Ordered macroporous titania photonic balls by micrometer-scale sphericalassembly templating

Hongyu Li,ab Haiqiao Wang,a Aihua Chen,a Bo Menga and Xiaoyu Li*a

Received 2nd February 2005, Accepted 11th April 2005

First published as an Advance Article on the web 26th April 2005

DOI: 10.1039/b502976g

Highly ordered macroporous photonic balls (i.e. inverse opaline structure) composed of titania

frameworks were fabricated by using a titania precursor templated around polystyrene spheres

which had been assembled into polymer photonic balls (i.e. opaline structure). Narrow disperse

polymer photonic balls consisting of monodisperse surface-modified polystyrene (PS) latex

particles were prepared by utilizing a suspension system. The diameters of the opaline balls can be

controlled in a range of a few or a few tens of micrometers. The macroporous titania structure

made by this method was well-defined because the PS spheres making up the polymer photonic

balls were close-packed and ordered in three-dimensions. Furthermore, crystalline types of titania

(anatase or rutile) were readily adjusted through tuning the calcination temperatures, so the

macroporous titania inverse opaline balls composed of anatase or rutile can be used for various

applications.

Instruction

Colloidal crystals (e.g., opal materials) are extensively used as

templates to synthesize ordered macroporous (e.g., inverse

opal) materials with varied composition. They have broad

potential applications, such as in catalysis, separation,

chemical sensors, and photonic band-gap materials. The

morphology and substrate of the macroporous materials

can be tuned by using different shape stacked opal templates

and precursors. A major barrier to technological application

of these materials is the lack of simple, easily controlled

methods for mounting or shaping the templates into usable

solid objects. The simplest examples of such templates are

face-centered cubic (fcc) colloidal crystals, formed sponta-

neously in all-identical spherical colloids, such as polymer

latexes or silica suspensions. Recently, more complex struc-

tures have been made by a variety of clever techniques, such as

altering the shape of the colloidal particles,1 directing their

assembly with patterned surfaces,2 or tuning the interaction

between particles.3 Furthermore, photonic crystals in the form

of colloidal clusters (uniform spherical colloidal aggregates),

that is so-called ‘‘opaline photonic balls’’, have received more

and more attention because of their unique photonic pro-

perties resulting from the arrangement of colloids with

spherical shape, for example, as light scatterers, light diffusers,

and pigments for electronic paper and electronic displays.4

Several research groups have reported some methods to

fabricate photonic balls. Velev and coworkers synthesized

spherical assemblies from polystyrene latex particles by grow-

ing colloidal crystals in aqueous droplets suspended on

fluorinated oil.5 Yang and coworkers did similar work to

assemble spherical colloidal templates through colloidal

crystallization of suspended polystyrene latex sphere particles

in aqueous droplets straddling an air–oil interface.6 These two

kinds of photonic balls are from a few hundreds of micro-

meters to a few millimeters in size, and the size distribution of

these balls can not be controlled easily. However, one of the

important issues in the design of photonic balls is the control

of their uniformity in size and shape, so as to meet the

requirements of practical use of these materials. Recently,

Yang and coworkers synthesized uniform photonic balls and

their inverse structures by injecting an aqueous suspension of

polymer latex spheres into a surfactant-laden oil phase at an

oil/water junction of capillary tubes or soft-microfluidic

devices7,8 and by electrospraying an aqueous colloidal suspen-

sion.9 The corresponding inverse opaline balls being composed

of close-packed hollow particles consisting of a low-refractive-

index air core and a high-refractive-index metal oxide shell

(such as titania) are suitable building blocks for photonic

crystals, so-called ‘‘inverse opaline photonic balls’’.

Here we describe a new method to generate narrow disperse

opaline photonic balls which are in the range of a few or a few

tens of micrometers in size, and then, to template the closely

packed colloidal spherical assemblies into macroporous titania

(titanium dioxide) photonic balls (inverse opaline photonic

balls) possessing ordered macropores having pore diameters in

the range of a few hundreds of nanometers comparable to

optical wavelengths. Our synthetic route for making the

macroporous titania photonic balls is a three-step template-

assisted fabrication process, illustrated in Scheme 1.

First, the polymer opaline photonic balls were fabricated by

the colloidal crystallization of aqueous emulsion droplets in

a suspension system similar to a suspension polymerization

system. A hydrophobic silicone liquid was selected to be a

dispersing medium (i.e. the continuous phase) of the suspen-

sion system. The building blocks of the crystal balls were

monodisperse polymer latex spheres. The size of the balls

could be controlled by tuning the concentration of the aqueous

latex, position of the stirrer in the system, rotation speed of

stirring of the suspension system and the volume ratio of the*lixy@mail.buct.edu.cn

PAPER www.rsc.org/materials | Journal of Materials Chemistry

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silicone liquid to the aqueous latex. Second, the polymer

photonic balls were infused with the titania precursor solution

which was converted into titania in the void spaces between the

polymer spheres. Finally, the polymer spheres were removed

by calcination, which left macropores at their sites. The pore

diameters were in the range of a few tens of nanometers to a

few hundreds of nanometers and could be adjusted by using

different sizes of polymer latex spheres. These macroporous

titania photonic balls possessed a narrow size distribution

and an ordered internal lattice of pores. Furthermore, titania

macroporous photonic balls consisting exclusively of crystal-

line rutile or anatase could be obtained by tuning the calcina-

tion temperature. The crystalline rutile titania has a sufficiently

high refractive index to lead to a complete photonic band gap

of the corresponding colloid crystals10 and the crystalline

anatase is of practical significance for potential applications in

photocatalysis.11

Experimental

Synthesis of monodisperse latex particles

To ensure the structure stabilization of our photonic balls

during the preparation procedure, the latex particles used

in our study were hydrophilic and cross-linked polystyrene.

Monodisperse cross-linked polystyrene (PS) latex particles

with carboxyl groups on their surface were synthesized by

emulsifier-free emulsion copolymerization using potassium

persulfate and Na2HSO3 as a redox initiating system,

tri(ethylene glycol) diacrylate (TEGDA) as cross-linker, and

methacrylic acid as functional monomer to provide carboxyl

groups on the surface of particles. During polymerization, a

definite amount of NaHCO3 was added to partially ionize

the carboxyl groups. The resulting emulsion was purified by

dialyzing in deionized water using a dialysis tube. Fig. 1 is the

TEM image of the monodisperse cross-linked polystyrene (PS)

latex particles with carboxyl groups on their surface.

Preparation of polymer spherical clusters (i.e. opaline photonic

balls)

Photonic balls consisting of monodisperse cross-linked poly-

styrene (PS) latex particles with carboxyl groups on their

surface were prepared by using a suspension dispersed system.

Scheme 2 is the schematic of this system.

A 3000 ml three-neck flask with 2300 ml hydrophobic

silicone oil (viscosity 50 cSt, as a continuous phase, purchased

from Beijing Chemical Factory) was fixed in a controllable

temperature water cell. The silicone oil was rotated at constant

angular velocity (y480 rpm) using a stirrer. The aqueous

emulsion (115 ml, concentration of 1.5–5.0%) synthesized as

above was uniformly injected into the stabilized stream silicone

oil using a constant air or N2 flow pressure through a tapered

container, which has a capillary tube with an inner diameter of

10–20 mm at the top. Narrow size distribution emulsion

droplets were formed in the suspension system. Then colloidal

crystallization of the emulsion droplets gradually occurred

when water in the droplets was evaporated under an constant

temperature (here, we selected 55 uC). About six hours later,

colloidal spherical assemblies were formed and the suspension

system was cured at 80 uC for another six hours to enhance the

structure. The polymer photonic balls (opaline structure) were

obtained after the colloidal spherical assemblies in the

suspension system were filtered and washed with n-hexane,

isopropanal, and ethanol, respectively, and dried in the air.

Subsequently, they were treated with stainless steel sieves with

different pore sizes (purchased from Shang Yu Instrument

Scheme 1 Schematic diagram of the preparation method for photonic

balls composed of titania building blocks.

Fig. 1 TEM image of the monodisperse cross-linked polystyrene (PS)

latex particles with carboxyl groups on their surface.

Scheme 2 Schematic diagram of the synthetic method for opaline

balls. The emulsion droplets are suspended in the silicone oil and

colloidal crystals form during the drying process, as described in

the text.

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Company, ZeJiang province, China) to remove some asym-

metric balls. Fig. 2 shows the optical micrographs and SEM

images of the polymer photonic balls prepared using the above

process, and the polystyrene latex spheres that formed the balls

were y200 and y400 nm in diameter, respectively.

Preparation of ordered macroporous titania spherical aggregates

(i.e. the inverse opaline photonic balls)

The opaline balls obtained as mentioned above were immersed

in a titania precursor for a week to allow the precursor

permeation sufficiently. The precursor was composed of

tetrabutyl titanate (TBT) and ethanol (the TBT : ethanol ratio

was 3 : 4). The precursor-soaked opaline balls were washed

carefully with ethanol to rinse off the precursor on the surface

of the balls, and then exposed to the air for several days to

undertake a hydrolyzation process with moisture from the

atmosphere to form polymer/titania composite balls. Finally,

the composite balls were calcined at 500 uC or 700 uC for

8–10 hours (the heating rate was fixed at 1 uC min21), resulting

in polymer removal and anatase or rutile titania crystallite

formation, respectively.

Characteristics

Transmission electron microscopy (TEM) images of the

monodisperse cross-linked polystyrene (PS) latex particles

with carboxyl groups on their surface were obtained by TEM

(H-800, Electron Microscope, Hitachi). The morphology of

the photonic balls and their inverse ones was observed by

scanning electron microscopy (SEM, Stereoscan 250 MK3,

Cambridge Instruments and JSM-6360LV, Jeol Ltd, Japan)

and optical microscopy (XJZ-1A, ChongQing, China).

Polydispersity of the photonic balls was measured using a

Zetasizer (3000HS, Malver Instruments). Thermogravimetric

analysis of the inorganic–organic composite balls was deter-

mined by using a thermoanalyzer (O2 atmosphere, heating rate

10 K min21, Perkin Elmer, TGS-2, America). Crystallite types

of titania were determined by X-ray diffraction (XRD) using a

X-ray diffractometer (D/Max 2500 VB2t/PC, Rigaku, Japan).

Results and discussion

Narrow size distribution opaline photonic balls

In this study, narrow size distribution opaline photonic balls

were prepared by a suspension system in which a silicone oil

(viscosity, 50 cSt)12 was the continuous phase, and polymer

emulsion droplets were the disperse phase. As we know, the

disperse phase droplets that were suspended in the continuous

phase should be monodisperse or narrow-dispersed in size as

the oil : water volume ratio was sufficiently high and the

stirring rate was constant, this is based on the fact that the

suspension droplet destruction caused by stirring should be

omitted in this case. According to this idea, a set of devices

was designed to obtain narrow-dispersed aqueous emulsion

droplets in a suspension system using synthetic conditions

of 20 : 1 volume ratio (silicone liquid : latex suspension) and

a constant stirring rate. Then, the polymer spheres in the

aqueous emulsion droplets which were suspended in the

silicone liquid were concentrated by evaporating the water

under evaporation temperature of 55 uC. This evaporation

temperature of 55 uC during crystallization was selected based

on a report of the optimum crystallization temperature for

Fig. 2 (a), (c) Optical micrographs of opaline photonic balls formed

by polymer latex spheres with two different diameters, y200 and

y400 nm respectively. (b), (d) SEM images of these two balls.

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opal fabrication.13 The droplet size decreased monotonically

with time during evaporation of water and the polymer sphere

concentration increased at the same time. As the concentration

exceeded a certain transition value, the polymer latex spheres

began to order into a face-centered cubic (fcc) structure to

form close-packed opaline photonic balls. It should be noted

here that the hydrophobic silicone liquid can cover the

hydrophilic polymer emulsion droplets to form a stabilized

suspension system because it is lighter than water and polymer

microspheres (specific gravity values: 0.97 g ml21, 1.00 g ml21

and .1.00 g ml21, respectively). The vapor pressure of the

silicone liquid is important, because low-viscosity silicon liquid

will evaporate before completion of the crystallization process

(viscosity 50 cSt). After crystallization, the opaline balls

were obtained through filtration, soaking and rinsing with

n-hexane, isopropanol, and ethanol, respectively. Finally, the

balls were dried and cured at 80 uC for several hours to

increase the degree of close-packing of the crystal balls and

evaporate substantially any residual silicone oil.

The size of the photonic balls obtained by using the above

method can be tuned through changing the initial polymer

latex concentration. Three different latex concentrations

(1.62%, 3.24% and 4.86%) were used, and three distinct balls

with average diameters (y40, y80, and y100 mm, respec-

tively) were obtained (based on the optical microscopy images)

(generation conditions: volume ratio 20 : 1, a constant stirring

rate y480 rpm).

The balls with average diameters y40 mm were treated with

stainless steel sieves with pore sizes of 50 and 30 mm. From the

mass ratios of photonic balls of various diameters, it can be

seen that the photonic balls are narrowly dispersed (the

percentage of balls with diameters larger than 50 mm and

smaller than 30 mm is totally less than 10%, however the balls

in the size range 30–50 mm account for over 90%). The poly-

dispersity of the balls with diameters of 30–50 mm is around 8%

based on the Zetasizer.

We found that in these systems the diameters of the

photonic balls will increase very little when the air flow

pressure rises and other parameters, such as latex concentra-

tions, volume ratios of oil/emulsion and stirring rate, are fixed.

Also, the diameters decrease with increasing stirring rate

while the other parameters are fixed. Besides, when the volume

ratio of silicone oil to emulsion decreases to 10 : 1, the

monodispersity of the photonic balls decreases greatly. For

instance, when we prepared photonic balls with diameters of

around 40 mm, the percentage of 30–50 mm photonic balls is

only 65%, which is much less than 90% obtained from the

preparation with the volume ratio 20 : 1. However, if the

volume ratio increased to 30 : 1, the percentage of photonic

balls in the 30y50 mm range has not changed greatly,

compared with that obtained at 20 : 1. Meanwhile, the yield

of photonic balls between 30 and 50 mm decreased dramati-

cally. Therefore, it is suitable to prepare photonic balls in the

range 30–50 mm at a volume ratio of 20 : 1.

From all the results mentioned above, we can conclude that

the optimal conditions for preparing photonic balls in the

range 10–100 mm are: volume ratio of silicon oil to emulsion

20 : 1, stirring rate 480 rpm, air or nitrogen flow pressure to

push the emulsion into the silicon system by using a capillary

with a mouth diameter of 10–20 mm is kept stable. Fig. 2–4

show the SEM and optical microscope images of photonic

balls obtained under these conditions. Fig. 2(b, d), Fig. 3 and

Fig. 4(a, b) indicate that the surface of the photonic ball has

a highly ordered hexagonal structure while the interior has a

close-packed fcc structure.

Macroporous titania inverse opaline photonic balls

The opaline balls prepared above were soaked in a solution of

titania precursor (TBT : ethanol 5 3 : 4), which went through

the void spaces between the polymer latex spheres by capillary

force. The precursor concentration is an important factor in

this process. When TBT was in excess, the precursor could not

fully infiltrate into the interstices, while when ethanol was in

excess, a solid oxide ceramic gel block should not be produced.

The precursor concentration of TBT : ethanol 5 3 : 4 in this

study was selected based on a series of systematically con-

ditional experimental results that we will report elsewhere.

After infiltration, the balls which were fully infiltrated by the

precursor were pulled out and exposed to the air. Finally, the

titania precursor was hydrolyzed into the oxide ceramic gel as

a result of its exposure to the moisture of the ambient air, and

organic–inorganic composite balls were obtained. Fig. 3 is the

SEM image of the fractional section morphology of a broken

composite ball. It clearly shows that the void spaces of the

opaline balls were filled up by inorganic material (titania).

It is especially noteworthy that before the hydrolysis

reaction proceeded, the residual precursor remaining on the

surface of the balls should be removed. Otherwise, a thick skin

of titania would be formed on the surface of the macroporous

balls. To avoid this titania skin formation, we rinsed the balls

with ethanol or isopropanol carefully in a moisture-free

environment. Finally, the organic–inorganic composite balls

were calcined at 500 uC for crystalline anatase titania or at

700 uC for crystalline rutile titania macroporous micro-

structure inverse opaline photonic balls, respectively. Fig. 4

is the SEM images of the opaline photonic balls (a, surface

and b, inner structures) and its inverse opaline balls (c and d).

Fig. 4(a) and (b) show that the opaline balls were close-packed

and highly ordered in three-dimensions. From the surface of

Fig. 3 SEM image of the fracture section of broken organic–

inorganic composite balls.

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the macroporous balls (Fig. 4(c) and (d)), we can see that the

void spaces whose sizes were about 130–150 nm in diameter

were interconnected in three dimensions through windows

which were 25–30% smaller than the diameters of the original

latex beads (y200 nm), indicating shrinkage during the

calcination process. The wall thickness of the windows was

about 40–60 nm.

Thermogravimetric analysis of the organic–inorganic com-

posite balls shows that the organic polymer spheres could be

completely decomposed at high temperatures above 450 uC.

According to the TG results, the calcinations of the composite

balls were performed at 500 or 700 uC for preparation of

anatase or rutile type titania, respectively. When the composite

balls were calcined at 500 uC, macroporous balls consisting

exclusively of crystalline anatase titania was obtained, while

calcinations at 700 uC yielded macroporous balls consisting

exclusively of crystalline rutile titania (Fig. 5). It should be

noted that the phase transformation from anatase to rutile

occurs in a narrow temperature range between 550–650 uC.

The anatase–rutile ratio changes with increasing calcination

temperature. As mentioned above, the crystalline type (anatase

or rutile) of the titania inverse opaline balls can be fully

controllable for various applications by adjusting the calcina-

tion temperature.

Conclusion

Opaline photonic balls and inverse macroporous balls were

fabricated in this study. The opaline photonic balls were

assembled by utilizing monodisperse cross-linked polystyrene

(PS) latex particles with carboxyl groups on their surface and

using a suspension dispersed system. The size of the balls can

be adjusted by tuning the following factors: the concentration

of the aqueous latex, the position of the stirrer in the system,

the rotation speed of stirring and the volume ratio of silicone

oil to aqueous latex. The modified monodisperse polystyrene

spheres used for the building blocks of opaline balls were

synthesized by conventional emulsifier-free emulsion copoly-

merization. Inverse opaline macroporous balls consisting of

titania frameworks were obtained from titania precursor

templated around polystyrene spheres making up close-packed

Fig. 4 SEM images of the opaline balls and inverse macroporous

balls. (a) Surface image of the opaline balls. (b) Fractional section

image of broken opaline balls. (c) Image of an inverse opaline ball. (d)

Surface image of the inverse opaline balls.

Fig. 5 X-Ray diffraction patterns of the macroporous titania

photonic balls at different calcination temperatures.

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opaline balls. The cavity diameter of the macroporous balls

could be tuned by using different sized PS spheres. Further-

more, the crystalline types of titania consisting of macroporous

balls were also controllable by adjusting the calcination

temperature. So, the macroporous titania photonic balls

obtained by this method can be used for various fields, such

as photonic band-gap materials and photocatalysts, through

controlling the titania crystalline types, anatase or rutile.

Acknowledgements

This work was financially supported by the Beijing

Foundation of Natural Science, P. R. China (Nos.Z012013)

Hongyu Li,ab Haiqiao Wang,a Aihua Chen,a Bo Menga and Xiaoyu Li*a

aThe Key Laboratory of Nanomaterials of Ministry of Education,College of Materials Science Engineering, Beijing University ofChemical Technology, Beijing, 100029, People’s Republic of China.E-mail: lixy@mail.buct.edu.cnbDepartment of Chemistry, Anyang Teachers College, Anyang, Henan,455000, People’s Republic of China

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