Oxygen Adatoms Research Articles

16O2 – 18O2 interface exchange study between gas phase and the BaFeO3–δ oxide

Studies of oxygen surface exchange kinetics for BaFeO3–δ oxide were performed using the oxygen isotope exchange method with pulsed supply of an isotopically enriched mixture (PIE) at the partial oxygen pressure 21.3 kPa in the temperature range of 350–600 °С. Oxygen surface exchange kinetics was considered in the framework of two-step model including two consecutive steps: dissociative adsorption of oxygen and incorporation of oxygen adatoms into the crystal lattice of the oxide. The rates of oxygen heterogeneous exchange (rH), as well as the rates of dissociative adsorption (ra) and oxygen incorporation (ri), have been calculated. The process of oxygen dissociative adsorption at the surface of BaFeO3–δ oxide was found to be the rate-determining step of the surface exchange. The appropriate models describing the oxygen exchange kinetics and possible mechanisms occurring in the system “gaseous oxygen – BaFeO3–δ oxide” were discussed.

Wavelength-Dependent Activity of Oxygen Species in Propane Conversion on Rutile TiO2(110).

Photocatalytic oxidative dehydrogenation of propane (C3H8) into propene (C3H6) under mild conditions holds great potential in the chemical industry, but understanding how active species participate in C3H8 conversion remains a significant challenge. Here, the wavelength-dependent activities of bridging oxygen (Ob2-) and the Ti5c-bound oxygen adatom (OTi2-) of model rutile (R) TiO2(110) in C3H8 conversion have been investigated. Under 257 and 343 nm irradiation, hole-trapped OTi- and Ob- can abstract the hydrogen atom of C3H8, forming the CH3CH•CH3 radical and C3H6. However, the rate of C3H8 conversion with hole-trapped Ob- is strongly dependent on the wavelength, primarily producing the C3H7• radical. In the case of hole-trapped OTi-, C3H6 is the main product, which is nearly independent of wavelength. The differences in the wavelength-dependent activity and product selectivity are likely due to dynamic control rather than thermodynamic control. The result provides a deeper understanding of the dynamic processes involved in the conversion of light alkanes in TiO2 photocatalysis.

Multiscale functionalized graphene origami metamaterials enable programming thermoelectric performance of flexible energy harvesters

Origami engineering stands out as one of the prominent approaches for fine tuning the multiphysical properties of energy materials. Employing first-principle calculations at the nanoscale, we explore the mechano-thermoelectric properties of graphene origami metamaterials functionalized by hydrogen (H), nitrogen (N), oxygen (O), and fluorine (F) adatoms. Our computational results reveal that functionalizing carbon (C) atoms induce buckling in the pristine graphene, transitioning carbon atoms from sp2 (for pristine graphene sheet) to hybridized sp3 orbital to fold the graphene origami. The functionalized graphene origami metamaterials, adopting the Miura-ori architecture, exhibit anisotropic mechanical properties and an unprecedented negative Poisson's ratio (NPR) of up to −0.8. Non-symmetrical functionalization breaks the inherent symmetry of graphene, resulting in a robust non-zero electronic band gap. This characteristic makes the graphene origami metamaterial a compelling contender for thermoelectric applications, since they offer an electronic figure of merit that exceed 200 at room temperature (Graphene-H), surpassing that of the majority of 2D thermoelectric materials can offer. Resorting to the finite element approach, we explore the origami-based thermoelectric legs in nanogenerators at mesoscale. The folding capability of origami architectures offer tunable heat insulation in order to realize next generation of thermoelectric legs. Compared to conventional cuboid leg thermoelectric nanogenerators, thermoelectric conversion efficiency of origami metamaterials can be two orders of magnitude higher; an enhancement that is significantly governed by their folding angle. The underlying principles governing the multiphysics performance of multiscale origami-based thermoelectric materials impart novel design concepts for customizing the thermoelectric performance of shape-changing low-dimensional metamaterials and energy-related devices.

Cu@CuOx/WO3 with photo-regulated singlet oxygen and oxygen adatoms generation for selective photocatalytic aromatic amines to imines

Photocatalysts can absorb light and activate molecular O2 under mild conditions, but the generation of unsuitable reactive oxygen species often limits their use in synthesizing fine chemicals. To address this issue, we disperse 1 wt% copper on tungsten trioxide (WO3) support to create an efficient catalyst for selective oxidative coupling of aromatic amines to imines under sunlight irradiation at room temperature. Copper consists of a metallic copper core and an oxide shell. Experimental and density functional theory calculations have confirmed that Cu2O is the primary activation site. Under λ < 475 nm, the light excites electrons of the valence bands in Cu2O and WO3, which activate O2 to superoxide radical •O2−. Then rapidly transforms into oxygen adatoms (•O) and oxygen anion radicals (•O−) species on the surface of Cu2O. Simultaneously, it is captured by holes in the WO3 valence band to generate singlet oxygen (1O2). •O bind to 1O2 promoting the coupling reaction of amines. When λ > 475 nm, intense light absorption due to the localized surface plasmon resonance excites numerous electrons in Cu to promote the oxidative coupling with the adsorbed O2. This study presents a promising approach towards the design of high-performance photocatalysts for solar energy conversion and environmentally-friendly oxidative organic synthesis.

Tip-activated single-atom catalysis: CO oxidation on Au adatom on oxidized rutile TiO2 surface.

Single-atom catalysis of carbon monoxide oxidation on metal-oxide surfaces is crucial for greenhouse recycling, automotive catalysis, and beyond, but reports of the atomic-scale mechanism are still scarce. Here, using scanning probe microscopy, we show that charging single gold atoms on oxidized rutile titanium dioxide surface, both positively and negatively, considerably promotes adsorption of carbon monoxide. No carbon monoxide adsorption is observed on neutral gold atoms. Two different carbon monoxide adsorption geometries on gold atoms are identified. We demonstrate full control over the redox state of adsorbed gold single atoms, carbon monoxide adsorption geometry, and carbon monoxide adsorption/desorption by the atomic force microscopy tip. On charged gold atoms, we activate Eley-Rideal oxidation reaction between carbon monoxide and a neighboring oxygen adatom by the tip. Our results provide unprecedented insights into carbon monoxide adsorption and suggest that the gold dual activity for carbon monoxide oxidation after electron or hole attachment is also the key ingredient in photocatalysis under realistic conditions.

Identifying and clearing individual oxygen impurities on graphene through the use of NO2 as a radical scavenger

We present a novel method to detect and eliminate oxygen adatoms on a graphene basal plane, which can be observed as individual single-electron transfer (SET) events through graphene Hall measurements. The central idea, supported by first-principles calculations, is to use NO2 as a radical scavenger, effectively targeting and removing oxygen impurities on graphene. The reaction between NO2 and the oxygen adatom produces a negatively charged NO3− species and creates a hole carrier in graphene. Subsequently, NO3− reacts with NO2 to form N2O5, accompanied by an electron back-donation to graphene. The thermal decomposition of N2O5 produces neutral NO3, which can accept an electron from graphene and return to the NO3− state. Under conditions of low oxygen impurity levels and NO2 partial pressures, these SET reactions induce intermittent, step-like changes in the graphene's Hall resistivity ρxy at room temperature. Notably, the simulated ρxy patterns during NO2 adsorption closely resemble previously observed patterns [Schedin et al., Nat. Mater. 6 (2007) 652–655], suggesting a connection to the proposed SET reactions for unintentionally introduced oxygen impurities. Furthermore, we find that NO2 also reacts with hydroxyl and peroxide groups on the graphene basal plane, removing them by forming HNO3 and N2O5, respectively. Our findings offer a promising approach to remove oxygen-containing functional groups from graphene, paving the way for obtaining high-quality graphene from reduced graphene oxide.

Atomic-Scale Insights into the Interlayer Characteristics and Oxygen Reactivity of Bilayer Borophene.

Bilayer (BL) two-dimensional boron (i.e., borophene) has recently been synthesized and computationally predicted to have promising physical properties for a variety of electronic and energy technologies. However, the fundamental chemical properties of BL borophene that form the foundation of practical applications remain unexplored. Here, we present atomic-level chemical characterization of BL borophene using ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS). UHV-TERS identifies the vibrational fingerprint of BL borophene with angstrom-scale spatial resolution. The observed Raman spectra are directly correlated with the vibrations of interlayer boron-boron bonds, validating the three-dimensional lattice geometry of BL borophene. By virtue of the single-bond sensitivity of UHV-TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres in UHV. In addition to providing fundamental chemical insight into BL borophene, this work establishes UHV-TERS as a powerful tool to probe interlayer bonding and surface reactivity of low-dimensional materials at the atomic scale.

Atomic‐Scale Insights into the Interlayer Characteristics and Oxygen Reactivity of Bilayer Borophene

AbstractBilayer (BL) two‐dimensional boron (i.e., borophene) has recently been synthesized and computationally predicted to have promising physical properties for a variety of electronic and energy technologies. However, the fundamental chemical properties of BL borophene that form the foundation of practical applications remain unexplored. Here, we present atomic‐level chemical characterization of BL borophene using ultrahigh vacuum tip‐enhanced Raman spectroscopy (UHV‐TERS). UHV‐TERS identifies the vibrational fingerprint of BL borophene with angstrom‐scale spatial resolution. The observed Raman spectra are directly correlated with the vibrations of interlayer boron–boron bonds, validating the three‐dimensional lattice geometry of BL borophene. By virtue of the single‐bond sensitivity of UHV‐TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres in UHV. In addition to providing fundamental chemical insight into BL borophene, this work establishes UHV‐TERS as a powerful tool to probe interlayer bonding and surface reactivity of low‐dimensional materials at the atomic scale.

Chemically identifying single adatoms with single-bond sensitivity during oxidation reactions of borophene

The chemical interrogation of individual atomic adsorbates on a surface significantly contributes to understanding the atomic-scale processes behind on-surface reactions. However, it remains highly challenging for current imaging or spectroscopic methods to achieve such a high chemical spatial resolution. Here we show that single oxygen adatoms on a boron monolayer (i.e., borophene) can be identified and mapped via ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) with ~4.8 Å spatial resolution and single bond (B–O) sensitivity. With this capability, we realize the atomically defined, chemically homogeneous, and thermally reversible oxidation of borophene via atomic oxygen in UHV. Furthermore, we reveal the propensity of borophene towards molecular oxygen activation at room temperature and phase-dependent chemical properties. In addition to offering atomic-level insights into the oxidation of borophene, this work demonstrates UHV-TERS as a powerful tool to probe the local chemistry of surface adsorbates in the atomic regime with widespread utilities in heterogeneous catalysis, on-surface molecular engineering, and low-dimensional materials.

Generalized Poisson-Nernst-Planck-Based Physical Model of the O2∣LSM∣YSZ Electrode

The paper presents a generalized Poisson-Nernst-Planck model of an yttria-stabilized zirconia electrolyte developed from first principles of nonequilibrium thermodynamics which allows for spatial resolution of the space charge layer. It takes into account limitations in oxide ion concentrations due to the limited availability of oxygen vacancies. The electrolyte model is coupled with a reaction kinetic model describing the triple phase boundary with electron conducting lanthanum strontium manganite and gaseous phase oxygen. By comparing the outcome of numerical simulations based on different formulations of the kinetic Eqs. with the results of EIS and CV measurements we attempt to discern the existence of separate surface lattice sites for oxygen adatoms and surface oxides from the assumption of shared ones. Moreover, we show that the mass-action kinetics model is sensitive to oxygen partial pressure unlike exponential kinetics models. The resulting model is fitted to a dataset of EIS and CVs spanning multiple temperatures and pressures, using various relative weights of EIS and CV data in the fitness function. The model successfully describes the physics of the interface around the OCV.

Charge State Tristability of Oxygen Adatom on a Rutile TiO2(110)–(1 × 1) Surface Controlled by Atomic Force Microscopy

Charge State Tristability of Oxygen Adatom on a Rutile TiO₂(110)–(1 × 1) Surface Controlled by Atomic Force Microscopy

Toward Functionalized Ultrathin Oxide Films: The Impact of Surface Apical Oxygen

AbstractThin films of transition metal oxides open up a gateway to nanoscale electronic devices beyond silicon characterized by novel electronic functionalities. While such films are commonly prepared in an oxygen atmosphere, they are typically considered to be ideally terminated with the stoichiometric composition. Using the prototypical correlated metal SrVO3 as an example, it is demonstrated that this idealized description overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not explicitly exposed to air, are shown to severely affect the intrinsic electronic structure of a transition metal oxide film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in the thin films. These findings highlight that it is important to take into account surface apical oxygen or—mutatis mutandis—the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of transition metal oxides near the single unit‐cell limit.

Electronic state changes of Cu nanoparticles on rutile TiO2(110) by O2 and CO exposure

In order to investigate the strong catalytic activities of Cu nanoparticles, we have measured the electronic states of oxygen and carbon monoxide-exposed Cu nanoparticles on the rutile TiO2(110) surfaces by synchrotron radiation photoelectron spectroscopy. After depositing Cu on the oxygen rich TiO2(110) (O-TiO2) surface, the Ti 3d state appeared due to the disappearance of the oxygen adatoms by reaction with Cu. After further oxygen exposure, Cu 3d state drastically changed. This result indicates that Cu2O is formed by oxidation of Cu nanoparticles. We then dosed a CO gas onto the oxidized Cu/O-TiO2(110) surface and found that the work function slightly decreases and elimination of O adatoms is confirmed by medium energy ion scattering. These results obtained clearly shows the reaction of CO with O adatoms adsorbed on Cu/TiO2 probably to form CO2.

Kinetic Assessment of the Dry Reforming of Methane over a Ni–La2O3Catalyst

The dry reforming of methane is a promising technology for the abatement of CH4 and CO2. Ni–La2O3 catalysts are characterized by their long-term stability (100 h) when tested at full conversion. The kinetics of dry reforming over these types of catalysts has been studied using both power-law and Langmuir–Hinshelwood-based approaches. However, these studies typically deal with fitting the net CH4 rate, hence disregarding competing and parallel surface processes and the different possible configurations of the active surface. In this work, we synthesized a Ni–La2O3 catalyst and tested six Langmuir–Hinshelwood mechanisms considering both single and dual active sites for assessing the kinetics of dry reforming and the competing reverse water–gas shift reaction and investigated the performance of the derived kinetic models. In doing this, it was found that: (1) all of the net rates were better fitted by a single-site model that considered that the first C–H bond cleavage in methane occurred over a metal–oxygen pair site; (2) this model predicted the existence of a nearly saturated nickel surface with chemisorbed oxygen adatoms derived from the CO2 dissociation; (3) the CO2dissociation can either be an inhibitory or an irrelevant step, and it can also modify the apparent activation energy for CH4 activation. These findings contribute to a better understanding of the dry reforming reaction’s kinetics and provide a robust kinetic model for the design and scale-up of the process.

Crystal plane‐dependent ethanol gas sensing of ZnO studied by low‐energy He+ ion scattering combined with pulsed jet technique

Although a semiconductor gas sensor is promising in the future gas sensing technology, the material design for highly sensitive and selective gas sensing is quite challenging. The crystal plane dependence of gas sensing has been studied using various nano‐crystal samples with a controlled crystal facet to obtain a clue for designing the sensing material, but the contact effect between nano‐crystal particles has often hindered the straightforward analysis. In the present study, the crystal plane dependence of ethanol (EtOH) sensing by ZnO, one of the most widely studied target gas‐sensing material combinations, was investigated from an analytical approach. We applied a newly developed low‐energy He+ ion scattering spectroscopy (He+ LEIS) combined with the pulsed jet technique for analyzing the surface structure of ZnO during the EtOH sensing. In this novel technique, the free gas jet is periodically blown onto the sample surface to simulate the gas‐sensing surface in a vacuum as one under the realistic working condition, while the background pressure is maintained low enough for operating He+ LEIS. It was found that the divalent oxygen adatom on the outermost surface is consumed during the EtOH sensing while no change occurs in the concentration of the lattice oxygen on the Zn‐terminated (0001) surface. The availability of this oxygen adsorbate determines the dissociative EtOH adsorption probability on the surface; hence it determines the EtOH sensing sensitivity, which is substantially higher on Zn‐terminated (0001) than other dominant low‐index surfaces, such as O‐terminated (000 ), (10 0), and (11 0).

Electromigration-triggered programmable droplet spreading

Controlled spreading of droplet on solid surface bursts out of strong applications from micro-fluidic sensors to artificial intelligence settings. Recently, to extend its application range, reversible spreading has been realized by introducing external stimulate to change the direction of wetting gradient. Here, we propose a new strategy for temporospatial control of droplet spreading (along or against the wetting gradient) by mediating current intensity (not current direction as previous reported) via the electromigration of oxygen adatom. The results indicate that, during the electromigration process, two gradients, i.e., wetting gradient and temperature gradient, are formed and the battle between these two gradients leads to the formation of an equilibrium position. No matter where droplet is dripped, it will spread toward this equilibrium position. More importantly, the equilibrium position could be easily adjusted by current intensity and predicted via our theoretical model, i.e., the spreading behaviour of droplet could be programed via current intensity. We envision this new strategy of programmable spreading of droplet could be valuable in practical applications involving fluidic transport, such as microreactor.

Guiding the design of oxidation-resistant Fe-based single atom alloy catalysts with insights from configurational space.

The high activity and selectivity of Fe-based heterogeneous catalysts toward a variety of reactions that require the breaking of strong bonds are offset in large part by their considerable instability with respect to oxidative deactivation. While it has been shown that the stability of Fe catalysts is considerably enhanced by alloying them with precious metals (even at the single-atom limit), rational design criteria for choosing such secondary metals are still missing. Since oxidative deactivation occurs due to the strong binding of oxygen to Fe and reduction by adsorbed hydrogen mitigates the deactivation, we propose here to use the binding affinity of oxygen and hydrogen adatoms as the basis for rational design. As it would also be beneficial to use cheaper secondary metals, we have scanned over a large subset of 3d-5d mid-to-late transition metal single atoms and computationally determined their effect on the oxygen and hydrogen adlayer binding as a function of chemical potential and adsorbate coverage. We further determine the underlying chemical origins that are responsible for these effects and connect them to experimentally tunable quantities. Our results reveal a reliable periodic trend wherein oxygen binding is weakened greatest as one moves right and down the periodic table. Hydrogen binding shows the same trend only at high (but relevant) coverages and otherwise tends to have its binding slightly increased in all systems. Trends with secondary metal coverage are also uncovered and connected to experimentally tunable parameters.

Stochastic model of oxygen adsorption

The authors propose a microscopic stochastic model of the process of oxygen adsorption on the surface of a solid object. As a first approximation, the constructed model describes the dynamics of the system in terms of a planar surface coating with oxygen adatoms. A simplified diagram of the adsorbent surface oxidation mechanism is presented in five stages. The reaction conditions are determined by the temperature of the crystal surface and the partial pressure of the reagent in the gas phase. The reaction is considered as a random Markov process with a discrete set of states for a stream of elementary events occurring on a fragment of an ideal square lattice of ferrous sulfide with periodic boundary conditions. For the constructed model, numerical calculations are done using a software solution and the results of computational experiments are presented. The research results analysis suggests that the presented mathematical process model of oxygen adsorption on the ferrous sulfide surface along with the software solution for simulation studies can be used to determine control parameters for various technological processes.

Seed layer mediated wettability and wettability transition of ZnO nano/micro-rod arrays

ZnO nano/micro-rod arrays are grown on metal (In, Sn) and metal oxide (In2O3, SnO and ZnO) thin films as seed layers, by hydrothermal processing. The ZnO nano/micro-rod arrays are aligned vertically at different angles and polycrystalline in nature. The hydrothermal processing time required to form nano/micro rods is shorter (30 min) on the oxide seed layers than on the metal layers (180 min). The rods on In, Sn, thermally oxidized In and Sn have horizontal dimensions of the order of 500–1000 nm with height up to 15 μm, indicating large aspect ratio. In contrast, the rods are 100–200 nm in diameter on ZnO seed layer with length up to 2 μm and higher density of packing. Wettability studies demonstrate that the arrays are hydrophobic in nature with water contact angle in the range of 90–110° on all seed layers except ZnO. The arrays are superhydrophobic on ZnO with a contact angle of 150.4°. Significantly, there is a transition to the hydrophilic/superhydrophilic state under the influence of UV irradiation. The transition is fastest on ZnO seed layers followed by oxidized Sn and In and slowest on the metal layers, suggesting promise for self-cleaning applications. The transition from hydrophobic to hydrophilic behaviour is correlated with the presence of oxygen vacancies, defects and oxygen adatoms on the surface, using photoluminescence and Raman spectroscopy studies.

Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy

Interaction of molecular oxygen with semiconducting oxide surfaces plays a key role in many technologies. The topic is difficult to approach both by experiment and in theory, mainly due to multiple stable charge states, adsorption configurations, and reaction channels of adsorbed oxygen species. Here we use a combination of noncontact atomic force microscopy (AFM) and density functional theory (DFT) to resolve [Formula: see text] adsorption on the rutile [Formula: see text](110) surface, which presents a longstanding challenge in the surface chemistry of metal oxides. We show that chemically inert AFM tips terminated by an oxygen adatom provide excellent resolution of both the adsorbed species and the oxygen sublattice of the substrate. Adsorbed [Formula: see text] molecules can accept either one or two electron polarons from the surface, forming superoxo or peroxo species. The peroxo state is energetically preferred under any conditions relevant for applications. The possibility of nonintrusive imaging allows us to explain behavior related to electron/hole injection from the tip, interaction with UV light, and the effect of thermal annealing.