Noble Metal Sites Modulated Cyano-COF for Boosted Photocatalytic O₂ to H₂O₂ Production

The pristine cyano-functionalized sp²-carbon conjugated COF (cyano-COF) was synthesized via Knoevenagel condensation between trialdehyde and diacetonitrile under solvothermal conditions. XRD pattern (Figure S1, Supporting Information, SI) confirms that the as-obtained cyano-COF is crystalline, which is in good agreement with that reported in a previous study [29]. Theoretical calculations confirm that cyano-COF exhibits AA overlapping stacking and AB staggered stacking characteristics. The cyano-COF-mediated Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF were prepared via a simple suspension impregnation with in-situ NaBH₄ reduction (Figure 1a). In contrast to previous studies focusing on the coordination effect of electron-donating groups with MNPs, this work proposes a synergistic interaction between electron-withdrawing groups and MNPs as a mechanism to enhance photogenerated charge separation (Figure 1b). These as-obtained metal/cyano-COF photocatalysts were subsequently characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). As shown in Figure 1c, the FT-IR spectra of cyano-COF and metal/cyano-COF exhibit a strong C≡N stretching vibration at ~2215 cm⁻¹ [29]. Additionally, prominent signal peaks at ~3230 cm⁻¹, corresponding to the C=C stretching vibration [27], are observed. These results confirm that the chemical structure of cyano-COF remains intact following the incorporation of Pt, Pd, Au, and Ag species. TEM images (Figure 1d–k) reveal that these noble MNPs are dispersed on the cyano-COF surface with varying sizes. Specifically, Pt NPs and Pd NPs exhibit uniform diameters of 2.35 ± 1.08 nm and 2.53 ± 0.80 nm, respectively, whereas Au NPs and Ag NPs tend to aggregate into larger particles, with size distributions centered at 3.62 nm and 2.94 nm, respectively. These particle size and distribution differences are primarily attributed to their distinct electronic properties and nucleation-growth kinetics. From the perspective of electronic structure, d-orbital electrons of Pt and Pd have high reactivity during bonding. The electron-withdrawing effect of cyano-groups could enhance the electronic interaction between the COFs and Pt (or Pd), which leads to more effective stabilization of the surface atoms of MNPs and a reduction in their surface energy, forming smaller NPs. In contrast, the electronic structures of Au and Ag are relatively stable, with lower d-orbital electron reactivity, resulting in weak electronic interactions between these metals and cyano-COF. Consequently, it might be much more challenging to stabilize the surface atoms of Au NPs and Ag NPs, making them more prone to aggregation during the growth process and forming larger particles. The surface chemical states of these four metal/cyano-COF materials were further characterized using X-ray photoelectron spectroscopy (XPS). As presented in Figure S2 (SI), the XPS survey spectra indicate the presence of C and N elements in both cyano-COF and metal/cyano-COF. The high-resolution C 1s spectra (Figure 2a) of cyano-COF and metal/cyano-COF exhibit two sets of doublets, which are ascribed to C-C/C=C (284.5 eV) and -C≡N (285.7–286.4 eV), corresponding to the carbon elements in the benzene ring and cyano-group, respectively [29]. Compared with pristine cyano-COF (286.4 eV), the C 1s characteristic peak centers of -C≡N in Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF are reduced by 0.3, 0.1, 0.7, and 0.3 eV, respectively, indicating that the incorporation of noble metals suppresses the electron transfer from carbon to nitrogen. The binding energy of N 1s (~399 eV) shows slight shifts of −0.11, −0.10, 0.02 eV upon the incorporation of Pt, Pd, and Au respectively, indicating the electron-withdrawing nature of the -C≡N group toward Pt NPs and Pd NPs, while a very weak electron-donating property is observed for Au NPs (Figure 2b). The binding energy of N 1s of Ag/cyano-COF was observed to be unchanged. This effect is notably distinct from the previously reported electronic influence of electron-withdrawing anchoring groups on metal species [30]. The deconvolution analysis of the doublet peaks in the Pt 4f, Pd 3d, and Au 4f regions reveals two distinct sets of doublets. A detailed analysis of the XPS spectra (Figure 2c) confirms the coexistence of two distinct species, Pt(0) and Pt(II), in Pt/cyano-COF. The peaks located at 71.5 eV and 75.1 eV are assigned to the 4f_7/2 and 4f_5/2 spin-orbit components of Pt(0), respectively [31]. Additionally, the higher binding energy peaks at 73.3 eV (4f_7/2) and 76.6 eV (4f_5/2) are ascribed to the Pt(II) species, which likely arises from surface oxidation during exposure to air. This phenomenon is commonly observed in supported nano-metal catalysts [32]. The deconvolution of the Pd 3d regions (Figure 2d) reveals two sets of doublets, each comprising a lower-energy band (Pd 3d_5/2) and a higher-energy band (Pd 3d_3/2). The doublet located at 335.9 eV and 341.1 eV is assigned to Pd(0) [33], confirming the successful reduction of Pd(II) to Pd(0). Another set of characteristic peaks at 338.2 eV and 343.4 eV is attributed to PdO [23]. The coexistence of Au(0) and Au(I) species is also observed in Au/cyano-COF (Figure 2e). The peaks centered at 80.0 eV and 87.7 eV are attributed to the 4f_7/2 and 4f_5/2 bands of Au(0), respectively. Additionally, the higher binding energy peaks at 84.6 eV (4f_7/2) and 88.3 eV (4f_5/2) are assigned to Au(I) species [34]. The Ag species in Ag/cyano-COF photocatalyst are exclusively present in the chemical states of Ag(0), as indicated by the binding energies of 368.3 eV (Ag 3d_3/2) and 374.2 eV (Ag 3d_5/2) (Figure 2f) [35]. It should be noted here that the binding energy of Ag(0) typically exhibits only a minor variation compared to that of Ag(I). The maximum shift reported in the literature is 0.4 eV for Ag₂O. Such negligible differences in binding energy render the determination of oxidation states particularly challenging, unless XAS measurements are performed [36]. Besides, in almost supported Ag NPs, the XPS region of Ag species is typically found to predominantly exist in the chemical state of Ag(0) [37,38,39]. Given the sp²-carbon conjugated framework structure, the electron-withdrawing nature of cyano-groups, and the distinctive electronic structures and properties of these four noble metals (Pt, Pd, Au, and Ag), these metal/cyano-COF materials were then utilized for photocatalytic H₂O₂ synthesis that is a promising and environmentally friendly pathway, offering a viable alternative to traditional energy-intensive technologies. The H₂O₂ photogeneration of metal/cyano-COF was carried out under simulated solar light. As shown in Figure 3a and 3b, Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, Ag/cyano-COF exhibit photocatalytic H₂O₂ production rates of 903 ± 24, 1073 ± 35, 963 ± 9, and 851 ± 56 μmol·g⁻¹·h⁻¹, respectively, representing 1.56, 1.86, 1.67 and 1.47 times higher than that of pristine cyano-COF (577 ± 69 μmol·g⁻¹·h⁻¹). Furthermore, the cycle durability of these supported metal/cyano-COF photocatalysts was systematically evaluated. As shown in Figure 3c, the production rates of H₂O₂ photosynthesized by the four metal/cyano-COF materials exhibit a slight decrease during the first cycle and remains almost constant in the subsequent three cycles. In the fourth cycle, the H₂O₂ yield of these metal/cyano-COF materials retained more than 66% of initial yields, demonstrating their good stability. To gain deep insights into the enhanced photocatalytic performance of H₂O₂ photogeneration, controlled experiments were conducted under different atmospheres (O₂, air, and N₂). As shown in Figure 3d, Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF exhibit much higher H₂O₂ production rates in an O₂ atmosphere compared to those in air and N₂. Specifically, under air condition, the H₂O₂ photosynthesis yields for Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF are 410 ± 24, 353 ± 13, 500 ± 54, and 525 ± 16 μmol·g⁻¹·h⁻¹, respectively. In contrast, under a N₂-saturated atmosphere, the corresponding yields are significantly lower, with yields of 109 ± 10, 46 ± 4, 87 ± 15, and 161 ± 46 μmol·g⁻¹·h⁻¹, respectively. These results indicate that O₂ is the key raw material for photocatalytic synthesis of H₂O₂ over these four metal/cyano-COF materials. To understand the photocatalytic mechanism of H₂O₂ synthesis, the light absorption properties of cyano-COF and metal/cyano-COF were studied using ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS). As shown in Figure 4a, all materials exhibit strong absorption across UV and visible regions. Based on Tauc plots, the optical band gaps of cyano-COF, Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF are determined to be 2.37, 2.39, 2.37, 2.38, and 2.36 eV, respectively (Figure S3, SI). Mott-Schottky (M-S) test (Figure S4, SI) was employed to determine the electronic band positions of these photocatalysts. The positive slopes of the M-S curves indicate that both cyano-COF and metal/cyano-COF are n-type semiconductors [40]. The corresponding flat band potentials of cyano-COF, Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF are −0.84, −0.83, −0.84, −0.74, and −0.73 V vs. Ag/AgCl, respectively. Given that the flat band potential of n-type semiconductors is typically more positive (by 0.1–0.3 V) than the conduction band (CB) potentials [41], the lowest unoccupied molecular orbital (LUMO) positions of cyano-COF, Pt/cyano-COF, Pd/cyano-COF, Au/cyano-COF, and Ag/cyano-COF are estimated to be −0.74, −0.73, −0.74, −0.64, and −0.63 V vs. NHE, respectively. Based on the band edges derived from Tauc plots, the relative positions of the LUMO and the highest occupied molecular orbital (HOMO) for cyano-COF and metal/cyano-COF are illustrated in Figure 4b. Since the LUMO of all materials is more negative than the potentials for both the concerted 1e⁻ oxygen reduction reaction (ORR) (O₂/∙O₂⁻, −0.33 V vs. NHE) and the 2e⁻ ORR (O₂/H₂O₂, 0.68 V vs. NHE), which indicates that all materials are favorable for the photoreduction of O₂ to produce ∙O₂⁻, followed by the formation of H₂O₂ [42,43]. Given the analogous optical properties of the four noble metals supported on cyano-COF and their performance in photocatalytic H₂O₂ synthesis, cyano-COF and Pd/cyano-COF were utilized as discussed subjects for subsequent experiments to further investigate the photocatalytic mechanism. The photocurrent responses of cyano-COF and Pd/cyano-COF are presented in Figure 4c. Pd/cyano-COF has a significantly stronger photocurrent response compared to pristine cyano-COF, indicating efficient photoinduced charge transfer between cyano-COF and Pd NPs. The Nyquist plots (Figure 4d) corroborate this observation. The smaller semicircle diameter in the high-frequency region for Pd/cyano-COF, relative to that of cyano-COF, reflects a reduced charge transfer impedance, thereby enhancing charge separation and transfer efficiency, which promotes the participation of more e⁻ and h⁺ during the photocatalytic H₂O₂ formation [44]. In addition, the substantial reduction in photoluminescence (PL) emission intensity (Figure 4e and Figure S5 (SI)) confirms the effective suppression of charge carrier recombination in Pd-modified cyano-COF. The enhanced charge separation is attributed to the synergistic interaction between noble MNPs and cyano-COF. Photocatalytic H₂O₂ production exhibits significant potential but encounters developmental challenges, primarily the rapid recombination of photogenerated charge carriers (e⁻ and h⁺) and the inefficiency of the water oxidation reaction in pure water systems. The application of sacrificial electron donors, such as methanol, EtOH, isopropanol, and benzyl alcohol, addresses these issues by undergoing oxidation more readily than water. This facilitates the effective consumption of photogenerated h⁺, suppresses charge recombination, and substantially enhances H₂O₂ yield. Organic photocatalysts benefit particularly from this approach, as efficient h⁺ scavenging prevents self-oxidation and improves long-term stability [45,46]. Compared with the pure water system, the introduction of EtOH consumes photogenerated h⁺ and facilitates the separation of e⁻-h⁺ pairs, thereby enhancing the catalytic performance. As exhibited in Figure 5a, cyano-COF and metal/cyano-COF have 1.40–2.17 times higher H₂O₂ yields in the presence of EtOH as a sacrificial agent of h⁺ compared to the pure water system. Control experiments were carried out to scavenge other active intermediates to elucidate the photocatalytic mechanism of H₂O₂ synthesis over cyano-COF and metal/cyano-COF. TBA, BQ, and KIO₃ are commonly utilized as scavengers for ·OH, ·O₂⁻, and e⁻, respectively [47,48]. The addition of TBA exerts no significant influence on the H₂O₂ yield, suggesting that ·OH is not the predominant active species in the photocatalytic reaction. The presence of KIO₃ resulted in a reduced rate of photocatalytic H₂O₂ production, which is attributed to its scavenging effect on photogenerated e⁻. Furthermore, KIO₃ is employed as an e⁻ capture agent to suppress H₂O₂ production under N₂ atmosphere, indicating that ORR plays a dominant role in this process [49]. Atmosphere experiments demonstrated that ORR is the primary pathway for photocatalytic H₂O₂ production over metal/cyano-COF (Figure 3d). The pronounced inhibitory effect of BQ on H₂O₂ production strongly suggests that·O₂⁻ serves as a critical intermediate in the formation mechanism of H₂O₂ on metal/cyano-COF. Furthermore, electron paramagnetic resonance (EPR) spectroscopy by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was employed to identify the presence of ·O₂⁻ during the photocatalytic process [16,50]. As depicted in Figure 5b, a characteristic EPR signal of DMPO-·O₂⁻ signals was detected for Pd/cyano-COF under illumination, whereas no such signals were observed in the dark. This result is consistent with the results of the above sacrificial agent experiments. Consequently, it can be concluded that the photocatalytic production of H₂O₂ proceeds via a sequential two-step 1e⁻-ORR process (Figure 5c) (O₂ → ·O₂⁻ → H₂O₂) [42]. During the photocatalytic process, photogenerated electrons are transferred from the conduction band of cyano-COF to the noble metals, forming a highly active electron-enriched region. These electrons can subsequently react with oxygen molecules adsorbed on the noble metal surface to generate ·O₂⁻, which further participates in reactions leading to the efficient formation of H₂O₂ [51,52,53].