One-pot in-situ hydrothermal synthesis of VSe2/MoSe2 nanocomposite for enhanced hydrogen evolution reaction

M. Alahmadi; Sami Ben Aoun

doi:10.1016/j.arabjc.2023.104846

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Original article

07 2023

:16;

104846

doi:

10.1016/j.arabjc.2023.104846

One-pot in-situ hydrothermal synthesis of VSe₂/MoSe₂ nanocomposite for enhanced hydrogen evolution reaction

M. Alahmadi^⁎, Sami Ben Aoun^⁎

Chemistry Department, College of Science, Taibah University, P.O. Box 30002, Al-Madinah Al-Munawarah, Saudi Arabia

⁎Corresponding authors. mnahmadi@taibahu.edu.sa (M. Alahmadi), sbenaoun@taibahu.edu.sa (Sami Ben Aoun)

Received: 2023-1-10, Accepted: 2023-3-21,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Hydrogen is the kind of pure, renewable energy that is required for the world to begin relying less and less on the fossil fuels that it currently consumes. The production of hydrogen by electrocatalytic water splitting is deemed to be preferable to the consumption of fossil fuels for the generation of clean and reliable energy. A catalyst's ability to catalyze is significantly influenced by the number of exposed active sites in the catalyst. Thus, the purpose of the current work is to offer a simple and inexpensive strategy to synthesize a double non-noble metal catalyst of VSe₂/MoSe₂ via one-step hydrothermal synthesis for catalytic Hydrogen evolution reaction (HER). The strong and unique orientation interaction between the nanosheet-like structure of VSe₂ and the nanoflower-like structure of MoSe₂ in the nanocomposite significantly improved the electron transfer kinetics. Subsequently, the electrochemical hydrogen production performance of the hybrid VSe₂/MoSe₂ is enhanced as compared to its constituent materials. Electrochemical characterizations prove that the VSe₂/MoSe₂ nanocomposite enhances the electrochemical activity performance with the lowest onset potential (330 mV) and a low value of Tafel plot (66 mV/decade) in comparison with sole MoSe₂ and VSe₂. In addition, the nanocomposite shows a low charge transfer resistance of 65 Ω, which further advocates the HER polarization curve and Tafel plot.

Keywords

Hydrothermal synthesis

VSe2

MoSe2

Nanocomposite

Hydrogen evolution reaction

Screen-printed electrodes

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1 Introduction

The development of clean, sustainable and eco-friendly energy has attracted a lot of attention. Due to the rise in environmental degradation and the depletion of fossil resources, this interest is escalating speedily. The environmentally friendly electrochemical production of hydrogen through electrolysis of water (2H₂O → 2H₂ + O₂) is a desirable option to produce a clean, sustainable and environmentally safe energy source compared to fossil fuels (Huang et al., 2018; Q. Zhang et al., 2020; Liang et al., 2015 ). Despite the fact that platinum and platinum-based alloys are the most efficient catalysts for the hydrogen evolution reaction (HER) in both acidic and alkaline conditions, their applicability in practical applications has been hampered by their intrinsic drawback of scarcity, high cost and low reserves (X.-D. Wang et al., 2016; Zhu et al., 2016; K. Zhang et al., 2015 ). Hence, the question of the development of alternative electrocatalysts that are non-precious, durable and highly HER active, as an alternative to Pt-based materials, is a crucial topic for scientists.

More recently, researchers have made enormous efforts to develop a variety of approaches to alternate precious electrocatalysts. Among these approaches, transition metal dichalcogenides (TMDC), including VSe₂, VS₂, WSe₂ and MoSe₂, have currently attracted a lot of interest due to their earth-abundant nature, unique structure and physical properties (Chen et al., 2018; Chhowalla et al., 2013; Alahmadi et al., 2021 ). TMDC consist of three layers (MX₂) bonded via Van der Waals forces, in which the middle layer is a transition metal (M) while the top and bottom layers are chalcogenide atoms (X) (Eichfeld et al., 2015; Fang et al., 2021; Fan et al., 2015 ).

Among various TMDC, molybdenum diselenide (MoSe₂) and its hybrids have been on the focus of research as electrocatalysts for HER due to their low cost, nontoxicity and ease of production. MoSe₂ material possesses two phases: the 2H (semiconducting) and the IT (metallic) set it apart from other TMDC materials (Kiran et al., 2014). Very recent reports have theoretically and experimentally demonstrated that the edge of materials controls the electronic structure. For instance, Jaramillo et al. and Helveg et al. have experimentally demonstrated through a scanning tunneling microscope (STM) that the active sites of MoS₂ layers are located along the edges of the domains while basal surfaces of the domains are inactive (Jaramillo et al., 2007; Helveg et al., 2000). In another study, Tang et al. showed that the atomic hydrogen adsorption on the MoSe₂ edges exhibits lower Gibbs free energy compared to MoS₂, which leads to greater coverage of hydrogen adsorption (Tang et al., 2014). Consequently, by tuning and manipulating the morphology of the nanoparticles or the sheets, by increasing the number of exposed active edge sites in TMDC, HER performance can be enhanced (Li et al., 2011; Poorahong et al., 2017; Liao et al., 2013 ). In addition, the influence of the electrocatalyst's electrical conductivity is a significant factor in HER activity. Thus, MoSe₂ is incapable of serving as a standalone and effective HER catalyst due to its poor conductivity and worse electroactive site exposure (Xu et al., 2018). Several attempts have been made to address this serious problem by employing strategies such as functional structure design (Ojha et al., 2017), synthesized hybrid materials (Tang et al., 2014) and doping with other materials (Deng et al., 2017; Qian et al., 2019). It is possible to alter the catalytic activity of MoSe₂ through the hybridization process, which is a simple technique for improving HER performance. Vanadium diselenide (VSe₂) has been cited in the literature as one of the most important TMDs, with special mention made of its exceptional properties, such as excellent electrocatalytic activities (Z.-L. Liu et al., 2018), high conductivity (Yi et al., 2022), charge density wave (Duvjir et al., 2018) and ferromagnetism (Bonilla et al., 2018). Moreover, VSe₂ is frequently used to modify the electronic structure when combined with other materials, thus reducing their overpotential (Kwon et al., 2022; Feng et al., 2022).

So far, successful techniques have been reported to increase the active edge sites of TMDCS including chemical deposition (H. Yang et al., 2016; Y. Zhang et al., 2015), electro-Fenton processing (Li et al., 2014), electrodeposition (Kuo et al., 2020), liquid exfoliation (Gopalakrishnan et al., 2014) and microwave heating process (Chen et al., 2019; Tang et al., 2021; Chen et al., 2020 ). These processes are frequently constrained because they require a lot of time, demand difficult circumstances, or involve the use of dangerous and expensive organic solvents (Ren et al., 2015). Among these methods, hydrothermal synthesis provides a lot of benefits for the economy and the environment (Chen et al., 2022).

In the current work, we demonstrated a facile one-pot synthesis strategy of the VSe₂/MoSe₂ nanocomposite by using a one-step hydrothermal method. In this method, we utilized ammonium molybdate hexahydrate precursor and vanadium nitrate as a metal source and selenium as a chalcogenide source for the one-pot growth of the VSe₂/MoSe₂ hybrid structure. The interaction between MoSe₂ and VSe₂, which is generally caused by the abundance of the MoSe₂ and VSe₂ interfaces, results in an increase in the number of catalytic active sites in the resulting hybrid heterostructure. The typical 3D heterostructure can enhance the active sites to fully expose in electrolyte due to the VSe₂ and MoSe₂ being well dispersed. In comparison to standalone pure VSe₂ and MoSe₂, the VSe₂/MoSe₂ hybrid displays better HER performances. The unique heterostructure of the VSe₂/MoSe₂ nanocomposite shows an excellent HER performance with a low onset potential of 330 mV, a small Tafel slope of 66 mV/decade, and excellent long-term stability in acidic medium. The structure, composition and morphology of the heterojunction nanocomposite were also studied using different characterization tools.

2 Materials and method

2.1

2.1 Chemical and reagents

Ammonium molybdate tetrahydrate ((NH₄) Mo₇O₂₄·6H₂O), ammonium metavanadate (NH₄VO₃), oxalic acid (C₂H₂O₄), selenium (Se) and nafion solution (5 wt%) were obtained from Sigma-Aldrich. There was no additional purification performed for any of the chemicals and reagents.

2.2

2.2 Synthesis of VSe₂/MoSe₂ nanocomposite

We describe a facile and low-cost one-step hydrothermal technique that was employed to produce the VSe₂/MoSe₂ nanocomposite. In a typical synthesis, 40 ml of distilled water was used to dissolve 0.16 g of NH₄VO₃, 1.12 g of C₂H₂O₄ and 0.25 g of (NH₄) Mo₇O₂₄·6H₂O. Following that, 0.16 g of Se powder was gradually added to the above solution as a source of a selenium precursor. Then, the resultant mixture was stirred for 1 h at ambient temperature. Subsequently, the solution was placed into a stainless steel autoclave and heated for 24 h at 200 °C. The mixture was allowed to cool at ambient temperature after the hydrothermal reaction. Finally, ethanol and distilled water were used to wash the synthesized products before being filtered using a filter paper. Finally, the synthesized product was dried for 12 h at 50 °C. For comparison, under the same conditions, both MoSe₂ and VSe₂ were also synthesized in their pure forms.

2.3

2.3 Materials characterization

The phase structure of the investigated samples was identified using X-ray powder diffraction (XRD) on a SHIMADZU-MAXima-XRD-7000 diffractometer equipped with Cu-Ka radiation (1.5418 A). Raman spectra of powder samples were performed on Senterra with a laser excitation wavelength of 532 nm. The structural phase of VSe₂/MoSe₂ was examined by X-ray photoelectron spectra (XPS) using K-alpha (Thermo fisher scientific, USA) with monochromatic X-ray Al K-Alpha radiation-10 to 1350ev spot size 400 µm at a pressure of 10⁻⁹ mbar. The morphology of the products was examined using filed emission scanning electron microscopy (FESEM-JSM-IT700HR). To examine the chemical composition of the developed hybrid nanocomposites, the energy dispersive X-ray (EDX) was also collected using the JSM-IT700HR. The hybrid VSe₂/MoSe₂ material was investigated using transmission electron microscopy (TEM) (QUANTA FEG 250).

2.4

2.4 Electrochemical measurements

All measurements of the electrochemistry were achieved by using Autolab (PGSTAT204) and recorded at room temperature. A disposable three-electrode conformation (screen printed carbon electrode (SPCE), Metrohm) consists of a working electrode (carbon), reference electrode (Ag/AgCl) and counter electrode (carbon). All experiments were performed in sulfuric acid (H₂SO₄, 0.5 M) as an electrolyte. All potentials in the present work are reported with respect to reversible hydrogen electrode RHE using the equation: E_RHE = E_Ag/AgCl + 0.1976 + 0.056pH. In order to make the catalyst ink, 55 mg of the catalyst material as it had been synthesized was dissolved in 4 ml solution of water-n-propanol (1:3). Then the catalyst ink was ultrasonicated for at least one hour in order to obtain a homogenous ink. Thereafter, 2 µL aliquot of the above slurry ink was directly drop-casted onto the 4 mm diameter working electrode and then covered with 2 µL of Nafion solution (1:3 water and propanol). Further, 10 ml of electrolyte (0.5 M H₂SO₄) was purged with nitrogen for at least 20 mins. Prior to evaluating the electrocatalytic activity of the VSe₂/MoSe₂ catalyst, a few cycles of cyclic voltammetry (CV) were performed to insure the electrode activation. Linear sweep voltammetry (LSV) was performed in 0.5 M H₂SO₄ by sweeping the potential in the range from 0.2 to −0.8V vs. RHE at a scan rate of 1 mV/s under nitrogen atmosphere. Electrochemical impedance spectroscopy (EIS) experiments were performed with frequency ranging from 80KHz to 1 Hz at a potential of 60 mV. To assess durability, 1000 cycles of the CV measurements were swept from 0.2 to −0.8 V at a scan rate of 100 mV/s.

3 Results and discussion

3.1

3.1 Structure and morphology of the nanocomposites

Using a simple one-pot hydrothermal procedure and common reagents like ammonium metavanadate, ammonium molybdate tetrahydrate, oxalic acid and selenium powder as precursors, the distinctive hierarchical solid blocks nanocomposite of VSe₂/MoSe₂ were synthesized. During our hydrothermal process, the VSe₂ prefers to produce ultra-thin nanosheets with random shapes while the MoSe₂ forms a nanoflower-like structure. Nevertheless, the VSe₂/MoSe₂ hybrid would lead to multidirectional solid blocks that are arranged in a hierarchy. Fig. 1 shows the XRD patterns and Raman spectra of the synthesized samples. Fig. 1(a) shows the XRD patterns of pure MoSe₂, VSe₂ and the hybrid mixture of MoSe₂/VSe₂. For the XRD pattern of the produced MoSe₂, all the diffraction peaks are perfectly consistent with the hexagonal MoSe₂ phase (JCPDS Card No. 77-1715). The reflection peaks localized at 14.6°, 37.06°, 44.4° and 66.2° corresponding to (0 0 2), (1 0 0), (1 0 3), (1 1 0), (1 0 5), and (1 1 0) planes, indicating excellent purity of the synthesized MoSe₂ (Liu et al., 2015). From the VSe₂ XRD patterns, the location of diffraction peaks is consistent with hexagonal VSe₂ nanosheets (DB Card No. 1538289, a = b = 3295 Å and c = 6031 Å). A few peaks at 2 theta of 14.4, 29.5, 32.4, 40.0, 43.5, 55.8, 60.0, 65.0, 71.4 and 86.3 originated from (0 0 1), (0 0 2), (0 1 1), (1 0 2), (0 0 3), (1 0 3), (0 0 4), (2 0 1), (0 0 2) and (0 0 5) planes (Sundaresan et al., 2023; He et al., 2017). For the VSe₂/MoSe₂ hybridized sample, all diffraction peaks of pure MoSe₂ and VSe₂ were observed. Further, Raman spectroscopy was employed to characterize the synthesized samples as shown in Fig. 1(b). The Raman spectra of MoSe₂ displays that the typical strong peak at 239 cm⁻¹ corresponds to the A_1g out-of-plane vibration mode of the MoSe₂ (Yang et al., 2017; Huang et al., 2014). From the Raman profile of pure VSe₂, the vibration modes at 280 and 406 cm⁻¹ are attributed to the vibration modes of E_1g and A_1g, respectively (Ming et al., 2018; Ratha et al., 2019). Raman spectra of the VSe₂/MoSe₂ mixture heterostructure exhibits the characteristic Raman bands of both sole MoSe₂ and VSe₂. The peaks at 280 and 406 cm⁻¹ correspond to E_2g and A_1g vibration modes of VSe₂, respectively, while the A_1g vibrational mode of MoSe₂ is at 239 cm⁻¹ implyes the existence of two materials in the heterostructure, which is also compatible with EDX and XPS analyses. These findings suggest that no phase shifts occurred during the synthesis of a nanocomposite comprised of MoSe₂ and VSe₂.

(a) XRD patterns (b) Raman spectra of the as-synthesized materials of MoSe2, VSe2, and VSe2/MoSe2.

The morphology and nanostructure of the pristine VSe₂ and MoSe₂ structures as well as the VSe₂/MoSe₂ nanocomposite were studied using field emission scanning electron microscopy (FESEM). As exhibited in Fig. 2(a), the pristine VSe₂ shows a nanosheet-like structure with a length of a few micrometers. On closer inspection, it becomes clear that these VSe₂ are made of a number of ultrathin nanosheets, with an estimated thickness of each nanosheet of roughly 10 nm (Fig. 2(b). Fig. 2(c) and Fig. 2(d) show the nanoflower-like structures of the pure MoSe₂ that were synthesized through the hydrothermal method from (NH₄) Mo₇O₂₄·6H₂O and Se. It is composed of ultrathin nanosheets with an average thickness of roughly 20 nm. The morphology of the hybrid VSe₂/MoSe₂ is illustrated in Fig. 2(e) and Fig. 2(f). It has been observed that when using combinations of V and Mo salts under the same experimental conditions, the produced VSe₂/MoSe₂ nanocomposites display morphologies that are completely different from either VSe₂ or MoSe₂. As clearly observed, the nanocomposite forms a hierarchical slab-stacking-like structure. Further, the elemental composition of the hybrid VSe₂/MoSe₂ nanocomposite was also examined using an EDX analysis, as revealed in Fig. 2(g), identifying Mo, V and Se as the primary constituents of the synthesized nanocomposite. The presence of these elements is further confirmed by EDX elemental mapping (Fig. 2(i-k)), which is also distributed throughout the hybrid composite material. Meanwhile, VSe₂ and MoSe₂ in the hybrid mixture seem to be in close contact based on the distribution of Mo, V, and Se. These findings indicate that the VSe₂/MoSe₂ hybrid nanostructure was successfully synthesized and contains both components. The hybrid VSe₂/MoSe₂ exhibits two distinct types of lattice fringes in high-resolution TEM analyses, as shown in Fig. 3(a) and (b). The interplanar spacing of the MoSe₂ (0 0 2) plane is 0.63 nm (Wu et al., 2020) while the interplanar spacing of the VSe₂ (0 1 1) plane is 0.263 nm (KA et al., 2020).

(a and B) FESEM images of pristine VSe2 nanosheets at different magnifications, (c and d) flower-like structure of MoSe2, (e and f) hybrid VSe2/MoSe2 material, (g) EDX spectra of VSe2/MoSe2 and (i-k) element mapping images of Mo, V, and Se, respectively.

(a and b) TEM images of the hybrid VSe2/MoSe2 material with the d spacing of VSe2 and MoSe2 and with the corresponding lattice scale views.

To confirm the surface chemical composition of the VSe₂/MoSe₂ heterostructure and the purity of its VSe₂ and MoSe₂ constituents, high-resolution X-ray photoelectron spectroscopy (XPS) measurements were made. The survey scan spectrum of MoSe₂, VSe₂, and VSe₂/MoSe₂ nanostructures is illustrated in Fig. 4(a). The synthesized samples mainly contain the five elements V, Mo, Se, C and O. The major source of the C 1S signal at 284.6 eV is the contamination from the carbon conductive tape that was used to hold the sample for the XPS analysis. The O 1 s signal in the XPS survey scan implies the existence of oxide phases that may be caused by the surface adsorption of oxygen. Moreover, as shown in Fig. 4(b), the pristine VSe₂ nanostructure shows two distinct peaks, which are attributed to V 2p_3/2 and V 2p_1/2, localized at binding energies of 517.15 and 523.5 eV, respectively. The peak at about 517.15 eV suggests the occurrence of V⁴⁺ (+4 oxidation state of vanadium) (Ulusoy Ghobadi et al., 2017; Karamat et al., 2016). In the VSe₂/MoSe₂ nanocomposite, this peak was also shifted to a low-binding energy region as a result of the incorporation of MoSe₂. The VSe₂/MoSe₂ spectrum, as shown in Fig. 4(c), has four dominant peaks. The two peaks at 229.7 and 232.8 eV, which correspond to Mo 3d_5/2 and Mo 3d_3/2, are assigned to the oxidation state of the Mo⁴⁺. The peaks located at 230.8 and 235.5 eV, which correspond to Mo 3d_5/2 and Mo 3d_3/2, are assigned to the oxidation state of the Mo⁶⁺(J. Yang et al., 2016; Zhang et al., 2018). These confirmed that Mo is mainly present in the nanocomposite mixture before being oxidized (L. Zhang et al., 2020). Additionally, the former small peak at around 235.5 eV indicates the presence of the hexavalent state of Mo, which is assigned to Mo 3d_3/2. On the other hand, in the case of pure MoSe₂, the hexavalent state peak becomes barely detectable. This peak indicates the reduction of Mo of the hexavalent state (Mo (VI) 3d_3/2, 235.5 eV) to the tetravalent state (Mo (IV) 3d_5/2, 232.5 eV) (Wang et al., 2014). Further, the binding energies of Mo (IV) 3d_3/2 shift by 0.5 eV, indicating that the electronic structure of MoSe₂ in the nanocomposite has been modified. The hexavalent state peak may be most likely caused by a combination of factors, including the reduction of $M {oO}_{4}^{- 2}$ during the hydrothermal reaction (Sakthivel et al., 2018), air oxidation of the MoSe₂/VSe₂ surface (Fan et al., 2015), or the formation of vacancies in the VSe₂ structure in the hybrid structure. It is noteworthy that by utilizing thermal treatment annealing at a high temperature of 600 °C, this peak can be completely eliminated (Zhang et al., 2017). In a comparable kind of this application, Zhang et al. have also realized this peak (L. Zhang et al., 2020). Further, Fig. 4(d) displays the XPS analysis of Se 3d. For hybrid VSe₂/MoSe₂ material, the characteristic doublet peaks are referred to Se 3d_5/2 at 55 eV and Se 3d_3/2 at 56.3 eV, indicating the −2 oxidation state of selenium (H. Yang et al., 2016). These findings support the successful synthesis of pure VSe₂, MoSe₂ and hybrid composites.

XPS spectra of the hybrid VSe2/MoSe2 product: (a) XPS survey spectra of MoSe2, VSe2 and VSe2/MoSe2, (b) XPS spectra of Mo 3d, (c) V 2p, and (c) Se 3d,respectively.

3.2

3.2 Electrocatalytic analysis

We evaluated the electrocatalytic performances of modified SPCE with the as-prepared MoSe₂, pristine VSe₂ and VSe₂/MoSe₂ nanocomposite in 0.5 M H₂SO₄ solution by LSV with a scan rate of 1 mVs⁻¹ at ambient temperature. The electrochemical performance of Pt was also studied as a control experiment. The prepared electrocatalysts will be denoted as MoSe₂@SPCE, VSe₂@SPCE and VSe₂/MoSe₂@SPCE, respectively. The polarization curves are shown in Fig. 5(a). Among all three electrocatalysts, the VSe₂/MoSe₂@SPCE exhibits the highest HER catalytic activity with an onset potential of 330 mV revealing robust interaction in the interface that could develop the activity. Comparatively, MoSe₂@SPCE and VSe₂@SPCE exhibit higher overpotentials. Due to the lower electrocatalytic reduction of proton to H₂, the cathodic current rose under more negative potential. Consequently, a relatively low hydrogen evolution activity was observed. It is noteworthy that the one-pot synthesed VSe₂/MoSe₂ can be stated as having better catalytic activity. With the one-pot synthesis method, we think there are unique possibilities for enhancing the catalytic activity of dual non-noble materials.

(a) The HER polarization curves of MoSe2@SPCE, VSe2 @SPCE and hybrid VSe2/MoSe2@SPCE in 0.5 M H2SO4. (b) Tafel Plots derived from the HER polarization curves.

The Tafel slope, which can use to understand the dynamics of hydrogen evolution and reaction mechanism, is extracted from the polarization curves (Shi et al., 2017; Yan et al., 2017). From the Tafel equation (η = b log (j) + a) (Begum et al., 2023), where η is the overpotential, b is the Tafel slope and j is the current density, the Tafel slope is derived. In order to estimate the Tafel slope of the various electrocatalysts, Tafel plots are reported in Fig. 5(b). A Tafel slope of 66 mV decade⁻¹ is determined for VSe₂/MoSe₂@SPCE. This value indicates that the hybrid nanocomposite increases the HER catalytic activity since it is substantially lower than the value observed for VSe₂@SPCE (99 mV decade⁻¹), MoSe₂@SPCE (196 mV decade⁻¹) and bare SPCE (235 mVdecade⁻¹). Interestingly, the Tafel slope value of VSe₂/MoSe₂@SPCE is lower compared to other TMDC previously reported; like MoSe₂/Graphene (80 mVdecade⁻¹; Najafi et al., 2018), MoS₂ nanosheets (90 mVdecade⁻¹; Muralikrishna et al., 2015) and WSe₂/rGO (85 mVdecade⁻¹; Li et al., 2018).

Understanding the interfacial processes of the VSe₂/MoSe₂ hybrid material requires an examination of the kinetics of HER. Conway et al. revealed a well-known conclusion regarding the participation of chemisorbed hydrogen, which may help in understanding how the cathodic H₂ evolution reaction occurs at electrodes (Conway and Tilak, 2002; Conway and Tilak, 1992; Tilak et al., 1977; X. Wang et al., 2016 ). Typically, the HER includes three main steps to convert H⁺ to H₂ in acidic electrolytes.

(1)

H_{3} O^{+} + e^{-} + M \to {MH}_{ads} + H_{2} O (s t e p 1, V o l m e r r e a c t i o n)

(2)

{MH}_{ads} + H_{2} O^{+} + e^{-} \to {H_{2} + H}_{2} O (s t e p 2, H e y r o v s k y r e a c t i o n)

(3)

{2 M H}_{ads} \to 2 M + H_{2} (s t e p 3, T a f e l r e a c t i o n)

Where M denotes the catalyst while M_Hads denotes the hydrogen atom adsorbed on the surface of the catalyst. The first step (1) is known as a discharge step (Volmer step, 120 mV dec⁻¹). Herein, hydrogen ions (H₃O⁺) and electrons interact to form adsorbed hydrogen intermediates (M_Hads) on the surface of the catalyst. The following steps (2 or 3) are desorption, also known as Heyrovsky electrochemical reaction (40 mV dec⁻¹) or Tafel chemical reaction (30 mV dec⁻¹). In the current work, the hybrid VSe₂/MSe₂ material has a Tafel slope of 66 mVdec⁻¹, which indicates that the Volmer and Heyrovsky reactions are taking place during the HER activity. In contrast to pure VSe₂ and MSe₂, the lower Tafel slope of VSe₂/MSe₂ should be the result of the H intermediate participating in a different state, which is promoted by the disordered state of the surface of VSe₂/MSe₂.

In order to further explore the relationship between the active surface of the electrocatalysts and catalytic HER activity, the charge-transfer processes of the various electrocatalysts were studied by (EIS) (Argoubi et al., 2015, 2019; Mars et al., 2016; Aoun, 2017 ). Fig. 6(a) shows the obtained Nyquist plot of MoSe₂@SPCE, VSe₂ @SPCE and VSe₂/MoSe₂@SPCE electrocatalysts along with bare SPCE. The charge transfer resistance (R_ct) of the electrocatalysts is a typical parameter reflecting the interfacial Faradic kinetics of the catalytic reaction of HER. Calculations were made based on the best fitting of the experimental data using a simple Randles-type equivalent circuit (Tanjila et al., 2021) (cf. inset of Fig. 6(a)). The VSe₂/MoSe₂@SPCE has the lowest R_ct value of 65 Ω while those of VSe₂@SPCE and MoSe₂@SPCE are around 4.84 and 12.50 kΩ, respectively. Additionally, the low resistance of the VSe₂/MoSe₂ hybrid material implies a highly effective electrical communication between the two catalytic edge sites. This tremendous decrease in charge transfer means a substantially greater conductivity of the VSe₂/MoSe₂@SPCE. This goes in line with the observed superior HER performance revealed from the polarization measurements. Moreover, along with its outstanding electrocatalytic activity, the durability is another essential factor to be considered. As presented in Fig. 6(b), the long-term stability of the VSe₂/MoSe₂@SPCE electrocatalyst was evaluated upon 1000 uninterrupted CV cycles in 0.5 M H₂SO₄ after 4 months of storage. In fact, the VSe₂/MoSe₂@SPCE electrocatalyst demonstrated excellent long-term stability with a minor drop in current densities and an even slightly positively shifted HER onset potential. This finding indicates the excellent stability of the VSe₂/MoSe₂ catalyst throughout HER performance. Table 1 provides a thorough comparison of several catalysts in terms of the synthesis process, electrolyte, charge transfer resistance and Tafel slope. As illustrated in Table 1, our nanocomposite nanomaterials' resistance and Tafel slope are comparable or even better than those of previously reported MoSe₂-based nanocomposite nanomaterials.

(a) Nyquist plots of MoSe2@SPCE, VSe2@SPCE and VSe2/MoSe2@SPCE in 0.5 M H2SO4. (b) Stability results of the VSe2/MoSe2@SPCE before and after 1000 CV cycles in 0.5 M H2SO4.

Table 1 Comparison Table of various MoSe2 and MoSe2-based electrocatalysts.

HER catalyst	Synthesis technique	Loading	Electrolyte	Tafel slope mV/dec	Overpotential η10 (mV)	Resistance	Reference
MoSe₂/GC	LPA	2 mg cm⁻²	0.5 M H₂SO₄	88	−340	–	(Najafi et al., 2018)
MoSe₂/CNFs	CVD	–	0.5 M H₂SO₄	107	−219	–	(H. Yang et al., 2016)
Porous MoSe₂	Liquid Exfoliation Method	0.45 mg cm⁻²	0.5 M H₂SO₄	80	−150	25	(Lei et al., 2016)
MoSe₂/WSe₂	Colloidal Method	0.28 mg cm⁻²	0.5 M H₂SO₄	238	−580	713	(Hwang and Shin, 2021)
MoSe₂-NiSe@C	Hydrothermal Synthesis	0.25 mg cm⁻²	0.5 M H₂SO₄	76.3	−154	–	(C. Liu et al., 2018)
H-NiSe₂/MoSe₂	Hydrothermal Synthesis	0.6 mg cm⁻²	0.5 M H₂SO₄	43.5	−147	188.2	(Dai et al., 2022)
Co doped MoSe₂	Hot Injection Technique	1.04 mg cm⁻²	0.5 M H₂SO₄	67	−232	388	(Zimron et al., 2020)
VSe₂/MoSe₂@SPCE	Hydrothermal Synthesis	0.2 mg cm⁻²	0.5 M H₂SO₄	66	−480	65	This Work

4 Conclusion

In summary, we have presented insights into the synthesis of a simple and cost-effective HER catalyst (VSe₂/MoSe₂) using a one-pot hydrothermal technique. Such technique led to maximize surface area by raising the number of exposed edges of the VSe₂/MoSe₂ nanocomposite, which in turn improved the HER catalytic activity. The proposed VSe₂/MoSe₂@SPCE electrocatalyst exhibits a drastically reduced charge transfer towards HER facilitated by the strong interaction between VSe₂ and MoSe₂, which also increases the conductivity of the nanocomposite as a whole. The prepared electrocatalyst presented an outstanding HER activity with a relatively low onset potential of 330 mV and a small Tafel slope of 66 mV decade⁻¹. In addition, impedance analysis reveals a low charge transfer resistance of 65 Ω for the prepared VSe₂/MoSe₂@SPCE, which suggests that active edge sites are more numerous. Due to its excellent stability in an acidic medium, VSe₂/MoSe₂@SPCE would be a potentially sophisticated HER electrocatalyst.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

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Introduction

Materials and method

Chemical and reagents
Synthesis of VSe2/MoSe2 nanocomposite
Materials characterization
Electrochemical measurements

Results and discussion

Structure and morphology of the nanocomposites
Electrocatalytic analysis

Conclusion

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[1] Alahmadi M., Mahvash F., Szkopek T., Siaj M., . A two-step chemical vapor deposition process for the growth of continuous vertical heterostructure WSe 2/h-BN and its optical properties. RSC Adv.. 2021;11:16962-16969.
[Google Scholar]

[2] Argoubi W., Rabti A., Aoun S.B., . Sensitive detection of ascorbic acid using screen-printed electrodes modified by electroactive melanin-like nanoparticles. RSC advances. 2019;9:37384-37390.
[Google Scholar]

[3] Argoubi W., Saadaoui M., Aoun S.B., . Optimized design of a nanostructured SPCE-based multipurpose biosensing platform formed by ferrocene-tethered electrochemically-deposited cauliflower-shaped gold nanoparticles. Beilstein Journal of Nanotechnology. 2015;6:1840-1852.
[Google Scholar]

[4] Begum H., Islam M.N., Ben Aoun, . Electrocatalytic reduction of nitrate ions in neutral medium at coinage metal-modified platinum electrodes. Environ. Sci. Pollut. Res.. 2023;30:34904-34914.
[CrossRef] [Google Scholar]

[5] Ben Aoun, S., 2017. Nanostructured carbon electrode modified with N-doped graphene quantum dots–chitosan nanocomposite: a sensitive electrochemical dopamine sensor. Royal Society open science. 4, 171199.

[6] Bonilla M., Kolekar S., Ma Y., Diaz H.C., Kalappattil V., Das R., Eggers T., Gutierrez H.R., Phan M.-H., Batzill M., . Strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates. Nat. Nanotechnol.. 2018;13:289-293.
[Google Scholar]

[7] Chen Z., Chen P., Xing P., Hu X., Lin H., Wu Y., Zhao L., He Y., . Novel carbon modified KTa0. 75Nb0. 25O3 nanocubes with excellent efficiency in photocatalytic H2 evolution. Fuel. 2018;233:486-496.
[Google Scholar]

[8] Chen P., Chen L., Ge S., Zhang W., Wu M., Xing P., Rotamond T.B., Lin H., Wu Y., He Y., . Microwave heating preparation of phosphorus doped g-C3N4 and its enhanced performance for photocatalytic H2 evolution in the help of Ag3PO4 nanoparticles. Int. J. Hydrogen Energy. 2020;45:14354-14367.
[Google Scholar]

[9] Chen L., Wang J., Li X., Zhao C., Hu X., Wu Y., He Y., . A novel Z-scheme Bi-Bi 2 O 3/KTa 0.5 Nb 0.5 O 3 heterojunction for efficient photocatalytic conversion of N 2 to NH. Inorg. Chem. Front. 2022
[Google Scholar]

[10] Chen P., Xing P., Chen Z., Hu X., Lin H., Zhao L., He Y., . In-situ synthesis of AgNbO3/g-C3N4 photocatalyst via microwave heating method for efficiently photocatalytic H2 generation. J. Colloid Interface Sci.. 2019;534:163-171.
[Google Scholar]

[11] Chhowalla M., Shin H.S., Eda G., Li L.-J., Loh K.P., Zhang H., . The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem.. 2013;5:263-275.
[CrossRef] [Google Scholar]

[12] Conway B.E., Tilak B.V., . Behavior and characterization of kinetically involved chemisorbed intermediates in electrocatalysis of gas evolution reactions. In: Advances in Catalysis. Elsevier; 1992. p. :1-147.
[Google Scholar]

[13] Conway B.E., Tilak B.V., . Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H. Electrochim. Acta. 2002;47:3571-3594.
[Google Scholar]

[14] Dai T., Sun J., Peng X., Gong J., Zhou Z., Wang X., . In situ synthesis of heterogeneous NiSe2/MoSe2 nanocomposite for high-efficiency electrocatalytic hydrogen evolution reaction. Energy Sci. Eng. 2022
[Google Scholar]

[15] Deng S., Zhong Y., Zeng Y., Wang Y., Yao Z., Yang F., Lin S., Wang X., Lu X., Xia X., . Directional construction of vertical nitrogen-doped 1T–2H MoSe2/graphene shell/core nanoflake arrays for efficient hydrogen evolution reaction. Adv. Mater.. 2017;29:1700748.
[Google Scholar]

[16] Duvjir G., Choi B.K., Jang I., Ulstrup S., Kang S., Thi Ly T., Kim S., Choi Y.H., Jozwiak C., Bostwick A., . Emergence of a metal–insulator transition and high-temperature charge-density waves in VSe2 at the monolayer limit. Nano Lett.. 2018;18:5432-5438.
[Google Scholar]

[17] Eichfeld S.M., Hossain L., Lin Y., Piasecki A.F., Kupp B., Birdwell A.G., Burke R.A., Lu N., Peng X., Li J., Azcatl A., Mcdonnell S., Wallace R.M., Kim M.J., Mayer T.S., Redwing J.M., Robinson J.A., . Highly scalable, atomically Thin WSe2 grown via metal-organic chemical vapor deposition. ACS Nano. 2015;9:2080-2087.
[CrossRef] [Google Scholar]

[18] Fan X., Xu P., Zhou D., Sun Y., Li Y.C., Nguyen M.A.T., Terrones M., Mallouk T.E., . Fast and efficient preparation of exfoliated 2H MoS2 nanosheets by sonication-assisted lithium intercalation and infrared laser-induced 1T to 2H phase reversion. Nano Lett.. 2015;15:5956-5960.
[Google Scholar]

[19] Fang B., Yao J., Zhang X., Ma L., Ye Y., Tang J., Zou G., Zhang J., Jiang L., Sun Y., . A large scaled-up monocrystalline 3R MoS 2 electrocatalyst for efficient nitrogen reduction reactions. New J. Chem.. 2021;45:2488-2495.
[Google Scholar]

[20] Feng W., Cheng M., Du R., Wang Y., Wang P., Li H., Song L., Wen X., Yang J., Li X., . Gram-scale synthesized two-dimensional VSe2 and SnSe2 for ultrahigh electrocatalytic sulfion recycling. Adv. Mater. Interfaces. 2022;9:2200060.
[Google Scholar]

[21] Gopalakrishnan D., Damien D., Shaijumon M.M., . MoS2 quantum dot-interspersed exfoliated MoS2 nanosheets. ACS Nano. 2014;8:5297-5303.
[Google Scholar]

[22] He S., Lin H., Qin L., Mao Z., He H., Li Y., Li Q., . Synthesis, stability, and intrinsic photocatalytic properties of vanadium diselenide. J. Mater. Chem. A. 2017;5:2163-2171.
[Google Scholar]

[23] Helveg S., Lauritsen J.V., Lægsgaard E., Stensgaard I., Nørskov J.K., Clausen B.S., Topsøe H., Besenbacher F., . Atomic-scale structure of single-layer MoS 2 nanoclusters. Phys. Rev. Lett.. 2000;84:951.
[Google Scholar]

[24] Huang J., Gao H., Xia Y., Sun Y., Xiong J., Li Y., Cong S., Guo J., Du S., Zou G., . Enhanced photoelectrochemical performance of defect-rich ReS2 nanosheets in visible-light assisted hydrogen generation. Nano Energy. 2018;46:305-313.
[Google Scholar]

[25] Huang C., Wu S., Sanchez A.M., Peters J.J.P., Beanland R., Ross J.S., Rivera P., Yao W., Cobden D.H., Xu X., . Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors. Nat. Mater.. 2014;13:1-6.
[CrossRef] [Google Scholar]

[26] Hwang Y., Shin N., . Colloidal synthesis of MoSe2/WSe2 heterostructure nanoflowers via two-step growth. Materials (Basel).. 2021;14:7294.
[Google Scholar]

[27] Jaramillo T.F., Jørgensen K.P., Bonde J., Nielsen J.H., Horch S., Chorkendorff I., . Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts. Science (80-.). 2007;317:100-102.
[Google Scholar]

[28] KA, S.R., Shajahan, A.S., Chakraborty, B., Rout, C.S., 2020. The role of carbon nanotubes in enhanced charge storage performance of VSe 2: experimental and theoretical insight from DFT simulations. RSC Adv. 10, 31712–31719.

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