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Optik 124 (2013) 2963– 2965

Contents lists available at SciVerse ScienceDirect

Optik

jou rn al homepage: www.elsev ier .de / i j leo

ptical properties and preparation of Bismuth Titanate (Bi12TiO20) usingombustion synthesis technique

roosa Subohia,∗, G.S. Kumarb, M.M. Malika

Department of Physics, Maulana Azad National Institute of Technology, Bhopal, M.P. 462003, IndiaDepartment of Physics, Osmania University, Hyderabad, A.P. 500007, India

a r t i c l e i n f o

rticle history:eceived 25 April 2012

a b s t r a c t

Bismuth Titanate (Bi12TiO20) with sillenite structures was prepared by combustion synthesis methodwith urea as fuel at much lower calcination temperature due to which volatilization of bismuth can be

ccepted 4 September 2012

eywords:ombustion synthesishotocatalyst

minimized. The complete phase formation in the as prepared samples was confirmed by X-ray diffraction(XRD) studies. Surface morphology of the powders were studied using scanning electron microscope(SEM). Chemical analysis of the powder was done with the help of TG-DTA studies. Its optical propertieswere studied with the help of UV–visible absorption spectra, and photoluminescence spectroscopy (PL).

© 2012 Elsevier GmbH. All rights reserved.

illenite

. Introduction

Photocatalytic oxidation of organic pollutants has proved to ben effective process for air and water decontamination. Amongsthe presently used photocatalyst, TiO2 is one of the most activehotocatalyst for decomposition of organic pollutants and its pho-ocatalytic behaviour [1]. However, a major disadvantage for titanias its large band gap which allows it to absorb UV light only. So, its important to develop new photo catalyst to extend the absorbed

avelength range into the visible region where less extensive lightources exist and for better utilization of solar energy [2].

Lately, sellinite compounds with general formula Bi12MO20here M represents tetravalent ion (M = Si, Ge, Ti) have drawnuch attention because of their unique crystal structures and high

hotoreactivities in visible region. [3] These sillenite type crys-als have attracted many researchers due to their photorefractive,lectro-optic, piezoelectric and photo-chromic properties apartrom these materials showing photocatalytic properties [1].

The Bi12TiO20 (BTO) crystals belongs to sillenite family withheir crystal structures being formed by Bi–O polyhedra, where Bions are coordinated with five oxygen ions that form an octahedralrrangement. A Bi–O polyhedron network connects to geometri-ally regular TiO4 tetrahedra. The overall structure of Bi12TiO20 can

e described in terms of seven oxygen coordinated Bi polyhedra,hich share corners with other identical Bi polyhedra and with

iO4 tetrahedra [2].

∗ Corresponding author. Tel.: +91 9755990799.E-mail address: oroosa@gmail.com (O. Subohi).

030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved.ttp://dx.doi.org/10.1016/j.ijleo.2012.09.017

Due to its importance in various optoelectronic applications BTOhas been synthesized by many chemical and also mechanical routes[1–3]. There is hardly any paper reporting BTO production by com-bustion synthesis and to our knowledge photocatalytic propertiesof this material synthesized by this method have not been reportedyet.

Combustion synthesis is an attractive technique to produceceramic materials since it is simple and efficient process. It alsohas substantial economic benefit and for this reason it is beingwidely used in synthesis of various compounds. It requires lowercalcinations temperature which can minimize volatilization if Bi3+

ions. The technique consists of heating a homogeneous mixtureof metal salts and a suitable organic fuel. The high temperatureis induced by self sustained exothermic chemical reaction whoseignition temperature is lower than the crystallization temperatureof the product [4].

In this paper we discuss the preparation, characterization andoptical properties of BTO.

2. Experimental

The sillenite crystals of Bi12TiO20 were prepared by combus-tion synthesis route. Bismuth Nitrate pentahydrate Bi(NO3)3·5H2O(Merck) and titanium dioxide TiO2 (Merck) were used as cation pre-cursors, and urea CO(NH2)2 was used as fuel. The reactants taken inthe mole ratio of Bismuth:Titanium = 12:1 were first mixed to dis-

tilled water and homogenized in a quartz crucible. The synthesiswas carried out by heating the mixture in air using electric plateat 450 ◦C. At this temperature the ignition took place and the reac-tion lasted for about less than 5 min producing a dry yellowish to

2964 O. Subohi et al. / Optik 124 (2013) 2963– 2965

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Fig. 2. SEM micrographs of BTO powder synthesized from SHS route after calcina-tions for 30 min at 750 ◦C.

light accounting for photoactivity under visible light irradiation.As shown in Fig. 4, the absorption band of Bi12TiO20 crystal has nostructure such as shoulders and posses steep edge suggesting that

ig. 1. XRD pattern of Bi12TiO20 powder (a) reference JCPDS file no: 78-1158 (b)ithout calcination (c) after calcination at 750 ◦C.

rownish powder. This powder was then crushed in an agate mor-or and then calcined at 750 ◦C for 30 min.

The crystalline phase obtained was identified by X-ray diffrac-ion (XRD) in continuous scanning mode using Rikagu. A scanninglectron microscope (SEM) JEOL was used to investigate particleize and surface morphology.

The thermal analysis of the produced powders was performedn DTA equipment simultaneous thermal Analyser Melter Toledo.

UV–visible absorption spectra of the as-prepared sample wasetermined using Systronics double beam U-V VIS spectropho-ometer: 2202.

Photoluminescence spectrum of the powder sample wasbtained using Hitachi spectrophotometer with the excitationavelength of 325 nm.

. Results and discussions

X-ray diffraction studies on the sample before and after calcina-ion are shown in the Fig. 1. The XRD pattern of the initial powderhows the amorphous nature indicating that the powder is non-rystalline. The as-prepared powder shows evident crystallizationfter calcinations at 750 ◦C for 30 min. The three strong peaks cor-esponding to Bi12TiO20 confirms its phase formation. The X-rayiffraction patterns of Bi12TiO20 crystals were indexed based onhose given in JCPDS data cards file no: 78-1158. Fig. 2 shows theEM micrographs of BTO synthesized by combustion synthesis afteralcination at 750 ◦C for 30 min.

.1. TG-DTA analysis

The relative weight loss and the differential thermal analysis ofhe preheated precursors as shown in the Fig. 3 indicate that theeat begins to evolve at about 250 ◦C. There is an initial weight lossf about 4% at 250 ◦C and the second weight loss of about 8% occursround 450 ◦C. The first weight loss is due to the decomposition andecarbonization of organic fuel that is urea used in the reaction.nd second weight loss is due to combustion process initiated byecomposition of urea and probably also corresponds to the firstucleation events as can be seen from XRD results as well. It is seenhat after 600 ◦C there is no weight loss so 750 ◦C was chosen asalcination temperature.

.2. UV–vis analysis

The UV–visible absorption spectrum of the prepared Bi12TiO20s shown in Fig. 4. As a comparison UV–vis absorption spectra of-25 TiO2 was also measured and shown in Fig. 4. It is seen that

Fig. 3. TG-DTA curves of as-synthesized powders.

the absorption edge of Bi12TiO20 crystal is at 500 nm, having a redshift of 60 nm in comparison to P-25 TiO2. The band gap energy Eg isestimated to be 2.361 eV based on the formula Eg = 1240/�g(nm)[9].This indicates that the prepared Bi12TiO20 is sensitive to visible

Fig. 4. UV–vis absorption spectrum of BTO powders after calcinations at 750 ◦C.

O. Subohi et al. / Optik 124

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ig. 5. Photoluminescence spectra of the prepared powders at the excitation wave-ength of 325 nm.

he absorption in the visible light region should be due to intrinsicand transition and not due to surface states [5]. Mizoguchi et al. [6]xamined the mixed valence state of bismuth and pointed out thati 6s level lies above O 2p level. The visible absorption in Bi12TiO20rystal may be due to the electron excitation from Bi 6s level intoi 3d level. In other words, the band gap of Bi12TiO20 crystal maye specified by Bi 6s level and Ti 3d level.

.3. Photoluminescence analysis

The photoluminescence (PL) emission spectra of Bi12TiO20 crys-al and P-25 TiO2 are shown in Fig. 5. The PL emission spectra cane used to disclose the efficiency of charge carrier trapping, immi-

ration and transfer and to understand the rate of recombinationf photo-generated electron–hole pair in the semiconductor, sinceL emission mainly results from the recombination of the excitedlectrons and holes [7]. Therefore less PL intensity indicates lower

[

(2013) 2963– 2965 2965

recombination rate of photo-generated electrons and holes [8]. Asseen in the Fig. 5, PL intensity of Bi12TiO20 crystal is substantiallyquenched as compared to P-25 TiO2, suggesting lesser recombina-tion rate of electron–hole pair in the as prepared BTO.

4. Conclusions

It can be concluded that Bismuth Titanate (Bi12TiO20) can beprepared with ease using combustion synthesis technique. It is seenthat pure phase of BTO is obtained only after calcination at 750 ◦C.The band gap of the as prepared sample was estimated to be about2.361 eV from the onset of UV–vis absorption spectra. Its PL Spectraindicates lesser recombination rate as compared to P-25 Titania.

References

1] B.-H. Kim, T.-H. Lim, J.-W. Roh, S.-G. Lee, C.-S. Ju, S.-S. Park, S.-S. Hong, G.D. Lee,Effect of Cr doping on the properties and photocatalytic activity of Bi12TiO20,Reac. Kinet. Mech. Catal. 99 (2010) 217–224.

2] W. Feng Yao, X. Hong Wang, X. Hong Xu, F. Cheng, S. Ji Huang, X. Xia Shang, N.Yang, M. Wang, Photocatalytic property of Bismuth Titanate Bi12TiO20 crystals,Appl. Catal. A: Gen. 243 (2003) 185–190.

3] Z. Aneva, D. Neshva, Recombination of photoexcited carriers in Bi12TiO20

monocrystals, J. Optoelectron. Adv. Mater. 7 (1) (2005) 513–516.4] Z.S. Macedo, C.R. Ferrari, A.C. Hernandes, Self-propagation high temperature

synthesis of Bismuth Titanate, J. Eur. Ceram. Soc. 24 (2004) 2567–2574.5] K. Akihiko, I. Mikami, Chem. Lett. 10 (1998) 1027.6] H. Mizoguchi, K. Ueda, H. Kawazoe, H. Hosono, T. Omata, S. Fujitsu, New

mixed-valence oxides of bismuth: Bi1−xYxO1.5+ı(x = 0.4), J. Mater. Chem. 7 (1997)943–946.

7] F.B. Li, X.Z. Li, The enhancement of photodegradation efficiency using Pt-TiO2

catalyst, Chemosphere 48 (2002) 1103–1111.8] Y.-L. Kuo, H.-W. Chen, Y. Ku, Analysis of silver particles incorporated on TiO2

coatings for photodecomposition of o-cresol, Thin Solid Films 515 (2007)3461–3468.

9] H. Zhang, M. Lü, S. Liu, Z. Xiu, G. Zhou, Y. Zhou, Z. Qiu, A. Zhang, Q. Ma, Preperationand photocatalytic properties of sillenite Bi12TiO20 films, Surf. Coat. Technol. 202(2008) 4930–4934.

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