preparation and characterization of high surface area nanosheet titania with mesoporous structure

Preparation and characterization of high surface area nanosheet titaniawith mesoporous structure Sorapong Pavasupreea,b, Supachai Ngamsinlapasathian a, Yoshikazu Suzukia, Susumu Yoshikaw

Preparation and characterization of high surface area nanosheet titania

with mesoporous structure

Sorapong Pavasupreea,b, Supachai Ngamsinlapasathian a,

Yoshikazu Suzukia, Susumu Yoshikawa a,⁎

aInstitute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan

bDepartment of Materials and Metallurgical Engineering, Faculty of Engineering, Rajamangala University of Technology Thanyaburi,

Klong 6, Pathumthani, 12110, Thailand Received 6 September 2006; accepted 23 October 2006 Available online 9 November 2006

Abstract

High surface area nanosheet TiO2with mesoporous structure were synthesized by hydrothermal method at 130 °C for 12 h The samples were characterized by XRD, SEM, TEM, SAED, and BET surface area The nanosheet structure was slightly curved and approximately 50 –100 nm in width and several nanometers in thickness The as-synthesized nanosheet TiO2had an average pore diameter about 3–4 nm The BET surface area and pore volume of the sample are about 642 m2/g and 0.774 cm3/g, respectively.

© 2006 Published by Elsevier B.V.

Keywords: Nanosheet; Mesoporous; High surface area; TiO2

1 Introduction

The synthesis and characterization of nanostructured

materi-als (nanotubes, nanorods, nanowires, and nanosheet) have

received considerable attention due to their unique properties

and novel applications [1–4] Much effort has concentrated on

the important metal oxides such as TiO2, SnO2, VO2, and ZnO

[1–10] Among them, TiO2and TiO2-derived materials are of

importance for utilizing solar energy and environmental

purification TiO2has been widely used for various applications

such as a semiconductor in dye-sensitized solar cell, water

treatment materials, catalysts, gas sensors, and so on [11–22]

Functional properties of TiO2are influenced by many factors

such as crystallinity, particle size, surface area, and preparation

[11] In the view point of surface area, nanosheet and nanotubes

(from nanosheet rolling technique) TiO2 (or titanate) offered

high surface area (about 100–400 m2

/g) [10,23–31] In our previous works, nanofibers TiO2were synthesized by

hydro-thermal and post heat-treatments from natural rutile sand,

however, nanofibers TiO2 had rather low surface area (10 –

20 m2/g) [32,33]

In this study, high surface area nanosheet TiO2 with me-soporous structure (with much higher surface area, 642 m2/g) obtained by hydrothermal at 130 °C for 12 h will be reported.

2 Experimental 2.1 Synthesis Titanium (IV) butoxide (Aldrich) was mixed with the same mole of acetyacetone (ACA, Nacalai Tesque, Inc., Japan) to slowdown the hydrolysis and the condensation reactions [14–16] Subsequently, 40 ml distilled water was added into the solution, and the solution was stirred at room temperature for 5 min After constant stirring, ammonia aqueous solution 28% (Wako Co., Ltd., Japan) 30 ml was added into the solution, then the solution was put into a Teflon-lined stainless steel autoclave and heated at

130 °C for 12 h with stirring condition After the autoclave was naturally cooled to room temperature, the obtained product was washed with HCl aqueous solution, 2-propanal and distilled water for several times, followed by drying at 100 °C for 12 h The

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⁎ Corresponding author Tel.: +81 774 38 3502; fax: +81 774 38 3508

E-mail address:s-yoshi@iae.kyoto-u.ac.jp(S Yoshikawa)

0167-577X/$ - see front matter © 2006 Published by Elsevier B.V

doi:10.1016/j.matlet.2006.10.056

samples were calcined for 4 h at 300–700 °C in air condition

( Fig 1 ).

2.2 Characterization

The crystalline structure of the samples was evaluated by

X-ray diffraction (XRD, RIGAKU RINT 2100) The

microstruc-ture of the prepared materials was analyzed by scanning

electron microscopy (SEM, JEOL JSM-6500FE), transmission

electron microscopy (TEM, JEOL JEM-200CX), and

selected-area electron diffraction (SAED) The Brunauer –Emmett–

Teller (BET) specific surface area was determined by nitrogen

adsorption (BEL Japan, BELSORP-18 Plus).

3 Results and discussion

Fig 2 (a–b) shows the low and high magnification SEM images of

the as-synthesized sample, indicating the flower-like morphology

composed of nanosheets The flower-like structure had a diameter

about 500 nm to 2 μm ( Fig 3 (a)) The nanosheet structure was slightly

curved and approximately 50 –100 nm in width and several nanometers

in thickness ( Fig 3 (b –c)) The electron diffraction pattern shown in the

inset of Fig 3 (b) supported that the nanosheet was anatase-type TiO2,

which corresponded to the XRD results (low crystallinity of anatase

TiO2, Fig 5 ) In addition, higher magnification TEM ( Fig 3 (c)) image

shows nanopores in the flower-like structure.

Fig 4 gives the nitrogen adsorption isotherm and the pore size

distribution of the as-synthesized nanosheet TiO2 The isotherm shows

a typical IUPAC type IV pattern with inflection of nitrogen adsorbed

volume at P/P0 about 0.45 (type H2hysteresis loop), indicating the

existence of mesopores The pore size distribution of the sample, as

shown in the inset of Fig 4 , showed that the nanosheet TiO with

narrow pore size distribution had an average pore diameter about 3–

4 nm The BET surface area and pore volume of the as-synthesized nanosheet TiO2are about 642 m2/g and 0.774 cm3/g, respectively The as-synthesized nanosheet TiO2 (prepared by this method) showed higher surface area than nanosheet and nanotubes TiO2(about 100 –

400 m2/g) [10,23 –31]

Fig 5 shows the X-ray diffraction pattern of the as-synthesized nanosheet TiO2, and nanosheet TiO2calcined for 1 h at 300–700 °C The nanosheet TiO2 calcined for 1 h at 300–500 °C consisted of anatase TiO2structure The peak intensity of anatase TiO2increased as the calcination temperature was increased The peaks were rather sharp, which indicated that the calcined nanosheet TiO2had relatively high crystallinity The peaks corresponding to rutile TiO2appeared at 600 °C and almost showed rutile TiO2structure at 700 °C.

Fig 6 (a –j) shows the SEM, TEM, and SAED images of the nanosheet TiO2 calcined for 1 h at 300 –700 °C The nanosheet structure after calcinations was destroyed and changed to nanorods/ nanoparticles composite with anatase TiO2structure at 300 –500 °C (10 –15 nm in rods diameter and about 5–10 nm in particles diameter,

Fig 6 (a –f)) The SEM, TEM, and SAED images of the nanosheet TiO2

calcined at 600 and 700 °C showed almost nanoparticles with a mixture

of anatase and rutile TiO2structure (about 10–50 nm in diameter, at

600 °C, Fig 6 (g–h)), and rutile TiO2structure (about 40–100 nm in diameter, at 700 °C, Fig 6 (i–j)).

Fig 1 Schematic representation of the experimental procedure

Fig 2 SEM images of the as-synthesized flower-like nanosheet TiO2(a) ×30,000 and (b) ×100,000 magnified

2974 S Pavasupree et al / Materials Letters 61 (2007) 2973–2977

From the above data we can deduce the growth process of the

nanosheet TiO2( Fig 7 ) At first, amorphous TiO2nanoparticles were

prepared by TiBu–ACA in 40 ml H2O There are many aggregations of

TiO2 nanoparticles (after added ammonia aqueous solution 28%),

which can work as nuclei for TiO2growth The nanosheet TiO2grow

under hydrothermal treatment because the nanotubes (from nanosheets

rolling technique) can be synthesized at 110 –120 °C for 48–72 h and

dilute base treatment generates thin, curled sheet materials [28 –31]

The nanosheet structure after calcinations was destroyed and changed

to nanorods/nanoparticles composite at high temperature [27,30]

4 Conclusions

In summary, high surface area nanosheet TiO2 with

mesoporous structure were synthesized by hydrothermal

method at 130 °C for 12 h The nanosheet structure was slightly curved and approximately 50–100 nm in width and several nanometers in thickness The as-synthesized nanosheet TiO2had an average pore diameter about 3–4 nm The BET

Fig 3 (a–c) TEM and (inset of (b)) SAED images of the as-synthesized

nanosheet TiO2

Fig 4 Nitrogen adsorption isotherm pattern of the as-synthesized nanosheet TiO2(BET surface is 642 m2/g), and the pore size distribution of the sample with pore diameter about 3–4 nm (inset)

Fig 5 X-ray diffraction pattern of the as-synthesized nanosheet TiO2 and nanosheet TiO calcined for 1 h at 300–700 °C

Fig 6 SEM, TEM, and SAED images of the nanosheet TiO2calcined for 1 h at 300–700 °C.

2976 S Pavasupree et al / Materials Letters 61 (2007) 2973–2977

surface area and pore volume of the sample are about 642 m2/g

and 0.774 cm3/g, respectively The nanosheet structure after

calcinations was changed into nanorods/nanoparticles

compos-ite with anatase TiO2structure at 300–500 °C (10–15 nm in

rods diameter and about 5–10 nm in particles diameter).

Acknowledgements

The authors would like to express their gratitude to Prof S.

Isoda and Prof H Kurata, Institute for Chemical Research,

Kyoto University for the use of TEM apparatus and to Prof T.

Yoko, Institute for Chemical Research, Kyoto University for the

use of XRD equipment This work was supported by

grant-in-aids from the Ministry of Education, Science Sports, and

Culture of Japan under the 21 COE program, the

Nanotechnol-ogy Support Project, and from NEDO under high-performance

dye-sensitized solar cell project.

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