Production Process for Titanium Dioxide Nanowires and Nanorods Revealed by Hong Kong University of Science and Technology Researchers

Hong Kong University of Science and Technology (Kowloon, CN) scientists Shihe Yang and Chenmim Liu describe a method for the synthesis of nano-products, such as atomic titanium oxide wires in U.S. Patent Application 20100069229. 

The method allows wires of anatase titanium oxide wires to be formed in a range of tunable diameters and aspect ratios in the nanometer and subnanometer size scales. The method also allows the titanium wires to be capped by oleic acid to enhance dispersing and solubility. The method allows the titanium wires to be surface doped with nitrogen species to enhance stability and functionality such as enhanced absorption in the visible wavelength region, which is useful for photodegradation of organic wastes in water by sunlight.

Nanoscale titanium dioxide, TiO₂, or titania, has outstanding properties which can be used in wide-ranging areas. For example, TiO₂ can be used in heterogeneous catalysis, photocatalysis, solar cells, gas sensor, corrosion-protective coating, electrical devices such as varistors and so on. Thus, many TiO₂ nanostructures, including hollow spheres, nanotubes, nanowires, and mesoporous structures have been synthesized.

The method of synthesizing or producing a nano-product includes the steps of a) providing a mixture of a metal (M) -alkoxide and an unsaturated carboxylic acid, b) heating the mixture for a pre-determined period of time to form an M-complex precursor, c) precipitating a nano-product of M oxide from the M-complex precursor, wherein M is an element, the oxide of which is suitable to form a nano-product.

Preferably, precipitating a nano-product of M-oxide from the M-complex precursor in step c) comprises heating the M-complex precursor at a pre-determined temperature for a pre-determined period of time. Typically, the M-complex precursor is an ester complex, and the M-alkoxide is titanium alkoxide, zirconium alkoxide, tin alkoxide or cerium alkoxide.

Advantageously, this process provides the possibility of controlling the size and structure of the nano-products by controlling the temperature and time during the formation of the nano-products, wherein the higher the temperature, the greater the diameter of the nano-products and the longer the period, the longer the lengths of the nano-products. This provides the possibility of slowly growing small and fine crystalline nano- or atomic wires, having diameters as small as 0.3 nanometers.

Furthermore, the esterification of the M-alkoxide in the presence of unsaturated carboxylic acid in ambient air provides the possibility of limiting the presence of water, thus preventing significant hydrolytic process forming amorphous TiO ₂ products. Preferably, the heating of the mixture for a pre-determined period of time in step b) comprises solvothermally treating the mixture.

Preferably, unsaturated carboxylic acids such as oleic acid are also used as a capping agent, capping onto the surface of the nano-products. Thus, the carboxylic acids act as a surfactant between the nano-product and the medium in which they are dispersed. This possibly improves the disperse-ability of the nano-product.

Preferably, the mixture includes an organic solvent having a boiling point of 180 degree C  at ambient pressure, such as 1-octadecene. This allows the mixture to be sustained at a high temperature such as 150 degree C without boiling. FIG. 1 is flowchart for the production of titanium dioxide in the process developed at Hong Kong University of Science and Technology. 

FIG. 1b is flowchart for the production of TiO2 nanowires and nanorods. 

FIG. 3 is a HRTEM image of exemplary nitrogen-doped TiO2 nanowires.

FIG. 7 is a typical TEM image of TiO2 nanorods produced by a variation of the process. FIG. 7 shows nanorods produced by another variation of the first embodiment in which stearic acid is used instead of oleic acid. The resulting nanorods have a uniform diameter of about 2 nm and uniform length around 30 nm with a tendency of branching.

FIG. 8 is typical TEM images showing the evolution of products, produced from Ti-complex precursor to anatase titania atomic wires and nanorods and their self-assembled networks.