MoSe₂-WS₂ Nanostructure for an Efficient Hydrogen Generation under White Light LED Irradiation

Abstract

In this work, MoSe₂-WS₂ nanocomposites consisting of WS₂ nanoparticles covered with few MoSe₂ nanosheets were successfully developed via an easy hydrothermal synthesis method. Their nanostructure and photocatalytic hydrogen evolution (PHE) performance are investigated by a series of characterization techniques. The PHE rate of MoSe₂-WS₂ is evaluated under the white light LED irradiation. Under LED illumination, the highest PHE of MoSe₂-WS₂ nanocomposite is 1600.2 µmol g⁻¹ h⁻¹. When compared with pristine WS₂, the MoSe₂-WS₂ nanostructures demonstrated improved PHE rate, which is 10-fold higher than that of the pristine one. This work suggests that MoSe₂-WS₂ could be a promising photocatalyst candidate and might stimulate the further studies of other layered materials for energy conversion and storage.

1. Introduction

Presently, energy shortage and environmental pollution are the major problems [1,2,3]. Intuitively, the current photocatalytic hydrogen (H₂) generation from water through semiconductor nanostructures is an ideal approach, and this technique is considered as a potential, economical way to overcome the issue of energy shortage [4,5,6,7]. Over the years, there has been substantial experimental data calibrated on transition-metal (TM) oxide-based photocatalysts for energy-storage applications [6,7,8] with minimal environmental pollution. Though these materials display excellent photocatalytic activity just beneath the UV illumination, they exhibit poor activity in visible light due to their wide bandgap. In order to stimulate photocatalytic activity in visible light, recent attempts on anion-doped TM oxides-, sulfides-, and selenides-based nanostructures have been reported [9,10]. However, these TMs are unstable, making them inert towards commercial applications. Henceforth, there is a challenge for the researchers to identify and develop potential, stable and economically novel photocatalysts for H₂ evolution.

In lieu of this, the two-dimensional (2D) MoSe₂ semiconductor is a compelling potential catalyst for next-generation hydrogen evolution due to its narrow bandgap (1.2 eV), specific surface area and more metallic nature, which prompts higher electrical conductivity that is more favorable to hydrogen evolution reactions. However, there are few reports on the preparation of MoSe₂-based composites through diverse approaches, which display an enhanced photocatalytic activity (PCA) when compared with the bare MoSe₂ [11]. In spite of this enhanced photocatalyst, there is provision to promote further the PCA in the water-splitting process by resolving the photo corrosion and faster charge recombination problems. In this context, an inclination towards the development of a nanostructure heterojunction, which can improve the visible light absorption, further altering the surface defects followed by reduction in the surface recombination. This may enhance the capability of water splitting both quantitatively and qualitatively. Recently, 2D-layered WS₂ has enticed significant attention in the field of energy and photocatalysis applications because of its promising characteristics towards photocatalytic performance [11,12]. There is a decent work available in the scientific literature on WS₂-based composites, which displayed significant enhancement in the PCA compared to pristine WS₂ [12,13,14]. Various methods have been reported in the literature to process these composites [15,16]. However, in addition to this, the interest of an advanced study combining two transition-metal dichalcogenides (TMDs) such as MoSe₂ and WS₂ possessing alike hexagonal structure, enables to simulation of the heterojunction formation.

The MoX₂/WX₂ (X = S, Se) heterostructures, which exhibit a type-II band configuration, are considered to be efficient systems for the production of optoelectronic and photovoltaic devices, in which the free electrons and holes are spontaneously isolated [17]. The lattice constants of MoX₂ and WX₂ being very close to each other indicate that MoX₂-WX₂ heterostructures can have minimum structural defects. According to the theoretical calculations, the conduction band minimum (CBM) of the WS₂ is only slightly lower than that of the MoSe₂ [18]. In 2018, Jin et al. reviewed experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures [19]. The dominant interlayer electron transport relaxation pathway in WS₂/MoSe₂ heterostructures proving the strong interlayer dipole−dipole interaction was identified by Kozawa et al. [20]. Photoluminescence excitation spectroscopy evidenced the fast interlayer energy transfer across the van der Waals interface of the MoSe₂/WS₂ heterostructures. Meng et al. investigated the ultrafast carrier transfer, which can efficiently separate electrons and holes in the intralayer excitons in a MoSe₂/WS₂ heterostructure [21]. Based on first-principles ab initio calculations, Amin et al. investigated the band structure of WS₂/MoSe₂ and showed the indirect electron transition semiconducting behavior [17]. The distinctive interactions between stacked layer are essential for solar cells because the confinement of electrons by MoSe₂ and holes by WS₂, leading to a spontaneous charge separation when excitons scatter to the WS₂/MoSe₂ junction [22]. Ceballos et al. evidenced highly efficient and anomalous charge transfer in the van der Waals MoSe₂/WS₂/MoS₂ trilayer semiconductors [23]. Wu et al. fabricated a WS₂/MoSe₂ hybrid semiconductor catalyst (WS₂ mass fraction of 20%) with a p-n heterojunction, which is composed of spherical WS₂ particles (2 µm diameter) mixed with flower-like granularMoSe₂ (50 nm in size). This product exhibits good photocatalytic performance with a photocurrent density of 35 μA cm⁻² at −0.6 V vs. SCE [24].

Moreover, experimental and theoretical investigations have shown that the unsaturated X-edges of TMDs are favorable to hydrogen evolution electrocatalytic activity [25]. Band offsets and heterostructures of monolayer and few-layer TMDs were calculated using the vacuum level as reference, and a simple model was proposed to explain the observed chemical trends [26]. This concept has been applied to the MoS₂/WS₂ and MoS₂/WSe₂ heterostructures, which exhibited robust electrocatalytic properties [27]. Recently, Vikraman et al. have constructed a MoSe₂/WS₂ heterojunction model by a chemical/physical process and have intricately examined its hydrogen evolution reaction performances [28]. The MoSe₂/WS₂ heterostructure displayed excellent electrocatalytic hydrogen evolution behavior with a 75 mV overpotential to drive a 10 mA·cm⁻² current density, a 60 mV·dec⁻¹. Such a type of structure is developed as an active electrode for hydrogen evolution to replace the nonabundant Pt.

To the best of our knowledge, both photocatalytic hydrogen evolution (PHE) and electrocatalytic hydrogen evolution (EHE) performance of the van der Waals two-layer MoSe₂-WS₂ heterostructure has not been studied yet. Only a mixture of WS₂ and MoSe₂ particles was considered. The main goal of the present work was to synthesize a MoSe₂-WS₂ nanocomposite through hydrothermal process and further elucidate its photocatalytic and electrocatalytic properties for hydrogen evolution. As anticipated, the MoSe₂-WS₂ nanostructure displayed better PHE rate that of pristine WS₂ sample. It is demonstrated that the PHE of MoSe₂-WS₂ is 10-fold higher than that of WS₂.

2. Materials and Methods

The raw materials sodium tungstate dehydrates, polyethylene glycol, and thioacetamide were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ammonium molybdate tetrahydrate was from Junsei Chemical Co. (Tokyo, Japan). Polyvinylpyrrolidone (PVP), Selenium powder and anhydrous oxalic acid were provided from Daejung (Daejeon, Korea) and used as received without further treatment.

2.1. Preparation of WS₂

The synthesis procedure for WS₂ followed that of Yuan et al. [29] with modification. Typically, 1.1 g of sodium tungstate dehydrate, 2.3 g of thioacetamide, and 5.5 mL of polyethylene glycol were dissolved in 25 mL of deionized water followed by stirring to obtain a homogeneous solution. Then, the pH of the solution was adjusted to 2 by adding the oxalic acid and transferred to sealed autoclave-reactor solution that was maintained at 210 °C for 48 h. After cooling, the precipitates were collected by centrifuge at 9000 rpm, washed with water and ethanol for several times, and dried in a vacuum oven for 100 °C for 48 h and then annealed at 600 °C for 1 h under Ar to obtain the final product.

2.2. Preparation of MoSe₂-WS₂

Typically, 0.025 mol of NH₄Mo₇O₂₄∙4H₂O and 0.12 g of PVP were dissolved in 12 mL of ammonium hydroxide solution under constant stirring. On the other hand, 0.05 mol of Se powder was dispersed in hydrazine hydrate under vigorous stirring for 30 min. Then, this solution was added dropwise to the above resultant solution at room temperature, which has a nominal Mo/Se molar ratio of 1:2. After that, 25 mg of WS₂ was added into resultant solution stirred for about 1 h. The resulting homogenous solution was irradiated by hydrothermal reaction at 220 °C for 48 h. The as-obtained precipitate (MoSe₂-WS₂) was collected by centrifugation at 9000 rpm, washed with a mixture of deionized water and ethanol, and dried in a vacuum oven at 130 °C for 48 h. To make a comparison, pure MoSe₂ was also synthesized from the similar procedure without WS₂.

2.3. Instruments

X-ray diffraction (XRD) patterns were analyzed on Shimadzu 6100 X-ray diffractometer (Shimadzu Corp., Tokyo, Japan) equipped with a CuK_α X-ray source (λ = 1.5406 Å). The morphology analysis was performed by transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) using a FEI Tecnai G2 F20 (FEI Company, Hillsboro, OR, USA). Initially, the synthesized MoSe₂-WS₂ nanocomposites were dispersed in ethanol and sonicated for 10 min, then the dispersed MoSe₂-WS₂ was dropped on a TEM grid (200 mesh Cu grid) and dried at 90 °C overnight.

The X-ray photoelectron spectra (XPS) were recorded using a Thermo Scientific k-α surface analyzer (ThermoFisher Scientific, Alachua, FL, USA), equipped with a AlK_α X-ray source (hν = 1486 eV). The ultraviolet–visible (UV-Vis) spectroscopy was carried out on a Cary 5000 (ThermoFisher Scientific, Alachua, FL, USA) in the 200–800 nm wavelength range using a transmittance and reflectance equipment. The infrared spectra were recorded in the spectral range of 400–4000 cm⁻¹ using an Avatar 370 Fourier transform infrared spectrometer (FTIR, Thermo Nicolet, Alachua, FL, USA). The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface area and pore size distribution measured by an ASAP 2420 surface area analyzer degassed for 1 h at 150 °C (Micromeritics, Norcross, GA, USA). High-resolution transmission electron microscope (HRTEM Titan G2 Chemi STEM Cs probe, FEI Company, Hillsboro, OR, USA) with EDS windowless (Super-X model) and physisorption analyzer (BET-Micromeritics, 3FLEX, Norcross, GA, USA) were used for both morphology and surface area studies.

2.4. Photocatalytic Hydrogen Tests

The photoreaction was conducted in a quartz top-irradiation reactor tightened with silicone rubber “Septa”. Then, 5 mg of photocatalysts and 5 mL of Na₂S/Na₂SO₃ were dispersed into 45 mL of water by sonication for 40 min. The reactor was evacuated for 45 min and N₂-bubbled for 30 min prior to irradiation. A light-emitting diode (LED, white light irradiation) was facilitated by 100 W lamp without an optical filter. The induced gases were measured using a gas chromatograph (YL-6500 with TCD detector) equipped with a 5-Å molecular sieve column under He carrier gas.

2.5. Electrochemical Measurements

Electrochemical characterizations were performed with a Bio-Logic (SP-200) workstation with a standard three-electrode cell using Ag/AgCl (3.5 mol L⁻¹ KCl solution), a platinum coil and fluorine-doped tin oxide (FTO) glass as reference, counter and working electrodes, respectively. The working electrode was prepared as follows: 5 mg of catalysts and 80 µL of 5 wt.% Nafion solution were dispersed in 1 mL of a solution of deionized water and ethanol, i.e., 4:1 in volume ratio, and then stirred for 45 min. A total of 5 µL of the resultant solution was drop-casted on the FTO glass. The working electrode was dried at 75 °C for 1 h. Photocurrent measurements and electronchemical impedance spectroscopy analyses were carried out in 0.5 mol L⁻¹ Na₂SO₄ solution with purging nitrogen gas.

3. Results and Discussion

3.1. XRD Analysis

The phase purity and crystal structure of the as-prepared nanostructures were analyzed through bulk X-ray diffraction. Figure 1 displays the XRD patterns of WS₂, MoSe₂ pristine materials and that of the MoSe₂-WS₂ nanocomposite in the 2θ range 10–80°. The diffraction peaks at 13.7°, 28.21°, 34.11°, 39.49°, and 58.89° corresponding to the (002), (004), (100), (103), and (008) crystal planes, respectively, match well with the hexagonal structure of WS₂ (JCPDS Card # 08-0237) [29,30,31].

The as-synthesized MoSe₂ similarly displays a hexagonal structure and XRD patterns are in good agreement with literature data (JCPDS Card # 29-0914) [32]. No other impurity peaks were detected. The peak positions of the as-prepared MoSe₂-WS₂ sample are identical to those of WS₂ showing that layered MoSe₂ does not substitute into the WS₂ lattice. Although, an obvious decrease in the peak intensity is noticed, while no peaks related to MoSe₂ are recognized due to its good dispersion and its low loading content.

3.2. Morphology and Elemental Analysis

HRTEM was employed to probe the morphology of the as-synthesized MoSe₂-WS₂ nanocomposite (Figure 2a–d). As depicted, the typical sheet-like structure of MoSe₂ can be distinctly noticed. Some MoSe₂ slabs with the characteristic layered structure are remarkable on the surface of WS₂ particles (see Figure 2b–d). In addition, the HAADF-STEM elemental mapping images shown in Figure 2e–i clearly depict that W (yellow), Mo (purple), S (dark blue), and Se (light blue) are uniformly distributed on the surface of nanostructures. These findings indicate that the MoSe₂-WS₂ nanocomposite heterostructures were effectively prepared.

3.3. XPS Analysis

Figure 3 presents the XPS magnified scans of (a) W, (b) Mo, (c) S and (d) Se of the MoSe₂-WS₂ sample and confirms the existence of MoSe₂ and WS₂. Figure 3a depicts the W 4f peaks at binding energies of 32.1, 34.1, 36.2, and 28.3 eV corresponding to the W⁴⁺ 4f_7/2, W⁴⁺ 4f_5/2, W⁶⁺ 4f_7/2, and W⁶⁺ 4f_5/2 core level, respectively, which indicates the existence of W⁴⁺, W⁶⁺ species in the synthesized MoSe₂-WS₂ [31]. The higher binding energy of 36.2 eV signifies the existence of the +6 oxidation state of W element, with a very low peak intensity specifying that only a small portion of W⁴⁺ cations is oxidized when the sample is in contact with the air [32]. Figure 3b shows the Mo 3d contribution with peaks located at 228.0 and 232.6 eV, belonging to the Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2 core level of MoSe₂. The valence states of the Mo element corresponding to the double peaks at 231.2 and 235.1 eV are assigned to Mo⁶⁺ 3d_5/2 and Mo⁶⁺ 3d_3/2 core levels specifying the oxidized states of Mo due to air contact [16,17]. The S 2p peak can be deconvoluted into two peaks at 163.6 and 161.8 eV (Figure 3c) corresponding to the S 2p_1/2 and S 2p_3/2 core levels with doublet positioned at 166.98 and 160.97 eV, which can be attributed to the Se 3p_1/2 and Se 3p_1/2 core levels [33]. Finally, Figure 3d displays the binding energies of Se 3d_5/2 at 54.3 eV and Se 3d_3/2 at 55.2 eV, corresponding to the divalent (−2) state of Se. All these values match well with those reported in previous investigations and indicate the anticipated chemical states of Mo⁴⁺, W⁴⁺, Se^2-, and S²⁻ in the MoSe₂-WS₂ nanostructure [31].

3.4. FTIR Analysis

Figure 4 shows the FTIR spectra of WS₂, MoSe₂ and MoSe₂-WS₂ samples recorded in the range 400–4000 cm⁻¹. In the FTIR spectrum of WS₂, the bands pointed at 743, 1739 and 2943 cm⁻¹ are attributed to the W-S bending and stretching vibrations, and hydroxyl groups related to O-H bonds, respectively [33,34]. In the MoSe₂ spectrum, the bands located at 523, 1218, 1327, 1744 and 3013 cm⁻¹ could be ascribed to the stretching vibration of Mo-Se and hydroxyl groups of MoSe₂ [35]. For the MoSe₂-WS₂, the band intensities corresponding to the hydroxyl group decrease significantly, and the infrared band of the Se-W-S bonds shifts from 743 to 622 cm⁻¹, which suggests the interface interaction between the MoSe₂ and WS₂.

3.5. Surface Area Analysis

Figure 5a shows the N₂ adsorption–desorption isotherms of pristine WS₂ and MoSe₂-WS₂ samples. Both samples show the standard type-IV isotherms with obvious hysteresis loops, endorsing the presence of hierarchical mesoporosity [36]. On the other hand, the sharp uptakes at high pressure 0.55–1.0 P/Po of WS₂ and MoSe₂-WS₂ indicate the presence of mesopores in both samples, which is in good agreement with the results of pore size distribution curves as shown in Figure 5b. Data are listed in

Table 1. BET analysis results of pristine WS₂ and MoSe₂-WS₂ nanostructures.

Sample	BET Surface Area (m2 g−1)	Pore Volume (cm3 g−1)	Pore Size (nm)
WS2	106.2	0.214	10.7(2)
MoSe2	35.07	0.11	8.3(2)
MoSe2-WS2	132.8	0.268	10.6(5)

. The MoSe₂-WS₂ nanocomposite has a high specific surface area of 132.79 m² g⁻¹, which is slightly higher than pristine WS₂ (106.23 m² g⁻¹). On the other hand, both samples exhibit almost the same pore size: 10.6(5) nm for MoSe₂-WS₂ and 10.7(2) nm for WS₂.

3.6. Optical Studies

To evaluate the electronic and optoelectronic properties of the MoSe₂-WS₂ nanocomposite, the optical bandgap has been determined using the UV-Vis spectroscopy in the vicinity of the fundamental transition, i.e., wavelength range of 200–800 nm, and compared with that of pristine WS₂ and MoSe₂ samples. The UV–Vis transmittance and reflectance spectra of samples are depicted in Figure 6a–f. Analyses of material bandgaps are presented in Figure 6g–i.

The optical absorption coefficient (α) is calculated taking into consideration the transmittance (T%) and reflectance (R%) of the sample using the standard equation [37]:

$$\alpha =\frac{1}{d} ln\left[\frac{{\left(1-R\right)}^2}{T}\right],$$

(1)

where d is the film thickness. The bandgap E_g of samples is calculated by the Tauc’s formula [38]:

αhν = C (E_g − hν)ⁿ,

(2)

where h is the Plank constant, ν is the photon energy, E_g is the average bandgap of the material, C is a constant depending on several intrinsic properties of the material, i.e., the effective mass of the electron and hole and the material refractive index, and n is the transition-type dependent. It is equal to 1/2, 3/2, 2 and 3 for the direct-allowed, direct-forbidden, indirect-allowed and indirect-forbidden transitions, respectively [39]. The average bandgap was calculated from the intercept of the linear part of the (αhν)² vs. hν plot on x-axis for all samples as shown in Figure 6g–i.

The evaluated optical bandgap values (±0.02 eV) are 1.35, 1.16 and 1.24 eV for WS₂, MoSe₂ and MoSe₂-WS₂, respectively. The knowledge of the bandgap is also useful for the phase identification of the samples. It has been demonstrated that 2D TMDs possess sizable bandgaps around 1–2 eV [40]. According to Wang et al. [41], the bandgap of TMDs has the following electronic properties: (i) the bulk has an indirect bandgap of 1.3 eV for WS₂ and 1.1 eV for MoSe₂ and (ii) the monolayer has a direct bandgap of 2.1 eV for WS₂ and 1.5 eV for MoSe₂. Thus, the bandgap of the MoSe₂-WS₂ nanocomposite, in which WS₂ is the core of the sample (bulk) and MoSe₂ is formed by few nanosheets covering the bulk is in good agreement with previous reports [36]. Both MoSe₂ and WS₂ layered compounds are expected to undergo a similar indirect-to-direct bandgap evolution with decreasing layer numbers [42]. Indeed, the optical properties of the samples critically depend on the physical properties, and these variations in the energy gap here are consistent with those of the crystallite size.

3.7. Photocatalytic Performance

The photocatalytic performances of the as-synthesized products were estimated through water splitting for H₂ evolution. Figure 7a displays the comparison of hydrogen evolution performance of MoSe₂, WS₂ and MoSe₂-WS₂ nanostructures for 5 h under LED irradiation. The achieved H₂ production is 600.1, 150.2 and 1600.2 µmol g⁻¹ h⁻¹ for MoSe₂, WS₂, and MoSe₂-WS₂, respectively. The obtained H₂ generation for MoSe₂-WS₂ is almost 3 and 10 times higher than that of bare MoSe₂ and WS₂, respectively. It is vital to consider that all the photocatalytic experimental results in this work were employed in an identical condition. The enhanced photocatalytic activity of MoSe₂-WS₂ nanostructure has the following characteristics: (i) it is due to the formation of heterostructures, which promote the separation of the photocarriers a well as their recombination, (ii) the coupling of MoSe₂ and WS₂ with diverse energy levels could engender the enhancement of the separation and transfer of photoinduced charges, which leads the photoinduced carriers to involve in the photo-redox reactions, and (iii) the MoSe₂ acts as a cocatalyst for generation of H₂. On the other hand, after a few hours, the volume of the reactor became insufficient to accommodate a large amount of H₂, which increased solubility of H₂ in the solution, suppressing the production of H₂ in solution, reducing the evolution of H₂ and total scavenger consumption, reducing the ability of H₂ evolution [43]. Thus, it needs to provide additional number of scavengers in the continuous time-on-stream activity of the catalysts. Figure 7b illustrates the continuous stability of MoSe₂-WS₂ nanostructure over 16 h. After 6 h of continuing H₂ production, it was observed that changing the scavenger and adding scavengers would increase H₂ production, then reduced H₂ production at 11 h and adding the scavenger again at 12 h, the increase in H₂ activity was maintained steadily until 16 h on MoSe₂-WS₂ nanostructure.

These results suggest that the H₂ evolution of the MoSe₂-WS₂ nanostructure harbors good stability up to three cycles. Therefore, MoSe₂-WS₂ nanostructures are promising and potential candidates for practical photocatalytic H₂ evolution. Note that the as-prepared MoSe₂-WS₂ nanocomposite shows better H₂ production yield (10 times greater than that bare WS₂) than that of the WS₂-MoS₂ heterostructure (6.5 times greater than that bare WS₂) [43].

3.8. Electrochemical Performance

To probe the enriched mechanism of the photocatalytic H₂ production, the excitation and transfer of photogenerated charge carriers of the as-synthesized products were studied. The photocurrent-time (PI) as well as the electrochemical impedance spectroscopy (EIS) were employed. The acquired PI profiles are displayed in Figure 8a, which portrays the periodic on-off photocurrent response of all the prepared products under visible light illumination.

Identically, the photocurrent response of MoSe₂-WS₂ is higher than that of bare WS₂, which is consistent with the photocatalytic activity. Measurements carried out in 0.5 mol L⁻¹ Na₂SO₄ solution show a photocurrent density of 0.75 µA cm⁻² for the MoSe₂-WS₂ heterostructure against 0.38 µA cm⁻² for bulk WS₂. This result demonstrated that the MoSe₂-WS₂ nanocomposite has brawny ability in transferring and generating the photo-excited charge carrier under the visible light illumination. On the other hand, recombination process and charge transfer of photo-induced electrons as well as holes can be displayed via EIS (Figure 8b). Compared with that of bare MoSe₂ and WS₂ materials, the Nyquist plot evidences clearly a depressed semicircle for MoSe₂-WS₂, which designates a fast charge-carrier transfer rate in the MoSe₂-WS₂. Hence, it stipulates that the effective transfer of photo-induced electrons among MoSe₂ and WS₂ enables the electron–hole separation.

3.9. Proposed Photocatalytic H₂ Mechanism

In pursuant with the results discussed above, a mechanism for water reduction by the use of MoSe₂-WS₂ nanostructure-based catalyst is proposed as illustrated by the scheme in Figure 9. The electron–hole pairs are usually generated on MoSe₂-WS₂ under the visible light illumination. In the development of visible-light-driven devices, the nanostructured MoSe₂-WS₂ composite is a so-called Z-scheme photocatalyst [44,45,46,47]. In this system, electrons are excited from the valence band (VB) to the conduction band (CB) of WS₂ upon visible light illumination, then transferred to VB of MoSe₂ and finally reach to CB of MoSe₂ during generation of H₂. The photocatalytic H₂ mechanism has been described as follows: in the H₂ evolution setup, the photoreduction of proton by CB electrons as:

2H⁺ + 2e⁻ → H₂,

(3)

and the oxidation of an electron donor (D) by VB holes yields an electron acceptor (A) as:

D + nh⁺ → A.

(4)

Thus, the water-splitting reaction occurs when a cycle of D and A redox pairs is achieved. Meanwhile, the photogenerated holes on the VB of WS₂ can reduce the scavengers, which considerably decrease the recombination process of electron–hole pairs and lead to improved stability of the MoSe₂-WS₂ nanostructure and the enhancement of hydrogen production rate. Consequently, the photocatalytic H₂ evolution activity is promoted for WS₂ modified with few MoSe₂ monolayers, which promotes a significant increase of photogenerated charge–hole separation efficiency [48].

4. Conclusions

In summary, we have investigated the structural, vibrational and electronic characteristics of the van der Waals interrelated MoSe₂-WS₂ nanostructure used as photocatalysts for H₂ production. We found that:

(1)

A cost-effective and simple chemical methodology was handled to fabricate the MoSe₂-WS₂ nanostructure using an easy one-step hydrothermal process without high-temperature annealing;

(2)

The MoSe₂-WS₂ nanocomposite has a high specific surface area of 132.79 m² g⁻¹ and a pore size of 10.6 nm, which are values favorable for an efficient photocatalytic activity. For the MoSe₂-WS₂ heterostructure, in which WS₂ is the core of the sample (bulk) and MoSe₂ is formed by a few nanosheets covering the bulk, the evaluated optical bandgap is 1.24 eV;

(3)

The use of MoSe₂ and WS₂ sheets with similar lattice parameters allows the fabrication of heterostructure without matching restriction;

(4)

The coupling of MoSe₂ with WS₂ led to a considerably enhanced surface area and higher photoinduced charge separation. It results a remarkably improved photocatalytic H₂ production, which was observed by photocurrent measurements and EIS studies;

(5)

Therefore, the resultant MoSe₂-WS₂ is a capable photocatalyst for the H₂ energy applications. Under LED light irradiation, the MoSe₂-WS₂ nanostructure demonstrated enhanced photocatalytic hydrogen evolution, which is approximately 3- and 10-times higher compare to bare MoSe₂ and WS₂. MoSe₂-WS₂ nanocomposite exhibits a high PHE rate of 1600 µmol g⁻¹ h⁻¹;

(6)

The photocatalytic activity of the MoSe₂-WS₂ nanostructure can be explained by Z-scheme carrier transfer pathways, which favor the production of reactive species;

(7)

The MoSe₂/WS₂ heterostructure displayed excellent electrocatalytic hydrogen evolution behavior. The demonstrated hydrogen evolution reaction performance attests to the capability of this nanohybrid to replace the high-cost and scarce Pt and will spark hybrid-based research toward the various future energy sectors. The edges of MoSe₂ and WS₂ present an ideal hydrogen-binding energy, which makes them promising to replace the Pt-based electrocatalysts for hydrogen generation. In addition, the MoSe₂/WS₂ heterostructure could be a new cost-effective electrode replacing carbon supported Pt and Pt/Ru electrodes in fuel cells.