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Dependence of Sensing Properties upon Metal Oxide Nanostructure

Randy L. Vander Wal1, Gordon M. Berger1, Gary W. Hunter2, and Jennifer C. Xu2. (1) NCSER c/o NASA-Glenn Research Center, 21000 Brookpark Drive, Cleveland, OH 44135, (2) The NASA-Glenn Research Center, 21000 Brookpark Rd., M.S. 77-1, Cleveland, OH 44135

Applications of chemical sensors include environmental monitoring, automotive applications, emission monitoring, and aerospace vehicle health monitoring. The superiority of nanomaterials for sensor applications is clear. High surface area and controlled structure are the hallmarks. Each aspect is particularly relevant to sensors. Surface area is critical to reactive gas adsorption and translates into high sensitivity. Relative to micron sized grains or powders or layers or films, nanoparticles offer a 10 to 100-fold increase in each parameter. Controlled structure provides the reactive sites for adsorption and their effect upon the overall conductance. Nanomaterials often possess enhanced reactivity due both to a strained surface lattice and/or increased prevalence of step, edge, corner and terrace sites. Such activity further enhances sensitivity and potentially lowers operational temperature. Operation at lower temperatures saves power, extends operating lifetime and maintains reproducibility by preventing sintering-induced grain growth. Finally, lower operational temperature, combined with structure control can advantageously yield selectivity.

In recent years two competing approaches have been developed for synthesizing 1-dimensional forms of metal oxide semiconductors (MOSs): electrospinning and thermal-evaporative-condensation (TEC) synthesis. Each method produces 1-d sensor elements that can be incorporated into next generation sensors. Each synthesis method and product has attendant advantages and limitations. Apart from device fabrication and manufacturing issues, these two methods produce elements that differ primarily in their crystallinity. TEC synthesis can produce single crystalline nanowires. Electrospinning produces polycrystalline elements of the (as-spun) sol-gel fiber upon calcination. Though each form is a nanoscale material, an additional parameter, crystallinity, will have profound consequences upon the viability of each 1-d element for reactive gas sensing and devices incorporating them.

With a diameter approaching twice the nominal charge depletion layer thickness (~ 10's of nanometers, dependent upon temperature), the full conducting channel of a nanorod can be affected by adsorption of oxidizing gases, analogous to the electrical conductivity modulation in a field effect transistor. Although somewhat thicker, the polycrystallinity and cylindrical structure of a nanofiber potentially permits deeper penetration of the depletion layer into the structure. Essentially the same narrowing of the “effective” conduction channel occurs. Carrier depletion (or replenishment) throughout the “bulk” nanostructure will expand the sensor dynamic range by the virtue of oxidizing (reducing) adsorbates leading to full charge depletion (or replenishment) with corresponding near-zero or high conductivity, respectively. With the entire bulk of the material responsive to surface adsorbed species, sensitivity gains may be achieved relative to 2-d films where often only the outer surface layer is affected while the underlying portion remains unchanged (maintaining constant conductance).

While it would appear that the nanorod is the limit of the summation describing the nanofiber case, in practice the nanorod diameter is generally larger than twice the depletion layer thickness. Moreover the uniform crystallographic surface planes may not support a high surface concentration of defect sites or chemisorbed oxygen species. Alternatively in the polycrystalline nanofiber not all junctions may be accessible to adsorbates. Such spots would correspond to a “short”. Moreover there could be a considerable variation in potential boundaries between grains, given the random orientation of single crystal nanoparticles with respect to each other. Therein experimental tests of the sensing performance of each material were conducted. The validity of such a comparison is facilitated by the one-dimensional morphology of each differently nanostructured metal oxide.

To further enhance the sensitivity of these morphologies to gases, catalyst nanoparticles were deposited upon the surface of the sensing elements. The utility of catalysts upon active oxide supports is well known in catalysis but has yet received little attention in gas sensors. Most approaches using catalysts of either noble metals or other transition metal doped the material. With a homogeneous distribution, most of the catalyst (whether atomically distributed or as nanophase particles) is inaccessible to the near-surface region, the region where active sensing occurs. Secondly, an atomic dispersion will alter lattice and grain structure, but not specifically create catalytic sites upon the sensor surface. The latter is the desirable structure from a catalysis viewpoint and is exploited for the 1-d sensing elements reported here. We note that this approach is frequently used in catalysis where the noble metal nanoparticles and/or the interfacial region between the particle and oxide support greatly accelerates the reaction compared to the bare oxide surface.

In general this particle coating will permit exposure of the underlying metal oxide support and most importantly establishes an interface between the particle and support oxide. These interfacial junctions will be self-polarized (forming a Schottky junction) by virtue of charge transfer, driven by the difference between the metal work function and oxide electron affinity. This interface is expected to be highly reactive for crystalline metal nanoparticles. Clearly this reactivity is critically dependent upon the nature of the oxide support material (composition, its crystallinity, chemisorbed oxygen sites and their binding energy, etc.) and availability of lattice oxygen from the semiconductor oxide surrounding the metal nanoparticle. Nevertheless such junctions are expected to be particularly reactive given the interfacial polarization. Additionally the catalyst nanoparticles will alter the free carrier charge density and band energy levels.

In summary, the central goal of this investigation was to operationally test the difference between these two nanostructured materials forms, with and without catalysts. In the fabrication of the prototype devices we gained practical knowledge of each synthesis method and realized caveats for integration of each element type towards commercial device manufacture. Advantages and limitations of each method towards practical sensor fabrication will also be presented.