Microdroplets Boosted Photocatalytic H₂O₂ Production Over Covalent Organic Frameworks via Tri‐Phase Interface Catalysis

ABSTRACT

Photocatalytic H₂O₂ production from H₂O/O₂ is a green solar energy conversion strategy, but the conventional bulk liquid systems suffer from poor mass transfer and limited active site accessibility. Here, by introducing sessile water microdroplets into the system using a covalent organic framework (DS‐OH‐COF) as a photocatalyst, the H₂O₂ production rate was significantly enhanced. The yield strongly depends on droplet size. At 1 µL under air atmosphere, H₂O₂ yield reached 11.11 mmol g⁻¹ h⁻¹, representing a 12.3‐fold increase over bulk water systems. Under O₂, the yield increases to 14.79 mmol g⁻¹ h⁻¹, outperforming most reported photocatalysts. The large specific surface area of microdroplets enhances O₂ mass transfer into the liquid phase, promoting interaction with catalyst active sites. Most importantly, the gas‐liquid‐solid tri‐phase interface plays a vital role in the catalytic process. Density functional theory calculations confirm that the O₂ adsorption behavior is modulated by the substrate, which regulates O₂ reduction at the tri‐phase interface. The microdroplet system also enabled efficient methyl orange degradation, demonstrating its practical potential. This microdroplet‐based catalytic path effectively overcomes the inherent limitations of insufficient oxygen mass transfer and low efficiency in bulk reactions, providing new insights for catalytic H₂O₂ generation.

The sessile water microdroplet system significantly boosts photocatalytic H₂O₂ production over a covalent organic framework through gas–liquid–solid tri‐phase catalysis. By reducing droplet volume and selecting appropriate substrates, O₂ adsorption is tuned to favor the two‐electron reduction pathway, achieving yields much higher than those in typical conventional bulk systems.

1. Introduction

Hydrogen peroxide (H₂O₂) is a critical chemical for contemporary sustainable development, occupying a crucial position in environmental geochemical cycles and material transformation processes [1, 2]. Meanwhile, as an indispensable industrial feedstock and laboratory reagent, H₂O₂ has been widely applied in diverse fields such as chemical synthesis, medical disinfection, and textile bleaching [3, 4]. However, the dominant anthraquinone‐based production process suffers from notable limitations. Specifically, it is characterized by high energy consumption, relies heavily on noble metal catalysts, and inevitably generates toxic organic by‐products [5]. Thus, the development of efficient, environmentally benign, and cost‐effective novel strategies for H₂O₂ synthesis is of great significance for replacing traditional anthraquinone methods and advancing sustainable development [6]. Against this backdrop, photocatalytic synthesis routes using H₂O/O₂ as green feedstocks have garnered widespread attention in recent years as highly promising alternative technologies, owing to their advantages in atomic economy and zero‐pollution emissions [7]. Photocatalytic H₂O₂ production can be achieved via three pathways, namely the water oxidation reaction (WOR), the oxygen reduction reaction (ORR), and the dual pathway integrating ORR and WOR [8, 9]. The dual pathway integrates the merits of both ORR and WOR, enabling the synthesis of H₂O from H₂O and O₂ without the use of sacrificial agents. This integration enhances atom utilization efficiency and energy conversion efficiency, thereby boosting the overall efficiency of H₂O₂ production. Therefore, the dual pathway is regarded as the ideal strategy for photocatalytic H₂O₂ generation.

Catalysts relevant to this photocatalytic H₂O₂ synthesis system encompass metal complexes, semiconductor materials, organic polymers, crystalline porous materials, metal‐organic frameworks (MOFs), and covalent organic frameworks (COFs) [10, 11, 12]. Specifically, COFs represent a type of crystalline porous material precisely constructed through covalent bonds between organic structural units [13, 14]. Their modular design enables precise tailoring of active sites and electronic band structures to match the thermodynamic requirements of H₂O₂ synthesis, while the abundant porous paths and large specific surface area facilitate mass transfer and enhance the adsorption of H₂O and O₂ molecules. Such structural and electronic merits render COFs promising candidates for overcoming crucial bottlenecks in photocatalytic H₂O₂ production. Nevertheless, current research on photocatalytic H₂O₂ synthesis is mainly focused on bulk solution systems, and fewer studies have been conducted in microdroplet systems [15].

Recently, microdroplet chemistry has attracted considerable attention in the scientific community due to its unique physicochemical properties [16, 17]. Zhang's group adopted WO₃·0.33H₂O as the photocatalyst to investigate the photocatalytic CO₂ reduction reaction. The results confirmed that the formic HCOOH yield in the microdroplets system is enhanced by two orders of magnitude compared with that in the bulk solution [18, 19, 20]. Such a significant acceleration effect is mainly attributed to the intrinsic properties of the microdroplet interface, including high electric field strength and extremely large specific surface area [21]. These characteristics synergistically modulate the chemical reaction pathways and reduce the activation energy, ultimately achieving performance that is unattainable in conventional bulk systems [22]. A pioneering example in this field is a series of studies conducted by Professor Richard N. Zare's research group: by dispersing water into micrometer‐scale microdroplets via spraying or vapor condensation techniques, they first discovered and reported the spontaneous generation of H₂O₂ (∼30 µM) from aqueous microdroplets [23]. Therefore, the utilization of microdroplets for H₂O₂ generation or the significant enhancement of its synthesis yield may represent a novel and promising strategy.

Herein, we report an efficient photocatalytic H₂O₂ synthesis within sessile microdroplets. Employing DS‐OH‐COF as the photocatalyst, the photocatalytic synthesis of H₂O₂ was achieved under visible light irradiation (λ ≥ 420 nm) through systematic regulation of the size of catalyst‐containing microdroplets. Experiments demonstrated that the H₂O₂ yield exhibits distinct size dependence, increasing with the decrease in microdroplets volume. The optimal photocatalytic performance was observed when the microdroplet volume was controlled at 1 µL, achieving an H₂O₂ yield of 11.1 mmol g⁻¹ h⁻¹ under an air atmosphere. This value substantially outperforms that obtained with the same catalyst in bulk systems (0.9 mmol g⁻¹ h⁻¹), representing a 12.3‐fold enhancement. Importantly, the performances can be regulated by the substrate of the microdroplets. Theoretical calculations verified that the O₂ adsorption capacity of the substrates directly determines the H₂O₂ generation. Meanwhile, in situ Raman spectroscopy and other characterization techniques confirmed that the gas‐liquid‐solid tri‐phase interface of microdroplets can effectively promote H₂O₂ generation, demonstrating the remarkable advantages of the microdroplets size regulation strategy. This strongly validates the unique merits of the tri‐phase interface in the sessile microdroplets system. This concise, efficient, and eco‐friendly method improves the application potential of H₂O₂ in green energy and opens up a new approach for the development of dual pathway photocatalytic H₂O₂ synthesis.

2. Results and Discussion

2.1. Synthesis and Characterizations of Photocatalysts

Prior to the photocatalytic experiments, DS‐OH‐COF was synthesized and comprehensively characterized. Specifically, DS‐OH‐COF was synthesized under a solvothermal condition of 120°C for 3 days (Figure 1a, detailed information in Section S2, Supporting Information). Firstly, the crystal properties of COF were characterized by powder x‐ray diffraction (PXRD). The experimental PXRD pattern of the COF was consistent with the simulation result of the AA stacking mode (Figure 1b,c). Next, a series of analytical techniques were used to conduct in‐depth studies on the structural characteristics of the synthesized COF. In the Fourier transform infrared (FT‐IR) spectra, distinct stretching vibration peaks corresponding to C = O (1618 cm⁻¹) and C‐N (1356 cm⁻¹) bonds were observed, confirming the enol‐keto tautomerization and the successful formation of the target COF structures (Figure 1d) [24]. The porosity of the material was explored by the N₂ adsorption‐desorption isotherm at 77 K (Figure 1e). The specific surface area of DS‐OH‐COF was 656 m² g⁻¹, with a microporous structure and an IV‐type adsorption isotherm. Subsequently, the morphology of the COF material was studied. Scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (HR‐TEM) confirmed the nanoparticle morphology of DS‐OH‐COF (Figures S1 and S2).

FIGURE 1.

(a) The synthesis scheme for DS‐OH‐COF. (b) AA stacking mode of the COF. (C: light blue; N: dark blue; O: red; S: yellow; H: white). (c) The Pawley refined PXRD patterns, experimental, and simulated PXRD patterns of the COF with the AA stacking mode. (d) FT‐IR spectra of the related ligands of DS‐OH‐COF. (e) N₂ adsorption‐desorption isotherms and pore size distribution of the DS‐OH‐COF.

2.2. Optical Properties and Band Structure of Photocatalysts

The photoelectric properties of the material were characterized by ultraviolet‐visible (UV‐vis) diffuse reflectance spectroscopy. As shown in Figure 2a, DS‐OH‐COF exhibits a broad light absorption ranging from 300 to 800 nm. The band gap (E _g) of DS‐OH‐COF was calculated to be 2.13 eV from the Tauc plot derived from UV‐vis (inset of Figure 2a) [25]. Meanwhile, the Mott‐Schottky (M‐S) curve of this COF has a positive slope, which is a characteristic of n‐type semiconductors (Figure 2b). The flat band potential (E _fb) of DS‐OH‐COF was calculated to be −0.22 V (vs. NHE). Based on this, the conduction band minimum (CBM) of DS‐OH‐COF is −0.32 to −0.42 V (vs. NHE), and the corresponding valence band maximum (VBM) is 1.71 to 1.81 V (Figure 2c). Therefore, DS‐OH‐COF has thermodynamic potential for both ORR and WOR, where both the two‐step single‐electron ORR reaction ($E_{O_2/•{\mathrm{O}}_2^{-}}$= ‐0.33 V [vs. NHE]), and $E_{•{\mathrm{O}}_2^{-}/H_2{O}_2}$ = 1.44 V [vs. NHE]) and the one‐step two‐electron ORR reaction ($E_{O_2/H_2{O}_2}$= 0.68 V [vs. NHE]), as well as the WOR reaction ($E_{H_{2}O/H_2{O}_2}$ = 1.76 V [vs. NHE]), are thermodynamically feasible for H₂O₂ formation [26]. DS‐OH‐COF generates a transient photocurrent under light irradiation demonstrating its ability to produce photoinduced charge carriers (Figure 2d). Subsequently, in the electrochemical impedance spectroscopy (EIS) results, DS‐OH‐COF was shown to have a semi‐circular diameter in the Nyquist plot, indicating its ability for charge separation and interface charge transport (Figure S3). Furthermore, photoluminescence (PL) measurements were also conducted, which further demonstrated that DS‐OH‐COF has a certain ability for charge separation (Figure S4). In addition, based on the fluorescence decay curves (Figure 2e,f), The average PL lifetime of DS‐OH‐COF was calculated to be 1.53 ns. This indicates that DS‐OH‐COF can prolong the lifetime of photogenerated charge carriers, and thus facilitate the subsequent photocatalytic H₂O₂ synthesis reaction [27].

FIGURE 2.

(a) UV‐Vis diffuse reflectance spectra of the DS‐OH‐COF measured in the solid state. Inset: Tauc–plot for the determination of the optical band gap. (b) Mott–Schottky plot, (c) Band structure diagram, (d) transient photocurrent spectra, (e) fluorescence decay curves (excited at 372 nm), and (f) normalized fluorescence decay fitting curves of DS‐OH‐COF.

2.3. Photocatalytic H₂O₂ Evolution With Microdroplets

The photocatalytic H₂O₂ production capacity of DS‐OH‐COF in the solution‐based bulk system was first verified, with the yield of H₂O₂ to be 0.9 mmol g⁻¹ h⁻¹ under an air atmosphere. Subsequently, we investigated the process of photocatalytic H₂O₂ generation in the microdroplet system. The photocatalyst was fully dispersed in deionized water and sonicated to ensure a homogeneous dispersion. Subsequently, the homogeneous mixture was uniformly dropwise added onto the superhydrophobic glass with a pipette to form uniformly distributed microdroplets. The glass was then transferred to a dedicated photocatalytic reactor, which was vacuumed and subsequently filled with O₂ to maintain a pressure of 90 kPa. Visible‐light irradiation was offered by a 300 W xenon lamp equipped with a cutoff filter (λ ≥ 420 nm). Under irradiation for 1 h, the H₂O₂ yield was significantly elevated to 11.11 mmol g⁻¹ h⁻¹. To clarify the origin of H₂O₂ and rule out potential interference, a series of control experiments were designed. Given the ultrasonic dispersion step in pretreatment, H₂O₂ concentration changes in the solution were measured before and after dispersion. Results showed that H₂O₂ concentration remained negligible both prior to and following 10 min of ultrasonic treatment, confirming negligible interference from ultrasonic pretreatment on the experimental outcomes. Similarly, no significant H₂O₂ production was observed in control experiments lacking the photocatalyst or conducted in the dark (Figure S5a). To elucidate the reaction pathway of photocatalytic H₂O₂ production in the microdroplets system, the H₂O₂ yield was investigated under different gas atmospheres. Reduced H₂O₂ production in the air atmosphere indicates the involvement of O₂ in H₂O₂ generation. However, H₂O₂ production was still achieved under Ar atmosphere (Figure S5b). This key observation clearly demonstrates that DS‐OH‐COF possesses a WOR pathway for H₂O₂ production [28].

Next, we evaluated the hydrophilicity of the photocatalyst. The contact angle of DS‐OH‐COF was measured to be 47°, indicating its excellent hydrophilicity, which provides a favorable aqueous environment for photocatalytic H₂O₂ production (Figure S6). Furthermore, to investigate whether the addition of COF affects the contact angle and thus the morphology of microdroplets, we performed corresponding contact angle measurements. The contact angle of pure water microdroplets was determined to be 92° (Figure S7a), whereas that of microdroplets containing COF (50 mg L⁻¹) was 95° (Figure S7b). These results demonstrate that the introduction of COF has a negligible effect on the contact angle and the shape of microdroplets.

To investigate the effect of microdroplet size on H₂O₂ production efficiency, the volume of microdroplets containing DS‐OH‐COF catalyst was systematically controlled. As shown in Figure 3a, the H₂O₂ yield exhibits a significant size dependence under air atmosphere. Given that there is a clear physical correlation between the volume and specific surface area of microdroplets, we calculated the specific surface area (S _V) corresponding to different microdroplets volumes (Table S2). The results reveal that reducing microdroplets volume leads to an increase in specific surface area, which further remarkably promotes the H₂O₂ yield. This phenomenon is also observed under O₂ atmosphere (Figure 3b), and the 1 µL microdroplets system further increases the H₂O₂ yield to 14.79 mmol g⁻¹ h⁻¹. This indicates that the introduction of the gas‐liquid interface generated by microdroplets promotes the production of H₂O₂. Repeatability experiments demonstrate that this microdroplets system maintains a high H₂O₂ generation activity of 14.7 mmol g⁻¹ h⁻¹ after multiple repeated photocatalytic runs, confirming its excellent reaction stability (Figure S8). The H₂O₂ yield achieved by the DS‐OH‐COF‐based microdroplets photocatalytic system in pure water reached the level of currently reported high‐performance catalysts (Figure 3c and Table S3).

FIGURE 3.

Relationship between photocatalytic H₂O₂ yield and microdroplets volume under (a) air and (b) O₂ atmosphere. (c) Summary of photocatalytic H₂O₂ production rates of as‐prepared COF and other reported photocatalysts. (d) Relationship between different substrates and H₂O₂ yield in the microdroplets system.

Furthermore, a cumulative 6 h experiment (Figure S9) further highlights the advantages of the microdroplets system. The microdroplets system achieved a cumulative H₂O₂ yield as high as 75.25 mmol g⁻¹ over 6 h, which was approximately 7.3‐fold higher than that of the bulk system with the same catalyst (10.26 mmol g⁻¹). Additionally, we explored the kinetic performance of H₂O₂ decomposition in both microdroplets‐based and bulk system. Results showed that over a 10 h reaction period, the H₂O₂ concentration in the microdroplets decreased by only 11% compared to the original system, whereas the concentration reduction in the bulk system reached 46% (Figure S10). The consumption of H₂O₂ in the bulk system is significantly higher than in the microdroplets system. This confirms that the microdroplets environment not only significantly promotes the in‐situ generation of H₂O₂ but also effectively suppresses its reverse reaction, thereby ensuring excellent long‐term sustained H₂O₂ generation performance. This could be attributed to the fact that the H₂O₂ production rate constant (k _f) in the microdroplet system is significantly higher than that in the bulk system, while the decomposition rate constant (k _d) is substantially lower [29]. Subsequently, the apparent quantum yields (AQY) of the microdroplet system were measured to be 0.0148%, 0.0151%, 0.0077%, and 0.0058% at 420, 450, 520, and 550 nm, respectively (Figure S11). It is noted that in AQY measurement the total volume of microdroplets in the reactor is low, and the microdroplets with gaps cannot fully cover the substrate. These factors lead to the inadequate absorption of light and thus the relatively low AQY values in the microdroplet system.

Next, we adjusted a series of substrate materials to investigate the substrate's regulatory effect on H₂O₂ synthesis. The substrates selected included metallic materials such as Fe, Pt, Zn and Au, non‐metallic materials like zirconium oxide (ZrO₂), as well as hydrophobic glass. As shown in Figure 3d, the H₂O₂ yields over Au, Pt, Fe, were generally low, all below 3 mmol g⁻¹ h⁻¹. In contrast, the H₂O₂ yield over Zn and hydrophobic glass substrates exceeded 11 mmol g⁻¹ h⁻¹. To deeply explore the relationships among substrate contact angle, microdroplet geometric dimension, and H₂O₂ yield, we first measured the water contact angles of Au, Pt, Fe, ZrO₂, Zn, and hydrophobic glass, which were 91°, 86°, 65°, 48°, 102°, and 95°, respectively. Combined with Figure S12 and Table S2, for microdroplets with a volume of 1 µL, the contact angle, tri‐phase line perimeter, and specific surface area on different substrates have no correlation with the H₂O₂ yield. Specifically, although Au and hydrophobic glass possess similar contact angles and nearly identical tri‐phase line perimeter and specific surface area, their H₂O₂ yield differ greatly, with glass exhibiting a much higher H₂O₂ yield than Au. Similarly, the tri‐phase line perimeter of microdroplets on ZrO₂ is evidently larger than that on Zn, while its H₂O₂ yield is far lower. It is considered that if the gas‐liquid interface functions independently, H₂O₂ yield should be positively correlated with the specific surface area. Therefore, the results indicated unique effect of the gas‐liquid‐solid tri‐phase interface of the sessile microdroplets.

To further verify the critical promoting effect of the gas‐liquid‐solid tri‐phase interface on H₂O₂ production, we designed a control experiment using hanging microdroplets. In this system, microdroplets only form a gas‐liquid interface without a tri‐phase interface. The volume of hanging microdroplets was controlled at approximately 5 µL, and photocatalytic experiments were performed under identical initial conditions. As shown in Figure S13, the H₂O₂ yield in the hanging microdroplets system was approximately 1.5 mmol g⁻¹ h⁻¹ across three repeated experiments. This value is slightly higher than 0.86 mmol g⁻¹ h⁻¹ measured in the bulk system, yet much lower than 3.67 mmol g⁻¹ h⁻¹ obtained from sessile microdroplets with an identical volume. These results reveal that the gas‐liquid interface can moderately facilitate H₂O₂ production, whereas the tri‐phase interface further plays a decisive role in substantially boosting catalytic performance.

2.4. Mechanism of Photocatalytic H₂O₂ Generation in Microdroplets

To gain deeper insights into the underlying mechanism of photocatalytic H₂O₂ generation in the microdroplet system, further systematic investigations were conducted. The ORR selectivity of DS‐OH‐COF was further quantitatively evaluated using rotating disk electrode (RDE) technology. The electron transfer number (n) derived from RDE measurements was approximately 1.90 (Figure 4a), a value very close to that of the two‐electron transfer reaction (n = 2). This clearly demonstrates that the 2e⁻ ORR pathway is prevalent in the DS─OH─COF catalyst system [30]. Subsequently, quenchers for photogenerated electrons (e ⁻), superoxide radicals (•O₂ ⁻) and hydroxyl radicals (•OH) were introduced in the control experiments (Figure 4b). Addition of the electron‐trapping agent potassium bromate (KBrO₃, 5 mM) decreased the H₂O₂ yield by 66.4%. Similarly, the introduction of the superoxide anion radical quencher benzoquinone (BQ, 5 mM) markedly reduced the H₂O₂ yield by 61.7%. In contrast, the addition of the hydroxyl radical quencher tert‐butanol (TBA, 10 vol%) reduced the H₂O₂ yield by 41.1%. This demonstrates that the contributions of ORR and WOR to H₂O₂ production in this system are 61.7% and 38.3%, respectively [28]. We further performed electron paramagnetic resonance (EPR) spectroscopic analysis using 5,5‐dimethyl‐1‐pyrroline N‐oxide (DMPO) as the spin trap to identify possible intermediates during the photocatalytic synthesis of H₂O₂. As shown in Figure S14, DS‐OH‐COF exhibited the characteristic six‐line signal corresponding to DMPO‐•O₂ ⁻ adducts and the characteristic four‐line signal assigned to DMPO‐•OH adducts under light irradiation, whereas no distinct signals were detected under dark conditions. These results confirm the photocatalytic H₂O₂ production via both the ORR and WOR pathways [31, 32]. To investigate the WOR process on DS‐OH‐COF, we performed rotating ring‐disk electrode (RRDE) measurements under an argon atmosphere, with the rotating disk electrode potential scanned from 1.2 to 2.4 V (vs. Ag/AgCl). As shown in Figure 4c,d, no distinct reduction current was observed at the Pt ring electrode when a constant potential of −0.23 V (vs. Ag/AgCl) was applied, ruling out the possibility of O₂ generation via the 4e⁻ WOR pathway; conversely, a significant oxidation current emerged when the Pt ring potential was switched to an oxidative potential of +0.6 V (vs. Ag/AgCl), confirming the 2e⁻ WOR pathway on DS‐OH‐COF [33]. Collectively, the results from RDE and RRDE measurements verify that DS‐OH‐COF follows both 2e⁻ ORR and 2e⁻ WOR pathways during the photocatalytic production of H₂O₂.

FIGURE 4.

(a) Linear‐sweep RDE voltammograms of DS‐OH‐COF (insets: corresponding Koutecky‐Levich plots). (b) Photocatalytic H₂O₂ generation over DS‐OH‐COF in KBrO₃, p‐BQ, and TBA solution. (c) RRDE voltammograms of DS‐OH‐COF with a potential of ‐0.23 V vs. Ag/AgCl on Pt ring electrode to detect O₂. (d) RRDE voltammograms of DS‐OH‐COF with a potential of 0.60 V versus Ag/AgCl on Pt ring electrode to detect H₂O₂.

Next, we employed in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to monitor the dynamic evolution of reaction intermediates (Figure 5a). Upon light irradiation of the catalyst, three characteristic peaks corresponding to adsorbed molecular oxygen (O₂, ads), hydroxyl groups (–OH), and water/hydroxyl groups (H₂O/–OH) emerged at 1477, 1638, and 3337 cm⁻¹, respectively, indicating that oxygen and water can undergo strong adsorption on the catalyst surface. Meanwhile, the intensities of the characteristic peak at 967 cm⁻¹ attributed to the stretching vibration of O‐O bonds, the characteristic absorption peak at 1189 cm⁻¹ assigned to •O₂ ⁻, and the peak at 1265 cm⁻¹ corresponding to peroxide intermediates (•OOH) increased progressively with extending illumination time. The above spectral features indicate that oxygen‐containing intermediate species are continuously generated during the reaction, and the ORR follows a two‐step single‐electron transfer mechanism. Notably, the characteristic peak at 1099 cm⁻¹ corresponding to C‐OH groups and the absorption peak at 1408 cm⁻¹ ascribed to •OH emerged sequentially with the proceeding of the reaction. This result provides direct spectroscopic evidence for the occurrence of WOR [28]. In summary, the generation of H₂O₂ is jointly mediated by the synergistic effect of the dual pathway system involving ORR and WOR.

FIGURE 5.

(a) In situ FT‐IR spectra of the DS‐OH‐COF photocatalytic H₂O₂ release process in a microdroplet system. (b) The three regions of the microdroplets. (c) Under the conditions of air atmosphere and xenon light irradiation, the in situ Raman spectra of O–O stretching vibration of H₂O₂ in the interior of microdroplets and at the tri‐phase interface were measured at different reaction times. (d, e) Adsorption configurations of O₂ on various substrates.

To elucidate the contribution of the tri‐phase interface to the reaction mechanism, in situ Raman spectroscopy was employed to real‐time monitor the O–O stretching vibration (876 cm⁻¹) within the microdroplets and at tri‐phase interface under air atmosphere and Xenon lamp irradiation at different reaction times [34]. The tri‐phase interface is annotated at two boundary positions in the microdroplets cross‐sectional diagram, representing the contact area between the microdroplets, air, and the solid substrate (Figure 5b). The interior region occupies the central portion of the schematic, corresponding to the area where the microdroplets contact air (gas‐liquid interface). As shown in Figure 5c, the O–O peak intensity both within the microdroplets and at the tri‐phase interface significantly increased with prolonged illumination, indicating continuous H₂O₂ accumulation in these regions. Notably, the signal intensity at the tri‐phase interface was consistently significantly higher than that in the interior regions. This indicates that the H₂O₂ generation rate is faster at the gas‐liquid‐solid tri‐phase interface.

To theoretically validate the correlation between substrate‐O₂ interaction and photocatalytic H₂O₂ production efficiency, systematic DFT calculations were performed focusing on O₂ adsorption configurations over various substrates. As shown in Figure 5d,e, the optimized results revealed a strict structure‐activity relationship between O₂ adsorption mode and H₂O₂ yield. For Au, Pt, Fe substrates with low H₂O₂ yields (below 3 mmol g⁻¹ h⁻¹) show strong adsorption interactions with O₂, belonging to the Yeager‐type adsorption [35]. During the adsorption process, the active sites of the substrates transfer electrons to the p orbital of O₂, fully occupying its π* antibonding orbital. This leads to the formation of the peroxide intermediate (•O₂ ²⁻) and the cleavage of the O‐O bond, accompanied by the generation of •OH. Due to the high energy barrier for the coupling of •OH, the synthetic pathway of H₂O₂ is significantly hindered. Meanwhile, the oxygen reduction reaction of O₂ is more prone to follow the 4e⁻ pathway, with H₂O as the primary product, thereby remarkably inhibiting H₂O₂ generation. The strong adsorption‐induced O‐O bond cleavage directly offsets the interfacial mass transfer advantage of the microdroplet system, resulting in low H₂O₂ yield. In contrast, for ZrO₂, Zn, and hydrophobic glass substrates (high‐yield substrates, over 11 mmol g⁻¹ h⁻¹), they exhibit weak adsorption toward O₂, with the adsorption mode tending to be Pauling‐type adsorption. This adsorption pattern enables partial filling of the π* antibonding orbital of O₂, facilitating the formation of the •O₂ ⁻ while keeping the O‐O bond intact without cleavage during the reaction. Subsequently, the protonation of •O₂ ⁻ generates the hydroperoxyl radical (•OOH), which is a key intermediate in the 2e⁻ ORR. This ultimately drives the ORR to proceed via the two‐electron pathway and thus realizes efficient H₂O₂ production. This weak adsorption characteristic is consistent with the interfacial advantage of microdroplets, which promotes O₂ mass transfer while maintaining the structural integrity of O₂ molecules. The detailed theoretical calculation parameters are shown in Table S4. These DFT results are in excellent agreement with experimental observations, providing atomic‐level insights into the regulatory role of substrate properties in O₂ activation. The calculations confirm that tuning O₂ adsorption mode via rational substrate selection is a key strategy to steer the ORR pathway toward efficient H₂O₂ synthesis, thus offering a theoretical basis for the design of high‐performance tri‐phase interface catalytic systems.

2.5. Practical Applications of Photocatalytic H₂O₂ Generation in Microdroplets

To evaluate the application prospects of H₂O₂ production in microdroplets systems, we first explored the universality of this method by employing the other two COF photocatalysts (DS‐OHOMe‐COF and DS‐OMe‐COF). They also exhibited an increase in H₂O₂ production with the reduction of microdroplets’ volume, consistent with the performances of DS‐OH‐COF (Figure S15). This verifies the universal feasibility of combining microdroplets and COFs in photocatalysis. Furthermore, we investigated the impact of different water sources (Figure 6a). The microdroplets system efficiently produces H₂O₂ across all water sources, demonstrating excellent universality and application potential. Further, to evaluate the feasibility of this microdroplets photocatalytic system for larger‐scale applications, the reaction system was scaled up to 18,000 cm³ for H₂O₂ synthesis in both the microdroplets system and conventional bulk system. As shown in Figure S16, under identical Xenon lamp irradiation intensity, the microdroplets system produced an H₂O₂ concentration of 183.4 µM, significantly higher than the 46.9 µM achieved by the bulk system (Figure S17). The highly efficient in situ generated H₂O₂ in this system was directly utilized for the degradation of a typical pollutant (methyl orange, MO) to validate its practical application potential [36]. Degradation kinetic analysis (Figure 6b,c) clearly demonstrates that the microdroplets system exhibits degradation efficiency far surpassing that of the bulk system. Specifically, the microdroplets system enabled the complete degradation of methyl orange within merely 300 s, with its characteristic visible absorption peaks vanishing entirely and the solution undergoing a distinct color transition from dark‐colored to pale before eventually becoming colorless. In contrast, the bulk system required twice as long (600 s) to achieve an equivalent degradation efficiency (Figure 6d). This further confirms the significant advantages of the microdroplets photocatalytic pathway in efficiently generating H₂O₂ and outstanding pollutant degradation performance.

FIGURE 6.

(a) H₂O₂ yield from microdroplets system in different water sources. MO was selected as a representative organic dye for degradation experiments conducted in two systems using the Fenton reaction method. (b,c) Changes in absorbance over time, (d) Corresponding color changes.

3. Conclusions

In conclusion, this work successfully demonstrates the feasibility of efficiently driving photocatalytic H₂O₂ synthesis via microdroplets size regulation. A microscale reaction environment was constructed using functionalized COFs as the catalyst. The tri‐phase interface at the microdroplets boundary effectively overcomes the inherent limitation of insufficient oxygen mass transfer in bulk systems. Furthermore, it was demonstrated that the photocatalyst generates H₂O₂ via the dual pathway involving both ORR and WOR. Experimental results clearly verify that the gas‐liquid interface of microdroplets remarkably enhances oxygen dissolution and transport. Meanwhile, the modified metal substrate directly modulates H₂O₂ generation by regulating the gas‐liquid‐solid tri‐phase interface. This strategy based on tri‐phase interface achieves an order‐of‐magnitude improvement in both the synthesis rate (particularly in small‐sized microdroplets) and cumulative yield of the target product (75.25 mmol g⁻¹), which represent 12.9‐fold and 7.3‐fold enhancements compared to bulk system, respectively. Its direct application in efficient pollutant degradation confirms significant practical potential of tri‐phase interface in photocatalytic H₂O₂ synthesis. The tri‐phase interface mechanism proposed in this work provides a crucial theoretical basis and technical approach for the development of a green, high‐throughput dual‐path H₂O₂ synthesis method. Notably, compared with bulk systems, the microdroplets system in this work has limited total volume and is hard to realize large‐scale preparation of H₂O₂. In future research, microdroplet printers, nebulizers, and microfluidic devices will be used to construct microdroplets arrays to expand reaction interfaces and total volume so as to promote the practical application of this interface strategy in large‐scale green synthesis of H₂O₂.

Author Contributions

Yuchun Xu: methodology, investigation, writing – original draft, writing – review and editing, formal analysis, data curation, visualization, software. Wanying Xie: writing – review and editing, software, investigation, data curation, writing – original draft, formal analysis. Ning Sun: investigation, writing – review and editing, data curation. Xiuqin Ci: investigation, writing – review and editing. Yunjie Lang: writing – review and editing, investigation, software. Changjiang Yang: writing – review and editing, investigation. Tianyi Liu: investigation, writing – review and editing. Li Yang: funding acquisition, writing – review and editing, software, investigation, methodology. Wei‐Qiao Deng: funding acquisition, writing – review and editing, supervision, project administration, resources. Zhen Li: conceptualization, writing – review and editing, supervision, project administration, funding acquisition, validation, resources.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie73237‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this manuscript.