Synthesis And Properties Of Crystalline Titanium Oxo Clusters

Developing excellent photocatalysts to make full use of solar energy is one of the effective ways to alleviate the global energy shortage and environmental pollution crisis.Titanium dioxide（Ti O₂）nanomaterials have attracted much attention due to their excellent photocatalytic properties,such as stability,high efficiency,biocompatibility and low toxicity.However,the structural inaccuracy of these nanomaterials greatly hinders the basic research of these important photocatalytic materials,such as catalytic mechanism,charge transfer and surface modification.Crystalline titanium oxo clusters（TOCs）with precise structural information can not only be used as compounds for the structure and reaction models of Ti O₂ but also have the advantages of adjustable structure and properties,which is one of the hot topics in the field of coordination chemistry.However,compared with other transition metal complexes,the types and quantity of TOC s are still relatively small.Exploring the synthesis methods of TOCs,developing novel TOCs and studying their photocatalytic properties can not only provide a basic model for the application of Ti O₂,but also have important application prospects.In this work,we maily focused on the synthesis methods,band gap tuning,metal doping and photocatalytic properties of TOCs,and 27 complexes were synthesized.The dissertation is divided into the following seven parts:1.In the introduction,we first summarize the research background of TOC s.Then the development of this field from the aspects of metal free doped TOCs and metal doped TOCs synthesis is introduced,and further the properties of TOCs in photocatalytic H₂evolution,photocatalytic degradation of organic dyes and so on are described.Finally,the topic selection and work progress of this aiticle are briefly summarized.2.Titanate is the most widely used titanium source for the synthesis of TOCs.Controlling the hydrolysis rate and degree of titanate is very important for the construction of TOCs with different structures.A certain amount of formic acid and isopropanol were added to the solvolthermal reaction system of the reported Ti₆-TOC,in which the in situ-produced water ia used to control the to the hydrolysis rate and degree of titanate,resulting from the Ti₆-core structure to Ti₁₀-core structure（Complex 1）.Furthermore,when we changed ligands of benzoic acid ligand for p-methylbenzoic acid or p-methoxybenzoic acid,the Ti₁₀-core structure to Ti₂₂-core structure（Complexes 2 and 3）.It is the first time to study the structural regulation of TOCs based on in situ water and ligand substituent strategy,which is important for the design and synthesis of TOCs with different structures.3.Band gap values determine the spectral absorption range of TOCs,but research on band gap tuning of TOCs at the large range of 3.7-2.0 e V by chromophore ligands（CLs）has been lacking.In this section,six new Ti₆ cores based TOCs（Ti₆-TOCs）（Complexes 4-9）were successfully synthesized by using CLs in a simple and general approach.To be noted,the band gap values were tuned at a wide range of 3.41-1.98 e V by controlling the type and number of CLs 2-hydroxypyridine,salicylaldoxime and catechol in the structure.The results not only provide a research model for the study of dye-sensitized Ti O₂,but also provide an important experimental basis for the band gap regulation of Ti O₂.4.Silver doped Ti O₂（Ag-Ti O₂）materials show high activity and good stability in photocatalysis,but the mechanism could not be illustrated clearly due to the imprecise and inhomogeneous characteristics.Ag-doped titanium-oxo clusters（Ag-TOCs）with exact crystal structure,which are rarely reported,are benifical for further understanding structure-property relationships.In this section,six new Ag-TOCs（Complexes 10-15）with butterfly-like Ti₈Ag₂core have been synthesized through facile solvothermal reactions,in which two Ag ions are successfully linked to the surface of Ti₈ core.To be interest,the Ti₆ unit from the Ti₈Ag₂ core is similar to that found in the anatase structure.The DFT calculation revealed the lowest energy bands of Ag-TOCs are attributed to the Ag-to-Ti O core charge transfer bands.Additionaly,all Ag-TOCs exhibit good photoelectric response and high photodegradation activity of organic dyes.This work provides a simple way to synthesize crystalline Ag-TOCs which is helpful to explain the mechanism of photocatalytic process of Ag-Ti O₂.5.There are few reports on the photocatalytic H₂evolution by TOCS,and the performance needs to be improved.In this section,nine metal doped TOCs（Complexes16-24）with Ti₁₀M₂（M=Mn,Fe,Co,Ni and Cd）cores were synthesized by solvothermal method with polydentate carboxylic acid and organic phosphonic acid ligands,and the properties of photocatalytic H₂ evolution were studied.This work shows the following advantages:（1）Using cheap and easily available transition metal chlorides as raw materials,the types of transition metal doped TOCs could be greatly enriched.（2）Both aliphatic and aromatic carboxylic acids can be used as polydentate ligands to construct the Ti₁₀M₂-type of M-TOCs,which can expand M-TOCs based on different carboxylic acid ligands.（3）All Ti（IV）atons are protected by oxegen atoms of multidentate ligands.All M（II）atoms are protected by four oxegen atoms of multidentate ligands,and the remaining two sites are coordinated by solvent molecules.These complexes have excellent structural stability in organic solvents and acid-base aqueous solutions,which is beneficial to be used as heterogeneous catalysts for catalytic reactions.（4）In the molecular structure,Ti O core can be used as the light absorption center,M（II）atoms can be used as the catalytic active center.Moreover,and Ti O core and M（II）atoms are connected by Ti-O-M bonds,which is beneficial for electron transmission.The photocatalytic H₂ evolution results show that the complexes19-24 based on benzoic acid ligands have good photocatalytic activities.The hydrogen production rate of complex 22 can reach 498μmol h^-1g^-1,which is the highest among the reported TOCs.In addition,the difference of photocatalytic activity further confirmed that the doped M（II）atoms were the photocatalytic active center.The work may provide good ideas for the further design and synthesis of TOCs with excellent photocatalytic performance.6.Flexible ligands are suitable for constructing novel clusters.In this section,one Ti₄-core TOCs（Complex 25）and two octanuclear cobalt clusters（Complexes 26 and 27）were synthesized by solvothermal method using N-methyldiethanolamine,diethanolamine and triethanolamine as flexible ligands.In the structure of complex 25,N-methyldiethanolamine and triethanolamine were both introduced to the structure,forming a novel tetranuclear TOCs.In the complex 26,the template Cl^-is derived from Co Cl₂ presenting a tetrahedralμ₄-Cl coordination mode.In the complex 27,the template S^2-comes from the decomposition of S-CN bond of thiocyanate.In spite of 26 and 27being derivatives of cubes,eight Co atoms in 26 consist of two co-centered tetrahedra of different sizes,whereas in 27 they appear as a rhombohedron formed via elongating a cube along the C3-axis direction.Strong intracluster antiferromagnetic interactions were found in 26 and 27.To be noted,S^2-ions generated by in situ decomposition of thiocyanate ions in solvothermal reaction were successfully captured for the first time.7.The overall work of the paper is summarized systematically,and the future work of the project is prospected.