OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Fig. 5. FTIR spectra of pure ZnO, nanographite material (NGM), and NGM-doped ZnO composites Doped ZnO showed characteristic Zn–O vibrations (400–600 cm−1) and O–H stretching (~3,400 cm−1). NGM spectra included C=C stretching (~1,600 cm−1), C–O (~1,400 cm−1), and C–H bending (~1,100–1,200 cm−1), characteristic of sp²-hybridized carbon structures. Composites of ZnO–NGM showed shifted and intensified peaks, especially for Zn–O and C=C, indicating strong chemical bonding and successful doping. The broadened O–H peak indicated enhanced hydrogen bonding, which provided enhanced water affinity. These microscopic and spectroscopic findings establish the successful integration of NGM into the ZnO structure. Increased surface area, lattice tension, and chemical functionalization enhance sensor performance and water adsorption. Decreased crystallite size and enhanced defect sites also enhance improved charge transport, which further increases the response rate and sensitivity of the humidity sensor. The characteristic peaks of ZnO, NGM, and their composite indicate the presence of functional groups corresponding to ZnO vibrations and carbon-based materials. The shift in peak positions and intensity variations suggests successful doping of NGM into ZnO. Electrical characterization and humidity sensing performance To compare the electrical performance of the ZnO–NGM humidity sensors, a controlled humidity chamber was utilized. The sensor was positioned in a sealed chamber where humidity was accurately controlled from 10% to 90% RH using a dual-path nitrogen gas system. One path supplied nitrogen gas through a reservoir of distilled water to humidify it, while the other provided dry nitrogen to dehumidify it. The relative humidity was controlled by varying the flow rates of the two paths of nitrogen. A digital hygrometer with ±0.8% RH accuracy was placed close to the sensor to continuously monitor RH, and the ambient temperature was held at 23 ± 1 °C. As shown in Fig. 6, the sensor was interfaced to a Fluke PM6304/023 precision LCR meter with shielded terminals to reduce electrical noise. Automated data acquisition with real-time plotting and saving of capacitance values was carried out using a Python-based script. The capacitance of the sensor was recorded at 10 kHz, 20 kHz, 50 kHz, 80 kHz, 100 kHz, and 1 MHz frequencies under different humidity conditions. The 2% ZnO–NGM sensor demonstrated a distinct monotonic increase in capacitance with RH. This was due to the adsorption of water molecules and increased dipolar polarization. The degree of doping made the sensor more sensitive than the pure ZnO because of the enhanced surface area and charge conduction pathways provided by the NGM flakes. The composite structure facilitated ionic conduction by adsorbed water layers and enhanced the dielectric constant via interfacial polarization. The response time was
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