Interfacial p–n coupling in Cu3N/1T-MoS2 heterojunctions drives efficient and durable acidic hydrogen evolution

Rational interfacial engineering is crucial to designing non-precious electrocatalysts for the hydrogen evolution reaction (HER). Here we report a Cu₃N/1T-MoS₂ heterojunction in which ultrathin 1T-MoS₂ nanosheets are conformally grown on conductive p-type Cu₃N nanocubes. Spectroscopy and microscopy reveal intimate lattice contact and interfacial strain, while XPS indicates charge redistribution across the junction. In 0.5 M H₂SO₄, the optimized 1:1 composite delivers an overpotential of 387 mV at 50 mA cm⁻² and a Tafel slope of 57 mV dec⁻¹, outperforming both constituents (MoS₂ ≈ 456 mV; Cu₃N inactive at this current density). Continuous operation for 150 h demonstrates excellent acid stability. Electrochemical impedance and double-layer capacitance analyses show the lowest charge-transfer resistance and the highest electrochemically active surface area among all samples (C_dl = 103.6 mF cm⁻²), corroborating rapid electron transport and abundant accessible sites. The activity enhancement arises from a p–n heterojunction with a built-in field that promotes directional electron flow to MoS₂ active edges, together with strain-induced defect exposure. This work identifies Cu₃N as an effective platform to stabilize conductive MoS₂ and provides design rules for interface-engineered HER catalysts.

GRAPHICAL ABSTRACT

1 Introduction

Hydrogen is a versatile clean-energy carrier crucial for deep decarbonization across chemicals, transportation, and grid storage (Ayodele et al., 2019; Li et al., 2020; McHugh et al., 2020; Sun et al., 2023). Electrochemical water splitting (Sun Y. et al., 2021) offers a modular, high-purity route to hydrogen (H₂) when powered by renewables, but cost-effective deployment still requires non-precious electrocatalysts that combine low overpotentials, high current densities, and long-term stability − requirements that remain a major challenge (Boretti et al., 2021; Li et al., 2016; Sun H. et al., 2021; Vasiliadou et al., 2018).

Among non-precious metal candidates, transition-metal dichalcogenides (TMDs) are promising for the hydrogen evolution reaction (HER) (Chia et al., 2015; Liu et al., 2019; Meshesha et al., 2023; Rao et al., 2021; Xue et al., 2022). Molybdenum disulfide (MoS₂) is particularly attractive owing to its tunable chemistry and edge-site activity, yet performance is constrained by the largely inert basal plane of semiconducting 2H-MoS₂ and by limited in-plane conductivity. Metallic 1T-MoS₂ improves conductivity and increases the population of active sites, but it is metastable under operating conditions and prone to activity decay (Amiinu et al., 2017; Chiu et al., 2024). Conventional remedies − defect/phase engineering, heteroatom doping, and conductive supports − can accelerate kinetics but often compromise stability or require complex syntheses (Gong et al., 2023; Jin et al., 2025; Liu et al., 2020; Miao et al., 2024).

To overcome these limitations, we couple p-type copper nitride (Cu₃N) with 1T-MoS₂ (Kwon et al., 2025). Cu₃N is an attractive yet underexplored semiconductor with good conductivity, nitrogen-vacancy-rich cubic lattices, and a moderate work function (Gan et al., 2022; Wang et al., 2023; Zhang et al., 2021; Zhu et al., 2020a), enabling rapid carrier transport and scalable synthesis, enabling a p-n heterojunction with a built-in field that directs electrons to MoS₂ edge sites while extracting holes into Cu₃N, thereby enhancing charge separation and accelerating HER kinetics. Strong interfacial coupling further stabilizes the 1 T phase of MoS₂, and favorable adsorption/transport characteristics support high-current, durable operation (Du et al., 2022; Jin et al., 2025; Wang et al., 2024). Compared with recent MoS₂-based heterojunctions (typically n–n or metal–semiconductor pairings, Supplementary Table S1), the Cu₃N/1 T-MoS₂ architecture combines directional charge transfer, noble-metal-free electronic wiring, improved 1T-phase stability, and robust acidic performance.

In this work, Cu₃N/1T-MoS₂ achieves η₅₀ = 387 mV (under 50 mA cm⁻² of current density) and a Tafel slope of 57 mV dec⁻¹ in 0.5 M H₂SO₄, maintaining performance for 150 h, demonstrating advantages in comprehensive performance. These results highlight the benefits of p-n interfacial coupling and the built-in field in funneling electrons to MoS₂ edge sites, delivering mechanism-anchored gains in both kinetics and durability, and offering a simple, non-precious route toward practical HER catalysts¹.

2 Materials and methods

2.1 Chemicals

All reagents were of analytical grade and used without further purification. Sodium hydroxide (NaOH, 95%) was purchased from Macklin Biochemical Technology Co., Ltd. Hydrochloric acid (HCl, 36%) and sulfuric acid (H₂SO₄, 98%) were obtained by Hunan Huihong Reagent Co., Ltd. Ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, 99%) and thiourea (CH₄N₂S) were supplied from Sinopharm Chemical Reagent Co., Ltd. Copper (II) acetate monohydrate (Cu(OAc)₂·H₂O, 99%) and potassium hydroxide (KOH, 95%) were obtained from Aladdin Biochemical Technology Co., Ltd. Urea (CH₄N₂O, 99%) was acquired from Xilong Scientific Co., Ltd.

2.2 Synthesis of Cu₃N

Cu₃N was synthesized via a chemical vapor deposition route. Specifically, 0.33 g of Cu(OAc)₂·H₂O and 0.50 g of urea were separately placed in two porcelain boats. The precursors were heated to 300 °C at a ramp rate of 5 °C·min^-1 under a continuous Ar flow (40 sccm) and maintained for 2 h. After cooling to room temperature, the black powder was collected, washed with deionized water (3 × 50 mL) and ethanol (3 × 50 mL) successively, and dried under vacuum at 60 °C for 12 h (Mondal and Raj, 2018).

2.3 Synthesis of MoS₂

1 T phase MoS₂ was synthesized following a modified hydrothermal procedure. Briefly, 1.25 g ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, 1.0 mmol) and 2.28 g thiourea (30.0 mmol) were dissolved in 35 mL deionized water and stirred for 30 min. The homogeneous solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 180 °C for 24 h. After cooling, the resulting black precipitate was separated by centrifugation (6,000 rpm, 10 min per cycle), washed thoroughly with deionized water and ethanol, and vacuum-dried at 60 °C for 12 h (Wang et al., 2020).

2.4 Synthesis of Cu₃N/MoS₂ composites

To synthesize the Cu₃N/MoS₂(1:1) composite, 0.21 g of Cu₃N (1.0 mmol), 0.54 g (NH₄)₆Mo₇O₂₄·4H₂O (0.43 mmol), and 0.98 g thiourea (12.87 mmol) were dispersed in 35 mL of deionized water with stirring for 30 min. The mixture was sealed in a 100 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 180 °C for 24 h. After natural cooling, the solid product was collected by centrifugation, washed with water and ethanol successively for three times, and dried under vacuum at 60 °C for 12 h. Additional composites with Cu₃N/MoS₂ of 1:0.5 and 1:2 were prepared under identical conditions.

2.5 Electrode preparation

Catalyst inks were prepared by dispersing 2.5 mg of Cu₃N/MoS₂ powder in 400 μL ethanol and 90 μL deionized water, followed by ultrasonication to achieve a homogeneous suspension. Then, 10 μL of Nafion® 117 solution (∼5 wt%) was added, and the mixture was sonicated again. A 6 μL aliquot of the resulting ink was drop-cast onto a glassy carbon electrode (GCE, 3 mm diameter), yielding a loading of ∼0.4 mg cm^-2. The electrode was dried at ambient temperature prior to electrochemical testing.

2.6 Characterization

X-ray diffraction (XRD) was performed on a TD-3500 diffractometer using Cu Kα radiation (λ = 0.15406 nm), scanned over a 2θ range of 5°–90° with a scan rate of 2.4°·min⁻¹. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific K-Alpha spectrometer equipped with an Al Kα source (hν = 1486.6 eV). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were conducted on a ZEISS Sigma 300 microscope using INLENS and ET detectors. Transmission electron microscopy (TEM) and EDS mapping were conducted using a JEOL JEM-F200 system equipped with JED-2300T.

2.7 Electrochemical measurements

Electrochemical HER measurements were conducted in 0.5 M H₂SO₄ (pH ≈ 0.34) using a three-electrode setup: the modified GCE as the working electrode, a graphite rod as the counter electrode, and an Ag/AgCl (3 M KCl) electrode as the reference. Linear sweep voltammetry (LSV) was recorded at 5 mV s^-1 with 90% iR compensation. Potentials were converted to the reversible hydrogen electrode (RHE) scale using related formula (E_RHE = E_Ag/AgCl + 0.0592 pH + 0.197 V). Tafel slopes were derived from LSV curves. Electrochemical impedance spectroscopy (EIS) was conducted at an overpotential of 387 mV over the frequency range of 0.1 Hz–100 kHz. The electrochemically active surface area (ECSA) was estimated from the double-layer capacitance (C_dl), obtained via cyclic voltammetry (CV) at scan rates of 5–25 mV s^-1 within the non-Faradaic potential window (0.2175–0.3175 V vs. RHE).

3 Results and discussion

Scanning-electron-microscopy (SEM) images for pristine Cu₃N and for Cu₃N/MoS₂ composites obtained with Cu₃N-to-Mo precursors of 1:0.5, 1:1 and 1:2 are compiled, while the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental maps for the 1:1 sample are presented in Figures 1c,d, Supplementary Figure S1B. The as-synthesized Cu₃N consists of smooth cubes. The narrow size distribution and sharp edges corroborate the phase purity inferred from XRD (Figure 2a) (Mallick et al., 2024; Mondal and Raj, 2018).

Figure 1

Figure 1. SEM of (a) Cu₃N, and (b) Cu₃N/MoS₂(1:1). (c) and (d) EDS of Cu₃N/MoS₂(1:1) with Cu, Mo, N and S elements.

Figure 2

Figure 2. (a) XRD patterns of Cu₃N, MoS₂ and Cu₃N/MoS₂(1:1). (b) TEM of Cu₃N. (c) TEM of Cu₃N/MoS₂(1:1). (d) HRTEM of Cu₃N/MoS₂(1:1) with characteristic lattice fringes.

Introducing a limited amount of Mo precursor leads to the nucleation of ultrathin MoS₂ nanosheets directly on the Cu₃N cubes. The flakes anchor preferentially on the vertices and edges, giving rise to a rough texture while preserving the underlying cubic morphology (Figures 1a,b, Supplementary Figures S1C, D). At a stoichiometric precursor ratio, MoS₂ growth becomes self-sustaining, producing densely packed, radially oriented nanosheets that coalesce into rose-like microflowers (overall diameter 0.8–2 µm). Each flower encloses a Cu₃N core that is completely obscured by the MoS₂ shell, suggesting a core-shell or yolk-shell heterostructure (Chiu et al., 2024; Gong et al., 2023). The hierarchical porosity created by the inter-flake voids is expected to facilitate electrolyte infiltration and accelerate mass transport in electrocatalytic operation. Further increasing the Mo precursor leads to excessive MoS₂ deposition. Thick, entangled nanosheets (∼20 nm) intergrow into open networks that protrude several hundred nanometers from the core. Although this architecture maximizes the content of MoS₂, it may compromise electronic coupling to the Cu₃N core and partially block ion channels, underscoring the need for stoichiometric optimization (Supplementary Figures S1C, D). Element-specific signals for Cu, N, Mo and S are co-distributed throughout the 1:1 heterostructure. The Cu and N maps coincide with the core region, whereas Mo and S extend uniformly over the entire flower-like particle, confirming the conformal growth of MoS₂ on Cu₃N. No extraneous elements are detected, attesting to the chemical integrity of the composite (Figures 1c,d). The systematic evolution from sparsely decorated cubes to fully encapsulated core-shell flowers demonstrates that MoS₂ nucleation and growth can be finely tuned by the Cu₃N: Mo precursor ratio. These morphological attributes rationalize the superior electrochemical performance discussed in next section.

The crystal structures of Cu₃N, MoS₂, and Cu₃N/MoS₂ heterojunction were analyzed by X-ray diffraction (XRD) (Figure 2a). Distinct reflections at 23.2°, 33.2°, 41.0° and 47.7° are assigned to the (100) (110), (111), and (200) planes of cubic Cu₃N (JCPDS No. 74-0242), confirming phase purity (Mallick et al., 2024; Mondal and Raj, 2018). The MoS₂ sample shows reflections at 33°, 49°, and 58° for the (100) (105), and (110) planes of the 1 T phase, with additional peaks at 19° and 43° further indicating 1 T formation. A broad basal reflection centred at 9.5° corresponds to an interlayer spacing of d₀₀₂ = 0.93 nm, markedly larger than the 0.62 nm of bulk 2H-MoS₂. Such expansion is characteristic of the 1 T polymorph generated by chemical exfoliation or intercalation-driven electron injection (Jayabal et al., 2018; Wang et al., 2020; Yu et al., 2018). All fingerprint peaks of both parent lattices are retained in Cu₃N/MoS₂ composite, indicating that hydrothermal assembly proceeds without alloying or oxide/sulfide by-products. The peak shifts of Cu₃N(111) (41.0° → 40.78°) and MoS₂(100) (33.0° → 33.26°), together with line broadening, indicate strong interfacial interactions between Cu₃N and MoS₂, further corroborated by TEM and XPS analyses.

Transmission-electron microscopy (TEM) was employed to visualize the crystal architecture of pristine Cu₃N (Figure 2b) and of the Cu₃N/MoS₂(1:1) heterostructures synthesised with Cu₃N: Mo precursor ratios of 1:1 (Figure 2c). Low-magnification images reveal monodisperse nanocubes (Figure 2b). Lattice fringes of 0.19 nm assigned to the Cu₃N (200) planes extend coherently before intersecting with 0.32 nm fringes originating from the MoS₂ (100) planes (Figure 2d) (Jayabal et al., 2018; Mallick et al., 2024; Mondal and Raj, 2018; Wang et al., 2020; Yu et al., 2018). The acute intersection angle (≈90°) reveals a semi-coherent interface, indicating epitaxial alignment despite the large symmetry mismatch between cubic Cu₃N and hexagonal MoS₂. No amorphous halos or disordered fringe terminations are detected, indicating that the hydrothermal overgrowth does not induce grain-boundary amorphisation. The absence of misfit dislocations within the examined field further supports coherent, low-defect bonding.

Supplementary Figures S3a, b displays the selected-area electron-diffraction (SAED) pattern acquired from an individual Cu₃N nanocube, fully corroborating the XRD results (Figure 2a). The pattern shows discrete, azimuthally discontinuous spots arranged on concentric rings rather than continuous Debye–Scherrer rings (Jiang et al., 2014; Mallick et al., 2024; Mondal and Raj, 2018). This indicates that the probed area contains a limited number of well-oriented crystallites–consistent with monocrystalline cubes observed by TEM–rather than a random powder. The SAED analysis substantiates that the as-synthesized Cu₃N nanocubes are highly crystalline, phase-pure and possess long-range order–prerequisites for achieving intimate electronic contact and efficient charge transfer in the Cu₃N/MoS₂ heterostructures discussed in the subsequent sections. Supplementary Figures S3b presents the SAED pattern collected from a single Cu₃N/MoS₂ particle synthesized at the optimal Cu₃N: Mo precursor ratio of 1:1. In contrast to the spot-like pattern of pristine Cu₃N (Supplementary Figures S3a), the composite exhibits a series of nearly continuous diffraction rings populated by discrete spots, reflecting the coexistence of a highly ordered Cu₃N core and a polycrystalline shell of few-layer MoS₂ nanosheets with random in-plane orientation. All diffraction maxima can be indexed exclusively to Cu₃N and metallic 1T-MoS₂, confirming the phase purity inferred from XRD. The innermost rings (2.6, 3.7, 4.5 nm⁻¹) are dominated by discrete spots rather than continuous arcs, indicative of a single-crystalline Cu₃N core whose zone axis is preserved across the illuminated area (Jayabal et al., 2018; Mallick et al., 2024; Mondal and Raj, 2018; Wang et al., 2020; Yu et al., 2018). In contrast, the outer rings (3.2, 5.2, 6.2 nm^-1) are quasi-continuous, signifying that the MoS₂ shell is composed of numerous nanoflakes with random azimuthal distribution–fully consistent with the radially oriented “rose-like” nanosheets observed in SEM/TEM. the SAED evidence validates the successful construction of a coherent Cu₃N/1T-MoS₂ core–shell heterostructure with intimate lattice contact and well-defined crystallographic orientation, underpinning the superior electrochemical performance.

The atomically intimate juxtaposition of the metallic 1T-MoS₂ flakes with the conductive Cu₃N core, together with the measured bidirectional strain, is expected to (i) enhance orbital overlap for interfacial electron transfer and (ii) modulate the d-band centre of MoS₂, thereby optimising the adsorption free energy of reaction intermediates. Increasing the Mo precursor to the stoichiometric ratio yields a conformal, flower-like MoS₂ shell composed of radially aligned nanosheets. The original cubic outline remains discernible as a bright core in TEM, suggesting a core–shell rather than a yolk–shell arrangement. The tight interface and hierarchical porosity of this architecture rationalize its optimal electrochemical behaviour. A further increase in Mo results in excessive MoS₂ deposition. Thick, intergrown sheets envelop the cubes and protrude outward, forming an open network that partially detaches from the core. Such over-growth is likely to impede charge transfer between the catalytic MoS₂ surface and the conductive Cu₃N core.

Across all hybrids, the observed lattice juxtaposition matches the compressive/tensile peak shifts detected by XRD, corroborating the presence of elastic lattice strain at the heterojunction. The TEM study verifies that controlled MoS₂ growth on Cu₃N proceeds via an edge-selective nucleation mechanism, allowing precise tuning of shell thickness. The 1:1 composite offers the optimal compromise between interfacial area, electronic coupling and mass transport, providing a structural basis for the superior catalytic metrics.

XPS analysis confirms the formation of the Cu₃N/MoS₂ heterojunction through distinct interfacial electronic interactions (Figure 3 and Supplementary Figures S4). For pristine Cu₃N, Cu 2p peaks at 933.87 eV (Cu⁺ 2p_3/2) and 935.93 eV (Cu²⁺ 2p_3/2), along with the N 1s signal at 399.13 eV, are observed (Amiinu et al., 2017; Mallick et al., 2024; Mondal and Raj, 2018; Zhu et al., 2020a), while in Cu₃N/MoS₂ these shift to 932.55 eV and 934.86 eV for Cu 2p_3/2 and 399.39 eV for N 1s, indicating electron redistribution at the interface. For MoS₂, Mo 3d peaks at 229.02 eV/232.22 eV (1 T-Mo⁴⁺ 3d_5/2 and 3d_3/2) and S 2p peaks at 161.98 eV/163.10 eV are characteristic of 1T-MoS₂, while in Cu₃N/MoS₂ the Mo 3d signals shift positively to 232.7 eV/235.7 eV and the S 2p peaks remain slightly perturbed. The opposite shifts of Cu and Mo binding energies demonstrate interfacial charge transfer between Cu₃N and MoS₂, consistent with p-n junction formation. These results, together with XRD and TEM evidence, confirm strong interfacial coupling and validate the construction of the Cu₃N/1 T-MoS₂ heterostructure. The observed charge transfer and chemical potential equilibration also align with the strain couple revealed by XRD/TEM, which is expected to modulate the d-band center of MoS₂, strengthen adsorption of intermediates, and facilitate interfacial electron delivery, thereby promoting HER kinetics.

Figure 3

Figure 3. XPS of Cu₃N, MoS₂ and Cu₃N/MoS₂. (a) Mo 3d spectrum of MoS₂. (b) Cu 2p spectrum of Cu₃N. (c) Mo 3d spectrum of Cu₃N/MoS₂. (d) Cu 2p spectrum of Cu₃N/MoS₂.

HER activity was evaluated in 0.5 M H₂SO₄ (pH ≈ 0.34) using a standard three-electrode configuration (Ag/AgCl reference, graphite counter). All potentials were converted to the RHE scale according to E_(RHE) = E_(Ag/AgCl) + 0.0592 pH + 0.197, and iR-drop was corrected by 90% unless otherwise noted. Polarization curves (LSV) and Tafel plots for pristine Cu₃N, pristine MoS₂ and Cu₃N/MoS₂ with feed ratios of 1:0.5, 1:1 and 1:2 are shown in Figure 4.

Figure 4

Figure 4. (a) LSV curves of Cu₃N, MoS₂ and Cu₃N/MoS₂ with different feed ratios. (b) Tafel slope of Cu₃N, MoS₂ and Cu₃N/MoS₂ with different feed ratios. (c) Overpotential of MoS₂ and Cu₃N/MoS₂ with different feed ratios at 50 mA cm⁻² of current density. (d) Chronopotentiometry at 387 mV overpotential for 150 h.

The overpotential required to deliver j = 50 mA cm⁻² (η₅₀) and the Tafel slope extracted from the low-overpotential region are summarized in Figure 4. Consistent with the LSV traces (Figure 4a), the Cu₃N/MoS₂ (1:1) heterostructure exhibits the lowest η₅₀ (387 mV), outperforming pristine MoS₂ (≈456 mV) and far exceeding Cu₃N, which shows negligible activity at this current density (Figures 4a,c). Increasing or decreasing the MoS₂ fraction from the 1:1 optimum degrades performance (η₅₀ = 417–455 mV) (Figure 4c), indicating that an appropriate Cu₃N-MoS₂ balance is crucial for maximizing accessible active sites and charge-transfer pathways. Long-term stability was assessed via chronopotentiometry at 387 mV of overpotential over 150 h (Figure 4d). The Cu₃N/MoS₂(1:1) electrode shows negligible performance decay, demonstrating excellent durability in acidic conditions.

In acidic media, canonical slopes of ≈120, ≈40 and ≈30 mV dec⁻¹ are typically associated with Volmer (H* adsorption), Heyrovsky (electrochemical desorption) and Tafel rate-determining steps, respectively (Jacobson et al., 2022). The value of 57 mV dec⁻¹ for the 1:1 sample points to a Volmer–Heyrovsky mechanism, whereas pristine MoS₂ (105 mV dec⁻¹) is largely limited by the Volmer adsorption step (Figure 4b). The substantial reduction in Tafel slope for the 1:1 heterostructure signifies faster interfacial charge transfer and more favorable H* adsorption free energy (ΔG_H*), in line with the electronic modulation inferred from XPS (electron flow from Cu₃N to MoS₂) and the coherent/strained interfaces resolved by XRD/TEM.

Electrochemical impedance spectroscopy (EIS) was carried out for Cu₃N, MoS₂, and Cu₃N/MoS₂ with varied ratios (Figure 5a). The Nyquist plots deviate from an ideal semicircle, likely due to the porous electrode, heterogeneous interfaces, and mixed kinetics-diffusion contributions (rather than a simple Randles circuit). Even so, the high-frequency intercepts and arc radii remain diagnostic of charge transfer. The Cu₃N/MoS₂ (1:1) sample shows the smallest arc radius, indicating the lowest Rct, which agrees with its lower overpotential, smaller Tafel slope, and superior durability. These results confirm that the 1:1 heterojunction affords the most effective interfacial coupling and fastest charge transport among the tested catalysts (Feng et al., 2022).

Figure 5

Figure 5. (a) EIS of Cu₃N, MoS₂ and Cu₃N/MoS₂ with different feed ratios. (b) C_dl of Cu₃N and Cu₃N/MoS₂ with different feed ratios.

The electrochemically active surface area (ECSA) was estimated from the double-layer capacitance (C_dl) obtained by cyclic voltammetry in a non-Faradaic potential window at various scan rates. The slope of the capacitive current density (Δj) versus scan rate plot yields C_dl values (Figure 5b). The C_dl of Cu₃N/MoS₂(1:1) is ∼25× larger than that of pristine Cu₃N and markedly higher than those of the other composites, signifying a dramatic increase in accessible active sites. This enhancement originates from the hierarchical core–shell morphology and ultrathin MoS₂ nanosheets observed in SEM/TEM, which expose abundant edge sites while preserving open ion-transport channels. To further evaluate the intrinsic catalytic activity, LSV curves normalized to the electrochemical surface area (ECSA) were recorded (Supplementary Figure S6). The ECSA-normalized activity sequence follows Cu₃N/MoS₂(1:1) < Cu₃N/MoS₂(1:0.5) < Cu₃N/MoS₂ (1:2) (under 0.1 mA cm⁻²), indicating that the 1:2 composite delivers the highest intrinsic activity per active site. However, the conventional LSV (Figure 4a) shows the order Cu₃N/MoS₂(1:2) < Cu₃N/MoS₂(1:0.5) < Cu₃N/MoS₂(1:1) (under 50 mA cm⁻²), demonstrating that the 1:1 composite exhibits the best overall apparent performance. This difference arises because ECSA normalization highlights intrinsic activity per site, whereas the conventional curves reflect both site activity and the density of accessible active sites. Thus, although Cu₃N/MoS₂(1:2) features highly active sites, its relatively limited surface area reduces the overall current density. In contrast, Cu₃N/MoS₂(1:1) achieves an optimal balance between intrinsic activity and accessible sites, leading to the best overall HER performance. These findings confirm that the synergistic p–n interfacial coupling not only enhances site activity but also optimizes charge transport and site accessibility, together underpinning the superior HER performance of Cu₃N/1T-MoS₂.

The EIS and C_dl results together demonstrate that the superior HER activity of Cu₃N/MoS₂(1:1) stems from a dual benefit: (i) enhanced interfacial charge transfer, facilitated by strong electronic coupling and lattice-matched interfaces between Cu₃N and MoS₂; and (ii) maximised ECSA, arising from the optimal MoS₂ coverage that prevents both under-exposure of catalytic edges (as in low-Mo samples) and blockage of conductive pathways (as in high-Mo samples). These synergistic effects explain why the 1:1 heterostructure achieves the best performance metrics across all electrochemical analyses.

The band-edge positions of p-type Cu₃N and n-type MoS₂ were determined by combining diffuse reflectance spectroscopy (DRS), Mott–Schottky (M–S) analysis, and valence band (VB) XPS measurements, enabling precise construction of their energy level diagrams relative to RHE.

From the Tauc plots (Figures 6a,b), the optical band gaps (E_g) were extracted as 1.12 eV for MoS₂ and 1.34 eV for Cu₃N (Feng et al., 2015; Makuła et al., 2018). Mott–Schottky plots (Figures 6c,d) recorded at different frequencies exhibit opposite slopes, confirming the p-type semiconductor of Cu₃N and the n-type semiconductor of MoS₂ (Gelderman et al., 2007; Sivula, 2021; Xu et al., 2022). The flat-band potentials (E_fb) were determined to be +1.59 V (vs. RHE) for Cu₃N and +0.116 V (vs. RHE) for MoS₂. For p-type Cu₃N, the VB maximum (VBM) lies slightly below E_fb (≈0.2 eV), giving E_VB ≈ +1.79 V vs. RHE. For n-type MoS₂, E_fb approximates the conduction band minimum (CBM), thus E_CB ≈ −0.08 V (vs. RHE). According to E_CB = E_VB − E.g., the E_CB of Cu₃N and E_VB of MoS₂ are 0.45 V and 1.04 V (vs. RHE), respectively.

Figure 6

Figure 6. Tauc plots of (a) MoS₂ and (b) Cu₃N. Mott-Schottky plots of (c) MoS₂ and (d) Cu₃N at different frequency.

On the basis of the foregoing analysis, a plausible catalytic mechanism was elucidated. The Cu₃N/MoS₂ heterojunction was constructed via intimate contact between Cu₃N nanostructures and layered MoS₂, forming a p-n charge transfer pathway (Scheme 1). (Dai et al., 2024; Li and Zhou, 2021; Li et al., 2025; Qin et al., 2023; Zhu et al., 2020a; Zhu et al., 2020b) Upon contact, the difference in Fermi levels drives electrons from MoS₂ to Cu₃N, resulting in electron accumulation on the Cu₃N side and hole accumulation on the MoS₂ side. This interfacial charge redistribution generates a built-in electric field across the junction, inducing upward bending of MoS₂ bands and downward bending of Cu₃N bands until thermodynamic equilibrium is reached. Such an internal field has been widely recognized to facilitate charge separation and directional migration in heterojunction electrocatalysts.

Scheme 1

Scheme 1. The mechanism of Cu₃N/MoS₂ of electrocatalytic hydrogen evolution.

Under cathodic polarization, electrons are preferentially injected into the MoS₂ conduction band, where they participate in the Volmer step (H⁺ + e⁻ → H*) on exposed S-edge/Mo-center sites, followed by either the Heyrovsky (H* + H⁺ + e⁻ → H₂) or Tafel (2H* → H₂) step. Simultaneously, bias-driven holes are directed towards Cu₃N and consumed via the external circuit, effectively suppressing charge recombination. The synergistic action of the internal field and external bias thus ensures spatial separation of electrons (localized in MoS₂-CB) and holes (localized in Cu₃N-VB).

Electrochemical impedance spectroscopy (EIS) measurements demonstrate that the Cu₃N/MoS₂ electrode exhibits a markedly reduced charge-transfer resistance (R_ct) compared to pristine components, while double-layer capacitance (C_dl) measurements reveal an increased electrochemically active surface area (ECSA). These results confirm accelerated charge transport and higher active site density, consistent with the lower Tafel slope and higher turnover frequency observed. The hierarchical architecture further facilitates electrolyte penetration and rapid release of evolved H₂, maintaining high catalytic activity and stability under large current densities.

The Cu₃N/1 T-MoS₂ heterojunction delivers a balanced acidic-HER performance, combining a low overpotential at practical current density (η₅₀ = 387 mV; Tafel = 57 mV dec⁻¹), 150 h durability, and robust operation in 0.5 M H₂SO₄ (comparison Supplementary Table S1). This co-optimization stems from a p–n built-in field that drives directional charge separation, noble-metal-free electronic wiring from Cu₃N, and interfacial stabilization of the 1 T phase. The straightforward synthesis and interfacial durability further support scale-up and device integration.

4 Conclusion

In conclusion, we have developed an interface-engineered Cu₃N/1 T-MoS₂ heterojunction that couples conductive p-type Cu₃N nanocubes with ultrathin MoS₂ nanosheets. The composite achieves 387 mV at 50 mA cm^-2 with a 57 mV dec⁻¹ Tafel slope in 0.5 M H₂SO₄ and maintains performance for 150 h. Structure–property correlation shows: (i) coherent interfaces and interfacial strain verified by XRD/TEM; (ii) XPS-evidenced charge redistribution; (iii) lowest R_ct and highest C_dl (up to 103.6 mF cm^-2) among all samples, indicating rapid charge transport and abundant active sites. The activity enhancement is thus attributed to a p–n heterojunction with a built-in electric field, which funnels electrons toward MoS₂ edge sites while suppressing recombination. This study positions Cu₃N as a versatile platform for stabilizing conductive MoS₂ and offers generalizable design guidelines for interfacial-coupled HER electrocatalysts operating in acidic media.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

TH: Validation, Methodology, Investigation, Writing – original draft. WX: Conceptualization, Investigation, Supervision, Funding acquisition, Writing – review and editing, Project administration, Data curation, Methodology, Validation. FJ: Writing – review and editing. VP: Writing – review and editing. QZ: Writing – review and editing, Methodology, Formal analysis. SK: Writing – review and editing, Methodology, Formal analysis. AY: Writing – review and editing.

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. This work was financially supported by the Natural Science Foundation of Hunan Province (No.2024JJ7407), Outstanding Youth Foundation of Hunan Provincial Education Department (No.23B0503), Scientific research start-up funding of Jishou university, which were greatly appreciated.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenrg.2025.1683318/full#supplementary-material

Footnotes

¹For Original Research articles, please note that the Material and Methods section can be placed in any of the following ways: before Results, before Discussion or after Discussion

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