2D/2D/2D heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity

Authors: Yujie Li, Zhaohua Yin, Guanrui Ji, Zhangqian Liang, Yanjun Xue, Yichen Guo, Jian Tian, Xinzhen Wang, Hongzhi Cui

DOI: https://doi.org/10.1016/j.apcatb.2019.01.051

School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China. Email: jiantian@sdust.edu.cn (J. Tian), xzwang@sdust.edu.cn (X. Wang), cuihongzhi1965@163.com (H. Cui)

Highlights

TiO2 nanosheets are in situ grown on highly conductive Ti3C2 MXene.

MoS2 nanosheets are deposited on (101) facets of TiO2 nanosheets with mainly exposed high-active (001) facets.

The Ti3C2@TiO2@MoS2 photocatalyst is highly active for water splitting to produce hydrogen at 6425.297 μmol g−1 h −1 .

The Ti3C2 MXene acts as a source of titanium and a pathway transferring photo-generated electrons.

The MoS2 on the (101) facets of TiO2 can capture photogenerated electrons of (101) facets and act as reduction active sites.

Abstract:

Exposing the highly active facets and hybridizing the photocatalyst with appropriate cocatalysts with right placement have been regarded as a powerful approach to high performance photocatalysts. Herein, TiO2 nanosheets (NSs) are in situ grown on highly conductive Ti3C2 MXene and then MoS2 NSs are deposited on the (101) facets of TiO2 NSs with mainly exposed high-active (001) facets through a two-step hydrothermal method. And a unique 2D-2D-2D structure of Ti3C2@TiO2@MoS2 composite is achieved. With an optimized MoS2 loading amounts (15 wt%), the Ti3C2@TiO2@MoS2 composite shows a remarkable enhancement in the photocatalytic H2 evolution reaction compared with Ti3C2@TiO2 composite and TiO2 NS. It also shows good stability under the reaction condition. This arises from: (i) the in situ growth of TiO2 NSs construct strong interfacial contact with excellent electronic conductivity of Ti3C2, which facilitates the separation of carriers; (ii) the coexposed (101) and (001) facets can form a surface heterojunction within single TiO2 NS, which is beneficial for the transfer and separation of charge carriers; and (iii) the MoS2 NSs are deposited on the electrons-rich (101) facets of TiO2 NSs, which not only effectively reduces the charge carriers recombination rate by capturing photoelectrons, but also makes TiO2 NSs expose more highly active (001) facets to afford high-efficiency photogeneration of electron-hole pairs.

1. Introduction

To solve the ever increasing global demands for energy crisis and environmental protection, the development of clean energy has attracted great attention recently [1, 2]. Hydrogen evolution via photocatalytic water splitting is a promising approach to alleviating the energy and environmental crisis [3-5]. TiO2 is one of the most important semiconductor-based photocatalysts and has been widely studied for photocatalytic H2 production [6, 7]. However, the application of TiO2 is restricted due to the rapid recombination of photo-generated electron-hole pairs, so massive scientific endeavors (for instance, surface modification, cocatalyst loading, impurity doping and heterojunction construction) have been devoted to promote photo-induced electrons and holes separation over the TiO2 -based photocatalysts [8, 9]. Since Yang et al. found that the 47% exposed active (001) facets of TiO2 nanosheets (NSs) showed the highest photocatalytic activity [10], morphology engineering of TiO2 that can attain exposed high-active crystal facets is a promising approach to enhance the photocatalytic activity [11]. TiO2 has three different exposed facets: (001), (100) and (101). The order of the average surface energies of crystal facets follows 0.90 J/m2 for (001) > 0.53 J/m2 for (100) > 0.44 J/m2 for (101) [12]. Besides, TiO2 NSs with exposed (101) and (001) facets can form a surface heterojunction, which is beneficial for the transfer of photogenerated electrons and holes to (101) and (001) facets [13].

Herein, we propose a new design of TiO2-based photocatalyst with dual co-catalysts, in which both two-dimensional (2D) Ti3C2 MXene and MoS2 NSs act as electron mediators and reduction cocatalysts. Recently, MXenes, a new family of 2D transition metal carbides, have been successfully synthesized by selective exfoliation of ternary carbides, nitrides, or carbonitrides with a general formula of Mn+1Xn, where M represents transition metals (such as Sc, Ti, Ta, Cr, Mo, etc.) and X is carbon and/or nitrogen [14-16]. The density functional theory (DFT) calculations indicate that MXenes exhibit metallic conductivity, which have been explored for catalysis, energy storage and conversion [17, 18]. The experimental characterizations have also been demonstrated crucial to be the effective electrocatalysts in the hydrogen evolution reaction (HER) [19, 20], indicating that MXenes may be a good cocatalyst for photocatalytic H2. These unique electronic properties suggest the potential of MXene as a promoter for the transfer and accumulation of charge carrier, which can cause a negative shift and alignment of the Fermi level and improve the photocatalytic water splitting [21, 22]. Molybdenum disulfide (MoS2) is also a typical 2D layered transition metal sulfide with a structure composed of three stacked atom layers, composed of Mo atoms sandwiched between two layers of hexagonally close-packed sulfur atoms (SMo-S) [23]. MoS2 can be used as a promising cost-effective cocatalysts and shows superior photocatalytic H2 production activity, due that the unsaturated S atoms on the exposed edges of MoS2 act as active sites and have a strong affinity to H+ in solution [24]. More importantly, the unique 2D structure of MXenes and MoS2 with rich surface groups favors the construction of 2D-2D-2D heterojunction based on MXenes, MoS2 and 2D semiconductors, establishing strong interface contact between cocatalyst and photocatalyst. Such 2D-2D-2D heterojunction with intimate contact can greatly improve the transfer and separation of photo-induced charge carriers across the heterojunction interface due to intense physical and electronic coupling effects [25].

In this paper, the design motif is the grafting of in situ growth of TiO2 NSs on the Ti3C2 MXene and MoS2 NSs on the (101) facets of TiO2 NSs with mainly exposed highactive (001) facets via a two-step hydrothermal method. In this design, electrons and holes are photogenerated on the (001) surfaces of TiO2, the most active surface for photocatalysis. Besides, the high electronic conductivity of Ti3C2 MXene acts as a source of titanium and a pathway transferring photo-generated electrons to enhance the separation efficiency of charges. Furthermore, the photogenerated electrons and holes can be respectively transferred onto (101) and (001) facets of TiO2 due to the presence of surface heterojunction. Moreover, the deposited of MoS2 NSs on the (101) facets of TiO2 NSs can capture photogenerated electrons of (101) facets and act as reduction active sites. Through this design, the photogenerated charge carriers are effectively separated, and the activity of photocatalytic H2 production is improved, showing the promise of dual cocatalyst strategy for photocatalysis.

2. Experimental procedure

2.1. Materials

The Ti3AlC2 powder was provided by Hello Nano Technology Co., Ltd., Changchun. Hydrofluoric acid (HF, 40 wt%), hydrochloric acid (HCl), sodium tetrafluoroborate (NaBF4), sodium molybdate dihydrate (Na2MoO4·2H2O), and thiourea (CN2H4S) were purchased from Sinopharm. All chemicals used were of analytical reagent grade.

2.2. Preparation of Ti3C2 MXene

Typically, 1 g Ti3AlC2 powders were slowly added to 200 mL 40 wt% HF solution. Then the reaction mixtures were stirred for 72 h at room temperature. After that, the mixed solutions were washed with deionized water to neutral, and the powders were collected after discarding the supernatant. Finally, Ti3C2 MXene was dried in vacuum oven at 50 ◦C for 12 h.

2.3. Preparation of Ti3C2@TiO2 composites and TiO2 nanosheets (NSs)

To synthesis of Ti3C2@TiO2 composites, 100 mg Ti3C2 MXene and 0.1M NaBF4 were added into 15 ml of 1.0 M HCl. After the solution had been stirred for 30 min, it was transferred into a 25 mL Teflon-lined stainless-steel autoclave, which was hydrothermally treated at 160 oC 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 vacuum oven at 60 oC for 12 h.

For comparison, pure TiO2 NSs were also synthesized with Ti(OBu)4 as the source of titanium. In the synthesis process, 0.5 mL Ti(OBu)4 and 0.15 g CFs were slowly dropped into HCl (9 mL, 5 M) solution. After the solution was stirred for 30 min, then 0.2 mL HF was added to the mixed solution. And the solutions were transferred into Teflon-lined stainless-steel autoclaves with a total volume of 25 mL. The hydrothermal synthesis was conducted at 180 °C for 4h in an electric oven. The obtained CF@TiO2 composites were heated at 800°C for 2h to remove the CF templates.

2.4. Preparation of Ti3C2@TiO2@MoS2 composites

To synthesis of Ti3C2@TiO2@MoS2 composites, 15 mg Na2MoO4·2H2O and 30 mg CN2H4S were dissolved in 20mL deionized water to form a transparent solution. Then, 60 mg Ti3C2@TiO2 composites was added into the above solution and stirred to form the suspension. The suspension was transferred into a 25 mL Teflon-lined stainlesssteel autoclave, which was hydrothermally treated at 200 oC for 24 h. After naturally cooling down to room temperature, the reaction solution was collected by vacuum filtration, and the resulting Ti3C2@TiO2@MoS2 composites (15 wt% MoS2) were washed with distilled water several times, and dried in vacuum oven at 50 oC for 12 h. Similarity, by changing the mass of Na2MoO4·2H2O (7.5 mg, 23 mg, and 30 mg) and CN2H4S (15 mg, 46 mg, 60 mg), Ti3C2@TiO2@MoS2 composites with other MoS2 loading amounts (10 wt%, 25 wt%, 30 wt%) were obtained, respectively.

2.5. Characterization

The phase constituents of the synthesized products were analyzed by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) with Cu Kα (λ =0.15406 nm) radiation source. The nanostructure and surface characteristic of the products were  observed with a high-resolution scanning electron microscope (FESEM, FEI Nova Nanosem 450, USA) with an energy-dispersive X-ray spectroscopy (EDS). High transmission electron microscopy (HRTEM) was carried out with 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 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 200–800 nm range and with BaSO4 as the reflectance standard. The surface area was measured using the Brunauer Emmett Teller (BET) method asexamined on a Micromeritics ASAP2020 nitrogen adsorption–desorption apparatus. The photoluminescence (PL) spectra were acquired at room temperature with 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. 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 experiments were performed in aqueous acetone with dissolved sacrificial reagent (TEOA), and suspended with 10 mg of catalysts powder following ultrasonic dispersion for 10 min. Then the reaction solution was evacuated several times to remove air and the reaction temperature of reactant solution was maintained at 25°C. The amount of the 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 under the same light source. The focused intensity on the flask was ca. 513 mW/cm2 . The AQE was calculated according to the following equations:

Transient photocurrent responses (PEC) and electrochemical impedance spectroscopy (EIS) curves were measured 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 Pt wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. 5 mg as-synthesized samples were mixed with 0.5 mL ethanol and terpilenol. After sonicated for 5min, 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

Scheme 1 describes the synthesis strategy of the hybrid structures of the Ti3C2@TiO2@MoS2 composites. Firstly, The Ti3AlC2 ceramics (MAX phase) are firstly etched by HF to remove the Al layers [26]. Secondly, the in-situ growth of TiO2 NSs with preferentially exposed (001) facets from the layered Ti3C2 MXene are got through the hydrothermal oxidation of Ti3C2 in the assistance of HCl and NaBF4. The formation of TiO2 NSs with abundant high-activity (001) surfaces can improve the photocatalytic activity. Subsequently, the obtained Ti3C2@TiO2 composites are immersed in a clear solution with Na2MoO4·2H2O and CN2H4S at 200 oC for 24 h in order to introduce MoS2 cocatalyst. Ultimately, MoS2 NSs are deposited onto (101) facets of TiO2 NSs to form the ternary Ti3C2@TiO2@MoS2 composites.

The etching of Ti3AlC2 and the formation of Ti3C2 and Ti3C2@TiO2 composites were clearly revealed by XRD analysis. The MAX phase exhibits intense peaks, which can be assigned to Ti3AlC2 according to previous reports [26, 27]. After Ti3AlC2 was etched by HF, as expected, the most intense (104) diffraction peak in Ti3AlC2 pattern located at 39o completely disappeared (Fig. 1a). The (002) at 9.58o and (004) at 19.17o of Ti3AlC2 were broadened and shifted toward lower angle side, indicating the transformation from Ti3AlC2 to Ti3C2 MXene [28]. The hydrothermal oxidation of Ti3C2 caused the growth of TiO2 NSs across the layered Ti3C2 sheets, evidenced by the SEM image in Scheme 1 and the emergence of XRD reflections from anatase TiO2 (Fig. 1a, JCPDS No. 21-1272). For Ti3C2@TiO2@MoS2 composites with different MoS2 loading amounts (Fig. 1b), all the diffraction peaks could be well-indexed to the Ti3C2 or anatase TiO2. No signals assignable to MoS2 are detectable. This can be explained by the fact that MoS2 is ultra-thin and is high dispersed on the Ti3C2@TiO2@MoS2 composites.

The morphology of Ti3C2 MXene, Ti3C2@TiO2 composites, and Ti3C2@TiO2@MoS2 composites with different MoS2 loading amounts were characterized by scanning electron microscopy (SEM) (Fig. 2, S1 and S2). As shown in Fig. 2a, as the Al atoms between layers are removed after HF etching, the formed loose accordion-shape layered structure reveals the typical MXene morphology. After hydrothermal oxidation of Ti3C2 MXene, a lot of nanosheets are sideling inserted across ACCEPTED MANUSCRIPT the stack of Ti3C2 MXene to form 2D-2D Ti3C2@TiO2 composites (Fig. 2b). These nanosheets are anatase TiO2 with exposed (001) surfaces which are in situ grown due to the introduction of HCl and NaBF4 during the delamination of Ti3C2. Under the acidic hydrothermal conditions, the layered Ti3C2 provides Ti sources for the growth of TiO2 [29]. Assisted by the directing reagent NaBF4, the formation of high-energy (001) facets is enhanced during the sequential crystal growth, because of the lower energy of (101) planes adsorbing F- [25, 30-32]. The TiO2 NSs with the most active (001) facets can be homogeneously distributed around the layered Ti3C2 to provide improved accessibility to light and reactants. More importantly, the 2D Ti3C2 sheets traverse TiO2 nanocrystals at the most active (001) facets, as shown in Fig. 2b. The intimate contact between these two phases might facilitate the separation of charge carriers photogenerated on the (001) surfaces, thereby improving the photocatalytic activity.