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Article

Hydrothermally Synthesized Ag@MoS2 Composite for Enhanced Photocatalytic Hydrogen Production

by
Anuja A. Yadav
1,†,
Yuvaraj M. Hunge
2,*,†,
Ananta G. Dhodamani
3 and
Seok-Won Kang
1
1
Department of Automotive Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan 38541, Republic of Korea
2
Research Institute of Science and Technology (RIST), Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Chiba, Japan
3
Department of Chemistry, Sadguru Gadage Maharaj College, Karad 415124, Maharashtra, India
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2023, 13(4), 716; https://doi.org/10.3390/catal13040716
Submission received: 8 March 2023 / Revised: 24 March 2023 / Accepted: 30 March 2023 / Published: 10 April 2023
(This article belongs to the Special Issue Nanomaterials for Photocatalysis II)
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Abstract

:
Photocatalytic hydrogen production is a green, cost-effective, simple, and pollution-free technology for the supply of clean energy, which plays an important role in alleviating the fossil fuel crisis caused by exponentially grown energy consumption. Therefore, designing highly visible-light-active novel photocatalyst materials for photocatalytic hydrogen production is a promising task. The production efficiency of photocatalyst can be improved by using noble metals, which are useful for the effective transfer of charge carriers. This study highlights the synergistic effect of the noble co-catalyst Ag on MoS2 during the investigation of photocatalytic hydrogen production. The hydrothermal method was used for the preparation of an Ag-MoS2 composite, and their structural and morphological characterizations were carried out using different physiochemical characterization techniques. The Ag-MoS2 composite shows an enhanced visible light absorption capacity and photocatalytic hydrogen production rate, as compared to that of pure MoS2, which proves that Ag nanoparticles (NPs) can act as efficient co-catalyst materials for photocatalytic hydrogen production with an improved rate of hydrogen production. Along with this, a possible working mechanism was proposed for visible-light-driven photocatalytic hydrogen production using the Ag@MoS2 composite.

1. Introduction

Nowadays, the rapid growth of industrialization and increased populations have led to an enhanced demand for energy. To fulfill this energy demand, more pressure will be placed on natural resources, such as fossil fuels. Thus, the burning of fossil fuels leads to increased environmental pollution, which causes major problems of increased scarcity of fossil fuels, therefore worsening the energy crises in the upcoming future. Thus, it is important to develop different energy sources, such as wind, solar, geothermal, biomass, etc., which are clean, renewable, pollution-free, and sustainable energy sources [1,2,3]. However, the energy sources mentioned above have some major drawbacks and limitations, such as difficulties in their storage and transport and require high costs, and some sources (wind and solar) are regional, depending on location, etc. Among the different renewable energy sources, hydrogen energy is considered one of the best energy sources, owing to its good efficiency, being clean and pollution free and easy to handle, and its transport and storage are convenient. Several different techniques have been adopted to produce hydrogen gas, which include the steam reforming of hydrocarbons, the electrolysis of water, fermentation, the gasification of biomass, photocatalytic hydrogen production, etc. As compared to the photocatalytic hydrogen production method, the steam reforming of hydrocarbons method is an endothermic, reversible, and high-temperature reaction that generates CO2, which further leads to the greenhouse effect [4]. Hence, photocatalytic hydrogen production is a green technology in which photo-generated hydrogen can easily be stored, and it is a potential approach to meet energy demands. Hydrogen energy can be substituted for fossil fuels in the near future. The photocatalytic hydrogen production process is more economical, energy-saving, and produces clean hydrogen [4,5,6,7]. In this process, no electrical energy is required to initiate the process. Photocatalytic hydrogen production, using semiconductor-based photocatalyst materials, is of growing research interest among the research community to overcome the global energy crises [1,2]. Many efforts have been made in the development of highly efficient semiconductor photocatalyst materials and their use in photocatalytic hydrogen production through water-splitting reactions, but the efficiency of this process remains unsatisfactory due to the narrow solar spectral response, the high recombination of charge carriers, etc. [3]. Honda and Fujishima, in the 1970s, reported photocatalytic hydrogen evolution using a TiO2 electrode for the first time [6]. Later, many semiconductor materials have been used to explore photocatalytic hydrogen production activity using metal oxides, sulfides, nitrides, etc. over the past few decades [8]. In the previous few decades, semiconductor materials, such as TiO2 and ZnO, etc., due to their unique properties, such as their low cost, high thermal stability, high catalytic activity, chemical stability, etc., have been widely studied and used for various applications, such as solar cells, water splitting, hydrogen production, photocatalysis, air purification, etc. In addition, ZnO and TiO2 possess direct bandgap energies of 3.37 and 3.30 eV, respectively, which restricts their photocatalytic activity under visible light illumination. Along with this, the recombination of photogenerated charge carriers is so high in both ZnO and TiO2 because of its high exciton binding energy. In addition, it is responsible for photo-corrosion. Thus, it is important to develop visible-light-active photocatalyst materials [9,10,11].
Recently, transition metal dichalcogenides (TMDs) have been widely used in different applications, such as supercapacitors, water splitting, photocatalysis, etc., due to their sufficient properties, such as their photoelectrochemical stability, good charge carrier mobility, fast nonlinear response, etc. [10]. Molybdenum sulfide (MoS2) is a two-dimensional material and exhibits a good response under visible light (450–700 nm) illumination. These properties of MoS2 help to improve its charge transfer properties and light absorption capacity and the performance of the heterojunction. As with graphene, MoS2 has a stratified structure of S-Mo-S atoms [9,12]. The lattice structure and morphology of MoS2 play an important role in its catalytic performance. However, a major drawback of MoS2 is the recombination of the photogenerated charge carriers in MoS2 being higher, which directly impacts its photocatalytic efficiency. According to the literature study, the noble metal nanoparticles had great advantages, such as the avoidance of charge recombination, as well as enhanced light absorption capacity [13,14]. The plasmonic NPs of noble metal (Ag, Au, and Pt) play an important role in photocatalytic hydrogen production as co-catalysts [15]. The addition or decoration of these noble metals onto semiconductor photocatalyst materials, such as MoS2, g-C3N4, TiO2, ZnO, etc., will result in the development of the space charge region. The space charge region is helpful for effective charge separation and ultimately improves the photocatalytic efficiency of the process [9]. Surface plasmon resonance (SPR) will be generated using noble metals under visible light illumination, and it forms a strong and non-homogeneous electric field near to the semiconductor surface. Through this process, plasmonic energy is created, which will be used to avoid the recombination of photo-excited electrons. Simultaneously, it creates a plasmonic heating zone that will be used in chemical transformation for breaking water molecules [10,16].
Zhao et al. synthesized a Ag-MoS2 nanohybrid composite using a laser-assisted technique and used it for gas sensing applications [17]. Liu et. al. prepared Ag-doted 1T-2H-MoS2 as a photocatalyst and studied the degradation of Cr (VI) and MB and observed that Ag nanoparticles could significantly enhance the charges’ separation efficiency [18]. Krishnan et al. prepared the Ag-MoS2 composite using a combination of hydrothermal and wet-chemical methods for electrochemical nitrobenzene sensing [19]. Cheah et al. reported the facile synthesis of Ag/MoS2 nanocomposite photocatalyst for visible-light driven photocatalytic hydrogen gas production and found that Ag NPs can act as an efficient co-catalyst for the MoS2 nanoflakes and later improve the hydrogen gas evolution rate. Additionally, they observed that, at 20 wt%, Ag-loading exhibits the highest photocatalytic activity with a hydrogen gas evolution of 179.5 μmol H2 gcat−1 [7]. Sun et al. prepared Ag-MoS2 as a photocatalyst and studied the chromium reduction (Cr (VI) reduced into Cr (III)) in an aqueous solution and observed that, in dark conditions, MoS2 acts as an electron donor through its self-oxidation and Cr (VI) anions were electron acceptors [20]. Therefore, noble metals can enhance the light adsorption capacity and catalytic performance of composite or heterojunction.
In this study, Ag-MoS2 composite photocatalyst was designed using the hydrothermal method. The prepared Ag-MoS2 composite photocatalyst was analytically characterized using different characterization techniques, such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-Vis spectroscopy, etc. The prepared Ag-MoS2 composite photocatalyst and commercialized MoS2 powder were used to study the photocatalytic hydrogen production activity. Photocatalytic hydrogen production activity shows that the Ag-MoS2 composite photocatalyst exhibits a better photocatalytic performance than the commercialized MoS2 powder. The effects of Ag on the light absorption capacity, photoluminescence, and photocatalytic properties of MoS2 were investigated in detail, and the possible mechanisms that contribute to the improvement of visible-light driven photocatalytic performance for the Ag-MoS2 composite are discussed. The current work exemplifies the potential of designing a composite photocatalyst material by integrating Ag NPs as co-catalyst onto MoS2 for improved visible-light-driven photocatalytic hydrogen production, which can serve as one of the feasible approaches for the advancement of renewable energy research.

2. Results and Discussion

Structural characterizations of MoS2 and Ag-MoS2 composite photocatalysts were performed using X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Figure 1 displays the XRD patterns for MoS2 and Ag@MoS2 composite photocatalysts. XRD patterns were recorded by varying the diffraction angle (2θ) from 10 to 70°. For both MoS2 and Ag@MoS2, composite XRD patterns are polycrystalline. In the case of MoS2, most prominent peaks were found at 2θ 14.40°, 32.80°, and 39.71°, which correspond to the (002), (100), and (103) planes, respectively. From the XRD pattern, the hexagonal crystal structure of MoS2 is confirmed using PDF file no. 77-1716 [21]. While, in the case of the Ag/MoS2 composite, some additional peaks were observed at 38.02°, as well as 64.36°, which were associated with the Ag. The peaks are well matched with the PDF file No. 04–0783, which confirms the face-centered cubic structure of Ag [22]. The symbol * denotes the Ag peaks. XRD results confirm the formation of the Ag-MoS2 composite without any impurities. Further, the prepared Ag-MoS2 composite is characterized using Raman spectroscopy and XPS spectroscopy.
The different types of bonding, vibrational modes, and defects were analyzed using Raman spectroscopy. Figure 2 displays the Raman spectra of MoS2 and Ag-MoS2 composite. For MoS2, the major peak positioned at 379.43 cm−1 corresponds to E12g mode, which is associated with the in-plane opposite vibration of two S atoms [23]. The peaks were observed at 821.13 and 991.61 cm−1 due to the oxidation of MoS2 by laser irradiation, leading to the formation of MoO3, which presents the vibrational energy of MoO3. Other major peaks found at 284.23 and 821.13 cm−1 are associated with MoS2. In the case of the Ag-MoS2 composite, the peaks found at 1066.33, 1301.89, and 1536.68 cm−1 are associated with Ag [24]. So, Raman investigation confirms the formation of a Ag-MoS2 composite.
To understand the chemical composition and valence states of synthesized materials, X-ray photoelectron spectroscopy (XPS) study was conducted. Figure S1 presents the XPS spectrum of MoS2. Figure S1a shows the high-resolution XPS spectrum of Mo 3d, which splits into Mo 3d 5/2 and Mo 3d 3/2 with binding energy at 229.12 and 232.3 eV. The S 2p spectrum (Figure S1b) shows two major peaks at binding energy 162.02 and 163.05 eV, while, in the case of oxygen (O 1s), peaks were found at binding energies of 529.89 and 532.32 eV (Figure S1c). The peaks at 529.89 and 532.32 eV correspond to lattice oxygen and OH− groups, respectively [3]. Figure 3 presents the XPS study of the Ag-MoS2 composite. Figure 3a presents the survey scan spectrum of the Ag-MoS2 composite. It confirms the occurrence of Ag 3d, Mo 3d, S2p, and O 1s elements. No other impurity elements were detected. Figure 3b presents the Ag 3d spectrum, which splits into two major peaks having binding energy 373.56 and 367.51 eV, which confirm Ag 3 d3/2 and Ag 3d 5/2, respectively [25]. Figure 3c displays the high-resolution Mo 3 d spectrum. The Mo 3d spectrum splits into Mo 3d 5/2 and Mo 3d 3/2 with binding energies (BEs) of 229.02 and 232.21 eV, respectively [26]. The S2p spectrum is shown in Figure 3d. The S2p spectrum splits into two major peaks, having BE at 161.90 and 163.02 eV, which are associated with the S 2p 3/2, and S 2p1/2, respectively [27]. The O 1s spectrum is shown in Figure 3e, and it is split into two major peaks with binding energies of 532.23 eV (O1) and 533.78 eV (O2), which correspond to the core level O2, and S-O bonds, respectively [1,11]. Not much difference was observed in the binding energy of MoS2, as compared to the MoS2 presented in the Ag-MoS2 composite [1].
The morphological study provides information on the size and shape (i.e., the overall surface characteristics of the synthesized nanomaterials). Figure 4 shows scanning electron microscopy images of MoS2 and Ag-MoS2 composites at different magnifications. Figure 4a presents the scanning electron microscopy (SEM) image of MoS2. For MoS2, irregularly shaped and sized particles were found, and, in some places, particles agglomerated. While, in the case of the Ag-MoS2 composite along MoS2 particles, some hexagonal-shaped Ag particles are observed and presented in Figure 4b,c. Suber [28] observed similar morphology of hexagonal tabular-shaped silver particles.
Optical properties are another significant parameter while investigating the photocatalytic properties of semiconductor materials. Figure 5a presents the absorbance spectra for MoS2 and Ag-MoS2 composites, scanned in the wavelength range of 350–1400 nm. MoS2 has an absorption edge of nearly 900 nm, while, in the case of the Ag-MoS2 composite, the absorption edge shifted towards a higher wavelength size (i.e., 925 nm). In the case of MoS2, two separated exciton peaks are observed at 695 and 660 nm. No significant difference is observed in the absorption spectra of MoS2 and the Ag-MoS2 composite except for a minor difference in terms of absorption intensity. For the Ag-MoS2 composite, enhanced visible light absorption was detected, which suggests the exclusive co-catalyst function of Ag. The following equation was used to calculate the band gap energy [29]:
αhν = A(hν − Eg)n
The band gap energy values were found to be 1.57 and 1.67 eV for Ag-MoS2 composites and MoS2, respectively, and they are presented in Figure 5b. “A” is constant, n is order, Eg is band gap energy, and hν is the photon energy. The reduced bandgap energy of MoS2, after its composite formation with Ag, suggests that there exists a change in the electronic structure of MoS2 [30].
Photoluminescence (PL) spectroscopy was used to determine the recombination of photo-generated charge carriers in the catalyst. Recombination of photo-generated charge carriers is determined using fluorescence intensity. The higher the fluorescence intensity, the more there is recombination of charge carriers, and vice versa [1]. These photogenerated charge carriers facilitate redox reactions. Figure S2 shows the photoluminescence (PL) spectra of the MoS2 and Ag-MoS2 composite photocatalysts. The PL spectra for both of the samples showed a principal emission peak centered at around 660 nm. The lower PL emission intensity of the Ag-MoS2 composite suggested that Ag nanoparticles hindered the recombination of the charge carrier (i.e., the highest charge separation efficiency) [11].

3. Photocatalytic Hydrogen Production Activity

Visible-light-driven photocatalytic hydrogen production activity of MoS2 and Ag-MoS2 composites were investigated and presented in Figure 6. Figure 6a presents the plot of hydrogen evolution as a function of illumination time. A hydrogen production experiment was conducted for 150 min. From the graph, it is revealed that the hydrogen evolution rate increased with increasing illumination time. For MoS2 and Ag-MoS2, the composite hydrogen gas production rates were found to be 24.8 and 43.4 µmol/h, respectively. Thus, the Ag-MoS2 composite exhibits a better hydrogen production rate than MoS2. The enhanced hydrogen production rate of the Ag-MoS2 composite is attributed to “effective charge generation, separation, and transfer, along with more active sites available for redox reactions and minimum charge recombination” [3]. In addition, this improvement is observed mainly due to the electron sink effect [31,32]. In the case of the Ag-MoS2 composite, Ag particles act as a sink of electrons for those photoexcited electrons from MoS2 and stop the recombination of charge carriers. Thus, photo-generated charge carriers trigger photocatalytic hydrogen evolution reactions [33]. To confirm the role of Ag nanoparticles for clarification, using the same experimental conditions, the photocatalytic activity of Ag NPs was also tested, and it was found that no trace amount of H2 gas was detected after 150 min of reaction. From the above observation, it is confirmed that Ag nanoparticles were able to trigger the photosensitization process, which can be excluded, and the hydrogen gas evolution improvement is indeed contributed by the Ag NP that serve as a co-catalyst. The Ag-MoS2 composite was selected for further study to observe the stability of the photocatalyst. The Table S1 comparison of photocatalytic hydrogen activity was proposed, using different photocatalysts.
A photostability study of the Ag-MoS2 composite was performed by recycling tests. The photostability study or recycling study is presented in Figure 6b. The stability study was conducted for up to three cycles and each cycle of four hours. From the plot, it is observed that the Ag-MoS2 composite exhibits good stability. During the first cycle of four hours, 181.2 µmol hydrogen gas evolved. Up to the third cycle, 169.2 µmol hydrogen gas evolution was observed. There is a slight decrease in hydrogen production activity, which was detected after the third cycle. Photo-corrosion of photocatalysts and the loss of the photocatalyst during the recovery process are responsible for the decrease in H2 production activity [34].
The photostability of the Ag-MoS2 composite catalyst is determined by characterization techniques, such as X-ray diffraction study before and after the photocatalytic reaction was presented in Figure S3. Similar photocatalytic experimental conditions were used to check the stability. This photostability was checked three times using the Ag-MoS2 composite photocatalyst. It showed that the recyclability of the Ag-MoS2 composite photocatalyst was good. From the results, it was observed that the structure of the catalyst after the third cycle did not change much compared to before the photocatalytic experiment, except for the slight change in the XRD peak intensity.
The electron sink effect is responsible for the improvement in hydrogen production [31]. In the case of the Ag-MoS2 composite, Ag nanoparticles act as a pool to gather photoexcited electrons from the MoS2 and to stop the recombination of photogenerated charge carriers. These photoexcited electrons will be responsible for initiating the photocatalytic reactions and, thereby, enhancing the hydrogen gas production rate. From the UV-Vis absorption spectroscopy, it is observed that the absorption of the Ag-MoS2 composites does not change much, but the hydrogen gas evolution rate appears to be quite different. Making a composite of Ag with MoS2 boosted the H2 gas production rate [33].

Photocatalytic Mechanism Discussion

Ag nanoparticles served as co-catalysts in the Ag-MoS2 composite, which is responsible for the improvement in the hydrogen gas evolution rate. Such a composite has encouraged the development of a space charge region and is presented in Figure S4. The space charge region will ease the effective charge separation. According to the literature, the Fermi energy level of MoS2 lies in the potential range of 4.7–4.0 eV [35], and the Fermi energy level for Ag nanoparticles is 5.5 eV [36]. Such differences in Fermi energy levels (0.8–1.5 eV) result in the bending of bands and the formation of a space charge region at the interface of the Ag-MoS2 composite [35]. The space charge region is a charge-free region and is occupied with a continuum of opposite charges. In this case, MoS2 and Ag co-catalyst sides are occupied by negative and positive charges, respectively [37]. The positive and negative charges are distributed in a parallel manner and are separated by a gap (space charge region) [38]. Due to the high energy barrier, the movement of charge carriers is prevented, which results from the internal electric field under thermal equilibrium conditions. Upon illumination, the thermal equilibrium condition is disturbed by the incident photons, which causes the electric field in the space charge region to strictly promote the photoexcited electrons to bypass the potential barrier. After that, these photoexcited electrons will underpass through the space charge region and gather on the surface of the co-catalyst (Ag). Holes are left on the MoS2 side. Hence, the formation of a space charge region can prevent the recombination of charge carriers and the effective separation of electrons–holes [39]. In the Ag-MoS2 composite, Ag nanoparticles act as an electron pool for photoexcited electrons for the conversion of hydrogen ions to H2 molecules. As compared to pure MoS2, the reduction in water molecules is greatly eased at the surface of Ag nanoparticles, while oxidation reactions will take place on the active edges of the MoS2. Na2S and Na2SO3 were used as hole sacrificial agents to avoid the degradation of the composite. These sacrificial agents are widely used in sulfide-based photocatalyst materials [40]. It was found that the sulfur ions (S2− and SO32−) from these sacrificial agents (Na2S and Na2SO3) provide an extra sulfur ion to enable surface reconstruction of the MoS2, as well as prevent further oxidation of the MoS2 [41]. Additionally, the sacrificial agents act as electron donors to react with the photo-generated holes [41,42], and they hence increase the tendency of the photo-generated electrons to transfer to the conduction band of the MoS2.
The addition or incorporation of metal nanoparticles to semiconductor material acts as an alternative way to enhance the hydrogen gas evolution in water due to the fast transfer of the excited electrons generated by photon absorption from the semiconductor to the metal, which results in undesired recombination by deexcitation to the ground state severely inhibited. The contribution of the metal nanoparticles is more an important due to effective charge transfer within the composite if compared to the MoS2. In the case of pure MoS2, there is a high possibility for the recombination of excitons to occur [43].

4. Experimental Details

4.1. Chemicals

Molybdenum disulfide powder (MoS2), sodium hydroxide (NaOH), silver nitrate (AgNO3), sodium sulfite (Na2SO3), and sodium sulfide (Na2S) were used. The above required chemicals were provided by Sigma Aldrich Company, Bengaluru, India.

4.2. Synthesis of the Ag@MoS2 Composite

Commercialized MoS2 was spread into a 20 mL mixture of deionized water (DI) and ethanol (2:8) for 30 min in the first flask. In another flask, 0.2 M of 9% weight percentage of AgNO3 solution was prepared. Both solutions were mixed, and the suspension was stirred. Using a NaBH4 aqueous solution, the reduction of Ag was achieved. The chemical reduction process was simple, cost-effective, environmentally friendly, and performed at room temperature. An aqueous solution of NaBH4 was added dropwise to the above solution. The reaction solution was moved to a sealed Teflon autoclave. The hydrothermal reactor was placed in a furnace for 12 h at 120 °C.

4.3. Characterizations

Crystal structures of the prepared materials were analyzed using X-ray diffraction (XRD; CuKα radiation (λ = 1.5406 Å) from a Bruker D2 Phaser, Mannheim, Germany). The Raman spectrum of the Ag-MoS2 sample was analyzed using a Renishaw in Via Raman spectrometer using a He–Ne (633 nm) laser excitation source. Elemental composition was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Waltham, MA, USA). The K-Alpha was set up by using a monochromatic Al Kα X-ray source. The morphology of the prepared material was observed by using the SEM instrument JSM-7600F of the Japan Electron Optics Laboratory (JEOL, Tokyo, Japan). Optical properties were studied using UV–Vis absorption spectroscopy (Shimadzu: UV-1800, Kyoto, Japan). PL measurements were carried out on a micro-confocal Raman spectrometer (Horiba HR Evolution) equipped with an Olympus BX 41 microscope. The 532 nm laser was used as the excitation source.

4.4. Photocatalytic Hydrogen Production

The photocatalytic hydrogen production study was carried out in a closed Pyrex reactor using commercialized MoS2 and Ag@MoS2 composites. An amount of 300 W of the xenon arc lamp 172 mW/cm2 with a UV cutoff filter (λ > 420 nm) was used as a visible light source. For hydrogen production measurement, 25 mg amount of each catalyst was added into 50 mL of DI water containing 0.1 M Na2S and 0.2 M Na2SO3. The formed solution was bubbled with N2 gas for 30 min to remove the oxygen traces. After that, the reaction mixture was exposed to a light source. At different time intervals, gas samples were collected using a syringe. The photocatalytic reaction was performed under ambient conditions. An amount of 1 mL of gas was sampled every hour with a gas syringe and analyzed using gas chromatography (GC; GC-2014AT, Shimadzu, Japan; thermal conductivity detector, Ar carrier gas, molecular sieve 5 Å column) to quantify the gas composition.

5. Conclusions

Visible light-activated Ag-MoS2 composite was successfully prepared using the hydrothermal method. The formation of the Ag-MoS2 composite was confirmed using different techniques, such as X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. X-ray diffraction study confirms the formation of Ag-MoS2 composite with a hexagonal and face-centered cubic crystal structure of MoS2 and Ag, respectively. Along with this, Raman spectroscopy and X-ray photoelectron spectroscopy confirm the formation of a Ag-MoS2 composite. From the morphological investigation, it was observed that hexagonal-shaped Ag particles formed. An optical study proves that Ag-MoS2 exhibits strong absorption in the visible region, with an energy bandgap of 1.57 eV. For MoS2 and Ag-MoS2 composites, hydrogen gas production rate was found to be 24.8 and 43.4 µmol/h, respectively. The Ag-MoS2 composite exhibits nearly double the hydrogen production rate of the MoS2, and this enhanced photocatalytic activity of the Ag-MoS2 composite due to the electron sink effect. During the stability study, 181.2 and 169.2 µmol hydrogen gas evolved for the first and third cycles, respectively, which proves that Ag@MoS2 exhibits good photostability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13040716/s1, Figure S1: XPS spectrum of MoS2; Figure S2: Photoluminescence spectra of MoS2 and Ag@MoS2 composite; Figure S3: XRD patterns Ag@MoS2 composite before and after (third cycle) the photocatalytic experiment; Figure S4: Schematic diagram of visible light hydrogen gas evolution over MoS2 in the presence of Ag NPs as co-catalyst; Table S1: Comparison of photocatalytic hydrogen activity using different photocatalysts [7,44,45,46].

Author Contributions

Conceptualization, S.-W.K.; methodology, Y.M.H. and A.G.D. formal analysis, A.A.Y.; investigation, A.A.Y., Y.M.H. and A.A.Y.; writing—original draft preparation, Y.M.H. and A.A.Y.; writing—review and editing, A.A.Y., Y.M.H. and S.-W.K.; supervision, S.-W.K.; validation, S.-W.K.; project administration, S.-W.K.; funding acquisition, S.-W.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data included in this study are available upon request by contact with the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction patterns of the MoS2 and Ag-MoS2 composites.
Figure 1. X-ray diffraction patterns of the MoS2 and Ag-MoS2 composites.
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Figure 2. Raman spectra of MoS2 and Ag-MoS2 composites.
Figure 2. Raman spectra of MoS2 and Ag-MoS2 composites.
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Figure 3. (a) X-ray photoelectron spectroscopy (XPS) spectrum of Ag/MoS2 composite, (b) Ag 3d spectrum, (c) Mo 3sd spectrum, (d) S 2p spectrum, and (e) O 1s spectrum.
Figure 3. (a) X-ray photoelectron spectroscopy (XPS) spectrum of Ag/MoS2 composite, (b) Ag 3d spectrum, (c) Mo 3sd spectrum, (d) S 2p spectrum, and (e) O 1s spectrum.
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Figure 4. SEM micrographs of (a) MoS2 and (b,c) Ag-MoS2 composites at different magnification.
Figure 4. SEM micrographs of (a) MoS2 and (b,c) Ag-MoS2 composites at different magnification.
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Figure 5. (a) Absorption spectra and (b) band gap plots for the MoS2 and Ag-MoS2 composites.
Figure 5. (a) Absorption spectra and (b) band gap plots for the MoS2 and Ag-MoS2 composites.
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Figure 6. Photocatalytic H2 production activity using MoS2 and Ag-MoS2 composites and Ag NPs: (a) cycling tests for photocatalytic H2 production; (b) H2 evolution in three consecutives for 4 h cycles using Ag-MoS2 composite. (★ indicate the amount of hydrogen produced.)
Figure 6. Photocatalytic H2 production activity using MoS2 and Ag-MoS2 composites and Ag NPs: (a) cycling tests for photocatalytic H2 production; (b) H2 evolution in three consecutives for 4 h cycles using Ag-MoS2 composite. (★ indicate the amount of hydrogen produced.)
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Yadav, A.A.; Hunge, Y.M.; Dhodamani, A.G.; Kang, S.-W. Hydrothermally Synthesized Ag@MoS2 Composite for Enhanced Photocatalytic Hydrogen Production. Catalysts 2023, 13, 716. https://doi.org/10.3390/catal13040716

AMA Style

Yadav AA, Hunge YM, Dhodamani AG, Kang S-W. Hydrothermally Synthesized Ag@MoS2 Composite for Enhanced Photocatalytic Hydrogen Production. Catalysts. 2023; 13(4):716. https://doi.org/10.3390/catal13040716

Chicago/Turabian Style

Yadav, Anuja A., Yuvaraj M. Hunge, Ananta G. Dhodamani, and Seok-Won Kang. 2023. "Hydrothermally Synthesized Ag@MoS2 Composite for Enhanced Photocatalytic Hydrogen Production" Catalysts 13, no. 4: 716. https://doi.org/10.3390/catal13040716

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