Room-temperature ionic liquids are a new class of liquids with many important uses in electrical and electrochemical devices. The liquids are composed purely of ions in the liquid state with no solvent. They generally have good electrical and ionic conductivity and are electrochemically stable. Since their applications often depend critically on the interface structure of the liquid adjacent to the electrode, a molecular level description is necessary to understanding and improving their performance. There are currently no adequate models or descriptions on the organization of the ions, in these pure ionic compounds, adjacent to the electrode surface. In normal electrolytic solutions, the organization of solvent and ions is adequately described by the Gouy-Chapman-Sterns model. However, this model is based on the same concepts as those in Debye-Huckel theory, that is a dilute electrolyte, where ions are well-separated and noninteracting. This is definitely not the situation for ionic liquids. Thus our goal was to investigate the ionic liquid-metal interface using surface-specific vibrational spectroscopy sum frequency generation, SFG. This technique can probe the metal-liquid interface without interference from the bulk electrolyte. Thus the interface is probed in situ while the electrode potential is changed. To compliment the vibrational spectroscopy, electrochemical impedance spectroscopy (EIS) is used to measure the capacitance and estimate the "double layer" thickness and the potential of zero charge (PZC). In addition, the vibrational Stark shift of CO adsorbed on the Pt electrode was measured to provide an independent measure of the "double layer" thickness. All techniques were measured as a function of applied potential to provide full description of the interface for a variety of imidazolium-based (cation) ionic liquids. The vibrational Stark shift and EIS results suggest that ions organize in a Helmholtz-like layer at the interface, where the potential drop occurs over the a range of 3-5 A from the metal surface into the liquid. Further, the SFG results imply that the "double layer" structure is potential-dependent; At potentials positive of the PZC, anions adsorbed to the surface and the imidazolium ring are repelled to orient more along the surface normal, compared with the potentials negative of the PZC, at which the cation is oriented more parallel to the surface plane and the anions are repelled from the surface. The results present a view of the ionic liquid-metal electrode interface having a very thin "double layer" structure where the ions form a single layer at the surface to screen the electrode charge. However, the results also raise many other fundamental questions as to the detailed nature of the interfacial structure and interpretations of both electrochemical and spectroscopic data.

The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously, but the use of photocatalysts to split water into stoichiometric amounts of H2 and O2 (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga1-xZnx)(N1-xOx)) do not exhibit good activity beyond 440 nm (refs 1,2,9) and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized from non-active CoO micropowders using two distinct methods (femtosecond laser ablation and mechanical ball milling), and the CoO nanoparticles that result can decompose pure water under visible-light irradiation without any co-catalysts or sacrificial reagents. Using electrochemical impedance spectroscopy, we show that the high photocatalytic activity of the nanoparticles arises from a significant shift in the position of the band edge of the material.

Vibrational spectroscopy, sum frequency generation, has been used to study the gas-liquid interface of imidazolium-based, room-temperature ionic liquids. The ionic liquids are divided into two categories on the basis of their miscibility with water, as hydrophobic (immiscible) and hydrophilic (miscible). The spectroscopy results indicate that water will reorient the cations of the hydrophobic ionic liquid, while in the hydrophilic ones they remain unaffected. Thus, under low water conditions the plane of the imidazolium ring remains parallel to the surface for both types of ionic liquid. However, at high water content the ring will reorient to along the surface normal for the hydrophobic ionic liquid but remain parallel to the surface for the hydrophilic one. This is a reversible process.

Sum frequency generation spectroscopy, SFG, was used for the surface characterization at the gas-liquid interface of the 1-butyl-3-methylimidazolium cation combined with the following anions: Br-, I-, PF6-, BF4-, (CF3SO2)2N- (imide), SCN-, CH3SO3- (MeSO3), CH3SO4- (MS), and (CN)2N- (DCN). The SFG spectra obtained for the different ionic liquids were similar independent of the anion selected; therefore, a comprehensive analysis for the surface characterization of the ionic liquids' cation was focused only on the PF6- and Br- anion combinations. For an accurate identification of the vibrational modes observed, FT-IR and Raman spectroscopy in combination with isotopic labeling with deuterium and polarized Raman spectroscopy was used. The cation orientation was determined by analysis of polarization-dependent SFG spectra. For a compound dried in a vacuum to < or = 2 x 10(-5) Torr, the cation appears to be oriented with the ring laying flat along the surface plane and the butyl chain projecting into the gas phase independent of the anion identity.

Sum frequency generation vibrational spectroscopy was used to determine the orientation of the cations in a room-temperature ionic liquid at the air-liquid interface. The ionic liquid that was studied was 1-butyl-3methylimidazolium bis-trifluoromethylsulfonimide, [BMIM] + [imide] -. The orientation of the cation, [BMIM] + , was studied as a function of water pressure in the vapor over the range of 10 -5 to 200 Torr. At water pressures below 10 -4 Torr, the imidazolium ring was oriented parallel to the surface plane. On increasing the water pressure to greater than 10 -4 Torr, the imidazolium ring tipped up from the surface with an angle along the surface normal of 40-55°. No water signal was observed in the spectra obtained at the water pressures studied here.