Synthesis of ZnO nanowires by thermal chemical vapor deposition technique: Role of oxygen flow rate

Novel catalyst-free synthesis of hierarchical zinc oxide (ZnO) nanowires (NWs) by atmospheric pressure thermal chemical vapor deposition (TCVD) technique and the impact of different oxygen (O2) flow rates (25, 50, 75, and 100 mL/min) on the optical behavior, crystalline structure, emission response, and surface morphology of the prepared samples were studied in detail. The hierarchical ZnO NWs were characterized by ultraviolet-visible (UV-Vis) spectrophotometer, Xray diffraction (XRD), photoluminescence (PL) spectroscopy, and field emission scanning electron microscope (FESEM) measurements. The physical characteristics of the samples were found to be intensely influenced by the O2 flow rate. UV-Vis spectrophotometer measurements indicated that the optical energy gap (Eg) of the samples varies from 3.24 eV to 3.10 eV with regulating O2 flow rates from 25 to 100 mL/min. XRD analyses confirmed that all the prepared samples were polycrystalline in nature with hexagonal wurtzite-type crystal structure and a strong preferential direction along the (002) plane. Photoluminescence results revealed that oxygen vacancies were the major defects causing the blue emissions. Besides, the higher flow rate of O2 (100 mL/min) was efficient in decreasing the number of oxygen vacancies accountable for wide visible emission band intensity. FESEM images demonstrated a high density of hierarchical ZnO NWs with diameters ranging from 33 & PLUSMN; 2 nm to 88 & PLUSMN; 5 nm. Based on the FESEM observations, the potential growth mechanism of the hierarchical ZnO NWs was a vapor-solid (VS) process. Our simple systematic process for growth and characterization may provide toward the development of the hierarchical ZnO NWs-based gas sensors.