Materials Science and Engineering

Nitrogen oxide (NOX) is considered as toxic molecule giving harmful influences not only on human body, but also on environment. Thus, to detect NOx, here, in this article we report hydrothermal synthesis of different morphologies of ZnO with gradual increase of CTAB concentration forming nano rods (0M), nano rods assembled structure (0.001M) to flower-like (0.005M) and studied morphology dependent gas sensing behavior to NOx gas at low temperature. The characterizations of the as-prepared samples were done in details by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, photoluminescence (PL) spectroscopy and Brunauer-Emmett-Teller (BET). The gas sensing performance of sensors fabricated with as-prepared ZnO structures to various concentrations of NOX, ammonia, toluene, carbon-monoxide, acetone and ethanol, at various operation temperatures (25 to 150°C) were noted. A clear trend showing effects of the morphology on the sensing behavior with temperature was demonstrated. The flower-like ZnO exhibited stability (up to 140 days) and excellent sensitivity with a high gas response of 29 which drops sharply with increase of temperature and is highly selective towards 0.74 ppm of NOX at 25 °C without additional use of UV irradiation or doping materials such as Pt or Pd. The ZnO sample with 0.001M of CTAB shows a similar trend but with much lower sensitivities at low temperatures. For the sample without CTAB, the temperature behavior switches to a volcano-type one, in which the sensitivity is low at a low temperature and increases gradually with the temperature increasing. Thus, we proposed possible reason for the evolution of overall characteristics by suggesting changing reactions between ZnO and reacting gases in various operating temperatures.

Figure 1: Effect of CTAB concentration variation (no CTAB, 0.001M, 0.1M, 0.35M and 0.5M) on the morphology of the ZnO microstructures for the formation of FZO.

Figure 2: Sensing response of (a) 0.005 M CTAB concentration sample and of (b) samples of all concentration variations of NOX gas (0.74 ppm) with different operating temperatures (25°C, 43°C, 72°C, and 150°C).