Recently, nanoparticles have been studied in various fields such as medical treatment, bioengineering, environment and energy, due to their high activity caused by large surface area. In special, metal oxide semiconductors have been used for gas sensors, photo- and electrocatalysts. These metal oxide semiconductor nanoparticles show distinct properties dependent upon morphology, sizes, and oxidation states, and thus numerous scientists have studied in many directions to enhance the sensitivity or selectivity for catalytic reactions.
In charter 2, we suggested Co-rich $ZnCo_2O_4$ hollow nanospheres for the sensitive detection of formaldehyde gas. The resulting nanospheres show the lowest value of the detection limit ever reported. In addition, the selectivity of formaldehyde and durability were high, too. The composition and nanostructure of this material would be a good reference for enhancing a sensitivity, selectivity and stability of gas sensors with other semiconducting materials.
In chapter 3, we synthesized ZnO-$Cu_2O$ hybrid nanostructures bearing uniform junctions and employed them to photochemical $CO_2$ reduction. The ZnO-$Cu_2O$ hybrid catalysts showed the methane generation with more than 99% selectivity and a high activity. This activity is the highest among photochemical $CO_2$ reduction studies in aqueous medium thus far. To identify the junction effect between ZnO and $Cu_2O$, the control experiments were conducted only with ZnO or $Cu_2O$ nanoparticles, confirming the fact that well-defined junctions could enhance the activity. Further, the mechanism studies were conducted to explain the photocatalytic properties of ZnO-$Cu_2O$ hybrid catalysts. For photochemical $CO_2$ conversion reactions, this result indicates that the photocatalysts other than TiO2 can exhibit high activity by the control of their structures and compositions. In addition, the Z-scheme like mechanism in this study would be a good reference for understanding other photocatalytic reactions using various semiconducting materials.
In charter 4, we synthesized copper oxide nanoparticles with two different morphology and oxidation states, i. e. branched CuO and cube $Cu_2O$, and employed them for the electrocatalytic $CO_2$ reduction reaction. Particularly, branched CuO nanoparticles show over the selectivity of ethylene generation more than 70%, which is one of the highest values ever reported. This result figures out that the disintegration of copper nanoparticles significantly affects the selectivity in electrochemical $CO_2$ reduction reactions.