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.
Selective detection of small amounts of toxic gases, such as ammonia and CO is very important to environmental monitoring as well as for medical diagnoses. MoO3 and WO3 have been identified as suitable materials for detecting these gases with high sensitivity. Sol-gel processed thin films of MoO3, WO3 and their combination have been prepared at SUNY Stony Brook by the hydrolysis of metal alkoxide precursors followed by spin coating and were deposited on alumina heater/electrode
containing substrates that were produced by the Brescia group. Sensing tests were carried out in the state-of-the-art gas
sensor testing facilities available in Brescia, where the electrical resistance of sensor arrays was recorded as a function
of gas concentration, for various combinations of gases (including ammonia, CO, NO2, Methanol, isoprene, etc) at 10%
relative humidity and at temperatures ranging from 400-500°C. The MoO3-WO3 composite system showed the best
stability at the highest testing temperature. The sensing results obtained are correlated with the structural characteristics
of the sensing films. This work has been carried out as a joint collaboration between the Advanced Materials Characterization Laboratory of SUNY Stony Brook (USA) and the Sensor Lab at the University of Brescia (Italy) and was funded by a NSF-AAAS (WISC) grant awarded to Perena Gouma.
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 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.
Nanocomposite materials have attracted considerable research effort in recent years both because of their potential technological applications and because of their relevance to the study of basic physical properties in this range of sizes. On decreasing the dimensions of a material to the nanometre scale, many new effects appear, such as quantum size effects, which can result in new physical properties so that nanostructured materials can show novel optical, electrical, magnetic and catalytic properties that are different from those of bulk materials. The electrical conductivity is also related to the grain size and separation and the gas sensing properties. Many techniques can be employed to fabricate nanocomposite films, such as sol-gel, sputtering and ion implantation. In this work we have deposited mixed oxide of Mo-W, Ti-W and Mo-Ti by sputtering. The production of chemoresistive semiconducting films through selective sublimation processing Ti-W-O, Mo-W-O has been obtained and furthermore the manipulation of a metal-oxide properties by doping Mo:TiO2 and Ti:MoO3. This technique consists of deposition of a mixed-oxide layer, the annealing of the film leads to depletion of the phase with lowest sublimation temperature. The method allows the achievement of films ideal for gas sensing via chemoresistive effect: grain-coalescence inhibition, i.e., nanostructured materials, porous films and modifiable composition. The gas sensing properties of these materials toward gases like CO, NO2 and ethanol have been studied.
Molybdenum trioxide - tungsten trioxide (MoO3-WO3) and titanium oxide - molybdenum oxide (TiO2-MoO3) binary metal oxide thin films have been prepared by the sol-gel process and by PVD. The films were deposited using the spin coating technique and by sputtering onto alumina substrates with interdigital electrodes. MoO3-WO3 film morphology is composed of MoO3 needle like grains and WO3 spherical grains when annealed at 450°C. MoO3-TiO2 film morphology consists of a well-developed crystal structure for the sol-gel film and a porous morphology for the sputtered films when annealed at 800°C. The films exhibited selective gas sensing characteristics at an operating temperature of 300°C towards nitrogen dioxide (NO2). The RF and SG fabricated MoO3-TiO2 possess different gas sensing properties attributed to the fabrication and resulting morphological difference of the thin films.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.