Growth of Long ZnO Nanowires with High Density on the ZnO Surface for Gas Sensors

Among the various approaches to grow semiconducting oxide nanowires, the thermal oxidation procedure is considered a simple, efficient, and fast method that allows the synthesis of micro and nanostructured arrangements with controlled size and morphology. In the work reported in this paper, long ZnO nanowires were synthesized on the surface of oxidized high-purity Zn foils by heating in air at different rates and temperatures. The size and morphology investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) for a sample heated at 620 degrees C with heating rate of 20 degrees C/min reveal the growth of long ZnO nanowires with length of similar to 50 mu m and average diameter of 74 nm grown along the < 11 (2) over bar0 > direction with high population density. Results with different heating determinant in tuning the size, morphology, and population density of nanowires. X-ray diffraction (XRD) shows patterns for both ZnO and metallic Zn with preferential orientation, whereas perturbed angular correlation (PAC) measurements using In-111(Cd-111) probe nuclei indicate that probe nuclei occupy only Zn sites in the preferential oriented metallic zinc. However, for samples submitted to high-temperature heating (820 and 1000 degrees C), XRD yields only the ZnO pattern and, amazingly, PAC continues showing probe nuclei only at metallic Zn sites indicating the presence of thin regions of highly oriented Zn trapped between grains of ZnO. Moreover, this strong preference of indium atoms (of parent radioactive In-111) here revealed helps to understand the oxidation mechanism and the growth of the nanowires.