Preparation And Gas-sensing Properties Of Perovskite-type Nanocrystalline (La, Nd)_y(Fe, Co)_1-xZn_xO₃(y≤1)

There has long been interest in perovskite-type compounds (of general formula ABO₃ ) because of their high sensitivity , good selectivity and good stability. Another excellence is that its characteristic can be controlled by selecting suitable A and B atoms or chemical doping in ABO₃,. The samples (La, Nd)_y(Fe,Co)_1-xZn_xO₃ were prepared using sol-gel citrate method. The structure, electrical and gas-sensing properties of the sample were studied, the results demonstrate that samples (La, Nd)_y(Fe, Co)_1-xZn_xO₃ exhibited high sensitivity and better selectivity than oxide, the substitution of Zn increase the sensitivity and conductance. Following are summary of our works: 1. the structure, electrical and sensing properties of LaFe_1-xZn_xO₃ and LaCo_1-xZn_xO₃ samples(1) The X-ray diffraction patterns (XRD) showed that LaZn_xFe_1-xO₃, and LaFe_1-xZn_xO₃ nanocrystalline were perovskite phases with the orthorhombic structure. With an increase of x, the mean crystallite size increase at first then reduce. Because Zn²⁺ is larger than Fe³⁺, when the Fe³⁺ is replaced by the Zn²⁺ at the B-site, the unit cell volume should become larger with increasing x. on the other hand, in the view of charge compensation, oxygen vacancies are produced to maintain a neutral charge. Lattice might be somewhat smaller in order to maintain normal perovskite structure.(2) They all showed p-type semiconducting properties and the charge carries are holes (h·) obviously. By using Kroger -Vink defect notations, formation of charge carriers can be expressed as follows:When Fe(Co) in La(Fe,Co)O₃ is replaced by Zn, the carrier concentration will depend on the holes produced by the ionization of [Zn_Fe^x]or [ZnZn_Co^x]:Zn_Fe^x → ZnZn_Fe' + h^· Zn_Co^x→Zn_Co'+h^·In this formula, Zn_Fe^x means a point defect produced when a Zn ionoccupies the sites of a Fe ion in the crystal Upon the addition of Zn ion, holes (h^·) will be generated based on this equation.As a result of [Zn_Fe^x]or [Zn_Co^x]>>[V_La^x], the concentration of h·increases, which results in the conductivity of Zn-doping samples considerably higher than that of LaFeO₃.(3) Results demonstrate that LaFe_0.77 Zn_0.23O₃showed higher sensitivity to HCHO among LaFe_1-xZn_xO₃, The LaFe_0.77 Zn_0.23O₃-based gas sensor exhibits short response and recovery times to HCHO, so it is a promising gas sensor for detecting HCHO. so we can say that LaFe_0.77 Zn_0.23O₃ is a promising sensor for detecting HCHO.(4)LaCo_1-xZn_xO₃ show high sensitivtity to ethanol, but low sensitivity to other gases.(5) Nd_yFe_1-xZn_xO₃ powders were prepared using sol-gel method, XRD showed that the powders was single orthogonal perovskite structure. The CH₄ gas-sensing properties of Nd_yFe_1-xZn_xO₃ nano-powders have been studied. Among all the investigated materials, Nd_0.85Fe_0.93Zn_0.07O₃ showed maximum sensitivity to CH₄ gas. The sensitivity S of Nd_0.85Fe_0.93Zn_0.07O₃at optimal working temperature 200℃ is 3.87, 16 for 4×10^-6mol/l, 2×10^-5 mol/L CH₄ respectively. The response and recovering times under 2×10^-5 mol/L CH₄ for Nd_0.85Fe_0.93Zn_0.07O₃ at 200℃ are 25s and 45s, respectively. The HCHO gas-sensing properties of Nd_yFe_1-xZn_xO₃ nano-powders have been studied. The results demonstrate that Nd_0.92Fe_0.96 Zn_0.04O₃ showed higher sensitivity and good selectivity to HCHO.(6) XRD showed that La_yFe_1-xZn_xO₃ powders were not single perovskite structure but with a little ZnFe₂O₄ phase. La_yFe_0.77Zn_0.23O₃ showed high sensitivity of 1300 to ethanol this is because La_yFe_0.77Zn_0.23O₃ is a p-type semiconductor, ZnFe₂O₄ is a n-type semiconductor, a lot of heterojunctions were formed, which results in the increasing of resistance. La_yZn_0.23Fe_0.77O₃-based sensor showed higher sensitivity forethanol.(7) XRD showed that La_yCo_1-xZn_xO₃ powders were not single perovskite structure but with a little ZnCo₂O₄ phase. The sensitivity of La_yCo_1-xZn_xO₃ to xylene is a little better than to acetone and formaldehyde, but it showed bad s-c linearity to xylene concentration.