Synthesis and Characterization of Nanostructured ZnO and SnOx for VOC Sensor Devices

Abstract

In this dissertation, nanostructured ZnO and SnOx with various forms of thin films, particles and rods were deposited and synthesized by combination of sputtering, thermolysis assisted chemical solution method, and/or dc applied electrodeposition. Different substrates such as alumina, silicon dioxide, and polyimide films were used to grow nanostructured materials in order to fabricate highly sensitive and selective VOC sensor devices. Synthesized ZnO and SnOx materials were characterized by FE-SEM, XRD, EDS, Raman spectroscopy and Keithley 2400 sourcemeter to examine the surface morphology, crystalline phase, atomic composition and electrical resistance change. Gas sensing properties of nanostructured metal oxides were studied as functions of the structural and compositional changes. Three different gases: acetone, ethanol, and ethylene, mixed with synthetic air were tested in a closed chamber by continuously flowing gases. SnO2 thin films were deposited by rf sputtering from a SnO2 ceramic target under different argon-to-oxygen ratios to investigate the effects of oxygen stoichiometry on ethylene sensing properties. Thin film sensors exhibited higher sensitivity compared with bulk from SnO2 sensors. Post-annealing of the fabricated thin films influenced gas sensitivity while the control of argon-to-oxygen ratio during the film deposition did not affect the properties significantly due to the effective formation SnO2 by a post-annealing process. An ethylene sensing mechanism for the SnO2 thin film sensor was also newly suggested.

Significant compositional effects of tin oxide were investigated by sputter deposition from a metallic tin target. Post-annealing of the films resulted in SnO and/or SnO2 phases depending on annealing temperature. Combinatorial phases of SnOx, i.e. gradual distribution of SnO and SnO2 on the sample substrate, were fabricated by co-sputtering of tin metal and tin oxide ceramic targets. Gas sensing properties of the films were investigated with an emphasis on tin phases and microstructure. Although SnO is a p-type semiconductor and SnO2 is a n-type semiconductor, the data on sensitivity using three different gases were similar except for the direction of resistance changes during the detection of the gases. Such a combinatorial approach would enhance the selectivity of a VOC sensor by merging two different types of semiconducting materials. Geometric effects of the oxides on the gas sensing properties were investigated by constructing ZnO nanorods on ZnO thin film seed layers. A series of devices were prepared with seed layers of different thickness upon which nanorods with tuned density were grown. Quantitative analysis of the sensing mechanism shows that volumetric geometry of the nanorods such as diameter and length is a more critical factor than the thickness of the seed layer. In addition to control of the nanorod structure, the transition metal ions such as nickel, cobalt, and copper were doped into ZnO nanorods during electrodeposition. Such doping can provide the ability to operate at room temperature and to use flexible polymer substrates. Nickel was successfully doped in-situ into ZnO nanorods in aqueous solution. A doped concentration of 6% nickel revealed the most enhanced sensing property at room temperature under UV illumination. A mechanism is proposed to explain how the transition metal ions in zinc oxide play an important role in the gas sensitivity under UV illumination.