Nanostructured semiconductor films: synthesis, surface functionalization and innovative applications

Environmental pollution is one of the great challenges of the 21st century. The WHO has recently estimated that air pollution causes 600,000 premature deaths each year in Europe. Another major health concern is water pollution by compounds that resist biodegradation and conventional purification processes. Photocatalytic degradation of pollutants is an emerging technology in environmental protection, that has been successfully applied to the degradation of numerous organic and inorganic pollutants, both in gas-phase and in solution. With respect to conventional purification processes, photocatalysis by nanostructured semiconductors can lead to the complete degradation of highly recalcitrant pollutants to harmless substances using only light to activate the process. Some photocatalytic concretes and paintings that degrade pollutants are already on the market, but many problems remain to be overcome to obtain commercially successful products. The ultimate aim of research in this field is using solar light as an environmental-friendly and inexpensive light source to activate photocatalysis. However, the most stable and active photocatalysts are activated only by UV light, which accounts for only about 5% of the solar spectrum. During her PhD, Daniela Meroni synthesized oxide-based photocatalysts active under visible light by doping with nonmetal ions. By an innovative combined approach involving both advanced characterization techniques and ab initio calculations, she clarified the dopant position within the semiconductor lattice and its influence on the electronic structure and photoactivity of the photocatalyst. These materials were successfully tested for the degradation of gas-phase pollutants, such as volatile organic compounds (VOCs), under solar light. She also increased the efficiency of these materials by tailoring their synthetic pathway using soft templates and mixed oxides, and improved their durability by combining the photocatalytic approach with other advanced oxidation processes. In this way, she was able to obtain for the first time the complete degradation of an important recalcitrant endocrine-disrupting pollutant (cumylphenol), shedding light on its degradation mechanism. With the aim of integrating nanostructured oxides in more complex devices, D.M. investigated their functionalization with alkylsilanes. In this way, she was able to tune their surface properties, such as surface energy, wetting and adhesion. The resulting materials were used to obtain self-cleaning films showing both superhydrophobic and superoleophobic properties. Furthermore, she exploited the oxide photocatalytic activity to create superhydrophobic/superhydrophilic patterns by irradiation with UV light through a photomask. The obtained wetting pattern was used to promote the site specific adsorption of molecules and nanoparticles. She also investigated another patterning technique, probe-based electrooxidative lithography, developing the first application of this patterning technique with nanometer-scale resolution to substrates other than silicon wafer. These studies laid the foundations for the integration of nanostructured oxides in complex devices, such as self-cleaning sensors for environmental monitoring.