Effect of pH on ZnO nanoparticle properties synthesized by sol–gel centrifugation
Introduction
Zinc oxide (ZnO) has recently attracted significant interest for both fundamental studies and technical applications. In the past few years, ZnO has been synthesized using electrochemical deposition [1], chemical vapor deposition [2], catalysis-driven molecular beam epitaxy [3], thermal evaporation [4], pulsed laser deposition [5], zinc–air [6] and solution–gelation (sol–gel) [7]. Sol–gel is one of the most commonly selected methods for the synthesis of ZnO nanoparticles [8], [9], [10] because this method produces good homogeneity and optical properties and has easy composition control, low processing temperatures, the ability to coat large areas, and low equipment costs.
The sol pH is one of the important factors influencing the ZnO properties when ZnO is synthesized by sol–gel. The pH affects the hydrolysis and condensation behavior of the solution during gel formation and therefore influences the morphology of the ZnO [11]. For instance, Li et al. [12] showed that the solution conditions have a particular effect on ZnO particle-size powders. The pH can also change the number of ZnO nuclei and growth units [13]. Sagar et al. [14] claimed that the increase in pH (from acidic to alkaline) of the sols results in the growth of a ZnO film.
Metal alkoxides are often used as raw materials for the sol–gel process. In early work on the sol–gel method, an n-ZnO/p-Si heterojunction was prepared by the sol–gel process using zinc diethoxide as a starting material for synthesizing the ZnO films [15]. In the sol–gel method, the nucleation of the metal oxide particles occurs by precipitation. This precipitation involves a reaction between the soluble metal salt and the hydroxide ions or water. The nucleation reaction of a divalent metal salt (MX2) and a solution containing hydroxide ions (YOH) can be describes as follows [16]:MX2 + 2YOH → MO (s) + 2Y+ + 2X− + H2Owhere X is an anion (CH3CO2−, Br−, ClO4−) and Y is a cation (Na+, Li+, K+).
ZnO nanoparticles can also be synthesized using divalent metal salts and an aqueous solution based on Eq. (1). Centrifuging is commonly used to separate species of different size, mass, or density. The mixture with denser components will migrate away from the axis of the centrifuge, and the less-dense components will migrate toward the axis. The Coriolis force influences crystal growth, and well-defined size characteristics can be generated by this processing method [17]. The goal of this work is to synthesis ZnO in aqueous solution with pH values ranging from 6 (acidic) to 11 (alkaline). The pH of the sol was controlled by a zinc acetate dihydrate [Zn(CH3COO)2·2H2O] precursor, and sodium hydroxide (NaOH) was used to control the aqueous pH. Centrifugation was used to improve the ZnO properties.
Section snippets
Synthesis of ZnO powder
The ZnO sols were prepared by adding 0.2 M Zn(CH3COO)2·2H2O (Sigma–Aldrich) to methanol (CH3OH, Merck) at room temperature. The solution was stirred for 2 h using a magnetic stirrer until a clear solution without turbidity was obtained. The pH of the sols was ∼5. The clear solution transformed into a milky white slurry solution after titration with 1.0 M NaOH (Merck). The addition of NaOH changed the pH values of the sol from 6 (acidic condition) to 11 (alkaline condition). The resulting milky
X-ray diffraction analysis
Fig. 1 shows the XRD profiles of ZnO powders prepared pH values of 6, 7, 8, 9, 10 and 11. The samples prepared at pH 6 and 7 have no intense ZnO peak. Normally, the ZnO structure cannot be synthesized well at pH 6 because of the high concentration of H+ ions and low concentration of OH− ions in the sol. For the neutral condition at pH 7, the number of H+ ions reacting with number of OH− from NaOH is equivalent. Therefore, both of the peaks are broad. The ZnO powder peaks for samples with pH ≥ 8
Conclusion
ZnO nanoparticles powders have been successfully synthesized by the sol–gel technique at different pH values using centrifugation. All of the samples have good structural, morphological and optical properties. ZnO synthesized at pH 9 has the best properties. At pH 9, the ZnO has an average particles size of about 48.31 nm, and the particles are nearly uniform and spherical. The influence of nanocrystallinity can be seen in the band gap enhancement of 3.24 eV.
Acknowledgements
SSA would like to thank MOSTI for the National Science Fellowship award and USM-RU-PRGS for Grant (8031020). AAM wish to thanks USM Short Term Grant (6039030) for their financial support in this study.
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