oxidation method involves the oxidation of species in solution from a lower ..... M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions; NACE,.
submitted to J. Electrochem. Soc.
Anodic Electrodeposition of Cerium Oxide Thin Films: II. Mechanism Studies
Teresa Diane Golden* and Adele Qi Wang
Department of Chemistry P.O. Box 305070 University of North Texas Denton, Texas 76203
*to whom correspondence should be addressed
Abstract Nanocrystalline cerium oxide films with grain sizes as small as 6 nm are deposited by anodic deposition. Cyclic voltammetry studies of a series of complexing ligands indicate that ligand complexation strength below 1 x 103 and a solution pH between 7 to 10 are needed for CeO2 film formation.
A mechanism is proposed for the deposition of
polycrystalline CeO2 films by anodic deposition.
2
Introduction Cerium oxide (CeO2) has been studied as a promising material in corrosion protection and solid fuel electrolyte industries.1 Processing techniques such as sputtering, chemical vapor deposition, sol-gel processing, and electrodeposition has been used for the formation of cerium oxide films. Electrodeposition of cerium oxide has received considerable attention since the deposition can be carried out at low temperatures and pressure and relatively low cost. Using electrodeposition, cerium oxide film or powder formation is possible with a base generation deposition method. Base generation deposition of ceramic films and powders was proposed by Switzer2 and later proved to be an attractive method for producing other ceramic powders such as ZrO2, TiO2, Al2O3, and Nb2O5.3-8 An in-depth study of the complete mechanism for CeO2 deposition using the base generation method, was done by Aldykiewicz9 and further supported by Li and coworkers.10, 11 Aldykiewicz proposed a mechanism involving oxygen to produce an oxidizing agent for Ce(III) to Ce(IV) formation. With an oxidant available in the system, cerium oxide film formation was accomplished by a two-electron process to produce a hydroxide intermediate. The final step was the precipitation of CeO2 onto the electrode surface. The mechanism was supported by rotation disk electrode experiments and XANES studies. Also Li studied the mechanism proposed by Aldykiewicz 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.
3
For anodic deposition, two methods are used for oxide film formation. One involves the oxidation of the metal electrode to form the metal oxide, such as the oxidation of metallic zirconium to produce zirconium oxide films.12, 13 The other anodic oxidation method involves the oxidation of species in solution from a lower oxidation state to a higher oxidation state, examples for this technique include the formation of hydrous IrO2 films, hydrated nickel oxide, PbO2/Co2O3 composites and single crystalline Bi2O3 films.14-17 There has been little study of the anodic deposition of cerium oxide films.
Balasubramanian and co-workers' studied electrochemically deposited cerium
related thin films and investigated the oxidation state of the cerium utilizing X-ray absorption fine structure technique (XAFS).18 The anodic deposition resulted in films with stoichiometry close to CeO2, whereas cathodic deposition produced a mixture of Ce(III)/Ce(IV) oxidation states in the films, with the Ce(III) phase oxidizing upon exposure to air. This result encourages the utilization of anodic deposition for the production of CeO2 films. In our lab, polycrystalline CeO2 film has been successfully deposited on metal and semiconductor substrates utilizing anodic oxidation.19 Our previous work reveals that both cerium ion complexation and pH are critical factors for successful formation of the film onto substrate in anodic electrodeposition of cerium oxide. To further understand the formation of the cerium oxide film, cyclic voltammetry combined with deposition was employed to study the mechanism of anodic deposition of CeO2 at various pHs and ligand conditions. In the present research, rotating disk electrode experiments are also used to study mass transfer at the electrode interface.
4
Experimental For this study a series of ligands was used as complexing agents, with the cerium ion and include lactic, acetic, citric and oxalic acids and EDTA. Ce(NO3)3 and ligand concentrations were 0.1 M throughout the study.
Solution pH was adjusted and
monitored throughout the deposition experiments. The rotating disk experiments were done using a Pine Instrument analytical rotator and stainless steel electrode as the substrate. Cyclic voltammetry of the ceriumligand solution systems were performed with an EG&G Princeton Applied Research (PAR)
Model
273A
potentiostat/galvanostat
and
Model
270/250
Research
Electrochemistry Software. The temperature was controlled with a Fisher Scientific Model 1016D circulator. A three-electrode setup was utilized in the experiment, which included a SCE reference electrode, platinum counter electrode and various working electrodes.
Additional experimental details about the deposition of nanocrystalline
cerium oxide films are described in a previous paper.19 X-ray diffraction analysis was done using a Siemens D-500 diffractometer. The tube source was CuKα radiation (λ = 0.15405 nm) operated at 40 kV and 30 mA.
Results and Discussion A simplified Pourbaix diagram for cerium species in solution is shown in Figure 1. Pourbaix diagram’s (E vs pH) consist of redox species and solubility products for an element in aqueous solutions. Single solid lines separate species related by acid-base equilibria and solid double lines separate species related by redox equilibria. The dotted lines frame the boundaries of oxidation and reduction for water. According to the
5
Pourbaix diagram for cerium,20 at pH’s above 7, Ce(OH)3 precipitate (gray-green) forms toward reducing voltages and CeO2 precipitate (yellow) forms at oxidizing voltages. In aqueous solutions below pH of 7, Ce3+ ions are stable between the reduction and oxidation limits of the electrolyte. There is a small region for oxidization of Ce3+ to Ce(OH)2++ ions. As seen from the simplified Pourbaix diagram, direct anodic deposition of CeO2 as a film is difficult in aqueous solution. If the solution pH is kept below 7, in a 0.1 M cerium nitrate solution, at an oxidation voltage above ~ 1.0 V, Ce3+ ions form a hydroxide species, Ce(OH)2++ turning the solution light orange.20 Ce3+ + 2H2O ----> Ce(OH)2++ + 2H+ + e-
(1)
This Ce(OH)2++ species can undergo a chemical reaction to form CeO2 in solution, which then drops to the bottom of the cell as a powder (yellow precipitate). Ce(OH)2++ ----> CeO2 + 2H+
(2)
Likewise, if the solution pH is above 7, then the Ce3+ ions in solution form a precipitate, Ce(OH)3 (gray-green precipitate), by a chemical reaction pathway,20 2Ce3+ + 3H2O ----> Ce2O3 + 6H+
(3)
which can be oxidized to form a CeO2 precipitate. Ce2O3 + H2O ----> 2CeO2 + 2H+ + 2e-
(4)
If it is possible to stabilize the Ce3+ ion in solution to prevent the formation of the Ce(OH)3 precipitate, then higher oxidation voltages can be applied for the conversion of Ce3+ to CeO2. We have used several organic acids as ligands to stabilize the Ce3+ in solution.
For the study, five organic acids were chosen which form metal-organic
complexes with Ce3+ and dissolve well in aqueous environment. These ligands are listed
6
in Table 1, and have formation constants that vary from a weakly held complex (i.e. acetic acid, log K = 1.68) to strongly bonded complex such as EDTA (log K = 16.80). Since CeO2 powder formation occurs at pH’s above 7, the solution pH for this study is kept between 7 to 12. Figure 2 shows the cyclic voltammetry for the ceriumEDTA complex (which represents the stronger complexes) in solution at pH 8. The reversible wave at ~1.05 V corresponds to the redox change of the metal center in the complex. The metal-ligand complex can be oxidized and reduced, however the metal is held tightly in the complex and free metal is not available to form cerium oxide precipitate or films from solution. By increasing the pH, the Ce(III)-L to Ce(IV)-L oxidation is pushed past the O2 evolution and no anodic peak is observed. In fact for pH’s 7 to 12, there is no CeO2 film deposition onto the electrode and no powder formed in the solution during electrolysis for the cerium-EDTA complex at applied anodic currents. Similar results for other complexes listed in Table 1, oxalic and citric acid, produced no films of CeO2 at anodic currents. In contrast, ligands with weaker formation constants, i.e. acetic and lactic acid are able to produce CeO2 powder and films at certain experimental conditions. Figure 3 shows the cyclic voltammogram at pH 12 for the cerium-acetic acid complex, which displays an interesting curve. Anodic deposition from this solution does not form a CeO2 film onto the electrode, however a variety of precipitates can be formed in the solution. If the solution is stirred with no voltage applied a gray-green precipitate forms. When this precipitate is collected, washed and dried, the x-ray diffraction pattern of the precipitate corresponds to Ce(OH)3.
However if the solution undergoes anodic
electrolysis at ~ 1.1 V, then a yellow precipitate forms in the cell. X-ray diffraction of
7
this collected powder corresponds to CeO2. Using this data a mechanism is proposed for the CV curve in Figure 3. The oxidation peak, at ~ 0.9 V is preceded by a chemical reaction as follows: 2Ce(III)-L 2Ce3+ + 2L 2Ce3+ + 3H2O ----> Ce2O3 + 6H+
(5)
Since the formation constant for the cerium-ligand complex in eq 5 is relatively small, then free metal ions in solution are available at the higher pH to form Ce2O3, this species can oxidize to CeO2 at voltages above ~ 0.8 V vs. SHE (Figure 1). Ce2O3 + H2O ----> 2CeO2 + 2H+ + 2e-
(6)
The thermodynamic voltage calculated for the reaction in eq. 6 is: E = 1.559 - 0.0591pH = 0.85 V vs. SHE or 0.61 V vs. SCE. On the reverse scan of the CV, a reduction peak positive of the anodic peak is seen in the CV. The CeO2 produced at the electrode during the anodic scan (eq. 6) also produces hydrogen ions, which lowers the local pH at the electrode surface. A chemical reaction can then form Ce(OH)2++ ions at the immediate vicinity of the electrode, which can then be reduced to Ce2O3. CeO2 + 2H+ ----> Ce(OH)2++
(7)
2Ce(OH)2++ + 2e- ----> Ce2O3 + H2O + 2H+
(8)
The thermodynamic potential for eq. 8 can be calculated as 1.13 V vs. SHE or 0.89 V SCE which is 0.28 V positive of the oxidation potential for the CV scan. The ∆V between the two peaks in figure 3 is ~ 0.25 V, which corresponds to the calculated thermodynamic potential difference for the equations above.
Experimentally, two
powder products, Ce2O3 and CeO2, form in this solution.
8
Only by adjusting the pH between 7 to 10 in the cerium-acetate solution, does a CeO2 film deposit onto the electrode. Figure 4 shows the CV of the Ce(III)-acetate complex at a pH of 8.0. At a potential of ~ 0.95 V there is a small peak for Ce3+ to Ce4+ oxidation. This peak is diminished at pH’s between 7.5 to 10 compared to the CV’s of the complex at all other pH’s. The small peak in figure 4 is attributed to a slow release of free Ce3+ in solution from the complex available for oxidation. An example of x-ray diffraction (XRD) for the formed films from the Ce-acetate solution at pH’s between 7.5 to 10 under galvanostatic control is shown in Figure 5. These films are polycrystalline with a preferred (111) orientation and have nanocrystallite sizes ranging from 6 to 20 nm.19 Linear sweep voltammetry for deposition of cerium oxide onto a CeO2 prelayer gives steady state currents at a potential range from 0.7 to 1.0 V. Levich plots (current density versus the square root of the rotation speed) for a series of temperatures (25 - 80 o
C) and rotation rates (0 - 3000 rpm) are shown in figure 6. The series of Levich plots at
different temperatures are relatively flat, demonstrating a non-dependence of current on the rotation rate. The deposition of cerium oxide is not under diffusion control, but there is an exponential increase in current with increasing temperature. A barrier height for the CeO2/solution interface can be determined from the following equation:21 ln( I / T 2 ) = ln( Ae A** ) − qφ b / kT
(9)
where I is the current density (A/cm2), T is the temperature (K), Ae is the electrically active area, A** is the effective Richardson’s constant (A/cm2K2), q is the elementary charge on the electron (1.60218 x 10-19C), φB is the barrier height (eV), and k is
9
Boltzman’s constant (1.38066 x 10-23J/K). Plotting ln(I/T2) versus 1/T at a set rotation rate, the barrier height can be determined from the slope, qφB/kT. From the slope of the plot of figure 7, the barrier height for the cerium oxide/solution interface is 0.30 eV. This temperature-dependent limiting current may be related to a CE mechanism, where C represents a homogeneous chemical reaction and E represents an electron transfer reaction. A slow chemical step (i.e. ligand dissociation) would generate an electrochemical species prior to electron transfer at the electrode. The rate constant of the chemical step would be temperature dependent and have an activation energy of ~0.3 eV. Based on the pH dependence CVs and rotating disk electrode results described above, we propose a possible reaction mechanism during the anodic formation of CeO2 films: Complexation and slow release of Ce(III) ions; Ce 3+ + xL → Ce( III ) L x
(10)
Ce( III ) Lx + H 2 O Slow → Ce 3+ + HL x + OH −
(11)
Oxidation at the electrode interface; Ce 3+ + 2OH − → Ce(OH ) 22+ + e −
(12)
Deposition of CeO2 films; Ce(OH ) 22+ + 2OH − → CeO2 + 2 H 2 O
(13)
In first step Ce3+ is complexed with a ligand, which helps stabilize Ce(III) in solution and prevent Ce2O3 precipitation. The dissociation of the Ce(III)Lx to Ce3+ is a slow step followed by electron transfer at the electrode. It is important to note that there is a
10
simultaneous formation of CeO2 powder during the deposition of the CeO2 film and other competing pathways such as in eq. 5 and 6 may occur at the same time. Other researchers have shown that the oxidation state of cerium ions in solution is sensitive to pH and aeration of the solution.22 In deaerated solutions, the cerium remains in a +3 oxidation state during precipitation on surfaces regardless of pH for cathodic deposition. Also, oxygen reduction in solution plays an important role for the deposition of cerium oxide using cathodic deposition.
For this reason, we checked anodic
deposition of cerium oxide films from deaerated solutions. For deaerated solutions, polycrystalline CeO2 films still form on the working electrode at the same rate. There is a possibility of a small amount of oxidant, i.e. H2O2, formed from the oxidation of water at this pH and voltage. However this would have the formation of the Ce(OH)22+ species by a chemical reaction and not support the CE mechanism.
Conclusions The anodic deposition of CeO2 films was studied with cyclic voltammetry and rotating disk electrode methods. The investigation suggests a three-step mechanism to explain the pH dependence on CV response.
The mechanism includes first the
association of Ce3+ with a weaker ligand to stabilize Ce(III) in solution and allow a slow dissociation of the cerium complex to give Ce3+ free ions in solution. This is followed by an electron transfer reaction to oxidize Ce(III) to Ce(IV), then nucleation and precipitation of the CeO2 onto the substrate.
11
Acknowledgements The work is supported by a grant from the Robert A. Welch Foundation and a UNT Faculty Research Grant.
12
References 1. H. Inaba and H. Tagawa, Solid State Ionics, 83, 1 (1996). 2. J. A. Switzer, Ceram. Soc. Bull., 66, 1521 (1988). 3. L. Gal-Or, I. Silberman, R. Chaim, J. Electrochem. Soc., 138, 1939 (1991). 4. C. Natarajan and G. J. Nogami, J. Electrochem. Soc., 143, 1547 (1996). 5. L. Aries, J. Appl. Electrochem., 25, 554 (1994). 6. M. Izaki and T. Omi, J. Electrochem. Soc., 143, L53 (1996). 7. G. R. Lee and J. A. Crayston, J. Mater. Chem., 6, 187 (1996). 8. G. H. A. Therese and P. V. Kamath, Chem. Mater., 12, 1195 (2000). 9. A. J. Aldykiewicz, A. J. Davenport and H. S. Isaacs, J. Electrochem. Soc., 143 (1), 147 (1996). 10. F.-B. Li, R. C. Newman and G. E. Thompson, Electrochim. Acta, 42 (16), 2455 (1997). 11. F.-B. Li, G. E. Thompson and R. C. Newman, Appl. Surf. Sci., 126, 21 (1998). 12. E. O. Bensadon, P. A. P. Nascente, P. Olivi, L. O. S. Bulhoes and E. C. Pereira, Chem. Mater., 11, 277 (1999). 13. L. Kavan, B. O’Regan, A. Kay and M. Gratzel, J. Electroanal. Chem., 346, 291 (1993). 14. V. I. Birss, C. Bock and H. Eizanowska, Can. J. Chem., 75, 1687 (1997). 15. Z. C. Orel, M. G. Huchins and G. McMeeking, Solar Energy Materials and Solar Cells, 30, 327 (1993). 16. M. Musiani, Chem. Commun., 21, 2403 (1996). 17. J. A. Switzer, M. G. Shumsky and E. W. Bohannan, Science, 284 (5412), 293 (1999). 18. M. Balasubramanian, C. A. Melendres and A. N. Mansour, Thin Solid Films, 347, 178 (1999). 19. A. Q. Wang and T. D. Golden, J. Electrochem. Soc., submitted, 2002.
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20. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions; NACE, Houston, TX, 1974. 21. S. M. Sze, Physics of Semiconductor Devices, 2nd ed.; John Wiley & Sons: New York, 1981. 22. A. J. Davenport, H. S. Isaacs and M. W. Kendig, in The Application of Surface Analysis Methods to Environmental/Materials Interactions, D. R. Baer, C. E. Clayton and G. D. Davis, Eds. PV 91-7, 433, The Electrochem. Soc. Proceed. Series, Pennington, NJ (1991).
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Figure Captions
Figure 1. Simplified Pourbaix diagram of cerium species.
Figure 2. Cyclic voltammetry of cerium-EDTA complex in 0.1 M Ce(NO3)3 and 0.1 M EDTA, pH = 8.0, T = 25 oC, and scan rate = 50 mV/sec.
Figure 3. Cyclic voltammetry of cerium-acetic acid complex in 0.1 M Ce(NO3)3 and 0.1 M acetic acid, pH = 12.0, T = 25 oC, and scan rate = 50 mV/sec.
Figure 4. Cyclic voltammetry of cerium-acetic acid complex in 0.1 M Ce(NO3)3 and 0.1 M acetic acid, pH = 8.0, T = 25 oC, and scan rate = 50 mV/sec.
Figure 5. X-ray diffraction of CeO2 film deposited on stainless steel substrate from 0.1 M Ce(NO3)3 and 0.1 M acetic acid solution, pH = 8.0, T = 25 oC.
Figure 6. Levich plots for Ce(III)/Ce(IV) oxidation at temperatures of 25, 40, 50, 60, 70 and 80 oC at rotation rates from 0 to 3000 rpm.
Figure 7. Activation plot from linear sweep voltammetry data at a rotation rate of 1000 rpm.
15
Ligands logKf
Acetic Acid 1.68
Lactic Acid 2.76
Citric Acid 6.18
Oxalic Acid 6.52
EDTA 16.80
Table 1. Cerium-Ligand Complex Formation Constant at 25oC.
0
4
8
2
12
++ 2
Ce(OH) (orange)
2
CeO2 (yellow) 1
1 3+
Ce
Ce(OH)3 (Ce2O3 hydr.) (grey-green)
E (V)
0
0
-1
-1
-2
-2
Ce
-3 0
-3
4
8
12
pH Figure 1, Golden & Wang "Anodic Oxidation of..."
0.02
0.01
Current (A)
0.00
-0.01
-0.02
-0.03
-0.04
-0.05 1.5
1.0
0.5
0.0
-0.5
-1.0
Potential (V) Figure 2, Golden & Wang "Anodic Oxidation of..."
0.00
Current (A)
-0.01
-0.02
-0.03
-0.04
-0.05 1.5
1.0
0.5
0.0
-0.5
Potential(V) Figure 3, Golden & Wang "Anodic Oxidation of..."
0.000
Current (A)
-0.003
-0.006
-0.009
-0.012
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Potential (V) Figure 4, Golden & Wang "Anodic Oxidation of..."
7000
(111) 6000
Intensity (cps)
5000
4000
3000
2000
1000
0 0
20
40
60
80
100
Two Theta (Degrees) Figure 5, Golden & Wang "Anodic Oxidation of..."
8 7 o
80 C
6
o
70 C o
60 C
-4
Current (10 A)
5
o
50 C o
40 C
4
o
25 C
3 2 1 0
0
10
20
30
40
50
60
70
80
1/2
ω
Figure 6, Golden & Wang "Anodic Electrodeposition..."
-18.5
-19.0
-19.5
2
ln(I/T )
-20.0
-20.5
-21.0
-21.5
-22.0 0.0028
0.0030
0.0032
0.0034
1/T Figure 7, Golden & Wang "Anodic Electrodeposition of..."
