Visible light induced efficient photocatalytic hydrogen production by graphdiyne/CoSe ohmic heterojunction

Abstract

The discovery of graphdiyne (GDY) represents a significant advancement in the field of carbon allotropes, and has garnered widespread attention for its potential applications in hydrogen production. Lamellar graphdiyne (GDY) synthesized via the ball milling method serves as a good carrier for preparing a composite photocatalyst. Modified with flower-ball CoSe particles, the GDY/CoSe ohmic junction composite photocatalyst has been reasonably designed. The GDY/CoSe-15 photocatalyst (15 wt% CoSe) exhibited stable photocatalytic H₂ evolution activity. It was capable of achieving a rate of 2.54 mmol h⁻¹ g⁻¹, which was 8.7 and 6.1 times higher than the respective rates of GDY (0.29 mmol h⁻¹ g⁻¹) and CoSe (0.42 mmol h⁻¹ g⁻¹). The experimental findings and density functional theory (DFT) calculations indicate that the GDY/CoSe photocatalyst demonstrates exceptional light absorption capacity and effectively separates photogenerated carriers. Furthermore, the CoSe cocatalyst has the potential to function as an electron acceptor, facilitating the efficient transfer and transportation of photogenerated electrons; as a result, this enhances the efficiency of photocatalytic hydrogen production.

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1. Introduction

Hydrogen serves as a crucial raw material in industrial progress and represents a promising alternative to non-renewable resources.^1,2 The conversion of water to hydrogen holds immense potential in the chemical industry.^3,4 However, the photocatalytic strategy is an appealing approach that enables the utilization of unlimited solar energy for generating chemical fuels.^5,6 An ideal photocatalyst should possess high light absorption capacity, and rapid photogenerated carrier transfer and separation efficiency, as well as maintain a sufficient redox potential.⁷

Graphdiyne (GDY) has emerged as a novel carbon material that has garnered significant attention in recent years.⁸ It is composed of sp and sp² hybridized carbon atoms, marking a significant advancement in the current carbon allotropes.^9,10 The structure of graphdiyne (GDY) consists of a benzene ring group connected by six acetylene bonds, forming a highly π-conjugated structure.^11,12 This characteristic gives graphdiyne (GDY) a uniform distribution of pores, an large surface area, and exceptional electrical conductivity.^13,14 Given these characteristics, graphdiyne (GDY) has emerged as a cutting-edge material and a prominent research hotspot within the realm of photocatalysis.

The introduction of metal-like materials to a semiconductor can modify its surface properties.¹⁵ Typically, semiconductors have a higher work function compared to metal-like materials. When combined, electrons spontaneously migrate from the metal-like material to the semiconductor until equilibrium is reached at the Fermi level, resulting in an internal electric field. This electronic interaction at the interface between metal-like materials and semiconductor is known as an ohmic heterojunction.¹⁶ The heterojunction facilitates efficient transfer of photoelectrons from the semiconductor to metal-like materials and promotes effective separation of electron–hole pairs within the semiconductor.^17,18 Given the low resistance of metal selenides, the charge transfer process in the hydrogen evolution reaction can be accelerated, suggesting that metal selenides are a promising co-catalyst.^19,20 The inherent characteristics of CoSe endow it with excellent electronic conductivity and proton adsorption capacity, while its cost-effectiveness and stable chemical properties render it a viable substitute to precious metals.²¹

In this work, the advantages of GDY and CoSe were combined to successfully design the three-dimensional GDY/CoSe heterostructure, which was subsequently used for the photocatalytic hydrogen evolution reaction under 5 W white light. As anticipated, the photocatalytic activity of the GDY/CoSe heterostructures increased significantly upon exposure to light compared to the original GDY and CoSe. The crystal structure, morphology, and photogenic charge transfer behavior of the synthesized photocatalyst were investigated using a range of characterization techniques. It was further confirmed that the photogenerated electrons of GDY were promptly transferred to CoSe within the GDY/CoSe heterostructure. This occurrence effectively curtails the photogenerated carrier recombination and enhances the separation efficiency. This research provides insights for the development of more efficient semiconductor heterostructures and showcases the potential of GDY/CoSe photocatalysts in energy conversion applications.

2. Experimental

2.1. Synthesis of graphdiyne (GDY)

The synthesis of graphdiyne (GDY) was executed using a conventional ball milling technique, in which 2.94 g of calcium carbide, 1.03 g of tribromobenzene, and 1.03 g of hexabromobenzene were incorporated into a ball mill tank containing 50 g of ball mill beads. The air within the ball mill tank was substituted with N₂, and the tank was secured within the ball mill, operating at a speed of 500 rmp for a duration of 10 h, pausing for 10 minutes every hour. Upon completion of the reaction, the precursor for GDY was obtained.

Into a three-neck flask containing 20 mL of high-purity toluene, add 1 g of the GDY precursor, 0.25 g of tetrapalladium, and 0.25 g of copper acetate. Subsequently, the flask is sealed, the air is replaced with N₂, and the reaction takes place in an oil bath at 75 °C for 3 days. Following the reaction, the solution is subjected to suspension and drying, and the acquired black material was calcined at 400 °C for 3 h in N₂ atmosphere at a heating rate of 3 °C min⁻¹. Thereafter, the calcined material is moved into an aqua regia solution and soaked for 12 h. Following filtration and washing, a black solid GDY is ultimately obtained.

2.2 Synthesis of CoSe

Co(NO₃)₂·6H₂O and SeO₂, in a molar ratio of 1 : 1, were introduced into 65 mL of glycol, stirred for 1 h, and the resulting mixture was transferred to a reactor for reacting at 180 °C for 12 h. Upon completion of the reaction, the resulting product was subjected to multiple washes with deionized water and ethanol, followed by overnight drying at 60 °C, to yield the black compound CoSe.

2.3. Synthesis of GDY/CoSe

CoSe was incorporated into GDY through the physical mixing method, and 20 mg of GDY was added to a mixture of 15 mL ethanol and 5 mL deionized water, which was uniformly dispersed using ultrasound. Subsequently, 3 mg of CoSe was introduced, and the GDY and CoSe mixture was processed using ultrasound, followed by magnetic stirring for a duration of 2 h. Subsequently, it was heated in an 80 °C water bath to facilitate evaporation and drying. This resulted in the production of the composite material GCS-15. GCS-x composites with different proportions (x = 5, 10, 15, 20, and 25) were synthesized (Scheme 1).

Scheme 1 Schematic diagram of the synthesis process of GDY, CoSe and GCS-x (x = 5, 10, 15, 20, 25).

2.4. Photocatalytic hydrogen evolution experiment

In the experiment for photocatalytic hydrogen evolution, initially, 10 mg of catalyst and 10 mg of photosensitizer eosin Y (EY) were added to a hydrogen evolution bottle containing a 30 mL TEOA (15%, pH = 9) solution, which was subsequently sealed, subject to ultrasonic treatment. This ensured the uniform dispersion of the catalyst within the solution. Subsequently, N₂ was utilized for the substitution of air within the sealing system. Following this, the hydrogen evolution bottle was placed in a multi-channel reaction system (PCX50A), and irradiated with a 5 W white light for a period of 5 hours. Ultimately, the process of hydrogen evolution was determined and analyzed using gas chromatography (TianmiGC00, TCD/13X column, N₂ as the carrier gas). The photocatalytic hydrogen evolution activity was evaluated hourly by extracting the gas generated from the bottle (0.5 mL).

2.5. Characterization

The crystal structure of the sample was examined using an X-ray diffractometer (XRD: Rigaku INTT-2000), while the surface characteristics and microstructure were examined via scanning electron microscopy (SEM, SIGMA 500, ZEISS) and transmission electron microscopy (TEM, JEOL JEM-F200, Japan). The elemental composition and valence states of the catalyst were analyzed by X-ray photoelectron spectroscopy (XPS, ThermoFisher ESCALAB Xi⁺). Photoluminescence (PL) and time-resolved photoluminescence (TRPL) were investigated by a fluorometer (Fluoromax-4). The optical properties of the catalyst was examined by UV-vis diffuse reflection spectroscopy (DRS, PerkinElmer, Lambda 750). Finally, for electrochemical measurements, we use a standard three-electrode battery for test experiments. A solution containing 5 mg of sample is dried on a conductive glass (1 × 2 cm²) and uniformly coated to prepare a working electrode. Saturated calomel electrode and metallic platinum electrode are used as reference electrode and opposite electrode, respectively. The 300 W xenon lamp was used as the light source (420 nm filter), and 0.2 M Na₂SO₄ solution was used as the electrolyte. Finally, the photocurrent response, electrochemical impedance spectroscopy (EIS), linear sweep voltammetry characteristic curve (LSV), and Mott–Schottky were tested by an electrochemical workstation (AMETEK, VersaSTAT4-400).

3. Results and discussion

3.1. Structure analysis

The crystal structure of the sample was analyzed utilizing XRD. The XRD pattern of GDY (Fig. 1a) displays a prominent peak at approximately 23°, similar to highly disordered carbon materials, suggesting that this sample exhibits low crystallinity.²² As illustrated in Fig. 1b, the distinctive characteristic diffraction peak of the synthesized CoSe matches well with its corresponding standard card (CoSe PDF#52-1008). It exhibits distinct characteristic diffraction peaks at 33.34°, 44.78°, 50.64° and 62.01°, which correspond to the (101), (102), (110) and (112) crystal planes of CoSe, respectively.^23,24 This confirms successful synthesis of CoSe. In the XRD pattern of GCS-x (x = 5, 10, 15, 20, 25) samples (Fig. 1c), an obvious diffraction peak of CoSe can be detected with an increase in its content. The findings indicate that the preparation of composite catalyst is successful. The molecular structure of GDY was analyzed using Raman spectroscopy, as depicted in Fig. 1d. Three peaks were detected within the Raman spectrum. The peak at 1344 cm⁻¹ was identified as peak D,²⁵ which is associated with defects or impurities within the carbon material. The peak observed at 1582 cm⁻¹ corresponds to peak G, which results from the vibration of sp² hybrid carbon–carbon bonds within the carbon material.²⁶ The peak at 2143 cm⁻¹ was caused by the conjugated diacetylene bond (–C≡C–C≡C–), thereby demonstrating the acquisition of GDY.²⁷

Fig. 1 XRD diagram of (a) GDY, (b) CoSe, (c) GCS-x (x = 5, 10, 15, 20, 25); (d) Raman diagram of GDY.

3.2. Morphology analysis

The morphology and microstructure of different catalysts were preliminarily characterized using SEM, TEM and HRTEM. As depicted in Fig. 2a and b, GDY is observed as a lamellar accumulation, while CoSe samples consist of nanoparticles aggregated into cauliflower-like clusters. By examining the morphology formed by the GCS-15 composite photocatalyst, it can be observed that CoSe nanoparticles and GDY lamellae are closely combined (Fig. 2c). The TEM images of the GCS-15 sample further revealed that the CoSe nanoparticles were in an aggregated state on the GDY sheet (Fig. 2d). It can be distinctly observed from the HRTEM image with local magnification (Fig. 2e) that CoSe exhibits uniform lattice stripes with a planar distance of 0.264 nm, which corresponds to the (101) crystal plane of CoSe.²⁸ Furthermore, element mapping images of C, Co and Se atoms provide additional evidence for the intimate contact between GDY and CoSe in GCS-15, as well as their position relationships (Fig. 2f–i).

Fig. 2 SEM image of (a) GDY, (b) CoSe, (c) GCS-15; (d) TEM and (e) HRTEM image of GCS-15; (f–i) mapping images of GCS-15.

3.3. Elemental composition and chemical state analysis

The chemical compositions and valence states of GDY, CoSe, and GCS-15 composite photocatalysts were further investigated through XPS analysis. As depicted in Fig. 3a, the elements C, Co, Se, and O are present in GCS-15. The XPS spectrum of GDY exhibits four distinct peaks, which are located at 284.32, 285.00, 286.25, and 288.88 eV, respectively, corresponding to C=C, C≡C, C–O, and C=O (Fig. 3b).^29,30 As illustrated in Fig. 3c, the peaks of binding energy at 781.19 and 797.24 eV are identified as the Co 2p_3/2 and Co 2p_1/2 orbits of Co²⁺, respectively. Meanwhile, the peaks at 785.88 and 802.95 eV are presumably ascribed to the satellite peaks of the Co 2p orbital.^31,32 As for the high-resolution spectrum of Se 3d (Fig. 3d), the peaks at 54.24 and 55.06 eV correspond to Se 3d_5/2 and Se 3d_3/2, respectively. Notably, in the absence of light, the binding energy of C 1s in the GCS-15 heterojunction is more negative than that of GDY. In contrast, the binding energy of Co 2p and Se 3d in the GCS-15 heterojunction is more correct than that of pure CoSe. These transfers of XPS binding energy support the migration of electrons from CoSe to GDY upon contact, resulting in the formation of an internal electric field along the interface between CoSe to GDY. Subsequent in situ XPS analysis revealed a positive shift in the C 1s binding energy of GCS-15 under light conditions compared to that under dark conditions (Fig. 3b), while a negative shift in the binding energy of Co 2p and Se 3d (Fig. 3c and d), indicating photoelectron migration from GDY to CoSe. These XPS results indicate that the close contact between GDY and CoSe leads to the formation of an ohmic junction.^33,34

Fig. 3 (a) Full-scan spectra; high-resolution XPS spectra of (b) C 1s, (c) Co 2p and (d) Se 3d in darkness and under irradiation.

3.4. Photocatalytic hydrogen evolution activity

The assessment of the photocatalytic hydrogen evolution activities of GDY, CoSe, and GCS-x was carried out through photocatalytic hydrogen evolution tests. To investigate the difference in photocatalytic hydrogen evolution activity, a comparison was conducted between the pure sample and the composite sample with an optimal loading ratio. As illustrated in Fig. 4a, both GDY and CoSe demonstrated low hydrogen evolution activity under visible light illumination. However, the hydrogen evolution activity of the composite catalyst GCS-x was notably enhanced. This is attributed to the intimate contact between GDY and CoSe, which forms an electronic interaction at the interface, thereby enhancing the hydrogen evolution capacity of the catalyst.³⁵ The hydrogen evolution activity of the composite catalyst GCS-x initially exhibits an upward trend followed by a subsequent decline as the content of CoSe increases. When the content of CoSe exceeds 15%, there is a reduction in hydrogen evolution activity. The excessive loading of CoSe may potentially affect the primary active sites of GDY.³⁶ The hydrogen production activity of the catalyst was optimized across varying pH environments (Fig. 4b). The hydrogen evolution activity of GCS-15 was maximized at a pH of 10. Due to the presence of a low pH environment, TEOA undergoes protonation, leading to a reduction in its ability to donate electrons.³⁷ In contrast, in a high pH system, the concentration of H⁺ is reduced, thereby decreasing the driving force for hydrogen evolution.³⁸ Therefore, the catalyst demonstrates the highest hydrogen evolution activity under mildly alkaline conditions.³⁹ The photosensitizer EY can adsorb onto the surface of the semiconductor, forming an excited state under light conditions, and subsequently transferring electrons into the conduction band (CB) of the semiconductor to participate in the photochemical reaction.⁴⁰ An investigation was conducted to examine the impact of the incremental quantity of EY on the hydrogen generation capability of the catalyst, as illustrated in Fig. 4c. At an EY content of 15 mg, GCS-15 exhibits the highest level of hydrogen evolution activity, indicating that the amount of EY added satisfies the active site requirements on the catalyst surface.⁴¹ However, excessive addition of EY result in a shielding effect on the catalyst surface, which diminishing its light absorption capacity and reduce the hydrogen evolution performance.⁴² The stability of the composite catalyst GCS-15 was assessed through a cyclic hydrogen evolution experiment (Fig. 4d). During the 20 h test for hydrogen evolution, a significant reduction in the rate of hydrogen evolution was detected for the catalyst when EY was absent. Nevertheless, with the introduction of 10 mg EY, the hydrogen evolution rate reached 67% of the maximum hydrogen production capacity, and the reduction in the hydrogen evolution capacity of the catalyst might be attributed to the depletion of EY. As illustrated in Fig. 4e, the XRD patterns of GCS-15 before and after photocatalytic hydrogen evolution remained essentially unchanged, indicating that the catalyst maintains strong structural stability.^43,44 When the incident light wavelength is 520 nm, the apparent quantum efficiency (AQE) of GCS-15 reaches its maximum value (Fig. 4f), which amounting to 1.17%.

Fig. 4 Hydrogen evolution diagram of (a) GDY, CoSe, GCS-x (x = 5, 10, 15, 20, 25); (b) hydrogen evolution testing in different pH environments; (c) effect of EY addition on hydrogen evolution activity; (d) stability experiment of GCS-15; (e) XRD patterns of GCS-15 before and after hydrogen evolution; (f) AQE of GCS-15 at different wavelengths.

3.5. Optical performance analysis

The optical and bandgap characteristics of the samples were assessed using UV-vis DRS.^45,46 The UV-vis DRS of GDY, CoSe and GCS-15 is displayed in Fig. 5a. It is evident that GDY, CoSe and GCS-15 exhibit significant light absorption across the entire UV-vis light spectrum. At the same time, the introduction of CoSe significantly enhances the light absorption capacity of GCS-15 within the visible spectrum, thereby promoting the generation of more photogenerated charge carriers and facilitating the photocatalytic reaction to a certain extent.⁴⁷ According to formula (1), the UV-vis DRS curve is transformed into the Tauc diagram depicted in Fig. 5b, indicating a band gap of approximately 1.56 eV for GDY.

(αhυ)^1/n = K(hυ − E_g) (1)

α is the absorption coefficient, hυ is the incident photon energy, K is an energy independent constant, E_g is the bandgap energy and n is the nature of transition. n usually choose one of the following 2 values: (1) n = 1/2 (direct allowed transition), (2) n = 2 (indirect allowed transition).

Fig. 5 (a) UV-vis DRS of GDY, CoSe, GCS-15; (b) Tauc diagram of GDY; (c) PL and (d) TRPL of EY, GDY, CoSe, GCS-15.

The efficiency of carrier separation in photocatalysts was assessed by analyzing the PL spectrum. As depicted in Fig. 5c, the PL signal of GCS-15 is notably attenuated compared to GDY due to the interface coupling between CoSe and GDY, which effectively inhibits the recombination of photogenerated carriers. The minimal PL signal of GCS-15 is associated with the minimal recombination and the most efficient carrier separation.^48,49 The carrier transfer dynamics are examined using TRPL spectroscopy. As illustrated in Fig. 5d and Table 1, the charge lifetimes for EY, GDY, CoSe and GCS-15 are 1.18, 1.09, 0.77 and 0.58 ns, respectively. The migration of photogenerated electrons between catalysts effectively suppresses the recombination of photogenerated carriers, subsequently shortening the fluorescence lifetime.⁵⁰ The minimal fluorescence lifetime of GCS-15 suggests that the formation of an ohmic junction between catalysts enhances the efficient charge transfer of electrons from the GDY conduction band to CoSe.

Table 1 TRPL fitted data of EY, GDY, CoSe, and GCS-15 samples

Sample	Lifetime, τ (ns)	Rel. (%)	〈τ〉 (ns)	χ ^2
EY	τ = 1.18	A = 100	1.18	1.27
GDY	τ 1 = 0.59	A 1 = 15.35	1.09	1.25
	τ 2 = 1.27	A 2 = 83.16
	τ 3 = 5.45	A 3 = 1.49
CoSe	τ 1 = 1.31	A 1 = 77.12	0.77	1.16
	τ 2 = 5.51	A 2 = 4.67
	τ 3 = 0.26	A 3 = 18.21
GCS-15	τ 1 = 4.05	A 1 = 2.28	0.58	1.19
	τ 2 = 1.24	A 2 = 72.26
	τ 3 = 0.22	A 3 = 25.46

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3.6. Photoelectric chemical performance test

The photocurrent response is utilized to illustrate the process of charge separation and transfer that occurs during photoexcitation. As illustrated in Fig. 6, it can be discerned that all test samples exhibit sensitivity to light irradiation during the continuous switching cycle under a 300 W Xe lamp. Moreover, the peak photocurrent density remains virtually unaltered throughout the cycle, suggesting that the synthesized catalysts exhibit remarkable structural stability. Among them, CoSe exhibits almost no photocurrent response, indicates that it exhibits metallic characteristics.

Fig. 6 (a) Photocurrent response, (b) EIS and (c) LSV of GDY, CoSe and GCS-15, (d) Mott–Schottky diagram of GDY.

The photocurrent response of GCS-15 exhibits a significantly higher level compared to GDY, suggesting effective suppression of electron–hole pair recombination.⁵¹ As a result, a substantial quantity of photogenerated electrons participate in the hydrogen evolution reaction. The utilization of EIS was applied to examine the resistance of interfacial charge transfer in photocatalysts. As depicted in Fig. 6b, upon fitting the equivalent circuit, it can be observed that the GCS-15 diameter possesses the minimal size, indicating that it registers the lowest resistance value and exhibits the fastest charge separation and transfer efficiency.⁵² The LSV demonstrates the hydrogen evolution overpotential of the sample, with a smaller overpotential indicating more favorable H₂ generation.⁵³ As depicted in Fig. 6c, the hydrogen evolution overpotential of CoSe, GDY and GCS-15 decreased successively, providing additional evidence that GCS-15 is a suitable photocatalyst for hydrogen generation. The flat band potential (E_fb) of a semiconductor can be determined by using the Mott–Schottky diagram (Fig. 6d). GDY exhibits a positive slope, suggesting that it is an n-type semiconductor with a flat band potential (E_fb) of −0.64 V (vs. SCE).^54,55 For n-type semiconductors, the conduction potential (vs.SCE) is 0.1–0.2 typically smaller than the flat band potential (E_fb), so the E_CB for GDY is −0.84 V. At a standard hydrogen electrode, the conduction band of GDY measures −0.6 V (E_NHE = E_SCE + 0.241).⁵⁶ Therefore, the conduction potential of GDY is −0.6 V. By considering the E_g value shown in Fig. 5b, the valence band potential of GDY can be determined to be 0.96 V (vs. NHE).

3.7. Density functional theory (DFT) calculations

The band gap, state density, and work function of the catalyst were further analyzed through theoretical calculation. Fig. 7a and b shows the model optimized by GDY and CoSe. The band structure and state density distribution (Fig. 7c–f) indicate that GDY exhibits a band gap of 0.38 eV, classifying it as a direct band gap semiconductors. The valence band and conduction band consist mainly of s and p orbitals.⁵⁷ However, the bandgap values computed based on theory are smaller than the experimental values derived from UV-vis diffuse reflection spectroscopy measurements, which is attributed to the limitations of the generalized gradient approximation.^58,59 Because there is no band gap in CoSe, it can be classified as having metallic properties. Fig. 7g and h display the work functions of GDY and CoSe, respectively. The size of the work function indicates the strength of electrons that are bound within the catalyst. A higher work function corresponds to a greater ability of the catalyst to bind electrons.⁶⁰ The figure demonstrates that the calculated work functions are 5.22 and 3.32 eV, respectively. According to eqn (2), it can be inferred that the Fermi level of GDY is lower than that of CoSe. Therefore, at the interface of GDY and CoSe, the free electrons in CoSe with higher Fermi energy migrate towards GDY along the tightly bound interface until the Fermi level reaches equilibrium.^61,62

Φ = E_v − E_f (2)

Φ is work function, E_v is vacuum level and E_f is Fermi level.

Fig. 7 Cell diagram of (a) GDY and (b) CoSe, band structure of (c) GDY and (d) CoSe, state density of (e) GDY and (f) CoSe, work function of (g) GDY and (h) CoSe.

3.8. Possible photocatalytic mechanisms

In situ XPS analysis showed that the photogenerated charge transfer mechanism of GDY/CoSe was consistent with ohmic junction. As depicted in Fig. 8, the formation process of ohmic junction within GDY/CoSe is presented. Due to the fact that the Fermi level of GDY is lower than that of CoSe, electrons tend to migrate from CoSe to GDY upon contact. This results in a bending of the Fermi levels within the interface region, where the equilibrium between E_f of GDY and CoSe is achieved at the contact point.⁶³ As a result of the loss of electrons, an internal electric field (IEF) is generated from CoSe towards GDY. Upon exposure to visible light, photogenerated electrons in the valence band of GDY can be excited to its conduction band, leaving photogenerated holes in the valence band. The photogenerated electrons from the conduction band can be transferred to the surface of the metal-like CoSe cocatalyst via the internal electric field, thereby initiating the hydrogen evolution reaction.^64,65 Under the influence of eosin Y (EY) photosensitizer, the hydrogen production diagram of GDY/CoSe photocatalyst is presented in Fig. 9. EY is stimulated to generate EY¹*, which is transformed into the more stable EY³* through inter-system crossing (ISC). EY³* then acquires electrons from TEOA to form EY⁻˙ radicals with strong reducing power. Finally, EY⁻˙ transfers electrons to the conduction band (CB) of photocatalyst GDY/CoSe and participating in hydrogen evolution.^66,67 The formation of an ohmic junction helps improve the separation efficiency of GDY photogenerated carriers, enabling a large amount of electrons to participate in the reduction reaction, thereby enhancing the catalytic activity of photocatalytic water splitting.

Fig. 8 GDY/CoSe ohmic junction electron flow diagram.

Fig. 9 Hydrogen evolution mechanism of GDY/CoSe ohmic junction in EY system.

4. Conclusions

In summary, the GDY/CoSe ohmic junction photocatalyst was successfully designed and synthesized by loading CoSe onto GDY using a physical mixing method. The optimized GDY/CoSe samples exhibited the highest photocatalytic efficiency, and the photocatalytic hydrogen evolution activity under 5 W white light was 2.54 mmol h⁻¹ g⁻¹ which was 8.7 and 6.1 times higher than that of GDY and CoSe under the same conditions, respectively. The systematic characterization and mechanism analysis revealed that the GDY/CoSe ohmic junction exhibits superior capabilities in visible light response and hydrogen evolution activity. The ohmic junction facilitates the efficient transfer of photogenerated electrons from GDY to CoSe, enhances the separation efficiency of photogenerated carriers, and thus achieves high photocatalytic performance. The novel charge transfer mechanism proposed in this study provides novel insights for the design of greatly efficient photocatalytic material systems.

Data availability

No data available in this work.

Author contributions

Bingzhu Li and Minjun Lei designed the experiments; Zhiliang Jin and Xiaohua Ma provided reagents and instruments; Bingzhu Li wrote the paper; Youlin Wu and Chunying Long assisted with a portion of the testing.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

This work was financially supported by the Ningxia Natural Science Foundation (2021AAC03194).

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