Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O₂ at low pressure (<5 × 10⁻⁶ mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu₂O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu₂O(1 1 1)-like structure.     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Nano-scale copper oxidation on leadframe surface

Copper oxidation studies were carried out by means of field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) techniques. The growth of copper oxide occurs as a copper surface comes in an oxygen containing environment. The reaction sequence leading to oxidation of the copper surface is generally accepted to be oxygen chemisorption, nucleation and growth of the surface oxide and bulk oxide growth. HRTEM examination of the cross section of the oxidized copper sample revealed the interface region in between the copper and copper oxide. At high oxidation temperature, formation of micro-voids and separations were observed along this interface region. Poor adhesion at this interface region due to micro-voids and separation were found to be the root cause of delamination issue. EELS analysis determined that for regions with intact interface the oxidation system is Cu/CuO/Cu₂O/CuO, however, in regions containing micro-voids or separation it is found to be Cu/Cu₂O/CuO.     

Surface segregation and oxidation studies on Cu-Ni-Sn and Ag-Cu-Ge ternary alloys

Surface segregation and oxidation of Cu-9.7at%Ni-2.3at%Sn and Ag-39at%Cu-1.6at%Ge ternary alloys have been carried out employing XPS and AES techniques. Segregation of both Ni and Sn have been observed on the surface of Cu-Ni-Sn alloy. On oxidation of this alloy at 500 K, Cu₂O, NiO and SnO have been observed and on heating the oxidized alloy in vacuum to 600 K, Cu₂O was reduced to Cu through a displacement reaction involving Ni. Further heating to 750 K showed only SnO₂ along with the reduction of NiO to Ni. The Ag-Cu-Ge alloy showed segregation of both Ag and Ge to the surface. On oxidation at 500 K, only Cu₂O and GeO have been observed. Heating of the oxidized surface to 700 K in vacuum showed desorption of GeO, coupled with the reduction of Cu₂O to Cu, thereby leaving the alloy surface clean.     

Shape evolution, photoluminescence and degradation properties of novel Cu2O micro/nanostructures

Three kinds of novel cuprous oxide (Cu₂O) micro/nanostructures are synthesized via a facile template-free hydrothermal method. Two factors are critical for the growth process of typical samples: the concentration of copper ions (Cu(II)) and the addition of Polyvinylpyrrolidone (PVP) as surfactant. It is found that the application of ethanol as solvent speeds up the reduction rate of Cu(II), and it promotes the aggregating of Cu₂O nanocrystals at the preliminary stage to form irregular spherical structures. Photoluminescence (PL) properties of the three kinds of samples and their photocatalytic activities for degradation of Methyl Orange (MO) are also measured. The sample with higher concentration of copper vacancy (V _Cu) defects has better photocatalytic ability, indicating that besides the morphology of Cu₂O nano/microcrystals, the defects in crystalline structures can also influence their electrical characteristics, and thus change their photocatalytic activity. This provides a potential method to improve the photocatalytic performances of Cu₂O crystals.     

One-pot synthesis of single-crystalline Cu2O hollow nanocubes

Single-crystalline Cu₂O hollow nanocubes have been successfully synthesized via a simple wet chemical route in the absence of any surfactants or templates. By studying the growth process of the Cu₂O hollow nanocubes, we found that the Cu₂O hollow nanocubes were formed through a reducing and simultaneously etching process. The speed of reducing Cu(OH)₂ into Cu₂O was much faster than the speed of etching Cu₂O. As a result, Cu₂O solid nanocubes were firstly formed, and then the solid nanocubes were gradually etched into hollow nanocubes.     

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.     

Initial oxidation kinetics and energetics of Cu0.5Au0.5 (0 0 1) film investigated by in situ ultrahigh vacuum transmission electron microscopy

The initial oxidation behavior of Cu_0.5Au_0.5 (0 0 1) thin film was investigated by in situ ultrahigh vacuum transmission electron microscopy to model nano-oxidation of alloys with one active component and one noble component. The formation of irregular-shaped octahedron Cu₂O islands with cube-on-cube crystallographic orientation to the substrate film was observed at all temperature studied. The energetics of Cu₂O nucleation for Cu and Cu_0.5Au_0.5 oxidation was compared. Cu_0.5Au_0.5 oxidation has lower nucleation activation energy due to the reduced mismatch strain between Cu₂O and Cu_0.5Au_0.5 films. On the other hand, the reaction kinetics for Cu_0.5Au_0.5 alloy oxidation is slower due to the higher diffusion activation energy of Cu.     

Copper-containing polyvinyl alcohol composite systems: Preparation,characterization and biological activity

The present investigation reports, the complex formation of Cu(II) with polyvinyl alcohol (PVA) and the synthesis of PVA-stabilized Cu₂O particles. This PVA–Cu₂O composite has been prepared via chemical reduction method using PVA–Cu(II) complex as precursor. At first, Cu(II) ions were stabilized in PVA matrix via complex formation with OH groups; subsequently, this PVA–Cu(II) macromolecular complex as precursor reacted with ascorbic acid as reducing agent at pH=12 to prepare PVA–Cu₂O composite. The products were characterized by FTIR, XRD, FE-SEM, HRTEM, Visible Spectroscopy and atomic absorption. In the following, the antibacterial properties of as-prepared composites were examined against Gram-positive (Bacillus thuringiensis) and Gram-negative bacteria (Escherichia coli), and the results showed excellent antibacterial activity of these materials.     

Optical characterization of thin thermal oxide films on copper by ellipsometry

A complete optical characterization in the visible region of thin copper oxide films has been performed by ellipsometry. Copper oxide films of various thicknesses were grown on thick copper films by low temperature thermal oxidation at 125 °C in air for different time intervals. The thickness and optical constants of the copper oxide films were determined in the visible region by ellipsometric measurements. It was found that a linear time law is valid for the oxide growth in air at 125 °C. The spectral behaviour of the optical constants and the value of the band gap in the oxide films determined by ellipsometry in this study are in agreement with the behaviour of those of Cu₂O, which have been obtained elsewhere through reflectance and transmittance methods. The band gap of copper oxide, determined from the spectral behaviour of the absorption coefficient was about 2 eV, which is the generally accepted value for Cu₂O. It was therefore concluded that the oxide composition of the surface film grown on copper is in the form of Cu₂O (cuprous oxide). It was also shown that the reflectance spectra of the copper oxide–copper structures exhibit behaviour expected from a single layer antireflection coating of Cu₂O on Cu. Received: 19 July 2001 / Accepted: 27 July 2001 / Published online: 17 October 2001     

A novel one-step electron beam irradiation method for synthesis of Ag/Cu2O nanocomposites

In this paper, a novel method for fabrication of silver/cuprous oxide (Ag/Cu₂O) nanocomposites is reported. The method involves the reduction of Ag⁺ and Cu²⁺ in the aqueous solution to Ag/Cu₂O without adding any reducing reagent under electron beam (EB) irradiation. Dye methyl orange is used as the pollutant model to investigate the photocatalytic properties of these nanocomposites. The results reveal that they have higher photocatalytic efficiencies than that of Cu₂O under visible light. These visible light-sensitive catalysts may have potential application in the field of environmental remediation.     

Identification of Cu(I) and Cu(II) oxides by electron spectroscopic methods: AES,ELS and UPS investigations

The externally prepared black-coloured copper oxide (T? 700 K, P_O2 ? 100 torr) on a Cu(100) surface is identified by electron spectroscopy as CuO. Compared to the red-coloured Cu(I) oxide (in situ oxidation at T ? 400 K, P_O2 ? 0.5 torr, ～ 10⁹ L), the He(I)- excited photoemisson from CuO reveals characteristic shake-up satellites 10–12 eV below E_F and a broadened emission from overlapping oxygen-induced 2p and Cu 3d states. From the AES and ELS results, in correlation with the data from core electron spectroscopy, chemical shifts of Cu 2p, Cu 3s and Cu 3p in CuO to higher binding energy and decreases in binding energy of the oxygen-induced states were deduced. The unoccupied electron states of Cu at 5 and 7.5 eV above E_F — postulated from the ELS results — are preserved in Cu₂O and CuO compounds. Annealing of the Cu(II) oxide at 670 K is accompanied by decomposition into Cu₂O due to the solid-state reaction following the scheme: 2CuO → 1/2 O₂ + Cu₂O.     

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ～ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ_Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H₂ and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ_Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H₂2H⁺ couple at platinum electrodes.     

Fast and selective Cu2O nanorod growth into anodic alumina templates via electrodeposition

The fast and selective growth of cuprous oxide (Cu₂O) nanorods into anodic aluminum oxide (AAO) templates is achieved under optimized alkaline conditions via electrochemical deposition. The growth rate of Cu₂O nanorods at room temperature reached 360 nm/min, the fastest rate reported to date. The synthesis of Cu₂O nanorods by applying a constant current by using Cu₂O nanotubes as a transition state is extensively discussed; a Pt pottery-shaped layer played a key role as a seed layer for the fast Cu₂O growth. We report here the existence of regions of nanostructured Cu₂O based on our studies and previous relevant works, which include potential-pH curves for Cu²⁺-lactate solutions.     

EPIR effect of Cu2O films by electrochemical deposition

Cuprous oxide (Cu₂O) films and Cu/Cu₂O/Cu/FTO sandwich structures were prepared by electrochemical deposition on conductive FTO substrates with different pH value conditions but constant deposition potential. The phase composition, crystal structure and microstructure of the Cu₂O films were characterized by XRD, SEM and EDS as well as by Electric–Pulse–Induced–Resistance (EPIR) perturbation. In particular, the switching effects of the Cu/Cu₂O/Cu/FTO device are examined in this work. The result shows that the EPIR-effect is large for the Cu/Cu₂O/Cu/FTO device at room temperature and strongly related to the pH value of the solution. In both acidic and neutral conditions, for example at pH = 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value. It disappears when the pH value goes further into the alkaline regime, i.e. pH = 8, 9 and 10. Space charge barriers at the interface of electrode and Cu₂O are used to explain the I–V characteristic of the layer structure and the EPIR-effect.     

Metal film growth on regular and defective MgO(0 0 1) surface: A comparative ab initio simulation and thermodynamic study

In order to understand the difference in metallic film growth modes on perfect and defective oxide substrates, we have combined ab initio B3LYP periodic calculations on the slab models of the corresponding Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. For a defectless magnesia surface, we confirm the experimentally observed submonolayer growth of 3D metallic islands (Ag possesses a higher trend than Cu). Formation of F_s centers (neutral O vacancies) on the substrate markedly enhances metal atom adsorption as compared to physisorption over regular sites on a defect-free substrate. For the first time, we predict that the presence of these surface defects (beginning with concentrations of 5% for Cu and 22% for Ag) can stimulate the growth of uniform 2D metallic sublayers.     

Fabrication of cuprous and cupric oxide thin films by heat treatment

Cuprous oxide (Cu₂O) and cupric oxide (CuO) thin films were prepared by thermal oxidation of copper films coated on indium tin oxide (ITO) glass and non-alkaline glass substrates. The formation of Cu₂O and CuO was controlled by varying oxidation conditions such as, oxygen partial pressure, heat treatment temperature, and oxidation time. The microstructure, crystal direction, and optical properties of copper oxide films were measured with X-ray diffraction, atomic force microscopy, and optical spectroscopy. The results indicated that the phase-pure Cu₂O and CuO films were produced in the oxidation process. Optical transmittance and reflectance spectra of Cu₂O and CuO clearly exhibited distinct characteristics related to their phases. The electrical properties indicated that these films formed ohmic contacts with Cu and ITO electrode materials. Multilayers of Cu₂O/CuO were fabricated by choosing the oxidation sequence. The experimental results in this paper suggest that the thermal oxidation method can be employed to fabricate device quality Cu₂O and CuO films that are up to 200–300 nm thick.     

Effective phototransformation in a heterostructure based on copper(I) oxide and cadmium tin oxide

We present a heterostructure consisting of anodic copper oxide Cu₂O on a copper substrate and a transparent Cd–Sn–O conducting film for use in solar cells. Focusing on simplicity and the availability of film fabrication techniques, we chose anodic oxidation for forming the Cu₂O film and the extraction-pyrolysis technique for forming the transparent Cd–Sn–O conducting layer. We demonstrate the possibility of considerable enhancement of the phototransformation efficiency in the Cu–Cu₂O/Cd–Sn–O structure over this parameter in the Cu–Cu₂O structure.     

Synthesis of quasi-1D cuprous oxide nanostructure and its structural characterizations

A wealth of superfine polycrystalline cuprous oxide (Cu₂O) nanowires have been synthesized with hydrazine hydrated (N₂H₄·H₂O), act as the reducing agent, and Cu(OH)₂ nanowires, act as a soft template and surfactant, at room temperature. Two methods were employed for the synthesis of these nanowires, i.e. with and without capping agent (polyethylene glycol Mw 8000). Techniques of powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) pattern, electron diffraction X-ray (EDX) spectroscopy, and UV-visible (UV-vis) spectroscopy have been used to characterize the morphology, structure, crystallinity, purity, and composition of nanowires. The average diameters of Cu₂O nanowires, prepared with and without capping agent, were observed to be 8-10 and 12-15 nm and lengths of several microns, respectively. It is found that capping agent (PEG) confines the dimensions of synthesized nanowires. In addition, the observed optical band gap of products show blue-shift effect compared to the bulk Cu₂O (E_g=2.17 eV), which ascribe it as a promising material for the conversion between solar energy and electrical or chemical energy.     

Caltrop particles synthesized by photochemical reaction induced by X‐ray radiolysis

X‐ray radiolysis of a Cu(CH₃COO)₂ solution was observed to produce caltrop‐shaped particles of cupric oxide (CuO, Cu₂O), which were characterized using high‐resolution scanning electron microscopy and micro‐Raman spectrometry. X‐ray irradiation from a synchrotron source drove the room‐temperature synthesis of submicrometer‐ and micrometer‐scale cupric oxide caltrop particles from an aqueous Cu(CH₃COO)₂ solution spiked with ethanol. The size of the caltrop particles depended on the ratio of ethanol in the stock solution and the surface of the substrate. The results indicated that there were several synthetic routes to obtain caltrop particles, each associated with electron donation. The technique of X‐ray irradiation enables the rapid synthesis of caltrop cupric oxide particles compared with conventional synthetic methods.