ORIGINAL PAPER

Novel water based cerium acetate precursor solution for thedeposition of epitaxial cerium oxide films as HTSC buffers

Tran Thi Thuy Æ S. Hoste Æ G. G. Herman ÆN. Van de Velde Æ K. De Buysser Æ I. Van Driessche

Received: 30 December 2008 / Accepted: 10 March 2009 / Published online: 21 March 2009

� Springer Science+Business Media, LLC 2009

Abstract A novel water based precursor solution using

ethylenediaminetetraacetic acid (H4EDTA) as a complex-

ant and acetic acid and ethylenediamine (EDA) as

additional components to obtain CeO2 buffer layers on Ni

(5%W) tapes is described in detail. The influence of com-

plexation behavior in the formation of transparent and

homogenous sols and gels by the combination of cerium

acetate, acetic acid and H4EDTA has been studied. The

optimal growth conditions for cerium oxide were Ar-5% H2

gas processing atmosphere, solution concentration levels of

0.2–0.4 M, a dwell time of 60 min at 900 �C and 5–30 min

at 1,050 �C. X-ray diffraction, SEM, spectroscopic ellips-

ometry and pole figures were used to characterize the CeO2

films. Highly textured CeO2 layers were obtained.

Keywords Species distribution simulation � CeO2 buffer

layers � Thin films � Water based � Sol–gel � EDTA

1 Introduction

Epitaxial ceramic buffer layers are deposited onto a Ni-W

substrate to prevent Ni diffusion into the superconducting

layer and transfer a strong biaxial texture from the substrate

to the superconducting layer. Oxide buffer layers such as

CeO2 have obtained a great deal of interest as a diffusion

barrier for growth of YBCO films [1]. Compared with the

other methods, the sol–gel process [2] has the potential

advantage not only of achieving homogeneous mixing of

the component cations on atomic scale, but also forming

films or fibers which are of great technological importance

[3]. In recent studies, it is shown that CeO2 films fabricated

from cerium acetate precursor in organic solvent have a

high texture quality and this may be a promising step in the

fabrication of YBCO—coated conductors [4]. The advan-

tage of cerium acetate is that the acetate ligands are

retained in the complex. It leads to a reduction of free

cerium ions in precursor solution. Meanwhile, CeO2 films

fabricated from cerium acetate water based precursor

solutions have not been documented yet. Besides this, as it

is a water based method, it is environmental friendly and

expected to leave less carbon residue, which is detrimental

for the superconducting properties.

We have selected and studied the combination of

H4EDTA and acetic acid in order to establish the prepa-

ration of stable precursor solutions which can be used for

dip coating of buffer layers. Furthermore, a species distri-

bution program was employed to assess the influence of

complexation behavior in the formation of transparent and

homogeneous sols and gels. From these stable precursor

solutions, cerium oxide buffer layers were synthesized. The

CeO2 layers were characterized by using X-ray diffraction

(XRD) and pole figures for phase purity and texture,

scanning electron microscopy (SEM) for homogeneity and

microstructure and atomic force microscopy (AFM) for

surface roughness analysis.

2 Experimental

2.1 Chemicals

Cerium acetate Ce(OAc)3, KNO3, HNO3, ethylenediamine

(EDA), and H4EDTA were purchased from Sigma–Aldrich

T. T. Thuy (&) � S. Hoste � G. G. Herman � N. Van de Velde �K. De Buysser � I. Van Driessche

Department of Inorganic and Physical Chemistry, Ghent

University, Krijgslaan 281-S3, 9000 Gent, Belgium

e-mail: thithuy.tran@Ugent.be

123

J Sol-Gel Sci Technol (2009) 51:112–118

DOI 10.1007/s10971-009-1949-7

(Germany). Glacial acetic acid (99 wt %) was obtained

from Chem.-Lab (Belgium).

Thermo gravimetric analysis/differential thermal analy-

sis (TGA/DTA) was performed on the starting cerium

acetate to determine the water content of the cerium acetate

in order to assure the concentration and the ratio of cerium

to ligands in the precursor solution.

2.2 EQUIL program

The EQUIL program [5] was used to calculate the distri-

bution of the different species which might form into the

precursor solution. The simulation was run for a solution

containing Ce(III), acetic acid, and H4EDTA in the molar

ratio 1:10:0.8 in the pH range 2–11. The stability constant

data needed for that purpose were taken from the literature

[6, 7]. The detailed model of species used with EQUIL is

shown in Table 1.

2.3 Preparation of precursor solution and substrates

The scheme to make CeO2 precursor solution is indicated

in Fig. 1. The CeO2 precursor solution was prepared by

dissolving cerium acetate in a mixture of water and acetic

acid with the stoichiometric ratio Ce3?: acetic acid of 1:10.

EDTA in acid form (H4EDTA) was dissolved in EDA

solution with the stoichiometric ratio Ce3?: EDTA of

1.0:0.8. The final pH of solution was adjusted by EDA up

to a value between 5.5 and 6.0. This combination of

compounds leads at pH 5.5–6.0 to clear and homogeneous

sol and gel as shown in Fig. 2. The solutions could be

stored at room temperature for a year without losing

stability.

The Ni 5 atomic % W tapes were heated and treated for

recrystallization at 1,000 �C for 1 h under a reducing

Ar-5%H2 atmosphere, these conditions constitutively pro-

duce a good biaxial texture formation [8]. It is also

effective to remove the native surface Ni oxide [9]. The

evolution of wettability of Ni-W surface was determined

by contact angle analysis using the sessile drop contact

technique. The drop has a contact angle of 54� when it is

untreated. This contact angle decreased to 39� after thermal

treatment. After this, the substrates were chemically

cleaned in different solvents. The details of the procedure

employed are as follows: the substrates were first dipped in

acetone and secondly in methanol for 5 min. This proce-

dure was followed by a rinse in deionized water for 5 min

[10]. After degreasing the contact angle is reduced insig-

nificantly from 39� to 33�. Then the substrates were dipped

in a hot mixture of H2O2:HCOOH = 1:1 for 15 min. The

mixture was heated to 55–57 �C. The temperature of the

mixture increased further automatically because of the

exothermic reaction of hydrogen peroxide in formic acid.

The contact angle is decreased radically from 33� to *0�indicating a perfect wettability. Finally, the clean substrates

were kept in methanol until use. These substrates were

taken out and dried at room temperature in the clean room

before dip coating.

2.4 Dip coating and heat treatment

The films were prepared using dip coating at room tem-

perature in a clean room (class 10000) with a computer

Table 1 The stability constant data of Ce3? with acetic acid and

EDTA taken from [6, 7] used as model for the EQUIL program

logb Stoichiometry

Ce3? HOAc EDTA H

4.56 0 1 0 1

10.07 0 0 1 1

16.68 0 0 1 2

18.96 0 0 1 3

20.96 0 0 1 4

22.46 0 0 1 5

22.46 0 0 1 6

1.91 1 1 0 0

3.09 1 2 0 0

3.68 1 3 0 0

15.94 1 0 1 0

-19.70 1 0 0 -3

-13.78 0 0 0 -1

The compositions of species are given with stoichiometric coeffi-

cients. A negative coefficient for H means bonded OH-

Glacial acetic acid (HOAc: Ce3+ = 10:1)

Reflux 90°C, 1h

Cerium acetate (aq)

Reflux 90°C, 1h

EDTA solution(EDTA: Ce3+ = 0.8:1.0) EDTA was dissolved in

Ethylenediamine solution

Clear light yellow solution 0.25M, pH=5.8, η = 3.2 cP

Clear yellow gel

Fig. 1 Schematic overview of the preparation of the aqueous CeO2

precursor solution

J Sol-Gel Sci Technol (2009) 51:112–118 113

123

controlled precision dip coater. After dipping into the

solution concentration levels of 0.2–0.4 M, they were held

immersed for 10 s and withdrawn at as speed of 10, 20, 30,

40, 50 mm/min.

The dip coated samples were converted to the gel in a

dust free furnace for 1 h at 60 �C. Subsequently, these

amorphous gels were transformed to the desired crystalline

CeO2 phase by an appropriate heat treatment in a quartz

tube furnace. The optimization of heat treatment was

shown in the Table 2. The data in the Table 2 shows that

the optimal growth conditions for this precursor were

Ar-5% H2 gas processing atmosphere, a dwell time 60 min

at 900 �C and another dwell time range of 5–30 min at

1,050 �C.

2.5 Characterization

The thermal decomposition behavior of the gel networks

was studied by TGA–DTA (STD 2960 Simultaneous DSC–

TGA). Identification of the different phases present in the

CeO2 films was performed by X-ray diffraction (Siemens

D5000, CuKa). Scanning electron microscopy (SEM)

(Philips 501), Atomic force microscope (AFM) Molecular

Imaging Picoplus with PicoScan 2100 Controller and XRD

pole figures were also employed to indicate the overall

morphology of the cerium oxide films. Spectroscopic

ellipsometry (JA Woollam Alpha-SE) was used to measure

the thicknesses of the films.

3 Results and discussion

3.1 Characterization of precursor solutions

To control the hydrolysis reactions upon gel formation and

to avoid precipitation of metal hydroxides suitable organic

ligands are usually added to the precursor solution [3]. The

basic idea behind that is the reduction of the concentration

of free Ce3? in the solution by formation of soluble chelate

complexes. EDTA and acetic acid could be considered as

complexing agents. Although acetic acid was present with

a ratio higher than 1 in comparison with the metal ions, it

can hardly play an important role in complexation of Ce3?

due to its very low stability constant value of 101.91 for the

mono acetate complex [Ce(CH3COO)]2? [6, 7]. Therefore,

the ideal precursor solution should contain a stronger

ligand such as EDTA4-. EDTA4- [11–13] forms strong 1:1

complex with Ce3?. The equilibrium constant for the

reaction, Ce3?? EDTA4-� [Ce(EDTA)]- is 1015.98

[6, 7]. But EDTA in its acid form is only sparingly soluble

in water. At room temperature its solubility is limited to

0.5 g/L (0.05 W/V) and this solution has a pH of 2.7. The

neutralization of EDTA with weak bases like ammonia or

ethylene diamine (EDA) results in the formation of

ammonium salts with a significantly increased solubility in

water. This now allows the preparation of EDTA solutions

in the concentration range of 5–30% when the pH is raised

to around 6 [14]. In our precursor solution EDA in stead of

Fig. 2 Pictures of clear and homogeneous sol and gel

Table 2 Optimization of heat treatment for deposition of CeO2 on Ni-W in Ar-5% H2 atmosphere

No Ramp1

(�C/min)

T1 (�C) Dwell1 (min) Ramp2 (�C/min) T2 (�C) Dwell2 (min) Ramp3 (�C/min) T3 (�C) Ratio (%)

(200/(111 ? 200)) 9 100

1 5 1000 120 10 25 75.0

2 10 900 60 10 950 30 5 25 76.9

3 10 900 60 10 1000 30 5 25 82.1

4 10 900 60 10 1050 30 5 25 85.0

5 10 900 60 10 1100 30 5 25 87.5

6 10 900 60 10 1100 60 5 25 58.6

7 5 900 60 10 1050 120 10 25 85.7

8 5 900 90 10 1050 5 10 25 88.5

9 5 900 60 10 1050 5 10 25 100.0

10 5 900 60 10 1050 20 10 25 100.0

11 5 900 60 10 1050 30 10 25 100.0

Bold values indicates that only one parameter was changed, the other was constant

114 J Sol-Gel Sci Technol (2009) 51:112–118

123

ammonia was used because of a greater stability of the

precursor solution at higher temperature. A clear, stable

precursor solution was obtained when the molar ratio

EDTA:Ce3? was adjusted to the value of 0.8.

With the molar ratio of Ce3?: acetic acid: H4EDTA of

1:10:0.8 as in the precursor, the distribution of different

species was calculated using the stability constant data

from literature. The result is shown in Fig. 3. One can see

that coordination by EDTA4- starts at very acidic medium.

At pH 2.2, 50% of [Ce(EDTA)]- is already formed. From

pH 3.0 on the formation of [Ce(OAc)]2? is detected, but its

presence is at most 4%. The concentration of free Ce3? is

reduced significantly from 46 to 22% due to the formation

of [Ce(EDTA)]-. The lowest concentration of free Ce3? in

solution is reached at about pH 5.5. From pH 6.0 on free

Ce3? starts to precipitate as Ce(OH)3. So the optimal pH

range of the precursor solution is 5.5–6.0. In this pH range

clear and stable sols and gels were obtained. The role of

acetic acid in the precursor solution is probably providing

more buffer capacity in a hydrogen bonding network of

solutes and water.

3.2 Characterization of CeO2 precursor gels

The thermal decomposition behavior of CeO2 precursor

gels was studied in order to obtain information on the

decomposition of the deposited precursor layers and

establish a suitable heat treatment schedule. Figure 4

shows the TGA–DTA curves in air and in Ar for CeO2

precursor gels in the range from 25 �C to 1,100 �C. At

temperatures below 200 �C, two large decomposition steps

occur at 85 and 160 �C in rapid succession. These two

steps can be associated with the release of water and acetic

acid due to their boiling points. Soon after, at temperatures

ranging from 200–500 �C, gases such as CO, CO2, NO,

NO2 escape from the gel due to its decomposition. The

mass loss of the gel in air and in Ar is almost the same from

room temperature to 300 �C. However, starting from a

temperature of 300 �C the mass loss of the gel in Ar

decreased much slower than the one in air, regardless of the

oxygen rates. From a temperature of 800 �C on the mass

loss of the gel in air and in Ar are also the same. As can be

seen from the TGA curve (curve 2), there is no mass loss

above 800 �C. This corresponds most likely to the crys-

tallization of cerium oxide. It is in agreement with the

nucleation and growth analysis by in situ high temperature

XRD given in [4].

3.3 Characterization of CeO2 films

The h–2h scans of the CeO2 films are shown in Fig. 5. The

growth conditions for cerium oxide films were Ar-5% H2

gas processing atmosphere, a dwell time 60 min at 900 �C

and another dwell time range of 5–30 min at 1050 �C. The

strong CeO2 (200) peaks and absence of non-(h00) peaks

indicate that the buffer layers have a strong cube texture.

The intensity of (200) peaks depends on the thicknesses of

the films. The thicker film was formed, the higher intensity

of (200) was obtained.

Ce3+

[Ce(OAc)]2+

[Ce(OAc)2]+

[Ce(EDTA)]-

Ce(OH)3

0

20

40

60

80

100

10.28.26.24.22.2

pH

%

Ce3+

[Ce(OAc)]2+

[Ce(OAc)2]+

[Ce(OAc)3]

[Ce(EDTA)]-

Ce(OH)3

pH area (5.5-6.0) to produce clear, stable and homogeneous sol and gel

Fig. 3 Distribution of species

which contain Ce(III) ion, acetic

acid, EDTA, at 25 �C, I = 0.1 M

(with ratio of Ce3? acetic acid:

EDTA in 1:10:0.8)

J Sol-Gel Sci Technol (2009) 51:112–118 115

123

To examine the in-plane orientation of a CeO2 buffer

layer on Ni-W, pole figures were measured on a treated

Ni-W and a CeO2 film. Figure 6 shows the pole figures of

Ni (111) and CeO2 (222) in comparison. The CeO2 poles

are rotated by 45� with respect to the underlying Ni-W tape

due to an improved lattice match in this orientation

between CeO2 and Ni-W. The CeO2 buffer layer shows a

very good in-plane alignment (U-scan) on Ni with full-

width-at-haft-maximum (FWHM) value of 6.76� (Ni: 6.00�).

This value means that an expitaxial CeO2 film has been

formed onto the biaxially textured Ni-W tape. The results

are shown in Fig. 7.

An AFM micrograph of a CeO2 buffer layer, presented

in Fig. 8, indicates a continuous, crack-free, and dense

surface morphology. The average roughness is around

3.5 nm, which is rougher than the surface roughness of the

Ni-W substrate (around 2.0 nm).

A SEM micrograph of a CeO2 buffer layer on Ni-W

with thickness of 21.0 nm, exposed in Fig. 9 also reveals

that the film is continuous as well as crack-free.

The surface roughness of CeO2 on Ni-W obtained by

AFM measurements with different thicknesses were given

in Fig. 10. The thicknesses of the films were calculated

from spectroscopic ellipsometry measurements by fitting

(1)

(2)

488.385

(3)

357.778

85.5497160.826

(4)

0.00

20.00

40.00

60.00

80.00

100.00

0 200 400 600 800 1000Temperature, °C

Wei

gh

t, %

-20

-10

0

10

20

30

Tem

per

atu

re d

iffe

ren

ce, °

C/m

g

(1)_weight%_in air

(2)_weight%_in Ar

(3)_temp. difference_in air

(4)_temp. difference_in Ar

Fig. 4 TGA–DTA curves of

CeO2 gels in air and in Ar

atmosphere

(200)

-20

180

380

580

780

980

1180

1380

454035302520

2- Theta (degrees)

Lin

(co

un

ts p

er s

eco

nd

), r

elat

ive

inte

nsi

ty 13.9 nm21.0 nm35.4 nm

Fig. 5 XRD of CeO2 thin films

on Ni-W

116 J Sol-Gel Sci Technol (2009) 51:112–118

123

the parameters of a reference (known thicknesses) CeO2–

NiW model. The roughness value increased steeply with

the increase of film thicknesses from 10 to 40 nm.

Increasing the thicknesses of the films resulted in a gradual

increase of the roughness and starting from a thickness of

50 nm the roughness tends to be a constant. Nevertheless,

the roughness values of films varied in very small values

from 2 to 3.5 nm. Based on this observation, we can con-

clude that the films are smooth even though the thicknesses

of the films were extensively varied.

Fig. 6 X-ray pole figures of aNi-W (111) and b CeO2 on a

Ni-W substrate

-200

3300

6800

10300

13800

40 90 140 190 240 290 340 390

Lin

(co

un

t)

Lin

(co

un

t)

Ni (111)FWHM 6.00

0

100

200

300

400

500

60 110 160 210 260 310 360 410

Phi-scale (°)Phi-scale (°)

CeO2 (222)FWHM 6.76°

Fig. 7 Phi scans of Ni-W and a CeO2 buffer layer on a Ni-W substrate

Fig. 8 AFM micrograph of

CeO2 film with thickness of

21.0 nm in 2d (4 9 4 lm) and

3d (4 lm 9 4 lm 9 41.2 nm)

J Sol-Gel Sci Technol (2009) 51:112–118 117

123

4 Conclusion

We have successfully synthesized a novel water based

cerium acetate precursor. It is very stable and is expected to

leave less carbon residue compared to the precursors based

on organic solvents. It was also favorably used for the

preparation of CeO2 films. The influence of complexation

behavior in the formation of transparent and homogenous

sols and gels by the combination of cerium acetate, acetic

acid and H4EDTA has been studied and interpreted using

simulated metal-ligands equilibriums with the EQUIL

program. The occurrence of different species at different

pH values could be related to stable gel formation condi-

tions. The optimal growth conditions for cerium oxide were

Ar-5% H2 gas processing atmosphere, solution concentra-

tion levels of 0.2–0.4 M, a dwell time of 60 min at 900 �C

and another dwell time range of 5–30 min at 1,050 �C.

These films were characterized by XRD, AFM, SEM and

pole figures. Spectroscopic ellipsometry was used to mea-

sure the thickness of films. Highly textured CeO2 layers

were finally obtained.

Acknowledgments The authors would like to acknowledge the

following people D. Vandeput (Ghent University) for AFM mea-

surements, O. Janssen (Ghent University) for XRD, SEM and pole

figure measurements, M. Backer (Zenergy Power, GmbH) for pro-

viding us with Ni–W tape and T. Wagner (Lot–Oriel Company) for

spectroscopic ellipsometry measurements.

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Fig. 9 SEM micrograph of CeO2 film with thickness of 21.0 nm

Fig. 10 Surface roughness of the CeO2 layers with various thick-

nesses on Ni-W substrates

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