Effects of pH on Grain Size and Structure of ZnO Nanoparticle Synthesized via Sol–Gel Method for Enhanced Thermoelectric Materials

Zinc Oxide (ZnO) emerges as a potential thermoelectric material with high thermoelectric performance suitable for enhancing power harvesting applications efficiently. However, its intrinsic high thermal conductivity poses a challenge to achieving optimal thermoelectric performance. To address this, the nanostructuring approach has been employed, leveraging the creation of nanometer-scale grains to effectively reduce thermal conductivity. This paper investigates the impact of pH on the size of ZnO nanoparticles grains. The sol–gel method was used to synthesize the ZnO nanoparticles with various pH levels (7, 9, and 12). Subsequently, the resulting powder was then calcined at 800 °C for 1 h to produce pure ZnO powder. X-ray diffraction (XRD) analysis revealed a consistent hexagonal wurtzite structure across all pH levels, with the smallest crystallite sizes observed at pH 12 (34.52 nm), followed by pH 9 (34.72 nm), and pH 7 (40.38 nm). At pH 7 and 12, field emission scanning electron microscopy (FESEM) pictures displayed a hexagonal-like structure, whereas pH 9 revealed a nanorod-like structure. The average particle sizes were determined to be 84.56 nm at pH 12, 97.22 nm at pH 9, and 118.70 nm at pH 7, respectively. Energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the high purity of the synthesized ZnO nanoparticles, with atomic percentages of Zn and O closely aligning with the stoichiometric composition. These results validate the substantial purity of the ZnO nanoparticles. Overall, the findings demonstrate that increasing the pH values during synthesis leads to a reduction in both crystallite and particle sizes. This decrease in size is associated with lower thermal conductivity, thus offering the potential for improved thermoelectric performance.