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

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Progress in Chemistry >

2024 , Vol. 36 >Issue 6: 851 - 866

DOI: https://doi.org/10.7536/PC240105

Review

The Electronic Principle of Nanomaterial Surface Chemistry

  • Guolei Xiang , *
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  • College of Chemistry, Beijing University of Chemical Technology, Beijing 100029, China
* e-mail: ;

Received date: 2024-01-08

  Revised date: 2024-03-11

  Online published: 2024-03-18

Supported by

National Natural Science Foundation of China(21801012)

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Abstract

Revealing The intrinsic electronic principles driving the surface chemistry of nanomaterials is a central goal in nanoscience;however,the concepts and theoretical frameworks have long remained incomplete and unsystematic.this review systematically introduces a theoretical framework to reveal the interaction mechanisms and trends of surface ligands with nanomaterials at the electronic level,on the basis of competitive orbital redistribution in chemisorption and a concept of orbital potential,the characteristic electronic attribute directly determining surface reactivity.Based on the competitive interactions between surface coordination bonds and bulk energy bands,This theoretical framework can provide coherent answers to these key scientific issues.(1)the opposite and uniform relation of surface activity and stability in nanomaterials originates from the normalization principle of wavefunctions.(2)the physical nature of enhanced surface activity by size reduction lies in two mechanisms:weakening the constrain strength to surface valence atomic orbitals by nanomaterial energy bands,and amplifying the effects of other structural parameters like defects.(3)Nanoscale cooperative chemisorption(NCC)model generally reveals the electronic-level mechanisms and common rules how ligand coverage regulates the energy band states and physical/chemical properties of nanomaterials.(4)the roles and interaction mechanisms of nanomaterial size(r),specific surface area(S/V),surface ligands,and ligand coverage(θ)in nanomaterial surface chemical reactions are elucidated.

Contents

1 Introduction

2 Nanomaterial surface chemistry

2.1 Key science issues

2.2 Three types of understanding viewpoints

2.3 Nanomaterial surface coordination chemistry

2.4 Four modes of nanomaterial surface effects

3 Electronic principle of structure-function relationships

3.1 Structure-function relationship in physical science

3.2 Electronic attributes

3.3 Quantum size effect

4 Chemisorption model based on competitive orbital redistribution

4.1 Chemisorption interaction

4.2 Competitive redistribution of surface valence orbitals

4.3 Orbital potential

4.4 Structure-function relationship of surface reactivity

5 Electronic principle of size-dependent surface reactivity

5.1 Meaning of surface activity

5.2 Mathematic model of surface reactivity

5.3 Dual roles of size reduction in enhancing surface reactivity

6 Nanoscale competitive chemisorption model

6.1 Relationship of energy band and surface reactivity

6.2 Nanoscale competitive chemisorption model

6.3 The roles of r,S/V,andθin nanosurface chemistry

6.4 Two-electronic-state competition model

6.5 The uniform principle of ligand effect on photoluminescence

7 Comparison of typical adsorption models

7.1 Adsorption isotherm model

7.2 Electronic model of chemisorption

7.3 Chemisorption model of nanomaterial

8 Summary and outlook

Key words： nanomaterial surface chemistry; orbital redistribution; size effect; surface effect; electronic principle; nanoscale cooperative chemisorption

Cite this article

Guolei Xiang . The Electronic Principle of Nanomaterial Surface Chemistry[J]. Progress in Chemistry, 2024 , 36(6) : 851 -866 . DOI: 10.7536/PC240105

1 Introduction

Nanomaterials have generally shown special physical and chemical properties due to their size reduction,which has promoted the vigorous development of nanoscience and technology in the past 40 years,and has become a research frontier of interdisciplinary integration[1⇓~3][4,5]。 Researchers have accumulated a lot of exploration in nanomaterial synthesis methodology,material system,surface and interface structure regulation,property and application exploration,and structure-activity relationship discovery and analysis,and initially established the foundation of nanochemistry as a new discipline.Nowadays,the scale concept of nanometer has developed into a cognitive concept and research paradigm,which has penetrated into all fields of basic and applied research related to materials,and expanded the research and innovation modes of many disciplines.the surface science of nanomaterials runs through the synthesis,preparation,processing,properties and applications,and is at the center of the whole nanoscience.On the one hand,the huge specific surface area(S/V)of nanomaterials makes it inevitable to interact with the surrounding environment through surface contact,especially for the adsorption of various molecules and ions.On the other hand,the surface chemical state can widely affect and even determine the structure and properties of nanomaterials,showing a Ligand-induced surface effect(LISE)[6]。 As a result,surface ligands can not only regulate and stabilize the surface and overall structure of nanomaterials,but also affect the properties and functions of nanomaterials,so they are the key structural and functional components of the whole nanostructure.In addition,the chemisorption between molecules and nanomaterials is the basis of the synthesis and modification of nanomaterials,surface adsorption,heterogeneous catalysis,electrochemistry,photoelectric conversion,optical properties and biomedical materials,so the surface science of nanomaterials is the basis of nanoscience and technology[7]。
Compared with macroscopic materials,the surface chemistry of nanomaterials has more structural variables,more practical application backgrounds and more complex structure-activity relationship models.As shown in Figure 1,the structural parameters that affect the process and results of nanosurface chemistry include the overall composition,phase,size,morphology,crystal plane and defects of the nanomaterial,the local coordination environment and electronic structure state of the active site atoms,and the coordination atoms,molecular structure,coverage and molecular orbital characteristics of the adsorbate[8]。 Exploring the structure-activity function relationship between these structural parameters and properties is the core of basic research In nanoscience.in nanoscale surface chemistry,size effect and surface effect are the two most typical nanoscale effects.the size effect is that when the size of the nanomaterial is less than a critical value,reducing the size can significantly enhance the chemical interaction between the adsorbed molecules and the surface of the nanomaterial,a typical example is the catalytic performance of gold nanoparticles[9][10,11]。 the surface effect is that the surface ligand can regulate and even determine the structure and properties of nanomaterials,a typical example is the regulation of the fluorescence properties of gold nanoclusters and quantum dots by ligands[12⇓~14][15⇓⇓~18]。 It is the most basic scientific goal of the basic research of nanoscience to reveal the law and trend of nanoscale surface chemical action and to decipher the principle of structure-activity relationship between structural parameters and properties from three aspects of experimental exploration,theoretical model and computational simulation。
图1 纳米材料表面科学领域构效关系

Fig. 1 Structure-function relationships in the surface science of nanomaterial

2 Nanosurface chemistry

2.1 Key Scientific Issues

For a specific combination of ligand and nanomaterial,regardless of its composition,structure and morphology,reducing the size of the material and increasing the coverage of the ligand can generally enhance the strength of the nano-surface chemical interaction and the effect of the surface effect[17,19⇓-21]。 this common trend means that there must be some universal law behind it,and revealing This basic principle is the core task of the basic research field of nanochemistry.At present,the concept and theoretical system in the field of nano-surface science are not perfect and systematic[22]。 By establishing new models,conceiving new concepts and revealing new principles,the mechanism and law of the interaction between ligands and nanomaterials at the electronic structure level are clarified,which is the basis of revealing the basic principles of nano-surface chemistry.Some key scientific issues to be clarified in the field of nanosurface chemistry are clues and guides for further exploration.Common typical issues include:
(1)What is the physical picture and key mechanism of the electronic structure level of the interaction between adsorbed molecules and nanomaterials through surface coordination bonds,and how to describe the strength and results of chemical adsorption。
(2)How do adsorbed molecules control the electronic structure and properties of nanomaterials,and what physical and chemical principles do ligand-induced surface effects follow。
(3)How do the structural parameters such as size(R),specific surface area(S/V),surface ligand and coverage(θ)of nanomaterials affect the strength and results of the interaction between ligands and nanomaterials,and what is the physical nature and significance of these parameters。
(4)How to establish a physical and chemical theoretical model that can uniformly describe the laws and principles of nano-surface chemistry。

2.2 Three cognitive levels

At present,the cognition of the structure-activity relationship mechanism of nano-surface chemistry can be divided into three levels,namely,the representation cognition at the geometric level,the chemical cognition at the molecular level,and the essential cognition at the electronic structure level.at the geometric level,the mechanism of nano-effect is explained from the perspective of small size(R)and large specific surface area(S/V),and the improvement of surface activity is mostly attributed to the decrease of size or the increase of specific surface area[4,9,23]。 Although these two geometric parameters are the direct cause of nano-effect,they can not really reveal the law and physical nature of the intensity and result of the chemical interaction between molecules and nano-materials.At the molecular level,surface coordination chemistry is used to explain the adsorption between molecules and nanomaterials[24,25]。 This chemical method focuses on changing the parameters of ligand atoms,surface bonding configuration and coverage of ligand molecules to control the strength of surface adsorption and the surface electronic state of nanomaterials,so as to control the catalytic and optical properties of nanomaterials[17,26,27]。 Although the cognition at the molecular level can explain the trend and law of nano-surface chemical action to a certain extent from the perspective of bonding strength and configuration,it can not explain the essential mechanism of nano-effects such as size effect and surface effect[24,25]。 At the electronic structure level,the mechanism and law of nano-surface chemical action are explained from the perspective of electronic structure[28,29]。 Because the structure and physical and chemical properties of a substance are essentially determined by the specific electronic structure characteristics of the system,the electronic structure is the basis for analyzing the mechanism and nature of nano-surface chemistry[30]。 However,the picture of electronic structure is more abstract and difficult to characterize than atomic structure,and it is difficult to fully understand the relevant concepts and principles。

2.3 Nanosurface coordination chemistry

Nanosurface chemistry focuses on various interactions between molecules/ions and nanomaterials based on chemisorption.Nanochemistry focuses on the stable chemisorption of ligand molecules on the surface of nanomaterials,while catalytic chemistry focuses on the dynamic reaction process of substrate molecules on the surface of nanocatalysts[24,25][31]。 the surface of nanomaterials prepared by liquid phase method usually adsorbs a large number of solvent and ligand molecules,and most of them are in saturated adsorption state.the chemical interaction between the ligand and the surface atoms of the nanomaterial is in the form of chemisorption,and its essence is the surface coordination bond formed by orbital overlap and electron sharing,so the nanosurface chemistry is essentially the heterogeneous coordination chemistry on the surface of the nanomaterial[24,25]。 As shown in fig.2,according to the classification method of ligands by Green et al.,the ligands are divided into three types of L,X,and Z according to the number of electrons provided by the coordination group to form a covalent bond when the charge is neutral[32,33]。 The L-type ligand can provide two electrons and has the characteristics of Lewis base,which can form typical coordination bonds with metal sites on the surface of nanomaterials,such as—OH,—COO-,—NH2and—SH.The X-type ligand can donate an electron,is a free radical in chemical nature,and can form a conventional covalent bond with a surface site,such as·Cl.Z-type ligands can accept electron pairs,have Lewis acid characteristics,and can form acceptor coordination bonds with electron-rich sites on the surface.The classification of these three types of ligands is widely used to study the surface chemical interaction between ligands and quantum dot materials 。
图2 基于Green的共价键分类方法的三种配体类型。在带电中性时，L型配体共享2个电子属于路易斯碱，X型共享1个电子属于自由基，Z型配体提供0个电子为路易斯酸[32]

Fig. 2 LXZ ligands based on Green’s Covalent Bond Classification (CBC) method. In their neutral forms, L-type ligands donate two electrons and are identified as Lewis bases, X-type ligands donate one electron and are radicals, and Z-type ligands donate 0 electron as Lewis acids[32]

2.4 Four modes of surface effect

Both the surface ligand and the inorganic core are integral components of the nanostructure,which together constitute the organic-inorganic hybrid nanostructure[34]。 Due to the structural differences,the two components are independent of each other to a certain extent,and can interact and influence each other through surface coordination bonds.as shown in Fig.3,the regulation of surface ligands on the properties of nanomaterials contains four modes.One is the ligand-dominated type,such As controlling the dispersion of nanomaterials in polar or non-polar solvents by changing the carbon chain length of surface ligand molecules[35]。 The other is the bulk properties of nanomaterials,such as the fluorescence of CdSe quantum dots,the catalytic performance of nanocatalysts and the magnetic properties of nanoparticles such as Fe3O4,which are mainly the embodiment of the properties of nanomaterials themselves[36,37][38]。 the third is the interfacial coordination bond-dominated type,such as the surface state luminescence of metal clusters(Au,Ag,Cu)protected by ligands such as thiols,which is affected by the type of coordination atoms,the strength of surface adsorption bonds and the coverage of ligands[13,14,39]。 the fourth is the combination of ligand and inorganic phase,such as the regulation of activity and selectivity of nanocatalyst by surface ligand,the change of adsorption properties,and the passivation of exciton capture sites on the surface of quantum dot materials by ligand[40⇓⇓⇓~44][45,46][15,16,47⇓~49]。
图3 表面配体控制或影响纳米材料结构与性质的四种模式

Fig. 3 Four modes for surface ligands to control or influence the structures and properties of nanomaterial

quantum dots and metal nanoclusters are two typical systems for the study of ligand-controlled electronic structure and properties of nanomaterials.Owen et al.Remove the carboxylate ligand on the surface of CdSe quantum dots by ligand exchange,which can cause a significant decrease in fluorescence quantum yield[17]。 By changing the coverage of glutathione(GSH)ligands on the surface of gold nanoclusters with a size of about 2.5 nm,Zheng et al.Found that reducing the coverage of GSH can simultaneously reduce the quantum yield and photon energy of surface state luminescence[21]。 Zheng Nanfeng's team took metal nanoclusters and metal nanocatalysts as model systems,systematically explored the molecular level mechanism and law of the interaction between ligands and metal nanomaterials from the perspective of surface coordination chemistry,and found that surface ligands can effectively improve catalytic activity and selectivity in Pt and other systems[26,30,50,51]。

3 Electronic structure principle of structure-activity relationship

3.1 Principle of structure-activity relationship

In The field of material science,the physical and chemical properties of substances such as molecules and solids are essentially determined by the specific electronic structure of the system,and the electronic structure is affected by the aggregation state of atoms.the mathematical expression of this structure-activity relationship principle is a composite function:
F(x) = F[E(x)]
In the QSAR function,F is a Descriptor parameter describing a certain property,E is a Characteristic electronic attribute(CEA)directly determining the property,and X is various structural parameters expressing the characteristics of the atomic aggregation state,such as size,coordination configuration,defect state,ligand coverage,and the like[52]。 to reveal the mathematical essence of structure-activity relationship is To analyze the expression of the composite function by means of experimental exploration,computational simulation or theoretical modeling。
Experimental exploration and computational simulation often change a single structural variable to obtain a series of dependent data,and then explore and summarize the trend of performance changes,that is,various parameter effects,such as size effect,ligand effect,etc.the mathematical expression of the parametric effect is the derivative of the structure-activity relationship function:
d F d x = ∂ F ∂ E ⋅ ∂ E ∂ x
In the formula,dF/dx is the derivative form of the QSAR parameter effect,which describes the influence mechanism of the structural parameter change on the performance;;F/E expresses the relationship between performance and characteristic electronic structure attributes,which belongs to the structure-activity relationship mechanism described by physical laws;。 E/X is the mechanism by which the structural parameter regulates the characteristic electronic structure properties。
Equations(1)and(2)show that a profound analysis of the principle of structure-activity relationship requires the clarification of two key concepts,one is a suitable descriptor(F)that can describe the essential properties of the property,and the other is a Characteristic electronic structure property(E)that can directly determine the property.characteristic electronic structure attributes are the bridge between certain properties and structural parameters,and are the key parameters to analyze the nature of structure-activity relationship。

3.2 Electronic structure property

electronic structure is an energy state formed by the combination of atomic orbitals and electrons under a certain atomic aggregation configuration,and a certain atomic aggregation configuration produces a Specific electronic structure state.specific electronic structure States have different properties,based on which scientists have defined different concepts to describe the characteristics of electronic structure from different perspectives,such as density of States,energy level,oxidation state,Fermi level and work function[53]。 as shown in Fig.4,these attribute parameters reflect the characteristics of the same electronic structure state of the system at different angles,and they are the same as the thermodynamic functions,which are all state functions.Fundamentally speaking,the physical and chemical properties of atomic aggregation matter are determined by the overall state of the electronic structure,but the specific properties are often directly related to a certain property of the electronic structure,such as the wavelength of light absorption and emission is determined by the energy level difference in the electronic state,and the magnetism is determined by the density of unpaired single electrons in the system.the electronic structure property that directly determines a property is the characteristic electronic structure property of the property。
图4 电子结构状态的不同属性。电子结构是一个全局的整体状态，从不同的角度与层次看表现出不同的局部属性，要用不同的参数描述。每一属性往往是某种性能的直接决定机制

Fig. 4 Attributes of electronic states. Electronic structure is the global state for a matter system, but it contains many different attributes when considered at different aspects. An attribute usually determines a certain property and function performance

physics and chemistry have different ways of understanding electronic structure.In Physics,the electronic structure is understood from the perspective of the whole,the electronic structure state of the system is regarded as the whole,and the concepts of energy band and density of States are used to describe the electronic structure characteristics of the solid as a whole and to analyze the electronic structure mechanism of various structure-activity relationships[54,55]。 Chemistry focuses on understanding the structure-activity relationship between the electronic structure characteristics and reactivity of the system from the perspective of the electronic structure characteristics and bonding of local atoms[56⇓~58]。 oxidation state is the most commonly used concept to describe the characteristics of local electronic structure In chemistry.in the experimental study of surface chemistry,photoelectron spectroscopy can be used to detect the Oxidation state of surface atoms to explain the change of surface activity[59]。 Although the oxidation state is widely used,it is only a manifestation of the electronic structure state,which can not directly reveal the physical nature of surface activity,nor can it clarify the law and mechanism of ligand regulation of nanomaterials。

3.3 Quantum size effect

Understanding the overall characteristics of the electronic structure of nanomaterials is the starting point for analyzing the generation and interaction mechanism of all nanoeffects,while quantum size effect and quantum confinement effect are the basic theories for describing the changes of the electronic structure of nanomaterials caused by size reduction[29,60]。 As shown in Fig.5,with the evolution of the material system from atom to crystal,the electronic structure gradually evolves from discrete energy level States to band States with continuous energy level distribution,while the decrease of the reverse system size leads to the gradual discretization of the band density of States[53]。 Therefore,the primary electronic structural feature of nanomaterials is the discretization of the band density of States.Kubo,a Japanese theoretical physicist,proposed the famous Kubo formula in 1962:
δ N = 4 E F 3 N
WhereδNis the energy level spacing in the band of the metal nanocluster,EFis the Fermi level,and N is the number of atoms in the nanocluster.Based on this,the effect of size on the electronic energy level of metal clusters is analyzed,and the mechanism of metal clusters from conductor to semiconductor is explained[61]。
图5 电子结构随体系尺寸变大的演化趋势与尺寸减小引起固体能带的量子尺寸效应

Fig. 5 The evolution trend of electronic state with increased system size and quantum size effect of energy band levels with size reduction

the quantization of the band density of States is the core feature of the electronic structure of nanomaterials.Based on this,nanomaterials are regarded as a mesoscopic transition state between microscopic molecules and macroscopic solids,which has the characteristics of both[22,62,63]。 the quantum size effect can reduce the width of the band density of States and increase the band gap,thus increasing the energy of the absorbed and emitted photons.This mechanism is the theoretical basis for the size control of the luminescence energy of semiconductor quantum dots[64]。 However,how the quantum size effect generally enhances the surface activity of nanomaterials is still a key scientific issue that needs to be clarified in depth[9]。 There are two basic questions that need to be clarified:one is how to describe the surface chemistry,and the other is what are the characteristic electronic structure properties that directly determine the surface chemistry。

4 Chemisorption model based on orbital competition reconstruction mechanism

4.1 Chemisorption

Chemisorption is the common basis of all surface chemistry and the chemical form of interaction between adsorbed molecules such as ligands and nanomaterials[6,7,24]。 as a basic chemical interaction mode,there are two core issues to be concerned:one is the intensity of interaction,and the other is the result of interaction.the strength of interaction reflects the degree of host-guest coupling,which is reflected by the strength of chemisorption and the activity of catalytic reaction in surface chemistry,and can be described by parameters such as adsorption energy and reaction rate.the result of interaction is the change of the existing state of the subject and object,including the structural state and the motion state;the amount of state change(degree of change)is positively correlated with the intensity of action,and the same state change can be described in different ways from different perspectives,that is,different descriptors.For example,the chemisorption of molecules on a surface can cause changes in parameters such as bond length,bond energy,bond polarity,electron distribution,and oxidation state,each of which is the result of chemisorption.Identifying the best descriptor that can reveal the essential characteristics of a state change from many parameters contained in a state change is the key link in establishing a theoretical model。
the chemical nature of chemisorption is the surface coordination bond,which is based on the overlap between the frontier molecular orbitals of the adsorbed molecules and the valence atomic orbitals of the surface active center atom[57,65]。 the common physical essence of all chemical actions is to break old bonds and form new bonds,and to change the state of atomic aggregation in the process of breaking old bonds and establishing new ones.the premise of the formation of covalent bond and coordination bond is the effective overlap between orbitals,so the basic result of the change process of breaking the old bond and forming the new bond is the recombination of the coupling state of atomic orbitals,that is,the reconstruction of orbital distribution,which is the core physical link of chemical interaction[66]。 the direct result of chemisorption is the reconstruction of surface valence atomic orbitals,and the difficulty of orbital reconstruction reflects the strength of chemisorption and surface activity,so the key of theoretical model analysis is how to describe the orbital reconstruction in the process of surface adsorption。

4.2 Surface valence orbital competition reconstruction

Solid surface atoms have residual bonding ability due to coordination unsaturation,and their valence atomic orbitals can overlap with the frontier molecular orbitals of adsorbed molecules to form surface coordination bonds.As shown in Fig.6a,the distribution States of surface atomic valence orbitals can be divided into two categories.Some of them overlap with the valence orbitals of other adjacent atoms in the bulk of nanomaterials and participate in the formation of bulk band electronic States.Distribution fraction can be expressed by fB.fBrepresents the intrinsic bonding strength between surface atoms and bulk atoms,which reflects the stability of surface atoms of nanomaterials.As shown in Fig.6 B,the other part is localized into the surface coordination bond formed with the adsorbed molecule,and the distribution coefficient is denoted by fS.The fSreflects the strength of chemisorption and the activity of surface atoms.The physical essence of fBand fSis the mode of wave function(electronic orbital),which means the distribution coefficient of surface atomic valence orbital in different electronic States in quantum chemistry,that is,the contribution rate to different electronic States.According to the normalization principle of wave function in quantum mechanics theory,the distribution coefficients of surface valence atomic orbitals in two electronic States satisfy:
fB + fS = 1
Since fBand fSare the square of the wave function,the range of their values is 0≤fB,fS≤1.∆fS=-∆fBcan be obtained by combining equation(4),so the values of fBand fSare in a competitive relationship with each other,which is antagonistic.At the same time,the fBand the fSare unified because they describe the distribution of the same surface valence atomic orbital between the bulk band state and the surface adsorption state.The normalization principle of wave function is the physical origin of the dialectical relationship of unity of opposites between surface activity and stability of nanomaterials,which reveals why the reduction of size increases the surface activity of nanomaterials while reducing the overall stability[52,67]。
图6 基于表面价原子轨道(SVAO)竞争重构机制的化学吸附模型：(a)表面配位键的形成将活性中心原子的价轨道束缚在表面态，减小其往体相拓展参与能带的形成[50]。(b)化学吸附前后体系电子态变化示意图。左图为未发生相互作用的吸附质前线轨道及表面的能带电子态，此时SVAO主要往体相拓展参与能带的形成；右图为化学吸附后体系的组合电子态，一部分SVAO (fS)与吸附质的前线轨道重叠形成表面吸附键的成键态，另一部分SVAO (fB)与表面中周围其他原子的价轨道重叠并在晶格内拓展参与能带的形成

Fig. 6 Chemisorption model based the competitive redistribution of surface valence atomic orbitals (SVAO). (a) The formation of surface coordination bond confines the valence orbitals of active center atoms into surface chemisorption states, reducing their extension into the bulk phase to participate in the formation of energy bands[50]. (b) Scheme showing the changes in electronic states before and after chemisorption. The left diagram illustrates the electronic states of the adsorbate's frontier orbitals and the surface energy bands without chemisorption. The right diagram shows the combined electronic states after chemisorption, in which a portion of SVAO (fS) overlaps with adsorbate's frontier orbitals to form the bonding states of chemisorption bonds, while other portion of SVAO (fB) overlaps with the valence orbitals of surrounding atoms in the surface phase to participate in the formation of energy bands

The core physical mechanism of surface chemistry is the competitive reconstruction of surface valence atomic orbitals between bulk band States and surface adsorption States.The more the surface valence orbital overlaps with the bulk atomic orbital(fB),the more tightly it is bound and the more stable the surface is;The stronger the overlapping tendency with the frontier molecular orbital of the adsorbed molecule(fS),the higher the surface chemical activity.Therefore,fSrepresents the distribution coefficient of surface valence atomic orbital in the surface bonding state,which is an electronic structure scale descriptor to describe the surface chemical activity(chemisorption strength),and has a clear physical meaning in quantum chemistry,and its value can be obtained by theoretical calculation.The strength of the surface valence orbital bound by bulk atoms and adsorbed molecules directly determines the difficulty of orbital reconstruction,which is a basic physical quantity that directly determines the reactivity of surface sites 。

4.3 Orbital potential concept

chemisorption is a steady state.For the Chemisorption state formed by the definite adsorbed molecules and the solid surface,there is a definite proportion between the distribution of the surface valence atomic orbitals in the bulk band state and the surface adsorption state.According to the steady-state model of chemical kinetics treatment of adsorption,the distribution ratio is determined by the ability of the adsorbed molecule to compete with the surface valence atomic orbital in the surface coordination bond with the solid surface as a whole,which can be expressed as:
f S f B = G S G B
In the formula,GSand GBrespectively represent the ability of the adsorbed molecule to compete with the surface valence Orbital in the adsorption bonding state,that is,the ability to bind the surface valence orbital,which is defined as the Orbital potential(G )[52]。 The orbital potential is a characteristic electronic structure attribute that determines the proportion of orbitals contributed by an atom in the bonding state,and its size is related to the specific chemical environment of the active center atom,that is,the strength of chemisorption on a surface atom is affected by its specific coordination environment,such as the type of coordination atom,coordination number,coordination configuration,defect state and other structural parameters.For a surface atom,the GBreflects the ability of the band electronic States of the surface phase as a whole to bind its surface valence orbital at the atomic site,and the larger the GBis,the more firmly the surface valence orbital is bound by the surface phase,and the more tightly the surface atom is bonded,resulting in a higher stability of the surface.Broadly speaking,orbital potential is a characteristic electronic structure attribute reflecting the binding orbital,competitive orbital and rebonding ability of bonding atoms in a certain chemical environment.In the extreme case,when both parties involved in the bonding interaction degenerate into isolated single atoms,the meaning of the orbital potential is the electronegativity proposed by Pauling,so the electronegativity is a special case of the concept of the orbital potential 。

4.4 Principle of structure-activity relationship of surface chemical activity

Combining equations(4)and(5)yields:
f S = G S G S + G B
The formula shows that the strength of chemisorption is determined by the relative magnitude of GSand GB,and the strength of chemisorption can be enhanced by increasing the value of GS/GBby increasing GSor decreasing GB.For a given adsorbed molecule and material,the GSis a constant,and the strength of the surface adsorption is completely determined by the GB.Therefore,for the surface chemistry of nanomaterials,the GBis the bridge between all the structural parameters such as size,coordination atom,coordination number and defect and the surface chemical activity,and the structure-activity relationship of the surface chemical activity of nanomaterials can be expressed as:
f S ( x ) = f S [ G B ( x ) ]
Then there are:
d f S d x = ∂ f S ∂ G B ⋅ ∂ G B ∂ x
GBis a characteristic electronic structure attribute that directly determines the reactivity of surface sites of nanomaterials,and the essence of revealing various structure-activity relationships and parametric effect mechanisms in nanoscale surface chemistry is to analyze the GB=GB(x).For example,size reduction can generally enhance the surface activity of nanomaterials,and its essence is to reveal how size reduction reduces the GB,that is,how nanomaterials as a whole reduce the binding strength of surface atomic valence orbitals 。

5 Electronic structure mechanism of size effect

5.1 The meaning of surface activity

In interaction,the essential meaning of activity is the ease of state change,while stability is the ability of the system to resist change.For different state changes,activity and stability have different specific meanings.the surface activity and stability of nanomaterials can be divided into three categories according to the way of surface structure change.the first type of change involves the reconstruction of surface atomic configuration,including phase transformation,ripening,agglomeration,sintering,etc.the meaning of this type of activity is the difficulty of surface reconstruction caused by thermal motion of surface atoms,and the description parameter is the change temperature[2,3,68,69]。 the second type of change involves The adsorption of molecules and ions on surface sites,including adsorption,surface coordination,ligand exchange,etc.The meaning of this type of activity is the strength of surface adsorption,which is often described by parameters such as adsorption energy[7,45,46]。 the third type of change involves chemical reactions between adsorbed molecules on the surface,including heterogeneous catalysis,electrochemistry,and photocatalysis.The nature of this type of activity is the rate of surface reaction processes,and the intrinsic catalytic activity is measured by Turnover frequency(TOF)in catalytic studies[10]。 Reducing the size can generally enhance the three types of surface activity of nanomaterials,and there must be an inevitable law behind this common trend,which has nothing to do with the specific composition and phase of the material.How to understand the physical and chemical nature of the origin of this size effect from the level of electronic structure is one of the core scientific issues in the field of nanoscience。

5.2 Mathematical model of surface chemical activity

Equation(7)can be expanded as:
f S = f S 0 + ∑ ∫ f S f B G ˙ S G S − G ˙ B G B d x i
In the formula,the f S 0 represents the adsorption strength of the adsorbed molecule on the perfect single crystal surface,reflects the intrinsic interaction strength between the frontier molecular orbital of the adsorbed molecule and the surface atomic valence orbital,and is related to factors such as orbital symmetry,orbital energy level,orbital binding strength and the like;Derivative G ˙ S =∂GS/∂xiand G ˙ B =∂GB/∂xiRepresents the effect of structural parameter changes on the GSand the GB.Definition:
X = G ˙ S G S − G ˙ B G B
Then dfS=fSfBXdxi.Since 0≤fB,fS≤1,the influence trend of any structural parameter xion the surface activity depends on the sign of X,and the overall trend is that positive G ˙ S or negative G ˙ B can enhance the surface chemical activity。
Since GSand GBrepresent the ability of the adsorbed molecule and the surface site to bind the valence atomic orbital of the surface atom in the surface coordination bond,respectively,and describe independent objects,they are orthogonal.For a certain adsorbed molecule,its GSis constant on a certain surface,that is,the change of surface structural parameters has no effect on the GS,so G ˙ S =0.Then X becomes:
X = − G ˙ B G B
Equation(11)expresses the mechanism by which the surface chemical activity is regulated by the change of surface phase structure parameters.For nanomaterials,the nature of surface chemical activity controlled by structural parameters such as size,composition,coordination atom and coordination configuration of active center,defect and surface adsorption state is to change the binding strength of nanomaterials to the valence orbital of surface atoms,that is,the size of GB.If increasing a parameter,such as size and coordination number,can increase the binding strength of nanomaterials to surface valence orbitals,that is , G ˙ B >0,then X<0,which will reduce the surface chemical activity.On the contrary,if the parameter is reduced,the surface chemical activity can be improved。

5.3 Two mechanisms of surface activity enhancement by size reduction

The GBdescribes the electronic structure property at a specific surface site,which is related to both the overall structure of the nanomaterial and the local coordination environment at the surface site,and thus varies with the chemical environment of the surface site.To understand the structure-activity relationship(SAR)between structural parameters and surface activity and the physical and chemical nature of structural effect is to reveal the functional correlation mechanism with GB,so the analysis of GB=GB(x)is the key to analyze the SAR mechanism of surface chemical interaction[52]。 Equation(6)can be rewritten as:
f S = f S 0 f S 0 + G B f B 0 G B m
Where GBmis the value of GBon the surface of a perfect single crystal and is the basic value of GB.For the origin of the size effect,its physical essence lies in revealing the GB=GB(r).Based on the theoretical exploration of the surface thermodynamic properties of nanomaterials,for spherical nanoparticles of non-layered structural materials,the GB(r)can be approximately expressed as:
G B = G B m 1 − k d 0 r
In this formula,d0is the atomic size,and K is a parameter related to the structure.From equations(12)and(13)it follows that:
f S = f S 0 1 − k f B 0 d 0 r
X = − G ˙ B G B = − G ˙ B G B m 1 − k d 0 r
Equations(14)and(15)show that there are two mechanisms for the enhancement of the surface activity of nanomaterials by size reduction.As shown in Fig.7,the first is to weaken the binding strength of the solid band States to the valence orbitals of the surface atoms and increase the tendency and strength of their reconstruction to form surface coordination bonds(Equations 12~14).The weak binding effect of nanomaterials on the surface atoms and valence orbitals is the primary electronic structure mechanism of the origin of the surface active size effect phenomenon.The other is that the weak-binding effect can reduce the orbital potential GBof the surface,thus magnifying the extent to which defects and other structural parameters such as coordination number can change the surface activity(Equations 11,13,15).The amplification effect makes the effect of defects and coordination number on the surface chemical activity of nanomaterials increase with the decrease of size,that is,the effect of the same defect state on the surface chemical activity of nanomaterials is more significant.Weak-binding effect and amplification effect are the two electronic structure mechanisms for the enhancement of surface activity of nanomaterials by size reduction,in which the weak-binding effect on surface atoms and their valence orbitals is the primary reason[52]。 For all kinds of nanomaterials,the key and breakthrough to deeply analyze the structure-activity relationship of surface activity is to explore the expression form of GB=GB(x )。
图7 纳米材料尺寸相关表面活性的电子结构原理。尺寸减小削弱了纳米材料整体对表面价原子轨道的束缚强度[50]

Fig. 7 The electronic principle of size-dependent surface reactivity of nanomaterials. The basic principle lies in reduced confinement degree of surface valence atomic orbitals by nanostructures [50]

6 Nanoscale cooperative chemisorption model.

6.1 The relationship between the energy band character and the surface activity

surface ligands can effectively control the electronic structure and properties of nanomaterials,and the degree of change increases with the decrease of nanomaterials size and the increase of ligand coverage.the basic scientific questions behind this common trend are:(1)How surface ligands regulate the electronic structure and properties of nanomaterials;(2)How to establish a theoretical model at the level of electronic structure to reveal the mechanism and law of this effect;(3)How to understand the mechanism of the size and specific surface area of nanomaterials,as well as the type and coverage of ligands in such surface effects.to reveal the mechanism and law of surface ligand regulating the properties of nanomaterials,the key is to clarify the electronic structure characteristics of nanomaterials before and after ligand adsorption,and to refine the basic principle of action by comparison。
the basic feature of the electronic structure of inorganic nanomaterials lies in the band electronic States of their bulk phases.the formation of solid energy bands is based on the overlapping interaction of valence orbitals between adjacent atoms,that is,the local chemical bonding interaction,and the expansion of this local overlapping interaction in the lattice forms energy bands[53]。 the density of States is the key concept to describe the characteristics of the energy band,which reflects the range of its energy distribution and the number of States.the energy distribution range,that is,the width of the energy band,is related to the closeness of the local orbital overlap.the larger the overlap of the valence orbitals of adjacent atoms,that is,the stronger the covalency,the larger the width of the energy band formed,and the more tightly each orbital is bound by the energy band state;On the contrary,if the overlap between valence orbitals is weak,a band with a narrow energy range is formed,in which each orbital is bound by the band state more weakly,thus showing a greater degree of localization and a stronger tendency to reconstruct to participate in the new chemical interaction mode[53]。 Therefore,the binding strength of the surface valence orbital to the band States,that is,the magnitude of the GB,is essentially positively correlated with the strength of the local chemical bonds and the width of the global band 。

6.2 Nanoscale cooperative chemisorption model.

When there is no surface ligand adsorption,the nanomaterial exhibits its intrinsic electronic state.As shown in Fig.8 a-i,the valence orbitals of atoms on the surface of nanomaterials preferentially overlap with the valence orbitals of other atoms in the lattice,participating in the formation of band States.At this time,the valence orbital of the surface atom has the largest tendency to expand into the interior of the lattice,showing the largest inward overlap,that is,the largest fB,and the largest band width in the density of States 。
图8 纳米尺度协同化学吸附过程(NCC)的电子结构变化机制与作用结果：(a)表面价原子轨道随配体覆盖度变化的分布趋势与协同重构机制。(b)表面配体调控纳米材料能带电子态与物理化学性质的物理化学图像与机制[65]

Fig. 8 The mechanism of nanoscale cooperative chemisorption (NCC) in inducing changes in the electronic states of nanomaterials. (a) Redistribution trends of surface valence atomic orbitals depending on the coverage of surface ligands on nanomaterials. (b) The physical and chemical mechanism depicting how surface ligands regulate the nanomaterial's energy band states and physical and chemical properties[65]

When a ligand molecule adsorbs on the surface of nanomaterials,its frontier molecular orbital overlaps with the valence orbital of the surface atom to form a surface coordination bond,which leads to the partial reconstruction of the surface valence orbital extended into the band state to the surface adsorption state(I→ii),that is,the fBdecreases and the fSincreases.The basic consequence of this surface-competing orbital reconstruction is that reducing the number of orbitals that overlap to form a band in the interior of the lattice leads to a reduction of the total number of States in the band.When the ligand coverage increases,the valence orbitals of more surface atoms are reconstructed into the adsorption bonds,which further reduces the total number of orbitals and the orbital overlap in the bulk band States.This process will weaken the correlation strength between the atomic orbitals in the band and reduce the strength of the bound surface valence orbital(GB),thus further improving the surface activity.The degree of surface valence orbital reconstruction is positively correlated with the ligand coverage,and the greater the coverage,the greater the degree of band state weakening.When the adsorption saturation is reached,the surface valence orbital is reconstructed to the surface localized state to the maximum extent,while the orbital correlation in the band state is the lowest,forming the band state with the narrowest energy distribution,and the energy level in the band is discretized to the maximum extent(Fig.8 a-ii→iii→iv )。
the overall mechanism of adjusting the electronic structure of nanomaterials by increasing the ligand coverage is to reconstruct the valence orbitals of more surface atoms into the surface adsorption bonds,weaken the coupling strength between the orbitals and other lattice atoms,reduce the density of electronic States in the bulk band States,and narrow the energy distribution of the band.the narrower the energy band is,the weaker the coupling strength between orbitals is,which can further improve the reconstruction ability of surface valence orbitals and obtain stronger rebonding activity.the coverage-dependent adsorption mechanism on the surface of nanomaterials is a positive feedback effect,and the trend is that the larger the ligand coverage is,the stronger the average adsorption is,so the saturation adsorption with the maximum adsorption strength will be achieved spontaneously.the trend of stronger adsorption with the increase of ligand coverage indicates that there is a synergistic effect between ligand molecules,so the chemical interaction mode related to ligand coverage on the surface of nanomaterials is called Nanoscale Cooperative Chemisorption(NCC)[67]。
the nanoscale cooperative chemisorption model describes the electronic structure hierarchy mechanism of ligand coverage regulating the intensity of nanosurface chemical interaction and the trend of surface effect,and the basic principle is that the surface ligand can effectively weaken the coupling strength between orbitals in the energy band of nanomaterials through the surface orbital reconstruction mechanism[67]。 the consequences of this mode of interaction include The following two aspects。
(1)For nanomaterials,the valence orbitals of surface atoms are partially reconstructed and localized to the surface adsorption bonds,which reduces the number of atomic orbitals combined to form bulk band electronic States,thus weakening the extended overlap and correlation strength between band constituent orbitals.As shown in Fig.8B,This weakened local orbital interaction narrows the energy distribution range of the band States from the root,thereby moving the energy level positions of the upper and lower band edges of the band,and further quantizing the energy levels of the orbitals in the band,thus improving the adsorption activity of the surface atoms.the change trend of band electronic structure of nanomaterials caused by ligand molecules increases with the increase of coverage,which fundamentally changes the various physical and chemical properties of nanomaterials.this mechanism is the common electronic structure principle of ligand-induced nanosurface effect。
(2)For the adsorbed molecules,the strength of chemical adsorption increases with the increase of coverage,forming a positive feedback effect of stronger adsorption and more stable adsorption.This mechanism leads to the fact that once the surface of nanomaterials is adsorbed by ligands,it will spontaneously achieve the most stable saturation adsorption,thus clarifying why the surface of nanomaterials always tends to be saturated by ligands,and why the surface ligands of metal clusters,quantum dots and sub-nanostructured materials synthesized in liquid phase are difficult to remove.the greater the coverage,the stronger the adsorption,which also means that the adsorbed molecules can be activated more effectively。

6.3 Interaction mechanism of R, S/V and θ

the competitive reconstruction of surface valence orbitals is the basic physicochemical mechanism of nanosurface chemical action and nanosurface effect,and the difficulty of orbital reconstruction depends on the ability of adsorbed molecules and nanomaterials to bind surface valence orbitals.the ability of atoms on the surface of nanomaterials to bind valence orbitals,that is,the size of orbital potential,is positively correlated with the coupling strength between atomic orbitals participating in the combination of energy bands.the general trend is that the greater the degree of orbital overlap coupling between adjacent atoms and the greater the number of atomic orbitals in the extended system,the wider the energy range of the formed band and the lower the overall energy level,so that each orbital is more tightly bound,resulting in a weaker reconstruction trend.on the contrary,the fewer the number of orbitals participating in the formation of energy bands,the weaker the coupling between orbitals,the narrower the energy bands,the higher the energy levels,and the more localized the orbitals,so they can show stronger reconstruction tendency and surface activity.Therefore,the factors affecting the activity of nano-surface are the number of States in the energy band of nano-materials,that is,the number of atoms in nano-materials,and the number of adsorbed molecules on the surface,that is,the coverage。
the smaller the size of nanomaterials,the less the number of orbitals involved in the formation of energy bands,resulting in the weaker the binding strength of surface valence orbitals,thus showing higher surface activity.this mechanism means that there is also a synergistic effect between valence orbitals in nanomaterials,and the more orbitals are coupled,the tighter each orbital is bound.Therefore,there are two groups of synergistic effects in nanosurface chemistry,the synergistic effect between ligands and the synergistic effect between atoms that make up nanomaterials.in This dual synergistic mode,the synergistic effect between ligands reconstructs the electronic state of the nanomaterial,while the synergistic effect within the nanomaterial system resists the change of the electronic state to maintain stability.the size(R or V)expresses the number of valence orbitals in the nanomaterial system,which reflects the resistance to the change of electronic state,that is,the larger the size,the tighter the valence orbitals are bound,and the higher the overall stability.the specific surface area(S/V)expresses the maximum number of ligands that can be accommodated,and the larger the S/V is,the greater the potential of synergistic effect between ligands is,and the higher the degree of regulating the electronic state of nanomaterials is.the ligand coverage(θ)expresses the dynamic regulation effect,which changes the actual number of ligands that can be regulated byθ,thus changing the degree of synergistic effect between ligands and the actual effect of changing the electronic state of nanomaterials。

6.4 Two-electron state competition model

A comprehensive understanding of the electronic structure of nanomaterials is the basis of analyzing the properties and interactions of nanomaterials,and is also the foundation of the whole nanoscience.For semiconductor nanomaterials such as quantum dots,the electronic state is mostly understood from the perspective of the energy level distribution of the density of States,and the surface adsorption state is regarded as a defect state.By comparing the position relationship between its energy level and the conduction band and valence band,the properties of its electron-donating or electron-accepting are analyzed.Research In the field of quantum dots suggests that surface ligands are passivators of surface defect States,which can inhibit the capture of photogenerated excitons and improve the quantum yield of fluorescence or the efficiency of photoelectric conversion devices of perovskite quantum dot materials.in contrast,the current understanding of the electronic structure of metal nanoclusters is not sufficient,and most of them are considered to be the transition state between atomic and crystalline States on the mesoscopic scale,which contains special physical mechanisms to be explored。
ligand-adsorbed inorganic nanomaterials include two structural components:inorganic phase core and surface ligand,and the interaction form of surface coordination bond.Therefore,the overall electronic structure characteristics of nanostructures can be divided into three parts:the first is the band electronic States of the inorganic core,the second is the molecular orbitals of the ligand,and the third is the surface coordination bonds formed between the ligand and the surface atoms.These three electronic States are unified In the whole nanostructure and together constitute the electronic structure state of the whole nanomaterial;However,the three States represent different attributes and are independent of each other.in a specific property,one of the electronic States often plays a dominant role,for example,the luminescence of quantum dots is the embodiment of the properties of band States,while the luminescence of Au nanoclusters is the embodiment of the properties of surface coordination bonds。
the nano-surface effect is mainly the regulation of ligands on the properties of nanomaterials,which is rooted in the change of band electronic States of nanomaterials through surface coordination bonds,so it mainly involves the interaction between band States and coordination bonds.the band States and the coordination bonds are coupled through the valence orbitals of the surface atoms,and they compete due to the normalization of the wave function.These two types of electronic States are spatially separated,located at the lattice of the inorganic core and the interface,respectively,and are therefore independent of each other。
the surface adsorption bond energy partially reconstructs the valence orbitals of the surface atoms from the band States to the surface States,which directly weakens the coupling strength between the surface valence orbitals and other valence orbitals of the neighboring lattice atoms.As shown in Fig.9a,during saturated adsorption,this competitive orbital reconstruction can greatly weaken the bonding strength between the topmost atoms and the subsurface atoms of the nanomaterial,forming a"core-shell structure"at the electronic structure level[62,70]。 As shown in Fig.9b,in this core-shell configuration,the core layer is composed of the subsurface and inner atoms of the nanomaterial,and the shell layer is composed of the surface coordination layer formed by the outermost atoms and ligands.the existence of the coordination sphere weakens the orbital overlap between the surface atoms and the subsurface atoms,resulting in weak orbital overlap,and then forms weak coupling surface adsorption States and band States.the degree of polarization separation between surface adsorption States and band States increases with the increase of coverage,which affects the physical and chemical properties of nanomaterials.At the atomic level,this weakened bonding makes the configuration of the outermost atoms deviate from the internal lattice structure,resulting in a certain degree of reconstruction。
图9 轨道竞争重构产生的轨道分布层面的纳米材料“核壳”结构图示：(a) 轨道竞争重构与纳米尺度协同吸附机制将表面价原子轨道由能带态重构到表面吸附态，形成由最表层原子与配体组成的表面配位层和内部体相原子组成的内核。表层原子与内核原子间的轨道耦合作用减弱。(b) 球形纳米颗粒的轨道分布“核壳”结构图示

Fig. 9 Schematic diagrams of orbital-level 'core-shell' structure of nanomaterials. (a) The polarization of surface valence atomic orbitals from band states to surface chemisorption state caused by competitive orbital redistribution and nanoscale cooperative chemisorption. This results in the formation of surface complex shell composed of outermost surface atoms and ligands, and the inner core composed of bulk atoms. The orbital couplings between surface atoms and core atoms are weakened. (b) Schematic 'core-shell' structure of spherical nanoparticles in orbital distribution

6.5 Principles of Ligand Modulation of Optical Properties of Nanomaterials

the absorption and emission in the ultraviolet and visible regions are the main optical properties to detect the ligand effect on the surface of nanomaterials[71]。 optical absorption is the superposition of absorption signals of three electronic States,while fluorescence properties have been widely studied in quantum dots,metal nanoclusters and rare earth nanomaterials.the fluorescence mechanism of nanomaterials includes three modes:one is the luminescence from the bulk band electronic state of the semiconductor,when the electrons excited from the valence band to the conduction band are deexcited back to the valence band;the other is the surface state luminescence from the surface adsorption bonds,such as the fluorescence of Au,Ag and Cu metal clusters coordinated on the surface of thiols and other molecules,where the transition of electrons between ligands and metal atoms leads to Optical absorption and emission[72]; the third is the light absorption and emission caused by the transition of electrons between different energy levels of doped atoms,such as rare earth ion-doped nanomaterials[73]。 the quantum yields of these three fluorescence modes are controlled by the type and coverage of the surface ligand atoms,and the general trend is that both reducing the particle size and increasing the ligand coverage can improve the quantum yield and brightness,while increasing the ligand coverage can also increase the fluorescence energy of Au and other metal nanoclusters[16,18][21]。 the nano-scale cooperative chemisorption model can explain The law and principle of these trends。
the surface adsorption state fluorescence of nanomaterials originates from the transition of electrons between the bonding state and the antibonding state of the surface coordination bond.Typical examples are metal nanoclusters such as Au coordinated by thiol molecules and oxide nanoparticles such as ZnO coordinated by amine molecules[72,74]。 In these systems,the surface metal site provides an empty surface valence orbital to form a surface coordination bond with the L-type ligand,and the bonding state is mainly composed of the frontier orbital of the Ligand molecule,while the antibonding state is mainly composed of the empty orbital of the metal site(as shown In fig.10).In the process of light absorption,the electron jumps from the ligand to the empty orbital of the metal site through the surface coordination bond,that is,ligand to metal electron transfer(LMET),and the electron fluoresces when it returns to the ligand from the empty orbital of the metal[14,39]。 Molecular orbital theory shows that the greater the degree of overlap between orbitals,the greater the energy difference between the bonding state and the antibonding state,which leads to the greater the energy of the absorbed and emitted photons.the nanoscale cooperative chemisorption model shows that the smaller the nanoparticle size and the larger the ligand coverage,the greater the degree of orbital overlap in the surface coordination bond,which leads to the greater the energy difference between the surface bonding state and the antibonding state,thus increasing the energy of the surface luminescence,that is,the shorter the wavelength。
图10 纳米材料的表面态荧光与体相能带态荧光图示。表面态光吸收与发射源于电子在表面吸附键的成键态和反键态间的跃迁，体相态光吸收与发射源于电子在能带态的价带和导带间的跃迁。表面配位层与内核层间的弱轨道耦合效应抑制了光生激子的扩散与捕获，从而提高两种发光过程的量子产率

Fig. 10 Photoluminescence modes of ligand-capped nanomaterials resulting from surface chemisorption states and bulk energy band states. Optical absorption and emission of surface states result from the transitions of electrons between the bonding and antibonding states of surface bonds. The absorption and emission of band states arise from the transitions of electrons between the valence band and the conduction band. The weakened orbital couplings at the interfaces of surface complex shells and inner cores can suppress the bidirectional diffusions and capture of photogenerated excitons, thereby enhancing the quantum yields of both luminescent processes

Photoluminescence quantum yield(PLQY),which is related to the dissipation of excited electrons,is used to express the photon utilization rate of fluorescence process.If the delocalization of the excited state electron is strong,it can quickly diffuse to other regions in the system,which leads to the effective separation of electron-hole pairs and reduces the probability of the excited state electron returning to the ground state to emit fluorescence,thus reducing the quantum yield of fluorescence.Whether band state fluorescence or surface state fluorescence,the key to improve the quantum yield is to tightly bind(or localize)the excited state electrons at the excitation site,inhibit their diffusion and migration,and thus improve the probability of in situ recombination fluorescence emission。
the surface coordination bond reconstructs the valence orbital of the surface atom from the band state to the surface state,which weakens the strength of the orbital coupling between the inner core layer and the coordination layer.the polarization separation of the two electronic states formed by saturated adsorption can effectively block the electron transfer channel between the inner core layer and the coordination layer,thus inhibiting the migration of excited electrons between the two States and improving the efficiency of the two fluorescence modes.For the surface state fluorescence of metal nanoclusters such as Au,the electrons in the ligand are excited and then jump to the unoccupied orbitals of the surface metal atoms.When the unoccupied orbitals are effectively polarized to the surface state,the barrier For the excited electrons to migrate to the inner core layer atoms increases,thus increasing the probability and quantum yield of returning to the ligand orbitals to emit fluorescence.For the band state luminescence of semiconductor quantum dots,excitons are generated in the inner core layer,while the empty orbitals of surface atoms can be used as Trap States to capture excited electrons,which inhibits the probability of falling back to the valence band state to emit fluorescence[75]。 the weak orbital overlap between the inner core layer and the coordination layer can also inhibit the migration of excited electrons to the unoccupied orbitals of the surface atoms,thereby reducing the probability of exciton capture by the surface States and improving the fluorescence efficiency.Therefore,the weak orbital coupling effect between the core layer and the coordination layer of nanomaterials can simultaneously improve the quantum yield of the surface state fluorescence of metal clusters and the band state fluorescence of semiconductor quantum dots.the nanoscale cooperative chemisorption model predicts that the enhancement effect increases with the decrease of particle size and the increase of ligand coverage,which is also consistent with many previous experimental results,thus proving the rationality of the NCC model。

7 Comparison of typical adsorption models.

in the field of material chemistry,adsorption refers to the heterogeneous interaction mode of adsorbate on solid surface,which can be divided into physical adsorption and chemical adsorption.Revealing the law and principle of adsorption is the core scientific goal of physical chemistry,surface science,catalytic chemistry,electrochemistry and many other fields.Since van Bemmelen proposed the Freundlich adsorption isotherm formula In 1888,scientists have continued to explore the theoretical model and mathematical description of adsorption phenomena,and developed a series of adsorption models[57,58,76]。 adsorption models are generally divided into two categories.One is The adsorption isotherm model which focuses on describing the relationship between adsorption capacity and adsorption conditions,and these models do not discuss the specific mechanism of adsorption;the other model focuses on the physical and chemical mechanism of adsorption,and analyzes the trend and law of adsorption,especially the structure-activity relationship between the strength of chemical adsorption and the structure of adsorption system.the representative isotherm models are Freundlich model,Langmuir model and BET model[76]; At present,the chemisorption mechanism is mainly studied by theoretical calculation,and the representative methods are Newns-Anderson model and d-band center model。

7.1 Adsorption isotherm model

the Freundlich isotherm model was proposed by van Bemmelen in 1888 and Boedecker in 1895,and was developed by Freundlich in 1907.This model is mostly used to fit the relationship between the adsorption capacity of adsorbate on the solid surface and its concentration,but due to the lack of a clear model and theoretical basis,it is only an empirical fitting model,and its parameters have no practical and specific physical meaning[76]。
The Langmuir isotherm model was established by Irving Langmuir,an American chemist,in three papers published from 1916 to 1918[77⇓~79]。 the model assumes that the solid surface structure is uniform,the adsorption capacity is the same everywhere on the surface,and the heat of adsorption does not change with the coverage.In addition,it assumes that the molecules adsorbed on the surface do not interact before and after adsorption,and the adsorption rate is proportional to the number of vacancies on the surface.Langmuir model can successfully describe the trend of monomolecular adsorption and chemisorption of real gases at low pressure,and has wide applicability,which is the basis for the development of subsequent adsorption isotherm models[80]。
The BET isotherm model was proposed by Brunauer,Emmett and Teller in 1938 to describe multilayer physical adsorption[81]。 Based on the Langmuir monolayer adsorption model,the model considers multilayer adsorption and introduces the interaction between molecules in different adsorption layers,which can better describe the adsorption behavior of gas on the surface of porous materials.Based on the BET model,the specific surface area of solid materials can be estimated by measuring the adsorption isotherms of N2and other molecules at different pressures 。

7.2 Electronic structure model of chemisorption

the Newns-Anderson model was proposed by Newns in 1969 on the basis of the Anderson localized state model of metals[54,55]。 This model theoretically describes the electron exchange process between the adsorbed molecule and the metal surface,especially the interaction between the localized States on the metal surface and the adsorbed molecular orbital.the Newns-Anderson model is helpful to understand the effect of the interaction between localized States and molecular orbitals on the adsorption behavior,and can also be used to perform electronic structure calculations.In the study of localized States and molecular adsorption on metal surfaces,information about the electronic structure of the adsorption system can be obtained[82]。
the d-band center model was gradually established by Nørskov et al.in the 1970s on the basis of the Newns-Anderson model[65,83,84]。 the model is based on solid state physics and electronic structure theory.the state of d-band on the surface of transition metal is calculated by density functional method.the energy level position and energy band width of d-electron orbital are analyzed,and the adsorption properties and catalytic activity on the surface of transition metal are explained.In this model,the width and central level position of the d-band reflect the electronic density of States of the metal surface and determine the adsorption strength of molecules on the metal surface,which has been widely used to explain the activity and selectivity of transition metal catalysts[85]。

7.3 Chemisorption model of nanomaterial system

the core characteristics of chemisorption on the surface of nanomaterials are size effect and ligand coverage effect.the theoretical models for analyzing these structure-activity relationship mechanisms are not perfect,and new concepts and mechanisms need to be explored extensively and deeply.The nanoscale cooperative chemisorption(NCC)model proposed by The authors based on the competitive reconstruction mechanism of surface valence orbitals reveals a universal principle for the regulation of band electronic structure and physical and chemical properties of nanomaterials by ligands and coverage[52,67]。 the model reveals the physical picture of nanoscale molecule-surface chemical interaction from the electronic structure level,and provides a new perspective for in-depth analysis of the mechanism of nanoscale surface chemistry and surface effects of nanomaterials.It is suitable for analyzing all systems involving molecule-nanomaterial surface chemical interaction,and expands the theoretical system of surface physical chemistry at the nanoscale.However,the establishment of comprehensive and profound electronic structure theoretical models in the field of nano-surface chemistry is still in its infancy and is still full of challenges and opportunities。

8 Conclusion and prospect

the surface chemistry of nanomaterials is the basis for the preparation,modification,performance and application exploration of nanomaterials,and plays a central role in the whole nanoscience system.Therefore,revealing the basic principles of the interaction between molecules and nanomaterials from the electronic structure level is the fundamental scientific goal of interdisciplinary integration.in the past 40 years,researchers have extensively explored the material system,phenomena,properties and applications,action trends and structure-activity relationships in the field of nanosurface chemistry through experimental exploration and computational simulation,but the theoretical cognitive system is not perfect,systematic and profound enough.based on the competitive reconstruction mechanism of surface valence orbitals,this paper introduces a chemisorption model in which the surface States compete with the band States,and Based on the orbital potential,which directly determines the characteristic electronic structure attribute of the surface site reactivity.A mathematical model for analyzing the structure-activity relationship of surface chemical activity was derived,and two mechanisms for the enhancement of surface chemical activity of nanomaterials by size reduction were revealed.the nanoscale cooperative chemisorption model with ligand coverage as the main variable is further systematically expounded,and the common trend and physicochemical mechanism of ligand and coverage regulating the electronic structure state and properties of nanomaterials are revealed;the model answers the physical origin that the smaller the size of nanomaterials,the higher the surface activity,endows the size,specific surface area and ligand coverage with specific physical meaning in surface chemistry,and can unify the physical and chemical principles of surface ligand regulating the fluorescence trend of nanomaterials in surface state and band state。
the surface valence orbital competition reconstruction mechanism and the nanoscale cooperative chemisorption model preliminarily reveal the electronic structure level mechanism and physical and chemical principles of the surface chemisorption of nanomaterials,and provide a new theoretical thinking paradigm for in-depth analysis of the structure-activity relationship mechanism of various nanosurface chemical interactions.Up to now,the electronic structure principles and theoretical system in the field of nanosurface chemistry are still in its infancy,and conceptual innovation and theoretical exploration are still needed from new perspectives,constantly improving the underlying theoretical framework in the field of nanoscience,and expanding the analysis of the nature of structure-activity relationship in specific action systems.the subsequent exploration of electronic structure principles and theoretical models of nano-surface chemistry includes the development of experimental analytical methods,the construction of concepts and theoretical models,and the development of Computational simulation methods.experimental analysis includes the design of material model system and the development of characteristic electronic structure signal characterization and analysis technology;to realize the innovation of concepts and theoretical models,it is necessary to fully integrate the concepts and theoretical achievements in the fields of solid state physics,quantum chemistry,surface and catalytic chemistry,coordination chemistry and spectroscopy,and to propose new cognitive perspectives;computational simulation can explore the mechanism and interaction between surface States and energy bands from the perspective of competitive reconstruction of surface chemical bonds and energy bands.the exploration of the electronic structure principle of surface chemical interaction of nanomaterials is expected to become a breakthrough to improve the theory of nanoscience.Although it is full of challenges in both experimental and theoretical aspects,it also contains a historical opportunity to realize the original concept and theoretical innovation。

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