Functionalized ZnO nanostructures for gas sensing and photovoltaic applications

Zinc oxide (ZnO) is an incredibly versatile material that in its nanostructured form is 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 only once they are functionalized with different organic or inorganic materials for tailoring their intrinsic properties towards the final application. In this work authors report some meaningful examples of surface functionalization of their 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 accurately by electron microscopy (SEM and TEM), x-ray diffraction (XRD) and optical measurements (optical absorption, photoluminescence, and cathodoluminescence). The heterostructures properties have been also studied by functional characterizations such as chemoresistive gas sensing tests, photocurrent and photocatalytic activity measurements. The correlation between structural and functional properties has then been discussed. In the case of functionalization with CdS, for example, a thickness dependent effect of charge transfer to the ZnO nanostructure has been demonstrated, together with an enhancement of the photocatalytic properties, and the modification of the gas sensing response mechanisms. In the case of functionalization with organics, different emission spectra have been observed together with different morphologies of the obtained depositions. These phenomena have been discussed in terms of distinct molecular bonding of phthalocyanines and porphyrins at the interface with the oxide nanostructures.