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Description
As a semiconductor metal oxide with perovskite structure, LaCoO3 is of interest for chemical sensors. The hole-type conduction occurs via Co-O framework. The surface of LaCoO3 nanostructures exhibits different adsorption sites (La3+ and Co3+) and active sites (chemisorbed oxygen, lattice anions) for gas molecules reception. The sensing mechanisms with LaCoO3 and its nanocomposites are unclear. In this work we obtained nanocrystalline LaCoO3 modified by Ag nanoparticles with improved sensitivity and selectivity to H2S, characterized the microstructure and surface sites of materials, and proposed the sensing routes during gas-solid interaction.
Nanocrystalline LaCoO3 with particle size 30-80 nm (Fig. 1) and specific surface area 5-10 m2/g was obtained by sol-gel synthesis using ethylenediamine as a coordination ligand. The samples were impregnated by Ag nanoparticles with the size increasing in the range 30-60 nm on increasing silver percentage 2-5 wt.%. XPS spectroscopy demonstrated the presence of La3+, Co3+, O2- ions in the bulk along with a large fraction of chemisorbed oxygen species. Metallic Ag nanoparticles were observed by XPS and XRD. The DC-resistance increased in presence of Ag due to electrons donation into p-type LaCoO3. The Ag/LaCoO3 nanocomposites demonstrated higher sensitivity to 0.2-5 ppm H2S at 200 ºC, in comparison to pure LaCoO3 (Fig. 2). Cross-sensitivity tests showed about 10-times higher sensor response of Ag/LaCoO3 to 2 ppm H2S, as opposed to 20 ppm CO and NH3 (Fig. 3). On DRIFT spectra of the samples Ag/LaCoO3 exposed to H2S at 200 ºC the evolution of peaks was observed relevant to adsorbed H2S, Ag2S and SO42- groups (Fig. 4a). Thus, the sensing process occurred via H2S adsorption favored by Ag nanoparticles and oxidation to sulfur oxide and sulfate species on the LaCoO3 surface. The reaction products, except SO42-, disappeared during further exposure in air, which accounts for sensor recovery (Fig. 4b). The persistent sulfate species were likely inactive by-products that did not affect the sensors behavior.
Summary
Nanocrystalline LaCoO3 was synthesized by sol-gel method and functionalized by Ag nanoparticles via impregnation. An improved sensitivity to H2S gas was detected for the Ag/LaCoO3. The nanocomposite sensors showed lower cross-sensitivity to CO and NH3, in comparison to pure LaCoO3. The role of Ag nanoparticles in promotion of the H2S adsorption and oxidation on the surface of LaCoO3 was elucidated using diffuse reflectance infrared Fourier-transformed (DRIFT) spectroscopy.
