Interfacial Synthesis of 2D COF Thin Films

Abstract

Two-dimensional covalent organic frameworks (2D COFs) are emerging crystalline 2D organic material comprising planar and covalent networks with long-ranging structural order. Benefiting from their intrinsic porosity, crystallinity, and electrical properties, 2D COFs have displayed great potential for separation, energy conversion, and electronic fields. For the most of these applications, large-area and highly-ordered 2D COFs thin films are required. As such, considerable efforts have been devoted to exploring the fabrication of 2D COF thin films with controllable architectures and properties. In this chapter, we aim to provide the recent advances in the fabrication of 2D COF thin films and highlight the advantages and limitations of different methods focusing on chemical bonding, morphology, and crystal structure.

1. Introduction

In 2005, the first covalently bonded crystalline porous polymer was successfully synthesized and named covalent organic frameworks (COFs). In subsequent developments, COFs linked by B∙O, C∙N, C∙C, and other bonds have been reported. The regular network structure of COFs can be fully characterized with the help of existing instruments, which is very beneficial to study the relationship between the performance and structure. COFs can also be divided into two-dimensional (2D) COFs and three-dimensional (3D) COFs according to the specific structure. 2D COFs are emerging crystalline 2D organic material comprising planar and covalent networks with long-ranging structural order [1, 2]. In recent years, 2D COFs have been rapidly developed due to their ease of synthesis and definite structure. Benefiting from their intrinsic porosity, crystallinity, and electrical properties, 2D COFs have displayed great potential for separation [3, 4], energy conversion [5, 6, 7], and electronic fields [8]. The preparation of COF materials as thin films is advantageous for most applications. Large-area and highly ordered 2D COFs thin films are widely studied [9, 10, 11, 12]. COFs are mostly connected by reversible covalent bonds. If the reaction conditions can be adjusted to make the structure of COFs in a dynamic self-repair process, highly ordered films can be obtained. Clever use of the interface can also give the film a good substrate for growth, and the COFs can be spread out along the interface to produce a smooth film. Common interfaces include gas/solid interface, liquid/liquid interface, liquid/solid interface, and gas/liquid interface. Hence, in this critical review, we aim to provide the recent advances in the fabrication of 2D COF thin films and highlight the advantages and limitations of different methods focusing on chemical bonding, morphology, and crystal structure.

2. Preparation methods of COF film

At present, the common preparation methods include top-down method and bottom-up method. The bottom-up method is mainly through the interface reaction, so that the formation and rupture of chemical bonds of small organic molecules occur at the interface, and the final product is spread along the interface. The interfaces include gas/solid interface, liquid/liquid interface, liquid/solid interface, and gas/liquid interface. 2D COF films obtained by this method have better orientation and fewer defects, and the relationship between properties and structure can be better studied in the application. But there are some problems such as low yield and slow reaction through interface reaction. Reaction time and efficient utilization of small molecular monomers are also important issues to be considered in the process of synthesizing materials. The top-down preparation method is to peel the powder COFs material into nanosheets by chemical treatment or physical method and then process the nanosheets into large-size films by vacuum extraction and filtration. The prepared materials via this method have many defects, weak orientation, and other problems. How to control the thickness and structure of materials to obtain 2D films with orderly atomic structure is the main challenge at present (Figure 1).

Figure 1.

Schematic illustration of interface synthesis of COF films. (a) Solid/liquid interface; (b) air/solid interface; (c) air/liquid interface; (d) Langmuir-Blodgett (LB) method; and (e) liquid/liquid interface.

2.1 Bottom-up strategy

2.1.1 Solid/liquid interface

COFs were first reported by Yaghi et al. in 2005. COF-1 and COF-5 connected by B-O bond were respectively prepared in Pyrex tubes by solvothermal method [1]. The reversible B-O bond gives the material excellent repairability, resulting in a highly regular pore structure and a high specific surface area. Solvothermal synthesis is still the main strategy for preparing COFs powder [13, 14, 15].

The preparation of COF film by gas/solid interface is mainly to add solid substrate in the solvent and let COFs grow into film on the surface of the substrate in situ. In 2011, Dichtel et al. prepared COF-5 films from the polymerization of two monomers, 2,3,6,7,10,11-hexahydroxytriphenyl (HHTP) and 1, 4-phenyldiboric acid (PBBA), using a single layer of graphene as a substrate [16] (Figure 2a). In this process, the concentration of monomer needs to be regulated. When the concentration of monomer is high, the system will nucleate and generate powder. The characterization of the COF-5 film by grazing incidence X-ray diffraction (GIXRD) and scanning electron microscope (SEM) showed that the films were parallel to the graphene surface and have good orientation (Figure 2b–d). In this work, COF powder was processed into film for the first time, which is of great significance for the application of COF as organic electronic devices. When COF is processed into a film, it can better characterize its optoelectronic properties. Inspired by this work, different research groups began to choose different substrate materials for the preparation of thin films [17, 18, 19]. Base materials include Indium Tin Oxides (ITO) and polyethersulfone (PES) [20, 21], etc. The synthetic conditions of BDT-ETTA COF and the transmission electron microscopy (TEM) images of the powder are shown in Figure 3a and b, respectively. SEM images and GID patterns analysis of the cross section found that the COF film was spread evenly on the ITO substrate (Figure 3c–f). The connecting units of COFs include B∙O bond and C∙N bond. These thin films can be well used for water splitting and molecular separation. Solid/liquid interface synthesis is considered to be an effective method for vertically growing high crystallinity films. Since the film grows on the substrate surface, it is easy to collect and characterize. However, there is another problem with this method, that is, the adhesion force between the film and the substrate is strong, and it is challenging to transfer it from the substrate. Since the film cannot be transferred between different substrates, it is only possible to select the appropriate substrate before each experiment [22].

Figure 2.

(a) Schematic illustration of chemical synthesis at solid/liquid interface using a single layer of graphene as a substrate; (b) X-ray scattering data obtained from COF-5 powder; (c) GID data from a COF-5 film on SLG/Cu; (d) top-view SEM image of the COF-5 thin film.

Figure 3.

Schematic illustration of chemical synthesis at solid/liquid interface using the indium tin oxides (ITO) as a substrate; (a) synthesis process of BDT-ETTA COF; (b) TEM image of BDT-ETTA COF powder; (c,d) SEM images (cross-section) of COF film; (e, f) GID patterns of COF film.

2.1.2 Liquid/liquid interface

Water and various organic solvents are incompatible, resulting in a phase interface at the junction of the two phases. If COFs can be spread out during growth, 2D COF films with excellent orientation can be obtained at the interface with the continuous self-repair of dynamic chemical bonds. The films obtained by this method can be easily transferred to different substrate materials, which is very beneficial in application [23, 24]. In 2016, Xinliang Feng et al. designed and synthesized wafer-sized 2D imine COFs with high mechanical stiffness. Porphyrin amino monomer and 2,5-dihydroxyterephthalaldehyde monomer are dissolved in trichloromethane and water, respectively [25] (Figure 4a). The amine group and aldehyde group contact at the interface to form imine bond, leading to 2D polymerization. The authors simulated the structure of COF film and characterized the surface topography of the material by atomic force microscopy (AFM) and TEM, in which a highly ordered arrangement of structures can be seen (Figure 4b–d). Photographic image of 2DP on 4-inch 300 nm SiO₂/Si wafer shows that the entire thin film is macroscopically flat (Figure 4e). Liquid/liquid interface synthesis is another method that can efficiently synthesize high-quality films. At present, the main problem is to prepare COF films with higher crystallinity by adjusting the amount of catalyst and the choice of solvent system. On the basis of this work, more efficient catalysts of Schiff base reaction catalysts have also been developed. Dichtel et al. prepared high crystallinity COF films efficiently using Sc(OTf)₃ as a catalyst [26, 27]. The catalyst is dissolved in the aqueous phase, and the monomers are dispersed in the organic phase. Amine and aldehyde monomers can be polymerized into films at the phase interface under the catalysis of Sc(OTf)₃ (Figure 4f–g). Bo Wang et al. also used this catalyst to covalently connect 2, 5-dihydroxy-1, 4-phthalate formaldehyde (DOBDA) and 1,3, 5-tri (4-aminophenyl)-benzene (TAPB) to prepare compact COF films with different thickness of 300 nm ~ 500 nm [28]. Besides, by removing C∙N bonds and using the defects in COF films, a vertical channel with hydrophilic gradient was fabricated.

Figure 4.

Schematic illustration of chemical synthesis at liquid/liquid interface. (a) Synthesis of a 2DP through Schiff-base condensation reaction; (b) molecular structure of the 2DP by DFTB calculation. (c) AFM, and (d) TEM images of 2DP; (e) photographic image of 2DP on 4-inch 300 nm SiO₂/Si wafer; (f) schematic explanation and photograph of TAPB-PDA COF; (g) photo of the TAPB-PDA COF film.

The reaction time of interfacial synthesis is another problem that needs to be considered. In order to obtain high crystalline films, the rate of reaction is usually controlled to slow down the polymerization kinetics. In 2019, wafer-scale synthesis of monolayer 2D porphyrin polymers was reported by Park Research Group (Figure 5a). The authors report that by growing the film at the pentane/water interface, a 2D porphyrin polymer film with sheet-level uniformity was synthesized at the limit of the thickness of a single layer, by growing the film at the pentane/water interface [29]. Films of different structures have different absorption spectra and colors (Figure 5b–d). The superposition of films of different colors also reveals different optical properties. The corresponding color and monomer are very close, which provides experience for future film designs of different colors.

Figure 5.

(a) Schematic of monolayer 2DPs and corresponding chemical structures of the molecular precursors; (b) absorption spectra of monolayer 2DPs on fused silica substrates; (c) hyperspectral transmission images and resulting false color images of 1 inch-square 2DP I on a 2-inch fused silica substrate. Transmission images taken at the wavelength of 405 nm, 420 nm, and 440 nm are assigned as red, green, and blue channel, respectively, to generate the false color image. (d) False color images of monolayer 2DPs covering entire 2-inch fused silica wafers.

In 2017, Banerjee et al. selected p-toluenesulfonic acid (PTSA) to form a self-supporting COF film at the interface between water and methylene chloride [30] (Figure 6a). The hydrogen bond network formed by PTSA can slow down the diffusion rate of monomer and increase the quality of the COF film. The content of catalyst and concentration of monomer have great influence on the thickness of thin films. SEM image and AFM image of Tp–Bpy COF thin film prove that the surface of the film is very smooth and the structural orientation is high (Figure 6b–c). Zhongyi Jiang et al. prepared ionic covalent organic framework membranes (iCOFMs) with ultrahigh ion exchange capacity via the double-activation interfacial polymerization strategy (Figure 6d). Brønsted acid and Brønsted base activate aldehyde monomers and amine monomers with sulfonic groups in aqueous and organic phases, respectively [31]. After the double activation of acid and base, the monomer can react quickly at the interface and form iCOFMs with high crystallinity. At present, the liquid/liquid interface synthesis is developing rapidly. The films synthesized by this method are widely used in the fields of separation and purification and seawater desalination. At the same time, this method has also been continuously improved in the process of development, which can obtain higher quality films in a shorter time.

Figure 6.

(a) Schematic illustration of the interfacial synthesis of Tp–Bpy COF thin film; (b) SEM images of Tp–Bpy COF thin film; (c) AFM image of Tp–Bpy COF thin film; (d) schematic illustration of the TpBD-(SO₃H)₂ iCOFMs fabrication process.

2.1.3 Air/liquid interface

Zhenan Bao et al. first reported the preparation of polyTB film via DMF/air interface reaction in 2015 [32]. The two types of monomers are 4,8-Bis(octyloxy)thieno[2,3-f][1]benzothiophene-2,6-dicarbaldehyde (BDTA) and Tris(4-Aminophenyl)amine (TAPA). Since the COFs powder is easily formed in the direct polymerization process, the surface of the film obtained by this method is very rough. To solve this problem, the team continued to grow the film using the solution from the first reaction. By controlling the reaction conditions, the polyTB film with different thickness was obtained. The average surface roughness of the material is only 0.2 nm. Lai et al. loaded ultra-thin TFP-DHF 2D COF film (2.9 nm) on porous substrate via the Langmuir-Blodgett (LB) method. This COF was formed through the reaction between 1,3,5-triformylphloroglucinol (TFP) and 9,9-dihexylfluorene-2,7-diamine (DHF).

In 2019, Xinliang Feng et al. reported surfactant-monolayer-assisted interfacial synthesis (SMAIS) as a general method to prepare 2D polymer films with high crystallinity (Figure 7a). Sodium oleyl sulfate (SOS) was used as a surfactant to induce aniline to align and polymerize on the surface of aqueous solution and obtain fully conjugated 2D polyaniline films with lateral size ~50 cm [2] and tunable thickness (2.6–30 nm) (Figure 7e–g) [33]. In another work, the researchers prepared polyimide COF film by the reaction of 4,4′,4′′,4′′′-(porphyrin-5,10,15,20-tetrayl)tetraaniline and disochromeno (Figure 6b) [34]. The polyimide COF film was characterized by X-ray scattering and AFM (Figure 7c and d). It is found that the thickness is about 2 nm and the average domain size is about 3.5 μm [2]. Later, on the basis of the above two works, Xinliang Feng et al. also synthesized a series of COF films connected by C∙N and B∙O bonds [35, 36]. Bien Tan et al. reported an aliphatic amine-assisted interfacial polymerization method to obtain independent covalent triazine frameworks (CTFs) films [37]. The structure was investigated by grazing incidence wide-angle X-ray scattering (GIWAXS) and small-angle X-ray scattering (SAXS). The lateral size of the film was up to 250 cm [2], and average thickness can be tuned from 30 to 500 nm. The unique conjugated structure of the material endows it with excellent photocatalytic performance for hydrogen evolution reaction. Zhikun Zheng et al. synthesized a series of imine COF films assisted by charged polymers [38]. The morphology and diffusion of preorganized monomers in charged polymers on water surface are very important for the formation of organic two-dimensional crystals. The monomers were 5,10,15,20-Tetrakis (4-aminophenyl)-21H,23H-porphyrin (TAPP) and four aldehyde monomers with different structures. Air/liquid interfacial synthesis is the most common method for preparing 2D COF films. The film is easy to operate and has good crystallinity and orientation. The air/liquid interface is very helpful for the preparation of single-layer or few-layer COF films. First, the monomers should be induced to arrange at the interface, and then the crystallinity of the film can be better guaranteed in the process of polymerization. So how to arrange the monomers on the surface is the main consideration in this method. The transfer and characterization of monolayers are also not easy. Generally, advanced electron microscope is needed to observe its morphology.

Figure 7.

Schematic illustration of chemical synthesis at air/liquid interface. (a) the synthetic procedure for the 2D polymers; (b) molecular structure of the 2DPI synthesized in the article; (c) optical microscope image of 2DPI film; (d) AFM image of the 2DPI film; (e) molecular structure of the q2D polyaniline synthesized in the article; (f) AC-HRTEM image of q2D polyaniline perpendicular to [001] axis; (g) simulated atomic structure of the q2D polyaniline.

2.1.4 Air/solid interface

In 2008, Porte et al. prepared monolayer surface covalent organic frameworks (SCOFs) on the surface of Ag (111) via chemical vapor deposition (CVD) [39]. The polymerization process is achieved by the reaction of 1, 4-Benzenediboronic acid (BDBA) and boronic acid. The experiment was performed under ultrahigh vacuum (UHV) conditions. Two monomers were sublimated from two heated molybdenum crucible evaporators to the clean Ag (111) surface to obtain a molecular array.

Dong Wang et al. reported the formation of highly ordered 2D COF film via dehydration reaction of boronic acid (Figure 8a). The molecular layers were imaged at room temperature using scanning tunneling microscopy (STM) (Figure 8b–c). In 2013, Dong Wang et al. reported another method for the synthesis of imine COF films at the air/solid interface [40]. The solution of the two monomers was first coated on the substrate and then sealed in a reactor with copper sulfate pentahydrate as a thermodynamic regulator. By heating the reactor to a specified temperature to control the evaporation of aldehyde monomers, the aldehyde monomers condense on the surface covered by amine monomers and polymerize with them, high-quality monolayer imine COF films can be prepared at the air/solid interface [41]. Recently, Yunqi Liu et al. reported the preparation of large area imine PyTTA-TPA COF film with controllable thickness by gas-phase induced conversion in a CVD system (Figure 8d) [42]. The assembly process is achieved by reversible reaction between 4,4′,4″,4″′-(1,3,6,8-Tetrakis(4-aminophenyl)pyrene (PyTTA) film and terephthalaldehyde (TPA) vapor. Driven by π-π superposition and catalyzed by acid vapors, a uniform organic frame film was formed on a growing substrate. The COF films obtained by air/solid interface synthesis have adjustable thickness and highly ordered structure, which is an effective method to grow high-quality thin films [43, 44, 45]. However, this method requires high temperature and vacuum environment and has high requirements for equipment.

Figure 8.

Schematic illustration of chemical synthesis at air/solid interface. (a) the synthesis route to SCOF-1; (b) STM image of SCOF-1 on HOPG formed after dehydration of BPDA precursors at 150°C; (c) a high-resolution STM image showing the hexagonal structure of SCOF-1; (d) schematic representation for the growth of imine-linked 2D COF films on SiO₂/Si substrates.

2.1.5 Vapor-assisted conversion

Some organic solvents with low boiling point can evaporate at room temperature. Combined with this feature, Bein et al. reported the strategy of steam-assisted conversion at room temperature in 2014 [46]. They succeeded in making three thin films named BDT-COF, COF-5, and pyrene-COF (Figure 9a). The monomers are first mixed into a solution of acetone/ethanol, which is then dripped onto the matrix. Finally, the material is transformed into crystalline porous COF films in a vapor atmosphere of homotrimethylbenzene/dioxane. The presence of vapor plays an important role in the growth of thin films.

Figure 9.

(a) Schematic representation of BDT-COF and COF-5; (b) top view SEM micrograph of BDT-COF film synthesized by room temperature vapor-assisted conversion, representing the surface morphology; (c) cross-sectional SEM micrograph shows a uniform film thickness.

This method can accurately prepare COF films from 100 nm to a few microns thick. SEM characterization showed that the structure of the film was composed of small particles stacked irregularly, and there were submicron intervals between the particles (Figure 9b and c).

2.1.6 Continuous flow condition synthesis

Because COFs are easy to form powder particles under thermodynamic conditions, the films synthesized by liquid/solid interface method may have the problem of high surface roughness. To remedy this problem, Dichtel et al. converted the solution from a static state to a flowing state [47]. The flow rate affects the surface morphology, thickness, and crystallization degree of films. High-quality films with different thickness can be prepared by adjusting this condition (Figure 10). A quartz crystal microbalance can be used as the base of the flow tank to monitor the quality of film deposition at any time.

Figure 10.

(a) Turbidity as a function of reaction time during the formation of COF from homogeneous conditions provides an induction period amenable to a flow cell configuration. (b) Schematic of flow setup designed with variable induction period.

2.2 Other promising preparation methods of COF films

COFs powders are formed by layer upon layer of planes through π stacking and the interlayer forces are weak relative to covalent bonds. Similar to graphene, a large area of smooth COF film can be obtained if the powdered COFs can be stripped into a single layer of nanosheets, which can then be self-assembled into films [48]. The common preparation methods of nanosheets include physical exfoliation and chemical exfoliation [49, 50].

The physical method is to disperse COFs powder in the solvent and then form nanosheets assisted by ultrasound. Chemical methods require the addition of a chemical agent to the solvent to promote lamellar abscission. Compared with COFs powder, COFs nanosheets show more advantages in photoelectric applications. In addition, the processing of nanosheets into films is also a key point in practical applications. In 2017, Tsuru et al. applied the obtained COF-1 nanosheet solution drops on an α-Al₂O₃ macroporous support with a SiO₂-ZrO₂ intermediate layer and obtained uniform and smooth COF films after several drops (Figure 11a) [51]. Dichtel et al. protonated imine bond using trifluoroacetic acid to promote the stripping of COFs powder into nanosheets dispersed in a solvent (Figure 11b and c) [52]. COF films with thickness ranging from 50 nm to 20 μm can be prepared by deposition on any substrate.

Figure 11.

(a) Schematic illustration of the preparation of a COF-1 membrane via the assembly of exfoliated COF-1 nanosheets; (b) overview of acid exfoliation and film casting procedures; (c) pore structure of the BND-TFB COF.

3. Conclusion

The above is a review of common methods of film synthesis, including interfacial synthesis and some other new and promising preparation methods. By replacing different organic monomers, COF films with different functions can be designed in a targeted manner. Combined with the final different applications, the appropriate preparation method can be selected. At present, the solid/liquid interface synthesis method is mature and suitable for most COFs. However, if organic solvents are used for solvothermal reaction at high temperature, only inorganic materials can be selected as the substrate. COF films grown on solid surfaces cannot be transferred to other substrates. The air/solid interface synthesis also has the problem that the grown film is difficult to transfer. For liquid/liquid interface synthesis, organic solvents and aqueous solutions will form an obvious contact surface, which is an ideal substrate for the efficient growth of films at the interface. Liquid/liquid interface synthesis is also the most promising method to grow single crystal thin films. This method usually requires dissolving aldehydes and amines in two solvents, respectively, and reacting at the interface to form a film. The growth of COF thin films depends on the diffusion of monomers in solvents. The process of film formation at the interface will affect the diffusion of molecules, and the subsequent polymerization reaction will also be limited. Therefore, the surface of the grown film will be rough. If the two monomers are dissolved in the same organic solvent, and then the catalyst is dissolved in water, the monomers are catalyzed at the interface, and the polymerization reaction takes place to form a film, which will significantly improve this problem. Air/solid interface synthesis is also a strategy to prepare large-scale single crystal films. This strategy can prepare ultrathin COF films with a thickness of several nanometers. Generally, surfactants are used to induce the regular arrangement of monomers on the surface of solvents. It usually takes a week to produce a better crystalline film.

Up to now, there are still few reports of the synthesis of wafer-scale films, and the methods are not necessarily universal. To obtain large-scale crystalline films, the reaction rate of monomers needs to be very slow. It usually takes a week. For imine COF films, it is very important to find a suitable reagent to inhibit the diffusion of monomers because of their high reactivity. In addition, surfactant is a good choice to induce crystallization of thin films. In future research, the efficiency of interfacial film formation and the crystallinity of the film are still important factors to be considered. Improving the existing methods, judging the catalyst activity in the reaction, and adjusting the diffusion rate are all promising research contents.

The currently reported COF linkage bonds mainly include boron-oxygen bonds, imide bonds, and carbon-carbon double bonds. COF films with higher reversibility of boron-oxygen bonds and imine bonds are reported the most. However, sp² carbon-carbon COF films have not been reported. This type of COF powder with a high degree of conjugate has been well applied in the field of photocatalysis. In future research, it is a good research direction to prepare this type of COF thin films by advanced interfacial methodologies. With its excellent light absorption ability and photoelectron migration ability, sp² carbon-carbon COF films can show excellent performance in seawater evaporation and photocatalysis.

Acknowledgments

T.Z. acknowledges the Excellent Youth Foundation of Zhejiang Province of China (Grant No. LR21E030001) and the National Natural Science Foundation of China (Grant No. 52005491 and No. 52003279).

Conflict of interest

The authors declare no conflict of interest.