Hydrothermal synthesis of MoS₂ nanosheet loaded TiO₂ nanoarrays for enhanced visible light photocatalytic applications

Abstract

A molybdenum disulfide (MoS₂) nanosheet-decorated titanium dioxide (TiO₂) NRA heterojunction composite was fabricated successfully through a two-step hydrothermal approach. Microstructures and optical properties of specimens were characterized by field-emission scanning electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and ultraviolet-visible spectroscopy. The gaps of the TiO₂ nanorods have been filled with tiny MoS₂ nanosheets, which can increase the surface area of MoS₂/TiO₂ NRA composite thin films. In addition, the photocatalytic activity of the thin films were measured and discussed in greater detail. The appropriate hydrothermal reaction temperature of MoS₂ is important for the growth of perfect MoS₂/TiO₂ NRA composites with significantly enhanced photocatalytic performance. The photodegradation rate and k value of MoS₂-220/TiO₂ are 86% and 0.0105 min⁻¹, respectively, which are much larger than those of blank TiO₂. The enhanced photocatalytic performance could be attributed to the higher visible light absorption and the reduced recombination rate of photogenerated electron–hole pairs.

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1. Introduction

In recent years, a large and growing literature has investigated the photocatalytic degradation and solar energy conversion to provide clean fuels^1–3 for addressing the energy crisis and environmental pollution. Titanium dioxide (TiO₂) as a semiconductor photocatalytic material has been broadly investigated due to its high photocatalytic activity, chemical stability, low cost and nontoxicity.⁴ In order to improve the photocatalytic activities, many efforts have been made for the fabrication of various morphologies of TiO₂, such as hollow spheres, hierarchical microspheres, nanorods, nanotubes and porous networks.^5–7 However, the fast recombination of photogenerated electron–hole pairs and large band gap (3.2 eV) of TiO₂ limit the use of sunlight as well as photocatalytic activity. In order to overcome these drawbacks, rational designs of TiO₂-based heterojunction photocatalysts with desirable architectural structure/morphology have been put into effect.^8–11 It is well-known that two-dimensional MoS₂ is a potential candidate for enhancing the visible light absorption and charge separation ability. Recently, MoS₂/TiO₂ heterostructures with diverse morphologies have been reported by researchers and have shown high photocatalytic activities in the visible light region.^12–14 However, most of the MoS₂/TiO₂ heterostructures were powder materials, which were either difficult to recycle or would require additives in order to prepare the electrodes. In order to overcome these drawbacks, researchers made great efforts to prepare the film-based heterostructure composites with enhanced photocatalytic performances. Zheng et al.¹⁵ prepared the hierarchical MoS₂ nanosheet@TiO₂ nanotube arrays via anodization and hydrothermal methods. Photocatalytic activity of the compositing MoS₂@TiO₂ hybrids can be enhanced by means of appropriate modification of MoS₂ on TiO₂ nanotube arrays. To the best of our knowledge, the studies on the synthesis of MoS₂ nanosheet coating on the TiO₂ nanoarrays are limited.

Herein, we demonstrate a facile two-step hydrothermal method to prepare MoS₂/TiO₂ NRA composite thin films with a highly-ordered three-dimensional hierarchical structure. The unique hierarchical structure of MoS₂/TiO₂ NRA heterojunctions was propitious to improve the light harvesting and charge separation rates. The novel MoS₂/TiO₂ NRAs exhibit enhanced photocatalytic activity due to the lower recombination rate and higher visible light absorption compared with the pure TiO₂ NRAs.

2. Experimental

A simple two-step hydrothermal approach was adopted to prepare MoS₂ nanosheet-loaded highly ordered TiO₂ nanoarrays (NRAs). TiO₂ NRAs were synthesized via the hydrothermal method. Then, 0.68 mL of tetrabutyl titanate was mixed with 20 mL of hydrochloric acid and 20 mL of deionized water (DI water). The mixed solution was magnetically stirred for 30 min and transferred into a 50 mL Teflon-lined stainless steel autoclave. Subsequently, a clean FTO glass was placed vertically in the autoclave, which was heated at 150 °C for 10 hours and then cooled to room temperature. The obtained TiO₂ NRAs were washed thoroughly with DI water and dried at 60 °C for several hours.

The preparation procedure of the MoS₂/TiO₂ NRA composite thin film is depicted as follows. 0.12 g thioacetamide (C₂H₅NS) and 0.06 g sodium molybdate (Na₂MoO₄·2H₂O) were dissolved in 40 mL DI water and magnetically stirred for 30 min to form a transparent solution. The mixed solution was transferred into a 50 mL-Teflon-lined stainless steel autoclave. The TiO₂ NRAs were placed vertically in the autoclave, which was heated at 200 °C and 220 °C for 24 h. The MoS₂/TiO₂ NRA composite thin film was obtained after washing the product with DI water and drying at 60 °C for 12 h.

Field-emission scanning electron microscopy (FE-SEM, S-4800) was adopted to observe the topologies of the samples. An X-ray diffractometer (XRD) was used to record the diffraction patterns of the samples. Chemical compositions of the samples were analyzed via X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The structural properties of the samples were examined using a laser micro-Raman spectrometer (Renishaw in Via-Reflex) at an excitation wavelength of 532 nm. Optical properties of the samples were characterized by an ultraviolet-visible spectrophotometer (UV-2550, Shimadzu). The photocatalytic performances of the samples were estimated by the degradation of methylene blue (MB) solution. A thin film (25 × 25 mm) was placed in a glass culture dish containing 10 mL MB solution (5 ppm), which was chosen as the simulated contaminant. Then, the glass culture dish was placed under the visible-light irradiation from a Xenon lamp with a 420 nm cut-off filter. The photocatalytic performance can be estimated by comparing the absorbance of MB at a certain light irradiation with that of the initial MB solution at a characteristic wavelength of 664 nm.

3. Results and discussion

Fig. 1 presents the top-view FE-SEM images of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂ on an FTO substrate. As shown in the FE-SEM image in Fig. 1a, the TiO₂ NRAs are well-aligned perpendicular to the FTO substrate. The nanorods have an average diameter of ∼100 nm. In MoS₂-200/TiO₂ (Fig. 1b), many tiny MoS₂ nanosheets exist on the side and top facets of TiO₂ nanorods. As the growth temperature of MoS₂ increases to 220 °C, the gap of the TiO₂ nanorods fills with tiny MoS₂ nanosheets, which form the well-stacked layered structure of MoS₂.

Fig. 1 Top-view FE-SEM images of (a) TiO₂ NRAs, (b) MoS₂-200/TiO₂, and (c) MoS₂-220/TiO₂.

Fig. 2 presents the XRD patterns of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂. The diffraction peaks for the TiO₂ NRAs match the standard peaks of rutile TiO₂ phase (JCPDS card no. 65-0191) with the lattice parameters of a = b = 0.4593 nm and c = 0.2958 nm. Five characteristic peaks at 2θ = 36.08°, 54.33°, 62.77°, 69.01° and 69.82° can be respectively assigned to the (101), (211), (002), (301) and (112) crystal facets of rutile TiO₂. With regard to MoS₂/TiO₂ NRA composite thin films, all diffraction peaks of the rutile TiO₂ phase still exist in the XRD patterns. These results indicate that the TiO₂ NRAs are well maintained in the hydrothermal process. Furthermore, the intensity of the diffraction peaks for rutile TiO₂ of MoS₂-200/TiO₂ is stronger than that of rutile TiO₂, which may be attributed to the recrystallization of rutile TiO₂ in the second step of the hydrothermal process. The decrease in the diffraction peak intensities for TiO₂ of MoS₂-220/TiO₂ may be due to the shielding effect of the large amounts of MoS₂ loaded onto TiO₂. No diffraction peak for MoS₂ has been observed in MoS₂-200/TiO₂. Absence of the diffraction peak indicates that the modificatory MoS₂ nanosheets on the TiO₂ NRAs may only contain a spot of MoS₂ nanosheets, which cannot be detected by XRD. Three characteristic peaks at 2θ = 14.40°, 33.52° and 58.35° have been observed in MoS₂-220/TiO₂, which can be indexed to (002), (101) and (110) crystal facets of MoS₂ (JCPDS card no. 65-7025), respectively. The stronger (002) diffraction peak that appeared in the XRD pattern of the sample indicates the formation of a well-stacked layered structure of MoS₂.¹⁶

Fig. 2 XRD patterns of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂.

In order to obtain the detailed topography of the binary composite thin film, TEM images of the MoS₂-220/TiO₂ composites have been recorded. Fig. 3a and b represent the typical transmission electron microscopy (TEM) images of the MoS₂-220/TiO₂ composite. It can be seen that the tiny MoS₂ nanosheets cover the surface of TiO₂ nanorods. Fig. 3c shows the high-resolution transmission electron microscopy (HRTEM) image of the composite thin film. The lattice fringes that are perpendicular to the growth direction have a lattice spacing of 0.32 nm, which is equal to the lattice parameter in the (110) facet of the rutile TiO₂. The lattice spacing is 0.62 nm, which corresponds to the (002) plane of MoS₂.

Fig. 3 (a and b) TEM images of MoS₂-220/TiO₂ composites. (c) High-resolution TEM (HRTEM) images of pristine TiO₂ NRAs.

As shown in Fig. 4, the survey XPS spectra show that the dominant elements in MoS₂-220/TiO₂ are Ti, O, S, Mo, and C (Fig. 4a). The high-resolution XPS spectra of Mo and S for MoS₂-220/TiO₂ are shown in Fig. 4b and c. The binding energies at 232.2 and 229.0 eV with a separation energy close to 3.2 eV are ascribed to the Mo3d_3/2 and Mo3d_5/2 of the Mo element in the tetravalent state.¹⁷ In addition, the binding energy at 235.9 eV is ascribed to the Mo(3d_3/2) signal of the Mo element in the hexavalent state, which results from the partial surface oxidation of MoS₂ in air.¹⁶ As shown in Fig. 4c, the S2p spectrum can be deconvoluted into two peaks. The binding energies at 163.3 and 161.9 eV are attributed to the S(2p_1/2) and S(2p_3/2) signals, respectively, of sulphide element in the divalent state. As shown in Fig. 4d, the C1s spectrum can be deconvoluted into three peaks, which correspond to C=O (288.4 eV), C–O (286.4 eV), and C–C (284.8 eV), respectively.¹⁸ As shown in Fig. 4e, the spin-energy separation between Ti2p_1/2 (464.3 eV) and Ti2p_3/2 (458.5 eV) peaks is about 5.8 eV, which is assigned to the Ti⁴⁺ state in the pure TiO₂ NRAs.¹⁹ The binding energies of Ti2p_1/2 (465.0 eV) and Ti2p_3/2 (459.2 eV) for MoS₂-220/TiO₂ shift to higher energies because of the existence of Ti³⁺ states by trapping electrons,¹⁶ which exhibit higher activity and form the radicals covalently on the surface of TiO₂. These trapped charge carriers can be transferred to the conduction or valence band by thermal excitation, which is beneficial to enhance the photocatalytic abilities.²⁰

Fig. 4 XPS survey spectra of (a) MoS₂-220/TiO₂, (b) Mo3d, (c) S2p, (d) C1s, and (e) Ti2p.

Fig. 5 shows the Raman spectra of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂. It can be seen from the Raman curve of bare TiO₂ NRAs that the two resonance peaks centered at 445 and 608 cm⁻¹ can be attributed to the B_1g and E_g of TiO₂ with rutile phase, respectively.²¹ There are three resonance peaks centered at 404, 445 and 608 cm⁻¹, which can be observed from MoS₂-200/TiO₂, which may be attributed to the out-of-plane E_2g¹ of MoS₂ and the B_1g and E_g of TiO₂, respectively.²² These results indicate that a spot of MoS₂ nanosheets deposited on the TiO₂ NRAs. The Raman curve of MoS₂-220/TiO₂ shows two strong resonance peaks centered at 375 and 404 cm⁻¹, which may be ascribed to in-plane A_1g and out-of-plane E_2g¹ of MoS₂, respectively. The other two weak resonance peaks centered at 445 and 608 cm⁻¹ may be assigned to B_1g and E_g of TiO₂, respectively, which weakened with the increase in MoS₂ nanosheet content.

Fig. 5 Raman spectra of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂.

Fig. 6 shows the UV-vis absorption spectra of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂. It can be seen that the spectrum of TiO₂ NRAs shows an abrupt absorption edge at about 400 nm and strong absorption in UV region. For MoS₂-200/TiO₂ and MoS₂-220/TiO₂, absorbance in the visible region has been enhanced due to the visible light absorbance arising from MoS₂, which possesses the considerably narrowed band gap. These results indicated that the TiO₂ NRAs were successfully modified by MoS₂ nanosheets. The enhanced visible light absorbance of MoS₂/TiO₂ can improve the visible photocatalytic activity of the samples.²¹ It can also be seen that the UV absorption is greatly dependent on the amount of MoS₂ loaded onto TiO₂. This may be attributed to the shielding effect of MoS₂ on TiO₂, which reduces the UV absorption of TiO₂.²³

Fig. 6 UV-vis absorbance spectra of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂.

The photocatalytic performances of TiO₂ NRAs, MoS₂, MoS₂-200/TiO₂ and MoS₂-220/TiO₂ were estimated by means of the photodegradation of MB under visible light. Fig. 7a shows the photodegradation rate of the samples. It is evident that the photodegradation rate of MoS₂-220/TiO₂ is far greater than that of TiO₂ NRAs, MoS₂, and MoS₂-200/TiO₂. The photodegradation rate of TiO₂ NRAs, MoS₂, MoS₂-200/TiO₂ and MoS₂-220/TiO₂ is about 44%, 53%, 57% and 86%, respectively, after visible light irradiation for 180 min.

Fig. 7 (a) Normalized absorption of MB in the presence of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂ under visible light irradiation. (b) The plots of ln(C₀/C) versus t of TiO₂ NRAs, MoS₂-200/TiO₂ and MoS₂-220/TiO₂.

The photodegradation of MB solution follows the pseudo-first-order reaction and its kinetics can be expressed as follows:¹⁷

ln(C₀/C) = kt (1)

where C₀ is the initial concentration of MB after adsorption equilibrium, C is the concentration after t min reaction, and k is the rate constant. The k values of TiO₂ NRAs, MoS₂, MoS₂-200/TiO₂ and MoS₂-220/TiO₂ can be calculated from Fig. 7b and are 0.0031, 0.0040, 0.0042, and 0.0105 min⁻¹, respectively. The k value of MoS₂-220/TiO₂ is almost 3.4 times greater than that of TiO₂ NRAs. These results indicate that the photocatalytic performances of TiO₂ NRAs can be considerably enhanced by the introduction of net-like MoS₂.

For the as-deposited TiO₂ NRAs, the photocatalytic degradation under visible light irradiation may be attributed to the existence of abundant defects, which will induce defect levels and narrow band-gaps and then improve the visible light absorption.²⁴ The schematic of the energy band structure and separation principle of carriers in MoS₂-220/TiO₂ is shown in Fig. 8. First, a small amount of electrons can be excited from the defect levels (or valence band) to the conduction band when TiO₂ is illuminated by visible light. Second, it is well known that MoS₂ is a narrow bandgap semiconductor with a band gap of about 1.8 eV. Upon visible light illumination, the electrons in the valence band (VB) of MoS₂ can be excited to conduction band (CB), thus leaving holes in the VB. As the CB of TiO₂ is lower than that of MoS₂, the photogenerated electrons in the CB of MoS₂ can be transferred to the CB of TiO₂ NRAs.¹⁷ TiO₂ is considered to be an electron receiver, which is conducive to the separation of charge carriers. The electrons may be captured by oxygen (O₂) to form superoxide radical anions (˙O₂⁻), which can react with a simulated pollutant. In addition, the holes left in the VB of MoS₂ can react with H₂O to form hydroxyl radicals, which can react with the dye on the sample.²⁵ Therefore, the electron and hole can be separated efficiently through the formation of a heterojunction between MoS₂ and TiO₂. Finally, the photocatalytic activities of MoS₂/TiO₂ could be enhanced by introducing MoS₂ onto the TiO₂ NRAs.

Fig. 8 Schematic of the energy band structure and separation principle of carriers in MoS₂-220/TiO₂.

4. Conclusions

A novel MoS₂ nanosheet decorating TiO₂ NRA heterojunction composite with outstanding photocatalytic performance for the degradation of organic pollutants under visible light illumination was deposited by a hydrothermal method on the FTO substrate. SEM, XRD and Raman results confirmed that MoS₂ nanosheets loaded on the surface of the TiO₂ NRAs and filled the gap between TiO₂ nanorods. Energy band structure of the MoS₂-220/TiO₂ heterojunction composite could promote charge transfer and prevent the recombination of photogenerated electron–hole pairs. The MoS₂-220/TiO₂ heterojunction composite exhibits the best photocatalytic activity, and the photodegradation rate and k value are 86% and 0.0105 min⁻¹, respectively.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (no. 51701001, 51102072, 51472003, and 51572002), Natural Science Foundation of Anhui Higher Education Institution of China (no. KJ2015ZD32, KJ2017A924, and KJ2017A002), Doctor Scientific Research Fund of Anhui University (no. J01001927), Youth Core Teacher Fund of Anhui University (no. J01005111), Foundation of Co-operative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration Anhui University (no. Y01008411, and WRXH201703), and the Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences (no. 2016KLMP01).

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