Zinc Oxide—From Synthesis to Application: A Review - MDPI
Effect of pH on Crystal Size and Photoluminescence Property of ZnO Nanoparticles Prepared by Chemical Precipitation Method
The mechanochemical method was proposed by Ao et al
The spectrum also indicates that the sample synthesized by chemical precipitation method has a very high level of purity. The presence of any contaminants would have acted as dopants in the ZnO, causing lower energy (higher wavelength) transitions, which would appear as small peaks or drops in the UV-Vis spectrum. But the spectrum shows a smooth line beyond the original drop. It proves that the precipitation method provides high purity ZnO nanoparticles.
A wide range of physical and chemical methods have been adopted by different research groups for the synthesis of ZnO nanoparticles such as chemical precipitation, sol-gel synthesis, hydrothermal reaction and electrochemical routes. Extensive literature assessment infers that chemical precipitation method is the foremost method for the synthesis of ZnO nanoparticles. As chemical precipitation is a low-temperature process, and its easy procedure to control the particle size and its low cost, this method finds suitable application in microelectronics and plastic electronics also. It is reported that  precipitation method yields the smallest nanoparticle with best optical and photocatalytic activity, compared with other methods mentioned above. Due to these reasons, precipitation method has been adopted for the present work.
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In conclusion, nanocrystalline ZnO powders have been successfully synthesized by chemical precipitation method at four different pH values. The result presented here confirms that pH value of the solution plays a major role in determining the structure, morphology and optical properties of the ZnO nanoparticles. The XRD pattern and SEM images confirmed the presence of crystalline wurtzite nanocrystals and also it confirmed that the particle size increases with pH value. TEM result also favors for the XRD analysis. The result of UV-Vis spectrum also justified that the prepared ZnO powders have particles in nanometer size. As the UV-Vis absorption of ZnO nanoparticles prepared at pH value of 6 is higher than the other particles, which concludes that ZnO nanoparticles with smaller size can be used as a UV absorber. From PL study, it is concluded that the pH value variation would affect the UV and Violet emission, but not the blue and green emissions. Finally, it is concluded that the ZnO nanoparticles prepared by chemical precipitation method at pH value of 13 is suitable for the construction of optoelectronic devices for violet emission.
The energy gap calculated to be 3.42, 3.40, 3.39 and 3.38 eV for the samples prepared at pH values of 6, 8, 12 and 13, respectively. As the sample prepared at pH value of 6 has smaller particles, its energy gap is greater than that of the sample prepared at pH value of 13. It proves that energy gap of the nanoparticle is inversely proportional to its radius. Therefore, from the XRD analysis, it is confirmed that chemical precipitation method is a suitable method for the synthesis of ZnO nanoparticles, and it provides a generic approach for producing the high-quality ZnO nanoparticles.
Journal of Nanoscience and Nanotechnology
We have shown that NiO nanoparticles can be readily produced by a rapid and efficient method. This method is based on oxidation of Ni2+ to NiO2 first and transforming of the latter to NiO by treating with ethanol in the presence of a surfactant at room temperature. The as-prepared NiO nanoparticles are obtained with high yield and purity. In our procedure there is no need for controlling the pH of the solution, as it usually required in most of the chemical precipitation methods. More importantly, calcinations at high temperature, which is used in other similar routes, have been eliminated in the present method. Therefore, this method can be introduced as an inexpensive, fast and reproducible process for the large-scale synthesis of NiO nanocrystals. The use of cationic surfactant CTAB which produced smaller nanosized NiO is recommended for this purpose.
In this context, an attempt has been made to investigate the effect of pH on crystalline size, microstructure and bandgap of ZnO nanoparticles synthesized via precipitation method. For this, pH value of solution has been adjusted in a wide range from 6 to 13. Our result favors for the increase in particle size with pH value. The crystallite size and microstructure of the synthesized nanopowders found to depend strongly on the pH value of solution. The novelty of the present work is that the chemical precipitation method is a suitable method for producing the ZnO nanoparticles of size around 14 nm. Our investigation carries further information that the preparation approach discussed here is suitable for the construction of violet emission optoelectronic and nano-photonic devices.
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