1 Introduction
Metal-organic frameworks (MOFs) are a class of crystalline porous materials assembled by metal ions or clusters and bridged organic ligands through coordination bonds, which have ultra-high specific surface area, great porosity and abundant active sites.Moreover, due to the diversity of metal and ligand selection, it also has the characteristics of rich and adjustable structure and function, and diverse pore channels, which has shown great application prospects in gas separation and storage, catalysis, energy, biomedicine and so on
[1]. However, due to the characteristics of organic ligands and the degree of overlap of metal ion orbitals, most of the traditional MOFs are insulators with low conductivity (lower than the 10
-10S·cm
-1), which seriously restricts their development and application in electrics-related fields
[2 ⇓~4].
In recent years, two-dimensional conducting metal-organic frameworks (2D ECMOFs) with planar structure, which are formed by self-assembly of well-designed organic linkers and metal nodes, have attracted much attention. They usually have π-π stacking or π-d conjugated structure, and can physically realize band-like transport or hopping transport.From the chemical point of view, they can also realize the carrier transfer path through space and through bond, so they not only have excellent conductive characteristics (conductivity can reach 1580 S·cm
-1[5]), but also have good conductivity.It is possible to realize superconductivity, and its designable structure and diverse chemical composition can also endow it with many unique physical and chemical properties, which has a wide range of research and application prospects in the fields of sensing, energy storage, energy conversion and electronic devices
[6][7,8][9 ⇓~11][10][12].
In this paper, we will first introduce the electron transport mechanisms in ECMOFs from the physical and chemical perspectives, respectively, and then discuss the structures and synthesis methods of some typical symmetric and asymmetric 2D ECMOFs reported in recent years.The potential applications of 2D ECMOFs with high porosity and conductivity in the fields of sensing, energy storage, energy conversion, electronic and spin devices are introduced, and the current problems and challenges faced by this kind of materials are also prospected.
2 Lectrical conduction mechanism of 2D ECMOF
The conductivity of the material is determined by the carrier concentration n and the carrier mobility V, and the current carrier transport process of 2D ECMOFs can be explained by two transport mechanisms of band-like and hopping from the physical point of view.Moreover, with the development of research and theory on conducting polymers, it can also be explained by chemical through-space and through-bond mechanisms, which have become important reference design principles for 2D ECMOFs.
2.1 Physical conduction mechanism of 2D ECMOFs
Band-like mechanism (upper figure of Figure 1) occurs in the conjugated system composed of metal ions and ligands with matched energy levels. The electronic orbitals of bonding atoms overlap and split into continuous energy levels to form energy bands. The carriers migrate in the delocalized energy bands in the form of approximate plane waves. The main influencing factor is the phonon scattering caused by lattice vibration. Hopping mechanism (lower figure of Figure 1) is common in glass and organic semiconductors. Molecules are bound by weak van der Waals forces. The mean free path of carriers is lower than the distance between molecules. Electrons are confined in local potential wells and need to cross the energy barrier E
a to achieve jumping transport
[13].
The main way to distinguish between these two mechanisms is to determine the temperature dependence of the conductivity. In the hopping mechanism, electrons are more likely to migrate across the energy barrier due to thermal excitation, and their mobility is improved, so the conductivity increases with the increase of temperature. As for the band-like mechanism, it is necessary to analyze it from the two aspects of thermal excitation and scattering. When the temperature rises, the carrier concentration will increase because of thermal excitation, but the phonon scattering process will also increase.Therefore, for metal materials with high intrinsic carrier concentration, the increase of temperature will enhance the scattering effect and reduce the mobility, thus reducing the conductivity. For semiconductor materials with low intrinsic carrier concentration, the increase of temperature will increase the carrier concentration and increase the conductivity. Therefore, if the conductivity of the sample decreases with the increase of temperature, it can be basically determined that the carrier transport follows the band-like mechanism, and if the conductivity of the sample increases with the increase of temperature,It is necessary to measure the temperature dependence of the mobility and carrier concentration in detail. The main feature of the band-like mechanism is the positive temperature dependence of the carrier concentration, while the main feature of the hopping mechanism is the positive temperature dependence of mobility
[14 ⇓~16].
In 2013, Nishihara et al. Combined hexaphenylmercapto (BHT) with Ni
2+ to achieve the controllable preparation of Ni
3(BHT)
2 nanosheets with 0.15 S·cm
-1 conductivity at room temperature, and then found that the conductivity could be adjusted with the change of oxidation state by regulating the oxidation state of the complex unit (the conductivity could be increased to 1.6×10
2S·cm
-1 at 300 K).Further theoretical calculation and photoelectron spectroscopy show that the Ni
3(BHT)
2stacked nanosheets have metal-like transport properties, but due to structural defects and other factors, their conductivity decreases slightly with the decrease of temperature in temperature-dependent tests, showing semiconductor characteristics
[17][18].
Similarly, the conductivity of the 2D ECMOF Ni
3(HITP)
2 prepared by mixing 2,3,6,7,10,11-hexaaminotriphenylene (HATP) and Ni
2+ can reach 40 S·cm
-1, and the temperature dependence test shows that it has semiconductor characteristics, which is considered to be a semiconductor metal organic graphene analog
[19]. In 2017, Marinescu et al. Found that the conductivity of Co
3(THT)
2 (Co
3(HTTP)
2) framework materials formed by coordination self-assembly of Co
2+ and 2,3,6,7,10,11-hexamercaptotriphenylene (THT or HTTP) would change from semiconductor to metal when the temperature decreased
[20]. Dinc Dincet al. Found that the prepared M
3(HIB)
2(M=Ni, Cu) framework materials only show metalloid transport properties in the ab plane, while they show semiconductor transport properties in the c-axis direction. The temperature dependence test found that they are dominated by intergranular transport and show semiconductor behavior
[21].
Using non-contact time-resolved terahertz spectroscopy, C Cánovas et al. Successfully revealed that the room-temperature mobility (220 cm
2·V
-1·s
-1) of the π-d conjugated semiconductor Fe
3(THT)
2(NH
4)
3 two-dimensional metal-organic framework thin film formed by van der Waals stacking layers exhibits band-like carrier transport properties, but due to the limitation of impurity scattering, thermally excited carriers play a major role in the temperature dependence of its conductivity
[22]. Furthermore, Marinescu et al. Successfully prepared a two-dimensional FeTHT metal-organic framework film without ammonia, which showed a transition from semiconductor to metal behavior when cooled, and the transition temperature (100-300 K) from semiconductor to metal behavior was significantly affected over time after being oxidized in air
[23].
In order to avoid the influence of defects such as grain boundaries, Dinc Dincet al. Used HATP and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) as ligands to prepare rod-like Ni
3(HITP)
2 and Cu
3(HHTP)
2 single crystals, whose conductivity is much higher than that of polycrystalline particles.Unlike the transport properties of polycrystalline thin film semiconductors reported in their previous work, the Ni
3(HITP)
2 single crystal shows metal-like conduction properties, which is consistent with the electronic band structure calculated by Talin et al. Through density functional theory, further indicating that structural defects are one of the important factors affecting the conductive properties of 2D ECMOFs
[24][19][25].
2.2 Chemical conduction mechanism of 2D ECMOFs
Similar to organic semiconductors, in 2D ECMOFs with interlayer non-covalent interactions (such as π-π stacking), there is also a through-space transfer mechanism (Fig. 2), in which carriers are transferred through space and orbital overlap, and this process can be effectively regulated by regulating the π-π stacking of ligands. The Through-bond mechanism relies on the formation of covalent bonds between energy-level matched metals and ligands, and continuous covalent interactions form a large conjugated system, thus delocalizing electrons.A narrower band gap and a higher mobility can be obtained, and the electron transfer process can be controlled by adjusting the symmetry and the overlap degree of the orbitals between the metal and the ligand (such as forming pi-d conjugation).
In 2D ECMOFs, a planar structure is formed by the chelation of a metal and a bridged ligand containing an aromatic ring, in which there is a π-π stacking interaction between the aromatic rings in different layers.By adjusting the number of aromatic rings in the ligand, the strength of the π-π stacking can be effectively changed, and the electrical properties of 2D ECMOFs can be controlled. For example, Yaghi et al. First reported a series of 2D ECMOFs based on HHTP ligands in 2012, in which the conductivity of Cu-CAT (i.e. Cu-HHTP or Cu
3(HHTP)
2) single crystal can reach the 0.21 S·cm
-1 (four-probe method)
[26]. On this basis, Bao et al coordinated a smaller tetrahydroxybenzoquinone (THQ) ligand with Cu to obtain a Cu-THQ 2D ECMOF with a conductivity of 7.3×10
-8S·cm
-1 (four-probe method, tabletting sample)
[27]. Further, Mirica et al. coordinated larger sized 2,3,8,9,14,15-hexahydroxyperylene (HHTN) with Cu to obtain a Cu
3(HHTN)
2 with a conductivity of 9.01×10
-8S·cm
-1 (two-probe method, tabletting sample)
[28]. Three kinds of 2D ECMOFs based on Cu-O
4 nodes show different conductivity, which indicates that the size of the aromatic system in the ligand has a regulatory effect on the electrical properties, but due to the differences in test methods and test sample types, the inherent law of this regulation needs to be further studied.
In addition to directly adjusting the size of the ligand, the electrical properties can also be adjusted by constructing double ligands or multiple ligands. For example, Kitagawa et al. Constructed a double ligand 2D ECMOF Cu
3(HHTP)(THQ) by mixing HHTP and THQ ligands with different sizes, and its conductivity is 10
-5S·cm
-3, which is between Cu
3(HHTP)
2(10
-3S·cm
-3) and Cu
3(THQ)
2(10
-8S·cm
-3) (Figure 3), which further indicates that the characteristics of the ligand itself and the π-π stacking effect are important factors affecting the electrical properties of 2D ECMOFs
[29].
The π-d conjugation between transition metal and ligand is one of the main characteristics of electron delocalization in 2D ECMOFs, and the electrical properties of materials can be effectively controlled by adjusting the covalent bond between metal and ligand atoms.For example, Lee et al. Constructed Mn-HIB-MOF and Fe-HIB-MOF with conductivity of 107.7 and 149.2 S·cm
-1, respectively, and alloy Mn/Fe-HIB MOF with conductivity of 359 S·cm
-1 by coordinating hexaaminobenzene (HAB) with Mn and Fe, respectively, and realized the regulation of the electrical properties of 2D ECMOFs through the selection of metals
[30]. Similarly, Dinc Dincet al coordinated HATP with Co, Ni and Cu respectively, and realized the continuous regulation of conductivity from 5.8×10
-3S·cm
-1((Co
2.47Cu
0.53)(HITP)
2) to 55.4 S·cm
-1(Ni
3(HITP)
2) by adjusting the stacking state and free carrier concentration between layers, which further showed that the coordination state between metal and ligand was an important factor affecting conductivity
[31].
To explore whether π-d conjugation is a necessary structural feature to determine the conductivity of MOFs, Dinc Dincet al. Prepared LnHHTP by coordinating lanthanide ions with HHTP, in which the 5d orbital of the Ln
3+ is empty, eliminating the interference of π-d conjugation between metal and ligand.The conductivity of 2D ECMOFs can also reach the 0.05 S·cm
-1, which indicates that the in-plane π-d conjugation is not necessarily required to achieve high conductivity in 2D ECMOFs, and the π-π stacking interaction between ligands outside the plane can also provide efficient charge transport channels
[32].
Although a series of theoretical and experimental progress has been made in the study of the electron transport mechanism in 2D ECMOFs, the polycrystalline nature of most of the tested samples (there are a large number of structural defects, such as vacancies, impurities, dislocations and grain boundaries, etc.) hinders the understanding of the intrinsic properties of these materials and the revelation of their conduction mechanism. From a physical point of view, structural defects can lead to the generation of shallow level trap States, which dominate the thermal excitation when the temperature rises, making the temperature dependence of most samples show semiconductor characteristics.Although a few single crystals have been tested to show band-like metal-like properties, more single crystal samples need to be prepared to further reveal their intrinsic properties. From the chemical point of view, the random orientation of polycrystalline samples does not show the anisotropy of the two electron transport pathways of π-π stacking out of plane and π-d conjugation in plane of 2D ECMOFs.A small number of rod-like single crystal devices are not suitable for in-plane and out-of-plane conductivity measurements at the same time, so the experimental verification of the conduction mechanism of 2D ECMOFs still needs to be achieved by synthesizing large single crystal samples.
3 2D ECMOFs structures
In 2D ECMOFs, bridged multidentate ligands chelate with the metal ions to form planar networks in which a single metal ion chelates with two ligands to form a dsp2 hybrid inner-orbital coordination polymer with a planar square geometry. The symmetry of the metal ion can be regarded as C2. To obtain a two-dimensional structure, the symmetry of the ligands can be C2, C3, C4 and C6 (Fig. 4). According to the structural symmetry, 2D ECMOFs can be divided into symmetric structures formed by the coordination of a single ligand with a metal ion and asymmetric structures formed by the coordination of multiple ligands with metal ions.
3.1 Symmetrical Structure
At present, C3 + C2 (Figure 5) is the most common symmetric structure type of 2D ECMOFs, and its ligands mainly include triphenylene derivatives (Figure 4,2) such as HHTP, THT, HATP, 1,3,5-trimercapto-2,4,6-triaminotriphenylene and 2,3,6,7,10,11-hexaselenophenyltriphenylene (TPHS), etc., and benzene derivatives (Figure 4,1) such as THQ, hexahydroxybenzene (HHB), BHT, HAB, benzenehexas
[26,33,34][35,36][19,37,38][39][40][27][27,41][5,17,35][21,42,43][44][28][45].
In recent years, 2D ECMOFs with C2 + C2, C4 + C2 and C6 + C2 structures have also attracted more and more attention. For example, Feng et al. Realized the controllable preparation of PTC-Fe 2D ECMOF based on C6 symmetric ligand 1,2,3,4,5,6,7,8,9,10,11,12-dodecylmercaptohexabenzene (PTC) (conductivity is about 10 S·cm
-1);Du et al.). NiPc-MOF 2D ECMOF was constructed by using C4 symmetric 2,3,9,10,16,17,23,24-octaamino nickel phthalocyanine (NiPc-NH
2) as ligand to coordinate with Ni (conductivity is about 0.2 S·cm
-1);Chen et al.)
[46][47][48].
The Symmetry between ligands and metal is one of the important factors affecting the structure and properties of 2D ECMOFs. For example, BHT ligands can exhibit both C3 and C6 symmetry when coordinating with metal, such as Ni-BHT with C3 + C2 symmetry when coordinating with Ni.In which each S atom is coordinated with one Ni atom, and the conductivity can reach the 160 S·cm-1[18], while in the case of BHT coordinated with Cu, the C6 + C2 type Cu-BHT is obtained, in which each S atom is coordinated with two Cu atoms, showing the characteristics of tight packing and low porosity, and the conductivity can reach the 1580 S·cm-1[5]. Therefore, the symmetry of metal and ligand coordination will directly affect the channel structure and packing state of 2D ECMOFs, and then play a significant role in regulating their electrical properties.
3.2 Asymmetric structure
Currently, 2D ECMOFs are mainly based on the coordination of transition metal elements such as Co, Ni, Cu and a few ligands such as benzene derivatives and triphenylene derivatives. In order to further realize the fine regulation of the pore and conductivity of 2D ECMOFs, researchers have introduced the asymmetric structure construction methods such as double-ligand MOFs and alloy MOFs into the preparation of 2D ECMOFs, which greatly expands and enriches the types and diversity of 2D ECMOFs and provides more possibilities for the regulation of structure and function.
3.2.1 Two-ligand 2D ECMOFs
Ligand design is a common strategy to regulate the structure and function of MOFs, and the use of mixed ligands can expand the diversity of ligands, which can be divided into two categories.One is the mixed ligands with the same skeleton but different coordination atoms, such as Xu et al., which assembled HHTP and HATP ligands with oxygen and nitrogen atoms respectively with Cu
2+ to form a two-ligand 2D ECMOF.Because the size of HATP and HHTP ligands is close, the structure of HATP and HHTP does not change obviously after doping, and X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy show that both HATP and the HHTP ligands participate in coordination,When the doping concentration of HATP is less than 1. 0 mol%, the conductivity increases by two orders of magnitude compared with the undoped film, but when the doping concentration continues to increase, the conductivity decreases due to the influence of competitive coordination and the decrease of crystal quality of the film
[49]. The other is the mixed ligands with different skeletons but the same coordinating atoms, such as Kitagawa et al., which mixed benzene derivative THQ and triphenylene derivative HHTP and coordinated with Cu
2+ to obtain Cu
3(HHTP)(THQ)2D ECMOF.Each Cu atom is linked to a THQ ligand and a HHTP ligand to form a hexagonal structure with a pore size of 1. 71 nm, and its conductivity and specific surface area are between those of Cu
3(HHTP)
2 and Cu
3(THQ)
2, which provides a new way to control the structure and function of 2D ECMOFs
[29].
3.2.2 Alloy 2D ECMOFs
The choice of metal is also one of the important factors affecting the structure and function of 2D ECMOFs. The structural diversity of 2D ECMOFs can be effectively expanded by constructing alloy MOFs, and the function of 2D ECMO can be further regulated by the synergistic effect between metals, such as Oh, which coordinates Co
2+ and Ni
2+ to HHTP at the same time.A series of Co
xNi
y-CAT were prepared by changing the ratio of reactant precursors, and the 2D ECMOFs of these alloys showed better electrocatalytic activity for oxygen reduction reaction (ORR) due to the combination of the high diffusion current density of Co-CAT and the high onset potential of Ni-CAT
[50]. In 2020, Dinc Dincet al. Constructed a series of 2D ECMOFs based on three kinds of alloys, namely, Cu
xCo
(3-x)(HITP)
2, Cu
xNi
(3-x)(HITP)
2 and Co
xNi
(3-x)(HITP)
2, by combining Co, Ni and Cu with HATP in pairs. They all have similar structures to monometallic 2D ECMOFs, but by precisely controlling the metal components and their proportions.Not only can the band gap of the 2D ECMOFs of the series of alloys be continuously and finely adjusted by more than 0.4 eV, but also the conductivity of the 2D ECMOFs can be continuously and precisely controlled by more than four orders of magnitude from the 5.8×10
-3((Co
2.47Cu
0.53)(HITP)
2) to the 55.4 S·cm
-1(Ni
3(HITP)
2), which are closely related to the change of free carrier concentration and the fine change of interlayer displacement or spacing caused by metal substitution
[31].
The crystal structure and function of 2D ECMOFs can also be controlled by mixing metals with different coordination States, for example, Peng et al. Coordinated Co
2+ and Ni
2+ with HATP to form M
3(HITP)
2(M=Co and Ni).The crystallinity and electrical conductivity of alloyed 2D ECMOFs decrease with the increase of Co content because of the different coordination States of Co
2+ and Ni
2+ due to the unpaired 3D electrons, but the Co
2+ in alloyed 2D ECMOFs become more active metal centers due to the existence of unpaired 3D electrons, which shows excellent electrocatalytic activity in oxygen evolution reaction (OER) and ORR
[38].
In addition to the direct mixing of metal ions, it is also a feasible method to directly coordinate metal-containing ligands such as phthalocyanine ligands with another metal to construct bimetallic 2D ECMOFs, such as Feng et al., which coordinated 2,3,9,10,16,17,23,24-octahydroxyphthalocyanine copper (PcCu-O
8) with Zn
2+ to construct PcCu-O
8-Zn bimetallic 2D ECMOFs.The Zn-O
4 node can be used as the active site of the CO
2 reduction reaction (CO
2RR), while the Cu-N
4 node can accelerate the proton and electron transport process, and the synergistic effect between the two endows the bimetallic 2D ECMOF with excellent selectivity, conversion and stability
[51].
Although much progress has been made in the regulation of the structure and function of 2D ECMOFs, in order to achieve a more sophisticated regulation of their structure and function, we still need to make continuous efforts from the selection of building blocks (such as ligand molecules, metal ions, etc.) And the regulation of the synergy between the building blocks.
4 2D ECMOFs synthesis method
2D ECMOFs are planar networks formed by organic conjugate-bridged multidentate ligands and metal ions, but in practice, due to the influence of reaction conditions and crystal growth orientation, 2D ECMOFs can also be represented as nanorods, wires and even hollow spheres in addition to two-dimensional morphologies such as thin films and nanosheets. The development of various synthesis methods, such as single-phase synthesis and interface-assisted synthesis, provides more possibilities to control the composition, crystalline state and morphology of 2D ECMOFs.
4.1 Ingle-phase synthesis method
The hydrothermal/solvothermal method is one of the most commonly used methods to synthesize MOFs. The high temperature and high pressure conditions in the reactor can improve the solubility and activity of the reactants, and also create a good environment for the nucleation and growth of crystals. At present, a series of 2D ECMOFs with different morphologies and compositions have been synthesized by single-phase synthesis method, such as Mn/Fe-HIB-MOF hollow spheres, Cu-BHS nanorods, Co-TPHS nanosheets, Ni
3(HITP)
2 and Cu
3(HHTP)
2 rod-like single crystals, NdHHTP nanorods, etc
[30][44][40][24][32]. The controllable growth of 2D ECMOFs with specific morphologies on substrates can also be achieved by single-phase synthesis methods, such as the controllable growth of Cu-CAT nanowires on the surface of polypyrrole films by Xia et al
[52].
The morphology and crystalline state of that 2D ECMOFs can be control by adjusting various reaction condition such as solvents, atmospheres, additives and the like in the single-phase synthesis method. The use of mixed solvents is one of the common strategies to obtain high-quality crystals. For example, in 2012, Yaghi et al. First successfully prepared a series of rod-like CATs 2D ECMOFs by single-phase synthesis, and synthesized Co-CAT (Co-HHTP or Co
3(HHTP)
2) and Cu-CAT single crystals by adding N-methylpyrrolidone to form a mixed solvent
[26]. Recently, Chen et al. Also realized the controllable preparation of Cu-DBC rod-like single crystals in the mixed solvent of dimethylformamide and water
[48].
Atmosphere is also one of the factors that affect the crystalline state of 2D ECMOFs. For example, Dinc Dincand others imitated the synthesis method of nickel di-o-phenylenediamine (Figure 6) and successfully prepared Ni
3(HITP)
2 powder polycrystalline samples by adding ammonia water and heating in an open reaction vessel under atmospheric environment. This condition is also applicable to the preparation of Cu
3(HITP)
2 polycrystalline samples
[19][37]. Moreover, in the controlled synthesis of Ni
3(HIB)
2 and Cu
3(HIB)
2, Dinc Dincet al. Further found that the presence of oxygen in the air was essential to obtain crystalline Ni
3(HIB)
2 and Cu
3(HIB)
2 products, and only amorphous powdery products could be obtained without oxygen
[21].
Appropriate addition of specific additives in the reaction system can also effectively adjust the coordination state of metal and ligand, thus realizing the regulation and control of the morphology and crystalline state of the product. As previously reported, 2D ECMOF Cu
3(HHTP)
2 usually present rod-like morphology, but Behrens et al., by exploring the effect of ethanol, N, N-dimethylformamide, tetrahydrofuran, pyridine and ammonia as additives on the morphology of Cu
3(HHTP)
2, found that when ammonia or pyridine was added as additives, not only the pH value of the solution could be raised, but also the solubility of HHTP ligands could be increased.The product with two-dimensional flake morphology can also be obtained, but only when ammonia is added as an additive, all the samples prepared show flake morphology, and when pyridine is added, the product with flake morphology can be obtained only when a specific equivalent is added.This is because the smaller size of ammonia than pyridine makes it easier to coordinate with the free coordination sites of Cu
2+ in the square environment of HHTP ligands, thus inhibiting the growth of crystals in the c-axis direction
[26][53]. In addition to adjusting the morphology, the addition of additives can also play a significant role in regulating the crystalline state of the product. For example, the Co
3(HITP)
2 synthesized by adding ammonia usually shows amorphous or low crystallinity, while Dinc Dincand others successfully prepared crystalline Co
3(HITP)
2 by replacing ammonia with sodium acetate
[38,54][31]. Similarly, Bao et al. realized the controllable preparation of Cu-HHB with high crystallinity by adding ethylenediamine, because compared with ammonia, ethylenediamine can chelate with Cu
2+ in the reaction system, which can not only avoid the formation of hydroxides under alkaline conditions, but also slow down the nucleation of 2D ECMOFs through coordination competition with ligands, thus improving their crystallinity
[27].
As the most commonly used synthesis method of 2D ECMOFs, the single-phase synthesis method has the advantages of low cost, high yield and simple process, but for the crystal preparation process of 2D ECMOFs, which includes several non-serial steps such as ligand deprotonation, partial oxidation, coordination and growth, it is difficult to obtain 2D ECMOF crystals.There are many variables to be controlled, and the repeatability of experiments is usually poor, so it is difficult to obtain large size and high quality crystals. In the future, it is necessary to study the internal mechanism of crystal growth kinetics in order to obtain materials with ideal composition, morphology and crystalline state.
4.2 Interface-assisted synthesis method
The formation of liquid/liquid, liquid/gas, solid/liquid interfaces as templates for the confined growth of 2D materials is a common method for the synthesis of 2D materials, which has been widely used in the synthesis of 2D polymers, 2D metal-organic frameworks and 2D covalent organic frameworks
[55 ⇓~57]. Similarly, the interface-assisted synthesis method is also suitable for the controllable preparation of high-quality 2D ECMOFs films
[58 ⇓~60].
4.2.1 Liquid/liquid interface
The liquid/liquid interface is an interface formed by the stratification of two immiscible solvents due to their different densities. When the ligand and the metal salt are dissolved in the two solvent layers respectively, they will react only at the liquid/liquid interface, thus realizing the localized growth of 2D ECMOFs on the interface. In 2013, Nishihara et al. Successfully prepared Ni-BHT film with thickness of 1 ~ 2 μm by standing (1 d) at the interface of dichloromethane solution containing BHT ligand and aqueous solution containing nickel acetate, which is also suitable for the preparation of Cu-BHT film (thickness of about 200 nm)
[17][5]. Moreover, the thickness of 2D ECMOFs films grown at the liquid/liquid interface can be further controlled by changing the reaction conditions. For example, the thickness of Fe
3(THT)
2(NH
4)
3 films grown at the interface of chloroform and water by C Cánovas et al. Increased by about 2 μm per 72 H
[22].
4.2.2 Liquid/gas interface
Liquid/gas interface is the interface between solution and air or other gases, which has been used in the synthesis of many ultra-thin materials. In 2017, Xu et al. Successfully obtained a Ni
3(HITP)
2 film at the interface between the mixed aqueous solution of the precursor for the preparation of 2D ECMOFs and air at 60 ℃, and the film thickness was about 100 nm after 3 minutes of reaction, and the film thickness increased with the extension of reaction time
[61]. In addition, under the same condition of 60 ℃, Hu et al. Successfully prepared a centimeter-scale Ni
3(HITP)
2 film at the water/triethylamine vapor interface within 3 minutes by quickly putting the mixed solution of HATP and Ni
2+ into a container containing triethylamine vapor, and achieved precise control of the thickness of the Ni
3(HITP)
2 film from 7 to 92 nm by adjusting the concentration of the precursor (Figure 7)
[62].
In order to obtain a thinner or even a monolayer film, a small amount of precursor dissolved in an organic solvent can be slowly added into a large-diameter container such as a Langmuir-Blodgett (LB) tank filled with the aqueous solution of the base phase, so that the precursor is uniformly coated on the water surface, and after the solvent is volatilized, a water/air interface is formed, and the preparation of a monolayer or a multilayer film can be realized by controlling the surface tension and the content of the precursor. For example, Nishihara et al. coated a small amount of BHT ethyl acetate solution over an aqueous solution of nickel acetate and sodium bromide, and allowed the ethyl acetate to evaporate completely for 2 H to obtain a single layer of Ni-BHT film with a thickness of 0.6 nm at the air/water interface (Figure 8)
[17]. In 2015, Feng et al. Dissolved a small amount of THT ligand in N, N-dimethylformamide/chloroform solvent by controlling surface tension in LB tank and coated it evenly on the surface of water. After evaporation of the organic solvent, a metal salt solution containing Ni
2+ was injected into the water. The metal ions reacted with the ligand at the water/air interface to prepare a single-layer Ni-THT film with a thickness of 0.7 ~ 0.9 nm
[36]. In addition, the number of layers and the thickness of the thin film can be controlled by transferring the thin film to the surface of the substrate through the Langmuir-Schaefer method
[63].
4.2.3 Solid/liquid interface
The solid/liquid interface is also commonly used in the confined synthesis of 2D ECMOFs, for example, in the layer-by-layer (LBL) method, the substrate is sequentially contacted with different precursor solutions and washed to remove excess reactants, and the layer-by-layer growth of thin films can be obtained after multiple cycles. In 2017, Xu et al. sprayed Cu
2+ solution and HHTP solution on the hydroxyl-modified functional substrate in sequence, washed with ethanol after each spraying, and repeated the process to achieve layer-by-layer self-assembly growth. The thickness of the film increased by about 2 nm per cycle, and the controllable preparation of samples with different thicknesses could be achieved by controlling the number of cycles (Figure 9)
[64]. Subsequently, Xu et al continued to prepare HITP-doped Cu-HHTP two-ligand 2D ECMOF with controllable thickness in the range of 20-70 nm by a similar method
[49].
4.2.4 Other interfaces
Solid/gas (vacuum) interfaces formed by solid surfaces with air or vacuum and solid/solid interfaces formed by solid surfaces with other solids have recently been explored for confined growth of 2D ECMOFs.For example, Lin et al. Deposited HATP on Au (111) surface in an ultra-high vacuum scanning tunneling microscope, then evaporated Ni atoms by electron beam, and finally annealed at 250 ℃ to form a single Ni
3(HITP)
2 film, which is also suitable for the controllable preparation of a single Cu
3(HHB)
2 film
[65][66][67]. In addition, Liu et al. Developed a two-temperature zone chemical vapor deposition method to achieve the controllable deposition of conductive Cu
3(HHTP)
2 nanowire arrays on the surface of Cu foil
[68].
Interface-assisted confined growth has been widely used in the synthesis of 2D ECMOFs, which can realize the controllable preparation of films with different thicknesses.However, compared with the single-phase synthesis method, it has many problems, such as low yield, difficult transfer and difficult to control defects, and many challenges need to be overcome in practical application.
4.3 Other synthetic methods
In addition to single-phase synthesis and interface-assisted synthesis, ball milling and ultrasonic-assisted methods have also been applied to the synthesis of 2D ECMOFs. For example, Oh et al. Successfully prepared Co
xNi
y-CAT bimetallic 2D ECMOFs nanorods by a simple ball milling method
[50]. Feng et al. Found that the addition of NaCl to the bulk sample of Ni
2[CuPc(NH)
8] for assisted ball milling could mechanically exfoliate the bulk crystal into larger size (about 160 nm in lateral dimension and about 7 nm in average thickness) nanosheets with high yield, because NaCl would intercalate between the layers of Ni
2[CuPc(NH)
8] during ball milling and achieve mild separation of stacked MOF layers by reducing shear force.They also successfully prepared HHB-Cu and HHB-Ni nanosheets with a thickness of 4 ~ 5 nm (about 8 ~ 10 layers) and HHTP-Cu nanosheets with a thickness of about (5.1 ± 2.6) nm by surfactant-assisted synthesis
[69][41]. Wu et al. Realized the preparation of Co/Ni-CAT powder samples by one-step ultrasound
[70].
5 Application of 2D ECMOFs
Compared with traditional carbon materials, 2D ECMOFs have both high porosity and electrical conductivity, and their functions can be precisely controlled by designing their composition and structure.2D ECMOFs have shown potential applications in gas sensing, energy storage, energy conversion, electronic devices and spin devices.
5.1 Sensing
2D ECMOFs have high porosity and abundant active sites, which can interact with gas molecules through adsorption to change their electrical properties, and have gradually become a new generation of porous conductive sensing materials. In 2015, Dinc Dincet al. First applied the 2D ECMOF Cu
3(HITP)
2 to the chemical resistance gas sensor, the conductivity measured by the two-probe method was 0.2 S·cm
-1, and the detection limit of the sensor prepared by the drop-coating method for NH
3 was 0.5 ppm.Compared with the NH
3 sensing properties of homogeneous 2D ECMOF Ni
3(HITP)
2, the copper junction is more conducive to NH
3 detection, which indicates that the sensing properties of 2D ECMOFs can be controlled by reasonable design
[37]. On this basis, Mart Martí-Gastaldo et al., taking another 2D ECMOF Cu-CAT as an example, further demonstrated that its NH
3 sensing characteristics originated from the interaction between gas molecules and Cu
II through theoretical calculations and experiments
[71]. In 2017, Xu et al. Found that high-quality Cu
3(HHTP)
2 thin films (fig. 9) with good crystallinity, high orientation, close packing, smooth surface and controllable thickness prepared by LBL method can achieve selective detection of NH
3.And has that advantage of high response strength (the average change of the resistance at the 100 ppm NH
3 is 129%), short response time (1.36 min), good stability (the response still keeps 88.4% aft 3 months) and the like, thereby providing an important reference for the selection of excellent room-temperature NH
3 sensor materials
[64].
In addition to NH
3, 2D ECMOFs have also been used to detect gases such as volatile organic compounds (VOCs) and CO
2. For example, based on two ligands of HHTP and HATP and two metals of Cu and Ni, Dinc Dincet al. constructed three kinds of 2D ECMOFs chemical resistance gas sensor arrays of Cu
3(HHTP)
2, Cu
3(HITP)
2 and Ni
3(HITP)
2, which can distinguish different kinds of VOCs (Figure 10); Behrens et al. found that the Cu
3(HHTP)
2 nanosheet coating showed good response and recovery characteristics to methanol gas
[33][53]; Dinc Dincet al applied a series of 2D ECMOFs to the atmospheric sensing of CO
2, in which the Cu
3(HIB)
2 has excellent CO
2 sensing performance in the 400-2500 ppm level, and has the highest selectivity reported at that time in the humidity range of 10% -80%. Theoretical calculations and experiments show that the hydration adsorption sites and charge trapping mechanism of the Cu
3(HIB)
2 are important factors for improving the CO
2 sensing performance
[72].
The sensitivity and selectivity of 2D ECMOFs-based sensing materials for specific gas detection can be further improved by compounding 2D ECMOFs with metal nanoparticles. Due to the high reactivity of metal nanoparticles and the high porosity of ECMOFs, the Cu
3(HHTP)
2 loaded with Pd or Pt nanoparticles constructed by Kim et al. Show significantly improved NO
2 sensing characteristics compared with Cu
3(HHTP)
2[73]. The performance of the material can also be improved by constructing heterojunction or two-ligand 2D ECMOFs, for example, Xu et al. realized the function that cannot be realized by single Cu-TCPP or Cu-HHTP by using Van der Waals force to integrate 2D ECMOFs Cu-HHTP and another MOF Cu-TCPP in sequence to form heterojunction.It not only greatly improves the sensing response characteristics of benzene (the highest response intensity at that time), but also reverses the selectivity of Cu-HHTP to benzene and strong interfering molecules NH
3[74]. In the same year, the detection limit of THQ and HHTP dual-ligand 2D ECMOFs constructed by Kitagawa et al. For NH
3 detection can be reduced to 0.02-0.35 ppm
[29].
In addition to gas sensing, 2D ECMOFs are also used in other types of sensing devices. In 2018, Mirica et al. Drop-cast three kinds of M
3HHTP
2(M=Co, Ni, Cu) on the glassy carbon electrode by drop-coating method and covered with ion-selective membrane to form a potentiometric sensor, which showed near-Nernst response and linear range sensing of more than 4 orders of magnitude for cations and anions
[75]. In the same year, Wang et al applied the Cu
3(HHTP)
2 to the electrochemical sensing of dopamine, which has a linear detection range of 5.0×10
-8~2.0×10
-4M and a minimum detection limit of 5.1 nM
[76]. In 2020, Xu et al. applied several 2D ECMOFs M
3(C
18H
6X
6)
2(M=Ni, Cu; X = O, NH) to the interlayer material of Schottky junction to construct a photodetection diode, and improved the separation and transport efficiency of photogenerated carriers by adjusting the height of Schottky barrier.The external quantum yield (84%) and the spectral detection range (250 ~ 1500 nm) of the self-powered photodetector were the best reported at that time, with an on-off ratio of about 10
3, a response time of 0.007 s and a recovery time of 0.03 s
[77].
5.2 Energy Storage
With the development of all kinds of electronic products and electric vehicles, the research of energy storage devices that can be cycled and charged has become an important topic.Compared with traditional conductive materials, 2D ECMOFs have not only high conductivity, but also large specific surface area and adjustable structure and function, which are potential active materials for energy storage.
5.2.1 Upercapacitor
Supercapacitor is a new type of energy storage device, which has the characteristics of both traditional capacitors and batteries. According to their different energy storage mechanisms, supercapacitors can be divided into three types: electric double layer capacitor, pseudocapacitor and hybrid capacitor.The electric double layer capacitance is due to the adsorption/desorption of electrolyte ions by electrode materials with large specific surface area, while the pseudocapacitance is due to the redox reaction between active solid electrodes and electrolyte ions to store charges. In 2017, Dinc Dincand others first applied 2D ECMOFs as electrode materials in supercapacitors. The Ni
3(HITP)
2 with high conductivity (5000 S·m
-1) and large specific surface area (630 m
2·g
-1) provided a fast channel for electrolyte ion diffusion, showing the behavior of electric double layer capacitance.When the current density is 0.05 A·g
-1, the capacitance per unit mass is about 111 F·g
-1, and the capacitance per unit area is about 18μF·cm
-2, which is higher than that of most carbon-based materials. After 10,000 cycles, more than 90% of the capacitance is still maintained, which can reach the level of commercial application
[78]. Subsequently, Bao et al. Used smaller HAB ligands to prepare compact Cu-HAB and Ni-HAB, whose unit mass capacitance can reach 215 F·g
-1(Cu-HAB) and 420 F·g
-1(Ni-HAB), because they show high redox activity in the electrolyte.The pseudocapacitance mechanism is dominant (Cu-HAB and Ni-HAB account for less than 20% and 10% of the electric double layer capacitance, respectively), and good cycle stability and 90% capacitance retention can be maintained even after 12 000 cycles
[42]. Recently, Chen et al. Found that Cu-DBC as an electrode material for the preparation of symmetric solid-state supercapacitors has excellent conductivity (conductivity is 1 S·m
-1), high specific surface area (271 m
2·g
-1) and redox active metal and ligand.The capacitor exhibits both electric double layer capacitance (62%) and pseudocapacitance (38%). At a current density of 0.2 A·g
-1, the capacitor exhibits a capacitance per unit mass of up to 479 F·g
-1 and good stability
[48].
The structure of 2D ECMOFs is one of the important factors affecting the performance of supercapacitors. Feng et al. studied the double layer capacitance behavior of 2D ECMOFs with Ni
3(HHB)
2 (0.81 nm), Ni
3(HITP)
2 (1.57 nm) and Ni
3(HITN)
2 (2.39 nm) in ionic liquids by molecular dynamics simulation and experiment.The results show that the crystallinity and porosity of 2D ECMOFs are important factors affecting the electrochemical performance. The higher the crystallinity and the larger the pore size of 1D channel, the higher the specific capacitance and energy density can be obtained
[45]. In addition to structural control, the performance of supercapacitors can be controlled and optimized by in-situ growth to improve contact and construction of 2D ECMOFs-based composite systems
[34,68][52,69,79,80].
5.2.2 Battery
Batteries are currently the most commonly used energy storage devices. 2D ECMOFs have both large porosity and high electrical conductivity, making them promising candidates for applications in lithium-sulfur, lithium-ion and other alkali metal batteries. In 2018, Fang et al. in-situ grew a Ni
3(HITP)
2 membrane with high porosity, high conductivity (3720 S·m
-1) and thiophilic characteristics on the surface of a commercial polypropylene separation membrane (fig. 11), which can be used as a barrier membrane to inhibit the shuttle of sulfides during the cycling of lithium-sulfur batteries.The capacity and the cycle stability of the battery are remarkably enhanced, and a high capacity per unit area of 7.24 mAh·cm
-2 can be provided after 200 cycles even when the sulfur loading of the cathode is as high as 8.0 mg·cm
-2 and 70 wt%
[81]. In addition to being used as separation membranes, 2D ECMOFs can also be directly used as electrode materials for lithium-sulfur batteries, such as Han et al., which directly load sulfur on Ni
3(HITP)
2 and composite with carbon nanotubes as electrode materials to avoid sulfide shuttle effect.The electrode has excellent electrochemical performance and cycle stability, has a high initial capacity of 1302.9 mAh·g
-1 at a discharge rate of 0.2 C, and can still maintain a capacity of 848.9 mAh·g
-1 after 100 cycles
[82].
In addition to lithium-sulfur batteries, 2D ECMOFs can also be used as electrode materials for lithium-ion batteries
[83 ⇓ ⇓~86]. In 2018, Nishihara et al. Realized lithium-ion batteries based on Ni-HAB electrodes, because Ni-HAB has multiple redox States, and cations (Li
+) and anions ($PF_{6}^{-}$) can be used as carriers during charging and discharging.When the current density is 10 mA·cm
-2, the capacity can reach 155 mAh·g
-1, the energy density can reach 434 Wh·kg
-1, and the performance is stable after 300 cycles at 250 mA·g
-1[87]. Recently, Chen et al took Cu-THQ electrode as an example to verify the lithium ion intercalation/deintercalation mechanism of 2D ECMOFs as lithium ion battery electrode materials through experiments and theoretical calculations. Lithium batteries based on Cu-THQ electrode have 387 mAh·g
-1 reversible capacity, 775 Wh·kg
-1 energy density, and maintain 340 mAh·g
-1 capacity after 100 cycles.The results of various spectroscopic experiments show that both the Cu ion and the ligand have significant redox activity during the charge-discharge process, and both the cation (Li
+) and the anion ($PF_{6}^{-}$) participate in the reaction process, in which a three-electron transfer process occurs on each basic structural unit, while a single-electron transfer process occurs on the Cu ion (Fig. 12)
[88].
Imilarly, 2D ECMOFs can also be apply to other alkali metal batteries,Uch as a Na-ion battery based on Co-HAB, a Zn-ion battery based on Cu
3(HHTP)
2, a battery using Fe
2-O
8-PcCu,Ni
2-O
8-PcCu and Zn
2-O
8-PcCu, three kinds of 2D ECMOFs and I
2 are used as Na-I
2 cells of electrode materials.All of them show excellent performance and have great application potential and prospects
[43][89][90].
5.3 Energy Conversion
Electrochemical energy conversion is a reliable way to obtain green and clean energy. 2D ECMOFs have high conductivity, high porosity and controllable structure, which can be used as catalysts for electrocatalytic hydrogen evolution reaction (HER), ORR, OER, electrocatalytic carbon dioxide reduction (CO2RR) and other electrochemical reactions, and have broad application prospects.
In 2015, Marinescu et al. First applied Co-BHT and Co-THT to HER electrode materials, which can maintain high catalyst loading (the maximum surface catalyst concentration of Co-BHT is 2.5×10
-6mol
Co·cm
-2 when the 3.7×10
-6mol
Co·cm
-2,Co-THT) and excellent stability even in strongly acidic (pH 1.3) solution, and only need 0.34 V (Co-BHT) and 0.53 V (Co-THT) overpotential to achieve 10 mA·cm
-2 current density
[35]. In the same year, Feng et al. Coated a single layer of Ni-THT film on a glassy carbon electrode for HER. When the current density was 10 mA·cm
-2, the Tafel slope of Ni-THT catalyst was 80.5 mV·dec
-1, and the overpotential was 333 mV, which was superior to the molecular catalyst supported on carbon nanotubes and the graphene catalyst doped with heteroatoms reported at that time
[36][91][92]. Subsequently, Marinescu et al compared the HER activity of Co-BHT, Ni-BHT and Fe-BHT, and found that Co-BHT showed the highest activity (overpotential was only 185 mV) due to its higher electrochemical effective area and lower charge transfer resistance, and the film thickness also affected the HER activity.With the increase of film thickness, the electrochemical active area of 2D ECMOFs increases and the charge transfer resistance decreases, resulting in the decrease of overpotential, but when the film thickness increases to 1000 nm, the overpotential increases to 213 mV due to the limitation of charge and proton transfer and the decrease of electrochemical active sites, which is of guiding significance for the actual electrode design
[93].
The application of 2D ECMOFs as ORR and OER catalysts began in 2016. Dinc Dincet al. Tested in alkaline solution with pH value of 13.0 and found that the ORR activity of Ni
3(HITP)
2 (starting potential of 0.82 V at a current density of -50μA·cm
-2) was comparable to that of non-Pt metal catalysts with the highest activity. The experiment combined with theoretical calculation showed that the ORR activity of Ni
3(HITP)
2 originated from ligands rather than metals
[94][95]. Moreover, by systematically studying the ORR activities of Co
3(HHTP)
2, Ni
3(HHTP)
2, Cu
3(HHTP)
2 and Cu
3(HITP)
2, it is found that the electronic band structure and chemical stability of 2D ECMOFs are regulated by ligands and metals, which leads to their different ORR activities
[96]. In addition, specific surface area, structural defects and crystallinity were also found to have significant effects on the ORR activity of 2D ECMOFs
[70][97].
The construction of alloy 2D ECMOFs or composites based on 2D ECMOFs is also an effective way to improve the activity of ORR. Oh et al. Constructed an alloy 2D ECMOF Co
xNi
y-CAT with a structure similar to that of a single metal MOF, which combines the diffusion current density of Co-CAT (-5.68 mA cm
-2) and the starting potential of Ni-CAT (0.46 V), and the ORR activity is significantly better than that of a single metal 2D ECMOF
[50]. Feng et al. Found that the 2D ECMOF PcCu-O
8-Co mixed with carbon nanotubes showed excellent ORR activity in alkaline medium, and the power density of Zn-air battery based on PcCu-O
8-Co electrode was as high as 94 mW·cm
-2, surpassing Pt/C electrode (78.3 mW·cm
-2)
[98].
2D ECMOFs have shown potential applications in OER. In 2018, Du et al. First used 2D ECMOFs as OER catalysts. Due to its two-dimensional planar structure and high conductivity, NiPc-MOF deposited on fluorine-doped tin oxide showed a starting potential lower than 1.48 V, a mass specific activity of 883.3 A·g
-1, a conversion frequency value of 2.5 s
-1 and excellent stability, which was the highest molecular based catalyst reported for OER activity at that time
[47]. Moreover, the OER activity and stability of 2D ECMOFs can also be improved by constructing alloy 2D ECMOFs. For example, Wang et al found that when Ni-HHTP nanowire arrays doped with Fe
3+ were used for OER, the existence of active sites of Fe
3+ would promote the formation of hydrogen bonds between OER intermediates and adjacent layers, thus reducing the free energy of OER process
[99]. Fu et al. Found that when the alloy 2D ECMOF CoNi-HAB constructed on the Co(OH)
2 was compounded with carbon fiber paper as an OER electrode, the synergistic effect between Co and Ni could enhance the OER activity of the material
[100]. A series of three-metal FeCo
xNi
1-x-CATs constructed by Lu et al. And the Cu
3(HITP)
2 grown in situ on copper foam by Peng et al. Have shown excellent catalytic activity and stability when used in OER
[101][102].
Recently, 2D ECMOFs have also been explored for application in CO
2RR, such as Feng et al.'S comparison of 2D ECMOFs based on Cu, Zn and PcCu-O
8, 2,3,9,10,16,17,23.PcCu-O
8-Cu and PcCu-O
8-Zn of two ligands of 24-octahydroxy phthalocyanine zinc (PcZn-O
8),PcZn-O
8-Zn and PcZn-O
8-Cu of the CO
2RR performance of four 2D ECMOFs,It was found that the PcCu-O
8-Zn had a CO selectivity of 88%, a conversion frequency of 0.39 s
-1 and a stable operation time of more than 10 H.And the H
2/CO mixed gas with the molar ratio from 1:7 to 4:1 can be obtained by selecting different metal centers and applying electric potentials, and has an important application prospect in the mixed gas industry,Theoretical calculations and experimental results reveal that there is a synergistic effect between the Zn-O
4 and the U Cu-O
4 sites, in which the U Zn-O
4 is the NKCO
2RR site, while the U Cu-O
4 promotes the proton and electron transport processes
[51].
5.4 Electronic device
Compared with traditional conductor or semiconductor materials, 2D ECMOFs have large specific surface area and porosity, and their structure and function can be controlled by ligand design.2D ECMOFs have many applications in electronic devices, including field effect transistors (FETs), topological insulators, spintronic devices and thermoelectric devices.
In 2015, Zhu et al measured and found that the room temperature conductivity of Cu-BHT was 1580 S·cm
-1, and when it was used in FET devices, it showed bipolar charge transport behavior and extremely high electron and hole mobility (electron mobility was 116 cm
2·V
-1·s
-1, hole mobility was 99 cm
2·V
-1·s
-1, Figure 13)
[5]; Because of the low porosity of Cu-BHT with C6 + C2 symmetry, Xu et al. Grown Ni
3(HITP)
2 films with larger pore size at the liquid/gas interface and constructed bottom-gate top-contact FETs, which showed P-type and had 48.6 cm
2·V
-1·s
-1 hole mobility.The current on-off ratio is about 2000, and the bottom-gate bottom-contact FET with in-situ grown Ni
3(HITP)
2 as the conductive channel can be used as a bipolar liquid gate device to detect gluconic acid in the 10
-6~10
-3g·mL
-1 range, which has the potential of biosensor application
[61][103].
Topological insulators are a class of materials with unique quantum transport properties, which have important applications in spintronics and quantum computation.2D ECMOFs have the potential to be used as topological insulators because their electronic band structures can be controlled by structural design and oxidation state adjustment. In 2013, Liu et al calculated the band structure, edge States, Chern number and spin Hall conductance of Ni-BHT by first principles, predicted that there were non-trivial topological States in the cage-like crystal structure of Ni-BHT, and explained the mechanism of spin-orbit coupling to expand the band gap by using single-orbit tight-binding model
[104]. Subsequently, Nishihara et al. Regulated the conductivity of Ni-BHT from the 2.8~160 S·cm
-1 by regulating the oxidation state of Ni-BHT monolayer, which showed that Ni-BHT had the potential to be a topological insulator from the experimental point of view
[18]. Nishihara et al. Further found that the Dirac band gap of Pt-BHT (0.054 eV) is larger than that of Ni-BHT (0.016 eV), indicating that Pt-BHT is more likely to become a 2D topological insulator at room temperature than Ni-BHT
[105]. Lin et al. Found that the self-supporting monolayer Ni
3(HITP)
2 adsorbed on Au surface has quantum spin Hall phase and can expand the band gap through spin-orbit coupling through density functional theory calculation, but because the substrate Au is a conductor in the experiment, quantum transport can not be observed in this system
[66].
Spin valve is a kind of prototype device with spin transport properties. In 2018, Feng et al. Found that Fe-PTC has ferromagnetism at low temperature (below 20 K), indicating that 2D ECMOFs are promising to be used as magnetic semiconductor materials in spin devices
[46]. Subsequently, Zhu et al. realized the controllable preparation of Co-TPHS with glassy magnetism at low temperature
[40]. Recently, Hu et al. Grown a Cu
3(HHTP)
2 thin film on the surface of a La
0.67Sr
0.33MnO
3(LSMO) ferromagnetic electrode by the LBL method and constructed an LSMO/Cu
3(HHTP)
2/Co sandwich spin valve device (Figure 14). The magnetoresistance can reach 25% at a temperature of 10 K, indicating that there may be a spin tunneling transport process in 2D ECMOFs, but the exact spin polarization transport mechanism has not yet been explained
[106].
Thermoelectric devices are important devices for the conversion of electrical and thermal energy. 2D ECMOFs have a crystalline porous structure with enhanced phonon scattering and intrinsic high conductivity. For example, Dinc Dincet al. Found that the Ni
3(HITP)
2 at room temperature has an extremely low thermal conductivity (0.21 W·m
-1·K
-1) and the highest thermoelectric figure of merit of MOFs reported at that time (1.19×10
-3);Kawai et al. Found that the Ni
3(HITP)
2 exhibits n-type thermoelectric transport behavior, and its thermoelectric properties are closely related to the crystallinity, and high crystallinity is more conducive to improving the thermoelectric power factor
[107][108]; Bradshaw et al. Prepared a Cu
3(HHTP)
2 film with a Seebeck coefficient of -121.4μV·K
-1 (a negative value indicates that it is an n-type semiconductor) by electrochemical deposition
[109]. These results indicate that 2D ECMOFs are a class of promising thermoelectric materials.
Although 2D ECMOFs have shown great potential in sensing, energy storage, energy conversion and electronic devices, there is still much room for improvement.The fine regulation of its function and the exploration of its versatility and application value can be further realized through the revelation of the relationship between its structure and function, the directional design of material components and structures, the utilization of synergistic effects between elements, the regulation and discovery of collective properties, and the in-depth study of the properties of composite materials.
6 Conclusions and outlook
The unique π-π stacking and π-d conjugated structure of 2D ECMOFs endow them with excellent conductivity.Moreover, 2D ECMOFs have the characteristics of high specific surface area, large porosity, rich and adjustable structure, diverse pore channels and multiple active sites, which make them have more potential and advantages in the fields of sensing, energy storage, energy conversion and electronic devices. Although a series of significant progress has been made in the research of 2D ECMOFs, there are still many challenges to be overcome: (1) Most of the 2D ECMOFs constructed so far are polycrystalline solids with serious defects and grain boundaries. In order to reveal the intrinsic transport properties and conduction mechanism of 2D ECMOF, it is essential to develop a controllable preparation method for general large size and high quality single crystal or monolayer 2D ECMOF; (2) The structure of 2D ECMOFs is mostly inferred roughly from powder X-ray diffraction data and theoretical simulation, which can not fully understand its formation mechanism and characteristics, so it is necessary to develop related technologies for accurate characterization of 2D ECMOFs structure to achieve accurate analysis of its structure; (3) The reported 2D ECMOFs ligands are mainly benzene, triphenylene and phthalocyanine derivatives, and the coordinated metals are mainly transition metals such as Co, Ni and Cu. It is necessary to introduce more ligands and metals into the 2D ECMOFs construction system to expand the material system to achieve further regulation and optimization of its function; (4) Although the excellent characteristics and application value of 2D ECMOFs have been recognized, more efforts are needed to optimize the performance and realize its application value.
In a word, the successful design and construction of 2D ECMOFs provide new opportunities for the basic and applied research of porous conductive materials, but the research is still in its infancy, and more in-depth research work needs to be carried out, hoping that more attention and efforts can be made to realize the application of this kind of materials in a broader field in the future.
the Strategic Priority Research Program of Chinese Academy of Sciences(XDB36000000)
the National Key Basic Research Program of China(2016YFA0200700)
the National Natural Science Foundation of China(92056204)
the National Natural Science Foundation of China(22073021)
the National Natural Science Foundation of China(21890381)
the National Natural Science Foundation of China(21721002)
the National Natural Science Foundation of China(21722301)
and the Frontier Science Key Project of Chinese Academy of Sciences(QYZDJ-SSW-SLH038)
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