Photocatalytic ZnO Nanopowder in Tetrapod Shape Is More Efficient than Round Shape

The Industrial Technology Research Institute (ITRI) (Hsinchu County, TW) earned U.S. Patent 7,666,385 for a nanofabrication method for improved zinc oxide (ZnO) powdered photocatalyst. The manufacturing method of the photocatalytic nanopowders is achieved by the non-transferred DC plasma apparatus in an atmosphere of nitrogen at around 1 atm. The nitrogen-containing gas is used as the plasma-forming gas. After the generation of the nitrogen-plasma in the non-transferred DC plasma apparatus, a plurality of solid zinc (Zn) precursors are introduced to the nitrogen-plasma for vaporization and oxidization. The solid Zn precursors are vaporized and oxidized through homogeneous nucleation and are rapidly cooled down by a large amount of cooling gas (i.e. mixture of nitrogen and oxygen). After the cooling process, the tetrapod-shaped and nitrogen-doped photocatalytic ZnO nanopowders with a wurtzite structure are formed, according to inventors Shih-Chieh Liao, Song-Wein Hong and Hsiu-Fen Lin

An electron microscope (EM) photograph of the photocatalytic ZnO nanopowders made through the ITRI method can be found in FIG. 1. The photocatalytic ZnO nanopowders shown in FIG. 1 are in a shape of tetrapod; wherein each crystalline rod of the tetrapod has a cross-section of hexagon. Moreover, each rod of the photocatalytic ZnO nanopowders grows along a direction of  and has a diameter less than 100 nm (the average diameter is about 25 nm, see FIG. 2).

Among the nanopowders produced through the Industrial Technology Research Institute method,  the photocatalytic ZnO nanopowders in a shape of tetrapod have better photocatalytic effect. The average diameter of each rod of the tetrapod-like ZnO nanopowders is about 25 nm. The photocatalytic ZnO nanopowders in the tetrapod shape can stand up and attach to the surface  by the assistance of binder0. Hence, ideally at least one of the four crystalline rods of the tetrapod-like ZnO nanopowders can protrude to perform photocatalysis. In other words, the photocatalytic efficiency of the tetrapod-like photocatalytic ZnO nanopowders is better than that of the photocatalytic ZnO nanopowders in a conventional ball shape.   

Photocatalysts such as ZnO and TiO2 nano-particles have drawn much attention due to their applications in antibacterials, water treatment, deodorants, NOX decomposition, self-cleaning and so on. Conduction-band electrons and valence-band holes are generated on its surface when a photocatalyst is illuminated by light with energy greater than its band gap energy. Holes can then react with water molecules adhering to the surface of the photocatalyst to form highly reactive hydroxyl radicals (OH.).

Oxygen here acts as an electron acceptor by forming a super-oxide radical anion (O₂^-.) on the surface. The super-oxide radical anions may act as oxidizing agents or as an additional source of hydroxyl radicals via the subsequent formation of hydrogen peroxide. The powerful oxidants associated with hydroxyl radicals are able to oxidize organic materials. When the cell membrane of bacteria is in contact with the powerful oxidants, it will be decomposed and consequently the bacteria will die. The body of the bacteria will eventually be decomposed into carbon dioxide and water. Photocatalysts not only kill the bacteria, but also clean their bodies. Therefore, the objective for cleaning and sterilization can be easily achieved by the assistance of photocatalyst. 

FIG. 1 is an EM photograph of the photocatalytic ZnO nanopowders made through the ITRI method. FIG. 2 is an EM photograph of the photocatalytic ZnO nanopowders in a shape of tetrapod made through the Industrial Technology Research Institute method.

FIG. 3 is a schematic view showing the photocatalytic ZnO nanopowders in a shape of tetrapod sprayed on the surface of a substrate