Content area
Full Text
Received Jul 6, 2017; Revised Oct 27, 2017; Accepted Nov 28, 2017
This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction
Zinc oxide (ZnO) is a n-type semiconducting material with a great application potential as electron conduit in (opto)electronic devices [1, 2], photocatalysts [3, 4], and photoluminescence materials [5, 6]. There are several methods for the synthesis of ZnO nanostructures, including sol-gel method, controlled precipitation, solvothermal techniques, microemulsion, and sonochemical treatments [7]. Among them, sonochemical synthesis has been emerged as an efficient technique to prepare ZnO NPs of different shapes [8, 9], ZnO polymer composites [10], and evenly doped ZnO NPs [11]. In the sonochemical synthesis, the application of an ultrasound with a frequency ranging from 15 kHz to 10 MHz induces the formation, growth, and implosive collapse of bubbles in the reaction mixture, also known as cavitations [12]. The collapse of bubbles induced by cavitations creates localized hotspots having extremely high temperature (5000 K), high pressure (500 atm), and rapid heating or cooling rate (109 K/s). These extreme conditions enable high-energy chemical processes such as formation of radical and high-surface-area nanocrystals and efficient diffusion of ionic dopants into crystals forming doped nanocrystals while keeping the apparent reaction conditions at room temperature and atmospheric pressure.
Many important applications of ZnO NPs rely on their optical properties. While ZnO absorb efficiently UV light with photon energy greater than its bandgap, of about 3.3 eV, it usually emits both UV and visible light. The UV emission is assigned to band-to-band recombination between electrons and holes residing at the edges of conduction and valence band, respectively. The visible emission is usually attributed to various recombination pathways between photoexcited charges being trapped at defect states within the bandgap. Although the chemical origin of defect states is still controversial [5, 13–15], surface modification has been widely used to control over the visible emission of ZnO, especially in ZnO nanostructures. Patrick Felbier and coworkers controlled air exposure of plasma-synthesized ZnO nanoparticles (NPs) to switch the emission from UV to yellow or green with a quantum yield of 60% [5]. The changes in photoluminescence properties were reasoned to surface OH groups created...