Atomic-scale interface engineering in Bi/Bi_2O_3 heterojunctions for selective CO_2 photoreduction to methanol

Domain: chemistry
Published: 2025-12-17
Editorial outcome: major_revision

Abstract
The strategic engineering of an Ohmic junction at the Bi/Bi2O3 (BBO) interface is demonstrated to synergistically enhance photocatalytic CO2-to-methanol conversion through precisely modulated charge behavior and interfacial energy alignment. This metallic Bi-semiconductor Bi2O3 Ohmic junction with local surface plasmon resonance effect induces a robust built-in electric field that promotes the unidirectional electron transfer from Bi2O3 to Bi while suppressing charge recombination. Theoretical calculations and experimental evidence reveal that the interfacial Bi sites within the Ohmic junction predominantly facilitate CO2 adsorption and activation to form *COOH, whereas ensuing protonation steps are favored on metallic Bi sites on BBO Ohmic junction. Furthermore, the Ohmic junction enhances interfacial electron density and strengthens orbital hybridization between Bi 6p and O 2p orbitals, thereby reducing the activation energy of the rate-limiting *CO → *COOH step by 0.6 eV, enabling a CH3OH production rate of 610 μmol g-1 under light irradiation. The work deciphers the dual role of Ohmic junctions in simultaneously resolving bulk charge transport limitations and tailoring surface catalytic landscapes, establishing a universal paradigm for metal-semiconductor heterojunction photocatalyst design.

Introduction
The photocatalytic reduction of CO2 into solar fuels like methanol represents a promising route to tackle both energy scarcity and carbon neutrality goals1–5. Nevertheless, the practical implementation of this photocatalytic reaction faces three fundamental challenges: rapid recombination of photogenerated charge carriers, high bond dissociation energy (≈750 kJ mol−1) for cleaving stable C=O bonds, and dependency on sacrificial agents for sustaining catalytic activity. Substantial research attention has focused on engineering the bulk phase configuration of photocatalysts, as this structural parameter fundamentally governs charge carrier separation dynamics, including constructing ultrathin architectures, introducing heteroatom dopants, and building heterojunctions6,7. Among these, Ohmic junctions formed, characterized by minimal contact resistance at material interfaces, offer a distinct advantage by enabling unimpeded charge injection while preserving catalytic sites8–10. However, achieving precise atomic-level modulation of Ohmic interfaces to concurrently enhance bulk charge migration and surface reaction kinetics remains a challenging.

Recent advances in Ohmic junction engineering, including metal-semiconductor systems (e.g., Bi/BiOCl, where the metallic Bi Nanoparticles formed a low-resistance contact with the n-type BiOCl photocatalyst to accelerate electron extraction)11,12 and semiconductor heterointerfaces (e.g., MoTiC2 MXene/ZnCdS ohmic junction)13, have demonstrated enhanced charge separation and catalytic activity in photocatalytic fields. To further improve the performance of Ohmic junction photocatalysts, leveraging the localized surface plasmon resonance (LSPR) effect presents a compelling approach. The LSPR effect confers unique advantages in photocatalytic CO2 reduction through multiple synergistic pathways: enhanced broadband light-harvesting capabilities, efficient hot-carrier manipulation, intensified electromagnetic near-fields, and thermally aided catalytic processes14,15. For example, Zhang et al. reported a plasmonic Fe-nanonecklace-hydrogenated titanium hybrid for advancing solar light-driven NH3 synthesis via a local heating effect-assisted photothermal pathway, circumventing the equilibrium limit of thermocatalysis16. He et al. synthesized anisotropic Ti3C2Tx MXene with strong LSPR features, which showed enhanced plasmonic charge carrier-induced photocatalytic water splitting activity17. These developments highlight the great potential of integrating LSPR effects into highly efficient Ohmic junction photocatalysts for CO2 photoconversion.

Herein, we demonstrate an atomic-level engineering strategy to construct Bi/Bi2O3 (BBO) Ohmic junctions with LSPR effect for high-efficiency CO2-to-methanol conversion. By in-situ growing metallic Bi nanodomains on nanosheets, we constructed an interfacial structure with aligned Fermi levels, thereby facilitating barrier-free electron migration from Bi2O3 to Bi. Combining X-ray absorption spectroscopy with density functional theory (DFT) simulations, we elucidate that the Ohmic contact facilitates electron accumulation on Bi and the polarized Biδ+ sites stabilize *CO2- intermediates via strong 6p-2p orbital hybridization (−ICOHP=1.85 eV). This interaction reduces the energy barrier for *CO2 protonation by 0.6 eV and stabilizes *CHO intermediates through orbital hybridization. This synergistic configuration achieves a methanol production rate of 610 μmol g−1, surpassing most reported photocatalytic systems. Our work establishes a paradigm for designing Ohmic interfaces that unify ultrafast charge transport and targeted surface catalysis, overcoming longstanding efficiency bottlenecks in artificial photosynthesis.

Results
CO2 adsorption and activation mechanism over Bi/Bi2O3 Ohmic junction

Figure 1a systematically elucidated the interfacial electronic engineering and catalytic mechanisms of Bi/Bi2O3 (BBO) Ohmic junction for enhanced CO2 reduction. The BBO Ohmic junction revealed higher photocatalytic CO2 reduction activity to CH3OH than that of Bi or Bi2O3, which was ascribed to the interfacial interaction inducing fast separation of photogenerated carriers and the strong adsorption to *CO2 for its protonation reaction produces. To further understand the mechanism of CO2 activation and conversion on BBO, we analyzed detailed molecular orbital interactions. The results of the partial density of states (PDOS) indicated that during CO2 adsorption and activation on Bi2O3, hybridization occurred between the Bi 6px orbital and the C 2pz orbital (Fig. 1b and Supplementary Figs. 1–3). This leaded to partial electron transfer from the Bi 6px orbital to the 2π anti-bonding orbital of CO2, weakening the CO2 molecule and causing its linear structure to bend. This structural change drove CO2 adsorption and activation on the Bi2O3 surface. In the case of CO2 adsorption and activation on metal Bi of BBO (Fig. 1c, and Supplementary Figs. 4 and 5), the electronic states of Bi’s 6px, 6py, and 6pz orbitals near the Fermi level nearly vanished. Meanwhile, tightly overlapping continuous hybrid peaks emerged in the electronic states of C 2px, 2py, and 2pz orbitals of CO2. This suggested significant electron transfer from 6px, 6py, and 6pz orbitals of Bi to the 2π anti-bonding orbital of CO2, facilitating CO2 adsorption and activation on metal Bi of BBO. The transferred electrons filled C 2px, 2py, and 2pz orbitals of CO2, resulting in degeneracy among these filled orbitals. This orbital degeneracy manifested as numerous continuous hybrid peaks in the corresponding PDOS. The projected crystal orbital Hamiltonian population (COHP) was also analyzed to understand the mechanism of *CO2 protonation. The rise of the antibonding state contributes to the bond strength, which can be seen from the computed integrated COHP (ICOHP) as shown in Fig. 1d, a more negtive ICOHP was observed for BBO Ohmic junction (−1.85 eV) than Bi2O3 (0.82 eV), indicating a much stronger bond coupling between BBO Ohmic junction and *CO2, favoring the further protonation reaction.

To clarify the interface charge transfer of BBO Ohmic junction, density functional theory calculation (DFT) calculations with Bader charge were calculated. Supplementary Fig. 6 exhibited the optimized crystal structure of Bi2O3 and BBO Ohmic junction. Figure 1e depicted the charge-density difference plots of BBO Ohmic junction, whereas the yellow and cyan areas represented electron density accumulation and deplete, respectively. Theoretical calculations indicated that the coupling of Bi and Bi2O3 promoted charge enrichment on Bi2O3 (Δq=1.557 e), which suggested that Bi atoms can adjust the surrounding chemical microenvironment of Bi2O3, thus affecting the adsorption of intermediate species. To verity the heterojunction type in theory, the theoretical work functions of Bi and Bi2O3 were calculated to be 4.78 and 5.15 eV, respectively (Fig. 1f). The much smaller work function of Bi promoted the charge transfer from Bi to Bi2O3 until their consistent Fermi levels after the coupling of Bi and Bi2O3, and at the same time, inducing the generation of a strong built-in electric field and band bending (Fig. 1g). This phenomenon can accelerate the transfer and separation of photogenerated carriers for CO2 photoreduction to CH3OH (Supplementary Fig. 7).

Structural characterizations

Figure 2a presents the synthetic procedures of BBO Ohmic junction via a hydrothermal and wet-chemical cascade strategy. The Bi19Br3S27 nanowires have been firstly synthesized by hydrothermal method, then followed by a NaBH4 reduction method to inducing the dissolution and crystallization of Bi species, and finally the BBO nanosheets were obtained. The X-ray diffraction (XRD) patterns of the samples were shown in Supplementary Fig. 8. The nanowire morphology of Bi19Br3S27 samples were shown in Fig. 2b and Supplementary Fig. 9. To validate the structure evolution of Bi19Br3S27 nanowires to form BBO nanosheets, the different reduction time for Bi19Br3S27 was conducted at the same conditions. Figure 2c–e and Supplementary Figs. 10–12 record the morphologies of BBO-2, BBO-4, and BBO-6. With the increasing of reduction time, the BBO samples exhibited a gradual increased nanosheet structure (Supplementary Figs. 13–15). Moreover, the progressive rise in Bi content of these samples indicated the existence of a rich bismuth environment (Supplementary Tables S2–4). The SEM, TEM and AFM tests of Bi2O3 were conducted. The scanning electron microscope (SEM) and transmission electron microscopy (TEM) images indicate that the Bi2O3 sample was composed of irregularly nanosheet (Supplementary Figs. S16a, b). The lattice spacings of 0.274 nm can be assigned to the (400) planes of Bi2O3 (Supplementary Fig. S16c). The AFM images and the corresponding height profile showed that the thickness of its nanosheet was ~5.06 nm (Supplementary Figs. S16d, e). These data provide a solid foundation for the subsequent comparison with BBO-4 composite materials. The excellent lattice matching and well-defined BBO-4 Ohmic junction were verified by high resolution TEM (HRTEM; Fig. 2f, g), demonstrating the success in fabricating the Ohmic junction. The high-quality and coherent heterointerface with a 0.328 nm lattice fringe spacing belonging to the (012) plane of Bi and a 0.274-nm lattice fringe spacing attributing to the (400) plane of Bi2O3. Furthermore, the element composition of BBO-4 Ohmic junction was detected by energy dispersivespectroscopy (EDS) element mapping (Fig. 2h–j). Moreover, the Bi content was gradually increased as time extended, and the XRD patterns of BBO samples can be indexed to hexagonal Bi (JCPDS: 05-0519) and Bi2O3 (JCPDS: 22-0515) (Fig. 2k, and Supplementary Fig. 17) and no other phase structures were detected (Supplementary Fig. 18)18,19. To precisely resolve the phase composition, Rietveld refinement of the XRD patterns (Supplementary Fig. 19) was performed to determine the phase composition of the BBO catalysts. The refinement results confirm the exact phase composition. The Bi/Bi2O3 mass ratios for BBO-2, BBO-4 and BBO-6 were 53.65/46.35, 91.75/8.25, and 97.74 wt%/2.26 wt%, respectively. The Bi content increased with increasing reduction time. The above results showed there was on other phase structures. The Raman peaks of metallic Bi at 71 and 98 cm−1 corresponded to the asymmetric coupling vibration and symmetric vibration of Bi atoms within the metal (Supplementary Fig. 20). The Raman peaks of Bi2O3 at 326 and 465 cm−1 were attributed to the stretching of Bi–O bonds and the symmetric stretching of Bi–O–Bi in Bi–O octahedral units within Bi2O3. In the Raman spectrum of BBO-4, characteristic peaks of both metallic Bi and Bi2O3 were observed. The above results confirmed the chemical reduction can achieve the structure reconstruction of Bi19Br3S27 for BBO nanosheet formation.

As shown in Fig. 2l, the absorption edge of BBO-4 in the X-ray absorption fine structure (XAFS) Bi L3-edge was between the Bi foil and Bi2O3, meaning that the valence state was Biδ+(0<δ<3), further confirming the existence of metal Bi. In addition, the absorption edge of Bi in the XAFS Bi L3-edge was between the Bi foil and Bi2O3, showing the occurrence of surface oxidation, which was accordance with the X-ray photoelectron spectroscopy (XPS) results. To obtain the quantitative structural parameters around Bi, Fourier transform of the least-squares curve fitting was performed on the EXAFS data of Bi, Bi2O3, and BBO-4 (Fig. 2m). BBO-4 exhibited a typical Bi/Bi2O3 structure. The presence of both Bi–O and Bi–O–Bi coordination shells confirmed the existence of Bi2O3 oxide, while the Bi-Bi metallic coordination indicated the presence of metallic Bi. In contrast, the Bi2O3 sample showed only a single Bi–O coordination shell, demonstrating its phase-pure Bi2O3 composition. The Bi metal sample, however, revealed both Bi-Bi and Bi-O bonds, suggesting the existence of surface oxidation. To investigate the electronic states and charge transfer of BBO-4, XPS analysis was employed. For Bi2O3, the XPS diffraction peaks at 159.00 eV (Bi 4f7/2) and 164.32 eV (Bi 4f5/2) were characteristic of Bi3+ species (Supplementary Fig. 21a)20,21. The characteristic Bi 4f peaks of Bi metals could be fitted into two pairs of doublets associated with Bi0 and Bi3+. The XPS signal of Bi oxidation states in Bi metals was derived from surface oxidation of the catalyst under exposure to air. For BBO-4, the Bi 4f signals deconvoluted into four peaks: two at 156.68 and 161.98 eV (assigned to Bi0 in metallic Bi) and two at 158.89 and 164.21 eV (attributed to Bi3+ in Bi2O3)22–24. Comparative analysis of binding energies across Bi and Bi2O3, BBO-4 revealed a 0.11 eV positive shift in the Bi0 binding energy for the heterojunction relative to pristine metallic Bi, indicating decreased electron density around Bi0. Conversely, the Bi3+ binding energy in BBO showed an 0.11 eV downshift compared to Bi2O3, signifying increased electron density around Bi3+. These XPS results collectively suggested electron transfer from metallic Bi to Bi2O3 within the heterojunction, consistent with the electron flow direction in Ohmic junction25–27. The O 1s XPS spectrum of BBO-4 was the same as that of pristine Bi2O3, with peaks at 529.52 and 530.85 eV, which were attributed to the chemical bonding of Bi–O and adsorbed oxygen (H–O), respectively (Supplementary Fig. 21b). The results were in accordance with the above work function analysis.

Local surface plasmon resonance effect and photocatalytic CO2 reduction over BBO

Prior to evaluating photocatalytic performance, we first investigated the material’s light utilization capability for photocatalytic applications. The UV-vis-NIR DRS (Fig. 3a) revealed that BBO-4 demonstrated significantly enhanced broad-spectrum light absorption (280–2100 nm) compared to its constituent materials (Bi and Bi2O3). This improved LSPR performance in the BBO Ohmic junction can be attributed to its unique nanosheet architecture. The abundant interfaces between metallic Bi and Bi2O3, along with their interfacial interactions, collectively modulated the LSPR intensity. Finite-difference time-domain simulations further corroborated the presence of a strong LSPR effect in BBO (Fig. 3b), demonstrating enhanced local electric field strength surpassing that of individual components28,29. Notably, the accumulation of LSPR-excited hot electrons at the interface spontaneously elevated the BBO surface temperature to 117.3 °C without external heating (Fig. 3b, and Supplementary Fig. 22). This LSPR-induced phenomenon simultaneously enhanced photon utilization efficiency and enabled localized photothermal energy conversion, thereby synergistically promoting photocatalytic activity. The corresponding band gaps Eg of Bi2O3 was determined to be 2.51 eV, according to Tauc plots: (αhν)1/2 = A(hν-Eg) (Supplementary Fig. 23). Mott–Schottky analysis indicated flat potentials of −0.20 V (Bi) and −0.57 V (Bi2O3) versus NHE (Fig. 3c). Given that semiconductor conduction band potentials typically located at more negative values than their flat potentials, the BBO band alignment satisfied the thermodynamic requirements for CO2 photoreduction to CH3OH.

The photocatalytic CO2 reduction performances of synthesized catalysts were evaluated under full spectrum (Fig. 3d, and Supplementary Fig. 24). The primary products of Bi, Bi2O3, and Bi2S3 resulting from photocatalytic CO2 reduction were mainly CO. The precursor Bi19Br3S27 nanowires revealed the generation of CO, CH4, and CH3OH. The series of BBO derived from Bi19Br3S27 nanowires displayed enhanced photocatalytic activity for CH3OH generation. The optimal BBO-4 samples reached the highest value of 610 µmol g−1, substantially surpassing the rates observed with unmodified Bi, Bi2O3, and precursor Bi19Br3S27 nanowires. The liquild products were further detected by nuclear magnetic spectrum (NMR). As shown in Supplementary Fig. 25, the proton spectrum exhibited a methanol peak at δ 3.28 ppm, and a water peak at δ 4.7 ppm. Based on the (NMR) analysis of the liquid products from photocatalytic CO2 reduction, it is confirmed that the liquid product generated by the BBO-4 heterojunction photocatalyst was CH3OH. No impurity peaks or other liquid products were detected.

In contrast, no reduction products were observed under dark conditions, in the absence of a catalyst or Ar atmosphere (Fig. 3e). The above results of controlled experiments demonstrated the generation of products as a result of reduced CO2 in the photocatalytic system. N2 adsorption/desorption isotherms and the pore-size distributions of Bi, Bi2O3, and BBO-4 were characterized (Supplementary Fig. 26), suggesting surface area was not the influence factor on photocatalytic activity. As depicted in Fig. 3f, the yields of products were basically no significant change after four cycles, and its structure had no obvious agglomeration and transform (Supplementary Fig. 27), demonstrating the excellent stability of BBO-4. Furthermore, precise detection of 13CH4 (m/z=17), 13CO peak (m/z=29) and 13CH3OH (m/z=33) utilizing 13C as the only carbon source, provided further evidence that CH3OH, CH4 and CO were derived from CO2 photoreduction (Supplementary Fig. 28, and Fig. 3g). To demonstrate the superior activity of BBO-4, we conducted comparative analyzes with over 20 different catalytic systems (Fig. 3h). The results conclusively showed that BBO-4 exhibited favorable photocatalytic performance among the reported photocatalyst.

Identification of CO2 photoreduction intermediates

To investigate the mechanistic pathway of photocatalytic CO2 reduction, we conducted in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) to monitor the dynamic evolution of adsorbed intermediates on pristine Bi, Bi2O3, and BBO-4 catalysts under light irradiation (Fig. 4a–f, and Supplementary Figs. 29–31). The characteristic peak at 1546 cm−1 (Fig. 4a) corresponds to combined bending and stretching vibrations of the *COOH intermediate on metallic Bi30–32, a feature consistently observed in both Bi2O3 and BBO-4 systems under illumination (Fig. 4b, c). Notably, comparative analysis revealed that BBO-4 exhibited weaker b-CO32− signal intensity but enhanced m-CO32− formation compared to pristine Bi and Bi2O3. This distinct behavior suggests that m-CO32− species generated through single oxygen-site adsorption at Bi centers serve as key intermediates in the CO2 reduction pathway on BBO-431,33,34. The diagnostic *CO intermediate (2076 cm−1) was detected across all three catalysts, with its presence indicating a critical branching point for subsequent hydrocarbon formation, particularly CH4 and CH3OH35–37. Temporal evolution studies (Fig. 4d–f) demonstrated progressive intensification of *COOH signals over 5–60 min, consistent with continuous CO2 activation and hydrogenation processes. Crucially, BBO-4 exhibited significantly stronger *CO adsorption compared to its constituent materials (Bi and Bi2O3), as evidenced by enhanced peak intensity. This pronounced adsorption capacity favored further hydrogenation steps over CO desorption, thereby directing the reaction pathway toward CH3OH products. Based on comprehensive analysis of these in situ DRIFTS results, we proposed a stepwise reaction mechanism for photocatalytic CO2 reduction on BBO-4 (Fig. 4g). The process initiates with CO2 chemisorption at Bi–O sites, followed by sequential protonation steps generating *COOH intermediates. Subsequent cleavage of C–O bonds yields adsorbed *CO species, which underwent progressive hydrogenation through surface-bound reactive hydrogen species to form CH4 and CH3OH.

Carrier dynamics

Through in situ Kelvin probe force microscopy (KPFM) coupled with controlled light illumination, we directly visualized the spatial-temporal dynamics of photoinduced charge carriers in BBO-4. Under dark conditions, surface potential line profiles revealed distinct differences among the materials: Bi2O3 and BBO-4 exhibited surface potentials of 0.50, 0.97 mV, respectively (Fig. 5a, b). Light irradiation induced dramatic surface potential reduction in BBO-4 (ΔV=0.62 mV, Fig. 5c), demonstrating efficient photoinduced electron migration to the material surface under built-in electric field guidance. Notably, BBO-4 demonstrated the much more pronounced surface potential heterogeneity, suggesting enhanced charge redistribution capabilities. Thickness-normalized analysis (Fig. 5c, and Supplementary Fig. 14) quantified this effect, showing normalized potential gradients of 0.21 and 0.13 mV nm−1 for BBO-4 with or without light irradiation, respectively. However, the normalized potential difference of BBO-4 decreased to 0.08 mV nm−1 (Supplementary Fig. 32), indicating that the electric field can be optimized by appropriate Bi content1. Ohmic junction generated favorable localized electric fields that facilitate electron accumulation. This light-responsive behavior confirmed that BBO-4 Ohmic junction enhanced charge separation through two synergistic mechanisms: (1) creation of strong interfacial electric fields and (2) directional charge transport mediation, which were also supported by the photocurrent and electrical impedance spectra (Supplementary Fig. 33).

To elucidate the interfacial charge transfer mechanisms in Ohmic junction systems, we conducted comparative femtosecond transient absorption (fs-TA) spectroscopy studies on Bi2O3 and BBO-4 heterostructures. Bi2O3 (Fig. 5d, e) and BBO-4 (Fig. 5g, h) demonstrated extended photoinduced absorption spanning 450–570 nm. The absorption intensity of BBO-4 was stronger than that of Bi2O3, indicating enhanced light-matter interactions in BBO-4 Ohmic junction system through interfacial electronic state hybridization. Figure 5f–i reveals the fs-TA kinetic profiles at 550 nm of Bi2O3 and BBO-4, and their signal can be well-fitted with triexponential results. The fs-TA spectra of Bi2O3 and BBO-4 were fitted by a tri-exponential function and the fitting parameters are τ1=0.191, τ2=6.53, τ3=1490 ps for Bi2O3, and τ1=0.366, τ2=44.1, τ3=994 ps for BBO-4. Photogenerated electron relaxation in the CB of Bi2O3 and BBO-4 occurred through three pathways: exciton-mediated states (τ1), defect trap states (τ2), and recombination of e- and h+ (τ3). Notably, the lifetime components of BBO-4 (τ1 and τ2) were much longer than that of Bi2O3 (τ1=2.95 ps), which was attributed to the ineffective utilization of photoelectrons within a single photocatalyst, hindering efficient photocatalytic activity38,39. The shorter lifetime of τ3 (994 ps) for BBO-4 compared with Bi2O3 (τ3=1490 ps) was favorable to the interfacial charge transfer of BBO-4 Ohmic junction (Supplementary Fig. 34).

Reaction mechanism of CO2 photoreduction

The interfacial engineering strategy significantly enhances gas-solid interactions in the BBO-4 Ohmic junction, as evidenced by complementary gas adsorption analyzes. CO2 adsorption isotherms (Fig. 6a) demonstrated the adsorption capacities of Bi, Bi2O3 and BBO-4 were 0.3402, 0.4826, and 0.5632 cm3 g−1. This indicated that BBO-4 may provide more active sites for adsorbing and converting CO2 molecules during the photocatalytic CO2 reduction process, thus facilitating the occurrence of the reaction. The enhanced CO2 adsorption effect favored the adsorption and activation of CO2. The CO2 and CO TPD profiles (Fig. 6b, c) revealed intensified desorption signals spanning 100–450 °C, which was indicative of optimized physisorption/chemisorption site distribution. Moreover, the hydrophilicity of Bi2O3 and BBO-4 was also observed by contact angle tests. The introduction of Bi reduced hydrophilicity of BBO-4, which was beneficial to inhibit the competing reaction of H2 generation (Fig. 6d, and Supplementary Figs. 35 and 36). The interfacial engineering of BBO-4 revealed improved CO2 activation capabilities through synergistic electronic modulation, as evidenced by comprehensive adsorption analyzes (Fig. 6e). Comparative structural studies revealed that CO2 chemisorption induced characteristic molecular distortion across all models: Bi2O3, BBO-4 surface, and BBO-4 interface. BBO-4 interface exhibited larger C=O bond elongation and smaller angular deformation compared with the surface of Bi2O3 and BBO-4, confirming effective CO2 polarization on BBO-4 Ohmic junction. The Bader charge analysis displayed that 0.381 e of BBO-4 interface transferred to CO2 during CO2 adsorption, which were much more than those of Bi2O3 (0.042 e) and BBO-4 surface (0.224 e). The CO2 adsorption energy of BBO-4 interface (−0.52 eV) was larger than that of Bi2O3 (−0.47 eV) and BBO-4 surface (−0.49 eV). These results confirmed that the interface effect was more conducive to CO2 activation in CO2 photoreduction process.

Figure 6f–i, and Supplementary Figs. 37–40 display the free energy profiles and the corresponding intermediate configurations. For the models of Bi, Bi2O3, BBO with metallic Bi site, and BBO with interfacial site, from a thermodynamic standpoint, COOH* formation represented a rate-limiting step in the CO2 reduction process with free-energy barriers of 2.07, 3.45, 1.76, and 1.51 eV, respectively. It is obvious that the Ohmic interface reduced the free-energy barrier of rate-determining step by 1.94 eV. The formation of Ohmic junction promoted charge redistribution at the interface, inducing the generation of built-in electric field, further accelerating the adsorption and activation of CO2. The *CO→CO (g) and *CO→*CHO were critical step which determine the selectivity of CO2 reaction. Due to the considerably lower free energy of *CO (g) compared to *CHO on both Bi and the metallic Bi sites in BBO, the formation of CO became the thermodynamically favored and primary product. For Bi2O3 and interface BBO, the *CHO formation energy was much lower than that of CO (g), suggesting the formation of *CHO was easier than that of CO (g). Moreover, the fabrication of the Ohmic junction interface greatly reduced the energy barrier of the rate-determining step. Experimentally and theoretically, BBO exhibited a better photocatalytic CO2 reduction performance into CO or CH3OH than Bi2O3.

Methods
Materials
Bismuth(III) nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), sodium hydroxide (NaOH, ≥96%), sodium borohydride (NaBH4, ≥98%), sodium carbonate (Na2CO3, 99.95–100.05%), nitric acid (HNO3, 65%), bismuth(Iii) Bromide (BiBr3, 99.99%), Thiocarbamide (CH4N2S, 99%), Ethylenediaminetetraacetic acid (EDTA, 99.5%), ethanol (CH3CH2OH, 99.7%) were all purchased from Beijing InnoChem Science & Technology Co., Ltd. All chemical reagents were used without further purification, and ultrapure water was used throughout the experiment.

Preparation of Bi2S3
In a typical synthetic procedure, First, 0.1455 g of Bi(NO3)3·5H2O was dissolved in 20 mL of ethylene glycol under vigorous stirring for 15 min. Subsequently, 0.114 g of CH4N2S was added to the solution and stirred vigorously for another 15 min. The mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave, sealed, and heated in an oven at 180 °C for 24 h. After natural cooling to ambient temperature, the resulting precipitate was washed separately with deionized water and ethanol, then dried at 70 °C for 8 h to obtain Bi2S3.

Preparation of Bi nanosheets
Firstly, 2.4254 g of Bi(NO3)3·5H2O was weighed and dissolved in 50 mL of deionized water under vigorous stirring for 15 min. Subsequently, 4 g of NaOH was added to the solution, and vigorous stirring was continued for another 15 min. The mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave, sealed, and heated in an oven at 120 °C for 10 h. After natural cooling to ambient temperature, the resulting precipitate was washed separately with deionized water and ethanol, then dried at 60 °C for 8 h. Next, 0.2 g of the dried sample was dissolved in a 50 mL mixed solution of deionized water and ethanol (volume ratio of ethanol to deionized water = 1:1) under vigorous stirring for 30 min. Then, 2 g of NaBH4 was added to the mixture, and stirring was continued vigorously for an additional 2 h. The final precipitate was washed separately with deionized water and ethanol, followed by drying at 60 °C for 6 h to obtain metallic Bi.

Preparation of Bi2O3 nanosheets
Firstly, 5.21 g of Bi(NO3)3·5H2O was dissolved in 27 mL of 1 M nitric acid to form solution A. Separately, 6.78 g of Na2CO3 was dissolved in 107 mL of deionized water to prepare solution B. Solution B was then added dropwise into vigorously stirred solution A, followed by continuous vigorous stirring for 15 min. The mixture was allowed to stand at 60 °C for 12 h. The resulting precipitate was washed with deionized water and ethanol, respectively, and dried at 60 °C for 6 h. The dried sample was calcined at 350 °C with a heating rate of 10 °C min−1 for 30 min to obtain Bi2O3.

Preparation of Bi19Br3S27 nanorods
2.2436 g of BiBr3, 0.3502 g of CH4N2S (thiourea), and 0.2 g of EDTA were weighed and dissolved together in 60 mL of ethanol under vigorous stirring for 1 h. The mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed, and heated in an oven at 180 °C for 72 h. After natural cooling to ambient temperature, the resulting precipitate was washed separately with deionized water and ethanol, then dried at 80 °C for 4 h to obtain Bi19Br3S27.

Preparation of Bi/Bi2O3
Bi/Bi2O3 (BBO) was synthesized via an in-situ reduction method. Specifically, 0.2 g of Bi19Br3S27 was dissolved in 50 mL of a mixed solution of deionized water and anhydrous ethanol (volume ratio of ethanol to deionized water = 1:1) under vigorous stirring for 30 min. Subsequently, 2 g of NaBH4 was dissolved in 10 mL of deionized water, and the NaBH4 solution was slowly added dropwise into the Bi19Br3S27 suspension while maintaining vigorous stirring. After complete addition, stirring was continued for 2, 4, or 6 h. The resulting precipitate was washed five times with deionized water and dried at 60 °C for 6 h to obtain Bi/Bi2O3 (BBO-2, BBO-4, and BBO-6).

Photocatalytic CO2 reduction activity measurement
In a typical reaction, 20 mg of the catalyst was dispersed in a centrifuge tube containing 2 mL of deionized water and sonicated for 5 min. The dispersed catalyst suspension was then drop-cast onto a quartz plate using a pipette, and the quartz plate was dried in a vacuum oven at 60 °C until completely dry. The catalyst-loaded quartz plate was placed on a quartz column inside a photoreactor (400 mL), followed by the addition of 5 mL of deionized water and sealing of the reactor.

High-purity CO2 (99.999%) was used as the carbon source. A multichannel atmosphere controller evacuated and purged the sealed photoreactor with high-purity CO2 10 times to ensure a CO2-saturated environment. The sealed photoreactor was then irradiated under a 300 W xenon lamp for 4 h. Gas samples (1 mL) were collected hourly using a microsyringe and analyzed by gas chromatography (GC) equipped with thermal conductivity (TCD) and flame ionization (FID) detectors. The CH3OH product quantitative analysis was employed on an on-line gas chromatograph (GC-Agilent Technologies 8860, USA) equipped with thermal conductivity detector (TCD) and a flame ionized detector (FID). Furthermore, FID detector was connected with a HP-PLOT Q capillary column (Agilent, length 30 m, ID 0.53 mm, film 40 μm) for separation of C1-C6 paraffin and olefin hydrocarbons, alcohols and oxygenated compounds. The quantification of the products was based on the external standard and the use of a calibration curve.

Electrochemical and photoelectrochemical measurements
Transient photocurrent response, electrochemical impedance spectroscopy, and Mott–Schottky measurements were performed using a CHI760E electrochemical workstation (CHI, China) in a three-electrode quartz cell. A 300 W xenon lamp (MC-PF300C, Beijing Merry Change Technology Co., Ltd, China) served as the light source. The three-electrode system consisted of an Ag/AgCl reference electrode, a Pt wire counter electrode, and a working electrode made from the photocatalyst-coated fluorine-doped tin oxide (FTO, 1.5 × 2.2 cm). The electrolyte was 0.5 M Na2SO4 aqueous solution. The working electrode was fabricated as follows: 10 mg of the as-prepared photocatalyst was dispersed in 1 mL of ethanol, followed by the addition of 40 μL of Nafion solution. The mixture was ultrasonicated for 10 min. Then, 400 μL of the resultant suspension was drop-cast onto the FTO substrate and dried under near-infrared (NIR) irradiation.

Computational method
The first-principle calculations were carried out within DFT, using the Vienna Ab Initio Simulation Package code40. The interaction between atomic cores and valence electrons was described by the frozen-core projector augmented-wave method41, while the electron-electron exchange and correlation interactions were treated with the generalized gradient approximation in the form of the Perdew–Burke–Ernzerhof functional42. A kinetic energy cutoff of 500 eV was adopted to ensure full convergence of the calculations. Supplementary Data 1–4 for details on the models of Bi, Bi2O3, and Bi/Bi2O3 heterojunction with different sites have been built based on a collection of characterization results. Owing to the large cell of Bi (012)/Bi2O3 (100) heterostructure, the Brillouin zone was sampled with the k-grid of 2 × 2 × 1. Structural relaxation was performed using the conjugate gradient (CG) method, by simultaneously minimizing the total energy and interatomic forces. The convergence threshold for the total energy was set to 10−5 eV. To properly describe van der Waals interactions, the dispersion-corrected DFT-D3 scheme was employed. In addition, a vacuum layer of 1.5 nm was introduced along the c-axis to avoid spurious interactions43. Gibbs free energies for each gaseous and adsorbed species were calculated at 298.15 K, according to the expression:

G = E_DFT + E_ZPE − TS

E_ZPE = Σ (1/2) ℏvi

S = Σ R [ln(1 − e−Θi/T)−1 + (Θi/T) eΘi/T − 1 − 1]

where E_DFT is the electronic energy calculated for specified geometrical structures, E_ZPE the zero-point energy, S is the entropy, ℏ is the Planck constant, vi is the computed vibrational frequencies, Θi is the characteristic temperature of vibration, k is the Boltzmann constant, and R is the molar gas constant. For adsorbates, all 3N degrees of freedom were treated as frustrated harmonic vibrations.

For each step, the ΔG is the difference of product or intermediate and reactant.

ΔG[CH3OH*] = G[CH3OH*] + G[H2O(g)] − (G[*] + G[CO2] + 6*G[H+ + e−])

ΔG[CH3O*] = G[CH3O*] + G[H+ + e−] + G[H2O(g)] − (G[*] + G[CO2] + 6*G[H+ + e−])

ΔG[CH2O*] = G[CH2O*] + 2*G[H+ + e−] + G[H2O(g)] − (G[*] + G[CO2] + 6*G[H+ + e−])

ΔG[CHO*] = G[CHO*] + 3*G[H+ + e−] + G[H2O(g)] − (G[*] + G[CO2] + 6*G[H+ + e−])

ΔG[CO*] = G[CO*] + 4*G[H+ + e−] + G[H2O(g)] − (G[*] + G[CO2] + 6*G[H+ + e−])

ΔG[COOH*] = G[COOH*] + 5*G[H+ + e−] − (G[*] + G[CO2] + 6*G[H+ + e−])

G[H+ + e−] = 1/2 G[H2] − eU

where * is the substrate, U is the applied overpotential and e is elementary charge. In this study, U=0 V versus reversible hydrogen electrode.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Bi19Br3S27 was used as the precursor to obtain Bi/Bi2O3 catalyst by reduction. The interfacial Bi sites facilitate CO adsorption and activation to form *COOH, lower the activation barrier by 0.6 eV through enhanced orbital hybridization between Bi 6p and O 2p states. This study is innovative in reducing CO2 to liquid products, but there are still some areas to be improved. I suggest accepting this article after the author addresses the following issues.\n1. whether Bi/Bi2O3 is a heterojunction or an ohmic junction, please confirm and explain.\n2. How catalyst of Bi/Bi2O3 inhibits reverse electron transfer?\n3. Syntax error in line 24.\n4. In the first paragraph of the results and discussion section, why did the authors first discuss the adsorption of *CO (Figure 1b) rather than CO2 (Figure 1c,d)?\n5. Language error on line 134.\n6. LSPR effect can improve the surface temperature of BBO. Is the CO2 reduction reaction in this work photocatalytic or photothermal catalysis?\n7. Whether 5 mL of water added during the catalytic reaction is in contact with the catalyst.\n8. Can high-resolution mass spectrometry of methanol be obtained?\n9. Some articles with strong relevance can be cited, DOI: 10.1021/acscatal.2c05724, 10.1039/d4ee01376j, 10.1016/j.apcatb.2023.123230.", "reviewer_2": "The manuscript constructed an Ohmic junction at the Bi/Bi2O3 interface, achieving an enhanced photocatalytic CO2-to-methanol conversion performance with a production rate of 610 μmol·g-1. The induced Ohmic junction with local surface plasmon resonance effect enhanced built-in electric field and drove the electron transfer. The interfacial Bi sites was active centers for CO2 activation toward *COOH intermediate formation, with m-CO32- being key intermediate. However, the evidence in some arguments is insufficient, and there are some deficiencies in the characterization and analysis. Therefore, in my opinion, the manuscript is currently not suitable for publication in Nature Communications unless the following issues are addressed.\n1.Heterojunctions such as S-scheme and Schottky junctions, etc have been reported to facilitate charge migration. How does the Ohmic junction proposed in this work differ from these established heterojunctions? The authors should provide precise definitions and evidence to demonstrate the formation mechanisms and inherent advantages of Ohmic junctions in charge transfer efficiency compared to conventional heterojunction systems.\n2.Bi@Bi2O3 composites have been previously reported (Adv. Funct. Mater. 2023, 33, 2301984). The innovations and scientific significance of this study in terms of synthetic methodology, structural characteristics, and catalytic performance enhancements should be clarified.\n3.The charge transfer direction in Bi@Bi2O3 was inferred through theoretical calculations (differential Bader charge and work function). However, direct experimental evidence of photo-induced electron-hole pair separation and directional migration under illumination was lacking.\n4.The XRD diffraction peaks of BBO-2, BBO-4, and BBO-6 did not exhibit satisfactory correspondence with the standard patterns of Bi or Bi2O3. Notably, the diffraction peaks below 2θ of 25° demonstrated a progressively enhanced intensity trend, suggesting the potential presence of an unidentified phase. Regarding the two peaks observed in the 30-35° range for BBO-2 (presumably attributed to Bi2O3), the significant negative shift required further explanation. To precisely resolve the phase composition, I recommend performing Rietveld refinement of the XRD patterns for quantitative phase analysis.\n5.Given the morphological similarities observed in BBO-2, BBO-4, and BBO-6 samples, quantitative EDS elemental analysis and ICP-OES are necessary to verify compositional variations, particularly regarding Bi content.\n6.The catalyst demonstrated visible-NIR light absorption. How about the photocatalytic performance under visible-light condition? In addition, the photocatalytic data and XRD patterns for BBO-3 and BBO-5 should be provided to obtain more refined results.\n7.The main product of photocatalytic CO2 reduction using Bi or Bi2O3 alone was CO, whereas their composite produced CH3OH. What caused this change in product selectivity?\n8.In the mass spectrum, why was there no signal at m/z 32? Please detailedly explain this issue.\n9.Quantitative 1H NMR analysis for methanol detection required comprehensive methodological description. In addition, NMR and GC spectra should be provided in supporting information.\n10.In addition to the CO, CH4, and CH3OH products discussed by the authors, other C1 products such as HCOOH and HCOH may also form in photocatalytic CO2 reduction. The authors need to further investigate the product selectivity through experiments and DFT calculations. Furthermore, how was the qualitative and quantitative analysis of CH3OH performed? The authors should provide more detailed descriptions and evidence regarding its identification and quantification methodology.\n11.The assignment of intermediate species in in situ DRIFTS spectra lacked sufficient evidence. Some intermediate species labeled by authors were not clearly discernible in the spectra. For example, the *CO species for Bi and Bi2O3, and the *CHO and b-CO32- species for BBO-4. In addition, what was the reason for the distinct absorption peak of the *CO species in BBO-4 at 0 min? Could this be attributed to impurities in the CO2 gas used?\n12.The author claimed that the m-CO32- species was the main intermediate for CO2 reduction by Bi@Bi2O3, with the proposed pathway shown in Figure 4g. However, these speculations required additional theoretical justification. Moreover, why is the *COOH species not directly formed through CO2 accepting protons and electrons at the reaction site?\n13.The SEM, TEM, and AFM data for Bi2O3 need to be provided. What does the difference in KPFM between Bi2O3 and BBO-4 indicate? The decrease in ΔV for BBO-4 under illumination suggests electron migration under light. However, how can the direction of electron migration be experimentally verified? If Bi@Bi2O3 exhibits a distinct interface, monitoring the potential differences at the interface under light and dark conditions via in situ KPFM could serve as more compelling evidence.\n14.The DFT results in Figures 6f-h showed gaseous CO and CH₃OH products at different energy levels. Thermodynamic said that Gibbs free energy is a state function. If one starts off with a CO2 reactant and ends with a CO product, the energy difference will be the same. Then why is the CO and CHOH products at different energy?\n15.It is recommended to perform synchrotron radiation characterization to thoroughly investigate the differences in coordination environments between Bi@Bi2O3 and single-phase materials, thereby further elucidating the structure-activity relationship.", "reviewer_3": "The authors synthesised a bismuth–bismuth oxide composite catalyst and evaluated its performance for methanol production via photocatalytic CO₂ reduction. The material appears attractive, demonstrating superior performance compared to other materials. However, I believe major revisions are required prior to publication.\nFirst, before addressing the scientific content, I would like to point out some issues regarding the overall presentation of the manuscript.\nThere are several instances where the font size in the figures is too small, which hampers readability. I recommend improving the clarity of the figures to enhance their utility in the manuscript. For example, in Figs. 1b, 1c, 1d, 1f, Figs. 2l, 3a, and 4a–f. In addition, the scale bars in Fig. 2c–e are not clearly visible. Figure 1 and Figure 6 both appear to describe the reaction mechanism of CO₂ reduction, but the differences between the two are unclear. I suggest clarifying these points.\nFurthermore, there is no explanation of the IPDOS in Fig. 1c, making it difficult to interpret. Although the caption of Fig. 1d,e refers to PDOS and IPDOS (line 127), they appear to be entirely different figures.\nRegarding Fig. 3a, it is unclear what “DRS” stands for in “UV-vis-NIR DRS” (line 224). Are they absorption spectra? The vertical-axis label does not make clear whether it refers to absorption or transmission. If the authors are insisting that BBO-4 exhibits the strongest absorption, then an arbitrary unit (a.u.) scale is insufficient. A quantitative evaluation is required.\nAdditionally, the caption and main text explanation of Fig. 3b appear insufficient. It would be helpful to show the sample used for the measurement and the meaning of the colours (they might be included in the figure, but the text is unreadable).\nPlease add a caption for Fig. 5c.\nFor Fig. 5d and 5g, the vertical axis labelled “ΔA” is undefined, which makes interpretation difficult. The main text states, “The absorption intensity of BBO-4 was stronger than that of Bi₂O₃,” (line 295) but it is unclear from which data this conclusion was drawn. Regarding Fig. 6a, is this a BET measurement? If so, further explanation is required. In Figs. 6b and 6c, the term “CO₂(CO) TPD intensity” is unclear—does this refer to the signal intensity of desorbed CO₂ (CO) species identified by a suitable method, or is it the total amount of evolved gases?\nFor Supplementary Figure S2a–c, an explanation of the differences between the data sets is needed, as the graphs are difficult to understand for various readers. The same applies to Supplementary Figures S3–S5. For the elemental mapping in Supplementary Figures S8–S11, please clearly indicate which elements are being mapped. Is there a reason for the different ordering between S9 and S10–S11? For the EDS mappings, it would be helpful to include spectra showing the mapped regions.\nSupplementary Figure S12 is labelled as an AFM image, but the presence of colourful features does not resemble a conventional AFM image. What do the colours represent? Please include a scale bar (AFM image). In Fig. S12b, which line in Figure S12a was used for the line profile? Although the instrument is identified as an Atomic Force Microscope (AFM, Bruker Dimension ICON), details of the substrate used and measurement conditions (e.g., cantilever type) should also be provided. The same applies to Supplementary Figures S13 and S14.\nIn Supplementary Figure S15, the sample is labelled as BBO-4 in the image but the caption refers to BBO-6. Please clarify which is correct.\nFor Supplementary Figure S16, an explanation of the sample's location and state within the photograph is needed. Is the entire image the sample? Please also include information on the sample’s thickness, substrate used, and the wavelength and intensity of the irradiation light.\nPlease add captions for Supplementary Figure S18a–h. What do these figures represent? In Supplementary Figure S21, the value “0.1” is shown—what does this number represent? Please include appropriate units.\nIn the main text and Fig. 2, the reducing agent is written as “NaHB₄.” Are they a typographical error for “NaBH₄”? Additionally, is “Na213CO₂” in the Supplementary Information correct?\nRegarding the scientific content, I have some concerns:\n1) The title refers to “Atomic-Scale Interface Engineering,” but are the samples truly engineered at the atomic level with controlled numbers of atoms and structures? Theoretical calculations (Figs. 1 and 6) appear to be based on Bi clusters of several atoms, but do these correspond to the experimental results? In the TEM images shown in Figs. 2f and 2g, the Bi metal particles appear to be quite large. Furthermore, in Fig. 2k, distinct XRD peaks corresponding to crystalline Bi metal are observed, which seems inconsistent with the amorphous Bi clusters described in Figs. 1 and 6. Also, in the XRD pattern of BBO-6 in Fig. 2k, there appear to be no observable peaks attributed to Bi₂O₃. Is this not a concern?\n2) Further explanation regarding the experimental methods appears necessary. According to the Supplementary Information, BBO was synthesised by the reduction of Bi₁₉Br₃S₂₇ using aqueous NaBH₄, added dropwise. However, NaBH₄ reacts quickly with water to produce hydrogen gas. Although both hydrogen and unreacted NaBH₄ may serve as reducing agents, the described method may lead to poor reproducibility. It may be helpful to provide a more detailed synthesis procedure for better reproducibility.\nConcerning phase identification, the presence of Bi metal and Bi₂O₃ is confirmed, but is sulphur absent? It seems plausible that a Bi–S compound such as Bi₂S₃ might be formed. Clarifying the presence or absence of sulphur in the sample, and, if possible, including a reaction equation, would make the synthetic process clearer.\nIn Fig. 3a, if Bi metal is included in the sample, it should not transmit light. The method used to obtain this spectrum needs to be described.\n3) The authors argue that Bi-metal part in BBO is negatively charged based on XPS (line 164-170), but the observed shift in binding energy appears to be in the positive direction (Figure 2l). Generally, contact with oxides results in positive shift and an increase in binding energy. Line 167 mentions a 0.06 eV shift, but it is unclear which data this refers to. In Fig. 2l, the shift appears to be either 0.12 eV or 0.07 eV. Although the authors discuss subtle changes, it is unclear whether charging effects were properly corrected. What calibration method was used? The charge accumulation characteristics differ significantly between Bi metal and Bi₂O₃, so binding energies are expected to change substantially. As it stands, I am not confident that the current data can support conclusions about charge transfer.\n4) In Figs. 5f and 5i, the authors evaluate lifetimes based on time-resolved decay curves and conclude that BBO-4 exhibits a longer lifetime. However, visually, Bi₂O₃ appears to have a longer lifetime. Please provide details on the fitting process. Also, the fitting was performed with three components—what is the basis for this approach?\n5) Regarding the discussion of ohmic contact:\n5-1) The authors state that electron transfer from Bi₂O₃ to metallic Bi within the ohmic junction was confirmed. However, when electrons flow from a semiconductor to a metal, I think a depletion region typically forms, making ohmic contact unlikely. Could the authors clarify this point?\n5-2) The authors further describe that well-defined BBO-4 Ohmic junction were verified by high resolution TEM. (line 143) However, it may be difficult for me to determine whether ohmic contact has been formed solely based on the TEM images. A more detailed explanation would be appreciated.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "This work demonstrates that constructing an Ohmic junction at the BBO interface synergistically enhances photocatalytic CO2-to-CH3OH conversion by optimizing charge dynamics and interfacial energetics. The metallic Bi–semiconductor Bi2O3 Ohmic junction induces a robust built-in electric field and unidirectional electron transfer, suppressing charge recombination while promoting CO2 activation and methanol synthesis. This study provides mechanistic insights into Ohmic junction engineering for photocatalytic CO2 conversion, emphasizing its dual role in charge management and surface reaction optimization. However, the methanol production rate of the optimized catalyst is only 610 μmol g-1, which is too low among the reported results, the analysis of some characterizations and reaction mechanisms remains unclear. The manuscript remains in need of significant improvement across several dimensions. It is not suitable for publication in the journal of Nature Commun.\n1.From line 143 to 145, the HRTEM analysis did not provide additional characterization of the formed Bi metal particles, including particle size distribution, etc.\n2.From line 164 to 170, \"These XPS results collectively suggest electron transfer from Bi₂O₃ to metallic Bi within the heterojunction, consistent with the electron flow direction in an Ohmic junction.\" However, a binding energy shift of less than 0.1 eV cannot serve as strong evidence for charge transfer.\n3.From line 171 to 176, why are there strong Bi2O3 peaks in the XPS spectrum of metallic Bi in Figure 2I? This needs to be clearly explained.\n4.From line 209 to 212, although the specific surface area of metallic Bi is much smaller than that of Bi2O3, why does the BBO-4 exhibit a higher specific surface area compared to pure Bi2O3?\n5.From line 253 to 265, Figure 4g shows that CO2 reacts with OH groups on Bi to form CO32-. It is necessary to clarify the origin of the OH groups: whether they arise from water splitting or protonation of O in Bi2O3.\n6.From line 276 to 280, the authours mentioned using normalized potential gradients, but Figures 5-7 do not present the normalized potential gradient data. Please specify the data source of the normalized potential gradient or the methodology used to derive the normalization.\n7.From line 322 to 326, Bi2O3 typically contains oxygen vacancies, which may also act as active sites influencing catalytic activity. Therefore, it is necessary to confirm whether the reactive sites are localized at the BBO-4 interface rather than at oxygen vacancies.\n8.The catalytic performance must be supplied in the abstract to clearly present the activity of the catalyst. Is it a high level of the methanol yield in the reported results, a detail comparison should be supplied."}, "author_response": "Response to Reviewers\nTitle: Atomic-Scale Interface Engineering in Bi/Bi O Heterojunctions for Selective\n2 3\nCO Photoreduction to Methanol\nGeneral response:\nWe thank the editor and reviewers for their valuable and insightful comments and\nsuggestions, which have significantly helped improve the quality of our manuscript. We\nhave revised the manuscript in response to these comments. Our point-by-point\nresponses are provided below.\nReviewer #1 (Remarks to the Author)\nIn this work, Bi Br S was used as the precursor to obtain Bi/Bi O catalyst by\n19 3 27 2 3\nreduction. The interfacial Bi sites facilitate CO adsorption and activation to form\n2\n*COOH, lower the activation barrier by 0.6 eV through enhanced orbital hybridization\nbetween Bi 6p and O 2p states. This study is innovative in reducing CO to liquid\n2\nproducts, but there are still some areas to be improved. I suggest accepting this article\nafter the author addresses the following issues.\nResponse: We sincerely thank the reviewer for their insightful and thorough evaluation\nof our work. Following their suggestions, we have carefully revised the manuscript to\nenhance its quality and clarity. Key improvements include a more thorough discussion\nof the samples' structure and chemical bonding states. All revisions are highlighted in\nyellow in the revised manuscript and Supplementary Information (SI). Our point-by-\npoint responses to the specific comments are detailed below.\nComment 1. whether Bi/Bi O is a heterojunction or an ohmic junction, please confirm\n2 3\nand explain.\nResponse: We thank the reviewer for raising this important point regarding the nature\nof the junction formed between Bi and Bi O in Bi/Bi O composites. The\n2 3 2 3\n1\nheterojunction is defined as the interface formed at the contact region between two\ndifferent semiconductors. The ohmic junction is a type of Mott-Schottky junction\nformed by the interface between a metal and a semiconductor (Chemical Reviews, 2022,\n123.1). The metals would form ohmic junction without any Schottky barriers if their\nwork functions aligned with the conduction or valence band edges of semiconductors.\nIn an ohmic junction, the Fermi levels of the metal and the semiconductor are nearly\naligned, resulting in negligible potential barriers for carrier transport.\nIn this manuscript, we verified the correctness of the Ohm junction structure\nthrough the calculation results of the work function, Fermi energy level and the charge\ndensity difference. The theoretical work functions of Bi and Bi O were calculated to\n2 3\nbe 4.78 and 5.15 eV, respectively (Figure R1). The much smaller work function of Bi\npromoted the charge transfer from Bi to Bi O . This electron transfer drives the\n2 3\nalignment of their Fermi levels and concurrently generates a strong built-in electric field\nwith associated band bending, which is the characteristic of the ohmic junction. This\nevidence confirms the establishment of an ohmic junction at the Bi/Bi O interface.\n2 3\nFigure R1 | (Figures 1f and g, in the revised manuscript). (f) Theoretical work\nfunction of the surface of Bi (012) and Bi O (100). (g) The electron transfer mechanism\n2 3\nof BBO Ohmic junction, thereby enabling the generation of CH OH.\n2 3\nResponse: Thank you for the detailed review. In Bi/Bi O Ohmic junction, metallic\n2 3\nbismuth (Bi0) has a relatively low work function (~4.78 eV) compared with Bi O , as\n2 3\n2\nevidenced by our density functional theory (DFT) calculations. Upon forming an\nintimate heterojunction, depicted in Figure R, the free electrons in Bi migrate to Bi O ,\n2 3\nreducing the Fermi level (E) in Bi and increasing the Fermi level of Bi O until\nf 2 3\nequilibrium is reached. This electron transfer results in a positive charge on Bi surface\ndue to electron depletion and a negative charge on Bi O due to electron accumulation.\n2 3\nThe charge distribution establishes an IEF from Bi to Bi O , facilitating the separation\n2 3\nand migration of photogenerated carriers (Supplementary Figure 8).\nFigure R2. | (Supplementary Figure S8, in the revised supporting information).\nSchematic illustration of IEF-induced Bi/Bi O Ohmic-junction for CO\n2 3 2\nphotoreduction.\nComment 3. Syntax error in line 24.\nResponse: Thank you for the constructive suggestion. The required descriptions in the\nmentioned manuscript have been carefully checked and corrected. The revised related\nsyntax have been added to the revised manuscript in line 24: “This electronic\nredistribution at the Ohmic junction interface concurrently stabilizes *CHO\nintermediates, enabling a CH OH production rate of 610 μmol g-1 under light irradiation,\n3\nwhich was much higher than that of Bi, Bi O , and most reported researches.”\n2 3\nComment 4. In the first paragraph of the results and discussion section, why did the\nauthors first discuss the adsorption of *CO (Figure 1b) rather than CO (Figure 1c,d)?\n3\nof photocatalytic reduction of CO to CH OH is summarized as follows (J. Am. Chem.\n2 3\nSoc. 2024, 146, 26478−26484):\n* + CO2 + H+ + eˉ → *COOH (1)\n*COOH + H+ + eˉ → *CO + H O (2)\n2\n*CO + H+ + eˉ → *CHO or *CO → CO ↑ (3)\n*CHO + H+ + eˉ → *CH O (4)\n2\n*CH O + H+ + eˉ → *CH O (5)\n2 3\n*CH O + H+ + eˉ → *CH OH (6)\n3 3\nIn photocatalytic CO reduction systems, the formation of *CO intermediate\n2\nconstitutes the pivotal kinetic gate for steering reaction pathways toward CH OH.\nthe selectivity and activity of the catalyst for CH OH generation.\nResponse: Thank you for the constructive suggestion. The required descriptions in the\nmentioned manuscript have been carefully checked and corrected. The following\nrevisions are made as suggested.\nThe revised related revision have been added to the revised manuscript in line 134:\n“The Bi Br S nanowires have been firstly synthesized by hydrothermal method, then\n19 3 27\nfollowed by a NaBH reduction method to inducing the dissolution and crystallization\n4\nof Bi species, and finally the BBO nanosheets were obtained.”\nComment 6. LSPR effect can improve the surface temperature of BBO. Is the CO\n2\nreduction reaction in this work photocatalytic or photothermal catalysis?\nResponse: We are grateful for the valuable feedback from the reviewer. In most\nplasmon-enhanced CO reduction systems, the LSPR effect primarily acts as a\n2\nphotocatalytic booster rather than a pure photothermal driver. The increased surface\ntemperature supplements photocatalysis by improving charge kinetics but rarely\nbecomes the sole energy input. Fundamentally, CO reduction reaction is a\n2\n4\nphotocatalytic process in this work.\nComment 7. Whether 5 mL of water added during the catalytic reaction is in contact\nwith the catalyst.\nResponse: We appreciate the reviewer’s attention to detail. The 5 mL of deionized\nwater added during the reaction did not directly contact the catalyst. The detailed\nexperimental procedure is as follows: A quartz sheet loaded with the catalyst was placed\non the quartz column inside the photoreactor (400 mL). Subsequently, 5 mL of\ndeionized water was introduced, and the system was sealed. High-purity CO (99.999%)\n2\nwas used as the carbon source. Using a multi-channel atmosphere controller, the sealed\nphotoreactor was evacuated and refilled with high-purity CO for 10 cycles to ensure\n2\nan inert atmosphere. In each set of experiments, photocatalytic reactions were\nconducted under irradiation from a 300 W xenon lamp for 4 hours. After the reactions\nwere completed, the resulting products were analyzed using gas chromatography\nequipped with both TCD and FID detectors (Supplementary Figure S25).\nIn response to the reviewer’s comments, now the more detailed descriptions of the\nphotocatalytic CO reduction process have been presented in Supplementary Figure\nFigure R3 | (Supplementary Figure S25, in the revised supporting information). (a)\nThe process of weighing using an analytical balance, (b) The ultrasonic dispersion\nprocess, (c) Catalyst droplet process, (d) Drying process, (e) The assembly procedure\n5\nof the photoreactor, (f) The testing process for photocatalytic CO reduction, (g) The\n2\nproduct collection process, (h) Analyze the production process using GC.\nComment 8. Can high-resolution mass spectrometry of methanol be obtained?\nResponse: Yes, high-resolution mass spectrometry of methanol (CH OH) is feasible\n3\nand well-documented. The absence of an m/z 33 signal is a direct consequence of our\nexperimental design employing 13C-labeled CO (13CO ) as the sole carbon source.\n2 2\nMethanol produced under these conditions is fully labeled as 13CH OH, with a\n3\nmolecular mass of 33 (m/z 33: 13C [13] + 3H [3] + 16O [16] + H [1]). Moreover, 13CO\nand 13CH were also detected. The isotopic labeling experiment demonstrated that the\n4\n13CH OH, 13CO and 13CH produced arose from the photocatalytic CO reduction\n3 4 2\nreaction.\nFigure R4 | (Supplementary Figure S29, in the revised supporting information;\nFigure 3g, in the revised manuscript). (a) Total ion chromatography and (b) Mass\nspectrum of CO photoreduction products using 13CO as the CO source.\n2 2 2\nComment 9. Some articles with strong relevance can be cited, DOI:\n10.1021/acscatal.2c05724, 10.1039/d4ee01376j, 10.1016/j.apcatb.2023.123230.\nResponse: We sincerely appreciate the reviewer’s valuable suggestion regarding the\ninclusion of highly relevant articles. We have carefully reviewed the recommended\nliterature with the following DOIs: 10.1021/acscatal.2c05724, 10.1039/d4ee01376j,\n10.1016/j.apcatb.2023.123230.\nsupport for mechanism discussed and catalyst design approach. We have therefore cited\nthese articles appropriately in the revised manuscript.\nThe manuscript constructed an Ohmic junction at the Bi/Bi O interface, achieving an\n2 3\nenhanced photocatalytic CO -to-methanol conversion performance with a production\n2\nrate of 610 μmol·g-1. The induced Ohmic junction with local surface plasmon resonance\neffect enhanced built-in electric field and drove the electron transfer. The interfacial Bi\nsites were active centers for CO activation toward *COOH intermediate formation,\n2\nwith m-CO 2- being key intermediate. However, the evidence in some arguments is\n3\ninsufficient, and there are some deficiencies in the characterization and analysis.\nTherefore, in my opinion, the manuscript is currently not suitable for publication in\nNature Communications unless the following issues are addressed.\nResponse: We sincerely appreciate the reviewer's positive feedback and recognition of\nour work. In response to their comments, we have carefully revised the manuscript and\nSupplementary Information. All changes have been clearly marked in the revised\ndocuments. Our detailed responses to each specific comment are provided below.\nComment 1. Heterojunctions such as S-scheme and Schottky junctions, etc have been\nreported to facilitate charge migration. How does the Ohmic junction proposed in this\nwork differ from these established heterojunctions? The authors should provide\nprecise definitions and evidence to demonstrate the formation mechanisms and inherent\nadvantages of Ohmic junctions in charge transfer efficiency compared to conventional\nheterojunction systems.\nResponse: We sincerely thank the reviewer for this insightful question, which allows\nus to clarify the fundamental distinctions and unique advantages of Ohmic junctions in\nour system. Below, we address the query through precise definitions, formation\nmechanisms, and comparative evidence:\n1. Definitions and key differences\nOhmic Junction: An Ohmic junction is characterized by negligible interfacial energy\nbarriers and bidirectional ohmic conductivity. Charge transfer occurs without\nrectification, enabling low-resistance pathways for both electrons and holes due to\nS-scheme heterojunction: Involves staggered band alignment where interfacial\nelectric fields selectively recombine low-energy carriers, preserving high-potential\ncharges for redox reactions. Charge flow is directional and rectified.\nSchottky junction: Forms a rectifying barrier at metal/semiconductor interfaces due\nto Fermi-level misalignment, creating a depletion region that impedes carrier flow.\nOhmic junctions minimize recombination losses by eliminating energy barriers,\nresulting in negligible potential barriers for carrier transport. Whereas S-\nscheme/Schottky junctions rely on built-in fields or barriers that inherently resist carrier\nflow.\nIn this manuscript, we verified the correctness of the Ohm junction structure\nthrough the calculation results of the work function, Fermi energy level and the charge\ndensity difference. The theoretical work functions of Bi and Bi O were calculated to\n2 3\nbe 4.78 and 5.15 eV, respectively (Figure R1). The much smaller work function of Bi\npromoted the charge transfer from Bi to Bi O . This electron transfer drives the\n2 3\nalignment of their Fermi levels and concurrently generates a strong built-in electric field\nwith associated band bending, which is the characteristic of the ohmic junction. This\nevidence confirms the establishment of an ohmic junction at the Bi/Bi O interface.\n2 3\nFigure R1 | (Figures 1f and g, in the revised manuscript). (f) Theoretical work\nfunction of the surface of Bi (012) and Bi O (100). (g) The electron transfer mechanism\n2 3\nof BBO Ohmic junction, thereby enabling the generation of CH OH.\n3\n2 3\n2023, 33, 2301984). The innovations and scientific significance of this study in terms\nof synthetic methodology, structural characteristics, and catalytic performance\nenhancements should be clarified.\nResponse: We appreciate the reviewer’s suggestion to help us highlight the innovation\nand advantages of this work.\n(1) Synthetic methodology: Bi@Bi O composites that been published in Adv. Funct.\n2 3\nMater. (2023, 33, 2301984) were synthesized by chemical replacement method. The\nobtained Bi@Bi O composites were composed of numerous several hexagonal and\n2 3\ntriangular sheets with the side length from 500 nm to 1 µm. In this work, Bi@Bi O\n2 3\n(BBO) composites were fabricated by wet-chemical cascade strategy. The Bi Br S\n19 3 27\nnanowires have been synthesized firstly by hydrothermal method, then followed by a\nNaBH reduction method to inducing the dissolution and crystallization of Bi species,\n4\nand finally the BBO composites were obtained. The TEM images (Figures 2d-e;\nSupplementary Figures S9-14) indicated the ultrathin nanosheet morphology of the\nsamples, showing lager specific surface area than Bi and Bi O (Supplementary Figure\n2 3\nS19), which was favorable to provide much more active sites.\n(2) Structural characteristics: In this work, we primarily investigated the\nphotocatalytic activity of the catalyst. Thus, the main structural characterization was\ndesigned to focus on its optical characterizations, such as: UV-Vis-NIR diffuse\nreflection spectrum, In-situ KPFM, and femtosecond transient absorption spectroscopy,\nto evaluate the light-harvesting ability and separation efficiency of photogenerated\ncarriers. Moreover, wo emphasized the function of BBO Ohmic junction. It showed\nstrong local surface plasmon resonance effect (LSPR) which facilitate photocatalytic\nCO reduction through three synergistic mechanisms: broadband light harvesting, hot\n2\ncarrier engineering, electromagnetic field amplification, and thermally assisted\nphotocatalysis. The reported work (Adv. Funct. Mater. 2023, 33, 2301984) mainly\nhighlighted in situ morphology reconstruction from Bi@Bi O nanodendrites\n2 3\n(Bi@Bi O -NDs) to Bi nanoflowers (Bi-NFs).\n2 3\n10\n(3) Catalytic performance enhancements: Moreover, the Ohmic junction optimized\nthe interfacial electron density, lowering the activation barrier for the *CO → *COOH\n2\nrate-determining step by 0.6 eV through enhanced orbital hybridization between Bi 6p\nand O 2p states. This electronic redistribution at the Ohmic junction interface\nconcurrently stabilizes *CHO intermediates, enabling a CH OH production rate of 610\n3\nμmol g-1 under light irradiation, which was much higher than that of Bi, Bi O , and\n2 3\nmost reported researches. The reported work (Adv. Funct. Mater. 2023, 33, 2301984)\nshowed extensive surface reconstruction of Bi@Bi O -NDs renders the realization of\n2 3\ntailored Bi-NFs electrocatalyst that maximized the number of exposed active sites and\nactive component (Bi0), which was conducive to the adsorption and activation of CO\n2\nand accelerated electron transfer process.\nThe above results demonstrated that structural reconstruction offers an effective\nstrategy to achieve enhancement of photo-/electro catalytic activity.\nComment 3. The charge transfer direction in Bi@Bi O was inferred through\n2 3\ntheoretical calculations (differential Bader charge and work function). However, direct\nexperimental evidence of photo-induced electron-hole pair separation and directional\nmigration under illumination was lacking.\nResponse: Thank you for the question. The Kelvin probe force microscopy (KPFM)\nallows the detection of the surface potential, which can be used to interpret surface\ncharge transfer. (Adv. Funct. Mater. 2016, 26, 3067; Phys. Rev. B 2001, 63, 125411;\nAppl. Phys. Lett. 2009, 9, 032907). Under dark conditions, surface potential line\nprofiles revealed distinct differences among the materials the BBO-4 exhibited surface\npotentials of 0.97 mV (Figures 5b). Upon illumination, the surface potential at the same\nposition (A-B) decrease to 0.62 mV as shown in Figure 5c. The decrease in surface\npotential following illumination indicates efficient photoinduced electron migration\nfrom Bi O to Bi surface under built-in electric field guidance.\n2 3\npotential analysis of (b-c) BBO-4 in the dark and under light irradiation.\nIn response to the reviewer’s comments, we have included this discussion on line\n322-329 of the manuscript. “Through in situ Kelvin probe force microscopy (KPFM)\ncoupled with controlled light illumination, we directly visualized the spatial-temporal\ndynamics of photoinduced charge carriers in BBO-4. Under dark conditions, surface\npotential line profiles revealed distinct differences among the materials: Bi O and\n2 3\nBBO-4 exhibited surface potentials of 0.50, 0.97 mV, respectively (Figures 5a-b). Light\nirradiation induced dramatic surface potential reduction in BBO-4 (ΔV = 0.62 mV,\nFigure 5c), demonstrating efficient photoinduced electron migration to the material\nsurface under built-in electric field guidance.”\nComment 4. The XRD diffraction peaks of BBO-2, BBO-4, and BBO-6 did not exhibit\nsatisfactory correspondence with the standard patterns of Bi or Bi O . Notably, the\n2 3\ndiffraction peaks below 2θ of 25° demonstrated a progressively enhanced intensity\ntrend, suggesting the potential presence of an unidentified phase. Regarding the two\npeaks observed in the 30-35° range for BBO-2 (presumably attributed to Bi O ), the\n2 3\nsignificant negative shift required further explanation. To precisely resolve the phase\ncomposition, I recommend performing Rietveld refinement of the XRD patterns for\nquantitative phase analysis.\nResponse: We thank to the reviewer for this suggestion. As shown in the figure, we\nprovide the XRD patterns of BBO-2, BBO-4, and BBO-6. It can be clearly observed\nthat with the increase of the reduction reaction time, the peak of Bi gradually\n12\nstrengthens and the peak of Bi O gradually weakens. During this process, an ohm\n2 3\njunction is formed in situ. The XRD pattern of the BBO sample can be composed of\nhexagonal Bi (JCPDS: 85-1329) and Bi O (JCPDS: 22-0515). We have updated the\n2 3\nXRD data in the main text accordingly.\nTo precisely resolve the phase composition, Rietveld refinement of the XRD patterns\n(Figure R6) was performed to determine the phase composition of the BBO catalysts.\nThe refinement results confirm the exact phase composition. The Bi/Bi O mass ratios\n2 3\nfor BBO-2, BBO-4 and BBO-6 are 53.65% wt/46.35 wt%, 91.75% wt/8.25 wt%, and\n97.74 wt%/2.26 wt%, respectively. The Bi content increases with increasing reduction\ntime. The above results show there is on other phase structures.\nWe have included this discussion on line 174-180 of the manuscript.\nFigure R6 | (Supplementary Figure S20, in the revised supporting information).\nThe Rietveld refinement of the XRD patterns of (a) BBO-2, (b) BBO-4 and (c) BBO-\n6, respectively.\nComment 5. Given the morphological similarities observed in BBO-2, BBO-4, and\nBBO-6 samples, quantitative EDS elemental analysis and ICP-OES are necessary to\nverify compositional variations, particularly regarding Bi content.\nResponse: We thank the reviewer for this valuable suggestion. We agree that while the\nobserved morphological similarities in BBO-2, BBO-4, and BBO-6 are significant,\nquantitative compositional analysis is essential to confirm whether subtle differences in\nBi content exist despite these structural parallels.\nQuantitative EDS: We will perform rigorous quantitative EDS point analysis and\nmapping on each sample (BBO-2, BBO-4, BBO-6) to obtain statistically robust\n13\nelemental ratios, specifically focusing on Bi content and its spatial distribution.\nTable R1 | (Supplementary Table S2, in the revised supporting information).\nContent of Bi element in different samples based on EDS analysis.\nSample Bi content\nBBO-2 72.57 %\nBBO-4 75.63 %\nBBO-6 78.19 %\nICP-OES: To complement EDS and provide bulk composition data, we conduct\nICP-OES analysis on digested samples. This will deliver precise quantitative\nmeasurements of the absolute Bi concentration in each sample, eliminating surface\neffects inherent to EDS.\nTable R2 | (Supplementary Table S3, in the revised supporting information).\nContent of Bi element in different samples based on ICP-OES analysis.\nSample Bi content\nBBO-2 71.42 %\nBBO-4 73.27 %\nBBO-6 81.91 %\nDespite morphological similarities across samples, the progressive rise in Bi content\n(increase from BBO-2 to BBO-6) indicates gradual accumulation of metallic bismuth\nduring extended reduction. This correlation is now discussed in Revised Manuscript\nLine 143-145), highlighting how composition drives phase evolution independently of\nmorphology.\nWe thank the reviewer for prompting this critical analysis, which strengthens our\nmechanistic interpretation of reduction dynamics.\nComment 6. The catalyst demonstrated visible-NIR light absorption. How about the\nphotocatalytic performance under visible-light condition? In addition, the\n14\nphotocatalytic data and XRD patterns for BBO-3 and BBO-5 should be provided to\nobtain more refined results.\nResponse: We sincerely appreciate the reviewer's insightful comments regarding the\nvisible-light photocatalytic performance and the request for additional data on BBO-3\nand BBO-5. These points are valuable for enhancing the completeness and clarity of\nour study.\nPhotocatalytic performance under visible-light irradiation: We agreed with the\nreviewer that evaluating the photocatalytic activity specifically under visible-light-only\nconditions is crucial to correlate with the observed visible-NIR light absorption\nspectrum. In our initial submission, we focused on the broad-spectrum solar-light-\ndriven performance (including UV, visible, and NIR components) to demonstrate the\ncatalyst’s potential for practical sunlight utilization. However, we recognize the\nimportance of isolating the visible-light contribution. To address this point directly, we\nhave now conducted additional photocatalytic experiments under visible-light\nirradiation (420 nm). As shown in the Figure R7, the data clearly demonstrate that BBO-\n4 maintains significant photocatalytic activity under visible-light (420 nm) irradiation.\nSpecifically, the optimal BBO-4 samples reached the highest value among these\nsamples.\nFigure R7. Photocatalytic CO activities of the as-prepared samples under visible light\n2\n(420 nm) irradiation.\nProvision of XRD Patterns for BBO-3 and BBO-5: We thank the reviewer for this\n15\nsuggestion to provide a more comprehensive dataset. We provide the XRD patterns of\nBBO-3 and BBO-5. It can be clearly observed that with the increase of the reduction\nreaction time, the peak of Bi gradually strengthens and the peak of Bi O gradually\n2 3\nweakens. During this process, an Ohm junction is formed in situ. The XRD pattern of\nthe BBO sample can be composed of hexagonal Bi (JCPDS: 85-1329) and Bi O\n2 3\n(JCPDS: 22-0515). As requested, the XRD patterns for BBO-3 and BBO-5 have now\nbeen added to Supplementary Figure S18.\nFigure R8 | (Supplementary Figure S18, in the revised supporting information).\nXRD patterns of BBO-3 and BBO-5.\nComment 7. The main product of photocatalytic CO reduction using Bi or Bi O alone\n2 2 3\nwas CO, whereas their composite produced CH OH. What caused this change in\n3\nproduct selectivity?\nResponse: We sincerely appreciate the reviewer’s insightful question regarding the\nsignificant shift in product selectivity from CO (over pure Bi and Bi O ) to CH OH\n2 3 3\n(over the BBO catalyst). This is indeed a crucial observation that highlights the unique\nadvantage of our designed composite material. The free energy diagram for CO\n2\nphotoreduction to CH OH displayed in Figure R9. In response to the reviewer’s\n3\ncomments, we have included this discussion on line 400-407 of the manuscript. “The\n*CO→ CO (g) and *CO→*CHO were critical step which determine the selectivity of\nCO reaction. The free energy of *CO (g) over Bi was much lower than that of *CHO,\n2\n16\nwhich result in the CO was the main product. For Bi O , metallic Bi site, and interface\n2 3\nBBO, the *CHO formation energy was much lower than that of CO (g), suggesting the\nformation of *CHO was easier than that of CO (g). Moreover, the fabrication of Ohmic\njunction interface greatly reduced the energy barrier for the subsequent protonation\nprocess of *CHO→*CH\nFigure R9 | (Figures 6f-h, in the revised manuscript; Supplementary Figures S38-\n41, in the revised supporting information). Free energy diagram for CO\n2\nphotoreduction over the surface of (a) Bi, (b) Bi O ,(c) Bi site of BBO, and interface\n2 3\nsite of BBO.\nComment 8. In the mass spectrum, why was there no signal at m/z 32? Please detailedly\nexplain this issue.\nResponse: Thank you for this insightful question. The absence of a signal at m/z 32 is\nnot due to instrumental limitations, but a direct consequence of our experimental design\nusing isotopically labeled precursors. Below we provide a detailed explanation:\nThe absence of an m/z 32 signal is a direct consequence of our experimental design\nemploying 13C-labeled CO (13CO ) as the sole carbon source. Methanol produced\n2 2\nunder these conditions is fully labeled as 13CH OH, with a molecular mass of 33 (m/z\n3\n17\n33: 13C [13] + 3H [3] + 16O [16] + H [1]). Consequently, the molecular ion peak shifts\nfrom m/z 32 (characteristic of natural-abundance 12CH OH) to m/z 33, leaving no\n3\ndetectable signal at m/z 32.\nThe isotopic labeling experiment demonstrated that the methanol produced arose\nfrom the photocatalytic CO reduction reaction.\nFigure 3g, in the revised manuscript). (a) Total ion chromatography and (b) Mass\nspectrum of CO photoreduction products using 13CO as the CO source.\n2 2 2\nComment 9. Quantitative 1H NMR analysis for methanol detection required\ncomprehensive methodological description. In addition, NMR and GC spectra should\nbe provided in supporting information.\nResponse: We thank the reviewer for raising these valuable points. We fully agree that\nmethodological transparency and data accessibility are essential. We have revised the\nmanuscript accordingly.\nTo further detect the liquid product CH OH, the liquid product obtained from\n3\nphotocatalytic CO reduction was subjected to NMR detection for verification. Using a\n2\npipette, 200 μL of the liquid product from photocatalytic CO reduction and 300 μL of\n2\ndeuterium oxide (D O) were transferred into an NMR tube. After sealing, the sample\n2\nwas sent for NMR analysis.\nAs shown in Figure R10, a methanol peak at δ 3.28 ppm, and a water peak at δ 4.7\n18\nppm. Based on the NMR analysis of the liquid products from photocatalytic CO\n2\nreduction, it is confirmed that the liquid product generated by the BBO-4 heterojunction\nphotocatalyst is CH OH. No impurity peaks or other liquid products were detected.\nNuclear magnetic spectrum of the liquid products of CO photoreduction.\nfrom the photocatalytic CO reduction reaction (Figure R4).\nFigure 3g, in the revised manuscript) (a) Total ion chromatography and (b) Mass\nspectrum of CO photoreduction products using 13CO as the CO source.\n2 2 2\nAs requested, representative NMR and GC chromatograms have been added to the\nSupporting Information.\n4 3\nauthors, other C1 products such as HCOOH and HCOH may also form in photocatalytic\nCO reduction. The authors need to further investigate the product selectivity through\n2\nexperiments and DFT calculations. Furthermore, how was the qualitative and\nquantitative analysis of CH OH performed? The authors should provide more detailed\n3\ndescriptions and evidence regarding its identification and quantification methodology.\nResponse: We thank the reviewer for raising this important point. We have now\nintegrated in situ DRIFTS experimental evidence and expanded DFT calculations to\naddress the selectivity toward minor C1 products:\nExperimental evidence from in situ DRIFTS: Our time-resolved in situ DRIFTS\nspectra during photocatalytic CO reduction show no detectable vibrational modes\n2\nassociated with HCOOH or HCHO across the reaction timeline (0-60 min). This critical\nobservation indicates that: These species are not generated as stable intermediates on\nthe catalyst surface under our experimental conditions. Their formation pathways are\nkinetically inhibited or thermodynamically unfavorable. Based on this direct\nexperimental evidence, we intentionally excluded HCOOH/HCHO pathways from\ninitial DFT investigations.\nCH OH product qualitative analysis: As shown in Figure R10, a methanol peak at\n3\nδ 3.28 ppm, and a water peak at δ 4.7 ppm. Based on the NMR analysis of the liquid\nproducts from photocatalytic CO reduction, it is confirmed that the liquid product\n2\ngenerated by the BBO-4 heterojunction photocatalyst is CH OH. No impurity peaks or\n3\nother liquid products were detected.\nNuclear magnetic spectrum of the liquid products of CO photoreduction.\n3\ngas chromatograph (GC-Agilent Technologies 8860, USA) equipped with thermal\nconductivity detector (TCD) and a flame ionized detector (FID). Furthermore, FID\ndetector was connected with a HP-PLOT Q capillary column (Agilent, length 30 m, ID\n0.53 mm, film 40 μm) for separation of C1-C6 paraffin and olefin hydrocarbons,\nalcohols and oxygenated compounds. The quantification of the products was based on\nthe external standard and the use of a calibration curve.\nComment 11. The assignment of intermediate species in in situ DRIFTS spectra lacked\nsufficient evidence. Some intermediate species labeled by authors were not clearly\ndiscernible in the spectra. For example, the *CO species for Bi and Bi O , and the\n2 3\n*CHO and b-CO 2- species for BBO-4. In addition, what was the reason for the distinct\n3\nabsorption peak of the *CO species in BBO-4 at 0 min? Could this be attributed to\nimpurities in the CO gas used?\nDRIFTS spectral assignments. Due to the deviation in the selection of the collection\ntime points, the results were inaccurate. We have adjusted the collection time and\nstrengthened the evidence for intermediate species identification and addressed the\n21\nconcerns as follows:\n(1) the *CO species for Bi and Bi O were not clearly discernible in the spectra,\n2 3\ndemonstrating the desorption of *CO species, which was favorable to the formation of\nCO not the CH and CH OH.\n4 3\n(2) *CHO species at 1095 cm-1 and b-CO 2- species at 1362 cm-1 for BBO-4 can be\n3\nclearly observed in Figure R11. The generation of *CHO species showed the\nprotonation reaction of *CO, which was the key intermediate for the generation of CH\n4\nand CH OH.\n3\ngeneration during photocatalytic CO reduction. In situ ATR-FTIR experiments for (a)\n2 3\n(3) The peaks of *CO at 0 min can be detected, which may be influenced by residual\ngas in the instrument. In order to further eliminate the interference from the background,\nthe in-situ infrared spectra were recorded. The spectrum date was recorded before light\nirradiation and as the light exposure time increased. As shown in Figure 3a-c, the\ndistinct absorption peaks of the *CO species in Bi, Bi O , and BBO-4 at 0 min were\n2 3\nnot undiscovered. Crucially, BBO-4 exhibited significantly stronger *CO adsorption\ncompared to its constituent materials (Bi and Bi O ), as evidenced by enhanced peak\n2 3\nintensity. This pronounced adsorption capacity favors further hydrogenation of *CO,\nthereby directing the reaction pathway toward CH OH generation.\n3\nfor CO reduction by Bi@Bi O , with the proposed pathway shown in Figure 4g.\n2 2 3\nHowever, these speculations required additional theoretical justification. Moreover,\n22\nwhy is the *COOH species not directly formed through CO accepting protons and\n2\nelectrons at the reaction site?\nResponse: We appreciate the reviewer’s insightful comment regarding the proposed m-\nm-CO 2- intermediate pathway. We acknowledge that mechanistic proposals benefit\n3\nstrongly from experimental support. Our assignment of m-CO 2- as the key intermediate\n3\nis based on the following experimental rationale.\nThe surface structure of the samples was investigated by XPS analysis. An shown\nin Supplementary Figure 20b, the O 1s XPS spectrum of BBO-4 was the same as that\nof pristine Bi O , with peaks at 529.52 and 530.85 eV, which were attributed to the\n2 3\nchemical bonding of Bi-O and adsorbed oxygen (H-O), respectively. The above result\nshowed that the surface of the catalyst contained a large number of hydroxyl groups.\nThe catalytic process typically initiates with the adsorption and activation of CO\n2\nmolecules on the catalyst surface, forming reactive intermediates such as carbonate\n(*CO ) and bicarbonate (*HCO ) species (via pathways like CO + *O/*OH →\n3 3 2\n*CO /*HCO )1-3. These intermediates then undergo a complex reaction network\n3 3\ninvolving multiple steps and numerous transient species, ultimately yielding the final\nproducts. Although the conversion of CO to CO appears to be a simple oxygen removal,\n2\nit actually proceeds through a pathway mediated by intermediates like formate (*HCO )\n2\nand carboxyl (*COOH) species.4-6 Specifically, *HCO can dissociate to form *HCO\n3 2\nand adsorbed oxygen (*HCO → *HCO + *O). Subsequently, *HCO decomposes to\n3 2 2\nyield adsorbed CO and hydroxyl (*HCO → *CO + *OH). Alternatively, *CO can\n2 3\nreact with adsorbed hydrogen (*H) to form a *COOH intermediate (*CO + *H →\n3\n*COOH + *O), which then decomposes to produce adsorbed CO and hydroxyl\n(COOH* → CO* + OH*).7-8\nReference:\n(1) H. Jiang, L. Wang, H. Kaneko, R. Gu, G. Su, L. Li, J. Zhang, H. Song, F. Zhu, A.\nYamaguchi, Light-driven CO methanation over Au-grafted Ce Ru O solid-\n2 0.95 0.05 2\nsolution catalysts with activities approaching the thermodynamic limit. Nat. Catal.\n2023, 6, 519-530.\n23\n(2) Y. Yang, Z. Chai, X. Qin, Z. Zhang, A. Muhetaer, C. Wang, H. Huang, C. Yang, D.\nMa, Q. Li, et al. Light-induced redox looping of a Rhodium/Ce WO photocatalyst\nx 3\nfor highly active and robust dry reforming of methane. Angew.Chem. Int.Ed. 2022,\n61, e202200567.\n(3) F. Marques Mota, D. H. Kim, From CO methanation to ambitious long-chain\n2\nhydrocarbons: alternative fuels paving the path to sustainability. Chem. Soc. Rev.\n2019, 4, 205-259.\n(4) B. Xie, R. J. Wong, T. H. Tan, M. Higham, E. K. Gibson, D. Decarolis, J. Callison,\nK. F. Aguey-Zinsou, M. Bowker, C. R. A. Catlow, et al. Synergistic ultraviolet and\nvisible light photo-activation enables intensified low-temperature methanol synthesis\nover copper/zinc oxide/alumina. Nat. Commun. 2020, 11, 1615.\n(5) Z. Zhang, C. Mao, D. M. Meira, P. N. Duchesne, A. A. Tountas, Z. Li, C. Qiu, S.\nTang, R. Song, X. Ding, et al. New black indium oxide-tandem photothermal CO -\n2\n(6) T. H. Tan, B. Q. Xie, Y. H. Ng, S. F. B. Abdullah, H. Y. M. Tang, N. Bedford, R. A.\nTaylor, K. F. Aguey-Zinsou, R. Amal, J. Scott, Unlocking the potential of the formate\npathway in the photo-assisted Sabatier reaction. Nat. Catal. 2020, 3, 1034-1043.\n(7) K. Zhao, L. Wang, E. Moioli, M. Calizzi, A. Zuttel, Identifying reaction species by\nevolutionary fitting and kinetic analysis: an example of CO hydrogenation in\n(8) Z. Q. Rao, Z. A. Huang, G. X. Chen, L. Z. Zhang, K. W. Wang, Y. H. Cao, Y. L.\nChen, Y. T. Yang, Q. Y. Feng, A. K. Weidenkaff, Y. Zhou, Atomically dispersed\npraseodymium-modified Ni active sites boost the direct cleavage of carbonate\nintermediates for photothermal CO conversion. 2025 ACS Nano.\n2\nhttps://doi.org/10.1021/acsnano.5c05699\nTo identify the generation of intermediate products in photocatalytic CO reduction\n2\nreaction over the samples, in situ DRIFTS measurements were employed. Notably,\ncomparative analysis revealed that BBO-4 exhibited weaker b-CO 2- signal intensity\n3\nbut enhanced m-CO 2- formation compared to pristine Bi and Bi O . This distinct\n3 2 3\n24\nbehavior suggests that m-CO 2- species generated through single oxygen-site\n3\nadsorption at Bi centers serve as key intermediates in the CO reduction pathway on\n*COOH signals over 5-60 minutes, consistent with continuous CO activation and\n2\nhydrogenation processes. Just as the reviewer pointed out, *COOH can also be formed\nby the protonation of CO . Therefore, in the CO reduction process, there are multiple\n2 2\npathways that can generate *COOH. A similar description of the same process is\nprovided in the following references.\n[1] J. P. Sheng, Y. He, J. Y. Li, C. W. Yuan, H. W. Huang, S. Y. Wang, Y. J. Sun, Z. M.\nWang, F. Dong, Identification of halogen-associated active sites on bismuth-based\nperovskite quantum dots for efficient and selective CO -to-CO photoreduction. ACS\n[2] J. Q. Tian, Y. Y. Zhang, Z. H. Shi, Z. Y. Liu, Z. W. Zhao, J. Li, N. Li, H. W. Huang,\nEnabling interfacial lattice matching by selective epitaxial growth of CuS crystals on\nBi WO nanosheets for efficient CO photoreduction into solar fuels. Angew. Chem.\n2 6 2\nInt. Ed. 2025, 64, e202418496.\nTherefore, according to analysis of the in-situ DRIFTS spectra, the possible\nphotocatalytic steps of CO reduction on BBO-4 were proposed in Figure 4g.\nmechanisms of CO adsorption and CO -to-CH OH photosynthesis on the surface of\n2 2 3\nBBO-4 nanosheets.\nComment 13. The SEM, TEM, and AFM data for Bi O need to be provided. What\n2 3\ndoes the difference in KPFM between Bi O and BBO-4 indicate? The decrease in ΔV\n2 3\nfor BBO-4 under illumination suggests electron migration under light. However, how\n25\ncan the direction of electron migration be experimentally verified? If Bi@Bi O\n2 3\nexhibits a distinct interface, monitoring the potential differences at the interface under\nlight and dark conditions via in situ KPFM could serve as more compelling evidence.\nResponse: We appreciate these insightful and constructive questions. They highlight\nthe need for additional data and a clearer elucidation of our mechanisms. Our responses\nto each question are provided below:\n1. Regarding the provision of SEM, TEM and AFM data for Bi O .\n2 3\nWe fully agree that providing morphological data of pure Bi O is of vital importance\n2 3\nfor subsequent comparative analysis. Following your suggestions, we have\nsupplemented the relevant characterization data of Bi O in the revised manuscript. The\n2 3\nSEM, TEM and AFM tests of Bi O were conducted as your suggestion. The SEM and\n2 3\nLow TEM images indicate that the Bi O sample is composed of irregularly nanosheet\n2 3\n(Figures R13a and b). The lattice spacings of 0.274 nm can be assigned to the (400)\nplanes of Bi O in the Figure R13c. The AFM images and the corresponding height\n2 3\nprofile show that the thickness of its nanosheet is approximately 5.06 nm (Figures R13d\nand e). These data provide a solid foundation for the subsequent comparison with BBO-\n4 composite materials.\nFigure R13. | (Supplementary Figure S17, in the revised supporting manuscript).\n26\n(a) SEM, (b) TEM, (c) HRTEM, (d) AFM images and (e) corresponding height profile\nof Bi O .\n2 3\n2. The significance of the differences between Bi O and BBO-4 in KPFM\n2 3\nThe differences between Bi O and BBO-4 on the Kelvin probe force microscope\n2 3\n(KPFM) mainly reveal the formation of the built-in electric field at the heterojunction\ninterface. Under dark conditions, our data (Figure 5a-b) show that the surface potential\nof pure Bi O is 0.50 mV, while the surface potential of BBO-4 is significantly higher,\n2 3\nat 0.97 mV. This difference indicates that the ground state electron distribution of the\nmaterial has already changed merely by constructing the Bi/Bi O heterojunction. This\n2 3\nis because the different Fermi levels between Bi and Bi O cause charges to\n2 3\nspontaneously redistribute at the interface, thereby forming an internal electric field\nfrom Bi to Bi O , resulting in a higher work function and surface potential on the\n2 3\nsurface of the BBO-4 composite material. Furthermore, BBO-4 exhibits more\nsignificant surface potential inhomogeneity, indicating that it has a stronger charge\nredistribution ability.\n3. Regarding how to experimentally verify the direction of electron migration\nThe question you raised is extremely crucial, that is, how to experimentally verify\nthe direction of electron migration. Our current core experimental evidence is that under\nlight exposure, the surface potential of BBO-4 significantly decreases (from 0.97 mV\nto 0.62 mV, as shown in Figure 5c). Our interpretation logic is that the reduction of\nsurface potential is a direct manifestation of the accumulation of photogenerated\nelectrons on the material surface (i.e., the photoionization effect). These accumulated\nnegative charges will reduce the work function on the material's surface, thereby\ncausing the surface potential measured by KPFM to decrease. According to the\nmanuscript’s discussion, this process is interpreted as “efficient photogenerated\nelectrons migrating from Bi O to the Bi surface under the guidance of an internal\n2 3\nelectric field.” Therefore, the reduction of surface potential is the experimental basis\nfor us to determine the migration of electrons to the material surface.\nWe fully agree with you that directly monitoring the potential difference changes\n27\nacross the Bi@Bi O interface through in-situ KPFM technology will undoubtedly\n2 3\nprovide more direct and convincing evidence for the direction of electron migration.\nThis is a very good suggestion, representing more cutting-edge and refined\ncharacterization methods. In our current work, we infer this process by analyzing the\naverage potential change across the entire material surface. Your suggestion has pointed\nout the direction for our future research, that is, to further visually verify this charge\ntransfer path at the nanoscale through higher-resolution interface potential imaging\ntechnology. Thank you again for your valuable opinions. These issues have greatly\nenhanced the depth and rigor of our research.\nComment 14. The DFT results in Figures 6f-h showed gaseous CO and CH₃OH\nproducts at different energy levels. Thermodynamic said that Gibbs free energy is a\nstate function. If one starts off with a CO reactant and ends with a CO product, the\n2\nenergy difference will be the same. Then why is the CO and CH OH products at\n3\ndifferent energy?\nResponse: We sincerely thank the reviewer for identifying this critical inconsistency.\nAfter careful re-examination, we confirm that the different CO energy levels in original\nFigures 6f-h resulted from an error in our computational post-processing. Your\nunderstanding of thermodynamics is entirely correct: for the reaction CO → CO +\n2\n1/2O under fixed conditions, the Gibbs free energy change (ΔG) must be path-\n2\nindependent. We provide a transparent explanation and correction below. The free\nenergy diagram for CO photoreduction to CH OH displayed in Figure R9. In response\n2 3\nto the reviewer’s comments, we have included this discussion on line 400-407 of the\nmanuscript: “The *CO→ CO (g) and *CO→*CHO were critical step which determine\nthe selectivity of CO reaction. The free energy of *CO (g) over Bi was much lower\n2\nthan that of *CHO, which result in the CO was the main product. For Bi O , metallic\n2 3\nBi site, and interface BBO, the *CHO formation energy was much lower than that of\nCO (g), suggesting the formation of *CHO was easier than that of CO (g). Moreover,\nthe fabrication of Ohmic junction interface greatly reduced the energy barrier for the\n28\nsubsequent protonation process of *CHO→*CH\nFigure R9. | (Figures 6f-h, in the revised manuscript; Supplementary Figures S38-\n41, in the revised supporting information). Free energy diagram for CO\n2\nphotoreduction over the surface of (a) Bi, (b) Bi O ,(c) Bi site of BBO, and interface\n2 3\nsite of BBO.\nComment 15. It is recommended to perform synchrotron radiation characterization to\nthoroughly investigate the differences in coordination environments between\nBi@Bi O and single-phase materials, thereby further elucidating the structure-activity\n2 3\nrelationship.\nResponse: We sincerely thank the reviewer for this highly pertinent suggestion.\nSynchrotron-based techniques would indeed provide deeper insights into the atomic-\nscale coordination environments of Bi@Bi O composites and single-phase materials.\n2 3\nFollowing this recommendation, we have conducted the proposed synchrotron\nexperiments (Figure R14). The acquired data provide complementary and direct\nevidence on the distinct local atomic arrangements surrounding the Bi sites in these\nmaterials. As shown in Figure R5, the absorption edge of BBO-4 in the XAFS Bi L -\n3\nedge was between the Bi foil and Bi O , meaning that the valence state was Biδ+ (0 < δ\n2 3\n29\n< 3) (Figure R14l), further confirming the existence of metal Bi. In addition, the\nabsorption edge of Bi in the XAFS Bi L -edge was between the Bi foil and Bi O ,\n3 2 3\nshowing the occurrence of surface oxidation, which is accordance with the XPS results.\nTo obtain the quantitative structural parameters around Bi, Fourier transform of the\nleast-squares curve fitting was performed on the EXAFS data of Bi, Bi O , and BBO-\n2 3\n4 (Figure R14m). BBO-4 exhibits a typical Bi/Bi O structure. The presence of both\n2 3\nBi-O and Bi-O-Bi coordination shells confirms the existence of Bi O oxide, while the\n2 3\nBi-Bi metallic coordination indicates the presence of metallic Bi. In contrast, the Bi O\n2 3\nsample shows only a single Bi-O coordination shell, demonstrating its phase-pure Bi O\n2 3\ncomposition. The Bi metal sample, however, reveals both Bi-Bi and Bi-O bonds,\nsuggesting the existence of surface oxidation.\nFigure R14 | (Figures 1l and m, in the revised manuscript). Normalized XANES\nspectra at the (l) Bi-L edge and (m) corresponding Fourier transform R space fitting\n3\nresults.\nThese structural differences offer a key explanation of the formation of Bi@ Bi O\n2 3\nOhmic junction for the observed variations in catalytic activity, thus greatly\nstrengthening our mechanistic understanding of the structure-activity relationship. The\ndetailed synchrotron characterization results are now included in Figures 1l and m, in\nthe revised manuscript.\nThe authors synthesized a bismuth-bismuth oxide composite catalyst and evaluated its\nperformance for methanol production via photocatalytic CO reduction. The material\n2\nappears attractive, demonstrating superior performance compared to other materials.\nHowever, I believe major revisions are required prior to publication.\nResponse: We appreciate the reviewer's detailed summary of our work and are pleased\nthat the novelty and strengths of our bismuth-bismuth oxide composite catalyst design,\nalong with its photocatalytic performance, were well recognized. We also thank them\nfor the suggestions and have responded point-by-point in our response.\nComment 1. First, before addressing the scientific content, I would like to point out\nsome issues regarding the overall presentation of the manuscript. There are several\ninstances where the font size in the figures is too small, which hampers readability. I\nrecommend improving the clarity of the figures to enhance their utility in the\nmanuscript. For example, in Figs. 1b, 1c, 1d, 1f, Figs. 2l, 3a, and 4a–f. In addition, the\nscale bars in Fig. 2c-e are not clearly visible. Figure 1 and Figure 6 both appear to\ndescribe the reaction mechanism of CO reduction, but the differences between the two\n2\nare unclear. I suggest clarifying these points.\nResponse: We sincerely thank the reviewer for their meticulous assessment of the\nmanuscript's presentation. All figures have been comprehensively revised with the\nfollowing improvements.\nComment 2. Furthermore, there is no explanation of the IPDOS in Fig. 1c, making it\ndifficult to interpret. Although the caption of Fig. 1d,e refers to PDOS and IPDOS (line\n127), they appear to be entirely different figures.\nResponse: Thank you for the constructive suggestion. The required descriptions in the\nmentioned manuscript have been carefully checked and corrected. The following\nrevisions are made as suggested on line 139-141 of the manuscript: “PDOS and IPDOS\nrelated to Bi 6p-O 2p interactions before (up) and after (down) CO adsorption on (c)\n2\n2 3\njunction.”\nComment 3. Regarding Fig. 3a, it is unclear what “DRS” stands for in “UV-vis-NIR\nDRS” (line 224). Are they absorption spectra? The vertical-axis label does not make\nclear whether it refers to absorption or transmission. If the authors are insisting that\nBBO-4 exhibits the strongest absorption, then an arbitrary unit (a.u.) scale is insufficient.\nA quantitative evaluation is required. Additionally, the caption and main text\nexplanation of Fig. 3b appear insufficient. It would be helpful to show the sample used\nfor the measurement and the meaning of the colours (they might be included in the\nfigure, but the text is unreadable).\nResponse: We sincerely appreciate the reviewer’s careful scrutiny of Figures 3a and 3b\nand for highlighting the areas needing clarification and improvement. Please find our\npoint-by-point responses and the corresponding modifications made in the revised\nmanuscript below:\nRegarding “UV-vis-NIR DRS” (Figure 3a): We apologize for the lack of clarity.\n“DRS” stands for “diffuse reflectance spectroscopy”. This technique measures the\nreflectance (R) of light scattered diffusely from a powder sample, and then directly\nconverting into absorbance (A) of the samples. Therefore, the vertical-axis label in\nFigure 3a is absorbance (a.u.) of the samples.\nWe agree that relying solely on reflectance spectra plotted in arbitrary units (a.u.) is\ninsufficient to quantitatively support the statement that “BBO-4 exhibits the strongest\nabsorption”. To address this, we have improved the vertical axis of the UV-vis-NIR\nDRS and obtained the absorbance data of the samples. The UV-vis-NIR DRS (Figure\n3a) revealed that BBO-4 demonstrated significantly enhanced broad-spectrum light\nabsorption (280-2100 nm) compared to its constituent materials (Bi and Bi O ).\n2 3\nRegarding the insufficient explanation of Figure 3b: We thank the reviewer for\npointing out the lack of detail in the original caption and text description for Figure 3b.\nResponse: Thank you for pointing this out. A caption for Figure 5c has now been added\nin line 345-347 of the revised manuscript: “KPFM images and surface potential analysis\nof (a) Bi O in the dark and (b-c) BBO-4 in the dark and under light irradiation,\n2 3\nrespectively.”\nComment 5. For Fig. 5d and 5g, the vertical axis labelled “ΔA” is undefined, which\nmakes interpretation difficult. The main text states, “The absorption intensity of BBO-\n4 was stronger than that of Bi O ,” (line 295) but it is unclear from which data this\n2 3\nconclusion was drawn. Regarding Fig. 6a, is this a BET measurement? If so, further\nexplanation is required. In Figs. 6b and 6c, the term “CO (CO) TPD intensity” is\n2\nunclear—does this refer to the signal intensity of desorbed CO (CO) species identified\n2\nby a suitable method, or is it the total amount of evolved gases?\nResponse: We sincerely apologize for the lack of clarity regarding the “ΔA” label and\nthe basis for the comparison in the text. We appreciate the reviewer pointing this out.\nDefinition of ΔA: ΔA stands for differential absorbance. These figures (5d, 5g)\npresent transient absorption (TA) spectroscopy data. We revise the Y-axis label on\nFigures 5d and 5g to explicitly state. We will also include a clear definition in the figure\ncaption: “Differential absorbance (ΔA) measured by transient absorption spectroscopy”\nWe will add a brief explanation of TA and ΔA in the main text or methods section where\nTA is first introduced.\nBasis for comparison: The statement regarding stronger absorption intensity for\nBBO-4 compared to Bi O specifically refers to the magnitude of the initial ΔA signal\n2 3\nobserved in the transient absorption spectra at comparable time delays and wavelengths.\nWe will revise the text in line 295 for clarity: “The initial differential absorbance\nmagnitude (|ΔA|) of BBO-4 at [650 nm, 100 ps] was significantly stronger than that of\nBi O (Figs. 5d and 5g), indicating enhanced light absorption.” We will ensure the\n2 3\nfigures are clearly referenced at the point of comparison.\nRegarding Figure 6a, this is a BET measurement. The CO adsorption capacity of a\n2\n33\ncatalyst serves as a crucial criterion for evaluating its photocatalytic CO reduction\n2\nperformance. Therefore, CO adsorption isotherms were employed to investigate the\n2\n6a, the BBO-4 exhibits the highest CO physisorption capacity of 0.4826 cm3 g-1,\n2\nsurpassing that of metallic Bi (0.3402 cm3 g-1) and BO (0.4826 cm3 g-1). This indicates\nthat BBO-4 may provide more active sites for adsorbing and converting CO molecules\n2\nduring the photocatalytic CO reduction process, thus facilitating the occurrence of the\n2\nreaction. In response to your suggestion, we have included a detailed discussion in the\nrevised manuscript in line 371-374: “CO adsorption isotherms (Figure 6a)\n2\ndemonstrated the adsorption capacities of Bi, Bi O and BBO-4 were 0.3402, 0.4826,\n2 3\nand 0.5632 m3 g-1. This indicated that BBO-4 may provide more active sites for\nadsorbing and converting CO molecules during the photocatalytic CO reduction\n2 2\nprocess, thus facilitating the occurrence of the reaction.”\nWe thank the reviewer for raising this important point regarding the clarity of the\n“CO (CO) TPD intensity” terminology in Figures 6b and 6c. We acknowledge that the\n2\noriginal label lacked sufficient specificity. The term “CO (CO) TPD intensity” refers\n2\nspecifically to the signal intensity (in arbitrary units, a.u.) measured by the detector for\nthe desorbed gases during the Temperature-Programmed Desorption (TPD) experiment.\nThis signal intensity is directly proportional to the relative amount of the corresponding\ngas species (CO or CO) evolving from the catalyst surface at a given temperature.\nbetween the data sets is needed, as the graphs are difficult to understand for various\nreaders. The same applies to Supplementary Figures S3–S5. For the elemental mapping\nin Supplementary Figures S8–S11, please clearly indicate which elements are being\nmapped. Is there a reason for the different ordering between S9 and S10–S11? For the\nEDS mappings, it would be helpful to include spectra showing the mapped regions.\n(1) “For Supplementary Figure S2a-c, an explanation of the differences between the\ndata sets is needed, as the graphs are difficult to understand for various readers. The\n34\nsame applies to Supplementary Figures S3-S5.”\nResponse: Thank you for this constructive suggestion. For clarity, we have also added\nannotations directly to the figures and included a concise summary in the figure legends.\nThese modifications will ensure the data is accessible to readers across disciplines.\n“Note 1: To elucidate the interaction between Bi 6p orbitals and CO , we calculated\n2\nthe partial wave density of states (PDOS) of relevant orbitals. The results demonstrated\nthat for CO adsorption and activation on Bi O , hybridization occurred between the Bi\n2 2 3\n6px orbital and the C 2pz orbital. Electrons were transferred from the Bi 6px orbital to\nthe 2π antibonding orbital of CO (π back-donation), weakening the CO molecule and\n2 2\ncausing its structure to transform from linear to bent. This drove CO adsorption and\n2\nactivation on the Bi O surface.\n2 3\nNote 2: For the CO adsorption and activation process on BBO, the electronic states\n2\nof Bi 6px, 6py, and 6pz orbitals nearly vanished near the Fermi level, while dense and\ncontinuous hybridized peaks emerged in the PDOS of C 2px, 2py, and 2pz orbitals of\nCO . This indicated substantial electron transfer from Bi 6px, 6py, and 6pz orbitals to\n2\nthe 2π antibonding orbitals of CO , facilitating CO adsorption and activation on BBO.\n2 2\nThese transferred electrons populate the C 2px, 2py, and 2pz orbitals of CO , leading\n2\nto significant reconstruction of the internal orbital structure within the CO molecule.\n2\nreactivity.”\n(2) For the elemental mapping in Supplementary Figures S8-S11, please clearly indicate\nwhich elements are being mapped. Is there a reason for the different ordering between\nS9 and S10–S11? For the EDS mappings, it would be helpful to include spectra\nshowing the mapped regions.\nResponse: Element identification: All images in Supplementary Figures S8-S11 have\ncolored element labels overlaid on mappings. The different ordering between Figures\nS9 and S10-S11 may be caused by the randomness. For clarity, the element order across\nall figures in Supplementary Figures S9-S11 has been standardized. In the Figures, The\nHAADF-STEM images show the mapped regions.\nof colourful features does not resemble a conventional AFM image. What do the colours\nrepresent? Please include a scale bar (AFM image). In Fig. S12b, which line in Figure\nS12a was used for the line profile? Although the instrument is identified as an Atomic\nForce Microscope (AFM, Bruker Dimension ICON), details of the substrate used and\nmeasurement conditions (e.g., cantilever type) should also be provided. The same\napplies to Supplementary Figures S13 and S14.\nResponse: We appreciate the reviewer’s attention to detail and the reviewer’s valuable\nsuggestion. We apologize for the incorrect description of the AFM images. Now we\nhave revised the Supplementary Figures S14-16 in the revised manuscript so that the\nsample information can be clearly identified.\nFigure R15 | (Supplementary Figure S14, in the revised supporting information).\n(a) AFM images of BBO-2 and (b) corresponding height profile.\nFigure R16 | (Supplementary Figure S15, in the revised supporting information).\n(a) AFM images of BBO-4 and (b) corresponding height profile.\n(a) AFM images of BBO-6 and (b) corresponding height profile.\nSurface topography analysis was performed using an Atomic Force Microscope\n(Bruker Model: SCM-PIT-V2, USA). Measurements were conducted on catalyst\npowders dispersed onto freshly cleaved mica substrates (10 mm × 10 mm). A silicon\ncantilever (Bruker RTESPA, nominal spring constant: 3 N•m-1, resonant frequency: ~75\nkHz, Thickness: 2.8 μm; Length: 225 μm; Width: 35 μm) was used with a tip radius <\n8 nm. Representative images were flattened using NanoScope Analysis v1.7 software\nto remove background.\nIn response to the reviewer’s comments, we have included this description on the\nCharacterization section of the revised supporting information.\nComment 8. In Supplementary Figure S15, the sample is labelled as BBO-4 in the\nimage but the caption refers to BBO-6. Please clarify which is correct.\nAuthor reply: Thank you for the question. The required descriptions in the mentioned\nmanuscript have been carefully checked and corrected. The original Supplementary\nFigure S15 corresponded to Supplementary Figure S21 in revised supporting\ninformation. The following revisions are made as suggested as follow: “Supplementary\nFigure S21. (a) Raman spectrum of Bi, Bi O , and BBO-4.”\n2 3\nComment 9. For Supplementary Figure S16, an explanation of the sample’s location\nand state within the photograph is needed. Is the entire image the sample? Please also\n37\ninclude information on the sample’s thickness, substrate used, and the wavelength and\nintensity of the irradiation light.\nResponse: We appreciate the reviewer’s attention to detail. Before the test, a certain\nmass of Bi, Bi O , and BBO-4 powders were dispersed in 3 mL of deionized water and\n2 3\nsubsequently sonicated for 10 minutes to form a homogeneous ink. This ink was then\ndropped onto a clean quartz substrate. After drying at 60°C, a uniform layer of catalyst\nwith an approximate thickness of 0.1 mm was formed on the quartz substrate. The entire\nimage in Figure S23 represents the surface temperature captured by a handheld infrared\nthermometer under Xenon lamp irradiation (full spectrum, wavelength range of 300-\n2500 nm, light intensity of 1.5 W/cm2) during the test.\nComment 10. Please add captions for Supplementary Figure S18a–h. What do these\nfigures represent? In Supplementary Figure S21, the value “0.1” is shown—what does\nthis number represent? Please include appropriate units.\nResponse: Thank you for the constructive suggestion. The required descriptions in the\nmentioned manuscript have been carefully checked and corrected. The original\nSupplementary Figure S18 and S21 corresponded to Supplementary Figure S25 and\nS30, respectively, in revised supporting information. The following revisions are made\nas suggested.\n“Supplementary Figure S25. (a) The process of weighing using an analytical balance,\n(b) The ultrasonic dispersion process, (c) Catalyst droplet process, (d) Drying process,\n(e) The assembly procedure of the photoreactor, (f) The testing process for\nphotocatalytic CO reduction, (g) The product collection process, (h) Analyze the\n2\nproduction process using GC.”\nThe scale of in-situ infrared absorbance is 0.1, with the unit expressed in a.u..\nComment 11. In the main text and Fig. 2, the reducing agent is written as “NaHB₄.”\nAre they a typographical error for “NaBH ”? Additionally, is “Na 13CO ” in the\n4 2 2\nSupplementary Information correct?\nmentioned manuscript have been carefully checked and corrected. The following\nrevisions are made as suggested.\n(1) “The Bi Br S nanowires have been synthesized firstly by hydrothermal method,\n19 3 27\nthen followed by a NaBH reduction method to inducing the dissolution and\n4\ncrystallization of Bi species, and finally the BBO nanosheets were obtained.”\n(2) “All the procedures were the same as those for the light-driven CO reduction\n2\ntests, except Na 13CO was used for the 13C labeling experiment.”\n2 3\nComment 12. The title refers to “Atomic-Scale Interface Engineering,” but are the\nsamples truly engineered at the atomic level with controlled numbers of atoms and\nstructures? Theoretical calculations (Figs. 1 and 6) appear to be based on Bi clusters of\nseveral atoms, but do these correspond to the experimental results? In the TEM images\nshown in Figs. 2f and 2g, the Bi metal particles appear to be quite large. Furthermore,\nin Fig. 2k, distinct XRD peaks corresponding to crystalline Bi metal are observed,\nwhich seems inconsistent with the amorphous Bi clusters described in Figs. 1 and 6.\nAlso, in the XRD pattern of BBO-6 in Fig. 2k, there appear to be no observable peaks\nattributed to Bi O . Is this not a concern?\n2 3\nResponse: Thank you very much for raising these profound and insightful questions.\nThey touch upon the core of our work, that is, how to relate theoretical calculations,\nmicrostructures and macroscopic performance. We are more than willing to provide\ndetailed clarification and explanation of the four key points you have raised here.\n1. Regarding the rationality of “Atomic-scale Interface Engineering” in the title\nWe use the term “atomic-scale interface engineering” to emphasize that we have\nsuccessfully constructed a high-quality, lattice-level matched heterogeneous interface\nbetween two different phases (metallic Bi and semiconductor Bi O ). Our core evidence\n2 3\nis the high-resolution transmission electron microscopy (HRTEM) image in Figure 2g.\nThis image clearly shows that the (012) crystal plane belonging to the hexagonal crystal\nsystem Bi and the (400) crystal plane belonging to Bi O have achieved seamless,\n2 3\n39\natomic-level “stitching” at the interface. This clear and coherent contact interface\nbetween lattices is precisely the result of what we refer to as “atomic-scale” engineering.\nOur aim is not to stack atoms one by one, but to successfully regulate and generate this\nclosely contacting and ordered interface structure at the atomic level through in-situ\nchemical reduction strategies, which is crucial for promoting the effective transfer of\ncharges between interfaces.\n2. Regarding the differences in “size” and “crystal form” between the theoretical model\nand the experimental results\nYou keenly pointed out the apparent differences between the theoretical model\n(small-sized clusters) and the experimental results (large-sized, crystalline particles).\nThe key here lies in clarifying the purpose and positioning of the theoretical calculation\nmodel. The purpose of the model: Our theoretical calculations (as shown in Figures 1\nand 6) aim to simulate and understand the reaction mechanism of local active sites at\nthe complex Bi/Bi O interface. In computational chemistry, to obtain reliable quantum\n2 3\nmechanical results (such as adsorption energy, energy barrier, and electronic structure)\nwithin an acceptable time frame, it is usually necessary to construct a representative\nsimplified model containing dozens to hundreds of atoms. The correspondence between\nthe model and reality: The “Bi cluster” we use in the model does not mean that it is\nessentially amorphous. This cluster was constructed based on the lattice parameters of\ncrystalline Bi, representing a surface of large-sized crystalline Bi nanoparticles\nobserved in the experiment. Therefore, the crystalline properties of Bi in the theoretical\nmodel (confirmed by XRD and HRTEM) are essentially consistent with those in the\nexperiment. The purpose of this model is to reveal the nature of the chemical reactions\noccurring at the interface active sites, rather than replicating the macroscopic\nmorphology of the entire nanoparticle. This is a standard and effective research method\nin the field of heterogeneous catalysis theoretical calculation.\nIn order to ensure the reliability of the models, the experimental and simulated\nXANES spectra of the BBO-4 were compared (Figure R18). Based on FDMNES\nabsorption spectrum simulations, the presence of Bi-O bonds at the interface primarily\n40\ncaused the differences in peak shapes between P1 and P2. At P1, the existence of the\nBi-Bi O interface led to the disappearance of the peak valley observed in Bi. At P2,\n2 3\nBi-Bi bonds resulted in the emergence of a distinct peak. This indicated that the model\nof Bi loaded on the Bi O surface can effectively resolve the structure-activity\n2 3\nrelationship of the samples discussed in this work. Moreover, the Bi-Bi bond lengths in\nBi were close to those in metal Bi, while the Bi-O bond lengths at the interface were\nlonger than those in Bi O (Figure R19). This phenomenon aligned with the XAS fitting\n2 3\nresults (Table RS3).\nFigure R18 | (Supplementary Figure S7, in the revised supporting information). (a)\nNormalized XANES spectra and (b) simulated normalized XANES spectra at the Bi-\nL edge of BBO-4.\nTheoretical simulation models of (a) Bi O and (b) BBO.\n2 3\nTable R3 | (Supplementary Table S1, in the revised supporting information).\nEXAFS fitting parameters at the Bi L -edge for various samples (s0 = 1.41 for Bi-foil;\n3\n41\ns = 1.00 for BBO-4, Bi O and Bi).\n0 2 3\nSample Shell CNa R(Å)b σ2 (Å2) c ΔE (eV)d R factor\nBi-foil -1.32±0.83 0.031\nBi-Bi1 4 3.51±0.01 0.031±0.004\nBi-Bi1 1.11±0.32 3.08±0.01 0.004±0.002\nBi-O1 0.16±0.08 1.77±0.02 0.002±0.001\nBBO-4 -2.44±1.56 0.046\nBi-O2 0.77±0.51 2.96±0.01 0.007±0.008\nBi-O3 0.30±0.01 2.13±0.01 0.001±0.001\nBi O Bi-O1 2.07±0.21 2.12±0.01 0.006±0.001 -3.32±1.31 0.022\n2 3\nBi-Bi1 0.83±0.12 3.04±0.01 0.001±0.001\nBi-O1 0.21±0.11 1.79±0.06 0.010±0.005\nBi 0.21±1.45 0.047\nBi-O2 0.29±0.18 2.16±0.01 0.006±0.005\nBi-Bi2 0.33±0.06 3.16±0.06 0.001±0.001\naCN, coordination number; bR, distance between absorber and backscatter atoms; cσ2,\nDebye-Waller factor to account for both thermal and structural disorders; dΔE , inner\n0\npotential correction; R factor indicates the goodness of the fit. Fitting range: 2 < k (/Å)\n< 12 and 1< R (Å)< 4 for Bi-foil; 3 < k (/Å) < 11 and 1< R (Å)< 4 for BBO-4; 3 < k\n(/Å) < 12 and 1< R (Å)< 3 for Bi O ; 3 < k (/Å) < 13 and 1< R (Å)< 4 for Bi.\n2 3\n3. Regarding the issue of the missing Bi O peak in the XRD pattern of BBO-6\n2 3\nWhat you have observed is very correct. The diffraction peaks of Bi O are indeed\n2 3\nalmost invisible in the XRD pattern of BBO-6. This is not an unexpected result but\nrather conforms to the logical evolution of our material synthesis process. Please\ncombine and observe the evolution trend of the series of graphs from BBO-2 to BBO-\n4 and then to BBO-6 in Figure 2k. This is a clear evolutionary process: as the reduction\ntime increases, Bi O , as a precursor, is continuously consumed and converted into\n2 3\nmetallic Bi. Therefore, the diffraction peak of Bi O gradually weakens, while that of\n2 3\nmetallic Bi gradually intensifies. BBO-6 is the sample with the longest reduction time\nin this series. Therefore, its reduction degree is the highest, and the vast majority of\n2 3 2 3\nalready fallen below the detection limit of conventional XRD instruments, or exist in\nthe form of extremely small nanocrystals or amorphous forms, thus failing to form\nclearly identifiable diffraction peaks.\nIn conclusion, we believe that the proposition of \"atomic-scale interface engineering\"\nis based on the direct evidence of HRTEM. The difference between theory and\nexperiment is a common practice of computational simulation methodology, and the\ntwo are unified on core chemical issues. The absence of the Bi O peak in BBO-6\n2 3\nprecisely confirms our controllable chemical reduction process.\nComment 13. Further explanation regarding the experimental methods appears\nnecessary. According to the Supplementary Information, BBO was synthesized by the\nreduction of Bi Br S using aqueous NaBH , added dropwise. However, NaBH\n19 3 27 4 4\nreacts quickly with water to produce hydrogen gas. Although both hydrogen and\nunreacted NaBH may serve as reducing agents, the described method may lead to poor\n4\nreproducibility. It may be helpful to provide a more detailed synthesis procedure for\nbetter reproducibility.\nResponse: We thank to the reviewer for this suggestion and agree such details should\nbe provided. Firstly, 2.2436 g of bismuth bromide (BiBr\n3\n), 0.3502 g of thiourea\n(CH N S), and 0.2 g of EDTA were weighed and placed in a beaker containing 60ml of\n4 2\nethanol, and stirred at room temperature for 1 hour. Then, the mixed solution was\ntransferred to a 100 mL inner lining of polytetrafluoroethylene, and finally placed in an\noven for a hydrothermal reaction at 180 oC for 72 hours. After the reaction was\ncompleted, it was naturally cooled to room temperature, then washed several times with\ndeionized water and ethanol, and dried at 80 oC for 4 hours to obtain Bi Br S (BBS).\n19 3 27\nIn order to obtain Bi-Bi O (BBO), we adopted the in-situ reduction method. Firstly, 0.2\n2 3\ng of BBS was weighed and placed in a mixture containing 50ml of a mixed solution of\ndeionized water and anhydrous ethanol (volume ratio of ethanol to deionized water =\n1:1), it was vigorously stirred at room temperature for 30 minutes. Then, 2 g of NaBH\n4\n43\nwas dissolved in 10ml of deionized water, and the solution was slowly dropped into the\nBBS solution that was being stirred. The precipitates were obtained by stirring for 2,\n4, and 6 hours respectively at room temperature. Then, they were washed and dried\nrespectively with deionized water to obtain Bi-Bi O (BBO-2, BBO-4, BBO-6).\n2 3\nComment 14. Concerning phase identification, the presence of Bi metal and Bi O is\n2 3\nconfirmed, but is sulphur absent? It seems plausible that a Bi–S compound such as\nBi S might be formed. Clarifying the presence or absence of sulphur in the sample,\n2 3\nand, if possible, including a reaction equation, would make the synthetic process clearer.\nResponse: We thank the reviewer for raising this critical point regarding the potential\nformation of Bi-S phases and the necessity to clarify sulfur’s fate in our synthesis\nprocess. We appreciate the opportunity to address these concerns and provide further\nmechanistic insights.\nFirstly, based on our comprehensive XRD analysis (Figure 2k in the manuscript), no\ncharacteristic peaks corresponding to crystalline Bi S or other bismuth sulfides were\n2 3\ndetected in the as-prepared samples. The dominant phases identified were metallic Bi\nand Bi O , consistent with our initial report.\n2 3\nTo explicitly confirm the absence of residual sulfur in the catalyst, the absorption\nedge of BBO-4 in the X-ray absorption fine structure (XAFS) Bi L -edge was employed.\nFigure R14 | (Figures 1l and m, in the revised manuscript). Normalized XANES\nspectra at the (l) Bi-L edge and (m) corresponding Fourier transform R space fitting\n3\nresults.\n(EDS) and point analysis on BBO-4 samples. The results show no detectable sulfur\nsignal within the instrument’s detection limit.\nThe above structural characterization clearly confirms the absence of sulfur in the\nsynthesized catalyst. These additions comprehensively reinforce the purity of the\nsynthesized Bi/Bi O Ohmic junction catalyst.\n2 3\nFigure R20 | (Supplementary Figure S19, in the revised supporting information).\nSEM and energy-dispersive X-ray spectroscopy (EDS) of BBO-4.\nComment 15. In Fig. 3a, if Bi metal is included in the sample, it should not transmit\nlight. The method used to obtain this spectrum needs to be described.\nResponse: We appreciate the reviewer’s insightful suggestion. UV-vis-NIR is an\nimportant testing method used to characterize the optical properties of solid materials\n(such as absorption, reflection, band gap, etc.). The following are the specific testing\nsteps for obtaining this spectrum: First, we prepare the sample by grinding the powder-\nlike sample to be tested evenly and setting it aside. The next step is to set up the\ninstrument. We use the high-reflectivity white standard BaSO to perform baseline\n4\ncalibration on the ultraviolet-visible-near-infrared spectrophotometer. At this time, the\n45\nwavelength range is usually within 200-2500 nm. Then, we perform background\nsubtraction to eliminate the interference of instrument noise and dark current. Finally,\nwe evenly pour the sample into the circular diffuse reflection sample cell, make its\nsurface smooth and then tighten it to ensure that the sample is not transparent before\nconducting the test. After completion, we process the data to obtain the spectrum shown\nin the figure.\nComment 16. The authors argue that Bi-metal part in BBO is negatively charged based\non XPS (line 164-170), but the observed shift in binding energy appears to be in the\npositive direction (Figure 2l).\nResponse: We thank the reviewer for highlighting this point. The apparent binding\nenergy shift in Bi 4f spectra stems from local charge polarization at the Bi/Bi O\n2 3\ninterface. We sincerely thank the reviewer for identifying this critical point. Our original\ndescription oversimplified the charge distribution. Revised analysis is added in Line\n202-217 in the revised manuscript: “To investigate the electronic states and charge\ntransfer of BBO-4, XPS analysis was employed. For Bi O , the XPS diffraction peaks\n2 3\nat 159.00 eV (Bi 4f ) and 164.32 eV (Bi 4f ) were characteristic of Bi3+ species\n7/2 5/2\n(Supplementary Figure 22a).20-21 The characteristic Bi 4f peaks of Bi metals could be\nfitted into two pairs of doublets associated with Bi0 and Bi3+. The XPS signal of Bi\noxidation states in Bi metals was derived from surface oxidation of the catalyst under\nexposure to air. For BBO-4, the Bi 4f signals deconvoluted into four peaks: two at\n156.68 and 161.98 eV (assigned to Bi0 in metallic Bi) and two at 158.89 and 164.21 eV\n(attributed to Bi3+ in Bi O ).22-24 Comparative analysis of binding energies across Bi\n2 3\nand Bi O , BBO-4 revealed a 0.11 eV positive shift in the Bi0 binding energy for the\n2 3\nheterojunction relative to pristine metallic Bi, indicating decreased electron density\naround Bi0. Conversely, the Bi3+ binding energy in BBO showed an 0.11 eV downshift\ncompared to Bi O , signifying increased electron density around Bi3+. These XPS\n2 3\nresults collectively suggest electron transfer from metallic Bi to Bi O within the\n2 3\nheterojunction, consistent with the electron flow direction in Ohmic junction.25-27 The\n2 3\n529.52 and 530.85 eV, which were attributed to the chemical bonding of Bi-O and\nadsorbed oxygen (H-O), respectively (Supplementary Figure 22b). The results are in\naccordance with the above work function analysis.”\nFigure R21 | (Figure S22, in the revised supporting information). XPS spectra of\nBi, Bi O , and BBO-4: (a) Bi 4 f and (b) O 1s.\n2 3\nComment 17. Generally, contact with oxides results in positive shift and an increase in\nbinding energy. Line 167 mentions a 0.06 eV shift, but it is unclear which data this\nrefers to.\nResponse: We thank the reviewer for highlighting this ambiguity. The 0.06 eV binding\nenergy shift specifically refers to the Bi 4f peak of metallic Bi in BBO-4 compared\n7/2\nto pure Bi metal. We acknowledge an inaccuracy in our initial analysis and have refined\nthe interpretation accordingly. We have included a detailed discussion in the revised\nmanuscript in line 169-175: “Comparative analysis of binding energies across Bi and\nBi O , BBO-4 revealed a 0.11 eV positive shift in the Bi0 binding energy for the\n2 3\nheterojunction relative to pristine metallic Bi, indicating decreased electron density\naround Bi0. Conversely, the Bi3+ binding energy in BBO showed an 0.11 eV downshift\ncompared to Bi O , signifying increased electron density around Bi3+. These XPS\n2 3\nresults collectively suggest electron transfer from metallic Bi to Bi O within the\n2 3\nheterojunction, consistent with the electron flow direction in Ohmic junction.25-27”\nauthors discuss subtle changes, it is unclear whether charging effects were properly\ncorrected. What calibration method was used? The charge accumulation characteristics\ndiffer significantly between Bi metal and Bi O , so binding energies are expected to\n2 3\nchange substantially. As it stands, I am not confident that the current data can support\nconclusions about charge transfer.\nResponse: Thank you very much for raising this extremely important question, which\ndirectly hits the core of the reliability of our XPS data. We fully understand your\nconcerns regarding the accuracy of the data, the method of charge calibration, and the\nstrength of the conclusion support, and thank you for the opportunity to elaborate on\nthese details.\n1. Clarification on the value of combined energy shift\nWe admit that the manually marked displacement arrows and values (such as 0.12\neV and 0.07 eV) in the original figure 2l are indeed inaccurate and prone to confusion.\nThis is our oversight and we apologize for it. In the revised manuscript, we have\nremoved these incorrect annotations to present the data in a clearer way. The conclusion\nin our manuscript is based on the precise comparison of the peak position centers of\neach sample. The correct displacement values are as follows: “For BBO-4, the Bi 4f\nsignals deconvoluted into four peaks: two at 156.68 and 161.98 eV (assigned to Bi0 in\nmetallic Bi) and two at 158.89 and 164.21 eV (attributed to Bi3+ in Bi O ).22-24\n2 3\nComparative analysis of binding energies across Bi and Bi O , BBO-4 revealed a 0.11\n2 3\neV positive shift in the Bi0 binding energy for the heterojunction relative to pristine\nmetallic Bi, indicating decreased electron density around Bi0. Conversely, the Bi3+\nbinding energy in BBO showed an 0.11 eV downshift compared to Bi O , signifying\n2 3\nincreased electron density around Bi3+. These XPS results collectively suggest electron\ntransfer from metallic Bi to Bi O within the heterojunction, consistent with the electron\n2 3\nflow direction in Ohmic junction.”\n2. Regarding the charge calibration method\nThe questions about charge calibration are very crucial. Here we clearly state that in\n48\norder to eliminate the overall peak position shift of the sample caused by surface\ncharging under X-ray irradiation, we have adopted the internationally recognized\nstandard calibration method: The binding energies of all spectra were precisely\ncalibrated with the C1s peak of the inevitably present Adventitious Carbon on the\nsample surface (set at 284.8 eV) as the internal standard. This method is the standard\noperation for XPS analysis, especially for samples containing semiconductor or\ninsulator components such as Bi O . It can effectively compensate for the charging\n2 3\neffect, ensuring that the minute chemical shifts we observe (such as 0.11 eV) are due to\nreal changes in different chemical states and electron cloud densities (i.e., charge\ntransfer), rather than artifacts produced by the instrument or the sample itself. XPS was\napplied used to adjust the charge transfer was reported in some top-tier, highly regarded\njournals, such as: Nature Chemistry, 2024, 16, 1250-1260; Nature Reviews Chemistry\n2025, 9, 328-342; Nature Communications 2024, 15, 4807.\nTherefore, we firmly believe that, under the premise of using the standard C 1s charge\ncalibration, our data are reliable and sufficient to support the conclusion in the\nmanuscript regarding the direction of charge transfer. We hope our detailed explanation\ncan dispel your doubts. Thank you again for your rigorous review, which prompts us to\npresent and interpret our data to a higher standard.\nComment 19. In Figs. 5f and 5i, the authors evaluate lifetimes based on time-resolved\ndecay curves and conclude that BBO-4 exhibits a longer lifetime. However, visually,\nBi O appears to have a longer lifetime. Please provide details on the fitting process.\n2 3\nAlso, the fitting was performed with three components—what is the basis for this\napproach?\nResponse: We sincerely thank the reviewer for this critical observation. Upon re-\nexamining our data and fitting procedures, we recognize an error in the original\nmanuscript’s conclusion regarding the τ lifetimes of BBO-4 and Bi O . The corrected\n3 2 3\nanalysis confirms that BBO-4 has a shorter lifetime (τ ) than Bi O , consistent with the\n3 2 3\nreviewer’s visual assessment and, importantly, indicative of accelerated interfacial\n49\ncharge transfer in BBO-4. We apologize for this oversight and provide the following\ndetailed clarification:\nAbout fitting procedure & tri-exponential model validation:\nAccording to transient absorption (TA) measurements, a three-stage kinetic model is\nproposed for the BBO-4 composite. Upon photoexcitation of Bi O , hole transfer (HT)\n2 3\noccurs from the excited state of Bi O (1Bi O *) to the Fermi level of metallic Bi\n2 3 2 3\nnanoparticles (NPs), generating charge-separated states (Bi+-Bi O -) with a rate\n2 3\nconstant k (Stage I). Subsequently, charge recombination (CR) of the Bi+-Bi O - pairs\nHT 2 3\nproduces triplet excitons (3Bi O *) at a rate k (Stage II). Finally, the triplets (3Bi O *)\n2 3 CR 2 3\ndecay to regenerate ground-state BBO-4 (Stage III). Therefore, we can write the\nfollowing expressions for the kinetics at these wavelengths:\nS (t) = A 1 e-t/τ HT + A 2 e-t/τ CR +A 3 e-t/τ T (650 nm)\nWhere τ , τ and τ are the time constants of the components of the HT, CR and\nHT CR T\ntriplet decay processes, respectively. Using the same set of HT, CR and triplet decay\nparameters, we simultaneously fit the kinetics at the above wavelengths, corresponding\nto τ , τ , and τ , respectively.\n1 2 3\nComment 20. The authors state that electron transfer from Bi O to metallic Bi within\n2 3\nthe ohmic junction was confirmed. However, when electrons flow from a\nsemiconductor to a metal, I think a depletion region typically forms, making ohmic\ncontact unlikely. Could the authors clarify this point?\nResponse: We sincerely thank the reviewer for this insightful comment, which\nhighlights an important theoretical consideration. We agree that for a conventional n-\ntype semiconductor interfacing with a higher-work-function metal, electron transfer\nfrom the semiconductor to the metal would indeed create a depletion region and a\nSchottky barrier, not an ohmic contact. However, in our specific system (Bi O /metallic\n2 3\nBi), metallic bismuth (Bi0) has a relatively low work function (~4.78 eV) compared\nwith Bi O , as evidenced by our density functional theory (DFT) calculations (Figure\n2 3\nR). Upon forming an intimate heterojunction, depicted in Figure R, the free electrons\n50\nin Bi migrate to Bi O , reducing the Fermi level (E) in Bi and increasing the Fermi\n2 3 f\nlevel of Bi O until equilibrium is reached. This electron transfer results in a positive\n2 3\ncharge on Bi surface due to electron depletion and a negative charge on Bi O due to\n2 3\nelectron accumulation. The charge distribution establishes an IEF from Bi to Bi O ,\n2 3\nfacilitating the separation and migration of photogenerated carriers. The accumulated\nelectron induces a downward band bending in Bi O , forming an Ohmic junction with\n2 3\nBi.\nWe appreciate the reviewer’s keen observation, which has prompted us to provide a\nmore detailed physical explanation of this Ohmic junction.\nFigure R2. | (Supplementary Figure S8, in the revised supporting information).\nSchematic illustration of IEF-induced Bi/Bi O Ohmic-junction for CO\n2 3 2\nphotoreduction.\nComment 21. The authors further describe that well-defined BBO-4 Ohmic junction\nwere verified by high resolution TEM. (line 143) However, it may be difficult for me\nto determine whether ohmic contact has been formed solely based on the TEM images.\nA more detailed explanation would be appreciated.\nResponse: Thank you for your insightful question. In this manuscript, we verified the\ncorrectness of the Ohm junction structure through the calculation results of the work\nfunction, Fermi energy level and the charge density difference. The theoretical work\nfunctions of Bi and Bi O were calculated to be 4.78 and 5.15eV, respectively (Figure\n2 3\nR2; Supplementary Figure S8, in the revised supporting information). The much\nsmaller work function of Bi promoted the charge transfer from Bi to Bi O . This\n2 3\n51\nelectron transfer drives the alignment of their Fermi levels and concurrently generates\na strong built-in electric field with associated band bending, which is the characteristic\nof the ohmic junction. This evidence confirmed the establishment of an ohmic junction\nat the Bi/Bi O interface. We have included this discussion on line 126-131 of the\n2 3\nrevised manuscript.\nAdditionally, the excellent lattice matching and well-defined BBO-4 Ohmic junction\nwere verified by high resolution TEM (HRTEM; Figures 2f-g), demonstrating the\nsuccess in fabricating the Ohmic junction. The high-quality coherent heterointerface\nwith a 0.328 nm lattice fringe spacing belonging to the (012) plane of Bi and a 0.274-\nnm lattice fringe spacing attributing to the (400) plane of Bi O . We have included this\n2 3\ndiscussion on line 164-168 of the revised manuscript.\nThe above results theoretically and experimentally demonstrate the formation of a\nBi/Bi O Ohmic junction.\n2 3\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: We sincerely appreciate the reviewer's valuable comments and positive\nfeedback. The insightful questions and constructive suggestions have been instrumental\nin enhancing the quality of our work. We have carefully addressed all raised issues and\nrevised the manuscript accordingly.\nThis work demonstrates that constructing an Ohmic junction at the BBO interface\nsynergistically enhances photocatalytic CO2-to-CH3OH conversion by optimizing\ncharge dynamics and interfacial energetics. The metallic Bi–semiconductor Bi2O3\nOhmic junction induces a robust built-in electric field and unidirectional electron\ntransfer, suppressing charge recombination while promoting CO activation and\n2\nmethanol synthesis. This study provides mechanistic insights into Ohmic junction\nengineering for photocatalytic CO conversion, emphasizing its dual role in charge\n2\nmanagement and surface reaction optimization. However, the methanol production rate\nof the optimized catalyst is only 610 μmol g-1, which is too low among the reported\nresults, the analysis of some characterizations and reaction mechanisms remains unclear.\nThe manuscript remains in need of significant improvement across several dimensions.\nIt is not suitable for publication in the journal of Nature Commun.\nResponse: Thank you for acknowledging the mechanistic insights into Ohmic junction\nengineering for photocatalytic CO conversion in our manuscript. Achieving efficient\n2\nphotocatalytic CO -to-CH OH reduction is a significant challenge. The metallic Bi-\n2 3\nsemiconductor Bi O Ohmic junction with local surface plasmon resonance effect\n2 3\ninduced a robust built-in electric field, as confirmed by in-situ Kelvin probe force\nmicroscopy and Mott-Schottky analysis, which drove the unidirectional electron\ntransfer from Bi O to Bi while suppressing back-recombination. The photocatalytic\n2 3\nCO reduction performances of synthesized catalysts were evaluated under full\n2\nspectrum (Figure 3d and Supplementary Figure 18). The primary products of Bi, Bi O ,\n2 3\nand Bi S resulting from photocatalytic CO reduction were mainly CO. The precursor\n2 3 2\nBi Br S nanowires revealed the generation of CO, CH , and CH OH. The series of\n19 3 27 4 3\nBBO derived from Bi Br S nanowires displayed enhanced photocatalytic activity for\n19 3 27\nCH OH generation. The optimal BBO-4 samples reached the highest value of 610 µmol\n3\ng-1, substantially surpassing the rates observed with unmodified Bi, Bi O ,\n2 3\nprecursor Bi Br S nanowires. In contrast, no reduction products were observed under\n19 3 27\ndark conditions, in the absence of a catalyst or Ar atmosphere (Figure 3e). The above\n54\nresults of controlled experiments demonstrated the generation of products as a result of\nreduced CO in the photocatalytic system. To demonstrate the superior activity of BBO-\n2\n4, we conducted comparative analyses with over 20 different catalytic systems (Figure\n3h and Supplementary Table S1). The results conclusively showed that BBO-4\nexhibited favourable photocatalytic performance among the reported photocatalytst.\nPhotocatalytic CO reduction performances of BBO-4 compared with reported\n2\nrepresentative photocatalysts (the detailed reaction conditions were listed in Table R1).\nTable R4 | (Supplementary Table S4, in the revised supporting information).\nPhotocatalytic CO reduction performances of BBO-4 compared with reported\n2\nrepresentative photocatalysts.\nNo. Photocatalysts Yields Additives Light Reference\n(μmol•h-1•g- source\n1)\n2 2\n2\n4 g-C N /ZnO 6 - 300 W Xe lamp [7]\n3 4\n2 2 3\n(λ > 420 nm)\n2 3 10\nUV-light\n2 2\n(Hg-125 W)\n2\n(λ > 420 nm)\nLED\n(λ = 435 nm 3.0\n3 4 3 2\nmW·cm-2 )\n2 2 3\n2 4 2 2\nLED\n(λ = 435 nm 3.0\n3 4 3 2\nmW·cm-2 )\n2\n(UV cut-off\nfilter)\n15 rGO-Cu O 35.96 H O 20 W white cold [18]\n2 2\nLED\n2\n(λ > 400 nm)\n55\n3\n2 6 2\n2\n(λ = 400 nm)\n2 2\n(CS@Pd)\nO\n2\n2 3 2\n2 2\nhigh pressure\nmercury lamp\n2\n(λ = 365 nm)\n3 4 2\n2\n2\n[4] B. An, J. Zhang, K. Cheng. J. Am. Chem. Soc. 2017, 139, 3834-3840.\n[5] H. Abdullah, N. A. Ismail, Z. Yaakob, Malaysian J. Anal. Sci. 2017, 21, 166-172.\n[6] G. Wang, C. T. He, R. Huang, J. J. Mao, D. S. Wang, Y. D. Li, J. Am. Chem. Soc.\n2020, 142, 19339-19345.\n[7] M. Sayed, B. C. Zhu, P.Y. Kuang, X. Y. Liu, B. C, A. A. A. Ghamdi, S. Wageh, L.\nY. Zhang, J. G. Yu, Adv. Sustainable Syst. 2022, 6, 2100264.\n[8] Y. Su, Z. Zhang, H. Liu, Y. Wang, Appl. Catal. B. 2017, 200, 448-457.\n[9] N. N. Vu, C. C. Nguyen, S. Kaliaguine, T. O. Do, Adv. Sustainable Syst. 2017, 1,\n1700048.\n[10] D. Permporn, R. Khunphonoi, J. Wilamat, P. Khemthong, P. Chirawatkul, T.\nButburee, W. Sangkhun, K. Wantala, N. Grisdanurak, J. Santatiwongchai, P. Hirunsit,\nW. Klysubun, M. Luna, Nanomaterials. 2022, 12, 474.\n[11] H. Guo, J. Ding, S. Wan, Y. Wang, Q. Zhong, Appl. Surf. Sci. 2020, 528, 146943.\n[12] T. Ohno, N. Murakami, T. Koyanagi, Y. Yin, J. CO2 Util. 2014, 6, 17-25.\n[13] S.Goyal, M. S. Shaharun, C. F. Kait, B. Abdullah, M. Ameen, Catalysts 2018, 8,\n581.\n[14] M. Tahir, B. Tahir, M. G. M. Nawawi, M. Hussain, A. Muhammad, Appl. Surf. Sci.\n2019, 485, 450-461.\n[15] R. Fiorenza, C. Crisafulli, G. G. Condorelli, F. Lupo, Catal. Lett. 2015, 145, 1691-\n1702.\n[16] B. Tahir, M. Tahir, N. A. S. Amin, Appl. Surf. Sci. 2017, 419, 875-885.\n[17] J. Meng, Q. Chen, J. Lu, H. Liu, ACS Appl. Mater. Interfaces 2019, 11, 550-562.\n[18] R. Gusain, P. Kumar, O. P. Sharma, S. L. Jain, O. P. Khatri, Appl. Catal. B. 2016,\n181, 352-362.\n[19] C. An, J. Wang, W. Jiang, M. Zhang, X. Ming, S. Wang, Q. Zhang, Nanoscale.\n2012, 4, 5646.\n[20] C. Wei, Y. Xue, C. Yan, H. Chenyao, J. Environ. Chem. Eng. 2022, 10, 107461.\n[21] N. Lia, X. Liu, J. Zhou, Chem. Eng. J. 2020, 399, 125782.\n[22] S. Liu, S. Zhou, Z. Chen, C. Liu, Catal. Commun. 2016, 73, 7-11.\n56\n[23] V. Sagar, Y. T. Prabhu, T. Anjana, T. Ranjit, B. Sreedhar, A. Polumati, Inorg. Chem.\n2023, 62, 7235-7249.\n[24] X. Li, H. Liu, D. Luo, J. Li, Y. Huang, H. Li, Y. Fang, Y. Xu, L. Zhu, Chem. Eng.\nJ. 2012, 180, 151-158.\n[25] X. Shang, G. Li, R. Wang, T. Xie, J. Ding, Q. Zhong, Chem. Eng. J. 2023, 456,\n140805.\n[26] T. Yang, Q. Yu, H. Wang, Catal. Lett. 2018, 148, 2382-2390.\n[27] Z. Zhen, C. Y. Chen, W. R. J. Wu, J. Chin. Chem. Soc. 2020, 67, 1654-1660.\n[28] M. M. Kandy, V. G. Gaikar, Mater. Res. Bull. 2018, 102, 440-449.\nComment 1. From line 143 to 145, the HRTEM analysis did not provide additional\ncharacterization of the formed Bi metal particles, including particle size distribution,\netc.\nResponse: Thank you for this important comment. Figure R22 confirms that BBO-4\nexhibits a continuous nanosheet morphology. No discernible Bi metal particles or Bi\noxide particles are observed in the high-resolution TEM images, indicating that\nBi/Bi O exists as interconnected sheets rather than discrete particles. Consequently,\n2 3\nparticle size distribution analysis was not applicable. The SAED pattern (corresponding\nto Bi (012) and Bi O (400) planes) further supports the coexistence of both phases\n2 3\nwithin the nanosheet framework.\nFigure R22. (a) TEM and (b) the corresponding SAED of BBO-4.\nComment 2. From line 164 to 170, \"These XPS results collectively suggest electron\ntransfer from Bi O to metallic Bi within the heterojunction, consistent with the electron\n2 3\n57\nflow direction in an Ohmic junction.\" However, a binding energy shift of less than 0.1\neV cannot serve as strong evidence for charge transfer.\nResponse: We thank the reviewer for highlighting this point. The apparent binding\nenergy shift in Bi 4f spectra stems from local charge polarization at the Bi/Bi O\n2 3\ninterface. We acknowledge an inaccuracy in our initial data analysis and have refined\nthe interpretation accordingly. Revised analysis is added in Line 202-217 in the revised\nmanuscript: “To investigate the electronic states and charge transfer of BBO-4, XPS\nanalysis was employed. For Bi O , the XPS diffraction peaks at 159.00 eV (Bi 4f )\n2 3 7/2\nand 164.32 eV (Bi 4f ) were characteristic of Bi3+ species (Supplementary Figure\n5/2\n21a).20-21 The characteristic Bi 4f peaks of Bi metals could be fitted into two pairs of\ndoublets associated with Bi0 and Bi3+. The XPS signal of Bi oxidation states in Bi metals\nwas derived from surface oxidation of the catalyst under exposure to air. For BBO-4,\nthe Bi 4f signals deconvoluted into four peaks: two at 156.68 and 161.98 eV (assigned\nto Bi0 in metallic Bi) and two at 158.89 and 164.21 eV (attributed to Bi3+ in Bi O ).22-\n2 3\n2 3\n0.11 eV positive shift in the Bi0 binding energy for the heterojunction relative to pristine\nmetallic Bi, indicating decreased electron density around Bi0. Conversely, the Bi3+\nbinding energy in BBO showed an 0.11 eV downshift compared to Bi O , signifying\n2 3\nincreased electron density around Bi3+. These XPS results collectively suggest electron\ntransfer from metallic Bi to Bi O within the heterojunction, consistent with the electron\n2 3\nflow direction in Ohmic junction.25-27 The O 1s XPS spectrum of BBO-4 was the same\nas that of pristine Bi O , with peaks at 529.52 and 530.85 eV, which were attributed to\n2 3\nthe chemical bonding of Bi-O and adsorbed oxygen (H-O), respectively\n(Supplementary Figure 21b). The results are in accordance with the above work\nfunction analysis.”\nBi O , and BBO-4: (a) Bi 4 f and (b) O 1s.\n2 3\nComment 3. From line 171 to 176, why are there strong Bi O peaks in the XPS\n2 3\nspectrum of metallic Bi in Figure 2I? This needs to be clearly explained.\nResponse: Thank you for your insightful question. We have addressed the concerns as\nfollows:\nThe characteristic Bi 4f peaks of Bi metals could be fitted into two pairs of doublets\nassociated with Bi0 and Bi3+. The XPS signal of Bi oxidation states in Bi metals was\nderived from surface oxidation of the catalyst under exposure to air. To ensure accurate\nresults of the analysis, we have reviewed several articles and found that the results were\nconsistent.\nReferences:\n1. S. Y. Yang, M. H. Jiang, W. J. Zhang, Y. Hu, J. C. Liang, Y. D. Wang, Z. X. Tie, Z.\nJin, In situ structure refactoring of bismuth nanoflowers for highly selective\nelectrochemical reduction of CO to formate. Adv. Funct. Mater. 2023, 33, 2301984.\n2\n(The same result can be seen in Figure 2a)\n2. Y. X. Li, D. P. Hui, Y. Q. Sun, Y. Wang, Z. J. Wu, C. Y. Wang, J. C. Zhao, Boosting\nthermo-photocatalytic CO conversion activity by using photosynthesis-inspired\n2\nelectron-proton-transfer mediators. Nat. Commun. 2021, 12, 123. (The same result\ncan be seen in Supplementary Figure 10)\n3. X. W. Wang, Y. Y. Zhang, S. Wang, Y. F. Li, Y. F. Feng, Z. C. Dai, Y. X. Chen, X.\naccelerated dynamics for CO electroreduction. Angew. Chem. 2024, 136,\n2\ne202407665. (The same result can be seen in Figure 2c)\nIn response to your suggestion, we have included a detailed discussion in the revised\nmanuscript on line 203-206: “The characteristic Bi 4f peaks of Bi metals could be fitted\ninto two pairs of doublets associated with Bi0 and Bi3+. The XPS signal of Bi oxidation\nstates in Bi metals was derived from surface oxidation of the catalyst under exposure\nto air.”\nComment 4. From line 209 to 212, although the specific surface area of metallic Bi is\nmuch smaller than that of Bi O , why does the BBO-4 exhibit a higher specific surface\n2 3\narea compared to pure Bi O ?\n2 3\nResponse: Thank you for this suggestion. In this work, BBO composites were\nfabricated by wet-chemical cascade strategy. The Bi Br S nanowires have been\n19 3 27\nsynthesized firstly by hydrothermal method, then followed by a NaBH reduction\n4\nmethod to inducing the dissolution and crystallization of Bi species, and finally the\nBBO composites were obtained. The TEM images (Figures 2d-e; Supplementary\nFigures S11-13) indicated the ultrathin nanosheet morphology of the samples, showing\nlager specific surface area than bulk Bi and Bi O (Supplementary Figure S27), which\n2 3\nwas favorable to provide much more active sites.\nComment 5. From line 253 to 265, Figure 4g shows that CO reacts with OH groups\n2\non Bi to form CO 2-. It is necessary to clarify the origin of the OH groups: whether they\n3\narise from water splitting or protonation of O in Bi O .\n2 3\nResponse: Thank you for your valuable suggestion. We have addressed the concerns as\nfollows:\nThe surface structure of the samples was investigated by XPS analysis. An shown in\nSupplementary Figure 20b, the O 1s XPS spectrum of BBO-4 was the same as that of\npristine Bi O , with peaks at 529.52 and 530.85 eV, which were attributed to the\n2 3\n60\nchemical bonding of Bi-O and adsorbed oxygen (H-O), respectively. The above result\nshowed that the surface of the catalyst contained a large number of hydroxyl groups.\nThe catalytic process typically initiates with the adsorption and activation of CO\n2\nmolecules on the catalyst surface, forming reactive intermediates such as carbonate\n(*CO ) and bicarbonate (*HCO ) species (via pathways like CO + *O/*OH →\n3 3 2\n*CO /*HCO )1-3. These intermediates then undergo a complex reaction network\n3 3\ninvolving multiple steps and numerous transient species, ultimately yielding the final\nproducts. Although the conversion of CO to CO appears to be a simple oxygen removal,\n2\nit actually proceeds through a pathway mediated by intermediates like formate (*HCO )\n2\nand carboxyl (*COOH) species.4-6 Specifically, *HCO can dissociate to form *HCO\n3 2\nand adsorbed oxygen (*HCO → *HCO + *O). Subsequently, *HCO decomposes to\n3 2 2\nyield adsorbed CO and hydroxyl (*HCO → *CO + *OH). Alternatively, *CO can\n2 3\nreact with adsorbed hydrogen (*H) to form a *COOH intermediate (*CO + *H →\n3\n*COOH + *O), which then decomposes to produce adsorbed CO and hydroxyl\n(COOH* → CO* + OH*).7-8 According to analysis of the in situ DRIFTS spectra, the\npossible photocatalytic steps of CO reduction on BBO-4 were proposed in Figure 4g.\nmechanisms of CO adsorption and CO -to-CH OH photosynthesis on the surface of\n2 2 3\nBBO-4 nanosheets.\nReference:\n(1) H. Jiang, L. Wang, H. Kaneko, R. Gu, G. Su, L. Li, J. Zhang, H. Song, F. Zhu, A.\nYamaguchi, Light-driven CO methanation over Au-grafted Ce Ru O solid-\n2 0.95 0.05 2\nsolution catalysts with activities approaching the thermodynamic limit. Nat. Catal.\n2023, 6, 519-530.\n61\n(2) Y. Yang, Z. Chai, X. Qin, Z. Zhang, A. Muhetaer, C. Wang, H. Huang, C. Yang, D.\nMa, Q. Li, et al. Light-induced redox looping of a Rhodium/Ce WO photocatalyst\nx 3\nfor highly active and robust dry reforming of methane. Angew.Chem. Int.Ed. 2022,\n61, e202200567.\n(3) F. Marques Mota, D. H. Kim, From CO methanation to ambitious long-chain\n2\nhydrocarbons: alternative fuels paving the path to sustainability. Chem. Soc. Rev.\n2019, 4, 205-259.\n(4) B. Xie, R. J. Wong, T. H. Tan, M. Higham, E. K. Gibson, D. Decarolis, J. Callison,\nK. F. Aguey-Zinsou, M. Bowker, C. R. A. Catlow, et al. Synergistic ultraviolet and\nvisible light photo-activation enables intensified low-temperature methanol synthesis\nover copper/zinc oxide/alumina. Nat. Commun. 2020, 11, 1615.\n(5) Z. Zhang, C. Mao, D. M. Meira, P. N. Duchesne, A. A. Tountas, Z. Li, C. Qiu, S.\nTang, R. Song, X. Ding, et al. New black indium oxide-tandem photothermal CO -\n2\n(6) T. H. Tan, B. Q. Xie, Y. H. Ng, S. F. B. Abdullah, H. Y. M. Tang, N. Bedford, R. A.\nTaylor, K. F. Aguey-Zinsou, R. Amal, J. Scott, Unlocking the potential of the formate\npathway in the photo-assisted Sabatier reaction. Nat. Catal. 2020, 3, 1034-1043.\n(7) K. Zhao, L. Wang, E. Moioli, M. Calizzi, A. Zuttel, Identifying reaction species by\nevolutionary fitting and kinetic analysis: an example of CO hydrogenation in\n(8) Z. Q. Rao, Z. A. Huang, G. X. Chen, L. Z. Zhang, K. W. Wang, Y. H. Cao, Y. L.\nChen, Y. T. Yang, Q. Y. Feng, A. K. Weidenkaff, Y. Zhou, Atomically dispersed\npraseodymium-modified Ni active sites boost the direct cleavage of carbonate\nintermediates for photothermal CO conversion. 2025 ACS Nano.\n2\nhttps://doi.org/10.1021/acsnano.5c05699.\nComment 6. From line 276 to 280, the authors mentioned using normalized potential\ngradients, but Figures 5-7 do not present the normalized potential gradient data. Please\nspecify the data source of the normalized potential gradient or the methodology used to\n62\nderive the normalization.\nResponse: Thank you for your valuable suggestion. Through in situ Kelvin probe force\nmicroscopy (KPFM) coupled with controlled light illumination, we directly visualized\nthe spatial-temporal dynamics of photoinduced charge carriers in BBO-4. Under dark\nconditions, surface potential line profiles revealed distinct differences among the\nmaterials: Bi O and BBO-4 exhibited surface potentials of 0.50, 0.97 mV, respectively\n2 3\n(Figures 5a-b). Light irradiation induced dramatic surface potential reduction in BBO-\n4 (ΔV = 0.62 mV, Figure 5c), demonstrating efficient photoinduced electron migration\nto the material surface under built-in electric field guidance. Notably, BBO-4\ndemonstrated the much more pronounced surface potential heterogeneity, suggesting\nenhanced charge redistribution capabilities. Thickness-normalized surface potential\nanalysis in Figures 5c and Supplementary Figure 13 quantified this effect, showing\nnormalized potential gradients of 0.21 and 0.13 mV nm-1 for BBO-4 with or without\nlight irradiation, respectively.1 However, the normalized potential difference of BBO-4\ndecreased to 0.08 mV nm-1, indicating that the electric field can be optimized by\nappropriate Bi content.\nNote: The thickness-normalized surface potential (mV nm-1) = surface potential /\nthickness.\nWe have updated the manuscript accordingly on line 332-336.\nReference:\n1. Y. T. Yu, Z. J. Zhu, F. Chen, T. Y. Ma, H. W. Huang, Triggering asymmetric layer\ndisplacement polarization and redox dual-sites activation by inside-out anion\nsubstitution for efficient CO photoreduction. Adv. Mater. 2024, 2413835.\n2\npotential analysis of (b-c) BBO-4 in the dark and under light irradiation.\nFigure R22 | (Supplementary Figure S15, in the revised supporting information).\n(a) AFM images of BBO-4 and (b) corresponding height profile.\nComment 7. From line 322 to 326, Bi O typically contains oxygen vacancies, which\n2 3\nmay also act as active sites influencing catalytic activity. Therefore, it is necessary to\nconfirm whether the reactive sites are localized at the BBO-4 interface rather than at\noxygen vacancies.\nResponse: We sincerely appreciate the reviewer’s comment regarding the potential role\nof oxygen vacancies (OVs) in Bi O photocatalysts. The reviewer rightly points out that\n2 3\nBi O is often associated with oxygen vacancy formation. However, our comprehensive\n2 3\nstructural characterization provides conclusive evidence that the specific Bi/Bi O\n2 3\nheterostructure within our BBO-4 catalyst, synthesized under controlled conditions,\nexhibits no detectable oxygen vacancies, confirming that the reactive sites are\nunequivocally localized at the Bi/Bi O interface. It was found through EPR testing that\n2 3\nthere were no oxygen vacancies in both Bi O and BBO-4 (Figure R7). It further\n2 3\nindicates that the reaction site is at the interface of BBO-4, not at the oxygen vacancy.\n2 3\nComment 8. The catalytic performance must be supplied in the abstract to clearly\npresent the activity of the catalyst. Is it a high level of the methanol yield in the reported\nresults, a detail comparison should be supplied.\nResponse: Thank you for the question. The required descriptions in the mentioned\nmanuscript have been carefully checked and corrected. The following revisions are\nmade as suggested.\n“This electronic redistribution at the Ohmic junction interface concurrently stabilizes\n*CHO intermediates, enabling a CH OH production rate of 610 μmol g-1 (152.67 g-1 h-\n3\n1) under light irradiation, which was much higher than that of Bi, Bi O , and most\n2 3\nreported researches.”\nTable R4 | (Supplementary Table S4, in the revised supporting information).\nPhotocatalytic CO reduction performances of BBO-4 compared with reported\n2\nrepresentative photocatalysts.\nNo. Photocatalysts Yields Additives Light Reference\n(μmol•h-1•g-1) source\n2 2\n2\n4 g-CN/ZnO 6 - 300 W Xe lamp [7]\n3 4\n2 2 3\n65\n(λ > 420 nm)\n2 3 10\nUV-light\n2 2\n(Hg-125 W)\n2\n(λ > 420 nm)\nLED\n(λ = 435 nm 3.0\n[12]\n3 4 3 2\nmW·cm-2 )\n2 2 3\n2 4 2 2\nLED\n(λ = 435 nm 3.0\n[16]\n3 4 3 2\nmW·cm-2 )\n2\n(UV cut-off filter)\n15 rGO-CuO 35.96 HO 20 W white cold LED [18]\n2 2\n2 3\n(λ > 400 nm)\n2 6 2\n2\n(λ = 400 nm)\n2 2\n2\n(CS@Pd)\n2 3 2\n2 2\nhigh pressure mercury\nlamp\n[26]\n2\n(λ = 365 nm)\n3 4 2\n2\nReferences:\n[4] B. An, J. Zhang, K. Cheng. J. Am. Chem. Soc. 2017, 139, 3834-3840.\n[5] H. Abdullah, N. A. Ismail, Z. Yaakob, Malaysian J. Anal. Sci. 2017, 21, 166-172.\n[6] G. Wang, C. T. He, R. Huang, J. J. Mao, D. S. Wang, Y. D. Li, J. Am. Chem. Soc.\n2020, 142, 19339-19345.\n[7] M. Sayed, B. C. Zhu, P.Y. Kuang, X. Y. Liu, B. C, A. A. A. Ghamdi, S. Wageh, L.\nY. Zhang, J. G. Yu, Adv. Sustainable Syst. 2022, 6, 2100264.\n[8] Y. Su, Z. Zhang, H. Liu, Y. Wang, Appl. Catal. B. 2017, 200, 448-457.\n[9] N. N. Vu, C. C. Nguyen, S. Kaliaguine, T. O. Do, Adv. Sustainable Syst. 2017, 1,\n1700048.\n66\n[10] D. Permporn, R. Khunphonoi, J. Wilamat, P. Khemthong, P. Chirawatkul, T.\nButburee, W. Sangkhun, K. Wantala, N. Grisdanurak, J. Santatiwongchai, P. Hirunsit,\nW. Klysubun, M. Luna, Nanomaterials. 2022, 12, 474.\n[11] H. Guo, J. Ding, S. Wan, Y. Wang, Q. Zhong, Appl. Surf. Sci. 2020, 528, 146943.\n[12] T. Ohno, N. Murakami, T. Koyanagi, Y. Yin, J. CO2 Util. 2014, 6, 17-25.\n[13] S.Goyal, M. S. Shaharun, C. F. Kait, B. Abdullah, M. Ameen, Catalysts 2018, 8,\n581.\n[14] M. Tahir, B. Tahir, M. G. M. Nawawi, M. Hussain, A. Muhammad, Appl. Surf. Sci.\n2019, 485, 450-461.\n[15] R. Fiorenza, C. Crisafulli, G. G. Condorelli, F. Lupo, Catal. Lett. 2015, 145, 1691-\n1702.\n[16] B. Tahir, M. Tahir, N. A. S. Amin, Appl. Surf. Sci. 2017, 419, 875-885.\n[17] J. Meng, Q. Chen, J. Lu, H. Liu, ACS Appl. Mater. Interfaces 2019, 11, 550-562.\n[18] R. Gusain, P. Kumar, O. P. Sharma, S. L. Jain, O. P. Khatri, Appl. Catal. B. 2016,\n181, 352-362.\n[19] C. An, J. Wang, W. Jiang, M. Zhang, X. Ming, S. Wang, Q. Zhang, Nanoscale.\n2012, 4, 5646.\n[20] C. Wei, Y. Xue, C. Yan, H. Chenyao, J. Environ. Chem. Eng. 2022, 10, 107461.\n[21] N. Lia, X. Liu, J. Zhou, Chem. Eng. J. 2020, 399, 125782.\n[22] S. Liu, S. Zhou, Z. Chen, C. Liu, Catal. Commun. 2016, 73, 7-11.\nTitle: Atomic-Scale Interface Engineering in Bi/Bi O Heterojunctions for Selective\n2 3\nCO Photoreduction to Methanol\nGeneral response:\nWe thank the editor and reviewers for their valuable and insightful comments and\nsuggestions, which have significantly helped improve the quality of our manuscript. We\nhave revised the manuscript in response to these comments. Our point-by-point\nresponses are provided below.\nReviewer #1 (Remarks to the Author)\nIn the manuscript, Bi Br S was used as the precursor to obtain Bi/Bi2O3 catalyst,\n19 3 27\nwhich has a certain degree of innovation in preparation. The utilization of light energy\nto convert CO into high value-added methanol is of great practical significance. And\n2\nthe authors have reasonably answered all questions and agreed to accept this manuscript.\nResponse: We sincerely thank the reviewer for their insightful and thorough evaluation\nof our work.\nReviewer #2 (Remarks to the Author):\nThe authors have clarified some points, but several critical issues persist. Thus, the\nmanuscript is not suitable for publication in its current form unless the following\nconcerns are comprehensively resolved.\nResponse: We greatly appreciate the reviewers’ positive feedback on our work. In\nresponse to their comments, we have carefully revised the manuscript and\nSupplementary Information, with all modifications clearly highlighted in the updated\ndocuments. Our point-by-point responses to each specific comment are detailed below.\nComment 1. The authors employed KPFM to address the issue of charge transfer.\nHowever, this method requires certain preconditions: First, an AFM image of BBO-4\nshould be provided, and combining this AFM image with corresponding SEM (or TEM)\nand elemental mapping data to precisely identify the Bi/Bi O interface locations within\n2 3\n1\nthe AFM image. Then, KPFM measurements be performed, under accurately\nidentifying these interface locations. Currently, the authors’ evidence for charge\ntransfer relies solely on an observed reduction in potential. This alone is insufficient\nproof.\nResponse: Thank you for this valuable feedback. We agree that precise localization of\ninterfaces and multi-modal validation are essential for robust interpretation of KPFM\nresults. We have thoroughly revised our study to address these concerns, with key\nimprovements outlined below:\n1. Proof of Bi/Bi O heterojunction\n2 3\n1) TEM measurements for confirming the formation of Bi/Bi O heterojunction\n2 3\nIn this work, Bi@Bi O (BBO) composites were fabricated by wet-chemical cascade\n2 3\nstrategy. The Bi Br S nanowires have been synthesized firstly by hydrothermal\n19 3 27\nmethod, then followed by a NaBH reduction method to inducing the dissolution and\n4\ncrystallization of Bi species, and finally the BBO composites were obtained. The TEM\nimages (Figures 2c-e) indicated the ultrathin nanosheet morphology of the samples,\nshowing lager specific surface area than Bi and Bi O (Supplementary Figure S26),\n2 3\nwhich was favorable to provide much more active sites. The excellent lattice matching\nand well-defined BBO-4 Ohmic junction were verified by high resolution TEM\n(HRTEM; Figures 2f-g), demonstrating the success in fabricating the Ohmic junction.\nThe high-quality coherent heterointerface with a 0.328 nm lattice fringe spacing\nbelonging to the (012) plane of Bi and a 0.274-nm lattice fringe spacing attributing to\nthe (400) plane of Bi O .\n2 3\n2) XRD Rietveld refinement for phase composition determination\nTo precisely resolve the phase composition, Rietveld refinement of the XRD patterns\n(Supplementary Figure 19) was performed to determine the phase composition of the\nBBO catalysts. The refinement results confirm the exact phase composition. The\nBi/Bi O mass ratios for BBO-2, BBO-4 and BBO-6 are 53.65% wt/46.35 wt%, 91.75%\n2 3\nwt/8.25 wt%, and 97.74 wt%/2.26 wt%, respectively. The Bi content increases with\nincreasing reduction time. The above results show there is on other phase structures.\ndirect observation of charge transfer direction using KPFM. However, because of the\nsimilar morphology of Bi and Bi O , KPFM only detected the overall potential change,\n2 3\ncannot observed the charge transfer process. To compensate for this characterization\ndeficiency, femtosecond transient absorption spectroscopy and theoretical calculations\nwere employed to study the charge carriers’ transfer and separation.\n2. Detection of charge transfer\n1) femtosecond transient absorption (fs-TA) spectroscopy\nTo elucidate the interfacial charge transfer mechanisms in the samples, fs-TA\nspectroscopy was conducted. Strikingly, Bi O (Figures 5d-e) and BBO-4 (Figures 5g-\n2 3\nh) demonstrated extended photoinduced absorption spanning 450-570 nm. The\nabsorption intensity of BBO-4 was stronger than that of Bi O , indicating enhanced\n2 3\nlight-matter interactions in BBO-4 Ohmic junction system through interfacial\nelectronic state hybridization. Photogenerated electron relaxation in the CB of Bi O\n2 3\nand BBO-4 occurred through three pathways: exciton-mediated states (τ ), defect trap\n1\nstates (τ ), and recombination of e- and h+(τ ). Notably, the lifetime components of\n2 3\nBBO-4 (τ and τ ) were much longer than that of Bi O (τ = 2.95 ps), which was\n1 2 2 3 1\nattributed to the ineffective utilization of photoelectrons within a single photocatalyst,\nhindering efficient photocatalytic activity. The shorter lifetime of τ (994 ps) for BBO-\n3\n4 compared with Bi O (τ = 1490 ps) was favorable to the interfacial charge transfer\n2 3 3\nof BBO-4 Ohmic junction (Supplementary Figure 34).\n2) DFT calculations\nDensity functional theory (DFT) calculations confirmed metallic bismuth (Bi⁰)\nexhibits a lower work function (~4.78 eV) than Bi O . Upon heterojunction formation,\n2 3\nelectrons spontaneously flow from Bi to Bi O to equilibrate the Fermi levels. This\n2 3\ntransfer creates: Positive surface charge on Bi due to electron depletion and negative\nsurface charge on Bi O through electron accumulation. The resultant interfacial charge\n2 3\ndistribution establishes an interfacial electric field (IEF) directed from Bi to Bi O ,\n2 3\nwhich drives photogenerated carrier separation and migration. Simultaneously, electron\n3\naccumulation in Bi O induces downward band bending, forming an Ohmic contact\n2 3\nwith metallic Bi.\nThus, the synergistic implementation of these analytical methods provides\ncompelling evidence for the charge transfer mechanism.\nComment 2. An additional suggestion regarding the KPFM presentation: The scale\nbars in the dark and light KPFM images (Figure R5) should be set to the exact same\nscale to allow for a direct and fair visual comparison. Currently, the scale bars in the\nleft and right images of Figure R5 differ significantly.\nResponse: We appreciate the reviewer’s valid concern regarding the comparative\nanalysis of KPFM images in Figure R5. After careful consideration, we determined that\nmaintaining distinct scales is technically necessary. If we adjust the scale to be the same,\none of the datasets would become indistinct. To ensure rigorous comparability while\npreserving data integrity, we have implemented a solution: Thickness-normalized\nsurface potential analysis in Figures 5c and Supplementary Figure 13 quantified this\neffect, showing normalized potential gradients of 0.21 and 0.13 mV nm-1 for BBO-4\nwith or without light irradiation, respectively.1 However, the normalized potential\ndifference of BBO-4 decreased to 0.08 mV nm-1, indicating that the electric field can\nbe optimized by appropriate Bi content.\nNote: The thickness-normalized surface potential (mV nm-1) = surface potential /\nthickness.\nReference:\n1. Y. T. Yu, Z. J. Zhu, F. Chen, T. Y. Ma, H. W. Huang, Triggering asymmetric layer\ndisplacement polarization and redox dual-sites activation by inside-out anion\nsubstitution for efficient CO photoreduction. Adv. Mater. 2024, 2413835.\nsuggested that EDS images also be included to more clearly demonstrate the content of\nBi.\nResponse: Thank you for this valuable suggestion. We agree that including elemental\nmapping images (EDS mappings) will provide a clearer visual representation of the Bi\n4\ndistribution within the sample. We have now included representative EDS mapping\nimages clearly showing the Bi content and its distribution in the revised supporting\ninformation (Supplementary Figures S10-12; Supplementary Tables S2-4) (Figures R1-\nR3, Tables R2-R4).\nFigure R1. | (Supplementary Figure S10, in the Supplementary Information). (a)\nTEM and (b) HAADF-STEM and (c-e) EDS mapping image of BBO-2.\nTable R1. | (Supplementary Table S2, in the Supplementary Information). Content\nof Bi element in different samples based on EDS analysis for BBO-2.\nSample Element Content (Mass%)\nC 11.48\nO 8.56\nAl 0.24\nBBO-2 Ca 1.14\nCr 0.80\nBi 77.77\nTEM and (b) HAADF-STEM and (c-d) EDS mapping image of BBO-4.\nTable R2. | (Supplementary Table S3, in the Supplementary Information).\nContent of Bi element in different samples based on EDS analysis for BBO-4.\nSample Element Content (Mass%)\nC 29.58\nO 9.21\nAl 0.28\nBBO-4\nCa 1.81\nCr 0.63\nAu 2.21\nBi 56.27\nTEM and (b) HAADF-STEM and (c-d) EDS mapping image of BBO-6.\nTable R3. | (Supplementary Table S4, in the Supplementary Information). Content\nof Bi element in different samples based on EDS analysis for BBO-6.\nSample Element Content (Mass%)\nC 18.88\nO 5.58\nBBO-6\nCr 0.93\nBi 74.61\nproducing CO instead of CH OH, a related issue remains unresolved. Specifically,\n2 3\n*CO. Why does Bi O still primarily produce CO rather than proceeding further to form\n2 3\nCH OH? Furthermore, why do both metallic Bi and Bi O phases favor CO production,\n3 2 3\nwhereas the composite phase shifts the selectivity predominantly towards CH OH?\nclusters rather than periodic structures. As rigorously suggested, we have re-performed\nall DFT calculations using slab models (Bi(012) and Bi O (100) surfaces) to ensure\n2 3\nphysical accuracy. Our revised results provide fundamental insights into the selectivity\nparadox:\n1) For Bi O : Kinetic dominance overrides thermodynamics. While *CHO formation\n2 3\nis thermodynamically favorable (ΔG < 0), the barrier for *CO desorption is 2.32 eV.\nThis kinetic bottleneck promotes *CHO generation before *CO desorption. Moreover,\nthe free energy of *CH OH desorption *CHO was 3.50 eV. Concurrently, *CH OH →\n3 3\nCH OH faces a 3.50 eV endergonic step, creating a thermodynamic trap that stalls\n3 3 2 3\nbarrier for CO generation is lower. Therefore, the Bi O system exhibited the generation\n2 3\nof CO rather than CH OH.\n3\n2) Regarding metallic Bi system: from a thermodynamic standpoint, COOH*\nformation represented a rate-limiting step in the CO reduction process with free-energy\n2\nbarriers of 2.07 eV. The free energy of CO (g) desorption over Bi was much lower than\nthat of *CHO formation, which result in the CO was the main product.\n3) The composite’s methanol selectivity emerges from interfacial synergy: the\nfabrication of Bi/Bi O (BBO) heterojunction greatly decreased the free energy of rate-\n2 3\nlimiting step. Further, the fabrication BBO with interfacial sites significantly lowered\nthe energy barrier for *CO hydrogenation and promoted the formation of *COOH\n2\ncompared with BBO with metallic Bi sites. Additionally, the *CHO formation energy\nwas much lower than that of CO (g), suggesting the formation of *CHO was easier than\n8\nthat of CO (g), which was favorable to the generation of CH OH.\nbarrier of the rate-determining step and facilitates CH OH production.\npeak for 13C-labeled CH OH is expected at m/z 33, there is a high probability that a\n3\nseries of fragment peaks will be generated in mass spectrometry. Then why are there no\ncorresponding fragment peaks in the mass spectrum? Moreover, the [13CH OH]+\n2\nfragment peak is usually more intense than the molecular ion peak.\nResponse: Response to Reviewer’s comment on Mass Spectral Fragmentation Patterns\nWe thank the reviewer for this technically astute observation. During our experimental\nprocess, we measured three products separately: CO, CH , and CH OH. For qualitative\n4 3\nanalysis, we provided the ion peaks of key products in signal processing (Figure R4).\nIn the revised version, we added additional ion peaks of methanol following the\nReviewer’s suggestions.\nFigure R4 | (Supplementary Figure S28, in the revised supporting information;\nFigure 3g, in the revised manuscript) (a) Total ion chromatography and (b) Mass\nspectrum of CO photoreduction products using 13CO as the CO source. (c) Mass\n2 2 2\nspectra of the product 13CH OH from 13CO photoconversion by the BBO-4\n3 2\nphotocatalyst.\nComment 6. The description of the quantitative analysis performed using 1H NMR\nspectroscopy lacks sufficient detail. Moreover, 1H NMR is suggested for the product\n9\nafter 13C-labeled test.\nResponse: We appreciate the reviewer’s valuable suggestions regarding quantitative\nmethodology. In the Revision 1, the CH OH product quantitative analysis were\n3\nemployed on an on-line gas chromatograph (GC-Agilent Technologies 8860, USA)\nequipped with thermal conductivity detector (TCD) and a flame ionized detector (FID).\nFurthermore, FID detector was connected with a HP-PLOT Q capillary column\n(Agilent, length 30 m, ID 0.53 mm, film 40 μm) for separation of C1-C6 paraffin and\nolefin hydrocarbons, alcohols and oxygenated compounds. The quantification of the\nproducts was based on the external standard and the use of a calibration curve. Due to\nexperimental constraints, in this work, 1H-NMR was used solely for qualitative analysis.\nWe thank the reviewer for the insightful suggestion to perform 1H NMR analysis on the\nproduct derived from a 13C-labeled test. This is indeed a powerful approach to gain\ndeeper mechanistic insights, such as tracing the fate of the labeled carbon atom or\nprobing kinetic isotope effects. However, due to significant experimental constraints\n(primarily the lack of dedicated facilities and funding required for conducting 13C-\nlabeling experiments), we were unable to incorporate 13C-labeled experiments or\nsubsequent detailed NMR analysis (including quantitative 1H NMR) within the scope\nof this study. We fully recognize the scientific merit of this suggestion. Future studies,\ncontingent upon resource availability, will prioritize employing 13C-labeling strategies\ncombined with comprehensive NMR analysis to provide more direct mechanistic\nevidence and a deeper understanding of the reaction pathway.\nIn this work, in order to further the generation of CH OH, the GC-MS was employed.\n3\nprecise detection of 13CH (m/z = 17), 13CO peak (m/z = 29) and 13CH OH (m/z = 33)\n4 3\nutilizing 13C as the only carbon source, provided further evidence that CH OH, CH\n3 4\nand CO were derived from CO photoreduction (Supplementary Figure 28, Figure 3g).\nshould be significantly expanded to include greater detail. For techniques like XRD,\nthis entails specifying the instrument model, key operating parameters (e.g., X-ray\n10\nwavelength, scan range, step size, scanning rate), and other relevant conditions. This\nlevel of detail should be applied consistently to all characterization methods described.\nFurthermore, it is recommended that the experimental section be included in the main\ntext of the manuscript.\nResponse: We thank the reviewer for this technically observation. We have fully\naddressed this comment as follows: “XRD was performed on a D/Max-RB X-ray\ndiffractometer (X’Pert-Pro-MPD) with Cu Kα radiation under the operation conditions\nof 40 kV and 50 mA, and the scanning range was set from 10 to 70o with a scanning\nspeed of 5o/min. X-ray Photoelectron Spectrometer was performed on a Thermo\nESCALAB 250XI instrument with Al Kα X-ray radiation and binding energies were\nreferred to the C 1 s peak at 284.6 eV of the surface adventitious carbon. The nitrogen\nadsorption-desorption curves (BET) of the Micromeritics ASAP 2420-4MP were used\nto measure the phase composition and calculate the specific surface area. TEM images\nwere observed on a Joel Jem 2100F electron microscope equipped with an EDX\nspectrometer. UV-vis-NIR DRS was collected on a SHIMADZU UV-3600\nspectrophotometer at room temperature in the wavelength range of 280-2500 nm with\nBaSO as the background. CO/CO -TPD experiments were performed on a\n4 2\nMicromeritics Auto Chem 2950HP. Atomic force microscopy (AFM) and in situ Kelvin\nprobe force microscopy (in situ KPFM) were recorded by a Bruker Dimension ICON-\nXR. The CO adsorption isotherms were acquired on a Micromeritics ASAP 2460-4MP.\nout using a regenerative amplified Ti: sapphire laser system (Coherent; 800nm, 100 fs,\n7mJ/pulse, and 1 kHz repetition rate), which is as the laser source and a HELIOS\nspectrometer (Ultrafast Systems LLC) is as the spectrometer. The light wavelength with\n380-650 nm probe pulse were produced by focusing the 800-nm beam onto CaF crystal.\nmeasurements were performed in diluted ethanol solution. with UV-vis absorption\nremained consistent before and after the fs-TA experiment. Surface topography analysis\nwas performed using an Atomic Force Microscope (Bruker Model: SCM-PIT-V2,\nFollowing the reviewer’s recommendation, we have moved the expanded\nExperimental Section from the Supporting Information to the main manuscript.\nComment 8. Similarly, the computational methods section requires more\ncomprehensive information, such as the specific procedure used for calculating Gibbs\nfree energy, the optimization parameters employed for the computational models, etc.\nResponse: Thank you for this insightful question. The first-principle based calculations\nwere carried out within density-functional theory (DFT), implemented in the Vienna\nAb Initio Simulation Package (VASP) code[1], using the frozen-core projector\naugmented-wave (PAW) method[2] to describe the interaction between the atomic cores\nand the valence electron density. The exchange-correlation potential was approximated\nwithin the generalized gradient approximation (GGA) using the Perdew-Burke-\nErnzerhof (PBE) functional[3] which was used to calculate electron-electron exchange\nand correlation interactions. A kinetic energy cutoff of 500 eV was chosen to ensure\ncomplete convergence. Owing to the large cell of Bi (012)@Bi O (100) heterostructure,\n2 3\nthe Brillouin zone was sampled with the k-grid of 2×2×1. The conjugate gradient (CG)\nmethod was used for structural relaxation, simultaneously minimizing total energy and\ninteratomic forces. The convergence tolerance for the total energy was set at 10-5 eV.\nAdditionally, dispersion-corrected DFT-D3 schemes were employed to account for van\nder Waals interactions[4]. A 1.5 nm vacuum layer was added along c axis to avoid\ninteractions.\nGibbs free energies for each gaseous and adsorbed species were calculated at\n298.15K, according to the expression:\nG = E +E -TS\nDFT ZPE\nZPE 𝑖\n2\nΘ= ℏ𝑣𝑖/k\ni\nS= ∑ R[ln(1-e-Θi/T)-1+ΘiT(eΘi/T-1)-1]\n𝑖\nwhere E is the electronic energy calculated for specified geometrical structures, E\nDFT ZPE\n12\nthe zero-point energy, S is the entropy, ℏ is the Planck constant, 𝑣 is the computed\nvibrational frequencies, Θ is the characteristic temperature of vibration, k is the\ni\nBoltzmann constant, and R is the molar gas constant. For adsorbates, all 3N degrees of\nfreedom were treated as frustrated harmonic vibrations.\nFor each step, the ΔG is the difference of product or intermediate and reactant.\nΔG[CH OH*]= G[CH OH *]+ G[H O (g)]-(G[*]+ G[CO ]+6*G[H++e-])\n3 3 2 2\nΔG[CH O*]= G[CH O *] + G[H++e-]+G[H O (g)]-(G[*]+ G[CO ]+ 6*G[H++e-])\n3 3 2 2\nΔG[CH O*]= G[CH O*] + 2*G[H++e-]+G[H O (g)]-(G[*]+ G[CO ]+ 6*G[H++e-])\n2 2 2 2\nΔG[CHO*]= G[CHO*] + 3*G[H++e-]+G[H O (g)]-(G[*]+ G[CO ]+ 6*G[H++e-])\n2 2\nΔG[CO*]= G[CO*] + 4*G[H++e-]+G[H O (g)]-(G[*]+ G[CO ]+ 6*G[H++e-])\n2 2\nΔG[COOH*]= G[COOH*] + 5*G[H++e-]-(G[*]+ G[CO ]+ 6*G[H++e-])\n2\nwhere * is the substrate, U is the applied overpotential and e is elementary charge. In\nthis study, U=0 V versus reversible hydrogen electrode.\nReferences:\n[1] G. Kresse andJ. Hafner, Ab initio molecular dynamics of liquid metals. Phys. Rev.\nB 1993, 47, 558.\n[2] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-\nwave method. Phys. Rev. B 1999, 59, 1758.\n[3] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made\nsimple. Phys. Rev. Lett. 1996, 77, 3865.\n[4] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio\nparametrization of density functional dispersion correction (DFT-D) for the 94\nelements H-Pu. J. Chem. Phys. 2010, 132, 154104.\nReviewer #3 (Remarks to the Author):\nThe authors have revised the manuscript in response to our comments. However, it\nappears that certain issues remain unresolved.\nResponse: We appreciate the reviewers’ summary of our work and are gratified that\n13\nthe novelty and strengths of our bismuth-bismuth oxide composite catalyst design, as\nwell as its photocatalytic performance, were clearly highlighted. We also thank them\nfor their suggestions and have provided detailed point-by-point responses below.\nComment 1. In particular, regarding the discrepancy between the theoretical model and\nthe actual material, the authors state that: “This cluster was constructed based on the\nlattice parameters of crystalline Bi, representing a surface of large-sized crystalline Bi\nnanoparticles observed in the experiment. Therefore, the crystalline properties of Bi in\nthe theoretical model (confirmed by XRD and HRTEM) are essentially consistent with\nthose in the experiment.” If that is the case, then the cluster is located on the surface of\ncrystalline bismuth, and it should exhibit an electronic structure entirely different from\na model situated on a bismuth oxide surface. As suggested by the calculations presented\nin the manuscript, the electronic state of the Bi cluster is likely to be significantly\ninfluenced by the supporting material, such as Bi O . Furthermore, the bond lengths in\n2 3\nsuch small clusters are not stabilized by a crystal lattice, and therefore should deviate\nfrom those in crystalline materials such as nanoparticles. For these reasons, it is difficult\nto accept the claim that a small cluster on an oxide support accurately reproduces the\nsurface state of bulk nanoparticles. Has this cluster actually been observed on the\nnanoparticle surface? In light of these points, we judge that the discussion does not\nprovide a sufficiently convincing explanation of the experimental results.\nResponse: We sincerely thank the reviewer for their meticulous assessment of the\nmanuscript’s presentation. We apologize for the initial oversight in modeling metallic\nBi as clusters rather than periodic structures. As rigorously suggested, we have re-\nperformed all DFT calculations using slab models (Bi(012) and Bi O (100) surfaces)\n2 3\nto ensure physical accuracy. The Bi (012)@Bi O (100) heterojunction was constructed\n2 3\nunder periodic boundary conditions. After lattice expansion, the lattice mismatch\nbetween the two surfaces was 2.58%, meeting the common requirement of being less\nthan 5% (Figure R5). To explain the origin of the superior performance of the\nheterojunction structure, two representative sites were selected for free energy\n14\ncalculations of the CO reduction reaction. These sites are the Bi surface of the\n2\nheterojunction and the interface, the latter of which was realized by adding a vacuum\nlayer along the b-direction based on the optimized heterojunction structure.\nFigure R5 | (Supplementary Figure S6, in the revised supporting information).\nTheoretical simulation models of (a) Bi O and (b) BBO surfaces.\n2 3\nThe free energies of the optimized structures for CO reduction were reassessed.\n2\n“Figures 6f-i and Supplementary Figures 37-40 displayed the free energy profiles and\nthe corresponding intermediate configurations. For the models of Bi, Bi O , BBO with\n2 3\nmetallic Bi site, and BBO with interfacial site, from a thermodynamic standpoint,\nCOOH* formation represented a rate-limiting step in the CO reduction process with\n2\nfree-energy barriers of 2.07, 3.45, 1.76, and 1.51 eV, respectively. It is obvious that the\nOhmic interface can greatly reduce the free-energy barrier of rate-determining step by\n1.94 eV. The formation of Ohmic junction promoted charge redistribution at the\ninterface, inducing the generation of built-in electric field, further accelerating the\nadsorption and activation of CO . The free energy of *CO (g) over Bi and BBO with\n2\nmetallic Bi site was much lower than that of *CHO, which result in the CO was the\nmain product. For Bi O and interface BBO, the *CHO formation energy was much\n2 3\nlower than that of CO (g), suggesting the formation of *CHO was easier than that of\nCO (g). Moreover, the fabrication of Ohmic junction interface greatly reduced the\nenergy barrier of rate-determining step. In experimentally and theoretically, BBO\nexhibited a better photocatalytic CO reduction performance into CO or CH OH than\n2 3\nBi O .”\n2 3\ntypographical errors have been corrected, some remain—for instance, in Figure 2,\n\"NaHB \" is still misspelled.\nidentifying the persistent typographical error (NaHB ) in Figure 2. This has now been\n4\ncorrected to “NaBH ” in the revised manuscript. We deeply regret this oversight and\n4\nhave taken steps to thoroughly proofread the entire manuscript again to ensure no\nsimilar errors remain. We appreciate the reviewer bringing this to our attention.\nComment 3. Regarding the sentence \"The UV-vis-NIR DRS (Figure 3a) revealed that\nBBO-4 demonstrated significantly enhanced broad-spectrum light absorption (280-\n2100 nm) compared to its constituent materials (Bi and Bi O ),\" it is not possible to\n2 3\ndetermine from Figure 3a whether the y-axis is given in arbitrary units.\nResponse: We sincerely apologize for the oversight in mislabeling the y-axis units in\nFigure R6. The reviewer is absolutely correct—the original figure incorrectly displayed\narbitrary units. The data were presented as Kubelka-Munk transformed reflectance\nvalues (F(R)), calculated from measured reflectance (R) using the standard equation:\nF(R) = (1-R)2/2R\nwhere R is the experimentally measured reflectance.\nFigure R6 | (Figure 3, in the revised manuscript). (a) UV-vis-NIR DRS spectra of\n800 nm of Bi, Bi O , and BBO-4.\n2 3\nKubelka-Munk Function (F(R)).\nThe absorption properties were quantified via the Kubelka-Munk transformation of\nreflectance data. All spectra are presented in absolute F(R) units, enabling direct\ncomparison of absorption intensities. The statement in our original text regarding the\nenhancement of light absorption intensity is inaccurate. It would be more appropriate\nto describe it as an extension of the spectral absorption range.\nReviewer #4 (Remarks to the Author):\nI co-reviewed the manuscript with reviewer #3.\nResponse: We greatly appreciate the reviewers’ constructive comments and positive\nfeedback. Their insightful questions and suggestions have significantly improved our\nmanuscript. We have addressed all concerns raised and revised the manuscript\naccordingly. The specific modifications are detailed below:\nThe authors have considered and revised reviewers' suggestions. The quality of this\nwork is obviously improved in this form. I would like to recommend to accept this\nmanuscript in its present form.\nResponse: We sincerely appreciate your time and valuable feedback, and are grateful\nfor your endorsement of our manuscript.\nTitle: Atomic-Scale Interface Engineering in Bi/Bi O Heterojunctions for Selective\n2 3\nCO Photoreduction to Methanol\nGeneral response:\nWe thank the editor and reviewers for their valuable and insightful comments and\nsuggestions, which have significantly helped improve the quality of our manuscript. We\nhave revised the manuscript in response to these comments. Our point-by-point\nresponses are provided below.\nReviewer #2 (Remarks to the Author)\nThe authors have made substantial revisions in this round, which have largely addressed\nmy concerns. I believe the manuscript is now suitable for acceptance after minor\nrevisions.\nResponse: We greatly appreciate the reviewers’ positive feedback on our work. In\nresponse to their comments, we have carefully revised the manuscript and\nSupplementary Information, with all modifications clearly highlighted in the updated\ndocuments. Our point-by-point responses to each specific comment are detailed below.\nComment 1. A minor error was noted in the revised manuscript: Fig.S10 (BBO-2) and\nFig.S11 (BBO-4) appear to be identical. Additionally, could the authors explain why\nthe Bi content in BBO-2 to BBO-6 shows an initial decrease followed by an increase?\nResponse: Thank you for this insightful question. We thank the reviewer for their\ncareful reading of our manuscript and for pointing out this oversight. Regarding the\nfirst point, we have carefully checked Figures S10 and S11 and confirmed that they\nwere indeed mistakenly duplicated during the final compilation of the supplementary\nmaterials. The correct Figure S11 has now been prepared and was uploaded accordingly.\nAs for the variation in Bi content from BBO-2 to BBO-6 in Tables S2-4, we appreciate\nthe reviewer’s insightful question. The observed discrepancies are likely attributable to\n1\nimpurities present in the carbon mesh used during testing, as well as the intrinsic carbon\ncontent of the mesh itself. Moreover, variations in the measurement range can also\ncontribute to fluctuations in the overall quantified content. Overall, the Bi content in\nthe three samples should be comparable. As evidenced by the XRD patterns, the\nmetallic Bi phase increases with prolonged reduction time, which is consistent with the\nexpected outcome.\nReviewer #3 (Remarks to the Author):\nThe authors have revised the manuscript, but many errors still remain. For example, if\nFigure 2l indeed shows the XANES spectra at Bi-L3 edge as described in the caption,\nan absorption edge should appear around 13.4 keV. The peaks at Figure 2l is about 159\neV. In Figure 2m, the x-axis is labeled as “Binding energy,” indicating that it is not\nan R-space fitting plot.\nDespite our numerous previous comments, the manuscript still fails to meet the\nstandards required for publication, and it is difficult to consider that the authors have\ncarefully prepared the manuscript. Therefore, this reviewer believes that the manuscript\nshould be rejected, at least by Nature Communications.\nResponse: We sincerely thank the reviewer for taking the time to re-evaluate our\nmanuscript and for providing these critical comments. We deeply apologize for the\nsignificant errors that remained in the previous version and for the carelessness in our\npreparation, which has clearly fallen short of the standards expected for publication.\nWe take these criticisms very seriously and have conducted a thorough, point-by-point\nrevision of the entire manuscript and figures in response.\nThe Figures 2l and m originally contained the XANES spectra in R1 version, but\nincorrectly displayed in R2 version. We have now replaced it with the correct Bi L -\n3\nedge XANES spectra and R-space fitting, which clearly shows the absorption edge at\napproximately 13.4 keV and the Fourier-transformed EXAFS data in R-space as\nexpected. We apologize for this grave oversight.\nIn light of these fundamental errors, we have undertaken a complete re-examination\n2\nof all data and figure labels throughout the manuscript to ensure no similar inaccuracies\npersist. We acknowledge that the presence of these errors was unacceptable, and we\nhave meticulously addressed every point from this and previous rounds of review to\nfully meet the journal’s high standards.\nWe are profoundly sorry for the frustrations our earlier revisions have caused. We\nhave now rigorously corrected the manuscript and believe it has been significantly\nimproved. We respectfully ask for your reconsideration of our work based on this\nthoroughly revised version.\nFigure 2. Structural characterization of BBO Ohmic junction. (a) Schematic\nillustration of the synthesis of BBO. TEM image of (b) BBS, (c) BBO-2, (d) BBO-4,\nand (e) BBO-6. (f) High-solution TEM image, (g) lattice fringes, and (h-j) EDS\nmapping images of BBO-4. (k) XRD patterns of Bi, Bi O , BBO-2, BBO-4, BBO-6.\n2 3\n3\ntransform R space fitting results.\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: We greatly appreciate the reviewer’s constructive comments. Their insightful\nquestions and suggestions have significantly improved our manuscript. We have\naddressed all concerns raised and revised the manuscript accordingly.\n4"}]

Decision letter