The effects of the surface morphology of hydrothermally grown WO 3 nanosheets (NSs) and sputtering WO 3 film on the performance of pH sensing electrodes are presented and compared in the pH range of 2-12. Using a separated electrode of an extended-gate field-effect transistor (EGFET) configuration, the WO 3 nanosheet (NS) pH sensor shows a sensitivity of 63.37 mV/pH, a good linearity of 0.9973, a low voltage hysteresis of 4.79 mV, and a low drift rate of 3.18 mV/h. In contrast, the film-type one shows a typical sensitivity of only 50.08 mV/pH and a linearity of 0.9932. The super-Nernstian response could be attributed to the significant increase in the number of surface ion adsorption sites of the NS structure and the occurrence of local electric field enhancement over the sharp edges and corners of WO 3 NSs.
In this work, the fabrication of gas sensors utilizing self-synthesized tungsten oxide nanowires (TONWs) and their response to NO 2 are reported. The gas sensor is based on a sputter-deposited WC x /SiO 2 /WC x triple-layer structure with the periphery of the SiO 2 layer etched chemically. Self-synthesized TONWs with crystalline W 18 O 49 (010) were grown by simple thermal annealing in nitrogen ambient, which linked, in parallel, the upper and lower WC x electrodes for gas sensing. The TONW-based sensors increased in resistance in NO 2 because the surface of TONWs comprised oxygen adsorbates and the adjacent space charge region was electron-depleted. The amount of enlargement in resistance increased with increasing temperature. To improve the detectability of the parallel-connected TONW-based sensor, a connection of several individual sensors in series was proposed to enlarge the number of TONWs for gas sensing. For the 8-series-connected sensor, a sensitivity as high as 9.3, a response time as low as about 9 s, and a detectability as low as 2 ppm for NO 2 were obtained.
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