Sunghwan Shin

Generation of strong electric fields in an ice film capacitor

We present a capacitor-type device that can generate strong electrostatic field in condensed phase. The device comprises an ice film grown on a cold metal substrate in vacuum, and the film is charged by trapping Cs(+) ions on the ice surface with thermodynamic surface energy. Electric field within the charged film was monitored through measuring the film voltage using a Kelvin work function probe and the vibrational Stark effect of acetonitrile using IR spectroscopy. These measurements show that the electric field can be increased to ∼4 × 10(8) V m(-1), higher than that achievable by conventional metal plate capacitors. In addition, the present device may provide several advantages in studying the effects of electric field on molecules in condensed phase, such as the ability to control the sample composition and structure at molecular scale and the spectroscopic monitoring of the sample under electric field.

Acidic Water Monolayer on Ruthenium(0001)

The ubiquity of water in natural environments makes the interaction of water with solid surfaces an important subject of study in a wide variety of scientific disciplines and technologies. One of the most intensively investigated systems for the interaction of water with metal surfaces is water on Ru(0001), which has become a test system for our understanding of this scientific field. Numerous experimental and theoretical studies conducted during the past decade have greatly improved our understanding of the structure and dynamics of water adsorption on Ru(0001). These studies have reached the consensus that water adsorption leads to the formation of an intact molecularwater layer on the surface at low temperature (less than about 155 K). As the surface is heated, H2O partially dissociates to form amixedOH+H2O+H adsorption layer, in competition with desorption of H2O. On the other hand, D2O does not dissociate on the surface and desorbs intact due to a kinetic isotope effect. Despite the wealth of theoretical and experimental research on this system, there are still many open questions, in particular concerning the acid–base properties of adsorbed water. This information is fundamentally important to heterogeneous catalysis, corrosion, and electrochemistry because it determines the proton transfer and acid–base characteristics of the water–solid interface. Therefore, it is highly desirable to investigate these properties for adsorbed water using a systematic surface science approach; this could be one way to unravel the intricacies of the acid–base chemistry at water–solid interfaces. In the present work, we study the proton-transfer ability of water molecules adsorbed on a Ru(0001) surface by using surface spectroscopic measurements and ammonia adsorption experiments. The study shows that the first monolayer of water is much more acidic than bulk water, with the ability to spontaneously transfer a proton to an ammonia molecule. We prepared a water layer on Ru(0001) in ultrahigh vacuum (UHV) by the adsorption of H2O vapor at 140 K to a monolayer saturation coverage, a condition that is known to produce an intact molecular-water layer on the surface. Then, NH3 was adsorbed onto the H2O monolayer surface for a small coverage [0.04 ML; 1 ML= 1.14 10 moleculescm 2 corresponding to the monolayer density of water on Ru(0001)]. The NH3 molecules served as a probe for the surface acidity. Figure 1 shows the results of low-energy sputtering (LES) and reactive-ion scattering (RIS) measurements for the H2O monolayer before and after the adsorption of NH3. The RIS and LES methods measure neutral and ionic species, respectively, on the surface. For a layer of pure H2O, spectrum a in Figure 1 shows the RIS signal of CsH2O + (m/z= 151 amu/charge), which was produced by the pickup of surface H2O molecules by scattering Cs + projectiles. The peak of elastically scattered Cs ions appeared at m/z= 133. After NH3 adsorption (spectrum b in Figure 1), a CsNH3 + (m/ z= 150) signal appeared with a small intensity, indicating the presence of neutral NH3 adsorbates on the surface. In addition, LES signals appeared for NH4 + (m/z= 18) and NH4(H2O) + (m/z= 36), indicating the presence of NH4 + and its hydrated species. These ammonium signals indicated that protons were transferred from the water monolayer to NH3 adsorbates to form NH4 . In additional experiments, we observed that the ammonium signals exhibited the following features. First, the NH4 + and NH4(H2O) + signals did not appear when NH3 was adsorbed onto a multilayer ice film grown on Ru(0001). This result showed that the first water monolayer was the proton donor to NH3. Second, in order to check if the ammonium signals originated from preformed ions on the surface, we measured the appearance threshold of the NH4 + signal as a function of Cs impact energy for the water monolayer (where the NH4 + was formed by proton transfer) and for the multilayer ice film (where only neutral NH3 was present). The two surfaces showed well-distinguished characteristics for the threshold energy and intensity of NH4 + emission. The NH4 + signal from the water monolayer exhibited a lower threshold energy (20–25 eV) and stronger intensity than that from the multilayer film, which is characteristic for the low-energy sputtering of preformed NH4 + species. On the other hand, on the multilayer ice film, Figure 1. LES and RIS mass spectra of positive ions obtained from a) H2O monolayer formed on Ru(0001) at 140 K, and b) after NH3 adsorption (ca. 0.04 ML) on surface at 80 K. The LES and RIS measurements were conducted at 80 K with a Cs beam energy of 25 eV.

Phase transitions of amorphous solid acetone in confined geometry investigated by reflection absorption infrared spectroscopy.

We investigated the phase transformations of amorphous solid acetone under confined geometry by preparing acetone films trapped in amorphous solid water (ASW) or CCl4. Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) were used to monitor the phase changes of the acetone sample with increasing temperature. An acetone film trapped in ASW shows an abrupt change in the RAIRS features of the acetone vibrational bands during heating from 80 to 100 K, which indicates the transformation of amorphous solid acetone to a molecularly aligned crystalline phase. Further heating of the sample to 140 K produces an isotropic solid phase, and eventually a fluid phase near 157 K, at which the acetone sample is probably trapped in a pressurized, superheated condition inside the ASW matrix. Inside a CCl4 matrix, amorphous solid acetone crystallizes into a different, isotropic structure at ca. 90 K. We propose that the molecularly aligned crystalline phase formed in ASW is created by heterogeneous nucleation at the acetone-water interface, with resultant crystal growth, whereas the isotropic crystalline phase in CCl4 is formed by homogeneous crystal growth starting from the bulk region of the acetone sample.

Zundel-like and Eigen-like hydrated protons on a platinum surface.

The nature of hydrated protons is an important topic in the fundamental study of electrode processes in acidic environment. For example, it is not yet clear whether hydrated protons are formed in the solution or on the electrode surface in the hydrogen evolution reaction on a Pt electrode. Using mass spectrometry and infrared spectroscopy, we show that hydrogen atoms are converted into hydrated protons directly on a Pt(111) surface coadsorbed with hydrogen and water in ultrahigh vacuum. The hydrated protons are preferentially stabilized as multiply hydrated species (H5 O2 (+) and H7 O3 (+) ) rather than as hydronium (H3 O(+) ) ions. These surface-bound hydrated protons may play an important role in the interconversion between adsorbed hydrogen atoms and solvated protons in solution.

Spectroscopic Monitoring of the Acidity of Water Films on Ru(0001): Orientation-Specific Acidity of Adsorbed Water

We examined the acid–base properties of water films adsorbed onto a Ru(0001) substrate by using surface spectroscopic methods in vacuum environments. Ammonia adsorption experiments combined with low-energy sputtering (LES), reactive ion scattering (RIS), reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) measurements showed that the adsorbed water is acidic enough to transfer protons to ammonia. Only the water molecules in an intact water monolayer and water clusters larger than the hexamer exhibit such acidity, whereas small clusters, a thick ice film or a partially dissociated water monolayer that contains OH, H2O and H species are not acidic. The observations indicate the orientation-specific acidity of adsorbed water. The acidity stems from water molecules with H-down adsorption geometry present in the monolayer. However, the dissociation of water into H and OH on the surface does not promote but rather suppresses the proton transfer to ammonia.

Entropy-Driven Spontaneous Reaction in Cryogenic Ice: Dissociation of Fluoroacetic Acids

Chemical reactions are extremely difficult to occur in ice at low temperature, where atoms and molecules are frozen in position with minimal thermal energy and entropy. Contrary to this general behavior, certain weak acids including fluoroacetic acids dissociate spontaneously and more efficiently in cryogenic ice than in aqueous solution at room temperaure. The enhanced reactivity of weak acids is an unexpected consequence of proton-transfer equilibrium in ice. The configurational entropy of protons in ice shifts the acid dissociation equilibrium forward. This configurational entropy, although a solid-state property, is comparatively large in magnitude with the entropy of vaporization and can effectively drive proton-transfer reactions in ice.