Understanding the Sensing Mechanism of ZnO Nanoparticles through Identifying Intrinsic Defects

Abstract

Progress in crystal growth made ZnO applicable as the next generation of inorganic light-emitting diodes and lasers, which has attracted much attention. Our previous work is a proof that Zn vacancies are occupied by three protons to form VZnH3 complexes in high-quality ZnO single crystals. In this work, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) studies on high-quality ZnO single crystals further confirm that three proton clusters or cations with three positive charges are the intrinsic defects and/or impurities related to doping in the host-lattice of high-quality ZnO single crystals. Unintentionally doped hydrogen (H) clusters might be relating to ZnO’s sensing capabilities. The sensor response of ZnO semiconductors is promoted by a decreasing size; however, the nature of the size effects has not been elucidated satisfactorily. In this work, broccoli-like ZnO nanoarrays, which can detect H2S gas in the parts per billion (ppb) region, has been synthesized under a hydrothermal method. High-resolution transmission electron microscopy (HRTEM) experimental results demonstrate that there are multiple polar faces exposed on the surfaces of ZnO nano crystals. The high catalytic active of oxygen species adsorbed on the multiple polar surfaces should respond to the high sensing performance and the high selectivity of ZnO toward reducing H2S gas. However, the enormous variations in electric current cannot be produced by releasing electrical charges from O– adsorbates into semiconductors. EPR results show that Zn+ interstitials have been produced after the reaction between H2S and O– adsorbates on ZnO. In contrast to the unintentionally doped hydrogen (H) clusters, our experimental results show that the Zn+ interstitials acting as shallow donors is what really offer the extra free-electron carriers.