Metal oxide-based photoanodes are critical components of dye sensitized solar cells (DSSCs), which are photoelectrochemical cells for the conversion of solar energy, promising to have several benefits as compared with their traditional counterparts. A careful engineering of the wide band gap metal oxide composing the photoanode, as well as their process design, is strategic for improving device performances and for planning a near future production scale up, especially devoted to reducing the environmental impact of the device fabrication. Herein, we present the application of ZnO hierarchical structures as efficient materials to be applied as photoanodes in DSSC, in the perspective of looking for alternative to TiO<sub>2</sub> nanoparticles, currently the most exploited metal oxide in these devices.
ZnO@SnO<sub>2</sub> multilayered network was deposited on fluorine doped tin oxide (FTO) glass and applied as photoanode in dye sensitized solar cells whose functional performances are compared with single oxide-based photoanodes made of SnO<sub>2</sub> nanoparticles and ZnO microparticles. Multi-oxide photoanodes provide for enhanced photoconversion efficiency (3.31%) as compared with bare SnO<sub>2</sub> nanoparticles (1.06%) and ZnO microparticles (1.04%). Improved functional performances of the ZnO@SnO<sub>2</sub> layered network are ascribable to partial inhibition of back electron transfer from SnO<sub>2</sub> to the redox electrolyte, guaranteed by the ZnO, which acts as a capping layer for the underlying SnO<sub>2</sub>.
SnO<sub>2</sub> 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 NH<sub>3</sub>. 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.
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. MoO<sub>3</sub> and WO<sub>3</sub> have been identified as suitable materials for detecting these gases with high sensitivity. Sol-gel processed thin films of MoO<sub>3</sub>, WO<sub>3</sub> 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, NO<sub>2</sub>, Methanol, isoprene, etc) at 10%
relative humidity and at temperatures ranging from 400-500°C. The MoO<sub>3</sub>-WO<sub>3</sub> 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 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
Molybdenum trioxide - tungsten trioxide (MoO<sub>3</sub>-WO<sub>3</sub>) and titanium oxide - molybdenum oxide (TiO<sub>2</sub>-MoO<sub>3</sub>) 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. MoO<sub>3</sub>-WO<sub>3</sub> film morphology is composed of MoO<sub>3</sub> needle like grains and WO<sub>3</sub> spherical grains when annealed at 450°C. MoO<sub>3</sub>-TiO<sub>2</sub> 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 (NO<sub>2</sub>). The RF and SG fabricated MoO<sub>3</sub>-TiO<sub>2</sub> possess different gas sensing properties attributed to the fabrication and resulting morphological difference of the thin films.
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:TiO<sub>2</sub> and Ti:MoO<sub>3</sub>. 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, NO<sub>2</sub> and ethanol have been studied.