Electronic transport and Electrocatalytic properties of RuO₂ thin films

This work aims to achieve insights into the roles that surface orientation plays in determining the electronic and electrocatalytic properties of single-crystalline rutile ruthenium oxide (RuO₂) thin films.  RuO₂ thin films with (110), (100), and (101) surface orientation were  controllably grown in situ using a pulsed laser deposition method on (110), (100), and (101)-oriented single-crystal titanium dioxide (TiO₂) and sapphire (r-plane and c-plane) substrates. While the four-probe resistance versus temperature measurements from 500 K to 77 K in Vander Pauw Geometry showed that all the films were metallic in nature, the sign of the Hall coefficient (R_H) of RuO₂ thin films was a function of the films’ surface orientation. The R_H for the (110) and (100) RuO₂ films was positive (2.44 ×10^-5 cm³/C versus 2.47 ×10^-5 cm³/C), the R_H was negative (-2.45 ×10^-5 cm³/C) for the (101) RuO₂ thin films. Thus, these measurements have established that the charge carriers are holes in (110) and (100) RuO₂ thin films, which were calculated to be 2.58×10²³/cm³ and 2.54×10²³/cm³, respectively. The reason for the negative coefficient, in the case of (101) RuO₂ thin films could be due to either a lower mobility of holes in comparison to that of electrons, as per the two-band model, or the predominance of electronic charge carriers over holes. The electrical transport studies were followed by detailed cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) investigations to understand the electrocatalytic property of the RuO₂ thin films as a function of surface orientation.  The electrolytic current density of these films, measured using LSV, and the surface charge density, measured from the area under the redox peak in the CV curve, were both found to depend linearly on the Ru-coordinatively unsaturated sites.