Hydrothermally Synthesized pH-Tailored Ag-Doped ZnO Nanostructure for Printed Room-Temperature NO₂ Gas Sensor

pH-tailored Ag-doped ZnO (AgZO) nanostructures with controlled morphologies are developed through a cost-effective hydrothermal technique. The nanostructure morphology evolves from flower-like structures at pH 7 (average petal length: 1.7 ± 0.65 μm), to star-like rods at pH 10 (length: 0.97 ± 0.33μm), and finally to needle-like self-assemblies at pH 12 (length: 4.4 ± 1.92 μm). Optical, chemical, and morphological investigations confirmed the presence of wurtzite ZnO, Ag incorporation, increased oxygen vacancies, and tunable bandgap from 3.51 eV (pH 7) to 3.09 eV (pH 12). Surface area analysis demonstrates a surface area variation from 37.18 m2/g at pH 7 to 5.15 m2/g at pH 12. Printable AgZO ink was formulated and printed on Ag-interdigitated electrodes to fabricate a miniature room temperature printed NO2gas sensor. Among all devices, pH 12 sensor exhibited improved performance, attributed to its anisotropic morphology, enhanced crystallinity, and optimized defect-assisted charge transport. The pH 12 sensor demonstrated a stable response (R) of 16.2% at 32 PPM, with a response and recovery time of 107 s and 130 s, consistent multicycle operation, and strong concentration-dependent resistance modulation from 16 - 40 PPM. Selectivity towards NO2was confirmed through negligible interference from CO2. These results demonstrate that pH-controlled Ag incorporation significantly influences surface reactivity and electron depletion dynamics, enabling efficient printed NO2sensor operating at room temperature. The approach provides a lowcost, scalable route for fabricating high-performing gas-sensing materials.