Current perspective of nano-engineered metal oxide based photocatalysts in advanced oxidation processes for degradation of organic pollutants in wastewater

Semiconductor-based photocatalysis attracts wide attention because of its ability to directly utilize solar energy for production of solar fuels, such as hydrogen and hydrocarbon fuels and for degradation of various pollutants. However, the efficiency of photocatalytic reactions remains low due to the fast electron-hole recombination and low light utilization. Therefore, enormous efforts have been undertaken to solve these problems. Particularly, properly engineered heterojunction photocatalysts are shown to be able to possess higher photocatalytic activity because of spatial separation of photogenerated electron-hole pairs. Here, the basic principles of various heterojunction photocatalysts are systematically discussed. Recent efforts toward the development of heterojunction photocatalysts for various photocatalytic applications are also presented and appraised. Finally, a brief summary and perspectives on the challenges and future directions in the area of heterojunction photocatalysts are also provided.

The regulation of engineered nanoparticles requires a widely agreed definition of such particles. Nanoparticles are routinely defined as particles with sizes between about 1 and 100 nm that show properties that are not found in bulk samples of the same material. Here we argue that evidence for novel size-dependent properties alone, rather than particle size, should be the primary criterion in any definition of nanoparticles when making decisions about their regulation for environmental, health and safety reasons. We review the size-dependent properties of a variety of inorganic nanoparticles and find that particles larger than about 30 nm do not in general show properties that would require regulatory scrutiny beyond that required for their bulk counterparts.