Oxygen Atoms In Sites Research Articles

Adsorption of naphthenic acids to the nitrogen-coordinated transition-metal embedded graphene: A DFT study

The adsorption properties of two types of naphthenic acids (NAs), benzoic acid and cyclohexane carboxylic acid on four-nitrogen coordinated transition-metal (Mn, Fe, Co, Ni, Cu, and Zn) embedded graphene (TMN4-G) were investigated in detail by means of density functional theory method. The calculation results indicate that NAs prefer the perpendicular adsorption configuration by bonding interactions between their carbonyl oxygen atom and TMN4 active site, and could be chemisorbed on FeN4-G, MnN4-G, and ZnN4-G. The FeN4-G gives the strongest adsorption to the NAs, indicating it is the best adsorbent among them. Electron density maps further confirm that NAs are chemically adsorbed on the FeN4-G surface, accompanied by electron transfer in the adsorption systems. The calculations indicate that benzoic acid has relatively stronger adsorption energy than that of cyclohexane carboxylic acid for the perpendicular adsorption on TMN4-G surface.

Two-dimensional electron gas in GaAs/SrHfO3 heterostructure

The III-V/perovskite-oxide system can potentially create new material properties and new device applications by combining the rich properties of perovskite-oxides together with the superior optical and electronic properties of III-Vs. The structural and electronic properties of the surface and interface are studied using first-principles calculations for the GaAs/SrHfO3 heterostructure. We investigate the specific adsorption sites and the atomic structure at the initial growth stage of GaAs on the SrHfO3 (001) substrate. Ga and As adsorption atoms preferentially adsorb at the top sites of oxygen atoms under different coverage. The energetically favorable interfaces are presented among the atomic arrangements of the GaAs/SrHfO3 interfaces. Our calculations predict the existing of the two-dimensional electron gas in the GaAs/SrHfO3 heterostructure.

Influences of the Pb 6s2 lone pair effect and quantum size effect on the diffusion of oxygen atoms on Pb(111) films

Based on our previous studies revealing quantum oscillations in the adsorption energetics of atomic oxygen on Pb(111) films, here we study all the possible on-surface and subsurface adsorption sites of oxygen atoms on Pb(111) films at different coverages.

The cluster compound In4Ti1.5Mo0.5Mo14O26 containing Mo14 clusters and the new mono- and bi-capped trioctahedral Mo15 and Mo16 clusters: Synthesis, crystal structure, and electrical and magnetic properties

Single crystals of the new quaternary compound In4Ti1.5Mo0.5Mo14O26 were obtained by solid state reaction. The crystal structure was determined by single-crystal X-ray diffraction. In4Ti1.5Mo0.5Mo14O26 crystallizes in the orthorhombic space group Pbca with unit-cell parameters a=9.4432(14)Å, b=11.4828(12)Å, c=20.299(4)Å and Z=4. Full-matrix least-squares refinement on F2 using 3807 independent reflections for 219 refinable parameters resulted in R1=0.0259 and wR2=0.0591. The crystal structure contains in addition to Mo14 clusters the first examples of mono- and bi-capped trioctahedral Mo14 i.e. Mo15 and Mo16 clusters. The oxygen framework derives from a stacking along the a direction of close-packed layers with sequence (…ABAC…). The Mo–Mo distances range between 2.6938(5) and 2.8420(6)Å and the Mo–O distances between 1.879(5) and 2.250(3)Å, as usually observed in molybdenum oxide clusters. The indium atoms form In46+ bent chains with In–In distances of 2.6682(5) and 2.6622(8)Å and the Ti atoms are in highly distorted octahedral sites of oxygen atoms with Ti–O distances ranging between 1.865(4) and 2.161(4)Å. Magnetic susceptibility measurements confirm the presence of Ti4+ cations and the absence of localized moments on the Mo network. Electrical resistivity measurements on a single crystal of In4Ti1.5Mo0.5Mo14O26 show a semimetallic behavior.

Direct Imaging of Pt Single Atoms Adsorbed on TiO2 (110) Surfaces

Noble metal nanoparticles (e.g., gold and platinum) supported on TiO2 surfaces are utilized in many technological applications such as heterogeneous catalysts. To fully understand their enhanced catalytic activity, it is essential to unravel the interfacial interaction between the metal atoms and TiO2 surfaces at the level of atomic dimensions. However, it has been extremely difficult to directly characterize the atomic-scale structures that result when individual metal atoms are adsorbed on the TiO2 surfaces. Here, we show direct atomic-resolution images of individual Pt atoms adsorbed on TiO2 (110) surfaces using aberration-corrected scanning transmission electron microscopy. Subangstrom spatial resolution enables us to identify five different Pt atom adsorption sites on the TiO2 (110) surface. Combining this with systematic density functional theory calculations reveals that the most favorable Pt adsorption sites are on vacancy sites of basal oxygen atoms that are located in subsurface positions relative to the top surface bridging oxygen atoms.

Study of Hydrothermal Stability and Water Sorption Characteristics of 3-Dimensional Zn-Based Trimesate

Zinc trimesate material with high hydrothermal stability was studied for water adsorption/desorption dynamics, and we explained the reason for material’s stability in water. A zinc trimesate Zn2(BTC)(OH)(H2O)·1.67H2O with the three-dimensional framework contains [Zn2O6(OH)(H2O)] chains with ZnO2(OH)2 tetrahedra and ZnO4(OH)(H2O) octahedra corner-shared through μ3-OH group. Inorganic chains are linked with 1,3,5-benzenetricarboxylates forming two types of parallel channels (open and closed) containing adsorbed water in different environments. Closed channels are occupied by free water molecules connected through strong hydrogen-bonds with coordinated water, whereas open-type channels contain water molecules with partially occupied oxygen atom sites. The dynamics of water adsorption/desorption was evaluated by complementary techniques of thermogravimetric (TG), infrared (IR), water sorption, and different magic-angle spinning nuclear magnetic resonance (MAS NMR) techniques. The removal of water from open channels occurs below 100 °C, whereas hydrogen-bonded water molecules and coordinated water are expelled at higher temperatures. 2H MAS NMR was employed to prove that the removal of water from closed channels is not entirely simultaneous and that adsorbed water begins to diffuse at slightly lower temperatures than the coordinated one. The investigated material shows high hydrothermal stability and withstands 40-cycle hydrothermal-stability test without any significant loss of the structure integrity. It also shows complete structural reversibility upon dehydration/rehydration process at 200 °C. The reason for high stability in water mainly lies in the stabilization of the inorganic chains established by the interaction between the adsorbed water molecules and coordinated ones (framework water) via hydrogen bonds. The material also exhibits notable sorption capacity for water (208 mg·g–1) adsorbed in a stepwise process.

Preparation and structure of BiCrTeO6: A new compound in Bi–Cr–Te–O system. Thermal expansion studies of Cr2TeO6, Bi2TeO6 and BiCrTeO6

A new compound BiCrTeO6 in Bi–Cr–Te–O system was prepared by solid state reaction of Bi2O3, Cr2O3 and H6TeO6 in oxygen and characterized by X-ray diffraction (XRD) method. It could be indexed on a trigonal lattice, with the space group P-31c, unit cell parameters a=5.16268(7)Å and c=9.91861(17)Å. The crystal structure of BiCrTeO6 was determined by Rietveld refinement method using the powder XRD data. Structure shows that there is one distinct site for bismuth (Bi) atom, one chromium rich (Cr/Te=68/32), and one tellurium rich (Te/Cr=68/32) sites, and one distinct site for oxygen (O) atom in the unit cell. All cations in this structure show an octahedral coordination with oxygen atoms at the corners. The thermogravimetric analysis (TGA) of the compound in air shows that it is stable up to 1103K and decomposes thereafter. The thermal expansion behavior of Cr2TeO6, Bi2TeO6 and BiCrTeO6 was studied using High Temperature X-ray diffraction (HTXRD) method from room temperature to 973, 873 and 923K respectively under vacuum of 10−8 atmospheres. All the compounds showed positive thermal expansion with the average volume thermal expansion coefficients of 14.38×10−6/K, 22.0×10−6/K and 16.0×10−6/K respectively.

Electron scavenger of thin layer Carbon coated and Nitrogen doped P25 with enhanced photocatalytic activity under visible light fluorescent lamp

The commercially available and relatively cheap Degussa P25, TiO2 (80% anatase and 20% rutile) was modified to produce Carbon coated and Nitrogen doped P25. The modified photocatalyst was produced by using peat as the carbon precursor. The results showed that a thin layer of carbon in graphite form was deposited on the surface of P25 by which N atoms were also incorporated into the lattice of TiO2 by substituting the sites of oxygen atoms. From PL analysis, it was shown that the Carbon coated and Nitrogen doped P25 improved photocatalytic activity by acting as electron scavengers as well as reduced the electron–hole pair recombination. The photocatalytic activity of Carbon coated and Nitrogen doped P25 was examined by degradation of anionic Cibacron Brilliant Red or otherwise known as reactive red 4 (RR4) dye under visible light fluorescent lamp irradiations. The results indicated that the Carbon coated and Nitrogen doped P25 had enhanced photocatalytic activity and excellent photocatalytic stability even up to the 5th cycle of reuse. Its photocatalytic activity was almost 2 times faster than pristine P25. Carbon coated and Nitrogen doped P25 was not visibly active under low intensity visible light source (45-W) but showed clear photoactivity under a higher intensity visible light source (125-W).

Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situ neutron diffraction

The layered perovskite NdBaCo2O5+δ (NBCO) was characterized using neutron powder diffraction under in situ conditions from 577–852 °C and in 10−1 to 10−4 atm oxygen. The best fit to the data was obtained in the tetragonal (P4/mmm) space group. No oxygen atom vacancy ordering was observed that warranted a lowering of the symmetry to orthorhombic (Pmmm). Two P4/mmm structural models were investigated: Model 1 (no split sites) and Model 2 (split Nd and O2 sites). Transport of oxygen through the material via the vacancy hopping mechanism likely involves the nearest-neighbor oxygen atom sites in the Nd layer. Total oxygen stoichiometry values were in the range 5.51 ≤ δ ≤ 5.11. The tetragonal lattice parameters increased with temperature as expected. However, the a-axis expands while the c-axis contracts with decreasing pO2 at a given temperature due to increasing vacancy concentration in the Nd layer.

Effect of carbon and nitrogen co-doping on characteristics and sonocatalytic activity of TiO2 nanotubes catalyst for degradation of Rhodamine B in water

Carbon and nitrogen co-doped titanium dioxide nanotubes (CN co-doped TiO2 NTs) catalysts were successfully synthesized using a hydrothermal method at N:Ti molar ratios between 0.1 and 1.0. Transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), X-ray photoelectron spectroscope (XPS), UV–Vis diffuse reflectance spectroscope (UV–Vis DRS) and surface area analyzer were used to characterize the synthesized catalysts. Urea doping at N:Ti=0.3 led to the highest sonocatalytic activity with an apparent-first-order rate constant of 0.0226min−1 as compared to 0.0098min−1 for the un-doped TiO2 NTs. The high performance was associated with the synergistic effects between the doped C and N atoms, higher surface area and lower band gap energy. It was also ascribed to the creation of surface oxygen vacancies due to the formation of Ti3+ species and better crystallinity of anatase phase as compared to the un-doped TiO2 NTs. Most of the nitrogen atoms were bonded to oxygen atoms in the interstitial sites (TiON and TiNO), but only minority of them could substitute the sites of oxygen atoms (NTiO). C atoms evidently presented in the form of TiC and a complex carbonate species on the catalyst surface.

Adsorption of atomic oxygen on HfC and TaC (1 1 0) surface from first principles

We investigated the initial adsorption of oxygen atom on the HfC (110) and TaC (110) surface using first principles. Both the carbides have the same crystal structure, a sodium–chloride structure. The (110) surfaces of the carbides were modeled with (2×1) supercells. Every supercell is composed of five atomic planes. Our results demonstrate that the preferred site for oxygen atom is the C–Hf bridge site and the Ta–Ta bridge site on the HfC (110) and TaC (110) surface, respectively. The adsorption sites are different from the one on the (100) surfaces of the carbides. For the carbides, the adsorption energies of oxygen on the (110) surfaces are larger than that on the (100) surfaces. There exists the CO bond in O/HfC (110), while no CO bond is found in O/TaC (110), indicating that the adsorption mechanism of atomic oxygen on the TaC (110) surface is different from that on the HfC (110) surface at initial adsorption stage.

Carbon Coated N-Doping of TiO<sub>2</sub> Nanotube Films with Enhanced Visible-Light Photocatalytic Activity

Carbon coated N-doping of TiO2 nanotube films were successfully prepared by a two-step process of anodization and gas thermal penetration. The morphologies and structures of composites were characterized by SEM, XRD, XPS and Raman spectroscopy analyses, respectively. The results show that graphitized carbon deposited on the surface of TiO2NTs nanotubes under gas thermal penetration conditions, by which N atoms are incorporated into the lattice of TiO2NTs through substituting the site of oxygen atoms. Such Carbon coated N-doping of TiO2 nanotube films presents the enhance absorption in visible region. The photo catalytic tests show that the prepared films at 600°C reveal higher photocatalytic activity for splitting of water than other samples.

Solvothermal synthesis ZnS–In2S3–Ag2S solid solution coupled with TiO2−xSx nanotubes film for photocatalytic hydrogen production

ZnS–In2S3–Ag2S solid solution coupled with TiO2-xSx nanotubes film catalyst has been successfully prepared by a two-step process of anodization and solvothermal methods for the first time. The as-prepared photo-catalysts are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Visible diffuse reflectance spectra (UV–Vis DRS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The results show that the ZnS–In2S3–Ag2S solid solution are deposited on the surface of TiO2NTs nanotubes under the solvothermal conditions, by which S atoms are incorporated into the lattice of TiO2 through substituting the sites of oxygen atoms. Such ZnS–In2S3–Ag2S@TiO2-xSx nanotubes composite presents the enhanced absorption in visible region and the efficient transfer of photoelectron between the solid solution and TiO2-xSx nanotubes, which determines the excellent photocatalytic activity for the photocatalytic hydrogen evolution from aqueous solutions containing the sacrificial reagents of Na2S and Na2SO3 under 500 W Xe lamp irradiation.

Preparation and Characterization of N Doped TiO<sub>2</sub>/ Sepiolite Composite Materials

Nitrogen doped TiO2/sepiolite composite materials (N-TiO2/sep) with different nitrogen contents were prepared by a sol-gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), respectively. XRD and SEM results showed that anatase-TiO2 nanoparticles were distributed homogenously on the surface of sepiolite. XPS revealed that N atoms could incorporate into the lattice of anatase TiO2 substituting the oxygen atoms sites of oxygen atoms.

Preparation and Photocatalytic Activity of N-TiO<sub>2</sub>/Sepiolite Nanocomposite

To utilize visible light and separate of TiO2 nanoparticles more efficiently in photocatalytic reactions, nitrogen doped TiO2/sepiolite composites (N-TiO2/sep) with different nitrogen contents were prepared by a sol-gel method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. XRD showed that anatase-TiO2 nanoparticles were loaded on the surface of sepiolite. XPS revealed that N atoms could incorporate into the lattice of anatase TiO2 substituting the sites of oxygen atoms. UV-vis spectroscopy showed that the visible light absorption of N-TiO2/sep samples decreased with the increase of calciantion temperature and increased with the increase of N content. The photocatalytic activities of obtained N-TiO2/sep samples were evaluated by methylene blue degradation under visible light irradiation. It was found that the N-TiO2/sep samples had the higher photocatalytic activity than that of TiO2/sep.

Preparation and enhanced visible light photocatalytic activity of N-doped titanate nanotubes by loaded with Ag for the degradation of X-3B

N-doped Titanate nanotubes (NTNTs) loaded with Ag (Ag-NTNTs) in order to improve photocatalytic activity of NTNTs under visible light irradiation were successfully synthesized by hydrothermal technique combined with photochemical reduction process. The physical and chemical properties of the prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectrum (DRS). XPS analysis indicated Ag species existed as metallic Ag and N atoms could incorporate into the lattice of titania by substituting the sites of oxygen atoms. UV–vis spectra indicated the remarkable red shift of absorption edge to the visible light region for Ag-NTNTs. The visible light photocatalytic activities of the samples were evaluated for the degradation of dye Reactive Brilliant Red X-3B (C.I. reactive red 2). It was found that the photocatalytic activities exhibited by Ag-NTNTs were higher than that of N-doped sample under visible light irradiation. The interaction between N species and metallic Ag may be responsible for the higher visible light photocatalytic activity.

Atomic oxygen adsorption on the silicon-doped hafnium carbide (0 0 1) surface from first principles

To improve the oxidation resistance of Hafnium carbide (HfC) surface, we investigated the adsorption of atomic oxygen on the silicon–doped HfC (0 0 1) surface by first principles. The O/HfC (0 0 1) system was also calculated for comparison. The (√2 × √2) R45° supercell was constructed to calculate the adsorption. In calculations, we treated the exchange and correlation potential with the revised version of the Perdew–Burke–Ernzerhof generalized-gradient approximation (GGA-RPBE). Our data demonstrate that the preference adsorption site for oxygen atom is the 4–fold hollow site on the silicon–doped HfC (0 0 1) surface. The oxygen on the silicon–doped surface receives more charges from silicon atoms than that in O/HfC (0 0 1) from carbon atoms. The Si–O bonds exhibit ionic and covalent characteristics, while the C–O bond exhibits primarily covalence. And the covalence of Si–Hf bonds is stronger than that of C–Hf bonds. The strong Si–O and Si–Hf bonds indicate the strong interactions of oxygen with the silicon–doped surface. The strong interactions can explain the possibility of improving the oxidation resistance of HfC surface via doping silicon.

Ab initiostudy of surface self-segregation effect on the adsorption of oxygen on theγ-TiAl(111)surface

The effects of surface self-segregation on the adsorption of oxygen on the $\ensuremath{\gamma}\text{-TiAl}(111)$ surface are investigated by density-functional theory calculations. The oxygen binding-energy results show that on the pure $\ensuremath{\gamma}\text{-TiAl}(111)$ surface, the most stable adsorption sites of oxygen atoms are the sites with more Ti sites as their neighbors, which is in agreement with previous calculations. The defect formation energies and the relative surface energies as functions of chemical potential of Al are calculated. Furthermore, the phase diagram of $\ensuremath{\gamma}\text{-TiAl}(111)$ surfaces with different defects and different oxygen coverage is also calculated. The calculated results show that Al atom can segregate at the $\ensuremath{\gamma}\text{-TiAl}(111)$ surface. Especially, at high Al chemical potential, the first surface layer of the $\ensuremath{\gamma}\text{-TiAl}(111)$ surface can be composed of Al atoms. In this case, the oxygen adsorption behavior is very similar to the case of Al(111) surface: the binding energy per oxygen atom decreases with the increase in the oxygen coverage due to an attractive interaction between O atoms. These results show that the occurrence of Al self-segregation at the $\ensuremath{\gamma}\text{-TiAl}(111)$ surface can enhance the interaction between O and Al atoms. The Al surface segregation effect may be the reason why the growth of pure alumina layer resulting from the selective oxidation of aluminum formed on $\ensuremath{\gamma}\text{-TiAl}(111)$ surface at the first stage in the experiments.

Density functional theory calculations of vibrational spectra of rhodium oxide clusters

Predicted infrared vibrational spectra of low-energy isomers of rhodium cluster oxides, Rh6Om+(m=1–4), are presented, based on structures calculated using density functional theory. The oxygen stretching frequencies are found to be characteristic of the oxygen binding mode. These simulated spectra provide a guide to future experimental measurements of the infrared multi-photon dissociation spectra, enabling assignment of the binding sites of oxygen atoms on the cluster. In favourable cases these calculations suggest that a determination of the underlying metal framework structure is possible.

Diffusion Path of Oxide Ions in an Apatite-Type Ionic Conductor La9.69(Si5.70Mg0.30)O26.24

Densities of coherent-scattering lengths in oxide-ion conductor La9.69(Si5.70Mg0.30)O26.24 with an apatite-type structure (space group P63/m) have been determined by a whole-pattern fitting approach based on the maximum-entropy method (MEM) using neutron powder diffraction data measured at 25 and 1558 °C. The Rietveld refinement suggested an interstitial oxygen (O5) atom site (12i; x, y, z; x ≈ 0, y ≈ 0.22, z ≈ 0.65) at the periphery of the hexagonal axis. The oxide ions located at 2a (O4) and 12i (O3) sites are largely distributed parallel and perpendicular to the c axis, respectively. The densities of coherent-scattering lengths derived from the MEM analysis revealed that the O4 oxide ions located at the 2a site (0, 0, 1/4) diffuse to nearest-neighbor 2a sites along the [001] direction. This [001] diffusion path is linearly linked with that of neighboring unit cell, which forms a long-range, one-dimensional oxide-ion diffusion pathway extending along the c axis of the hexagonal P63/m framework. The prob...