**6. Electrochemical synthesis of other rare earth oxides**

The remaining literature for electrodeposition of pure rare earth oxides is sparse. Most of the research is on doping of CeO2 with another rare earth oxide (i.e., PrO2, Sm2O3, Gd2O3, and Tb2O3) to increase the oxygen vacancies, ionic conductivity, or catalytic activity [106-109]. However, there are a few reports of individual REO coatings prepared by electrodeposition using the base generation method. Lair et al. prepared Sm2O3 by electrodepositing Sm(OH)3 from a nitrate solution and then sintering at 600 o C for 1 h [110]. The as-deposited Sm-based films were thick and adherent to the substrate even though cracking was observed in the film. Raman spectra of the as-deposited films had peaks at 1054 and 741 cm-1 attributed to the internal vibration modes of nitrate ions. Both nitrate-related peaks greatly diminished after annealing and major peaks for Sm2O3 increased in the spectrum.

Other researchers prepared Gd2O3 by electrodepositing Gd(OH)3 from a nitrate solution and then sintering at 700 o C for 3 h [111, 112]. The authors used a cathodic pulse current method where ton = 10 ms and toff = 10 ms. The deposition mechanism was still a base generation method:

Electrochemical step:

$$\rm NO\_3^- + H\_2O + 2e^- \rightarrow NO\_2^- + 2OH^- \tag{15}$$

$$\rm O\_2 + 2H\_2O + 4e^- \rightarrow 4OH^- \tag{16}$$

$$2\text{H}\_2\text{O} + 2\text{e}^\cdot \rightarrow \text{H}\_2 + 2\text{OH}^\cdot \tag{17}$$

Chemical step:

$$\text{Cd}^{3+} + 3\text{OH}^{\cdot} + \text{yH}\_{2}\text{O} \rightarrow \left[\text{Cd} \text{(OH)}\_{3}\text{yH}\_{2}\text{O}\right] \tag{18}$$

$$\mathrm{^1Gd^{3+}} + \mathrm{3OH^{-}} + \mathrm{xNO\_3^{-}} + \mathrm{yH\_2O} \rightarrow \left[ \mathrm{Cd(OH)\_{\mathrm{(3\times)}}} \left( \mathrm{NO\_3} \right)\_{\mathrm{x}} \mathrm{yH\_2O} \right] \tag{19}$$

The reduction of nitrate ions and water in the electrochemical step causes the electrogeneration of base at the cathode surface due to the increase of local pH, which leads to the formation of gadolinium hydroxide deposit on the electrode. Analysis of the films showed a high nitrogen content indicating nitrate ions had been intercalated into the deposit during electrodeposition. SEM images of the sintered coatings did show formed nanorods ~20–30 nm in diameter and up to 1 μm in length.

Lu et al. electrodeposited TbO2-x coatings onto copper substrates from a nitrate solution using the base generation method [113]. SEM images of the electrodeposited TbO2−x samples showed flower-like structures made up of numerous leaf-like nanosheets, in which the leaf-shaped nanosheets were approximately 100 nm in thickness and 1 μm in width. The crystal structure was analyzed by powder XRD. The values of the lattice constant were close to those of TbO1.75.
