Robust P–N heterojunction polymer nanocomposites: advanced pH-universal electrocatalysts for highly efficient and stable water-splitting hydrogen production

Abstract

When combined, P-type polyaniline (PANI) and N-type exfoliated molybdenum diselenide (MoSe₂) nanosheets form a P–N heterojunction at their interface that significantly enhances electrocatalytic performance. Herein, an organic–inorganic PANI/MoSe₂ composite with a P–N heterojunction interface was successfully developed as an efficient electrocatalyst for the hydrogen evolution reaction (HER) under universal pH conditions. This newly developed system demonstrates exceptional long-term electrocatalytic stability in acidic, alkaline, and simulated seawater solutions and also shows promising potential to replace commercial noble metal platinum–carbon (Pt/C) catalysts in energy-related applications. The development of hydrogen energy is key in order to move toward net-zero carbon emissions; however, efficient, low-cost, pH-universal hydrogen evolution reaction (HER) catalysts remain a significant challenge. Organic polyaniline (PANI) and inorganic exfoliated molybdenum diselenide (MoSe₂) nanosheets are P-type and N-type semiconductors. Their combination facilitates formation of an organic–inorganic P–N heterojunction interface, which promotes efficient charge transfer from PANI to MoSe₂ and enhances HER catalysis under universal pH conditions. Here, we successfully constructed a MoSe₂/PANI composite catalyst with a P–N heterojunction interface and excellent electrocatalytic performance by integrating biopolymer-functionalized exfoliated MoSe₂ nanosheets with PANI on a conductive nickel foam (NF) substrate through electropolymerization and electroactivation. Compared to the commercial platinum–carbon catalyst, MoSe₂/PANI/NF exhibits superior electrocatalytic HER performance in acidic, alkaline, and simulated seawater electrolyte solutions, with similar Tafel slopes and lower overpotential and resistance values. Importantly, after 24 h of operation (100 mA cm⁻²) or 1000 cycles of cyclic voltammetry scanning in acidic and simulated seawater conditions, MoSe₂/PANI/NF demonstrated excellent catalytic stability and environmental tolerance, indicating potential to achieve efficient and highly stable water electrolysis for hydrogen production. Therefore, this emerging system may enable the development of pH-universal, non-precious metal electrocatalysts for various energy applications.

---

Introduction

Due to the widespread use of traditional energy sources such as petroleum, coal, and natural gas, research into environmentally friendly and feasible energy solutions has become increasingly important to meet global energy demands and mitigate environmental degradation.^1,2 In recent years, research institutions and industrial sectors in various countries have been paying increased attention to environmental protection and renewable energy. Hydrogen has shown great potential as an alternative to traditional fuels due to its superior energy density and zero emissions. Water electrolysis is a promising method for producing sustainable hydrogen using green energy sources such as solar, geothermal, and wind energy.^3,4 Water electrocatalysis provides a direct and effective method for hydrogen production that may help to address global energy demands and environmental issues; this electrochemical method of splitting water to generate hydrogen molecules is attractive due to its high energy content and environmental benefits.^5,6

The hydrogen evolution reaction (HER) process requires active catalysts to enhance the efficiency of hydrogen production. Platinum (Pt) and its compounds are considered the most advanced HER catalysts because their Gibbs free energy is close to zero, which is an ideal characteristic for efficient HER catalysts.^7,8 However, the high cost, low availability, and poor stability of platinum catalysts in corrosive environments—as well as their suboptimal performance under alkaline and neutral conditions due to slow water dissociation kinetics—have significantly impeded their widespread industrial adoption.⁹ Therefore, there is an urgent need to develop an electrocatalyst that can effectively function across a wide pH range and under different electrolyte conditions, as currently only a few materials exhibit such excellent pH universality and performance in neutral and diverse environments.^10–12

Developing efficient electrocatalysts for neutral or alkaline conditions remains a significant challenge, primarily due to the higher energy required for water dissociation and the strong adsorption of hydroxyl species. However, recent advancements in catalyst engineering have revealed promising approaches to enhance catalytic performance in these environments.^13,14 For example, Tan and colleagues developed an approach to enhance HER catalyst activity and durability by assembling low-load Pt nanoparticles on a copper foam substrate. This strategy is based on a strong interaction between the Pt nanoparticles and the copper substrate, which promotes proton adsorption and enhances the overall catalytic performance.¹⁵ Nevertheless, this system employs a precious metal as the primary catalytic substrate. Therefore, development of non-precious metal electrocatalysts that are low-cost, resource-abundant, environmentally benign, and exhibit high catalytic performance and durability for HER remains an urgent and challenging goal.

Various types of transition metal-based materials, such as dichalcogenides,^16–19 phosphides,²⁰ carbides, and nitrides, have been widely studied for the HER.^21,22 Transition metal dichalcogenides (TMDs)—such as molybdenum diselenide (MoSe₂)—are a promising alternative that have garnered attention due to their cost-effectiveness, abundance, low Gibbs free energy, and tunable structure.²² Although MoSe₂ exhibits high HER activity, its performance is hindered by poor conductivity and limited active sites, particularly in alkaline and neutral solutions, where strong hydroxyl adsorption and high kinetic barriers can impede efficiency.¹⁴ To address these challenges, researchers have explored strategies such as doping,^16,23 phase engineering, and formation of composite materials²⁴ to enhance the electronic structure, increase active site exposure, and improve the catalytic performance of MoSe₂.²⁵ In addition, strategies involving the introduction of heterostructures have been adopted to optimize conductivity and site accessibility, and enabled MoSe₂-based electrocatalysts to exhibit desirable HER catalytic activity in acidic, neutral, and alkaline conditions, which makes them promising candidates for universal pH water electrolysis applications.²⁶

Catalysts based on heterojunctions have become a key approach for advancing the design and implementation of new catalytic systems. By rearranging electrons at the heterojunction interfaces, the active sites of these catalysts can be modified to enhance reaction kinetics through the synergistic combination of multiple active sites.²⁷ Compared to single-component catalysts, heterojunction catalysts often exhibit higher electrocatalytic activity. However, despite extensive research on their photocatalytic applications, the potential of heterojunction catalysts in the field of electrocatalysis remains underexplored.^26–28 P–N heterojunction materials for HER are composed of two tightly coupled semiconductor materials with different doping types (P-type or N-type). P–N heterojunctions in electrocatalysts enable charge redistribution and strengthen electronic interactions, and thereby offer a promising approach to improve the catalytic performance of the HER.²⁹

MoSe₂ is an N-type semiconductor with a layered structure, in which the layers are connected by weak van der Waals forces.^30,31 When combined with materials exhibiting P-type semiconductor properties, MoSe₂ can form P–N heterojunctions that could potentially improve electrocatalytic efficiency and structural reliability. For example, Gu et al. developed and fabricated a core–shell structure by coating ultrathin MoSe₂ nanosheets on copper sulfide hollow nanoboxes to form an open P–N heterojunction structure.³² This heterojunction was utilized as a versatile electrocatalyst for water oxidation across a wide pH range. Similarly, Zhao et al. developed a heterojunction composed of cobalt diselenide quantum dots deposited on MoSe₂ nanosheets, which exhibited enhanced HER activity in alkaline media compared to bare MoSe₂ nanosheets.³³ Nevertheless, although the approach of forming P–N heterojunctions can enhance the HER electrocatalytic performance of MoSe₂, there are still significant deficiencies in the electrocatalytic properties of MoSe₂ nanosheets compared to commercial Pt catalysts due to heterophase issues between the P-type and N-type materials. Therefore, overcoming the challenges of heterophase formation (i.e., improving the compatibility between the two materials) will be crucial to advance the development of P–N heterojunction catalysts for HER.

Based on a previous study, selenides are superior to sulfides due to their lower linewidth, narrower bandgap, and tunable exciton charging effects.³⁴ The bandgap of MoSe₂ ranges from 1.10 eV for bulk materials to approximately 1.58 eV for monolayer structures. Monolayer or exfoliated MoSe₂ nanosheets possess a high surface area, and exhibit excellent physicochemical and optical properties.³⁵ Notably, their unique electrocatalytic capability, which is primarily attributed to the uncoordinated Se edge atoms, makes MoSe₂ nanosheets highly suitable as electrocatalysts for HER.³⁶ Recently, we successfully developed an efficient system for exfoliating bulk MoSe₂ crystals in water with the assistance of water-soluble sodium ion-functionalized chitosan (Na-CMC).³⁷ Due to a high affinity between Na-CMC and the surface of MoSe₂, the resulting exfoliated MoSe₂ nanosheets exhibit uniform nanoscale dimensions, a high specific surface area, and excellent dispersion stability in water. Furthermore, the nanosheets can be stably and reversibly dried into solid form and redispersed in water. These intriguing findings and properties can primarily be attributed to the stable attachment of Na-CMC onto the surface of the exfoliated MoSe₂ nanosheets, which imparts high structural stability to the nanosheets and prevents them from restacking or self-aggregating in aqueous or solid-state conditions. Therefore, based on the development of this Na-CMC/MoSe₂ composite system, we boldly hypothesized that combining Na-CMC-functionalized exfoliated MoSe₂ nanosheets with a P-type semiconductor material would both promote the formation of a P–N heterojunction at their interface and also significantly enhance heterostructure compatibility due to the presence of Na-CMC. We speculated that these features would confer desirable HER catalytic performance and structural stability in aqueous solutions across different pH conditions and thus offer a promising pathway to replace traditional precious metal electrocatalysts.

Herein, we successfully developed a combination of P-type acid-doped polyaniline (PANI) and N-type exfoliated MoSe₂ nanosheets, to form a new organic–inorganic P–N heterojunction catalyst with excellent pH-universal catalytic activity (Scheme 1). By including Na-CMC-functionalized exfoliated MoSe₂ nanosheets during the electropolymerization (EP) and electroactivation (EA) of PANI on conductive nickel foam (NF), we successfully constructed a Na-CMC/MoSe₂/PANI/NF composite catalyst with a P–N heterojunction. The P–N heterojunction at the interface between the PANI matrix and the MoSe₂ nanosheets results in an efficient, smooth charge transfer pathway from PANI to MoSe₂, which effectively facilitates and promotes HER catalysis by the MoSe₂ nanosheets. Compared to a commercial, precious metal-based Pt/carbon (Pt/C) catalyst, Na-CMC/MoSe₂/PANI/NF demonstrated superior electrocatalytic performance in a series of HER evaluations conducted in acidic, alkaline, and simulated seawater; Na-CMC/MoSe₂/PANI/NF exhibited comparable Tafel slopes, lower overpotential and resistance values, as well as excellent prolonged stability under high current densities and repeated cyclic voltammetry cycles. These results confirm that this innovative system offers a feasible strategy to enable the development of high-performance, low-cost, non-precious metal-based electrocatalysts with strong potential for future applications in various energy fields.

Scheme 1 The schematic illustrates how Na-CMC-functionalized exfoliated MoSe₂ nanosheets are incorporated with PANI through electropolymerization and electroactivation to construct the composite electrocatalyst Na-CMC/MoSe₂/PANI/NF with outstanding HER electrocatalytic performance. The dashed frame in the upper-right corner highlights how the combination of P-type acid-doped PANI and N-type exfoliated MoSe₂ nanosheets leads to the formation of a P–N heterojunction at their interface, which facilitates stable charge transfer from PANI to MoSe₂.

Results and discussion

We recently successfully developed an efficient system for exfoliating bulk MoSe₂ crystals in water with the assistance of water-soluble Na-CMC.³⁷ The resulting exfoliated MoSe₂ nanosheets exhibit a high yield, uniform nanoscale dimensions, and excellent long-term dispersion stability in water. The nanosheets can also be stably freeze-dried into a solid form and subsequently redispersed in water while retaining the same structure and physical properties.

Based on these findings and the unique electrocatalytic activity of MoSe₂,^35,38 we further explored the catalytic HER capabilities of these exfoliated MoSe₂ nanosheets in different aqueous conditions (Scheme 1). Before evaluating the HER performance of the exfoliated MoSe₂ nanosheets, we first confirmed their long-term dispersion stability in water by varying the Na-CMC and MoSe₂ ratio. We monitored the percentage transmittance at 550 nm over time for 1/1, 1/3, and 1/5 Na-CMC/MoSe₂ nanosheets in aqueous solutions with different pH values using ultraviolet-visible (UV-vis) spectroscopy at 25 °C. As shown in Fig. S1 and S2,† under acidic (pH 1.0) and neutral (pH 7.4) conditions, the percentage transmittance of 1/3 and 1/5 Na-CMC/MoSe₂ solutions remained unchanged over more than one month of monitoring. In contrast, the transmittance of the 1/1 Na-CMC/MoSe₂ solution increased by over 12%, suggesting that 1/3 and 1/5 Na-CMC/MoSe₂ nanosheets demonstrate high dispersion stability in acidic and neutral aqueous solutions.³⁷ In contrast, under alkaline conditions (pH 13.5), the percentage transmittance of all Na-CMC/MoSe₂ solutions increased over time (Fig. S3†). After 29 days of monitoring, the transmittance values for 1/1, 1/3, and 1/5 Na-CMC/MoSe₂ solutions were 45%, 16%, and 29%, respectively; these values indicated that the 1/3 Na-CMC/MoSe₂ nanosheets possess the most optimized composition and dispersion characteristics in alkaline conditions. Therefore, based on these results, we selected the 1/3 Na-CMC/MoSe₂ solution for subsequent evaluations of HER catalytic performance in acidic, alkaline, and simulated seawater conditions.

We speculated that the exfoliated Na-CMC/MoSe₂ nanosheets, with their uniform nanoscale structure, high catalytic surface area, and excellent long-term structural stability, could significantly enhance HER catalytic performance. Nevertheless, as the HER must be conducted in various aqueous conditions, the stable adhesion of hydrophilic, exfoliated MoSe₂ nanosheets onto conductive electrode surfaces posed a significant challenge. To overcome this barrier, we propose a feasible solution by introducing the acid-doped conjugated polymer PANI onto the electrode surface as a binder that effectively anchors the exfoliated MoSe₂ nanosheets. In addition, acid-doped PANI and exfoliated MoSe₂ nanosheets, as P-type and N-type semiconductors respectively, exhibit complementary properties;^30,39 thus we anticipated that a stable P–N heterojunction would form at the interface between the PANI matrix and MoSe₂ nanosheets, and in turn subsequently potentially enhance the stability of the MoSe₂ nanosheets within the PANI matrix and significantly improve charge transport and HER catalytic performance (Scheme 1). To confirm the formation of a P–N heterojunction interface between the acid-doped PANI and exfoliated MoSe₂ nanosheets, we verified the properties of the semiconductors before and after their combination using Mott–Schottky plots.⁴⁰ As shown in Fig. S4a and b,† the exfoliated MoSe₂ nanosheets and acid-doped PANI exhibit positive and negative linear slopes, respectively, indicating that MoSe₂ is an N-type semiconductor and PANI is a P-type semiconductor. The two materials were electropolymerized onto indium tin oxide (ITO) to form Na-CMC-MoSe₂/PANI composite film. The Mott–Schottky plot in Fig. 1a clearly showed the inflection points of the positive and negative linear slopes, suggesting the formation of a heterojunction between the MoSe₂ nanosheets and PANI. In addition, the flat band potentials of the MoSe₂ nanosheets and PANI in the composite matrix were −0.60 eV and 0.81 eV, respectively. Both of these values are slightly different to those of pristine PANI and MoSe₂ nanosheets (Fig. S4a and b†), which may be attributed to the formation of the P–N heterojunction altering their affinities with the surrounding medium.⁴¹

Fig. 1 (a) Mott–Schottky plot for the 1/3 Na-CMC/MoSe₂/PANI composite coated on indium tin oxide (ITO) glass. HER characteristics of commercial Pt/C, blank NF, PANI/NF, and EP-treated Na-CMC/MoSe₂/PANI/NF in 0.5 M H₂SO₄ solution: (b) LSV curves, (c) Tafel plots, and (d) EIS Nyquist plots. The inset on the right side of (d) illustrates the equivalent circuit model employed for fitting the experimental data. (e–g) SEM images of EP-treated Na-CMC/MoSe₂/PANI/NF at different magnifications. (g) SEM images of EP-treated Na-CMC/MoSe₂/PANI/NF, and (h) the corresponding EDX spectrum with the weight and atomic percentage of elements (shown in the table on the right). (i) SEM image of EP-treated Na-CMC/MoSe₂/PANI/NF and the corresponding EDX images showing the elemental distribution for (j) C, (k) N, (l) O, (m) Ni, (n) Mo, and (o) Se.

After confirming the semiconductor properties of the Na-CMC-MoSe₂/PANI composite film, we analyzed the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using UV-vis spectroscopy and cyclic voltammetry (CV).³⁷ As presented in Fig. S4c,† the direct energy band gap (E_g) distribution revealed a significant increase in E_g for the 1/3 Na-CMC/MoSe₂ nanosheets to 2.0 eV compared to the bulk MoSe₂ crystal, which has an E_g of 1.5 eV (HOMO and LUMO at −5.3 and −3.8 eV, respectively).^37,42 This increase in E_g can be attributed to the presence of the nanosheet structure, which enhances quantum confinement. In addition, compared to the LUMO value of −3.1 eV for acid-doped PANI,⁴³ the 1/3 Na-CMC/MoSe₂ nanosheets exhibited a slightly lower LUMO value of −3.4 eV, indicating that charge can be smoothly transferred from PANI to the exfoliated MoSe₂ nanosheets, which would effectively minimize charge transport losses. Therefore, these results indicate a stable P–N heterojunction-based organic–inorganic PANI–MoSe₂ composite was formed.

Subsequently, we extended the study to the preparation of HER catalytic electrodes and evaluated their catalytic performance. First, a 40-minute EP process was conducted using a 1 cm × 1 cm NF conductive substrate to construct the Na-CMC/MoSe₂/PANI/NF composite electrode.⁴³ The resulting electrode was characterized directly using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in 0.5 M sulfuric acid (H₂SO₄) electrolyte solution to establish the Tafel slope and resistance. As shown in Fig. 1b–d, compared to the pristine NF and PANI/NF electrodes, the Na-CMC/MoSe₂/PANI/NF composite electrode exhibited significantly lower overpotential (130.2 mV at −10 mA cm⁻²), Tafel slope (95.3 mV dec⁻¹), and resistance (7.6 Ω) values, which demonstrated that the exfoliated MoSe₂ nanosheets effectively drive the HER process on the electrode. Nevertheless, these results are significantly inferior to the HER catalytic performance of the noble metal Pt/C catalyst, which has an overpotential of 20.8 mV at −10 mA cm⁻², a Tafel slope of 34.2 mV dec⁻¹, and a resistance of 1.3 Ω.

To identify the possible reasons for these differences, we further investigated the correlation between surface morphology and HER characteristics. As shown in Fig. S5† and 1e–g, the pristine NF substrate exhibited a uniform surface morphology with clearly defined grain boundaries in scanning electron microscopy (SEM) images. In contrast, the Na-CMC/MoSe₂/PANI/NF composite presented an uneven, rough surface morphology with cracks, indicating poor compatibility between the exfoliated MoSe₂ nanosheets and the PANI matrix. This also implies that the two materials form a phase-separated morphology, which would limit the formation of extensive P–N heterojunction interfaces. Nevertheless, through SEM and energy-dispersive X-ray (EDX) spectroscopy for elemental analysis and mapping at the microstructural level, the EDX spectra (Fig. 1g and h) and mapping images (Fig. 1i–o) confirmed that all characteristic elements were detectable and uniformly distributed on the surface of the Na-CMC/MoSe₂/PANI/NF composite. These results further demonstrate that the EP process effectively immobilizes and uniformly distributes exfoliated MoSe₂ nanosheets within the PANI matrix. Overall, although the HER performance of Na-CMC/MoSe₂/PANI/NF was not equivalent to Pt/C, EP effectively facilitated immobilization of the MoSe₂ nanosheets, promoted the formation of a P–N heterojunction interface between MoSe₂ and PANI, and enhanced their structural stability and catalytic efficiency for HER.

Based on the above results, the compatibility of the interface between the organic PANI matrix and the inorganic MoSe₂ nanosheets appeared to be a key factor that influences the formation of a high-level P–N heterojunction and overall HER catalytic performance. We speculated that high-energy external environmental parameters may improve the compatibility between the two interfaces. Therefore, we applied a fixed current density of 500 mA cm⁻² to Na-CMC/MoSe₂/PANI/NF in 0.5 M H₂SO₄ solution and monitored the time-dependent changes in potential.⁴³ As presented in Fig. 2a, surprisingly, the corresponding potential decreased during the 12 h observation period, stabilizing after 8 h. This implies that the high current density environment promotes the interaction between the organic PANI matrix and the inorganic MoSe₂ nanosheets, which would stabilize the formation of a high-level P–N heterojunction at their interface and also effectively enhance the overall catalytic performance. In view of this, this treatment process is hereafter referred to as EA.

Fig. 2 (a) Chronoamperometric curve at a constant current density of 500 mA cm⁻² over 12 h in 0.5 M H₂SO₄ solution for EP-treated Na-CMC/MoSe₂/PANI/NF. HER characteristics of commercial Pt/C, EP-treated Na-CMC/MoSe₂/PANI/NF, and EA-treated Na-CMC/MoSe₂/PANI/NF after different durations of EA treatment in 0.5 M H₂SO₄ solution: (b) LSV curves, (c) Tafel plots, and (d) EIS Nyquist plots. The inset on the right side of (d) illustrates the equivalent circuit model employed for fitting the experimental data. (e–g) SEM images of EA-treated Na-CMC/MoSe₂/PANI/NF at different magnifications. (g) SEM images of EA-treated Na-CMC/MoSe₂/PANI/NF, and (h) the corresponding EDX spectrum with the weight and atomic percentage of elements (shown in the table on the right). (i) HRTEM image of EA-treated Na-CMC/MoSe₂/PANI/NF, (k) the corresponding SAED pattern showing its lattice characteristics, and (j) a high-magnification HRTEM image of the EA-treated Na-CMC/MoSe₂/PANI/NF from the red dashed box area in (i). (l) HAADF-STEM image of EA-treated Na-CMC/MoSe₂/PANI/NF and the corresponding elemental distribution mappings for (m) C, (n) N, (o) O, (p) Ni, (q) Mo, and (r) Se.

Subsequently, the electrocatalytic performance of Na-CMC/MoSe₂/PANI/NF subjected to EA for different periods of time was characterized through LSV and EIS. As shown in Fig. 2b–d, compared to the results obtained after EP treatment, the overpotential, Tafel slope, and resistance of Na-CMC/MoSe₂/PANI/NF all significantly decreased after different durations of EA. For instance, after 4 h of EA treatment, the overpotential was 56.7 mV (at −10 mA cm⁻²), the Tafel slope was 39.9 mV dec⁻¹, and the resistance was 4.9 Ω. After 12 h of EA treatment, the overpotential was 11.3 mV (at −10 mA cm⁻²), the Tafel slope was 25.7 mV dec⁻¹, and the resistance was approximately 2.0 Ω, confirming that the catalytic performance and conductivity of Na-CMC/MoSe₂/PANI/NF gradually improved as the duration of EA increased. In addition, evaluation of electrochemical double-layer capacitance (C_dl) further confirmed that after 12 h of EA treatment, the C_dl value of Na-CMC/MoSe₂/PANI/NF was 7.48 mF cm⁻² (Fig. S6a–c†), which is approximately 6.1 times higher than that after EP treatment. This confirms that the EA treatment effectively increased the electrochemically active surface area of Na-CMC/MoSe₂/PANI/NF, thereby resulting in sufficient catalytic sites on its surface and ultimately significantly strengthening the overall catalytic performance.⁴⁴ It is particularly noteworthy that, after 12 h of EA treatment, the HER catalytic properties of EA-treated Na-CMC/MoSe₂/PANI/NF not only outperformed the precious metal Pt/C but also resulted in an overpotential of only 57.6 mV at −500 mA cm⁻² (compared to 127.4 mV for Pt/C, as shown in Fig. 2b). Therefore, EA-treated Na-CMC/MoSe₂/PANI/NF can maintain stable and efficient electrocatalytic performance even under high current density conditions and holds potential as a non-precious metal electrocatalyst for hydrogen production.

To understand how EA treatment can significantly enhance the electrocatalytic performance of Na-CMC/MoSe₂/PANI/NF, we examined the transformations in its surface morphology and microstructure. As shown in Fig. 2e–g, SEM images showed that—compared to the results obtained after EP treatment (Fig. 1e–g)—Na-CMC/MoSe₂/PANI/NF after 12 h of EA treatment exhibited a surface with uniformly distributed textures, clearly indicating that EA enhanced the interface compatibility between PANI and MoSe₂ and thereby promoted the formation of a stable high-level P–N heterojunction at their interface. The mapping images in Fig. S7† and surface elemental analysis for Fig. 2g shown in Fig. 2h using EDX spectroscopy revealed that all characteristic elements could be observed on the surface of EA-treated Na-CMC/MoSe₂/PANI/NF. Nevertheless, the total proportion of the MoSe₂ component was less than 0.4%, which may be due to the difficulty of accurately quantifying the content of two-dimensional MoSe₂ nanosheets through surface analysis methods (see table on right of Fig. 2h). Therefore, to further confirm the distribution of the MoSe₂ component within the matrix, we prepared thin slices of EA-treated Na-CMC/MoSe₂/PANI/NF using cryo-ultramicrotomy and observed its microstructure using high-resolution transmission electron microscopy (HRTEM). The HRTEM, selected area electron diffraction (SAED), and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images in Fig. 2i–r clearly revealed the microstructure of the EA-treated Na-CMC/MoSe₂/PANI/NF. Low-magnification and high-magnification HRTEM images showed that the exfoliated MoSe₂ nanosheets, with an interlayer thickness of approximately 5–10 nm, were uniformly distributed within the matrix (Fig. 2i) and exhibited a nearly amorphous microstructure (Fig. 2j), indicating that the MoSe₂ nanosheets were firmly integrated with the PANI matrix. In contrast, the Na-CMC aggregates were non-covalently attached to the surface of the MoSe₂ nanosheets, and thus could effectively prevent re-stacking (or self-aggregation) of the nanosheets and thereby maintain the exfoliated nanosheet structure within the PANI matrix.^43,45 Similar results were also observed in the SAED pattern shown in Fig. 2k, in which the characteristic (100) and (110) diffraction spots of the exfoliated MoSe₂ nanosheets are clearly visible, while no (002) diffraction spot was observed. This further confirms that—even after EP and EA treatment—the Na-CMC aggregates were still firmly adhered to the surface of the MoSe₂ nanosheets and promote stable distribution of the nanosheets within the PANI matrix. In addition, the HAADF-STEM images (Fig. 2l–r) further verified that all characteristic elements were uniformly distributed within the composite matrix. Collectively, these observations clearly indicate that the EA treatment process not only effectively enhances the compatibility between PANI and MoSe₂ but also promotes the formation of a high-level organic–inorganic P–N heterojunction at their interface, which helps to facilitate smooth charge transfer from PANI to MoSe₂ during the electrocatalytic process and, ultimately, achievement of high HER catalytic performance.

After exploring the morphological characteristics of EA-treated Na-CMC/MoSe₂/PANI/NF, we utilized Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) to confirm the structural features of exfoliated MoSe₂ nanosheets before and after EA treatment. As shown in Fig. S8,† compared to the original exfoliated MoSe₂ nanosheets, characteristic E¹_2g and B¹_2g peaks were clearly observed for both EP- and EA-treated Na-CMC/MoSe₂/PANI/NF. This indicates that the EP and EA treatments did not affect the structure of the exfoliated MoSe₂ nanosheets.³⁷ Notably, the intensity of the characteristic E¹_2g peak significantly increased in both composites, especially after EA treatment, suggesting that the formation of high-degree P–N heterojunctions isolated the exfoliated MoSe₂ nanosheets within the PANI matrix and therefore imposed more pronounced exfoliated structural characteristics.⁴⁶ In the XPS spectra, the characteristic Mo 3d and Se 3d peaks for EP-treated Na-CMC/MoSe₂/PANI/NF exhibited broader distributions (Fig. S9 and S10†) compared to those of pristine exfoliated MoSe₂ nanosheets.³⁷ This implies that incompatibility between PANI and MoSe₂ significantly affects these characteristic peaks and thereby leads to the formation of low-degree P–N heterojunction interfaces. Interestingly, the characteristic Mo 3d and Se 3d peaks of EA-treated Na-CMC/MoSe₂/PANI/NF shifted toward significantly higher binding energies. This result is in stark contrast to the peaks for pristine exfoliated MoSe₂ nanosheets, which shifted toward lower binding energies compared to the bulk MoSe₂ crystals.³⁷ This phenomenon may be attributed to the enhanced interaction between the PANI matrix and Mo/Se atoms in MoSe₂ after EA treatment, which leads to the formation of new heteronuclear non-covalent bonds at the Mo and Se sites. Overall, the spectral characteristics clearly confirmed that PANI forms a high-degree P–N heterojunction interface with exfoliated MoSe₂ nanosheets after EA treatment.

Zero emissions are a current global concern, and the production of hydrogen using green energy has garnered significant attention and is rapidly emerging as a promising solution.^47,48 Green hydrogen is primarily produced through water electrolysis powered by renewable energy, which offers the potential to achieve zero-emission goals and foster the development of sustainable energy solutions.^49,50 Therefore, based on the HER performance achieved by EA-treated Na-CMC/MoSe₂/PANI/NF in acidic conditions, we further explored its hydrogen production capabilities in alkaline and simulated seawater conditions. First, we focused on evaluating the HER catalytic activity of Na-CMC/MoSe₂/PANI/NF after 12 h of EA treatment by conducting LSV measurements in alkaline (1.0 M) KOH electrolyte solution. In 1.0 M KOH, the C_dl value of EA-treated Na-CMC/MoSe₂/PANI/NF reached as high as 16.74 mF cm⁻², which was 2.4 times higher than the EP-treated sample (Fig. 3a) and indicates that EA-treated Na-CMC/MoSe₂/PANI/NF retained a high catalytic active surface area in alkaline conditions. Moreover, the C_dl value of EA-treated Na-CMC/MoSe₂/PANI/NF was significantly higher in alkaline conditions than under acidic conditions (Fig. S6a†); this suggests that NF inherently possesses a certain degree of catalytic capability in alkaline conditions, which may effectively exert a synergistic action that enhances the overall catalytic surface area of Na-CMC/MoSe₂/PANI/NF.^51,52 As shown in the LSV profile in Fig. 3b, the EA-treated Na-CMC/MoSe₂/PANI/NF demonstrated an overpotential of 35.2 mV at −10 mA cm⁻², a Tafel slope of 64.4 mV dec⁻¹ (Fig. 3c), and a resistance of approximately 1.7 Ω (Fig. 3d), indicating excellent HER catalytic performance under alkaline conditions. Although the Tafel slope of EA-treated Na-CMC/MoSe₂/PANI/NF was not as low as that of Pt/C in 1.0 M KOH (38.8 mV dec⁻¹), EA-treated Na-CMC/MoSe₂/PANI/NF had an overpotential of 204.2 mV at a current density of −350 mA cm⁻² (Fig. 4b), which is lower than the 248.6 mV observed for Pt/C. This suggests that EA-treated Na-CMC/MoSe₂/PANI/NF may exhibit long-term stable catalytic performance under alkaline conditions and thus has potential to serve as a non-precious metal electrocatalyst for rapid, large-scale hydrogen production under high current density conditions.⁵³

Fig. 3 (a) C_dl results in 1.0 M KOH solution for Na-CMC/MoSe₂/PANI/NF after EP and EA treatments. HER characteristics of commercial Pt/C, blank NF, PANI/NF, EP-treated Na-CMC/MoSe₂/PANI/NF, and EA-treated Na-CMC/MoSe₂/PANI/NF in 1.0 M KOH solution: (b) LSV curves, (c) Tafel plots, and (d) EIS Nyquist plots. (e) C_dl results in 2/1 1 M KOH/0.5 M NaCl solution for Na-CMC/MoSe₂/PANI/NF after EP and EA treatments. HER characteristics of commercial Pt/C, blank NF, PANI/NF, EP-treated Na-CMC/MoSe₂/PANI/NF, and EA-treated Na-CMC/MoSe₂/PANI/NF in 1.0 M KOH solution: (f) LSV curves, (g) Tafel plots, and (h) EIS Nyquist plots. (i) Chronoamperometric curve of commercial Pt/C in 2/1 1 M KOH/0.5 M NaCl solution at a fixed current density of 100 mA cm⁻² for 24 h. HER characteristics of commercial Pt/C before and after 24 h testing at 100 mA cm⁻² in 2/1 1 M KOH/0.5 M NaCl solution: (j) LSV curves, (k) Tafel plots, and (l) EIS Nyquist plots. The insets on the right side of (d), (h), and (l) illustrate the equivalent circuit models employed for fitting the experimental data.

Fig. 4 (a) Chronoamperometric curve for EA-treated Na-CMC/MoSe₂/PANI/NF in 0.5 M H₂SO₄ solution at a fixed current density of 100 mA cm⁻² for 24 h. HER characteristics of EA-treated Na-CMC/MoSe₂/PANI/NF before and after 24 h testing at 100 mA cm⁻² in 0.5 M H₂SO₄ solution: (b) LSV curves, (c) Tafel plots, and (d) EIS Nyquist plots. (e) Chronoamperometric curve of EA-treated Na-CMC/MoSe₂/PANI/NF in 2/1 1 M KOH/0.5 M NaCl solution at a fixed current density of 100 mA cm⁻² for 24 h. HER characteristics of EA-treated Na-CMC/MoSe₂/PANI/NF before and after 24 h testing at 100 mA cm⁻² in 2/1 1 M KOH/0.5 M NaCl solution: (f) LSV curves, (g) Tafel plots, and (h) EIS Nyquist plots. The insets on the right side of (d and h) illustrate the equivalent circuit models employed for fitting the experimental data. SEM images of EA-treated Na-CMC/MoSe₂/PANI/NF after 24 h operation at 100 mA cm⁻² in (i–k) 0.5 M H₂SO₄ and (l–n) 2/1 1 M KOH/0.5 M NaCl solutions, respectively.

After confirming the electrocatalytic performance of EA-treated Na-CMC/MoSe₂/PANI/NF in 1.0 M KOH, we extended the HER catalytic evaluation to simulated seawater. To explore the impact of alkaline and saline aqueous conditions on the HER catalysis potential of EA-treated Na-CMC/MoSe₂/PANI/NF, simulated seawater tests were conducted under three different conditions: pure 0.5 M aqueous sodium chloride (NaCl) and 1/1 and 2/1 mixtures of 1 M KOH/0.5 M NaCl aqueous solutions. The 0.5 M NaCl solution corresponds to the salinity content of natural seawater, approximately 3.5 wt%.⁵⁴ In 0.5 M NaCl, the catalytic performance of EA-treated Na-CMC/MoSe₂/PANI/NF was significantly passivated. LSV revealed an overpotential of 303.6 mV at −10 mA cm⁻² (Fig. S11a†), a Tafel slope of 176.8 mV dec⁻¹ (Fig. S11b†), and a resistance exceeding 6 Ω (Fig. S11c†). This decline in catalytic efficiency is likely due to surface corrosion by chloride ions and deposition of ions on the catalytic electrode.⁵⁵ Nevertheless, in 1/1 and 2/1 1 M KOH/0.5 M NaCl mixed aqueous solutions, the catalytic performance of EA-treated Na-CMC/MoSe₂/PANI/NF approached similar values as those obtained in alkaline 1 M KOH solution as the volumetric ratio of 0.5 M NaCl decreased. Notably, in 2/1 1 M KOH/0.5 M NaCl solution, EA-treated Na-CMC/MoSe₂/PANI/NF exhibited an overpotential of 19.1 mV at −10 mA cm⁻² (Fig. S11a†), a Tafel slope of 57.1 mV dec⁻¹ (Fig. S11b†), resistance below 1.5 Ω (Fig. S11c†), and a C_dl value of 14.88 mF cm⁻² (Fig. 3e). These values suggest EA-treated Na-CMC/MoSe₂/PANI/NF possesses a certain level of salt resistance, and can thereby achieve favorable catalytic performance in mixed KOH/NaCl conditions. In contrast to the results for EA-treated Na-CMC/MoSe₂/PANI/NF, the HER catalytic performance of Pt/C was significantly impacted in the saline solutions. Compared to the results obtained in 1 M KOH (Fig. 3b–d), Pt/C showed a considerably higher overpotential (123.5 mV at −10 mA cm⁻², Fig. 3f), Tafel slope (107.0 mV dec⁻¹, Fig. 3g), and resistance (approximately 10 Ω, Fig. 3h) in 2/1 1 M KOH/0.5 M NaCl. This decline in catalytic efficiency could be attributed to two reasons. Firstly, the simulated seawater could significantly affect the surface chemistry of the carbon in Pt/C by altering its anchoring sites for Pt and leading to degradation/destruction of the Pt/C structure.⁵⁶ Secondly, the simulated seawater may inhibit the active catalytic sites on Pt and thus significantly decrease its electrocatalytic performance.⁵⁷ Further assessments using chronopotentiometry for 24 h at a constant current density of 100 mA cm⁻² revealed the catalytic performance of Pt/C continuously deteriorated in 2/1 1 M KOH/0.5 M NaCl, as evidenced by increased Tafel slope and resistance values (Fig. 3i–l), which indicates that Pt/C does not possess long-term catalytic stability and environmental tolerance in simulated seawater. Overall, these results confirm that EA-treated Na-CMC/MoSe₂/PANI/NF demonstrates good electrocatalytic hydrogen production capability in both alkaline (1 M KOH) and simulated seawater (1 M KOH/0.5 M NaCl) conditions. In addition, a particularly noteworthy finding is that the LSV profile showed EA-treated Na-CMC/MoSe₂/PANI/NF exhibited an overpotential of 211.6 mV in 2/1 1 M KOH/0.5 M NaCl at a current density of −350 mA cm⁻² (Fig. 3f), which is very close to the result obtained in 1 M KOH (Fig. 3b). These results clearly confirm that, even with the presence of NaCl in the solution, EA-treated Na-CMC/MoSe₂/PANI/NF can maintain stable catalytic performance at high current densities and thus holds potential for efficient, large-scale hydrogen production from seawater.

After confirming the electrocatalytic performance of EA-treated Na-CMC/MoSe₂/PANI/NF in acidic, alkaline, and simulated seawater conditions, the long-term catalytic stability was evaluated in 0.5 M H₂SO₄ (acidic) and 2/1 1 M KOH/0.5 M NaCl (simulated seawater) solutions through continuous 24 h operation under a constant current density of 100 mA cm⁻² using chronoamperometry. As shown in Fig. 4a–h, the LSV curves, Tafel slopes, and resistance values of EA-treated Na-CMC/MoSe₂/PANI/NF remained almost unchanged in both of these solutions before and after the 24 h test at a current density of 100 mA cm⁻². Moreover, SEM images (Fig. 4i–n) and Raman spectra (Fig. S12†) further indicated that, compared to the original EA-treated composite samples (Fig. 2e–g and S8†), the structure of EA-treated Na-CMC/MoSe₂/PANI/NF remained essentially unchanged after testing under a current density of 100 mA cm⁻². These results clearly demonstrate that EA-treated Na-CMC/MoSe₂/PANI/NF exhibits long-term catalytic stability and tolerance to the extreme conditions in both the 0.5 M H₂SO₄ and 2/1 1 M KOH/0.5 M NaCl solutions. Moreover, EA-treated Na-CMC/MoSe₂/PANI/NF exhibited virtually no changes in catalytic performance even after 1000 CV cycles (Fig. S13†). This further suggests that Na-CMC in the EA-treated Na-CMC/MoSe₂/PANI/NF structure plays a crucial role in enhancing HER catalytic performance under various conditions by preventing restacking of the exfoliated MoSe₂ nanosheets and facilitating the connection between P-type MoSe₂ and N-type PANI, and thereby helps to maintain a stable P–N heterojunction interface at the MoSe₂/PANI junction.

Conclusions

We successfully constructed a Na-CMC/MoSe₂/PANI/NF composite system with an organic–inorganic P–N heterojunction that exhibits excellent and highly stable pH-universal HER activity by integrating exfoliated MoSe₂ nanosheets functionalized with Na-CMC into the EP reaction for PANI and subsequent EA treatment. As a non-noble metal-based electrocatalyst, this system holds promising potential to replace the widely used commercial Pt/C catalyst in diverse energy applications. Acid-doped PANI and exfoliated MoSe₂ nanosheets are P-type and N-type semiconductors, respectively, thus EP in the presence of Na-CMC effectively connects PANI and MoSe₂ in the medium and integrates them onto the conductive NF substrate. Subsequently, EA further enhances the P–N heterojunction interface between PANI and MoSe₂, resulting in an electrocatalytic composite catalyst with a high catalytically active surface area and smooth charge transfer pathways. Following EA, the Na-CMC/MoSe₂/PANI/NF catalytic system demonstrates superior and stable HER electrocatalytic performance compared to the noble metal Pt/C in various electrolytes, including 0.5 M H₂SO₄ (acidic), 1 M KOH (alkaline), and 2/1 1 M KOH/0.5 M NaCl (simulated seawater). These results confirm that EA-treated Na-CMC/MoSe₂/PANI/NF possesses exceptional pH-universal HER activity, achieving overpotentials of only 11.3, 35.2, and 19.1 mV a current density of −10 mA cm⁻² under acidic, alkaline, and simulated seawater conditions, respectively. In addition, EA-treated Na-CMC/MoSe₂/PANI/NF exhibits desirable Tafel slopes of 25.7, 64.4, and 57.1 mV dec⁻¹ in acidic, alkaline, and simulated seawater, respectively, as well as resistance values below 2.0 Ω across all tested conditions. More importantly, EA-treated Na-CMC/MoSe₂/PANI/NF maintained its catalytic performance even after 24 h of operation at 100 mA cm⁻² or 1000 CV cycles in both 0.5 M H₂SO₄ and 2/1 1 M KOH/0.5 M NaCl, which indicates this catalyst system offers exceptional environmental resistance and long-term catalytic stability, and has potential to achieve efficient and stable electrocatalytic hydrogen evolution performance. Overall, this newly developed system with an organic–inorganic P–N heterojunction interface holds significant potential for the future development of non-precious metal-based high-performance electrocatalysts and the advancement of energy-related applications.

Experimental section

The Experimental section is presented in the ESI,† and includes detailed descriptions of the chemicals, experimental equipment, material preparation, analysis and characterization, and electrochemical experiments.

Data availability

The author confirms that all data generated or analysed during this study are included in this published article.

Author contributions

Kumasser Kusse Kuchayita: data curation, investigation, methodology, validation. Chih-Chia Cheng: conceptualization, funding acquisition, investigation, methodology, resources, supervision, visualization, writing – original draft, writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This study was supported financially by the National Science and Technology Council, Taiwan (contract no. NSTC 110-2221-E-011-003-MY3 and NSTC 113-2221-E-011-007-MY3).

References

1. L. Huang, G. Wei, J. Wang, D. Li, S. Jia, S. Wu, T. Jiang, Y. Guo, Y. Liu and F. Ren, Adv. Energy Mater., 2023, 13, 2300651.
2. L. Jia, X. Sun, Y. Jiang, S. Yu and C. Wang, Adv. Funct. Mater., 2015, 25, 1814–1820.
3. B. You and Y. Sun, Acc. Chem. Res., 2018, 51, 1571–1580.
4. K. L. Zhou, Z. Wang, C. B. Han, X. Ke, C. Wang, Y. Jin, Q. Zhang, J. Liu, H. Wang and H. Yan, Nat. Commun., 2021, 12, 1–10.
5. L. Quan, H. Jiang, G. Mei, Y. Sun and B. You, Chem. Rev., 2024, 124, 3694–3812.
6. A. I. Osman, N. Mehta, A. M. Elgarahy, M. Hefny, A. Al-Hinai, A. H. Al-Muhtaseb and D. W. Rooney, Environ. Chem. Lett., 2022, 20, 153–188.
7. J. Zhu, L. Hu, P. Zhao, L. Y. S. Lee and K. Y. Wong, Chem. Rev., 2020, 120, 851–918.
8. W. Sheng, H. A. Gasteiger and Y. Shao-Horn, J. Electrochem. Soc., 2010, 157, B1529.
9. J. Liang, Z. Zhao, N. Li, X. Wang, S. Li, X. Liu, T. Wang, G. Lu, D. Wang, B. J. Hwang, Y. Huang, D. Su and Q. Li, Adv. Energy Mater., 2020, 10, 2000179.
10. S. Ye, F. Luo, Q. Zhang, P. Zhang, T. Xu, Q. Wang, D. He, L. Guo, Y. Zhang, C. He, X. Ouyang, M. Gu, J. Liu and X. Sun, Energy Environ. Sci., 2019, 12, 1000–1007.
11. Z. Cao, H. Hu, M. Wu, C. Tu, D. Zhang and Z. Wu, Sustainable Energy Fuels, 2019, 3, 2409–2416.
12. J. Ying, G. Jiang, Z. Paul Cano, L. Han, X. Y. Yang and Z. Chen, Nano Energy, 2017, 40, 88–94.
13. J. Zhang, T. Wang, P. Liu, S. Liu, R. Dong, X. Zhuang, M. Chen and X. Feng, Energy Environ. Sci., 2016, 9, 2789–2793.
14. L. Liao, L. Yang, G. Zhao, H. Zhou, F. Cai, Y. Li, X. Wang and F. Yu, Chin. J. Chem., 2021, 39, 288–294.
15. Y. Tan, R. Xie, S. Zhao, X. Lu, L. Liu, F. Zhao, C. Li, H. Jiang, G. Chai, D. J. L. Brett, P. R. Shearing, G. He and I. P. Parkin, Adv. Funct. Mater., 2021, 31, 2105579.
16. C. Xu, S. Peng, C. Tan, H. Ang, H. Tan, H. Zhang and Q. Yan, J. Mater. Chem. A, 2014, 2, 5597–5601.
17. Y. Kim, D. H. K. Jackson, D. Lee, M. Choi, T. W. Kim, S. Y. Jeong, H. J. Chae, H. W. Kim, N. Park, H. Chang, T. F. Kuech and H. J. Kim, Adv. Funct. Mater., 2017, 27, 1701825.
18. D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. B. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda and M. Chhowalla, Nat. Mater., 2013, 12, 850–855.
19. P. K. Maurya and A. K. Mishra, ACS Appl. Energy Mater., 2024, 7, 487–498.
20. D. Y. Chung, S. W. Jun, G. Yoon, H. Kim, J. M. Yoo, K.-S. Lee, T. Kim, H. Shin, A. K. Sinha, S. G. Kwon, K. Kang, T. Hyeon and Y.-E. Sung, J. Am. Chem. Soc., 2017, 139, 6669–6674.
21. H. Ang, H. Wang, B. Li, Y. Zong, X. Wang and Q. Yan, Small, 2016, 12, 2859–2865.
22. J. Wang, X. Yue, Y. Yang, S. Sirisomboonchai, P. Wang, X. Ma, A. Abudula and G. Guan, J. Alloys Compd., 2020, 819, 153346.
23. P. Li, J. Liang, Z. Zhu, P. Jiang, L. Zhang, Z. Gao, X. Hou, X. Zheng, Y. Yao, Q. Sun and T. Dong, Mater. Lett., 2024, 365, 136413.
24. N. Dogra and S. Sharma, Int. J. Hydrogen Energy, 2024, 55, 78–87.
25. G. Zhao, X. Wang, S. Wang, K. Rui, Y. Chen, H. Yu, J. Ma, S. X. Dou and W. Sun, Chem.–Asian J., 2019, 14, 301–306.
26. G. Zhao, K. Rui, S. X. Dou and W. Sun, Adv. Funct. Mater., 2018, 28, 1803291.
27. Z. Li, M. Hu, P. Wang, J. Liu, J. Yao and C. Li, Coord. Chem. Rev., 2021, 439, 213953.
28. B. Zhu, J. Sun, Y. Zhao, L. Zhang and J. Yu, Adv. Mater., 2024, 36, 2310600.
29. Y. Cao, ACS Nano, 2021, 15, 11014–11039.
30. Y. Jin, D. H. Keum, S. J. An, J. Kim, H. S. Lee and Y. H. Lee, Adv. Mater., 2015, 27, 5534–5540.
31. Md. S. Hassan, S. Bera, D. Gupta, S. K. Ray and S. Sapra, ACS Appl. Mater. Interfaces, 2019, 11, 4074–4083.
32. M. Gu, L. Jiang, S. Zhao, H. Wang, M. Lin, X. Deng, X. Huang, A. Gao, X. Liu, P. Sun and X. Zhang, ACS Nano, 2022, 16, 15425–15439.
33. G. Zhao, P. Li, K. Rui, Y. Chen, S. X. Dou and W. Sun, Chem.–Eur. J., 2018, 24, 11158–11165.
34. Y. Zhang, T. R. Chang, B. Zhou, Y. T. Cui, H. Yan, Z. Liu, F. Schmitt, J. Lee, R. Moore, Y. Chen, H. Lin, H. T. Jeng, S. K. Mo, Z. Hussain, A. Bansil and Z. X. Shen, Nat. Nanotechnol., 2014, 9, 111–115.
35. A. Eftekhari, Appl. Mater. Today, 2017, 8, 1–17.
36. D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao and Y. Cui, Nano Lett., 2013, 13, 1341–1347.
37. K. K. Kuchayita, H. S. Li, M. Tokita and C. C. Cheng, FlatChem, 2025, 51, 100844.
38. I. S. KwonIn, H. Kwak, T. T. Debela, H. G. Abbas, Y. C. Park, J. Ahn, J. Park and H. S. Kang, ACS Nano, 2020, 14, 6295–6304.
39. H. K. Chaudhari and D. S. Kelkar, Polym. Int., 1997, 42, 380.
40. A. Fattah-Alhosseini and S. Vafaeian, J. Alloys Compd., 2015, 639, 301–307.
41. A. Hankin, F. E. Bedoya-Lora, J. C. Alexander, A. Regoutz and G. H. Kelsall, J. Mater. Chem. A, 2019, 7, 26162–26176.
42. J. Gusakova, X. Wang, L. L. Shiau, A. Krivosheeva, V. Shaposhnikov, V. Borisenko, V. Gusakov and B. K. Tay, Phys. Status Solidi A, 2017, 214, 1700218.
43. C. Y. Tsai, H. S. Li, K. K. Kuchayita, H. C. Huang, W. N. Su and C. C. Cheng, Adv. Sci., 2024, 11, 2407061.
44. P. Sharma and T. S. Bhatti, Energy Convers. Manage., 2010, 51, 2901–2912.
45. A. Z. Melaku, W. T. Chuang, C. W. Chiu, J. Y. Lai and C. C. Cheng, Chem. Mater., 2022, 34, 3333–3345.
46. P. Tonndorf, R. Schmidt, P. Böttger, X. Zhang, J. Börner, A. Liebig, M. Albrecht, C. Kloc, O. Gordan, D. R. T. Zahn, S. M. de Vasconcellos and R. Bratschitsch, Opt. Express, 2013, 21, 4908–4916.
47. I. Staffell, D. Scamman, A. V. Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah and K. R. Ward, Energy Environ. Sci., 2019, 12, 463–491.
48. P. J. Megía, A. J. Vizcaíno, J. A. Calles and A. Carrero, Energy Fuels, 2021, 35, 16403–16415.
49. L. Yu, Q. Zhu, S. Song, B. McElhenny, D. Wang, C. Wu, Z. Qin, J. Bao, Y. Yu, S. Chen and Z. Ren, Nat. Commun., 2019, 10, 5106.
50. S. Wang, A. Lu and C. J. Zhong, Nano Convergence, 2021, 8, 4.
51. N. K. Chaudhari, H. Jin, B. Kim and K. Lee, Nanoscale, 2017, 9, 12231–12247.
52. X. Hu, X. Tian, Y. W. Lin and Z. Wang, RSC Adv., 2019, 9, 31563–31571.
53. Y. Luo, L. Tang, U. Khan, Q. Yu, H. M. Cheng, X. Zou and B. Liu, Nat. Commun., 2019, 10, 269.
54. S. Dresp, F. Dionigi, M. Klingenhof and P. Strasser, ACS Energy Lett., 2019, 4, 933–942.
55. Y. Liu, Y. Wang, P. Fornasiero, G. Tian, P. Strasser and X. Y. Yang, Angew. Chem., Int. Ed., 2024, 63, e202412087.
56. A. Zadick, L. Dubau, N. Sergent, G. Berthomé and M. Chatenet, ACS Catal., 2015, 5, 4819–4824.
57. H. Jin, J. Xu, H. Liu, H. Shen, H. Yu, M. Jaroniec, Y. Zheng and S. Z. Qiao, Sci. Adv., 2023, 9, eadi7755.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section and relevant characteristic data. See DOI: https://doi.org/10.1039/d5ta03626g

---