Gas Sensing Behaviors of Hierarchically Structured Nickel Oxide Film by Electrophoretic Deposition

Over the past few years, the interest in wearable devices has increased dramatically. A highly miniaturized and wearable gas sensor system is one of interests. Integration of fabric and gas sensors for a wearable breath monitoring system would allow continuous monitoring patients’ breathe gases such as acetone gas for diabetes [1], ammonia for renal disease [2], and ethanol for hepatic steatosis [3]. Thus, there has been demands to fabricate wearable gas sensing system operating at daily environment.

 Semiconducting nano-scale metal oxides such as tin oxide [4], zinc oxide [5], and nickel oxide [6] have been developed to apply gas sensors. Among them, nickel oxide, which shows p-type behavior, has outstanding electrical and optical properties as well as chemical stability. There have been different approaches to synthesize nickel oxide films, such as physical and chemical vapor deposition. In this study, the particles with different morphology were first prepared by solution based synthesis method by using different precursors. Electrophoretic deposition (EPD) was then used to construct hierarchically structured NiO films.

 The nickel oxide was prepared by precipitation followed by calcination of nickel hydroxide-based precursors prepared by nickel acetate, nickel chloride or nickel nitrate. Morphology-tunable NiO nanoparticles were designed and prepared by heat treating different forms of nickel hydroxide precursors. By intercalating different size of anions (Cl^- < NO₃^- < CH₃COO^-) [7], different size and morphologies of precipitates are expected. The crystal structure and morphology by calcination are shown in Fig. 1 and Fig. 2, respectively. Each nickel oxide nanoparticles were coated to films by EPD process. Parameters in EPD were controlled to construct different hierarchical structures. The structural properties of the deposited nickel oxide layers are analyzed by using SEM and XRD. Then, gas sensing behaviors to detect ethanol by morphological variation of layers are discussed in detail.

 This research was partially supported by Agency for Defense Development (ADD) as global corporative research for direct urine fuel cell and Auburn University IGP.

Figure Caption

Fig. 1 XRD patterns of synthesized (a) as-precipitated Ni(OH)₂ and (b) calcined NiO at 300 °C for 1 hr and 3 hr.

Fig. 2. Morphology of (a) the as-precipitated nickel hydroxide particles and calcined NiO particles at 300 °C for (b) 1 hr and (c) 3 hr.

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