Oscar Steenhaut

Auger Electron Spectroscopy Element Profiles and Interface with Substrates of Electroless Deposited Ternary Alloys

Electroless NiMeP alloys (Me = Cu, Sn, Sb) with high phosphorus content (∼11 weight percent) and a low weight percent of the third component (Me) have been plated in acidic baths onto aluminum, iron, and nickel. Scanning Auger electron spectroscopy is applied to study the element profiles and interface with the substrates. Generally the third component follows the profiles of Ni and P, which proves the alloy formation. A surface enrichment in the third element (Me) is observed in all cases. It is very strong when tin is the third component and very weak in the case of antimony, which is more or less uniformly distributed through the thickness. Decrease of surface concentration of phosphorus is noticed when NiSbP is plated. In Cu profiles three different zones exist : a surface enrichment zone, a groove (a zone of reduced concentration), and a plateau (a zone of almost constant concentration). There is no enrichment in the third element at the interface with the substrate for all three alloys and three substrates. At the interface with aluminum prepared for electroless plating with double zincate pretreatment complete dissolution of the zincate layer has been established.

Electroless deposited Ni-Re-P, Ni-W-P and Ni-Re-W-P alloys

Coatings of electroless Ni–W–P, Ni–Re–P and Ni–W–Re–P alloys were plated in alkaline citrate baths containing amino alcohols, but not free ammonia ions. The reference Ni–P alloy was used as an intermediate layer in the sandwich: Ni–Me–P/Ni–P/substrate. An extremely homogeneous thickness distribution of all alloy components was found by applying scanning Auger electron spectroscopy (SAES(. The inclusion of refractory metals at the expense of nickel and without substantial change in phosphorus content was established. A non-oxidized state of the codeposited Re and W in Ni–W–P, Ni–Re–P and Ni–W–Re–P alloys was determined by means of X-ray photoelectron spectroscopy examination, as well as by SAES profiles, revealing the absence of oxygen throughout the coatings. All alloy films are amorphous and paramagnetic.

Crystalline and Amorphous Electroless Co-W-P Coatings

Electroless deposition onto polycrystalline (Cu, Au) and amorphous (Ni-P) substrates was applied to prepare Co-W-P coatings of two types: crystalline (hexagonal close packed, hcp), with low phosphorus content about 2.4-2.7 atom %, and amorphous, with P concentration within 7.4-8.3 atom %. Tungsten content varied typically in the narrow range of 2.9-3.7 atom % in both types of coatings. Atomic force microscopy revealed substantial difference in their morphology. Polycrystalline Co-W-P coatings consist of grains of stacked plates (lamellas), confirmed by transmission electron microscopy also. Amorphous films are smoother and uniform. The crystalline structure promotes the surface oxidation to a higher extent than the amorphous structure, as shown by the X-ray photoelectron spectroscopy (XPS). Auger electron spectroscopy depth profiles display oxidation, smoothly diminishing toward the inside of the crystalline films. Amorphous coatings are oxidized only at the surface. Inside both types of coatings, however, all alloy components are in nonoxidized form, according to XPS data. Differential scanning calorimetry (DSC) studies of amorphous coatings revealed three transformation peaks, ascribed to a crystallization of hypoeutectic alloy and a transition of Co-based hcp phase into face-centered cubic. Magnetic properties variation with temperature is in agreement with DSC results.

Pt-Ni carbon-supported catalysts for methanol oxidation prepared by Ni electroless deposition and its galvanic replacement by Pt

Pt–Ni particles supported on Vulcan XC72R carbon powder have been prepared by a combination of crystalline Ni electroless deposition and its subsequent partial galvanic replacement by Pt upon treatment of the Ni/C precursor by a solution of chloroplatinate ions. The Pt-to-Ni atomic ratio of the prepared catalyst has been confirmed by EDS analysis to be ca. 1.5:1. No shift of Pt XPS peaks has been observed, indicating no significant modification of its electronic properties, whereas the small shift of the corresponding X-ray diffraction (XRD) peaks indicates the formation of a Pt-rich alloy. No Ni XRD peaks have been observed in the XRD pattern, suggesting the existence of very small pockets of Ni in the core of the particles. The surface electrochemistry of electrodes prepared from the catalyst material suggests the existence of a Pt shell. A moderate increase in intrinsic catalytic activity towards methanol oxidation in acid has been observed with respect to a commercial Pt catalyst, but significant mass specific activity has been recorded as a result of Pt preferential confinement to the outer layers of the catalyst nanoparticles.

Comparison of the Structure and Chemical Composition of Crystalline and Amorphous Electroless Ni-W-P Coatings

Electroless deposition was applied to prepare Ni-W-P coatings of two types: crystalline, with low P and W, and amorphous with high W content. Their morphology was studied by atomic force microscopy. Polycrystalline Ni-W-P alloys consist of grains of stacked plates (lamellas), as it was revealed by transmission electron microscopy. The coatings exhibit a (100) texture. X-ray diffraction and electron diffraction analysis demonstrated the unit cell parameter of the crystalline phase in Ni-W-P is practically equal to that of pure Ni. This implies W and P are localized along the grain boundaries. The Warren/Averbach method was applied to determine microstrain and size of coherent scattering domains. The crystalline structure promotes the surface oxidation of Ni-W-P to higher extent in comparison with the amorphous structure. X-ray photoelectron spectroscopy analysis demonstrated the presence of oxygen and carbon in the bulk of the crystalline Ni-W-P coatings in larger quantities than in the amorphous. In the crystalline coatings in addition to oxygen and carbon, scanning Auger electron spectroscopy showed the presence of nitrogen. It is supposed that all these elements come from ligand residues adsorbed at the grain boundaries during coating growth. Nanoindentation tests indicated that amorphous Ni-W-P samples display more uniform surface mechanical properties at the nanometer scale than the crystalline ones.

Carbon-supported Pt(Cu) electrocatalysts for methanol oxidation prepared by Cu electroless deposition and its galvanic replacement by Pt

Bimetallic Pt–Cu carbon-supported catalysts (Pt(Cu)/C) were prepared by electroless deposition of Cu on a high surface area carbon powder support, followed by its partial exchange for Pt; the latter was achieved by a galvanic replacement process involving treatment of the Cu/C precursor with a chloroplatinate solution. X-ray diffraction characterization of the Pt(Cu)/C material showed the formation of Pt-rich Pt–Cu alloys. X-ray photoelectron spectroscopy revealed that the outer layers are mainly composed of Pt and residual Cu oxides, while metallic Cu is recessed into the core of the particles. Repetitive cyclic voltammetry in deaerated acid solutions in the potential range between hydrogen and oxygen evolution resulted in steady-state characteristics similar to those of pure Pt, indicating the removal of residual Cu compounds from the surface (due to electrochemical treatment) and the formation of a compact Pt outer shell. The electrocatalytic activity of the thus prepared Pt(Cu)/C material toward methanol oxidation was compared to that of a commercial Pt/C catalyst as well as of similar Pt(Cu)/C catalysts formed by simple Cu chemical reduction. The Pt(Cu)/C catalyst prepared using Cu electroless plating showed more pronounced intrinsic catalytic activity toward methanol oxidation than its counterparts and a similar mass activity when compared to the commercial catalyst. The observed trends were interpreted by interplay between mere surface area effects and modification of Pt electrocatalytic performance in the presence of Cu, both with respect to methanol oxidation and poisonous CO removal.

Interface with Substrates of High‐Phosphorus Electroless NiP and NiCuP Deposited from Nonammonia Alkaline Solutions

Electroless NiP and NiCuP alloys with high phosphorus content (11 to 13 weight percent) are plated from nonammonia alkaline baths onto aluminum and iron substrates. Energy-dispersive spectroscopy analysis is used to examine the chemical composition of the coatings. Scanning Auger electron spectroscopy is applied to study elemental profiles and the interface with the substrates. The zincate layer transformation at the interface with the aluminum substrate is investigated at two bath temperatures. Nickel, phosphorus, and copper concentration is plotted against the coating thickness to examine the chemical homogeneity and to obtain information about the mechanism of electroless ternary alloy formation on both types of substrates.

Electroless Deposition of Ni–Sn–P and Ni–Sn–Cu–P Coatings

The surface morphology and the elemental distribution of low- and high-tin Ni-Sn-P coatings have been investigated. It is shown that in low-tin Ni-Sn-P coatings there is a uniform distribution of the alloy components, both on the surface and through the thickness. The main mechanism of electroless alloy deposition in this case is based on the well-known hypophosphite oxidation as a source of electrons for the metals (Ni and Sn) and phosphorus reduction. In high-tin coatings, a nonuniform distribution of the components is observed, both on the surface and through the coating thickness. Three-dimensional areas enriched in tin and impoverished in Ni and especially in P have been observed using scanning electron microscopy with energy-dispersive X-ray spectroscopy, scanning Auger electron spectroscopy, and Auger electron spectroscopy. The disproportionation reaction of Sn(II) is suggested as being predominant over the hypophosphite oxidation in these three-dimensional areas and is responsible for their formation. The introduction of copper in the solution is giving an additional opportunity to reveal the role of the oxidation of Sn(II) into Sn(IV) as a source of electrons.

Incorporation of Zinc in Electroless Deposited Nickel-Phosphorus Alloys II. Compositional Variations Through Alloy Coating Thickness

The zinc state in Ni-Zn-P coatings electrolessly plated from alkaline solution with citrate as the main complexing agent was examined by combining X-ray photoelectron spectroscopy (XPS) and scanning Auger electron spectroscopy (SAES). It was demonstrated that zinc at the surface of Ni-Zn-P films was in an oxidized state. The SAES sputter-etch depth profiling showed that zinc incorporated within the coating was always accompanied by oxygen with an almost parallel profile, independent of the substrate. Analyzing the observed chemical shift of the LMM peaks in SAES and XPS spectra, it seems reasonable to assume that zinc is incorporated in the coating in a partially oxidized state. The parallel profiles of all components of the alloy indicated their actual codeposition and suggested almost homogeneous distribution through the film thickness in this type of Ni-Zn-P coatings. The elemental profiles in specially designed two-layer samples (Ni-Zn-P over Ni-P) demonstrated the significant reduction in phosphorus content caused by zinc codeposition, which started at the interface between Ni-P and Ni-Zn-P.

Electroless Deposited Co–Re–P and Co–Re–Ni–P Coatings: Elemental State of the Alloy Components and Their Localization

The effect of Re on the composition distribution and structure of Co-Re-P and Co-Re-Ni-P electroless coatings is studied. The participation of disproportionation reactions during perrhenate reduction to the elemental state, coupled with the two-electron reduction of Re(VII) to Re(VI) by hypophosphite, is proposed as an explanation for the mechanism of Re codeposition. This also gives an explication of the increased efficiency of hypophosphite in the presence of perrhenate, and the observed extremely low P percentage in the coatings. The deposits have a hexagonal close-packed (hcp) lattice with a texture (1010). Uniform cross sections of the films, prepared by the focused ion beam technique, are used to characterize the nanocrystalline columnar structure in relation to the composition distribution by applying high-resolution analytical electron microscopy. Besides the partial localization of Re at the grain boundaries, it is alloyed with Co and Ni within the crystalline grains. This could be considered as evidence of two possible ways of Re inclusion in the alloys. The parameters of the hcp lattice increase with the Re content in the alloys. The impact of Re and its distribution in the nanocrystalline structure on the magnetic properties of the coatings is demonstrated.