Heterojunction Design between WSe₂ Nanosheets and TiO₂ for Efficient Photocatalytic Hydrogen Generation

Abstract

Design and fabrication of efficient and stable photocatalysts are critically required for practical applications of solar water splitting. Herein, a series of WSe₂/TiO₂ nanocomposites were constructed through a facile mechanical grinding method, and all of the nanocomposites exhibited boosted photocatalytic hydrogen evolution. It was discovered that the enhanced photocatalytic performance was attributed to the efficient electron transfer from TiO₂ to WSe₂ and the abundant active sites provided by WSe₂ nanosheets. Moreover, the intimate heterojunction between WSe₂ nanosheets and TiO₂ favors the interfacial charge separation. As a result, a highest hydrogen evolution rate of 2.28 mmol/g·h, 114 times higher than pristine TiO₂, was obtained when the weight ratio of WSe₂/(WSe₂ + TiO₂) was adjusted to be 20%. The designed WSe₂/TiO₂ heterojunctions can be regarded as a promising photocatalysts for high-throughput hydrogen production.

1. Introduction

Green and sustainable energy is urgently required for the rapid development of human beings. Hydrogen, as a type of green and renewable energy carrier, is considered to be an ideal substitute for fossil fuels in the future [1,2,3]. Solar energy conversion to hydrogen via photocatalysts holds great promise for hydrogen generation owing to the advantages of being environmentally friendly and high product purity [4,5,6]. Numerous semiconductors, including metal oxides, sulfides, phosphides, and polymers, are applied to photocatalytic hydrogen generation, and impressive achievements have been made in the past decades [7,8,9,10]. TiO₂, as a prototype photocatalyst, has been widely studied for solar water splitting owing to its chemical stability, nontoxicity, and low cost [11,12,13,14,15]. For example, (001)-facet-exposed ultrathin anatase TiO₂ nanosheets was designed for hydrogen generation [16]. However, single TiO₂ exhibits poor photocatalytic efficiency owing to its inability to absorb visible light, fast charge carrier recombination, and a slow interfacial hydrogen-production reaction [17,18,19]. Many approaches, such as doping, sensitizing, and surface hydrogeneration, are devoted to improving the photocatalytic performance and loading cocatalysts has been demonstrated as an effective way to boost the photocatalytic activity of TiO₂ [20,21,22].

Noble metals such as Pt, Au, Pd, etc. are widely applied as cocatalysts to improve the photocatalytic efficiency of TiO₂ owing to their low overpotentials and superior conductivity [23,24,25]. For example, Pt decorated anatase-TiO₂/H-rutile TiO₂ heterophase homojunctions displayed excellent photocatalytic performance with an apparent quantum yield of 45.6% at 365 nm [15]. However, given that the scalable application of photocatalytic water splitting, low-cost and earth-abundant cocatalysts are critically welcomed [26,27,28]. Two-dimensional layer transition metal dichalcogenides–known as MX₂ where M and X are transition metal and chalcogen, respectively–have attracted increasing attention in the field of photocatalytic water splitting due to their fascinating intrinsic features [29,30,31,32]. For example, MoS₂-tipped CdS nanorods were prepared for hydrogen generation by Du and co-workers [33]. Interestingly, our previous work suggested that WSe₂ nanosheets were an efficient cocatalyst for photocatalytic hydrogen generation and accelerated charge separation are expected in WSe₂-semiconductor photosystems [34,35]. Reddy et al. loaded layer-dependent WSe₂ nanosheets on CdS nanorods and the designed CdS/WSe₂ heterojunction displayed enhanced photocatalytic hydrogen generation [36]. In addition, WSe₂-PANI nanohybrids were achieved via a sonication-assisted solution method, and they showed stable and photosensitive hydrogen evolution [37]. Compared with other transition metal dichalcogenides, WSe₂ nanosheets owned superior electrical conductivity and abundant active sites [38,39]. In addition, WSe₂ semiconductors displayed excellent photostability as well [40]. All these made it a promising cocatalysts for hydrogen evolution. However, WSe₂ decorated TiO₂ photocatalysts have been rarely reported for water splitting so far and they might exhibit exciting photocatalytic performance if they were coupled together. In addition, many approaches have been applied to construct cocatalysts–semiconductor photosystems and the mechanical grinding method is regarded as the simplest and scalable way to achieve efficient and stable heterojunctions, which is favorable for practical application of photocatalytic water splitting [41,42,43,44].

Inspired by these findings, in this work, we applied a facile mechanical grinding method to decorate TiO₂ nanoparticles with WSe₂ nanosheets. Transmission electron microscope, X-ray diffraction, X-ray photoelectron spectrometer, UV-Vis, Photoluminescence, and time-resolved Photoluminescence were applied to study the morphology, crystal structure, composition, and optical properties of the prepared nanocomposites. It was discovered that as-prepared WSe₂/TiO₂ heterojunctions displayed boosted photocatalytic activity and a highest hydrogen generation rate of 2.28 mmol/g·h, which was 114 times higher than pristine TiO₂ and was achieved with an apparent quantum yield of 43.8% at 365 nm. The improved photocatalytic performance was attributed to the efficient charge separation and abundant active sites. This work paves the way for exploitation of TiO₂-based catalysts for photocatalytic water splitting.

2. Results and Discussions

The synthesize procedure was schematically illustrated in Figure S1. WSe₂ nanosheets were prepared via a hot injection method and then they was mixed with TiO₂ nanoparticles. The powders were mechanically ground for 30 min and the micro-structures of as-prepared products were studied using TEM and HR-TEM. The obtained TiO₂ catalysts displayed heterogeneous nanoparticles as Figure 1a shows. The lattice spacing of 0.35 nm, which was attributed to the (101) plane of TiO₂, was recorded in HR-TEM images. The prepared WSe₂ displayed nanosheet morphology (about ~20 nm) with only a few layers, and a lattice spacing of 0.24 nm was characterized. After mechanical grinding, WSe₂ nanosheets could be characterized on the surface of TiO₂ nanoparticles. In addition, the corresponding lattice spacing of 0.35 nm (TiO₂) and 0.65 nm (WSe₂) could be clearly observed as Figure 1f shows. Furthermore, STEM and EDX elemental mappings were carried out to obtain the spatial distribution of WSe₂ and TiO₂ in the prepared nanocomposites. It was discovered that WSe₂ nanosheets were homogeneously distributed over the area of TiO₂ nanoparticles with some aggregations. Thus, we can conclude that WSe₂/TiO₂ heterostructures with an intimate contact were successfully obtained by the facile mechanical grinding method.

The crystal structure and components of prepared samples were investigated by X-ray diffraction patterns. As Figure 2 shows, the typical diffraction peaks observed at 25.31°, 37.95°, 48.08°, 53.86°, 55.02°, 62.74°, 68.79°, 70.34°, and 74.93° were corresponding to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes of anatase TiO₂ (JCPDS No.: 01-071-1166). The prepared WSe₂ nanosheets displayed broad and low diffraction peaks centered at about 13.6°, 32.1°, 37.8°, 47.20°, and 56.60°, which were attributed to the (002), (101), (103), (105), and (008) planes of hexagonal WSe₂ (JCPDS No.: 00-038-1388). In the case of the designed TW-x heterojunctions, both the diffraction peaks of TiO₂ and WSe₂ could be characterized with no clear shift, indicating that the mechanical grinding method did not alter the crystal structure of TiO₂ and WSe₂. Therefore, it can be concluded that the prepared samples were composed of TiO₂ and Wse₂ semiconductors.

In order to investigate the surface chemical composition and valence states of prepared catalysts, XPS studies were conducted, and the spectra are shown in Figure 3. Doublet peaks centered at 458.33 eV (Ti 2p_3/2) and 464.03 eV (Ti 2p_1/2) were observed, suggesting a predominant chemical state of Ti⁴⁺ in TiO₂ [12,45,46]. The O 1s spectrum contained two peaks centered at 529.56 eV and 531.41 eV. The former one was attributed to the lattice oxygen in TiO₂, while the latter was assigned to surface hydroxyl (O–H) groups [47,48,49]. Furthermore, the W 4f spectrum could be deconvoluted into four peaks appearing at 31.52 eV, 33.62 eV, 35.32 eV, and 37.50 eV. The former doublet peaks were ascribed to the +4 chemical state of the W element, which was in accordance with previous reports [50,51]. The latter two small peaks were attributed to the +6 chemical state of the W element, suggesting an oxidation of WSe₂ during the synthesis procedure [34,52]. The appearance of peak at 54.10 eV in Se 3d spectrum suggested the Se²⁻ in WSe₂ nanosheet [53,54]. Comparatively, the binding energy of Ti and O elements was slightly higher than pristine TiO₂, while the binding energy of W and Se was lower than single WSe₂ when they were coupled together. The shifted binding energies reveal the change of electron density around the atoms, and the results suggested TiO₂ was an electron donor and that WSe₂ was an electron acceptor in the prepared TW-x samples [6,55]. Moreover, it can be seen that the W⁶⁺ was apparently increased, indicating a strong interaction of the W–O bond in prepared TW-2 heterojunctions, which would be favorable to the interfacial charge transfer from TiO₂ to WSe₂.

The light absorption properties of TiO₂, WSe₂ nanosheets, and TW-x samples were recorded by UV-Vis absorption spectra. It was discovered that the absorption edge of TiO₂ was centered at about 390 nm with weak absorption in the visible light region as Figure 4 shows. The bandgap of TiO₂ was determined to be 3.21 eV using the Kubelka–Munk method (Figure S4), which is similar to previous reports [15,24]. A strong light absorption spectrum in the entire visible region with two broad bands at about ~528 nm and ~730 nm was recorded for the prepared WSe₂ nanosheets (Figure S3), indicating the 2H phase of WSe₂ [39,40]. The absorption edge of the prepared TW-x catalysts exhibited a slightly red shift and gradually enhanced light absorbance in the visible region, which was ascribed to the light absorption of WSe₂ cocatalysts in TW-x. As expected, the enhanced light absorption properties would benefit the photocatalytic performance of WSe₂/TiO₂ heterojunctions.

To unveil the photo-induced charge carriers transfer and recombination, photoluminescence spectra were analyzed. Commonly, the intensity of PL emission represents the utilization of photo-induced electron holes, and a strong emission band means severe charge carrier recombination [56,57]. As Figure 5a shows, steady-state PL emission of TiO₂ nanoparticles exhibited a broad and strong band centered at about ~450 nm under the excitation of 375 nm, which was associated with electron–hole recombination near the band-edges of TiO₂ [15,58]. The PL emission was obviously quenched while WSe₂/TiO₂ heterostructures were constructed via the mechanical grinding method, indicating that the recombination of photo-induced charges was largely inhibited [58,59,60]. In order to reveal the charge separation in depth, time-resolved PL spectra was carried out and a biexponential function fitting was applied to analyze the decay kinetics. The average lifetime of TiO₂ was calculated to be 1.48 ns according to the following Equation (1). Comparatively, the average lifetime was shorter for TW-2 (1.27 ns) than that of TiO₂ alone. The fast decay indicated that photo-induced electrons could quickly transfer from TiO₂ to WSe₂ for water reduction according to the above analysis.

$$t_A=\frac{A_1{\tau }_1^2+A_2{\tau }_2^2}{A_1{\tau }_1+A_2{\tau }_2}$$

(1)

Mott–Schottky analysis was applied to determine the flat band potentials of TiO₂ and WSe₂ nanosheets. It was known that the flat band potential was approximated to the conduction band position for n type semiconductors while the flat band potential was approximated to valence band position for p type semiconductors [9,36]. As Figure 6 shows, the Mott–Schottky curves of TiO₂ and WSe₂ exhibited positive slopes, which suggested an n-type semiconductor for the obtained TiO₂ and WSe₂. The conduction band positions were determined to be −0.131 and −0.016 V vs. NHE for TiO₂ and WSe₂ according to the flat band potentials based on Mott–Schottky curves. The higher conduction band position of TiO₂ than WSe₂ indicated that photo-induced electrons could transfer from TiO₂ to WSe₂ nanosheets for water reduction during the photocatalytic reaction.

The photocatalytic performance of the as-obtained samples was assessed by adding 10 mg of catalysts into 30 mL of an aqueous solution containing 6 mL of methanol as the hole scavenger. As Figure 7 shown, the pristine TiO₂ displayed a low hydrogen evolution activity with a rate of 0.02 mmol/g·h. No hydrogen evolution could be detected for WSe₂ nanosheets because of the strong binding energies of photo-induced excitons [61]. Notably, TW-x nanocomposites, with a coupled WSe₂ nanosheet with TiO₂ nanoparticle, all exhibited the evidently boosted photocatalytic hydrogen generation activity. The remarkable promotion effect of TW-x for photocatalytic performance should benefit from the efficient charge transfer from TiO₂ to WSe₂ and the abundant active site provide by WSe₂. However, the excessive amount of WSe₂ nanosheets led to a decreased hydrogen evolution rate. On one hand, the photoactive sites of TiO₂ might be blocked by the agglomerated WSe₂. On the other hand, the excessive amount of WSe₂ would encroach on the light absorption of TiO₂ nanoparticles and reduce the photoexcitation of TiO₂ because the bare WSe₂ shows no hydrogen evolution activity [35]. Consequently, the optimized TW-2 catalysts showed the highest hydrogen evolution rate of 2.28 mmol/g·h, 114 times higher than that of TiO₂ alone. The corresponding apparent quantum yield was estimated to be 43.8% at 365 nm, which was substantially greater than that of most reported cocatalysts-TiO₂ photocatalysts. Furthermore, the photocatalytic stability was also investigated, and no significant decrease was observed during the four cycles tests. The superior photocatalytic performance and excellent stability made WSe₂/TiO₂ nanocomposites a promising material for photocatalytic water splitting.

Based on the above results, the photocatalytic mechanism of the prepared TW-x heterostructures was proposed and schematically elucidated in Figure 8 and Equations (2)–(6). Under simulated solar light irradiation, both the TiO₂ and WSe₂ were excited. WSe₂ nanosheets served as electron acceptors to extract photo-induced electrons from TiO₂ and then reduced water to hydrogen. The holes on TiO₂ would be consumed by the added methanol. Moreover, the strong interaction and intimate contact between TiO₂ and WSe₂ was favorable for accelerating the interfacial charge transfer and separation [49,62]. As a result, the photocatalytic efficiency was greatly enhanced compared with TiO₂ alone. It is well known that the morphology of TiO₂, phase of WSe₂ nanosheets, and contact manners between TiO₂ and WSe₂ would have an important effect on the photocatalytic efficiency of WSe₂/TiO₂ heterojunctions. This will be investigated in the future.

TiO₂ + hν → TiO₂(e) + TiO₂(h)

(2)

WSe₂ + hν → WSe₂(e) + WSe₂(h)

(3)

TiO₂(e) + TiO₂(h) + WSe₂ → TiO₂ + TiO₂(h) + WSe₂(e)

(4)

2H⁺ + WSe₂(e) → WSe₂ + H₂

(5)

TiO₂(h) + WSe₂(h) + CH₃OH → TiO₂ + WSe₂ + CO₂ + H₂O

(6)

3. Materials and Methods

3.1. Chemicals

All chemicals were used as received without further purification. W(CO)₆, Ph₂Se₂ (diphenyl diselenide), and TOPO (trioctylphosphine oxide) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Hexane, methanol, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). TiO₂ was purchased from Nanjing XFNANO Materials Tech Co. LTD. The water used in the synthesis and reaction was deionized with a resistivity of 18.2 MΩ·cm.

3.2. Sample Preparation

WSe₂ semiconductors. WSe₂ semiconductors were synthesized via a hot-injection method according to our previous reports with slight modifications [34]. Specifically, 48 mmol of TOPO and 0.5 mmol of W(CO)₆ were added into a 100 mL three-neck flask. The flask was degassed at 120 °C for 1 h. Then, it was heated to 330 °C under N₂ flow to dissolve the chemicals. At this point, 2 mmol of TOPO and 1 mmol of Ph₂Se₂ were added into a vial and heated to dissolve the chemicals completely by using a dryer. The Se-TOPO solution was rapidly injected into the reaction solution, and the flask was kept at 330 °C for 1 h. After cooling to room temperature, the precipitates were centrifuged with hexane and ethanol 5 times, and WSe₂ nanosheets were obtained by drying in vacuum at 60 °C for 8 h.

WSe₂/TiO₂ catalysts. The WSe₂/TiO₂ catalysts were prepared via a facial mechanical grinding method. Namely, a certain amount of the prepared WSe₂ and obtained TiO₂ was mixed and ground gently for 30 min at room temperature, and then the powder was collected. The weight ratio of WSe₂ and (WSe₂ + TiO₂) was adjusted to be 1:10, 2:10, 3:10, and 4:10, and the samples were labeled TW-1, TW-2, TW-3, and TW-4, respectively.

3.3. Sample Characterization

The morphology of the prepared samples was characterized on a field-emission transmission electron microscope (FE-TEM, JEOL JEM 2100 microscope, 300 kV) equipped with energy dispersing X-ray spectroscopy (EDX). The acceleration voltage was 300 kV. The crystal structure of the studied samples was characterized using a Rigaku Smartlab-9 kW X-ray diffractometer with Cu Kα radiation working at 40 kV/40 mA. X-ray photoelectron spectroscopy spectra (XPS) were collected using Escalab Xi+ X-ray photoelectron spectroscopy (Thermo Fisher Scientific, Waltham, MA, USA), and the binding energies were calibrated using adventitious carbon (C 1s peak, 284.8 eV) as a reference. The light absorption features of the materials were recorded on a Perkin-Elmer Lambda 950 spectrophotometer with BaSO₄ as reference. Steady state and time-resolved photoluminescence spectra at room temperature were recorded on a PicoQuant FT-300 and FT-100 fluorescence spectrophotometer under 375 nm irradiation. Mott–Schottky analysis was carried out on a CHI 760E electrochemical workstation using a standard three-electrode cell. The reference electrode and the counter electrode were Ag/AgCl (saturated KCl solution) and Pt plate. A total of 1 mg of the samples was dispersed in a mixed solution containing 250 μL of water, 250 μL of ethanol, and 10 μL of Nafion solution. It was ultrasonicated for 30 min, and 3.5 μL of the suspension was coated on the glassy carbon rotating disk electrode. After being dried at room temperature for 12 h, the prepared electrode was used as working electrode. N₂-saturated Na₂SO₄ solution (0.5 M, pH = 6.8) was used as the electrolyte.

3.4. Photocatalytic Hydrogen Generation

The photocatalytic hydrogen evolution reaction was conducted using a homemade side-irradiation Pyrex glass reactor. The temperature for the photocatalytic reaction was maintained at 35 °C by thermostatic circulating water. The side irradiation area was about 7.06 cm². A PLS-SXE 300+/UV Xe lamp (Perfect Light) was employed as light source (the spectrum was shown in Figure S5) and it worked at 12 mA. A total of 10 mg of the as-prepared photocatalysts was added into 30 mL of deionized water containing 6 mL of methanol as the sacrificial reagent. The solution was constantly stirred and sonicated for 15 min and then bubbled with N₂ to eliminate the air completely before light irradiation. The amount of hydrogen evolution was measured by a gas chromatograph (Bruker GC-450, Nax Zeolite column, TCD detector, and N₂ carrier). Apparent quantum yield (AQY) was defined by the following equation using a 365 nm band-pass filter, in which N_H was the number of evolved H₂ molecules and N_P was the number of incident photons [8,42].

$$\mathrm{AQY}(\%)=\frac{2N_H}{N_P}\times 100\%$$

(7)

4. Conclusions

To summarize, we demonstrated that mechanical grinding is a facile way to construct WSe₂/TiO₂ nanocomposites and that loading of WSe₂ can not only inhibit electron–hole recombination of TiO₂ but also provide active sites for water reduction. With the optimization of the weight ratio of WSe₂/(WSe₂ + TiO₂), the prepared photocatalysts displayed the highest hydrogen evolution rate of 2.28 mmol/g·h, which corresponds to an apparent quantum yield of 43.8% at 420 nm. These findings shed light on rational design and construction of noble-metal-free cocatalysts decorated TiO₂ semiconductors for water splitting.