Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110)

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from 10⁻⁸ to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu₂O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu₂O on Cu(110) is about two orders of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu₂O formation indicate the distinct growth modes of Cu₂O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.     

Oxidation‐induced nanostructures on Cu{100}, Cu(Ag) and Ag/Cu{100} studied by photoelectron spectroscopy

Initial surface oxidation and nanoscale morphology on Cu{100}, Cu(Ag) and Ag/Cu{100} have been investigated in situ by X‐ray photoelectron spectroscopy (XPS), X‐ray induced Auger electron spectroscopy (XAES) and the inelastic electron background analysis as a function of oxygen exposure at 3.7 × 10^?2 and 213 mbar pressures at a surface temperature of 373 K. Relative Cu₂O concentrations have been quantified by analysis of the peak shape of the XAES Cu LMM transition. The surface morphology of Cu₂O islands and the Ag layer has been characterized by inelastic electron background analysis of XAES O KLL and Ag 3d transitions. Oxygen‐induced segregation of Cu, as well as the subsequent Cu₂O island formation on Cu(Ag) and Ag/Cu{100} surfaces, has been investigated quantitatively. Our results indicate that Ag has a clear inhibitive effect on the initial oxidation and Cu₂O island formation on Cu(Ag) and Ag/Cu{100} surfaces. The Cu₂O islands are also observed to remain highly strained on Ag/Cu{100} even at higher O₂ exposures. The results suggest that strained Cu₂O islands eventually penetrate through the buried Ag layer, and in conjunction with segregating Cu atoms enable the oxidation to proceed at a similar rate to or even faster than on the unalloyed Cu surface. Copyright © 2007 John Wiley & Sons, Ltd.     

A Polarographic Method Determing Cu(II), Cu(I) and Metallic Copper Contents in Copper Oxide Samples

Abstract

A polarographic procedure was developed which permits the analysis of powdered cupric and cuprous oxides in the presence of metallic copper. To determine CuO, Cu₂O and metallic copper content in the sample two weight aliquots were used. The first aliquot was dissolved in medium of 50 % ethanol + 3 M hydrochloric acid + saturated ascorbic acid solution. Insoluable metallic copper was determined polarographically after its' separation and additional dissolving in concentrated nitric acid.

The second sample aliquot was dissolved in 6 M hydrochloric acid and the ratio of Cu(I) / Cu(II) in the solution was determined from the polarographic curves. To calculate CuO, Cu₂O and Cu content in a sample the proposed procedure was applied. The developed method provides the accurate results of the determination of CuO, Cu₂O and Cu content in a powdered mixture. The reproducibility expressed as the relative standard deviation is from 1 % to 5 %.     

Preparation of cuprite (Cu2O), paramelaconite (Cu32+Cu21+O4) and tenorite (CuO) with magnetron sputtering ion plating: characterization by EPMA, XRD, HEED and SEM

Cu-O layers were deposited on Si-<100> wafers at 90°?C by means of reactive magnetron sputtering ion plating (R-MSIP). A Cu-target was sputtered in rf-mode by an oxygen/argon plasma, and the influence of the oxygen partial pressure on composition, structure, texture and morphology of the Cu-O layers was investigated. The analysis with EPMA, XRD, HEED and SEM yielded the following results: with an appropriate setting of the oxygen partial pressure, the oxygen content of the films could be controlled between 0 and 50 at-%. XRD bulk structure analysis shows changes in the crystal structure of the films with increasing oxygen content from the fcc structure of Cu, followed by the sc structure of Cu₂O (cuprite), the tetragonal structure of Cu₃ ²⁺Cu₂ ¹⁺O₄ (paramelaconite) to the monoclinic structure of CuO (tenorite). As revealed by HEED, the structure of the near-surface region of the latter two is the same as that of the bulk, whereas in the case of the films with fcc bulk structure, due to oxidation by air, the surface has the sc structure of Cu₂O, and in the case of the film with the sc structure, a monoclinic surface structure of CuO is observed. SEM analyses detected a disordered columnar growth of all Cu-O films.     

XPS,UPS, and STM studies of nanostructured CuO films

A Cu₁O_1.7 oxide film containing a large amout of superstoichiometric oxygen was obtained by low-temperature oxidation of metallic copper in the oxygen plasma. An STM study of the film structure showed that ～10 nm planar copper oxide nanocrystallites with particles packed parallel to the starting metal surface. In an XPS study, the spectral characteristics of the Cu2p and O1s lines indicated that particles with a CuO lattice formed (E _bnd(Cu2p _3/2) = 933.3 eV and a shake-up satellite, E _bnd(O1s) = 529.3 eV). The additional superstoichiometric oxygen is localized at the sites of contact of nanoparticles in the interunit space and is characterized by a state with the binding energy E _bnd(O1s) = 531.2 eV. Due to the formation of a nanostructure in the films during low-temperature plasma oxidation, the resulting copper oxide has a much lower thermal stability than crystalline oxide CuO.     

Derivatives (Cu/CuO,Cu/Cu2O,and CuS) of Cu superstructures reduced by biomass reductants

Metallic Cu is considered as the promising functional material owing to its high conductivity and harmlessness. Here, metallic Cu which presents a unique interconnected and continuous structure (Cu superstructure) is prepared using Magnolia grandiflora leaves as the biomass reductant, a green process which avoided the release of harmful gases and massive energy consumption. What's more, Cu/CuO, Cu/Cu₂O, and CuS nanosheets with different sizes were fabricated using Cu superstructure as the substrate via facile methods, and the morphology is regulated by controlling the relevant factors. The electrochemical sensors based on the three derivations were fabricated to study the sensing performance of glucose. The unique structure of nanosheets encapsulating Cu superstructure guarantees the excellent conductivity of Cu/CuO and Cu/Cu₂O composites. Moreover, the electrochemical stability is improved owing to the nanosheet protective layer. Although no metallic Cu was maintained in CuS, the integrated multilayer nanosheets endow CuS with short channels for fast interlayer electronic transmission and with structural stability.     

Electronic properties of Cu clusters and islands and their reaction with O2 on SnO2(110) surfaces

The deposition of Cu on SnO₂(110) surfaces, and its oxidation to Cu_xO, have been studied by low-energy electron diffraction (LEED) and angle-integrated photoemission using synchrotron radiation photoemission spectroscopy (SRPES). With the growth of copper on SnO₂(110), which was found to follow the Volmer-Weber (“islanding”) growth mode, a small amount of metal-phase Sn segregates to the surface, and even when the copper thickness reaches several tens of Å, Sn metal still is seen at the surface. But when this surface is annealed at 800 K in 5 × 10^?6 mbar O₂ for 20 min, the Sn atoms are totally converted to SnO₂. Simultaneously, the deposited Cu atoms become oxidized. The surface charges up both during LEED and SRPES data acquisition. The clean SnO₂(110) surface shows a 1 × 1 structure. With Cu deposition, the substrate LEED pattern gradually becomes weaker. With even more copper deposited, a Cu(111)-1 × 1-oriented particle structure appears, indicating coalescence of the Cu islands to 3-dimensional Cu(111) epitaxy. After subsequent heating to 500 K, the substrate signal appears again, and we see the SnO₂ 1 × 1 pattern. In conclusion, Cu atoms quite easily form clusters on the SnO₂(110) surface already after a slight heat treatment. The results show that this system is quite active towards O₂ gas exposure, and that the surface conductivity changes during O₂ exposure.     

Mechanochemical Activation of Cu–CeO<Subscript>2</Subscript> Mixture as a Promising Technique for the Solid-State Synthesis of Catalysts for the Selective Oxidation of CO in the Presence of H<Subscript>2</Subscript>

A new ecologically clean method for the solid-phase synthesis of oxide copper–ceria catalysts with the use of the mechanochemical activation of a mixture of Cu powder (8 wt %) with CeO₂ was developed. It was established that metallic copper was oxidized by oxygen from CeO₂ in the course of mechanochemical activation. The intensity of a signal due to metallic Cu in the X-ray diffraction analysis spectra decreased with the duration of mechanochemical activation. The Cu¹⁺, Cu²⁺, and Ce³⁺ ions were detected on the sample surface by X-ray photoelectron spectroscopy. The application of temperature-programmed reduction (TPR) made it possible to detect two active oxygen species in the reaction of CO oxidation in the regions of 190 and 210–220°C by a TPR-H₂ method and in the regions of 150 and 180–190°C by a TPR-CO method. It is likely that the former species occurred in the catalytically active nanocomposite surface structures containing Cu–O–Ce bonds, whereas the latter occurred in the finely dispersed particles of CuO on the surface of CeO₂. The maximum conversion of CO (98%, 165°C) reached by the mechanochemical activation of the sample for 60 min was almost the same as conversion on a supported CuO/CeO₂ catalyst.     

Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O₂ to propylene oxide and acrolein is of great interest and importance. We report the crystal‐plane‐controlled selectivity of uniform capping‐ligand‐free Cu₂O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O₂: Cu₂O octahedra exposing {111} crystal planes are most selective for acrolein; Cu₂O cubes exposing {100} crystal planes are most selective for CO₂; Cu₂O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One‐coordinated Cu on Cu₂O(111), three‐coordinated O on Cu₂O(110), and two‐coordinated O on Cu₂O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO₂, respectively. These results reveal that crystal‐plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.     

CO oxidation on inverse CeO(x)/Cu(111) catalysts: high catalytic activity and ceria-promoted dissociation of O2

A Cu(111) surface displays a low activity for the oxidation of carbon monoxide (2CO + O(2) → 2CO(2)). Depending on the temperature, background pressure of O(2), and the exposure time, one can get chemisorbed O on Cu(111) or a layer of Cu(2)O that may be deficient in oxygen. The addition of ceria nanoparticles (NPs) to Cu(111) substantially enhances interactions with the O(2) molecule and facilitates the oxidation of the copper substrate. In images of scanning tunneling microscopy, ceria NPs exhibit two overlapping honeycomb-type moire? structures, with the larger ones (H(1)) having a periodicity of 4.2 nm and the smaller ones (H(2)) having a periodicity of 1.20 nm. After annealing CeO(2)/Cu(111) in O(2) at elevated temperatures (600-700 K), a new phase of a Cu(2)O(1+x) surface oxide appears and propagates from the ceria NPs. The ceria is not only active for O(2) dissociation, but provides a much faster channel for oxidation than the step edges of Cu(111). Exposure to CO at 550-750 K led to a partial reduction of the ceria NPs and the removal of the copper oxide layer. The CeO(x)/Cu(111) systems have activities for the 2CO + O(2) → 2CO(2) reaction that are comparable or larger than those reported for surfaces of expensive noble metals such as Rh(111), Pd(110), and Pt(100). Density-functional calculations show that the supported ceria NPs are able to catalyze the oxidation of CO due to their special electronic and chemical properties. The configuration of the inverse oxide/metal catalyst opens new interesting routes for applications in catalysis.     

Selective oxidation of styrene on an oxygen‐adsorbed Cu(111): A comparison with Au(111)

The reaction mechanism for the styrene selective oxidation on the oxygen preadsorbed Cu(111) surface has been studied by the density functional theory calculation with the periodic slab model. The calculated result indicated that the process includes two steps: forming the oxametallacycle intermediate (OMMS) and then producing the products. In addition, it was found that the second step, from OMMS to the product, is the rate‐controlling step, which is similar to the previous work of ethylene selective oxidation. The present result indicated that the selectivity towards the formation of styrene epoxide on Cu(111) is much higher than that on Au(111). More importantly, we found that the mechanism via the OMMS (2) (i.e., the preadsorbed atomic oxygen bound to the CH₂ group involved in C₆H₅? CH?CH₂) to produce styrene epoxide is kinetically favored than that of OMMS (1). We also found that the selectivity toward the styrene epoxide formation on Cu₂O is similar to that of Cu(111). © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

CO2 Reduction Mechanism on the Cu2O(110) Surface: A First-Principles Study

Cu₂O is an attractive catalyst for the selective reduction of CO₂ to methanol. However, the mechanism of the reaction and the role of the Cu species in different oxidation states are not well understood yet. In this work, by first-principles calculations, we investigate the mechanism of the reaction on the Cu₂O(110) surface, which is the most selective for methanol, in different degrees of reduction: ideal surface, slightly reduced surface (SRS), and partially reduced surface (PRS). The most favorable reaction pathways on the three surfaces were identified. We found that Cu(I) on the ideal surface is not capable of chemisorbing CO₂, but surface oxygen serves as the active site which selectively converts CO₂ to CH₃OH with a limiting potential of −0.77 V. The Cu(0) on the SRS and PRS promotes the adsorption and reduction of CO₂, while the removal of the residue O* becomes potential/rate limiting with a more negative limiting potential than the ideal surface. The SRS is selective to methanol while the PRS becomes selective to methane. The result suggests that the key to high methanol selectivity is to avoid the reduction of Cu(I), which provides a new strategy for the design of more efficient catalysts for selective CO₂ reduction to methanol.     

Probing enantioselectivity on chirally modified Cu(110), Cu(100), and Cu(111) surfaces

Temperature programmed desorption methods have been used to probe the enantioselectivity of achiral Cu(100), Cu(110), and Cu(111) single crystal surfaces modified by chiral organic molecules including amino acids, alcohols, alkoxides, and amino-alcohols. The following combinations of chiral probes and chiral modifiers on Cu surfaces were included in this study: propylene oxide (PO) on L-alanine modified Cu(110), PO on L-alaninol modified Cu(111), PO on 2-butanol modified Cu(111), PO on 2-butoxide modified Cu(100), PO on 2-butoxide modified Cu(111), R-3-methylcyclohexanone (R-3-MCHO) on 2-butoxide modified Cu(100), and R-3-MCHO on 2-butoxide modified Cu(111). In contrast with the fact that these and other chiral probe/modifier systems have exhibited enantioselectivity on Pd(111) and Pt(111) surfaces, none of these probe/modifier/Cu systems exhibit enantioselectivity at either low or high modifier coverages. The nature of the underlying substrate plays a significant role in the mechanism of hydrogen-bonding interactions and could be critical to observing enantioselectivity. While hydrogen-bonding interactions between modifier and probe molecule are believed to induce enantioselectivity on Pd surfaces (Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240-15249), such critical interactions may be missing on Cu surfaces where hydrogen-bonding interactions are believed to occur between adjacent modifier molecules, enabling them to form clusters or islands.     

The composite catalysts of Cu/CuxO nanoparticles supported on the carbon fibers were prepared for styrene oxidation reaction

In this paper a novel simple method for preparing two different catalysts with various‐valences copper was reported. Carbon nanofibers supported copper‐cuprous oxide nanoparticles (Cu‐Cu₂O NPs/CNFs) and copper oxide nanoparticles (CuO NPs/CNFs) through electrospinning, adsorption and reduction in the high‐pressure hydrogenation and the high‐temperature calcination methods. These catalysts were investigated by a series of characterizations and were applied in reaction in nitrogen atmosphere, which had a good catalytic activity and selectivity of benzaldehyde for the reaction. Above all, the new study has been certified clearly, in which Cu‐Cu₂O NPs/CNFs and CuO NPs/CNFs composite catalysts enhanced the generation of benzaldehydeand the excellent catalytic properties were exhibited.     

Reaction of imidazole in gas phase at very low pressure with Cu foil and Cu oxides studied by X‐ray photoelectron spectroscopy

The gas‐phase reactions of imidazole with Cu foil, CuO and Cu₂O powders at a pressure of 10^?5 Torr are studied by means of X‐ray photoelectron spectroscopy (XPS) at room temperature for 1 h. The reactivity of gaseous imidazole is very intense with Cu foil, weak with CuO and nil with Cu₂O. The reaction product on the Cu foil is cuprous imidazolate of oligomeric nature and with CuO is cupric imidazolate of very low molecular weight. Copyright © 2006 John Wiley & Sons, Ltd.     

Selective electrodeposition of ZnO onto Cu2O

The co-deposition of cuprous oxide (Cu₂O) and zinc oxide (ZnO) on indium tin oxide (ITO) substrate was executed by two different electrochemical methods and the formation mechanism of ZnO onto Cu₂O was investigated by ex-situ SEM, XRD, and XPS. The single galvanostatic electrodeposition step in a mixed nitrate electrolyte offered a useful method in preparing ZnO onto triangular Cu₂O islands formed. On the other hand, hexagonal shaped ZnO phase was electrodeposited on ITO substrate as well as on Cu₂O islands when two steps of the galvanostatic and potentiostatic process were applied.     

Nanoscale oxidation of Cu100: oxide morphology and surface reactivity

Surface oxidation of Cu(100) in O(2) has been investigated in situ by x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy (XAES), and scanning tunneling microscopy (STM) as a function of surface temperature (T(S)=303-423 K) and O(2) pressure (p(O(2) )=3.7 x 10(-2)-213 mbars). Morphology of the oxide on the surface and in the near surface layers is characterized by utilizing STM and the inelastic electron background of the XAES O KLL signal. Analysis of the peak shape of the XAES Cu LMM facilitates the quantification of Cu, Cu(2)O, and CuO surface concentrations. The authors conclude that the surface oxidation of Cu(100) proceeds in three distinct steps: (1) Dissociative adsorption of O(2) and the onset of Cu-(2 square root 2 x square root 2)R45 degrees -O (theta(O)=0.5 ML) surface reconstruction, (2) initial formation of Cu(2)O and the appearance of 1.8 A high elongated islands that also adopt the Cu-(2 square root 2 x square root 2)R45 degrees -O structure, and (3) formation of highly corrugated Cu-O islands which together with the surface reconstruction strongly enhance the reactivity of the surface towards further oxide formation. Both Cu(2)O and CuO formations are enhanced by increased surface temperature, but no pressure dependence can be seen.     

Island morphology statistics and growth mechanism for oxidation of the Al(111) surface with thermal O2 and NO

Scanning tunneling microscopy (STM) was employed to study the mechanism for the oxidation of Al(111) with thermal O2 and NO in the 20%-40% monolayer coverage regime. Experiments show that the islands formed upon exposure to thermal O2 and NO have dramatically different shapes, which are ultimately dictated by the dynamics of the gas surface interaction. The circumference-to-area ratio and other island morphology statistics are used to quantify the average difference in the two island types. Ultrahigh-vacuum STM was employed to make the following observations: (1) Oxygen islands on the Al(111) surface, formed upon exposure to thermal oxygen, are elongated and noncompact. (2) Mixed O/N islands on the Al(111) surface, formed upon exposure to thermal nitric oxide (NO), are round and compact. (3) STM movies acquired during thermal O2 exposure indicate that a complex mechanism involving chemisorption initiated rearrangement of preexisting oxygen islands leads to the asymmetric and elongated island shapes. The overall mechanism for the oxidation of the Al(111) surface can be summarized in three regimes. Low coverage is dominated by widely isolated small oxygen features (<3 O atoms) where normal dissociative chemisorption and oxygen abstraction mechanisms are present. At 20%-40% monolayer coverage, additional oxygen chemisorption induces rearrangement of preexisting islands to form free-energy minimum island shapes. At greater than approximately 40% monolayer coverage, the apparent surface oxygen coverage asymptotes corresponding to the conversion of the 2D islands to 3D Al2O3 surface crystallites. The rearrangement of oxygen islands on the surface to form the observed islands indicates that there is a short-range oxygen-oxygen attractive potential and a long-range oxygen-oxygen repulsive potential.     

An Effective Non-Enzymatic Glucose Biosensor Based on Nanostructured CuxO/Cu Electrodes Synthesized in Situ by Copper Anodization

Potentiostatic anodization was developed to synthesize copper oxide/copper (Cu_xO/Cu, x=1,2) electrode with nano structure for sensitive non-enzymatic glucose detection. At a catalytic potential of 0.55 V, the CuO/Cu electrode presented a high sensitivity of 2954.38 μA mM⁻¹ cm⁻² to glucose and a linear range of 0.1 mM to 1.3 mM. The response time is less than 3 s with addition of 0.1 mM glucose. The CuO/Cu electrode above was anodized in 1M KOH solution at −100 mV and the morphology was compact nanoparticles and sparsely dispersed nanosheets, which enlarged the surface area and provided abundant electrocatalytic active sites. Compared the sensing property of electrodes with different morphologies, it indicated that nanostructure was significant to the efficient glucose catalytic oxidation process and it could be regulated by changing the potential and electrolyte concentration during anodization.     

Das Kupfer(II)-oxidphosphat Cu4O(PO4)2 in einer neuen,orthorhombischen Modifikation durch Oxidation einer Tl/Cu/P-Legierung

The Copper(II) Oxide Phosphate Cu₄O(PO₄)₂ in a New, Orthorhombic Modification by Oxidation of a Tl/Cu/P Alloy Single crystals of Cu₄O(PO₄)₂ in a new, orthorhombic modification were prepared by reaction of a Tl/Cu/P alloy with oxygen. The compound crystallizes in the space group Pnma with Z = 4 and lattice constants a = 808.8(5) pm, b = 627.0(7) pm, c = 1338.3(1) pm. It is isotypic with the orthorhombic form of Cu₄O(AsO₄)₂. The copper atoms are five-coordinated by oxygen. The distorted square-pyramidal and trigonal-bipyramidal polyhedra are connected by edges and vertices to form [Cu₈O₁₈] ribbons running along [010] and forming slabs perpendicular to the c-axis interconnected by phosphate groups.