Ho‐Loaded ZIF‐L/COF Heterojunctions as Efficient Photocatalysts for H₂O₂ Production

Abstract

Covalent organic frameworks (COFs) exhibit considerable potential as photocatalysts for H₂O₂ production through the oxygen reduction reaction (ORR) in water. However, their catalytic efficiency is often hindered by sluggish charge separation and a scarcity of sites for O₂ activation. Here, a heterojunction of Ho‐loaded ZIF‐L/triazine‐based COF (Ho‐ZIF‐L/TTA‐COF) is designed and synthesized through the electrostatically induced π–π assembly for efficient photocatalytic H₂O₂ production. The optimized Ho‐ZIF‐L/TTA‐COF heterojunction achieves an impressive H₂O₂ production rate of 7.19 mmol g⁻¹ h⁻¹ without any sacrificial agents under visible‐light irradiation, representing a 7.6‐fold enhancement compared to individual TTA‐COF. Based on the results of in situ X‐ray photoelectron spectroscopy and Kelvin probe force microscopy, the enhanced performance is mainly ascribed to the favorable electrons transfer from TTA‐COF to ZIF‐L then to the loaded Ho species for greatly enhancing charge separation. Noteworthily, Ho‐ZIF‐L possesses fully exposed Ho active sites and a large specific surface area, accounting for the efficient 2e⁻ ORR dominated by the produced •O₂ ⁻ radicals for H₂O₂ formation.

Ho‐anchored ZIF‐L is assembled with a triazine‐based COF through the electrostatically induced π–π interaction to form a Ho‐ZIF‐L/TTA‐COF heterojunction. It achieves 7.19 mmol g⁻¹ h⁻¹ H₂O₂ in pure water under visible light without sacrificial agents. The Ho‐ZIF‐L as a thermodynamic energy platform is capable of not only enhancing the charge transfer and separation of TTA‐COF but also providing the microenvironment conducive to O₂ activation.

1. Introduction

Hydrogen peroxide (H₂O₂) is a sustainable and clean energy source, widely used in various fields, including pulp and textile bleaching, chemical synthesis, wastewater treatment, and mining [1, 2, 3, 4]. In addition, it serves as a carbon‐free fuel, providing 2.7 kJ g⁻¹ of energy, with only H₂O and O₂ as by‐products, making it highly eco‐friendly [5, 6]. Industrially, H₂O₂ is mainly produced via the anthraquinone process, which accounts for over 95% of the total global output [7]. However, this method involves multistep hydrogenation and oxidation reactions that demand high energy input and organic solvents, leading to substantial environmental harm and energy inefficiency [8, 9, 10]. Solar‐driven H₂O₂ production uses only abundant water and oxygen as raw materials and has gained considerable attention over the past decade [11, 12]. This process is primarily achieved by employing semiconductors to absorb photons and generate reactive electron–hole pairs, which can, respectively, drive the two‐electron oxygen reduction reaction (ORR) and the two‐electron water oxidation reaction (WOR) for H₂O₂ production. Compared with the thermodynamically uphill reaction of WOR, the two‐electron ORR pathway shows potential for efficient H₂O₂ production [13]. Therefore, there is an urgent need to design and fabricate photocatalysts with efficient charge separation and excellent ORR performance.

Covalent organic frameworks (COFs) are a class of porous crystalline materials connected by covalent bonds, which have attracted significant interest due to their ordered pore structures and tunable electronic properties [14, 15, 16]. Among them, triazine‐based COFs are particularly attractive owing to their donor–acceptor (D–A) structure and high specific surface area, and have been widely applied in the field of photocatalytic H₂O₂ production [17, 18]. However, triazine‐based COFs still suffer from limited photogenerated charge separation and a lack of O₂ active reaction sites. Constructing heterojunction has been accepted as a common strategy to enhance the charge separation. To date, some triazine‐based COF heterojunctions have been reported such as the construction of the COF/g‐C₃N₄ [19], TpPa/TpDz [20], and In₂S₃/TpBpy [21] heterojunction, which facilitates photogenerated charge separation and thereby promotes photocatalytic H₂O₂ production. In our previous work, it has been demonstrated that a wide‐bandgap semiconductor, such as TiO₂, can serve as an energy platform for accepting electrons from a narrow‐bandgap semiconductor like g‐C₃N₄ to prolong the lifetimes of visible‐light‐induced photoelectrons and then promote photogenerated charge separation [22]. Moreover, the prolonged lifetime of photogenerated electrons is beneficial for the cocatalyst loaded on the platform to initiate subsequent reduction reactions [23]. Therefore, introducing a wide‐bandgap electron platform with integrated catalytic sites can simultaneously facilitate charge transfer and serve as active sites for ORR.

Metal organic frameworks (MOFs) are also a class of porous framework materials, characterized by uniformly distributed catalytic centers, abundant pore structures, and a large surface area [24]. ZIF‐L is an important and unique crystalline morphology of the zeolitic imidazolate framework (ZIF‐8) [25]. The “L” denotes its 2D leaf‐like morphology, which is a distinctive feature that significantly distinguishes it from ZIF‐8. This 2D leaf‐like structure also gives it a sheet‐like morphology similar to that of triazine‐based COF materials. Notably, ZIF‐L is a wide‐bandgap semiconductor with the negative conduction band (CB) to reduce O₂ [26]. This encourages us to select ZIF‐L as a thermodynamically appropriate energy platform to couple with triazine‐based COF. The ZIF‐L can accept the visible‐light‐excited electrons from the COFs for enhancing photogenerated charge separation, and the electrons subsequently transfer to the well‐defined Zn sites for O₂ activation. Significantly, the abundant pore structure of ZIF‐L is conducive to concentrating O₂. However, metal center Zn with its fully filled d‐orbitals remains inherent limitations in O₂ activation.

Rare earth metals have emerged as promising candidates for O₂ activation owing to their unique electronic configurations and tunable 4f orbital characteristics. Holmium (Ho), with a [Xe]4f ¹¹6s ² configuration, features partially filled 4f orbitals [27]. This allows for favorable electronic interactions with O₂, potentially enabling electron donation into the π* antibonding orbital, which weakens the O=O bond strength and lowers the dissociation barrier of the O₂ molecule, facilitating O₂ activation [28]. Interestingly, Chen et al. reported that when using ZIF‐8 as support, the active metal Ni was incompatible with the tetrahedral framework but could be selectively anchored to the particle surface [29]. In other words, the ZIF‐L with a higher specific surface area will offer more planar coordination sites for Ho, and the fully exposed Ho active sites will effectively promote the adsorption and activation of O₂. Therefore, the incorporation of Ho into ZIF‐L would enable electron extraction from ZIF‐L, which significantly accelerates the ORR.

Herein, the Ho‐loaded ZIF‐L/triazine‐based COFs heterojunction (Ho‐ZIF‐L/TTA‐COF) is constructed by an electrostatically induced π–π assembly process. The optimal Ho‐ZIF‐L/TTA‐COF heterojunction exhibits a high H₂O₂ production rate of 7.19 mmol g⁻¹ h⁻¹ in pure water without sacrificial agents, representing a 7.6‐fold enhancement over pristine TTA‐COF. Experimental results reveal that the Ho‐ZIF‐L as an energy platform is capable of not only accepting the photogenerated electrons of TTA‐COF which largely enhances its charge separation, but also concentrating and activating O₂ which promotes 2e⁻ ORR for efficient H₂O₂ production.

2. Results and Discussion

The preparation of Ho‐ZIF‐L/TTA‐COF heterojunction was schematically shown in Scheme 1. TTA‐COF was synthesized through a Schiff base reaction using 2,4,6‐triformylphloroglucinol (TPG) and 4,4′,4″‐(1,3,5‐triazine‐2,4,6‐triyl)trianiline (TTA) and was subsequently coupled with Ho‐ZIF‐L the electrostatically induced π–π assembly, where the Ho‐ZIF‐L was synthesized by a facile aqueous‐phase methods, followed by an impregnation with Ho(NO₃)₃.

SCHEME 1.

Schematic illustration of the synthesis process for Ho‐ZIF‐L/TTA‐COF heterojunction.

2.1. Structural Characterizations

The characteristic diffraction peaks observed at 5.5° and 27° are correspond to the (100) and (002) crystal planes of TTA‐COF, respectively, based on simulation and Pawley refinement (Figure S1). The detailed chemical structure was further corroborated by solid‐state ¹³C NMR spectroscopy (Figure S2), in which the signals of carbonyl carbon, triazine carbon, C‐NH in TTA‐COF are observed, further confirming the successful synthesis of TTA‐COF. On the other hand, the XRD diffraction peaks of ZIF‐L are consistent with those reported in the literature [27], which remained unchanged upon the introduction of Ho (Figure S3a), indicating that the incorporation of Ho does not alter the parent ZIF‐L structure. Meanwhile, an absorption band at 2933 cm⁻¹ is attributed to the stretching vibrations of C—H bonds from both the methyl group and the imidazole ring, and the peaks observed at 1145 and 990 cm⁻¹ originated from the C—N stretching vibrations of the 2‐methylimidazole ligand can be observed from the Fourier transform infrared (FT‐IR) spectra (Figure S3b). Additionally, the characteristic Zn–N₄ stretching vibration of ZIF‐L is also detected at 423 cm⁻¹. And the incorporation of Ho does not significantly alter the organic ligand framework structure of ZIF‐L or the coordination environment of the central Zn metal. The UV–Vis absorption spectra presented in Figure S4 reveal that pristine ZIF‐L possesses a sharp absorption edge at ≈240 nm, indicative of its wide‐bandgap nature. An additional broad absorption enhancement of Ho‐ZIF‐L appears between 400 and 700 nm, likely arising from the incorporation of Ho species. By analyzing the X‐ray photoelectron spectroscopy (XPS) spectra of N 1s for ZIF‐L and Ho‐ZIF‐L (Figure S5a), the binding energy (BE) are deconvoluted into two components, assigned to Zn–N₄ and C—N=C, respectively. Upon the introduction of Ho, the characteristic peak of C—N=C shifts to a higher BE, while the BE of Zn–N₄ remained almost unchanged, indicating that the introduced Ho is likely anchored to the ZIF‐L surface, coordinated with imidazole groups from the framework of ZIF‐L [30]. Meanwhile, the Ho 4d spectrum is obtained as shown in Figure S5b, confirming the successful incorporation of Ho.

Notably, the XRD patterns of the ZIF‐L/TTA‐COF and Ho‐ZIF‐L/TTA‐COF heterojunctions exhibit characteristic diffraction peaks corresponding to both TTA‐COF and ZIF‐L (Figure 1a), confirming the successful integration of the two components. The light absorption properties were characterized using UV–Vis absorption spectroscopy. Both TTA‐COF and heterojunctions display a broad optical absorption band ranging from 400 to 700 nm (Figure S6). N₂ adsorption–desorption isotherm measurements reveal the high specific surface areas of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF, originating from their unique pore architectures, which will benefit mass transfer during the subsequent reaction (Figure 1b). The Brunauer–Emmett–Teller specific surface area decreases from 811.17 to 800.12 m² g⁻¹ after Ho introduction, which may be due to Ho species partially occupying the pores. Furthermore, FT‐IR spectra (Figure 1c) detect stretching vibration bands corresponding to C=O bonds at ≈1624 cm⁻¹, C=C bonds at ≈1576 cm⁻¹, and C—N bonds at 1288 cm⁻¹, confirming the occurrence of the Schiff base reaction and enol‐to‐keto tautomerism during the COFs synthesis. And the stretching mode of characteristic Zn–N₄ from ZIF‐L was also be detected at 423 and 756 cm⁻¹. Importantly, the FT‐IR spectra of the heterojunctions exhibit characteristic stretching vibrations from both TTA‐COF and ZIF‐L. In the enlarged regions of 400–2000 cm⁻¹ (Figure S7), the characteristic vibrations of the C=O group in TTA‐COF and the Zn–N₄ in ZIF‐L exhibit slight shifts, which are assignable to π–π interactions between TTA‐COF and ZIF‐L. With increasing the loading amounts of ZIF‐L, the C=O stretching band of TTA‐COF gradually shifts to a more noticeable extent, and the Zn–N₄ stretching peak shows a progressive enhancement in intensity along with a slight peak shift (Figure S8), further suggesting the chemical interactions between TTA‐COF and ZIF‐L. However, these two peaks show no obvious further shifts upon Ho loading, indicating that the introduction of Ho does not alter the π–π interactions and the Ho is preferentially anchored on the surface of ZIF‐L via coordination with imidazole groups of its framework, rather than by substituting Zn. These findings are consistent with the XPS results.

FIGURE 1.

(a) XRD patterns and (b) N₂ adsorption–desorption isotherm of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF, (c) FT‐IR spectra of ZIF‐L, TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF. XPS analyses of (d) C 1s of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF, and (e) Zn 2p of ZIF‐L, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF, (f) HRTEM image of TTA‐COF, (g) STEM image and the corresponding EDX mapping of Ho‐ZIF‐L, (h) TEM image, and (i) AFM image and corresponding height profiles of Ho‐ZIF‐L/TTA‐COF.

The interaction and chemical environment within the materials were further explored using XPS. In the C 1s spectra (Figure 1d), BE observed at 288.73, 286.4, and 284.67 eV is assigned to C=O, C—N, and C—C groups of TTA‐COF, respectively. After coupling with ZIF‐L, the BE of C=O group shifts to a higher position, while the BE of C—N and C—C groups remains unchanged, suggesting the possible formation of π–π interactions between TTA‐COF and ZIF‐L. The O 1s spectra of TTA‐COF show peaks at 532.78 and 531.45 eV, attributed to C=O and —OH groups, respectively (Figure S9a). And the N 1s spectra display peaks at 398.46 and 399.95 eV, corresponding to C—N=C and C—NH–, respectively (Figure S9b). The BE of O 1s and N 1s for ZIF‐L/TTA‐COF is higher than the TTA‐COF, which illustrates the electron diffusion from TTA‐COF to ZIF‐L. However, no further BE shift was observed between ZIF‐L/TTA‐COF and Ho‐ZIF‐L/TTA‐COF, suggesting that the introduction of Ho does not affect the interaction between them. In Figure 1e, the BE of Zn 2p at 1023.03 and 1046.11 eV is assigned to Zn 2p _3/2 and Zn 2p _1/2, respectively, which illustrates that Zn mainly exists in valence state of Zn²⁺. After combined with TTA‐COF, the peaks of Zn 2p shift toward lower BE values, and no further shift was observed upon Ho introduction. It manifests that the electrons diffuse from TTA‐COF to ZIF‐L after they come into close attachment. These XPS results indicate the successful formation of a well‐defined interface between Ho‐ZIF‐L and TTA‐COF, promoting efficient charge carrier migration across their interface.

The TEM image of TTA‐COF reveals a 2D nanosheet morphology (Figure 1f). Furthermore, high‐resolution transmission electron microscopy (HRTEM) images of TTA‐COF display distinct lattice fringes with an interplanar spacing of ≈0.72 nm, corresponding to the (002) crystal plane. SEM image (Figure S10) and STEM images (Figure 1g) confirm that both ZIF‐L and Ho‐ZIF‐L exhibit a 2D leaf‐like nanosheet structure, indicating that the introduction of Ho does not alter the pristine ZIF‐L morphology. Energy‐dispersive spectroscopy elemental mapping clearly shows the distribution of C, N, O, Zn, and Ho elements within the material, confirming the presence of Ho in the Ho‐ZIF‐L. The TEM image of Ho‐ZIF‐L/TTA‐COF (Figure 1h) distinctly shows smaller TTA‐COF nanosheets supported on larger Ho‐ZIF‐L nanosheets. AFM images shown in Figures S11–S13 further corroborate their ultrathin structure, and the average thicknesses for TTA‐COF, ZIF‐L, and Ho‐ZIF‐L are ≈4.1, 4.3, and 4.4 nm, respectively. Notably, the Ho‐ZIF‐L/TTA‐COF composite exhibits an average thickness of 8.6 nm, which corresponds well to the individual thicknesses of TTA‐COF and Ho‐ZIF‐L.

2.2. Charge Separation and Photocatalytic Performance

Steady‐state photoluminescence (PL) spectroscopy was employed to probe the charge separation properties of the materials. A significant quenching of the PL signal over TTA‐COF is observed upon the introduction of ZIF‐L, indicating suppressed electron–hole recombination and thus enhanced charge separation efficiency resulting from heterojunction formation. Notably, the PL intensity further quenches after the subsequent introduction of Ho (Figure S14), demonstrating that Ho further promotes charge separation. Photoelectrochemical (PEC) measurements corroborated these findings. Compared to pristine TTA‐COF, the ZIF‐L/TTA‐COF composite exhibits a substantially increased photocurrent density, with the Ho‐ZIF‐L/TTA‐COF composite displaying the highest value (Figure S15), demonstrating that the heterojunction construction and Ho incorporation synergistically enhance the charge separation and transfer. Electrochemical impedance spectroscopy (EIS) shows that the radius of the curves follows the order: Ho‐ZIF‐L/TTA‐COF < ZIF‐L/TTA‐COF < TTA‐COF, indicating that Ho‐ZIF‐L/TTA‐COF exhibits the lowest charge transfer resistance (Figure S16). This demonstrates that the construction of the heterojunction and the introduction of Ho reduce the charge transfer resistance. EIS results are consistent with PL and PEC results.

The photocatalytic H₂O₂ production performance of the as‐prepared samples was evaluated under visible‐light irradiation (λ ≥ 420 nm, 100 mW cm⁻²) in O₂‐saturated pure water without any sacrificial agent, with the standard curves shown in Figure S17. The results (Figure 2a) show that TTA‐COF exhibits a H₂O₂ production rate of 0.92 mmol g⁻¹ h⁻¹. After coupling with ZIF‐L, the performance is significantly enhanced. The loading amount of ZIF‐L in the ZIF‐L/TTA‐COF composite has also been optimized, and the optimal 10ZIF‐L/TTA‐COF achieves a H₂O₂ production rate of 3.22 mmol g⁻¹ h⁻¹ (Figure S18). Notably, Ho incorporation into ZIF‐L further boosts the performance. Optimization of the Ho content yields a H₂O₂ production rate of 7.19 mmol g⁻¹ h⁻¹ (Figure S19). Concurrently, a certain amount of O₂ is detected as the oxidant product (Figure S20). As shown Figure S21, the photocatalytic activity of TTA‐COF modified only with Ho (denoted as Ho‐ZIF‐L) is higher than that of pristine TTA‐COF, but lower than that of ZIF‐L/TTA‐COF, indicating that the construction of a heterojunction between TTA‐COF and ZIF‐L plays a critical role in boosting the photocatalytic performance. In parallel, the influence of incorporating other metals into ZIF‐L on the heterojunction system has also been investigated. The introduction of different metals generally enhances the H₂O₂ production rate, in which Ho exhibits the highest photocatalytic activity for H₂O₂ generation. Compared with recently reported studies on heterojunction photocatalysts based on triazine COFs and triazine‐based COFs (Figure 2b) [17, 23, 31, 32, 33, 34, 35, 36, 37, 38, 39], the Ho‐ZIF‐L/TTA‐COF demonstrates superior H₂O₂ production performance in comparable reaction conditions. The time‐dependent performance of the investigated catalysts for H₂O₂ production was further investigated (Figure 2c). For all three materials, the produced H₂O₂ increases continuously with prolonged irradiation time. The H₂O₂ decomposition behavior of the catalysts was also evaluated (Figure S22). The generation rate constants (K _f, μmol L⁻¹ min⁻¹) and decomposition rate constants (K _d, 10⁻² min⁻¹) were calculated (Figure 2d). Notably, Ho‐ZIF‐L/TTA‐COF exhibits the highest K _f value and the lowest K _d value among these three samples, indicating that the introduction of both ZIF‐L and Ho not only promotes H₂O₂ generation but also effectively inhibits its decomposition. Stability tests (Figure 2e) demonstrate the H₂O₂ yield of the Ho‐ZIF‐L/TTA‐COF retains after five consecutive cycles.

FIGURE 2.

(a) Photocatalytic activities for H₂O₂ production of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF, (b) comparison of the H₂O₂ production of Ho‐ZIF‐L/TTA‐COF and recently reported photocatalysts, (c) the H₂O₂ production rate and (d) the formation rate constant (K _f) and decomposition rate constant (K _d) of TTA‐COF, ZIF‐L/TTA‐COF and Ho‐ZIF‐L/TTA‐COF, and (e) photocatalytic cycling test of H₂O₂ production on Ho‐ZIF‐L/TTA‐COF.

2.3. Mechanism Discussion

Based on the Kubelka–Munk function, the optical band gaps of TTA‐COF and ZIF‐L were calculated to be 2.33 and 5.16 eV, respectively (Figure S23). Subsequent Mott–Schottky measurements were employed to determine the energy band structures of the materials (Figure S24). The CB positions of TTA‐COF and ZIF‐L are located at −0.75 and −0.66 V (vs. Ag/AgCl). The results indicated that the CB position of ZIF‐L is lower than that of TTA‐COF, which facilitates electrons transfer from TTA‐COF to ZIF‐L. The schematic diagram of the band structure is shown in Figure S25. Notably, the CB positions of both materials satisfy the thermodynamic requirements for H₂O₂ production, demonstrating their feasibility for photocatalytic H₂O₂ generation. The mechanism of charge transfer was investigated by time‐resolved photoluminescence (TR‐PL) spectroscopy. The average PL lifetimes of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF are 1.33, 0.65, and 0.54 ns, respectively (Figure 3a). The shortened lifetime detected on ZIF‐L/TTA‐COF in comparison to TTA‐COF indicates the establishment of efficient charge transfer pathways between TTA‐COF and ZIF‐L. Further introducing of Ho facilitates electrons transfer. In situ irradiated XPS measurements were then performed to determine the charge transfer direction. For the ZIF‐L/TTA‐COF photocatalyst, upon light irradiation, the Zn 2p BE shifts negatively, indicating an increase in electron density. While the O 1s BE shifts positively, demonstrating a decrease in electron density (Figure S26). This confirms that under illumination, electrons transfer from TTA‐COF to ZIF‐L. In the case of the Ho‐ZIF‐L/TTA‐COF photocatalyst, a similar charge transfer behavior is observed. Upon light irradiation, the BE of Ho 4d shifts negatively compared to that under dark conditions, while the BE of C 1s shifts positively (Figure 3b,c). This result demonstrates that the electrons further transfer to the incorporated Ho.

FIGURE 3.

(a) TR‐PL spectra of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF, in situ irradiated XPS analyses for (b) C 1s and (c) Ho 4d of Ho‐ZIF‐L/TTA‐COF, (d) topography and KPFM images of Ho‐ZIF‐L/TTA‐COF in the (e) dark and (f) under 460 nm irradiation and (g) the corresponding contact potential differences, (h) O₂‐TPD curves, electrochemical reduction curves in (i) N₂‐saturated electrolyte and (j) O₂‐saturated electrolyte of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF.

Kelvin probe force microscopy (KPFM) was employed to further elucidate the electrons transfer mechanism within the Ho‐ZIF‐L/TTA‐COF heterojunction. The topography and the corresponding surface potential variations acquired before and after light irradiation of Ho‐ZIF‐L/TTA‐COF are shown in Figure 3d–g. Under 460 nm light illumination, the surface potential of Ho‐ZIF‐L decreased by 27.8 mV, whereas that of TTA‐COF decreased by 9.1 mV. When only TTA‐COF is excited and ZIF‐L remains unexcited, the greater decrease in surface potential for Ho‐ZIF‐L than for TTA‐COF suggests photogenerated electron transfer from TTA‐COF to Ho‐ZIF‐L, in agreement with the aforementioned in situ XPS results. The ORR is a critical step in photocatalytic H₂O₂ production. The O₂ adsorption and activation on the photocatalysts were investigated by O₂ temperature‐programmed desorption (O₂‐TPD). As shown in Figure 3h, the desorption peak at ≈200°C corresponds to physically adsorbed O₂, while the peak at around 360°C is attributed to chemisorbed O₂. Ho‐ZIF‐L/TTA‐COF exhibits the strongest signal for both peaks, particularly the chemisorption one, indicating that the introduction of Ho significantly enhances the O₂ adsorption. The catalytic function of Ho was further validated by electrochemical reduction experiments. The O₂ activation on the samples was investigated through electrochemical I–V curve measurements. Under N₂ condition as shown in Figure 3i, the reduction potential required for the photocatalyst followed the order Ho‐ZIF‐L/TTA‐COF < ZIF‐L/TTA‐COF < TTA‐COF, indicating that the reduction reaction occurs most readily on Ho‐ZIF‐L/TTA‐COF. A similar trend is observed under O₂ atmosphere (Figure 3j), where Ho‐ZIF‐L/TTA‐COF also exhibits the lowest reduction potential. The above results illustrate that Ho incorporation significantly boosts the O₂ activation.

To determine the selectivity of the ORR pathway and the average number of transferred electron numbers (n) during H₂O₂ generation over the photocatalysts, rotating ring‐disk electrode (RRDE) measurements were conducted. As shown in Figure 4a, the disk currents for TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF increase as the applied potential decreased versus the reversible hydrogen electrode. Concurrently, H₂O₂ generated on the disk electrode diffuses to the platinum ring electrode, where it is further oxidized, producing a positive ring current. The ring current generated over the Ho‐ZIF‐L/TTA‐COF catalyst is substantially higher than those over TTA‐COF and ZIF‐L/TTA‐COF, further suggesting the improved H₂O₂ production by Ho‐ZIF‐L/TTA‐COF. The H₂O₂ selectivity and the average electron transfer number for the H₂O₂ generation process were further calculated based on the RRDE results. As shown in Figure 4b, the Ho‐ZIF‐L/TTA‐COF catalyst exhibits a selectivity of up to 81%, surpassing that of TTA‐COF and ZIF‐L/TTA‐COF. The the calculated n for Ho‐ZIF‐L/TTA‐COF is close to 2, which indicated a highly selective two‐electron ORR pathway (Figure 4c).

FIGURE 4.

(a) The ring current and disk current, (b) H₂O₂ selectivity as a function of the applied potential, (c) the average number of transferred electrons, (d) photocatalytic activities for H₂O₂ production under different reaction gases or different sacrificial agents, (e) DMPO spin‐trapping EPR spectra recorded for •O₂ ⁻ under visible‐light irradiation of TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF in methanolic solution (the concentration of DMPO 50 mM). (f) H₂O₂ generation rates of Ho‐ZIF‐L/TTA‐COF with different superoxide dismutase (SOD) concentrations, (g,h) In situ DRIFT spectra of Ho‐ZIF‐L/TTA‐COF recorded during photocatalytic H₂O₂ evolution, (i) the intensity of the peaks at 1184 and 2885 cm⁻¹ versus illumination time.

Subsequently, trapping experiments were performed to identify the reactive species involved in the ORR pathway. As shown in Figure 4d, when the reaction atmosphere is switched from O₂ to N₂, the H₂O₂ yield over all catalysts decrease dramatically, confirming that the ORR is the primary pathway for H₂O₂ production. Upon the addition of isopropyl alcohol (IPA) as a hydroxyl radical (•OH) scavenger, no significant change in H₂O₂ yield is observed for any of the three catalysts, which suggests that •OH radicals are barely involved in H₂O₂ generation. In contrast, when benzoquinone (BQ), a superoxide radical (•O₂ ⁻) scavenger, is introduced into the reaction system, the H₂O₂ yields over TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF decrease substantially. This indicates that •O₂ ⁻ radicals are key intermediates in the H₂O₂ production pathway. Electron paramagnetic resonance (EPR) spectroscopy was employed to further detect the generation of •O₂ ⁻ radicals, using 5,5‐dimethyl‐1‐pyrroline N‐oxide (DMPO) as a spin‐trapping agent. The EPR spectra clearly show characteristic signals corresponding to the DMPO‐•O₂ ⁻ adduct for TTA‐COF, ZIF‐L/TTA‐COF, and Ho‐ZIF‐L/TTA‐COF under light irradiation (Figure 4e). Notably, the Ho‐ZIF‐L/TTA‐COF heterojunction exhibits the strongest signal intensity, indicating its superior ability to generate •O₂ ⁻ radicals. To investigate the role of the •O₂ ⁻ in the reaction pathway, superoxide dismutase (SOD), which catalyzes the dismutation of •O₂ ⁻ and thereby promotes H₂O₂ formation, was introduced into the reaction system. As shown in Figure 4f, the H₂O₂ yield of the Ho‐ZIF‐L/TTA‐COF catalyst increases with rising SOD concentration, further confirming that •O₂ ⁻ radicals act as intermediates in the photocatalytic H₂O₂ production pathway. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was conducted to monitor the H₂O₂ production process under visible‐light irradiation. The signals observed at 1185 and 2884 cm⁻¹ that attributed to •O₂ ⁻ species and the typical (ν ₂ + ν ₆)/2ν₆ O—H bending mode of H₂O₂, respectively, gradually increased in intensity with prolonged irradiation time for TTA‐COF, indicating a stepwise single‐electron reduction route for H₂O₂ production (Figure S27). Similarly, these signals are also observed on ZIF‐L/TTA‐COF and Ho‐ZIF‐L/TTA‐COF, with a more pronounced increase in intensity (Figures 4g,h and S28). This indicates that neither the incorporation of ZIF‐L nor the introduction of Ho alters the ORR route. Moreover, the formation of the heterojunction effectively enhances charge separation, which facilitates the •O₂ ⁻ radicals and H₂O₂ generation. As expected, the Ho‐ZIF‐L/TTA‐COF exhibits the highest generation rates of both •O₂ ⁻ radicals and H₂O₂, as revealed by the normalized relative peak intensities at 1185 and 2884 cm⁻¹, respectively (Figure 4i). Accordingly, the enhanced charge separation and preferable O₂ activation brought by the introduced ZIF‐L as a thermodynamic platform for accepting photogenerated electrons from TTA‐COF, along with the unique Ho sites, synergistically enable high‐efficiency photocatalytic H₂O₂ production. The proposed mechanism for photocatalytic H₂O₂ production over the Ho‐ZIF‐L/TTA‐COF heterojunction is shown in Figure 5. Upon visible‐light irradiation, photogenerated electrons in TTA‐COF are excited to the CB, and would first transfer to ZIF‐L and then to the introducted Ho sites, consequently initiating O₂ activation to produce •O₂ ⁻ and then product H₂O₂. Meanwhile, the photogenerated holes remaining in the VB of TTA‐COF participate in the WOR, continuously supplying protons and electrons, which collectively drive the efficient synthesis of H₂O₂.

FIGURE 5.

Schematic of the charge transfer and involved redox reactions on Ho‐ZIF‐L/TTA‐COF heterojunction.

3. Conclusion

In summary, a Ho‐ZIF‐L/TTA‐COF heterojunction is constructed via an electrostatically‐induced π–π assembly strategy. The optimized heterojunction achieves an impressive photocatalytic H₂O₂ production rate of 7.19 mmol g⁻¹ h⁻¹ with excellent stability. The high activity is mainly attributed to the enhanced charge transfer and separation achieved by coupling ZIF‐L as a thermodynamic energy platform, as well as the microenvironment conducive to O₂ activation arising from the porous structure and the incorporated Ho sites. In addition, it has been confirmed that the sequential single‐electron ORR pathway involving •O₂ ⁻ radicals is the dominant route for H₂O₂ generation. This work provides a feasible strategy for developing efficient MOF/COF heterojunction photocatalysts with improved charge separation and tailored active sites for H₂O₂ production.

Funding

This work was supported by the National Natural Science Foundation of China (U23A20576, 22579049); National Key Research and Development Program of China (2024YFF0506202); Outstanding Science Foundation of Heilongjiang University (JCL202401).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

The authors have cited additional references within the Supporting Information [30, 31, 32, 33, 34, 35, 36, 37, 38, 39].

Data Availability Statement

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