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Covalently Tethering Atomically Precise Au25 Nanoclusters onto Covalent Organic Framework for Visible-Light-Driven CO2 Reduction
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Ping Fu1, Biao Meng1, 2, 3, Qizhi Hu1, Jiao Wei1, Qihong Yue1, Mingdong Sun1, Qing Xu1, Xiaoling Liu1, Shibo Xi4, Wen Yin2, 3, Yu Zhou1, *, Jun Wang1, *
Research. Vol 9 Article ID 1152
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Research. Vol 9 Article ID 1152
• RESEARCH ARTICLE •
Covalently Tethering Atomically Precise Au25 Nanoclusters onto Covalent Organic Framework for Visible-Light-Driven CO2 Reduction
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Ping Fu1, Biao Meng1, 2, 3, Qizhi Hu1, Jiao Wei1, Qihong Yue1, Mingdong Sun1, Qing Xu1, Xiaoling Liu1, Shibo Xi4, Wen Yin2, 3, Yu Zhou1, *, Jun Wang1, *
Affiliations
  • 1State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China.
  • 2University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 101408, China.
  • 3Spallation Neutron Source Science Center, Dongguan 523803, China.
  • 4Institute of Sustainability for Chemicals, Energy and Environment, A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Singapore 627833, Singapore.
Published: 2026-02-20 doi: 10.34133/research.1152
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Solar-driven carbon dioxide (CO2) reduction can sustainably produce chemicals and fuels but is often limited by rapid charge recombination and slow CO2-to-product kinetics, typically necessitating homogeneous photosensitizers and cocatalysts. Here, we reported an integrated photocatalyst, TF-COF-CONH-Au25-Co, constructed by immobilizing atomically precise Au25 nanoclusters (NCs) on a covalent organic framework (COF) incorporating [Co(bpy)3]2+ complex (Co–N6 coordination). Under visible-light illumination, this hybrid catalyzes CO2 conversion to syngas without external photosensitizers or cocatalysts, delivering a CO formation rate of 2,321.9 μmol·g−1·h−1 (turnover number of 171.9 and turnover frequency of 7.2 h−1). The Au25 NCs enhance light responsiveness and charge transfer efficiency, thereby enriching long-lived photogenerated electrons, while concurrently modulating the electronic state of Co sites to reduce the energy barrier for CO2 reduction. This study illustrates a molecular-level strategy to synergistically integrate metal NCs, COFs, and [Co(bpy)3]2+, showing a promising platform for high-performance photocatalytic CO2 conversion.

Ping Fu, Biao Meng, Qizhi Hu, Jiao Wei, Qihong Yue, Mingdong Sun, Qing Xu, Xiaoling Liu, Shibo Xi, Wen Yin, Yu Zhou, Jun Wang. Covalently Tethering Atomically Precise Au25 Nanoclusters onto Covalent Organic Framework for Visible-Light-Driven CO2 Reduction[J]. Research, 2026 , 9 (2) : 1152 - . DOI: 10.34133/research.1152
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Introduction
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Solar-driven reduction of carbon dioxide (CO2) represents a compelling strategy for addressing both climate change and global energy challenges [1–4]. Nevertheless, the formidable CO2 thermodynamic stability and rapid charge recombination remain important obstacles to the development of efficient photocatalytic systems [5,6]. Homogeneous organometallic photosensitizers, typically Ru- or Re-based complexes, are normally involved in bridging the gap between sluggish CO2 reduction kinetics and the short lifetimes of excited states [3,5,7]. Nevertheless, their high price, limited durability, and recycling difficulties restrict their practical application [8], making the urgent need for photosensitizer-free heterogeneous CO2 photoreduction systems.
Atomically precise metal nanoclusters (NCs) are ligand-protected metal assemblies characteristic of well-defined chemical composition and structure [9–12]. Their subnanometer dimensions endow distinctive geometric and electronic properties, including abundant surface unsaturated sites, quantum confinement, and pronounced size effects [11,13–16]. In photocatalysis, the discrete energy level and the molecule-like behavior of metal NCs render excellent light harvesting across the ultraviolet–visible (UV–vis) near-infrared region [13,17,18]. They can function as narrow-bandgap semiconductors to generate photoexcited carriers and as the electron acceptors to suppress charge recombination [13,14,17]. However, their high surface free energy will easily lead to deactivation due to aggregation during the catalytic process [19,20]. Immobilization on well-defined porous supports via electrostatic interactions, encapsulation, coordination, or covalent bonding is therefore crucial to retain their structure and activity [20–24].
Covalent organic frameworks (COFs) are attractive supports due to the well-defined ordered porosity, tunable composition, ordered porosity, and predictable design [6,8,25–30]. Moreover, COF-based single-atom catalysts enable site-specific structural and functional control at the atomic level [8,31–34]. Here, we report the first example of immobilizing atomically precise Au25(Cys)18 NCs (Au25 NCs) on a COF scaffold bearing [Co(bpy)3]2+ complex. A 2-dimensional COF, named TF-COF, was synthesized via the condensation of 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TTA) and tris(4-formylphenyl)amine (TFA), followed by quinoline-4-carboxylic acid functionalization through an irreversible Doebner reaction to yield TF-COF-COOH [35,36]. Au25 NCs were covalently anchored through amidation between COF’s carboxyl groups and NCs’ amino groups, affording TF-COF-CONH-Au25. Subsequent incorporation of Co sites produced TF-COF-CONH-Au25-Co. The Au NCs promote the light absorption and act as electron sinks to prolong carrier lifetimes. Meanwhile, their strong electronic interaction with neighboring Co sites modulates the local environment, thereby reducing the activation energy of the key step in CO2 reduction. These synergistic effects enable TF-COF-CONH-Au25-Co to drive visible-light-mediated CO2 transformation into syngas, a valuable mixture of CO and H2 that has numerous industrial applications [32,37], without extra photosensitizers or cocatalysts.
Results and Discussion
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Construction of Au25 NCs tethered to COF
Figure 1 illustrated the overall strategy for the construction of Au25 NCs tethered to COFs, with the detailed synthetic route of the COF support shown in Fig. 2A. Au25 NCs were synthesized, and the structure was confirmed by UV–vis spectrum (Fig. S1) and high-resolution electrospray ionization mass spectrometry (Fig. S2) [20,38]. Powder x-ray diffraction (PXRD) analysis of TF-COF (Fig. 2B) displayed characteristic reflections at 4.39°, 7.66°, 8.88°, 11.74°, and 22.54°, indexing to the (100), (210), (200), (310), and (001) planes [39], respectively. TF-COF-COOH exhibited a nearly identical diffraction profile (Fig. 2C), indicating that the quinoline-4-carboxylic acid functionalization preserved the original COF framework. Simulated PXRD patterns based on an overlapping (AA) stacking model resembled the experimentally observed data (Fig. 2B and C and Figs. S3 and S4). Pawley refinements resulted in good agreement factors and provided the unit cell parameters (Fig. 2B and C). The chemical robustness of TF-COF-COOH was evaluated by exposure to harsh conditions, including strong base (3 M NaOH), strong acid (6 M HCl), oxidizing (0.5 M H2O2), and reducing (1 M NaBH4) environments for 24 h. PXRD patterns validated the retention of crystallinity (Fig. 2D), whereas TF-COF suffered framework collapse under the same treatments (Fig. S5), demonstrating that conversion of imine linkages to quinoline units enhanced chemical stability. Importantly, TF-COF-CONH-Au25 exhibited a PXRD pattern essentially identical to TF-COF-COOH (Fig. S6), verifying that covalent anchoring of Au25 NCs did not disrupt the COF lattice.
Fourier transform infrared (FT-IR) spectrum of TF-COF showed a characteristic band at 1,626 cm−1, assignable to the C═N stretching vibration of the imine unit (Fig. S7A) [40,41]. This band disappeared in TF-COF-COOH and TF-COF-CONH-Au25, accompanied by new absorptions at 1,722 and 1,608 cm−1, attributable to the C═O stretching of the quinoline-4-carboxylic acid moiety and the pyridyl group vibration [35,36], respectively. The C═O peak in the spectrum of TF-COF-CONH-Au25 was red-shifted relative to TF-COF-COOH (Fig. S7B), indicative of the amide bond generation [42,43]. Solid-state 13C nuclear magnetic resonance (NMR) spectra confirmed the structural transformation (Figs. S8 to S10). TF-COF displayed a resonance at 157 parts per million (ppm) from imine carbon, which disappeared in TF-COF-COOH, alongside the emergence of a –COOH signal at 172 ppm [35,36]. TF-COF-CONH-Au25 showed a C═O resonance at 174 ppm from the amide linkage and an additional peak at 64 ppm assignable to the methylene group of the Au25 NC ligand (Fig. S10). Nitrogen sorption measurement revealed the type I isotherms for TF-COF and TF-COF-COOH (Fig. S11A) [44]. The surface areas were 1,510 and 1,215 m2·g−1, respectively, with the most probable pore sizes of 21 Å (Fig. S11B and Table S1). TF-COF-CONH-Au25 exhibited a type IV isotherm with a reduced surface area of 640 m2·g−1, consistent with partial pore occupancy by bulky Au25 NCs. CO2 sorption isotherms at 298 K demonstrated that the uptake at 1 bar increased from 36.7 cm3·g−1 for TF-COF to 48.6 and 41.4 cm3·g−1, respectively, for TF-COF-COOH and TF-COF-CONH-Au25 (Fig. S12). The Au content in TF-COF-CONH-Au25 was approximately 5.56 wt %. Scanning electron microscopy (SEM) images of these COFs showed similar coral-bar morphology (Fig. S13), while elemental imaging confirmed homogeneous distribution of C, N, O, S, and Au in TF-COF-CONH-Au25 (Fig. S14). Transmission electron microscopy (TEM) images of TF-COF and TF-COF-COOH revealed well-resolved lattice fringes, indicative of high crystallinity (Fig. 2E and F). High-angle annular dark-field scanning TEM (HAADF-STEM) and TEM images of TF-COF-CONH-Au25 visualized uniformly distributed NCs (Fig. 2G and Fig. S15), confirming the incorporation of Au25 NCs.
X-ray photoelectron spectroscopy (XPS) was conducted to elucidate the surface chemical states (Figs. S16 to S20). The survey scan XPS spectra revealed the C 1s, N 1s, and O 1s signals at 291.4, 404.4, and 540.1 eV, respectively, while TF-COF-CONH-Au25 exhibited additional Au 4f (87.0 eV) and S 2p (162.0 eV) peaks (Fig. S16) [20,42]. High-resolution N 1s XPS spectra (Fig. S17) corroborated the integration of quinoline-4-carboxylic acid units in both TF-COF-COOH and TF-COF-CONH-Au25, with a characteristic quinoline nitrogen signal at 400.2 eV, alongside contributions from sp3-N (399.3 eV) and imine/triazine nitrogen (C═N; 400.3 eV) [39]. TF-COF-CONH-Au25 exhibited an extra component at 401.7 eV from the amide nitrogen [20]. The C 1s and O 1s spectra (Figs. S18 and S19) of TF-COF-COOH and TF-COF-CONH-Au25 presented the peaks at 288.8 and 532.4 eV, respectively, arising from the –COOH moieties [35,36]. The Au 4f and S 2p peaks of TF-COF-CONH-Au25 showed a positive binding energy shift compared to pristine Au25 NCs (Fig. S20), suggesting reduced electron densities on Au and S atoms due to the NC–COF interaction [20].
The local metal coordination environment was measured by x-ray absorption fine structure (XAFS) analysis (Fig. 3A to F). The Au L3-edge x-ray absorption near-edge structure (XANES) spectrum of TF-COF-CONH-Au25 displayed a rising edge of TF-COF-CONH-Au25 close to that of Au foil (Fig. 3A), indicating that the Au25 NCs in TF-COF-CONH-Au25 retained a metallic state with a slight positive charge [45]. Fourier-transformed L3-weighted extended XAFS (FT-EXAFS) spectrum of TF-COF-CONH-Au25 (Fig. 3B) demonstrated a dominant peak at 1.8 Å for Au–S coordination in the outermost layer of Au25 NCs [46]. Two weak peaks were observed at 2.5 and 3.0 Å for Au–Au contribution in the inner core of Au25 NCs [46,47]. Multishell EXAFS fitting (Fig. 3C, Figs. S21 to S23, and Table S2) displayed Au–S coordination number (CN) of 1.6 at 2.3 Å and Au–Au CNs of 1.5 (2.8 Å), 1.6 (3.1 Å), and 1.9 (3.3 Å) in TF-COF-CONH-Au25, consistent with reported structures of Au25 NCs [48,49].
Neutron total scattering combined with pair distribution function (PDF) analysis was performed on TF-COF-COOH and TF-COF-CONH-Au25 to gain atomic-level structure insights [32,50]. Covalent bonding of Au25 NCs intensified several peaks and introduced new features in the PDF profiles (Fig. 3G and Figs. S24 to S26). Differential neutron PDF (d-PDF) was obtained by subtracting the neutron PDF of TF-COF-COOH from TF-COF-CONH-Au25, showing the correlations at r = 1.3, 1.5, 1.7, 2.3, 2.9, and 3.3 Å for C−O, C═C, C═S, Au−S, Au−Au, and Au~Au (inner core Au13 to outer Au atoms), respectively. The experimentally obtained d-PDF profile agreed with the refined one (Figs. S25 and S26), confirming the covalent tethering and structural integrity of the Au25 NCs within the COF frameworks.
Anchoring of [Co(bpy)3]2+ complex
Co species were anchored by complexing the COFs with [Co(bpy)3]2+ cations via Coulombic interaction and pore confinement (Fig. 1) [51,52]. Zeta potential tests revealed that the potentials of TF-COF, TF-COF-COOH, and TF-COF-CONH-Au25 were negatively charged, while [Co(bpy)3]2+ exhibited a positive potential of +18.2 mV (Fig. S27), enabling their assembly through electrostatic attraction. The resulting TF-COF-CONH-Au25-Co had a Co loading of 1.91 wt %, while TF-COF-Co and TF-COF-COOH-Co exhibited a Co content of 1.76 and 2.13 wt %, respectively. PXRD, FT-IR, nitrogen sorption, and SEM analyses verified the preservation of crystalline structure, porosity, and morphology (Figs. S28 to S31). TEM image verified the existence of Au NCs (Fig. S32).
Survey scan XPS spectra revealed the Au and Co signals (Fig. S33). High-resolution Au 4f XPS spectra showed a negative shift of the Au 4f signals of TF-COF-CONH-Au25-Co relative to TF-COF-CONH-Au25 (Fig. S34), reflecting the decrease in the valence state of Au species due to the interaction between Co species and Au25 NCs [17]. High-resolution Co 2p XPS spectrum of COF-CONH-Au25-Co displayed peaks at 797.4 and 781.4 eV, confirming Co(II) species (Fig. S35) [53]. The Co 2p signals of TF-COF-COOH-Co and TF-COF-CONH-Au25-Co were red-shifted relative to TF-COF-Co, revealing a higher Co oxidation state (Fig. S35). XANES analysis further supported these findings. The Au L3-edge profile of TF-COF-CONH-Au25-Co resembled that of TF-COF-CONH-Au25 (Fig. 3A), with a slight negative shift suggesting reduced Au valence [54], while the Au coordination environment remained unchanged, confirming the structural integrity of Au25 NCs. In the Co K-edge XANES (Fig. 3D), the absorption edges of TF-COF-Co, TF-COF-COOH-Co, and TF-COF-CONH-Au25-Co align closely with CoO, indicating Co valence near +2. The subtle edge shifts were attributable to ligand-field/covalency differences from bpy-N coordination. A weak pre-edge at ~7,707 eV further supported a distorted octahedral Co(II), consistent with the Co 2p XPS result [55,56]. The Co valence increased in the sequence of TF-COF-Co < TF-COF-COOH-Co < TF-COF-CONH-Au25-Co, reflecting enhanced Co–COF interactions promoted by –COOH groups and Au25 NCs (Figs. S36 and S37) [55,57,58]. FT-EXAFS spectra of these Co-loaded COFs exhibited the Co–N contribution at 1.6 Å (Fig. 3E). No distinct peak corresponding to the Co–Co coordination was observed at ~2.1 Å, reflecting the atomically dispersed Co species [59,60]. EXAFS fitting gave the CN of ~6 and bond length of 2.1 Å for Co–N contribution (Fig. 3F, Figs. S38 to S42, and Table S3). The d-PDF profiles of TF-COF-CONH-Au25 and TF-COF-CONH-Au25-Co showed the presence of correlations at r = 1.3, 1.5, 2.1, and 2.3 Å, respectively, for C═N, C−C, Co−N, and C~N (pyridyl C–N) of loaded [Co(bpy)3]2+. This result revealed the existence of immobilized [Co(bpy)3]2+, which is further confirmed by the good match between the refined d-PDF profile and the experimentally obtained one (Fig. 3H and Figs. S43 to S45).
Photocatalytic CO2 reduction
We performed the photocatalytic CO2 conversion under visible light irradiation (≥400 nm). TF-COF-CONH-Au25-Co demonstrated outstanding activity, continuously generating syngas (CO + H2) with linear product accumulation over time (Fig. 4A and Figs. S46 to S49). CO and H2 were produced at rates of 2,321.9 and 1,717.4 μmol·g−1·h−1, respectively, corresponding to the turnover number (TON) and turnover frequency (TOF) of 171.9 and 7.2 h−1 for CO generation. An ~1- to 2-h induction preceded steady state, arising from photoactivation of Co centers and triethanolamine (TEOA)-assisted charge separation build-up, as evidenced by the reversible growth of a 500- to 700-nm band in quasi in situ UV–vis spectra (Figs. S46 and S47) [61]. By contrast, TF-COF-COOH-Co achieved CO and H2 production rates of 1,042.7 and 124.8 μmol·g−1·h−1, respectively, while TF-COF-Co exhibited minimal activity (103.9 μmol·g−1·h−1 for CO and 60.4 μmol·g−1·h−1 for H2) (Fig. 4B). The superior performance of TF-COF-CONH-Au25-Co highlighted the synergy of Au25 NCs and Co centers. The inertness of Co-free analogs (TF-COF, TF-COF-COOH, and TF-COF-CONH-Au25) confirmed that Co species served as the active sites (Fig. S50). Au25 NCs produced no detectable CO and only trace H2 (76 μmol·g−1·h−1; Fig. S51), excluding Au25 as a carbon dioxide reduction reactioin (CO2RR) site. The physically mixed Au25+TF-COF-CONH-Co delivered a lower CO production rate of 1,313.0 μmol·g−1·h−1 (Fig. S51), underscoring that covalent tethering is critical for efficient interfacial charge transfer. The apparent quantum yield for TF-COF-CONH-Au25-Co reached 0.58% at 400 nm (Fig. S52). A comprehensive comparison with previous photosensitizer-free systems under comparable reaction conditions revealed that TF-COF-CONH-Au25-Co is advanced in terms of CO production rate, TON, and TOF (Fig. 4C and Table S4) [29,51,52,55,59,62].
Control experiments further verified the reaction pathway (Fig. S53). Certain H2 and trace CO were detected without the sacrificial agents. No product was detectable under dark conditions, illustrating the light dependence. Ar atmosphere led to the sole H2 production, suggesting the formation of CO from CO2. Photoreduction of 13CO2 (Fig. 4D) caused the formation of 13CO, as demonstrated by the 13CO signal (mass/charge ratio [m/z] = 29) in gas chromatography–mass spectrometry, substantiating CO2 as the carbon source [63]. 1H NMR spectrum excluded the generation of potential liquid products (Fig. S54). Over 5 consecutive 8-h cycles (40 h total), COF-CONH-Au25-Co sustained a CO rate of 1,798.8 μmol·g−1·h−1 (~77% retention; Fig. 4E). Postreaction PXRD of the recovered catalyst showed intact COF reflections without peak broadening or shifts (Fig. S55), while XPS confirmed preservation of the Au0 4f7/2 and Co2+ 2p3/2 features and the absence of new oxide signatures (Fig. S56). Inductively coupled plasma mass spectrometry showed the recovered catalyst retained 1.62 wt % Co (~85% of the starting 1.91 wt %), showing only minor leaching and quite robust immobilization of Co complex. By contrast, the physically mixed Au25+TF-COF-CONH-Co exhibited poorer activity retention (Fig. S57), indicating that covalent anchoring markedly enhances durability. In addition, we further evaluated the photocatalytic performance of TF-COF-CONH-Au25-Co under natural sunlight (Fig. 4F). The CO and H2 production rate depended on the weather conditions, exhibiting the highest activity (891.6 and 637.5 μmol·g−1·h−1 for CO and H2) on 2025 October 10, a sunny day.
Photoelectrochemical properties
We systematically evaluated the photoelectrochemical properties of TF-COF, TF-COF-COOH, TF-COF-CONH-Au25, and their Co-loaded analogs. UV–vis spectra revealed that the introduction of 4-carboxyquinoline units into TF-COF induced a red shift in the absorption edge (Fig. 5A), attributed to enhanced π-conjugation [35,62]. The optical bandgap energy (Eg) changed from 2.38 eV for TF-COF to 1.95 eV for TF-COF-COOH and 1.86 eV for TF-COF-CONH-Au25 (Figs. S58 and S59). Mott–Schottky analysis indicated n-type semiconducting behavior for all 3 COFs, with conduction band (CB) potentials of −0.86, −0.99, and −1.12 V versus the normal hydrogen electrode (NHE) [64], sufficiently negative for CO2-to-CO transformation (−0.53 V versus NHE) (Figs. S60 to S63) [32,40]. Co loading resulted in little effect on the band alignment but slightly enhanced visible-light absorption (Fig. 5A and Fig. S59).
Steady-state photoluminescence (PL) spectra showed a pronounced emission for TF-COF, while TF-COF-COOH, TF-COF-CONH-Au25, and TF-COF-CONH-Au25-Co demonstrated progressively quenched signals, reflecting the inhibited radiative recombination (Fig. 5B) [62]. Time-resolved PL spectra revealed prolonged carrier lifetimes by introducing –COOH (1.42 ns) and Au25 NCs (1.44 ns) relative to TF-COF (1.29 ns), with the longest lifetime for TF-COF-CONH-Au25-Co (1.68 ns) (Fig. 5C and Table S5) [20,35,62]. Transient surface photovoltage (TPV) measurements showed an increasing trend in both signal intensity and accumulated charge (A) from TF-COF to TF-COF-CONH-Au25-Co, along with shorter charge extraction times (Tmax) and larger decay constants (τ), with τ increasing from 0.104 μs (TF-COF) to 1.140 μs (TF-COF-CONH-Au25-Co) (Fig. 5D to F and Table S6), suggesting a obviously reduced recombination rate [65,66]. Electrochemical impedance spectroscopy (EIS) spectra showed the smallest semicircle for TF-COF-CONH-Au25-Co, corresponding to the lowest charge transfer resistance (Fig. S64) [28,55,67]. The photocurrent density followed the order of TF-COF < TF-COF-COOH < TF-COF-CONH-Au25 < TF-COF-CONH-Au25-Co (Fig. S65), in agreement with the improved charge separation and transport efficiency [55,62].
The photoelectrochemical properties of the other 2 Co-loaded materials, TF-COF-Co and TF-COF-COOH-Co, were compared with each parent COF. Co loading enhanced the charge separation and transfer as shown by the declined peak intensity in the PL spectra, extended carrier decay lifetime in the time-resolved PL spectra, increased TPV peak intensity, elongated decay constants, declined charge transfer resistance, and boosted photocurrent response (Figs. S66 to S70 and Tables S5 and S6) [55,68,69]. All these measurements indicate that the cooperation of Au25 NCs, COFs, and Co sites enhances light harvesting, facilitates charge separation and transfer, and thereby provides an abundance of long-lived photoelectrons.
Femtosecond transient absorption (fs-TA) spectra were collected to investigate the electron transfer dynamics. All samples exhibited a broad negative bleaching signal, indicative of ground-state bleaching, suggesting the excitation of carriers leading to exciton formation (Fig. 5G and H and Figs. S71 to S73). As the probe time progressed, this negative bleaching signal gradually transitioned into an excited-state absorption (ESA) signal, indicating the occurrence of charge transfer processes. Notably, TF-COF-CONH-Au25-Co displayed a obviously stronger ESA signal than TF-COF-CONH-Au25, TF-COF-COOH, and TF-COF [70]. Global fits to the TA kinetics resolved biexponential decays for TF-COF, TF-COF-COOH, and TF-COF-CONH-Au25 and a triexponential response for TF-COF-CONH-Au25-Co (Fig. 4I, Figs. S74 to S76, and Table S7) [71]. We assigned the fast component (τ1) to electron diffusion/dissociation within the COF domain and the slower component (τ2) to trap-assisted recombination [56,72]. TF-COF showed τ1 = 3.99 ps (k1 ≈ 2.5 × 1011 s−1) and τ2 = 365.2 ps (k2 ≈ 2.7 × 109 s−1), while carboxylation modestly lengthens both τ1 (4.89 ps) and τ2 (437.6 ps) over TF-COF-COOH, consistent with slightly improved carrier separation [33]. Incorporating Au25 further extended lifetimes (τ1 = 5.87 ps and τ2 = 518.9 ps), indicating efficient exciton harvesting by the cluster yet recombination that still dominates on the subnanosecond scale in the absence of Co. By contrast, TF-COF-CONH-Au25-Co exhibited τ1 = 0.93 ps (k1 ≈ 1.1 × 1012 s−1), τ2 = 34.3 ps (k2 ≈ 2.9 × 1010 s−1), and an emergent long-lived component τ3 = 982.3 ps (k3 ≈ 1.0 × 109 s−1). We attributed these to ultrafast COF → Au25 exciton capture (τ1), rapid Au25 → Co electron relay and stabilization at the cobalt center (τ2), and a persistent charge-separated state (τ3) that suppresses recombination [72]. Thus, covalent integration of Au25 (electron sink) with Co sites transforms a 4- to 6-ps/300- to 500-ps exciton recombination sequence into subpicosecond harvesting, tens-of-picosecond interfacial transfer, and ~1-ns charge separation, quantitatively accounting for the stronger time-resolved PL quenching, reduced EIS semicircles/TPV shifts, and higher CO formation rates.
In situ spectral and theoretical study
Quasi in situ XPS spectra of TF-COF-CONH-Au25-Co under light irradiation (Fig. 6A) indicated that the Au 4f level underwent a −0.5 eV shift, whereas the Co 2p level shifted by +0.3 eV (Fig. 6B). The negative shift on Au signifies transient electron accumulation, while the positive shift on Co indicates electron depletion (upward band bending), collectively evidencing a directional, light-driven COF → Au25 → Co electron-transfer cascade [73,74].
Photochromism further corroborated carrier generation and shuttling. Dispersions of TF-COF-CONH-Au25 in acetonitrile (MeCN)/TEOA/1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) turned yellow to green under light. In situ electron paramagnetic resonance (EPR) revealed a strong growth of the paramagnetic signal after illumination (Fig. 6C). In contrast, the EPR intensity diminished when [Co(bpy)3]2+ is present (TF-COF-CONH-Au25-Co), indicating electron transfer from the COF/Au25 ensemble to the Co complex with the reduction of Co sites [75,76]. Cyclic voltammetry resolved 2 irreversible reductions assignable to [Co(bpy)3]2+ → [Co(bpy)2]+ and [Co(bpy)2]+ → [CoI(bpy)(bpy)•−]. Currents were enhanced under CO2 versus Ar, consistent with engagement of the reduced Co species in CO2 activation (Fig. S77) [55,56].
In situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements were used to track the CO2 photoreduction pathway over TF-COF-CONH-Au25-Co (Fig. 6D). Characteristic bands at 1,721 and 1,402 cm−1 were assignable to HCO3−, formed via coadsorption of CO2 and H2O [41]. Signals at 1,669, 1,593, and 1,352 cm−1 corresponded to the *COOH intermediate [77], while additional peaks at 1,559/1,281 cm−1 and 1,531/1,486 cm−1 indicated bidentate (b-CO32−) [78] and monodentate (m-CO32−) [37,79]. A distinct band at 2,134 cm−1 was assignable to *CO [34,79]. The gradual increase in peak intensities with irradiation time confirmed continuous formation of these intermediates, supporting a reaction sequence of CO2 adsorption, *COOH generation, *CO formation, and CO release. The Co-free control (TF-COF-CONH-Au25) showed only bicarbonate/carbonate signals (no *COOH/*CO; Fig. S78), whereas TF-COF-COOH-Co (no Au25) exhibited the same assignment but with lower *COOH/*CO intensities (Fig. S79). Together with EPR, these results establish Co complexes as the catalytic reduction sites and demonstrate that Au25 funnels electrons to Co, amplifying intermediate coverage and turnover.
Theoretical study using density functional theory (DFT) was conducted on representative frameworks of TF-COF-Co, TF-COF-COOH-Co, and TF-COF-CONH-Au25-Co (Fig. S80). As illustrated in Fig. 6E, the rate-determining step was the *COOH formation [34,80], with the barrier sequence of TF-COF-CONH-Au25-Co (1.17 eV) < TF-COF-COOH-Co (1.48 eV) < TF-COF-Co (1.76 eV). The lowest energy barrier of TF-COF-CONH-Au25-Co was thermodynamically responsive to its best activity. This sequence cohered with the more negative conduction band from Mott–Schottky analysis for TF-COF-CONH-Au25-Co, which provided a greater thermodynamic driving force for interfacial electron transfer, and with its longest TA carrier lifetime, indicating a larger steady-state electron population reaching the surface [81]. Together, these factors complemented the lowest computed kinetic barrier and rationalize the highest activity. Consistently, hydrogen evolution reaction (HER) energetics (Fig. 6F) revealed that TF-COF-CONH-Au25-Co has a smaller barrier for *H generation (1.48 eV) than TF-COF-Co (1.61 eV) and TF-COF-COOH-Co (1.97 eV), explaining its superior H2 evolution rate and syngas productivity.
Differential charge density and Bader charge analyses (Fig. S81) revealed that –COOH functionalization boosted electron transfer from the COF framework to the Co center, increasing from 0.65 e in TF-COF-Co to 0.94 e in TF-COF-COOH-Co [53,82]. Incorporation of Au25 NCs further increased electron transfer to 0.98 e, indicating that covalent Au–amide bonding strengthens Co–COF coupling and facilitates interfacial charge transport. Moreover, a near-linear correlation between Co 2p binding energy shifts and Bader charges (Fig. S82) placed TF-COF-CONH-Au25-Co at the largest positive shift/charge depletion, quantitatively evidencing strong Au25-induced electronic modulation at the Co center [22,83]. A mechanistic model is thus proposed in Fig. 6G. Under illumination, photogenerated holes are consumed by the sacrificial agent, while electrons migrate via the amide linkage to the lowest unoccupied molecular orbitals (LUMOs) of Au25 NCs, which act as electron reservoirs [74,84]. These electrons are subsequently transferred to the CO2-adsorbed [Co(bpy)3]2+ site through the COF framework (Fig. 6G), driving CO formation [55]. The synergistic effect of covalent anchoring and interfacial electrostatics not only stabilizes Au25 NCs but also optimizes the Co electronic environment, thereby lowering the barrier of the rate-limiting *COOH step and enhancing overall CO2 reduction efficiency.
Conclusion
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Atomically precise Au25 NCs were covalently immobilized on a carboxyl-functionalized COF via amide bond formation. Sequential incorporation [Co(bpy)3]2+ fabricated a highly efficient, photosensitizer-free heterogeneous catalyst for photocatalytic CO2 conversion into syngas under visible-light illumination. The immobilized Au NCs serve a dual function. First, they promote light-harvesting and accelerate charge separation and transfer by serving simultaneously as broadband absorbers and efficient electron sinks. Second, they adjust the electronic structure of nearby Co sites, optimizing intermediates binding and lowering the activation barrier of the rate-limiting step. This work presents a molecularly engineered photocatalyst that integrates [Co(bpy)3]2+, COFs, and atomically precise metal NCs into a unified platform, highlighting the great prospect of COFs as functional supports for metal NCs toward high-performance solar-to-chemical energy conversion.
Materials and Methods
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Synthesis of TF-COF
To synthesize TF-COF, a 25-ml Pyrex tube was prepared with TTA (0.17 mmol, 67 mg), TFA (0.17 mmol, 56 mg), n-butanol (2.7 ml), o-dichlorobenzene (0.9 ml), and 3 M acetic acid (0.45 ml). The mixture was sonicated for 10 min and subsequently degassed at 77 K through 3 freeze–pump–thaw cycles. The tube was then sealed and heated at 120 °C for 3 d. The solid product formed was isolated using centrifugation, followed by successive washing with ethanol (3 × 10 ml) and tetrahydrofuran (THF; 3 × 10 ml). The resultant powder was dried under vacuum, yielding approximately 87% of yellow TF-COF.
Synthesis of TF-COF-COOH
Similarly, a 25-ml Pyrex tube containing TF-COF (63 mg), n-butanol (2.7 ml), o-dichlorobenzene (0.9 ml), 3 M acetic acid (0.45 ml), p-toluenesulfonic acid (TsOH) (20 mg), and pyruvic acid (60 μl) was prepared. The mixture was sonicated for 10 min and degassed at 77 K through 3 freeze–pump–thaw cycles. After sealing, the mixture was heated at 120 °C for 3 d. The solid was collected via centrifugation and subjected to washing with ethanol (3 × 10 ml) and THF (3 × 10 ml). The final product, yellow TF-COF-COOH, was dried under vacuum, yielding an approximate 92% yield.
Synthesis of TF-COF-CONH-Au25
To synthesize TF-COF-CONH-Au25, TF-COF-COOH (40 mg), N-hydroxysuccinimide (11.4 mg), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide·HCl (37.8 mg) were combined in a 10-ml 2-(n-morpholino)ethanesulfonic acid (MES) solution (ethanol/H2O = 1/5, 4.2 mg·l−1) and stirred for 10 min. Subsequently, a 10-ml solution of Au25 NCs (0.57 mg·ml−1) was added to the mixture, which was then stirred for 2 h at room temperature. The resultant solid was isolated via centrifugation, washed with deionized water, and dried under vacuum to obtain the final product.
Preparation of Co-loaded COF TF-COF-CONH-Au25-Co
To prepare the Co-loaded COF, CoCl2·6H2O (2 mg, 5.5 μmol) and 2,2′-bipyridine (4.3 mg, 27.5 μmol) were dissolved in 5 ml of MeCN. TF-COF-CONH-Au25 (10 mg) was then added to this solution. The mixture was stirred for 12 h under a nitrogen atmosphere at room temperature. Afterward, the solid was isolated through filtration, washed with water, and dried overnight in an oven at 60 °C to yield the Co-loaded product, TF-COF-CONH-Au25-Co. The same method was used for preparing TF-COF-Co and TF-COF-COOH-Co, utilizing the respective COF materials.
Photocatalytic CO2 reduction
For the photocatalytic CO2 reduction, a gas-tight quartz reactor (90 ml) was filled with a powder sample (1 mg), MeCN (4 ml), TEOA (1 ml), and a solution of BIH (20 mg). High-purity CO2 (99.999 vol %) was then bubbled through the reaction mixture for 20 min to completely displace air. A 300-W xenon lamp (Microsolar 300, Beijing Perfectlight Technology Co. Ltd.) equipped with a UV cutoff filter (λ ≥ 400 nm) served as the light source to initiate the reaction at room temperature. The reaction setup was stirred vigorously using a magnetic stirrer throughout the photocatalytic process. After the reaction, gas samples were collected and analyzed using gas chromatography (GC-9860-5CNJ, Nanjing Hope Analytical Equipment Co. Ltd., China), using calibrations with standard gas mixtures. Hydrogen was detected using a thermal conductivity detector, while a flame ionization detector was utilized for measuring carbon monoxide and other hydrocarbons. The liquid phase products underwent identification through 1H NMR analysis. To evaluate the recycling potential of the catalyst, a 4-run recycling test was conducted, where, after each cycle, the catalyst was cleaned with water and subsequently isolated by centrifugation for the next cycle. To investigate the practical applicability, the catalytic efficiency of TF-COF-CONH-Au25-Co was evaluated in an open-air environment on the rooftop of a laboratory building from 2025 October 9 to 12.
XAFS analysis
XAFS spectra were measured at the XAFCA beamline of Singapore Synchrotron Light Source with an electron energy of 0.7 GeV. Athena program was used for the energy calibration, background removal, and Fourier transform of the XAFS spectra. EXAFS fitting was conducted using Artemis software.
Neutron total scattering
Neutron total scattering data were collected on a Multi-Physics Instrument at China Spallation Neutron Source (CSNS), Dongguan, China. The samples were pretreated under vacuum at 393 K for 3 h and then cooled to 10 K for the measurement. The total scattering S(Q) data were converted to PDF G(r) data through Fourier transformation. d-PDF spectrum was calculated from the PDF data of the COFs before and after Au25 NCs or [Co(bpy)3]2+ loading. PDF refinement was carried out on the program Topas-academic v.7. The structural models involved in the refinements were constructed according to the results of PXRD refinement, structure optimization by DFT calculation, and XANES simulation.
In situ spectral and theoretical study
In situ ATR-SEIRAS spectra were collected on a Thermo Fisher Scientific Nicolet iS50 spectrometer (USA). DFT calculation was performed with the Vienna Ab initio Simulation Package. Experimental procedures for the in situ spectroscopy and computational details, together with other characterizations, are available in the Supplementary Materials.
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doi: 10.34133/research.1152
  • Receive Date：2025-10-31
  • Online Date：2026-10-16
  • Published：2026-02-20
Article Data
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  • Received：2025-10-31
  • Revised：2026-01-11
  • Accepted：2026-01-27
Affiliations
    1State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China.
    2University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 101408, China.
    3Spallation Neutron Source Science Center, Dongguan 523803, China.
    4Institute of Sustainability for Chemicals, Energy and Environment, A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Singapore 627833, Singapore.

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* Address correspondence to: (Y.Z.);
* Address correspondence to: (J. W.)
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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