van der Waals Epitaxial Growth of 2D Metal–Porphyrin Framework Derived Thin Films for Dye‐Sensitized Solar Cells

Abstract

nature, which cannot meet the increasing demand for its broad applications in DSSCs. Thus, there is a huge interest in Pt-free materials for CE fabrication, such as inorganic semiconductors, [5] carbon materials, [6] conductive organic polymers, [7] and so on. [8,9] Metal-organic frameworks (MOFs) represent a class of porous coordination polymers that consist of organic ligands linker by metal ions to form crystalline assemblies, [10,11] which have been widely investigated for their attractive physical and chemical properties. [12,13] Recently, the tunable functionality of MOFs has allowed these porous materials as precursors or sacrificial templates for the preparation of electrode materials in DSSCs. [14,15] In particular, porphyrin-based MOFs (also called metal-porphyrin frameworks) [16] have been demonstrated to exhibit tunable optical, electrical, and photophysical properties, [17][18][19] which in particular suggest applications in DSSCs. However, up to now, employing metal-porphyrin frameworks for CE materials in DSSCs has not been reported.When aiming to use the potential of MOFs for electrical, electrochemical, and electronic applications, MOF thin films [20][21][22] prepared by a layer-by-layer (lbl) liquid-phase epitaxial (LPE) procedure have attracted huge interest. These surfacemounted MOFs or SURMOFs are assembled by a sequence of simple immersion processes, subsequently into solution of the metal source and the organic ligand. They exhibit a number of interesting properties, including controllable thickness, crystallinity, high degree of orientation, and homogeneous surface. So far, the lbl-approach has been successfully used to realize several types of SURMOFs, [23] e.g., HKUST-1 (Cu 3 (BTC) 2 , BTC = benzene-1,3,5-tri-carboxylate), SURMOF-2, Zn(2-methylimidazolate) 2 (ZIF-8), etc. [23][24][25] These SUR-MOFs are epitaxially grown on appropriately functionalized substrates by coordination bonding between metal ions and organic ligands. The growth of SURMOFs based on lbl protocols involving noncovalent interactions (such as van der Waals, hydrogen bonding, and other weak interactions, etc.) has not yet been studied. When using porphyrinic ligands, such SUR-MOFs are expected to have interesting applications in catalysis, electronic devices, and in photovoltaics. [26][27][28][29][30][31][32][33][34] Herein, we report a layer-by-layer epitaxial growth strategy for preparing SURMOFs with a layered structure. 2D sheets consisting of paddle-wheel (PW) units linked by porphyrin linkers are stacked by the interaction between layers being of van der Waals type. X-ray diffraction (XRD) data reveal that layer-by-layer deposition on appropriately functionalized substrates yields In this work, monolithic, crystalline, porous, and oriented porphyrin thin films are grown using a novel van der Waals layer-by-layer (lbl) epitaxial growth protocol, yielding an unusual AB-stacking motif of these interesting macrocycles units. Subsequently, these surface-mounted metal-organic frameworks (SURMOFs) are transformed by th...

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

“…As shown in Figure 24a, this method has also been used to epitaxilly grow other 2D SURMOFs such as Zn-TCPP, in which consequtive layers are connected by van der Waals forces. 370 In addition, Quartz Crystal Microbalance (QCM) and Surface Plasmon Resonance (SPR) have been widely reported as complementary methods to monitor the amount of material deposited during the process. For example, Shekhah has recorded the sequential injection of Cu(Ac)2 (Ac = Acetyl group) and H3BTC and then rinsed with ethanol on a functionalized QCM electrode.…”

Proximity Effect in Crystalline Framework Materials: Stacking‐Induced Functionality in MOFs and COFs

“…[94,95] In contrast to COFs, where the 2D versions are more prominent than their 3D counterparts, 2D MOFs with atomically thin layers coupled by van der Waals interactions are-to date-rather exotic and have been reported in a few cases only. [24,[96][97][98][99][100][101][102] However, the instances of such layered system reported so far promise an interesting bridge between chemistry and physics, offering materials with interesting electronic and magnetic properties.…”

“…To date, a huge variety of MOFs has been synthesized and characterized, with the most frequent applications in the fields of gas adsorption and separation, sensing, drug‐release, proton conductance in fuel cells, catalysis, water splitting, and many others 94,95. In contrast to COFs, where the 2D versions are more prominent than their 3D counterparts, 2D MOFs with atomically thin layers coupled by van der Waals interactions are—to date—rather exotic and have been reported in a few cases only 24,96–102. However, the instances of such layered system reported so far promise an interesting bridge between chemistry and physics, offering materials with interesting electronic and magnetic properties.…”

Proximity Effect in Crystalline Framework Materials: Stacking-Induced Functionality in MOFs and COFs

“…[94,95] In contrast to COFs, where the 2D versions are more prominent than their 3D counterparts, 2D MOFs with atomically thin layers coupled by van-der Waals-interactions areto daterather exotic and have been reported in few cases only. [24,[96][97][98][99][100][101][102] However, the instances of such layered system reported so far promise an interesting bridge between chemistry and physics, offering materials with interesting electronic and magnetic properties.…”