Surface Oxygen Atoms Research Articles

HRTEM‐EELS cross‐sectional structural analyses of glassy carbon substrate irradiated by platinum ions using a coaxial arc plasma gun

A glassy carbon (GC) substrate was irradiated by Pt ions using a coaxial arc plasma gun in vacuum. The structure of the substrate was studied in the atomic scale using high‐resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) with a thin foil specimen prepared by cross‐sectioning using focused ion beam milling. HRTEM combined with the EELS measurements of the cross‐sectioned sample indicated a disordering of the GC substrate surface and the detection of phenol‐type species, carbon atoms in aliphatic chain molecules, and carboxyl groups, which are considered to have been formed by chemical reaction between the carbon atoms of the GC substrate surface and hydrogen or oxygen atoms. Penetrated Pt atoms were observed inside the GC substrate, and the surface position of the GC substrate was located from the depth profiles of the EELS spectra. STEM‐EELS analyses of a pristine GC substrate without Pt ion irradiation were also conducted for reference.

Density Functional Theory and Molecular Dynamics insights into the site-dependent adsorption of hydrogen fluoride on kaolinite

The removal of fluoride from water using adsorption on a low-cost mineral such as kaolinite is an important research topic. The present communication uses Density Functional Theory (DFT) and Molecular Dynamics (MD) based simulations to comprehensively investigate the adsorption behaviour of HF, one of the most predominant species of fluoride in the aqueous solutions, on the (0 0 1) surface of kaolinite. The optimum geometric configurations of adsorption of HF on different possible sites on the mineral surface have been established, and the corresponding adsorption energies, bond distances, Partial Density of States (PDOS), Mulliken charges and electron density difference plots have been analyzed to understand the interaction at each site. To explore the site-dependent adsorption behaviour, the interaction of HF has been investigated on different sites such as aluminium centres, surface oxygen atoms (both lying and upright) and cavity site (void space encircled by six aluminium centres). The analysis shows that the F atom has a strong tendency to form hydrogen bonds with the surface H atom on kaolinite. Among the three aforementioned types of adsorption sites considered, the “cavity” site is found to offer the greatest adsorption strength. Further, MD simulations have been undertaken to explain the effect of water on the adsorption and substantiate the bonding mechanism.

Passivating Surface States on Water Splitting Cuprous Oxide Photocatalyst with Bismuth Decoration.

To enhance the visible light photocatalystic activity of Cu2O(100) surface, we performed first-principles calculations on the structural, electronic and optical properties of a bismuth (Bi)-decorated Cu2O(100) surface (Bi@Cu2O(100)). It is shown that the Bi prefer to be loaded to the hollow sites among four surface oxygen atoms and tend to individual dispersion instead of aggregating on the surface due to the lowest formation energy and larger distance between two Bi atoms at the surface than the Bi clusters; the coverage of around 0.25 monolayer Bi atoms can effectively eliminate the surface states and modify the band edges to satisfy the angular momentum selection rules for light excited transition of electrons, and the loaded Bi atoms contribute to the separation of photogenerated electron-holes. The relative positions between the band edges and the redox potentials are suitable for photocatalytic hydrogen production from the redox water, and moreover, the optical absorption spectrum indicates a positive response of the Bi0.25@Cu2O(100) to visible light, implying that the Bi0.25@Cu2O(100) is a promising visible light photocatalyst.

Aggregation Patterns in Low- and High-Charge Anions Define Opposite Solubility Trends.

Molecular dynamics simulations in aqueous solution reveal the existence of two distinct patterns of aggregation in low and high charge density Lindqvist-type polyoxometalates (POMs). Our results indicate the presence of contact and solvent-shared ion pairs and specific and preferential interactions of alkalis with POMs. Highly charged POMs are capable of breaking apart the Li+ and Cs+ solvation shell, thus enhancing the formation of long-lived alkali-POM contact ion pairs, where alkalis act as an electrostatic "glue" forming large oligomers. Stronger ion pair interactions for Li+ than for Cs+ promote lower solubility for Li+ than for Cs+, evoking anomalous solubility trends. Lower charge density POMs are not capable of disrupting the Li+ solvation shell and only solvent-shared ion pairs are formed, whereas for Cs+, contact ion pairs exist. The large number of oxygen atoms in the POM surface enhances the hydrogen bonds between POM and water, thus promoting aggregation. In this case, aggregation follows normal solubility trends. Thus, aggregation depends on the strength of ion pair interactions, the capacity of POM to disrupt alkali's solvation shell, and the contact surface area between the solvent and POM.

Investigating Charge Transfer Interactions in AuCe2On Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory.

The properties of small cerium oxide and gold-cerium oxide clusters were explored as analogues for gold deposition at defect sites on a cerium oxide surface. Ce2On (n = 0-2) and AuCe2On (n = 0-2) clusters were prepared in the gas phase and investigated using photoionization efficiency spectroscopy complemented by spectral simulations based on DFT calculations; purely theoretical investigations were conducted on the Ce2O3, Ce2O4, AuCe2O3, and AuCe2O4 clusters due to these species not being detected. The optimized AuCe2On (n = 0-3) cluster geometries are consistent with Au adsorption to oxygen vacancy sites while the AuCe2O4 cluster correlates with Au adsorption to a CeO2 vacancy site. The electronic properties of the adsorbed Au atom depend strongly on the nature of the ceria adsorption site: O vacancy-adsorbed Au is negatively charged with a Ce → Au charge transfer occurring at the adsorption interface, whereas Au adsorbed to a CeO2 vacancy is positively charged with an Au → Ce charge transfer. The adsorbed Au atom is proposed to enhance the catalytic properties of the AuCe2On cluster by (i) stabilizing the negatively charged Au atom on reduced AuCe2On clusters to enhance nucleophilicity; (ii) increasing the electron accepting capability of the AuCe2O4 species; (iii) destabilizing the HOMO of the AuCe2O4 cluster; and (iv) facilitating the abstraction of additional surface oxygen atoms by reactants.

Initial Stages of Metal Films Growth on a SiO2-Cristobalite Surface

Using the density functional theory approach and pseudopotentials we studied energetics and electron structures of metal layers (Al, Cu, Ni, and Cr) deposited on a cristobalite surface. We have found that the properties of the first adsorbed layers decide by interaction of metal atoms with oxygen atoms of a substrate surface. Aluminum as an easily oxidized metal, is characterized by the high adhesion, it is followed by nickel and chromium, and copper closes the studied group of metals. Further plating is characterized by significant reduction of binding energy; the properties of films tend to properties of bulk metals

Atomistic Simulations of Plasma-Enhanced Atomic Layer Deposition.

Plasma-enhanced atomic layer deposition (PEALD) is a widely used, powerful layer-by-layer coating technology. Here, we present an atomistic simulation scheme for PEALD processes, combining the Monte Carlo deposition algorithm and structure relaxation using molecular dynamics. In contrast to previous implementations, our approach employs a real, atomistic model of the precursor. This allows us to account for steric hindrance and overlap restrictions at the surface corresponding to the real precursor deposition step. In addition, our scheme takes various process parameters into account, employing predefined probabilities for precursor products at each Monte Carlo deposition step. The new simulation protocol was applied to investigate PEALD synthesis of SiO2 thin films using the bis-diethylaminosilane precursor. It revealed that increasing the probability for precursor binding to one surface oxygen atom favors amorphous layer growth, a large number of –OH impurities, and the formation of voids. In contrast, a higher probability for precursor binding to two surface oxygen atoms leads to dense SiO2 film growth and a reduction of –OH impurities. Increasing the probability for the formation of doubly bonded precursor sites is therefore the key factor for the formation of dense SiO2 PEALD thin films with reduced amounts of voids and –OH impurities.

Adsorption characteristics of glycols on calcite and hematite

AbstractMonoethylene glycol, diethylene glycol, and triethylene glycol can be used for hydrate prevention during transport of liquids and gases, in dehumidifiers, for controlling growth of hematite nanoparticles and in the precipitation of different calcium carbonate morphologies. In all these applications, glycols may interact with calcite or hematite or both. Molecular dynamics and electronic structure calculations were used to investigate glycol adsorption on these surfaces. All glycols primarily adsorbed with hydroxyl hydrogen atoms to surface oxygen atoms. Glycol hydroxyl oxygen atoms coordinated toward surface calcium atoms and iron atoms of calcite and hematite, respectively. Glycols generally had a stronger affinity toward un‐hydroxylated hematite. However, hydroxylation of hematite reduced the absolute value of glycol adsorption free energies considerably. Molecular conformations changed during adsorption, which affected the free energy profiles. Adsorbed glycols resulted in a dipole moment pointing away from the surfaces.

Interaction Mechanism of Ferric Ions with Celestite Surface and Implications for Flotation Recovery

In practical celestite flotation, iron contamination is commonly found on celestite surfaces. The effect of ferric ions on celestite flotation was assessed by a combination of ion release experiments, DFT calculation, X-ray photoelectron spectroscopy (XPS) analysis, adsorption isotherm study, and flotation experiments in this work. The ion release experiments showed that the associated limonite released ferric ions to solution. According to DFT calculation and surface complexation theory, we found that ≡SrOH0 and ≡SO4H0 are primary functional groups on celestite surface in aqueous environments. The XPS analysis and adsorption isotherm study revealed that ferric ions mainly adsorbed on celestite surface by complexing with two oxygen atoms of surface ≡SrOH0 groups to form ≡Sr–O–Fe–OH precipitates. Flotation results showed that ferric ions strongly depressed celestite flotation. Combined with the change in surface properties determined by XPS, it can be concluded that the adsorption of ferric ions on celestite surface decreased adsorption sites for the collector, and hence, led to depression on celestite flotation.

Molecular Dynamics Simulations of the DNA Radiation Damage and Conformation Behavior on a Zirconium Dioxide Surface

This work is aimed on a complex study of the DNA immobilization and conformation processes on the zirconium dioxide (ZrO2) surface. The DNA+ZrO2 nanoparticles and nanosized films were investigated with the molecular dynamics (MD) modeling, experimental spectral and integral methods, including nuclear physics. Using the MD hybrid classical and quantum chemistry potentials, for the DNA solvated with water the DNA+ZrO2 surface interactions were simulated We have generated series MD models, thereby simulating a different scenario of the DNA with possible charge modifications. The DNA charge modification were introduced in the DNA central region via its two phosphorus atoms, Pa and Pb, and for several set of MD models for the relaxed DNA structures we have estimated the positional changes of the distance D[DNA(Pa,Pb) - ZrO2(O)] between the phosphorus atoms (Pa,Pb) and selected oxygen atoms of the ZrO2 surface. The work is aimed to the development of functional heterojunctions such as a biological molecule - wide-gap dielectric. These heterojunctions are intended for useing in the field of molecular electronics, in particular, for the creation of biochips, memory arrays and computer architectures of the future.

Specific ion effects of Cu2+, Ca2+ and Mg2+ on montmorillonite aggregation

Differences in ion-surface reactions among various divalent cations may result in large differences in clay particle interaction behaviors. Specific ion effects of Cu2+, Ca2+ and Mg2+ in montmorillonite (Mt) aggregation were studied by laser light scattering. The aggregation rate in the presence of Cu2+ was much higher than that in the presence of Ca2+ and Mg2+. The critical coagulation concentrations (CCCs) were Cu2+ (0.650 mM)<<Ca2+ (2.38 mM)<Mg2+ (7.99 mM). The differences in aggregation rates and CCC values were attributed to non-electrostatic interaction forces between the cations and the clay mineral surface. These non-electrostatic forces generated from (i) non-classical polarization of cations in the strong electric field near the clay mineral surface; and (ii) the quantum-mechanical interaction between the strongly polarized cations, especially for the d-type orbital electronic configuration of Cu2+, and the strongly polarized surface oxygen atoms. It was assumed that a new, polarization-induced covalent bond formed between Cu2+ and the surface O.

Lithium Doping Y2O3: A Highly Efficient Solid Base Catalyst for Biodiesel Synthesis with Excellent Water Resistance and Acid Resistance

A series of nLi–Y2O3 catalysts with different proportion of lithium oxide were prepared by wet impregnation method, the 0.1Li–Y2O3 catalyst exhibited highly efficient catalytic activity in biodiesel synthesis. The as-prepared catalysts were characterized by XRD, SEM, TEM, CO2–TPD and XPS techniques. The results indicated that the significant change in the basicity of Y2O3 with the addition of Li was attributed to the strong interaction produced by electron transfer between the components of the 0.1Li–Y2O3 catalyst and the distortion areas formed at the interface of crystal grains (grain boundaries) in Y2O3 nanocrystals. The load of lithium enhanced electron pair donating ability of the surface oxygen atom which made the catalyst easier to capture H+ from CH3OH, and promoted the yield of biodiesel eventually. At the same time, the experiment verified that the catalyst had outstanding reusability and universality on transesterification of different oils into biodiesel. When the acid value was 8.1 mg KOH/g, the yield of biodiesel still exceeded 80%. Furthermore, the yield of biodiesel was maintained above 90% after adding 2 wt% of water to palm oil, which indicated that 0.1Li–Y2O3 catalyst had good water resistance. The 0.1Li–Y2O3 catalyst was believed as an exceptional competitive catalyst for future commercial biodiesel production applications.

Reduction and Oxidation of Maghemite (001) Surfaces: The Role of Iron Vacancies

The knowledge of surface reduction and oxidation energetics in reducible oxides is essential for the design of improved catalysts for oxidation reactions. This is particularly true for iron oxides, a very attractive material system, because of the availability and biocompatibility of its constituents. In this work, by means of the density functional theory, we have thoroughly studied the γ-Fe2O3(001) maghemite surfaces, taking full account of iron vacancies beyond a mean field approach. Despite the structural similarity with the more studied magnetite Fe3O4 surfaces from which maghemite differs only for the presence of iron vacancies in the octahedral sites and for the absence of reduced Fe2+ cations, the surface properties are quite different. Our investigation shows that the presence of Fe vacancies is responsible for an increase in the surface reducibility. Also, it favors surface oxidation. The main reason is that the Fe vacancies cause a decrease of electronic charge of the surface oxygen atoms, whic...

Water dissociation on the reduced PuO2(110) surface from first principles

We present a first-principles investigation on the dynamics and mechanism of the oxidation reaction between water molecules and the reduced PuO2(110) surface using ab initio molecular dynamics (AIMD) simulations in combination with density functional theory (DFT) + U calculations. We find a dominating dissociation preference of water molecules for the vacancy defect sites on the PuO2(110) surface, irrespective of the water or vacancy coverage. Due to hybridizations between the frontier orbitals of water molecule and the electronic states of the vacancy vicinity, partial water dissociation at the vacancy sites is exothermic and barrierless. The dissociation product, an OH group, further hydrogenates the PuO2(110) surface by occupying the vacancy site. We also observe surface vacancy diffusion induced by the interactions between the water molecules and the surface oxygen atom in the proximity of the defect sites.

Correction to Reduction of N2O by CO via Mars-van Krevelen Mechanism over Phosphotungstic Acid Supported Single-Atom Catalysts: A Density Functional Theory Study.

In general, reduction of N2O by CO is first performed by N2O decomposition over a catalyst surface to release N2 and form an active oxygen species, and subsequently CO is oxidized by the active oxygen species to produce CO2. However, the strong adsorption behavior of CO on the catalyst surface usually inhibits adsorption and decomposition of N2O, which leads to a low activity or poisoning of catalysts. In the present paper, a Mans–van Krevelen (MvK) mechanism has been probed based on a series of phosphotungstic acid (PTA) supported single-atom catalysts (SACs), M1/PTA (M = Fe, Co, Mn, Rh, Ru, Ir, Os, Pt, and Pd). Although the calculated adsorption energy of CO is exceedingly higher than N2O for our studied systems, the adsorbed CO could react with the surface oxygen atom of the PTA support through the MvK mechanism to form an oxygen vacancy on the PTA surface. N2O acts as an oxygen donor to replenish the PTA support and release N2 in the whole reaction process. This proposed reaction mechanism avoids comp...

Mechanistic Investigations on Thermal Hydrogenation of CO2 to Methanol by Nanostructured CeO2(100): The Crystal-Plane Effect on Catalytic Reactivity

As a key structural parameter, a crystal plane has a distinguished impact on the catalytic performance. Different exposed crystal planes exhibit different reactivities. CeO2 nanostructured powders are usually exposed to three low-index surfaces, which are (111), (110), and (100) surfaces. Because of the unique structure and low stability of the (100) surface, the investigation of the catalytic reaction mechanism on this surface is rarely involved. Here, density functional theory calculations suggest that the CeO2(100) surface exhibits the strongest reactivity for H2 oxidation, attributed to the coordination unsaturation of surface oxygen atoms. For the hydrogenation of CO2 to methanol on the defective CeO2(100) surface, CO2 is prone to adsorb at the oxygen vacancy in a nearly linear configuration and the formate pathway was verified as the dominant one. The bi-H2COO* can easily convert to bi-H2CO* with the vacancy site filled, in which bi-H2CO* serves as the key intermediate in the methanol synthesis. Thi...

Selectivity Control of the Photo-Catalytic Water Oxidation on SrTiO3 Nanocubes via Surface Dimensionality.

The role of surface dimensionality in photo-electrochemical water oxidation was studied for different-sized SrTiO3 nanocubes. The band gap illumination of strontium titanate electrodes results in anodic current; the photo-current appears at a bias of ca. 220 mV with respect to flat-band potential. The bias needed to record anodic photo-current increases with pH, reflecting the change in the protonation of surface oxygen atoms. The photo-electrochemical activity of SrTiO3 nanocubes is size-dependent and increases with increasing particle size. Semiquantitative analysis of the observed photo-currents combined with mass spectrometric detection of the reaction products shows that the contact of water with illuminated SrTiO3 nanocubes leads to the formation of oxygen, hydrogen peroxide, and ozone. Oxygen and ozone are the primary products of the water oxidation proceeding on {100}-oriented SrTiO3 faces and their fractions increase with increasing particle size. The hydrogen peroxide is simultaneously produced via oxygen reduction at the low-dimensionality sites (crystal edges, vertices), the abundance of which increases with decreasing particle size.

Reduction of N2O by CO via Mars-van Krevelen [corrected] Mechanism over Phosphotungstic Acid Supported Single-Atom Catalysts: A Density Functional Theory Study.

In general, reduction of N2O by CO is first performed by N2O decomposition over a catalyst surface to release N2 and form an active oxygen species, and subsequently CO is oxidized by the active oxygen species to produce CO2. However, the strong adsorption behavior of CO on the catalyst surface usually inhibits adsorption and decomposition of N2O, which leads to a low activity or poisoning of catalysts. In the present paper, a Mars-van Krevelen (MvK) [correction] mechanism has been probed based on a series of phosphotungstic acid (PTA) supported single-atom catalysts (SACs), M1/PTA (M = Fe, Co, Mn, Rh, Ru, Ir, Os, Pt, and Pd). Although the calculated adsorption energy of CO is exceedingly higher than N2O for our studied systems, the adsorbed CO could react with the surface oxygen atom of the PTA support through the MvK mechanism to form an oxygen vacancy on the PTA surface. N2O acts as an oxygen donor to replenish the PTA support and release N2 in the whole reaction process. This proposed reaction mechanism avoids competitive adsorption and poisoning of the catalyst caused by CO. The calculated adsorption energy, oxygen vacancy formation energy, and the free energy profiles show that the catalytic activity of Pd1/PTA, Rh1/PTA, and Pt1/PTA SACs is quite high, especially for Pt1/PTA and Pd1/PTA systems. Meanwhile, molecular geometry and electronic structure analysis along the favorable reaction pathway indicates that the metal single atom not only plays the role of adsorbing CO and activating surface atoms of the PTA support but also works as an electron transfer media in the whole reaction process. We expect that the present calculated results could provide some clues for the search for appropriate catalyst for reduction of N2O to N2 by CO at low temperature.

The role of hydrogen in the nitriding of anatase TiO2 films in the N2-H2 microwave afterglows

Nitriding in the afterglows can be beneficial in the surface-selective nitridation of oxide materials for the control of surface defects or active sites in electronic devices or catalysts. Here we introduce our results on the nitriding of anatase TiO2 films in the afterglows of N2-H2 microwave plasmas at the substrate temperatures up to 300 °C. We employed optical emission spectroscopy (OES) and X-ray photoemission spectroscopy (XPS) to determine active species in the afterglows and the N species incorporated into the film, respectively. Adding a few percent of H2 into the N2 gas flow generates active species such as NH and H atoms in the afterglow region in addition to the N2-derived active species such as N atoms and excited N2 species. The XPS results revealed that such changes in the type and densities of active species in the afterglows can significantly enhance the surface nitriding performance. The origin for the enhanced surface nitriding is attributed to the efficient removal of surface oxygen atoms by reactive H atoms via the formation of H2O to result in an efficient surface uptake of nitrogen. Our results show that the N2-H2 afterglows can be effectively used for a selective surface nitriding of oxide films such as TiO2 at the relatively low temperature range.

Recycled photocatalyst and available photodetector based on ternary Bi6Mo2O15 sub-microcrystals

Abstract Monoclinic phase Bi6Mo2O15 sub-microwires were fabricated by one-step molten salt method. These sub-microwires were found to be much more active than Bi2MoO6 in photocatalytic degradation of rhodamine B (RhB) under sunlight irradiation. The degradation velocity of Bi6Mo2O15 is 98% four times than Bi2MoO6 which might be attributed to its surface oxygen atoms surrounding Bi elements. In particular, the individual ultra-long (∼60 µm) Bi6Mo2O15 sub-microwire was employed as an ultraviolet photodetector, which shows an ultrahigh quantum efficiency of 2.95 × 107% under a wavelength of 254 nm illumination. This study provides a feasible alternative to development of nanocomposite photocatalysts with multi atom unit which manifest important photocatlytic and photoelectochemical applications.