1T-MoS₂ nanopatch/Ti₃C₂ MXene/TiO₂ nanosheet hybrids for efficient photocatalytic hydrogen evolution†

Yujie Li , Shaorui Yang , Zhangqian Liang , Yanjun Xue , Hongzhi Cui and Jian Tian *
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China. E-mail: jiantian@sdust.edu.cn

First published on 28th October 2019

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1. Introduction

The increasing global demands for energy consumption and consequent environmental pollution have aroused considerable concern to develop clean energy technologies.¹ The generation of hydrogen (H₂) from water using solar energy through semiconductor-based photocatalysis is regarded as a promising strategy for solving global energy problems.^2–5 Among various semiconductor photocatalysts, TiO₂ is currently one of the most extensively used due to its stability against photo and chemical corrosion, low cost and nontoxicity.^6,7 However, the application of TiO₂ is restricted due to the rapid recombination of photoinduced electron–hole pairs, and thus massive endeavors (for instance, surface modification, co-catalyst loading and heterojunction construction) have been devoted to promoting photoexcited electron and hole separation.^8,9 Among these, the co-catalyst strategies have proven to be one of the most effective ways to improve the photocatalytic hydrogen evolution reaction (HER) performance.¹⁰ Noble metals such as Ag, Pt, Pd and Au are outstanding co-catalysts in photocatalytic H₂ evolution.¹¹ Nevertheless, the high price and extreme scarcity restrict their application in photocatalytic water splitting.¹² Therefore, seeking an inexpensive and highly active co-catalyst is of paramount significance for achieving photocatalytic H₂ production in the future.¹³

As a new series of two-dimensional (2D) materials, MXenes obtained by the removal of the intermediate element from the MAX phases have attracted tremendous attention because of their excellent electrical conductivity like graphene.^14,15 MAX phases, the precursors of MXenes, have the general formula of M_n+1AX_n, where M is an early transition metal, A is a group IIIA or IVA element, X is carbon and/or nitrogen, and n = 1, 2, or 3.¹⁶ For example, a 2D material with an accordion-liken structure of layered Ti₃C₂ MXene can be produced by etching the Al layers in HF solution for a certain time from the MAX phase Ti₃AlC₂.¹⁷ And the presence of hydrophilic functionalities (–OH and –O) on its surface is due to the etching process, enabling it to easily construct strong connections with various semiconductors and water molecules.⁴ Thus, due to its high electrical conductivity and unique layer morphology, Ti₃C₂ MXene is a promising co-catalyst candidate to replace Pt for photocatalytic H₂ production.¹⁸

MoS₂, as a two-dimensional (2D) layered structure, has received a great deal of attention for its potential application as a promising electrocatalyst for H₂ evolution and has been considered as a promising alternative for noble-metal co-catalysts due to its abundance and low cost.¹⁹ Generally, MoS₂ mainly exists in two different polymorphs, including trigonal prismatic semiconducting phase MoS₂ (2H-MoS₂, a bandgap of ∼1.8 eV) and octahedral metallic phase MoS₂ (1T-MoS₂).²⁰ Both theoretical and experimental research have demonstrated that catalytically active sites of 2H-MoS₂ are only located along the edges of the MoS₂ layers, and the inherent poor conductivity character of 2H-MoS₂ restricts the fast electron transfer during the catalytic HER process.^21,22 While 1T-MoS₂ is different from 2H-MoS₂, it possesses dense active sites (both the basal plane and edges) and a much higher conductivity than 2H MoS₂. So 1T-MoS₂ has been applied to photocatalysis as an efficient co-catalyst and an electron acceptor.^23,24

In this work, we report an innovative 2D heterojunction by taking advantage of the metallic features of Ti₃C₂ MXene and 1T-MoS₂. A two-step procedure is used to design a new type of photocatalyst Ti₃C₂/TiO₂/1T-MoS₂ where Ti₃C₂ MXene and 1T-MoS₂ play important roles as electron acceptors. First, TiO₂ nanosheets (NSs) are in situ grown on the surface of Ti₃C₂ MXenes to construct Ti₃C₂/TiO₂ composites by a facile hydrothermal method. Secondly, we intentionally employ the 1T-MoS₂ nanopatches evenly distributed on the surface of Ti₃C₂/TiO₂ composites using a hydrothermal process, in which octahedral MoO₃ is a source of molybdenum and potassium thiocyanate is a source of sulfur. This process subsequently leads to the formation of an efficient photocatalytic system with close contact between the metallic Ti₃C₂ MXene, 1T-MoS₂ nanopatches and TiO₂ NSs. The newly designed Ti₃C₂/TiO₂/1T-MoS₂ composites not only exhibit drastically improved photocatalytic H₂ evolution performance, but also demonstrate much better stability because of the unique 2D–2D–2D structure.

2. Experimental procedure

2.1. Materials

The Ti₃AlC₂ powder was provided by 11 Technology Co., Ltd, Changchun. Hydrochloric acid (HCl), hydrofluoric acid (HF, 40 wt%), sodium tetrafluoroborate (NaBF₄), molybdenum oxide (MoO₃) and potassium thiocyanate (KSCN) were purchased from Sinopharm. All chemicals used were of analytical reagent grade.

2.2. Preparation of Ti₃C₂ MXene nanosheets

In a typical synthesis, 0.5 g Ti₃AlC₂ powder was slowly added to 100 mL 40 wt% HF solution. Then the reaction mixture was stirred for 72 h at room temperature. After that, the mixed solution was separated by vacuum filtration, and washed with deionized water to neutral. Finally, Ti₃C₂ MXene nanosheets were dried in a vacuum oven at 50 °C for 12 h.

2.3. Preparation of Ti₃C₂/TiO₂ composites

For the synthesis of Ti₃C₂/TiO₂ composites, 100 mg Ti₃C₂ MXene and 165 mg NaBF₄ were added into 15 mL of 1.0 M HCl. After the solution was stirred and ultrasonically treated for 30 min, it was transferred into a 25 mL Teflon-lined stainless-steel autoclave, which was hydrothermally treated at 160 °C for 12 h. After naturally cooling down to room temperature, the reaction solution was collected by vacuum filtration, and the resulting Ti₃C₂/TiO₂ composites were washed with distilled water several times, and dried in a vacuum oven at 60 °C for 12 h.

2.4. Preparation of Ti₃C₂/TiO₂/1T-MoS₂ composites and Ti₃C₂/TiO₂/2H-MoS₂ composites

For the synthesis of Ti₃C₂/TiO₂/1T-MoS₂ composites, 8.1 mg MoO₃ and 23.15 mg KSCN were dissolved in 30 mL deionized water to form a transparent solution. Then, 60 mg Ti₃C₂/TiO₂ composites were added into the above solution and stirred to form the suspension. After the suspension was stirred, it was ultrasonically treated for 60 min. The solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave, which was hydrothermally treated at 200 °C for 18 h. After naturally cooling down to room temperature, the reaction solution was collected by vacuum filtration, and the resulting Ti₃C₂/TiO₂/1T-MoS₂ composites (15 wt% MoS₂) were washed with distilled water several times, and dried in a vacuum oven at 50 °C for 12 h. Similarly, by changing the mass of MoO₃ (5.4 mg, 13.5 mg, and 16.2 mg) and KSCN (15.4 mg, 38.6 mg, 46.3 mg), Ti₃C₂/TiO₂/1T-MoS₂ composites with other MoS₂ loading amounts (10 wt%, 25 wt%, and 30 wt%) were obtained, respectively. Ti₃C₂/TiO₂/2H-MoS₂ composites with different MoS₂ loading amounts (10 wt%, 15 wt%, 25 wt%, and 30 wt%) were prepared by a hydrothermal method.²²

2.5. Characterization

The phase constituents of the synthesized products were analyzed by X-ray diffraction (XRD, H-12 KURARAY, Japan). The nanostructure and surface characteristics of the products were observed via high-resolution scanning electron microscopy (FESEM, FEI Nova NanoSEM 450, USA) with energy-dispersive X-ray spectroscopy (EDS). High-resolution transmission electron microscopy (HRTEM) was carried out using a JEOL JEM 2100F field emission transmission electron microscope. The chemical states of the composite were tested using X-ray photoelectron spectrometry measurements (XPS, Thermo ESCALAB 250XI, USA). The pore size distribution was measured using the Brunauer Emmett Teller (BET) method as-examined on a Micromeritics ASAP 2020 nitrogen adsorption–desorption apparatus. The UV-vis diffuse reflectance spectra (DRS) of the samples were tested on a UV-vis spectrophotometer (Hitachi UV-3101) with an integrating sphere attachment within the 200–800 nm range. The photoluminescence (PL) spectra were acquired at room temperature using a FLS920 fluorescence spectrometer under the ultraviolet excitation of 325 nm.

2.6. Photocatalytic and photoelectrochemical activity test

The photocatalytic reaction was implemented in a Pyrex glass vessel, with a top quartz window suitable for vertical illumination, connected to a gastight circulation system. The stir rate of the vessel is 550 rpm. A 300 W Xe arc lamp (CELHXF300, Beijing China Education Au-light Co., Ltd) with an AM-1.5 filter was used as the light source. The wavelength range of the AM-1.5 filter is from 200 to 1200 nm. The focused intensity of the flask was ca. 180 mW cm⁻². The experiments were performed in aqueous acetone which dissolved TEOA (15 mL acetone + 5 mL TEOA + 80 mL deionized water), and suspended with 10 mg of catalyst powder following ultrasonic dispersion for 2 min. Then the reaction solution was evacuated several times to remove air and the reaction temperature of the reactant solution was maintained at 25 °C. The amount of generated hydrogen was analyzed by a gas chromatograph (Techcomp GC-7920) equipped with a thermal conductivity detector (TCD). The apparent quantum efficiency (AQE) was measured using the same light source with a CUT420 filter.

The transient photocurrent response and electrochemical impedance spectroscopy (EIS) curves of the samples were measured by using an electrochemical workstation (Shanghai Chenhua CHI660D) under a 300 W Xe arc lamp with an AM-1.5 filter with light on–off switches of 100 s in a three-electrode electrochemical cell in the 0.5 M Na₂SO₄ electrolyte, in which the Pt wire and the Ag/AgCl electrode were used as the counter and reference electrodes, respectively. 2 mg of the as-synthesized samples were mixed with 0.5 mL ethanol and terpilenol. After being sonicated for 5 min, the mixture was dropped onto fluoride-tin oxide (FTO) conductor glass and dried at 50 °C for 6 h to form a working electrode.

3. Results and discussion

A typical synthesis route of the Ti₃C₂/TiO₂/1T-MoS₂ composites is schematically depicted in Scheme 1. First, the Ti₃C₂ MXene nanosheets are synthesized by selectively etching Al layers in Ti₃AlC₂ MAX phase with HF acid.²⁵ Second, the layered Ti₃C₂ MXene nanosheets providing Ti sources for the growth of TiO₂ NSs across the layered Ti₃C₂ MXene are achieved through the hydrothermal oxidation of Ti₃C₂ with the assistance of HCl and NaBF₄. Then, the obtained Ti₃C₂/TiO₂ composites are immersed in a clear solution with MoO₃ and KSCN at 200 °C for 18 h in order to introduce the MoS₂ cocatalyst. The 1T-MoS₂ nanopatches are evenly distributed over the surface of the Ti₃C₂/TiO₂ composites to form ternary Ti₃C₂/TiO₂/1T-MoS₂ composites.

The phase structure and crystallization of the obtained powder samples were characterized by Xray diffraction (XRD), as shown in Fig. 1 and Fig. S1 (ESI†). The XRD patterns of Ti₃AlC₂, Ti₃C₂ and Ti₃C₂/TiO₂ composites are revealed in Fig. S1 (ESI†). The XRD diffraction peaks of Ti₃AlC₂ are similar to the previous literature.⁷ After the etching of Ti₃AlC₂ by HF, the most intense (104) diffraction peak in the Ti₃AlC₂ pattern located at 39° completely disappeared and the (002) plane at 9.58° and (004) plane at 19.17° of Ti₃AlC₂ were broadened and shifted toward the lower angle side due to the expansion of the interlayer spacing, which indicates the successful removal of the Al layers and formation of Ti₃C₂ MXene.²⁶ The growth of TiO₂ NSs across the layered Ti₃C₂ sheets by the hydrothermal oxidation of Ti₃C₂ is evidenced by the emergence of diffraction peaks of anatase TiO₂ (JCPDS No. 21-1272). Fig. 1 shows the XRD patterns of Ti₃C₂/TiO₂/1T-MoS₂ composites with different MoS₂ loading amounts. The diffraction peaks appearing at 8.9°, 18.5°, 27.6° and 60.4° are indexed to the (002), (004), (006) and (110) planes of Ti₃C₂, and the peaks observed at 25.3°, 37.8°, 48.1°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3°, 75.38° and 82.7° are indexed to the (101), (004), (200), (105), (211), (204), (116), (220), (107) and (224) planes of anatase TiO₂ (JCPDS No. 21-1272). No signals assignable to MoS₂ are detectable. This can be explained by the fact that MoS₂ is ultra-thin and is highly dispersed on the Ti₃C₂/TiO₂/1T-MoS₂ composites.

The morphology and structure of the obtained samples are characterized by scanning electron microscopy (SEM), as exhibited in Fig. 2 and Fig. S2 (ESI†). As shown in Fig. 2a, Ti₃C₂ has an accordion-like multilayer structure with a smooth surface, which reveals the typical MXene morphology. Fig. 2b shows that the bulk MoS₂ is about 3–4 microns in size, and is made up of sheets (Fig. S2, ESI†). The EDS mapping of bulk MoS₂ is shown in Fig. S3 (ESI†). After hydrothermal oxidation of Ti₃C₂ MXene, the layered Ti₃C₂ MXene nanosheet acts as the Ti source for the growth of TiO₂ NSs inserted across the layered Ti₃C₂ MXene nanosheet to form Ti₃C₂/TiO₂ composites (Fig. 2c). For the Ti₃C₂/TiO₂/1T-MoS₂ composites (15 wt% MoS₂), the 1T-MoS₂ nanopatches are evenly distributed on the surface of the Ti₃C₂/TiO₂ composites (Fig. 2d), exposing rich active edge sites for H₂ evolution reactions.²⁰ Furthermore, Ti₃C₂/TiO₂/1T-MoS₂ composites with other different MoS₂ loading amounts (10 wt%, 25 wt%, and 30 wt%) are also obtained, and the corresponding SEM images are shown in Fig. S4 (ESI†).

The morphology and microscopic structure of the Ti₃C₂/TiO₂/1T-MoS₂ composites have been further elucidated by TEM analysis. As shown in Fig. 3a, b and Fig. S5a (ESI†), ultrathin large lamellar Ti₃C₂ MXene sheets, square TiO₂ NSs and small MoS₂ nanopatches can be clearly seen in the Ti₃C₂/TiO₂/1T-MoS₂ composites, which is consistent with the presentation of SEM images in Fig. 2d. The size of the MoS₂ nanopatches in the Ti₃C₂/TiO₂/1T-MoS₂ composites is about 10–20 nm, and the formation of Ti₃C₂/TiO₂/1T-MoS₂ composites could efficiently decrease the particle size of MoS₂ and greatly improve the dispersibility of MoS₂, which further increases the specific surface area of the photocatalyst, facilitates sufficient contact of the catalyst with the reactants, and enhances photocatalytic activity. The HRTEM image, as displayed in Fig. 3c and d, provides more detailed information about the interface between 1T-MoS₂ and TiO₂–Ti₃C₂. In detail, the lattices with d spaces of 0.98 and 0.26 nm are attributed to the (002) and (0110) planes of Ti₃C₂ MXene, which are the same as the description of the literature.^1,27 The lattice spacing of 0.35 nm is indexed to the (101) plane of anatase TiO₂ (Fig. 3d).²⁸ Most importantly, the trigonal lattice of the octahedral 1T MoS₂ phase is found, as shown in the inset of Fig. 3d, visualizing and demonstrating the formation of 1T-MoS₂ phase,²⁹ and the number of plies is mostly about 2–5 layers as shown in Fig. 3c and Fig. S5b (ESI†). The EDX elemental mappings of the composite are shown in Fig. 3e, and the Ti, O, Mo and S elements are distributed accordantly within the selected area. The as-fabricated photocatalyst with superior metallic quality of Ti₃C₂ MXene and 1T-MoS₂ has faster charge separation and more efficient carrier transfer compared with pure TiO₂ NSs or Ti₃C₂/TiO₂ composites, and hence, the photocatalytic activities are improved.

The X-ray photoelectron spectroscopy (XPS) analysis of Ti₃C₂/TiO₂/1T-MoS₂ composites is investigated to further confirm the chemical composition and states of elements, and the results are shown in Fig. 4. The full-scale XPS pattern of the Ti₃C₂/TiO₂/1T-MoS₂ composites shown in Fig. 4a reveals that Ti, C, O, S, and Mo are the predominant elements, while the F element can be attributed to the F⁻ ions physically adsorbed on the surface of composites derived from the HF solution. The C 1s XPS spectrum presents two peaks at 284.8 and 282.1 eV, designated to sp² carbon (CC) and C–Ti, respectively,²⁵ which are attributed to the adventitious elemental carbon and Ti₃C₂ (Fig. 4b). The high-resolution XPS spectrum of Ti 2p is deconvoluted into four peaks as shown in Fig. 4c.⁴ The two peaks at binding energies of 459.3 eV (Ti 2p_3/2) and 465.1 eV (Ti 2p_1/2) are attributed to the lattice Ti–O bond in TiO₂,³⁰ while the other two peaks at binding energies of 455.7 eV (Ti 2p_3/2) and 461.8 eV (Ti 2p_1/2) correspond to the lattice Ti–C bond in Ti₃C₂.²⁶ The O 1s XPS spectrum (Fig. 4d) exhibits two peaks centered at 530.1 eV and 532.1 eV, corresponding to the bonds of Ti–O–Ti (lattice O) and surface hydroxyl groups,³¹ respectively. The Mo 3d and S 2p XPS spectra of Ti₃C₂/TiO₂/1T-MoS₂ composites (Fig. 4e and f) can further verify the identity of 1T MoS₂ and calculate the relative content. In the Mo 3d region, the doublet peaks of 2H phase correlate to Mo 3d_5/2 and Mo 3d_3/2 resonated at around 230.0 eV and 233.0 eV, respectively (Fig. 4e). However, two additional peaks are found to have shifted to lower binding energies at around 228.7 and 232.2 eV, which suggests the presence of metallic 1T phase.^32–34 The peak at 235.9 eV is ascribed to Mo⁶⁺ arising from MoO₃.²⁵ Similarly, two peaks located at around 162.3 and 164.0 eV depicted in Fig. 4f correlate strongly to the binding energies of S 2p_3/2 and S 2p_1/2, further clarifying the existence of 2H phase.³⁵ Meanwhile, additional peaks with a shift to lower binding energies at 161.6 and 162.8 eV have been observed, which once again demonstrates the presence of metallic 1T phase in Ti₃C₂/TiO₂/1T-MoS₂ composites.³⁶ The high-resolution Mo 3d and S 2p XPS spectra of the composite suggest the predominance of 1T phase (∼84% based on the area ratio) over the 2H counterpart (Fig. 4e and f). All of the above results prove the successful generation of 1T-MoS₂ at the surface of Ti₃C₂/TiO₂ composites.

To further analyze the textural properties of the Ti₃C₂/TiO₂/1T-MoS₂ composites, the isotherms and the pore size distributions are studied by N₂ adsorption–desorption measurements (Fig. 5). All the samples possess type IV isotherms with H3 hysteresis loops, suggesting the existence of mesopores.^37,38 The pore-size distribution curve (Fig. 5a–d inset) of the Ti₃C₂/TiO₂/1T-MoS₂ composites with different MoS₂ loading amounts reveals that the size of the dominant mesopores ranges from 3 to 30 nm, and the pore size is mainly distributed about 4 nm. The BET surface area of the as-prepared Ti₃C₂/TiO₂/1T-MoS₂-15%, as shown in Table S1 (ESI†), reveals a higher surface area of 22.138 m² g⁻¹, compared with Ti₃C₂ MXene, Ti₃C₂/TiO₂/1T-MoS₂ composites with other MoS₂ loading amounts, and 1T-MoS₂. A larger surface area is favorable in photocatalysis because it can possibly offer more adsorption and active sites, and thus the photocatalytic activity is improved.^39,40

To investigate the optical absorptivity, the UV-vis diffuse refectance spectra (DRS) of the samples were measured. As shown in Fig. 6, TiO₂ NSs (blue curves) possess an obvious absorption in the UV region, and no optical absorption in the visible region, which is consistent with antase TiO₂.⁴¹ Ti₃C₂ MXene (black curves) shows an enhanced visible absorption due to the black color nature (Fig. S6, ESI†). The bulk 1T-MoS₂ is found to exhibit a monotonic increase in the optical absorption as the wavelength decreased from 800 to 400 nm (red curves), indicating the characteristics of a nonplasmonic metallic nanostructure of predominant 1T phases.⁴² Compared with TiO₂ NSs, the Ti₃C₂/TiO₂/1T-MoS₂ composites present a significant red shift in the absorption edge and enhanced absorption in the visible-light region, which can be ascribed to the optical absorption of Ti₃C₂ MXene and 1T-MoS₂. Besides, the Ti₃C₂/TiO₂/1T-MoS₂ composites display stronger optical absorption with increasing MoS₂ content. And the derived band gap energy (E_g) from the Kubelka–Munk function (in which (F(R)hν)² is plotted versus the photon energy (hν)) is 3.15 eV for TiO₂ (Fig. S7, ESI†).

The introduction of Ti₃C₂ MXene and 1T-MoS₂ in Ti₃C₂/TiO₂/1T-MoS₂ composites would be expected to influence the separation of photoinduced charge carriers, which could be characterized by steady-state and time-resolved PL spectroscopy (Fig. 7).⁴³ In general, a lower steady-state PL intensity indicates a longer lifetime and higher separation efficiency of photo-generated electron–hole pairs, leading to higher photocatalytic activity.⁴⁴ As illustrated in Fig. 7a, it shows that TiO₂ NSs possess a fast recombination of photoinduced charge carriers. When Ti₃C₂ MXene and 1T-MoS₂ are loaded, the PL is drastically quenched (Fig. 7a). Obviously, the recombination of photo-generated e⁻–h⁺ pairs in TiO₂ can be suppressed by transferring electrons to Ti₃C₂ and 1T-MoS₂ as the electron acceptor. An increased lifetime of charge carriers can be observed by incorporating Ti₃C₂ MXene and 1T-MoS₂ (Fig. 7b), which is in agreement with the steady-state PL measurement results. The intensity-average lifetime (τ) of TiO₂ NSs is 0.1138 ns, which is much lower than that of Ti₃C₂/TiO₂/1T-MoS₂ composites (1.4238 ns). It is believed that the increased lifetime of charge carriers in Ti₃C₂/TiO₂/1T-MoS₂ composites is associated with improved electron transport and charge-separation efficiency.

To gain deep insights into the charge transfer behaviors in the composites, transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements were carried out. The photocurrent responses of the samples were prompted by several on–off cycles of an intermittent 300 W Xe arc lamp with an AM-1.5 filter irradiation in 0.5 M Na₂SO₄ aqueous solution (Fig. 8a and Fig. S8, ESI†). As shown in Fig. S8 (ESI†), pure 1T-MoS₂ and Ti₃C₂ exhibit no photocurrent responses on each illumination, which is consistent with their metallic properties. The TiO₂ NSs and Ti₃C₂/TiO₂/1T-MoS₂ composites exhibit reversible photocurrent responses on each illumination. The photocurrent intensity of the Ti₃C₂/TiO₂/1T-MoS₂ composites is much higher than that of pure TiO₂ NSs, which can be ascribed to the enhanced absorption of visible light as well as improved charge separation and migration efficiency resulting from the addition of 1T-MoS₂ nanopatches inducing the improved light absorption, as well as Ti₃C₂ and metallic 1T-MoS₂ as a co-catalyst more efficiently accepting photo-generated electrons from TiO₂.²⁷ The EIS measurement is also an effective approach to study the charge transfer process. The Ti₃C₂/TiO₂/1T-MoS₂ composites (Fig. 8b) exhibit a smaller arc radius compared with TiO₂ NSs under light irradiation, suggesting that the Ti₃C₂/TiO₂/1T-MoS₂ composite has smaller charge transfer resistance, ultimately resulting in a high separation and transfer efficiency of photogenerated charge carriers.^45–47

The photocatalytic performance of Ti₃C₂/TiO₂/1T-MoS₂ composites was evaluated using H₂ evolution under simulated sunlight irradiation in an aqueous acetone solution at room temperature (Fig. 9a and b). Control experiments (Fig. S9, ESI†) indicate that no appreciable H₂ production is detected in the absence of either irradiation or photocatalysts, suggesting that H₂ is produced by a photocatalytic reaction of the photocatalyst. As shown in Fig. 9, the pure TiO₂ NSs show a poor photocatalytic H₂ evolution activity (74 μmol g⁻¹ h⁻¹), arising from its poor charge separation and transfer capability (Fig. S10a, ESI†). As expected, the photocatalytic H₂ evolution rate of the Ti₃C₂/TiO₂ composites is significantly enhanced (898 μmol g⁻¹ h⁻¹), nearly 12 times that of pure TiO₂ NSs. Conductive Ti₃C₂ MXene can serve as a co-catalyst for the efficient capture of photo-generated electrons (Fig. S10b, ESI†), which is crucial to the photocatalytic H₂ production activity of the heterostructure. Notably, after the assembly of 1T-MoS₂ nanopatches, a significantly improved photocatalytic performance is observed, with the highest photocatalytic H₂ evolution rate of 9738 μmol g⁻¹ h⁻¹ occurring for the Ti₃C₂/TiO₂/1T-MoS₂ composites (15 wt% MoS₂), which is nearly 132 and 11 times higher than that of pure TiO₂ NSs and Ti₃C₂/TiO₂ composites. Moreover, it is worth pointing out that the Ti₃C₂/TiO₂/1T-MoS₂ composite (10 wt% MoS₂), which is composed of a low amount of MoS₂, displays a lower photocatalytic performance compared with the Ti₃C₂/TiO₂/1T-MoS₂ composite (15 wt% MoS₂), because of the relatively poor light absorption (Fig. 6). Furthermore, with the increasing content of MoS₂ from 15 wt% to 30 wt%, a decrease in the photocatalytic activity of Ti₃C₂/TiO₂/1T-MoS₂ composites is discovered. This is because excess black MoS₂ nanopatches absorb photons in the photocatalytic system, probably blocking light from entering into TiO₂, which could be called a “shielding effect”.⁴⁸

The stability and durability of a photocatalyst are crucial for practical photocatalytic H₂ production applications. We then further conducted a cycling test of Ti₃C₂/TiO₂/1T-MoS₂ composites (15 wt% MoS₂) for 24 h under the same conditions. As observed in Fig. S11 (ESI†), no obvious decrease in H₂ evolution rate is observed within the three cycles in 24 h. Besides that, the SEM images (Fig. S12, ESI†) and XRD patterns (Fig. S13, ESI†) of the Ti₃C₂/TiO₂/1T-MoS₂ composites after 3 cycles show no obvious difference between those of the fresh sample. This further demonstrates the stability of the Ti₃C₂/TiO₂/1T-MoS₂ composites. We further evaluated the apparent quantum efficiency (AQE) of the samples under a 300 W Xe arc lamp with a CUT420 optical filter. Table S2 (ESI†) shows the comparison of AQE values over TiO₂ NSs, Ti₃C₂/TiO₂ composites and Ti₃C₂/TiO₂/1T-MoS₂ composites with different MoS₂ loading amounts (10 wt%, 15 wt%, 25 wt%, and 30 wt%): 0.0529% (TiO₂ NSs) < 0.644% (Ti₃C₂/TiO₂ composites) < 4.17% (Ti₃C₂/TiO₂/1T-MoS₂-30 wt%) < 5.07% (Ti₃C₂/TiO₂/1T-MoS₂-25 wt%) < 6.24% (Ti₃C₂/TiO₂/1T-MoS₂-10 wt%) < 6.86% (Ti₃C₂/TiO₂/1T-MoS₂-15 wt%). By comparing with the other catalysts (Table S3, ESI†), such as MoS₂/TiO₂ and 1T-MoS₂/TiO₂, our synthetic Ti₃C₂/TiO₂/1T-MoS₂ composites present the best photocatalytic H₂ evolution activity, wherein the presence of Ti₃C₂ MXene and 1T phase MoS₂ shows excellent electronic conductivity, resulting in an increase in efficiency for electron transfer. The results further demonstrate that Ti₃C₂/TiO₂/1T-MoS₂ composites can serve as promising photocatalysts for H₂ evolution from water.

To investigate the effect of 1T phase versus the 2H counterpart on the photocatalytic activity, the Ti₃C₂/TiO₂/2H-MoS₂ composites were prepared and compared. The EDS mapping and TEM images of the Ti₃C₂/TiO₂/2H-MoS₂ composites (15 wt% MoS₂) are shown in Fig. S14 and S15 (ESI†). Ultra-thin 2H-MoS₂ NSs are mainly distributed on the surface of TiO₂ NSs and Ti₃C₂ MXene. For the S 2p XPS spectra of Ti₃C₂/TiO₂/2H-MoS₂ composites and Ti₃C₂/TiO₂/1T-MoS₂ composites (Fig. 10a), the peaks of 1T-MoS₂ shifted to lower binding energies (about 0.8 eV), as compared with that of 2H-MoS₂. The AQE and photocatalytic H₂ production activity of the Ti₃C₂/TiO₂/1T-MoS₂ composites are also significantly improved (Fig. 10b and c). Specifically, with the same MoS₂ loading of 15%, the H₂ evolution rate of the Ti₃C₂/TiO₂/1T-MoS₂ composites reaches 9738 μmol g⁻¹ h⁻¹ (Fig. 10d), which is 1.5 times higher than that of Ti₃C₂/TiO₂/2H-MoS₂ composites (6425 μmol g⁻¹ h⁻¹). This suggests that 2H-MoS₂ can not act well as an electron transport channel and efficient active sites. The photocatalytic H₂ production activity constrained on the edge sites of 2H-MoS₂ also partly contributes to the decrease of performance. However, 1T-MoS₂ can serve as a co-catalyst for the efficient capture of photo-generated electrons and active sites for H₂ production.

A possible photocatalytic mechanism for H₂ generation with the Ti₃C₂/TiO₂/1T-MoS₂ composites is proposed and illustrated in Scheme 2. Under light irradiation, TiO₂ NSs can be excited to generate electron–hole pairs. Most of the photo-generated electrons in the conduction band (CB) of TiO₂ can immediately transfer to Ti₃C₂ MXene and 1T-MoS₂ due to the metal characteristic. As a photoelectron receiver, Ti₃C₂ MXene and 1T-MoS₂ act as active sites for H₂ evolution.^26,49 Meanwhile, the holes in the VB of TiO₂ are consumed by the sacrificial reagents. Thus, the photoinduced electron–hole pairs can effectively be separated and transferred in the presence of double co-catalysts Ti₃C₂ and 1T-MoS₂.

4. Conclusions

In summary, we have successfully developed an efficient Ti₃C₂/TiO₂/1T-MoS₂ composite photocatalyst by a two-step hydrothermal method. The design concept is the in situ growth of TiO₂ NSs on the Ti₃C₂ MXene and then 1T phase MoS₂ nanopatches are uniformly distributed on the Ti₃C₂/TiO₂ composites. The obtained Ti₃C₂/TiO₂/1T-MoS₂ composite exhibits high photocatalytic H₂ evolution activity with a rate as high as 9738 μmol g⁻¹ h⁻¹ for the sample with 15 wt% MoS₂ loading, which is nearly 132, 11 and 1.5 times higher than that of pure TiO₂ NSs, Ti₃C₂/TiO₂ composites and Ti₃C₂/TiO₂/1T-MoS₂ composites. The high H₂ production activity of Ti₃C₂/TiO₂/1T-MoS₂ composites can be attributed to the following: (1) the introduction of MoS₂ nanopatches induces enhanced specific surface area and more active sites on the edges, and the photogenerated electron will react at these active sites; (2) both Ti₃C₂ MXene and 1T-MoS₂ possess extraordinary conductivity, which greatly enhances the electron transfer ability and thus achieves highly-efficient spatial charge separation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful for funding from the National Natural Science Foundation of China (No. 51872173 and 51772167), the Taishan Scholarship of Young Scholars (no. tsqn201812068), the Taishan Scholarship of Climbing Plan (no. tspd20161006), the Key Research and Development Program of Shandong Province (no. 2018GGX102028) and the Natural Science Foundation of Shandong Province (no. ZR2017JL020).

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns of Ti3AlC2, Ti3C2 MXene and Ti3C2/TiO2 composites; SEM images of bulk 1T phase MoS2; energy dispersive X-ray spectroscopy (EDS) mapping images of bulk 1T phase MoS2; TEM images of Ti3C2/TiO2/1T-MoS2 composites (15 wt% MoS2); SEM images of Ti3C2/TiO2/1T-MoS2 composites with different MoS2 loading amounts of 10 wt%, 25 wt% and 30 wt%; the BET surface area of Ti3C2 MXene, Ti3C2/TiO2/1T-MoS2 composites with different MoS2 loading amounts (10 wt%, 15 wt%, 25 wt% and 30 wt% MoS2) and the mixed phase MoS2; optical photographs of TiO2 NSs, Ti3C2 MXene, Ti3C2/TiO2/1T-MoS2 composites with different MoS2 loading amounts (10 wt%, 15 wt%, 25 wt% and 30 wt% MoS2) and 1T-MoS2; band gap values of TiO2; photocatalytic H2 production of control experiments in the absence of irradiation and photocatalysts; schematic photocatalytic mechanisms for TiO2 NSs and Ti3C2/TiO2 composites; stability and recyclability of the Ti3C2/TiO2/1T-MoS2 composites (15 wt% MoS2); SEM images of Ti3C2/TiO2/1T-MoS2 composites (15 wt% MoS2) after 3 cycles; XRD patterns of Ti3C2/TiO2/1T-MoS2 composites (15 wt% MoS2) after 3 cycles; SEM images and EDS mapping images of Ti3C2/TiO2/2H-MoS2 composites (15 wt% MoS2). See DOI: 10.1039/c9qm00608g

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