Fe3O4 surface electronic structures and stability from GGA + U

The electronic structures, stabilities and magnetic properties of the Fe₃O₄(111), (110) and (001) surfaces have been computed at the level of density functional theory by including the Hubbard parameter (U) for describing the on-site Coulomb interaction of iron 3d electrons. Among the six Fe₃O₄(111) terminations, the Fe_tet1 (exposing tetrahedral coordinated iron) and Fe_oct2 (exposing octahedral coordinated iron) terminations are more stable and have metallic character. For the Fe₃O₄(110) surface, strong surface distortion has been found; the A-layer termination (exposing tetrahedral coordinated iron) has metallic character, while the B-layer termination (exposing tetrahedral and octahedral coordinated iron) has half-metal character. For the Fe₃O₄(001) surface, both A-layer (exposing tetrahedral coordinated iron) and B-layer (exposing octahedral coordinated iron) terminations have half-metal character. The surface stability of (111) > (001) > (110) on the basis of the computed surface energies agrees well with the experimental findings, and explains reasonably the observed diversity and complexity of the experiments.     

Adsorption of H2O on clean and on boron-contaminated Rh surfaces

The adsorption and dissociation of H₂O on Rh(111) and Rh foil surfaces have been studied in UHV using Auger electron, electron energy loss (in the electronic range) and thermal desorption spectroscopy. H₂O adsorbs weakly on clean Rh samples at 110 K. The adsorption is accompanied by the appearance of a broad loss feature at 14–14.5 eV. At higher exposures new losses appeared at 8.6 and 10.5 eV. The desorption of H₂O took place in two stages, with T_p = 183 K (β, chemisorption) and 158 K (α, multilayer formation). There was no indication of dissociation of H₂O on a clean Rh(111) surface. Similar results were obtained for a clean Rh foil. However, when small amounts of boron segregated on the surface of Rh, they exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociation of H₂O. This was exhibited by the formation of H₂, by the buildup of surface oxygen, by the appearance of an intense new loss at 9.4 eV, identified as B-O surface species, and by the development of “boron-oxide”-like Auger fine structure.     

Oxide formation and reduction on rhodium surfaces

The growth of thin oxide layers on Rh and their reduction by CO has been investigated by imaging atom-probe mass spectroscopy and field-ion microscopy. Surface oxides were produced by heating Rh field-emitter tips in oxygen at pressures between 0.01 and 1.0 Torr and temperatures between 400 and 650 K. The oxidized samples were transferred under ultrahigh vacuum to an imaging atom-probe/field-ion microscope for compositional and structural analysis. Oxygen uptake was found to follow a logarithmic law with an initial activation energy of 3.1 kcal/mol. Imaging atom-probe analysis indicated that the oxide formed in 1 Torr O₂ was stoichiometric Rh₂O₃ for temperatures of 500 K and above. The onset pressure for oxidation at 500 K was found to be ≈0.01 Torr, with only a weak pressure dependence in the range from 0.01 to 1 Torr. Field-ion microscope images of the oxide showed ring structures suggestive of epitaxial growth above the (111) plane, and images of the substrate after removal of the oxide indicated that the oxide was thicker above the stepped regions of the surface than above the low-index planes. The oxide was quickly reduced in 1 Torr CO at temperatures above 420 K, and partially-reduced oxides were found to be substoichiometric throughout the oxide region. CO reduction exhibited a much stronger temperature dependence than surface oxide formation indicating a different rate-controlling step for the two processes. The time dependence for CO reduction at 418 K suggested that the COO surface reaction was rate-determining in the reduction process.     

Partial and complete reduction of O2 by hydrogen on transition metal surfaces

The metal-catalyzed reduction of di-oxygen (O₂) by hydrogen is at the heart of direct synthesis of hydrogen peroxide (HOOH) and power generation by proton exchange membrane fuel cells. Despite its apparent simplicity, how the reaction proceeds on different metals is not yet well understood. We present a systematic study of O₂ reduction on the (111) facets of eight transition metals (Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) based on periodic density functional theory (DFT-GGA) calculations. Analysis of ten surface elementary reaction steps suggests three selectivity regimes as a function of the binding energy of atomic oxygen (BE_O), delineated by the opposite demands to catalyze O–O bond scission and O–H bond formation: The dissociative adsorption of O₂ prevails on Ni, Rh, Ir, and Cu; the complete reduction to water via associative (peroxyl, peroxide, and aquoxyl) mechanisms prevails on Pd, Pt, and Ag; and HOOH formation prevails on Au. The reducing power of hydrogen is decreased electrochemically by increasing the electrode potential. This hinders the hydrogenation of oxygen species and shifts the optimal selectivity for water to less reactive metals. Our results point to the important role of the intrinsic reactivity of metals in the selectivity of O2 reduction, provide a unified basis for understanding the metal-catalyzed reduction of O₂ to H₂O and HOOH, and offer useful insights for identifying new catalysts for desired oxygen reduction products.     

Inelastic electron tunneling spectroscopy of a model catalyst: CO on Rh/Al2O3

Using tunneling spectroscopy we have studied the preparation and behavior of dispersed rhodium model catalysts supported on alumina. Samples were prepared by vacuum evaporation from Rh metal or Rh₂O₃ sources onto an oxidized Al film and CO was adsorbed in-situ. The tunnel junctions were formed by adding a Pb top electrode and the vibrational spectra of the adsorbed species were measured. We observed qualitatively different spectra when the preparation procedure was varied. Special care was taken to monitor and control background gases. We obtained different results from Rh of Rh₂O₃ sources and the presence of oxygen or water affects the vibrational spectra of the adsorbed CO. We also study the effect of the Rh thickness on the spectral intensity. Other experiments were measurement of the superconducting tunneling spectra of the Pb and a TEM study of Rh particle size. Previously reported data from tunneling and IR measurements are compared with the present work. Based on these results, we conclude that there are two species present, either a linear Rh-CO or doubly (geminal) adsorbed Rh(CO)₂ depending upon the degree of dispersion and oxidation of the Rh. The evidence also indicates that in both instances a dispersed form of Rh, rather than relatively large Rh metal particles, is responsible for the observed spectra.     

Adhesion at the interfaces between BCC metals and α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Ab initio calculations of the atomic and electronic structures of Me(111)/α-Al₂O₃(0001) interfaces (Me = V, Cr, Nb, Mo, Ta, W) in the framework of density functional theory are reported. The energies of separation of metal films from oxide surfaces have been calculated. The structural and electronic factors responsible for the strong adhesion of bcc metal films on the oxygen termination of the surface of aluminum oxide have been analyzed.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

First-principles study of the adhesive and mechanical properties of the O-terminated α-Al2O3(0001)/Cu(111) interfaces

Adhesive and mechanical properties of the O-terminated (O-rich) α-Al₂O₃(0001)/Cu(111) interface have been examined by the first-principles pseudopotential method. Strong Cu–O covalent and ionic interactions exist, such as Cu3d–O2p hybridization and substantial electron transfer from Cu to O, which result in larger adhesive energy, greater tensile strength and larger interfacial Young's moduli than the Al-terminated (stoichiometric) interface with electrostatic–image and Cu–Al hybridization interactions. Substantial effects of interfacial Cu–O coordination are also present. Changes in the interface electronic structure for cleavage have been examined. Cu–O interlayer potential curves have been analyzed using the universal binding energy relation and compared with Cu–Al and Cu–Cu curves, which is valuable for the development of effective interatomic potentials in large-scale simulations.     

Some coadsorption effects on Rh(111)

ESD energy analysis is used to study the reaction products produced during coadsorption of CO-O₂ and CH₄-O₂ on Rh(111). Residence of CO₂ on the surface is confirmed by detection of a CO₂⁺ ionic component with energy of 1.8 eV. Adsorption and dissociation of CH₄ on oxygen- covered Rh(111) is inferred as a result of a low energy component present in the energy spectra of desorbed O⁺.     

Oxygen plasma activation of Cr(CO)6 on α-Fe2O3(0001)

The chemistry of Cr(CO)₆ on the Fe₃O₄(111) surface termination of α-Fe₂O₃(0001) was explored using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED) both with and without activation from an oxygen plasma source. No thermal decomposition of Cr(CO)₆ was detected on the surface in the absence of O₂ plasma treatment, with first layer molecules desorbing in TPD at 215 K from a close-packed overlayer. The interaction of first layer Cr(CO)₆ with the Fe₃O₄(111)-termination was weak, desorbing only ～ 30 K above the leading edge of the multilayer state. Activation of multilayer coverages of Cr(CO)₆ with the O₂ plasma source at 100 K resulted in complete conversion of the outer Cr(CO)₆ layers, presumably to a disordered Cr oxide film, with Cr(CO)₆ molecules near the surface left unaffected. Absence of CO or CO₂ desorption states suggests that all carbonyl ligands are liberated for each Cr(CO)₆ molecule activated by the plasma. AES and SSIMS both show that O₂ plasma activation of Cr(CO)₆ results in a carbon-free surface (after desorption of unreacted Cr(CO)₆). LEED, however, shows that the Cr oxide film was disordered at 600 K and likely O-terminated based on subsequent water TPD. Attempts to order the film at temperatures above 650 K resulted in dissolution of Cr into the α-Fe₂O₃(0001) crystal based on SSIMS, an observation linked to the Fe₃O₄(111) termination of the surface and not to the properties of α-Cr₂O₃/α-Fe₂O₃ corundum interface. Nevertheless, this study shows that O₂ plasma activation of Cr(CO)₆ is an effective means of depositing Cr oxide films on surfaces without accompanying carbon contamination.     

Density-functional study of CO adsorbed on RhN （N = 2-19） clusters

We investigate the structural, electronic and adsorption properties of one single CO molecule adsorbed on Rh_N (N = 2-19) clusters, using the density-functional theory in the spin-polarized generalized gradient approximation. It is found that the structural growth model of the Rh_N clusters transforms from double layers (N = 12-16) to three layers (N = 17-19). Three different adsorption types are the atop site adsorption for N = 6, 8, 9, 11, 12, the bridge site adsorption for N = 2-5, 7, 10, 13-15, 17 and the face adsorption for N = 16, 18, 19. The adsorption abilities of Rh_N clusters are related to C-O bond length, vibrational frequency, adsorption energy and the charge transfer between CO and Rh clusters as well as the electronic density of state. With the increase of Rh cluster size, the adsorption energy of CO adsorbed on Rh_N clusters tends to be 2.2 eV-2.3 eV, which is 0.2 eV-0.3 eV larger than the theoretical value (about 2.0 eV) of CO molecule adsorption on clean Rh (111) surface.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

Fabrication of Rh based solid-solution bimetallic alloy nanoparticles with fully-tunable composition through femtosecond laser irradiation in aqueous solution

Many late transition binary alloy nanoparticles (NPs) have been fabricated through a wide variety of techniques. Various steps are involved in the fabrication of such NPs. Here, we used a simple and green route to fabricate solid-solution Rh–Pd and Rh–Pt bimetallic alloy NPs through femtosecond laser irradiation in a solution without any chemicals like reducing agents. X-ray diffraction (XRD) peaks of NPs obtained in the solutions with different ratios of Rh–Pd and Rh–Pt ions monotonically varied from the position of pure Rh to those of Pd and to Pt which respectively indicated that these NPs were alloy. Composition of fabricated NPs was fully tuned over the entire range of Rh_1?x –Pd _x , and Rh_1?x –Pt _x with varying the mixing ratio of metal ions in the solution. Studies of Rh–Pd and Rh–Pt solid-solution system suggest that the alloy formation occurs through the nucleation of Rh and then followed by the diffusion of Rh, Pd and Rh, Pt to form a homogeneous alloy. The variety of average size of the alloy NPs for different compositions could be attributed to different reduction rate and surface energies of metal ions. Our result implies that femtosecond laser irradiation in aqueous solution is one of the potential methodologies to form multimetallic solid-solution alloy NPs with fully tunable composition.     

Surface termination of an ultrathin V2O3-film on Au(111) studied via ion beam triangulation

The structure of the topmost layer of thin V₂O₃ films on a Au(111) substrate is studied via Ion Beam Triangulation. From electron emission induced by fast H atoms scattered from the film surface under a grazing angle of incidence as function of azimuthal rotation of the target, we find evidence for a reconstructed O₃ termination as proposed from DFT-calculations and recent experimental work using methods based on large angle impact of fast ions. From our studies we derive detailed lateral positions for the topmost O atoms.     

Adsorption and desorption of no from Rh{111} and Rh{331} surfaces

The adsorption and desorption chemistry of NO on the clean Rh{111} and Rh{331} single crystal surfaces was followed with SIMS, XPS, and LEED. Results suggest dissociative NO adsorption occurs at step and/or defect sites. At saturation coverage there was ～ 10 times more dissociated species on the Rh{331} surface at 300 K than on the Rh{111} surface. On both surfaces two molecular states of NO_ads have been identified as β₁, and β₂ which possess different chemical reactivity. Under the condition of saturation coverage the β₁ and β₂ states are populated on the Rh{111} surface in a different proportion than on the Rh{331} surface. Further, their population on both surfaces is coverage and temperature dependent. When the sample is heated to desorb the saturation overlayer formed on the Rh{111} and Rh{331} crystal surfaces, approximately 50% of the overlayer is found to desorb below ? 400 K primarily from the β₂ state, molecularly as NO(g). Between 300 and 400 K the β₁ state dissociates as binding sites necessary to coordinate N_ads and O_ads are freed by desorption of NO(g).     

Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

La solution solide Cr1−xRhxO2

A new solid solution Cr_1?x,Rh_xO₂ (0?x?1) has been prepared using oxidizing acids or ammonium perchlorate under pressure. The magnetic study suggests that chromium (IV) and rhodium (IV) are present in the structure at x ? 0.2 and chromium (V) and rhodium (III) at 0.2 < x ? 0.5. Forx > 0.5 increasing rhodium (IV) content gives rise to properties similar to those of RhO₂.     

Spectroscopic study on thermal reaction and desorption of sulfur/Rh(1 0 0) induced by oxygen-containing molecules

The surface reaction and desorption of sulfur on Rh(1 0 0) induced by O₂ and H₂O are investigated with X-ray photoelectron spectroscopy (XPS) technique. The Rh(1 0 0) sample covered with atomic sulfur is prepared by means of the exposure to H₂S gas, and subsequently the sample is annealed under O₂ or H₂O atmosphere. The XPS results show that atomic sulfur adsorbed on Rh(1 0 0) reacts with O₂ and desorbs from the surface at 473 K or more. On the other hand, atomic sulfur can not be removed from Rh(1 0 0) surface by H₂O at any temperature.     

HREELS and LEED studies on Cu deposited on 1 × 1 reconstructed α-Fe2O3(0001) surfaces

High-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction, and X-ray photoelectron spectroscopy have been used to study clean 825 K-preannealed α-Fe₂O₃-1 × 1 (haematite) surfaces, an α-Fe₂O₃-(0001)-1 × 1 surface reconstructed with Fe₃O₄(111)-1 × 1 and to study Cu deposited on room-temperature surfaces of those. Three pronounced losses, at 47.5, 55.5 and 78.0 meV, of the surface phonons for the clean α-Fe₂O₃(0001) were observed. By deposition of copper, Cu---O vibrational features observed by HREELS indicate formation of a Cu(I) state for the very low coverages. Increased submonoloayer amounts of Cu result in clustering of the copper, leading for both the α-Fe₂O₃(0001)-1 × 1 and the reconstructed composite substrate surfaces to Cu(111) epitaxial growth.