Amorphous Aluminum Oxide Clusters Regulating Oxygen Reduction for Photocatalytic H2 O2 Production over Carbon Nitride

Amorphous Aluminum Oxide Clusters Regulating Oxygen Reduction for Photocatalytic H2 O2 Production over Carbon Nitride | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Amorphous Aluminum Oxide Clusters Regulating Oxygen Reduction for Photocatalytic H 2 O 2 Production over Carbon Nitride Hiba Elmansoura, Donghui Wang, Ganglu Taojin, Feng Chen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7444053/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 25 Sep, 2025 Read the published version in Research on Chemical Intermediates → Version 1 posted 9 You are reading this latest preprint version Abstract The rational design of catalysts for photocatalytic H 2 O 2 production remains a major challenge, as most photocatalysts for H 2 O 2 production also tend to decompose H 2 O 2 . Here, density functional theory (DFT) calculations reveal that amorphous Al 2 O 3 clusters anchored on crystalline carbon nitride (CCN) provide efficient oxygen adsorption sites, facilitate electron transfer, and stabilize key *OOH intermediates. Guided by these insights, CCN composites loaded with Al 2 O 3 clusters (CCN-Al-x) were synthesized using an ionothermal treatment strategy. Experimental studies confirm that the introduction of Al 2 O 3 clusters enhances charge separation, suppresses H 2 O 2 decomposition and improves the selectivity of the 2e − oxygen reduction reaction (ORR). The optimized CCN-Al-2 achieves an H 2 O 2 production rate of 50.2 mmol g − 1 h − 1 , with an apparent quantum yield (AQY) of 21.6% under 420 nm irradiation. This work highlights the critical impact of Al 2 O 3 clusters modification on CCN, providing new opportunities for efficient and selective photocatalytic H 2 O 2 production. Photocatalysis Hydrogen Peroxide Production Crystalline Carbon Nitride Amorphous Aluminum Oxide Clusters Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Highlights Unveil by DFT calculations the end-on adsorption of O 2 on Al 2 O 3 clusters and the promoted selectivity toward the 2e - ORR pathway. Anchor Al 2 O 3 clusters uniformly on crystalline carbon nitride through an ionothermal strategy. Enhance charge separation and suppress H 2 O 2 decomposition via the effect of Al 2 O 3 clusters loading. Achieve outstanding performance, with CCN-Al-2 delivering 50.2 mmol g -1 h -1 H 2 O 2 production and an AQY of 21.6% at 420 nm. 1. Introduction Currently, over 95% of the total H 2 O 2 production in industry is achieved through the anthraquinone oxidation process, which is characterized by high energy consumption and the release of toxic byproducts. [ 1 , 2 ] In contrast, the solar-driven water oxidation or oxygen reduction in the presence of water and oxygen or air provides a promising avenue for environmentally friendly H 2 O 2 production under mild and safe conditions. [ 3 , 4 ] To realize high selectivity and a rapid rate of H 2 O 2 production, it is essential to promote the 2e − ORR (Eq. 1) [ 5 , 6 ] or the 2e − water oxidation reaction (WOR) (Eq. 2). [ 7 ] However, H 2 O 2 produced by photocatalytic 2e − WOR is susceptible to decomposition at high oxidation potentials (1.76 V vs. RHE). [ 8 ] Therefore, the key to improving the rate of H 2 O 2 production lies in facilitating the 2e − ORR reaction while suppressing the 4e − ORR (Eq. 3). Graphitic carbon nitride, as a metal-free, non-toxic, and chemically stable material, is particularly intriguing due to its excellent potential in H 2 O 2 generation. However, its application in photocatalytic H 2 O 2 production is limited by its relatively low surface area, rapid recombination of photogenerated electron-hole pairs, and challenges related to achieving selectivity in the 2e − ORR reaction. O 2 + 2H + +2e − →H 2 O 2 (0.695 V vs. NHE) (1) 2HO + 2h →HO + 2H (1.76 V vs. NHE) (2) O 2 + 4H + +4e − →2H 2 O(1.23 V vs. NHE) (3) In order to address these drawbacks of carbon nitride, numerous material modification strategies have been developed over the past few decades. These strategies include doping, constructing heterojunctions, modulating crystallinity, and altering morphology.[ 9 – 12 ] Among these strategies, the crystallinity of carbon nitride can be adjusted through ionothermal treatment, promoting the periodic extension of hexazine/triazine units in-plane. High crystallinity significantly enhances charge carrier mobility. [ 11 ] On the other hand, introducing co-catalysts onto the surface of carbon nitride is an effective strategy to enhance photocatalytic performance by improving charge separation capability. Various noble metal co-catalysts, such as Pt, Pd, and Au, have been demonstrated to promote catalytic performance.[ 13 – 15 ] Additionally, co-catalysts consisting of abundant and low-cost elements like MoS 2 , Ni, or CoP offer efficient and high-rate photocatalytic H 2 O 2 production.[ 16 – 18 ] However, conventional co-catalysts mentioned above typically contain transition metals, which can facilitate the decomposition of H 2 O 2 at high concentrations. Therefore, they can enhance H 2 O 2 production efficiency at low H 2 O 2 concentrations, but they can have a counterproductive effect in photocatalytic systems that are capable of efficiently producing hydrogen peroxide.[ 19 ] Previous reports suggest that aluminum, as a p-block metal, can inhibit the decomposition of H 2 O 2 on carbon nitride.[ 20 ] As a compound of aluminum, Al 2 O 3 is commonly used as an insulating material. Due to its excellent stability, dispersion, higher surface area, and outstanding adsorption properties, Al 2 O 3 has gained favor among many researchers.[ 21 , 22 ] Moreover, a thin amorphous Al 2 O 3 layer on metal oxide semiconductors has been proved to suppress charge recombination. Coating TiO 2 with a thin layer of Al 2 O 3 reduces charge recombination rates[ 23 ], passivates surface trap states, and thereby enhances charge transfer performance.[ 24 ] Amorphous Al 2 O 3 -modified SnO 2 also demonstrates increased electron lifetimes.[ 25 ] Water readily dissociates on the Al 2 O 3 surface to form adsorbed –OH groups, which facilitate proton transfer via hydrogen bonding.[ 26 ] In summary, introducing an appropriate amorphous Al 2 O 3 clusters onto the photocatalyst can suppress H 2 O 2 degradation while offering excellent adsorption properties and charge recombination inhibition. In this work, we combine theoretical calculations and experimental studies to investigate the effects of Al 2 O 3 clusters on photocatalytic H 2 O 2 production. DFT results reveal that Al 2 O 3 clusters anchored on CCN lower the adsorption energy of O 2 , favor an end-on adsorption mode, and promote the formation of *OOH intermediates, thereby enhancing the selectivity of the 2e − ORR. Guided by these insights, CCN-Al-x were synthesized using an ionothermal treatment strategy. The optimized CCN-Al-2 exhibits superior photocatalytic activity and selectivity, achieving a high H 2 O 2 production rate and AQY. This study not only clarifies how Al 2 O 3 clusters regulate photocatalysis but also establishes a rational pathway to design CN for efficient H 2 O 2 production. 2. Experimental section 2.1 Photocatalyst preparation 10.0 g melamine was placed in a 30 ml ceramic crucible with a lid and subsequently calcined for 4.0 h at 550°C in a muffle furnace at a heating rate of 5°C/min. After cooling to room temperature, the intermediate product (referred to as CN) was collected for further use. Subsequently, 0.5 g of CN was combined with a predetermined amount of Al(OH) 3 and added to 5 ml of deionized water. After 30 minutes of ultrasound treatment, the sample was rapidly frozen using liquid nitrogen, followed by freeze-drying for 24 hours. The resulting powder was mixed with 1.0 g of lithium chloride (LiCl) and thoroughly ground in an agate mortar. The sample was then transferred to a quartz boat and heated at a rate of 5°C/min under a nitrogen atmosphere to 550°C for 2 hours. The obtained sample was washed five times with deionized water and ethanol, and subsequently dried overnight in an oven at 60°C. The resulting catalyst was denoted as CCN-Al-x (where x represents the mass ratio of Al element to CN). Additionally, a crystalline carbon nitride sample without the addition of Al(OH) 3 was prepared following the aforementioned procedure and named CCN. Chemical materials, sample characterization, photocatalytic performance measurement, AQY measurement, photo(electro)chemical measurement and computational method were supplemented in the Supplementary Material. 3. Results and discussion 3.1 Theoretical prediction of efficient photocatalyst To predict the ORR activity of CCN-Al, DFT calculations were performed. The CCN-Al models were optimized by DFT, and the most stable structure was selected (Fig. S1 ). Previous studies have indicated that the activity of 2e − ORR for H 2 O 2 production crucially depends on the formation of *OOH and subsequent hydrogenation steps.[ 27 , 28 ] However, the adsorption of O 2 is a prerequisite for the formation of *OOH intermediates on the catalyst surface in the 2e − ORR pathway. Therefore, considering the adsorption of O 2 on the CCN surface in the 2e − ORR pathway is particularly important. The adsorption energies of O 2 on CCN, Al 2 O 3 clusters, and CCN-Al were calculated (Fig. 1 a-c). As shown in Fig. 1 a, on the model of single-layer CCN, O 2 molecules preferentially adsorb at the hexazine unit within the triazine ring, consistent with previous reports. [ 4 , 29 ] In the models of Al 2 O 3 clusters and CCN-Al, O 2 molecules adsorb end-on to aluminum atoms of Al 2 O 3 clusters. Given previous reports, this end-on adsorption mode is favorable for avoiding O-O bond cleavage, thus reducing the selectivity of 4e − ORR.[ 7 , 8 ] The adsorption free energy of O 2 on Al 2 O 3 clusters is 0.08 eV, significantly lower than that on CCN, which is 0.79 eV, indicating that Al 2 O 3 clusters can promote O 2 adsorption. After introducing Al 2 O 3 clusters onto CCN, the adsorption energy of O 2 decreases to 0.15 eV, significantly enhancing the adsorption capacity for O 2 . Differential charge density and Bader charge analysis for the adsorption models reveal that after loading Al 2 O 3 clusters onto CCN, more electrons are transferred to O 2 (∆q = 0.40 e), which is much greater than the electron transfer from CCN to O 2 (∆q = 0.19 e). Additionally, the O-O bond length of adsorbed oxygen on CCN-Al is longer than that on Al 2 O 3 clusters and CCN (Fig. S2). Combining the above analysis, it can be concluded that CCN-Al is more capable of adsorbing and activating O 2 molecules. Furthermore, the free energy of 2e − ORR process to produce H 2 O 2 on CCN, Al 2 O 3 , and CCN-Al was also calculated (Fig. 1 e). Comparing the free energy changes along the entire reaction pathway in Fig. 1 d, the hydrogenation of O 2 on CCN exhibits a positive reaction free energy, indicating that it is thermodynamically less feasible. On Al 2 O 3 clusters, the reaction free energy for the formation of *OOH is -1.41 eV, indicating that Al 2 O 3 clusters can stabilize *OOH. Relative to CCN, the free energy for the formation of *OOH on CCN-Al decreases to -1.11 eV, indicating that the interaction between Al 2 O 3 clusters and CCN promotes the formation of *OOH intermediates. In the ORR process, H 2 O would be also produced via a 4e − pathway, which could be a significant constraint on H 2 O 2 production. As shown in Fig. 1 e, the free energy changes for the 2e − ORR and 4e − ORR reaction pathways of the CCN-Al catalyst were studied to confirm the preferred reaction pathway. Clearly, in the 4e − ORR process, the reaction free energy from *OOH to *O is positive, preventing the progress of 4e − ORR, whereas the 2e − ORR reaction processes are all exothermic reactions. This indicates that CCN-Al is more inclined toward the 2e − ORR reaction pathway. Collectively, the theoretical results from these calculations suggest that Al 2 O 3 clusters, as a non-photocatalytic material, can theoretically serve as a co-catalyst to enhance the activity and selectivity of CN for oxygen reduction and H 2 O 2 production. 3.2 Morphology and Structure The CCN-Al-x was prepared through adsorption of Al(OH) 3 onto bulk carbon nitride, followed by an alkali metal ion thermal treatment. Heat treatment under the coexistence of alkali metal ions is conducive to obtaining CN with high in-plane polymerization.[ 30 ] Firstly, XRD analysis was performed on the prepared catalysts. As shown in Fig. 1 a, the CN exhibited two characteristic peaks at 13.1° and 27.9°, attributed to the in-plane repeating units of heptazine and the interlayer stacking of conjugated aromatic systems, respectively, confirming the presence of the layered structure[ 31 ]. However, significant structural changes were observed upon mixed calcination with LiCl. Compared to CN, the (100) peak of CCN and CCN-Al-x decreased from 13.1° to 8.2°, indicating an increase in the spacing between in-plane repeating units from 0.675 nm to 1.05 nm, significantly enhancing the crystallinity of CN.[ 32 ] The (002) peak of CCN and CCN-Al-x significantly decreased, indicating a reduction in the number of stacked layers of carbon nitride. SEM images of the samples (Fig. S3) showed that under the influence of LiCl, CCN exhibited a flake-like structure, and the introduction of Al 2 O 3 did not lead to significant morphological changes. Further testing of the catalysts was conducted using FT-IR (Fig. 2b). Apart from CN, the other catalysts exhibited an absorption peak at 2170 cm⁻¹, attributed to the asymmetric stretching vibration of cyanide groups (-CN), indicating that ionothermal treatment induces cyanide defects at the edges of carbon nitride.[ 33 ] Both XRD and FT-IR spectra suggest that the introduction of Al 2 O 3 has little effect on the CCN.</p X-ray photoelectron spectroscopy (XPS) was employed to investigate the catalysts' chemical composition and electron transfer behaviors. The Al content of the prepared samples determined by XPS was significantly higher than that measured by ICP-AES (Fig. S4), indicating that a substantial portion of Al elements resides on the catalyst's surface. The survey scans (Fig. 3 a) demonstrated the presence of O, C, and N elements on the surface of all catalysts. Notably, CCN-Al-2 exhibited peaks corresponding to Al 2s and Al 2p, confirming the successful incorporation of Al elements into the catalyst, as shown in Fig. 3 a. The C 1s spectra, depicted in Fig. 3 c, exhibited similar features across all catalysts, with peaks at 284.8 eV, 286.6 eV, and 288.2 eV attributed to sp 2 -hybridized C in C = C, C − NH x , and N = C-N moieties of contaminating carbon.[ 34 ] The N 1 s spectra in Fig. 3 b revealed distinctive peaks for CN, corresponding to C = N-C, N-(C) 3 , and N-H.[ 35 ] However, the N-H peak originating from terminal amino groups of the CN framework vanished in the N 1 s spectra of CCN and CCN-Al-2, implying that the introduction of LiCl significantly facilitated the condensation of reactants. Furthermore, relative to CN, the N–(C) 3 peak in CCN and CCN-Al-2 appeared at noticeably higher binding energies. Comparative analysis of the Al 2 p peak positions in Fig. 3 d showed that, compared to the Al 2 p peak of pure Al 2 O 3 (74.4 eV), the Al 2 p peak of CCN-Al shifted by 0.3 eV towards higher binding energy. Similarly, compared to the N-(C) 3 N 1 s peak of CCN, the corresponding peak of CCN-Al shifted by 0.2 eV towards higher binding energy, indicating electron transfer from the surface Al 2 O 3 to CCN. To further validate the charge transfer properties between Al 2 O 3 and CCN, differential charge density calculations were performed on the CCN-Al model, as shown in Fig. 3 e. The electrons were found to transfer from the surface Al 2 O 3 clusters to CCN (q = 0.07 e), consistent with the characterization results obtained from XPS. Such electron transfer establishes an electrostatic field from Al 2 O 3 to CCN, thereby facilitating the transfer of photogenerated electrons from CCN to the Al 2 O 3 clusters. Moreover, the catalysts were subjected to TEM characterization, revealing lattice fringes with d-spacings of 0.34 nm (Fig. 4 a) and 1.05 nm (Fig. 4 c), attributed to the (002) and (100) crystal planes of polyheptazine imide, respectively.[ 36 , 37 ] Lattice fringes with d-spacings of 0.28 nm (Fig. 4 b) and 0.42 nm (Fig. 4 d) were attributed to the (210) and (110) crystal planes of polytriazine imide, respectively. [ 38 ]Notably, the diffraction peaks of polytriazine imide were not observed in XRD, likely due to its lower content. In Fig. 4 e, lattice fringes of CCN are observed within the red box, while spindle-like features appear in the white box. Figure 4 f and the corresponding Fourier transform image in the upper right corner indicate that these features are amorphous. HADDF-STEM (Fig. 4 g) and elemental mapping (Fig.s h-k) of the same region further confirm that the amorphous phase corresponds to Al 2 O 3 , which is also found to be uniformly distributed across the CCN matrix. Combined TEM and XRD analyses reveal that CCN-Al-x is a heterostructure formed by amorphous Al 2 O 3 clusters anchored onto CCN. 3.3 Catalytic Activity Evaluation and Stability Under the conditions of ethanol serving as a proton source, the catalytic activity for H 2 O 2 production using different photocatalysts was assessed under white light LED irradiation (See Fig. S5 for the white light LED spectrum). As depicted in Fig. 5 a, the catalytic activities of CCN and CCN-Al-x were significantly higher than that of CN, highlighting that the enhancement in crystallinity greatly improved the efficiency of visible-light-driven H 2 O 2 synthesis. Furthermore, the introduction of Al 2 O 3 further augmented the activity of crystalline carbon nitride. Among these, CCN-Al-2 exhibited the highest activity, achieving a H 2 O 2 production rate of 50.2 mmol g − 1 h − 1 , which is 185.9 times and 1.6 times higher than CN and CCN, respectively. Figure 5 b illustrates the influence of varying catalyst masses on the rate of H 2 O 2 production. It can be observed that as the catalyst mass increases from 10 mg to 20 mg, the H 2 O 2 yield increases by 41.7%. Further increasing the mass from 20 mg to 30 mg results in a 12.7% increase in yield, and from 30 mg to 50 mg, there is a 3.7% increase in yield. Therefore, 20 mg of catalyst was chosen for the experiment to determine the AQY for H 2 O 2 production. (Except for the AQY experiment, all other photocatalytic activity experiments used 10 mg of catalyst.) The stability of CCN-Al-2 was evaluated through four consecutive cycles of photocatalytic H 2 O 2 production, totaling 12 hours. As shown in Fig. 5 c, after four cycles, the catalytic activity of CCN-Al-2 declined to 79% of its initial value. Additionally, the trend of AQY values for CCN-Al-2 with changing absorbance (Fig. 5 d) corresponded well with the UV-vis diffuse reflectance spectroscopy (DRS) spectrum. An AQY value of 21.6% was measured for CCN-Al-2 under 420 nm light irradiation. The in-situ decomposition rate of H 2 O 2 on the catalysts was also studied and is presented in Fig. 5 e. The H 2 O 2 decomposition rate followed the sequence of CN, CCN, and CCN-Al-2. The performance of CCN-Al-2 was compared with that of recently reported g -C 3 N 4 -based materials in photocatalytic H 2 O 2 production (Fig. 5 f and Table S1 ). The results indicate that, under identical conditions, CCN-Al-2 exhibits a superior AQY for H 2 O 2 production compared to most photocatalysts, highlighting its high activity as a photocatalyst for H 2 O 2 generation. 3.3 Charge Carrier Dynamics and Optical Properties The efficient migration and separation of photo-generated electrons and holes play a crucial role in photocatalytic processes. Therefore, the catalysts were characterized using PL spectroscopy to investigate the recombination rates of free charge carriers. As shown in Fig. 6 a, the PL intensity of CCN and CCN-Al-x is significantly lower compared to CN, indicating that the improved crystallinity significantly suppresses the recombination of electrons and holes. The PL intensity of CCN-Al-x is lower than that of CCN, suggesting that the loading of Al 2 O 3 clusters further prevents charge carrier recombination. Furthermore, EIS (Fig. 6 b) and photocurrent response (Fig. 6 c) further demonstrate the impact of crystallinity and loaded Al 2 O 3 clusters on charge separation. CCN-Al-x shows lower photocurrent density and electrochemical impedance. However, with an increase in Al 2 O 3 content, CCN-Al-x exhibits higher impedance and lower photocurrent. The results of EIS and photocurrent response indicate that an appropriate loading of Al 2 O 3 clusters promotes catalyst charge separation, but excess Al 2 O 3 clusters increases the impedance of the catalyst and reduces photocurrent density. Based on the fitted fluorescence decay components (Table S2), the τ values for CN, CCN, and CCN-Al-2 are calculated to be 4.36, 1.79, and 1.66 ns, respectively (Fig. 5 d). Consistent with previous results, the significantly reduced average PL lifetime of CCN and CCN-Al-2 samples implies enhanced charge carrier separation. Combining optical and photoelectrochemical results, the high crystallinity of CCN-Al and the loading of Al 2 O 3 clusters can broaden the light absorption range and promote the separation and transfer of photo-generated charge carriers, contributing to an increased yield in the photocatalytic production of hydrogen peroxide. The optical properties of the samples are shown in Fig. S6. UV–vis DRS (Fig. S6a,b) shows that enhanced crystallinity and Al 2 O 3 clusters loading narrowed the bandgap. VB-XPS (Fig. S6c) reveals that CCN-Al has a higher VB potential (1.86 eV) than CN and CCN (1.6 eV). The calculated band structures (Fig. S6d) indicate that CCN-Al has a CB position closer to the O 2 / • O 2 − potential. The expanded optical absorption and favorable band alignment can promote photocatalytic H 2 O 2 production activity. 3.4 Mechanism of Photocatalytic H 2 O 2 Evolution The selectivity of the oxygen reduction reaction (ORR) is crucial for the production of H 2 O 2 . Electrochemical tests using a rotating ring-disk electrode (RRDE) were conducted to investigate the impact of crystallinity and Al 2 O 3 clusters loading on the selectivity of electron transfer during O 2 reduction. The disk current of the RRDE originates from the reduction of oxygen (O 2 + 2H + +2e − → H 2 O 2 or O 2 + 4H + +4e − → 2H 2 O), while the ring current arises from the oxidation of H 2 O 2 to O 2 and two protons (H 2 O 2 →O 2 + 2H + + 2e − ). The selectivity of H 2 O 2 production on CN, CCN, and CCN-Al-2 was monitored in O 2 -saturated 0.1 M KOH electrolyte. As the potential was lowered from − 0.25 V (vs. Ag/AgCl), the reduction disk currents of CN, CCN, and CCN-Al-2 gradually increased (Fig. 7 a, bottom). Simultaneously, the generated H 2 O 2 at the disk electrode could rapidly diffuse to the ring electrode and undergo further oxidation, leading to a positive oxidation current. In Fig. 7 a (top), the trends in ring currents were similar for all three catalysts, but the intensity of the ring current increased in the order CN < CCN < CCN-Al-2, indicating that CCN-Al-2 produced the most H 2 O 2 . The selectivity of H 2 O 2 production and the average number of transferred electrons (n) within the potential range of -1 to -0.5 V (vs. Ag/AgCl) are presented in Fig. 7 b and Fig. 7 c. Clearly, CCN-Al-2 exhibited a transfer of electrons closer to 2 under the same conditions, indicating higher selectivity for H 2 O 2 . Specifically, CCN-Al-2 transferred 2.56 electrons (-0.5 V vs. Ag/AgCl), achieving a H 2 O 2 selectivity of 72.0%. Under the same potential, CN transferred 2.94 electrons with an H 2 O 2 selectivity of only 53.2%. The combination of RRDE and photocatalytic H 2 O 2 performance tests demonstrated that the improvement in crystallinity and the loading of Al 2 O 3 together enhanced the 2e − ORR selectivity of the catalyst, effectively promoting the surface reaction to generate H 2 O 2 . To further understand the mechanism of H 2 O 2 production via photocatalysis with CCN-Al-2, experiments capturing active species were conducted using AgNO 3 and benzoquinone (BQ) as scavengers for e − and • O 2 − . As shown in Fig. 7 d, when O 2 was replaced with N 2 in the reaction solution, almost no H 2 O 2 was detected, indicating that O 2 is a necessary reactant. The addition of AgNO 3 to the reaction system led to a rapid decrease in H 2 O 2 yield, suggesting that H 2 O 2 formation occurs via the electron reduction of O 2 . Notably, the addition of BQ to the system resulted in a very low H 2 O 2 yield, indicating that • O 2 − is an important intermediate for H 2 O 2 generation. Furthermore, electron paramagnetic resonance (EPR) capture experiments with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were used to confirm the presence of O 2 (Fig. 7 e). As shown in Fig. S7, no • O 2 − signals were observed in the three samples in the dark. After opening the visible light for 5 minutes, • O 2 − signals were observed in all samples except for CN, indicating that O 2 is the key radical in this reaction. Additionally, the • O 2 − signal of CCN-Al-2 was higher than that of CCN, suggesting that CCN-Al-2 can produce • O 2 − more rapidly, consistent with the trend in catalytic H 2 O 2 production rates observed earlier. Therefore, based on the above tests, the photocatalytic mechanism of CCN-Al-2 involves 2e − ORR for H 2 O 2 production. Upon visible-light irradiation, CCN-Al absorbs photons to generate electron-hole pairs, which are subsequently transferred to the surface-anchored Al 2 O 3 clusters. The Al 2 O 3 clusters not only adsorb and activate O 2 but also facilitate rapid proton transfer between surface oxygen atoms and adsorbed hydroxyl groups, thereby promoting the formation of *OOH intermediates and ultimately H 2 O 2 . 4. Conclusions In conclusion, this study demonstrates that the incorporation of amorphous Al 2 O 3 clusters on CN plays decisive roles in boosting photocatalytic H 2 O 2 production. Upon visible-light excitation, photogenerated charge carriers in CCN-Al are efficiently transferred to the surface-anchored Al 2 O 3 clusters, which not only adsorb and activate O 2 but also facilitate proton transfer via surface hydroxyl groups. It promotes the formation of *OOH intermediates while suppressing competing 4e − pathways, thereby enhancing the activity and selectivity of the 2e − ORR. At the same time, the decomposition of H 2 O 2 is also inhibited. As a result, CCN-Al-2 achieves a significantly improved H 2 O 2 generation rate of 50.2 mmol g − 1 h − 1 , with an apparent quantum yield of 21.6%. These findings provide both mechanistic insight and practical guidance for the rational design of non-transition-metal photocatalyst, offering a promising strategy toward efficient and selective H 2 O 2 production. Declarations Declarations The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contribution H. Elmansour contributes to the data curation, methodology, and writing – original draft. D. Wang contributes to the software and visualization. G. Taojin contributes to the Writing – review & editing. F. Chen contributes to the funding acquisition and validation. All authors discussed the results and contributed to the final manuscript. Acknowledgement This work was supported by National Key R&D Program of China No.2024YFA1211004. Additional support was provided by the Feringa Nobel Prize Scientist Joint Research Center at East China University of Science and Technology. Data Availability Data will be made available on request. References J.M. Campos-Martin, G. Blanco-Brieva, J.L. Fierro, Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process, Angew. Chem. Int. Ed. 45(42) (2006) 6962–84. https://doi.org/10.1002/anie.200503779 . Y. Yi, L. Wang, G. Li, H. Guo, A review on research progress in the direct synthesis of hydrogen peroxide from hydrogen and oxygen: noble-metal catalytic method, fuel-cell method and plasma method, Catal. Sci. Technol. 6(6) (2016) 1593–1610. https://doi.org/10.1039/c5cy01567g . Y. Wu, J. Chen, H. Che, X. Gao, Y. Ao, P. 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The isosurface value is 0.003 eV Å\u003csup\u003e−3\u003c/sup\u003e and the yellow represents the electron accumulation area, and the light blue is the electron dissipation area.\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/a3bfad60c2b60e3fab9f3b30.jpg"},{"id":91347546,"identity":"4822d46c-6c57-42b4-a801-5f84c4dd84a7","added_by":"auto","created_at":"2025-09-15 14:07:54","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":24047054,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c) HRTEM images of CCN-Al-2 (Inset shows the magnified area); (d) HRTEM image of the red box area in Fig. e (Inset shows the magnified area); (e) TEM image of CCN-Al-2; (f) HRTEM image of the white box area in Fig. e (Inset is FFT pattern). (g) HAADF-STEM image and (h–k) corresponding elemental mappings of Al, O, N and C distribution in CCN-Al-2.\u003c/p\u003e","description":"","filename":"Figure4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/e919e6c51d36f57fc8a71266.jpg"},{"id":91346833,"identity":"ceb28761-8bec-4bdd-8878-d9e5d8931cc7","added_by":"auto","created_at":"2025-09-15 13:59:54","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2398737,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rate of all samples. (b) Photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rates over different amounts (10, 20, 50, 100 mg) of CCN-Al-2. (c) Cycling photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production activity over CCN-Al-2. (d) Wavelength dependent photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rate over CCN-Al-2. (e) The photocatalytic decomposition of high concentration H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (0.1 M) on CN, CCN and CCN-Al in the O\u003csub\u003e2\u003c/sub\u003e-saturated solution with 10 vol% EtOH under visible light. (f) Comparison of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production AQY between CCN-Al-2 and recently reported \u003cem\u003eg\u003c/em\u003e-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e-based photocatalysts[\u003ca href=\"#_ENREF_4\" title=\"Chen, 2021 #13\"\u003e4\u003c/a\u003e, \u003ca href=\"#_ENREF_16\" title=\"Du, 2022 #65\"\u003e16\u003c/a\u003e, \u003ca href=\"#_ENREF_39\" title=\"Zhang, 2022 #58\"\u003e39-46\u003c/a\u003e].\u003c/p\u003e","description":"","filename":"Figure5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/bb42da9e3153bb8080da8831.jpg"},{"id":91346839,"identity":"955cec2d-5cbe-4cb0-bf8b-4967a9521fc2","added_by":"auto","created_at":"2025-09-15 13:59:54","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":3490774,"visible":true,"origin":"","legend":"\u003cp\u003e(a) PL spectra (excitation wavelength is 365 nm), (b) EIS plots, (c) transient photocurrent density and (d) TRPL decay of samples.\u003c/p\u003e","description":"","filename":"Figure6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/4877b341726cfbc3d8accb43.jpg"},{"id":91346837,"identity":"58999d33-70f0-478c-8b01-7ebb831881a3","added_by":"auto","created_at":"2025-09-15 13:59:54","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":2297906,"visible":true,"origin":"","legend":"\u003cp\u003e(a) RRDE polarization curves over CN, CCN and CCN-Al-2 at 1600 rpm in O\u003csub\u003e2\u003c/sub\u003e -saturated 0.1 M KOH with ring current (upper part) and disk current (bottom part). (b) H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e selectivity as a function of the applied potential. (c) The calculated average number of transferred electrons (n) as a function of the applied potential. (d) The photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation rates of CCN-Al-2 under different reaction gases or different sacrificial agents. (e) EPR signals of \u003csup\u003e•\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003eover CN, CCN, and CCN-Al-2 in the presence of DMPO under five minutes of visible light.\u003c/p\u003e","description":"","filename":"Figure7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/2a4cb5ea0b812fc08dd888d1.jpg"},{"id":92430952,"identity":"37990116-b215-46c7-83c9-780331bf1d23","added_by":"auto","created_at":"2025-09-29 16:08:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":41861552,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/14a72fd1-8aed-42e4-b816-49b022a188e7.pdf"},{"id":91348780,"identity":"f92ed160-8906-4be0-9b25-705bcd41c914","added_by":"auto","created_at":"2025-09-15 14:15:54","extension":"jpg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4704956,"visible":true,"origin":"","legend":"","description":"","filename":"TOC.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/d9eed2f98cff86419481e29e.jpg"},{"id":91347544,"identity":"40be7ae7-3d74-4404-9441-f1e0305fc719","added_by":"auto","created_at":"2025-09-15 14:07:54","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":3306993,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7444053/v1/1d16342e93c375e2b9b6be50.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eAmorphous Aluminum Oxide Clusters Regulating Oxygen Reduction for Photocatalytic H\u003csub\u003e2\u003c/sub\u003e O\u003csub\u003e2\u003c/sub\u003e Production over Carbon Nitride\u003c/p\u003e","fulltext":[{"header":"Highlights","content":"\u003col\u003e\n \u003cli\u003eUnveil by DFT calculations the end-on adsorption of O\u003csub\u003e2\u003c/sub\u003e on Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters and the promoted selectivity toward the 2e\u003csup\u003e-\u003c/sup\u003e ORR pathway.\u003c/li\u003e\n \u003cli\u003eAnchor Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters uniformly on crystalline carbon nitride through an ionothermal strategy.\u003c/li\u003e\n \u003cli\u003eEnhance charge separation and suppress H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition via the effect of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters loading.\u003c/li\u003e\n \u003cli\u003eAchieve outstanding performance, with CCN-Al-2 delivering 50.2 mmol g\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production and an AQY of 21.6% at 420 nm.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eCurrently, over 95% of the total H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production in industry is achieved through the anthraquinone oxidation process, which is characterized by high energy consumption and the release of toxic byproducts. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] In contrast, the solar-driven water oxidation or oxygen reduction in the presence of water and oxygen or air provides a promising avenue for environmentally friendly H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production under mild and safe conditions. [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] To realize high selectivity and a rapid rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production, it is essential to promote the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR (Eq.\u0026nbsp;1) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] or the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e water oxidation reaction (WOR) (Eq.\u0026nbsp;2). [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] However, H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e produced by photocatalytic 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e WOR is susceptible to decomposition at high oxidation potentials (1.76 V vs. RHE). [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] Therefore, the key to improving the rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production lies in facilitating the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR reaction while suppressing the 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR (Eq.\u0026nbsp;3). Graphitic carbon nitride, as a metal-free, non-toxic, and chemically stable material, is particularly intriguing due to its excellent potential in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation. However, its application in photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production is limited by its relatively low surface area, rapid recombination of photogenerated electron-hole pairs, and challenges related to achieving selectivity in the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR reaction.\u003c/p\u003e\u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2H\u003csup\u003e+\u003c/sup\u003e +2e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr;H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (0.695 V vs. NHE) (1)\u003c/p\u003e\n\u003cp\u003e2HO + 2h →HO + 2H (1.76 V vs. NHE) (2)\u003c/p\u003e\n\u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4H\u003csup\u003e+\u003c/sup\u003e +4e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr;2H\u003csub\u003e2\u003c/sub\u003eO(1.23 V vs. NHE) (3)\u003c/p\u003e\u003cp\u003eIn order to address these drawbacks of carbon nitride, numerous material modification strategies have been developed over the past few decades. These strategies include doping, constructing heterojunctions, modulating crystallinity, and altering morphology.[\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] Among these strategies, the crystallinity of carbon nitride can be adjusted through ionothermal treatment, promoting the periodic extension of hexazine/triazine units in-plane. High crystallinity significantly enhances charge carrier mobility. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eOn the other hand, introducing co-catalysts onto the surface of carbon nitride is an effective strategy to enhance photocatalytic performance by improving charge separation capability. Various noble metal co-catalysts, such as Pt, Pd, and Au, have been demonstrated to promote catalytic performance.[\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] Additionally, co-catalysts consisting of abundant and low-cost elements like MoS\u003csub\u003e2\u003c/sub\u003e, Ni, or CoP offer efficient and high-rate photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.[\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] However, conventional co-catalysts mentioned above typically contain transition metals, which can facilitate the decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at high concentrations. Therefore, they can enhance H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production efficiency at low H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e concentrations, but they can have a counterproductive effect in photocatalytic systems that are capable of efficiently producing hydrogen peroxide.[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] Previous reports suggest that aluminum, as a p-block metal, can inhibit the decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e on carbon nitride.[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] As a compound of aluminum, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is commonly used as an insulating material. Due to its excellent stability, dispersion, higher surface area, and outstanding adsorption properties, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e has gained favor among many researchers.[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] Moreover, a thin amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e layer on metal oxide semiconductors has been proved to suppress charge recombination. Coating TiO\u003csub\u003e2\u003c/sub\u003e with a thin layer of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e reduces charge recombination rates[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], passivates surface trap states, and thereby enhances charge transfer performance.[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] Amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-modified SnO\u003csub\u003e2\u003c/sub\u003e also demonstrates increased electron lifetimes.[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] Water readily dissociates on the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface to form adsorbed \u0026ndash;OH groups, which facilitate proton transfer via hydrogen bonding.[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] In summary, introducing an appropriate amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters onto the photocatalyst can suppress H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e degradation while offering excellent adsorption properties and charge recombination inhibition.\u003c/p\u003e\u003cp\u003eIn this work, we combine theoretical calculations and experimental studies to investigate the effects of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters on photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production. DFT results reveal that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters anchored on CCN lower the adsorption energy of O\u003csub\u003e2\u003c/sub\u003e, favor an end-on adsorption mode, and promote the formation of *OOH intermediates, thereby enhancing the selectivity of the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR. Guided by these insights, CCN-Al-x were synthesized using an ionothermal treatment strategy. The optimized CCN-Al-2 exhibits superior photocatalytic activity and selectivity, achieving a high H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rate and AQY. This study not only clarifies how Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters regulate photocatalysis but also establishes a rational pathway to design CN for efficient H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Photocatalyst preparation\u003c/h2\u003e\u003cp\u003e10.0 g melamine was placed in a 30 ml ceramic crucible with a lid and subsequently calcined for 4.0 h at 550\u0026deg;C in a muffle furnace at a heating rate of 5\u0026deg;C/min. After cooling to room temperature, the intermediate product (referred to as CN) was collected for further use. Subsequently, 0.5 g of CN was combined with a predetermined amount of Al(OH)\u003csub\u003e3\u003c/sub\u003e and added to 5 ml of deionized water. After 30 minutes of ultrasound treatment, the sample was rapidly frozen using liquid nitrogen, followed by freeze-drying for 24 hours. The resulting powder was mixed with 1.0 g of lithium chloride (LiCl) and thoroughly ground in an agate mortar. The sample was then transferred to a quartz boat and heated at a rate of 5\u0026deg;C/min under a nitrogen atmosphere to 550\u0026deg;C for 2 hours. The obtained sample was washed five times with deionized water and ethanol, and subsequently dried overnight in an oven at 60\u0026deg;C. The resulting catalyst was denoted as CCN-Al-x (where x represents the mass ratio of Al element to CN). Additionally, a crystalline carbon nitride sample without the addition of Al(OH)\u003csub\u003e3\u003c/sub\u003e was prepared following the aforementioned procedure and named CCN.\u003c/p\u003e\u003cp\u003eChemical materials, sample characterization, photocatalytic performance measurement, AQY measurement, photo(electro)chemical measurement and computational method were supplemented in the Supplementary Material.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Theoretical prediction of efficient photocatalyst\u003c/h2\u003e\u003cp\u003eTo predict the ORR activity of CCN-Al, DFT calculations were performed. The CCN-Al models were optimized by DFT, and the most stable structure was selected (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Previous studies have indicated that the activity of 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production crucially depends on the formation of *OOH and subsequent hydrogenation steps.[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] However, the adsorption of O\u003csub\u003e2\u003c/sub\u003e is a prerequisite for the formation of *OOH intermediates on the catalyst surface in the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR pathway. Therefore, considering the adsorption of O\u003csub\u003e2\u003c/sub\u003e on the CCN surface in the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR pathway is particularly important. The adsorption energies of O\u003csub\u003e2\u003c/sub\u003e on CCN, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters, and CCN-Al were calculated (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-c). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, on the model of single-layer CCN, O\u003csub\u003e2\u003c/sub\u003e molecules preferentially adsorb at the hexazine unit within the triazine ring, consistent with previous reports. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] In the models of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters and CCN-Al, O\u003csub\u003e2\u003c/sub\u003e molecules adsorb end-on to aluminum atoms of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters. Given previous reports, this end-on adsorption mode is favorable for avoiding O-O bond cleavage, thus reducing the selectivity of 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR.[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] The adsorption free energy of O\u003csub\u003e2\u003c/sub\u003e on Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters is 0.08 eV, significantly lower than that on CCN, which is 0.79 eV, indicating that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters can promote O\u003csub\u003e2\u003c/sub\u003e adsorption. After introducing Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters onto CCN, the adsorption energy of O\u003csub\u003e2\u003c/sub\u003e decreases to 0.15 eV, significantly enhancing the adsorption capacity for O\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003eDifferential charge density and Bader charge analysis for the adsorption models reveal that after loading Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters onto CCN, more electrons are transferred to O\u003csub\u003e2\u003c/sub\u003e (∆q\u0026thinsp;=\u0026thinsp;0.40 e), which is much greater than the electron transfer from CCN to O\u003csub\u003e2\u003c/sub\u003e (∆q\u0026thinsp;=\u0026thinsp;0.19 e). Additionally, the O-O bond length of adsorbed oxygen on CCN-Al is longer than that on Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters and CCN (Fig. S2). Combining the above analysis, it can be concluded that CCN-Al is more capable of adsorbing and activating O\u003csub\u003e2\u003c/sub\u003e molecules. Furthermore, the free energy of 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR process to produce H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e on CCN, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and CCN-Al was also calculated (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). Comparing the free energy changes along the entire reaction pathway in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, the hydrogenation of O\u003csub\u003e2\u003c/sub\u003e on CCN exhibits a positive reaction free energy, indicating that it is thermodynamically less feasible. On Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters, the reaction free energy for the formation of *OOH is -1.41 eV, indicating that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters can stabilize *OOH. Relative to CCN, the free energy for the formation of *OOH on CCN-Al decreases to -1.11 eV, indicating that the interaction between Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters and CCN promotes the formation of *OOH intermediates.\u003c/p\u003e\u003cp\u003eIn the ORR process, H\u003csub\u003e2\u003c/sub\u003eO would be also produced via a 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e pathway, which could be a significant constraint on H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, the free energy changes for the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR and 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR reaction pathways of the CCN-Al catalyst were studied to confirm the preferred reaction pathway. Clearly, in the 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR process, the reaction free energy from *OOH to *O is positive, preventing the progress of 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR, whereas the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR reaction processes are all exothermic reactions. This indicates that CCN-Al is more inclined toward the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR reaction pathway. Collectively, the theoretical results from these calculations suggest that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters, as a non-photocatalytic material, can theoretically serve as a co-catalyst to enhance the activity and selectivity of CN for oxygen reduction and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Morphology and Structure\u003c/h2\u003e\u003cp\u003eThe CCN-Al-x was prepared through adsorption of Al(OH)\u003csub\u003e3\u003c/sub\u003e onto bulk carbon nitride, followed by an alkali metal ion thermal treatment. Heat treatment under the coexistence of alkali metal ions is conducive to obtaining CN with high in-plane polymerization.[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] Firstly, XRD analysis was performed on the prepared catalysts. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, the CN exhibited two characteristic peaks at 13.1\u0026deg; and 27.9\u0026deg;, attributed to the in-plane repeating units of heptazine and the interlayer stacking of conjugated aromatic systems, respectively, confirming the presence of the layered structure[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. However, significant structural changes were observed upon mixed calcination with LiCl. Compared to CN, the (100) peak of CCN and CCN-Al-x decreased from 13.1\u0026deg; to 8.2\u0026deg;, indicating an increase in the spacing between in-plane repeating units from 0.675 nm to 1.05 nm, significantly enhancing the crystallinity of CN.[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] The (002) peak of CCN and CCN-Al-x significantly decreased, indicating a reduction in the number of stacked layers of carbon nitride. SEM images of the samples (Fig. S3) showed that under the influence of LiCl, CCN exhibited a flake-like structure, and the introduction of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e did not lead to significant morphological changes. Further testing of the catalysts was conducted using FT-IR (Fig.\u0026nbsp;2b). Apart from CN, the other catalysts exhibited an absorption peak at 2170 cm⁻\u0026sup1;, attributed to the asymmetric stretching vibration of cyanide groups (-CN), indicating that ionothermal treatment induces cyanide defects at the edges of carbon nitride.[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e] Both XRD and FT-IR spectra suggest that the introduction of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e has little effect on the CCN.\u003c/p\u003cp\u003eX-ray photoelectron spectroscopy (XPS) was employed to investigate the catalysts' chemical composition and electron transfer behaviors. The Al content of the prepared samples determined by XPS was significantly higher than that measured by ICP-AES (Fig. S4), indicating that a substantial portion of Al elements resides on the catalyst's surface. The survey scans (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) demonstrated the presence of O, C, and N elements on the surface of all catalysts. Notably, CCN-Al-2 exhibited peaks corresponding to Al 2s and Al 2p, confirming the successful incorporation of Al elements into the catalyst, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. The C 1s spectra, depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, exhibited similar features across all catalysts, with peaks at 284.8 eV, 286.6 eV, and 288.2 eV attributed to \u003cem\u003esp\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e-hybridized C in C\u0026thinsp;=\u0026thinsp;C, C\u0026thinsp;\u0026minus;\u0026thinsp;NH\u003csub\u003ex\u003c/sub\u003e, and N\u0026thinsp;=\u0026thinsp;C-N moieties of contaminating carbon.[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] The N 1\u003cem\u003es\u003c/em\u003e spectra in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003eb revealed distinctive peaks for CN, corresponding to C\u0026thinsp;=\u0026thinsp;N-C, N-(C)\u003csub\u003e3\u003c/sub\u003e, and N-H.[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] However, the N-H peak originating from terminal amino groups of the CN framework vanished in the N 1\u003cem\u003es\u003c/em\u003e spectra of CCN and CCN-Al-2, implying that the introduction of LiCl significantly facilitated the condensation of reactants. Furthermore, relative to CN, the N\u0026ndash;(C)\u003csub\u003e3\u003c/sub\u003e peak in CCN and CCN-Al-2 appeared at noticeably higher binding energies. Comparative analysis of the Al 2\u003cem\u003ep\u003c/em\u003e peak positions in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003ed showed that, compared to the Al 2\u003cem\u003ep\u003c/em\u003e peak of pure Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (74.4 eV), the Al 2\u003cem\u003ep\u003c/em\u003e peak of CCN-Al shifted by 0.3 eV towards higher binding energy. Similarly, compared to the N-(C)\u003csub\u003e3\u003c/sub\u003e N 1\u003cem\u003es\u003c/em\u003e peak of CCN, the corresponding peak of CCN-Al shifted by 0.2 eV towards higher binding energy, indicating electron transfer from the surface Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to CCN. To further validate the charge transfer properties between Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and CCN, differential charge density calculations were performed on the CCN-Al model, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003ee. The electrons were found to transfer from the surface Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters to CCN (q\u0026thinsp;=\u0026thinsp;0.07 e), consistent with the characterization results obtained from XPS. Such electron transfer establishes an electrostatic field from Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to CCN, thereby facilitating the transfer of photogenerated electrons from CCN to the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eMoreover, the catalysts were subjected to TEM characterization, revealing lattice fringes with d-spacings of 0.34 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) and 1.05 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), attributed to the (002) and (100) crystal planes of polyheptazine imide, respectively.[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] Lattice fringes with d-spacings of 0.28 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) and 0.42 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) were attributed to the (210) and (110) crystal planes of polytriazine imide, respectively. [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]Notably, the diffraction peaks of polytriazine imide were not observed in XRD, likely due to its lower content. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, lattice fringes of CCN are observed within the red box, while spindle-like features appear in the white box. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ef and the corresponding Fourier transform image in the upper right corner indicate that these features are amorphous. HADDF-STEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003eg) and elemental mapping (Fig.s h-k) of the same region further confirm that the amorphous phase corresponds to Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, which is also found to be uniformly distributed across the CCN matrix. Combined TEM and XRD analyses reveal that CCN-Al-x is a heterostructure formed by amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters anchored onto CCN.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Catalytic Activity Evaluation and Stability\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eUnder the conditions of ethanol serving as a proton source, the catalytic activity for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production using different photocatalysts was assessed under white light LED irradiation (See Fig. S5 for the white light LED spectrum). As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, the catalytic activities of CCN and CCN-Al-x were significantly higher than that of CN, highlighting that the enhancement in crystallinity greatly improved the efficiency of visible-light-driven H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis. Furthermore, the introduction of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e further augmented the activity of crystalline carbon nitride. Among these, CCN-Al-2 exhibited the highest activity, achieving a H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rate of 50.2 mmol g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is 185.9 times and 1.6 times higher than CN and CCN, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eb illustrates the influence of varying catalyst masses on the rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production. It can be observed that as the catalyst mass increases from 10 mg to 20 mg, the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yield increases by 41.7%. Further increasing the mass from 20 mg to 30 mg results in a 12.7% increase in yield, and from 30 mg to 50 mg, there is a 3.7% increase in yield. Therefore, 20 mg of catalyst was chosen for the experiment to determine the AQY for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production. (Except for the AQY experiment, all other photocatalytic activity experiments used 10 mg of catalyst.)\u003c/p\u003e\u003cp\u003eThe stability of CCN-Al-2 was evaluated through four consecutive cycles of photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production, totaling 12 hours. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, after four cycles, the catalytic activity of CCN-Al-2 declined to 79% of its initial value. Additionally, the trend of AQY values for CCN-Al-2 with changing absorbance (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ed) corresponded well with the UV-vis diffuse reflectance spectroscopy (DRS) spectrum. An AQY value of 21.6% was measured for CCN-Al-2 under 420 nm light irradiation. The in-situ decomposition rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e on the catalysts was also studied and is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ee. The H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition rate followed the sequence of CN, CCN, and CCN-Al-2. The performance of CCN-Al-2 was compared with that of recently reported \u003cem\u003eg\u003c/em\u003e-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e-based materials in photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ef and Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The results indicate that, under identical conditions, CCN-Al-2 exhibits a superior AQY for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production compared to most photocatalysts, highlighting its high activity as a photocatalyst for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Charge Carrier Dynamics and Optical Properties\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe efficient migration and separation of photo-generated electrons and holes play a crucial role in photocatalytic processes. Therefore, the catalysts were characterized using PL spectroscopy to investigate the recombination rates of free charge carriers. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the PL intensity of CCN and CCN-Al-x is significantly lower compared to CN, indicating that the improved crystallinity significantly suppresses the recombination of electrons and holes. The PL intensity of CCN-Al-x is lower than that of CCN, suggesting that the loading of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters further prevents charge carrier recombination. Furthermore, EIS (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003eb) and photocurrent response (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003ec) further demonstrate the impact of crystallinity and loaded Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters on charge separation. CCN-Al-x shows lower photocurrent density and electrochemical impedance. However, with an increase in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content, CCN-Al-x exhibits higher impedance and lower photocurrent. The results of EIS and photocurrent response indicate that an appropriate loading of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters promotes catalyst charge separation, but excess Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters increases the impedance of the catalyst and reduces photocurrent density. Based on the fitted fluorescence decay components (Table S2), the τ values for CN, CCN, and CCN-Al-2 are calculated to be 4.36, 1.79, and 1.66 ns, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Consistent with previous results, the significantly reduced average PL lifetime of CCN and CCN-Al-2 samples implies enhanced charge carrier separation. Combining optical and photoelectrochemical results, the high crystallinity of CCN-Al and the loading of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters can broaden the light absorption range and promote the separation and transfer of photo-generated charge carriers, contributing to an increased yield in the photocatalytic production of hydrogen peroxide.\u003c/p\u003e\u003cp\u003eThe optical properties of the samples are shown in Fig. S6. UV\u0026ndash;vis DRS (Fig. S6a,b) shows that enhanced crystallinity and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters loading narrowed the bandgap. VB-XPS (Fig. S6c) reveals that CCN-Al has a higher VB potential (1.86 eV) than CN and CCN (1.6 eV). The calculated band structures (Fig. S6d) indicate that CCN-Al has a CB position closer to the O\u003csub\u003e2\u003c/sub\u003e/\u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e potential. The expanded optical absorption and favorable band alignment can promote photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production activity.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Mechanism of Photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e Evolution\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe selectivity of the oxygen reduction reaction (ORR) is crucial for the production of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Electrochemical tests using a rotating ring-disk electrode (RRDE) were conducted to investigate the impact of crystallinity and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters loading on the selectivity of electron transfer during O\u003csub\u003e2\u003c/sub\u003e reduction. The disk current of the RRDE originates from the reduction of oxygen (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2H\u003csup\u003e+\u003c/sup\u003e +2e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e or O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4H\u003csup\u003e+\u003c/sup\u003e +4e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; 2H\u003csub\u003e2\u003c/sub\u003eO), while the ring current arises from the oxidation of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e to O\u003csub\u003e2\u003c/sub\u003e and two protons (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e \u0026rarr;O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2H\u003csup\u003e+\u003c/sup\u003e + 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e). The selectivity of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production on CN, CCN, and CCN-Al-2 was monitored in O\u003csub\u003e2\u003c/sub\u003e-saturated 0.1 M KOH electrolyte.\u003c/p\u003e\u003cp\u003eAs the potential was lowered from \u0026minus;\u0026thinsp;0.25 V (vs. Ag/AgCl), the reduction disk currents of CN, CCN, and CCN-Al-2 gradually increased (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ea, bottom). Simultaneously, the generated H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at the disk electrode could rapidly diffuse to the ring electrode and undergo further oxidation, leading to a positive oxidation current. In Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ea (top), the trends in ring currents were similar for all three catalysts, but the intensity of the ring current increased in the order CN\u0026thinsp;\u0026lt;\u0026thinsp;CCN\u0026thinsp;\u0026lt;\u0026thinsp;CCN-Al-2, indicating that CCN-Al-2 produced the most H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The selectivity of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production and the average number of transferred electrons (n) within the potential range of -1 to -0.5 V (vs. Ag/AgCl) are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003eb and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ec. Clearly, CCN-Al-2 exhibited a transfer of electrons closer to 2 under the same conditions, indicating higher selectivity for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Specifically, CCN-Al-2 transferred 2.56 electrons (-0.5 V vs. Ag/AgCl), achieving a H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e selectivity of 72.0%. Under the same potential, CN transferred 2.94 electrons with an H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e selectivity of only 53.2%. The combination of RRDE and photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e performance tests demonstrated that the improvement in crystallinity and the loading of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e together enhanced the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR selectivity of the catalyst, effectively promoting the surface reaction to generate H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003eTo further understand the mechanism of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production via photocatalysis with CCN-Al-2, experiments capturing active species were conducted using AgNO\u003csub\u003e3\u003c/sub\u003e and benzoquinone (BQ) as scavengers for e\u003csup\u003e\u0026minus;\u003c/sup\u003e and \u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ed, when O\u003csub\u003e2\u003c/sub\u003e was replaced with N\u003csub\u003e2\u003c/sub\u003e in the reaction solution, almost no H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was detected, indicating that O\u003csub\u003e2\u003c/sub\u003e is a necessary reactant. The addition of AgNO\u003csub\u003e3\u003c/sub\u003e to the reaction system led to a rapid decrease in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yield, suggesting that H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e formation occurs via the electron reduction of O\u003csub\u003e2\u003c/sub\u003e. Notably, the addition of BQ to the system resulted in a very low H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yield, indicating that \u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is an important intermediate for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation. Furthermore, electron paramagnetic resonance (EPR) capture experiments with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were used to confirm the presence of O\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ee). As shown in Fig. S7, no \u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e signals were observed in the three samples in the dark. After opening the visible light for 5 minutes, \u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e signals were observed in all samples except for CN, indicating that O\u003csub\u003e2\u003c/sub\u003e is the key radical in this reaction. Additionally, the \u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e signal of CCN-Al-2 was higher than that of CCN, suggesting that CCN-Al-2 can produce \u003csup\u003e\u0026bull;\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e more rapidly, consistent with the trend in catalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rates observed earlier. Therefore, based on the above tests, the photocatalytic mechanism of CCN-Al-2 involves 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.\u003c/p\u003e\u003cp\u003eUpon visible-light irradiation, CCN-Al absorbs photons to generate electron-hole pairs, which are subsequently transferred to the surface-anchored Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters. The Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters not only adsorb and activate O\u003csub\u003e2\u003c/sub\u003e but also facilitate rapid proton transfer between surface oxygen atoms and adsorbed hydroxyl groups, thereby promoting the formation of *OOH intermediates and ultimately H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn conclusion, this study demonstrates that the incorporation of amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters on CN plays decisive roles in boosting photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production. Upon visible-light excitation, photogenerated charge carriers in CCN-Al are efficiently transferred to the surface-anchored Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters, which not only adsorb and activate O\u003csub\u003e2\u003c/sub\u003e but also facilitate proton transfer via surface hydroxyl groups. It promotes the formation of *OOH intermediates while suppressing competing 4e\u003csup\u003e\u0026minus;\u003c/sup\u003e pathways, thereby enhancing the activity and selectivity of the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e ORR. At the same time, the decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e is also inhibited. As a result, CCN-Al-2 achieves a significantly improved H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e generation rate of 50.2 mmol g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with an apparent quantum yield of 21.6%. These findings provide both mechanistic insight and practical guidance for the rational design of non-transition-metal photocatalyst, offering a promising strategy toward efficient and selective H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eDeclarations\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eH. Elmansour contributes to the data curation, methodology, and writing \u0026ndash; original draft. D. Wang contributes to the software and visualization. G. Taojin contributes to the Writing \u0026ndash; review \u0026amp; editing. F. Chen contributes to the funding acquisition and validation. All authors discussed the results and contributed to the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by National Key R\u0026amp;D Program of China No.2024YFA1211004. Additional support was provided by the Feringa Nobel Prize Scientist Joint Research Center at East China University of Science and Technology.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eJ.M. Campos-Martin, G. Blanco-Brieva, J.L. Fierro, Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process, Angew. Chem. Int. 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J. 519 (2025) 165557. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/https://doi.org/10.1016/j.cej.2025.165557\u003c/span\u003e\u003cspan address=\"10.1016/j.cej.2025.165557\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"research-on-chemical-intermediates","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"rint","sideBox":"Learn more about [Research on Chemical Intermediates](http://link.springer.com/journal/11164)","snPcode":"11164","submissionUrl":"https://submission.nature.com/new-submission/11164/3","title":"Research on Chemical Intermediates","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Photocatalysis, Hydrogen Peroxide Production, Crystalline Carbon Nitride, Amorphous Aluminum Oxide Clusters","lastPublishedDoi":"10.21203/rs.3.rs-7444053/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7444053/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe rational design of catalysts for photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production remains a major challenge, as most photocatalysts for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production also tend to decompose H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Here, density functional theory (DFT) calculations reveal that amorphous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters anchored on crystalline carbon nitride (CCN) provide efficient oxygen adsorption sites, facilitate electron transfer, and stabilize key *OOH intermediates. Guided by these insights, CCN composites loaded with Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters (CCN-Al-x) were synthesized using an ionothermal treatment strategy. Experimental studies confirm that the introduction of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters enhances charge separation, suppresses H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition and improves the selectivity of the 2e\u003csup\u003e\u0026minus;\u003c/sup\u003e oxygen reduction reaction (ORR). The optimized CCN-Al-2 achieves an H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rate of 50.2 mmol g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with an apparent quantum yield (AQY) of 21.6% under 420 nm irradiation. This work highlights the critical impact of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e clusters modification on CCN, providing new opportunities for efficient and selective photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e","manuscriptTitle":"Amorphous Aluminum Oxide Clusters Regulating Oxygen Reduction for Photocatalytic H2 O2 Production over Carbon Nitride","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-15 13:59:49","doi":"10.21203/rs.3.rs-7444053/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-10T05:25:29+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-10T00:40:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"56885258778688663249394039360022505831","date":"2025-09-09T07:01:44+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-09T01:57:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"339615719833480537598641664008157586730","date":"2025-09-08T11:12:06+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-08T10:41:55+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-25T06:11:15+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-25T06:10:14+00:00","index":"","fulltext":""},{"type":"submitted","content":"Research on Chemical Intermediates","date":"2025-08-24T04:18:13+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"research-on-chemical-intermediates","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"rint","sideBox":"Learn more about [Research on Chemical Intermediates](http://link.springer.com/journal/11164)","snPcode":"11164","submissionUrl":"https://submission.nature.com/new-submission/11164/3","title":"Research on Chemical Intermediates","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"babb57f7-7af4-42b0-9c24-e4839fa0857a","owner":[],"postedDate":"September 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-09-29T16:06:38+00:00","versionOfRecord":{"articleIdentity":"rs-7444053","link":"https://doi.org/10.1007/s11164-025-05750-x","journal":{"identity":"research-on-chemical-intermediates","isVorOnly":false,"title":"Research on Chemical Intermediates"},"publishedOn":"2025-09-25 15:57:03","publishedOnDateReadable":"September 25th, 2025"},"versionCreatedAt":"2025-09-15 13:59:49","video":"","vorDoi":"10.1007/s11164-025-05750-x","vorDoiUrl":"https://doi.org/10.1007/s11164-025-05750-x","workflowStages":[]},"version":"v1","identity":"rs-7444053","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7444053","identity":"rs-7444053","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}