A comprehensive study on doping ZnO nanowires with transition metals using the hydrothermal synthesis method

This work presents a systematic investigation of hydrothermally grown Mn-, Y-, V-, and In-doped ZnO nanowires, with emphasis on how bottom electrode material, precursor molarity, and dopant chemistry influence nanowire morphology and elemental incorporation. A comparison between aluminium (Al) and titanium nitride (TiN) bottom electrodes showed that TiN provides a more suitable growth platform for controlled ZnO nanowire formation. Nanowires grown on TiN exhibited narrower dimensional distributions, with height ranges of 2.10–2.70 µm and 1.90–2.20 µm at 25 mM and 50 mM, respectively, compared with broader ranges of 4.54–5.73 µm and 3.98–4.65 µm on Al. This improved uniformity is attributed to stronger compatibility between the ZnO seed layer and the TiN surface, which promotes more homogeneous nucleation and stable anisotropic growth. Using TiN as the selected growth platform, the doped ZnO nanowires exhibited distinct dopant-specific morphologies. Pure ZnO grown at 25 mM showed heights of 4.07–5.96 µm and diameters of 0.33–0.46 µm, whereas growth at 50 mM reduced the height to 2.89–3.55 µm and broadened the diameter to 0.35–0.78 µm. Y incorporation suppressed nanowire development at 25 mM but produced larger faceted structures at 50 mM, with heights of 6.32–7.51 µm and diameters of 0.84–1.67 µm. Mn produced a more moderate morphological modification, while V generated the strongest structural amplification at 50 mM, reaching heights of 10.41–14.30 µm and diameters of 1.83–2.15 µm. In contrast, In consistently restricted both axial and radial growth, yielding the finest nanowire arrays, with dimensions as low as 1.42–2.18 µm in height and 0.12–0.18 µm in diameter at 50 mM. Energy-dispersive X-ray spectroscopy confirmed successful incorporation of all investigated dopants into the ZnO nanowires. The measured dopant atomic percentages increased from 0.28 to 0.34 Atm.% for Y, 0.15–0.30 Atm.% for Mn, 0.20–0.31 Atm.% for V, and 0.30–0.41 Atm.% for In when the precursor molarity was increased from 25 mM to 50 mM. These results show that ZnO nanowire architecture can be tuned through coordinated control of electrode selection, growth chemistry, and dopant species. The findings provide practical guidance for selecting dopant/electrode combinations for ZnO nanowire-based sensing, piezoelectric, and other functional device applications.