Effect of process conditions (withdrawal rate and coating repetition) on morphological characteristics of sol–gel TiO2 film during dip coating

Abstract

The effects of the process conditions (withdrawal rate and coating repetition) on the morphological characteristics of sol–gel TiO2 film during dip coating were studied. We carried out dip-coating experiments to observe the effect of the withdrawal rate, coating repetition, and heat treatment on morphological development (homogeneity, thickness, and defects) in coated film. Sol was prepared with acetic acid catalyst and ethyl acetoacetate chelating agent. Withdrawal rate varied between 0.4 and 3.2 mm/s. Multilayered coatings were prepared by conducting 1, 2, 3, 5, and 10 cycles of coating. Heat treatment was carried out for 30 min at 450°C in air. The properties of the TiO2 film were characterized by visual observation, SEM, FIB, FTIR, and electrochemical impedance spectroscopy (EIS). As the withdrawal rate increased, the homogeneity of the film decreased, while the thickness of the film increased to a certain limit without cracking. A homogeneous coating, without edge effect, formed at an intermediate withdrawal rate of 0.1–1 mm/s, conditions under which both viscous drag and capillarity feeding were active. As the number of coatings increased, the coating thickness increased and cracking increased. EIS tests were used to analyze internal defects in coatings. With repeated coating, defects in coatings increased. After a 30-min heat treatment at 450°C in air, pore resistance (Rpore) decreased approximately 20–30% compared to Rpore of specimens without heat treatment. This study confirmed that it is important to control the morphology to obtain optimum properties of TiO2 film as a coating for corrosion barriers, photocatalysts, and photoelectric and optical applications.