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Phase-enabled metal-organic framework homojunction for highly selective CO2 photoreduction

Results . Synthesis and characterization of Co-MOF . The dynamic feature of supramolecular coordination bonds in MOFs allows metal NPs to direct MOFs’ synthesis through metal node exchange. One of the determining factors of the node exchange consists in the relative strength of coordination bonds between metal nodes and organic linkers. Generally, stronger coordination bonds can gain a competitive edge over weaker coordination bonds 28 , 29 . In MOFs, metal ions interact with –COOH in the organic linker of tetrakis (4-carboxyphenyl) porphyrin (TCPP) in the most common form of –(COO) 4 M 2 (M=Co, Au, Ag) (Fig.? 1a ). It is well known that the d-band position of transition metals plays an important role when metals interplay with oxygen (O) atoms or molecules 36 . Hence, density-functional theory (DFT) was used to investigate the d-band position of transition metals in Co-MOF, Ag-MOFs, and Au-MOFs. The d orbital projected densities of states of M in –(COO) 4 M 2 are plotted in Fig.? 1b . A higher d -band center ( E d ) of Co (?1.20?eV) in –(COO) 4 Co 2 than Au (?2.93?eV) in –(COO) 4 Au 2 and Ag (?2.69?eV) in –(COO) 4 Ag 2 suggests a decrease in?the filling of the ( d–σ ) state and stronger bonding of Co–O than Au–O and Ag–O. Moreover, the crystal orbital Hamilton population (COHP) was taken to chop the total band-structure energy of –(COO) 4 M 2 into specified M–O interactions (Fig.? 1c ). Co–O shows the highest intensity of integrated COHP (1.086 eF) than Ag–O (0.487 eF) and Au–O (0.637 eF), further revealing the stronger bonding of Co–O and less destabilized feature of –(COO) 4 Co 2 37 . Thus we decided to use weaker-interacting, transition metal Au/Ag NPs as potential inducers. Further considering cost effectiveness, we designed hollow, instead of solid, Au/Ag nanocubes to induce the growth of cobalt MOFs in this work. Fig. 1: Structural and elemental analysis of Co-MOF-3 nanostacks. a Schematic illustrations of –(COO) 4 M 2 (M=Co, Au, Ag). b PDOS of d -band of M in –(COO) 4 M 2 (up: spin up, down: spin down) and corresponding caculated d-band center values. Ef?=?0. c COHP bonding analysis of M–O interactions in –(COO) 4 M 2 . Ef?=?0. The unit is eV/f.u.(eF), standing for eV per formula unit. ICOHP represents the intensities of COHP. d A typical TEM image of Co-MOF-3. e The SEM image of Co-MOF-3. f 3D TEM image of Co-MOF-3 nanostack and 3D electron tomography reconstruction model of the small squared plate ( g , h ) and large squared plate in Co-MOF-3 ( i , j ) shown in f . Scale bar, 500?nm. k HAADF image of Co-MOF-3 and the corresponding EDS elemental mapping images of l C, m Co, o O, p N and q Ag in Co-MOF-3. r EDS peak-area ratios of K series for Co/N, Co/O, and Co/C between the central (center) and the peripheral area (edge) of Co-MOF-3. Full size image Hollow Au/Ag nanocubes were prepared following a previously reported method 38 . They were ca. 45?nm in size and had a narrow size distribution (Supplementary Fig.? 1 ). From the TEM image (Supplementary Fig.? 2 ), we can clearly see the hollow structure. X-ray photoelectron spectroscopy (XPS) (Supplementary Fig.? 3 ) and element dispersive spectroscopy (EDS) (Supplementary Fig.? 4 ) confirmed the co-existence of Au and Ag in the nanocubes and the Ag/Au ratio of 47:53. The preset amount of hollow Au/Ag nanocubes was then added into the MOF precursor solution to initiate the MOF synthesis reaction. As a result, three different samples, here named Co-MOF-1, Co-MOF-2, and Co-MOF-3, with different morphologies were formed by just adjusting the amount (1, 2, and 5?μmol) of hollow Au/Ag nanocubes. The red shift of the S?ret band in the ultraviolet–visible (UV–vis) absorption spectra of three MOFs (Supplementary Fig.? 5 ) suggested the formation of coordination polymers 39 , 40 . As shown in Supplementary Fig.? 6 , the Raman spectra of Co-MOFs are similar but quite different from that of the TCPP monomer ranging from 1500 to 1800?cm ?1 , suggesting the coordination of -COOH in TCPP with Co atoms in the Co-MOFs, and similar chemical bonding in Co-MOFs. A typical TEM image of Co-MOF-1 (Supplementary Fig.? 7 ) reveals its morphology, mainly single square nanoplates plus a small number of rectangular ones with their lateral size ranging from 200 to 300?nm, and its high-resolution TEM (HR-TEM) and a?single set of selected-area electron diffraction (SAED) pattern suggested single crystal feature (Supplementary Fig.? 8 ). After we increased the Ag/Au NPs to 2?μmol, a few stacked MOFs appeared in resultant Co-MOF-2, but most of the MOFs were still single unstacked small nanoplates (Supplementary Fig.? 9 ). When the Ag/Au NPs reached 5?μmol, Co-MOF-3 with two well-stacked concentric-aligned MOF nanoplates with identical orientation were formed, as shown in Fig.? 1d and Supplementary Fig.? 10 . Scanning electron microscopes (SEM) image (Fig.? 1e ) and 3D TEM image (Fig.? 1f ) confirmed the stacked structure and excluded the possibility of a core-shell structure. 3D topography reconstruction (Fig.? 1g–j and Supplementary Movie 1 ) suggested that the smaller nanoplate was partially embedded into the large nanoplate. Such a well-defined unique structure is very rare in either heterojunctions or homojucntions, since it is still very challenging to control the relative orientations of the two components at their interface. An AFM image of Co-MOF-3 nanoplates implies that the lateral sizes were much larger than the heights of both large and small nanoplates (Supplementary Fig.? 11 ). AFM (Supplementary Fig.? 12 ) and SEM (Supplementary Fig.? 13 ) images of a?single Co-MOF-3 particle with the small nanoplate removed, showed a reduction in height ca. 27?nm in the center of the large nanoplate, suggesting that the smaller nanoplate was partially embedded into the large nanoplate, instead of merely stacking on the surface, thereby producing an intimate and robust junction interface between the two. Moreover, a high-angle annular dark-field?(HADDF) image of a representative Co-MOF-3 nanostack (Fig.? 1k ) and the corresponding EDS element mapping analysis of C (Fig.? 1l ), Co (Fig.? 1m ), O (Fig.? 1o ), N (Fig.? 1p ), and Ag (Fig.? 1q ) were conducted. The C, Co, O, and N elements were found to be evenly distributed in both large and small nanoplates in Co-MOF-3, suggesting a uniform Co-MOF composition. However, no detectable signal from Ag or Au elements indicates that the hollow Au/Ag nanocubes only acted as an inducer for the synthesis of MOF nanostacks, in line with EDS spectra (Supplementary Fig.? 14 ). Meanwhile, the XPS spectrum of Co-MOF-3 (Supplementary Fig.? 15 ) suggested the presence of Co element, but no apparent Au and Ag elements left in the samples. Besides, the ratios of integrated peaks of Co K series to N K series (Co/N), O K series (Co/O), and C K series (Co/C) from mapping images of different Co-MOF-3 nanoplates were calculated and are listed in Fig.? 1r and Supplementary Table? 1 . It can be seen that the ratios of Co/N, Co/O, and Co/C in the central part of three randomly selected Co-MOF-3 nanostacks were consistently higher than those in the peripheral part, suggesting a slightly higher Co content in smaller nanoplates than large ones. Crystal structure, and optical and electronic properties of Co-MOF-3 nanostacks . The atomic structure of one typical Co-MOF-3 nanostack was investigated by SAED, as shown in Fig.? 2a . The SAED patterns from the central part (Fig. 2b ) and peripheral part (Fig.? 2c ) of nanostack. The fourfold symmetry diffraction spots indicate the tetragonal crystal structure of Co-MOF-3 nanoplates 4 , 40 , 41 . Interestingly, we observed two different interplanar distances of 1.64 and 1.43?nm for the (100) diffraction from the central part, while only one spacing of 1.64?nm from the peripheral part, suggesting their slightly different crystal structures of two nanoplates in one nanostack, and?a similar phenomenon was noticed in epitaxial heterostructures 42 . From the HR-TEM image (Fig.? 2d ) of the interface between the central and peripheral parts, a certain dislocation at the interface was observed. The lattice fringes of a peripheral and central region near the?interface were observed, and the lattice distances measured to be 1.64?nm (Fig.? 2e ) and 1.43?nm (Fig.? 2f ), respectively, in accord with the SAED results. Moreover, two sets of interplanar distances were also noticed for (110), (200), (210), (220) crystal lattice planes (Fig.? 2e, f and Supplementary Fig.? 16 ) 43 , 44 . Besides, the TEM image of one Co-MOF-3 nanoplate with a missing central part (Supplementary Fig.? 17a, b, d–g ) shows continuous crystal lattice fringes with the?same distance, and the corresponding Fast Fourier Transform image (Supplementary Fig.? 17c ) exhibits only a single set of (100) diffraction spots. Furthermore, we have done extensive searching under TEM and finally found several small plates with a?similar size of MOF(s). Their corresponding SAED (Supplementary Fig.? 18 ) also agrees well with that of the MOF(s), suggesting that they may be the small plates falling down from the Co-MOF-3 stacks. All these observations further verified our proposed structure. All these data confirmed that two nanoplates in the Co-MOF-3 nanostacks had slightly different crystal cell parameters. Although we had also tried the XRD characterization of MOFs (Supplementary Fig.? 19 ), unfortunately, the XRD patterns are noisy and not good enough to verify?the crystal structure. There may be two reasons; one is that the long-range order in MOF or COF NPs is often not good as the bulk materials. The other possible explanation is that although the Ag/Au NPs and polyvinylpyrrolidone (PVP) in this system are very effective for regulating and controlling MOFs’ morphology, they inevitably decreased the crystalline degree of MOF NPs. Fig. 2: Crystal structure of Co-MOF-3 nanostacks. a TEM image and b , c the corresponding SAED from the central (position 1 in a ) and peripheral (position 2 in a ) areas of Co-MOF-3, respectively. d HR-TEM image of the epitaxial inteface between the central and peripheral parts (yellow polygons represent interfacial dislocation). e , f Enlarged lattice image of position 1 in d and position 2 in d , respectively. g Cystal structural model of the smaller MOF, named MOF(s), in nanostacked Co-MOF-3. h Cystal structural model of the larger MOF, named MOF(l). The rectangular boxes in g and h highlight the unit cells. i Proposed top-view structure of Co-MOF-3 nanostacks. j , k TDOS plots of monolayer MOF(s) and MOF(l) in nanostacks, respectively. l Calculated energy levels of monolayer MOF(s) and MOF(l) in nanostacks (Potential vs. NHE (pH?=?0)). Full size image DFT simulations were conducted to reveal the crystal structure of two stacked nanoplates in Co-MOF-3, according to SAED diffractions 44 , 45 . The smaller MOF nanoplate, with lattice constants of a ?=? b ?=?14.3?nm and α ?=? β ?=? γ ?=?90°, in the nanostacked Co-MOF-3 was named MOF(s), while the crystal structural model of the larger MOF nanoplate, with lattice constants of a ?=? b ?=?16.4?nm and α ?=? β ?=? γ ?=?90°, was named MOF(l). Their crystal structural models are displayed in Fig.? 2g, h , respectively. Figure? 2i shows the proposed top-view structure of Co-MOF-3 nanostacks composed of MOF(s) and MOF(l). Moreover, the three-dimensional (3D) crystal structural models of MOF(l) and MOF(s) are shown in Supplementary Figs.? 20 – 23 . Considering the charge transfer mainly happens at the contact interface, the interfacial electronic structures of monolayer MOF(l) and MOF(s) were investigated using total densities of states (TDOS) (Fig. 2j, k ), respectively 45 . Moreover, the bandgap of 2D MOF(s) and 2D MOF(l) was calculated to be 1.48 and 1.92?eV, respectively. The higher conduction band minimum (CBM, ?0.99?eV) and lower valence band maximum (VBM, 0.93?eV) of MOF(l) than those of MOF(s) indicate the formation of the type-II homojunction (Fig.? 2l ). Thus, MOF(l) can transfer electrons to MOF(s) while holes transport in the opposite direction, so as to promote the charge separation in a photochemical reaction. Besides, the bandgap of 3D MOF(l) and MOF(s) are only a little smaller than corresponding 2D ones, and their relative band alignment is very similar (Supplementary Fig.? 24 ). It should be noted that MOF(s) was partially embedded into MOF(l). Thus, photogenerated charges cannot only transport in the vertical direction but also the horizontal direction, as shown in Supplementary Fig.? 25 , which is rare for epitaxial heterojunctions and homojunctions 40 . The experimental bandgap of Co-MOF-3 was determined to be 1.46?eV from UV–vis diffuse reflectance spectra and the Tauc plot (Supplementary Fig.? 26 ), which is close to the calculated bandgap of 2D MOF(s) (1.28?eV). CV curves of Co-MOF-3 at a scan rate of 10?mV?s ?1 were further collected in acetonitrile and 0.1?M Tetrabutylammonium hexafluorophosphate (Supplementary Fig.? 27 ), by using the Ferrocenium/ferrocene ( F c / F c + ) redox couple to calibrate the pseudo reference electrode 46 . From this experiment, the VBM and CBM were determined to be ?0.55 and 0.91?eV, respectively, in line with DFT calculation results and suggesting that the energy band of Co-MOF-3 satisfies the requirement of CO 2 reduction to CO. Moreover, synchrotron XAS was used to analyze the coordination geometry, the local chemical environment, and the metal valence. Figure? 3a presents Co K -edge X-ray absorption near-edge structure (XANES) spectra of the Co-MOFs and references, including Co foil, CoO, and Co 3 O 4 . The pre-edge peak signal at 7712?eV of Co foil is not detected in Co-MOFs due to the fully occupied 3d orbital of Co 2+ , excluding the existence of Co (0) in Co-MOFs 47 . The absorption edge position at 7728?eV for Co-MOFs implies that the Co atoms in Co-MOFs are present as Co 2+ (the same position as Co 2+ in CoO), which is consistent with our proposed Co-MOF structure. Fig. 3: Elemental analysis of Co-MOF-3 nanostacks. a Normalized XANES spectra at Co K -edge for all the Co-MOFs and references of Co 2 O 3 , CoO, and Co foil. b Co K -edge Fourier transform k 2 -weighted EXAFS (not phase corrected) of Co-MOF-3, CoO, Co foil, and Co-porphyrin (Co-por). c HADDF image of of Co-MOF-3. d The low-loss EELS spectra of edge and center (after Gaussian smoothing). The inset is the raw data and Guassian fitting curve of EELS spectrum of edge. Low-loss EELS mapping in the range of e 20–35?eV, f 7.5–20?eV and g 2–4?eV. The low-loss EELS mapping was normalized by the integral signal, extracted with background removal, and smoothed by the Gaussian fitting model. Non-negative matrix factorization (NMF) was applied to the 41?×?41 pixels low-loss EELS spectra dataset. h – j Super-resolution multiphoton confocal images of Co-MOF-3. The excitation of wavelength is 405?nm and the emission was collected using ( h ) a 412–472-nm bandpass filter (blue) or ( i ) a 559–682-nm bandpass filter (red), respectively, and j merged image of h and i . k The FL intensity profile of the inset Co-MOF-3 image; the scale bar is 500?nm. Full size image From the Fourier-transformed k 2 -weighted extended X-ray absorption fine structure (EXAFS) results (Fig.? 3b ), it can be seen that the Co-MOF-3 has a similar peak position with porphyrin Co coordination center (containing CoN 4 ) and CoO at ca. 1.44??, which is mainly ascribed to the Co–N/O coordination at the first shell. Besides, there is neither the Co–Co interaction peak at 2.15?? nor other high-shell peaks in the EXAFS spectra of Co-MOF-3, further confirming that no heavy backscattering atoms (Co) are connected to Co sites. Moreover, the local environment of the Co atoms was determined using the phase shift, and amplitude functions of Co–N contribution and Co–C contribution generated using MOF(s) structure, and the coordination number of Co atoms in Co-MOF-3 is found to be 5. It should be noted that it is very challenging to distinguish first-row ligands (carbon, nitrogen, oxygen, and fluorine) using EXAFS since the photoelectron scattering function is weakly dependent on the Z number. Therefore, we can conclude that the samples present approximately five neighbors in the first shell, but we cannot affirm which atoms (O or N) are present since they would give a similar result. Thus the pillar structure of Co-MOF-3 was confirmed. Besides, the carbon K-edge core-loss mapping (Supplementary Fig.? 28a–c ) and spectra (Supplementary Fig.? 28d ) were obtained from electron energy loss spectroscopy (EELS) 48 . The stronger carbon element signal of raw data in the central part of position 1 than in the peripheral part of position 2 is mainly due to the center part’s relatively higher thickness (see 3D TEM reconstruction, AFM, and SEM images). To eliminate the effect of thickness, the carbon K-edge core-loss EELS image and spectra were normalized by the integral signal, extracted with background removal, and smoothed by the Gaussian fitting method. Normalized carbon K -edge from the core-loss EELS mapping and spectra are shown in Supplementary Fig? 28e, f , and it is possible to observe the differences between the edges’ response intensity. Less carbon element signal was observed in the central part of position 1 than in the peripheral part of position 2, which suggested that the C content in the former is smaller than in the latter, in good agreement with our EDS elemental analysis in Fig.? 1r . Besides, an energy loss-peak at 285?eV induced by transitions from 1 s to π molecular orbital in carbon due to the presence of sp 2 carbon, and a transition from 1 s to σ in carbon at 295?eV 49 , were observed in both central and peripheral parts. It indicated a similar carbon environment in these two parts, consistent with our proposed structures. Furthermore, the low-loss EELS spectra of Co-MOF-3 nanostack (Fig.? 3c ) were collected pixelwise (41?×?41 pixels) via using scanning transmission electron microscopy (STEM). Bulk plasmon bands of edge and center regions were extracted from the corresponding pixels in the raw dataset and then smoothed by the Gaussian method (Fig.? 3d ). As shown in the inset of Fig.? 3d , the raw curve and fitting curve of edge EELS spectrum matched very well, indicative of a very-good fitting. The bulk plasmon peak positions of the edge and center regions were found slightly shifted, suggesting the difference of the valence electron density between MOF(s) in center and MOF(l) in center and edge in Co-MOF-3 nanostack. We further mapped the low-loss EELS dataset of Co-MOF-3 in 2–35-eV energy range by applying non-negative matrix factorization (NMF) 48 . Here, the 20–35?eV (Fig.? 3e ) and 7.5–20?eV (Fig.? 3f ) are both attributed to the bulk plasmon band, and the thickness effect was removed by the Fourier-log deconvolution method. It should be noted that although 2–4?eV (Fig.? 3g ) is due to the tail of the zero loss-peak not bulk plasmon, it supports the three components (center, edge, and substrate) in a whole STEM/EELS view. The different variation trend of relative intensity of two target components (center and edge) in these energy ranges straightforwardly confirmed center/edge difference in terms of valence electron density. As shown in Supplementary Fig.? 29 , there are two peaks in the fluorescence (FL) spectrum of a single Co-MOF-3 nanostack excited at 450?nm. One is at 430–470?nm, which should be ascribed to triplet-triplet annihilation produced S2 emission (S2–S0), and the other is at 600–730?nm of S1–S0 emission. Furthermore, much lower intensity of S2 emission than S1 emission suggests the efficient charge transfer between porphyrins in MOFs. From Fig. 3h–j , Supplementary Fig.? 30 , and Supplementary Movie? 2 , we can see that the FL intensity for both S1 and S2 emission in the center is much higher than the edge part in the Co-MOF-3. To exclude the effect of thickness, we have quantitatively measured the FL intensity profile from one Co-MOF-3 nanostack. It was found that the intensity ratio of the center to edge in the nanostack reached ca. 85.2:28.1 (Fig.? 3k ), which is much higher than the thickness ratio (1:1.3–1.6) of center to edge in the Co-MOF-3 based on the AFM measurements and 3D reconstruction. Therefore, the FL intensity difference should not mainly result from the thickness difference. Higher FL intensity of MOF(s) than MOF(l) further suggested the difference in quenching rates attributed to differences in the free energies of electron transfer between porphyrin molecules in MOFs. A much remarkable quenching of red FL than blue FL in MOF(l) compared with MOF(s) suggested the more S1–S0 state quenching than S2–S0 in the former, which may be affected by the charge transfer between MOF(s) and MOF(l) at the junction and self-quenching properties of these two MOFs due to the different molecular arrangement. Formation mechanism of Co-MOF-3 nanostacks . The formation mechanism of asymmetric (in the thickness direction) Co-MOF-3 nanostacks was revealed by tracking intermediate evolution in time 50 . At the very beginning, organic porphyrin molecules are easily absorbed onto the surface of Au/Ag nanocubes through the interaction of Au/Ag with COO– or Au/Ag and π electron interaction in porphyrin and then prearrange through self-assembly 51 . The porphyrins are generally prearranged on the surface of Au or Ag as a form of inclined configuration with shorter center distance than free self-assembly due to restricted arrangement, and it is the same as what we have observed in MOF(s). The role of hollow Au/Ag nanocubes as self-assembly inducer was confirmed by more TEM observations of Co-MOF-3 nanostacks (Supplementary Figs.? 31 and 32 ). Subsquently, primary hollow square-shaped Co-MOF nanoplates (Fig.? 4a and Supplementary Fig.? 34 ) were formed through metal exchange from Ag/Au to Co due to stronger bonding strength of Co–O than Au–O and Ag–O. It was supported by the computer simulation of d -band center and COHP of Co–O, Ag–O, and Au–O (Fig.? 1b, c ). And then primary solid MOF(s) nanoplates were formed after the small hollow Co-MOF nanoplates grew along lateral directions (Fig.? 4b ). Meanwhile, free TCPP molecules and Co 2+ in the solution formed the second nanoplate of MOF(l) on the 001 facet of the primary nanoplate, which act as a substrate due to similar composition (Fig.? 4c and Supplementary Fig.? 34 ). Due to the crystal structure of MOF(l) is thermodynamically more stable than MOF(s), the growth speed of MOF(l) should be faster than MOF(s). Fig. 4: Morphological evolution of Co-MOF-3 nanostacks in time. TEM images of Co-MOF-3 at various reaction time of 15?min ( a ), 30?min ( b ), 1?h ( c ), and 2?h ( d ) (bar is 50?nm). TEM and HE-TEM images of top ( e ) and side ( g ) Co-MOF-3 at 3-h reaction time, and the corresponding FFT images of f and h , respectively. Top ( i ) and side ( k ) TEM and HR-TEM images of Co-MOF-3 at 6?h and corresponding FFT images of j and l , respectively. m Schematic diagram of the formation mechanism of Co-MOF-3 nanostacks directed by the hollow Au/Ag nanocubes. Full size image Thus, following the fast growth around the nucleation sites, the MOF(l) layer gradually grow on the (001) facet of the primary MOF(s) (Fig.? 4d and Supplementary Fig.? 35 ) at 2?h. Furthermore, it reached a similar lateral size and orientation with the primary layer at 3?h, confirmed by both the top view (Fig.? 4e and Supplementary Fig.? 36 ) and side view (Fig. 4g ), and the corresponding TEM images of the two facets are shown in Fig.? 4f, h , respectively. It was found that two sets of SAED diffraction spots already emerged (Fig.? 4e–h ). Considering the more thermodynamically stable crystal structure of MOF(l), the growth speed of MOF(l) should be faster than MOF(s). Subsequently, both layers continued to grow, mainly in two dimensions due to the attachment of PVP in (100) surface. For the 6-h sample, one nanolayer was already slightly larger than the other (Fig.? 4i, k and Supplementary Fig.? 37 ). Consistently, two sets of (100) diffraction spots were observed (inset in Fig.? 4j, l ). And the smaller layer was partially embedded into the large layer, likely resulting from different growth rates along the [001] axis of both nanoplates. As a result, Co-MOF-3 nanostacks with slightly different crystal phases were prepared. The schematic diagram of the formation mechanism was summarized in Fig.? 4m , which was also supported by time-dependent UV–vis spectra of the reaction solutions (Supplementary Fig.? 38 ). Thus, we named this method “transition metal NPs-directed MOF growth.” Moreover, when we increase the amount of Au/Ag to 15?μmol, the homojunction still can be formed (Supplementary Fig.? 39 ), but a?larger diameter of MOF(s) in Co-MOF nanostacks can be observed, which?is in line with our proposed mechanism. Furthermore, we have tuned the ratio of Au/Ag in transition metal NPs and investigated the formation of nanostacks. First, we prepared the hollow Au/Ag nanocubes with an Ag/Au molar ratio of 72:28 (Supplementary Fig.? 40 ), different from that (Ag/Au molar ratio of 47:53) shown in our initial manuscript, and used them to induce Co-MOF formation. Once again, stacked nanoplates with two SAED diffraction sets were observed (Supplementary Fig.? 41 ). We then further increased the Ag/Au ratio of Au/Ag nanocubes to 89:11, MOF nanostacks can also be successfully obtained (Supplementary Figs.? 42 , 43 ). In two extreme cases, we used pure metals, the solid Ag nanocubes (Supplementary Fig.? 44 ) and solid Au NPs (Supplementary Fig.? 46 ), as inductors and in both cases, the stacked nanoplates with two sets of SAED diffraction were observed (Supplementary Figs.? 45 , ? 47 – 49 ), similar to the use of hollow alloy NPs. We can already reasonably conclude that our method for the synthesis of Co-MOF nanostacks is universal. SPVM characterization of Co-MOF-3 homojunction . We adopted SPVM technique 33 , 34 to visualize the spatial charge transfer and separation between the two stacked nanoplates in Co-MOF-3 by integrating light illumination with the KPFM to map the surface photovoltage (SPV). The AFM topography image of one typical Co-MOF-3 nanostack is shown in Fig.? 5a and its corresponding height profile in Fig.? 5b . The edge lengths of two nanoplates were 0.69 and 1.67?μm, respectively. When the measurement was done without illumination, the Fermi levels of MOF(s) and MOF(l) were aligned at a thermal equilibrium state. Therefore, the contact potential difference (CPD) represents the local vacuum energy level variations relative to Fermi level positions at phase junction. The CPD of MOF(l) is about 15?mV lower than that of MOF(s), suggesting that the work function of MOF(l) is 15?eV higher than that of MOF(s). Thus, it can be deduced the formation of an internal built-in electric field and an upward band bending from MOF(s) to MOF(l) across the interface. It suggested that photogenerated electrons could transfer from MOF(l) to MOF(s) across the phase junction interface, according to the surface potential and the direction of the built-in electric field 52 . Fig. 5: SPVM analyses of stacked phase-enabled Co-MOF-3 homojunction. a AFM topography image and b the corresponding height profile of Co-MOF-3. c KPFM image of Co-MOF-3 under no illumination (dark condition). KPFM image of Co-MOF-3 under the irradiation of 420-nm LED light with the power of d 5?W, e 15?W, and f 50?W. g The corresponding normalized CPD profiles across the center of Co-MOF-3 (see the arrow line in c ) under the dark and irradiation conditions. h The statistical SPV values ( n ?=?5, SPV, ΔCPD?=?CPD light ???CPD dark ) collected from the center (purple) and peripheral (orange) regions, respectively, in the (001) facet of Co-MOF-3 stacked nanoplate under the illumination with various power densities (correction of SPV values was done by subtracting the corresponding CPD vaules of the HOPG support). Data are presented as mean values ±S.D. i SPVM image obtained by subtracting the potential under the dark condition from that under 50-W illumination and j the corresponding SPV profile. Full size image The CPD curves across the center and the peripheral parts were profiled from the KPFM images of Co-MOF-3 under the dark condition (Fig.? 5c ) and the irradiation of a 420-nm LED with radiation power of 5?W (Fig.? 4d ), 15?W (Fig.? 5e ), and 50?W (Fig.? 5f ), respectively. Under the dark condition, the CPD values of the small nanoplate (MOF(s)) were lower than those of the large one (MOF(l)) in the Co-MOF-3 nanostack, suggesting a higher work function of the smaller nanoplate 35 . To investigate the impact of illumination, a LED (420?nm) lamp with tunable power was applied in the direction parallel to the (001) facet of the Co-MOF-3 nanostack. It was found that the CPD values decrease with increasing light intensity, thus the decrease of CPD yields a negative SPV (Fig.? 5g ). However, the increase of MOF(s) was more significant, increasing the CPD difference between the smaller and large nanoplates. In other words, the ΔCPD values of the MOF(s) became higher than those of MOF(l) under illumination. The SPV values, obtained from five sets of data and after subtraction of the background CPD values, are shown in Fig.? 5h . Moreover, the SPV image (SPV, ΔCPD?=?CPD light ???CPD dark , Fig.? 5i ) of the nanostack was constructed by subtracting the KPFM image in the dark from the image (Fig.? 5c ) under illumination (Fig.? 5f ), and the corresponding SPV profile is shown in Fig.? 5j . A remarkably higher SPV value of ca. 40?mV was observed for MOF(s) nanoplate. Combining the results of direction of the built-in electric field and calculated relative energy band alignment of MOF(l) and MOF(s), we infer that the most possible reason for this phenomenon could be charge separation and transfer between homojunction in Co-MOF-3 53 , 54 . To the best of our knowledge, it is the first report on MOF-based homojunction, innovatively demonstrated by the SPVM method. The charge-carrier dynamic of Co-MOF-3 nanostacks is revealed by steady-state FL (Supplementary Fig.? 50a ) and time-resolved FL characterizations (Supplementary Fig.? 50b ). The FL spectrum of the Co-MOF-3 nanostacks presented a more remarkable FL quenching with respect to Co-MOF-1 and Co-MOF-2. Along with the shorter average lifetime ( τ avg ) of Co-MOF-3 (2.74?ns) than those of Co-MOF-2 (4.05?ns) and Co-MOF-1 (10.74?ns), it could be reasonably concluded that the increase of transfer and separation efficiency of charge carriers in Co-MOF-3 resulted from the short charge-transport distance, large interfacial contact area (both side-to-side and face-to-face), and strong interfacial bonding in the as-prepared homojunction 25 . Moreover, the Co-MOF-3 nanostacks manifested the smallest semicircle in the Nyquist plots (Supplementary Fig.? 51 ), highest transient photocurrent (Supplementary Fig.? 52 ), and most significant amplitude of open circuit potential (OCP) values (Supplementary Fig.? 53 ) under irradiation among the three Co-MOFs, further supporting the most efficient photoinduced electron-hole transport and separation of Co-MOF-3 nanostacks due to the structural homojunction. Photocatalytic CO 2 reduction performance of Co-MOF-3 homojunction . The photocatalytic CO 2 reduction performance of the Co-MOF-3 (mainly composed of homojunction nanostacks) with Co-MOF-1 (mainly composed of single nanoplates) as control was evaluated in aqueous solution under visible-light irradiation ( λ ?≥?400?nm, 1-atm CO 2 ). Figure? 6a shows that CO 2 was light reduced by Co-MOF-1 to CO with a generation rate of 10.7?μmol?h ?1 ?g ?1 , which is close to the performance of Zn-porphyrin MOFs reported 54 . As expected, the Co-MOF-3 with the homojunction presented a much higher CO production rate of 27.1?μmol?h ?1 ?g ?1 , which is ca. 2.5 times higher than that of the Co-MOF-1. We believe that the increase in performance is mainly due to the promoted charge separation driven by the presence of the homojunction. Such a CO 2 to CO conversion rate is moderate in the family of MOFs 55 , but higher than?the rate of pure g-C 3 N 4 (10.3??mol?h ?1 ?g ?1 ) 56 and that of most Z-scheme heterojunctions 16 . No evident decrease of CO evolution rate was perceived after 22-h photocatalytic reaction (Fig.? 6b ), and no significant change of structure reflecting the high stability of the Co-MOF-3 as a photocatalyst. It should be noted that we did not use organic solvent of acetonitrile, which was used in most photocatalytic CO 2 reduction to lift the efficiency of photocatalytic CO 2 reduction and contributing to the stability of photocatalysts. More importantly, no H 2 or liquid products could be detected in this reaction system, indicating a superior selectivity (~100) of Co-MOF-3 toward CO production. Such a high CO selectivity of our Co-MOF systems should be closely related to the chemical composition and pore structure of MOFs 6 , 7 , 14 . Till now, among numerous literature reports on CO 2 reduction, only a very few systems could achieve 100% selectivity for CO 19 , 21 . Fig. 6: Photocatalytic CO 2 reduction performance of Co-MOF-3 homojunction. a Evolution of CO and H 2 as a function of reaction time during the CO 2 reduction in aqueous solution under visible-light illumination for the Co-MOF-1 and Co-MOF-3 photocatalysts. b Stability of Co-MOF-3 during the continuous CO 2 reduction reaction. c Isotope labeling of Co-MOF-3 system carried out by using 13 CO 2 gas. d In situ DRIFTS measurement of Co-MOF-3. e – h ?Schematic illustration of intermediate COOH adsorbed onto MOF(l)_N 5 Co, MOF(l)_O 4 Co, MOF(s)_N 5 Co, and MOF(s)_O 4 Co in the Co-MOF-3 homojunction during CO 2 reduction, respectively. Calculated free energy (Δ G ) diagram of i CO 2 reduction to CO and j HER on Co sites in varied units of MOF(l)_N 5 Co, MOF(l)_O 4 Co, MOF(s)_N 5 Co, and MOF(s)_O 4 Co, respectively ( U ?=?0). Full size image The control experiments with inert gas (Ar), without light irradiation and in?the absence of catalyst of Co-MOF-3 were carried out, and it was found that no CO or other products were detected during these processes (Supplementary Fig.? 54 ). To further validate the origin of CO product, the 13 CO 2 isotope labeling experiment was further carried out 57 , 58 . As shown in Supplementary Fig.? 55 , the total ion chromatographic peaks at ca. 6.0 and ca. 15.8?min can be assigned to the CO (inset of Supplementary Fig.? 55 ) and CO 2 , respectively. The mass spectrum (MS) with the signal peaks at m/z ?=?45 and m/z ?=?29 ( 13 CO), m/z ?=?16 ( 16 O) and m/z ?=?13( 13 C) from gas CO 2 (ca. 15?min) confirmed the successful labeling (Fig.? 6c and Supplementary Fig.? 56 ). MS with the signal peak at m/z ?=?29 ( 13 CO), from gas at ca. 6.0?min unambiguously indicated that 13 CO indeed originated from light-driven 13 CO 2 reduction during the reaction. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suggested that the primary intermediates of CO 2 photoreduction in Co-MOF-3 are the –COO and –COOH species (Fig.? 6d ) 58 , 59 . The mechanism of the CO 2 to CO conversion in the Co-MOF-3 homojunction system was investigated using DFT. It is well known that the photocatalytic reduction depends on the energy level of photocatalysts and the adsorption and desorption energies of the starting material, intermediate, and the product. Generally, the CO 2 -to-CO reduction reaction includes the following four steps: (i) the CO 2 adsorption onto catalysts to form CO 2 , (ii) the H + and e ? coupled transfer to CO 2 and producing an intermediate of COOH, (iii) second electron and proton transfer to COOH and generating CO and H 2 O, (iv) the desorption of CO from catalysts to release the product of CO 22 . For porphyrins or analogs, the metal centers were demonstrated to be the preferably active sites as compared to C, N, and O elements in the porphyrin skeleton 60 . For our Co-MOF-3 homojunction, there are four different possible active sites of MOF(l)_N 5 Co, MOF(l)_O 4 Co, MOF(s)_N 5 Co, and MOF(s)_O 4 Co, and the schematic illustration of examples of intermediate COOH adsorbed onto them are shown in Fig.? 6i . Theoretical calculations showed that the N 5 Co presents a lower energy barrier in the path of COOH and CO intermediates than O 4 Co for either central MOF(s) or peripheral MOF(l) in Co-MOF-3 homojunction (Fig.? 6j ). It indicates that the CO 2 reduction tends to occur on the N 4 Co sites. Meanwhile, because it protects O 4 Co as the node and linker of MOFs from damage in the process of catalysis, the photocatalytic durability of Co-MOF-3 is thus achieved. Besides, the CO 2 adsorption energies onto the N 5 Co sites are quite small (ca. 0.08?eV), suggesting good adsorption capacility of Co-MOF-3. Meanwhile, the required H + transfer energy from an intermediate of CO 2 to COOH is about 0.42?eV over MOF(s)_N 5 Co, which is much smaller than the energy barrier of H + reduction to H 2 (0.87?eV, Fig.? 6e ), which well explains the observed high CO selectivity for Co-MOF-3 over the hydrogen evolution reaction. .

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