full papers

2286

Nanoparticle arrays

DOI: 10.1002/smll.200800428

A Hierarchically Ordered TiO2 Hemispherical Particle Arraywith Hexagonal-Non-Close-Packed Tops: Synthesis andStable Superhydrophilicity Without UV IrradiationYue Li, Takeshi Sasaki, Yoshiki Shimizu, and Naoto Koshizaki*

Keywords:� hexagonal-non-close-

packed

� hierarchical arrays

� superhydrophilicity

� UV irradiation

A hierarchical TiO2 ordered hemispherical particle array with

hexagonal-non-close-packed (hncp) tops is prepared by pulsed laser

deposition (PLD) using a polystyrene colloidal monolayer as a template.

Compared with conventional lithography, the route presented has the

advantage of low cost for producing hncp nanostructured arrays.

This hierarchical particle array exhibits excellent superhydrophilicity

with a water contact angle of 08 without further UV irradiation. The

superhydrophilic property originates from oxygen defects or vacancies

on the surface of the TiO2 nanoparticles produced by PLD and the

increased roughness of the hierarchical particle arrays. More importantly,

this property is very stable for half a year and could be used in self-cleaning

surfaces and microfluidic devices.

1. Introduction

The structures of ordered arrays have recently attracted

much attention due to their potential applications in photonic

crystals,[1] catalysis,[2] as active substrates for surface-

enhanced Raman spectroscopy,[3] data-storage media,[4]

biosensors,[5] and so on. The traditional methods for obtaining

ordered arrays are lithographic techniques including optical

lithography,[6a] X-ray lithography,[6b] electron-beam lithogra-

phy,[6c] and soft lithography.[6d] However, most laboratories

cannot afford the high costs of these technologies, and they

also have low sample throughput. Recently, many researchers

have devoted themselves to other parallel strategies to

synthesize the ordered structured arrays, which are primarily

based on self-assembly processes.[7] Most of these ordered

arrays take on a hexagonal-close-packed (hcp) arrangement.

Hexagonal-non-close-packed (hncp) colloidal sphere arrays

could be obtained by different methods such as reactive ion

[�] Dr. Y. Li, Dr. T. Sasaki, Dr. Y. Shimizu, Dr. N. Koshizaki

Nanoarchitectonics Research Center (NARC)

National Institute of Advanced Industrial

Science and Technology (AIST)

Central 5, 1–1–1 Higashi, Tsukuba, Ibaraki 305–8565 (Japan)

E-mail: koshizaki.naoto@aist.go.jp

� 2008 Wiley-VCH Verlag GmbH & Co

etching, self-assembly of hydrogel spheres by dip coating,

post removal of particle surface coating from hcp particle

arrays, and substrate stretching by solvent swelling or

mechanical deformation.[8] Furthermore, hierarchically struc-

tured arrays have attracted much interest due to their unique

properties and applications in optoelectronic devices, biome-

dical science, field emission, self-cleaning surfaces, and so

on.[9] These hierarchical structures could be synthesized by

replication induced by an electric field,[9a] chemical vapor

deposition,[9b–c] template techniques,[9d] electron irradia-

tion,[9e] and so on.

TiO2 is a very useful functional material due to its wide

range of applications in the fields of photocatalysis,[10] optical

materials,[11] dye-sensitized solar cells,[12] lithium-ion bat-

teries,[13] and superhydrophobic[14] and superhydrophilic

materials.[15] Superhydrophilicity of TiO2, generally obtained

by UV irradiation, is especially useful in antifogging and self-

cleaning surfaces. When TiO2 film is irradiated by UV light, a

photo-induced redox reaction produces active oxygen on the

film surface, and the UV irradiation creates surface oxygen

defects (oxygen vacancies) on the TiO2 surface. This makes

the surface favorable for the dissociative adsorption of water,

resulting in the superhydrophilicity. The formation of dangling

bonds is also responsible for the superhydrophilicity due to the

easy adsorption of water molecules to the dangling bonds on

. KGaA, Weinheim small 2008, 4, No. 12, 2286–2291

A Hierarchically Ordered TiO2 Hemispherical Particle Array

Scheme 1. Schematic illustration of fabrication process for the

the TiO2 surface.[16] Producing a TiO2 superhydrophilic

surface without UV irradiation is a challenge and the

development of such a surface has seldom been reported.

In this study, we synthesize hierarchical TiO2 ordered

particle arrays with hncp tops by pulsed laser deposition

(PLD) and subsequent annealing. In this process, a monolayer

polystyrene colloidal array was used as a template and TiO2

was used as a target. Interestingly, this TiO2 hierarchical

particle array exhibited superhydrophilicity with a water

contact angle of nearly 08 without further UV irradiation.

More importantly, this superhydrophilicity was so stable that

the water contact angle (CA) of the as-prepared sample only

increased slightly, even when it was kept in air for about half a

year.

hierarchical TiO2 ordered particle arrays with a top hncp

arrangement.

2. Results and Discussion

Polystyrene (PS) monolayer colloidal crystals were

fabricated on cleaned Si substrates by spin coating using a

self-assembly process.[17] The colloidal monolayer, on a

supporting substrate, was placed in a PLD chamber for

TiO2 deposition at room temperature. After deposition, the

sample was moved from the PLD chamber to an oven for

heating at 650 8C for 2 h in air. The hncp, hierarchically

ordered TiO2 particle arrays were thus prepared on the

substrate, as illustrated in Scheme 1.

Figure 1 presents field emission scanning electron micro-

scopy (FE-SEM) images of the sample obtained by the PLD

process using a colloidal monolayer as the template and

subsequently heated at 650 8C for 2 h in air. From Figure 1a

and b, one can clearly see that this particle-ordered array takes

on a top hncp arrangement. Each particle in the ordered array

Figure 1. FE-SEM images of the as-prepared sample. a) Large area hierarchical structures

at low magnification. b) Same at high magnification. c) One unit in the ordered array

composed of small nanoparticles. d) Cross section.

exhibits a hemispherical shape with an average

size of 240 nm, as depicted in Figure 1b and c.

Interestingly, such particles are composed of

small nanoparticles (Figure 1c). These results

indicate that this hncp-ordered particle array

possesses a hierarchical structure.

A X-ray photoelectron spectroscopy (XPS)

survey of the as-prepared sample demonstrates

that titanium, oxygen and carbon are present

(Figure 2). The titanium and oxygen should

have originated from the hncp hierarchical

array and thus correspond well to the target in

the PLD process. Carbon is always detected in

XPS measurements as an impurity from air.

These particles were scratched from the

substrate using a blade and transferred to a

transmission electron microscope (TEM)

sample grid. The TEM image (Figure 3) of a

single particle also clearly shows that the

particle was composed of much smaller

nanoparticles, even after the PS spheres

were removed in the heating process and

an inverted bowl-like cave was left at the

bottom of each large particle. The electron

diffraction pattern (inset in Figure 3) demon-

small 2008, 4, No. 12, 2286–2291 � 2008 Wiley-VCH Verlag Gmb

strates that each large particle consisted of polycrystalline

anatase TiO2.

The wettability of the as-prepared film was measured

with a contact angle meter. A 2 mL water droplet placed on

this hierarchical particle array film spread rapidly along the

film, producing a flat surface of water on it. The water

CA became close to 08 within a fairly short time after

having been dropped (1.25 s), indicating the superhydrophi-

licity of the film. The change of water droplet shape with time

is plotted in Figure 4. More importantly, the as-prepared

sample possessed a very stable superhydrophilicity. Figure 5

presents the water CA change with time when the as-prepared

sample was kept in air. The water CA increased just

slightly but still retained its superhydrophilicity, even after

six months.

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full papers N. Koshizaki et al.

Figure 2. XPS survey spectra of the hierarchical nanoparticle arrays.

Figure 3. TEM image of hierarchical particles (scratched from the

supporting substrate by a knife and transferred to TEM copper grid for

observation) and the corresponding SAED pattern (inset).

Figure 4. Time course of water contacting behavior on the hncp hier-

archically ordered TiO2 particle array film.

Figure 5. Water contact angle change with time when the as-prepared

sample was kept in air.

2288 www.small-journal.com � 2008 Wiley-VCH Verlag Gm

2.1. Formation of the TiO2 hncp Hierarchical Particle

Array

Figure 6 presents the FE-SEM images of the sample

produced by PLD without further heating. Observation from

the top indicated that this ordered structured array displayed

an hcp alignment (Figure 6a). TiO2 was deposited on the PS

sphere surfaces and grew along the vertical direction, as can be

seen from the cross-sectional image in Figure 6b. Each particle

is composed of two parts: the PS sphere at the bottom and the

deposited TiO2 layer consisting of smaller particles on the top.

In this method, the films directly deposited by PLD are usually

amorphous due to their deposition at room temperature and

exhibit a porous structure, as in our previous reports.[18] The

Figure 6. FE-SEM images of the as-deposited sample produced by

the PLD in ambient atmosphere without heating. a) Top-view image.

b) Cross-sectional image.

bH & Co. KGaA, Weinheim small 2008, 4, No. 12, 2286–2291

A Hierarchically Ordered TiO2 Hemispherical Particle Array

Figure 8. FE-SEM images of annealed TiO2 nanoparticle film on a

silicon wafer prepared by PLD and subsequently annealed at 650 8C for

2 h without using the PS colloidal monolayer. Inset is a photo of water

contact angle measurement.

amorphous materials crystallizes after being annealed at high

temperature. In our case, when the amorphous TiO2 with its

supporting PS sphere was heated at 650 8C for 2 h, the PS

spheres were burned out. The TiO2 particles on top of the PS

sphere were changed to anatase TiO2 polycrystals composed

of smaller nanoparticles of �30 nm and were moved vertically

down to the original position of the PS sphere. Additionally,

the volume of TiO2 particles decreased during the change from

the amorphous to the crystalline phase and therefore an hncp

hierarchical particle array was formed from the hcp amor-

phous TiO2 array. The hierarchical-particle-array film adhered

tightly to the substrate after annealing at 650 8C for 2 h and

could not be detached from the supporting substrate even

when it was washed ultrasonically in water for 30min.

2.2. Origin of the Superhydrophilicity

In general, a TiO2 film with a large convexo-concave

structure exhibits hydrophobicity.[19] However, it can be made

superhydrophilic by UV irradiation due to hydroxyls gener-

ated by oxygen defects and dangling bonds on its surface

induced by photochemical processes.[15] Interestingly, in our

case, the superhydrophilicity of the TiO2 film was obtained

without further UV irradiation.

In the PLD process, the TiO2 target is irradiated with

energy exceeding its threshold using a laser beam. The ions

(Ti4þ, O2� etc.,) and electrons will be released into the

chamber and some oxygen species will be lost in the vacuum

environment. Although oxygen is chosen as the background

gas to supplement oxygen elements in the deposition process,

it is still easy to produce oxygen defects or vacancies in

deposited TiO2 during the PLD process, which can be

confirmed by the XPS spectrum of the O1s core level (Figure

7). Generally, O1s peaks at about 531 and 533–534 eV are due

to the lattice oxygen surface OH groups related to oxygen

defects or vacancies, respectively.[20] Figure 7 shows a strong

peak at 533 eV, indicating the existence of oxygen defects or

vacancies on surface of the as-prepared sample. These oxygen

defects or vacancies will make the TiO2 surface hydrophilic.

TheWenzel mode is usually used to explain the wettability of a

rough surface for such a hydrophilic surface.[21]

cos ur ¼ r cos u (1)

Figure 7. XPS spectrum of the hierarchical nanoparticle arrays: O1s

core level.

small 2008, 4, No. 12, 2286–2291 � 2008 Wiley-VCH Verlag Gmb

Here, r is the surface roughness, which is the ratio of the

total surface area to the projected area on the horizontal plane,

and ur and u are the CAs on a particle film and a native film

with a smooth surface, respectively. This equation suggests

that hydrophilicity will be enhanced with increasing roughness

factor for hydrophilic materials. To further identify the origin

of the superhydrophilic property of the synthesized film, a

TiO2 nanoparticle film was prepared by PLD under the same

experimental conditions as for the hierarchical particle array,

but without using the PS colloidal monolayer. The film was

subsequently annealed at 650 8C for 2 h (Figure 8). The

nanoparticle film had almost the same particle size distribution

as that of the smaller nanoparticle in the hierarchical particle

array (Figure 1). The water CA of the nanoparticle film

without the PS colloidal monolayer was 388. This indicates

that, although oxygen defects or vacancies were produced in

the PLD process, making the TiO2 surface hydrophilic, there

were too few such defects to directly produce superhydro-

philicity. Compared with the nanoparticle film (Figure 8)

obtained without using a colloidal monolayer, the hncp

hierarchical TiO2 particle array possesses a much higher

roughness, which improves the wettability from hydrophilicity

to superhydrophilicity. The superhydrophilicity of the

annealed sample thus originates from the combination of

the hydrophilicity produced by the PLD process and the

special rough structures of hncp hierarchical particle arrays.

3. Conclusions

In conclusion, we present a new process for fabricating

hncp, hierarchically ordered particle arrays composed of TiO2

nanoparticles, by using the PLD process with a PS colloidal

monolayer as the substrate. Compared with traditional

lithographic techniques, the strategy we presented here has

the advantage of low cost. The hierarchical particle array

displays excellent and stable superhydrophilicity without

further UV irradiation, which is attributed to the combination

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full papers N. Koshizaki et al.

2290

of the hydrophilicity produced by the PLD and the enhanced

roughness on the surface of the hncp hierarchical particle

array. The outstanding superhydrophilicity could be used in

self-cleaning surfaces and microfluidic devices.

4. Experimental Section

A 2.5 wt% suspension of monodispersed PS spheres with

diameters of 350 nm was purchased from the Alfa Aesar Co. PS

monolayer colloidal crystals were fabricated on cleaned Si

substrates by spin coating, as previously described.[17] Briefly, a

droplet of 350 nm PS sphere suspension was placed on a cleaned

substrate with an area of 1.5�1.5 cm2 fixed on the spin coater.

The rotating speed was kept at 1500 rpm for 5 min and a

monolayer colloidal crystal with an area of �2 cm2 was formed

on the substrate by a self-assembly process.

The colloidal monolayer on the supporting substrate was

placed in a chamber for PLD in an off-axis configuration. A

laser beam with a wavelength of 355 nm from a Q-switched

Nd:YAG laser (Continuum, Precision 8000), operated at 10 Hz with

100 mJ pulse�1 and a pulse width of 7 ns, was applied and

focused on the target surface with a diameter of 2 mm. The rutile

TiO2 was used as a target. The substrate and target were rotated at

40 and 30 rpm, respectively. The PLD was performed for 30 min at

a base pressure of 2.66�10�4 Pa and a background O2 pressure

of 6.7 Pa. The sample was removed from the PLD chamber and put

into an oven for heating at 650 8C for 2 h in air. The hncp,

hierarchically ordered TiO2 particle arrays were thus prepared on

the substrate, as illustrated in Scheme 1.

The morphologies of the as-prepared samples were observed

using an FE-SEM (Hitachi S-4800) and a TEM (JEOL JEM-2000FX). The

composition and chemical states of samples were examined using

X-ray photoelectron spectroscopy (XPS, PHI 5600ci). The water CA

was measured with a VCA Optima XE from AST Products Inc.

Acknowledgements

This work was supported by the Japan Society for the

Promotion of Science (JSPS) fellowship. The authors are

grateful to Dr. Haoshen Zhou, Dr. Eiji Hosono, and Mr. Wataru

Imano from National Institute of Advanced Industrial Science

and Technology for measuring the water contact angle and for

helpful discussions.

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H & Co. KGaA, Weinheim

Received: March 25, 2008Revised: May 14, 2008

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