SrTiO3 Surface Research Articles

Controllable and fast growth of high-quality atomically thin and atomically flat Bi2O2Se films

As a promising 2D material, bismuth oxyselenide (Bi2O2Se) has demonstrated significant potential to overcome existing technical barriers in various electronic device applications due to its unique physical properties like high symmetry, adjustable electronic structure, and ultra-high electron mobility. However, the rapid growth of Bi2O2Se films down to a few atomic layers with precise control remains a significant challenge. In this work, the growth of two-dimensional (2D) Bi2O2Se thin films by the pulsed laser deposition (PLD) method is systematically investigated. By controlling temperature, oxygen pressure, laser energy density, and laser emission frequency, we finally prepare atomically thin and flat Bi2O2Se (001) thin films on the (001) surface of SrTiO3. Importantly, we provide a fundamental and unique perspective toward understanding the growth process of atomically thin and flat Bi2O2Se films, and the growth process primarily proceeds in four steps. Moreover, the combined results of the crystallinity quality, surface morphology, and the chemical states demonstrate the PLD-growth of high-quality Bi2O2Se films in a controllable and fast mode.

A-Site Doping Promotes CO2 Activation and Prolongs Charge Carrier Lifetimes in SrTiO3: Insight from Quantum Dynamics.

Using time-dependent density functional theory and nonadiabatic molecular dynamics, we systematically investigated the effect of A-site doping on the CO2 activation and charge carrier lifetimes in SrTiO3(STO). Our simulations revealed that A-site doping significantly enhances the chemical adsorption of CO2 on SrTiO3 surfaces, which is beneficial for promoting CO2 activation. Moreover, we found that A-site doping can efficiently stabilize the lowest unoccupied molecular orbital (LUMO) of CO2 near the conduction band minimum of STO, promoting the photogenerated electron transfer from the conduction band of STO to the CO2 LUMO. Importantly, A-site doping causes a significant nonadiabatic coupling reduction and prolongs the charge recombination time by a factor of 1.86 compared to the pristine STO. Our study clarifies the influencing mechanism of A-site doping on CO2 activation and charge carrier lifetimes and suggests important principles for the design of high-performance photocatalytic semiconductors.

Enhanced Electromechanical Properties of Nitrile Rubber Dielectric Elastomer Composites by SrTiO3‐PPy‐PCPA Multilayer Core‐Shell Hybrids Construction

AbstractIn order to obtain high dielectric properties nitrile rubber (NBR) dielectric elastomer composites, in this work, polypyrrole (PPy) was encapsulated on the surface of SrTiO3 (STO) ceramic particles, and an insulating layer of poly (catechol/polyamine) (PCPA) was also covered on the outer surface of the STO‐PPy core‐shell particles (denoted as STO‐PPy‐PCPA particles), then the dielectric elastomer composites were prepared by adding STO and STO‐PPy‐PCPA to NBR, respectively. Due to the electrical conductivity of PPy, the dielectric constant of the STO‐PPy‐PCPA particles increases substantially. At the same time, the insulating layer of PCPA further prevents current leakage from the composite, which not only avoids premature electrical breakdown but also effectively reduces the dielectric loss. Compared with pure NBR, the dielectric constant of the prepared dielectric elastomer composites increased from 13 to 17.19 (20 phr STO‐PPy‐PCPA/NBR), and the actuated strain increased from 3.28 % to 13.86 % (5 phr STO‐PPy‐PCPA/NBR), which is 4.2 times that of pure NBR. This work successfully solves the current problems of requiring a large amount of ceramics to improve the dielectric constant and the tendency of conductive particles to agglomerate and premature breakdown.

The catalytic enhancement of the hydrogen evolution reaction facilitated by Pd coating on SrTiO3

The wide bandgap structure of 3.2 eV has made SrTiO3 (STO) well-known for its exclusive responsiveness to ultraviolet (UV) light. By anchoring Pd to the surface oxygen atoms of STO, we have successfully reduced the forbidden bandwidth of STO to 2.75 V, thereby extending its light absorption range to visible light. Simultaneously, trace amounts of low-valent Pd were distributed on the surface, significantly enhancing the photocatalytic hydrogen precipitation rate to 7366.07 μmol·g-1·h-1. This innovative approach led to the development of a novel nanocatalyst with numerous tailored active sites specifically designed for efficient photocatalytic hydrogen evolution. X-ray fine spectroscopy confirms Pd was successfully anchored to the STO surface and coordinated with surface O and the occurrence of Pd and Pd coordination at 2.0 Å from the Pd center in the R-space. Density functional theory (DFT) calculations further validate that anchoring Pd onto the STO surface reduces the free energy of adsorption for H* on these active sites. This discovery presents a promising strategy for immobilizing metal catalysts at perovskite STO’s unit sites.

Symmetry change in LaNiO3 films caused by epitaxial strain from LaAlO3, SrTiO3, and DyScO3 pseudocubic (001) surfaces

To elucidate the epitaxial strain effect over a wide range of lattice mismatch, we investigated the structures of ∼25 nm thick LaNiO3 films grown on the pseudocubic (001) surfaces of three different substrates, namely, LaAlO3 (LAO), SrTiO3 (STO), and DyScO3 (DSO). Such structural information had been inferred from the intensities of a small number of Bragg reflections that relate to the NiO6 octahedral tilting in previous studies. Here, we measured more than 100 reciprocal lattice points to derive reliable structural information. The procedure of ordinary crystal structure analysis is hampered by the multidomain structure and limited volume of measurable reciprocal space, both caused by a huge, highly symmetric substrate. To overcome this difficulty, we employed the Bayesian inference to obtain the detailed atomic positions in film samples. Octahedral tilting about the c axis was dominant for the compressively strained film grown on LAO, whereas tilting about the a and b axes was dominant for the tensile strained films grown on STO and DSO. The film lattice parameters of the samples grown on STO and DSO were nearly identical, whereas additional twofold lattice modulation, including cation displacement, was only observed in the latter.

(Invited) Resolving Catalytic Mechanism at an Electrode Surface: Thermodynamics & Kinetics of Reaction Steps

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution. Yet, when compared to experiment, kinetic models are applied. For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms. A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO3/aqueous interface. I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO3 surface. The intermediate population is of trapped holes measured by emission from the conduction band into unoccupied states in the middle of the semi-conductor bandgap. The isotherm is tuned by the pH of the electrolyte. The pH-dependent formation of these intermediates are connected to their pH-dependent decay at microsecond timescales, presumably related to later reaction steps of OER. I'll also present work on the side of vibrational spectroscopy that looks at how the potential energy surface of a particular trapped hole, the terminal oxyl radical, arises in time through the spectral kinetics of its normal mode. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely polymorphs of TiO2.

Study of CO2 Adsorption Properties on the SrTiO3(001) Surface with Ambient Pressure XPS.

The adsorption properties of CO2 on the SrTiO3(001) surface were investigated using ambient pressure X-ray photoelectron spectroscopy under elevated pressure and temperature conditions. On the Nb-doped TiO2-enriched (1 × 1) SrTiO3 surface, CO2 adsorption, i.e., the formation of CO3 surface species, occurs first at the oxygen lattice site under 10-6 mbar CO2 at room temperature. The interaction of CO2 molecules with oxygen vacancies begins when the CO2 pressure increases to 0.25 mbar. The adsorbed CO3 species on the Nb-doped SrTiO3 surface increases continuously as the pressure increases but starts to leave the surface as the surface temperature increases, which occurs at approximately 373 K on the defect-free surface. On the undoped TiO2-enriched (1 × 1) SrTiO3 surface, CO2 adsorption also occurs first at the lattice oxygen sites. Both the doped and undoped SrTiO3 surfaces exhibit an enhancement of the CO3 species with the presence of oxygen vacancies, thus indicating the important role of oxygen vacancies in CO2 dissociation. When OH species are removed from the undoped SrTiO3 surface, the CO3 species begin to form under 10-6 mbar at 573 K, thus indicating the critical role of OH in preventing CO2 adsorption. The observed CO2 adsorption properties of the various SrTiO3 surfaces provide valuable information for designing SrTiO3-based CO2 catalysts.

Unidirectional Charge Orders Induced by Oxygen Vacancies on SrTiO3(001).

The discovery of high-mobility two-dimensional electron gas and low carrier density superconductivity in multiple SrTiO3-based heterostructures has stimulated intense interest in the surface properties of SrTiO3. The recent discovery of high-Tc superconductivity in the monolayer FeSe/SrTiO3 led to the upsurge and underscored the atomic precision probe of the surface structure. By performing atomically resolved cryogenic scanning tunneling microscopy/spectroscopy characterization on dual-TiO2-δ-terminated SrTiO3(001) surfaces with (√13 × √13), c(4 × 2), mixed (2 × 1), and (2 × 2) reconstructions, we disclosed universally broken rotational symmetry and contrasting bias- and temperature-dependent electronic states for apical and equatorial oxygen sites. With the sequentially evolved surface reconstructions and simultaneously increasing equatorial oxygen vacancies, the surface anisotropy reduces and the work function lowers. Intriguingly, unidirectional stripe orders appear on the c(4 × 2) surface, whereas local (4 × 4) order emerges and eventually forms long-range unidirectional c(4 × 4) charge order on the (2 × 2) surface. This work reveals robust unidirectional charge orders induced by oxygen vacancies due to strong and delicate electronic-lattice interaction under broken rotational symmetry, providing insights into understanding the complex behaviors in perovskite oxide-based heterostructures.

Ag-doped SrTiO3: Enhanced water splitting for hydrogen production

This study investigates the effect of Ag-doping on the (001) surface of SrTiO3 (STO) to enhance hydrogen production via water-splitting employing density functional theory (DFT) simulations. It was found that the heterolytic water dissociation on Ag-doped (001)-STO occurs via a first-order reaction with an energy barrier of 5.74 kJ/mol, representing a reduction of 94% compared to undoped (001)-STO. The Gibbs free energy of H2 formation catalyzed by Ag-doped (001)-STO is approximately −302 kJ/mol, indicating the spontaneity of this process. Additionally, the surface band gap energy decreases by ∼2 eV after water-splitting, suggesting charge transfer from the Ag-doped STO to the water and leading to new electron-hole recombination states. This research underscores the potential of Ag-doping in enhancing the efficiency of hydrogen production through water-splitting, contributing to the development of greener and more efficient energy technologies.

Regular perovskite SrTiO3 and monoclinic layered perovskite Nd2Ti2O7 for oxidative coupling of methane: Elucidating the role of chemisorbed oxygen species and lattice oxygen

Herein, regular perovskite SrTiO3 and monoclinic layered perovskite Nd2Ti2O7 have been prepared by the sol-gel method and calcined at different temperatures. As the calcination temperature of SrTiO3 increases, the Ti element on their surface is enriched, and the amount of Sr element that provides basic sites is reduced, resulting in a decrease in the number of basic sites on the catalyst surface. For Nd2Ti2O7, its surface is enriched with Nd elements, but the enrichment amount does not change with the increase in calcination temperature, and it contains less surface basic sites less than SrTiO3. Although the enrichment of Ti on the surface of SrTiO3 is conducive to the formation of oxygen vacancies, the oxygen vacancies on the surface of Nd2Ti2O7 are more abundant than those on SrTiO3. The amount of electron-deficient oxygen species on their surfaces is influenced by the presence of basic sites, which make it more favorable to stabilize electrophilic oxygen species. The C2 selective oxygen species are chemisorbed oxygen (O2-, O2δ-, O22-) for SrTiO3 and surface lattice oxygen for Nd2Ti2O7. This is because the [TiO6] octahedra in the crystal structure of SrTiO3 are tightly stacked, whereas the [TiO6] units in the Nd2Ti2O7 structure are loosely stacked, resulting in a weaker Ti-O bond strength in Nd2Ti2O7 compared with SrTiO3. In addition, the lattice oxygen of SrTiO3 has strong basicity, which is beneficial for activating gas-phase oxygen to generate chemisorbed oxygen species and stabilizing them on the catalyst surface, while the nucleophilic lattice oxygen is more likely to cause deep oxidation of methane and coupling products. However, the and basicity of lattice oxygen in Nd2Ti2O7 are weaker than those in SrTiO3, making it more favorable for C2 selectivity.

First-principles study on the Te/Po doped SrTiO3 (100) surface for photocatalytic overall water splitting to produce hydrogen

The (100) surface of SrTiO3 is inefficient for solar photocatalytic performance because its wide bandgap restricts the response to visible light. The effects of the different surface termination and the doped atoms are investigated to manipulate the solar photocatalytic behavior of the surface. The results reveal that the band edges for the Ti/O-terminated (100) surface of SrTiO3 (SS-I) can straddle the redox potentials for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), but this is not true for the Sr/O-terminated one (SS-II). However, we successfully make the band edges of SS-II meet the HER and OER conditions by the doped Te and Po atoms (Te-SS-II and Po-SS-II), after trying the doped elements of the IIIA-VIA group. The solar-to-hydrogen efficiency are 14.50 %, 9.89 %, and 12.75 % for SS-I, Te-SS-II, and Po-SS-II, respectively. The Gibbs free energies are calculated for each HER and OER. The result of -0.34 eV for the HER on SS-I indicates that this reaction can spontaneously proceed. Therefore, these surfaces are promisingly used for photocatalytic water splitting. The present findings provide helpful guidance to manipulate the photocatalytic water splitting on the (100) surfaces of SrTiO3 by choosing the terminal atoms or doped atoms on the surface.

Atomic-Scale Interface for Pt Nanoparticles on SrTiO3 (001).

The interfacial structure formed by Pt nanoparticles grown epitaxially on a SrTiO3 (001) surface by pulsed laser deposition was studied by X-ray standing-wave (XSW) excited core-level photoelectron emission. The XSW-generated 3D atomic map of the Pt and interfacial oxygens for the oxidized Pt/SrTiO3 interface differs significantly from that of the as-deposited interface. After oxidation, the Pt atoms shifted upward and their atomic occupation at fcc-like sites evolved as the oxidation temperature increased. Interfacial oxygen atoms were differentiated from bulk O atoms by the chemical shift in the binding energy of their 1s electrons. After oxidation, the interfacial oxygen atoms rearranged to form a TiO2 bilayer at the interface. These results provide a more complete description of the strong metal-support interaction process at the interface.

Distinct differences in surface transport between electrolyte-gated (110) and (001) SrTiO3

Electrolyte gating technology has been widely used as an effective tool to study novel physics at complex oxide surfaces and interfaces. Certain emergent phenomena reported in electro-gated SrTiO3 (STO) surfaces are particularly interesting. Here, we report on the disparate electron transport behaviors of electrolyte-gated STO (001) and (110) oriented surfaces. In contrast to the anomalous transport from an insulating state to a Kondo-like state and then metallic states on the (001) surface, with increasing carrier density, the (110) surface always behaves as an insulating state. A comparison study suggests that the oxygen vacancies and localized Ti3+ ions on the (001) surface are scatter carriers, but no such defects are found on the (110) surface. This suggests that these surfaces are intrinsically insulating, and the observed anomalous effects are likely induced by the Ti3+ ions and oxygen vacancies introduced during electrolyte gating. This work sheds light on complicated phenomena in electrolyte-gated STO-based systems.

Formation of the oxyl's potential energy surface by the spectral kinetics of a vibrational mode.

One of the most reactive intermediates for oxidative reactions is the oxyl radical, an electron-deficient oxygen atom. The discovery of a new vibration upon photoexcitation of the oxygen evolution catalysis detected the oxyl radical at the SrTiO3 surface. The vibration was assigned to a motion of the sub-surface oxygen underneath the titanium oxyl (Ti-O●-) created upon hole transfer to (or electron extraction from) a hydroxylated surface site. Evidence for such an interfacial mode is derived from its spectral shape, which exhibited a Fano resonance-a coupling of a sharp normal mode to continuum excitations. Here, this Fano resonance is utilized to derive precise formation kinetics of the oxyl radical and its associated potential energy surface (PES). From the Fano lineshape, the formation kinetics are obtained from the anti-resonance (the kinetics of the coupling factor), the resonance (the kinetics of the coupled continuum excitations), and the frequency integrated spectrum (the kinetics of the normal mode's cross-section). All three perspectives yield logistic function growth with a half-rise of 2.3 ± 0.3ps and a time constant of 0.48 ± 0.09ps. A non-equilibrium transient associated with photoexcitation is separated from the rise of the equilibrated PES. The logistic function characterizes the oxyl coverage at the very initial stages (t ∼ 0) to have an exponential growth rate that quickly decreases toward zero as a limiting coverage is reached. Such time-dependent reaction kinetics identify a dynamic activation barrier associated with the formation of a PES and quantify it for oxyl radical coverage.

Spontaneous Atomic-Scale Polar Skyrmions and Merons on a SrTiO3 (001) Surface: Defect Engineering for Emerging Topological Orders.

The emergence of nontrivial topological order in condensed matter has been attracting a great deal of attention owing to its promising technological applications in novel functional nanodevices. In ferroelectrics, the realization of polar topological order at an ultimately small scale is extremely challenging due to the lack of chiral interaction and the critical size of the ferroelectricity. Here, we break through these limitations and demonstrate that the ultimate atomic-scale polar skyrmion and meron (∼2 nm) can be induced by engineering oxygen vacancies on the SrTiO3 (001) surface based on first-principles calculations. The paraelectric-to-antiferrodistortive phase transition leads to a novel topological transition from skyrmion to meron, indicating phase-topology correlations. We also discuss accumulating and driving polar skyrmions based on the oxygen divacancy model; these results and the recent discovery of defect engineering techniques suggest the possibility of arithmetic operations on topological numbers through the natural self-organization and diffusion features of oxygen vacancies.

Modulating the electron interaction of Cu-Ni hybrid sites for boosting photocatalytic hydrogen evolution over Al-doped SrTiO3

Cocatalysts play a pivotal role in enhancing the efficiency of photocatalytic water splitting. In this study, we introduced a novel and straightforward pre-impregnation strategy to anchor a CuNi dual cocatalyst onto the surface of Al-doped SrTiO3 (Al-STO), coupled with photodeposition. This approach led to a remarkable 11.07-fold (77.5 µmol/h) increase in hydrogen evolution performance compared to the conventional photodeposition method for decorating with CuNi dual cocatalysts. Not only that, after the 18 h cycle experiment, the catalyst still maintains high activity and high stability similar to that of precious metals, which overcomes the defects of poor stability and short life of non-precious metals. This strategy capitalizes on the pre-adsorption of cocatalyst precursor ions on the Al-STO support via impregnation, enhancing both the adsorption energy and capacity between the precursor ions and the active sites. Detailed characterizations have revealed a strong electronic interaction within the Cu-Ni structure and improved interfacial coupling between CuNi and Al-STO. This not only significantly promotes charge separation and transfer but also provides a plethora of active sites for the photocatalytic generation of H2. Moreover, extensive theoretical calculations have provided deep insights into the reaction mechanism of the CuNi system and elucidated the reasons behind the varying degrees of activity enhancement across different cocatalyst architectural systems.

A circular photogalvanic effect in two-dimensional electron gas on the surface of SrTiO3

A spin-splitting state due to a Rashba-type spin–orbit interaction is investigated using two-dimensional electron gas (2DEG) at the surface of SrTiO3. The circular photogalvanic effect is utilized to detect the spin-splitting state. Both the polarization and incident light angle dependence of the measured photocurrent generated in the 2DEG unequivocally show the presence of surface spin splitting in the 2DEG, and variation of the carrier densities of the 2DEG provides further supporting evidence. This finding could pave the way for investigating spin textures and spin physics in two-dimensional carrier gas systems.

Coverage-sensitive mechanism of electrochemical NO reduction on the SrTiO3(001) surface: a DFT investigation.

Due to its adverse environmental and human health hazards, addressing the elimination of nitric oxide (NO) has become a pressing concern for modern society. Currently, electrochemical NO reduction provides a new alternative to traditional selective catalytic reduction technology under mild reaction conditions. However, the complexity and variability of products make the coverage of NO an influencing factor that needs to be investigated. Hence, this study delves into the coverage-sensitive mechanism of electrochemical NO reduction on cost-effective perovskite catalysts, using SrTiO3 as an example, through density functional theory calculations. Phase diagrams analysis reveals that the coverage range from 0.25 to 1.00 monolayer (ML) coverage is favorable for NO adsorption. Gibbs free energy results indicate that the selectivity is significantly influenced by NO coverage. NH3 is likely to be generated at low coverage, while N2O and N2 are more likely to be produced at high coverage through a dimer mechanism. Charge analysis suggests that the charge transfer and Ti-O bond strength between reactants and catalysts are crucial factors. This work not only provides deep insights into coverage-sensitive reaction mechanisms but also is a guideline towards further rational design of high-performance perovskite catalysts.

An in situ study on the depth-resolved chemical states of undoped SrTiO3(001) surfaces during Ar+ sputtering and annealing processes with XPS

During Ar+ sputtering and UHV annealing processes on undoped SrTiO3(001) surfaces, the interactions between SrO and impurity ions can be witnessed from their Sr 3d spectra.

Exceptional behavior of a high-temperature superconductor in proximity to a ferromagnet in a bilayer film, La0.67Sr0.33MnO3/YBa2Cu3O7.

We studied the electronic properties of a high-temperature superconductor in proximity to a ferromagnetic material in a bilayer film of La0.67Sr0.33MnO3 (LSMO)/YBa2Cu3O7 (YBCO). High-quality single-crystalline films of YBCO and LSMO/YBCO were grown epitaxially on an SrTiO3 (001) surface. Magnetization data of the LSMO/YBCO bilayer exhibit ferromagnetic transition at about 255 K, which is much smaller than the Curie temperature of bulk LSMO. Experimental data show the emergence of magnetic anisotropy with cooling, which becomes significantly stronger in the superconducting phase. The onset temperature of diamagnetism is observed at 86 K in the YBCO sample for the out-of-plane magnetization and at 89 K in the in-plane data. Interestingly, the diamagnetism sets in at about 86 K for both magnetization directions in the LSMO/YBCO film despite the presence of the ferromagnetic LSMO layer underneath. Ba 4d and Y 3d core-level spectra show different surface and bulk electronic structures. Surface contribution is reduced significantly in the LSMO/YBCO sample, suggesting enhanced bulk-like behavior due to an enhancement of electron density near the surface arising from charge transfer across the interface. These results reveal an outstanding platform for on-demand tuning of properties without affecting the superconductivity of the system for the exploration of fundamental science and applications in advanced technology.