Effect Of Surface Orientation Research Articles

Effect of surface orientation on the corrosion behavior of Ni-based single-crystal alloy in a simulated marine atmosphere at 750 °C

DD5 single-crystal alloy samples with different surface orientations were obtained by machining the alloy ingot perpendicular (PP) and parallel (P), respectively, to its solidification direction (SD). The corrosion behaviors of the PPSD and PSD alloy samples were investigated in the atmosphere of moist air + NaCl aerosol at 750 °C. Results showed that during initial corrosion, γ′ phase developed a thin NiO layer with Al internal corrosion product. However, a NiO nodule with an internal Cr2O3 layer was formed on γ phase. As corrosion progressed, the corrosion product gradually grew to a NiO/(Al, Cr)2O3 bilayer, accompanied by internal corrosion consisting of Al2O3. It was also found that the PSD sample with a lower fraction and smaller size of γ’ phases exhibited better corrosion resistance than the PPSD sample. The effect of alloy surface orientation on the corrosion mechanism was discussed.

Chemical vapor deposition of hexagonal boron nitride on germanium from borazine.

The growth of hexagonal boron nitride (hBN) directly onto semiconducting substrates, like Ge and Ge on Si, promises to advance the integration of hBN into microelectronics. However, a detailed understanding of the growth and characteristics of hBN islands and monolayers on these substrates is lacking. Here, we present the growth of hBN on Ge and Ge epilayers on Si via high-vacuum chemical vapor deposition from borazine and study the effects of Ge sublimation, surface orientation, and vicinality on the shape and alignment of hBN islands. We find that suppressing Ge sublimation is essential for growing high quality hBN and that the Ge surface orientation and vicinality strongly affect hBN alignment. Interestingly, 95% of hBN islands are unidirectionally aligned on Ge(111), which may be a path toward metal- and transfer-free, single-crystalline hBN. Finally, we extend the growth time and borazine partial pressure to grow monolayer hBN on Ge and Ge epilayers on Si. These findings provide new insights into the growth of high-quality hBN on semiconducting substrates.

Anisotropic Effects in Local Anodic Oxidation Nanolithography on Silicon Surfaces: Insights from ReaxFF Molecular Dynamics.

Fully understanding the anisotropic effect of silicon surface orientations in local anodic oxidation (LAO) nanolithography processes is critical to the precise control of oxide quality and rate. This study used ReaxFF MD simulations to reveal the surface anisotropic effects in the LAO through the analysis of adsorbed species, atomic charge, and oxide growth. Our results show that the LAO behaves differently on silicon (100), (110), and (111) surfaces. Specifically, the application of an electric field significantly increases the quantity of surface-adsorbed -OH2 while reducing -OH on the (111) surface, and results in a higher charge on a greater number of Si atoms on the (100) surface. Moreover, the quantity of surface-adsorbed -OH plays a pivotal role in influencing the oxidation rate, as it directly correlates with an increased formation rate of Si-O-Si bonds. During bias-induced oxidation, the (111) surface appears with a high initial oxidation rate among three surfaces, while the (110) surface underwent increased oxidation at higher electric field strengths. This conclusion is based on the analysis of the evolution of Si-O-Si bond number, surface elevation, and oxide thickness. Our findings align well with prior theoretical and experimental studies, providing deeper insights and clear guidance for the fabrication of high-performance nanoinsulator gates using LAO nanolithography.

The effect of surface orientation on band alignment and carrier transfer at WS2/CdS interface: Insight from first-principles calculations

Loading of WS2 can greatly improve water splitting H2 generation efficiency of CdS in experiments. Here, we constructed WS2/CdS(100) and WS2/CdS(110) heterostructures with smaller mismatches and explored their interaction energy and band offset by first-principles calculations. Our calculation suggests that the WS2/CdS(100) interface with a stronger binding energy is more active in experiments, while the WS2/CdS(110) interface is metastable. The band alignment between CdS and WS2 is highly dependent on the orientation of the interfaces, and WS2/CdS(100) and WS2/CdS(110) belong to type-I and type-II band alignments, respectively. Therefore, a metal electrode and hole scavenger may be essential in experiments to help WS2/CdS(100) efficiently trap electrons, and a suitable substrate and an appropriate growth temperature are also needed to composite the CdS(110) surface to achieve a higher photocatalytic efficiency. In addition, we performed a detailed analysis of the macroscopic average potential and found that the calculated accuracy of potential difference across the heterostructures due to slab thickness is less than 80 meV at WS2/CdS interfaces. In total, our calculations not only explain the physical reasons for the increased efficiency of WS2/CdS, but also provide a detailed guideline for the design of a more efficient synergistic catalyst.

Unexpected Electronic Features of NiO Quantum Dots Produced by Femtosecond Pulsed Laser Ablation in Water.

This study examines the effect of quantum confinement and surface orientations on the electronic properties of NiO quantum dots. It compares NiO nanocrystals produced via atmospheric-pressure microplasma and femtosecond laser (fs-laser) ablation in water, finding that both methods yield quantum-confined nanocrystals with a defined face-centered cubic lattice. Notably, fs-laser synthesis generates crystalline nanocrystals from both crystalline and amorphous targets. While the electronic properties, i.e., energy of the highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO), of microplasma-synthesized NiO nanocrystals are consistent with the literature, the electronic characteristics of NiO nanocrystals produced by a fs-laser, particularly the high-lying LUMO level, are unusual for NiO quantum dots. Supported by density functional theory calculations, we show that the observed level positions are related to the different polar and nonpolar faces of the nanocrystal surface.

Surface crack effect on frequency and vibration mode switching of solar-powered aerospace structures

In the engineering design of modern wing panels and their optimal control system, the first step is to find and predict their dynamic characteristics. These characteristics are dependent on material properties, panel geometry, and boundary conditions. In addition, possible fracture existence, imperfections and environmental conditions may unpredictably change the frequency and modes shape of the panel and then the parameters of a control system. In the present paper, the effect of surface fracture depth and orientation on frequency and vibration mode switching of perovskite solar cells-based aircraft wing panels is investigated for the first time. The PSC (perovskite solar cell) panel is made of a polymer substrate reinforced with even-distributed graphene platelets (GPLs) and thin solar layers to enhance the overall electromechanical performance of the whole structure. Equations of motion of the panel are obtained by the Hamilton approach based on the properties of Reddy's third-order shear deformation theory and linear components of the Green-Lagrange strain tensor. The novel aspect is a line-spring model of surface crack by Cartmell et al. adapted to the model of the PSC panel under consideration. The discretization of the established equations of motion for the cracked panel is carried out by the element-free IMLS-Ritz method with mesh-free features. Validation of the obtained solutions is carried out by comparisons with results from previous studies for cracked laminated plates.First, material and geometric properties effect on vibration characteristics of the PSC structure was investigated. Next, partial surface crack depth and orientation angle are examined to show changes in frequency and vibration modes shape of the panel. It is shown, that there is a mode swapping between the second and third modes when transitioning from an intact panel to a cracked one. As the crack length increases, higher-order mode variations become more noticeable, potentially leading to their swapping or the excitation of new ones. Additionally, the findings demonstrate that the surface crack orientation angle significantly impacts the higher-order modes of the panel. When the orientation angle is 90°, the first six modes align with those of the intact plate. Mode shapes and frequency shifts are significant for crack orientation angles between 0° and 90°.

Pool boiling heat transfer of Novec 649 on sandblasted surfaces

Pool boiling represents a potentially efficient mechanism for thermal management of power electronic components. The present study investigates pool boiling heat transfer using Novec 649 on copper substrates with varying surfaces. Sandblasting, which is a relatively simple surface modification technique, has been used to obtain different copper surface roughness levels ranging from 0.1 µm to 9 µm arithmetic mean surface roughness values. The effects of surface modifications and orientation on boiling heat transfer coefficient (HTC), and critical heat flux (CHF), of Novec 649 have been experimentally investigated by performing pool boiling experiments at atmospheric pressure and saturation pool temperature. The experimental results showed that the size and orientation of the boiling surface do not have a significant impact on the CHF, whilst the HTC improves in the nucleate boiling regime with vertical orientation. In addition, for the range of surface roughness investigated improvements of ∼3X and ∼1.5X in HTC and CHF, respectively, were found relative to the original smooth surface. The HTC in the film boiling regime relative to the nucleate boiling regime was also quantified and found to be ∼20X for our geometries.

Reactive Molecular Dynamics Simulation Study on Atomic-Scale Adhesive Wear Mechanisms of Single Crystalline Body-Centered Cubic Iron

The adhesive wear of steel is a crucial issue in many industrial fields because it can lead to serious machine failure. However, the adhesive wear mechanism is still under debate owing to its complexity. Therefore, in this work, we performed reactive molecular dynamics-based sliding simulations of single crystalline body-centered cubic iron and investigated the fundamental atomic-scale adhesive wear mechanism for improving the wear resistance of steel. The effects of surface orientation, sliding direction, and humid atmosphere on the adhesive wear property were analyzed. In the sliding simulation, we observed two adhesive wear types. One is the wear accompanying surface deformation, in which the surface asperities gradually deform by slip and adhere severely. The other is the wear accompanying surface fracture with crack generation. The former can lead to seizures, whereas the latter can lead to wear debris formation. We propose that the rubbing surface orientation and sliding direction alter the atomic-scale adhesive wear type. Wear with surface deformation occurred when the deformation by slip was favorable, whereas wear with surface fracture occurred when slip was not favorable. Understanding the adhesive wear mechanism of iron in humid atmospheres is also important in many industrial fields. When water molecules were present at the sliding interface, both types of adhesive wear were suppressed. At the sliding interface, Fe–OH and Fe–O–Fe groups were formed on the scars through the tribochemical reaction with water. These groups passivated the nascent Fe surfaces and suppressed adhesion to the counter surface, thereby reducing adhesive wear. Therefore, we conclude that the surface orientation and sliding direction determine the atomic-scale adhesive wear type, whereas a humid atmosphere affects the wear amount at the atomic scale.

Effect of surface orientation on blistering of copper under high fluence keV hydrogen ion irradiation

Copper and hydrogen are among the most common elements that are widely used in industrial and fundamental research applications. Copper surfaces are often exposed to hydrogen in the form of charged ions. The hydrogen ions can accelerate towards the surface, resulting in an accumulation of hydrogen below the surface. Harmless in low concentrations, prolonged hydrogen exposure can lead to dramatic changes on copper surfaces. This effect is visible to the naked eye in the form of blisters densely covering the exposed surface. Blisters are structural modifications that can affect the physical properties of the surface including, for example, vacuum dielectric strength.Using scanning electron microscopy we found that the blistering of the irradiated polycrystalline copper surface does not grow uniformly with ion fluence. Initially, only some grains exhibit blisters, while others remain intact. Our experiments indicate that grains with the {100} orientation are the most prone to blistering, while the grains oriented in the {110} are the most resistant to it. Moreover, we noticed that blisters assume different shapes correlating with specific grain orientation.Good agreement of experiments with the atomistic simulations explains the difference in the shapes of the blisters by specific behavior of dislocations within the FCC crystal structure. Moreover, our simulations reveal the correlation of the delay in blister formation on surfaces with certain orientations compared to the others with the dependence of the hydrogen penetration depth and the depth and amount of vacancies in copper on the orientation of the irradiated surface.

Long-Range Uniform Deposition of Ag Nanoseed on Cu Current Collector for High-Performance Lithium Metal Batteries.

Uniform lithium deposition is essential to hinder dendritic growth. Achieving this demands even seed material distribution across the electrode, posing challenges in correlating the electrode's surface structure with the uniformity of seed material distribution. In this study, the effect of periodic surface and facet orientation on seed distribution is investigated using a model system consisting of a wrinkled copper (Cu)/graphene structure with a [100] facet orientation. A new methodology is developed for uniformly distributed silver (Ag) nanoparticles over a large area by controlling the surface features of Cu substrates. The regularly arranged Ag nanoparticles, with a diameter of 26.4nm, are fabricated by controlling the Cu surface condition as [100]-oriented wrinkled Cu. The wrinkled Cu guides a deposition site for spherical Ag nanoparticles, the [100] facet determines the Ag morphology, and the presence of graphene leads to spacings of Ag seeds. This patterned surface and high lithiophilicity, with homogeneously distributed Ag nanoparticles, lead to uniform Li+ flux and reduced nucleation energy barrier, resulting in excellent battery performance. The electrochemical measurements exhibit improved cyclic stability over 260 cycles at 0.5mAcm-2 and 100 cycles at 1.0mAcm-2 and enhanced kinetics even under a high current density of 5.0mAcm-2.

Revealing the Effects of Weak Surfactants on the Dynamics of Surface Fluctuations by Surface Light Scattering.

Surfactant monolayers at liquid interfaces induce a viscoelastic behavior that influences the dynamics of surface fluctuations probed by surface light scattering (SLS). Recent thermophysical property research on viscosity and interfacial tension of liquid organic hydrogen carrier (LOHC) systems based on diphenylmethane suggested that such viscoelastic effects may also be present here, although not being expected a priori. To prove the hypothesis that the LOHC intermediate cyclohexylphenylmethane (H6-DPM) can induce a surfactant-like behavior, binary mixtures of diphenylmethane (H0-DPM) or dicyclohexylmethane (H12-DPM) with small amounts of H6-DPM were studied by SLS in combination with conventional viscometry and tensiometry and molecular dynamics simulations between (303 and 473) K. Only in mixtures with H0-DPM which has a slightly larger surface tension than H6-DPM, the presence of the latter compound causes a significant effect on the dynamics of surface fluctuations, especially on their damping. In analogy to the concentration-dependent behavior observed for a monolayer of a highly amphiphilic ionic surfactant on the surface of water at ambient temperature, the orientation of H6-DPM molecules with respect to the surface appears to change from a preferentially perpendicular to a parallel alignment with increasing temperature. This demonstrates that viscoelastic effects including accompanied surface orientation effects can be resolved by SLS even for weakly asymmetric surface-active molecules such as H6-DPM in its diluted mixtures with very similar species.

Mixture modeling to simulate helium boiling: Helium gas bubble trapped in high magnetic field

Multi-phase flow hydrodynamics is an essential theoretical foundation for modernizing engineering and is important in guiding and developing modernizing engineering. In the realm of superconducting power technology, it is imperative to submerge and cool high-field superconducting magnets and superconducting cables, constructed from superconducting materials, in cryogenic fluids in order to maintain a superconducting state. Even minor thermal disturbances can induce boiling of the cryogenic fluid, leading to a two-phase flow regime. In the case of high-field superconducting magnets, the presence of liquid helium as the cooling medium gives rise to a diamagnetic effect. Consequently, the formation of a bubble resulting from boiling is confined in close proximity to the central aperture of the magnet owing to the influence of the magnetic field force. This confinement adversely affects the heat transmission properties of the magnet. This work presents a comprehensive analysis of the flow state during the boiling process of liquid helium, using the finite element approach and the mixture model. The heat transfer in a two-phase flow is analyzed by treating the system as a single pseudo-fluid. The model is consistent with empirical formulations of liquid helium boiling in large vessels and is also suitable for modeling the effect of heating surface orientation on the critical heat flux. Consequently, the analysis of the retention of helium bubbles generated by boiling in a high-field magnet has been further examined, using this particular model. The results of numerical hydrodynamic studies show that the width of the helium bubble stagnation zone is larger compared to the results of hydrostatic calculations. This poses a major challenge for superconducting magnets operating at high magnetic fields and high temperatures. The paper further analyses the effect of supercooled liquid helium on helium bubble stagnation, thus providing valuable insight into the construction of superconducting magnets with high magnetic fields.

Atomistic Wear Mechanisms in Diamond: Effects of Surface Orientation, Stress, and Interaction with Adsorbed Molecules.

Despite its unrivaled hardness, diamond can be severely worn during the interaction with others, even softer materials. In this work, we calculate from first-principles the energy and forces necessary to induce the atomistic wear of diamond and compare them for different surface orientations and passivation by oxygen, hydrogen, and water fragments. The primary mechanism of wear is identified as the detachment of the carbon chains. This is particularly true for oxidized diamond and diamonds interacting with silica. A very interesting result concerns the role of stress, which reveals that compressive stresses can highly favor wear, making it even energetically favorable.

The effect of surface orientation on early successional fouling communities in the southern gulf of Maine

Artificial environments have hard surfaces positioned at different orientations that attract a wide diversity of sessile invertebrate species, forming fouling communities. Fouling communities play a large role in the spread of a species introduction, as organisms gain purchase in artificial environments and use them as a stepping-stone into neighboring natural systems. These man-made systems harbor a large diversity of species, and there are often vastly different communities present on both vertical and horizontal surfaces. We used a species abundance dataset at three time periods spanning 30 years, collected from fouling panels in a high tidal flow estuary at the mouth of the Piscataqua River (New Castle, New Hampshire) in the southern Gulf of Maine to measure differences in community composition between horizontal and vertical panels. Early successional communities on settlement panels were photographed one year after installment in late summer to capture the highest diversity seasons. Organisms were then identified to the lowest taxonomic level, and communities on the underside of horizontal and vertical panels were compared. Vertical and horizontal communities from 1980 and 2004 were similar, while those from 2009 statistically differed from one another. Vertical and horizontal surfaces in the 1980s were dominated by the blue mussel Mytilus edulis, which declined in later years, and was replaced by invasive ascidians. Species diversity increased between the 1980 and 2004 sampling, but declined by the 2009 sampling. This study captures changes of species composition in early successional communities on vertical and horizontal surfaces at a single locale from three time periods spanning over 30 years.

The effect of surface treatment and orientation on fatigue crack growth rate and residual stress distribution of wire arc additively manufactured low carbon steel components

The directed energy deposition (DED) processes, such as laser metal deposition or Wire Arc Additive Manufacturing (WAAM), are gradually becoming the preferred method for fabrication of large-scale components using metal additive manufacturing (AM) technology. In this work, the possibility of fatigue life enhancement in WAAM built low carbon steel components, by means of rolling and laser shock peening surface treatment techniques, was investigated. A series of fatigue crack propagation tests were performed on surface treated ER70S-6 and ER100S-1 WAAM built specimens, and the results were analysed and compared with the untreated materials tested under the same loading conditions. The obtained results were interpreted in terms of the sensitivity of the cracking behaviour to the specimen orientation and extraction location. Furthermore, the residual stress profiles were measured, before and after applying the surface treatment techniques, and the effects of locked-in residual stresses on the fatigue performance of WAAM built components were discussed. Finally, a detailed texture analysis was performed on the surface treated and untreated regions of both WAAM built materials considered in this work. The obtained results from this study provide an insight into the advantages and disadvantages of various surface treatment techniques for fatigue life enhancement of WAAM built components with the view to extend the application of this advanced manufacturing technology to a wider range of industrial applications.

Visualization of Surface Charge Carrier Diffusion Lengths in Different Perovskite Crystal Orientations Using 4D Electron Imaging

AbstractUnderstanding charge carrier dynamics on the surface of materials at the nanometer and femtosecond scales is one of the key elements to optimizing the performance of light‐conversion devices, including solar cells. Unfortunately, most of the pump‐probe characterization techniques are surface‐insensitive and obtain information from the bulk due to the large penetration depth of the pulses. However, ultrafast scanning electron microscopy (USEM) is superior in visualizing carrier dynamics at the surface with high spatial‐temporal resolution. Here, the authors successfully used USEM to uncover the tremendous effect of surface orientations and termination on the charge carrier of MAPbI3 perovskite single crystals. Time‐resolved secondary electrons snapshots and density functional theory calculations clearly demonstrate that charge carrier diffusion, surface trap density, surface work function, and carrier concentration are strongly facet‐dependent. The results display a diffusion length of 22 micrometers within 6.0 nanoseconds along (001) orientation. While (100) facet forms defect states that prevent carrier diffusion and shows an increase in the surface work function leading to dark contrast and fast charge carrier recombination. These findings provide a new key component to optimizing the surface of perovskites, thus paving the way for even more efficient and stable solar‐cell devices based on perovskite single crystals.

Preferential adhesion of bacterial cells onto top- and bottom-mounted nanostructured surfaces under flow conditions.

The bactericidal effect of biomimetic nanostructured surfaces has been known for a long time, with recent data suggesting an enhanced efficiency of the nanostructured surfaces under fluid shear. While some of the influential factors on the bactericidal effect of nanostructured surfaces under fluid shear are understood, there are numerous important factors yet to be studied, which is essential for the successful implementation of this technology in industrial applications. Among those influential factors, the orientation of the nanostructured surface can play an important role in bacterial cell adhesion onto surfaces. Gravitational effects can become dominant under low flow velocities, making the diffusive transport of bacterial cells more prominent than the advective transport. However, the role of nanostructure orientation in determining its bactericidal efficiency under flow conditions is still not clear. In this study, we analysed the effect of surface orientation of nanostructured surfaces, along with bacterial cell concentration, fluid flow rate, and the duration of time which the surface is exposed to flow, on bacterial adhesion and viability on these surfaces. Two surface orientations, with one on the top and the other on the bottom of a flow channel, were studied. Under flow conditions, the bactericidal efficacy of the nanostructured surface is both orientation and bacterial species dependent. The effects of cell concentration, fluid flow rate, and exposure time on cell adhesion are independent of the nanostructured surface orientation. Fluid shear showed a species-dependent effect on bacterial adhesion, while the effects of concentration and exposure time on bacterial cell adhesion are independent of the bacterial species. Moreover, bacterial cells demonstrate preferential adhesion onto surfaces based on the surface orientation, and these effects are species dependent. These results outline the capabilities and limitations of nanostructures under flow conditions. This provides valuable insights into the applications of nanostructures in medical or industrial sectors, which are associated with overlaying fluid flow.

Computational study of surface orientation effect of wurtzite GaN on CH4 and CO sensing mechanism

The adsorption behaviors of gas molecules CH4 and CO on polar (0001), (000–1) facets and nonpolar (10-10), (11–20) facets of wurtzite GaN were explored in details by first-principles calculations. Among these important low-index facets, CH4 on the (0001) facet exhibits the lowest adsorption energy of −1.57 eV, signifying the highest density of adsorbed CH4 molecules under the same methane concentration in air. Meanwhile, the C–H bond is weakened revealed by the bond length elongation from 1.0956 Å to 1.1020 Å during adsorption, consistent with charge redistribution results. The adsorption energies for CO on the (0001) and (000–1) facets are calculated to be −2.17 eV and −3.89 eV, much lower than those on nonpolar facets. Obviously, the corresponding C–O bond is activated during adsorption, proved by the bond length elongated from 1.1531 Å to 1.2003 Å and 1.1715 Å, respectively, in line with charge redistribution results. Particularly, the co-adsorption simulation verifies the surface redox reaction between CO and O2 on + c-plane GaN. Therefore, we predict superior performance of (0001) GaN in detection for high concentration CH4 and (000–1) GaN in selective detection for low concentration CO.

The effect of Pt surface orientation on the oscillatory electro-oxidation of glycerol

• The anion (bi)sulfate of the electrolyte exerts a strong influence on the electro-oxidation of glycerol on basal and stepped Pt single crystal surfaces. • Cyclic voltammetry reveals that when there is an increase in sulfate concentration, the glycerol oxidation current density decreases. • Chronoamperometric studies showed that Pt(110) and stepped surfaces exhibited greater ability to catalyze the glycerol electro-oxidation than Pt(111). • Emergence of potential oscillations obtained during glycerol oxidation over Pt( hkl ) and stepped surfaces highlights different mechanisms of the reaction. In the present paper, we have studied the influence of (bi)sulfate anion (0.1 and 0.5 M) on the electro-oxidation of glycerol on basal Pt( hkl ) and stepped surfaces belonging to the series of Pt(S)[ n (111) x (111)]. Cyclic voltammograms and derivative voltammetry pointed out that the catalytic activity decreases for Pt(111) and Pt(110) and, to a minor extent, for stepped surfaces in 0.5 mol L -1 H 2 SO 4. Chronoamperometric curves demonstrated that above 0.60 V ( vs. RHE), for both concentrations (0.1 and 0.5 mol L -1 H 2 SO 4 ), stepped surfaces and Pt(110) showed greater ability to catalyze the glycerol electro-oxidation in comparison with Pt(111). Potential oscillations were mapped along with slow galvanodynamic sweeps and studied at constant current. For Pt(111), no oscillations were found in the galvanodynamic regime, however, under the galvanostatic regime, period 1 oscillations were observed after a long induction period. The oscillations showed a very similar profile for stepped surfaces, even for the Pt(332) surface, which has a high density of (110) steps. Pattern changes were observed only for Pt(110) compared to other surfaces. Therefore, we conclude that (110) step sites influence the oscillatory behavior, thus the insertion of the steps favors the path of formation of inactive species, which compete for the same catalytic sites in a given potential region. The extinction of the mechanism oscillatory occurs differently due to the intrinsic characteristics of each surface electrode for the formation of (hydro)oxides.

Surface Space‐Charge Potential and Oxygen Surface Exchange Coefficient of SrTiO3: The Effect of Surface Orientation

Abstract18O2/16O2 exchange experiments on terminated (100), (110), and (111) oriented samples of single‐crystal SrTiO3 are used to study the effect of surface orientation on the oxygen surface exchange coefficient and the surface space‐charge potential Φ0. Isotope exchanges are carried out at temperatures 922 < T/K < 1073 at an oxygen activity aO2 = 0.2 in dry conditions (aH2O < 10−7.7); isotope profiles are measured subsequently by secondary ion mass spectrometry. Isothermal values are found to vary by less than one order of magnitude, with . All data are consistent with a change in the activation enthalpy of surface exchange from 3.0 eV at high temperatures to 1.8 eV at lower temperatures. The transition is attributed to a change from a charge‐transfer mechanism at high temperatures to a mechanism involving surface OH species. The transition temperature depends on the surface orientation, Ttr(111) ≈ Ttr(110) > Ttr(100). Values of Φ0 obtained varied between 0.50 V and 0.37 V, with isothermal values obeying Φ0(111) ≈ Φ0(110) < Φ0(100). Φ0 data is analysed to obtain the OH− surface coverage for the low‐temperature regime. Similarities between the behavior of Φ0 and that of are highlighted.