Molecular shape-selective detection by tin oxide film sensor modified with chemical vapor deposition of molecular-sieving silica overlayer using organic template

Abstract

A film (several hundred micrometers in thickness) made of tin oxide powder was loaded on an insulator plate, and the particle surface was covered with a molecular-sieving silica overlayer by a chemical vapor deposition method using a molecular template. Electrical response as a chemical sensor was then examined to hexane isomers. The SiO 2/SnO 2 sensor prepared using benzaldehyde as a template showed a high response to linear hexane, while quite low responses were observed to branched isomers (2-methylpentane and 2,2-dimethylbutane). It is suggested that the branched isomers could not react with the tin oxide surface because they could not penetrate into the cavities in the silica overlayer templated by the pre-adsorbed molecules. SiO 2/SnO 2 prepared using a butanal template also showed quite low responses to the branched alkanes, and in addition, the response to hexane was slow. Use of a 1-naphthaldehyde template allowed the reactions of all the examined isomers. These are in agreement with molecular models, showing that the response was controlled by the shapes of alkane molecules and surface cavities; the latter had been controlled by the molecular shape of the template. Thus, clear shape selectivity was demonstrated for the sensing of hydrocarbons. The selectivity was low on a bulky pellet sensor (5 mm thickness and 10 mm diameter), where the branched alkanes showed relatively high responses even in the case that benzaldehyde was used as a template. X-ray photoelectron spectroscopy showed that the concentrations of the template and silica were low inside the pellet, resulting in the high responses to the branched alkanes of the pellet. The high selectivity was obtained by changing the sensor morphology from the pellet to the film.