Wettability of ZnO: A comparison of reactively sputtered; thermally oxidized and vacuum annealed coatings

▶ The wettability of reactively sputtered ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings was studied by using various probe liquids. ▶ The wettability studies demonstrated that reactively sputtered ZnO coatings were hydrophobic, whereas thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings were superhydrophobic with static water contact angles of 110.3°, 158.5° and 155.2°, respectively. ▶ The dynamic contact angle measurements indicated advancing and receding contact angles of 2° and 4°, respectively for thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. ▶ The work of adhesion of water was very low for thermally oxidized Zn-ZnO coating (5.06 mJ/m2) when compared to reactively sputtered ZnO coating (90.41 mJ/m2). Superior hydrophobic properties of thermally oxidized Zn-ZnO coatings are attributed to nanorod cluster like morphology along with the presence of high fraction of micron scale air pockets. ▶ The apparent surface free energy values for reactively sputtered ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings were 32.95, 23.21 and 18.78 mJ/m2, respectively.