Synthesis, characterization and photocatalytic activity

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ScienceDirect Journal of Taibah University for Science xxx (2015) xxx–xxx

Short Communication

Synthesis, characterization and photocatalytic activity of ␣-Bi2O3 nanoparticles Hicham Oudghiri-Hassani a,c , Souad Rakass a , Fahd T. Al Wadaani a , Khalaf J. Al-ghamdi a , Ahmed Omer a , Mouslim Messali a , Mostafa Abboudi a,b,∗ b

a Taibah University, College of Science, Chemistry Department, Almadinah 30002, Saudi Arabia Université Abdelmalek Essadi, Faculté des Sciences et Techniques de Tanger, Département de Génie Chimique, B.P. 416 Tangier, Morocco c Cégep de Drummondville, 960 rue Saint-Georges, Drummondville J2C 6A2, Québec, Canada

Abstract Monoclinic bismuth oxide ␣-Bi2 O3 nanoparticles were synthesized starting from a mixture of oxalate complexes of bismuth Bi(C2 O4 )OH and Bi2 (C2 O4 )3 ·xH2 O obtained by a direct solid-state reaction between a nitrate salt of bismuth and oxalic acid. The starting oxalate mixture precursors were studied by thermal gravimetric analysis (TGA) and characterized by Fourier transform infrared spectroscopy (FTIR). After heat treatment of the oxalate precursors, the obtained oxide ␣-Bi2 O3 was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). These ␣-Bi2 O3 nanoparticles show low efficiency in photodegradation under UV light irradiation of the dye rhodamine B. © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: ␣-Bi2 O3 ; Photocatalysis; Nanoparticles

1. Introduction Bismuth-based semiconductors are receiving increased attention as photocatalysts that degrade organic pollutants under UV–vis light [1–8]. In particular, bismuth oxide (Bi2 O3 ) [9–11] has a variety of desirable properties, including a high band gap ∗ Corresponding author at: Taibah University, College of Science, Chemistry Department, Almadinah 30002, Saudi Arabia. Tel.: +966 541123908; fax: +212 39393953. E-mail address: [email protected] (M. Abboudi). Peer review under responsibility of Taibah University.

(2–3.96 eV), a high refractive index and photoluminescence, and this oxide is implicated in a range of fields such as solid oxide fuel cells, gas sensors, high temperature superconductor materials, functional ceramics and catalysis [2,3]. As a consequence of the high band gap of bismuth oxide, this material produces electron–hole pairs when subjected to a beam of photons with equal or greater energy, generating free radicals that undergo secondary reactions [5]. Bismuth oxide exhibits well-known polymorphism with five modifications: ␣, ␤, ␥, ␦, and ␻-Bi2 O3 . Monoclinic ␣-Bi2 O3 and cubic ␦-Bi2 O3 are low- and high-temperature stable forms, respectively [12,3]. A growing number of methods are available for the preparation of Bi2 O3 , including precipitation, flame spray pyrolysis, sol–gel methods [2], atomization, magnetron sputtering deposition [5] and low-temperature

http://dx.doi.org/10.1016/j.jtusci.2015.01.009 1658-3655 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: H. Oudghiri-Hassani, et al. Synthesis, characterization and photocatalytic activity of ␣-Bi2 O3 nanoparticles, J. Taibah Univ. Sci. (2015), http://dx.doi.org/10.1016/j.jtusci.2015.01.009

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electrodeposition [3]. Recently, Chen et al. prepared mesh-like bismuth oxide single crystalline nanoflakes (monoclinic ␣-Bi2 O3 and tetragonal ␤-Bi2 O3 ) via a bismuth oxalate precursor and tested the photocatalytic activity of these nanoflakes using the photodegradation of rhodamine B under visible light irradiation. The study showed that the photocatalytic activity of bismuth oxide was related to its crystalline phase and morphology [13]. Other studies have shown that bismuth oxide films annealed at 550 ◦ C (containing a higher proportion of the tetragonal phase) have high photocatalytic activity in the photodegradation of rhodamine B [4]. In addition, degradation experiments using rhodamine 6G and bismuth oxide nanowires produced by magnetron sputtering deposition report a direct relationship between nanowire density and photocatalytic activity [5]. In this work, we report a photodegradation study of rhodamine B in aqueous solution using a ␣Bi2 O3 catalyst prepared via a solid-state reaction of Bi(NO3 )3 ·5H2 O and oxalic acid. The catalysts are characterized by a combination of thermogravimetric analysis (TGA), X-ray diffraction (XRD), IR spectroscopy and transmission electron microscopy (TEM).

and θ is the diffraction angle. For transmission electron microscopy (TEM), a JEOL 1400 microscope was used to study the morphology and particle size of the prepared bismuth oxide. 2.3. Photocatalytic experiments The photocatalytic experiments were carried out in a quartz cell cooled with cycled water. In a typical photocatalytic experiment, 0.1 g of Bi2 O3 was added to 250 ml of an aqueous solution of rhodamine B (rhodB) in the concentration 10 mg/L. The solution was maintained under magnetic stirring in darkness for one hour to reach rhodB adsorption–desorption equilibrium. The solution with the suspended nano-photocatalyst was irradiated by UV light from a 450 W Ace-Hanovia medium-pressure quartz mercury vapour arc lamp with a wavelength peak centred at 366 nm, and periodically, 3 ml of the solution was taken with a syringe equipped with a micro filter and analyzed in a Varian Cary 100 UV-visible spectrophotometer at a wavelength of 554 nm, corresponding to the highest absorbance by rhodB. 2.4. Synthesis of materials

2. Experimental 2.1. Reagents Bismuth (III) nitrate hexahydrate [Bi(NO3 )3 ·6H2 O] and oxalic acid dihydrate [H2 C2 O4 ·2H2 O] were purchased from Sigma–Aldrich and used as received. 2.2. Materials characterization The thermal gravimetric analysis study (TGA) was conducted on a SDT Q 600 analyser. Fourier transform infrared (FTIR) spectroscopy analysis was conducted on a Shimadzu 8400S instrument using standard KBr pellets. Powder X-ray diffraction (XRD) patterns to determine the phase purity and the structure of the end product were performed on an XRD P-6000-Shimadzu X-ray diffractometer (40 kV/20 mA) in a conventional θ–2θ reflection geometry and using Cu K␣ radiation ´˚ The Scherrer equation was used to esti(λ = 1.5406 A). mate the particles size from the XRD pattern of the as prepared nanoparticles: DXRD =

kλ β cos θ

where DXRD is the average particle diameter, λ is the used wavelength of Cu k␣, k is equal to 0.9, β is the fullwidth at half-maximum (FWHM) of the diffraction peak

In a typical procedure, a mixture of bismuth nitrate and oxalic acid in the molar ratio 1:3 was well mixed and ground in an agate mortar and pestle. Then, the mixture was placed in a large ceramic crucible and heated on a hot plate at 160 ◦ C. An oxido-reduction reaction occurred in which the nitrate anions were reduced to nitrogen monoxide (NO), which reacted with oxygen to form nitrogen dioxide (NO2 ) and produced its characteristic brown-red colour [14–16]. At the end of this reaction, we obtained an oxalate precursor. This precursor is then heated in the range of 280–500 ◦ C in a tubular furnace open from both sides where the precursor is decomposed thermally to form the final oxide. 3. Results and discussion The obtained precursor was characterized by infrared spectroscopy FTIR. Fig. 1 shows the obtained infrared spectra. The coordinated oxalate group has absorption bands at 1589, 1454, 1385, and 794 cm−1 , confirming the presence of a chelating oxalate group in the complex as previously studied by Rivenet et al. [17]. These bands are assigned to C O stretching, C O stretching, C C stretching and O C O in plane bending vibrations [18,19]. A band at 1628 cm−1 corresponding to δ(H2 O) is also observed, and this bland is clarified by deconvolution of the large peak at 1589 cm−1 [20]. A broad band



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100

20 36003200 1800

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Wavenumber (cm ) Fig. 1. Infrared spectrum of the complex precursor.

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90 85 80 75

o

Mass loss weight (%)

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70 65 0

50 100 150 200 250 300 350 400 450 500 550 600 o

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Fig. 2. Thermal gravimetric curves of the as-prepared precursors.

at 3459 cm−1 , attributed to O H vibration, is observed indicating the presence of the water molecule and the hydroxyl group. This result is confirmed by the thermal analysis study, which reproduces the results obtained by Rivenet et al. [17], where the bismuth oxalate hydroxide (Bi(C2 O4 )OH) is obtained. The hydroxyl OH group appears at a lower wave number because this group is bonded to two bismuth atoms in the molecular structure, which explains the observed shift. In Fig. 2, the thermal gravimetric curve shows a weight loss of 34% (the theoretical loss is 25%). A small mass loss is observed at 300 ◦ C (2.5%), as is observed for bismuth hydroxy oxalate. The 9% difference between the theoretical and the observed of mass loss values is due to the existence of hydrated bismuth oxalates Bi2 (C2 O4 )3 ·xH2 O formed at the same time as an impurity. Neither hydration water nor an excess of oxalic acid can cause this difference because hydration water would have evaporated and oxalic acid would have sublimed during prior

Fig. 3. XRD patterns of (a) the as-prepared bismuth oxide ␣-Bi2 O3 at 500 ◦ C and (b) the as-prepared bismuth oxide ␣-Bi2 O3 at different temperatures.

heating at 160 ◦ C on the hot plate. Thus, the unique source of the impurity should be the formation of different complexes. Conversely, the thermal decomposition of bismuth oxalate Bi2 (C2 O4 )3 ·xH2 O did not show any final small weight loss at 300 ◦ C that is characteristic of an existing hydroxyl bismuth [17]. This result permits us to confirm the existence of the mixture of Bi2 (C2 O4 )3 ·xH2 O and Bi(C2 O4 )OH. The XRD pattern of the obtained oxide after decomposition at 500 ◦ C is shown in Fig. 3a. All of the peaks are assigned and indexed in accordance with the J.C.P.D.S. file #41-1449. The observed oxide is of the ␣-Bi2 O3 variety, which crystallizes in the monoclinic system with the linear parameters a = 5.8499(3), b = 8.1698(4), and c = 7.5123(3). The angular parameters are α = 90◦ , β = 112.988(4)◦ and γ = 90◦ . This result is in agreement with the work of Rivenet et al. [17], which discusses how the different allotropic varieties are obtained depending on the temperature range used for the synthesis of bismuth oxide Bi2 O3 . To detect the phase transition, several decompositions of the precursor were conducted at different temperatures. The comparison of the XRD patterns obtained for these different compounds did not show any differences, as the same pattern is observed for different temperatures ranging from 280 to 500 ◦ C, as is shown in Fig. 3b.


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Fig. 4. TEM micrographs of the as-prepared bismuth oxide at (a) 380 ◦ C and (b) 400 ◦ C.

absence of the photocatalyst, the degradation reaction takes 16 min. In the presence of the ␣-Bi2 O3 photocatalyst prepared at 500 ◦ C, the pollutant is completely degraded after 12 min. From these curves, assuming that we have a first-order reaction, the rate constants of the reactions with and without the photocatalyst were 0.73 and 0.39 min−1 , respectively.

6.0 5.5 5.0 4.5

-Ln(C/C0)

4.0 3.5 3.0 2.5 2.0 1.5

4. Conclusion

Without Catalyst With Bi2O3

1.0 0.5 0.0 0

1

2

3

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8

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Time (min) Fig. 5. Plot of ln(C/C0 ) versus the reaction time for the photodegradation of rhodamine B. The variation of the absorbance is shown in the inset.

Crystallite size estimation was obtained using the Debye-Scherrer formula applied to the separated peak (0 0 2), and this technique yields values varying from 120 to 170 nm with the temperature of the heat treatment ranging from 280 to 500 ◦ C. A comparable value is observed in the TEM micrographs for samples prepared at 380 and 400 ◦ C shown in Fig. 4a and b, respectively, which is approximately 140 nm. However, these micrographs show an agglomerated powder where smaller, very agglomerated nanocrystallites are forming approximately 140-nm particles. Fig. 5 depicts the photocatalytic performance results. The concentration of RhodB vs time is decreasing, indicating the photocatalytic activity of the photocatalyst. In the

Nanoparticles of the ␣ variety of bismuth oxide Bi2 O3 were successfully prepared, characterized and tested in the photodegradation of rhodamine B. The synthesis of this oxide is via a precursor, a mixture of oxalate complexes Bi(C2 O4 )OH and Bi2 (C2 O4 )3 ·xH2 O. The ␣Bi2 O3 crystallization in the monoclinic structure begins at 280 ◦ C. The size of the obtained particles was in the range of 150 nm, as confirmed from the Debye Scherrer equation and observed with transmission electron microscopy. These particles are efficient in the degradation of a standard pollutant, rhodamine B, under UV radiation. An interesting topic for future work is to modify the Bi2 O3 oxide by the inclusion of elements to enhance its efficiency in the photodegradation of pollutants. Acknowledgement This work was supported financially by the Research Deanship of Taibah University under the grant no (433/795).



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