전기방사법을 이용한 1차원 금속산화물 나노구조체의 제조 및 화학 센서의 응요에 관한 연구

Abstract

Increasing demands for ever more sensitive sensors for global environmental monitoring, food inspection and medical diagnostics have led to an upsurge of interests in nanostructured materials such as nanofibers and nanowebs. Electrospinning exhibits the unique ability to produce diverse forms of fibrous assemblies. The remarkable specific surface area and high porosity bring electrospun nanomaterials highly attractive to ultrasensitive sensors and increasing importance in other nanotechnological applications. In this thesis, we present a new processing strategy using electrospinning process for the fabrication of unique sensing device architectures comprising highly porous metal oxide nanofibers with typical length diameters ranging from 50 nm up to few micrometers and length of up to several centimeters. These porous inner structures can be assemble in different ways enabling to construct nanoengineered device architectures with tailored functional properties. Chapter 3~5 demonstrate application of this fabrication method as a means to produce highly sensitive chemical sensors comprising porous or hollow nanofibers of metal oxide (SnO2, ZnO and Zn2SnO4), an important material with potential applications in chemical gas sensors. In Chapter 3, porous and dense Zn2SnO4 nanofibers were synthesized via a controlled electrospinning method including an in situ phase separation process between the polymer matrix and inorganic precursors. The morphology of Zn2SnO4 fibers was strongly affected by the miscibility between zinc acetate, tin(IV) acetate and the matrix polymer. The porous and dense Zn2SnO4 fibers applied to semiconducting gas sensors. Remarkably high selectivity for C2H5OH against CO and H2 gases was observed in both porous and dense Zn2SnO4 sensors. In particular, the porous Zn2SnO4 sensor exhibited approximately 4-fold higher C2H5OH sensitivity compared to the dense Zn2SnO4 sensor, leading to a new player for application in volatile organic compound sensors. The proposed synthetic method in this work is simple and versatile, providing fascinating opportunities to control the morphology of various complex metal-oxide nanofibers, particularly optimized for applications in gas sensors. In Chapter 4, SnO2 nanofiber mats were prepared by a combined electrospinning-thermo-compression procedure. The resultant SnO2 fiber mats exhibit attractive NO2 and CO sensing characteristics. In particular, as determined by extrapolation of actual sensor tests, sub-ppm levels of NO2 can be detected at temperatures below 200°C, attributed to the unique surface structure of the nanofiber mats. This point to the potential of accessing reduced operating temperatures with the present synthesis method, a significant advantage when compared to conventional microscaled gas sensor devices requiring higher temperature operation. In particular, this unique process modification, i.e., thermo-compression step for electrospinning, opens up new opportunities for processing novel materials with enhanced surface activity, important for technological applications such as gas and chemical sensors as demonstrated in this work. Moreover, the ability to control the geometrical structure of such electrospun nanofiber mats will enable systematic investigation of property microstructure correlations in different classes of semiconducting metal oxides for various applications. In Chapter 5, thin (0.5 to 1 μm) layers of nonaligned or quasi-aligned hollow ZnO fibers were prepared by sputtering ZnO onto sacrificial templates comprising poly(vinylacetate) (PVAc) fibers deposited by electrospinning on silicon or alumina substrate. Subsequently, the ZnO/PVAc composite fibers were calcined to remove the organic components and crystallize the ZnO overlayer, resulting in hollow fibers comprising nanocrystalline ZnO shells with an average grain size of 23 nm. The NO2 gas sensing properties were examined using DC conductivity measurements under exposure to residual (2 ~ 10 ppm) of NO2 in air at elevated temperatures (200~400°C). The hollow ZnO fibers were much more sensitive compared to reference ZnO thin film specimens, displaying even larger sensitivity enhancement that the 2-fold increase in their surface to volume ratio. The quasi-aligned fibers were more sensitive that their non-aligned counterparts