Article pubs.acs.org/JPCC
Transport and Surface Accumulation of Hydroniums and Chlorides in an Ice Film. A High Temperature (140−180 K) Study Eunhee Park, Du Hyeong Lee, Sooyeon Kim, and Heon Kang* Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 151-742, Republic of Korea ABSTRACT: We studied the transport of hydronium, chloride, and water in an ice film at 140−180 K, a temperature range that encompasses the onset of ice sublimation, crystallization, and roughening transition of the ice film. The ice film was grown in vacuum to have the structure of a bottom H2O layer, an upper D2O layer, and hydronium and chloride ions embedded at the H2O/D2O interface, with a total sample thickness ≥100 BL. The transport of hydronium and chloride ions from the interface layer to the ice surface was monitored as a function of the sample temperature using a low energy sputtering (LES) method. The transport of water molecules and their H/D exchange reactions were monitored using a Cs+ reactive ion scattering (RIS) method. Temperature-programmed desorption (TPD) experiments measured the desorption of water and hydrogen chloride from the surface. The study revealed that hydroniums and chlorides accumulate at the ice surface at an elevated temperature due to their thermodynamic affinity for the surface. The ion transports may occur via isotropic diffusion in the interior region of the ice. Near the ice surface, thermodynamic forces accelerate the directional migration of ions toward the surface. The accumulation of hydroniums and chlorides at the surface facilitates the recombinative thermal desorption of hydrogen chloride gas. These observations support the idea that the heterogeneous reactions of hydroniums and chlorides in ice particles in the atmosphere, such as the chlorine activation reactions in polar stratospheric clouds, occur preferentially at the surface of ice rather than in the bulk phase. solution,24−31 which suggests that the segregation properties are determined largely by ion specificity. An experimental approach widely used in the study of ice surface properties is to grow thin films of ice on metal substrates at low temperature in an ultrahigh vacuum (UHV) and to investigate the film surface with surface spectroscopic techniques.2,8−13,20,21,32−36 Such experiments offer a clear advantage over the experiments conducted under atmospheric conditions, because in vacuum an ice sample can be prepared in a controlled way and kept clean, avoiding contamination from environmental gases. However, the disadvantage is that the temperature range of the investigation is usually limited to below ∼150 K in UHV because the rate of ice surface sublimation becomes significant at higher temperatures. There is a substantial gap between the temperature range accessible for UHV experiments and that of the earth’s atmosphere (>180 K). Therefore, it is highly desirable to bridge the gap by expanding the temperature range of UHV study, but still utilizing its advantage of ice sample preparation and characterization. As part of such an experimental effort, it was demonstrated that thermal desorption spectrometry employing a rapid heating (∼105 deg s−1) method can be used to examine the H/D exchange kinetics in an ice film near the melting temperature of ice.37 In the present work we study the transport properties of hydronium, chloride, and water in an ice film for temperatures
1. INTRODUCTION Information about the mobility of small molecules in ice and their partition behavior between the ice surface and interior is very important to understand the heterogeneous atmospheric chemistry of ice particles.1,2 A few well-known examples of heterogeneous atmospheric reactions include ozone depletion reactions catalyzed by ice particles in polar stratospheric clouds3−5 and the halogen chemistry of sea-salt aerosols in the marine boundary layer.6 In interstellar space, icy dust particles are considered a birth place of many extraterrestrial molecules.7 The transport of hydronium ions in ice has been extensively studied in recent years.2,8−21 These studies revealed that hydronium ions move via a proton-hopping relay along the hydrogen-bond chain of water (Grotthuss mechanism). At elevated temperatures (≥ca. 130 K), the proton-hopping relay channel is coupled with Bjerrum defect motion to increase the rate and distance of the hydronium transport.8,9,14−16,18,19 An interstitial diffusion mechanism of hydroniums was also suggested in the experiments performed at 160−260 K.17 The partition behavior of hydroniums between the surface and interior of ice was investigated for ice samples prepared in the form of thin films or nanoparticles at low temperature (≤140 K).8−12,20,21 These studies observed that hydroniums prefer to reside at the ice surface rather than in the interior. The partition behaviors of other simple ions were also examined. Studies of the ionic dissociation of alkali halide salts on ice films showed that Na+ and F− ions migrate from the ice surface to its interior at temperatures above ∼130 K, while Cl− and Br− ions stay on the ice surface.22,23 Interestingly, each of these ions exhibits similar surface or bulk segregative behaviors in ice and aqueous © 2012 American Chemical Society
Received: June 21, 2012 Revised: September 17, 2012 Published: September 19, 2012 21828
dx.doi.org/10.1021/jp3061416 | J. Phys. Chem. C 2012, 116, 21828−21835
The Journal of Physical Chemistry C
Article
Figure 1. (a) Positive-ion LES spectrum, (b) negative-ion LES spectrum, and (c) RIS spectrum measured at each preparation step of an HClsandwich ice sample. The sample was prepared in the following sequence: (I) a crystalline H2O film (∼60 BL thick) was grown on Pt(111). (II) HCl was adsorbed onto the film for a coverage of 0.3 MLE. (III) Addition of an amorphous D2O overlayer (∼60 BL thick) at 95 K completed an HClsandwich sample. The spectra III(a)−(c) were taken after brief heating of the HCl-sandwich sample at ∼160 K. All the spectra were measured at 95 K with a Cs+ beam energy of 35 eV.
ature-programmed desorption (TPD).1,49 Ice films were grown on a Pt(111) crystal. The Pt surface was cleaned by standard sputtering and annealing procedures, and surface cleanliness was checked by TPD and RIS. It is reported43 that growth of a crystalline ice film on a Pt(111) surface follows a Stranski− Krastanov mechanism, forming initially isolated ice clusters on a wetting water monolayer on Pt(111).44−48 When an ice film grows above a certain thickness, typically ∼50 BL, a continuous film growth occurs with an incommensurate bulk ice structure.46−48 We prepared a thick, continuous ice film by depositing H2O vapor on Pt(111), maintained at a temperature of 140 K, through a microcapillary doser50 at a growth rate of ∼0.1 BL s−1 (bilayer; 1 BL = 1.1 × 1015 water molecules cm−2). H2O partial pressure inside the chamber was below 2 × 10−9 Torr during the film growth. The use of a relatively low temperature (140 K) in crystalline ice growth favors the formation of small ice clusters and abundant growth nuclei initially,47,48 which is advantageous for the growth of continuous multilayer ice films with Ih(0001) surfaces in the later stage.47 The ice film was annealed briefly at 150−155 K to ensure its crystallization. HCl was adsorbed onto the sample surface through a separate leak valve and a tube doser. A D2O overlayer was grown by backfilling the chamber with D2O vapor at a partial pressure of ∼1 × 10−7 Torr. TPD experiments were used to characterize an ice sample and to estimate its thickness. The TPD of water for an ice film showed a pseudo-first-order desorption curve starting slowly at a temperature above ∼150 K and dropping to zero at the point of desorption of the last water layer,39−41,43,51 which occurred around ∼177 K for the present ice films at a heating rate of 0.5 deg s−1. A small bump appeared at 150−160 K in the water TPD, which indicated a phase transition from an amorphous ice overlayer to a crystalline structure, as reported previously.39−41 This bump structure faded away when the amount of HCl dopants in the sample was increased. The ice film surface was analyzed by RIS and LES methods, which have been widely used for identifying neutral and ionic species, respectively, present on ice surfaces.1,49 In these
in the range 140−180 K in UHV. The investigation of ice surfaces at such high temperatures faces several new challenges. Owing to the rapid occurrence of diffusion and desorption of the surface water, the structure of the ice surface is no longer considered static at the molecular scale.38 The population of intrinsic point defects in the ice lattice increases exponentially with temperature, as does the rate of self-diffusion.38−41 In addition, an ice film undergoes an amorphous-to-crystalline phase transition in this temperature region,39−42 which accompanies diffusion of water and molecular mixing in the ice.39−41 A roughening transition of an ice film may also occur dewet the metal substrate.43−48 All these phenomena may occur concurrently with the transport processes of interest in the study. For this reason, meticulous care will be needed for hightemperature experiments in order to properly extract the desired kinetic information, while discriminating between the intervening effects of the other parallel processes. As an optimized compromise for conducting a high-temperature experiment, we employ a thick (≥100 BL) ice film so that the gradual evaporation of the ice surface does not significantly alter the whole sample dimension. This enables us to perform kinetic measurements up to a sample temperature of ∼170 K at a ramping rate higher than 0.5 deg s−1. The ice sample comprises a bottom H2O layer, an upper D2O layer, and excess hydroniums and chlorides trapped at the H2O/D2O interface (“HCl-sandwich” sample). Surface spectroscopic methods measure the transport of hydroniums, chlorides, and various water isotopomers through the ice layer and their appearance at the sample surface. Gaseous species desorbing from the surface are also measured. This study expands upon the previous knowledge about the transport behavior of these species examined with thin (≤10 BL) ice films at low temperature (
