Contact Angle Relaxation and Long-Lasting Hydrophilicity of Sputtered Anatase TiO Thin Films by Novel Quantitative XPS Analysis.

The contact angle relaxation of TiO surfaces is an important problem that must be understood, particularly for long-lasting hydrophilicity under dark conditions. The relaxation of sputtered anatase TiO thin films over a long time (∼22 days) in an atmospheric environment was observed using quantitative XPS analysis. A new peak was identified as HO within a donor-acceptor complex at ∼2.57 eV above the lattice oxygen peak. This donor-acceptor complex turns out to be a key factor for long lasting hydrophilicity, and our model is presented. Adventitious carbon contamination was not the main cause of the contact angle relaxation. Instead, samples with lower amounts of donor-acceptor complexes ( I/ I ≤ ∼5%) underwent contact angle relaxation over time, and samples with a high density of donor-acceptor complexes ( I/ I ≥ ∼10%) showed good hydrophilicity (contact angle ≤20°) over 22 days. Larger amounts of basic Ti-OH relative to acidic OH ( I/ I ≥ 1) resulted in greater amounts of donor-acceptor complexes ( I/ I ≥ ∼10%). Thus, basic Ti-OH groups interact with HO by forming a strong electrostatic donor-acceptor complex, leading to long-lasting hydrophilicity. Indeed, TiO was transformed to show long lasting hydrophilicity by high-density oxygen plasma treatment by forming sufficient Ti-OH groups and HO molecules in the donor-acceptor complexes. Contact angle relaxation is closely related to the interactions between water molecules and the TiO surface in the dark. It is suggested that the relaxation depends on the number of electrostatic donor-acceptor complexes. This study provides new insight by linking theoretical studies with the experimental contact angle at the TiO surface in an ambient environment and is the first study that provides the presented relaxation mechanism.