We demonstrate that conductometric gas sensing at room temperature with SnO2 nanowires is enhanced by visible and supra bandgap UV irradiation when and only when the metal oxide nanowires are decorated with Ag nanoparticles (r < 20 nm); no enhancement is observed for the bare SnO2 case. We combine spectroscopic techniques with conductometric gas sensing to study the wavelength dependency of the sensors response, showing a strict correlation between the Ag loaded SnO2 optical absorption and its gas response as a function of irradiation wavelength. Our results lead to the hypothesis that the enhanced gas response under UV-vis light is the effect of plasmonic hot electrons populating the Ag nanoparticles surface. Finally we discuss the chemoresistive properties of Ag loaded SnO2 sensor in parallel with the theory of Plasmon-Driven Catalysis, to propose an interpretative framework that is coherent with the established paradigma of these two actually separated fields of study.
The modification of the surface reception properties of nanocrystalline structures is of great interest in environmental, catalysis and energy related applications. For instance, an oxide surface covered with a layer of another oxide opens the possibility of creating the nanosized counterparts of bulk catalytic systems. A relevant example is the TiO2-WO3, which is an active catalysts in a broad range of reactions. The chemical synthesis of the colloidal, nanocrystalline version of such system will first be exposed, by coupling suitable sol-gel chemistry with solvothermal processing. Then, the range of obtained structures will be discussed, ranging from WOx-surface modified TiO2 to TiO2-WO3 heterojunctions. The complex structural evolution of the materials will be discussed, depending on the W concentration. A summary of the acetone sensing properties of these systems will be shown. In particular, the surface activation of the otherwise almost inactive pure TiO2 by surface deposition of WO3-like layers will be highlighted. Addition of the smallest W concentration boosted the sensor response to values comparable to those of pure WO3, ranging over 2-3 orders of magnitude of conductance variation in presence of ethanol or acetone gases. Simple analysis of the sensing data will evidence that the combination of such nanocrystalline oxides results in catalytic activation effects, with exactly opposite trend, with respect to pure TiO2, of the activation energies and best responses.
SnO2 and ZnO and metal oxide nanowires were synthesized by vapor transport process in a horizontal tube furnace. The
peculiar characteristic of these materials is the emission of visible photoluminescence (PL) when they are excited with
UV light. The visible photoluminescence of tin and zinc oxide nanowires is quenched by nitrogen dioxide at ppm level in
a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity
and other polluting species, such as CO and NH3. We believe that adsorbed gaseous species that create surface states can
quench PL by creating competitive nonradiative paths.
The material properties of the nano-structured materials show remarkable improvement or deviation from the properties
exhibited by the coarser grained material. These unique properties are attributed to the significant increase in grain
boundary area due to the small grain size. The possibility to manipulate the properties of a nanosized thin film simply
through annealing appears to be of widespread interest for material science. In the gas sensing field of application there
is a great effort in reducing the grain dimension and increasing the surface area exposed to the interaction with gaseous
species. One of the strategies used is the addition of a second element, which can inhibit the grain growth. Furthermore,
there may be a coexistence of two phases and one phase can act as a receptor while the other can act as transducers and
an effect on film porosity is also expected, depending on the extent of oxide segregation from the nanosized film. Thin
films made of Mo-Ti, Mo-W, Ti-W, Ti-Nb mixed oxides were achieved by reactive sputtering, assisted by thermal
treatments. These layers were characterized by means of the electrical measurements in presence of different pollutants
and alcohols and with the Kelvin probe at different working temperatures; the good sensing capabilities registered with
these mixed oxide compared to their single oxides have to be ascribed to the nanosized structure of these layers. In
particular different p-type sensing materials were produced, the opposite behavior of these layer is attractive to ease data
processing in sensors arrays.
The work function of nano Porous Silicon (PS) has been studied by the kelvin probe method as a function of the exposure to different gaseous species. Characterisation has been performed n dark and in presence of sub band and supra band gap light Surface Photovoltage (SPV)measurements. Traces of ammonia and nitrogen dioxide change drastically the shape of SPV as a function of photon energy:light induces transitions from and to surface states produced by gas adsorption. The results foresee the possibility to improve semiconductor sensor selectivity by using monochromatic light at well defined frequency able to activate/deactivate surface states where species are adsorbed
The work function of tin oxide has been studied by the kelvin probe method as a function of the exposure to different gaseous species. Characterisation has been performed in dark and in presence of sub band and supra band gap light (Surface Photovoltage measurements). The light changes the response towards gases in particular at room temperature. The results foresee the possibility to improve semiconductor sensor selectivity by using monochromatic light at well defined frequency able to activate/deactivate surface states where species are adsorbed.
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