1. Titanium-Oxo Clusters (TOCs), the intermediates of TiO₂ nanocrystalline formation

Fig. 1    A Pentagonal-Prismatic 18-nuclear Titanium-Oxo Cluster, [Ti₁₈O₂₇(OH₂)₃₀(SO₄)₆]⁶⁺. For clarity, the SO₄^2- ligands are omitted.

 

Polyoxometalates are nano-size molecules, and can be viewed as mimics of transition-metal-oxides of ca. 2-nm. Due to their excellent reversible redox behaviors, catalytic capabilities, and cation-mediated self-assembly behaviors, there have been vast interests in their syntheses, structure, catalysis, self-assembly, medical and biological applications. Among the known types, titanium-oxo clusters, also known as polyoxotitanates, are molcules comprised of many Ti- and O-atoms, in which TiO_n moieties share their edge-O atoms. A TiO₂nanoparticle smaller than 3-nm can be views as a Ti-oxo cluster. However, because large portions of the atoms are located on the surface of the nano-structure, their properties are very different from that of the typical phases of TiO₂ like anatase.

 

Ti-oxo clusters can be crystallized from solutions. Thus owning to the advantage of single-crystal XRD solved structures, they are very good models for studying the interactions and electron transfer (ET) processes between TiO₂­ cluster surfaces and adsorbates, ET processes between TiO₂ NPs and molecules in solutions, mechanisms for (cation-mediated) electron/hole migration, and recently-reported proton-coupled electron transfer during photocatalysis. Moreover, as TiO₂ minerials like anatase, brookite and rutile are synthezied va sol-gel method or hydrothermal method, the Ti-oxo clusters are regarded as intermediates or building blocks during self-assembly of Ti-precusors into these minerals and other materials such as Ti-containing metal-organic frameworks (MOFs).

 

Our aims are to synthesize a series of new Ti-oxo clusters, for the goal of:

(1) Using the precise structures to address issues associated with TiO₂ nanocrystalline formation. Notably, TiO₂ is a widely explored photocatalyst which has attracted extensive studies during the last four decades. Meanwhile, TiO₂ is a typical metal oxide nano-material, and its molecular level growth mechanism via the sol-gel approach is representitive for all the oxide-materials. Hence, this project, based mainly on single-crystal XRD and HR-MS, is expected to reveal the detailed mechanism in nanocrystalline formation, and thereby to modulate the nano-material synthesis.

(2) Functionalization of nanomaterials with the well-defined structures. The TOCs can be considered as quantum dots with precise structures. They can be used to modify the nanocrystal surfaces to realize new morphologies and new activities.

(3) Exploring their applications in molecular catalysis. The TOCs are molecular forms of titania, and hence are expected to find applications in photocatalysis. We recently find that these Ti-oxo clusters are actually very active catalysts for a variety of organic syntheses.

 

2. Photocatalytic Organic Synthesis

 

Light has the potential to serve as an inexpensive, abundant, renewable, and nonpolluting reagent for chemical synthesis. The increasingly environmentally conscious has aroused the chemical community extensive interests on application of photocatalysis for chemical reactions.

However, photochemical syntheses have enjoyed only limited application on industrial scales. One fundamental obstacle is the inability of most common organic molecules to absorb the solar irradiation, especially the longer wavelengths which carry most of the solar energy. Efficient organic photochemical reactions typically require high-intensity UVC light (wavelength < 280 nm) generated in specialized photoreactors; however, this wavelength region of solar spectrum hardly reaches the earth surfaces and as a result this compromises the benefits of utilizing direct solar photocatalysis. Hence, the development of new strategies for efficient solar light photocatalysis of synthetic transformations is a particularly important goal.

 

Our research in this field focuses on harnessing solar energy to address the photosynthetic demands. For this, developing new photocatalytic systems and exteding the reaction scope of the known photocatalysts are two obvious and fundamental directions:

 

(1) Exploring new photocatalytic systems

 

(2) Extending the scope of TiO₂ photocatalysis