Metal-oxide-based nanocomposites comprising advanced gas sensing properties

Abstract

Abstract. Using the sol-gel technique, nanocomposite SiO 2 -SnO 2 thin films comprising a designed hierarchical pore structure were prepared on cost-efficient oxidized silicon substrates. These films exhibit advanced gas sensing properties both for reducing gases and oxygen due to an efficient analyte transport by mesoporores. IntroductionGas sensitivity of metal oxide gas sensors increases rapidly when the dimensions of oxide sensing materials become comparable or smaller than the typical thickness of the electron depletion layer [1]. Therefore, nanosized materials exhibit outstanding properties in sensor application, particularly low operation temperature and high sensitivity, attributed to their large surface area to volume ratio.Hierarchical structures are higher dimensional structures composed of many, lower dimensional building blocks. Because they are generally larger than the individual nanostructures, their van der Waals attraction is relatively weak. On the other hand, hierarchical microstructures provide an effective gas diffusion path via well aligned mesoporous structures without sacrificing a high surface area [2]. Consequently, they exhibit high gas sensitivity and fast gas response. Hierarchical nanostructures for gas sensors were prepared by vapour phase growth, hydrothermal or hydrothermal/solvothermal reactions [2], by electrospinning and subsequent oxygen plasma etching [3], by ultrasonic spray pyrolysis [4], by flame-spray-pyrolysis [5,6], and by pulsed laser deposition techniques [7]. Well defined hierarchical pore structures were prepared by sol-gel method in alkoxyderived silica systems [8] as well as in aluminium salt solutions [9] utilizing a phase separation mechanism as a pore forming principle. Recently, one of our laboratories has applied this approach for the fabrication of SnO 2 -SiO 2 composites comprising hierarchical macro-, meso-and micropores (according to IUPAC notation [10]). This minimises the build up of thermal stresses during sensor fabrication and, therefore, improves adhesion. Moreover, our technology is based on inexpensive precursors such as SnCl 2 and tetraethylorthosilicate (TOES). In the first case, advantages compared to alkoxides are besides the precursor costs a reduced sensitivity of the precursor to heat, moisture and light, and an improved time stability of the sol against the increase in viscosity. On the other hand, the presence of Cl -increases the gelling time. In this work, we investigate gas sensing properties of sol-gel deposited nanocrystalline SnO 2 -SiO 2 composites in relation to their microstructure and demonstrate gas sensitivities at the state-of-art level.

Band alignment and depletion zone at ZnO/CdS and ZnO/CdSe hetero-structures for temperature independent ammonia vapor sensing

“…[1][2][3] The metal oxide based sensors are capable of sensing a wide range of hazardous gas molecules (methane, liquefied petroleum gas (LPG), hydrogen peroxide (H 2 O 2 ), ethanol, ammonia, hydrochloric acid (HCl), trimethylamine, hydrogen sulphide (H 2 S), toluene) with varying response time. 4,5 To accomplish high sensitivity, high selectivity, and fast responsivity many proposals and techniques have been adopted, such as doping the metal oxide with additives, 6 incorporating a noble metal catalyst, 7 controlling the sensor operating temperature, 8 applying an external driving force (electric field and magnetic field), 9 incorporating conducting polymers, 10 irradiating the sensor with ultraviolet and visible radiation, 11 nanocomposite formation 12 and assembling small sized particles on the material surface. 13 Among them, the hierarchical structural assembly of the nanoparticles on the metal oxide surface for tuning the surface activities/properties is gaining great insight into the present sensor fabrication.…”