機能性金/二氧化錫核/衛星異質組裝結構之氣體感測應用

SnO2 nanoparticles (NPs) and their assemblies have been widely designed, synthesized and applied for gas sensing by screening the drastic changes of electrical resistance while absorbing or desorbing targeting gases. In this work, we mainly focus on the chemical preparation and optic-based gas-sensing application of functional Au-SnO2 core-satellite heteroassemblies (Au-SnO2 CSHA). The fabrication of such heteroassemblies involving two steps, i.e. the fabrication of Au cores (~13 nm) by direct reduction method and the following formation and assembling of SnO2 NPs (~2 nm) on the suspended Au cores, is especially designed and precisely optimized to approach purposes of both CO sensing and optical observation. From high resolution transmission electron microscopy (HRTEM) images, it is found that the assembled thickness as well as the morphology of the tiny SnO2 NPs can be precisely controlled by varying pH value and concentration of Sn precursors. The prepared Au-SnO2 CSHA were repeatedly washed with de-ionized water and then deposited onto plasma-treated glass substrates for a series of UV-visible absorption and CO sensing characterizations at various CO concentrations and substrate temperatures. From the UV-visible spectra measured at atmosphere at room temperature, as the thickness of the SnO2 NPs assemblies increased from 5 nm up to 9 nm, the absorption peak of the prepared Au-SnO2 CSHA greatly red shifted from 520 nm to 540 nm, where the solution color changed from red to purple. Based on classical Mie theory and the assumption of a simplified Au-SnO2 core-shell spherical model, the calculated spectrum peak of a 13 nm Au core coated with a dense 7 nm SnO2 shell located at 556 nm which is greatly red-shifted compared with the experimental one with a porous 7 nm SnO2 shell at 540 nm. In order to approach the real structural parameters for obtaining a peak at 540 nm, we further introduced the effective medium theory (EMT) for a theoretical estimation of the Au-SnO2 CSHA with a porous 7 nm SnO2 and found that with the assumption of a mixture of 50% (volume ratio) SnO2 NPs and 50% water, the calculated peak is coincident with the experimental one. The Au-SnO2 CSHA (7 nm SnO2) was applied for the in-situ investigation of CO sensing properties under controlled CO concentration from 5 ppm to 10000 ppm at various temperatures from room temperature to 300 oC. It was found that with the increase of CO concentration, the absorption intensity accordingly decreased over the full measured range from 200 nm to 1100 nm and a maximum change was found at around 544 nm which is the characteristic absorption of Au NPs. By analyzing the CO concentration dependent variation of the Au absorption intensity, CSHA exhibit a significant CO sensing sensitivity below 1000 ppm where an excellent absorbance change ratio (ACR) as high as 0.01885 can be approached at room temperature.