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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 6: 954 - 967

DOI: https://doi.org/10.7536/PC230102

Review

Condensed Matter Chemistry: The Defect Engineering of Porous Materials

  • Yuenan Zheng 1, 2 ,
  • Jiaqi Yang 1 ,
  • Zhen-An Qiao , 1, *
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  • 1 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun,Jilin 130031, China
  • 2 State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian,Liaoning 116024, China
*Corresponding authore-mail:

Received date: 2022-12-01

  Revised date: 2023-03-19

  Online published: 2023-04-30

Supported by

The 1000 Talents Plan for Young Talents and the National Natural Science Foundation of China(21671073)

The 1000 Talents Plan for Young Talents and the National Natural Science Foundation of China(21621001)

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Abstract

Condensed matter chemistry is mainly concerned with the multilevel structure, chemical properties and chemical reactions of various states of materials, and frontier scientific issues in condensed matter construction chemistry. Porous materials with high surface area and adjustable pore structure, show great potential in a variety of applications. With the continuous exploration of defect engineering strategies for porous materials, the research scope of condensed matter chemistry has been greatly expanded. In the construction of defect sites and the functionalization application of porous materials, condensed matter chemistry permeates every process. The formation and regulation of phase, pore structure and defect sites involved in the synthesis of defective porous materials and the effective transformation of guest species on surface active sites in performance application, which fully reflects various chemical reactions, surface and interface interactions between the microstructure of porous materials and different species in condensed matter chemistry. This paper takes defective porous materials as the research object to discuss, including the suitable inorganic porous materials for defect engineering strategies, the types of defect structures in porous materials, the construction and regulation of defect sites of porous materials in condensed matter chemistry, the characterization of defect sites in porous materials, and applications of defect-rich porous materials in the field of energy storage and catalysis. In is desired to deepen the understanding of porous material defect engineering from the perspective of condensed matter chemistry, and it is expected to further promote the development of functional porous materials under the guidance of condensed matter chemistry.

Contents

1 Introduction

2 Porous materials suitable for defect engineering

3 Defect types of porous materials

3.1 Vacancy defect

3.2 Doping defect

3.3 Other type defects

4 Construction methods for defect engineering of porous materials

4.1 In-situ synthesis method

4.2 High temperature heat treatment method

4.3 Chemical reduction method

4.4 Vacuum activation method

4.5 Other methods

5 Characterization method of defect structure in porous materials

5.1 Micromorphology characterization

5.2 X-ray photoelectron spectroscopy

5.3 Raman spectrum

5.4 Electron paramagnetic resonance spectroscopy

5.5 Synchrotron radiation X-ray fine structure spectrum

6 Applications of defect-rich porous materials

6.1 Effect of defect engineering on porous materials

6.2 Application of defect-rich porous materials in catalysis and energy fields

7 Conclusion and perspective

Key words： condensed matter chemistry; porous materials; mesoporous materials; defect engineering

Cite this article

Yuenan Zheng , Jiaqi Yang , Zhen-An Qiao . Condensed Matter Chemistry: The Defect Engineering of Porous Materials[J]. Progress in Chemistry, 2023 , 35(6) : 954 -967 . DOI: 10.7536/PC230102

1 Introduction

With the continuous development of global economy and the gradual depletion of non-renewable resources, in order to meet the growing energy demand, the development of efficient energy storage and catalytic materials has become the frontier and hot spot of research. Porous material is a kind of very important functional material with wide application value[1⇓⇓⇓~5]. From the perspective of condensed matter chemistry, the high specific surface area and abundant surface active sites of inorganic porous materials have become the key factors to achieve high performance in a variety of applications. With the increasing recognition of important intermediates and rate-determining steps in chemical reactions, defect sites in materials have gradually become active centers in various reaction processes, and the high density of defect reaction sites provided by porous structures accelerates the reaction process. Therefore, it is very important to construct porous materials containing defect sites. From the perspective of condensed matter chemistry, the multi-level condensed structure of materials changes during the construction of defect sites. In the catalytic reaction, there are also some changes between the defect sites and the reactant molecules, which accelerate the overall chemical reaction and promote the reaction efficiency. That is to say, the properties and functions of the prepared materials are closely related to their multi-level condensed structure[6,7]. Therefore, the reasonable design of defect engineering and the controllable preparation of defect porous materials have a great impact on the structure and properties of materials. From the point of view of condensed state chemistry, this paper mainly focuses on the condensed state chemical construction of defect sites in inorganic porous materials and the application of defect-rich porous materials, including: (1) condensed state chemical construction of defect sites in porous materials; (2) characterization of porous materials rich in defect sites; (3) Applications of porous materials rich in defect sites in catalysis and energy storage and conversion.

2 Porous materials for defect engineering

With the continuous development of materials science and synthetic chemistry, the family of porous materials has expanded, such as porous carbon materials, molecular sieve materials, metal-organic frameworks (MOF) materials, porous metal oxide materials, etc., and they have shown great application prospects in catalysis, adsorption, separation and other fields. Porous materials have the characteristics of high specific surface area, adjustable pore size and various pore structures, which make them suitable for different application environments. In order to further improve its application performance, a variety of defect engineering strategies have been developed, which can optimize the physical and chemical properties of porous materials by changing the atomic composition of the skeleton or the functional groups on the surface of the pores. This chapter will focus on the inorganic porous materials suitable for the construction of defect sites, including porous carbon materials, molecular sieve materials, MOF materials and mesoporous metal oxide materials.
Porous molecular sieves are usually crystalline nanoporous materials with different silica-alumina ratios, diverse crystal structures, open pore structures, adjustable pore window sizes, and multiple dimensions of pore channels, which make molecular sieve materials play an important role in catalysis, adsorption, and ion exchange[2,8,9]. In particular, the petrochemical industry has shown great interest in the synthesis of new molecular sieve catalysts with high catalytic activity and selectivity. In order to further promote the application and development of molecular sieve materials, molecular sieve materials with diversified components have been designed and synthesized. For example, other atoms (such as gallium, germanium, iron, boron, phosphorus, chromium, vanadium, titanium and arsenic) are introduced into the framework of the molecular sieve to replace silicon or aluminum to form a heteroatom molecular sieve[10⇓~12]; Some atoms in the molecular sieve framework can also be selectively removed to form defect sites for the introduction of other active components, so as to realize the microstructure regulation of the composite material catalyst. From the perspective of condensed matter chemistry, the defect engineering strategy has been used to regulate the type and quantity of ions in the framework of molecular sieves and the active groups in the pores, which has promoted the performance of molecular sieves in the fields of in-pore catalysis, shape-selective catalysis and bifunctional catalysis.
Metal-organic frameworks (MOFs) are a kind of inorganic-organic hybrid porous crystalline materials, which are usually assembled into three-dimensional network materials by metal ions and organic ligand molecules through coordination bonds. MOFs exhibit excellent properties in gas storage, heterogeneous catalysis, chemical sensors, drug delivery and other applications due to their ultra-high specific surface area, controllable pore size and modifiable pore environment[13⇓⇓~16]. In order to show good performance in specific application areas, it is particularly important to design and adjust the physical and chemical characteristics of MOFs. Directional introduction of defects into the structure of MOFs has been shown to be an effective way to improve their performance for intended applications. For example, (1) improved gas diffusion and mass transport properties due to the lack of bridging ligands or metal vacancies; (2) enhancement of catalytic performance due to open metal sites; (3) optimization of MOFs electrode materials by grafting metal ions or counter ions at defects; (4) Other effects on electronic, magnetic, or optical functions, etc[17⇓⇓⇓⇓~22].
Porous carbon materials are an attractive class of functional porous materials with controllable pore structure, low density, excellent electrical and thermal conductivity, and good chemical stability, which are widely used.Such as porous carbon fibers, porous carbon spheres, aerogels, biomass porous carbon materials, porous graphite carbon nitride and so on, are important members of the porous carbon family[23⇓~25]. However, according to the second law of thermodynamics, carbon materials generally have inherent defects inside, but their density is too low to produce high catalytic performance at the macroscopic level. And the carbon skeleton of a single component is inherently chemically inert and is generally regarded as an inactive material to drive certain reactions. Theoretically, the introduction of heterogeneous atoms or vacancy defects can disturb the electronic symmetry in aromatic rings, thus providing heterogeneous composition and active sites by adjusting the charge and spin density of carbon atoms, and then showing extraordinary potential in important fields such as energy storage and photo-/electrocatalysis[26⇓⇓~29]. Luo et al. Used zinc phthalocyanine as a precursor to effectively overcome the serious loss of pyridine nitrogen and prepared a three-dimensional porous nitrogen-doped carbon framework[30]. The as-prepared catalyst is an excellent ORR electrocatalyst with a half-wave potential up to 33 mV.
Mesoporous metal oxides have attracted much attention in the fields of catalysis and energy storage and conversion. Because of their relatively simple preparation method, good redox characteristics, high specific surface area and large pore volume, which can provide a large number of surface active sites, increase the contact area of reactant molecules, and the interconnected skeleton network improves the mobility of electrons and ions in solids[31⇓~33]. In recent years, defect engineering has emerged as an effective way to tune active sites in mesoporous metal oxide materials. A number of theoretical calculations and experimental results have shown that defect engineering breaks the electronic neutrality of inorganic crystals, produces unsaturated active sites, adjusts the electronic structure of metal oxides or the geometric environment of atoms, and regularizes the surface adsorption energy of key reaction intermediates, thus greatly enhancing the catalytic conversion performance[34,35].

3 Defect types of porous materials

The construction of various types of defects can control the characteristics of porous materials, such as surface chemical characteristics, electronic structure, atomic coordination number, carrier concentration, conductivity and so on. According to the dimension, the defect structure can be divided into four categories: zero-dimensional point defects (such as vacancies, doping), one-dimensional line defects (such as dislocations), two-dimensional surface defects (e.g. grain boundaries), and three-dimensional bulk defects (e.g. spatial lattice disorder)[36][37][38][39][40]. At present, the research on vacancy and doped defects is more extensive and mature. This chapter mainly summarizes the vacancy, doped and other types of defects and their positive effects. Targeted construction of defect sites can enrich the functional properties of porous materials and improve their application performance. On the contrary, porous materials are also a good platform to show that defect engineering is an effective strategy to improve the performance of porous materials.

3.1 Vacancy defect

3.1.1 Anion vacancy defect

As one of the most common anion vacancies, oxygen vacancy has been widely studied in the field of transition metal oxides due to its low formation energy[41]. Especially for two-dimensional materials whose thickness is only atomic scale, their electronic structure and physical and chemical properties can be effectively controlled by the structure of oxygen vacancies. For example, oxygen vacancies can be constructed on the surface of In2O3 with atomic thickness by rapid heating phase transformation strategy[42]. Density functional theory (DFT) calculation results show that, compared with bulk In2O3, a significant increase in the density of States at the conduction band edge can be observed in the ultrathin In2O3 materials rich in oxygen vacancies, indicating that more carriers can be effectively transferred to the conduction band, and the photocatalytic water splitting performance is significantly improved. In addition to oxygen vacancies, other anions such as sulfur, carbon and nitrogen vacancies have also been found[43]. For example, carbon vacancies can be introduced into graphitic carbon nitride nanosheets by heat treatment of carbon nitride in an NH3 atmosphere[44]. After the introduction of carbon vacancy, the lone pair electrons are generated on the carbon atoms of the heptazine ring, and the optical absorption of the defective carbon nitride nanosheets is extended to the near-infrared region. At the same time, the mobility of the photo-generated carrier and the conductivity of the material are improved.

3.1.2 Cation vacancy defect

Cation vacancy refers to the removal of a small amount of cations from a material, and the vacancy at the lattice site that should be occupied by metal cations, which is called cation vacancy. Due to their different electronic configurations and orbital distributions, cation vacancies can greatly regulate the electronic structure and physicochemical properties of metal compounds[43]. However, the formation energy of cation vacancy is higher, and the construction of cation vacancy is more difficult than that of anion vacancy, which also brings greater challenges to the exploration and realization of cation vacancy function. Cation vacancy defects in metal oxides can significantly adjust the electronic structure, increase the density of States near the Fermi level, and improve their conductivity. The relatively low atomic escape energy of ultrathin two-dimensional materials makes it easier to construct cationic defects. Song et al. Found that Ti vacancies in monolayer H1.07Ti1.73O4·H2O nanosheets can lead to the formation of abundant active O species around the vacancy sites, and combine with water molecules through hydrogen bonds to form surface coordination[45]. The results showed that the photocatalytic activity of hydrogen evolution was 10. 5 times higher than that of the layered catalyst.

3.1.3 Vacancy association compound

In addition to single-site atomic vacancies, multi-atom vacancy coupling will produce vacancy associates, which can change the physicochemical properties of semiconductors more significantly[45]. It has been reported that the exposure of the high-energy surface is beneficial to the formation of surface defects, such as vacancy associates. For example, the exposed (113) and (100) crystal faces of the peony-like aggregate and the nano-double pyramid-like Bi2WO6 materials prepared by morphology control can be obtained, respectively. The formation of "Bi — O" vacancy association in the (100) high energy plane of Bi2WO6 nanobipyramids is confirmed by characterization and theoretical calculation. The results show that the constructed "Bi — O" vacancy association is helpful to reduce the band gap, improve the charge separation efficiency, and enhance the photocatalytic activity for pollutant degradation[46]. At the same time, the thickness of nanosheets and the existence of mesoporous structure also play an important role in the generation of vacancy associates. For example, Guan et al. Found that when the thickness of BiOCl nanosheets synthesized using the solvothermal method was reduced to the atomic scale, the defects present in their structure would change from isolated defects V‴Bi to trivacancy-associated V‴BiVÖV‴Bi (Fig. 4 (a))[47]. Guo et al. Found that the type of vacancy association on mesoporous BiOCl nanosheets not only changes with the thickness of the material, but also the existence of mesopores promotes the formation of large vacancy association, which balances the thermodynamic instability caused by the incomplete coordination of Bi atoms and O atoms near the mesopores[48].
图4 Vs-M-ZnIn2S4纳米片的:(a)原子力显微镜图片,(b)透射电子显微镜图片,(c)高倍率透射电子显微镜虚拟彩色图片。MoS2QDs@Vs-M-ZnIn2S4纳米片的:(d)透射电子显微镜图片,(e)高倍率透射电子显微镜虚拟彩色图片与(f)高倍率透射电子显微镜图片[78]

Fig.4 (a) AFM image (inset is the height profiles of lines 1 and 2), (b) TEM image (inset is HRTEM image), and (c) false-color image of the HRTEM image of Vs-M-ZnIn2S4 nanosheets. (d) TEM image, (e) false-color image of the HRTEM image, and (f) HRTEM image of MoS2QDs@Vs-M-ZnIn2S4 nanosheets[78]

3.2 Doped defect

As another common defect in porous materials, heteroatom doping means that foreign atoms enter the original crystal structure to occupy normal lattice sites or interstitial sites. According to the different chemical elements, it can be further divided into three categories: non-metal atom doping, metal atom doping and co-doping. Due to the difference between the radii of the two ions in the doping process, the incorporation of the doping ions will inevitably lead to the expansion or contraction of the lattice, resulting in lattice distortion and defects. In addition to different ionic radii, the incorporation of doping ions with different valence States can also cause a variety of oxygen defects or cation defects. For example, when the valence state of the dopant ion is less than the valence state of the metal ion in the host to be doped, i.e., p-type doping, holes are generated, and at the same time, anion vacancies are generated to maintain overall electrical neutrality. In addition, the doping of low-valence ions makes the surface of the dopant present more Lewis acid sites, so the oxygen on the surface is easy to combine with Lewis base species, which improves the surface activity of the porous material[49]. When high-valence ions are doped, that is, n-type doping, additional electrons will be generated, followed by cation defects or interstitial oxygen defects to compensate the charge of the main body of the material. In a study, Chen et al. Prepared N-doped MoP under plasma treatment, which not only enriched the active sites of Mo-P, but also increased the electron density near the doping sites in the material, thus improving the surface activity of the material[50].
The synthesis of heteroatom-doped porous carbon materials by polymer derivation is a very efficient method, and heteroatom-doped porous carbon materials with high doping amount and stable structure can be obtained (fig. 1)[51]. The physical and chemical properties of porous carbon materials can be significantly promoted by doping non-metallic heteroatoms N, P, B, S and Se in the synthesis process of porous carbon materials. N and B are easy to be incorporated into graphite lattice because their diameters are very similar to those of carbon atoms, so the development of N-rich or B-rich porous carbon materials has attracted much attention. Doping N can improve the surface electron transfer ability of carbon materials and adjust the surface basicity of carbon materials. At the same time, the electrons in N may delocalize to neighboring carbons or functional groups, changing their chemical properties.
图1 不同杂原子掺杂的多孔碳材料的微观结构示意图[51]

Fig.1 Schematic diagram of microstructure of porous carbon materials doped with differentheteroatoms[51]

3.3 Other types of defects

As another defect of porous metal oxides, pits are formed due to the escape of adjacent surface atoms of ultrathin two-dimensional materials from the lattice[42]. Atomic escape enables the formation of different vacancy defects and many other types of lattice defects, such as lattice dislocation, distortion, and disorder, and can also have a significant regulatory effect on the electronic structure and physicochemical properties of materials[52]. In general, the surface energy of a crystal always tends to reach a minimum to maintain a thermodynamically stable state. From this point of view, ultrathin two-dimensional crystals usually suffer from surface distortion to reduce the surface energy and stabilize the crystal structure. During the surface distortion, many parameters of the local atomic arrangement will undergo changes, such as bond length, bond angle, interatomic distance, coordination number, etc[53]. Obviously, the electronic configuration of the host will inevitably be affected by these distorted structures, and its performance will be regulated. Zhao et al. Found that Ti3+ with lower coordination number could be obtained in NiTi-LDH nanosheets with a thickness of about 2 nm, compared with bulk NiTi-LDH containing only Ti4+[54]. And serious structural distortion and adjusted electronic structure can be observed in the NiTi-LDH nanosheet.

4 Construction of Defects in Porous Materials

Defects can effectively adjust the local atomic structure, optical properties, electronic structure and conductivity of materials, and then affect the physical and chemical properties of materials. Therefore, it is necessary to summarize the effective strategies of defect construction and deeply understand the internal mechanism of defect formation.

4.1 In situ synthesis

Porous materials contain more surface defect sites than bulk non-porous materials, so making the target component materials porous is one of the simplest methods to construct defect engineering. Doping defects and anion/cation defects can be directly realized through the synthesis of porous materials in one step, and the proportion of each component in the precursor needs to be well controlled, such as sol-gel method, soft template method and hard template method.
Hydrothermal/solvothermal method is not only an important method for the synthesis of porous materials, but also can control the oxygen vacancy defects on the surface of the product materials, so it has become an efficient defect engineering strategy. Oxygen vacancies are directly obtained in the preparation of porous materials by hydrothermal/solvothermal methods. By adjusting the reaction conditions such as reaction temperature, time and precursor ratio, the valence state of metal ions in the oxide is adjusted, resulting in the loss of some lattice oxygen in the metal oxide crystal, and the metal oxide containing oxygen vacancies is directly obtained. For example, Shao et al. Prepared Sn2+ self-doped SnO2 nanocrystals with abundant oxygen vacancies by hydrothermal method using SnCl4·5H2O and Sn as reaction raw materials[55]. Due to the substitution of low-valent Sn2+ for high-valent Sn4+, oxygen vacancies will be generated in the self-doped SnO2 lattice to maintain the overall electrical neutrality of the material. In addition, a series of studies have reported the preparation of heteroatom-doped oxides by hydrothermal/solvothermal methods to introduce doping defects. In hydrothermal/solvothermal reaction systems, high temperature and pressure provide the driving force for the dispersion of the reactants. Therefore, it is easy to enter the lattice of the product to form doping defects by adding heteroatom precursors to the raw materials. For example, Liu et al. Designed and synthesized oxygen-doped MoS2 composite (O-MoS2/rGO) supported on graphene oxide (rGO) by solvent-assisted hydrothermal method, and oxygen doping improved its intrinsic conductivity and the synergistic effect between MoS2 and rGO, based on which the superior catalyst showed significantly improved hydrogen evolution reaction activity[56].
The synthesis of heteroatom-doped porous carbon materials by in situ polymerization is a very efficient method. Compared with the post-treatment method, the in situ synthesis method has better control over the structure and composition of the doped porous carbon materials, and can obtain heteroatom-doped porou carbon materials with high doping amount and stable structure. Starting from the polymerization reaction, porous carbon materials with different morphologies and pore structures can be prepared by designing reasonable polymer precursors and pore-forming agent or template molecules that can be well co-assembled (Fig. 2). The realization of new polymers based on chemical reactions can not only introduce abundant heteroatom doping into the polymer/carbon skeleton, but also regulate the type of heteroatom. For example, conducting polymers such as polyaniline (amino nitrogen) and polypyrrole (pyrrole nitrogen) synthesized by oxidative polymerization were directly used as carbon precursors to obtain different kinds of nitrogen-doped porous carbon materials. New nitrogen-rich backbone polymers, such as melamine resin based on aldehyde-amine polymerization, polydopamine and polyaminopyridine based on oxidative polymerization, introduce better nitrogen sites for carbon products to control the structure[51,57].
图2 聚合物衍生法合成杂原子掺杂的多孔碳材料[51]

Fig.2 Heteroatom-doped porous carbon materials were synthesized by polymer derivation method[51]

4.2 High temperature heat treatment

High temperature heat treatment mainly refers to a method of defect construction by heating porous materials in reducing or inert atmosphere, especially in the construction of oxygen vacancies. Chen et al. Reduced the Ti4+ in the TiO2 to Ti3+ and formed oxygen vacancies by means of hydrogen heating treatment, and found that the introduction of oxygen vacancies greatly expanded the effective light absorption range of the material and improved the photocatalytic performance[58]. In addition to oxygen vacancies, high temperature hydrogen heating treatment can also realize the formation of other ion vacancies, such as N vacancies in C3N4. In addition to reducing atmosphere, high temperature treatment under inert atmosphere can also produce defects. Ding et al. Used soybean meal as raw material and CO2 as protective gas to prepare N-doped porous carbon materials by pyrolysis method, in addition, they further coated MnO2 nanoparticles on the surface of the porous carbon materials, these new electrode materials extracted from soybean meal have great application potential in energy storage, when the current density is 0.25 A·g-1, the specific capacitance of BPC@MnO2 can reach 288.3 A·g-1[59].

4.3 Chemical reduction method

In addition to hydrogen, other reducing agents such as NaBH4, CaH2, N2H4, ethylene glycol, etc. Can also be used to construct defects on the surface of semiconductor materials. Bi et al. Prepared K4Nb6O17 ultrathin porous nanosheets with defects by chemical reduction using NaBH4 as a reducing agent[60]. During the reaction, the strongly reducing NaBH4 would react with the lattice oxygen in the K4Nb6O17 ultrathin nanosheets, thus leaving oxygen vacancies on the surface. The introduced surface oxygen vacancies narrow the band gap by lowering the conduction band edge and improve the light harvesting efficiency. At the same time, it can also act as a surface electron capture site to promote charge separation. The generated oxygen vacancy not only extends the optical absorption band to 650 nm, but also endows the material with the ability to effectively capture photogenerated electrons and molecular oxygen, thus producing superoxide anion radicals, which ultimately promotes the improvement of hydrogen production activity.

4.4 Vacuum activation process

Vacuum activation is also an effective method to construct defects in porous materials. Xing et al. Used a simple and inexpensive low-temperature vacuum activation method to produce defects in TiO2[61]. One of the advantages of this method is that Ti3+ and oxygen vacancies are introduced into the TiO2 without changing its crystal structure and crystallinity. When properly heated under vacuum conditions, the O atoms on the surface of TiO2 lack external pressure constraints and are easy to escape from the lattice. Because vacuum activation is a relatively mild surface treatment process, the limitation is that the number of defect sites will gradually decrease during the catalytic process, and the defect sites need to be treated again. This activation method can also be applied to other metal oxides such as ZnO, WO3, MoO3, etc. Chu et al. Successfully synthesized a CoO@Co3O4/C composite with abundant oxygen vacancies[62]. The control of oxygen vacancy concentration was achieved by varying the degree of vacuum during the heat treatment. The porous structure in the composite exposes more oxygen vacancies and reaction sites on the heterointerface, which provides an efficient path for the transport of charge and gas and significantly improves the electrocatalytic oxygen evolution performance. At a current density of 10 mA·cm-2, the OER overpotential of the CoO@Co3O4/C is 287 mV with good stability.

4.5 Other methods

UV irradiation was confirmed to be suitable for the construction of oxygen vacancies in multiple systems. Zhang and collaborators separately demonstrated that UV irradiation can induce the formation of oxygen vacancies in BiOCl[63]. Because of the low bond energy of the Bi — O bond in BiOCl, it is easy to break under the irradiation of high-power ultraviolet light, and O atoms escape, leaving oxygen vacancies on the surface of BiOCl nanosheets. However, the energy provided by ultraviolet light is not enough to break the metal-oxygen bond and produce oxygen vacancies in various oxides, so this method is not applicable to all metal oxides.
In addition to the above defect construction strategies, plasma etching has also been proved to be an effective method to introduce defects. Wei et al. Employed a simple and effective plasma activation strategy to synthesize oxygen-vacancy deficient MOF-based single-atom copper catalysts[64]. The bombardment of the plasma forms abundant oxygen vacancies and significantly increases the number of low-coordinated active copper sites. In addition, plasma treatment can make the material further produce hierarchical porous structure and promote the effective adsorption of reactant molecules. The synergistic effect of the porous structure and low-coordinated copper sites enhanced the electroreduction activity of CO2 with a maximum faradaic efficiency of 75.3%. Density functional theory calculations confirm that the low-coordinated copper sites favor the formation and further reduction of the key intermediate to CH4.

5 Characterization of defect structure in porous materials

Some advanced characterization techniques can directly observe or indirectly prove the existence of various defects, which can help the rational design of defect structures and better understand the structure-activity relationship. Therefore, it is necessary to summarize the advanced characterization techniques for defect research to provide useful guidance for further exploration of defective materials.

5.1 Micromorphology characterization

As the most common method to observe the morphology of materials, electron microscopy has been greatly applied and developed in scientific research and industrial production. Generally speaking, high-dimensional defects can be directly observed by high-resolution transmission microscopy (HRTEM). The anatase TiO2 synthesized by Ren et al. Can be characterized by HRTEM, and a large number of edge dislocation defects can be observed in its lattice[65]. Conventional transmission microscopic imaging technology is limited by its relatively low resolution, and it is difficult to image and characterize vacancies and doping defects, so it is necessary to use atomic resolution electron microscopy for observation. The development of spherical aberration correctors in scanning transmission electron microscopy (STEM) has made it possible to obtain images in the sub-angstrom range. In addition, STEM can obtain images showing the square scale of atomic number when a high-angle annular dark field (HAADF) detector is used. Spherical aberration corrected high angle annular dark field scanning electron microscopy (HAADF-STEM) is a powerful imaging tool, and the intensity of the image is closely related to the atomic number, according to which the arrangement of each atom in the crystal structure can be directly determined. In addition, based on the observed relative atomic brightness, atomic arrangement in the crystal, and spot defects, defect types can be distinguished and even defects can be counted.

5.2 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a sensitive spectroscopic tool for identifying the chemical and electronic States of elements on the surface of materials. The presence of defects can change the bond energy between elements or even add new bond energy, resulting in a slight shift of peaks or the appearance of new peaks in the XPS spectrum. Because the detection depth of XPS is only a few nanometers, it usually only reflects the surface information of materials, but for ultra-thin two-dimensional materials with atomic scale thickness, XPS detection technology is perfect. According to the relative element content, bond energy position and peak intensity ratio, the detailed defect information of ultra-thin two-dimensional materials can be determined. The XPS C1s spectrum of a Br/Co/N Co-doped porous carbon framework material has an additional C-Br peak, which provides evidence for the successful introduction of functional groups into the porous carbon framework, indicating that the main chain of carbon is rearranged by the addition of bromine heteroatoms[66].

5.3 Raman spectrum

In Raman spectroscopy, the Raman shift depends on the change of molecular vibrational levels. Different chemical bonds or ground States have different vibrational modes, which determine the energy change between levels, so the Raman spectrum is characteristic. Raman shifts are also related to lattice vibration modes and can be used to study the structural properties of crystalline materials. The presence of defects in the material can modify the vibrational modes so that the shift of Raman peaks or the appearance of new peaks can be observed. Raman spectroscopy can probe disorder in carbon and other solid materials through the shift of peaks or the appearance of new peaks caused by the presence of defects[66,67]. The ratio of the intensity of the D peak to the G peak (ID/IG) is usually used to define the degree of disordered structure of carbon materials. The ID/IG ratios of various N-doped porous carbons prepared by Huang et al. Are 1.340, 1.428, and 1.465, respectively[68]. The results show that the ID/IG ratio increases with the increase of N doping content, which also confirms that N doping increases more structural defects in porous carbon.

5.4 Electron paramagnetic resonance spectrum

Electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR), is a powerful tool for the sensitive detection of single-electron trapping vacancy-type defects in materials and for the study of materials containing unpaired electrons. EPR spectroscopy can provide valuable fingerprint information for unpaired electrons in materials based on transitions between the resulting quantized States of magnetic moments. The test principle is that by applying an external magnetic field to the unpaired electrons in the material, the unpaired electrons will be arranged along a certain orientation under the action of the magnetic field, and Zeeman splitting will occur in proportion to the intensity of the external magnetic field. If an electromagnetic wave with the same energy as the energy difference of Zeeman splitting is added in the direction perpendicular to the magnetic field, resonance will occur[69]. According to different G values and signal intensities, information about the type and relative concentration of defects can be obtained. The most common defect characterized by electron paramagnetic resonance is oxygen vacancy defect, which usually appears at G = 2. 004 in the spectrum, and the signal increases with the increase of oxygen vacancy density in the material.

5.5 Synchrotron radiation X-ray fine structure spectrum

Synchrotron radiation X-ray fine structure spectroscopy (XAFS) is a sensitive tool used to determine the local atomic and electronic structure of materials. It can provide more accurate information than XPS, and can detect the bonding type, oxidation state, bond length, bond angle and coordination number of atoms, which shows that XAFS has strong advantages in the study of atomic defects, coordination number of metal atoms and disordered structure in materials. There are two main types of XAFS: one is extended X-ray absorption fine structure spectroscopy (EXAFS), and the other is X-ray absorption near edge structure spectroscopy (XANES). Different microscopic information can be obtained from the two types of structural spectra. XANES is extremely sensitive to the valence and bonding morphology of the absorbing atoms, which can be used to determine the electronic structure of the absorbing atoms. EXAFS can provide precise microscopic arrangement information between absorbing atoms and neighboring atoms, including bond length, coordination number, coordination species, etc[69].

6 Application of defect-rich porous materials.

6.1 Effect of Defect Engineering on Porous Materials

6.1.1 Tuning the band structure of porous

When a semiconductor material absorbs a photon whose energy is greater than or equal to the band gap, driven by the photon energy, the electrons that jump from the valence band to the conduction band are photogenerated electrons, and the holes that remain in the valence band are photogenerated holes, that is, photogenerated electron-hole pairs. The absorption properties of materials are closely related to the band structure of semiconductors. However, most oxide photocatalysts have the disadvantages of narrow light absorption range and limited intensity due to their wide band gap, which makes them unable to efficiently utilize sunlight. The valence band and conduction band of metal oxide semiconductor are composed of 2p orbital of O and d or p orbital of metal atom respectively, and the positions of conduction band, valence band and Fermi level are usually affected by the defect structure, so the introduction of defects can adjust the band structure of oxide semiconductor, which is one of the methods to improve its photocatalytic performance[70].

6.1.2 Adjust the conductivity, magnetism,

The physical properties of porous metal oxide semiconductor, such as conductivity and magnetism, are affected by the existence of defects. On the one hand, the formation of defects will lead to the change of carrier concentration and charge transport properties of oxide semiconductors, and then adjust the conductivity and magnetism. On the other hand, the defect structure can affect the conductivity and magnetism of oxide semiconductors by adjusting the spin properties of electrons[71].

6.1.3 Geometrical Effect, Electronic Effect and Surface Characteristics of Tuning Material

The geometric environment of the active atoms in the material, including the coordination number, the atomic arrangement, or the size of the active cluster, will be changed in the process of defect formation by hetero-ion doping, which will lead to changes in catalytic or adsorption behavior. At the same time, the electronic state of the material also changes, such as the charge state of the local atom and the d-band structure. Therefore, the surface acid-base characteristics and conductivity of materials can be achieved by heterogeneous atom doping. These methods can be used as an effective means to regulate the functionalization of porous materials.

6.1.4 Construct or act as an active site

In general, the metal atoms in metal-based materials are in a saturated coordination state, so it is difficult to activate the reactant molecules by chemical adsorption, while the introduction of anion vacancies can change the metal atoms from saturated coordination to unsaturated coordination. When the metal atom is unsaturated coordination, it will become the electron aggregation center, which promotes the interaction and electron transfer between the adsorption site and the adsorbed molecules, has an important impact on the adsorption and activation of the adsorbed molecules on the surface of the material, and enhances the surface reaction[72]. Moreover, the recombination of photogenerated electron-hole pairs in semiconductor materials is limited, thereby improving the separation efficiency of electrons and carriers.
Several research reports have shown that a variety of materials undergo catalyst reconstruction during electrocatalytic oxygen production, because the real active sites are the species formed after surface reconstruction. The presence of defects can lead to the formation of highly active species, further facilitating the reconstruction process in electrocatalysis. Therefore, we can guide or promote the process of catalyst reconstruction in a targeted way to facilitate the catalytic reaction.
In addition, the defect can serve as an active site directly involved in the catalytic reaction. These defect sites themselves are reactive and can directly participate in the reaction of catalytic substrate molecules. A certain element in the target molecule can establish a strong interaction with the defect site to promote the catalytic reaction.

6.2 Applications of Defective Porous Materials in Catalysis and Energy

The lack of active sites makes it difficult for nanomaterials to effectively activate substrate molecules, which seriously hinders the application and development of nanomaterials. In order to overcome the above difficulties, the strategy of combining defect engineering with porous structure has been gradually used by researchers to change the electronic and geometric structure of the central atom by doping different atoms or creating vacancy defects, thereby changing the electronic properties and chemical reactivity of materials and promoting them to play an important role in application fields.

6.2.1 Catalyze

Porous materials have been widely used in electrocatalysis due to their abundant accessible active sites and large solid-liquid contact area. Changing the intrinsic electron distribution of materials can promote the transfer of electrons or protons and improve the conductivity and catalytic activity of electrocatalysts[73]. Based on this, the introduction of defect sites in porous electrocatalysts to control the redistribution of charge is an effective strategy to improve the catalytic performance, and the porous structure will promote the exposure of more active sites in the catalyst. Dilpazir et al. Used dimethyl octacosyl ammonium bromide (DODAB) to modify ZIF-67 to realize heteroatom doping and introduce a large number of defect sites, and finally obtained a Br/Co/N Co-doped porous carbon framework[74]. The catalyst showed excellent ORR/OER performance with E1/2=0.90 V vs RHE for ORR and lower overpotential η10=254 mV for OER.
Wang et al. Used a simple template method to prepare a three-dimensional ordered porous Co3O4/CeO2 heterostructure catalyst (3DOM-Co3O4/CeO2) rich in oxygen vacancies, and in this synergistic interfacial catalyst, Ce3+ and Ce4+ coexist, resulting in more oxygen vacancies[75]. It is due to the combination of abundant oxygen vacancies, the interfacial electronic effect between Co3O4/CeO2 and the three-dimensional ordered layered porous conductive network that the catalyst achieves good OER catalytic performance.
Zhu et al. Used microwave-induced plasma treatment to create defects on its surface and modify the coordinatively unsaturated metal sites of Co-MOF-74 (Fig. 3) without destroying the crystal structure of the material[76]. The electrochemical properties of the samples were optimized by adjusting the plasma gas source, intensity and treatment time. Hydrogen plasma treated Co-MOF-74(MOF-H2) rich in defect and coordinatively unsaturated metal sites exhibited the highest OER performance, even better than the commercial catalyst RuO2. In general, the higher the atomic ratio of Co-Ox (coordinatively unsaturated cobalt atom) to Co-O5 (cobalt atom coordinated with five oxygen atoms), the better the electrocatalysis. The quantitative correlation between overpotential and Co-Ox/Co-O5 further confirms the important role of defect sites generated by plasma treatment in promoting OER.
图3 (a)等离子体刻蚀法制备的含缺陷的Co-MOF-74催化剂。(b)不同微观结构的富缺陷Co-MOF-74的电催化性能[76]

Fig.3 (a) Schematic diagram of the preparation of catalysts by plasma engraving. (b) Electrocatalytic performance of Co-MOF-74 with different microstructures[76]

Different defect structures have a significant impact on metal oxide photocatalysts. For example, photocatalysts with surface defect structure can effectively control the migration of light-induced carriers, adjust the electronic structure, light absorption range, conductivity and other characteristics of the catalyst, and can affect the interfacial catalytic process, showing great potential in a variety of photocatalytic reactions. Xie Yi's team constructed WO3·2H2O ultrathin nanosheets into WO3 nanosheets with a large number of pit defects on the surface by rapid heating method[77]. Compared with bulk WO3 and non-porous WO3 nanosheets, the defect-rich porous WO3 nanosheets exhibited significantly improved photocatalytic oxygen evolution performance. Some researchers have found that the coordinatively unsaturated structure and delocalized electrons on the defect surface can be used as anchoring sites for other semiconductor materials.
The application of defect-induced strategy to construct heterostructured photocatalysts can not only overcome the metastable problem of defect structures, but also help to form a unique tight interface structure, thus promoting the improvement of catalytic activity. Zhang et al. Found that sulfur vacancies confined in the ZnIn2S4 monolayer (VS-M-ZnIn2S4) induced the growth of MoS2 QDs, forming an atomic-scale heterostructure (Fig. 4)[78]. Under visible light irradiation, the electrons excited by VS-M-ZnIn2S4 are transferred to MoS2 through the S atom) bond of Zn-S(MoS2 QDs, thus enhancing the photocatalytic activity.
Qiao Zhenan's research group has developed a high-entropy metal oxide Co0.2Ni0.2Cu0.2Mg0.2Zn0.2O(HEO) with porous layered structure[79]. The porous layered HEO catalyst exhibited 98% conversion in solvent-free oxidation of benzyl alcohol within 2 H (fig. 5), which is superior to various precious metal or alloy catalysts reported in the literature. By adjusting the catalytic reaction conditions, the selective regulation of different catalytic products can be realized. Through further exploration of the structure-activity relationship, it is concluded that the ultra-high catalytic performance of porous layered HEO is mainly attributed to the following three aspects: (1) HEO has a variety of cations randomly distributed, rich redox valence, excellent chemical stability and thermal stability, and this unique structure endows porous HEO materials with excellent catalytic ability; (2) The porous structure shrinks the HEO particles to nanoscale, increasing the number of active sites that can be exposed. The porous layered HEO has a large number of surface oxygen defect sites, which greatly improves the adsorption energy of the catalyst for benzyl alcohol and makes the catalytic reaction easier to be triggered. (3) The interconnected HEO porous layered framework prepared by the anchor-fusion method has a high specific surface area, which provides abundant catalytic active sites for the catalytic reaction and promotes the catalytic efficiency to be significantly improved.
图5 锚固融合法合成多孔层状高熵金属氧化物作为苯甲醇无溶剂氧化的高效催化剂[79]

Fig.5 The holey lamellar high entropy oxide material is prepared by an anchoring and merging process, which exhibits ultra-high catalytic activity for solvent-free oxidation of benzyl alcohol[79]

In 2021, Qiao Zhenan's research group designed a universal phenolic resin-assisted cation coordination co-assembly strategy to construct mesoporous ABO3 perovskite metal oxides rich in oxygen vacancies[80,81]. The phenolic resin as an important synthetic unit in the co-assembly process and the citric acid molecule coordinated with both La3+ and M2+(M=Mn,Co,Ni,Fe) are the two key factors to maintain the integrity of the pore structure and avoid phase separation. The authors further explored the structure-activity relationship between the number of surface oxygen vacancies and the porous framework structure of LaMnO3 catalysts and their hydrogenation catalytic activity. It was confirmed that Mn was the main active site in the LaMnO3 catalyst, and it was found that the increase of oxygen vacancy content on the catalyst surface was beneficial to the exposure of Mn sites. The density functional theory calculation results showed that the catalytic substrate furfural molecules showed strong interaction and interfacial electron transfer with the mesoporous LaMnO3 rich in oxygen vacancies.This further verifies that the oxygen vacancies on the surface of mesoporous LaMnO3 can better balance the adsorption and activation process of furfural molecules on the catalyst surface, and can effectively reduce the activation energy required for hydrogenation.
The defective molecular sieve catalyst containing boron active species showed excellent activity at low temperature in the catalytic oxidative dehydrogenation of propane[82,83]. Among them, the boron-containing MFI nanosheets exhibited the highest propylene formation rate and good stability when the B2O3/SiO2 molar ratio was 0.12. By means of two-dimensional NMR and in situ infrared spectroscopy, the authors observed the dynamic self-dispersion of polymeric boron clusters (B — O — B groups) into oligomeric boron species (Si — O — B groups) during the catalytic induction period. This in situ dynamic structural evolution improves the oxidative dehydrogenation activity, and the boron species can be anchored in the framework of the molecular sieve nanosheets, further ensuring the stability of the catalyst under oxidative dehydrogenation conditions.

6.2.2 Energy storage and conversion

Controllable construction defects in electrode materials can not only effectively promote ion diffusion and charge transfer, but also provide more storage sites/adsorption sites/active sites for metal ions or intermediates, and maintain high structural flexibility and stability, thus improving the electrochemical performance of materials.
Yuan et al. Used a simple NaCl-assisted ball milling method to prepare a novel graphene material (BSG, Fig. 6) with large density of self-doped defects, abundant pores, balanced conductivity, and high density (0.83 g·cm-3)[84]. The optimized ion and electron transfer pathways facilitate the efficient utilization of self-doping defects in BSG, contributing to the improved mass capacitance, volume capacitance, and rate capability (52.2% capacity retention at 20 A·g-1). Xi et al. Synthesized defect-rich iron disulfide nanoflowers by self-assembly on hierarchically porous heteroatom-doped carbon matrix[85]. Electrochemical experiments and density functional theory simulations show that the iron defect in the cathode helps to reduce the diffusion barrier of ions, and can promote lithium ions to enter the crystal and structure through the interface, thus accelerating the electrochemical conversion process of lithium batteries. At the same time, the hierarchical porous structure can also adapt to the volume change of the electrode during charge and discharge, which is conducive to the immersion of electrolyte and shows excellent electrochemical stability.
图6 (a)不同扫描速率下的CV曲线;(b)质量比电容;(c)与报道的多孔碳的重量电容和比表面积的对比结果;(d)与商用活性炭YP-50F相比,在2和10 mg·cm-2的质量负载下的体积电容;(e)5 A·g-1时BSG的循环性能曲线[84]

Fig.6 (a) CV curves at various scan rates, (b) gravimetric capacitances, (c) gravimetric capacitance and specific surface area compared with the reported porous carbons (the dotted line is the specific surface area capacitance (Cs)), (d) volumetric capacitances at the mass loadings of 2 and 10 mg·cm-2 compared with commercial activated carbon YP-50F, and (e) cycling performance of BSG at 5 A·g-1[84]

Chen et al. Prepared Co-Zn heterometallic imidazole framework (ZIF) with ligand vacancy and hierarchical pore structure as air cathode to overcome the bottleneck of rechargeable zinc-air battery development[86]. The authors slow down the energy barrier of the oxo electrocatalytic process by tuning the ligand coordination and creating an unoccupied 3D orbital at the metal site. At the same time, the synergistic effect of Co-Zn heterometals increases the energy level of unsaturated d-orbitals and optimizes their adsorption/desorption process with oxygenated intermediates. In addition, the hierarchical porous structure ensures fast charging and mass transfer processes.
Our group used a surfactant-induced confined polymerization strategy to synthesize N-doped multi-chamber carbon microspheres (MCC) with fine structure[87]. The precursor of this carbon sphere is a novel multichamber polymer (MCP) based on 2,6-diaminopyridine (DAP). The authors used a one-pot synthesis of MCP involving two polymerization steps. In the first step, DAP and formaldehyde were grown into prepolymer DAP-F with cytoskeleton-like structure in alkaline solution. In the second step, formic acid was added to polymerize and further crosslink the prepolymer. The polymerization of the prepolymer in the confined space of the microspheres resulted in the separation of large chambers and the formation of abundant small chambers. The multi-chamber MCC can be obtained by activation calcination in a N2/CO2 environment. The MCC consists of an exquisite multi-level structure with an outer shell with abundant micropores, an inner core with multiple chambers, and an N-doped carbon skeleton. The experimental results show that the specific surface area of MCC activated by CO2 for 6 H is as high as 1797 m2·g-1, and the specific capacitance of MCC as an electrode material for capacitors can reach 301 F·g-1, which is much higher than that of other porous carbon materials, thanks to the hierarchical structure of fully interconnected multi-chambers.
During the synthesis of highly N-doped mesoporous carbon materials by selecting new monomer molecules, there is often a lack of appropriate driving force between the monomer molecules and the surfactant molecules, resulting in the direct synthesis of N-doped mesoporous carbon materials with non-porous or sparse pore structure. Based on this, our research group proposed a multi-level self-assembly method to prepare mesoporous carbon materials with high pyridine N doping (Fig. 7). In the method, a series of high-pyridine N-doped mesoporous carbon materials are prepared by taking DAP containing a pyridine nitrogen configuration as a monomer, taking a micelle formed by a surfactant PS-b-PEO as a template, and taking graphene oxide as a structure directing agent and a platform for enriching the micelle[88]. The precise regulation of mesopore size was achieved by adjusting the length of PS block in surfactant PS-b-PEO. After calcination at 700 ℃, the N-doped mesoporous carbon (GO @ NMC) still has a high nitrogen content of nearly 19%, and the nitrogen content of pyridine and pyrrole configuration is as high as 49. 9% and 35. 3%. It exhibits excellent area-normalized capacitance (15~25μF·cm-2) of theoretical electrochemical double layer capacitance and area-normalized capacitance of most carbon materials as electrode materials for capacitors. A pseudocapacitive contribution of up to about 45% was obtain by both Trasatti and Dunn analysis. Such excellent capacitance per unit surface area is attributed to the high proportion of pseudocapacitance provided by the material itself, which benefits from its unique two-dimensional structure, high nitrogen content of pyridine and pyrrole configuration, and controllable mesoporous structure.
图7 (a)高N掺杂的二维介孔碳材料的合成示意图;(b)GO@NMC的电化学双层电容和赝电容示意图;(c)不同电流密度下GO@NMC-1的恒流充电/放电曲线[88]

Fig.7 (a) Schematic illustration of the synthetic procedure for the development of GO@NMC-1. (b) Schematic illustration of the electrochemical double-layer capacitance (EDLC) and pseudocapacitance of GO@NMC. (c) Galvanostatic charges/discharge curves of GO@NMC-1 at different constant currents[88]

7 Conclusion and prospect

Defect site-rich functionalized inorganic porous materials have been used in various research fields and exhibit excellent properties. In this paper, we summarize the defect engineering of different porous materials from the perspective of condensed matter chemistry. The principles and strategies of condensed matter chemistry in the construction of defect sites in porous materials are mainly described, and the application fields showing good performance potential are summarized. Based on the principle of condensed matter chemistry, the construction and regulation of porous structure and defect sites in inorganic porous materials are guided, so that the porous materials can better meet the application requirements. Although some progress has been made in the development of porous materials with defect-rich sites, there are still some areas to be further explored and improved:
(1) Create new preparation methods for porous materials with rich defect sites, explore the chemical reaction mechanism from the perspective of condensed state, conduct in-depth understanding at the micro level, explore possible new structures of materials and their change control laws, and strive to develop efficient, simple and green synthesis strategies.
(2) To accurately characterize and explore the defect site structure, micro-morphology, surface and interface groups of porous materials and the resulting changes in electronic structure, which are the key to determine the external properties of materials.
(3) It is necessary to reveal the source of the active component and the catalytic reaction mechanism when it shows good performance in application. Advanced characterization techniques and computational chemistry methods are used to reveal the relationship between porous structure, active component and catalytic behavior of materials, and to analyze the reasons for their high performance from the perspective of condensed matter chemistry.

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