Synthesis And Properties Of Ti-oxide Clusters In Aqueous Solution

Over the past four decades,scientists have conducted extensive and in-depth studies on controlling synthesis of TiO2 to improve the properties,which is one of the most important semiconductor nanomaterials.The key to achieve controlled synthesis of TiO2 nanomaterials is to understand the formation mechanism.At present,there are mainly sol-gel and solvothermal methods for the preparation of TiO2.However,there are few reports about important issues such as the structure of the intermediates for the hydrolysis of Ti4+ precursors,the gradual conversion of intermediates,and the nucleation formation of TiO2.Titanium-oxo clusters are intermediates during synthesizing TiO2 via sol-gel and solvothermal methods,so the separation and synthesis of titanium-oxo clusters is the key to explore the nucleation and growth mechanism of TiO2 nanocrystals.Titanium-oxo cluster is a molecular type of TiO2 whose structure and composition are similar to those of TiO2.Therefore,it may have similar or even superior properties than TiO2.And it may has important applications in photocatalysis,energy storage,synthesizing inorganic-organic hybrid materials.At present,most of titanium-oxo clusters are synthesized by solvothermal method in organic solvents,which are sensitive to water and oxidizing environment,that greatly limits their practical application.Since titanium precursor is easily hydrolyzed to produce TiO2 precipitate,it is very difficult to separate and synthesize titanium-oxo clusters in aqueous solution,which greatly restricts the nucleation and crystal growth mechanism of TiO2 in sol-gel process.In this thesis,a series of titanium-oxo clusters were obtained through a new synthetic strategy that captures intermediates in the sol-gel process to prepare TiO2 in aqueous solution.For the solution chemistry of these titanium-oxo clusters,this thesis studies systematically,establishing a foundation for further elucidating the mechanism of nucleation and crystal growth of TiO2.This thesis also discusses in detail the solution chemistry,surface modification applications,photocatalytic applications and host-guest exchange properties of these titanium-oxo clusters.These compounds demonstrate their application in surface modification,photocatalysis,and guest capture applications.The contents of this thesis are divided into five chapters as follows?In Chapter ?,the development of metal-oxo clusters,the synthesis methods and formation mechanism of TiO2 are reviewed.The development of titanium-oxo clusters is mainly introduced.Finally,scientific significance of this thesis is highlighted.In Chapter ?,three novel titanium-oxo clusters isolated from the aqueous solutions of TiOSO4 and TiCl4 were reported,and their separation,characterization and electronic properties were studied.The three clusters provide precise structural information for future characterization of the solution species and structural evolution of Ti4+ in water.In Chapter ?,a novel titanium oxide complex {Ti6O8} with similar Lindqvist structure containing only Ti,O and H elements was isolated from aqueous solution,and its synthesis,structure and solution stability were systematically studied.In this chapter,we have conducted detailed studies and fully discussed the role of counter anions in the synthesis of titanium-oxo clusters,structural stability and solution behavior.In Chapter ?,by using solubility control to crystallize the prenucleation clusters of Ti4+/SO42-system,a family of titanium-oxo clusters possessing the {Ti18O27} core in which the 18 Ti(?)-ions are uniquely connected with ?-oxo ligands into a triple-decked pentagonal prism was obtained.The cluster cores are wrapped by external sulfate and aqua ligands,showing good solubilities and stabilities in a variety of solvents including acetonitrile and water and allowing their solution chemistry being studied by means of electrospray ionization mass spectroscopy,17O NMR,and vibrational spectroscopy.Furthermore,this study provides new titanium oxide candidates for surface modifications and homogeneous photocatalysis.In Chapter ?,a class of titanium oxide cation host-guest cluster {Ti12O18} was synthesized.{Ti12O18} possesses a hexagonal,cryptate-like Ti12O1812+ framework.We have succeeded in obtaining single-crystal structures in which monovalent cationic guest such as K+,Rb+,Cs+,and H3O+ are internally encapsulated.The solution properties,host-guest exchange properties,and bridging-O/solvent exchange properties of these molecules were systematically studied using 133Cs NMR and 17O NMR.We demonstrated that the {Ti12O18} host possesses a flexible pore allowing the passage of larger cations,and the structure remains unchanged during this process.We find that there is an ionic-dipole attractive force between the host and guest.The force depends on the size matching between the host and guest,and determines the ability of the host recognize guest,the ability to capture the guest,and the stability of the host-guest compound.We have also demonstrated that this type of molecule can change the type of surface ligand through ligand exchange reaction,so it is easy to achieve surface functionalization.The titanium-oxo cluster family has a potential application as a model supramolecular compound in the study of host-guest chemistry and assembly of functional inorganic-organic hybrid materials.