Fabrication of amorphous subnanometric palladium nanostructures on metallic transition metal dichalcogenides for efficient hydrogen evolution reaction

Abstract

Fabricating solution-processable composite materials of transition metal dichalcogenides (TMDs) with ultrasmall noble metal structures employing an easy preparation method poses a significant challenge. In this study, we utilized a green, one-step synthetic method by directly employing electrochemical lithium intercalation-based exfoliated metallic TMD nanosheets (MoS₂, WS₂, and TiS₂) to reduce palladium ions (Pd²⁺) to metallic Pd⁰, leading to the deposition on their surfaces. The resulting Pd nanoparticles (Pd NPs) in composites (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) were found to be amorphous, with a size ranging from 0.81 to 1.37 nm. The impact of Pd NP size on hydrogen evolution reaction (HER) activity was elucidated. Among the fabricated composites, Pd-MoS₂ exhibits the best HER performance, attributed to its smallest Pd NP size (0.81 nm). It shows an overpotential of 70 mV at a current density of 10 mA cm⁻², along with a Tafel slope of 43 mV dec⁻¹. These HER performance metrics surpass those of most Pd-decorated 2D catalysts.

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1. Introduction

The electrocatalytic hydrogen evolution reaction (HER) is a crucial process in electrochemistry and renewable energy technology involved in the production of clean and sustainable hydrogen fuel.^1–3 It involves the reduction of protons (H⁺) from solution to generate molecular hydrogen (H₂) using an external electrical energy source. This reaction holds significant importance as hydrogen is considered a promising clean energy carrier due to its high energy density and environmentally friendly combustion, producing only water as a byproduct.^4–6 The HER typically occurs at the cathode of an electrochemical cell, where an electrocatalyst facilitates the reaction by lowering the energy barrier required for the proton reduction. This catalyst enhances the kinetics of the reaction, making it more efficient and economically viable.^7–9 The design and development of efficient electrocatalysts for the HER are critical for advancing hydrogen production technologies.^10–12 Various materials, including noble metals as well as Earth-abundant and cost-effective catalysts have been explored to catalyze the HER.^13–15 Researchers aim to improve the performance of electrocatalysts by enhancing their activity, stability, and cost-effectiveness. Tailoring the catalyst's surface structure, morphology, composition, and electronic properties is vital in optimizing its efficiency for hydrogen evolution.

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), transition-metal carbides and/or nitrides (MXenes), have garnered significant attention for HER applications.^14,16–18 These 2D materials possess atomically thin layers that provide high surface-to-volume ratios, exposing a significant number of active sites for catalysis.¹⁹ Their electronic band structures can be tailored by manipulating the number of layers, defects, or doping, allowing precise control over their catalytic activity.^20–22 The fabrication of composites involving 2D materials and noble metals stands as one of the most effective strategies for the HER. At present, the methods used to prepare these composites often necessitate stringent conditions, such as toxic reducing agents,²³ stabilizers,²⁴ high temperature treatments,^25,26 or complex procedures.²⁷ In this study, we introduce a green preparation method that circumvents the need for using reducing agents or stabilizers and follows a one-step approach for preparing 2D TMDs (MoS₂, WS₂, and TiS₂) decorated with amorphous subnanometric palladium nanostructures at room temperature. The metallic TMD nanosheets act as both a reducing agent and a stabilizer. Metallic TMD nanosheets, being electron-rich and possessing a matched redox potential, are capable of reducing Pd²⁺ to Pd⁰.^28,29 The reduced Pd nanoparticles are stabilized on the TMD surface,³⁰ ensuring their stability during the HER test. The resulting Pd-TMD composites demonstrate comparable HER performance to that of commercial Pt/C.

2. Experimental section

2.1 Materials

Molybdenum disulfide powder (MoS₂, Innochem), tungsten disulfide powder (WS₂, Macklin), titanium disulfide powder (TiS₂, Sigma-Aldrich), N-methylpyrrolidone (NMP, Aladdin), polyvinylidene difluoride (PVDF, Sigma-Aldrich), natural graphite (Bay Carbon), copper foil (Aritech Chemazone Private), lithium foil (DodoChem), polypropylene (pp) film (Celgard 2300), lithium hexafluorophosphate dissolved in an ethylene carbonate, dimethyl carbonate and methylene carbonate mixture (1 M LiPF₆ in EC : DMC : EMC, 1 : 1 : 1 vol%, DodoChem), platinum acetate (Pd(OAc)₂, Macklin), and acetone (99.5%, Anaqua Global International Inc. Limited) were used. Milli-Q water (Millipore, Billerica, MA) was used in all experiments.

2.2 Synthesis of TMD (MoS₂, WS₂, and TiS₂) nanosheet dispersions

2D TMD (MoS₂, WS₂, and TiS₂) nanosheets were synthesized using a battery intercalation-based exfoliation method that we developed.^31,32 In this process, lithium intercalation was carried out within a coin cell assembly, with bulk TMD-coated copper film serving as the cathode, lithium foil as the anode, and a 1 M lithium hexafluorophosphate (LiPF₆) solution dissolved in a mixture of ethyl carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a 1 : 1 : 1 volume ratio as the electrolyte. The optimization of lithium intercalation in bulk TMDs was achieved by carefully controlling the cutoff voltage and current during the galvanostatic discharge process. Subsequently, the lithiated TMDs were extracted from the battery and subjected to sonication in deionized (DI) water, completing the exfoliation process. The resulting dispersions underwent multiple washes with DI water to eliminate any residual electrolyte and potential impurities. These TMD nanosheet solutions were employed as the basis for further characterization or subsequent integration with Pd nanostructures.

2.3 Synthesis of Pd-TMD (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) composites

TMD nanosheets (MoS₂, WS₂, and TiS₂) were individually dispersed in DI water at a concentration of approximately 40 mg L⁻¹. To each TMD nanosheet solution, 100 μL of a 0.8 mg mL⁻¹ palladium acetate solution was added. The solution was sonicated for 30 min to ensure thorough mixing and interaction. The resulting mixture underwent centrifugation at 9000 rpm for 5 min. Subsequently, the supernatant was carefully removed. The collected precipitate was re-dispersed in fresh DI water. This washing process was repeated three times to eliminate any residual reactants and impurities effectively. The purified precipitate containing Pd-TMD composites was re-dispersed in DI water for subsequent characterization and electrode preparation.

2.4 Material characterization

A transmission electron microscope (TEM, JEM 2100F) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector was utilized for the examination of the morphology and elemental composition of the materials. The chemical state of the samples was characterized using an X-ray photoelectron spectrometer (XPS, Thermo Fisher ESCALAB XI XPS). The phase purity of the materials was confirmed by X-ray diffraction (XRD, D2 PHASER XE-T). Investigation of atomic vibration patterns was carried out using a Raman spectrometer (WITec alpha300 confocal Raman microscope, wavelength 532 nm). The surface charge of the TMD and composites was determined via zeta potential measurements (Malvern Zetasizer Nano series). These characterization techniques were employed to comprehensively analyze and understand the morphology, elemental composition, chemical state, phase purity, and atomic vibration patterns of the synthesized materials.

2.5 Electrocatalytic measurements

TMD and Pd-TMD composites were evaluated for their HER performance. For the catalyst ink preparation, each sample was mixed with ethanol and 5% Nafion perfluorinated resin solution (v/v = 25 : 1), forming the electrode coating materials. This mixture was subjected to a 30 min ultrasonication process to achieve optimal dispersion. Following this, 50 μl of the well-dispersed material was drop coated onto a 1.5 × 0.5 cm carbon paper electrode. The coated carbon paper electrode was air-dried at room temperature. The completely dried coated carbon paper electrode was then evaluated for its electrocatalytic properties using a typical three-electrode setup at room temperature. Linear sweep voltammetry (LSV) was performed in 0.5 M H₂SO₄ at a scan rate of 2 mV s⁻¹. The setup included an Ag/AgCl electrode as the reference electrode, a graphite rod as the counter electrode, and carbon paper-coated electrocatalysts as the working electrode. All potentials were converted to values relative to a reversible hydrogen electrode, with the Ag/AgCl electrode consistently used as the reference in all measurements.

2.6 Computational methods

Spin-polarized density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP) code within the projected-augmented wave method.^33,34 The exchange–correlation interactions were described by the generalized gradient approximation (GGA)³⁵ within the Perdew–Burke–Ernzerhof (PBE) functional framework. The long-range van der Waals (vdW) interaction is treated with the empirical correction in Grimme's scheme (DFT-D3).³⁶ The cutoff energy for plane-wave basis sets is 450 eV. The convergence thresholds for force and total energy are 0.05 eV Å⁻¹ and 10⁻⁵ eV, respectively. The vacuum thickness was set to 16 Å to avoid the electronic interaction of periodic images. The Monkhorst–Pack scheme was adopted to sample the Brillouin region with a (3 × 3 × 1) k-point mesh grid for WS₂, MoS₂, and Pd(111), (2 × 2 × 1) for TiS₂ and (4 × 3 × 1) for the Pd(211) supercell. The projected crystal orbital Hamilton population (pCOHP)³⁷ analysis was employed to analyze the bonding/anti-bonding population of H–S and H–Pd. The Bader charge method was performed to analyze the changes in atomic charges.³⁸ The Pd(111) slab and the Pd(211) slab contain four atomic layers; the atoms in the bottom two layers were fixed at a bulk truncated position, while the other atomic layers and the adsorbates were allowed to relax completely until meeting the convergence criteria.

The change in Gibbs free energy (ΔG) for each elementary step was calculated by exploiting the standard hydrogen electrode (SHE) model suggested by Nørskov et al.,³⁹ given by:

ΔG_H = ΔE_H + ΔE_ZPE − TΔS_H + ΔG_pH

Here, ΔE_ZPE and ΔS represent the change in zero-point energies and entropy respectively, and T is the system temperature (298.15 K). ΔG_pH = 2.303k_BTpH represents the free energy contribution due to the variations in H concentration, where k_B is the Boltzmann constant and the pH value was assumed to be zero for an acidic medium. ΔE_H represents the energy required to increase the coverage by one hydrogen atom, which is calculated as follows:

ΔE_H = E[base + nH] − E[base + (n − 1)H] − ½E[H₂]

The base is the MoS₂, WS₂, TiS₂, Pd(111), and Pd(211) catalyst. E[base + nH], E[base + (n − 1)H], and E[H₂] are the total energy of the catalyst system with n and n − 1 adsorbed hydrogen atoms on the surface and of a gas phase H₂ molecule, respectively.

3. Results and discussion

In this work, the metallic 2D MoS₂, WS₂, and TiS₂ nanosheets were prepared from their bulk counterparts using an electrochemical lithium intercalation-based exfoliation method (refer to Experimental section and Fig. S1†). The lithium intercalation process induces the phase transition because lithium intercalation involves the electron injection from the s orbitals of guest lithium to the d orbitals of the host transition metal atoms (Mo, W, and Ti), to maintain overall charge neutrality.⁴⁰ Consequently, this method enables the fabrication of metallic phase MoS₂, WS₂, and TiS₂ nanosheets with negative charge after exfoliation (Fig. S2–S4†). We previously systematically optimized the electrochemical lithium intercalation conditions, including the discharging current and cutoff voltage, to ensure appropriate amounts of intercalated lithium in the interlayer of bulk TMDs for high-yield mono- or few-layer nanosheets upon exfoliation. Insufficient or excessive intercalated lithium ions can lead to the production of thick nanosheets or structural decomposition after exfoliation. For further details, see ref. 32. These metallic TMD nanosheets become enriched with additional electrons, capable of effectively reducing Pd²⁺ to Pd⁰ nanoparticles (Pd NPs) that are deposited on their surface (Fig. 1a). These fabricated composites (Pd-TMDs) were then employed to assess their electrocatalytic properties for the HER (Fig. 1b).

Fig. 1 (a) Schematic illustration of the fabrication of Pd-TMD (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) composites by the reduction of the Pd(OAc)₂ precursor using the corresponding metallic TMD nanosheets as the reducing agent. (b) Pd-TMD composites used for the hydrogen evolution reaction.

After the exfoliation and washing steps, the TMD (MoS₂, WS₂, and TiS₂) materials exhibit a nanosheet morphology characterized by a smooth surface and excellent crystallinity, as depicted in Fig. S5.† Upon composite formation with Pd(OAc)₂, TEM analyses of the Pd-modified TMD composites were conducted, and the results are presented in Fig. 2 and Fig. S6.† Specifically, Fig. 2a, b, e, f, i and j emphasize the dense loading of monodisperse Pd nanoparticles (NPs) onto the MoS₂, WS₂, and TiS₂ nanosheets, respectively. A detailed examination through high-resolution TEM images (Fig. 2c, g and k) revealed the amorphous structure of the Pd NPs. The measured d-spacing of 0.27 nm, shown in Fig. 2c and g, corresponds to the (100) plane of MoS₂ and WS₂, respectively.³¹ High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pd-MoS₂, Pd-WS₂, and Pd-TiS₂ are shown in Fig. 2d, h and l, respectively. The EDS mapping images directly validate the homogeneous dispersion of Pd NPs on MoS₂, WS₂, and TiS₂ nanosheets (Fig. 2d, h, l and Fig. S7†).

Fig. 2 The morphologies of Pd-TMDs (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂). TEM images of (a and b) Pd-MoS₂, (e and f) Pd-WS₂, and (i and j) Pd-TiS₂. The corresponding high-resolution TEM images are shown in (c) Pd-MoS₂, (g) Pd-WS₂, and (k) Pd-TiS₂. EDX-mapping results are presented in (d) (Pd-MoS₂, HAADF-STEM image and elemental mapping of Mo, S, and Pd), (h) (Pd-WS₂, HAADF-STEM image and elemental mapping of W, S, and Pd), and (l) (Pd-TiS₂, HAADF-STEM image and elemental mapping of Ti, S, and Pd).

The average sizes of Pd NPs in Pd-MoS₂, Pd-WS₂, and Pd-TiS₂, as illustrated in Fig. 3a–c, were determined through statistical analysis to be 0.81 nm, 0.90 nm, and 1.37 nm, respectively. These findings confirm the successful fabrication of subnanometric Pd NPs on MoS₂ and WS₂ nanosheets. The XRD patterns exclusively exhibit the (002) peak for MoS₂ and WS₂, as well as the (001) peak for TiS₂, respectively. Notably, no discernible peaks corresponding to Pd NPs were observed, further confirming the amorphous nature of the Pd NPs formed on these TMD nanosheets. These findings align consistently with the observations from the high-resolution TEM images depicted in Fig. 2c, g and k. The chemical states of pristine TMD nanosheets (MoS₂, WS₂, and TiS₂) and those after Pd NP decoration (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) were assessed through XPS measurements. All the Pd-TMD samples exhibit a strong Pd 3d signal in the XPS full spectra (Fig. S8†), confirming the successful decoration of Pd NPs. As shown in Fig. 3e, Pd-MoS₂, Pd-WS₂, and Pd-TiS₂ exhibit distinct doublets at approximately 335.4 and 340.7 eV, corresponding to Pd⁰ 3d_5/2–3/2 in the Pd 3d spectrum.⁴¹ This unequivocally indicates the reduction of Pd²⁺ to Pd⁰ within the system. Following the reaction, the Mo 3d_5/2–3/2 spectrum (Fig. 3f), the W 4f_7/2–5/2 spectrum (Fig. 3g), and the Ti 2p_3/2–1/2 spectrum (Fig. 3h) within Pd-MoS₂, Pd-WS₂, and Pd-TiS₂, respectively, exhibit discernible shifts toward higher binding energies when compared to their pristine TMD nanosheets. These observed peak shifts serve as direct evidence for electron transfer from the TMD nanosheets to Pd²⁺, facilitating the reduction of the latter to form Pd NPs. This charge transfer is also revealed through their Raman and zeta potential results. As shown in Fig. S4,† after growing Pd NPs on TMD nanosheets, all the Raman peaks of Pd-TMDs shift to higher wavenumbers compared to the pristine TMDs. This shift in each peak is attributed to the charge transfer between the Pd NPs and TMDs. The peaks shift to higher frequencies, indicating direct charge transfer from TMDs to Pd NPs.^41,42 Additionally, the zeta potential results of Pd-TMDs (Fig. S9†) show that following the growth of Pd NPs, the zeta potential values of Pd-MoS₂, Pd-WS₂, and Pd-TiS₂ increased to −1.57 mV, −3.07 mV, and −2.36 mV, respectively. This decrease in negative charge on the TMD surface further supports the observed charge transfer from TMDs to Pd nanoparticles after the reaction. Besides, the dominant metallic phase MoS₂ and WS₂, as well as a pure metallic phase in TiS₂, are revealed through their XPS analysis.^43–45

Fig. 3 Pd NP size and chemical states of Pd-TMDs. The Pd NP size distribution analyses of (a) Pd-MoS₂, (b) Pd-WS₂, and (c) Pd-TiS₂ were performed by counting over 100 Pd NPs using ImageJ software, and the data were fitted with the Gaussian function. (d) XRD patterns of Pd-TMDs (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂). (e) XPS Pd 3d spectra of Pd-MoS₂, Pd-WS₂, Pd-TiS₂, and Pd(OAc)₂. (f) XPS Mo 3d spectra of Pd-MoS₂ and pristine MoS₂. (g) XPS W 4f spectra of Pd-WS₂ and pristine WS₂. (h) XPS Ti 2p spectra of Pd-TiS₂ and pristine TiS₂.

The electrocatalytic performance of Pd-MoS₂, Pd-WS₂, and Pd-TiS₂ composites was evaluated in a 0.5 M H₂SO₄ electrolyte using linear sweep voltammetry (LSV). As illustrated in Fig. 4a, d and g, in comparison with pristine MoS₂, WS₂ and TiS₂ nanosheets, all Pd-decorated composites (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) exhibited enhanced activity, as evidenced by the decreased overpotentials at a current density of 10 mA cm⁻² in the polarization curves. This enhancement can be attributed to the synergistic coupling effect between TMD nanosheets and Pd NPs. From the Tafel plots, it is evident that after depositing the Pd NPs, the Tafel slope decreases from 101 mV dec⁻¹ for MoS₂ to 43 mV dec⁻¹ for Pd-MoS₂ (Fig. 4b). A similar enhancement in the catalytic activity is also observed for Pd-WS₂ and Pd-TiS₂ composites, with the Tafel slope lowering from 105 mV dec⁻¹ for WS₂ to 49 mV dec⁻¹ for Pd-WS₂ (Fig. 4e) and from 97 mV dec⁻¹ for TiS₂ to 61 mV dec⁻¹ for Pd-TiS₂ (Fig. 4h). Electrochemical impedance spectroscopy (EIS) was employed to assess the charge transfer resistance of Pd-TMDs and the pristine TMD electrodes. As demonstrated in the Nyquist plots with the equivalent circuit presented in Fig. 4c, f and i, the radii of the semicircles in the high-frequency region reflect the charge-transfer resistance, which signifies the resistance of charge transfer at the interface between the electrolyte and the electrode.⁴⁶ This parameter is crucial for understanding the electrode kinetics under specific operating conditions. All Pd NP decorated composites (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) exhibit smaller charge transfer resistance values compared to those of pristine MoS₂, WS₂, and TiS₂. These further confirm the efficient charge transfer kinetics and agree with their polarization curves (Fig. 4a, d and g) and Tafel plots (Fig. 4b, e and h). Upon analyzing the various Pd NP sizes in Pd-MoS₂, Pd-WS₂, and Pd-TiS₂, we observed a correlation between the particle size and the catalytic activity, revealing that smaller Pd NPs exhibit enhanced catalytic performance,⁴⁷ as depicted in Fig. 4j. Cyclic stability is a crucial consideration for practical applications. Herein, all the Pd-TMD composites display long-term stability with no obvious decline in the current density at an overpotential of 130 mV (vs. RHE) for over 10 h (Fig. 4k). After the HER stability test, the peak positions in the XPS spectra of Pd 3d, Mo 3d, W 4f, and Ti 2p (Fig. S10†), along with characteristic peaks in the Raman spectra (Fig. S11†) of Pd-TMDs (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂), exhibit negligible differences after the durability test, indicating their structural stability post-testing. Furthermore, SEM (Fig. S12a–c†) and HRTEM (Fig. S12d–f†) images of Pd-MoS₂, Pd-WS₂, and Pd-TiS₂ after the HER stability test reveal no obvious changes in morphology, with the monodisperse Pd nanoparticles remaining on the nanosheets without aggregation. Overall, the Pd-TMD catalysts fabricated in this study demonstrate excellent structural stability after the HER test. Besides, comparing the HER performance of Pd-TMDs with those of previously reported Pd-decorated 2D catalysts (Fig. 4l and ESI Table S1†), it is evident that the Pd-MoS₂ and Pd-WS₂ composites developed in this study exhibit much better activity than most of the other catalysts, considering factors like the Tafel slope and overpotential at a current density of 10 mA cm⁻². The superior performance can be attributed to the presence of subnanometric Pd NPs and the amorphous Pd structure in these composites.⁴⁸ Consequently, they hold promise as alternatives to commercial Pt–C catalysts for HER applications.

Fig. 4 HER performance of Pd-TMD composites. Polarization curves of Pd-TMD composites, pristine TMDs, and the commercial Pt/C catalyst are shown in (a) Pd-MoS₂, (d) Pd-WS₂, and (g) Pd-TiS₂. The corresponding Tafel plots of (b) Pd-MoS₂, (e) Pd-WS₂, and (h) Pd-TiS₂. EIS Nyquist plots of (c) Pd-MoS₂, (f) Pd-WS₂, and (i) Pd-TiS₂ composites with the corresponding equivalent circuits shown in the insets. (j) The relationship of overpotentials (η) at 10 mA cm⁻² and the Tafel slopes with the Pd NP size in Pd-MoS₂, Pd-WS₂, and Pd-TiS₂. (k) Long-term stability test of Pd-TMDs under an overpotential of 130 mV (vs. RHE) in 0.5 M H₂SO₄. (l) Comparison of the overpotentials (η) at 10 mA cm⁻² and the Tafel slopes between previously reported Pd decorated 2D catalysts and the Pd-MoS₂, Pd-WS₂, and Pd-TiS₂ developed in this work. The details can be found in ESI Table S1.†

To gain a more comprehensive understanding of the experimental phenomena, we use DFT calculations to elucidate the underlying reaction mechanism. The theoretical model of the Pd cluster, designed to be of equivalent size to the experimental system, is excessively large, resulting in substantial computational requirements in terms of time and cost. (111) and (211) are the low-index and high-index faces of Pd, and (111) has a lower surface energy (Fig. S13d†), which is also consistent with the fact that the (111) face is generally considered to be the most stable face in the face-centered cubic structure (Pd). In addition, as the particle size increases, the structure begins to resemble more closely to the bulk Pd, predominantly exposing the representative (111) plane. Therefore, we choose the (211) and (111) planes to approximate the transition trend of particles from small (unstable surface) to large (stable surface).

Fig. S13a–c† shows the structural models and possible adsorption sites of 1T-TMD (MoS₂, WS₂ and TiS₂), Pd(111), and Pd(211) systems. TMD has 3 adsorption sites, and the ontop (T1) site is a stable adsorption site (Fig. S14†). Pd(111) has 4 adsorption sites, with the most stable adsorption site being the FCC (F) site (Fig. S15a†), and Pd(211) has 14 adsorption sites, with the most stable adsorption sites being H1 and H2 (Fig. S15b†). Fig. 5a displays the thermodynamic Gibbs free energy diagrams of TMDs. H adsorption on MoS₂ and WS₂ exhibits significantly negative ΔG, suggesting that H has strong interactions with MoS₂ and WS₂ substrates, allowing the second H to absorb on the surface. The formation of the second H* is an uphill process with ΔG values of 0.91 eV and 0.52 eV for WS₂ and MoS₂, respectively, and is the rate-determining step (RDS) of the reaction. On TiS₂, in contrast, the first H* formation is endothermic, which requires less energy compared to that of the RDS of WS₂ and MoS₂. Consequently, TiS₂ exhibits better catalytic activity. The formation of H* is energetically supported on Pd(111) (Fig. 5b), resulting in a higher concentration of H atoms occupying the surface. The ΔG of H* with a coverage of 1 (9H*) is closest to 0, and it will be very difficult to adsorb an additional H atom (10H*), so the overpotential of Pd(111) is 0.21 V. Similarly, the first H* is easily formed on Pd(211) (Fig. 5c), and 9H* coverage is the RDS of the reaction with an extremely low ΔG value of 0.07 eV. In summary, the presence of Pd enhances the activity of HER and Pd(211) exhibits the highest catalytic performance, which provides a solid explanation for the improved catalytic activity observed in experiments using small-sized Pd particles. To gain a deeper understanding of the H adsorption mechanism, we present the relevant adsorption energy diagrams and electronic properties. Fig. 5e illustrates the charge density difference of H* at different coverages absorbed on various substrates. Larger electron clouds imply more charge transfer and thus exhibit strong interactions. MoS₂ with one H* and WS₂ with 2H* exist in the largest and smallest electron clouds, respectively, corresponding to the strongest and weakest interactions, which further elucidates the adsorption energy of H and the substrate (Fig. 5d). Quantitative relationships are also calculated via Bader charges (Fig. 5f). H bonded to Pd gains electrons from Pd, while H bonded to TMD loses electrons to S. The electron transfer between the second H* and S is minimal, implying their weak interaction, as shown in Fig. 5e. Furthermore, the integrated crystal orbital Hamilton population (ICOHP) analysis was also performed to quantify the population of electrons in bonding and antibonding molecular orbitals between H and S atoms and Pd atoms. As displayed in Fig. 5g, the ICOHP value for H–Pd in Pd(111) is more negative than that for H–Pd in Pd(211) (Fig. 5h), implying that H adsorption on Pd(111) is more energetically supported, which further explains the free energy diagram. Also, the trend of the bonding strength shown in Fig. S17–19† aligns with the energy. In conclusion, the introduction of Pd improves the catalytic activity compared to TMD, and smaller particle sizes demonstrate better catalytic performance; these calculated results well support the experimental data.

Fig. 5 DFT calculations for clarifying the mechanism. Gibbs free energy diagram for the HER in (a) MoS₂, WS₂, and TiS₂, (b) Pd(111), and (c) Pd (211). (d) The adsorption energy values of H* with a low average and a high coverage on different substrates and (e) the corresponding charge density difference diagrams. (f) Charge transfer of RDS H* on TMDs and different coverage H* on excellent Pd(211). (g) Crystal orbital Hamilton population (COHP) analysis of the interactions between H–Pd in Pd(111) and (h) Pd(211). Pd1, Pd2, and Pd3 are the three atoms bonded to H* on the Pd surface, as shown in Fig. S16.†

4. Conclusions

In this study, amorphous subnanometric Pd NPs have been successfully fabricated on metallic TMD nanosheets (MoS₂, WS₂, and TiS₂) through redox reactions between them. The metallic TMD nanosheets serve dual roles as both a reducing agent and a stabilizer, facilitating the reduction of Pd²⁺ to Pd⁰ and anchoring these ultrasmall Pd NPs on their surface using this solution-processable method. The resulting composites (Pd-MoS₂, Pd-WS₂, and Pd-TiS₂) exhibited HER performance comparable to that of commercial Pt/C. This study introduces a novel and environmentally friendly synthetic strategy for fabricating cost-effective, easily producible amorphous ultrasmall noble metal-decorated TMD composites, contributing to the efficient generation of clean energy.

Author contributions

Liang Mei: conceptualization, methodology, investigation, data curation, writing – original draft, and writing – review & editing. Yuefeng Zhang: investigation, data curation, and writing – original draft. Zimeng Ye: investigation and data curation. Ting Han: investigation and data curation. Honglu Hu: data curation. Ruijie Yang: writing – review & editing. Ting Ying: data curation. Weikang Zheng: data curation. Ruixin Yan: data curation. Yue Zhang: data curation. Zhenbin Wang: methodology and investigation. Zhiyuan Zeng: conceptualization, methodology, writing – review & editing, supervision, project administration, and funding acquisition.

Conflicts of interest

The authors declare that there are no competing interests.

Acknowledgements

Z. Y. Zeng acknowledges the General Research Fund (GRF) support from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. CityU11308923), the Basic Research Project from Shenzhen Science and Technology Innovation Committee in Shenzhen, China (no. JCYJ20210324134012034), the Applied Research Grant of City University of Hong Kong (project no. 9667247) and the Chow Sang Sang Group Research Fund of City University of Hong Kong (project no. 9229123). Z. Y. Zeng also acknowledges the funding support from the Seed Collaborative Research Fund Scheme of the State Key Laboratory of Marine Pollution, which receives regular research funding from the Innovation and Technology Commission (ITC) of the Hong Kong SAR Government. However, any opinions, findings, conclusions or recommendations expressed in this publication do not reflect the views of the Hong Kong SAR Government or the ITC.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi00622d

‡ These authors contributed equally.

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