| Recently, with the progress in quantum chemistry theory and its numerical methods, first-principles calculation based density functional theory (DFT) has become a routine method for condensed matter theory, quantum chemistry and material science. For the critical problems in the structure control of the nanomaterials, our work was performed around the structure design of the nanomaterials, aiming at realizing the controlled synthesis and functionalization of the nanomaterials. Starting by studying the surface/interface microscopic structure and properties of the nanostructures, this thesis has investigated the effect of surface chemistry on the morphology and phase stability of the nanocrystals, and the interface structure of the nanocomposites by combining experiment and first-principles calculations. It revealed the morphology evolution microscopic mechanism and phase transition of the nanomaterials, as well as the interface microstructure and properties of the nanocomposites, which can provide theoretical guidance for the structural design of nanomaterials. The main researches are listed as follows:The first chapter introduced the research background of this thesis, including the characteristics and applications of nanomaterials, and the importance and approaches of the structure control of the nanomaterials. In this chapter, it put forward revealing the underlying relationship between the growth environment and morphology, phase of the nanomaterials, and also the interface micro structure and properties of nanocomposites. On this basis, we briefly introduced the research ideas and content of this thesis.In the second chapter, we have introduced the basic theoretical methods of density functional theory and its development and applications, including the development of exchange-correlation functional, Local Density Approximation (LDA), Generalized Gradient Approximation (GGA) and self-consistent field theory. Then, the software packages used in our work have been introduced.Starting from chapter3, starting by studying the surface microscopic structure and thermodynamic properties (surface energy and surface tension) of the nanostructures, we investigated the morphology evolution microscopic mechanism of two kinds of inorganic nanomaterials-ZnWO4and tetragonal phase LaVO4(t-LaVO4). For ZnWO4, ZnW04nanorods with different aspect ratio were synthesized by tuning the pH of the solution using the hydrothermal method. It had been found that the basicity of the solution had an important role in the aspect ratio of the nanocrystals. The calculated results indicated that the surface energies of the (010) and (011) surfaces were lowest in the weakly-basic conditions, while they were increased with the fraction of hydroxyl in the adsorbates increased. The (100) surface was consistently the highest energy surface except for the most-basic conditions, and the (010) surface became the most unstable in energy. According to the Wulff law, the equilibrium shapes of ZnWO4nanocrystals are obtained from the respective surface energies. The aspect ratio of the nanocrystals changed with the basicity of surfaces conditions varied, which was consistent with our experimental findings. A modified Wulff construction model which considered the effect of surface tension was used to optimize the nanoparticle shapes as a function of size. It drew the same conclusions about the law of morphology transformation of ZnWO4nanocrystals with the Wulff construction model. For t-LaVO4, we found that the acidity of the growth solution had a very important effect on the aspect ratios and exposed facets of the nanocrystals. First-principles calculations found that the surface energies of (100) and (101) surfaces increased firstly and then decreased, while the (001) surface continued to decrease with the fraction of hydrogen in the adsorbates decreased. The predicted equilibrium shapes of t-LaVO4nanocrystals for each type of surface acidity conditions obtained from the respective surface energies was in good agreement with our experimental findings. The modified Wulff construction model drew the same conclusions about the law of morphology transformation of t-LaVO4nanocrystals with the Wulff construction model. Our work showed that the surface chemistry has an important effect on the surface energy of the nanocrystals, the change of relative stability among the surfaces of the nanocrystals results in the different shapes of the nanocrystals. Our work is beneficial to get a profound understanding of how to achieve shape-controlled nanocrystal synthesis.In chapter4, starting by studying the surface microscopic structure and thermodynamic properties (surface energy, surface tension, and Gibbs free energy) of the nanostructures, we investigated the phase stability of two kinds of inorganic nanomaterials-In2O3and AgInSe2. For In2O3, our experimental work found that the solvent has an important role in the resulting phase of the nanocrystals. The calculated results indicated that the surface energies of cubic phase In2O3(C-In2O3) can be greatly decreased in the conditions of water adsorption; while the surface energies of hexagonal phase (H-In2O3) were more stable in the conditions of methanol and ethanol adsorption. The surface tension of C-In2O3(111) surface was negative, while the surface tensions of the other surfaces were positive. We used a shape and size-dependent model to examine the phase stability of In2O3. The phase transition size of the nanocrystals can be obtained by bringing the appropriate surface energies and tensions into the expression of the total free energy of the nanocrystals. The results showed that in the conditions of methanol and ethanol-adsorption, the phase transition size from H-In2O3to C-In2O3was bigger than that in the conditions of water-adsorption, indicating that the hexagonal phase was more readily be obtained in the methanol and ethanol solvents, which was in good agreement with our experimental findings. For AgInSe2, Abazovic et. al. found that the ligand has an important role in the resulting phase of the nanocrystals. Our first-principles calculations showed that when using oleylamine as ligand, the surface energies of tetragonal phase AgInSe2(T-AgInSe2) were most stable in the case that the surface coverage was75%, while for orthorhombic phase (O-AgInSe2), the surface energies were most stable when the surface coverage was100%. While using trioctylphosphine as ligand, the surface energies were all most stable when the surface coverage was100%. When the surface coverage was lower (25%-75%), the surface energies of T-AgInSe2can be more greatly decreased in the conditions of using oleylamine as ligand than using trioctylphosphine as ligand; while the surface coverage was100%, the surface energies of O-AgInSe2can be more greatly decreased in the conditions of using oleylamine as ligand. The surface tension of T-AglnSe2(001) surface was negative, while the surface tensions of the other surfaces were positive. The phase transition size of the AgInSe2nanocrystals can be obtained by bringing the appropriate surface energies and tensions into the expression of the total free energy of the nanocrystals. The results showed that when under the same ligand, the phase transition size from O-AgInSe2to T-AgInSe2was biggest as the surface coverage was100%using oleylamine as ligand, indicating that the orthorhombic phase was more readily to be obtained; while using trioctylphosphine as ligand, the phase transition size from O-AgInSe2to T-AgInSe2was smallest as the surface coverage was100%. When under the same surface coverage, the orthorhombic phase was more readily to be obtained using trioctylphosphine as ligand as the surface coverage was lower (25%-75%); while the surface coverage was100%, the phase transition size from O-AgInSe2to T-AgInSe2was bigger when using oleylamine as ligand than that in the conditions of trioctylphosphine, which was in good agreement with Abazovic’s experimental findings. Our work showed that the surface chemistry has an important effect on the surface thermodynamic properties (surface energy and surface tension) of the nanocrystals, the change of relative stability among the phases of the nanocrystals results in the different resulting phases of the nanocrystals. Our work is beneficial to get a profound understanding of how to achieve phase-controlled nanocrystal synthesis.In chapter5, starting by studying the interface microscopic structure and properties of the nanocomposites, we investigated the interface structures and properties of nanocomposites-ZnWO4/BiOI heterostructurcs. ZnWO4/BiOI heterostructures were synthesized via a mild chemical bath approach. The obtained ZnWO4/BiOI heterostructures displayed high photocatalytic activities in degradation of MO and photocurrent response under visible light irradiation. First-principles calculations are used to investigate the interface lattice and energy level match by examining the surface geometry structures and the work function of the (011) and (010) surfaces of the ZnWO4phase and (001) surface of the BiOI phase. The results showed that the lattice mismatch between ZnWO4and BiOI was small, which was beneficial to form stable heterostructures. The energy level between the ZnWO4and BiOI phases match well, for BiOI has larger work function, the Fermi level of BiOI is lower than that of ZnW04. When BiOI contacts ZnWO4to form a p-n junction, the CBM of BiOI is higher than ZnWO4, the photo generated electrons would easily migrate from BiOI to the CB of ZnWO4. Moreover, the built-in electric field could further facilitate the separation of the electron-hole pairs. This good interface lattice and energy level match resulted in the excellent photocatalytic performance of the ZnWO4/BiOI heterostructures. Our work is beneficial to get a profound understanding of the interface structure of the nanocomposites, and for designing and developing more effcient heterostructured photocatalysts.In the last chapter, we summarized the conclusions and innovative points of this thesis, and gave prospects of the future research. |
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