Plasma Polymerization on Mesoporous Surfaces: <i>n</i>‑Hexane on Titanium Nanoparticles

Plasma treatment of porous materials has huge potential in many applications where chemical modification of interior structure or its loading with polymeric matrices is required. Plasma polymers are often assumed to grow conformally on solid supports; however, their innate roughness may interfere with the surface topography at mesoscale. Here, model mesoporous coatings were prepared by deposition of titanium nanoparticles with the average size of 50 nm onto flat silicon substrates. The nanoparticles were fabricated by magnetron sputtering in the configuration of a gas aggregation source. The porous coatings were subsequently subjected to deposition of soft hydrocarbon plasma polymers from <i>n</i>-hexane. In the early stage of the deposition, negligible increase of thickness is observed as the plasma polymer fills the inner free space of the coatings. The topographical features expand in lateral dimension, but the RMS roughness does not change. In the late growth regime, the inner voids become filled and the plasma polymer grows on top of the coatings. The growth proceeds with preferential filling of the valleys between the surface asperities, thus leading to smoothening of the surface. The decrease of roughness is manifested in negative growth exponent. Equality between the absolute values of the local and global roughness exponent evidences about self-affine growth dynamics. Nevertheless, the set of the critical exponents (the growth exponent β = −0.16, the dynamic exponent 1/<i>z</i> = 0.16, the roughness exponent |α| = 1.0) indicates that the deposition does not match any of universality classes of local growth. Local diffusion coupled with nonlocal mass transport due to re-emission of the incoming species can explain the kinetic smoothening observed. The technology allows for fabrication of mesoporous coatings with precisely adjustable pore size or straightforward production of nanocomposite thin films with nanoparticles homogeneously embedded in the matrix of plasma polymer