In recent years, engineered nanostructure assemblies such as nanowire arrays have attracted much research attention due to their unique chemical and functional characteristics collectively. The engineered nano-assemblies usually carry the characteristics distinct from bulk as a result of a size effect in their comprised elemental building blocks. The nanoscale size induced high surface-to-volume ratio is a fundamental attribute responsible for various chemical and physical properties required in various technologically important applications such as catalysts and sensors. This review article surveys the latest progress in engineered metal oxide nanostructure arrays, i.e., nanoarrays, for advanced chemical sensors’ design and application. It starts with an overview of gaseous chemical sensors followed by surveys of various fabrication methods and routes for metal oxide nanoarrays. Different sensing modes and corresponding applications have been highlighted in the mixed gaseous chemical sensing, which provides new approaches and perspectives to meet the challenges of selective gas sensing, such as the cross-sensitivity and inter-correlation of multiple sensing signals.

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Metal Oxide Nanoarrays for Chemical Sensing: A Review of Fabrication Methods, Sensing Modes, and Their Inter-correlations

Intercalative hybrid materials, comprised of polypyrrole (PPy) chains interleaved with the layers of MoO3, have been synthesized using concomitant ion exchange reaction. The (PPy)xMoO3-pressed pell...

Synthesis and gas sensing properties of polypyrrole/MoO3-layered nanohybrids

An adsorption/combustion-type gas sensor with low heat capacity employing Pd/γ-Al2O3 as a sensing material was fabricated by MEMS technology on a silicon substrate. The sensor was driven by low–high pulse heating at two temperatures of 200 °C and 400 °C. The sensor showed a large response peak to toluene upon high temperature combustion for a period of 0.4 s after low temperature adsorption for 17.6 s. The peak response detected toluene of low concentrations in the range of 10–1000 ppb. On indoor field tests of new residential houses using the adsorption/combustion-type sensor, the total volatile organic compound concentration determined by the sensor well agreed with that obtained by GC/MS or GC/FID analysis using active carbon trapping.

Development of a ppb-level sensor based on catalytic combustion for total volatile organic compounds in indoor air

A perovskite group of materials (ABO3) has a wide range of applications due to their structural diversity, adaptability including exceptional physical and chemical properties. They can accommodate around 90% of metallic elements of the periodic table at positions A and/or B without destroying the matrix structure. This gives an extraordinary possibility of various combinations and scope of complete or partial substitutions of cations resulting incredible large number of compounds with enhanced properties. These materials have been extensively explored for their magnetic, optical, catalytic and electrical properties. Thermal stability and catalytic properties enable them to serve as promising candidates for the control of environmental pollution. The methods of synthesis greatly influence the catalytic properties of perovskites. They have been exploited as oxidants, reductants and photocatalysts due to their enhanced catalytic properties. In addition to catalytic uses, the role of perovskites has also been explored for sensing and adsorption of various aqueous and gas phase species. This article provides an overview including the techniques for synthesis of perovskite materials, properties favouring the catalytic activities and the environmental applications of perovskites towards remediation of various organic pollutants through adsorption and photocatalytic degradation.

Application of perovskites towards remediation of environmental pollutants: an overview

The influence of the substrate on the structure and the microstructure of Ca2Fe2O5 (CFO) thin films has been investigated by transmission electron microscopy. Ca2Fe2O5 films have been deposited by pulsed laser deposition on (001)-oriented SrTiO3 (STO), LaAlO3 (LAO), and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. Minor variations in the lattice mismatch of the CFO film with the three different substrates strongly determine the growth direction of the film. The misfit stress also promotes the development of strain accommodation mechanisms. (101) “orthogonal” twins are formed to accommodate the misfit in epitaxial films of CFO on STO(001) and LSAT(001), while CFO films on LAO(001) develop 90° rotation twins leading to film fragmentation into grains. The growth direction of the CFO film on LAO not only depends on the lattice mismatch but also on the lattice energy involved at the initial stages of the growth process. Our results also evidence the formation of an unexpected phase with composition Ca3Fe2...

Structure of epitaxial Ca2Fe2O5 films deposited on different perovskite-type substrates

Suspended microstructures consisting of a thin oxide/nitride diaphragm with embedded polysilicon heaters were designed and fabricated using a standard complementary metal–oxide–semiconductor process and simple postprocessing. Thin films of gas sensitive materials based on the SrFeO2.5+x nonstoichiometric perovskite family were deposited onto the diaphragms by room-temperature pulsed excimer laser deposition. Successful chemical sensor functionality was demonstrated. With applied power up to 30 mW, estimated temperatures of the gas sensor film up to 900 °C were reached. When the device was exposed to volatile organic compounds (VOCs) such as acetone and methanol, a reversible ten to 100-fold increase in sensor film resistance was observed, with response times from less than 1 s to a few minutes. Sensor response sensitivity depended on applied power and on the nature of the VOC analyte. This sensor device has the potential for use in multiarray configurations such as in an electronic nose.

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J. D. N. Cheeke

Papers

High-temperature gas sensor using perovskite thin films on a suspended microheater