Hydrophobic surfaces by duplex treatments of titanium combining nanosecond laser processing and chemical functionalization

Duplex treatment of titanium surfaces that consists in a nanosecond laser functionalization on air producing a rough oxide film and its subsequent chemical modification was investigated with the goal of obtaining highly hydrophobic surfaces suitable for self-cleaning and microfluidics applications. A pulsed nanosecond Nd:YAG laser with λ = 532 nm was used for the creation of anatase-rich sawtooth patterned substrates with a maximum roughness of 45 μm. The chemical modification of laser-treated surfaces with octylphosphonic acid (OPA) in the form of tetrahydrofuran (THF) or boiling water solutions was performed. The effect of modifier concentrations ranging from 10⁻² to 100 mM on the resulting organic load, layer structure and contact angles was investigated. The duplex-treated surfaces were characterized by SEM-EDS/WDS, XPS, XRD and Raman spectroscopy.

It was found that OPA solutions in THF resulted exclusively in the formation of monolayers (∼13 Å), with the completion of surface coverage at around 4.8 at.% P load according to XPS. Concentrated aqueous solutions of OPA produced an additional titanium (IV) octylphosphonate overlayer which brought the total thickness of organic layer to 25–29 Å at a maximum P amount approaching 8 at.%. Despite the different nature of the organic layers, all surfaces bearing at least 2.2 at. % P showed similar static contact angle of about 155°. Based on dynamic contact angle measurements, superhydrophobic behavior was confirmed both for the surfaces bearing complete OPA monolayers and titanium (IV) octylphosphonate overlayers.