Inorganic Control of Interfacial Charge-Transfer Transitions in Catechol-Functionalized Titanium Oxides Using SrTiO3, BaTiO3, and TiO2

Charge-transfer transitions at the interface between wide band gap semiconductors and organic compounds have new potentialities in optical functions such as chemical sensing and solar energy conversion. A feature of interfacial charge-transfer (ICT) transitions is the flexible wavelength controllability based on the combination of various organic and inorganic materials. The organic control of ICT transitions in the visible to near-IR region has been demonstrated with chemical modification of organic compounds. On the other hand, the inorganic control of ICT transitions has not yet been studied systematically. In order to demonstrate the inorganic control of ICT transitions, we comparatively study ICT transitions in catechol (CA)-functionalized titanium oxides using SrTiO3, BaTiO3, and anatase TiO2 nanoparticles. It has been reported that SrTiO3 and BaTiO3 have the higher-energy conduction band edge by ca. 0.4 and 0.3 eV, respectively, than that of anatase TiO2. CA-functionalized SrTiO3 and BaTiO3 show a broad ICT band in the visible region. The ICT bands are blue-shifted by 0.29 and 0.24 eV as compared to that of TiO2 CA. Ionization potential measurements reveal that the highest occupied molecular orbital level of CA chemisorbed on SrTiO3 and BaTiO3 is higher in energy by ca. 0.1 eV than that on TiO2, which gives no explanation of the blue shift observed for SrTiO3 and BaTiO3. From the experimental results, the blue shift of the ICT band in SrTiO3 and BaTiO3 is attributed to the higher-energy conduction band edges of SrTiO3 and BaTiO3 than that of TiO2. Our research demonstrates the inorganic controllability of ICT transitions based on the energy position of the conduction band edge of inorganic semiconductors.