Photostability and Thermal Decomposition of Benzoic Acid on TiO2

Carboxylic acid moieties are used to anchor organic dyes to TiO2 in many applications; therefore, their structure, distribution on the surface, and thermal and photochemical stability are extremely important. Herein, we investigate the thermal and photochemistry of benzoic acid monolayers on TiO2. Benzoic acid deprotonates to form benzoate monomers on the TiO2 surface, and thermally decomposes to benzene and CO. Titanium interstitials migrate to the surface and react with oxygen atoms from the benzoate molecules to form new TiOx islands. A combination of X-ray photoelectron spectroscopy and scanning tunneling microscopy shows that the benzoate layer on TiO2 is stable under UV illumination for several hours. The stability of benzoate under illumination is in contrast to the photochemical instability of other small carboxylic acid containing molecules on TiO2 and demonstrates the importance of substituents on the stability of the molecular layer.