The Effect of Brighteners on the Fabrication of Electroplated Bright Aluminum Films Using an AlCl3-EMIC-Toluene Bath

Aluminum (Al) is the second most abundant metallic element in the Earth's crust after silicon. Al has been widely used as a coating material, because it has the ability to resist corrosion due to passivation and has high visible-light reflectivity. Al has many uses in the transportation, packaging, construction and consumer goods industries. Thin layers of Al are typically deposited onto a flat surface by physical vapor deposition, chemical vapor deposition or a hot-dip coating process. However, electrochemical deposition of Al at ambient temperatures has attracted much interest in corrosion-resistive and decorative coatings, electro-refining processes and Al-ion batteries, because the deposition processes do not need expensive equipment, the size of Al films can be easily scaled-up or scaled-down, and Al films of uniform thickness can be prepared on various surface geometries. However, as understood from the Pourbaix Diagram of Al, at any pH, the reduction of proton preferentially occurs in aqueous solutions and the electrodeposition of Al is not feasible. Aprotic electrolytes, such as inorganic molten salts, organic molten salts (i.e., ionic liquids), and molecular organic solvents, have been applied for the electrodeposition of Al. There are many examples in the literature in which various electrolytes and conditions were used. Among aprotic electrolytes, ionic liquids that are composed solely of ions and have melting points below 100℃ have increasingly been replacing organic solvents in the research field of metals that are unable to be electrodeposited from aqueous solutions. Numerous ionic liquid electrolytic systems have been investigated for the electrodeposition of Al. These studies have focused on the formation of smooth Al films that exhibit high visible-light reflectivity on the Al surface. It was reported that in order to obtain a smooth surface for the Al films, trace amount of brighteners, such as 1,10-phenanthroline (PH) and nicotinic acid were added to the ionic liquid electrolytes. However, the electrodeposition of high bright Al has not been realized even with the presence of additives in the electrolytes. In this study, we found a more effective brightener: isonicotinic acid hydrazine (IAH) for the preparation of high bright Al films and report that the effectiveness of the two additives, IAH and PH, for smoothing the deposited Al surfaces was compared in terms of brightness, surface roughness, and film thickness. The Al films prepared with ionic liquid containing IAH showed brighter surfaces in the film thickness range of 2 to 250 mm, although generally an increase in the thickness of the deposited films caused an increase in the surface roughness.We found that the addition of IAH to EMIC/AlCl₃/toluene improved the brightness of deposited Al films. The Al films clearly and legibly reflected the characters on the Al surface. Although the EMIC/AlCl₃/toluene/IAH and EMIC/AlCl₃/toluene/PH baths exhibited high current efficiencies close to 100%, as expected from the high brightness of the Al films prepared with the EMIC/AlCl₃/toluene/IAH bath, both the grain size and average roughness of the Al films prepared with the EMIC/AlCl₃/toluene/IAH bath were smaller than that of the Al films prepared with the EMIC/AlCl₃/toluene/PH bath. In addition, the Al films prepared with the EMIC/AlCl₃/toluene/IAH bath have the perpendicularly preferred [100] orientation on the Cu plate when compared with the EMIC/AlCl₃/toluene/PH bath sample. The strong orientation preference in the [100] plane likely causes flattening of the deposited Al surface in the EMIC/AlCl₃/toluene/IAH. The mechanism for flattening the deposited Al surface with IAH will be discussed in our presentation.