催化学报  2019, Vol. 40 Issue (1): 60-69   PDF    
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Yongchuan Wu
Zhongmin liu
Yaru Li
Jitao Chen
Xixi Zhu
Ping Na
Construction of 2D-2D TiO2 nanosheet/layered WS2 heterojunctions with enhanced visible-light-responsive photocatalytic activity
Yongchuan Wua, Zhongmin liub, Yaru Lia, Jitao Chena, Xixi Zhuc, Ping Naa     
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China;
b. College of Chemistry and Chemical Engineering, Dezhou University, Dezhou 253023, Shandong, China;
c. College of Chemistry and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China
* Corresponding author. Na Ping, Tel/Fax: +86-22-87405961; E-mail: naping@tju.edu.cn
Foundation item: This work was supported by the National High Technology Research and Development Program of China (863 Program, 2012AA063504), the Na-tional Natural Science Foundation of China (U1407116, 21511130020, 21276193), and the Tianjin Municipal Natural Science Foundation (13JCZDJC35600)
Abstract: Constructing nanocomposites that combine the advantages of composite materials, nanomaterials, and interfaces has been regarded as an important strategy to improve the photocatalytic activity of TiO2. In this study, 2D-2D TiO2 nanosheet/layered WS2 (TNS/WS2) heterojunctions were prepared via a hydrothermal method. The structure and morphology of the photocatalysts were systematically characterized. Layered WS2 (~4 layers) was wrapped on the surface of TiO2 nanosheets with a plate-to-plate stacked structure and connected with each other by W=O bonds. The as-prepared TNS/WS2 heterojunctions showed higher photocatalytic activity for the degradation of RhB under visible-light irradiation, than pristine TiO2 nanosheets and layered WS2. The improvement of photocatalytic activity was primarily attributed to enhanced charge separation efficiency, which originated from the perfect 2D-2D nanointerfaces and intimate interfacial contacts between TiO2 nanosheets and layered WS2. Based on experimental results, a double-transfer photocatalytic mechanism for the TNS/WS2 heterojunctions was proposed and discussed. This work provides new insights for synthesizing highly efficient and environmentally stable photocatalysts by engineering the surface heterojunctions.
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Key words: WS2    TiO2    Nanosheet    Heterojunction    Photocatalysis    Visible-light responsive    
构建2D-2D TiO2纳米片/层状WS2异质结用以增强可见光响应光催化活性
吴勇川a, 刘中敏b, 李亚茹a, 陈继涛a, 主曦曦c, 那平a     
a. 天津大学化工学院, 天津 300350;
b. 德州大学化学与化学工程学院, 山东德州 253023;
c. 山东科技大学化学与环境工程学院, 山东青岛 266590
摘要:自Fujishima等首次报道以来,TiO2作为一种重要的光催化剂引起了人们的广泛关注.迄今为止,研究人员已经开发出了各种形貌的具有不同晶型结构的TiO2,并用于光催化降解有机污染物.然而,TiO2的宽禁带(3.2eV)使其难以被可见光激活,导致对太阳光的利用效率低下.而且,在光催化反应中,低的量子效率无法满足实际应用.因此,开发具有可见光响应的高催化活性的TiO2基催化剂具有重要意义.集成复合材料、纳米材料和界面的优势构建纳米复合材料已成为提高TiO2光催化活性的重要策略.WS2具有典型的类石墨烯层状结构和窄的带隙(1.35eV),且其导带高于TiO2的导带,适合作为助催化剂修饰TiO2,使其具备可见光响应光催化活性.本文采用一步水热法,以二维(2D)TiO2纳米片作基质材料,直接在其表面原位生长WS2层,制得了2D-2D TiO2纳米片/层状WS2(TNS/WS2)异质结.XRD及Raman结果表明,层状WS2与TiO2纳米片紧密结合在一起,且两者之间形成了W=O键.TEM结果显示,层状WS2以面-面堆叠方式均匀地包覆在TiO2纳米片表面,包覆层数约为4层.光催化性能测试结果表明,可见光照射下,TNS/WS2异质结对RhB的光催化降解能力高于原始TiO2纳米片和层状WS2,光催化活性得到明显增强.紫外可见光谱试验结果显示,层状WS2的引入极大地增强了异质结的光吸收性能.PL光谱测试表明,TNS/WS2异质结具有更高效的载流子分离效率.为了进一步证实是光吸收性能的提升还是载流子分离效率的增强对光催化性能提起其主要作用,本文还研究了3D-2D TiO2空心微球/层状WS2(THS/WS2)复合材料.结果表明,TNS/WS2异质结比THS/WS2复合材料具有更高效的光生电子和空穴的分离能力.从而证明了TiO2纳米片与层状WS2之间完美的2D-2D纳米界面和紧密的界面结合,显著增加了载流子分离效率,因此光催化活性得到明显提高.为了研究TNS/WS2异质结光催化剂的光催化机理,采用重铬酸钾、草酸铵、叔丁醇和对苯醌作自由基猝灭剂进行了自由基捕捉剂实验.结果表明,空穴在RhB降解过程中起主要作用,超氧自由基起次要作用.基于自由基猝灭实验结果和带隙结构分析,提出了TNS/WS2异质结对RhB的光催化机理为双转移光催化机理.可见,界面异质结工程化可能是制备高效和环境稳定的光催化剂的新思路.
关键词WS2    TiO2    纳米片    异质结    光催化    可见光响应    

1 Introduction

Photocatalysis has been regarded as a promising strategy for environmental remediation because it is a clean process and has high degradation ability as well as low energy consumption [1, 2]. As one of the most important photocatalysts, TiO2 has attracted considerable attention since the first report by Fujishima et al. [3]. To date, various TiO2 nanostructures with different crystalline phases have been developed and used to photodegrade organic pollutants [4-6]. However, the wide band gap of TiO2 (3.2 eV) makes it difficult for activation by visible light, which leads to inefficient utilization of sunlight [7], and moreover, the low quantum efficiency in photocatalytic reactions is not satisfactory for practical applications [8]. Therefore, it is highly desirable to overcome these inherent shortcomings of TiO2 to enhance its photocatalytic activity.

Coupling TiO2 with co-catalysts to construct hybrid photocatalysts is an effective approach to achieve this goal. Compared to single-component photocatalysts, hybrid photocatalysts combine individual advantages and show better photocatalytic performance [9, 10]. Furthermore, the presence of heterojunctions between host and guest materials changes the band bending at the interfaces and provides a driving force to separate photogenerated electrons and holes [11, 12]. Therefore, combining these aspects, i.e., morphology, phase structure, and heterojunctions, may offer an ideal way to fabricate stable and efficient photocatalysts.

Transition metal dichalcogenides (TMDs) have been a focus of research for a long time owing to their extraordinary optical and electrical properties [13, 14]. A typical TMD, WS2, has been extensively studied for multiple applications in solar cells, electrocatalysis, photocatalysis, and bioimaging labels [15, 16]. WS2 has a typical graphene-like layered structure and a tunable bandgap [17, 18]. The bandgap of WS2 is about 1.35 eV, which is close to the bandgap value for the optimum utilization of solar radiant energy [19]. More importantly, the conduction-band minimum of WS2 is higher than that of TiO2, indicating that it is a possible co-catalyst candidate to photosensitize TiO2 in visible light. Guo et al. [20] reported that high photocatalytic activity and low recombination of photogenerated carriers were achieved by the deposition of nanosized WS2 on the surface of mesoporous TiO2. Liu et al. [21] synthesized nanoscale WS2/TiO2 composites and predicted that they may have potential applications in visible-light photocatalysis. More recently, Qi et al. [22] prepared WS2/TiO2 heterostructures and confirmed their good photoresponse activity and structural robustness. However, these studies only focused on the nanosize effect and ignored the crystal surface effect of the TiO2 substrate. Moreover, the TiO2 substrates used in these WS2/TiO2 composites were three-dimensional nanospheres or nanoparticles, which led to loose and non-uniform contact between WS2 and TiO2 and reduced the separation efficiency of photogenerated carriers. Hence, it is highly desirable to develop new WS2/TiO2 composites with higher photocatalytic activity and stability.

In this work, we chose 2D TiO2 nanosheets as the substrate to fabricate 2D-2D TiO2 nanosheets/layered WS2 (2D-2D TNS/WS2) heterojunctions via a facile hydrothermal method. The structure and morphology of the TNS/WS2 heterojunctions were systematically characterized. As expected, the layered WS2 were tightly coated on the surface of TiO2 nanosheet with a plate-on-plate stacked structure and connected to each other by W=O chemical bonds. The introduction of layered WS2 improved the light absorption properties of the TNS/WS2 heterojunctions and the heterogeneous interface enhanced the separation efficiency of the photo-induced carriers. As a result, the TNS/WS2 heterojunctions showed higher photocatalytic activity than pure TiO2 nanosheets and layered WS2. The photodegradation mechanism and degradation process were also discussed.

2 Experimental
2.1 Preparation of TNS/WS2 heterojunctions

Anatase TiO2 nanosheets with exposed (001) and (101) facets were prepared by a facile hydrothermal process as previously reported [23]. The synthesis process of TNS/WS2 heterojunctions is described below. Typically, 2.5 mmol sodium tungstate (Na2WO4·2H2O) was dissolved in 80 mL deionized water under magnetic stirring and the pH value of the solution was adjusted to 3 by dropping 2 mol/L hydrochloric acid. Then 7.5 mmol L-cysteine was added into the mixture, which was stirred until it became transparent again. Subsequently, a certain amount of TiO2 nanosheets was added to the above solution under vigorous stirring. The solution was transferred to a Teflon-lined stainless steel autoclave and reacted at 200 ℃ for 24 h. The black products were collected, washed, and dried in a vacuum oven at 60 ℃ for 12 h. The TNT/WND heterojunctions with different W:Ti molar ratios (0.20, 0.10, 0.07, and 0.05) were prepared by adjusting the amount of TiO2 nanosheets and were named TNS/WS2-0.20, TNS/WS2-0.10, TNS/WS2-0.07, and TNS/WS2-0.05. For comparison, pure WS2 was synthesized under the same conditions without adding TiO2 nanosheets. A TiO2 hollow microsphere/layered layered WS2 (THS/WS2) composite was synthesized under the same conditions by adding TiO2 hollow microspheres instead of TiO2 nanosheets.

2.2 Characterization

X-ray diffraction (XRD) patterns were recorded using a Bruker AXS D8 Focus X-ray diffractometer (Germany) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was conducted using a PerkinElmer PHI 1600 ESCA X-ray photoelectron spectroscope with monochromatic Mg Kα radiation and binding energies normalized to the C 1s peak at 284.6 eV. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were performed on a Hitachi S-4800 instrument. Transmission electron microscopy (TEM) was performed with a JEM-2100F (JEOL, Japan) instrument. Raman spectra were gathered from a confocal microscope-based Raman spectrometer (Renishaw InVia) in ambient air environment with an excitation laser line of 532 nm. The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joiner-Halenda (BJH) pore-size distribution measurements were obtained using a NOVA-2000 volumetric gas sorption instrument (Quantachrome, USA). UV-Vis diffuse reflectance spectra (DRS) of the samples were obtained using a Shimadzu UV-2550 spectrophotometer using BaSO4 as a reference. Photoluminescence (PL) spectra were obtained with a Fluorolog3 spectrofluorometer (Horiba Jobin Yvon) with an excitation wavelength of 325 nm. High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was performed on Bruker Daltonics micrOTOF-QII.

2.3 Photoreaction procedures

In the photocatalytic test, the light source was a 300-W Xe lamp with a 420 nm cut-off filter (to remove all incoming wavelengths shorter than 420 nm) placed 15  cm above the liquid surface. 20 mg of photocatalyst was dispersed in 100 mL of a rhodamine B (RhB) solution (20 mg/L). Prior to light irradiation, the suspensions were vigorously stirred for 60 min in the dark to establish an adsorption-desorption equilibrium. During the photocatalytic reaction, at given intervals, 5 mL samples were collected to test the concentration of RhB. The RhB concentration was calculated by UV-vis spectroscopy (Perkin Elmer, lambda 35).

2.4 Electrochemical measurement

Electrochemical tests were carried out on a three-electrode electrochemical workstation (CHI 660E, Chenhua, China) equipped with an FTO electrode deposited with the samples as a photoanode, a Pt wire as counter electrode, and Hg/Hg2Cl2/saturated KCl (SCE) as the reference electrode. In a typical experiment, 0.05 mol/L Na2SO4 (40 mL, pH = 6.8) purged with N2 was used as the electrolyte. For the fabrication of the photoanode, 10 mg of the solid sample was homogeneously mixed with 2 mL of absolute ethanol by using a vortex oscillator. The obtained sample was deposited onto the as-washed FTO glass with a controlled area of 1 cm2 by dip-coating to form a film electrode and then dried in air. Impedance-potential scanning (I-P) was carried out in the potential scanning range of ‒1.5‒1.0 V with an increase potential of 0.05 V and a frequency of 1000 Hz.

3 Results and discussion
3.1 Crystal structure and morphology

Fig. 1 shows the XRD patterns of layered WS2, TiO2 nanosheets, and TNS/WS2 heterojunctions. For pure layered WS2, there are three diffraction peaks at 17.7°, 32.5°, and 56.9°, which are ascribed to the (002), (101), and (110) lattice planes of hexagonal-phase WS2 (JCPDS 084-1398) [24]. The diffraction peaks of TiO2 nanosheets and corresponding heterojunctions matched the standard diffraction patterns of anatase TiO2 (JCPDS: 73-1764) [25]. No diffraction patterns associated with WS2 are observed in the TNS/WS2 heterojunctions, indicating that WS2 coated on the surface of TiO2 nanosheets may contain too few layers and are too thin to be detected by XRD [26]. In addition, the (101) peak experiences a chemical shift from 25.5° to 25.2° after WS2 is incorporated into TiO2, revealing the intimate contact and strong interaction between TiO2 nanosheets and layered WS2 [27]. Furthermore, the photos of photocatalysts show that increasing the WS2 content changes the color of the TiO2 nanosheets from white to black, implying an increased absorption of light.

Fig. 1. XRD patterns of pure WS2 (a), TNS/WS2-0.20 (b), TNS/WS2-0.10 (c), TNS/WS2-0.07 (d), TNS/WS2-0.05 (e), and pure TiO2 (f). Insets are the photos of corresponding photocatalysts.

To further explore the properties of the hybrid nanostructure, phonon spectra of the layered WS2, TiO2 nanosheets, and TNS/WS2-0.10 were measured by Raman scattering (Fig. 2). TiO2 nanosheets were characterized by well-defined peaks at 148 (Eg(1)), 201 (Eg(2)), 398 (B1g(1)), 519 (A1g + B1g(2)), and 643 cm−1 (Eg(3)), which corresponded to the anatase structure [28]. For pure WS2, the two dominant Raman scattering peaks can be observed at 377 and 404 cm−1, and are attributed to E2g1 and A1g, respectively [29]. When the layered WS2 was deposited on the surface of TiO2 nanosheets, characteristic peaks of WS2 and TiO2 can be detected in TNS/WS2-0.10. Thus, layered WS2 was successfully introduced into the TiO2 nanosheets. Interestingly, there is an extra peak at 972 cm−1, which may be attributed to the W=O bond of tungsten trioxide [30]. This extra peak suggests that the layered WS2 and TiO2 nanosheets are tightly combined, which leads to high reliability.

Fig. 2. Raman spectra of pure WS2 (a), TNS/WS2-0.10 (b), and pure TiO2 (c).

The SEM images and corresponding EDX images of layered WS2, TiO2 nanosheets, and TNS/WS2-0.10 are presented in Fig. 3. As shown in Fig. 3(a), the pure WS2 nanosheets are stacked together to form WS2 microspheres, with petal-like features observed on the surface. For TiO2 nanosheets (Fig. 3(c)), the SEM image shows well-dispersed sheets. After the introduction of WS2, the surface of TNS/WS2-0.10 (Fig. 3(e)) becomes rough, owing to the layered WS2 coating on the surface of the TiO2 nanosheets. The corresponding EDX spectrum shows that Ti, O, W, and S simultaneously exist in TNS/WS2-0.10; the atomic ratio of W:Ti is about 1:20, which indicates that the actual content of WS2 in TNS/WS2-0.10 is about 13.4 wt%.

Fig. 3. SEM images and EDS spectra of WS2 (a, b), TiO2 (c, d) and TNS/WS2-0.10 (e, f).

The microstructures of layered WS2, TiO2 nanosheets, and TNS/WS2-0.10 were further investigated by TEM. As shown in Fig. 4(a), WS2 is a well-layered structure and the number of layers ranges from one to six. The TEM image of the TiO2 nanosheets indicates that the product consists of well-defined sheet-shaped structures and that the (101) slab exhibits a clear crystal lattice. To gain a clear understanding of the interface between WS2 and the TiO2 nanosheets, the morphology of the TNS/WS2-0.10 nanostructure was vividly studied by TEM. As shown in Fig. 4(c), the TEM images of TNS/WS2-0.10 present layered and sheet-shaped morphologies, which are attributed to layered WS2 and TiO2 nanosheets, respectively. The top-view (Fig. 4(d)) image reveals that WS2 forms a homogeneous coat on the surface of the TiO2 nanosheets and that the crystal lattice can hardly be distinguished, except for the uncovered part. The side views shown in Fig. 4(e) and (f) clearly show that the layered WS2 is coated closely on the (004) and (101) facets of TiO2 nanosheets and that the number of coating layers is about four. Furthermore, the EDX elemental mapping of TNS/WS2-0.10 (Fig. 4(g)) show that O dovetails well with Ti and S dovetails well with W. The intensity of W (or S) is lower than that of Ti (or O), consistent with the content of WS2, which was in the minority. The uniform color and luster of the S and W mappings imply that the layered WS2 are homogeneously wrapped around the TiO2 nanosheets.

Fig. 4. HRTEM images of WS2 (a) and TiO2 (b), inset is TEM image. TEM image (c), HRTEM images (d-f), and EDX elemental mapping (g) of TNS/WS2-0.10.

The surface chemical states and components of the as-prepared heterojunctions were recorded by XPS. The full range XPS spectrum shown in Fig. 5(a) reveals that the dominant elements are Ti, O, W, and S in TNS/WS2-0.10. A typical high-resolution XPS spectrum of Ti 2p is shown in Fig. 5(b). Two peaks at 458.8 and 464.6 eV are ascribed to Ti 2p3/2 and Ti 2p1/2, respectively. The separation energy is close to 5.8 eV between Ti 2p1/2 and Ti 2p3/2, indicating a normal state of Ti4+ in TiO2 [31]. Fig. 5(c) shows the XPS spectrum of W 4f, which can be fitted to three peaks. The peaks at 35.6, 37.6, and 40.6 eV are assigned to W 4f7/2, W 4f5/2, and W 5p3/2, respectively. This result is in accordance with previous reports [32]. As for the high-resolution spectrum of S 2p (Fig. 5(d)), the fitted peaks at 161.3 and 163.4 eV are attributed to S 2p3/2 and S 2p1/2, respectively [33]. The XPS analysis further confirms that the layered WS2 was successfully introduced into the TiO2 nanosheets.

Fig. 5. XPS spectra of TNS/WS2-0.10. (a) full scan; (b) Ti 2p3/2; (c) W 4f; (d) S2p. Inset in (a) is local enlarged picture

The nitrogen adsorption-desorption isotherms and BJH pore-size distribution plots of the TiO2 nanosheets, TNS/WS2-0.10, and WS2 are shown in Fig. 6. The appearance of distinct hysteresis loops at high relative pressure suggests that these materials consist of mesopores. From the textural parameters present in the inset, we can see that the surface areas of TNS/WS2-0.10 sharply increase, which is beneficial for the dye adsorbed on the surface. The high surface areas TNS/WS2-0.10 validate their highly porous structure, which could provide a more flexible response to active sites.

Fig. 6. N2 adsorption-desorption isotherms and textural parameters (inset) of TiO2, WS2, and TNS/WS2-0.10.
3.2 UV-vis diffuse reflectance spectra and PL spectra analysis

The light-absorption property of the as-prepared heterojunctions was determined by UV-vis diffuse reflectance spectra and is displayed in Fig. 7(a). For comparison, the spectra of pure TiO2 nanosheets and layered WS2 are also plotted. As we can see, pure TiO2 nanosheets exhibit a sharp fundamental absorption band edge at about 380 nm, suggesting that they are excited by UV light. In contrast, the layered WS2 display strong absorption from the ultraviolet to visible-light regions as well as in the near-infrared light region. Notably, when WS2 is introduced into TiO2, the photoabsorption regions of the TNS/WS2 heterojunctions are significantly broadened and the absorption intensity is enhanced.

Fig. 7. (a) UV-vis spectra of TiO2, WS2 and TNS/WS2 heterojunctions; (b, d) Tauc's plots of TiO2 and WS2; (c, e) Corresponding Mott-Schottky curves; (f) PL spectra of TiO2, WS2, and TNS/WS2-0.10.

The band gaps, conduction band (CB), and valence band (VB) of TiO2 and WS2 were estimated by Tauc's plots and Mott-Schottky (M-S) measurements. The band gaps of the photocatalysts can be calculated using the following formula [34]:

Where α is the absorption coefficient, A is a constant, h is Planck's constant, Eg is the band energy, and ν is the frequency of the incident light. The value of n is decided by the property of the semiconductor. Anatase TiO2 is an indirect-band-gap semiconductor and its value of n is 1/2 [35]; layer-like WS2 is a direct-band-gap semiconductor and its value of n is 2 [36, 37].

The corresponding calculated results for TiO2 nanosheets and layered WS2 are displayed in Fig. 7(b) and (d), where the slopes of the tangents are band gap energies. The band gaps of TiO2 and WS2 are 3.23 and 1.43 eV, respectively. Fig. 7(c) and (e) shows the CB for TiO2 and WS2. The CBs of TiO2 and WS2 are determined to be ‒0.40 and ‒0.73 eV vs. NHE, respectively. Therefore, according to the empirical formula EVB = ECB + Eg, the VBs of TiO2 and WS2 are calculated to be 2.83 and 0.7 eV vs. NHE, respectively. Therefore, type-Ⅱ band alignment is produced at the interface of the TNS/WS2 heterojunctions, which is beneficial for the separation of electron-hole pairs.

The PL spectra of TiO2, WS2, and TNS/WS2-0.10 are shown in Fig. 7(f). Clearly, the PL intensity of TNS/WS2-0.10 is lower than that of either TiO2 or WS2. These results directly confirm that TNS/WS2-0.10 possesses more efficient carrier separation, which is a result of the heterojunctions between the TiO2 nanosheets and layered WS2 suppressing the excited electron-hole pair recombination.

3.3 Photocatalytic studies

To evaluate the photocatalytic activity, photocatalytic degradation of RhB was carried out under visible-light irradiation. As illustrated in Fig. 8(a), the self-degradation of RhB is negligible without photocatalysts. The TiO2 nanosheets show low adsorption capacity and unsatisfactory activity. In contrast, despite the unsatisfactory photocatalytic activity of WS2, its adsorption capacity is higher than that of TiO2 nanosheets. The excellent absorption capacity of WS2 was due to the TNS/WS2 heterojunctions. In addition, the photocatalytic performance of TNS/WS2 heterojunctions was obviously enhanced, especially TNS/WS2-0.10, which showed the highest photocatalytic activity as almost 100% RhB was decomposed after 90 min irradiation under visible light. It is noteworthy that the TNS/WS2 composites still showed higher photocatalytic activity than pure TiO2 nanosheets under UV irradiation (Fig. S1).

Fig. 8. Photocatalytic degradation of Rh B (a), first-order kinetic model fitting curves (b) and degradation rate constant of RhB (c) with as-prepared samples. (d) Time-dependent absorption spectra of RhB solution over TNS/WS2-0.10 (d). Inset shows the photos of RhB at different irradiation times.

The RhB photodegradation kinetics was fitted by applying a first-order model. The ‒ln(C/Co) plot is shown in Fig. 8(b) and the fitted degradation rates (k) of RhB are illustrated in Fig. 8(c). It is obvious that the TNS/WS2-0.10 has the highest degradation rate constant, which is about 9.8 and 4.6 times higher than that of TiO2 nanosheet and layered WS2, respectively. The notably facilitated photocatalytic performance of TNS/WS2 heterojunctions may be derived from the synergistic effect of the improvement of light-harvesting properties and the effective separation of photogenerated electrons and holes.

In order to confirm this conclusion, TiO2 hollow microsphere/layered layered WS2 (THS/WS2) composites were prepared by the same method for comparison. The TEM images (Fig. S2) indicate that the THS/WS2 composites are core/shell-structured. Exposed (004) and (101) facets of the TiO2 hollow microsphere are also found in THS/WS2-0.10. The surface area of THS/WS2-0.10 (Fig. S3(a)) is 72.43 m2·g−1, which is close to that of TNS/WS2-0.10 (76.10 m2·g−1). Furthermore, the light-harvesting properties of THS/WS2-0.10 (Fig. S3(b)) are greatly improved compared to the pristine TiO2 hollow microsphere. These experimental data showed that the exposed facets, surface areas, or light-harvesting properties did not primarily contribute to improving the photocatalytic activity. The charge separation efficiency (Fig. S4) of TNS/WS2-0.10 is higher that of THS/WS2-0.10. Meanwhile, the photocatalytic activity of THS/WS2-0.10 (Fig. S5(a)) is higher than that of the pristine TiO2 hollow microsphere, but is still lower than that of TNS/WS2-0.10. The reaction rate of TNS/WS2-0.10 (Fig. S5(b)) is about five times that of THS/WS2-0.10. Hence, the 2D-2D heterojunction nanostructure is key to improving the photocatalytic activity of TNS/WS2 heterojunctions.

Fig. 8(d) shows the time-dependent absorption spectra of the RhB solution under visible-light irradiation in the presence of TNS/WS2-0.10. The continuous decrease in the absorption intensity at 553 nm indicates the gradually decreasing concentration of RhB with prolonged irradiation time. Interestingly, the major absorption peak of RhB exhibits a small blue-shift from 553 to 530 nm, corresponding to the stepwise formation of a series of N-de-ethylated intermediates [38]. With increasing irradiation time, the peak located at 530 nm continues to shift and decrease, which indicates that the RhB molecules are further decomposed into smaller molecular fragments and the structure of RhB is also destroyed in the end.

The degradation products of RhB over TNS/WS2-0.10 were further analyzed by HPLC-MS/MS and showed in Fig. 9. The peak at a retention time of 7.5 min is attributed to RhB, and the area of peak decreases with increasing irradiation time. Finally, the peak for RhB almost disappears, demonstrating that RhB is completely decomposed. The results confirm the conclusion of Fig. 8(d). As is well known, the photocatalytic degradation of RhB involves a series of complex chemical reactions, mainly including diethyl, ring-opening, and redox reactions [39, 40]. With increasing reaction time, some new peaks appear in the HPLC chromatograms. The new peaks appear at elution times of 4.4, 2.6, 1.9, and 1.6 min, and were further analyzed by MS/MS. The mass-to-charge ratios (m/z) were 415.2, 387.2, 359.1, and 331.1, respectively (Fig. S6). The difference between two successive m/z values is 28, indicating that the diethyl reaction occurs during photocatalytic degradation. After 90 min of irradiation, the intensity of the peak at 1.1 min is obviously enhanced. The m/z for this peak is 89.5, and the chemical composition may be ethylene diacid. The results show that RhB molecules are further decomposed into smaller molecular fragments by the ring-opening reaction [41]. This conclusion could serve as a reference for the degradation of other organic contaminants over the TNS/WS2 heterojunction photocatalyst.

Fig. 9. HPLC-MS/MS of degradation products toward RhB under different irradiation times over TNS/WS2-0.10.
3.4 Reusability studies

The renewable stability is an important index for practicality of the photocatalysts. Here, we investigated the photocatalytic cycle stability of TNS/WS2-0.10. As shown in Fig. 10(a), the photocatalytic activity of TNS/WS2-0.10 is effectively maintained, except for a 7% decrease from 96% to 89% after three cycles. To further understand the stability of the samples, the XRD pattern of the sample reused for three cycles was measured. The XRD pattern of TNS/WS2-0.10 in Fig. 10(b) shows no obvious change. This confirms that TNS/WS2-0.10 has relatively good stability.

Fig. 10. Recyclability (a) and XRD patterns (b) of TNS/WS2-0.10 after three cycles.
3.5 Photocatalytic mechanism studies

In order to further determine the main active species involved in the degradation process and possible photodegradation mechanism, trapping experiments of TNS/WS2-0.10 were conducted. For this, K2Cr2O7, ammonium oxalate (AO), tert-butyl alcohol (TBA), and benzoquinone (BQ) were employed as scavengers for the reactive species of e, h+, ·OH, and O2·, respectively. A comparative experiment with no quencher was also performed under the same conditions. As presented in Fig. 11, in the presence of different quenchers, the corresponding photocatalytic efficiency is partly restrained. The degradation of RhB is slightly inhibited by the addition of K2Cr2O7 or TBA, implying that ·OH or e is not the main active species responsible for the decomposition of RhB. However, when AO or BQ was added into the reactive solution, the photocatalytic activity of TNS/WS2-0.10 was remarkably lowered. The degradation efficiency of RhB decreased to 33% in the presence of BQ. Surprisingly, RhB was not entirely degraded after adding AO adding to the reactive solution. Given the above results, holes (h+) and superoxide anion radicals (O2·) were the main active species in the reaction process, and h+ played a dominant role for RhB degradation under visible-light irradiation.

Fig. 11. Effects of a series of scavengers on RhB degradation over TNS/WS2-0.10. Conditions: irradiation time = 90 min, K2Cr2O7 and AO dosage = 0.2 g, TBA dosage = 4 mL, BQ dosage = 2 mg.

On the basis of the experimental results discussed above and the band positions of TiO2 and WS2, the plausible photocatalytic mechanism of TNS/WS2 heterojunctions was proposed and is illustrated in Fig 12. From analyses in the above section, we could conclude that the n-n heterojunction is formed and is type-Ⅱ. When the TNS/WS2 heterojunctions are exposed to visible light, WS2 can be excited and yield electron-hole pairs. Because the CB level of WS2 is more negative than that of TiO2, the photoexcited electrons in the CB of WS2 can be injected easily into the CB of TiO2 and reacted with O2 in the aqueous solution to form O2· and thus degrade RhB. Meanwhile, because the potential of ·OH/H2O (2.32 eV vs NHE) is more positive than that of WS2 (0.70 eV vs NHE), photogenerated holes accumulate in the VB of WS2 and directly react with RhB. The large number of holes accelerate the decomposition of RhB into a non-toxic substance. This is why the hole plays a dominant role in RhB degradation, rather than O2·-. Finally, the photoinduced electrons and holes are effectively separated through the traditional heterojunction structure, so as to solve the high recombination rate of the photogenerated electron-hole of TiO2 and greatly enhance its photocatalytic activity under visible light.

Fig. 12. Schematic illustration of RhB degradation mechanism over TNS/WS2 heterojunctions.
4 Conclusions

In summary, 2D-2D TiO2 nanosheets/layered WS2 (TNS/WS2) heterojunctions were successfully fabricated via a simple hydrothermal reaction. Physical characterization revealed that the layered WS2 was well-coated on the surface of the TiO2 nanosheet, which resulted in the formation of a perfect heterojunction structure. There were four coating layers, and WS2 was connected with TiO2 by a W=O bond. The as-prepared heterojunction composite exhibited excellent photocatalytic activity and reusability for the degradation of RhB under visible-light irradiation. The degradation products were traced by HPLC-MS/MS and the reaction process was explored. In addition, the typical type-Ⅱ mechanism was confirmed, demonstrating that holes and superoxide anion radicals (O2·) played specific roles in the photodegradation process. This work could provide insights toward understanding TiO2/WS2-based composites.

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