HIGH-PRESSURE OXIDATION OF RUTHENIUM AS PROBED BY SURFACE-ENHANCED RAMAN AND X-RAY PHOTOELECTRON SPECTROSCOPIES

Surface-enhanced Raman spectroscopy (SERS) combined with X-ray photoel ectron spectroscopy (XPS) has been utilized to study the oxidation of ruthenium at ambient pressure (1 atm) and elevated temperatures (25-30 0 degrees C). The SERS probe provides in-situ vibrational information regarding surface oxide bonding. While the XPS probe necessarily invol ves ex-situ measurements (i.e., transfer to and from ultrahigh vacuum) , it provides valuable complementary information on the metal and oxyg en electronic states. Ruthenium surfaces were prepared by electrodepos iting ultrathin films (about three monolayers) onto electrochemically roughened (i.e., SERS-active) gold substrates. Insight into the in-sit u oxidation process was obtained by probing the changes of surface spe ciation by SERS upon heating Ru in flowing O-2. A pair of SERS bands a t 470 and 670 cm(-1) appear in the spectrum acquired for a freshly ele ctrodeposited film, which are assigned to different stretching modes o f hydrated RuO2 formed 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 electroche mical cell followed by heating to 200 degrees C in vacuum so to therma lly desorb the CO. While the initially oxidized (i.e., RuO2) surface w as stable to further oxidation upon heating in O-2, adsorbed atomic ox ygen was detected at 200 degrees C from the appearance of a SERS band at 600 cm(-1) and a XPS O(1s) peak at 531.7 eV. In contrast, the highe r oxides RuO4 and possibly RuO3 were produced only upon thermal oxidat ion of the fully reduced Ru surface. Evidence for RuO3 formation inclu des the appearance of a 800 cm(-1) SERS band at 200 degrees C which co rrelates with the advent of a Ru(3d(5/2)) peak at 282.6 eV. The surfac e was further oxidized to RuO4 at 250 degrees C, as deduced from the f ormation of a 875 cm(-1) band and a Ru(3d(5/2)) peak at 283.3 eV. Whil e RuO3 and RuO4 were exclusively formed at temperatures higher than 25 0 degrees C, RuO2 was produced upon cooling to room temperature, possi bly via the decomposition of RuO4. (C) 1997 Academic Press.