Ethanol photosynthesis from CO_2 and H_2O via a formate intermediate pathway

Domain: biology
Published: 2026-03-25
Editorial outcome: minor_revision

Abstract
Photocatalytic CO2-to-ethanol conversion offers an interesting route for intermittent solar energy utilization. However, the performance is impeded by slow mass-transfer and high dimerization barrier (>1.0 eV) of *CO intermediates, coupled with a kinetic mismatch between the short-lived carriers and the 12e⁻ reaction. Herein, we develop a Z-scheme Cuδ+O/floral-like BN photocatalyst that achieves CO2-to-ethanol conversion with near 100% selectivity. The photocatalyst utilizes the three-dimensional structure and alkaline surface of BN to concentrate CO2 and directs transfer of long-lived carriers to unsaturated Cu⁺ sites, stabilizing them for flexible electron donation to the π* orbital of CO2. Enhanced ethanol photosynthesis activity/selectivity stems from a new pathway: CO2 is first reduced to HCOOH at Cuδ+O sites, lowering the C–C coupling barrier to 0.44 eV. Liquid formate is then concentrated and protonated to C2 precursor, minimizing side reactions for high selectivity. Here, we show the feasibility of selective ethanol photosynthesis and uncovers a new formate intermediate pathway.

Introduction
Photocatalytic transformation of CO2 to high value-added chemicals is a promising strategy for addressing the global energy crisis and mitigating the rising CO2 concentration in the atmosphere1-5. Direct photoreduction of CO2 to ethanol is of particular interest due to its high energy density (~1366.8 kJ mol−1) and ease of storage, provising a complement to traditional industrial catalysis6-9 (Supplementary Fig. 1). Unfortunately, the photocatalytic carbon dioxide reduction reaction (PCO2RR) to produce ethanol remains inefficient, primarily due to the inherent slow reaction kinetics10-13 and a mismatch between transient photogenerated charge carriers and the multi-step (12 e⁻) reaction pathway14-16.

The slow kinetics of carbon-carbon (C-C) coupling leads to limited selectivity and efficiency in CO2 conversion to C2 products17-21. The CO2-to-ethanol conversion usually follows an asymmetric C-C coupling pathway, primarily involving *COCHO and/or *COCOH as reactive intermediates22-24. Numerous studies have reported copper-based catalysts for the selective conversion of CO2 to specific C2 products25-28. These strategies generally rely on increasing the surface coverage of *CO intermediates and enhancing their mass transfer to facilitate C-C coupling. However, it is evident that regardless of whether the coupling occurs between *CO and another *CO, *CHO, or *COH species, the energy barrier typically exceeds 1 eV29-31. Theoretically, such a high energy barrier (~1 eV) renders C-C coupling challenging, and the resultant slow kinetics are fundamentally incompatible with the short lifetime of photogenerated electrons in PCO2RR (Supplementary Table 1).

In recent studies of liquid-phase electrochemical CO2RR, it is found that S-modified copper-based catalysts preferentially reduce adsorbed *CO to formate rather than CO32, 33. This selectivity originates from thermodynamics: the conversion of *CO to *OCHO, a key formate intermediate, is typically spontaneous, while the pathway via *COOH generally involves an uphill energy barrier34. Additionally, our previous research demonstrated that formate, as a C1 intermediate, participated in C-C coupling to form ethanol with a low energy barrier of 0.13 eV7. These findings prompt an intriguing question: can formate be utilized as a key intermediate in liquid-phase PCO2RR to establish an asymmetric C-C coupling route for ethanol synthesis (Supplementary Fig. 2)? Unlike electrocatalysis, which operates under an applied bias, photocatalysis depends solely on separated photogenerated electrons and holes. This requires reactive intermediates to rapidly migrate and accumulate on the surface of photocatalyst, allowing timely electron/hole injection to initiate photoreduction/photooxidation. Hence, constructing localized high-concentration formate centers in proximity to the C-C coupling active sites becomes crucial for ethanol photosynthesis via a formate intermediate pathway.

Herein, we design a Cuδ⁺O/BNF heterostructured photocatalyst by growing Cuδ⁺O on boron nitride flower (BNF), which enables CO2-to-ethanol conversion in pure water with near quantitative ethanol selectivity (100%) and competitive stability (maintaining 95% of the initial activity after 25 hours) under sunlight illumination. CO2 is first photocatalytically reduced to formic acid on the Cuδ⁺O surface, which is subsequently enriched at the three-dimensional Cuδ⁺O/BNF interface. The adjacent Cu-(O)-Cu sites then promote formic acid dehydration coupling via a proton-coupled electron transfer (PCET) mechanism, producing ethanol. Density functional theory (DFT) calculations indicate an energy barrier for formic acid dehydration coupling on Cuδ⁺O/BNAF of only 0.44 eV, providing a highly efficient C-C coupling pathway for ethanol synthesis via PCO2RR. Furthermore, femtosecond transient absorption spectroscopy and in-situ photoelectric measurements verify the Z-scheme catalytic interface of Cuδ⁺O/BNF, which effectively enhances charge separation and electron accumulation, promoting the 12 e- transfer ethanol synthesis. In this work, we show a novel photocatalytic CO2-formate-ethanol reaction pathway, which offering a feasible approach to synthesize ethanol from CO2 and H2O powered by sunlight.

Results
Structural characterization of Cuδ+O/BNF

The Cuδ+O/BNF heterostructured photocatalyst was prepared by a two-step synthesis route as illustrated in Supplementary Fig. 3. In the first step, a crystalline BNF was prepared through a high-temperature calcination process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements clearly reveal the three-dimensional flower-like morphology of BNF formed by orderly stacking of one-dimensional nanofibers (Fig. 1a and Supplementary Figs. 4a, 4b). In the subsequent step, Cuδ+O was grown onto BNF via reduction of Cu2+ ions using hydrazine hydrate, thereby constructing a Cuδ+O/BNF composite. SEM and TEM measurements reveal that the Cuδ+O particles are embedded within the nanofibrous matrix of BNF (Supplementary Figs. 4c, 4d and Fig. 1b). The selected area electron diffraction (SAED) pattern depicts coexistence of both crystalline Cu2O and BNF. As displayed in Fig. 1c and Supplementary Fig. 5a and 5b, a clear interface between Cu2O and BNF can be observed in the high-resolution transmission electron microscopy (HRTEM) image with lattice fringes of 0.246 and 0.333 nm, corresponding to the (111) plane of cubic Cu2O and the (002) plane of BNF, respectively35, 36. Energy dispersive X-ray spectroscopy (EDS) mapping image shows clear boundaries between the Cu2O and BNF phases (Supplementary Fig. 5c).

To investigate the structure of the as-prepared photocatalysts, a collection of characterizations was conducted, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). As demonstrated in the XRD patterns, compared to Cuδ+O, the diffraction peaks of Cuδ+O in the Cuδ+O/BNF composite shift towards larger angles, indicating lattice compression, which may be attributed to the close interaction at the interface between Cuδ+O and BNF (Supplementary Fig. 6 and Figs. 1d, 1e)37, 38. The FT-IR spectra recorded over Cuδ+O/BNF and BNF display similar -NH2/-OH and B-O/B-N vibrational peaks, suggesting that the growth of Cuδ+O on BNF has little influence on the chemical structure of the BNF matrix (Supplementary Fig. 7)39.

XPS was employed to elucidate the chemical states (Supplementary Fig. 8). In the high-resolution B 1s XPS spectrum, the peaks centered at ~191.9 eV and ~190.7 eV can be ascribed to the B-O bond and the B-N bond, respectively (Supplementary Fig. 9a). As demonstrated in Supplementary Fig. 9b, the high-resolution N 1s XPS spectrum displays two peaks centered at 398.9 eV and 398.0 eV, which are attributable to B-N-C and B-N, respectively.40. As illustrated in Figs. 1f, 1g and Supplementary Fig. 9c, the minor peaks centered at approximately 954.6 and ~934.5 eV, and the major peaks centered at approximately 951.7 and 931.8 eV, are attributed to Cu(II) and Cu(I), respectively. It is noteworthy that the characteristic B 1s XPS peak for Cuδ+O/BNF shifted to lower binding energies in comparison with that for BNF, indicating electron transfer from Cuδ+O to BNF upon depositing Cuδ+O onto BNF due to their different Fermi levels41. Concurrently, this electron transfer increased the binding energy of the main peak in the Cu 2p XPS spectrum of Cuδ+O/BNF. The Auger signals are consistent with the aforementioned findings (Supplementary Fig. 9d).

To further probe the coordination structure of Cu atoms, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were conducted. As illustrated in the Cu K-edge XANES spectra (Fig. 1h), the absorption edge for Cuδ+O/BNF is positioned between that for CuO and Cu2O, indicating that the Cu species in Cuδ+O/BNF possess an oxidation state between +1 and +2 (Cuδ+, 1 < δ < 2). In particular, the Cuδ+O/BNF shows two signals A and B (Fig. 1i), which can be ascribed to the 1s → 4p (continuum) electron transition of Cu+ and Cu2+, respectively, suggesting that the electron orbital of the Cu atom in Cuδ+O/BNF lies between that in Cu2O and CuO21, 42, further corroborated by the signal within the pink region.

Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) measurement in R-space was performed to ascertain the local structure of Cu (Supplementary Figs. 10–12). The Cu K-edge k3-weighted FT-EXAFS spectrum of Cuδ+O/BNF displays a single prominent peak at 1.46 Å and a secondary minor peak at 2.54 Å, which can be ascribed to the Cu-O bond and the Cu-Cu bond, respectively (Supplementary Table 2). The wavelet transform EXAFS (WT-EXAFS) plot of Cuδ+O/BNF (Fig. 1j) exhibits a single maximum at (6.45 Å–1, 1.46 Å), derived from the Cu-O contribution of the first coordination shell. The other WT signal (6.87 Å–1, 2.54 Å) is situated between that of CuO and Cu2O, possibly due to local lattice distortion43.

Photocatalytic CO2 reduction reaction

The CO2 photoreduction performance was evaluated in a batch reactor filled with pure water under simulated sunlight illumination from a Xe lamp equipped with a reflector to provide irradiation mainly in the wavelength region of 320–780 nm. The gaseous and liquid products were analyzed by gas chromatography and nuclear magnetic resonance (NMR), respectively. As shown in Fig. 2a, in comparison with Cuδ+O and BNF, the optimized Cuδ+O/BNF (with 10 wt.% Cuδ+O, Supplementary Fig. 13a) displays a promising photocatalytic performance in reducing CO2 to ethanol, achieving an ethanol production rate of 68.5 µmol g–1 h–1 with ~100% ethanol selectivity (Figs. 2b, 2c). The ethanol production rate could be maintained constant with increasing reaction time, yielding 1.63 mmol of ethanol per gram of photocatalyst after 25 hours of PCO2RR (Figs. 2d, 2e). Additionally, after 100 hours of cycling tests, the ethanol yield remained above 85% (Supplementary Fig. 13b). Compared to the published data in literature, the Cuδ+O/BNF photocatalyst displays competitive performance in artificial ethanol photosynthesis via PCO2RR in pure water (Supplementary Fig. 13b and Table 3). The structure of Cuδ+O/BNF after PCO2RR showed negligible changes, as demonstrated in Supplementary Fig. 14. Control experiments conducted in the absence of a photocatalyst, in dark condition, or replacing CO2 with Ar did not result in the production of ethanol, suggesting that ethanol was produced via a photocatalytic process (Supplementary Fig. 15a). 13CO2 isotope experiment further confirmed that ethanol was produced via PCO2RR (Supplementary Fig. 15b)44.

To demonstrate the practical application potential of Cuδ+O/BNF for PCO2RR, the CO2 reduction performance was evaluated in a panel reactor with a 0.3  0.3 m2 panel under Xe lamp and natural sunlight illumination (Fig. 2f)45. It is evident that the ethanol yield in the panel reactor is always higher than that in the tank reactor under both laboratory and outdoor conditions, reaching 1.44 and 1.059 µmol h–1, respectively. Additionally, a continuous outdoor PCO2RR experiment was conducted over a period of 7 days. The average ethanol yield achieved 1.05 µmol h–1, indicating the reliable photocatalytic CO2 reduction to ethanol performance in actual ambient environments (Figs. 2g, 2h and Supplementary Fig. 16).

PCO2RR mechanism

To probe the PCO2RR mechanism over the Cuδ+O/BNF photocatalyst, the chemical state and electronic structure at the Cuδ+O and BNF interface under steady state was first examined (Supplementary Figs. 17–22). Figs. 3a and 3b display the XPS elemental mapping images of Cuδ+O/BNF, which reveal uniform elemental distribution within a 200 m region at the Cuδ+O and BNF interface46. To probe charge distribution across Cuδ+O/BNF, density functional theory (DFT) calculation was performed (Supplementary Data 1). Fig. 3c shows the charge-density difference plot for Cuδ+O/BNF, from which, a pronounced accumulation of charge is observed on Cuδ+O (pink isosurface)47. Subsequently, in-situ XPS measurement was conducted to elucidate the charge transfer pathway across the Cuδ+O/BNF interface under light illumination. As depicted in Figs. 3h, 3i and Supplementary Figs. 23, 24, under light irradiation, a positive shift in the binding energies of B 1s and N 1s was observed. In contrast, the Cu 2p XPS spectrum showed an increased peak intensity ratio of Cu(I)/Cu(II), indicating that the photoexcited electrons in BNF were transferred to Cuδ+O under light irradiation. It is noteworthy that the identified charge transfer pathway follows a Z-scheme mechanism (Supplementary Fig. 25)48, 49. To directly probe charge transfer across the Cuδ+O−BNF interface, in-situ Kelvin probe force microscopy (KPFM) measurement was conducted. As shown in Fig. 3j, the Cuδ+O in Cuδ+O/BNF displays a larger dimensionality than BNF. The VCPD mapping of Cuδ+O/BNF performed in dark gives a contact potential difference of 16.9 mV, indicating that the work function of BNF is higher than that of Cuδ+O, which shall induce electron flow from Cuδ+O to BNF to create an interfacial electric field (IEF) across Cuδ+O/BNF. Under light illumination, the VCPD of both Cuδ+O and BNF increased, whereas BNF showed a more significant enhancement as compared to Cuδ+O (Fig. 3m). On the contrary, for the tests of single components (Figs. 3k, 3l), the Cuδ+O exhibited a higher photoelectric voltage response (33.5 mV) in comparison to BNF (3.6 mV). These results indicate formation of a Z-scheme heterojunction in Cuδ+O/BNF (Fig. 3n)50.

To investigate the photogenerated charge carrier dynamics, a series of photochemical spectroscopic characterizations were conducted. As displayed in Supplementary Fig. 26a, the Cuδ+O/BNF sample exhibits a distinctive steady-state photoluminescence (PL) quenching at 400 nm, indicating a significant suppression of charge recombination. This conclusion is further corroborated by time-resolved photoluminescence spectroscopy (TRPL) measurement (Supplementary Fig. 26b)51. The reduced lifetime of Cuδ+O/BNF indicates charge transfer between Cuδ+O and BNF, induced by IEF, which can be further verified in the photoelectric characterization (Supplementary Fig. 27). Femtosecond transient absorption (fs-TA) spectroscopy measurement was performed to gain a deeper understanding of the charge carrier dynamics in Cuδ+O/BNF. The TA spectra of BNF, Cuδ+O and Cuδ+O/BNF were recorded at varying time points following a 300 nm pump pulse, which displayed pronounced ground-state-bleach features at 600 nm (Figs. 3d, f and Supplementary Fig. 28). As shown in the fitted decay profiles (Figs. 3e, g and Supplementary Fig. 28), the rapid decay component τ0 (1.20 ps) observed in Cuδ+O can be attributed to the electron diffusion over Cuδ+O lattices. The short lifetime component, τ1, is indicative of electron trapping from the conduction band (CB) into the trap states, while the long lifetime component, τ2, corresponds to electron-hole recombination. It is noteworthy that the significantly smaller τ1 (1.23 ps) of Cuδ+O/BNF as compared to BNF indicate that the photoexcited charge carriers in BNF can be effectively transferred to Cuδ+O. The considerably longer τ2 (169.1 ps) of Cuδ+O/BNF indicates greatly reduced charge recombination in Cuδ+O/BNF52.

To unravel the mechanistic origin of the high selectivity towards ethanol production in photocatalytic CO2 reduction over Cuδ+O/BNF, the distinct roles of individual catalytic components were systematically investigated, and the complete reaction pathway was mapped through a combination of experimental and theoretical investigations. The adsorption isotherms of CO2 on various photocatalysts were first acquired. In comparison with BNF and Cuδ+O, Cuδ+O/BNF possesses a higher capacity for CO2 uptake (Supplementary Fig. 29). The CO2 temperature programmed desorption (CO2-TPD) curve of Cuδ+O/BNF shows three distinct regions: ɑ, β and γ, corresponding to weak adsorption sites, medium adsorption sites and strong adsorption sites, respectively. As displayed in Fig. 4a, it is clear to see that Cuδ+O/BNF exhibits a pronounced adsorption for CO2, attributed to the adsorption and enrichment effect of BNF on CO2, which is beneficial for PCO2RR.

To determine the reaction pathway of PCO2RR on Cuδ+O/BNF, in-situ FT-IR spectroscopy measurements were conducted. As displayed in Fig. 4b and Supplementary Fig. 30, the peaks at 1541.8 cm–1, 1507.6 cm–1 and 1339.3 cm–1 are assigned to *m-CO32–. The peaks at 1313.7 cm–1 and 1515.0 cm–1 are ascribed to *b-CO32– and *CO32–, respectively53. The peaks at 1456.8 cm–1, 1417.4 cm–1 and 1386.6 cm–1 are attributed to *HCO3–. The peaks at 1536.4, 1433.9 and 1361.9 cm–1 are assigned to *HCOO–54, 55. Importantly, the essential signals of *C-C can be observed at 1488.9 cm–1 and 1374.0 cm–1, and their intensities gradually increase with increasing light irradiation time, clarifying the photoinduced C-C coupling process56. The vibrational peaks located at 1396.2, 1472.6, 1488.9 and 1374.0 cm–1 are indexed to *OC2H5, *CH3O, and *CH3, respectively, which are associated with the formation of ethanol44, 57.

The photocatalytic reaction was further performed by replacing CO2 with CO or formic acid (HCOOH) as the potential reactant. As demonstrated in Fig. 4c, little ethanol could be produced in CO + H2O system. In contrast, similar ethanol yield could be detected in Ar + HCOOH and CO2 + HCOOH systems, close to that detected in CO2 + H2O system. These results suggest that HCOOH may act as a key C1 intermediate in C-C coupling for the synthesis of ethanol via PCO2RR. Together with in-situ FT-IR observation of adsorbed *CO32– species (Fig. 4d) – the key intermediate for CO2 reduction to generate HCOOH58, we deduce that CO2 may undergo initial activation at the Cu-O sites to form HCOOH, followed by further reduction and C-C coupling to eventually produce ethanol under light illumination over Cuδ+O/BNF. DFT calculation of CO2 → HCOOH (Fig. 4e and Supplementary Tables 4, 5) reveals a significantly lower Gibbs free energy barrier on Cuδ+O surface (ΔG = 0.36 eV) than BNF surface (ΔG = 1.93 eV), tending to support that Cu-O sites are responsible for CO2 reduction to generate HCOOH (Fig. 4f and Supplementary Figs. 31–34). It should be noted that the DFT model represents a simplified approximation of the Cuδ+O-BNF catalytic interface, and discrepancies may arise in accounting for surface coverage, support effects, solvation effects, and the temperature-pressure conditions of heterogeneous catalysis.

To determine the active species responsible for C-C coupling in PCO2RR over Cuδ+O/BNF, photocatalytic reaction was conducted in Ar + HCOOH system over Cuδ+O and BNF alone. It was found that ethanol could be produced over Cuδ+O instead of BNF (Fig. 4g and Supplementary Fig. 35), suggesting that the C-C coupling is likely facilitated by Cuδ+O sites. In PCO2RR, mechanically mixed Cuδ+O and BNF could produce ethanol at 15 µmol g–1 h–1, while Cuδ+O alone failed to produce ethanol, indicating that the Cuδ+O-BNF interface plays a critical role in ethanol photosynthesis (Fig. 4h). DFT calculations suggest a stronger *HCOOH adsorption (Figs. 4i, 4j, Supplementary Figs. 36, 37 and Table 6) and a lower *HCOOH-*HCOOH coupling barrier over Cuδ+O/BNF than Cuδ+O. To reveal the reaction energetics of HCOOH reduction to ethanol, two possible *HCOOH coupling mechanisms were assessed: the proton-coupled electron transfer (PCET) dehydration coupling pathway and the step-by-step hydrogenation coupling pathway. Fig. 4k, Supplementary Fig. 38 and Tables 7, 8 compare the free energy diagrams of the two *HCOOH coupling pathways. It is evident that, for the PCET dehydration coupling pathway, two *HCOOH can be easily dehydrated and coupled with a free energy barrier of only ~0.44 eV. In contrast, HCOOH hydrodehydration necessitates to pass a higher energy barrier of approximately 0.65 eV. Following the reduction of all carbonyl groups to aldehyde groups, the formation of CHO-CHOH intermediate is extremely difficult with a free energy barrier of ~1.24 eV, implying that dehydration coupling is a more likely reaction pathway for ethanol photosynthesis over Cuδ+O/BNF. Compared to the spontaneous ethanol formation pathway, the formation of C2H4 is endothermic (~0.37 eV). It is thus concluded that the *CH3CH2OH intermediate tends to desorb to release ethanol product rather than undergo further dehydration to form ethylene. Additionally, to check whether the produced ethanol could be re-oxidized by photogenerated holes over Cuδ+O/BNF, the adsorption energies of ethanol and H2O at varying catalytic sites on Cuδ+O/BNF were calculated (Supplementary Figs. 39-42 and Tables 9, 10). The calculation results indicated poor ethanol adsorption on Cuδ+O/BNF, explaining the observed accumulation of ethanol during ethanol photosynthesis over Cuδ+O/BNF. Furthermore, the kinetic limitations of water oxidation are also mitigated to some extent, which may be attributed to the enhanced hole separation and transport enabled by the designed Z-scheme heterojunction and the three-dimensional flower-like morphology of BNF (Supplementary Fig. 43).

In summary, a Z-scheme Cuδ+O/BNF heterostructured photocatalyst has been successfully developed for PCO2RR to synthesize ethanol with near quantitative ethanol selectivity. Based on a collection of experimental characterizations and theoretical calculations, it is found that CO2 is first photocatalytically reduced to formic acid over Cuδ⁺O, which undergoes further dehydration coupling to eventually produce ethanol. The formate intermediate involved pathway significantly reduces the C–C coupling barrier. Furthermore, the Z-scheme catalytic interface of Cuδ⁺O/BNF effectively enhances photogenerated charge separation. Both of which promote the 12e- transfer ethanol synthesis under sunlight illumination. This work uncovers a new formate intermediate pathway for PCO2RR to make value-added chemicals.

Methods
Materials

Melamine (99%), boric acid (99.50%), and Cu(NO3)2·3H2O (99.99%) were analytical reagents and purchased from Aladdin chemical reagent Co., Ltd. (Shanghai, China). Methenamine (99%) and hydrazine hydrate (50%) were obtained from Sinopharm chemical reagent Co., Ltd. (China). All chemicals were used without further purification.

Synthesis of BNF

In a standard synthetic procedure, 1 mmol of melamine, 1 mmol of boric acid, and 5 mmol of methenamine were placed in a beaker and dissolved in 40 mL of deionized water. The resulting solution was heated at 80 °C in a water bath under continuous stirring for 10 minutes. Afterwards, the beaker was removed from the water bath and stirred for an additional 10 minutes at room temperature. The system was then transferred to an ice bath for cooling, followed by deep-freezing treatment with liquid nitrogen. Finally, the precursor product was obtained via freeze-drying. Subsequently, the precursor was placed into a tube furnace and calcined at 550 °C for 2 h at a heating rate of 5 °C min–1, and at 950 °C for 3 h at a heating rate of 5 °C min–1 in N2 gas. After naturally cooling to room temperature, the BNF sample was obtained.

Synthesis of Cuδ+O

In a standard synthesis process, Cu(NO3)2·3H2O was dissolved in deionized water to form a transparent blue solution. Subsequently, N2H4·H2O was added dropwise, and the resultant solution was kept stirred for 20 min. Afterwards, the product was collected by centrifugation, washed with deionized water three times, and dried in a vacuum oven at 60 °C.

Synthesis of Cuδ+O/BNF

Cuδ+O/BNF was prepared via in-situ chemical reduction. Specifically, 50 mg of BNF was dispersed into 50 ml of deionized water under sonication, followed by dropwisely adding an appropriate amount of Cu(NO3)2·3H2O. Then the resultant solution was stirred for 1 hour. Afterwards, N2H4·H2O was added dropwise, and the solution was stirred for 20 min until it turned pink. Finally, the product was collected by centrifugation and washed with deionized water three times. The collected product was redispersed in deionized water, frozen by liquid nitrogen, and freeze-dried to obtain Cuδ+O/BNF. Theoretical load amounts were named as 5%, 10%, 20% and 30% Cuδ+O/BNF.

Photocatalytic CO2 reduction measurement

Photocatalytic CO2 reduction reaction (PCO2RR) was conducted within a 500-mL reactor equipped with a quartz glass window. A 300 W Xe lamp (Beijing Perfectlight, 320–780 nm) was used as the light source with light density of 300 mW cm–2. The light density of the light source is measured by a light power meter. Following photoreaction, the gaseous products were analyzed using a gas chromatograph (GC9790 II (PLF-01), Fuli, Zhejiang) equipped with thermal conductivity and flame ionization detectors, and the liquid products were quantified by nuclear magnetic resonance (NMR, Bruker AVANCE AV III 400). In a typical PCO2RR experiment, 10 mg of photocatalyst was dispersed uniformly into 10 mL of H2O under constant stirring (400 rpm min−1). After evacuation, the reactor was filled with high-purity CO2 gas (99.999%) until the internal pressure reached approximately 1 atm. Following three cycles of evacuation and refilling, the PCO2RR was initiated40. During the photoreaction, a circulation cooling system was used to maintain the reaction system at room temperature (20 °C).

Characterizations

The crystalline information was analyzed by X-ray diffraction (XRD, Bruker D8 diffractometer) with Cu Kα radiation (λ = 1.5418 Å). Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Bruker Vetext 80V from 4000 to 400 cm−1. Transmission electron microscopy (TEM) images were obtained on a Tecnai G2 F20 transmission electron microscope. Scanning electron microscopy (SEM) measurements were performed on a JSM7800F equipped with an energy dispersive X-ray spectroscopy detector. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 220Xi spectrometer with an Al Kα source. X-ray absorption spectroscopy (XAS) of Cu was performed at beamline 4B9A of the Beijing Synchrotron Radiation Facility (BSRF). The powder samples were coated onto the Kapton tape (3M Co., Ltd) examined in a transmission mode. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were processed using the Athena software package. Ultraviolet-visible spectroscopy (UV-vis) absorption spectra were recorded on a Shimadzu solidSpec-3700/3700DUV system. Photoluminescence spectra and time-resolved fluorescence decay spectra were recorded on a QuantaMaster 40 (QM-40) spectrophotometer (Horiba-PTI). Kelvin probe force microscopy (KPFM) was recorded by a Bruker dimension ICON-XR. CO2 physical adsorption measurements were performed on Micrometrics ASAP 2460 at 298 K. The CO2 temperature-programmed desorption was measured on a MicroActive AutoChem II 2920 instrument.

Photoelectrochemical measurements

All photoelectrochemical measurements were performed in a three-electrode setup using a CHI660E electrochemical workstation (Chenhua CHI660E). First, an ink solution was prepared with a volume ratio of isopropanol to Nafion of 1:0.02. Then, 5 mg of photocatalyst powder was added to 1 mL of this solution and ultrasonicated for 0.5 hours to form a homogeneous dispersion. Subsequently, the uniformly dispersed suspension was drop-cast onto the surface of indium tin oxide (ITO) conductive glass to fabricate the working electrode59. Pt and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. During the photocurrent and EIS tests, 0.5 M Na2SO4 solution and 5 mmol·L−1 K4[Fe(CN)6]/K3[Fe(CN)6] were applied as electrolytes, and the illumination source was a Xenon lamp (Beijing Perfectlight, PLS-FX300HU). The electrolytes are prepared and used immediately.

In-situ XPS measurement

In-situ XPS measurement was conducted on an Escalab 220Xi spectrometer with an Al Kα source. The light source was a 300 W Xe lamp. An optical fiber was used to transmit light to irradiate the sample during XPS measurement.

In-situ KPFM characterization

In-situ KPFM characterization was used to simultaneously measure the surface morphology and surface potential of the catalyst under dark and illumination (420 nm LED, 6.67 mW cm−2) conditions. 1 mg catalyst was dispersed in 10 mL H2O by ultrasonication for 5 min. The 100 μL of the dispersion was taken and diluted 100 times with H2O. Then 200 μL of the diluted liquid was dropped on the surface of a clean silicon wafer substrate and dried in a vacuum drying oven. After that, the silicon wafer substrate with the sample was placed on the KPFM sample stage for scanning.

SPV = VCPD-light - VCPD-dark (1)

The contact potential difference (VCPD) arises due to the work function (W) disparity between the sample and the tip.

VCPD = Vsample - Vtip = (Wtip - Wsample)/e (2)

where VCPD stands for the contact potential difference between the sample and the tip, while Vsample and Vtip are the absolute potentials of the sample and the probe, respectively. Besides, Wtip and Wsample are the work functions of the tip and the sample, respectively.

The Fermi level (Ef) and work function (W) follows the below equation:

Ef = -W (3)

In-situ FT-IR spectroscopy measurement

In-situ FT-IR spectrometry measurement was performed on a Nicolet iS–50 spectrometer (Thermo Scientific) equipped with an attenuated total reflection component (ATR). The catalyst was dispersed into water solution and then dropped on the ATR crystal. The baseline was obtained before irradiation. After that the system was kept in the dark for 10 mins and signal was collected at 10 mins, which was used as the baseline. Finally, the Xe lamp was turned on to irradiate the sample for 60 mins and data was collected at 10, 20, 30, 40, 50, and 60 mins.

Ultrafast transient absorption (TA) spectroscopy measurements

The TA spectra were recorded on a femtosecond spectroscopy system composing of a regenerative-amplified Ti: sapphire laser system and a Helios pump-probe system (Ultrafast Systems). The regenerative-amplified Ti: sapphire laser system (Legend Elite-1K-HE, center wavelength of 800 nm, pulse duration of 25 fs, pulse energy of 4 mJ, repetition rate of 1 kHz) was seeded with a mode-locked Ti: sapphire laser system (Vitara) and pumped with a Nd: YLF laser (Evolution 30). In the transient absorption spectroscopy system, the pump light is provided by a sapphire laser and its output wavelength is set to 300 nm through an optical parametric amplifier (TOPAS-C). The pump pulses are modulated by a 500 Hz mechanical chopper, enabling the alternating recording of transient absorption spectra under pumped and unpumped conditions. The probe light is generated by splitting a portion of the output from a regenerative amplified Ti:sapphire laser. After passing through a motor-controlled optical delay line, the 800 nm beam is focused onto a sapphire crystal to produce continuous white-light probe pulses covering the wavelength range of 430–850 nm. By adjusting the optical path difference between the pump and probe beams, precise control over their relative time delay is achieved. To correct for intensity fluctuations in the white-light pulses, a reference beam is split from the continuous white light for real-time normalization. The instrument response function of the system is measured to be approximately 150 fs (full width at half maximum) 60.

Density functional theory calculations

Density functional theory (DFT) employing the generalized-gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was utilized for first-principles calculations61, 62. The Vienna Ab Initio Simulation Package (VASP) was employed with a plane-wave basis set spanning reciprocal space and the projector-augmented wave (PAW) method. Valence configurations for B, N, O, C, H, and Cu atoms, respectively, were considered as follows: 2s2 2p1, 2s2 2p3, 2s2 2p4, 2s2 2p2, 1s1, and 3d10 4s1. Utilizing a slab model, a vacuum spacing of approximately 15 Å along the z-direction was incorporated to separate neighboring interactions. The basis set parameters included an energy cutoff of 450 eV, an energy convergence threshold of 1×10−4 eV, and a forced convergence of 0.01 eV/Å.

For the BN (001) slab model, a 5×5 single atomic layer containing 25 B atoms and 25 N atoms was constructed. To determine the favorable O atomic doping site in BN, various defect models were established. The formation energy (Ef) for the adsorption structure was calculated according to Ef = EO-BN + μv + μa - (EBN + μO), where EO-BN and EBN represent the energies for pristine BN and the structure with O doping-induced defects, respectively. μa, μv, and μO denote the energies of the substituting atom, vacancy, and O atom, respectively. The energies of O and N atoms were derived from N2 and O2, while the energy of C atoms was derived from diamond. A 3×3×1 Monkhorst-Pack k-point grid was utilized.

For the BN (100) slab model, a 2×5 4-layer atomic layer containing 32 B atoms and 32 N atoms was constructed. For the Cu2O (111) slab model, a 4×4 4 atomic layer containing 64 Cu atoms and 32 O atoms was built. A 2×2×1 Monkhorst-Pack k-point grid was utilized. For the Cu2O/O-BN model, a hexagonal prism model simultaneously exposing the (111) equivalent crystal plane of Cu2O was placed atop the O-BN model, containing 24 B atoms, 22 N atoms, 22 O atoms, and 22 N atoms. A 2×2×1 Monkhorst-Pack k-point grid was employed.

Gibbs free energy was calculated using the following formula:

ΔG = ΔE - TΔS + ΔEZPE (4)

where ΔE represents the zero-point energy and S denotes the entropy of intermediate products.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Liu and co-workers developed a Z-scheme Cuδ+O/floral-like boron nitride (BNF) photocatalyst that enabled efficient 20 photocatalytic CO2-to-ethanol conversion with near 100% ethanol selectivity. Given that the photocatalytic reduction of CO2 to ethanol is 12 e- process, which is quite difficult in kinetics. This work is interesting, which may provide a potential route for the CO2 reduction with solar energy. In this regard, I think this work can be accepted after addressing the following comments.\n1. The atuhors stated the strong interaction in Cu-BNF. However, the size particle of Cu is above 500 nm, how this large particles enable the so-call strong synergy between Cu and BNF?\n2. The TEM image in Figure 1c is insufficient to observe the interface. The authors should provide a high-resoultion TEM image to repalce the old one.\n3. The authors used H2O as the reductant to realize CO2-to-ethanol, and indentified HCOOH was a crucial intermediate. In this case, the authors should provide the detailed process for CO2+H2O to HCOOH. In this process, CO2 was reduced by photogenerated elctrons (2e-), which part goes to consume holes?\n4. The authors should provide the AQY data under differet wavelength.\n5. What is the structural model for DFT calculations?\n6. In the EXAFS spectra, only Cu-O coordination was fitted. Will Cu-N coordination exist in Cu-BNF？If surface Cu atoms were not coordinated with N or B atoms in BNF, how can the electrons migrate between Cu and BNF?\n7. The authors should provide the structure of the spent Cu-BNF after cycled test.\n8. Some recent literatures should be referred.\n9. The authors should carefully check the manuscript to avoid any typos.", "reviewer_2": "In this manuscript, Liu and co-workers reported a Z-scheme Cuδ+O/BNF heterostructure photocatalyst that achieves nearly 100% selectivity toward ethanol production from CO₂ and H₂O under sunlight illumination. The key point lies in a proposed formate-intermediate pathway, which reduces the C–C coupling energy barrier to 0.44 eV, addressing the kinetic limitations of traditional *CO-mediated routes. The catalyst also demonstrates remarkable stability and scalability under natural sunlight. The study is suitable for Nature Communications based on its conceptual advance and importance to the field, however the authors need to address the following comments prior to acceptance.\n1. The well-established advantages of nanomaterials in catalysis, could the authors clarify the rationale for prioritizing microscale cubic copper oxide over a nanostructured counterpart? A discussion on the potential benefits (e.g., enhanced stability, distinct facet exposure) or specific catalytic mechanism enabled by the larger particle size would strengthen the manuscript.\n2. Could the valence states of Cu be unambiguously identified, and can the specific functions of Cu⁺ and Cu²⁺ in the catalytic reduction be clearly defined?\n3. The promising performance of the Cuδ+/BNF catalyst in outdoor tests is noteworthy. To better gauge its practical potential, a preliminary assessment of the catalyst's cost and the scalability prospects of the entire reaction system would be highly valuable for the readership.\n4. The proposal of the formate coupling pathway in this manuscript as an alternative to the traditional CO coupling pathway is the innovation of this paper. Why are the differences between CO coupling and formate coupling not mainly compared in the Gibbs free energy calculation? Instead, why are the differences among different formate coupling pathways compared?\n5. The radar chart in Figure 2e should be supplemented with specific numerical labels (such as ethanol yield and selectivity), so as to facilitate direct comparison with the benchmark in the literature. The test results of the photocurrent (Supplementary Figure 25a) are abnormal. Please provide some explanations.\n6. While the 25-hour test shows promising initial stability with 95% activity retention, a more in-depth analysis of the catalyst's stability is warranted given the known susceptibility of copper-based catalysts to deactivation. Post-reaction characterization is needed to discuss potential deactivation mechanisms and project long-term durability.", "reviewer_3": "Ethanol Photosynthesis from CO2 and H2O via a Formate Intermediate Pathway\nBy Zheyang Liu et al.\nThis study achieves ~100% ethanol selectivity in photocatalytic CO2 reduction using\npure water as proton source without electrolytes or sacrificial agents, it is rare and\nsuperior to most reported systems. The authors identified a new HCOOH intermediate\npathway for C-C coupling, reducing the kinetic barrier to 0.44 eV for ethanol formation.\nThis is di(cid:431)erent from the conventional CO dimierization pathway with barriers\ngenerally > 1.0 eV. The z-scheme junction between the Cuδ+O and floral-like BNF\nenhances charge separation and extends carrier lifetime to 169.1 ps, thus promotes\nelectron accumulation at Cu+ site. The authors applied multiple in-situ techniques (FT-\nIR, XPS, KPFM, fs-TA, XAS) combined with DFT calculations to elucidate the reaction\npathways, providing strong evidence for the reaction mechanism, charge transfer and\nactive site roles. This study demonstrates outdoor photocatlytic CO2 reduction under\nnatural sunlight over 7 consecutive days showing stable ethanol production of 1.05\nµmol·h⁻¹, significantly enhances the practical relevance of the study. 68.5 µmol·g⁻¹·h⁻¹\nethanol production in pure water without sacrificial agents or electrolytes is an\nadvantageous for practical solar-driven CO₂ reduction. The catalyst shows negligible\nstructural change (retains 95% of initial activity after 25 hours) after reaction suggesting\ngood stability.\nDespite these advantages, there are a few limitations:\n1. The light source (320-780 nm) is broad, it seems that the authors did not discuss\nabout the catalysts’ bandgap, light absorption range or action wavelength, which\nlimits understanding of the solar utilization e(cid:431)iciency.\n2. While the formate pathway is strongly supported, the exact role of interfacial\nwater and the proton transfer mechanism during PCET are not deeply explained.\nThe source of protron for the multistep reduction (12 e⁻ transfer) is pure water is\nnot explicitly explained, kinetics of water oxidation on BNF could be a bottleneck.\n3. The study does not compare with high performance electrocatalytic CO2 to\nethanol systems (Cu based GDE cells), which achieved higher rates (>100\nmA·cm⁻²)\n4. No discussion of catalyst cost, recyclability or energy payback is provided, BN is\nmore expensive than common photocatalyst supports (TiO2 and Carbon)\n5. While 25 hour stability is good, longer term testing (100-500 h) would strengthen\nthe stability claim, since practical applications require thousands of hours of\noperation.\n6. The produced ethanol is in aqueous solution at low concentration (µmol). No\ndiscussion is provided on product extraction or separation for practical\napplications.\n7. The preparation of 3D floral-like BN involves high temperature calcination and\nfreeze drying and the controlled deposition of Cuδ+O requires careful reduction\nsteps, which may pose challenges for large scale cost e(cid:431)ective applications.\nIn summary, this study presents a significant advance in photocatalytic CO2 to ethanol\nconversion by using a novel Z-scheme heterojunction catalyst, and discover a novel\nformate mediated C-C coupling pathway with lower energy barrier. The reviewer\nrecommends for publication after addressing the above limitations."}, "author_response": "Response Letter\nReviewer #1\nIn this work, Liu and co-workers developed a Z-scheme Cuδ+O/floral-like boron nitride (BNF)\nphotocatalyst that enabled efficient photocatalytic CO -to-ethanol conversion with near 100%\n2\nethanol selectivity. Given that the photocatalytic reduction of CO to ethanol is 12 e- process, which\n2\nis quite difficult in kinetics. This work is interesting, which may provide a potential route for the\nCO reduction with solar energy. In this regard, I think this work can be accepted after addressing\n2\nthe following comments.\n1. The authors stated the strong interaction in Cu-BNF. However, the size particle of Cu is above\n500 nm, how this large particles enable the so-call strong synergy between Cu and BNF?\nResponse: We thank the reviewer for raising the valuable question. The \"strong interaction\"\ndiscussed in our work mainly refers to electron-level coupling, which can be effectively realized\nthrough interface engineering even when one component is at the micrometer scale. In our initial\ndesign, to achieve strong synergistic effect between micron-sized copper oxide and the BN substrate,\nwe deliberately employed an in-situ growth method.\nThe strong interaction between micron-sized copper oxide and the BN substrate is demonstrated\nby multiple lines of evidence:\n(1) Morphology (Figs. R1a, b): Cubic copper oxide particles are grown in-situ and tightly wrapped\nby 3D flower-like BN.\n(2) Structural Stress (Figs. R1c, d): XRD peak shifts indicate stress exerted by the BN substrate\nduring the growth of copper oxide particles.\n(3) Electronic Coupling (Figs. R1e, f): XPS peak shifts in Cu+ and B–O signals confirm a strong\ninterfacial electronic coupling.\nFig. R1 | (a) TEM image of Cuδ+O/BNF. (b) SEM image of Cuδ+O/BNF. (c) and (d) XRD patterns\nof Cuδ+O and Cuδ+O/BNF. (e) B 1s and (f) Cu 2p high resolution XPS spectra of BNF, Cuδ+O and\nCuδ+O/BNF samples.\n2. The TEM image in Figure 1c is insufficient to observe the interface. The authors should provide\na high-resolution TEM image to replace the old one.\nResponse: We thank the reviewer for the nice suggestion. In response, we considered that the color\noverlay might have caused the lattice to appear less distinct. In Fig. R2 (HRTEM), we have\nre-demarcated and labeled the different regions. The revised HRTEM image was added to Figure\nS3 (Supporting Information).\n(1) The regular linear lattice in the bottom region, with a spacing of 0.246 nm, corresponds to the\n(111) plane of cuprous oxide (Cuδ+O).\n(2) The less-ordered lattice in the top right region, with a spacing of 0.333 nm, corresponds to the\n(002) plane of boron nitride (BN).\n(3) The selected-area electron diffraction (SAED) pattern from the central demarcated region\nexhibits characteristics of both a single crystal and an amorphous structure, corresponding to Cuδ+O\nand BN, respectively.\nThis figure clearly reveals the interface structure between the Cuδ+O species and BNF, confirming\nthe intimate contact between cuprous oxide and BN at their phase boundary.\nFig. R2 | HRTEM image of Cuδ+O/BNF.\n3. The authors used H O as the reductant to realize CO -to-ethanol, and identified HCOOH was a\n2 2\ncrucial intermediate. In this case, the authors should provide the detailed process for CO +H O\n2 2\nto HCOOH. In this process, CO was reduced by photogenerated electrons (2e-), which part\n2\ngoes to consume holes?\nResponse: We thank the reviewer for prompting us to clarify this key mechanistic detail. As shown\nin Fig. R3a, under illumination, photogenerated carriers are directionally transported and\naccumulated at the Cuδ+O side under the influence of the built-in electric field, where CO is reduced\n2\nto HCOOH and other intermediates. Meanwhile, holes remain on the BN side, oxidizing H O to\n2\nproduce O and protons (H+). These protons serve as the proton source for all subsequent\n2\nhydrogenation steps, including the 12-electron transfer process.\nFurthermore, we have added kinetic energy barrier calculations for the hole-oxidation side to the\nSupporting Information. As shown in Fig. R3b, through DFT calculation, the free energy changes\nfor the water oxidation reaction (2H O → O + 4H+ + 4e-) catalyzed by cuprous oxide supported on\n2 2\nboron nitride framework (Cuδ+O/BNF) and pristine cuprous oxide (Cuδ+O) are compared. Both\ncatalysts follow the typical four-electron water oxidation pathway: H O → *H O → *OH → *O →\n2 2\n*OOH → *O . In the Cuδ+O system, the step from *O to *OOH is the rate-determining step, with\n2\nan energy barrier of 2.23 eV. In the Cuδ+O/BNF system, the energy barrier for the same reaction step\nis reduced to 1.41 eV, indicating that the introduction of BNF can significantly lower the kinetic\nbarrier for water oxidation. The Cuδ+O/BNF curve lies entirely below that of Cuδ+O, suggesting that\nthe free energies of all intermediate states are generally lower, making the reaction\nthermodynamically more favorable.\nFig. R3 | (a) Band structure of Cuδ+O/BNF heterojunction under UV–visible light irradiation: BIEF\ninduced charge transfer, separation, and the formation of Z-scheme heterojunction. (b) Reaction\nenergy profiles and the corresponding intermediate structures for H O-to-O conversion over\n2 2\nCuδ+O/BNF.\n4. The authors should provide the AQY data under different wavelength.\nResponse: We sincerely appreciate the reviewer for the helpful suggestion. Accordingly, we\nperformed AQY tests using a bandpass filter (390 nm) and a longpass filter (>400 nm). The results,\nhowever, showed that no ethanol product was detected under illumination with wavelengths ≥ 390\nnm. Based on this, we conducted a thorough analysis.\n(1) The overall reaction from CO and H O to ethanol (C H OH) is a complex multi-electron/proton\n2 2 2 5\nprocess requiring the absorption of multiple photons. In particular, the step driving C–C bond\nformation—specifically, the dehydration coupling of the formic acid intermediate that we propose—\nmay require an even relatively higher activation energy.\n(2) According to the Z-scheme heterojunction mechanism, under illumination with wavelengths ≥\n390 nm, the built-in electric field in Cuδ+O/BNF does not function effectively. In detail, light in the\nvisible range fails to excite electrons in BN sufficiently to consume the holes on the Cuδ+O side,\nthereby preventing effective accumulation of photogenerated carriers at the reductive Cuδ+O end.\nConsequently, light ≥ 390 nm is unlikely to provide sufficient driving force to enable the conversion\nto ethanol.\n5. What is the structural model for DFT calculations?\nResponse: We thank the reviewer for raising the question. Here, we provide a detailed description\nof how each model was constructed. For the BN (001) slab model, a 5×5 single atomic layer\ncontaining 25 B atoms and 25 N atoms was constructed. To determine the favorable O atomic doping\nsite in BN, various defect models were established. For the BN (100) slab model, a 2×5 four-layer\natomic layer containing 32 B atoms and 32 N atoms was constructed. For the Cuδ+O (111) slab\nmodel, a 4×4 four-atomic layer containing 64 Cu atoms and 32 O atoms was built.\nFor the Cuδ+O/O-BN model, a hexagonal prism was placed atop the O-BN model to\nsimultaneously expose the (111) equivalent crystal plane of Cu O. This model contains 24 B atoms,\n22\nthe O B type BN is negative compared to other O-doped BN models, indicating spontaneous\n2\nformation. Therefore, this model is identified as the BN (001) surface. In Figs. R4(f–h), the O-BN₁\ntype BN exhibits a lower formation energy and is therefore determined to be the BN(100) surface.\nBased on the dominant Cuδ+O (111) facet in combination with the BN models, a Cuδ+O/BN\nheterostructure was designed and constructed as shown in Fig. R4(i). These structural models can\nbe found in the Supporting Information (Supplementary Figures 29–32).\nFig. R4 | (a-d) O-BN model. (e) Formation energy of O-BN model. (f-g) Edge O atom substitution\nof the BN (100) model. (h) Formation energy of edge O atom substitution of the BN (100) model.\n(i) Cuδ+O/O-BN model. (pink: Cu, red: O, brown: C, white: H, gray: N, and blue: B)\n6. In the EXAFS spectra, only Cu-O coordination was fitted. Will Cu-N coordination exist in Cu-\nBNF? If surface Cu atoms were not coordinated with N or B atoms in BNF, how can the\nelectrons migrate between Cu and BNF?\nResponse: We thank the reviewer for raising these questions. As shown in Figs. R5a and b, we\nhave re-analyzed the XAFS spectra and performed the refinements that include a possible Cu–N\nscattering path. The results show no appreciable contribution from Cu–N coordination, suggesting\nthat the primary coordination environments consist of Cu–O and Cu–Cu. This implies that the\ndominant electron-transfer mechanism is likely mediated by space-charge transfer or the interfacial\nelectric field, rather than through the formation of direct Cu–N chemical bonds.\nIn accordance with the Z‑scheme heterojunction mechanism, effective interfacial charge transfer\ncan be realized via the built‑in electric field established through band alignment. As illustrated in\nFigs. R5c and d, in-situ XPS measurements indicate that under illumination, the binding energy of\nthe B–O bond decreases, while Cu2+ is significantly reduced to Cu+. These observations confirm\nthat the built‑in electric field effectively drives the directional migration of photogenerated carriers.\nFig. R5 | (a) Fourier-transformed k2-weighted EXAFS spectra of Cu foil, Cu O, CuO and\n2\nresolution XPS spectra for (c) Cu 2p and (d) B 1s recorded over Cuδ+O/BNF.\nTable R1. The radial distance of chemical bonds over Cuδ+O/BNF, Cu O, CuO and Cu foil.\nCuδ+O/BNF 1.45 2.53 /\nCu O 1.46 2.74 /\nCu foil / / 2.24\n7. The authors should provide the structure of the spent Cu-BNF after cycled test.\nResponse: We thank the reviewer for the nice suggestion. As illustrated in the Supporting\nInformation (Supplementary Figure 12), XRD and FTIR characterizations of the post-reaction\ncatalyst were conducted to analyze the structural changes after reaction. As demonstrated in Figure\nR6, the characteristic signals in the XRD patterns and FTIR spectra remain almost unchanged before\nand after the reaction, indicating the stable catalyst structure.\nFig. R6 | Structure characterizations of Cuδ+O/BNF before and after PCO RR: (a) XRD patterns.\n2\n(b) FTIR spectra.\n8. Some recent literatures should be referred.\nResponse: We thank the reviewer for the nice suggestion. Accordingly, we have revised the\nintroduction and discussion sections, incorporating additional references from recent literature\n(2023-2025) as detailed below:\nPhotocatalytic transformation of CO to high value-added chemicals has been regarded as a\n2\npromising strategy for addressing the global energy crisis and mitigating the rising CO\n2\nconcentration in the atmosphere1-5.\nUnfortunately, the PCO RR to produce ethanol remains inefficient, primarily due to the inherent\n2\nslow reaction kinetics6-9 and a critical mismatch between transient photogenerated charge carriers\nand the multi-step (12 e⁻) reaction pathway10-12.\nReferences\n4. Cui, Y. et al. A chiral mesostructured photocatalyst for efficient solar-driven CO reduction to\n2\nethanol. Nat. Synth. 10.1038/s44160-025-00908-2 (2025).\n5. Hu, Y. et al. Identifying a highly efficient molecular photocatalytic CO reduction system via\n2\ndescriptor-based high-throughput screening. Nat. Catal. 8, 126-136 (2025).\n9. He, Y. et al. Metastable dual-defect states drive deep protonation for selective CO photomethanation.\n12. Lyu, W., Liu, Y., Chen, D., Wang, F. & Li, Y. Engineering the electron localization of metal sites on\nnanosheets assembled periodic macropores for CO photoreduction. Nat. Commun. 15, 10589 (2024).\n2\n9. The authors should carefully check the manuscript to avoid any typos.\nResponse: We thank the reviewer for the nice suggestion. We have conducted a thorough review of\nthe entire manuscript and corrected the typos.\nReviewer #2\nIn this manuscript, Liu and co-workers reported a Z-scheme Cuδ+O/BNF heterostructure\nphotocatalyst that achieves nearly 100% selectivity toward ethanol production from CO and H O\n2 2\nunder sunlight illumination. The key point lies in a proposed formate-intermediate pathway, which\nreduces the C–C coupling energy barrier to 0.44 eV, addressing the kinetic limitations of traditional\n*CO-mediated routes. The catalyst also demonstrates remarkable stability and scalability under\nnatural sunlight. The study is suitable for Nature Communications based on its conceptual advance\nand importance to the field, however the authors need to address the following comments prior to\nacceptance.\n1. The well-established advantages of nanomaterials in catalysis, could the authors clarify the\nrationale for prioritizing microscale cubic copper oxide over a nanostructured counterpart? A\ndiscussion on the potential benefits (e.g., enhanced stability, distinct facet exposure) or specific\ncatalytic mechanism enabled by the larger particle size would strengthen the manuscript.\nResponse: We thank the reviewer for raising the valuable question. Our selection of micron-sized\ncopper oxide was based on the considerations of stability and reactivity.\nPrioritizing stability: The initial motivation for choosing micron-sized Cuδ+O was to prioritize\ncatalyst stability. Nanoparticles tend to migrate and agglomerate readily due to their high surface\nenergy, whereas micron-scale particles exhibit greater structural stability, which is consistent with\nthe excellent cycling performance we have observed (95% activity retention over 100 hours).\nEnsuring activity: Despite the micron-scale dimension of Cuδ+O, the construction of a\nheterojunction with BNF nanoflowers gives rise to a substantial number of interfacial active sites,\nthereby ensuring high reactivity.\n2. Could the valence states of Cu be unambiguously identified, and can the specific functions of\nCu+ and Cu2+ in the catalytic reduction be clearly defined?\nResponse: We thank the reviewer for raising the valuable question. Through the integration of XPS\nand XANES analyses, the mixed valence state of Cuδ+(1<δ<2) has been elucidated. As demonstrated\nin Figs. R7a and b, it is proposed that Cu+ sites, with their unsaturated electronic structure, are likely\nto be the key active sites for CO activation and the reduction process towards ethanol.\nmaterial under steady-state conditions, thereby ensuring its long-term preservation and usability. As\nillustrated in Fig. R7c, the transformation of Cu2+ to Cu+ under light exposure directly substantiates\nthat the unsaturated Cu+ sites function as the principal active centers for catalytic reduction.\nConcurrently, the reversion of Cu+ to Cu2+ following the cessation of light illumination indicates\nthat Cu2+ effectively stabilizes the material structure.\nFig. R7 | (a) Cu K-edge XANES spectra of Cu foil, Cu O, CuO and Cuδ+O/BNF. (b) XANES spectra\n2\nof Cu foil, Cu O, CuO and Cuδ+O/BNF. (c) In-situ high-resolution XPS spectra for Cu 2p recorded\n2\nover Cuδ+O/BNF.\n3. The promising performance of the Cuδ+O/BNF catalyst in outdoor tests is noteworthy. To better\ngauge its practical potential, a preliminary assessment of the catalyst's cost and the scalability\nprospects of the entire reaction system would be highly valuable for the readership.\nResponse: We thank the reviewer for the nice suggestion. Accordingly, we have conducted a\ndetailed cost analysis of the catalyst and the overall scale-up production, along with comparative\nevaluations (Supplementary Scheme 1).\nCost considerations: In consideration of the commercial production scale and the process\ntechnology of TiO , the utilization of BN as a support material entails a higher cost. However, given\n2\nthat the unique properties of BN (such as stability and CO adsorption) are essential for achieving\n2\nthe high catalytic ethanol conversion performance reported, and considering the potential for cost\nreduction through large-scale production in the future, this trade-off is justified.\nScale-up potential: As illustrated in Figs. R8a and b, a conceptual framework and design have\nbeen developed for a prospective scaled-up photocatalytic system. The successful 7-day continuous\noutdoor experiment represents a crucial step toward practical application. While further\nbreakthroughs in catalyst synthesis, reactor design, and product separation are required in order to\nachieve true industrialization, the economic and environmental advantages of a scaled-up\nphotocatalytic system are substantial. As demonstrated in Figs. R8c and d, when compared with\ntraditional thermal catalytic technologies, the photocatalytic system offers a significant advantage\nin terms of overall cost-effectiveness, taking into account carbon emissions, integrated energy\nconsumption, catalyst cost, and raw material costs.\nFig. R8 | (a) Schematic diagram of modular photochemical synthesis. (b) Ethanol yield during 7\nconsecutive days of photoreaction from 01/07/2025 to 07/07/2025 in an outdoor photoreaction\nsystem under natural sunlight illumination. The carbon emissions (c) and economic costs (d) per ton\nof ethanol produced by industrial thermochemical systems and clean photochemical systems.\n4. The proposal of the formate coupling pathway in this manuscript as an alternative to the\ntraditional CO coupling pathway is the innovation of this paper. Why are the differences\nbetween CO coupling and formate coupling not mainly compared in the Gibbs free energy\ncalculation? Instead, why are the differences among different formate coupling pathways\ncompared?\nResponse: We thank the reviewer for raising the valuable questions. Accordingly, in the discussion\nsection of the manuscript, we have conducted a thorough and detailed investigation into the reaction\nintermediates and coupling mechanisms.\n(1) As shown in Fig. R9d, within the CO + H O reaction system, the production of ethanol is\n2\nminimal. This observation suggests that, over Cuδ+O/BNF, the pathway involving CO as an\nintermediate for C-C coupling to ethanol is improbable.\n(2) To date, the reaction pathway involving CO as an intermediate for C-C coupling to ethanol has\nbeen extensively studied and reported (Figs. R9a-c)13-15. In contrast, the reaction pathway involving\nformic acid as a C1 intermediate in the process of producing ethanol has received minimal research\nattention. It is hypothesized that, in contrast to a mechanistic analysis of the CO coupling pathway,\na more profound and scientifically meaningful research endeavor would be constituted by the\nelucidation of the formic acid coupling mechanism. This would include the resulting C2\nintermediates and potential hydrogenation steps (Fig. R9e).\nFig. R9 | (a) Gibbs free energy diagrams for the CO to ethanol photoreduction pathways on Cu/S-\n(1 to 9) for the formation of C H OH via the CO RR on the TiO /BNF13. (c) Gibbs free energy of\n2 5 2 2−x\neach step during the CO reduction to ethylene over the CZTS-S slab, in which the energy barrier\n2 v\nof the rate-determining step (RDS) is 1.52 eV15. (d) Ethanol production under different reaction\nconditions over Cuδ+O/BNF. (e) Reaction energy profiles and the corresponding intermediate\nstructures for HCOOH-to-ethanol conversion over Cuδ+O/BNF (pink: Cu, red: O, brown: C, white:\nH, gray: N, and blue: B).\n5. The radar chart in Figure 2e should be supplemented with specific numerical labels (such as\nethanol yield and selectivity), so as to facilitate direct comparison with the benchmark in the\nliterature. The test results of the photocurrent (Supplementary Figure 25a) are abnormal. Please\nprovide some explanations.\nResponse: We thank the reviewer for the valuable suggestions.\nRadar Chart: We have added specific numerical labels for ethanol production rate and selectivity\non the radar chart (Fig. 2e and Fig. R10a).\nPhotocurrent Diagram: The photocurrent is fundamentally attributable to the directional migration\nof photogenerated charges. When a photon with sufficient energy irradiates a semiconductor, an\nelectron-hole pair is generated. The movement of these charge carriers, resulting from the separation\ncaused by an electric field, generates a current. However, in practical measurement, the photocurrent\nvalue reflects the number of photogenerated carriers collected on the ITO glass. Specifically, in the\nZ-scheme mechanism, the built-in electric field formed at the interface drives the directional\nmigration of carriers, enhancing the photocurrent density. As illustrated in Figures Figs. R10b and\nc, the SEM and TEM analyses provide unequivocal evidence that the Cuδ+O particles are closely\nwrapped by BN. In photocurrent testing, BN—rather than the two-phase interface—is in direct\ncontact with the ITO glass. Consequently, the photocurrent value of Cuδ+O/BNF is not significantly\nhigher than that of BN or Cuδ+O alone, but rather is comparable to that of the standalone BNF.\nFig. R10 | (a) Radar chart showing performance comparison. (b) TEM image of Cuδ+O/BNF. (c)\nSEM image of Cuδ+O/BNF.\n6. While the 25-hour test shows promising initial stability with 95% activity retention, a more in-\ndepth analysis of the catalyst's stability is warranted given the known susceptibility of copper-based\ncatalysts to deactivation. Post-reaction characterization is needed to discuss potential deactivation\nmechanisms and project long-term durability.\nResponse: We thank the reviewer for the constructive suggestion. Accordingly, we have performed\nthe post-reaction characterization of the catalyst.\nFirstly, in the in-situ XPS measurements, as demonstrated in Fig. R11a, the conversion of Cu2+\nto Cu+ under illumination directly confirms that the activated, unsaturated Cu+ serves as the primary\nreactive site in the reaction (metastable state). Conversely, the reversion of Cu+ to Cu2+ after the\ncessation of light irradiation indicates that, once the reaction ceases (steady state), the activated Cu+\non the surface is re-oxidized to the more stable, saturated Cu2+. This behavior is pivotal to the long-\nterm functionality of the catalyst.\nSecondly, XRD and FTIR characterizations of the post-reaction catalyst were conducted to\nanalyze its structural changes. As shown in Figs. R11b and c, the characteristic signals in the XRD\npatterns and FTIR spectra remain almost unchanged before and after the reaction, thereby\nconfirming the structural stability of the catalyst.\nTo conclude the experimental phase of the study, multiple long-term cycling tests were performed,\nwith each test lasting 25 hours. As demonstrated in Fig. R11d, following cumulative testing over\n100 hours, the ethanol yield retained over 85% of its initial value, thereby evidencing the material's\nlong-term durability.\nFig. R11 | (a) In-situ high-resolution XPS spectra for Cu 2p recorded over Cuδ+O/BNF. Structure\ncharacterizations of Cuδ+O/BNF before and after PCO RR: (b) XRD patterns. (c) FTIR spectra. (d)\nEthanol Photosynthesis from CO and H O via a Formate Intermediate Pathway\n2 2\nBy Zheyang Liu et al.\nThis study achieves ~100% ethanol selectivity in photocatalytic CO reduction using pure water as\n2\nproton source without electrolytes or sacrificial agents, it is rare and superior to most reported\nsystems. The authors identified a new HCOOH intermediate pathway for C-C coupling, reducing\nthe kinetic barrier to 0.44 eV for ethanol formation. This is different from the conventional CO\ndimerization pathway with barriers generally > 1.0 eV. The z-scheme junction between the Cuδ+O\nand floral-like BNF enhances charge separation and extends carrier lifetime to 169.1 ps, thus\npromotes electron accumulation at Cu+ site. The authors applied multiple in-situ techniques (FTIR,\nXPS, KPFM, fs-TA, XAS) combined with DFT calculations to elucidate the reaction pathways,\nproviding strong evidence for the reaction mechanism, charge transfer and active site roles. This\nstudy demonstrates outdoor photocatalytic CO reduction under natural sunlight over 7 consecutive\n2\ndays showing stable ethanol production of 1.05 μmol·h⁻1, significantly enhances the practical\nrelevance of the study. 68.5 μmol·g⁻1·h⁻1 ethanol production in pure water without sacrificial agents\nor electrolytes is an advantageous for practical solar-driven CO reduction. The catalyst shows\n2\nnegligible structural change (retains 95% of initial activity after 25 hours) after reaction suggesting\ngood stability.\n1. The light source (320-780 nm) is broad, it seems that the authors did not discuss about the\ncatalysts’ bandgap, light absorption range or action wavelength, which limits understanding of\nthe solar utilization efficiency.\nResponse: We thank the reviewer for the constructive comment. In the Supporting Information\n(Supplementary Figures 15-20), we have provided the UV-vis DRS spectra of the catalysts and\ncalculated the band gaps. As shown in Fig. R12, the band gap of pristine BN is 3.54 eV, with an\nabsorption edge at around 350 nm. The band gap of pristine Cuδ+O is 1.93 eV, with an absorption\nedge at around 640 nm. In contrast, the composite Cuδ+O/BNF exhibits two distinct band gaps, 3.4\neV and 2.01 eV, corresponding to the band gaps of BNF and Cuδ+O, respectively.\nFig. R12 | (a) UV-visible diffuse reflectance spectroscopy spectra of Cuδ+O, BNF and Cuδ+O/BNF.\n(b) Tauc plot of Cuδ+O/BNF. (c) Tauc plot of Cuδ+O. (d) Ultraviolet photo-electron spectroscopy\nspectrum of Cuδ+O. (e) Local area enlarged image of Cuδ+O. (f) Tauc plot of BNF. (g) Ultraviolet\nphoto-electron spectroscopy spectrum of BNF. (h) Local area enlarged image of BNF. (i) Energy\nband structure of Cuδ+O/BNF.\n2. While the formate pathway is strongly supported, the exact role of interfacial water and the\nproton transfer mechanism during PCET are not deeply explained. The source of proton for the\nmultistep reduction (12 e⁻ transfer) is pure water is not explicitly explained, kinetics of water\noxidation on BNF could be a bottleneck.\nResponse: We thank the reviewer for the nice comments. Your insights into the proton source and\nwater oxidation kinetics are highly valuable and have prompted us to provide a clearer explanation\nof the mechanism.\nProton source: According to the Z-scheme mechanism, on the water oxidation side, photogenerated\nholes (h+) on BNF are used to oxidize H O molecules (2H O + 4h+ → O + 4H+). The protons (H+)\n2 2 2\ngenerated in this process serve as the sole proton source for the multi-step hydrogenation reactions\non the CO reduction side (requiring a total of 12 electrons and 12 protons).\nformic acid coupling step, we propose that water molecules and hydroxyl species enriched at the\ninterface likely act as proton donors. Driven by the electric field, the H+ produced from water\noxidation is rapidly transferred to the formic acid intermediate on the Cuδ+O surface, thereby\ncooperatively completing the dehydration-coupling reaction.\nKinetic bottleneck: We fully agree with the reviewer’s assessment that slow water oxidation\nkinetics is often the rate-limiting step for the overall reaction. We have further investigated this by\ncalculating the reaction energy barrier for water oxidation. As shown in Fig. R13, through density\nfunctional theory (DFT) calculations, the free energy changes for the water oxidation reaction (2H O\n2\n→ O + 4H+ + 4e-) catalyzed by cuprous oxide supported on boron nitride framework (Cuδ+O/BNF)\n2\nand pristine cuprous oxide (Cuδ+O) are compared. Both catalysts follow the typical four-electron\nwater oxidation pathway: H O → *H O → *OH → *O → *OOH → *O . In the Cuδ+O system, the\n2 2 2\nformation step from *O to *OOH is the rate-determining step, with an energy barrier of 2.23 eV. In\nthe Cuδ+O/BNF system, the energy barrier for the same reaction step is reduced to 1.41 eV,\nindicating that the introduction of BNF can significantly lower the kinetic barrier for water oxidation.\nThe Cuδ+O/BNF curve lies entirely below that of Cuδ+O, suggesting that the free energies of all\nintermediate states are generally lower, making the reaction thermodynamically more favorable.\nFurthermore, compared to recently reported classical water oxidation catalysts (such as WO , TiO ,\n3 2\nCN, and COFs), the rate-limiting energy barrier for water oxidation in this study remains relatively\nlower.\nCuδ+O\n4\n)\nV\ne\ny\n( 3\ng\nr\ne\nn 2\ne\ne 2.23 eV\ne\nF r 1\n1.41 eV\n0\n-1\nH O *H O *OH *O *OOH *O\n2 2 2\nReaction coordinates\nFig. R13 | The free energy diagram of H O-to-O conversion over Cuδ+O and Cuδ+O/BNF.\n2 2\nTable R2. The rate-limiting step and energy barrier in water oxidation.\nCatalyst C-C coupling step Energy barrier References\nCuδ+O/BNF *OH→*O 1.41 eV\nThis work\nCuδ+O *OH→*O 2.23 eV\nCOF/Ov-WO *O→*OOH 1.76 eV 16\nT 2\nZn-gZIS *OOH→O 2.03 eV 17\nT 2\nS -gZIS *OOH→O 3.68 eV\nv T 2\n1L C N *O→*OOH 2.60 eV\n6 6\n4L C N *O→*OOH 1.97 eV 18\n7 6\nGh-C N H O→*OH 2.70 eV\n3 4 2\nβ-NiS/TiO *O→*OOH 1.83 eV 19\n2-x\nTCN *O→*OOH 2.0 eV 20\nTSCN *O→*OOH 1.8 eV\nSCN *OOH→O 2.6 eV\nCOF-C2-single *O→*OOH 2.55 eV\nCOF-C2-dual *H O→*OH 1.77 eV 22\n3. The study does not compare with high performance electrocatalytic CO to ethanol systems (Cu\n2\nbased GDE cells), which achieved higher rates (>100 mA·cm-2)\nResponse: We thank the reviewer for raising this important comparative perspective. Firstly, at the\ncurrent stage of technological development, high-performance electrocatalytic systems (particularly\nthose utilizing gas diffusion electrode reactors) are indeed capable of achieving higher carbon-based\nproduct formation rates (with current densities >100 mA·cm-2). However, we emphasize the unique\nadvantages of the photocatalytic pathway presented in this study, and we provide a comparative\ndiscussion regarding photocatalytic selectivity and electrocatalytic Faradaic efficiency:\n(1) Simplified energy input: Our process directly utilizes sunlight as the sole energy input,\neliminating the need for external electrical power and complex infrastructure. This demonstrates\nsignificant potential for distributed solar fuel production.\n(2) Mild reaction conditions: The reaction proceeds under ambient temperature and pressure, using\npure water as the proton source, without requiring additional electrolytes or sacrificial agents. This\nenhances the sustainability and safety of the process.\n(3) Scientific value: The core contribution of this work lies in the discovery of a novel, low-energy-\nbarrier pathway involving a formate intermediate, which provides a new paradigm for understanding\nC–C coupling and offers valuable insights for both the photocatalytic and electrocatalytic fields.\n(4) Comparison with electrocatalytic systems: In accordance with the reviewer’s suggestion, we\nhave summarized the performance of a series of electrocatalytic CO -to-ethanol studies based on\n2\nthe criterion of >100 mA·cm-2. As shown in Table R3, under conditions exceeding 100 mA·cm-2,\nthe Faradaic efficiency for ethanol does not surpass 60%. In contrast, in our photocatalytic\nCO -to-ethanol system using Cuδ+O/BNF, the selectivity approaches 100%. With regard to the\n2\nproduction rate of ethanol, however, it is acknowledged that the yields achieved by electrocatalysis\nare currently much higher, which represents a significant challenge for future development in the\nfield of photocatalysis.\nConsequently, we regard photocatalysis and electrocatalysis to be complementary technological\npathways, each appropriate to different application scenarios. The present study offers novel\nmaterial design and mechanistic insights for solar-driven \"artificial photosynthesis.\"\nTable R3. Performance comparison of various CO RR to ethanol catalysts.\nEthanol Faradaic\nCatalyst Electrode density (mA/cm2)/ References\nefficiency/selectivity (%)\nYield (mol g-1 h-1)\nCu/Au 0.1 M KHCO 60 300 23\n3\n24\ndCu O/Ag2.3% 1 M KOH 40.8 326.4\n2\nx\n0.14 0.86\nCu V Se 1 M KOH 68.3 207.9 27\n1.22 0.19\nCu DMAP/P-H 1 M KOH 47.3 189.2 28\nH Cu O@C/N 1 M KOH 43.2 141.6 29\nCu/CuPO 2 M KOH 30.7 107.45 31\ncAA-CuNW 1 M KOH 20 278 32\nM Cu1/CuNP 5 M KOH 30 115 33\nR-8-Cu-16 1 M KOH 45 144 34\n3D Cu-CS GDL 1 M KOH 36 324 35\nCuδ+O/BNF H O Almost 100 68.5 This work\n2\n4. No discussion of catalyst cost, recyclability or energy payback is provided, BN is more\nexpensive than common photocatalyst supports (TiO and Carbon)\nCost and Materials: At present, hexagonal boron nitride (h-BN) indeed has a higher cost than\nconventional supports such as titanium dioxide (TiO ) or carbon-based materials. However, the\n2\nunique properties of h-BN—such as its excellent chemical stability, thermal conductivity, and\nsurface basicity—are key to achieving high selectivity and stability in ethanol artificial\nphotosynthesis. We believe that as synthesis techniques advance (e.g., progress in chemical vapor\ndeposition, CVD) and the abundance of the constituent non-metal elements (boron, nitrogen, oxygen,\netc.), the cost of h-BN is expected to decrease and may even become comparable to that of\ncarbon-based materials.\nRecyclability: h-BN has a layered, graphite-like structure and exhibits hydrophobic characteristics.\nEven when exfoliated into few-layer BN or engineered into porous forms, h-BN can retain sufficient\nmechanical strength to enable repeated use. Moreover, BN is resistant to acids, alkalis, and high\ntemperatures, and it does not readily dissolve or collapse in most reactive environments, making it\neasy to separate from liquid-phase systems via centrifugation or filtration. Overall, compared to\nTiO and carbon materials, h-BN remains an ideal material in terms of recyclability and reusability.\nanalysis will be essential as photocatalytic research progresses toward industrial-scale\nimplementation. This work provides foundational performance data for such analyses, and the 7-day\ncontinuous outdoor testing, along with large-scale planar catalytic studies, represents preliminary\nexploration toward the scale-up of photocatalytic technology.\n5. While 25 hour stability is good, longer term testing (100-500 h) would strengthen the stability\nclaim, since practical applications require thousands of hours of operation.\nResponse: We thank the reviewer for the nice suggestion. As requested, a series of extended cycling\ntests were conducted, with durations reaching 25 hours. The results, depicted in Figure R14,\ndemonstrate that after 100 hours of cumulative cycling, the ethanol production rate retained over\n85% of its initial value. This finding substantiates the viability of the material for practical\napplications.\nFig. R14 | PCO RR to produce ethanol over Cuδ+O/BNF in 100 hours.\n2\n6. The produced ethanol is in aqueous solution at low concentration (μmol). No discussion is\nprovided on product extraction or separation for practical applications.\nResponse: We thank the reviewer for the nice suggestion. It is acknowledged that the separation of\nethanol from dilute aqueous solutions represents a prevalent challenge encountered by all aqueous-\nphase CO reduction technologies. Potential solutions to this issue include the use of membrane\n2\nseparation technology, solvent adsorption technology, or extraction-coupled reactors. Furthermore,\nthe development of efficient integrated reaction-separation systems represents a crucial step towards\nadvancing the scale-up and application of photocatalytic technologies.\n7. The preparation of 3D floral-like BN involves high temperature calcination and freeze drying\nand the controlled deposition of Cuδ+O requires careful reduction steps, which may pose\nchallenges for large scale cost effective applications.\nResponse: We thank the reviewer for the valuable comment. The current synthesis method indeed\npresents challenges in terms of energy consumption and procedural steps for large‑scale production.\nIn future work, we aim to develop milder and more scalable synthesis strategies. For the multi‑step\nsynthesis of BNF, established commercial‑scale BN preparation processes could be leveraged.\nMeanwhile, the loading and modification of Cuδ+O could be shifted to simpler metal deposition\ntechniques. In summary, developing a large‑scale, cost‑effective catalyst synthesis process is a key\nstep toward industrializing photocatalytic research, and we will place significant emphasis on this\naspect in subsequent studies.\nIn summary, this study presents a significant advance in photocatalytic CO to ethanol conversion\n2\nby using a novel Z-scheme heterojunction catalyst, and discover a novel formate mediated C-C\ncoupling pathway with lower energy barrier. The reviewer recommends for publication after\naddressing the above limitations."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns, and this manuscript can be accepted with the current version.", "reviewer_2": "The authors have answered what I am concerned, and the manuscript is ready for the publication in Nature\nCommunications.", "reviewer_3": "The authors have addressed the raised questions correctly, adding new Suppl. Figure 11b and Figure 41 in the\nsupplementary information to suport their conclusions. The manuscript has been improved by these revisions. The reviewer\nis satisfied with the improved manuscipt and recommends it for publication."}, "author_response": ""}]

Decision letter