The effect of oxygen vacancies on water wettability of a ZnO surface

Abstract

In this study, the effect of oxygen vacancies on the water wettability of a hydrated ZnO(100) surface has been examined via molecular dynamics simulations with a reactive force field (ReaxFF). The results show that the oxygen vacancies on the ZnO surface change the structures of the ZnO surface and subsequently its water adsorption capability. While a 1 : 1 ratio of water to hydroxyl is observed for a water monolayer absorbed on ZnO(100) without oxygen vacancies, additional water adsorption as coordinate hydroxyl that resides on the vacancy site and bonds with three lattice zinc atoms is observed on the surfaces with oxygen vacancies. The results also show that the energy of the interaction per unit area between water and the hydrated ZnO surface is 55.1% higher in the presence of the oxygen vacancies than that without oxygen vacancies. This leads to a water contact angle of ~115° for the hydrated ZnO(100) surface in the absence of vacancies and ~21° with vacancies. The wetting kinetics of a water droplet on a ZnO(100) surface with and without oxygen vacancies are compared with the diffusion-limited reactive wetting and molecular kinetics models, respectively. In addition, the ordering of the vacancy sites is found not to significantly affect the wettability of the ZnO(100) surface.

UV-activated ZnO films on a flexible substrate for room temperature O2 and H2O sensing

“…The simulations show a different adsorption geometry of water molecules interacting with an oxygen vacancy lattice than with an ideal lattice, which corresponds more closely to the coordinate OH configuration described by Hu et al . 47 .…”

“…We also find that the adsorption energy on a surface with oxygen vacancies is 55% higher than on one without vacancies, which is in agreement with Hu et al . 47 . Adsorption of a single water molecule on a hydroxylated ZnO surface with an oxygen vacancy is 69% more energetically favorable than that on an ideal hydroxylated surface, which is in agreement with the 58% higher adsorption energy shown by Hu et al ., and supports their conclusion that additional binding sites are created due to oxygen vacancies 47 .…”

“…47 . Adsorption of a single water molecule on a hydroxylated ZnO surface with an oxygen vacancy is 69% more energetically favorable than that on an ideal hydroxylated surface, which is in agreement with the 58% higher adsorption energy shown by Hu et al ., and supports their conclusion that additional binding sites are created due to oxygen vacancies 47 . As previously discussed, the adsorption of either O 2 or H 2 O on the ZnO surface will alter the electronic properties of the ZnO film, but the differences in adsorption energy between H 2 O and O 2 result in resistive and capacitive signatures unique to each adsorbate.…”

Effect of nitrogen doping on wetting and photoactive properties of laser processed zinc oxide-graphene oxide nanocomposite layers

“…Hydrophobic to hydrophilic conversion was related to defects creation on ZnO surfaces. 9,38 The contact angle decrease on the UV illuminated surfaces was attributed to dissociative adsorption of water molecules on the photogenerated surface defective oxygen vacancy sites 27 and as reported, N doping of metal oxides enhances the formation of oxygen vacancies. 39,40 Recently, molecular dynamic simulations 38 confirmed this assumption: the contact angles on the surface with oxygen vacancies was found to be lower as compared with those without oxygen vacancies for the entire simulation time.…”

Microstructure, wettability and electrical properties of n-ZnO/ZnO-SL/p-Cu2O heterojunction

“…A large number of compact and vertically aligned ZnO nanorods on the Sample 2 result in higher total interface area between water and ZnO nanorods. This leads to an effective increase in light-induced surface oxygen vacancies, which are kinetically more favorable for hydroxyl adsorption than oxygen adsorption [15,16]. Thus, the Sample 2 has the most obvious hydrophobic-to-hydrophilic transition.…”