Application of the Multi-Step EPD Technique to Fabricate Thick TiO₂ Layers: Effect of Organic Medium Viscosity on the Layer Microstructure

Abstract

In the present study, electrophoretic deposition (EPD) was used to obtain dense layers of TiO(2) in four organic media-methanol, ethanol, 1-propanol, and butanol-with different TiO(2) nanoparticle concenterations of 1-8 g/L. Microstructural study of the obtained layers by scanning electron (SEM) and optical microscope (OM) revealed that the multistep EPD technique could effectively prevent crack formation across the layer compared with the single-step method and will consequently increase the critical cracking thickness (CCT). The quality of EPD layers was also affected by viscosity. According to SEM and atomic force microscope (AFM) results, as the viscosity of the medium increased, more compact layers were formed which can be attributed to the lower deposition rates in heavier alcohols. High deposition rate in methanol and ethanol was also confirmed by zeta potential results. Suspension viscosity was interestingly observed to control the threshold concentration above which crack formation would occur. These values were measured to be 3 and 5 g/L for methanol and ethanol, respectively. However, in suspensions based on more viscous alcohols, the threshold concentration increased to 8 g/L which implied the decisive role of medium on concentration limits. It indicates that by employing organic vehicles of higher viscosity it is possible to maintain the CCT values obtained in less viscous media with no need to decrease the colloidal concentration of the suspension.

Electrophoretic deposition of nanoscale TiO2: technology and applications

“…The stability of suspensions in terms of zeta potential was found to be around +40 mV for the three systems and therefore, the electrophoretic mobility of the particles was confirmed to depend on the ratio between the dielectric constant and the viscosity of the solvent. Under these conditions, and as observed also by Farrokhi-Rad et al [42] and Sadeghi et al [43,44], the lower the dielectric constant ( = 18.3 for isopropanol) and the higher the viscosity (Á = 1.945 mPa s for isopropanol), the lower would be the electrophoretic mobility and therefore, the deposition yield.…”

“…The employment of different alcoholic media has been studied by different authors [42][43][44], focusing on the influence of pK a values, vapor pressures, viscosities, or dielectric constants of the alcohols on both the suspension stability and the final quality of the coatings. In this sense, Farrokhi-Rad and Ghorbani, [42] studied the deposition of titania nanoparticles with an average size of 5 nm (Sigma-Aldrich Corporation, USA) using methanol, ethanol and butanol in the presence of triethanolamine (TEA) as a dispersant, and reported that the lower the pK a of the alcohol, the higher the absolute value of zeta potential, which implies higher stability of the particles.…”

“…In related investigations, Sadeghi et al, [43,44] studied the influence of methanol, ethanol, 1-propanol, butanol, pentanol, hexanol, and heptanol on the EPD of titania P25 . P25 is a common commercially available TiO 2 nanopowder (Evonik Industries) of 21 nm in diameter, which is a mixture of anatase and rutile polymorphs.…”

“…In the study involving simple alcohols, i.e., methanol, ethanol, 1-propanol, and butanol, the authors observed that there was a critical concentration of titania above which the coatings suffered cracking and that crack formation was affected by the viscosity of the solvent. It was observed that for higher viscosities, the solid content in the starting suspension that led to crack-free coatings was higher (3 g/L for methanol (Á = 0.544 mPa s) and 8 g/L for butanol (Á = 3.0 mPa s)) [43]. In the case of higher alcohols, i.e., pentanol, hexanol, and heptanol, it was also observed that the viscosity of the dispersion media plays an important role in the colloidal stability of the particles while the quality of the final coatings was confirmed to be affected by its dielectric constant.…”

Effect of polyethylenimine addition and washing on stability and electrophoretic deposition of Co3O4 nanoparticles

“…For repeatable EPD results an additive usually should be used as a charging agent. Polyethyleneimine (PEI) is a cationic polyelectrolyte that has been widely used in EPD processes in aqueous and nonaqueous suspensions …”