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Fabrication of nanoporous MTiO3 (M ¼ Pb, Ba, Sr) perovskite array filmswith unprecedented high structural regularity†

Badro Im, Hwichan Jun, Kyung Hee Lee and Jae Sung Lee*

Received 14th July 2011, Accepted 12th October 2011

DOI: 10.1039/c1ce05891f

Nanoporous MTiO3 (M ¼ Pb, Ba, Sr) array films with unprece-

dented high structural regularity are fabricated on Ti substrate by

hydrothermal treatment of anodic TiO2 film prepared by two-step

anodization. The films contain a high density of uniform cylindrical

pores that are aligned perpendicular to the Ti substrate. One-step

anodization, however, produces individually-separated nanotubes.

Patterned thin film with regular nanoarrays has attracted consider-

able interest because of their utilization for nanoscale electronics such

as magnetic, electronic, and optoelectronic devices.1–8 As a typical

method to fabricate self-organized one-dimensional (1-D) nanoarray

structures, anodic oxidation has received increasing attention lately.

In contrast to conventional methods such as the lithographic process,

the anodization method is inexpensive, easy and simple. Above all, it

is possible to obtain nano-patterned thin films of large area and high

aspect ratio.5–9 So far, the best-known material prepared from the

anodization is highly-regular arrays of anodic aluminum oxide

(AAO) nanochannels.3–7,10,11 Recently, however, Ti anodization has

attracted increasing attention because of a variety of useful properties

of TiO2 as a functional material. In addition, the Ti anodization

technique has improved recently to fabricate high quality materials

with uniform morphology.12–20 For example, fabrication of self-

organized regular TiO2 arrays via two-step anodization of Ti foil has

been reported.21

Various perovskite 1-D nanostructures of BaTiO3, (Ba,Sr)TiO3

and PbTiO3, could be synthesized by reacting the anodic TiO2

nanotubes with respective cation-containing solutions.1,2,22–28 These

perovskite materials have been widely studied because of their

ferroelectric, piezoelectric, dielectric, pyroelectric, electro-optic,

photoelectric and catalytic properties. Also SrTiO3 is an important

n-type semiconductor used as a photocatalyst or a photoelectrode.29

Thus these titanate-based perovskite materials with regular patterns

on a conducting substrate could find many applications such as

capacitors, actuators, electrochromics, gas-sensors, photocatalysts,

bio-templates and various electronic applications.10,11,29–33 It is

Department of Chemical Engineering, Division of Advanced NuclearEngineering, Pohang University of Science and Technology(POSTECH), Pohang, Korea. E-mail: jlee@postech.ac.kr; Fax: +82 54279 8299; Tel: +82 54 279 2266

† Electronic supplementary information (ESI) available: Experimentaldetails and morphology of typical perovskite titanate films reported inthe literature. See DOI: 10.1039/c1ce05891f

7212 | CrystEngComm, 2011, 13, 7212–7215

critically important to fabricate these materials with high structural

and morphological regularity to take full advantage of their physi-

cochemical properties.34 Yet, the majority of synthesized perovskite

materials reported so far have failed to maintain the high-quality

morphology of the original anodic titanate. For example, in the

synthesis of BaTiO3, (Ba,Sr)TiO3, highly alkaline solution from

hydroxide precursors caused morphology collapse and dissolution of

nanotubes.1,2,17,23,25,33

In the present work, we report a new fabrication method of tita-

nate-based perovskite nanoporousmaterials with unprecedented high

structural regularity on a large area completely maintaining the

morphology of initial anodic TiO2. As-synthesized materials are

highly-ordered hexagonal nanoporous PbTiO3, BaTiO3 and SrTiO3

with a high aspect ratio of ca. 50 prepared via hydrothermal reaction

of nanoporous TiO2 in respective cation-containing solutions. In the

synthesis, the use of stable, high-quality anodic TiO2 as the starting

material is essential because it determines the quality of titanate-based

perovskite nanoarrays. We prepared highly-organized nanoporous

anodic TiO2 arrays of densely packed hexagonal pore structures

(honeycomb-like) from electropolishing and two-step anodization

procedures. In general, the hexagonal structure has a superior

morphological stability to withstand harsh hydrothermal condi-

tions.3,4 Sowe fabricated the highly-ordered hexagonal titanate-based

perovskite nanoporous array from hydrothermal treatment main-

taining the morphology of the initial anodic TiO2. And we also

demonstrated the generality of the synthesis procedure by fabricating

common perovskite nanoarrays of PbTiO3, BaTiO3 and SrTiO3, all

with highly-regular nanoporous structures.

The two-step anodization procedure to obtain highly regular TiO2

nanoporous array films is made up of the following steps: electro-

polishing step, first anodization, peeling-off the first anodization

layer, and second anodization.21 Bare Ti foil (1 � 1.5 cm, 0.25 mm

thickness) was cleaned by ultrasonication in acetone, isopropyl

alcohol, and ethanol. The electropolishing of cleaned Ti foil was

carried out in a solution with perchloric acid, butanol, and ethanol at

20 V for 5min below �20 �C. The two-step anodization process was

performed in a two-electrode electrochemical cell equipped with Ti

foil (anode) and platinum mesh (cathode) under constant voltage.

The first potentiostatic anodization process was performed at 60 V

for 3 h at 20 �C in a solution of 0.3 wt%NH4F and 2 vol% deionized

water dissolved in ethylene glycol. As-prepared first anodic TiO2 was

removedmechanically and the second anodizationwas carried out on

the textured Ti surface at 60 V for 30 min at 20 �C in the same

This journal is ª The Royal Society of Chemistry 2011

Fig. 2 FESEM images of anodized TiO2 nanotube film on Ti substrate

prepared by single-step anodization; (a) top-view, (b) cross-sectional

view.

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solution. An electropolishing step is helpful to obtain a uniform

electric field distribution on the entire metal surface during anod-

ization. After removing the first anodized TiO2 layer, the formed

textured Ti surface induces a narrow pore diameter distribution.

Fig. 1a–c present top-surface and cross-sectional images of field

emission scanning electron microscopy (FE-SEM, JEOL JSM-

7401F) for two-step anodized TiO2. As seen from the representative

high magnification image given in Fig. 1a, as-prepared TiO2 exhibits

high quality arrays of regular hexagonally-ordered cylindrical pore

structure with average center-to-center interpore distance of ca. 120

nm, inner pore diameter of ca. 80 nm, and pore wall thickness of ca.

20 nm. The formation of the hexagonal nanopore structure with

extraordinary regularity is the unique feature of the two-step anod-

ization process. A low magnification SEM image in Fig. 1b shows

that the pore arrangement is uniform and closely packed with little

size distribution. The structure wasmaintained over the whole area of

the film (1cm x1.5 cm) except for a few cracks on the surface. The

thickness of the anodic TiO2 film is uniform at ca. 6 mm, indicating

the nanopores have a high aspect ratio (ca. 50) as shown in Fig. 1c.

When we reduced the second anodization time by half (for 15min),

we obtained the thickness of 3.4 mm. Thus by adjusting the anod-

ization time, the thickness of the metal oxide thin film could be

tuned.12–15,21

To demonstrate the uniqueness of the two-step anodization

process, we tried the conventional single step anodization under

otherwise the same conditions. This process produced individually

separated TiO2 nanotubes as shown in Fig. 2 with an average tube

diameter of 120 nm, wall thickness of 20 nm, and length of about 10

mm. This is a typical feature of anodic TiO2 nanotubes prepared by

the usual single step anodic oxidation of Ti foil. The main difference

between the single and two-step anodization is the surface texture of

the substrate during the final anodization as schematically shown in

Scheme 1. The bare Ti has a flat surface and the electric field is

uniform during the first step anodization. As a result, individually

Fig. 1 (a–c) FESEM images of nanoporous TiO2 film on Ti substrate

prepared by two-step anodization; (a,b) top-view, (c) cross-sectional

view. (d–f) FESEM images of anatase TiO2 film obtained by calcination

of the amorphous TiO2. (g–i) FESEM images of nanoporous PbTiO3 film

obtained by hydrothermal treatment of anatase TiO2 film. The white

arrows represent relative structural regularity of these nanoporous

materials.

This journal is ª The Royal Society of Chemistry 2011

separated TiO2 nanotubes are produced from the single step anod-

ization. After peeling off this nanotube layer, the embossing textured

Ti surface is created, which induces a uniformly undulated electro-

static field that produces uniform pores of little size distribution by

the 2nd step anodization. In any case, this represents an interesting

display of morphology control of the anodic oxidation process.

This amorphous anodic TiO2 film on Ti substrate was annealed at

400 �C for 3 h to transform amorphous titanate into crystalline TiO2

structure. In the X-ray diffraction (XRD, PANalytical X’Pert

diffractometer with an X’Celerator detector) pattern shown later, the

peaks of anatase TiO2 appeared after calcination (JCPDS No.21-

1272). As shown in Fig. 1d–f, both as-anodized titanate (amorphous)

and annealed anatase TiO2 showed almost the same regular hexag-

onal nanopore morphology despite the phase change from armor-

phous titanate to anatase TiO2.

The next step of the procedure is the hydrothermal reaction to

convert the anatase TiO2 into MTiO3 (M ¼ Pb, Ba, and Sr). The

hydrothermal treatment was carried out in a Teflon-vessel fitted

stainless steel reactor containing 0.002MPb acetate trihydrate, 0.05M

Ba hydroxide octahydrate, or 0.05M Sr hydroxide dissolved in 80ml

CO2-free water under a N2 atmosphere. The reactor was placed in

a convection oven at 280 �C (PbTiO3), 200�C (BaTiO3), and 200 �C

(SrTiO3), for 6 h without disruption. After the reaction, as-prepared

thin film was washed with deionized water and dried in a vacuum

oven.

Fig. 1g–i show FESEM images of the transformed nanoporous

PbTiO3 nanostructure array. These SEM images of well-defined

nanoporous PbTiO3 reveal the hexagonal array structure of TiO2 has

been preserved during the hydrothermal reaction at 280 �C for 6 h in

a Pb acetate trihydrate solution.As shown in Fig. 1d,e, as-synthesized

PbTiO3 has an average center-to-center interpore distance of ca. 120

nm and inner pore diameter of ca. 60 nm. The decreased inner pore

diameter of nanoporous PbTiO3 arrays after hydrothermal treatment

from that of TiO2 indicates the increased pore wall thickness. Thus

the conversion of TiO2 into PbTiO3 leads to the swelling of the pore

wall.21,24,25 The cross-sectional view of the TiO2 nanopore array in

Scheme 1 Schematics of one-step and two-step anodization processes

and obtained morphologies of TiO2. The distributions of the electric field

are indicated in bare Ti and textured Ti.

CrystEngComm, 2011, 13, 7212–7215 | 7213

Fig. 3 FESEM images of highly-ordered nanoporous BaTiO3 (a,b) and

SrTiO3 (c,d) on Ti substrate; (a,c) top-view (inset in a high magnification)

and (b,d) cross-sectional view.

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Fig. 1c shows a smooth surface along the whole thickness of the

vertically aligned film. In contrast, as-prepared PbTiO3 film in Fig. 1i

has a polygrain structure showing periodic features on the surface

along the thickness. The structural regularity along the extended cells

is also reduced slightly as indicated by the broken arrow in Fig. 1g.

This represents residual structural strain involved in the phase

change. Apart from these differences, the overall morphology of the

perovskite material matches well to that of the starting TiO2. Thus

thismethod proves to be an effective process to obtain highly-ordered

nanoporous titanate perovskite films of large area (Fig. 1h). The high

quality of the perovskite nanoporous films synthesized here is

obvious when we compare them with those reported in the litera-

ture.1,24,27 All these materials synthesized by the usual single step

anodization show highly distorted nanotube bundles instead of

Fig. 4 XRD patterns of (a) calcined anatase TiO2 (JCPDS 21-1276) (b)

nanoporous PbTiO3 (JCPDS 06-452) (c) BaTiO3 (JCPDS 31-174) and (d)

SrTiO3 (JCPDS 73-0661) array thin films on Ti.

7214 | CrystEngComm, 2011, 13, 7212–7215

hexagonal honeycomb-like array structures of our samples (see also

ESI†). Thus, initial formation of highly-organized nanoporous

anodic TiO2 of densely-packed hexagonal pore structures from the

two-step anodization procedure seem responsible for the high quality

of our perovskite films.

We extended the scope of this fabrication strategy to other

common perovskite materials, BaTiO3 and SrTiO3. Thus we also

hydrothermally treated the nanoporous TiO2 film with Ba or Sr

hydroxide solution at 200 �C for 6 h. Despite the harsh alkaline

conditions caused by the hydroxide precursors, highly-ordered

nanoporous BaTiO3 and SrTiO3 array films were also obtained just

like in the synthesis of PbTiO3 film discussed above. As shown in

Fig. 3, they have an average interpore distance of ca. 120 nm and an

inner pore diameter of ca. 60 nm. This corresponds to a packing

density of 4.7 � 1010 pores cm�2. Again the hydrothermal treatment

changes only the inner pore diameter by expansion of the pore wall.

Thus the morphological change from TiO2 film to perovskite films of

MTiO3 (M¼ Pb, Ba, Sr) is almost the same, but the regularity of the

nanopore structure varies slightly following the order of TiO2 >

PbTiO3 > BaTiO3 > SrTiO3. But all of them showed highly-ordered

uniform hexagonal nanopore structures over the large area. The film

thickness of BaTiO3 and SrTiO3 is also the same as that of TiO2 and

PbTiO3 (ca. 6 mm) as shown in Fig. 3b and d. They show very

uniform, densely-packed and perpendicular film structure deposited

on Ti substrate.

The X-ray diffraction (XRD) was performed to identify the crys-

talline phase of the as-prepared samples. From XRD patterns in

Fig. 4, it is clear that hydrothermal treatment transform anatase TiO2

completely to PbTiO3, BaTiO3, and SrTiO3. These perovskite

materials are in pure single phases without any impurity except for Ti

peaks originated from the Ti substrate. The XRD pattern in Fig. 4b

could be indexed to the tetragonal PbTiO3 according to JCPDS No.

06-452. Fig. 4c and d represent the cubic BaTiO3 and SrTiO3

according to JCPDS No. 31-0174 and JCPDS No. 73-0661,

respectively.

Thus it has been demonstrated that we can synthesize highly-

ordered nanoporous MTiO3 (M ¼ Pb, Ba, Sr) perovskite array film

on Ti substrate from two-step anodic TiO2. The morphology of the

starting TiO2 is preserved in the product perovskite film except for the

swelling of the pore wall. We can tune the properties of the nano-

porous perovskite films, i.e. pore size, spacing, length and thickness

by adjusting the operation conditions of anodization step such as

solvent, voltage, time, and temperature, which are known to deter-

mine the properties of anodic TiO2,12–15,21 which is the precursor to

the perovskite titanates.

Conclusions

In this study, for the first time, we fabricated highly-ordered,

hexagonal array films of nanoporous PbTiO3, BaTiO3, and SrTiO3

perovskite on Ti substrate with unprecedented structural regularity

by hydrothermal reaction in appropriate cation-containing solutions

of anodic anatase TiO2 prepared by two-step anodization. In

contrast, conventional one-step anodization produced individually-

separated nanotubes. The films contained a high density of uniform

cylindrical pores (4.7 � 1010 pores cm�2) that were aligned perpen-

dicular to the Ti substrate with high aspect ratio (ca. 50) and regu-

larity on a large area. It was demonstrated that themorphology of the

starting TiO2 could be preserved in the product perovskite film except

This journal is ª The Royal Society of Chemistry 2011

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for the swelling of the pore wall. These vertical, highly-ordered

structures are suitable for applications to various device elements.

And the nanoporous structure provides a good architecture as a hard

template by filling the various substances into the pore. The prop-

erties of the nanoporous perovskite films were tunable by adjusting

the operation conditions of the anodization step. Themethod is quite

general, so that the other titanate-based perovskite materials could be

synthesized by changing the metal precursor in the hydrothermal

process.

Acknowledgements

The authors would like to acknoledge funding from theKoreaCenter

for Artificial Photosynthesis (KCAP) (NRF-2009-C1AAA001-2009-

0093879) and BK-21 program and WCU program (R31-30005)

funded by the National Research Foundation of Korea.

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