Mechanism of wettability conversion on sprayed Zn2SnO4 thin films surfaces modified by thermal annealing in air

In this work, zinc stannate (Zn2SnO4) thin films were synthesized by the spray-pyrolysis technique. The effects of air annealing on photoluminescence (PL) and wettability properties were investigated. The Xray diffraction patterns revealed that the deposited thin films, belonging to cubic structure, crystallize preferentially along (311) plane. It has also been found that air annealing at 500 degrees C improves the crystallinity of this ternary oxide by releasing the internal strain. Furthermore, the contact angle (CA) values of Zn2SnO4 thin films decreased significantly (from 90.5 to 26.5 degrees) indicating a change in the wetting characteristics of the surfaces: from hydrophobic state (before annealing) to hydrophilic one (after annealing). Besides, PL measurements showed that the near band edge (NBE) emission-to-visible emission ratio (I-UV/I-Visible) was enhanced significantly after air annealing showing an improvement of the crystal quality. The increase in UV emission is attributed to the reduction in native defects, the presence of which in sample provides a competing route to the UV emission. The decrease in the concentration of oxygen vacancy from 71% to 31% allowed us to conclude that the wettability conversion phenomenon is not due to oxygen vacancy but can be attributed to the presence of oxygen adatoms on the sample surfaces. These findings demonstrate that the thermal treatment approach can switch the wettability of semiconductor oxide surfaces promising interesting applications in surface engineering. (C) 2017 Elsevier B.V. All rights reserved.