High-Pressure Oxidation of Ruthenium as Probed by Surface-Enhanced Raman and X-Ray Photoelectron Spectroscopies

Abstract

Surface-enhanced Raman spectroscopy (SERS) combined with X-ray photoelectron spectroscopy (XPS) has been utilized to study the oxidation of ruthenium at ambient pressure (1 atm) and elevated temperatures (25–300°C). The SERS probe providesin-situvibrational information regarding surface oxide bonding. While the XPS probe necessarily involvesex-situmeasurements (i.e., transfer to and from ultrahigh vacuum), it provides valuable complementary information on the metal and oxygen electronic states. Ruthenium surfaces were prepared by electrodepositing ultrathin films (about three monolayers) onto electrochemically roughened (i.e., SERS-active) gold substrates. Insight into thein-situoxidation process was obtained by probing the changes of surface speciation by SERS upon heating Ru in flowing O2. A pair of SERS bands at 470 and 670 cm−1appear in the spectrum acquired for a freshly electrodeposited film, which are assigned to different stretching modes of hydrated RuO2formed during sample transfer to the gas-phase reactor. However, a fully reduced Ru surface (i.e., devoid of oxide features) could be formed by adsorbing a protective CO adlayer in an electrochemical cell followed by heating to 200°C in vacuum so to thermally desorb the CO. While the initially oxidized (i.e., RuO2) surface was stable to further oxidation upon heating in O2, adsorbed atomic oxygen was detected at 200°C from the appearance of a SERS band at 600 cm−1and a XPS O(1s) peak at 531.7 eV. In contrast, the higher oxides RuO4and possibly RuO3were produced only upon thermal oxidation of the fully reduced Ru surface. Evidence for RuO3formation includes the appearance of a 800 cm−1SERS band at 200°C which correlates with the advent of a Ru(3d5/2) peak at 282.6 eV. The surface was further oxidized to RuO4at 250°C, as deduced from the formation of a 875 cm−1band and a Ru(3d5/2) peak at 283.3 eV. While RuO3and RuO4were exclusively formed at temperatures higher than 250°C, RuO2was produced upon cooling to room temperature, possibly via the decomposition of RuO4.