Flow-directed assembly of nanostructured thin films from suspensions of anisotropic titania particles

Abstract

Nanostructured thin films are fabricated by directing the assembly of anisotropic titanium dioxide particles from a colloidal suspension by fixed blade flow coating. The titania particles (equatorial radius a = 24 ± 4 nm, polar radius b = 130 ± 31 nm, aspect ratio b/a = 5.4 ± 0.9) undergo an isotropic–nematic (I–N) transition at a volume fraction ϕ* ≈ 0.40. For ϕ > ϕ* the deposited film has three structures depending on the substrate velocity, v. At a blade angle α = 25° and a gap d = 200 μm, nanoparticles orient either isotropically in the direction of the coating flow (v ≥ 350 μm s⁻¹), exhibit partially oriented “wavy” structures (100 < v < 300 μm s⁻¹), or are isotropically oriented (v < 100 μm s⁻¹). Below the I–N transition (ϕ < ϕ*), nanoparticles assemble into domains of uniform orientation with dimensions that increase with increasing v. Increasing the blade angle shifts these structural transitions to lower substrate velocities, consistent with an increasing extensional component of the flow. Established scaling relationships describe the film height dependence on ϕ and v. Overall, these results enable nanostructured films to be deposited with desired thickness and structure.