Preparation and properties of polyoxo-titanium clusters

Abstract

Effectively harvesting solar energy plays a crucially important role in supplying sustainable energy and addressing the environmental pollutants. Titanium dioxide (TiO2) nanoparticles are recognised as the best promising photocatalysts for solar energy conversion, photovoltaic cells and degradation of organic pollutants, due to its advantages of low cost, high efficiency and environmental friendliness. The strong oxidative potential of the photogenerated holes TiO2 nanoparticles can oxidize water to create hydroxyl radicals and oxidize most organic pollutants into carbon dioxide. Honda-Fujishima effect was described firstly by Fujishima and Honda in 1972, that promote the field of photocatalysis extensively. The first example that TiO2 nanoparticles was used for photocatalyst of photocatalytic hydrogen production under UV excitation, was pubulished in 1977. However, the defects of strong imprecise structural information at the atomic level, including uncertain surface composition, uneven sizes, and unclear inorganic-organic interface information, have led to a great limitation for deeper research on such vital photocatalytic materials. Therefore it would be a great challenge to investigate the theoretical catalytic mechanism, explore the charge transfer path, modify the surface as well as understand the structure-property relationship at the molecular level. Serving as the structure and reactivity model, crystalline titanium-oxo clusters (TOCs) with accurate stuctures have attracted considerable attention in recent decades. A growing numbers of TOCs with various structures have been synthesized attributed to the rapid development of single-crystal X-ray diffraction analysis. As a most special class of oxo-clusters-based materials between molecular compounds and nanoparticles, it did have made tremendous progress, although there is still large space for the development of TOCs, compared with polyoxometalate clusters like polyoxomolybdate complexes and polyoxotungstate compounds. Accurate atomic location, clear ligand-nuclearity connectivity, highly tunable cluster size and high solubility of TOCs give great chances to investigate the optimal reaction condition through simply varying the synthesized conditions of solution-based prepared methods, and analyze the structure-property relationship at the molecular level, as well as provide an excellent research platform for the molecular simulation and theoretical calculation of the surface modification of TOCs. This review is directed to cover the main advances on the development of TOCs. Firstly, the representative synthetic and design approaches are classified and discussed including the conventional and latest successful synthetic routes, such as strict inert condition synthesis, solvothermal synthesis, two-step synthesis, ionothermal synthesis, etc. In addition, a large proportion of TOCs with diverse novel structures are reviewed, as the result of successful synthetic and design methods, for example, the first fullerene-like Ti42 cluster, the highest nuclearity of Ti52 cluster, microporous cyclic Ti32 clusters, etc. Moreover, tunable properties are also outlined by modifying different coordinating ligands and dopping metals or non-metals. They contribute to get rid of UV-light dependence and enhance their visible-light absorption. Furthemore, applications in a variety of different fields, e.g. photocatalytic hydrogen generation, photoelectrochemical water oxidation, recognition and separation, are also discussed.