arXiv subjects

Jan Balajka

Publications and source records attributed to Jan Balajka.

10 recordsLinked to original sources

arXiv2026

The Unreconstructed {\alpha}-Al$_{2}$O$_{3}$(0001) Surface is Inhomogeneous and Rough

Alumina (Al$_{2}$O$_{3}$) is a key material for thin-film growth and heterogeneous catalysis, where the atomic surface structure critically impacts performance. Using noncontact atomic force microscopy (nc-AFM) combined with density functional theory (DFT) calculations, we challenge the common assumption that the unreconstructed $\alpha$-Al$_{2}$O$_{3}$(0001) surface is atomically flat and uniformly Al-terminated. This widely accepted bulk termination satisfies polarity compensation requirements but results in highly undercoordinated surface Al cations at the surface. Despite substantial inward relaxation of these Al cations, we find that the (1 ${\times}$ 1) surface remains inherently metastable, relative to the thermodynamically stable $(\sqrt{31} \times \sqrt{31})R\pm9\deg$ surface reconstruction that forms at high temperatures above 1000 {\deg}C. Nc-AFM imaging of the unreconstructed surface reveals a rough and disordered morphology, with only nanometer-scale regions exhibiting the ordered Al-terminated (1 $\times$ 1) structure. Our results show that the unreconstructed Al$_{2}$O$_{3}$(0001) surface is intrinsically inhomogeneous, reconciling conflicting experimental observations and challenging the validity of commonly used atomistic models.

cond-mat.mtrl-sci↗

arXiv2025

Multi-Technique Characterization of Rhodium Gem-Dicarbonyls on TiO$_2$(110)

Gem-dicarbonyls of transition metals supported on metal (oxide) surfaces are common intermediates in heterogeneous catalysis. While infrared (IR) spectroscopy is a standard tool for detecting these species on applied catalysts, the ill-defined crystallographic environment of species observed on powder catalysts renders data interpretation challenging. In this work, we apply a multi-technique surface science approach to investigate rhodium gem-dicarbonyls on a single-crystalline rutile TiO$_2$(110) surface. We combine spectroscopy, scanning probe microscopy, and Density Functional Theory (DFT) to determine their location and coordination on the surface. IR spectroscopy shows the successful creation of gem-dicarbonyls on a titania single crystal by exposing deposited Rh atoms to CO gas, followed by annealing to 200-250 K. Low-temperature scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) data reveal that these complexes are mostly aligned along the [001] crystallographic direction, corroborating theoretical predictions. Notably, x-ray photoelectron spectroscopy (XPS) data reveal multiple rhodium species on the surface, even when the IR spectra show only the signature of rhodium gem-dicarbonyls. As such, our results highlight the complex behavior of carbonyls on metal oxide surfaces, and demonstrate the necessity of multi-technique approaches for the adequate characterization of single-atom catalysts.

physics.chem-ph↗

arXiv2024

A Multi-Technique Study of C2H4 Adsorption on a Model Single-Atom Rh1 Catalyst

Single-atom catalysts are potentially ideal model systems to investigate structure-function relationships in catalysis, if the active sites can be uniquely determined. In this work, we study the interaction of C2H4 with a model Rh/Fe3O4(001) catalyst that features 2-, 5-, and 6-fold coordinated Rh adatoms, as well as Rh clusters. Using multiple surface-sensitive techniques in combination with calculations of density functional theory (DFT), we follow the thermal evolution of the system and disentangle the behavior of the different species. C2H4 adsorption is strongest at the 2-fold coordinated Rh1 with a DFT-determined adsorption energy of -2.26 eV. However, desorption occurs at lower temperatures than expected because the Rh migrates into substitutional sites within the support, where the molecule is more weakly bound. Adsorption at the 5-fold coordinated Rh sites is predicated to -1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C2H4 desorption shoulder in TPD above room temperature.

cond-mat.mtrl-sci↗

arXiv2024

Stoichiometric reconstruction of the Al$_{2}$O$_{3}$(0001) surface

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. The basal (0001) plane of ${\alpha}$-Al$_{2}$O$_{3}$ was imaged with noncontact atomic force microscopy with an atomically-defined tip apex. The surface forms a complex $({\sqrt31} {\times} {\sqrt31})R{\pm}9{\deg}$ reconstruction. The lateral positions of the individual O and Al surface atoms come directly from experiment; how these connect to the underlying crystal bulk was determined based on computational modeling. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al$_{2}$O$_{3}$.

cond-mat.mtrl-sci↗

arXiv2023

Interaction of surface cations of cleaved mica with water in vapor and liquid forms

Natural minerals contain ions that become hydrated when they come into contact with water in vapor and liquid forms. Muscovite mica -- a common phyllosilicate with perfect cleavage planes -- is an ideal system to investigate the details of ion hydration. The cleaved mica surface is decorated by an array of K$^+$ ions that can be easily exchanged with other ions or protons when immersed in an aqueous solution. Despite the vast interest in the atomic-scale hydration processes of these K$^+$ ions, experimental data under controlled conditions have remained elusive. Here, atomically resolved non-contact atomic force microscopy (nc-AFM) is combined with X-ray photoelectron spectroscopy (XPS) to investigate the cation hydration upon dosing water vapor at 100 K in ultra-high vacuum (UHV). The cleaved surface is further exposed to ultra-clean liquid water at room temperature, which promotes ion mobility and partial ion-to-proton substitution. The results offer the first direct experimental views of the interaction of water with muscovite mica in UHV. The findings are in line with previous theoretical predictions.

cond-mat.mtrl-sci↗

arXiv2023

Resolving the intrinsic short-range ordering of K$^+$ ions on cleaved muscovite mica

Muscovite mica, KAl$_2$(Si$_3$Al)O$_{10}$(OH)$_2$, is a common layered phyllosilicate with perfect cleavage planes. The atomically flat surfaces obtained through cleaving lend themselves to scanning probe techniques with atomic resolution and are ideal to model minerals and clays. Despite the importance of the cleaved mica surfaces, several questions remain unresolved. It is established that K$^+$ ions decorate the cleaved surface, but their intrinsic ordering -- unaffected by the interaction with the environment -- is not known. This work presents clear images of the K$^+$ distribution of cleaved mica obtained with low-temperature non-contact atomic force microscopy (AFM) under ultra-high vacuum (UHV) conditions. The data unveil the presence of short-range ordering, contrasting previous assumptions of random or fully ordered distributions. Density functional theory (DFT) calculations and Monte Carlo simulations show that the substitutional subsurface Al$^{3+}$ ions have an important role for the surface K$^+$ ion arrangement.

cond-mat.mtrl-sci↗

arXiv2023

Oxygen-Terminated (1x1) Reconstruction of Reduced Magnetite Fe$_3$O$_4$(111)

The (111) facet of magnetite (Fe$_3$O$_4$) has been studied extensively by experimental and theoretical methods, but controversy remains regarding the structure of its low-energy surface terminations. Using density functional theory (DFT) computations, we demonstrate three reconstructions that are more favorable than the accepted Fe$_{\rm oct2}$ termination in reducing conditions. All three structures change the coordination of iron in the kagome Fe$_{\rm oct1}$ layer to tetrahedral. With atomically-resolved microscopy techniques, we show that the termination that coexists with the Fe$_{\rm tet1}$ termination consists of tetrahedral iron capped by three-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.

cond-mat.mtrl-sci↗

arXiv2022

Rapid oxygen exchange between hematite and water vapor

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline r-cut (1-102) surface of hematite ({\alpha}-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 {\deg}C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.

cond-mat.mtrl-sci↗

arXiv2019

Self-limited Growth of an Oxyhydroxide Phase at the Fe3O4(001) Surface in Liquid and Ambient Pressure Water

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g. dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the UHV-compatible dosing of liquids to explore the stability of the Fe3O4(001)-c(2x2) surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) data show that extensive hydroxylation causes the surface to revert to a bulk-like (1x1) termination. However, scanning tunnelling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers, and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM (EC-STM) studies lead us to believe a similar structure exists at the solid-electrolyte interface during immersion at pH 7.

physics.chem-ph↗

arXiv2018

Apparatus for dosing liquid water in ultrahigh vacuum

The structure of the solid-liquid interface often defines function and performance of materials in applications. To study the interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows to bring a sample in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice, whose vapor pressure is reduced to UHV range by cooling it to cryogenic temperatures. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. First experiments carried out on rutile TiO2(110) single crystals using this apparatus exhibit unprecedented purity, as tested by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). These results enabled to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed.

cond-mat.mtrl-sci↗