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Materials Letters 62 (2

Synthesis and luminescence properties of hand-like α-Bi2O3 microcrystals

Ying Xiong a, Mingzai Wu b, Jing Ye a, Qianwang Chen a,⁎

a Hefei National Laboratory for Physical Sciences at Microscale, University of Science & Technology of China, Hefei 230026, PR Chinab Key Laboratory of Opto-Electronic Information Acquisition and Manipulation, School of Physics and Materials Science, Anhui University,

Hefei 230039, PR China

Received 4 February 2007; accepted 1 August 2007Available online 7 August 2007

Abstract

This letter reports a facile hydrothermal approach to the preparation of novel hand-like α-Bi2O3 microcrystals that are assembled by severalshort rods with the diameter of about 500 nm. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM) were employed to characterize the structure and morphology of these novel structures. The cooperative effect between thesurfactant (oleic acid) and the coordinating agent (EDTA) was an important key to obtain these novel hand-like structures of α-Bi2O3. Theluminescence properties of α-Bi2O3 microcrystals exhibited an important dependence on the surface states and the shape feature of products,which were mainly determined by the preparation conditions.© 2007 Elsevier B.V. All rights reserved.

Keywords: Bismuth oxide; Electron microscopy; Hand-like; Luminescence

1. Introduction

Many properties of inorganic materials, such as optical,electronic, magnetic and catalytic properties, are size and shape-dependent. In recent years, much effort had been devoted to thedesign of rational methods for synthesizing nanostructuredmaterials with the novel shapes, such as nanorings [1],nanobowls [2], nanocombs [3], due to the unique physicalproperties and potential applications.

As the starting material for synthesizing high temperaturesuperconductor and Bi-containing ferroelectric compounds,bismuth oxide has been widely studied in the fields of opticcoating [4], sensor technology [5], electrolytes materials [6] andcatalysts [7]. When combined with other elements to formglasses and films, bismuth oxide also has important optical andelectrical applications [8]. Bismuth oxide is a complex systemthat has four different polymorphs, with significantly differentstructure and properties: α, β, γ, and δ [9]. The presentsynthesis of bismuth oxide is focused on the α and δ forms,which, unlike the others, are the stable structures [10,11].Recently, α-Bi2O3 microrods have been synthesized by a

⁎ Corresponding author. Tel./fax: +86 551 3607292.E-mail address: cqw@ustc.edu.cn (Q. Chen).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.08.004

conventional precipitation method [7], and needle-like bismuthoxides were obtained using hydrothermal process [12]. α-Bi2O3

nanoparticles coated with stearic acid have been preparedthrough a simple microemulsion method [13].

Herein, we present a facile hydrothermal approach to prepareα-Bi2O3 microcrystals with novel hand-like morphologieswherein several short rods with ca. 500 nm in diameter areassembled. The cooperation effect between the surfactant (oleicacid) and the coordinating agent (EDTA) on the formation ofhand-like structures is proposed based on the experimentalresults.

2. Experimental section

All chemicals were of analytical grade and used withoutfurther purification. In a typical procedure, a homogeneousacidic solution of bismuth nitrate ([Bi3+]=0.5 mol/L) wasemployed as the source of bismuth. 2.0 mL of the solution wastook out and diluted to 10.0 mL by distilled water. Then,1.0 mmol of EDTA and 5.0 mL of oleic acid were added to thediluted solution with magnetically stirring. After half an hour, acertain amount of precipitation agent (NaOH) was slowlyadded, with vigorous stirring to obtain a precursor suspension.Keep stirring for 1 h, the precursor suspension was transferred

Fig. 1. X-ray diffraction patterns of three products prepared under the differentreaction, aging 12 h at 120 °C: a) S1; b) S2; c) S3.

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to the Teflon-lined stainless-steel autoclave with the totalvolume of 50.0 mL and diluted by distilled water to ensure theultimate concentration of NaOH to be 1.0 mol/L. The autoclavewas sealed and maintained at 120 °C for 12 h. After theautoclave was cooled to room temperature naturally, theproducts were collected by centrifugation, washed severaltimes with distilled water and absolute ethanol, and then dried at60 °C for 6 h in air. The sample obtained was labelled as S3. Toinvestigate the effects of oleic acid and EDTA on the

Fig. 2. TEM and SEM images of α-Bi2O3 under the different reaction, aging 12 h at 1(e) SEM and (f) TEM images of S1.

morphology of products, two samples, which were named asS1 and S2, were prepared under the conditions of adding onlycoordinating agent EDTA and only surfactant (oleic acid) intothe precursor solution, respectively.

3. Results and discussion

The X-ray diffraction patterns of three products are shown in Fig. 1.All reflection peaks can be well indexed with a pure monoclinic phaseof crystalline α-Bi2O3, which are in good agreement with the literaturevalue (JCPDF card number 71-2274). Furthermore, it is notable that inthe absence of surfactant oleic acid or coordinating agent EDTA, thephase of the products (S1 and S2) remains unchanged.

Fig. 2a is a representative SEM image of S3, which shows that theproducts consist of novel hand-like microcrystals. High-magnificationTEM image of a typical hand-like microcrystal (Fig. 2b) clearly revealsthat this structure is assembled by several short rods with the diameterof about 500 nm. It was noticeable that these hand-like structures of α-Bi2O3 microcrystals could not be destroyed even through the strongultrasonic processing (250Wultrasonic power) before the TEM samplepreparation. Accordingly, It could be rationally concluded that thesenovel hand-like structures were not simply stacked by several shortrods, but were in situ assembled in a hand-sharp manner during thehydrothermal reaction. Special details about how to connect the“fingers” and the “palm”, to organize the “fingers” and to joint the“fingers” in these hand-like structures must be further characterized byhigh-resolution transmission electron microscopy and electronicdiffraction.

The influences of surfactant (oleic acid) and coordinating agent(EDTA) on the morphologies of products were also investigated. The

20 °C: (a) SEM and (b) TEM images of S3; (c) SEM and (d) TEM images of S2;

Fig. 3. Excitation and emission spectra of three samples measured at room temperature: (a) excitation spectrum of S1 detected with emission wavelength of 440 nm; (b)excitation spectrum of S2 and S3 detected with emission wavelength of 440 nm and 410 nm; (c) emission spectrum of S1 excited at 367 nm and 275 nm; (d) emissionspectrum of S2 and S3 excited at 367 nm and 275 nm.

Table 1Data of fluorescence spectra of S1, S2 and S3 measured at room temperature

Samples Excitation spectra Emission spectra

λema (nm) λex

b (nm) λexc (nm) λem

d (nm)

S1 440 370 367 440275 442

S2 440 367, 260 367 440, 415410 367, 260 275 440, 415

S3 440 367, 275 367 440, 415410 367, 275 275 440, 415

a Detected emission wavelength for the excitation spectra.b Position of excitation maximum.c Excitation wavelength for the emission spectra.d Position of emission spectra.

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SEM and TEM images of sample S2 are shown in Fig. 2c and d. Asseen in SEM image, only isolated sub-micron rods with the averagediameter of ∼0.7 μm are presented and no hand-like structures can beobserved. The SEM and TEM images of S1 are shown in Fig. 2e and f,respectively, which reveal that the products also consist of short rodswith the diameter in a board range of 200 nm–3 μm. The appearance ofbroad diameter distribution could be attributed to the formation ofcoordination compounds Bi-EDTAwith high stability constant, whichcould change the rates of crystal nucleation and crystal growth.Similarly, no hand-like structures of Bi2O3 can be found in SEM andTEM observations. Based on these results, it was rationally concludedthat the cooperative effect of surfactant and coordinating agent was animportant key to obtain these novel hand-like structures of α-Bi2O3.The detailed mechanism of the formation of these novel structurescould be proposed after the HRTEM and ED characterization.

The excitation and emission spectra of above-mentioned threesamples measured at room temperature are shown in Fig. 3, and thefluorescent characteristics are summarized in Table 1. During themeasurement of luminescent properties, all experimental parametersincluding the sample mass were the same in all the three samples.Compared with S2 and S3, S1 showed vastly lower fluorescenceintensity. Considering that residual oleic acid could be absorbed on thesurface of samples S2 and S3, probably resulting in the change of thesurface states [14], the large difference in fluorescence intensity ofthree samples could be tentatively attributed to the different surfacestates.

As seen in Table 1 and Fig. 3c and d, the emission peaks of S2 andS3 are located at 440 nm and 415 nm, but only an emission peak at

440 nm can be observed in S1. The bulk α-Bi2O3 was a directsemiconductor with wide band gap of 2.85 eV [15,16]. The emissionpeak at 440 nm could be indexed as the band-to-band recombination ina direct transition manner. However, an additional peak at 415 nmappeared in S2 and S3 might be relative to the some defects inthe surface originated from the absorbed oleic acid. Moreover, thefluorescence intensity of S3 is obviously stronger than that of S2 inboth excitation and emission spectra, which could possibly be related tothe novel hand-like structure of S3, as demonstrated by Yu et al. [17].

In the excitation spectra of S2 and S3 (Fig. 3b), two main excitationpeaks exist: One is at about 367 nm, which is similar to the reported

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value in literatures [13], and the other is at about 275 nm, which hasnever been observed previously by other authors. The reason for theappearance of excitation peaks at 275 nm is not clear and needs furtherinvestigation.

4. Conclusions

In summary, novel hand-like structures of α-Bi2O3, whichwere assembled by several short rods with the diameter of∼500 nm, were synthesized on a large scale via a mildhydrothermal process at 120 °C for 12 h, under the assistance ofsurfactant and coordinating agent. The surfactant oleic acid andthe coordinating agent EDTA played a crucial role in theformation of these novel structures. The fact that luminescentproperties of different samples exhibited obvious difference inthe fluorescence intensity and peak position could be related tothe difference in the surface states and novel structure of α-Bi2O3.

Acknowledgements

This work is supported by the National Natural ScienceFoundation of China (NSFC) under grant numbers 20125103and 90206034.

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