Contact Angle Relaxation and Long-Lasting Hydrophilicity of Sputtered Anatase TiO₂ Thin Films by Novel Quantitative...

The contact angle relaxation of TiO₂ surfaces is an important problem that must be understood, particularly for long-lasting hydrophilicity under dark conditions. The relaxation of sputtered anatase TiO₂ thin films over a long time (∼22 days) in an atmospheric environment was observed using quantitative XPS analysis. A new peak was identified as H₂O 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_DAC/ I_bulk ≤ ∼5%) underwent contact angle relaxation over time, and samples with a high density of donor-acceptor complexes ( I_DAC/ I_bulk ≥ ∼10%) showed good hydrophilicity (contact angle ≤20°) over 22 days. Larger amounts of basic Ti-OH relative to acidic OH_bridge ( I_Ti-OH/ I_bridge ≥ 1) resulted in greater amounts of donor-acceptor complexes ( I_DAC/ I_bulk ≥ ∼10%). Thus, basic Ti-OH groups interact with H₂O by forming a strong electrostatic donor-acceptor complex, leading to long-lasting hydrophilicity. Indeed, TiO₂ was transformed to show long lasting hydrophilicity by high-density oxygen plasma treatment by forming sufficient Ti-OH groups and H₂O molecules in the donor-acceptor complexes. Contact angle relaxation is closely related to the interactions between water molecules and the TiO₂ 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 TiO₂ surface in an ambient environment and is the first study that provides the presented relaxation mechanism.