Surface Of Strontium Titanate Research Articles

NiS modified SrTiO3:Al bifunctional photocatalyst for H2 generation and cathodic protection

Developing low-cost and high-efficiency non-precious metal cocatalysts is the key to the practical application of photocatalysis technology. Here, nickel sulfide (NiS) nanoparticles were synthesized on the surface of strontium titanate (SrTiO3) via a hydrothermal method, identifying the optimal loading molar concentration of NiS as 10 %. The solar-to-hydrogen efficiency (STH) of 10%NiS/SrTiO3:Al was enhanced by 97 times compared to SrTiO3:Al and by 1.62 times compared to RhCrCoOx/SrTiO3:Al. This improvement is primarily attributed to the metallic-like properties of NiS, which enhance the photocatalyst's light absorption capacity and charge transfer capability. Additionally, the STH value of samples loaded with both NiS and RhCrCoOx reached 0.436 %, indicating a positive synergistic effect in photocatalysis between NiS and RhCrCoOx. Their photocathodic protection capability was also studied in simulated seawater conditions, showing a reduced self-corrosion potential (−0.31V) and a low interface charge impedance (5962 KΩ). This work reveals the potential of NiS as an alternative to precious metal cocatalysts, offering new possibilities for developing cost-effective and high-performance materials for photocatalytic water splitting and photocathodic protection.

Charge Transfer and Orbital Reconstruction at an Organic-Oxide Interface.

The two-dimensional electron system (2DES) located at the surface of strontium titanate (STO) and at several other STO-based interfaces has been an established platform for the study of novel physical phenomena since its discovery. Here we report how the interfacing of STO and tetracyanoquinodimethane (TCNQ) results in a charge transfer that depletes the number of free carriers at the STO surface, with a strong impact on its electronic structure. Our study paves the way for efficient tuning of the electronic properties, which promises novel applications in the framework of oxide/organic-based electronics.

The interaction mechanism of cesium with water on the SrTiO3(100) surface at room temperature

The interaction of water with cesium on the strontium titanate surface SrTiO3(100), was studied, mainly by means of work function measurements and thermal desorption spectroscopy. The catalytic role of cesium with respect to the dissociation of water on surface was investigated, by applying two different adsorption processes at room temperature (RT): (1) The adsorption of water on the cesium covered surface (sequential adsorption), and (2) the co-adsorption process (simultaneous adsorption) on surface. Based on the results and by adopting the Lewis acid–base model, we conclude that during the sequential adsorption the water molecules are mostly adsorbs non-dissociatively on surface, without oxidizing the alkaline overlayer. This seems to be due, first to the strong interaction between the alkaline adatoms and the substrate, and secondly to the limited maximum pre-deposited amount of cesium (≤ 0.45 ML). Instead, water dissociation appears to merely occur on defective sites of the substrate in accordance with previous studies. For a full cesium layer covered surface, the adsorbed water retracts the metallicity of cesium due to electrostatic interactions. In contrast to the sequential adsorption, during the co-adsorption process the oxidation of cesium takes place above a critical coverage of cesium (≥ 0.45 ML). It appears that the co-adsorbed cesium with water modifies the surface potential providing an effective template for cesium oxide, Cs2O development. Based on that, we suggest a catalytic reaction of water dissociation according to the Langmuir–Hinshelwood mechanism. Finally, we propose atomistic adsorption models for both processes of cesium with water adsorption.

Correction to: Photocatalytic hydrogen production on chemically etched strontium titanate surfaces

Correction to: Photocatalytic hydrogen production on chemically etched strontium titanate surfaces

Interfacial acidity on the strontium titanate surface: a scaling paradigm and the role of the hydrogen bond.

A fundamental understanding of acidity at an interface, as mediated by structure and molecule-surface interactions, is essential to elucidate the mechanisms of a range of chemical transformations. While the strength of an acid in homogeneous gas and solution phases is conceptually well understood, acid-base chemistry at heterogeneous interfaces is notoriously more complicated. Using density functional theory and nonlinear vibrational spectroscopy, we present a method to determine the interfacial Brønsted-Lowry acidity of aliphatic alcohols adsorbed on the (100) surface of the model perovskite, strontium titanate. While shorter and less branched alkanols are known to be less acidic in the gas phase and more acidic in solution, here we show that shorter alcohols are less acidic whereas less substituted alkanols are more acidic at the gas-oxide interface. Hydrogen bonding plays a critical role in defining acidity, whereas structure-acidity relationships are dominated by van der Waals interactions between the alcohol and the surface.

Photocatalytic hydrogen production on chemically etched strontium titanate surfaces

This work aims to investigate the photocatalytic hydrogen evolution over acid (citric acid and hydrochloric acid) etched strontium titanate (SrTiO3), which is promoted by 1 wt% platinum (Pt) as co-catalyst. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible spectroscopy (UV–Vis) analyses of the un-etched and etched SrTiO3 are presented; the results indicate that etching affects the surface structure while it does not change the bulk properties of semiconductor. The results of the photocatalytic activity tests performed in a methanol solution using a semi-batch reaction system equipped with argon flow to collect hydrogen, 300 W xenon lamp as light source and gas chromatograph for the analysis, are also reported. The etching with 4 mol L−1 citric acid increases the hydrogen evolution rate about 20% while the highest rate of hydrogen production over etched Pt-SrTiO3 is about 550 µmol/gcat.h at the 6 h of the reaction test.

Non-volatile electric control of spin-charge conversion in a SrTiO3 Rashba system.

After 50years of development, the technology of today's electronics is approaching its physical limits, with feature sizes smaller than 10nanometres. It is also becoming clear that the ever-increasing power consumption of information and communication systems1 needs to be contained. These two factors require the introduction of non-traditional materials and state variables. As recently highlighted2, the remanence associated with collective switching in ferroic systems is an appealing way to reduce power consumption. A promising approach is spintronics, which relies on ferromagnets to provide non-volatility and to generate and detect spin currents3. However, magnetization reversal by spin transfer torques4 is a power-consuming process. This is driving research on multiferroics to achieve low-power electric-field control of magnetization5, but practical materials are scarce and magnetoelectric switching remains difficult to control. Here we demonstrate an alternative strategy to achieve low-power spin detection, in a non-magnetic system. We harness the electric-field-induced ferroelectric-like state of strontium titanate (SrTiO3)6-9 to manipulate the spin-orbit properties10 of a two-dimensional electron gas11, and efficiently convert spin currents into positive or negative charge currents, depending on the polarization direction. This non-volatile effect opens the way to the electric-field control of spin currents and to ultralow-power spintronics, in which non-volatility would be provided by ferroelectricity rather than by ferromagnetism.

The Facet Structure and Photochemical Reactivity of Arbitrarily Oriented Strontium Titanate Surfaces

AbstractSrTiO3 polycrystalline ceramics with polished surfaces are annealed at 1250 °C in air. This treatment causes the flat surfaces to break up into facets meeting at sharp edges and corners. An analysis of the orientations and topography of the faceted surfaces demonstrates that all are either {100} or {110} oriented. The {100} surfaces are photocathodically active and reduce Ag+ to Ag metal. The {110} surfaces are photoanodically active and oxidize Mn2+ and Pb2+ to Mn4+ and Pb4+, respectively. The chemical properties of both surfaces appear to be uniformly photocathodic or photoanodic. However, after annealing at 1100 °C with Sr3Ti2O7, the {110} facets have a combination of photocathodic and photoanodic terraces. The results show that the photocathodic‐to‐photoanodic surface area ratio, which influences the overall rate of a photochemical reaction, can be controlled for arbitrarily oriented SrTiO3 surfaces by using thermal treatments to create low index facets and to control the chemical terminations on these facets.

Ethanol-Quenching Introduced Oxygen Vacancies in Strontium Titanate Surface and the Enhanced Photocatalytic Activity.

Modification of the surface properties of SrTiO3 crystals by regulating the reaction environment in order to improve the photocatalytic activity has been widely studied. However, the development of a facile, effective, and universal method to improve the photocatalytic activity of these crystals remains an enormous challenge. We have developed a simple method to modify the surface environment of SrTiO3 by ethanol quenching, which results in enhanced UV, visible and infrared light absorption and photocatalytic performance. The SrTiO3 nanocrystals were preheated to 800 °C and immediately quenched by submersion in ethanol. X-ray diffraction patterns, electron paramagnetic resonance spectra, and X-ray photoelectron spectra indicated that upon rapid ethanol quenching, the interaction between hot SrTiO3 and ethanol led to the introduction of a high concentration of oxygen vacancies on the surface of the SrTiO3 lattice. Consequently, to maintain the regional charge balance of SrTiO3, Sr2+ could be substituted for Ti4+. Moreover, oxygen vacancies induced localized states into the band gap of the modified SrTiO3 and acted as photoinduced charge traps, thus promoting the photocatalytic activity. The improved photocatalytic performance of the modified SrTiO3 was demonstrated by using it for the decomposition of rhodamine B and production of H2 from water under visible or solar light.

Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate

For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.

Controlling the magnetism of oxygen surface vacancies in SrTiO3 through charging

We discuss, based on first principles calculations, the possibility to tune the magnetism of oxygen vacancies at the (001) surface of strontium titanate $(\mathrm{SrTiO_3}\!)$. The magnetic moment of single and clustered vacancies stemming from Ti-O broken bonds can be both quenched and stabilized controllably by chemical potential adjustment associated with doping the system with electrons or holes. We discuss to what extent this route to magnetization state control is robust against other external influences like chemical doping, mechanical action and electric field. Such control of vacancy state and magnetization can conceivably be achieved experimentally by using local probe tips.

Microstructure and magnetic properties of strontium titanate implanted with iron

The magnetic phase composition of strontium titanate surface layers implanted with iron ions is studied by means of Mossbauer spectroscopy, and by measuring alternating current magnetic susceptibility. It is shown that the interaction between α-Fe nanoclusters at high concentrations of the implanted admixture produces ferromagnetic order in samples at room temperature.

ALD synthesis of platinum nanoparticles on single-crystal SrTiO3 pretreated with wet chemical etching

Formic acid-hydrogen peroxide and buffered hydrogen fluoride-hydrogen peroxide solutions were used to etch single-crystal strontium titanate to remove carbon contamination and increase hydroxyl group density to improve atomic layer deposition onto these materials. X-ray photoelectron spectroscopy indicated that both are effective for carbon contamination removal. However, for increasing hydroxyl group density on the strontium titanate surface, the buffered hydrogen fluoride-hydrogen peroxide is more effective. Transmission electron microscopy and x-ray photoemission show enhanced platinum deposition on the etched strontium titanate surface. These results provide the basis for optimizing atomic layer deposition for other technologically relevant materials.

Transition from Reconstruction toward Thin Film on the (110) Surface of Strontium Titanate.

The surfaces of metal oxides often are reconstructed with a geometry and composition that is considerably different from a simple termination of the bulk. Such structures can also be viewed as ultrathin films, epitaxed on a substrate. Here, the reconstructions of the SrTiO3 (110) surface are studied combining scanning tunneling microscopy (STM), transmission electron diffraction, and X-ray absorption spectroscopy (XAS), and analyzed with density functional theory calculations. Whereas SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n × 1 to 2 × n. At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium. This transition from the n × 1 to 2 × n reconstructions is a transition from a pseudomorphically stabilized tetrahedral network toward an octahedral titania thin film with stress-relief from octahedral strontia units at the surface.

Localized versus itinerant states created by multiple oxygen vacancies in SrTiO3

Oxygen vacancies in strontium titanate surfaces (SrTiO3) have been linked to the presence of a two-dimensional electron gas with unique behavior. We perform a detailed density functional theory study of the lattice and electronic structure of SrTiO3 slabs with multiple oxygen vacancies, with a main focus on two vacancies near a titanium dioxide terminated SrTiO3 surface. We conclude based on total energies that the two vacancies preferably inhabit the first two layers, i.e. they cluster vertically, while in the direction parallel to the surface, the vacancies show a weak tendency towards equal spacing. Analysis of the nonmagnetic electronic structure indicates that oxygen defects in the surface TiO2 layer lead to population of Ti states and thus itinerancy of the electrons donated by the oxygen vacancy. In contrast, electrons from subsurface oxygen vacancies populate Ti eg states and remain localized on the two Ti ions neighboring the vacancy. We find that both the formation of a bound oxygen-vacancy state composed of hybridized Ti 3eg and 4p states neighboring the oxygen vacancy as well as the elastic deformation after extracting oxygen contribute to the stabilization of the in-gap state.

Imaging the evolution of d states at a strontium titanate surface.

Oxide electronics is a promising alternative to the conventional silicon-based semiconductor technology, owing to the rich functionalities of oxide thin films and heterostructures. In contrast to the silicon surface, however, the electronic structure of the SrTiO3 surface, the most important substrate for oxide thin films growth, is not yet completely understood. Here we report on the electronic states of a reconstructed (001) surface of SrTiO3 determined in real space, with scanning tunneling microscopy/spectroscopy and density functional theory calculations. We found a remarkable energy dependence of the spectroscopic image: Theoretical analysis reveals that symmetry breaking at the surface lifts the degeneracy in the t2g state (dxy, dyz, and dzx) of Ti 3d orbitals, whose anisotropic spatial distribution leads to a sharp transition in the spectroscopic image as a function of energy. The knowledge obtained here could be used to gain further insights into emergent phenomena at the surfaces and interfaces with SrTiO3.

Universal conductance fluctuations in electrolyte-gated SrTiO3 nanostructures

We report low-temperature magnetoconductance measurements of a patterned two-dimensional electron system at the surface of strontium titanate, gated by an ionic liquid electrolyte. We observe universal conductance fluctuations, a signature of phase-coherent transport in mesoscopic devices. From the universal conductance fluctuations, we extract an electron dephasing rate linear in temperature, characteristic of electron-electron interaction in a disordered conductor. The dephasing rate has a temperature-independent offset, which could possibly be explained by the presence of unscreened local magnetic moments in the sample.

Single crystal strontium titanate surface and bulk modifications due to vacuum annealing

Vacuum annealing is a widely used method to increase the electric conductivity of SrTiO3 single crystals. The induced oxygen vacancies act as intrinsic donors and lead to n-type conductivity. Apart from the changed electronic structure, however, also crystal structure modifications arise from this treatment. Hence, electronic properties are determined by the interplay between point defects and line defects. The present paper provides a survey of the real structure of commercially available SrTiO3 single crystals and the changes induced by reducing vacuum heat-treatment. Therefore, all investigations were performed ex situ, i.e., after the annealing process. Used characterization methods include atomic force microscopy, transmission electron microscopy, spectroscopic ellipsometry, infrared spectroscopy, and photoluminescence spectroscopy. Besides the expected variation of bulk properties, especially surface modifications have been detected. The intrinsic number of near-surface dislocations in the samples was reduced by vacuum annealing. X-ray photoelectron spectroscopy proves the existence of a layer of adsorbed molecules, which influences the SrTiO3 work function. Also, the interaction between adsorbates and surface point defects as well as laser annealing due to local oxygen absorption are discussed.

Quasi-ordered nanostructures on the surface of strontium titanate

The results of the investigation of the surface of strontium titanate single crystals after treatment with high-energy plasma are presented. The surface morphology of the strontium titanate single crystals and the change in its characteristics after plasma treatment have been studied using electron scanning and atomicforce microscopy. A change in the electronic state of a part of the titanium ions and a change in the stoichiometry in the modified near-surface layer have been found by the method of valence shift of X-ray lines. A preliminary analysis has been made of the conditions providing the formation of single- and two-level systems of ordered crystallites with sizes of 10−7–10−10 m on the surface of single-crystal strontium titanate with impurities of ions of the iron and lanthanum groups.

Surface nanostructuring of SrTiO 3 single crystals by slow highly charged ions and swift heavy ions

Single crystals of strontium titanate have been irradiated with both slow highly charged Xe ions extracted from an Electron Beam Ion Trap and swift heavy Xe ions. After irradiation, the crystals were investigated by scanning force microscopy in air. In both cases nanohillocks due to impact of individual projectiles were observed. This similarity originates from the fact that both swift heavy ions and slow highly charged ions initially transfer their energy to the electronic system of the target, leading to a localized region of high electronic excitation. This electronic excitation is subsequently transferred to the lattice atoms by electron–phonon coupling, leading to pronounced lattice heating. The formation of surface hillocks can then be ascribed to a melting process. We also present first evidence for the existence of a potential energy threshold for nanohillock formation on strontium titanate surfaces by slow highly charged ions.