Surface structure and reactivity of
multi-component oxides at the atomic scale

@article{Ryan_2024a,

title = {Quantitative Measurement of Cooperative Binding in Partially Dissociated Water Dimers at the Hematite “R-Cut” Surface},

author = {Paul T. P. Ryan and Panukorn Sombut and Ali Rafsanjani-Abbasi and Chunlei Wang and Fulden Eratam and Francesco Goto and Ulrike Diebold and Matthias Meier and David A. Duncan and Gareth S. Parkinson},

url = {https://arxiv.org/abs/2406.18264

https://pubs.acs.org/doi/10.1021/acs.jpcc.4c04537},

year  = {2024},

date = {2024-09-30},

urldate = {2024-09-30},

journal = {The Journal of Physical Chemistry C},

volume = {128},

issue = {40},

pages = {16977–16985},

abstract = {Water–solid interfaces pervade the natural environment and modern technology. On some surfaces, water–water interactions induce the formation of partially dissociated interfacial layers; understanding why is important to model processes in catalysis or mineralogy. The complexity of the partially dissociated structures often makes it difficult to probe them quantitatively. Here, we utilize normal incidence X-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H_{2}O–OH) at the α-Fe_{2}O_{3}(012) surface (also called the (11̅02) or “R-cut” surface): a system simple enough to be tractable yet complex enough to capture the essential physics. We find the H_{2}O and terminal OH groups to be the same height above the surface within experimental error (1.45 ± 0.04 and 1.47 ± 0.02 Å, respectively), in line with DFT-based calculations that predict comparable Fe–O bond lengths for both water and OH species. This result is understood in the context of cooperative binding, where the formation of the H-bond between adsorbed H_{2}O and OH induces the H_{2}O to bind more strongly and the OH to bind more weakly compared to when these species are isolated on the surface. The surface OH formed by the liberated proton is found to be in plane with a bulk truncated (012) surface (−0.01 ± 0.02 Å). DFT calculations based on various functionals correctly model the cooperative effect but overestimate the water–surface interaction.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Rafsanjani_2024a,

title = {Digging Its Own Site: Linear Coordination Stabilizes a Pt_{1}/Fe_{2}O_{3} Single-Atom Catalyst},

author = {Ali Rafsanjani-Abbasi and Florian Buchner and Faith J. Lewis and Lena Puntscher and Florian Kraushofer and Panukorn Sombut and Moritz Eder and Jiří Pavelec and Erik Rheinfrank and Giada Franceschi and Viktor Birschitzky and Michele Riva and Cesare Franchini and Michael Schmid and Ulrike Diebold and Matthias Meier and Georg K. H. Madsen and Gareth S. Parkinson},

url = {https://doi.org/10.1021/acsnano.4c08781},

year  = {2024},

date = {2024-09-18},

urldate = {2024-09-18},

journal = {ACS Nano},

volume = {18},

issue = {39},

pages = {26920–26927},

abstract = {Determining the local coordination of the active site is a prerequisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems and much emphasis is placed on density functional theory computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (11̅02) facet of α-Fe_{2}O_{3}; a common support material in SACs. Using a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudolinear coordination to surface oxygen atoms. Despite breaking three surface Fe–O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O–Pt–O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase. Moreover, we conclude that extensive structural searches are necessary to determine realistic active site geometries in single-atom catalysis.},

keywords = {P02, P04, P07, P09},

pubstate = {published},

tppubtype = {article}

}

@article{Wanzenboeck2024b,

title = {Exploring Inhomogeneous Surfaces: Ti-rich SrTiO_{3}(110) Reconstructions via Active Learning},

author = {Ralf Wanzenböck and Esther Heid and Michele Riva and Giada Franceschi and Alexander M. Imre and Jesús Carrete and Ulrike Diebold and Georg K. H. Madsen},

url = {https://doi.org/10.1039/D4DD00231H},

year  = {2024},

date = {2024-09-16},

urldate = {2024-09-16},

journal = {Digital Discovery},

volume = {3},

pages = {2137-2145},

abstract = {The investigation of inhomogeneous surfaces, where various local structures co-exist, is crucial for understanding interfaces of technological interest, yet it presents significant challenges. Here, we study the atomic configurations of the (2×m) Ti-rich surfaces at (110)-oriented SrTiO_{3} by bringing together scanning tunneling microscopy and transferable neural-network force fields combined with evolutionary exploration. We leverage an active learning methodology to iteratively extend the training data as needed for different configurations. Training on only small well-known reconstructions, we are able to extrapolate to the complicated and diverse overlayers encountered in different regions of the heterogeneous SrTiO_{3}(110)-(2×m) surface. Our machine-learning-backed approach generates several new candidate structures, in good agreement with experiment and verified using density functional theory. The approach could be extended to other complex metal oxides featuring large coexisting surface reconstructions.},

keywords = {P02, P09},

pubstate = {published},

tppubtype = {article}

}

@article{Huetner2024,

title = {Stoichiometric reconstruction of the Al_{2}O_{3}(0001) surface},

author = {Johanna I. Hütner and Andrea Conti and David Kugler and Florian Mittendorfer and Georg Kresse and Michael Schmid and Ulrike Diebold and Jan Balajka},

url = {https://www.science.org/doi/10.1126/science.adq4744

https://arxiv.org/abs/2405.19263},

issn = {1095-9203},

year  = {2024},

date = {2024-09-12},

urldate = {2024-09-12},

journal = {Science},

volume = {385},

pages = {1241--1244},

publisher = {American Association for the Advancement of Science (AAAS)},

abstract = {Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of α–aluminum oxide (α-Al_{2}O_{3}) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex (√31 × √31)R±9° reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. 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}.},

keywords = {P02, P03},

pubstate = {published},

tppubtype = {article}

}

@article{Wang_2024b,

title = {A Multitechnique Study of C_{2}H_{4} Adsorption on a Model Single-Atom Rh_{1} Catalyst},

author = {Chunlei Wang and Panukorn Sombut and Lena Puntscher and Manuel Ulreich and Jiri Pavelec and David Rath and Jan Balajka and Matthias Meier and Michael Schmid and Ulrike Diebold and Cesare Franchini and Gareth S. Parkinson},

url = {https://doi.org/10.1021/acs.jpcc.4c03588},

year  = {2024},

date = {2024-09-05},

journal = {The Journal of Physical Chemistry C},

volume = {128},

issue = {37},

pages = {15404–15411},

abstract = {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 C_{2}H_{4} with a model Rh/Fe_{3}O_{4}(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. C_{2}H_{4} adsorption is strongest at the 2-fold coordinated Rh_{1} 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. The adsorption energy at the 5-fold coordinated Rh sites is predicated to be −1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C_{2}H_{4} desorption shoulder in TPD above room temperature.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Rath2024,

title = {Infrared Reflection Absorption Spectroscopy Setup with Incidence Angle Selection for Surfaces of Non-Metals},

author = {David Rath and Vojtěch Mikerásek and Chunlei Wang and Moritz Eder and Michael Schmid and Ulrike Diebold and Gareth S. Parkinson and Jiří Pavelec},

url = {https://arxiv.org/abs/2403.19263},

doi = {https://doi.org/10.1063/5.0210860},

year  = {2024},

date = {2024-06-07},

urldate = {2024-06-07},

journal = {Review of Scientific Instruments},

volume = {95},

issue = {6},

pages = {065106},

publisher = {arXiv},

abstract = {Infrared Reflection Absorption Spectroscopy (IRAS) on dielectric single crystals is challenging because the optimal incidence angles for light-adsorbate interaction coincide with regions of low IR reflectivity. Here, we introduce an optimized IRAS setup that maximizes the signal-to-noise ratio for non-metals. This is achieved by maximizing light throughput, and by selecting optimal incidence angles that directly impact the peak heights in the spectra. The setup uses a commercial FTIR spectrometer and is usable in ultra-high vacuum (UHV). Specifically, the design features sample illumination and collection mirrors with a high numerical aperture inside the UHV system, and an adjustable aperture to select the incidence angle range on the sample. This is important for p-polarized measurements on dielectrics, because the peaks in the spectra reverse direction at the Brewster angle (band inversion). The system components are connected precisely via a single flange, ensuring long-term stability. We studied the signal-to-noise (SNR) variation in p-polarized IRAS spectra for one monolayer of CO on TiO_{2}(110) as a function of incidence angle range, where a maximum signal-to-noise ratio of 70 was achieved at 4 cm^{-1} resolution in five minutes measurement time. The capabilities for s-polarization are demonstrated by measuring one monolayer D_{2}O adsorbed on a TiO_{2}(110) surface, where a SNR of 65 was achieved at a delta_R/R0 peak height of 1.4x10-4 in twenty minutes.},

keywords = {P02, P04},

pubstate = {published},

tppubtype = {article}

}

@article{sokolovic2024duality,

title = {Duality and degeneracy lifting in two-dimensional electron liquids on SrTiO_{3}(001)},

author = {Igor Sokolović and Eduardo B. Guedes and Thomas P. van Waas and Samuel Poncé and Craig Polley and Michael Schmid and Milan Radović and Martin Setvín and J. Hugo Dil},

url = {https://arxiv.org/abs/2405.18946},

year  = {2024},

date = {2024-05-29},

urldate = {2024-05-29},

journal = {arXiv},

abstract = {Two-dimensional electron liquids (2DELs) have increasing technological relevance for ultrafast electronics and spintronics, yet significant gaps in their fundamental understanding are exemplified on the prototypical SrTiO_{3}. We correlate the exact SrTiO_{3}(001) surface structure with distinct 2DELs through combined microscopic angle-resolved photoemission spectroscopy and non-contact atomic force microscopy on truly bulk-terminated surfaces that alleviate structural uncertainties inherent to this long-studied system. The SrO termination is shown to develop a 2DEL following the creation of oxygen vacancies, unlike the intrinsically metallic TiO_{2} termination. Differences in degeneracy of the 2DELs, that share the same band filling and identical band bending, are assigned to polar distortions of the Ti atoms in combination with spin order, supported with the extraction of fundamental electron-phonon coupling strength. These results not only resolve the ambiguities regarding 2DELs on},

keywords = {P02},

pubstate = {published},

tppubtype = {article}

}

@article{Birschitzky_2024a,

title = {Machine learning-based prediction of polaron-vacancy patterns on the TiO_{2}(110) surface},

author = {Viktor Birschitzky and Igor Sokolovic and Michael Prezzi and Krisztian Palotas and Martin Setvin and Ulrike Diebold and Michele Reticcioli and Cesare Franchini},

url = {https://www.nature.com/articles/s41524-024-01289-4},

doi = {https://doi.org/10.1038/s41524-024-01289-4},

year  = {2024},

date = {2024-05-06},

urldate = {2024-05-06},

journal = {npj Computational Materials},

volume = {10},

number = {89},

abstract = {The multifaceted physics of oxides is shaped by their composition and the presence of defects, which are often accompanied by the formation of polarons. The simultaneous presence of polarons and defects, and their complex interactions, pose challenges for first-principles simulations and experimental techniques. In this study, we leverage machine learning and a first-principles database to analyze the distribution of surface oxygen vacancies (V_{O}) and induced small polarons on rutile TiO_{2}(110), effectively disentangling the interactions between polarons and defects. By combining neural-network supervised learning and simulated annealing, we elucidate the inhomogeneous VO distribution observed in scanning probe microscopy (SPM). Our approach allows us to understand and predict defective surface patterns at enhanced length scales, identifying the specific role of individual types of defects. Specifically, surface-polaron-stabilizing V_{O}-configurations are identified, which could have consequences for surface reactivity.},

keywords = {P02, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2024,

title = {CO‐Induced Dimer Decay Responsible for Gem‐Dicarbonyl Formation on a Model Single‐Atom Catalyst},

author = {Chunlei Wang and Panukorn Sombut and Lena Puntscher and Zdenek Jakub and Matthias Meier and Jiri Pavelec and Roland Bliem and Michael Schmid and Ulrike Diebold and Cesare Franchini and Gareth S. Parkinson},

doi = {10.1002/anie.202317347},

issn = {1521-3773},

year  = {2024},

date = {2024-01-31},

journal = {Angewandte Chemie - International Edition},

number = {e202317347},

publisher = {Wiley},

abstract = {The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh_{1} adatoms and minority Rh_{2} dimers on Fe_{3}O_{4}(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh_{1} sites results exclusively in stable Rh_{1}CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh_{1}(CO)_{2} gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh_{2} dimers via an unstable Rh_{2}(CO)_{3} intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Gamba2023,

title = {Formation and stability of Fe-rich terminations of the Fe_{3}O_{4}(001) surface},

author = {Oscar Gamba and Moritz Eder and Matthias Poglitsch and Jiri Pavelec and Panukorn Sombut and Matthias Meier and Ulrike Diebold and Michael Schmid and Gareth S. Parkinson},

doi = {10.1088/2053-1591/ad0ac5},

year  = {2023},

date = {2023-11-22},

urldate = {2023-11-22},

journal = {Materials Research Express},

volume = {10},

number = {116517},

issue = {44},

publisher = {IOP Publishing},

abstract = {Understanding how the structure of iron oxide surfaces varies with their environment is essential for rationalizing their role in (geo-)chemistry and optimizing their application in modern technologies. In this paper, we create Fe-rich terminations of Fe_{3}O_{4}(001) by depositing iron directly onto the 'subsurface cation vacancy'-reconstructed surface, which is the most stable surface under ultrahigh vacuum conditions. Scanning tunneling microscopy and x-ray photoelectron spectroscopy data reveal that the excess iron is initially accommodated as two-fold coordinated adatoms and later incorporates into the subsurface cation vacancies. As the coverage increases, small patches of the octahedral pair termination (also known as the 'Fe dimer' termination) nucleate, eventually covering the entire surface after the deposition of 2 iron atoms per (√2×√2)R45° unit cell. This conclusion effectively rules out some existing models for the termination and provides support for the model proposed by Rustad \textit{et al.} (Surface Science 432, L583-L588, 1999), highlighting the need for further theoretical work to complete the Fe_{3}O_{4}(001) surface phase diagram. The octahedral pair termination is found to be unstable above 523 K and upon exposure to molecular O2 because the excess iron atoms agglomerate to form small FeO_{x} clusters.},

keywords = {P02, P04},

pubstate = {published},

tppubtype = {article}

}