www.rsc.org/materials Volume18|Number43|21November2008|Pages5169–5308

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PAPERBoLiuandRichardThorntonBakerFactorsaffectingthepreparationoforderedmesoporousZrO2usingthereplicamethod

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Factors affecting the preparation of ordered mesoporous ZrO2 using thereplica method†

Bo Liu and Richard Thornton Baker*

Received 6th May 2008, Accepted 12th August 2008

First published as an Advance Article on the web 18th September 2008

DOI: 10.1039/b807620k

SBA-15 was employed as the hard template in the preparation of ordered mesoporous ZrO2 by the

replica method. The resultant ZrO2-SR product was characterized by XRD, TEM, EDX, SEM and

nitrogen physisorption. The starting material, intermediate and product were studied by FT-IR. The

formation of Zr–O–Si crosslinks made it impossible to obtain a pure ZrO2 product, although the

material did have a similar ordered mesoporous structure to the SBA-15 template. The results showed

that the replica method itself could be successfully used to prepare ordered mesoporous ZrO2, but that

the use of mesoporous silica as the hard template meant that the silicon could not be completely

removed from the product. The product was seen to contain both the normally stable monoclinic form

of zirconia as well as the metastable tetragonal phase. The presence of the latter was considered to

be related to the constriction of particle size by the SBA-15 template and by the Si-containing surface

layer. These could both hinder sintering of the zirconia particles and prevent crystallite growth to sizes

above the critical size, where the tetragonal phase would not be expected. The mesoporous zirconia

product had a specific surface area of 220 m2 g�1 and a pore volume of 0.57 cm3 g�1, making it of great

interest for applications as a support in catalysis and electrocatalysis.

1. Introduction

Since Mobil scientists first reported the MS41 family in 1992,1,2

mesoporous materials have attracted a great deal of attention,

and many mesoporous materials with different structures, such

as 2-D hexagonal p6mm (MCM-41, SBA-15, SBA-3, FDU-15),

cubic Ia3d (MCM-48, KIT-6, FDU-5, FDU-14) and cubic Im3m

(SBA-16, FDU-16), have been synthesized.1–10 Currently, there

are two main routes for the synthesis of mesoporous materials.

The first route is the cooperative self-assembly of inorganic

precursors and amphiphilic surfactants through either a ‘hydro-

thermal’1–4,10 or an evaporation-induced self-assembly (EISA)

pathway;11–14 most mesoporous silicas have been synthesized

using this route. Recently, the group of Zhao extended this

method to synthesise ordered mesoporous carbons.9,10,14 The

EISA method can also be successful in the preparation of some

mesoporous transition metal oxides, such as ZrO2, TiO2, TiO2–

SiO2, Al2O3, CeO2–ZrO2 and Y2O3–ZrO2.15–18 The second route

takes place via a replica step in which mesoporous starting

materials are used as hard templates.19 In this route, the pores of

the mesoporous starting material are first filled with inorganic

precursors. The precursors inside the pores are converted to the

desired final composition, for example, by a heating step, and the

parent template material is removed, usually by dissolution, to

leave a mesoporous structure with a negative replica structure

(Fig. 1) of the desired composition. This route was pioneered by

EaStChem, School of Chemistry, University of St Andrews, North Haugh,St Andrews, Fife, UK KY16 9ST. E-mail: rtb5@st-andrews.ac.uk; Fax:+44 1334 463808; Tel: +44 1334 463899

† Electronic supplementary information (ESI) available: AdditionalTEM results and elemental maps taken by SEM. See DOI:10.1039/b807620k

5200 | J. Mater. Chem., 2008, 18, 5200–5207

Ryoo and co-workers, who demonstrated the synthesis of

a series of mesoporous carbons (CMK series) using different

mesoporous silicas as hard templates.19–22 Later, it was widely

used to prepare mesoporous nonsiliceous materials, such as

CeO2, Cr2O3, MnO2, NiO, Co3O4 and Fe2O3.23–28 Besides

mesoporous silicas, mesoporous carbon (CMK-3) has also been

applied as a hard template to prepare mesoporous SiO2, CuO

and MgO.29–34

Because of their excellent chemical resistance, refractory

character, oxygen ion conductivity and polymorphous nature,

ZrO2-based oxides have found application in a wide range of

technologies, including automotive three-way exhaust catal-

ysis,35 solid oxide fuel cells (SOFCs)36–39 and gas sensors.40 If

ZrO2-based oxides with a mesoporous structure could be

synthesised, some new and promising properties relevant to these

applications, and to others, would likely result. These include

high specific surface area, high porosity and the existence of

mesopore networks allowing good gas transport into the

materials. The preparation of mesoporous ZrO2 using the

self-assembly method has been reported,15,16,41–44 but the meso-

porous structure of the resultant materials was either disordered

or unstable on removal of the surfactants. The replica method

has advantages for the preparation of mesoporous materials with

stable, predictable and controllable structure. Although much

Fig. 1 Schematic representation of the replica process.

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work has been done to prepare other metal oxides using the

replica method, no work has so far been reported on ZrO2. Here,

for the first time, we report the application of the replica method

with the aim of preparing mesoporous ZrO2 using SBA-15 as the

hard template.

Fig. 2 XRD patterns of ZrO2/SBA-15 calcined at (a) 873 K, (b) 1373K

and (c) ZrO2-SR.

2. Experimental section

Mesoporous SBA-15 silica was prepared according to a literature

procedure.3 Mesoporous ZrO2 (denoted ZrO2-SR) was

synthesised by a conventional impregnation method. 1.0 g of

SBA-15 was added to 6.0 g of a clear aqueous solution containing

1.8 g of ZrOCl2$8H2O, and the mixture was kneaded three times.

After drying at 373 K, the ZrOCl2/SBA-15 obtained was calcined

from room temperature to 873 K with a ramp rate of 1 K min�1

and maintained at 873 K for 5 h in order to decompose the

inorganic salts completely, thus forming ZrO2/SBA-15. To

remove the silica template, the ZrO2/SBA-15 was reacted with

2 M aqueous NaOH solution for 12 h and washed with deionised

water at least three times. The resultant solid was centrifuged and

washed with deionised water, washed with ethanol and dried at

373 K to obtain the ZrO2-SR product.

The X-ray diffraction (XRD) patterns were collected using

a Philips XRD system using CuKa radiation with a step size of

0.01� and a scanning rate of 1.0� min�1 from a 2q of 10� to 90�.

The small-angle (SA)XRD patterns were recorded in trans-

mission mode with sample spinning on a Hecus X-ray generation

1 instrument using CuKa radiation. Nitrogen physisorption and

desorption measurements were conducted using a Hiden IGA

porosimeter at 77 K. The samples were degassed prior to the

measurements. Scanning electron microscopy (SEM) was carried

out using a JEOL 5600 SEM to study the morphology of the

samples. Transmission Electron Microscopy (TEM) was carried

out on a JEOL JEM 2011 fitted with a LaB6 filament, operating

at 200 kV, and TEM images were recorded using a Gatan CCD

camera. Energy-dispersive X-ray spectroscopy (EDX) analysis

and elementary mapping were carried out using an Oxford

Instruments ISIS EDX 300 system integrated with the TEM.

Fourier transform infrared (FT-IR) spectroscopy was performed

on a Perkin Elmer Spectrum GX spectrometer using KBr to

make the sample wafers by the addition of 8 scans at a 1.0 cm�1

resolution. The samples and the KBr were dried at 433 K in

advance and the spectra were recorded under air and at room

temperature.

Fig. 3 SAXRD patterns of SBA-15, ZrO2/SBA-15 and ZrO2-SR (on

same intensity scale and with latter two traces shifted upwards from

baseline for clarity).

3. Results and discussion

Fig. 2 shows the powder XRD patterns of ZrO2/SBA-15 calcined

at 873 K and 1373 K and the pattern of ZrO2-SR. Except for the

broad diffraction peaks at 2q � 22.5� attributed to amorphous

SiO2, the XRD patterns of both ZrO2/SBA-15 samples exhibit

diffraction peaks at 2q � 30.1�, 34.9�, 50.2�, 59.7�, 62.7�, 73.0�,

81.7� and 84.4� corresponding, respectively, to the (111), (200),

(220), (311), (222), (400), (331) and (420) diffraction planes of

cubic ZrO2. (It should be noted here that although these peaks

are in accordance with cubic ZrO2, it is more reasonable to

attribute them to the tetragonal phase rather than cubic phase, as

discussed below). A very small amount of monoclinic ZrO2 seems

to have been present because of the appearance of an indistinct

This journal is ª The Royal Society of Chemistry 2008

peak at 2q� 28.1�, corresponding to the (�111) plane, which is the

strongest diffraction peak for monoclinic ZrO2. As the calcina-

tion temperature was increased, the diffraction peaks became

more intense and sharper. No zirconium silicate was detected,

even after the sample had been calcined at 1373 K. By compar-

ison, in the XRD pattern of ZrO2-SR, the broad diffraction peak

of SiO2 was not seen, but more intense peaks corresponding to

monoclinic ZrO2 were observed, for example at 2q � 28.1� and

31.1�, which correspond to the (�111) and (111) planes, respec-

tively. The SAXRD patterns of SBA-15, ZrO2/SBA-15 and

ZrO2-SR are shown, on the same intensity scale, in Fig. 3. The

three well-resolved peaks in the pattern of SBA-15 can be

indexed to the (100), (110) and (200) reflections of the p6mm

hexagonal structure.1,3 In the SAXRD pattern of ZrO2-SR,

although the intensities are low, and the main peak is partly

J. Mater. Chem., 2008, 18, 5200–5207 | 5201

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masked by the slope of the non-diffracted peak, the (100) and

(200) peaks can be identified. These are shifted to slightly higher

angles, indicating some contraction of the structure on removal

of the silica template. The decreasing peak intensities in SAXRD

as ZrO2 content increased may be caused in part by peak

broadening related to a reduction in long-range order because of

decreasing mesoporous particle size. A further contributing

factor may be that, according to the pore structure of the SBA-

15, ZrO2 ‘nanorods’ are connected together only by thin bridges.

These may allow some variation in inter-rod distance and angle,

once the silica template is removed, which would broaden the

SAXRD peaks and reduce their height. Although the diffraction

peaks for the ZrO2-SR are small, the corresponding TEM images

do reveal the ordered mesoporous structure to be widespread, as

discussed below.

Fig. 4(a) presents a TEM image of the starting template, SBA-

15, viewed in a direction perpendicular to the pore direction. The

distance between the adjacent layers is about 9.0 nm and the unit

cell length is about 10.4 nm. In order to avoid forming a high-

curvature end-cap between neighbouring layers, the layers are

terminated in a rounded structure. After incorporation of the

ZrO2, no obvious pores can be seen in the ZrO2/SBA-15 sample,

as is seen in Fig. 4(b), indicating that all of the pores in the

SBA-15 had been filled with ZrO2. Fig. 4(c) and (d) present

representative TEM images of the sample after removal of the

template, ZrO2-SR. Here, pore channels and a mesoporous

structure with long-range order are observed once more. It is

thought that the reappearance of the mesopores in the ZrO2-SR

was caused by the removal of the walls of the SBA-15 by reaction

with the 2M NaOH solution. This mesoporous structure was

evident for nearly all of the material examined in the TEM study

(more information is available as ESI†). In the high-resolution

TEM image of the as-prepared ZrO2-SR (Fig. 4(e)), it is clear

that the ZrO2 material is crystalline and, interestingly, there is

Fig. 4 TEM images of (a) SBA-15; (b) ZrO2/SBA-15; (c),(d) ZrO2-SR (e) Z

SBA-15, and (ii) ZrO2-SR.

5202 | J. Mater. Chem., 2008, 18, 5200–5207

evidence for the presence of single crystal nanorods, in which the

crystal planes are parallel over relatively large distances. The

digital diffraction pattern (DDP) taken from the area indicated

in Fig. 4(e) shows one main diffraction pattern (with non-

arrowed spots arising from the background) associated with the

rod-like particle in the image. This implies that the particle is

a single crystal. A particle of such morphology would be

energetically unfavourable and would be very unlikely to form

outside the constraining structure of the SBA-15 template.

Furthermore, single-crystalline ZrO2 is unlikely to be formed via

the self-assembly method. Comparing the EDX results of ZrO2/

SBA-15 and ZrO2-SR (Fig. 4(f)), there was much more silicon in

ZrO2/SBA-15 than in ZrO2-SR, meaning that most of silica is

removed during the template removal process. This is in

agreement with the observed reappearance of the mesopores in

ZrO2-SR. However, the presence of some residual silicon in the

ZrO2-SR sample (Si/Zr ¼ 0.11 w/w, compared to Si/Zr ¼ 0.49 w/

w in the SBA-15/ZrO2) is unusual in that no analogous effect has

been reported in the preparation of other mesoporous metal

oxides using the same method. The presence of Si in the ZrO2-SR

was further investigated by the element mapping technique, and

the results are shown in Fig. 5. Comparing the element maps with

the TEM image, the oxygen, silicon and zirconium are all seen to

be present throughout the sample. It should be noted that the

mesopores are not observed in the element maps because the size

of the sample volume probed at any one time by the electron

beam is larger than the mesopore dimension. In spite of this, the

elementary maps do indicate that the Si is highly dispersed in

the ZrO2-SR.

Fig. 6 shows the SEM images of SBA-15, ZrO2/SBA-15 and

ZrO2-SR. As reported in a pioneering study,3 SBA-15 is

composed of rope-like domains with a length of about 1 mm, and

these domains aggregate and give rise to what has been described

as a wheat-like morphology. The SEM image in Fig. 6(a) clearly

rO2-SR at high resolution with DDP inset; (f) EDX spectra for (i) ZrO2/

This journal is ª The Royal Society of Chemistry 2008

Fig. 5 (a) TEM image, and elemental maps of the ZrO2-SR sample: (b)

O Ka1, (c) Si Ka1 and (d) Zr Ka1. Note that correspondence between the

image and the maps is not exact because of the difference between EDX

probe and TEM geometries.

Fig. 6 SEM images of: (a) SBA-15; (b) ZrO2/SBA-15; (c) ZrO2/SBA-15

at higher magnification; and (d) ZrO2-SR.

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shows this wheat-like morphology in the as-prepared parent

material. After impregnation and calcination, ZrO2 is present in

the mesopores and in the voids between the wheat-like particles,

not only making the material appear more dense than SBA-15,

but also connecting the wheat-like particles to each other, as

shown in Fig. 6(b). It is worth noting that the rope-like domains

are still evident in ZrO2/SBA-15 (Fig. 6(c)). In Fig. 6(d), the

as-prepared ZrO2-SR is seen to be composed of small particles

resembling cotton and therefore presenting a totally different

morphology from that of the SBA-15-containing samples. This

indicates that the silica template skeleton had been destroyed in

the reaction with the 2 M NaOH(aq). It can be predicted that

there are many interparticle voids that can be detected by

nitrogen physisorption in the ZrO2-SR sample. Although these

particles were not organised in a wheat-like morphology, as the

particles of SBA-15 were, the particles are still composed of

rope-like domains, as is shown in the low-magnification TEM

image (Fig. 4(d)).

The N2 physisorption isotherm of the SBA-15 and ZrO2-SR

samples are given in Fig. 7. Fig. 7(a) presents an isotherm typical

of the mesoporous SBA-15 materials. The sharp increase in

adsorption at relative pressure between 0.7 and 0.8 can be

attributed to the capillary condensation in the mesopores. The

inset BJH pore size distribution (PSD) shows only one narrow

peak at a pore diameter of about 8 nm. Fig. 7(b) shows the N2

physisorption isotherm of ZrO2-SR. A gradual increase in slope

started at a relative pressure of ca. 0.4, and was followed by

a sharp increase from relative pressure of ca. 0.8 up to ca. 1.0 in

both the adsorption and the desorption branches. The more

gentle upward slope from ca. 0.4 also corresponds to capillary

condensation, typical of mesoporous materials with uniform

pore systems, while the further increase at higher relative

This journal is ª The Royal Society of Chemistry 2008

pressures indicates substantial interparticle porosity. The pres-

ence of both regular mesopores and interparticle voids is

confirmed by the PSD (inset) which shows a sharp peak at 2.9 nm

and a rather broad peak at about 27 nm. The existence of

substantial interparticle porosity is a common phenomenon in

mesoporous materials synthesized via the replica method.28,32,33

The specific BET surface area and the total pore volume of

the ZrO2-SR product were calculated from the physisorption

results to be 220 m2 g�1 and 0.57 cm3 g�1, respectively. Both of

these values are higher than those of ZrO2 with long-range

J. Mater. Chem., 2008, 18, 5200–5207 | 5203

Fig. 7 Nitrogen physisorption isotherm and the pore size distribution

(inset) calculated from the adsorption branch using the BJH method for:

(a) SBA-15 and (b) ZrO2-SR.

Fig. 8 FT-IR spectra of (a) SBA-15, (b) ZrO2/SBA-15, (c) the difference

between the spectra of SBA-15 and ZrO2/SBA-15, and (d) ZrO2-SR.

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structural order synthesized via the self-assembly method.15,16

This result is clearly of great interest for catalysis applications.

The FT-IR spectra of SBA-15, ZrO2/SBA-15 and ZrO2-SR are

given in Fig. 8. In the region from 1600 cm�1 to 400 cm�1 five

peaks were observed in the spectrum of SBA-15 (Fig. 8(a)). The

bands at 1220 cm�1 and 1080 cm�1 can be attributed to Si–O–Si

asymmetric stretching; 945 cm�1 to Si–OH stretching; 796 cm�1

to Si–O–Si symmetric stretching and 462 cm�1 to O–Si–O

bending.45,46 When SBA-15 is loaded with ZrO2, an additional

absorption peak at around 1000 cm�1 caused the features at 1080

cm�1 and 945 cm�1 to appear to merge into one broad peak

(Fig. 8(b)). In order to further investigate the effect of ZrO2

loading on the spectra, the difference between the spectra of

SBA-15 and ZrO2/SBA-15 was calculated, and the result is

shown in Fig. 8(c). Besides a relatively intense absorption peak at

999 cm�1, there are four weak peaks at 740 cm�1, 581 cm�1,

507 cm�1 and 428 cm�1. The peak at about 999 cm�1 has been

found in other ZrO2–SiO2 systems and is attributed to the

presence of Zr–O–Si crosslinking,47–54 while the other four weak

peaks correspond to monoclinic ZrO2.55–59 After ZrO2/SBA-15

was treated with the NaOH solution, all of the characteristic

peaks of SBA-15 were seen to disappear, leaving only the four

weak peaks corresponding to monoclinic ZrO2 and a broad

peak at 971 cm�1 attributed to the presence of the Zr–O–Si

crosslinks (Fig. 8(d)).

It is well known that pure ZrO2 has three crystalline phases.

The monoclinic phase is the thermodynamically stable phase up

5204 | J. Mater. Chem., 2008, 18, 5200–5207

to 1373 K. The tetragonal phase exists in the temperature range

of 1373–2643 K, and the cubic phase is found above 2643 K up to

the melting point of 2953 K. Transitions between these phases

cause unavoidable volume changes and resulting cracking which

restricts the use of ZrO2. Much work has been focused on

stabilizing tetragonal and cubic ZrO2 at low temperature by

substituting some Zr4+ cations with cations of a lower charge,

such as Ca2+, Mg2+, La3+ and Y3+.60,61 Doping with 8 mol% Y2O3

is found to be effective for obtaining stabilised cubic YSZ, which

is widely used as the electrolyte in SOFCs because of its thermal

stability and high oxygen ionic conductivity. Although tetrag-

onal ZrO2 is stable only above 1373 K, it has been found to exist

at much lower temperatures without doping in some cases.

Clearfield first reported a ‘cubic’ form of ZrO2 at room temper-

ature obtained by precipitation, but Garvie soon pointed out that

the ‘cubic’ phase was in fact the tetragonal form with the peak

doublets in XRD masked by peak broadening.62 The same

phenomenon was generally found in fine-grained ZrO2.63–66 In all

of the systems, the phase transformation observed was: amor-

phous / tetragonal / tetragonal + monoclinic / monoclinic.

Garvie proposed that the occurrence of the metastable tetragonal

ZrO2 at such a low temperature was attributed to the very small

crystallite size in these samples which would give rise to a large

specific area and high surface energy.62 Later it was established

that the critical particle size of tetragonal ZrO2 was around

30 nm. Besides the particle-size effect, the silica matrix is also

found to be effective for stabilising the tetragonal phase of ZrO2.

It has been suggested that there are two possible mechanisms

for stabilising tetragonal ZrO2 in the presence of silica. In

non-mesoporous SiO2–ZrO2 systems with high SiO2 content, the

stabilisation of tetragonal zirconia and relatively slow crystal

growth is due to the constraining effect of the silica matrix

surrounding the ZrO2 particles, making zirconia diffusion within

the matrix difficult.67,68 For systems with a low SiO2 content,

however, stabilization of tetragonal zirconia is mainly attributed

to the formation of Si–O–Zr crosslinks. The Si–O–Zr crosslinks

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can be viewed as chemical impurities which are able to stabilise

the tetragonal phase.69 For these reasons, the ‘cubic’ form of

ZrO2 in the XRD patterns (Fig. 1) should be attributed to the

tetragonal phase rather than the cubic phase.

FT-IR has proved to be a useful and powerful method to

detect the presence of the Si–O–Zr crosslinks. By investigating

the FT-IR spectra of a series of (non-mesoporous) ZrO2–SiO2

samples with SiO2 content lower than 20% prepared by the

glycothermal method,47 Praserthdam et al. found that: (1) the

Zr–O–Si crosslinks gave IR absorption peaks over the range

from 965 cm�1 to 1050 cm�1; (2) the peak attributed to the Zr–O–

Si crosslinks gradually shifted to higher wave number with

increasing SiO2 content; (3) the Zr–O–Si crosslinks decomposed

at high temperature, giving rise to a peak of progressively higher

wave number with increasing temperature. Zeng and co-workers

studied similar systems, but focused on the samples with higher

SiO2 content (> 50%).48 It was found that the absorption wave

number of the Si–O band (1095 cm�1) was shifted to lower wave

numbers with increasing ZrO2 content because of the formation

of Zr–O–Si crosslinks. For example, the absorption peak

appeared at 1065 cm�1 and 1040 cm�1 for samples with 20% and

50% ZrO2, respectively. Reports on the IR absorption peaks

of Zr–O–Si crosslink can be found in many other papers. Some

of these give the peak position at lower wave number (e.g. �970

cm�1)51–54 and other contributions give higher wave numbers

(�1010 cm�1).49,50 These variations are understandable bearing in

mind the range of compositions and preparative conditions.

However, despite these variations, it is clear from the literature

that the absorption peak of the Zr–O–Si crosslinks is always

located between the absorption peaks attributed to the Si–O–Si

asymmetric stretch and the Si–OH stretch.

In this connection, Arnal et al.70 report the preparation of

ZrO2 layers on monodispersed silica spheres and the removal

of the silica, as here, by treatment with NaOH(aq) to leave hollow

spheres of ZrO2. They performed 29Si solid state NMR on the

precursor, the ZrO2–SiO2 composite and the final ZrO2 spheres.

The NMR results indicated the retention of a small amount of

Si-containing material in the final product, as also found in

the current paper. This had a much lower level of condensation

(Q# 2) than the starting SiO2 or the composite material (Q$ 3).

This might be explained by the formation of a thin layer of

hydrated Si-containing material at the surface of the ZrO2.

Formation of such a layer would also explain the findings of

the current work.

In the case of ZrO2/SBA-15, the constraining effect appears to

have been the main reason for the stabilization of the metastable

ZrO2, because of the large silica content of this material. Firstly,

during the impregnation process, ZrOCl2 would be highly

dispersed on the SBA-15, either within the mesopores or on the

external surfaces. When the precursor was heated, ZrOCl2 would

have decomposed to form fine ZrO2 particles, which generally

would have existed as tetragonal ZrO2. The silica matrix would

have prevented the fine ZrO2 particles from aggregating,

explaining why the tetragonal ZrO2 phase was maintained in this

sample, even after calcination at 1373 K. Secondly, because the

pore size of SBA-15 is only about 8 nm, the cross-sectional

diameter of the resultant ZrO2 particles in the mesopores would

have been no more than 8 nm, although they could possibly have

reached a length of some micrometers. The mean particle size of

This journal is ª The Royal Society of Chemistry 2008

ZrO2/SBA-15 calcined at 873 K and 1373 K were found to be

5.5 nm and 6.3 nm, respectively, as estimated from the (111) peak

in the XRD data using the Scherrer equation,

t ¼ 0:9l

B cosq

where t is the particle diameter (nm), l is the wavelength of the

X-rays (nm), q is the diffraction angle and B is the XRD peak

width (rad). Such small particle sizes caused the difficulty in

distinguishing the doublets expected in the XRD pattern of

tetragonal ZrO2, because of the resulting peak broadening.

Since SiO2 can be easily reacted with NaOH solution to form

soluble salts, SiO2 is a good candidate as a hard-template

material because it can be completely removed from the

precursor by contacting with NaOH solution. Many mesoporous

materials, such as C, NiO, Cr2O3 and Co3O4, have been

successfully prepared using mesoporous SiO2 as the hard

template. In the preparation of ZrO2-SR reported here, some

specific findings can be emphasised. First, it was confirmed that

the walls of the SBA-15 were destroyed by reaction with NaOH

solution, giving rise to a different morphology in the ZrO2-SR

than in the starting SBA-15, as seen in the SEM images. This is

also evident in the FT-IR and XRD results. The FT-IR results

show that all of the absorption peaks corresponding to SBA-15

disappear on formation of the ZrO2-SR, and the XRD patterns

indicate the disappearance of the broad peak at about 22.5� –

corresponding to amorphous SiO2 – in ZrO2-SR. Second, most

of the siliceous material was removed, but about 10 wt% residual

silicon remained in the ZrO2-SR and was highly dispersed in the

sample, as shown in the element maps. The authors have

attempted to remove the silicon by increasing the reaction

temperature up to 353 K, extending the reaction time and using

NaOH solution with different concentrations, but the results

were the same. Third, the amount of the monoclinic phase of

ZrO2 increased after the removal of the walls of SBA-15, as

shown in the XRD patterns. Fourth, the FT-IR results not only

showed that the removal of the silica walls of SBA-15 had been

successful and indicated the presence of the monoclinic ZrO2, but

also revealed that Zr–O–Si crosslinks were formed in ZrO2-SR.

Therefore, in the case of ZrO2-SR, the constraining effect, which

would have been caused by silica walls, seems not to have been

present. The fine ZrO2 particles, originally separated by the pore

walls, would have a tendency to aggregate. On the one hand,

some of the fine ZrO2 particles would aggregate to form large

particles, causing the appearance of the thermodynamically

stable monoclinic ZrO2 phase. On the other hand, not all of the

fine ZrO2 particles would have been able to aggregate because of

the presence of the Zr–O–Si crosslinks at their surfaces. This

would lead to the stabilization of some of the metastable

(tetragonal) ZrO2 in the ZrO2-SR. Formation of the Zr–O–Si

crosslinks may have been favoured by the presence of the NaOH

solution, which could catalyse the formation of Zr–O–Si cross-

links via the formation of Zr–OH and Si–OH which would then

couple in a condensation reaction.

Although some residual silicon was found in ZrO2-SR, as

opposed to the reports of the preparation of other mesoporous

materials using the same method, in the ZrO2-SR the structure of

the parent SBA-15 material was accurately replicated, as was

shown by the presence of the rope-like domains, the mesopores

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and nanorods. According to the TEM and N2 adsorption results,

the parent SBA-15 had the common p6mm symmetry with unit

cell parameter about 10.4 nm and pore diameter about 8 nm,

meaning that the wall thickness was about 2.4 nm. The BJH pore

size distribution of the resultant ZrO2-SR had a sharp peak at

about 2.9 nm, demonstrating the existence of the mesopores

generated by the removal of the silica wall of SBA-15. The

pore diameter of ZrO2-SR is about 0.5 nm larger than the wall

thickness of SBA-15. This might be related to structural

shrinkage during the decomposition process and perhaps to the

existence of a certain distance between the SBA-15 pore walls

and the ZrO2 framework in the ZrO2/SBA-15 composite.

Alternatively, the difference might be related to the formation of

a thin Si–O–Zr reaction layer at the ZrO2 surface.

4. Conclusions

In summary, the ZrO2-SR prepared via the replica method with

SBA-15 as the hard template had the same rope-like domains and

p6mm symmetry as the parent SBA-15. The silica walls were

successfully removed after reacting with NaOH solution, leaving

mesopores with a diameter of 2.9 nm, which were a little larger

than the wall thickness of SBA-15 (2.4 nm). Some residual silicon

remained in the ZrO2-SR. It was found that this was because

of the formation of Zr–O–Si crosslinks, rather than retention of

bulk silica or formation of zirconium silicate. These crosslinks

were found to stabilise the tetragonal phase of ZrO2. This work

demonstrates that the replica method itself was successfully

applied to prepare ordered mesoporous ZrO2, but that meso-

porous silica is not the appropriate hard template because

Si cannot be removed completely from the product.

5. Acknowledgement

This work is funded by the European Community’s Sixth

Framework Programme through a Marie Curie Incoming

International Fellowship for BL (No. 40457).

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