RESEARCH PAPER

Nanocrystalline oxide (Y2O3, Dy2O3, ZrO2, NiO) coatingson BaTiO3 submicron particles by precipitation

Alessio Bassano Æ Vincenzo Buscaglia ÆMohamed Sennour Æ Maria Teresa Buscaglia ÆMassimo Viviani Æ Paolo Nanni

Received: 25 November 2008 / Accepted: 30 March 2009 / Published online: 18 April 2009

� Springer Science+Business Media B.V. 2009

Abstract Nanocoatings (5–20 nm) of different com-

pounds on fine BaTiO3 particles were obtained by

means of precipitation processes. Homogeneous and

smooth shells of Y(OH)CO3 and Dy(OH)CO3 were

grown from nitrate solutions in the presence of urea.

An irregular coating consisting of zirconia nanoparti-

cles was produced from zirconyl nitrate solution using

ammonia as a precipitating agent after adsorption of a

polymeric polyelectrolyte on the BaTiO3 surface.

Composite particles with a peculiar morphology were

obtained by inducing heterogeneous nucleation and

growth of Ni(OH)2 lamellae on the BaTiO3 surface.

The different shells can be transformed in a nanocrys-

talline coating of the corresponding oxide (Y2O3,

Dy2O3, ZrO2, NiO) by calcination at moderate

temperatures (400–700 �C). The overall results indi-

cate that precipitation from solution represents a

versatile process to grow a second-phase layer on the

surface of BaTiO3 particles. This approach can be used

as an alternative to mechanical wet mixing for

controlled doping of ferroelectric materials and for

the fabrication of composite materials with specific

geometry of the two-phase assembly.

Keywords Core–shell particles �Coating � Dielectrics � Ferroelectrics �Barium titanate � Nanolayer � Composite materials

Introduction

Barium titanate (BaTiO3) is a ferroelectric and

piezoelectric material which has a variety of com-

mercial applications, including multilayer ceramic

capacitors (MLCCs), embedded capacitance in

printed circuit boards, underwater transducers

(sonars), thermistors with positive temperature coef-

ficient of resistivity (PTCR), and electroluminescent

panels (Moulson and Herbert 1990; Rae et al. 1999).

The properties of barium titanate ceramics can be

tailored to the specific application by changing the

chemical composition, through the formation of

solid solutions and doping, and controlling the

dopant distribution and the microstructure. Some

A. Bassano � P. Nanni

Department of Chemical and Process Engineering,

University of Genoa, Fiera del Mare, P.le Kennedy,

16129 Genoa, Italy

A. Bassano � V. Buscaglia (&) � M. T. Buscaglia �M. Viviani � P. Nanni

Institute for Energetics and Interphases, National

Research Council, Via De Marini 6, 16149 Genoa, Italy

e-mail: v.buscaglia@ge.ieni.cnr.it

M. Sennour

Centre des Materiaux, Ecole des Mines de Paris, BP 87,

91000 Evry, France

123

J Nanopart Res (2010) 12:623–633

DOI 10.1007/s11051-009-9631-0

homovalent ions (Sr2?, Ca2?, Pb2?) can replace

barium and act as shifters of the Curie temperature

(TC). Zirconium, tin, and hafnium replace titanium

and have a similar effect. BaTiO3 ceramics doped

with a small amount of donor elements (Nb, La)

have semiconducting properties and show the PTCR

effect. The dielectric permittivity–temperature curve

and other characteristics can be greatly modified by

addition of homovalent (Zr, Ce) or aliovalent (La,

Nd, Y, Dy, Ho, Nb) elements. A substantial amount

of some elements (La, Zr, Ce, etc.) determines a

transition from ferroelectric to relaxor behavior. A

rather flat permittivity–temperature relation and

temperature-stable capacitors can be obtained by a

non-homogeneous distribution of the dopant inside

the grains (core–shell structure) which determines a

distribution of the Curie temperature (Kahn 1971;

Hennings and Rosenstein 1984; Armstrong et al.

1989, 1990). For example, the relative dielectric

constant of the so-called X7R materials does not

change by more than ±15% from the 25 �C value

over the temperature range -55 to 125 �C. Rare-

earth oxides such as Y2O3, Dy2O3, and Ho2O3

together with MgO are important dopants for the

sintering of BaTiO3 in reducing conditions because

they suppress the rise of electrical conductivity as

well as the grain growth. Their use has allowed the

development of the base-metal electrode (BME)

technology, in which the expensive Pd–Ag alloy has

been replaced with cheaper metals, such as Ni and

Cu.

Traditional manufacturing of BaTiO3 dielectric

ceramics is performed by mechanical wet mixing of

the BaTiO3 powder with the dopant oxides using a

ball-milling process followed by tape casting and

liquid-phase sintering. With the progressive minia-

turization of MLCCs, the thickness of a single

dielectric layer is approaching 1 lm (Reynolds

2001). Considering that the thickness of a single

dielectric layer should comprise, as empirical rule,

about 5–7 grains, it is evident that the fabrication of

these very thin layers requires well-dispersed, small,

and uniform BaTiO3 particles with a diameter of

100–200 nm. With such fine particles it is difficult to

obtain an homogeneous dopant distribution around

the particles by the ball-milling process. In turn, this

produces a non-optimal dopant distribution in the

ceramic grains and poor dielectric properties. Chem-

ical coating is an alternative to the conventional

mechanical mixing process and can produce a very

homogeneous dopant shell irrespective of particle

size. Recent examples include the synthesis of

BaTiO3–Al2O3 and BaTiO3–SiO2 particles (Huber

et al. 2003; Aymonier et al. 2005; Mornet et al.

2005) as well as the coating of BaTiO3 particles with

SrTiO3 and BaZrO3 (Buscaglia et al. 2006).

While the chemical coating with MgO and Nb2O5

has been extensively studied (Bruno and Swanson

1993; Wang and Dayton 1999; Kim et al. 2005; Park

and Han 2006, 2007; Park et al. 2007), the formation

of a core–shell structure with rare-earth oxides and

other transition metal oxides has received much less

attention. Different approaches have been proposed

to obtain core–shell particles or, more generally, to

decorate a solid surface with nanoparticles of a

different compound (Caruso and Antonietti 2001;

Caruso 2001). These include the direct growth of the

shell from solution, the controlled assembly of one or

more layers of preformed nanoparticles on the surface

of the cores, the heterogeneous nucleation of the

second-phase, the chemical reaction between a suit-

able soluble precursor and specific functional groups

on the solid surface. The assembly of the coating

nanoparticles on the core surface is often driven by

electrostatic interactions. By choosing appropriate

experimental conditions (pH, temperature, ionic

strength) at which the two types of particles have

opposite charge, formation of a core–shell structure

can be readily obtained. The surface charge of one or

both solids can be controlled by the adsorption of

polymeric polyelectrolytes. Chemical coating can be

also useful for the preparation of composite materials

with specific geometries of the two-phase assembly.

Recently, it has been reported that a negative index of

refraction could be achieved in multiferroic BaTiO3

(ferroelectric)–NiO (antiferromagnetic) composites

by tuning the resonant frequencies of the two phases

(Kirby et al. 2007). The mechanical coupling

between the two phases could be improved by means

of a core–shell geometry.

This study reports on the use of chemical precip-

itation methods for coating BaTiO3 fine particles

suspended in aqueous solution with oxide nanopar-

ticles. The study explores different strategies for the

growth of the shell, including the control of electro-

static interactions between particles and the hetero-

geneous nucleation of a second phase on the solid

surface.

624 J Nanopart Res (2010) 12:623–633

123

Experimental

Materials

A submicron BaTiO3 powder (N41125, electronic

grade purity, SBET: 3.14 m2 g-1, density: 6.01 g cm-3,

equivalent BET diameter: 320 nm) was provided by

Yageo Ltd. The powder was produced via the oxalate

decomposition route and consisted of partly faceted,

equiaxed non-agglomerated single-phase BaTiO3 par-

ticles (diameter: 200–400 nm) with tetragonal struc-

ture. Deionized water was used in all the preparations.

The coating experiments were performed in closed

polypropylene bottles.

Y2O3 and Dy2O3 coating

The synthesis procedure is similar to that described

for the coating of polystyrene submicron spheres with

yttrium basic carbonate (Kawahashi and Matjievic

1990). In a typical synthesis, 5 g of BaTiO3 powder

was suspended in 214 mL of deionized water while

stirring. Then, 1.64 g of Y(NO3)3 * 6H2O (Aldrich)

or 1.88 g of Dy(NO3)3 * 5H2O (Aldrich) and 12.87 g

of urea (Aldrich) were dissolved in the suspension.

The final concentration of M3? (M = Y, Dy) and

urea was 0.02 mol L-1 and 1.0 mol L-1, respec-

tively. The [M3?]/[BaTiO3] molar ratio was 0.2. The

suspension was heated to 95 �C at 2 �C min-1 in a

thermostatic bath and kept at the final temperature for

3 h under stirring. The suspension was then cooled

down and the solid phase separated by centrifugation.

The powder was thoroughly washed with deionized

water and finally freeze-dried.

ZrO2 coating

A quantity of 0.5 g of BaTiO3 was dispersed in 100 mL

of a solution containing 1 g L-1 of a polymeric

polyelectrolyte. A stable suspension was obtained by

ultrasonication at room temperature for 30 min. Two

polymers were used: polydiallyldimethylammonium

chloride (PDADMAC, MW \ 200,000, Aldrich) and

polyethyleneimine (PEI, MW = 55,000, Aldrich).

Adsorption of these polymers on the BaTiO3 surface

produces positively charged particles, and the resulting

electrostatic repulsion stabilizes the suspension. The

suspension was then centrifuged and the solid phase

washed twice with 100 mL water to remove the excess

polyelectrolyte. The powder was then dispersed in

107 mL of a solution containing 0.002 mol L-1 of

ZrO(NO3)2 * 6H2O (Aldrich). The [Zr]/[BaTiO3]

molar ratio was 0.1. Subsequently, 107 mL of a

0.008 mol L-1 NH4OH solution was added dropwise

to the suspension under stirring. After 12-h aging at

room temperature the solid phase was separated by

centrifugation. The resulting powder was thoroughly

washed with water and finally freeze-dried.

Ni(OH)2 coating

Appropriate amounts of Ni(NO3)2 * 6H2O (Aldrich)

and NH4OH (25% in water, Aldrich) were dissolved

in 50 mL of water obtaining a clear blue solution

with [Ni] = 0.2 mol L-1 and [NH4OH]/[Ni] = 4

(where [Ni] denotes the overall nickel concentration).

The blue color is due to the formation of the

Ni(NH3)62? amino complex. The suspension obtained

by dispersing 2 g of BaTiO3 powder in the precursor

solution ([Ni]/[BaTiO3] = 1.2) was heated at

1 �C min-1 up to 90 �C and kept at the final

temperature for 8 h while stirring. The final precip-

itate was repeatedly washed with water and then

freeze-dried.

Characterization

Morphology and internal structure of the particles

were investigated by scanning electron microscopy

(SEM, LEO 1450VP) and transmission electron

microscopy (TEM, JEOL J2010 and FEI Tecnai

F20). The TEM was equipped with an energy-

dispersive electron microprobe (EDS, Oxford Instru-

ments) and electron diffraction (ED). Phase compo-

sition and crystal structure were studied using X-ray

diffraction (Philips PW1710, Co Ka radiation, graph-

ite monochromator, 2h range: 20–130�, 2h step:

0.025�, sampling time: 10 s).

Results

Y2O3 coating

TEM observation (Fig. 1a) shows the formation of a

homogeneous and smooth coating with a thickness of

about 20 nm. The shell is composed of a Y

compound, as indicated by the EDS spectrum of

J Nanopart Res (2010) 12:623–633 625

123

Fig. 1c. When precipitation is performed without the

BaTiO3 particles in the same experimental condi-

tions, it produces spherical particles with a uniform

diameter of 300 nm. The XRD pattern of these

particles only displays three broad humps, suggesting

the presence of a predominantly amorphous material.

The thermogravimetric and differential thermal anal-

ysis (TG-DTA) indicates, according to previous

results (Aiken et al. 1988), that the particles are

composed of hydrated yttrium basic carbonate,

Y(OH)CO3 * H2O. Thus, it can be assumed that the

shell grown on the BaTiO3 cores consists of the same

compound. Only the reflections of BaTiO3 can be

observed on the XRD pattern (Fig. 2) of the as-coated

powder, indicating that the coating is mainly amor-

phous or poorly crystalline, as also suggested by the

faint circles observed in the ED pattern reported in

the inset of Fig. 1b. According to the high-resolution

image of Fig. 1b, the shell region is composed of

small nanocrystals with a size of the order of 5 nm

embedded in an amorphous matrix. Calcination at

700 �C for 2 h results in the complete decomposition

of the yttrium basic carbonate with formation of

crystalline Y2O3 with cubic bixbyte structure,

2 4 6 8 10Energy (keV)

0.0

0.2

0.4

0.6

cps

C

O

Cu

Y

Y

Ba

TiBa

Cu

Cu

(a)

(c)

(d) (e)

(b)Fig. 1 BaTiO3 particles

coated with yttrium basic

carbonate. a, b As-coated

particles. Part a: low-

resolution TEM image. Part

b: high-resolution TEM

image of the shell region.

The inset shows the ED

pattern of the shell. c EDS

spectrum of the shell

region. The peak at 1.9 keV

corresponds to Y. d, eParticles calcined at

700 �C. Part d: low-

resolution TEM image. Part

e: high-resolution TEM

image of the shell region

626 J Nanopart Res (2010) 12:623–633

123

as illustrated by the XRD pattern of Fig. 2. Typical

high-resolution TEM images of the shell region after

calcination are reported in Fig. 1d–e. The continuous

and relatively smooth coating consists of Y2O3

nanocrystals with sizes in the range of 5–15 nm.

Formation of a reaction product layer between the

BaTiO3 core and the oxide shell was revealed neither

by XRD nor by HRTEM. However, solid-state

diffusion of Y3? within the perovskite lattice cannot

be excluded.

Dy2O3 coating

The morphology of the coating is slightly different

from that observed in the case of yttrium basic

carbonate. According to the high-resolution TEM

images of Fig. 3, the shell consists of a mixture of

amorphous nano regions and nanocrystals with a size

of the order of 5 nm. Often, the lattice fringes of

these nanocrystals are oriented parallel to the surface

of the BaTiO3 particles (Fig. 3b). As the BaTiO3

particles are single crystals (see Fig. 3e), the lattice

fringes displayed in Fig. 3a, b correspond to the

nanocrystals of the coating material. EDS analysis

(Fig. 3c) indicates that the shell consists of a

dysprosium compound. Previous studies (Matjievic

and Hsu 1987; Aiken et al. 1988; Tok et al. 2006)

suggest that the precipitation product, obtained by

urea hydrolysis from solutions containing trivalent

rare-earth ions such as Gd3?, Sm3?, Eu3?, and Tb3?,

is the hydrated rare-earth basic carbonate. In agree-

ment, the weight loss (&27%) and the TG-DTA

analysis performed on the precipitate obtained in the

absence of BaTiO3 are compatible with the compo-

sition Dy(OH)CO3 * H2O. Because of the small

crystal size and the relatively small amount of

coating material, the corresponding peaks are not

observed in the XRD pattern of the as-coated powder

(Fig. 4). Again, after calcination at 700 �C for 2 h,

the shell transforms in nanocrystalline Dy2O3 with

cubic bixbyte structure. Typical TEM images of the

calcined particles are shown in Fig. 3d–e. The surface

of the BaTiO3 particles is coated with Dy2O3

nanocrystals with size of 5–20 nm. High-resolution

observation indicate a random orientation of the

dysprosia nanocrystals. The manifest lattice fringes

of the core correspond to the 100 planes of BaTiO3

and indicate the single crystal nature of the particles.

ZrO2 coating

The TEM images of Fig. 5 illustrate the morphology

of the coated particles. The shell is very irregular and

composed of small nanoparticles with a diameter of

3–7 nm forming larger irregular agglomerates. The

crystal structure of the coating depends on the

polymer which was adsorbed at the surface of

BaTiO3. An amorphous phase (most likely hydrous

zirconia, as indicated by EDS analysis, Fig. 5d) is

obtained in the presence of PDADMAC, while

nanocrystalline zirconia is observed with PEI. The

interplanar distances calculated from the lattices

fringes were in good agreement with those of ZrO2,

although it was not possible to discriminate between

the monoclinic and the tetragonal polymorphs. It is

worth noting that the precipitate obtained in the

absence of the BaTiO3 particles consisted of hydrous

amorphous zirconia. In the case of PDADMAC, the

high-resolution TEM images of the BaTiO3 surface in

the regions where no coating was apparently visible

in the low-resolution observation, showed the pres-

ence of a very thin continuous layer of amorphous

zirconia with a thickness of few nm, as illustrated in

Fig. 5c.

Ni(OH)2 coating

As shown in Fig. 6, the as-coated particles do not

have a conventional core–shell structure. Relatively

large and irregular hexagonal lamellae (diameter:

100–200 nm, thickness: 10–50 nm) with different

0

500

1000

20 40 60 802theta

Inte

nsi

ty (

a.u

.)

YY Y Y

BT

BTBT

BT

BT

BT

BT

#1

#2

Fig. 2 XRD patterns (Co Ka radiation) of BaTiO3 particles

coated with Y(OH)CO3. Pattern #1: as-coated powder. Pattern

#2: powder calcined at 700 �C. BT tetragonal BaTiO3, Y cubic

Y2O3

J Nanopart Res (2010) 12:623–633 627

123

orientations have grown on the BaTiO3 surface. EDS

analysis and measurements of the lattice fringe

spacing indicate that the lamellae consist of b-

Ni(OH)2 with hexagonal brucite structure, in agree-

ment with the XRD pattern of the sample. The basal

surfaces of the lamellae correspond to the (001)

lattice planes. It has been observed that the hydroxide

lamellae have the tendency to grow with the [001]

direction perpendicular to the BaTiO3 surface. In

several cases, the (002) planes form an angle of about

71� with the (100) planes of the substrate, as shown in

Fig. 6c. Although most of the nickel hydroxide

lamellae are located onto the BaTiO3 particles,

isolated agglomerates of Ni(OH)2 nanocrystals are

also observed. Calcination of the powder for 2 h at

400 �C results in the decomposition of nickel

hydroxide to NiO.

Discussion

The above results indicate that BaTiO3 submicron

particles can be coated with different compounds,

including Y(OH)CO3, Dy(OH)CO3, ZrO2, and

2 4 6 8 10Energy (keV)

0

2

4

6

8

cps

O

Cu

Dy

Dy

Ba

Ba

Ti

Ba Dy

Dy

Dy

Dy

Dy

Cu

DyCu

(a) (b)

(c)

(d) (e)

Fig. 3 BaTiO3 particles

coated with dysprosium

basic carbonate. a, b High-

resolution TEM images of

the shell region of the as-

coated particles. c EDS

spectrum of the shell

region. Peaks at 1, 1.3, 6.5,

7.2, 7.6, and 8.4 keV

correspond to Dy. d, eParticles calcined at

700 �C. Part d: low-

resolution TEM image. Part

e high-resolution TEM

image of the shell region

628 J Nanopart Res (2010) 12:623–633

123

Ni(OH)2, using simple and relatively inexpensive

precipitation methods. Precipitation with urea is

particularly suitable to produce very homogeneous

and smooth coatings of rare-earth compounds. It is

well established (Matjievic and Hsu 1987; Aiken

et al. 1988) that hydrolysis of urea in aqueous media

yields ammonia and cyanate ions.

ðNH2Þ2CO! NHþ4 þ OCN�

In acidic solution, cyanate ions react rapidly accord-

ing to

OCN� þ 2Hþ þ H2O! CO2 þ NHþ4

whereas, in neutral and alkaline solutions, carbonate

ions and ammonia are formed

OCN� þ OH� þ H2O! NH3 þ CO2�3

Yttrium and lanthanide ions are slightly hydrolyzed

in water to [MOH(H2O)n]2?. The precipitation of the

basic carbonate can, therefore, be described by the

reaction

MOH H2Oð Þn� �2þþCO2 þ H2O

! M(OH)CO3 � H2Oþ 2Hþ þ ðn� 1ÞH2O

The deposition of the basic carbonate shell is

probably driven by electrostatic interactions. The

pH of the urea solution decreases from &6 to 5

during heating and then, at the final constant

temperature of 95 �C, it slowly increases up to 7.2

after 3 h. This rise in pH is related to the hydrolysis

of urea. The isoelectric point (iep) of BaTiO3 is in the

pH range 3–10 depending on the surface chemistry

and the solid loading (Tripathy and Raichur 2008;

Neubrand et al. 2000). Dissolution of Ba2? ions from

the BaTiO3 surface occurs progressively in water

with formation of a TiO2-like surface and the iep

shifts to acidic pH. Therefore, a relatively low iep is

expected for the BaTiO3 particles suspended in the

urea solution at 95 �C. The iep of Y(OH)CO3 is at

pH & 8.5 (Sprycha et al. 1992). As a consequence,

the two kinds of particles are likely to have opposite

surface charge in the pH windows 5 to 7 correspond-

ing to urea decomposition, and the precipitation of

Y(OH)CO3 can directly occur on the surface of

BaTiO3. Formation of BaCO3 was not detected in the

XRD patterns of the as-coated powders (Figs. 2 and

4) meaning that barium leaching from the BaTiO3

surface is not severe.

The above approach cannot be applied to the

coating of BaTiO3 with zirconia. The iep of ZrO2 is at

pH of 5–6 (Fengqiu et al. 2000; Rao et al. 2007).

This is the same pH of the urea solution during the

initial hydrolysis. Consequently, the fine zirconia

particles originated by hydrolysis will have the

tendency to agglomerate in bigger particles rather

than forming a layer on the BaTiO3 surface. Indeed,

formation of relatively large (1–10 lm) hydrous

zirconia particles with irregular shape was observed

using urea as precipitating agent. The surface charge

and iep can be modified by adsorption of suitable

polyelectrolytes. Both PEI and PDAMAC are cat-

ionic polyelectrolytes. Thus, adsorption of the poly-

mers on the BaTiO3 particles should impart a positive

charge to the surface and let the iep move to values

[10. Accordingly, both polymers provided a stable

suspension. When PEI or PDADMAC are not

adsorbed on the BaTiO3 particles, zirconia precipita-

tion leads to the separate formation of irregular

amorphous particles rather than a coating. Thus, the

adsorption of a cationic polyelectrolyte is quite

effective to induce adhesion of the ZrO2 particle on

the BaTiO3 surface. The pH of the suspension after

addition of the ZrO(NO3)2 solution was &2.8 and

gradually increased up to &9.6 as a consequence of

ammonia addition. In acidic conditions, aqueous

solutions of zirconyl salts spontaneously hydrolyze

with formation of [Zr4(OH)8(H2O)16]8? tetramers

(Brinker and Scherer 1990). At higher pH,further

hydrolysis and condensation first yield stable nuclei

and then small (3 nm) nanoparticles. Deposition of

0

500

1000

20 30 40 50 60 70 80

2theta

Inte

nsi

ty (

a.u

.)

BT

BT

BT

BT

BT

BT

BT

Dy

Dy Dy Dy#2

#1

Fig. 4 XRD patterns (Co Ka radiation) of BaTiO3 particles

coated with Dy(OH)CO3. Pattern #1: as-coated powder. Pattern

#2: powder calcined at 700 �C. BT tetragonal BaTiO3, Dy

cubic Dy2O3

J Nanopart Res (2010) 12:623–633 629

123

ZrO2 nanoparticles on the precoated BaTiO3 should

be possible in the pH range 6–10, when the two kinds

of particles are oppositely charged. However, the

obtained coating is rather irregular (Fig. 5) and

suggests that agglomeration of the zirconia nanopar-

ticles during precipitation occurred to a significant

Fig. 5 BaTiO3 particles

coated with ZrO2. a–c After

PDADMAC adsorption.

Part a: Low-resolution

TEM image. Parts b–c:

high-resolution TEM

images of the surface region

of the particles. d EDS

spectrum of the shell

region. The peak at

&2.0 keV corresponds to

Zr. e, f After PEI

adsorption. Part a: low-

resolution TEM image. Part

b: high-resolution TEM

image of the shell region

630 J Nanopart Res (2010) 12:623–633

123

extent. This indicates that the zeta potential of

zirconia was negative but not strong enough to

prevent agglomeration.

The peculiar morphology of the BaTiO3–Ni(OH)2

particles (Fig. 6) suggests a different coating mech-

anism. Indeed, given the strongly alkaline pH (11.2)

of the precursor solution, both kinds of particles are

expected to have a negative surface charge and

repulsion should dominate. The starting Ni2?-ammo-

nia solution with a total nickel concentration of

0.2 mol L-1 is stable at room temperature because of

the formation of the amino complex Ni(NH3)62?.

However, the stability of this complex decreases with

increasing temperature and the supersaturation of the

solution will gradually rise during heating, eventually

leading to crystallization of Ni(OH)2. In general,

heterogeneous nucleation of a solid phase from

solution requires a lower activation energy than

homogeneous nucleation. This is because the solid–

solid interfacial energy is smaller than the solid–

liquid interfacial energy. Nucleation of Ni(OH)2 can

therefore take place at a lower supersaturation on the

BaTiO3 particle surface than in solution. After

formation of the heteronuclei on the solid substrate,

crystal growth occurs along the directions of faster

crystallization, resulting in the formation of thin

lamellar crystals (Fig. 6). The larger hexagonal basal

faces correspond to the (001) crystallographic planes.

This is the most common morphology observed for b-

Ni(OH)2 particles obtained by precipitation in alka-

line environment (Liang et al. 2004; Meyer et al.

2004; Wang et al. 2006).

Summary and conclusions

The process of precipitation from solution is partic-

ularly well suited for coating BaTiO3 submicron

particles with a thin shell of a different compound.

Homogeneous and smooth nanocoatings of

Y(OH)CO3 and Dy(OH)CO3 were obtained by using

urea as precipitating agent. Formation of the coating

Fig. 6 TEM images of

BaTiO3 particles coated

with Ni(OH)2. a Typical

morphology corresponding

to a BaTiO3 particle

surrounded by Ni(OH)2

lamellae with different

orientations. b High-

resolution image of the

basal surface of a Ni(OH)2

lamellar crystal. The lattice

fringes correspond to the

(100) planes. c Ni(OH)2

lamellar crystal grown on

the BaTiO3 surface with the

[001] direction

perpendicular to the BaTiO3

surface

J Nanopart Res (2010) 12:623–633 631

123

is mainly driven by electrostatic interactions. This

method can be probably extended to basic carbonate

coatings of other rare-earth elements. Irregular

zirconia coatings were prepared by precipitation

from zirconyl nitrate solution using ammonia after

adsorption of a cationic polymeric polyelectrolyte

(PEI or PDADMAC) on the BaTiO3 particles. The

polymer imparts a positive charge to the surface,

promoting the adhesion of the zirconia nanoparticles.

Nanocomposite Ni(OH)2–BaTiO3 particles were syn-

thesized by inducing heterogeneous nucleation and

growth of Ni(OH)2 lamellar crystals on the BaTiO3

surface. The different shell compounds can be

converted in the corresponding nanocrystalline oxi-

des by calcination. The described approach can be

used as an alternative to mechanical wet mixing for

controlled doping of dielectric and ferroelectric

materials and for the fabrication of composite mate-

rials with specific geometry of the two-phase assem-

bly. More generally, chemical coating of fine

particles allows for the design of materials with

specific structure and properties.

Acknowledgment Partial financial support from the Ministero

dell’Universita e della Ricerca (FISR Project ‘‘Nanosistemi

inorganici ed ibridi per lo sviluppo e l’innovazione di celle a

combustibile’’) is gratefully acknowledged.

References

Aiken B, Hsu WP, Matjievic E (1988) Preparation and properties

of monodispersed colloidal particles of lanthanide com-

pounds: III, yttrium(III) and mixed yttrium(III)/cerium(III)

systems. J Am Ceram Soc 71:845–853. doi:10.1111/j.1151-

2916.1988.tb07534.x

Armstrong TR, Morgens LE, Maurice AK, Buchanan RC

(1989) Effect of zirconia on microstructure and dielectric

properties of barium titanate ceramics. J Am Ceram Soc

72:605–611. doi:10.1111/j.1151-2916.1989.tb06182.x

Armstrong TR, Young KA, Buchanan RC (1990) Dielectric

properties of fluxed barium titanate ceramics with zirconia

additions. J Am Ceram Soc 73:700–706. doi:10.1111/j.1151-

2916.1990.tb06575.x

Aymonier C, Elissalde C, Reveron H, Weill F, Maglione M,

Cansell F (2005) Supercritical fluid technology of nano-

particle coating for new ceramic materials. J Nanosci

Nanotechnol 5:980–983. doi:10.1166/jnn.2005.147

Brinker CJ, Scherer GW (1990) Sol–gel science. Academic

Press, Boston

Bruno AS, Swanson DK (1993) High-performance multilayer

capacitor dielectrics from chemically prepared powders. J

Am Ceram Soc 76:1233–1241. doi:10.1111/j.1151-2916.

1993.tb03747.x

Buscaglia MT, Viviani M, Zhao Z, Buscaglia V, Nanni P

(2006) Synthesis of BaTiO3 core-shell particles and fab-

rication of dielectric ceramics with local graded structure.

Chem Mater 18:4002–4010. doi:10.1021/cm060403j

Caruso F (2001) Nanoengineering of particle surfaces. Adv

Mater 13:11–22. doi:10.1002/1521-4095(200101)13:1

\11::AID-ADMA11[3.0.CO;2-N

Caruso RA, Antonietti M (2001) Sol–gel nanocoating: an

approach to the preparation of structured materials. Chem

Mater 13:3272–3282. doi:10.1021/cm001257z

Fengqiu T, Xiaoxian H, Yufeng Z, Jingkun G (2000) Effect of

dispersants on surface chemical properties of nano-

zirconia suspensions. Ceram Int 26:93–97. doi:10.1016/

S0272-8842(99)00024-3

Hennings D, Rosenstein G (1984) Temperature-stable dielec-

trics based on chemically inhomogeneous BaTiO3. J Am

Ceram Soc 67:249–254

Huber C, Treguer-Delapierre M, Elissalde C, Weill F, Magli-

one M (2003) New application of the core-shell concept to

ferroelectric nanopowders. J Mater Chem 13:650–653.

doi:10.1039/b300991b

Kahn M (1971) Influence of grain growth on dielectric prop-

erties of Nb-doped BaTiO3. J Am Ceram Soc 54:455–457.

doi:10.1111/j.1151-2916.1971.tb12384.x

Kawahashi N, Matjievic E (1990) Preparation and properties of

uniform coated colloidal particles V. Yttrium basic car-

bonate on polystyrene latex. J Colloid Interface Sci

138:534–542. doi:10.1016/0021-9797(90)90235-G

Kim J-N, Byun T-S, Kim C-S, Kim Y-J, Choi C-S (2005)

Preparation of core-shell BaTiO3 particles coated with

MgO. J Chem Eng Jpn 38:553–557. doi:10.1252/jcej.

38.553

Kirby SD, Lee M, van Dover RB (2007) An approach to

achieving a negative index of refraction using coincident

resonances. J Phys D 40:1161–1166. doi:10.1088/0022-

3727/40/4/038

Liang Z-H, Zhu Y-J, Hu X-L (2004) b-Nickel hydroxide

nanosheets and their thermal decomposition to nickel

oxide nanosheets. J Phys Chem B 108:3488–3491. doi:

10.1021/jp037513n

Matjievic E, Hsu WP (1987) Preparation and properties of

monodispersed colloidal particles of lanthanide com-

pounds I. Gadolinium, europium, terbium, samarium and

cerium(III). J Colloid Interface Sci 118:506–523. doi:

10.1016/0021-9797(87)90486-3

Meyer M, Bee A, Talbot D, Cabuil V, Boyer JM, Repetti B,

Garrigos R (2004) Synthesis and dispersion of Ni(OH)2

platelet-like nanoparticles in water. J Colloid Interface Sci

277:309–315. doi:10.1016/j.jcis.2004.04.034

Mornet S, Elissalde C, Hornebecq V, Bidault O, Duguet E,

Brisson A, Maglione M (2005) Controlled growth of silica

shell on Ba0.6Sr0.4TiO3 nanoparticles used as precursors of

ferroelectric composites. Chem Mater 17:4530–4536. doi:

10.1021/cm050884r

Moulson AJ, Herbert JM (1990) Electroceramics. Chapman

and Hall, London

Neubrand A, Lindner R, Hoffmann P (2000) Room-tempera-

ture solubility behavior of barium titanate in aqueous

media. J Am Ceram Soc 83:860–864

Park JS, Han YH (2006) Effects of oxide additives coating on

microstructure and dielectric properties of BaTiO3.

632 J Nanopart Res (2010) 12:623–633

123

J Electroceram 17:867–873. doi:10.1007/s10832-006-

5413-6

Park JS, Han YH (2007) Effects of MgO coating on micro-

structure and dielectric properties of BaTiO3. J Eur Ceram

Soc 27:1077–1082. doi:10.1016/j.jeurceramsoc.2006.

05.073

Park JS, Han MH, Han YH (2007) Effects of MgO coating on

the sintering behavior and dielectric properties of BaTiO3.

Mater Chem Phys 104:261–266. doi:10.1016/j.match

emphys.2007.02.092

Rae A, Chu M, Ganine V (1999) Barium titanate—past, present

and future. In: Nair KM, Bhalla AS (eds) Dielectric cera-

mic materials, ceramic transactions, vol 100. The Ameri-

can Ceramic Society, Westerville (OH), pp 1–12

Rao SP, Tripathy SS, Raichur AM (2007) Dispersion studies of

sub-micron zirconia using Dolapix CE 64. Colloids Surf

A 302:553–558. doi:10.1016/j.colsurfa.2007.03.034

Reynolds TG III (2001) Application space influences electronic

ceramic materials. Am Ceram Soc Bull 80:29–33

Sprycha R, Jablonski J, Matjievic E (1992) Zeta potential and

surface charge of monodispersed colloidal yttrium(III)

oxide and basic carbonate. J Colloid Interface Sci

149:561–568. doi:10.1016/0021-9797(92)90443-P

Tok HAIY, Boey FYC, Huebner R, Ng SH (2006) Synthesis of

dysprosium oxide by homogeneous precipitation. J Elec-

troceram 17:75–78. doi:10.1007/s10832-006-9940-y

Tripathy SS, Raichur AM (2008) Dispersibility of barium

titanate suspension in the presence of polyelectrolytes: a

review. J Dispers Sci Technol 29:230–239. doi:10.1080/

01932690701707423

Wang S-F, Dayton GO (1999) Dielectric properties of fine-

grained barium titanate based X7R materials. J Am Ceram

Soc 82:2677–2682

Wang X, Li L, Zhang Y, Wang S, Zhang Z, Fei L, Qian Y

(2006) High-yield synthesis of NiO nanoplatelets and

their excellent electrochemical performance. Cryst

Growth Des 6:2163–2165. doi:10.1021/cg060156w

J Nanopart Res (2010) 12:623–633 633

123