Influence of tannic acid pH on the crystallographic structure and electrical properties of ZnO nanostructured thin films

This study presents a comprehensive investigation into the structural, chemical, and electronic properties of zinc oxide (ZnO) and tannic acid-functionalized ZnO (ZnO–TA) nanostructures, synthesized via a green hydrothermal route under controlled pH conditions. FESEM revealed that pristine ZnO crystallized into well-defined nanorods, whereas incorporation of TA at acidic pH values induced a morphological transition towards interconnected networks, concomitant with elevated carbon incorporation. FTIR confirmed the presence of Zn–O vibrational modes alongside characteristic peaks corresponding to carbonyl, ester, and hydroxyl functionalities, evidencing successful surface modification by TA. The four-point probe I–V measurements demonstrated linear ohmic behaviour and a significant enhancement in lateral conductivity, reaching up to 10⁻⁸ S·cm⁻¹, surpassing previously reported benchmarks for similar systems. Hall effect measurements consistently indicated n-type conductivity across all samples. Notably, carrier mobility peaked at pH 5, attributed to effective passivation of surface trap states by TA’s functional groups. In contrast, excessive TA loading at pH 3 resulted in increased carrier concentrations but diminished mobility, likely due to nanoparticle aggregation and enhanced grain boundary scattering. These findings demonstrate the pivotal influence of pH and TA concentration in tailoring the morphology, surface chemistry and charge transport characteristics of ZnO nanostructures.