Ordered mesoporous materials have great potential in the field of gas sensing. Today various template-assisted synthesis methods facilitate the preparation of silica (SiO2) as well as numerous metal oxides with well-defined, uniform and regular pore systems. The unique nanostructural properties of such materials are particularly useful for their application as active layers in gas sensors based on various operating principles, such as capacitive, resistive, or optical sensing. This review summarizes the basic aspects of materials synthesis, discusses some structural properties relevant in gas sensing, and gives an overview of the literature on ordered mesoporous gas sensors.
The synthesis and characterization of ordered mesoporous In2O3 materials by structure replication from hexagonal mesoporous SBA‐15 silica and cubic KIT‐6 silica is presented. Variation of the synthesis parameters allows for different pore sizes and pore wall thicknesses in the products. The In2O3 samples turn out to be stable up to temperatures between 450 °C and 650 °C; such high thermal stability is necessary for their application as gas sensors. Test measurements show a high sensitivity to methane gas in concentrations relevant for explosion prevention. The sensitivity is shown to be correlated not only with the surface‐to‐volume ratio, but also with the nanoscopic structural properties of the materials.
An approach for the size measurement of particulate (nano)materials by transmission electron microscopy was evaluated. The approach combines standard operating procedures for specimen preparation, imaging, and image analysis, and it was evaluated on a series of certified reference materials and representative test materials with varying physical properties, including particle size, shape, and agglomeration state. The measurement of the median value of the minimal external particle diameter distribution was intra-laboratory validated. The validation study included an assessment of the limit of detection, working range, selectivity, precision, trueness, robustness, and ruggedness. An uncertainty that was associated to intermediate precision in the range of 1–7% and an expanded measurement uncertainty in the range of 7–20% were obtained, depending on the material and image analysis mode. No bias was observed when assessing the trueness of the approach on the certified reference materials ERM-FD100 and ERM-FD304. The image analysis method was validated in an inter-laboratory study by 19 laboratories, which resulted in a within-laboratory precision in the range of 2–8% and a between-laboratory precision of between 2% and 14%. The automation and standardization of the proposed approach significantly improves labour and cost efficiency for the accurate and precise size measurement of the particulate materials. The approach is shown to be implementable in many other electron microscopy laboratories.
We present the preparation and characterization of a novel copper(II)oxide (CuO) nanofiber based sensor with very high sensitivity and selectivity to hydrogen sulfide (H 2 S). The working principle is based on the phase transition of semiconducting p-type CuO to strongly degenerated p-type copper sulfide (CuS) with metallic conductivity. Electrospun polymer fiber networks of polyvinyl butyrate (PVB) and Cu(NO 3 ) 2 were attached on standard gas sensing substrates and calcined to CuO at 600 8C in ambient air for 24 h. Continuous exposure to H 2 S (0.1-5 ppm) as well as a sequence of 1 ppm H 2 S pulses result in a dosimeter type behavior of the nanofiber sensors. Triggered by a certain dose (gas concentration multiplied by time) a steep conductance increase of the sensitive layer over several orders of magnitude is observed. After reaching this percolation threshold only small conductance changes were observed. These fiber based sensors show a remarkably high specificity, there is no response to carbon monoxide, hydrogen, and methane at 160 8C. The fiber network can be regenerated by raising the operating temperature to 350 8C for 30 min in absence of H 2 S.Conductance change under exposure to various gases. Continuous recording with one measurement per second.
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