room-temperature synthesis of nanometric α-bi2o3

Materials Letters 64 (2010) 2247–2250

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Materials Letters

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Room-temperature synthesis of nanometric α-Bi2O3

Marija Prekajski a,⁎, Aleksandar Kremenović b, Biljana Babić a, Milena Rosić a, Branko Matović a,Ana Radosavljević-Mihajlović a, Marko Radović c

a Materials Science Laboratory, Institute of Nuclear Sciences, 170 Vinča, P.O. Box 522, 11001 Belgrade, Serbiab Faculty of Mining and Geology, Laboratory for Crystallography, University of Belgrade, Djusina 7, 11000 Belgrade, Serbiac Institute of Physics, Pregrevica, 11080 Belgrade, Serbia

⁎ Corresponding author.E-mail address: prekajski@vinca.rs (M. Prekajski).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.06.052

a b s t r a c t

Article history:Received 23 April 2010Accepted 23 June 2010Available online 5 August 2010

Keywords:Bi2O3

NanomaterialsSelf-propagating reaction

Nanometric Bi2O3 powder was successfully synthesized by applying the method based on self-propagatingroom temperature reaction (SPRT) between bismuth nitrates and sodium hydroxide. X-ray powderdiffraction (XRPD) and Rietveld's structure refinement method were applied to characterize preparedpowder. It revealed that synthesized material is a single phase monoclinic α-Bi2O3 (space group P21/c withcell parameters a=5.84605(4)Å, b=8.16339(6) Å, c=7.50788(6) Å and β=112.9883(8)). Powderparticles were of nanometric size (about 50 nm). Raman spectral studies conformed that the obtainedpowder is single phase α-Bi2O3. Specific surface area of obtained powder was measured by Brunauer–Emmet–Teller (BET) method.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Bismuth(III) oxide presents great interest for numerous tech-nological applications in electronics and electro-optics [1–4],catalysis [5] and gas sensors [6]. Also it is used in fireproofing ofpapers and polymers, in enameling cast iron ceramic and indisinfectants [7], but the most important application of Bi2O3 is asmaterial for solid oxide fuel cells (SOFCs) since it is an ionicconductor [8].

Bi2O3 appears in five polymorphic modification, as α-, β-, γ-, δ-Bi2O3 [9] and ε-Bi2O3 (the last one is recently reported [10]). Stablephases are α- and δ-Bi2O3, while β-, γ- and ε-phases are metastable.α-Bi2O3 is room-temperature stable phase and it occurs as mineralbismite in nature. It has a monoclinic crystal structure. δ-Bi2O3 hasface centered cubic crystal structure, and it is stable at temperatureabove 730 °C until melting point at 824 °C [9,11]. During cooling, δ-Bi2O3 transforms to one of the metastable phases, depending oncooling rate. Tetragonal β-Bi2O3 forms at high cooling rates, while γ-Bi2O3 with body-centered cubic structure forms at low cooling rates[9].

There are various methods for synthesis bismuth(III) oxide thatare described in literature [7,11,12] and most of them includessintering at high temperatures, and it is known that the sinteringtemperature decreases when a nanosized powder is used. One of themajor challenges for the preparation of these nanostructured oxides is

to develop cost and time effective method with precise control ofparticle size.

In respect of this, we synthesized α-Bi2O3 nanopowder with self-propagating room temperature reaction (SPRT), which is verypromising low cost, simple and effective method and it was alreadysuccessfully used for preparation of nanoscale cerium oxides [13–16].However, nanocrystalline Bi2O3 have not been obtained by thismethod so far.

The goals of this work were: to apply SPRT method on synthesis ofnanocrystalline Bi2O3 for the first time and to characterize obtainedpowder using XRD, Raman spectroscopy and Brunauer–Emmet–Teller(BET) methods (specific surface area measurement).

2. Experimental

Nanocrystalline Bi2O3 was synthesized by a SPRT method usingbismuth nitrate hexahydrate (Riedel-de Haën, 99% purity) andsodium hydroxide (Lach-Ner, 99% purity) as starting materials.Preparation of Bi2O3 powder was performed according to reaction:

2Bi2O3ðNO3Þ3:5H2O þ 6NaOH→Bi2O3 þ 6NaNO3 þ 13H2O ð1ÞThe above reaction proceeds at room temperature after the

mixture of reactants was mechanically activated by hand mixing inalumina mortar for 5–7 min, until the mixture gets light yellow.After being exposed to air for 3 h, the mixture was suspended inwater in order to eliminate NaNO3. Rinsing was performed incentrifuge — Centurion 1020D, at 3500 rpm for 10 min. Thisprocedure was performed four times with distilled water and

Table 1Refined structural factors, atomic parameters and corresponding agreement factors.

Space group symbol P21/c

Lattice parameters (Å)a 5.84605(4)b 8.16339(6)c 7.50788(6)α, γ 90β 112.9883(8)

Bi1 sitex 0.5255(5)y 0.1836(4)z 0.3623(3)

Bi2 sitex 0.4074(4)y 0.4249(3)z 0.7763(3)

O1 sitex 0.786(5)y 0.300(4)z 0.693(4)

O2 sitex 0.258(5)y 0.044(4)z 0.114(4)

O3 sitex 0.271(5)y 0.043(4)z 0.514(3)

Bov (Å) 0.75(3)Agreement factors

RB 4.87χ2 5.05

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twice with ethanol, and after that material was dried out at 60 °C inambient atmosphere.

The structure of obtained powder was determinate by X-raypowder diffraction on a Siemens D-500 XRPD diffractometer with CuKα1,2 radiation, at room temperature. Data for structural refinementwere taken in the 2θ range 10–104.56°, with the step of 0.03° andscanning time of 14 s per step. The refinement was performed withthe FullProf computer program [17–19] which adopts the Rietveldcalculation method. The TCH pseudo-Voigt profile function was used.To take instrumental broadening into account, the XRD pattern of astandard specimen CeO2 was fitted by the convolution of theexperimental TCH pseudo-Voigt function [20]. The size–strain micro-structural parameters were analyzed by the Breadth computerprogram [21,22]. The X-ray line broadening of the h00 (h=1,…, 5),0k0 (k=2,…, 8) and 00l (l=2,…, 6) reflections was analyzed. Inputdata for the Breadth program was taken from the output of theFullProf program [23].

The Raman spectra were obtained using a U-1000 (Jobin-Ivon)double monochromator in back scattering geometry. The Ramanspectra were excited by the 514 nm line of an Ar+ ion laser and takenat room temperature. Because of the strong coupling betweensensitive materials and the laser light, careful attention was paid tothe used power of illumination. Examination was made on smallparticle aggregates, a procedure preferentially used for black material[24]. In order to avoid sample heating we used a cylindrical focus andthe laser power was kept lower than 10 mW [25].

Adsorption and desorption isotherms of N2 were measured onobtained powder at −196 °C using the gravimetric McBainmethod. From the isotherms, the specific surface area, SBET, poresize distribution, mesopore including external surface area, Smeso,micropore volume, Vmic, for the samples were calculated. Pore sizedistribution was estimated by applying BJH method [26] to thedesorption branch of isotherms and mesopore surface andmicropore volume were estimated using the high resolution αs

plot method [27–29]. Micropore surface, Smic, was calculated bysubtracting Smeso from SBET.

3. Results and discussion

The X-ray diffraction studies showed that synthesized powder issingle phase α-Bi2O3. Data reported by Ivanov at al. [30] was used asstarting structural model for Rietveld refinement. Graphic result of

Fig. 1. XRD pattern after structural refinement procedure using Rietveld's method. Adifference (observed−calculated) plot is shown beneath. Tick marks above thedifference data indicate the reflection position.

Rietveld structural refinement as the best fit between calculated andobserved X-ray diffraction pattern is shown in Fig. 1.

Refined crystallographic parameters and refined positions of allatoms in structure of α-Bi2O3 are given in Table 1. Correspondingagreement factors are also given in Table 1. Reasonable values of Rfactors and χ2 are an indication of good refinement.

All atoms occupy the 4e general positions according toWyckoff's notation. The structure consists of oxygen atoms, parallelto the y and z axes, which are separated by layers of bismuthatoms. There is five-fold coordination of oxygen atoms around Bi(1) while it is six-fold around Bi(2) atoms. Bi―O bond lengths varyfrom 2.08 to 2.80 Å and that is completely in agreement withliterature data [31].

Results obtained by the Breadth program show that crystallitesize is in nanometric range, about 50 nm, and that strain size isnegligibly small. In additional, it shows that crystals are isometricbecause size distribution is almost the same in directions h00, 0k0and 00l. The distribution of crystallite size among volume-weighted-length function (pV) in h00 direction is presented in Fig. 2. Ascrystallite size decreases the distribution of pV becomes increasinglynarrow, indicating a uniform crystallite distribution for a smallercrystallite size.

Raman spectral studies conformed that obtained powder is singlephase α-Bi2O3 (Fig. 3). Raman spectra agree well with those inliterature [31–34]. Room-temperature Raman spectra show broadbends in the higher-frequency region which matches well for α-Bi2O3 [31].

Nitrogen adsorption isotherms (as the amount of adsorbed N2) as afunction of relative pressure at−196 °C, are shown in Fig. 4. Accordingto the IUPAC classification isotherms are of type-IVwith hysteresis loopwhich is associated with mesoporous materials. Specific surface areascalculated by BET equation, SBET, showed 64m2 g−1, Smeso, 53m2 g−1,Smic, 11m2 g−1 andVmic, 0.0039cm3 g−1, respectively. SBET values showthat sample has microporous surface.

Fig. 4. Nitrogen adsorption isotherms. Solid symbols — adsorption, open symbols —

desorption.

Fig. 2. Crystallite size distribution in h00 direction.

Fig. 3. Raman spectrum of α-Bi2O3.

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4. Conclusions

Monoclinic α-Bi2O3 was successfully synthesized by the self-propagating room temperature reaction, what is confirmed byRietveld refinement of X-ray data and Raman spectroscopy mea-surement. It was found that the crystallite size lies in the nanometricrange (50 nm). Synthesized powder is mesoporous material withmicroporous surface. This simple and effectivemethod can be used toprepare nanometric α-Bi2O3 in large scale.

Acknowledgement

This work was financially supported by the Ministry of Science ofthe Republic of Serbia.

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