Platinum-gold alloy films

Tungsten trioxide thin films activated by gold layers have also been used for NOj. detection.115 This material possesses excellent sensitivity towards nitric oxide and nitrogen dioxide. An automobile exhaust gas NO sensor that uses a gold-platinum alloy electrode has also been reported.116... 

Xiao F, Mo Z, Zhao F, Zeng B (2008) Ultrasonic-electrodeposition of gold-platinum alloy nanoparticles on multi-walled carbon nanotubes-ionic liquid composite film and their electrocatalysis towards the oxidation of nitrite. Electrochem Commnn 10 1740-1743... 

Xiao F, Zhao F, Zhang Y, Guo G, Ztaig B (2009) Ultrasmiic electrodeposition of gold-platinum alloy nanoparticles on ionic liqnid-chitosan cmnposite film and their applicatimi in fabricating nonenzyme hydrogen peroxide sensors. J Phys Chcm C 113 849-855... 

Other metals, such as copper, nickel, or silver, have been used as electrode materials in connection with specific applications, such as the detection of amino acids or carbohydrates in alkaline media (copper and nickel) and cyanide or sulfur compounds (silver). Unlike platinum or gold electrodes, these electrodes offer a stable response for carbohydrates at constant potentials, through the formation of high-valence oxyhydroxide species formed in situ on the surface and believed to act as redox mediators (40,41). Bismuth film electrodes (preplated or in situ plated ones) have been shown to be an attractive alternative to mercury films used for stripping voltammetry of trace metals (42,43). Alloy electrodes (e.g., platinum-ruthenium, nickel-titanium) are also being used for addressing adsorption or corrosion effects of one of their components. The bifunctional catalytic mechanism of alloy electrodes (such as Pt-Ru or Pt-Sn ones) has been particularly useful for fuel cell applications (44). 

Another important problem is the elimination of the chemical interactions between contacting phases and also of the diffusion of metal atoms into the oxide bulk [487 89], One example is the operation of commonly used indium junctions, which are convenient because films of this soft metal and its alloys can be applied mechanically [490], This fact stimulates the quest for low-temperature techniques for junction fabrication. It is known that silver, gold, and copper, and also probably platinum [202] and palladium [487], are most suitable because of their weak interaction with HTSCs. 

WO3 thin films activated by An layers have been used [625]. This type of material possesses excellent sensitivity towards NO and NO2 gases. An exhaust gas NOj sensor that uses a platinum-gold alloy electrode to selectively remove oxygen but not NO was also reported [626]. 

The alloy catalysts used in these early studies were low surface area materials, commonly metal powders or films. The surface areas, for example, were two orders of magnitude lower than that of platinum in a commercial reforming catalyst. Hence these alloys were not of interest as practical catalysts. The systems emphasized in these studies were combinations of metallic elements that formed continuous series of solid solutions, such as nickel-copper and palladium-gold. The use of such systems presumably made it possible to vary the electronic structure of a metal crystal in a known and convenient manner, and thereby to determine its influence on catalytic activity. Bimetallic combinations of elements exhibiting limited miscibility in the bulk were not of interest. Aspects of bimetallic catalysts other than questions related to the influence of bulk electronic structure received little attention in these studies. 

Thin film metallization is often used to establish contact with a surface. In many cases, the contact involves reactions that form a layer of compound material between the metallization and the surface. For instance, in the deposition of platinum on silicon, a layer of platinum silicide is formed. The contact resistance involves not only the resistance between the metallization and the surface but the effect of the reaction layer, which can be a high resistivity material or present a potential barrier. In the extreme, the junction may be rectifying (i.e. current can flow in one direction easily but in the other with difficulty). The metallization material can also alloy with the substrate material. For example, gold will diffuse into silicon to some extent and the higher the temperature, the higher the solubility of gold in the silicon. 

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