Interplay between external strain and oxygen vacancies on a rutile TiO2(110) Surface

Direct investigation of the atomic structure and decreased magnetism of antiphase boundaries in garnet

The ferrimagnetic insulator iron garnets, tailored artificially with specific compositions, have been widely utilized in magneto-optical (MO) devices. The adjustment on synthesis always induces structural variation, which is underestimated due to the limited knowledge of the local structures. Here, by analyzing the structure and magnetic properties, two different antiphase boundaries (APBs) with individual interfacial structure are investigated in substituted iron garnet film. We reveal that magnetic signals decrease in the regions close to APBs, which implies degraded MO performance. In particular, the segregation of oxygen deficiencies across the APBs directly leads to reduced magnetic elements, further decreases the magnetic moment of Fe and results in a higher absorption coefficient close to the APBs. Furthermore, the formation of APBs can be eliminated by optimizing the growth rate, thus contributing to the enhanced MO performance. These analyses at the atomic scale provide important guidance for optimizing MO functional materials.

Iron garnets are widely used in magneto-optical devices, but knowledge of the effects of common defects on performance is limited. Here, using high-resolution microscopy and spectroscopy, the authors find that magnetism is weakened near these defects causing reduced performance, but can be avoided by tuning the growth rate.

The influence of the oxygen vacancies on the Pt/TiO2 single-atom catalyst-a DFT study

The titanium dioxide (TiO₂) surface is suitable as a substrate for single-atom catalysts (SACs) for oxygen reduction reaction (ORR). As a common defect on TiO₂, oxygen vacancies may have a significant impact on the adsorption and activity of the adatoms. This work aims to investigate whether titanium dioxide containing surface oxygen vacancies is more suitable as a base material for SACs. This paper calculates the changes in the adsorption energy of the Pt atom and the energy of the d-band center on the perfect surface and the surface containing oxygen vacancies. Concerning the perfect surface, the surface containing oxygen vacancies fixes the Pt atom more firmly and increases the center energy of the d-band of Pt, thereby improving the performance of the Pt atom as SACs. Consequently, the (110) surface of rutile TiO₂ with oxygen vacancies may be the best substrate for SACs.

Probing Permanent Dipole Moments and Removing Exciton Fine Structures in Single Perovskite Nanocrystals by an Electric Field

Single perovskite nanocrystals have emerged as a novel type of semiconductor nanostructure capable of emitting single photons with rich exciton species and fine energy-level structures. Here we focus on single excitons and biexcitons in single perovskite CsPbI_{3} nanocrystals to show, for the first time, how their optical properties are modulated by an external electric field at the cryogenic temperature. The electric field can cause a blueshift in the photoluminescence peak of single excitons, from which the existence of a permanent dipole moment can be deduced. Meanwhile, the fine energy-level structures of single excitons and biexcitons in a single CsPbI_{3} nanocrystal can be simultaneously eliminated, thus preparing a potent platform for the potential generation of polarization-entangled photon pairs.

The effect of strain on water dissociation on reduced rutile TiO2(110) surface

The effect of external uniaxial strain on water dissociation on a reduced rutile TiO₂(110) surface has been theoretically studied using first-principles calculations. We find that when the tensile strain along [11̄0] is applied, the energy barrier of water dissociation substantially decreases with the increase of strain. In particular, water almost automatically dissociates when the strain is larger than 3%. Besides, the water dissociation mechanism changes from indirect to direct dissociation when the compressive strain is larger than 1.3% along [11̄0] or 3% along [001]. The results strongly suggest that it is feasible to engineer the water dissociation on the reduced rutile TiO₂(110) surface using external strain.

The tensile strain along [11̄0] on the reduced TiO₂(110) surface can greatly promote the dissociation of water, the compressive strain along [001] and [11̄0] can change the dissociation mechanisms.

Superionic Conductivity in Ceria-Based Heterostructure Composites for Low-Temperature Solid Oxide Fuel Cells

Ceria-based heterostructure composite (CHC) has become a new stream to develop advanced low-temperature (300-600 °C) solid oxide fuel cells (LTSOFCs) with excellent power outputs at 1000 mW cm-2 level. The state-of-the-art ceria-carbonate or ceria-semiconductor heterostructure composites have made the CHC systems significantly contribute to both fundamental and applied science researches of LTSOFCs; however, a deep scientific understanding to achieve excellent fuel cell performance and high superionic conduction is still missing, which may hinder its wide application and commercialization. This review aims to establish a new fundamental strategy for superionic conduction of the CHC materials and relevant LTSOFCs. This involves energy band and built-in-field assisting superionic conduction, highlighting coupling effect among the ionic transfer, band structure and alignment impact. Furthermore, theories of ceria-carbonate, e.g., space charge and multi-ion conduction, as well as new scientific understanding are discussed and presented for functional CHC materials.

Engineering of self-rectifying filamentary resistive switching in LiNbO3 single crystalline thin film via strain doping

The abilities to fabricate wafer scale single crystalline oxide thin films on metallic substrates and to locally engineer their resistive switching characteristics not only contribute to the fundamental investigations of the resistive switching mechanism but also promote the practical applications of resistive switching devices. Here, wafer scale LiNbO₃ (LNO) single crystalline thin films are fabricated on Pt/SiO₂/LNO substrates by ion slicing with wafer bonding. The lattice strain of the LNO single crystalline thin films can be tuned by He implantation as indicated by XRD measurements. After He implantation, the LNO single crystalline thin films show self-rectifying filamentary resistive switching behaviors, which is interpreted by a model that the local conductive filaments only connect/disconnect with the bottom interface while the top interface maintains the Schottky contact. Thanks to the homogeneous distribution of defects in single crystalline thin films, highly reproducible and uniform self-rectifying resistive switching with large on/off ratio over four order of magnitude was achieved. Multilevel resistive switching can be obtained by varying the compliance current or by using different magnitude of writing voltage.

Interfacial Lattice-Strain-Driven Generation of Oxygen Vacancies in an Aerobic-Annealed TiO2 (B) Electrode

Oxygen vacancies play crucial roles in defining physical and chemical properties of materials to enhance the performances in electronics, solar cells, catalysis, sensors, and energy conversion and storage. Conventional approaches to incorporate oxygen defects mainly rely on reducing the oxygen partial pressure for the removal of product to change the equilibrium position. However, directly affecting reactants to shift the reaction toward generating oxygen vacancies is lacking and to fill this blank in synthetic methodology is very challenging. Here, a strategy is demonstrated to create oxygen vacancies through making the reaction energetically more favorable via applying interfacial strain on reactants by coating, using TiO₂ (B) as a model system. Geometrical phase analysis and density functional theory simulations verify that the formation energy of oxygen vacancies is largely decreased under external strain. Benefiting from these, the obtained oxygen-deficient TiO₂ (B) exhibits impressively high level of capacitive charge storage, e.g., ≈53% at 0.5 mV s^-1 , far surpassing the ≈31% of the unmodified counterpart. Meanwhile, the modified electrode shows significantly enhanced rate capability delivering a capacity of 112 mAh g^-1 at 20 C (≈6.7 A g^-1 ), ≈30% higher than air-annealed TiO₂ and comparable to vacuum-calcined TiO₂ . This work heralds a new paradigm of mechanical manipulation of materials through interfacial control for rational defect engineering.

Dynamic Tuning of a Thin Film Electrocatalyst by Tensile Strain

We report the ability to tune the catalytic activities for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by applying mechanical stress on a highly n-type doped rutile TiO₂ films. We demonstrate through operando electrochemical experiments that the low HER activity of TiO₂ can reversibly approach those of the state-of-the-art non-precious metal catalysts when the TiO₂ is under tensile strain. At 3% tensile strain, the HER overpotential required to generate a current density of 1 mA/cm² shifts anodically by 260 mV to give an onset potential of 125 mV, representing a drastic reduction in the kinetic overpotential. A similar albeit smaller cathodic shift in the OER overpotential is observed when tensile strain is applied to TiO₂. Results suggest that significant improvements in HER and OER activities with tensile strain are due to an increase in concentration of surface active sites and a decrease in kinetic and thermodynamics barriers along the reaction pathway(s). Our results highlight that strain applied to TiO₂ by precisely controlled and incrementally increasing (i.e. dynamic) tensile stress is an effective tool for dynamically tuning the electrocatalytic properties of HER and OER electrocatalysts relative to their activities under static conditions.

Intrinsic interaction between in-plane ferroelectric polarization and surface adsorption

The chemical properties of a ferroelectric surface are polarization dependent, the underlying nature of which is, however, far from being completely understood. One of the reasons is that when the polarization direction is perpendicular to the surface, the depolarization field may induce electronic or atomic reconstruction and thus change the chemistry of the ferroelectric surface in complicated ways. Instead, the in-plane polarization results in no depolarization field. Therefore, the chemical properties of a ferroelectric surface can be more intrinsically reflected by the interplay between the in-plane polarization and the surface adsorption. By using first-principles calculations, we study the effect of the strain-induced in-plane polarization on the adsorption of a series of molecules on the reduced rutile TiO2(110) surface. We reveal that it is the surface doping caused by the charge transfer between the adsorbates and the TiO2(110) surface that dominates the polarization-induced change of the adsorption energy, as a result of screening long-range Coulomb interactions. The electrostatic interaction between the polarization of the substrate and the polar molecule is of relatively less importance. We propose that charge transfer effects generally occur for ferroelectric surfaces with no localized surface states.

Controllable dissociation of H2O on a CeO2(111) surface

The lattice strain is an effective approach to tune the adsorption states of H₂O on metal oxide surfaces.

Influence of strain on water adsorption and dissociation on rutile TiO2(110) surface

The influence of externally applied strain on water adsorption and dissociation on a defect-free rutile TiO₂(110) surface is studied by using first-principles calculations.

The unexpectedly rich reconstructions of rutile TiO2(011)-(2 × 1) surface and the driving forces behind their formation: an ab initio evolutionary study

In this paper, we employ state-of-the-art theoretical approaches to elucidate the structures of the (011) surface of rutile (R-)TiO₂.

Hydrogen Impurity Defects in Rutile TiO2

Hydrogen-related defects play crucial roles in determining physical properties of their host oxides. In this work, we report our systematic experimental and theoretical (based on density functional theory) studies of the defect states formed in hydrogenated-rutile TiO₂ in gaseous H₂ and atomic H. In gas-hydrogenated TiO₂, the incorporated hydrogen tends to occupy the oxygen vacancy site and negatively charged. The incorporated hydrogen takes the interstitial position in atom-hydrogenated TiO₂, forming a weak O-H bond with the closest oxygen ion, and becomes positive. Both states of hydrogen affect the electronic structure of TiO₂ mainly through changes of Ti 3d and O 2p states instead of the direct contributions of hydrogen. The resulted electronic structures of the hydrogenated TiO₂ are manifested in modifications of the electrical and optical properties that will be useful for the design of new materials capable for green energy economy.

Controlling surface reactions with nanopatterned surface elastic strain

The application of elastic lattice strain is a promising approach for tuning material properties, but the attainment of a systematic approach for introducing a high level of strain in materials so as to study its effects has been a major challenge. Here we create an array of intense locally varying strain fields on a TiO2 (110) surface by introducing highly pressurized argon nanoclusters at 6-20 monolayers under the surface. By combining scanning tunneling microscopy imaging and the continuum mechanics model, we show that strain causes the surface bridge-bonded oxygen vacancies (BBOv), which are typically present on this surface, to be absent from the strained area and generates defect-free regions. In addition, we find that the adsorption energy of hydrogen binding to oxygen (BBO) is significantly altered by local lattice strain. In particular, the adsorption energy of hydrogen on BBO rows is reduced by ∼ 35 meV when the local crystal lattice is compressed by ∼ 1.3%. Our results provide direct evidence of the influence of strain on atomic-scale surface chemical properties, and such effects may help guide future research in catalysis materials design.

Tuning the area percentage of reactive surface of TiO2 by strain engineering

Surfaces with high reactivity usually have a low area percentage, which greatly limits the efficiency of surface reactivity. In this Letter we demonstrate a generic way of increasing the percentage of the highly reactive surface by using external strain. Bulk and surface elastic properties of TiO2 are studied via density functional theory calculations. The equilibrium shape of anatase TiO2 under applied strain is discussed based on the elastic properties. We find that when 5% compressive strain is applied biaxially along [100] and [010]; directions, the area percentage of the anatase (001) surface can be increased by ~5 times in comparison with the case when no strain is applied. Since the moderate strain does not introduce extrinsic defects into the material, we propose that it is an ideal way to increase the reactivity of titanium dioxide crystallites by applying biaxial compressive external strain along the a axis.

Stoichiometry engineering of monoclinic to rutile phase transition in suspended single crystalline vanadium dioxide nanobeams

Coexisting monoclinic M(1) (insulating) and rutile (metallic) domains were observed in free-standing vanadium dioxide nanobeams at room temperature. Similar domain structures have been attributed to interfacial strain, which was not present here. Annealing under reducing conditions indicated that a deficiency of oxygen stabilizes the rutile phase to temperatures as low as 103 K, which represents an unprecedented suppression of the phase transition by 238 K. In a complementary manner, oxygen-rich growth conditions stabilize the metastable monoclinic M(2) and triclinic T (or M(3)) phases. A pseudophase diagram with dimensions of temperature and stoichiometry is established that highlights the accessibility of new phases in the nanobeam geometry.

The effects of antimony doping on the surface structure of rutile TiO2(110)

Titanium dioxide represents a very important wide bandgap photocatalyst that is known to be sensitized to visible light by transition metal doping. Antimony doping has been demonstrated to provide photocatalytic activity when codoped with chromium at an optimum dopant ratio [Sb]/[Cr] of about 1.5. Here, the role of antimony doping on the surface structure of rutile TiO(2)(110) is studied using non-contact atomic force microscopy (NC-AFM) under ultra-high vacuum conditions. At first glance, the surface structure of antimony-doped TiO(2)(110) resembles the structure of pristine TiO(2)(110). However, in contrast to what is found in pristine TiO(2)(110), a dense layer of protruding features is observed upon antimony doping, which is tentatively ascribed to antimony-rich clusters. Moreover, homogeneously distributed holes are found on the surface, which differ in depth and shape depending on the preparation conditions. Holes with depths ranging from a few up to more than a hundred monatomic steps are observed. These holes are explained by surface segregation of antimony during annealing, as the ionic radius of Sb(3+) is considerably larger than the ionic radius of Ti(4+). Our finding provides an indication of why an antimony concentration larger than the optimum ratio results in decreased photocatalytic activity. Moreover, controlling annealing temperature seems to constitute a promising strategy for creating nanosized holes on TiO(2) surfaces.