(Sn,Ti,Nb)xO2 Solid Solution: An Innovative Nanostructured Material and Its Chemoresistive Properties

Abstract

Introduction Recently, great attention has been paid to solid solutions and composite materials based on metal oxides in order to obtain nanostructures with enhanced sensing performance with respect to those of single-oxide counterparts [1]. Tin and titanium dioxides (SnO2 and TiO2) are wide-gap n-type semiconductors extensively investigated for the fabrication of solid-state devices for gas sensing applications. Albeit SnO2-based gas sensors exhibit high response to reducing gases, they suffer of poor selectivity and degradation of electrical properties at low oxygen partial pressure and high operating temperatures upon prolonged exposure to reducing gases [2]. Instead, TiO2 is more thermally stable than SnO2, but it has a lower sensitivity due to a higher density of surface states, that entails the pinning of the Fermi level [3]. SnO2 and TiO2 would easily form solid solutions because they can exhibit a rutile type structure where octahedrally coordinated Ti4+ and Sn4+ have similar ionic radii (i.e. 0.605 Å and 0.69 Å, respectively) [4,5].In an earlier investigation, the sensing properties of Sn1-x TixO2 (0.2<x<0.7) oxides prepared through sol-gel route from stoichiometric solutions of the two metal-alkoxides have been explored [6]. Sn1-xTixO2 solid solution showed better sensing performances than SnO2 and TiO2 under different target gases, with the Sn0.7Ti0.3O2 solid solution overperforming with respect to the other investigated compounds [6].Despite the Sn1-x Ti x O2 good sensing performances, a further improvement on the sensing aptitude of these materials has been attempted by means of a Nb doping. The incorporation of Nb5+ within the Sn0.7Ti0.3O2 lattice would increase the conductivity of the material, as niobium acts as donor dopant in n-type semiconductors and it can inhibit grain growth [7]. Scope and Methods The aim of this work was to study the sensing properties and the structural features of solid solutions based on Sn, Ti and Nb oxides. According to what reported in a previous investigation [6], different sample of (Sn,Ti,Nb)xO2 were synthesized keeping constant the Sn/Ti stoichiometric ratio (70/30) at different Nb doping of 2, 3.5 and 5% of the total metal amount, and hereafter labelled as STN 2, STN 3.5 and STN 5. As synthetized powders were heated at 650 and 850 °C for 2h (STN Nb% 650, STN Nb% 850) to investigate the possible influence of the temperature on grain size, Nb percolation and sensing properties. Observations at SEM microscopy revealed that the morphology of the samples consists of rounded nanoparticles. Samples heated at 650 °C are composed by grains with diameters from 10 to 30 nm, while those heated at 850 °C showed a more uniform grain size distribution (30-40 nm). Along with a small fraction of TiO2 nanocrystalline phase (about 2.5% on average), X-ray powder diffraction analyses confirm that the (Sn,Ti,Nb)xO2 with rutile-type structure is the main phase in all the investigated samples. On the base of the refined unit-cell parameters a 17% substitution of Ti4+ (i.r. 0.605Å) and Nb5+ (i.r.=0.64Å) for the bigger Sn4+ (i.r.=0.69Å) was estimated.The sensing performance of (Sn,Ti,Nb)xO2 sensors were investigated toward various gases (H2, CO, ethanol, NO2, CH4 and acetaldehyde) at their optimal working temperature of 450 °C. As shown in figure 1a, it was found that the STN 2 650 sample exhibit a response to hydrogen 6 to 8 times higher than STN 2 850 and SnO2 650, respectively. Moreover, STN 2 650 displays a better stability in wet conditions (Fig 1b). Despite the increase of relative humidity (RH%) affects the response to hydrogen, the dynamic response to H2 of the sample heated at 650 °C remains still high (Fig. 1c).Thanks to its outstanding characteristics, (Sn,Ti,Nb)xO2 solid solution is a promising candidate for hydrogen detection. The sensing properties towards other gases will be considered.