(General Student Poster Session Winner - 2nd Place) Molybdenum Electroplating Using Concentrated Chloride Aqueous Solution
Authors: Takai Ryota , Kitada Atsushi , Fukami Kazuhiro , Murase Kuniaki 
Published: December 2020

Molybdenum (Mo) is a metal that has excellent wear resistance and high-temperature strength, and is used in various fields such as aircraft and automotive industries. Since Mo is difficult to be molded due to its high melting point (2625 °C), Mo coating techniques are of interest and importance. We focused on electroplating from a concentrated aqueous solution in search of a process that can replace conventional processes such as chemical vapor deposition (CVD) and sputtering, which require high temperature or vacuum equipment. It has been believed that metallic Mo cannot be obtained by electrodeposition from aqueous solutions, although it can be co-deposited with Fe-group elements. Recently, there has been several reports on the electrodeposition of metallic Mo from a concentrated acetate aqueous solution, aiming for suppressing hydrogen generation reaction during electrodeposition [1]. In this study, we worked on Mo electroplating from concentrated chloride aqueous solutions: they enabled chromium electrodeposition using Cr3+ ion without additional chelating agents [2].

First, a concentrated aqueous solution of 11.1 mol kg–1 LiCl was prepared. Then, as Mo source, 1.0 mol kg–1 of MoCl5 or 0.0014 mol kg–1 of (NH4)6Mo7O24 were dissolved to prepare the bath. Electrochemical measurements were performed at room temperature in air. A brass plate with a matte Ni plating was used as a working electrode. Linear sweep voltammetry (LSV), cyclic voltammetry (CV), and potentiostatic electrodeposition were performed. The composition and surface morphology of the obtained electrodeposits were investigated by SEM-EDX, XRD and XPS.

The results of LSV and CV show that the reduction current decreased as bulk LiCl concentration increases, evidencing that the generation of hydrogen is suppressed. Fig. 1(a) shows the surface SEM image of a sample obtained by potentiostatic electrodeposition from the LiCl–MoCl5 bath. Spherical deposits of several micrometers were observed, and EDX analysis revealed that the main component was Mo and O. Such behaviors were also observed in the case of the LiCl–(NH4)6Mo7O24 bath. It is suggested that the Mo source and Mo concentration do not so much affect the surface morphology. Fig. 1(b) shows the XRD patterns of a sample, in which a weak peak appeared at a diffraction angle of 40.5°. The peak corresponds to 110 diffraction of Mo, indicating that the phase composed of nanocrystals. A heat treatment of the sample at 800°C for 30 min in vacuum gave a highly crystalline sample with the conventional bcc structure of metallic Mo. XPS analysis detected metallic Mo in the sample after sputtered by Ar+ ion.

In conclusion, we succeeded in electrodepositing metallic Mo from a concentrated chloride aqueous solution for the first time. It showed that hydrogen generation reaction during electrodeposition, which is a current problem, can be suppressed by coordinating and stabilizing many water molecules around ions. Therefore, the use of concentrated aqueous solution may play a key role in Mo electroplating. The deposits contained considerable oxygen impurities, as is the case of acetate bath [1]. Further studies on additives are required.

References:

1) S. Hasan et al., Can. Metall. Q., 58, 1-18 (2019).

2) K. Adachi et al., Electrochim. Acta, 338, 135873 (2020).

Figure 1