In this study, we will report that the effectiveness of 4-pyridinecarboxylic acid hydrazide (4-PCAH) to smooth the deposited Al surfaces were compared with 3-pyridinecarboxylic acid hydrazide (3-PCAH), 2-pyridinecarboxylic acid hydrazide (2-PCAH) and PH in the view point of brightness, surface roughness and the film thickness. The Al films prepared with ionic liquid containing 4-PCAH showed brighter surfaces in the film thickness from 20 to 100 micrometer although generally the increase in the thickness of the deposited films causes the increase in the surface roughness.

Figure 1 shows photographs of Al film obtained by electrodeposition of Al from (A) EMIC/AlCl₃/toluene/4-PCAH, (B) EMIC/AlCl₃/toluene/3-PCAH, (C) EMIC/AlCl₃/toluene/2-PCAH  and (D) EMIC/AlCl₃/toluene/PH baths. In the baths, a constant current of 8 mAcm^-2 (A-D) was used for electrodeposition of Al. In order to recognize the difference in brightness of Al surfaces, the characters of “KU UNIV” were reflected in the Al film surface. Whether the characters that were reflected in the Al surfaces can be seen or not is an index of surface brightness, in other words, surface smoothness. The Al films obtained with the EMIC/AlCl₃/toluene/4-PCAH bath exhibits the highest brightness among four brighteners used in this study although the brightness became degrade as the deposition time became longer (Fig. 1). On the other hand, the Al films obtained with EMIC/AlCl₃/toluene/PH bath and the deposition times of 2 h barely reflect the characters on its surface. In the deposition time which is longer 2 h, dull characters can be barely seen in the partial area. PH has been reported as a representative brightener for Al electrodeposition as long as we know in several papers.  4-PCAH which was applied to Al deposition for the first time as brightener by us exhibits remarkable effect for improvement of brightness of Al surface when compared with that obtained with EMIC/AlCl₃/toluene/3-PCAH, EMIC/AlCl₃/toluene/2-PCAH and EMIC/AlCl₃/toluene/PH bath. The electrodeposition of Al from EMIC/AlCl₃/toluene/4-PCAH bath for 2 h with 4 and 12 mAcm^-2, and from EMIC/AlCl₃/toluene/PH bath with 4 mAcm^-2 showed degradation of the brightness. Employment of current of 12 mAcm^-2 in the EMIC/AlCl₃/toluene/PH bath naturally leaded to the same brightness as the deposited Al films prepared with 8 mAcm^-2 from the EMIC/AlCl₃/toluene/PH bath (these results are not shown here).

Figure 2 shows reflectance spectra of Al films deposition from AlCl₃-EMIC-toluene-4-PCAH (b), AlCl₃-EMIC-toluene-3-PCAH (c), AlCl₃-EMIC-toluene-2-PCAH (d) and AlCl₃-EMIC-toluene-PH (e). The spectrum (a) was obtained with a commercial Al foil (A1N80H-H18, Mitsubishi Aluminum, Japan). Preparation conditions of Al films are deposition time of 2 h and constant current of 8 mAcm^-2. The Al film obtained with AlCl₃-EMIC-toluene-4-PCAH (b) exhibits the reflectivity similar to one observed with the Al foil, followed by the Al film obtained with AlCl₃-EMIC-toluene-PH (e) in the view point of the reflectivity. Although the brighteners of 4-PCAH, 3-PCAH and 2-PCAH are positional isomer, the difference in the reflectivity of the Al films obtained with 4-PCAH, 3-PCAH and 2-PCAH are interesting result, indicating the importance of adsorption of brightener to Al surfaces.

Fig. 1  Micrographs of the Al film surfaces obtained on a Cu substrate by coulometeric electrodeposition from (a) AlCl₃-EMIC-toluene-4-PCAH, (b) AlCl₃-EMIC-toluene-4-PCAH, (c) AlCl₃-EMIC-toluene-2-PCAH and (d) AlCl₃-EMIC-toluene-PH baths. Deposition time: 2 h; constant current: 8 mAcm^-2.

Fig. 2  Reflectance spectra of Al films deposition from AlCl₃-EMIC-toluene-4-PCAH (b), AlCl₃-EMIC-toluene-3-PCAH (c), AlCl₃-EMIC-toluene-2-PCAH (d) and AlCl₃-EMIC-toluene-PH (e). The spectrum (a) was obtained with a commercial Al foil.