Photochemical sensing of NO(2) with SnO(2) nanoribbon nanosensors at room temperature.

A major area of application for nanowires and nanotubes is likely to be the sensing of important molecules, either for medical or environmental health purposes. The ultrahigh surface-to-volume ratios of these structures make their electrical properties extremely sensitive to surface-adsorbed species, as recent work has shown with carbon nanotubes,[1, 2] functionalized silicon nanowires and metal nanowires.[3, 4] Chemical nanosensors are interesting because of their potential for detecting very low concentrations of biomolecules or pollutants on platforms small enough to be used in vivo or on a microchip. Here we report the development of photochemical NO2 sensors that work at room temperature and are based on individual single-crystalline SnO2 nanoribbons. Tin dioxide is a wide-bandgap (3.6 eV) semiconductor. For n-type SnO2 single crystals, the intrinsic carrier concentration is primarily determined by deviations from stoichiometry in the form of equilibrium oxygen vacancies, which are predominantly atomic defects.[5] The electrical conductivity of nanocrystalline SnO2 depends strongly on surface states produced by molecular adsorption that results in space-charge layer changes and band modulation.[6] NO2, a combustion product that plays a key role in tropospheric ozone and smog formation, acts as an electron-trapping adsorbate on SnO2 crystal faces and can be sensed by monitoring the electrical conductance of the material. Because NO2 chemisorbs strongly on many metal oxides,[7] commercial sensors based on particulate or thin-film SnO2 operate at 300 ± 500 C to enhance the surface molecular desorption kinetics and continuously TMclean∫ the sensors.[8] The high-temperature operation of these oxide sensors is not favorable in many cases, particularly in an explosive environment. We have found that the strong photoconducting response of individual singlecrystalline SnO2 nanoribbons makes it possible to achieve equally favorable adsorption ± desorption behavior at room temperature by illuminating the devices with ultraviolet (UV) light of energy near the SnO2 bandgap. The active desorption process is thus photoinduced molecular desorption (Figure 1).[9] In conclusion, we have succeeded in the development of the ruthenium-based metathesis catalyst 4, which exhibits excellent metathesis activity, without any loss of stability in air. These findings once again demonstrate that seemingly small variations in ligand structure can result in significant improvements in catalysis.