ZnO HoMS@ZIF-8 Nanoreactors for Efficient Enrichment and Photoreduction of Atmospheric CO 2

Open AccessCCS ChemistryRESEARCH ARTICLES19 Feb 2024ZnO HoMS@ZIF-8 Nanoreactors for Efficient Enrichment and Photoreduction of Atmospheric CO2 Yanze Wei, Juan Li, Decai Zhao, Yasong Zhao, Qinghua Zhang, Lin Gu, Jiawei Wan and Dan Wang Yanze Wei State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 Key Laboratory of Biopharmaceutical Preparation and Delivery, Chinese Academy of Sciences, Beijing 100190 , Juan Li State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Decai Zhao State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 Key Laboratory of Biopharmaceutical Preparation and Delivery, Chinese Academy of Sciences, Beijing 100190 , Yasong Zhao State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 Key Laboratory of Biopharmaceutical Preparation and Delivery, Chinese Academy of Sciences, Beijing 100190 , Qinghua Zhang University of Chinese Academy of Sciences, Beijing 100049 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 , Lin Gu University of Chinese Academy of Sciences, Beijing 100049 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 , Jiawei Wan State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 Key Laboratory of Biopharmaceutical Preparation and Delivery, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 and Dan Wang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 Key Laboratory of Biopharmaceutical Preparation and Delivery, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.024.202303604 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The ultimate goal of photocatalytic CO2 reduction is to efficiently convert atmospheric CO2 into value-added products by designing hierarchical photocatalysts that combine effective harvesting and accelerate activation capabilities of CO2 molecules. In this work, we demonstrated direct air-level CO2 reduction by hollow multishell structure (HoMS) nanoreactors with zeolitic imidazolate framework-8 (ZIF-8)-modified ZnO heteroshells, which offers a promising solution to optimize the mass transfer process via selective sieving and effective enrichment of CO2 molecules from the atmospheric environments. Specifically, heteroshells with ZIF-8 matrix act as pumps that effectively capture CO2 molecules, while the cavities serve as tanks to hold these molecules, thus significantly enhancing the transfer kinetics for CO2 photoreduction. The periodic shell-cavity configuration allows the nanoreactor to effectively gather CO2 molecules near the catalytic sites, thus increasing conversion opportunities for activated molecules. Simultaneously, the surface atomic steps on the ZnO/ZIF-8 heterointerfaces with staggered geometry expedite charge transfer and adsorption efficiency, which synergistically boosts the CO2-CO yield up to 3.7 μmol/h under CO2 flow and 0.7 μmol/h under atmospheric conditions with almost complete CO selectivity. Hopefully, the HoMS nanoreactors offer a potential strategy for solving the "last mile" challenge in practical CO2 valorization. Download figure Download PowerPoint Introduction The potential of direct air-level CO2 conversion into value-added solar fuels via photocatalytic process offers a promising solution to combat the greenhouse effect and achieve carbon neutrality.1–4 Natural photosynthesis systems give an excellent example by realizing the delicate assembly of organelles with varying nano-/microstructures and chemical composition, which enables the efficient capturing and continuous conversion of CO2.5–8 Research on mimicking photosynthesis systems has sparked significant interest in developing hierarchical mesostructured complex photocatalysts with diverse compositions for the purpose of highly efficient photocatalytic CO2 reduction.9–13 Among them, hollow multishelled structures (HoMS) nanoreactors with adjustable shell architecture and adaptable shell compositions have sparked pioneering explorations in CO2 valorization.14–16 Notably, the distinctive temporal-spatial ordering characteristic that ensures the proper sequence of mass transfer through shells, either from the interior to the exterior or opposite direction would significantly leverage the multiscale benefits of HoMS for mass and energy transfer processes in CO2 reduction, leading to enhanced incident light absorption, accelerated charge transfer, and improved reactant enrichment, thereby increasing conversion opportunities for CO2 molecules.17 Boosted photocatalytic CO2 reduction activities have been achieved by Co3O4,18 SnO2/SnS2,19 ZnS/CdS,20 and TiO2/CeO2 HoMS photocatalysts,21 demonstrating the effectiveness of structural regulation in promoting CO2 adsorption ability, improving interfacial charge transfer, and increasing active sites.22 Despite these advancements, the sluggish mass transfer kinetics in the photocatalytic CO2 reduction reaction still hinders their catalytic activities under a lowly concentrated atmospheric environment. To realize the effective conversion of CO2 under low concentrations or even atmospheric CO2, practical scenarios for CO2 reduction with more stringent requirements are proposed for HoMS-based photoreactors.23 Effectively capturing CO2 molecules from the atmosphere necessitates a heightened affinity between HoMS and CO2, accompanied by a significantly enlarged surface area with improved kinetic selectivity for CO2/N2.24,25 Additionally, it is crucial to have a fast conversion process for CO2, achieved through accelerated charge transfer and boosted surface reaction.26 To meet these requirements, it has become imperative to meticulously customize the local environment of HoMS-based photoreactors, ensuring optimal conditions for efficient CO2 conversion. Accordingly, through the construction of heterointerfaces on multishells, the resulting composite HoMS can be synergistically optimized to enhance its performance in targeted CO2 harvesting and electron transfer processes in photocatalytic CO2 reduction reactions. Building upon metal oxide (MO) HoMS photocatalysts, the integration of metal organic frameworks (MOFs) can significantly enhance the surface of HoMS through periodic pores and provide additional active sites for efficient adsorption.27–30 The structural characteristics of HoMS elevate the utilization of pores of MOFs in the self-supported and well-aligned MO/MOFs hierarchical heterostructure. The tunable micropores present within MOFs offer a promising opportunity for molecular sieving, enabling the efficient capture of CO2 within the mesopores of the shells and cavities of HoMS.31 The complementary roles of two counterparts may improve the mass transfer kinetics, thereby significantly contributing to the overall effectiveness of HoMS-based photoreactors. Herein, we select a prototypical zeolitic imidazolate framework-8 (ZIF-8), also known as metal-azolate framework-4 as initially reported by the Chen group,32 which possesses high intrinsic chemical stability, catalytic activity, and enhanced kinetic selectivity for CO2/N2, to modify the surface of ZnO HoMS. Through controlled in-situ growth, ZIF-8 layers are uniformly formed on both the inner and outer ZnO shells, resulting in a composite HoMS structure with ZnO/ZIF-8 heterointerfaces. Enriched surface atomic steps in ZnO/ZIF-8 heterointerfaces are observed in ZnO HoMS@ZIF-8 with a more abundant supply of Zn2+ ions by multiple shells. Significantly, the increased pores of the shell surface and cavities of HoMS boost the CO2 enrichment and mass transfer kinetics. Synergistically, the accelerated charge transfer strengthens the photocatalytic activity, affording 3S ZnO HoMS@ZIF-8 the highest CO yield of 3.7 μmol/h under CO2 atmospheres and 0.7 μmol/h with airflow with almost 100% selectivity, reaching the highest CO production under atmospheric environment. We also demonstrate the potential of ZnO HoMS@ZIF-8 in practical applications with low concentrations of CO2 in the environment. The efficient enrichment of CO2 and accelerated electron transfer ability ensure a CO supply for a tandem catalytic system, which also demonstrates the CO2-to-C2 production under low CO2 feeding (Scheme 1). Scheme 1 | Illustration of the composited ZnO HoMS@ZIF-8 nanoreactor with enhanced mass transfer kinetics toward atmospheric CO2 reduction. Download figure Download PowerPoint Experimental Commercially available reagents and starting materials were utilized without any additional purification or were prepared using reported methods as specified. All reactions, unless stated otherwise, were carried out using dry solvents in autoclaves or vials that had been dried and sealed in an air atmosphere. Briefly, the synthesis of ZnO HoMS follows the sequential templating approach, in which Zn precursor-enriched carbonaceous spheres were adopted as the sequential templates. The fabrication of ZnO HoMS@ZIF-8 is then applied in a DMF-based liquid phase, in which the dissolution of Zn2+ ions and the growth of ZIF-8 layers on ZnO shells are controlled via the concentration of ligands. The complete experimental details regarding the syntheses and characterization of compounds mentioned in this study can be accessed in the Supporting Information. Results and Discussion Fabrication and characterization of ZnO HoMS@ZIF-8 nanoreactors To enhance the affinity of the MO-based surface of HoMS toward CO2 molecules, it is essential to carry out appropriate modifications that enrich the pore distribution and increase the active surface area.33 Considering the kinetic diameters of CO2 (3.30 Å) and N2 (3.64 Å) molecules, ZIF-8 with a micropore size of 3.4 Å may induce molecule sieving and enhance the selective harvesting for CO2 molecules.34 Further compositing of ZIF-8 with hierarchical MO substrates is reported to fix the ligand rotation, resulting in enhanced lattice rigidity.35 This specific characteristic offers significant advantages in achieving precise molecular sieving of CO2 molecules, particularly when dealing with low concentrations. To achieve the goal, starting from the photoactive ZnO HoMS, direct growth of ZIF-8 counterparts on the inner and outer surfaces of shell structures is conducted following the reaction: ZnO + 2 ( C 4 H 6 N 2 ) → Zn ( C 4 H 5 N 2 ) 2 + H 2 O(1) A diluted 2-MIM solution is adopted for fabricating ZnO HoMS@ZIF-8, which allows the controlled dissolution of ZnO and nucleation of ZIF-8 near the surface of ZnO substrate, thus avoiding the homogeneous nucleation of ZIF-8 crystals ( Supporting Information Figure S1).36 By optimizing reaction time, the full coverage of the ZIF-8 layer with good crystallinity is realized ( Supporting Information Figure S2). This development is applicable for ZnO HoMS with varying shell numbers. As shown in Figure 1a–c, the single-shelled ZnO hollow spheres @ZIF-8 (ZnO HS@ZIF-8), double-shelled (2S) ZnO HoMS@ZIF-8, and triple-shelled (3S) ZnO HoMS@ZIF-8 maintain the integrity of the nano-microstructure after transformation ( Supporting Information Figure S3). The thickness of the covered ZIF-8 layers was measured to be approximately 53 nm for all structures, providing a consistent platform for studying the merits brought about by the HoMS scaffold and the MO/MOF heterointerfaces in uniform ZnO/ZIF-8 heterostructures ( Supporting Information Figure S4). The high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image in Figure 1d of a single 3S ZnO HoMS@ZIF-8 clearly illustrates the different contrasts that result from the rigid ZnO shells and flexible ZIF-8 layer. Moreover, the carbon/nitrogen (C/N) element distribution from energy dispersive spectral (EDS) mapping images in Figure 1e indicates that the growth of the ZIF-8 layer happens on both sides of the ZnO shells, resulting in sandwich-like heteroshells. The crystal structures of the two counterparts are investigated by powder X-ray diffraction (XRD) analysis, which shows well-crystalized hexagonal wurtzite ZnO and cubic ZIF-8 phases (Figure 1f and Supporting Information Figure S5). X-ray EDS results in selected areas further confirmed the different chemical compositions of the heteroshells (Figure 1g). As compared with area-1 with red dotted lines, the atomic percent of O and Zn dramatically decreased from 9.0% to 2.3% and 7.7% to 1.5%, suggesting that ZIF-8 is the main counterpart on the surface of heteroshells (Figure 1h and Supporting Information Table S1). The electronic structures and coordination environments of Zn central ions in ZnO HoMS@ZIF-8 are then studied by X-ray absorption near edge structure (XANES) spectra. Figure 1i demonstrates a negative shift in the Zn K-edge for 3S ZnO HoMS@ZIF-8, which is positioned between the adsorption edges of ZnO and ZIF-8. The altered coordination environment of Zn ions is attributed to the incorporation of N atoms, which increases the number of electrons. The changes in the Zn-O and Zn-N shells are revealed in the extended X-ray absorption fine structure shown in Supporting Information Figure S6. The peak positions are quite similar, except for a slight decrease in bond length in 3S ZnO HoMS@ZIF-8, indicating the distorted coordination environment of Zn atoms around the ZnO/ZIF-8 heterointerfaces. Figure 1 | Transmission electron microscopy (TEM) images of (a) ZnO HS@ZIF-8; (b) 2S ZnO HoMS@ZIF-8; and (c) 3S ZnO/ZIF-8 HoMS. (d) HAADF-STEM image and (e) X-ray EDS mapping images of a single 3S ZnO/ZIF-8 HoMS. (f) XRD patterns of 3S ZnO HoMS and 3S ZnO HoMS@ZIF-8. (g) HAADF-STEM image of shell structure and (h) corresponding EDS results of the selected area. (i) XANES spectra of Zn K-edge of corresponding samples, with the inset showing the magnified adsorption edges. Download figure Download PowerPoint Despite the compositional variation, surface-sensitive X-ray photoelectron spectroscopy (XPS) results give additional information about the change of chemical environments of corresponding elements ( Supporting Information Figure S7). The high-resolution XPS peaks of Zn 2p, which are located at 1021.8 eV (Zn 2p3/2) and 1044.9 eV (Zn 2p1/2), have shifted to a higher binding energy of 0.7 eV after the growth of the ZIF-8 layer. This shift is due to the decrease of valence electrons of Zn ions from the alteration of Zn-O bonds to Zn-N bonds, which involves more electrons in the hybridized orbitals.29 The significant increase in peak intensity centered at 284.8 and 286.0 eV corresponding to sp2 C–C bonding and characteristic sp2-hybridized carbon of N–C=N bonding verifies the existence of 2-MIM ligands on the surface, which is further confirmed by Fourier transform infrared (FTIR) spectroscopy and Raman results. As shown in Supporting Information Figure S8, the characteristic peaks corresponding to Zn–N, –C–N, and C–C vibrations in the Raman spectra demonstrate the introduction of ZIF-8 to ZnO HoMS. Compared to 2-MIM and ZnO HoMS, the peaks in the 800 to 1500 cm−1 range, which are attributed to the typical skeletal vibrations of aromatic heterocycles, are well-preserved in ZnO HoMS@ZIF-8. Additionally, the broad peak at around 3400 cm−1, as assigned to the surface hydroxyl groups, explains the O 1s signal observed in the XPS results for ZnO HoMS@ZIF-8 ( Supporting Information Figure S9). HoMS have proven to be an efficient hierarchical scaffold for fabricating ZnO/ZIF-8 composites, owing to their exceptional mass transfer properties.37 It is noteworthy that the confinement effect of HoMS on mass transfer may potentially influence the properties of heterointerfaces during the synthesis of composite structures. The initiation of ZIF-8 nucleation is triggered by the dissolution of ZnO substrates, and is controlled by the Zn2+ flux on the liquid–solid interface. In the solution system, the growth of the ZIF-8 layer occurs concomitantly with the dissolution of ZnO, which is accompanied by the progression of dissolution pits (Figure 2a). According to the crystal dissolution theory, the local environment of the nanostructures influences the ZnO dissolution kinetics, the interplay between 2-MIM and substrates, and the ZIF-8 crystallization kinetics. These factors then shape the distinct physicochemical properties of the ZnO/ZIF-8 heterointerfaces. As a comparison, we disrupted the structure of 3S ZnO HoMS through grinding and obtained a batch of ZnO NPs@ZIF-8 samples using the afforded ZnO debris as raw materials under the same experimental conditions. We thoroughly investigated the chemical composition, physicochemical properties, and influence of heterojunction interfaces on the resulting products after transformation, with a focus on the impact of the HoMS structure. Initially, we analyzed the chemical composition and crystal structure of two distinct materials through macroscopic characterizations. Compared to the XRD patterns of 3S ZnO HoMS@ZIF-8, the distinctive peaks attributed to the ZIF-8 layer exhibited a significant reduction in intensity in the ZnO NPs@ZIF-8 samples ( Supporting Information Figure S10). The content of ZnO and ZIF-8 components could be estimated by the ratios of intensities of ZIF-8 (010) and ZnO (101) peaks. The significant decrease from 1.65 for 3S ZnO HoMS@ZIF-8 to 0.57 for ZnO NPs@ZIF-8 confirms the suppressed conversion, which is further evidenced by the thermogravimetric analysis in Supporting Information Figure S11 ( Supporting Information Table S2). Therefore, the variations in chemical compositions can be attributed to the limited flux of Zn2+ caused by the broken shell structures. Additionally, these variations in composition can lead to different ZnO/ZIF-8 heterointerfaces, which possess distinct chemical, interfacial energy, and strain energy properties. Focusing on the advancement of the dissolution pits, the step rate v could be described as: v = ω β a e { exp ( σ ) − 1 }(2)where ω is the volume of the dissolution unit, ae is the equilibrium dissolution unit activity, and β is the kinetic coefficient (remains constant at a given temperature and additive concentration).38 σ is defined as the undersaturation degree, which is described as: σ = ln a a e (3) Figure 2 | (a) Illustration of the formation of Zn flux-driven surface atomic steps. (b) TEM image of the ZnO/ZIF-8 heterointerface. (c, d) High-resolution spherical aberration-corrected STEM images of ZnO/ZIF-8 heterointerface viewed from [001] and [100] faces with red arrows indicating the surface atom steps. Insets are FFT patterns of selected areas. (e, f) Simulated atomic STEM images and illustrations of surface atomic steps from (c, d). (g, h) EELS acquired from the selected area in (c, d). Download figure Download PowerPoint Hence, we can deduce from eqs (2) and (3) that v is proportional to a, suggesting that the ZnO/ZIF-8 heterointerface could potentially be influenced by the distinct mass transfer characteristics of HoMS, which result in exceptional nanostructures, arising from the enrichment of Zn2+ ions on the surface.39 The details of ZnO/ZIF-8 heterointerfaces in the selected area in Figure 2b are then inspected from high-resolution spherical aberration-corrected STEM images. Two surface states of the ZnO/ZIF-8 heterointerfaces are observed. The red arrows in Figure 2c indicate indented configurations accompanied by pitted crystal surfaces, suggesting the presence of abundant atomic steps on the heterointerfaces. The simulation results from the fast Fourier transformation (FFT) patterns clearly illustrate the typical wurtzite structure with a crystal orientation of [001]. The interface geometry with surface atomic steps is verified, designated by (100)/(010) (Figure 2e). Additionally, another representative exposed facet of ZnO is viewed from the ZnO [100] orientation, providing additional crystal structure (Figure 2d). The atomic steps are also witnessed on the coarse surface of the ZnO grind, resulting in a blurry ZnO/ZIF-8 heterointerface. The geometry of the interface is verified by the simulated atomic crystal configuration, suggesting the presence of ZnO (010)/(001) pits on the surface suggesting the presence of ZnO (010)/(001) pits on the surface (Figure 2f). Moreover, electron energy loss spectroscopy (EELS) is utilized to monitor the structural impact of heterointerfaces on electron structures. Figure 2a illustrates the collection of EELS data within three continuous regions: the ZnO crystal, ZnO/ZIF-8 interface, and ZIF-8 counterparts. The peak position of Zn L2,3 in Figure 2g exhibits a slight shift towards a lower energy direction, indicating a reduction in valence state due to the transition from a [ZnO4] tetrahedron to a [ZnN4] tetrahedron, which provides extra electrons ( Supporting Information Figure S12).40 The staggered geometry of heterointerfaces, resulting from a large number of surface atom steps, creates close interactions between the two counterparts, generating an unbalanced electric field that enhances charge transfer. Additionally, the increased edge and corner sites at atomic steps where the atoms have a lower coordination number further activate CO2 and stabilize the electric dipole of intermediates (e.g., *CO2).25 Similar observations are made with EELS for ZnO (010)/(001) pits in Figure 2h, indicating the overall improved charge transfer properties of ZnO HoMS@ZIF-8. In contrast, the ZnO NPs@ZIF-8 samples exhibit different conditions at the ZnO/ZIF-8 heterointerfaces. As depicted in Supporting Information Figure S13, the surface of ZnO grinds appears much smoother when viewed from both [001] and [100] perspectives. Surface steps in the ZnO/ZIF-8 heterointerface are typically limited to 2–3 atomic layers, and staggered interface structures are rarely observed. The results indicate limited step propagation, which can be attributed to the inability of ZnO NPs to localize the dissolved Zn2+ ions and the resulting insufficient Zn2+ flux. These findings are in line with the compositional characterizations. It is noteworthy that although the simulated crystal configurations confirm quite similar interface geometries, the EELS results of the selected regions at the interfaces of ZnO NPs@ZIF-8 samples exhibit different trends. Specifically, the peaks of the L2,3-edge show no significant change but there is a decrease in intensity due to the more regular ZnO/ZIF-8 interface structure. Accordingly, during the preparation of ZnO/ZIF-8 composites, the HoMS structure significantly impacts the macroscopic chemical composition and microscopic interface structure of the product by influencing the interfacial reactions between ZnO substrate dissolution and ZIF-8 growth. These factors are crucial in defining the physicochemical properties and catalytic performance of the as-synthesized hybrid materials, thereby enhancing the activation efficiency toward CO2 molecules through increased affinity.41 Atmospheric photocatalytic reduction of CO2 to CO Next, we proceed to assess the photocatalytic CO2 reduction activity of the ZnO HoMS@ZIF-8 and investigate the underlying mechanism for the observed enhancement in activity. The introduction of the ZnO/ZIF-8 heterointerface significantly enhances the rate of CO evolution, with the promotion becoming more prominent as the shell number increases, as depicted in Supporting Information Figure S14. All samples exhibit nearly 100% CO selectivity, and the highest CO yield is achieved by the 3S ZnO HoMS@ZIF-8, reaching approximately 161 μmol/h/g under H2O vapor and 393 μmol/h/g with the sacrificial reagent. The apparent quantum yield (AQY) of 3S ZnO HoMS@ZIF-8 is then studied. The highest AQY of 0.091% is achieved under 300 nm monochromatic light irradiation, which drops sharply as the wavelength of incident light increases ( Supporting Information Figure S15). The origin of the produced CO has been verified to stem from the reduction of CO2, as evidenced by the use of 13CO2 as an isotopic tracer ( Supporting Information Figure S16). The beneficial impact of improved mass transfer kinetics on the photocatalytic performance of ZnO HoMS@ZIF-8 is further validated by increasing the catalyst loading in the photocatalytic reactor. As depicted in Supporting Information Figure S17, the CO yield exhibits a linear increase with the augmented loading amount of the HoMS-based nanoreactors. This trend continues until a plateau is observed at a loading amount of 120 mg, which surpasses the catalyst usage reported in most reported studies. In a fixed-bed photocatalytic reactor, the CO conversion rate of 3S ZnO HoMS@ZIF-8 could reach 3.7 μmol/h in a CO2 atmosphere and 0.76 μmol/h with air flow, exhibiting the best performance compared to similar composited photocatalysts under similar conditions (Figure 3a and Supporting Information Table S3). Moreover, the integrity of the HoMS scaffold, the composition, crystallinity, and coordination structure of heteroshells are confirmed to remain intact after photocatalysis, providing strong evidence for the robustness of these nanoreactors ( Supporting Information Figure S18). The findings highlight the potential utilization of the composited ZnO HoMS@ZIF-8 in low CO2 abundance environments. In addition, the conversion of CO2 to CO and subsequently to C2 products can be achieved through tandem reactions using an H-cell fitted with a copper electrode ( Supporting Information Figure S19), thus providing a viable alternative route for converting CO2 to C2 products from feeding atmospheres with relatively low CO2 concentrations.42 The contribution of HoMS and ZIF-8 layer in the atmospheric CO2 reduction process was investigated by evaluating the CO evolution activity of grated 3S ZnO HoMS under different CO2 volume fractions, as illustrated in Figure 3b. The results indicate that the grated HoMS exhibits CO2 reduction activity after mixing 60 vol % CO2 in the N2 atmosphere, confirming that the enrichment effect of multistructures contributes to the atmospheric CO2 reduction process. Importantly, the increase in CO yield for grated ZnO HoMS is more sensitive to the increased CO2 concentration, revealing the increased CO2 harvesting ability due to the ZIF-8 layer.25 Figure 3 | (a) CO evolution activity of 3S ZnO HoMS@ZIF-8 with different gas flows. (b) CO evolution activity of 3S ZnO HoMS@ZIF-8 and grated samples under different CO2 volume fractions. (c) The CO2/N2 adsorption selectivity analysis of corresponding samples. (d) CO2 adsorption isotherms, (e) TPD analysis, and (f) Raman spectra of CO2-saturated samples with 1 wt% Au loading. Download figure Download PowerPoint To understand the promotion mechanism for atmospheric CO2 photoreduction, the CO2/N2 system was employed to validate the precise sieving that occurs within the ZnO HoMS@ZIF-8 nanoreactors, which feature shells with ZIF-8 layers and ZnO/ZIF-8 heterointerfaces (Figure 3c). The selectivity is obtained by observing linear sorption isotherms using the barometric pressure decay method for both CO2 and N2 ( Supporting Information Figure S20), in accordance with Henry's law.43 The 3S ZnO HoMS@ZIF-8 exhibited a high CO2/N2 selectivity of 39.4 compared with 22.0 of the ZnO NPs@ZIF-8 sample, also significantly surpassing ZnO HoMS with a selectivity of only 5.7. The Maxwell–Stefan equation regarding chemical potential helps to further explain the selective harvesting of CO2 molecules: N i = − ρ D i q i R T d μ i d x (4)where Ni is the molecular flux of species, ρ is the zeolite framework density, Di is the Maxwell-Stefan diffusivity, qi is the molar loading, R is the gas constant, T is the absolute temperature, and d