Vibrational Spectroscopy of the Adsorbates on Nb-doped TiO2 Single-crystal Electrodes (Supporting Information)

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) 2(110) ∼ Nb/TiO2(111). However, the order of the activity for the oxygen reduction reaction (ORR) was Nb/TiO2(100) 2(111) 2(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.