**3. Electrochemical synthesis of lanthanum oxide (La2O3)**

Lanthanum oxide (La2O3) has been utilized in several technological applications, such as lightemitting phosphors, solid oxide fuel cells, catalysis, automobile exhaust-gas converters, and sorbent materials [35-39]. There are only a few papers reporting the attempt to electrodeposit lanthanum oxides from aqueous solutions [39-42]. In practice, La2O3 has not been deposited directly using electrodeposition. However, lanthanum hydroxide, La(OH)3, has been electro‐ chemically deposited. Bocchetta et al. first showed that it was possible to obtain La(OH)3 from a solution of lanthanum nitrate using galvanostatic deposition at a cathodic current of 1 mA/cm2 [40]. They obtained nanowires on an Al substrate, and the authors proposed that deposition occurred through a base generation mechanism. Yao et al. did a similar type of deposition using lanthanum nitrate and ammonia nitrate to obtain La(OH)3 nanorods on a copper substrate [41]. Like previous authors, they also proposed a base generation mechanism in which hydroxide ions are formed from the nitrate and water reduction:

$$\rm NO\_3^- + H\_2O + 2e^- \rightarrow NO\_2^- + 2OH^- \tag{9}$$

$$2\text{H}\_2\text{O} + 2\text{e}^\cdot \rightarrow \text{H}\_2 + 2\text{OH}^\cdot \tag{10}$$

However, by studying the SEM images along the potential-time curve, they also proposed that the evolution of hydrogen (reaction 10) was important in obtaining the nanorod formation. The H2 bubbles acted as a dynamic template forcing the nanorods in a vertical growth direction through the pressure of the bubbles.

The formation of La2O3 from La(OH)3 was first done by Liu et al. [42]. They fabricated La(OH)3 nanospindles and nanorods on F-doped SnO2 substrates using galvanostatic deposi‐ tion from a bath containing 0.01 M La(NO3)3 and 30-50% DMSO. After obtaining La(OH)3 nanorods, the coating was sintered at 690 o C. A pure hexagonal structure of La2O3 was obtained as revealed by the XRD pattern. The percentage of DMSO in the deposition solution affected the nanostructure of the deposits. Lower concentration produced nanorods, higher concen‐ tration produced nanospindles.

Other researchers also obtained nanorods, nanospindles, and nanocapsules of La2O3 by electrodeposition of La(OH)3 using the base generation method and then sintering [43-45]. However, it was shown by CHN analysis and FTIR that nitrates were codeposited into the La(OH)3 hexagonal lattice. A sharp peak at 1383 cm-1 in the FTIR spectra for the hydroxide sample is due to the vibration modes of NO3 - ions intercalated in the deposit structure during electrodeposition. After sintering, no nitrates were present in the coating. Very nice vertically aligned uniform nanorods were prepared of La(OH)3 and La2O3 using a pulsed electrodepo‐ sition method followed by heat treatment [43]. In conclusion, while La2O3 has not been directly electrodeposited onto various substrates, La(OH)3 can be deposited from a nitrate solution using the base generation method and then heat treated (typically ~600 o C) to obtain La2O3.
