Tungsten trioxide (WO3), as a typical transition metal oxide, receives extensive attention because of its great potential applications in chromogenic, gas sensor , photocatalysis and batteries. Electrochromism is the phenomenon that a material can change its optical properties reversible by application of an electric field.

Nitrogen oxides (NOx) have become one of the main atmospheric pollutants with the increasing consumption of fossil fuels as well as the rapid growth of the automobile exhaust emissions. NOx causes serious environmental problems such as acid rain, photochemical smog, and ozone depletion.

Trivalent lanthanide ions (Ln3+) have caught a lot attention due to its applications as upconverting luminescent materials in biological analysis, counterfeit in printed documents, and back-lighting in liquid crystal displays.

Vehicle emissions by burning fossil fuels had produce serious air pollutions. Some major pollutants include itrogen oxides (NOx), sulfur dioxide (SO2) and volatile organic compounds (VOC). As the indoor environment has attracted more attentions, the indoor air purification device is becoming more popular in the market.