Multifunctional ZnO-based nanostructures

Zinc oxide (ZnO) nanostructures are studied for a large number of applications in optoelectronics, photovoltaics, gas and bio-sensing, piezoelectric devices, photocatalysis, spintronics, etc. Recently this group has successfully reported the optimized not-catalyzed synthesis processes for producing single morphologies of ZnO nanostructures (i.e. nanowires [1], aligned nanorods [2], and nanotetrapods [3]) among the many different ones that are generally obtained in vapor phase reactions. The proper combination of metal thermal evaporation and controlled oxidation has produced good quality nanostructures, whose properties were not affected by contamination from catalysts or precursors. The great potential of these nanostructures, however, can be fully explored when they are functionalized with different organic or inorganic materials for tailoring ZnO intrinsic properties towards the final application. In this work authors report some meaningful examples of surface functionalization of ZnO nanostructures by inorganic (calcogenides) and organic (phthalocyanines and porphyrins) semiconductors for applications in gas sensing, photovoltaics and photocatalitic degradation of water and gas pollutants. Although different techniques were used (an optimized chemical bath deposition - CBD - process and a supersonic molecular beam deposition - SuMBD - apparatus), in both cases particular attention has been devoted to the interface formation in the coupled compounds, in order to obtain working heterostructures where charge carriers can be efficiently separated and transferred. The obtained functionalized ZnO nanotructures have been characterized by electron microscopy (SEM and TEM), x-ray diffraction (XRD) and optical measurements (optical absorption, photoluminescence, and cathodoluminescence). The heterostructure properties have been also studied by functional characterizations such as chemo-resistive gas sensing tests, photocurrent and photocatalytic activity measurements. In particular the CdS/ZnO interface show good optical properties (exciton separation and quench of CdS luminescence), unexpected gas-sensing behavior (enhanced selectivity by inversion of chemoresistive response) and enhanced photocatalytic activity