Situ Surface X-ray Diffraction Research Articles

In Situ Surface X-Ray Diffraction Studies of the Electrochemical Double Layer on Pt(111)

The molecular structure of the electrochemical double layer is an important fundamental topic in interfacial electrochemistry. It has received renewed interest in the last years, because of its influence on many electrochemical reactions and the emergence of ab initio molecular dynamics simulations that allow a detailed description of the interface structure and dynamics [1,2]. In the measurements presented here, we investigated the properties of the electrochemical double layer on Pt(111) via in situ surface X-ray diffraction (SXRD), a technique that is well-established for single crystal studies [3-7].Platinum electrodes are widely used as electrocatalysts, for example as cathodes in proton exchange membrane fuel cells due to their high ORR reactivity. The single-crystalline Pt(111) surface, which is a well studied model system, is especially interesting for studies of the electrochemical double layer. Recent electrochemical measurements revisited this system and found evidence for a double layer structure that strongly deviated from the traditional Gouy-Chapman-Stern model and indicated a complex structural influence of water [8,9]. The water structure within the electrochemical double layer is also of major interest in computational studies. For non-adsorbing, aqueous electrolyte the electrode surface charge can be partly compensated by a layer of oriented water molecules due to their dipole moment. For potentials around the potential of zero charge (PZC), a potential-dependent orientation of water molecules with respect to the electrode surface was suggested [1,2,10].To investigate the arrangement and orientation of the water molecules near the Pt(111) surface we used in situ SXRD methods, focussing on X-ray reflectivity (XRR) and crystal truncation rod (CTR) analysis. These methods allow to acquire detailed information about the electrode surface structure as well as the electrochemical double layer. By measuring multiple CTR sets at different surface potentials, information on the potential-dependence of the interface processes can be obtained. The experiments were performed at beamline ID31 of the European Synchrotron Radiation Facility using high photon energies of 75 keV, which provide very large data sets. This allows significantly improved structural modeling as compared to conventional SXRD. The studies employed an electrochemical hanging-meniscus X-ray diffraction cell that allows for SXRD measurements of fast dynamic systems in a well-defined electrochemical environment. We will report results on the water structure in the double layer obtained in perchloric acid at potentials around the PZC.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1.

In Situ Studies of the Electrochemical Reduction of a Supported Ultrathin Single-Crystalline RuO2(110) Layer in an Acidic Environment

With in situ surface X-ray diffraction (SXRD) and X-ray reflectivity (XRR) in combination with ex situ characterization by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry, the electrochemical reduction of an ultrathin (1.66 nm thick) single-crystalline RuO2(110) layer supported on Ru(0001) is studied in an acidic environment, providing clear-cut evidence and mechanistic details for the transformation of RuO2 to hydrous RuO2 and metallic Ru. The reduction process proceeds via proton insertion into the RuO2(110) lattice. For electrode potentials (0 to −50 mV vs standard hydrogen electrode), the layer spacing of RuO2(110) increased, maintaining the octahedral coordination of Ru (SXRD). Continuous proton insertion at −100 to −150 mV leads to the transformation of the lattice oxygen of RuO2 to OH and water, which destroys the connectivity among the Ru-O6 octahedrons and eventually leads to the loss of crystallinity (SXRD) in the RuO2(110) film at −200 mV accom...

Potential dependence of the structure and magnetism of electrodeposited Pd/Co/Au(111) layers

In this work we investigate the magnetic properties and structure of ultrathin Pd/Co/Au(111) stacks prepared by sequential electrodeposition. A combination of in situ real-time magneto optical Kerr effect (MOKE), in situ surface X-ray diffraction (SXRD), and complementary in situ scanning tunneling microscopy, electrochemical studies, and optical reflectivity measurements is employed to characterize the formation of the Pd/Co deposit and its potential-dependent structure. For the preparation of the Pd/Co film a specific procedure is used that prevents Co dissolution. The SXRD results reveal a 2D growth of a (111)-oriented Pd layer, which is in epitaxy with the underlying Co(001)/Au(111). A potential dependent study demonstrates that the Co magnetic properties are reversibly modified upon hydrogen absorption in and release out of the Pd capping film, respectively. Analysis of the SXRD data indicates a Pd lattice expansion upon H-absorption, which is strongly inhibited within the surface plane due to clamping by the substrate. The transmission of hydrogen-induced strain to the Co layer is measured and the resulting impact on the magnetic properties is estimated and discussed.

Lithium Intercalation and Structural Changes at the LiCoO2 Surface Under High Voltage Battery Operation

LiCoO2 electrode exhibits severe capacity fading when charged to 4.4 V, although it delivers a higher capacity of over 150 mAh g-1 at the initial cycles [1]. Surface coating with metal oxides and phosphates has been one of the effective methods to improve cycle performance [1,2]. The coating layer with very small thickness of ca. 0.5−1 nm improves the stability of LiCoO2 more efficiently than the thick coating layer [2], although a small region of coverage by coating species on LiCoO2. These experimental evidences bring up a question about the role of the surface coating, because the surface coating is widely considered to stabilize the electrode by suppressing interfacial reactions between electrodes and the electrolyte species. In this study, the origin of reversible intercalation into the surface-coated LiCoO2 is investigated using uncoated and Li3PO4-coated epitaxial LiCoO2 films which have intercalation-active (104) plane. The role of the surface coating is discussed based on the surface structural changes characterized using in situsurface X-ray diffraction (XRD). An epitaxial-film model electrode of LiCoO2(104) was fabricated on SrRuO3(100)/Nb:SrTiO3(100) using pulsed laser deposition. The LiCoO2 surface was modified by the deposition of amorphous Li3PO4 using radio-frequency (RF) magnetic sputtering with a target of g-Li3PO4. Charge-discharge measurements were performed with the 2032-type coin cell composed of a Li anode and 1 mol dm-3 LiPF6 in a 3:7 mixture of ethylene carbonate and diethyl carbonate as the electrolyte. To clarify the effects of the Li3PO4 coating on the crystal structure at the LiCoO2 surface, in situsurface XRD measurements were performed using grazing angle synchrotron X-rays at the bending-magnet BL14B1 beamline at SPring-8. The 50 nm thick LiCoO2(104) film exhibited lithium (de-)intercalation activity with a first discharge capacity of 119 mAh g-1 between 3.0 and 4.4 V, followed by a gradual capacity fading with subsequent charge-discharge cycles. In contrast, a 3.2 nm thick Li3PO4-coated film exhibited a higher intercalation capacity of 148 mAh g-1 with superior cycle retention than the uncoated film. In situ surface X-ray diffraction measurements revealed a small lattice change at the coated surface during the (de-)intercalation processes compared to the uncoated surface. The surface modification of LiCoO2 by the Li3PO4 coating could lead to improvement of the structural stability at the surface region during lithium (de-)intercalation at high voltage. The surface structure changes during the electrochemical reactions is one of the factors for the reversible lithium (de-)intercalation reaction [3].

Film and Interface Atomic Structures of Electrodeposited Co/Au(111) Layers: An in Situ X-ray Scattering Study as a Function of the Surface Chemistry and the Electrochemical Potential

The growth and atomic structure of thin Co films electrodeposited onto Au(111) have been studied using in situ surface X-ray diffraction (SXRD) and scanning tunneling microscopy (STM). SXRD is used to monitor the growth in real-time and, following Co deposition, the strain of the Co film is characterized as a function of the Co thickness. In-plane SXRD measurements show that Co deposition leads to the formation of a reconstructed Au layer at the Au–Co interface that is composed of an unfaulted, single Au monolayer uniformly compressed with respect to the underlying bulk Au lattice. The surface normal structure of the Au–Co interface and Co films are examined by measurement of the extended X-ray reflectivity (or specular crystal truncation rod (CTR)) for a nominally two monolayer thick (2 ML) Co deposit. In situ STM has also been used to obtain the morphology of the Co films and the Au(111) reconstruction pattern both before and after Co dissolution. The STM data give complementary information to the SXRD ...

Competitive Anion/Water and Cation/Water Interactions at Electrified Copper/Electrolyte Interfaces Probed by in Situ X-ray Diffraction

The full 3D structure of a copper/electrolyte interface is studied by means of in situ surface X-ray diffraction (SXRD) methods. Chloride anions chemisorb on Cu(100) in 10 mM HCl at high potentials under formation of a p(1 × 1)-Cl adlayer. This anionic chemisorption layer serves as a structural template for the lateral ordering of water molecules and hydronium cations in the near-surface liquid electrolyte. Evidence for this interfacial geometry is mainly derived from the intensity distribution of surface-sensitive X-ray diffraction data along the (10L)-adlayer rod. The characteristic oscillating intensity distribution along the (10L) rod is due to a centered bilayer system consisting of the anionic inner Helmholtz layer (IHL) of chemisorbed chloride and the cationic outer Helmholtz layer (OHL). The latter is constituted in the present case by hydronium cations that preferentially populate 4-fold hollow sites of the underlying chloride lattice. IHL and OHL are separated by an extra interfacial water layer...

In situ studies of the oxidation of HCl over RuO2 model catalysts: Stability and reactivity

Structure–activity experiments were performed for the HCl oxidation reaction (Deacon-like process) over RuO2 model catalysts – RuO2(1 1 0) and RuO2(1 0 0) – applying in situ surface X-ray diffraction (SXRD) combined with on-line mass spectrometry. The studied model catalysts turned out to be long-term stable under reaction conditions with gas feed ratios p(HCl):p(O2) ranging from 1:4 to 4:1 in the mbar pressure regime and temperatures as high as 685 K. Even pure HCl exposure in the mbar regime was not able to reduce RuO2 below 600 K; above 650 K chemical reduction of the oxide sets in. Under strongly oxidizing reaction conditions, the (surface) oxides grow slowly in thickness. On-line reactivity experiments of both types of model catalysts in a batch reactor yield a mean turn-over frequency (TOF) of 0.6 Cl2 molecules per second and active site for the HCl oxidation at 650 K and initial partial pressures of p(HCl) = 2 mbar and p(O2) = 0.5 mbar. The HCl oxidation over RuO2 is therefore considered to be structure insensitive.

In situ structure–activity correlation experiments of the ruthenium catalyzed CO oxidation reaction

The complex structure–activity correlation of the CO oxidation on ruthenium has been studied in a batch reactor by using in situ surface X-ray diffraction (SXRD) and on-line mass spectrometry. Two distinct active phases are identified at higher pressures in the mbar range depending on the reaction conditions: a non-oxidic phase and a RuO 2(1 1 0) layer of variable thickness ranging from 1.5 nm to 10 nm. For reaction temperatures lower than 520 K the experimental turnover frequency (TOF) numbers are shown to be almost identical for the two types of active phases. Above 520 K the RuO 2(1 1 0) layer turned out to be much more active than the non-oxidic phase. Kinetic reaction experiments on the RuO 2(1 1 0) phase reveal an activation energy of 78 ± 10 kJ/mol which is in perfect agreement with corresponding reactivity experiments on supported and powder RuO 2 catalyst. Under oxidizing reaction conditions and high concentration of CO 2 in the gas mixture, the RuO 2(1 1 0) model catalyst shows reversible product-poisoning.

Direct comparison of the reactivity of the non-oxidic phase of Ru(0 0 0 1) and the RuO 2 phase in the CO oxidation reaction

Applying in situ surface X-ray diffraction (SXRD) together with on-line mass spectrometry during the CO oxidation over Ru(0 0 0 1) allows a direct comparison of the reactivity of the non-oxidic state with that of the RuO 2(1 1 0) covered surface. This comparison reveals that the RuO 2(1 1 0) surface is a catalytically active phase at least as active as the non-oxidic phase. At high CO and O 2 pressures of 200 mbar and temperatures above 550 K, the CO oxidation reaction does not proceed isothermally on the RuO 2(1 1 0) surface. The released reaction heat leads rather to an increase of the sample temperature of up to 130 K accompanied by a self-acceleration of the CO oxidation reaction.

Oxidation of Ir(111): From O−Ir−O Trilayer to Bulk Oxide Formation

The oxidation of the Ir(111) surface was studied by in situ surface X-ray diffraction (SXRD) in an oxygen pressure range of 10−6−100 mbar at temperatures from 575 to 875 K, in combination with dens...

In situ X-ray analysis under controlled potential conditions: An innovative setup and its application to the investigation of ultrathin films electrodeposited on Ag(1 1 1)

An innovative setup to combine electrochemical and in situ surface X-ray diffraction (SXRD) measurements is described. This electrochemical cell has a different design from the other ones commonly used for X-ray diffraction studies. It allows the sample surface to stay always completely immersed into the solution under controlled potential conditions even during the SXRD measurements. The X-ray beam crosses the liquid (about 1 cm) and the cell walls. Because of the high X-ray energy, the beam attenuation is negligible and by an appropriate positioning of the detector arm slits it is possible to minimize the diffuse scattering induced by the liquid and cell walls in order to still detect the minima of the crystal truncation rods (CTRs). The liquid solution in the cell is managed by a special device, which allows the controlled exchange of the electrolyte solutions necessary in the electrochemical atomic layer epitaxy (ECALE) growth. The whole setup can be remotely controlled from outside the experimental hutch by a dedicated computer. As an example we report measurements on S layers deposited at underpotential on the Ag(1 1 1) surface, and on CdS films of increasing thickness.