Enhanced photoelectrochemical performance of MoS2 nanobelts-loaded TiO2 nanotube arrays by photo-assisted electrodeposition

Accepted Manuscript Title: Enhanced photoelectrochemical performance of MoS2 nanobelts-loaded TiO2 nanotube arrays by photo-assisted electrodeposition Authors: Wei Teng, Youmei Wang, HuiHui Huang, Xinyong Li, Yubin Tang PII: DOI: Reference:

S0169-4332(17)31952-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.297 APSUSC 36497

To appear in:

APSUSC

Received date: Revised date: Accepted date:

15-5-2017 16-6-2017 28-6-2017

Please cite this article as: Wei Teng, Youmei Wang, HuiHui Huang, Xinyong Li, Yubin Tang, Enhanced photoelectrochemical performance of MoS2 nanobeltsloaded TiO2 nanotube arrays by photo-assisted electrodeposition, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.297 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.

Enhanced photoelectrochemical performance of MoS2 nanobelts-loaded TiO2 nanotube arrays by photo-assisted electrodeposition

Wei Tenga* [email protected], Youmei Wanga, HuiHui Huanga, Xinyong Lib, Yubin Tanga

a

Department of Environmental and Chemical Engineering, Jiangsu University of

Science and Technology, Zhenjiang, 212003, P. R. China b

Key Laboratory of Industrial Ecology and Environmental Engineering and State Key

Laboratory of Fine Chemical, School of Environmental Sciences and Technology, Dalian University of Technology, Dalian, 116024, PR China

*

Corresponding author at: Tel.: +86-511-8560-5157; Fax.: +86-511-8563-5850

1

Graphical Abstract The proposed reaction schemes of visible light induced processes at interface of the

hν

Potential eV vs. NHE

MoS2/TiO2 NTs heterostructure photoelectrodes. -1.6 -0.8

+0.8 eV

-0.5 eV

0 2.4 eV

+0.8

3.2 eV

+1.6 eV

+1.6 +2.4

+2.7 eV

+3.2

O2

TiO2

MoS2

hν

e-

e-

+e−

O2·−

+h+ h+

1O

2

2

MoS2

TiO2

EF

Highlights 

MoS2/TiO2 was synthesized by photo-assisted electrodeposition method.



The MoS2/TiO2 have enhanced photoelectrocatalytic activity for organics degradation.



The holes and 1O2 were the main active species involved in the reaction process.

3

Abstract Novel MoS2 sensitized TiO2 nanotube arrays with high photoelectrocatalytic activity under photo-assisted

visible

light

irradiation were successfully synthesized

electrodeposition

procedure.

The

photoelectrocatalytic

via

(PEC)

performance of the composite electrode was examined by the photoelectrocatalytic oxidation of methylene blue (MB) and sulfadiazinmu (SD) under a 500 W Xe lamp with a UV light cutoff filter (λ ≥ 410 nm). The MoS2/TiO2 heterostructure photoelectrode presented a significantly enhanced PEC activity than the pure TiO2 NTs owing to its stronger light-harvesting ability and improved separation of photogenerated electrons and holes in comparison with TiO2 nanotubes. The obviously

reduced

electron-hole

recombination

rates

of

MoS2/TiO2

were

demonstrated from PL spectroscopy measurements and the photoelectrochemical evaluation. The degradation rate of MoS2/TiO2 NTs photoelectrode for PEC degradation of MB and SD was 3 times that of TiO2 NTs photoelectrode. It was found that holes and single oxygen act as the main oxidative species.

Keywords: TiO2 nanotube arrays; MoS2 nanobelts; photoelectrocatalytic

4

1. Introduction One-dimensional (1D) nanomaterials such as nanotubes, nanobelts and nanowires have attracted much interest in the last decade due to their high surface-to-volume ratios and other different properties compared to their bulk counterparts. TiO2 is one of the most widely used photocatalytic materials due to its special ability of oxidative decomposition of organic pollutants, corrosion resistance, nontoxicity and inexpensive [1]. The highly-ordered, vertically oriented TiO2 nanotube arrays (NTs), achieved by Ti anodization, have been shown to be promising structures for water photoelectrolysis [2, 3]. Compared with other TiO2 structures, nanotube arrays with smooth surface possess advantageous one-dimensional geometry for efficient charge transfer [4, 5]. This improved configuration affords significantly shorter carrier-diffusion paths along the tube walls and thereby minimizes the occurrence of charge losses due to the electron hopping between nanoparticles [6]. 1D TiO2 nanotube arrays inherited all the typical features of TiO2 nanoparticles and displayed a large specific surface area which is easy for photogenerated carriers to transfer along the axial direction, allowing 1D TiO2 nanotube arrays materials to be widely used in photocatalysis for generation of hydrogen [7, 8], dye-sensitized solar cells [9, 10]. lithium ion batteries[11, 12] , sensors [13] and biological applications [14]. However, the wide application of 1D TiO2 nanostructured materials in some fields was limited due to two main disadvantages. One is the lack of effective absorption in the visible light which contributes more than 52% of the total solar spectrum region [15] due to the wide band gap of TiO2 (anatase: 3.2 eV, rutile: 3.0 eV) [16]. The other is that the electron-hole pairs could easily recombine [17, 18]. Various strategies have been developed to overcome the problems of the above-mentioned. For example, doping with metal [19-21], non-metal [22-24] and coupling with narrow-bandgap semiconductors [25, 26]. Especially the coupling of TiO2 NTs to suitable narrow band gap semiconductors has been widely reported to be an efficient method to enhance its photoresponse under visible light. Several semiconductors, such as CdS [27], CdTe [28], CdSe [29, 30], Cu2O [31], Cu2ZnSnS4 [32], Ag-based semiconductor [33] and 5

SrTiO3 [34, 35] have been adopted to improve the utilization of visible light for TiO2 NTs. However, most of these sensitizers are either highly poisonous or expensive, thus limiting their practical applications. Molybdenum disulfide (MoS2), a transition metal dichalcogenides, has attracted considerable attention due to its important role in lithium ion battery [36], flexible electronic device [37], photoluminescence,[38] and catalysts [39-42]. MoS2 is a graphene-like transition metal dichalcogenides with an appreciable bandgap of 1.89 eV [43, 44]. It has good electronic, optical, thermal and mechanical properties, especially in the high electron mobility at room temperature and excellent conductive properties. Until now, the MoS2-based semiconductor heterostructures, such as MoO3/MoS2[45]. ZnO/MoS2 [46], and MoS2@graphene [47, 48] with good photocatalytic properties have been reported. The efficient charge separation could be obtained by coupling two semiconductor structures with the matched energy levels. Therefore, MoS2 is a good candidate for interfacing with TiO2 for constructing hybrid photocatalytic systems for photocatalytic degradation. Recently, there are several reports of MoS2/TiO2 heterostructure with remarkable photocatalytic activity have been successfully synthesized [49, 50]. In this paper, the highly ordered n-type TiO2 NTs decorated with n-type MoS2 was prepared by employing a photo-assisted electrodeposition method. The nanoscale n-n heterojunction formed at the crystallites’ interface. The structure and optical properties over the prepared MoS2/TiO2 NTs were systematically investigated. The visible light photoelectrocatalytic activity of the MoS2/TiO2 heterostructure photoelectrodes was evaluated in degradation of MB and SD in aqueous solution. The MoS2/TiO2 NTs exhibited higher activities than pure TiO2 NTs under the same reaction conditions.

2. Experimental 2.1. Preparation of MoS2/TiO2 nanotube arrays The highly ordered TiO2 NTs was synthesized by anodic oxidation in a NH4F electrolyte, similar to that described previously [33]. In a typical procedure, the 6

titanium foil with a purity of 99.8% (1 mm thick, specifications: 20 mm×40 mm) was mechanically polished with different abrasive papers, and then rinsed in an ultrasonic bath of ethanol for 15 min and deionized water for 15 min in turn. And then the pretreated Ti foils were chemically etched for 40 s by immersing in a mixture of HF and HNO3 (HF:HNO3:H2O=1:4:5 in volume ratio), and finally rinsed with deionized water and ethanol, respectively. The electrochemical anodization was carried out in a two-electrode electrochemical cell connected to a DC power supply (Gwinstek PSW80-13.5). Ti foil served as anodic electrode and Pt foil as the cathode. The anodizing voltage varied from 0 to 60 V with an increasing rate of 0.5 V s-1 and was kept at 60 V for 60 min. The electrolyte composed of 0.3 wt% NH4F and 2 vol% H2O solution in ethylene glycol. After anodic oxidation, the samples were annealed at 755 K in oxygen for 2 h with heating and cooling rates of 2 ℃ min-1 to convert the amorphous phase to the crystalline one. Figure 1 presents the incorporation procedure of MoS2 nanobelts on TiO2 NTs in detail. MoS2 nanobelts were deposited onto the crystallized TiO2 NTs by reducing the MoS42- ions using (NH4)2[MoS4] as the precursor and methanol solution as hole scavenger [51]. For photo-assisted electrodeposition of MoS2 catalyst, a freshly prepared 0.1 mM (NH4)2[MoS4] solution in pH 5.0 Na2SO4 buffer was used as a deposition bath. Prior to use, the electrolyte solution was saturated with nitrogen gas to remove oxygen for 15 min. The MoS2 nanobelts was then deposited onto TiO2 electrode held at -0.4 V versus SCE and under simulated sunlight irradiation for different time (5, 10, 20 and 30 min). According to the irradiation time of electrode, the obtained MoS2/TiO2 heterojunction were labeled as MoS2/TiO2-5, MoS2/TiO2-10, MoS2/TiO2-20 and MoS2/TiO2-30, respectively. After that, the MoS2/TiO2 NTs was rinsed with distilled water. The discussion of the effect of irradiation time on the catalyst is shown in the supplementary material, the results indicated that the MoS2/TiO2-20 electrode performance is the best. And the MoS2/TiO2-20 electrode was used in the characterization and photoelectrocatalytic activity test.

2.2. Characterization 7

The surface morphology of the as prepared electrode was observed using a field-emission scanning electron microscope (HITACHI, S-4800, Japan) with an accelerating voltage of 30.0 kV. The dopant concentration was examined by energy dispersive X-ray analysis (EDX, Horiba 7593H) to verify the elemental concentration distribution on the electrode using Link Isis Series 300 software. The composition and crystal structure of the obtained electrode was characterized on a X-ray diffractometer (D8 Advance, Bruker-AXS, Germany) with a Cu Ka radiation source. Chemical states of these electrodes were analyzed with X-ray photoelectron spectroscopy (XPS) on an ESCALAB250Xi Thermo Fisher Scientific apparatus. All the binding energies were calibrated by using the contaminant carbon (C 1s) 284.6 eV as a reference. The chemical bonding characteristics was verified by using a Raman microscope Renishaw 300. The light absorption properties were measured using UV-Vis diffuse reflectance spectrophotometer (SHIMADZU, UV-2550) with a wavelength range of 200-800 nm. The photoluminescence (PL) characteristics were probed using a FS5 spectrofluorometer from Edinburgh Instruments. The photoelectrochemical measurements of the as prepared TiO2 and MoS2/TiO2 NTs electrode was performed on a electrochemical workstation (CHI 660D, CH Instruments Inc., Shanghai) with conventional three-electrode cell system. A Pt electrode and a saturated calomel electrode served as the counter and reference electrode, respectively. The as prepared electrode was employed as the working electrode. Meanwhile, The working electrode was irradiated by visible light (410 nm < λ) through a UV-cutoff filter (Shanghai Seagull Colored Optical Glass Co., Ltd.) from a high-pressure xenon short arc lamp (a Phillips 500 W Xe lamp). The incident light intensity of the visible light was 75.6 mW cm−2, which was measured with a radiometer (Photoelectric Instrument Factory Beijing Normal University, model FZ-A). The electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 10−2 to 105 Hz with an ac voltage amplitude of 10 mV at a bias of 0.3 V vs. SCE in a 0.01 M Na2SO4 electrolyte.

2.3. Photoelectrocatalytic Activity Test 8

The photoelectrocatalytic activity were evaluated using the degradation of aqueous solutions of MB and SD under visible light irradiation. The as prepared electrode, Pt sheet and SCE were employed as the working electrode, counter electrode and reference electrode, respectively. The experiments were performed with magnetic stirring, using 0.01 M Na2SO4 as the electrolyte. The initial concentration of the MB and SD aqueous solution was 20 mg L−1 and 10 mg L-1 during the experiment, respectively. Before the light irradiation, the suspensions were magnetically stirred for 20 min to reach the adsorption-desorption equilibrium. The concentration of MB was determined by an UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). High performance liquid chromatography (HPLC Shimadzu LC-20A) was adopted for analysis of SD concentration. The mobile phase was composed of methanol and water (volume ratio: 80/20), and the elution time was 2.3 min at a flow rate of 0.9 mL min-1. The detector was set at the wavelength of 270 nm, and C18 reversed phase column (Inertsil ODS-SP 4.6 nm×150 nm) was used for chromatographic analysis. The active species generated in the photoelectrocatalytic system were detected with an in-situ capture

technique

using

the

tert-butanol

(tBuOH),

Benzoquinone

(BQ),

ethylenediaminetetraacetic acid (EDTA) and β-carotene tests. The generation of ·OH and 1O2 radicals was investigated using the electron spin resonance (ESR Bruker spectrometer E 500).

3. Results and discussion 3.1 Preparation and Characterization of Photocatalysts Self-organized and vertically oriented TiO2 NT arrays were obtained by anodization of Ti film. The low and a high magnification SEM images showing the morphologies of the TiO2 NT arrays are displayed in Figure 2a and b. It can be observed that well-ordered, and uniform TiO2 nanotubes are fabricated on the pure titanium sheet and the average diameter of the TiO2 nanotubes range from 80 to 100 nm. The wall thickness of nanotube could be reach 15~20 nm. The high-magnification SEM image of the sample shows the nanotube morphology more clearly in Figure 2b. The morphologies of TiO2 NTs after the modification by MoS2 nanobelts were 9

illustrated in Figure 2c and d. It can be observed that the MoS2 nanobelts with a length of approximate 200 nm are successfully grown on the top surface of vertically oriented TiO2 nanotube arrays. In some areas, clusters of MoS2 nanobelts were formed in the entrances of the tube openings. The amount of MoS2 formed on the surface of TiO2 NTs is related to the time of electrodeposition. The SEM of MoS2/TiO2 samples with different deposition time were depicted in the supporting information (Figure S1-S4). It is found that there are more MoS2 nanobelts deposited on the TiO2 NTs surface with the deposition time extended. Moreover, the as prepared MoS2/TiO2 NTs was also investigated by the elemental signature in the EDX spectrum (Figure S5). The results further confirm that the electrode was composed of O, Mo, S and Ti elements. The weight proportions of O, Mo, S and Ti elements were 0.4%, 1.12%, 0.33% and 98.15%, respectively. The results confirmed that most of MoS2 nanobelts were loaded on the top surface of TiO2 NTs. The XRD patterns of as prepared pure TiO2 NTs and MoS2/TiO2 NTs are shown in Figure 3. All the samples display good crystalline. Qualitative analysis of this pattern shows that all peaks in the pattern match the standard peaks of Ti metal phase (JCPDS file No. 65-6231) and the anatase phase of the TiO2 (JCPDS file No. 21-1272). The peak located at 25.3° could be indexed to (101) facet of anatase TiO2. There is no obvious changes between the pure TiO2 NTs and MoS2/TiO2 NTs in crystal structure. The results confirm that the intrinsic structure of TiO2 NTs are well retained during the MoS2 electrodeposition process. It can be observed from the result of XRD for the MoS2/TiO2 NTs electrode that there exhibit a new peak at 2θ of 14.5◦. The peak can be assigned to (003) crystal planes (JCPDS file No. 17-0744) of the MoS2. The results confirmed the existence of MoS2 nanobelts on the surface of TiO2 nanotube array electrode. To further investigate the surface chemical composition and valence state of the MoS2/TiO2 NTs, XPS study was performed, and the spectra are illustrated in Figure 4a-d. Elements of Ti, Mo, S, O and adventitious C existed in the MoS2/TiO2 NTs composite (Figure 3a). A typical high resolution XPS spectrum of Mo 3d is shown in Figure 4b, two peaks at 232.4 and 229.4 eV are attributed to Mo(+4) 3d3/2 and Mo(+4) 10

3d5/2, respectively [51]. Figure 4c shows the XPS spectrum of the S 2p region, which can be fitted into two peaks: S 2p1/2 and 2p3/2, which appear at 162.5 and 163.9 eV, respectively [52, 53]. The XPS spectra of O 1s in both TiO2 and MoS2/TiO2 NTs were also fitted by the nonlinear least-squares program using Gaussian-Lorentzian peak shapes. The major peak at 530.2 eV and 530.6 eV are attributed to lattice oxygen, whereas the weak peak at 531.6 eV and 532.4 eV are due to the presence of a surface hydroxyl group [54, 55]. As shown in Figure 4f, a comparison of the observed spin-energy separation of about 5.8 eV between Ti 2p1/2 and Ti 2p3/2 peaks. The result suggests the predominant state of the Ti element is Ti4+ in both pure TiO2 and MoS2/TiO2 NTs. The binding energies of O 1s and Ti 2p for MoS2/TiO2 NTs shifted to negative higher energies due to the electronic interaction between MoS2 and TiO2 [56]. The XPS results further confirmed the MoS2 was loaded onto the TiO2 NTs successfully, which agree well with the XRD results. Raman spectra were adopted to identify the existence of MoS2 due to the strong Raman characteristic signals. As depicted in Figure 5, there are a intense peak located at 144 cm-1 was observed, which could be assigned to the main Eg anatase vibration mode of TiO2 [49]. Moreover, three peaks at 393, 515 and 637 cm-1 could be assigned to the Raman active modes B1g, A1g and Eg of anatase TiO2, respectively. In the spectrum of MoS2/TiO2 NTs, a broad peak centered at 400 cm-1 is fitted into two dominant scattering peaks at 379 and 400 cm-1 (inset in Figure 5), corresponding to the B1g of TiO2 and E2g and A1g of MoS2, respectively. The results confirm that the MoS2 was successfully introduced into the TiO2 NTs. Furthermore, a small blue shift was observed in the mode of E1g in the MoS2/TiO2 heterostructure compared with pure TiO2. This phenomenon could be ascribed to the surface strain induced by the deposition of MoS2 nanobelts on the surface of TiO2 [57]. External surface strain is a potential way to control the physical and chemical properties of the surfaces of the transition metal oxide [58]. The strain on the surface of TiO2 could hence the area ratio of anatase crystallite has been reported [59]. Furthermore, the formation energy of various types of oxygen vacancy on the surface of rutile TiO2<110> also depends on external surface strain [60]. Therefore, we believe that the surface strain could 11

enhance the photocatalytic activity of MoS2/TiO2 NTs, which could increase the area of the anatase crystal and the oxygen hole on the surface of the catalyst. Figure 6a shows the UV-vis diffuse reflection spectra of pure TiO2 NTs and MoS2/TiO2 NTs samples. Compared with pure TiO2, MoS2/TiO2 heterostructures show evident red shifts in the absorbance peak edges. It is concluded that the MoS2 nanobelts could absorb photons around 450 nm. The absorption peak of MoS2 (d = 4.5 nm) was at ~470 nm in the visible light region [61]. The MoS2/TiO2 NTs used in this work showed absorption peaks at ~450 and ~380 nm. Peak at ~380 nm in Figure 6a can be ascribed to the absorption of TiO2 in the UV region, which is consistent with the result reported in everywhere [62]. The MoS2 nanobelts in the composite exhibited varying sizes (including length and thickness) from 2 to 10 nm, which led to the appearance of the absorption peak in the visible light region. The optical band gap energy was calculated from the Kubelka-Munk function. For a crystalline semiconductor, the optical band gap is determined by the following equation using the optical absorption data near the band edge [63]. ahν = A(hν - Eg)n/2 where a, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the equation, n decides the characteristics of the transition in a semiconductor, that is, direct transition (n=1) or indirect transition (n=4). According to the literature, monolayer MoS2 was a direct-gap material. However, the bulk MoS2 crystal became an indirect-gap semiconductor [61, 64]. It is clear that the as prepared MoS2 is not monolayer. Therefore, the value of n is 4. The results were shown in Figure 6b and was found to be 3.2 eV and 2.4 eV for TiO2 and MoS2/TiO2 NTs, respectively.

3.2 Photoelectrochemical measurements To give further evidence to support the above suggested visible light photocatalytic mechanism, the representative current density versus voltage (I-V) characteristics of the TiO2, and MoS2/TiO2 NTs electrode under visible light irradiation. As comparatively studied in Figure 7a. the maximum photocurrent density 12

of MoS2/TiO2 and TiO2 NTs is about 3.55 and 0.12 mA cm-2, respectively. Compared with the pure TiO2 NTs, a significant increase in the photocurrent density was observed for the MoS2/TiO2 NTs under visible light irradiation. The transient photocurrent of TiO2 nanotube array electrode and MoS2/TiO2 NTs electrodes measured at a fixed bias potential of 0.2 V vs. SCE with a visible light pulse of 20 s is also shown in Figure 7b. The results demonstrate that MoS2/TiO2 and TiO2 photoelectrodes have a fast photoresponse when the light is turned on and off. The photocurrent density of MoS2/TiO2 NTs is larger than 0.3 mA cm−2, whereas that of TiO2 NTs is only 0.05 mA cm-2. The short-circuit photocurrent of the MoS2/TiO2 composite array electrode was more than five times that of pure TiO2 NTs. The remarkably increased photocurrent density in MoS2/TiO2 NTs could be attributed to the sensitivity of MoS2 to visible light and the high separation efficiency of the photogenerated electron-hole pairs. Electrochemical impedance spectroscopy (EIS) is an effective measure for probing the properties of MoS2/TiO2 NTs electrode and was further employed to analyze the features of separation and transfer of electron and hole pairs [65]. The EIS tests were carried out to further unveil the reasons for superior performance of the MoS2/TiO2 NTs. The radius of the arc on the EIS (Nyquist plots, corresponding to the imaginary part Z′′ vs the real part Z′ of the complex impedance Z) plot reflects the reaction rate occurring at the surface of electrode. The equivalent circuit model of the studied system is shown in Figure 7c inset according to the studies reported by others [66]. The smaller semicircle in the high frequency region represents the resistance of the surface film formed on the electrodes (Rf) within interfacial capacitance (CPE1), while the charge transfer resistance (Rct) and interfacial capacitance (CPE2) at the electrode/electrolyte interface are related to the larger one in the middle frequency region. Moreover, Rs is attributed to a total electrical resistance with the electrode. As shown in Figure 6c, arc radius on the EIS Nynquist plot of MoS2/TiO2 NTs is smaller than that of TiO2 NTs both in dark and under visible light irradiation, indicating that the MoS2/TiO2 NTs has lower charge transfer resistance (Rct). The fact illustrates that the uniform and tight coverage of the MoS2 nanobelts onto the surface of TiO2 and the 13

intimate interfacial contact between MoS2 and TiO2 can greatly enhance rapid electron transport and reduces the contact and charge-transfer resistance, leading to a much better rate performance of MoS2/TiO2 than that of the TiO2 NTs [67]. Mott-Schottky (MS) analysis is based on the assumption that the capacitance of the space charge layer is much less than that of the Helmholtz layer. Mott-Schottky plots were performed by using the impedance technique at 10 Hz. Figure 7d shows the corresponding MS plots for the MoS2/TiO2 NTs and the unmodified TiO2 NT electrode. Both samples exhibit reversed sigmoidal plots with an overall shape consistent with n-type semiconductors. The MS plots are linear in the potential range from -0.1 to +0.3 V vs. SCE. Moreover, compared to pristine TiO2 NTs, the MoS2/TiO2 NTs electrode shows much smaller slopes in the Mott-Schottky plots under visible light irradiation near the flat band potential. The carrier density ND can be calculated from Figure 7d using the following equation [68]:    2 dE    ND  e0   1   d   C 2  

where C is the space charge capacitance in the semiconductor, ND is the electron carrier density, e is the elemental charge (e=1.6×10−19 C), ε0 is the permittivity of a vacuum (ε0=8.86×10−12 F/m), ε is the relative permittivity of the semiconductor (ε = 48 for anatase TiO2), the ND values of TiO2 and MoS2/TiO2 NTs were determined to be 2.02×1017 and 3.60×1017 cm−3, respectively. Consequently, the higher ND of MoS2/TiO2 NTs signified a faster carrier transfer than in TiO2 NTs electrode, and thus an enhanced PEC performance [69]. These results prove that the n-type MoS2 nanobelts deposited on the surface of TiO2 NTs could promote the transfer of the photo-generated electrons. And the recombination of the electrons and holes were inhibited effectively. PL emission spectrum of TiO2 and the MoS2/TiO2 NTs are displayed in Figure 8. The peaks located at 466 nm and 523 nm are likely attributed to the defect levels

14

formed by oxygen vacancies [70]. The PL intensity of the MoS2 nanobelts modified TiO2 NTs is obviously much lower than that of pure TiO2 NTs. As far as we know, the intensity of PL emission spectrum related to the efficiency of charge trapping and recombination of photoinduced electron-hole pairs in the semiconductor. According to the result, the recombination of photogenerated electrons and holes is inhibited effectively for the MoS2/TiO2 heterostructures. The results confirm that the efficient migration of charge carries between TiO2 NTs and MoS2 nanobelts lead to the superior photocatalytic activity.

Photocatalytic activity The visible light photocatalytic activity of MoS2/TiO2 NTs was evaluated by degradation of MB and SD in aqueous solution (Figure 9). Under visible light irradiation, over 60% of MB and 64% of SD was degraded by the MoS2/TiO2 NTs in the photoelectrocatalytic process after 240 min, respectively. However, only 25% of MB and 22% of SD was degraded by the non-loaded TiO2 NTs. The corresponding kinetic constants of MB and SD degradation are given in Figure 9b. The pseudo-first-order kinetic model using the linear transformations：−ln (Ct/C0) = F(t) = kt (k is the kinetic constant). The values of kinetic constants represent a good measure of the overall photodegradation rate. The final and stable pseudo-first-rate kinetic constant of TiO2 NTs is 0.0011 (degradation of SD) and 0.0014 (degradation of MB), respectively. And the final pseudo-first-rate kinetic constant of MoS2/TiO2 NTs is 0.0039 (degradation of SD) and 0.0036 (degradation of MB), respectively. Which is more than 3.0 times of those from pure TiO2 NTs. The final and stable pseudo-first-rate kinetic constant of MB and SD degradation by MoS2/TiO2 NTs composite is higher than the unmodified TiO2 electrode. The MB removal in the various degradation processes including PEC, the photocatalytic (PC), electrochemical process (EC), and direct photolysis processes is summarized in Figure 10. The results indicate that the PEC process provides the most powerful approach to degrade the MB in aqueous solution. It is obvious that only 46% of MB removal was obtained in the PC process with the same illumination time. 15

However, the removal with EC was insignificant, by which 33% of MB was decomposed. In addition, the degradation efficiency of MB only reached 24% during the direct photolysis process. A synergetic effect was observed on the MoS2/TiO2 NTAs during PEC degradation. The values of the corresponding kinetic constants of different degradation processes are given in Figure 10d. The kinetic constants of MB oxidation in the direct photolysis, EC, PC, and PEC processes were 5.2×10−4, 1.9×10−3, 2.7×10−3, and 3.6×10−3 min−1, respectively. Therefore, the MoS2/TiO2 NTs electrode had much higher performance in the PEC process than in either the photocatalytic or electrochemical process alone. The repeatability and stability of catalysts is a key factor in actual application of photocatalytic. The photoelectrocatalytic repeatability of the MoS2/TiO2 NTs, as typical representative, was studied by cycling test and the experiment result is presented in Figure 11. As plotted in Figure 11, after five recycles for the degradation of MB, the photodegradation ability of the MoS2/TiO2 NTs was slightly decreased when compared to the fresh electrode. The degraded MB decreased from 55.3% to 46.6% in five successive experimental run. The results confirm that the MoS2/TiO2 NTs electrode was stable under visible light irradiation. The main active species in the process of photoelectrocatalytic degradation of MB were investigated by in situ capture technique. As shown in Figure 12a, the degradation of MB has been suppressed slightly by the addition of tBuOH (hydroxyl radicals scavenger) under visible light irradiation, indicating that the free hydroxyl radicals were not main active oxidizing species in the photoelectrocatalytic process. However, it was intensively suppressed when EDTA (holes scavenger) was introduced. This result indicates that holes were the main active oxidizing species involved in the photoreaction

process.

The

direct

hole

transfer

mainly

governs

the

photoeletrocatalytic process. Meanwhile, BQ and β-carotene was used to confirm the active species of O2•− and singlet oxygen (1O2) in the reaction, respectively. The results are shown in Figure 12a. The degradation rate was inhibited when BQ (superoxide radicals scavenger) was introduced, indicating that O2•− was also the active species in the photoelectrocatalytic progress. When β-carotene (1O2 scavenger) 16

was introduced, the degradation was also intensively suppressed. The results indicated that the singlet oxygen was also the main active species. The ESR spin-trap technique was also applied to further confirm the active species. The results are displayed in Figure 13. Under visible light irradiation, there are four weak peaks of DMPO-·OH were observed and the relative intensities of the four peaks are 1:2:2:1 [71]. The results indicate that a small amount of hydroxyl radicals are produced during the PEC process. However, no such signals were detected in the dark. In addition, the characteristic ESR spectrum DMPO- 1O2 species with relative intensities of 1:1:1 were detected. These results mean that 1O2 radicals are produced on the surface of the MoS2/TiO2 electrode in the PEC process. These results are consistent with the above obtained by in situ capture technique. Based on the above discussion, a possible photocatalytic mechanism was proposed. The energy band structure and relevant band positions of MoS2 and TiO2 are shown on a comparable scale in Figure 14. Upon visible light illumination (λ > 420 nm), the photo-generated electron-hole pairs came from MoS2 semiconductor. The flat band potential of TiO2 corresponds to the minimum energy required for a photoexcited electron in MoS2 to inject into TiO2 conduction band. The flat band position of TiO2 has been shown to be strongly dependent on the pH of the penetrating electrolyte solution [72]. We can estimate the flat band potential of our TiO2 to be at approximately -0.50 V (vs. NHE). And the flat band potential of MoS2 was estimated be at approximately -0.80 V (vs. NHE) [61]. As far as we know that the excitons in the lower conduction band of MoS2 are of sufficient energy to inject into the TiO2 conduction band. Thus the excited electron transferred to conduction band of TiO2 along the n-n heterojunction from the conduction band of MoS2. Some of the photo-generated holes accumulated in MoS2 could interact with the contaminants adsorbed on the electrode surfaces. Furthermore, the photoelectrons could react with the adsorbed molecular oxygen to yield O2•－. And 1O2 could be generation as a result of superoxide oxidation by valence band holes. Therefore, the high separation efficiency of electron-hole pairs is advantageous to producing reactive species, such as 1O2 and holes, which could react with the organic pollutants in the solution. 17

4. Conclusions In summary, we have successfully prepared a novel MoS2/TiO2 heterostructure photoelectrodes by using a photo-assisted electrodeposition method. The prepared MoS2/TiO2 photocatalyst exhibits larger photocurrent densities and enhanced visible-light photoelectrocatalytic activity toward degradation of MB and SD compared with pure TiO2 arrays. The enhanced photoelectrocatalytic activity could be ascribed to the strong absorptions in the visible light region of MoS2 and the matching band structure between TiO2 and MoS2. Considering the excellent properties of MoS2/TiO2 NTs, the composite heterojunction described herein presents a new material which is expected to lead to a wide range of applications in photoelectrocatalytic degradation of organic pollutants in water and solar cells.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (No. 1407067), the National Natural Science Foundation of Jiangsu province (No. BK20140506), the Key Laboratory of Environmental Materials and Environmental Engineering, Jiangsu Province and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

18

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Figures Legend Figure 1. Schematic illustration of a photo-assisted electrodeposition technique. Figure 2. (a) A typical top-view SEM image of the TiO2 NTs; (b) The image of TiO2 NTs at high magnification; (c) Top-view SEM image of the MoS2/TiO2 NTs; (d) The image of MoS2/TiO2 NTs at high magnification. Figure 3. XRD patterns of the TiO2 NTs and MoS2/TiO2 NTs. Figure 4. (a) XPS survey spectrum of MoS2/TiO2 NTs. (b) XPS spectrum of Mo 3d. (c) XPS spectrum of S 2p. (d) XPS spectrum of O 1s. (f) XPS spectrum of Ti 2p. Figure 5. Raman spectra of pure TiO2 NTs and MoS2/TiO2 heterostructure. Inset shows the fitting peaks of the MoS2/TiO2 sample. Figure 6. (a) The UV-vis diffuse reflectance spectra of the as-prepared TiO2 and MoS2/TiO2 NTs. (b) The Energy band gaps for TiO2 NTs and MoS2/TiO2 NTs. Figure 7. (a) I-V curves of the as-prepared TiO2 NTs and MoS2/TiO2 NTs under visible light irradiation. (b) Short circuit photocurrent density vs. time plotted for TiO2 NTs and MoS2/TiO2 NTs in 0.01 M Na2SO4 solution under visible light irradiation. (c) EIS Nynquist plots of TiO2 NTs and MoS2/TiO2 NTs under visible light irradiation. (d) Mott-Schottky plots of the different electrodes. The MS plots were obtained at a frequency of 1 kHz in an aqueous solution of Na2SO4 (0.01 M). Figure 8. The PL spectra of the as-prepared TiO2 and MoS2/TiO2 NTs (excitation wavelength: 370 nm for the samples) Figure 9. (a) Decontamination of MB and SD solutions by photoelectrocatalysis with the as-prepared TiO2 NTs and MoS2/TiO2 NTs electrode under visible light irradiation. (b) The variation of ln(Ct/C0) of MB and SD by different electrode. Figure 10. (a) Degradation of MB by MoS2/TiO2 NTs with different process. (b) The variation of ln(Ct/C0) of MB by MoS2/TiO2 NTs with different process. Figure

11.

Degradation

of

MB

molecules

for

5

successive

reactions

photoelectrocatalyzed with the same MoS2/TiO2 NTs electrode under visible light irradiation. Figure 12. (a) Photogenerated active species trapped in the system of photodegradation of MB by MoS2/TiO2 NTs under visible light irradiation; (b) 26

Degradation efficiency of MB during different active species trapped process. Figure 13. DMPO spin-trapping ESR spectra recorded at ambient temperature in aqueous dispersion: MoS2/TiO2 NTs in dark and under visible light irradiation. Figure 14. Schematic illustration of the charge transfer process for MoS2/TiO2 NTs under visible light irradiation.

Figure 1

MoS2

ee-

MoS42MoS42-

Ti

TiO2

27

Figure 2

(a)

(b)

100 nm

100 nm

(c)

(d)

500 nm

100 nm

28

(002)Ti

Figure 3 TiO2 NTs

Intensity (a. u.)

(101)TiO2

MoS2/TiO2 NTs Ti (PDF#65-6231) TiO2 (PDF#21-1272)

10

20

30

40

50

(103)Ti (110)Ti

(105)TiO2 (211)TiO2

(200)TiO2

(101)Ti

<003>MoS2

MoS2 (PDF#17-0744)

60

2 Theta (degree)

29

70

80

Mo 3d

Figure 4 (b)

1200

1000

800

600

400

200

0

Mo 3d3/2 (232.4 eV)

S 2s (226.1 eV)

240

236

S 2p5/2 (163.9 eV)

532.4 eV

MoS2/TiO2 NTs 530.2 eV

168

165

162

159 540

538

Binding Energy (eV)

(f) Intensity (a.u.)

171

531.6 eV

536

534

2p3/2 (459.2 eV)

2p1/2 (465.0 eV)

2p3/2 (458.8 eV)

TiO2 NTs

470

532

530

Binding Energy (eV)

Ti 2p

MoS2/TiO2 NTs

475

224

O 1s

TiO2 NTs

174

228

530.6 eV

(d)

S 2p3/2 (162.5 eV)

Intensity (a.u.)

Intensity (a.u.)

S 2p

232

Binding energy (eV)

Binding energy (eV)

(c)

Mo 3d5/2 (229.4 eV)

Mo 3d

Intensity (a.u.)

C 1s

O 1s Ti 2p

Intensity (a.u.)

S 2p

(a)

2p1/2 (464.6 eV)

465

460

Bingding Energy (eV)

30

455

450

528

526

Figure 5 MoS2/TiO2 A1g E2g

1

Intensity (a.u.)

B1g B1g

MoS2/TiO2 NTs TiO2

E1g

350

TiO2 NTs

100

200

300

B1g

400

375

A1g

500

425

450

Eg

600 -1

Ranman shift (cm )

31

400

700

800

Figure 6 4.5

Absorbance (a.u.)

(a)

200

(b)

(Ahν)

2

3.0 MoS2/TiO2 NTs

1.5 TiO2 NTs

300

400

500

600

700

800

Wavelength (nm)

0.0 1.5

2.0

2.5

3.0

3.5

Eg (eV)

32

4.0

4.5

5.0

5.5

Figure 7 4

0.6

(a) J (mA cm )

0

-2

-2

J (mA/cm )

(b)

-2

MoS2/TiO2 NTs TiO2 NTs

-4

TiO2 NTs light on

0.4

light off

0.3 0.2 0.1

-6

0.0

-8 -1.0

-0.5

0.0

0.5

0

1.0

Voltage (V)

4000

(c)

4

MoS2/TiO2 NTs dark

×10

3000

3

C (cm F )

TiO2 NTs dark

40

60

80

100

120

140

160

Time (s) TiO2 NTs MoS2/TiO2 NTs

4

-2

TiO2 NTs visible light

20 10

(d)

MoS2/TiO2 NTs visible light

2000

Rf

Rct

CPE1

CPE1

Zw

Re

2

-2

-Z'' (ohm)

MoS2/TiO2 NTs

0.5

2

1

1000

0 0

400

800

1200

1600

0 -1.0

-0.5

0.0

E (V vs SCE)

Z' (ohm)

33

0.5

1.0

Intensity (a.u.)

Figure 8

466 TiO2 NTs

523

MoS2/TiO2 NTs

450

500

550

600

650

Wavelength (nm)

34

700

Figure 9 1.0

1.0

(a)

(b)

0.8

MoS2/TiO2 NTs+MB TiO2 NTs+MB

-Ln(C/C0)

C/C0

0.8

0.6 TiO2 NTs+SA

0.4

y=0.0039x

MoS2/TiO2 NTs+SA

TiO2 NTs+MB MoS2/TiO2 NTs+MB

y=0.0036x

TiO2 NTs+SA

0.6 0.4

y=0.0014x

0.2

y=0.0011x

MoS2/TiO2 NTs+SA

0.2 -20 0

0.0

40

80

120

160

200

240

0

40

80

120

Time (min)

Time (min)

35

160

200

240

Figure 10 1.0

(a)

C/C0

0.8

0.6 Photolysis Dark Photocatalytic Electrochemical Photoelectrocatalytic

0.4

0.2 -20

1.0

40

80

120

160

200

240

200

240

Time (min)

(b) Photolysis Dark absorbance Photolcatalytic Electrochemical Photoelectrocatalytic

0.8

-Ln(C/C0)

0

0.6 0.4 0.2 0.0 0

40

80

120

160

Time (min)

36

Figure 11

1.0 0.9

C/C0

0.8 0.7 0.6 0.5 0.4 0

120 240 0

120 240 0

120 240 0

Time (min)

37

120 240 0

120 240

Figure 12 1.0

(a)

0.8

C/C0

0.6 MoS2/TiO2 NTs

0.4

MoS2/TiO2 NTs+1mM EDTA MoS2/TiO2 NTs+ 1mM tBuOH

0.2

MoS2/TiO2 NTs+1mM BQ MoS2/TiO2 NTs+0.1mM β-carotene

0.0 -20 0

40

120

160

200

240

Time (min)

1.0 No scavenger

0.8

80

(b) tBuOH

(C0-C)/C0

BQ

0.6 0.4

EDTA

0.2

β-carotene

0.0

38

Figure 13

* * *

Intensity (a.u.)

Irradiation

Irradiation *

* *

*

In dark

3200

3300

3400

Magnetic field (G)

39

3500

hν

Potential eV vs. NHE

Figure 14 -1.6 -0.8

+0.8 eV

-0.5 eV

0 2.4 eV

+0.8

3.2 eV

+1.6 eV

+1.6 +2.4

+2.7 eV

+3.2

O2

TiO2

MoS 2

hν

e-

e-

+e−

O2·−

+h+ h+

1O

40

2

MoS2

TiO2

EF