Electrodeposition metal substrates

Electroplating—the process of electrodeposition onto a metallic substrate of a thin adherent layer of a metal or alloy having desirable chemical, physical and/or mechanical properties. 

Non-epitaxial electrodeposition occurs when the substrate is a semiconductor. The metallic deposit cannot form strong bonds with the substrate lattice, and the stability conferred by co-ordination across the interface would be much less than that lost by straining the lattices. The case is the converse of the metal-metal interface the stable arrangement is that in which each lattice maintains its equilibrium spacing, and there is consequently no epitaxy. The bonding between the met lic lattice of the electrodeposit and the ionic or covalent lattice of the substrate arises only from secondary or van der Waals forces. The force of adhesion is not more than a tenth of that to a metal substrate, and may be much less. 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

Epitaxial effects are not limited to single-crystalline substrates. The possibility for substrate-induced epitaxial development in the difficult case of ZnSe (cf. conventional electrodeposition) has been established also by using strongly textured, albeit polycrystalline, zinc blende (111) CdSe electrolytic films to sustain monolithic growth of ZnSe in typical acidic selenite baths [16]. Investigation of the structural relations in this all-electrodeposited ZnSe/CdSe bilayer revealed that more than 30-fold intensification of the (111) ZnSe XRD orientation can be obtained on the textured (111) CdSe films, compared to polycrystalline metal substrates (Fig. 4.2). The inherent problems of deposition from the Se(IV) bath, i.e., formation of... 

There has been an increasing number of studies of the UPD of main group elements, including S, Se, Te, I, Br, Cl, and As, on metal substrates, whereas studies of UPD processes on the surface of semiconductors and semimetal substrates are significantly less. Presently, most interesting in this connection is the combined use of photoexcitation of a semiconductor substrate and/or an immobilized precursor, and electrodeposition, as will be discussed in a subsequent paragraph. 

Electrodeposition of Nanostructures Size-Quantized Films on Metal Substrates... 

Electrodeposition was used to prepare a biaxially textured Gd2Zr207 (GZO) buffer layer on Ni-W substrates.129 Buffer layers provide chemically inert, continuous, and smooth bases for the growth of the superconductor oxide films. They also prevent both the diffusion of metal to the high-temperature superconductor (HTS) layer and the oxidation of the metal substrate when superconductor oxide films are processed at high temperature (-800 °C) in an oxygen atmosphere (100ppm or more). 

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

In industrial applications of metal deposition a metal M is deposited either on the native metal substrate M or on a foreign metal substrate S. As an example of the former, Cu is electrodeposited on a Cu substrate formed by electroless Cu deposition on an activated nonconductor in the fabrication of printed circuit boards. As an example of the latter, Ni is electrodeposited on Cu in the fabrication of contact pads in the electronics industry. 

Cladding may be less expensive than sdective electrodeposition when coatings greater than 1 Jim of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electrodeposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

Underpotential deposition (UPD) is the electrochemical adsorption and (partial) reduction of a submonolayer or monolayer of cations on a foreign metal substrate at potentials more positive than the reversible potential of the deposited metal [141]. The UPD phenomenon is used in many fundamental and applied studies because it offers a means of controlling coverages during electrodeposition in a very concise manner. Until recently, most of the information obtained about the structure of the overlayers deposited on single crystal surfaces has come from indirect means such as current-voltage analysis or by analysis of the deposited films after transfer to a UHV chamber [141]. 

PTFE aqueous dispersions are applied onto metal substrates by spraying, dipping, flow coating, electrodeposition, or coagulation to provide chemical resistance, nonstick, and low-friction surfaces. Nonstick cookware and bakeware are made from dispersion specifically formulated for that purpose with the use of a primer for the metal. After coating, the parts are dried and sintered. 

The mechanism of electrodeposition or electrocrystallization28 29 involves, as a first step, the reduction of a cation on the substrate surface (aided by an applied potential or current) to form an adatom, and its migration over the surface to an energetically favourable site. Other atoms of the electrodeposit aggregate with the first, forming the nucleus of a new phase. The nucleus grows parallel and/or perpendicular to the surface. Clearly, a number of nuclei can form and grow on the surface. When all the electrode surface is covered with at least a monolayer, deposition is on the same metal rather than on a different metal substrate. As is to be expected, the formation of the first layers determines the structure and adhesion of the electrodeposit. 

Fig. 16.11. Corrosion at the contact between electrodeposits and metal substrates (a) Corrosion of the electrodeposit, e.g. zinc on iron (b) Corrosion of the metal substrate, e.g. tin on iron.

Much of metal electrodeposition is carried out with the aim of minimizing corrosion, the most common electrodeposits being tin, zinc, nickel and chromium on a cheaper metal substrate, such as iron. Since there is chemical bonding between substrate and electrodeposit, this is better than covering with paint (except electrophoretic painting, see Chapter 15) and additionally the surface generally becomes harder, as it does in nickel electroless plating. 

Electroplating — (or - electrodeposition) is the process of depositing a usually thin layer of metal upon a usually metallic substrate (or any other conductor, e.g. graphite),... 

The H and OH" ions are supplied by the simultaneous electrolysis of water during electrodeposition. Electrode reactions involving the resins are of little significance. However, oxidation of the metal substrate does play a role in anodic electrocoating. 

Figure 7. The influence of metal substrate on electrodeposition performance. , p-nitrobenzylmethyldo-decylsulfonium chloride = —0.64. A, Flfm-triflu-oromethylbenzyl) - N - dodecyl - N, N-dimethylammonium chloride = —1.64. p-fluorobenzyldodecyldi-methylammonium chloride = —1.90.

It must be mentioned that composite materials produced by reduction of oxide mixtures in the Hj at 500°C (161) are not, by any means, true intermetallic phases such as could be prepared by melting and cooling the pure metal components in the same nominal composition ratios. For example, it is found that such composites produced thermally from oxides, reductively at a metal substrate from oxides, or by electrodeposition at a metal substrate from appropriate salts in solution, for example, NiS04 plus (NH4)2Mo04, do not show well-defined X-ray diffraction patterns, and their mode of preparation almost certainly results in important quantities of H remaining in a sorbed hydridic state in such preparations (75). 

Figure 1. SEM images of different electrodeposited metal oxide nanoparticles Ti02 nanotube arrays grown on Ti substrate(a) cobalt oxide nanoparticles onto glassy carbon electrode (b) nickel oxide nanoparticles(c) and zinc oxide nanoparticles Reproduced from references [ 138],[ 102],[ 137] and [135] with permission from Elsevier.

Ottova et al. looked at two-compartment semiconductor-septum electrochemical photovoltaic cells with cadmium selenide and cadmium selenide telluride for water photolysis [126], They used cells consisting of two chambers separated by a CdSe or CdSe/CdTe bipolar electrode. The bipolar electrodes were prepared by painting a CdSe slurry on a metal substrate or by ultrasound-aided electrodeposition from CdSe solution in ZnCl2. The photoresponse (voltage and current output) and hydrogen yield from photo-induced electrolysis of H20 in the dark chamber of the cell were evaluated as a function of CdSe preparation method. The ultrasound-aided deposition technique gave excellent coatings of CdSe. 

Quite different adsorption characteristics are formed in the case of the underpotential deposition of metal ions [49]. As described in section 10.2, underpotential deposition (UPD) is the process by which a metal ion adsorbs on a different metal substrate at a potential more positive than that at which it is electrodeposited on itself. UPD occurs when there is a significant difference between the work functions of the depositing metal phase M and the substrate metal S. Consider as an example the electrodeposition of Pb on Au. The work function of polycrystalline Au is approximately 300 mV greater than that of polycrystalline Pb (see table 8.2). This also means that the PZC of the Au electrode is positive of that for Pb. As a result, Pb adsorbs on Au more readily than it does on Pb. The adsorption process is accompanied by significant charge transfer. In fact, an estimate of pad for Pb " " on Au is close to zero, indicating that the cation is essentially discharged. 

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