Repository logo

Elucidating structure-property-performance relationships of plasma modified tin(IV) oxide nanomaterials for enhanced gas sensing applications




Stuckert, Erin P., author
Fisher, Ellen R., advisor
Barisas, B. George, committee member
Prieto, Amy L., committee member
Krummel, Amber T., committee member
Ma, Kaka, committee member

Journal Title

Journal ISSN

Volume Title


This dissertation examines structure-property-performance relationships of plasma modified tin(IV) oxide (SnO2) nanomaterials to successfully and efficiently create sensitive targeted gas sensors. Different project aspects include (1) materials characterization before and after plasma modification, (2) plasma diagnostics with and without a SnO2 nanomaterial, (3) sensor performance testing, and ultimately (4) elucidation of gas-surface relationships during this project. The research presented herein focuses on a holistic approach to addressing current limitations in gas sensors to produce desired capabilities for a given sensing application. Strategic application of an array of complementary imaging and diffraction techniques is critical to determine accurate structural information of nanomaterials, especially when also seeking to elucidate structure-property relationships and their effects on performance in specific applications such as gas sensors. In this work, SnO2 nanowires and nanobrushes grown via chemical vapor deposition (CVD) displayed the same tetragonal SnO2 structure as revealed via powder X-ray diffraction (PXRD) bulk crystallinity data. Additional characterization using a range of electron microscopy imaging and diffraction techniques, however, revealed important structure and morphology distinctions between the nanomaterials. Tailoring scanning transmission electron microscopy (STEM) modes and combining these data with transmission electron backscatter diffraction (t-EBSD) techniques afforded a more detailed view of the SnO2 nanostructures. Indeed, upon deeper analysis of individual wires and brushes, we discovered that despite a similar bulk structure, wires and brushes grew with different crystal faces and lattice spacings. Had we not utilized multiple STEM diffraction modes in conjunction with t-EBSD, differences in orientation related to bristle density would have been overlooked. Thus, it is only through methodical combination of several analysis techniques that precise structural information can be reliably obtained. To begin considering what additional features can affect gas sensing capabilities, we needed to understand the driving force behind SnO2 sensors. SnO2 operates widely as a gas sensor for a variety of molecules via a mechanism that relies on interactions with adsorbed oxygen. To enhance these interactions by increasing surface oxygen vacancies, commercial SnO2 nanoparticles and CVD-grown SnO2 nanowires were plasma modified by Ar/O2 and H2O(v) plasmas. Scanning electron microscopy (SEM) revealed changes in nanomaterial morphology between pre- and post-plasma treatment using H2O plasma treatments but not when using Ar/O2 plasmas. PXRD patterns of the bulk SnO2 showed the Sn4+ is reduced by H2O and not Ar/O2 plasma treatments. X-ray photoelectron spectroscopy (XPS) indicated Ar/O2 plasma treatment increases oxygen adsorption with increasing plasma power and treatment time, without changing Sn oxidation. With the lowest plasma powers and treatment times, however, H2O plasma treatment results in nearly complete bulk Sn reduction. Although both plasma systems increased oxygen adsorption over the untreated (UT) materials, there were clear differences in the tin and oxygen species as well as morphological variations upon plasma treatment. Given that H2O plasma modification of SnO2 nanomaterials resulted in reduction of Sn+4 to Sn0, this phenomenon was further explored. To develop a deeper understanding of the mechanism for this behavior, gas-phase species were detected via optical emission spectroscopy (OES) during H2O plasma processing (nominally an oxidizing environment), both with and without SnO2 substrates in the reactor. Gas-phase species were also detected in the reducing environment of H2 plasmas, which provided a comparative system without oxygen. Sn* and OH* appear in the gas phase in both plasma systems when SnO2 nanowire or nanoparticle substrates are present, indicative of SnO2 etching. Furthermore, H2 and H2O plasmas reduced the Sn in both nanomaterial morphologies. Differences in H* and OH* emission intensities as a function of plasma parameters show that plasma species interact differently with the two SnO2 morphologies. The H2O plasma gas-phase studies found that under most plasma parameters the ratio of reducing to oxidizing gas-phase species was ≥1. The final consideration in our holistic approach relied on sensor performance studies of SnO2 nanomaterials. Resistance was recorded as a function temperature for UT, Ar/O2 and H2O plasma treated nanoparticles and nanowires exposed to air, carbon monoxide (CO), or benzene (C6H6). Resistance data were then used to calculate sensor response (Rair/Rgas) and sensitivity (Rair/Rgas > 1 or Rgas/Rair > 1). Specifically, Ar/O2 and H2O plasma modification increase CO and C6H6 sensitivity under certain conditions, but H2O plasma was more successful at increasing sensitivity over a wider range of plasma parameters. In particular, certain H2O plasma conditions resulted in increased sensitivity over the UT nanomaterials at 25 and 50 °C. Overall, H2O plasma appears to be more effective at increasing sensitivity than Ar/O2 plasma. Furthermore, although certain treatments and temperatures for nanoparticles had greater CO or C6H6 sensitivity than nanowires, nanowire sensitivity was less temperature dependent than nanoparticle sensitivity. Prior materials characterization data were combined with resistance data to elucidate specific structure-property-performance relationships for the different UT and plasma treated materials.


Rights Access


tin oxide
plasma surface modification
gas sensing


Associated Publications