OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Ta b l e 1 Effect of NGM doping on capacitance sensitivity, response time, and recovery time of ZnO-based humidity sensors NGM Doping (%) Capacitance sensitivity (pF/% RH) Response time (s) Recovery time (s) 0% (pure ZnO) 18.5 5 7 1 % 25.2 4.8 6.9 2 % 38.7 4.5 6.9 4 % 47.3 4.0 6.2 5 % 53.9 4.2 6.6 7 % 56.8 4.4 6.6 10 % 62.1 6 8 a b Fig. 8. (a) Response and recovery times as a function of NGM doping level; (b) Capacitance sensitivity as a function of NGM doping level Although 10% doping resulted in maximum sensitivity (62.1 pF/% RH), it came at the expense of slower kinetics. The 4% and 5% ZnO-NGM sensors presented the best compromise between high sensitivity and rapid response/recovery behavior, making them ideal candidates for real-time humidity detection. ZnO-NGM-based humidity sensor response and recovery behavior was investigated to analyze real-time performance (Fig. 8, b). The best response (4.0 s) and recovery (6.2 s) times were obtained at 4% ZnO– NGM, due to the synergy between ZnO’s high surface area and NGM’s superior charge transport properties. For doping above 5%, performance decreased because NGM agglomeration decreased active adsorption sites and hampered electron mobility. At 10% doping, response and recovery times were elevated to 6.0 s and 8.0 s, respectively. In general, controlled NGM doping greatly improves ZnO sensor performance, outperforming the drawbacks of conventional metal oxide sensors. The optimized ZnO-NGM composites surpass or match current sensors, showing great potential for use in environmental monitoring, industrial systems, and biomedical diagnostics. The cyclic sensing performance is indicated in Fig. 9, illustrating capacitance and RH levels against time over a period of 1,000 seconds. The capacitance (dotted red-blue line) closely follows the humidity (red squares), with evidence of synchronized and repeatable behavior. The rapid slopes at times of humidity transition reflect rapid response and recovery behaviors. The regular periodic behavior supports sensor stability, repeatability, and suitability for dynamic environmental monitoring.
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