Metallic thin-film electrodes

Fig. 14 Main characteristics of (a) carbon-based electrodes and (b) metallic thin-film electrodes for DEAs (Rosset and Shea 2013)...

For technical purposes (as well as in the laboratory) RuOz and Ru based thin film electrodes are prepared by thermal decomposition techniques. Chlorides or other salts of the respective metals are dissolved in an aqueous or alcoholic solution, painted onto a valve metal substrate, dried and fired in the presence of air or oxygen. In order to achieve reasonable thicknesses the procedure has to be applied repetitively with a final firing for usually 1 hour at temperatures of around 450°C. A survey of the various processes can be found in Trasatti s book [44], For such thermal decomposition processes it is dangerous to assume that the bulk composition of the final sample is the same as the composition of the starting products. This is especially true for the surface composition. The knowledge of these parameters, however, is of vital importance for a better understanding of the electrochemical performance including stability of the electrode material. 

Batley [28] examined the techniques available for the in situ electrodeposition of lead and cadmium in estuary water. These included anodic stripping voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen and in situ electrodeposition on mercury coated graphite tubes. Batley [28] found that in situ deposition of lead and cadmium on a mercury coated tube was the more versatile technique. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals were then determined by electrothermal atomic absorption spectrometry, Hasle and Abdullah [29] used differential pulse anodic stripping voltammetry in speciation studies on dissolved copper, lead, and cadmium in coastal sea water. 

The photoelectrochemical production of chlorine at nanocrystalline titanium dioxide thin film electrodes exposed to U V light has been reported [96]. In this process, the energy from photons substantially reduces the overpotential required for the chlorine evolution process and therefore less harsh conditions are required. Metal doping of the Ti02 photoelectrocatalyst was explored but found to be not beneficial for this process. In future, this kind of process could be of practical value, in particular, for water treatment and disinfection applications requiring low levels of chlorine. 

Figure 9.9 Assembly of sandwich-type optically transparent thin-layer electrochemical cell, a, Glass or quartz plates b, adhesive Teflon tape spacers c, minigrid working electrode d, metal thin-film working electrode, which may be used in place of (c) e, platinum wire auxiliary electrode f, silver-silver chloride reference electrode g, sample solution h, sample cup. [Adapted with permission from T.P. DeAngelis and W.R. Heineman, J. Chem. Educ. 53 594 (1976), Copyright 1976 American Chemical Society.]...

There are two main approaches or configurations for the AFM-assisted detection of the local piezoelectric activity (pfm) in ferroelectric thin films for ferroelectric memory applications (FeRAM). The most used one was introduced in the early 90s and uses a conductive afm-tip as both top electrode and sensor for the induced vibration [2-4]. The second and more recent one [15,16], uses a normal metallic thin top electrode to apply the electric field and the vibration signal is detected by the AFM-tip above the top electrode. Both approaches present numerous advantages and disadvantages, as widely discussed in the literature, and are quite complementary. 

In addition to thin-film electrodes, compact diamond single crystals grown at high temperature and high pressure have become the object of electrochemical study in recent years. These so-called HTHP crystals can be obtained by crystallization from a carbon solution in a metal melt (e.g., based on the Ni-Fe-Mn system) at /arranges that correspond to the conditions of thermodynamic stability of diamond. These crystals can be also doped with boron in the course of growth. 

Typical values of transfer coefficients a and ji thus obtained are listed in Table 4 for single crystal and polycrystalline thin-film electrodes [69] and for a HTHP diamond single crystal [77], We see for Ce3+/ 41 system (as well as for Fe(CN)63 /4 and quinone/hydroquinone systems [104]), that, on the whole, the transfer coefficients are small and their sum is less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, a reaction proceeding via the valence band has transfer coefficients a = 0, / =l a + / = 1 [6], Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials fabricated by advanced technologies. Departure from the ideal semiconductor behavior is likely because the interfacial potential drop is located in part in the Helmholtz layer (due e.g. to a high density of surface states), or because the surface states participate in the reaction. As a result, the transfer coefficients a and ji take values intermediate between those characteristic of a semiconductor (0 or 1) and a metal ( 0.5). 

As noted above, EP involves controlling the coverage of an electro-active species, supplied from a solid electrolyte, on the surface of a metal thin-film catalyst electrode with which it is in contact. In our case, the electro-active species is alkali ions and the electrolyte is Na (or K) alumina. A schematic of the experimental arrangement is shown in Figure 1. The thin-film metal catalyst must be both continuous and porous. 

It may be worthwhile to point out that the term abnormal infrared effects refers to the phenomenon of different spectral properties of adsorbates at thin film electrodes in comparison with those at massive metal electrodes. The origin of the AIREs relates certainly to the particular properties of thin film material and the interaction of adsorbates with the nanometer-scale thin film. A change in optical properties of the adsorbate-thin film system may be consequently expected. Bjerke et al. [21] have simulated the variation of IR features of CO adsorption on platinized platinum electrodes, which may have thrown a light on interpreting the phenomenon. However, thoroughly understanding the origin of the AIREs is yet... 

Metal oxide nanostructures have been fabricated using different methods and preparation conditions. The most promising technique is sol-gel processing in combination with dipcoating technique.This method enables us to prepare spinel oxide thin film electrodes at ambient temperature with high level of doping and large surface area [117,118], The physical and chemical vapor deposition is another technique for metal oxide preparation [119,120],... 

A thin-film electrode is relatively dense, as the metallic film does not have the electrocatalytic properties that a porous electrode has. Therefore, in many instances, the surface of the thin film is chemically or electrochemically modified to enhance its electrocatalytic activity. For instance, thin platinum film electrodes can be platinized electrochemically forming a porous platinum black layer. This platinum black layer is electrocatalytically more active than the thin platinum film. Thin-film processes are more capital and labor intensive and the process is more complicated than thick-film processes. Thin-film deposition is also a batch process which may produce sensors of limited numbers of silicon substrates. This is very desirable in prototype development, for it allows modification on prototypes with minimum cost. 

Etch thin-film electrode and sacrificial metal layer... 

Thin-film electrodes that exhibit the SEIRA effect can be deposited on the total-reflecting plane of the prism by vacuum evaporation or electroless plating of the desired metal. Electrochemical deposition cannot be used for non-doped (and thus non-conducting) Si windows, but is possible for Ge. 

Vacuum-evaporated very thin ( 20 run) Au and Ag electrodes suitable for SEIRAS show pale blue to purple colors, while chemically deposited film electrodes show the colors of the corresponding massive metals and are as shiny as a well-polished surface. Both thin-film electrodes have enough conductivity for electrochemistry. These film electrodes are usually polycrystaUine. In the case of Au films,... 

Klofla, T Rieke, P., Unkous, C., Buttner, W.J., Nanthakumar, A., Mewbom, T.D., and Armstrong, N.R. (1985) Tri- and tetravalent phthalocyanine thin film electrodes comparison with other metal and demetaUated phthalocyanine systems. 

To study the earliest stages of repassivation where the current densities are the highest, it is necessary to create the fresh area as quickly as possible. A very small area of fresh metal can be created extremely quickly (on the order of microseconds) by the thin-film-brealdrig experiment [48]. In this approach, a thin film deposited onto a brittle substrate such as glass or Si is suspended into the solution. Breaking of the thin-film electrode results in the creation of a fresh metal area of size equal to the cross-section of the thin film. Current densities on the order of 1000 A cm were measured using this technique on A1 thin films [48]. 

Recent studies have been performed on alternative electrode materials. Nano-sonic has developed low modulus, highly conducting thin film electrodes by molecular level self-assembly processing methods capable of maintaining conductivity up to strains of 100% [217, 218]. Recent developments have enabled the reduction of the modulus to less than 1 MPa and an increase in the strain to rupture to 1000% [219]. A version of the material is commercially available under the name Metal Rubber . Delille et al. have developed novel compliant electrodes based on a platinum salt reduction [220]. The platinum salt is dispersed into a host elastomer and immersed in a reducing agent. A maximum conductivity of 1 S cm was observed and conductivity was maintained for strains up to 40%. 

Chapter 8 concentrates on the physical incorporation of metal complexes or clusters in macromolecules. Techniques for the inclusion of metal complexes in polymers in order to get thin-film electrodes are well-investigated (Section 8.1, and Chapters 9, 10, 13 and 14). There are also several methods for forming and stabilizing metal particles in macromolecules, especially organic polymers (Section 8.3). Up to now it has been difficult to establish a correlation between the molecular structure and the properties of the generated material. 

FIGURE 4.4 Examples of different skin electrodes (a) metal plate electrodes, (b) suction electrode for EGG, (c) metal cup EEG electrode, (d) recessed electrode, (e) disposable electrode with electrolyte-impregnated sponge (shown in cross section), (f) disposable hydrogel electrode (shown in cross section), (g) thin-film electrode for use with neonates (shown in cross section), and (h) carbon-filled elastomer dry electrode. 

After deposition, the system must achieve an equilibrium distribution of the deposited metal. This is only possible if the metal A has enough mobUity within the matrix of B, which is only possible at higher temperatures. This also limits the thickness of an alloy sample so that either a thin wire or a thin film electrode is used. With ultra thin metal films or with metal clusters, new applications of this method seem possible at lower temperatures. 

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