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

@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}

}

@article{Gericke_2023a,

title = {Effect of Different In_{2}O_{3}(111) Surface Terminations on CO_{2} Adsorption},

author = {Sabrina M. Gericke and Minttu M. Kauppinen and Margareta Wagner and Michele Riva and Giada Franceschi and Alvaro Posada-Borbón and Lisa Rämisch and Sebastian Pfaff and Erik Rheinfrank and Alexander M. Imre and Alexei B. Preobrajenski and Stephan Appelfeller and Sara Blomberg and Lindsay R. Merte and Johan Zetterberg and Ulrike Diebold and Henrik Grönbeck and Edvin Lundgren},

url = {https://doi.org/10.1021/acsami.3c07166},

year  = {2023},

date = {2023-09-13},

journal = {ACS Applied Materials & Interfaces},

volume = {15},

issue = {38},

pages = {45367–45377},

abstract = {In_{2}O_{3}-based catalysts have shown high activity and selectivity for CO_{2} hydrogenation to methanol; however, the origin of the high performance of In_{2}O_{3} is still unclear. To elucidate the initial steps of CO_{2} hydrogenation over In_{2}O_{3}, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO_{2} on the In_{2}O_{3}(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO_{2} adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO_{2} is hindered by hydroxyl groups on the hydroxylated surface.},

keywords = {P02},

pubstate = {published},

tppubtype = {article}

}

@article{Puntscher2023,

title = {A Multitechnique Study of C_{2}H_{4} Adsorption on Fe_{3}O_{4}(001)},

author = {Lena Puntscher and Panukorn Sombut and Chunlei Wang and Manuel Ulreich and Jiri Pavelec and Ali Rafsanjani-Abbasi and Matthias Meier and Adam Lagin and Martin Setvin and Ulrike Diebold and Cesare Franchini and Michael Schmid and Gareth S. Parkinson},

doi = {10.1021/acs.jpcc.3c03684},

year  = {2023},

date = {2023-09-11},

urldate = {2023-09-11},

journal = {Journal of Physical Chemistry C},

volume = {127},

issue = {37},

pages = {18378--18388},

publisher = {American Chemical Society (ACS)},

abstract = {The adsorption/desorption of ethene (C_{2}H_{4}), also commonly known as ethylene, on Fe_{3}O_{4}(001) was studied under ultrahigh vacuum conditions using temperature-programmed desorption (TPD), scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT)-based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C_{2}H_{4} adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C_{2}H_{4} binds atop surface Fe sites up to a coverage of 2 molecules per (√2 × √2)R45° unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1–0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C_{2}H_{4} adsorption is found to be close to 4 molecules per (√2 × √2)R45° unit cell.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}

@article{Redondo2023,

title = {Hematite α-Fe_{2}O_{3}(0001) in Top and Side View: Resolving Long-Standing Controversies about Its Surface Structure},

author = {Jesús Redondo and Jan Michalička and Florian Kraushofer and Giada Franceschi and Břetislav Šmid and Nishant Kumar and Ondřej Man and Matthias Blatnik and Dominik Wrana and Benjamin Mallada and Martin Švec and Gareth S. Parkinson and Martin Setvin and Michele Riva and Ulrike Diebold and Jan Čechal},

doi = {10.1002/admi.202300602},

year  = {2023},

date = {2023-08-18},

urldate = {2023-08-18},

journal = {Advanced Materials Interfaces},

number = {2300602},

publisher = {Wiley},

abstract = {Hematite is a common iron oxide found in nature, and the α-Fe_{2}O_{3}(0001) plane is prevalent on the nanomaterial utilized in photo- and electrocatalytic applications. The atomic-scale structure of the surface remains controversial despite decades of study, partly because it depends on sample history as well as the preparation conditions. Here, a comprehensive study is performed using an arsenal of surface techniques (non-contact atomic force microscopy, scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoemission spectroscopy) complemented by analyses of the near surface region by high-resolution transmission electron microscopy and electron energy loss spectroscopy. The results show that the so-called “bi-phase” termination forms even under highly oxidizing conditions; a (1 × 1) surface is only observed in the presence of impurities. Furthermore, it is shown that the biphase is actually a continuous layer distorted due to a mismatch with the subsurface layers, and thus not the proposed mixture of FeO(111) and α-Fe_{2}O_{3}(0001) phases. Overall, the results show how combining surface and cross-sectional imaging provides a full view that can be essential for understanding the role of the near-surface region on oxide surface properties.},

keywords = {P02, P04},

pubstate = {published},

tppubtype = {article}

}

@article{Kraushofer2023,

title = {Oxygen-Terminated (1 × 1) Reconstruction of Reduced Magnetite Fe_{3}O_{4}(111)},

author = {Florian Kraushofer and Matthias Meier and Zdeněk Jakub and Johanna Hütner and Jan Balajka and Jan Hulva and Michael Schmid and Cesare Franchini and Ulrike Diebold and Gareth S. Parkinson},

doi = {10.1021/acs.jpclett.3c00281},

year  = {2023},

date = {2023-03-28},

urldate = {2023-03-28},

journal = {The Journal of Physical Chemistry Letters},

volume = {14},

number = {13},

pages = {3258--3265},

publisher = {American Chemical Society (ACS)},

abstract = {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 Feoct2 termination under reducing conditions. All three structures change the coordination of iron in the kagome Feoct1 layer to be tetrahedral. With atomically resolved microscopy techniques, we show that the termination that coexists with the Fetet1 termination consists of tetrahedral iron capped by 3-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.},

keywords = {P02, P04, P07},

pubstate = {published},

tppubtype = {article}

}