**4. Electrochemical synthesis of cerium oxides**

to produce the desired stoichiometry. We will make note of this in our discussions when

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

[40]. They obtained nanowires on an Al substrate, and the authors proposed that


2 2 2H O + 2e H + 2OH ® (10)

C. A pure hexagonal structure of La2O3 was obtained


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


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

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

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‐

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

electrodeposition. After sintering, no nitrates were present in the coating. Very nice vertically

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

in which hydroxide ions are formed from the nitrate and water reduction:

applicable for each section.

88 Advanced Ceramic Processing

through the pressure of the bubbles.

nanorods, the coating was sintered at 690 o

sample is due to the vibration modes of NO3

tration produced nanospindles.

mA/cm2

Cerium oxide (CeO2) is of interest in the area of catalysis [46]. Much effort has been dedicated to studying the role of ceria in several well-established industrial processes such as three-waycatalysis (TWC) systems and fluid catalytic cracking (FCC) systems, where CeO2 is a key component in catalyst formulation. Cerium oxide is used to remove automotive exhaust gases such as NO, CO, and CHx, and to eliminate SOx and NOx from fuel gases [47]. Ceria also demonstrates catalytic function in the removal of soot from diesel engine exhaust, elimination of organics from wastewaters (catalytic wet oxidation), and cracking of heavy oil in zeolite. In addition, CeO2 is a semiconducting and ionic-conducting oxide. It can substitute for ZrO2 as the electrolyte material in solid oxide fuel cell (SOFC) systems and has been regarded as a model system for mixed ionic/electronic conductor investigations [48].

Particle size plays a significant role in the unique properties and applications of cerium oxide. Generally speaking, the smaller the particle size, the lower the packing porosity and the higher the surface area. Catalytic activity and electrical conductivity of crystalline cerium oxide are dependent on particle size, where electronic conductivity predominates in the nanocrystalline phase and ionic conductivity mainly controls the microcrystalline structure [49]. The nano‐ crystalline phase of cerium oxide is favorable for formation of a nonstoichiometric structure due to reduced enthalpy of defect formation and propensity for oxygen vacancies. The fluorite structure of CeO2 favors oxygen vacancy formation in the lattice, enhancing the catalytic reactivity of CeO2 in TWC, FCC, and other gas phase oxidation and reduction reactions [50]. Nanocrystalline cerium oxide improves catalytic properties significantly, leading to higher conversion of carbon monoxide to dioxide and sulfur dioxide to elemental sulfur at lower temperatures. The improved catalytic reactivity was also demonstrated in the oxidation of methane [51-53]. The sinterability of crystalline cerium oxide increases with decreasing particle size. The reduction of sintering temperature for nanocrystalline cerium oxide overcomes processing temperature difficulty and makes it a promising candidate as an electrolyte in solid oxide fuel cells [52, 53]. Ceria can also be used for corrosion protection on a number of different substrates [54-56].

Cerium typically forms two types of oxides, cerium dioxide (CeO2) and cerium sesquioxide (Ce2O3). Cerium sesquioxide (Ce2O3) has two structural forms, hexagonal (A-type) and cubic (C-type). Cerium oxide (CeO2) has a fluorite (CaF2) structure (fcc) with space group Fm3m. Figure 1 shows the structure of the stoichiometric CeO2 with the oxygen (represented by circles) four coordinated and the cerium (represented by solid balls) eight coordinated. The cerium atom is at the center of the tetrahedron and the tetrahedral corners are occupied by oxygen atoms. Cerium oxide can exist as a non-stoichiometric oxide, that is a mixture of Ce(III) oxide and Ce(IV) oxide, while still retaining the fluorite cubic structure. The coatings can be easily identified by x-ray diffraction (XRD) analysis.

**Figure 1.** Fluorite structure of cerium oxide.

For the electrodeposition of rare earth oxides, cerium oxide is by far the most studied. It is also one of the few that has been deposited by both electrolytic (base generation) and direct electrodeposition. Cathodic electrodeposition (i.e., base generation electrochemical method) was first introduced for the plating of cerium oxide films [32]. Switzer et al. used this method to produce cerium oxide films and powders [57, 58]. Crystalline, randomly oriented cerium oxide coatings were deposited galvanostatically from a cerium nitrate solution. During the synthesis, the pH changed from ~ 4.5 to 7.5, showing that base was generated during deposition.

An in-depth study was done by Aldykiewicz et al. to understand the base generation mechanism for CeO2 deposition [59]. They proposed a mechanism involving oxygen to produce an oxidizing agent (i.e., H2O2) for Ce(III) to Ce(IV) formation. With an oxidant available in the system, cerium oxide film formation was accomplished through a four- or two-electron process to a hydroxide intermediate. A critical pH value above 8.7 was needed to keep the cerium hydroxide ions stable in solution. The final step was the precipitation of CeO2 onto the electrode surface. This mechanism was supported by rotation disk electrode experiments and XANES studies. Li et al. studied the mechanism proposed by Aldykiewicz with in situ atomic force microscopy technique (AFM) and concluded that a cerium hydroxide species is produced at the electrode surface with CeO2 forming nuclei out of this hydroxide "gel". The rate-determining step for this mechanism is then the nucleation and growth of the CeO2 crystals [60-62].

Zhitomirsky also proposed that hydrogen peroxide plays a dominant role in the two-electron reduction process for the earlier mentioned mechanism [63-65]. He used H2O2 as an additive

for the cathodic electrodeposition of CeO2 and Gd-doped ceria films from aqueous and mixed alcohol–water solutions of cerium chloride or nitrate. He then proposed that a CeO2.nH2O or Ce(OH)3OOH deposit formed via two- or four-electron reduction, owing to the participation of hydrogen peroxide in the oxidation step. Zhitomirsky noticed that there was always a lot of cracking of the films, probably due to dehydration of the film and/or mismatch of the linear thermal expansion coefficients for the coatings and substrate. For example, stainless steel, a common substrate used for ceria deposition, has a linear thermal expansion coefficient of ~12.5 x 10-6 K-1. For ceria, the linear thermal expansion coefficient has been reported to range from 9 to 18 x 10-6 K-1 at 298 K [66-69]. In addition, α increased as the oxygen vacancies increased for ceria, indicating that the +3/+4 ratio of cerium in the coatings is important. It is reasonable to assume that the thermal expansion coefficient values for thin coatings or nanocrystalline films will be different than that measured for bulk cerium oxide. This cracking or "stain-glass" effect that occurs for deposition of CeO2 can be seen in Figure 2. Zhitomirsky added polymers (PVB or PVP) into the deposition solution, which were electrochemically intercalated into the deposit, so that the resultant films exhibited better adherence and crack-proof properties [63, 70]. Switzer took a different approach, in which he used anodic deposition at different applied voltages to directly oxidize Ce(III) to Ce(IV) and obtained crack free films [25]. A XANE study on electrodeposited cerium oxide thin films revealed that anodic deposition led to higher percentage of Ce(IV) species while cathodic base generation method led to the formation of high percentage of Ce(III) species in the composition [71]. In fact, no matter what electrode‐ position method is used, the coatings obtained typically have a mixed stoichiometry of CeO2 x. Much of the work for deposition of cerium oxide has been done using the base generation method; however several researchers have studied the deposition using direct oxidation of Ce(III) to Ce(IV) to produce the films [25, 27, 28, 72].

and Ce(IV) oxide, while still retaining the fluorite cubic structure. The coatings can be easily

For the electrodeposition of rare earth oxides, cerium oxide is by far the most studied. It is also one of the few that has been deposited by both electrolytic (base generation) and direct electrodeposition. Cathodic electrodeposition (i.e., base generation electrochemical method) was first introduced for the plating of cerium oxide films [32]. Switzer et al. used this method to produce cerium oxide films and powders [57, 58]. Crystalline, randomly oriented cerium oxide coatings were deposited galvanostatically from a cerium nitrate solution. During the synthesis, the pH changed from ~ 4.5 to 7.5, showing that base was

An in-depth study was done by Aldykiewicz et al. to understand the base generation mechanism for CeO2 deposition [59]. They proposed a mechanism involving oxygen to produce an oxidizing agent (i.e., H2O2) for Ce(III) to Ce(IV) formation. With an oxidant available in the system, cerium oxide film formation was accomplished through a four- or two-electron process to a hydroxide intermediate. A critical pH value above 8.7 was needed to keep the cerium hydroxide ions stable in solution. The final step was the precipitation of CeO2 onto the electrode surface. This mechanism was supported by rotation disk electrode experiments and XANES studies. Li et al. studied the mechanism proposed by Aldykiewicz with in situ atomic force microscopy technique (AFM) and concluded that a cerium hydroxide species is produced at the electrode surface with CeO2 forming nuclei out of this hydroxide "gel". The rate-determining step for this mechanism is then the

Zhitomirsky also proposed that hydrogen peroxide plays a dominant role in the two-electron reduction process for the earlier mentioned mechanism [63-65]. He used H2O2 as an additive

identified by x-ray diffraction (XRD) analysis.

90 Advanced Ceramic Processing

**Figure 1.** Fluorite structure of cerium oxide.

generated during deposition.

nucleation and growth of the CeO2 crystals [60-62].

Direct anodic deposition of CeO2 as a film is difficult in aqueous solutions. A simplified Pourbaix diagram, as shown in Figure 3, can help elucidate the different species that are stable at various pHs and voltages [34]. The dotted lines frame the boundaries of oxidation and reduction for water. At pHs below 7, Ce3+ ions are stable between the reduction and oxidation limits of the electrolyte; however, as the pH increases above 7, Ce(OH)3 precipitates.

Golden et al. proposed a deposition route in which the Ce3+ ion in solution was first stabilized using a ligand [27, 28]. Several weakly to strongly bound cerium complexes were studied for the direct anodic deposition of CeO2. The deposition proceeded best when a ligand such as acetate or lactate was complexed with cerium in solution. The deposition was found to be pH and temperature dependent. Figure 4 shows the x-ray diffraction patterns for films deposited at different pHs. As seen from the XRD pattern, for a solution pH between 7 and 9, the deposited CeO2 film exhibits a preferred (111) orientation, but at a pH of 9 to 11, the CeO2 films exhibit a random orientation. At solution pHs higher than 11, no CeO2 deposits on the electrode surface, although CeO2 powder (confirmed by XRD) is generated and settles at the bottom of the reaction cell. Golden et al. found that the preferred oriented CeO2 films could be obtained by tailoring the deposition conditions. The optimized deposition parameters included anodic deposition at current densities lower than -0.06 mA/cm2 and temperatures higher than 50 o C [1].

**Figure 2.** Optical Micrograph of an electrodeposited cerium oxide film on stainless steel. 800x magnification. (from T. Golden).

**Figure 4.** X-ray diffraction patterns of cerium oxide films deposited at a pH of (A) 7.5, (B) 8.5 and (C) 10.5. Deposition temperature, 70 o C; janodic = -0.06 mA/cm2 . Y-axis represents x-ray intensity in cps.

Electrochemical Synthesis of Rare Earth Ceramic Oxide Coatings http://dx.doi.org/10.5772/61056 93

**Figure 3.** Simplified Pourbaix diagram for the Ce system in aqueous solution.

**Figure 4.** X-ray diffraction patterns of cerium oxide films deposited at a pH of (A) 7.5, (B) 8.5 and (C) 10.5. Deposition

**Figure 2.** Optical Micrograph of an electrodeposited cerium oxide film on stainless steel. 800x magnification. (from T.

. Y-axis represents x-ray intensity in cps.

temperature, 70 o

Golden).

92 Advanced Ceramic Processing

C; janodic = -0.06 mA/cm2

The electrodeposition of cerium oxide has an approximate linear relationship with fixed current density and deposition time up to a point. Typically, the faradaic yield drops off 35% after a certain film thickness due to the insulating quality of the cerium oxide coating [73]. After long periods of deposition time (~24 hrs), the current becomes negligible. [27, 28].

Cerium oxide has been deposited from electrolytic solution containing either cerium nitrate or cerium chloride salts. Using chloride salts poses a problem, in that the deposits tend to be amorphous and incorporate chloride ions into the films except under certain conditions. Creus et al. found that to deposit using cerium chloride salts required either an addition of H2O2 to the aqueous electrolyte solution or use of a mixed water–ethyl alcohol solution [74]. Others also found that the base generation method could be used for the deposition from chloride salts when the plating solution contained a mixture of water and ethanol [70, 75, 76].

A major thrust in recent years has been to electrodeposit cerium oxide onto various substrates for corrosion protection. Successful deposition of cerium oxide has been accomplished on surfaces such steels [77-79], zinc [15, 80], aluminum [81], and nickel superalloys [82, 83]. Linear polarization and tafel analysis were applied to test the corrosion protection effect of the asdeposited cerium oxide–oriented films [1]. The corrosion current decreased from 7.94 x 10-9 for the substrate to 7.59 x 10-10 A∙cm-2 for the CeO2 film coated substrate in a 0.1 M NaCl solution. Film coated substrate in a 0.1 M NaCl increased from 2.63 x 106 for the substrate to 6.69 x 107 Ω∙cm<sup>2</sup> for the CeO2 film showing increased corrosion protection for the coating.
