Nanoscale greenhouse effect for promoting solar-driven CO_2 reduction with water to CH_4

Domain: biology
Published: 2026-03-28
Editorial outcome: major_revision

Abstract
Improvement of solar-to-chemical energy conversion in photocatalytic CO2 reduction remains fundamentally constrained by insufficient utilization of solar energy, particularly low-energy photons. Here we report a nanoscale greenhouse structure (Bi@Fe2O3) that enables cascaded utilization of full solar spectrum. The Bi nanocore primarily absorbs low-energy photons, generating localized nanoheating via non-radiative heating through localized surface plasmon resonance effects and energetic hot electrons. Meanwhile, the oxygen-vacancies rich loose Fe2O3 shell absorbs high-energy photons and serves as the catalytic bed, where injected hot electrons and confined heat synergistically promote CO2 activation and deep hydrogenation. Benefiting from the interplay between photochemical and photothermal effects, the system achieves a CH4 production rate of 273.81 μmol g–1 h–1 with 98.60% selectivity and an apparent quantum efficiency of 0.64% at 850 nm illumination without any external heating or sacrificial agents. This work paves the way to the technique for the efficient utilization of the entire solar spectrum.

Introduction
Photocatalytic CO2 reduction (PCOR), a pivotal technology by directly converting CO2, H2O and sunlight into hydrocarbon compounds to store intermittent solar energy, represents a critical pathway toward achieving carbon neutrality and enabling renewable energy economy1-3. Consequently, a vast array of semiconductors and metallic nanocatalysts have been studied, demonstrating the feasibility of solar-driven CO2 reduction4-7. Despite recent progress, the advancement of PCOR technologies toward technoeconomic applications is still restricted by two key obstacles. One is the low utilization efficiency of solar energy, especially in the low-energy photon region8, and the other is the sluggish kinetics of transferring photogenerated energy into reactant molecules9. Furthermore, the selective synthesis of high-value hydrocarbons remains a formidable challenge, due to complex multi-electron and multi-proton transfer steps during PCOR10.

As is well known, the solar spectrum is composed of approximately 6.8% ultraviolet (UV), 38.9% visible (vis), and 54.3% near-infrared (NIR) light. However, most photocatalysts commonly developed for PCOR are only capable of harvesting the solar spectrum in the UV and visible ranges for a long time11. Therefore, developing NIR-responsive photocatalysts is considered essential for achieving broad-spectrum-driven PCOR. To date, conventional NIR-responsive photocatalysts have mainly relied on some narrow-bandgap semiconductors such as lead12 and mercury chalcogenides13. While their practical application in PCOR is severely constrained, which is ascribed to high toxicity, poor stability, and the diminished redox capability associated with bandgap narrowing. Recent research has demonstrated that noble metal plasmonic nanoparticles can interact strongly with incident light through localized surface plasmon resonance (LSPR)14-16. Adjusting the geometric characteristics of these nanostructures, such as their symmetry and size, allows precise modulation of the LSPR absorption peak of these materials across a wide spectral range from ultraviolet (UV) to near-infrared (NIR)17-19. This spectral versatility is the cornerstone of their application as broadband light-harvesting antennas, offering immense potential for advanced solar-to-chemical energy conversion systems20-24. Moreover, some studies have exhibited that with the assistance of organic additives, specific proton nanostructures can convert CO2 under laser irradiation, indicating their potential in PCOR25,26. However, in the rapid development of such protonic materials, it has been found that the mismatch in timescales between fast LSPR relaxation (up to approximately 100 ps) and slow chemical reduction kinetics (milliseconds or seconds) severely hinders solar-to-chemical energy conversion27-29. To overcome this bottleneck, researchers have hybridized or coupled plasmonic materials with semiconductor nanostructures (referred to as “antenna reactors”30-32) to capture hot carriers and accelerate chemical reduction kinetics. Nevertheless, such structures may cause plasmonic nanoparticles to block reaction sites on the semiconductor, resulting in sluggish chemical reduction kinetics and inadequate utilization of solar energy, thereby limiting their enhancement capabilities. Additionally, the surface atoms of plasmonic metals (primarily gold (Au), silver (Ag), and palladium (Pd), among others) exhibit high mobility under incident light, leading to the rapid degradation of such nanostructures33,34.

Inspired by the Earth’s greenhouse effect, we herein realize nanoscale greenhouse effect for promoting highly selective CO2 reduction to CH4 based on a noble-metal-free Bi@Fe2O3 core@shell structure (Fig.1). In this design, the Bi nanocore functions as a nanoscale transducer, converting solar energy into two synergistic driving forces: energetic hot electrons and intense localized heat. The loose Fe2O3 shell plays a dual, critical role by acting as a thermal insulator to confine the localized heat while simultaneously accepting the injected hot electrons from Bi nanocore to serve as a catalytic platform. Thus, effectively activating CO2 and water molecules, enhancing the local electric field and dramatically accelerating the chemical reaction kinetics. This unique coupling of photochemical and photothermal effect achieve remarkable performance for solar-to-chemical energy conversion without any external energy (e.g., thermal, electric) or sacrificial agents, CH4 production rate of 273.81 μmol g–1 h–1 with 98.60% selectivity, demonstrating a record-high apparent quantum efficiency (AQE) of 0.64% at 850 nm NIR light illumination compared with the current reports. Our work provides an insightful pathway for efficiently utilizing the full solar spectrum to drive CO2 reduction with water into hydrocarbons.

Results
Synthesis and microstructural analyses
The 5-Bi@Fe2O3 core@shell photocatalyst was obtained via a two-step solvothermal strategy followed by a controlled calcination process under an inert atmosphere. (Supplementary Fig. 1), yielding an intimately coupled metal/oxide heterostructure. As illustrated in Fig. 2a, a metallic Bi nanocrystal (NC) core is conformally encapsulated by a porous Fe2O3 shell, which maximizes interfacial contact while maintaining open diffusion pathways. Schematic illustration of the interfacial electronic contact is exhibited in Fig. 2b. Fermi-level equilibration upon contact establishes band bending and an internal electric field between the Bi core and the Fe2O3 shell, providing a driving force for directional transfer of plasmon-excited electrons from Bi core to the Fe2O3 shell. According to the results in Fig. 2c, Powder X-ray diffraction (XRD) further confirms the coexistence of Fe2O3 (JCPDS No. 87-1164) and metallic Bi (JCPDS No. 85-1329) without discernible impurities. The scanning electron microscope (SEM) and transmission electron microscope (TEM) directly evidence the morphology of the obtained spherical core@shell structure. The TEM image shows a clear contrast between the dense Bi core and the surrounding Fe2O3 shell (Fig. 2d), in which a sphere Bi nanocrystal (Bi NC) was surrounded by a loose Fe2O3 shell, with an average size of 101 ± 31 nm (Supplementary Fig. 2). It can also be seen from the SEM images of 5-Bi@Fe2O3 possesses a spherical core@shell nanostructure (Supplementary Fig. 3). The high-resolution TEM (HRTEM, Fig. 2e, Supplementary Fig. 4) images indicated that the main exposed lattice planes of Bi NC in 5-Bi@Fe2O3 were (012) and (104) facets, corresponding to measured fringes of 0.33 nm and 0.24 nm, respectively. The lattice fringe of 0.25 nm was also exhibited, indexed to (110) facets of Fe2O3 in 5-Bi@Fe2O3. Selected area electron diffraction (SAED) (inset in Fig. 2e) showed two sets of diffraction patterns, with the (012) or (104) and (110) spots still indexed to Bi and Fe2O3. Moreover, HAADF-STEM with EDX elemental mapping measurements (Fig. 2f) and SEM with EDX elemental mapping measurements of 5-Bi@Fe2O3 (Supplementary Fig. 5) further verify the spatially separated distribution of Bi (core) and Fe/O (shell), corroborating the uniform encapsulation of Bi NCs by the Fe2O3 shell and the formation of a well-defined core@shell junction. Finally, it is worth noting that this sphere core@shell structure exhibited a larger specific surface area (175.5 m2/g) with a uniform mesoporous structure (5.8 nm) (Supplementary Fig. 6, 7, and Supplementary Table 1), which has the advantage of adsorbing more reactant molecules, exposing a greater number of active sites, and providing convenient channels for mass transport35.

To gain deeper insight into the atomic and electronic structure of 5-Bi@Fe2O3 core@shell, synchrotron radiation XAS was further applied to test the Bi L3-edge and Fe K-edge. The Bi L3-edge X-ray absorption near-edge structure (XANES) spectrum of 5-Bi@Fe2O3 closely resembles that of Bi foil, indicating Bi of 5-Bi@Fe2O3 exists predominantly in the metallic Bi0 state (Fig. 2g), which is consistent with X-ray photoelectron spectroscopy (XPS) results (Supplementary Fig. 8a). The results of the Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) further corroborates this metallic nature by showing two obvious Bi–Bi coordination features (Fig. 2h), and quantitative EXAFS fitting resolves two Bi–Bi shells at 3.07 ± 0.02 Å and 3.50 ± 0.03 Å (Supplementary Table 2). In addition to the dominant Bi–Bi coordination, an extra Bi–O contribution is resolved at 2.16 ± 0.04 Å (Supplementary Table 2), indicating interfacial Bi–O bonding between the metallic Bi core and the Fe2O3 shell. The coexistence of Bi–Bi and Bi–O scattering paths is further supported by wavelet-transform analysis and EXAFS fitting (Fig. 2i and Supplementary Fig. 7b–d). This directly confirmed that 5-Bi@Fe2O3 is a chemically coupled core@shell interface that can shorten the transfer distance and facilitate efficient transport of plasmon-excited carriers from Bi to the Fe2O3 shell.

Furthermore, the Fe K-edge XAS spectra were analyzed to confirm the nature of the shell. The Fe K-edge EXAFS fitting reveals an Fe–O first-shell distance of 2.01 ± 0.03 Å and a higher-shell Fe–Fe contribution at 3.33 ± 0.02 Å (Supplementary Table 3, Supplementary Fig. 9). This is consistent with a defect-rich Fe2O3 shell framework36, while indicating a locally distorted coordination environment associated with defect formation in the shell. The Fe XANES spectrum and the Fe XPS result of 5-Bi@Fe2O3 also exhibit the same results about Fe state (Supplementary Fig. 10), which suggests the possible formation of oxygen vacancies (Vo) within the Fe2O3 shell. The presence of oxygen vacancies is further verified by the EPR signal centered at g ≈ 2.003 and O 1s XPS (Supplementary Fig. 11). Collectively, these characterizations fully demonstrate the core@shell structure of 5-Bi@Fe2O3, featuring a metallic Bi0 core, a Vo-enriched Fe2O3 shell, and an interfacial Bi–O linkage, which together provide a robust structure for efficient interfacial charge transfer and defect-mediated CO2 reduction. As a comparison, a series of x-Bi@Fe2O3 was prepared by adjusting the amount of Bi precursor (Supplementary Fig. 12). On the other hand, Bi NCs, pure Fe2O3, and Bi-Fe2O3 were also prepared and characterized in Supplementary Fig. 13.

Light and thermal effects over the unique core@shell structure
After confirming the unique structure, we further explore the fundamental optical properties and solar-to-heat energy conversion ability. Three representative catalysts are shown in Supplementary Fig. 14. As displayed in Fig. 3a, the Fe2O3 catalyst exhibits a strong, broad absorption band below ~600 nm. Bi NCs display strong, broadband absorption covering the UV-vis-NIR region, which belongs to characteristic of bismuth plasmonic and interband transitions. Compared to pure Fe2O3, the Bi-Fe2O3 catalyst and Bi@Fe2O3 catalyst maintain this excellent broadband light-harvesting ability of Bi NCs (300–2000 nm). Moreover, increasing the Bi content effectively enhances the light absorption of x-Bi@Fe2O3, particularly in the near-infrared (NIR) region (Supplementary Fig. 15). This enhanced NIR absorption is crucial for the efficient cascaded utilization of the full solar spectrum.

The heat generated by the non-radiative relaxation of hot electrons can cause an increase of temperature in the catalyst bed (without any external heating), thereby accelerating the reaction37, especially in gas-solid phase reaction systems without temperature control. Under the reaction conditions of the following photocatalytic performance tests, the equilibrium temperature at the center of the illuminated surface of the catalyst was measured using an embedded contact thermocouple attached to the surface of the photocatalyst (Supplementary Fig. 16a). As the light intensity increased, the surface equilibrium temperature of Fe2O3, Bi-Fe2O3 and 5-Bi@Fe2O3 continued to rise (Fig. 3b). Numerical simulation results also revealed that light power density significantly affects the average temperature of 5-Bi@Fe2O3 (Supplementary Fig. 17a). Specially, after 20 minutes of illumination, the temperature of the 5-Bi@Fe2O3 reached above 170 ℃, higher than that of the blank comparsion (56 ℃) (Fig. 3c). Compared to Fe2O3 and Bi-Fe2O3, the temperatures of 5-Bi@Fe2O3 are obviously higher, indicating that the core@shell structure exhibits the best photothermal effect38-40. The average temperature of specific regions, which is in the infrared images measured using SmartView software (Supplementary Fig. 16b), is very close to the temperatures measured by thermocouples in Figs. 3c and 3d. It is worth noting that the values measured by the thermocouple and infrared thermal imager represent the macroscopic surface temperatures of different catalysts, which are lower than the local temperatures of Bi NCs.

Importantly, compared to Bi-Fe2O3, the 5-Bi@Fe2O3 catalyst effectively suppresses heat dissipation from the Bi core to the surrounding environment after illumination (Fig. 3e). COMSOL Multiphysics simulations further reveal obvious enhancement of local electric fields and surface thermal energy in 5-Bi@Fe2O3 under near-infrared excitation, accompanied by a temperature increase of ~135 K (Fig. 3f, Supplementary Fig. 17b and Supplementary Figs. 18–20). Additionally, to decouple thermal confinement from electronic effects, a Bi@SiO2 core@shell structure with an optically transparent and electronically insulating shell was synthesized as a control (Supplementary Fig. 21). Although Bi@SiO2 exhibits a certain photothermal temperature rise due to suppressed heat dissipation (Supplementary Fig. 22), its heating efficiency remains lower than that of Bi@Fe2O3 under identical illumination conditions (as shown in Supplementary Fig. 22 b and Fig. 3c). These results indicate that the elevated temperature of 5-Bi@Fe2O3 originates not only from thermal insulation by the shell, but also from enhanced broadband light absorption and interfacial energy conversion enabled by the Fe2O3 shell. Based on simulation and experimental results, the observed temperature increase is more likely attributed to the enhanced broadband absorption of the Bi core after core@shell structure construction, as well as the intrinsically low thermal conductivity of the Fe2O3 shell layer. The resulting localized high-temperature environment promotes molecular collision frequency and reaction kinetics, thereby enhancing the solar-to-chemical energy conversion efficiency41,42.

PCOR driven by coupling photo-/photothermal effect
This unique structure design, composed of Bi spheres and loose Fe2O3 shell, aims to offer sufficient reaction driving force and to provide a channel for the mass transfer of reactants and products. To reveal the advantages of the unique structure design, PCOR performance was tested under full-Arc Xe lamp irradiation for all photocatalysts (Supplementary Fig. 23). Detailed experimental procedures for PCOR are shown in the Supporting Information. By optimizing the volume of H2O and the partial pressure of CO2 (Supplementary Fig. 24, with the entire system operating at atmospheric pressure), PCOR was performed under full-spectrum illumination at different light intensities without any external heating and any sacrificial agent or photosensitizer. Firstly, to distinguish the individual contributions of the core and the shell of unique structure in photocatalytic CO2 to CH4, a series of control experiments were performed under light power 50–455 mW cm–2 for pure Fe2O3 and Bi NCs. At different light powers, nearly no products can be detected for pure Bi NCs, clearly indicating that Bi NCs itself is not active for PCOR (Supplementary Fig. 25). While pure Fe2O3 produces only a small amount of products and intermediates, which may be associated with defect-derived surface states rather than intrinsic band-edge reduction (Supplementary 26, 27). Interestingly, after introducing Bi nanoparticles, CH4 becomes the dominant product instead of CO (Fig. 4a and Supplementary 28), which may be attributed to an additional driving force from Bi nanoparticles to break the thermodynamic barrier for CO2 to hydrocarbons. More importantly, constructing the Bi@Fe2O3 core@shell structure further markedly increases the CH4 selectivity (Fig. 4a and Supplementary 29). Moreover, by investigating the effect of light intensity on the catalytic activity, it is found that the catalytic activity of the Bi-Fe2O3 increases with increasing light power, reaching a maximum (20.69 μmol g–1 h–1) at a critical value of 455 mW cm–2 (Fig. 4b). In sharp contrast, the spherical 5-Bi@Fe2O3 exhibits much higher CH4 production rate at each light power compared to Bi-Fe2O3, the maximum value of 273.81 μmol g–1 h–1 with a selectivity 98.60% at 455 mW cm–2 (Fig. 4c). Besides these gaseous products (CO, CH4), only trace amounts of H2 (0.27~0.58 μmol g–1 h–1) were detected over these photocatalysts, indicating that the competing hydrogen evolution reaction (HER) was effectively suppressed43.

Verifying the origin of the product is another critical step for PCOR. As shown in Supplementary Fig. 20 and Supplementary Table 4, the presence of the catalyst, light, CO2, and H2O is indispensable for the PCOR over 5-Bi@Fe2O3. To further corroborate the carbon source, we performed isotopic labeling experiments using 13CO2 as the feedstock. As shown in Supplementary Fig. 31a, when 12CO2 and 13CO2 were used as the respective feed gases, mass signals at m/z = 28 and 29 were assigned to 12CO and 13CO, respectively. Similarly, signals at m/z = 16 and 17 were attributed to 12CH4 and 13CH4. These results confirm that the reduced carbon products were only generated by PCOR rather than other reactions. Furthermore, previous studies indicated that the reduced carbon products were difficult to generate in the photocatalytic system without adding water44, thus the water was indispensable in the PCOR. Therefore, an H218O isotope-labeling experiment was performed to investigate whether water participates in the overall reaction (Supplementary Fig. 32a). Although there might be oxygen atom exchange between CO2 and H2O, the production of 18O implied the involvement of water in the whole PCOR process. Moreover, quantitative analysis of the evolved gases revealed the concurrent formation of O2 throughout the CO2 reduction process (Supplementary Fig. 32b, Supplementary Fig. 32c), with no detectable by-products such as H2O2. This confirms that the photocatalytic conversion between CO2 and H2O proceeds through a complete and closed redox cycle. Moreover, deuterium oxide (D2O) was employed in the reaction to trace the origin of the H element in the produced CH4. The product peak at m/z = 18 was indexed to CD4 (Supplementary Fig. 31b) when using D2O in the reaction, indicating that the proton in CH4 comes from H2O. This result further manifests that the formation of CH4 is a multistep hydrogenation process and that the proton is provided by H2O45-47.

To explore the high stability of the core@shell structure with the proposed design, a comprehensive characterization was performed. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results demonstrate that the 5-Bi@Fe2O3 catalyst exhibits excellent high-temperature tolerance (Supplementary Fig. 33). More importantly, the 5-Bi@Fe2O3 catalyst nearly retained its original morphology rather than collapsing after PCOR reaction (Supplementary Fig. 34a, b). The XRD patterns of 5-Bi@Fe2O3 after PCOR reaction exhibited no discernible changes (Supplementary Fig. 34c), confirming the strong thermal stability of the catalyst structure. Furthermore, according to the XPS and XAS spectra of the 5-Bi@Fe2O3 catalyst after PCOR reaction (Supplementary Figs. 35, 36; Supplementary Tables 5, 6), the valence state and coordination environment of Bi and Fe in the core@shell catalyst did not clearly change, indicating that the Bi core does not undergo oxidation during the reaction, and the Fe-Vo active sites keep stable in the reaction process (Supplementary Fig. 37). Concurrently, 21-h consecutive testing and another 11-h consecutive testing for 5-Bi@Fe2O3 were carried out (Fig. 4d). Between the two tests, the 5-Bi@Fe2O3 was kept in Ar gas at room temperature for 45 days. During the first 21-h test, there is almost no significant degradation of PCOR activity. Surprisingly, the next 11-h test still exhibits stable selectivity in spite of the slightly fluctuating yield of CH4. These results clearly confirm that the outer Fe2O3 shell effectively protects the encapsulated Bi nanoparticles from thermal sintering at elevated temperatures48. The core@shell design not only provides a super-photothermal effect through the thermal insulation and the electronic-structure modulation of the nanoporous shell, but also enhances the thermal stability of the Bi nanoparticles against sintering due to spatial confinement within the Fe2O3 sheath. This enhanced photothermal effect and thermal stability are expected to synergistically improve the photothermal catalytic CO2 activity. We compared the performance in this work to typical reports of PCOR without an external heater. Impressively, to the best of our knowledge, this represents a record-high performance for noble-metal-free photocatalysts in the reduction of CO2 with water to CH4, substantially surpassing most previously reported systems (Fig. 4e and Supplementary Table 7). And the apparent quantum efficiency (AQE) at 850 nm is promoted to 0.64% (Supplementary Fig. 38).Such a superior photoconversion efficiency under low-energy NIR light illumination, which is even comparable with the efficiencies of the representative works in PCOR with high-energy photons, achieves the record-high AQE of PCOR under NIR light illumination (Supplementary Table 8).

To distinguish the respective contributions of light and heat, the PCOR over the 5-Bi@Fe2O3 catalyst was conducted under four distinct conditions: (i) in the dark at room temperature; (ii) under 300 W Xe lamp illumination with a broadband pass filter (λ < 800 nm) to exclude infrared light; (iii) under full-spectrum irradiation from the 300 W Xe lamp; and (iv) in the dark at 220 ℃. As shown in Fig. 4f, PCOR can not occur in the dark at either room temperature or at an elevated temperature of 220 ℃, which unambiguously confirms that the PCOR over 5-Bi@Fe2O3 is exclusively light-driven. Under full-spectrum irradiation from the 300 W Xe lamp, the 5-Bi@Fe2O3 catalyst exhibited a remarkable average CH4 production rate of 273.81 μmol g–1 h–1 over a 60-minute reaction period, during which the catalyst surface temperature reached approximately 212 ℃. This rate is 2.7 times higher than that achieved under purely photocatalytic conditions (i.e., with IR light excluded, λ < 800 nm), where the surface temperature only reached ~73 ℃. These results powerfully demonstrate a synergistic enhancement between photocatalysis and photothermal catalysis, leading to a superior solar energy utilization efficiency.

Further investigation into the wavelength-dependent activity revealed the distinct roles of different spectral regions. The relative spectral distribution for this series of tests, labeled I through IV, was shown in Supplementary Fig. 39 for (I) no filter (UV-vis-NIR), (II) l >420 nm (vis-NIR), (III) l > 800 nm (NIR), and (IV) l < 800 nm (UV-vis). The temperature variations under different spectral regions were visualized using an infrared (IR) camera (Fig. 3d). As exhibited in Fig. 4g, both visible and NIR light can independently drive the reaction, while filtering out either the NIR portion (λ < 800 nm) or the UV-vis portion (λ > 800 nm) leads to a dramatic drop of the activity. The UV-vis photons are primarily responsible for initiating the photochemical pathway via bandgap excitation in the Fe2O3 shell. Meanwhile, the NIR photons are selectively harvested by the Bi core to generate energetic hot electrons and strong photothermal effect. This coupling of a semiconductor component, confirmed as indispensable by control experiments with Bi NCs, and a plasmonic nanoheater is the key to the catalyst’s exceptional performance in utilizing low-energy photons. To further reveal the influence of thermal effects, temperature-dependent photocatalytic tests were conducted by systematically varying the reaction temperature under 520 nm illumination while tuning the light intensity. As shown in Fig. 4h, under the same light intensity, the catalytic performance of 5-Bi@Fe2O3 exhibited a clear correlation with the reaction temperature, indicating that light-induced heat can promote the PCOR process. Furthermore, a similar positive correlation between performance and reaction temperature was observed across various light intensities, indicating that at a constant reaction temperature, increasing the light intensity enhances the catalytic activity. This finding aligns with the observed temperature varying trends (Fig. 3b) and catalytic performance changes (Fig. 4a and 4b) as a function of light intensity.

Charge carrier dynamics
To elucidate the charge carrier dynamics over 5-Bi@Fe2O3 for PCOR process, femtosecond transient absorption (fs-TA) spectroscopy was performed under dual excitation at 560 nm and 870 nm. Under 560 nm excitation, pristine Fe2O3 exhibits a broad photoinduced absorption (PIA) in the 500–800 nm region (Fig. 5a)49, which is attributed to the absorption of trapped photogenerated charge carriers. In contrast, 5-Bi@Fe2O3 displays a modified transient spectral profile with a weakened PIA at short wavelengths and a notable ground-state bleach (GSB) around ~700 nm (Fig. 5b), consistent with interfacial electronic coupling between the Bi core and Fe2O3 shell. Upon 870 nm excitation, a distinct plasmon-induced bleach in the near-infrared region (800–1000 nm) is observed for 5-Bi@Fe2O3 (Fig. 5c), whereas negligible signals are detected for pristine Fe2O3, confirming that the NIR response originates exclusively from the localized surface plasmon resonance (LSPR) of Bi nanospheres. Kinetic fitting analysis further reveals that the carrier lifetimes of 5-Bi@Fe2O3 are markedly longer than pristine Fe2O3 (Supplementary Fig. 40), particularly under NIR illumination. This lifetime extension indicates suppressed electron-hole recombination enabled by ultrafast interfacial charge separation at the Bi/Fe2O3 interface, which stabilizes plasmon-generated hot electrons and promotes their participation in subsequent surface reactions.

To further visualize the interfacial charge redistribution, light-assisted Kelvin probe force microscopy (KPFM) was employed to investigate the surface potential distribution of 5-Bi@Fe2O3 (Supplementary Fig. 41). As shown in Figs. 5d–5g, the surface potential (CPD) of both the Bi core and Fe2O3 shell under UV-vis-NIR irradiation exhibits a significant positive shift compared to that under dark or UV-vis illumination, indicating enhanced electron accumulation under full-spectrum excitation. This enhanced CPD response is attributed to the additional absorption of NIR photons, which activates the LSPR of Bi and generates a larger population of energetic hot electrons. Additionally, the transient photocurrent of 5-Bi@Fe2O3 under full-spectrum illumination was markedly higher than that under UV-vis irradiation (Supplementary Fig. 42). These results demonstrate that UV-vis and near-infrared light synergistically enhance charge separation and carrier transport in 5-Bi@Fe2O3, thereby providing an optimal electronic environment for plasmon-assisted CO2 reduction.

Based on the above complementary spectroscopic analysis results, a simplified PCOR model driven by the cascaded utilization of full-solar spectrum-driven over the 5-Bi@Fe2O3 core@shell catalyst is depicted in Fig. 5h and 5i. Due to the favorable work function alignment (Supplementary Figs. 43, 44), photoexcited electrons in the Bi core tend to transfer toward the Bi/Fe2O3 junction and accumulate in an interfacial electron-rich region. PCOR preferentially occurs on the interfacial electron-accumulation layer associated with Fe-Vo active sites. Specifically, under UV-vis illumination (Fig. 5h), the electrons generated from Bi interband excitation are transferred to the interfacial accumulation layer (rate constant kET)50, which is corroborated by theoretical computation electron redistribution and electrostatic potential (Supplementary Fig. 45). These energetic hot electrons drive CO2 reduction toward CO/CH4 (rate constant kCO2R), which competes with the electron-hole recombination (rate constant k(e-h)1) of Bi NC and in the Fe2O3 region (rate constant k(e-h)2). The corresponding holes from the Fe2O3 are mainly through water oxidation (rate constant kHET2) or hole-trapping processes (maintaining charge balance, rate constant kHET1). Under UV-vis-NIR illumination (Fig. 5i), the Bi LSPR effect triggers more energetic hot electrons (kˊET). These hot electrons are efficiently injected and strongly trapped with reactant molecules in the interfacial accumulation layer, creating a significantly higher electron density than under UV-vis illumination. And the large localized electric field in 5-Bi@Fe2O3 promotes charge separation and transfer, thereby enhancing the reduction potential at the reaction interface. Moreover, the prolonged lifetime of photogenerated electrons under NIR excitation enables efficient utilization of the full solar spectrum, while concurrently creating a “self-heating greenhouse bed” effect at the nanoscale (Fig. 3e). This synergistic photothermal enhancement is particularly critical for overcoming the high kinetic barriers associated with multielectron CO2 activation and multielectron-proton CH4 production. Therefore, the CH4 yield under full-spectrum irradiation is significantly higher than that obtained under UV-vis or near-infrared light alone, which is consistent with the PCOR performance (Fig. 4g).

Proposed reaction mechanism for enhanced photoactivity
After elucidating the charge carrier dynamics mechanism in this unique core@shell structure, it was necessary to explore deep insight into the origins of the enhanced activity of 5-Bi@Fe2O3 in PCOR with the support of in situ experimental results and theoretical calculations. As widely reported, oxygen vacancies often serve as the primary active sites for CO2 adsorption and activation51. Generally, the generated oxygen vacancies can act as reactive centers that modulate the local charge density distribution, thereby increasing the number of adsorbed and activated CO2 molecules and lowering the energy barrier for the CO2 reduction reaction. Firstly, to validate this enhanced CO2 adsorption capacity, CO2 adsorption isotherms and temperature-programmed desorption (TPD) experiments were performed. CO2 adsorption isotherms demonstrated that the adsorption capacity of 5-Bi@Fe2O3 is stronger than that of Bi NCs, Fe2O3, and Bi-Fe2O3 (Supplementary Fig. 46a), which is likely due to the defect-enriched shell chemistry and the porous curved shell microenvironment. This result indicates that 5-Bi@Fe2O3 may provide more active sites for adsorbing and converting CO2 molecules during the PCOR process, thus facilitating the occurrence of the reaction. As the shell composition was optimized, the overall CO2 adsorption capacity increased significantly, consistent with the independent CO2 adsorption results (Supplementary Fig. 46b). Notably, the similarity in CO2 adsorption isotherms among the core@shell samples with different core loadings suggests that CO2 activation on the interfacial sites is sufficiently efficient and does not constitute the rate-determining step in the catalytic reaction. The CO2 adsorption properties of the various catalysts were further evaluated by CO2-TPD, as shown in Fig. 6a. The desorption profiles can be broadly divided into three temperature regions, namely 50 °C–300 °C, 300 °C–450 °C, and above 450 °C, which are generally associated with weak, moderate, and strong CO2 adsorption sites, respectively. Compared with the Fe2O3 and Bi-Fe2O3 catalysts, the 5-Bi@Fe2O3 catalyst showed markedly enhanced CO2 adsorption after the construction of the spherical core@shell structure and active component incorporation. In particular, the appearance of obvious medium- and high-temperature desorption features indicates the generation of stronger CO2 binding sites (a broad peak at 300 °C–450 °C and a sharp peak at >450 °C). Strong CO2 adsorption led to a high CO2 conversion rate (Fig. 4a). These sites are mainly attributed to the abundant interfacial regions formed at the core@shell junction, which effectively promote CO2 adsorption and activation. These results powerfully underscore the critical importance of a CO2-activating support (Fe-Vo active centers) for promoting efficient PCOR.

CO is a key reaction intermediate during CO2 conversion. As shown in Fig. 6b, the evolution of CO adsorption on the spherical core@shell catalysts closely followed the trend observed for CO2 adsorption, indicating that the construction of the core@shell structure effectively increased both the number and strength of CO adsorption sites. These enhanced CO-binding sites, mainly originating from the abundant core@shell interfacial regions (Fe-Vo active centers), stabilized the *CO intermediates and favored their subsequent hydrogenation or dissociation rather than desorption. Consequently, CO selectivity was suppressed, while CH4 formation was significantly promoted. To gain deeper insight into the protonation behavior of *CO, the projected crystal orbital Hamilton population (COHP) analysis was also performed. As shown in Supplementary Fig. 47, the spherical 5-Bi@Fe2O3 catalyst exhibits a markedly more positive integrated COHP (ICOHP) value than the Fe2O3 catalyst (3.92 eLV vs 0.72 eV), indicating a stronger interfacial electronic coupling between the core@shell junction and the *CO intermediate. This strengthened bond interaction arises from the unique core@shell interface and effectively stabilizes *CO, thereby facilitating its subsequent protonation and promoting the downstream hydrogenation reaction.

Combined with charge carrier dynamics in the above section, to further confirm the proposed mechanism, in situ near-ambient pressure XPS (NAP-XPS) was conducted to deeply explore the specific reconstruction mechanism of the Fe-Vo active centers at the reaction interface. As shown in Fig. 6c, the Fe 2p1/2 and 2p3/2 peaks of the Fe2O3-based catalysts shifted toward lower binding energies under light irradiation, accompanied by an obvious increase in the surface Fe2+ concentration compared to the dark condition. This observation indicates that hot electrons generated in the Bi core were efficiently transferred to the interface between the Fe2O3 shell and the Bi core. Notably, the 5-Bi@Fe2O3 sample exhibited a substantially higher Fe2+/Fe3+ proportion than the Bi-Fe2O3 catalyst (Supplementary Table 6), confirming that plasmon-induced hot-electron transfer from Bi nanoparticles promoted the formation of abundant Fe-Vo active centers. These sites provided higher electron density for CO2 adsorption and activation (Supplementary Figs. 48), thereby accelerating the superior catalytic activity observed (Fig. 4c). The In-situ EPR measurements further supported this conclusion (Supplementary Fig. 49). The Vo-associated signal increased slightly as photoexcited electrons were trapped, and a characteristic peak at g = 2.003 appeared under illumination, demonstrating the presence of excited electrons. In contrast, the EPR spectra of Bi-Fe2O3 before and after Xe lamp irradiation were nearly identical, suggesting less efficient photoinduced electron migration. These results highlight the pivotal role of Fe-Vo centers in PCOR by simultaneously enhancing the intrinsic reactivity of electron-deficient Fe sites and promoting charge separation and transfer. Moreover, under light irradiation, the Bi 4f peaks exhibited an evident positive shift (Supplementary Fig. 50). Combined with the Fe XPS results, this demonstrates that hot electrons in the Bi core are injected into the Fe2O3 shell during PCOR. Subsequently, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were employed to fundamentally study the reaction intermediates involved in PCOR. The infrared signals of intermediates for photoreduction CO2 over 5-Bi@Fe2O3 after illumination demonstrated more abundant characteristic fingerprints compared to pure Fe2O3 and Bi-Fe2O3 (Fig. 6d, e and Supplementary Fig. 27, Supplementary Fig. 51), which is well in line with its higher performance and selectivity toward photoreduction CO2 (Fig. 4a). After introducing CO2, a peak at 1378 cm−1 appeared, which could be attributed to the monodentate carbonate ion (m-CO32−)52. The asymmetric stretching vibration peaks of *HCO3 were observed at 1318 cm−1. Multiple new peaks appeared after illumination. The peak at 1269 cm−1 can be attributed to the stretching vibration of CO2−. The infrared signals at 1590 cm−1 and 1719 cm−1 can be attributed to the *COOH group, which is a critical intermediate in the conversion of CO2 to other solar fuels53,54. The increase in the peak intensity of the absorption band at 2082 cm−1 and 2134 cm−1 confirmed the formation of *CO intermediates in the reaction. Importantly, the absorption bands at 1011, 1473, and 1112 cm−1 correspond to the *CHO, *CH2O, and *CH3O species, respectively, which are key intermediates in CH4 formation. Furthermore, the peak at 3026 cm−1 is attributed to *CH3 stretching vibrations, clearly indicating CH4 formation.

The density functional theory (DFT) calculations were performed to elucidate the thermodynamic origins of the enhanced activity and selectivity in the PCOR to CH4 over 5-Bi@Fe2O3. To explicitly probe the interfacial electronic effects and solvent participation, the simplified slab models with explicit water molecules were constructed (details in Supporting Information; optimized structures of pure Fe2O3 and 5-Bi@Fe2O3 in Supplementary Figs. 52−54). The calculated Gibbs free‐energy for CO2 reduction with H2O toward CH4 reveals a markedly more favorable thermodynamic advantage over 5-Bi@Fe2O3 than over Fe2O3 (Fig. 6f; Supplementary Figs. 55, 56). In specific, CO2 was adsorbed on the surface of 5-Bi@Fe2O3 and Fe2O3 to produce *CO2, and then *CO2 was hydrogenated to produce *COOH. Owing to the high energy barrier, the first proton-coupled electron transfer for *COOH generation was considered a rate-limiting step. Importantly, the formation energy of *COOH on 5-Bi@Fe2O3 is 0.67 eV, distinctly lower than that on pristine Fe2O3 (0.85 eV), indicating that the electron-rich Bi/Fe2O3 interface significantly decreases the energy barrier for CO2 activation. After that, *COOH was then protonated and dehydrated to form a stable intermediate *CO, which was associated with the C−O bond breakage through a proton-assisted electron transfer process50. After *CO formation, pristine Fe2O3 tends to favor CO desorption rather than further hydrogenation (0.20 eV vs 0.98 eV). In contrast, the *CO intermediate is more strongly stabilized for 5-Bi@Fe2O3, and the subsequent hydrogenation to *CHO becomes thermodynamically preferred over CO desorption, thereby resulting in the pathway toward deep hydrogenation. The subsequent hydrogenation steps (*CHO → *CH2O → *CH3O → *+CH4 ↑) proceed with overall downhill energetics and small thermodynamic barrier, consistent with the enhanced CH4 selectivity. Notably, these theoretical trends are in good agreement with the experimental results. Specifically, the increased Fe2+/Fe3+ proportion under irradiation suggests an electron-enriched, defect-associated surface environment, while time-resolved DRIFTS shows the continuous accumulation of *CHO/*COOH-related bands on 5-Bi@Fe2O3. This is consistent with the predicted stabilization of key hydrogenation intermediates and the suppression of premature CO desorption. Therefore, the combined DFT and experiment analyses provide a clear logical framework for highly selective CH4 formation on the Bi@Fe2O3 core@shell catalyst.

Based on these results, we therefore proposed a CO2-to-CH4 mechanism driven by the synergistic interplay between photochemical effect and photothermal effect over the Bi@Fe2O3 catalyst (Fig. 6g). Specifically, the Bi core absorbs vis-NIR light to excite LSPR hot electrons, while the Fe2O3 shell absorbs ultraviolet-visible light to generate electron-hole pairs. The non-radiative relaxation of these excited electrons generates localized heat, elevating the temperature at the interface between the Bi core and Fe2O3 shell. The Fe2O3 shell, possessing intrinsically low thermal conductivity, effectively confines this energy to establish a super-photothermal environment. Crucially, the hot electrons injected from Bi are continuously trapped by Fe-Vo sites (Fe3+ to form transient Fe2+ species), which serve as highly active centers for CO2 activation. These Fe-Vo sites are not only the platform of the initial activation for CO2 to the *COOH intermediate but also support the subsequent proton-coupled electron transfer deep reduction steps (*COOH → *CO → *CHO → *CH2O → *CH3O → *CH4). Under continuous illumination, plasmon-induced hot-electron injection and localized photothermal heating sustain a dynamic Fe3+/Fe2+ redox cycle, ensuring the persistent availability of Fe-Vo sites during multi-electron proton-coupled transfer steps toward CH4 formation. Consequently, the synergistic coupling of plasmonic nanoheating and hot-electron injection based on the Bi@Fe2O3 structure not only accelerates charge-carrier dynamics but also promotes the deep reduction of CO2 and H2O to CH4, as confirmed by in situ spectroscopy and control experiments.

Methods
Materials and Synthesis
Chemicals. All chemicals are of analytical grade and used without further purification. Polyvinyl pyrrolidone (PVP, molecular weight of 360,000) was purchased from Aladdin Biochemical Technology Co. Ltd. (Shanghai, China); sodium borohydride (NaBH4), ethanol, bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The deionized water was supplied by a Millipore system (Outlet water resistivity > 18 MΩ cm at 25 ℃) in all experiments.

Preparation of Bi@Fe2O3 spherical Nanostructures. In a typical procedure, 10 mmol of urea was dissolved in ethylene glycol (EG) under continuous Ar purging. Subsequently, 0.5 mmol of Fe(NO3)3·9H2O was added under stirring for 2 h, obtaining solution A. Additionally, 1 mmol of Bi(NO3)3·5H2O was dispersed in 100 mL EG in a three-necked flask equipped with a reflux condenser, thermometer, and gas inlet/outlet. An appropriate amount of polyvinyl pyrrolidone (PVP) was then added. The mixture was ultrasonicated for 30 min and then vigorously stirred for 1 h. Ar was continuously bubbled through the system for an additional 1 h. The reaction mixture was heated to 150 ℃ and maintained for 4 h under an Ar atmosphere. After cooling to 70 ℃, the resulting solution was marked as solution B. Subsequently, solution A was added dropwise into solution B under Ar atmosphere. After addition, the reaction was maintained at 150 °C for 8 h under continuous stirring and Ar flow. Then, the mixture was cooled to room temperature and allowed to stand for several hours. The obtained mixture was washed sequentially with ethanol and deionized water, followed by vacuum drying at 60 ℃ for 12 h. The dried powder was then transferred into a tubular furnace and annealed at 250 ℃ for 4 h under Ar atmosphere with a ramping rate of 1 ℃/min. The final product, Bi@Fe2O3, featuring a core@shell architecture, was obtained. By varying the amount of Bi(NO3)3·5H2O precursor, a series of samples with different Bi (1-Bi@Fe2O3, 3-Bi@Fe2O3, 5-Bi@Fe2O3, and 7-Bi@Fe2O3) were synthesized accordingly.

Preparation of Fe2O3 catalyst. Pristine Fe2O3 sample was synthesized under identical conditions but without adding the Bi precursor. All products were stored in a desiccator before further characterization and catalytic evaluation.

Preparation of Bi-Fe2O3 flat Nanostructures. Fe(NO3)3·9H2O (2 mmol) and urea (6 mmol) were dissolved in 40 mL of deionized water and subsequently treated hydrothermally at 120 ℃ for 2 h, obtaining FeOOH precursor. The precursor mixture was washed, dried, and calcined at 350 ℃ in air for 2 h to obtain crystalline Fe2O3. Then, 100 mg Fe2O3 was dispersed in 50 mL EG with PVP under ultrasonication. After N2 purging (30 min), Bi(NO3)3·5H2O (0.5 mmol, dissolved in 5 mL EG) was added, followed by dropwise addition of NaBH4 (0.1 M, 10 mL) at 80 ℃. After stirred for 2 h, the mixture was washed alternately with ethanol and water, then vacuum-dried at 60 ℃. Finally, the dried mixture was annealed at 250 °C for 1 h under flowing Ar with a ramping rate of 2.5 ℃/min to stabilize the Bi nanoparticles.

Fundamental characterizations
The powder X-ray diffraction (PXRD) spectra were acquired on a PANalytical X’pert MPD Pro diffractometer with Ni-filtered Cu Kα radiation (λ = 0.1538 nm, 40 kV  40 mA). The microcrystalline structure and surface characteristics were examined by a JEOL JSM-7800F field emission scanning electron microscope (FE-SEM) and an FEI Tecnai G2 F30 S-Twin transmission microscope (TEM). The energy-dispersive X-ray spectroscopy (EDS) analysis was performed using the installed energy-dispersive X-ray detector, OXFORD MAX-80. SEM and EDS images were recorded on the XL-30 ESEM-FEG Scanning Electron Microscope. XPS measurements were conducted using a Kratos spectrometer (Axis Ultra DLD) with monochromatic Al Kα radiation (hν = 1486.69 eV). The in situ XPS was recorded using United States-Thermo Fisher SCIENTIFIC ESCALAB 250Xi. The X-ray source was an aluminum target (Al Kα line energy of 1486.69 eV after monochromator filtration), and the detection pressure was 2 × 10−8 Pa. The UV-vis diffuse reflectance spectrum (DRS) was acquired using a Hitachi double-beam UV4100 UV-vis-NIR spectrophotometer equipped with an integrating sphere in which BaSO4 acted as the background. Electron paramagnetic resonance (EPR) measurements were performed on a Bruker A300 spectrometer.

Ultrafast spectroscopy characterization
Femtosecond transient absorption spectroscopy was carried out using a PHAROS PH2 Femtosecond laser system. The excitation source was a Ti:sapphire regenerative amplifier (LIGHT CONVERSION, Lithuani, 35 fs pulse width, 1 kHz repetition rate, 3 mJ per pulse). The output beam was divided into two paths to feed a pair of optical parametric amplifiers (TOPAS-C) for generating the pump and probe pulse. A white light continuum (350–950 nm) was produced by focusing the 1300 nm signal from one OPA (TOPAS1) onto a sapphire plate, then filtered with a short-pass filter to limit the spectral window. The pump pulses at 560 and 870 nm were generated from the second OPA (TOPAS2), with fluences adjusted to 5.0 mJ cm–2 and 0.7 mJ cm–2, respectively. Temporal resolution was achieved by introducing a delay line into the pump beam path, allowing the capture of differential transmission spectra over time. To correct for spectral dispersion in the white light probe, the optical Kerr effect of the substrate was measured following a standard polarization-resolved method. This involved setting the pump polarization at 45° relative to the probe and analyzing the probe polarization changes after transmission through the sample. These polarization shifts reflected pump-induced anisotropy in the substrate’s refractive index. For nanosecond-scale measurements in the near-infrared (NIR), a separate nanosecond TAS system was employed. Here, excitation was provided by 560 and 870 nm pulses from a Nd:YAG laser, while probe light came from a tungsten–halogen lamp. Transient NIR absorption spectra were recorded using a liquid nitrogen-cooled InSb detector, operating in single-channel mode with spectral scanning via a grating. Decay kinetics at each wavelength were captured using a Ni-Scope digital oscilloscope (8-bit, 100 MHz bandwidth, 30,000 data points).

Kelvin Probe Force Microscopy Measurements
The atomic force microscope (AFM) and Kelvin probe force microscope (KPFM) signals were detected by an SPM-9700HT (SHIMADZU, Japan) AFM instrument. The powder sample was dispersed in ethanol and then dropped on a Silicon chip. Au was used for the potentiometric calibration. CPD changes under dark and illumination were recorded to explore photoinduced charge dynamics. According to the following equation (1), the CPD can be calculated as:
CPD= ( sample − tip ) e−1 (1)
where sample is the work function of the sample, tip is the work function of the AFM tip.

In-situ DRIFTS characterization
In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurement was performed on a Thermo Fisher Nicolet iS50 FTIR spectrometer equipped with a mercury-cadmium-telluride (MCT) detector cooled by liquid nitrogen and a DRIFTS accessory. Each spectrum was collected over the range of 4000 to 600 cm−1 at a resolution of 0.964 cm−1. The as-prepared samples were compressed at the bottom of an in situ ATR cell and then degassed for 1 h under an Ar atmosphere at 120 ℃ to remove the absorbed species before the photocatalytic experiments. After cooling down to room temperature, humidified CO2 (CO2/H2O = 4/1) was flowed into the specimen chamber to saturate the surface absorption of the catalyst. The DRIFTS spectra were collected after a certain irradiation duration. The simulated light source (UV-vis and UV-vis-NIR) was consistent with that in the photocatalytic experiment above.

Photocurrent and Mott-Schottky measurements
First, the original ITO glass was washed with acetone (30 min), ethanol (30 min), and water (30 min) using ultrasonic waves. The 5.0 mg catalyst was dispersed in 0.5 mL of ethanol and then treated with ultrasonic for 5 h. The obtained suspension was added to the 1.5 cm × 2 cm ITO glass drop by drop. Before the photoelectrochemical measurement, the ITO glass was treated in a vacuum at 150 ℃ for 2 hours. The CHI 660B electrochemical workstation equipped with three electrodes was used for photochemical experiments. An aqueous solution of 0.5 M Na2SO4 was used as the electrolyte. Using standard three electrodes, the catalyst-coated ITO was used as the working electrode, Pt foil was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. The Mott-Schottky diagram was obtained at the frequencies of 1000 and 1500 Hz in the range of −1~1 V vs. Ag/AgCl.

Photocatalytic CO2 reduction measurement
The photocatalytic CO2 reduction experiments were carried out in a custom-made reactor with a quartz glass top (illuminated area approximately 1 cm²). 5 mg catalyst was uniformly spread on a glass fiber cloth after plasma cleaning and then placed into the reactor. Except for the temperature comparison experiment, all other experiments were conducted at room temperature (without any external heat source). A PLS-SXE300 Xe lamp (Beijing Perfect-Light Technology Co. Ltd.) with different reflectors and cut filters was used to simulate UV-vis, NIR, and full-spectrum. The light intensities can be adjusted continuously by changing the current of the xenon lamp and the optical path to the reactor. The light intensity on the sample surface was measured by an optical power meter (CEL-NP2000-2A, Beijing China Education Au-light Co., Ltd.), and the measuring range of wavelength was 190−11000 nm. Before irradiation, the whole reaction system was purged with CO2 and H2O vapor for 1 h to remove the gaseous impurity. High-purity CO2 was introduced into the reaction chamber through a mass flow controller, while H2O was injected into the chamber using a syringe pump, which can precisely control the amount of each reactant. The temperature of the catalyst bed was measured by a K-type thermocouple (DT-8891E) and an infrared thermometer (Fisher Scientific). The gas products were qualitatively detected using an Agilent GC-8890 gas chromatograph equipped with a TDX-01 column, a thermal conductivity detector (TCD), and a flame ionization detector (FID) with ultra-high purity argon as the carrier gas. The liquid products were quantified by a liquid chromatography (1260 Infinity II LC System, Agilent Technologies, Inc, Santa Clara, CA, USA) with a refractive index detector (RID) and a column from Bio-Rad.

To detect the difference between photothermal and traditional thermal catalysis, an IKA heating plate equipped with a thermocouple was used as a heat source for both the heating zone and reaction area to test the thermal catalytic activity.

Isotopic labeling experiments
13CO2 gas (isotopic purity 99%) was used as the carbon source instead of pure 12CO2 gas (chemical purity 99.999%), and the same photocatalytic process was used for isotope labeling measurement. The high-purity Ar was purged at a flow rate of 8 sccm for 60 min to remove any existing CO2 in the reactor. The reactor was then exposed to a 300 W Xe lamp for 1 h while continuously bubbling and stirring at a flow rate of 13CO2 and 12CO2 at 30 mL/min. Photocatalytic species were separated into individual substances via gas chromatography columns. The origin of products was analyzed by gas chromatography-mass spectrometry (GC-MS) (Trace 1310 GC-ISQ quadrupole MS, ThermoFisher, USA).

To confirm the oxygen source of O2, the labelled H218O experiment was performed as follows: CO2 was purged continuously into the reactor for 60 min. Then, 1 mL of H218O was introduced into the reactor. The temperature was controlled at room temperature. Then, the photocatalytic reaction experiment was conducted as before by irradiating the mixture for 1 h. The gas samples from the reactor were collected and analyzed by mass spectroscopy.

Equations evaluating photocatalytic CO2 reduction
The production rate, the selectivity, and the apparent quantum efficiency (AQE) were calculated as follows.
Production rate. The mass-normalized production rate of product i was calculated as55:
ri ( μmol g-1 h-1 ) = Ni / (mcat × t) (2)
where Ni is the amount of product i (μmol), mcat is the catalyst mass (g), and t is the irradiation time (h).

Amount of gaseous products. For continuous-flow analysis, the amount of product i was obtained from the outlet flow rate and the measured volume fraction:
Ni (mol)= (Fout × (Vi/100)) / Vm (3)
where Fout is the total outlet flow rate (sccm), Vi is the volume percentage of product i (vol%), and Vm = 22.414 L mol–1 is the molar volume of an ideal gas at 273.15 K and 1 atm.

Selectivity. The electron-based selectivity toward the product i was calculated as:
Si ( % ) = (ni × Ni) /  (nj × Nj) × 100% (4)
where ni is the number of electrons required to form one molecule of product i, and the summation index j refers to all detected reduction products.

Apparent quantum efficiency (AQE). The light was filtered by different monochromatic filters to test the PCOR, and the AQE was calculated as:
AQE ( % ) = (ne × Nprod) / Nph × 100% (5)
where ne is the electron number per product molecule (for CH4, ne = 8); Nprod is the amount of the target product formed (mol).
Nph = (I × A × t × ) / (h × c) (6)
where Nph is the total number of incident photons during the irradiation time t; I is the incident light intensity (W m–2); A is the illuminated area (m2); t is the irradiation time (s);  is the incident wavelength (m); and h and c are Planck’s constant and the speed of light, respectively.

COMSOL Multiphysics simulations
The finite element modeling (FEM)-based numerical framework in COMSOL Multiphysics 6.3 was used to numerically solve the frequency domain form of Maxwell’s equation:
 × (r−1  × E) − k02 (r − j / 0) E = 0 (7)
where k0 represents the wavenumber,  is the angular frequency, E is the scattered field; 0 is the vacuum permittivity, r, r, and  represent the properties of the material, namely, relative permeability, relative permittivity, and electrical conductivity, respectively.

The frequency domain module and thermal conduction module are coupled in one model, in which single spherical Bi@Fe2O3 nanoparticle is illuminated by the linear polarized plane wave, with a power density of 2.5 mW μm–2. The core radius and shell thickness of the Bi@Fe2O3 nanoparticle are 50 and 5 nm, respectively, in line with the TEM and SEM images. A spherical computational domain with a perfectly matched layer (PML) was implemented. To characterize the thermal effects of the plasma itself, the heat-transfer analysis was carried out by solving the steady-state form of the heat equation:
 Cp U T =  (k T) + q (8)
where T is the temperature, U is the velocity vector, k, , and Cp are the local properties, namely, thermal conductivity, density, and heat capacity, respectively. The heat source term q(r) induced by LSPR is given by the equation:
q(r) = 0.5  0 Im(r) |E(r)|2 (9)
where  is the angular frequency of the incident light, 0 is the vacuum permittivity of the material, and r is the complex relative permittivity, and E(r) is the local electric field. Optical constants and heat-transfer parameters for Bi, Fe2O3, and water were taken from the built-in Optical Materials Database. Due to the small length scale of the nanoparticle, the Grashof number is negligible, and the convective contribution to heat transfer can be neglected. Thus, heat transfer was modeled using the conduction equation in a sufficiently large domain that approximates an infinite medium.

Note: i) The perfectly matched layer (PML) is a non-reflective boundary condition that absorbs outgoing electromagnetic waves, minimizing spurious reflections to accurately model wave propagation in unbounded domains.
ii) Due to the high symmetry, only a quarter of the model is simulated to reduce computational costs.
iii) Due to the low air mass fraction, water’s properties are applied to the medium.

Density functional theory (DFT) calculations
The spin-polarized first-principles calculations in this study were performed using density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP, version 5.4.4)56. The interactions between valence electrons and ionic cores were described using the projector augmented wave (PAW) method57,58, which ensures accurate treatment of the core-valence interactions. The exchange-correlation energy was treated with the revised Perdew-Burke-Ernzerhof (RPBE) functional within the framework of the generalized gradient approximation (GGA)59. A kinetic energy cutoff of 500 eV was employed for the plane-wave basis set to ensure convergence of the total energy.

All structural optimizations were conducted using the conjugate gradient algorithm until the residual force on each atom was less than 0.05 eV/Å, and the electronic self-consistent field (SCF) convergence criterion was set to 10–6 eV. Brillouin zone integrations were carried out using a Γ-centered 2 × 3 × 1 Monkhorst-Pack k-point grid for the slab models. Gaussian smearing with a width of 0.05 eV was applied for electronic occupations. Where necessary, dipole corrections were applied to eliminate spurious interactions between periodic images. For surface and adsorption calculations, a vacuum layer of at least 15 Å was added along the direction perpendicular to the slab to avoid image interactions. To illustrate the long-range dispersion interactions between the adsorbates and catalysts, we employed the D3 correction method by Grimme et al60. All calculations were carried out under periodic boundary conditions.

The free energy change (G) of each elementary step was estimated using the following thermodynamic correction:
G = E + ZPE + T0 Cp dT − TS (10)
where E is the DFT total energy difference between initial and final states, ZPE is the zero-point energy correction, T0 Cp dT represents the thermal contribution to the enthalpy, and S is the entropy change. All thermodynamic corrections were obtained from vibrational frequency analysis and processed using the VASPKIT package61.

Note: It should be noted that although the experimentally synthesized Bi@Fe2O3 catalyst exhibits a core@shell architecture, constructing a full-scale core@shell model is hardly possible in DFT calculations. Therefore, as commonly reported in previous studies62-65, simplified slab-based models are employed to approximate the essential physicochemical features of core@shell systems. In a conventional stacked slab model (a layered “vertical” slab model, Supplementary Fig 57), the Bi/Fe2O3 interface is buried and inaccessible to reactant molecules, so that it is extremely difficult to investigate the interfacial electronic effects under CO2/H2O coexisting conditions. To better represent the realistic reaction microenvironment at the interface, a new Bi@Fe2O3 interfacial model was constructed by loading Bi nanoparticles onto the Fe2O3 slab. A layer of 16 H2O molecules was introduced above the surface to account for solvation and interfacial electric-field effects66.

Although this model represents a geometric simplification of the ideal core@shell structure, it captures the intrinsic electronic interaction at the Bi@Fe2O3 heterojunction. Importantly, the charge redistribution and activation mechanisms observed in this interfacial model reflect the intrinsic chemical nature of the Bi@Fe2O3 heterojunction. The resulting theoretical insights are therefore directly relevant to understanding the experimentally observed high selectivity for CO2 reduction to CH4 under CO2 and H2O coexisting conditions. Considering all computational constraints and mechanistic requirements, we believe that the adopted modeling approach provides more physically meaningful and experimentally consistent theoretical description of the catalytic mechanism proposed in this work.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors synthesized a Bi@Fe2O3 core@shell structure and applied it to driven photoreduction of CO2 to CH4 under Uv-Vis-NIR light irradiation. The Bi@Fe2O3 obtained a CH4 production rate of 273.81 μmol g–1 h–1 with 98.6% selectivity. It is found that the Fe2O3 as a catalytic bed to accept the hot-electrons and confine the localized heat. This synergistic effect of photochemical and photothermal effects has achieved a result in the conversion of CO2 to CH4. Overall, the catalyst is well characterized and the system appears promising. However, there are a number of missing information and inconsistencies about mechanism for the catalytic procedure and selectivity that need to be addressed. Therefore, a major revision is suggested. Below are my comments and concerns.\n1. The reduction of CO2 to CH4 is an 8-electron transfer process, which is significantly slower in kinetics compared to the pathway leading to CO formation. Why do current systems primarily detect CH4 as the main product while yielding minimal CO? In addition, the underlying mechanism lacks corresponding experimental validation.\n2. The conduction band position of Fe2O3 is typically positive (more positive than 0 V vs. NHE), posing a significant thermodynamic challenge for driving CO2 reduction to CO or CH4. It is therefore difficult to reconcile this with the experimental observation that the authors detected these products on pristine Fe2O3. In contrast, if the product is originated from the hot electrons in Bi@Fe2O3, what’s the specific role of Fe2O3 in such a structure? The mechanism that overcomes this thermodynamic barrier needs to be addressed.\n3. The authors propose that Fe2O3 functions as the photo-absorber while Bi provides LSPR-induced heat effect. However, under NIR light irradiation (which does not excite Fe2O3), a substantial CH4 yield (about 50% of the full-spectrum yield) is still observed on the Bi@Fe2O3. This notable NIR activity, which contradicts a mechanism solely dependent on Fe2O3's photo-absorption, remains unexplained.\n4. Regarding the activity data, the authors have only provided the production rates for the reduction products (e.g., CH4, CO). For a complete assessment of the reaction, it is essential to also identify and quantify the oxidation products. Furthermore, what about the stoichiometry between the oxidation and reduction products?\n5. The author claims an enhanced CO2 adsorption for the Bi@Fe2O3 core-shell composite compared to that of Fe2O3 and Bi-Fe2O3. However, if Bi is encapsulated as the core, the adsorption should be dominated by the shell properties. This apparent contradiction requires clarification.\n6. The stability and potential evolution of the catalysts after reaction require further investigation. It is essential to characterize the post-reaction samples in terms of their morphology, composition, and chemical states. Analyses such as XRD, XPS, and TEM should be performed to confirm whether the core-shell structure remains intact, and to identify any potential phase transitions or changes in the oxidation states of Fe and Bi.\n7. The stability of the Fe2O3 shell and the potential formation of bismuth oxide require thorough investigation. It is crucial to determine whether the surface defects of Fe2O3, proposed as active sites, are repaired or altered during the reaction. Concurrently, the chemical state of the Bi core must be verified to rule out its oxidation, which would fundamentally change the proposed LSPR and thermal mechanism.", "reviewer_2": "The study presents an innovative strategy for highly efficient, full-spectrum photocatalytic CO2 reduction PCO2R to CH4 using a noble-metal-free Bi@Fe2O3 coreshell nanostructure. The core concept of a \"nanoscale greenhouse effect\" is interesting to the community, enabling synergistic photochemical and photothermal catalysis by confining locally generated heat. The performance metrics, particularly the CH4 production rate and NIR-light AQE, are superior to current literature, making this work a significant contribution to sustainable energy and catalysis. The overall manuscript has been organized with good logic flow and well-written. However, before considering publication in the international edition of Nature Communications, we recommend the following minor revisions to enhance the clarity and rigor of the work.\n1. Please ensure consistent terminology is used throughout the entire manuscript (including the main text and Supporting Information) when referring to the Bismuth components. For instance, standardize the usage to one term (e.g., sometimes Bi nanocore, sometimes Bi NCs, sometimes Bi NPs) instead of interchanging them..\n2. The prominent defect visible in the upper-left corner of the Bi core in Figure 2E (main text) and Figure 2b (Supporting Information) is clearly noticeable. Please provide an explanation for the cause of this defect and discuss its potential impact, if any, on the catalyst's performance.\n3. Supplementary Figure 14 shows that Bi-NC exhibits better light absorption properties, which are not presented in Figure 4a of the main text. Could you include this in Figure 4a and further analyze the mechanism behind the enhanced near-infrared absorption performance of 5-Bi@Fe2O3?\n4. Figure 5 demonstrates the superior photocatalytic reduction performance of 5-Bi@Fe2O3. Could you include photocatalytic experiments under different atmospheres (N2, Air, pure CO2) for further comparison?\n5. The in situ XPS clearly shows the formation of a higher concentration of Fe2+ under illumination, suggesting hot-electron trapping and the formation of oxygen vacancies are critical for CO2 activation. The final proposed mechanism (Fig. 6h/6i) primarily focuses on electron transfer. The authors must explicitly integrate the transient Fe2+ active site into the reaction mechanism, particularly for the activation of CO2 to the first intermediate (*COOH). Is the Fe2+ state the actual reaction site? Does the high temperature stabilize or destabilize this state?\n6. What specific mechanism of the 5-Bi@Fe2O3 photocatalyst led to its selection in this study? Additionally, discuss the key differences between your core-shell system and previously reported photocatalytic dual-metal cases (e.g., AgCu bimetal photocatalyst in Ag2Cu2O3, or other bimetal photocatalyst), emphasizing the novelty of your \"nanoscale greenhouse effect\" strategy.\n7. Supplementary Figures 6 and 7 display the physical adsorption capacity. Could you add the pore size distribution for Bi, Fe2O3, and Bi-Fe2O3, and further explain the influence of the core-shell structure of 5-Bi@Fe2O3?\n8. The main text mentions that the specific surface area of the 5-Bi@Fe2O3 material has been enhanced. Please provide the BET data (table/figure) to quantitatively validate this claim.", "reviewer_3": "This manuscript reports a Bi@Fe2O3 core-shell photocatalyst described as exhibiting a “nanoscale greenhouse effect” for full-spectrum CO2 reduction with H2O to CH4. The study is experimentally comprehensive, integrating fs-TA, KPFM, NAP-XPS, DRIFTS, and DFT analyses to correlate photothermal and photochemical contributions. The catalytic performance is impressive for a noble-metal-free system, and the proposed photothermal synergy represents a practical approach for harvesting the full solar spectrum for fuel production. Nevertheless, despite the solid experimental effort, the conceptual novelty and mechanistic interpretation require significant clarification. The current evidence for the proposed “nanoscale greenhouse effect” is not fully convincing, and several data inconsistencies, figure errors, and methodological gaps need to be addressed.\n1. The manuscript attributes the higher temperature of Bi@Fe2O3 to an “infrared shielding” effect of the Fe2O3 shell. However, this description is too general and should specify the relevant infrared wavelength range. If the shell is proposed to “trap infrared heat,” the authors should clarify in which IR band this occurs and provide corresponding optical or thermal data (e.g., absorption or emissivity spectra) for Fe2O3. Without this information, the observed temperature increase is more plausibly due to enhanced broadband absorption or the low thermal conductivity of the shell rather than true infrared reflection or confinement. To further distinguish thermal confinement from electronic effects, the authors are encouraged to prepare a Bi@SiO2 reference sample. SiO2 is optically transparent and electronically insulating but provides similar thermal insulation owing to its low thermal conductivity. Comparing the heating behavior of Bi@Fe2O3 and Bi@SiO2 under identical illumination would help separate the pure photothermal (“greenhouse-like”) contribution from the electronic and catalytic roles of Fe2O3, thereby clarifying whether the observed temperature rise genuinely reflects an IR-shielding mechanism.\n2. The size of the core-shell structure is mentioned more than once in the text ~100 nm. Is there any discrepancy between the text and the pictures? For example, in Figure 2c, the size is significantly larger than 200 nm.\n3. There are numerous typographical and graphical errors throughout the manuscript. Please carefully check all figures and labels. For instance, the first XRD label in Figure 2f should be “5-Bi@Fe2O3.” In Figure S19, the image corresponding to 5-Bi@Fe2O3 should be labeled as S19c. The diagrams in Figures 6b and 6c appear to be inverted and should be placed the other way around. In the product diagram of Figure S22, both lines are labeled as “CO,” which should be corrected.\n4. In most Fe 2p XPS peak-fitting diagrams, the Fe2+ peaks are almost completely obscured.\n5. Infrared thermal imagers can only record the surface temperature of objects. Thus, the infrared image shown in Figure 4d, which was measured through a quartz glass window, represents the temperature of the glass surface rather than the actual catalyst surface. In addition, in Figure S16b the infrared camera appears to be positioned too far from the sample, which may further reduce temperature accuracy.\n6. Only SEM, TEM, and XRD were used to characterize the morphology and structure of 5-Bi@Fe2O3 after reaction. However, the changes in the valence states of Bi and Fe were not analyzed. It is recommended to include post-reaction XPS characterization to clarify these electronic-state changes.\n7. In Figure 5f, the temperature for condition 4 is shown as 170°C, whereas the main text states 220°C. Please verify and correct the specific temperature value.\n8. The photocurrent data of Bi@Fe2O3 under full-spectrum illumination shown in Figures S33a and S33b appear to be inconsistent, and the sample labeling in Figure S33b seems to contain an error. In addition, the presence of a measurable current under dark conditions requires clarification. The authors should explain whether this dark current arises from instrumental background, electrochemical leakage, or intrinsic conductivity of the sample.\n9. The DFT calculations for both Fe₂O₃ and 5-Bi@Fe2O3 are performed on the bare Fe₂O₃ surface, focusing solely on CO2 reduction. However, since the experimental reaction involves both CO2 and H2O, it is recommended that the authors construct a model of Fe2O3 supporting Bi nanoparticles and perform interfacial calculations in the presence of H2O. Simulating the reaction pathways of CO2 and H2O coadsorption and activation at the Bi-Fe2O3 interface and comparing these results with those on bare Fe₂O₃ would provide a more realistic understanding of the catalytic mechanism and clarify the cooperative role of the Bi-Fe2O3 interface in the observed activity."}, "author_response": "We would like to thank all the reviewers for carefully evaluating our manuscript and providing their expert opinion. We are grateful for the constructive feedback, which helped us to further improve the manuscript. We have addressed all of the reviewers’ points below and have updated the contents in the revised manuscript and Supporting Information accordingly: Reviewers’ comments in black; authors’ response in blue; changes to the manuscript Supporting Information in red. Please see below our point-by-point responses.\nPlease note: In response to the reviewers’ comments, Fig. R is the figure in the Response to Reviews. Fig. is the figure in the Revised Manuscript. Supplementary Fig. S is the figure in the Supporting Information.\nReviewer #1 (Remarks to the Author):\nGeneral comment: In this work, the authors synthesized a Bi@Fe O core@shell structure and applied it to driven photoreduction of CO to CH under Uv-Vis-NIR light irradiation. The Bi@Fe O obtained a CH production rate of 273.81 μmol g–1 h–1 with 98.6% selectivity. It is found that the Fe O as a catalytic bed to accept the hot-electrons and confine the localized heat. This synergistic effect of photochemical and photothermal effects has achieved a result in the conversion of CO to CH . Overall, the catalyst is well characterized and the system appears promising. However, there are a number of missing information and inconsistencies about mechanism for the catalytic procedure and selectivity that need to be addressed. Therefore, a major revision is suggested. Below are my comments and concerns.\nResponse: We sincerely thank the reviewer for the careful evaluation of our manuscript and favorable recognition of the proposed photochemical-photothermal synergy over the Bi@Fe O core@shell catalyst. We also appreciate the reviewer’s constructive suggestion for missing information and mechanistic consistency. In response, we have performed additional experiments, DFT calculations, and corresponding analyses to address all comments. We have revised the manuscript accordingly to strengthen the mechanistic interpretation and improve clarity throughout the main text and Supporting Information. We hope that our responses adequately address all concerns to the reviewer’s satisfaction, with the authors’ response in blue; changes to the manuscript in red. We address each comment point-by-point below:\nComment 1: The reduction of CO to CH is an 8-electron transfer process, which is significantly slower in kinetics compared to the pathway leading to CO formation. Why do current systems primarily detect CH as the main product while yielding minimal CO? In addition, the underlying mechanism lacks corresponding experimental validation.\nResponse: We sincerely thank the reviewer for this insightful comment regarding the fundamental kinetic challenge. We agree with the reviewer that the 8-electron reduction of CO to CH involves more complex steps and typically exhibits slower kinetics compared to the 2-electron pathway leading to CO formation.\nThe photocatalytic CO reduction to CO:\n2 → *CO\n2 2\n*CO + e− + H+ → *COOH\n2\n*COOH + e− + H+ → *CO\n*CO + e− → CO ↑\nThe photocatalytic CO reduction to CH :\n2 4\nCO → *CO\n2 2\n*CO + e− + H+ → *COOH\n2\n*COOH + e− + H+ → *CO\n*CO + e− + H+ → *CHO\n*CHO + e− + H+ → *CH O\n2\n*CH O + e− + H+ → *CH O\n2 3\n*CH O + e− + H+ → *CH ↑+ *O\n3 4\n*O + e− + H+ → *OH\n2\nwhere the “*” denotes catalytically active sites and the “↑” represents the release of gas.\nHowever, in our work, pristine Fe O only yields an extremely small amount of CO (a 2-electron reduction product), and the formation of CH (requiring an eight-electron transfer) and H is negligible (Fig. R1a). This result indicates that, in the absence of sufficient driving force, the reaction is kinetically inclined to terminate at the CO stage due to the high energy barriers associated with the subsequent hydrogenation steps. It is worth noting that after introducing Bi nanoparticles, CH becomes the dominant product instead of CO (Fig. R1a). Importantly, after constructing the Bi@Fe O core@shell structure, the selectivity toward CH is markedly enhanced (Fig. R1a). This enhancement was ascribed to the unique role of the Bi core in providing a strong driving force to overcome these kinetic limitations.\nSpecifically, the Bi core contributes the driving force based on two synergistic effects: (i) high-energy hot electrons generated via localized surface plasmon resonance (LSPR) in Bi nanocaore possess a higher reduction potential than the intrinsic conduction band electrons of Fe O , which enable the reduction pathway to proceed beyond the *CO intermediate; (ii) a localized photothermal effect, which increases the local surface temperature and thereby accelerates the sluggish rate-determining hydrogenation steps (e.g., *CO → *CHO).\nFig. R1. (Fig. 4a, in the revised manuscript; Supplementary Fig. 29, in the revised Supporting Information) (a) The PCO R performance over different samples at 455 mW cm–2. (b) Calibration curve for GC-MS quantification of CH and CO. The absolute peak area versus the concentration of (c) CO and (d) CH over 5-Bi@Fe O .\nTo further address the reviewer’s concerns and validate the mechanism in our work, we have performed additional experiments (in-situ DRIFTS and CO-TPD) and theory calculations (DFT). As shown in Fig. R2, Fig. R3 and Fig. R4, the evolution of surface intermediates over Fe O , Bi-Fe O , and 5-Bi@Fe O was monitored under full-spectrum irradiation. Upon irradiation, bands corresponding to *COOH and *CO intermediates were clearly observed over Bi-Fe O and 5-Bi@Fe O , while the peaks of these intermediates over Fe O are too weak to be almost unobservable. In contrast, the intensities of the intermediate-related peaks observed on Bi-Fe O are much lower than those detected on 5-Bi@Fe O . And key intermediates involved in CH formation, such as *CH O and *CH O, are almost undetectable on Bi-Fe O . While the subsequent emergence and strengthening of bands attributed to oxygen-bound intermediates such as *CHO, *CH O, and *CH O were observed on 5-Bi@Fe O . This indicates that the *CO species are strongly adsorbed to hydrogenate rather than desorbing as CO gas over 5-Bi@Fe O . In addition, CO temperature-programmed desorption (CO-TPD) measurements were performed to further investigate the CO adsorption strength on the surfaces of the three catalysts (Fig. R5). The 5-Bi@Fe O sample exhibits a CO desorption peak at a significantly higher temperature compared to pristine Fe O and Bi-Fe O , indicating a much stronger binding affinity for CO. Moreover, the projected crystal orbital Hamilton population (COHP) analysis exhibited that the spherical 5-Bi@Fe O catalyst exhibits a markedly more positive integrated COHP (ICOHP) value than the Fe O catalyst (3.92 eV vs 0.72 eV) (Fig. R6), indicating a stronger interfacial electronic coupling between the core@shell junction and the *CO intermediate. To further explore the mechanism underlying the high selectivity toward CO to CH over the core@shell structure, we calculated the Gibbs free energy diagrams for CO reduction pathways on the 5-Bi@Fe O and Fe O catalysts (Fig. R7). The hydrogenation of *CO to *CHO exhibits a higher energy barrier (0.98 eV) than that of CO desorption (0.20 eV) for Fe O catalyst, which indicates the *CO intermediate tends to desorb as CO gas instead of hydrogenation. So the construction of Bi@Fe O fundamentally alters the reaction pathway. The hydrogenation step from *CO to *CHO becomes thermodynamically downhill (–1.09 eV). This dramatic decrease in energy barrier means that once *CO is formed, it is spontaneously and rapidly protonated to *CHO rather than desorbing as CO gas. This “downhill” thermodynamic driving force effectively drives the reaction forward through the 8-electron pathway to CH .\nFig. R2. (Supplementary Fig. 27, in the revised Supporting Information) In-situ DRIFTS over Fe O catalyst.\nFig. R3. (Fig. 6d, in the revised manuscript; Supplementary Fig. 51c, in the revised Supporting Information) In-situ DRIFTS of Bi-Fe O catalyst.\nFig. R4. (Fig. 6e, in the revised manuscript; Supplementary Figs. 51a and b, in the revised Supporting Information) In-situ DRIFTS of 5-Bi@Fe O catalyst.\nFig. R5. (Fig. 6b, in the revised manuscript) CO-TPD profiles of different catalysts.\nFig. R6. (Supplementary Fig. 47, in the revised Supporting Information) COHP for the Fe-C of absorbed *CO on the surface of (a) Fe O catalyst and (b) 5-Bi@Fe O catalyst.\nFig. R7. (Fig. 6f, in the revised manuscript) Gibbs Free energy diagram for PCO R over Fe O and 5-Bi@Fe O model.\nTherefore, in our work, the high selectivity for CH originates from the unique property of the Bi@Fe O interface. The Bi core generates high-energy hot electrons via LSPR, which possess sufficient overpotential to drive the difficult multi-electron transfer steps that Fe O alone cannot sustain. And the localized heat confined by the Fe O shell significantly lowers the activation energy for the rate-determining hydrogenation steps (e.g., *CO → *CHO), effectively accelerating the deep reduction process. Consequently, the *CO intermediate is rapidly consumed (hydrogenated) on the surface rather than desorbing as CO gas, leading to more CH detection.\nFinally, previous studies have demonstrated that product selectivity can depend upon reaction environment such as catalyst structure, sacrificial electron donor, and solvent media (Angew. Chem. Int. Ed., 2022, 61, e202116094; Inorg. Chem., 2015, 54, 5096-5104; Coord. Chem. Rev., 2018, 373, 333-356; Nat. Chem., 2020, 12, 180–185; Energy Environ. Sci., 2021,14, 2429-2440). Additionally, near-100% selectivity toward single products such as CH , C H , and CH COOH has been widely reported in recent photocatalytic CO reduction studies (Nat. Commun., 2024, 15, 4675; Energy Environ. Sci., 2024, 17, 5060-5069; Adv. Mater. 2025, e12480).\nThank you once again for your careful review and constructive feedback. We believe these revisions have significantly improved the manuscript’s readability and quality.\nChanges in Manuscript:\nWe have added the revised figures along with the discussion to the “Proposed reaction mechanism for enhanced photoactivity” section:\n“CO is a key reaction intermediate during CO conversion. As shown in Fig. 6b, the evolution of CO adsorption on the spherical core@shell catalysts closely followed the trend observed for CO adsorption, indicating that the construction of the core@shell structure effectively increased both the number and strength of CO adsorption sites. These enhanced CO-binding sites, mainly originating from the abundant core@shell interfacial regions (Fe-V active centers), stabilized the *CO intermediates and favored O their subsequent hydrogenation or dissociation rather than desorption. Consequently, CO selectivity was suppressed, while CH formation was significantly promoted. To gain deeper insight into the protonation behavior of *CO, the projected crystal orbital Hamilton population (COHP) analysis was also performed. As shown in Supplementary Fig. 47, the spherical 5-Bi@Fe O catalyst exhibits a markedly more positive integrated COHP (ICOHP) value than the Fe O catalyst (3.92 eV vs 0.72 eV), indicating a stronger interfacial electronic coupling between the core@shell junction and the *CO intermediate. This strengthened bond interaction arises from the unique core@shell interface and effectively stabilizes *CO, thereby facilitating its subsequent protonation and promoting the downstream hydrogenation reaction.”\nFig. 6 Mechanism analysis of photoactivity enhancement. a CO -TPD profiles of different catalysts. b CO-TPD profiles of different catalysts. c In-situ XPS analysis of the Fe 2p signals in 5-Bi@Fe O under full-spectrum light irradiation. d In-situ DRIFTS of Bi-Fe O catalyst. e In-situ DRIFTS of 5-Bi@Fe O catalyst. f Gibbs Free energy diagram for PCO R over the pristine Fe O surface and the Bi/Fe O interfacial model, both with adsorbed CO and an explicit H O solvent layer. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C). g Mechanism of PCO R driven by the synergy of photochemical and photothermal effects based on the cascaded utilization of full-solar spectrum.\nChanges in Supporting Information:\nSupplementary Fig. 29 has been added, along with the discussion:\nSupplementary Figure 29. The absolute peak area versus the concentration of (a) CO and (b) CH over 5-Bi@Fe O catalyst. (c) Calibration curve for GC-MS quantification of CH and CO.\nSupplementary Discussion: The calibration and quantification basis for the CO and CH yields in this work were presented. The peak area shows a good linear relationship with gas concentration for both CO and CH over the relevant range, enabling reliable conversion of chromatographic signals into absolute product amounts. A representative chromatogram further illustrates the clear separation and identification of CH and CO peaks, supporting accurate integration without peak overlap. These calibrations are essential for rigorously comparing product yields and selectivity among Fe O catalyst, Bi@Fe O catalyst, and the 5-Bi@Fe O core@shell catalyst.\nComment 2: The conduction band position of Fe O is typically positive (more positive than 0 V vs. NHE), posing a significant thermodynamic challenge for driving CO reduction to CO or CH . It is therefore difficult to reconcile this with the experimental observation that the authors detected these products on pristine Fe O . In contrast, if the product is originated from the hot electrons in Bi@Fe O , what’s the specific role of Fe O in such a structure? The mechanism that overcomes this thermodynamic barrier needs to be addressed.\nResponse: We thank the reviewer for this important comment. Firstly, we fully agree that the conduction band (CB) edge of bulk Fe O is generally located at a relatively positive potential (more positive than 0 V vs. NHE), which is thermodynamically unfavorable for CO reduction to CO/CH via a classical band-edge photoreduction mechanism. In the revised manuscript, we therefore clarify that the trace CO/CH detected on pristine Fe O should not be explained as arising from a pure CB-driven pathway. Moreover, we have provided a detailed explanation of the specific role of Fe₂O₃ in the core@shell architecture and made the corresponding supplement in the revised manuscript.\n(i) Why can pristine Fe O show almost negligible PCO R activity?\nThe amounts of the detected products of PCO R over pristine Fe O catalyst in our experiments were extremely low (near the GC-MS quantification limit in our setup), which may be associated with defect-derived surface states and minor local heating rather than intrinsic band-edge reduction. Oxygen-vacancy-related defects are evidenced by EPR (Fig. R8a) and by the O 1s XPS deconvolution showing a distinct oxygen-vacancy component (Fig. R8b). These V surface defect states can locally enrich electrons (Fig. R8c), stabilize early activation intermediates (e.g., *COOH), and elevate surface temperature (Fig. R8d), enabling trace CO reduction under broadband irradiation even though the CB edge of bulk Fe O is not sufficiently negative (Fig. R9).\nMoreover, very small amounts of PCO R products associated with non-band-edge processes, such as defect-related states, surface-trapped electrons, or localized thermal effects under illumination, have been reported in previous works (such as, J. Mater. Chem. A, 2021, 9, 22353-22363; Nano Lett., 2024, 24, 696-702).\nFig. R8. (a) EPR spectra of the Fe O catalyst. (b) O 1s for XPS spectra of the Fe O catalyst. (c) DFT-calculated interfacial charge redistribution (charge-density difference) at the Fe O interface. (Orange: Fe; Red: O; White: H; Grey: C), in which yellow and cyan represent electron accumulation and depletion, respectively. (d) Monitoring of temperatures over different Fe O -based catalysts under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2.\nFig. R9. (Supplementary Fig. 55, in the revised Supporting Information) Schematic illustration of the reaction pathway in PCO₂R for the pristine Fe O surface model, with an explicit H O solvent layer. (Orange: Fe; Red: O; White: H; Grey: C)\n(ii) If the products originate from hot electrons in Bi@Fe O , what is the specific role of Fe O ? How is the thermodynamic barrier overcome?\nFig. R10. (Supplementary Figs. 43, 44, in the revised Supporting Information) The work function from the theoretical calculation of (a) Bi (012) and (b) Fe O (110). (c) UPS secondary electron cutoff (UPS-SEC) spectra of Bi NCs and Fe O . The incident photon energy (hν) is 21.22 eV. The work function (WF) is deduced by subtracting the photon energy with the cut off energy. (d) Schematic illustration of the heterojunction in core@shell nanospheres.\nFig. R11. (Supplementary Fig. 45, in the revised Supporting Information) (a) Charge density distribution of 5-Bi@Fe O model. The iso-surface value is 0.01e Å- 3. (b) Electrostatic potential energy along the x-axis for 5-Bi@Fe O model. (c) Electric field distribution under different excitation wavelengths that was excited at an incident power density of 2.5 mW μm–2 over Bi@Fe O . Scale bars, 50 nm.\nFirstly, Fe O can modulate the electronic distribution at the core@shell interface. The ultraviolet photoelectron spectroscopy (UPS) analysis and theoretical calculation results (DFT) indicate Bi has a lower work function than Fe O (Fig. R10a–c). As a result, the energetic hot electrons generated in the Bi NC can be efficiently transferred to the Fe O shell. This creates an electron accumulation layer (Fig. R10d), acting as an electron sink to efficiently extract and store hot electrons from Bi NC (Fig. R11). Because these are non-equilibrium interfacial hot electrons and trapped surface states, the CO reduction reaction is not strictly constrained by the CB position of bulk Fe O .\nFig. R12. (Fig. 6c, in the revised manuscript; Supplementary Fig. 49, in the revised Supporting Information). (a) In-situ XPS analysis of the Fe 2p signals in 5-Bi@Fe O under full-spectrum light irradiation. (b) In-situ EPR spectra over Fe O , Bi-Fe O , and 5-Bi@Fe O catalysts.\nFig. R13. (Supplementary Fig. 56, in the revised Supporting Information). Schematic illustration of the reaction pathway in PCO R for the Bi/Fe O interfacial model, with an explicit H O solvent layer. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C)\nMoreover, the exposed Fe O shell provides a suitable reaction bed where CO adsorption, activation, and subsequent hydrogenation occur. In situ XPS over Bi@Fe O catalyst exhibited a substantially higher Fe2+/Fe3+ proportion than the Bi-Fe O catalyst (Fig. R12), consistent with the formation of electron-rich Fe-V sites O that facilitate the multistep proton-coupled electron transfer (as depicted in Fig. R13).\nFig. R14. (Fig. 3, in the revised manuscript). (a) Schematics of the nanoscale greenhouse effect for Bi@Fe O . (b) Monitoring of temperatures over different Fe O -based catalysts under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (c) spatial distribution of temperature rise under different excitation wavelengths that were excited at an incident power density of 2.5 mW μm–2 over Bi@Fe O and Fe O catalyst. Scale bars, 50 nm.\nFinally, the shell acts as a thermal insulator to confine the LSPR-induced heat, which further lowers kinetic barriers for CO activation and intermediate hydrogenation. The Fe O shell in the core@shell structure effectively suppresses heat dissipation from the Bi core to the surrounding environment after illumination (as depicted in Fig. R14a). Consistently, temperature measurements show that 5-Bi@Fe O reaches markedly higher catalyst-bed temperatures than Bi-Fe O under identical illumination (Fig. R14b). COMSOL Multiphysics simulations further reveal obvious enhancement of local electric fields and surface thermal energy in 5-Bi@Fe O under near-infrared excitation, accompanied by a temperature increase of ~135 K (Fig. R14c). The resulting localized high-temperature environment promotes molecular collision frequency and reaction kinetics, thereby accelerating proton-coupled multi-electron steps required for CO -to-CH conversion.\nIn summary, the CO -to-CH conversion for Bi@Fe O is not limited by the band-edge position of the Fe O shell. Instead, plasmonic Bi supplies non-equilibrium hot electrons and localized nanoheating, while the exposed Fe O shell serves as the accessible catalytic bed that captures these hot electrons and provides Fe-V –associated O activation sites for CO adsorption, activation, and subsequent hydrogenation. The coupled effects of interfacial hot-carrier transfer, defect-mediated surface activation, and photothermal acceleration synergistically overcome the apparent thermodynamic barrier implied by the Fe O band edge, thereby enabling efficient CO activation and deep hydrogenation toward CH . We have revised the mechanistic discussion accordingly to clearly distinguish the intrinsic band-edge limitation of pristine Fe O from the actual operative pathway in Bi@Fe O .\nChanges in Manuscript:\nWe have added the revised discussion to the “PCO R driven by coupling photo-/photothermal effect” section:\n“While pure Fe O produces only a small amount of CO, which may be associated with defect-derived surface states rather than intrinsic band-edge reduction (Supplementary Figs. 26 and 27).”\nWe have added the revised discussion to the “Proposed reaction mechanism for enhanced photoactivity” section:\n“Based on these results, we therefore proposed a CO -to-CH mechanism driven by the synergistic interplay between photochemical effect and photothermal effect over the Bi@Fe O catalyst (Fig. 6g). Specifically, the Bi core absorbs vis-NIR light to excite LSPR hot electrons, while the Fe O shell absorbs ultraviolet-visible light to generate electron-hole pairs. The non-radiative relaxation of these excited electrons generates localized heat, elevating the temperature at the interface between the Bi core and Fe O shell. The Fe O shell, possessing intrinsically low thermal conductivity, effectively confines this energy to establish a super-photothermal environment. Crucially, the hot electrons injected from Bi are continuously trapped by Fe-V sites (Fe3+ to form transient Fe2+ species), which serve as highly active centers for CO activation. These Fe-V sites are not only the platform of the initial activation for CO to the *COOH intermediate but also support the subsequent proton-coupled electron transfer deep reduction steps (*COOH → *CO → *CHO → *CH O → *CH O → *CH ).\nUnder continuous illumination, plasmon-induced hot-electron injection and localized photothermal heating sustain a dynamic Fe3+/Fe2+ redox cycle, ensuring the persistent availability of Fe-V sites during multi-electron proton-coupled transfer steps toward CH formation. Consequently, the synergistic coupling of plasmonic nanoheating and hot-electron injection based on the Bi@Fe O structure not only accelerates charge-carrier dynamics but also promotes the deep reduction of CO and H O to CH , as confirmed by in situ spectroscopy and control experiments.”\nChanges in Supporting Information:\nSupplementary Fig. 26 and 28 have been added, along with the discussion:\nSupplementary Fig. 26 Effect of light intensity on product yields for photocatalytic CO reduction over Fe O catalyst.\n“Supplementary Discussion: Pristine Fe O exhibits only trace product formation under increasing light intensity, with the yields remaining close to the detection limit compared with the Bi-containing catalysts. This observation is consistent with the fact that the conduction band edge of bulk Fe O is too positive to drive CO reduction via a conventional band-edge photoreduction pathway. Therefore, the minor CO/CH signals detected on Fe O should not be interpreted as intrinsic CB-driven activity, but are more plausibly associated with non-band-edge processes such as defect-derived surface states or localized photothermal effects under high-intensity irradiation. This can locally enrich electrons, facilitate the formation of early intermediates (e.g., *COOH), and thus enable only marginal CO reduction on pristine Fe O despite its unfavorable bulk band energetics.”\nSupplementary Fig. 28. The evolution rate of CH , CO, and H products over Fe O and different Bi content catalysts. Error bars represent standard deviations obtained from three independent measurements.\n“Supplementary Discussion: The product distribution over the x-Bi@Fe O catalysts, highlighted the critical role of the core@shell architecture in tuning both activity and selectivity. Pristine Fe O and the Bi-Fe O catalyst exhibit only low photocatalytic activity, whereas the core@shell structure (x-Bi@Fe O ) leads to a significant increase in photocatalytic activity, accompanied by higher CH -dominated selectivity. Moreover, the CH yield increases with Bi content from 1-Bi@Fe O to 5-Bi@Fe O and then approaches a plateau at higher loadings, suggesting an optimal balance between plasmon-enabled hot-electron generation in the Bi core and the accessible V -rich O Fe O surface sites for CO adsorption/activation. In this mechanism ascribed to these phenomena, the Bi core strengthens light harvesting and provides energetic carriers, while the porous Fe O shell supplies defect-mediated activation sites and promotes interfacial charge utilization, together enabling efficient deep CO hydrogenation to CH .”\nSupplementary Fig. 44 has been added.\nSupplementary Fig. 44 UPS secondary electron cutoff (UPS-SEC) spectra of Bi NCs and Fe O . The incident photon energy (hν) is 21.22 eV. The work function (WF) is deduced by subtracting the photon energy with the cut off energy.\nΦ = hv−(𝐸 − 𝐸 )\n𝑐𝑢𝑡𝑜𝑓𝑓 𝐹\nComment 3: The authors propose that Fe O functions as the photo-absorber while Bi provides LSPR-induced heat effect. However, under NIR light irradiation (which does not excite Fe O ), a substantial CH yield (about 50% of the full-spectrum yield) is still observed on the Bi@Fe O . This notable NIR activity, which contradicts a mechanism solely dependent on Fe O 's photo-absorption, remains unexplained.\nResponse: We thank the reviewer for this insightful comment. We agree that the activity of PCO R to CH under NIR irradiation cannot be explained by a mechanism solely relying on the photo-absorption of Fe O , because its bandgap does not allow direct excitation by NIR photons. We have revised the manuscript to clarify that CO was reduced to CH over Bi@Fe O via a photochemical-photothermal synergistic mechanism under NIR irradiation, in which NIR photons are utilized predominantly through plasmonic Bi even when Fe O is not directly photoexcited.\nFig. R15. (Fig. 3, in the revised manuscript) (a) UV-vis-NIR DRS spectra of Bi NCs, Fe O , Bi-Fe O , and 5-Bi@Fe O . (b) The infrared thermal images of 5-Bi@Fe O under UV-vis, NIR, vis-NIR, and UV-vis-NIR light under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (c) The spatial distribution of temperature rise under different excitation wavelengths that were excited at an incident power density of 2.5 mW μm–2 over Bi@Fe O . Scale bars, 50 nm. (d) Synchronous illumination XPS analysis of the Fe 2p signals in 5-Bi@Fe O under NIR light irradiation.\nUnder NIR irradiation, the Bi core is efficiently excited via localized surface plasmon resonance (LSPR), as evidenced by the enhanced vis-NIR absorption of Bi-containing samples in the diffuse reflectance spectra (Fig. R15a). The plasmonic Bi generates non-equilibrium hot carriers, and the non-radiative relaxation of Bi plasmonic excitation creates strong localized photothermal effects, which are directly supported by the significantly higher catalyst-bed temperatures of 5-Bi@Fe O under illumination (Fig. R15b, c). The high-energy hot electrons produced in Bi core were injected into Fe O shell to directly participate in CO activation and reduction, bypassing the band-edge limitations of Fe O . Meanwhile, the localized temperature rise at the Bi/Fe O interface significantly accelerates the multi-electron proton-coupled steps required for CH formation. Accordingly, the robust NIR activity primarily originates from the synergy of plasmon-enabled hot-carrier and photothermal effects rather than Fe O bandgap excitation (explained in the Response to Comment 2).\nImportantly, Fe O remains indispensable under NIR irradiation because it provides the catalytic reaction bed and CO activation sites. In situ XPS shows a clear illumination-induced Fe3+ → Fe2+ evolution (Fig. R15d), indicating that plasmon-derived hot electrons can be injected into the Fe O shell and trapped as defect-associated electrons. These Fe-V sites facilitate CO activation (e.g., * CO → O 2 2 *COOH) and promote deep hydrogenation steps toward CH . Therefore, even without direct Fe O photoexcitation, NIR-driven plasmonic hot-electron injection together with localized photothermal heating can activate CO and promote the multi-electron proton-coupled steps to CH formation.\nFig. R16. (Supplementary Fig. 42, in the revised Supporting Information; Fig. 4g, in the revised manuscript). (a) Photocurrent response of 5-Bi@Fe O with light on/off cycles under different light irradiation in 0.5 M Na SO electrolyte solution at an applied potential of 0.5 V vs. Ag/AgCl electrode. (b) The surface potential analysis of the 5-Bi@Fe O . (c) Electric field distribution under different excitation wavelengths that were excited at an incident power density of 2.5 mW μm–2 over Bi@Fe O . Scale bars, 50 nm.\nIt is noted that the difference between NIR irradiation and full-spectrum irradiation performance can be explained by the additional photochemical contribution under UV-visible irradiation. Under full-spectrum irradiation, Fe O can additionally contribute via bandgap excitation (UV-vis), supplying extra photogenerated carriers and further promoting the interfacial charge separation/transfer, while the Bi core still provides more photogenerated electrons and strong photothermal heating (Fig. R15 and Fig. R16), increasing the steady-state population of reactive carriers and key intermediates. As a result, the overall activity under full-spectrum irradiation is higher, whereas the NIR-only activity remains significant because the plasmonic hot-electron and photothermal effect alone already accounts for a large fraction of the total contribution.\nIn conclusion, the notable NIR activity does not contradict our mechanism; instead, this result is ascribed to a synergistic pathway. Here, plasmonic Bi core primarily supplies hot carriers and nanoheating, while Fe O acts as the catalytic reaction surface and provides Fe-V sites, enabling CO activation and deep reduction to CH . We thank the reviewer for this comment, which helped us refine and strengthen the mechanistic interpretation of the NIR-driven CO reduction behavior.\nChanges in Manuscript:\nWe have added the following discussion to the “Charge carrier dynamics ”section:\n“As shown in Figs. 5d–5g, the surface potential (CPD)of both the Bi core and Fe O shell under UV-vis-NIR irradiation exhibits a significant positive shift compared to that under dark or UV-vis illumination, indicating enhanced electron accumulation under full-spectrum excitation. This enhanced CPD response is attributed to the additional absorption of NIR photons, which activates the LSPR of Bi and generates a larger population of energetic hot electrons. Additionally, the transient photocurrent of 5-Bi@Fe O under full-spectrum illumination was markedly higher than that under UV-vis irradiation (Supplementary Fig. 42). These results demonstrate that UV-vis and near-infrared light synergistically enhance charge separation and carrier transport in 5-Bi@Fe O , thereby providing an optimal electronic environment for plasmon-assisted CO reduction.”\nComment 4: Regarding the activity data, the authors have only provided the production rates for the reduction products (e.g., CH , CO). For a complete assessment of the reaction, it is essential to also identify and quantify the oxidation products. Furthermore, what about the stoichiometry between the oxidation and reduction products?\nResponse: We thank the reviewer for this critical comment regarding the overall reaction stoichiometry. We fully acknowledge the reviewer’s concern about the oxidation products when studying photocatalytic CO reduction. Since our system was conducted in pure water without any sacrificial agents, identifying the oxidation products is indeed crucial to validate the source of electrons and protons (J. Mater. Chem. A., 2021, 9, 87-110; J. Am. Chem. Soc., 2023, 145, 422-435). As mentioned, pure water (H O) serves as the electron donor in our system, and CH was produced as the main product, along with trace CO and H . Therefore, the theoretical stoichiometric oxidation product is O (No other oxidized species were detected, such as H O ). In response to the key issues raised, we have carefully considered them and conducted further experiments to address these points.\nWe qualitate and quantify the O evolution by TCD detector in Agilent GC-8890 gas chromatograph under the calibration of standard pure O to monitor O evolution under the same photocatalytic conditions used for CH and CO quantification (Fig. R17a–c). In the experiment, argon gas was used as a carrier gas, and sample injection temperature and auxiliary heating temperature were set at 200 ℃ and 375 ℃, respectively, with the constant flow rate being 10 mL min-1. The initial temperature of the column box is 80 ℃, and the temperature is heated to 280 ℃ at 10 ℃ min-1, and the storage temperature is kept for 1 min. From the chromatographic signals analysis, the chromatographic peak observed at approximately 4–5 min can be attributed to O (Fig. R17c). Therefore, in this photocatalytic and photothermal-assisted reaction system, CO is reduced to CO and CH , while water is oxidized to O .\nFig. R17. (Supplementary Fig. 32, in the revised Supporting Information). (a) O production (oxidation product from water oxidation) over different samples as a function of time of irradiation in water. Error bars in these graphs represent standard deviations obtained from three independent measurements. (b) Mass spectrum of produced O under visible light over 5-Bi@Fe O catalyst. (c) Gas chromatogram of O in water over 5-Bi@Fe O catalyst.\nTo strictly assess the reaction stoichiometry, we performed a comprehensive electron balance analysis based on the detected production rates. The principle of charge conservation requires that the total rate of electrons consumed by the reduction half-reactions equals the rate of electrons released by the oxidation half-reaction.\nThe total electron consumption rate accounting for all reduction products (CH , CO, and H ) is calculated as:\nTotal reduction consumption electron = 8e– × R + 2e– × R + 2e– × R\nCH4 CO H2\nwhere R represents the production rate in PCO R.\nThe total electron generation rate from water oxidation to O is calculated as:\nTotal oxidation generation electron = 4e– × R\nO2\nBased on the gas chromatography data obtained from PCO R experiment (Fig. R17a), O evolution related to photogenerated holes consumption is stoichiometrically approximate to products of photogenerated electrons reduction (holes : electrons ≈ 1: 1). The slight deviation from the theoretical value is likely due to the strong adsorption of oxygen species on the catalyst surface or the H produced during the reaction reacted with O . This observation is consistent with many reported works (Nat. Commun., 2022, 13, 4592). These results together confirmed that the reaction in our system follows the theoretical stoichiometry of overall photocatalytic CO reduction with water.\nChanges in Manuscript:\nWe have added the following discussion to the “PCO R driven by coupling photo-/photothermal effect” section:\n“Therefore, an H 18O isotope-labeling experiment was performed to investigate whether water participates in the overall reaction (Supplementary Fig. 32a). Although there might be oxygen atom exchange between CO and H O, the production of 18O implied the involvement of water in the whole PCO R process. Moreover, quantitative analysis of the evolved gases revealed the concurrent formation of O throughout the CO reduction process (Supplementary Fig. 32b, Supplementary Fig. 32c), with no detectable by-products such as H O . This confirms that the photocatalytic conversion between CO and H O proceeds through a complete and closed redox cycle. Moreover, deuterium oxide (D O) was employed in the reaction to trace the origin of the H element in the produced CH . The product peak at m/z = 18 was indexed to CD (Supplementary Fig. 31b) when using D O in the reaction, indicating that the proton in CH comes from H O. This result further manifests that the formation of CH is a multistep hydrogenation process and that the proton is provided by H O45-47.”\nChanges in Supporting Information:\nSupplementary Fig. 32 has been added.\nSupplementary Fig 32. (a) Mass spectrum of produced O under visible light over 5-Bi@Fe O catalyst. (b) O production (oxidation product from water oxidation) over different samples as a function of time of irradiation in water. Error bars in these graphs represent standard deviations obtained from three independent measurements. (c) Gas chromatogram of O in water over 5-Bi@Fe O catalyst.\nComment 5: The author claims an enhanced CO adsorption for the Bi@Fe O core-shell composite compared to that of Fe O and Bi-Fe O . However, if Bi is encapsulated as the core, the adsorption should be dominated by the shell properties. This apparent contradiction requires clarification.\nResponse: We thank the reviewer for this thoughtful comment. We agree that, for a core@shell architecture, CO adsorption occurs predominantly on the exposed Fe O shell surface, and the encapsulated Bi core is not directly accessible as an adsorption surface. However, for the Bi@Fe O core@shell structure in this work, the enhanced CO adsorption cannot only be attributed to the intrinsic surface chemistry of Fe O , but also arises from interfacial electronic modulation and structural uniformity enabled by the spherical core@shell architecture.\nFig. R18. (Supplementary Fig. 11, in the revised Supporting Information). (a) O 1s for XPS spectra of different samples. (b) EPR spectra of different samples.\nFirstly, the core@shell configuration promotes the formation of oxygen-vacancy-related Fe-V -associated sites on the shell surface, which act as strong CO adsorption and activation centers. This is supported by the defect characterizations (Fig. R18), which show an increased contribution of oxygen-vacancy-related features for Bi@Fe O , compared with Bi-Fe O and pristine Fe O .\nFig. R19. (Supplementary Fig. 6, in the revised Supporting Information). N adsorption–desorption isotherm of (a) 5-Bi@Fe O , (b) Bi-Fe O , (c) Fe O , and (d) Bi NCs catalysts. (e) Schematic illustration of the Fe O , flat Bi-Fe O structure, Bi@Fe O core@shell nanosphere.\nMoreover, the spherical and porous Fe O shell for Bi@Fe O core@shell structure provides a higher density of under-coordinated surface sites and confined nanoscale environments(Fig. R19a-d), which strengthen local CO adsorption through enhanced collision and retention near the active surface. In contrast, for the flat Bi-Fe O structure, only a limited portion of Fe O surface is affected by Bi NCs due to point-contact interfaces (Fig. R19e).\nFig. R20. (Fig. 6c, in the revised manuscript; Supplementary Fig. 48, in the revised Supporting Information). (a) Synchronous illumination XPS analysis of the Fe 2p signals in 5-Bi@Fe O under full-spectrum light irradiation. (b) DFT-calculated interfacial charge redistribution (charge-density difference) at the Bi@Fe O core@shell interface. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C), in which yellow and cyan represent electron accumulation and depletion, respectively.\nAlthough Bi is partially encapsulated, plasmon-induced hot electrons and localized heating can modulate the electronic structure of the Fe O shell and facilitate defect generation under illumination, thereby increasing the population of Fe-V sites O on the shell surface for CO adsorption. This is consistent with the illumination-induced Fe3+ → Fe2+ evolution observed by in situ XPS (Fig. R20a). And charge redistribution on the Bi/Fe O interface also increases the electron density of surface active sites, thereby strengthening the interaction with CO and stabilizing adsorbed intermediates (Fig. R20b). These defects act as key active sites for CO activation, which are much denser in the strained shell than in relaxed pristine Fe O .\nFig. R21. (Supplementary Fig. 46, in the revised Supporting Information; Fig. 6a, in the revised manuscript). (a) CO adsorption isotherms of different control samples. (b) CO -TPD profiles of the Fe O , Bi-Fe O , Bi@Fe O catalysts.\nAs a result, despite the similar composition, Bi@Fe O exhibits a higher effective CO adsorption capacity than both pristine Fe O and Bi-Fe O (Fig. R21). We have revised the manuscript to clarify that the enhanced CO adsorption originates from not only the direct adsorption capability of Bi and Fe O themselves, but also the synergistic interfacial electronic effects and the uniform spherical shell geometry.\nChanges in Manuscript:\nWe have added the following discussion to the “Synthesis and microstructural analyses” section:\n“Finally, it is worth noting that this sphere core@shell structure exhibited a larger specific surface area (175.5 m2/g) with a uniform mesoporous structure (5.8 nm) (Supplementary Fig. 6, 7, and Supplementary Table 1), which has the advantage of adsorbing more reactant molecules, exposing a greater number of active sites, and providing convenient channels for mass transport35.”\nWe have added the following discussion to the “Proposed reaction mechanism for enhanced photoactivity” section:\n“Firstly, to validate this enhanced CO adsorption capacity, CO adsorption isotherms and temperature-programmed desorption (TPD) experiments were performed. CO adsorption isotherms demonstrated that the adsorption capacity of 5-Bi@Fe O is stronger than that of Bi NCs, Fe O , and Bi-Fe O (Supplementary Fig. 46a), which is likely due to the defect-enriched shell chemistry and the porous curved shell microenvironment. This result indicates that 5-Bi@Fe O may provide more active sites for adsorbing and converting CO molecules during the PCO R process, thus facilitating the occurrence of the reaction. As the shell composition was optimized, the overall CO adsorption capacity increased significantly, consistent with the independent CO adsorption results (Supplementary Fig. 46b). Notably, the similarity in CO adsorption isotherms among the core@shell samples with different core loadings suggests that CO activation on the interfacial sites is sufficiently efficient and does not constitute the rate-determining step in the catalytic reaction. The CO adsorption properties of the various catalysts were further evaluated by CO -TPD, as shown in Fig. 6a. The desorption profiles can be broadly divided into three temperature regions, namely 50 °C–300 °C, 300 °C–450 °C, and above 450 °C, which are generally associated with weak, moderate, and strong CO adsorption sites, respectively.\nCompared with the Fe O and Bi-Fe O catalysts, the 5-Bi@Fe O catalyst showed markedly enhanced CO adsorption after the construction of the spherical core@shell structure and active component incorporation. In particular, the appearance of obvious medium- and high-temperature desorption features indicates the generation of stronger CO binding sites (a broad peak at 300 °C–450 °C and a sharp peak at >450 °C). Strong CO adsorption led to a high CO conversion rate (Fig. 4a). These sites are mainly attributed to the abundant interfacial regions formed at the core@shell junction, which effectively promote CO adsorption and activation. These results powerfully underscore the critical importance of a CO -activating support (Fe-V active centers) O for promoting efficient PCO R.”\nChanges in Supporting Information:\nSupplementary Fig. 6 has been added, along with the discussion:\nSupplementary Fig. 6. Physical adsorption capacity. N adsorption–desorption isotherm of (a) 5-Bi@Fe O , (b) Bi-Fe O , (c) Fe O , and (d) Bi NCs catalysts.\nSupplementary Discussion: The results of N adsorption–desorption isotherm showed that the construction of the 5-Bi@Fe O core@shell structure has larger surface area (175.4603 m2/g), compared to the Bi-Fe O catalyst (38.6021 m2/g), Fe O (26.8375 m2/g), and Bi NCs (14.2943 m2/g). The unique core@shell structure can exposing abundant active sites and adsorb more CO molecules, thereby facilitating the further progression of multi-electron proton coupling.\nComment 6: The stability and potential evolution of the catalysts after reaction require further investigation. It is essential to characterize the post-reaction samples in terms of their morphology, composition, and chemical states. Analyses such as XRD, XPS, and TEM should be performed to confirm whether the core-shell structure remains intact, and to identify any potential phase transitions or changes in the oxidation states of Fe and Bi.\nResponse: We sincerely thank the reviewer for pointing out this critical issue. It is essential for evaluating the potential in practical applications to ensure the stability of the catalysts after the CO reduction reaction, especially under the photothermal conditions induced by the Bi core. To systematically address this concern, a series of comprehensive characterizations was conducted.\nFig. R22. (Supplementary Fig. 33 and 34, in the revised Supporting Information). The thermogravimetric analysis (a) and differential scanning calorimetry analysis curves (b) of the 5-Bi@Fe O and Bi-Fe O catalysts. Morphological characterizations before (c) and after (d) 5-Bi@Fe O catalyst undergoes the PCO R. (e) XRD pattern before and after 5-Bi@Fe O undergoes the PCO R.\nThe thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results demonstrate that 5-Bi@Fe O has excellent high-temperature tolerance (Fig. R22a, b). Transmission electron microscopy (TEM) images clearly show that the core@shell structure of the 5-Bi@Fe O catalyst remains intact after the photocatalytic CO reduction reaction, with no observable shell cracking (Fig. R22c). Furthermore, high-resolution TEM (HRTEM) images reveal that the lattice fringes of the Bi nanocore remain clearly discernible (Fig. R22d). According to the XRD pattern of the catalyst after the reaction, the characteristic diffraction peaks corresponding to Bi (core) and Fe O (shell) remain sharp and unchanged in position (Fig. R22e). Moreover, no peaks corresponding to other species such as Bi O or FeO and Fe O were observed, which indicates that no phase change occurred during the reaction.\nFig. R23. (Supplementary Fig. 35 and 36, in the revised Supporting Information). XPS spectra for (a) Fe 2p and (b) Bi 4f before and after 5-Bi@Fe O catalyst undergoes the PCO R. High-resolution XPS analysis of Fe 2p indicates that Fe3+ is still the predominant oxidation state and no noticeable shift in binding energies or emergence of new oxidation-state signals (Fig. R23a). This confirms that Fe O remains stable against photocorrosion during the reaction. More importantly, high-resolution XPS spectra of Bi 4f demonstrate almost no shift in binding energies, and Bi is still mainly present in the zero-valent state (Fig. R23b). This result verifies that the Fe O shell effectively prevents the Bi core from being oxidized in the PCO R process.\nThese characterization results clearly demonstrate that the Bi@Fe O core@shell structure exhibits excellent stability under the reaction conditions, validating its strong potential for long-term CO photoreduction applications.\nChanges in Manuscript:\nWe have added the following discussion to the “PCO R driven by coupling photo-/photothermal effect” section:\n“Firstly, the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results demonstrate that 5-Bi@Fe O has excellent high-temperature tolerance (Supplementary Fig. 33). More importantly, the 5-Bi@Fe O catalyst nearly retained its original morphology rather than collapsing after PCO R reaction (Supplementary Fig. 34a, b). The XRD patterns of 5-Bi@Fe O after PCO R reaction exhibited no discernible changes (Supplementary Fig. 34c), confirming the strong thermal stability of the catalyst structure. Furthermore, according to the XPS and XAS spectra of the 5-Bi@Fe O catalyst after PCO R reaction (Supplementary Figs. 35, 36; Supplementary Tables 5, 6), the valence state and coordination environment of Bi and Fe in the core@shell catalyst did not clearly change, indicating that the Bi core does not undergo oxidation during the reaction, and the Fe-V active sites keep stable in the reaction process (Supplementary Fig. 37).”\nChanges in Supporting Information:\nSupplementary Fig. 33 has been added, along with the discussion:\nSupplementary Fig. 33 The thermogravimetric analysis (a) and differential scanning calorimetry analysis curves (b) of the 5-Bi@Fe O and Bi-Fe O catalysts.\n“Supplementary Discussion: The Bi-Fe O sample exhibits a significant mass loss of 6.82% at a temperature of 220 °C. In contrast, the 5-Bi@Fe O sample shows a mass loss of only 1.43%, indicating that the dense Fe O shell effectively protects the inner Bi cores. Correspondingly, the DSC curves reveal that Bi-Fe O undergoes a continuous endothermic process associated with water evaporation and exhibits a distinct endothermic event (decalescence) around 220 °C. Conversely, the 5-Bi@Fe O presents a smooth and flat baseline without significant thermal peaks. These results collectively demonstrate the superior thermal stability of the core-shell 5-Bi@Fe O architecture compared to the uncoated counterpart.”\nSupplementary Figs. 34, 35, 36 have been added.\nSupplementary Fig. 34. Morphological characterizations before (a) and after (b) 5-Bi@Fe O undergoes the PCO R. (c) XRD pattern before and after 5-Bi@Fe O undergoes the PCO R.\nSupplementary Fig. 35 (a) XPS spectra for Bi 4f before and after the 5-Bi@Fe O catalyst undergoes the PCO R.\nSupplementary Fig. 36 (a) XPS spectra for Fe 2p before and after the 5-Bi@Fe O catalyst undergoes the PCO R.\nComment 7: The stability of the Fe O shell and the potential formation of bismuth oxide require thorough investigation. It is crucial to determine whether the surface defects of Fe O , proposed as active sites, are repaired or altered during the reaction. Concurrently, the chemical state of the Bi core must be verified to rule out its oxidation, which would fundamentally change the proposed LSPR and thermal mechanism.\nResponse: We thank the reviewer for raising this critical comment. We fully agree that it is crucial for the reliability of the proposed mechanism in this work to verify (i) the chemical state stability of the Bi core (to retain LSPR-enabled nanoheating and hot-carrier generation) and (ii) the persistence of the Fe O -shell surface defects (proposed activation sites) during the reaction. To address this concern, we have employed synchrotron radiation X-ray absorption fine structure (XAFS) and electron paramagnetic resonance (EPR) spectroscopies to probe the atomic-level chemical states of the 5-Bi@Fe O catalyst.\nFig. R24. (Supplementary Fig. 35, in the revised Supporting Information). (a) Normalized X-ray absorption near-edge spectra at the Bi L -edge. (b) k -weighted Fourier transform extended X-ray absorption fine structure spectra (EXAFS) in r-space. Wavelet Transformation for the k -weighted EXAFS signal of (c) the 5-Bi@Fe O catalyst, (d) Bi foil, and (e) Bi O after reaction.\nBased on the Response to Comment 6, the post-reaction XRD patterns show no detectable formation of bismuth oxide phases, and microscopic analyses confirm that the core@shell morphology remains intact after reaction. More importantly, we further conducted a detailed analysis using bulk-sensitive Bi L -edge XAS. The absorption edge position and white-line features of the sample after reaction were close to the Bi foil, while being clearly distinct from the Bi O (Fig. R24a). This result clearly confirms that the Bi core still retains its metallic valence state (Bi0) after the reaction, which is consistent with the XPS analysis results. Moreover, the Fourier-transformed R-space EXAFS spectrum (Fig. R24b) and Wavelet Transformation for the k -weighted EXAFS signal (Fig. R24c−e) further exclude the possibility of Bi oxidation. These results demonstrate that the Fe O shell acts as a highly effective protective barrier, preventing oxidation of the Bi core in the core@shell architecture, thereby ensuring the continuous operation of the LSPR-driven photothermal mechanism throughout the reaction.\nFig. R25. (Supplementary Fig. 36, in the revised Supporting Information). (a) Normalized X-ray absorption near-edge spectra at the Fe K-edge. (b) k -weighted Fourier transform extended X-ray absorption fine structure spectra (EXAFS) in r-space. Wavelet Transformation for the k -weighted EXAFS signal of (c) the 5-Bi@Fe O catalyst, (d) Fe O , and (e) Fe foil after reaction. (f) EPR spectra before and after 5-Bi@Fe O undergoes the PCO R. (g) O 1s for XPS spectra before and after 5-Bi@Fe O undergoes the PCO R.\nTo explore whether the surface defects of Fe O as active site components are repaired or altered during the reaction, we compared defect signals before and after reaction and also monitored them under illumination. The Fe K-edge XAS (Fig. R25a−e) exhibited that the Fe−O coordination number of the catalyst after reaction remains significantly lower than that of bulk Fe O , indicating that the local coordination environment of Fe atoms remains unsaturated. That is to say that oxygen vacancies are structurally preserved. The XPS O 1s and EPR measurements show that the characteristic defect-related signals are retained without obvious loss (Fig. R25f, g). These results indicate that oxygen-vacancies-associated active sites could maintain a steady-state population of active sites for CO adsorption, activation and conversion.\nTaken together, these results demonstrate that (i) the Bi core largely maintains its metallic state and (ii) the Fe O shell and its defect-related active sites are preserved during the reaction. This result powerfully supports the reliability of the mechanism in this work, in which interfacial charge transfer (photochemical effects) and plasmon-induced nanoheating (photothermal effects) synergistically drive the highly selective production of CH from CO and H O.\nChanges in Manuscript:\nWe have added the following discussion to the “PCO R driven by coupling photo-/photothermal effect” section:\n“More importantly, the 5-Bi@Fe O catalyst nearly retained its original morphology rather than collapsing after PCO R reaction (Supplementary Fig. 34a, b). The XRD patterns of 5-Bi@Fe O after PCO R reaction exhibited no discernible changes (Supplementary Fig. 34c), confirming the strong thermal stability of the catalyst structure. Furthermore, according to the XPS and XAS spectra of the 5-Bi@Fe O catalyst after PCO R reaction (Supplementary Figs. 35, 36; Supplementary Tables 5, 6), the valence state and coordination environment of Bi and Fe in the core@shell catalyst did not clearly change, indicating that the Bi core does not undergo oxidation during the reaction, and the Fe-V active sites keep stable in the reaction process O (Supplementary Figs. 37).”\nChanges in Supporting Information:\nSupplementary Figs. 35, 36, and 37 have been added.\nSupplementary Fig. 35. Stability of the metallic Bi core before and after photocatalytic CO reduction. (a) XPS spectra for Bi 4f before and after the 5-Bi@Fe O catalyst undergoes the PCO R. (b) Normalized X-ray absorption near-edge spectra at the Bi L -edge collected before and after the 5-Bi@Fe O catalyst undergoes the PCO R, compared with Bi foil and Bi O references. (c) k -weighted Fourier transform extended X-ray absorption fine structure spectra (EXAFS) in r-space before and after the 5-Bi@Fe O catalyst undergoes the PCO R. Wavelet Transformation for the k -weighted EXAFS signal of (d) the 5-Bi@Fe O catalyst, (e) Bi foil, and (f) Bi O after reaction.\nSupplementary Fig. 36. Stability of the Fe O shell before and after photocatalytic CO reduction. (a) XPS spectra for Fe 2p before and after the 5-Bi@Fe O catalyst undergoes the PCO R. (b) Normalized X-ray absorption near-edge spectra at the Fe K-edge collected before and after the 5-Bi@Fe O catalyst undergoes the PCO R, compared with Bi foil and Bi O references. (c) k -weighted Fourier transform extended X-ray absorption fine structure spectra (EXAFS) in r-space before and after the 5-Bi@Fe O catalyst undergoes the PCO R. Wavelet Transformation for the k -weighted EXAFS signal of (d) the 5-Bi@Fe O catalyst, (e) Fe O , and (f) Fe foil after reaction.\nSupplementary Fig. 37. Stability of oxygen-vacancy–related species in 5-Bi@Fe O catalyst before and after photocatalytic CO reduction. (a) EPR spectra of 5-Bi@Fe O catalyst before and after reaction, showing the characteristic signal at g = 2.003 attributed to oxygen vacancies. (b) High-resolution O 1s XPS spectra and peak deconvolution of the catalyst before and after reaction, with components assigned to lattice oxygen (O ), oxygen-vacancy–related oxygen (V ), and adsorbed oxygen (O ), evidencing the preservation of the defect/active-site environment during catalysis.\nReviewer #2 (Remarks to the Author):\nGeneral comments: The study presents an innovative strategy for highly efficient, full-spectrum photocatalytic CO reduction PCO R to CH using a noble-metal-free Bi@Fe O coreshell nanostructure. The core concept of a \"nanoscale greenhouse effect\" is interesting to the community, enabling synergistic photochemical and photothermal catalysis by confining locally generated heat. The performance metrics, particularly the CH production rate and NIR-light AQE, are superior to current literature, making this work a significant contribution to sustainable energy and catalysis. The overall manuscript has been organized with good logic flow and well-written. However, before considering publication in the international edition of Nature Communications, we recommend the following minor revisions to enhance the clarity and rigor of the work.\nResponse: We deeply appreciate the reviewer for the positive evaluation of our manuscript and helpful suggestions. In response, we have carefully revised the manuscript and Supporting Information by adding several experiments and more detailed discussions, and correcting the minor issues accordingly. Additionally, we have made further efforts to enhance the clarity and rigor of the manuscript, such as merging “Fig. 2” and “Fig. 3” (both exhibiting the core@shell structure information about the 5-Bi@Fe O catalyst) in the previous manuscript into a combined figure (now “Fig. 2” in the revised manuscript), and updating the corresponding language part in the manuscript. We address each of the reviewer’s comments point-by-point below, with authors’ response in blue; changes to the manuscript in red.\nComment 1: Please ensure consistent terminology is used throughout the entire manuscript (including the main text and Supporting Information) when referring to the Bismuth components. For instance, standardize the usage to one term (e.g., sometimes Bi nanocore, sometimes Bi NCs, sometimes Bi NPs) instead of interchanging them.\nResponse: Thank the reviewer very much for this helpful suggestion. We have carefully revised the entire manuscript, including the main text, figures, captions, and Supporting Information, to ensure consistent and unified term for the bismuth components. Specifically, all alternative expressions such as “Bi nanocore”, “Bi NPs”, and other interchangeable descriptions have been systematically replaced by “Bi NC (a sphere Bi nanocrystal core in the 5-Bi@Fe O catalyst)” or “Bi NCs (Bi nanocrystals, representing a catalyst as a comparison)” to avoid ambiguity.\nAdditionally, we carefully reviewed the entire manuscript (including the main text and Supporting Information) to ensure that all terms are consistent and appropriate. This will help improve the overall clarity of the manuscript and ensure it adheres to standard academic writing conventions.\nComment 2: The prominent defect visible in the upper-left corner of the Bi core in Figure 2E (main text) and Figure 2b (Supporting Information) is clearly noticeable. Please provide an explanation for the cause of this defect and discuss its potential impact, if any, on the catalyst's performance.\nResponse: We thank the reviewer for careful observation regarding the morphological details. The feature observed in the upper-left region of the Bi core in Fig. 2e and Supplementary Fig. 2b (Supplementary Fig. 3b, in the revised Supporting Information) may originate from the synthesis and core@shell formation process. Specifically, on the one hand, this feature observed in the morphology may be attributed to the anisotropic growth kinetics or volume contraction and interfacial strain induced during the encapsulation of the Bi core by the Fe O shell. On the other hand, it may result from minor localized etching at the Bi surface during the initial nucleation stage of the Fe O shell, which involves subsequent thermal treatment. Similar localized feature has been widely reported in metal-oxide core@shell systems formed under comparable conditions (Nat. Commun., 2025, 16, 7559; Nat. Commun., 2024, 15, 413; Adv. Mater., 2024, 36, 2406088; ACS Nano, 2022, 16, 5839−5850). Importantly, this defect does not compromise the structural integrity or continuity of the Bi core, nor does it disrupt the overall core@shell architecture. TEM analysis reveals that while such defects may appear locally, the majority of Bi cores maintain a well-defined morphology without extensive void formation or fragmentation.\nBased on the results of PCO R, we emphasize that the feature observed in the morphology is not considered as the dominant active site responsible for the observed CO reduction activity. The catalytic performance in this work is ascribed to the synergistic interplay between photochemical and photothermal effects, which is evidenced by the consistent activity trends across the catalysts with varying local microstructural features. Moreover, the reproducible catalytic performance and narrow error bars obtained from multiple independent batches indicate that the activity is not sensitive to the presence or absence of such a feature.\nComment 3: Supplementary Figure 14 shows that Bi-NC exhibits better light absorption properties, which are not presented in Figure 4a of the main text. Could you include this in Figure 4a and further analyze the mechanism behind the enhanced near-infrared absorption performance of 5-Bi@Fe O ?\nResponse: We thank the reviewer for this valuable suggestion. Accordingly, we have included the UV-vis-NIR absorption spectrum of Bi NCs in Fig. R25, enabling a direct comparison with Fe O and 5-Bi@Fe O . The comparison clearly shows that while pristine Fe O has negligible absorption beyond 600 nm, both pure Bi NCs and 5-Bi@Fe O exhibit strong broadband absorption extending into the NIR region. The corresponding discussion has also been expanded in the revised manuscript.\nFig. R26 (Fig. 3a, in the revised manuscript) UV-vis-NIR DRS spectra of Bi NCs, Fe O , Bi-Fe O , and 5-Bi@Fe O .\nThe pronounced NIR absorption of the 5-Bi@Fe O core@shell structure is not only a superposition of the two components but involves a synergistic plasmonic effect. Specially, the broad NIR absorption originates from the Localized Surface Plasmon Resonance (LSPR) of the metallic Bi core. Unlike noble metals (Au/Ag) which resonate in the visible, the lower carrier density of Bi naturally positions its LSPR in the vis-NIR range. Crucially, coating the Bi core with an Fe O shell significantly modifies the plasmonic resonance. According to the Mie theory, the LSPR frequency is highly sensitive to the refractive index (n) of the surrounding medium. Fe O has a high refractive index (n ≈ 3.0 ). Encapsulating the Bi core in this high-dielectric shell induces a strong red-shift and broadening of the LSPR band compared to Bi in air/solvent, thereby enhancing the light harvesting capacity specifically in the NIR region (800–2000 nm).\nIn addition, the spherical core@shell geometry promotes multiple light-scattering and confinement effects, which increase the effective optical path length within the composite and further strengthen NIR light harvesting. Importantly, Fe O itself does not exhibit intrinsic near-infrared bandgap absorption. Instead, NIR photons are primarily absorbed by the plasmonic Bi core, generating hot carriers and localized photothermal effects, while the Fe O shell facilitates the interfacial transfer, confinement, and utilization of these plasmon-derived energetic carriers. Therefore, the 5-Bi@Fe O composite effectively integrates the NIR-harvesting capability of the Bi core, which is further optimized by the high-refractive-index Fe O shell, enabling efficient full-spectrum solar utilization.\nChanges in Manuscript:\nWe have added the following figure and discussion to the “Light and thermal effects over the unique core@shell structure” section:\nFig. 3 a UV-vis-NIR DRS spectra of Bi NCs, Fe O , Bi-Fe O , and 5-Bi@Fe O .\n“After confirming the unique structure, we further explore the fundamental optical properties and solar-to-heat energy conversion ability. Three representative catalysts are shown in Supplementary Fig. 14. As displayed in Fig. 3a, the Fe O catalyst exhibits a strong, broad absorption band below ~600 nm. Bi NCs display strong, broadband absorption covering the UV-vis-NIR region, which belongs to characteristic of bismuth plasmonic and interband transitions. Compared to pure Fe O , the Bi-Fe O catalyst and Bi@Fe O catalyst maintain this excellent broadband light-harvesting ability of Bi NCs (300–2000 nm). Moreover, increasing the Bi content effectively enhances the light absorption of x-Bi@Fe O , particularly in the near-infrared (NIR) region (Supplementary Fig. 15). This enhanced NIR absorption is crucial for the efficient cascaded utilization of the full solar spectrum.”\nComment 4: Figure 5 demonstrates the superior photocatalytic reduction performance of 5-Bi@Fe O . Could you include photocatalytic experiments under different atmospheres (N , Air, pure CO ) for further comparison?\nResponse: We thank the reviewer for this valuable suggestion. We agree that atmosphere-controlled tests are essential to validate the CO origin of the carbon-containing products and to exclude possible interference from adventitious carbon sources or other side reactions. Following the reviewer’s recommendation, we have performed additional photocatalytic experiments under different atmospheres, including N₂, air, and pure CO (Fig. R27), under otherwise identical conditions (light intensity, catalyst mass, gas flow, and reaction time).\nFig. R27 The photocatalytic performance for 5-Bi@Fe O under different atmospheres.\nUnder N atmosphere, no detectable carbon-containing reduction products (CO or CH ) were observed for 5-Bi@Fe O , indicating that the detected products under CO atmosphere do not originate from residual carbon species or catalyst decomposition. The presence of CO is a prerequisite for fuel production. When the reaction was conducted in air, the formation of reduction products was strongly suppressed, which can be attributed to the limited adsorption and reaction rate by the extremely low partial pressure of CO in air or competitive oxygen reduction reactions. In contrast, under pure CO atmosphere, both CO and CH were produced with significantly enhanced rates. These control experiments clearly demonstrate that the observed photocatalytic activity of 5-Bi@Fe O is CO -dependent and confirm that CO is the sole carbon source for the reduction products. High-purity CO provides sufficient reactant coverage on the active sites, allowing the Bi@Fe O system to efficiently drive the CO -to-CH conversion.\nFig. R28 (Supplementary Fig. 31, in the revised Supporting Information) (a) Isotope labeling mass spectra of CH and CO after PCO R were obtained from the PCO R over 5-Bi@Fe O in 12CO and 13CO atmosphere. (b) Isotope labeling mass spectra of CH and CO after PCO R were obtained from the PCO R over 5-Bi@Fe O using H O, D O as the feedstock. The reactions are conducted in CO saturated water and 8 sccm CO atmosphere under 455 mW cm–2 full-spectrum illumination. The spectra are collected by a GC-MS.\nIn addition, to further identify the carbon source of the reduction products, we carried out isotopic labeling experiments using 13CO and D O (Fig. R28). The corresponding mass spectrometry analysis clearly confirms that the carbon in the generated CO and CH originates exclusively from CO and H O rather than from any extra carbon-containing impurities. These results provide direct evidence for the CO -dependent nature of the photocatalytic reduction process.\nChanges in Supporting Information:\nSupplementary Fig. 31 has been added.\nSupplementary Fig. 31 (a) Isotope labeling mass spectra of CH and CO after PCO2R were obtained from the PCO R over 5-Bi@Fe O in 12CO and 13CO atmosphere. (b) Isotope labeling mass spectra of CH and CO after PCO R were obtained from the PCO R over 5-Bi@Fe O using H O, D O as the feedstock. The reactions are conducted in CO saturated water and 8 sccm CO atmosphere under 455 mW cm–2 full-spectrum illumination. The spectra are collected by a GC-MS.\nComment 5: The in situ XPS clearly shows the formation of a higher concentration of Fe2+ under illumination, suggesting hot-electron trapping and the formation of oxygen vacancies are critical for CO activation. The final proposed mechanism (Fig. 6h/6i) primarily focuses on electron transfer. The authors must explicitly integrate the transient Fe2+ active site into the reaction mechanism, particularly for the activation of CO to the first intermediate (*COOH). Is the Fe2+ state the actual reaction site? Does the high temperature stabilize or destabilize this state?\nResponse: We thank the reviewer for this constructive comment. We agree that the Fe2+ species plays a crucial role in CO activation and should be clearly integrated into the reaction mechanism. We would like to clarify that the results in Fig. R29a and b are intended to illustrate the carrier dynamics for the Bi@Fe O catalyst in this work, including plasmon-induced hot-electron generation in Bi, interfacial electron transfer, and carrier trapping in the Fe O shell. In contrast, the overall reaction mechanism of PCO R in this work was explicitly presented in Fig. R29c (Fig 6g in the revised manuscript), which includes carrier transfer, energy transfer, and the synergy of photochemical effect and photothermal effect.\nFig. R29 (Fig. 5h, i and Fig. 6g in the revised manuscript) Schematic illustration of the charge transfer processes of the 5-Bi@Fe O catalyst in PCO R under (a) UV-vis and (b) UV-vis-NIR excitation conditions. (c) Mechanism of PCO R driven by the synergy of photochemical and photothermal effects based on the cascaded utilization of full-solar spectrum.\nFig. R30 (Fig. 6c, in the revised manuscript) In situ XPS analysis of the Fe 2p signals in 5-Bi@Fe O .\nFig. R31 (Supplementary Fig. 56, in the revised Supporting Information) Schematic illustration of the reaction pathway in PCO₂R for 5-Bi@Fe O catalyst.\nBased on the in situ XPS results (Fig. R29), combining other experimental and theoretical analyses in this work, we propose that the transient Fe2+ species generated under illumination (Fe3+ + e– → Fe2+) and oxygen vacancies adjacent to these species serve as the actual catalytically active centers (Fe-V sites) responsible for the initial CO activation and the subsequent hydrogenation steps. These Fe-V sites possess enhanced local electron density, which facilitate CO adsorption, molecular bending, and charge transfer into the antibonding orbitals of CO , thereby lowering the energy barrier for the formation of the first key intermediate (*COOH). The regenerated Fe2+ species continues to donate electrons to the surface-bound intermediates, driving the subsequent proton-coupled electron transfer steps (*COOH → *CO → *CHO → *CH O → *CH O → *CH ) (Fig. R31). Under continuous illumination, plasmon-induced hot-electron injection and localized photothermal heating sustain a dynamic Fe3+/Fe2+ redox cycle, ensuring persistent availability of Fe-V sites during multi-electron proton-coupled transfer steps toward CH formation.\nFig. R32. (Supplementary Fig. 36, in the revised Supporting Information) (a) Normalized X-ray absorption near-edge spectra at the Bi L -edge. (b) k -weighted Fourier transform extended X-ray absorption fine structure spectra (EXAFS) in r-space. Wavelet Transformation for the k -weighted EXAFS signal of (c) the 5-Bi@Fe O catalyst, (d) Bi O , and (e) Bi foil after reaction. (f) EPR spectra before and after 5-Bi@Fe O undergoes the PCO R. (g) O 1s for XPS spectra before and after 5-Bi@Fe O undergoes the PCO R.\nImportantly, the Fe2+ species observed in this work is a dynamic surface catalytic state that is continuously generated and consumed during the reaction. Under operating conditions, the localized photothermal heating (∼220 °C) induced by plasmonic Bi does not lead to irreversible reduction or structural degradation of Fe O (Fig. R32). Instead, the elevated temperature promotes interfacial charge transfer, V -mediated electron trapping, and rapid Fe3+/Fe2+ redox cycling. As a result, the Fe-V sites are kinetically sustained under reaction conditions, enabling their persistent participation in CO activation rather than being thermodynamically stabilized or destroyed.\nChanges in Manuscript:\nWe have added the following discussion to the “Charge carrier dynamics” section:\nFig. 5 Charge carrier dynamics. Schematic illustration of the charge transfer processes of the 5-Bi@Fe O catalyst in PCO R under (h) UV-vis and (i) UV-vis-NIR excitation conditions.\n“Based on the above complementary spectroscopic analysis results, a simplified PCO R model driven by the cascaded utilization of full-solar spectrum-driven over the 5-Bi@Fe O core@shell catalyst is depicted in Fig. 5h and 5i. Due to the favorable work function alignment (Supplementary Figs. 43, 44), photoexcited electrons in the Bi core tend to transfer toward the Bi/Fe O junction and accumulate in an interfacial electron-rich region. PCO R preferentially occurs on the interfacial electron-accumulation layer associated with Fe-V active sites. Specifically, under UV-vis illumination (Fig. 5h), the electrons generated from Bi interband excitation are transferred to the interfacial accumulation layer (rate constant k ), which is corroborated by theoretical computation electron redistribution and electrostatic potential (Supplementary Fig. 45). These energetic hot electrons drive CO reduction toward CO/CH (rate constant k ), which competes with the electron-hole recombination (rate constant k ) of Bi NC and in the Fe O region (rate constant k ). The corresponding holes from the Fe O are mainly through water oxidation (rate constant k ) or hole-trapping processes (maintaining charge balance, rate constant k ). Under UV-vis-NIR illumination (Fig. 5i), the Bi LSPR effect triggers more energetic hot electrons (kˊ ). These hot electrons are efficiently injected and strongly trapped with reactant molecules in the interfacial accumulation layer, creating a significantly higher electron density than under UV-vis illumination. And the large localized electric field in 5-Bi@Fe O promotes charge separation and transfer, thereby enhancing the reduction potential at the reaction interface. Moreover, the prolonged lifetime of photogenerated electrons under NIR excitation enables efficient utilization of the full solar spectrum, while concurrently creating a “self-heating greenhouse bed” effect at the nanoscale (Fig. 3e). This synergistic photothermal enhancement is particularly critical for overcoming the high kinetic barriers associated with multielectron CO activation and multielectron-proton CH production. Therefore, the CH yield under full-spectrum irradiation is significantly higher than that obtained under UV-vis or near-infrared light alone, which is consistent with the PCO R performance (Fig. 4g).”\nComment 6: What specific mechanism of the 5-Bi@Fe O photocatalyst led to its selection in this study? Additionally, discuss the key differences between your core-shell system and previously reported photocatalytic dual-metal cases (e.g., AgCu bimetal photocatalyst in Ag Cu O , or other bimetal photocatalyst), emphasizing the novelty of your \"nanoscale greenhouse effect\" strategy.\nResponse: We thank the reviewer for giving us the opportunity to clarify our rational design and highlight the novelty of our strategy. The selection of this core@shell architecture was not empirical but was guided by a mechanism-oriented design strategy aimed at overcoming the intrinsic thermodynamic and kinetic limitations of CO reduction on Fe O .\nSpecifically, Fe O possesses abundant surface defects and strong CO affinity but suffers from an unfavorable conduction band position and limited light utilization. However, metallic Bi exhibits a broad LSPR response covering the vis-NIR region, enabling full-spectrum solar harvesting but lacks intrinsic catalytic activity toward multi-electron CO reduction. By constructing a Bi core fully encapsulated by a Fe O shell, the light harvesting, heat generation, and chemical activation were cooperatively coupled. In this system, the Bi core serves as a plasmonic nanoheater and hot-electron source, while the Fe O shell provides defect-rich catalytic sites and a confined reaction microenvironment (reaction bed).\nFig. R33. (Fig. 1, in the revised manuscript) Contrast of this work with previous works. Diagram illustrating the integration of intermittent sunlight and residual heat for on-demand CO conversion with water.\nThis design fundamentally differs from previously reported dual-metal or bimetallic photocatalysts (e.g., Ag-Cu systems), where performance enhancement is typically attributed to alloying effects, Schottky junctions, or static charge redistribution between adjacent metals. Additionally, due to high thermal conductivity of the alloy or bimetallic catalysts, the LSPR-generated heat is rapidly dissipated to the surroundings or the support, making it difficult to establish a high local temperature gradient (i.e., “heat is wasted”). In this work, the Bi@Fe O system exhibits a dynamic interfacial mechanism: plasmon-induced hot electrons and localized photothermal heating continuously generate and regenerate Fe-V active states on the interface between the Bi core and Fe O shell (Fig. R33). Importantly, the low thermal conductivity and closed geometry of the shell suppress heat dissipation, leading to a sustained local temperature elevation at the nanoscale.\nBeing similar to a greenhouse traps solar heat to maintain a warm earth, the Fe O shell traps the LSPR heat from the Bi core in this work. This synergistic light-heat-matter coupling was regarded as a “nanoscale greenhouse effect” which is fundamentally distinct from conventional photothermal enhancement or bimetallic charge transfer. Rather than only increasing carrier density or extending light absorption, this strategy creates a confined super-photothermal microenvironment that simultaneously accelerates reaction kinetics and stabilizes transient active states essential for deep CO reduction to CH . This provides a new paradigm for designing photothermal catalysts for photocatalytic CO reduction systems: moving from simple “light-to-heat conversion” to advanced “heat management and confinement”.\nComment 7: Supplementary Figures 6 and 7 display the physical adsorption capacity. Could you add the pore size distribution for Bi, Fe O , and Bi-Fe O , and further explain the influence of the core-shell structure of 5-Bi@Fe O ?\nResponse: We appreciate the reviewer for this useful suggestion. As suggested, we have added the N adsorption–desorption isotherm and the pore size distribution curves for Bi NCs, Fe O , and Bi-Fe O catalysts (Supplementary Fig. 6 and Supplementary Fig. 7). As shown in the BET results, the Bi-Fe O catalyst exhibits a low specific surface area of 38.6021 m2/g, similar to pure Bi NCs and Fe O . In contrast, the construction of the 5-Bi@Fe O core@shell structure remarkably increases the surface area to 175.4603 m2/g. This confirms that the Fe O shell effectively isolates the Bi cores, preventing their aggregation and exposing significantly more surface sites.\nThe pore size distribution (Figure S7) reveals that the control samples (Bi NCs, Fe O , and Bi-Fe O catalysts) possess disordered piling pores with random sizes (ranging from ~4.5 to ~14.7 nm). However, 5-Bi@Fe O displays a focused and sharp peak centered at 5.8 nm. This indicates that the core@shell structure is not merely a physical coating but forms a uniform mesoporous framework, providing convenient channels for mass transport.\nThese results demonstrate that the Fe O shell not only encapsulates the Bi cores to prevent aggregation but also creates a robust, mesoporous framework. This uniform mesoporous structure (5.8 nm) with high surface area is highly beneficial for exposing abundant active sites and facilitating the mass transport of CO and products during the catalytic reaction.\nChanges in Manuscript:\nWe have added the following discussion to the “Synthesis and microstructural analyses” section:\n“It's worth noting that this sphere core@shell structure has a larger specific surface area (175.5 m2/g) with uniform mesoporous structure (5.8 nm) (Supplementary Fig. 6, 7 and Supplementary Table 1), which has the advantage of adsorbing more reactant molecules, exposing a greater number of active sites, and providing convenient channels for mass transport35.”\nChanges in Supporting Information:\nSupplementary Figs. 6 and 7, Supplementary Table 1 have been added, along with the discussion:\nSupplementary Fig. 6. Physical adsorption capacity. (a) N adsorption–desorption isotherm of 5-Bi@Fe O , Bi-Fe O , Fe O , and Bi NCs catalysts.\nSupplementary Discussion: The results of N adsorption–desorption isotherm showed that the construction of the 5-Bi@Fe O core@shell structure has larger surface area (175.4603 m2/g), compared to the Bi-Fe O catalyst (38.6021 m2/g), Fe O (26.8375 m2/g), and Bi NCs (14.2943 m2/g). The unique core@shell structure can exposing abundant active sites and adsorb more CO molecules, thereby facilitating the further progression of multi-electron proton coupling.\nSupplementary Fig. 7. Physical adsorption capacity. The pore size distribution of 5-Bi@Fe O , Bi-Fe O , Fe O , and Bi NCs catalysts.\nSupplementary Discussion: The pore size distribution exhibited that 5-Bi@Fe O a focused and sharp peak centered at 5.8 nm. However, the control samples (Bi NCs, Fe O , and Bi-Fe O catalysts) possess disordered piling pores with random sizes (ranging from ~4.5 to ~14.7 nm). This indicates that the core@shell structure is not merely a physical coating but forms a uniform mesoporous framework, providing convenient channels for mass transport.\nSupplementary Table 1. Textural parameters of as-synthesized samples.\nBET surface area Pore Volume Pore size\nSamples\n[m2/g]a [cm3/g]b [nm]c\n5-Bi@Fe O 175.4603 0.53004 5.8367\nBi-Fe O 38.6021 0.1186 9.4203\nFe O 26.8375 0.10143 14.7012\nBi NCs 14.2943 0.07296 4.534\na Surface area calculated by BET Surface Area.\nb Single point total pore volume calculated by the nitrogen amount adsorbed.\nc Average pore size calculated by BJH desorption average pore diameter (4V/A).\nComment 8: The main text mentions that the specific surface area of the 5-Bi@Fe O material has been enhanced. Please provide the BET data (table/figure) to quantitatively validate this claim.\nResponse: We thank the reviewer for this comment. To quantitatively support the discussion of surface area, we have now provided the N adsorption-desorption isotherms and the corresponding BET surface area values for Fe O , Bi-Fe O , 5-Bi@Fe O . The results clearly show that 5-Bi@Fe O exhibits a higher specific surface area (175.4603 m2/g) than the reference samples, thereby quantitatively validating the statement made in the main text. We would also like to emphasize that, while the increase in surface area contributes to improved reactant accessibility, the enhanced CO reduction performance of 5-Bi@Fe O cannot be solely attributed to surface area effects.\nChanges in Manuscript:\nWe have added the following discussion to the “Synthesis and microstructural analyses” section:\n“It's worth noting that this sphere core@shell structure has a larger specific surface area (175.5 m2/g) with uniform mesoporous structure (5.8 nm) (Supplementary Fig. 6, 7 and Supplementary Table 1), which has the advantage of adsorbing more reactant molecules, exposing a greater number of active sites, and providing convenient channels for mass transport35.”\nChanges in Supporting Information:\nSupplementary Table 1 has been added.\nSupplementary Table 1. Textural parameters of as-synthesized samples.\nBET surface area Pore Volume Pore size\nSamples\n[m2/g]a [cm3/g]b [nm]c\n5-Bi@Fe O 175.4603 0.53004 5.8367\na Surface area calculated by BET Surface Area.\nb Single point total pore volume calculated by the nitrogen amount adsorbed.\nc Average pore size calculated by BJH desorption average pore diameter (4V/A).\nReviewer #3 (Remarks to the Author):\nGeneral comment: This manuscript reports a Bi@Fe O core-shell photocatalyst described as exhibiting a “nanoscale greenhouse effect” for full-spectrum CO reduction with H O to CH . The study is experimentally comprehensive, integrating fs-TA, KPFM, NAP-XPS, DRIFTS, and DFT analyses to correlate photothermal and photochemical contributions. The catalytic performance is impressive for a noble-metal-free system, and the proposed photothermal synergy represents a practical approach for harvesting the full solar spectrum for fuel production. Nevertheless, despite the solid experimental effort, the conceptual novelty and mechanistic interpretation require significant clarification. The current evidence for the proposed “nanoscale greenhouse effect” is not fully convincing, and several data inconsistencies, figure errors, and methodological gaps need to be addressed.\nResponse: We sincerely appreciate the reviewer for the thorough evaluation of our manuscript and for acknowledging the comprehensive experimental and theoretical analyses. We also appreciate the reviewer’s constructive concerns regarding the mechanism of the proposed “nanoscale greenhouse effect,” as well as the noted figure inconsistencies. In response, we have revised the relevant sections to clarify the definition and physical origin of the nanoscale thermal confinement in the manuscript, combined with the additional experiments, numerical simulations, and DFT calculations. We have also carefully rechecked all data and figures to correct these inconsistencies. We address each specific comment point-by-point, with authors’ response in blue; changes to the manuscript in red. Detailed responses are given below:\n1) New DFT model geometry to reflect the real composition of the Bi@Fe O core@shell structure.\n2) The corrected size of the core@shell structure and other typographical and graphical errors throughout the manuscript;\n3) The revised Fe 2p XPS peak-fitting diagrams;\n4) XPS and XAFS for 5-Bi@Fe O photocatalyst before and after reaction;\n5) New temperature measurements;\n6) The new photocurrent data of all photocatalysts and the corresponding explanation;\n7) Enhanced COMSOL Multiphysics simulations.\nComment 1: The manuscript attributes the higher temperature of Bi@Fe O to an “infrared shielding” effect of the Fe O shell. However, this description is too general and should specify the relevant infrared wavelength range. If the shell is proposed to “trap infrared heat,” the authors should clarify in which IR band this occurs and provide corresponding optical or thermal data (e.g., absorption or emissivity spectra) for Fe O . Without this information, the observed temperature increase is more plausibly due to enhanced broadband absorption or the low thermal conductivity of the shell rather than true infrared reflection or confinement. To further distinguish thermal confinement from electronic effects, the authors are encouraged to prepare a Bi@SiO reference sample. SiO is optically transparent and electronically insulating but provides similar thermal insulation owing to its low thermal conductivity. Comparing the heating behavior of Bi@Fe O and Bi@SiO under identical illumination would help separate the pure photothermal (“greenhouse-like”) contribution from the electronic and catalytic roles of Fe O , thereby clarifying whether the observed temperature rise genuinely reflects an IR-shielding mechanism.\nResponse: We thank the reviewer for this insightful comment. Following the reviewer’s recommendation, we have synthesized a Bi@SiO core@shell structure as a reference sample (Fig. R34a−d) and compared the heating behavior of Bi@Fe O and Bi@SiO under identical illumination. This additional comparison has helped us to explicitly decouple pure thermal confinement from electronic and catalytic effects, and refine the mechanistic description of the “nanoscale greenhouse effect”. We have revised the manuscript to clarify that the temperature enhancement in Bi@Fe O originates from nanoscale thermal confinement rather than selective infrared reflection.\nFig. R34 (Supplementary Figs. 21 and 22, in the revised Supporting Information) (a) TEM images and (b-d) the corresponding element maps of Bi@SiO . (e) UV-vis-NIR DRS spectra of Bi@SiO . (f) The infrared thermal images of Bi@SiO under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. Images were acquired sequentially at 0, 300, 600, and 1500 s. (g) Spatial distribution of temperature rise that was excited at an incident power density of 2.5 mW μm-2 of Bi@SiO . Scale bars, 50 nm. The excitation wavelengths were 420, 560, 870, and 1200 nm, respectively. (h) Thermal power generation excited at an incident power density of 2.5 mW μm-2 of Bi@SiO . Scale bars, 50 nm. The excitation wavelengths were 420, 560, 870, and 1200 nm, respectively. (i) Monitoring of temperatures over different Bi-based catalysts under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (j) Simulation spectra of Bi@SiO and Bi@Fe O .\nFirstly, SiO is optically transparent across the UV-vis-NIR region (Fig. R34e and Fig. R34j) and electronically insulating. Bi@SiO exhibits an obvious temperature rise under illumination (Fig. R34f−i), confirming that the core@shell structure itself can induce photothermal confinement by suppressing heat dissipation from the plasmonic Bi core. This observation indicates that the temperature enhancement of Bi@SiO core@shell structure originates from a greenhouse-like thermal trapping effect at the nanoscale, rather than from selective infrared reflection.\nFig. R35 (Fig. 3, in the revised manuscript; Supplementary Fig. 19, in the revised Supporting Information) (a) UV-vis-NIR DRS spectra of Bi NCs, Fe O , Bi-Fe O , and 5-Bi@Fe O . (b) Schematics of heat dissipation for Bi-Fe O and the nanoscale greenhouse effect for Bi@Fe O . (c) Monitoring of temperatures over different Fe O -based catalysts under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (d) The infrared thermal images of 5-Bi@Fe O under UV-vis, NIR, vis-NIR, and UV-vis-NIR light under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (e) Thermal power generation and spatial distribution of temperature rise excited at an incident power density of 2.5 mW μm-2 of Bi@SiO . Scale bars, 50 nm.\nMoreover, as shown in the UV-vis-NIR absorption spectra (Fig. R35a) and simulation transmittance spectra (Fig. R34j), 5-Bi@Fe O exhibits enhanced broadband absorption extending from the visible to the near-infrared region (300–2000 nm), which is primarily enabled by the plasmonic response of the Bi core. Importantly, Fe O itself does not act as an efficient reflector in any specific IR band. Instead, under illumination, the absorbed photon energy, particularly from the vis-NIR region, is predominantly converted into heat through non-radiative relaxation of plasmonically excited carriers. Therefore, the elevated temperature observed for Bi@Fe O is not attributed to infrared reflection but to suppressed heat dissipation (Fig. R35b). As shown in Fig. R35c, Bi@Fe O consistently reaches significantly higher steady-state temperatures than Fe O and Bi-Fe O under identical illumination conditions. Infrared thermal images (Fig. R35d) and COMSOL Multiphysics simulations (Fig. R35e) further confirm rapid heat accumulation and sustained temperature localization for the core@shell structure.\nImportantly, however, a direct comparison reveals that the steady-state temperature achieved by Bi@Fe O is higher than that of Bi@SiO under the same irradiation intensity and spectral conditions (Fig. R34i and Fig. R35c). This result demonstrates that thermal confinement alone from the shell cannot fully account for the enhanced heating behavior of Bi@Fe O . In contrast to SiO , the Fe O shell enables strong interfacial electronic coupling with the Bi core, allowing plasmon-induced hot electrons to be injected into Fe O and subsequently trapped at Fe sites adjacent to oxygen vacancies. These electronic processes not only promote non-radiative energy dissipation but also dynamically generate Fe-V active centers, further reinforcing localized heat accumulation.\nTherefore, we have replaced the term “infrared shielding” with “nanoscale thermal confinement” throughout the manuscript, which more accurately describes the plasmon-induced photothermal effect. Specifically, the observed temperature rise in Bi@Fe O reflects a synergistic photothermal-electronic process: the core@shell geometry provides nanoscale thermal confinement, while the semiconducting Fe O shell actively participates in trapping hot electrons and suppressing heat dissipation. We emphasize that this behavior is fundamentally different from passive thermal insulation, as demonstrated by the Bi@SiO reference. Accordingly, we have revised the manuscript to clarify that the “nanoscale greenhouse effect” originates from suppressed heat dissipation coupled with interfacial electronic interactions, rather than from a true infrared shielding or reflection mechanism.\nChanges in Manuscript:\nWe have added the following discussion to the “Spectroscopy characterization and temperatures measurements” section:\n“After confirming the unique structure, we further explore the fundamental optical properties and solar-to-heat energy conversion ability. Three representative catalysts are shown in Supplementary Fig. 14. As displayed in Fig. 3a, the Fe O catalyst exhibits a strong, broad absorption band below ~600 nm. Bi NCs display strong, broadband absorption covering the UV-vis-NIR region, which belongs to characteristic of bismuth plasmonic and interband transitions. Compared to pure Fe O , the Bi-Fe O catalyst and Bi@Fe O catalyst maintain this excellent broadband light-harvesting ability of Bi NCs (300–2000 nm). Moreover, increasing the Bi content effectively enhances the light absorption of x-Bi@Fe O , particularly in the near-infrared (NIR) region (Supplementary Fig. 15). This enhanced NIR absorption is crucial for the efficient cascaded utilization of the full solar spectrum.”\n“Importantly, compared to Bi-Fe O , the 5-Bi@Fe O catalyst effectively suppresses heat dissipation from the Bi core to the surrounding environment after illumination (Fig. 3e). COMSOL Multiphysics simulations further reveal obvious enhancement of local electric fields and surface thermal energy in 5-Bi@Fe O under near-infrared excitation, accompanied by a temperature increase of ~135 K (Fig. 3f, Supplementary Fig. 17b and Supplementary Figs. 18–20). Additionally, to decouple thermal confinement from electronic effects, a Bi@SiO core@shell structure with an optically transparent and electronically insulating shell was synthesized as a control (Supplementary Fig. 21). Although Bi@SiO exhibits a certain photothermal temperature rise due to suppressed heat dissipation (Supplementary Fig. 22), its heating efficiency remains lower than that of Bi@Fe O under identical illumination conditions (as shown in Supplementary Fig. 22 b and Fig. 3c). These results indicate that the elevated temperature of 5-Bi@Fe O originates not only from thermal insulation by the shell, but also from enhanced broadband light absorption and interfacial energy conversion enabled by the Fe O shell. Based on simulation and experimental results, the observed temperature increase is more likely attributed to the enhanced broadband absorption of the Bi core after core@shell structure construction, as well as the intrinsically low thermal conductivity of the Fe O shell layer. The resulting localized high-temperature environment promotes molecular collision frequency and reaction kinetics, thereby enhancing the solar-to-chemical energy conversion efficiency41,42.”\nChanges in Supporting Information:\nSupplementary Figs. 21 and 22 have been added.\nSupplementary Fig. 21. (a) TEM images and (b-d) the corresponding element maps of Bi@SiO . (e) UV-vis-NIR DRS spectra of Bi@SiO .\nSupplementary Fig. 22. (a) The infrared thermal images of Bi@SiO under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (b) Monitoring of temperatures over different Bi-based catalysts under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2. (c) Thermal power generation and (d) Spatial distribution of temperature rise excited at an incident power density of 2.5 mW μm-2 of Bi@SiO . Scale bars, 50 nm.\nComment 2: The size of the core-shell structure is mentioned more than once in the text ~100 nm. Is there any discrepancy between the text and the pictures? For example, in Figure 2c, the size is significantly larger than 200 nm.\nResponse: We thank the reviewer for the rigorous check. We sincerely apologize for this confusion due to the typographical and graphical errors in the manuscript, as well as the presented these errors in Comment 3. In this work, the characteristic size of the Bi@Fe O core@shell particles is approximately ~100 nm, which corresponds to the diameter of the Bi core together with the dense Fe O shell. To provide a more accurate and comprehensive description, we presented the results of the particle size statistical analysis that had been performed previously (Fig.R36). This analysis yields an average primary particle diameter of 100.66 nm ± 31.96 nm. We note that the obvious size difference in certain TEM images originates from the initial scale-bar annotation, which has now been carefully re-checked and corrected to ensure consistency with the actual magnification. After calibration, the TEM images confirm that the overall size of the core@shell structures is indeed around 100 nm. To avoid any ambiguity, we have revised the scale bars in the relevant TEM/SEM/COMOSOL Multiphysics simulations figures and unified the size description throughout the manuscript and Supporting Information.\nFig. R36 (Supplementary Fig. 2, in the revised Supporting Information) Morphology and particle-size distribution of the spherical core@shell catalyst. (a) Representative TEM image. (b) Particle-size distribution histogram with Gaussian fitting (average size: 100.66 ± 31.96 nm).\nThese revisions will provide a more accurate and robust description of the physical characteristics of the core@shell structure in this work. We thank the reviewer again for prompting this important clarification to improve the rigor and scientific soundness of the manuscript.\nChanges in Supporting Information:\nSupplementary Fig. 2 has been added.\nSupplementary Fig. 2. Morphology and particle-size distribution of the spherical core@shell catalyst. (a) Representative TEM image. (b) Particle-size distribution histogram with Gaussian fitting (average size: 100.66 ± 31.96 nm).\nComment 3: There are numerous typographical and graphical errors throughout the manuscript. Please carefully check all figures and labels. For instance, the first XRD label in Figure 2f should be “5-Bi@Fe O .” In Figure S19, the image corresponding to 5-Bi@Fe O should be labeled as S19c. The diagrams in Figures 6b and 6c appear to be inverted and should be placed the other way around. In the product diagram of Figure S22, both lines are labeled as “CO,” which should be corrected.\nResponse: We sincerely thank the reviewer for the careful check. We are very sorry for these problems that occurred in our manuscript. We have conducted a thorough and systematic revision of the entire manuscript, including all figures, captions, labels, and the Supporting Information, to correct typographical and graphical errors. Specifically, the following corrections have been implemented:\n(i) The first XRD label has been corrected to “5-Bi@Fe O ” in Figure 2f (Fig. 2c, in the revised manuscript).\nFig. 2 Morphological and structural characterizations. c XRD patterns of 5-Bi@Fe O .\n(ii) The image corresponding to 5-Bi@Fe O has been correctly relabeled as Figure S19c in Figure S19 (Supplementary Fig. 19, in the revised Supporting Information).\nSupplementary Fig. 19. Thermal power generation excited at an incident power density of 2.5 mW μm-2 of (a) Bi NCs, (b) Fe O , (c) 5-Bi@Fe O , and (d) Bi-Fe O . Scale bars, 50 nm.\n(iii) The schematic diagrams of Figures 6b and 6c have been reoriented and placed correctly in Figure 6 (Figs. 5b and 5c in Fig. 5, in the revised manuscript).\nFig. 5 Charge carrier dynamics. 2D pseudo-color plot for (a) pure Fe O pumped at 560 nm, (b) 5-Bi@Fe O measured pumped at 560 nm and (c) 870 nm, respectively. KPFM images of 5-Bi@Fe O (d) in the dark, (e) under UV-vis, and (f) under UV-vis-NIR illumination. g Corresponding to the surface potential analysis of the 5-Bi@Fe O . Schematic illustration of the charge transfer processes of the 5-Bi@Fe O catalyst in PCO R under (h) UV-vis and (i) UV-vis-NIR excitation conditions.\n(iv) The duplicated product label “CO” in Figure S22 (Supplementary Fig. 24, in the revised Supporting Information) has been corrected, and all product assignments have been carefully verified.\nSupplementary Fig. 24. Feeding ratios V(CO )/V(H O) for photocatalytic CO R performance at room temperature. The product was tested continuously for 1 h at each ratio. Error bars represent standard deviations obtained from three independent measurements.\nIn addition to these specific revisions, we carefully reviewed all figures, labels, axis titles, legends, and captions throughout both the main text and the Supporting Information to ensure consistency and accuracy. We appreciate the reviewer’s careful reading and constructive comments, which have helped improve the clarity and overall quality of the manuscript.\nComment 4: In most Fe 2p XPS peak-fitting diagrams, the Fe2+ peaks are almost completely obscured.\nResponse: We thank the reviewer for this expert observation regarding the XPS peak fitting details. Due to the high ratio of bulk Fe3+ to surface Fe2+, the intensity of the Fe2+ component is intrinsically low and indeed often appears visually obscured or heavily overlapped by the low-energy tail of the prominent Fe3+ peak. To address the reviewer’s concern and improve clarity, we have carefully re-performed the peak fitting for the Fe 2p spectra.\nChanges in Manuscript:\nWe have added the following figure to the “Proposed reaction mechanism for enhanced photoactivity” section:\nFig. 6 Mechanism analysis of photoactivity enhancement. c In-situ XPS analysis of the Fe 2p signals in 5-Bi@Fe O under full-spectrum light irradiation.\nChanges in Supporting Information:\nSupplementary Fig. 36 (a) XPS spectra for Fe 2p before and after the 5-Bi@Fe O catalyst undergoes the PCO R.\nComment 5: Infrared thermal imagers can only record the surface temperature of objects. Thus, the infrared image shown in Figure 4d, which was measured through a quartz glass window, represents the temperature of the glass surface rather than the actual catalyst surface. In addition, in Figure S16b the infrared camera appears to be positioned too far from the sample, which may further reduce temperature accuracy.\nResponse: We thank the reviewer for this thoughtful comment. Following the reviewer’s suggestion, we repositioned the infrared thermal imager closer to the reactor to improve measurement accuracy (Fig. R37). In addition, a thermocouple was placed in direct contact with the catalyst surface to directly monitor the catalyst temperature during reaction. The catalyst surface temperature was found to be approximately 20 °C higher than the value recorded by infrared thermography, which is reasonable considering heat transfer through the quartz window and distance between the infrared camera and the catalyst surface. These results have now been included and discussed in the revised manuscript and Supporting Information.\nFig. R37 (Supplementary Fig. 16a, in the revised Supporting Information; Fig. 3d, in the revised manuscript) (a) An infrared camera records the surface temperature of the catalyst undergoing the reaction. (b) The infrared thermal images of 5-Bi@Fe O under UV-vis, NIR, vis-NIR, and UV-vis-NIR light under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2.\nIt is a very important issue to accurately measure temperature of the active center for photothermal systems. However, measurement of the temperature at the atomic scale is still a challenging task. Currently, available methods for temperature measurement at microscale mainly relies on the utilization of infrared thermometer or a microsized thermocouple (Nat. Commun., 2024, 15, 4675; Nat. Commun., 2024, 15, 1154; Nat. Commun., 2024, 15, 10604; Joule, 2021, 5, 3235–3251), while both of which are inherently limited by micrometer-scale spatial resolution. Recent advances have demonstrated atomic-scale temperature probing by combining in situ scanning Raman microscopy with temperature mapping, but such techniques remain experimentally demanding and are not yet broadly accessible. Unfortunately, currently we don’t have condition to conduct such experiments. Although accurately measuring the temperature of atomic sites is important to this study, our understanding toward the photothermal coupling mechanism at the atomic level is achieved through current characterization and computational calculation, which together provide a reliable description of the system. Anyway, we agree that direct atomic-scale temperature measurements would be highly valuable and represent an important direction for future research.\nChanges in Manuscript:\nWe have added the following figure to the “Light and thermal effects over the unique core@shell structure” section:\nFig. 3 Full-spectrum light harvesting and nanoscale heat-localization behavior of 5-Bi@Fe O . d The infrared thermal images of 5-Bi@Fe O under UV-vis, NIR, vis-NIR, and UV-vis-NIR light under a full-Arc 300 W Xe lamp with an optical density of 455 mW cm–2.\nChanges in Supporting Information:\nSupplementary Fig. 16 have been added, along with the discussion:\nSupplementary Fig. 16. Surface temperature evolution of the catalyst under light irradiation. (a) Thermocouple measurement of the temperature of the catalyst undergoing the reaction. (b) An infrared camera records the surface temperature of the catalyst undergoing the reaction.\nComment 6: Only SEM, TEM, and XRD were used to characterize the morphology and structure of 5-Bi@Fe O after reaction. However, the changes in the valence states of Bi and Fe were not analyzed. It is recommended to include post-reaction XPS characterization to clarify these electronic-state changes.\nResponse: We sincerely thank the reviewer for this constructive suggestion. As recommended, we performed high-resolution X-ray Photoelectron Spectroscopy (XPS) on the 5-Bi@Fe O sample after long-term photocatalytic testing. The comparative spectra (before reaction vs. after reaction) are presented in Fig. R38. The XPS results of Fe 2p after 5-Bi@Fe O undergoes the PCO R show that the Bi core remains predominantly in the metallic Bi0 state, indicating that the plasmonic component is chemically stable under the reaction conditions. Meanwhile, the Fe 2p spectra reveal that the overall Fe3+-dominated framework is preserved, accompanied by a small fraction of Fe2+ species, which is consistent with the reversible light-induced reduction process observed in the in situ XPS experiments under illumination.\nFig. R38. (Supplementary Fig. 35 and 36, in the revised Supporting Information). XPS spectra for (a) Bi 4f and (b) Fe 2p before and after 5-Bi@Fe O undergoes the PCO R.\nImportantly, no irreversible oxidation of the Bi core or permanent reconstruction of the Fe O shell is observed after reaction, corroborating the structural stability inferred from post-reaction SEM, TEM, and XRD analyses. These results further confirm that more active Fe-V sites are transiently generated under illumination rather than arising from permanent chemical transformation of the catalyst.\nChanges in Manuscript:\nWe have added the following discussion to the “PCO R driven by coupling photo-/photothermal effect” section:\n“Furthermore, according to the XPS and XAS spectra of the 5-Bi@Fe O catalyst after PCO R reaction (Supplementary Figs. 35, 36; Supplementary Tables 5, 6), the valence state and coordination environment of Bi and Fe in the core@shell catalyst did not clearly change, indicating that the Bi core does not undergo oxidation during the reaction, and the Fe-V active sites keep stable in the reaction process (Supplementary Figs. 37).”\nChanges in Supporting Information:\nSupplementary Fig. 35 and 36 have been added.\nSupplementary Fig. 35. XPS spectra for (a) Bi 4f before and after the 5-Bi@Fe O catalyst undergoes the PCO R.\nSupplementary Fig. 36. XPS spectra for (a) Fe 2p before and after the 5-Bi@Fe O catalyst undergoes the PCO R.\nComment 7: In Figure 5f, the temperature for condition 4 is shown as 170°C, whereas the main text states 220°C. Please verify and correct the specific temperature value.\nResponse: We sincerely thank the reviewer for this helpful comment. After carefully rechecking the part text and the corresponding figures, we confirm that the correct label in Fig. 5f (Fig. 4, in the revised manuscript) should be “Dark 220 °C”. The different testing conditions were varied to distinguish the effect of light and heat on the PCO R in this work, respectively, thereby elucidating the kinetic mechanism by which the synergistic photochemical and photothermal effects drive the conversion of CO to CH . We have verified and corrected the corresponding label as well as the related descriptions in the manuscript. To further improve the rigor and scientific soundness of the manuscript, we also conducted a careful check of the entire manuscript to avoid similar errors.\nChanges in Manuscript:\nWe have added the following figure to the “PCO R driven by coupling photo-/photothermal effect” section:\nFig. 4 Photo-/photothermo-synergistic catalysis of PCO R. f Average CH production rates over 5-Bi@Fe O catalyst under different conditions at 455 mW cm–2.\nComment 8: The photocurrent data of Bi@Fe O under full-spectrum illumination shown in Figures S33a and S33b appear to be inconsistent, and the sample labeling in Figure S33b seems to contain an error. In addition, the presence of a measurable current under dark conditions requires clarification. The authors should explain whether this dark current arises from instrumental background, electrochemical leakage, or intrinsic conductivity of the sample.\nResponse: We are grateful to the reviewer for the rigorous examination. And We sincerely apologize to the reviewer for the confusion caused by the inconsistent photocurrent data and the incorrect sample labeling in Supplementary Fig. 33 (Supplementary Fig. 42, in the revised Supporting Information).\nAfter carefully revisiting the experimental records, we found that the apparent discrepancy in the full-spectrum photocurrent responses of 5-Bi@Fe O shown in Supplementary Fig. 33a (Supplementary Fig. 42a, in the revised Supporting Information) originates from the fact that these two measurements were conducted on independently prepared electrodes and tested in different environments. Photocurrent measurements are known to be sensitive to multiple experimental factors, such as electrode film thickness, catalyst loading, interfacial contact resistance, electrolyte conditions, and measurement environment. As a result, variations in the absolute photocurrent magnitude can occur when experiments are conducted at different times.\nIn addition, we acknowledge that the sample labeling in Supplementary Fig. 33a (Supplementary Fig. 42a, in the revised Supporting Information) was incorrect due to our oversight during figure preparation.\nTo eliminate these ambiguities, we have re-measured the photocurrent measurements by testing all samples under identical conditions and within the same experimental session. The re-measured photocurrent data, now consistently labeled, have been updated in the Supporting Information (Supplementary Fig. 42, in the revised Supporting Information). The new results show consistent and reproducible photocurrent responses under full-spectrum illumination, confirming the enhanced photoresponse of 5-Bi@Fe O compared with the reference samples.\nIn the newly acquired data, a weak current is also observed under dark conditions. This dark current is not associated with photoexcited charge carriers, but is mainly attributed to the intrinsic electrical conductivity of the Bi-containing catalyst film and the Fe O shell, together with minor electrochemical background currents arising from the electrode–electrolyte interface. Importantly, the dark current remains stable and does not exhibit any light-on/off response, and its magnitude is significantly lower than that of the photocurrent under illumination. Therefore, the light-induced photocurrent enhancement can be clearly distinguished from the dark-state background contribution. These points have been clarified in the revised Supporting Information.\nChanges in Supporting Information:\nSupplementary Fig. 42 has been added.\nSupplementary Fig. 42. Transient photocurrent densities. (a) Photocurrent response of 5-Bi@Fe O with light on/off cycles under different light irradiation in 0.5 M Na SO electrolyte solution at an applied potential of 0.5 V vs. Ag/AgCl electrode. (b) Photocurrent response of different samples with light on/off cycles under full spectrum in 0.5 M Na SO electrolyte solution at an applied potential of 0.5 V vs. Ag/AgCl electrode. All photocurrent measurements were conducted using freshly prepared electrodes under identical conditions in a single experimental process.\nComment 9: The DFT calculations for both Fe O and 5-Bi@Fe O are performed on the bare Fe O surface, focusing solely on CO reduction. However, since the experimental reaction involves both CO and H O, it is recommended that the authors construct a model of Fe O supporting Bi nanoparticles and perform interfacial calculations in the presence of H O. Simulating the reaction pathways of CO and H O coadsorption and activation at the Bi-Fe O interface and comparing these results with those on bare Fe O would provide a more realistic understanding of the catalytic mechanism and clarify the cooperative role of the Bi-Fe O interface in the observed activity.\nResponse: We sincerely thank the reviewer for this constructive suggestion. It is indeed crucial for elucidating the cooperative catalytic mechanism to construct a realistic model that includes the Bi/Fe O interface and solvent molecules in DFT calculations. Following the reviewer’s recommendation, we have performed new DFT calculations by constructing a new model of Fe O supporting Bi nanoparticles (Bi/Fe O interfacial model) in the presence of explicit water molecules (Fig. R39).\nFig. R39 (Supplementary Figs. 52–54, in the revised Supporting Information). Optimized DFT calculation models. (a) The pristine Fe O surface and (b) the Bi/Fe O interfacial model, both with adsorbed CO and an explicit H O solvent layer. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C)\nSpecifically, Bi nanoparticles were supported on the Fe O surface to simulate the heterogeneous interface, and both CO and H O molecules were introduced to simulate the experimentally relevant reaction microenvironment. These new calculations reveal that the Bi/Fe O interface plays a critical cooperative role by modulating charge redistribution and stabilizing key reaction intermediates under CO /H O coexisting conditions. In particular, the interfacial electronic interaction facilitates CO activation at Fe-V sites while simultaneously promoting proton-coupled electron transfer steps associated with H 2 O (Fig. R40), which is consistent with the experimentally observed enhancement in CH selectivity.\nFig. R40 (Fig. 3 in the revised manuscript) Gibbs Free energy diagram for PCO R over the pristine Fe O surface and the Bi/Fe O interfacial model.\nOverall, these newly added calculations provide a more realistic description of the synergistic role of the Bi/Fe O interface under CO /H O coadsorption conditions, and further elucidate the cooperative catalytic mechanism proposed in this work, in which the core@shell architecture enables highly selective CO reduction to CH .\nThe corresponding models, computational details, and results have been added to the revised manuscript and Supplementary Information.\nNote: It should be noted that although the experimentally synthesized Bi@Fe O catalyst exhibits a core@shell architecture, constructing a full-scale core@shell model is hardly possible in DFT calculations. Therefore, as commonly reported in previous studies (Nat. Commun., 2025, 16, 7559; Nat. Commun., 2024, 15, 413; Adv. Mater. 2024, 36, 2406088; Adv. Mater. 2025, 2501091), simplified slab-based models are employed to approximate the essential physicochemical features of core@shell systems.\nFig R41 (in the previous manuscript). Traditional slab structure of the Fe O shell (top) supported on metallic Bi nanoparticles (bottom). (Blue: Bi; Orange: Fe; Red: O)\nIn a conventional stacked slab model (a layered “vertical” slab model, Fig R41), the Bi/Fe O interface is buried and inaccessible to reactant molecules, so that it is extremely difficult to investigate the interfacial electronic effects under CO /H O coexisting conditions. To better represent the realistic reaction microenvironment at the interface, a new Bi@Fe O interfacial model was constructed by loading Bi nanoparticles onto the Fe O slab. A layer of 16 H O molecules was introduced above the surface to account for solvation and interfacial electric-field effects.\nAlthough this model represents a geometric simplification of the ideal core@shell structure, it captures the intrinsic electronic interaction at the Bi@Fe O heterojunction. Importantly, the charge redistribution and activation mechanisms observed in this interfacial model reflect the intrinsic chemical nature of the Bi@Fe O heterojunction. The resulting theoretical insights are therefore directly relevant to understanding the experimentally observed high selectivity for CO reduction to CH under CO -H O coexisting conditions. Considering all computational constraints and mechanistic requirements, we believe that the adopted modeling approach provides more physically meaningful and experimentally consistent theoretical description of the catalytic mechanism proposed in this work.\nChanges in Manuscript:\nWe have added the figure with following discussion to the “Proposed reaction mechanism for enhanced photoactivity” section:\nFig. 6 Mechanism analysis of photoactivity enhancement. f Gibbs Free energy diagram for PCO R over the pristine Fe O surface and the Bi/Fe O interfacial model, both with adsorbed CO and an explicit H O solvent layer. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C).\n“The density functional theory (DFT) calculations were performed to elucidate the thermodynamic origins of the enhanced activity and selectivity in the PCO R to CH over 5-Bi@Fe O . To explicitly probe the interfacial electronic effects and solvent participation, the simplified slab models with explicit water molecules were constructed (details in Supporting Information; optimized structures of pure Fe O and 5-Bi@Fe O in Supplementary Figs. 52−54). The calculated Gibbs free‐energy for CO reduction with H O toward CH reveals a markedly more favorable thermodynamic advantage over 5-Bi@Fe O than over Fe O (Fig. 6f; Supplementary Figs. 55, 56). In specific, CO was adsorbed on the surface of 5-Bi@Fe O and Fe O to produce *CO , and then *CO was hydrogenated to produce *COOH. Owing to the high energy barrier, the first proton-coupled electron transfer for *COOH generation was considered a rate-limiting step. Importantly, the formation energy of *COOH on 5-Bi@Fe O is 0.67 eV, distinctly lower than that on pristine Fe O (0.85 eV), indicating that the electron-rich Bi/Fe O interface significantly decreases the energy barrier for CO activation. After that, *COOH was then protonated and dehydrated to form a stable intermediate *CO, which was associated with the C−O bond breakage through a proton-assisted electron transfer process50. After *CO formation, pristine Fe O tends to favor CO desorption rather than further hydrogenation (0.20 eV vs 0.98 eV). In contrast, the *CO intermediate is more strongly stabilized for 5-Bi@Fe O , and the subsequent hydrogenation to *CHO becomes thermodynamically preferred over CO desorption, thereby resulting in the pathway toward deep hydrogenation. The subsequent hydrogenation steps (*CHO → *CH O → *CH O → *+CH ↑) proceed with overall downhill energetics and small thermodynamic barrier, consistent with the enhanced CH selectivity. Notably, these theoretical trends are in good agreement with the experimental results. Specifically, the increased Fe2+/Fe3+ proportion under irradiation suggests an electron-enriched, defect-associated surface environment, while time-resolved DRIFTS shows the continuous accumulation of *CHO/*COOH-related bands on 5-Bi@Fe O . This is consistent with the predicted stabilization of key hydrogenation intermediates and the suppression of premature CO desorption. Therefore, the combined DFT and experiment analyses provide a clear logical framework for highly selective CH formation on the Bi@Fe O core@shell catalyst.”\nChanges in Supporting Information:\nSupplementary Figs. 52–56 have been added, along with the discussion:\nSupplementary Fig. 52. Optimized theoretical model. The DFT optimization model for the CO to CO pathway over the pristine Fe O , with an explicit H O solvent layer. (Orange: Fe; Red: O; White: H; Grey: C)\nSupplementary Fig. 53. Optimized theoretical model. The DFT optimization model for the CO to CH pathway over the pristine Fe O , with an explicit H O solvent layer. (Orange: Fe; Red: O; White: H; Grey: C)\nSupplementary Fig. 54. Optimized theoretical model. The DFT optimization model for the Bi/Fe O interfacial model, with an explicit H O solvent layer. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C)\nSupplementary Discussion: Due to the computational infeasibility of constructing a full-scale core@shell model, a simplified Bi nanoparticle/Fe O interfacial model was employed. Unlike conventional stacked slab models with buried interfaces, this configuration provides an accessible Bi/Fe O boundary, allowing explicit investigation of interfacial electronic effects under CO /H O coexisting conditions. A solvation layer of H O molecules was included to account for interfacial electric-field effects. Despite geometric simplification, the model captures the essential electronic interaction of the Bi@Fe 2 O 3 heterojunction relevant to CO 2 -to-CH 4 conversion. Further details and justification of the modeling strategy are provided in the “DFT simulations” section.\nSupplementary Fig. 55. Schematic illustration of the reaction pathway in PCO₂R for the pristine Fe O surface model, with an explicit H O solvent layer. (Orange: Fe; Red: O; White: H; Grey: C)\nSupplementary Fig. 56. Schematic illustration of the reaction pathway in PCO₂R for the Bi/Fe O interfacial model, with an explicit H O solvent layer. (Blue: Bi; Orange: Fe; Red: O; White: H; Grey: C)\nIn addition, we have added the following discussion to the “3. DFT simulations”section:\n“To illustrate the long-range dispersion interactions between the adsorbates and catalysts, we employed the D3 correction method by Grimme et al.”\n“Note: It should be noted that although the experimentally synthesized Bi@Fe O catalyst exhibits a core@shell architecture, constructing a full-scale core@shell model is hardly possible in DFT calculations. Therefore, as commonly reported in previous studies (Nat.Commun., 2025, 16, 7559; Nat. Commun., 2024, 15, 413; Adv. Mater. 2024, 36, 2406088; Adv. Mater. 2025, 2501091), simplified slab-based models are employed to approximate the essential physicochemical features of core@shell systems.\nIn a conventional stacked slab model (a layered “vertical” slab model, Supplementary Fig 57), the Bi/Fe O interface is buried and inaccessible to reactant molecules, so that it is extremely difficult to investigate the interfacial electronic effects under CO /H O coexisting conditions. To better represent the realistic reaction microenvironment at the interface, a new Bi@Fe O interfacial model was constructed by loading Bi nanoparticles onto the Fe O slab. A layer of 16 H O molecules was introduced above the surface to account for solvation and interfacial electric-field effects.\nAlthough this model represents a geometric simplification of the ideal core@shell structure, it captures the intrinsic electronic interaction at the Bi@Fe O heterojunction. Importantly, the charge redistribution and activation mechanisms observed in this interfacial model reflect the intrinsic chemical nature of the Bi@Fe O heterojunction. The resulting theoretical insights are therefore directly relevant to understanding the experimentally observed high selectivity for CO reduction to CH under CO -H O coexisting conditions. Considering all computational constraints and mechanistic requirements, we believe that the adopted modeling approach provides more physically meaningful and experimentally consistent theoretical description of the catalytic mechanism proposed in this work.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns. Therefore, I recommend that the manuscript be published in its current form.", "reviewer_2": "The authors have carefully revised the manuscript according to my advice, It could be accepted for publication.Congurations to the authors.", "reviewer_3": "The comments have been appropriately addressed, and the revised manuscript can be accepted for publication."}, "author_response": ""}]

Decision letter