==== Front Nat Commun Nat Commun Nature Communications 2041-1723 Nature Publishing Group UK London 37386039 39630 10.1038/s41467-023-39630-y Article Hierarchical conductive metal-organic framework films enabling efficient interfacial mass transfer http://orcid.org/0000-0002-2747-4930 Huang Chuanhui 1 Shang Xinglong 2 Zhou Xinyuan 3 Zhang Zhe 14 Huang Xing 1 Lu Yang 1 http://orcid.org/0000-0001-9979-3503 Wang Mingchao 1 Löffler Markus 5 Liao Zhongquan 6 Qi Haoyuan 17 http://orcid.org/0000-0003-0582-4044 Kaiser Ute 7 Schwarz Dana 8 http://orcid.org/0000-0001-6692-3762 Fery Andreas 18 Wang Tie 3 http://orcid.org/0000-0003-0268-519X Mannsfeld Stefan C. B. 4 http://orcid.org/0000-0001-9451-5336 Hu Guoqing ghu@zju.edu.cn 2 http://orcid.org/0000-0003-3885-2703 Feng Xinliang xinliang.feng@tu-dresden.de 19 http://orcid.org/0000-0002-4125-9284 Dong Renhao renhaodong@sdu.edu.cn 110 1 grid.4488.0 0000 0001 2111 7257 Center for Advancing Electronics Dresden (Cfaed) and Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany 2 grid.13402.34 0000 0004 1759 700X Department of Engineering Mechanics & State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou, 310027 China 3 grid.265025.6 0000 0000 9736 3676 Tianjin Key Laboratory of Drug Targeting and Bioimaging, Life and Health Intelligent Research Institute, Tianjin University of Technology, Tianjin, 300384 People’s Republic of China 4 grid.4488.0 0000 0001 2111 7257 Center for Advancing Electronics Dresden (cfaed) and Faculty of Electrical and Computer Engineering, Technische Universität Dresden, 01062 Dresden, Germany 5 grid.4488.0 0000 0001 2111 7257 Dresden Center for Nanoanalysis, Center for Advancing Electronics Dresden, Technische Universität Dresden, 01062 Dresden, Germany 6 grid.461622.5 0000 0001 2034 8950 Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), Maria-Reiche-Strasse 2, 01109 Dresden, Germany 7 grid.6582.9 0000 0004 1936 9748 Electron Microscopy of Materials Science, Central Facility for Electron Microscopy Universität Ulm, 89081 Ulm, Germany 8 grid.419239.4 0000 0000 8583 7301 Leibniz-Institut für Polymerforschung Dresden e.V. (IPF), Hohe Str. 6, Dresden, 01069 Germany 9 grid.450270.4 0000 0004 0491 5558 Department of Synthetic Materials and Functional Devices, Max Planck Institute for Microstructure Physics, D-06120 Halle (Saale), Germany 10 grid.27255.37 0000 0004 1761 1174 Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100 China 29 6 2023 29 6 2023 2023 14 385014 11 2022 19 6 2023 © The Author(s) 2023 https://creativecommons.org/licenses/by/4.0/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Heterogeneous reactions associated with porous solid films are ubiquitous and play an important role in both nature and industrial processes. However, due to the no-slip boundary condition in pressure-driven flows, the interfacial mass transfer between the porous solid surface and the environment is largely limited to slow molecular diffusion, which severely hinders the enhancement of heterogeneous reaction kinetics. Herein, we report a hierarchical-structure-accelerated interfacial dynamic strategy to improve interfacial gas transfer on hierarchical conductive metal-organic framework (c-MOF) films. Hierarchical c-MOF films are synthesized via the in-situ transformation of insulating MOF film precursors using π-conjugated ligands and comprise both a nanoporous shell and hollow inner voids. The introduction of hollow structures in the c-MOF films enables an increase of gas permeability, thus enhancing the motion velocity of gas molecules toward the c-MOF film surface, which is more than 8.0-fold higher than that of bulk-type film. The c-MOF film-based chemiresistive sensor exhibits a faster response towards ammonia than other reported chemiresistive ammonia sensors at room temperature and a response speed 10 times faster than that of the bulk-type film. Heterogeneous reactions associated with porous films are vital in nature and industry. A hierarchical-structure-accelerated interfacial dynamic strategy is reported to improve interfacial gas transfer on conductive metal-organic framework films. Subject terms Sensors and biosensors Metal-organic frameworks Coordination polymers https://doi.org/10.13039/100010663 EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council) 852909 Dong Renhao https://doi.org/10.13039/501100001809 National Natural Science Foundation of China (National Science Foundation of China) 22272092 Dong Renhao issue-copyright-statement© Springer Nature Limited 2023 ==== Body pmcIntroduction Porous solids, ranging from classic inorganic zeolites to molecular organic and hybrid frameworks1–3, have fascinating properties for various heterogeneous reactions, such as electrochemical energy storage4–6, heterogeneous catalysis7–10, and sensing11–14. Regarding heterogeneous reactions involving porous solid films, mass transfer at solid/liquid or solid/gas interfaces plays an important role in reaction kinetics, and the overall rate of reaction is usually limited by the interfacial mass transfer15–17. However, subject to the no-slip boundary condition in pressure-driven flows18,19, mass transfer near the surface of the solid film is largely limited to the relatively slow process of molecular diffusion since the convective movement of fluids approaches zero (Fig. 1a). The critical bottleneck to promote convective movement at the boundary layer results from high hydraulic resistance, i.e., the low permeability of solid film samples20,21. Such a low permeability induced stagnant boundary layer poses a grand challenge for further promotion of interfacial mass transfer, necessitating the design of innovative porous nanointerfaces to boost interfacial mass transfer.Fig. 1 Schematic of interfacial mass transport on a solid porous film. a Flow velocity vectors and slow mass transfer (concentration gradient-induced molecular diffusion) on the surface of a solid porous film (left) and the corresponding concentration field on the surface of the solid porous film (right). b Flow velocity vectors and fast mass transfer (molecular diffusion plus convection) on the surface of a hierarchical porous film (left) and the corresponding concentration field on the surface of the hierarchical porous film (right). The black arrow indicates the flow direction, and the length of the arrow indicates the magnitude of the velocity. The red spots represent the molecules that reach the surface of the film. Conductive metal–organic frameworks (c-MOFs), which are electroactive crystalline porous coordination polymers22–24, are emerging as promising electronic materials and have exhibited potential for broad applications such as electronic devices25, electrocatalysis26,27 and energy storage28,29. Due to their high porosity, intrinsic electrical conductivity and abundant functional sites, c-MOF films have been widely utilized as active layers for chemiresistive gas sensing30–32. In a c-MOF-based chemiresistive gas sensing system, the sensing relies heavily on the adsorption of target gas molecules on the surface of c-MOF films, and the kinetics of this kind of heterogeneous reaction are mainly influenced by interfacial mass transfer33,34. Therefore, the c-MOF-based chemiresistive gas sensing can be considered a model for establishing the permeability-interfacial mass transfer correlation, providing guidelines to address the challenge of accelerating the interfacial mass transfer for porous solid systems. Herein, we demonstrate a hierarchical-structure-accelerated interfacial dynamic (HSAID) strategy for the promotion of interfacial mass transfer to boost heterogeneous reactions. Hierarchical c-MOF films (denoted Zn-HHTP-H, PcCu-Zn-H and Co-HHTP-H) were constructed by an in-situ transformation of three-dimensional (3D) ZIF-8 (Zn(MeIM)2, where MeIM = 2-methylimidazole) or ZIF-67(Co(MeIM)2) film precursors using π-conjugated ligands (2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) or 2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato copper (PcCu-(OH)8) on a solid surface. The resultant crystalline c-MOF films possess hierarchical structures with a nanoporous shell (~1.2 nm) and hollow interior voids (~500 nm). Systematic gas permeability tests and computational fluid dynamics (CFD) simulations revealed that the permeability of hierarchical hollow Zn-HHTP-H films was 8.4-fold greater than that of a bulk-type Zn-HHTP film, greatly increasing the interfacial mass transfer rate (Fig. 1b). The prepared c-MOF films were further integrated as chemiresistors for ammonia sensing to elucidate the influence of their hollow nature on interfacial mass transfer. Compared to the bulk-type films (MOF films without hierarchical porous structures synthesized by the hydrothermal method), the hierarchical c-MOF film-based sensors displayed a 10-fold improvement in the response speed of ammonia. In particular, the Zn-HHTP-H-based chemiresistive ammonia sensor exhibited the fastest response speed (response time of 9.1 s) at room temperature, superior to those of previously reported chemiresistive ammonia sensors (with response time ≥35 s)32,33,35. Our work provides a general synthetic strategy for constructing hierarchical porous structures, thus improving the interfacial mass transfer to accelerate heterogeneous reactions, which can be further employed to achieve high-performance devices. Results Synthesis and characterization of hierarchical c-MOF films Hierarchical c-MOF films were constructed on a silicon substrate based on an accessible insulating MOF-to-c-MOF transformation approach (Fig. 2a). In the first step, a typical ~500 nm thick ZIF-8 film was synthesized on silicon substrate36, which was utilized as the sacrificial precursor (Supplementary Fig. 1). Second, upon immersion into an HHTP solution (ethanol:water = 7:1, v/v) at room temperature, the as-prepared ZIF-8 film was in-situ transformed into a hierarchical c-MOF (Zn-HHTP-H) film (Fig. 2b, e). Through this sacrificial-template synthetic method, the white ZIF-8 films were gradually decomposed while dark purple Zn-HHTP-H films were formed after 24 h (Supplementary Figs. 2, 3). DFT calculations revealed that the transformation reaction from ZIF-8 crystals to Zn-HHTP crystals is thermodynamically spontaneous (Supplementary Table 1). This should be attributed to the greater stability of the square planar linkages of ZnO4 in Zn-HHTP than the Zn-N coordination bonds in ZIF-8 (Supplementary Fig. 4)37. Following the same sacrificial template synthetic method, the PcCu-Zn and Co-HHTP films with hierarchical hollow nanostructures were synthesized (named PcCu-Zn-H and Co-HHTP-H, respectively, Fig. 2c–g, Supplementary Figs. 5–8). All these hierarchical c-MOF films exhibited intrinsic electrical conductivity, while the inside was hollow (Fig. 2f, g, Supplementary Table 2).Fig. 2 Synthetic strategy and structural characterization. a Schematic overview of the transformation of insulating 3D MOF film precursors to hierarchical c-MOF films. b–d Schematic structures of Zn-HHTP, PcCu-Zn, and Co-HHTP, respectively. e–g The corresponding cross-sectional scanning electron microscope (SEM) images of the Zn-HHTP-H, PcCu-Zn-H, and Co-HHTP-H films, respectively. The scale bars represent 500 nm for (e), 1 μm for (f), and 500 nm for (g). As shown in the transmission electron microscope (TEM) image (Fig. 3a and Supplementary Fig. 9), the thickness of the Zn-HHTP-H hollow shell was approximately 20 nm, suggesting that the Zn-HHTP-H film possessed a very thin upper surface. The selected-area electron diffraction (SAED) pattern verified the polycrystalline feature of the hollow Zn-HHTP film (Fig. 3b). The lattice fringes at interplanar spacings of 1.8 nm and 0.31 nm corresponded to the (100) and (022) planes of Zn-HHTP crystals, respectively (Fig. 3c)38. The honeycomb pattern observed by high-resolution transmission electron microscopy (HRTEM) illustrated the highly ordered hexagonal MOF frameworks along c-axis (Fig. 3d). Elemental mappings by electron energy loss spectroscopy (EELS) confirmed the homogeneous distribution of C, O, and Zn throughout Zn-HHTP-H (Supplementary Fig. 9).Fig. 3 Characterization and gas permeability properties. a TEM image of the Zn-HHTP-H scraped off from the film. b SAED pattern (white circle in (a)). c, d High-resolution TEM image of the red square shown in (a). e Nitrogen adsorption and desorption isotherms measured at 77 K for different Zn-HHTP films. f Comparison of the N2 flux values of different Zn-HHTP films at the same operating pressure. g–i Contour plots of velocity through the film in the plane of symmetry. The insets show the models for three different Zn-HHTP films. The scale bars represent 500 nm for (a) and 10 nm for (c, d). Morphology control of c-MOF films We suggest that the transformation followed a “dissolution-recrystallization” mechanism39,40, and the following possible reaction pathway was proposed (Supplementary Figs. 10–13a):1 2HHTP⇌2(HHTP*)3−+6H+ 2 3Zn(MeIM)2+6H+⇌6HMeIM+3Zn2+ 3 3Zn2++2(HHTP*)3−⇌Zn3(HHTP*)2 The total reaction is represented as follows:4 3Zn(MeIM)2+2HHTP=Zn3(HHTP*)2+6HMeIM A proper balance between the etching and coordination rates was found to be pivotal for achieving a well-defined hollow nanostructure (Supplementary Figs. 13b–19). Herein, the ethanol was introduced to reduce the water-dependent etch rate. The Zn-HHTP-H film with ~500 nm cavities was obtained only at low water fractions (25%) and low temperatures (40 °C) (Supplementary Figs. 13b, S13d and S15). When the water fraction and temperature were increased (e.g., 37.5% and 40 °C, respectively), a Zn-HHTP film was formed with small cavities of ~175 nm (denoted as Zn-HHTP-HS film, Supplementary Figs. 13b, S13e and S16, 17). For comparison, a bulk-type Zn-HHTP film (denoted Zn-HHTP-B) with a thickness of ~500 nm was also synthesized directly on a silicon wafer via a procedure similar to that described previously (Supplementary Figs. 20–21)41. The Zn-HHTP-HS and Zn-HHTP-B films share identical crystalline structures with Zn-HHTP-H (Supplementary Fig. 22). Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analyses (TGA) further confirmed the identical compositions of these Zn-HHTP films (Supplementary Figs. 23–25). The Brunauer−Emmett−Teller (BET) measurements revealed that the surface areas of the Zn-HHTP-H (614.0 m2 g−1) and Zn-HHTP-HS (479.4 m2 g−1) films were much higher than that of the Zn-HHTP-B film (165.3 m2 g−1), although they all possessed 1.1 nm micropores (Fig. 3e and Supplementary Fig. 26). To determine the difference in the gas permeability, the abovementioned Zn-HHTP films were further synthesized on a porous nylon 66 membrane via the MOF-to-c-MOF transformation approach (Supplementary Fig. 27). As shown in the permeability test, although the three Zn-HHTP film samples presented a similar film thickness of ~500 nm, much different gas permeabilities were observed (Supplementary Fig. 28). The N2 fluxes of the Zn-HHTP-H film (70.65 L m−2s−1) and the Zn-HHTP-HS film (19.21 L m−2s−1) were 8.4 and 2.3 times higher than that of the bulk-type Zn-HHTP film (8.45 L m−2s−1), respectively (Fig. 3f). Apparently, the hollow interior of the Zn-HHTP films prevented the pressure loss along the path through the c-MOF films, leading to high permeability of the c-MOF films42. Next, the theoretical pressure-driven flow was analyzed to understand the film permeability via commercial CFD software Ansys Fluent (Fig. 3g–i, Supplementary Figs. 29, 30). The simulated N2 fluxes of the Zn-HHTP-H and Zn-HHTP-HS films were 10.2 and 2.2 times higher than that of the Zn-HHTP-B film, respectively, consistent with the experimental permeability results (Fig. 3g–i). Numerical simulations of the film structure–gaseous fluid interaction The flow in the vicinity of the solid surface was facilitated by introducing hollow cavities into the bulk-type Zn-HHTP films (Fig. 4, Supplementary Figs. 31, 32 and Supplementary Table 3). Mass transfer mainly depended on the slow molecular diffusion onto the surface of bulk-type film, while extra nonzero convection velocities were generated on the hollow film (Fig. 4a). The surface convection induced by the nonzero velocity enhanced the mass transport, thus largely reducing the time required to achieve equilibrium concentration fields for the hollow films (Fig. 4b, Supplementary Videos 1–3). The simulation results suggested that the mass transfer efficiency followed the order Zn-HHTP-B film