Hydrothermal Synthesis of La-MoS₂ and Its Catalytic Activity for Improved Hydrogen Evolution Reaction

Abstract

Herein, we report the synthesis and characterization of lanthanum-doped MoS₂ (La-MoS₂) via a hydrothermal route. The synthesized La-MoS₂ was characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) techniques. The band gap of La-MoS₂ was observed to be 1.68 eV, compared to 1.80 eV for synthesized MoS₂. In the photoluminescence (PL) spectra, a decrease in the intensity was observed for La-MoS₂ compared to MoS₂, which suggests that due to doping with charged La³⁺, separation increases. The as-synthesized MoS₂ and La-MoS₂ were used for photocatalytic hydrogen evolution reactions (HERs), exhibiting 928 µmol·g⁻¹ evolution of H₂ in five hours for a 10 mg dose of La-MoS₂, compared to 612 µmol·g⁻¹ for MoS₂. A 50 mg mass of the catalyst (La-MoS₂) exhibited enhanced H₂ production of 1670 µmol·g⁻¹ after five hours. The higher rate of the HER for La-MoS₂ is because of doping with La³⁺. The photocatalytic hydrogen evolution performance of La-MoS₂ was also evaluated for different doses of La-MoS₂ exhibiting reusability up to the fourth cycle, showing potential applications of La-MoS₂ in hydrogen evolution reactions. Mechanistic aspects of the HER on the surface of La-MoS₂ have also been discussed.

1. Introduction

In the past few decades, rapid expansion of industries and a growing global population have posed significant challenges for humanity in meeting its energy needs [1]. Over the past few years, energy demand has been risen exponentially [2]. At present, the primary source of energy is fossil fuels, which are rapidly depleting and have harmful environmental effects when utilized [2]. To overcome this challenge, researchers and scientists are working on the development of sustainable technologies designed to reduce environmental impact. Solar energy has been proved to be one of the most efficient and green of all energy sources. The sun provides an enormous amount of solar energy, around 10²² joules, which can contribute to the development of next generation energy technologies [3]. Thus, it is essential to utilize solar energy for the development of green and renewable energy technology for the future. In connection with this, hydrogen energy is considered one of the most efficient energy technologies to fulfill energy requirements [4,5]. Photocatalytic hydrogen generation by the use of solar energy on the surface of a catalyst (semiconductor) is a very propitious alternative for resolving the present energy and environmental crisis [6,7,8]. Hydrogen fuel provides a high yield of energy upon combustion, i.e., 122 kJ/mol, which is far better than any other fossil fuel (gasoline, coal, etc.) [9,10]. The process of hydrogen generation by photocatalysis is environmentally benign, producing no harmful byproducts [11]. The use of photocatalytic materials for the generation of hydrogen was demonstrated for the first time in 1972 [12]. Thereafter, a great number of semiconductor materials have been designed and demonstrated for the efficient generation of hydrogen [13,14,15]. A series of MoS₂-TiO₂ photocatalysts have been designed by Zhu et al. [16], using a ball-milling process. Due to the introduction of MoS₂, a decrease in the recombination of photogenerated charge carriers was observed, enhancing the photocatalytic performance compared to pure TiO₂. The rate of the HER was enhanced to 150.7 μmol/h for the 4.0 wt% MoS₂-TiO₂ catalyst. Kumar et al. designed defect-rich MoS₂ nanosheets and decorated them with nitrogen-doped ZnO nanorod composites [17]. For this photocatalyst, optimal hydrogen evolution (17.3 mmol h⁻¹ g⁻¹) was recorded for 15 wt% defect-rich MoS₂ nanosheets coated on N-ZnO. The results also demonstrated that a defect-induced interfacial contact region developed on encapsulation of defect-rich MoS₂ across N-ZnO, resulting in a composite that further increased the photocatalytic activity of the designed semiconductor system.

Doping of MoS₂ with a metal or nonmetal may generates defects along with alteration of the optical bandgap. For a Co-doped MoS₂/g-C₃N₄ composite, a very good photocatalytic reduction rate of water has been reported by Hu et al. [18] with a 0.31 mmol g⁻¹h⁻¹ rate of H₂ evolution due to the generation of an edge-enriched 1T phase as a result of Co doping. Additionally, the rate of photocatalysis can be enhanced by depositing a metal on MoS₂. With metal deposition on MoS_2, the interfacial charge transfer rate is enhanced along with the generation of a local electric field via the Schottky junction. It may also broaden the absorption spectrum to the near-infrared region. Scientists have studied the deposition of several metals, viz., copper [19], silver [20] gold [21], platinum [22], and palladium [23], on MoS₂, showing the creation of plasmonic photocatalysts absorbing strong visible light along with size- and shape-dependent surface plasmon resonance (SPR) effects.

Metal dichalcogenide compounds have received a great deal of attention because of their excellent optoelectronic properties. Metal dichalcogenide compounds including metal sulfides (such as MoS₂), selenides, and tellurides exhibit tunable band gaps and high surface activity, making them suitable for semiconductors, photocatalysis, and energy storage. Metal chalcogenides, especially in two-dimensional forms, have gained attention for hydrogen evolution reactions (HERs) and other electrochemical applications, as they offer excellent electron mobility and catalytic efficiency. MoS₂ is a 2D layered material characterized by a large surface area, tunable electronic properties, and structural flexibility, which make it suitable for catalytic processes. Its catalytic activity in HERs is primarily driven by the active edge sites of the MoS₂ layers, where sulfur vacancies and exposed Mo atoms enhance electron transfer and improve reaction kinetics. One key property of MoS₂ is its relatively low overpotential, especially when engineered with additional defects or doped with metals such as Ni or Co, which introduce more active sites and increase conductivity. The basal plane, which is generally inert, can also be activated through exfoliation, creating more active edge sites. Additionally, MoS₂ has excellent stability at acidic and neutral pH values, making it a durable option for water-splitting applications. Recent research focuses on synthesizing MoS₂ heterostructures with materials such as graphene or carbon nanotubes, further enhancing conductivity and stability. These modifications make MoS₂-based catalysts a promising alternative to noble metals in efficient and cost-effective H₂ production. MoS₂, a layered transition metal dichalcogenide, is generated by the stacking of S–Mo–S atomic layers that are held together by van der Waals forces present among them [24]. When MoS₂ is changed from bulk to the nanoscale, it exhibits unique physiochemical and optical properties and becomes a potential candidate for photocatalysis [25]. Bulk MoS₂ is optically inactive with an indirect bandgap of 1.29 eV; it also demonstrates a low photoluminescence response [26]. However, when the thickness of bulk MoS₂ is reduced to a single layer or a few layers, it exhibits a peak at 1.8–1.9 eV for the direct band gap [27]. In MoS₂, speedy/fast transfer of charge carriers occurs when visible light strikes its surface because of the narrow band gap; therefore, it is a promising and valuable photocatalyst candidate. However, the fast rate of recombination of charge carriers limits it applications in photocatalysis reactions [27,28].

Herein, we report the fabrication of a lanthanum-doped MoS₂ photocatalyst for a hydrogen evolution reaction. La-MoS₂ was prepared using a hydrothermal method, and its photocatalytic activity was examined for a hydrogen evolution reaction under visible light irradiation. The proposed photocatalyst La-MoS₂ also demonstrated decent stability for hydrogen evolution under visible light irradiation.

2. Results and Discussion

2.1. Material Characterization

To characterize the generated phase, the powder X-ray diffraction patterns of pure MoS₂ and La-MoS₂ were recorded and are shown in Figure 1. Four distinctive peaks at 2θ values of 13.1°, 32.4°, 35.8°, and 57.9° were observed in the samples, corresponding to the (002), (100), (103), and (110) planes of MoS₂. The PXRD patterns confirm the presence of a hexagonal crystal structure (2H-MoS₂), which is in accordance with the existing literature (JCPDS No.: 0037-1492). On comparing the PXRD patterns of both the samples of pure MoS₂ and La-MoS₂, it was confirmed that almost the same peak position could be observed in both cases, with a slight shift in La-MoS₂. The slight shift in the peak position after doping may be due to the larger ionic radius of lanthanum (La³⁺) as compared to molybdenum (Mo⁴⁺), which results in the expansion of the crystalline lattice and is consistent with Bragg’s equation [29]. In the PXRD pattern of La-MoS₂, a higher peak intensity could be observed as compared to MoS₂; this increase in the peak intensity may be ascribed to the different electron densities of La³⁺ (dopant) and MoS₂ (host), which primarily relate to the scattering factor, structure factor, etc. In the case of La-MoS₂, the increase in the structure factor may be attributed to increased crystallite size, which results in an increase in the intensity of PXRD peaks on doping with La³⁺ [30]. The absence of peaks for elemental La³⁺ confirmed that the crystal structure of MoS₂ had not been changed due to doping. To calculate the particle sizes of both MoS₂ and La-MoS₂, the Debye–Scherrer formula (D = K λ/β cosθ, where K = 0.89, λ = 0.154, D is the average crystalline size, is β is full width half maxima (FWHM) and θ is the Bragg angle) has been used in a previous study. The average crystallite sizes of MoS₂ and La-Mos₂ samples have been calculated as 17.5 and 21.2 nm, respectively, showing an increase in the crystallite size of MoS₂ on doping with La³⁺ [31].

Figure 2 presents surface morphology of synthesized MoS₂ (Figure 2a) and La-MoS₂ (Figure 2c). For both of the samples, spherical morphologies were observed, with an average diameter of approximately one nanometer (Figure 2b,d). The average sizes of the MoS₂ and La-MoS₂ particles were found to be 648.26 nm and 676.01 nm, respectively, using particle size distribution curves (Figure 2b,d).

From the FESEM images, it appeared that the surfaces of the MoS₂ and La-MoS₂ microspheres were made up of nanoflakes. Hollow microspheres of MoS₂ have also been reported by Afanasiev and Bezverkhy [32], who produced them via a heat treatment method, and by Chen et al. [33], who produced them via a direct sulfidization route; these results are similar to ours. Moreover, in these synthesized microspheres, numerous thin-stretched, folding flakes have also been observed, which are helpful in hydrogen evolution reactions.

When these active sites are exposed to light, they assist in the transportation of charge carriers along with participating in oxidation and reduction reactions in the course of the photocatalytic procedure [34]. EDX spectra of MoS₂ and La-MoS₂ sub-microspheres are shown in Figure S1. As shown in the EDX spectrum of MoS₂, two signals indicating the elements Mo and S in the elemental composition were observed, while in the EDX spectrum of La-MoS₂, signals for the element La were also detected. The obtained results again indicated the phase purity of the synthesized samples.

In order to check the distribution of the elements in the samples, elemental mapping was also performed. Images of element mapping of MoS₂ revealed that the sample contained Mo and S as the main elements, and both elements were uniformly distributed in the sample (Figure 3a–c). Images of elemental mapping along with the corresponding overlay and FESEM images for La-MoS₂ sample are presented in Figure 3d–g, showing the presence of the element La in the MoS₂ sample. It is evident from the images that the element La was distributed in lower amounts than Mo and S.

To calculate the optical bandgaps of the prepared samples, UV–visible spectra were been recorded for both the as-prepared MoS₂ and La-MoS₂ samples. Figure 4a depicts the UV–visible spectra of both the samples. The results of UV–visible spectroscopy indicated that there was a significant shift in the absorption band towards visible region in the case of La-MoS₂. To calculate the band gap energy of both the samples, a Tauc plot was created using the formula (αhυ)² = A (hυ − Eg). A plot of (αhν)² against hν is shown in Figure 4b. The band gap of as-prepared MoS₂ was calculated as 1.80 eV, similar to the band gap of MoS₂ monolayer material [35], whereas a significant reduction in the band gap (1.68 eV) was noticed after doping with La³⁺, which further affirms that upon doping with La³⁺, the visible light absorption property of the synthesized photocatalyst was enhanced.

PL spectra of MoS₂ and La-MoS₂ are shown in Figure S2. The main role of photoluminescence (PL) spectroscopy is to investigate migration, electron transfer efficiency, and electron trapping in semiconductor materials [36]. When the rate of recombination of electron–hole pairs is high, the peaks in the PL spectrum are intense. Thus, it is clear from the recorded PL spectra of MoS₂ and La-MoS₂ that on doping with La³⁺, the rate of recombination of electron–hole pairs decreases, as the intensity La-MoS₂ is less than that of pure MoS₂. The decrease in the recombination rate is due to the introduction of a discrete energy level due to doping with La³⁺.

2.2. Photocatalytic Performance of La-MoS₂

To analyze the photocatalytic efficiency, the synthesized MoS₂ and La-MoS₂ samples were suspended in water using lactic acid as a sacrificial agent, and the whole suspensions were stimulated with solar irradiation. Figure 5a shows the results of hydrogen production using 10 mg of photocatalyst. With increasing time, the photocatalytic efficiency increases for both MoS₂ and La-MoS₂, but for La-MoS₂, the rate of the photocatalytic hydrogen evolution rection (HER) was much higher than for MoS₂. After 5 h, the amount of H₂ was found to be 612 µmol·g⁻¹ for MoS₂ and 918 µmol·g⁻¹ for La-MoS₂. Thus, it may be concluded that La-MoS₂ works better for the HER as compared to MoS₂ due to charge separation. Hence, for further experiments, La-MoS₂ was taken into consideration. The effects of different doses of La-MoS₂ are shown in Figure 5b. The doses of La-MoS₂ were varied between 10–50 mg. The results were collected at intervals of one hour for a total of five hours. As shown in Figure 5b, with an increasing dose of the photocatalyst, the amount of H₂ production also increased.

The highest amount of H₂, 1670 µmol·g⁻¹, was obtained for 50 mg catalyst. The rates of H₂ evolution for different catalyst doses are summarized in Figure 5c. Figure 5c shows that when the dose of La-MoS₂ was increased, the rate of H₂ production increased more than 1.5-folds. It reached from 185.6 µmol·h⁻¹·g⁻¹ to 334 µmol·h⁻¹·g⁻¹. In order to use a catalyst in real-time applications, it is essential to test the stability and reusability of the photocatalyst; accordingly, for La-MoS₂, a reusability test was performed, and it showed almost consistent results up to four consecutive cycles (Figure 5d). The XRD results after the stability test also reflected the acceptable stability of the La-MoS₂ (Figure 1).

A potential mechanism for the hydrogen evolution reaction on the surface of La-MoS₂ is shown in Scheme 1. The H₂ evolution is initiated when visible light strikes the surface of La-MoS₂ and it absorbs photons with energy (hυ) either equal to or greater than its energy band gap [1]. On absorbing the energy from solar radiation, electrons jump from the valence band to the conduction band, creating electron–hole pairs and leaving the holes in the valence band. The photogenerated electrons that are present in the conduction band reduce H⁺ into H₂, whereas the holes present in the valence band combine with H₂O and break it down into O₂ and H⁺ [37]. These holes also react with the scavenger lactic acid, changing it to pyruvic acid. The enhanced rate of photocatalytic production over the surface of La-MoS₂ may be attributed to the narrow band gap and synergistic interactions. This may improve the electron transport process, and enhanced H₂ evolution activity can be observed. Therefore, it can be concluded that controlling the transfer and migration of charge carriers may enhance the efficiency of photocatalysts, such as the spatial separation of carriers, elongating their lifetime and thus increasing their photocatalytic performance.

In previous years, various photocatalysts have been reported for photocatalytic H₂ evolution. In particular, MoS₂ exhibited acceptable performance for the generation of H₂ under visible light irradiation. Xin et al. [38] reported the use of MoS₂ as a photocatalyst; it achieved a demonstrated H₂ evolution rate of 99.4 µmol·h⁻¹·g⁻¹ with lactic acid as a sacrificial reagent. The authors further adopted P-doped MoS₂ as a photocatalyst and explored it for H₂ evolution. An enhanced H₂ evolution rate of 278.8 µmol·h⁻¹·g⁻¹ was observed with a similar environment and conditions. The authors stated that doping with the element P significantly improved the catalytic activity of P-MoS₂. In another published article, MoS₂ was used as a photocatalyst, and its photocatalytic activity was evaluated in presence of SO₃²⁻ sacrificial reagent [39]. The authors found that pristine MoS₂ had lower photocatalytic activity for H₂ evolution and a low H₂ evolution rate of 39 µmol·h⁻¹·g⁻¹. The authors also used pristine ZnO as a photocatalyst, which achieved a demonstrated H₂ evolution rate of 22 µmol·h⁻¹·g⁻¹. However, an MoS₂/ZnO composite demonstrated a significant improvement in its H₂ evolution rate, and a decent H₂ evolution rate of 235 µmol·h⁻¹·g⁻¹ was obtained. Pristine MoS₂ also exhibited H₂ evolution at a rate of 185 µmol·h⁻¹·g⁻¹ in the presence of methanol as a sacrificial reagent [40]. In another work, MoS₂/g-C₃N₄ composite-based investigations revealed that H₂ evolution at a rate of 441.3 µmol·h⁻¹·g⁻¹ could be observed in presence of triethanolamine as a sacrificial reagent [41]. Wei et al. [42] also proposed the synthesis of MoN₁._2xS₂₋₁._2x@g-C₃N₄ as a photocatalyst for H₂ production applications. An interesting H₂ evolution rate of 360.4 µmol·h⁻¹·g⁻¹ was observed for the photocatalyst MoN₁._2xS₂₋₁._2x@g-C₃N₄ in the presence of triethanolamine as a sacrificial reagent. In 2023, Wang et al. [43] proposed the hydrothermal synthesis of O, P–MoS₂ for H₂ production application under visible light. The authors used triethanolamine/acetonitrile as sacrificial reagents and reported that the H₂ evolution rate was 339.3 µmol·h⁻¹·g⁻¹. The aforementioned studies showed that MoS₂-based photocatalysts have a significant role in photocatalytic H₂ evolution studies. The results obtained in the present study are compared with previously reported articles in

Table 1. Comparison of H₂ evolution rate of La-MoS₂ with published articles.

Photocatalysts	H2 Evolution Rate (µmol·h−1·g−1)	Light Source	Sacrificial Agent	Reference
La-MoS2	334	300 W; Xe lamp (λ = 420 nm)	Lactic acid	This study
P-MoS2	278.8	300 W; Xe lamp (λ = 420 nm)	Lactic acid	38
MoS2	99.4	300 W; Xe lamp (λ = 420 nm)	Lactic acid	38
MoS2	39	300 W; Xe lamp (λ = 420 nm)	SO32−	39
MoS2/ZnO	235	300 W; Xe lamp (λ = 420 nm)	SO32−	39
ZnO	22	300 W; Xe lamp (λ = 420 nm)	SO32−	39
g-C3N4	54	300 W; Xe lamp (λ = 420 nm)	Methanol	40
MoS2	185	300 W; Xe lamp (λ = 420 nm)	Methanol	40
MoS2/g-C3N4	441.3	300 W; Xe lamp (λ = 420 nm)	Triethanolamine	41
MoN1.2xS2−1.2x@g-C3N4	360.4	300 W; simulated solar light	Triethanolamine	42
O, P–MoS2	339.3	LED light; λ = 420 nm	Triethanolamine/acetonitrile	43

. The comparison of results showed that La-MoS₂ showed a decent response for hydrogen production compared to the many previously reported photocatalysts (Table 1).

3. Materials and Methods

3.1. Materials

Ammonium molybdate tetrahydrate (NH₄)₆Mo₇O₂₄·4H₂O; 99.98% trace metal basis), thiourea (ACS reagent, ≥99.0%), and lanthanum (III) nitrate hexahydrate (99.999% trace metal basis) were purchased from Merck (Mumbai, India). All the other used chemicals and materials were of analytical grade and used as received from Sigma (Mumbai, India), Alfa Aesar (Indore, India), and Merck (Mumbai, India). No further treatment or purification was carried out.

3.2. Synthesis of La-MoS₂

In this study, we adopted hydrothermal synthesis method for the preparation of pristine MoS₂ and La-MoS₂ materials. In brief, hydrothermal treatment of the reaction solution of NH₄)₆Mo₇O₂₄·4H₂O and thiourea yielded MoS₂. In brief, 0.6 g of thiourea was slowly added to an aqueous solution of 0.5 g ammonium molybdate (30 mL), and this reaction solution was stirred for 25–30 min at room temperature. A clear solution was obtained, which was then poured into a Teflon reactor and sealed tightly in a stainless steel autoclave (the autoclave needed to be carefully tightened to prevent the melting of the Teflon cup). This autoclave was heated at 180 °C for 18 h in a muffle furnace. After cooling down the furnace, the autoclave was opened; the MoS₂ was then collected, washed with DI water and ethanol, and dried at 70 °C for 8 h in a vacuum oven. In further investigations, La-MoS₂ was also prepared using a hydrothermal method as illustrated in Scheme 2.

Typically, 0.6 g of thiourea was added to an aqueous solution of 0.5 g ammonium molybdate. Furthermore, 5 wt% (0.055 g) lanthanum nitrate was added to the above reaction solution and stirred for 25–30 min at room temperature. After stirring, this solution was transferred to a Teflon reactor, sealed tightly in a stainless steel autoclave, and heated at 180 °C for 18 h in a muffle furnace (Scheme 2). After cooling down the furnace, the autoclave was opened, and La-MoS₂ was collected using centrifugation, washed with DI water and ethanol, and dried at 70 °C for 8 h in a vacuum oven.

3.3. Instruments

A Rigaku powder X-ray diffractometer (model Rint 2500, manufactured in Rigaku, Tokyo, Japan) with wavelength of 1.5406 Å and Cu/Kα radiation was used to record the PXRD results of the synthesized samples. A Supra Zeiss-55 field-emission scanning electron microscope was used to record the surface morphological images of the synthesized samples (Zeiss, Jena, Germany). The energy-dispersive X-ray spectroscopy (EDX) results were obtained on a Horiba EDX spectroscope (Horiba, Kyoto, Japan). An ultraviolet–visible spectrophotometer (UV-vis spectrophotometer model Cary 100 (Varian, Palo Alto, CA, USA) was used to capture the UV-vis spectra of the synthesized samples. The photoluminescence (PL) spectra of the samples were obtained on a Dongwoo Optron PL spectrometer. The hydrogen evolution results were obtained on a gas chromatograph with a thermal conductivity detector (TCD).

3.4. Photocatalytic H₂ Evolution

We used a quartz tube reactor as the photocatalytic H₂ production set-up. A mass of 10 mg of the catalyst (La-MoS₂ or MoS₂) was added to a mixture of 80 mL DI water and 20 mL lactic acid. The reaction solution was purged with nitrogen gas for 40 min to remove unnecessary gases or oxygen. The quartz tube containing the catalyst and reaction solution was closed with an airtight seal and used for photocatalytic H₂ evolution processes under visible light irradiation (Figure S3). A 300 W xenon lamp with wavelength = 420 nm was used as the light source. Different doses of the catalyst (La-MoS₂; 10, 20, 30, 40, and 50 mg) were used to optimize the performance of the photocatalyst for improved H₂ evolution. The generated H₂ was taken out using a syringe and measured by employing a gas chromatograph (in combination with a TCD).

4. Conclusions

In conclusion, lanthanum-doped molybdenum disulfide (La-MoS₂) was synthesized via _a hydrothermal route. To characterize La-MoS₂, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) techniques were used. The band gap of La-MoS₂ was calculated as 1.68 eV, which is less than the band gap of synthesized MoS₂ (i.e., 1.80 eV). In the photoluminescence (PL) spectra, a decrease in intensity was observed for La-MoS₂ compared to MoS₂, indicating increased charge separation for La-MoS₂. The as-synthesized MoS₂ and La-MoS₂ were used for photocatalytic hydrogen evolution reactions (HERs), and La-MoS₂ exhibited an improved H₂ production rate compared to the MoS₂. The increased photocatalytic performance may be because of the generation of another discrete energy level due to doping with La³⁺. Mechanistic aspects of the HER on the surface of La-MoS₂ have also been discussed.