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C: Energy Conversion and Storage; Energy and Charge Transport

Experimental and Theoretical Evidence of Enhanced Visible-light Photoelectrochemical and Photocatalytic Properties in MoS2/TiO2 Nanohole Arrays Feng Nan, Ping Li, JianKang Li, Tianyi Cai, Sheng Ju, and Liang Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01574 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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The Journal of Physical Chemistry

Experimental and Theoretical Evidence of Enhanced Visible-light Photoelectrochemical and Photocatalytic Properties in MoS2/TiO2 Nanohole Arrays Feng Nana, Ping Lia, Jiankang Lib, Tianyi Cai*a, Sheng Jua, and Liang Fang*a a

College of Physics, Optoelectronics and Energy and Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou, 215006, People’s Republic of China b

Institute of Electronic Information Engineering, Suzhou Vocational University, Suzhou, 215104, People’s Republic of China

ABSTRACT Two dimensional (2D) ordered micro/nanostructured arrays have attracted intense interest for potential applications in a wide variety of fields, such as surface-enhanced Raman scattering, chemical and biological sensing, catalysis, energy storage and conversion. In this paper, MoS2/TiO2 nanohole arrays are prepared by combing colloidal lithography and dip-coating method. The decorating of MoS2 nanosheets can significantly improve the photoelectrochemical and photocatalytic activity of TiO2 nanohole arrays in the visible light region. The photocurrent density of the optimized MoS2/TiO2 sample is 17.8 times higher than that of the pristine TiO2 film, while the enhancement factor of the corresponding kinetic constant in photocatalytic degradation is about 5.2. Our first-principles calculation results are in good agreement with the experimental observations. Based on the experimental and theoretical results, such enhanced performances can be assigned to two reasons: one is the improved visible light harvesting owing to MoS2 nanosheets, and the other is the efficient separation of photogenerated carriers due to band alignment between MoS2 and TiO2.

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1. Introduction Since the pioneering work of Fujishima and Honda in 1972,1 semiconductor photocatalysis technology has attracted great attention due to the energy crisis and environmental problems. Titanium dioxide (TiO2) has been regarded as one of the most promising materials for photocatalysts due to its high chemical stability, favorable band edge positions, low cost, and low toxicity.2-4 However, as a typical wide band gap semiconductor photocatalyst, TiO2 can only absorb the UV light, which accounts for less than 5% of the total sunlight power. Moreover, the high recombination rate of photogenerated electron-hole pairs of TiO2 leads to a low quantum efficiency and poor photocatalytic activity. Nano-structuring the TiO2 is believed to be effective technology for overcoming the disadvantages of TiO2 due to geometric–dependence of its properties. For instance, different TiO2 geometric structures, such as mesoporous,5 nanotube,6 nanobelt,7 and inverse opal structure,8 have demonstrated enhanced photocatalytic activity compared to normal TiO2 nanoparticles or films. Recently, two dimensional (2D) ordered micro/nanostructured arrays, including nanodome, nanobowl and nanohole arrays also have attracted intense interest in a wide variety of fields, such as surface-enhanced Raman scattering, chemical and biological sensing, catalysis, energy storage and conversion.9-11 For example, Wang et al. fabricated heterostructured TiO2 nanorod@nanobowl arrays by using interfacial nanosphere lithography followed by hydrothermal growth.12 Owing to the two-dimensionally arrayed structure of nanobowls and the nearly radial alignment of nanorods, the composite arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the nanobowls arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Xiong’s group also developed a facile surfactant-free nanofabrication approach to confine Au nanoparticles in N-doped TiO2 nanobowl arrays.13 Considering the advantage of the ordered structure and noble metal, the composite showed the improved photocatalytic water splitting performance in full spectrum. Molybdenum disulfide (MoS2), a member of the layered transition metal 2


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dichalcogenides family, has attracted great attention for its application sensing, lithium battery, phototransistor and photocatalytic area.14-17 As a noble metal free co-catalysts, MoS2 has be demonstrared as electrocatalyst for the hydrogen evolution reaction due to its high H2 activation ability. 18 Most importantly, MoS2 is a narrow band gap semiconductor, which can be coupled with TiO2 to improve efficient charge separation as well as expand the light absorption.19-22 In this study, few layers MoS2 nanosheets were fabricated by a moderate disintegration-exfoliation approach using diluted H2SO4 as an ‘‘efficient knife’’,23 and MoS2/TiO2 nanohole arrays were then prepared by collodial lithography method followed by dip-coating technique. The decoration of the MoS2 nanosheets could significantly improve the photoelectrochemical and photocatalytic activity of TiO2 nanohole arrays in the visible light region, which could be ascribed to the improved light absorption and faster charge separation rate. The corresponding electronic structures and charge transfer mechanisms between TiO2 and MoS2 were also systematically discussed by the first-principles calculation. Our results provide a platform for the design and application of two dimensional materials/nanohole arrays heterostructure in energy/environment applications. 2. Experiment Section Preparation of MoS2 nanosheets: The suspension of MoS2 nanosheets was obtained by the treatment of bulk-MoS2 with H2SO4. Typically, bulk-MoS2 was dispersed into 30 ml of H2SO4 (70 wt%) in a 100 mL beaker and stirred for 24 h at room temperature. Then, the mixture was slowly poured into 60 mL of deionized water and sonicated with 4 h for exfoliation. After that, the above compound was centrifuged at 10000 rpm, and the deposition was washed by deionized water again and again. The fabrication process of MoS2 nanosheets is shown in Figure S1. Preparation of MoS2/TiO2 nanohole arrays: Fluorine-doped SnO2 (FTO) substrates were cleaned by acetone, ethanol and deionized water for 30 min each with sonication. Polystyrene (PS) sphere (1 µm, SuZhou Nanomicro Tech.) template was prepared by self-assembled method on the FTO substrates. Then the self-assembled PS template was immersed vertically in TiO2 precursor to form TiO2 nanohole arrays. 3



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TiO2 precursor was prepared using sol-gel technique. A mixed solution of 0.5 ml tetrabutyl titanate, 0.2 ml HCl, 1 ml ethanol and 0.4 ml deionized water was prepared as the precursor sol. The template was dipped into the precursor sol for 5 min and then dried for 1 h in an oven (at 80°C) to transform the sol into a gel. To ensure that the voids in the template were completely filled with the gel, the above procedures were repeated at least 3 times. Finally, the sample was annealed at 450 °C in air for 2 h. The MoS2/TiO2 nanohole arrays were prepared by a dip-coating method: Appropriate as-prepared MoS2 nanosheets was dispersed in distilled water to form a 1mg/10ml dispersion, MoS2 nanosheets suspension was then dip-coated onto the surface of the TiO2 nanohole arrays. In addition, the dip-coating content of MoS2 were controlled at 0, 5, 10, 15 and 20 µg/cm2, respectively. The obtained samples were labeled

as

n-TiO2,

5-MoS2/n-TiO2,

10-MoS2/n-TiO2,

15-MoS2/n-TiO2

and

20-MoS2/n-TiO2, respectively. The fabrication process of MoS2/TiO2 nanohole arrays is shown in Figure S2. For comparison, TiO2 film was also prepared by sol-gel method, which is labeled as f-TiO2. Characterization: The X-ray diffraction (XRD, Rigaku D/MAX 3C) were used to the purity of the samples equipped with Cu Kα radiation (λ=1.540598Å). Raman spectrometer (JY- HR800) was used to obtain Raman spectra at room temperature with a wavelength of 633 nm as the excitation source. Scanning electron microscopy (SEM, HITACHI S-4700) and Atomic force microscope (AFM, Asylum Research MFP-3D) were used to investigate the morphologies of samples. The optical absorption spectra of the films were measured using a UV/Vis/NIR spectrometer (Perkin Elmer Lambda 750). The surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000). All the binding energy values were calibrated by using C1s (284.6 eV) as a reference. Ultraviolet photoelectron spectroscopy (UPS) measurement was conducted using a ESCALAB 250xi X-ray photoelectron spectrometer (ThermoFisher Scientific) with Helium I (21.21 eV) as the radiation source. Density functional theory (DFT) calculation: Our ab initio calculations are performed using the projector augmented wave (PAW) method, as implemented in the 4


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Vienna ab initio Simulation Package (VASP).24,25 The generalized-gradient approximation (GGA) in the form proposed by Perdew, Burke, and Ernzerhof is used.26 The PAW potentials are adopted to describe the electron-ion interaction, with 14 valence electrons for Mo (4s24p64d55s1), 12 for Ti (3s23p63d24s2), and 6 O (2s22p4). A plane-wave cutoff of 500 eV is used. The calculated in-plane lattice constant a = 3.18 Å for MoS2, which is in good agreement with the experimental value 3.160 Å,27 and 3.78 Å for anatase TiO2. The MoS2

3 × 3 unit cell is adjusted to 1×1 unit

cells of TiO2 (110) surfaces. The lattice constant mismatches with respect to that of TiO2 (110) are 3.18 %. For the ultrathin films, a vacuum layer larger than 16 Å is adopted. We relax the ions toward equilibrium positions until the Hellman-Feynman forces are less than 0.01 eV/ Å. The equilibrium interlayer spacing between the TiO2 (110) layer and the MoS2 is 2.82 Å. Photocurrent measurement: Photocurrent measurements were performed by electrochemical analyzer (CHI-660D, Shanghai Chenhua Instrument Co. Ltd.) in a standard three-electrode configuration with 0.1M Na2SO4 solution as the electrolyte. The nanohole arrays, a Pt wire and an Ag/AgCl electrode were used as working, the counter, and the reference electrodes, respectively. The illumination source was a 300 W Xe lamp. The light intensity was 100 mW/cm2 measured by a spectroradiometer (Newport 1918-C, USA). Visible light irradiation was performed using the above Xe lamp equipped with a UV cut off filter (λ > 420 nm). Photocatalytic

degradation of organic pollutant: The photocatalytic

degradation of organic pollutant of the samples was evaluated by measuring the decomposition rates of 4-chlorophenol (4-CP) in water at ambient temperature. Experiment was as follows: the samples were placed into 10 ml of 4-CP aqueous solution in a rectangular beaker. Before photodegradation reaction, the solution was magnetically stirred in dark for 2 h to achieve the adsorption-desorption equilibrium among the photocatalyst, 4-CP, and water. The 4-CP concentration after equilibration was determined by measuring the absorbance at 225 nm using a Perkin Elmer Lambda 750 UV/Vis/NIR Spectrometer, and taken as the initial concentration (C0). 5



The above Xe lamp positioned above 10 cm away from the beaker was used as a light source to trigger the photodegradation reaction. The light intensity in this experiment was 100 mW/cm2, which was determined by an optical power meter (Newport 1918-C, USA). One surface of the samples was irradiated along the normal direction. The reaction temperature was kept at room temperature by using cooling fans to prevent any thermal catalytic effect. After the elapse of a period of time, a small quantity of the solution was taken, and the concentration of 4-CP (C) was measured. 3. Results and Discussion The morphologies and the thicknesses of the MoS2 nanosheets were measured by atomic force microscopy (AFM). Figures 1a and 1b show MoS2 nanosheets with lateral dimensions of 100-350 nm and heights in the range of 10±2 nm, which is similar to that of other MoS2 nanosheets prepared by liquid exfoliation technique.28 Raman spectroscopy has been widely used to characterize structural and electronic properties of 2D layered nanomaterials. Figure 1c illustrates the Raman spectrum of the pure MoS2 nanosheets. Three Raman scattering peaks observed at 383.8, 411.5 and 460.6 cm-1 can be assigned to the representative modes of E2g1, A1g and E1g2 modes of the MoS2 phase, which is in agreement with previous report.29 A typical SEM image of a self-organized polystyrene (PS) colloidal sphere monolayer is presented in Figure 2a. The monolayer is composed of PS spheres (1 µm in diameter) in a hexagonal close-packed arrangement. The edge region of the PS spheres (inset of Figure 2a) selected purposely shows clearly single layer characteristics, indicating the successful preparation of the PS sphere monolayer. Figures 2b and 2c show SEM images of the pure TiO2 and 15-MoS2/n-TiO2 nanohole arrays. The pure TiO2 nanohole arrays (n-TiO2) show highly ordered macroporous structure, while the 15-MoS2/n-TiO2 sample has a similar morphology as that of the n-TiO2 sample, indicating that the decoration process of MoS2 does not damage the porous TiO2 structure. Figure 2d shows the EDX spectrum of the 15-MoS2/n-TiO2 sample. The EDX spectrum indicates that the composite sample contains Mo, S, Ti and O elements. The atomic ratio of Mo/Ti is 1:17.3. The XRD patterns of the different samples are also shown in Figure 2e. The peaks of the n-TiO2 sample match 6


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the standard peaks of anatase TiO2 phase (JCPDS card No. 21-1272).30,31 As for the 15-MoS2/n-TiO2 sample, a minor peak centered at 14.3° can be observed, which is assigned to (002) planes of MoS2 (JCPDS card No. 37-1492).31,32 Figure 2f shows the Raman spectra of the corresponding two samples. For the n-TiO2 sample, four Raman peaks located at 140.1, 394.3, 516.4 and 639.9 cm-1 can be observed, which agree well with E1g, B1g, A1g and Eg modes of anatase TiO2, 30 respectively. Besides the Raman peaks of TiO2, three feature MoS2 Raman peaks can also be observed in the 15-MoS2/n-TiO2 sample. In addition, compared with that the pure TiO2 nanohole arrays, the E1g, A1g and Eg Raman peaks of TiO2 in the MoS2/TiO2 composite samples undergo 4, 4.6 and 1 cm-1 blue shift, respectively. Such phenomena can be ascribed to a surface strain induced by the decorated MoS2 nanosheets on the TiO2 surface.33,34 X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the chemical states of different samples. As shown in Figure 3a, the survey XPS spectra reveal that the dominant elements are Ti, O, Mo, S, and C in 15-MoS2/n-TiO2 sample. The high-resolution XPS spectra of Ti for the different samples are indicated in Figure 3b. The binding energies of Ti 2p3/2 and Ti 2p1/2 for the n-TiO2 sample are about 458.2 and 464.2 eV. Such a separation energy close to 6.0 eV is in good agreement with the Ti4+ state,20 which is also existed in the 15-MoS2/n-TiO2 sample with corresponding binding energies of 457.2 and 464.7, respectively. Figure 3c and 3d show the Mo 3d and S 2p high-resolution XPS spectra of the different sample. For pure MoS2 nanosheets, the Mo XPS spectrum shows two main peaks located at 228.5 and 231.7 eV, which typically correspond to the Mo 3d5/2 and Mo 3d3/2 components of MoS2, respectively. It should be mentioned that the low intensity at 225.7 eV is derived from S 2s.35 The S 2p spectrum indicates two major peaks at 163.5 eV and 162.4 eV, corresponding to S 2p1/2 and S 2p3/2 spin orbits, respectively. After MoS2 nanosheets decorating on the TiO2 nanohole arrays, Mo 3d5/2 and Mo 3d3/2, S 2p1/2 and S 2p3/2 peaks shifted to 229.2, 232.4, 163.7 and 162.6 eV, respectively. Such shift of the peaks can be attributed to the electronic interaction between MoS2 and TiO2, where the electrons are injected from TiO2 to the MoS2 nanosheet, as described in the previous reports.36,37 For transition metal dichalcogenide like MoS2, it is known that 7



injected electrons will be transfer to the d orbital of the transition metal center.38 Therefore, an enhanced shift of the binding energies of Mo 3d peaks than that of S 2p can be observed in the 15-MoS2/n-TiO2 sample.39 Moreover, the atomic ratio of Mo/Ti in the surface region is 1:4.7, which is larger than that of the EDX result. It is known that the EDX detection depth is up to several micrometers, while the XPS detection depth is only several nanometers. Since the MoS2 nanosheets is decorated on the surface of TiO2, such atomic ratio difference can be attributed the different depths detection of EDX and XPS. From the above SEM, XRD, Raman and XPS results, we confirm the successful decoration of MoS2 nanosheets into the TiO2 nanohole arrays. To evaluate the photoelectrochemical performance of the samples, the short-circuit photocurrent densities of different samples were measured under UV-visible light irradiation, as shown in Figure 4a. Photocurrent densities of all samples appear rapidly when the light is turned on, and decrease to zero as soon as the light is turned off. This pattern of photocurrent was highly reproducible for numerous on/off cycles of illumination. A small photocurrent density of ~2.8 µA/cm2 can be observed in the pure TiO2 film (f-TiO2). Such small value is not surprising, since the photoactive material amount of pure TiO2 film is too small. However, the n-TiO2 sample exhibits higher photocurrent intensity (~4.3 µA/cm2) than that of the f-TiO2 sample, which may be attributed to the following reasons: First, the electrolyte or organic pollutant can easily infiltrate the pores and result in more effective redox reactions at the nanohole arrays/liquid interface. However, it is difficult to accurately measure the surface areas of the samples. The surface area of the porous electrode can be evaluated by measuring the capacitance in the dark from Mott–Schottky curves.40 The capacitance of the n-TiO2 is around 11.18 µF/cm2 at 0 V vs. Ag/AgCl, which is larger than the value of f-TiO2 sample (~ 9.62 µF/cm2). If the enhanced photocurrent density is totally resulted from the change of the surface area, the ratio of the photocurrent density for the n-TiO2 and f-TiO2 should be 1.16. However, the actual ratio is 1.54 from Figure 4a, which means the enhanced photocurrent density cannot be explained by the increase in surface area alone. The second reason is that the porous structure of the nanohole arrays can improve light harvesting. Such enhanced 8


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light scattering ability can be ascribed to the multiple scattering inside the nanoholes, which resulted in the higher intensity of light scattering and the longer pathways of light in the structure.41 By adding MoS2 nanosheets, both two composite samples exhibit the improved photocurrent densities. Remarkably, this MoS2 enhancement is much stronger in the case of n-TiO2, and the photocurrent density of the 15-MoS2/n-TiO2 sample is amplified to 27 µA/cm2, which is 6.3 times higher than that of the n-TiO2 sample. Further investigation shows that the effect of MoS2 amount has an optimum amount window, less or more the optimized MoS2 amount (15 µg/cm2) will lead to decreasing photocurrent density (Figure S3). The photocurrent density results of the samples under visible light irradiation (λ > 420 nm) are similar to those under UV-visible light irradiation, as show in Figure 4b. Although the photocurrent densities of the f-TiO2 and n-TiO2 samples are nearly zero, the MoS2/TiO2 composite samples show obvious visible light photoelectric response. The highest photocurrent density is found to be 16 µA/cm2 in the 15-MoS2/n-TiO2 sample, which is 17.8 times than that of the f-TiO2 sample. The inset of Figure 4b shows the photocurrent density of 15-MoS2/n-TiO2 sample under visible-light irradiation during 4 h measurement, demonstrating the high stability of the photoelectrode. The UV-visible light photocatalytic activities of the samples were then evaluated by measuring the decomposition rates of 4-chlorophenol (4-CP) aqueous solution, as shown in the inset of Figure 4c. Blank test (4-CP without any catalysts) under illumination exhibits little photolysis, which indicates that the self-degradation of 4-CP is extremely slow. The degradation of 4-CP with the f-TiO2 in dark for 2 h is similar to that of the blank test, implying that the absorption of 4-CP on TiO2 is limited after the adsorption-desorption equilibrium is reached. After 2 h irradiation, the 4-CP degradation rate of the f-TiO2 and n-TiO2 sample is about 13% and 20%, while that of the 15-MoS2/f-TiO2 and 15-MoS2/n-TiO2 sample is about 41% and 62%, respectively. The temporal evolution of the spectral changes taking place during the photodegradation of 4-CP over the 15-MoS2/n-TiO2 sample is displayed in Figure 4c. The absorption peaks corresponding to 4-CP steadily decrease with increasing 9



reaction time, demonstrating the effective photodegradation activity of the 15-MoS2/n-TiO2 sample. Figure 4d shows the photocatalytic activities of the samples under visible light irradiation, which are also similar to those under UV-visible light irradiation. In order to quantify the reaction kinetics of 4-CP degradation rate, the pseudo-first order model was employed using the linear transformation: ln(C/C0)= k×t, where k is the photocatalytic reaction rate constant. As shown in the Figure S4, the k value extracted from the plots of ln(C0/C) vs time for f-TiO2, n-TiO2, 15-MoS2/f-TiO2 and 15-MoS2/n-TiO2 are 0.035 h-1, 0.046 h-1, 0.112 h-1 and 0.181 h-1, respectively. The visible light photodegradation stability of the 15-MoS2/n-TiO2 sample is also investigated by the cycling experiment (inset in the Figure 4d). After four cycles of photodegradation of 4-CP, the sample does not exhibit obvious loss of activity, indicating that the sample is relatively stable during the photodegradation reaction. There are at least two factors resulting in the improved visible-light photoelectrochemical and photocatalytic performance of the 15-MoS2/n-TiO2 sample, especially in visible-light region. The first factor is the visible light absorption ability of MoS2. Figure 5a shows UV–vis absorption spectra of the different samples. The f-TiO2 sample shows the characteristic spectrum with its fundamental absorption edge rising at 380 nm, while the n-TiO2 sample shows higher absorption intensity, which confirm the enhanced light-matter interaction resulting from the porous structure. However, the spectrum of the MoS2 sample shows a wide absorption band covering a broader visible light region owing to the intrinsic band gap of MoS2.42 After decorating with MoS2 nanosheets, the absorption edge of the composite samples is substantially extended to a longer wavelength, and an enhanced absorption intensity in the visible light region is clearly observed. The corresponding energy band gap of the different samples can be subsequently estimated from the (αhν)2-(hν) plot by extrapolating the linear portion of (αhν)2 to zero, where α, h, ν are absorption coefficient, Planck constant and light frequency, respectively. As shown in the inset of Figure 5a, the calculated band gap energies for the f-TiO2, n-TiO2 and MoS2 samples are about 3.2, 3.18 and 1.75 eV, which is consistent with the previous reports.20,32 The above results reinforce the assumption that the presence of MoS2 is benefit for the 10


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visible light absorption, and the MoS2/TiO2 composite can be excited to generate many more electron-hole pairs under visible light irradiation. The second factor is the improved charge-transfer process in the MoS2/TiO2 composite samples. We then use electrochemical impedance spectroscopy (EIS) to investigate the change in the charge transfer resistance.43 The inset of Figure 5b shows the Nyquist plots of the different samples measured without illumination. All the samples show only one arc, corresponding to the large resistance of direct charge transfer between the samples and electrolyte. In addition, the arc radius shows similar trend as the photoelectrochemical and photocatalytic properties of the sample, i.e., f-TiO2>n-TiO2>15-MoS2/f-TiO2>15-MoS2/n-TiO2. Since a smaller radius of the arc in the EIS spectra indicates a smaller electron transfer resistance at the surface of sample, a more effective separation of photogenerated electron-hole pairs and faster interfacial charge transfer are expected in the 15-MoS2/n-TiO2 sample. Mott–Schottky measurements were performed to determine the semiconductor types and capacitances of the different samples, as shown in Figure 5b. The positive slopes confirm all the photoelectrodes are n-type semiconductors. For n-type semiconductor, it is well known that the flat band potential (Efb) measured according to Mott–Schottky plots is close to the conduction band of the semiconductor.44 The 15-MoS2/f-TiO2 and 15-MoS2/n-TiO2 samples exhibit Efb values of 0.03 and -0.01 vs. RHE, calculated from the x intercepts of the linear region. Compared to the values of 0.11 and 0.08 V for f-TiO2 and n-TiO2, there are 0.08 V and 0.09 V negative shifts, respectively. For n-type semiconductor, the conduction band edge (ECB) can be calculated from the following the equation, ECB=Efb+kTln(ND/NC), where ND is the donor concentration, NC is the effective density of states (typically~1020) at the conduction band edge, k is the Boltzmann constant, and T is temperature. The donor density (Nc) of 15-MoS2/f-TiO2, 15-MoS2/n-TiO2, f-TiO2, and n-TiO2 is 5.11×1018, 8.08×1018, 1.46 ×1018 and 2.19×1018m-3, respectively. Based upon the Mott–Schottky and band gap values, a band diagram of the composite is constructed, as shown in Figure S5. The staggered gap structure represents a kind of well-aligned band configuration for an efficient

separation

of

photo-generated

carriers,

11


which

can

enhance

the


photoelectrochemical and photocatalytic activity. Density functional theory (DFT) calculations were further carried out to study the electronic structure and electron transfer properties. Figures 6a-c show the side and top view of the relaxed structure of MoS2 and TiO2. Bulk anatase TiO2 is an indirect-gap semiconductor with an optical gap of 3.2 eV.45 The top of the valence band is O-2p in character and the bottom of the conduction band comprises mainly Ti-3d sates.46,47 MoS2 is also an indirect-gap semiconductor with an optical 1.6 eV.48 The valence band and conduction band are dominated by Mo-3d. Our calculation indicates that the heterojunction effect is formed in the interface, and the Local density of states (LDOS) is shown in Figure 6d. The lowest conduction bands of TiO2 are found to lie just above the highest valence bands of MoS2. Therefore, a type-II heterointerface is formed, which is very suitable for the separation of photo-excited electron-hole pairs. Hence, the MoS2/TiO2 interface is a type II staggered gap heterostructure and is suitable for charge transfer. To further confirm the relative band positions and experimentally demonstrate the type II band alignment, ultraviolet photoelectron spectroscopy (UPS) was used in addition to Mott−Schottky measurements.49 As shown in Figures 7a-c, The work function (ϕ) is determined by intersection of the secondary electron cutoff occurring at high binding energy, and the value is 4.63 eV for the f-TiO2 sample. The energy level of EF-EV is 3.08 eV, which is derived by extrapolating the linear portion of the low binding energy edge of the peak to the energy axis. For the 15-MoS2/f-TiO2 sample, the 0.12 eV increase in secondary electron cutoff binding energy reflects the reduction of the work function, which agrees with the previous report.50 This vacuum level shift is attributed to the formation of an interfacial dipole between MoS2 and TiO2 resulting from a charge transfer interaction. Moreover, noticeable difference is observed in the edge of the valence band state (VB). Based on the above values, the energy band diagram is schematically shown in the inset of Figure 7d. Consistent with our photoelectrochemical and photocatalytic data, the results of both our spectroscopic and electrochemical measurements demonstrate that TiO2 and MoS2 form a matched band alignment. When MoS2 and TiO2 are in contact, free electrons will redistribute 12


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across the interface to realign the Fermi levels until an equilibrium state is achieved. After equilibration, band bending at the interface of the two semiconductors will determine the ease with which charge carriers can cross the interface. Since the position of CB and VB for TiO2 lie below the corresponding energy bands of MoS2, the photoexcited electrons can easily cross the interface, and then transfer to the CB of TiO2 driven by the built-in electric field in the heterojunction. Meanwhile, the holes in the VB of TiO2 can be transferred to the VB of MoS2. Therefore, such high interfacial charge transfer and separation ability suppressed the recombination of electron-hole pairs, resulting in the enhanced visible light photoelectrochemical and photocatalytic activity. 4. Conclusions In conclusion, MoS2 nanosheets decorated TiO2 nanohole arrays was prepared by the nanosphere lithography method combined the dip-coating method. The visible light photoelectrochemical and photodegradation activity of the composites sample can be significantly improved, which can be attributed to two reasons: one is the improved visible light absorption of MoS2 nanosheets; another is that the matched band alignment between MoS2 and TiO2. Our results maybe help for using MoS2/TiO2 composite nanostructure in visible-light response photoelectrode and photocatalytic materials.

ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publications Website at xxx. Schematic diagram of the fabrication procedures for MoS2 nanosheets, schematic diagram of the fabrication procedures for the MoS2 /TiO2 nanohole arrays, photocurrent density-potential curves of the composite samples with different amount MoS2 nanosheets, and first order kinetics data of photocatalytic degradation for different samples.

AUTHOR INFORMATIOM Corresponding Authors 13



a

E-mail: [email protected]

a

E-mail: [email protected]

ACKONWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program 2014CB920900), the National Natural Science Foundation of China (11374220, 11774249), the Natural Science Foundation of Jiangsu Province (BK20171209), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). L. Fang acknowledges the China Postdoctoral Science Foundation for supporting this research. S. Ju also wishes to acknowledge Dongwu Scholar Project of Soochow University.

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FIGURE CAPTION Figure 1 (a) AFM image, (b) the section analysis and (c) Raman scattering spectrum of the MoS2 nanosheets.

Figure 2 The SEM images of top view for (a) the colloidal crystal template self-assembled from PS spheres with diameters of 1µm, inset is the edge region of the PS spheres; (b) the n-TiO2 sample; (c) the 15-MoS2/n-TiO2 sample; (d) the EDX spectrum of 15-MoS2/n-TiO2; (e) The XRD patterns of different samples; (f) Raman scattering spectra of different samples.

Figure 3 (a) XPS survey spectra of the different samples. A high-resolution XPS spectra of the different samples. (b) Ti 2p; (c) Mo 3d; (d) S 2p.

Figure 4 Photocurrent density-potential curves of the different samples at an external potential of 0 V vs. Ag/AgCl under (a) UV-Visible light irradiation; (b) visible light irradiation (λ>420nm),

the inset shows the time-photocurrent density of

15-MoS2/n-TiO2 sample; (c) The changes of temporal spectral of 4-CP aqueous solution over 15-MoS2/n-TiO2 sample, the inset is the photocatalytic degradation efficiencies of 4-CP using different samples under UV-Visible light irradiation; (d) the photocatalytic degradation efficiencies of 4-CP using different samples under visible light irradiation, the inset is the cycling degradation curve of the 15-MoS2/n-TiO2 sample.

Figure 5 (a) The absorption spectra of the different samples, the inset is the plot analysis of optical band gap of different samples; (b) Mott−Schottky plots of the different samples without light irradiation, the inset is EIS Nyquist plots of the different samples.

Figure 6 (a)-(c) Top and side views of the relaxed structure of MoS2 and TiO2, Gray,

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light blue, and red balls correspond to Mo, Ti and O, respectively. (d) Total, MoS2， and TiO2 densities of sates in MoS2/TiO2 heterojunction.

Figure 7 (a)-(c) The UPS spectra of different samples. (d) Schematic drawing of the band alignment between MoS2 and TiO2.

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