Organic–Inorganic Interfacial Charge‐Directed Migration for Enhanced Photocatalytic Hydrogen Evolution Over Cu _x O/Hydrogen‐Bonded Organic Frameworks/Persistent Luminescence Nanoparticle Heterojunctions

Abstract

Hydrogen‐bonded organic frameworks (HOFs) have attracted increasing attention as promising photocatalysts owing to their well‐defined crystalline structures and highly tunable architectures. However, their application in photocatalytic sacrificial hydrogen evolution remains constrained by inefficient separation of photogenerated charge carriers. Herein, we designed and synthesized a covalently bonded organic–inorganic type‐II heterojunction composite comprising Cu _x O, HOFs, and persistent luminescence nanoparticles (PLNPs). The optimized Cu_xO/HOFs/PLNPs (3:1) sample delivered a high H₂ evolution rate of 70.62 mmol g⁻¹ h⁻¹ under visible‐light irradiation (λ ≥ 420 nm) without any co‐catalyst, which was 3.57‐fold higher than that of the pristine HOFs. It also exhibited an apparent quantum efficiency of 3.09% and excellent long‐term cycling stability. The enhanced photocatalytic activity is mainly attributed to the synergistic effect of efficient interfacial charge transfer and electron trapping. Specifically, the type‐II band alignment drives the directional migration of photogenerated electrons, while the intrinsic electron‐storage states of PLNPs effectively suppress charge recombination and facilitate charge transport. Meanwhile, variable‐valence Cu _x O species provide abundant active sites for proton reduction. This work offers an effective interfacial engineering strategy for constructing efficient HOF‐based photocatalysts.

An inorganic–organic type‐II heterojunction photocatalyst Cu _x O/hydrogen‐bonded organic frameworks/persistent luminescence nanoparticles (PLNPs) featuring interfacial charge‐directed migration is rationally constructed. Benefiting from synergistic band alignment and the intrinsic electron‐trapping states of PLNPs, the composite exhibits markedly enhanced charge separation and transport, delivering a remarkably high photocatalytic hydrogen evolution rate of 70.62 mmol g⁻¹ h⁻¹ along with excellent stability.

1. Introduction

The rapid growth of global energy demand has intensified the pressure on conventional energy systems, driving the urgent development of sustainable and renewable energy technologies. Among various candidates, hydrogen has been widely recognized as an ideal clean energy carrier due to its high energy density and environmentally benign combustion product (water) [1, 2, 3, 4]. However, current industrial hydrogen production processes, including steam reforming, coal gasification, methane decomposition, and water electrolysis, are still dominated by fossil‐derived pathways and are therefore associated with high energy consumption and substantial carbon emissions [5, 6, 7, 8]. In this context, solar‐driven photocatalytic hydrogen evolution has emerged as a promising alternative, as it enables direct conversion of solar energy into chemical fuels. In semiconductor photocatalysis, photon absorption induces electron excitation from the valence band to the conduction band, generating electron‐hole pairs that drive surface redox reactions [9, 10, 11, 12]. Nevertheless, the overall efficiency of photocatalytic systems remains severely limited by two intrinsic bottlenecks: rapid recombination of photogenerated charge carriers and insufficient density of accessible catalytic active sites [13]. Addressing these coupled limitations is therefore crucial for the rational design of high‐performance photocatalytic materials capable of efficient charge separation, abundant active‐site utilization, and practical energy conversion.

Hydrogen‐bonded organic frameworks (HOFs) are an emerging class of microporous crystalline materials constructed via directional hydrogen‐bonding interactions between organic building blocks [14, 15]. Owing to their high crystallinity, large specific surface area, and structural reversibility, HOFs have demonstrated broad potential in gas storage and separation [16, 17], chemical sensing [18], photoluminescence [19], and catalysis [20]. However, compared with covalent organic frameworks (COFs) [21] and metal–organic frameworks (MOFs) [22], their exploration in catalytic applications remains relatively limited, leaving considerable room for further development. Although recent studies have shown that some HOFs can achieve improved chemical stability through rational structural design [23, 24], their practical application in photocatalysis still faces challenges, particularly in achieving efficient charge transport and utilization under photoexcitation. In this context, HOFs can serve as an ordered organic electronic framework; however, their intrinsically strong exciton binding energy arising from hydrogen‐bonded assemblies significantly hinders charge separation and accelerates electron‐hole recombination, thereby limiting photocatalytic efficiency [25]. To overcome these limitations, a common strategy is to integrate HOFs with functional components, such as metal ions, semiconductors, or redox‐active units [26, 27, 28]. Among them, noble metals such as Pt can act as efficient electron‐trapping cocatalysts, significantly enhancing hydrogen evolution activity and stability [29]. In addition, non‐precious transition metals (Fe, Zn, and Cu) have attracted increasing attention due to their cost‐effectiveness [30]. These metal species can be anchored onto HOF surfaces via electrostatic interactions and coordination bonding, forming porous composite structures with abundant interfacial sites. In particular, metastable Cu _x O species generated via photoreduction and immobilized on HOFs have been demonstrated to effectively promote photocatalytic hydrogen evolution [31, 32]. In such systems, HOFs function as the organic electronic framework governing charge migration, while Cu _x O serves as the key catalytic center that directly determines proton‐reduction activity through its surface active sites. However, in conventional Cu _x O/HOF systems, insufficient interfacial charge management often leads to incomplete utilization of photogenerated carriers, indicating that charge separation and surface catalysis remain poorly synergized. To address this issue, constructing heterostructures has emerged as an effective strategy to enhance both charge separation and interfacial coupling. Nevertheless, achieving efficient interfacial charge transfer while simultaneously maximizing catalytic site utilization remains a key challenge in Cu _x O/HOF‐based photocatalytic systems [33, 34].

PLNPs (Persistent luminescence nanoparticles) possess the ability to absorb, store, and slowly release light energy, and their intrinsic trap states can effectively capture and delay the recombination of photogenerated charge carriers [35, 36, 37]. This unique property provides a new strategy to regulate charge separation in composite systems such as Cu _x O/HOFs by extending carrier lifetime and enabling sustained charge utilization. Moreover, our previous studies have demonstrated that PLNPs may enable round‐the‐clock photocatalytic hydrogen evolution [38]. From a functional perspective, HOFs serve as the organic electronic framework governing charge migration and structural assembly, while PLNPs function as the light‐harvesting and electron‐storage component that determines charge persistence. Although PLNPs‐particularly rare‐earth‐doped systems‐have been explored for photocatalytic hydrogen production, their practical application is still limited by harsh synthesis conditions and low specific surface areas, which hinder their efficient integration and interfacial utilization in composite photocatalytic systems [39].

Based on the above research background, an organic–inorganic type‐II heterojunction Cu _x O/HOFs/PLNPs was rationally designed and constructed via covalent bonding. In this architecture, HOFs function as the organic electronic framework governing charge migration pathways, PLNPs serve as the light‐harvesting and electron‐storage component that regulates charge persistence, and Cu _x O acts as the key catalytic center determining the proton‐reduction reaction sites. Upon visible‐light irradiation, Cu _x O is uniformly generated and anchored within the channels of HOFs (1,3,6,8‐tetrakis(4‐carboxyphenyl)pyrene) via in situ photoreduction, thereby increasing the density of accessible active sites and enabling efficient capture and utilization of photogenerated electrons for hydrogen evolution. To overcome the inherent limitation that conventional Cu _x O/HOF systems cannot simultaneously optimize charge separation and surface catalytic reactions, Zn₃Ga₂Ge₂O₁₀/0.5%Mn (PLNPs) were further introduced and covalently integrated with the Cu _x O/HOF interface. The deep trap states in PLNPs enable effective electron capture and long‐term storage, thereby constructing an internal electron reservoir that prolongs carrier lifetime and significantly suppresses electron‐hole recombination. As a result, the optimized Cu _x O/HOFs/PLNPs (3:1) heterojunction delivers a hydrogen evolution rate of 70.62 mmol g⁻¹ h⁻¹ under visible‐light irradiation (λ ≥ 420 nm), which is 3.57 and 1.94 times higher than those of pristine HOFs (19.76 mmol g⁻¹ h⁻¹) and Cu _x O/HOFs (36.49 mmol g⁻¹ h⁻¹), respectively. Notably, this performance surpasses that of most reported systems employing noble‐metal cocatalysts such as Pt or Pd. This work demonstrates a rational strategy for integrating charge transport, electron storage, and catalytic activation within a unified heterostructure, providing new insights into the design of efficient HOF‐based photocatalytic systems.

2. Experiment

2.1. Preparation of Cu _x O/HOFs

Certain amounts of anhydrous copper chloride (0.5 mg, 1 mg, or 2 mg) and HOFs (20 mg) were dispersed in 10 mL of deionized water, followed by the addition of 40 mL of methanol under continuous stirring. The resulting mixture was degassed under vacuum and irradiated with light at 5 °C for 2 h while stirring. The resulting products, corresponding to 2.5%, 5%, and 10% Cu _x O/HOFs, were collected by centrifugation and dried.

2.2. Synthesis of Cu _x O/HOFs/PLNPs Composites

PLNPs and Cu _x O/HOFs were combined at various mass ratios to prepare a series of composites, denoted as Cu _x O/HOFs/PLNPs (1:1), (1:2), (3:1), and (5:1). The surface of PLNPs was first functionalized (Scheme 1) by dispersing 100 mg of PLNPs in sodium hydroxide solution and stirring overnight to obtain PLNPs‐OH. Subsequently, 400 μL of APTES was added to a DMF suspension of PLNPs‐OH (100 mg), followed by overnight stirring and heating at 80 °C to yield PLNPs‐NH₂. In parallel, 300 mg of Cu _x O/HOFs was suspended in water with EDC·HCl and NHS and stirred in the dark for 2 h. Finally, 100 mg of PLNPs‐NH₂ was added, and the mixture was stirred for 48 h to produce the Cu _x O/HOFs/PLNPs (3:1) composite. Composites with other mass ratios were prepared following the same procedure.

SCHEME 1.

Schematic illustration for the preparation of Cu_xO/HOFs/PLNPs composites.

Physical mixture for comparison. To verify the role of covalent bonding, a physically mixed composite (denoted as Cu _x O/HOFs + PLNPs) with the same mass ratio (3:1) was prepared by simply grinding the corresponding amounts of Cu _x O/HOFs and PLNPs together.

2.3. Sacrificial Photocatalytic Hydrogen Evolution

The photocatalytic hydrogen production experiment was conducted in a closed‐loop reactor (CEL‐PAEM‐D8 PLUS, Beijing Zhongjiao Jinyuan Technology Co., Ltd.) with a total volume of 100 mL. During the reaction, the reactor temperature was maintained at 25 °C using a circulating water cooling system. 10 mg of freshly prepared photocatalyst was dispersed in 10 mL of deionized water containing 0.2 mol L⁻¹ ascorbic acid (AA) as a sacrificial agent. Prior to irradiation, the reactor was evacuated to remove air. Subsequently, the reaction mixture was irradiated by a 300 W xenon lamp (CEL‐HXF300‐T3) equipped with a 420 nm cut‐off filter. During irradiation, gas samples were collected every 30 min, and the amount of evolved hydrogen was analyzed using a gas chromatograph (GC‐7920) equipped with a 5 Å molecular sieve column and a thermal conductivity detector (TCD).

3. Results and Discussion

3.1. Structural Characterization

The morphology and structure of the HOFs, Cu _x O/HOFs, PLNPs, and Cu _x O/HOFs/PLNPs (3:1) composites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and EDS mapping images. The SEM image of HOFs shows a spontaneously stacked rod‐like morphology (Figure S1), suggesting the presence of π–π stacking interactions, which can provide favorable anchoring platforms for the immobilization of Cu _x O and PLNPs. Cu _x O is successfully anchored on the surface of HOFs (Figure S2). The HOFs preserve their intrinsic rod‐like morphology with ordered conjugated stacking, and no apparent structural collapse is observed, suggesting that Cu_xO is primarily surface‐loaded without significantly affecting the HOFs framework [40, 41]. TEM clearly shows that PLNPs are uniformly sized nanoparticles with an average size of around 100 nm (Figure S3). Furthermore, PLNPs are loaded onto the surface of Cu _x O/HOFs, thereby reinforcing the interfacial architecture (Figure 1a–c). Figure S4a,b TEM images of Cu _x O/HOFs/PLNPs with mass ratios of 1:1 and 5:1, respectively. The optimized Cu _x O/HOFs/PLNPs (3:1) sample facilitates the formation of a well‐defined and intimate interfacial contact between the components. EDS analysis demonstrated that the Cu_xO/HOFs/PLNPs (3:1) composite exhibits a uniform distribution of the elements C, O, Cu, Zn, Mn, Ge, and Ga (Figure 1d). These observations preliminarily reveal the formation of an interfacial structure between Cu_xO/HOFs and PLNPs.

FIGURE 1.

(a) SEM, (b,c) TEM, and (d) EDS mapping images of Cu _x O/HOFs/PLNPs (3:1).

The X‐ray diffraction (XRD) analysis (Figure 2a) revealed that the as‐synthesized PLNPs exhibit characteristic peaks at 32.0°, 39.5°, 61.0°, and 66.8°, corresponding to the (220), (222), (511), and (440) crystal planes, respectively, which are consistent with the standard pattern (PDF#38‐1240). Notably, these characteristic peaks remain clearly observable in the Cu _x O/HOFs/PLNPs composite, indicating the successful construction of a Cu _x O/HOFs/PLNPs heterojunction interface. Meanwhile, the HOFs and composites exhibit characteristic diffraction peaks at 4.2°, 8.5°, and 12.8°, which can be assigned to the (011), (022), and (033) planes of the HOFs framework, demonstrating that the highly ordered microporous structure of HOFs remains intact during the composite construction process. In addition, for both Cu _x O/HOFs and Cu _x O/HOFs/PLNPs, only a weak diffraction peak located at approximately 24.5° is detected, which is consistent with the standard diffraction pattern of Cu _x O (PDF#44‐0706). The weak intensity of this diffraction peak indicates that Cu _x O is present at a low content within the composites, and this depends on the Cu _x O/HOFs ratio in the composite (Figure S5). In contrast, the physically mixed Cu _x O/HOFs + PLNPs (3:1) sample exhibits only the superimposed diffraction peaks of the individual components, with no additional peaks observed, confirming the absence of interfacial formation under simple physical mixing conditions. These results are consistent with the TEM and EDS analysis.

FIGURE 2.

(a–e) PLNPs, HOFs, Cu _x O/HOFs, and Cu _x O/HOFs/PLNPs (3:1) XRD spectra, IR spectra, BET, UV–vis DRS, and bandgap curves, and (f) Cu _x O/HOFs and PLNPs energy band curves.

The chemical structures and surface functional groups of the as‐prepared samples were characterized by FTIR spectroscopy (Figure 2b). HOFs exhibit a characteristic C=O stretching vibration at 1704 cm⁻¹ [42], which reflects abundant hydrogen‐bonding interactions, in good agreement with the XRD results. After Cu _x O loading, the C=O stretching band shows a slight decrease in intensity, likely attributable to coordination interactions between Cu _x O species and C=O functional groups. For PLNPs‐NH₂, the FTIR spectrum displays an N—H stretching vibration at 3440 cm⁻¹ and an N—H bending vibration at 1648 cm⁻¹ [43]. These characteristic bands are observed in all APTES‐treated samples, and their intensities increase with increasing APTES dosage (200–800 μL), confirming successful grafting of —NH₂ groups onto the PLNP surface (Figure S6) [44]. In the FTIR spectrum of the Cu _x O/HOFs/PLNPs (3:1) composite, distinct amide I and amide II bands appear at 1650 and 1540 cm⁻¹ [45], respectively, indicating the formation of covalent amide linkages between components. In contrast, no amide‐related signals are observed in the physically mixed Cu _x O/HOFs + PLNPs (3:1) sample, confirming the absence of interfacial covalent bonding in the physically blended system. Furthermore, the N 1s XPS spectrum (Figure S7) reveals characteristic components at 399.44, 400.32, 401.25, and 404.23 eV, assignable to N—C=O, N—C, N—H, and π–π interactions, respectively [46], further supporting the successful formation of a chemically coupled interfacial structure.

The textural properties of the samples, including adsorption behavior, specific surface area, and pore structure, were subsequently evaluated by N₂ adsorption–desorption isotherms and pore size distribution analyses (Figure 2c). After loading with Cu _x O, the specific surface area of HOFs decreases from 198.28 to 187.43 m² g⁻¹ (Table S1 and Figure S8), which can be attributed to partial pore blockage and surface coverage, i.e., the occupation of pore entrances or external surface sites of the framework by Cu _x O nanoparticles [47, 48]. In contrast, PLNPs exhibit a typical mesoporous structure (Type IV isotherm) with a relatively low specific surface area of 3.75 m² g⁻¹. After further integration to form the Cu _x O/HOFs/PLNPs (3:1) composite, the specific surface area decreases to 174.15 m² g⁻¹, representing a reduction of 13.28 m² g⁻¹compared with Cu _x O/HOFs. This further decrease indicates that PLNPs are effectively anchored and uniformly distributed on the surface of the Cu _x O/HOFs support, leading to partial surface coverage and interfacial assembly within the composite structure. The microporosity of the materials was further confirmed by I₂ adsorption experiments (Figure S9). This composite strategy effectively addresses the low surface area of PLNPs and optimizes the adsorption and mass transfer performance of the materials.

The samples optical absorption properties were characterized by UV–vis DRS (Figures 2d and S10). The PLNPs exhibit weak absorption in the 200–350 nm range, whereas HOFs show pronounced visible‐light absorption between 300 and 500 nm. The incorporation of Cu _x O leads to a further enhancement in the light‐harvesting capability of Cu _x O/HOFs, attributed to the role of Cu _x O as active sites that improve the photogenerated electron‐capturing capability. After further compositing with PLNPs, the Cu _x O/HOFs/PLNPs (3:1) composite displays a significant increase in both absorption intensity and range, with the absorption edge redshifting to 540 nm, indicating an enhanced light‐harvesting ability [49]. These results further demonstrate that the organic–inorganic heterointerface may facilitate photogenerated electron transfer, thereby suppressing electron‐hole recombination. To further investigate the type of heterojunction formed between PLNPs and Cu _x O/HOFs, their band structures were analyzed. The optical band gaps (E _g) of HOFs, PLNPs, Cu _x O/HOFs, and Cu _x O/HOFs/PLNPs (3:1) were determined as 2.3, 2.14, 2.32, and 2.32 eV, respectively (Figures 2e and S11), according to the following equation [25]:

$$ahv=A{(hv-E_{\mathrm{g}})}^{n/2}$$ (1)

where α denotes the absorption coefficient, hv represents the photon energy, and A is a proportionality constant. All measured potentials were converted to the normal hydrogen electrode (NHE) scale using the equation [50]:

$$E(\mathrm{vs}.\mathrm{NHE})=E_{\mathrm{Ag}/\text{AgCl}}+0.197\mathrm{V}$$ (2)

On the basis of the optical bandgaps and Mott–Schottky analyses (Figure S12), the conduction band (CB) edges of Cu _x O/HOFs and PLNPs were estimated to be –1.06 and –1.66 eV (vs. NHE), respectively, while their corresponding valence band (VB) positions were located at 1.24 and 0.48 eV (vs. NHE), respectively, using the relationship VB = CB + E _g. As illustrated in the band alignment diagram (Figure 2f), the relative energy levels exhibit a staggered configuration, indicative of a type‐II heterojunction. Such an electronic structure is conducive to directional charge transfer and efficient spatial separation of photogenerated charge carriers.

The XPS survey spectra of the Cu _x O/HOFs/PLNPs (3:1) composite before and after photocatalytic hydrogen evolution are nearly identical (Figure S13), exhibiting only signals corresponding to C, N, O, Cu, Zn, Mn, Ge, and Ga, with no detectable impurity peaks, indicating the chemical purity and stability of the composite system. The high‐resolution C 1s spectrum (Figure 3a) can be deconvoluted into four components assigned to C—C/C=C, C—O, C=O, and π–π* transitions, which are associated with the aromatic framework, carboxyl groups, and conjugated structure of the HOF ligands [51], confirming that the HOF framework is well preserved after composite construction. In addition, as shown in Figure S14, the characteristic features of the HOF structure remain essentially unchanged after 3 h of photocatalytic hydrogen evolution, further demonstrating its structural stability under reaction conditions.

FIGURE 3.

(a) C 1s, (b) Cu 2p, (c) Zn 2p, and (d) O 1s XPS spectra of HOFs, Cu _x O/HOFs, PLNPs, and Cu _x O/HOFs/PLNPs (3:1).

For Cu species, the Cu 2p spectrum (Figure 3b) indicates that Cu exists predominantly in a mixed‐valence state of Cu²⁺/Cu⁺. In addition, the Cu LMM Auger analysis yields a kinetic energy of approximately 570.5 eV, corresponding to a binding energy of ~916.1 eV using the relation KE = hν – BE (hν = 1486.6 eV for Al Kα excitation). This value is consistent with reported standards for Cu⁺ species in Cu _x O, further confirming the coexistence of Cu⁺ in the composite (Figure S15) [52]. Compared with Cu _x O/HOFs, the Cu‐related binding energies in Cu _x O/HOFs/PLNPs show a slight shift, which can be attributed to interfacial electronic interactions induced by PLNP coupling. Such electronic modulation reflects changes in the local chemical environment and is beneficial for promoting multivalent Cu active sites and facilitating interfacial charge transfer.

The Zn 2p and O 1s spectra (Figures 3c,d and S16) show that, after composite formation, the O 1s binding energy shifts to lower values by 0.13 eV, while the Zn 2p binding energy shifts to higher values. Such opposite shifts indicate interfacial electronic redistribution, consistent with directional electron transfer from PLNPs to the Cu _x O/HOFs component, thereby effectively suppressing charge recombination and supporting the formation of a type‐II heterojunction [53]. In addition, under photocatalytic conditions, the increased binding energy associated with PLNP‐related signals further confirms their role as electron donors, with photogenerated electrons being transferred to the Cu _x O/HOFs domain (Figures S17 and S18). The N 1s spectrum (Figure S7) provides further evidence for the formation of amide bonds, which serve as covalent linkages between PLNPs and Cu _x O/HOFs. The observed negative shift of the amide‐related peak is attributed to interfacial electronic coupling and photoinduced electron redistribution across the covalent bridge [54]. During the photocatalytic hydrogen evolution process, the Cu 2p binding energy shifts toward lower values (Figure S19), indicating partial reduction of Cu²⁺ to Cu⁺ [55]. This result confirms that Cu sites act as electron‐accepting centers during the reaction, consistent with the proposed type‐II heterojunction charge‐transfer mechanism.

3.2. Characterization of Optical Properties

The photoinduced charge separation and transfer capabilities were investigated via photoelectrochemical analysis. Moreover, transient photocurrent measurements show that Cu _x O/HOFs/PLNPs (3:1) exhibit a significantly higher photocurrent density compared to HOFs, Cu _x O/HOFs, and PLNPs (Figures 4a and S20), confirming that the Cu _x O/HOFs/PLNPs (3:1) heterojunction enables more efficient charge carrier separation [56]. This enhanced separation likely provides a greater number of photogenerated electrons for the photocatalytic process, thereby improving hydrogen evolution efficiency. The electrochemical impedance spectroscopy (EIS) Nyquist plots of the samples are shown in Figures 4b and S21. Notably, Cu _x O/HOFs/PLNPs (3:1) exhibit the smallest semicircle, indicating the lowest charge‐transfer resistance and most efficient interfacial electron transport. Steady‐state photoluminescence (PL) spectroscopy further elucidates the separation efficiency of photogenerated charge carriers (Figures 4c and S22). Under excitation at 254 nm, Cu _x O/HOFs/PLNPs (3:1) show the weakest PL emission intensity, suggesting that photogenerated electron‐hole recombination is significantly suppressed, which is favorable for efficient charge separation. These photoelectrochemical characterization results demonstrate that the Cu _x O/HOFs/PLNPs (3:1) composite exhibits superior charge transfer and separation capabilities, which are fully consistent with its enhanced photocatalytic hydrogen evolution performance.

FIGURE 4.

(a–c) PLNPs, HOFs, Cu _x O/HOFs, and Cu _x O/HOFs/PLNPs (3:1) photocurrent, EIS and photoluminescence spectra. (d) Transient fluorescence lifetime of HOFs, Cu _x O/HOFs and Cu _x O/HOFs/PLNPs (3:1), (e) PLNPs excitation and emission fluorescence spectra, and (f) Cu _x O/HOFs/PLNPs band structure curves.

To elucidate the photocatalytic performance of Cu _x O/HOFs/PLNPs (3:1) and the electron‐storage capability of PLNPs, the charge transfer dynamics were investigated by time‐resolved photoluminescence spectroscopy [57]. Upon doping with Cu _x O, the fluorescence lifetime of Cu _x O/HOFs dramatically decreases to 5.5 ns, indicating that Cu active sites can efficiently capture photogenerated electrons from HOFs and provide additional nonradiative decay pathways for the excited states. After further coupling with PLNPs, the lifetime of Cu _x O/HOFs/PLNPs (3:1) is slightly reduced to 5.2 ns (Table S2 and Figure 4d), demonstrating that the introduction of PLNPs markedly promotes interfacial electron transfer. This process provides a more stable platform for charge transport, enabling more photogenerated electrons to participate in the hydrogen evolution reaction and thereby enhancing the overall efficiency.

To further evaluate the energy storage and sustained‐release properties of PLNPs, their persistent luminescence decay behavior was systematically investigated (Table S3 and Figure S23). In addition, PLNPs exhibit excitation bands at 274 and 421 nm, together with an emission peak at 516 nm (Figure 4e). Notably, the presence of the 421 nm excitation band indicates that PLNPs can be directly excited under visible‐light irradiation (λ ≥ 420 nm). PLNPs are activated via the 421 nm excitation band, with photoinduced carriers trapped in intrinsic deep levels acting as an internal electron reservoir that suppresses recombination. These stored electrons are subsequently released in a delayed manner and transferred across the covalently coupled interface to the Cu _x O/HOFs component, where they drive proton reduction to generate H₂ [58]. Based on these spectroscopic results, a type‐II charge transfer mechanism is proposed (Figure 4f). In this process, PLNPs function as an electron storage and relay platform, facilitating charge transfer toward HOFs and effectively suppressing electron‐hole recombination, while Cu active sites act as electron‐accepting centers that capture photogenerated electrons from HOFs and subsequently drive proton reduction to generate hydrogen. In summary, an efficient photocatalytic system featuring a well‐defined type‐II heterojunction was successfully constructed through organic–inorganic interfacial coupling.

3.3. Photocatalytic H₂ Evolution Performance

In the absence of any cocatalyst, the photocatalytic hydrogen evolution activity of the prepared catalysts was evaluated in an aqueous ascorbic acid (AA, 0.2 mol L⁻¹) solution with a catalyst concentration of 1 g L⁻¹ (10 mg of catalyst dispersed in 10 mL of solution) under visible‐light irradiation (λ ≥ 420 nm). The time‐dependent pressure profiles recorded under initial negative pressure conditions indicate that system pressure has a negligible effect on the hydrogen evolution rate (Figures S24 and S25). Initially, the hydrogen evolution performance of Cu _x O/HOFs photocatalysts with different Cu _x O loadings was investigated. As shown in Figure 5a, using 20 mg of HOFs as the support, increasing the Cu _x O loading from 2.5% to 5% raised the hydrogen evolution rate from 25.75 to 36.49 mmol g⁻¹ h⁻¹, indicating that an increased number of active sites can effectively enhance the hydrogen production rate. However, further increasing the copper loading to 10% led to a decrease in hydrogen evolution to 30.58 mmol g⁻¹ h⁻¹, likely due to aggregation of Cu _x O species, which reduces interfacial electron transfer efficiency with HOFs and negatively impacts the catalytic performance. Subsequently, PLNPs were composited with Cu_xO/HOFs to form a heterojunction. The hydrogen evolution performance (Figure 5b,c) indicates that pristine PLNPs exhibit negligible activity. Upon compositing, the Cu _x O/HOFs/PLNPs photocatalysts with different mass ratios (1:1, 1:2, 3:1, and 5:1) achieved hydrogen evolution rates of 39.70, 56.26, 70.62, and 54.8 mmol g⁻¹ h⁻¹, respectively, with Cu _x O/HOFs/PLNPs (3:1) displaying the highest activity of 70.62 mmol g⁻¹ h⁻¹. This value is 1.94‐fold higher than that of Cu _x O/HOFs and 3.57‐fold higher than that of HOFs, surpassing most recently reported HOF‐based hydrogen evolution materials (Figure 5d and Table S4). When the PLNPs content was further increased to mass ratios of 1:2 and 1:3, the hydrogen evolution rate decreased from 56.26 to 29.96 mmol g⁻¹ h⁻¹, which can be attributed to excessive PLNP coverage that partially blocks the accessible active sites (Figure S26) [59]. In sharp contrast, the physically mixed sample (Cu _x O/HOFs + PLNPs, 3:1) exhibited a much lower hydrogen evolution rate of only 15.08 mmol g⁻¹ h⁻¹. This pronounced activity difference clearly demonstrates that the performance enhancement cannot be achieved through simple physical blending, highlighting the critical role of interfacial coupling in the Cu _x O/HOFs/PLNPs composite system (Figure S27). To further verify the charge separation behavior, hole‐scavenging experiments were performed using 0.2 mol L⁻¹ AA supplemented with either 1 mmol L⁻¹ EDTA or 10% IPA. Compared with the pure AA system, the hydrogen evolution rate varied by only 5%–10% upon addition of EDTA or IPA, indicating that photogenerated holes are already efficiently consumed in the AA‐based system. The negligible enhancement upon introducing stronger hole scavengers further confirms that hole consumption is not the rate‐limiting step, and that efficient charge separation is achieved via the type‐II heterojunction (Figure S28) [60]. To evaluate the round‐the‐clock photocatalytic capability of the system, the catalysts were first pre‐irradiated under UV–vis light for 1 h, followed by hydrogen evolution measurements in the dark. As shown in Figure S29, all Cu _x O/HOFs/PLNPs composites with different mass ratios (1:1, 2:1, 3:1, and 5:1) exhibited measurable hydrogen evolution rates of 6.55, 5.07, 5.90, and 3.50 mmol g⁻¹ h⁻¹, respectively, after the light source was switched off, although the hydrogen evolution efficiency is relatively low, the Cu _x O/HOFs/PLNPs composite exhibits the potential for round‐the‐clock hydrogen production. This behavior can be attributed to the ability of PLNPs to store photogenerated electrons under illumination and gradually release them in the dark, where they are subsequently captured by Cu _x O active sites to drive proton reduction. Furthermore, as shown in Figure S30, the PLNPs exhibit persistent luminescence that remains detectable by a CCD camera for up to 2 days even in the absence of external illumination, further confirming their long‐lived energy storage capability.

FIGURE 5.

(a) HOFs and Cu _x O/HOFs different content performance test, (b,c) HOFs, Cu _x O/HOFs, and Cu _x O/HOFs/PLNPs different ratio performance test and histogram, (d) a comparison of performance between Cu _x O/HOFs/PLNPs and HOFs‐based photocatalysts, and (e,f) Cu _x O/HOFs/PLNPs (3:1) cycle test and AQY test.

Moreover, the Cu _x O/HOFs/PLNPs (3:1) composite maintained high photocatalytic activity over four consecutive cycles (Figure 5e) and exhibited excellent chemical stability, remaining structurally intact after exposure to 12 mol L⁻¹ HCl and 5 mol L⁻¹ NaOH for over 168 h (Figure S31). This remarkable stability can be mainly attributed to strong π–π interactions within the framework [61]. As shown in Figures S32 and S33, the XRD patterns of Cu _x O/HOFs/PLNPs (3:1) remain essentially unchanged after 3 h of illumination, indicating that the crystal structure of the composite is well preserved under photocatalytic conditions. Consistently, FT‐IR spectra show only a slight decrease in peak intensity without the appearance of new bands, while SEM and TEM images further confirm that the morphology remains intact without structural collapse. In addition, under monochromatic irradiation at 420 nm, the Cu _x O/HOFs/PLNPs (3:1) composite delivers an apparent quantum yield (AQY) of 3.09% (Figure 5f), which decreases to 1.6% at 365 nm. The wavelength‐dependent AQY closely follows the UV–vis absorption profile, indicating that the constructed heterojunction effectively enhances light harvesting and promotes efficient separation of photogenerated charge carriers under visible‐light irradiation [62].

4. Conclusion

In this work, an organic–inorganic interfacial charge‐directed strategy was developed to construct a Cu_xO/HOFs/PLNPs heterojunction photocatalyst. Through rational interfacial coupling, favorable energy‐level alignment and efficient charge‐transport pathways were established, effectively alleviating the intrinsic limitations of HOFs in terms of rapid electron‐hole recombination, PLNPs in terms of limited surface accessibility, and their restricted light‐utilization capability. The incorporation of Cu _x O introduces abundant catalytic active sites on the HOF surface, enabling efficient capture and utilization of photogenerated electrons for proton reduction, thereby significantly enhancing hydrogen evolution performance. Upon integration with PLNPs, the resulting heterojunction exhibits further improved sacrificial hydrogen evolution activity, which is primarily attributed to efficient interfacial charge transshfer among the three components. This synergistic effect accelerates carrier separation and migration while effectively suppressing charge recombination. The optimized Cu _x O/HOFs/PLNPs (3:1) composite delivers a high hydrogen evolution rate of 70.62 mmol g⁻¹ h⁻¹, which is 3.57 and 1.94 times higher than those of pristine HOFs (19.76 mmol g⁻¹ h⁻¹) and Cu _x O/HOFs (36.49 mmol g⁻¹ h⁻¹), respectively. Moreover, the composite maintains excellent catalytic stability over four consecutive cycles, highlighting the effectiveness of heterojunction engineering in enhancing both activity and durability. In conclusion, this work provides a rational strategy for the design of highly efficient HOF‐based heterojunction photocatalysts through integrated interfacial charge management.

Author Contributions

Shuqing Wang: writing – original draft, investigation, data curation. Ailijiang Tuerdi: writing – original draft, data curation. Weicheng Yao: data curation. Peng Yan: data curation. Xiao Liu: supervision, writing – review editing. Abdukader Abdukayum: writing – review and editing, supervision, resources, methodology, investigation, funding acquisition, conceptualization.

Funding

This work is supported by the Natural Science Foundation of Xinjiang Uygur Autonomous Region, China (2022D01E16), the Xinjiang Tianshan Talent Training Program, China (2024TSYCCX0109), the Open Research Project of Xinjiang Key Laboratory of New Functional Materials Chemistry (XJLNFMC‐202610), the Kashi Science and Technology Plan, China (KS2024005) and the Tianshan Innovation Team Plan of Xinjiang Uygur Autonomous Region, China (2023D14002), international Science and Technology Cooperation Project of Hubei Province (No. 2024EHA060).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.