Covalent-organic framework film: From synthesis and structure to energy, catalysis and sensor applications

Films are used a lot in nature, especially with animals and plants. With the development of film technology, it is becoming more popular and being used applied in many fields. There has been a growing trend to make covalent-organic framework (COF) films, which was inspired by the development of COF. Compare with traditional polymer, COF is a new polymer form that not only has high stability, but also functional diversity and designability. With these characteristics, COF film will create new possibilities for many applications. In this review, we summarize the major progress made on the synthetic strategies of COF film and their applications in energy, catalysis, and sensor application. Finally, we discuss the existing issues and future direction.


Introduction
With society growing, the film has attracted a lot of attention because of its structural properties, such as its flexibility, light weight, and small thickness.Film preparation is an essential technology in the development of novel materials because it can make the material's properties as good as it can be to dig the film's application direction.[1][2][3][4][5] The primary source of film is polymer, which penetrates many fields.COF, a new polymer form, which dates back to 2005, has become a popular material in recent years as the technology develops and the materials are refined.[6] COF is a periodic network structure assembly based on self-condensation or co-condensation.COF is different from the classical short-range ordered amorphous polymers formed by irreversible condensation.The COF with long-range ordered crystal structure was formed by the reversible reaction.[7][8] Compared to traditional crystalline porous solids and polymer, COF can be pre-designed and achieve a controllable specific functional structure.Moreover, they have the advantage of structural diversity, low density, high thermal stability, and permanent porosity.Their applications include semiconductors, catalysis, gas adsorption, and energy storage.Since 2005, related works about COF films have been extensively reported.[9] Figure . 1 shows the development of publications in the field.COF is the excellent primary material of film in many applications because (1) COF films have high stability and facilitate their wide application.(2) COF with a controllable structure can render their film with predictable functions.(3) COF was given diverse possibilities of post-modification by the abundant organic building blocks.
Herein, we summarize the structural characterization and synthetic strategy of the COF film.We focus on the functions specific to COF film, including energy, catalysis, and sensor.We also discuss the bottleneck and challenging issues and provide future directions.

Synthesized method (1) Bottom-up Strategy
The most widely used strategy for synthesis of COF film materials is bottom-up, which is mainly through the deposition of COF on different substrates, appropriate interfaces, or surfaces for controlled polymerization.Dictel's group took the lead in using the bottom-up method to grow COF film on singlelayer graphene under solvothermal conditions.[10] Solvothermal synthesis involves immersing substrates into the monomer system, then washing them with organic solvent and drying them under inert gas flow.This method was successful in making continuous COF film.Recently, COF film has been synthesized by using different kinds of substrates, such as graphene [10], hexagonal boron nitride [11], glass [12], polyamine-coated capillary [13], and highly oriented pyrolytic graphite [14].The range of synthetic film materials includes COF-5, COF-366, BDT-COF, etc. Lei's group synthesized a two-dimensional covalent organic framework (2D COF) on a highly oriented pyrolytic graphite (HOPG) surface with benzene-1,3,5-tricarbaldehyde and pphenylenediamine as the precursors.[15] This method can also be used to make atomic layered COF on the substrates such as copper foil (SLG-copper), Au(111) and Ag(111).Porte's group presented an example of ordered hexagonal covalent nanoporous structures deposited in extended arrays of near monolayer coverage across Ag(111) surface.(Figure2) [16] Films with thickness ranging from nanometers to millimeters have been prepared from the above materials.Such methods are restricted by their defects and the degree of cross-linking.Copyright 2008 American Chemical Society.
Solution-based synthesis methods may result in insoluble powders or fragile films on the solution surface.Chemical vapor deposition (CVD) is a good solution for solving these types of issues under certain circumstances.The Liu's group reported large-area -C=Nlinked 2D COF films with controllable thicknesses via vapor-induced conversion.Using a Schiff base polycondensation process, PyTTA film and TPA vapor form a uniform organic framework film directly on the growth substrate.This process is driven by π -π stacking interactions with the aid of water and acetic acid.[ Alternatively, COF film can be synthesized by the interfacial method, which reacts at the interface of single or few layers, including the liquid/gas interface and the liquid/liquid interface.Su's group proposed a potential difference-modulated biphasic strategy for fabricating large-area, self-standing COF films under ambient conditions.[18] Since the potential difference across the water/DCE interface can be facilely modulated by dissolving proper electrolyte ions in two phases, the fabrication of largearea COF film is made possible in beakers.The controlled release of monomers to the interface is crucial for the formation of crystalline films.For example, controlled release can be achieved by replacing free amine with an amine salt containing p-toluenesulfonic acid.The hydrogen bond in p-toluenesulfonate causes amine to be slowly released to the interface.[19] In addition, a microfluidic system was further developed to prepare COF-5, DPB-COF, TP-COF, and COF-10 films based on this synthesis principle.In this system, COF film grew at a constant mass deposition rate.Because the reaction depends on the interface, it is important to select the reaction environment and reaction conditions, including the selection of solvents and catalysts.
(2) Top-down Strategy The top-down strategy is to directly exfoliate the bulk COF in order to form an independent COF film, using solvent assisted exfoliation, mechanical exfoliation and self-exfoliation.Bulk COF is stacked by van der Waals force through ππ interaction between atomic layers.Felix's group firstly exfoliated COF-8 into COF film with a thickness of 4 nm-8 nm, inspired by the excellent performance of graphene.[20] Exfoliation is mainly achieved though ultrasonic treatment in solvent and mechanical grinding.For example, the borate ester linked COF-8 suspension was ultrasonically treated with dry dichloromethane for 15 min to make nanoflakes.The grinding method can exfoliate COF, such as TpPa-1, TpPa-2, TpPa-NO2, TpPa-F4, TpBD, TpBD-(NO2)2, TpBD-Me2 and TpBD(OMe)2 into nanosheets.(Figure3) [21] Due to the strong ππ interaction between monolayer structures, it is difficult to prepare real monolayer COF film, and the yield of pure monolayer or thin layer COF is often not very high.Currently, the synthesis of COF thin films is mainly based on the reaction substrate, interface and interlayer interaction force.However, it remains a challenge to transform crystalline COF powder into high yield, independent, and solid films.The main methods for making macro-sized film materials are mainly direct mixed method, in-situ growth method and layer-by-layer stacking method.
(3) Others The direct mixed method of film material is combining COF and polymer because COF has an organic skeleton and relatively good compatibility with polymer matrix.Michael's group prepared a new type of nanocomposite films for organic solvent nanofiltration by adding COF nanoparticles with secondary amine groups to the polyamide.The films have high separation and solvent resistance.However, the discontinuous COF base film prepared by this method cannot well reflect the structural properties of COF.[22] Jiang's group research that the bulk COF was exfoliated into two-dimensional COF nanosheets (CONs) with the chemical properties like their bulk COF.[23] In situ growth is the process of making film materials by using molecules with reactive functional groups as surface modifiers for the reaction between monomers and substrates.The Caro's group showed a novel two-dimensional double-layer stacked COF composite film that was made by continuously adjusting the growth of imine based COF-LZU1 and nitrogen based ACOF-1 layers on the porous carrier through the temperature-changed solvothermal method.[24] COF-LZU1-ACOF-1 bilayer film also has high thermal stability and time stability.The layer by layer stacking principle is based on the above exfoliated method.Firstly, the bulk material is exfoliated in water or solvent to obtain a nano-sheet dispersion system.Next, the nano-sheets are stacked on the porous substrate to form a continuous film by pressure, vacuum assisted filtration or dip coating.Toshinori's group successfully prepared a new COF-1 film by assembling exfoliated microporous COF-1 nanosheets on porous ceramic carriers.[25] This film has low yield, fragile and poor continuity.
(4) Structural characteristic In order to realize the application of film materials, the composition, surface state, morphology, crystallinity and other relevant properties of COF film should be characterized, so as to guide the optimization of development.
The chemical structure of COF film has been characterized by solid-state 13 C NMR, 1 H NMR and infrared spectroscopy which can uncover the bond formed between the knot and linker in the COF film structure.[7] For determining the surface morphology of COF film, optical microscope and scanning electron microscope are widely used.Among them, scanning electron microscopy has a wide range of applications, which can magnify the sample by 10 3 ~10 4 times for observation of fine tissues.The microstructure and structure of the film materials can also be characterized by transmission electron microscopy.In combination with electron diffraction analysis, we can further understand the defect or the atomic arrangement state of the boundary of the materials.In addition, atomic force microscope is used to investigate the surface roughness.
For bulk materials, the crystalline state and crystal structure of materials are mainly characterized by X-ray diffraction.The film thickness can only be effectively measured at the micron level.However, the conventional X-ray of about 0.1 nm is easy to enter the substrate when test micron-sized films.That will make the test signal include the signal of the substrate.Therefore, the sample should be as large as possible in order to obtain a sufficiently strong signal.
The element analysis of the film material can be carried out through the signal of the interaction between X-ray or electron beam and the sample.At present, the characteristic X-ray and other energies of various elements have been systematically measured.By comparing the test value with the reference value, the type of elements contained can be determined based on the wavelength and energy.The strength can also determine the content and concentration of elements under certain circumstance.

Application (1) Energy
As technology develops and the population grows, society has increasingly high requirements for the diversification of scientific and technological products, followed by the rapid growth of the demand for mobile power.Battery and supercapacitors are attractive to researchers because of their performance.The high specific surface area of COF film provides advantages in electrolyte infiltration and ion storage.The thickness of nano-scale will reduce the distance between ion/electron and promotes ion diffusion/electron transmission.COF can be made with a special structure to adapt to different conditions for clean energy.[26][27][28][29] COF has strong electron delocalization, high surface area and tunable porosity, which makes it good for applications like hydrogen, optical energy, osmotic power, etc.
COF films materials have potential in the field of energy storage due to their structural composition and physical and chemical properties.The 2D COF is stacked by ππ interaction.The ultra-thin COF film can reduce the migration distance of ions/electrons and further promote their diffusion and transmission.The designability of COF allows them to have a large number of available lithium sites and redox sites to assist in the rapid charge-discharge cycle.The nano-sheet materials obtained by the top-down exfoliated method have more advantages compared to ultra-thin COF films.Wang's group obtained the DAAQ-ECOF film material with a thickness of 5 nm by mechanical grinding and exfoliating.[30] Compared to the original block COF, it has a shorter Li + diffusion path, improves the utilization efficiency of redox sites, and achieves faster lithium storage dynamics.Its charge-discharge performance is up to 1800 times, and 98% of the fast charge-discharge capacity is retained.The DABQ-ECOF has peak voltage of 2.8 V and specific capacity of 210 mAh -1 g -1 .In addition, COF film has the advantages of rich adsorption sites and channels, which facilitate ion transport at the electrodeelectrolyte interface.These advantages have potential in the field of supercapacitors.Catherine's group synthesized the 2D COF structure connected by alkenoalanine, which contains anthraquinone subunits, and has a reversible electrochemical process.[31] After 5000 charge-discharge cycles, the electrode modified with redox active COF showed higher capacitance than the electrode modified with non-redox active COF.Based on the all organic framework structure of COF, the way of composite materials can further improve the application performance.Talyzin's group has prepared a mixed 2D-2D film material composed of vertically oriented COF and graphene by using diboric acid molecules modified graphene oxide.[32] This film demonstrates high performance of the supercapacitor in energy storage application.Tang's group synthesized COF films with atomic layers that features well-ordered pore arrangement, which can achieve can achieve an extremely low film resistivity and ultrahigh ion conductivity.When used as osmotic power generators, these films produce an unprecedented output power density over 200 W m -2 .[33] In the energy field, there is a high demand for the thickness of COF film materials.The COF film materials are easily stacked again, which driven by the ππ interaction force after obtaining ultrathin film materials in the subsequent reaction.Moreover, the thickness of COF film has a greater influence on defect: the thicker the COF film is, the more defect the material has.
(2) Catalysis As an important research direction, catalysis is involved in many fields, including biology, energy, chemical industry and other fields.Film is used to perform both separation and catalysis functions in the catalysis field.COF film has advantages in their designability and rich composition sources increase the possibility of composition and number of catalytic active sites.[34][35][36][37] The COF film materials with polymer structure, highly ordered electronic channels and accessible active sites provide a platform for their application in the field of catalysis.Rajesh's group prepared triazine based 2D COF film materials by the polycondensation of cyanuric chloride and perylene imide.[38] With the ability to capture visible light, proper band gap and highly ordered electronic channels, the film material has excellent photocatalytic performance.COF film is nearly 3.7 times higher than monomer in the regeneration of photo catalytic reduced coenzyme I, and 3.4 times higher in formic acid formation.At the same time, based on the designability of COF, porphyrins and phthalocyanines monomers can be selected for polymerization when metal ions are needed for catalysis due to their ability to chelate with metal ions.In Cao's group, they showed the synthesis of large metalloporphyrinbased COF films and their applications for oxygen electrocatalysis.[39] The Co-porphyrin-based films are efficient for electrocatalytic O2 reduction and evolution reactions.The films were used to make a flexible, all-solid-state Zn-air battery that had a high performance with a charge-discharge voltage gap of 0.88 V at 1 mA/cm 2 and high stability under bent conditions (0° to 180°).Christia's group grew COF-366-Co, COF-366-(OMe)2-Co, COF-366-F-Co and COF-366-(F)4-Co on the surface of highly oriented pyrolytic graphite by bottom-up method, and prepared a series of COF films with different electron absorption characteristics for electrocatalytic carbon dioxide reduction.(Figure4) [14] These materials are rich in accessible metal centers with electrochemical activity, which significantly improve the electrocatalytic performance of COF films.The all organic framework structure of COF film materials provides stability, but they limit the reactions that need metal ions to catalyze.Therefore, in addition to the existing specific monomers for metal ion chelation, other ways of modification and the use of interlayer space are worth of further research.
(3) Sensor The three pillars of information technology are sensor technology, together with computer technology and communication.Sensor technology has been widely used in many industries and has become the basic condition for the development of modern science and technology, such as temperature measurement, pressure measurement, molecular recognition and ion detection.The adjustable structures of COF film can be changed on organic ligands to get more specific responses.[40][41][42][43] Comparing with bulk/powder materials, COF film has larger transverse dimensions and thinner thickness, which is easy to achieve more sensitive response.
COF film has good optical and physical properties.The problem of aggregation layer formed by π-π stacking and poor electron mobility is relatedly solved with the help of film structure.COF has made a big step forward in the field of sensing and detection of nitroaromatic explosives.Gobinda's group successfully synthesized two new imide based block COF by solvothermal crystallization method.[44] This method was used to obtain thin layer COF film materials from these block COF by simple liquid phase exfoliation.TfpBDH CONs can turn-on detection capability for 2,4,6-trinitrophenol in solid state.On the contrary, they also can close detection capability in dispersed state.In the biological field, COF film materials have a certain selective adsorption capacity for biological molecules, such as RNA molecules and proteins.Fang's research group has constructed an imine linked COF material on an amino functionalized silicon substrate, which allows proteins or DNA to be adsorbed on the surface of the amino COF due to their strong interaction.[45] The high roughness of the COF film surface and its internal interactive pore structure enhance the electrochemical response activity of the COF film.Zhang's group reported a nanofluidic film based on an ionic COF that is capable of intelligently monitoring temperature variations and expressing it in the form of continuous potential differences.The high density of the charged sites present in the sub-nanochannels renders superior permselectivity to the resulting nanofluidic system, leading to a high thermosensation sensitivity of 1.27 mV K −1 .[46]

Conclusions
The research of film has been around for a long time and is important for social development.In this review, we summarize the major progress made on the synthetic strategies of COF film and their applications in energy, catalysis, and sensor application.Finally, we discuss the existing issues and future direction.COF film have structural characteristics such as light weight and small thickness, as well as technological characteristics such as economy and tailorability, which make the current film technology one of the necessary means for developing new materials.COF film has the advantages of flexible coordination mode, rich monomer types can be synthesized by top-down method and bottomup method, and can be used in energy, catalysis and sensor.However, there still has some challenge for the COF film, which is explained in further detail below.
(1) The growth mechanism of COF prevents condense and defect-free film formation.Kitagawa et al. said that it is hard to make films from traditional processing techniques because they were insoluble in most organic solvents, although such polymers with nano-sized channels or cavities. [47-48]The inside of most COF film is the joint crystals.Just like laying floor tiles, the boundary between crystals is very fragile.The more tiles there are, the more gaps will be generated, which will lead to defects.The synthetic strategy of COF film is still a challenge in their development (2) The limitation of COF film size has an effect on their industrial applications, such as seawater desalination, waste-water treatment and so on.COF film has achieved a certain level of laboratory research.More and more researchers are paying attention to these fields, which is making them gradually develop towards commercial scale.However, increased the size of COF film may have a negative impact on their quality, including their porosity and chemical structure.
(3) More studies are needed to find it out if COF film can be post-modified.For structure, COF was built by π-π interaction.There is hard to overcome this force and modify interlayer.COF film is hard to anchor with metal nanoparticle or metal ion unless using a specific monomer like porphyrin.The increased machinability of COF film is good for their further use.
(4) Materials with good crystallinity and pore characteristics have strict requirements for the reaction environment and the amount of reactants.The small amount of feed per batch, coupled with a certain time cost, makes the cost of COF film higher.Therefore, it is particularly important to study a highyield method for preparing COF films based on the growth mechanism of COF.
In summary, we provide an overview of the research progress in the synthetic strategy and application of COF film.This review hopes to guide the future research direction of the COF film.We believe that along with the development of material science, COF film will further grow in both depth and broadness.

Figure 3 .
Figure 3. Schematic Representation of the Formation of CONs from As-synthesized COFs via Mechanical Grinding.[21]Adapted with permission from ref21.Copyright 2013 American Chemical Society.

Figure 4 .
Figure 4. Structure and electrocatalytic performance of porphyrin COFs for carbon dioxide reduction.[14]Adapted with permission from ref14.Copyright 2018 American Chemical Society 17]