OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 a b c d e Fig. 7. Capacitance as a function of RH for ZnO–NGM sensors with (a) 2 wt.%, (b) 4 wt.%, (c) 1 wt.%, (d) 5 wt.%, and (e) 10 wt.% NGM doping The dynamic behavior of the sensor was evaluated in terms of response and recovery time, which are defined as the time to achieve 90% of the maximum capacitance change when exposed to humidity and the time to recover to 10% of the initial value when moisture is removed, respectively (Table 1). The undoped ZnO had a response time of 5.0 s and a recovery time of 7.0 s. After doping, these values were significantly enhanced, and the 4% ZnO–NGM sensor realized an optimal trade-off: a quick response time of 4.0 s and a recovery time of 6.2 s. This is a characteristic of good water molecule adsorption–desorption kinetics facilitated by the synergy between ZnO’s porous nature and NGM’s charge transport ability. At elevated NGM doping concentrations (≥5%), response and recovery times started to grow. For example, the 10% ZnO–NGM sensor had slower response (6.0 s) and recovery (8.0 s) times due to probable NGM agglomeration. Agglomeration decreases the number of active adsorption sites and inhibits water diffusion paths, thereby reducing sensing speed even though high capacitance sensitivity exists. Capacitance variation with RH for different doping levels is shown in Fig. 8, a. While pure ZnO exhibited a baseline sensitivity of 18.5 pF/% RH, the introduction of NGM significantly enhanced the response.
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