ZnO Nanorods Growth by Chemical Vapor Transport: Precision-timed Introduction of O₂ Reaction Gas

In this study, ZnO nanorods (NRs) were synthesized on FTO substrates using the chemical vapor transport (CVT) technique with Zn powder at a reaction temperature of 600°C. Notably, the heating rate of the reaction chamber was approximately 20°C/min, requiring about 30 min to reach 600°C from room temperature. The introduction of O₂ gas during heating significantly impacted the resulting ZnO samples. Three samples (S1, S2, and S3) were grown by varying the timing of O₂ gas introduction at reaction temperatures of 400, 450, and 600°C, respectively. The findings revealed that early introduction of oxygen gas led to Zn powder oxidation before melting, while late introduction resulted in the formation of defective ZnO crystals due to excess Zn vapor on the substrate surface. Evaluation of the samples’ photoelectrochemical (PEC) performance in water splitting reactions showed photocurrent densities of 2.2, 4.5, and 0.7 mA/cm² for S1, S2, and S3, respectively, under UV illumination (365 nm, 6 mW/cm²). Corresponding photoconversion efficiencies were calculated as 34.1, 69.7, and 10.1%, respectively. This study highlights the significant influence of O₂ gas introduction timing during the CVT heating process on the morphology and crystal quality of ZnO nanostructures, thereby affecting their PEC efficiency.