Zinc, underpotential deposition

The change in surface layer structure formation observed at the potential range of <130 mV is due to two processes zinc underpotential deposition (UPD) on the gold surface and HEDP-zinc complex adsorption. Modification of the gold surface... 

Zinc and tin The electrodeposition of Zn [52] has been investigated in acidic chloroaluminate liquids on gold, platinum, tungsten, and glassy carbon. On glassy carbon only three-dimensional bulk deposition was observed, due to the metal s underpotential deposition behavior. At higher overvoltages, codeposition with A1... 

In electroplating industrial iron metals, zinc metal electrodeposition is accompanied by the formation of Zn-Ni, Zn-Co, and Zn-Fe alloys, where zinc electrodeposition is known to be anomalous in some cases. The ratio of zinc metal to iron metal in those alloys is sometimes higher than that of the electroplating bath solution, and zinc ions occasionally deposit at potentials positive to the equilibrium potential of zinc ions on zinc metal although is very negative to the equilibrium potentials of iron metals. It can be seen from the study of underpotential deposition of zinc ions " that this is not anomalous, but could be explained as an underpotential deposition phenomenon, to be clarified in further work. 

S. Taguchi, T. Fukuda, and A. Aramata, Kinetic study of underpotential deposition of zinc ions on pt( 111) in acidic phosphate solution, in press (1996). 

The electrodeposition of zinc on polycrystalline Au, Pt, and tungsten electrodes was preceded by underpotential deposition (UPD) [172]. The formal potential of Zn(II)/Zn(0) couple and diffusion coefficient of Zn(II) were also determined [172]. 

There was significant interest in zinc UPD, since zinc plays a major role in plating technology. Underpotential deposition of zinc in acidic solutions on polycrystaUine Pt electrode was shown by using cyclic voltammetry [190]. Later, Zn UPD was also demonstrated on Au and Pd electrodes in acidic and alkaline solutions [191, 192]. 

Zn UPD on Pt Underpotential deposition of zinc was observed on Pt(l 11) in alkaline solution as a sharp cyclic voltammetric (CV) peak, in contrast to the behavior on polycrystalline Pt, when several broad UPD peaks were observed [193]. The changes of the peak potential with concentration of Zn02 were equal to 60 mV/log [Zn02 ] and led to the apparent electron transfer number ria = 1. 

More than at mercury, it makes a difference whether the electrode is inert or not. In the first case, the electrode reaction is of the type Fe3+/ Fe2+ etc. and the modelling of processes is the same as with mercury. However, if the electrode reaction is of the type Zn2+/Zn, e.g. at a gold electrode, at least the electrode surface will be modified by the deposited zinc, Frequently, it is observed that the first monolayer of the foreign metal is deposited at a potential substantially positive to its standard potential. This phenomenon is named underpotential deposition and bears some resemblance to an electrode reaction that involves adsorption of the reacting species (see Sect. 6). 

In the Lewis acidic melts underpotential deposition (UPD) of zinc was observed on Pt and Ni electrodes. The potential window and UPD of zinc in Lewis acidic choline chloride (ChClfZnCh was found to be exactly the same as the corresponding [EMIM]Q system, suggesting that the cation has little or nothing to do with the electrochemistry of the liquid. 

The mixtures of ZnCh-EMIC also exhibit a lower melting temperature than the conventional inorganic molten salts. The Lewis acidity of ZnCh-EMIC ionic liquid can be adjusted by varying the molar ratio of ZnCh to EMIC [174]. The ionic liquids that have a ZnCh mole fraction higher than 33 mol % are acidic because there are not enough chloride ions to fully coordinate with Zn(II). The melts containing less than 33 mol % ZnCb are basic. In such liquids, the major zinc species is probably ZnCU . Underpotential deposition of zinc was observed on Pt and Ni electrodes in acidic liquids. 

G. Zheng, B.N. Popov, R.E. White, Use of underpotential deposition of zinc to mitigate hydrogen absorption into Monel-K500, J. Electrochem. Soc. 141 (1994) 1220—1224. 

B.N. Popov, G. Zheng, R.E. White, The underpotential deposition of zinc for mitigation of hydrogen absorption and penetration into HY-130 steel, Corros. Sci. 36 (1994) 2139-2153. 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

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