SrO Termination Research Articles

Surface Geometry, Electronic and Thermodynamic Properties of SrCeO3 (001): First-Principles Calculation

This study uses first-principles density functional theory (DFT) to calculate the SrCeO3 (001) surface with CeO and SrO2 terminals. The influence of vacuum layer thickness on the geometric structure and energy stability is studied. The calculation by Materials Studio software shows that the surface with SrO as the terminal has obvious surface wrinkles compared with the surface with CeO2 as the terminal. The surface stability evaluation shows that the SrO termination surface is more stable in energy than CeO2. The band gaps of CeO2 and SrO terminals were determined to be 0.971eV and 1.733eV, respectively, which were smaller than those of SrCeO3 protocells.

Correlating surface stoichiometry and termination in SrTiO3 films grown by hybrid molecular beam epitaxy

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO3 (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy, and low-energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich–Schwoebel barrier consistent with A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO2 termination or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain the chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique.

Schottky barrier formation at theFe/SrTiO3(001) interface: Influence of oxygen vacancies and layer oxidation

Schottky barrier formation at metal/insulating oxide interfaces relies on complex mechanisms which are difficult to unravel. We propose a detailed numerical study of the atomic, magnetic, and electronic properties of the $\mathrm{Fe}/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ interface, in which we focused our discussion on different parameters which can affect the Schottky barrier height (SBH). The interface termination appears to be the most critical aspect to be controlled for this interface: While an ideal $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated interface would guarantee a $n$-type barrier of about 1.2--1.6 eV, the presence of a SrO termination can drastically decrease its value down to few meV. The oxidation state of the interface is also an important criterium to maintain a high barrier value. Oxygen vacancies are always cited as the source of a deterioration of the SBH. For a $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated interface, we found that in their most stable position, i.e., in the interface layer, the oxygen vacancies do not affect the value of the SBH; when moving some atomic layers away from the interface, the SBH on the contrary decreases regularly. We propose that oxidizing the interface would allow us to improve the $n$-type SBH by healing the oxygen vacancies and forming an interfacial FeO layer, which seems favorable to the formation of a higher SBH.

Model-dependent polar catastrophe scenario in ABO3/SrTiO3 (001) (A = La, Y, Sc, B = Al, Ga) systems

We have theoretically studied the structural and physical properties of ABO3 (A = La, Y, Sc, B = Al, Ga) films on SrTiO3 (001) crystals with SrO and TiO terminations under the assumption of epitaxially strained growth. To loosen the epitaxial condition, we have studied periodic √2×√2 in-plane superstructures to accommodate strain by tilting of octahedra as well as distortion. We also have studied spin-independent and spin-dependent calculations to monitor the possible magnetic ground states. The data show that spin-polarized Ti ions promote metal–insulator transition toward thinner ABO3/SrTiO3 compared to the nonmagnetic cases. Besides, octahedral distortions in the grown films favor a−a−c− orientations to accommodate lattice mismatch, reducing the ferroelectric polar nature by antisymmetric octahedral ionic coordinates, as well as overall monoclinic-like distortion along the growth direction to accommodate strains as A atoms get smaller.

Influence of crystalline order and defects on the absolute work functions and electron affinities of TiO2 - and SrO-terminated n−SrTiO3(001)

We explore how the structure and composition of the terminal layer of $n\text{\ensuremath{-}}\mathrm{SrTi}{\mathrm{O}}_{3}(001)$ determine key surface electronic properties. We have measured and calculated from first principles the absolute work functions and electron affinities of bulk $\mathrm{SrN}{\mathrm{b}}_{0.01}\mathrm{T}{\mathrm{i}}_{0.99}{\mathrm{O}}_{3}(001)$ terminated along the $\mathrm{Ti}{\mathrm{O}}_{2}$ and SrO planes. The match between theory and experiment is quite satisfactory for the $\mathrm{Ti}{\mathrm{O}}_{2}$ termination if an ideal, bulk-truncated surface structure is assumed. In contrast, the ideal SrO termination leads to a calculated work function considerably lower than the experimental value. We show that this discrepancy can be associated with defects on the SrO surface that act as electron scavengers. These defects deplete the concentration of itinerant electrons in the subsurface region and increase the negative charge density on the surface, thus increasing the work function. Several different surface defect configurations were modeled; the ones that yield the best agreement with experiment involve Sr vacancies in the terminal layer along with ${\mathrm{O}}^{2\ensuremath{-}}$, $\mathrm{O}{\mathrm{H}}^{\ensuremath{-}}/{\mathrm{H}}^{\ensuremath{-}}$ pairs or ${{\mathrm{O}}_{2}}^{2\ensuremath{-}}$ occupying anion sites adjacent to the Sr vacancies.

Dichotomy of the photo-induced 2-dimensional electron gas on SrTiO3 surface terminations

Oxide materials are important candidates for the next generation of electronics due to a wide array of desired properties, which they can exhibit alone or when combined with other materials. While SrTiO3 (STO) is often considered a prototypical oxide, it, too, hosts a wide array of unusual properties, including a 2-dimensional electron gas (2DEG), which can form at the surface when exposed to ultraviolet (UV) light. Using layer-by-layer growth of high-quality STO films, we show that the 2DEG only forms with the SrO termination and not with the TiO2 termination, contrary to expectation. This dichotomy of the observed angle-resolved photoemission spectroscopy (ARPES) spectra is similarly seen in BaTiO3 (BTO), in which the 2DEG is only observed for BaO-terminated films. These results will allow for a deeper understanding and better control of the electronic structure of titanate films, substrates, and heterostructures.

Effect of SrO termination on electron transport of MoS2/SrTiO3 heterostructure: A DFT approach

In the present density functional theory based study, we have investigated the interfacial electronic properties and adsorption curve along with chemical bonding with interface of MoS2/SrO. Our calculations indicates that the electronic properties are highly interface dependent as the heterostructure (HS) shows metallic behavior in MoS2/SrO interface. The equilibrium interlayer distance deq between MoS2 and SrTiO3 (STO) is found within 2.62 to 2.67 . From our adsorption curve analysis, we have found that MoS2 is chemisorbed over STO substrate. Chemical bonds at the interface shows ionic bonding in MoS2/SrO interface. A unique behavior of cubic confined MoS2 monolayer is also observed in our investigation i.e. an indirect band gap of ∼1.63 eV is found. This behavior may lead to fabricate and investigate Si in the form of hexagonal honeycomb like monolayer which could present a direct band gap. Conclusively we can use the above investigated HS in metallic applications.

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Perovskite oxides attract increasing attention due to their broad potential in many applications. Understanding their surfaces is challenging, though, because the ternary composition of the bulk allows for multiple stable surface terminations. We demonstrate a simple procedure for preparing the bulk-terminated (001) surface of SrTiO3, a prototypical cubic perovskite. Controlled application of strain on a SrTiO3 single crystal results in a flat cleavage with micrometer-size domains of SrO and TiO2. Distribution of these two terminations is dictated by ferroelectric domains induced by strain during the cleavage process. Atomically-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) measurements reveal the presence of point defects in a well-defined concentration of (14+-2)%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the TiO2 termination. These intrinsic defects are induced by the interplay between ferroelectricity, surface polarity, and surface charge.

Water oxidation chemistry of oxynitrides and oxides: Comparing NaTaO3 and SrTaO2N

The oxygen evolution reaction (OER) plays an important role in evaluating a photocatalyst and to understand its surface chemistry. In this work we present a comparative study of the OER on the oxide NaTaO3 (113) surface and the oxynitride SrTaO2N (001) surface. Oxynitrides are highly promising photocatalysts due to their smaller band gap and resulting better visible light absorption compared to oxides but our knowledge about their surface structure and chemistry is still very limited. With the goal to compare the surface chemistry of oxides and oxynitrides, we perform density functional theory calculations to obtain the free energy changes associated with the OER reaction steps. For the OER at the Ta site of the clean surfaces, our results predict the overpotential-determining step (ODS) for both materials to be the formation of the *OOH intermediate, with a larger overpotential for the oxide than the oxynitride (1.30 V vs 1.01 V). The Na site is found to be more active than the Ta site on the oxide surface with an OER overpotential of 0.88 V, whereas the OER at the Sr site on the oxynitride has an overpotential of 1.14 V. For the A sites, contrary to the Ta site, the deprotonation of *OH was found to be the ODS. Computed Pourbaix diagrams show that at relevant (photo)electrochemical conditions all surfaces are covered with oxygen adsorbates. Oxygen adsorbates at A (Na, Sr) sites are however found to couple and desorb as O2, leaving these sites empty under typical operating conditions. Following this desorption, we find the OER to proceed by the conventional *OOH mechanism on the SrO termination of the oxynitride but by a direct coupling of neighbouring *O at Na sites on the oxide surface. This coupling mechanism on the oxide has the smallest overpotential of 0.79 V compared to 0.88 V for the oxynitride, implying that the oxide is a better OER catalyst. Since it however absorbs light only in the UV part of the solar spectrum this leads to a tradeoff between light absorption and the catalytic activity.

Adsorption of Water onto SrTiO3 from Periodic Møller-Plesset Second-Order Perturbation Theory.

Adsorption of water onto metal oxide surfaces is a long-standing problem motivated by relevance to many promising technological applications. In this work, we compute the adsorption energy of water on SrTiO3 using periodic Møller-Plesset second-order perturbation theory (MP2). We compare our MP2 results to density functional and hybrid density functional theory calculations with and without the widely used D3 dispersion correction. The MP2 ground-state adsorption energy of water on SrTiO3 (001) at one monolayer coverage is 0.9 eV on the TiO2 termination in the molecular configuration and 0.6 eV in the dissociative configuration, the corresponding results on the SrO termination being 0.9 eV for both modes of adsorption. These results are reproduced well by the PBE and PBE0 exchange-correlation functionals. Correcting for dispersion effects through the D3 dispersion correction leads to significantly overestimated adsorption energies for both PBE and PBE0 with respect to MP2. The D3 correction also fails to capture the difference in electron correlation between the molecular and dissociative adsorption states, similarly to the optB86b van der Waals density functional.

(Invited) Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides

It is generally agreed that the oxygen reduction reaction on oxide surfaces is strongly affected by defects, and the same is true for the counter reaction of oxygen evolution from an oxide. Relevant defects include the “standard” point defects also present in the bulk, i.e. oxygen vacancies and electrons or holes, but also defects within the ideal termination of an oxide, e.g. cation segregates, and higher dimensional defects such as dislocations or grain boundaries. However, it is far from trivial to clearly identify the role of specific defects on the oxygen reduction or release reaction. Often, one neither knows exact defect concentrations nor can one vary those in a defined manner without affecting the entity of all defects. This also hampers development of theoretical models describing the current voltage characteristics of oxygen reduction or evolution, i.e. oxygen exchange, at gas/solid interfaces. In this contribution, two types of experiments are employed to shed further light on the role of defects in oxygen exchange of mixed conducting oxides such as Sr doped LaCoO3 (LSC) and LaFeO3 (LSF). i) The first approach makes use of the fact that both oxygen partial pressure p(O2) and overpotential affect the oxygen chemical potential and thus defect concentrations in oxides. However, the oxygen reduction or evolution kinetics may still be affected very differently by overpotential and p(O2), since only the latter also changes the concentration of reacting molecules. By properly combining oxygen partial pressure changes and overpotential, we could tune the chemical potentials of LSF and gas phase independently and thus we extracted “true” reaction orders of O2 and defects in the oxygen reduction and evolution reaction. Additional bias dependent three point impedance measurements revealed the chemical capacitance of LSF and this was used for calculating the absolute bulk defect concentration in dependence of the oxygen chemical potential. This analysis also revealed that defect interaction strongly affects the thermodynamic data of LSF and a new defect chemical model including non-dilute defects is suggested. ii) The second type of experiments deals with the effect of certain cations in the topmost layer of LSC. It is already known that Sr segregation to the surface of mixed conducting oxides may reduce their activity towards oxygen reduction. A kind of SrO termination layer seems to be unavoidable on LSC at higher temperatures but still additional degradation may take place and reasons are not fully understood. A novel type of experiment was thus developed, with impedance spectroscopic analysis of samples inside a PLD (pulsed laser deposition) chamber (in-situ impedance PLD). In this manner, the surface of an LSC electrode in an YSZ based electrochemical cell could be decorated by very tiny amounts of other oxides (SrO, Co3O4, La2O3) and the effect on oxygen exchange kinetics could be detected immediately by impedance measurements. Already small fractions of monolayers of Sr strongly modified the polarization resistance, thus indicating that only a small number of very active sites was present on LSC and those became deactivated by Sr. Co, on the other hand, can significantly activate the LSC electrode surface.

Antiferromagnetic and xy ferro-orbital order in insulating SrRuO3 thin films with SrO termination

By means of first-principles calculations we study the structural, magnetic and electronic properties of SrRuO3 surface for the SrO termination. We find that the RuO6 octahedra and the structure of the SrO layers at the surface are strongly modified as well as the Ru–O–Ru bond angles. We find in the thin films a dxy ferro-orbital order. The dxy orbital becomes the lowest in energy as in other quasitwodimensional ruthenates. Such structural rearrangement, together with a band reduction, leads to a modification of the magnetic properties. We compare the Jahn–Teller effect between the ferromagnetic and antiferromagnetic phases. We show that an insulating G-type antiferromagnetic phase takes place in SrRuO3 thin films, substituting the metallic phase experimentally found in every bulk Sr-ruthenates. The single layer SrRuO3 presents many similarities with the Ca2RuO4 low temperature phase, these similarities disappear with a larger number of layers. A study of the ground state of the as function of the number of layers is presented, the competition between bandwidth and Coulomb repulsion determines the ground state. We propose the disorder as responsible for the exchange bias effect observed.

The interface of SrTiO3 and H2O from density functional theory molecular dynamics.

We use dispersion-corrected density functional theory molecular dynamics simulations to predict the ionic, electronic and vibrational properties of the SrTiO3/H2O solid-liquid interface. Approximately 50% of surface oxygens on the planar SrO termination are hydroxylated at all studied levels of water coverage, the corresponding number being 15% for the planar TiO2 termination and 5% on the stepped TiO2-terminated surface. The lateral ordering of the hydration structure is largely controlled by covalent-like surface cation to H2O bonding and surface corrugation. We find a featureless electronic density of states in and around the band gap energy region at the solid-liquid interface. The vibrational spectrum indicates redshifting of the O-H stretching band due to surface-to-liquid hydrogen bonding and blueshifting due to high-frequency stretching vibrations of OH fragments within the liquid, as well as strong suppression of the OH stretching band on the stepped surface. We find highly varying rates of proton transfer above different SrTiO3 surfaces, owing to differences in hydrogen bond strength and the degree of dissociation of incident water. Trends in proton dynamics and the mode of H2O adsorption among studied surfaces can be explained by the differential ionicity of the Ti-O and Sr-O bonds in the SrTiO3 crystal.

Mechanisms of Performance Degradation of La0.6Sr0.4Co0.2Fe0.8O3 Solid Oxide Fuel Cell Cathodes

In recent years, mixed ionic-electronic conducting (MIEC) materials such as (La,Sr)(Fe,Co)O3-δ(LSCF) have been studied and developed as solid oxide fuel cell (SOFC) cathodes, due to their activity for the oxygen reduction reaction at intermediate temperatures (< 800°C) [1-4]. On the other hand, a number of such MIEC materials including LSCF exhibit Sr surface segregation, which has been proposed to hinder the oxygen exchange reaction at the surface [5-8]. In this work, we present the study of long-term degradation mechanisms of porous La0.6Sr0.4Co0.2Fe0.8O3 (LSCF6428) cathodes under thermal annealing. LSCF6428 symmetric electrode cells with Gd0.1Ce0.9O1.95 (GDC) electrolytes were maintained at an elevated SOFC operating temperature of 800°C for ~ 800 hours in ambient air, without current/polarization. As illustrated in Fig. 1, electrochemical impedance spectroscopy (EIS) measurements taken periodically at 700°C showed a polarization resistance increase of ~ 120%, from 0.15 to 0.33 Ω∙cm2. The electrode morphological changes and Sr surface segregation were examined using a combination of three-dimensional (3D) tomography via focused ion beam-scanning electron microscopy (FIB-SEM) and surface composition measurements using XPS and a chemical etching procedure with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection [9]. The 3D imaging showed that there was no coarsening or sintering of the LSCF6428 electrode microstructure that could affect electrochemical performance during annealing. However, the ICP-OES analysis found an increased amount of water-soluble Sr on the surface of annealed samples, from 1.3 nmol Sr/cm2 to 3.9 nmol Sr/cm2, when normalized to the LSCF6428 particle surface measured from 3D image data. Assuming that the measured Sr phase is SrO, the water-soluble Sr amount on freshly prepared cells would correspond to 1.04±0.22 atomic layers and agree well with reports on Sr-doped perovskite-type model thin films suggesting a SrO termination [9]. The Adler-Lane-Steel (ALS) model was then applied, again making use of 3D image data, to examine the effect of Sr surface segregation on the oxygen surface exchange process. Fig. 1. EIS results showing polarization resistance measured for a single LSCF6428 cathode during 800°C anneal. Temperature was temporarily reduced to 700°C during EIS measurements and back to 800°C afterwards.

Large Spatial Spin Polarization at Benzene/La2/3Sr1/3MnO3 Spinterface: Toward Organic Spintronic Devices

The fascinating spinterfaces between benzene and La2/3Sr1/3MnO3 (LSMO) are investigated based on the first-principles calculations. LSMO is a traditional high spin-polarized material used as the electrode in organic spintronic devices. However, the understanding of spin characteristics at organic/LSMO interfaces is unclear. Therefore, we study the benzene/LSMO spinterfaces to clarify the spin behaviors at organic/LSMO interfaces because benzene is the basic unit of organic molecules. It is found that SrO terminations have weak binding with benzene, showing the inert properties. More importantly, the positive spatial spin polarization expands into the benzene layer and vacuum layer in all of the models. To further make sure the credibility of the results, both the effect of the ordering of Sr atoms and the adsorption density of benzene are considered in our calculations. There is no spatial spin-polarization inversion above the benzene plane in all of the models. The results indicate that the organic layer...

Comparative ab initio calculations of SrTiO3/BaTiO3 and SrZrO3/PbZrO3 (0 0 1) heterostructures

Using a B3PW hybrid exchange–correlation functional within the density functional theory (DFT) we calculated from the first principles the electronic structure of BaTiO3/SrTiO3 and PbZrO3/SrZrO3 (001) interfaces. The optical band gap of both BaTiO3/SrTiO3 and PbZrO3/SrZrO3 (001) interfaces depends mostly from BaO or TiO2 and SrO or ZrO2 termination of the upper layer, respectively. Based on the results of our calculations we predict increase of the Ti–O and Zr–O chemical bond covalency near the SrTiO3/BaTiO3 and SrZrO3/PbZrO3 (001) interfaces as compared to the BaTiO3 and PbZrO3 bulk.

Intrinsic origin of two-dimensional electron gas at the (001) surface ofSrTiO3

It is generally assumed that two-dimensional electron gas (2DEG) recently observed at the (001) ${\text{SrTiO}}_{3}$ surface can be solely derived by oxygen vacancies introduced during ultrahigh vacuum annealing or through ultraviolet irradiation exposure. However, 2DEG entirely due to defect formation may be at odds with the characteristics of high mobility and easy field-effect manipulation required for applications; to that aim, an intrinsic formation mechanism should be preferred. Using advanced ab initio simulations we give evidence that 2DEG at the (001) ${\text{SrTiO}}_{3}$ surface may even result from purely intrinsic properties of the pristine surface, provided that the surface is SrO terminated. The key concept is that the SrO termination is electron-attractive as a consequence of both the surface-induced polarity and the specific electronic reconstruction, whereas the ${\text{TiO}}_{2}$ termination is electron-repulsive. It follows that in vacuum-cleaved samples where both terminations are present, 2DEG can result from the structurally ordered superposition of the two kinds of domain, even in the absence of any extrinsic source. On the other hand, in etching-prepared single-terminated ${\text{TiO}}_{2}$ samples 2DEG should be assumed as entirely derived by extrinsic factors.

Reversible modulation of electric transport properties by oxygen absorption and releasing on Nb:SrTiO3 surface

Pt Schottky contacts on (001)-orientated Nb-doped SrTiO3 (NSTO) in both ambient air and vacuum were investigated by the conductive atomic force microscope. The co-existed TiO2 and SrO termination layers were identified on the terrace-structured NSTO surface, where the former possessed a higher forward current than the latter. In ambient air, the barrier height of Pt/NSTO Schottky junction exhibited periodical variation with cyclic terrace plane and step sites, whereas it became homogeneous in ambient vacuum. We suggested that the oxygen absorption and releasing of surface dangling bonds were the origin for reversible changes in transport properties, which indicates a feasible approach for the surface modulation and band structure tailoring of NSTO based heterojunctions.

Understanding the interactions between oxygen vacancies atSrTiO3(001) surfaces

We examine the role of neutral divacancies on the electronic and atomic structure at ${\mathrm{SrTiO}}_{3}$ (001) surfaces using a density functional theory + $U$ approach. Our results show that the interactions between divacancies are significantly less repulsive at the SrO-terminated surface (0.05 eV) than at the ${\mathrm{TiO}}_{2}$-terminated one (0.38 eV), mainly due to the increased electrostatic screening at the ionic SrO layer compared to the covalently bonded ${\mathrm{TiO}}_{2}$ layer. The interaction energies are a nonmonotonic function of distance, with the fourth-nearest-neighbor oxygen-oxygen divacancy showing a significantly reduced repulsion at 0 K on the ${\mathrm{TiO}}_{2}$-terminated surface where the defects are in the equatorial oxygen plane. This enhanced reduction in the repulsive interaction is a consequence of the much larger reduction in local symmetry relative to other divacancy arrangements arising from strong coupling with in-plane octahedral distortions. On the SrO-terminated surface, due to increased electrostatic screening, the interaction energy begins to decrease beyond the third-nearest neighbor. On both surfaces, the reduced repulsion (0.05 and 0.38 eV) should permit oxygen vacancy ordering at finite temperatures. Finally, we discuss the emergence of a two-dimensional electron gas due to oxygen divacancies at both the ${\mathrm{TiO}}_{2}$- and SrO-terminated ${\mathrm{SrTiO}}_{3}$ (001) surfaces and contrast them with the case of a single oxygen vacancy. Neutral oxygen vacancies on the SrO termination lead to more electron localization than on the ${\mathrm{TiO}}_{2}$ surface. These results suggest an explanation for the local ordering observed in experiment, thereby highlighting the importance of ordering both for enhanced conductivity and carrier densities at oxide surfaces and at heterostructure interfaces.

Synthesis-Dependent Atomic Surface Structures of Oxide Nanoparticles

Using SrTiO3 nanocuboids as a model system, we show with aberration-corrected high resolution electron microscopy at sub-Å resolution that surface relaxations or reconstructions are present on the nanocuboids, depending on the synthetic process. Oleic acid synthesis, acetic acid synthesis, and microwave-assisted acetic acid synthesis result in a SrO termination, TiO2-rich reconstruction, and mixed termination, respectively. The experimental atomic positions are in better agreement with density functional theory calculations using an exact-exchange corrected PBEsol functional than the Perdew-Burke-Ernzerhof (PBE) functional.