| In the process of preparation and application of inorganic sol, for multicomponent complex system, studying on the key factors such as solvation interface, surface adsorption, clusters aggregation and chemical reactions, etc., not only can we accurately understand the relationship of structure and properties but achieve the controllable synthesis of inorganic sol with desired structure and properties. Among them, the formation and transformation of nanosheet-like sol of layered metal hydroxide and oxyhalides have great influence on their construction and properties. From the atomic or molecular scale, conducting in-depth research on the interface behavior of sol particles has great significance both to the basic theory of sol and to the development of new materials.1. Insight into the segregation phenomenon of doped metal ions in the drying process of γ-AlOOH sol with NO3- as counterionsThe segregation mechanism of the doped metal ions (Na+, K+, Mg2+, Ca2+, Co2+ Ni2+, La3+and Y+) in y-AlOOH was investigated during sol-gel process. Experimental results revealed that the segregation indeed appeared during the drying process. The segregation rate of metal cations in y-AlOOH xerogel shows the sequence of Y3+> La3+> Ni2+> Co2+> Mg2+> Ca2+> K+> Na+ after drying at 25℃ while La3+> Y3+> Ni2+> Co2+> Mg2+> Ca2+> K+> Na+ at 100℃. The segregation of metal cations in y-AlOOH sol during drying process mainly occurred at the late period of drying with the removal of the adsorbed water. Theoretically, the interaction between metal cations and γ-AlOOH surface in neutral and acidic conditions was calculated by using density functional theory (DFT) calculations. The DFT results showed that the adsorption energy of H3O+ was bigger than all of the metal ions in neutral condition and could be preferentially adsorbed on the surface. The metal ions even could not be adsorbed on protonated γ-AlOOH surface due to electrostatic repulsion. However metal ions should be in double electric layer and close to Stern layer due to the partial protonation on (010) face of colloidal particles and electrostatic attraction between metal cations and NO3-. The segregation of metal ions in γ-AlOOH sol during drying process was caused by weak adsorption of metal ions on γ-AlOOH surface in acid condition. In order to strengthen the interaction on the surface of the metal cation and colloidal particles, we found that adding metal ion chelating agent (citric acid) into sol system can effectively reduce the segregation during drying. The intensive study on the segregation mechanism and the guidance based on the control of segregation will be of great help to improve the performance of metal ion doped alumina materials.2. Effects of inorganic acids and divalent hydrated metal cations (Mg2+, Ca2+, Co2+, Ni2+) on γ-AlOOH sol-gel processThe effects of different inorganic acids (HCl, HNO3 and H2SO4) and divalent hydrated metal cations (Mg2+, Ca2+, Co2+, Ni2+) on y-AlOOH sol-gel process were studied based on experiments and density functional theory (DFT) calculations. Experimental results revealed that the sol originating from y-AlOOH suspension only formed by adding HCl and HNO3 instead of adding H2SO4. The effect on the stability of γ-AlOOH sol by divalent hydrated metal cations can’t be reasonable explained by traditional DLVO theory. Therefore, the DFT calculations were carried out to calculate the interaction of inorganic acid and metal cation on the surface of and colloidal particles. The results showed that the surface adsorption of the sol particles should be one of the important factors that affect the sol-gel process. Owing to the strong adsorption of polybasic acid anion on the surface of the colloidal particles and hydrogen atoms in polybasic acid radical towards solvent may form a strong adsorption to anions in the solvent, leading to electric double layer compression and it is difficult to form sol. When ionic strength is small, the gelation time is long (>200 s) and ionic strength plays the key role at this time. When the ionic strength increased (≥0.198 mol/kg for divalent metal cations), the gelation time shortened (<200 s) and the effect of electrostatic attraction and adsorption energy on γ-AlOOH sol-gel process significantly enhanced. Researching on γ-AlOOH sol-gel process based on experiments and density functional theory (DFT) calculations, we can have an in-depth understanding of the stability of sol. Thus, it will be conducive to the design and preparation of new material.3. Liquid Exfoliation of FeOCl Nanosheets and Their Enhanced Catalytic Activity as Fenton-like CatalystFor the purpose of studying the exfoliation possibility of FeOCl, we firstly calculated the cleavage energy which is 340 mJ m-2 and deduced that the exfoliation of bulk FeOCl is feasible in experiment. Few-layer FeOCl nanosheets were prepared by a facile liquid-phase exfoliation method in acetonitrile. At room temperature, neutral solution and sunlight conditions, FeOCl nanosheets showed outstanding properties in catalytic degradation of phenol in water compared with that of FeOCl plates. In order to study the catalytic mechanism of FeOCl nanosheets, we also prepared nanosheets with different thickness (1.9 nm,5.88 nm,10 nm and 20 nm). Apart from increased surface area in nanosheets, the surface state change of the nanosheets also plays a great role in improved catalytic performance. The increased delocalization in nanosheets and the reduction of Fem (Feâ…¢â†'Feâ…¡) in nanosheets promote the generation of OH-radical and catalytic degradation of phenol. The study on FeOCl nanosheets is beneficial to accurately understand the catalytic mechanism of phenol degradation. In addition, the study can also be of help to design new two-dimensional nanosheets used in Fenton reaction and provide a new approach on preparation and application of 2D nanosheets.4. Density functional theory study on the exfoliation of layered BiOX (X=Cl, Br, I) and thickness-dependent propertiesWe first study the exfoliation possibility of BiOCl, BiOBr and BiOI by calculating the cleavage energy. The results reveal that the exfoliation of BiOBr and BiOI is feasible in experiment due to the smaller exfoliation energies compared with graphite, while the exfoliation of BiOCl is difficult due to the bigger exfoliation energies than graphite. Then, we further calculate the layer-dependent band gap, which increases following reducing layer number in both of BiOBr and BiOI. The layer-dependent surface state also closely related to layer number. The electronegativity of Br atoms decreases while I atoms increase with decreasing layer number. The results on layer-dependent adsorption of azo groups show that the adsorption energy of CH3NNCH3 is bigger than H2O when the layer numbers are smaller than 6 for BiOBr while bigger than 2 for BiOI. In addition, the adsorption of azo groups on BiOBr and BiOI surface has selectivity. The photocatalytic properties of BiOBr and BiOI are closely related to the layer number of nanosheets. |
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