Hydrothermal synthesis and characterization of bismuth titanate regular tetrahedron crystallites

Article abstract of DOI:10.1016/j.materresbull.2006.05.023

Bi₁₂TiO₂₀ (BTO) single crystals were synthesized by a hydrothermal process in KOH solution at 180 °C for 4 h from Ti(OC₄H₉)₄ and Bi(NO₃)₃·5H₂O compounds. The effects of processing parameters on the growth and morphology of BTO single crystals were investigated. The KOH concentration, reaction time and temperature had a great effect on the phase composition and morphology of the resultant crystals. BTO phase could not be obtained with KOH concentrations lower than 3 M or higher than 5 M. XRD, SEM and TEM were used to characterize the samples. BTO single crystals were regular tetrahedron in morphology and 10 μm or so in size.

Full text of DOI:10.1016/j.materresbull.2006.05.023

Materials Research Bulletin 42 (2007) 389–393
www.elsevier.com/locate/matresbu
Short communication
Hydrothermal synthesis and characterization of bismuth titanate
regular tetrahedron crystallites
b
a
a
Lijin Xie a, Junfeng Ma a,b, , Pingwei Wu , Hua Tian , Zhongqiang Zhao ,
*
Jun Zhou a, Yingmo Hu a, Yonggang Wang b, Jiantao Tao b, Xiaoyi Zhu b
a College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266003, China
b Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266003, China
Received 29 September 2005; received in revised form 21 February 2006; accepted 26 May 2006
Available online 16 June 2006
Abstract
Bi12TiO20 (BTO) single crystals were synthesized by a hydrothermal process in KOH solution at 180 8C for 4 h from
Ti(OC4H9)4 and Bi(NO3)3Á5H2O compounds. The effects of processing parameters on the growth and morphology of BTO single
crystals were investigated. The KOH concentration, reaction time and temperature had a great effect on the phase composition and
morphology of the resultant crystals. BTO phase could not be obtained with KOH concentrations lower than 3 M or higher than 5 M.
XRD, SEM and TEM were used to characterize the samples. BTO single crystals were regular tetrahedron in morphology and
10 mm or so in size.
# 2006 Elsevier Ltd. All rights reserved.
Keyword: B. Crystal growth
1. Introduction
Since the discovery of the photorefractive effect, a great deal of research has been devoted to identify materials of
photorefractive effect such as the sillenite family crystals [1]. Sillenites are attracted increasing attention due to their
excellent photorefractive effect without the double refraction induced by an applied external magnetic field [2].
Bi12TiO20 (BTO) is one of the most important sillenite materials, having the highest refractive index [3] and electro optic
coefficient [4]. It exhibits highest sensitivity [5] in the red among all the presently known photorefractive materials. In
addition, the photocatalytic reaction has attracted much interest owing to energy and environmental issues [6–8]. The
maximal absorption edge of BTO powder was determined to be 520 nm or so, corresponding to the band gap energy of
2.39 eV [9]. Therefore, BTO has prodigious potential value in photocatalytic field, and it has been used as photocatalyst
due to its high photocatalytic activity for the decoloration of methyl orange and 4-nitrophenol [9,10]. However, the
growth of good quality single crystals is rather difficult owing to the incongruent phase. There have been some reports on
the growth of BTO using self flux technique [11,12], TSSG techniques [13], hydrothermal method [14,15] and chemical
solution decomposition (CSD) [16]. It is noteworthy that the powders prepared by the self flux, TSSG and previous
hydrothermal technique are so big in size that they are not good for photocatalytic reaction. On the other hand, the CSD
* Corresponding author. Tel.: +86 53 2203 1623; fax: +86 53 2203 1623.
E-mail address: majf@mail.ouc.edu.cn (J. Ma).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.05.023

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L. Xie et al. / Materials Research Bulletin 42 (2007) 389–393
method needs not only tedious procedures but also high reaction temperature though their products were small in size. In
recent years, the low temperature, relatively inexpensive feedstock and simplicity of hydrothermal process are attractive
for the preparation of high-quality single crystal materials. It has the obvious advantages over other methods in
controlling the growth and morphology of single crystals. Nevertheless, to our knowledge, there have been few reports on
the morphology and growth of the BTO single crystals by a hydrothermal process yet.
In present paper, we report a synthetic method based on a low temperature hydrothermal process, and discuss the
effects of preparation conditions on the formation of regular tetrahedron BTO single crystals.
2. Experimental procedure
2.1. Preparation of the BTO single crystals
Bismuth nitrate [Bi(NO3)3Á5H2O] and tetrabutyl titanate [Ti(OC4H9)4], both of which were analytically pure, were
used as starting materials with the mole ratio of bismuth:titanium = 12:1. Solution A was prepared by dissolving
4.17 Â 10À3 mol Bi(NO3)3Á5H2O in 6 M HNO3. Solution B was prepared by adding ammonia to 3.48 Â 10À4 mol
[Ti(OC4H9)4] to form a white precipitate, which then dissolved in the mixed solution of 6 M HNO3 and 30% H2O2 to
produce a reddish brown solution containing titanium cation. Solution A and B were mixed well together by constant
stirring at room temperature, and then a yellow coprecipitation was formed with adding ammonia. The yellow
coprecipitation was washed with deionized water for several times, and directly used as a precursor for hydrothermal
treatment. The hydrothermal reactions were carried out in a 100 ml Teflonlined autoclave. Aqueous solution of KOH
(75 ml) and an appropriate amount of precursor were put into the autoclave. After the hydrothermal treatment at
180 8C for 4 h, the products were removed out and filtered, washed with deionized water up to the pH value of 7.0, and
then dried at 70 8C for 6 h.
2.2. Characterization
The phase composition of the obtained products was identified by the X-ray diffractometer (D8 Advance
diffractometer) with the Cu Ka radiation (l = 1.5406 A). The scan rate of 0.108 sÀ1 was applied to record the XRD
˚
patterns in the 2u range of 20–708. The morphology of single crystals was observed by scanning electron microscope
(SEM) on a JSM-840. Electron diffraction patterns were taken on a JEM-1200EX.
3. Results and discussion
3.1. Effects of pH, crystallization temperature and time on the crystallization
The influences of the processing parameters such as KOH concentration in solution, reaction temperature and time
on the formation of BTO were summarized in Table 1.
Fig. 1 showed the XRD patterns of the powders prepared at 180 8C for 4 h with different KOH concentration by
hydrothermal process. It indicated that the formation of BTO was greatly affected by KOH concentration, and a lower
or higher KOH concentration (ꢀ3.0 or ꢁ4.5 M) would lead to trace amount of Bi2O3, including monoclinic phase and
cubic phase. Only between 3.0 and 4.5 M, did a single phase of sillenite BTO existed. As shown in Fig. 1, the sharp
peaks in the XRD patterns confirmed a well crystallinity of the as-prepared BTO powders. The corresponding X-ray
˚
diffraction patterns could be indexed to cubic structure (a = 10.174 A), well consistent with the datum reported [17].
The effects of the reaction temperatures were also investigated in the range from 160 to 210 8C, for 4 h in 4.5 M
KOH aqueous solutions. As shown in Table 1, the pure phase BTO powders could be obtained by the hydrothermal
process at different temperature ranging from 180 to 210 8C.
Fig. 2 showed SEM micrographs of the BTO powders prepared in 4.5 M KOH solution at 180 8C for different
reaction time and inset images of the figure were the electron diffraction patterns of the particles arrowed. The
prepared powders for 1 h contained a few of regular tetrahedron particles and a mass of the amorphous phase. As the
reaction time increased, the crystallinity was gradually improved, and particles with regular tetrahedron shape
obviously increased in numbers. The BTO powders prepared for 2 h had a large amount of regular tetrahedrons about
10 mm in size in addition to spherical particles, as shown in Fig. 2(b). However, when reaction time increased, the

L. Xie et al. / Materials Research Bulletin 42 (2007) 389–393
391
Table 1
The effect of hydrothermal synthesis conditions on the formation of Bi12TiO20
Starting materials
Ti
KOH (M)
Time (h)
Temperature (8C)
Crystal structure
Bi
Ti(OC4H9)4
Ti(OC4H9)4
Ti(OC4H9)4
Ti(OC4H9)4
Ti(OC4H9)4
Ti(OC4H9)4
Ti(OC4H9)4
Ti(OC4H9)4
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
2
3
2
2
2
4
4
4
4
1
210
210
210
180
180
180
180
180
Bi2O3, BTO
BTO
4.5
ꢀ1
5
BTO
Bi2O3, BTO
Bi2O3, BTO
Bi2O3, BTO
Bi2O3, BTO
BTO,
6
8
4.5
amorphous
BTO,
Ti(OC4H9)4
Bi(NO3)3Á5H2O
4.5
2
180
amorphous
BTO
Ti(OC4H9)4
Ti(OC4H9)4
Bi(NO3)3Á5H2O
Bi(NO3)3Á5H2O
4.5
4.5
4
4
180
160
Bi2O3,
amorphous
Bi2O3, amorphous
Ti(OC4H9)4
Bi(NO3)3Á5H2O
4.5
6
160
particles did not have a significant increase in size. The single crystal nature of the regular tetrahedron particle could be
confirmed by electron diffraction pattern characteristic of typical single crystals (Fig. 2(c-1)), and spherical particle
corresponded to several diffuse rings, typical of the amorphous (Fig. 2(b-2)). When the reaction time increased to 4 h,
the BTO powders consisted of only perfect regular tetrahedron single crystals, and spherical particles disappeared
nearly (Fig. 2(c)). It implied that a long reaction time favored the growth of regular tetrahedron single crystals in the
hydrothermal process.
3.2. Formation and growth mechanisms of BTO single crystal
The growth mechanism is useful in predicting the growth and evolution of the single crystals. There have been two
formation mechanisms proposed for the hydrothermal reaction [18]. One of them is dissolution and recrystallization
mechanism in which crystallites form by dissolution of the precursors followed by recrystallization from
supersaturated solution. The other formation mechanism is in situ transformation process in which amorphous
Fig. 1. XRD patterns of the powders prepared at 180 8C for 4 h by the hydrothermal process with different KOH concentrations; a—monoclinic
phase Bi2O3 and b—cubic phase Bi2O3.

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Fig. 2. SEM micrographs of the Bi12TiO20 single crystals prepared at 4.5 M KOH (M = 4.5) at 180 8C for (a) 1 h, (b) 2 h and (c) 4 h; and electron
diffraction patterns of (1) regular tetrahedron and (2) micro-sphere.
precursors crystallize into polycrystalline particles by the removal of structure water. The formation mechanism of the
BTO single crystals may be belongs to dissolution and recrystallization process. The precursor used in the present
study was a coprecipitation containing titanium and bismuth cations. Under the suitable processing condition of
hydrothermal treatment, the amorphous phase with sphere shape formed firstly, and about 500 nm in size. The
amorphous was an intermediate phase to crystalline BTO phase. It is well known that the amorphous phase, in general,
should have higher free energy than the crystalline phase in the case of the same chemical composition. As the reaction
temperature and/or time increased, the crystalline regular tetrahedron appeared in the expense of the redissolution of
the amorphous micro-sphere owing to their higher free energy than the crystalline phase of the BTO, corresponding to
higher solubility in the same media. Therefore, there were only pure regular tetrahedron shape single crystals
ultimately.
4. Conclusions
Bi12TiO20 regular tetrahedron single crystals about 10 mm in dimension were easily grown by the low temperature
hydrothermal method in KOH solution. The concentration of KOH, reaction time and temperature had a significant
effect on the phase composition and morphology of the resultant single crystals. The pure BTO phase could not be
obtained with KOH concentrations lower than 3.0 or higher than 4.5 M. The morphology of BTO powders changed
from spherical to uniform regular tetrahedron particles with reaction time in the range of 1–4 h.
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