Growth of Two-Dimensional Edge-Rich Screwed WS₂ with High Active Site Density for Accelerated Hydrogen Evolution

Abstract

Two-dimensional transition metal dichalcogenides have attracted considerable attention in electrocatalytic hydrogen evolution due to their unique layered structures and tunable electronic properties. However, prior research has predominantly focused on the intrinsic catalytic activity of planar few-layer structures, which offer limited exposure of edge-active sites due to their restricted two-dimensional geometry. Moreover, van der Waals interactions between layers impose substantial barriers to electron transport, significantly hindering charge transfer efficiency. To overcome these limitations, this study presents the innovative synthesis of high-quality single-screw WS₂ with a 5° dislocation angle via physical vapor deposition. Second harmonic generation measurements revealed a pronounced asymmetric polarization response, while the selected area electron diffractionand atomic force microscopy elucidated the material’s distinctive screwed dislocation configuration. In contrast to planar monolayer WS₂, the conical/screw-structured WS₂—formed through screw-dislocation-mediated growth—exhibits a higher density of exposed edge-active catalytic sites and enhanced electron transport capabilities. Electrochemical performance tests revealed that in an alkaline medium, the screwed WS₂ nanosheets exhibited an overpotential of 310 mV at a current density of −10 mA/cm², with a Tafel slope of 204 mV/dec. Additionally, under a current density of 18 mA/cm², the screwed WS₂ can sustain this current density for at least 30 h. These findings offer valuable insights into the design of low-cost, high-efficiency, non-precious metal catalysts for hydrogen evolution reactions.

1. Introduction

Energy serves as the driving force behind the advancement of modern civilization, playing a pivotal role in human production and societal development. At present, the global energy supply system remains heavily reliant on non-renewable fossil fuels. However, the excessive consumption of these resources has intensified environmental pressures and raised concerns about resource depletion, presenting humanity with the dual challenges of environmental pollution and energy security.Among renewable energy technologies, hydrogen production via water electrolysis has garnered significant attention as a promising alternative to fossil fuels. The key to the widespread adoption of this technology lies in the development of efficient catalysts capable of lowering the reaction energy barrier. In this context, the design of electrode materials that combine high catalytic performance with cost-effectiveness has become a central focus of research [1,2,3,4]. Traditional noble metal catalysts, such as platinum (Pt), offer excellent catalytic activity but suffer from high costs and limited availability. This has driven the exploration of low-cost substitutes. Two-dimensional transition metal dichalcogenides (2D-TMDs)—including materials such as MoS₂, WS₂, and CrSe₂—have emerged as promising alternatives due to their tunable electronic structures, high surface-to-volume ratios, and abundant active sites [5,6,7,8,9]. Theoretical studies have revealed that the Gibbs free energy values for hydrogen evolution in 2D-TMDs are comparable to those of platinum-based catalysts, indicating their strong potential to replace noble metals [10,11,12,13]. For instance, monolayer or multilayer homogeneous stacked transition metal dichalcogenides exhibit hydrogen evolution reaction catalytic performance comparable to that of platinum, despite relying solely on their limited edge-active catalytic sites [10,14,15,16,17,18]. Despite this potential, these semiconductor materials face two major limitations in practical applications: (1) the limited exposure of active edge sites restricts overall catalytic efficiency, and (2) their inherently low electrical conductivity hampers effective charge transport [19,20,21,22,23].

To overcome these challenges, researchers have pursued strategies such as increasing the density of active sites, enhancing the intrinsic activity of catalytic centers, and improving charge transport properties through structural engineering and performance optimization [20,21,24,25,26,27,28,29,30,31,32,33]. One particularly promising structural configuration is the “screwed” growth architecture observed in WS₂ during the early stages of formation. Unlike conventional planar epitaxial growth, this unique structure—driven by screw dislocations—results in vertically stacked, spiral architectures that expose a significantly higher number of catalytically active sites. Density functional theory (DFT) calculations combined with experimental measurements in acidic electrolytes demonstrate that introducing dislocations, interlayer twisting, and stacking faults into screwed WS₂ architectures substantially increases the density of edge sites and sulfur vacancies, thereby enhancing catalytic activity [28,29,30,31,32]. These sites often contain unsaturated sulfur coordination, which facilitates hydrogen adsorption and makes them efficient reactive centers [33,34,35,36,37,38,39,40]. Furthermore, the local lattice strain induced by the screwed architecture can modulate the electronic band structure of WS₂, bringing the Gibbs free energy of hydrogen adsorption (ΔG_H*) closer to the ideal value of zero. This reduces the reaction energy barrier, promotes proton reduction to molecular hydrogen, and enhances the overall HER efficiency in water electrolysis [39,40,41,42]. Additionally, the interlayer coupling and defect engineering within the screwed structure can enhance electrical conductivity, expedite charge transfer, and reduce electrochemical polarization [43,44,45,46,47,48]. The architecture also improves mechanical strength, helping to mitigate structural degradation during prolonged catalytic operations [41,42,43,44]. Given these advantageous physical and chemical properties, exploring the catalytic performance of screwed WS₂ structures is of considerable scientific and technological interest.

In this study, high-quality screwed WS₂ structures were successfully synthesized via bidirectional gas-flow physical vapor deposition. Atomic force microscopy (AFM) confirmed the characteristic screwed morphology of the resulting WS₂. Selected area electron diffraction (SAED) and second harmonic generation (SHG) analyses revealed the presence of interlayer dislocations and structural asymmetry within the screwed WS₂, highlighting its unique internal architecture. Unlike previous studies that typically evaluated the hydrogen evolution reaction (HER) performance of WS₂ in acidic media, this work employs an alkaline electrolyte as the catalytic environment. In acidic conditions, HER primarily proceeds through the reduction of H⁺ ions, which can result in the deactivation of edge sites due to excessive proton adsorption. In contrast, alkaline media allow for surface charge modulation, mitigating this issue and preserving catalytic activity. Thanks to its abundant edge-active sites and interlayer distortion-induced defects, screwed WS₂ exhibits significantly enhanced catalytic performance and electrical conductivity compared to conventional monolayer WS₂. These improvements address the common limitations of low active site density and poor conductivity observed in typical WS₂ catalysts. This advancement offers a promising strategy for the development of highly efficient, low-cost, non-precious metal catalysts for hydrogen evolution.

2. Results and Discussion

2.1. Growth Mechanism and Characterization

A schematic diagram illustrating the preparation process is shown in Figure S1. The simulated growth process of monolayer WS₂ is depicted in Figure 1a. During the synthesis of monolayer WS₂, growth initiates from a single nucleation site and proceeds outward in a lateral, planar manner—commonly referred to as in-plane growth. In contrast, the formation of screwed WS₂ follows a screw dislocation-driven vertical growth mechanism, as shown in the dynamic evolution illustrated in Figure 1b. When two WS₂ crystalline domains intersect, lattice distortion occurs at their interface, leading to the formation of a localized protrusion. At the edges of these protrusions, unsaturated sulfur atoms with dangling bonds accumulate, serving as catalytically active sites. By optimizing the growth conditions—specifically, employing low-temperature deposition and reducing precursor concentration—in-plane growth is effectively suppressed, while dislocation-driven vertical (screwed) growth is promoted. Optical microscopy images of monolayer WS₂ and screwed WS₂ nanostructures are shown in Figure 1c,d, respectively. Growth terminates once the precursor materials are depleted. X-ray photoelectron spectroscopy (XPS) was used to confirm the chemical composition of WS₂, with results presented in Figure S2. The XPS spectra exhibit peaks at 32.9 eV and 35.1 eV, corresponding to the W 4f_7/2 and W 4f_5/2 states of tungsten, respectively. Peaks at 162.2 eV and 163.5 eV correspond to the S 2p_3/2 and S 2p_1/2 states of sulfur [49,50]. Raman spectroscopy was also conducted for both screwed and monolayer WS₂ samples, as shown in Figure S3. The spectra exhibit characteristic peaks at ~350 cm⁻¹ and ~420 cm⁻¹, corresponding to the E¹_2g and A_1g phonon modes of WS₂, respectively—confirming the successful synthesis of WS₂ [51,52]. Atomic force microscopy (AFM) was employed to investigate the topography of the screwed WS₂ structures. As shown in Figure 1e,f, the AFM images clearly reveal the single-screw morphology. Each screw layer is smooth and well-defined, with a measured interlayer thickness of approximately 0.56 nm [53,54]. For comparison, the AFM-measured thickness of monolayer WS₂ is approximately 0.66 nm (Figure S4), consistent with known monolayer thickness values. Furthermore, the AFM profile in Figure 1g shows that the angle between the origin point of the screw structure and the base layer is approximately 5°. Selected area electron diffraction (SAED), shown in Figure 1h, reveals the lattice distortion associated with screw dislocations. The diffraction pattern obtained along the (100) crystallographic zone axis displays a shift of approximately 5°, aligning with the model of continuous rotational displacement of crystal planes induced by screw dislocations.

2.2. Optical Properties of Screwed Nanomaterials

SHG, a nonlinear optical phenomenon highly sensitive to crystalline orientation, serves as a powerful tool for probing the symmetry of two-dimensional layered materials. When a material lacks spatial inversion symmetry, its second-order nonlinear optical susceptibility becomes non-zero, leading to a pronounced SHG response [55,56]. Monolayer WS₂ is known to exhibit a non-centrosymmetric crystal structure, as reported in previous studies [57,58]. AFM topography reveals that screwed WS₂ possesses a structure resembling 3R-type stacking, which can be approximated as an “AA”-type stacking configuration. To further confirm the non-centrosymmetric structure induced by interlayer dislocations in screwed WS₂, SHG characterization was performed. Systematic SHG measurements were conducted to probe the symmetry properties of the material, and the results are shown in Figure 2a. In the edge regions of the screwed steps—where non-centrosymmetry is more pronounced—the SHG signal intensity is approximately ten times higher than that of planar monolayer WS₂, confirming a significant local symmetry breaking caused by the screw-dislocation-induced stacking. As shown in Figure 2b, the SHG intensity of screwed WS₂ increases with rising laser power, further supporting the presence of a non-centrosymmetric structure. The extracted SHG intensities at various laser powers were plotted on a logarithmic scale (Figure 2c), yielding a slope of 2.07—closely matching the theoretical expectation of 2 for a second-order nonlinear optical process. Polarization-resolved SHG measurements were also carried out to investigate anisotropic structural features. As illustrated in Figure 2d, the SHG response of screwed WS₂ displays a periodic modulation with a polarization angle offset of approximately 5°, consistent with the interlayer rotational angle induced by screw dislocations. Monolayer WS₂ typically adopts a hexagonal 2H phase, with a sandwich-like S–W–S atomic configuration that lacks a center of inversion symmetry. This inherent asymmetry allows for the emergence of SHG signals. Additionally, the in-plane anisotropy of monolayer WS₂ causes the SHG intensity to exhibit periodic modulation as a function of the incident light’s polarization angle (Figure S5). Comparative SHG measurements between screwed WS₂ and monolayer WS₂ reveal that the SHG intensity of screwed WS₂ is approximately an order of magnitude higher. This enhancement is attributed to a higher density of interlayer defects and symmetry-breaking features, which are beneficial for catalytic performance. The pronounced SHG response of screwed WS₂ thus correlates with its potential for significantly improved hydrogen evolution activity.

2.3. Electrocatalytic Testing

To evaluate the electrocatalytic performance of screwed WS₂ and monolayer WS₂, devices were fabricated by transferring pre-patterned electrodes onto the WS₂ surfaces. Compared with conventional metal deposition methods such as magnetron sputtering or thermal evaporation, this transfer technique effectively mitigates interfacial issues, including Fermi level pinning and parasitic interfacial capacitance, thereby preserving the intrinsic catalytic and electrical characteristics of the WS₂ nanostructures. For the fabrication of screwed WS₂ device, a mechanically exfoliated few-layered mica flake was precisely transferred onto the central-to-edge region of the screwed WS₂, serving as a dielectric spacer. This spatial configuration isolates the central and edge regions of WS₂, allowing direct and reliable measurements of charge transport efficiency from the core to the periphery. Such a setup provides valuable insights into the anisotropic electron transport behavior of the screwed structure. The electronic transport properties of monolayer and screwed WS₂ were measured, and the results are shown in Figure 3a,b, respectively. The corresponding optical image of the fabricated device is provided in the lower-right corner. Under an applied voltage sweep from −1 V to 1 V, the maximum current for monolayer WS₂ reaches 0.9 nA, whereas the screwed WS₂ achieves a significantly higher current of 5400 nA. This marked difference indicates that the screwed WS₂ possesses superior electron transport efficiency, facilitating faster charge transfer and enhanced hydrogen evolution performance. For electrochemical testing, both WS₂ samples (grown on SiO₂/Si substrates) were transferred onto copper sheets via a wet-transfer method. A standard three-electrode configuration was employed: the WS₂ samples served as the working electrodes, a mercury/mercuric oxide (Hg/HgO) electrode was used as the reference, and a graphene rod functioned as the counter electrode. Figure 3c,d present the linear sweep voltammetry (LSV) curves and the corresponding Tafel slopes for both WS₂ morphologies. At a current density of −10 mA cm⁻², the overpotential of screwed WS₂ is 310 mV with a Tafel slope of 204 mV dec⁻¹, while monolayer WS₂ exhibits a higher overpotential of 390 mV and a Tafel slope of 225 mV dec⁻¹. These results clearly demonstrate the superior HER performance of the screwed WS₂ architecture. Electrochemical impedance spectroscopy (EIS) was further employed to assess the charge transfer kinetics of the two morphologies. As shown in Figure 3e, the charge transfer resistance (Rct) for screwed WS₂ is 72 Ω, compared to 92 Ω for monolayer WS₂, confirming improved interfacial charge transport in the screwed structure. The ECSA is a key metric for evaluating the number of active sites on a catalyst’s surface, and it is directly proportional to the electrochemical double-layer capacitance (ECSA = C_dl/C_s). In this work, the specific capacitance value (C_s) was taken as 40 μF cm⁻². Cyclic voltammetry at different scan rates (5–25 mV s⁻¹) was employed to determine the C_dl value by fitting and analyzing the relationship between current density and scan rate. The test results are shown in Figure S6. From the data, it can be observed that the screwed WS₂ and copper substrate exhibit a C_dl of 403.8 mF cm⁻², corresponding to an ECSA of 10,095 cm², whereas the monolayer WS₂ and copper substrate show a C_dl of 313.2 mF cm⁻², yielding an ECSA of 7830 cm². The results clearly demonstrate that the ECSA of screwed WS₂ is significantly higher than that of monolayer WS₂. To investigate the structural durability and catalytic stability, long-term performance retention tests were conducted. As shown in Figure 3f, the chronoamperometric experiments revealed that at a fixed overpotential, the current densities of screwed WS₂ and monolayer WS₂ remained stable at 18 mA cm⁻² and 10.3 mA cm⁻², respectively, for 30 h without degradation.

2.4. Electrocatalytic Mechanism Analysis

The electrocatalytic process for hydrogen evolution begins with the electrochemical adsorption step, which follows the Volmer reaction. In this step, hydrogen ions (protons) react with electrons on the catalyst surface, resulting in the adsorption of a hydrogen atom (H*) on the material surface. This corresponds to the following reaction (1). As the hydrogen content on the surface increases, the Tafel desorption reaction occurs (2), where two adjacent adsorbed hydrogen intermediates combine to form a hydrogen molecule. The mechanistic simulation diagrams for hydrogen evolution catalysis in alkaline solution (KOH) for screwed WS₂ and monolayer WS₂ are presented in Figure 4a,b, respectively.

H₂O + e⁻ + *→H* + OH⁻

(1)

2H*→H₂

(2)

WS₂ with a screwed structure possesses a continuous network of dislocations, providing a high density of surface active sites. This unique structural feature endows the material with exceptional catalytic properties. Compared to conventional two-dimensional materials, the screwed WS₂ exhibits superior catalytic kinetics in the hydrogen evolution reaction (HER). Specifically, it demonstrates a smaller Tafel slope and a lower onset potential, both of which are indicators of enhanced electrocatalytic efficiency. Studies have shown that the current intensity generated by the screwed WS₂ structure is approximately four orders of magnitude higher than that of the monolayer system. This significant enhancement is attributed to the fact that the screwed configuration allows the current to flow along the screw-dislocation edge, bypassing the need to overcome interlayer potential barriers. This geometry accelerates electron movement and optimizes the hydrogen evolution performance. In addition, lattice distortion at the interface of the screwed structure creates a localized bulging region, where unsaturated sulfur (S) atoms are suspended. These unsaturated sulfur atoms improve the adsorption of hydrogen atoms, facilitating the formation of H₂ molecules when sufficient hydrogen is accumulated on the material surface. This characteristic further boosts the catalytic hydrogen evolution ability of the material. Moreover, the screwed structure not only enhances the catalytic activity but also improves mechanical strength, reducing the likelihood of structural collapse during the catalytic process. This ensures a more reliable and stable hydrogen evolution process, making screwed WS₂ an effective and durable catalyst for hydrogen production.

3. Experimental Section

3.1. Synthesis of Screwed WS₂ Nanosheets

Screwed WS₂ and monolayer WS₂ nanosheets with atomic thickness were synthesized on SiO₂/Si substrates using the physical vapor deposition (PVD) method. A quartz tube (1 m length, 2.2 cm inner diameter) was placed in a horizontal high-temperature tube furnace (KSY controllable silicon temperature tube furnace, Shenyang Energy-saving Electric Furnace Factory, Shengyang, China). Using an electronic balance, 1 g of WS₂ (99.8%, Alfa Aesar, Haverhill, MA, USA) solid powder was weighed and placed in a ceramic boat. The ceramic boat was positioned in the central high-temperature zone of the tube furnace. A SiO₂/Si substrate (2 cm in length) was placed in a half-cut ceramic boat and pushed to the downstream deposition zone of the furnace using a small cart. The quartz tube was sealed with a rubber stopper. The purpose of the small cart was to immediately remove the substrate from the deposition zone after the temperature rise, allowing the WS₂ to grow and then cool abruptly, which enhanced the crystallinity of the WS₂ nanosheets. Additionally, extracting the substrate prevented re-nucleation of already grown nanosheets. For monolayer WS₂ nanosheets, the target heating temperature was set to 1100 °C, with an argon gas flow rate of 60 sccm (99.999%, Baoding Jinglian Gas Co., Ltd., Baoding, China). Reverse annealing was performed for 60 min, followed by forward annealing at 60 sccm for 10 min to prepare the monolayer WS₂ nanosheets. For screwed WS₂ nanosheets, the target heating temperature was set to 1070 °C, with reverse annealing for 10 min, followed by forward annealing at 60 sccm for 10 min to prepare the screwed WS₂ nanosheets.

3.2. Catalytic Sample Preparation

The WS₂ nanosheets (screwed/monolayer form) prepared on SiO₂/Si substrates were transferred onto copper sheets using the wet transfer method. The specific steps are as follows: The SiO₂/Si substrate with the materials was placed on an adhesive machine, and 0.2 g/mL PMMA solution was dropped onto its surface. The adhesive parameters were set to 3000 rpm for 60 s. After applying the adhesive, the substrate was placed on a heating table at 110 °C for 3 min to improve adhesion between the materials and the PMMA film. The substrate was then immersed in a 10 M KOH solution to etch away the SiO₂ layer, allowing the PMMA film with the materials to detach and float off spontaneously. After removing the residual KOH solution, the PMMA film was washed with deionized water. The clean copper sheet and PMMA film were placed on a heating table at 80 °C to ensure the materials adhered completely to the copper sheet. Finally, the PMMA glue was removed by placing the film in acetone, leaving only the materials adhered to the copper sheet.

3.3. Material Characterization

The morphology and thickness of the nanosheets were observed and measured using atomic force microscopy (AFM, MFP-3D, Oxford Instruments, Santa Barbara, CA, USA). Zone-selective electron diffraction (SAED) tests were conducted with transmission electron microscopy (TEM, JEOL-2100Plus, Japan Electronics, Tokyo, Japan). Raman spectroscopy and second harmonic generation (SHG) tests were carried out using a high-resolution Raman spectrometer (HR Evolution, Horiba Jobin Yvon, Glasgow, UK). Elemental composition was analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).

3.4. Electrochemical Tests

Current–voltage (I–V) tests were performed using a semiconductor parameter analyzer (B1500A, KEYSIGHT, Santa Rosa, CA, USA). Catalytic hydrogen evolution tests were conducted using an electrochemical workstation with a three-electrode system (CHI 760E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China), which consisted of a reference electrode, counter electrode, and working electrode. A mercury–mercuric oxide (Hg/HgO) electrode was used as the reference electrode, and a graphite rod was used as the counter electrode. Before the tests, 200 mL of 1 M KOH solution was prepared as the electrolyte. The actual contact area between the working electrode and the electrolyte was 0.5 cm². All tests were conducted at room temperature.

4. Conclusions

In this study, an improved bidirectional gas flow system was utilized to fabricate WS₂ nanosheets with high-density edge active sites in a screwed cone shape and single layer on SiO₂/Si substrates via the PVD method. Through I-V tests on two types of WS₂, it was found that due to the screwed structure, where electron transport occurs through the edge sites of the helix, there is no need to overcome the van der Waals potential barriers between layers. As a result, the current of screwed WS₂ is 10⁴ times higher than that of monolayer WS₂, significantly enhancing charge transport efficiency. In contrast to conventional hydrogen evolution reaction (HER) testing in acidic media, this study employed an alkaline environment to prevent edge site deactivation caused by excessive H⁺ adsorption. The screwed WS₂ structure significantly increases the density of edge sites and sulfur vacancies by introducing dislocations, interlayer distortions, and stacking defects. These sites exhibit unsaturated coordination of sulfur atoms, making them more prone to adsorbing hydrogen atoms, and thus acting as efficient reaction centers. The interlayer coupling and defects in the screwed structure enhance electron conductivity, accelerate charge transfer, and reduce electrochemical polarization. Furthermore, the screwed structure improves mechanical strength, reducing the risk of structural collapse during the catalytic process.