Design Of Nanomaterials And Tuning Of Charge Polarization

Material innovations are not only important driving forces for the progress of human civilization, but also the basis for the development of new industries. In recent years, the emerging new nanomaterials have shown many excellent properties, such as high specific surface area, multi-scale size effect, interface effect, surface effect and quantum confinement effect, etc., and thus are widely used in many key areas such as energy, environment and semiconductor industry, etc.However, the complexity of nanomaterials has brought many difficulties to both experimental and characterization techniques, leading to huge difficulties in catching many complex processes. Therefore the understandings of structure-function relationships and working mechanisms of nanomaterials are limited, which restricts the rational design of new nanomaterials. With the rapid development of high-performance computing and the continuous improvement of computational theories in recent years,the first-principles calculations provide a powerful tool for material analysis from atomic scale and the level of electronic structure. It can help people to better rational design and test the feasibility of the design. Meanwhile, theoretical calculations have a lot of advantages, for instance, short development cycle, low-cost, environmentally friendly and so on. Therefore, theoretical calculations combined with experimental characterization have become a new trend in the design and development of new materials.Regulating the compositions, sizes and surface morphologies of nano-materials are effective ways to control the properties of nanomaterials. Moreover, based on the understandings of the structure-activity relationships, rational design of composite materials can also achieve synergies. All these design strategies, in the final analysis,are based on the control of electrons. By jumping in different electronic states, the polarizated charge that result from electronic excitations and charge transfers are thus be able to drive the corresponding physical and chemical processes.Based on the first-principles theoretical calculations, in this dissertation we studied the electronic state structures and population behaviors of a series of complex systems.From the perspective of the formation and evolution of charge polarization, we expounded the structure-activity relationships and synergistic mechanisms in these complex systems (see Chapters 3 to 5). In addition, we exploratory proposed that dipole moment can be a descriptor of charge polarization in complex systems, and therefore can be used to study the influences of charge polarization on the surface reactivities(Chapter 6). This dissertation could be divided into six chapters, see follows:In the first chapter, based on the areas involved in the work and the problems we focused on, the background knowledge and research status of two areas were introduced.The first one is tunning conductivity of nano-materials. Here we took the family of vanadium oxides and graphene as examples to introduce the research statuses of tunning conductivities of strongly correlated systems and two-dimensional materials respectivity. In the vanadium oxides, geometric structures play an important role in the regulation of conductivity, making it possible to control the electronic structure by precisely controlling the geometric structure. Meanwhile, tunning the bandgap of graphene has always been a difficulty in its application in the semiconductor industry.Despite a bunch of achievements, there are still many challenges in the precisely control at atomic scale. In addition, how to maintain the inherent advantages of graphene such as high carrier mobility, high mechanical strength, etc. during the control of bandgap is another challenge. Next section, we then gave a brief introduction of photocatalysis.The research background, the main process and mechanisms, as well as the material screening and composites in photocatalysis were introduced. Traditional semiconductor materials have many shortcomings, and the rational design of semiconductor-metal or semiconductor-semiconductor composites should be a way to break the limitations of a single material and achieve synergies. Hence the nanocomposites are drawing more and more attention in the recent years, and become the focus of current photocatalysis field.In the second chapter, we briefly introduced the development history, theoretical framework, common approximations and mainstream quantum chemistry calculation softwares of the first-principles density functional theory respectively. Based on the quantum mechanics, density functional theory (DFT) considers the electronic density of the system as the basic object. By solving the Kohn-Sham equation, the interactional multi-particle system problem can be transformed to noninteracting single-particle problem, combined with exchange-correlation functional approximation, the ground state charge density of the system can be calculated. And then, almost every basic properties can be obtained including the ground state energy of the system. In the actual calculation, we choose the exchange-correlation functional and the quantum chemistry calculation softwares according to the characteristics and research purpose of the specific research system.Then, in order to investigate the charge polarization effect of the internal, interface and surface of the material, as well as their effects on the properties of the nanomaterials,we have studied a series of complex systems (see Chapter 3,4 and 5 respectively for further details). In Chapter 3, we described two examples of tunning internal charge polarization of nanomaterials by doping and defects, see below:(1) The introduction of oxygen vacancy defects in VO2 has been shown to modulate the metal-insulator transition (MIT) at room temperature. However, as the geometric and electronic structure reorganizations occur simultaneously, the origin of MIT is still unclear. In order to distinguish the contributions of geometrical and electronic reorganizations to MIT separately, here we performed first-principles calculations. It was found that oxygen vacancy defects can induce 3d orbital polarized electrons, which causes electronic reorganization, while the geometrical reorganization makes the conduction band edge be partially occupied. These polarized electrons, which occupy the conduction band, are delocalized in space and thus bewtowing conductivity to VO2. In addition, we revealed the linear proportional relationship between the number of polarized charges and the defect concentration, pointing out the direction of controlling MIT behavior and conductivity by defect engineering in VO2.(2) Electrocatalytic hydrogen evolution (HER) is an important reaction in the field of clean energy, and so far the precious metal Pt is still the most efficient material in HER. In order to reduce the amount of Pt and improve the electrocatalytic efficiency,here we proposed a rational design of transition metal Fe and Co bimetallic doping with a unique tristar nanostructure. This work systematically expounded the synergistic mechanism of chemical composition, electronic structure and surface morphology from both experimental and theoretical aspect. By adjusting the relative content of Fe and Co, it can effectively adjust the d-band center of Pt to the optimal position, so as to achieve the optimal activity. Moreover, by synthesizing the unique tristar nanostructure, we obtained more active sites and decreased the contact resistance with the electrode, and therefore further enhanced the HER activity. The design ideas that proposed in this work open up a new way for rational design and construction of low-cost and high-performance alloy materials.In Chapter 4, we design rationally for the main problems in the field of photocatalysis, such as the separation and transport of photogenerated electron-hole pairs. By studing two kind of semiconductor-metal composite photocatalytic materials,we discussed the promotion of interface effect and polarization charge on the photocatalytic process, as follows:(1) In this work, we proposed a unique Ag-CuO/Cu2O composite nanomaterial, which successfully accumulated polarized charge on CuO (100) facet through interfacial polarization and electron transport inside the material, and thus improved the efficiency of CO oxidation on this facet. This work opens up a new thinking to design efficient and economical semiconductor-metal composite catalyst through surface and interface engineering.(2) By utlizing the difference of the two-phase work function of composite materials,we designed g-C3N4 to combine with Pd {100} and {111} facets respectively. Both experimental and computational results demonstrated that the unique conjugate structure of C3N4 nanosheets ensures equivalent charge polarization to various Pd facets. This provides a way to examine the catalytic selectivity of crystal faces to gas molecules. It was found that CO2 reduction was more likely to occur on Pd ｛111｝facet while H2O reduction was more likely to occur on Pd {100} facet. This work provides a reliable structural design to study the selectivity of crystal facet in photocatalytic reaction, opening the door for the design of high-selectivity photocatalysts.In Chapter 5, we introduced two examples of improving the reactivity by controlling the surface charge polarization of the materials respectively, as follows:(1) Due to the structural characteristics of graphene single layer, the polarized charge formed at the interface between the metal and the graphene can easily migrate to graphene surface, forming surface polarization, which then can cooperate with the metal itself to promote the adsorption of H atoms. More importantly, theoretical calculations demonstrated that by regulating the pattern of metal clusters, it is possible to precisely control the H adsorption energy of different sites on graphene at atomic-scale, so as to control the H adsorption pattern on graphene surface. This work deepens our understanding of the generation and working mechanism of surface polarization, expands our ability to regulate the structure of materials at the atomic-scale, and also promots the development of graphene-based materials in the semiconductor industry.(2) The charge transfer between material surface and adsorbed molecules or clusters is a key process in the surface catalytic reaction. Here, we proposed a design strategy that grafting the homogeneous catalyst Cp*Cr(CH2SiMe3)2 onto an amorphous SiO2 substrate. Then we expounded the enhancement mechanism of activity and thermal stability induced by the charge polarization between material surface and catalyst. Results show that the polarized holes can be injected into the Cr center of the catalyst, which can thus enhance the interaction of Cr to ethylene and reduce the reaction energy barrier, improving the activity. Meanwhile, these holes injection can also enhance the Cr-C bonding energy and thus improves the energy barrier required for homolysis, improving the thermal stability. These findings help to deepen the understanding of the improved catalytic performance induced by charge polarization, which is good for the design of novel high active and stable catalysts.In Chapter 6, we choose the dipole moment as the descriptor of charge polarization to explore the relationship between the dipole moment included angle of material surfaces and adsorbates and adsorption energis and reaction barriers. Results show that the chemical adsorption configuration tends to have the smallest dipole moment included angle, and the adsorption energy can be codetermined by both the magnitude and the included angle of dipole moments. In the meantime, we also revealed the linear inverse relationship between the included angle and dot-multiply of the dipole moments and the barrier of the CO co-catalyzed oxidation reaction, indicating that with smaller included angle of surface and molecules, interactions can be stronger. Then the charge exchange and energy delivery should be more likely to occur, making the reactivity of catalyst better. Of course, the limited number of the systems in this work can not prove the universality of this law, in the future work we will study more systems from the perspective of dipole moments to find more unknown laws and develop nanomaterials design.