, the
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TiO2 nanobelts were obtained by heat treatment for 2 h at 600°C.
2.2 Synthesis of MoS2 nanoparticles modified TiO2 nanobelts
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MoS2 nanoparticles modified TiO2 nanobelts (TiO2-MoS2) (40wt% of MoS2, TM-4) was prepared via a simple hydrothermal method. Typically, 0.04g of TiO2 nanobelts were mixed with 0.06g of Na2MoO4·H2O and 0.12g of thiourea in 70mL of deionized
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water. The mixed solution was stirred and then transferred into Teflon-lined autoclave with 100 mL capacity. The autoclave was maintained at 180°C for 24 h. Finally, TM-
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4 were obtained by washing with deionized water and absolute ethanol several times
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and then dried at 80°C for 12 h.
2.3 Characterization
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The crystalline phase of the products were characterized using X-ray diffraction (XRD, D/max2200, Japan) technique with Cu K radiation of wavelength of λ=0.154 nm. The morphology and characterization of the as-prepared samples were performed by a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800 & Hiroba EDX electron microscopy) operated at 5 kV. TEM images were conducted on a JEM 2010 transmission electron microscope operated at 200 kV. UV-visible absorbance spectra were obtained for the dry-pressed disk samples with a UV-visible spectrophotometer (Lambda-950, Perkin Elmer, USA). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi. Photocurrent was measured on an electrochemical workstation (CH1660D instruments, shanghai) in a standard three-electrode system using the prepared samples as the working electrodes, a Pt wire as the auxiliary electrode, and a silver chloride electrode (Ag/AgCl) as a reference electrode. 4
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2.4 Photocatalytic activity measurement The photocatalytic tests of the different photocatalysts were performed by the degradation of Rhodamine B (Rh B) under visible light irradiation. In a typical process, 70 mL RhB solution (10 mg/L) was equally transferred into seven quartz
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beakers, and 70 mg of the sample was equally dispersed in every quartz beaker. The suspension was allowed to reach an adsorption-desorption equilibrium among the photocatalyst, RhB and water before light irradiation. A 500 W xenon lamp was used
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as light source to trigger the photocatalytic reaction. During illumination, a circulation
of water through an external cooling coil was conducted to maintain the temperature
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of suspension at about 25°C. After irradiation for an appropriate interval, the reaction solution was filtrated and concentration of RhB was determined by an UV-visible
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spectrophotometer (Lambda-950, Perkin Elmer, USA).
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3. Results and Discussions
Fig. 1 XRD patterns of samples: (a) pure TiO2 NBs; (b) TM-4 (c) pure MoS2 microspheres.
The crystal structure of the as-prepared samples was studied by XRD. Fig. 1 shows
the XRD patterns of TiO2 nanobelts (curve a), TM-4 (curve b) and pure MoS2 microspheres (curve c), respectively. The pattern of the TiO2 nanobelts (curve a) is a typical XRD profile showing that the feature diffraction peak of the standard pure anatase TiO2 (JCPDS: 21-1272) [24], a major peak centered at 25.28˚ can be ascribed to the (101) facet of anatase TiO2, which is corresponding to the interlayer space of 0.35 nm [25]. Moreover, there are some miscellaneous peaks (Ti6O11) in curve a, 5
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which may be caused by defects (Oxygen vacancies) introduced during the calcination process. About TM-4 (curve b), all diffraction peaks of TiO2 nanobelts are still present in the XRD pattern of TM-4, which indicate the intrinsic structure of TiO2 nanobelts are not destroyed during the fabrication of TM-4 [26]. As for pure MoS2
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microspheres (curve c), the (002), (100), (106) and (110) planes corresponding peaks can be observed. The strong peak at 14.38˚, is corresponding to the (002) facet. In addition, the peaks of TM-4 with lower intensity than pure TiO2 and MoS2 would be
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caused by pure TiO2 suppressed the growth of MoS2 along the (002) facet during the
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hydrothermal growth [27].
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Fig. 2 SEM images of samples: (a) pure TiO2 nanobelts; (b) TM-4 in low-magnification (c) TM-4 in high-magnification; (d) pure MoS2.
Fig. 2 presents the corresponding scanning electron microscopy (SEM) images of
pure TiO2 nanobelts, TM-4 in low-magnification and high-magnification, and pure MoS2, respectively. The result shows that TiO2-MoS2 heterojunction structure is
prepared successfully and the as-prepared samples have a good monodispersity. As shown in Fig. 2 (a), the TiO2 nanobelts are with a uniform morphology and a well
dispersion and the surface of pure TiO2 nanobelts is very smooth. Furthermore, they are of up to hundreds of micrometers in length, and 50-250 nm wide. Fig. 2 (b) is the low-magnification SEM image of TM-4, which indicates that the heterojunction structure is prepared successfully, and the high-magnification SEM image of TM-4 is shown in fig. 2 (c). It can be seen that the MoS2 nanoparticles are clearly observed on
TiO2 nanobelts because the MoS2 are grown on the surface of TiO2 nanobelts evenly, which have a good monodispersity. In addition, after the hydrothermal and calcination 6
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process, the MoS2 nanoparticles are grown on the surface of TiO2 nanobelts to form a heterojunction structure, which is beneficial to its adsorption ability. As can be seen from Fig. 2 (d) that the MoS2 microspheres are obtained by MoS2 nanosheets selfassembled under the same experimental conditions without TiO2, which had been
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described in our previous report [28].
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Fig. 3 TEM images of TM-4 in low magnification (a); TEM images of TM-4 in high
magnification (b) and (c); EDX spectrum and elemental mapping of TM-4 (d) and (e).
The typical transmission electron microscopy (TEM) images of TM-4 in low
magnification are illustrated in Fig.3 (a). As shown in the image, the surface of TiO2 nanobelts is uniformly modified by MoS2 nanoparticles successfully, and MoS2 nanoparticles with an average size of about 10 nm and an average thickness of about 5 nm are distributed on the surface of TiO2. Fig.3 (b) and (c) are the TEM images of
TM-4 in high magnification. It can be seen in Fig.3 (b) and (c), the lattice fringes of MoS2 nanoparticles can be clearly observed, suggesting the well-defined crystal structure. The crystallographic spacing of 0.27nm is equal to the lattice parameter in the (100) facets of MoS2 in Fig.3 (b). The growth direction of TiO2 nanobelts is decided by (101). This is supported by the HRTEM image in Fig.3 (b). The crystallographic spacing of 0.35nm in Fig.3 (b) matches well with the (101) 7
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crystallographic plane of anatase TiO2 [25]. What’s more, the crystallographic plane of the major exposed surfaces of the nanobelts has been determined to be the (101) facet, which is the most thermodynamically stable crystal facet of anatase TiO2 [29, 30]. In addition, there are some cross lattice fringes in Fig.3 (b), it can be seen that the
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TiO2 combine with MoS2 very well, and form a heterojunction structures, which are beneficial to the photocatalytic activity. In Fig.3 (c), the fringes with a lattice spacing of 0.6nm correspond to the (002) plane of MoS2 [31]. The structure of TiO2-MoS2
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composites are further studied by the energy dispersive X-ray (EDX). The results are
shown in Fig. 3 (d) and (e), from which one could clearly see the Ti, O, S and Mo
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elements distribution within the selected area. The EDX elemental mapping results
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also can confirm the heterojunction struvture of the selected sample.
Fig. 4 SEM images of TiO2-MoS2 heterostructures with different MoS2 loading amount: (a) and (b) 20wt%; (c) and (d) 40wt%; (e) and (f) 60wt%; (g) and (h) 80wt%.
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For comparison, TiO2-MoS2 heterojunction structure with different MoS2 loading amount (20wt%, 40wt%, 60wt%, 80wt%) were obtained, and the corresponding scanning electron microscopy (SEM) images are shown in Fig. 4. It can be seen that MoS2 nanoparticles on the surface of TiO2 are increasing with the amount of sulfur
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source and molybdenum source increasing. As a result, no obvious MoS2 nanoparticles can be observed in Fig. 4 (a) and (b), which may due to the reason that
the MoS2 nanoparticles formed here are very little and could not be observed clearly.
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When the MoS2 content is 40wt% (Fig. 4 (c) and (d)), MoS2 with appropriate amount can be observed, and it can just right form a heterojunction structures, which can
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effectively promote the separation of photogenerated electrons and holes, so as to improve the photocatalytic activity. However, when the content of MoS2 is slight
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excessive (60wt%), redundant MoS2 nanoparticles cover on the surface of TiO2 nanobelts, and a small quantity of MoS2 nanoflowers can be seen in Fig. 4 (e) and (f). When the content of MoS2 is 80wt% (Fig. 4 (g) and (h)), it can self assemble into
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flowers, but not form a heterojunction structures.
Fig. 5 The XPS spectra of the TM-4. (a) The wide scan XPS spectrum of the TM-4. (b) Ti 2p XPS spectrum. (c) O 1s XPS spectrum. (d) S 2p XPS spectrum. (e) Mo 3d XPS spectrum.
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The chemical components and the states S of and Mo in the TM-4 heterostructure were investigated by the XPS. As shown in Fig. 5 (a), the wide scan XPS spectrum reveals that the predom-inant elements are Ti, O, S, Mo and C. Among these elements, Ti, O, S and Mo elements are from the prepared TM-4 heterostructure and the C
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element is from the XPS instrument itself, and no other elements are detected. The high-resolution XPS spectra (Fig. 5 (b) - (e)) show that the binding energies of Ti
2p1/2, Ti 2p3/2, O 1s, S 2p1/2, S 2p3/2, Mo 3d3/2 and Mo 3d5/2 peaks are located at 464.4
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eV, 458.5 eV, 530.4 eV, 162.9 eV, 161.8 eV, 232.2 eV and 228.8 eV, respectively.
The results suggest that the prepared TM-4 heterostructure are consisted of Ti4+, S2-
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and Mo4+. In addition, it can be seen that one asymmetric pesk located at 236 eV in
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Fig. 5 (e), this is likely due to the existence of a small amount of Mo6+.
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Fig. 6 UV-vis absorption spectra of TiO2 NBs, TM-2, TM-4, TM-6 and TM-8.
The UV-vis absorption spectra of TiO2 NBs, TM-2, TM-4, TM-6 and TM-8 were
recorded in Fig. 6. In Fig. 6 the spectra of the pure TiO2 NBs shows a significant absorption edge at wavelength shorter than 400 nm, which can be assigned to the intrinsic bandgap absorption. Compared with the pure TiO2 NBs, after the MoS2 nanoparticles grew on the TiO2 NBs, the enhanced absorption in the visible light
regionis are clearly observed. The absorption edge of MoS2 coated samples are red-
shifted, which can be attributed to the chemical bonding between TiO2 and MoS2. For the TM-4, an enhanced absorption can be seen because of the heterojunction structure between TiO2 and MoS2. Furthermore, with the increasing content of MoS2, the absorption of samples is more and more strong. However, another absorption edge can be seen in the TM-6 and TM-8, which may attribute to the excess MoS2. The UVvis absorption spectra also confirmed the argument in Fig. 4. 10
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Fig. 7 The transient photocurrent responses of TiO2 NBs, TM-2, TM-4, TM-6, TM-8.
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The transient photocurrent responses of TiO2 NBs, TM-2, TM-4, TM-6 and TM-8 electrodes under intermittent illumination were shown in Fig. 7. The test was
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performed in 0.1 M L-1 KHCO3 solution [26]. The photocurrent response is reversible as the illumination was turned on and off [32]. Pure TiO2 nanobelts show a very low photocurrent density, which can be attributed to the fast recombination of
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photogenerated electrons and holes in TiO2 [33]. The photocurrent density (7.6
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μA/cm2) of TM-4 composites reach to the highest value rapidly under visible light illumination compared with other samples, which probably attribute to the formation
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of heterojunction structures between TiO2 and MoS2 resulting in the most effective separation of photogenerated electron-hole pairs in TiO2. In addition, TM-8 show an
obviously damped trend, which may be caused that too much MoS2 loading result in a
process of slight photocorrosion and it suppress the separation of photogenerated electrons-holes pairs.
Fig. 8 (a) The degradation rate of RhB, (C/C0) as the function of irradiation time and (b) the plots of ln(C0/C) vs t of TiO2 NBs, TM-2, TM-4, TM-6, TM-8, TiO2+MoS2. 11
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The photocatalytic performance of different photocatalysts was evaluated by degradation of Rh B. Before the light irradiation, an adsorption-desorption equilibrium would be established between the TiO2-MoS2 heterojunction structures and Rh B under stirring in dark. Fig. 8 (a) shows the degradation rate of Rh B, (C/C0)
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as the function of irradiation time, where C0 and C are its initial concentration and the concentration after light irradiation, respectively. It can be seen that the photocatalytic activity of different photocatalysts follows the order of TM-4 ＞TM-6＞ TM-8＞
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TiO2+MoS2 ＞TM-2＞TiO2 NBs, indicating that the appropriate amount MoS2 can
enhance the photocatalytic activity by forming heterojunction structures with TiO2
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nanobelts [27]. In addition, as shown from the areas before 0 min, an adsorption reaction carried out between the photocatalysts and Rh B in dark, that TM-4, TM-6,
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TM-8, TM-2 and TiO2+MoS2 show better adsorption performance than that of pure TiO2 nanobelts, which are due to the existence of MoS2, it has a strong absorption ability. Furthermore, the relatively poor absorption ability of mechanical mixing
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TiO2+MoS2 compared to TM-4 is ascribed to the weak interfacial contact. The photocatalytic degradation of Rh B is a pseudo-first-order reaction and its
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kinetics can be expressed as: ln (C0/C) =k×t, where k is the photocatalytic reaction rate constant [32, 34]. The k value can be calculated from the plots of ln (C0/C) vs t,
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and the results are given in Fig. 8 (b). It can be seen that the k values of TiO2 NBs, TM-2, TM-4, TM-6, TM-8 and TiO2+MoS2 are 0.005 min-1, 0.008 min-1, 0.0239 min1
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, 0.0146 min-1, 0.0088 min-1 and 0.0078 min-1, respectively. The k value of TM-4 is
the highest, which is about 4.78 times than that of pure TiO2 NBs. It is because that
appropriate loading of MoS2 and TiO2 can form heterojunction structures, which can promote the separation of photogenerated electrons and holes. In addition, the results reveal that insufficient or excess MoS2 loading can both result in a lower k value
compared with TM-4, which will decrease the photocatalytic activity of TiO2 nanobelts.
The schematic diagram illustrating the energy band structure and occurrence of vectorial electrons and holes transfer in the TiO2-MoS2 composites was shown in Fig. 9. The band gap of TiO2 nanobelts and MoS2 nanoparticles is closely 3.2 and 1.9eV, respectively [35, 36]. Under visible light irradiation, the photogenerated electrons from the valence band (VB) of MoS2 nanoparticles are directly transferred to conduction band (CB) of MoS2, and leaving behind holes in the VB. As the CB of 12
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TiO2 is lower than that of MoS2, the TiO2 can be used as a photoelectronic receiver, the photogenerated electrons of the MoS2 CB will be transferred to the CB of TiO2 nanobelts [27]. The photogenerated electrons can be trapped by oxygen molecules in the aqueous solution to form singlet oxygen [37]. Simultaneously, the holes moved in
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the opposite direction from the electrons, photogenerated holes will be captured within the MoS2 nanoparticles. The photogenerated electrons and holes can be separated effectively and improve the photocatalytic activity of TiO2-MoS2
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composition by this way [38].
Fig. 9 The schematic diagram illustrating the energy band structure and occurrence of vectorial
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electron and hole transfer in the TiO2-MoS2 composites.
4. Conclusions
In summary, MoS2 nanoparticles modified TiO2 nanobelts with enhanced visible-
light-driven photocatalytic activity were successfully synthesized via a two-step hydrothermal method and their photocatalytic activity for the degradation of Rh B were investigated. The experimental results indicate that the heterojunction structural composites present a uniform morphology. Sample TM-4 (with MoS2 content of 40%) possesses the highest photocatalytic activity since it has the appropriate amount of MoS2 for prohibiting the recombination of photogenerated electrons and holes. In addition, highly apparent photocatalytic reaction rate constant of TM-4 is about 4.78 times than that of pure TiO2 nanobelts. So it is believed that this research can be extended to improve the quality and performance for variety of photocatalyst.
Acknowledgements 13
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We acknowledge financially supported from the National Science Foundation of China (51272147), the Academic Backbone Cultivation Program of Shaanxi University of Science & Technology (XSGP201203), and the Graduate Innovation
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Found of Shaanxi University of Science and Technology.
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S1381-1169(14)00446-4 http://dx.doi.org/doi:10.1016/j.molcata.2014.10.002 MOLCAA 9304
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
30-7-2014 30-9-2014 3-10-2014
Please cite this article as: H. Liu, T. Lv, C. Zhu, X. Su, Z. Zhu, Efficient Synthesis of MoS2 Nanoparticles modified TiO2 Nanobelts with Enhanced Visible-lightdriven Photocatalytic Activity, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.10.002 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.
Efficient Synthesis of MoS2 Nanoparticles modified TiO2 Nanobelts with Enhanced Visible-light-driven Photocatalytic Activity
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Hui Liu*, Ting Lv, Chunkui Zhu, Xing Su and Zhenfeng Zhu
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[*] Prof. Hui Liu, Zhenfeng Zhu, Ms. Ting Lv, Chunkui Zhu, Xing Su School of Materials Science and Engineering
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Shaanxi University of Science and Technology Xi’an 710021, P. R. China
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Fax: 86-29-86177018
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E-mail: [email protected]
Highlights
TiO2-MoS2 heterojunction structures were synthesized via a simple twostep hydrothermal method.
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The photocatalytic activity of the product on removing of Rh B was evaluated.
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The TM-4 shows the highest photocatalytic activity under visible light
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irradiation.
Abstract
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MoS2 nanoparticles modified TiO2 nanobelts (TiO2-MoS2) with tunable decoration amount of MoS2 nanparticles have been successfully synthesized via a two-step
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hydrothermal method, which involves preparation of TiO2 nanobelts and decoration
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of the MoS2 nanoparticles. The as-prepared samples were carefully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
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microscopy (TEM), energy dispersive X-ray and detector (EDX), X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic degradation of Rhodamine B (Rh B) under visible light irradiation. The photocurrent measurement was also tested to confirm the results of photocatalytic activity. It can be found that TiO2-MoS2 heterojunction
structures with MoS2 decoration amount of 40wt% (TM-4) possess the highest photocatalytic activity since it has the suitable amount of MoS2 for prohibiting the
recombination of photogenerated electrons and holes. In addition, highly apparent photocatalytic reaction rate constant of TM-4 is about 4.78 times than that of pure TiO2 nanobelts.
Keywords:
TiO2-MoS2;
Hydrothermal
method;
Heterojuction
structure;
Photocatalytic activity
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1. Introduction Over the past decades, wastewater treatment techniques have attracted extensive attention in the world due to the water pollution, especially pollution by synthetic dyes and pigments from industries severely disturb the aquatic ecosystem [1, 2]. In
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order to solve these problems, many solutions have been put into effect [3, 4]. TiO2, as one of the most promising semiconductor photocatalysts, is used for degradation of organic pollutants. However, there are two major barriers limit the widespread use of
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TiO2 in the photocatalytic process. The one is its relatively large optical band gap,
about 3.2eV, which limits its photoresponse to the visible light; the other is its rapid
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recombination of photogenerated electron-hole pairs, which results in a low quantum efficiency and poor photocatalytic activity [5]. In order to overcome these limitations,
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various strategies have been developed to improve its photoresponse in the visiblelight and suppress the recombination of photogenerated electron-hole pairs, such as doping with metal and nonmetal ions [6], modified with noble metals [7-11] and
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compositing with other semiconductors [12,13], including Bi2WO6, ZnO, WO3, CdS and MoS2. Coupling TiO2 with these semiconductors to form heterojunction structure is an effective approach to improve the photocatalytic activity of TiO2 [5].
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MoS2, as a two-dimensional structure, has a large specific surface area, strong
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absorption ability and the weak van der Waals gap, and shows many excellent properties such as intercalation, lubrication, anisotropy, chemically inertness, catalysis
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and so on [14,15]. As for its catalytic property, MoS2 has been widely used to remove
S and N from crude oil [16-18]. What’s more, it has a potential application in photocatalysis [19-23]. It is always used for combining with other semiconductors to form heterojunction structure with the purpose of improve the photocatalytic activity of photocatalyst.
Hence, in this paper, MoS2 nanoparticles modified TiO2 nanobelts composites
(TiO2-MoS2) with enhanced visible-light-driven photocatalytic activity were successfully synthesized via a simple two-step hydrothermal method. Owning to the introduction of MoS2 nanoparticles, the adsorption performance of dye molecules and photogenerated electron-hole pairs separation enhanced obviously, and the asprepared TiO2-MoS2 composites show higher photocatalytic efficiency for the photocatalytic decolorization of Rhodamine B (Rh B) aqueous solution under visible light irradiation. 3
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2. Experimental section 2.1 Synthesis of TiO2 nanobelts The TiO2 nanobelts were synthesized via a typical hydrothermal method. In detail, 0.4g of P25 (commercial TiO2) were dissolved in 70 mL of 10M NaOH aqueous
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solution under continuous stirring for 30 min; the resulting mixture was transferred into Teflon-lined autoclave with 100 mL capacity. The autoclave was maintained at 180°C for 24 h and then cooled to room temperature naturally. The obtained powder
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were washed with 0.2M HCl aqueous solution to pH=7, Then washed with deionized water and absolute ethanol several times and then dried at 80°C for 12 h. Finally, the
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TiO2 nanobelts were obtained by heat treatment for 2 h at 600°C.
2.2 Synthesis of MoS2 nanoparticles modified TiO2 nanobelts
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MoS2 nanoparticles modified TiO2 nanobelts (TiO2-MoS2) (40wt% of MoS2, TM-4) was prepared via a simple hydrothermal method. Typically, 0.04g of TiO2 nanobelts were mixed with 0.06g of Na2MoO4·H2O and 0.12g of thiourea in 70mL of deionized
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water. The mixed solution was stirred and then transferred into Teflon-lined autoclave with 100 mL capacity. The autoclave was maintained at 180°C for 24 h. Finally, TM-
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4 were obtained by washing with deionized water and absolute ethanol several times
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and then dried at 80°C for 12 h.
2.3 Characterization
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The crystalline phase of the products were characterized using X-ray diffraction (XRD, D/max2200, Japan) technique with Cu K radiation of wavelength of λ=0.154 nm. The morphology and characterization of the as-prepared samples were performed by a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800 & Hiroba EDX electron microscopy) operated at 5 kV. TEM images were conducted on a JEM 2010 transmission electron microscope operated at 200 kV. UV-visible absorbance spectra were obtained for the dry-pressed disk samples with a UV-visible spectrophotometer (Lambda-950, Perkin Elmer, USA). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi. Photocurrent was measured on an electrochemical workstation (CH1660D instruments, shanghai) in a standard three-electrode system using the prepared samples as the working electrodes, a Pt wire as the auxiliary electrode, and a silver chloride electrode (Ag/AgCl) as a reference electrode. 4
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2.4 Photocatalytic activity measurement The photocatalytic tests of the different photocatalysts were performed by the degradation of Rhodamine B (Rh B) under visible light irradiation. In a typical process, 70 mL RhB solution (10 mg/L) was equally transferred into seven quartz
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beakers, and 70 mg of the sample was equally dispersed in every quartz beaker. The suspension was allowed to reach an adsorption-desorption equilibrium among the photocatalyst, RhB and water before light irradiation. A 500 W xenon lamp was used
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as light source to trigger the photocatalytic reaction. During illumination, a circulation
of water through an external cooling coil was conducted to maintain the temperature
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of suspension at about 25°C. After irradiation for an appropriate interval, the reaction solution was filtrated and concentration of RhB was determined by an UV-visible
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spectrophotometer (Lambda-950, Perkin Elmer, USA).
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3. Results and Discussions
Fig. 1 XRD patterns of samples: (a) pure TiO2 NBs; (b) TM-4 (c) pure MoS2 microspheres.
The crystal structure of the as-prepared samples was studied by XRD. Fig. 1 shows
the XRD patterns of TiO2 nanobelts (curve a), TM-4 (curve b) and pure MoS2 microspheres (curve c), respectively. The pattern of the TiO2 nanobelts (curve a) is a typical XRD profile showing that the feature diffraction peak of the standard pure anatase TiO2 (JCPDS: 21-1272) [24], a major peak centered at 25.28˚ can be ascribed to the (101) facet of anatase TiO2, which is corresponding to the interlayer space of 0.35 nm [25]. Moreover, there are some miscellaneous peaks (Ti6O11) in curve a, 5
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which may be caused by defects (Oxygen vacancies) introduced during the calcination process. About TM-4 (curve b), all diffraction peaks of TiO2 nanobelts are still present in the XRD pattern of TM-4, which indicate the intrinsic structure of TiO2 nanobelts are not destroyed during the fabrication of TM-4 [26]. As for pure MoS2
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microspheres (curve c), the (002), (100), (106) and (110) planes corresponding peaks can be observed. The strong peak at 14.38˚, is corresponding to the (002) facet. In addition, the peaks of TM-4 with lower intensity than pure TiO2 and MoS2 would be
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caused by pure TiO2 suppressed the growth of MoS2 along the (002) facet during the
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hydrothermal growth [27].
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Fig. 2 SEM images of samples: (a) pure TiO2 nanobelts; (b) TM-4 in low-magnification (c) TM-4 in high-magnification; (d) pure MoS2.
Fig. 2 presents the corresponding scanning electron microscopy (SEM) images of
pure TiO2 nanobelts, TM-4 in low-magnification and high-magnification, and pure MoS2, respectively. The result shows that TiO2-MoS2 heterojunction structure is
prepared successfully and the as-prepared samples have a good monodispersity. As shown in Fig. 2 (a), the TiO2 nanobelts are with a uniform morphology and a well
dispersion and the surface of pure TiO2 nanobelts is very smooth. Furthermore, they are of up to hundreds of micrometers in length, and 50-250 nm wide. Fig. 2 (b) is the low-magnification SEM image of TM-4, which indicates that the heterojunction structure is prepared successfully, and the high-magnification SEM image of TM-4 is shown in fig. 2 (c). It can be seen that the MoS2 nanoparticles are clearly observed on
TiO2 nanobelts because the MoS2 are grown on the surface of TiO2 nanobelts evenly, which have a good monodispersity. In addition, after the hydrothermal and calcination 6
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process, the MoS2 nanoparticles are grown on the surface of TiO2 nanobelts to form a heterojunction structure, which is beneficial to its adsorption ability. As can be seen from Fig. 2 (d) that the MoS2 microspheres are obtained by MoS2 nanosheets selfassembled under the same experimental conditions without TiO2, which had been
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described in our previous report [28].
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Fig. 3 TEM images of TM-4 in low magnification (a); TEM images of TM-4 in high
magnification (b) and (c); EDX spectrum and elemental mapping of TM-4 (d) and (e).
The typical transmission electron microscopy (TEM) images of TM-4 in low
magnification are illustrated in Fig.3 (a). As shown in the image, the surface of TiO2 nanobelts is uniformly modified by MoS2 nanoparticles successfully, and MoS2 nanoparticles with an average size of about 10 nm and an average thickness of about 5 nm are distributed on the surface of TiO2. Fig.3 (b) and (c) are the TEM images of
TM-4 in high magnification. It can be seen in Fig.3 (b) and (c), the lattice fringes of MoS2 nanoparticles can be clearly observed, suggesting the well-defined crystal structure. The crystallographic spacing of 0.27nm is equal to the lattice parameter in the (100) facets of MoS2 in Fig.3 (b). The growth direction of TiO2 nanobelts is decided by (101). This is supported by the HRTEM image in Fig.3 (b). The crystallographic spacing of 0.35nm in Fig.3 (b) matches well with the (101) 7
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crystallographic plane of anatase TiO2 [25]. What’s more, the crystallographic plane of the major exposed surfaces of the nanobelts has been determined to be the (101) facet, which is the most thermodynamically stable crystal facet of anatase TiO2 [29, 30]. In addition, there are some cross lattice fringes in Fig.3 (b), it can be seen that the
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TiO2 combine with MoS2 very well, and form a heterojunction structures, which are beneficial to the photocatalytic activity. In Fig.3 (c), the fringes with a lattice spacing of 0.6nm correspond to the (002) plane of MoS2 [31]. The structure of TiO2-MoS2
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composites are further studied by the energy dispersive X-ray (EDX). The results are
shown in Fig. 3 (d) and (e), from which one could clearly see the Ti, O, S and Mo
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elements distribution within the selected area. The EDX elemental mapping results
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also can confirm the heterojunction struvture of the selected sample.
Fig. 4 SEM images of TiO2-MoS2 heterostructures with different MoS2 loading amount: (a) and (b) 20wt%; (c) and (d) 40wt%; (e) and (f) 60wt%; (g) and (h) 80wt%.
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For comparison, TiO2-MoS2 heterojunction structure with different MoS2 loading amount (20wt%, 40wt%, 60wt%, 80wt%) were obtained, and the corresponding scanning electron microscopy (SEM) images are shown in Fig. 4. It can be seen that MoS2 nanoparticles on the surface of TiO2 are increasing with the amount of sulfur
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source and molybdenum source increasing. As a result, no obvious MoS2 nanoparticles can be observed in Fig. 4 (a) and (b), which may due to the reason that
the MoS2 nanoparticles formed here are very little and could not be observed clearly.
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When the MoS2 content is 40wt% (Fig. 4 (c) and (d)), MoS2 with appropriate amount can be observed, and it can just right form a heterojunction structures, which can
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effectively promote the separation of photogenerated electrons and holes, so as to improve the photocatalytic activity. However, when the content of MoS2 is slight
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excessive (60wt%), redundant MoS2 nanoparticles cover on the surface of TiO2 nanobelts, and a small quantity of MoS2 nanoflowers can be seen in Fig. 4 (e) and (f). When the content of MoS2 is 80wt% (Fig. 4 (g) and (h)), it can self assemble into
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flowers, but not form a heterojunction structures.
Fig. 5 The XPS spectra of the TM-4. (a) The wide scan XPS spectrum of the TM-4. (b) Ti 2p XPS spectrum. (c) O 1s XPS spectrum. (d) S 2p XPS spectrum. (e) Mo 3d XPS spectrum.
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The chemical components and the states S of and Mo in the TM-4 heterostructure were investigated by the XPS. As shown in Fig. 5 (a), the wide scan XPS spectrum reveals that the predom-inant elements are Ti, O, S, Mo and C. Among these elements, Ti, O, S and Mo elements are from the prepared TM-4 heterostructure and the C
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element is from the XPS instrument itself, and no other elements are detected. The high-resolution XPS spectra (Fig. 5 (b) - (e)) show that the binding energies of Ti
2p1/2, Ti 2p3/2, O 1s, S 2p1/2, S 2p3/2, Mo 3d3/2 and Mo 3d5/2 peaks are located at 464.4
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eV, 458.5 eV, 530.4 eV, 162.9 eV, 161.8 eV, 232.2 eV and 228.8 eV, respectively.
The results suggest that the prepared TM-4 heterostructure are consisted of Ti4+, S2-
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and Mo4+. In addition, it can be seen that one asymmetric pesk located at 236 eV in
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Fig. 5 (e), this is likely due to the existence of a small amount of Mo6+.
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Fig. 6 UV-vis absorption spectra of TiO2 NBs, TM-2, TM-4, TM-6 and TM-8.
The UV-vis absorption spectra of TiO2 NBs, TM-2, TM-4, TM-6 and TM-8 were
recorded in Fig. 6. In Fig. 6 the spectra of the pure TiO2 NBs shows a significant absorption edge at wavelength shorter than 400 nm, which can be assigned to the intrinsic bandgap absorption. Compared with the pure TiO2 NBs, after the MoS2 nanoparticles grew on the TiO2 NBs, the enhanced absorption in the visible light
regionis are clearly observed. The absorption edge of MoS2 coated samples are red-
shifted, which can be attributed to the chemical bonding between TiO2 and MoS2. For the TM-4, an enhanced absorption can be seen because of the heterojunction structure between TiO2 and MoS2. Furthermore, with the increasing content of MoS2, the absorption of samples is more and more strong. However, another absorption edge can be seen in the TM-6 and TM-8, which may attribute to the excess MoS2. The UVvis absorption spectra also confirmed the argument in Fig. 4. 10
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Fig. 7 The transient photocurrent responses of TiO2 NBs, TM-2, TM-4, TM-6, TM-8.
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The transient photocurrent responses of TiO2 NBs, TM-2, TM-4, TM-6 and TM-8 electrodes under intermittent illumination were shown in Fig. 7. The test was
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performed in 0.1 M L-1 KHCO3 solution [26]. The photocurrent response is reversible as the illumination was turned on and off [32]. Pure TiO2 nanobelts show a very low photocurrent density, which can be attributed to the fast recombination of
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photogenerated electrons and holes in TiO2 [33]. The photocurrent density (7.6
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μA/cm2) of TM-4 composites reach to the highest value rapidly under visible light illumination compared with other samples, which probably attribute to the formation
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of heterojunction structures between TiO2 and MoS2 resulting in the most effective separation of photogenerated electron-hole pairs in TiO2. In addition, TM-8 show an
obviously damped trend, which may be caused that too much MoS2 loading result in a
process of slight photocorrosion and it suppress the separation of photogenerated electrons-holes pairs.
Fig. 8 (a) The degradation rate of RhB, (C/C0) as the function of irradiation time and (b) the plots of ln(C0/C) vs t of TiO2 NBs, TM-2, TM-4, TM-6, TM-8, TiO2+MoS2. 11
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The photocatalytic performance of different photocatalysts was evaluated by degradation of Rh B. Before the light irradiation, an adsorption-desorption equilibrium would be established between the TiO2-MoS2 heterojunction structures and Rh B under stirring in dark. Fig. 8 (a) shows the degradation rate of Rh B, (C/C0)
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as the function of irradiation time, where C0 and C are its initial concentration and the concentration after light irradiation, respectively. It can be seen that the photocatalytic activity of different photocatalysts follows the order of TM-4 ＞TM-6＞ TM-8＞
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TiO2+MoS2 ＞TM-2＞TiO2 NBs, indicating that the appropriate amount MoS2 can
enhance the photocatalytic activity by forming heterojunction structures with TiO2
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nanobelts [27]. In addition, as shown from the areas before 0 min, an adsorption reaction carried out between the photocatalysts and Rh B in dark, that TM-4, TM-6,
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TM-8, TM-2 and TiO2+MoS2 show better adsorption performance than that of pure TiO2 nanobelts, which are due to the existence of MoS2, it has a strong absorption ability. Furthermore, the relatively poor absorption ability of mechanical mixing
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TiO2+MoS2 compared to TM-4 is ascribed to the weak interfacial contact. The photocatalytic degradation of Rh B is a pseudo-first-order reaction and its
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kinetics can be expressed as: ln (C0/C) =k×t, where k is the photocatalytic reaction rate constant [32, 34]. The k value can be calculated from the plots of ln (C0/C) vs t,
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and the results are given in Fig. 8 (b). It can be seen that the k values of TiO2 NBs, TM-2, TM-4, TM-6, TM-8 and TiO2+MoS2 are 0.005 min-1, 0.008 min-1, 0.0239 min1
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, 0.0146 min-1, 0.0088 min-1 and 0.0078 min-1, respectively. The k value of TM-4 is
the highest, which is about 4.78 times than that of pure TiO2 NBs. It is because that
appropriate loading of MoS2 and TiO2 can form heterojunction structures, which can promote the separation of photogenerated electrons and holes. In addition, the results reveal that insufficient or excess MoS2 loading can both result in a lower k value
compared with TM-4, which will decrease the photocatalytic activity of TiO2 nanobelts.
The schematic diagram illustrating the energy band structure and occurrence of vectorial electrons and holes transfer in the TiO2-MoS2 composites was shown in Fig. 9. The band gap of TiO2 nanobelts and MoS2 nanoparticles is closely 3.2 and 1.9eV, respectively [35, 36]. Under visible light irradiation, the photogenerated electrons from the valence band (VB) of MoS2 nanoparticles are directly transferred to conduction band (CB) of MoS2, and leaving behind holes in the VB. As the CB of 12
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TiO2 is lower than that of MoS2, the TiO2 can be used as a photoelectronic receiver, the photogenerated electrons of the MoS2 CB will be transferred to the CB of TiO2 nanobelts [27]. The photogenerated electrons can be trapped by oxygen molecules in the aqueous solution to form singlet oxygen [37]. Simultaneously, the holes moved in
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the opposite direction from the electrons, photogenerated holes will be captured within the MoS2 nanoparticles. The photogenerated electrons and holes can be separated effectively and improve the photocatalytic activity of TiO2-MoS2
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composition by this way [38].
Fig. 9 The schematic diagram illustrating the energy band structure and occurrence of vectorial
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electron and hole transfer in the TiO2-MoS2 composites.
4. Conclusions
In summary, MoS2 nanoparticles modified TiO2 nanobelts with enhanced visible-
light-driven photocatalytic activity were successfully synthesized via a two-step hydrothermal method and their photocatalytic activity for the degradation of Rh B were investigated. The experimental results indicate that the heterojunction structural composites present a uniform morphology. Sample TM-4 (with MoS2 content of 40%) possesses the highest photocatalytic activity since it has the appropriate amount of MoS2 for prohibiting the recombination of photogenerated electrons and holes. In addition, highly apparent photocatalytic reaction rate constant of TM-4 is about 4.78 times than that of pure TiO2 nanobelts. So it is believed that this research can be extended to improve the quality and performance for variety of photocatalyst.
Acknowledgements 13
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We acknowledge financially supported from the National Science Foundation of China (51272147), the Academic Backbone Cultivation Program of Shaanxi University of Science & Technology (XSGP201203), and the Graduate Innovation
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Found of Shaanxi University of Science and Technology.
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