Vibrational Spectroscopy of the Adsorbates on Nb-doped TiO₂ Single-crystal Electrodes

抄録

Titanium dioxide (TiO₂) is one of the candidates for nonplatinum electrocatalysis of the cathode of a fuel cell. Adsorbates on the electrode surface affect the activity of electrochemical reactions. However, adsorbates on TiO₂ single crystal electrodes have not been determined in electrochemical environments. In this study, we have studied the adsorbates on the low index planes of 1 % Nb-doped rutile TiO₂ single crystal electrodes (Nb/TiO₂) in 0.1 M (mol L⁻¹) HClO₄ using nanoparticle surface-enhanced Raman spectroscopy (NPSERS) and infrared reflection absorption spectroscopy (IRAS). A NPSERS band was observed on Nb/TiO₂(110) surface at 585 cm⁻¹ above 0.5 V (vs. RHE). The band was shifted 15 cm⁻¹ to lower frequency in D₂O, indicating that the corresponding adsorbed species contains hydrogen. This band was assigned to a symmetric stretching vibration of Ti–OH–Ti (bridged OH) according to the density functional theory (DFT) calculations. The onset potential of the 585 cm⁻¹ increased as Nb/TiO₂(100) < Nb/TiO₂(110) ∼ Nb/TiO₂(111). However, the order of the activity for the oxygen reduction reaction (ORR) was Nb/TiO₂(100) < Nb/TiO₂(111) < Nb/TiO₂(110). No direct correlation was found between the onset potential of the 585 cm⁻¹ band and the ORR activity. IRAS spectra found adsorbed water on Pd doped Nb/TiO₂(110) (Pd/Nb/TiO₂(110)), the most active plane for the ORR, whereas no adsorbed water was observed on Pd/Nb/TiO₂(100) that has no ORR activity. These findings suggest that adsorbed water, which is undetectable by NPSERS, may induce the ORR activity on Nb/TiO₂.