OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 device fabrication. Copper and silver electrodes were used for device fabrication. Before application, all the substrates and components were properly cleaned with a Liquinox detergent solution (Alconox Inc.) and then rinsed with analytical-grade acetone (Sigma-Aldrich) to remove organic impurities and provide a clean surface free of contaminants. Synthesis of zinc oxide (ZnO) nanoparticles via chemical precipitation 10 ml of 1.5 mol/L zinc nitrate (Zn(NO₃)₂) was slowly added to 10 ml of 2.25 mol/L ammonium carbonate ((NH₄)₂CO₃) solutions with magnetic stirring. A white ZnCO₃ precipitate confirmed successful synthesis [1]. The chemical reactions involved are: Zn(NO₃)₂S + (NH₄)₂CO₃ → ZnCO₃ ↓ + 2 NH₄NO₃ The precipitate was collected through vacuum filtration with an appropriate pore filter paper to remove liquid by-products and unreacted precursors. It was washed three times with deionized water. The high volatility of ethanol allowed for quicker drying, yielding a pure precipitate to be used in subsequent ZnO nanoparticles synthesis. The ZnCO₃ was filtered and dried at 80°C to evaporate residual solvents without agglomeration, maintaining the nanostructure. The dried precursor powder was calcined at 550°C for 2 hours using a muffle furnace, promoting the thermal decomposition of ZnCO₃ to ZnO. ZnCO₃ → ZnO + CO₂ Doping of ZnO nanoparticles with nanographite (NGM) Both ZnO and NGM were initially in powdered form. Both were dispersed separately in ethanol. The NGM suspension was then added dropwise to the ZnO solution with continuous stirring for 25 minutes to achieve good mixing. ZnO to NGM weight ratios were varied to obtain 99:1, 98:2, 97:3, 96:4, 95:5, and 90:10 composites. Next, 10 mL of ethanol was further added as a dispersant to facilitate dispersion and to lower surface tension. For the powder form, the paste was dried at 50–70 °C for 1–2 hours to prevent nanoparticle agglomeration and ensure equal NGM distribution. Fabrication of the humidity sensor FTO glass substrates were sequentially sonicated in detergent, deionized water, acetone, and ethanol to clean them for the best adhesion and consistent sensor performance. ZnO nanoparticles doped with NGM were combined with ethanol to produce a viscous paste appropriate for doctor-blading. The paste was cast on cleaned FTO substrates by the doctor-blade method and thermally treated at 150 °C for 1 hour for the improvement of film adhesion, evaporation of the residual solvents, and durability. Fig. 1 shows the ZnO doped with NGM paste on a FTO glass substrate. Figs. 1, a and 1, b show the ZnO doped with NGM paste on a FTO glass substrate. For further stabilization of the film and to increase its stability, ethyl cellulose (2–10 wt %) was dissolved in ethanol with constant stirring to produce a uniform solution. This was added progressively into the ZnO doped NGM paste to achieve optimal coating. Incorporation of ethyl cellulose enhanced adhesion, surface smoothness, and mechanical strength to yield stable, uniform nanocomposite layers for humidity sensing [1, 30]. Results and Discussion Structural and morphological analysis The optical properties of the as-synthesized ZnO nanoparticleswere investigated by UV–Vis spectroscopy. The absorption spectrum, shown in Fig. 2, shows a sharp and intense absorption edge at around 367 nm, which is consistent with the intrinsic bandgap transition of ZnO. The UV–Vis was performed using a UV-
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