X-ray diffraction studies of ordered chloride and bromide monolayers at the Au(111)-solution interface

Abstract

We present in situ surface x-ray scattering observations of ordered chloride and bromide monolayers on Au(111) electrodes in aqueous solutions as an example of in situ x-ray diffraction from adsorbates of the lighter elements (Z &50). At a critical surface density both halides form incommensurate, hexagonal-close-packed monolayers that compress uniformly with increasing potential. Our structural results provide quantitative data on the residual adsorbate charge, the two-dimensional melting, and the two-dimensional compressibilities of halide monolayers.The adsorption of halogen gases on metal surfaces and of halide anions on metal electrodes in aqueous solutions has been extensively studied in the past. In a vacuum environment, chemisorbed halogens dissociate and form ordered monolayers. ' These adlayers are often incommensurate (no lateral registry) with the metal substrate and change continuously with increasing halogen coverage, not unlike the two-dimensional phases of physisorbed noble gases. However, the equilibrium properties of halide adlayers are difticult to obtain in vacuum experiments due to the strong chemical halide-metal interaction. In contrast, equilibrium is readily established at the liquidsolid interface where the chemical potential of the adsorbate can be controlled via the electrode potential. With the recent advent of modern in situ techniques the structure of these interfaces can now be determined. Studies by in situ scanning tunneling microscopy (STM) and surface x-ray scattering revealed that the strongly bound iodide ions form close-packed, ordered monolayers on gold and platinum electrodes.Much less is known about the lighter halides. Ordered bromide adlayers were found in only one preliminary STM study and ordered phases of chloride and fluoride anions at a solid-liquid interface have not been observed to our knowledge. Here we report in situ observations of ordered chloride and bromide monolayers on Au(111) by grazing incident angle x-ray diffraction. The evaluated equilibrium properties of halide monolayers will be compared to those of noble gas and metallic adlayers.G'razing incident angle x-ray diffraction measurements of the interface structure of Au(111) electrodes were carried out at beamline X22B at the National Syncrotron Light Source at a wavelength 1=1.54 A. The experiments were performed on Au(111) single crystals sputtered and annealed at 1000 K in vacuum or prepared by Game annealing.The sample was then mounted in an electrochemical cell that was filled with deaerated suprapur 0.1M HC104 solutions containing NaC1 or NaBr in the range 0.001 -1M. Subsequently the cell was deAated leaving an electrolyte film of =10 pm thickness between the Au(111) surface and the 4-pm-thick prolene (Chemplex) x-ray window. During the experiment the cell was surrounded by a N2 atmosphere to keep the electrolyte free of oxygen. The electrode potential was controlled versus a Ag/AgC1 (3M KC1) reference electrode.For further experimental details see Refs. 6 and 7. First we will describe the expe...

Interaction of colloidal nanoparticles with their local environment: the (ionic) nanoenvironment around nanoparticles is different from bulk and determines the physico-chemical properties of the nanoparticles

“…for gold electrodes in the presence of different electrolytes. A significant accumulation and adsorption of the halogens Cl 2 , Br 2 and I 2 , and hence changes of the nanoenvironment have been reported and verified by several methods, including atomic force microscopy [68], X-ray diffraction analysis [67], scanning tunnelling microscopy [66] and surface enhanced Raman scattering [65]. Additionally, it has been known for a very long time that bare gold surfaces dipped in pure water suppositionally collect a negative surface charge owing to accumulation of OH 2 [69].…”

“…These effects have long been known for example in biological systems, in which they are responsible for the wellknown Hofmeister effect [61][62][63][64], describing stabilization and precipitation tendencies in proteins. Another example is the well-known fact that specific adsorption and monolayer formation of anions occurs in flat charged gold surfaces dipped in electrolytes [65][66][67][68][69]. Hence, ion-specific effects must not be ignored when the nanoenvironment of NPs is studied, particularly at low salinities in case the previously discussed screening of charges described by continuum models is not dominant.…”

Copper Electrodeposition on Alkanethiolate Covered Gold Electrodes*

“…The azimuthal tilt direction lies between the directions of the next and nearest next sulphur neighbours. The above-mentioned c(4 X 2) lattice appears to be based on an ordered distribution of alkyl chain twist angles (presumably a twist around the S-C axes); structures based on ordered distributions of alkyl twist angles exist also in bulk alkanes [4].…”

“…In electrochemical terms, the substances adsorb specifically. Such adlayers are often densely packed, hence the atoms or molecules prefer (quasi)-hexagonal arrangements [1][2][3][4][5][6][7].…”

“…Electrodeposition has been studied as a function of the thiolate chain length, the deposition potential and the temperature. Time-resolved [1][2][3][4][5][6][7]. …”