Morphology Controlled Fabrication and Application of Colloidal Fine Particles of Zinc Compounds

Abstract

Zinc compounds nanostructures with controlled morphological features were synthesized in aqueous solutions through simple and economical route without using any type of surfactant or template. The resulting powders were subjected to SEM analysis which revealed that morphology of the prepared powders was strongly dependent upon the applied experimental conditions like pH, reaction time, reaction temperature and reactants composition. As such synthesis conditions were optimized out in a systematic manner to obtain nanostructures of uniform morphological characteristics. Various morphologies ranging from nanorods, nano/microspheres, nanoellipsoids, nano/micro flowers, cubes, sea urchin like hierarchical microspheres composed of nanoneedles and hexagonal nanorods, sea shells and trigonal pyramidal shape were synthesized. Selected batches of the prepared powders were also investigated by XRD, FT-IR, TG/DTA and BET surface area analysis. Test samples of as-prepared powders were subjected to calcination under controlled heat treatment. XRD results illustrated the crystalline nature of the as-prepared and calcined powders. Various crystallographic parameters i.e., crystallite sizes, lattice constants, x-ray density and specific surface area were calculated from XRD results. Selected samples were then employed for the fabrication of room temperature gas sensor of industrial importance as well as for the application of effective antibacterial agent. The gas sensing behavior of selected batches of zinc oxide (Z1cal– Z4cal) and zinc phosphate (ZP1–ZP3) nanostructures were evaluated in specially designed gas sensor setup. The sensor response was evaluated towards ammonia, acetone and ethanol vapors. The effect of operating temperature, gas concentration and nanostructure morphology on the sensing performance of the desired systems were studied. Sensors based on the synthesized samples showed superior and v reproducible performance with high selectivity and stability towards 1 ppm ammonia at room temperature (29 oC). This was attributed to the unique morphology and remarkable uniformity in shape and size of the synthesized nanostructures. For instance, ZP1 sensor showed highest room temperature gas sensing response of 89% with response recovery time 31/12 s, towards 5 ppm ammonia. This can be attributed to highly porous and hierarchical surface characteristics of synthesized powders. Moreover, the lowest detection limit investigated was <1ppm, which demonstrated excellent ammonia sensing characteristics of the synthesized nanostructures. In addition, plausible reaction mechanisms for gas sensing of ZnO and ZP sensors were studied. The superior gas response with excellent reproducibility was due to novel hierarchical surface characteristic, considerable uniformity in shape and size and high specific surface area of synthesized structures. It is mentioned that up to our knowledge, no literature report is available concerning the gas sensing properties of zinc phosphate micro/nanostructures. Because of the excellent gas sensing performance, the studied samples could be employed as promising candidates for developing highly sensitive and selective room temperature ammonia gas sensor. Furthermore, selected ZnO powders (Z1cal –Z4cal) and commercial ZnO were then employed for in-vitro evaluation of antibacterial activity against various pathogenic bacteria (Staphylococcus aureus, Streptococcuss mutans, Escherichia coli, Pseudomonas aeruginosa and Enterobactor cloacae) of clinical importance. The synthesized nanostructures were found to exhibit a promising anti-bacterial activity by producing inhibition zones to the tested bacterial strains. Z4cal exhibited highest antibacterial activity compared to other ZnO samples (Z1cal –Z3cal) due to high surface area (95.20 m 2 /g) of its hierarchical porous structure. vi In addition, concentration dependent antibacterial study unfolded that size of the inhibition zones increased from ~28 mm to 32 mm with increasing ZnO concentration. However, ZnO-Com showed no antibacterial response in the employed concentration range. Moreover, the synthesized nanostructures significantly enhanced the antibacterial activity of ciprofloxacin, a standard antibiotic when employed in combination. The present study suggests that the application of synthesized ZnO nanostructures as antibacterial agent in biomedical sides may be effective at inhibiting certain pathogenic bacteria. Key Words: Zinc compounds, Zinc oxide, Monodispersed, Controlled morphologies, Hierarchical structures, Gas sensor, Sensor response, Response time, Recovery time, antibacterial activity, Pathogenic bacteria.