Hans-Henning Strehblow

In situ STM study of the duplex passive films formed on Cu(111) and Cu(001) in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (ECSTM) investigations of the anodic Cu(I)/Cu(II) duplex passive layers grown on Cu(1 1 1) and Cu(0 0 1) in 0.1 M NaOH are reported. The outer Cu(II) part of the duplex film formed on both substrates is crystalline with a terrace and step topography. The observed lattices are consistent with a bulk-like terminated CuO(0 0 1) surface on both substrates. This common crystallographic orientation is explained by the hydroxylation of the otherwise polar and unstable oxide surface at the passive film/electrolyte interface. The epitaxy of the oxide layers is governed by the parallel alignment of the close packed directions of the CuO outer layers and Cu2O inner layers on both substrates. A granular and amorphous layer covering the crystalline CuO(0 0 1) oxide has been observed on Cu(0 0 1) but not on Cu(1 1 1). It is assigned to a film of copper hydroxide corrosion products formed by a dissolution–precipitation mechanism. Its absence on the passivated Cu(1 1 1) surface is explained by the higher stability of the Cu2O(1 1 1) precursor oxide formed on this substrate in the initial stages of growth of the duplex passive film, resulting in a lower amount of dissolved copper.

Initial and later stages of anodic oxide formation on Cu, chemical aspects, structure and electronic properties

Abstract The formation of OH adsorption layers and the initial and later stages of the growth of anodic oxide have been studied on Cu(111) single crystal surfaces with STM. In 0.1 M NaOH Cu forms OH− adsorption layers for E>−0.57 V (SHE) whereas oxide formation occurs at E>−0.22 V. The adsorption of OH− and oxide formation can be followed by in situ electrochemical STM. At sufficiently high negative potentials even time resolved investigations become possible. OH adsorption goes along with a surface reconstruction. Oxide formation starts with small non-crystalline grains and with oxide crystals in a later stage. In borax buffer pH 9.2 no stable image is obtained during the growth of the Cu2O films with d>1 nm whereas the images of the duplex layer formed at higher potentials are stable again. These details are discussed on the basis of previous results on the chemical composition of the passive layer in alkaline solutions and its semiconducting properties obtained from XPS, UPS, and electrochemical and photoelectrochemical investigations.

The behavior of stainless steels in physiological solution containing complexing agent studied by X-ray photoelectron spectroscopy.

The passive film formed by electrochemical oxidation on two different stainless steels differing in molybdenum (Mo) content in physiological solution with and without the addition of complexing agent, i.e., citrate, was studied using X-ray photoelectron spectroscopy. The alloys were polarized at different oxidation potentials in the electrochemical chamber attached to the spectrometer. Thus, the composition of the film formed by oxidation was analyzed by X-ray photoelectron spectroscopy without prior exposure to air (quasi in situ). The passive film formed in physiological solution consists of two predominant oxides, i.e., chromium and iron oxides. Oxides of alloying elements nickel and Mo are also detected in the film. It seems that the strong enrichment of oxidized chromium and Mo in the passive layer, and strong enrichment of Mo and depletion of iron at the metal surface underneath the passive layer, are responsible for the outstanding corrosion resistance of orthopedic stainless steel in physiological solution. Commercial AISI 304 is not suitable for orthopedic applications. The addition of complexing agent affects significantly the passivation behavior of orthopedic stainless steel, because it changes the distribution of the elements within the passive layer and at the metal surface underneath.

The formation of Cu2O layers on Cu and their electrochemical and photoelectrochemical properties

Abstract Cu 2 O films up to 30 nm have been formed on Cu 2 O passivating layers on Cu by cathodic reduction of (CuO 2 ) 2- in 5 M KOH solutions. These films grow linearly with time. At E > 0.2 V they may be partially oxidized to Cu(II) oxide which grows according to a high field mechanism. With XPS a Cu 2 O composition is found for the cathodically formed films and a Cu(II) oxide overlayer at E > 0.2 V with large amounts of hydroxide. Only negative photocurrents are obtained. The results follow best a simple semiconductor model with p-type Cu 2 O. The band gap of Cu 2 O decreases slightly with the oxide thickness and fits well the data of passive oxides and the crystalline material. The flat-band potential equals the value of the passive film. The Cu(I)/Cu(II) duplex layer yields a much more positive flat-band potential, which is a consequence of the potential drop at the interfaces between both oxides and the Cu(II) oxide and the electrolyte.

In situ STM study of the anodic oxidation of Cu(0 0 1) in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy measurements of the anodic oxidation of Cu(0 0 1) in 0.1 M NaOH are reported. Adsorption-induced surface reconstruction is observed in the underpotential range of oxidation with the formation of dimers of superimposed Cu atoms ejected from the substrate and stabilized by adsorbed OH groups, presumably in bridging positions. The reconstruction causes the reorientation of the substrate step edges and the formation of holes and ad-islands of monoatomic height. The dimers of superimposed Cu atoms are alternatively aligned along the � 100 � directions to form zig-zag arrangements. Long range ordering is observed in areas of limited lateral extension with c(2/6) and c(6/2) domains. In the potential range of Cu(I) oxide formation, a facetted Cu2O layer grows with a Cu2O(0 0 1) /[1 ¯ 1 0] / jj Cu(0 0 1)[1 0 0] epitaxial relationship. The 458 rotation between the close-packed directions of the oxide lattice and metal lattice results from the orientation of the dimers of superimposed Cu atoms in the precursor adsorbed OH layer. The surface of the oxide layer is facetted due to a tilt of � /3% between oxide and metal lattices. Its (0 0 1) terraces have an identical chemical termination and are presumably hydroxylated. # 2003 Elsevier Science B.V. All rights reserved.

XPS and UPS examinations of passive layers on Ni and FE53Ni alloys

Passive layers on Ni and Fe53Ni formed in alkaline and acidic electrolytes have been studied by XPS. Both metals form an oxide barrier with NiO as constituent growing according to a high field mechanism. In alkaline solutions a Ni(OH) 2 containing overlayer is found. Characteristic shifts of the XPS and UPS signals in the transpassive range are interpreted by a change of Ni(OH) 2 to NiOOH on the basis of a simple semiconductor model. The features of the polarization curves can be closely related to the the surface analytical results

In situ STM study of the effect of chlorides on the initial stages of anodic oxidation of Cu(111) in alkaline solutions

Abstract In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the mechanisms of growth of passive layers on Cu(111) in NaOH solutions in the presence of chlorides. For [Cl−]/[OH−]=0.01, the same ordered precursor phase of adsorbed OH is observed in the underpotential region of oxidation as in Cl−-free solutions. Atomically resolved images reveal the structure of the reconstructed topmost metal plane and the threefold hollow adsorption site of the hydroxide. The induced reconstruction causes the ejection of Cu atoms that contribute to the observed lateral growth of the terraces and to the formation of 2D Cu ad-islands in the final stages of the adsorption process. For [Cl−]/[OH−]=0.1, threadlike nanostructures resulting from the reaction of the ejected Cu atoms with chlorides are formed before agglomeration with the 2D Cu ad-islands formed in the final stage of the hydroxide adsorption process. For [Cl−]/[OH−]=10, the step edges, which are normally the preferential sites of the reaction with hydroxide, are blocked by the formation of non-ordered surface chloride complexes. Hydroxide adsorption still predominates the surface reaction on the terraces but the 2D ad-islands form immediately due to the blocking of the step edges. In the potential range of Cu(I) oxide formation, crystalline Cu(I) oxide layers are formed with a high density of steps and (111) terraces. Their step edges are rougher in the presence of chlorides which indicates a Cl−-enhanced localized dissolution reaction of the oxide layers at step edges.

Passivity of cobalt in borate buffer at pH 9.3 studied by x-ray photoelectron spectroscopy

Passive layers on cobalt were prepared under potentiostatic control in borate buffer pH 9.3 and examined with x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed in a closed system under protection of purified argon. The XPS signals were evaluated quantitatively on the basis of standard spectra, which yields the composition of the passive layer, i.e. the contribution of the cobalt and oxygen species Co(0), Co(II), Co(III), OH−, O2− and H2O. Depending on the potential, cobalt forms two different passive layers. The primary passive film consists of Co(II) oxide and Co(II) hydroxide and is formed at low potentials. At high potentials a secondary passive layer is formed consisting of Co(II)/Co(III) mixed oxides. The oxidation to Co(III) is related to a significant increase of the layer thickness from 1 to 4 nm. Time-resolved XPS investigations show that the primary passive film grows within <1 s, whereas the Co(III) formation of the secondary passive film requires at least 300 s. Copyright

In Situ STM Study of the Initial Stages of Anodic Oxidation of Cu(111) in the Presence of Sulfates

In situ scanning tunneling microscopy (STM) has been applied to study adsorption and oxide formation on Cu(111) in weakly alkaline, neutral, and acidic sulfate-containing solutions. In 5 × 10 4 M NaOH + 5 × 10 3 M Na 2 SO 4 (pH ∼ 10.5), a hexagonally structured OH layer is formed at potentials well below oxide formation despite the tenfold larger sulfate concentration showing a stronger interaction of OH with the Cu surface than SO 2 4. In 5 X 10 3 M Na 2 SO 4 (pH ∼ 7), a partially structured and highly mobile adlayer is formed at potentials quite near the onset of oxide formation with coexisting (3 × 3)-ordered domains and nonordered domains. This is assigned to the competitive adsorption of water molecules with sulfate anions. In contrast, a highly ordered and stable (√3 x √7) adsorbed layer is formed in 5 x 10 - 3 M H 2 SO 4 . It is assigned to the coadsorption of sulfate anions and cations (H 3 O + or H + ), the latter compensating the repulsive forces between sulfate anions and thus stabilizing the adlayer. The anodic Cu(1) oxide is (111)-oriented and has a faceted surface in solutions of alkaline, weakly alkaline. and neutral pH.

The structure of Cu- and Cd-UPD-layers on a stepped Pt(533) single crystal surface studied by grazing incidence XAFS, XRD and XPS

Small two-dimensional clusters and linear nanostructures of copper and cadmium were prepared electrochemically on the stepped surface of a (533) Pt single crystal. The deposition was performed under potentiostatic conditions in a solution of 0.001 M CuSO4 or 0.005 M CdSO4 in H2SO4 in the potential range of under potential deposition (UPD). The surface was examined with X-ray photoelectron spectroscopy (XPS) for quantification of the deposited amount. Grazing incidence X-ray absorption fine structure (GIXAFS) and grazing incidence X-ray diffraction (GIXRD) were used for determination of the adsorbate structure. The X-ray absorption and X-ray diffraction experiments were performed in-situ within the electrolyte at X-ray beam lines at HASYlab (DESY, Hamburg). Combining the results of the different methods, it was possible to reveal the structures of the metal adsorbates at several characteristic potentials in the UPD range.