Effect of Surface pH on Electrodeposited Ni Films

Abstract

Ni metal was potentiostatically electrodeposited on a vertical plane cathode in a Watts bath in order to determine the effect of the cathode surface pH on the Ni film microstructure. Polarization curves and current efficiencies were measured at pH values of 1.5, 3.4, and 5.5. The surface pH value ͑pH s ͒ was estimated from the measured partial current density for hydrogen gas evolution based on a steady-state one-dimensional mass transfer model. Any species relating to the buffering function through the dissociation reactions ͑HSO 4 − , H 3 BO 3 , Ni 4 ͑OH͒ 4 4+ , H 3 O + , OH − ͒ were taken into account. The pH s abruptly rose to above 6 as the partial current density for H 2 gas evolution increased. The preferred orientation of electrodeposited Ni thin film was plotted in electrode potential-pH s diagram. It was found that the transition boundary between ͕110͖ and ͕100͖ preferred orientations was located along a ridge 500 mV below the H + /H 2 equilibrium potential line. This relationship suggests that the dissolved hydrogen atoms in Ni metal are partly responsible for the evolution of structural texture of the Ni films.Electrodeposited Ni metal films are used for protective coatings on carbon steels. Such films also have been applied to microelectromechanical systems ͑MEMS͒ 1,2 as well as magnetic recording heads 3,4 and ultrathin films for magnetic sensors. 5 The textured structure of electrodeposited Ni metal films is known to evolve as a result of nucleation and growth processes, depending on the electrolytic conditions, e.g., the electrolyte composition, pH, bath temperature, current density, stirring condition, and organic additives. The specific structure determines the physicochemical and mechanical properties of Ni films. Much more effort to improve the microcrystalline structure of high-area electrodeposited films is also necessary in the field of advanced energy science. Doing so leads to a highly functionalized catalytic electrode by tailoring a unique interface that is required for efficient energy conversion.In spite of many works, 6-17 the mechanism of Ni electrodeposition has not been thoroughly understood. Because the Ni 2+ /Ni system has a more negative equilibrium potential ͑E o = −0.23 V͒ than the H + /H 2 couple, H 2 gas evolution inevitably accompanies Ni metal electrodeposition. The pH value in the neighborhood of the cathode increases as the H + ions are reduced to evolved H 2 gas.The Centre National de la Recherche Scientifique ͑CNRS͒ research group investigated the correlations between the microstructure of Ni films electrodeposited from a Watts bath and the electrolytic conditions. 6-10 They considered the inhibiting influences of hydrogen adatoms ͑H ads ͒, H 2 molecules, and the stabilization of Ni͑OH͒ 2 on the cathode surface. The texture evolution phenomena were then discussed. Nakahara and Felder examined electrodeposited Ni films with transmission electron microscopy ͑TEM͒. 15 They emphasized the possible formation of metal hydrides as well as the inclusion of hydroxide.Dahms and...

Comparison of Ni–Cu alloy films electrodeposited at low and high pH levels

“…This may lead to the formation of different dendrites. Very recent studies showed that electrochemical conditions such as surface pH and overpotential affect the structure of electrodeposited metal films [13,14].…”

Corrosion tests of nickel coatings prepared from a Watts-type bath

“…Ni electrodeposition is usually performed at high temperatures (50-70°C) and at high current densities. [17][18][19][20] Electrodeposition of Ni has been reported both through direct current [21][22][23][24][25] and pulse 26 methods. The literature reveals that in direct current electrodeposition, an increase in current density leads to a corresponding decrease of the grain size of the deposits, this behavior being attributed to the evolution of more hydrogen at the cathode interface at higher current densities.…”

Role of electrolyte pH on structural and magnetic properties of Co–Fe films

“…The change of grain size may be explained by the hydrogen evolution, which is favoured at low pH. Recent studies [5,6] showed that deposition parameters, for example pH, affect the microstructure of electrodeposited metal films.…”