Phosphonic Acid Adsorbates Tune the Surface Potential of TiO2 in Gas and Liquid Environments

Controlled attachment of molecules to the surface of a material can alter the band structure energies with respect to the surrounding environment via a combination of intrinsic and bonding-induced dipoles. Here, we demonstrate that the surface potential of an application-relevant material, anatase TiO2, can be tuned over a broad energy range of ∼1 eV using a family of dipolar phosphonic acid-based adsorbates. Using TiO2 as an example, we show with photoelectron spectroscopy that these adsorbates are stable in a liquid environment (propylene carbonate). More interestingly, the tunability is substantially retained and follows trends in the computed bound dipole. The electrochemical surface potential is shown to vary over 600 meV, the highest range in electrolytes to the best of our knowledge. Using density functional theory calculations, we rationalize the measured trends and show that the effective dipole upon molecular adsorption and not the intrinsic dipole of the isolated molecules correlates with observed changes in surface potential. Control of the effective dipole, through judicious choice of robust surface species, can allow in situ tuning of energy levels and functionality at active surfaces for energy conversion and storage, biosensing, and molecular electronics.