Preparation of quasi-ordered hollow $SnO_2$ hemispheres using colloidal templating route and its application to gas sensors

Abstract

A great deal of worldwide attention has been paid to the ultra-sensitive gas sensors with ability to detect low concentration of gas in sub-ppb (parts per billion) range. Therefore, there have been diverse approaches for developing desirable architectures to obtain high sensitive gas sensors. For example, 1-D nanostructures such as nanowires, nanofibers, nanotubes, and nanobelts have been considered as suitable material architectures in terms of their high surface-to-volume ratio, ideal small building blocks for nanodevice, their specific chemical and physical properties induced by size reduction. In addition, nano-crystalline films and porous structures like hollow microspheres and inverse opals prepared by templating methods have been utilized in the gas sensor field. However, it is difficult to control parameters such as dimension and morphology and resulting microstructure flexibly, therefore, it has been considered as limitation and challenge in the practical application of gas sensors. In this work, we used 2-D close packed layers of poly(methyl methacrylate) (PMMA) microspheres as templates to fabricate quasi-ordered macroporous structure. By utilizing hybrid processing strategy, combined organic colloidal templating process and RF sputtering of $SnO_2$ thin films on templated substrates followed by calcination at $500^\circ C$, the resultant macroporous $SnO_2$ films consisting of highly ordered hollow hemispheres could be obtained. Such unique morphology can be attractive building blocks of sensing material in that their some advantages as compared to conventional thin films gas sensors. For conventional thin film gas sensors, the only external surface of film is exposed to gas species leading to the formation of gas sensitive surface depletion layer deeper inside sensing film from the external surface in the range of 1~10nm. However, overall resistance of thin film gas sensor is dominantly controlled by gas insensitive bulk resistance since its thi...