Effect of pH on ZnO nanoparticle properties synthesized by sol–gel centrifugation

Abstract

ZnO nanoparticles were synthesized at different pH values by the sol–gel method and centrifuged at 3000 rpm for 30 min. The ZnO powders agglomerate when synthesized in acidic and neutral conditions (pH 6 and 7). Fine powders were obtained when the pH of the sols was increased to 9. The maximum crystallite size (25.36 nm) of the ZnO powder was obtained at pH 9. The particles sizes of the ZnO synthesized between pH 6 and 11 were in the range of 36.65–49.98 nm. Ultraviolet–visible analysis (UV–vis) also demonstrated that ZnO synthesized from pH 8 to 11 has good optical properties with band gap energy (E_g) between 3.14 and 3.25 eV.

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⁻ + H₂O

where 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.